Various material design strategies have been developed to enhance photocatalytic performance of TiO₂. However, no report is available on applications of the photopiezocatalysis strategy on TiO₂ due to its lack of piezoelectricity. Here we developed a low-temperature molten salt etching process to create rutile TiO₂ nanoparticles by etching [MgO₆] octahedrons away from MgTiO₃ by molten NH₄Cl, during which a lattice distortion occurred in TiO₂. The lattice distortion broke the structure symmetry of rutile TiO₂ and subsequently endowed these rutile TiO₂ nanoparticles with an unusual piezoelectric response with the maximum effective piezoelectric coefficient (d₃₃) of ~41.6 pm/V, which had not previously been found in TiO₂ photocatalysts. Thus, the photopiezocatalysis strategy was applied for the first time to enhance the photocatalytic performance of these TiO₂ nanoparticles. The creation of lattice distortion to induce piezoelectricity could be extended to other photocatalysts that the photopiezocatalysis strategy has not been applied to and may generate novel functionalities for various technical applications.

Photocatalytic CO₂ reduction to valuable chemical compounds could be a promising approach for carbon-neutral practice. In this work, a simple and robust thermal decomposition process was developed with ammonium carbonate ((NH₄)₂CO₃) as both precipitation agent and sacrificial template to produce fine Nb₂O₅ nanoparticles with the rich existence of surface hydroxyl (–OH) groups. It was found by density functional theory (DFT) calculations and experiments that the rich existence of the surface –OH groups enhanced the adsorption of both reactants (CO₂ and H₂O molecules) for the photocatalytic CO₂ reduction on these fine Nb₂O₅ nanoparticles, and the highly selective conversion of CO₂ to the high-value chemical compound of ethylene (C₂H₄, ~68 μmol·g⁻¹·h⁻¹ with ~100% product selectivity) was achieved under simulated solar illumination without usage of any sacrificial agents or noble metal cocatalysts. This synthesis process may also be readily applied as a surface engineering method to enrich the existence of the surface –OH groups on various metal oxide-based photocatalysts for a broad range of technical applications.

As an effective means to improve charge carrier separation efficiency and directional transport, the gradient doping of foreign elements to build multi-homojunction structures inside catalysts has received wide attentions. Herein, we reported a simple and robust method to construct multi-homojunctions in black TiO₂ nanotubes by the gradient doping of Ni species through the diffusion of deposited Ni element on the top of black TiO₂ nanotubes driven by a high temperature annealing process. The gradient Ni distribution created parts of different Fermi energy levels and energy band structures within the same black TiO₂ nanotube, which subsequently formed two series of multi-homojunctions within it. This special multi-homojunction structure largely enhanced the charge carrier separation and transportation, while the low concentration of defect states near the surface layer further inhibited carrier recombination and facilitated the surface reaction. Thus, the B-TNT-2Ni sample with the optimized Ni doping concentration exhibited an enhanced hydrogen evolution rate of ~ 1.84 mmol·g⁻¹·h⁻¹ under visible light irradiation without the assistance of noble-metal cocatalysts, ~ four times higher than that of the pristine black TiO₂ nanotube array. With the capability to create multi-homojunction structures, this approach could be readily applied to various dopant systems and catalyst materials for a broad range of technical applications.

It is important to develop green and sustainable approaches to enhance electrochemical charge storage efficiencies. Herein, a two-step in-situ growth process was developed to fabricate carbon fiber paper-supported CeO₂/MnO₂ composite (CeO₂/MnO₂–CFP) as a binder-free photoelectrode for the photo-assisted electrochemical charge storage. The formation of CeO₂/MnO₂ type II heterojunction largely enhanced the separation efficiency of photo-generated charge carriers, resulting in a substantially enhanced photo-assisted charging capability of ~20%. Furthermore, it retained a large part of its photo-enhanced capacitance (~56%) in dark even after the illumination was off for 12 h, which could be attributed to its slow release of stored photo-generated electrons from its specific band structure to avoid their reaction with O₂ in dark. This study proposed the design principles for supercapacitors with both the photo-assisted charging capability and its long-lasting retainment in dark, which may be readily applied to other pseudocapacitive materials to better utilize solar energy.