Impact of clusters Ni-B on the photocatalytic properties of TiO2 nanopowdersw

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IMPACT OF CLUSTERS Ni-B ON THE PHOTOCATALYTIC PROPERTIES OF TiO2 NANOPOWDERSW

M.M. Nadareishvili, T.O. Gegechkori,

G.I. Mamniashvili, T.I. Zedginidze,

T.G. Petriashvili, S.J. Tsakadze

Tbilisi State University,

E. Andronikashvili Institute of Physics

Аннотация

Была изучена возможность изменения спектров поглощения фотокаталитических нанопорошков TiO_2, с целью увеличения эффективности фотокаталитической реакции и тем самым более эффективного использования энергии солнечного излучения. Было найдено, что нанесение кластеров Ni-B на поверхность наночастиц порошков TiO₂ с помощью оригинальной нанотехнологии, разработанной авторами статьи, существенно увеличивает поглощение солнечной энергий.

Ключевые слова: нанотехнология, фотокатализаторы, диоксид титана.

Abstract

The possibilities for changing the absorption spectra of the anatase modification of the photocatalytic TiO₂ nanopowder were studied with the aim enhancing the efficiency of the photocatalytic reaction and to use more efficiently the energy of solar radiation for photocatalysis. It was found that the deposition of Ni-B clusters on the TiO₂ powder by the unique nanotechnology developed by the authors increases significantly the absorption of solar energy.

Keywords: nanotechnology, photocatalisators, titanium dioxide

photocatalytic nanopowder cluster nanotechnology

Introduction

It is well known that gas and fuel supply on the Earth under the conditions of current consumption will soon be exhausted. The current fuel consumption intensity is defined by the industrialization of mankind, and practically it is impossible to slow it down. For this reason the only way out is finding of alternative energy sources: one of the most promising trends on this way is the water dissociation into hydrogen and oxygen using the solar energy and the use of produced hydrogen as fuel, the final product of which under burning is again water.

The nowadays actual problem hindering the wide application of the above-described method in practice is the low efficiency of the photocatalytic reaction - the reaction of water dissociation into hydrogen and oxygen with solar radiation energy by using catalysts. Hence the pressing problem many investigations are devoted to is the enhancement of the efficiency of the photocatalytic reaction. Considering the importance of the problem, enhancement of the efficiency of photocatalysts attracts the attention of many researchers throughout the world [1-3].

It is believed currently that the most promising substance for the photocatalysis is titanium dioxide TiO₂. Photocatalysis over a semiconductor oxide such as TiO₂ is initiated by the absorption of a photon with energy equal to or greater than the band gap of the semiconductor (3.2 eV for TiO₂) producing the electron - hole (e-/h⁺) pairs [2].

(1)

Consequently, after irradiation, the TiO₂ particle can act as either an electron donor or as an acceptor for molecules in the surrounding media. However, the photoinduced charge separation in bare TiO₂particles has a very short lifetime because of charge recombination. Therefore, it is important to prevent the hole-electron recombination before the above-mentioned chemical reaction occurs on the TiO₂ surface.

Having recognized that charge separation is a major problem, numerous techniques were developed to minimize this effect. One such technique is to scavenge photogenerated charges with strongly absorbed substances. The increase of the distance of charge separation can be obtained by adding metal and metal oxide clusters to the surface of the semiconductor particle.

The electrons produced upon band-gap excitation are injected into the metal particles, and positively charged holes are injected into the metal oxide. Various substances can be used to capture either holes or electrons, allowing them to react.

Results and discussion

In this paper we investigated the optical absorbtion spectra of the TiO₂ nanopowders in order to study the possibility of improvement of their photocatalitic properties by increasing the light absorption and of enhancement of photocatalysis reaction efficiency. We performed some investigations in the above-mentioned field, namely, the nanotechnology making it possible to change the optical properties of TiO₂ powders as desired were found. First of all, at Andronikashvili Institute of Physics a unique nanotechnology of coating of nanoparticles with clusters of different sizes from different materials (as example, Ni-B) was developed [4]. These nanostructures were fabricated using electroless deposition of metals and alloys. The peculiarity of the method consists in the maintaining of low temperature during the coating reaction (58-60^oC), which preserves the physical - technical properties of the substance to be coated. The proposed nanotechnology has a number of other advantages: it is simple, low-cost and hence competitive of production process purposes.

For studying the possibilities of nanotechnology we developed, the experiments were conducted by the following procedure: the optical absorption spectra of the distillate suspension of the photocatalyst powder was studied, two crystallographic modifications of the TiO₂ powder (anatase and rutile) were used. The light absorption of distilled water over the entire working spectral range was preliminarily studied. It appeared to be rather low. However, in order to exclude the distortion of absorption spectra of the powders under study as a result of the distillate effect, we recorded the absorption spectra of the powders dissolved in the distillate in reference to pure distillate instead of the air. For this purpose, the cell with the solution of the powder under study was placed in one compartment of the 4-compartment spectrophotometer ф-46 and an identical cell with pure distillate - in other compartment. To check the validity of the procedure used, we prepared several similar reference aqueous solutions of the same powder. Then their absorption curves were taken. The obtained spectra coincided very closely with the maximum difference of ± 5%.

Fig.1 - Absorption spectra of the TiO₂ nanopowders

curve 1 corresponds to the absorption spectra of the TiO₂ nanopowders of the Rutile modification.

curve 2 corresponds to the absorption spectra of the TiO₂ nanopowders of the Anatase modification.

curve 3 corresponds to the absorption spectra of the TiO₂ nanopowders of the Anatase modification after their coating with Ni-B nanoclusters.

For testing the efficiency of the developed nanotechnology of deposition of Ni clusters on photocatalytic TiO₂ nanopowders, we took two modifications of the TiO₂ powder, anatase and rutile, with the grains ? 200nm in size. We prepared their identical suspensions in distillate and recorded their absorption spectra over the wavelength range from 200 to 800 nm. The measurements showed that the absorption spectra of the nanopowders under study were very much alike, without any particular absorption at any wavelength. Then we deposited Ni-B clusters on the surface of powder particles under study and recorded again their absorption spectra over the same wavelength range by the same technique. The measurements showed that the absorption spectrum of rutile changed only slightly, while that of anatase altered dramatically: there appeared a clear and wide maximum at the wavelength 350nm. This implies a sharp increase in the absorption of light energyat the given wavelength, which in turn would enhance the efficiency of photocatalysis in the result of treatment of photocatalytic TiO₂ nanopowders by the nanotechnology we developed. Figure1 shows the absorption spectra of the TiO₂ nanopowders of rutile (curve 1) and anatase (curve 2) modification, curve 3 shows the absorption spectra of the TiO₂ nanopowders of anatase modification coated with Ni-B nanoclusters.

Acknowledgment: The authors express their very deep gratitude to Prof. J.J.Ramsden (University of Buckingham - UK) for his very useful discussions,

References

1. Kaneko. I. Okuna. Photocatalysis Science and Technology. Kodansha Springer. Tokyo 2002.

2. J.Ramsden, Nanotechnology, Copenhagen: Ventus (2009).

3. Vishnu Prabhakar, Tahira Bibi. Nanotechnology: Future Tools for Water Remediation. International Journal of Emerging Technology and Advanced Engineering, vol.3 Iss.7, p. 54-59, 2013.

4. Teimuraz Khoperia, Grigor Mamniashvili, Malkhaz Nadareishvili and Tinatin Zedginidze. Competitive nanotechnology for deposition of films and fabrication of powder-like particles. ECS Trans. vol.35, Iss.10, 17 (2011)

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