Preparation of Bi2S3 thin films with a nanoleaf structure by electrodeposition method

Nanoleaf-like Bi₂S₃ thin films were deposited on indium tin oxide (ITO) glass using Bi(NO₃)₃ and Na₂S₂O₃ as precursors by a cathodic electrodeposition process. The as-deposited thin films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and photoluminescence spectrum (PL). The influence of precursor solution mole concentration ratios [Bi(NO₃)₃]/[Na₂S₂O₃] on the phase compositions, morphologies and photoluminescence properties of the obtained thin films were investigated. Results show that a uniform Bi₂S₃ thin film with nanoleaf structure can be obtained with the precursor solution concentration ratio [Bi(NO₃)₃]/[Na₂S₂O₃] = 1:7. The as-prepared thin films exhibit blue-green photoluminescence properties under ultraviolet light excitation. With the increase of concentration ratios [Bi(NO₃)₃]/[Na₂S₂O₃] in the deposition solution, the crystallizations and PL properties of Bi₂S₃ thin films are obviously improved.     

Hydrogen gas-sensing properties of Pt/WO<Subscript>3</Subscript> thin film in various measurement conditions

A well-known gasochromic material is Pt particle-dispersed tungsten trioxide (Pt/WO₃). Its optical properties could make it effective as a hydrogen gas sensor. In this study, Pt nanoparticle-dispersed WO₃ thin films were prepared using the sol–gel process, and their optical and electrical properties dependent on the working environment (i.e., temperature, hydrogen gas concentration, oxygen partial pressure, etc.) were investigated. The Pt/WO₃ thin films prepared at 400 °C showed the largest change in optical transmittance and electrical conductivity when exposed to hydrogen gas compared with the films prepared at other temperatures. The optical absorbance and electrical conductivity were found to be dependent on the hydrogen and oxygen gas concentration in the atmosphere because generation and disappearance of W⁵⁺ in the thin films depend on the equilibrium reaction between injection and rejection of H⁺ into and from the thin films. In addition, the equilibrium reaction depends on the hydrogen and oxygen gas concentrations.     

Porous WO3 from anodized sputtered tungsten thin films for NO2 detection

In this paper, porous WO₃ films were prepared by anodic oxidation of metallic tungsten (W) films deposited on alumina substrates. The structural and morphological properties of the porous WO₃ films were investigated using field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD). A large number of cracks appeared on the surface of films after anodization, which makes the films porous. The porous WO₃ sensors achieved their maximum response values to NO₂ at a low operating temperature of 150 °C. The porous WO₃ sensors showed high response values, great stability and fast response-recovery characteristics to different concentration of NO₂ gas due to the high specific surface area and special structural and morphological properties.     

Effect of the excimer laser irradiation on sol–gel derived tungsten–titanium dioxide thin films

A novel technique based on the excimer laser induced crystallization and modification of TiO₂ thin films is being reported. W⁺⁶ ions loaded TiO₂ (WTO) precursor films were prepared by a modified sol–gel method and spin-coated onto microscopic glass slides. Pulsed KrF (248 nm, 13 ns) excimer laser was used to irradiate the WTO amorphous films at various laser parameters. Mesoporous and nanostructured films consisting of anatase and rutile were obtained after laser irradiation at room temperature. The effect of varying W⁺⁶ ions concentrations on structural and optical properties the WTO films was analyzed by X-ray diffraction, field-emission scanning electron microscope, UV-Vis spectrophotometer and transmission electron microscope before and after laser treatment. Films irradiated for 10 pulses at 65–75 mJ/cm² laser fluence, exhibited anatase whereas higher parameters promoted the formation of rutile. XPS results revealed WO₃ along with minor proportion of WO₂ compounds after laser irradiation. Photo-absorbance of the WTO films was increased with increase in W⁺⁶ ions concentration in the film. TEM results exhibited a crystallite size of 15 nm which was confirmed from SEM results as well.     

Effect of calcination atmosphere on photoluminescence properties of nanocrystalline ZrO2 thin films prepared by sol–gel dip coating method

Highly transparent and homogeneous nanocrystalline ZrO₂ thin films were prepared by the sol–gel dip coating method. The X-ray diffraction (XRD) pattern of ZrO₂ thin films calcined in air, O₂ or N₂ shows the formation of tetragonal phase with varying crystallite size. X-ray photoelectron spectroscopy (XPS) gives Zr 3d and O 1s spectra of thin film annealed in air, which reveal zirconium suboxide component (ZrO_x, 0<x<2), Zr–O bond and surface defects. An average transmittance greater than 85% (in UV–vis region) is observed in all calcined samples. Photoluminescence (PL) reveals an intense emission peak at 379 nm and weak peaks at 294, 586 and 754 nm for ZrO₂ film calcined in air. An enhancement of PL intensity and red-shift is observed in films calcined in O₂ and N₂ atmosphere. This is due to the reconstruction of zirconium nanocrystal interfaces and vacancies, which help passivate the non-radiative defects. The oxygen deficient defect, which is due to the distorted Zr–O bond, is suggested to be responsible for photoluminescence. The defect states in the nanocrystalline zirconia thin films play an important role in the energy transfer process. The luminescence defects in the film make it suitable for gas sensors development and tunable lasers.     

Soft X-ray spectroscopic study on the electronic structure of WO3 thin films fabricated under various annealing temperature and gas flow conditions

The electrical properties of WO₃ thin films vary significantly depending on the growth conditions. In this work, the influence of O₂ gas on the band gap of WO₃ thin films during growth was investigated via electronic structure characterization using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and X-ray emission spectroscopy (XES). A substantial decrease in the electrical conductivity of the WO₃ films was observed with an increase in the O₂ partial pressure during growth. Spectral differences in the peak energy and intensity were apparent for WO₃ films grown under only Ar and those grown in Ar:O₂. It is difficult to explain the acquired spectrum of WO₃ with oxygen defects through the rigid-band model in terms of the simple addition of electrons to the conduction band of WO₃. Our results show that an oxygen deficiency in WO₃ moves the conduction band to the Fermi edge.     

Effect of oxygen partial pressure and VO<Subscript>2</Subscript> content on hexagonal WO<Subscript>3</Subscript> thin films synthesized by pulsed laser deposition technique

We report on the effect of oxygen partial pressure and vacuum annealing on structural and optical properties of pulsed laser-deposited nanocrystalline WO₃ thin films. XRD results show the hexagonal phase of deposited WO₃ thin films. The crystallite size was observed to increase with increase in oxygen partial pressure. Vacuum annealing changed the transparent as-deposited WO₃ thin film to deep shade of blue color which increases the optical absorption of the film. The origin of this blue color could be due to the presence of oxygen vacancies associated with tungsten ions in lower oxidation states. In addition, the effects of VO₂ content on structural, electrochemical, and optical properties of (WO₃)_1−x (VO₂) _x nanocomposite thin films have also been systematically investigated. Cyclic voltammogram exhibits a modification with the appearance of an extra cathodic peak for VO₂–WO₃ thin film electrode with higher VO₂ content (x ≥ 0.2). Increase of VO₂ content in (WO₃)_1−x (VO₂) _x films leads to red shift in optical band gap.     

Correlation between crystallinity and resistive switching behavior of sputtered WO3 thin films

The as-deposited WO₃ thin films were post-annealed at different temperatures (300 °C and 600 °C) in air to investigate a correlation between crystallinity and switching behavior of WO₃ thin films. Associating the results of XRD, FTIR, XPS and FESEM measurements, the annealing-caused crystallinity change contributes to the variation of the switching behaviors of the WO₃ thin films. The as-deposited WO₃ films with low crystalline structure are preferred for random Ag conducting path, resulting in large switching ratio but fluctuating I–V hysteresis, whereas the annealed WO₃ films with crystallized compact structure limits Ag conducting path, favoring the stable I–V hysteresis but small switching ratio. It is therefore concluded that electrochemical redox reaction-controlled resistance switching depends not only on electrode materials (inert and reactive electrodes) but also on crystallinity of host oxide.     

Enhanced NO2 gas sensing properties of WO3 nanorods encapsulated with ZnO

The influence of the encapsulation of WO₃ nanorods with ZnO on the NO₂ gas sensing properties was examined. WO₃-core/ZnO-shell nanorods were fabricated by a two-step process comprising the catalyst-free thermal evaporation of a mixture of WO₃ and graphite powders in an oxidizing atmosphere and atomic layer deposition of ZnO. Multiple networked WO₃-core/ZnO-shell nanorod sensors showed the response of 281?% at 5 ppm NO₂ at 300?°C. This response value was approximately 9 times larger than that of bare WO₃ nanorod sensors at 5 ppm NO₂. The response values obtained from the WO₃-core/ZnO-shell nanorods in this study were more than 5 times higher than those obtained previously from the SnO₂-core/ZnO-shell nanofibers at the same NO₂ concentration range. The significant enhancement in the response of WO₃ nanorods to NO₂ gas by encapsulating them with ZnO can be accounted for based on the space-charge model.     

From beads-to-wires-to-fibers of tungsten oxide: electrochromic response

Suitable host lattice and morphology for easy intercalation and deintercalation process are crucial requirements for electrochromic device. In this investigation, the evolution of structural and morphological changes and their effect on electrochromic (EC) properties of spray-deposited WO₃ thin films are studied. Films of different morphologies were deposited from an ammonium tungstate precursor solution using a novel pulsed spray pyrolysis technique (PSPT) on tin-doped indium oxide (ITO) coated glass substrates by varying quantity of spraying solution. Interesting morphological transition from beads-to-wires-to-fibers as a function of quantity of sprayed solution has been demonstrated. The porosity, crystallinity and “open” structures in the films consisting of beads, wires, and fiber-like morphology enabled us to correlate these aspects to their EC performance. WO₃ films comprising wire-like morphology (20 cc spraying quantity) exhibited better EC properties both in terms of coloration efficiency (42.7 cm²/C) and electrochemical stability (10³ colored/bleached cycles) owing to their adequate open structure, porosity, and amorphicity, compared with the films having bead/fiber-like morphology.     

Enhancement of photoluminescence emission intensity of zirconia thin films via aluminum doping for the application of solid state lighting in light emitting diode

Al doped ZrO₂ thin films were deposited on quartz substrates by sol–gel dip coating technique. X-ray diffraction pattern showed the deterioration of the crystallinity of the films with increase in Al doping concentration due to the formation of stress in the film. Scanning electron microscope images showed crack free surface. An average transmittance of >80% (in UV–vis region) was observed for all samples. Optical band gap was found to vary as a function of doping concentration. Photoluminescence spectra of the films exhibited an increase in the emission intensity with increase in Al doping concentration which is due to the increase in oxygen vacancies in the Al doped films. The “Blueshift” and “Redshift” of the PL spectra with increase in Al concentration originates from the change of stress in the films. The enhancement of PL intensity in the Al doped ZrO₂ thin films make it suitable for generation of solid state lighting in light emitting diode.     

Promotion effect of Pt on a SnO2–WO3 material for NOx sensing

Metal-oxide nanocomposites were prepared over screen-printed gold electrodes to be used as room-temperature NO_x (nitric-oxide (NO) and nitrogen dioxide (NO₂)) sensors. Various weight ratios of SnO₂–WO₃ and Pt loadings were used for NO sensing. The sensing materials were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM) and BET surface analysis. The NO-sensing results indicated that SnO₂–WO₃ (1:2) was more effective than other materials were. The sensor response (S=resistance of N₂/resistance of NO=R_N2/R_NO) for detecting 1000 ppm of NO at room temperature was 2.6. The response time (T₉₀) and recovery time (TR₉₀) was 40 s and 86 s, respectively. By further loading with 0.5% Pt, the sensor response increased to 3.3. The response and recovery times of 0.5% Pt/SnO₂–WO₃ (1:2) were 40 s and 206 s, respectively. The linearity of the sensor response for a NO concentration range of 10–1000 ppm was 0.9729. A mechanism involving Pt promotion of the SnO₂–WO₃ heterojunction was proposed for NO adsorption, surface reaction, and adsorbed NO₂ desorption.     

Luminescence mechanism of ZnO thin film investigated by XPS measurement

The effects of annealing environment on the luminescence characteristics of ZnO thin films that were deposited on SiO₂/Si substrates by reactive RF magnetron sputtering were investigated by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). An analysis of the O 1s peak of ZnO film revealed that the concentration of oxygen vacancies increased with the annealing temperature from 600 °C to 900 °C under an ambient atmosphere. The PL results demonstrated that the intensity of green light emission at 523 nm also increased with temperature. Under various annealing atmospheres, the analyses of PL indicated that only one emission peak (523 nm) was obtained, indicating that only one class of defect was responsible for the green luminescence. The green light emission was strongest and the concentration of oxygen vacancies was highest when the ZnO film was annealed in ambient atmosphere at 900 °C. The results in this investigation show that the luminescence mechanism of the emission of green light from a ZnO thin film is associated primarily with oxygen vacancies. PACS 81.15.Cd; 81.40.Ef; 78.55.-m; 78.55.Et     

Intricate photocatalytic decomposition behavior of gaseous methanol with nanocrystalline tungsten trioxide films in high vacuum

Gas phase photocatalytic decomposition of methanol with nanocrystalline tungsten trioxide (WO₃) thin films in high vacuum was investigated. WO₃ thin films were prepared from a novel precursor prepared using peroxo-tungstic acid and polyethylene glycol (PEG300) in water. Uniform thin films of WO₃ with different morphologies such as micro-sheets, platelets, nanorods and nanoparticles were fabricated by varying the concentration of PEG300 in the precursor solution and by optimizing other preparative parameters. Nanocrystalline thin films were obtained with 20% of PEG300 in the precursor solution and at a calcination temperature of 350 °C, followed by post annealing in air at 500 °C. Photocatalytic decomposition of gaseous methanol in high vacuum was examined with nanocrystalline WO₃ thin films using a quadrupole mass spectrometer at a real-time scale under visible (400-700 nm) and UVA (300-400 nm) illumination. Methanol was first decomposed to formaldehyde via direct hole transfer mechanism. Subsequently formaldehyde was decomposed to CO and finally to CO₂. As a result, the partial pressures of CH₂O, CO and CO₂ showed a switching phenomenon according to the ON/OFF of light illumination. A rapid decrease in the photocatalytic activity was observed due to photo-induced desorption of methanol during the initial light pulse and gradual decrease at longer times was observed because of formation of tungsten bronze. Thus, the overall process of methanol decomposition over WO₃ films is complex convolution of elementary steps that involve several intermediates.     

Effect of process parameters on surface morphology and characterization of PE-ALD SnO2 thin films for gas sensing

Tin dioxide (SnO₂) thin films were deposited by plasma enhanced-atomic layer deposition (PE-ALD) on Si(1 0 0) substrate using dibutyl tin diacetate (DBTA) ((CH₃CO₂)₂Sn[(CH₂)₃-CH₃]₂) as precursor. The process parameters were optimized as a function of substrate temperature, source temperature and purging time. It is observed that the surface phenomenon of the thin films was changed with film thickness. Atomic force microscopy (AFM) images and X-ray diffraction (XRD) pattern were used to observe the texture and crystallanity of the films. The films deposited for 100, 200 and 400 cycles were characterized by XPS to determine the chemical bonding properties. XPS results reveal that the surface dominant oxygen species for 100, 200 and 400 cycles deposited films are O₂⁻, O⁻ and O²⁻, respectively. The 200 cycles film has exhibited highest concentration of oxygen (O⁻) species before and after annealing. Conductivity studies revel that this film has best adsorption strength to the oxygen ions forming on the surface. The sensor with 200 cycles SnO₂ thin film has shown highest sensitivity to CO gas than other films. A correlation between the characteristics of Sn3d_5/2 and O1s XPS spectra before and after annealing and the electrical behavior of the SnO₂ thin films is established.     

Preparation of WO3 nanoparticles and application to NO2 sensor

WO₃ nanoparticles were prepared by evaporating tungsten filament under a low pressure of oxygen gas, namely, by a gas evaporation method. The crystal structure, morphology, and NO₂ gas sensing properties of WO₃ nanoparticles deposited under various oxygen pressures and annealed at different temperatures were investigated. The particles obtained were identified as monoclinic WO₃. The particle size increased with increasing oxygen pressure and with increasing annealing temperature. The sensitivity increased with decreasing particle size, irrespective of the oxygen pressure during deposition and annealing temperature. The highest sensitivity of 4700 to NO₂ at 1 ppm observed in this study was measured at a relatively low operating temperature of 50 °C; this sensitivity was observed for a sensor made of particles as small as 36 nm.     

Studies on the effect of nozzle-to-substrate distance on the structural, electrical and optical properties of spray deposited CdIn2O4 thin films

The physical, chemical, electrical and optical properties of as-deposited and annealed CdIn₂O₄ thin films deposited using spray pyrolysis technique at different nozzle-to-substrate distances are reported. These films are characterized by X-ray diffraction, XPS, SEM, PL, Hall effect measurement techniques and optical absorption studies. The average film thickness lies within 600-800 nm range. The X-ray diffraction study shows that films exhibit cubic structure with orientation along (3 1 1) plane. The XPS study reveals that CdIn₂O₄ films are oxygen deficient. Room temperature PL indicates the presence of green shift with oxygen vacancies. The typical films show very smooth morphology. The best films deposited with optimum nozzle-to-substrate distance (NSD) of 30 cm, has minimum resistivity of 1.3 × 10⁻³ Ω cm and 2.6 × 10⁻⁴ Ω⁻¹ figure of merit. The band gap energy varies from 3.04 to 3.2 eV with change in NSD for annealed films. The effect of NSD as well as the annealing treatment resulted into the improvement of the structural, electrical and optical properties of the studied CdIn₂O₄ thin films.     

n型有序多孔硅基氧化钨室温气敏性能研究

利用电化学腐蚀方法制备了n型有序多孔硅, 并以此为基底用直流磁控溅射法在其表面溅射不同厚度的氧化钨薄膜. 利用X射线和扫描电子显微镜表征了材料的成分和结构, 结果表明, 多孔硅的孔呈柱形有序分布, 溅射10 min的WO₃薄膜是多晶结构, 比较松散地覆盖在整个多孔硅的表面. 分别测试了多孔硅和多孔硅基氧化钨在室温条件下对二氧化氮的气敏性能, 结果表明, 相对于多孔硅, 多孔硅基氧化钨薄膜对二氧化氮的气敏性能显著提高. 对多孔硅基氧化钨复合结构的气敏机理分析认为, 多孔硅和氧化钨薄膜复合形成的异质结对良好的气敏性能起到主要作用, 氧化钨薄膜表面出现了反型层引起了气敏响应时电阻的异常变化. 关键词： 有序多孔硅 氧化钨薄膜 二氧化氮 室温气敏性能     

Influence of oxygen vacancies on optical properties of anatase TiO2 thin films

This paper presents an investigation into the relaxation process of photo-excited carriers in anatase TiO₂ thin films, in which the concentration of oxygen vacancy is controlled by annealing at various temperatures in an oxygen atmosphere. The influence of oxygen vacancies on absorption spectra, photoluminescence (PL) spectra, and PL decay dynamics are discussed.     

Electrochromic performance of the mixed V2O5–WO3 thin films synthesized by pulsed spray pyrolysis technique

Vanadium pentoxide (V₂O₅) mixed tungsten trioxide (WO₃) thin films have been synthesized by a novel pulsed spray pyrolysis technique (PSPT) on glass and fluorine doped tin oxide (FTO) coated glass substrates at 400 °C. Aqueous solutions of equimolar vanadium chloride and ammonium tungstate were mixed in volume proportions (5%, 10% and 15%) for the deposition of V₂O₅–WO₃ thin films. The structural, morphological, optical and electrochemical properties of V₂O₅–WO₃ thin films were investigated by FT-IR, XRD, SEM, cyclic voltammetry, chronoamperometry and chronocoulometry techniques. The results showed that the electrochemical properties of V₂O₅ were altered by mixing WO₃. All the films exhibited cathodic electrochromism in lithium containing electrolyte (0.5 M LiClO₄ + propylene carbonate (PC)). Maximum coloration efficiency (CE) of about 49 cm² C⁻¹ was observed for the V₂O₅ film mixed with 15% WO₃. The electrochemical stability of the sample was examined and it was found to be stable up to 1000 cycles.