Jijian Xu

A Robust and Conductive Black Tin Oxide Nanostructure Makes Efficient Lithium-Ion Batteries Possible

SnO2 -based lithium-ion batteries have low cost and high energy density, but their capacity fades rapidly during lithiation/delithiation due to phase aggregation and cracking. These problems can be mitigated by using highly conducting black SnO2-x , which homogenizes the redox reactions and stabilizes fine, fracture-resistant Sn precipitates in the Li2 O matrix. Such fine Sn precipitates and their ample contact with Li2 O proliferate the reversible Sn → Li x Sn → Sn → SnO2 /SnO2-x cycle during charging/discharging. SnO2-x electrode has a reversible capacity of 1340 mAh g-1 and retains 590 mAh g-1 after 100 cycles. The addition of highly conductive, well-dispersed reduced graphene oxide further stabilizes and improves its performance, allowing 950 mAh g-1 remaining after 100 cycles at 0.2 A g-1 with 700 mAh g-1 at 2.0 A g-1 . Conductivity-directed microstructure development may offer a new approach to form advanced electrodes.

Black Titania for Superior Photocatalytic Hydrogen Production and Photoelectrochemical Water Splitting

To utilize visible‐light solar energy to meet environmental and energy crises, black TiO2 as a photocatalyst is an excellent solution to clean polluted air and water and to produce H2. Herein, black TiO2 with a crystalline core–amorphous shell structure reduced easily by CaH2 at 400 °C is demonstrated to harvest over 80 % solar absorption, whereas white TiO2 harvests only 7 %, and possesses superior photocatalytic performances in the degradation of organics and H2 production. Its water decontamination is 2.4 times faster and its H2 production was 1.7 times higher than that of pristine TiO2. Photoelectrochemical measurements reveal that the reduced samples exhibit greatly improved carrier densities, charge separation, and photocurrent (a 4.5‐fold increase) compared with the original TiO2. Consequently, this facile and versatile method could provide a promising and cost‐effective approach to improve the visible‐light absorption and performance of TiO2 in photocatalysis.

Conductive Carbon Nitride for Excellent Energy Storage

Conductive carbon nitride, as a hypothetical carbon material demonstrating high nitrogen doping, high electrical conductivity, and high surface area, has not been fabricated. A major challenge towards its fabrication is that high conductivity requires high temperature synthesis, but the high temperature eliminates nitrogen from carbon. Different from conventional methods, a facile preparation of conductive carbon nitride from novel thermal decomposition of nickel hydrogencyanamide in a confined space is reported. New developed nickel hydrogencyanamide is a unique precursor which provides self-grown fragments of ⋅NCN⋅ or NCCN and conductive carbon (C-sp2 ) catalyst of Ni metal during the decomposition. The final product is a tubular structure of rich mesoporous and microporous few-layer carbon with extraordinarily high N doping level (≈15 at%) and high extent of sp2 carbon (≈65%) favoring a high conductivity (>2 S cm-1 ); the ultrahigh contents of nongraphitic nitrogen, redox active pyridinic N (9 at%), and pyrrolic N (5 at%), are stabilized by forming NiN bonds. The conductive carbon nitride harvests a large capacitance of 372 F g-1 with >90% initial capacitance after 10 000 cycles as a supercapacitor electrode, far exceeding the activated carbon electrodes that have <250 F g-1 .

Molten salt assisted synthesis of black titania hexagonal nanosheets with tuneable phase composition and morphology

A facile, high yield ZnCl2/KCl molten-salt route is developed to fabricate black titania hexagonal nanosheets under atmospheric pressure and low temperature (400 °C). After post-annealing, the black titania possesses a tunable phase composition and enhanced visible light photocatalytic activity, accompanied with a controllable morphology transformation from hexagonal nanosheets to nanorods.

High-quality single-layer nanosheets of MS2 (M = Mo, Nb, Ta, Ti) directly exfoliated from AMS2 (A = Li, Na, K) crystals

Layered transition metal dichalcogenides (TMDs) such as MoS2 have attracted considerable interest as two-dimensional materials because of their unique physical and chemical properties. Single or few-layer TMD nanosheets can be achieved by conventional Li intercalation methods through organolithium chemistry or electrochemistry. However, these methods are hampered by the low yield, mixed phases and a lot of defects inside the nanosheets. Here we develop a novel general strategy to prepare single-layer TMD nanosheets by using AMS2 (A = Li, Na, K; M = Mo, Nb, Ta, Ti) crystals as ideal precursors. The crystal structure of these compounds ensures the robust S–M–S layers and fully filled alkali atoms between them, which lead to a high-yield production of high-quality single-layer nanosheets by following chemical exfoliation. Surprisingly, MoS2 nanosheets obtained by LiMoS2 crystals show a high-quality 1T′ phase, while the widely used n-butyl lithium method can only prepare phase-mixed (2H, 1T, 1T′) nanosheets with abundant defects. The as-prepared 1T′ MoS2 nanosheets exhibit a remarkable electrical conductivity (618 S cm−1), which is much higher than that of MoS2 nanosheets (35.4 S cm−1) obtained using the n-butyl lithium method, also the highest value in MoS2 related materials and superior to the best value of the reported 2D films of graphene (550 S cm−1).

Black rutile (Sn, Ti)O2 initializing electrochemically reversible Sn nanodots embedded in amorphous lithiated titania matrix for efficient lithium storage

Binary oxides MO2 (M = Ti, Sn) are promising anode materials for Li-ion batteries, but they suffer from rather low capacities (TiO2: ∼340 mA h g−1) and poor cycling stability (SnO2: <50 cycles). Here, the black (Sn, Ti)O2 solid solution of a core–shell structure SnxTi1−xO2@SnxTi1−xO2−yHy is first designed to simultaneously harvest a large reversible capacity, high rate performance and superior cycling stability. The conductive amorphous shell of the new material obtained from hydrogen plasma reduction leads to a significant improvement of conductivity from 0.1 to 35.7 μS cm−1. The rutile solid solution with a homogenous mixing of Sn and Ti helps to form a uniform distribution of Sn nanodots in an amorphous lithiated titania matrix after lithiation, and subsequently maintains a sub 10 nm scale nanostructure even after long-term cycling. The lithiated titania matrix prevents the aggregation of tin nanodots, accommodates the volume change, and provides a stable conductive network for ion kinetics, which consequently results in excellent lithium-ion battery performance. The black (Sn, Ti)O2 achieves a remarkable reversible capacity of 583.4 mA h g−1 after 100 cycles at 0.2 A g−1, retaining stable specific capacities of 419.2 mA h g−1 at 2 A g−1 after 500 cycles and 335.3 mA h g−1 at 5 A g−1. The overall performances of this material, including capacity, high-rate performance and cycling stability, are among the best for transition metal oxide anode materials. The ability to fundamentally improve the electrical conductivity and structure stability of the black material should open up new opportunities for high-performance Li-ion batteries.

Efficient Conversion of CO2 to Methane Photocatalyzed by Conductive Black Titania

One of the major challenges encountered in CO2 utilization is the development of available and cost‐efficient catalysts with sufficient activity, selectivity, and stability for the generation of useful methane. Here, conductive black titania, TiO2−x, is found to be efficient in photocatalyzing the reduction of CO2 to CH4. This unique material comprises a crystalline core–amorphous shell structure (TiO2@TiO2−x) with numerous surface oxygen vacancies, which facilitates the adsorption and chemical activation of CO2 molecules. Under full solar irradiation, the optimized 500‐TiO2−x material with narrowed band gap and intermediate states below the conduction band tail exhibits a high space‐time yield of CH4 of 14.3 μmol g−1 h−1, with 74 % selectivity and excellent photostability. The present findings can make a significant contribution, not only to develop the surface electron‐modified black TiO2 catalyst to boost photocatalytic efficiency, but also to establish a really viable and convenient CH4 production process for CO2 conversion and renewable solar energy storage.

Nano Titanium Monoxide Crystals and Unusual Superconductivity at 11 K

Nano TiO2 is investigated intensely due to extraordinary photoelectric performances in photocatalysis, new-type solar cells, etc., but only very few synthesis and physical properties have been reported on nanostructured TiO or other low valent titanium-containing oxides. Here, a core-shell nanoparticle made of TiO core covered with a ≈5 nm shell of amorphous TiO1+x is newly constructed via a controllable reduction method to synthesize nano TiO core and subsequent soft oxidation to form the shell (TiO1+x ). The physical properties measurements of electrical transport and magnetism indicate these TiO@TiO1+x nanocrystals are a type-ІІ superconductor of a recorded Tconset = 11 K in the binary Ti-O system. This unusual superconductivity could be attributed to the interfacial effect due to the nearly linear gradient of O/Ti ratio across the outer amorphous layer. This novel synthetic method and enhanced superconductivity could open up possibilities in interface superconductivity of nanostructured composites with well-controlled interfaces.

Effects of Iron Doping on the Physical Properties of Quaternary Ferromagnetic Sulfide: Ba2Fe0.6V1.4S6

The mixed-metal sulfide compound with the formula Ba2Fe0.6V1.4S6 was successfully synthesized via solid-state reaction. Ba2Fe0.6V1.4S6 has a quasi-one-dimensional structure and crystallizes in the hexagonal space group P63/mmc. The structure is composed of face-sharing anion octahedron [MS6]8- (M = V or Fe) units to construct infinite chains along the c axis, in which the Fe atoms randomly occupy the V sites. The Ba2+ ions reside between adjacent chains. Magnetic susceptibility measurements reveal a transition between paramagnetism and ferromagnetism around 25 K. The small polaron hopping (SPH) conduction behavior has been observed in the higher temperature region (75-300 K), while in the lower temperature region (25-74 K), the resistivity features a variable range hopping mechanism (VRH). The analysis of density of states indicates that Fe-3dz2 and S-3p states mainly dominate the valence band maximum, while Fe-3dz2 states contribute significantly to the magnetic susceptibility.