Oxygen Bridge Formed by Doping Nonmetal Atoms into Cationic Vacancies To Enhance the Photoelectrochemical Oxygen Evolution Reaction

To enhance photoelectrochemical (PEC) water splitting for renewable energy conversion, the conventional strategy is doping nonmetals into anionic vacancies. Compared to anionic vacancies, cationic vacancies are theoretically more effective and reliable for anchoring nonmetals owing to their larger radii and unique advantages. The current research mainly focuses on anionic vacancies, while there are few studies on cationic vacancies due to high formation energy and challenging characterizations by convenient techniques. To overcome the current limitations, nonmetallic S and P atoms are successfully doped into cationic vacancies on the TiO₂ surface for tuning local electronic structures. In contrast to the traditional strategy of reducing the bandgaps, nonmetallic atom doping into cationic vacancies facilitates efficient electronic regulation for PEC enhancement without changing the bandgap. The enhanced performance is attributed to the formation of an oxygen bridge, which can accumulate electrons from surrounding S/P atoms. Significantly, the electron-enriched oxygen bridge efficiently transfers electrons to activate reaction site Ti, which can promote the oxygen evolution reaction performance. Density functional theory calculations reveal that the decrease of reaction energy barriers and the optimization of local electron distribution are conducive to electronic transmission. This would provide a high-efficiency electronic tuning strategy for improving PEC performance.

Recent Advances in TiO2-based Photoanodes for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting has attracted a great attention in the past several decades which holds great promise to address global energy and environmental issues by converting solar energy into hydrogen. However, its low solar-to-hydrogen (STH) conversion efficiency remains a bottleneck for practical application. Developing efficient photoelectrocatalysts with high stability and high STH conversion efficiency is one of the key challenges. As a typical n-type semiconductor, titanium dioxide (TiO 2 ) exhibits high PEC water splitting performance, especially high chemical and photo stability. But, TiO 2 has also disadvantages such as wide band gap and fast electron-hole recombination rate, which seriously hinder its PEC performance. This review focuses on recent development in TiO 2 -based photoanodes as well as some key fundamentals. The corresponding mechanisms and key factors for high STH, and controllable synthesis and modification strategies are highlighted in this review. We conclude finally with an outlook providing a critical perspective on future trends on TiO 2 -based photoanodes for PEC water splitting.

N-Rich Doped Anatase TiO2 with Smart Defect Engineering as Efficient Photocatalysts for Acetaldehyde Degradation

Nitrogen (N) doping is an effective strategy for improving the solar-driven photocatalytic performance of anatase TiO2, but controllable methods for nitrogen-rich doping and associated defect engineering are highly desired. In this work, N-rich doped anatase TiO2 nanoparticles (4.2 at%) were successfully prepared via high-temperature nitridation based on thermally stable H3PO4-modified TiO2. Subsequently, the associated deep-energy-level defects such as oxygen vacancies and Ti3+ were successfully healed by smart photo-Fenton oxidation treatment. Under visible-light irradiation, the healed N-doped TiO2 exhibited a ~2-times higher activity of gas-phase acetaldehyde degradation than the non-treated one and even better than standard P25 TiO2 under UV-visible-light irradiation. The exceptional performance is attributed to the extended spectral response range from N-rich doping, the enhanced charge separation from hole capturing by N-doped species, and the healed defect levels with the proper thermodynamic ability for facilitating O2 reduction, depending on the results of ∙O2− radicals and defect measurement by electron spin resonance, X-ray photoelectron spectroscopy, atmosphere-controlled surface photovoltage spectra, etc. This work provides an easy and efficient strategy for the preparation of high-performance solar-driven TiO2 photocatalysts.

High content anion (S/Se/P) doping assisted by defect engineering with fast charge transfer kinetics for high-performance sodium ion capacitors

The rate-determining process for sodium storage in TiO₂ is greatly depending on charge transfer happening in the electrode materials owing to its inferior diffusion coefficient and electronic conductivity. Apart from reducing the diffusion distance of ion/electron, the increasement of ionic/electronic mobility in the crystal lattice is also very important for charge transport. Here, an oxygen vacancy (OV) engineering assisted in high-content anion (S/Se/P) doping strategy to enhance charge transfer kinetics for ultrafast sodium-storage performance is proposed. Theoretical calculations indicate that OV-engineering evokes spontaneous S doping into the TiO₂ phase and achieves high dopant concentration to bring about impurity state electron donor and electronic delocalization over S occupied sites, which can largely reduce the migration barrier of Na⁺. To realize the speculation, high-content anion doped anatase TiO₂/C composites (9.82 at% for S in A-TiO_2-x-S/C) are elaborately designed. The optimized A-TiO_2-x-S/C anode exhibits extraordinarily high-rate capability with 209.6 mAh g^-1 at 5000 mA g^-1. The assembled sodium ion capacitors deliver an ultrahigh energy density of 150.1 Wh kg^-1 at a power density of 150 W kg^-1 when applied as anode materials. This work provides a new strategy to realize high content anion doping concentration, and enhances the charge transfer kinetics for TiO₂, which delivers an efficient approach for the design of electrode materials with fast kinetic.

Surface domain heterojunction on rutile TiO2 for highly efficient photocatalytic hydrogen evolution

Compared with the highly active anatase TiO2, rutile TiO2 usually presents poor photocatalytic performance due to high electron-hole recombination. Herein, we propose a surface domain heterojunction (SDH) structure between adjacent micro-domains with and without chemisorbed chlorine on rutile TiO2, which utilizes the potential difference between these domains to form a built-in field that promotes charge separation. Single-crystal rutile TiO2 nanorods assembled into radial microspheres with SDHs were fabricated, and these exhibited excellent solar-driven photocatalytic hydrogen evolution, ∼8-fold higher than that of the pristine one. Experimental results and density functional theory calculations reveal that the exceptional photocatalytic performance can be attributed to the in situ formation of chemisorbed chlorine, which forms SDHs that separate electrons and holes efficiently and results in surface reconfiguration, exposing the tri-active sites, increasing the O-site active centers and enhancing the catalytic activity of the 4-coordinated (Ti4c) and 5-coordinated Ti sites (Ti5c). This SDH strategy can extend to other halogen elements and thus provides an universal approach for the rational design of high-efficiency TiO2 photocatalysts toward sustainable solar-fuel evolution.