Extensive attention has been paid to vanadium oxide polymorphs because of their potential to be used in applications in information and optoelectronic devices, as well as in energy harvesting technologies. However, vanadium oxides always form a very complex phase diagram; in particular, it is still challenging to synthesize pure vanadium oxide epitaxial polymorphs on low-cost, transparent and wafer-scale substrates. Here, we demonstrate the growth of epitaxial polymorphs of vanadium oxide (VO2 (R, M1 and their mixed phase), and V2O3) and (001)-textured VO2 (B) thin films on the (0006) surface of sapphire without selecting specific substrate orientations. This is achieved by controlling the vanadium arrival rate via sputtering power and oxidation of vanadium atoms through the partial pressure of oxygen using magnetron sputtering techniques, which enables wafer-scale production of the vanadium oxide thin films on the sapphire substrates. Growth phase diagrams of the various polymorphs are also developed for guiding device design based on the vanadium oxide thin films. This work paves a way towards practical applications of vanadium oxide thin films on chemically stable, transparent and low-cost sapphire substrates.

A series of Mo-doped Ca2NiWO6 (Ca2NiW1−xMoxO6, x = 0–0.05) samples were synthesized by a solid-state reaction. The physical and optical properties of these photocatalysts were characterized by X-ray diffraction, X-ray absorption fine structure, UV-visible diffuse reflectance spectra, and the band structure along with density of states were calculated by the plane-wave-based density functional theory. The photocatalytic activity of oxygen evolution from water was evaluated under visible light irradiation. The Mo doping significantly increased the photocatalytic activity of Ca2NiWO6. The optimal Ca2NiW0.97Mo0.03O6 showed an oxygen evolution rate approximately 2 times higher than that of the pure Ca2NiWO6. The characterization results indicated that the Mo-doped Ca2NiWO6 maintained the double perovskite structure. The Mo6+ ions were substituted into the W6+ sites in the lattice, causing a certain amount of lattice distortion and promoting the formation of oxygen vacancies. Furthermore, the Mo doping introduced impurity energy levels into the band structure of Ca2NiWO6. Consequently, the conduction band changed from discrete to continuous and the bottom of the conduction band was shifted to more positive potential, compared to that of pure Ca2NiWO6, leading to a lower band gap and higher absorbance in the wider visible light region. These characteristics are responsible for the high photocatalytic activity of the oxygen evolution reaction.

•The MIT temperatures are anisotropic along in-plane directions of (110)-VO2 films.•The resistivity has a pronounced anisotropy in the vicinity of the MIT.•Epitaxial strain-induced stripe domains contribute to anisotropy of the MIT.This work reports the thickness dependence of the anisotropic electronic transport in strained VO2/TiO2 (110) epitaxial thin films with the aim of exploring the strain-modulated metal-insulator transition (MIT). The MIT temperature varies from around 349 K–341 K while increasing the film thickness, which is higher than the bulk MIT temperature of ∼340 K. More importantly, the electrical transport property is anisotropic along the in-plane [100] and [1-10] directions and the anisotropy is further improved in thicker films. Using synchrotron radiation X-ray reciprocal space mapping (RSM) and atomic force microscopy, the thickness-dependent strain induces different stripe phase states in the VO2 thin films and thus the emergence of the distinctive anisotropic electronic transport in the vicinity of the MIT. These results provide a better understanding of the crystalline-facet-dependent strain modulation of the MIT and offer a guide for designing metamaterials based on the anisotropic features of VO2 thin films.Download high-res image (307KB)Download full-size image

•The (0 1 0)-VO2 film is epitaxially grown on functional ferroelectric substrate.•Metal-insulator transition temperature is reduced about 2.8 K by electric fields.•Electric-field-induced non-volatile resistance states can encode information.High-quality (0 1 0) VO2 thin films were epitaxially grown on functional ferroelectric (1 1 1)-oriented Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-0.3PT) substrates by reactive magnetron sputtering. The VO2/PMN-0.3PT heterostructures demonstrated metal–insulator transition (MIT) hysteresis with a resistance change of the order of ∼350. Structural characterization of the heterostructures at varying temperatures confirmed that a structural phase transition accompanies the MIT. Moreover, the dynamic strain induced by the converse piezoelectric effect lowers the critical temperature of the MIT from 341.9 K at 0 kV/cm to 339.1 K at 6 kV/cm in the VO2/PMN-0.3PT heterostructures. The resistance of the VO2 thin films could be dynamically modulated by electric field-induced strain, with a change ratio of up to 9.8% near the ferroelectric coercive field. Moreover, the heterostructures displayed non-volatile resistance switching, providing the potential to encode binary information at room temperature by proper electric-field cycling. These functional heterostructures based on correlated electron materials may realize dynamic and non-volatile manipulation of the MIT and resistance switching, thus demonstrating great potential for use in energy-efficient and non-volatile oxide electronic devices.

Improving the C2+ alcohols selectivity is the most difficult challenge in higher alcohol synthesis (HAS) from syngas. Herein, three effective strategies were combined to develop a Mo-based catalyst for HAS. The sol–gel method produced a highly homogeneous distribution of components, which ensured intimate and sufficient contact between different active sites. The incorporation of Mn oxide enhanced the interaction between Co and Mo and thus promoted the growth of the alcohol chain. More importantly, the reduction degrees of Co and Mo can be tuned precisely. The prepared Mn/K/Co/Mo catalysts show unusual activity for HAS.

•Angle-dependent anisotropic magnetoresistance is developed for the multiferroics.•Magnetization rotation is quantitatively obtained by angle-dependent AMR.•Electric-field-controlled coercive field and MR enables magnetoelectronic devices.We report that magnetization switching behaviors in multiferroic Ni/SiO2/Ti/(011)-PMN-PT hetrostructures with Hall-bar geometry can be easily understood by measuring angle-dependent anisotropic magnetoresistance (MR). The MR along the in-plane [100] and [011¯] directions of the Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) substrate were −0.175% and 0.165% at 200 Oe, respectively. The coercive field along the in-plane [100] and [011¯] directions of the PMN-PT substrate were both increased from 30 Oe at 0 kV/cm to 75 Oe at 4 kV/cm. However, we observed the anisotropic tunability of the AMR in this multiferroic heterostructure. While the AMR with magnetic field H (~200 Oe) parallel to j was 0.165% at the zero electric field and decreased to 0.032% at 4 kV/cm, the AMR with H perpendicular to j increased from −0.174% at the zero electric field to −0.339% at 4 kV/cm. Furthermore, based on angle-dependent AMR measurements using rotating in-plane external magnetic fields, the easy axis was determined to lie along the [011¯] direction and was rotated 15° by the 4 kV/cm electric field. Our work provides a facile method to study magnetization switching behaviors in multiferroic heterostructures by measuring angle-dependent AMR.

•The performances of large-size GL-LSCs were studied.•The mechanism of transport process was proposed.•The fabricated GL-LSC obtained a gain of 1.38 with PV cell coverage of 4.54%.•The fabricated GL-LSC achieved a PCE of 2.28% with PV cell coverage of 11.32%.•The lowest cost of $ 1.25/WP was optimized with a PCE of 2.02% and a gain of 1.27.Luminescent solar concentrators (LSCs) offer an attractive approach to combine spectral and spatial concentration of both direct and diffuse light without the expensive tracking system. In this study, different size glass laminated luminescent solar concentrators (GL-LSCs) of 7.8 × 7.8 cm2, 15.6 × 15.6 cm2, 31.2 × 31.2 cm2 and 61 × 122 cm2 were fabricated by using fluorescent dyes Lumogen Red 305 and Yellow 083, ultra-white glass and commercial mono-crystalline silicon solar cells. With the size increase, the output power of the GL-LSCs increases, but the power conversion efficiency (PCE) decreases rapidly due to the transport losses and re-absorption of luminescent materials. As a result, a series of GL-LSCs with bottom-mounted PV cells were fabricated to enhance the PCE. The relationship between the area of luminescent waveguide and the gain of the bottom-mounted PV cells was investigated for the optimization of the GL-LSCs performance. Among them, a highest gain of 1.38 in power over the bare PV cells was obtained with PV cell coverage of 4.54%. A highest PCE of 2.28% was achieved with PV cell coverage of 11.32%. It was found that the design with PV cell coverage of 9.07% have the lowest cost of $ 1.25/WP with a PCE of 2.02% and a gain of 1.27.

A series of high-quality vanadium dioxide (VO2) epitaxial thin films on (0001)-oriented sapphire substrates with various thicknesses were fabricated using radio frequency (RF) magnetron sputtering techniques. Structural analysis revealed that an out-of-plane tensile strain (∼+0.035%) in the thinner VO2 epitaxial films was induced by epitaxial lattice mismatch between the monoclinic VO2 films and Al2O3 substrates. However, an anomalous compressive strain (∼−0.32%) was accumulated along the out-of-plane direction in the thicker VO2 films. This result contradicts with the conventional epitaxial lattice-mismatch mechanism for strain formed in epitaxial films. We attribute this anomalous strain to the surface growth mode (island growth) in the thicker VO2 films, especially those sputtered from the metal target at low pressure. Furthermore, the metal–insulator transition (MIT) temperature shifted to lower temperature with decreasing thickness, which is attributed to modulation of the orbital occupancy through the epitaxial strain and growth-mode-induced strain in the VO2 epitaxial films. Moreover, the very large resistance change (on the order of magnitude ∼103) in the VO2/Al2O3 epitaxial heterostructures is promising for electrical switch applications.

Effective adsorption is of great importance to the photocatalytic degradation of volatile organic compounds. Herein, we succeeded in the preparation of anatase TiO2 with clean dominant {001} and {101} facets. By using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) equipped with a homemade reaction system and a coupling gas-dosing system, we found that TiO2 with dominant {001} facets exhibits higher toluene adsorption capacity than TiO2 with dominant {101} facets, which may be attributed to the different number of unsaturated 5c-Ti capable of forming the main active adsorption sites (terminal Ti–OH species). TiO2 with dominant {001} facets shows a significantly high photocatalytic degradation performance, with its degradation rate being 6 times higher than that of dominant {101} facets. Combined with simulation results, it is suggested that the synergetic effects of the formation of specific active adsorption sites, the low adsorption energy for toluene, and preservation of the free molecularly adsorbed water on the surface promote the degradation of gaseous toluene on the dominant {001} facets. This study exemplifies that the facet-dependent adsorption of volatile organic compounds is one of the most important factors to effectively engineer photocatalysts for air purification.

Vanadium dioxide (VO2) epitaxial thin films on (0001)-oriented Al2O3 substrates were prepared using radio frequency (RF) magnetron sputtering techniques. To study the metal-insulator-transition (MIT) mechanism and extend the applications of VO2 epitaxial films at terahertz (THz) band, temperature-dependent X-ray diffraction (XRD) and THz time domain spectroscopy of the VO2 epitaxial films were performed. Both the lattice constants and THz transmission exhibited a similar and sharp transition that was similar to that observed for the electrical resistance. Consequently, the MIT of the VO2/Al2O3 epitaxial films should be co-triggered by the structural phase transition and electronic transition. Moreover, the very large resistance change (on the order of ~103) and THz response (with a transmission modulation ratio of ~87%) in the VO2/Al2O3 epitaxial heterostructures are promising for electrical switch and electro-optical device applications.

Visible-light-driven hydrogen evolution with high efficiency is important in the current photocatalysis research. Here we report for the first time the design and synthesis of a new graphene–semiconductor nanocomposite consisting of YInO3 nanoparticles and two-dimensional graphene sheets as efficient photocatalysts for hydrogen evolution under visible light irradiation. The graphene/YInO3 nanocomposites were synthesized using a facile solvothermal method in which the formation of graphene and the deposition of YInO3 nanoparticles on the graphene sheets can be achieved simultaneously. The addition of graphene as a cocatalyst can narrow the band gap of YInO3 to visible photon energy and prolong the separation and lifetime of electron–hole pairs by the chemical bonding between YInO3 and graphene. The photocatalytic reaction with this nanocomposite reaches a high H2 evolution rate of 400.4 μmol h−1 g−1 when the content of graphene is 0.5 wt%, over 127 and 3.7 times higher than that of pure YInO3 and Pt/YInO3, respectively. This work can provide an effective approach to the fabrication of graphene-based photocatalysts with high performance in the field of energy conversion.

Ultrathin Sm0.6Nd0.4NiO3−δ epitaxial films were deposited by pulsed laser deposition (PLD) onto LaAlO3 (LAO) single crystal substrates. The influence of growth oxygen pressure on the metal–insulator transition (MIT) was investigated. It was found that the MI transition temperature (TMI) of the films decreases remarkably with the decrease of the growth oxygen pressure, while the films' strain state stays almost the same. The increased oxygen vacancies induced by lower growth oxygen pressure, verified by X-ray photoelectron spectroscopy, seem to be the main cause of such phenomena.

•G/CIO composites were synthesized by a facile solvothermal method.•The photocatalytic activities were studied under visible light irradiation.•The introduction of graphene can enhance the photocatalytic activity of CIO.•G1/CIO composite exhibited the best photocatalytic activity.•G1/CIO composite could evolve H2 stably for 32 h without deactivation.A series of graphene/CaIn2O4 composites were synthesized using a facile solvothermal method to improve the photocatalytic performance of CaIn2O4. The reduction of graphene oxide to graphene and the deposition of CaIn2O4 nanoparticles on the graphene sheets can be achieved simultaneously during the solvothermal process. The photocatalytic activities of as-prepared graphene/CaIn2O4 composites for hydrogen evolution from CH3OH/H2O solution were investigated under visible light irradiation. It was found that graphene exhibited an obvious influence on the photocatalytic activity of CaIn2O4. The graphene/CaIn2O4 composite reached a high H2 evolution rate of 62.5 μmol h−1 from CH3OH/H2O solution when the content of graphene was 1 wt%. Furthermore, the 1 wt% graphene/CaIn2O4 composite did not show deactivation for H2 evolution for longer than 32 h. This work could provide a new insight into the fabrication of visible light driven photocatalysts with efficient and stable performance.

In order to study the factors affecting superconductivity of Sr2CuO3 + δ, polarized X-ray absorption spectra (XAS) were measured at Cu L23 edge and O K edge of an over-doped single crystal specimen. This polarized XAS study reveals a distinctive anisotropic local electronic structure of Cu coordination: the unoccupied states are dominated by the in-plane components of Cu 3dx2 − y2 and O 2px,y. However, the signal from the unoccupied out-of-plane O 2pz and Cu 3dz2 states also appear. Furthermore, the detailed analysis of Cu L3-edge XAS at 300 K and 17.3 K indicates that the conductive carrier partly shifts from out-of-plane to in-plane in the superconducting state. Theoretical calculation of Cu L23-edge core level spectra based on Charge Transfer Multiplet theory is fairly consistent with above experimental results.

The development of far infrared spectroscopy offers a powerful method for comprehensive study in adsorption structure and photocatalytic degradation mechanism of photocatalysis. This study presented an improved in situ diffuse reflectance infrared Fourier transform spectroscopy technique in far infrared region for investigation of weak-bond adsorption and photocatalytic degradation of gaseous toluene on the surface of TiO2. It was found that toluene tends to be adsorbed on the hydroxyl group via three possible sites, the ortho-, meta-, and para-adsorption site, instead of ipso-structure. The methyl group of toluene is consumed first during the process of toluene photocatalytic degradation. Based on these, a reaction route for the photocatalytic degradation of gaseous toluene on TiO2 surface was proposed.

A series of CaIn2O4/Fe-TiO2 composite photocatalysts with tunable Fe-TiO2 contents were prepared in which Fe-TiO2 nanoparticles were uniformly deposited onto the surface of CaIn2O4 nanorods. The photocatalytic activities of Pt-loaded CaIn2O4/Fe-TiO2 composites were evaluated for H2 evolution from aqueous KI solution under visible light irradiation. It was found that the composites showed higher H2 evolution rates in comparison with pure CaIn2O4 or Fe-TiO2, which could be attributed to the increased surface area and enhanced visible light absorption. A high H2 evolution rate of 280 μmol h–1 g–1 was achieved when the mass ratio of Fe-TiO2 to CaIn2O4 was 0.5, which was 12.3 and 2.2 times higher than that of pure CaIn2O4 and Fe-TiO2, respectively. Furthermore, the interfaces between CaIn2O4 nanorods and Fe-TiO2 nanoparticles facilitated efficient charge separation that also led to the improved photocatalytic activity. This study may provide some inspiration for the fabrication of visible-light-driven photocatalysts with efficient and stable performance.

•Nanostructured CaIn2S4 was synthesized by hydrothermal method.•CaIn2S4 has an optical band gap of 1.76 eV.•The photocatalytic activities were studied under visible light irradiation.•CaIn2S4 exhibited excellent photocatalytic activity.•The photocatalyst could evolve H2 stably without deactivation.A novel visible-light-driven photocatalyst CaIn2S4 was synthesized using a facile hydrothermal method followed by a post-calcination process. The influence of the calcination temperature and time on the activities of the photocatalyst was investigated. CaIn2S4 exhibits optical absorption predominantly in visible region with an optical band gap of 1.76 eV. Considerable activity for hydrogen evolution from pure water was observed without any sacrificial agents or cocatalysts under visible light irradiation. The maximum hydrogen evolution rate achieved was 30.92 μmol g−1 h−1 without obvious deactivation of the photocatalytic activity for four consecutive runs of 32 h.

The adsorption and photocatalytic oxidation of formaldehyde on the pure TiO2 under dry and humid conditions were studied by in situ diffuse reflectance Fourier transform infrared spectroscopy. It was found that the formaldehyde molecules can be adsorbed on the hydroxyl groups on the TiO2 surface via hydrogen bonding. With UV irradiation, the adsorbed formaldehyde rapidly converts to the formate species even on the pure TiO2 at room temperature and atmospheric pressure. In the dry environment, the superoxide radical anion O2−•, formed by adsorbed oxygen reacting with electrons, is suggested to play an important role in the formaldehyde oxidation. The introduction of water vapor provides a large amount of water and hydroxyl groups on the catalyst surface. Oxidation of water and hydroxyl groups by the photogenerated holes produces very active OH• radicals, which take part in the redox reactions and improve significantly the mineralization rate of formaldehyde on the TiO2 due to its high redox potential.Open image in new window

A combinatorial approach was used to systematically investigate the photocatalytic activities of different kinds of ABO3-type oxides (A = Y, La, Nd, Sm, Eu, Gd, Dy, Yb; B = Al, In). Two novel photocatalysts, cubic YInO3 and perovskite YAlO3, were identified rapidly. Scale-up experiments confirmed that the two photocatalysts, especially the YInO3, had excellent photocatalytic activity for toluene oxidation and water splitting under visible-light irradiation.

Recent applications of combinatorial methodology to the investigation of optical functional materials are reviewed on the basis of the authors’ work. The content includes: basic concepts of combinatorial materials science, combinatorial investigations of UV/VUV excited photoluminescence, luminescent rear earth complex doped polymer and magneto-optical materials. The fundamental synthesis techniques and characterization methods in combinatorial methodology are also illustrated in the text.

A combinatorial approach was used to systematically investigate the effect of trace Pr3+, Tb3+, or Sm3+ on the VUV photoluminescence of Eu3+ in the Pr3+, Tb3+, or Sm3+ co-doped (Y0.65Gd0.35)BO3:Eu3+0.05. We found that Pr3+ and Tb3+increases the VUV photoluminescent efficiency, while Sm3+ decreases the efficiency. The optimized composition was identified to be between 7 × 10−6 and 3 × 10−4, and the corresponding efficiency improvement is about 15%. Scale-up experiments confirmed the results in the combinatorial materials libraries.

Pure gadolinium gallium garnet (Gd3Ga5O12, GGG) powders had been synthesized from a mixed solution of Gd and Ga nitrates with stoichiometric mole ratio of 3:5 (Gd/Ga) via co-precipitation using ammonium hydrogen carbonate (NH4HCO3, AHC) as the precipitant. XRD, TEM, and thermal analysis were used to characterize the phase evolution and morphology of the final powders. The synthesized GGG polycrystalline materials were loosely agglomerated and showed good dispersity and excellent sinterability.

•The performances of large-size GL-LSCs were studied.•The mechanism of transport process was proposed.•The fabricated GL-LSC obtained a gain of 1.38 with PV cell coverage of 4.54%.•The fabricated GL-LSC achieved a PCE of 2.28% with PV cell coverage of 11.32%.•The lowest cost of $ 1.25/WP was optimized with a PCE of 2.02% and a gain of 1.27.Luminescent solar concentrators (LSCs) offer an attractive approach to combine spectral and spatial concentration of both direct and diffuse light without the expensive tracking system. In this study, different size glass laminated luminescent solar concentrators (GL-LSCs) of 7.8 × 7.8 cm2, 15.6 × 15.6 cm2, 31.2 × 31.2 cm2 and 61 × 122 cm2 were fabricated by using fluorescent dyes Lumogen Red 305 and Yellow 083, ultra-white glass and commercial mono-crystalline silicon solar cells. With the size increase, the output power of the GL-LSCs increases, but the power conversion efficiency (PCE) decreases rapidly due to the transport losses and re-absorption of luminescent materials. As a result, a series of GL-LSCs with bottom-mounted PV cells were fabricated to enhance the PCE. The relationship between the area of luminescent waveguide and the gain of the bottom-mounted PV cells was investigated for the optimization of the GL-LSCs performance. Among them, a highest gain of 1.38 in power over the bare PV cells was obtained with PV cell coverage of 4.54%. A highest PCE of 2.28% was achieved with PV cell coverage of 11.32%. It was found that the design with PV cell coverage of 9.07% have the lowest cost of $ 1.25/WP with a PCE of 2.02% and a gain of 1.27.Download full-size image