The influence of precursors and additives on the hydrothermal synthesis of VO2: A route for tuning the metal–insulator transition temperature

Thermochromic materials have attracted the attention of scientific and technological researchers due to their ability to change color depending on the temperature. Vanadium dioxide (VO₂) is capable of considerable polymorphs and has aroused interest mainly because its metal–insulator transition (MIT) presents a thermochromic characteristic at a relatively low temperature. This work aimed to obtain vanadium oxide nanostructures using hydrothermal synthesis to tune the MIT temperature. Ammonium metavanadate or vanadium pentoxide was used as a precursor of vanadium, oxalic acid as a reducing agent, and sodium molybdate as an additive. The starting materials were homogenized and inserted in a hydrothermal reactor at 180 °C. After 24 h of synthesis, part of the resulting product was heat-treated at 400 °C for 3 h. The powders obtained were characterized by their structure, morphology, and thermal properties. The results showed a fiber/rod-shaped VO₂ (M) morphology. Distinct strategies were used to obtain the crystalline phase of interest (VO₂(M)), and the presence of a reversible change occurring at ~68 °C was evaluated according to the parameters from the VO₂ phase transition. The addition of sodium molybdate favored a 22% reduction in the MIT temperature when the precursor used was vanadium pentoxide, indicating possible doping in the structure increased the effects of smaller crystallite size and the presence of crystalline phases. This work opens new perspectives for applications of the vanadium oxides obtained, such as in thermal sensors and/or intelligent materials.     

Preparation and characterisation of titania-supported vanadium–phosphorus oxide catalysts

A series of vanadium–phosphorus oxides (mainly with V P ) supported on pigmentary anatase (10 m² g^-1) has been prepared using aqueous NH₄VO₃ and (NH₄)H₂PO₄ solutions, with loadings up to 11.3 wt%, equivalent to about 12.7 monolayers. Characterisation by X-ray diffraction, laser Raman spectroscopy, X-ray photoelectron spectroscopy and temperature-programmed reduction suggests that the main phase present at loadings below about 10 wt% is an amorphous V–P oxide which exists chiefly as blocks of disordered material. The presence of small amounts of crystalline -VOPO₄ and of V₂O₅ is indicated at the highest loadings, especially when and V P ratios are used. The two materials having the lowest loadings are active for methanol oxidation at 473–533 K, and show high selectivity to formaldehyde. This revised version was published online in June 2006 with corrections to the Cover Date.     

Low temperature catalytic oxidation of nitric oxide over the Mn–CoOx catalyst modified by nonthermal plasma

Nonthermal plasma (NTP) treatment was investigated to modify the Mn–CoO_x catalyst for the low-temperature oxidation of nitric oxide. The catalysts were characterized by XRD, BET, TGA and XPS techniques. The results showed that the activity of NTP-treated catalysts improved significantly, and that NTP treatment has the advantage of changing the structural and morphological properties (higher surface areas and pore volume) and varying the relative surface concentration and oxidation states of surface species over catalysts. High surface areas and pore volume, high concentration of chemisorbed oxygen, Mn^4 + and Co^2 +, and the efficient synergetic catalytic effect between Co and Mn ions were thought to be the main reasons for the high activity of NTP-treated catalysts.     

Direct multipulse laser processing of titanium oxide–graphene oxide nanocomposite thin films

Nanocomposite thin films consisting of titanium oxide (TiO₂) nanoparticles (NPs) and graphene oxide (GO) platelets were deposited by a spin-coating technique. The obtained films were submitted to direct laser irradiation using a frequency quadrupled Nd:YAG (λ=266 nm, τ_FWHM≅3 ns, ν=10 Hz) laser source. The effect of the laser processing conditions, as laser fluence value and number of subsequent laser pulses incident onto the same target location, on the surface morphology, crystalline structure, and chemical composition of the TiO₂/GO nanocomposite thin films was systematically investigated. The laser fluence values were maintained below the vaporization threshold of the irradiated composite material. With the increase of the laser fluence and number of incident laser pulses melting and coalescence of the TiO₂ NPs into inter-connected aggregates as well as rippling of the GO platelets take place. The gradual reduction of GO platelets and the onset of anatase to rutile phase transition were observed at high laser fluence values.     

In-situ generation of metal–metal oxide catalysts for the growth of highly oriented graphitic nanowiggles

In this work we demonstrate an in situ generated growth catalyst that produces highly oriented graphitic nanowiggles (GNWs). GNWs are a new form of disordered nanocarbons with graphite domains stacked perpendicular to the filament axis. They had been prepared by acetylene decomposition at ∼525 °C on Al–Mg mesh coated with iron-based nanoparticles. During CVD, inter-diffusion of Mg, Fe, and O between the electrodeposited Fe particles and the mesh generate flakes which become the active site for nanowiggle growth. The properties of this unique form of carbon were evaluated by TEM, HRTEM, SEM, Raman spectroscopy, XPS and cyclic voltammetry.     

The novel preparation method of thermochromic VO2 films with a low phase transition temperature by thermal oxidation of V–Mo cosputtered alloy films

Vanadium dioxide (VO₂) has been studied extensively for its unique insulator-metal transition characteristics and potential applications in thermochromic smart windows, switching devices, and infrared detectors. However, how to balance the metal-insulator transition temperature, luminous transmittance (T_lum) and solar modulation ability (ΔT_sol) of VO₂ thin films remains a challenge. In this work, high-quality thermochromic VO₂ thin films were prepared by a two-step method of magnetron sputtering and thermal oxidation annealing. Metallic and alloyed V–Mo layers were first deposited by direct-current reactive magnetron sputtering, and then a thermal oxidation annealing process was used to obtain pure and Mo-doped VO₂ thin films. The Mo content in the films was regulated by changing the sputtering power of the vanadium target, and the effect of Mo doping on the crystallinity, microstructure, phase transition temperature and optical properties of VO₂ thin films was studied. The shift of the VO₂(011) peak to a lower 2θ angle in the XRD patterns showed that Mo was successfully diffused into vanadium dioxide films. The phase transition temperatures were decreased continuously from 57.4 to 32.7 °C by decreasing the sputtering power of vanadium. The thinner Mo-doped VO₂ thin films showed higher luminous transmittance and lower transition temperature. Our results were shown to be an innovative preparation method to fabricate thermochromic VO₂ films with a low phase transition temperature, balanced luminous transmittance and solar modulation ability by thermal oxidation of V–Mo cosputtered alloy films.     

Hydration of ethene over W–P mixed metal oxide catalysts

W–P mixed metal oxide catalysts are active and selective for the gas-phase hydration of ethene to ethanol. The activity and selectivity of this catalytic reaction depend on the W/P atomic ratio. However, ethene conversion slightly decreases at higher W/(W + P) atomic ratio. The selectivity for ethanol increases with the W/P atomic ratio and reaches the highest value (92%) at W_0.81P_0.19O_x. The W_0.81P_0.19O_x catalyst is less active than the conventional H₃PO₄/SiO₂ catalyst, but the activity is maintained for a long time without the supply of any catalyst components. The reaction temperature does not affect substantially the rate of ethene hydration over the W_0.81P_0.19O_x catalyst. The H₂O/ethene molar ratio of 0.4 is the most appropriate for both reaction rate and selectivity. The active species of W–P mixed metal oxide are amorphous. But there is Keggin structure of W–P oxide species (PW₁₂O₄₀ ³⁻) in the presence of steam. And the species are the active sites for the hydration of ethene, confirmed by in situ Raman spectroscopy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stabilized bismuth oxide–noble metal mixed conducting composites as high temperature oxygen separation membranes

Oxygen concentration cells, with dense dual phase composite membranes made from erbia-stabilized bismuth oxide and a noble metal (Au, Ag), were investigated in the temperature range 650–850°C under controlled oxygen partial pressure gradients. An electrochemical treatment was applied to interpretation of the oxygen permeation data. It is found that the composite membranes exhibit high oxygen permeability relative to the single phase bismuth oxide, since oxygen ions and electrons are allowed to transport through the oxide and metal phase, respectively. The oxygen permeability of the silver-containing composite is at least one order higher than that of the gold-containing one, which can be explained by the fact that silver has a higher catalytic activity than gold for the surface oxygen exchange reaction and thus less limitations are exerted on the overall oxygen transport.     

Low temperature firing of Co2Y–NiCuZn ferrite composites

Hexagonal structure magnetoplumbite ferrites have revealed a higher dispersion frequency than that of nickel ferrites because of the magnetoplumbite's magnetic anisotropy. The magnetoplumbite ferrite densification temperature always exceeds 1000 °C and the initial low temperature firing permeability of magnetoplumbite ferrites with added glass is too low (μ_i = 2–4). Therefore, it is desirable to develop a material that has a higher permeability at above 300 MHz and can be densified at temperatures below 900 °C. The Bi₂O₃–B₂O₃–ZnO–SiO₂ (BBSZ) glass addition effects on the densification and magnetic properties of Co₂Y–NiCuZn ferrite composites with various Co₂Y/NiCuZn ferrite ratios were investigated. The densification of Co₂Y–NiCuZn ferrite composites was enhanced by the addition of glass at low sintering temperatures (<900 °C) due to the liquid phase sintering. Co₂Y–NiCuZn ferrite composites with 4 wt% BBSZ glass sintered at 900 °C show a relative density above 90%, a high-initial-permeability of 5–6, a quality factor of above 30 in the 200–300 MHz frequency and a resonance frequency above 1 GHz, which can be used in high frequency multilayer chip inductors.     

Observation of the semiconductor–metal transition behavior in monolayer graphene

We have observed that during temperature-dependent four-terminal resistance measurement of monolayer graphene, the resistance exhibits anomalous rising and falling behavior at different temperature regions. At lower temperature region (2–200 K) the resistance decreases gradually, but when the temperature rise further it turn to a sudden increase, and after 280 K it resumes gradual decrease. The rising and falling resistance behavior is characteristic of semiconductor or metal property. Consequently, the resistance transition follows a phase of semiconductor–metal–semiconductor. However, when a perpendicular magnetic field is applied, the resistance shows reverse transition behavior which follows a sequence of metal–semiconductor–metal. The novel transition property is attributed to the competition between the disorder of lattice defects as a short-range scattering in monolayer graphene and the Landau levels interaction. Magneto-transport measurement reveals that the excitonic gap induced by magnetic field in the monolayer graphene show an anomalous thermally activated property.     

Low temperature preparation and electrical properties of sodium–potassium bismuth titanate lead-free piezoelectric thick films by screen printing

Sodium–potassium bismuth titanate (NKBT) thick films with thickness of 40 μm were prepared by screen printing. To improve the homogeneity, the sintering aids were added into the pastes as a chemical liquid-phase doping method. The results show that the addition of Bi–Li sintering aids was beneficial for both the reduction of the sintering temperature and the improvement of the electrical performance of the thick films. The thick films containing 5 wt.% Bi–Li sintering aids demonstrated optimal dielectric properties with the maximum dielectric constant of 725 and minimum dielectric loss of 2.5%. Moreover, the NKBT thick films containing 3 wt.% Bi–Li sintering aids sintered at 950 °C exhibited the remanent polarization of 19.6 μC/cm², room-temperature pyroelectric coefficient of 1.56 × 10^?4 C/(m² °C), figure of merit for specific detectivity of 0.48 × 10^?5 Pa^?0.5, and effective longitudinal piezoelectric coefficient of 88 pm/V, which are comparable to that of the high-temperature sintered thick films without sintering aids.     

Metal–insulator transition in boron-ion implanted type IIa diamond

Electrical conductivity measurements for selected boron-ion dopant concentrations have been made on type IIa diamond specimens in the temperature range 1.5–30 K. Samples have been implanted using the CIRA (cold implantation–rapid annealing) process, in which small implantation increments were used followed by high-temperature annealing to achieve a significant reduction in the levels of implantation-induced radiation damage and to obtain maximum boron activation. Further post anneals at temperatures up to 1700°C were carried out. Using this procedure, we have recorded, for the first time, metallic conductivity behaviour in implanted surface layers in single-crystal diamond specimens with boron concentrations measured by secondary-ion mass spectrometry to be of the order of n=10²¹ cm⁻³. The occurrence of a metal–insulator transition in this system is discussed.     

Effect of annealing temperature on ferroelectric electron emission of sol–gel PZT films

In this work, the influence of annealing temperature on the ferroelectric electron emission behaviors of 1.3-μm-thick sol–gel PbZr_0.52Ti_0.48O₃ (PZT) thin film emitters was investigated. The results revealed that the PZT films were crack-free in perovskite structure with columnar-like grains. Increasing annealing temperature led to the growth of the grains with improved ferroelectric and dielectric properties. The remnant polarization increased slightly from 35.3 to 39.6 μC/cm² and the coercive field decreased from the 56.4 to 54.6 kV/cm with increasing annealing temperature from 600 to 700 °C. The PZT film emitters exhibited remarkable ferroelectric electron emission behaviors at the threshold voltage above 95 V. The film annealed at 700 °C showed a relatively lower threshold voltage and higher emission current, which is related to the improved ferroelectric and dielectric properties at higher annealing temperature. The highest emission current achieved in this work was around 25 mA at the trigger voltage of 160 V.     

Promoting role of transition metal oxide on ZnTiO3–TiO2 nanocomposites for the photocatalytic activity under solar light irradiation

Transition metal oxide (Fe₂O₃, Co₃O₄ and CuO) loaded ZnTiO₃–TiO₂ nanocomposites were successfully prepared by solid state dispersion method. The structural, morphological and optical properties of samples were characterized by TGA/DTA, XRD, BET, FT-IR, DRS, PL, XPS and SEM techniques. The photocatalytic activity of samples was investigated by degradation of 4-chlorophenol in water under sunlight. The Fe₂O₃ loaded sample was found to exhibit much higher photocatalytic activity than the other composite powders. 7Fe₂O₃/ZnTi sample has the highest percentage of 4-chlorophenol degradation (100%) and highest reaction rate (1.27 mg L⁻¹ min⁻¹) was obtained in 45 min. The enhancement of photocatalytic activity for ZnTiO₃–TiO₂ sample with Fe₂O₃ addition may be attributed to its small particle size, the presence of more surface OH groups, lower band gap energy than other samples in this paper and the presence of more hexagonal ZnTiO₃ phase in the morphology.     

HDS, HDN and HYD activities and temperature‐programmed reduction of unsupported transition metal sulfides

Several transition metal sulfides have been shown to exhibit high catalytic activities in various hydrotreating reactions. The simultaneous catalytic activities for hydrodesulfurization of dibenzothiophene, hydrodenitrogenation of indole and hydrogenation of naphthalene have been studied using high surface area bulk sulfides: NbS₂, MoS₂, ReS₂, RuS₂ and Co₉S₈. The metal–sulfur bond strengths have been assessed by temperature‐programmed reduction of H₂S‐pretreated sulfides, and it is demonstrated that this pretreatment is necessary to obtain reliable TPR data for the sulfides. The catalytic HDS activity is shown to follow the trend in the metal–sulfur bond strengths estimated by this improved method. Furthermore, the metal–sulfur bond strengths determined this way follow the trend estimated previously from theory. The TPR data suggest that a large number of sulfur vacancies exist in active catalysts under industrial reaction conditions. Such vacancies have recently been observed on model catalysts by use of STM. This revised version was published online in July 2006 with corrections to the Cover Date.     

Low voltage resistive memory devices based on graphene oxide–iron oxide hybrid

Low cost resistive switching memory devices using graphene oxide–iron oxide (GF) hybrid thin films, sandwiched between platinum (Pt) and indium-tin-oxide (ITO) electrodes, were demonstrated. The fabricated devices with Pt/GF/ITO structure exhibited reliable and reproducible bipolar resistive switching performance, with an ON/OFF current ratio of 5 × 10³, excellent retention time longer than 10⁵ s, SET voltage of 0.9 V, and good endurance properties. In all aspects of the device characteristics, the GF based devices outperformed graphene oxide (GO) based devices. Ohmic conduction was found to be dominant current conduction mechanism in all switching regions except for the high voltage regime where space charge limited conduction and trap charge limited conduction were found to be the main current conduction mechanism. X-ray photoelectron spectroscopy and transmission electron microscopy/selected area diffraction analysis revealed γ-Fe₂O₃ and Fe₃O₄ iron oxide phases coexist in the hybrid films. While the desorption/adsorption of oxygen-related functional groups on the GO sheets is the dominant resistive switching mechanism in Pt/GO/ITO devices, the formation/rupture of multiple highly conducting Fe₃O₄ filaments at the iron oxide/GO interface additionally facilitate the switching in the present Pt/GF/ITO devices. Thereby, excellent electrical switching performance was achieved.     

Simultaneous two-step assisted growth of aluminium zirconium oxide from Al–Zr films

Temperature-dependent oxidation of Zr–Al at different temperatures (400, 600, 800, and 1000 °C) was studied. Formation of oxygen vacancies for charge compensation and occupants of oxygen at interstitial sites in the films have contributed to changes in terms of band transition and capacitance-voltage (C–V) flatband voltage shift. Dielectric constant (k) values (12.6–17.6) were obtained by the functional metal-oxide-semiconductor (MOS) capacitor. The acquisition of an abnormal C–V curve at 400 °C might be due to the occurrence of an inadequate oxidation at this temperature. The highest k value was demonstrated at 600 °C and beyond which the k value was decreasing. Though the film at 1000 °C possessed the lowest interface trap density, its low k value and compressive strain induced film buckling have weakened the overall film quality and thus was deemed not suitable for use in MOS application. Corresponding relationship with regards to structural, morphological and optical characteristics was also discussed.     

Temperature hysteresis of AgI phase transition in AgI–chalcogenide glass nanolayered films

Using a method of laser ablation, a number of AgI–chalcogenide glass nanolayered films has been obtained with different thicknesses of the layers (10, 25, 50, and 100 nm). In order to study α ? β phase transition in AgI, X-ray phase analysis has been carried out in the temperature range from 30 to 200°C. A correlation between the layer thickness and the temperature of the α → β phase transition during the lowering of the temperature is found. An explanation of the correlation is proposed.