Qinglin Deng

Optoelectronic properties and polar nano-domain behavior of sol–gel derived K0.5Na0.5Nb1−xMnxO3−δ nanocrystalline films with enhanced ferroelectricity

High-quality lead-free piezoelectric K0.5Na0.5Nb1−xMnxO3−δ (KNNMx, 0 ≤ x ≤ 0.10) films have been successfully deposited on Pt(111)/Ti/SiO2/Si(100) substrates by a modified sol–gel method. The effects of Mn substitution on the microstructure, morphology, lattice vibrations, and optical and ferroelectric properties of the KNNMx films have been investigated in detail. All films are polycrystalline, crack-free and show a pseudo-cubic (pc) structure with a thickness of about 215 nm. Raman analysis indicates that the characteristic frequency of ν1, ν5 and ν1 + ν5 modes shifts towards lower wavenumbers with increasing Mn concentration. The optimal ferroelectric properties were obtained in the film doped with x = 0.06, whose remnant polarization (2Pr) and coercive field (2Ec) values at the applied electric field of 1000 kV cm−1 are 51 μC cm−2 and 265 kV cm−1, respectively. The increased valence of Mn2+, which is substituted at the Nb5+ site as Mn3+, plays an important role in reducing the amount of both oxygen vacancies and holes. In addition, the dielectric functions of the KNNMx films have been uniquely extracted by fitting ellipsometric spectra with the Adachi dielectric function model and a four-phase layered model (air/surface rough layer/KNNMx/Pt) in the photon energy range of 1.5–5.5 eV. The optical band gap (Eg) slightly decreases, while the high-frequency dielectric constant (e∞) linearly increases with increasing Mn concentration. Moreover, temperature dependent optical dispersion behavior of the KNNM0.06 film has been investigated from 300 K to 800 K. The analysis of Eg and the extinction coefficient (κ) reveals the correlation between optical properties and structural phase transition. Furthermore, a distinct in-plane (180°) polar nano-domain pattern with a well-defined rectangular phase hysteresis loop has been observed in the KNNM0.06 film from piezoresponse force microscopy (PFM) experiments. The present results could be crucial for potential multifunctional KNN-based device applications.

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).

Manipulations from oxygen partial pressure on the higher energy electronic transition and dielectric function of VO2 films during a metal–insulator transition process

Optical properties and metal–insulator transition (MIT) of vanadium dioxide (VO2) films grown by pulsed laser deposition with different oxygen pressures (5 to 50 mTorr) have been investigated by temperature dependent transmittance spectra. Three interband critical points (E1, E2 and E3) can be obtained via fitting transmittance spectra and the hysteresis behavior of the center transition energies E1 and E2 is presented. The VO2 film grown at optimized oxygen pressure exhibits the well-defined resistivity drop (∼103 Ω cm) across the MIT process. It is found that the metal–insulator transition temperature (TMIT) increases with the oxygen pressure and the complex dielectric functions are drastically affected by oxygen pressure. It is believed that the oxygen pressure can lead to lattice defects, which introduce the donor level and the acceptor level in the forbidden gap produced by oxygen vacancies and vanadium vacancies, respectively. The donor level provides electrons for higher empty π* bands, which can make the energy barrier lower and decrease critical temperature. On the contrary, electrons jumping from the d∥ band can be recombined by holes on the acceptor, impeding the MIT occurrence. It is claimed that the electronic orbital occupancy is closely related to oxygen pressure, which changes the energy barrier and manipulates the phase transition temperature. The present results are helpful to understand the fundamental mechanism of VO2 films and practical applications for VO2-based optoelectronic devices.

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.

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.

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.

Controllable interlayer space effects of layered potassium triniobate nanoflakes on enhanced pH dependent adsorption-photocatalysis behaviors

Despite the extensive study of two-dimensional layered KNb3O8 (KN), there still remains some vital problems need to be clarified for future applications in environmental purification. Here we demonstrated the successful preparation of interlayer-controlled KN nanoflakes using alkaline hydrothermal conditions by adjusting the amount of thiourea in the reaction. This process resulted in KN nanoflakes with a larger specific surface area than previously reported. Moreover, the initial pH of dye solution and discrepant preferential orientation of interlayer peak have been proved to significantly influence the adsorption and photocatalysis performances of KN. In addition, relevant photocatalysis mechanisms have been expounded, by combined the first-principles calculation. The present work could be helpful in revealing the intrinsic adsorption-photocatalysis features of KN and other similar niobates.

Facile fabrication of 3D porous MnO@GS/CNT architecture as advanced anode materials for high-performance lithium-ion battery

To overcome inferior rate capability and cycle stability of MnO-based anode materials for lithium-ion batteries (LIBs), we reported a novel 3D porous MnO@GS/CNT composite, consisting of MnO nanoparticles homogeneously distributed on the conductive interconnected framework based on 2D graphene sheets (GS) and 1D carbon nanotubes (CNTs). The distinctive architecture offers highly interpenetrated network along with efficient porous channels for fast electron transfer and ionic diffusion as well as abundant stress buffer space to accommodate the volume expansion of the MnO nanoparticles. The MnO@GS/CNT anode exhibits an ultrahigh capacity of 1115 mAh g-1 at 0.2 A g-1 after 150 cycles and outstanding rate capacity of 306 mAh g-1 at 10.0 A g-1. Moreover, a stable capacity of 405 mAh g-1 after 3200 cycles can still be achieved, even at a large current density of 5.0 A g-1. When coupled with LiMn2O4 (LMO) cathode, the LMO [Formula: see text] MnO@GS/CNT full cell characterizes an excellent cycling stability and rate capability, indicating the promising application of MnO@GS/CNT anode in the next-generation LIBs.