Remarkable improvement of photoelectrochemical water splitting in pristine and black anodic TiO2 nanotubes by enhancing microstructural ordering and uniformity

Efficient photoelectrochemical (PEC) water splitting is crucial for future energy and sus-tainable world. We here report on the improvement of PEC activity of anodic TiO2 nano-tubes (TNTs) by enhancing tube ordering and subsequent electrochemical reduction. TNTs were prepared by two-step anodic oxidisation from an organic electrolyte containing fluoride ions. The effects of first-step anodisation time on the ordering of TNTs and sub-sequent electrolytic reduction were investigated on the PEC performance under simulated solar light spectrum. The photocurrent densities of TNTs anodised for 1 h, 4 h and sub-sequently reduced are about 25.12 mu A cm-2, 51.76 mu A cm-2 and 126.89 mu A cm-2, respec-tively, at 1.23 V vs RHE and their conversion efficiency of light to electrical energy achieved are about 0.016%, 0.04% and 0.08% respectively. Electrochemical impedance spectroscopy (ESI) curves revealed the improved PEC water splitting confirmed by sharper charge carrier separation and enhanced charge transfer in highly ordered pristine and black TNTs. This improvement of PEC in dopant-free TNT is at the first instance interpreted by enhancing TNT ordering and uniformity achieved by prolonging of the first-step anodisation time and its effect on the electronic band structure of TNTs. This significant effect on PEC perfor-mance of pristine TNT under visible light absorption takes place due to the induced surface defects and slower recombination rates of hole and electron. This demonstrates an effi-cient economic materials production appraoch for PEC hydrogen production.(c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.