Jingguo Hu

Photogenerated electron reservoir in hetero-p–n CuO–ZnO nanocomposite device for visible-light-driven photocatalytic reduction of aqueous Cr(VI)

The development of visible-light-responsive catalysts with efficient recyclability is important for solar energy conversion applications. Here, we report a three-dimensional (3D) heterohierarchical device consisting of two-dimensional (2D) p-type narrow bandgap semiconductor CuO nanosheets and one-dimensional n-type wide bandgap semiconductor ZnO nanorods that was fabricated via hydrothermal reaction after in situ crystallization on Cu foil. This heterostructured composite exhibited significantly enhanced visible light photocatalytic reduction capacity with stable recyclability to hexavalent chromium (Cr(VI)) compared with pure CuO nanosheets and ZnO nanorods. This improved photocatalysis was attributed to the synergistic actions of CuO and ZnO consisting of their formed p–n hetero-interface for the extension of solar absorption and the anti-recombination of photogenerated electron–hole pairs, demonstrating the potential for p–n heterostructures to be used for solar photocatalytic pollutant degradation or energy conversion. Moreover, the control of the photogenerated electron reservoir cultivated on ZnO nanorods over the photocatalytic reduction activity was demonstrated, further clarifying the photocarrier dynamics and the photocatalytic mechanism in the CuO–ZnO heterojunction. The preparation of the 3D CuO–ZnO p–n junction device could provide a brand new approach to design versatile devices for solar energy conversion.

Photogenerated Carriers Transfer in Dye–Graphene–SnO2 Composites for Highly Efficient Visible-Light Photocatalysis

The visible-light-driven photocatalytic activities of graphene-semiconductor catalysts have recently been demonstrated, however, the transfer pathway of photogenerated carriers especially where the role of graphene still remains controversial. Here we report graphene-SnO2 aerosol nanocomposites that exhibit more superior dye adsorption capacity and photocatalytic efficiency compared with pure SnO2 quantum dots, P25 TiO2, and pure graphene aerosol under the visible light. This study examines the origin of the visible-light-driven photocatalysis, which for the first time links to the synergistic effect of the cophotosensitization of the dye and graphene to SnO2. We hope this concept and corresponding mechanism of cophotosensitization could provide an original understanding for the photocatalytic reaction process at the level of carrier transfer pathway as well as a brand new approach to design novel and versatile graphene-based composites for solar energy conversion.

Well–Steered Charge–Carrier Transfer in 3D Branched CuxO/ZnO@Au Heterostructures for Efficient Photocatalytic Hydrogen Evolution

Multi-component hetero-nanostructures exhibit multifunctional properties or synergistic performance and are thus considered as attractive materials for energy conversion applications. There is a long-standing demand to construct more sophisticated heterostructures for steering charge-carrier flow in semiconductor systems. Herein we fabricate a large-scale quantity of three-dimensional (3D) branched CuxO/ZnO@Au heterostructure consisting of CuO nanowires (NWs) and grafted ZnO nanodisks (NDs) decorated with Au nanoparticles via sequential hierarchical assemblies. This treelike hetero-nanostructure ensures well-steered transfer of photogenerated electrons to the exposed ZnO NDs, while holes to the CuO backbone NWs with concerted efforts from multi-node p-n junctions, polar ZnO facets, and Au plasmon, resulting in the significantly enhanced photocatalytic hydrogen evolution performance.

Photoanode Current of Large–Area MoS2 Ultrathin Nanosheets with Vertically Mesh–Shaped Structure on Indium Tin Oxide

A large area of hydrothermally grown MoS2 ultrathin nanosheets (NSs) with a vertically mesh-shaped structure on indium tin oxide (ITO) substrate was directly used as the photoanode of a potoelectrochemical (PEC) cell. The photoelectrocatalytic capacity of ultrathin MoS2 NSs was demonstrated, which was attributed not only to the excellent electrocatalytic activity originating from the exposed preferentially active edge sites but also to the superior photoelectric response resulting from the large light absorption of ultrathin MoS2 NSs and from the efficient separation of electron-hole pairs at the ITO/MoS2 interfaces. The significantly enhanced photocurrent indicates that the MoS2 ultrathin NSs can be a promising photoelectrocatalyst for PEC cells, unveiling the potential of MoS2-based PEC cells for solar energy absorption and conversion.

Surface photoluminescence and magnetism in hydrothermally grown undoped ZnO nanorod arrays

ZnO nanorod arrays were synthesized by a hydrothermal method on the Si substrate with ZnO thin film as seed layer prepared by magnetron sputtering. The presence of -OH ligands on the surface of the as-grown sample was confirmed, and its dominant role in both suppressing the visible emission and boosting the room-temperature ferromagnetism (FM) was revealed. Through alternative H2 and O2 annealing to remove the -OH ligands, reconstruct surface-states and tune the oxygen occupancy in ZnO nanorods, the clear correlation between the characteristic green emission and ferromagnetism was established.

Resistive switching memories in MoS2 nanosphere assemblies

A resistive switching memory device consisting of reduced graphene oxide and indium tin oxide as top/bottom two electrodes, separated by dielectric MoS2 nanosphere assemblies as the active interlayer, was fabricated. This device exhibits the rewritable nonvolatile resistive switching with low SET/RESET voltage (∼2 V), high ON/OFF resistance ratio (∼104), and superior electrical bistability, introducing a potential application in data storage field. The resistance switching mechanism was analyzed in the assumptive model of the electron tunneling across the polarized potential barriers.

Control mechanism behind broad fluorescence from violet to orange in ZnO quantum dots

ZnO quantum dots with tunable size in the range of 2.0–7.8 nm were synthesized through a facile sol–gel route. Photoluminescence and photoluminescence excitation spectral examinations revealed that two types of transition mechanisms might occur under control during excitation regulation corresponding to the coexistence of deep and shallow levels in energy band structure. The broad-color emission from violet to orange could be tuned by adjusting the excitation energy and quantum dot size. Moreover, ZnO quantum dots with tunable and broad luminescence were demonstrated to be promising in the anti-fake labeling applications.

Spin-polarized transport in diluted GaMnAs/AlAs/GaMnAs ferromagnetic semiconductor tunnel junctions

Taking into account the basic physics of diluted ferromagnetic semiconductors (DMS), in which the variation of the splitting energy with temperature is included, we apply a quantum-mechanical approach to studying the spin-polarized transport in GaMnAs/AlAs/GaMnAs DMS tunnel junctions. It is shown that tunneling magnetoresistance first rapidly increases and then decreases with increasing barrier thickness, exhibiting a peak at an optimum value of barrier thickness. We also find that the normalized conductance difference decreases with the enhancement of temperature. The theoretical results can reproduce the main feature of the experiments.Taking into account the basic physics of diluted ferromagnetic semiconductors (DMS), in which the variation of the splitting energy with temperature is included, we apply a quantum-mechanical approach to studying the spin-polarized transport in GaMnAs/AlAs/GaMnAs DMS tunnel junctions. It is shown that tunneling magnetoresistance first rapidly increases and then decreases with increasing barrier thickness, exhibiting a peak at an optimum value of barrier thickness. We also find that the normalized conductance difference decreases with the enhancement of temperature. The theoretical results can reproduce the main feature of the experiments.

Identification of visible emission from ZnO quantum dots: Excitation-dependence and size-dependence

ZnO quantum dots (QDs) with uniform shape and different sizes were synthesized by a simple sol-gel method. The visible emission of the ZnO QDs displays highly both excitation-dependent and size-dependent behaviors. The results indicate that the green emission should be attributed to the transition of electrons from the conduction band to a certain deep trap related mainly to defects on the surface, while the violet emission may correspond to the transition of electrons from the shallow donor levels to the valence band. This work is favor to clarify the transition mechanism of visible emission and to extend optical and electronic applications. Particularly, the importance of combining the excitation effect with quantum size effect for investigating photoluminescence of QDs is first highlighted.

Vertically aligned MoS2/MoOx heterojunction nanosheets for enhanced visible-light photocatalytic activity and photostability

Vertically aligned nanosheet heterostructures with partly reduced MoO3 cores and adjustable MoS2 shells were fabricated via two-step chemical vapor deposition (CVD). The as-synthesized MoS2/MoOx heterostructures exhibit enhanced visible-light photocatalytic activity and good compatibility in a wide range of PH values (e.g. 2–12) for the degradation of organic dyes, both of which are of significance for practical applications. The vertically grown nanosheets form a three-dimensional (3D) mesh structure, creating a large specific surface area for the optical absorption and the catalytic redox reaction. In particular, the sulfidation-produced MoS2 coating layer provides an effective protective against photocorrosion in a wide PH window, and meanwhile modulates the energy band structure to promote the absorption of visible-light photons and the separation of photo-generated electron–hole pairs.