Review on the Versatility of Tungsten Oxide Coatings

a FEATURE ARTICLE solidi status physica Tungsten Oxides www.pss-a.com Review on the Versatility of Tungsten Oxide Coatings

Cezarina C. Mardare* and Achim W. Hassel

in Figure 1. The importance of this material Tungsten oxide is a versatile material with many advantages such as large is reflected by the steep increase of pub- availability and low fabrications costs. Additionally, it is suitable to be lications, with more than 1500 being produced in the form of thin films or coatings, which makes it very attractive published in 2017 and summing up to for many applications. In this Feature Article, the material properties such as hundreds of thousands of citations over the crystallography, chemical stability, and semiconducting properties are pre- years. sented, followed by examples of coating technologies. Furthermore, the As mentioned before, there is a wide- range of applications in which tungsten relation between its structure and some of the very important applications oxides are used and one of the first, fields such as electro- and photochromism, as well as pH sensing are discovered in 1920s, is the pH sensitivity.[2] analyzed in detail. Its auspicious semiconducting properties make it attractive Until 1960s a detailed research on stoi- to be used as photo(electro)catalyst, therefore applications of thin films based chiometric and substoichiometric tungsten [3–6] on WO3 are emphasized such as photoanodes for water splitting, in oxides was carried on by A. Magneli healthcare as antimicrobial material and for degradation of pollutants or for dealing with the production and character- < < CO2 reduction. The article is concluded by a short overview of the current ization of WOx phases (2.625 x 2.92), status of research employing tungsten oxide. later known as Magneli phases (W32O84, W3O8,W18O49,W17O47,W5O14,W20O58, fi and W25O73). Due to their oxygen de cit, these phases have a high electric conduc- – 1. Introduction tivity[7 9] and are nowadays studied as photocatalysts, for pollutants reduction Tungsten and its oxides are materials that have been studied and as materials for electrodes.[10] In late 1960s three important since early 1900s. In the last 50 years, there has been an upsurge applications involving tungsten trioxide emerged. Platinum- in the research related to tungsten alloys and oxides with various activated tungsten trioxide was used for hydrogen detection[11] stoichiometries due to their suitability for a large variety of and for fuel cell electrodes,[12] and in 1973 Deb[13] discovered the applications. This transition metal oxide is naturally abundant; it electrochromic properties of WO3. A few years later, in 1976, two has low costs and very low toxicity toward living organisms, and parallel independent studies showed for the first time the it is environmentally friendly. These features together with a suitability of WO3 for photoanodes in photoelectrochemical cells high chemical stability in a pH range of relevance for many for water splitting.[14,15] Due to increased energy requirements, applications[1] and its semiconductor properties led to large fi new research elds have emerged, and WO3 has been recently number of publications in the last 15 years. The number of articles investigated for applications in dye-sensitized solar cells, for CO “ ” 2 published per year when the topic tungsten oxide was used in the reduction, and batteries, as well as for sensors, for pollutants search engine from https://apps.webofknowledge.com is shown degradation, air purification, and as antimicrobial agent.[16] From the diversity of applications and the number of studies dedicated to it, it can be inferred that WO has an auspicious Dr. C. C. Mardare, Prof. Dr. A. W. Hassel x Institute for Chemical Technology of Inorganic Materials (TIM) combination of bandgap, crystalline structures, and chemical Johannes Kepler University Linz stability, optical and electrical properties that make it very Altenberger Str. 69, 4040 Linz, Austria versatile and therefore very attractive. In general, properties E-mail: [email protected] optimization for the desired application is achieved by nano- – Dr. C. C. Mardare, Prof. Dr. A. W. Hassel structuring,[17 20] doping[21] or mixing it with different – Christian Doppler Laboratory for Combinatorial Oxide Chemistry at the oxides,[22 28] and even by utilizing an amorphous phase instead Institute for Chemical Technology of Inorganic Materials [29] Johannes Kepler University Linz of crystalline phases. Altenberger Str. 69, 4040 Linz, Austria In this Feature Article, we focus on tungsten oxides coatings together with properties tuning achieved by mixing WOx with a The ORCID identification number(s) for the author(s) of this article fi can be found under https://doi.org/10.1002/pssa.201900047. suitable material. Since the number of elds in which this oxide is used is vast, we will focus on some applications utilizing © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.. This is an open access article under the terms of the WO3x for electrochromic and photo(electro)catalytic applica- Creative Commons Attribution-NonCommercial-NoDerivatives License, tions, in pH sensing, and as antimicrobial agent. Some which permits use and distribution in any medium, provided the important deposition methods such as chemical synthesis and original work is properly cited, the use is non-commercial and no physical vapor deposition (PVD) are reviewed. Furthermore, the modifications or adaptations are made. fundamental properties of pristine tungsten trioxide are DOI: 10.1002/pssa.201900047 presented, including crystallography, physical and chemical

Cezarina C. Mardare is a physicist working in the field of functional materials for various applications. She studied physics at University of Bucharest, Romania and conducted her doctoral work at Max-Planck Institute for Iron Research in Düsseldorf, Germany. She received her Ph.D. degree in materials science in 2009 from Ruhr University Bochum (RUB) Germany. She then completed a post-doctoral training at RUB, and in 2010 she joined the group of Prof. Achim W. Hassel at Johannes Kepler University Linz, Austria. Her current research interests are focused on development of new materials for healthcare application, with emphasis on combinatorial materials science. Figure 1. Number of publications per year, as reported by Web of Science (https://apps.webofknowledge.com) when using the topic “tungsten oxide” in the search engine. Achim W. Hassel received his Ph.D. in 1997 from University of Düsseldorf, Germany. After that, he characteristics, as well as the importance of doping/mixing with was an Alexander von Humboldt- different chemical elements for increased functionalization. and JSPS-fellow until 1999 at Hokkaido University (Sapporo, Japan). Between 2000 and 2009 he 2. Properties of Tungsten Oxide was head of the Electrochemistry 2.1. Crystallography and Corrosion group at the Max Planck Institute for Iron Research Stoichiometric tungsten trioxide shows structural polymorphism and the scientific director of the IMPRS Surmat. Since and phase transitions occur at different temperature during heating 2009 he holds a chair in Chemistry at the Johannes Kepler or cooling. The most commonly found room temperature stable University Linz, Austria. Moreover he is head of the γ Christian Doppler Laboratory for Combinatorial Oxide phase is the monoclinic I ( -WO3). This phase is present in a temperature range from 17 to 330 C. Below room temperature, two Chemistry. His research interests are in the field of e combinatorial and electrochemical materials science. other crystallographic phases exist: monoclinic II ( -WO3)for < δ < < T 43 C and triclinic ( -WO3)for 43 C T 17 C. When γ β heated above 330 C, -WO3 is converted to orthorhombic -WO3 > α (stable up to 740 C) and for T 740 C, tetragonal -WO3 is oxygen atoms present. Figure 3 presents the O-W phase diagram found.[30,31] However,thesetwophasesarestableonlyathigh γ as a function of temperature, with emphasis of different oxygen temperatures and upon coolingtheyareconvertedbackto -WO3.A content ranges where the Magneli phases are located.[36] metastable phase, hexagonal WO3 (h-WO3) also exists and it can be [32,33] For the stoichiometric WO3, the WO6 octahedra share only the synthesized by different chemical methods. Upon heating to corners, whereas for the substoichiometric oxides shared edges T > 400 C and cooling, the hexagonal phase is not retained and h- γ and even surfaces progressively form as the oxygen content WO3 is converted to -WO3. Another phase, cubic c-WO3 was also decreases. Edge-sharing WO6 octahedra with channels forming found in powders when impurity atoms such as H, Na or Li were pentagonal columns and hexagonal tunnels are characteristic to [34] ﬁ present or in thin lms where it developed along with the these oxides.[7] An example of such crystallographic structure for monoclinic phase.[24,28] This cubic phase was not reported for bulk, W18O49 is shown in Figure 4. but it is considered as the ideal high temperature phase and consequently used as reference for the structure of WO3.Theatoms 2.2. Chemical and Physical Properties arrangement for c-WO3 is of ReO3-type and it consists of corners and edge sharing WO6 octahedra as shown in Figure 2.Alltheother polymorphs are built up according to tilting angles and rotation of Crystalline tungsten trioxide is obtained by calcination of these WO6 octahedra and referred to as distorted ReO3-structures. ammonium paratungstate tetrahydrate (Reaction 1) or of Furthermore, all octahedral units are arranged in a perovskite-like tungstic acid (Reaction 2) in air at temperatures exceeding structure. An exception is h-WO3,forwhichtheWO6 octahedra are 400 C. Chemical transformations occur via the following sharing corner oxygen in a six- and three-membered rings decomposition reactions [35] arrangement, and as a result tunnels along c-axis are formed. ðÞðÞT>400! C þ Substoichiometric oxides, like Magneli phase, are formed by NH4 10 H2W12O42 4H2O 12WO3 10NH3 þ ðÞ restructuring of the crystal structures due to lower number of 10H2O Reaction 1

and

T>400 C WO3 xH2O ! WO3 þ xH2OðÞ Reaction 2

If stoichiometric WO3 is obtained, the resulting powder has a yellow color that turns toward green when oxygen deficit is present. Magneli phases of WO3 develop upon reduction in moist H2 at elevated temperatures and they can have different colors as a function of stoichiometry changing from bluish-violet [37] for WO2.9 to reddish-violet for WO2.72 and brown for WO2. Tungsten trioxide has a large window of pH values in which it is stable ranging from neutral to acidic as indicated by the Pourbaix diagram and additionally, it possesses a high (photo) electrochemical stability in acidic media, being hardly soluble.[1] It has a relatively high melting point (1473 C), but it starts to sublimate below this temperature, especially if water vapors are present.[37] Regarding its physical properties, tungsten trioxide is a d0- transition metal oxide and a wide bandgap n-type semiconductor. The electron concentration is determined mainly by the 2þ intrinsic defects such as oxygen vacancies (VO ) with a 13– 17 3 [38,39] Figure 2. 3D cell projections together with the views along the [010] concentration in the range of 10 10 cm . The crystallographic direction for ideal cubic structure (c-WO3) and for the electronic bandgap (Eg) is represented by the difference between γ room temperature stable phase (monoclinic, -WO3); the red small the energy levels of the valence and conduction bands. The spheres represent the oxygen atoms and the black large spheres inside the valence band is formed by filled O 2p orbitals, and the polyhedra represent the tungsten atoms. conduction band, by empty W 5d orbitals[40] as shown in γ Figure 5. The electronic bandgap value for the -WO3 bulk is 2.6–2.8 eV. This value changes with the degree of distortion from the cubic structure, with amorphous WO3 showing a much larger bandgap of 3.2 eV. If the size of WO3 is reduced into the nanometer range, the bandgap values are even larger due to quantum-confinement.[41] The range of bandgap values confers [42] WO3 the ability to absorb 12% from the solar radiation from UV up to wavelengths of 400 nm. The edge of conduction band (CB) lies below the photocatalytic hydrogen evolution threshold as reported to the standard hydrogen electrode (SHE), thus WO3 is not suitable for one-step hydrogen generation. However, the valence band (VB) potential is located at 3.2 eV versus SHE, therefore WO3 can promote oxygen evolution. This feature makes it an attractive material as photoanode for photoelectrochemical water splitting. The VB position enables also the oxidation of organic pollutants via hydroxyl radicals generation. The same mechanism is also responsible for the antimicrobial properties, since the presence of hydroxyl radicals has been related to the damage of cell membranes, thus inhibiting bacterial growth.[43] Ab initio calculations for determining the electric properties of Magneli phases were performed by Migas et al.[9] These materials show metallic-like character and the metallic properties are governed by the position of Fermi levels which, for each of the aforementioned substoichiometric oxides, cross some of the energy bands. Optical properties of WO3 are directly related to its bandgap. In the visible region of the spectrum and for light wavelengths higher than 400 nm there is no absorption taking place therefore [44] the appearance of WO3 is transparent to light yellow. For Figure 3. Oxygen–tungsten phase diagram (redrawn from Ref. [36] and lower wavelengths with photon energy values exceeding the extended down to 0 C for the visualization of different WO3 polymorphs). energy of the bandgap, interband transitions take place. The

Figure 4. Example of the crystalline structure of W18O49 Magneli phase in the 3D cell projection and along the [010] crystallographic direction; the red small spheres represent the oxygen atoms and the black large spheres inside the polyhedra represent the tungsten atoms. absorption threshold is located in the ultraviolet to blue region of doping (e.g., Ag, Cr, Hf, Mo, Pt, Ti, Zr) has been reported to the spectrum, as reported by Gullapalli et al.[41] In the case of create energy states into the bandgap by the introduction of oxygen deﬁcient phases (i.e., Magneli phases), the oxygen impurities or a reduction of the bandgap by an upward shift of vacancies create new energy bands below the conduction band. the valence band edge.[47] Anion doping (e.g., N, S) can also have They exhibit an absorption band in the near infra-red region and a beneﬁcial effect by reducing the bandgap via band edge as a result, their color appears as green to bluish.[45,46] shifting due to the introduction of new p-states of the anion into [42] the O 2p states of WO3.

2.3. Doping 3. (Electro)chemical Synthesis Energy increasing demands require the development of 3.1. Sol-Gel Synthesis materials that can harvest and utilize a large portion of the solar spectrum. Due to its attractive properties and availability, extensive studies have been conducted concerning bandgap Among wet-chemistry coating technologies, sol-gel has gained an increased attention in the last 20 years due to its low- engineering of tungsten trioxide. As mentioned before, the production costs and large coating area capability. These features electronic properties can be tuned by doping with suitable make sol-gel a very attractive method for commercial applica- elements in order to achieve the reduction of the bandgap or [42] tions. The principle relies on formation of an oxide network via shifting of bandgap edges. Depending of the element, cation polymerization reactions of molecular precursors.[48] Metal alkoxides dissolved in a suitable organic solvent (e.g., chloro- [49] alkoxides: WOCl4 in isopropyl alcohol or tungsten oxo-tetra-n- n [50] butoxide (WO(O Bu)4) dissolved in ethanol ) and inorganic precursors (metal salts) in aqueous solutions (e.g., tungstic acid powder in hydrogen peroxide[51]) are used as molecular precursors. After the formation of sols, layers are formed onto the substrate by dip-coating, spin-coating or spraying. Drying, hydrolysis and calcination are the final steps in order to achieve fi [52] stoichiometric and crystalline WO3 lms. Yang et al. studied the formation of mesoporous films from aqueous precursor containing peroxopolytungstic acid as a function of pH and performed photoelectrochemical investigations. By using different organic additives (polyethylene glycol, glycerol, mannitol or ethylene glycol) and by varying the post-deposition heat treatment, surface features, electrochromic properties and [53] photoactivity of WO3 could be tuned.

3.2. Hydrothermal Synthesis

Figure 5. Edges of valence and conduction bands of WO versus the 3 Hydrothermal method is used for synthesizing/precipitating standard hydrogen electrode (SHE) potential, together with the oxygen [54] and hydrogen evolution reaction potentials for water splitting, and the compounds from simple and to extremely complex (oxides, oxidation potential of hydroxyl radicals and reduction potential of carbonates, silicates, etc.) in liquids at high temperature and superoxide radicals relevant for oxidation of organic pollutants, as well as pressure. The apparatus consists of a steel vessel (autoclave) for inactivation of microbes. lined with PTFE or alloys that withstand corrosion and in which

Phys. Status Solidi A 2019, 1900047 1900047 (4 of 16) © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. a solidi status physica www.advancedsciencenews.com www.pss-a.com the liquid containing the precursors is fed. The autoclave is ability, and precise thickness control. The setup consists of an hermetically sealed and at the lower side heating is applied electrochemical-cell filled with the electrolyte containing the resulting in a temperature gradient between the lower and the metal species to be deposited and two (or three) electrodes: the upper ends. Consequently, due to supersaturation of the solute cathode, which is a conducting substrate onto which the film in the cooler zone, nucleation, followed by precipitation and grows, the anode (in general Pt) and a reference electrode if a growth of the compounds occurs.[55] three-electrode arrangement is used. A DC current is supplied to Different morphologies, sizes or phases can be achieved by the circuit in such a manner that the metal species from the changing the solution pH, reaction temperature, and concen- solution are driven to the cathode where they are reduced and tration of solute species or the types of additives. For the consequently, electrodeposited to form the film.[58] By tailoring particular case of WO3 coatings grown by this method, several the process parameters and the electrolyte composition, the studies have been performed in order to tune the surface resulting film microstructure can be precisely modified and properties and the crystallography. For example, Ding et al.[56] controlled. Extended research has been dedicated to the study of fi fi obtained WO3 coatings with different hierarchical structures electrodeposition of WO3 lms after the publication of the rst (ordered nanorods, peeled-orange-like and cauliflower-like article in 1987 by Yamanaka.[59] A comprehensive study on the arrays) by changing the capping agents and they showed good mechanism of WO3 electrodeposition from peroxytungstate photochromic properties (Figure 6). Nanobrick-like structured solution was conducted 10 years later by Meulenkamp.[60] Kwong fi [61] fl thin lms of WO3 with large surface area suitable for et al. studied the in uence of the peroxotungstic acid electrochromic windows applications were also achieved using concentration and deposition time on the structural and [57] fi fl hydrothermal growth. photoelectrochemical properties of WO3 lms. Nano ake array films with enhanced electrochromic and sensing to H2S properties were electrodeposited on FTO coated with a 3.3. Electrochemical Deposition seed layer.[62] Baeck et al.[63] studied the formation of nano- particulate films as a function of the pulses characteristics Electrochemical deposition is a very attractive coating technique variation in a pulsed electrodeposition process and they showed due to several advantages such as low cost, large area coating enhanced photoactivity as compared to coatings obtained by

Figure 6. Tungsten oxide coatings with different morphologies obtained by hydrothermal synthesis: (a and b) nanorods arrays, (c and d) peeled-orange- like hierarchical structures, and (e and f) cauliflower-like hierarchical structures, together with the grow mechanism. Reproduced with permission.[56] Copyright 2012, Elsevier B.V.

Phys. Status Solidi A 2019, 1900047 1900047 (5 of 16) © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. a solidi status physica www.advancedsciencenews.com www.pss-a.com continuous electrodeposition. Habazaki et al.[64] electrodepos- 4. Physical Vapor Deposition ited WO films and studied the effect of annealing temperature 3 4.1. Thermal Evaporation on their crystallinity, and they were further used for as anodes for electrochemical decomposition of phenols for wastewater treatment. Thermal evaporation is one of the physical vapor deposition (PVD) techniques used for coatings deposition. The source of material (e.g., metal, metal oxide) is heated in vacuum, and when 3.4. Anodization enough thermal energy has been supplied, the material evaporates or sublimes. Atoms of the heated source are than directionally released and collected onto a substrate.[74] To obtain Anodization process of W represents the growth of a WO3 layer fi by passing a DC current through the electrolyte solution. tungsten oxide lms, the source material can be either Hydrogen release takes place at the cathode (e.g., platinum foil, metallic W or WO3 in the form of powder or pellets. If metallic wire, or mesh) while at the anode (metallic tungsten) oxygen is tungsten is used as evaporation source, the resulting coating is formed leading to the formation of a tungsten oxide layer. metallic. A post-deposition annealing in oxygen containing Cristino et al.[65] proposed that the oxidation occurs via multiple environment is further required in order to obtain tungsten reactions involving at first formation of oxides with mixed oxide. Another possibility is to use directly tungsten trioxide as an evaporation source. During heating in vacuum, tungsten stoichiometries (WO2 and W2O5), followed by WO3 only if the applied voltages exceed 2 V. Due to the low-cost setup require- trioxide sublimates, but some oxygen is released due to the ments, flexibility of electrolytes availability, and precise growth thermal energy provided and therefore substoichiometric control conditions, a great amount of work has been dedicated to tungsten oxide species are being formed the formation of WO3 having various 2D and 3D structures. sublimate x Verge et al. studied the formation in 0.1 M H SO þ 0.4 M WO ðÞs ! WO ðÞþg O ðÞ"!g WO ðÞs ðÞReaction 3 2 4 3 3 x 2 2 3 x Na2SO4 electrolyte of anodically grown oxide on both bulk and fi sputter deposited W lms using stationary and rotating disk Consequently, the resulting coating can have different electrodes. Various analytical techniques were employed in order stoichiometries (x ¼ 2–3) as a function of deposition parameters to acquire in depth information about the growth/dissolution of and additionally, due to the low energy of the species in thermal [66,67] oxides. Photoelectrochemical anodization of a W foil in a evaporation processes, oxide films are in general amorphous. In two-electrode setup at 50 V under illumination up to several order to achieve crystalline and stoichiometric WO3, annealing at μ hours led to the formation of a thick (2.6 m) nanoporous WO3 temperatures exceeding 400 C is required.[75] Evaporated WO fi [68] 3 lm with enhanced photocurrent response. films having a rough surface proved themselves to be suitable for Kalantar-zadeh and co-workers obtained 3D nanoporous WO3 gas sensing applications.[76] Smart glass windows combining fi layers by anodizing sputter deposited W lms in an electrolyte energy storage and electrochromic properties with outstanding containing ethylene glycol with NH4F(Figure 7). Their WO3 [77] properties were fabricated using thermally evaporated WO3. films showed remarkable electrochromic properties due to crystallinity combined with high surface area.[69] The study of galvanostatic anodization of W in oxalic acid revealed the [70] formation of porous structures. Self-organized WO3 nano- 4.2. Sputtering tubes formation in different concentrations of NaF electrolyte for achieving regular pores arrangement and diameter was studied Sputtering is another PVD technique that is used for producing by Tsuchiya et al.[71] An extensive work on formation of W- thin and thick films. It is already strongly implemented in nanowires from directionally solidified NiAl-W eutectic alloys industry due to its versatility and large-scale coating capability. exposed to different electrolytes in order to either create nano- The process takes place in a vacuum chamber and it is conducted sized filters or W-based nanostructures for different applications at pressures in the range of 10 1–10 Pa achieved by the was conducted by Hassel and his co-workers.[19,20,72,73] introduction of a sputtering gas (Ar, Xe, He, etc.).[74] The source

Figure 7. SEM images of the surface and cross-section of a sputter-deposited W film, followed by anodization and subsequent annealing at 450 C. Reproduced with permission.[69] Copyright 2012, Royal Society of Chemistry.

Phys. Status Solidi A 2019, 1900047 1900047 (6 of 16) © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. a solidi status physica www.advancedsciencenews.com www.pss-a.com of material (the target) is bombarded with energetic ions from reproduce the target stoichiometry and its versatility concerning þ the sputtering gas (e.g., Ar ) and as a result the target atoms are tuning from amorphous to crystalline structures, PLD has been ejected and coated onto a substrate. Similar to the thermal also used for deposition of WO3. The principle is based on evaporation process, the source of material can be either ablating a ceramic target surface with the use of an excimer laser, fi fi metallic W or WO3, in DC- or RF-con gurations. In the case of having as a result plasma generation. The lm is deposited onto sputtering, reactive processes are also possible by introducing O2 a substrate placed in front of this plasma plum, which contains in different proportions together with the sputtering gas. atom clusters and molten droplets of material. As a function of Consequently, tungsten oxide coatings can be realized both substrate temperature and oxygen content during the process, fi from metallic and oxide targets directly during the deposition amorphous or crystalline WO3 lms can be obtained, as well as process with controlled stoichiometry. Heat treatments during various substoichiometric oxides.[80] deposition or post-deposition can be used in order to obtain fully The structure of the coating can be tuned from continuous crystalline and/or stoichiometric films. By changing the film to nanowires by changing the laser fluence and energy sputtering parameters such as pressure, power density, substrate (Figure 8). This change came along with the formation of temperature, or oxygen content, several features such as the different polymorphs such as monoclinic and the stabilization of [81] crystallinity, stoichiometry, and quality of the coatings can be orthorhombic WO3. controlled.[78,79]

5. Applications 4.3. Pulsed Laser Deposition Nowadays tungsten oxide is employed in various applications For studying the fundamental properties of materials, pulsed from different technological ﬁelds such as electrochemistry, laser deposition (PLD) is one of the most suitable coating photo(electro)catalysis, for sensors or as antimicrobial agent. Its methods.[74] Due to its ability to form ﬁlms that reliably electrochromic and photochromic properties make it an

Figure 8. SEM images of different morphology obtained for WO3 coatings deposited by PLD using various laser energy values and the change of crystalline structure corresponding to these morphologies as evidenced by XRD patterns. Reproduced with permission.[81] Copyright 2016, Royal Society of Chemistry.

Phys. Status Solidi A 2019, 1900047 1900047 (7 of 16) © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. a solidi status physica www.advancedsciencenews.com www.pss-a.com important candidate for smart windows applications, whereas section, we would rather like to focus on the implementation of photocatalytic properties enable its use as photoanodes for water WO3 into devices that utilize its electrochromic properties. splitting. An overview of some important application including A recent technological development is found in fenestration, latest developments and market implementation is further namely the concept of “smart windows.” Figure 9 schematically presented. shows the layered structure of such an electrochromic (EC) device (a and b), together with photographs from Boeing 787 Dreamline airplane electrochromic windows (c). 5.1. Electrochromism and Photochromism The typical support material is glass, but different flexible plastics that enable the fabrication of devices using the roll-to-roll Since the discovery by Deb in early 1970s[13] of electrochromic technology can also be used. The outer parts of the stack are and photochromic properties of WO3, many publication have panes of glass, onto which a transparent conducting oxide (TCO) been dedicated to the study of these effects and to the tailoring of layer is deposited. The most common material employed as TCO fi properties to the nal goal of large scale implementation. To this is Sn-doped In2O3 (ITO), but doping with other different subject, it can be said that electrochromism is a truly successful elements (e.g., Zn or Nb) showed also appropriate electric story, because nowadays electrochromic windows for airplanes properties. However, due to the constantly increasing price of or rear-view mirrors in cars are already in everyday use. indium caused by its scarcity, other TCOs such as doped-ZnO Electrochromic materials are materials that reversibly switch and doped-SnO2 were studied, and they also show promising their optical properties when an external potential is applied to features for being implemented in EC devices. Additionally, very them. Simply explained, the electrochromic property of thin high conductive metallic layers (Cu, Ag, or Au) or organic amorphous WO3 is based on the intercalation of a small cation materials such as PEDOT are also currently under consider- þ þ þ (e.g., H ,Na ,orLi ) into its network, which has as a result the ation.[85,86] In between these two glass panes coated on inner change of material color from transparent to blue. For example, sides with transparent electrodes, there is a three layer structure þ upon insertion of protons (H ) and electrons (e-), the color of consisting of an ion storage thin film, an ion conductor fi WO3 changes from transparent (bleached) to colored (Reaction (electrolyte) and the electrochromic (WO3) thin lm. By applying 4).[82] The process is partially reversible, up to a maximum a small potential difference between the electrodes (<5 V DC), amount of protons x < 0.5. the cations from the ion storage layer (e.g., also an electrochromic material such as NiOH:Li) are transported through the þ ðÞWO3 þ H þ e Ð ðÞHxWO3 ðÞReaction 4 ion conducting thin film. As a function of the type of cation used, bleached colored þ the ion conducting film can be for example Ta oxide for H In more details, the mechanism relies on the localization of transport or polymeric electrolytes such as PEO-PEGMA:Li for þ [87] electrons on tungsten sites, resulting in a change of oxidation Li . Cations are injected in the WO3 active layer and þ þ state from W6 to W5 . These localized electrons may attain coloration occurs via combination with the electrons supplied enough energy to hop to a neighboring site if photon absorption by the external circuit (Reaction 4). Simultaneously, the same takes place. Under these conditions, the lattice is slightly process takes place at the ion storage layer where coloration distorted by the interaction between the electrons and phonons occurs as well, but it occurs when the ions leave the material. that lead to the formation of polarons.[83] Furthermore, photon Consequently, the overall coloration effect is strengthened activated polaron hopping occurs and consequently, the (Figure 9b). When the reverse potential is applied, bleaching coloration occurs due to photons absorption. occurs and the window becomes once again transparent Extended reviews on the literature published on topics related (Figure 9a). Different levels of coloration can be achieved by [21,82,84] to the electrochromism of WO3 already exist. In this applying any intermediary voltage values, since the EC materials

Figure 9. Schematic drawing of a smart window based on electrochromic layers (a) transparent and (b) colored. c) Photographs of a dimmable electrochromic window from Boeing 787 Dreamliner airplane.

Phys. Status Solidi A 2019, 1900047 1900047 (8 of 16) © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. a solidi status physica www.advancedsciencenews.com www.pss-a.com can preserve their properties (i.e., coloration) under open-circuit 5.2. Photo(electro)catalytic Properties conditions. As mentioned before, the study and development of devices The semiconducting property of WO3 and its bandgap value and using electrochromic materials for everyday life is one of the positioning in relation to the conduction and valence bands successful stories and there are already many large-scale enables the absorbance of solar radiation ranging from UV to producers for smart windows based on WO3, such as visible. Based on this feature, there are many investigations for EControl-Glas GmbH &Co. KG (Germany), GESIMAT GmbH applications with some prominent ones such as intensive (Germany), ChromoGenics AB (Sweden) or SAGE Electro- studies as photoanodes for water splitting, as material for chromics, Inc. (USA). degradation of pollutants, for CO2 reduction, or as a biocidal An interesting application of electrochromism involving WO3 agent (Figure 10). [88] was developed by Marques et al. which employed this property Upon light irradiation of WO3, electrons from the VB are to create a colorimetric sensor. This sensor was based on regular exited into the CB leading to the formation of photogenerated fi of ce paper that was impregnated with WO3 nanoparticles. It holes in the VB (Reaction 5). If the recombination of electron– was used to detect the presence of an electrochemically active hole pairs is hindered (e.g., by nanostructuring or introduction bacterium (Geobacter sulfurreducens) using the bioelectrochromic of dopants), photogenerated electrons can be further used for response achieved by electron-transfer between the bacteria and water splitting, or in CO2 reduction processes, whereas the holes WO3 resulting in the color change. can be involved in pollutants degradation, as exemplified in the Similar to electrochromism, photochromism also refers to the following reactions (Reaction 6–11). It is worth mentioning that reversible change of color, but in this case upon photoirradiation. the process of CO2 reduction into fuels (Reaction 10 and 11) by The mechanism of photochromism in WO3 has been intensively mesoporous WO3 is in reality more complex, involving several studied and to the date, though not fully understood, there is processes such as thermocatalysis, photocatalysis, and photo- some generally accepted explanation. Tungsten oxide being a thermal coupling.[91] semiconductor, upon light irradiation electron–hole pairs are generated, and, under certain conditions, the optical absorption !hv þ þ ðÞ is modified and color change from bleached to blue occurs. The WO3 eCB hVB Reaction 5 photogenerated holes enter into the conduction band, react with þ the humidity adsorbed on the surface and as a result H is Water splitting generated. These protons and electrons lead to the formation of tungsten bronze H WO . Simultaneously, the electron transfer þ þ ! þ þ " ðÞ x 3 hVB 2H2O 4H O2 Reaction 6 between neighboring tungsten ions with different valent states occurs and optical transitions take place.[89] To switch back to the bleached state, a heat treatment is normally required in order to þ þ ! " ðÞ eCB 4H 2H2 Reaction 7 eliminate the defects introduced into the structure, but new strategies (e.g., surface nanostructuring[90]) are developed to Degradation of pollutants overcome this drawback. Even though not fully exploited to the date, materials showing þ þ ! • ðÞ photochromism are foreseen to be used for commercial hVB OH OH Reaction 8 applications, such as photochromic pigments or nanoinks.[89,90]

Figure 10. Presentation of various applications of WO3 based on its semiconducting nature together with the photo(electro)catalytic properties under light irradiation.

þ ! • ðÞ eCB O2 O2 Reaction 9 and subsequently annealed. Photocurrent measurements were performed using photoelectrochemical-scanning droplet cell mi-

CO2 reduction croscopy (PE-SDCM Figure 11a and b) and the values were mapped along the entire library. Avery strong photocurrent density peak with þ μ 2 fi 2H2O ! 4H þ O2 þ 4e ðÞReaction 10 25.9 Acm was identi ed at 21.9 at.% Fe, and its occurrence was correlated to an increased amount of hematite phase formation (Figure 11c and d). A similar investigation on Fe-W-O combinatorial þ CO2 þ 8H þ 8e ! CH4 þ 2H2OðÞ Reaction 11 libraries with various compositionsandthicknessesshowedthe highest photocurrent density value at 65 μAcm 2.Thisvaluewas Decline in fossil fuel provisions triggered intensive inves- obtained in the region of the library having 1060 nm thickness and [25] tigations for the exploitation of renewable energy in order to consisting of Fe15W85Ox with W5O14 phases. An additional overcome the foreseen future energy shortage. Since the report of publication form the same research group, screening additional [92] fi Fujishima and Honda in 1972 for the production of H2 through properties of Fe-W-O co-sputtered thin lms, revealed a region with photoelectrochemical (PEC) water splitting a wide variety of increased photocurrents in the composition range of 45–72 at.% Fe materials has been studied for the conversion of water molecule to with the peak maximum at 55 at.% Fe, as well as an increase of – [95] H2 and O2 under photons irradiation (Reaction 5 7). Among other bandgap in the same compositional zone. Another material oxides, tungsten trioxide thin films have also been studied for suitable for photoanodes, which was extensively studied as stand- photoanode applications and the tuning of properties has been alone but also in combination with WO3,isTiO2.ContinuousTi-W- fi achieved by doping, nanostructuring, or altering the stoichiome- Othin lm libraries deposited on SiO2/Si wafers were screened and 2 try.[93,94] Here, we would like to focus the discussion on materials the highest peak photocurrent density of 70.3 μAcm was achieved fi discovery using combinatorial thin lm libraries based on WO3. at 94.4 at.% Ti. One promising candidate to be mixed with WO3 is another Screening of various dopants via high-throughput combina- α cheap and abundant oxide, -Fe2O3, to the end of extending the torial methods revealed also that Ni and Co are suitable [96] light absorption width and simultaneously decreasing the elements in order to increase the photocurrent response. electron–hole recombination rate. Studies on NiO-WO3 co-evaporated libraries showed that at [27] Kollender et al. investigated a wide range WO3-Fe2O3 6.2 at.% Ni, outstanding values of photocurrent density of 2 [24] combinatorial thin film library deposited by thermal co-evaporation 2.5 mA cm were achieved (Figure 11e), and, a more

Figure 11. Photoelectrochemical‐scanning droplet cell microscopy (PE‐SDCM) setups (a) [Reprinted from[24], Copyright (2014) with permission from Elsevier] and (b) [Reprinted with permission from[27]. Copyright (2013) American Chemical Society]. Selected results obtained when scanning different combinatorial thin film libraries from WO3 mixed with Fe (c and d) [Reprinted with permission from[27]. Copyright (2013) American Chemical Society], Ni (e) [Republished with permission of Electrochemical Society, Inc, from[28], Copyright (2015), permission conveyed through Copyright Clearance Center, Inc.] and Co (f) [Reprinted from[24], Copyright (2014) with permission from Elsevier].

Phys. Status Solidi A 2019, 1900047 1900047 (10 of 16) © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. a solidi status physica www.advancedsciencenews.com www.pss-a.com detailed electrochemical investigation on the same system the CO production. The correlation between the electrochromic correlated the changes of the semiconducting properties within effect observed on the WO3 electrodes and CO2 reduction was for the same compositional region to the photoelectrochemical the first time reported. [97] behavior. Libraries of CoO-WO3 deposited by thermal co- evaporation and investigated using the PE-SDCM showed in the compositional range of 7–15 at.% Co a remarkable 5.3. Antibacterial Properties photocurrent with values higher than 110 μAcm 2 (Figure 11f), which was attributed to a complementary contribution A newly emerging field of application involving tungsten oxide- from an enhanced electrocatalytic activity for oxygen evolution based compounds is as antimicrobial agents in healthcare on the surface and increased bulk radiation absorption.[28] industry. There are a few different mechanisms through which Another critical problem that nowadays the environment and tungsten oxides acts. One of them is related to the bandgap value humankind are facing is the constantly increasing water and band edges positions which allow for carrier generation pollution. Industrial, agricultural, and household pollution have under light irradiation. The photogenerated electrons are as a result wastewater production containing pesticides, dyes, transferred to the conduction band and, under favorable organic and inorganic compounds, and heavy metals.[98] conditions may oxidize the molecular oxygen dissolved into Consequently, the degradation of toxic compounds is of utmost the environmental humidity leading to the formation of • importance, and it triggered the search of materials having the superoxide anion radical ( O2 ). Also, the photogenerated hole ability to decontaminate the wastewater at a low-cost, and in an can react with the H2O molecule leading to the formation of • eco-friendly manner. hydroxyl radicals ( OH). These species, known as reactive As already mentioned, WO3 absorbs only a small part of the oxygen species (ROS), are responsible for microorganisms visible spectrum, hence the increased effort to expand its inactivation via oxidative stress inflicted upon the cell’s absorption to wavelengths higher than 400 nm for photo- membrane.[102,103] Most of the research studies have been catalytic effects under visible light. However, for the particular performed with powders, rather than coatings. Gondal et al.[104] application of pollutants degradation, the irradiation using the synthesized nano-WO3 with different values of the bandgap UV part of the spectrum can be employed, since the water (2.61 and 2.71 eV) and under laser irradiation they achieved transmits most of the UV range and UV lamps are cheap water disinfection from Escherichia coli (E. coli). The same group nowadays. Tungsten trioxide in pure form or doped with other performed a similar study with nano-tungsten oxide loaded with elements can be used as a photocatalyst for decomposition of nano-palladium which showed increased water disinfection pollutants (Reactions 8 and 9) under light irradiation. Oxidation capability against E. coli under laser irradiation. This effect was of water/OH molecules by the holes present in the VB leads to attributed to a blue shift in the Eg from 2.71 for the undoped WO3 • the formation of hydroxyl radicals ( OH), whereas the electrons to 3.5 eV for the doped one.[43] Another study, this time involving react to the dissolved O2 resulting in the generation of coatings, used tungsten oxide nano- and microrods grown on • superoxide radicals ( O2 ). Both these species are very active a W foil as antibacterial material. The tests were conducted by toward degradation of pollutants. For example, studies on exposing to visible light irradiation the coatings with the E. coli or fi degradation of phenol by electrodeposited WO3 lms revealed by holding them in the dark. The mechanism was related to the relatively high decomposition rates in chloride-containing acid formation of photogenerated carriers, change in the morphology [64] solutions. Another study showed the ability of WO3 caused by the different heat treatments, as well as to the amount deposited by spray pyrolysis to act as a photoelectrocatalyst of oxygen from O-H bonds and H2O together with an oxidation for the degradation of methyl blue (MB) dye with a 98.56% number of W lower than 6þ.[105] In order to shift the light degradation in 2 h under visible light irradiation.[99] Successful absorption in the visible region, photocatalysts of more complex decomposition of the Orange II dye by electrodeposited WO3 systems utilizing tungstic acid and Ag/AgBr were synthesized films via a photoelectrochemical process in a potential range and tested against E. coli. The composite formed of Ag/AgBr fl from 1.0 to 1.2 V in NaCl electrolyte and under UV-irradiation nanoparticles on the surface of WO3 H2O akes showed good was also demonstrated.[100] photocatalytic activity under visible light irradiation for Carbon dioxide capture and conversion to fuels is a very hot inactivation of E. coli due to plasmon resonance effects of topic due to the increase awareness toward global warming and metallic Ag on AgBr.[106] forecasted energy crises. Recently, a new application of WO3 for A newly studied property of WO3 which triggers antimicrobial [91] catalytic conversion of CO2 emerged. Wang et al. developed effects is its ability to absorb near-infrared (NIR) radiation. This ordered-mesoporous WO3x structures for photocatalytic con- feature was used to develop recyclable photothermal nano- version of CO2 to CH4. The process is based on the presence of particles that have antibacterial properties against both E.coli and oxygen vacancies together with light and thermal irradiation. Streptococcus aureus (S. aureus).[107] The authors synthesized Furthermore, by an elevated temperature photothermal cou- Fe3O4 and WO3 immobilized in exfoliated montmorillonite pling, reaction of CH4 with O2 may be promoted leading to the (MMT) clay and treat them with the aforementioned two generation of CH3OH as hydrocarbon fuel. An interesting bacterial strains in various concentrations, followed by irradia- approach has been pursued by Mendieta-Reyes et al.[101] tion with an NIR laser working at 808 nm. The antibacterial consisting of fabrication of nanostructured WO3 electrodes mechanism was explained based on the cells lysis caused by the and the study of CO2 reduction in both humid and dry heat release in a very short period of time (2 min). Based on this acetonitrile solution. This study revealed that the presence of hyperthermia effect, the same group developed a polydimethyl- water traces was critical to the formation of formate along with siloxane (PDMS) continuous flow microreactor with tungsten

[108] bronze Cs0.33WO3 nanoparticles as coatings. Results showed oxide enrichment which was held responsible for the local fl fi very good inactivation toward both E. coli and S. aureus at a ow acidi cation (Figure 12), together with the presence of W18O49 • rate of 50 μl min 1 in the reactor. Magneli phase, which generates OH free radicals under visible A different antimicrobial mechanism is connecting the light. Recently, an antimicrobial study was conducted on release of protons in the humid neighboring environment powders from the Mo-W-O system having a systematic variation under specific pH conditions.[109] This is the so-called medium of Mo/W ratio. The authors demonstrated experimentally that fi [110] fi acidi cation and it is best known to occur in MoO3. The the self-acidi cation properties are responsible for the growth acidification (local drop of pH) is the result of hydroxonium ions inhibition of E. coli. The presence of W was crucial to decrease þ (H3O ) release concurrent with the formation of tungstate the solubility of the oxides, as well as for the formation of 2 [1] (WO4 ) species. Surface properties of MoO3,WO3 and ZnO Mo0.6W0.4O3 mixed oxide, which also showed antimicrobial embedded into polymers (polypropylene and thermoplastic properties. These compounds are envisaged to be implemented polyurethane) were investigated at nano-scale by different as coatings on touch surfaces used in healthcare facilities.[112] scanning probe microscopic techniques in order to clarify the Another approach to prevent the spreading of bacterial antimicrobial activity.[111] No biofilm formation was observed on infections is to design materials that hinder biofilm formation by the active surfaces and large variations of the surface potential reducing the microorganism adhesion. In a recent study, and capacitance gradient, as well as high surface roughness as electrodeposition of nanoporous hydrated tungsten oxide compared with non-active surfaces were found. A thin film hierarchical films was utilized to create mechanically resistant combinatorial study on antibacterial properties of the Mo-W-O anti-fouling steel surfaces.[113] The as-deposited coating is system was performed by Mardare and Hassel.[22] This initially hydrophilic and by surface modification with perfluor- investigation screened compositional regions (91–74 at.% Mo, oalkyl-bearing phosphate can be converted to superhydrophobic, and complementary 9–26 at.% W, as well as 55–44 at.% Mo, and and even to omniphobic-slippery liquid-infused porous surface complementary 45–56 at.% W) where antibacterial properties by the use of perfluoropolyether lubricants. Very good repellent against E. coli were found. The antibacterial activity was related to properties were proven for the coated scalpel blades, with no the surface structuring of large grains showing molybdenum blood adhesion being observed. A decreased number of E. coli

Figure 12. Agar petri dishes showing photographs of E.coli colonies developed as a function of compositional region of the thin film combinatorial library, together with atomic force microscopy images of the oxide film surface, and the crystalline phases found by X-ray diffractometry. Reproduced with permission.[22] Copyright 2014, American Chemical Society.

Phys. Status Solidi A 2019, 1900047 1900047 (12 of 16) © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. a solidi status physica www.advancedsciencenews.com www.pss-a.com colonies as compared to the control sample was also promoted and additionally, anti-biofouling properties against Chlamydo- E þ E RT þ H ¼ þ O þ ln½H monas reinhardtii were demonstrated. H2 H H2 F

E þ E RT 5.4. pH-Sensing H ¼ þ O þ ðÞ2:303 pH ) H2 H H2 F The pH sensitivity of tungsten oxide was one of the ﬁrst [2] discovered and studied properties of this material. The pH E þ E þ H ¼ þ O 0:05916pH sensing is based on the intercalation of H ions into the ReO3- H2 H H2 type structure of the WO3 (Figure 13a). This type of structure can þ where E is the measured potential, E0 corresponds to the also be seen as a defect perovskite, with the H occupying the A þ standard potential, R is the universal gas constant (8.314 J K 1- site (ABO3-structure). The intercalation of H leads to the mol 1), T is the temperature (K), z is the number of electrons formation of tungsten bronze (HxWO3) via the same chemical reaction exemplary shown for the electrochromic effect (Reac- transferred in the reaction, and F is the Faraday constant 1 tion 4). The mechanism for pH sensing is related to the (96485.34C mol ). þ simultaneous introduction of both cations (H ) and electrons This property of WO3 was widely explored for developing pH sensors. For example, in our group, different approaches have into the WO3 structure, with the resulting HxWO3 phase having electric conductivity higher than WO . been undertaken to fabricate miniaturized electrodes based 3 ﬁ The dependence between the electrode potential and the pH on W nanowires. Directional solidi cation of a NiAl-W eutectic value is based on the Nernst equation. If this dependence is alloy led to the formation of W nanowires with the diameter of linear, from the linear regression the sensitivity of the sensor in 200 nm, which were employed to fabricate a pH sensor via [114] mV pH 1 can be obtained. The theoretical sensitivity is given by formation of an oxide layer on the surface (Figure 13b). [115] the slope and the maximum value is 59.1 mV pH 1 (at Drensler et al. further studied the behavior of macro-, micro- standard conditions of temperature and pressure) demonstrated and nano-electrodes based on W/WO3. These developments below.[114,115] enabled the possibility of measuring the pH in very limited ! volumes, down to nanoliter using the nano-electrodes, whereas ½þ 2 the micro-electrodes could for instance be employed for E þ E RT H H ¼ þ O þ ln ) measuring the pH values inside cavities or bubbles. A solid- H2 H H2 zF p H2 state composite pH sensor was fabricated by Zhang and Xu and

þ Figure 13. a) 3D cell projection of the monoclinic γ-WO3 showing the interstitial intercalation of H responsible for the pH sensing properties; the red small spheres represent the oxygen atoms and the black large spheres inside the polyhedra represent the tungsten atoms, and (b) nano-pH sensor based on an oxidized W nanowire obtained by directional solidification of a NiAl-W eutectic alloy. Reproduced with permission.[114] Copyright 2008, Elsevier B.V.

it was consisting of WO3 nanoparticles deposited by sputtering Keywords on vertically aligned multiwalled carbon nanotubes. This type of applications, coatings, properties, tungsten oxide sensor showed good stability in a pH range from 2 to 12, as well as selectivity and fast response time.[116] More recent research showed the possibility of using sputter deposited columnar Received: January 16, 2019 nanoporous WO for fabrication of a full solid-state pH sensor Revised: March 14, 2019 3 Published online: based on commercial screen printed electrodes which showed good stability and Nernstian behavior in a pH range from 1 to 12.[117] Development of flexible pH sensors for specific – applications has recently become of great importance.[118 120] [1] E. Deltombe, N. de Zoubov, M. Pourbaix, in Atlas of Electrochemical Equilibria in Aqueous Solution (Ed: M. Pourbaix), National Recently, a tungsten oxide conformable sensor based on coatings fl Association of Corrosion Engineers, Houston, TX, USA 1974, Ch. 4. from WO3 nanoparticles electrodeposited on exible substrates [2] J. R. Baylis, Ind. Eng. Chem. 1923, 15, 852. was developed. The authors showed the possibility to achieve in [3] A. Magneli, Acta Crystallogr. 1953, 6, 495. such configuration a near-Nernstian response of 56.7 [4] A. Magneli, Acta Chem. Scand. 1949, 3, 88. 1 [121] 1.3 mV pH a pH range of 5–9. [5] A. Magneli, Nature 1950, 165, 356. [6] A. Magneli, G. Andersson, B. Blomberg, L. Kihlborg, Anal. Chem. 1952, 24, 2000. 6. Conclusion [7] K. Viswanathan, K. Brandt, E. Salje, J. Solid State Chem. 1981, 36, 45. This feature article gives an up to date overview of the [8] D. B. Migas, V. L. Shaposhnikov, V. N. Rodin, V. E. Borisenko, development of several applications based on WO3 coatings. J. Appl. Phys. 2010, 108, 093713. 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Quinn, Solid State Commun. 1976, thin film and coatings technology, with details for tungsten oxide 19, 1011. deposition methods being reviewed. [16] Z. F. Huang, J. Song, L. Pan, X. Zhang, L. Wang, J. J. Zou, Adv. Mater. In the light of the focus of this paper, an overview of the main 2015, 27, 5309. applications of tungsten oxide in the form of coatings is [17] H. Zheng, J. Z. Ou, M. S. Strano, R. B. Kaner, A. Mitchell, K. Kalantar-zadeh, Adv. Funct. Mater 2011, 21, 2175. presented. Electrochromic windows using WO3 structural properties, which enabled its use as a successful electrochromic [18] A. Mozalev, V. Khatko, C. Bittencourt, A. W. Hassel, G. Gorokh, E. Llobet, X. Correig, Chem. Mater. 2008, 20, 6482. material, are already a huge marketing success. Sensors for pH [19] A. W. Hassel, S. Milenkovic, A. J. Smith, Phys. Status Solidi A 2010, sensing can be customized to work down to nanoliter volumes. 863, 858. The outstanding chemical stability combined with the semicon- [20] V. Cimalla, C.-C. Röhlig, J. Pezoldt, M. Niebelschütz, O. Ambacher, ducting properties facilitate its use as photoanode for water K. Brückner, M. Hein, J. Weber, S. Milenkovic, A. J. Smith, splitting and for water purification, or in healthcare application A. W. Hassel, J. Nanomater. 2008, 2008, 638947. due to its antimicrobial properties, especially when some tuning [21] S. K. Deb, Sol. Energy Mater. Sol. Cells 2008, 92, 245. is done by doping or mixing with other oxides. New various fields [22] C. C. Mardare, A. W. Hassel, ACS Comb. Sci. 2014, 16, 631. [23] C. Khare, K. Sliozberg, R. Meyer, A. Savan, W. Schuhmann, of applications are fast emerging, with WO3 being employed for A. Ludwig, Int. J. Hydrogen Energy 2013, 38, 15954. CO2 conversion to hydrocarbon fuels or as anti-fouling coating. [24] J. P. Kollender, B. Gallistl, A. I. Mardare, A. W. Hassel, Electrochim. Acta 2014, 140, 275. [25] R. Meyer, K. Sliozberg, C. Khare, W. Schuhmann, A. Ludwig, Acknowledgments ChemSusChem 2015, 8, 1279. [26] K. Sliozberg, D. Schäfer, T. Erichsen, R. Meyer, C. Khare, A. Ludwig, The financial support by the Austrian Federal Ministry for Digital and W. Schuhmann, ChemSusChem 2015, 8, 1270. Economic Affairs and the National Foundation for Research, Technology 2013 and Development in the frame of the Christian Doppler Laboratory for [27] J. P. Kollender, A. I. Mardare, A. W. Hassel, ACS Comb. Sci. , 15, Combinatorial Oxide Chemistry (COMBOX) is gratefully acknowledged. 601. The authors gratefully acknowledge financial support of the NO€ [28] J. P. Kollender, A. I. Mardare, A. W. Hassel, J. Electrochem. Soc. 2015, Forschungs- und Bildungsges.m.b.H. (NFB) and the provincial govern- 162, H187. ment of Lower Austria through the Life Science Calls (Project ID: LSC15- [29] S.-H. Lee, H. M. Cheong, J.-G. Zhang, A. Mascarenhas, 026). D. K. Benson, S. K. Deb, Appl. Phys. Lett. 1999, 74, 242. [30] E. K. H. Salje, S. Rehmann, F. Pobell, D. 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