Sol-Gel Electrophoretically Deposited TiO2-Multiwalled Carbon Nanotube-SiO2 Thin-Film Electrode with High Photoelectrochemical Activity

Abstract

Hydrogen production via water splitting has been extensively researched for its environmental friendliness, energy efficiency, and renewability. This study describes the development of TiO2-multiwalled carbon nanotube (MWCNT)-SiO2 composite thin-film electrodes via electrophoretic deposition (EPD) from a 2-propanol solution of MWCNTs including TiO2 and SiO2 gels. The TiO2 and SiO2 gels were prepared via the sol-gel method and by mixing in varying weight ratios to enhance the efficiency of photoelectrochemical water splitting. Dual sol-gel EPD incorporates MWCNTs with a C/TiO2 molar ratio of ≥0.25 while varying the TiO2/SiO2 molar ratio from 5 to 14; the electronic conductivity is improved owing to the pristine graphene structure of the MWCNTs along with hydrophilicity imparted by SiO2. In addition, the volume of SiO2 sol influences the anatase-to-rutile ratio, the TiO2 crystal size, and chemical bonds, thereby affecting the formation of new energy levels. The optimal volume of SiO2 sol results in elevated ultraviolet-visible absorbance, attributed to midgap states generated by a high anatase-to-rutile ratio and Ti-O-Si formation, further leading to a substantial effective carrier density for the photoelectrochemical water-splitting reaction. Furthermore, the valence band maximum (VBM) and conduction band minimum, estimated using ultraviolet photoelectron and ultraviolet-visible spectroscopies, exhibited a downward shift with increasing SiO2 sol volume, followed by an upward shift; meanwhile, the Fermi level in a Na2SO4 solution under stimulated solar light deepened. The highest photoelectrochemical performance is achieved at the optimal SiO2 sol volume, where the VBM is deep enough to minimize the water-splitting overpotential, and the flat-band potential aligns with the set potential, thereby reducing band bending with a negligible hole depletion layer at the TiO2-solution interface. The best TiO2-MWCNT-SiO2 composite exhibits a photocurrent ∼7.4 times higher than that of a TiO2-MWCNT electrode.