Application of Nanostructures in Electrochromic Materials and Devices: Recent Progress

The recent progress in application of nanostructures in electrochromic materials and devices is reviewed. ZnO nanowire array modified by viologen and WO3, crystalline WO3 nanoparticles and nanorods, mesoporous WO3 and TiO2, poly(3,4-ethylenedioxythiophene) nanotubes, Prussian blue nanoinks and nanostructures in switchable mirrors are reviewed. The electrochromic properties were significantly enhanced by applying nanostructures, resulting in faster switching responses, higher stability and higher optical contrast. A perspective on the development trends in electrochromic materials and devices is also proposed.


Introduction
Electrochromism, a reversible change in a material's optical properties (transmittance, absorbance and reflectance) under an applied voltage [1][2][3], is an old phenomenon which was discovered 40 years ago [4]. Since its discovery, considerable progress has been achieved in the syntheses of electrochromic (EC) materials, the fabrications of EC devices, the improvements of EC properties and OPEN ACCESS the applications of EC materials that have been extended to smart windows, displays, antiglare mirrors and active camouflages [5][6][7][8][9][10][11][12]. Among them, smart windows represent an important application because they can effectively save energy by regulating solar heat gain, and provide indoor comfort by reversible color changes. EC smart windows have become more and more significant because the warming climate and energy crisis require a marked and substantial energy-saving to combat conventional energy source consumption.
Many inorganic and organic materials exhibit EC properties. Common inorganic EC materials include WO 3 [13], NiO [14], TiO 2 [15], V 2 O 5 [16] and PB [17]; while viologens [18], polyaniline (PANI) [19], poly(3,4-ethylenedioxythiophene) (PEDOTs) [20] are common organic and polymer EC materials. For the inorganic EC materials, WO 3 , TiO 2 , V 2 O 5 films show cathodic coloration states under negative potentials and bleaching states under positive potentials; the coloration/bleaching of these EC materials results from the insertion/extraction of electrons and charge balancing ions (H + , Li + , Na + , K + ions) accompanied by the reduction/oxidation reactions [21]. In contrast, nickel oxide [22] and iridium oxide [23,24] show anodic coloration under a positive potential because their reduced states are colorless and oxidized states are colored. Prussian blue (PB), with a blue color in its original state, can be bleached to a colorless state (reduced state) under a negative potential and can be recovered to its coloration state (oxidized state) under a positive potential [25].
Electrochromism involves the ions insertion/extraction into/out of EC materials, so nanostructures with small sizes and large specific surface areas, are expected to facilitate the ion insertion/extraction process, and then to enhance the properties of EC materials and devices. EC materials, devices and their applications have been well reviewed by experts in the field, especially by Granqvist [26][27][28][29][30][31] and Deb [3]. In this review, we emphasize recent progress in applying nanostructures in EC materials and devices. Some interesting nanostructures, including ZnO nanorod array, crystalline WO 3 nanoparticles and nanorods, mesoporous WO 3 , mesoporous TiO 2 , PEDOT nanotubes prepared from anodic aluminum oxide (AAO) template, V 2 O 5 nanowires, PB nanoinks and nanostructures in switchable mirrors, are briefly reviewed and a perspective is proposed.

ZnO Nanowire Array
ZnO nanowire arrays have been fabricated by both gas phase and solution phase methods. Their applications were mostly focused on photoluminescence, electroluminescence, gas sensors and transistors. Recently, Vayssieres [32] reported a simple aqueous thermal decomposition method to fabricate uniform and large-area ZnO nanowire arrays on indium tin oxide (ITO) coated glass at a low temperature (<100 °C), which does not affect the conductivity of the substrate.
Sun and Wang [33] successfully applied ZnO nanowire array in EC device and fabricated a fast-switching display ( Figure 1). ZnO nanowire array was modified with viologen molecules which act as the EC material ( Figure 2a). The ZnO nanowire electrochromic device exhibited fast switching time (170 and 142 ms for coloration and bleaching, respectively, which is faster than those of the EC devices based on viologen molecules. The coloration time is defined as the time it takes for the reflectance to decrease by 2/3 of the reflectance difference between the steady bleached and the colored state.), high coloration efficiency (196 C −1 cm 2 ) and good stability ( Figure 3). The improved EC properties of the ZnO nanowires EC device is attributed to the large surface area, well-crystalline structure and good electron transport properties of the ZnO nanowire array. The fabricated ZnO nanowire electrochromic device is promising for application in low-cost -electronic papers‖ (e-papers).

Crystalline WO 3 Nanostructures
Most of the EC investigations on WO 3 were focused on its amorphous films [35][36][37]. Compared with the amorphous structure, crystalline WO 3 is much more stable due to the denser structure and slower dissolution rate in electrolytes. However, crystalline WO 3 bulk material usually has slow switching response. To improve the switching response, nanocrystalline WO 3 were applied in EC materials and devices in recent years.
Dillon et al. [38] has synthesized crystalline WO 3 nanoparticles and nanorods ( Figure 5a) by hot-wire chemical vapor deposition and fabricated an electrochromic film by electrophoresis deposition. The fabricated crystalline WO 3 nanoparticle film has greater charge density for H + ions intercalation, which is attributed to a larger active surface area and the low density of the films. Compared with amorphous WO 3 film, the crystalline WO 3 nanoparticle film exhibited better cycling stability in H 2 SO 4 solution ( Figure 5b). Wang et al. [39,40] hydrothermally synthesized uniform crystalline WO 3 nanorods ( Figure 6) with NaCl as the capping agent firstly, and fabricated a transparent EC film by a drop-casting process. The synthesized WO 3 nanorods were assembled on the surface of the film, showing tunable coloration (green, green-blue, and blue colors) at different voltages (Figure 7), high stability ( Figure 8) both in LiClO 4 and H 2 SO 4 electrolytes, comparable contrast and switching coloration/bleaching responses. The assembly of WO 3 nanorods allows sufficient Li + ions to be intercalated into WO 3 nanorods, resulting in a high contrast. The displayed colors of the WO 3 nanorod film can be tuned by changing the applied voltages to control the amount of intercalated Li + ions into WO 3 nanorods. The crystalline structure of the synthesized WO 3 nanorods greatly enhances the stability of the EC film. The coarse surfaces of nanorods and interstices among nanorods increase the specific surface area of the film and accelerate the intercalation/deintercalation of Li + ions, resulting in fast coloration/bleaching switching. The WO 3 nanorod film can be used in H 2 SO 4 electrolyte due to the crystalline structure. Compared with the EC switching characteristics of WO 3 nanorod film in lithium-based electrolyte, faster response, much higher charge density, comparable coloration efficiency and contrast were achieved in H 2 SO 4 electrolyte.  Figure 6a shows the assembly of WO 3 nanorods. Reproduced with permission from Ref. [39]; Copyright 2008 American Chemical Society.  Recently, Jiao et al. [41] prepared crystalline plate-like WO 3 nanostructures ( Figure 9) on fluorine-doped tin oxide (FTO) coated glass by a crystal-seed-assisted hydrothermal method. The hydrothermally grown film is well adhesive to the substrate, which is attributed to the use of crystal seeds. The film, consisting of crystalline WO 3 nanostructures, exhibited tunable transmittance modulation (Figure 10a) under different voltages, high cycling stability (Figure 10b), and high optical contrast ( Figure 11).  According to the above results, we draw a conclusion that crystalline WO 3 nanostructures improve the cycling stability (due to the crystalline structure) of the EC materials and devices without degrading the switching responses, contrast and coloration efficiency (due to the nanoscale structure). This is important progress towards the practical application of high-performance EC devices.

Mesoporous WO 3
Mesoporous EC materials have quite thin walls and large specific surface areas, which makes them interesting for enhancing EC properties. Baeck et al. [42] have successfully synthesized mesoporous tungsten oxide films with lamellar structure (Figure 12) by electrodeposition using a templating agent. Lamellar mesoporous tungsten oxide shows greater current density for hydrogen intercalation and faster switching responses ( Figure 13) compared to nonporous structures. The functional improvements may be due to the larger surface area of mesoporous tungsten and facilitated charge transport.  Brezesinski et al. [43] synthesized mesoporous WO 3 EC thin film ( Figure 14) by evaporation-induced self-assembly using a block-copolymer template. The three-dimensional mesoporosity significantly improves the electrochromic response times, because of shorter diffusion pathlengths compared to the dense materials ( Figure 15). The combination of mesoporosity and crystallinity leads to improved reversibility of ion insertion/extraction and cycling stability. Amorphous mesoporous WO 3 films suffer from irreversible degradation because of their structural modifications. By annealing at 550 °C in air, fully crystalline mesoporous WO 3 thin films are obtained. Sallard et al. [44] also prepared crystalline mesoporous WO 3 films ( Figure 16) by annealing amorphous mesoporous WO 3 films. The crystallinity degree ranging from fully amorphous to crystalline can be adjusted without mesostructural collapse. Electrochemical results show that only the highly crystalline mesoporous WO 3 films exhibit long-term EC stability.   Though mesoporous WO 3 films have faster responses due to the large specific surface area, the EC stability is dependent on the crystallinity of the mesoporous structure. The film coating process using the WO 3 mesoporous structures also needs to be developed to achieve good-adhesion EC films on transparent substrates.

Mesoporous TiO 2
EC device based on dye-modified semiconductor electrodes has much improved switching response and enhanced contrast, especially for displays such as electronic paper and billboards [45][46][47][48][49]. The key part of these devices is a working electrode composed of a nanocrystalline semiconductor modified with electrochromophoric molecular species, such as redox active viologen derivatives. TiO 2 nanostructures have been successfully used for this purpose, resulting in high-performance EC devices.   Choi et al. [50] synthesized viologen modified mesoporous TiO 2 film ( Figure 17) with hexagonally close-packed mesopores and channel walls made of 8-10 nm anatase nanostructures. The film exhibited similar switching speed and reversibility as nanocrystalline titania but better contrast (Figure 18 and 19). The higher contrast can be attributed to a contiguous pathway of well-connected anatase nanocrystallites arranged into a well-defined mesoporous architecture, which results in a greater volume density of loaded viologen molecules.
Weng et al. [51] demonstrated a high-speed passive-matrix EC display using a leuco modified mesoporous TiO 2 electrode with vertical porosity (Figure 20). The device exhibited better background whiteness, which improves readability and reduces eyestrain. Imaging and erasing can be carried out by applying a potential of ±3.0 V. The thickness of the mesoporous layer critically affects the contrast of the displayed image. Upon writing, the clear images can remain for a few minutes without becoming blurry because the vertical pores of the electrode can support effective diffusion of leuco dyes perpendicular to the electrode and prevent the diffusion of the dye around the electrode. The leuco modified mesoporous TiO 2 electrode shows potential for the realization of a full-color reflective display for use in e-papers.

PEDOT Nanotubes
PEDOT and its derivatives are good polymer EC materials for e-paper due to their high color contrast, good mechanical stabilities, and facile fabrication. Cho et al. [52] have synthesized PEDOT nanotubes using AAO as the template by electrochemical polymerization within the pores of the alumina films ( Figure 21). The thin walls of PEDOT nanotubes (Figure 22b) can provide ions with short diffusion distances (10-20 nm), which allows very fast switching response, while the micrometer length of the PEDOT nanotubes can produce clear coloration contrast for the EC display system. The wall thickness of PEDOT nanotubes can be adjusted by controlling both the applied potential and the monomer concentration.
The EC device can be switched from pale blue (bleached and oxidized state of PEDOT) to deep blue (colored and reduced state of PEDOT) by applying alternating square potentials between 1.0 V and −1.0 V (Figure 23). Both the coloration and bleaching processes show quite fast switching responses of less than 10 ms, which are fast enough for display applications. However, the contrast in this EC device is not high enough. To realize a practical application in EC display devices, the contrast still needs to be further enhanced.

V 2 O 5 Nanowires
Xiong et al. [53] prepared layered silver vanadium oxide (SVO) nanowires and V 2 O 5 nanowires by hydrothermal process. The as-made nanowires are over 30 µm in length and 10-20 nm in diameter, corresponding to an aspect ratio of over 1500 ( Figure 24). The nanowires with diameters of 10-20 nm have a short Li + ions diffusion distance. The EC device displayed the coloration/bleaching between green (colored state) and red-brown (bleached state) ( Figure 25). The coloration/bleaching switching responses for half of the total transmittance change, are about 0.1 s (coloration time) from red-brown to green and 0.2 s (bleaching time) for the reverse process at 633 nm (Figure 26a). For V 2 O 5 nanowires, the coloration and bleaching times are 1 s from brown to green and 6 s for the reverse process (Figure 26b), respectively. The SVO nanowire-based EC device displayed color-switching responses over 20 times faster than those of the V 2 O 5 nanowire-based EC device. These enhancements in EC properties of SVO nanowires should be attributable to their better electrical conductivity (0.5 S/cm) than V 2 O 5 nanowires (0.08 S/cm) and the introduction of Ag atoms in the V 2 O 5 framework further enlarged the interlayer spacing, resulting in a 7 times higher Li + ions diffusion coefficient than that of V 2 O 5 nanowires.

PB Nanoinks
PB, iron (III) hexacyanoferrate (ІІ), a synthetic coordination-compounded pigment that has a three century history [54], is the prototype of a number of polynuclear transition metal hexacyanometallates which form an important class of insoluble mixed-valence compounds [55]. They have the general formula M' k [M''(CN) 6 ] l (l, k integral), where M' and M'' are transition metals with different formal oxidation numbers with the possibility of change/electron transfer between their different oxidation states. For PB, the transition metal M is iron (Fe), i.e., ferric ferrocyanide.
Generally, PB thin films are fabricated by an electrodeposition method through a bath containing iron (III) and hexacyanoferrate (ІІ) ions [65][66][67]. The detailed formation mechanism, corresponding reactions and chemical composition of electrodeposited PB films have been well summarized in previous reports [68,69]. Recently, water-soluble and organic-solvent-soluble PB nanoparticles inks have been successfully prepared by surface modification [70][71][72][73]. The strategies for the process are schematically shown in Figure 27. By surface modification of the insoluble PB pigment (agglomerated nanoparticles) with [Fe(CN) 6 ] 4− ions and oleylamine, water-soluble and organic-solvent-soluble PB inks could be obtained, respectively. A similar ink preparation is also applicable to nickel hexacyanoferrates (Ni-PBA) and cobalt hexacyanoferrates (Co-PBA). The PB (blue), Ni-PBA (yellow), and Co-PBA (red) nanoinks function as the three primary colors for color displays and more colors can be obtained by mixing different nanoinks ( Figure 28), which is an important progress for achieving full-color displays.  The soluble PB nanoinks can be uniformly spread on substrates, and PB films composed of aggregated nanoparticles can be fabricated after solvent evaporation. The EC thin films fabricated by spin-coating of PB nanoinks are usually not well-adhesive to transparent conductive substrates, resulting in elution from electrolytes. To overcome this difficulty, Omura et al. [74] successfully developed a method for controlling the elution using a simple electrochemical treatment. The PB nanoinks can be further patterned as desired by conventional lithography. Due to the solubility of the nanoinks, micro-fabrication processes, such as spin-coating, ink-jet printing and photolithography can be used in the fabrication of PB-based EC devices, which is cost-efficient, facile and easily scaled-up.

Nanostructures in Switchable Mirrors
Switchable mirrors can reversibly change their optical properties between reflective and transparent states as a result of hydrogenation and dehydrogenation [75][76][77][78][79]. Almost all heat and light can be reflected in their metal states, which is significant in energy-saving smart windows. Rare earth metals [80], magnesium lanthanide alloys [81], Mg-Ni alloys [82,83], Mg-Ti alloys [84] have been used as switchable mirror materials. However, rare earth metals are easily oxidized; Mg-Ni alloys are not color neutral; Mg-Ti alloys have low visible transmittance in their transparent states. Thin overlayers of palladium have been successfully used to protect the switchable mirrors against oxidation and enhance the kinetics of hydrogen insertion and extraction (acting as a catalyst for introduction and removal of hydrogen) [85,86].
Recently, Yamada et al. prepared switchable mirrors based on Mg-Ca alloy thin films by magnetron sputtering [87]. The switchable mirrors looked completely color neutral with high visible transmittance in the transparent state, which may be attributed to the grayish neutral hydrogenated state of MgH 2 and MgCaH 3.72 .
Metallic copper exhibits very high near-infrared reflectance and much less reflectance below ~600 nm; Cu 2 O has low reflectance in both the visible and near-IR spectra. Metallic thin films of antimony and bismuth, deposited on glass substrates by vacuum evaporation, can be reversibly converted to transparent, semiconducting lithium pnictides by cathodic polarization in a nonaqueous lithium electrolyte. These characteristics make copper-copper (I) oxide, Sb-Li, Bi-Li [88] and Sb-Cu-Li [89] attractive in switchable mirrors.

Conclusions and Perspective
Since the discovery of EC materials, extensive investigations have been carried out to improve their EC properties, including enhancement of the switching responses; contrast; coloration efficiency and cycling stability; as well as EC device fabrications and applications. ZnO nanowire/nanorod arrays modified by EC materials, mesoporous and tubular EC materials significantly enhance the switching responses of EC devices, for potential application in fast-switching displays and e-papers. Crystalline WO 3 nanoparticles, nanorods and crystalline mesoporous WO 3 exhibit enhanced EC stability and contrast, especially in acidic electrolytes. Soluble PB and its analogues Ni-PBA, Co-PBA nanoinks exhibit blue, yellow and red colors, promising the possibility of achieving full-color displays. Due to the solubility of the nanoinks, micro-fabrication processes, such as spin-coating, ink-jet printing and photolithography, can be used in the fabrication of PB-based EC devices, which is cost-efficient, facile and easily scaled-up. Switchable mirrors are the most effective energy-saving EC devices in all kinds of smart windows. However, the metal alloys are easily oxidized, resulting in a poor long-term stability.
Further efforts are needed to accelerate practical applications of EC devices. For large-area EC devices, the difficulties may be in slow responses, poor stability and high cost. To overcome these difficulties, the synthesis of new nanostructured EC materials needs to be developed. The ideal nanostructures for EC materials may include ultrathin crystalline nanorods, nanowires or nanotubes, crystalline mesoporous structures and quantum dots. These nanostructures with large specific surface areas, are expected to possess fast and stable EC switching. Low-cost synthesis routes for these EC nanostructures are favorable. The coating process for EC nanostructures also needs to develop to obtain well-adhesive EC films on transparent conductive substrates. The combination of different kinds of EC materials is also worthwhile, for example, to exhibit multi-colors and to enhance the coloration modulation and stability. Complimentary EC devices need further development to enhance the contrast, coloration efficiency and stability of EC devices. Inorganic-organic hybrid EC devices are also promising for practical application, for example in e-papers. Besides the developments of EC materials and devices, transparent conductive substrates with better conductivity, solid-state electrolytes, advanced sealing and packaging technologies, are also critical and deserve intensive investigations for the practical applications of EC materials and devices. Gallium doped ZnO (GZO), antimony doped tin oxide (ATO), carbon nanotubes or graphene coated glass are promising substitutes to replace ITO. Low-cost processes for the fabrication of EC devices must be developed to realize their practical applications, which need the cooperation of researchers and technicians to solve both scientific and technical challenges.