Iron-based oxides are promising bifunctional electrocatalysts. The energy conversion efficiency of water splitting is limited by the scarcity of active sites and sluggish surface reactions in Fe₂O₃. Therefore, we prepared one-dimensional Fe₂O₃ nanobelt arrays (HNBs-V_O(LRO)-S) with ordered oxygen vacancy (V_O) structures via Pd-catalyzed oxygen reduction and sulfide thermal treatment. While preserving the ordered oxygen vacancy structure, sulfur (S) atoms selectively fill the trap-state oxygen vacancies to improve the bifunctional electrocatalytic activity and stability of Fe₂O₃. Fe₂O₃ nanobelt arrays with synergistic interactions between S atoms and ordered oxygen vacancies have low overpotentials for the anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER). Under 1 M potassium hydroxide (KOH), HNBs-V_O(LRO)-S exhibited excellent electrocatalytic performance for both the HER (226 mV@100 mA·cm⁻²) and the OER (262 and 306 mV@100 mA·cm⁻²). In addition, the HNBs-V_O(LRO)-S bifunctional catalyst only requires a low cell voltage of 1.92 V to deliver a current density of 100 mA·cm⁻² and exhibits excellent long-term durability for over 100 h. The long-range ordered oxygen vacancies serve as fast channels for electron transfer and as active sites for the catalytic reaction. The S atoms only fill the trap-state oxygen vacancies (TS-V_O) in the Fe₂O₃ nanobelts, which eliminates the negative effect of TS-V_O in the reaction. Moreover, the formed Fe–S coordination structure both stabilizes the ordered oxygen vacancy structure of HNBs-V_O(LRO)-S and provides more reactive active sites for the electrocatalytic reaction. Theoretical calculations show that the filling of S atoms lowers the free energy barrier such that the formation of OOH^* from O^* optimizes the free energy of uptake of the hydrogen intermediate H* (∆G_H*) of the Fe₂O₃ surface. This ingenious synergistic mechanism of vacancy filling provides new insights into the defective design of catalysts.

Photocatalytic non-oxidative coupling of methane (PNOCM) is a mild and cost-effective method for the production of multicarbon compounds. However, the separation of photogenerated charges and activation of methane (CH₄) are the main challenges for this reaction. Here, single crystal-like TiO₂ nanotubes (V_O-p-TNTs) with oxygen vacancies (V_O) and preferential orientation were prepared and applied to PNOCM. The results demonstrate that the significantly enhanced photocatalytic performance is mainly related to the strong synergistic effect between preferential orientation and V_O. The preferential orientation of V_O-p-TNT along the [001] direction reduces the formation of complex centers at grain boundaries as the form of interfacial states and potential barriers, which improves the separation and transport of photogenerated carriers. Meanwhile, V_O provides abundant coordination unsaturated sites for CH₄ chemisorption and also acts as electron traps to hinder the recombination of electrons and holes, establishing an effective electron transfer channel between the adsorbed CH₄ molecule and photocatalyst, thus weakening the C–H bond. In addition, the introduction of V_O broadens the light absorption range. As a result, V_O-p-TNT exhibits excellent PNOCM performance and provides new insights into catalyst design for CH₄ conversion.

Effective charge separation and transfer is deemed to be the contributing factor to achieve high photoelectrochemical (PEC) water splitting performance on photoelectrodes. Building a phase junction structure with controllable phase transition of WO₃ can further improve the photocatalytic performance. In this work, we realized the transition from orthorhombic to monoclinic by regulating the annealing temperatures, and constructed an orthorhombic–monoclinic WO₃ (o-WO₃/m-WO₃) phase junction. The formation of oxygen vacancies causes an imbalance of the charge distribution in the crystal structure, which changes the W–O bond length and bond angle, accelerating the phase transition. As expected, an optimum PEC activity was achieved over the o-WO₃/m-WO₃ phase junction in WO₃-450 photoelectrode, yielding the maximum O₂ evolution rate roughly 32 times higher than that of pure WO₃-250 without any sacrificial agents under visible light irradiation. The enhancement of catalytic activity is attributed to the atomically smooth interface with a highly matched lattice and robust built-in electric field around the phase junction, which leads to a less-defective and abrupt interface and provides a smooth interfacial charge separation and transfer path, leading to improved charge separation and transfer efficiency and a great enhancement in photocatalytic activity. This work strikes out on new paths in the formation of an oxygen vacancy-induced phase transition and provides new ideas for the design of catalysts.

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.