Preparation of high performance pure single phase VO2 nanopowder by hydrothermally reducing the V2O5 gel

The most important potential application for VO₂ is “smart windows”. Producing polymer—VO₂ composite film is a practicable alternative for PVD film for windows retrofitting. Here the main work should be concentrated on preparing functional filler—VO₂. In this paper, high quality single crystal VO₂ nanopowder and the composite film coated with this powder had been prepared by a novel and facile route. A new vanadium precursor—V₂O₅·nH₂O sol was fabricated by the dissolution reaction of V₂O₅ in H₂O₂. Adopting this sol, VO₂ powder with an average size less than 20 nm and narrower size distribution was obtained by pre-reduction and hydrothermal treatment. The as-prepared powder showed excellent optical properties in the composite film. The films reached a sufficiently high luminous transmittance (45-60%) and keeping thermochromic effect in infrared area very well—an abrupt transmittance change greater than 50% with the phase transition was observed. Its optical properties are comparable with high quality single layer VO₂ film deposited by PVD method. The as-prepared nanopowder has a phase transition temperature (τ_c) about 55.5 °C and it can be tuned down to ambient by slight tungsten doping without deteriorating the thermochromic properties, which made this powder very suitable for practical application. Based on our experiments, the τ_c reduction efficiency of tungsten was about −21.96 K/at.%w in the doping range from 0 to 2.5 at.%w.     

Significant changes in phase-transition hysteresis for Ti-doped VO2 films prepared by polymer-assisted deposition

This study is one of a series of researches on polymer-assisted deposition (PAD) of thermochromic VO₂ film. This paper describes the synthesis of Ti-doped VO₂ films (V_1−xTi_xO₂, x=0-0.167) and a systematic investigation of the effects of Ti doping on morphology, crystalline phase and optical properties (visible transmittance, transmittance changes in near infrared (NIR) light across the metal-insulator transition (MIT), absorption edge, MIT temperatures and sharpness) of V_1−xTi_xO₂ films. The films showed excellent visible transmittance and large changes (∼50% at 2000 nm) in the NIR-light transmittance before and after MIT for samples with a wide range of Ti contents (0-10%). The width of the hysteresis loop was severely reduced, which is in agreement with a qualitative analysis using the nucleation theory, while the MIT sharpness was not obviously influenced. An increase in the MIT temperature was observed for all samples with Ti doping, in particular 18.5 °C for 2% Ti doping, which is quite large compared with those in the literature. However, the MIT seemed saturated at around 80 °C. The morphology evolution of Ti-doped VO₂ films was investigated for the first time. Interestingly, Ti doping could remarkably reduce the particle size of VO₂ films due to Ti enhanced heterogeneous nucleation process of VO₂ particles, indicating a new method for the growth of nanostructured VO₂ films.     

Thermochromic properties and low emissivity of ZnO:Al/VO2 double-layered films with a lowered phase transition temperature

We studied the optical and semiconductor-metal (S-M) transition properties of ZnO:Al/VO₂/substrate double-layered films that consisted of a ZnO:Al top layer and a VO₂ bottom layer. ZnO:Al and VO₂ films were grown on fused silica substrates by radio frequency magnetron sputtering and polymer-assisted deposition, respectively. The ZnO:Al/VO₂/substrate films displayed low emissivity (0.31-0.32) with integrated luminous transmittance (T_lum>46%) and thermochromic properties (ΔT_sol>4.1%). The low emissivity and thermochromic properties were independently introduced by the transparent conductive ZnO:Al layer and the VO₂ layer. In addition, the S-M transition temperatures for VO₂ shifted to lower temperatures after the ZnO:Al deposition process, which was due to the formation of surface nonstoichiometry—oxygen deficiency that was induced by the ZnO:Al deposition process.     

VO2-based double-layered films for smart windows: Optical design, all-solution preparation and improved properties

An all-solution process was developed to prepare VO₂-based double-layered films containing SiO₂ and TiO₂ antireflection layers. These double-layered films were optimized to improve luminous transmittance (T_lum) and switching efficiency (ΔT_sol). The substrate/VO₂/TiO₂ double-layered structure showed the largest improvement of 21.2% in T_lum (from 40.3% to 61.5%). T_lum could be further improved to the maximum of 84.8% by combining film thickness optimization and antireflection layer deposition. ΔT_sol (usually below 10% for single VO₂ films) could be improved by adjusting the position of antireflection peaks (the highest ΔT_sol was 15.1%). A sample with balanced T_lum and ΔT_sol showed T_lum of about 58% (20 °C) and 54% (90 °C), and ΔT_sol of 10.9%. This work is an important technical breakthrough toward the practical application of VO₂-based smart windows.     

Efficient indium tin oxide/Cr-doped-TiO2 multilayer thin films for H2 production by photocatalytic water-splitting

Cr-doped-TiO₂ thin films, with three different Cr concentrations (2, 5.5, and 9 at.%), have been synthesized by radio-frequency magnetron sputtering in order to sensitize TiO₂ in visible light. UV–visible spectra showed that maximum narrowing (2.1 eV) of the TiO₂ band gap is obtained for the highest Cr concentration. However, negligible photocurrent was measured with Indium Tin Oxide (ITO)/Cr-doped-TiO₂ (9 at.%) single bilayer sample due to the increased recombination rate of the photo-generated charges on the defects associated to the Cr³⁺ ions. To lower the charge recombination rate in the Cr-doped-TiO₂, multilayer films with different numbers of ITO/Cr-doped-TiO₂ (9 at.%) bilayers (namely, 3-, 4-, 5-, 6- and 7-bilayers) were deposited by keeping the total thickness of TiO₂ constant in each multilayer film. When the multilayer films were exposed to visible light, we observed that the photocurrent increases as function of the number of bilayers by reaching the maximum with 6-bilayers of ITO/Cr-doped-TiO₂. The enhanced photocurrent is attributed to: 1) higher absorption of visible light by Cr-doped-TiO₂, 2) number of space charge layers in form of ITO/TiO₂ interfaces in multilayer films, and 3) generation of photoelectrons just in/or near to the space charge layer by decreasing the Cr-doped-TiO₂ layer thickness. The reduced charge recombination rate in multilayer films was also confirmed by studying the photocurrent kinetic curve. The superior photocatalytic efficiency of the 6-bilayers film implies higher hydrogen production rate through water-splitting: we obtained indeed 24.4 μmol/h of H₂ production rate, a value about two times higher than that of pure TiO₂ (12.5 μmol/h).     

TiO2 and TiO2–SiO2 thin films and powders by one-step soft-solution method: Synthesis and characterizations

Simple soft-solution method has been developed to synthesize films and powders of TiO₂ and mixed TiO₂–SiO₂ at relatively low temperatures. This method is simple and inexpensive. Furthermore, reactor can be designed for large-scale applications as well as to produce large quantities of composite powders in a single step. For the preparation of TiO₂, we used aqueous acidic medium containing TiOSO₄ and H₂O₂, which results in a peroxo-titanium precursor while colloidal SiO₂ has been added to the precursor for the formation of TiO₂–SiO₂. Post annealing at 500 °C is necessary to have anatase structure. Resulting films and powders were characterized by different techniques. TiO₂ (anatase) phase with (1 0 1) preferred orientation has been obtained. Also in TiO₂–SiO₂ mixed films and powders, TiO₂ (anatase) phase was found. Fourier transform infrared spectroscopy (FTIR) results for TiO₂ and mixed TiO₂–SiO₂ films have been presented and discussed. The method developed in this paper allowed obtaining compact and homogeneous TiO₂ films. These compact films are highly photoactive when TiO₂ is used as photo anode in an photoelectrochemical cell. Nanoporous morphology is obtained when SiO₂ colloids are added into the solution.     

TiO2 thin films - Influence of annealing temperature on structural, optical and photocatalytic properties

Nanostructured TiO₂ thin films were deposited on glass substrates by sol-gel dip coating technique. The structural, morphological and optical characterizations of the as deposited and annealed films were carried out using X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), and UV-vis transmittance spectroscopy. As-deposited films were amorphous, and the XRD studies showed that the formation of anatase phase was initiated at annealing temperature close to 400 °C. The grain size of the film annealed at 600 °C was about 20 nm. The lattice parameters for the films annealed at 600 °C were a = 3.7862 ? and c = 9.5172 ?, which is close to the reported values of anatase phase. Band gap of the as deposited film was estimated as 3.42 eV and was found to decrease with the annealing temperature. At 550 nm the refractive index of the films annealed at 600 °C was 2.11, which is low compared to a pore free anatase TiO₂. The room temperature electrical resistivity in the dark was of the order of 4.45 × 10⁶ ohm-cm. Photocatalytic activity of the TiO₂ films were studied by monitoring the degradation of aqueous methylene blue under UV light irradiation and was observed that films annealed above 400 °C had good photocatalytic activity which is explained as due to the structural and morphological properties of the films.     

Efficient H2 production by water-splitting using indium–tin-oxide/V-doped TiO2 multilayer thin film photocatalyst

In order to sensitize TiO₂ in visible light and to reduce photo-induced charge recombination, the multilayer films of Indium-Tin Oxide (ITO)/V-doped TiO₂ were synthesized by radio-frequency magnetron sputtering. V-doped TiO₂ thin films showed red shift in TiO₂ absorption edge with increasing dopant concentration and, most importantly, the dopant energy levels are formed in the TiO₂ band gap due to V⁵⁺/V⁴⁺ ions as confirmed by UV-Visible and XPS spectra. Multilayer films with different numbers of ITO/V-doped TiO₂ (6 at.%) bilayers (namely, 2-, 3-, 4-, 5-, 6- and 7-bilayers) were deposited, in order to reduce the charge recombination rate, by keeping the total thickness of TiO₂ constant in each multilayer film. In multilayer films, when exposed to visible light the photocurrent increases as function of the number of bilayers by reaching the maximum with 6-bilayers of ITO/V-doped TiO₂. The measured enhanced photocurrent is attributed to: 1) ability of V-doped TiO₂ to absorb visible light, 2) number of space-charge layers in form of ITO/TiO₂ interfaces in multilayer films, and 3) generation of photoelectrons just in/or near to the space-charge layer by decreasing the V-doped TiO₂ layer thickness. The reduced charge recombination rate in multilayer films was also confirmed by the photocurrent kinetic curves. The superior photocatalytic efficiency of the 6-bilayers film is also reflected in hydrogen production rate through water-splitting: we obtained indeed 31.2 μmol/h of H₂ production rate.     

Low-temperature synthesis of TiO2 nanoparticles and preparation of TiO2 thin films by spray deposition

Low-temperature (180–240 °C) synthesis of nanocrystalline titanium dioxide (TiO₂) by surfactant-free solvothermal route is investigated. Titanium iso-propoxide is used as the precursor and toluene as the solvent. Different precursors to solvent weight ratios have been used for the synthesis of TiO₂ nanoparticles. For the weight ratios 15/100, 25/100 and 35/100 the X-ray diffractograms show the formation of nanocrystalline TiO₂. The X-ray diffraction and transmission electron microscopy studies shows that the product has anatase crystal structure (for temperatures <200 °C) with average particle size below 15 nm. The films deposited by spray deposition method using these nanoparticles show the crystalline and porous nature of the films. The present method of deposition also avoids the post-treatment (sintering) of the films. The nanoparticles thus prepared and the films can be used for gas sensing and biological applications and also as photo-electrodes for dye-sensitized solar cells.     

Sol-gel SiO2/TiO2 bilayer films with self-cleaning and antireflection properties

We report here that a facile sol-gel dip-coating technique can be used to fabricate a SiO₂/TiO₂ bilayer film with self-cleaning and antireflection properties. The bottom SiO₂ layer acts as an antireflection coating due to its lower refractive index; the top TiO₂ layer acts as a self-cleaning coating generated from its photocatalysis and photo-induced superhydrophilicity. The maximal transmittance of SiO₂/TiO₂ bilayer film at normally incident light can be reached 96.7%, independent of the high refractive index and coverage of TiO₂ nanoparticles. However, the photocatalytic activity of the bilayer film shows a close dependence on coverage of TiO₂ nanoparticles. After illuminated by ultraviolet light, the SiO₂/TiO₂ bilayer films are superhydrophilic with water contact angle less than 2°, which favors greatly the self-cleaning function of the films.     

Formation and metal-to-insulator transition properties of VO2-ZrV2O7 composite films by polymer-assisted deposition

VO₂-ZrV₂O₇ composite films were prepared on silica glass substrates by polymer-assisted deposition using a V-Zr-O solution. The coexistence of ZrV₂O₇ and VO₂ was confirmed by Raman spectroscopy, glance angle X-ray diffraction, and X-ray photoelectron spectroscopy. The composite films with similar thickness of about 95 nm showed decreased particle sizes and significantly enhanced luminous transmittances (from 32.3% at Zr/V=0 to 53.4% at Zr/V=0.12) with increasing Zr/V rations. The enhancement in the luminous transmittance was ascribed to the absorption-edge changes in the composite films. This feature benefits the application of VO₂ to smart windows.     

A comparative study on photoelectrochemical activity of ZnO/TiO2 and TiO2/ZnO nanolayer systems under visible irradiation

TiO₂/ZnO and ZnO/TiO₂ nanolayer thin films were synthesized using sol-gel method. Optical analysis revealed high transmittance of the films in the visible range with almost the same bandgap energy for the both systems. XPS technique shows stoichiometric formation of TiO₂ and ZnO on the surface of TiO₂/ZnO and ZnO/TiO₂ layers, respectively. According to AFM observations and its data analysis, the TiO₂/ZnO films exhibited a higher surface roughness and more effective interfaces with electrolyte during redox reactions. Based on photoelectrochemical measurements, TiO₂/ZnO nanolayer photoanode possesses a lower charge transfer resistance and higher transient time for charge carriers (e⁻ and h⁺) and hence a higher photocurrent density under visible light irradiation as compared with the ZnO/TiO₂ nanolayer system.     

Preparation and characterization of spray-deposited Cu2ZnSnS4 thin films

Thin films of Cu₂ZnSnS₄ (CZTS), a potential candidate for absorber layer in thin film heterojunction solar cell, have been successfully deposited by spray pyrolysis technique on soda-lime glass substrates. The effect of substrate temperature on the growth of CZTS films is investigated. X-ray diffraction studies reveal that polycrystalline CZTS films with better crystallinity could be obtained for substrate temperatures in the range 643-683 K. The lattice parameters are found to be a=0.542 and c=1.085 nm. The optical band gap of films deposited at various substrate temperatures is found to lie between 1.40 and 1.45 eV. The average optical absorption coefficient is found to be >10⁴ cm⁻¹.     

Hybrid solar cells based on MEH-PPV and thin film semiconductor oxides (TiO2, Nb2O5, ZnO, CeO2 and CeO2–TiO2): Performance improvement during long-time irradiation

Performance improvement of hybrid solar cells (HSC) applying five different thin film semiconductor oxides has been observed during long-time irradiation in ambient atmosphere. This behavior shows a direct relation between HSC and oxygen content from the environment. Photovoltaic devices were prepared as bi-layers of thin film semiconducting oxides (TiO₂, Nb₂O₅, ZnO, CeO₂–TiO₂ and CeO₂) and the polymer MEH-PPV, with a final device configuration of ITO/Oxide_{thin film}/MEH-PPV/Ag. The oxides were prepared as thin transparent films from sol–gel solutions. The photovoltaic cells were studied in ambient atmosphere by recording the initial values of open circuit voltage (V_oc) and current density (I_sc). Solar decay curves presented as the measurement of the short circuit current as a function of time, IV curves and photophysical analyses were also carried out for each type of device. Solar cells with TiO₂ thin films showed the best performance with maximum V_oc as high as −0.74 V and I_sc of 0.4 mA/cm². Solar decay analyses showed that the devices require a stabilization period of several hours in order to reach maximum performance. In the case of TiO₂, Nb₂O₅ and CeO₂–TiO₂, the maximum current density was observed after 15 h; for CeO₂, the maximum performance was observed after 30 h. The only exception was observed with devices applying ZnO in which the current density decreased drastically and degraded the polymer in just a couple of hours.     

Ni/CeO2 catalysts with high CO2 methanation activity and high CH4 selectivity at low temperatures

CO₂ methanation was performed over 10 wt%Ni/CeO₂, 10 wt%Ni/α-Al₂O₃, 10 wt%Ni/TiO₂, and 10 wt%Ni/MgO, and the effect of support materials on CO₂ conversion and CH₄ selectivity was examined. Catalysts were prepared by a wet impregnation method, and characterized by BET, XRD, H₂-TPR and CO₂-TPD. Ni/CeO₂ showed high CO₂ conversion especially at low temperatures compared to Ni/α-Al₂O₃, and the selectivity to CH₄ was very close to 1. The surface coverage by CO₂-derived species on CeO₂ surface and the partial reduction of CeO₂ surface could result in the high CO₂ conversion over Ni/CeO₂. In addition, superior CO methanation activity over Ni/CeO₂ led to the high CH₄ selectivity.     

Significantly improved dehydrogenation of LiAlH4 catalysed with TiO2 nanopowder

The effects of TiO₂ nanopowder addition on the dehydrogenation behaviour of LiAlH₄ have been studied. The 5 wt.% TiO₂-added LiAlH₄ sample showed a significant improvement in dehydrogenation rate compared to that of undoped LiAlH₄, with the dehydrogenation temperature reduced from 150 °C to 60 °C. Kinetic desorption results show that the added LiAlH₄ released about 5.2 wt% hydrogen within 30 min at 100 °C, while the as-received LiAlH₄ just released below 0.2 wt.% hydrogen within same time at 120 °C. From the Arrhenius plot of the hydrogen desorption kinetics, the apparent activation energy is 114 kJ/mol for pure LiAlH₄ and 49 kJ/mol for the 5 wt.% TiO₂ added LiAlH₄, indicating that TiO₂ nanopowder adding significantly decreased the activation energy for hydrogen desorption of LiAlH₄. X-ray diffraction and Fourier transform infrared analysis show that there is no phase change in the cell volume or on the Al-H bonds of the LiAlH₄ due to admixture of TiO₂ after milling. X-ray photoelectron spectroscopy results show no changes in the Ti 2p spectra for TiO₂ after milling and after dehydrogenation. The improved dehydrogenation behaviour of LiAlH₄ in the presence of TiO₂ is believed to be due to the high defect density introduced at the surfaces of the TiO₂ particles during the milling process.     

Sprayed CeO2 thin films for electrochromic applications

Cerium dioxide (CeO₂) thin films were prepared by spray pyrolysis using hydrated cerium chloride (CeCl₃·7H₂O) as source compound. The films prepared at substrate temperatures below 300°C were amorphous, while those prepared at optimal conditions (T_s=500°C,s=5 ml/min) were polycrystalline, cubic in structure, preferentially oriented along the (2 0 0) direction and exhibited a transmittance value greater than 80% in the visible range. The cyclic voltammetry study showed that films of CeO₂ deposited on ITO pre-coated glass substrates were capable of charge insertion/extraction when immersed in an electrolyte of propylene carbonate with 1 M LiClO₄.These films also remained fully transparent after Li+ intercalation/deintercalation.     

Incorporating carbon nanotube in a low-temperature fabrication process for dye-sensitized TiO2 solar cells

Dye-sensitized solar cells (DSSCs) incorporating TiO₂ porous films, prepared at a low temperature (150 °C), along with multi-wall carbon nanotubes (MWCNTs) were studied using two different electrolytes, namely LiI and THI. Electrochemical impedance spectroscopy (EIS) was employed to quantify the charge transport resistance and electron lifetime (τ_e) under different levels (wt%) of MWCNTs and electrolytes. The charge transport resistance at the TiO₂/dye/electrolyte interface (R_ct2) increased as a function of the MWCNT concentration, which ranged 0.1-0.5 wt%, due to a decrease in the surface area and decreased dye adsorption. The characteristic peak shifted to a lower frequency at 0.1 wt% of MWCNT, indicating a longer electron lifetime. The DSSC with the TiO₂ electrode containing 0.1 wt% of MWCNT resulted in a higher short-circuited current density (J_SC) of 9.08 mA/cm², an open-circuit voltage (V_OC) of 0.781 V, and a cell conversion efficiency of 5.02%. EIS was also conducted under dark conditions. The large value at a middle frequency represented electron transport at the TiO₂/dye/electrolyte interface (R_rec). The R_rec for 0.1 wt% MWCNT/TiO₂ was found to be 114 Ω, and for those with 0.3 and 0.5 wt% were 35 and 30 Ω, respectively. The significantly higher value of R_rec suggested that the charge recombination between injected electrons and electron acceptors in the redox electrolyte, I₃⁻, was remarkably retarded. Finally, electrolytes with LiI and THI were used to compare the cell conversion performance under the same conditions. It was found that more electrons were injected in the TiO₂ electrode and the electron recombination reaction was faster in the DSSC with THI than that with LiI.     

Growth of highly c-axis oriented Mg:ZnO nanorods on Al2O3 substrate towards high-performance H2 sensing

Highly c-axis oriented Mg:ZnO films were fabricated on Al₂O₃ substrate by radio frequency sputtering for different substrate temperatures. The crystal structure revealed that the Mg dopants are well integrated into ZnO wurtzite lattice. X-ray photoelectron spectroscopy measurements also confirmed the successful incorporation of Mg into ZnO. The substrate temperature exhibit significant influence on the optical absorbance and band gap of Mg:ZnO films. Scanning electron microscope images revealed the formation of Mg:ZnO nanorods with good crystalline quality. The films prepared at 1200 °C show well grown rods of Mg:ZnO due to strengthening of the preferred orientation of ZnO along the c-axis. The Mg:ZnO/Al₂O₃ films prepared at different temperature were tested for its sensing performance towards 200 ppm of H₂ at room temperature. The Mg:ZnO sensor prepared at 1200 °C revealed fast response and recovery time of about 85 s and 70 s, respectively. The response of the sensor was linear to H₂ concentration in the range of 100–500 ppm. It can be summarized that this high performance H₂ sensor has potential for use as a portable room temperature gas sensor.     

Highly-efficient electrochromic performance of nanostructured TiO2 films made by doctor blade technique

Electrochromic TiO₂ anatase thin films on F-doped tin oxide (FTO) substrates were prepared by doctor blade method using a colloidal solution of titanium oxide with particles of 15 nm in size. The films were transparent in the visible range and well colored in a solution of 1 M LiClO₄ in propylene carbonate. The transmittances of the colored films were found to be strongly dependent on the Li⁺ inserted charges. The response time of the electrochromic device coloration was found to be as small as 2 s for a 1 cm² sample and the coloration efficiency at a wavelength of 550 nm reached a value as high as 33.7 cm² C⁻¹ for a 600 nm thick nanocrystalline-TiO₂ on a FTO-coated glass substrate. Combining the experimental data obtained from in situ transmittance spectra and in situ X-ray diffraction analysis with the data from chronoamperometric measurements, it was clearly demonstrated that Li⁺ insertion (extraction) into (out of) the TiO₂ anatase films resulted in the formation (disappearance) of the Li_0.5TiO₂ compound. Potential application of nanocrystalline porous TiO₂ films in large-area electrochromic windows may be considered.