Oxygen vacant gray shell and interior ordered TiO₂ core for enhanced visible-light photocatalysis

Oxygen vacancy in metal oxides serves a key function in improving their (photo)(electro)catalytic activity, though quantifying and understanding the exact location and making the oxygen vacancy functional is challenging. Here, we used the depth profiling technique during the X-ray photoelectron spectroscopy (XPS) measurement to investigate the oxygen vacancy in titania created by treating it with a borohydride compound at 350 °C. By etching the surface through the sputtering Ar ions on the sample, the first few layers were removed from the surface of the titania to understand the interior compositions. The results show that by etching only the first few layers, the oxygen ratio is enhanced by 16.5 % compared to the pristine form, but further etching and going into more depth almost show the same ratio. Results also show that these defects are initially covered by NaBO₂, blocking the surface photoactivity. The obtained titania is a gray-shaded titania that shows enhanced photocatalytic activity than the pristine P25 under visible light in degradation tests, achieving 81.47 % degradation efficiency (DE) for oxytetracycline (OTC), with the increased surface area and active defect sites.