Metal-semiconductor photocatalysts

After some general remarks on the relations between semiconductor properties and their use as photocatalysts, this text will first deal with oxidations of organic compounds.The interactions of illuminated semiconductors with gaseous C>2 (and, for comparison, with gaseous NO) will be then presented, whereas the last part will consider metal/semiconductor photocatalysts and the organic reactions they allow. In this presentation, the results of this laboratory will be highlighted. 

Recently was estimated an expected impact on the global chemistry of the atmosphere of the indirect heterogeneous photocatalytic reactions under the much more abundant near ultraviolet, visible and near infrared solar light [2]. As photocatalysts may serve atmospheric aerosols, i.e. ultrasmall solid particles that sometimes are embedded into liquid droplets. Aerosols are known to contain Ti02, Fc203, ZnO and other natural oxides, as well as metal sulfides of volcanic or antropogenic origin, that may serve as semiconductor photocatalysts (see Fig.5). Aerosols are known to be concentrated mainly in the air layers near the surface of the Earth, i.e. in the troposphere, rather than stratosphere. 

Y. Nosaka, K. Norimatsu, H. Miyama, The function of metals in metal-compounded semiconductor photocatalysts, Chem. Phys. Lett. 106 (1984) 128-131. 

Various pairs of inorganic ions such as lOsVr, Fe /Fe, and Ce /Ce have been used as redox mediators to facilitate electron-hole separation in metal loaded oxide semiconductor photocatalysts [105-107], Two different photocatalysts, Pt-Ti02 (anatase) and Ti02 (rutile), suspended in an aqueous solution of Nal were employed to produce H2 and O2 under, respectively, the mediation of 1 (electron donor) and IOs (electron acceptor) [105]. The following steps are involved in a one-cell reaction in the presence of UV light. 

Zou Z, Ye J, Sayama K, Arakawa H (2001) Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst. Nature 414 625-627 Konta R, Ishii T, Kato H, Kudo A (2004) Photocatalytic activities of noble metal ion doped SrTiOs under visible light irradiation. J Phys Chem 108 8992-8995 Kato H, Kudo A (2002) Photocatalytic activities of noble metal ion doped SrTiOs under visible light irradiation. J Phys Chem B 106 5029-5034... 

Even without deposition of a metal island, such powders often maintain photoactivity. The requirement for effective photoelectrochemical conversion on untreated surfaces is that either the oxidation or reduction half reaction occur readily on the dark material upon application of an appropriate potential, so that one of the photogenerated charge carries can be efficiently scavenged. Thus, for some photoinduced redox reactions, metallization of the semiconductor photocatalyst will be essential, whereas for others platinization will have nearly no effect. 

We have studied photocatalytic properties of nanostructured metal-semiconductor composites, made from mesoporous samples of Ti02 and a number of metals (Cu2+, Ni2+, Co2+, Cd2+, Fe2+, Ag+, Zn2+, Pb2+) in hydrogen evolution from water-ethanol solutions. Correlations between the quantum yields of the photoreaction and various parameters of the reacting system (such as the metal nature and concentration, photocatalyst quantity, light intensity, temperature) have been found and discussed. It has been shown, that maximal quantum yield of hydrogen production (y = 0.44) could be achieved in case of Ti02/Cu composite. 

Thin-lilm photoelectrodes are needed in photoelectrocatalytic systems to apply a bias potential, either for the photoelectrode characterization or to facilitate the photocatalytic reactions. However, to be able to present a more comprehensive view on the performance of different materials, our subsequent discussions will focus on particulate semiconductor photocatalysts since the latter have been much more extensively investigated. Their electronic band structure (i.e., both the bandgap energy and the positions of CB and VB) is the key factor to determine whether or not a semiconductor material is suitable for a specific photocatalytic reaction, as will be demonstrated by reviewing a number of selected metal oxides and cou-pled/composite materials based on various semiconductors. 

The above-mentioned aqueous-phase methods have shown their ability to produce structure-controlled (on the nanometer scale) photoelectrodes. In this section, several other methods operated in aqueous phase will be briefly discussed with a focus on the synthesis of composite photocatalysts such as bimetal oxides and metal/semiconductor nanocomposite materials. 

Given the importance of the oxidation states of the actinides in reprocessing and the capability of heterogeneous semiconductor photocatalysts to manipulate the valence states of a wide range of metal ions, attention has been given to the potential applications of photocatalysis in nuclear reprocessing scenarios [59-65]. The attractions of this are twofold, and lead to a further minimisation of the environmental impact associated with fuel use and processing ... 

When deployed on-line, the semiconductor photocatalyst may be required to photoreduce more than one type of actinide metal ion simultaneously. Figure 9 shows the effect of illuminating U(VI) with light of wavelength 350 nm in the presence of colloidal SnCh, nitric acid (pH 0) and ethanol as an electron scavenger for the semiconductor photocatalyst and Ce(IV) as a non-radioactive, thermodynamic analogue for Pu(IV). Comparison of the data in Fig. 9 with the data recorded under similar conditions as shown in Fig. 7 indicates that the presence of Ce(IV) has no effect on the rate of photocatalysed reduction of U(VI) to U(IV). Furthermore, spectroscopic analysis indicates that virtually all of the Ce(IV) has been reduced to Ce(III) over the same timescale, suggesting that the simultaneous photocatalysed reduction of two or more different types of (actinide) metal ion can be accomplished with no loss of yield for either reaction. 

Figure 9. Metal-modified semiconductor photocatalyst particle.

Emeline A. V., Frolov A., Ryabchnk V. K and Serpone N. (2003), Spectral dependencies of the quantum yield of photochemical processes on the surface of nano-/microparticulates of wide bandgap metal oxides. IV. Theoretical modelling of the activity and selectivity of semiconductor photocatalysts with inclusion of a snbsnrface electric field in the space-charge region , J. Phys. Chem. B 107, 7109-7119. 

Nakato Y., Shioji M. and Tsubomura H. (1982), Photoeffects on the potentials of thin metal-films on a n-TiOa crystal wafer. The mechanism of semiconductor photocatalysts , Chem. Phys. Lett. 90, 453-456. 

Therefore, significant improvement in the photoactivity is possible only at low dopant concentrations. It further activates the competitive processes on the photocatalyst surfaces, however, the exact involvement of these processes are still not known. The surface metal ion activates the carbon dioxide molecule in a different way as that of semiconductors, leading to a relatively high product yield compared to the bare photocatalyst. The earth s abundance and cost of the metals are the main problems which adversely affect the process economics, hence the metal doping increases the overall cost of the process. Hence the practical implementation is not possible with the metal doped photocatalyst even though it s a promising route for the conversion of carbon dioxide into chemical and fuels. 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

Keywords Electron transfer Metal complex photocatalyst Photocatalytic CO2 reduction Semiconductor photocatalyst Supramolecular chemistry... 

Recently, the number of reports about CO2 reduction using semiconductor photocatalysts has been increasing rapidly. Some metal oxide semiconductors, typically Ti02, have advantages over molecular catalysts, in that they can use water as a reductant. However, the efficiencies of many of the semiconductor photocatalysts for CO2 reduction remain quite low, mainly because of competition... 

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