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.