Hui Yan

One-step approach to novel Bi4V2O11 hierarchical hollow microspheres with high visible-light-driven photocatalytic activities

New visible-light-sensitive hierarchical Bi4V2O11 hollow microspheres have been successfully synthesized by a facile template-free solvothermal route. The hierarchical Bi4V2O11 superstructure was constructed of single-crystalline nanoplates with a porous surface. A possible growth pattern and formation mechanism of hierarchical Bi4V2O11 hollow microspheres was proposed. The Brunauer–Emmett–Teller (BET) surface area of the hollow sample was 84.613 m2 g−1, which was much higher than other prepared Bi4V2O11 powders. The hierarchical Bi4V2O11 hollow microspheres exhibited excellent visible-light-driven photocatalytic activity for the degradation of Rhodamine-B (RhB). The improved photocatalytic performance could be ascribed to the high specific surface area, the narrow band gap and unique hierarchical hollow structure. The resulting hierarchical Bi4V2O11 hollow microspheres are very promising photocatalysts for degrading organic pollutants and other applications.

Electrochemical performance of binder-free carbon nanotubes with different nitrogen amounts grown on the nickel foam as cathodes in Li–O2 batteries

Although studies have been done on nitrogen doped carbon materials as lithium–oxygen (Li–O2) battery cathodes, few of them focus on the binder-free electrode structure, although they have been proved to bring improved performance. To fill this gap this work not only studies the nitrogen doped binder-free carbon cathode but also determines the performance of these cathodes with different levels of nitrogen doping. To make binder-free electrodes, these CNTs and N-CNTs were synthesized on nickel foam by a floating catalyst chemical vapor deposition method. The study found that the electrochemical performance of binder-free N-CNT cathodes in Li–O2 batteries improves as the level of nitrogen doping increases. To further study the reason why the electrodes with higher nitrogen amounts deliver better electrochemical properties, the morphology of discharge products on the different nanotubes are detected by scanning electron microscopy (SEM). The scan shows that the distribution of discharge products on the surface of CNTs become more and more uniform as the level of nitrogen doping increases and the discharge capacity and cycle performance are subsequently improved. Therefore, these binder-free N-CNT electrodes could be further explored as high capacity cathode materials for Li–O2 battery applications.

Wurtzite-type CuInSe2 for high-performance solar cell absorber: ab initio exploration of the new phase structure

CuInSe2 (CIS) has been widely studied because of its potential applications in photovoltaics, and the phase structure is believed to significantly affect its electronic and optical properties. A new wurtzite-type phase of CIS is predicted by density functional theory calculations combined with evolutionary methodology. In contrast to the common chalcopyrite CIS, Cu atoms of the predicted phase form new bonds with Se atoms derived from the interaction with the second nearest neighbor due to symmetry, and such new bonding results in beneficial band structure for both electron transition and transport because of the delocalized Cu-d electrons. The calculated absorption spectrum of the new phase further reveals an improvement in light absorption index over that of the chalcopyrite phase under near-infrared and visible light. Thus the wurtzite-type CIS has advantageous electronic and optical properties and is a highly efficient active layer material for high-performance solar cells.

Bandgap engineering of GaN nanowires

Bandgap engineering has been a powerful technique for manipulating the electronic and optical properties of semiconductors. In this work, a systematic investigation of the electronic properties of [0001] GaN nanowires was carried out using the density functional based tight-binding method (DFTB). We studied the effects of geometric structure and uniaxial strain on the electronic properties of GaN nanowires with diameters ranging from 0.8 to 10 nm. Our results show that the band gap of GaN nanowires depends linearly on both the surface to volume ratio (S/V) and tensile strain. The band gap of GaN nanowires increases linearly with S/V, while it decreases linearly with increasing tensile strain. These linear relationships provide an effect way in designing GaN nanowires for their applications in novel nano-devices.

Si doping at GaN inversion domain boundaries: an interfacial polar field for electron and hole separation

Using first-principles calculations, we investigated the phenomenon of Si doping at the GaN inversion domain boundaries (IDB) perpendicular to the wurtzite [0001] axis. The results reveal that the half monolayer Si doped GaN IDB is more stable than the abrupt monolayer Si doped IDB. This finding is vital to understanding the unique growth mechanism of Si-induced IDBs in N-polar GaN nanowires that are embedded in a Ga-polar layer [Nano Lett., 2012, 12, 6119]. The lower-energy boundary exhibits the characteristics of intrinsic semiconductor and fulfils the electron counting rule. Charge neutrality is achieved by transferring electrons from the Si–N to the Ga–Ga bonds. Moreover, a potential step is induced by the asymmetric substitution of Si for Ga atoms at the interface, which facilitates the spatial separation of excited carriers at this neutral boundary. Our results suggest an alternative strategy for designing novel and highly efficient photovoltaic devices.

Engineering of hydrogenated two-dimensional h-BN/C superlattices as electrostatic substrates

Hybridized two-dimensional materials incorporating domains from the hexagonal boron nitride (h-BN) and graphene is an interesting branch of materials science due to their highly tunable electronic properties. In the present study, we investigate the hydrogenated two-dimensional (2D) h-BN/C superlattices (SLs) with zigzag edges using first-principles calculations. We found that the domain width, the phase ratio, and the vertical dipole orientation all have significant influence on the stability of SLs. The electronic reconstruction is associated with the lateral polar discontinuities at the zigzag edges and the vertically polarized (B2N2H4)(m) domains, which modifies the electronic structures and the spatial potential of the SLs significantly. Furthermore, we demonstrate that the hydrogenated 2D h-BN/C SLs can be applied in engineering the electronic structure of graphene: laterally-varying doping can be achieved by taking advantage of the spatial variation of the surface potential of the SLs. By applying an external vertical electric field on these novel bidirectional heterostructures, graphene doping levels and band offsets can be tuned to a wide range, such that the graphene doping profile can be switched from the bipolar (p-n junction) to unipolar (n(+)-n junction) mode. It is expected that such bidirectional heterostructures provide an effective approach for developing novel nanoscale electronic devices and improving our understanding of the fundamentals of low-dimensional materials.

Why Clorine Is an Inefficient n-Type Dopant in CuInSe2?

To unveil the physical origin of clorine (Cl) as the low effectiveness n-type dopant, the effects of Cl defects in CuInSe2 (CIS) with the different doping sites and defect-pairs are systematically investigated by first-principles calculations. The results exhibit that the Cl energetically prefers the interstitial site instead of generally believed substitutional site with both Perdew–Burke–Ernzerhof (PBE) and Heyd–Scuseria–Ernzerhof (HSE06) functionals. The electronic structure calculations further show that doping would be p-type when Cl occupies the interstitial site, which is greatly different from the n-type doping in substitutional site. Such results clarify the intrinsic mechanism of the low effectiveness of n-type Cl-doping in CIS.