Bin Shan

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.

First-principles study of metal-graphene interfaces

Metal-graphene contact is a key interface in graphene-based device applications, and it is known that two types of interfaces are formed between metal and graphene. In this paper, we apply first-principles calculations to twelve metal-graphene interfaces and investigate the detailed interface atomic and electronic structures of physisorption and chemisorption interfaces. For physisorption interfaces (Ag, Al, Cu, Cd, Ir, Pt, and Au), Fermi level pinning and Pauli-exclusion-induced energy-level shifts are shown to be two primary factors determining graphene’s doping types and densities. For chemisorption interfaces (Ni, Co, Ru, Pd, and Ti), the combination of Pauli-exclusion-induced energy-level shifts and hybridized states’ repulsive interactions lead to a band gap opening with metallic gap states. For practical applications, we show that external electric field can be used to modulate graphene’s energy-levels and the corresponding control of doping or energy range of hybridization.

First principles study of work functions of single wall carbon nanotubes

We perform first principles calculations on work functions of single wall carbon nanotubes, which can be divided into two classes according to tube diameter (D). For class I tubes (D > 1 nm), work functions lie within a narrow distribution (approximately 0.1 eV) and show no significant chirality or diameter dependence. For class II tubes (D < 1 nm), work functions show substantial changes, with armchair tubes decreasing monotonically with diameter, while zigzag tubes show the opposite trend. Surface dipoles and hybridization effects are shown to be responsible for the observed work function change.

Mixed-Phase Oxide Catalyst Based on Mn-Mullite (Sm, Gd)Mn2O5 for NO Oxidation in Diesel Exhaust

Cleaning Diesel Exhaust One strategy for removing pollutants from diesel engine exhaust is to trap the unburned carbon soot and then to combust the soot with the NO2 that is generated from NO; the two pollutants are then converted to N2 and CO2. Diesel exhaust is relatively cold, compared to gasoline engine exhaust, and conversion of NO to NO2 has required the use of platinum catalysts. W. Wang et al. (p. 832) now report that a more earth-abundant catalyst, based on Mn-mullite (Sm, Gd)Mn2O5 metal oxides was able to oxidize NO in simulated diesel exhaust at temperatures as low as 75°C. Spectroscopic studies and quantum chemical modeling suggested that Mn-nitrates formed on Mn-Mn dimer sites were the key intermediates responsible for NO2 formation. Costly platinum catalysts for removing nitrogen oxide pollutants could potentially be replaced with metal oxide catalysts. Oxidation of nitric oxide (NO) for subsequent efficient reduction in selective catalytic reduction or lean NOx trap devices continues to be a challenge in diesel engines because of the low efficiency and high cost of the currently used platinum (Pt)–based catalysts. We show that mixed-phase oxide materials based on Mn-mullite (Sm, Gd)Mn2O5 are an efficient substitute for the current commercial Pt-based catalysts. Under laboratory-simulated diesel exhaust conditions, this mixed-phase oxide material was superior to Pt in terms of cost, thermal durability, and catalytic activity for NO oxidation. This oxide material is active at temperatures as low as 120°C with conversion maxima of ~45% higher than that achieved with Pt. Density functional theory and diffuse reflectance infrared Fourier transform spectroscopy provide insights into the NO-to-NO2 reaction mechanism on catalytically active Mn-Mn sites via the intermediate nitrate species.

Metal-graphene-metal sandwich contacts for enhanced interface bonding and work function control

Only a small fraction of all available metals has been used as electrode materials for carbon-based devices due to metal-graphene interface debonding problems. We report an enhancement of the bonding energy of weakly interacting metals by using a metal-graphene-metal sandwich geometry, without sacrificing the intrinsic π-electron dispersions of graphene that is usually undermined by strong metal-graphene interface hybridization. This sandwich structure further makes it possible to effectively tune the doping of graphene with an appropriate selection of metals. Density functional theory calculations reveal that the strengthening of the interface interaction is ascribed to an enhancement of interface dipole-dipole interactions. Raman scattering studies of metal-graphene-copper sandwiches are used to validate the theoretically predicted tuning of graphene doping through sandwich structures.

Visible-light-driven photocatalytic and photoelectrochemical properties of porous SnSx(x = 1,2) architectures

By using a facile and template-free polyol refluxing process, we reported the successful synthesis of porous SnS and SnS2 architectures on a large scale. The as-synthesized samples were characterized by using XRD, SEM, TEM, UV-vis DRS, Raman and N2 adsorption–desorption analyses. Studies revealed that the as-synthesized SnS and SnS2 products mainly consist of porous flower-like microstructures with reasonable BET surface areas of 66 m2 g−1 and 33 m2 g−1, respectively. Photocatalytic properties of trace amounts of samples were investigated by photodegradation of MB and RhB under visible light irradiation. The photoelectrochemical properties of both samples were also studied by configuring the samples as photoelectrochemical (PEC) cells, exhibiting excellent photosensitivity and response with greatly enhanced Ion/off as high as 1.4 × 103, three orders of magnitude higher than previous work. The results indicate the potential applications of the SnSx nanostructures in visible-light-driven photocatalysts, high response photodetectors and other optoelectronic nanodevices.

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.

First-principles study of band-gap change in deformed nanotubes

The effects of cross-sectional deformation and bending on the electronic structures of single-wall carbon nanotubes (SWNTs) are examined. Upon increasing the deformation, semiconducting SWNTs undergo semiconductor-metal transition, and the conduction band and valence band show asymmetric response to the deformation. The metallic tubes’ electronic structures are relatively insensitive to similar mechanical deformation. Using the properties of deformed nanotubes, we propose a conceptual design of SWNT-based single-electron quantum-well devices.

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.