water

Review
N–TiO2 Photocatalysts: A Review of Their
Characteristics and Capacity for Emerging
Contaminants Removal
João Gomes *, João Lincho, Eva Domingues, Rosa M. Quinta-Ferreira and Rui C. Martins
CIEPQPF—Chemical Engineering Processes and Forest Products Research Center, Department of Chemical
Engineering, Faculty of Sciences and Technology, University of Coimbra, Rua Sílvio Lima, 3030-790 Coimbra,
Portugal; uc2015249034@student.uc.pt (J.L.); evadomingues@eq.uc.pt (E.D.); rosaqf@eq.uc.pt (R.M.Q.-F.);
martins@eq.uc.pt (R.C.M.)
* Correspondence: jgomes@eq.uc.pt; Tel.:+35-1239798723




Received: 2 January 2019; Accepted: 15 February 2019; Published: 21 February 2019


Abstract: Titanium dioxide is the most used photocatalyst in wastewater treatment; its semiconductor
capacity allows the indirect production of reactive oxidative species. The main drawback of the
application of TiO2 is related to its high band-gap energy. The nonmetal that is most often used as
the doping element is nitrogen, which is due to its capacity to reduce the band-gap energy at low
preparation costs. There are multiple and assorted methods of preparation. The main advantages
and disadvantages of a wide range of preparation methods were discussed in this paper. Different
sources of N were also analyzed, and their individual impact on the characteristics of N–TiO2 was
assessed. The core of this paper was focused on the large spectrum of analytical techniques to detect
modifications in the TiO2 structure from the incorporation of N. The effect of N–TiO2 co-doping was
also analyzed, as well as the main characteristics that are relevant to the performance of the catalyst,
such as its particle size, surface area, quantum size effect, crystalline phases, and the hydrophilicity
of the catalyst surface. Powder is the most used form of N–TiO2 , but the economic benefits and
applications involving continuous reactors were also analyzed with supported N–TiO2 . Moreover,
the degradation of contaminants emerging from water and wastewater using N–TiO2 and co-doped
TiO2 was also discussed.

Keywords: N–TiO2 ; preparation methods; co-doping; emerging contaminants; sunlight degradation;

catalysts characterization techniques; photocatalytic oxidation

1. Introduction
Titanium dioxide is the most widely used photocatalyst material due to its semiconductor
characteristics, which make it a remarkable option for photocatalysis applications involving solar or
other sources of radiation [1–3]. Along time, several uses were found for titanium dioxide such as
hydrogen production, photovoltaic cells, self-cleaning surfaces, air purification, anti-fogging surfaces,
adjuvant on heat transfer and dissipation, anticorrosion applications, environmentally-friendly surface
treatment, photocatalytic lithography, photochromism, etc. [4–7]. However, the application of this
material as the catalyst for wastewater treatment through heterogeneous photocatalysis gained force
in the last years [7–10].
Water scarcity is one of the major global problems of this century. The exponential growth of the
world population and global warming are the main reasons for this problem. Conventional wastewater
treatments alone are not suitable for water reclamation, since the presence of chemical and biological
emerging contaminants that are refractory to these methodologies can be detected in the effluents of
wastewater treatment plants as well as at the rivers and lakes where this treated water is disposed [11].

Water 2019, 11, 373; doi:10.3390/w11020373 www.mdpi.com/journal/water

Water 2019, 11, 373 2 of 35

In a preferred scenario, the future of water supply must go through water reuse from wastewater
treatment plants. However, focus is needed on new treatment technologies involving low investment
and operational costs to attract stakeholders. The scientific community is working on the development
of new technologies for this purpose. The use of TiO2 is an interesting alternative in these studies,
since there is the possibility of using it with sunlight radiation to promote the removal of pollutants
from water, which would reduce water treatment costs.
Titanium dioxide is an easy-to-handle material that is very chemically stable when compared to
other catalysts, and can be found at low cost. Titanium dioxide can present three different phases of
polymorphs: anatase, rutile, and brookite [12]. Anatase and rutile are the most commonly studied
phases in the photocatalytic experiments, since anatase is the most photocatalytically active, whereas
rutile is thermodynamically more stable [12,13]. Regarding brookite, few studies exist related to its
surface structure characterization [12].
Titanium dioxide is a semiconductor. Thus, a suitable source of energy can promote the transfer
of electrons from the valence band to the conduction band. The energy needed to promote the
photogeneration of electron–holes pairs is usually called a band gap. If this band gap is small enough,
solar energy can be used as the primary source of radiation to activate this kind of catalyst. Reversely,
if the band gap is high, ultra-violet (UV) radiation shall be needed. In terms of the process, when this
band gap is broken or disrupted, leading to the previously described electron transfer, the titanium
dioxide can promote the degradation of contaminants from the wastewater [1]. The photogenerated
electron–hole pair will allow the oxidation of water and/or organic matter as well as the reduction
of oxygen and other reactive oxidative species, leading to radical moieties that are able to oxidize
pollutants at the liquid bulk [2,3]. For instance, in real wastewater, some other species are present
(such as Cl− , HCO3 − , CO3 2− , and SO4 2− ), which can be reduced, producing radicals that can help
with the degradation process of organic contaminants [14].
The typical band-gap energy of titanium dioxide is between 3–3.2 eV. Thus, UVA radiation
(wavelength <400 nm) is needed for its activation. Sunlight radiation just comprises 4–5% of UVA
radiation. This means that low performance is expected when natural light is applied. Therefore, to take
advantage of the remaining spectrum of sunlight radiation, the visible region, the TiO2 band gap must
be reduced. This is usually achieved by using a suitable dopant. Doping appears as a good alternative
for changing the activity of TiO2 catalysts through the optoelectrical modification of this material by
the introduction of dopants with different energy levels between the conduction and valence band [15].
The applied dopants can be metals, such as noble and transition metals, or nonmetals (N, B, S, F, and C).
The noble metals (such as Ag, Au, Pd, and Pt) present some advantages such as the possibility to
absorb the visible light due to the surface plasmon resonance [16–18], but the high cost associated with
these materials should be considered a disadvantage. The transition metals can also enhance their
photocatalytic activity. However, their leaching behavior leads to the fast deactivation of the catalyst
and constitutes a second source of pollution, requiring the removal of dissolved metals from treated
water [3]. Therefore, nonmetals present some advantages compared to the metal dopants. Besides
the effective activation of TiO2 in the visible spectrum of sunlight due to the narrowing of the band
gap that allows avoiding the recombination phenomenon, the low cost and environmentally-friendly
character of this material appear as tempting features [19]. As nonmetal doping elements, some anionic
species such as nitrogen, carbon, sulfur, and boron have been studied due to the beneficial advantages
of substituting oxygen in a TiO2 lattice and given activity to these catalysts at visible light radiation,
maintaining its maximum efficiency [15,20,21]. Among the above-mentioned nonmetals, the most
suitable and commonly used is nitrogen. This element can introduce few states at the valence band
edge, tuning the optical band gap and acting as superficial donors [15,22]. Moreover, N instead of O in
the TiO2 lattice allows the band gap to narrow due to the 2p states of the N atom mixed with O 2p
states [23,24]. In fact, it was proved that for both the anatase and rutile phases of TiO2 , the N 2p states
were located just above the top of the O 2p valence band, which means a red shift of the absorption
band edge to the visible region [23]. Therefore, it indicates that nitrogen is the best nonmetal dopant
Water 2019, 11, 373 3 of 35

to TiO2 , improving the photocatalytic activity at the visible light radiation, which means a low-cost
energy source for the degradation of contaminants.
In this context, N–TiO2 catalysts are interesting in this field, and lots of information about them
can be found in the literature. However, data about the main changes provided by nitrogen doping or
co-doping in TiO2 is not well established and defined. In fact, to the best of our knowledge, there is
no overview on the subject. In the present study, the main objective is to critically overview the
information about N doping and co-doping changes in the characterization analysis of the typical
catalysts. Then, we can understand the advantages of using these catalysts in advanced oxidation
technologies for wastewater treatment, especially in what regards emerging contaminants removal.
Another feature that will be analyzed is the advantage between using powder and supported TiO2 for
wastewater treatment. Finally, future perspectives regarding this kind of catalyst will be discussed
considering the wastewater treatment applications.

2. Effect of the Catalyst Preparation Method

A wide range of preparation methods for doped forms of titanium dioxide can be found
in the literature (Table 1). The most common are ion implantation [25], sintering at high
temperature [20,26,27], plasma processes [28], the hydrothermal method [29–32], the oxidation
of titanium nitride [33–35], the sol–gel method [15,21,36,37] hydrolysis [4,8,38], chemical
modifications [39], and low-temperature nitridization [40]. The effect of the preparation methods on
the N–TiO2 activity for green energy applications was revised by Devi and Kavitha [41]. Thus, this
section will focus on the effect of the catalyst preparation procedure, especially for water/wastewater
treatment through the photocatalytic oxidation of contaminants.
Ion implantation consists of ion bombardment on the TiO2 powder or film. Therefore, a special
implanter will be needed which, besides the energy required for ion bombardment, can represent
a negative impact regarding the cost of the catalyst production. Ghicov et al. [25] prepared N–TiO2
nanotubes using an ion bombardment of N at the nominal dose of 1 × 1016 ions/cm2 with an
accelerating energy of 60 keV. The TiO2 nanotubes were previously prepared by anodization. In this
study, the ion implantation promotes a significant band gap decrease from 3.15 to 2.20 eV, but during
the implantation, anatase changes to amorphous phase. Thus, a calcination step was then required
(450 ◦ C for three hours) to convert it again into anatase [25].
The sintering of TiO2 powders at high temperatures in the presence of N2 and NH3 gas is
the conventional method to produce N–TiO2 . Nevertheless, the amount of nitrogen that can be
doped with this method is limited [20]. Besides, the operational conditions imply more restrictive
equipment [20,26]. Asahi et al. [26] produced N–TiO2 by sputtering the TiO2 with an N2 (40%)/Ar
gas mixture annealed at 550 ◦ C over four hours, and as comparison, pure TiO2 was obtained through
the sputtering with an O2 (20%)/Ar gas mixture at the same conditions. The high temperatures
promoted agglomeration and pore structure collapse, leading to the reduction of the active surface area.
This can decrease the performance when photocatalysis is considered. Another example of sintering at
high temperatures is the annealing method [27]. In this method, the titanium and nitrogen precursor
solids are mixed together, heated, and calcinated at the same time for three hours at 350 ◦ C. The heat
treatment can promote a change in the physical and chemical properties of the crystal, allowing the
incorporation of nitrogen into a TiO2 lattice. This method allows the production of N–TiO2 in a single
step, and different amounts of nitrogen can be used. However, it is important to evaluate the yield
of the process regarding the level of nitrogen incorporation. On the other hand, the use of NH3 may
represent an environmental issue due to air pollution through its erosive and toxic characteristics [4].
In the same way, the plasma process involves a high consumption of energy in the plasma reactor
and during the annealing of the material at high temperatures [28]. Chen et al. [28] mixed titanium
tetraisopropoxide, water vapor, and N2 stream before feeding it into a plasma reactor. This plasma
reactor has two electrodes and an electrical field of 9.6 kV/cm at atmospheric pressure, which represents
a power consumption of about nine W [28]. Afterwards, the resulting powder from the plasma process
Water 2019, 11, 373 4 of 35

was annealed at 500 ◦ C for three hours to promote the formation of the anatase phase [28]. This process
needed some special conditions such as a gas stream and a reactor with the suitable characteristics,
which can represent an important budget in terms of implantation costs. On the other hand, the amount
of catalyst produced is very dependent upon the reactor size.
The hydrothermal method implies the utilization of high temperature and pressure conditions,
requiring suitable equipment for these kinds of reactions. This process was truly believed as being
the best over other methods, since high amounts of nitrogen can be doped in TiO2 [30]. With up to
5% mol of nitrogen incorporated, in the past, it was believed that other methods would not be able
to dope such amounts of nitrogen in a TiO2 lattice [20,29]. However, Burda et al. [29], using a direct
amination of the titania particles were able to dope 8% mol of nitrogen by reducing the doping reaction
speed. Later, Peng et al. [30] produced N–TiO2 via the hydrothermal method using triethanolamine
as the N precursor and Degussa P25 as the TiO2 source. The reaction inside an autoclave at 140 ◦ C
was run over 24 h. The catalyst did not suffer significant modifications in terms of the constitution
of phases compared to the typical ones of P25 (80% anatase and 20% rutile). This study proved that
the doping can be up to 21% mol in N, corresponding to a Ti:N:O ratio of 1:0.86:2.25, respectively.
The new molecular form produced by Peng et al. [30], TiO2.25 N0.86 , disagreed with the other proposed
Nx TiO2−x structures [20]. Therefore, it was concluded that the hydrothermal method does not just
substitute oxygen atoms by nitrogen, but it can also substitute titanium by nitrogen due to the high
pressure applied with this methodology [30].
As previously described, the predominant phases of TiO2 are anatase and rutile. Normally,
the produced titania catalysts are mainly constituted by the anatase phase. Hu et al. [31] synthetized
N–TiO2 using titanium tetrachloride and urea via hydrothermal conditions at 105 ◦ C over six hours.
After this process, the ratio between the anatase and rutile phases was about 50:50. However, just 0.7%
mol of nitrogen in the titanium dioxide was obtained. The authors verified impurity levels above
the valence band, which allowed the usage of visible light irradiation [31]. In order to reduce
the high pressure and temperature costs related with the conditions of the typical hydrothermal
method, the UV-assisted thermal method was proposed as a cost-effective alternative [27]. In this
method, the UV-C light was applied to assist the synthesis of the photocatalyst. For this, an ordinary
glassware was used with a condenser coupled at normal pressure and low temperature. However,
after that, the catalyst needs to be calcinated over three hours at 350 ◦ C. Nasirian and Mehvrar [27]
noted that this method produces a N–TiO2 catalyst with a large surface area and high photocatalytic
activity. Parallelly, alternative sources of energy can be used for the preparation of catalysts, such
as a microwave. This application requires a special reactor that can be suitable to work inside of a
microwave generator [27,42]. One of the main problems of this method is the high investment cost
with the microwave reactor and the reduced amount of catalyst that can be prepared. These parameters
are dependent on the capacity of the microwave reactor. However, the time of reaction will be reduced
compared to typical hydrothermal methods [27].
Another perspective of the application of the hydrothermal method is the production of
hollow-structured spheres in one step [32]. The presence of a chelating agent to give a microsphere
structure can be advantageous in terms of the photocatalytic activity, since the decomposition of
rhodamine-B was better with those materials than with P25 powder under sunlight radiation [32].
These preparation method conditions allowed obtaining mainly the anatase phase, as was confirmed
from N–TiO2 X-Ray Diffraction (XRD) analysis [32]. Comparing to the previous study, Li et al. [32]
used a higher temperature for a longer time, which implies higher costs of production. However,
compared to other methods such as sol–gel, this preparation does not need the calcination step.
Looking for the phase transformation during the catalysts’ preparation, one that is the responsible
for the presence of anatase and rutile in the final material can be the type of titanium precursor [30,31].
The oxidation of titanium nitride (TiN) at hydrothermal conditions can represent a substantial
investment cost, since it couples the hydrothermal process with the oxidation under ultrasound
energy. Zhou et al. [35] produced N–TiO2 using TiN as a precursor and different amounts of hydrogen
Water 2019, 11, 373 5 of 35

peroxide (1 wt %, 2.5 wt %, 5 wt %, 7.5 wt %, and 10 wt %) as oxidant. For this, TiN was dispersed
into a H2 O2 solution and mixed using ultrasonification at 700 W over 10 min. After two hours of
stirring, the white solid was transferred to a Teflon-lined autoclave over 24 h at 170 ◦ C [35]. After the
hydrothermal step, the powder was washed and dried in air over 24 h at 110 ◦ C [35]. The increase
in the amount of the H2 O2 solution promoted a decrease in the N content in the catalyst. It was
concluded that the incorporation of N occurs by substitution and NO chemisorption at the catalyst
surface. However, this process presents high-energy consumption to achieve a photoactive N–TiO2 .
Zhu et al. [34] prepared a nitrogen-doped titanium dioxide thin film through the oxidation of sputtered
TiN. The amount of nitrogen doped onto thin films was easily controlled by changing the ratio of N2 on
the gas mixture that was used for the magnetron-sputtering process with a reactive direct current [34].
The sol–gel method for doped and undoped titania catalysts is the most used approach,
due to the simplicity of the process. However, different types of sol–gel can be applied with wide
changes regarding the method of application [42]. One of the major advantages is the use of low
temperature, allowing a reduction on the production costs. Moreover, it can be done without any
special equipment [15,43,44]. In this method, the conditions of preparation can be widely different,
so we will focus on a few to describe it.
Barkul et al. [15] produced N–TiO2 from titanium tert-butoxide and urea. After initial preparation
of the titanium solution, urea was added at different loadings, and the pH was adjusted to 10
with ammonia. The obtained solution was stirred over three hours at 60 ◦ C, and then was cooled.
The resulting precipitate was dispersed in distilled water and stirred again under the same conditions.
After that, the final solution was dried at 110 ◦ C and calcinated at 400 ◦ C over five hours [15].
This method presents a low aging time, but a solution to correct the pH is required. Besides,
the calcination step must be a little bit longer. Sun et al. [6] produced N–TiO2 from tetrabutyl titanate
and urea using the sol–gel method with 24 h of aging at room temperature; then, it was dried at 70 ◦ C.
A study that analyzes N–F-TiO2 preparation by the sol–gel method concluded that the preparation
conditions lead to changes in the material characteristics such as those that concern the crystallinity,
size, dispersion and band gap. Thus, this will influence the material efficiency regarding photocatalytic
degradation [45].
As previously described, the limiting step of the sol–gel process is the time needed to promote
the aging of the catalyst. This step can usually go from eight to 48 h. Therefore, ultrasounds can be
considered a suitable technology, as they are easy to operate and environmentally friendly, which will
allow the reduction of the synthesis time [19,44]. However, compared to some sol–gel methods, the use
of ultrasounds can present higher energy costs [19,44]. Ramandi et al. [19] produced N–TiO2 from
titanium butoxide and urea using ultrasound energy (20 kHz). The sonication time in a Rosset cell
totaled 15 min at 25 ◦ C. After that, the sonicated solution was placed aside to promote the formation
of gel. At the end of the process, after the gel drying, the catalyst was calcinated with different
temperatures [19]. On the other hand, Lee et al. [44] produced N–TiO2 from titanium butoxide and
urea using a modified sol–gel method with ultrasound irradiation just for the doping step. Firstly,
TiO2 powder was obtained after six hours of aging, and then, ultrasound energy (20 kHz) was applied
for 40 min. This method allows obtaining N–TiO2 without a calcination step [44].
Hydrolysis is comparable to sol–gel; the main difference is the absence of an aging step.
In the hydrolysis method, the powder is obtained instantaneously at low temperatures. Normally,
the solution was stirred for 30 min [21]. Sacco et al. [38] prepared different aqueous solutions of
ammonia to add on the titanium tetraisopropoxide at 0 ◦ C, and the solution was maintained under
stirring until the formation of a white precipitate. This precipitate is washed, centrifuged, and dried
before calcination for 30 min at 450 ◦ C [38]. Rizzo et al. [8] prepared N–TiO2 by the hydrolysis
method and different calcination times (10 min, 20 min, 30 min, and 40 min) at 450 ◦ C using titanium
tetraisopropoxide and ammonia solution (30 wt %) as titanium and ammonia sources, respectively.
After characterization and degradation experiments, it was concluded that the most active and suitable
catalyst was the one prepared using a calcination temperature of 450 ◦ C over 30 min [8]. Yuan et al. [4],
Water 2019, 11, 373 6 of 35

instead of using low temperature, added ammonia to the colloid solution obtained from the hydrolysis
of TiCl4 , where OH− neutralized the H+ that was provided from water, leading to the precipitation
of TiO2 . However, the N present in the ammonia did not work as the N precursor. To promote the
doping process, those authors added urea in agate mortar a few minutes after the centrifugal step [4].
Chemical modifications are based on the hydrolysis process, but with some modifications, since
instead of working at low temperatures [8,38], it works at low pressure [39]. Nosaka et al. [39] used
as N precursors urea, guanidine hydrochloride, and guanidine carbonate on the aqueous form with
different TiO2 powder sources. The two precursors were mixed at room temperature and were kept in
dark conditions for one day. Afterwards, the mixture was dried at vacuum conditions until obtaining a
white powder. The powder was then calcinated at different temperatures (350 ◦ C, 450 ◦ C, and 550 ◦ C)
during the period of 30 min, one hour, and five hours. After that, the calcinated powder was washed
and dried under reduced pressure [39].
In the same way, the low-temperature nitridization method is based on the incorporation of N
into the TiO2 lattice after hydrolysis during the crystallization step, and the peptization time can have
an important role on the content of the different phases. Hu et al. [40] used the low-temperature
nitridization method using triethylamine as the nitrogen source and tetrabutyltitanate dissolved in
ethanol as the titanium precursor. These precursors were stirred at room temperature for 24 h, and the
pH value was 12. At those conditions, condensation and crystallization occurred. Then, different
amounts of nitric acid (0.1 M, 0.5 M, and 1 M) were added into the solution and peptization took
place at 70 ◦ C for different periods of time (four hours, eight hours, 20 h, 36 h, and 48 h) for further
crystallization. The concentration increase of acid and peptization times enhanced the rutile phase
production. The authors noted that the formation of N–Ti-O bonds is related with the occurrence of
nitridization before crystallization. The peptization time of 20 h and nitric acid concentration of 0.5 M
provided the most active photocatalyst on the methylene blue decolorization using UV radiation or
visible light [40].
As it can be seen, the methods of doped catalyst preparation can present some advantages or
disadvantages in terms of photoactivity under visible light for the degradation of organic compounds.
One of the main conclusions that should be withdrawn from this section is that the temperature during
the preparation has real impact in terms of phase development. The catalysts prepared without high
temperature or calcination present, in general, anatase as the predominant phase. Exception goes to
the materials prepared by the hydrothermal process, since the high pressures can also influence the
phase production. Table 1 summarizes some of the pros and cons for each preparation method, as well
as the main operating conditions applied in each work for obtaining N–TiO2 .

Table 1. Advantages and disadvantages of N–TiO2 preparation methods.

Ref. Method Advantages Disadvantages

-Needs to be washed;
-Centrifugation step;
-Fast production;
[4] Hydrolysis -Drying at 200 ◦ C;
-Does not need low temperature;
-Calcination step (350–700 ◦ C);
-Agate mortar usage to promote the doping;
-Fast production; -Low temperature (4 ◦ C) for 30 min;
-Diethanolamine allows anatase -Calcination step (800 ◦ C for 30 min).
[21] Hydrolysis production; -Triethylamine and urea produced rutile, which
-Band gap decreases from 3.16 to decrease the photoactivity;
2.85 eV for diethanolamine. -Drying at 100 ◦ C for 90 min.
-Fast production; -Low temperature (0 ◦ C);
-Anatase production; -Needs to be washed;
[8,38] Hydrolysis
-Significant reduction of band gap (3.3 -Centrifugation step;
to 2.5 eV); -Calcination step (450 ◦ C for 30 min).
Sol–gel to TiO2 after
-Aging step at room temperature; -Two steps;
[44] ultrasound irradiation
-Absence calcination step. -Aging step time (six h);
for doping
Water 2019, 11, 373 7 of 35

Table 1. Cont.

Ref. Method Advantages Disadvantages

Sol–gel to TiO2 after
-Aging step at room temperature; -Two steps;
[44] ultrasound irradiation
-Absence calcination step. -Aging step time (six h);
for doping
-Rosset cell needed;
-Ultrasound energy (20 kHz);
Sol–gel method coupled -Room temperature can be used;
-Calcination step (three hours at 300 ◦ C, 400 ◦ C,
[19] with ultrasound -Low time of aging (15 min);
and 500 ◦ C);
treatment -Reduced production time;
-Two hours after sonication to obtain a gel;
-Drying step (10 h at 80 ◦ C)
-Aging time (24 h);
-Aging at room temperature; -Drying step (70 ◦ C);
[6] Sol–gel method
-Low band gap (2.58 eV); -Calcination step (400 ◦ C, 500 ◦ C, 600 ◦ C, and
700 ◦ C).
-Aging temperature (60 ◦ C);
-Needed additional step (stirring three hours at
-Low time of aging (three hours);
60 ◦ C);
[15] Sol–gel method -High band-gap reduction from 3.21 to
-Calcination step (five hours at 400 ◦ C);
2.34 for the 7% mol of N doping;
-pH correction to basic conditions;
-Drying step (110 ◦ C).
-Two steps of vacuum drying;
-Needs to be washed (H2 SO4 /H2 O);
Chemical modifications
[39] -Reduced time of production; -Calcination step (30 min, one hour, and five
(such as hydrolysis)
hours at 350 ◦ C, 450 ◦ C, and 550 ◦ C);
-High costs with vacuum;
Sintering at high -High-temperature process (550 ◦ C);
[26] -Significant reduction of the band gap
temperatures -Gas streams needed;
-Autoclave;
-Allows a great amount of N
-High pressure and temperatures;
[30] Hydrothermal process introduction;
-Reaction time (24 h at 140 ◦ C);
-21% of N concentration;
-Drying step (10 h at 200 ◦ C).
-Stainless steel autoclave (30 mL);
-Reduced time of production;
-High pressure and temperatures;
-Low particle size (five nm);
[31] Hydrothermal process -Reaction time (six hours at 105 ◦ C);
-Produced rutile amount without any
-Drying step under vacuum conditions;
high temperature treatment;
-Calcination step (10 h at 500 ◦ C).
-Drying step (12 h at 60 ◦ C);
- Teflon-lined autoclave (50 mL);
-Does not need a calcination step; -High pressure and temperatures;
[32] Hydrothermal method
-Hollow spheres production; -Reaction time (eight hours at 180 ◦ C);
-Chelating agent (ethylenediaminetetraacetic
acid, or EDTA);
-Plasma reactor;
[28] Plasma process -Calcination at high temperatures;
-High voltage for reactor;
-Ion implanter;
-Reduction of band gap from 3.15 eV
[25] Ion implantation -Energy costs;
to 2.2 eV;
-Calcination step;
-Hydrothermal step for 24 h at 170 ◦ C;
Oxidation of titanium -Presence of hydrogen peroxide;
[35] -N-substituitional;
nitride -Ultrasonification energy (700 W);
-Need to be washed and dried (24 h at 110 ◦ C)
-Different proportions of the rutile
Low temperature of -Higher times for peptization (20 h);
[40] and anatase phase;
nitridization -Temperature of 70 ◦ C for peptization;
-Absence of calcination step;

3. Type of Nitrogen Source

The nitrogen source should be selected according to the precursor for titanium dioxide. These
precursors can change the catalytic properties and photoactivity of the catalysts [15,21,45]. Also,
the method chosen for the production of catalysts is very dependent upon the nitrogen and titanium
dioxide precursors. For example, the most common nitrogen precursors for the sol–gel method are
ammonia hydroxide, urea, diethanolamine, and triethylamine [21,46]. Ananpattarachai et al. [21]
studied three different types of nitrogen precursors for TiO2 doping—urea, diethanolamine,
and triethylamine—using the sol–gel method and titanium tetraisopropoxide as the titanium source.
Water 2019, 11, 373 8 of 35

For all the catalysts produced, the incorporation of nitrogen into the TiO2 lattice was interstitial. From
the different N precursors studied, the most active under visible light for 2-chlorophenol degradation
was diethanolamine. This dopant led to the smallest band gap and the smallest anatase crystal size,
which will increase the surface area of the catalyst, and consequently the availability of the active
sites [21]. In fact, for the materials prepared using other dopant precursors, the band gap was higher,
as well as the crystal size. Moreover, the N–TiO2 that is produced presents some fraction of the rutile
phase, which decreases the photocatalytic activity [21]. The use of primary and secondary amines
seems to be able to provide a more suitable N–TiO2 due to the structure and the accessibility of nitrogen
atoms to react with the titanium precursor [21]. Moreover, triethylamine can prevent the aggregation
of N–TiO2 particles, assuring the good dispersibility of the grains [40].
On the other hand, the ammonium hydrogen carbonate can also be used as an N precursor,
as described in the work by Mekprasart and Pecharapa [47], where N–TiO2 was produced from
ammonium hydrogen carbonate over P25 (Aeroxide) nanopowders with nitrogen doping amounts
of 10 wt %, 30 wt %, and 50 wt %. The authors indicate that this precursor is a good option, but also
concluded that the crystallite size was reduced compared to P25 due to the presence of hydroxyl
carbonate onto the N–TiO2 lattice. Moreover, an absorption spectrum at the UV region instead of
the visible region was verified for the highest (50 wt %) and lowest (10 wt %) amount of nitrogen.
The authors explained that for 10 wt %, this can be related to the low availability of nitrogen atoms
and for 50 wt %, the reason was related to the difficulty of decomposing such a huge amount of
precursor to combine with TiO2 . Two other reasons—the method selected to produce this catalyst and
the precursor used—can support these results, since other precursors with a higher amount of nitrogen
promoted significant differences on the absorption spectrum, pushing it to the visible region compared
to undoped TiO2 [30,48].
Anther nitrogen source is 1,3-diaminopropane. Nolan et al. [13] produced N–TiO2 using the
sol–gel method from the titanium tetraisopropoxide and 1,3-diaminopropane as the titanium and
nitrogen precursors, respectively. For the best calcination temperature (500 ◦ C), where the predominant
phase was anatase, N–TiO2 revealed greater photocatalytic activity using visible light when compared
to Degussa P25. The presence of diaminopropane promoted the formation of a monomeric titanium
amine structure which through hydrogen bonding can interact with the neighbor’s titanium amines.
However, these monomeric structures with little or no cross-linking are weak, and will easily collapse
during the calcination, promoting the formation of the rutile phase [13].

4. Effect of Nitrogen Incorporation

The incorporation or not of nonmetals such as nitrogen can produce significant changes to the
properties of TiO2 , which will affect its photocatalytic activity under visible or sunlight radiation [12].
One of the major doubts in this kind of catalyst preparation is related to how N is really incorporated
into TiO2 [48–50]. There are some controversial opinions regarding whether nitrogen is really in
the TiO2 lattice. In fact, related to nitrogen structure incorporation, some authors have argued
that N doping into the TiO2 structure can be interstitial or substitutional [49,50]. This subject can
be discussed based on the variations in the diffraction peaks in the XRD pattern, UV-Vis Diffuse
Reflectance Spectroscopy (DRS) spectra, Raman spectra, Fourier-Transform infrared spectroscopy
(FT-IR), photoluminescence, and X-Ray photoelectron spectroscopy (XPS) analysis compared to pure
TiO2 . The incorporation of nitrogen will change TiO2 photoactivity mechanisms. Zaleska [48] gave
three main explanations for the photoactivity enhancement to visible light with the incorporation of N:
the band gap narrowing, the creation of an impurity energy level, or even oxygen vacancies.
The best way to characterize the incorporation of nitrogen into the TiO2 lattice is the compilation
of the maximum number of characterization analytical technologies. However, sometimes, it is difficult
to gather such a huge amount of data. Therefore, the application of visible light in the photocatalytic
degradation of organic compounds can be also helpful to understand if the nitrogen doping was well
achieved. In fact, the way that N is incorporated will strongly influence the catalyst band gap [12].
Water 2019, 11, 373 9 of 35

As it can be seen in Figure 1, the band-gap narrowing can occur, which will facilitate the visible light
activity due to the introduction of intrabands resulting from the mixing of the 2p states of oxygen from
TiO2 with the 2p states of the dopant [51]. Another important feature is the temperature during the
preparation
Water 2019, 11,of this PEER
x FOR kindREVIEW
of catalysts [52]. 9 of 35

Figure 1. Explanation for the band-gap reduction for (a) N–TiO2 when compared with undoped
Figure 1. Explanation for the band-gap reduction for (a) N–TiO2 when compared with undoped (b)
(b) TiO2 .
TiO2.
The nitrogen presence on the TiO2 structure will affect the material phase’s crystallinity.
The nitrogen
For instance, a typicalpresence
anataseon XRDthepattern
TiO2 structure
will suffer willsomeaffect the material
deviations. One phase’s crystallinity.
sign of high crystallinityFor
is the presence of just one phase on the XRD diffractograms. The amorphous phase of TiO2 does notis
instance, a typical anatase XRD pattern will suffer some deviations. One sign of high crystallinity
the presence
present of just one
photocatalytic phase
activity. on can
This the be XRD diffractograms.
improved The amorphous
by the calcination phase which
of the powder, of TiO2promotes
does not
present photocatalytic activity. This can be improved by the calcination
the formation of the crystalline phase. This inactivity of the amorphous phase can be related to of the powder, which
the
promotes the formation of the crystalline phase.
recombination between photogenerated electron–hole pairs [53,54]. This inactivity of the amorphous phase can be related
to the recombination
During betweendifferent
nitrogen doping, photogenerated
temperatures electron–hole pairs can
of calcination [53,54].
be used. Thus, this can affect
During nitrogen doping, different temperatures of
the crystallinity of the final material [6,13]. Nolan et al. [13] verified that calcination canthebepresence
used. Thus, this can affect
of N enhanced the
the crystallinity of the final material [6,13]. Nolan et al. [13] verified that
transformation of anatase into rutile. During pure TiO2 production using a calcination temperature of the presence of N enhanced
the◦ C,
600 transformation of anatase
only anatase remains. into rutile.
The increase During pure
of calcination TiO2 production
temperature from 600 ◦ Cusingto 700 a◦ Ccalcination
promotes
a significant transformation into the rutile phase in both catalysts, but this is higher from
temperature of 600 °C, only anatase remains. The increase of calcination temperature in the600 °Cofto
case
700 °C .promotes
N–TiO a significant transformation into the rutile phase in both catalysts, but this is higher
2 In fact, while the increase for N–TiO2 was from 46% to 94%, for pure TiO2 , the transformation
in the case of N–TiO
was from 0% to 81% 2[13]. . In fact,
Sunwhile
et al. the
[6] increase
verified thatfor N–TiO
with the 2 was from 46% to 94%, for pure TiO
temperature increase from 500 ◦ C 2, the
to
transformation
◦ was from 0% to 81% [13]. Sun et al. [6] verified
◦ that
600 C, rutile formation began (about 7.4%), and from 600 C to 700 C, the amount of rutile phase ◦ with the temperature increase
from 500 until
increases °C to 73.2%,
600 °C,becoming
rutile formation began (about
the predominant 7.4%),Moreover,
phase. and from the 600 crystallite
°C to 700 °C, sizethe amount
also suffersof
rutile phase increases until 73.2%, becoming the predominant phase. Moreover,
from modifications due to the calcination temperature. Sun et al. [6] verified a sharpening related to an the crystallite size
also suffers
increase of thefrom modifications
crystalline size forduetheto the calcination
most temperature.
significant diffraction Sunofetthe
peak al. anatase
[6] verified
phase a sharpening
(1 0 1).
related to an increase of the crystalline size for the most significant diffraction
The presence of N onto the TiO2 structure will promote a peak broadening in XRD diffractograms peak of the anatase
phase (1 0to1).
compared pure TiO2 , which indicates a lower crystallite size of nanoparticles [44]. On the other hand,
The presence
Barkul et al. [15] verified of N an onto
increasetheinTiO structuresize
the2crystallite willafterpromote
nitrogena doping,
peak broadening
and concluded in that
XRD
diffractograms compared to pure TiO 2, which indicates a lower crystallite size of nanoparticles [44].
the increasing of the dopant load to at least 7% mol promoted higher crystallite sizes. This increase of
Oncrystalline
the the other hand,
size was Barkul et al. [15]
attributed to the verified
higheran increase
intensity of in
thethe crystallite
most sizepeak
significant after(1nitrogen doping,
0 1). Therefore,
and
no concluded
consensus can that
bethe increasing
found relativeoftothe thedopant
effect of load
thetopresence
at least 7% mol promoted
of nitrogen on thehigher crystallite
crystallite size.
sizes. This increase of the crystalline size was attributed
The crystallite size (D) is normally determined through the Scherrer equation: to the higher intensity of the most significant
peak (1 0 1). Therefore, no consensus can be found relative to the effect of the presence of nitrogen on
the crystallite size. The crystallite size (D) is Dnormally
= kλ/βcosθ determined through the Scherrer equation:(1)
D = kλ/βcosθ (1)
where k is a shape factor that has a typical value between 0.89–0.94 [6,15], λ is the X-ray wavelength,
βwhere k is awidth
is the full shapeatfactor that has apeak
half maximum typical
(in value between
radians), and θ 0.89–0.94 [6,15], λangle
is the diffraction is theatX-ray
whichwavelength,
(1 0 1) and
(1β 1is0)the full width
intensity at half
peaks appearmaximum
[6]. Thispeak
can (in radians),
be one and θ isfor
explanation the diffraction
these angleinatthe
differences which (1 0 1)
crystallite
sizes, since the k value can be attributed, and this equation just allows an estimate value forinthis
and (1 1 0) intensity peaks appear [6]. This can be one explanation for these differences the
crystallite sizes, since the k value can be attributed, and this equation just allows an estimate value
for this parameter. Another explanation is the criteria used in these studies to describe the crystallite
size: one focused on the peak broadening, and the other on the peak intensity. In order to obtain a
complete and more correct approach, the crystallite size must be also analyzed through High-
Resolution scanning electron microscope (HR-SEM) or High-Resolution transmission electron
microscope (HR-TEM. These two technologies can give more information about the correct crystallite
Water 2019, 11, 373 10 of 35

parameter. Another explanation is the criteria used in these studies to describe the crystallite size: one
focused on the peak broadening, and the other on the peak intensity. In order to obtain a complete and
more correct approach, the crystallite size must be also analyzed through High-Resolution scanning
electron microscope (HR-SEM) or High-Resolution transmission electron microscope (HR-TEM. These
two technologies can give more information about the correct crystallite size of nitrogen-doped
titanium dioxide particles.
Other changes are related to the shift of diffraction peaks compared to the diffractogram of
undoped TiO2 . Lee et al. [44] verified that a shift of 0.3◦ on the higher diffraction peak (1 0 1) can be
related to a substitution of oxygen from the TiO2 lattice by a nitrogen atom. This is possible due to the
atomic radius of nitrogen, which is higher than oxygen. Ata et al. [55] verified the same shift on this
diffraction peak for the sol–gel method of N–TiO2 preparation. The authors noted the typical angle
from XRD analysis for anatase as 25.53◦ , which is a value that decreases with the doping to 25.25◦ .
The same reason as the abovementioned was considered.
The presence of nitrogen with different loads does not change the phases from the pure TiO2 [15].
Barkul et al. [15] concluded that the formation of patterns of nitrogen precursors does not occur.
This can be due to the proper substitution of a few sites of oxygen by nitrogen and the low amount of
nitrogen usually incorporated [15,56] by the preparation methods.
According to Katoueizadeh et al. [57], the addition of N into the TiO2 structure decreased the
crystallization temperature. The authors verified that for the same conditions (100 ◦ C during the
sol–gel preparation method), the addition of triethylamine as the N precursor enhanced the formation
of the crystalline phase.
Yuan et al. [4] studied different molar ratios of urea/TiO2 (as N dopant)—1:2, 1:1, 3:1, and 5:1—and
with different calcination temperatures. The XRD patterns obtained for ratios lower than three was
mainly the anatase phase and for ratios 3:1 and 5:1, the phase changes totally to amorphous using the
calcination temperature of 350 ◦ C. The authors noted that such a huge amount of urea can block the
growth of the crystalline phase during the reaction process. However, the increase of the calcination
temperature to 500 ◦ C promoted anatase transformation from the amorphous phase considering the
molar ratio of 3:1 and converting it to rutile for temperatures above 600 ◦ C [4]. In the same way,
Li et al. [32] verified that the increase in the amount of ethylenediamine as the N precursor promoted
the broadening and weakness of XRD peaks, which indicated that N can decrease the crystallinity
to enhance the amorphous phase formation. The most significant change was observed in the TiO2
FT-IR spectrum when N is incorporated, which can be due to the presence of bands related to the
N–Ti bond vibrations. Normally, these bands can suggest the incorporation of nitrogen into the TiO2
lattice. The typical bands are 1450 cm−1 , 1365 cm−1 , 1250 cm−1 , and 1080 cm−1 [44,58,59]. These
bands can be found at 1420 cm−1 , 1270 cm−1 , and 1170 cm−1 [32,60]. Moreover, a sharp tip observed
at 455 cm−1 can correspond to the O–Ti–O bonding in the anatase phase of TiO2 [61,62]. However,
the previous typical bands of pure TiO2 can suffer a shift, which can indicate the presence of N–Ti–N
or O–Ti–N [61,63]. Etacheri et al. [61], while preparing N–TiO2 using ethylenediaminetetraacetic
acid (EDTA) as the nitrogen precursor, verified a shift from 456 cm−1 (undoped TiO2 ) to 508 cm−1 .
The preparation method was sol–gel and the calcination temperature was 400 ◦ C. The shift was
attributed to the incorporation of N into the TiO2 lattice. In the same way, Nolan et al. [13] noted
the 508 cm−1 peak as a sign of O substitution by N. In this case, the calcination temperatures were
600 ◦ C and 700 ◦ C. Whereas, the authors verified that such a shift did not occur when the calcination
temperature was lowered to 500 ◦ C. Thus, Ti–N was not produced at those conditions. This can be
due to the different methods or the different nitrogen sources that were used as precursors. On the
other hand, sometimes, if the production occurs at room temperature or if the calcination is not well
performed, some precursor residues can remain adsorbed on the catalyst surface. These compounds
can also be detected in the FT-IR spectrum. For instance, Yang et al. [58], to produce visible-light
active N–TiO2 using ethylenediamine as the N precursor, verified the presence on the FT-IR spectra
of a vibration band at 1390 cm−1 , which can be related to NO3 being surface-absorbed. Moreover,
Water 2019, 11, 373 11 of 35

Nolan et al. [13] using the 1,3-diaminopropane as the nitrogen precursor attributed the signals at
1612 cm−1 and 1503 cm−1 to NH2 vibrations. That data was collected before the catalyst calcination,
and the authors noted that this occurs due to the chelating of nitrogen into the titanium metal center.
The calcination temperature appears to be responsible for the removal of organic residues
adsorbed on the catalyst surface. However, some authors have noted that high calcination temperatures
can promote the elimination of N onto TiO2 . Through FT-IR analysis, Li et al. [64] proposed a structure
change with the increase of the calcination temperature from 300 ◦ C to 600 ◦ C. At 550 ◦ C, most of the
nitrogen was removed, and for 600 ◦ C, the N was totally depleted [64]. According to the XPS analysis
mentioned below, at higher calcination temperatures (600 ◦ C), it is still possible to identify interstitial
and substitutional N incorporation [4,13]. However, these differences can be related to the type of N
precursor that is used.
Other typical bands can appear in this kind of analysis at 1634 cm−1 and 3367 cm−1 . These
are related to the bending and stretching vibrations of hydroxyl groups, respectively [13,15]. These
vibrations can be associated with the physiosorbed water on the catalyst surface [13,15].
Barkul et al. [15], using two different loads of nitrogen (1% mol and 7% mol), concluded that
the high doping amount increased the transmittance (%) due to the higher surface hydroxylation,
and noted the presence of hydroxyl groups as an advantage regarding the efficient photodegradation
of dye molecules.
XPS analysis allows understanding whether the doped nitrogen was incorporated into the TiO2
structure as substitutional or interstitial [21,50]. The typical binding energy peaks from N 1s are in the
range of 396–404 eV for N–TiO2 [50,58,65]. The peaks observed at binding energies between 396–397 eV
were attributed to substitutional nitrogen, while peaks between 400–406 eV were related to interstitial
nitrogen [13,21,49,50].
Hu et al. [31] verified for the N-doped TiO2 that the N 1s peak was about 399.5 eV. This peak is in
accordance with the interstitial nitrogen introduction, since it is near the typical value of 400 eV [50].
In fact, 399.5 eV is significantly above the binding energy corresponding to substitutional N–Ti–N,
which is in the range of 396 to 397 eV [49]. Moreover, if nitrogen replaces oxygen, the N 1s binding
energy in O–Ti–N is higher than that of N–Ti–N, since the electron density will be reduced [31].
Valentin et al. [50] analyzed the way that N can dope TiO2 and how it reduces the catalyst’s band gap.
With the interstitial inclusion of N, orbitals associated to the binding energy of 401 eV can appear
(O–Ti–N) above the valence band, which will work as support for excited electrons between the valence
and conduction bands, allowing visible light absorption [50]. Moreover, Ananpattarachai et al. [21],
using three different dopants and the sol–gel method, verified that the incorporation of nitrogen into
the TiO2 lattice was interstitial for all of them, with N 1s peaks at the binding energies of 402.5 eV,
406.1 eV, and 402.5 eV for triethylamine, diethanolamine, and urea, respectively. The N 1s peaks
attributed to 402.5 eV and 406.1 eV have been considered for nitric oxide (NO) and nitrite (NO2− ),
respectively [50,66].
Nolan et al. [13] verified the effect of the calcination temperature on the N 1s XPS spectra.
The increase of temperature from 500 ◦ C to 600 ◦ C did not promote significant changes on the main
peak of the XPS spectra for the binding energy of 401 eV, which indicates the interstitial placing
of O–Ti–N [13]. On the other hand, Yuan et al. [4] concluded that the increase on the calcination
temperature from 350 ◦ C to 600 ◦ C promotes the disappearance of the interstitial peak (401 eV), which
means that the high temperatures favored the substitutional introduction of nitrogen. In fact, for
the calcination temperature of 350 ◦ C, XPS analysis revealed that the high intensity of the interstitial
peak was attributed to absorbed N2 and the small intensity was related to the substitutional peak
(396 eV) [4]. Li et al. [64] verified that when ethylenediamine is the N precursor, and 500 ◦ C is used as
the calcination temperature, N incorporation is both substitutional (398.3 eV) and interstitial (406.5 eV).
The amount of dopant used in the preparation of the catalyst can also have a relevant impact on
the doping processes. The low amount of nitrogen may not be enough to present modifications on
Water 2019, 11, 373 12 of 35

the XPS analysis of N 1s levels. The ratio of urea/TiO2 was analyzed in this ambit, and for the lowest
value tested (0.1), no N 1s peaks were observed [4].
XPS analysis can also allow the determination of the real content of nitrogen onto the TiO2 lattice.
During the doping processes, different amounts of nitrogen and different ratios between the dopant
and the titanium source can be used, but it does not mean that the yield is total, which means that the
real amount of dopant should be determined for a better comparison. However, this information is not
commonly given.
Yuan et al. [4] verified that for an urea/TiO2 ratio of three, using the calcination temperature
of 350 ◦ C over two hours, the doped nitrogen amount was about 20 wt %, and with the increase
of the calcination temperature to 500 ◦ C and 600 ◦ C, it is reduced to 2.2 wt % and 0.3 wt %,
respectively. Nolan et al. [13] verified the same reduction of N content with the increase of the
calcination temperature from 400 ◦ C to 600 ◦ C, which promoted a reduction of nitrogen percentage
from 2.09% to 0.14%.
The precursor selected for nitrogen has effects in the real amount that can be incorporated in
the N–TiO2 . Ananpattarachai et al. [21] concluded that from the initial molar ratio of 1:1 of N:Ti,
the percentage of nitrogen content was 4.1%, 0.6%, and 5% when using diethanolamine, triethylamine
and urea, respectively. This result is related to the type of amine that is used in the dopant process,
since the primary and secondary amines present more N content than the tertiary amine.
A typical undoped TiO2 is active at UV radiation regions below 400 nm. Whereas, typically,
catalysts that are well-doped with nitrogen show the maximum absorption for wavelengths above
400 nm. The doped catalyst must show the strongest absorption edge at 400-nm to 530-nm wavelengths
to be considered a good photoactive option to be used in the photocatalytic degradation under sunlight
radiation [6,67,68]. The energy gap, which is the typical energy of the band gap, is determined by the
equation Eg = 1239.8/λ, where λ is the wavelength (nm) of the absorption edge [6,68]. Asahi et al. [26]
proposed the N–TiO2 preparation through TiO2 anatase treating under NH3 (67%)/Ar atmosphere at
600 ◦ C for three hours. The authors verified for this catalyst visible light absorption when λ < 500 nm.
Sun et al. [6], using the sol–gel method to dope tetrabutyltitanate with urea, achieved a band gap
of 2.58 eV, which represents a significant reduction compared to pure TiO2 (3.02 eV). In the same
way, Rizzo et al. [8], using the sol–gel method with titanium tetraisopropoxide and ammonia solution
(30 wt %), observed a significant reduction of the band gap from 3.3 eV to 2.5 eV. Moreover, the effect
of the addition of different Ti:N ratios can influence the catalyst absorption capacity under visible light
and decrease the band gap energy as a consequence. Pérez et al. [69] for the N–TiO2 prepared by the
sol–gel method showed that the increase of the theoretical molar ratio of Ti:N from 2:1 to 1:2 enhanced
the absorption of light to higher wavelengths. The increase of ratios until 1:2 promoted a decrease on
the band gap from 3.19 to 2.51 eV. However, for the molar ratio of 1:1, two different absorption edges
appear, where the first corresponds to the undoped fraction, and the second one corresponds to the
presence of N onto TiO2 [69]. In fact, Chainorong et al. [70] also identified both UV-Vis absorption
edges for N–TiO2 prepared by the hydrothermal method. The first edge was associated with the oxide
presence (390–400 nm), and the second one, which was weaker than the first one, was related to the N
bonding on the TiO2 lattice [70].
The crystalline phases can have an important role in the visible light absorption due to the
wideness of the band gap. Anatase presents a higher band gap compared to rutile. So, rutile is almost
able to absorb the visible light [2,3]. As mentioned above, the incorporation of nitrogen into the anatase
phase can promote the reduction of the band gap due to the presence of orbitals between the valence
and conduction bands [50]. However, the incorporation of N into the rutile crystalline phase promotes
the wideness of this band gap [13,50]. The incorporation of N 2p levels reduces the absorption of pure
rutile, since it presents a lower energy than the rutile valence band, and the top of the valence band for
N-doped rutile was lower than that for pure rutile [50].
Li et al. [64] concluded that the absorption of N–TiO2 was much higher than the pure TiO2 on
the visible region due to the presence of nitrogen. This suggests a rearrangement of the energy levels
Water 2019, 11, 373 13 of 35

of TiO2 . Hu et al. [31] showed distinct shifts of the absorption bands in the visible light. This can
indicate the presence of nitrogen in the TiO2 lattice. In the same way, Nolan et al. [13] verified that
for the calcination temperature of 500 ◦ C, it was possible to achieve 100% anatase and a material with
high absorption under visible light, which indicates the successful incorporation of N into the TiO2
lattice. However, in this study, the application of high temperatures of calcination (above 600 ◦ C) led
to materials with no visible light absorption. This can be related to the band-gap expansion or the
nitrogen leaving the TiO2 lattice at those harsh conditions [13]. Besides the calcination temperature,
another feature can be relevant to the absorption of visible light or the reduction of the band gap: the
calcination time. Rizzo et al. [8] used different times of calcination at 450 ◦ C, and concluded that the
time increase from 10 to 30 min led to the band gap decreasing. Contrarily, when the calcination time
reached 40 min, the band-gap energy increased again. Therefore, the time of calcination can be an
important parameter to enhance the incorporation of nitrogen atoms into the TiO2 lattice and control
the band-gap energy of the catalyst.
Yuan et al. [4] revealed that, using an urea/TiO2 ratio of 3:1, an increase in the temperature
of calcination promoted the material absorption edge for lower wavelengths. The increase of the
calcination temperature may lead to nitrogen desorption or replacement by oxygen onto the TiO2
lattice [4]. The above-mentioned XRD analysis revealed that an increase of the urea/TiO2 ratio
decreased the crystallinity of the TiO2 , transforming the anatase into an amorphous phase. However,
in terms of the absorption edge, the urea/TiO2 ratio increase allowed the use of higher wavelengths [4]
to activate the catalyst. The type of introduction of nitrogen onto the TiO2 lattice seems to not affect the
band gap. Yuan et al. [4] concluded that the insertion and substitution of nitrogen, which was replaced
in the TiO2 lattice, could both contribute to the visible light absorption of the catalyst.
The typical Raman bands of the anatase phase appear at 144 cm−1 , 197 cm−1 , 399 cm−1 , 515 cm−1 ,
519 cm−1 , and 639 cm−1 , whereas those related to the rutile phase have been detected at 235 cm−1 ,
447 cm−1 , and 612 cm−1 [8,31,71,72]. Hu et al. [31] verified shifts on the typical bands to 149 cm−1 ,
402 cm−1 , 521 cm−1 , 615 cm−1 , and 642 cm−1 for the anatase and rutile phases (50:50 content).
However, there are some deviations in the literature regarding the characteristic wave numbers
for those crystalline phases in Raman spectra. Rizzo et al. [8] noted that the typical anatase peaks
were the ones detected at 141 cm−1 , 194 cm−1 , 394 cm−1 , 515 cm−1 , and 636 cm−1 . These values
present some deviation from those previously described. Therefore, in order for Raman analysis
to be a good technique for analyzing N doping into TiO2 , it will be important to compare N–TiO2
Raman spectra with that obtained for pure TiO2 produced by the same method. The quantum size
effect of nanoparticles of N–TiO2 can promote slight shifts on the Raman band [73]. This is the most
typical response found in the literature that mentioned the incorporation of N into a TiO2 lattice.
Pérez et al. [69] analyzed the Raman spectra for the different molar ratios of Ti:N for N–TiO2 produced
by the hydrothermal method. These authors concluded that for 2:1 and 1:1, the shift of the peaks
to lower values correspond to the incorporation of N into the TiO2 lattice [69]. On the other hand,
Rizzo et al. [8] verified a shift from the typical signal of anatase from 141 cm−1 to 144 cm−1 when
characterizing N–TiO2 , and related this to the change of the oxygen stoichiometry due to the presence
of nitrogen. Some controversy can be found regarding this characterization. In fact, the shift of typical
peaks to the lower or higher values may be a sign of N incorporation in the TiO2 lattice [8,31,69,74,75].
N incorporation in TiO2 can reduce the electron–hole recombination effect, which improves its
photoactivity. In fact, Lee et al. [44] verified that the undoped TiO2 presents a higher intensity of
photoluminescence compared to N–TiO2 . The Raman peaks can also help identify the crystal size of
doped TiO2 , working as complement of the XRD analysis. Kassahun et al. [75] verified a decrease in
the intensity of the 144 cm−1 peaks of N–TiO2 compared to pure TiO2 , which means a lower crystal
size in accordance to the XRD analysis.
Since in the literature, the way that N is incorporated into TiO2 is controversial, there is the need
to combine the characterization data (as discussed in this section) from several analytical techniques
to assure the success of the catalyst preparation method. Besides, in the wastewater treatment field,
Water 2019, 11, 373 14 of 35

the best way to select a catalyst preparation process should rely on the screening of several materials by
testing their efficiency and stability on the photocatalytic degradation of organic contaminants under
visible/sunlight irradiation. These data should complement the characterization of the materials.
In this context, Table 2 shows some typical modifications that can occur with the incorporation
of nitrogen in the TiO2 structure through several analytical methodologies that are used for the
characterization of catalysts.

Table 2. Main changes identified through different characterization techniques due to N incorporation
in TiO2 .

Analytical Technique Main Characteristics Reference

-N enhances anatase transformation into rutile
[13]
-N promoted 0.3◦ shift on (1 0 1) diffraction peak
[55]
XRD -No N diffraction peak is detected
[15,56]
-N precursor and N:TiO2 ratio have complex relation
[4,25,32,43]
with crystallinity
-TiO2 typical bands: 1450 cm−1 , 1365 cm−1 ,
1250 cm−1 , 1080 cm−1
-N–TiO2 typical bands: 1420 cm−1 , 1270 cm−1 , [32,43,58–60]
FT-IR 1170 cm−1 (suggest N in TiO2 lattice) [61–63]
-455 cm−1 : O–Ti–O (anatase) can be shifted due to [61]
N–Ti–N and O–Ti–N
-TiO2 : 456 cm−1 shifts to 508 cm−1 (N–TiO2 )
-Binding energy: 396–397 eV (substitutional N) and
XPS [13,21,48,50,58,65]
400–406 eV (interstitial N)
-TiO2 absorption λ ≤ 400 nm
[6,67]
-N–TiO2 absorption 400 ≤ λ ≤ 500 nm
UV-Vis DRS [69]
-N:TiO2 ratio affects band gap
[8,31,52,76,77]
-Calcination temperature and time affects band gap
Shift of the typical TiO2 bands to lower values
Raman [8,31,69,74]
related to N incorporation into lattice
Photoluminescence N–TiO2 shows lower photoluminescence than TiO2 [44]

5. Effect of N–TiO2 Co-Doping

N–TiO2 shows, as described above, some advantages regarding band-gap reduction, which would
allow the utilization of visible light and/or sunlight radiation in photocatalytic degradation processes.
The presence of other dopants can place a mid-gap between the valence and conduction band, which
permits the reduction of the typical band gap of TiO2 [78,79]. On the other hand, some dopants are
not able to reduce the band-gap width [45]. For example, fluorine does not change the TiO2 band
gap, since the p orbitals of F present lower energy than those from oxygen atoms [45]. However,
F shows higher electronegativity when compared to O, which will allow a better trapping of electrons,
promoting the separation of electron–holes pairs and minimizing the recombination phenomena.
This may enhance photocatalytic degradation [45,80]. Moreover, the synergetic effect of different
ions from two or more nonmetals can enhance the separation of photogenerated holes and electrons,
reducing the recombination phenomena [19,59]. In fact, some works have even reported some cases
of tri-doped materials [19,62]. In this case, the efficiency improvement at visible light should be
related with the increase of the surface acidity, band-gap narrowing, and the reduction of electron–hole
recombination [62,81,82].
The effect of the co-doping process can be analyzed in terms of the degradation of contaminants,
since sometimes, no significant differences are found in the characterization of the catalyst compared
to N–TiO2 . Thus, normally, in order to understand the real effect of co-doping, N–TiO2 should be
compared with the co-doped catalysts regarding their performance related to the removal of pollutants.
N–TiO2 , S–TiO2 , and N, S–TiO2 were successfully prepared using the sol–gel method at room
temperature for the degradation of methylene blue and phenol [59]. Syafiuddin et al. [59] verified that
Water 2019, 11, 373 15 of 35

N, S–TiO2 was the best catalyst using the visible light for methylene blue and phenol degradation after
six hours. Besides the higher photoactivity, another reason for the best performance can be related
with the high specific surface area of this catalyst [59].
N, S co-doped TiO2 can be very often found, since during the catalyst preparation method,
thiourea (a source of both S and N) is generally used as a precursor. This can also present advantages in
terms of band-gap reduction [12,76], allowing the best photocatalytic performance when using visible
light. Brindha and Sivakumar [79] found that the N, S co-doped TiO2 prepared by the hydrothermal
method verified no significant changes on the XRD patterns when compared to the anatase phase of
TiO2 . In the same way, Li et al. [32] verified that the incorporation of N and S onto the TiO2 structure
using thiourea as an N and S source does not promote modifications on the crystalline phase of
pure TiO2 .
Sun et al. [6] co-doped N–TiO2 with different amounts of Ce (0.3% mol, 0.6% mol, and 1.2% mol)
using cerium nitrate as the precursor. The initial change in the N–TiO2 is the disappearance of the
rutile phase with the presence of Ce. In fact, Ce has an inhibitory activity for the transition from the
anatase phase to the rutile phase [6]. The radii of Ce4+ or Ce3+ (0.093 nm and 0.103 nm, respectively)
are larger than that for Ti4+ (0.064 nm), which will promote lattice distortion and expansion when
Ce is in the TiO2 lattice. Moreover, a reduction of the crystallite size is also verified for increasing Ce
amounts [6]. In fact, the increase of the cerium amount on N–TiO2 reduces the crystalline size, since the
most significant peak of anatase suffers from broadening [6]. The same response was verified with the
presence of nitrogen compared to pure TiO2 [44]. Shen et al. [83] verified the inhibition of the anatase
transition to the rutile phase during the calcination of N, Fe–TiO2 produced via the sol–gel method.
Although N–TiO2 calcinated at a temperature of 600 ◦ C reveals a fraction transition of the anatase
phase to the rutile phase, only the anatase phase is detected for N, Fe–TiO2 , even when calcinated at
higher temperature. Thus, the presence of the co-dopant inhibits phase transition. Nkambule et al. [84]
prepared N, Pd–TiO2 by the sol–gel method using ammonium hydroxide and palladium diamine
dichloride as precursors of N and Pd, respectively. The presence of a PdO peak at the 2θ value of
33.8◦ was verified through XRD. Moreover, sharp and intense anatase peaks revealed crystallinity
improvement. On the other hand, the typical anatase peaks from Raman analysis do not suffer changes,
which can be related to the low amount of dopant used [84].
FT-IR analysis is important to verify whether the co-doping occurs at the lattice, establishing
bonds with Ti or O. The presence of S on the N–TiO2 lattice can be identified by the peak detected
at 1055 cm−1 , which can be attributed to the Ti–O–S bond [79,85]. On the other hand, the peak at
1217 cm−1 can be attributed to the surface-adsorbed SO4 2− , which is the result of some unreacted
precursor [59].
XPS analysis enables drawing conclusions about the chemical states of the different species
considered in the N co-doped TiO2 . Sun et al. [6] analyzed the peaks of Ce4+ and Ce3+ for different
states of Ce such as Ce3d5/2 and Ce3d3/2 . For Ce3+ , the typical binding energy peaks for Ce3d5/2 and
Ce3d3/2 can be detected at 885.3 eV and 904.1 eV [86]. Whereas for Ce4+ , the binding energy peaks
for Ce3d5/2 and Ce3d3/2 can be found at 881.9 eV and 898.2 eV [86]. After the photocatalytic reaction,
the peak area of Ce3+ was reduced significantly, which means that the majority of these ions were
oxidized to Ce4+ and worked as hole traps, minimizing the recombination, and therefore allowing
the enhancement of the photocatalytic activity for the degradation of contaminants [6]. Nevertheless,
this conversion does not mean a reduction of the amount of Ce ions; on the contrary, in this study,
the Ce amount remains almost the same, which means that this co-doped catalyst presented a good
stability [6] in what is regarded as the Ce-leaching effect. Sun et al. [6] also analyzed the binding
energies of N 1s states for this co-doped catalyst. The peaks were found for 369.9 eV, 399 eV, 400.3 eV,
and 402.4 eV. The signal of 396.9 eV is normally related to the substitution of O by N atoms, ascribing
to Ti–N bonds [6,26]. These results are in accordance with the FT-IR analysis for the Ce, N–TiO2 ,
as described above [6]. Moreover, the peak at the binding energy of 399 eV corresponds to the O–Ti–N.
Thus, this is related to the interstitial introduction of N onto the TiO2 lattice [6,87]. This peak (399 eV)
Water 2019, 11, 373 16 of 35

also presents the highest intensity of N 1s due to the lowest electron density around N, since the
presence of oxygen, which has a higher electronegativity, will attract the electrons more intensively [6].
On the other hand, the remaining peaks (400.3 eV and 402.4 eV) can be related to the accidental
presence of N atoms onto the surface of the samples, which could represent the N–N, N–O, or NHx
bonds [6,88].
Guo et al. [45] produced N, F–TiO2 by three different sol–gel methods with changes to the
operating conditions, and analyzed the XPS spectra for the N 1s and F 1s states. For all the methods,
the N 1s has its main peak at the binding energy of 396 eV, which suggests that N incorporation
occurs via the substitutional way with the formation of O–Ti–N bonds. In the case of F 1s, the binding
energy is located between 681.1–680.4 eV, which indicates a substitutional placement of F onto the TiO2
lattice [45]. Moreover, the three different methods do not promote significant changes on the binding
energy peaks, but rather can influence the amount of N and F that is doped. Guo et al. [45] verified
an increase in the N and F doping doses when using the first method compared with the two other
methods, as described above. N, Fe–TiO2 presents peaks for N 1s states at binding energies of 399.7 eV
and 396.7 eV, which means that the incorporation of N onto the TiO2 lattice occurs via interstitial and
substitutional mechanisms [83]. On the other hand, Fe was not incorporated into the TiO2 lattice, since
the peak of Fe 2p was about 710.6 eV, and corresponded to Fe 2p3/2 for Fe2 O3 , which means that the
ferric nitrate was converted into iron (III) oxide during the calcination [83]. Nkambule et al. [84] used
XPS to analyze the incorporation of Pd and N onto the TiO2 lattice. According to the authors, the N 1s
state was not observed due to the low amount of N used, which might have been below the detection
limit. Alternatively, it is even possible that Pd can be at the N defect sites, blocking the detection. The
peaks of Pd 3d5/2 (335.6 eV) and 3d3/2 (341.6 eV) reveal that Pd can be in metallic Pd and/or PdO
form (in accordance with the XRD analysis). Besides, the Pd 3s signal (671.2 eV) indicates a small
amount of Pd in the catalyst. Therefore, the authors cannot assure whether the Pd is interstitially or
substitutionally placed onto the catalyst lattice.
UV-Vis DRS should be analyzed in order to understand the modifications imposed by the
co-doping in the light absorption capacity. Using this method, Li et al. [32] compared the absorption of
N–TiO2 and N, S–TiO2 , revealing that the absorption at the visible region was higher for the N–TiO2
catalyst. The presence of S reduced the absorption edge, which means a lower reduction of the band
gap in the N, S–TiO2 . One of the explanations for this can be that S belongs to the same group as O.
This substitution of O by S on the TiO2 lattice may be preferred when compared to N replacement.
Thus, S will compete with N for O replacement on the TiO2 lattice. Moreover, N presents a more
similar atomic radius with O (65 pm and 60 pm, respectively), unlike S (100 pm), which will inhibit the
substitution of nitrogen on the TiO2 lattice.
Sun et al. [6] used Ce as the co-dopant of N–TiO2 , and verified a significant reduction of the
band gap compared to pure TiO2 . In fact, the band-gap energy decayed from 3.02 eV (pure TiO2 ) to
2.52 eV (Ce, N–TiO2 ). Still, the difference is not so high compared with the band-gap energy of N–TiO2 ,
in which the value was 2.58 eV. The substitution of O by N to promote the formation of Ti–N (nitride)
and the synergistic effect between the 4f orbital of Ce were identified as the main factors responsible for
the visible region absorption enhancement in this catalyst [6]. Shen et al. [83] verified that the presence
of Fe as the co-dopant does not interfere on the N–TiO2 absorption spectra. In fact, the band gap of the
co-doped materials was 2.22 eV, while for the N-doped materials, it was 2.24 eV. Nkambule et al. [84]
observed that the presence of N and Pd onto TiO2 promotes the decrease of the band gap from 3.15 eV
to 2.48 eV. However, the authors did not analyze the impact of N and Pd separately, which means that
the band-gap reduction can be related to the presence of N onto the TiO2 lattice.
As it happens for N–TiO2 , the use of a co-dopant may lead to changes in terms of the material
Raman spectrum. The typical changes are the shifting of the common Raman peaks of TiO2 . Brindha
and Sivakumar [79] verified Raman peaks shifts from 519 to 513 cm−1 , 399 to 396 cm−1 , and 197 to
194 cm−1 for N, S co-doped TiO2 prepared by the hydrothermal method. The authors related this
change to N, S-doping, but did not discuss the roles of N and S on this perturbation. As noted before,
Water 2019, 11, 373 17 of 35

N also promotes a shift on TiO2 Raman peaks, but for higher instead of lower values [31]. Thus,
the shift for lower values can be due to the presence of S on N–TiO2 .
The co-doping with noble metals (Pd, Pt, Au, and Ag) can enhance the photocatalytic activity, since
these metals have the capacity to absorb the visible light due to the surface plasmon resonance [16].
Moreover, Pt onto the TiO2 lattice acts as an electron sink from the TiO2 conduction band, facilitating
electron transfer to adsorbed oxygen [89]. Nkambule et al. [84] verified that Pd co-doped onto N–TiO2
enhances the photocatalytic oxidation of natural organic matter from water. Jurek et al. [90] prepared
N–TiO2 co-doped with Pt using a sol–gel method with different atomic ratios of Ti:N (1:1, 1:2, 1:4, 1:10).
For this, N–TiO2 was firstly prepared from urea and titanium tetraisopropoxide using different Ti:N
ratios. Then, K2 PtCl4 was added and stirred with the N–TiO2 suspension for 12 h, followed by further
calcination at 400 ◦ C for one hour. The increase of the Ti:N ratio to 1:4 promoted the decrease of the
band gap from 3.1 eV to 3.05 eV. Nevertheless, for higher ratios, no significant modifications were
detected. On the other hand, the addition of Pt promoted a decrease of the band gap to 2.95 eV for a
Ti:N ratio of 1:1. The further increase in the Ti:N ratio increased the band gap [86]. The incorporation
of N onto TiO2 was interstitial, due to the chemisorbed species such as N2 or NO. This was determined
by XPS analysis. However, many studies can be found in the literature where authors have noted that
higher photocatalytic activity can be achieved in the absence of substitutional N incorporation, but
just only with the interstitial placement [39,91]. The higher photoactivity of Pt–N–TiO2 was related
to the easier electron transfer from the conduction band to oxygen, which enhanced the superoxide
generation [90].
Shen et al. [83] concluded through data from multiple analytical techniques that in the preparation
of N, Fe–TiO2 , N was incorporated in the TiO2 , while Fe was adsorbed on the catalyst surface. Despite
this, the authors verified the positive effect of co-doping into methyl orange conversion, and concluded
that the presence of Fe on the TiO2 surface enhanced its photocatalytic activity, since it reduces the
electron–hole recombination phenomenon and increases the photo quantum efficiency. Teh et al. [92]
produced co-doped N, Cl–TiO2 via a sonochemical sol–gel method. The authors verified the effect of
the N, Cl:Ti ratio (from 0.5 to 4), and concluded that the increase on the N, Cl load until two promotes
the decolorization of Reactive Black Five. According to the authors, this improvement is specifically
related to the increase on the Cl amount onto the TiO2 lattice, since chlorine enhances the surface
acidity [92]. Moreover, for N, Cl:Ti ratios from 2:1 to 4:1, a decrease of the decolorization rate was
observed. The pointed reason was related to the high load of N, which can decrease the number of
active sites onto the catalyst surface [92]. Nevertheless, these authors never analyzed the doping effect
of N and Cl onto TiO2 separately to understand the real impact of each dopant on the decolorization,
as well as the catalyst characterization.

6. Relevant Parameters of N–Doped TiO2 Photoactivity

Several studies have indicated that the most representative characteristics for the definition of a
good N–TiO2 semiconductor catalyst is the reduced particle size, a good dispersion of nitrogen onto
the structural geometry of a TiO2 lattice, a high surface area, the quantum size effect, crystalline phases,
and the hydrophilicity of the catalyst surface [31,44]. These parameters are typically evaluated using
XRD analysis.
As mentioned above, the doped nitrogen can be regarded as interstitial or substitutional according
to the N position on the crystalline structure. Peng et al. [30] verified that the interstitial N–TiO2 is
more visible light-active than the substitutional N–TiO2 .
A quantum size effect for particles smaller than 10 nm promotes the band-gap extension, which
would allow a better exploitation of the potential of photogenerated electron–hole pairs [31,93]. The
increasing of active sites on the surface can occur through the reduction of the particle size or an
increase of the specific surface area [31,94]. The crystallinity can be improved with the calcination
temperature and with other preparation steps such as the use of ultrasound irradiation [43,95–97].
Water 2019, 11, 373 18 of 35

A hydrothermal method, using urea as the nitrogen precursor and titanium tetrachloride as
the titanium dioxide source, can produce a well-dispersed N–TiO2 with ultrafine particle sizes
(about five nm) [31]. This catalyst has also the same amount of anatase and rutile phases as a
particularity, since anatase is normally the predominant phase [32]. This ultrafine catalyst revealed
greater performance than the commercial P25 on the decolorization of a reactive Brilliant Blue KN–R
aqueous solution under visible light and UV light using a 110-W high-pressure sodium lamp and a
125-W high-pressure mercury lamp, respectively. According to the results, this high performance is
related with N doping and the usage of PEG-4000, which improves the dispersion of TiO2 particles [31].
Another important feature that can also be related with the good results of N-doped TiO2 is the
hydrophilicity of its surface [5,44,98]. The surface hydrophilicity makes the photocatalytic activity
under visible light irradiation possible [20,44,58,99]. In order to improve the hydrophilicity of the
catalyst surface, some different methods can be used, such as UV radiation, visible light radiation,
plasma treatments, and acid treatments [5,44,98]. As an example, visible light radiation was analyzed
to test the potential of this parameter on the photocatalytic activity enhancement. A sol–gel method
process aided by ultrasound irradiation was used to prepare a nanoporous N–TiO2 catalyst (with
titanium butoxide and urea as the TiO2 and N precursors, respectively). After preparation and drying,
the N–TiO2 was irradiated for one hour using a solar simulator equipped with a 150-W Xe lamp [44].
Regarding catalyst characterization, no significant changes were verified regarding the XRD pattern
and crystallite size when comparing N–TiO2 with and without visible light radiation. However,
a significant change was verified on the surface energies obtained through the contact angle data of
test liquids (deionized water, ethylene glycol, and n-hexane) using the extended Fowkes’ equation
to transform the data. The calculations reveal that the surface free energy of N–TiO2 radiated with
visible light was 154.71 mJ/m2 , while for unirradiated N–TiO2 , this value was significantly lower
(91.47 mJ/m2 ). This difference is related to the formation of surface hydroxyl groups during this
process [44]. Another aspect is the low intensity of photoluminescence analysis, since the high amount
of hydroxyl groups at the surface will accept more holes to form hydroxyl radicals and minimize the
recombination of electron–hole pairs [44]. Therefore, the high photocatalytic activity of this hydrophilic
catalyst is correlated with the larger amount of hydroxyl groups at the catalyst surface.
Powder catalysts are still widely used in photocatalytic processes to reduce mass transference
limitations, which is how they also increase the reactors’ performance. However, powder dispersion
inside the reactors should be as perfect as possible to take advantage of the reactor volume so that the
photocatalytic process performance for the contaminant’s degradation may be maximized. Moreover,
these small particles, which typically have large surface areas, can agglomerate due to the attraction
of Van der Waals forces coupled with electrostatic and steric stabilization, since the adsorbed organic
substance at the nanoparticle surface can attract other nanoparticles [100,101]. In fact, other factors,
such as ions, pH, and the amount of catalyst, may lead to modifications at the material surface and
change the stability of the nanoparticles. The aggregated size that can occur at these specific conditions
can promote a reduction in the photocatalytic efficiency. Indeed, besides leading to the reduction of the
active surface area, it can also promote light scattering and consequently reduce light propagation [100].
Some authors have noted the aggregate size as the factor that is most responsible for the reduction
of the degradation ability of a catalyst [8,38,100]. Vaiano et al. [100] analyzed the trimodal aggregate
distribution, with aggregate sizes ranging between 0–3000 µm, and compared the effect with that of
the presence of an organic species working as a dispersing agent. This dispersant changed the size
distribution to monomodal, ranging from 0.1 to 2 µm. The authors verified that at the presence of the
dispersant agent (10 ppm), N–TiO2 (3 g/L after 120 min using visible light irradiation), it was possible
to reach a better methylene blue degradation (80%) when compared to the results obtained when no
dispersant was applied (50%) [100]. The amount of catalyst can have a relevant impact on the aggregate
size. Vaiano et al. [100] verified that the increase of the catalyst amount from 3 to 6 g/L, using the
same amount of dispersant agent (10 ppm), promoted a decrease on the methylene blue decolorization.
In fact, the aggregate size increased with the increasing catalyst concentration. Moreover, after four
Water 2019, 11, 373 19 of 35

cycles of 15 min of catalyst reuse at the presence of the dispersant agent, no efficiency reduction was
observed on the methylene blue decolorization using visible light irradiation [100]. Therefore, another
feature that was taken in consideration in these processes is the amount of catalyst for the degradation
of photocatalytic contaminants. The optimal concentration must consider the beneficial ratio between
amount and efficiency. This is also substantially important regarding light scattering. Another way of
minimizing the aggregation of nanoparticles can be the use of carbon nanotubes and graphene over the
TiO2 structure due to the strong and intimate interaction between titania and carbon materials [20,102].
The use of ultrasounds during the preparation of catalysts through the sol–gel method can
influence the formation, growth, and crystallization of the anatase phase through the increase or
decrease of cavitation activity [92,103,104]. The power and time of ultrasonication were analyzed by
Nepollian et al. [103]. These authors verified that the increase of the ultrasonication time from 30 to
90 min decreased the anatase phase, and thus increased the rutile and brookite phases. Alongside
these effects, the crystalline size decreases as well, which promotes an increase of the catalyst-specific
surface area. However, 120 min of sonication seems to have the opposite effect. The increase of
the power density from 18 to 42 W/L endorses a decrease of the anatase fraction to form rutile and
brookite, while also increasing the crystalline size and decreasing the specific surface area. Moreover,
the changes promoted on these parameters are intimately related with the degradation efficiency of
4-chlorophenol. The increase of the ultrasonication time (up to 90 min) and the power density both
enhance the degradation of 4-chlorophenol [103].
Teh et al. [92] used co-doped N, Cl–TiO2 to verify the effect of a wide range of preparation
parameters such as the presence or absence of ultrasonication on the decolorization of Reactive Black
Five. The preparation time and power density increase the decolorization capacity up to a certain point.
The presence of ultrasonication during the sol–gel method seems to enhance the catalyst capacity of
visible light absorption when compared with the sol–gel preparation involving stirring. This can be
related to the higher capacity of ultrasonication to recover more N and Cl onto the TiO2 lattice due to
the production of hotspots from the collapse of cavitational bubbles during sonication [92]. Moreover,
the surface acidity of the catalyst can be a relevant parameter for this type of material, depending
on the type of organic molecule tested [105,106]. High surface acidity can increase the photocatalytic
activity for the polar molecules, since it enhances their adsorption on the catalyst surface, which will
favor their oxidation [92,106].
Teh et al. [92] verified that the catalyst calcination temperature had a large impact on the
decolorization of Reactive Black Five when ultrasonication was used as the catalytic material in
the co-preparation method. Increasing the calcination temperature from 200 ◦ C to 600 ◦ C promotes a
significant decrease of the decolorization efficiency. This could mean that the Cl in interstitial positions
on the TiO2 lattice disappear when the temperature increases. Moreover, for stirred prepared samples,
the decolorization efficiency increases when the calcination temperature increases from 200 ◦ C to
400 ◦ C [92], which means that the incorporation of N and Cl onto the TiO2 lattice occurs. This confirms
that the stirring method without calcination was not able to introduce the Cl onto the TiO2 lattice;
meanwhile, for ultrasonication, the placement of Cl would be interstitial, and therefore was removed
as the temperature increased.

7. More than Powder

One of the main questions when investigating potential industrial applications for this kind
of catalyst is related to its recovery and reuse. When using powder materials, this will imply
expensive particle-separation processes after treatment. Therefore, considering the recovery of
the catalyst, in the literature, studies can be found that are related to the immobilization of
doped TiO2 on suitable supports or even TiO2 nanostructures, such as nanotubes arrays supported
on titanium foils [106–109]. The immobilization of doped TiO2 can occur in different types of
substrates such as hollow glass spheres [110], reactor walls [111], membrane reactors [112], synthetic
polymers [55,113,114], and graphene [79]. The most used methods for supporting titanium dioxide
Water 2019, 11, 373 20 of 35

particles on a suitable substrate is dip coating (or the sol–gel method) [115], chemical vapor
deposition [116], the hydrothermal method [117], and the solvent-casting method [55]. This kind
of catalyst can present advantages for continuous processes, which will allow the industrial application
of photocatalytic processes, and reduce the implementation of separation processes and thus operating
costs. However, in terms of efficiency, decay may occur when compared to the powder forms of TiO2 .
In fact, the number of active sites will be reduced compared to powder, since the support will cover
only one side of the TiO2 surface.
The above analysis characterized the dopant presence, and can help understand whether
the application of the support was successful or not. This means analyzing whether or not the
photocatalytic characteristics were lost during the preparation of the support. As noted, the support
will generally lead to an efficiency decrease compared to unsupported N–TiO2 . One way to keep
the supported catalyst very active is using higher amounts of powder to totally cover the support.
Ata el al. [55] applied a solvent-cast method to support N–TiO2 on a polystyrene surface after using a
sol–gel method to prepare the doped titanium dioxide, which involved applying ammonia as the N
precursor. In this study, significant changes on the Raman and UV-Vis DRS spectra were verified upon
the presence of a polystyrene support, when considering a low of amount of N–TiO2 near the support.
For example, the polystyrene has Raman typical bands at 998 cm−1 , 1026 cm−1 , and 1599 cm−1 ,
which suffer a decrease in their intensity upon using 0.015 g of N–TiO2 . However, this decrease
disappears when using higher loads of catalyst (0.05 g, 0.1 g, 0.2 g, and 0.3 g) on the support [55].
This indicates that the support surface was totally covered by the N–TiO2 particles for those powder
loads. Horovitz et al. [112], using a membrane coating, also verified differences with the amount of
N–TiO2 that is coated. The authors used a porous commercial Al2 O3 membrane and coated 78% to
84% of its surface with N–TiO2 , which enabled the use of sunlight radiation for the photocatalytic
degradation of carbamazepine. Surface characterization revealed that the incorporation of N into the
coated structure was mainly interstitial, and the maximum atomic N content that was obtained was
0.9%. This coating was performed using a pipette drop-coating method, which promoted an about
50% reduction in the membrane permeability for the 200-nm pore size. However, this placement of
N–TiO2 inside the pores can present advantages regarding the contact of the contaminants, in which
the active sites increase the photocatalytic degradation [112]. To improve the mass transfer onto the
catalyst surface inside the membrane pores, the authors proposed a water flux and/or residence time
increase [112]. However, an increase in water flux can be a problem due to the limitations of the
membrane, and the residence time can be a problem when treating high-flow rates, since big reservoirs
would be required to avoid the threat of excess volume.
The need to increase the amount of powder to upload onto the support is a drawback of these
kinds of processes, since the support is usually a very porous material, and will need a great amount of
powder so that the supported catalyst can be active for contaminants degradation under visible light.
In this way, the amount of N–TiO2 that is used can be higher compared to the amount needed for the
degradation of contaminants when it is used in powder form. However, Ata et al. [55] verified for the
same amount (0.2 g) of N–TiO2 that the presence of the support did not influence the decolorization
of methylene blue (MB) when compared to powder N–TiO2 . This means that the support did not
introduce diffusional limitations to the process, which is an advantage. The reuse capacity of this
supported catalyst should also be analyzed as an important feature, since the main advantage of
this kind of catalyst is avoiding the processes that separate particles from liquid after treatment.
The number of cycles should be carefully selected to prove the stability of the supported catalyst.
Besides, the possibility of N–TiO2 leaching from the support must be considered, as well as the
leaching of nitrogen from titanium dioxide nanoparticles, if the doping was not well established.
Ata et al. [55] verified a slow decrease in the efficiency of the decolorization process after four cycles
of reuse, and noted that this decrease was a consequence of the weakening of the dye absorption
ability. Therefore, the reuse stability should be analyzed for a higher number of cycles. Moreover,
the changes to the characteristics of the catalyst after use must also be assessed. For instance, besides
Water 2019, 11, 373 21 of 35

understanding the effect of some contaminants or the degradation of dyes, it will be important to
characterize the used catalyst regarding UV-Vis DRS, since changes in the catalyst absorption spectrum
can occur after use. Another way to detect the activity of this kind of catalyst is to study their effect on
the degradation of contaminants. This way, it is possible to understand whether the support inhibits
the photocatalytic degradation of doped TiO2 more than the powder. Ata et al. [55] verified the effect
of different amounts of N–TiO2 (0.015 g, 0.05 g, 0.1 g, 0.2 g, and 0.3 g) on the support regarding the
photocatalytic decolorization of methylene blue (MB) under visible light with three eight-W lamps
over three hours, and compared it with an empty support. The profiles of degradation were enhanced
as the mass of the N–TiO2 that was used increased. The empty support did not promote a significant
decolorization of MB. Otherwise, the supported catalysts prepared with 0.015 g of N–TiO2 presented
the worst result (about 50% decolorization), which can be related to the low amount of N–TiO2 at the
surface of the support. This is in accordance with the characterization of catalysts through Raman and
UV-Vis DRS analysis [55], since, as shown before, the characteristic peaks of the support were still
detected for this load. Brindha and Sivakumar [79] detected a significant reduction of the efficiency
of the decolorization of dyes as the amount of graphene that was used as a support of N, S–TiO2
increased (7.5% and 10%). This was related to the masking of the active surface of TiO2 , which inhibits
the active sites. This is also in accordance with the results of UV-Vis DRS, in which it was possible
to observe an increase of the supported catalyst band gap when a higher amount of graphene was
applied [79].
Moreover, the supporting procedure is an additional step that involves costs. The supporting
method, as well as the support material, should be selected in accordance with the application.
For instance, most of the polymers that can be used for supporting N–TiO2 did not work with
photocatalytic ozonation. Ozone can destroy the polymer, leading to the decomposition of the catalyst
and the contamination of water. Therefore, the catalyst application technology should be taken into
consideration when the support is selected.
Alternative methods can be found to address the problems regarding the preparation and
stability of the polymeric-supported N–TiO2 . TiO2 nanotubes resulting from the anodization of
Ti foils is one example. In this preparation method, Ti foils work as anode, and platinum mesh
works as cathode [108,109]. Then, these foils can be used in continuous processes, thus avoiding a
posterior separation technique. However, a major drawback of TiO2 nanotubes is their large band
gap, which typically requires UVA radiation, and does not enable suitable activity under visible
light or even using sunlight radiation. One possibility is doping it with nitrogen in one or two
steps. The synthesis by two steps consists firstly of the production of structures of TiO2 nanotubes,
and then, as a second step, the incorporation of nitrogen by annealing the nanotubes with ammonia
at high temperatures [116,117]. Also, the hydrothermal treatment of TiO2 nanotubes may be a way
of incorporating N [118]. Alternatively, with one-step synthesis, nitrogen is incorporated during
the anodization method, which is used in the production of nanotubes [7,108,109,119]. In this case,
the production of nitrogen-doped TiO2 nanotubes is based on the presence of urea, ammonium nitrate,
and hydroxide; alternatively, they are produced by the anodization of TiN alloy [7,119] coupled with
the electrolyte. However, the most common nitrogen precursor is urea, due to the low cost of its
effective preparation method [108]. Antony et al. [7] produced N–TiO2 nanotubes using different
amounts of urea coupled with an electrolyte containing NH4 F, water, and ethylene glycol. The authors
observed a slight shift in the positions of XRD peaks for values higher than 2θ, indicating the lattice
deformation of TiO2 due to nitrogen doping. Moreover, the XPS analysis for the N–1s state indicated
the existence of different N environments. This means that nitrogen was incorporated into the lattice
by both interstitial and substitutional methods. However, the authors noted that the substitutional
method is predominant [7]. In this study, the catalyst activity under visible light was not tested, but
the effect of different nitrogen loads on the optical band gap were analyzed, which decreased while
increasing the N concentration [7]. Maziesrki et al. [108] produced N–TiO2 in a similar way, and
verified some changes on the electronic properties of the material. Besides, the authors also studied the
Water 2019, 11, 373 22 of 35

effect of this kind of catalyst on phenol degradation under visible light irradiation. In terms of catalyst
characterization, the increase of N concentration (0.1%, 0.3%, and 0.5%) onto N–TiO2 nanotubes
promoted the decrease of the band gap from 3.0 to 2.75 eV.
Other authors have supported TiO2 with graphene and carbon nanotubes [20,79,102].
These supports can also present advantages regarding charge separation in the semiconductor
catalysts [20,102]. For example, graphene sheets can work as electron acceptors, thus reducing
electron–hole recombination [120]. Moreover, the reduced graphene oxide can offer the possibility of
using the hydrothermal method to incorporate N and S onto its lattice using thiourea as the precursor.
This increases its photocatalytic efficiency using visible light [79]. In terms of catalyst characterization,
this kind of material can present significant differences compared to the typical behavior for TiO2 .
For example, Brindha and Sivakumar [79] verified the presence of graphene on the Raman and
FT-IR spectra through SEM analysis. Therefore, the characterization of supported N–TiO2 should be
compared with an analysis of the support alone, as well as those observed for other supports. The main
limitation or problem that can be found is when one or more of the typical analyses and spectrums
of TiO2 coincide with the support profiles, and relevant conclusions cannot be taken. Typical band
peaks of Raman spectra for graphene can be detected at 1351 cm−1 and 1593 cm−1 , but these peaks’
intensity increased when higher amounts of graphene were applied for the preparation of N, S–TiO2
graphene-supported catalysts [79]. On the other hand, Sacco et al. [121] immobilized N–TiO2 over the
polystyrene spheres, and verified that one polystyrene peak of Raman spectra was coincident with the
peak of N–TiO2 . However, since the aim of these catalysts is the photodegradation of contaminants,
their photocatalytic performance and the possibility of recovery and reuse should be considered the
main target when comparing different catalytic materials. In this way, Brindha and Sivakumar [79]
studied the photocatalytic degradation of Congo red, methylene blue, and reactive orange 16 using
N, S co-doped TiO2 supported on different amounts of graphene sheets (2.5%, 5%, 7.5%, and 10%).
The most photoactive catalyst under visible light was the N, S–TiO2 onto 5% of graphene, which was
able to achieve decolorization rates of 93%, 94%, and 96% for Congo red (in 50 min), methylene blue
(in 120 min), and reactive orange 16 (in 120 min), respectively. Moreover, after four cycles of reuse,
the same efficiency regarding the decolorization of dyes was obtained, and no changes were detected
regarding the crystalline phase [79], proving the stability of the catalyst. Still, results regarding the
long-term operation of continuous reactors for wastewater treatment are lacking in the literature. Only
in this way may reliable data be acquired regarding the stability of catalysts, which may boost their
full-scale application. Another factor must be considered when comparing powder with supported
catalysts: the mass of powder cannot be compared with the mass of the supported catalyst that is used.
Instead, the best method of comparison is to consider the mass of the support as negligible, and just
contemplate the amount of supported powder [79]. However, in the case of nanotubes rising from the
Ti foil, the specific surface area or even the tube diameter and length can be compared [108,109].
Some studies can be found in the literature regarding supporting TiO2 with other materials, such as
pumice rock and zeolites [122]. The main reason for choosing these materials is their high porosity,
which may correspond to a high surface area that can incorporate titanium dioxide nanoparticles [122].
This is important, since during the catalyst preparation, a significant reduction of the specific surface
area may occur with the increase of calcination temperature. Huang et al. [122] verified an important
reduction in the specific surface area with the increase of calcination temperatures from 110 to
500 ◦ C, and consequently, a decrease in the decolorization rate when loading TiO2 onto natural
zeolite. These solids can present some disadvantages due to their leaching behavior, since this kind
of material has metals and metal oxides in its composition [122,123]. In the photocatalytic processes,
these characteristics can be very advantageous to the oxidative activity related to the degradation of
contaminants. Otherwise, these processes normally promote rock leaching, which results in water
contamination, since some of the metals that are leached can be toxic to aquatic organisms [124]. Still,
these natural materials can be considered low-cost, and may reduce the costs associated with the
supported catalysts’ preparation.
Water 2019, 11, 373 23 of 35

8. N–TiO2 Applications for the Removal of Emerging Contaminants

Normally, after the preparation and characterization of the catalyst, its photocatalytic activity is
tested on the degradation of model compounds, such as for example phenols and dyes. However, when
looking for applications related to real wastewater treatment, the use of a single contaminant may not be
representative of the system behavior when applied to actual water treatment conditions. The meaning
of the results obtained in terms of photocatalytic activity can be misrepresented when applied to real
emerging contaminants. The performance of prepared catalysts should be evaluated against their
undoped form and/or the best titanium dioxide catalyst. Considering the problematic of wastewater
reclamation, the application of this methodology to remove emerging pollutants should be exploited,
since these catalysts can present good activity under sunlight radiation, which is a very low-cost
resource [125]. This would largely reduce the operating costs for water and wastewater treatment.
New paradigms related to water recovery and reuse point out the need to seek suitable depuration
strategies that are able to remove the so-called emerging contaminants from water, safeguarding
ecosystems and human health. Among them, chemical compounds (such as pharmaceutical and
personal care products) and biological species (such as bacteria, virus and protozoa) whose impact
over public health is still not very well-defined ought to be highlighted. The precautionary principle
imposes that their removal from water should be promoted. Thus, the application of N–TiO2 catalysts
for that purpose is now critically revised.

8.1. Chemical Emerging Contaminants Removal

P25 is a commercial TiO2 form that is found in the literature as active regarding the photocatalytic
degradation of contaminants under sunlight or visible light radiation; thus, it should be used for
comparative purposes when selecting an alternative photocatalyst. Some studies can be found about
P25 doping with nitrogen. Authors have stated that doping can enhance P25 activity. However,
improvement should also be measured in terms of both the preparation costs and the contaminants’
degradation rate when using these novel catalysts versus undoped catalysts.
Mekprasart and Pecharapa [47] did not verify significant differences regarding the XRD patterns
of P25 when doped with N. In the same way, rhodamine B photocatalytic decolorization under visible
light after 60 min showed no significant differences between the application of P25 or N-doped P25.
Still, the authors noted an increase in the decolorization kinetic constant from 0.051 to 0.105 min−1
when changing the catalyst from P25 to N-doped P25 [47]. This difference regarding the reaction
rate can be related to the higher initial dye absorption when N-doped P25 was used. However,
in a general way, total decolorization was achieved at the same time for both catalysts. In this case,
with the significant amount of nitrogen that was used (which was provided from ammonium hydrogen
carbonate), it seems that there was an increase in the cost and time regarding the catalyst’s preparation
without a significant beneficial effect.
The degradation of 2-chlorophenol (2-CP) was tested using N–TiO2 under visible light through
a 150-W lamp applied together with 1 M of sodium nitrite working as a UV filter [21]. To study the
photocatalytic oxidation of 2-CP, three different N precursors (diethanolamine, triethylamine, and urea)
were used to prepare the N–TiO2 by the sol–gel method [21]. The N–TiO2 that was prepared using
diethanolamine as the precursor presented the best 2-CP degradation compared to the other precursors.
As described in Section 4, this was due to the smallest band gap, and the anatase crystal size was due
to its better interstitial placement onto the TiO2 lattice [21]. These authors observed almost double the
kinetic rate for the catalyst prepared with diethanolamine compared to the other precursors that were
used, and after 50 min of irradiation, it achieved 66% 2-CP removal. Moreover, the other catalysts that
were prepared with the remaining dopant precursors presented higher efficiency on 2-CP removal than
the Degussa P-25 (TiO2 ). Ananpattarachai et al. [126] analyzed the effect of N–TiO2 and N, C–TiO2
produced by the sol–gel method using diethanolamine and triethanolamine as N and N, C precursors,
respectively on the degradation of 2-chlorophenol (2-CP). The photocatalytic oxidation of 2-CP under
visible light (420 to 700 nm for a 150-W lamp) occurred using one g/L of catalyst and pH 7. The best
Water 2019, 11, 373 24 of 35

result for 2-CP (initial concentration of 100 mg/L) degradation was obtained with the N,C–TiO2
catalyst (95% of removal) while for N–TiO2 , only 70% of 2-CP removal was obtained after 30 min
of irradiation.
Nolan et al. [13] studied the degradation of 4-chlorophenol (4-CP) using the N–TiO2 prepared by
the sol–gel method using 1,3-diaminopropane at different calcination temperatures from 400 to 800 ◦ C.
4-CP degradation was obtained after two hours of irradiation. Parallelly, the authors analyzed the
decolorization of methylene blue using a 60-W lamp, and concluded that the best performance was
achieved for the catalyst prepared using 500 ◦ C as the calcination temperature, where the anatase phase
appears as unique. The increase of calcination temperature leads to a decrease in the decolorization
kinetic rate, which increases again for the calcination temperature of 700 ◦ C [13]. The authors noted
that this increase was due to the appearance of rutile, which will reduce the band gap. However, for the
degradation of 4-CP, the degradation profiles for these two calcination temperatures were not shown,
prolonging the doubt of whether the presence of rutile can also enhance the N–TiO2 photocatalytic
activity of contaminant degradation.
Petala et al. [127] prepared N–TiO2 by the sol–gel method under ammonia flow at calcination
temperatures of 450 ◦ C and 800 ◦ C. Ethylparaben (EP) degradation was studied under simulated
sunlight radiation using N–TiO2 with different calcination temperatures, and the most active was the
N–TiO2 prepared for 600 ◦ C. The efficacy could be attributed to the appearance of the rutile phase,
which can help reduce the band gap. The degradation rate as a function of the catalyst load was also
analyzed for 0.3 mg/L of EP, with the best result achieved for 750 mg/L. The increase from 100 mg/L
to 750 mg/L enhanced the EP degradation rate, while for 1000 mg/L, the value was similar to the
catalyst load of 750 mg/L.
Ramandi et al. [19] used a modified sol–gel method coupled with ultrasound irradiation to
produce a tridoped C, N, S–TiO2 to promote the degradation of diclofenac. The photocatalytic
oxidation of diclofenac (25 mg/L) was performed using natural sunlight irradiation with a catalyst
load of one g/L. Total degradation and mineralization were both achieved after 180 min of irradiation.
The main factor responsible for the degradation efficiency was the oxidative species that were produced,
such as superoxide and hydroxyl radicals, as proven by scavengers’ usage.
Aba-Guevara et al. [128] verified the efficiency of Fe–N co-doped onto TiO2 prepared by two
different methods (sol–gel and microwaves) for amoxicillin, streptomycin, and diclofenac degradation
under visible light. The main difference between these two preparation methods occurs during the
hydrolysis, which for the sol–gel (SG) method happens at room temperature, and for the microwave
(MW) method takes place under 100 W and 300 psi of pressure over 15 min. Photocatalytic oxidation
occurs under a visible light lamp (23 W) with wavelength emissions above 400 nm, and each
pharmaceutical compound was degraded in separate conditions at the initial concentration of 30 mg/L
with a catalyst load of one g/L. After 240 min of irradiation, the amoxicillin degradation efficiencies
were 58.6% (SG) and 46.1% (MW) at pH 3.5, while streptomycin obtained 49.7% (SG) and 39.9% (MW)
removals at pH 8. Regarding diclofenac, the authors only noted 72.3% of removal after 300 min of
irradiation for the MW method and pH 5. For this irradiation time, the authors also indicated for the
MW method an increase of the degradation of amoxicillin and streptomycin, which reached about
69.2% and 72.3% for pH 3.5 and 8, respectively.
The degradation of 4-acetamidophenol was used to demonstrate the photocatalytic activity
differences between N–TiO2 prepared by conventional and ultrasonication sol–gel methods [129].
UV light (composed by UVB and UVA), aided by a N–TiO2 concentration of 2 g/L, promoted 60% and
17% of 4-acetamidophenol (initial concentration of 50 mg/L) removal using the catalyst prepared by
ultrasound and conventional methods, respectively [129]. Moreover, the authors concluded that the
efficiency of the material prepared by the ultrasound method was related to the smaller particle sizes
and higher surface area.
Pedrosa et al. [130] produced two different graphene oxides (based on the Hummers and Brodie
methods), which through thermal treatment supported N–TiO2 . The effect of the N–TiO2 support
Water 2019, 11, 373 25 of 35

on the photocatalytic performance of graphene oxide was evaluated regarding the degradation of
diphenhydramine using UV/Vis radiation (wavelength 350 to 700 nm and photon flux of 140 W/m2 ).
After 60 min of irradiation and using 1 g/L of catalyst, it was possible to achieve about 65% and 70%
diphenhydramine removal (initial concentration of 100 mg/L) for the Hummer and Brodie methods,
respectively. Moreover, graphene oxides without N-doping presented the best performance regarding
photocatalytic oxidation. For the N-doping process, the reduction of graphene oxide was needed
through thermal treatment, which promoted a negative effect for photocatalysis [130].
N–TiO2 produced by the sol–gel method using triethylamine as the N source was used through
photocatalysis under blacklight lamps (which emit in the UVA range) to promote the degradation of a
mixture of three herbicides (Picloram, Clopyralid and Triclopyr). The experimental conditions were:
an initial concentration of each herbicide of 5 mg/L, a catalyst load of 0.5 g/L, and pH 4. The best
result was achieved for the catalyst prepared with a Ti:N ratio of 1:1.6. At those conditions, 95% of
picloram, 45% of clopyralid, and 90% of triclopyr were removed after 180 min of irradiation. Higher
and lower ratios decreased the degradation rate [131].
Abramović et al. [132] produced N–TiO2 by hydrolysis and verified its photocatalytic efficiency
for the degradation of the following herbicides: RS-2-(4-chloro-o-tolyloxy), propionic acid (mecoprop),
and (4-chloro-2-methylphenoxy) acetic acid (MCPA). Multiple ranges of catalyst loads from 1 g/L to
8 g/L with different radiations sources were used, and the initial concentration of each herbicide was
2.7 mM. A mercury lamp, which emits UVA and UVB rays, presented better results than a halogen
lamp (visible light) or sunlight regarding the degradation of herbicides. On the other hand, the N–TiO2
was more efficient than P25 for the degradation of herbicides under visible light, and for UV radiation,
the opposite happens. A comparison between the prepared catalyst and P25 (a TiO2 benchmark
material) is essential in order to assess the feasibility of proposing a new catalytic material to the
market. In this case, that N–TiO2 is more efficient than P25 under visible light is quite relevant, since the
use of less energetic radiation may reduce the treatment costs. Thus, this makes N–TiO2 advantageous
compared with P25.
Table 3 summarizes some of the literature results regarding the application of the N–TiO2 catalyst
for the removal of chemical emerging contaminants from water.

Table 3. Emerging contaminants degradation through photocatalytic oxidation.

Ref. Preparation Method Pollutant Conditions Radiation Results

Sol–gel method with
66% of 2-CP removal
diethanolamine,
-Batch reactor (1.1 L) Visible light after 50 min of
[21] triethylamine, and 2-CP (25 mg/L)
-1 g/L of catalyst load; (150-W lamp) irradiation (N source,
urea as different N
diethanolamine;
precursors
Sol–gel method using -N,C–TiO2 catalyst,
Visible light
diethanolamine and -1 g/L of catalyst load; 95% of removal;
[126] 2-CP (100 mg/L) (420–700 nm),
triethanolamine as N -30 min of irradiation; -N–TiO2 , only 70% of
150 W
and N, C precursors removal.
Q-Sun solar
-Batch-stirred reactor; Total degradation of
chamber
-0.5 g/L of catalyst 4-CP after two hours
[13] Sol–gel method 4-CP (10 mg/L) (average
load; for calcination
intensity 0.66
-Calcination (500 ◦ C) temperature of 500 ◦ C.
W/m2 );
-Total EP removal was
-Cylindrical glass achieved in 60 min
reactor; with ultrapure water;
-Catalyst load -Total EP removal was
Solar simulator
Sol–gel method with Ethylparaben (EP) 750 mg/L; achieved in 60 min
[127] (100 W Xenon
flowing of ammonia (0.3 mg/L) -Calcination with wastewater;
lamp)
temperature 600 ◦ C; -Toxicity of treated
-Different matrix wastewater was higher
conditions. than ultrapure water
for the 60 min.
Water 2019, 11, 373 26 of 35

Table 3. Cont.

Ref. Preparation Method Pollutant Conditions Radiation Results

C, S, N–TiO2 allows
-Water-jacketed total diclofenac
Sol–gel method Natural
reactor–Catalyst load degradation and total
[19] modified coupled with Diclofenac (25 mg/L) sunlight
1g/L; COD mineralization
ultrasound irradiation irradiation;
-C, S, N–TiO2 used. after 180 min of
irradiation.
-Amoxycilin removal:
58.6% (sol–gel, SG)
and 46.1% (microwave,
MW) at pH 3.5 (240
Visible light
min of irradiation);
lamp (23 W)
Amoxycilin, -Streptomycin removal:
-Sol–gel method; -Stirred reactor with
[128] streptomycin, and 49.7% (SG) and 39.9%
-Microwave method; -Fe,N–TiO2 load 1g/L; wavelength
diclofenac (30mg/L) (MW) at pH 8 (240 min
emission above
of irradiation);
400 nm;
-Diclofenac removal:
72.3% of removal
(MW) at pH 5 (after
300 min of irradiation).
-60% of removal for
UV light
the catalyst prepared
Conventional and -Stirred reactor with (composed by
4-acetamidophenol by ultrasound method;
[129] ultrasound sol–gel lamp placed axially. UVB: 280–315
(50 mg/L) -17% of removal for the
method -Catalyst load 1g/L; nm and UVA:
catalyst prepared by
315 to 400 nm);
conventional method.
-Hummers and Brodie
UV/Vis -65% of removal for
method for graphene
(wavelength Hummers graphene
oxide. -Stirred glass reactor
Diphenhydramine 350 to 700 nm oxide N–TiO2 ;
[130] -Thermal treatment in with gas bubbling;
(100 mg/L); and photon -70% of removal for
a furnace of NH3 /N2 -1g/L of catalyst load;
flux of 140 Brodie graphene oxide
to N–TiO2 reduced
W/m2 ); N–TiO2 ;
graphene oxide
-Three herbicides -95% of picloram,
mixture (Picloram, 45% of clopyralid, and
Sol–gel method using Blacklight
Clopyralid, and -Catalyst load: 0.5 g/L; 90% of triclopyr were
[131] triethylamine as N lamps (UVA
Triclopyr); -Ratio Ti:N—1: 1.6. removed (at pH 4 and
source range);
-Initial concentration: after 180 min of
5 mg/L of each irradiation).

8.2. Action on Some Pathogens

Lee et al. [44] revealed that the N–TiO2 prepared by the modified sol–gel method coupled with
ultrasound irradiation presents a strong disinfection capacity over Escherichia coli and Staphylococcus
aureus comparing to the undoped material. Two forms of N–TiO2 were produced; one was irradiated
with visible light for one hour, and the other one was without irradiation. The irradiated one revealed
an increase in the surface hydrophilicity, as noted above. The bactericidal effect of irradiated N–TiO2
was higher than the unirradiated one, since after one hour of visible light irradiation, no bacteria
survival was verified, while for the unirradiated sample, a survival rate of 12.5% was still detected.
Rizzo et al. [8] concluded that the increase of the catalyst amount from the 0.025 to 0.2 g/L
enhanced the Escherichia coli inactivation rate, which was probably due to the increased availability
of active sites. However, the catalyst loads above 0.2 g/L until 0.5 g/L decreased the inactivation
rate, which could be related with the increasing agglomeration of particles and/or due to the light
scattering promoted by the higher N–TiO2 loading [8]. Using a simulated sunlight radiation (250-W
lamp) from an initial bacteria concentration (107 CFU/100mL), it was possible to achieve the complete
disinfection after 60 min of irradiation with the N–TiO2 load of 0.2 g/L.
Sacco et al. [121] used the solvent cast method to immobilize N–TiO2 on polystyrene spheres, and
evaluated the photocatalytic oxidation impact over E. coli provided from the real municipal wastewater
after secondary settling. The packing catalyst was used (325 g) with a visible light radiation source
from a strip of light-emitting diodes (LEDs) (81.6 W). The laboratory scale reactor was able to treat 500
Water 2019, 11, 373 27 of 35

mL of volume using a recirculation rate in the range of 6.3–74 mL/min. The authors also performed
experiments under sunlight radiation, where the volume that was treated was 1.2 L, with a recirculation
rate of 625 mL/min. Besides, 434 g of catalyst were used. In terms of results, inactivations of 92.6%
and 87% of the initial E. coli concentration (300 CFU/mL) were achieved after 120 min of irradiation at
the laboratory conditions and outdoor conditions, respectively.
Ata et al. [55] prepared N–TiO2 by the sol–gel method supported on a polystyrene surface by
the solvent-casting method to promote the inactivation of an antibiotic-resistant E. coli strain from
the effluent provided from the biological process of a municipal wastewater treatment plant. The
initial E. coli concentration was 105 CFU/mL. The best result was achieved with 0.2 wt % of N–TiO2
supported on the polystyrene support, since under visible light (emitting radiation between 400–800
nm), the inactivation of E. coli was about 97% in 30 min.
Table 4 summarizes some of the data concerning the application of N–TiO2 catalysts for the
removal of pathogens from water.

Table 4. Disinfection through photocatalytic oxidation.

Preparation
Ref. Pathogen Conditions Radiation Results
Method
-Surface hydrophilicity
increases the antibacterial
capacity;
Modified
-0% of survival bacteria
sol–gel method
E. coli and S. aureus -25 mg/L of catalyst Visible light (150-W after one hour of
[43] coupled with
(2 × 105 CFU/mL) load; Xe lamp) irradiation;
ultrasound
-12.5% of survival bacteria
irradiation
after one hour of
irradiation for absence of
surface hydrophilicity.
-0.025–0.5 g/L of Higher inactivation rate
Simulated sunlight
Hydrolysis E. coli catalyst; 8.5 × 105 CFU/100
[8] radiation (250-W
method (107 CFU/100 mL) -10 min of mL/min, for 0.2 g/L of
lamp);
irradiation; catalyst load.
-325 g of catalyst and
-92.6% of E. coli
500 mL of volume
Simulated sunlight inactivation after 120 min
treated (lab
radiation (LED, of irradiation (lab
Hydrolysis conditions);
[121] E. coli (300 CFU/mL) 81.6 W); conditions);
method -434 g of catalyst and
Natural sunlight -87% of E. coli inactivation
1.2 L of volume
(outdoor conditions); after 120 min of irradiation
treated (outdoor
(outdoor conditions).
conditions);
-0.2 wt.% of N–TiO2
Sol–gel method
supported; -inactivation of E. coli was
supported on Visible light (400 nm
[55] E. coli (105 CFU/mL) -diameter of about 97% after 30 min of
polystyrene and 800 nm).
polystyrene 8.5 cm; irradiation.
surface
-Real wastewater;

The improvement of the photocatalytic oxidation that was verified through using this kind of
catalyst for the degradation of chemical and biological contaminants is usually related to the availability
of reactive oxygen species due to the band-gap reduction. The production of radical species such as
hydroxyl and superoxide may enhance decontamination and disinfection.

9. Conclusions and Future Perspectives

N–TiO2 can be prepared in different ways; its advantages are related the production rate, and its
disadvantages are related to the energetic costs and the requirement of special equipment. There are
other production technologies with slower production rates, but that also have lower operating
costs and do not require special equipment. The production methods can be modified in order to
achieve a material with the best photocatalytic performance. In fact, the preparation procedure was
shown to have a large influence on the final catalyst properties. The selection of the best preparation
methodology must bear in mind the obtained material properties (particularly photoactivity under
Water 2019, 11, 373 28 of 35

visible/sunlight radiation), including its stability as well as the costs associated with its production.
The preparation procedure must not have prohibitive implementation/operating costs when scaled-up
to the industrial level. Among the described methodologies, the sol–gel method seems to be the easiest
to implement, and it is flexible enough to change the obtaining catalyst properties by manipulating the
operating conditions. Still, this method requires more chemicals, as well as a calcination step.
Some characteristics such as surface hydrophilicity and the type of crystalline phases can be
helpful for enhancing the photocatalytic activity of the material. Thus, the preparation method should
be designed to enhance those features. Still, the way that N is incorporated in TiO2 is controversial,
and seems to depend on the preparation procedure. It seems hard to rely on the data obtained from a
single characterization analysis to draw any conclusions regarding the effectiveness of the preparation
procedure. Rather, several analytical techniques should be joined together to characterize the catalysts.
Besides, the selection of a suitable catalyst should consider the screening of several materials and
compare their activity and stability on the photocatalytic degradation of the contaminants under
visible/sunlight radiation.
Co-doping N–TiO2 can improve its photocatalytic activity under visible or sunlight radiation,
since the presence of the co-dopant can further reduce the catalyst band gap. Thus, some co-dopants
such as F, Ce, and Pt can enhance the degradation of contaminants during photocatalytic oxidation.
The selection of the best catalyst must consider its activity, stability, and production costs, so that a
cost-effective water treatment process can be obtained.
While most of the studies involving N–TiO2 consider it in the powder form, this is not the best
way to operate a photocatalytic reactor under real water treatment conditions. Thus, supports have
been exploited to reduce the cost of their real application, since this way, the catalysts could be used in a
continuous reactor without requiring a posterior separation process. Alternatively, N–TiO2 nanotubes
can grow from Ti foils’ support. The design of a reactor to work with supported N–TiO2 is a challenge,
since mass-transfer limitations and lower effective light absorption by the catalyst will reduce treatment
efficiency. Still, research efforts must be put into action in order to reach suitable solutions to boost the
wide real-scale application of this treatment technology. Thus, more studies considering the emerging
degradation of contaminants using supported N–TiO2 should be performed to ensure that the decrease
in the catalyst activity resulting from the support will not make the process unviable. To attest to the
economic viability of supported N–TiO2 applications involving real wastewaters in pilot, solar reactors
are required. This way, the effects of factors such as hydrodynamics, reactor dimensions, and the
type of support could be analyzed. On the other hand, the reuse of these supported catalysts must be
evaluated in order to attest their stability regarding water treatment conditions. Another important
point is related to P25, which is a benchmark photocatalyst. Thus, the efficiency of the catalysts that
are developed must be compared with this material to verify whether the new catalysts are truly
an alternative.
The selection of a suitable treatment for emerging contaminants removal must involve the
screening of several technologies [133,134], and must consider their economic aspects.

Author Contributions: Conceptualization, J.G.; R.C.M., data curation, J.G.; E.D.; J.L.; writing—original draft
preparation, J.G., E.D. writing—review and editing, R.M.Q.-F. and R.C.M.; supervision, R.M.Q.-F. and R.C.M.;
project administration, R.C.M.; funding acquisition, R.C.M.
Funding: João Gomes and Rui C. Martins gratefully acknowledge Fundação para a Ciência e Tecnologia by the
financial support under IFCT2014 programme (IF/00215/2014) with financing from the European Social Fund
and the Human Potential Operational Programme.
Conflicts of Interest: The authors declare no conflict of interest.
Water 2019, 11, 373 29 of 35

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