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

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.