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TiO2 Nanoparticles for Photocatalytic Applications

   

ABSTRACT

        TiO2 photocatalysis is been widely studied for air and water purification applications. Titanium Dioxide (TiO2) has been considered an ideal photocatalyst due to factors such as its low cost, stability and chemical properties. However, its application has been limited to ultraviolet (UV) environments due to its high band gap (3.2 eV). This high band gap limits the harvesting of photons to approximately 4% of sunlight radiation. Recently it has been found that titanium dioxide, when doped with transition metals or non-metal anion, it could reduce the band gap and expanding its effectiveness well into visible range. This web page is discuss how TiO2 nanoparticle work as photocatalytic and how doping increase photocatalytic active in visible range.

Introduction

        Water and air pollution are two of  main problems affecting the environment due to waste products generated by industries and households. Detoxification and purification of water and air has become the main focus of today’s scientific research. The main causes of air and groundwater contamination are the industrial effluents (even in small amounts), excessive use of pesticides, fertilizers (agrochemicals) and domestic waste landfills. The pollution is caused mainly by non-degradable organic pollutants that are not treatable by conventional techniques due to their high chemical stability and/or low biodegradability[1].

        Solar photochemical technology can be defined as the technology that efficiently collects solar photons and uses them to promote specific chemical reactions[2]. In other words, this technology will allow us to take advantage of the energy from the sun by means of a photochemical process called photocatalysis.

        Titanium dioxide (TiO2) has been considered an ideal photocatalyst due to factors such as its photocatalytic properties, chemical stability, impact on the environment and cost[3]. Decomposing organic pollutants using TiO2 nano-particle is currently considered a possible decontamination process that could relieve much of the world’s problems with potable water.

        However, TiO2 is a wide band gap (3.2 eV) semiconductor and as such its application has been primarily limited to ultraviolet (UV) environments. This high band gap limits the harvesting of photons to approximately 4% of the sun’s available radiation, which is far too small for practical use[4]. Research today is focused on lowering the band gap of TiO2 by doping or coupling TiO2 with other semiconductors, transition metals and non-metal anions, and thereby expanding its effectiveness well into the visible range.

Photocatalys

A catalyst is a substance that promotes an increase in the rate of reaction of a chemical process, which otherwise is thermodynamically favored but kinetically slows, keeping the catalyst unchanged after the reaction. The process can be described as:

where A and B represent the reactants and products respectively.

        Photocatalysis is a reaction where a substance and a source of light are needed to influence a response in a reactant where the chemical structures of the reactants are modified and the catalyst remains unaltered[5]. The process can be described as:

where hν is a quantum of energy from the incident photons that cause the reaction.

        The photocatalyst materials include TiO2, tungsten oxide (WO3), tungsten sulfide (WS2), cadmium sulfide (CdS), zinc oxide (ZnO), and zinc sulfide (ZnS) etc.

TiO2 in photocatalysis

         TiO2 is the most importance photocatalyst materials because its ability to oxidize a large number of organic compounds into harmless compounds such as carbon dioxide and water using ultraviolet light, and potentially, visible-light. Degussa P-25 is generally considered the most photoactive commercially available form of TiO2 structure. The average particle sizes of Degussa P-25 particles are 20-30nm. The surface area is about 50 m2/g[6].

        When TiO2 absorbs the energy of impinging photons having equivalent or excess energy to the band gap, electron-hole pairs are generated. This means that an electron in the valance band earns sufficient energy to overcome the band gap and reach the conduction band, with the concomitant vacancy in the valance band (the hole). The band gap, is the void energy region which separates the valence band from the conduction band (see Figure 1). For TiO2, the band gap can be overcome by energy from UV-A photons (350-400 nm). The absorption of energy and the subsequent generation of the electron-hole pair is the initiating step and may be represented as follows[11]:

where ecb is the conduction band electron and hvb is the valence band hole.

Figure 1 Scheme of photocatalytic process over TiO2 surface[11]

       n-type semiconductor TiO2 has an electric field, which forms spontaneously at the semiconductor-electrolyte interface. The electron-hole pair generated in the region of the electric field, i.e. the space-charge region, is separated efficiently rather than undergoing immediate recombination. This forces the photo-generated electron towards the bulk of the semiconductor, where it can be transferred through a surface site to a point where an electron acceptor can be reduced. The photo-generated hole, under the influence of the electric field, migrates towards the surface of the semiconductor to a site where it can oxidize a suitable electron donor[11]. In order to photocatalyst to be efficient, the different interfacial processes involving the electron and hole must compete effectively with the major deactivation process referred to as recombination.

        The interaction between the hole and water molecules or hydroxide ions produces the very reactive hydroxyl radicals. These radicals are bound to the surface of the hydrated metal oxide and act as the primary oxidants in the photocatalytic system[7,8]. The formation of the radicals is illustrated below.

Oxidation of compounds could also occur directly via the valence band hole before it is trapped, either within the particle or at the particle’s surface. Nevertheless, researcher already confirmed that the presence of hydroxyl radicals in aqueous solutions of illuminated TiO2 and many intermediates are consistent with those found when organic compounds react with a known source of hydroxyl radicals[9,10,13,14]. The chemical properties of the compound and the reaction conditions largely determine which mechanism will dominate. However, the presence of hydroxyl radicals is the most important for the photocatalytic destruction of organic compounds and the inactivation of pathogens.

        TiIV sites trapped the photo-generated conduction band electrons and result in TiIII sites. Oxygen adsorbed at TiIII sites may result in reactive oxygen species such as the superoxide radical from a charge transfer reaction;

        Superoxide could also contribute to the oxidation of organic and inorganic electron donors by acting as a direct electron acceptor or interact with.

Modifications of TiO2 for Improved Photocatalytic Performance

        In order to use of TiO2 as a visible-light photocatalyst. To achieve this goal it is necessary to modify TiO2 to improve the visible-light absorption, prevent or delay charge carrier recombination and improve its surface properties. To reach these goals, it is necessary to modify TiO2 to achieve each particular goal.

1. Surface Modification Using Metals

        TiO2 could be Modified by using a metal to change the surface properties to improve the photocatalytic reaction rate, and also change the intermediate products[15,16]. Figure 2 showing a Schottky barrier that is created at the interface of the catalyst and metal. Platinum has been studied extensively and found to form particle clusters on the catalyst surface covering of the surface area allowing for a large TiO2 surface area for adsorption of the pollutant to the catalyst surface.

Figure 2 Metal as an Electron Trap[12]

        A electron migrating to the surface required quantum of energy creates an electron-hole pair and the metal acts as an electron trap. This suppresses the recombination of the electron-hole pair. The hole is then able to migrate to the surface of the catalyst and oxidize the adsorbed organic compound[13]. Further, metals such as platinum and silver have their own catalytic performance.

        Metals could change the photocatalytic properties of TiO2 by changing its electrical properties due to the distribution of electrons that occur. At the heterojunction, the Fermi levels of the metal and semiconductor align resulting in a flow of electrons from the catalyst to the metal. This leads to an increase in hydroxyl groups that play an important role in the photocatalytic reaction[13].

2. Transition Metal Doping

        Doping TiO2 by using transition metals ions has also been widely studied. Similar to metal doping, the use of transition metal ions allows for electron traps that suppress electron-hole recombination. It was concluded that a doping threshold exists where only small concentrations produce a positive effect on the photocatalytic rate[12]. It should be pointed out that not all transition metals produce a positive result. Some transition metals actually decrease the photocatalytic rate due to an increase in electron-hole recombination by creating recombination centers[14].

3. Coupled Photocatalysts

        Coupling or co-doping TiO2 was also been studied. This method is used to exploit the lower band gap of material to produce a photocatalytic effect in a wider gap material such as TiO2 by increasing the charge separation and extending the energy range of photoexcitation for the system. Figure 3 shows the valence band and conduction band positions for TiO2 and WO3 prior to contact. If a photon that is not enough energy to excite TiO2, but is enough energy to excite WO3 is incident, the hole that is created in the WO3 valence band is excited to the conduction band of TiO2, while the electron is transferred to the conduction band of TiO2. It is this electron transfer that increases the charge separation and increases the efficiency of the photocatalytic process[12]. After separation, the electron is free to reduce the adsorbed organic compound and the hole is available to oxidize[13].

Figure 3 Coupled TiO2-WO3[12]

 

Conclusion

        Overall, Titanium dioxide (TiO2) is an ideal photocatalyst due to factors such as its photocatalytic properties, chemical stability, impact on the environment and cost. Decomposing organic pollutants using TiO2 is currently considered a possible decontamination process that could relieve much of the world’s problems with potable water. However, TiO2 is a wide band gap (3.2 eV) semiconductor and its application has been limited to ultraviolet (UV) environments. The band gap of TiO2 could be reduced by doping or coupling TiO2 with other semiconductors, transition metals and non-metal anions, and thereby expanding its effectiveness well into the visible range.

 References

  1. S. Malato, J. Blanco, D. Alarco´n, M. Maldonado, P. Fernández-Ibáñez, W. Gernjak, “Photocatalytic decontamination and disinfection of water with solar collectors,” Catalysis Today vol. 122, pp.137–149, 2007
  2. M. Kositzi, I. Poulios, S. Malato, J. Caceres, A. Campos, “Solar photocatalytic treatment of synthetic municipal wastewater,” Water Research, vol. 38, pp. 1147-1154, 2004
  3. A. Fujishima, X. Zhang, “Titanium dioxide photocatalysis: present situation and future approaches,” C.R. Chimie, vol. 9, pp. 750-760, 2006
  4. O. Carp, C. L. Huisman, and A. Reller, “Photoinduced reactivity of titanium dioxide,” Progress in Solid State Chemistry, vol. 32, pp. 33-177, 2004
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  6. T. Ohno, K. Sarukawa, K. Tokieda, and M. Matsumura, “Morphology of a TiO2 Photocatalyst (Degussa, P-25) Consisting of Anatase and Rutile Crystalline Phases,” Journal of Catalysis, vol. 203, pp. 82-86, 2001
  7. Wong, C.C. and W. Chu, The hydrogen peroxide-assisted photocatalytic degradation of alachlor in TiO2 suspensions. Environ Sci Technol, 2003. 37(10): p. 2310-2316.
  8. Herrmann, J.-M., Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants. Catalysis Today, 1999. 53(1): p. 115-129.
  9. Turchi, C.S. and D.F. Ollis, Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack. Journal of Catalysis, 1990. 122(1): p. 178.

10.  Klare, M., et al., Degradation of short-chain alkyl- and alkanolamines by TiO2- and Pt/TiO2-assisted photocatalysis. Chemosphere, 2000. 41(3): p. 353-362.

11. Dalrymple, K. , design and optimization of photocatalytic air disinfection systems. PhD proposal, 2009.

12. Schmidt, M., Thermochemical treatment of TiO2 Nanoparticles for photocatalytic applications. Master thesis, 2007.

13.  Dutta, P.K., et al., Photocatalytic oxidation of arsenic(III): evidence of hydroxyl radicals. Environ Sci Technol, 2005. 39(6): p. 1827-1834.

14.  Chen, C., et al., Photocatalysis by titanium dioxide and polyoxometalate/TiO2 cocatalysts. Intermediates and mechanistic study. Environ Sci Technol, 2004. 38(1): p. 329-37.

15.  A. L. Linsebigler, G. Lu, and J. T. Yates, “Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results,” Chemical Reviews, vol. 95, pp. 735-758, 1995.

16.  U. Diebold, “The surface science of titanium dioxide,” Surface Science Reports, vol. 48, pp. 53-229, 2003.

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