Junyong Wang

Superior adsorption and photoinduced carries transfer behaviors of dandelion-shaped Bi 2 S 3 @MoS 2 : experiments and theory

The enhanced light-harvesting capacity and effective separation of photogenerated carriers in fantastic hierarchical heterostructures enjoy striking attention for potential applications in the field of solar cells and photocatalysis. A three-dimensional (3D) dandelion-shaped hierarchical Bi2S3 microsphere compactly decorated with wing-shaped few layered MoS2 lamella (D-BM) was fabricated via a facile hydrothermal self-assembly process. Especially, polyethylene glycol (PEG) has been proven as the vital template to form D-BM microsphere. Importantly, the as-prepared D-BM microsphere presents pH-dependent superior adsorption behavior and remarkable visible light photocatalytic activity for degradation of organic dyestuffs (Rhodamine B/RhB and Methylene blue/MB), far exceeding those for the pure Bi2S3 and MoS2. It is understandable that D-BM with high surface area possesses more active sites and promotes light utilization due to the unique porous structure with outspread wings. Besides, based on the experiments and theoretical calculations, the staggered type II band alignment of D-BM permits the charge injection from Bi2S3 to MoS2, subsequently accelerates the separation and restrains the recombination of carriers, leading to excellent photocatalytic activity, as well as the photoconductance and photoresponse performance (with Ilight/Idark ratio 567).

Copper ferrites@reduced graphene oxide anode materials for advanced lithium storage applications

Copper ferrites are emerging transition metal oxides that have potential applications in energy storage devices. However, it still lacks in-depth designing of copper ferrites based anode architectures with enhanced electroactivity for lithium-ion batteries. Here, we report a facile synthesis technology of copper ferrites anchored on reduced graphene oxide (CuFeO2@rGO and Cu/CuFe2O4@rGO) as the high-performance electrodes. In the resulting configuration, reduced graphene offers continuous conductive channels for electron/ion transfer and high specific surface area to accommodate the volume expansion of copper ferrites. Consequently, the sheet-on-sheet CuFeO2@rGO electrode exhibits a high reversible capacity (587 mAh g−1 after 100 cycles at 200 mA g−1). In particular, Cu/CuFe2O4@rGO hybrid, which combines the advantages of nano-copper and reduced graphene, manifests a significant enhancement in lithium storage properties. It reveals superior rate capability (723 mAh g−1 at 800 mA g−1; 560 mAh g−1 at 3200 mA g−1) and robust cycling capability (1102 mAh g−1 after 250 cycles at 800 mA g−1). This unique structure design provides a strategy for the development of multivalent metal oxides in lithium storage device applications.

Exploring optoelectronic properties and mechanisms of layered ferroelectric K 4 Nb 6 O 17 nanocrystalline films and nanolaminas

Two-dimensional layered K4Nb6O17 (KN) was easily formed as a secondary phase caused by the volatilization of alkali metal ions, when preparing ferroelectric KxNa1−xNbO3 based ceramics and films. In this work, it was believed that KN film is with weak ferroelectricity and has a little effect on the ferroelectric properties of KxNa1−xNbO3 based films. Moreover, temperature dependent (77–500 K) dielectric functions of KN film have been firstly extracted by fitting ellipsometric spectra with the Adachi dielectric function model and a four-phase layered model. The high-frequency dielectric constant linearly increases and optical band gap slightly decreases with increasing the temperature. We also research its photoelectrochemical properties and its application in high-efficient light-induced H2 evolution. In addition, X-ray photoelectron spectroscopy, Raman scattering, temperature dependent transmittance and infrared reflectance spectra, and first-principles calculation were conjointly performed to further reveal the intrinsic optoelectronic features and relevant mechanisms of KN.

Electronic transitions of the transparent delafossite-type CuGa1−xCrxO2 system: first-principles calculations and temperature-dependent spectral experiments

The structure and optical properties of the CuGa1−xCrxO2 (CGCOx) system with 0 ≤ x ≤ 1 have been investigated by combining theoretical calculations and optical experiments. Density functional theory within the generalized gradient approximation (GGA) was utilized to calculate the electronic structure of the CGCOx system. It reveals strong hybridization between the 3d states of the transition metal ions and 2p states of the O element, which has an important effect on the electronic transitions of CGCOx materials. Moreover, to confirm the theoretical results, CGCOx films with different Cr compositions were deposited via a sol–gel method and the optical properties were measured directly by temperature dependent UV-Vis transmittance and infrared reflectance spectroscopy. The frequency of two acoustic modes (Eu and A2u) gradually increases, whereas the values of the electronic band gap decrease linearly with increasing Cr composition, which can be attributed to the stronger Cr–O covalent interaction. Remarkably, an additional direct electronic band gap has been observed for the CuGa0.75Cr0.25O2 film, which shows an abnormal behavior in a low temperature region. It can be assigned to the p–d electron hybridization at the top of the valence band. These results show that the first-principles calculations agree well with the experimental data and can be used to explain the microscopic origin of the interband transitions for CGCOx films. The present work further improves the potential applications of delafossite-type oxides in the field of optoelectronic devices.

Tuning Coupling Behavior of Stacked Heterostructures Based on MoS 2 , WS 2 , and WSe 2

The interlayer interaction of vertically stacked heterojunctions is very sensitive to the interlayer spacing, which will affect the coupling between the monolayers and allow band structure modulation. Here, with the aid of density functional theory (DFT) calculations, an interesting phenomenon is found that MoS2-WS2, MoS2-WSe2, and WS2-WSe2 heterostructures turn into direct-gap semiconductors from indirect-gap semiconductors with increasing the interlayer space. Moreover, the electronic structure changing process with interlayer spacing of MoS2-WS2, MoS2-WSe2, and WS2-WSe2 is different from each other. With the help of variable-temperature spectral experiment, different electronic transition properties of MoS2-WS2, MoS2-WSe2, and WS2-WSe2 have been demonstrated. The transition transformation from indirect to direct can be only observed in the MoS2-WS2 heterostructure, as the valence band maximum (VBM) at the Γ point in the MoS2-WSe2 and WS2-WSe2 heterostructure is less sensitive to the interlayer spacing than those from the MoS2-WS2 heterostructure. The present work highlights the significance of the temperature tuning in interlayer coupling and advance the research of MoS2-WS2, MoS2-WSe2, and WS2-WSe2 based device applications.

Spin-manipulated phonon dynamics during magnetic phase transitions in triangular lattice antiferromagnet CuCr1−xMgxO2 semiconductor films

Raman scattering and infrared reflectance spectra of CuCr1−xMgxO2 films (x = 0.03, 0.06 and 0.09) in the temperature range of 5–300 K have been studied, combined with first-principles calculations. The abnormal redshift of the Eg Raman mode center with decreasing temperature below 100 K for the lightly doped film (x = 0.03) is ascribed to the Cr 3d–O 2p–Cu 3d interaction. Strong disturbance of the local spin fluctuation at Cr sites in heavily Mg-doped films (x = 0.06 and 0.09) drives the Eg mode center to a normal blueshift with decreasing temperature. With further decreasing temperature, the out-of-plane structure increases the internal spin-charge coupling. It enhances the spin-flip splitting and brings back the abnormal redshift of the Eg Raman mode center. A similar but more obvious trend can be found from temperature-dependent Eu infrared mode center shifts. Two successive magnetic transitions were observed at the corresponding Neel temperatures TN1 around 24.7 K and TN2 around 23.0 K, as manifested by magnetoresistance measurements. The interesting phenomena of phonon dynamics are suggested to be manipulated by the spin structures during the magnetic transitions.

Free-anchored Nb2O5@graphene networks for ultrafast-stable lithium storage

Orthorhombic Nb2O5 (T-Nb2O5) has structural merit but poor electrical conductivity, limiting their applications in energy storage. Although graphene is frequently adopted to effectively improve its electrochemical properties, the ordinary modified methods cannot meet the growing demands for high-performance. Here, we demonstrate that different graphene modified routes play a vital role in affecting the electrochemical performances of T-Nb2O5. By only manual shaking within one minute, Nb2O5 nano-particles can be rapidly adsorbed onto graphene, then the free-anchored T-Nb2O5@graphene three-dimensional networks can be successfully prepared based on hydrogel method. As for the application in lithium-ion batteries, it performs outstanding rate character (129 mA h g-1 (25C rate), 110 mA h g-1 (50C rate) and 90 mA h g-1 (100C rate), correspond to 79%, 67% and 55% capacity of 0.5C rate, respectively) and excellent long-term cycling feature (∼70% capacity retention after 20000 cycles). Moreover, it still maintains similar ultrafast-stable lithium storage performances when Cu foil is substituted by Al foil as current collector. In addition, relevant kinetics mechanisms are also expounded. This work provides a versatile strategy for the preparation of graphene modified Nb2O5 or other types of nanoparticles.

The electro-optic mechanism and infrared switching dynamic of the hybrid multilayer VO2/Al:ZnO heterojunctions

Active and widely controllable phase transition optical materials have got rapid applications in energy-efficient electronic devices, field of meta-devices and so on. Here, we report the optical properties of the vanadium dioxide (VO2)/aluminum-doped zinc oxide (Al:ZnO) hybrid n-n type heterojunctions and the corresponding electro-optic performances of the devices. Various structures are fabricated to compare the discrepancy of the optical and electrical characteristics. It was found that the reflectance spectra presents the wheel phenomenon rather than increases monotonically with temperature at near-infrared region range. The strong interference effects was found in the hybrid multilayer heterojunction. In addition, the phase transition temperature decreases with increasing the number of the Al:ZnO layer, which can be ascribed to the electron injection to the VO2 film from the Al:ZnO interface. Affected by the double layer Al:ZnO, the abnormal Raman vibration mode was presented in the insulator region. By adding the external voltage on the Al2O3/Al:ZnO/VO2/Al:ZnO, Al2O3/Al:ZnO/VO2 and Al2O3/VO2/Al:ZnO thin-film devices, the infrared optical spectra of the devices can be real-time manipulated by an external voltage. The main effect of joule heating and assistant effect of electric field are illustrated in this work. It is believed that the results will add a more thorough understanding in the application of the VO2/transparent conductive film device.

Evaluation of lattice dynamics, infrared optical properties and visible emissions of hexagonal GeO2 films prepared by liquid phase deposition

GeO2 films with thicknesses from 10 to 22 μm have been deposited on polished Si(100) substrates using liquid phase deposition. The effects of deposition time on the thickness, crystallographic and vibrational properties, as well as optical characteristics have been investigated by X-ray diffraction (XRD), scanning electron microscopy, Raman scattering and infrared (IR) reflectance/transmittance. The XRD analysis indicates the presence of the pure hexagonal GeO2 phase, and the space group P3121, in the films, which is confirmed by the frequencies and intensities of the Raman- and IR-active phonon modes. Furthermore, the optical constants n and k in the mid- and far-IR regions for the α-GeO2 films have been extracted using IR variable angle spectroscopic ellipsometry. Interestingly, we observe that the refractive index n in certain mid- and far-IR regions is below unity (n < 1), which explains the abnormal phenomena observed in the IR transmittance spectra. Based on this unique characteristic, the GeO2 films can be used as attenuated total reflection mid- and far-infrared hollow waveguides for infrared waves. In addition, the photoluminescence spectra were collected in the visible range to investigate the related defects in the GeO2 films. The emission band near 2.1 eV originates from the X3Ge–GeX2 related defects, while the bands located at around 2.4 and 2.8 eV are related to the Ge–O related defects, which are dominant in the as-deposited GeO2 films. These findings provide further information for developing GeO2-based optical devices.

Boosted adsorption–photocatalytic activities and potential lithium intercalation applications of layered potassium hexaniobate nano-family

Two-dimensional layered K4Nb6O17 (KN) possesses two different types of interlayer regions, which is of great interest for applications in energy conversion, environmental purification, etc. Although the photocatalytic properties of KN have been extensively studied, there still remain some pivotal problems that need to be clarified for future applications. Here we demonstrate that the KN nano-family (including KN nanolaminas and nano hollow spheres) can be derived from the same Nb2O5-based hydrothermal reaction. Different morphologies of KN show unique microstructures and optoelectronic properties. Remarkably, the initial pH of a dye solution has been proven to play a vital role in affecting the adsorption and photocatalytic performances of KN. Due to the effects of dye sensitization, KN shows superior photodegradation performance under both ultraviolet and visible light. Based on these crucial results, a highly-efficient and feasible scheme has been proposed to deal with dye wastewater. In addition, KN as a potential anode material for lithium ion batteries has been investigated for the first time. The present work could be helpful in broadening the multifunctional applications of KN and other layered niobate materials.