Electrochromic tungsten oxide films: Review of progress 1993–1998

This feature article summarizes recent research on and development of semiconductor-based photocatalyst materials that are applicable to environmental remediation and/or chemical synthesis purposes. A wide variety of TiO2 particles and/or films have been studied during the past 30 years because they are the most stable and powerful photocatalysts leading to the degradation of various organic pollutants. The photocatalytic performance of other semiconductor materials such as ZnO, SnO2, WO3, Fe2O3 and CdS has also been intensively investigated. A general limitation in the efficiency of any photocatalytic process is the recombination of the photogenerated charge carriers, i.e., of electrons and holes, following bandgap illumination. Considerable efforts have been made to suppress this recombination and hence to enhance the charge carrier separation and the overall efficiency by means of coupling of different semiconductors with desirable matching of their electronic band structures, or incorporation of noble metal nanoclusters onto the surface of semiconductor photocatalyst particles. Modification of the physicochemical properties, such as particle size, surface area, porosity and/or crystallinity of the semiconductor materials, and optimization of the experimental conditions, such as pH, illumination conditions and/or catalyst loading, during photocatalytic reactions have also been carefully addressed to achieve high reaction rates or yields. To utilize solar energy more efficiently, i.e., to extend the optical absorption of the mostly UV-sensitive photocatalysts into the visible light range, numerous research groups have contributed to the development of novel visible light active photocatalysts. With the application of semiconductors with narrower bandgaps such as CdS, Fe2O3 and WO3 being straightforward choices, doping of wide bandgap semiconductors like TiO2 has been the most popular technique to enhance the catalysts' optical absorption abilities. Research on mixed-oxide-based semiconductor photocatalysts with deliberately modulated band structures has also attracted tremendous attention in the past decade, concentrating on, for example, the generation of H2 and/or O2 from H2O splitting, and the degradation of organic pollutants under visible light irradiation. Both theoretical calculations and experimental results have convincingly shown that the developed materials can serve as highly efficient photocatalysts that are both environmentally and economically significant.

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Photoelectrocatalytic materials for environmental applications

Electrochromic (EC) materials and polymer electrolytes are the most imperative and active components in an electrochromic device (ECD). EC materials are able to reversibly change their light absorption properties in a certain wavelength range via redox reactions stimulated by low direct current (dc) potentials of the order of a fraction of volts to a few volts. The redox switching may result in a change in color of the EC materials owing to the generation of new or changes in absorption band in visible region, infrared or even microwave region. In ECDs the electrochromic layers need to be incorporated with supportive components such as electrical contacts and ion conducting electrolytes. The electrolytes play an indispensable role as the prime ionic conduction medium between the electrodes of the EC materials. The expected applications of the electrochromism in numerous fields such as reflective-type display and smart windows/mirrors make these materials of prime importance. In this article we have reviewed several examples from our research work as well as from other researchers’ work, describing the recent advancements on the materials that exhibit visible electrochromism and polymer electrolytes for electrochromic devices. The first part of the review is centered on nanostructured inorganic and conjugated polymer-based organic-inorganic hybrid EC materials. The emphasis has been to correlate the structures, morphologies and interfacial interactions of the EC materials to their electronic and ionic properties that influence the EC properties with unique advantages. The second part illustrates the perspectives of polymer electrolytes in electrochromic applications with emphasis on poly (ethylene oxide) (PEO), poly (methyl methacrylate) (PMMA) and polyvinylidene difluoride (PVDF) based polymer electrolytes. The requirements and approaches to optimize the formulation of electrolytes for feasible electrochromic devices have been delineated.

Hybrid Materials and Polymer Electrolytes for Electrochromic Device Applications

Electrochemical methods for the codeposition of Cr or Mo with α-Fe2O3 (hematite) have been developed to produce doped iron oxide films with varying compositions of Cr and Mo which are active photoanodes for photoelectrochemical (PEC) water decomposition (“water splitting”). The films were characterized by scanning electron microscopy, X-ray diffraction, UV−vis optical spectroscopy, and X-ray photoelectron spectroscopy to determine the effect of the dopants on the hematite structure and PEC performance. Upon doping, the microstructures of the films varied; however, no preferred crystallographic orientation or dopant phase segregation was observed. The best performing samples were 5% Cr and 15% Mo doped which had Incident Photon Conversion Efficiencies (IPCE’s) at 400 nm of 6% and 12%, respectively, with an applied potential of 0.4V vs Ag/AgCl. These IPCE values were 2.2× and 4× higher than the undoped sample for the 5% Cr and 15% Mo samples, respectively. The increase in performance is attributed to an imp...

Electrodeposition of α-Fe2O3 Doped with Mo or Cr as Photoanodes for Photocatalytic Water Splitting

Tungsten oxide exhibits pronounced photochromism upon bandgap photoexcitation, which makes it attractive and promising for applications in many areas. Some advances have been achieved during the past decades. The research on nanocrystalline films and single crystals indicates the critical importance of defects in tungsten oxide to its photochromism. Based on energy-band engineering of semiconductors, enhancement of photochromism has been achieved, for instance, extension of the photoresponse from UV to visible light by cathodic polarization, improved change in absorption before and after coloration through modification by a noble metal or another metal-oxide semiconductor, and increased photochromic reversibility via hybridization with organic amines. Nanocrystalline oxide films exhibit controllable wettability, which is coherent in nature with photochromism.

Photochromic materials based on tungsten oxide

Electrochromic and photoelectrochemical behavior of electrodeposited tungsten trioxide films

An all-solid-state electrochromic display device of prussian blue and WO3 particulate film with a PMMA gel electrolyte

Photochromic and photoelectrochemical behavior of thin semiconductor WO3 films

A new all-solid-state electrochromic device (SED) employing prussian blue and WO3 particulate film with poly (vinyl chloride) (PVC) gel electrolyte has been fabricated. A reversible color change between blue and colorless was observed when an appropriate potential was applied repeatedly to the SED. When the potential cycled more than 1000 times, less than 2% of the electrochromic characteristics were lost.

All-Solid-State Electrochromic Display Device of Prussian Blue and WO3 Particulate Film

An all-solid-state electrochromic window (SEW) employing electrodeposited WO3 and prussian blue with polymer gel electrolyte has been fabricated. The conductivity of poly(ethylene oxide) (PEO)-based gel electrolyte was 4.6×10-4 S/cm at room temperature. The W 4f core level spectra of WO3 film at different levels of coloration have been investigated using XPS. W 4f peaks become broader after coloration. The SEW has been found to have excellent electrochromism and memory characteristics. A reversible color change between blue and colorless was observed when an appropriate potential was applied repeatedly to the electrochromic window.

Jinghuai Fang

Papers

Electrochromic and photoelectrochemical behavior of electrodeposited tungsten trioxide films

An all-solid-state electrochromic display device of prussian blue and WO3 particulate film with a PMMA gel electrolyte

Photochromic and photoelectrochemical behavior of thin semiconductor WO3 films

All-Solid-State Electrochromic Display Device of Prussian Blue and WO3 Particulate Film

All-solid-state Electrochromic Window of Electrodeposited WO3 and Prussian Blue with Poly(ethylene oxide) Gel Electrolyte