Highly crystalline WO3 thin films with ordered 3D mesoporosity and improved electrochromic performance

WO3 thin layers with nanometer-scale periodicity were prepared by evaporation-induced self-assembly (EISA) using a novel amphiphilic block-copolymer template (poly(ethylene-co-butylene)-block-poly(ethylene oxide)). The evolution of the mesoporous ordered network and the crystallinity of the framework were monitored by 2D-SAXS, WAXS, SEM, XPS, and porosimetry. By annealing the films, the pore-wall crystallinity is adjusted between fully amorphous and highly crystalline without mesostructural degradation. Thus, the crystalline-film framework is composed of phase-pure monoclinic WO3 nanoparticles (12-14 nm in size). Furthermore, heat treatment transforms the originally spherical mesopores into ellipsoids, resulting in a unidirectionally shrunken, but still well-defined and fully accessible bcc mesopore architecture. The influence of mesoporosity and crystallinity on electrochemical/electrochromic characteristics was addressed by monitoring electrochemical features and the absorption changes during Li insertion/extraction (repetitive potentiostatic cycling). Both the amorphous and crystalline mesoporous films possess electrochromic response times on the order of only seconds, which are attributable to the facilitated insertion of guest ions due to shortening of the diffusion path lengths. Also, the insertion/extraction reversibility of crystalline WO3 layers with 3D mesoporosity is improved compared to amorphous ones and reaches values close to 100%.     

Amorphous tungstate precursor route to nanostructured tungsten oxide film with electrochromic property

Electrochromic tungsten oxide (WO3) films on ITO glass were fabricated by spin-coating with a tungsten peroxy acid solution, which was prepared by adding an equivolume mixture of hydrogen peroxide and glacial acetic acid to tungsten metal powder. The structural evolution of the tungstate precursor upon heat treatment was studied by X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES) analyses, which indicated that the as-synthesized tungstate transformed into nanocrystalline WO3 upon heating. It is, therefore, quite clear that as-synthesized tungstate can be a good precursor for electrochromic WO3 films. A series of WO3 thin films were prepared on ITO glass by spin-coating with different concentrations of tungsten peroxy acid solution and then post-annealing at various temperatures. Depending on the concentration of the tungstate coating solution (200-500 mg mL(-1)) and the annealing temperature (100-300 degrees C), the thickness and WO3 content as well as the electrochromic properties of WO3 films can be controlled. As a result, the optimum fabrication conditions were determined to be a tungstate solution concentration of 300-400 mg mL(-1) and a post-annealing temperature of 200 degrees C. Finally, an inorganic-inorganic hybrid electrochromic device (ECD) composed of optimized WO3 and Prussian Blue (PB) with desirable coloration efficiency was successfully developed.     

以聚合物凝胶为电解质的多层全固态电变色器件的研究

本文在掺ＬｉＣｌＯ４与碳酸丙烯酯的聚乙二醇固体电解质离子电导特性与ＷＯ３、ＮｉＯ薄膜离子插入性能研究了基础上，设计并制备了互补型ＷＯ３／ＮｉＯ全固态电变色器件。同时研究了该器件变色过程的循环伏安特性与不同状态的可见、近红外透过特性。结果表明，互补型的ＷＯ３／ＰＥＧ－ＬｉＣｌＯ４－ＰＣ／ＮｉＯ器件在可见与近红外均具有良好的电变色特性，其漂白态在波长６００ｎｍ的透过率为７０％，着色态为２０％。     

溶胶-凝胶-热解法制备介孔WO3薄膜及其电致变色性能研究

采用溶胶-凝胶-热解工艺,以PEG-400为结构导向模板剂,合成出介孔WO3薄膜,通过XRD、SEM、BET等对其晶相组成,显微结构,比表面积和孔径分布等进行了测试,结果表明所制备的介孔WO3薄膜材料为立方晶相,其平均孔径在6.37nm,比表面积高达20.69 m2/g;电化学循环伏安和光谱测试分析表明该薄膜具有良好的电致变色性能,对可见光具有很好的调制功能,平均最大透过率调制幅度可达69.96%.     

基于氧化钨和氧化镍的电致变色器件研究进展

电致变色器件(Electrochromic Devices, ECDs)是一种颜色变化受电压调控的智能装置,具有工作温度范围宽、光学对比度高、可逆双稳态性能好、驱动电压低和能耗低等优点,在智能动态调光窗、全彩色电子显示屏、防眩光护目镜、自适应双隐身伪装以及可视化储能等领域展现出了巨大的应用潜力。阴极着色材料氧化钨(WO₃)和阳极着色材料氧化镍(Ni O)是两种被广泛研究的无机电致变色材料,由WO₃和NiO薄膜组成的互补型电致变色器件在大规模智能窗的应用中具有极高的商业价值。改善电致变色器件的综合性能如光学调制范围、响应速度、循环寿命和耐候性等问题一直备受关注。本文围绕互补型电致变色器件的结构组成,综述了基于WO₃和NiO的电致变色全器件的近期研究进展。首先分别阐述了WO₃和NiO薄膜的电致变色机理和衰退机制,讨论了改进制备工艺、元素掺杂改性、设计纳米结构和引入复合材料这四种薄膜性能优化策略的作用和最新研究进展,其次,按照器件的组成成分和结构设计介绍了互补型电致变色全器件的分类体系,总结了各组分材料的...     

Electrochromic properties of N-doped tungsten oxide thin films prepared by reactive DC-pulsed sputtering

In depositing nitrogen doped tungsten oxide thin films by using reactive DC-pulsed magnetron sputtering process, nitrous oxide gas (N₂O) was employed instead of nitrogen (N₂) as the nitrogen dopant source. The nitrogen doping effect on the structural and electrochromic properties of WO₃ thin films was investigated. X-ray diffraction (XRD) results show that the films are amorphous. Morphological images reveal that the films are characterized by a hybrid structure comprising nanoparticles embedded in amorphous matrix and open channels between the agglomerated nanoparticles, which promotes rapid charge transport through the film. Increasing the nitrogen doping concentration is found to decrease the nanoparticle size and the band gap energy. The electrochromic properties were studied using cyclic voltammetric and spectroeletrochemical techniques. The film with N content of ~ 5 at.% exhibits higher optical modulation and coloration efficiency as well as faster ion transport kinetics. The results reveal that electrochromic and lithium ion transport properties are moderately enhanced relative to the un-doped tungsten oxide thin films by appropriate content of dopant, due to the effects of nitrogen doping.     

Visible-light active photocatalytic WO3 films loaded with Pt nanoparticles deposited by sputtering

Visible-Light active photocatalytic tungsten trioxide (WO3) films were deposited at a substrate temperature of 800 degrees C by dc reactive magnetron sputtering using a W metal target. In addition, Platinum (Pt) was deposited on the WO3 film surfaces at room temperature, also by sputtering. In the early stages of Pt growth, formation of Pt nanoparticles could be expected because of the island structure observed in Volmer-Weber-type growth mode. The surface coverage of Pt on the WO3 films was estimated quantitatively by X-ray photoelectron spectroscopy and was found to be approximately 60% after 7 s deposition. High resolution electron microscopy (HREM) demonstrated that Pt nanoparticles with a diameter of about 2.5 nm were generated and dispersed uniformly on the entire surface area of the columnar polycrystalline WO3 films. These Pt-loaded films exhibited high photocatalytic activity in the decomposition of acetaldehyde (CH3CHO) under visible light irradiation.     

Low-temperature ozone exposure technique to modulate the stoichiometry of WOx nanorods and optimize the electrochromic performance

A low-temperature ozone exposure technique was employed for the post-treatment of WO(x) nanorod thin films fabricated from hot-wire chemical vapor deposition (HWCVD) and ultrasonic spray deposition (USD) techniques. The resulting films were characterized with x-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, UV-vis-NIR spectroscopy and x-ray photoelectron spectroscopy (XPS). The stoichiometry and surface crystallinity of the WO(x) thin films were subsequently modulated upon ozone exposure and thermal annealing without particle growth. The electrochromic performance was studied in a LiClO(4)-propylene carbonate electrolyte, and the results suggest that the low-temperature ozone exposure technique is superior to the traditional high-temperature thermal annealing (employed to more fully oxidize the WO(x)). The optical modulation at 670?nm was improved from 35% for the as-deposited film to 57% for the film after ozone exposure at 150?°C. The coloration efficiency was improved and the switching speed to the darkened state was significantly accelerated from 18.0?s for the as-deposited film to 11.8?s for the film after the ozone exposure. The process opens an avenue for low-temperature and cost-effective manufacturing of electrochromic films, especially on flexible polymer substrates.     

Comparative study of gasochromic and electrochromic effect in thermally evaporated tungsten oxide thin films

Substoichiometric tungsten oxide films (WO_3 − y, 0.49 ≥ y ≥ 0.15) were prepared by non-reactive thermal evaporation of WO₃ powder in vacuum. The thin film composition, structure and optical properties were investigated with the purpose to establish their dependence on the deposition conditions and to prove a possible correlation between electrochromic and gasochromic colouration. An analogy in the dependencies of the maximum achievable optical density on the thin film oxygen content for gasochromically and electrochromically coloured films was observed.In-situ performed XPS measurements suggested that the main mechanism of gasochromic colouration is charge transfer between W⁶⁺ and W⁵⁺ states, i.e., similar to the electrochromic effect.     

Room-temperature resistive H₂ sensing response of Pd/WO₃ nanocluster-based highly porous film

Hydrogen- (H?-) induced resistive response of palladium (Pd) coated tungsten oxide (WO?) films prepared by using supersonic cluster beam deposition (SCBD) was investigated. An SCBD WO? film is found to be constructed of WO? nanoclusters of diameters of 3-5 nm. The nanoclusters are loosely connected to form a structure of high porosity around 66%. With this structure, the film exhibits many excellent room-temperature H? sensing properties, including high sensitivity, broad detectable range of H? concentration, low detection limit, fast response rate, excellent cyclic stability (>2400 cycles), high selectivity against vapor of many organic compounds, mild ambient pressure dependence and many other advantages such as low power consumption, miniaturizability and high batch-to-batch reproducibility. These findings are useful for making new high-quality H? sensors for monitoring the leakage of H? and ensuring safe use of this gas.     

Resistive switching behavior and multiple transmittance states in solution-processed tungsten oxide

In this work, a tungsten oxide (WO(x)) film is prepared using a thiourea-assisted solution process. We demonstrate a device composed of fluorine doped tin oxide (FTO)-glass/WO(x)/electrolyte/indium-tin oxide (ITO)-glass stacking electrochromic (EC) structure and Al electrodes that are locally patterned and interposed between the WO(x) film and electrolyte, which form an Al(top electrode)/WO(x)/FTO(bottom electrode) resistance random access memory (RRAM) unit. According to transmission electron microscopy and X-ray photoelectron spectroscopy analyses, the WO(x) film contains nanosize pores and metallic-tungsten nanoclusters which are scattered within the tungsten oxide layer and concentrated along the interface between the Al electrode and WO(x) film. With application of voltage to the ITO electrode, multiple transmittance states are achieved for the EC unit due to the different quantity of intercalated Li ions in the WO(x) film. As for the Al/WO(x)/FTO RRAM unit, a bipolar nonvolatile resistive switching behavior is attained by applying voltage on the Al top electrode, showing electrical bistability with an ON/OFF current ratio up to 1 × 10(4).     

Ta2O5-Incorporated WO3 nanocomposite film for improved electrochromic performance in an acidic condition

We report electrochromic and electrochemical properties of a WO3-Ta2O5 nanocomposite electrode that was fabricated from co-sputtering. Transmission electron microscopy (TEM) images of the WO3-Ta20 nanocomposite electrode revealed that morphology of the WO3 film was changed by incorporation of Ta2O5 nanoparticles, and their chemical states were confirmed to be W6+ and Ta5+ oxides from X-ray photoelectron spectroscopy (XPS). The introduction of Ta2O5 to the WO3 film played a role in alleviating surface roughness increase during continuous potential cycling; whereas the surface roughness of the WO3 film was increased from ca. 3.0 nm to ca. 13.4 nm after 400 cycles, the roughness increase on the WO3-Ta2O5 was significantly reduced to 4.2 nm after 400 cycles, as investigated by atomic force microscopy (AFM). This improvement of the stability by adding Ta2O5 may be responsible for the enhanced electrochemical and optical properties over long-term cycling with the WO3-Ta2O5 nanocomposite electrode.     

Synthesis and photocatalytic activity of WO(3) nanoparticles prepared by the arc discharge method in deionized water

In this paper, we discuss the synthesis and characterization of tungsten trioxide nanoparticles prepared by the arc discharge method in deionized (DI) water. The size and morphology of WO(3) nanoparticles prepared using different arc currents (25, 35 and 45?A) were studied. Dynamic light scattering (DLS) and scanning electron microscopy (SEM) results indicate that at an arc current of 25?A, the size of the particles is about 30?nm, and this increases to 64?nm by increasing the arc current. This size increase caused a decrease of optical band gap from 2.9 to 2.6?eV. X-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) spectra demonstrate the formation of the WO(3) phase. Photodegradation of Rhodamine B shows that samples prepared at the lowest current have more photocatalytic activity due to having the smallest particle size and highest surface area. The results demonstrate the ability of the arc discharge method for direct formation of WO(3) nanoparticles in DI?water medium.     

Electrochromic properties of self-assembled nanoparticle multilayer films

Hexagonal tungsten bronze (HTB) nanocrystal and TiO₂ nanoparticles were assembled into thin films by layer-by-layer self-assembly method. HTB nanocrystals were synthesized by hydrothermal route at 155 °C. UV-Vis spectra showed that the HTB/TiO₂ films exhibit a linear increase in film thickness with assembly exposure steps. The electrochromic property of the film was carefully investigated. Cyclic voltammetry indicated that the redox peak was around −0.5 V. The electrochromic contrast, coloration efficiency, switching speed, stability and optical memory were carefully investigated. The films vary from white to blue and finally dark brown. The electrochromic contrast is 63.9% at 633 nm. The coloration efficiency of the films is relatively high. The response time is less than 3 s.     

WO3薄膜制备及其电致变色性能研究

采用直流反应磁控溅射方法在室温下制备WO3薄膜。研究溅射功率对WO3薄膜结构及电致变色性能的影响规律,考察退火后WO3薄膜的结构演变及电致变色性能变化。结果表明溅射功率为270W时薄膜表现出较好的电致变色性能,其调制幅度达78.5%,着色时间为9s,褪色时间为3.2s。将该功率下制备的WO3薄膜进行退火处理,其结构由非晶态转变为晶态,但调制幅度、响应时间特性都发生一定程度的退化。非晶态WO3薄膜相比晶态结构具有更快的响应时间和更宽的调制幅度,但晶态薄膜具有更好的循环稳定性。     

Periodically ordered nanoscale islands and mesoporous films composed of nanocrystalline multimetallic oxides

Innovative strategies to produce well-defined nanoparticles and other nanostructures such as nanofibres, quantum wells and mesoporous materials have revitalized materials science for the potential benefit to society. Here, we report a controlled process, involving soft-chemistry-based deposition, template-assisted mesostructured growth, and tuned annealing conditions that allows the preparation of ordered mesoporous crystalline networks and mesostructured nano-island single layers, composed of multicationic metal oxides having perovskite, tetragonal or ilmenite structures. This strategy to obtain meso-organized multi-metal-oxide nanocrystalline films (M(3)NF) bridges the gap between conventional mesoporous materials and the remarkable properties of crystalline ternary or quaternary metallic oxides. Nanocrystalline mesoporous films with controlled wall thickness (10-20 nm) of dielectric SrTiO(3), photoactive MgTa(2)O(6) or ferromagnetic semi-conducting Co(x)Ti(1-x)O(2-x) were prepared by evaporation-induced self-assembly (EISA) using a specially designed non-ionic block-copolymer template. A tuned thermal treatment of the mesoporous films permits the transfer of the wall structure into nanocrystallites, with all tectonic units being tightly incorporated into mechanically stable ordered tri- or bidimensional nanocrystalline networks. This methodology should allow multifunctionalization, miniaturization and integration during development of devices such as smart sensors and actuators, better-performing photocatalysts, and fast electrochromic devices. On the other hand, organized arrays of dispersed ferromagnetic or ferroelectric nanoparticles are promising materials for spintronics and for cheap, non-volatile 'flash' memories.     

Electrochromic performance of hybrid tungsten oxide films with multiwalled-CNT additions

In this study, tungsten oxide films were prepared by sol–gel technique. Various amounts of multiwalled carbon nanotubes (MWCNTs) were added during sol–gel process to obtain hybrid WO₃/MWCNT films. The original and hybrid films were characterized by thermogravimetric analysis, X-ray diffraction analysis, and scanning electron microscopy analysis, whereas the electrochromic performance was evaluated by measuring changes in the optical transmittance caused by potentiostatic charge–discharge intercalation. The influence on the structure and properties of tungsten oxide film due to MWCNT addition was also investigated. The results showed that all of the films were amorphous and exhibited porous microstructure. The electrochromic performance of pristine WO₃ film was improved by adding MWCNTs that served as a template for the growth of WO₃ and resulted in more porous microstructure. The hybrid tungsten oxide films with 0.1 wt.% MWCNT addition exhibited the best electrochromic performance.     

Synthesis and electrochromic study of sol-gel cuprous oxide nanoparticles accumulated on silica thin film

In this study, electrochromic properties of cuprous oxide nanoparticles, self-accumulated on the surface of a sol-gel silica thin film, have been investigated by using UV-visible spectrophotometry in a lithium-based electrolyte cell. The cuprous oxide nanoparticles showed a reversible electrochromic process with a thin film transmission reduction of about 50% in a narrow wavelength range of 400-500 nm, as compared to the bleached state of the film. Using optical transmission measurement, we have found that the band gap energy of the films reduced from 2.7 eV for Cu₂O to 1.3 eV for CuO by increasing the annealing temperature from 220 to 300 °C in an N₂ environment for 1 h. Study of the band gaps of the as-deposited, colored and bleached states of the nanoparticles showed that the electrochromic process corresponded to a reversible red-ox conversion of Cu₂O to CuO on the film surface, in addition to the reversible red-ox reaction of the Cu₂O film. X-ray photoelectron spectroscopy indicated that the copper oxide nanoparticles accumulated on the film surface, after annealing the samples at 200 °C. Surface morphology of the films and particle size of the surface copper oxides have also been studied by atomic force microscopy analysis. The copper oxide nanoparticles with average size of about 100 nm increased the surface area ratio and surface roughness of the silica films from 2.2% and 0.8 nm to 51% and 21 nm, respectively.     

Nickel oxide sol-gel films from nickel diacetate for electrochromic applications

Nickel diacetate tetrahydrate, [Ni(acetate)₂·4H₂O] and nickel diacetate dimethylaminoethanol, [Ni(acetate)₂(dmaeH)₂] were successfully used to deposit NiO_x thin films on conductive glass substrates by sol-gel techniques for large area electrochromic applications. Homogeneous one layer films 100 nm thick were deposited by spin coating 0.5 M [Ni(acetate)₂·4H₂O] in dmaeH at 1000 rpm and by dip coating methods. The NiO_x films were characterised by X-ray diffraction, transmission electron microscopy and atomic force microscopy. The thin film electrochromic performances were characterised by means of optical (transmittance) and electrochemical (cyclic voltammetry) methods. On early cycling NiO_x thin films present an activation period, related to an increase in capacity. The electro-optical data show an increase in the electrochromic response (i.e. an increase in contrast and colouration efficiency) upon cycling. Following this initial activation period a steady state is reached in which the thin films reversibly switch from transparent to brown. The anodically coloured NiO_x thin films are therefore suitable for use in a complete electrochromic cell with tungsten oxide as the cathodic colouring layer. However, the films are not fully stable with long cycling.     

掺杂氧化钨薄膜的电致变色特性

掺杂氧化钼对氧化钨薄膜的电致变色特性有一定的影响,本文介绍了用电子束蒸发制备不同MoO3掺杂比例的氧化钨膜,对其着色态与漂白态的光学特性以及循环伏安特性进行了实验研究,通过对循环伏安曲线的分析,对杂氧化钨薄膜电致变色的机理进行了讨论。