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

Facet-engineered anatase TiO 2 with NiO nanoparticles heterocontacts were successfully prepared by selective photodeposition of NiO nanoparticles onto the {101} facet of the top-truncated bipyramidal TiO 2 anatase nanocrystals coexposed with {001} and {101} facets. The morphology and electronic properties of the resulting 0.1–10 wt % NiO-decorated TiO 2 were investigated by X-ray diffraction, high-resolution electron microscopy, N 2 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−TiO 2 heterostructure. The model experiment was performed by stepwise deposition of NiO onto oriented TiO 2 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 TiO 2 at the NiO–TiO 2 interface. Furthermore, 0.1 wt % NiO–TiO 2 exhibited 50% higher activities than the pure TiO 2 for methylene blue (MB) photodecomposition under UV irradiation. This enhanced photocatalytic activity of NiO–TiO 2 nanocomposites was related to the internal electric field developed at the p–n NiO−TiO 2 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–TiO 2 photocatalysts while electrons played the main role in photocatalysis for the pure TiO 2 materials.