Journal of Environmental Chemical Engineering

Volume 9, Issue 5, October 2021, 106078

Preparation and photocatalytic properties of titanium

dioxide modified with gold or silver nanoparticles
E.V. Salomatina b 1 , D.G. Fukina a 2, A.V. Koryagin a 3, D.N. Titaev a, E.V. Suleimanov a 4, L.A. Smirnova b 5 6

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Highlights

• A non-energy-consuming method for modifying the TiO2 surface of with Au and

Ag nanoparticles using chitosan enzymatic destruction has been developed.

• Gold and silver nanoparticles with sizes of 5 and 22 nm are fully depositing to
the TiO2 surface.

• TiO2/Au and TiO2/Ag are more photocatalytic active than TiO2 in the para-
nitrophenol and methylene blue conversion under UV and visible light action.

Abstract

A nontrivial and simple method for modifying the surface of anatase polymorphic particles with gold or silver
nanoparticles has been developed. The method involves the UV-induced formation of Au (Ag) nanoparticles
from the corresponding precursors – HAuCl4 or AgNO3 – in a solution of a stabilizing polymer – chitosan, than
dispersing TiO2 particles in the resulting colloidal solution, and subsequent enzymatic destruction of chitosan.
As a result, Au or Ag nanoparticles, the size of which is 5.0 ± 0.1 nm and 22.0 ± 0.25 nm, respectively, completely
settle on the TiO2 surface. It was found that in the reactions of decomposition of methylene blue and para-
nitrophenol in an aqueous solution under UV irradiation, modified TiO2 is 2–2.5 times more photocatalytically
active than the initial titanium dioxide. It is essential that the photocatalytic properties of TiO2 modified with
Au (Ag) nanoparticles also manifest themselves under the action of visible light.

Graphical Abstract

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Introduction

Many industrial processes of oil refining, petrochemistry, basic and fine organic and inorganic synthesis are
based on catalytic technologies. In industrially developed countries, their contribution to GDP is 20–25%, with
the largest "specific weight" in heterogeneous catalysis due to the possibility in this case of carrying out high-
tech synthesis in a continuous mode and the ease of reaction products separation from the catalyst [1], [2].
Modern catalytic chemistry, along with synthetic problems, solves another important mission of protecting
the environment from the technogenic, negative impact on its existing industrial production, utilities, and
vehicles [3], [4], [5], [6], [7]. At the same time, in both cases, the development of new environmentally friendly
processes of waste-free technologies for the production of new types of highly efficient catalysts remains
relevant.

The most promising types of heterogeneous catalysts in light of the aggravation of environmental problems
are photocatalysts, which are based on redox processes [6], [7], [8]. Photocatalysis provides a wide range of
applications, including the degradation of organic substances and dyes – water and air pollutants, the
conversion of organic compounds under mild conditions, antibacterial action, and fuel production through
water splitting and carbon dioxide recovery [9], [10], [11], [12], [13]. In this regard, the use of photocatalytic
systems has acquired particular social and economic significance in recent years. The advantages of
photocatalytic methods for the oxidation of organic compounds, including those involving the purification of
water and air, are well known:
1) the possibility of oxidation of almost any organic substances and also many inorganic substances, such as
CO, H2S, HCN, NH3, NOx, and others, under mild conditions – at room temperature and atmospheric
pressure [14], [15], [16], [17], [18], [19];

2) economic advantages in comparison with other methods associated with the possibility of decomposition
of organic pollutants of wastewater from enterprises, even in the case of low concentrations of these
substances [3], [4], [5], [6], [7];

3) the implementation of the photocatalytic oxidation method does not require additional reagents, since the
oxidizing agent is atmospheric oxygen [3], [4], [5], [6], [7], [20].
A special place among photocatalytic materials is occupied by nanocomposites containing semiconductor TiO2
[21], [22], [23], [24]. Composites based on it are capable of a reversible UV-induced reaction Ti4+ + e− ⇄ Ti3+,
accompanied by the breaking of the Ti O bond, the transition of an electron from the valence band to the
conduction band, and the generation of electron-hole pairs [25] (Fig. 1).

Free electrons in the conduction band are good reducing agents, "holes" in the valence band are strong
oxidants, which, in the presence of atmospheric oxygen or water vaporous, lead to the formation of
superoxide-hydroxyl radicals. The latter are involved in the transformation of various organic substances,
decomposition of dirt, soot, toxic drugs, cigarette smoke, destruction of bacteria, and can cause the death of
many microorganisms in water, air, or accumulating on surfaces [27].

It is known [28], [29], [30] that the photocatalytic activity of titanium dioxide depends on its polymorphic
modification, particle size, and the bandgap varies from 3.0 to 3.3 eV. Significant differences in the behavior of
TiO2 of different crystal structures under light irradiation are associated with the unequal character of the
bandgap in the space charge region at the semiconductor/electrolyte interface. The mechanism of formation of
electron-hole pairs is described in detail in the literature [31], [32]. A highly dispersed powdered anatase TiO2
is regarded one of the widely studied inorganic environmentally friendly photocatalysts. In some works, it is
noted that a more active photocatalyst, in comparison with anatase, is a mixture of anatase (70–75%) and
rutile (30–25%) [33].

Despite all the advantages, titanium dioxide has several disadvantages:

– the quantum yield does not exceed 20% [34], [35];

– light scattering by large particles;

– absorption of only ultraviolet radiation, which limits the use of all the energy of sunlight to activate
photocatalytic processes [3], [4], [5], [6], [7].

It is known that the introduction of insignificant amounts of active additives into the structure of titanium
dioxide can lead to an increase in the quantum yield of the reaction Ti4+ + e− ⇄ Ti3+ and a shift of its absorption
spectrum to the visible wavelength range, which contributes to an increase in the concentration of electrons
and "holes" in the surface layer of the oxide [36]. One of the options for improving the photocatalytic
properties of TiO2 is the modification of its surface with nanoparticles (NPs) of various metals, such as Pt, Ag,
Au, Pd, Ni, Cu, and Rh [37], [38], [39], [40]. Since the Fermi levels of these metals are lower than those of TiO2,
the electrons photoexcited by UV light can pass from the conduction band of TiO2 to metal particles deposited
on its surface, while the photogenerated "holes" remain in the valence band of TiO2. This significantly reduces
the possibility of recombination of electrons and holes, as a result of which there is an efficient separation of
charge carriers and an increase in photocatalytic activity. In the case of using visible radiation, a different
picture is observed, namely, metal NPs, absorbing light quanta with a wavelength corresponding to their
plasmon resonance (Ag – 400–430 nm or Au – 519–540 nm), will transfer the energy of an excited state
(plasmon electrons) to the conduction band of titanium dioxide, according to the scheme proposed earlier
[41], [42], [43].

It is known that the photocatalytic activity of catalysts containing NPs changes in the following order: Au/TiO2
> Ag/TiO2 ≥ Pt/TiO2 > TiO2 [44]. Thus, the most effective is the use of materials in which titanium dioxide is
modified with gold or silver NPs. The final result largely depends on the size of NPs and the method of their
deposition on titanium oxide. A striking example of this is the work of M. Haruta on the study of TiO2/Au
catalysts in CO oxidation reactions [45], [46]. An important role is played by Au (Ag) nanoparticles deposited
on TiO2 for the gas-phase condensation of ethanol [47]. The presence of Au/Ag in the catalyst promotes
dehydrogenation with the formation of acetaldehyde and condensation products [48]. Also, using these
catalysts, it is possible to carry out NO reduction reactions, selective hydrogenation of CO2 to CO, and oxidation
reactions of CO and H2 [49], [50]. The Me/TiO2 catalysts showed a high photocatalytic activity in the
photodegradation reaction of tartrazine, even when irradiated to visible light. The authors of [51] obtained a
photocatalyst by irradiating a TiO2 film immersed in a solution of a noble metal salt with an electron beam. It
was shown that the oxidation rate of methyl orange upon visible light irradiation in the presence of TiO2/NPs
from the series – Ag, Pt, Pd – is significantly higher than on pure TiO2. The use of TiO2/Au (Ag) as a
photocatalyst makes it possible to purify water and air under the action of visible light radiation [52].
However, it should be noted that the presence of some impurities in the TiO2 structure can adversely affect its
photocatalytic activity [53]. Thus, metal ions can additionally serve as recombination centers for electrons and
“holes”, reducing the overall activity of the photocatalyst [54].

Many existing methods for modifying TiO2 require a lot of energy, for example, high-temperature sputtering of
TiO2 and other substances in a vacuum. For example, Cu2O-Ag nanoparticles were deposited on TiO2
nanotubes by the hydrothermal method [55]. Dong and coauthors used cold plasma treatment of a dielectric
barrier discharge at atmospheric pressure to deposit Ag nanoparticles on TiO2 nanotubes doped with nitrogen
[56]. This method is environmentally friendly and efficient without the use of any polluting and biohazardous
reducing chemicals. In [57], the synthesis of TiO2 nanoparticles doped with particles (1 wt%) of noble metals
(Me/TiO2, Me = Ag, Au, and Pt) was proposed by the sol-gel method. Alternative method for modifying the
surface of titanium dioxide with gold or silver nanoparticles is the laser electrodispersion, in which the metal
target is ablated under the influence of a powerful pulsed-periodic laser [58], [59]. Recently, there have also
been works, in which the deposition of gold or silver nanoparticles on titanium dioxide occurs when it is
introduced into an aqueous solution of the precursor, which is then reduced [60]. However, it should be noted
that in this method it is difficult to control the size of the resulting metal particles. In article [61] a
photocatalyst was obtained by irradiating a TiO2 film immersed in a solution of a noble metal salt with an
electron beam.

It can be seen from the given examples that the currently used methods of modifying titanium dioxide with
nanoparticles are quite laborious, none of them combine the simplicity of preparation, the uniformity of
distribution of nanoparticles in the material, and, accordingly, the entire reserve of photocatalytic activity is
not used. In this regard, the development of non-laborious methods for the modification of TiO2 anatase
polymorphic modification of NPs of various metals remains urgent. Currently, methods are being developed
for obtaining noble metal nanoparticles in aqueous dispersion of titanium dioxide in the absence of any
stabilizer by citrate method or electron beam irradiation for example [62], [63]. The absence of a stabilizer in
titanium dioxide aqueous suspension containing HAuCl4 or AgNO3 during the formation of metal
nanoparticles is able to cause the agglomeration of both – TiO2 particles and metal NPs. Along with this, NPs
deposition not only on TiO2 surface, but also their direct precipitation is possible in the methods in the
absence of a stabilizer described by the authors.

This work aims to develop a method for deposition of Au and Ag nanoparticles on the TiO2 surface using a
polymer carrier of NPs – chitosan, to study the photocatalytic activity of the obtained systems in the
decomposition reactions of methylene blue and para-nitrophenol, as imitators of organic impurities in water.
The advantages of obtaining TiO2/Au and TiO2/Ag composites in the presence of chitosan are its ability to
perform a double function:

– chitosan is a stabilizer of both formed metal nanoparticles and titanium dioxide, providing their uniform
distribution in the volume and high dispersion.

– due to the ability of chitosan to fermentative destruction under mild conditions (at a temperature of 30–40
°C) the shielding layer between TiO2 particles and gold or silver nanoparticles is destructed. This can lead to
the effective deposition of the latter on the oxide surface and direct contact of their surfaces, which is
important from the point of view of activating the photocatalytic properties of composites.

Section snippets

Modification of powdered TiO2

We used titanium dioxide TiO2 of anatase polymorphic modification (KRONOS 1001 brand from KRONOS
TITAN GmbH & Co. OHG (Germany), ρ = 4.05 g/cm3, average particle size 2.1 ± 0.2 µm). Silver nitrate, analytical
grade (Sibproekt; AgNO3 content not less than 99.8%) was a precursor for the formation of silver NPs, and
chloroauric acid (JSC Aurat, gold content 48–50%) was for gold NPs. Chitosan (CTS) with a molecular weight of
1 × 105 and a degree of deacetylation of 80%, obtained from crab shells…

Surface modification of titanium dioxide with metal nanoparticles

A promising method for modifying the TiO2 surface with gold and silver nanoparticles from the point of view
of the possibility of practical use is their formation in situ followed by deposition on titanium dioxide
particles, which eliminates the need to clean the products from impurities. The process was carried out in two
stages. At the first stage, gold or silver NPs were formed in aqueous acetic acid solutions of chitosan during UV
reduction of precursors — HAuCl4 or AgNO3 — in the absence…

Conclusions

Thus, a low-temperature method has been developed for doping the surface of titanium dioxide particles with
gold or silver nanoparticles formed in situ by UV irradiation of aqueous solutions of the corresponding
precursors in the presence of chitosan as a polymer stabilizer. The advantages of the developed method of
deposition of gold or silver nanoparticles on the surface of titanium dioxide during the enzymatic destruction
of the nanoparticle carrier polymer are the availability and ease of…

CRediT authorship contribution statement

E.V. Salomatina: Methodology, Investigation, Writing – original draft, Writing – review & editing. D.G. Fukina:
Methodology, Investigation, Visualization. A.V. Koryagin: Methodology, Investigation. D.N. Titaev: Formal
analysis, Visualization. E.V. Suleimanov: Funding acquisition, Supervision. L.A. Smirnova: Conceptualization,
Supervision.…
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that
could have appeared to influence the work reported in this paper.…

Acknowledgments
This work was supported by the Ministry of Education and Science of the Russian Federation (assignment
0729-2020-0053) on the equipment of the Collective Usage Center “New Materials and Resource-saving
Technologies” (N.I. Lobachevsky State University of Nizhny Novgorod). The authors express their gratitude to
N.I. Kirillova, an employee of the Research Institute of Chemistry, for conducting research on the materials’
porosity by the nitrogen adsorption method.…

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1 https://orcid.org/0000-0002-7480-1324.

2 https://orcid.org/0000-0001-8375-6863.

3 https://orcid.org/0000-0002-4858-3351.

4 https://orcid.org/0000-0001-9292-4355.

5 https://orcid.org/0000-0001-9207-1524.

6 Department of Polymers and Colloidal Chemistry, Lobachevsky State University of Nizhny Novgorod, Gagarin ave 23/5, 603950
Nizhny Novgorod, Russia.

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