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Sodium ion batteries have a huge potential for large-scale energy storage for the low cost and abundance of sodium resources. In this work, a novel structure of ultrafine polycrystalline TiO2 nanofibers is prepared on nickel foam/carbon cloth by a simple vapor deposition method. The as-prepared TiO2 nanofibers show excellent performance when used as anodes for sodium-ion batteries. Specifically, the TiO2 nanofibers@nickel foam electrode delivers a high reversible capacity of 263.2 mAh g−1 at 0.2 C and maintains a considerable capacity of 144.2 mAh g−1 at 10 C. The TiO2 nanofibers@carbon cloth electrode also shows excellent high-rate capability, sustaining a capacity of 148 mAh g−1 after 2000 cycles at 10 C. It is believed that the novel nanofibrous structure increases the contact area with the electrolyte and greatly shortens the sodium ion diffusion distance, and meanwhile, the polycrystalline nature of nanofibers exposes more intercalation sites for sodium storage. Furthermore, the density functional theory calculations exhibit strong ionic interactions between the exposed TiO2 (101) facets and sodium ions, leading to a preferable sodiation/desodiation process. The unique structural features endow the TiO2 nanofibers electrodes great advantages in rapid sodium storage with an outstanding high-rate capability.

To address the bottlenecks of sluggish sodium-ion transfer processes, the electrode materials of sodium-ion battery (SIB) are always designed from the viewpoints of sodium-ion diffusion kinetics or reaction thermodynamics for high-performance sodium storage. Herein, starting with spongiform TiO2/C composite derived from zinc alginate, a NH4Cl-assisted annealing strategy is developed to prepare chlorine-doped spongiform TiO2/C composite (ZATC-Cl). Acting as the anode of SIBs, the obtained ZATC-Cl delivers excellent sodium storage performance with a high reversible capacity of 352.4 mAh g−1 at 50 mA g−1, a superior rate capability of 246.8 mAh g−1 at 2 A g−1 and a considerable high-rate cycling performance of 248.5 mAh g−1 at 2 A g−1 for 1000 cycles. It is believed that the unique spongiform structure and ultra-small sized TiO2 particles in ZATC-Cl can achieve fast sodium-ion insertion/extraction, realizing rapid sodium-ion diffusion kinetics. Moreover, as well evidenced by experiment results and theoretical calculations, the Cl dopants introduced in ZATC-Cl can provide more active sites for robust sodium-ion chemical adsorption, optimizing sodium-ion reaction thermodynamics. This study demonstrates an alternative approach to improve the sodium storage capability of TiO2 anodes by the synergistically engineering the morphological and structural properties and manipulating the surface active sites, from both kinetic and thermodynamic viewpoints of sodium storage processes.

With the properties of high theoretical capacity and excellent structural stability, TiO2 has been widely studied as anode material for sodium-ion batteries (SIBs). In this work, a composite structure of ultra-small TiO2 nanoparticles embedded in carbon nanosheets (TCNS) is obtained by annealing polyvinylpyrrolidone coated Zn-Ti layered double hydroxide under inert atmosphere, with zinc species removed by hydrochloric acid. As sodium-ion battery anode, the resultant TCNS shows a high reversible capacity of 271.2 mAh g−1 at 50 mA g−1 and considerable cycling stability (maintaining 196.7 mAh g−1 after 2000 cycles at 2 A g−1). Ex-situ XRD and TEM investigations clearly illustrate the structure changes of anatase during the sodium storage process. Specifically, ultra-small TiO2 nanoparticles in TCNS show obvious crystal distortion as triggered by the initial insertion of sodium ions, with the reversible sodium storage happening at the (1 0 1) active plane of anatase. As experimentally and theoretically evidenced, the twisted crystal structure is maintained in the subsequent cycles, which can effectively promote the sodium diffusion rate in anatase, resulting in the excellent rate and cycle performances of TCNS anode. This study provides informative guidance to explore high-performance TiO2 anodes for SIBs, with novel insights into the sodium ion insertion/extraction mechanism comprehensively elucidated during reversible sodium storage process.

光生载流子分离和表面催化反应是光催化分解水制氢过程的2个主要步骤,协同提高这两步速率必然能极大促进催化剂的制氢效率。本文以g-C3N4为研究对象,通过负载铁酸盐CoFe2O4或NiFe2O4,g-C3N4的光催化制氢性能得到大幅提高。研究结果表明,(Co, Ni)Fe2O4不仅能够有效地促进g-C3N4中的光生载流子的分离,而且能够有效地促进表面催化氧化半反应;与此同时,负载Pt作为产氢助催化剂,能促进表面催化还原产氢半反应。在光催化反应中,g-C3N4中的光生电子和空穴分别流向Pt和(Co, Ni)Fe2O4,电子在Pt上还原反应产生氢气,而空穴转移到(Co, Ni)Fe2O4上与牺牲剂反应。进一步研究结果发现,CoFe2O4对g-C3N4的载流子分离与氧化半反应催化效果均优于NiFe2O4。通过CoFe2O4和Pt共负载,Pt/g-C3N4/CoFe2O4光催化剂的催化制氢量子效率在420 nm处达到3.35 %,在可见光区(λ > 420 nm)的光催化制氢速率是未负载铁酸盐的Pt/g-C3N4的3.5倍。

The present study combines the advantages of chemical exfoliation and elemental doping to have enhanced surface area, better charge carrier separation and extended light absorption in N3−/Nb4+ co-doped nanosheets, which were employed for the first time to study their O2 evolution response. The Dion-Jacobson phase, KCa2NaNb4O13 perovskite was calcined under NH3 environment at various reaction durations (5 hr, 6 hr, 7 hr) to yield N3−/Nb4+ co-doped layered structures and subsequently exfoliated into ultrathin nanosheets. The N3−/Nb4+ co-doped nanosheets realized superior visible light absorption and bandgap narrowing as determined from UV–visible spectroscopy profiles. The synthesis of bulk materials and ultrathin morphology of exfoliated nanosheets were confirmed through XRD, SEM and AFM. The chemical states of the elements were examined by XPS measurements. The optimized N3−/Nb4+ co-doped CNNO--6hr nanosheets demonstrated excellent O2 evolution of 903 μmol g-1 after 4 h compared to N3−/Nb4+ co-doped CNNO--5hr (510 μmol g-1), N3−/Nb4+ co-doped CNNO--7hr (528 μmol g-1) and non-doped CNNO- nanosheets (381 μmol g-1). Our study paves a way on the feasibility of combining various chemical strategies for advanced photocatalyst design.