Visible light induced hydrogen production with cu(II)/Bi2O3 and Pt/Bi2O3/RuO 2 from aqueous methyl viologen solution

Abstract

Gamma-Bi203, a rarely studied oxide semiconductor, was doped individually with Cu(II) in different concentrations, Pt and RuO2 (0.5 wt%) and used as a fine powder for photocatalytic production of hydrogen from water in the presence of methyl viologen (MV 2+) as an electron relay. The diffuse absorption spectroscopy of the samples revealed that the doping process improved the absorption of ~-Bi203 in the visible region while the XRD spectra indicated no change in crystal structure, but an enhancement in crystallinity. For 90 min irradiation (X >_ 400 nm) of the aqueous slurry of the catalyst in presence of MV 2+, 0.79 and 0.66 ml of hydrogen were generated by 4 at% Cu(II)/Bi:O3 and ~/Bi203/RuO2 respectively. A suitable mechanism involving the reactions of photogenerated e- and h + has been proposed for the photodecomposition of water. The effects of dopant concentration and the catalyst amount on hydrogen production have also been explained. INTRODUCTION A great deal of research on the conversion of solar to chemical energy has been directed towards the development of photocatalytic systems capable of splitting water into hydrogen and oxygen [ 1 ]. The cleavage of water has been achieved by photocatalytic [2] and photoelectrochemical [3] means. In all these researches, it is emphasized that semiconductors with band-gap energies matching the solar spectrum could play a prominent role, as they are able to act as photocatalysts for the cleavage of water into hydrogen and oxygen in the presence of an electron relay, e.g. methyl viologen (MV2+). A serious problem associated with such semiconductors, for example, the chalcogenide semiconductors (CdS; Eg = 2.4 eV) is their inherent instability [4], because these semiconductors are subject to undesirable photocorrosion by the formed h + itself upon irradiation. The oxide semiconductors (e.g. WO3, Fe203 and TiO2 etc.), on the other hand, have exceptional stability both in acidic and alkaline media [5], but their disadvantage is their band-gaps are usually so wide that they absorb only a very small portion of the solar spectrum. Hence, in order to make them good light-absorbing catalysts, they must be sensitized "internally" by doping with transition metal ions or some external photosensitizer such as Ru(bpy)3 2+ must be used. Copper(II)-doping is particularly attractive [6- 8 ] because it sensitizes the semiconductors to absorb more light in the visible region as well as enhancing their catalytic efficiency through its ability to trap eca and to transfer it efficiently to the adsorbed redox species.