Enhanced catalytic activity and hydrogen production of Ho-TiO₂ nanotubes prepared under various pH conditions

The photocatalytic hydrogen production range of anatase TiO₂ lies in the ultraviolet region but can be extended to the visible-light region via lanthanide-doping-based bandgap tailoring. Holmium exhibits the highest photocatalytic activity among the lanthanides; however, the effects of the pH of the Ho precursor solution—an important surface charge attribute—on the photoelectrochemical properties of Ho-doped TiO₂ nanotubes (TNTs) are unexplored. Optimal conditions for photoanode in photoelectrochemical water splitting were established by observing the effects of pH on the charge of Ho species in aqueous solutions and the surface charge of pure TNTs. Here, Ho-doped TiO₂ nanotubes (Ho-TNT) were fabricated by electrochemical deposition using variable-pH Ho solutions and anodization and characterized in terms of their physicochemical and electrochemical properties and photocatalytic water splitting based hydrogen production performance. The highest photocatalytic activity (hydrogen production rate = 79.44 μmol cm⁻²h⁻¹, photocurrent density = 8.48 mA cm⁻², photoconversion efficiency = 7.97 % at 0 V vs. the reversible hydrogen electrode) was observed for the sample fabricated at pH 7 (H7). The research revealed that the catalytic activity of TiO₂ nanotubes decorated with holmium was influenced by the pH of the Ho solution, with Ho-TNTs prepared at pH 7 demonstrating the potential of this strategy to enhance the efficiency in photoelectrochemical hydrogen production.