Single Atom Catalyst-Like Strontium Doping of Titania Nanotube Arrays for CO₂ Photoreduction

This study focuses on the details concerning the adsorption mechanisms, surface electronic band structure and structural disorder in anodically formed TiO₂ nanotube arrays (TNTAs) due to the surface doping of strontium Sr²⁺ metal cations that end up being widely dispersed on the surface of the TNTAs. The surface doping was achieved using an electrochemical cathodic treatment in a Sr²⁺-containing aqueous electrolyte. Sr²⁺ surface dopants also modified the crystallographic texture of the nanotubes.³ X-ray diffraction patterns and Raman spectra confirmed the induced lattice strain, reduced crystallite size, and phonon confinement in the TNTAs due to Sr-doping.² The absence of segregated phases of Sr or SrO or SrTiO₃ led to the conclusion that Sr atoms were high entropy surface dopants in TiO₂, widely dispersed on the surface of the TNTAs and approaching single atom catalysts insofar as the surface distribution of dopant Sr atoms is concerned. Fourier transformed infrared (FTIR) spectra of the Sr cathodized TNTAs indicated the presence of a small population of bridging bidentate carbonate adsorbate on Sr doped TNTA surfaces exposed to CO₂.³ Sr-doping reduced the work function of TNTAs and caused appreciable downward band-bending of 0.25 eV resulting in an electron accumulation layer at the surface. We hypothesize that such an electronic band-structure makes it easier for photogenerated electrons to be transferred to surface adsorbates for CO₂ reduction. The electrochemically cathodized Sr-doped TNTAs exhibited a more than 3-fold enhancement in the rate (25 µmol g^-1h ^-1) of CO₂/CO transformation in surpassing the output of bare TNTA (7.7 µmol g^-1h ^-1), thereby impressively improving the photocatalytic performance.<br/><b>References:</b><br/>1. Kisslinger, R.; Askar, A. M.; Thakur, U. K.; Riddell, S.; Dahunsi, D.; Zhang, Y.; Zeng, S.; Goswami, A.; Shankar, K., Preferentially oriented TiO₂ nanotube arrays on non-native substrates and their improved performance as electron transporting layer in halide perovskite solar cells. <i>Nanotechnology </i><b>2019,</b> <i>30</i> (20), 204003.<br/>2. Hamedani, H. A.; Allam, N. K.; Garmestani, H.; El-Sayed, M. A., Electrochemical Fabrication of Strontium-Doped TiO₂ Nanotube Array Electrodes and Investigation of Their Photoelectrochemical Properties. <i>The Journal of Physical Chemistry C </i><b>2011,</b> <i>115</i> (27), 13480-13486.<br/>3. Luo, C.; Zhao, J.; Li, Y.; Zhao, W.; Zeng, Y.; Wang, C., Photocatalytic CO₂ reduction over SrTiO₃: Correlation between surface structure and activity. <i>Applied Surface Science </i><b>2018,</b> <i>447</i>, 627-635.