In Situ Tuning of Defects and Phase Transition in Titanium Dioxide by Lithiothermic Reduction

A fast-charging/discharging and long-term stable artificial electrode enabled by space charge storage mechanism

Lithium-ion batteries with fast-charging/discharging properties are urgently needed for the mass adoption of electric vehicles. Here, we show that fast charging/discharging, long-term stable and high energy charge-storage properties can be realized in an artificial electrode made from a mixed electronic/ionic conductor material (Fe/LixM, where M = O, F, S, N) enabled by a space charge principle. Particularly, the Fe/Li2O electrode is able to be charged/discharged to 126 mAh g-1 in 6 s at a high current density of up to 50 A g-1, and it also shows stable cycling performance for 30,000 cycles at a current density of 10 A g-1, with a mass-loading of ~2.5 mg cm-2 of the electrode materials. This study demonstrates the critical role of the space charge storage mechanism in advancing electrochemical energy storage and provides an unconventional perspective for designing high-performance anode materials for lithium-ion batteries.

Control of metal oxides' electronic conductivity through visual intercalation chemical reactions

Cation intercalation is an effective method to optimize the electronic structures of metal oxides, but tuning intercalation structure and conductivity by manipulating ion movement is difficult. Here, we report a visual topochemical synthesis strategy to control intercalation pathways and structures and realize the rapid synthesis of flexible conductive metal oxide films in one minute at room temperature. Using flexible TiO2 nanofiber films as the prototype, we design three charge-driven models to intercalate preset Li+-ions into the TiO2 lattice slowly (µm/s), rapidly (mm/s), or ultrafast (cm/s). The Li+-intercalation causes real-time color changes of the TiO2 films from white to blue and then black, corresponding to the structures of LixTiO2 and LixTiO2-δ, and the enhanced conductivity from 0 to 1 and 40 S/m. This work realizes large-scale and rapid synthesis of flexible TiO2 nanofiber films with tunable conductivity and is expected to extend the synthesis to other conductive metal oxide films.

Three-in-one oxygen-deficient titanium dioxide in a pomegranate-inspired design for improved lithium storage

Defects engineering has played an ever-increasing important role in electrochemistry, especially secondary lithium batteries. TiO₂ is regarded as a promising anode due to its attractive cycling stability, low volume strain and great abundance, while challenges of intrinsic poor electrical and ionic conductivity remain to be addressed. Herein, we report a three-in-one oxygen vacancy (V_O)-involved pomegranate design for TiO_2-x/C composite anode, which provides highly improved electrical conduction, shortened Li⁺ pathway and promoted Li⁺ redox. N-doped mesoporous carbon acts as a robust scaffold to support the whole structure, electron highway and efficient reductant to generate V_O on TiO₂ nanoparticles during crystallization. Theoretical calculations reveal the crucial role of surface V_O on TiO₂ in Li electrochemistry. Resultantly, the optimal TiO_2-x/C anode achieves significantly enhanced cycling performance (203 mAh/g retained after 2000 cycles at 1 A/g). Postmortem analysis reveals the robustness of V_O and reasonable structure stability upon cycles for improved battery performance.

Hexagonal NiMoO4-MoS2 nanosheet heterostructure as a bifunctional electrocatalyst for urea oxidation assisted overall water electrolysis

A hexagonal NiMoO4-MoS2 nanosheet heterostructure on nickel foam (NiMoO4-MoS2/NF) was synthesized by simple hydrothermal and annealing treatment.

Defect Engineering of Photocatalysts towards Elevated CO2 Reduction Performance

Photocatalytic CO₂ reduction provides a promising solution to address the crises of massive CO₂ emissions and fossil energy shortages. As one of the most effective strategies to promote CO₂ photoconversion, defect engineering shows great potential in modulating the electronic structure and light absorption properties of photocatalysts while increasing surface active sites for CO₂ activation and conversion. This Review summarizes the recent progress in defect engineering of photocatalysts to promote CO₂ reduction performances from the following four aspects: 1) Approaches to defect (mainly vacancy and dopant) generation in photocatalysts; 2) defect structure characterization techniques; 3) physical and chemical properties of defect-engineered photocatalysts; 4) CO₂ reduction performance enhancements in activity, selectivity, and stability of photocatalysts by defect engineering. This Review is expected to present readers with a comprehensive view of progress in the field of photocatalytic CO₂ reduction through defect engineering for elevated CO₂ -to-fuels conversion efficiency.

Defective TiO2 for photocatalytic CO2 conversion to fuels and chemicals

Photocatalytic conversion of CO2 into fuels and valuable chemicals using solar energy is a promising technology to combat climate change and meet the growing energy demand. Extensive effort is going on for the development of a photocatalyst with desirable optical, surface and electronic properties. This review article discusses recent development in the field of photocatalytic CO2 conversion using defective TiO2. It specifically focuses on the different synthesis methodologies adapted to generate the defects and their impact on the chemical, optical and surface properties of TiO2 and, thus, photocatalytic CO2 conversion. It also encompasses theoretical investigations performed to understand the role of defects in adsorption and activation of CO2 and identify the mechanistic pathway which governs the formation and selectivity of different products. It is divided into three parts: (i) general mechanism and thermodynamic criteria for defective TiO2 catalyzed CO2 conversion, (ii) theoretical investigation on the role of defects in the CO2 adsorption-activation and mechanism responsible for the formation and selectivity of different products, and (iii) the effect of variation of physicochemical properties of defective TiO2 synthesized using different methods on the photocatalytic conversion of CO2. The review also discusses the limitations and the challenges of defective TiO2 photocatalysts that need to be overcome for the production of sustainable fuel utilizing solar energy.

Catalytic Metal-Accelerated Crystallization of High-Performance Solution-Processed Earth-Abundant Metal Oxide Semiconductors

As an alternative strategy for conventional high-temperature crystallization of metal oxide (MO) channel layers, the catalytic metal-accelerated crystallization (CMAC) process using a metal seed layer is demonstrated for low-temperature crystallization of solution-processed MO semiconductors. In the CMAC process, the catalytic metal layer plays the role of seed sites for initiating and accelerating the crystallization of amorphous MO films. Generally, the solution-processed crystalline-TiO₂ (c-TiO₂) films required high-temperature crystallization conditions (≥500-600 °C), showing low electrical performance with a high defect density. In contrast, the suggested CMAC process could effectively lower crystallization temperature of the a-TiO₂ films, enabling high-quality c-TiO₂ films with well-aligned anatase grains and low-defect density. The various crystalline catalytic layers were deposited over the earth-abundant n-type amorphous titanium oxide (a-TiO₂) films. Also, then, the CMAC process was performed for facile low-temperature translation of solution-processed a-TiO₂ to a highly crystallized state. In particular, the Al-CMAC process using the crystalline thin-aluminum (Al) catalytic metal seed layer facilitates low-temperature (≥300 °C) crystallization of the solution-processed a-TiO₂ films and the fabrication of high-performance solution-processed c-TiO₂ thin-film transistors with superior field-effect mobility, good on/off switching behavior, and improved operational stability.

Oxygen-Deficient Ferric Oxide as an Electrochemical Cathode Catalyst for High-Energy Lithium-Sulfur Batteries

Lithium-sulfur batteries, as one of promising next-generation energy storage devices, hold great potential to meet the demands of electric vehicles and grids due to their high specific energy. However, the sluggish kinetics and the inevitable "shuttle effect" severely limit the practical application of this technology. Recently, design of composite cathode with effective catalysts has been reported as an essential way to overcome these issues. In this work, oxygen-deficient ferric oxide (Fe₂ O_{3- x} ), prepared by lithiothermic reduction, is used as a low-cost and effective cathodic catalyst. By introducing a small amount of Fe₂ O_{3- x} into the cathode, the battery can deliver a high capacity of 512 mAh g^-1 over 500 cycles at 4 C, with a capacity fade rate of 0.049% per cycle. In addition, a self-supporting porous S@KB/Fe₂ O_{3- x} cathode with a high sulfur loading of 12.73 mg cm^-2 is prepared by freeze-drying, which can achieve a high areal capacity of 12.24 mAh cm^-2 at 0.05 C. Both the calculative and experimental results demonstrate that the Fe₂ O_{3- x} has a strong adsorption toward soluble polysulfides and can accelerate their subsequent conversion to insoluble products. As a result, this work provides a low-cost and effective catalyst candidate for the practical application of lithium-sulfur batteries.