Source: https://pubs.rsc.org/en/content/articlehtml/2018/nh/c8nh00016f?page=search
Timestamp: 2019-04-18 14:21:46+00:00

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Zhongqiu Tong received MS and PhD degrees under the supervision of Prof. Yao Li in the Center for Composite Materials and Structure, Harbin Institute of Technology (HIT). In 2016, he joined the School of Materials Science and Engineering as a faculty member at Southwest Petroleum University in Chengdu, China. His research interests focus on fabrication of metal oxide nanomaterials and their applications in electrochemical energy storage and electrochromism.
Shikun Liu received his Master degree from QiLu University of Technology. He is currently pursuing his PhD degree under the co-supervision of Prof. Jiupeng Zhao and Prof. Yao Li at School of Chemistry and Chemical Engineering, Harbin Institute of Technology. His research interests mainly focus on the design and synthesis of nanomaterials for electrochemical energy storage and electrochromism.
Yao Li has been a Professor of Materials Science in the Center for Composite Materials and Structure, Harbin Institute of Technology (HIT) and a council member of Chinese Materials Research Society. He was named as “New Century Excellent Talents Program Scholar” in 2006, “Youth leader in Science and Technology Innovation” in 2015, “Yangtze River Scholar” in 2016, and “Ten Thousand Talent Program Scholar” in 2017. He was awarded “Science and Technology Award for the Youth of China” in 2013 and “National Award of the outstanding Scientific and Technological Workers” in 2014. His research interests include fabrication of inorganic materials and conductive polymers with well-defined nano/micro structures for energy storage and electrochromism. He is the author or co-author of over 130 papers in peer-reviewed journals, 62 patents, and 3 books in the field of materials science. His international standing can be substantiated by his delivery of over 30 keynotes or invited talks in international conferences including the International Meeting on Electrochromism and IUMRS-ICA, and organization of >10 international conferences/symposia, etc.
Over the past decades, chromism-related phenomena have received immense research attention due to their broad display and energy-related applications.1,2 Chromism is regarded as reversible color and optical changes of a material or composite materials derived from an external stimulus. Based on the types of stimulus, chromogenic technologies involving electrochromic, photochromic, thermochromic and gasochromic technologies can be used in various different fields.3 Electrochromism can be defined as color and optical parameter changes in the visible spectrum controlled by a temporarily applied electrical voltage.4,5 In some cases, the optical parameter changes in the near-infrared (NIR) and infrared (IR) spectrum regions are also used. Compared to other chromogenic types, electrochromism demonstrates some unique advantages, such as low energy consumption and operating voltage, multiple and high chromogenic states, high and reversible cycling stability, and reasonable memory effect. Thus, electrochromism has been demonstrated in various commercial applications, such as in smart windows, display devices, anti-glazing mirrors and spacecraft thermal control.
Because the electrochromism in materials is from reversible electric-field-induced redox processes, nanostructuring is an effective method to improve the performance.4,5 Among various nanostructures, one-dimensional (1D) morphologies are very applicable for electrochromism.6 For basic 1D nanostructures such as nanorods, nanofibers and nanoribbons, their width and thickness (or diameter when the 1D nanostructures exhibit a cylindrical morphology) are confined to the nanoscale range between 1 and 100 nm, while their lengths can be several micrometers, even up to hundreds of micrometers or a few millimeters. The small diameter scale of 1D nanostructures is rather suitable for accelerated electrochromic redox kinetics, while the large scale of length in 1D nanostructures reasonably matches the macroscopic world for many electrochemical and physical measurements, including electrochromic tests.7 In addition, the long length but short diameter characters of basic 1D nanostructures indicate the ease and high efficiency of fabricating nanorod, nanofiber and/or nanoribbon-knitted porous complex nanoarchitectures, such as nest- and urchin-like morphologies.
Various self-supported 1D nanostructured morphologies of electrochromic materials have been developed and investigated. This review will focus on the recent advancements on the self-supported 1D nanostructures for electrochromic devices. The first part of the review is centered on electrochromic advantages of self-supported 1D nanostructures. Then these nanostructures are separately discussed in two main categories including template-derived and template-free morphologies, followed by discussion about 1D core/shell nanostructures, which are a special type of complex 1D nanostructure with two components or phases. The emphasis is to correlate the morphologies, components and interfacial interactions of the electroactive materials to their electrochromic properties and illustrate how these nanostructures influence the electrochromic redox kinetics and offer advantages. A brief discussion about the application of self-supported 1D nanostructures in electrochromism-involving multifunctional devices is further presented. A future outlook for the self-supported 1D electrochromic nanomaterials will also be presented.
The origin of studies on electrochromism is usually traced to the pioneering work by Deb in 1969.9 He found that tungsten oxide (WOx) films can be blued in acid solution once a negative electric stimulus was applied, while the blue color was bleached under positive electric stimulus. This facile optical parameter modulation aroused worldwide research enthusiasm. Electrochromism was found in many transition metal oxides, such as titanium dioxide (TiO2), vanadium pentoxide (V2O5), molybdenum trioxide (MoO3), nickel oxide (NiO), cobalt oxide (Co3O4), niobium pentoxide (Nb2O5), and tantalum pentoxide (Ta2O5).10,11 For example, in 1989 Cogan et al. found that the color of a vanadium oxide film can be changed from yellow to deep blue in a lithium salt organic electrolyte.10 The authors attributed the color changes to the double injection of lithium ions and electrons into the vanadium oxide crystalline lattice.
The merits of transition metal oxides for electrochromism typically include: high electrochromic memory effects, long-term cycling stability, good mechanical strength, desirable environmental durability, and especially high durability under ultraviolet exposure outdoors when used as smart window electrode materials. However, their electrochromic kinetics is rather unsatisfactory when the transition metal oxides are not in the nano-region. In addition, the relatively low coloration efficiency of bulk transition metal oxides is a non-negligible obstacle for electrochromic applications, because a low coloration efficiency means that a high energy consumption is needed to fulfill coloration state switching.
Conductive polymers are another large family of materials used for electrochromism.12,13 The typical structure of these polymers includes a conjugated π bond on the main chain. The optical modulation arises when electrochemical doping/de-doping occurs on the π bonds. For example, in 1984 Kobayashi et al. found that a polyaniline film exhibited four color changes (transparent yellow at −0.2 V, green at 0.5 V, dark blue at 0.8 V, and black at 1.0 V) in 1 M HCl solution.12 These polymeric electrochromic materials demonstrate the advantages of fast redox kinetics, vivid color versatility, high optical modulation, rapid response times, low power consumption, and ease of manipulation of properties through structural modifications. However, the cycling stability, mechanical strength, and environmental durability are not as desirable as for transition metal oxides. Commonly used conductive polymers for electrochromism include polyaniline (PANI), polypyrrole (PPy), polythiophene (PT), poly(3,4-ethylenedioxythiophene) (PEDOT), polycarbazole, and their derivatives.
Hybrid materials, including inorganic–inorganic and organic–inorganic categories, have been a hotspot of the research community.4,14 The prime reasons behind this popularity are the synergetic properties offered by the resulting hybrids. They often combine the elasticity and functionality of each component to overcome the drawbacks of components. Hybrid materials are not simple physical mixtures. The interactions holding the two components together include weak interactions such as van der Waals forces and hydrogen bonds, and/or strong chemical interactions, which are beneficial for improving the structural integrity for long-term measurements. For inorganic–inorganic hybrids, their redox kinetics could be enhanced due to the efficient electron and ion transport derived from the interactions between electron bands of the two components. While for organic–inorganic hybrids, they can exhibit high optical modulation and rapid switching response derived from the conductive polymers as well as high thermal and chemical stability derived from the mechanical strength of the inorganic component. In this review, the main hybrids considered are self-supported 1D core/shell nanostructures.
Such redox-based optical response makes nanostructuring an effective approach to boost the electrochromic performance. The preparation of the electrochromic materials directly grown on TCO substrates (“self-supported” electrodes) is rather favorable for electrochromism due to the elimination of the incorporation of conductive additives or electrode binders and the added step of slurry casting during the electrode fabrication process.15,16 Furthermore, the formation of 1D nanoarchitectures, such as nanoneedle, nanorod, nanofiber, nanowire, and nanotube arrays, could significantly improve the electrochromic performance due to the electrochemical superiorities of these morphologies.6–8,17 Thus, the synthesis of 1D nanomaterials employing a self-supported strategy has become fascinating for electrochromism. Self-supported 1D nanostructures possess the general nanostructure-derived electrochemical and electrochromic advantages such as large surface area providing plenty of accessible electroactive sites for high coloration contrast, short ion diffusion distance for fast switching response, and enough voids for efficient strain relaxation, etc., while self-supported 1D nanoarchitectures also demonstrate several unique electrochemical and electrochromic advantages. Fig. 1 shows a schematic representation of an idealized self-supported 1D electrode.
Fig. 1 Schematic diagram illustrating the advantages of an ideal self-supported 1D nanostructure for electrochromism.
(ii) Strong attachment to the substrate and efficient 1D electron transport: for the majority of methods used to fabricate self-supported 1D nanostructured films, the electrochromic materials are directly grown on substrates, resulting in strong physical and chemical attachment between the electroactive materials and substrate, as well as continuous conductive pathways to the substrate, guaranteeing efficient electron transport for electrochromism. In addition, strong bonds ensure the structural integrity of the whole electrode films, efficiently preventing the detachment of electrochromic materials from the substrate, beneficial for the long-term cycling performance.
(iv) Ease of realizing the surface modification or fabrication of hybrid nanostructures: reactive liquids and gases (such as hydrogen, ammonia, and plasma) can easily penetrate and flow in the interconnected voids among the nanofibers or nanorods. Then reactions occur and may lead to mixed valences, new phases, defects, changes in the band structures, and/or production of new functional groups on the surfaces of the materials.
Using pre-prepared self-supported nanofibers and/or nanorods as templates to deposit another electrochromic material gives rise to self-supported core/shell hybrid nanostructures, which could exhibit enhanced coloration contrast and cycling stability, shortened switching response time, and increased coloration states. The coating layer can be the same material as that of the core but in a different phase, generating a “crystalline/amorphous core/shell” nanostructure.26 However, in most cases, the coating material is different from that of the core.27 As for the anodic preparation method, mixed metal oxides or doped nanotube arrays can be facilely prepared from anodization treatment on alloys.
(v) Ease of characterization and assembly of electrochromic devices: the strong structural integrity of self-supported 1D nanostructured electrode films indicates that these films can be directly used for electrochemical-optical characterization or device assembly.
(vi) Ease of realizing multifunctional electrodes or devices. Both electrochemical energy storage and electrochromism are from redox reactions, causing bifunctional integration into one film or device to be possible.28,29 However, the existing conflicts between electrochromism and electrochemical energy storage (such as fast switching response for electrochromism and high energy density for electrochemical energy storage) result in the necessity of delicately designed nanostructured morphologies. Fabrication of self-supported 1D nanostructures with high ion insertion/extraction kinetics is an ideal approach to solve these problems, as described in later sections. Furthermore, given the significance of self-supported 1D nanostructures for the emerging miniaturization of power sources aimed at integration into micro- and nano-electronic devices,8 fabrication of such self-supported 1D morphologies is becoming more and more vital and important for miniaturized electrochromic energy storage devices.
The templates widely used to fabricate self-supported 1D nanostructures are membranes with 1D parallel nanochannels. There are two types of typically used templates: polymetric (such as PC and nitrocellulose) and oxide-based (AAO) membranes. Their high pore density (up to 1011 cm−2) and wide pore diameter range (from tens to hundreds of nanometers) could ensure membrane-derived self-supported 1D nanostructures with a desirable areal density of electroactive materials, a controlled diameter scale and enough interspacing voids for efficient electrolyte penetration to achieve satisfactory coloration saturation. Meanwhile, the large thickness range of the membranes (from several to hundreds micrometers) gives feasible choice to prepare 1D nanostructures with controlled length.
Typical membrane-assisted preparation of metal oxide self-supported nanofiber and nanorod arrays is shown in Fig. 2a. Attaching the template to the TCO substrate, filling the nanochannels with precursors and then transforming them into the targeted metal oxides with subsequent annealing treatment whilst removing the membrane leads to the successful synthesis of self-supported metal oxide nanorod, nanowire, or nanofiber arrays (Fig. 2b).30 The tight attachment of the membrane to the substrate could effectively ensure strong physical and chemical contacts between the electroactive materials and the TCO substrate, resulting in efficient electron transport and long-term cycling stability. Meanwhile using different experimental details can control the morphologies of the 1D nanostructures, simply from nanorod arrays to nanofiber arrays. Notably, when the mechanical strength of the targeted 1D electroactive materials is not high enough or the length-to-diameter ratio of the targeted 1D electroactive materials is rather high, the tops of the nanorods or nanofibers could aggregate together, forming nanorod or nanofiber bunches instead of free standing and isolated nanorod or nanofiber arrays.
Fig. 2 (a) Schematic diagram illustrating the template-assisted synthesis of self-supported 1D metal oxide nanostructures. Step 1 is attachment of membranes on the TCO substrates, followed by a precursor filling process. Step 2 is the removal of the membranes and transformation of the precursor into the targeted metal oxide. (b) SEM image of TiO2 nanorod arrays prepared using an AAO membrane. Reprinted from ref. 30. Copyright 2015 Springer. (c and d) SEM image of open-ended (nanorod bunches) and close-ended Ni(OH)2 nanorod arrays prepared using an AAO membrane. (e) Comparison of the transmittance modulation for the annealed Ni(OH)2 thin film, and close-ended and open-ended nanorod structures. Reprinted from ref. 35. Copyright 2014 Wiley.
Zheng et al. prepared arrays of WO3 nanorods with lengths of ca. 1.8 μm by direct current (DC) magnetron sputtering of WO3 on an aluminum lattice membrane (an AAO membrane with short thickness).32 The confined growth of WO3 in the 1D nanochannels led to the diameter of the self-aligned parallel WO3 nanorods being almost the same as the pores of the aluminum lattice membrane (about 200 nm). In 1 M NaCl solution, sodium-ion insertion under −0.8 V (vs. SCE) led to blue coloration, while bleaching under 1.2 V polarization resulted in a transparent state. The WO3 nanorod arrays demonstrated obvious transmittance modulation in the visible spectrum range with a maximum value of about 50% at λ = 600 nm. By using a two-step-oxidation prepared AAO membrane with a pore size of 80 nm as a template and a colloidal suspension as a precursor, arrays of WO3 nanorods with nanosized pores distributing regularly along the length of the nanowires were prepared.33 A two-step electrochemical anodization on Al–W overlapped metal layers to prepare Al-doped WO3 nanorod arrays was proposed by Park et al.34 The outer Al layer was anodized into AAO, followed by anodization of the W layer creating Al-doped WOx in the pre-formed nanochannels of AAO. The removal of the AAO template and annealing treatment produced self-supported nanorod arrays on a W substrate. With the assistance of AAO membranes on ITO substrates, Guo et al. prepared two types of self-supported Ni(OH)2 nanorod arrays by electrodeposition filling and removal of the membranes with 10% NaOH solution. Over deposition gave rise to a dense film on the top of the nanorod arrays, producing a close-ended morphology (Fig. 2c and d).35 Open-ended nanowire arrays demonstrated good electrochemical activity with superior transmittance modulation of ∼35% at λ = 635 nm, better than the close-ended nanowire arrays and dense film (∼20% and 17%, respectively) (Fig. 2e). Also, by comparison with other literature reports, the authors also found that the open-ended nanowire arrays exhibited higher coloration efficiency (50.5 cm2 C−1) than many other Ni(OH)2 nanostructures, further demonstrating the efficient redox kinetics for low energy consumption in electrochromic nanowire arrays. Yamada et al. used a two-step process to prepare branched Au/NiO nanorod arrays for accelerated electrochromic color changes.36 After Au nanorod arrays were prepared by AAO membrane-assisted electrodeposition, a branched NiO layer was deposited on the Au surface. Due to the high reflectivity, mechanical strength and electrical conductivity of Au, as well as the thin thickness of the NiO layer (15 nm), the branched Au/NiO electrode exhibited high and stable reflectance contrast over 0.4 at λ = 600 nm.
Polymetric membranes (PC membrane as representative) are another category of widely used templates to prepare self-supported 1D nanostructures. The preparation process also can be reflected in Fig. 2a. These polymetric membranes can be easily wiped out by annealing treatment in air or dissolution by organic solvents. Limmer et al. demonstrated a general technique to synthesize metal oxide nanorod arrays using sol electrophoretic deposition with appropriate track-etched hydrophilic PC membranes attached on the ITO substrates.37,38 Electrochromic TiO2, V2O5, and Nb2O5 polycrystal nanorods with diameters of ∼100 nm and a length of ∼10 μm were prepared. These oxide nanofiber arrays exhibited absorbance modulation in the visible spectrum range with obvious color contrast during the Li-ion insertion/extraction processes. Furthermore, Limmer et al. also found that the diameter of the V2O5 nanorods can decrease to 50 nm; meanwhile these nanorods were changed to be single crystals by using delicately designed experimental parameters.39 These single-crystal V2O5 nanorod arrays exhibited higher transmittance modulation and faster switching response speed compared to polycrystal V2O5 nanorod arrays and a sol–gel-derived V2O5 dense film. Li-ion storage properties detected by charge/discharge tests showed that single-crystal V2O5 nanorod arrays exhibited superior lithium storage.39,40 Higher lithium storage of the single-crystal V2O5 nanorod arrays confirmed their electrochromic advantages because the larger amount of inserted Li-ions indicated that a larger amount of V5+ ions took part in the redox reactions for electrochromism. Furthermore, compared to the polycrystalline V2O5 (10−3–10−2 S cm−1), the high electrical conductivity of single-crystal V2O5 nanofibers (0.5 S cm−1)41 positively influenced the Li-ion insertion/extraction kinetics, which was beneficial for improving the electrochromic performance. By using PC membrane-assisted sol electrophoretic deposition, single-crystal TiO2 nanofiber arrays were also prepared.42 Furthermore, self-supported mixed transition metal oxide nanorod or nanofiber arrays, such as V2O5–TiO2,43 can also be prepared using a mixed metal oxide sol solution. Self-supported 1D nanostructures of mixed transition metal oxides were believed to possess better electrochromic performance compared to the corresponding 1D nanostructures of a single metal oxide component, because of the multiple types of color centers and enhanced redox kinetics derived from doping effects.
The uniformly preferential growth of metal oxide precursors on the surface of nanochannels is a prerequisite for synthesizing nanotubes employing PC or AAO membranes. Atomic layer deposition (ALD) combining AAO membranes is a well-developed process to prepare self-supported metal oxide nanotube arrays.46,47 In an ALD process, the gaseous precursors travel into the voids of the membrane and are preferentially absorbed on the surface of the nanochannels to decrease the system energy, producing nanotubes attached on the surface of the nanochannels (Fig. 3a). Furthermore, a surface layer covering on the membrane is usually produced in the ALD process. The removal of the membrane and the surface layer produces metal oxide nanotube arrays. Fig. 3b and c demonstrate typical SEM and TEM images of TiO2 nanotube arrays prepared by the approach combining an AAO template and ALD.46 The capability to prepare perfectly ordered nanotube arrays with a uniform pore size, length and wall thickness through the entire film is an outstanding advantage of this method. On the other hand, further excessive deposition of precursors in the nanochannels could make the voids fully filled, producing nanorod or nanofiber arrays. Preparation of nanotube arrays employing low concentration of metal oxide sol solution and AAO membranes is also due to the preferential absorption of colloid particles on the surface of the nanochannels.44,48 Physical and chemical interactions between the metal oxide sol solution (precursors) and the templates, such as capillary forces, hydrophobic interactions and chemical bonds could also enable the successful preparation of nanotubes. These chemical and physical interactions were also the main forces for the production of nanotubes during the filling process of membranes using hydrolysis49 and layer-by-layer (LBL) coating50 methods.
Fig. 3 (a) Schematic diagram illustrating metal oxide nanotube arrays prepared using a membrane-assistant ALD process. (b and c) SEM and TEM images of TiO2 nanotube arrays prepared by the approach combining an AAO template and ALD method. Reprinted from ref. 46. Copyright 2015 American Chemical Society. (d) Schematic diagram illustrating metal oxide nanotube arrays by using a membrane-assistant electrodeposition process. A sputtered Au layer is used as a conductive binder between the membrane and substrate. (e) SEM image of TiO2 nanotube arrays prepared from an AAO template-assisted electrodeposition method. Reprinted from ref. 52. Copyright 2014 American Chemical Society.
Electrodeposition with the assistance of PC and AAO membranes under constant current density is another well-developed approach to fabricate self-supported metal oxide nanotube arrays. Because of the insulated characteristic of the membranes, a conductive layer needs to be coated on one surface of the membrane as a substrate to realize electrodeposition. Usually, a thin gold layer is sputtered or evaporated on the surface of the membrane to produce a conductive substrate. Researchers found that the deposition speed around the joint sites between the membrane and substrate was faster than other positions, which finally created nanotube arrays after the removal of the membrane (Fig. 3d).51–53 A rational theory explaining the fast growth speed was the high electric field around the joint sites. Additionally, by-products produced during the electrodeposition, such as H+, OH−, and hydrogen bubbles, could also be beneficial factors for producing nanotubes. Fig. 3e shows the SEM image of TiO2 nanotube arrays prepared by an AAO membrane-assisted galvanostatic deposition process.52 The bottom-up growth of TiO2 led to nanotube arrays tightly bonded to the substrate; while uniform current density on the whole substrate ensured that the nanotubes showed relatively uniform diameter and length. In addition, when the deposition time is enough long or the deposition current density is rather high, excessive deposited metal oxide could be filled in the nanochannels, resulting in nanorod or nanofiber arrays.
Other 1D nanostructures, such as carbon nanotube arrays54 and ZnO nanorod arrays55 could also be used as templates to prepare metal oxide nanotube arrays. Notably, the mechanism using ZnO nanorod arrays as templates to prepare metal oxide nanotube arrays was sacrificial template-accelerated hydrolysis, a process of gradual dissolution of ZnO and slow deposition of amorphous metal oxide particles on the surface of the nanorods. The nanotubes prepared by this hydrolysis process were mesoporous and composed of several nanometer-sized crystals, leading the as-prepared nanotube arrays to demonstrate high optical modulation and fast switching response, as well as high-rate Li-ion storage, as shown in the following part about electrochromic energy storage devices.
Self-supported 1D conductive polymer nanostructures can also be fabricated employing PC and AAO membranes. The preparation process of these conductive polymer 1D nanostructures is similar to the synthesis process used to fabricate transition metal oxide 1D nanostructures. Electropolymerization is widely used to fill the nanochannels due to the advantages of bottom-up growth and ease of producing uniform conductive polymer 1D nanostructures. Similar to the AAO membrane-assisted electrodeposition of metal oxides, conductive polymer nanorod (or nanofiber) and nanotube arrays can be selectively synthesized by controlling the experimental parameters.58Fig. 4a and b demonstrate SEM images of self-supported PPy nanofiber59 and poly(3-methylthiophene) nanotube60 arrays prepared by AAO membrane-assisted electropolymerization, respectively.
Fig. 4 (a) SEM image of PPy nanofiber arrays prepared by AAO-membrane assisted electropolymerization. Reprinted from ref. 59. Copyright 2016 by Wiley. (b) SEM image of poly(3-methylthiophene) nanotube arrays prepared by AAO-membrane assisted electropolymerization. Inset top-view SEM image indicating the hollow character. Reprinted from ref. 60. Copyright 2016 by Elsevier. (c) Schematic representation of an ultrafast electrochromic device based on PEDOT nanotube arrays. (d) Plots of reflectivity of the PEDOT electrochromic window monitored at λ = 530 nm for coloration and bleaching. Patterned letter “N” on the device to demonstrate the color contrast between coloration and bleaching states. Reprinted from ref. 65. Copyright 2005 by Wiley. (e) The nanotube array architecture, double-walled structure, and high conductivity in the electrode provide ion and electron “highways” and a high utilization rate of the electrode. Reprinted from ref. 80. Copyright 2013 American Chemical Society.
Besides the above mentioned electrochemical advantages of the nanostructures, 1D nanostructuring can give the conductive polymers two interesting merits for electrochromism. (1) It was found that the nanofibers and nanotubes fabricated with AAO membranes demonstrated higher electric conductivity than dense films, and the value increased with the decrease of the diameter for nanofibers (or nanorod, and nanowire) or the wall thickness for nanotubes.61 For example, detected by using four-probe technology on platinum micro-electrodes, when the diameter of PEDOT nanofibers decreased from 190 to 25 nm, the electric conductivity increased from ca. 11 to 550 S cm−1.62p-Toluene sulfonic acid doped PPy microtubes with 560–400 nm outer diameters exhibited poor conductivity of only 0.13–0.29 S cm−1 (the inner pore diameter was 80 nm), while a high conductivity of 73 S cm−1 was achieved when the outer thickness was about 130 nm.63 Research implied that confined growth in the narrow nanochannels of the membranes altered the extent of disorder and polarons on the main chains of the conductive polymers, resulting in high electrical conductivity.61–63 (2) The AAO membrane-derived conductive polymer 1D nanostructures demonstrated enhanced mechanical strength due to the extent of arrangement of the conductive polymer main chains.61 For instance, research indicated that PPy nanotubes with thicker walls demonstrated higher elastic modulus.64 When the PPy nanotube thickness was between 20 and 16 nm (outer diameter of nanotube: between 100 and 70 nm), the elastic modulus of a single nanotube was around 5 GPa. When the nanotube wall thickness decreased under 16 nm (outer diameter of nanotube: <70 nm), the elastic modulus strongly increased with decreasing wall thickness. It reached a value close to 60 GPa for a thickness of 6.5 nm (outer diameter of nanotube: 35 nm). The high electric conductivity and mechanic strength are beneficial for fast electrochromic redox kinetics and long-term cycling performance.
Liquid crystals,72 porous block copolymer films,73 Au nanorod arrays,74 silicon nanowire arrays,75 nanopatterned polydimethylsiloxane (PDMS) molds,76 anodic metal oxide nanotube arrays,77 ZnO nanorod arrays,78 and MnO2 nanostructures79 have also been used to fabricate self-supported conductive polymer 1D nanostructures with high electrochemical and electrochromic performance. In particular, the ZnO nanorod arrays were also used to fabricate mixed conductive polymer double-walled nanotube arrays (PPy@PANI hybrid NTAs) (Fig. 4e).80 The PPy@PANI hybrid NTAs exhibited extraordinarily higher electrochemical activity and stability compared to the nanotube arrays with a single component due to the underlying synergistic effect.81,82 Such mixed double-walled nanotube arrays bring interesting insights into the design of nanostructures for electrochromism.
The solution process-based preparation process of self-supported 1D metal oxide nanostructures typically includes two types: hydrothermal and ambient pressure methods. Hydrothermal (or solvothermal when the solvent is not water) growth of metal oxides on substrates usually operates in a sealed autoclave above ambient temperature (typically from 120 to 200 °C) in a solution with metal-element-containing precursors (such as metal salts and organometallic compounds). Using a suitable synthesis temperature and pressure, fully crystallized even single-crystal metal oxide nanostructures could be produced, so post heat treatment is often not necessary. The hydrothermal method possesses many advantages including: (1) relatively lower temperature than that of solid reactions; (2) ease of preparing single phase nanomaterials; (3) ease of preparing various morphologies by simply using different tunable experimental parameters, such as temperature, type and concentration of the precursors, and solvents; (4) ease of preparing uniformly doped materials. However, the high sensitivity of the morphology and phase of the products on the experimental details as well as the still not fully understood nanostructure growth mechanism under hydrothermal processes could bring some difficulties for the reproduction of the morphology and/or performance. Furthermore, the relatively not large volume of autoclaves may limit the areal size of the as-synthesized electrodes, which could bring some unpleasant difficulties for practical applications.
An ambient pressure solution processes to prepare nanostructures are conducted in open vessels. Similar to the hydrothermal process, metal oxide nanostructures are also deposited on the TCO substrates from the metal-element-containing precursor solution, but the synthesis temperature under ambient pressure is lower than that of the hydrothermal method due to the limitation of the boiling point of the solvents. The growth of metal oxide nanostructures in an ambient pressure solution process is derived from the hydrolysis of metal ions in the solution or crystallization process of the sol–gel-based solution. In addition, the as-deposited nanostructures usually either contain crystallographic and/or absorbed water, or are amorphous and poorly crystallized, or are metal hydroxides, indicating that a subsequent annealing is needed. However, nanostructured electrodes with a large size can be facilely prepared using an ambient pressure solution process because of no requirement on the high experimental temperature and pressure as well as the size of experimental vessels. In addition, flexible electrochromic film electrodes can be prepared using an ambient pressure solution method, because typically used flexible transparent conductive substrates are polymer films (such as polyethylene terephthalate (PET)) coated by ITO or FTO layers. These flexible substrates are not stable under hydrothermal and solid state reaction conditions.
5.1.1 Hydrothermal preparation of self-supported 1D nanostructures.
Fig. 5 (a) SEM images of self-supported WO3 nanorod arrays. Reprinted from ref. 86. Copyright 2013 by Royal Society of Chemistry. (b) SEM images of self-supported WO3 nanofiber arrays. Reprinted from ref. 92. Copyright 2014 Elsevier. (c) SEM images of self-supported W18O49 nanofiber arrays. Reprinted from ref. 93. Copyright 2016 Royal Society of Chemistry. (d and e) SEM image and electrochromic performance of a nanofiber stacked honeycomb WO3 architecture. Reprinted from ref. 101. Copyright 2015 Royal Society of Chemistry.
Self-supported WO3 nanofiber arrays were also prepared as electrochromic electrodes. Fig. 5b demonstrates typical SEM images of WO3 nanofiber arrays on an FTO substrate prepared in hydrothermal solution containing similar reagents to that used to prepare nanorod arrays.92 The production of nanofibers can be regarded as the further growth stage of nanorods. As discussed above, the preferred orientation leads to the growth of nanorods along the length direction being faster than that along the diameter direction, finally giving rise to a nanofiber array morphology. The as-prepared WO3 nanofiber arrays showed remarkable enhancement of the transmittance modulation in the visible spectrum (66.5% at λ = 633 nm) and IR region (73.8% and 57.7% at λ = 0.2 and 8 μm, respectively), as well as high cycling stability. To further improve the length-to-diameter ratio, especially decrease the scale of the diameter, chemicals restricting the growth along the diameter direction were added to the hydrothermal solution. For instance, Zhang et al. developed a sulfate-assisted hydrothermal method to prepare arrays of WO3 nanofibers with a long length of ∼1.5 μm with a small diameter of 20–40 nm.90 The as-prepared nanofiber films exhibited a high coloration efficiency of 102.8 cm2 C−1 and fast switching speeds (7.6 and 4.2 s for coloration and bleaching, respectively). Employing mixed solvents or pure organic solvent was another effective method to prepare nanofibers with high length/diameter ratio. Hung et al. prepared nanofiber network films on an FTO substrate in a water/isopropyl alcohol mixture solution.91 The fast Li-ion insertion/extraction kinetics with a Li-ion diffusion coefficient of 2.14 × 10−9 cm2 s−1 led to a desirable transmittance modulation of 57% at λ = 632 nm and a high coloration efficiency of 120.3 cm2 C−1. A solvothermal process seems to be more favorable to synthesize tungsten oxide nanofibers with a smaller diameter. Lu et al. prepared W18O49 nanofiber arrays with diameter <25 nm in polyethylene glycol as a solvent.89 The as-prepared W18O49 nanostructures exhibited a high transmittance modulation of 49.64% at λ = 632.8 nm with high cycling stability (>3000 cycles). Lu et al. found that the solvothermal preparation in methanol solvent could further downsize the diameter of W18O49 nanofibers to ∼6 nm (Fig. 5c).93 A complementary electrochromic device based on self-supported W18O49 nanofiber arrays and a Prussian blue thin film showed a high transmittance contrast (59.05% at λ = 632.8 nm) and fast switching response (coloration time of 6.9 s and a bleaching time of 1.2 s). A nanofiber stacked honeycomb WO3 architecture with high Li+ diffusion coefficient of 3.091 × 10−9 cm2 s−1 was prepared by using a seed and sulfate co-assisted hydrothermal process (Fig. 5d).101 A assembled half-cell electrochromic device (bare TCO substrate as counter electrode) exhibited continuous color changes with high optical modulation of 60.74% at λ = 630 nm and fast switching response time (4.29 s for coloration and 3.38 s for bleaching) (Fig. 5e).
Titanium oxide. Titanium oxide (TiOx) is a type of cathodic electrochromic material with color changes between blue (coloration) and transparent (bleached). TiO2, the most common oxide type of hydrothermal or heat-treated product, has been regarded as a promising electrochromic material due to its stable crystal structure, low cost, high mechanical stability, environmental friendliness, safety, and fast charge transport and collection abilities.112,113 TiO2 naturally exhibits three common types of polymorphs, i.e., rutile, anatase, and brookite. All these three TiO2 phases use Ti–O octahedrons as the fundamental building block and share hollow lattice channels for facile ion (H+ and Li+) insertion/extraction. In addition, the significantly improved electron diffusion coefficient114,115 and enhanced surface redox contribution116 in TiO2 nanorods and nanofibers make the hydrothermally fabricated TiO2 self-supported 1D nanostructures expected to be high-performance electrochromic electrode films.
Using delicately designed experimental parameters, Liu et al. reported a three-step hydrothermal process to prepare arrays of TiO2 single crystalline anatase nanofibers oriented in the  direction with a diameter of 105 ± 10 nm and length of 12.16 ± 0.56 μm on FTO substrates.117 Such  direction oriented nanofibers were believed to possess fast electron transport.112,113 Anatase TiO2 nanofiber arrays with a high-porosity cross-linked geometry directly grown onto FTO substrates were prepared through hydrothermal processes under mild alkali conditions.118 The TiO2 nanofiber array-based half-cell electrochromic device demonstrated a desirable transmittance change of 28.20% at λ = 600 nm and an acceptable coloration efficiency of 13.87 cm2 C−1 with fast switching response (11.3 s for coloration and 14.3 s for bleaching). Electroanalysis indicated that the enhanced Li+ diffusion coefficient is an important feature. In addition, a low refractive index of 1.37 made the as-prepared TiO2 nanofiber arrays become ideal electrode films for transmittance-mode electrochromic devices. Qiang et al. hydrothermally fabricated densely packed rutile TiO2 nanorod arrays on the FTO substrate.119 The high areal density of nanorods led the electrode film to exhibit large transmittance modulation in the visible spectrum range with a maximum value of approximately 64% at λ = 600 nm. Furthermore, a self-powered system integrating an electrochromic device and a dye-sensitized solar cell was also assembled and tested. To further improve the electrochromic performance, Liu et al. fabricated a micro-/nanostructured film of self-supported rutile TiO2 nanorod arrays decorated by anatase TiO2 nanoparticles (TiO2 NR-NP).120 This micro-/nano-structured film simultaneously possessed the high electron transport and mechanical stability properties from the nanorod arrays, and fast and reversible redox kinetics from the small decorated nanoparticles, giving rise to enhanced transmittance modulation as well as accelerated switching responses, compared to pure TiO2 nanorod arrays (TiO2 NR) (Fig. 6a). Using a two-step hydrothermal process and pre-fabricated self-supported TiO2 nanofiber arrays as skeletons, hierarchical TiO2 nanofiber arrays with densely-packed and omnidirectional branches were synthesized (Fig. 6b).121 A typical synthesis process was via high-concentration TiCl4 treatment of upright backbone TiO2 nanofibers to produce seeds on the surface followed by hydrothermal growth, giving rise to the growth of secondary TiO2 nanobranches (such as nanoneedles and nanosheets) on the TiO2 nanofiber backbone and in all directions (Fig. 6c). Such a preparation method brings new insights to prepare novel hierarchical nanofiber arrays where the nanofiber and nanobranches could be the same oxides or different types like WO3 nanostructures on a TiO2 nanofiber backbone. Hierarchical nanofiber arrays are desirable for electrochromic applications.
Fig. 6 (a) Comparison of the transmittance modulation in the visible spectra of the TiO2 NW and NW-NP films. Inset figures demonstrate the coloration and bleaching colors of the TiO2 NW-NP film. Reprinted from ref. 120. Copyright 2014 Royal Society of Chemistry. (b and c) Schematic diagram illustrating preparation and SEM images of densely-branched TiO2 NWs. Reprinted from ref. 121. Copyright 2013 Royal Society of Chemistry.
Nickel oxide and cobalt oxide. Nickel oxide (NiOx) and cobalt oxide (CoOx) are two representative anodic electrochromic metal oxides. Crystalline NiOx and CoOx are formed by NiO6 or CoO6 octahedra connected by sharing common corners and/or by sharing common edges.10 Because of the high electrical conductivity of NiOx and CoOx,122,123 as well as their stable lattice structure, crystalline NiOx and CoOx nanomaterials exhibit faster coloration/bleaching switching and higher cycling stability compared to many other electrochromic metal oxides.
NiO, the common electrochromic nickel oxide, has been widely investigated in alkali solutions. In electrochromic devices, NiO has been successfully explored as a competent anodic counter electrode (also called an ion storage electrode) in conjunction with WO3 as a cathodic working electrode.124 However, practical applications of NiO as a promising electrochromic material are still difficult because of the low color contrast, poor cycling durability and unsatisfactory ion storage capacity when the NiO is in bulk particles or a dense film. Fabrication of self-supported 1D NiO nanostructures on the TCO substrate is an ideal method to solve these problems. Typically, the initial hydrothermal crystalline products are hydrous nickel oxides or nickel hydroxides; an annealing treatment is needed to transform the hydrothermal products to NiO. However, because the initial generated hydrothermally synthesized crystalline products on TCO substrates were usually in morphologies of nanosheet arrays or nanosheet-stacked complex nanoarchitectures (such as rose-like), the annealing-produced NiO nanostructures were inevitably in nanosheet arrays or nanosheet-stacked complex nanoarchitectures.125–129 These NiO nanosheet-based architectures exhibited desirable electrochromic performance. For instance, mesoporous nickel oxide nanosheet arrays demonstrated a high transmittance modulation of 77% at λ = 550 nm, fast switching response (2 and 2.5 s for coloration and bleaching, respectively), and highly stable cycling performance of negligible degradation after 3000 cycles.127 But the hydrothermal fabrication of self-supported NiO 1D nanostructures is still a challenge.
Fig. 7 (a) SEM image of Co3O4 nanorod arrays on an ITO substrate prepared by a seed-assisted hydrothermal process. Inset digital photos demonstrate the color contrast between coloration and bleaching. (b) Transmittance modulation response of Co3O4 nanorod arrays under alternative voltages. Reprinted from ref. 136. Copyright 2010 Elsevier.
Vanadium oxide and molybdenum oxide. Crystalline vanadium oxide and molybdenum oxide have been widely studied for electrochemical redox-based applications because of their attractive layered structure.138 The two-dimensional crystalline sheet structures are formed by MO6 (VO6 or MoO6) octahedra sharing corners and/or edges. Due to the layered structure, vanadium oxide and molybdenum oxide can demonstrate fast ion insertion/extraction kinetics in the interlamination of (001) planes. In addition, as typical extrinsic pseudocapacitor materials, 1D vanadium oxide and molybdenum oxide nanomaterials could exhibit enhanced pseudocapacitive effect, high Li-ion diffusion coefficient, and short characteristic relaxation process,139 which are beneficial for high optical modulation and fast switching response.
V2O5, as the common hydrothermally prepared vanadia type showing both cathodic and anodic coloration, has been widely investigated as a counter electrode material in electrochromic devices,10,139,140 because of not only its unique electrochromic performance but also its high Li-ion storage capacity. 1D V2O5 nanostructures can be facilely prepared using a hydrothermal method due to the strong preferential growth derived from the obvious lattice anisotropy of the layer crystalline structure.141–143 Chu et al. hydrothermally prepared V2O5 nanorod array films on FTO substrates and investigated their electrochromic performance (Fig. 8a).144 It was found that the V2O5 nanorod array films demonstrated obvious absorbance modulation in the visible spectrum range and vivid color changes between pale blue (cathodic coloration) and yellow-green (anodic coloration) (Fig. 8b).
Fig. 8 (a) SEM image of V2O5 nanorod arrays directly grown on an FTO substrate by using a hydrothermal method. (b) Optical modulation of V2O5 nanorod arrays derived from a Li-ion insertion/extraction process. Inset digital photos and plot of absorbance vs. time under alternative voltages indicate the electrochromic color contrast and switching response performance. Reprinted from ref. 144. Copyright 2016 Elsevier.
Molybdenum oxide can be used as a cathodic electrochromic material with color changes between transparent and blue.10,138 MoO3 is the most common form of hydrothermally prepared molybdenum oxide. To date, self-supported MoO3 nanorod arrays have been successfully prepared using hydrothermal methods.145,146 MoO3 nanorods, nanofibers, and nanobelts powders were also hydrothermally prepared for high-performance electrochemical energy storage and electrochromism.147–149 However, research about the fabrication and investigation of self-supported MoO3 1D nanostructures on TCO substrates for electrochromism is still rare.
A hydrothermal process is an effective and facile method to synthesize 1D uniformly doped metal oxide nanostructures, broadening the research horizon of electrochromism. For instance, the Ni-doping significantly increased the optical modulation of WO3 nanorods.157 When the Ni atomic concentration was 1.5%, the Ni-doped WO3 nanorods exhibited maximum transmittance modulation and charge density of 86.0% (at λ = 600 nm) and 24.6 mC cm−2, compared to the corresponding relatively low values of pure WO3 nanorods (50.9% and 18.2 mC cm−2, respectively). In addition, the switching response time was shortened to around 2 s. The detected increase of electrical conductivity was believed to be an important feature. Doping also can significantly enhance the optical modulation of metal oxides in the NIR and IR spectrum range. Zhou et al. reported that 2% Mo-doped WO3 nanofibers showed a high transmittance modulation in the visible and NIR spectrum range (56.7%, 83% and 48.5% at λ = 750 nm, 1600 nm and 10 μm, respectively), while the pure WO3 nanofibers only exhibited 44.4%, 52.6% and 25.1% at the three above mentioned wavelengths.158 Additionally, the enhanced electrical conductivity accelerated the switching speed to around 4 s. Nevertheless until now, the majority of these hydrothermally prepared 1D doped metal oxide nanostructures were in the powder form, and the fabrication of electrochromic 1D metal oxide nanostructures directly grown on TCO substrates for electrochromism deserves to be investigated.
5.1.2 Ambient pressure solution preparation of self-supported 1D nanostructures. Chemical bath deposition (CBD) is a typical and effective ambient pressure solution method to prepare metal oxide nanostructures on solid substrates. The CBD process refers to the thermohydrolysis (or “forced hydrolysis”) of metal salts involving two steps, nucleation and particle growth at low temperatures.159 Typical electrochromic metal oxide films or nanostructures can be easily prepared on the substrates using CBD,160–163 while the synthesis of well-defined self-supported 1D nanostructures seems to be difficult. For instance, electrochromic NiO nanostructures prepared by CBD are usually in the morphology of nanosheet arrays on TCO substrates.164,165 However, self-supported 1D nanostructures of some metal oxides can be obtained by using delicately designed experimental reagents and procedures. Hosono et al. found that employing a mixed solution of water–methanol as solvents could produce cobalt oxide nanorod arrays on glass slides, compared to nanosheet arrays prepared in a solvent of water.166 The reason that nanorods were produced in the water–methanol solvent was due to the increased amount of dissolved oxygen molecules, which converted some of the divalent cobalt ions into trivalent cobalt ions through oxidation. Li et al. proposed a modified CBD process to synthesize vertically aligned H2Ti5O11·3H2O nanowire arrays on arbitrary substrates of stainless steel, glass, silicon wafer and carbon cloth at a low temperature of 80 °C and in an open atmosphere (Fig. 9a and b).167 Anatase TiO2 nanowire arrays can be obtained by a further annealing treatment. The authors attributed the successful preparation of H2Ti5O11·3H2O nanowire arrays in the modified CBD process to the addition of an appropriate concentration of NaOH in the solution. In the presence of NaOH, Ti(IV) derived from the Ti–H2O2 interaction was transformed to protonated titanate nanowires after an alkali-hydrothermal-like process followed by a proton exchange, giving rise to H2Ti5O11·3H2O nanowires at a low temperature. Dhara et al. employed a seed-based chemical bath deposition to successfully synthesize single-crystal MoO3 nanorod arrays vertically grown on glass (Fig. 9c).168 The pre-coated seed layer was a key factor to obtain MoO3 nanorods vertical to the substrate because they defined the vertical direction as the only preferred growth direction for MoO3 nanorods (Fig. 9d). It is meaningful to note that these CBD methods were substrate-independent, providing the possibility of using flexible ITO/PET films as substrates.
Fig. 9 (a and b) Cross-sectional and top-view SEM images of self-supported H2Ti5O11·3H2O nanowire arrays prepared by a modified CBD method. Reprinted from ref. 167. Copyright 2014 Royal Society of Chemistry. (c and d) Top-view SEM image of MoO3 nanorod arrays and the corresponding schematic diagram illustrating the preparation process of a seed-based CBD. Reprinted from ref. 168. Copyright 2014 Royal Society of Chemistry.
Vapor process deposition is a direct and bottom up method to prepare self-supported 1D metal oxide nanostructures on substrates.169 Because the metal oxide nanostructures are grown from gaseous precursors, solid substrates are needed to provide low energy sites for nucleation and subsequent crystal growth. In a vapor deposition process, thermal activation under high temperature is usually employed to facilitate crystal nucleation and growth on the TCO substrate. The high experimental temperature makes the as-prepared self-supported 1D metal oxide nanostructures fully crystallized, even in single-crystal nature. Similar to a crystal growth mechanism under hydrothermal conditions, the production of 1D crystalline metal oxide nanostructures on TCO substrates is naturally formed due to the preferential direction of the anisotropic crystal lattice with or without external physical forces. In addition, employing catalysts or surface modification on substrates can affect the nucleation and crystal growth processes, controlling the morphology or phase of the produced self-supported 1D metal oxide nanostructures.170,171 Typically, vapor process deposition can be categorized into physical and chemical vapor deposition (PVD and CVD) processes.
Physical vapor deposition. PVD is a process in which the solid metal oxide and substrate are placed in a chamber; meanwhile, the solid metal oxide is volatilized into a vapor phase precursor and then transferred onto the surface of the solid substrate with lower temperature. These precursors condense on the surface of the substrate followed by nucleation and crystal growth processes, generating self-supported 1D metal oxide nanostructures (Fig. 10a). Based on the methods employed to volatilize solid metal oxides or to assist the metal oxide nucleation and crystal growth, PVD technologies include thermal evaporation, sputtering, laser ablation, plasma assistant, confinement growth, lithography, glancing angle deposition, etc.169 All these PVD technologies can be employed to fabricate self-supported 1D metal oxide nanostructures.172–178 To prepare electrochromic film electrodes, FTO or ITO substrates are usually used. Cheng et al. successfully prepared V2O5 arrays of nanowires on ITO substrates by thermal evaporation PVD.176 The diameter of the V2O5 nanowires was from 10 to 100 nm and the length was up to several hundred nanometers. Between −1 and +1 V (vs. Ag/AgCl) in 1 M LiClO4/propylene carbonate (PC), V2O5 nanowire arrays exhibited vivid color changes between pale blue (−1 V) and yellow (+1 V) with the maximum transmittance modulation of 37.4% at λ = 415 nm and fast switching response (6 and 5 s for cathodic and anodic coloration, respectively). The as-deposited V2O5 nanowires demonstrated high structural stability. The morphology and redox current response of the nanowire arrays did not alter after 1000 Li-ion insertion/extraction cycles observed by SEM and CV examinations. Liao et al. reported the successful preparation of W18O49 nanofiber arrays on the ITO substrate using a similar PVD process.177 The average diameter and length of the nanofibers were about 52 ± 13 nm and up to 5 μm, respectively. The high areal density of the nanofibers (about 1.68 × 1010 per square centimeter) led to a high transmittance modulation in the visible spectrum range with a maximum value of 34.5% at λ = 700 nm. The W18O49 nanofiber arrays also demonstrated fast switching response of less than 2 s both for coloration and bleaching with long-term cycling stability. Xiao et al. prepared three-dimensional, high-porous, and oriented WO3 nanocolumn layers on ITO/PET and FTO substrates employing a glancing angle PVD process when the WO3 layer was prepared with a deposition angle of 75° (Fig. 10b).178 By dynamically varying the substrate incline and revolution regarding the incoming tungsten oxide vapor in the glancing angle PVD process, the WO3 layer morphological and physical properties were precisely adjusted. The WO3 nanocolumn film anchoring ITO/PET substrate exhibited high flexibility and high transmittance modulation of 52% at λ = 700 nm with fast switching response (4.2 and 7.8 s for coloration and bleaching, respectively). It is interesting to note that the as-deposited WO3 nanocolumns can improve the broadband antireflective performance of the substrates due to the high porosity of WO3 nanocolumns, which can increase coloration/bleaching contrast.
Fig. 10 (a) Schematic diagram of tube furnace for synthesis of self-supported 1D metal oxide nanostructures by a PVD process. (b) Cross-sectional SEM image of three-dimensional, high-porous, and oriented WO3 nanocolumn layer prepared by a glancing angle PVD method. Left inset figure demonstrates the optical transmittance spectrum of bare ITO and ITO/WO3 films at normal incidence. Right inset figure demonstrates the transmittance modulation switching response curves of dense and three-dimensionally porous PET/ITO/WO3 films. Reprinted from ref. 178. Copyright 2016 American Chemical Society. (c) Electrochemical and electrochromic performance of self-supported NiO nanorods prepared by the HFMOVD method. Inset figure is the SEM image of the NiO sample. Reprinted from ref. 184. Copyright 2013 Elsevier. (d) Transmittance modulation of self-supported brookite TiO2 nanoneedles prepared by the HFMOVD method. Inset figure is the SEM image of the TiO2 sample. Reprinted from ref. 185. Copyright 2015 Elsevier.
Chemical vapor deposition. CVD is a process in which the condensed precursors for metal oxide crystal growth are derived from gaseous chemical reactions. Thus, the ability to produce compounds with high purity is a unique advantage of CVD over PVD. Based on differences in the used reactants and assisting energy sources, CVD technologies can be divided into various categories such as atmospheric-pressure, hot-filament, metal–organic, microwave-plasma, plasma-enhanced, and photo-assisted types, etc.169 To prepare 1D metal oxide nanostructures for electrochromism, an oxygen atmosphere is a vital factor. Until now, various electrochromic self-supported 1D nanostructures of metal oxides (such as WO3,179,180 Ta2O5,181 MoO3,182 V2O5,183 NiO,184 and TiO2185) on TCO substrates have been fabricated by a hot-filament metal-oxide vapor deposition (HFMOVD) technique. In the HFMOVD process, the metal source is placed on a heater in a vacuum chamber. The substrate is placed apart from the heater at an appropriate distance. When the metal source is heated to be volatilized, the volatilized metal atoms react with the residual oxygen molecules in the chamber or low oxygen flow supplied by external equipment, creating highly active metal–oxygen-containing gaseous precursors. These precursors travel to the TCO substrates, followed by metal oxide 1D crystal growth. The crystal growth mechanism in the HFMOVD process is quite different from other methods, resulting in the preparation of some unique self-supported 1D metal oxide nanostructures for high electrochromism. For instance, self-supported single-crystal NiO nanorod arrays which were hard to synthesize using a hydrothermal method, were simply prepared on an ITO substrate using the HFMOVD method (inset SEM image in Fig. 10c).184 The NiO nanorods exhibited a length of ∼500 nm and a width of ∼100 nm, with an areal density of ∼100 nanorods per square micrometer. In 0.5 M KOH electrolyte, the NiO nanorod arrays exhibited outstanding electrochromic properties, including great optical transmittance modulation (60% at λ = 630 nm) and fast coloration and bleaching times (1.55 and 1.22 s, respectively). Electrochemical analysis indicated that the NiO nanorod arrays showed fast redox kinetics with a large diffusion coefficient of ∼6.33 × 10−8 cm2 s−1. 1D TiO2 nanostructures (nanoneedles with length of ∼650 nm and width of ∼30 nm) in a pure brookite phase which is really hard to prepare by common synthesis processes such as hydrothermal methods and electrodeposition, were successfully prepared by the HFMOVD process (inset SEM image in Fig. 10d).185 Besides the high transmittance modulation and fast switching response, the TiO2 nanoneedles exhibited large optical density of 0.85, great coloration efficiency of 226 cm2 C−1, and high cycling retention of ∼99% after 250 cycles in a Li-ion organic electrolyte. Also, a fair diffusion coefficient of 1.56 × 10−11 cm2 s−1 was believed to be an advantageous electrochemical feature.
Aerosol-assisted chemical vapor deposition (AACVD) is another widely used CVD process employing metal-containing compounds as sources to prepare self-supported 1D metal oxide nanostructures for high electrochromism. In AACVD, metal-containing reactants are transported in solution as a mist, and therefore, compound volatility is less critical than in conventional CVD, which has an important influence on the morphology of the products.186 Various electrochromic self-supported 1D metal oxide nanostructures, such as NiO nanorod arrays,186 WO3 nanoneedle arrays187 and WO3 nanorod arrays,188 were prepared using AACVD on TCO substrates.
Fig. 11 (a) Typical SEM image of anodic TiO2 nanotube arrays. (b) Reflectance modulation curves of mixed Ti–V–O nanotube arrays with different vanadium atom concentrations. Reprinted from ref. 196. Copyright 2011 Elsevier. (c) Schematic diagram illustrating preparation of anodic metal oxide nanotube arrays directly grown on TCO substrates. (d) The influence of coated amount of MoO3 on the transmittance modulation of anodized Nb2O5/electrodeposited α-MoO3 binary films. Reprinted from ref. 208. Copyright 2014 American Chemical Society. (e and f) Digital photos of free anodic TiO2 nanotube arrays transferred on the FTO substrate and their transmittance modulation performance. Reprinted from ref. 213. Copyright 2016 Elsevier.
The initial electrochromic applications of anodic metal oxide nanotube arrays were in reflectance-mode because of the difficulty in peeling off the nanotube arrays from the metal substrate.191–198 For instance, H+ insertion/extraction led to a reversible reflectance modulation of over 50% and a fast switching response of 2 s at λ = 480 nm in the TiO2 nanotube arrays (film thickness of 1 ± 0.1 μm with an individual tube diameter of 100 ± 10 nm and a tube wall thickness of 10 ± 2 nm),191 while Li+ insertion/extraction could result in a reflectance modulation of approximately 35% at the same wavelength with a switching time of 5 s.192 CV measurements indicated that the TiO2 nanotube arrays showed improved H+ and Li+ chemical diffusion coefficients. Yang et al. reported that the V2O5 nanotube arrays with a length of 20 μm and pore diameter of 12 nm could demonstrate a reflectance modulation of over 30% with vivid color change from yellow to green and then dark accompanied by an increased amount of inserted Li-ions.193 In addition, the as-prepared V2O5 nanotube arrays also exhibited high cycling stability with negligible degradation of reflectance contrast after 250 cycles.
Anodic oxidation of alloy foils produced doped metal oxide (or mixed metal oxides) nanotube arrays. Ghicov et al. synthesized tungsten-doped titanium oxide nanotube arrays using Ti–W alloys.194 It was found that the doped products exhibited higher reflectance modulation and charge density than the pure TiO2 nanotube arrays. Ghicov et al. also found that the Ti–Nb–O nanotube arrays anodically prepared from β-Ti45Nb alloy exhibited a pure phase of anatase TiO2, while the lattice interlamellar spacing was widened due to the Nb-doping.195 Such widened lattice interlamellar spacing facilitated the Li+ insertion/extraction process, leading to increased reflectance modulation of approximately 80% at λ = 600 nm and high charge density of 126.1 mC cm−1, compared to the corresponding values of approximately 50% and 76.4 mC cm−1 for pure TiO2 nanotube arrays. Yang et al. anodically synthesized mixed TiO2–V2O5 nanotube arrays from Ti–V alloys.196 Maximum values of reflectance modulation and charge density were achieved for the nanotube arrays prepared from a Ti–V alloy with 3 at% vanadium during the Li-ion insertion/extraction process (Fig. 11b). In addition, the doping of vanadium also significantly enhanced the color contrast by increasing color saturation under cathodic polarization. Anodically prepared mixed Ti–Mo–O and W–Ta–O nanotube arrays also demonstrated enhanced reflectance modulation performance.197,198 In addition, the nanotube arrays provided large surface area for decoration of a second type of electrochromic material, such as silver phosphate crystals199 and WO3 nanocrystals,200 resulting in enhanced reflectance modulation and multicolor changes. Furthermore, mesoporous NiO201 and WO3202 films were also prepared by anodic oxidation processes for reflectance-mode electrochromic electrodes.
Anodic oxidation of metal layers deposited on the TCO substrates is an easy approach to directly prepare nanotube array electrodes for transmittance-mode electrochromic devices (Fig. 11c). The typical process to deposit metal layers was magnetron sputtering, and meanwhile heat-stable TCOs, such as ITO and FTO, were used as substrates.203,204 Barredo-Damas et al. prepared anodic Nb2O5 nanotube arrays with a thickness of 1 μm from a thin Nb-layer on FTO glass.205 The as-prepared nanotube arrays exhibited a high transmittance modulation of ca. 90% at λ = 600 nm with obvious color changes in 0.1 M HClO4. Yao et al. synthesized self-supported TiO2 nanotube arrays for electrochromic applications by anodic oxidation of an RF sputtered titanium layer on an FTO substrate.206 To further increase the optical modulation, α-MoO3 of ∼5 to 15 nm thickness was electrodeposited on the wall surfaces of the TiO2 nanotube arrays. The MoO3/TiO2 system showed a 4-fold increase in optical density over bare TiO2 when the thickness of the MoO3 coating was optimized. The enhancement was ascribed to (I) the desirable electron band coupling effect between α-MoO3 and TiO2 for feasible charge carrier transfer, (II) the increased amount of inserted Li-ions derived from the layered structure of α-MoO3 and (III) enhanced electron transport derived from the high electrical conductivity of α-MoO3 layers acting as efficient pathways for charge carrier transfer.206,207 Yao et al. also synthesized anodized Nb2O5/electrodeposited α-MoO3 binary films for high electrochromism.208,209 The thickness of the MoO3 layer was controlled by different electrodeposition cycles (a cycle was a chronoamperometric deposition of 60 s). It was found that the increased thickness of the MoO3 layer led to an increase of transmittance modulation, reaching a favorable value of approximately 45% at λ = 550 nm between ±1 V in a 0.1 M LiClO4/PC electrolyte (Fig. 11d). WO3 mesoporous film electrodes were prepared by anodic oxidation treatment on the corresponding metal layers deposited on FTO substrates.210 These films also exhibited high transmittance modulation with fast switching response.
Phase transformation-induced self-supported 1D nanostructures. For metal oxides demonstrating a layered crystalline structure, annealing treatment on their amorphous or poorly crystallized metal oxide films could produce self-supported 1D nanostructures. By annealing treatment on the 3D macroporous amorphous vanadium oxide films prepared by electrodeposition with the assistance of polystyrene colloidal crystals as templates (Fig. 12a),215–217 V2O5 nanorod architectures218,219 and a V2O5 nanofiber grassland220 can be prepared by using different annealing temperatures. Colloidal sphere-assisted heterogeneous nucleation during electrodeposition and the anisotropic bonding of the V2O5-layered structure were two important factors for the generation of the V2O5 nanorod architecture at 350 °C (Fig. 12b), while for the growth of the V2O5 nanofiber grassland morphology obtained under a higher annealing temperature of 450 °C (Fig. 12c), coalescence of adjacent nanorods with a similar crystallographic orientation, atom rearrangement, and Ostwald ripening were responsible for the production of nanofibers under thermal activation. The V2O5 nanorod architecture exhibited a five-color change in the voltage range of ±1 V (yellow, yellow-green, dark slate gray, steel blue, and Prussian blue) with transmittance modulation of 27.43% at λ = 450 nm and acceptable switching response time (8.8 s for coloration and 9.3 s for bleaching).215 The V2O5 nanofiber grassland displayed promising transmittance modulation (34% and 25% at λ = 460, 1000 nm, respectively) and high color contrast (yellow and dark green) upon the Li-ion insertion/extraction processes (Fig. 12d).220 Alsawafta et al. also reported a similar process to prepare high-performance electrochromic V2O5 nanorod arrays using stacked polystyrene spheres as a template.221 Self-supported nanorod arrays of MoO3222–224 and Nb2O5225 which showed an intrinsic layered structure, also have been prepared by annealing treatment on the corresponding metal oxide amorphous films. These films could exhibit high electrochromic performance.
Fig. 12 (a) SEM image of 3D macroporous amorphous vanadium oxide films prepared by electrodeposition with the assistance of polystyrene colloidal crystals as templates. (b and c) SEM images of V2O5 nanorods and nanofiber grassland prepared by annealing treatment on 3D macroporous amorphous vanadium oxide films. Reprinted from ref. 218. Copyright 2015 Wiley. Reprinted from ref. 220. Copyright 2015 Macmillan Publishers Limited. (d) Electrochromic performance of the V2O5 nanofiber grassland. (e) Cross-sectional SEM image of a flexible WOx–NbOx composite film electrode. Inset digital photos demonstrate the three-stage transmittance modulation in the visible and NIR regions. Reprinted from ref. 232. Copyright 2017 American Chemical Society. (f and g) SEM image and electrochromic performance of coassembled Ag/W18O49 nanofiber networks on the PET substrate. Reprinted from ref. 239. Copyright 2017 American Chemical Society.
Self-supported nanorod arrays of other electrochromic metal oxides, including Co3O4,226 NiO227 and WO3228 have also been prepared by annealing treatment on the corresponding amorphous films, while the morphology and phase transformation mechanisms are still not clear.
Drop-assembly of self-supported 1D nanostructures. When the 1D metal oxide nanomaterials are in a powder form, they can be assembled into porous films on TCO substrates by a drop-assembly process. Typically, the nanomaterials are uniformly dispersed into solvents, creating a suspension with high concentration. Then the suspension is dip-coated onto a clean TCO substrate and dried, producing a porous 1D metal oxide nanomaterials-stacked film.229–232 When efficient electron transport between the nanomaterials and substrate was achieved, these drop-assembled films could exhibit high electrochromic performance. For example, Xiong et al. prepared silver vanadium oxide and V2O5 nanofibers with a length of over 30 μm and a diameter of 10–20 nm by a hydrothermal process, and then fabricated the corresponding nanofiber-stacked porous films with a thickness of ∼500 nm on ITO substrates by a drop-assembly method.230 The as-prepared silver vanadium oxide film exhibited a higher transmittance modulation (60% at λ = 1020 nm) and faster switching response (0.2 and 0.1 s for coloration and bleaching, respectively) than the V2O5 film (corresponding values of 33.7%, 6 s and 5 s, respectively). The high electrical conductivity (0.5 S cm−1) and chemical Li+ diffusion coefficient (5.3 × 10−10 cm2 s−1) of silver vanadium oxide nanofibers were the important features, compared to the corresponding values of V2O5 nanofibers (0.08 S cm−1 and 7.5 × 10−11 cm2 s−1, respectively). Heo et al. drop-assembled WOx nanorods on ITO/PET substrates then coated NbOx nanoparticles on the WOx nanorods, forming flexible electrochromic WOx–NbOx composite films (Fig. 12e).232 The as-prepared film showed high and multi-stage transmittance modulation in the visible and NIR regions in the voltage range from 1.5 to 4 V vs. Li+/Li (inset figures in Fig. 11d). At 4 V (“Bright mode”), transmission of the film was high in both the visible and NIR regions as WOx and NbOx are in their oxidized states. When 2.4 V (“Cool mode”) was applied, transmittance in the NIR region was lowered due to charging of WOx, blocking the solar heat gained while largely maintaining the visible transmittance which was useful for daylighting. At 1.5 V (“Dark mode”), NbOx was also intercalated with Li+ so the visible transmittance also decreased.
Yoo et al. solvothermally prepared 1D W18O49 nanobundles formed by parallelly stacking tens of W18O49 nanowires in the diameter direction.235 Then a series of nanobundle-assembled porous films on ITO substrates were prepared by LB technology under different maximum surface pressures. A superior transmittance of ca. 33% at λ = 633 nm with a coloration efficiency of 47.5 cm2 C−1 was achieved when the proper maximum surface pressure was used. Wang et al. assembled the hydrothermally prepared WO3 nanorods (diameters of ∼100 nm and lengths of ∼2 μm) into a film on ITO substrates using an LB process.237 The as-assembled WO3 film exhibited multicolor changes (green, green-blue, and blue), high transmittance modulation of ∼ 66% at λ = 632.8 nm, and high intercalated charge of ∼133 mC cm−2 mg−1 compared to many other WO3 nanostructured films. The authors also found that the assembly was not affected by the surface properties of the substrates, but was dependent on the drying rate of the film, the concentration of the suspension, and the aspect ratio of the nanorods. Liu et al. found that the use of a surfactant was the key factor to effectively assemble metal oxide nanofibers with high length-to-diameter ratios.238 By using LB assembly with W18O49 nanofibers (sub 5 nm in diameter and tens of micrometers in length) and poly(vinyl pyrrolidone) (PVP), the authors achieved high electrochromic films on ITO substrates. Interestingly, W18O49 nanowires and Ag nanowires can be co-assembled as hybrid networks on PET substrates by an LB process, creating ITO-free flexible electrochromic electrodes (Fig. 12f).239 The mAg : mW18O49 mass ratio was 14 : 4. The hybrid film with optimized thickness exhibited high conductivity (7 Ω per square), high transmittance modulation of 68% at λ = 632.5 nm, fast switching response (3.92 and 7.78 s for coloration and bleaching, respectively) and good bending strength stability (Fig. 12g).
1D conductive polymer nanostructures were much easier to be synthesized when chemical oxidative polymerization occurred under low temperature such as from 0 to 5 °C.244,245 Zhang et al. prepared randomly stacked PANI nanotubes about 80–180 nm in diameter in the presence of D-10-camphorsulfonic acid as the dopant under ice bath conditions.244 Vertically aligned PANI nanowires with ∼50 nm average diameter and ∼600 nm length were synthesized by Wang et al. using a similar polymerization process.245 Chiou et al. developed a novel dilute polymerization method to produce a large quantity of interconnected and networked PANI nanofibers in bulk solutions on various substrates, including conducting and non-conducting substrates.246–248 In a typical dilute polymerization process, substantially lower concentrations of aniline and oxidant than the usually employed values were used. The diameters of the tips of the nanofibers can be controlled within the range of 10–40 nm, and the average length can be controlled within the range of 70–360 nm (Fig. 13a).248 These PANI nanofiber arrays grown on TCO substrates exhibited high redox kinetics, leading them to be desirable for bi-functional electrochromic energy storage applications, as shown in the following part about electrochromism-containing multifunctional devices.
Fig. 13 (a) SEM image of PANI nanofiber arrays prepared by using a dilute polymerization method. Reprinted from ref. 248. Copyright 2018 Elsevier. (b) SEM image of PPy nanofiber arrays electropolymerized by a galvanostatic method. Reprinted from ref. 253. Copyright 2016 Elsevier. (c and d) SEM image and electrochromic color changes of a PANI/inorganic salt binary nanofiber network. Reprinted from ref. 255. Copyright 2016 Springer.
Galvanostatic, potentiostatic, and cyclic voltammetry methods are three common technologies for electropolymerization of conductive polymers.249–251 The morphologies of electropolymerized nanostructures varied a lot from the used techniques. Due to the ease of 1D nucleation, a galvanostatic method was widely used to prepare vertically aligned conductive polymer 1D nanostructures, such as PANI and PPy nanorod and/or nanowire arrays on various conductive substrates.251–253 The morphology characters, such as areal density, length and diameter of 1D active materials can be easily controlled by varying the experimental parameters such as electropolymerization current densities and times. Fig. 13b showed a typical SEM image of PPy nanofiber arrays prepared using galvanostatic electropolymerization.253 By simply changing the electropolymerization time, the length of the PPy nanofibers varied from 1.5 to 4 μm accompanying the increase of diameter from 80 to 100 nm.252,253 When the conductive polymers were polymerized using potentiostatic, and cyclic voltammetry methods, the products usually exhibited a porous nanofiber network morphology built by random twisting of nanofibers.250,254–256Fig. 13c demonstrates a typical SEM image of a PANI/inorganic salt (LiClO4) binary nanofiber (diameter of 80–110 nm) network electropolymerized at a constant potential of 0.9 V versus Ag/AgCl.255 This nanofiber network exhibited three-color changes and high transmittance modulation in the visible spectrum range with a maximum value of ca. 50% at λ = 560 nm (Fig. 13d). Osuna et al. prepared a PEDOT nanofiber network on a flexible transparent single-wall carbon nanotube/PET electrode.256 This flexible electrochromic electrode showed a high transmittance modulation of ca. 38% at λ = 550 nm, fast switching response time (2.4 and 1.1 s for coloration and bleaching, respectively), and high cycling stability with negligible performance degradation after 1000 cycles.
Crystalline/amorphous metal oxide (MOx) core/shell 1D nanostructures. Crystalline/amorphous metal oxide (MOx) core/shell 1D nanostructures are attractive for high electrochromism due to the simultaneous possession of the electrochemical advantages of crystalline and amorphous MOx (here MOx means that the materials of the core and shell are the same oxide). Amorphous MOx electrodes exhibit fast coloration/bleaching response, high coloration efficiency, high ion diffusion coefficient, and fast redox kinetics on the surface or near surface layer derived from the surface defects and disordered atom arrangements, while crystalline MOx electrodes demonstrate fast electron transport, high mechanical strength and slow dissolution rate in electrolytes.257–260 Furthermore, there is a significant difference in the electrochromic kinetics between crystalline and amorphous MOx. Taking WO3 as an example, redox-reduced W-ions (i.e. W5+) and interstitial oxygen ions in amorphous WO3 can be separated and frequently hop from one site to another between two adjacent sites of W5+ and W6+ accompanying transfer of electrons between these sites, while no spatial separation occurs in crystalline WO3. This difference in the diffusion of ions and electrons gives rise to different electrochromic kinetics and performance in amorphous and crystalline WO3.11,84 Therefore, developing an electrochromic MOx with balanced electrochromic properties has become a major target pursued by researchers. Antonaia et al. proposed an amorphous/crystalline WO3 double layer and reported that this double layer showed a faster coloration response and a higher transmittance asymptotic value for the bleaching phase than the amorphous or crystalline single layer.257 Qu et al. demonstrated that the amorphous/crystalline WO3 double layer exhibited higher coloration efficiency than the amorphous or crystalline single layer.260 Analysis of the color centers of electrochromism indicated that the double layer demonstrated a higher number of surface and body color centers than the amorphous or crystalline single layer. Given the structural advantages, fabrication of self-supported crystalline/amorphous core/shell 1D nanostructures is a rational strategy to further enhance the electrochromic performance of MOx.
Her et al. synthesized crystalline/amorphous WO3 core/shell nanorod powders using a two-step hydrothermal process and then fabricated a porous film by the drop assembly method.26 Compared to pure nanorod and amorphous films, the crystalline/amorphous WO3 core/shell nanorods assembled film exhibited increased transmittance modulation of 44% at λ = 550 nm with acceptable switching response (41 and 6 s for coloration and bleaching respectively). Crystalline/amorphous WO3 core/shell nanorod arrays directly grown on FTO substrates exhibited better electrochromic performance than the film fabricated by drop assembly.261 Because of efficient electron transport as well as the enhanced synergistic effect between the amorphous shells and crystalline cores, the nanorod array electrode showed improved optical modulation both in the visible and IR regions (70.3% at λ = 750 nm, 42.0% at λ = 2000 nm and 51.4% at λ = 10 μm), fast switching speed (3.5 and 4.8 s for coloration and bleaching, respectively), and excellent cycling performance (maintained transmittance modulation of 68.5% after 3000 cycles). Self-supported core/shell nanorod arrays of crystalline/amorphous TiO2 on FTO substrates fabricated by hydrothermal then layer-by-layer methods were demonstrated by Chen et al. (Fig. 14a).262 The layer-by-layer method was beneficial to produce uniform and thin amorphous shells as well as high-quality chemical and physical contacts at the phase interfaces (Fig. 14b). The arrays of crystalline/amorphous TiO2 core/shell nanorods with an optimal amorphous layer thickness of 13 nm possessed notably larger optical contrast (43%) and higher coloration efficiency (16.2 cm2 C−1) than crystalline TiO2 nanorod arrays (18%, 8.8 cm2 C−1) at λ = 800 nm. In addition, it was interesting and meaningful to note that the coating of an amorphous shell significantly enhanced the redox stability of crystalline TiO2 nanorods in a larger potential window.
Fig. 14 (a) Schematic diagram illustrating preparation of self-supported crystalline/amorphous TiO2 core/shell nanorod arrays. (b) HRTEM image of a crystalline/amorphous TiO2 core/shell nanorod. Reprinted from ref. 262. Copyright 2017 Elsevier. (c and d) Cross-sectional SEM images of TiO2 nanorod arrays and TiO2/Prussian blue core/shell (TiO2@PB) nanorod arrays. (e) In situ transmittance response at λ = 700 nm of the PB-based and TiO2@PB-based electrochromic devices (ECDs). Reprinted from ref. 280. Copyright 2017 Elsevier.
Based on whether the oxides of the cores are electrochromic or not, M1Ox/M2Oy core/shell 1D nanostructures can be separated into two categories. In the first category, the 1D M1Ox oxide cores act as strong mechanical and highly conductive skeletons to deposit M2Ox shells with large surface area and efficient redox reactions. To obtain high color contrast in the M2Ox shells, the 1D M1Ox skeletons are expected to demonstrate high transparency and negligible redox reactions when the M2Ox shells show electrochromism. Thus, 1D ZnO and SnO2 nanostructures are ideal skeletons. Wang et al. presented a flexible electrochromic display electrode based on amorphous WO3 nanoparticle-modified ZnO nanorod arrays on ITO/PET substrates prepared by using a facile hydrothermal process and pulsed laser deposition (PLD) method.264 The as-prepared ZnO/WO3 core/shell nanorod arrays exhibited faster switching response time of ca. 5 s for both coloration and bleaching with higher coloration efficiency than many WO3 nanorod arrays. Bi et al. investigated the influence of WO3 shell thickness and morphology on the electrochromic performance of ZnO/WO3 core/shell nanorod arrays prepared on AZO/PET substrates employing a similar synthesis method.265 By using an optimized thickness and morphology of the WO3 shells, the ZnO/WO3 core/shell nanorod arrays exhibited a high transmittance modulation of 68.2% at λ = 633 nm and a large coloration efficiency of 80.6 cm2 C−1 with unusual color changes between transparent and black, compared to transparent/blue changes for pure WO3 nanostructures. Zhang et al. prepared hierarchical SnO2/NiO core/shell nanostructures on FTO substrates by a two-step hydrothermal process.266 Compared to the pure NiO nanostructures, the SnO2/NiO core/shell nanostructures exhibited higher transmittance modulation in the whole visible spectrum range, better cycling stability and a notably better memory effect under open-circuit conditions due to the enhancements of charge storage in NiO derived from the electron band structure and high electrical conductivity of SnO2 cores.
As for the second category where M1Ox and M2Oy are two electrochromic oxides, the M1Ox/M2Oy core/shell 1D nanostructures could demonstrate enhanced and/or new electrochromic performance compared to the pure M1Ox and M2Oy nanoelectrodes. Until now, NiO/V2O5,267 WO3/TiO2,53,268,269 TiO2/NiO,270,271 TiO2/WO3,272,273 lithium-titanate/WO3,274 TiO2/V2O5,275 and TiO2/MoO3276 core/shell 1D nanostructures have been fabricated for electrochromic applications. The electrochromic performance of these M1Ox/M2Oy core/shell 1D nanostructures was highly dependent on the mass ratio of the two oxides. For example, as shown in Table 1, TiO2/WO3 core/shell nanorod arrays prepared by a two-step approach including hydrothermal and electrodeposition processes exhibited better performance than pure WO3 films and TiO2 nanorod arrays. Their transmittance modulation and switching response time varied a lot when the thickness of the WO3 shell changed.272 The M1Ox/M2Oy core/shell 1D nanostructures could also exhibit better cycling stability and higher coloration efficiency than the pure M1Ox and M2Oy nanostructures as demonstrated in the electrochromic performance of WO3/TiO2,53,269 TiO2/NiO,270 TiO2/WO3,272 and TiO2/V2O5275 core/shell nanorod arrays. Electrochemical analysis indicated that the enhanced redox kinetics including the improved Li+ diffusion coefficient and efficient interfacial electron transport were important features.
Metal oxide/polynuclear compound core/shell 1D nanostructures. Hexacyanoferrates, Prussian blue as a representative, are redox-derived electrochromic compounds.277–279 Because the electrochromism of hexacyanoferrates is from the ion insertion/extraction process, nanostructuring is an effective process to improve the performance. Chen et al. prepared TiO2/Prussian blue core/shell nanorod arrays through two-step soft chemical methods (Fig. 14c and d).280 The remarkably enlarged surface area, notably shortened ion diffusion length and porous character of Prussian blue shells resulted in highly increased storage capacity of K+ ions in electrochromic processes. Compared with the half-cell electrochromic device based on dense Prussian blue film, higher coloration efficiency (131.5 cm2 C−1), higher optical contrast (48% at λ = 700 nm) and faster response speed (coloration and bleaching time of 6.2 and 2.2 s, respectively) were achieved in the device fabricated by TiO2@ Prussian blue core/shell structures (Fig. 14e).
Polyoxometalates are a newly emerging class of transition metal-containing nanoclusters with intriguing structures and diverse redox properties for electrochromism.281,282 Coating polyoxometalates on metal oxide 1D nanostructures can significantly enlarge their surface area and shorten the ion diffusion distance, resulting in enhanced electrochromic performance. Liu et al. synthesized Dawson-type polyoxometalate K6P2W18O62@TiO2 hybrid nanowire arrays on FTO substrates by combining hydrothermal and layer-by-layer assembly methods.283 The as-prepared hybrid nanowire arrays exhibited higher transmittance modulation (45.1% at λ = 650 nm) and faster switching times (1.9 and 6.7 s for coloration and bleaching, respectively) than the K6P2W18O62 dense film (corresponding values of 31.2%, 5.3 s and 10.5 s, respectively).
Fig. 15 (a and b) SEM images of coaxial and branched V2O5/PANI core shell nanorod arrays, respectively. The color of the samples indicates the different loaded mass of PANI shells. Reprinted from ref. 287. Copyright 2017 Elsevier. (c) Transmittance modulation of TiO2/PANI core shell nanorod arrays under different voltages in 0.1 M HCl solution. Inset digital photos indicate the color changes under different voltages. Reprinted from ref. 286. Copyright 2013 American Chemical Society. (d and e) Schematic and electrochromic color contrast of electrochromic devices with viologen-modified ZnO NT (or NR) arrays as the working electrode. Reprinted from ref. 299. Copyright 2016 Royal Society of Chemistry.
Various electrochromic metal oxide/conductive polymer core/shell 1D nanostructures including NiO/PANI (or PEDOT),286,288 Co3O4/PANI (or PEDOT),286 TiO2/PANI (or PEDOT),286,289,290 WO3/PANI (or PEDOT),291–293 ZnO/PEDOT (or poly(3-methylthiophene) (PMeT)),294,295 and MoO3/PANI296 have been fabricated for electrochromism. When the metal oxide 1D cores only acted as skeletons, the conductive polymer coating layers (i.e. shells) exhibited enlarged surface area and shortened ion diffusion distance, leading to enhanced electrochromism. For instance, the TiO2/PANI core/shell nanorod arrays showed similar but enhanced electrochemical/elecrochromic performance to pure PANI films in a 0.1 M HCl aqueous electrolyte.12,286 The as-prepared hybrid nanorod arrays exhibited high transmittance modulation in the whole visible spectrum range, four distinct color changes, fast switching speeds (1.3 and 1.2 s for coloration and bleaching, respectively), and ultra-high cycling stability (Fig. 15c). Using metal oxides with higher electrical conductivity such as ZnO and SnO2 nanorod arrays, the metal oxide/conductive polymer core/shell 1D nanostructures can exhibit much enhanced electrochromic performance. Kateb et al. prepared a ZnO/PEDOT core/shell nanorod array film as an electrochromic electrode.294 This electrode showed an almost homogeneous transmittance modulation of ca. 48% in the whole visible spectrum range, ultrafast switching response time of <2.2 ms to achieve 100% transmittance saturation modulation (faster than PEDOT nanotube arrays of 50 and 70 ms for bleaching and coloration, respectively68), and ultrahigh cycling stability with no transmittance modulation degradation after 100 000 cycles. Electrochemical analysis indicated that the ZnO/PEDOT core/shell nanorods exhibited a significantly improved Li+ diffusion coefficient of 2.01 × 10−4 cm2 s−1 which was 4 orders of the value of the PEDOT dense film.
When both metal oxide cores and conductive polymer shells demonstrate electrochromism, the hybrid core/shell 1D structures can exhibit unusual optical modulation characteristics. For instance, the WO3/PANI nanorod arrays exhibited color changes differing from the pure WO3 and PANI electrodes.291 The PANI showed three colored redox states under positive voltages and a light yellow or transparent state under negative voltages,250 while the WO3 presented a transparent state under positive voltages and deep blue state under negative voltages.86 This meant that the WO3/PANI nanorod arrays had a complementary dual-electrochromic effect to each other, giving rise to a new coloration phenomenon, i.e. deep blue color under both positive and negative voltages while bleaching under voltages around 0 V accompanying unique transmittance modulation characters.291 Similar electrochromic phenomena were also found in the NiO/PANI288 and MoO3/PANI296 core/shell nanostructures.
Viologen-modified metal oxide 1D nanostructures. Viologen and its derivatives are a category of bipyridilium-containing molecules. Electrochromism is derived from the electrochemical doping/de-doping process of π bonds in their chains.297 To improve the electrochromic performance, adsorption of viologen molecules on metal oxide 1D nanostructured skeletons is an effective method. Technically speaking, these viologen-modified metal oxide 1D nanostructures do not demonstrate core/shell morphology because the absorbed viologen layer is very thin, around molecular scale in most cases. However, we regard them as core/shell nanostructures due to their theoretical and operating similarities to that of the above mentioned MOx/conductive polymer core/shell 1D nanostructures.
ZnO nanorod or nanofiber arrays are desirable skeletons due to their large surface area, high transparency, high electrical conductivity, electrochemical stability in the electrochromic voltage window of viologens, and easy fabrication. Sun et al. assembled an electrochromic device (ECD) using viologen-modified ZnO nanowire arrays on FTO glass as the electrochromic electrode and TiO2 nanoparticles on FTO glass as the counter electrode.297 The as-assembled ECD demonstrated obvious transparent/blue color changes, fast switching time (170 and 142 ms for coloration and bleaching respectively) and desirable cycling stability. Hu et al. fabricated a flexible viologen-based electrochromic electrode using ZnO nanowire arrays on an ITO/PET substrate as a skeleton.298 Li et al. made a comparative study of the electrochromic performance of methyl-viologen modified ZnO nanotube (NT) and nanorod (NR) arrays (Fig. 15d).299 Due to the larger surface area of the NTs than that of the NRs, the electrochromic device based on ZnO NTs exhibited higher transmittance modulation of 70% at λ = 608 nm, higher coloration efficiency of 93.5 cm2 C−1, faster switching response (6 and 3 s for coloration and bleaching, respectively), and better coloration/bleaching color contrast than the electrochromic device based on ZnO NRs (Fig. 15e).
Wang et al. fabricated a symmetric electrochromic supercapacitor using PANI nanowire arrays (prepared by dilute polymerization method) on flexible PEDOT:PSS/PET substrates with gel electrolyte (Fig. 16a and b).301 In the voltage range from 0 to 1 V, the as-assembled device exhibited light yellow green (0 V)/dark blue (1 V) color changes with high areal capacitance rate performance (0.017 and 0.004 F cm−2 at scan rate of 5 and 100 mV s−1 under CV examination) (Fig. 16c).
Fig. 16 (a) SEM images of PANI nanowire arrays on a flexible PEDOT:PSS/PET substrate. (b and c) Electrochromic color contrast and energy storage performance of a symmetrical electrochromic supercapacitor using PANI nanowire arrays as electroactive materials and polymer gel as an electrolyte. Reprinted from ref. 301. Copyright 2012 Royal Society of Chemistry. (d) Schematic diagram illustrating preparation of mesoporous TiO2 nanotube arrays using STAH process. Step I was electrodeposition of ZnO nanorod arrays. Step II was immersion of ZnO into STAH solution. Step III was preparation of TiO2 mesoporous nanotube arrays by STAH and annealing treatment. (e and f) SEM and TEM images of mesoporous TiO2 nanotube arrays. (g) Transmittance modulation during the charge/discharge process of the TiO2 nanotube array electrode at a current density of 15C. Reprinted from ref. 302. Copyright 2018 Royal Society of Chemistry.
Fig. 17 (a) Schematic design and working principle of a PECD employing a dye-adsorbed TiO2 nanoparticle film as the photoanode and an electropolymerized PProDOT-Me2 film as a cathode. (b) SEM image of the electropolymerized PProDOT-Me2 film. Reprinted from ref. 305. Copyright 2017 Springer. (c) Transmittance modulation and long-term cycling performance of the as-assembled PECD. Inset digital photos demonstrate the color contrast of the as-assembled PECD. Reprinted from ref. 304. Copyright 2016 Elsevier.
Electrochromism has a wide range of applications in the various fields such as antireflection coatings, smart thermal control coatings, displays, and smart windows. Due to the fact that the electrochromism of metal oxides, hexacyanoferrates and conductive polymers comes from redox reactions, fabrication of self-supported 1D nanostructures becomes an effective category of morphologies to improve their performance due to their structural, electrochemical and processible advantages. The past few years have witnessed fast development and remarkable achievements of various morphologies and materials of self-supported 1D nanostructures for high electrochromism. Furthermore, apart from the basic function of electrochromism, additional functions have been successfully incorporated into electrochromic devices to broaden the devices’ functionality.
We thank the National Natural Science Foundation of China (No. 51572058 and 51502057), the National Key Research & Development Program (2016YFB0303903), the Foundation of Science and Technology on Advanced Composites in Special Environment Laboratory, the Science and Technology Foundation of Guizhou Province of China (No. qian ke he ji chu  1065), the Scientific Research Starting Project of SWPU (2017QHZ019), and the Young Scholars Development Fund of SWPU (201799010003).
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