Effective photoelectrocatalytic reduction of CO₂to formic acid using controllably annealed TiO₂nanoparticles derived from porous structured Ti foil

The rate of global warming and unfavorable climate changes caused by the drastic upsurge of carbon dioxide (CO₂) emission has necessitated the development of approaches to limit the significantly high concentration of CO₂ in the atmosphere. The photoelectrochemical reduction of CO₂ results in a reduction of the energy required to transform this greenhouse gas into valuable end products. In this study, we fabricated cost-effective and novel 3D nanoporous structured (3DNS) TiO2 nanoparticles (T-NPs) on the surface of a thin titanium foil (T-foil) by chemical treatment with hydrogen peroxide (H2O2) followed by calcination at high temperatures in the range of 400-800 °C. The as-proposed samples were analyzed by several characterizations such as XRD, XPS, TEM, and Raman spectroscopy. At 600 °C, the anatase-dominated mixed phases of calcinated T-foil (TO600) were seen, and a maximum photocurrent density of 71.5 μA/cm2 was obtained, in comparison to the T-foils treated at other temperatures (TO400, TO500, TO700, and TO800). Because of the better photocurrent density, TO600 was selected as the photocathode material for photoelectrochemical CO₂ reduction performed with or without the presence of solar light. The lowest CO₂ reduction onset potential (-1.191 V) was observed on the TO600 sample in the presence of light with Ag/AgCl as the reference electrode. 1H NMR analysis of the product solution revealed the formation of formic acid as the major product of the CO₂ reduction reaction after the chronoamperometric electrolysis was performed for more than 25 h. The maximum faradaic efficiency (64%) and formic acid yield (165 μmol cm-2 h-1) were obtained at an applied potential of-1.3 V (vs. Ag/AgCl reference electrode) for TO600.