Yanwei Wen

Na+ intercalation pseudocapacitance in graphene-coupled titanium oxide enabling ultra-fast sodium storage and long-term cycling

Sodium-ion batteries are emerging as a highly promising technology for large-scale energy storage applications. However, it remains a significant challenge to develop an anode with superior long-term cycling stability and high-rate capability. Here we demonstrate that the Na(+) intercalation pseudocapacitance in TiO2/graphene nanocomposites enables high-rate capability and long cycle life in a sodium-ion battery. This hybrid electrode exhibits a specific capacity of above 90 mA h g(-1) at 12,000 mA g(-1) (∼36 C). The capacity is highly reversible for more than 4,000 cycles, the longest demonstrated cyclability to date. First-principle calculations demonstrate that the intimate integration of graphene with TiO2 reduces the diffusion energy barrier, thus enhancing the Na(+) intercalation pseudocapacitive process. The Na-ion intercalation pseudocapacitance enabled by tailor-deigned nanostructures represents a promising strategy for developing electrode materials with high power density and long cycle life.

Co3O4-modified TiO2 nanotube arrays via atomic layer deposition for improved visible-light photoelectrochemical performance.

Composite Co3O4/TiO2 nanotube arrays (NTs) were fabricated via atomic layer deposition (ALD) of Co3O4 thin film onto well-aligned anodized TiO2 NTs. The microscopic morphology, composition, and interfacial plane of the composite structure were characterized by scanning electron microscopy, energy dispersion mapping, X-ray photoelectron spectra, and high-resolution transmission electron microscopy. It was shown that the ultrathin Co3O4 film uniformly coat onto the inner wall of the high aspect ratio (>100:1) TiO2 NTs with film thickness precisely controlled by the number of ALD deposition cycles. The composite structure with ∼4 nm Co3O4 coating revealed optimal photoelectrochemical (PEC) performance in the visible-light range (λ > 420 nm). The photocurrent density reaches as high as 90.4 μA/cm(2), which is ∼14 times that of the pristine TiO2 NTs and 3 times that of the impregnation method. The enhanced PEC performance could be attributed to the finely controlled Co3O4 coating layer that enhances the visible-light absorption, maintains large specific surface area to the electrolyte interface, and facilitates the charge transfer.

Electrospun sillenite Bi12MO20 (M = Ti, Ge, Si) nanofibers: general synthesis, band structure, and photocatalytic activity.

Sillenite Bi12MO20 (M = Ti, Ge, Si) nanofibers have been fabricated through a facile electrospinning route for photocatalytic applications. Uniform Bi12MO20 (M = Ti, Ge, Si) nanofibers with diameters of 100-200 nm and lengths of up to several millimeters can be readily obtained by thermally treating the electrospun precursors. The photocatalytic activities of these nanofibers for degradation of rhodamine B (RhB) were explored under UV-visible light. The band structure and the degradation mechanisms were also discussed. The fibrous photocatalysts of Bi12TiO20, Bi12SiO20 and Bi12GeO20 exhibit different photocatalytic behaviours, which are attributed to the microstructure, band gap, and electronic structures.

High-performance lithium–selenium batteries promoted by heteroatom-doped microporous carbon

A novel microporous N-doped carbon confined Se composite was developed as a cathode material for advanced Li–Se batteries. The microporous N-doped carbon was synthesized by carbonization of a ratio-fixed mixture of polypyrrole and KOH. The Se composite cathode is able to deliver a discharge capacity as high as 303 mA h g−1 at 20 C and a reversible capacity of 506 mA h g−1 at 1 C, even after 150 cycles. The superior electrochemical performance can be ascribed to the high electrical conductivity promoted by the N-doping and the unique microporous structure of carbonized polypyrrole, which creates additional active sites for Li-ion storage. More importantly, we used a first-principles calculation to evaluate the influence of heteroatom doping on the electrochemical performance, further confirming that the existence of heteroatoms in the carbon framework greatly facilitates the interaction between carbon and Li2Se, which could well explain the excellent cycling performance and rate capability.

Interfacial charge transfer and enhanced photocatalytic performance for the heterojunction WO3/BiOCl: first-principles study

First-principles calculations based on density functional theory were carried out to explore the interfacial properties of the WO3/BiOCl heterojunction aiming at gaining insights into the roles the interface played in the overall photocatalytic performance. The interfacial effects of the WO3 combination with BiOCl on electronic properties, charge transfer and visible-light response were investigated in detail. The density of states analysis showed that the interfacial structures resulted in a suitable band alignment to separate the excited carriers into two sides of the interface and thus, the electrons–holes recombination could be effectively suppressed. Moreover, excited holes could be readily transferred across the interface from the valence band maximum (VBM) of BiOCl to the VBM of WO3 under visible-light irradiation without being trapped in interfacial mid-gap states.

The atomic origin of high catalytic activity of ZnO nanotetrapods for decomposition of ammonium perchlorate

Distinct from the common well faceted ZnO nanorods (R-ZnO), ZnO nanotetrapods (T-ZnO) exhibited a remarkable catalytic activity for the thermal decomposition of ammonium perchlorate (AP): the activation energy at high temperature decomposition (HTD) was significantly decreased to 111.9 kJ mol−1, much lower than 162.5 kJ mol−1 for pure AP and 156.9 kJ mol−1 for AP with R-ZnO. This was attributed to more abundant atomic steps on the surface of T-ZnO than that of R-ZnO, as evidenced by HRTEM and density function theory (DFT) calculations. It was shown that the initiation step of perchloric acid (PA) decomposition happened much faster on stepped T-ZnO edges, resulting in the formation of active oxygen atoms from HClO4. The formed oxygen atoms would subsequently react with NH3 to produce HNO, N2O and NO species, thus leading to an obvious decrease in the activation energy of AP decomposition. The proposed catalytic mechanism was further corroborated by the TG-IR spectroscopy results. Our work can provide atomic insights into the catalytic decomposition of AP on ZnO nanostructures.

Modulation of contact resistance between metal and graphene by controlling the graphene edge, contact area, and point defects: An ab initio study

A systematic first-principles non-equilibrium Greens function study is conducted on the contact resistance between a series of metals (Au, Ag, Pt, Cu, Ni, and Pd) and graphene in the side contact geometry. Different factors such as the termination of the graphene edge, contact area, and point defect in contacted graphene are investigated. Notable differences are observed in structural configurations and electronic transport characteristics of these metal-graphene contacts, depending on the metal species and aforementioned influencing factors. It is found that the enhanced chemical reactivity of the graphene due to dangling bonds from either the unsaturated graphene edge or point defects strengthens the metal-graphene bonding, leading to a considerable contact resistance reduction for weakly interacting metals Au and Ag. For stronger interacting metals Pt and Cu, a slightly reduced contact resistance is found due to such influencing factors. However, the wetting metals Ni and Pd most strongly hybridize with graphene, exhibiting negligible dependence on the above influencing factors. This study provides guidance for the optimization of metal-graphene contacts at an atomic scale.

Promotional role of La addition in the NO oxidation performance of a SmMn2O5 mullite catalyst

A series of LaxSm1−xMn2Oδ (x = 0, 0.1, 0.3, 0.5) catalysts were synthesized through a co-precipitation method. The catalytic activity for NO oxidation was enhanced with La substitution, and the maximum activity was achieved at x = 0.3. XRD and HRTEM results revealed the formation of a multiphase oxide as well as the interface structure between the mullite (SmMn2O5) phase and Mn-rich perovskite (La0.96MnO3.05) phase. The main impact of different La/Sm molar ratios is the amount of surface adsorbed oxygen (Oads) and surface Mn4+ ions as revealed by XPS results. The NO oxidation performance was enhanced through La addition by promoting the decomposition of nitrate/nitrite species and desorption of NO2, improving the reducibility of surface adsorbed oxygen, as determined by H2-TPR, NO + O2-TPRD and in situ DRIFTS studies. Mono-, bi-dentate and bridged nitrates formed on the surface were determined to be the primary reaction intermediates.

Surface stabilities and NO oxidation kinetics on hexagonal-phase LaCoO3 facets: a first-principles study

LaCoO3 perovskite has recently received great attention as a potential alternative to precious metal based NO oxidation catalysts. We report here a comprehensive first-principles study of the NO oxidation kinetics on differently re-constructed hexagonal-phase LaCoO3 facets. Among the 42 low-index facets considered, the (102) LaO-, (104) O2- and (0001) LaO3-terminated facets were found to be thermodynamically stable and likely to be exposed in LaCoO3 oxide nanoparticles. Among these stable facets, the (0001) LaO3-terminated surface is catalytically most active towards NO oxidation, with the reaction proceeding through the mono-vacancy Mars–van Krevelen mechanism. Our study shed light on the atomistic scale NO oxidation mechanism on LaCoO3 facets and can aid further optimization of the catalyst.

First-principles study of the structural, energetic and electronic properties of C20-carbon nanobuds

The structural, energetic and electronic properties of carbon nanobuds (CNBs) with the smallest fullerene C20 covalently attached to the sidewall of single-walled carbon nanotubes (SWNTs) are studied by first-principles calculations. Due to the high curvature of C20 and the resulting chemical activity, the binding between C20 and SWNTs is quite strong. Among different CNB configurations, bond cycloaddition is energetically most favorable. The activation barrier for C20–CNB formation is only one-fourth that of C60 and it would maintain good stability once formed. Our results also reveal that C20–CNB stability depends on the chirality of the SWNTs, and they exhibit tunable band gaps that can be modulated by the density of C20 attached to the SWNTs.