Nickel Oxide Selectively Deposited on the {101} Facet of Anatase TiO₂ Nanocrystal Bipyramids for Enhanced Photocatalysis

Facet-engineered anatase TiO2 with NiO nanoparticles heterocontacts were successfully prepared by selective photodeposition of NiO nanoparticles onto the {101} facet of the top-truncated bipyramidal TiO2 anatase nanocrystals coexposed with {001} and {101} facets. The morphology and electronic properties of the resulting 0.1–10 wt % NiO-decorated TiO2 were investigated by X-ray diffraction, high-resolution electron microscopy, N2 sorption analysis, and UV–vis spectroscopy. Furthermore, a careful determination of the energy band alignment diagram was conducted by a model experiment using XPS and UPS to verify charge separation at the interface of the NiO−TiO2 heterostructure. The model experiment was performed by stepwise deposition of NiO onto oriented TiO2 substrates and in-situ photoelectron spectroscopy measurements without breaking vacuum. Core levels showed shifts of 0.58 eV toward lower binding energies, meaning an upward band bending in TiO2 at the NiO–TiO2 interface. Furthermore, 0.1 wt % NiO–TiO2 exhibited 50% higher activities than the pure TiO2 for methylene blue (MB) photodecomposition under UV irradiation. This enhanced photocatalytic activity of NiO–TiO2 nanocomposites was related to the internal electric field developed at the p–n NiO−TiO2 heterojunction, leading to vectorial charge separation. Finally, mechanistic studies conducted in the presence of carrier or radical scavengers revealed that holes dominantly contributed to the photocatalytic reactions in the case of NiO–TiO2 photocatalysts while electrons played the main role in photocatalysis for the pure TiO2 materials.