Photochemistry of ZnO/GeO2 film for H2 production

Abstract

In this study, ZnO/GeO2 compound was prepared by the sol-gel method and deposited onto ITO conductive glass substrate (Indium Tin Oxide) and non-conductive glass by spin coating technique to be used as active materials for H2 generation by means of photoelectrochemical and photocatalytic processes. Films were characterized by X-ray diffraction, which crystallized in ZnO type-structure. By UV-Vis tests, films showed an improvement in the light absorption and a shift towards the visible spectrum; Eg value of ZnO/GeO2 is slightly lower than ZnO value (3.15 vs. 3.25 eV, respectively). PL analysis exhibited a lower intensity of ZnO/GeO2 curve than ZnO, indicating low electron-hole pair recombination. ZnO/GeO2 films presented spherical grains and high roughness as revealed by AFM micrographs, which is beneficial to increase the number of active sites on the surface and reduces the light scattering for better catalytic performance. Photoelectrochemical characterization demonstrates that ZnO/GeO2 showed excellent photocurrent response due to the better charge transfer, low resistance and low impedance observed in the Nyquist curves and Bode diagrams, as well as an increase in donor density as Mott-Schottky measurements indicated. In this case, ZnO/GeO2 film generates five times higher photocurrent than ZnO under solar light simulated (0.5 vs. 0.1 mA/cm2). Furthermore, Incident Photon to Current Conversion Efficiency (IPCE) for ZnO/GeO2 was 63% at 350 nm in wavelength. The Solar-To-Hydrogen (STH) conversion for ZnO/GeO2 was six times higher than ZnO (0.24 vs. 0.04%, respectively). This conversion is due to the presence of GeO2, which is causing a lower depletion layer, favoring minor recombination of the electron-hole pair in the semiconductor. On the other hand, photocatalytic activity for H2 production showed a 6-fold increase for ZnO/GeO2 (31.6 vs 5.6 µmol H2, respectively). The presence of GeO2 increases the number of active sites, and the hole-electron pair separation, avoiding photocorrosion, a common problem for ZnO. Then, ZnO/GeO2 film could be a promising candidate for H2 generation under photoinduced processes.