Cathodically Inserted, Widely Dispersed Sr2+ Surface Dopants Produce a Threefold Improvement in the CO2 Photoreduction Activity of TiO2 Nanotube Arrays

Strontium surface doping of anodic TiO2 nanotube arrays (TNTAs) is performed using an electrochemical cathodization method in an Sr2+-containing electrolyte. The doped strontium does not result in either phase-segregated Sr, SrO, or SrTiO3. Instead, Sr-doping results in modification of the crystallographic texture of anatase phase TNTAs, reduced crystal size, and increased lattice strain. Subtle changes are observed in the Fourier-transform infrared spectra (FTIR) of the carbon dioxide (CO2) adsorbed on the Sr-cathodized TNTA (Sr-C-TNTA), indicating a larger prevalence of monodentate carbonate and bidentate bicarbonate species on the surface. This is attributed to the higher alkalinity of surface hydroxyls bound to Sr in comparison to that of the Ti-bound hydroxyls. In addition to linearly adsorbed CO2, a population of bridging bidentate carbonate adsorbate is observed, suggesting an enhanced stabilization of the CO2 anion radical on the Sr-C-TNTA surfaces. Under AM1.5G 1 sun illumination, the Sr-C-TNTAs produce 25.0 μmol g–1 hr–1 carbon monoxide (CO), a greater than threefold improvement over the amount of CO generated by bare TNTAs (7.7 μmol g–1 hr–1). The spectroscopic characterization data are consistent with high-entropy surface-doping, creating relatively isolated and thermally stable Sr atoms on the surface of TiO2. These results highlight the potential of electrochemical cathodization in achieving high-entropy surface-doped semiconductors and single-atom catalysts.