Optical Materials 127 (2022) 112259

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Optical Materials
journal homepage: www.elsevier.com/locate/optmat

Solar light-driven photocatalytic degradation of methyl blue by

carbon-doped TiO2 nanoparticles
Ali S. Alkorbi a, Hafiz Muhammad Asif Javed b, *, Shahid Hussain c, **, Sana Latif d,
M. Shabir Mahr b, M. Salman Mustafa e, Raiedhah Alsaiari a, Nabil A. Alhemiary a
a
Empty Quarter Research Unit, Department of Chemistry, College of Science and Art in Sharurah, Najran University, Sharurah, Saudi Arabia
b
Department of Physics, University of Agriculture Faisalabad, 38000, Faisalabad, Pakistan
c
School of Materials Science and Engineering, Jiangsu University, 212013, Zhenjiang, China
d
Department of Physics, Bahauddin Zakariya University, Multan, 60000, Pakistan
e
Department of Mechanical Engineering, COMSATS University Islamabad, Sahiwal, Pakistan

A R T I C L E I N F O A B S T R A C T

Keywords: Sol-gel method was used to synthesize carbon doped TiO2 nanoparticles with excellent photocatalytic activity
Photocatalytic degradation under solar light irradiation. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Fourier
Methyl blue transform infrared spectrophotometer (FTIR), and UV–vis spectrometer were used to analyze the absorption
TiO2 nanopowders
spectra and methyl blue concentration in water at different time intervals throughout the photodegradation
Sol-gel
experiment. The results revealed that pure TiO2 and carbon doped TiO2 nanopowders are anatase phase with
crystallite sizes ranging from 8 to 13 nm. The doping was found to decelerate the grain development of the
nanopowder. Although carbon doping in TiO2 may effectively expand the light absorption capabilities towards
visible light, too much doping is deleterious to light absorption ability. The optimal doping quantity for
maximum photocatalytic degradation of MB in our experiment was 2 atoms %; above this doping, the catalyst’s
photocatalytic activity diminishes.

1. Introduction more efficiently [4].

Many techniques have been investigated by researchers to accom­
Titanium dioxide (TiO2) and titanium-based materials have been plish this goal, one of which is the composite metal oxides obtained by
drawing an ongoing interest of researchers because of their high doping TiO2 with transition metal oxides such as Cr, Fe, Mn, and V, or
chemical stability, nontoxicity, and low cost [1]. TiO2 is strong semi­ their ions, which can absorb visible light by narrowing the band gap or
conductor photocatalyst for environmental protection, water purifica­ introducing intra-band gap energy states [5–8]. The substitution of Ti+4
tion, decompositions of harmful organic compounds and generation of by metal ions in TiO2 lattice creates allowed energy states in the band
hydrogen gas [2]. Titanium dioxide can exist in three crystallographic gap of TiO2, which becomes recombination centers for photogenerated
structures, such as anatase (tetragonal), rutile (tetragonal) and brookite charge carriers, thereby decreasing the photocatalytic activity.
(orthorhombic) [3]. Anatase phase shows the higher photoactivity than Furthermore, Transition metal ion-doped TiO2 nanoparticles can suffer
rutile or brookite crystal phases because of low recombination rate of its several serious drawbacks, such as thermal instability and low quantum
photogenerated electrons and holes on the surface of TiO2. The anatase efficiencies of their photogenerated charge carriers because several
TiO2 can be activated under UV light of wavelength <387 nm irradiation preparation methods and operating variables exist, there is no general
due to its lager bandgap of 3.2 eV, but unfortunately solar spectrum only agreement on the influence of each doped transition metal on the visible
containing about 4% UV light. Therefore, the optical response of TiO2 light activity of TiO2 [9]. Nonmetal elements, such as nitrogen, sulfur,
Shifting into the visible-light region will enhance its photocatalytic ac­ and carbon doped TiO2 nanoparticles can harvest visible light and
tivity. Consequentially, attempts to enable TiO2 to absorb visible light exhibit enhanced visible light activity [10–12]. Visible light activity of N
deserve greater attention from the point of view of using solar energy doped TiO2 for the formation of Ti–N or N–Ti–O bonds is due to

* Corresponding author.
** Corresponding author.
E-mail addresses: m.asif.javed@uaf.edu.pk (H. Muhammad Asif Javed), shahid@ujs.edu.cn (S. Hussain).

https://doi.org/10.1016/j.optmat.2022.112259
Received 28 January 2022; Received in revised form 9 March 2022; Accepted 21 March 2022
Available online 28 March 2022
0925-3467/© 2022 Elsevier B.V. All rights reserved.
A.S. Alkorbi et al. Optical Materials 127 (2022) 112259

substituting of oxygen atoms with nitrogen atoms to form TiO2− xNx

species leads to energetic perturbations of the N 2p and O 2p electrons,
introducing new intra-energy states within the TiO2 band gap [8,9].
Both cationic and anionic sulfur doped TiO2 exhibit visible light activity
through a mechanism like that of nitrogen doping [11,12]. The theory of
how non-metal element doping increases the photocatalytic activity of
TiO2 in visible light has still been controversial. Chemically modified
carbon substitutes of TiO2 have been used to mediate efficient photo­
chemical water splitting under illumination with visible light [13,14]. In
addition to the increased visible light activity of TiO2, carbon doping
also results in higher photocatalytic activity with respect to that of un­
modified TiO2 under UV illumination. Obviously, this carbon doped
TiO2 were all synthesized at high temperature. So, it is still a challenge to
prepare carbon-doped TiO2 at a low temperature, especially for the
energy-saving production of visible-light driven photocatalyst in a large
scale for pollutants removal. The synthesis of semiconductor nano­
particles via solution routes is advantageous in many ways as the
growth, size and morphology can be controlled. Although the sol-gel
process is widely accepted for the preparation of titania nanoparticles,
Sol-gel method is very simple and does not require any special equip­
Fig. 1. XRD patterns of (a) as prepared TiO2, (b) TiO2 (500 ◦ C), (c) 0.5% C/
ment. TiO2 prepared by this method have well crystalline phase and
TiO2 (500 ◦ C), (d) 1% C/TiO2 (500 ◦ C), (e) 2% C/TiO2 (500 ◦ C), (f) 4% C/
small crystalline size, which benefit to thermal stability and photo­ TiO2 (500 ◦ C).
catalytic activity.
In this study, we prepared carbon doped TiO2 nanoparticles by sol
(Hitachi U-2001) by checking the absorbance at 510 nm. The degrada­
gel method with various carbon concentrations using glucose as carbon
tion rate of methyl blue was calculated based on the following formula:
source. Photocatalytic activity of the synthesized samples was evaluated
via the degradation of methyl blue in aqueous solutions under solar light Degradation (%) = [(C0 – C)/C] × 100
irradiation.
where C0 is the initial concentration of MO mg L− 1, C is the concen­
2. Experimental section tration at each interval of time in mg L− 1.

A series of carbon doped TiO2 nanoparticles in this study have been 3. Results and discussion
prepared by simple sol gel method using titanium isopropoxide (TTIP,
98%) as the main starting material without any further purification. Fig. 1 (a) represents the XRD pattern for as-prepared TiO2 in alkaline
TTIP diluted with ethanol was added drop wise into water under solution by sol-gel method that was prepared by hydrolysis of TTIP, in
vigorous stirring at room temperature. The molar ratio of ethanol to which no sharp peaks appear due to poor crystallinity or amorphous
water and TTIP were 1.5 and 150 respectively. 0.1 g of Poly­ nature. It clears the fact that crystallinity improves with calcinations
vinylpyrrolidone (PVP) was added as surface modifier in the solution. temperature. Fig. 1 (b) represent the XRD pattern for undoped TiO2
After 15-min stirring, I M solution of NaOH was added drop wise to the nanopowder calcined at 500 ◦ C with broad peaks of anatase structure,
above solution to adjust the pH 9. The solution was aged for 24 h at room which was confirmed by (101), (112), (200), (211), (204), (220), (215)
temperature. Then the mixed solution was kept at constant temperature peak values (JCPDS 21–1272). There is no indication of rutile or
(150 ◦ C) for 5 h. Subsequently, the gel was formed and dried at 120 ◦ C brookite phases because alkaline environment favors the anatase phase.
for 12 h in an oven. Similarly, different concentration (0.5, 1, 2 and 4 The grain size was estimated using the full width at half maximum
atomic %) of carbon using glucose (C6H12O6) as source material was (FWHM) of high intensity peak (101) appear at 2θ = 25.31◦ using the
used for doped TiO2 nanoparticles. Finally, the dried powder was grin­ Scherer equation:
ded for 1 h and calcined at 500 ◦ C for 2 h for further characterization of D = kλ/βcosθ
the material.
Powder X-ray diffraction (XRD was used for crystal phase identifi­ where D is the crystallite size in nm, k = 0.89 which is a constant, λ is the
cation and estimation of the particle size. The X-ray diffraction (XRD) wavelength of X-ray radiation in nm, θ is the Bragg angle in radians and
measurements were carried out using Shimadzu X-ray diffractometer β is the FWHM in radians. The estimated crystallite size of the samples
equipped with CuKα radiation (λ = 1.5406 Å). A Fourier transform before doping of carbon was 10.3 nm. By using analytical method, lat­
infrared spectrophotometer (FTIR) PerkinElmer System 2000 was used tice parameter of anatase TiO2 (tetragonal in shape) are calculated and
to determine the specific functional groups. The morphology of the the values of a = b = 3.7878 Å and c = 9.4542 Å and c/a = 2.4959 using
samples was inspected with scanning electron microscopy (SEM) JEOL (101, 112) diffraction peaks.
Japan, JSM-5910). Light absorption spectra of the catalyst samples were Fig. 1(c) represents the XRD pattern for 0.5% carbon doped TiO2
obtained using a UV–visible spectrophotometer (Hitachi U-2001 beam nanopowders calcined at 500 ◦ C. The doping has no significant effect on
Spectrophotometer). Photocatalytic activities of the obtained samples structure and peaks intensity of TiO2 and does not change the lattice
were measured by the decomposition of methyl blue in an aqueous so­ parameter “a”, “b”, “c” and crystallite size of the composite. Fig. 1(d and
lution at ambient temperature. In each experiment, a 0.1 g amount of e) represents the XRD patterns for 1 and 2% carbon doped nano­
photocatalyst was added into beaker (Pyrex glass containing 100 ml of crystalline TiO2 that shows an apparent decrease in the peaks intensity
methyl blue solution with an initial concentration of 0.02mg/100 ml of C/TiO2 and increases in FWHM of C/TiO2. These additions of carbon
water. Prior to solar irradiation, each suspension was magnetically also decrease the lattice parameter “c” of the composites because ionic
stirred in the dark for 30 min to established adsorption-desorption radii difference of Ti (covalent radii 1.6 Å) and C (covalent radii 0.77 Å).
equilibrium. Then the solution was exposed to sunlight along with Due to these reasons the crystallite size of C/TiO2 nanopowders de­
stirring. At irradiation time of every 30 min, the concentration of the creases to 9.16 and 8.01 nm respectively. Although carbon was added as
methyl orange was monitored using a UV–vis spectrophotometer a dopant, but no peaks of carbon were appeared due to its amorphous

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A.S. Alkorbi et al. Optical Materials 127 (2022) 112259

of carbon. Carbon may reside itself to the surface rather than substitute
for oxygen that causes the crystal size to increase [16].
FTIR spectra of pure TiO2 and C/TiO2 samples in the region 4000-
400 cm− 1 are shown in Fig. 2. All the IR spectra of pure TiO2 and carbon
doped TiO2 samples show a characteristics peak of titania at about 540
cm− 1 due to the stretching and bending modes of Ti–O and O–Ti–O. It is
seen that the strong absorption peaks at 3420 cm− 1 and 1630 cm− 1 are
induced by stretching vibration and bending vibration of surface hy­
droxyl group for uncalcined TiO2 and become weak for heat treated
samples due to removal of trace amount of water. The band arising at
2350 and 1390 cm− 1 are due to the absorption of atmospheric CO2 on
the metallic cations and carbonyl group (C– – O) stretching mode [17].
SEM micrographs of the TiO2 nanopowders calcined at 500 ◦ C are
shown in Fig. 3(a–d). It can be seen that particles are irregularly shaped
as aggregates with a range of size distribution. However, we can identify
between different SEM micrographs on the basis of addition of different
dopant concentrations. As is cleared from the micrograph, that first the
particle size decreased with dopants concentration (0.5, 1, and 2%) and
beyond this at 4% particle size increases. It is also cleared that increase
Fig. 2. FTIR spectra of (a) as prepared TiO2, (b) TiO2 (500 ◦ C), (c) 1% C/TiO2 in the carbon concentration also result in high porosity in micrographs.
(500 ◦ C), (d) 4% C/TiO2 (500 ◦ C). The presence of more porosity is the indication of agglomeration of the
grains of different sizes in irregular manner. The morphologies of all
nature and the minute quantities of addition [15]. XRD pattern for 4% powders are similar but aggregate particle size of pure TiO2 and C/TiO2
carbon doped TiO2 nanopowder is shown in Fig. 1(f) which shows an are shown in Table 1 and are in the range of ~30–80 nm [18]. The
increase in the peaks intensity of the composite that led to an increase in corresponding low and high magnification TEM images of 2%C@TiO2
the crystallite size to 12.59 nm because of comparatively large amount nanoparticles are shown in Fig. 4.

Fig. 3. SEM images of TiO2 nanopowders calcined at 500 ◦ C, (a) 0%, (b) 1%, (c) 2% and (d) 4%.

Table 1
Results of XRD, SEM and Band gaps of photocatalysts calcined at 500 ◦ C.
Photo catalysts Crystal Phase Crystal size (nm) Particle size (nm) Lattice Parameter a(Ǻ) b(Ǻ) c(Ǻ) c/a V (nm) a2c Band Gap (eV)

Uncalcined TiO2 Amorphous – – – – – – – –

TiO2 Anatase 10.3 50 3.7878 3.7878 9.4542 2.4959 135.643 3.15
0.5%C/TiO2 Anatase 10.3 …. . 3.7868 3.7868 9.4538 2.5007 135.595 3.10
1%C/TiO2 Anatase 9.16 30 3.7832 3.7832 9.4344 2.4937 135.031 3.06
2%C/TiO2 Anatase 8.01 40 3.7762 3.7762 9.2609 2.4521 135.085 2.82
4%C/TiO2 Anatase 12.59 80 3.7862 3.7862 9.3978 2.4821 134.720 2.87

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A.S. Alkorbi et al. Optical Materials 127 (2022) 112259

Fig. 4. Low and high magnification TEM images of 2%C@TiO2 nanoparticles.

spectra using the relation:

1240
Ebg = (eV)
λ
The absorption against band gap energy plot is shown in Fig. 5. The
band gap energy obtained for pure TiO2 calcined at 500 ◦ C has the value
(3.15 eV). Also, it can be seen that there is significant decrease in the
band gap energy for the carbon doped titania. Particularly, the 2%
carbon doped TiO2 nanopowder exhibits greater shift towards minimum
energy of band gap (2.87 eV) than the other samples. It may be attrib­
uted to the small particle size of the photocatalyst, or it may be due to
appearance of a new electronic state in the middle of TiO2 band gap
[14]. Further increase in carbon concentration results in increase of
band gap and the calculated values of band gap energy for all prepared
samples are shown in Table 1.
Fig. 6 shows photodegradation of methyl blue (MB) using synthe­
sized nanopowder TiO2 and C/TiO2 catalysts in solar light. It is obvious
that all the C/TiO2 composite samples exhibit greater photoactivity than
pure TiO2. The carbon content influences the photocatalytic activity
markedly under solar light. The photocatalytic activity of the catalyst
Fig. 5. Band gap energy Vs absorbance of TiO2 and C/TiO2 composites calcined increases with the increase in content of carbon and then decreases as
at 500 ◦ C. carbon concentration increased beyond 2%. This concentration is the
optimum condition to achieve the synergism between carbon content
and TiO2 and is due to small crystallite size and low recombination rate
of electrons and holes at the surface of TiO2 [19]. The effect of higher
doping concentration on the lifetime of the charged carrier has been
discussed [20]. Increasing doping concentration above an optimal value
would result in forming the recombination centers for trapping the
charged carriers for a long time, thereby decreasing the photo­
degradation efficiency of the catalyst. It is concluded that an appropriate
amount of carbon concentration can acts as an effective adsorbent,
dispersing agent and photosensitizer, which degrades the dye. C/TiO2
composite has high adsorption capacity for pollutants and obviously
benefits the transfer of pollutants and accordingly the reaction activity
on the catalysts surface. Additionally, it is proved that the TiO2 or
C/TiO2 shows unique surface properties and ability to adsorb pollutant
in aqueous solution. It was also found that effect of different concen­
tration of carbon contents was different on different pollutants, increase
in the carbon content decrease the photocatalytic activity of C/TiO2
nanocomposites [15,20–23].

4. Conclusions

Fig. 6. Methyl blue degradation under sunlight (initial concentration of MB =

Carbon doped TiO2 nanopowders were successfully obtained by sol
20 mg/L, and catalyst = 1 g/L, pH = 9).
gel method. The XRD patterns revealed that samples obtained were
anatase phase with the crystallite size in (8–13 nm) range and showed
It is the fact that photocatalytic activity of the semiconductor is dependence on carbon concentration. This data is also supported by
related to its band gap structure. Absorption spectra of TiO2 and C/TiO2
evidence from SEM micrographs which show the indication of non
samples were obtained from UV–vis spectrophotometer. The band gap uniform grains of (~30–80 nm) size. Light absorption measurements
energy was calculated form the wavelength (200–800 nm) of absorption

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A.S. Alkorbi et al. Optical Materials 127 (2022) 112259

also confirmed that the presence of 2% carbon doping in TiO2 caused Saudi Arabia. This work was supported the National Natural Science
significant absorption shift into visible light region beyond this con­ Foundation of China Grant No. 51950410596.
centration absorption shifts towards lower wavelength. The experi­
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