Hongwei Tian

Atomic-level energy storage mechanism of cobalt hydroxide electrode for pseudocapacitors

Cobalt hydroxide is a promising electrode material for supercapacitors due to the high capacitance and long cyclability. However, the energy storage/conversion mechanism of cobalt hydroxide is still vague at the atomic level. Here we shed light on how cobalt hydroxide functions as a supercapacitor electrode at operando conditions. We find that the high specific capacitance and long cycling life of cobalt hydroxide involve a complete modification of the electrode morphology, which is usually believed to be unfavourable but in fact has little influence on the performance. The conversion during the charge/discharge process is free of any massive structural evolution, but with some tiny shuffling or adjustments of atom/ion species. The results not only unravel that the potential of supercapacitors could heavily rely on the underlying structural similarities of switching phases but also pave the way for future material design for supercapacitors, batteries and hybrid devices.

One-step synthesis of band-tunable N, S co-doped commercial TiO2/graphene quantum dots composites with enhanced photocatalytic activity

N, S co-doped commercial TiO2/N, S-GQDs graphene quantum dots (NSTG) composites with band tunability are synthesized via a facile solvothermal treatment in the presence of thiourea, which acts as a precursor for the dopants. The as prepared nanocomposites are characterized via X-ray diffraction (XRD), Raman spectroscopy (Raman), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform-infrared spectroscopy (FT-IR) and UV-vis diffuse reflectance spectroscopy (UV-vis DRS). The photocatalytic activity of NSTG is evaluated through the degradation of methylene blue (MB) under visible light irradiation (λ > 400 nm). Compared with commercial TiO2 (P25) and N, S co-doped TiO2 (NST), the NSTG composites exhibit the highest photodegradation efficiency. The apparent rate constant of NSTG is about 2.4 times and 50.7 times higher than that of NST and commercial TiO2, respectively. Furthermore, the band gaps of the NSTG composites can be tuned by changing the molar ratio of citric acid (CA) : thiourea. Our work demonstrates that this innovative synthetic approach could provide an effective approach for industrial applications due to its low cost and scalability. Furthermore, the NSTG composites are a more promising photocatalytic material than the well studied N doped TiO2 for potential applications in environmental protection.

A growth mechanism for graphene deposited on polycrystalline Co film by plasma enhanced chemical vapor deposition

Graphene is deposited on polycrystalline Co film by radio-frequency plasma enhanced chemical vapor deposition (RF-PECVD), and the effect of deposition time on the crystallinity of graphene, such as graphitic degree and in-plane crystallite size, is explored. The findings are that graphene can be obtained on polycrystalline Co film for only 15 s, suggesting that a direct growth mechanism plays an important role in the formation of graphene. The first-principles density functional theory (DFT) results also reveal that the graphene is more easily formed on Co via a surface direct growth mechanism than that via a precipitation mechanism. Our studies are critical in guiding the graphene growth process as we try to achieve the highest quality graphene for electronic devices.

Improving Photocatalytic Performance from Bi 2 WO 6 @MoS 2 /graphene Hybrids via Gradual Charge Transferred Pathway

The charge transfer from the main catalyst to the cocatalyst is a key factor to enhance catalytic activity for photocatalytic nanocomposite materials. In order to enhance the charge transfer between Bi2WO6 and graphene, we inlet MoS2 as a “stepping-stone” into Bi2WO6 and graphene. Here, we report an effective strategy to synthesize ternary Bi2WO6@MoS2/graphene nanocomposite photocatalyst by a facile two-step hydrothermal method, which is afforded by assembling two cocatalysts, graphene and MoS2, into the Bi2WO6 matrix with a nanoparticle morphology as a visible light harvester. Compared with Bi2WO6/graphene, Bi2WO6/MoS2 and pure Bi2WO6, the Bi2WO6@MoS2/graphene ternary composites exhibit superior photocatalytic activity owing to an enhanced charge carrier separation via gradual charge transferred pathway. This work indicates a promising cocatalyst strategy for designing a more efficient graphene based semiconductor photocatalyst toward degradation of organic pollutants.

High-speed creep process mediated by rapid dislocation absorption in nanocrystalline Cu

A high-speed creep process mediated by rapid dislocation absorption was found in the nanoindentation creep test on nanocrystalline Cu. The creep strain and creep strain rate depend strongly on the loading strain rate and are far higher than those predicted by the models of Coble creep and thermally activated grain boundary sliding. Our analysis revealed that grain boundary dislocation sources can be activated and emitted dislocations from grain boundaries can be stored effectively at a high loading strain rate, but cannot at a low loading strain rate. The observed high-speed creep process is mediated mainly by the rapid absorptions of the stored dislocations and the dislocations newly nucleated during the holding period. An implication of our experimental finding is that dislocation structure in nanocrystalline metals is highly unstable and dislocation activity can proceed after loading and lead to a significant post-loading plasticity.

Construction of a ternary hybrid of CdS nanoparticles loaded on mesoporous-TiO2/RGO for the enhancement of photocatalytic activity

We successfully fabricated a ternary hybrid of meso-TiO2/RGO/CdS via an efficient electrostatic self-assembly approach and photo-assisted treatment. In the illumination process, GO nanosheets were reduced to RGO and simultaneously, CdS nanoparticles were uniformly loaded onto the surface of the meso-TiO2/RGO. Compared with bare meso-TiO2 (unitary component) and meso-TiO2/RGO (binary components), the ternary meso-TiO2/RGO/CdS exhibited superior photocatalytic activity and stability. In the experiment of the degradation of methylene orange (MO) in the presence of simulated solar light, the ternary hybrid showed the highest degradation rate (0.0228 min−1), which was almost 2 and 17 times the degradation rate of meso-TiO2/RGO and meso-TiO2, respectively. This enhanced photoactivity is ascribed to the synergistic effect of meso-TiO2, RGO and CdS, including a relatively high surface area, extension of the absorption spectrum range and the highly efficient separation and transfer rate of the charge carriers. The establishment of a ternary hybrid thus provides a promising way to improve the performance of photocatalysts.

Nitrogen mediated electronic structure of the Ti(0001) surface

The unusual ability of nitrogen in functionalizing transition metals has tremendous implications for the nitride compounds for chemical, electronic, optical, mechanical, and tribological applications yet a consistent insight into the underlying mechanism remains yet a challenge. A combination of density function theory and photoelectron spectroscopy revealed that the nitrogen atom prefers tetrahedron bonding geometry in the Ti(0001) surface, which derives four additional valence density-of-states:bonding electron pairs, nonbonding lone pairs, electronic holes, and antibonding dipoles. Dipole formation modulates the work function, electron–hole generation opens the bandgap and nonbonding interaction ensures the superlubricity of the N–Ti(0001) skin.

TiO2 Band Restructuring by B and P Dopants

An examination of the effect of B- and P-doping and codoping on the electronic structure of anatase TiO2 by performing density functional theory calculations revealed the following: (i) B- or P-doping effects are similar to atomic undercoordination effects on local bond relaxation and core electron entrapment; (ii) the locally entrapped charge adds impurity levels within the band gap that could enhance the utilization of TiO2 to absorb visible light and prolong the carrier lifetime; (iii) the core electron entrapment polarizes nonbonding electrons in the upper edges of the valence and conduction bands, which reduces not only the work function but also the band gap; and (iv) work function reduction enhances the reactivity of the carriers and band gap reduction promotes visible-light absorption. These observations may shed light on effective catalyst design and synthesis.

Ultrathin Carbon Film Protected Silver Nanostructures for Surface-Enhanced Raman Scattering

In this paper, ultrathin carbon film protected silver substrate (Ag/C) was prepared via a plasma-enhanced chemical vapor deposition (PECVD) method. The morphological evolution of silver nanostructures underneath, as well as the surface-enhanced Raman scattering (SERS) activity of Ag/C hybrid can be tuned by controlling the deposition time. The stability and reproducibility of the as-prepared hybrid were also studied.

X-ray reflectivity and diffraction investigation on TiN/SiNx nanolayered coatings deposited by magnetron sputtering

Polycrystalline TiN/SiN x multilayer coatings were deposited by reactive magnetron sputtering from Ti and Si targets. Interfaces, structures, and mechanical properties of the multilayers were characterized using X-ray reflectivity (XRR), X-ray diffraction (XRD), and nanoindentation analyses. Results showed that substrate bias voltage had a significant influence on the structures and mechanical properties of the multilayer coatings, in which sharp interfaces are responsible for an enhancement of mechanical properties of the multilayer coatings. The maximum hardness occurs at the −80 V coating with the sharpest interface and the strongest [200] preferred orientation.