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Section 1. Materials science
https://doi.org/10.29013/ESR-21-9.10-3-6
Nadareishvili Malkhaz, PhD, Tbilisi State Universityy E. Andronikashvili Institute of Physics, Senior Researcher, Georgia
E-mail: malkhaz.nadareishvili@tsu.ge
IMPACT OF COMPLEX PROCESSING ON THE PHOTOCATALYTICPROPERTIES OF TITANIUM DIOXIDE
Abstract. Complex treatment of titanium dioxide nanopowders was carried out and the effect of this processing on its photocatalytic properties was studied. Complex treatment included the decoration of nanopowders grains surfaces with cobalt clusters and subsequent heat treatment in vacuum. The study of the properties of these nanopowders showed that the absorption of light in the ultraviolet region increases after decorating, however, it remains almost unchanged in the visible region. After additional heat treatment in a vacuum, a strong increase was observed in absorption spectra in the visible area. Studies of the photocatalytic efficiency of complex treated nanopowders with the use of methylene blue solutions showed a strong increase in the efficiency of these photocatalysts towards to solar radiation. There was also studied the disinfection properties of the suspension of these nanopowders.
Keywords: modernmaterials, nanotechnology, photocatalysts.
Introduction organic particles in the water, bacteria and viruses
The rapid growth of the global population and [2] etc. is also possible. industrialization has led to a concomitant increase Nowadays a topical problem that hinders the in environmental pollution and depletion of energy widespread use of photocatalysis in practice is its low resources. It becomes crucial to find ways: 1) to miti- efficiency in terms of solar energy conversion; conse-gate pollution to provide a clean and safe environ- quently, the main task facing this field is to increase the ment for humans; and 2) to provide a renewable, efficiency of the photocatalytic reaction. Therefore, sustainable, and ecologically valid energy source. intensive studies are currently underway to improve
The production and application of photocata- the effectiveness of photocatalysts [1, 3-6]. lytic nanopowders are considered the most prom- To explain the reasons for the low efficiency of the ising direction for resolving the abovementioned reaction, it is necessary to briefly consider the phenom-two current global problems. With the help of enon ofphotocatalysis. It is initiated by the absorption photocatalysts, it is possible to decompose water of a photon by a semiconductor nanoparticle. If the into oxygen and hydrogen using solar energy. The resulting energy of this photon is equal to or greater final product combustion of hydrogen will be wa- than the band gap of the semiconductor, it produces ter again [1]. By photocatalysis, the destruction of electron-hole (e-/h +) pairs. The gap width of the
stable photocatalysts currently available is quite large (more than 3 eV), therefore, only ultra-violet rays ofthe sun participate in the catalytic reaction. The share of these rays is about 4% in the solar radiation spectrum. The necessary energy for water dissociation is 1.23 eV. The energy of corresponding photons is situated in the visible (red) part of the spectrum. Therefore, one of the ways to improve the catalytic reaction efficiency is to increase the contribution of the visible part of the light spectrum ofthe sun in the photocatalytic process.
Created electron-hole pairs could reach the surface of TiO2 particles and act as either an electron donor or acceptor for molecules in the surrounding media, hence increase redox reactions in it (Figure 1) [7].
Figure 1. Electron-hole pair production in cluster-deposited particles of TiO2 powder under the effect of sunlight quanta and their interaction with surrounding molecules
However, the photo-induced charge separation in bare photocatalytic particles has a very short life duration because of these charges' fast recombination. Consequently, a significant part of the charges generated by the light does not have time to participate in the reaction. Therefore, the second way for increasing photocatalytic reaction efficiency is to prevent hole-electron recombination. For this purpose, it is used the method of placing clusters (co-catalysts) of different substances on the surface of nanoparticles, which will capture electrons and holes and reduce recombination (Figure 1) [8].
Because photocatalysis reaction takes place on the surface of the photocatalysts, they are used in the form of nanopowders to increase the surface area and therefore the reaction rate. The thin photocatalytic films are used also to obtain self-cleaning surfaces,
e.g. to create lamps that do not attach soot in automobile tunnels, smart window glasses, etc. [9].
Results and discussion
In studies, the so-called P25 powders (mixture of 86% anatase-modified TiO2 and 14% rutile-modified TiO2) were used, the size of the grain was 30 nm, the purity of the powder was 99%, the manufacturer was PlasmaChem, Berlin.
The photocatalytic particles were suspended in distilled water. Although the absorption of the pure water investigated over the entire spectral range appears to be low, absorption spectra of the powder suspensions were nevertheless recorded with distilled water in the reference compartment of the DU8200 spectrophotometer. Particle concentration was 5 mg P25 in 600 mL water in order to optimize the ratio of absorptionto scattering.
A vacuum furnace VA16 with a maximum temperature of 1300 °C was used for thermal processing. We equipped the furnace with an additional liquid nitrogen trap to improve the vacuum so that the vacuum during thermal processing was 10-5 mmHg.
The untreated P25 absorption spectrum was initially measured. Absorption is increased towards the short waves and it reaches a maximum in the ultraviolet area (Figure 2, curve1).
Then cobalt clusters were deposited on the surface of P25 nanoparticles. Coating of the P25 nanoparticles by Co clusters was carried out using original technology for the deposition of metallic clusters on fine powders developed at the Andronikashvili Institute of physics [10; 11]. The technology is electroless and inexpensive. Since it proceeds at non-high temperatures (75-80 °C), it does not generate changes in the properties of the substrate material, nor in the clusters themselves. The technology was adapted to the deposition of cobalt nanoclusters on TiO2.
Cobalt clusters deposition on the P25 nanograins surfaces and their percentage were recorded with a scanning electron microscope (SEM) VEGA3 equipped with EDS of Oxford Instruments.
As a result of the cluster deposition, the absorption increased in the ultraviolet region, but the pho-tocatalyst was not sensitized to visible light (Figure 2, curve 2). However, the picture changed dramatically after of heat treatment of these decorated powders at a temperature of700 °C for 8 hours. Specifically, light absorption sharply increases in the visible area(Figure 2, curve 3).
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blue solution were used for the experiment. The concentration of the solutions was 4.5 mg/L. The first vessel contained only a solution of methylene blue while the second vessel contained the solution of methylene blue with a P25 powder without treatment and the third vessel contained the same solution of methylene blue but with a P25 powder decorated with Co clusters and treated thermally in a vacuum. These three vessels were placed under the summer sunlight at 30 ° C. 5-, " 1
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Figure 2. Exchange of the absorption spectra of the P25 during complex treatment:
1 - Absorption spectra of the P25 before complex treatment, 2 - absorption spectra
of the P25 after deposition of the Co clusters, 3 - absorption spectra of the P25 after heat treatment in the vacuum during 8 house at the temperature 700 oC
We suggest that this is caused by the diffusion ofco-balt atoms in the volume oftitanium dioxide nanopar-ticles, which leads to the emergence of impurity levels and modification of the energy gap of the P25 photo-catalyst. This is confirmed by the fact that nothing like this happens during the processing in a similar mode of undecorated with clusters P25 nanoparticles.
Figure 3 shows the results of the investigation of the photocatalytic activity of the P25 before and after complex treatment. Methylene blue solution has been used to evaluate the photocatalytic activity towards organic compounds [12]. By changing the methylene absorption peak in this solution at 665 nm wavelength, methylene degradation has been observed. Three quartz vessels with equal amount of methylene
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Figure 3. Exchange of the photocatalytic
efficiency of the P25 during complex treatment:1 - methylene degradation in pure methylene solution, 2 - methylene degradation in methylene solution with untreated P25, 3 - methylene degradation in methylene solution with complex treated P25
After every 10 minutes of solar irradiation, the 4ml methylene blue solution from the vessels was placed in the spectrophotometer cuvette and the concentration of methylene blue was determined in the solution using methylene absorption peak value at 665 nm wavelength. The spectrophotometer was pre-calibrated and the methylene concentration dependence on the absorption peak value was known. The experiments showed (Figure 3) that the concentration of methylene remained almost unchanged in the vessel without the P25 nanopowder (curve 1), whereas the concentration of methylene gradually decreased in the vessels containing the P25, and this happened much faster in the vessel containing
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complex treated P25 (curve 3) than in the vessel with untreated P25 (curve 2).
A phenomenon should be also noted, which was observed in the covering of solid surfaces, including glasses, with a suspension of P25 nanoparticles previously decorated with cobalt clusters and then treated thermally in a vacuum. After such surfaces dry, they have protective properties from dust. This suggests that the photocatalyst nanoparticles that
remain on the surface use water molecules from the air for photocatalytic decomposition of dust particles. This property of covered surfaces retained
for several months.
In addition, it was observed that the said surfaces decomposed the dry particles of the methylene powder poured on them. This suggests that they can destroy bacteria and various harmful gases in the air and therefore improve air purity in the room.
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