Zheng, Adv.funct.mater.2011, Nano Tungston Oxide - Properties & Applications

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Haidong Zheng , * Jian Zhen Ou , Michael S. Strano , Richard B. Kaner , Arnan Mitchell , and Kourosh Kalantar-zadeh *

Nanostructured Tungsten Oxide – Properties, Synthesis, and Applications

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Metal oxides are the key ingredients for the development of many advanced functional materials and smart devices. Nanostructuring has emerged as one of the best tools to unlock their full potential. Tungsten oxides (WO x ) are unique materials that have been rigorously studied for their chromism, pho-tocatalysis, and sensing capabilities. However, they exhibit further important properties and functionalities that have received relatively little attention in the past. This Feature Article presents a general review of nanostructured WO x , their properties, methods of synthesis, and a description of how they can be used in unique ways for different applications.

1. Introduction

Tungsten oxide (WO x ) is a transition metal oxide with wide-ranging applications. Interest in WO x can be dated back to the 17 th century when the properties of LiWO 3 and the techniques for the synthesis of WO 3 and NaWO 3 were fi rst studied. [ 1 ] More recently, renewed research interest in WO x was sparked by the discovery of its electrochromic (EC) effect. [ 2 ]

With the advent of nanotechnologies, the synthesis and analysis of WO x nanostructures has become increasingly prominent. Nanostructuring of WO x can enhance the perform-ance of this important functional material and provides it with unique properties that do not exist in its bulk form. Exceptional qualities of nanostructured WO x compared to the bulk material include: i) increased surface-to-volume ratio, which provides more surface area for both chemical and physical interactions; ii) signifi cantly altered surface energies that allow tuning and engineering of the material’s properties, as atomic species near the surface have different bond structures than those embedded in the bulk; and iii) quantum confi nement effects, due to the

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Funct. Mater. 2011, 21, 2175–2196

DOI: 10.1002/adfm.201002477

H. Zheng , J. Z. Ou , Prof. A. Mitchell , Prof. K. Kalantar-zadeh School of Electrical and Computer EngineeringRMIT UniversityMelbourne, Victoria 3001, Australia E-mail: [email protected]; [email protected] Prof. M. S. Strano Department of Chemical EngineeringMassachusetts Institute of TechnologyCambridge, MA 02139, USA Prof. R. B. Kaner Department of Chemistry and BiochemistryUniversity of CaliforniaLos Angeles, CA 90024, USA

inherently small size of nanostructured materials, that signifi cantly infl uences charge transport, electronic band structure and optical properties.

Nanostructured WO x is exceptionally versatile and offers unique characteristics. It has become one of the most investigated functional metal oxides impacting many research fi elds ranging from condensed-matter physics to solid-state chemistry.

WO x has exceptional chromic proper-ties and thin fi lms made of nanostructured WO x have been increasingly investigated and applied for the development of elec-

trochromic devices. In comparison to many other functional metal oxide nanostructures, such as TiO 2 , ZnO, NiO, and their substoichiometric forms, the study of WO x chromic properties is much more advanced. [ 2 ] Nanostructured WO x is also a well-studied material for photocatalysis and sensing. [ 1 ] While metal oxides such as SnO 2 , TiO 2 , ZnO, as well as their various sub-stoichiometric forms, are commonly used in photocatalytic sys-tems and sensors, the importance of WO x is growing equally in these fi elds. [ 1 ] For solar-cell research, nanostructured WO x in its fully oxidized form, WO 3 , has received relatively little attention compared to materials such as TiO 2 and ZnO. However, the application of WO 3 in solar cells should be encouraged, since it offers similar functional properties to TiO 2 and ZnO, which are now widely used in the relevant industries.

In this Feature Article, we present a general, yet compre-hensive, review on WO x . In particular, we focus on the various nanostructured forms of WO x that have been presented in the literature to date. We describe the fundamental properties of nanostructured WO x and compare the nanomaterials with the bulk form. We summarize the different reported methods for the synthesis of nanostructured WO x and divide these syn-thesis methods into two major categories: vapor phase and liquid phase. Finally, we present a selection of the more popular and interesting applications that exploit WO x and illustrate the enhancements made possible by using nanostructures of this material.

2. Fundamental Properties

This section focuses on the fundamental properties of nano-structured WO x and start with its various crystal structures and the conditions for phase transitions between these structures. The structures of nonstoichiometric WO x and WO 3 hydrates

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Haidong Zheng is cur-rently in the fi nal stage of his Ph.D. study in the School of Electrical and Computer Engineering, RMIT Uni-versity, Australia. Prior to his Ph.D. candidature, he received a bachelor degree of electronic engineering from the same school in 2006. Haidong’s research in-terest includes nanostructure

synthesis for TiO2 and WO3 and their use in dye-sensitized solar cells as well as chromic devices.

Kourosh Kalantar-zadeh is an Associate Professor at RMIT University, Australia. He received his B.Sc. (1993) and M.Sc. (1997) degree from Sharif University of Technology, Iran, and Tehran University, Iran, respectively, and a Ph.D. at RMIT University, Australia (2001). His research interests include chemical and biochem-ical sensors, nanotechnology,

microsystems, materials sciences, electronic circuits, and microfl uidics.

are also discussed. Further fundamental properties including optical, electrical, and photocatalytic characteristics, which are presented next. This section also descripbe the thermoelectric and ferroelectric properties as well as doping effects.

2.1. Crystal Structures and Phase Transition

WO 3 crystals are generally formed by corner and edge sharing of WO 6 octahedra. The following phases are obtained by corner sharing: monoclinic II ( ε -WO 3 ), triclinic ( δ -WO 3 ), monoclinic I ( γ -WO 3 ), orthorhombic ( β -WO 3 ), tetragonal ( α -WO 3 ), and cubic WO 3. However, cubic WO 3 is not commonly observed experi-mentally. The detail of the polyhedral representations of these six structures is shown in Figure 1 . [ 3 ] The phase classifi cation is based on the tilting angles and rotation direction of WO 6 octa-hedra with reference to the “ideal” cubic structure (ReO 3 type). Lattice constant data for WO 3 crystal phases are presented in Table 1 .

Like other metal oxides, WO 3 crystal phase transitions can take place during annealing and cooling. It has been widely reported that for WO 3 , in bulk form, phase transformation occurs in the following sequence: [ 6 , 7 ] monoclinic II ( ε -WO 3 , < − 43 ° C) → triclinic ( δ -WO 3 , − 43 ° C to 17 ° C) → monoclinic I ( γ -WO 3 , 17 ° C to 330 ° C) → orthorhombic ( β -WO 3 , 330 ° C to 740 ° C) → tetragonal ( α -WO 3 , > 740 ° C)

The above phase transitions of WO 3 has been reported to be partially reversible. At room temperature, monoclinic I ( γ -WO 3 ) has been reported as the most stable phase, with triclinic ( δ -WO 3 ) also being observed. [ 6 ] When annealed at high tempera-ture, WO 3 transforms to other crystal phases (usually β -WO 3 and α -WO 3 ). However, WO 3 is generally unable to retain these alternate phases when it is returned to room temperature. The monoclinic II phase ( ε -WO 3 ), is only stable at subzero temper-atures, and is thus rarely encountered outside the laboratory.

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Figure 1 . Tilt patterns and stability temperature domains of the different poReproduce with permission. [ 3 ] Copyright 2000, International Union of Crysta

Hence, unless specifi ed, the term “monoclinic” will be used to refer to the monoclinic I ( γ -WO 3 ) phase throughout the remaining text.

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lymorphs of WO 3 . llography.

In addition to the aforementioned crystal phases, another possible stable phase for WO 3 is hexagonal (h-WO 3 ). Observation of this phase was fi rst reported by Gerand et al. in 1979, [ 8 ] and was originally obtained from the slow dehydration of tungstite. Figure 2 presents a diagram of the crystal structure of h-WO 3 . The crystal is again obtained from WO 6 octahedra, but with the form of three- and six- membered rings in the a b -plane. These three- and six-membered rings result in the appearance of trigonal cavities and hex-agonal windows, respectively. [ 9 ] In the c -axis, these octahedra stack by sharing the axial oxygen and form 4-coordinated square win-dows. However, this hexagonal crystal phase is metastable and reported to be transformed into a monoclinic structure when annealed at temperatures exceeding 400 ° C. The lattice constants reported for h-WO 3 are a = 7.298 Å and c = 3.899 Å. [ 8 ]

The phase transition behavior in nano-structured WO x can be quite complex, as it mainly depends on the material’s

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Table 1. Lattice constant data for different WO 3 crystal phases.

lattice constant (Å)

ε -WO 3 [ 4 ] δ -WO 3 [ 4 ] γ -WO 3 [ 4 ] β -WO 3 [ 5 ] α -O 3 [ 5 ] “ideal” cubic-WO 3 [ 2 ]

a 7.378 7.309 7.306 7.384 5.25 3.84

b 7.378 7.522 7.540 7.512 N/A N/A

c 7.664 7.686 7.692 3.846 3.91 N/A

morphology, which is greatly affected by the nanostructure syn-thesis process and the initial precursors used. Proposed by the Gibbs–Thomson expression, [ 10 ] the reduction of the size of WO x crystallites enhances the surface energy of the system, and this enhanced surface energy decreases melting and sublimation temperatures. Therefore, generally lower annealing tempera-tures, compared to the ones mentioned for the bulk WO x , are needed to induce the crystal phase transitions in the nanostruc-tured form. [ 11 ]

Boulova et al. [ 11 ] used in situ Raman spectroscopy to observe phase transitions in WO 3 nanoparticles with diameters in the range of 2 − 500 nm. It was found that samples with average diameter of 60 nm began the transformation from γ -WO 3 to β -WO 3 at ≈ 270 ° C and then to α -WO 3 at ≈ 670 ° C. These phase transition temperatures were both lower than that of bulk WO 3 . In their work, a 35 nm sample, which was initially in the β form, started to transform to α -WO 3 below 530 ° C. However, a 16 nm sample underwent a phase transition from an initial pseudo β -WO 3 structure to α -WO 3 form at 430 ° C, which is signifi cantly lower than that of bulk WO 3 as well as the WO 3 nanoparticles with larger dimensions. Similar fi ndings were also reported by Lu et al. [ 12 ] using in situ Raman spectroscopy. They reported that monoclinic I ( γ -WO 3 ) nanowires with an average diameter of 40–80 nm and a length of 1 μ m, started to transform revers-ibly into β -WO 3 at 230 ° C instead of ≈ 330 ° C for bulk WO 3 .

The morphology of the nanostructures can have a signifi -cant effect on obtaining stable phases at room temperature.

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Figure 2 . a) The structure of h-WO 3 is shown with the c -axis perpen-dicular to the plane. b) The structure of h-WO 3 with the c- axis parallel to the plane. Reproduce with permission. [ 9 ] Copyright 2009, American Chemical Society.

We recently synthesized nanoplatelet WO 3 with thicknesses of 20–60 nm via electrochemical anodization at 50 ° C, and the as-prepared products were post-annealed at 400 ° C. [ 13 ] Room-temperature material characterization showed that the majority of the crystal phase was orthorhombic instead of the expected monoclinic phase. In some recent literature [ 14 , 15 ] it was also reported that nanostructured WO 3 with an orthorhombic crystal phase (obtained by post-annealing) can retain its phase stability at room temperature.

2.2. Structures of Nonstoichiometric WO x

WO 3 is a transition metal oxide made up of perovskite units, which is well-known for its nonstoichiometric properties, as the lattice can withstand a considerable amount of oxygen defi ciency. [ 16 ] Only a partial loss of the WO 3 oxygen content is needed to affect its electronic band structure and increase its conductivity by a large amount. [ 17 ] However, the reduction of WO 3 is usually accompanied by structural changes. [ 16 ] Some of the better known nonstoichiometric WO x compositions are W 20 O 58 , W 18 O 49 and W 24 O 68 . Such oxides are formed by corner-sharing WO 6 , which alternate with octahedra that are partially established by edge-sharing. [ 16 ] Elimination of oxygen occurs according to the crystal-shear (CS) mechanism. [ 16 ] As the x value in WO x decreases, groups of edge-sharing WO 6 octahedra form pockets of shear planes. For x values closer to 3, these shear planes are considered as extended defects, if they are isolated or disordered. With further reduction in x , the shear planes tend to interact with each other and align in parallel, fi lling the space between the planes with corner-sharing WO 6 . If these shear planes are equidistant, a crystalline phase with a defi ned structure arises that belongs to the (103) CS series. For x smaller than or equal to 2.87 (i.e., W 18 O 49 and W 24 O 68 ), the above mentioned structures become unstable and further restructuring takes place involving the formation of pentagonal columns (PCs) parallel to the monoclinic b axis, which are either single or paired by edge-sharing (PC–PC). [ 16 ]

2.3. WO 3 Hydrates

Investigations of WO 3 hydrates (WO 3 · nH 2 O) or “tungstic acids” are important as they are closely related to WO x . The importance of WO 3 · n H 2 O stems from the fact that generally in liquid-phase synthesis routes, WO 3 hydrates are fi rst produced and subsequently annealed to obtain the desired crystal phase of WO x . The reports on these hydrates can be traced back to almost a century ago, [ 18 , 19 ] with the four most studied classes presented as WO 3 · 2 H 2 O (dihydrate), WO 3 · H 2 O (mono-hydrate), WO 3 · 0.5 H 2 O (hemihydrate), and WO 3 · 0.33 H 2 O. The crystal structures of WO 3 hydrates are highly dependent on their water content. [ 19 ] WO 3 · 2 H 2 O possesses a layered structure, which is composed of WO 5 (OH) 2 single sheets in corner-sharing mode. The second water molecule of the dihy-drate is found to be positioned between the layers. WO 3 · H 2 O consists of highly distorted corner-sharing WO 5 (OH) 2 units, which are coordinated by fi ve oxygen atoms and one water mol-ecule. As for WO 3 · 0.5 H 2 O, its existence is less documented

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Figure 3 . a) Transmittance spectra of the RF-sputtered WO 3 thin fi lms at different substrate temperatures. b) T s – E g relationship of RF-sputtered WO 3 fi lms. Reproduce with permission. [ 23 ] Copyright 2010, American Institute of Physics.

but the crystal structure is believed to be of a cubic pyrochlore-type, where the water mol-ecules are present in a tunnel constructed by the six-membered WO 6 corner-sharing octahedra. [ 19 ] Finally, WO 3 · 0.33 H 2 O was found by Gerand et al. in 1981. [ 20 ] Recently, a study conducted by Zhou et al. [ 21 ] revealed that orthorhombic WO 3 · 0.33 H 2 O actually contains two types of corner-sharing WO 6 octahedra. The fi rst type is constructed by a central tungston atom that is surrounded by six oxygen atoms, while in the second type, two of the oxygen atoms are replaced by a shorter terminal W = O bond and a longer W–(OH) 2 bond, respectively. Ultimately, the WO 3 · 0.33 H 2 O lattice is formed by stacking
up layers consisting of these two structural units. [ 21 ]
2.4. Electronic Band Structure

WO 3 is a wide-bandgap n-type semiconductor, with an elec-tronic bandgap ( E g ), corresponding to the difference between the energy levels of the valence band, formed by fi lled O 2p orbitals and the conduction band formed by empty W 5d orbitals. [ 22 , 23 ] As mentioned earlier, the crystal phase of WO 3 transits in a sequence that is determined by the degree of dis-tortion from the ideal cubic phase, and in principle, this tran-sition is also accompanied by a change in E g , as the occupied levels of the W 5d states change. [ 22 ] Amorphous WO 3 , with the most distorted structure, normally possesses a relatively large E g on the order of ≈ 3.25 eV, [ 22 ] whereas monoclinic WO 3, in bulk form, has been reported to show a typical E g of ≈ 2.62 eV at room temperature. [ 22 ] Density of states (DOS) calculations for the cubic WO 3 band structure [ 24 ] can be used to precisely calculate the lattice constant (3.84 Å). However, these calcu-lations generally fail to estimate E g correctly. For instance, Granqvist et al. calculated E g as 0.6 eV, which is signifi cantly smaller than the experimental values. [ 24 ] Similar discrepancies are also reported for other crystal phases of WO 3 .

In nanostructured WO 3 , the bandgap generally increases with reducing grain size. [ 23 ] Experimentally, this is often observed as a blue shift of the optical absorption bandedge as the nanos-tructure dimensions are reduced. It is widely accepted that this observed blue shift can be attributed to the quantum confi ne-ment (QC) effect. [ 25 ] The QC effect is divided into two regimes, namely strong and weak. [ 25 ] The strong QC effect occurs when the size of the crystal is reduced to much smaller than the Bohr radius for the material ( ≈ 3 nm for WO 3 [ 26 ] ). This causes direct changes to the electron wavefunctions and hence signifi cantly alters the E g . The weak QC effect occurs when the crystal size is larger than the Bohr radius. This causes indirect perturba-tion of the electron wavefunction due to Coulomb effects and results in more subtle changes in the bandgap energy. [ 25 ] In a recent study conducted by Gullapalli et al., [ 23 ] different scales of nanotextured WO 3 thin fi lms were obtained by radio frequency (RF) sputtering, where the crystallite size is controlled by the substrate temperature ( T s ) during deposition. The crystallite sizes obtained ranged from 9 nm ( T s = 100 ° C) to 50 nm ( T s =


500 ° C). The observed optical transmission spectra and derived bandgap energy for each value of T s is presented in Figure 3 . As seen in Figure 3 a, reduction in T s , and hence reduction in crys-tallite size, results in a blue shift of the transmission spectrum corresponding to a widening of E g . For each of the γ -WO 3 crys-tallites obtained at 300–500 ° C, the observed bandgap energy is higher than E g = 2.62 eV of the typical bulk γ -WO 3 (Figure 3 b). Further, the progressive increase of this bandgap energy with reduced crystal dimensions suggests that these observations can be ascribed to the weak QC effect. However, it should be noted that phase transformations caused by the increasing T s can also contribute to the E g shift. [ 23 ]

2.5. Optical Properties and Chromism

The optical properties of WO 3 in the visible region are domi-nated by the absorption threshold, which is defi ned by the bandgap energy of the material. [ 27 ] The bandgap of nanostruc-tured WO 3 is blue shifted compared to the bulk form, with reported values ranging widely from E g = 2.60 to 3.25 eV. [ 28 ] Therefore, WO 3 (stoichiometric) is essentially transparent to most visible wavelengths with a slightly yellow tint for smaller bandgap samples, which absorb part of the blue spectrum. [ 27 ] For photon energies greater than the bandgap energy, the light absorption α can be approximated by the equation: [ 27 ]

g" ∝ (g − Eg)0 (1)

where ε is the photon energy and η = 2, indicating that for WO 3 indirect transitions are allowed. A typical optical transmission spectra for nanostructured WO 3 can be seen in Figure 3 a with a clear absorption edge evident at ultraviolet (UV) to blue wave-lengths. [ 23 ] The oxygen-defi cient WO x (nonstoichiometric) are mostly found to be light green, which is caused by an additional broad absorption peak in the red to infrared with a transmis-sion window remaining in the blue-green range. This additional absorption peak is due to the electron transfer from W 6 + to W 5 + . [ 4 ] It is also worth mentioning that the refractive index of WO 3 is large, in the range of 2–2.5, and hence it is important to consider this parameter in the design of WO 3 -based optical devices. [ 29 ]

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Figure 4 . Illustration of “generation of oxygen vacancies” mechanism for the gasochromic effect.

It is well-known that the WO 3 optical transmittance can be modulated, which means its physical color can revers-ibly change from the initial state to dark blue in the presence of external stimuli. The two most reported effects are color change in response to applied voltage (electrochromism) [ 30–33 ] and reducing gases (such as H 2 , gasochromism); [ 34–37 ] however, heat- or light-induced chromic effects of WO 3 have also been studied. [ 12 , 38 ]

The electrochemical reaction that results in the chromic response of WO 3 can be described by considering the injection of a quantity ( x ) of positive ions (M + ) and an equal quantity of electrons (e − ). Symbolically, this reaction can be represented as follows: [ 2 ]

WO3 + xM+ + xe− ↔ MxWO3 (2)

Typical ions can be M + = H + , Li + and Na + among others, and the quantity x becomes the stoichiometric parameter of the product and can vary between 0 and 1.

The electrochromic effect of WO x is strongly dependent on crystal stoichiometry, with signifi cantly different properties being reported for amorphous and crystalline fi lms. [ 39–41 ] For the amorphous case, the absorption peak observed in the vis-ible optical spectrum resulting from the injection of ions and electrons can be attributed to polaron absorption. In the amor-phous fi lm, inserted electrons are localized in the W 5 + sites and polarize their surroundings to induce lattice vibration. [ 42 , 43 ] The inserted cations (M + ) either lie in the centers of the perovskite units or are chemically bonded with the lattice oxygen and are thus spatially separated from the electrons. This spatial separa-tion creates a polaron. [ 44 ]

In the crystalline case, there is no spatial separation between the inserted cations and electrons, as the inserted electrons behave as Drude-model-like free electrons, which enter the extended states in the WO 3 band structure and undergo scat-tering by impurities. [ 24 ] This makes the material slightly metallic with a small increase in absorption across the spectrum and a slight increase in refl ection at infrared wavelengths. Therefore, the crystalline WO 3 fi lms tend to have a smaller degree of color change compared to their amorphous counterpart. [ 45 ]

In contrast to electrochromism, there are still many unknown issues regarding the fundamental understanding of WO 3 gaso-chromism. Two models are widely accepted for describing the gasochromic effect in WO 3 , which both agree that the coloration of the WO 3 fi lm is caused by the transition of the W valence state from 6 + to 5 + . One model is called the “double injection of ions”, [ 36 ] which is the same as the electrochromic mecha-nism. The other is the “generation of oxygen vacancies”, [ 35 ] which is illustrated in Figure 4 . For a reducing gas such as H 2 , this mechanism can be briefl y described as follows (generally a catalytic material such as Pt or Au is required to enhance the effect): i) adsorption and dissociation of H 2 onto the catalyst layer; ii) diffusion of H atoms along surfaces; iii) formation of an oxygen vacancy and H 2 O; iv) diffusion of oxygen vacancies into WO 3 ; and v) eventually escape of H 2 O.

This model argues that the fi nal gasochromic products are WO 3 − x together with surface water molecules instead of the hydrogen tungsten bronze (H x WO 3 ), which is suggested by the “double injection of ions”. Interestingly, both models are backed by experimental results to some extent. [ 35 , 36 ]

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Chromic effi ciency is often observed to have a strong dependency on surface morphology. [ 46 ] Nanostructured WO 3 fi lms improve the surface-to-volume ratio, allowing more sur-face area available for reactions, hence producing superior optical modulation as compared to the compact fi lms. [ 31 , 32 ] Additionally, the open structure enhances species diffu-sion within the system, which greatly improves the chromic response time. [ 47 ] The intercalation behavior of nanostruc-tured WO 3 can be quite different from its bulk counterparts. We have recently shown that thin tungstite platelets on the order of several unit cells, greatly enhance the propen-sity for lithium intercalation compared to that of thick WO 3 fi lms. [ 48 ] The effect of morphology should be considered with care, as it has been reported that high surface irregulari-ties and the effect of the size of nanostructured WO 3 might conversely suppress the optical modulation during chromic interactions. [ 49 ]

Chen et al. [ 49 ] studied the electrochromic properties and relative near-infrared absorption mechanism of thermally evaporated WO 3 nanowire fi lms with an average diameters of 50–200 nm. They found that the colored fi lm had a relatively high transmittance compared to bulk material in the wave-length range higher than 800 nm. In addition, an extra absorp-tion band (when compared to bulk WO 3 ), which was fi t to a cat-ion-polaron absorption, was found in the near-infrared region. The appearance of this band suggested the existence of the interactions between inserted cations and compensating elec-trons, which remain spatially separated forming a polaron and modify host–lattice vibration energies.

Luo et al. [ 37 ] conducted Raman spectroscopic studies on gasochromism of monoclinic WO 3 nanowires with an average diameter of 50 nm to prove the coexistence of oxygen vacancies together with water molecules on the nanowires after exposure to pure H 2 . Based on their observations, they proposed a modi-fi ed mechanism for low-dimensional WO 3 interactions with H 2 similar to the “generation of oxygen vacancies” model, which describes the coloration without the presence of oxygen vacancy diffusion.

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Figure 5 . Carrier mobility, concentration, and resistivity of WO 3 fi lms grown at different substrate temperatures. Reproduce with permission. [ 52 ] Copyright 2008, Elsevier.

2.6. Electrical Conductivity

For n-type metal oxide semiconductors, electrical conduction relies on a signifi cant concentration of free electrons being present in their conduction bands. The free-electron concentra-tion in such materials is mainly determined by the concentra-tion of stoichiometric defects, such as oxygen vacancies. [ 50 ] The electrical conductivity of single-crystal WO 3 ranges from 10 to 10 − 4 S cm − 1 depending on the stoichiometry. [ 17 ] In addition, structural factors such as grain size, grain boundary, fi lm thick-ness, specifi c phase and dopants (if any) also have a great infl u-ence on the material’s conductivity. [ 51 ] Therefore, the electrical properties of WO 3 are strongly dependent on the synthesis

Figure 6 . CB and VB energy levels of a number of semiconductors. Reproduce with permis-sion. [ 58 ] Copyright 2001, Nature Publishing Group.

techniques and the growth conditions. For example, it has been reported that a relatively high carrier concentration (5 × 10 19 cm − 3 ) and electron mobility (6.5 cm 2 V − 1 s − 1 ) have been observed for WO 3 fi lms synthesized using an elevated substrate temperature during sputtering and thermal evaporation depositions. [ 52 ] These results are presented in Figure 5 . A high substrate temperature tends to detach oxygen ions under vacuum conditions, allowing more free electrons for conduction. It has been suggested that by confi ning the free carriers’ movement to within well oriented one- or two-dimensional crystal structures, with smooth boundaries, the carrier mobility increases as the scattering effects are reduced. This increase in mobility has been successfully shown in other metal oxides such as ZnO nanorods. [ 53 ] For WO 3 such reports are rare. However, recently, the carrier mobility of a single W 18 O 49 nanowire was measured by Rui et al. [ 54 ] to be as high as 40 cm 2 V − 1 s − 1 .

Increasing the roughness of the structure, however, increases resistivity. The conduc-tivity was reported to decrease within the WO 3 nanoparticles, due to an increasing volume of the grain boundaries, which contributes

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to more trapping and scattering of free charge carriers. [ 55 ] If the size of the crystallites is smaller than the electron mean free path, grain boundary scattering dominates and hence the electrical conductivity decreases. In addition, lattice strain and crystal distortions can also affect the motion of charge, causing a decrease in conductivity. [ 56 , 57 ]

2.7. Photocatalytic Properties

In principal, photocatalytic activity is achieved by illuminating a semiconductor material in a solvent (usually water) to photons that have energies equal to or larger than the semiconductors’ bandgap. This creates electron–hole pairs, which generates free radicals (e.g., OH · ) to undergo further reactions.

As a photocatalytic material, stoichiometric WO 3 has a con-duction band (CB) edge, which is positioned slightly more posi-tive (versus NHE (normal hydrogen electrode)) than the H 2 /H 2 O reduction potential ( Figure 6 ), [ 58 ] and a valence band (VB) edge much more positive than the H 2 O/O 2 oxidation potential, which makes WO 3 capable of effi ciently photo-oxidising a wide range of organic compounds [ 59 , 60 ] such as textile dyes [ 61–65 ] and bacterial pollutants. [ 64 , 65 ] When compared to TiO 2 , which is the most studied photocatalytic material so far, another advantage of WO 3 is that it can be irradiated by the blue region of the visible solar spectrum, as its bandgap energy is in the ≈ 2.6 to ≈ 3.2 eV range. Furthermore, WO 3 has a remarkable stability in acidic environments, making it a promising candidate for treat-ment of water contaminated by organic acids. [ 66 ]

Nanostructuring of WO 3 can greatly enhance its photo-catalytic capability. The large surface-to-volume ratio, which

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is provided by the nanostructures, signifi cantly increases the effective surface area available for photocatalytic reactions. Nev-ertheless, one should be cautious when comparing the photo-catalytic performances reported for WO 3 nanoparticles of dif-ferent sizes, obtained by calcinations at various temperatures. Hong et al. [ 67 ] found that 500-nm WO 3 nanoparticles obtained at 800 ° C could induce better O 2 evolution compared to 30-nm ones that were obtained at 500 ° C, as better crystallinity (as a result a smaller E g ) of larger particles offset their smaller sur-face-to-volume ratio. In addition to this, the charge-carrier sepa-ration and transport mechanisms of the nanocrystalline semi-conductors under illumination are believed to be different from the bulk material. [ 68 ] It is generally accepted that minimal band bending and small space–charge regions are developed within nanocrystalline semiconductors, allowing the photogenerated charge carriers to travel towards their surface without the need for excessive applied energies. [ 68 ] It is well known that the intro-duction of dopants is a common method for enhancing photo-catalysis. Nanostructured WO 3 allows effective intercalation of dopants to achieve optimum band structures, while it is more diffi cult for dopants to diffuse into bulk WO 3 .

2.8. Thermoelectric Properties

The thermoelectric effect involves the direct conversion of a temperature difference to a voltage gradient or vice versa. The thermoelectric effect in WO x is a relatively less studied area, and measurements have usually been conducted to evaluate the thermochromism of WO 3. [ 69 ] The thermoelectric effect has been shown to exist for stoichiometric WO 3 [ 69 ] and WO 3 hydrates, [ 70 ] and their Seebeck coeffi cients were found to be highly dependent on temperature. From subzero temperatures to approximately 50 ° C, the Seebeck coeffi cients increase lin-early with temperature. However, the Seebeck coeffi cient of the hydrates increases sharply when the temperature exceeds 50 ° C, and then drops exponentially after the temperature sur-passes 100 ° C. [ 70 ] The largest Seebeck coeffi cient (480 μ V K − 1 ) is obtained at 90 ° C. In contrast, experimental data show that the Seebeck coeffi cient of stoichiometric WO 3 increases to a maximum value of 600 μ V K − 1 by increasing the temperature to 200 ° C. [ 69 ] The thermoelectric effect has also been found in non-stoichiometric WO x [ 16 ] , however, the temperature-dependent Seebeck coeffi cient is only observed in subzero conditions. Interestingly, a linear relationship between Seebeck coeffi -cient and temperature has been found for WO 2.9 and WO 2.95 , whereas, a nonlinear relationship is present for WO 2.72 and WO 2.83 . [ 16 ] There is no report on the effects of nanostructuring WO x on its thermoelectric properties, which could be an inter-esting subject for future investigations.

2.9. Ferroelectric Properties

The ferroelectric effect is usually not found in WO x , except for the monoclinic II ε -WO 3 , which is the only acentric WO x phase and stable only at cryogenic temperatures. [ 4 ] There are only few reports in this fi eld over the last two decades as the instability of ε -WO 3 at room temperature has encumbered the research for its use as a ferroelectric material. [ 4 ] The ε -WO 3 was found to


have a spontaneous electric dipole moment. The polarity comes from the displacement of tungsten atoms from the centre of each WO 6 octahedra. [ 4 ] Recently Wang et al. [ 71 ] obtained stable Cr-doped WO 3 nanoparticles (15–30 nm), which consisted of both γ and ε phases at room temperature. It is suggested that the Cr acts as a structure stabilizing agent for the ε -WO 3 . [ 71 ]

2.10. Doping

It is possible to alter the material properties of WO x through doping. Typical dopants include Li, Na, and H which can be intercalated into the structure of WO 3 during or after the synthesis process. [ 27 ] Intercalation of H, Li, or Na into WO 3 is often used to induce chromic effects, which were discussed in Section 2.5. Intercalation of dopants leads to intricate phase changes that are not commonly observed in pure WO 3 . [ 27 ] For instance, cubic WO 3 has rarely been reported at room tem-perature; however, cubic phases can be observed for H 0.5 WO 3 , Li x WO 3 with 0.1 < x < 0.4, and Na x WO 3 with 0.3 < x < 1. [ 27 ] Structurally, it is believed that the Li and Na atoms are posi-tioned at the centres of the perovskite octahedral units, while the H atoms are attached to oxygen atoms to form hydroxyl groups at the corners of the perovskite octahedra. [ 27 ] WO 3 also experiences an insulator-to-metal transition after the interca-lation of Li, Na, or H. This can be attributed to the energy of the s level of the dopants, which are above the W 5 d level of the WO 3 crystal conduction band. For Li and Na, the s energy level is likely to be placed well above the conduction band edge and, and thus the contributed electrons signifi cantly increase the free-electron density in the conduction band. [ 27 ] When the dopant is H, the mechanism of electron donation is different,as the dopant bonds with oxygen to form a hydroxide unit. However, the net result of H intercalation is still the con-tribution of an electron to the bottom of the conduction band, which resembles the cases for Li or Na intercalation, and thus physical properties should not be too different.

In addition to Li, Na, or H doping, N doping has also been reported. It has been found that the E g of WO 3 nanowires decreases with N doping, and that the resistivity of a single N-doped WO 3 nanowire is approximately one order of magnitude less than that of the corresponding undoped crystalline WO 3 nanowire. [ 28 ] It has also been found that the conductivity of WO 3 can be improved by a factor of ≈ 000 through addition of Al 2 O 3 . [ 17 ] Other metal oxide dopants such as TiO 2 , [ 72 ] Co 3 O 4 , MnO 2 , and LiO 2 [ 17 ] have also observed to enhance the electrical conductivity of WO 3.

Similar to the chromic effect in nanostructured WO x , dis-cussed in Section 2.5, doping of nanostructured WO x can be more effi cient than in its bulk form due to the enhanced sur-face area and ease of diffusion of dopants through the struc-ture. The doping should be conducted with care, as the surface morphology of nanostructured WO x can be signifi cantly altered during the process, and deterioration of the performance may result due to the reduced structural porosity. [ 73 ]

3. Nanostructure Synthesis

Many different approaches for the syntheses of nanostruc-tured WO x have been implemented using both vapor- and


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liquid-phase-based methods. In this Feature Article, we present the most common synthetic methods and describe how they can be employed for engineering and tuning the morphology and properties of WO x .

3.1. Vapor-Phase Synthesis

The category of vapor-phase deposition includes a wide range of material synthesis techniques. In general, it involves the condensation of vaporized source material onto the targeted substrates. [ 74 ] To obtain nanostructured WO x , physical vapor deposition (PVD) is more commonly used. The PVD process can be purely physical, starting with the WO x material source in the form of a solid target or powder, which is energetically evap-orated (sublimed) by many possible techniques. Ion bombard-ment, [ 75–78 ] heat, [ 79 ] electron beam, [ 80 ] and laser irradiation [ 81 ] are among the most commonly reported energetic sources. With careful control on the process parameters, the evaporated species can condense into a nanostructured form with desired dimensions, crystallinity, and nanoscale morphology. The starter material source is not necessarily WO x , it can also be W metal in the form of powder or fi lament. [ 82 ] In this case, oxygen gas or oxidants must be added to the vapor to form WO x . In many cases, the as-synthesized material resulting from PVD is WO x rather than WO 3 . The color of the deposited material is a good indicator of the species stoichiometry. [ 82 , 83 ] Usually black fi lms are obtained because of severe lack of oxygen content within the material; translucent dark blue fi lms are observed for WO x with x = 2.7 ± 0.2; [ 82 ] and a pale green-yellow color is observed for WO 3 . [ 83 ] Generally post-annealing can be utilized to obtain the desired oxygen content as well as crystal phase and stoichiometry.

3.1.1. Sputtering

Sputtering, a process in which atoms are ejected from a solid target material by bombarding it with energetic particles, is a well established PVD process with a high degree of control-lability. The high energy and controllable parameters of sput-tering can result in the growth of well-structured and crystalline fi lms. Further, sputtering can be easily implemented as a roll-to-roll process for large-scale manufacturing. [ 24 ] Therefore, sput-tering is among the most popular methods for the deposition of WO x in industry.

Conventionally, quality anhydrous WO x ( x = 3 ± 0.1) thin fi lms can be obtained using direct current (DC) or RF sput-tering techniques with metallic W or WO x targets in an oxygen-rich environment. The as-synthesized materials usually exhibit a columnar structure that consists of micro- or nanometer-sized grains that are tightly packed. [ 75–78 ] The process parameters that can be adjusted during deposition include the applied sput-tering power, substrate type and temperature, and gas content and pressure. These parameters can be used to precisely con-trol the fi lm deposition rate, thickness, grain size, crystallinity, and nanostructure morphology. Dopants can also be introduced during the sputtering process by either adding material to the solid target, adding a secondary target, or introducing addi-tional species to the gas environment. The fi lms resulting from


sputtering can be controlled to become oxygen defi cient or non-stoichiometric. [ 24 , 76 ] Post-annealing in a controlled gas environ-ment can be used to achieve enhanced crystallinity. [ 76 ]

3.1.2. Thermal Evaporation

Thermal evaporation deposition is achieved by vaporizing a source material using heat either in a vacuum or in a controlled gaseous environment at low pressure. For the deposition, the source material can be either W metal or WO 3 in either powder or condensed form. Due to the low-pressure environment, vaporization of the source material can usually be conducted at a temperature that is lower than the melting point of WO 3 ( ≈ 1470 ° C). [ 79 ] The vaporized W or WO x emanates from the material source and may interact with the gas environment within the deposition chamber before condensing on a sub-strate located at a distance from the heated material source. Processing parameters such as evaporation temperature, sub-strate temperature, gas environment and pressure, and the type of substrate play an important role in achieving the desirable outcomes. Generally post-annealing can be utilized to obtain the desired oxygen content as well as the crystal phase and the species stoichiometry.

In general, WO x nanowires can be obtained under optimum thermal evaporation conditions, and the physical mechanism for the nanostructure condensation has been described using the vapor–solid growth mechanism. Thangala et al. [ 79 ] describe the vapor–solid process as follows: Firstly, the supersaturated WO x vapor condenses to form solid WO x clusters in a nuclea-tion step. Then, the WO x clusters are further oxidized to form a nanocrystalline ‘tip’ on the substrate. Finally this crystalline tip acts as a seed for further crystal growth and exhibits enhanced adsorption of the WO 2 /WO 3 species. Depending on the crystal-linity of this seed, the nucleation point will either grow prefer-entially in the vertical dimension, forming a one-dimensional (1D) nanowire structure, or can grow in both lateral and vertical dimensions forming small aggregated nanoparticles through the use of higher substrate temperature. [ 79 ] Most examples found in the literature [ 82 , 84 , 85 ] observe that the as-synthesized WO x nanostructures tend to grow along the (010) and (001) plane, which are more favorable for axial nanowire growth.

Zhu et al. [ 82 ] presented the early studies on WO x thermal evaporation, in which they obtained treelike structures of micrometer dimensions by heating a W plate pressed against a SiO 2 plate. The SiO 2 plate acted as the collection substrate as well as the source of oxygen ions. Materials such as B 2 O 3 can also be used as the oxygen source. [ 86 ] Thermal evaporation was further experimented with to produce 1D WO x nanostruc-tures in the forms of nanowires or nanorods by heating W metal or WO 3 powder in the presence of oxygen gas. [ 79 , 84–87 ] Figure 7 shows the typical morphology of such nanowires. Thangala et al. [ 79 ] found that the density and diameters of ther-mally evaporated WO x nanowires were inversely proportional to the substrate temperature and both increased by boosting the oxygen partial pressure during the process. The as-synthesized 1D WO x nanostructures were mostly reported to be monoclinic W 18 O 49 , and the oxygen defi ciency was minimized by increasing the oxygen content during the process or by post-annealing. [ 83 , 84 , 86 ] Further to the previous reports, water vapor


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Figure 7 . SEM images of WO x nanowires on quartz substrate synthe-sized by thermal evaporation at 1950 K. a) Low-magnifi cation image of individual nanowires with diameters in the range of 30 to 60 nm. b) Cross-sectional image of the as-synthesized vertical arrays of nanowires on a fl u-orine-doped tin oxide (FTO) substrate. Reproduce with permission. [ 79 ]

Figure 8 . a) AFM image showing the topography of a typical WO 3 fi lms obtained using spray pyrolysis at 250 ° C substrate temperature. Repro-duce with permission. [ 100 ] b) SEM image of the WO 3 fi lm with nanofi bre network obtained using pulsed spray pyrolysis (inset: magnifi ed image). Reproduce with permission. [ 108 ] Copyright 2007, Elsevier.

was introduced during the thermal evaporation of WO x as the oxygen source. [ 88 , 89 ] In this case, relatively denser and longer WO x nanowires were obtained, and a slightly lower tempera-ture (650 ° C) was required to evaporate the source material. It was suggested that water favors the formation of highly mobile species, such as WO x hydrates, allowing faster spreading of the material for nanostructural growth. [ 89 ]

3.1.3. Other PVD Methods

Apart from the abovementioned vapor-phase methods, elec-tron-beam (e-beam) deposition, [ 80 , 90 , 91 ] pulsed-laser deposition (PLD) [ 81 , 92–95 ] and arc-discharge [ 96 , 97 ] deposition techniques are the alternatives available for synthesizing nanostructured WO 3 . For the e-beam and PLD methods, WO x targets are gen-erally used, while W metal electrodes are employed in the arc-discharge technique. The as-synthesized WO x from these three methods are usually compact thin fi lms, which are tex-tured by nanoparticles (10–800 nm). The size of the parti-cles is greatly affected by the oxygen pressure and substrate temperature as the two most important parameters for these processes. [ 93 , 98 ]

Spray Pyrolysis : Spray pyrolysis is a typical aerosol-assisted chemical vapor deposition (CVD), which is well utilized in the glass industry and in solar cell production to deliver thin or thick fi lm coatings. [ 99 ] A typical spray pyrolysis process can be conducted either in vacuum or under atmospheric pressure, and the set-up usually consists of an atomizer, a precursor solu-tion, substrate heaters, and a temperature controller. [ 99 ] During the deposition, the precursor solution is pumped to an atomizer, and then sprayed through the carrier gas as a fi ne mist of very small droplets onto heated substrates. Subsequently the drop-lets undergo evaporation, solute condensation, and thermal decomposition, which then result in fi lm formation. [ 100 ]

WO x fi lms prepared by this method are generally produced from tungsten chloride or ammonium tungstate solutions, and oxygen or air is used as the carrier gas. [ 101–105 ] Such fi lms are typically of compact structure, consisting of 100–200-nm


grains as seen in Figure 8 a. [ 103 , 104 ] Substrate temperature is considered to be the most important parameter, as it affects both the crystallinity and thickness of the as-produced fi lms, and also infl uences the size of the deposited particles. [ 99 ] Many researchers [ 101–107 ] have found that amorphous or polycrystal-line WO 3 is formed on substrates with temperatures less than 300 ° C, while monoclinic WO 3 starts to dominate as the sub-strates’ temperature is raised above 300 ° C. Further improve-ment of the material crystallinity can also be achieved by post-annealing at ≈ 500 ° C. Regragui et al. [ 103 ] found that larger grain sizes were obtained when the substrates were heated to 400–500 ° C, due to more severe solvent evaporation and pre-cursor decomposition. More recently, Bathe et al. [ 108 ] obtained WO 3 fi lms of interconnected nanofi bres using pulsed spray pyrolysis to avoid the unwanted substrate cooling caused by continuous spraying of mist (Figure 8 b). The same research group [ 109 ] later presented the growth of nanofi bre networks by using different quantities of precursor solutions. It was found that beads (100–200 nm) were initially formed on the substrate; afterwards, nanowires (diameter: ≈ 250 nm) started to grow; and fi nally a network of interconnected nanofi bres (width: ≈ 500 nm) was obtained as the process continued. [ 109 ]

Modifi cations to the standard spray pyrolysis of WO 3 were also found to enhance the material’s structural morphology. Ghimbeu et al. [ 110 ] applied static voltages between 7.5–8 kV to the nozzle, electrically charging up the precursor droplets. Con-sequently, extremely porous WO 3 fi lms were created using this method. Additionally, a laser was applied to the mist during the process, and fi ne nanowires of WO 3 were found scattered in different densities on the substrate. [ 111 ] The application of the laser was believed to induce some photochemical reactions to the precursor droplets in the mist, which resulted in the self-assembled growth. Finally, one of the latest reports presented fl ame-assisted WO 3 synthesis by spray pyrolysis. [ 112 ] The com-bustion of the precursor droplets was supported by CH 4 and O 2 as carrier gases, and ultrafi ne monoclinic WO 3 nanoparticles (5–15 nm) were produced.

3.2. Liquid-Phase Synthesis

Liquid-phase synthesis (LPS) methods are generally chosen when the high capital cost of vapor-phase deposition equipment


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Figure 9 . SEM images of WO 3 · n H 2 O: a) left to dry at low humidity; b) dried in a humid environment. The insets show low magnifi cation optical images. Reproduce with permission. [ 119 ] Copyright 2010, Amer-ican Chemical Society.

Figure 10 . Surface morphology of a WO 3 fi lm prepared by peroxotungstic acid sol–gel method, the fi lm was annealed at 500 ° C. Reproduce with permission. [ 120 ] Copyright 2007, American Chemical Society.

cannot be justifi ed. LPS offers better control of the material morphology when compared to the vapor-phase methods. LPS can also be implemented at relatively low temperatures, hence not deteriorating the quality of low-heat-tolerance substrates during the deposition process. Over the years, a vast number of liquid-phase synthesis routes have been developed for synthe-sizing WO x in various nanostructured forms. Hydrolysis, con-densation, etching, and oxidation are the four most important chemical processes in LPS.

3.2.1. Sol–Gels

The sol–gel process is a well-known, intensively studied wet-chemical technique that is widely used in materials synthesis. This method generally starts with a precursor solution (the “sol”) to form discrete particles or a networked gel structure. During the course of gelation (aging process), various forms of hydrolysis and polycondensation take place. Film deposition is generally conducted during the gelation process, where the “sol–gel” is dip-coated, spun, or drop-cast onto the substrates. In most WO x sol–gel syntheses, hydrated WO 3 is produced. Sanato et al. [ 113 ] conducted a series of Raman analyses concerning the change of the sol–gel-produced WO 3 crystal structure induced by various annealing temperatures. They found that the as-synthesized material was a highly crystalline tungsten oxide hydrate; however, no specifi c type of hydrate could be deter-mined. The typical Raman peaks of hydrated WO 3 were still present even after calcination at 300 ° C, and anhydrous WO 3 in a monoclinic phase started to be dominant above 400 ° C.

For the “sol” preparation, a diverse range of precursor solu-tions are available. Early work mainly focused on dissolving W(OEt) 6 [ 114 ] and WOCl 4 [ 115 ] in various kinds of alcohols. How-ever, these precursors are expensive and highly volatile, and the WO 3 fi lms produced from them were rather compact and fea-tureless. [ 116 ] WCl 6 can also be used by dissolving it in organic solvents, but the resulting product was found to have tungsten bronze impurities instead of pure WO 3 , as the organic sol-vents partially reduce W 6 + to W 5 + . Therefore, such a precursor needs to be aged for a long period of time (3–7days at room temperature or 48 h at 50–60 ° C) to obtain a transparent solu-tion indicating that the amount of W 5 + has been minimized. [ 117 ] The WO 3 fi lms obtained were reported to lack porosity unless a templating method was also used. [ 117 , 118 ]

Aqueous solutions of tungstic acid are also widely used as precursors and are typically obtained by acidifi cation of aqueous sodium tungstate solutions. These clear precursor solutions are more stable and easier to handle than the ones mentioned above. [ 116 ] During the tungstic acid solution aging process precipitates are found as the hydrolysis of H 2 WO 4 proceeds, condensing to form crystalline WO 3 · n H 2 O parti-cles. [ 119 ] The as-prepared precursor can be immediately used for fi lm deposition; however, the resultant fi lms are made up of large crystallites. In contrast, by exerting some control over the tungstic acid preparation and the precursor aging process, interesting nanomorphologies can be realized. We recently reported [ 119 ] a new technique for preparing a tungstic acid solu-tion in which the mixing of Na 2 WO 4 solution and HNO 3 was conducted using a microfl uidic “Y” connector with a controlled fl ow rate rather than mixing the two solutions at once. As can

© 2011 WILEY-VCH Verlag Gmwileyonlinelibrary.com

be seen from Figure 9 , micro- and nanotextured tungsten oxide in a randomly aligned fl akelike structure (thickness: 10–30 nm) was obtained using a slow fl ow rate. The humidity of the aging process was also found to affect the material’s morphology, as enhanced fi lm porosity was obtained in a more humid environ-ment (Figure 9 b). As expected, the as-produced material was found to be a mixture of WO 3 · n H 2 O where n = 1/3, 1, or 2. A mixed phase of monoclinic and orthorhombic WO 3 formed after annealing at 550 ° C.

Peroxotungstic acid solutions are probably the most widely used precursors in WO 3 sol–gel syntheses due to their excel-lent stability in an ambient environment. [ 116 ] The precursor is typically obtained by dissolution of tungsten metal in concentrated hydrogen peroxide solution, which produces [WO 2 (O 2 )H 2 O] · n H 2 O complexes. The gelation of peroxotung-stic acid generally takes place with the addition of some organic acids at 50–60 ° C for 24–48 h, during which time WO 3 · n H 2 O is formed. [ 30 , 120 , 121 ] A typical surface morphology of the WO 3 fi lms obtained using peroxotungstic acid is a highly mesoporous network of interconnected particles (diameter: 100–200 nm), as shown in Figure 10 . [ 120 ] Humidity present during the precursor aging process affects the structural morphology in a similar way to using aqueous tungstic acid, as mentioned above. [ 122 ] However, the mechanism behind this is still unclear. Addition-

bH & Co. KGaA, Weinheim Adv. Funct. Mater. 2011, 21, 2175–2196

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Figure 12 . SEM (a,b,d) and TEM (c) images of WO 3 prepared using a colloidal crystal template. a) PMMA diameter, 490 nm; calcination temperature, 773 K. b,c) 180 nm; 773 K. d) 180 nm, 873 K. Reproduce with permission. [ 127 ] Copyright 2008, Royal Society of Chemistry.

ally, like the case of aqueous tungstic acid, hydrated WO 3 with a degree of crystallinity is normally the direct product of the syn-thesis, and the WO 3 formed is able to retain its hydrated nature after the post-annealing process at temperatures less than 500 ° C.

3.2.2. Templating

Templating is a modifi cation of the sol–gel synthesis technique and can be very effec-tive for the preparation of meso- or nano-structured WO x . Templating can be divided into two main categories, namely “hard tem-plating” and “soft templating”. In “hard tem-plating”, templates such as an anodic alumina membrane (AAM) with self-organized nano-pores [ 123 , 124 ] as well as a meso- or nanoporous silica substrate [ 5 ] are immersed in the WO 3 precursor solution, which is prepared in a similar fashion to the sol–gel method such that the precursor solution infi ltrates the hard template structures. The infi ltrated templates are then aged to achieve gelation. After aging, the WO x gels, together with the substrates, are annealed to induce solid WO 3 nanostruc-tures and crystallinity. Finally the templates
are gently removed by chemical etching. Figure 11 a presents a template nanostructured WO 3 fi lm with wire morphology achieved by infi ltrating an AAM fi lm with precursor, aging, and then dissolving the AAM template.
For the “soft templating” method, carbon-based structures (e.g., carbon microspheres [ 125 ] ) and organic compounds such as polyethylene glycol (PEG), [ 113 , 126 ] polymethyl methacrylate (PMMA), [ 127 ] and other block copolymers, [ 128–132 ] are mostly used as the templates. These template materials are either dissolved or dispersed in solutions, which are then added to the WO 3 pre-cursor. After thorough stirring, the mixture is aged under var-ious conditions, and then heated at elevated temperatures in an


Figure 11 . a) SEM images of the tungsten oxide nanowires synthesized using AAM template where the infi ltration of WO 3 precursor solution was assisted by gas-fi lled methods. Reproduce with permission. [ 124 ] Copy-right 2006, Institute of Physics. b) Surface morphology of a WO 3 fi lm prepared by the deposition of a tungstic acid/PEG 300 colloidal solution after annealing at 400 ° C. Reproduce with permission. [ 113 ] Copyright 2001, American Chemical Society.

oxygenated environment to remove the templates. Figure 11 b and Figure 12 show the WO 3 morphologies achieved by soft-templating using PEG and PMMA microballs, respectively. As is evident from Figure 12 , through careful control of gela-tion and annealing processes, it is possible to achieve highly ordererd meso-structures. To prevent structural collapse during the annealing treatment, it was suggested by Yang et al. [ 132 ] that the conditions of the precursor-template aging process needed to be controlled to promote slow hydrolysis. This is particu-larly important for the metal oxide–organic template assembly, where rapid crystallization can lead to phase separation and dis-sociation that degrade the integrity of the fi lm.

3.2.3 Hydrothermal

Hydrothermal treatment is a facile, cost-effective and well-studied liquid-phase technique, which has the capability of producing WO x of different nanomorphologies. In most cases, the hydrothermal synthesis of nanostructured WO x starts with the preparation of a tungstic acid solution (H 2 WO 4 ) as the precursor. This solution is then kept at an elevated temperature (120–300 ° C) for a certain period of time, [ 133 ] allowing the nucle-ation and growth of crystallites. Such synthetic processes gener-ally produce layered WO 3 · n H 2 O fl akes of lateral dimensions in the range of several tens of nanometers to several micrometers and thicknesses in the nanometre range, as can be seen from the TEM images in Figure 13 . [ 21 , 134 ]

Recent reports have shown that high-aspect-ratio 1D WO x nanostructures, such as nanowires, nanorods, and other inter-esting confi gurations, can be synthesized using the hydro-thermal technique by adding sulfates and certain types of organic acid to the tungsten acid precursor solution as structure-


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Figure 13 . TEM images of WO 3 · 0.33 H 2 O: hexagonal nanofl akes at low (A) and high (B) mag-nifi cation. The inset of (B) shows the SAED pattern. HRTEM images of the samples are shown in top (C) and cross-section (D). Reproduce with permission. [ 21 ] Copyright 2008, American Chemical Society.

directing and dispersing agents. The type of nanostructure and crystallinity can be controlled through the use of different kinds of sulfate and organic acid. Table 2 presents a brief summary of the nanostructured WO x types that result from hydrothermal treatment with different additives. Figure 14 presents a selection of the high-aspect-ratio structures that have been achieved using this technique.

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinwileyonlinelibrary.com

Table 2. Summary of WO x nanostructures obtained using hydrothermal process and their synth

Morphology Dimensions [nm] a) Precursor Additives

nanowire bundles [ 135 ] D: 5-15 L: > 5 × 10 3 H 2 WO 4 Li 2 SO 4 H 2 C 2 O 4

nanorods [ 136 ] D: 5-25 L: 200-500 H 2 WO 4 Rb 2 SO 4 H 2 C 2 O 4

nanoribbon [ 136 ] W: 100-1000 L: > 1 × 10 4 T: 10-25 H 2 WO 4 K 2 SO 4 H 2 C 2 O 4

cylindrical nanowire

bundles [ 136 ]

N/A H 2 WO 4 Na 2 SO 4 H 2 C 2 O 4

scattered nanorods [ 136 ] N/A H 2 WO 4 (NH 4 ) 2 SO 4 H 2 C 2 O 4

nanobelts [ 137 ] W: 30-100 L: > 1 × 10 3 T: ∼ 15 H 2 WO 4 cetyltrimethylammonium bromide

nanowire bundles [ 138 ] N/A H 2 WO 4 ethylenedi minetetraaetic Na 2 S

nanoplate [ 139 ] W: 500 H 2 WO 4 citric acid

nanowire, hollow sphere,

nanotube, solid ball [ 140 ]

Various WCl 6 urea

a) D: diameter, T: thickness, L: length, W: width; b) The crystal phase is determined in the as-synthesized materi

Many reports claim that hydrothermal synthesis with sulfates and organic spe-cies produce hexagonal or orthorhombic WO 3 phases and rely solely on X-ray diffrac-tion (XRD) patterns as the evidence for this claim. However, it is important to consider that hexagonal WO 3 is formed by dehydra-tion of orthorhombic WO 3 · 1/3 H 2 O, [ 8 ] thus they share similar crystal structures, which are refl ected by the nearly identical XRD pat-terns. [ 9 , 21 ] With the lack of detailed Raman and FTIR analysis, which are very sensitive to the presence of –OH groups, [ 141 ] it is dif-fi cult to draw a decisive conclusion on the exact WO x crystal phases obtained from hydrothermal synthesis with additives.

Regardless of crystallinity, the general morphologies of hydrothermally synthesized WO x , produced with and without additives, are undisputedly different, however it is still unclear what exact structural assembly mecha-nisms are infl uenced by the addition of sulfates and acids. Some literature [ 136 , 142 ] suggests that certain ions in the additives are adsorbed onto the surface of seed crystals of WO x . This effect locally decreases the crystal surface energy that alters the growth mechanism. In addi-tion, a recent report by Phuruangrat et al. [ 143 ] found that the application of microwave power during the hydrothermal process for WO x nanostructure synthesis shortens the reaction time by more than 50%.

3.2.4. Electrochemical Anodization

Electrochemical anodization has long been used on an indus-trial scale to passivate metal surfaces for corrosion resistance; it also helps to color and glue metal surfaces more effectively by producing microscopic textures on the surfaces. In the early 90s, nanoporous structures were discovered on silicon [ 144 ] and

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esis conditions.

Temperature [ ° C] Duration [h] Crystal phase b)

180 2–24 h-WO 3

180 2–72 h-WO 3

180 2–72 h-WO 3

180 8 N/A

180 8 N/A

Na 2 S 180 12 β -WO 3

140–180 4–12 h-WO 3

120 24 β -WO 3

180 12 crystalline

H 2 W 1.5 O 5.5 · H 2 O

al without post-annealing treatment.

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Figure 14 . SEM and TEM (inset) images show the morphologies of the hydrothermally synthe-sized WO x with different amounts of added urea in the WCl 6 precursor solution: a) 0 mmol, b,c) 5mmol, d,e) 10 mmol, f,g) 20 mmol, and h,i) 40 mmol. Reproduce with permission. [ 140 ] Copyright 2009, American Chemical Society.

aluminium [ 145 ] by anodizing the respective materials. This method was then extended to other metals including Ti, Zr, Ta, Hf, Cd, and Bi. [ 146–154 ]

In a typical anodization apparatus, potential is applied between two samples (working electrode and the counter elec-trode), which are both immersed in a liquid electrolyte. An electrical current is then applied between them which results in electrochemical reactions occurring at the surface of both samples. Grimes et al. [ 155 ] reported the fi rst use of anodization to produce WO x nanostructures. This demonstration used a W foil in an oxalic acid electrolyte resulting in the production of a thin nanoporous WO x layer on the W foil surface. This experi-ment was followed by the demonstration of more uniform and thicker fi lms (with thickness up to 500 nm) by anodizing W foils in fl uorine-containing electrolyte, where the fl uorine spe-cies were found to greatly enhance the electrochemical etching and chemical corrosion processes. [ 156–158 ] The authors of this review later reported on the nanoporous WO 3 fi lms obtained from anodizing the W fi lms, which were RF sputtered on FTO glass substrates. The resulting structure is presented in Figure 15 . [ 159 ]

The anodization current, composition, and temperature of the electrolyte are among the most important parameters affecting the WO x growth. [ 60 ] The anodic current density transient can be interpreted as a direct measurement of the electrochemical ano-dization. A typical anodic current density transient is presented in Figure 15 a. The as-anodized WO x fi lms are reported to be hydrated and mainly amorphous. [ 155 , 158 , 159 ] however, a more recent report by Ng et al. [ 14 ] claimed that subtle crystallinity peaks corresponding to monoclinic WO 3 · 2 H 2 O were observed in the XRD patterns of the as-anodized WO x fi lms. Nonetheless, the as–anodized WO x can be converted to the highly crystalline and stoichiometric WO 3 by post-annealing, with monoclinic or orthorhombic phase being present depending on the annealing

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, WeinhAdv. Funct. Mater. 2011, 21, 2175–2196

temperature. Apart from the nanoporous morphology, other nanostructures can also be obtained by W anodization. We recently reported on the synthesis of a bilayered WO 3 nanoplatelet structured fi lm of a thickness up to a few μ m by anodizing W foils in 1.5 M HNO 3 at 50 ° C. [ 160 , 161 ] Observation of fl ower-shaped WO 3 nanostructures achieved by ano-dization has also been reported recently. [ 14 ]

3.2.5. Electrodeposition

Electrodeposition can be considered the reverse of electrochemical anodization. For anodization, a metal is oxidized by ions present in the analyte, for electrodeposition, in contrast, a metal oxide fi lm forms by accu-mulating metal ions present in the electro-lyte. For the electrodeposition of WO x , the most popular choice of electrolyte is the per-oxotungstic acid solution. [ 162 ] Alcohol (isopro-panol or ethanol) can be added to the solution to increase the stability. [ 162 ] The deposition mechanism can be summarized as: [ 163 ] i) the formation of isolated nuclei and their growth

to larger grains; ii) the aggregation of grains; and iii) the forma-tion of crystallites.

Typically the process requires only a small applied voltage (e.g., − 0.5 V versus Ag/AgCl), and the deposition duration is in the range of 1–30 min. Excessive applied voltages and prolonged processing time can result in creating compact fi lms with low porosity. [ 164 , 165 ] Under optimal conditions, electrodeposition can produce porous WO x fi lms with grains of 20–100-nm diame-ters. An example of such a fi lm is presented in Figure 16 . [ 166 , 167 ] Analyses of electrodeposited fi lms using XRD indicate that they are predominantly amorphous with a small degree of nano-crystallinity. [ 164–166 , 168 , 169 ] As with most liquid-phase synthesis methods, the as-electrodeposited WO x is usually in a hydrated form. This fact has been confi rmed by numerous FTIR and Raman studies. [ 164–166 ] Post-annealing above 400 ° C is usu-ally conducted to transform the WO 3 · n H 2 O to monoclinic WO 3 . However, annealing tends to increase the grain size and decreases the porosity of the fi lms. [ 169 ]

Apart from the above-mentioned peroxotungstic acid elec-trolyte, Baeck et al. [ 170 ] introduced sodium dodecyl sulfate into the electrolyte solution, and obtained WO x fi lms with lamellar crystallites, which enhanced the photocatalytic and electro-chromic performance. In addition to this, WO x fi lms which exhibit a “pebble-beach”-like morphology, made up of nanoballs with diameters in the 40–350-nm range, can also be obtained using an aqueous sodium tungstate solution directly as the electrolyte. [ 171 ] To achieve these results, however, it is necessary to precisely control the electrolyte’s pH.

3.3. Other Methods

In 2008 Widenkvist et al. [ 172 ] reported a facile route for pro-ducing a WO 3 fl ake-like structure. The synthesis was conducted

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Figure 15 . a) Current transient curves recorded during anodization at different voltages with 0.5% (wt) NH 4 F/ethylene glycol electrolyte, and the resulting surface morphologies of anodized W fi lms at b) 20 V, c) 40 V, d) 60 V (inset: cross-sectional SEM image). Reproduce with permission. [ 159 ] Copyright 2009, Elsevier.

by immersing a W metal plate in a HNO 3 solution at 50–60 ° C to form layer-structured tungstite platelets. Monoclinic WO 3 was obtained by a post-annealing treatment at 350 ° C.


Figure 16 . Surface morphology of an electrodeposited WO 3 fi lm using peroxotungstic acid solution. Reproduce with permission. [ 167 ] Copyright 2008, Elsevier.

Recently, we have presented an innovative method for the synthesis of atomically thin 2D WO 3 structures using a three-step process that involves a wet-chemical synthesis of hydrated-WO 3 , mechanical exfoliation of the fundamental layers and dehydration by annealing. [ 48 ] We obtained the minimum resolv-able thickness of the hydrated fl akes as 1.4 nm, which corre-sponds to one unit-cell height.

4. Applications

In this section, some of the applications of WO x materials are presented. Particular emphasis is placed on the enhancements that can be achieved by exploiting the nanostructured forms of WO x . Common applications of WO x include electrochromic, photocatalytic, and gas sensing. These applications have been thoroughly investigated and reported in the literature. The exploitation of WO x for dye-sensitized solar cells, optical data storage and fi eld-emission displays as well as the cation-doped WO 3 high- T c superconductor are also discussed.

4.1. Electrochromic Devices

WO 3 -based electrochromic (EC) devices, which are normally seen in smart windows and EC displays, have been widely


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Figure 17 . a) Schematic image of a typical sandwich structure EC device. b) Samples of the PEC cell with solid electrolyte in the bleached state (left, short-circuit) and colored by illumina-tion equivalent to one sun (right, open-circuit). Reproduce with permission. [ 38 ] Copyright 2006, Elsevier.

studied over the past few decades. These devices exhibit a good memory effect with low power consumption, high contrast and long-term stability. There are a few types of confi gurations for EC devices. [ 173 ] The most widely used and simplest confi gura-tion is the sandwich structure, shown in Figure 17 a. It includes an EC layer (WO 3 ), an electrolyte for ion storage, and two trans-parent conductors, which are utilized to establish electrical con-tacts. Application of a voltage drives the ion into the WO 3 where it is intercalated, causing the chromic effect. Reversing the voltage withdraws the ions from the WO 3 matrix and returns them to the electrolyte. The electrolytes are usually H 2 SO 4 and HClO 4 with H + serving as the intercalation ion, while LiClO 4 is widely used for Li + intercalation. Five factors are normally used

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, WeinAdv. Funct. Mater. 2011, 21, 2175–2196

Table 3. Summary of nanostructured WO x -based EC devices featuring their typical performance

Crystal phase Surface morphology

Thickness [nm]

Optical modulation

H + containing-electrolyte

Monoclinic [ 32 ] Mesoporous with a pore size of 70 nm 350 60%

Orthorhombic [ 15 ] Mesoporous with a pore size of 50 nm 300 40%

Orthorhombic [ 174 ] Mesoporous with a poresize of 100 nm 400 50%

Hexagonal [ 175 ] Mesoporous with a pore size of 3–5 nm 100 N/A

Hexagonal [ 33 ] Nanorod with a diameter of 100 nm, and length

of 2 μ m

N/A 34%

Monoclinic [ 31 ] Nanospheroids (diameter of 10–20 nm) mixed

with nanorods (diameter of 40–60 nm)

N/A N/A

Li + containing-electrolyte

Monoclinic [ 176 ] Nanowires with a diameter of 40–60 nm, and a length

of 5 μ m

N/A 65%

Monoclinic [ 177 ] Nanowires with a diameter of 60–70 nm N/A 65%

Triclinic [ 164 ] Mesoporous with a pore size of 50–100 nm 220 50%

Monoclinic [ 129 ] Mesoporous with a pore size of 7 nm 190 85%

Monoclinic [ 178 ] Mesoporous 240 85%

Hexagonal [ 179 ] Nanorod with a diameter of 100 nm, and a length

of 2 μ m

N/A 66%

to evaluate the EC device performance in the literature: cyclic stability, optical modulation, color/bleach time, charge density, and colora-tion effi ciency. [ 31 ]

Most of the WO 3 EC devices investi-gated so far have focused on amorphous fi lms due to their high coloration effi ciency ( ≈ 55 cm 2 m C − 1 for H + intercalation) and fast color/bleach time. [ 30 , 31 ] Unfortunately, amorphous WO 3 has poor structural and chemical stability, resulting in poor EC sta-bility. Crystalline WO 3 is much more stable due to its denser structure and slower disso-lution rate in acidic electrolytes. [ 33 ] However, crystalline bulk WO 3 has a relatively lower charge density of 3 mC cm − 2 mg − 1 com-pared to ≈ 9 mC cm − 2 mg − 1 in the amorphous form, [ 31 ] as well as poor coloration effi ciency ( ≈ 25 cm 2 m C − 1 for H + intercalation) [ 15 ] and

slow switching times. Recently, a nanostructured WO 3 fi lm with a high degree of porosity has been utilized to overcome the drawbacks of crystalline WO 3 for EC applications. The details are summarized in Table 3 .

These nanostructured WO 3 -based devices show signifi cant improvement in charge density, [ 15 , 30–33 , 175 , 178 ] coloration effi -ciency, [ 174 , 176 ] and coloration/bleach time [ 32 , 175 , 176 ] compared to bulk amorphous and crystalline WO 3 . More importantly, it is observed that nanostructured hexagonal WO 3 -based devices have superior charge densities when compared to triclinic, monoclinic, and orthorhombic WO 3 . [ 9 ] This can be due to the unique hexagonal rings and trigonal cavities of hexagonal WO 3 , which provide more available sites for cation intercalation.

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data.

Charge density [mC cm − 2 nm − 1 ]

Coloration effi ciency [cm 2 m C − 1 ]

Coloration/bleach time [s]

0.054 N/A 2.8 9.5

0.05 N/A 10 25

0.033 58 8 8.1

0.082 49 11 2

114.5 mg − 1 37.6 25 18

32 mg − 1 42 N/A N/A

N/A 61.3 3 1.5

N/A 56 1 4.2

0.068 N/A 55 30

N/A 64 N/A N/A

0.06 N/A N/A N/A

133 mg − 1 N/A 38 42

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Figure 18 . I–V characteristics of WO 3 -DSC and TiO 2 -modifi ed WO 3 -DSC. Inset: SEM image of the WO 3 nanoparticle photoanode used in the DSC, nominal particle size = 40 nm.

However, the downside of using hexagonal WO 3 is slower color-ation/bleach time and lower coloration effi ciency, which could be due to the existence of surface defects. These effects are still under investigation.

In recent years, self-sustained EC devices such as photo-electrochromic (PEC) cells were developed as a new approach for smart windows with no external power. [ 1 ] Typically, a PEC cell can be a standard WO 3 -based-EC device fabricated on top of a translucent photovoltaic (PV) cell. Due to the complexity of this side-by-side device fabrication, current research on PEC cells mainly incorporate the WO 3 into a dye-sensitized solar cell (DSSC) with added lithium ions to the electrolyte, taking advan-tage of the ease of assembly of DSSCs. Since, currents as small as 0.1 mA cm − 2 are required to darken the WO 3 -based EC layer, the sensitized-photoanode layer can afford to be thin enough to achieve high transparency. [ 1 ] Such a PEC cell was fi rst dem-onstrated in 1996, using thermally evaporated WO 3 as the cathode. [ 180 ] Under short-circuited conditions, this kind of PEC cell is colored in ≈ 100 s under illumination. [ 180 ] Furthermore, a different cell confi guration was explored by Georg et al., [ 38 ] in which the WO 3 nanoparticle electrochromic layer was depos-ited between the transparent working electrode and the photo-anode. The coloration of the cell was achieved by open-circuit conditions, and inversely bleached by short-circuit conditions, as seen in Figure 17 b. The porous structure of the nanotextured WO 3 fi lms reduced the response time and enhanced the colora-tion effi ciency.

4.2. Dye-Sensitized Solar Cells

Dye-sensitized solar cell (DSSC) technology is currently a hot topic in the area of photovoltaics, and its advantages and oper-ating principles has been reported previously. [ 58 ] TiO 2 is the most investigated photoanode material for DSSCs, with an overall effi -ciency of ≈ 12%. [ 181 ] Although WO 3 was mentioned by Deb [ 1 ] as a suitable alternative for replacing TiO 2 due to their comparable electronic properties, few reports on WO 3 -based DSSCs exist. Very recently, authors of this text [ 182 ] presented the fi rst detailed investigation of nanoparticle WO 3 -based DSSC. The open-cir-cuit voltage ( V oc = 390 mV), short-circuit current density ( J sc = 4.6 mA cm − 2 ), and effi ciency ( η = 0.75%) obtained are still not comparable to the TiO 2 -based DSSCs. This is due to the unfavo-rable CB position of WO 3 , which limits the V oc , along with the very acidic surface of WO 3 that reduces dye adsorption, leading to severe charge recombination. However, covering the WO 3 surface with a layer of TiO 2 can improve the overall performance of the cell with η reaching 1.46%, as seen in Figure 18 . It is also worth noting that the dimension of the WO 3 nanoparticles (40 nm) used in this work was twice the size of the optimum TiO 2 nanoparticles in standard DSSCs. Further studies of WO 3 -DSSCs need to be conducted to investigate the effects of increasing sur-face area and changing the morphology of the WO 3 nanostruc-tures (e.g., nanowires can increase the carrier mobility).

4.3. Photocatalytic Applications

A main goal in the area of photocatalysis is to fi nd suitable materials for effi cient solar hydrogen production and organic


pollutant degradation. In 1969, Fujishima and Honda [ 183 ] reported the fi rst photoelectrolysis of water using single-crystal rutile-structured TiO 2 under UV irradiation. Since then, TiO 2 and other semiconductor materials including WO 3 were intensively explored for their photocatalytic abilities. Recent comprehensive reviews on photocatalytic materials and their industrial applications have been previously published [ 59 , 184 , 185 ] In searching for the optimum semiconductor material for effi -cient photocatalytic applications, Zhang et al. [ 59 ] have summa-rized the key aspects as: i) nanostructured forms can provide the optimum surface to volume ratio; ii) a lower band-gap energy that can utilize the visible spectrum of solar light; iii) an optimum band structure for the desired redox reactions; and iv) material stability to withstand harsh operating environments.

As mentioned earlier, nanostructuring of WO x increases its photocatalytic capability. We [ 159 ] and a number of other research groups [ 126 , 156 , 157 ] have reported more than 100% increase of photocurrent or incident photon-to-electron conversion effi -ciency (IPCE) for WO x in various nanostructured forms in com-parison to the compact WO x . This enhanced photoresponse has been mainly ascribed to a much larger effective WO x surface area provided by the nanostructures. IPCE of up to 190% was also recorded [ 126 , 184 ] by adding organic species (e.g., methanol) into the electrolyte due to the particularly favorable photo-oxidation kinetics between WO x and the organic species ( Figure 19 ). In addition to the enhanced surface-to-volume ratio, the charge-carrier separation and transport mechanisms in nanostructured WO x may also play an important role. Augustynski et al. [ 126 , 186 ] compared the photocurrent generated by a nanocrystalline WO 3 (Figure 11 b) and compact WO 3 fi lms, which can be seen in Figure 19 . They found that much higher photocurrents were achieved by a nanocrystalline WO 3 fi lm at a relatively small applied potential and that they saturated at even higher onset potentials. On the other hand, the photocurrent generated by the compact WO 3 fi lm did not saturate and increased with higher applied potential. Less power is required to operate a nanocyrstalline WO 3 based photocatalytic device as it is easier for the charge carriers to travel to the oxide surface. Other groups have now reported similar experimental results. [ 66 , 67 , 156 ]


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Figure 19 . Incident photon-to-current conversion effi ciency–potential curves for a nanocrystalline (about 2.5 μ m thick) and a bulk WO 3 photo electrode (fi lled and open circles, respectively) recorded in a 0.1 M CH 3 OH/1 M H 2 SO 4 solution: under 380 nm illumination from a 500 W Xe lamp equipped with a monochromators. Reproduce with permission. [ 126 ]

This unique charge separation and transport mechanism is still subject to further investigation, and more detailed explanations and arguments are reported elsewhere. [ 59 , 68 , 187 , 188 ]

Although WO x is capable of absorbing in the short wave-length region of visible light, which has much higher energy than the UV region of the solar spectrum, it is thermodynami-cally unfavorable to reduce water to produce hydrogen, as its CB edge is more positive (versus NHE) than the H 2 /H 2 O reduc-tion potential (Figure 6 ). This can be overcome by applying an external potential to help inject the photo-generated electrons into the water molecules. [ 113 ] However, passive photocatalytic systems are preferable given today’s energy issues. In such sys-tems, WO x nanoparticles are usually mixed with other metal oxides to form an overall complementary band structure, or doped with materials that can shift the WO x CB to a more negative position. Stoichiometric water splitting to H 2 and O 2 using Pt-loaded WO 3 mixed with Pt-loaded SrTiO 3 and an I 3 − /I − redox mediator, under visible light irradiation, was fi rst reported by Sayama et al. [ 189 ] The more negative CB of SrTiO 3 facilitates H 2 production, while O 2 is mainly produced by water oxidation from photogenerated holes on the VB of WO 3 . This work was followed by investigating mixtures of WO 3 and TaON, which achieved similar results. [ 190 ] There are also a number of reports on doping WO x with Mg, [ 191 ] Fe, Co, Ni, Cu, Zn, [ 192 ] Bi, Ag, [ 193 ] and Au, [ 194 ] which all show enhanced photocatalytic performance compared to the undoped WO x . However, it was found that not all metal ion dopings shift the band positions of WO x towards the H 2 /H 2 O energy level. In particular, Ni [ 192 ] and Ag [ 193 ] doping were found to be promising for H 2 evolu-tion, while the band positions of WO x doped with Fe, Cu [ 192 ] and Au [ 194 ] shifted downwards, which improved the oxidation of water.

© 2011 WILEY-VCH Verlag GAdv. Funct. Mater. 2011, 21, 2175–2196

4.4. Optical Recording Devices

Driven by the need for high-density and reversible informa-tion storage, optical recording is an advanced technology, which has seen application in everyday life. [ 195 ] The initial explora-tion of WO x for this fi eld used a combination of photochromic and electrochromic effects to record digital images. [ 1 ] Later Lu et al. [ 196 ] showed that an amorphous WO 3 thin fi lm could be colored by a single pulse of KrF excimer laser light at 248 nm and bleached by a single pulse of Nd-Y-Al-garnet laser light at 1.06 μ m in air. Experiments have also been conducted that demonstrate the use of laser light to change the crystal phase of WO x fi lms with associated changes in optical transmission. [ 197 ] This technique has been used to achieve a storage capacity of 25 GB on a single platter.

4.5. Sensing Applications

Precise and economical monitoring of chemical gases is critical for human health, industrial processes, and environmental pro-tection. WO x has a long history for sensing applications, espe-cially in gas sensing. [ 198 ] The response of WO x gas sensors can be based on a range of effects such as electrochemically induced changes in resistance and alteration of optical properties. An elevated operation temperature is crucial to the gas sensing capability of WO x , as it enhances the sensitivity and reduces the response and recovery times. [ 199 ] Noble metal catalysts such as Pt, Pd, and Au, which are coated on the oxide surface, are found to improve the sensing capabilities and tend to promote chemical reactions by reducing the activation energy between the oxide and the target gas. [ 200 ]

The sensing mechanism of a conductometric type-WO x gas sensor is well established. [ 199 ] Upon exposure to oxidizing gases, such as O 3 , NO 2 , [ 201 ] and CO 2 , [ 202 ] the surface free elec-trons are captured to reduce these gas species forming oxygen ions which adsorb onto the oxide surface. Therefore, the con-ductivity of WO x is decreased proportionally to the amount of target gas adsorbed. Conversely, exposure to reducing gases such as H 2 , H 2 S, NH 3 , CS 2 , CO and some alcoholic vapors, [ 125 ] increases the conductivity of the WO x sensing layer.

Nanostructured WO x has been intensively studied for gas sensing applications as it enhances sensor capabilities; reduces power consumption, and produces excellent reproducibility. [ 199 ] Many of the enhanced features of the nano-WO 3 sensor are mainly ascribed to the dramatically increased surface area for reactions; single crystallinity, and possible complete depletion of carriers within the nanostructure when exposed to the target gas. [ 125 , 199 , 203 , 204 ]

For a WO x gas sensor based on optical modulation, the fun-damentals lie within the gasochromic effect of WO x , which was explained in Section 2.5. We have reported a comprehensive study on optical modulation of nanotextured-WO 3 thin fi lm based sensors towards H 2 exposure, [ 205 ] for which the WO 3 surface morphology, the absorbance spectra and the dynamic response to H 2 are shown in Figure 20 . Examples of WO 3 based-fi bre-optic sensors can also be found in a report by Ito et al. [ 206 ] Based on the principles of H 2 sensing, WO x was also explored in biological sensing. A study was conducted by Deb and his

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Figure 20 . SEM images of (a) RF sputtered WO 3 surface, (b) absorbance versus optical wavelength for Pt/WO 3 fi lms exposed to 1% H 2 at room temperature. and (c) dynamic response when exposed to different con-centrations of H 2 at 100 ° C for a single wavelength 660nm. Reproduce with permission. [ 205 ] Copyright 2010, Elsevier.

co-workers [ 1 ] using WO 3 for selecting particular mutants of green algae that produced H 2 in water with increased O 2 pres-sure. On a less reported front, sensing based on perturbation in permittivity was explored by Dong et al. [ 207 ] as a capacitive humidity sensor fabricated using WO 3 deposited on a Si nano-template. A sensitivity of over 16 000 was achieved at an optimal operating frequency of 1000 Hz.

4.6. Field-Emission Applications

Field-emission, also known as Fowler–Nordheim tunneling, is usually considered to be a form of quantum tunneling. It

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is highly utilized in X-ray generators, electron microscopy and spectroscopy as well as in display devices. [ 208 ] In the fi eld-emission process, electrons travel from an emitting material (cathode) to the anode with the assistance of a high electric fi eld. The emitting capability is believed to be highly dependent on both the properties of the material and on the confi gura-tion of the cathode. It is know that materials with higher aspect ratios and sharp edges generally produce higher fi eld-emission currents. [ 209 ] Nanostructured materials are under intensive investigation for fi eld-emission applications due to their high current densities, fast response times, compactness, and sus-tainability. [ 209 ] So far mostly elongated WO x nanostructures such as nanowires and nanorods have been explored for fi eld emission applications. Zhou et al. [ 85 ] reported fi eld-emission measurements using a 3D nanowire network of WO 3 with some planar oxygen defects, with which a turn-on fi eld (to pro-duce 10 μ A cm − 2 current density) of 13.85 V μ m − 1 at 200 μ m distance between the cathode and the anode was achieved. This result is comparable to the reported fi eld-emission properties of MoO 3 nanobelts, [ 210 ] ZnO nanowires, [ 209 ] and In 2 O 3 nanopyra-mids. [ 211 ] Similar fi eld-emission results for WO x nanorods and nanowires were obtained by a number of research groups, [ 28 , 212 ] and it was found that the diameters and the chemical composi-tion of the materials played important roles in enhancing the fi eld-emission results. In a recent report, Chen et al. [ 213 ] suc-cessfully fabricated a double-gated fi eld-emission display (FED) based on WO 3 nanowire pixels, which showed high contrast and brightness. Figure 21 presents such FED devices in opera-tion as well as the morphology of the WO 3 nanowires used for the application.

4.7. High- T c Superconductor

High-temperature superconductors (HTS) are materials that have a superconducting transition temperature ( T c ) above 30 K. They are used in mass spectrometers, particle accel-erators, and possibly in power transmission applications to reduce loss. [ 1 ] Sheet superconductivity at 3 K in twin walls of a non-superconducting tetragonal WO 3 − x was fi rst reported by Aird et al. in 1998. [ 214 ] This was followed by the discovery of sodium doped WO 3 (Na 0.05 WO 3 ) possessing superconductivity at 91 K. [ 215 ] Although this work attracted less attention than high- T c cuprates, it was the fi rst non-cuprate HTS material. [ 1 ] Recently Reich’s group obtained a possible 2D H x WO 3 super-conductor with a T c of 120 K, which is very close to the highest T c (135 K) of HTS material reported so far. [ 216 ] To date, all the reports for superconducting WO x and its compounds are based on bulk materials; hence, it will be interesting to see the effect of nanostructured WO x or its compounds in superconductor applications.

5. Conclusion and Outlook

We have presented an overview and explanation of the funda-mental properties of nanostructured WO x , have surveyed a broad selection of methods of synthesis and observed material charac-teristics, and have reviewed a number of possible applications

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Figure 21 . a) Arabic numerals and Chinese characters displayed by the double-gated FED, b) the cross-sectional view of the WO 3 nanowires used for the FED fabrication. Reproduce with permission. [ 213 ] Copyright 2007, American Institute of Physics.

for this material. The key points that can be summarized from this review are as follows.

It was shown that WO x can be obtained in a range of crystal phases and that its phase transitions are rather complex. Reports indicate that by nanostructuring WO x , the energy required to transform the phases was considerably less than that of the bulk material, and that uniquely the orthorhombic phase can be retained at room temperature. This is a very important property of nanostructured WO x , as in addition to amorphous, triclinic, monoclinic and hexagonal phases, which are stable at room temperature, the orthorhombic phase can also be evaluated as a possible candidate for the inclusion in practical devices.

There have been few studies on the effects of reducing the dimensions of nonstoichiometric WO 3 and WO 3 hydrates and this may represent an important area for future research.

Although the electronic band structure, electrical conduc-tivity, optical properties, chromic effect, and photocatalytic properties of WO x as well as the respective changes induced by nanostructuring are well documented, there are still many fundamental issues that need to be addressed. For instance, the high carrier mobility observed for WO x nanowires requires thorough experimental confi rmation, and it will be interesting to see computational modelling of the electronic behavior within the nanostructures of WO x .

There are fascinating properties that WO x possesses such as thermoelectricity and ferroelectricity. Despite the fact that there is a very limited number of reports are available, the authors believe that the study of nanostructured WO x might provide possibilities to gain new insights into the nature of such prop-erties and present opportunities for the advancement of these very active fi elds.

Doping of WO x in nanostructured form is another important area worthy of further exploration. The recently reported capa-bility to obtain atomically thin WO x layers using exfoliation, presents new opportunities for ion intercalation with signifi -cantly enhanced chromic response.

Investigating the synthesis techniques of nanostructured WO x is equally important for understanding its fundamental properties as well as its practical applications. We have presented a variety of synthesis techniques for producing nano structured

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, WeinhAdv. Funct. Mater. 2011, 21, 2175–2196

WO x . A vast range of WO x nanostructures have already been reported, including nano-particles, nanoplates, nanowires, nanorib-bons, and nanoporous thin fi lms, among others. In fact, this variety is vital for the commercialization of nanostructured WO x -based devices, since certain morphologies and dimensions are better suited for par-ticular applications than others, and dif-ferent synthesis techniques provide fl exibility within the constraints of particular industrial circumstances. Therefore, it is very important for researchers to continue the exploration of nanostructured WO x synthesis in more inno-vative routes as well as the investigation of the growth control and mechanism.

Finally, for the applications of WO x , we have detailed the traditionally popular areas such as electrochromic devices, photoca-

talysis, and sensing devices. The studies in these areas are relatively advanced. However, there is still signifi cant room for improvement of the capabilities of these devices through exploi-tation of nanostructured WO x . In addition, we have reviewed some pioneering studies of WO x , which are used in dye-sen-sitized solar cells, optical data storage, fi eld-emission displays, of high- T c superconductors. Although WO x may not be the optimal material for each of these applications, it can provide an effective alternative in circumstances where the optimal material is simply not practical. Furthermore, since WO x can be well dispersed in liquid in a variety of nanostructured forms, one should consider the opportunity for its use in drug delivery and sensors in microfl uidic systems.

Received: November 24, 2010 Revised: January 31, 2011

Published online: May 24, 2011

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