Recent Advances in Fabrication of Flexible, Thermochromic Vanadium Dioxide Films for Smart Windows

Abstract

Monoclinic-phase VO₂ (VO₂(M)) has been extensively studied for use in energy-saving smart windows owing to its reversible insulator–metal transition property. At the critical temperature (T_c = 68 °C), the insulating VO₂(M) (space group P21/c) is transformed into metallic rutile VO₂ (VO₂(R) space group P42/mnm). VO₂(M) exhibits high transmittance in the near-infrared (NIR) wavelength; however, the NIR transmittance decreases significantly after phase transition into VO₂(R) at a higher T_c, which obstructs the infrared radiation in the solar spectrum and aids in managing the indoor temperature without requiring an external power supply. Recently, the fabrication of flexible thermochromic VO₂(M) thin films has also attracted considerable attention. These flexible films exhibit considerable potential for practical applications because they can be promptly applied to windows in existing buildings and easily integrated into curved surfaces, such as windshields and other automotive windows. Furthermore, flexible VO₂(M) thin films fabricated on microscales are potentially applicable in optical actuators and switches. However, most of the existing fabrication methods of phase-pure VO₂(M) thin films involve chamber-based deposition, which typically require a high-temperature deposition or calcination process. In this case, flexible polymer substrates cannot be used owing to the low-thermal-resistance condition in the process, which limits the utilization of flexible smart windows in several emerging applications. In this review, we focus on recent advances in the fabrication methods of flexible thermochromic VO₂(M) thin films using vacuum deposition methods and solution-based processes and discuss the optical properties of these flexible VO₂(M) thin films for potential applications in energy-saving smart windows and several other emerging technologies.

1. Introduction

To address the rapidly increasing energy demand and growing environmental concerns, the development of renewable resources and smart-energy materials is receiving widespread attention [1]. Building energy consumption is estimated to account for 30–40% of the total global energy consumption, and this proportion is expected to continue increasing [2,3]. Windows are the most energy-inefficient component of a building; in this regard, smart windows offer the potential to reduce energy consumption by reducing the air-conditioning load via modulation of solar radiation [4]. Researchers have extensively studied the development of energy-efficient materials for smart windows to address the increasing energy needs. Monoclinic-phase VO₂ (VO₂(M)) was first reported by Morin in 1959 and is the most widely studied inorganic material owing to its switchable thermochromic properties [5]. VO₂ exhibits a first-order insulator–metal phase transition at the critical temperature (T_c = 68 °C), accompanied by reversible phase-change properties in the transition from the insulating monoclinic (P21/c) phase to the metallic rutile (P42/mmm) phase [6,7]. Figure 1 shows the crystal structure and band diagram of monoclinic and rutile phase VO₂. The vanadium ions in the monoclinic phase dimerize to form zigzag atomic chains with two V-V distances of ≈3.12 and ≈2.65 Å. Conversely, in the rutile phase, straight and evenly distanced vanadium chains are formed along the c-axis with ≈2.85 Å of distance and V⁴⁺ ions surrounded by O^2- are located at the center and corner positions [8,9]. Dimerization of the vanadium ion causes the d_ll band to split into a filled bonding (d_ll) and an empty antibonding (d_ll*). Furthermore, the π* orbitals shift to higher energies and make a forbidden band of approximately 0.7 eV between the d_ll and π* [10,11]. The Fermi level is located within the forbidden band, thereby forming the insulating VO₂(M). When the temperature is higher than T_c, the density of the Fermi energy states in VO₂(R) is formed by a mixture of π* and d_ll orbitals [9,12]. The electrons at the d_ll state exhibit a behavior similar to that of free electrons, accomplishing a half-filled metallic state. The Fermi level form between the π* and d_ll bands, indicating an enhanced electrical conductivity of the VO₂(R) [13]. Therefore, electrical and optical properties are considerably modulated during the phase transition. The phase transition of VO₂ can be induced by different types of stimuli, such as heat [5], electric fields [14], and mechanical strain [15]. The phase change in VO₂(M) has also been utilized in various emerging technologies, including optical switches [16], thermoelectrics [17], hydrogen storage [18,19], sensors [20,21,22], transistors [23,24,25], active metamaterials [26,27,28], and photoelectric devices [29]. The application of VO₂(M) in smart windows was investigated in the 1980s by Jorgenson et al. [7] and Babulanam et al. [30]. When the external temperature is lower than the phase-transition temperature, which is approximately 68 °C, VO₂(M) exists in the insulating phase, exhibiting high transmittance of near-infrared (NIR) wavelengths in the solar spectrum. Conversely, when the temperature is higher than the phase-transition temperature, the crystal and band structures change because of the transition from the insulator phase (VO₂(M)) to the metallic phase (rutile VO₂ (VO₂(R))), which significantly reduces the optical transmittance of NIR wavelengths. Therefore, thermochromic smart windows can reversibly modulate their solar transmittance at different temperatures and can reduce the room temperature during hot weather conditions; this will reduce the total energy consumption of the building. VO₂-based thermochromic smart windows offer characteristic advantages over other types of energy-saving windows, such as low-emissivity (low-e) glass [31,32] and electrochromic (EC) windows, owing to their ability to self-regulate solar transmission/reflection according to the external environment without utilizing an external energy supply [33,34,35]. Moreover, thermochromic windows have a relatively simple structure when compared with low-e or EC glass, thereby exhibiting potential for large-area installation and mass production for commercialization [36].

The performance of VO₂ for smart windows is evaluated in terms of the luminous transmittance (T_lum) and solar modulation ability. Luminous transmittance refers to the integrated optical amount of visible-light transmittance, which is determined from the following equation:

$${\mathrm{T}}_{\mathrm{lum}}={{\displaystyle \int }}^{ }{\mathsf{\Phi }}_{\mathrm{lum}}\left(\mathsf{\lambda }\right)\mathrm{Td}\mathsf{\lambda }/{{\displaystyle \int }}^{ }{\mathsf{\Phi }}_{\mathrm{lum}}\left(\mathsf{\lambda }\right)\mathrm{d}\mathsf{\lambda }, \left(380 \mathrm{to} 780 \mathrm{nm}\right)$$

T(λ) and Φ_lum characterize the transmittance of the wavelength λ and photopic luminous efficiency function in the visible region, respectively [30,37]. Solar-energy modulation ability (ΔT_sol) is also a critical feature for determining the energy-saving capability of material. ΔT_sol is defined as the difference in the solar-energy transmittance (T_sol) values before and after phase transition in the 240 to 2500 nm spectrum, which is estimated using the follow equations [38]:

$${\mathrm{T}}_{\mathrm{sol}}={{\displaystyle \int }}^{ }{\mathsf{\Phi }}_{\mathrm{sol}}\left(\mathsf{\lambda }\right)\mathrm{Td}\mathsf{\lambda }/{{\displaystyle \int }}^{ }{\mathsf{\Phi }}_{\mathrm{sol}}\left(\mathsf{\lambda }\right)\mathrm{d}\mathsf{\lambda }, \left(250 \mathrm{to} 2600 \mathrm{nm}\right)$$

$$\Delta {\mathrm{T}}_{\mathrm{sol}}={\mathrm{T}}_{\mathrm{sol},\mathrm{low} \mathrm{temperature}} - {\mathrm{T}}_{\mathrm{sol},\mathrm{high} \mathrm{temperature}}$$

where Φ_sol denotes the solar irradiance spectrum for an air mass of 1.5, which is equivalent to the presence of the sun at an angle of 37° from the horizon [37]; moreover, T_{sol,low temperature} and T_{sol,high temperature} represent the solar transmittance of VO₂ films at a low temperature in the monoclinic phase and at a high temperature in the rutile phase, respectively. T_lum should be greater than 40% to indicate the requirement for daylight across windows, and ΔT_sol should be sufficiently high, at least 10%, for energy saving [39]. Furthermore, the phase-transition temperature of VO₂ (T_c = 68 °C) should be reduced from 68 °C for efficient regulation of solar energy during daytime [40]. Therefore, a reduced phase-transition temperature (T_c), high luminous transmittance (T_lum), and strong solar-energy modulation ability (ΔT_sol) are important characteristics for energy-efficient smart windows. To fulfill the demand for practical applications of energy-saving smart windows, VO₂-based thermochromic thin films should possess the following features: the phase-transition temperature (T_c) should be reduced to near-ambient temperature, and a high luminous transmittance (T_lum > 40%) accompanied by a strong solar-energy modulation ability (ΔT_sol > 10%) should be available [41,42].

Several studies have been conducted to improve the energy-saving performance of VO₂-based smart windows. For example, reductions in T_c have been achieved by doping with metal ions [43,44,45], or by utilizing nonstoichiometric compounds [46], strains [47], and nano-size effects [48]. Among the aforementioned methods, doping with metal ions, such as W⁶⁺ [49], Al³⁺ [50], Mg²⁺ [51], Sn⁴⁺ [52], and Mo⁶⁺ [53,54], is considered the most efficient. However, an increase in the dopant content results in the deterioration of phase-transition behaviors, such as a reduction in ΔT_sol and a broadened phase-transition temperature range [55,56]. High values of T_lum and ΔT_sol are also required to accomplish high-energy modulation efficiency for smart windows; however, these parameters involve a tradeoff, and thus, it is difficult to enhance them simultaneously [57]. Various strategies have been suggested to improve T_lum and ΔT_sol simultaneously, such as doping with Mg²⁺ [56] and F⁻ [55], or utilizing nano-size thermochromic materials [58], photonic crystals [59], antireflective overcoating [60], porous films [60], and multilayered structures [60,61]. However, the fabrication of VO₂(M) films with high T_lum (> 40%) and ΔT_sol(>10%) values as well as a sufficiently reduced T_c remains challenging, which limits the utilization of VO₂(M) in practical applications [56,57,62]

Recently, the fabrication of flexible VO₂(M) films has attracted widespread attention [39,56]. Flexible thermochromic films demonstrate significant potential for large-scale fabrication and commercialization [63,64,65,66]. For example, flexible VO₂(M) films can be instantly applied to the windows of existing buildings and easily integrated onto curved surfaces, such as automobile windows. Moreover, flexible VO₂(M) thin films show the potential for application in actuators and optical switches for future optical and electronic devices [63,67]. Thus far, high-quality VO₂(M) thin films have been fabricated using vacuum-chamber-based techniques, such as chemical vapor deposition (CVD) [68,69,70], physical vapor deposition [56], radiofrequency (RF) magnetron sputtering [71], and pulsed laser deposition [72]. These deposition methods provide high-quality and highly crystalline VO₂(M) films; however, they often require high-temperature deposition conditions or an additional thermal annealing process to yield phase-pure crystalline VO₂(M) films [63]. The deposition temperature is typically higher than 400 °C, which exceeds the thermal resistance of most flexible polymeric substrates [51,73,74,75]. Therefore, chamber-based deposition processes are predominantly performed on rigid inorganic substrates with high thermal resistance, which limits the fabrication of crystalline VO₂(M) films on flexible substrates. Flexible VO₂(M) films can also be obtained via colloidal deposition using VO₂(M) nanoparticles (NPs) [56,65,76]. Colloidal dispersion of VO₂(M) enables solution-based deposition onto polymeric substrates or the formation of flexible composite films through mixing in a polymer matrix. Hydrothermal synthesis of colloidal VO₂(M) NPs was reported in a recent study, which demonstrated the feasibility of producing flexible VO₂(M) films through the solution-based deposition of NPs at room temperature [77,78]. However, for colloidal VO₂(M) NPs synthesized hydrothermally, lowering T_c while maintaining favorable optical properties, such as a high T_lum and ΔT_sol, remains difficult [79,80]. Therefore, the fabrication of flexible VO₂ thin films using colloidal VO₂(M) NPs with a reduced T_c, high T_lum, and high ΔT_sol is still significantly challenging. In this review, we focus on the recent advances in the fabrication methods for flexible thermochromic VO₂(M) thin films. We systematically review the fabrication process, including chamber-based vacuum deposition on flexible substrates that possess high thermal resistance. In addition, we introduce film-transfer techniques used to transfer VO₂(M) layers deposited on rigid substrates onto flexible polymer substrates. Finally, we introduce the solution-based deposition process using colloidal VO₂(M) NPs. The optical properties and phase transition behaviors are discussed to investigate the potential of flexible VO₂(M) films for application in energy-saving smart windows and other emerging technologies.

2. Fabrication Methods

2.1. Fabrication of Flexible Monoclinic-Phase VO₂ (VO₂(M)) Films via Chamber-Based Deposition

As discussed, the fabrication of stoichiometric and highly crystalline VO₂ films using vacuum deposition requires high-temperature conditions or an additional calcination process [81]. Therefore, rigid inorganic substrates, which have high thermal stability, such as SiO₂ [82], MgF₂ [83], and Al₂O₃ [84], are generally used for growing VO₂(M) films. The fabrication of flexible VO₂(M) films through chamber-based deposition of VO₂(M) films has also demonstrated using flexible substrates with high thermal stability. For example, muscovite sheets were first used as substrates for the fabrication of VO₂(M) films because such sheets possess a high thermal stability of over 500 °C and superior chemical resistance, which enable the formation of highly crystalline VO₂(M) films through high-temperature sintering. High-quality, single-phase VO₂(M) films can be grown epitaxially on (001) muscovite substrates with high crystallinity, leading to superior phase transition behaviors in terms of resistivity and infrared (IR) transmittance [85]. Li et al. also developed a process for depositing a VO₂ film directly on a flexible muscovite substrate [86]. First, V₂O₅ films were deposited on a native muscovite substrate through pulsed-laser deposition for 20 min; then, the films were annealed at 650 °C under a 5 mTorr oxygen atmosphere to obtain highly crystalline VO₂(M) films (Figure 2a). The electrical resistance of the VO₂(M) thin films was measured under various bending radii. During the phase transition, the electrical resistance of the films varied by an order of 10³ or more (ΔR/R > 10³), and the change in luminous transmittance was higher than 50% (ΔT_r > 50%) (Figure 2b). Owing to the intrinsic transparency and flexibility of muscovite sheets, the VO₂/muscovite heterogeneous structures also exhibited superior flexibility and visible-light transparency. The electrical resistance of the VO₂/muscovite films remained the same even after the films were bent 1000 times; this confirmed the high mechanical stability of the films (Figure 2c). Thus, considering their enhanced electrical, thermal, optical, and mechanical properties, VO₂/muscovite films demonstrate considerable potential for application in flexible electronic devices, especially optical switches.

VO₂(M) thin films grown on substrates, such as TiO₂, Al₂O₃, diamond, and SiO₂, have strong chemical bonds (ionic or covalent) between the VO₂(M) layers and the substrates. Thus, the VO₂ lattice is constrained, which is known as the substrate-clamping effect; this complicates the lattice rearrangement during phase transition [82,87,88]. Therefore, VO₂ films deposited on inorganic substrates typically require a higher energy to drive the metal–insulator transition (MIT). Conversely, VO₂(M) films deposited on mica sheets typically have weak van der Waals (vdW) bonds (0.1–10 kJ mol^–1) between VO₂(M) and the mica layer, which is 2–3 times weaker than the aforementioned ionic or covalent bonds (100–1000 kJ mol^–1) [89]. This weak vdW bonding between the VO₂ film and the mica sheet does not induce any significant lattice strain in the VO₂ layer. Therefore, the VO₂ film behaves as a nearly freestanding film on the mica sheet, which enables MIT with exceedingly low energy stimuli [90]. Moreover, owing to the weak vdW bonding between adjacent mica sheets, the thin mica sheet can be peeled off from the substrate, creating transparent and flexible VO₂(M)/mica sheets. Wang et al. also employed a mica sheet as a support for VO₂(M) to fabricate a mechanically flexible and electrically tunable flexible phase-change material for IR absorption [91]. First, 100-nm-thick Au thin films were deposited on a mica sheet through magnetron sputtering. Then, a 100-nm-thick vanadium film was deposited on the Au film via electron-beam evaporation and was thermally annealed in an oxygen atmosphere at temperatures of 430–470 °C. Au and mica sheet can withstand high-temperature annealing conditions. Finally, graphene thin films were transferred onto the VO₂ thin film to deposit the conductive electrode that induces the phase transition of the VO₂(M) thin films through Joule heating (Figure 3a). The IR absorption of this device can be continuously adjusted from 20% to 90% by changing the current applied to the graphene film. Moreover, this structure exhibited superior bending durability when it was bent up to 1500 times, without any noticeable deterioration in the optical properties (Figure 3b). Such tunable and flexible VO₂ devices have various application prospects in flexible photodetectors and active wearable devices.

Chen et al. fabricated a flexible VO₂(M) thin film on a muscovite (mica) sheet directly through RF-plasma-assisted oxide molecular beam epitaxy (rf-OMBE) [92]. First, the VO₂ layer was grown using rf-OMBE on the (001) plane of mica sheets at 550 °C. Then, a layered single-walled carbon nanotube (SWNT) films was deposited using CVD on the high-quality VO₂/mica thin film. The SWNT layer exhibited superior conductivity and flexibility and can be employed as an efficient heater when a current/bias voltage is applied. The almost freestanding SWNTs/VO₂/mica (SVM) film was fabricated by peeling off the thin-layered SVM film from the substrates. Two Au electrodes were deposited on the flexible SVM thin film to provide a two-terminal electrode. The MIT process of the flexible VO₂(M) thin film can be easily controlled by heating SVM films with a bias current on Au electrodes, thereby enabling reversible modulation of IR transmission. When a bias current was applied, the transmittance decreased sharply from 70% and maintained an almost constant value of approximately 30% thereafter. When the input current was turned off, the transmittance quickly returned to its highest value of 70%; this confirms that direct modulation of the transmittance by applying a current is possible (Figure 4a,b). The MIT temperatures were 71 and 62 °C during the heating and cooling cycles, respectively (Figure 4c). Such ultrathin flexible SVM films with superior flexibility and transparency can be used for various applications involving future electrical devices.

In addition to mica sheets, carbon-based substrates, such as graphene sheets and networks of carbon nanotubes (CNTs), have also been utilized as flexible substrates for VO₂ deposition owing to their high thermal resistance. Xiao et al. reported the fabrication of VO₂/graphene/CNT (VGC) flexible thin films [93]. First, the graphene/CNT thin film was prepared by depositing graphene on a Cu substrate via low-pressure CVD. Then, the aligned CNT thin films were stacked on graphene substrates, followed by etching of the Cu substrate to form flexible graphene/CNT flexible thin films. The VO_x thin film was deposited on the graphene/CNT film through DC magnetron sputtering and was then thermally annealed at 450 °C in a low-pressure oxygen environment to obtain crystalline VO₂(M) thin films (Figure 5a). The phase transition of the VGC freestanding thin film can be induced by applying a current. The VGC films exhibited fast switching with low power consumption and highly reliable phase transition (Figure 5b,c). The drastic change in IR transmittance during the phase transition can potentially enable the application of VGC films in IR thermal camouflage, cloaking, and thermal optical modulators.

Chan et al. reported the fabrication of flexible VO₂(M)/Cr₂O₃/polyimide (PI) films using Cr₂O₃ as a buffer layer [94]. The Cr₂O₃ layer allows an epitaxial growth of the VO₂(M) layer, typically at approximately 300 °C, which enables the deposition of VO₂(M) on the PI polymer substrate at a relatively lower temperature (Figure 6a). The lattice constants for Cr₂O₃ are a = 0.496 nm, b = 0.496 nm, and c = 1.359 nm, and those for VO₂(R) are a = 0.455 nm, b = 0.455 nm, and c = 0.286 nm [95]. Therefore, Cr₂O₃ can act as a buffer layer owing to the similarity of its lattice constants with those of VO₂(R). Therefore, highly crystalline VO₂/Cr₂O₃ films can be successfully fabricated even under relatively low deposition conditions from 250 to 350 °C. Moreover, the refractive index of Cr₂O₃ is 2.2–2.3; hence, Cr₂O₃ behaves as an antireflective coating on top of the VO₂(M) layers, leading to a higher optical performance with T_lum and ΔT_sol. The VO₂ film fabricated at 275 °C showed 42.4% of T_lum and 0.4% of ΔT_sol; in contrast, the VO₂ film deposited with a 60-nm Cr₂O₃ buffer layer exhibits a high ΔT_sol value of 6.7% at a similar T_lum (43.7%). To fabricate flexible VO₂/Cr₂O₃/PI films, thin Cr₂O₃ layers were deposited on colorless PI films through magnetron sputtering; then, the VO₂ layers were directly deposited on Cr₂O₃/PI films using magnetron sputtering (Figure 6b). VO₂/Cr₂O₃/PI films exhibit minimal strain owing to the similar lattice parameters of the two layers. Therefore, flexible VO₂(M) films have a narrow and sharp hysteresis loop. The VO₂/Cr₂O₃/PI films exhibited superior IR modulation properties, i.e., approximately 60% variation at 2500 nm, when the VO₂ film thickness was approximately 80 nm (Figure 6c). The T_c values of the films calculated during the heating and cooling cycles were 71.8 and 71.3 °C, respectively, and the transition width of the hysteresis loop was approximately 0.5 °C, which is significantly low for a phase transition (Figure 6d,e). Furthermore, the resistivity decreased by more than two orders of magnitude during the phase transition, indicating the high crystallinity of VO₂(M) films. However, the deposition temperature of >250 °C is still higher than the temperature that typical polymer films can withstand, which limits the utilization of various flexible polymeric substrates other than PI.

Although direct deposition of VO₂(M) on substrates with high thermal resistance is a simple single-step process, only a limited number of substrates can be used under high-temperature deposition conditions. In contrast, the film-transfer process offers opportunities to utilize various types of polymeric substrates for the fabrication of flexible films [96]. In this process, VO₂(M) films are deposited on rigid substrates via high-temperature vacuum deposition and thermal annealing; then, the VO₂(M) thin films are transferred onto flexible polymeric substrates using the film-transfer process. As the VO₂(M) films are deposited under high-temperature conditions, they become highly crystalline, achieving enhanced optical properties (high T_lum and ΔT_sol) and improved stability under ambient conditions that persists for several months [97]. Moreover, polymer supports can impart enhanced mechanical stability and flexibility to films. The fabrication of flexible VO₂(M) films using the film-transfer process was first performed by Kim et al. [98]. In this process, an atomically thin, flexible graphene film was used to deposit a VO₂(M) layer for the transfer process. An amorphous VO_x layer was first deposited on a graphene/Cu substrate through RF magnetron sputtering. Then, the VO_x film on the graphene/Cu substrate was thermally annealed at 500 °C to transform VO_x into crystalline VO₂ films. The Cu substrate was selectively etched, and the remaining VO₂(M)/graphene film was transferred to a polyethylene terephthalate (PET) film to fabricate flexible VO₂(M)/graphene/PET films. Because of the deposition on polymer films, the VO₂(M)/graphene/PET films exhibited high mechanical stability and flexibility while maintaining their reversible phase-transition property. These flexible VO₂/graphene/PET films exhibited a transmittance of 65.4% at a 550-nm wavelength; moreover, the variation in the transmittance during phase transition reached 53% at a wavelength of 2500 nm, with the transition band width being 9.8 °C (Figure 7a,b). The VO₂(M)/graphene/PET was integrated onto glass in a model house to investigate its ability to regulate the indoor temperature when functioning as a smart window. The VO₂/graphene films reduced the indoor room temperature by 5.8 °C compared with bare glass, thereby exhibiting the potential to function as an energy-efficient smart window (Figure 7c,d).

The fabrication of flexible VO₂(M) thin films with a reduced T_c is still challenging owing to the difficulty in doping during the deposition process. Chae et al. reported a solution-based process to deposit VO₂(M) films using W-doped colloidal NPs, followed by film transfer, to fabricate flexible W-doped VO₂(M) films [74]. Colloidal VO_x NPs were synthesized via high-temperature thermal decomposition with vanadium precursors, which were used for the deposition of VO₂(M) layers [99]. During the synthesis, W precursors were added into the reaction mixture for efficient doping of W during the formation of VO_x NPs. Then, VO_x NPs were deposited on mica substrates using the solution-based process and thermally annealed to form highly crystalline VO₂(M) films. Subsequently, the VO₂(M)/mica films were transferred onto polymeric substrates using adhesive-coated PET films. During the transfer process, a thin layer of mica sheet was peeled off and transferred to the polymer film to form transparent and flexible mica/VO₂(M)/PET films. As mica sheets are brittle, the polymer substrate can provide high mechanical strength and ensure reliable bending (Figure 8a). The W dopants were effectively doped into the VO₂(M) thin films, which resulted in the systematic reduction in T_c depending on the different possible doping concentrations. The T_c of flexible mica/VO₂(M)/PET films can be easily controlled at 25.6 °C when 1 at% W doping is used (Figure 8b). These flexible films exhibit superior optical properties—a T_lum of 53% and a ΔT_sol of 10%—at a T_c of 29 °C when 1.3 at% Tungsten (W) doping is used. Thus, such films can be viable for use in energy-saving smart windows (Figure 8c,d).

2.2. Fabrication of Flexible VO₂(M) Films through Solution-Based Deposition Process

Although the vacuum chamber-based deposition and film-transfer processes are highly effective for the fabrication of crystalline VO₂(M) films on flexible substrates, these processes are significantly complex, involving multiple deposition steps and often requiring an etching process, which can potentially limit large-scale fabrication and commercialization [100]. In contrast, the solution-based process enables simple, low-cost, and large-area fabrication of flexible VO₂(M) films [101]. Early examples of solution-processed VO₂(M) films were demonstrated via a sol–gel process [102]. Speck et al. were the first to demonstrate the sol–gel deposition of VO₂(M) films using molecular vanadium precursors [103]. In general, the sol–gel process of VO₂(M) thin films have been performed on thermally stable substrates, such as quartz, mica, or silicon wafers, owing to the high temperature thermal annealing process, typically above 400 °C [104]. Recent literature demonstrates that low temperature sol–gel deposition of VO₂(M) film processes can be achieved using deep ultraviolet photoactivation chemistry, which enable the fabrication of flexible VO₂(M)/Al₂O₃/PI films at 250 °C [105]. Not only has the sol–gel process been widely studied for the fabrication of flexible smart windows, but solution-based deposition using colloidal VO₂(M) NPs has too. Among a variety of synthetic methods based on colloidal VO₂(M) NPs, hydrothermal synthesis has attracted considerable attention owing to the high phase purity of the as-synthesized VO₂(M) NPs [38]. Hydrothermal synthesis involves a chemical reaction that yields high-quality crystals in a sealed pressurized reactor under high pressure and temperature. Hydrothermal growth of VO₂(M) films on the substrates has also been reported in the literature [106,107]. For example, VO₂(M) films have been fabricated via hydrothermal reactions by placing r-Al₂O₃ substrates in a hydrothermal reactor containing a solution mixture of ammonium metavanadate and oxalic acid [108]. The self-organized VO₂(M) films were formed with T_lum of 65% and ΔT_sol of ~11.82% [109]. However, for the direct hydrothermal deposition of VO₂(M) films on flexible substrates, the substrates should have high thermal and chemical resistance to ensure that they can withstand hydrothermal reaction conditions and calcination temperature [110,111,112]. Therefore, the use of VO₂(M) NPs for film depositions could have potential for large-area fabrication by mass-production processes using various substrate types. The single-step hydrothermal synthesis of VO₂(M) NPs was first demonstrated by Théobald et al. using a V₂O₃–V₂O₅–H₂O system, and the reaction was performed at a temperature of 20–400 °C under supercritical pressure [113]. There exist several stable vanadium oxide structures, such as VO₂, V₂O₅, V₂O₃, V₅O₉, V₆O₁₃, and V₆O₁₁, with various nonstoichiometric compounds [114]. Even in the stoichiometric compound, i.e., VO₂, several polymorphs exist, such as VO₂(A) [115], VO₂(B) [59], VO₂(D) [116], VO₂(P) [117], and VO₂(M) [118]. Therefore, hydrothermal synthesis of phase-pure and highly crystalline VO₂(M) is significantly challenging. Strong phase transition behaviors and favorable optical properties, including high values of T_lum and ΔT_sol, can be obtained using high-purity VO₂(M) NPs, in the absence of nonstoichiometry and impurities of metastable polymorphs. Therefore, careful control of synthetic procedures, including the hydrothermal reaction conditions, types of metal precursors, solvents, and additives, is a prerequisite for obtaining phase-pure VO₂(M) NPs.

To enhance phase purity and crystallinity, a two-step hydrothermal synthesis process to synthesize VO₂(M) NPs has widely studied. In this process, metastable VO₂ NPs are first synthesized hydrothermally and then thermally annealed for the conversion into the VO₂(M) phase. Phase-pure VO₂(M) NPs are obtained from various types of metastable VO₂ NPs and under different annealing conditions. Xie et al. first reported the hydrothermal synthesis of VO₂(D) with a size of 1–2 μm, using NH₄VO₃ and H₂C₂O₄. Hydrothermal synthesis was performed at 210 °C for 24 h, followed by a calcination process to transform the VO₂(D) into VO₂(M) [116]. Calcination of VO₂(D) was performed at temperatures as low as 300 °C for 2 h under a flow of high-purity nitrogen to obtain VO₂(R) NPs. These NPs also exhibit MIT near 68 °C. A two-step hydrothermal synthesis using VO₂(B) NPs has also been reported; however, the phase transformation from VO₂(B) to VO₂(M) occurs at a significantly higher annealing temperature, typically higher than 500 °C [119]. Corr et al. also studied the hydrothermal synthesis of VO₂(B) nanorods using V₂O₅ and formaldehyde solution at 180 °C for two days [120]. Then, thermal annealing was performed to convert VO₂(B) to VO₂(R) at 700 °C for 1 h in an argon atmosphere. Sun et al. reported the hydrothermal synthesis of VO₂(P) using VO(OC₃H₇)₃ and oleylamine at 220 °C for 48 h; then, they obtained VO₂(M) after thermal annealing at 400 °C for 40 or 60 s in a nitrogen or air atmosphere [121]. The size-dependent MIT property of VO₂(M) NPs was demonstrated through in situ variable-temperature IR spectroscopy. The authors observed that the variation in the transmittance of single-domain VO₂(M) NPs during phase transition systematically increased with a reduction in the size of the VO₂(M) NPs. Zhong et al. reported star-shaped VO₂(M) NPs that were hydrothermally synthesized using NH₄VO₃ and formic acid for two days at 200 °C. Then, the as-synthesized NPs were thermally annealed at 300–450 °C for 1 h to obtain VO₂(M) NPs. The VO₂(M) NP thin films were 325 nm thick and exhibited a T_lum and ΔT_sol of 44.18% and 7.32%, respectively [122]. Song et al. reported the hydrothermal synthesis of VO₂(D) using NH₄VO₃ and H₂C₂O₄·2H₂O at ~220 °C for ~18 h, followed by thermal annealing of VO₂(D) at 250–600 °C for 3 h, to obtain VO₂(M) nanoaggregates [123]. The as-synthesized VO₂(M) exhibited a low T_c of approximately 41.0 °C and a thermal hysteresis width of approximately 6.6 °C. Li et al. demonstrated the electrothermochromicity of VO₂(M) NPs/Ag nanowire (NW) thin films deposited on glass and flexible PET substrates [124]. VO₂(M) NPs were hydrothermally synthesized using V₂O₅ and an oxalic acid dehydrate via at 220 °C for 36 h, followed by additional thermal annealing at 400 °C for 1 h in a vacuum chamber. The VO₂(M) NPs were deposited on top of Ag NW heaters. The optical response of the VO₂(M) NP films was then dynamically modulated by applying voltage on Ag NW. The infrared (IR) transmittance variation of the films from 0 V to 8 V of applied voltage is approximately 50% at 1500 nm. Li et al. demonstrated the two-step hydrothermal synthesis of VO₂(M) NPs using V₂O₅, H₂C₂O₄, and polyvinyl alcohol precursors [125]. The hydrothermal synthesis was performed at 220 °C for over 36 h, and the calcination was performed at 300–450 °C under vacuum. The VO₂(M) film had a thickness of 463 nm and exhibited a high T_lum of over 70% at 700 nm; moreover, its IR transmittances at 1500 nm were approximately 89.5% and 53.8% before and after phase transition, respectively. The IR modulation exceeded 35%, which represents favorable optical properties for application in smart windows.

A single-step hydrothermal synthesis without a calcination process has also been reported. This method is a potentially simple, convenient, and low-cost process because it involves no additional post-annealing to obtain phase-pure VO₂(M) NPs [126]. An additional thermal annealing processes induces grain growth in VO₂(M) films. Size dependence of VO₂(M) NPs on thermochromic properties have also been reported. Notably, a decrease in the size of VO₂(M) NPs improves T_lum and ΔT_sol values [80]. Narayan et al. reported a phase-transition model in which the hysteresis width is directly proportional to the grain boundary area per unit volume [127]. Therefore, the hysteresis width is inversely correlated to the particle radius, and as the particle size increases, the phase transition temperature reduces, and the hysteresis width decreases. The smaller the nanoparticle size, the wider the hysteresis, and the VO₂ thermochromic performance is improved [128,129]. Therefore, single-step hydrothermal synthesis is more preferable to prevent particle coarsening by an additional thermal annealing process, hence sustaining a high thermochromic performance [130,131]. Gao et al. first demonstrated single-step hydrothermal synthesis of W-doped snowflake-shaped VO₂(R) using V₂O₅ and H₂C₂O₄. The reaction was performed for seven days at 240 °C, and VO₂(M) NPs were synthesized without a thermal annealing step [78]. The width of the VO₂(R) nanocomposites was 200–300 nm, and the thickness was approximately 200–800 nm. Alie et al. also demonstrated single-step hydrothermal synthesis of star-shaped and spherical VO₂(M) particles using H₂C₂O₄ and V₂O₅ in a molar ratio of 3:1 at 260 °C for 24 h [132]. The highly crystalline star-shaped VO₂(M) particles exhibited a high thermal stability of up to ~300 °C and a >10% transmittance variation in the IR region during phase transition. Li et al. reported one-step hydrothermal synthesis of VO₂(M) NPs using V₂O₅, TiO₂, and H₂C₂O₄∙2H₂O at 240 °C for 24 h [133]. The VO₂(M) NPs, with a size of approximately 50–100 nm, were further modified using Zn(CH₃COO)₂ to obtain a VO₂–ZnO structure. The VO₂(M)–ZnO films exhibited a low T_c of approximately 62.6 °C and a T_lum and ΔT_sol of approximately 52.2% and 9.3%, respectively. Ji et al. demonstrated the synthesis of VO₂(M) using V₂O₅, N₂H₄, and H₂O₂ through a one-step hydrothermal process performed at 260 °C for 24 h (Figure 9a,b). The as-prepared VO₂(M) NPs exhibited a transmittance change of approximately 50% at a wavelength of 2000 nm [134]. Moreover, as the concentration of the W dopant increased from 0% to 1%, the T_c of the VO₂(M) NPs decreased from 55.5 to 37.1 °C (Figure 9c). Chen et al. reported the synthesis of phase-pure V_1-xW_xO₂ nanorods using H₂C₂O₄∙2H₂O and V₂O₅ precursors. For W doping, (NH₄)₅H₅[H₂(WO₄)₆]H₂O was added, and the reaction was performed at 260–280 °C for 6–72 h [135]. The T_lum of the 0.5 at% W-doped VO₂(M) films was 60.6% at 20 °C, and ΔT_sol was 8.1%. Whittaker et al. reported the synthesis of W-doped VO₂(M) nanobelts using V₂O₅ and H₂C₂O₄ precursors with H₂WO₄ for W doping [43]. The reaction was performed at 250 °C for 12 h to 7 days. W doping (0.90%) led to remarkable modulation of the T_c of VO₂(M) films, from 68.0 to 33.8 °C. Shen et al. demonstrated that Zr doping significantly enhances optical properties while reducing T_c. [118]. Moreover, Zr doping of VO₂(M) reduces T_c while improving T_lum and ΔT_sol. However, T_c is only reduced from 68.6 to 64.3 °C with 9.8% Zr doping; conversely, Zr-doped VO₂ flexible films exhibit high values of T_lum (60.4%) and T_sol (14.1%). The optical bandgap, which is 1.59 eV for undoped VO₂(M), increases to 1.89 eV after 9.8% Zr doping, resulting in a change in the apparent color of the VO₂(M) films. Accordingly, the color of the Zr-doped VO₂(M) flexible films is affected; the brown-yellow color of flexible VO₂(M) film is brightened, along with an increase in T_lum. In addition, T_c is further reduced to 28.6 °C, and T_lum and ΔT_sol values of 48.6% and 4.9%, respectively, are achieved through W-Zr-co-doping.

Several studies have been conducted to optimize the conditions for single-step hydrothermal synthesis to enhance the phase purity of VO₂(M) NPs and their optical properties, including T_lum and ΔT_sol. Guo et al. performed a one-step hydrothermal synthesis process using VOSO₄ and N₂H₄·H₂O in the presence of H₂O₂ [77]. H₂O₂, a strong oxidizing agent, is separated after the reaction with the vanadium solution in a hydrothermal autoclave reactor. Then, H₂O₂ decomposes and evaporates at 150 °C to provide a moderately oxidizing environment. This facilitates the synthesis of stoichiometric and highly crystalline VO₂(M) NPs. The as-synthesized VO₂(M) NPs exhibited an average size of ~30 nm, with significant size uniformity (Figure 10a,b). For the preparation of flexible VO₂(M) films, the VO₂(M) NPs were dispersed in N,N-dimethylformamide with polyacrylonitrile polymers. Then, the solution was deposited on a flexible PET substrate. The flexible VO₂(M) films attained favorable optical properties, with a T_lum of 54.26% and a ΔT_sol of 12.34% (Figure 10c,d). In addition to optimizing the hydrothermal reaction conditions, the enhancement in the purity of vanadium precursors also produces VO₂(M) NPs with improved optical properties.

Kim et al. demonstrated single-step hydrothermal synthesis of VO₂(M) NPs using phase-pure vanadium precursors [136]. After mixing the vanadium precursors, size-selective purification was performed to enhance the phase purity of the precursors, resulting in the formation of VO₂(M) NPs with enhanced optical properties. The obtained phase-pure VO₂(M) NPs exhibited an enhanced T_lum (55%) and ΔT_sol (18%), and the ΔT_sol value is one of the highest reported for hydrothermally synthesized VO₂(M). Furthermore, W-doped VO₂(M) NPs have been reported to exhibit superior phase-transition behaviors, while T_c is systematically reduced depending on the W doping concentration (Figure 11a,b). Flexible VO₂(M) films were fabricated and deposited on PET polymer substrates over a large area using a spray coater (15 cm × 15 cm) (Figure 11c,d). In model house experiments under daytime solar irradiation, the W-doped VO₂(M) films applied onto glass provided a significant reduction in the indoor temperature; thus, these films are potentially viable for practical applications.

Colloidal NPs enable convenient, large-scale fabrication of flexible VO₂(M) film through the solution process, which is beneficial considering the expected requirement for large-scale fabrication techniques [63,65,76,137]. For the fabrication of flexible VO₂(M) films, VO₂(M) NPs have been coated on flexible polymer films or embedded into a polymer matrix [138]. Shen et al. reported a process for blade coating of VO₂(M) NPs on indium tin oxide (ITO)-coated PET substrates to form flexible VO₂(M) films [139]. Applying a current along the ITO layer induced ohmic heating, which resulted in the phase transition of VO₂(M) layers and a change in IR transmittance. The obtained film showed well-controlled IR switching properties upon changing the input voltage, as well as superior thermochromic properties (T_lum of 57.3% and ΔT_sol of 13.8%). Under ohmic heating, the IR conversion properties did not show any evident deterioration, even after 10,000 bending cycles, which indicates superior stability and flexibility. Chen et al. demonstrated the preparation of VO₂(M)/polymer composite films by embedding VO₂(M) NPs into a polymeric matrix. VO₂(M) NPs were synthesized hydrothermally using V₂O₅ and N₂H₄ at approximately 180–400 °C for 15 h [80]. The size of the VO₂(M) NPs ranged from approximately 25 to 45 nm. The synthesized VO₂(M) NPs were dispersed in polyurethane (PU) and coated onto PET to form flexible VO₂(M) films. These films achieved high optical performance, with a ΔT_sol of 22.3% and a T_lum of 45.6%. Similarly, Zhou et al. reported the roll-coating of Mg-doped VO₂(M) NPs that were hydrothermally synthesized using V₂O₅ and H₂C₂O₄ [140]. Mg-doped VO₂(M) composite foils were prepared by mixing NPs with PU solutions and were deposited on a PET substrate using a roll-coater. The flexible composite foils exhibited a high T_lum and ΔT_sol of 54.2% and 10.6%, respectively. Liang et al. reported the bar-coating of W-doped VO₂(M) nanorods; the nanorods were prepared via one-step hydrothermal synthesis for 48 h at 240 °C using V₂O₅, C₄H₆O₆, and ammonium tungstate (Figure 12a,b) [49]. The nanorods were then mixed with the tetraethyl orthosilicate and poly(ethyl methacrylate) solution. The solution mixture was cast on PET substrates using a stainless-steel coating bar to fabricate large-area, flexible VO₂(M) films (Figure 12c). The T_c of the flexible VO₂(M) films could be systematically modulated by approximately 24.52 °C for 1 at% of W doping, and the mid-infrared transmission could be modulated by 31% at a T_c of 37.3 °C. Inkjet printing has also been widely utilized as a useful direct-write technology to fabricate high-resolution, low-cost, large-area, and uniform-surface films on flexible substrates [141,142]. Haining et al. reported the fabrication of VO₂(M) smart windows via inkjet printing using hydrothermally synthesized VO₂(M) NPs [143,144]. Large-area VO₂(M) films were fabricated on polyethylene substrates with a T_lum of 56.96% and a ΔT_sol of 5.21%.

The chemical instability of VO₂(M) NPs can potentially limit their long-term usage as smart windows in real-world environments [145]. To enhance the chemical stability of VO₂(M) NPs, core–shell structures, in which VO₂(M) NPs are overcoated with chemically inert shells, have been developed. Gao et al. reported a core–shell structure with VO₂@SiO₂ NPs. VO₂(M) was synthesized through a hydrothermal reaction, and SiO₂ shells were overcoated using the Stöber method [56]. SiO₂ is chemically inert and optically transparent, which is ideal for protecting VO₂(M) NPs. VO₂@SiO₂ NPs exhibit improved chemical resistance to oxidation. The SiO₂ shell of VO₂ NPs serves as an oxygen diffusion barrier layer, which can prevent the VO₂ from changing to V₂O₅. This phenomenon was confirmed through experiments conducted with VO₂ NPs and VO₂@SiO₂ NPs after annealing in an air atmosphere for 2 h at 300 °C. Flexible films were fabricated by embedding VO₂@SiO₂ NPs into a PU matrix; then, the VO₂@SiO₂ NPs/PU cast on a PET matrix were dispersed to fabricate flexible VO₂@SiO₂/PU composite films. These films exhibited a high T_lum (55.3%) and ΔT_sol (7.5%). In addition to SiO₂, various types of oxides, such as ZnS [146], TiO₂ [11], and ZrO₂ [147], have been utilized for overcoating to prepare core–shell NPs. Saini et al. demonstrated an approach to improve the thermal stability and thermochromic properties of VO₂(M) NPs by overcoating with CeO₂ [148]. VO₂(M)@CeO₂ NPs were observed to be thermally stable for up to 320 °C in air, which confirmed the enhancement in stability after overcoating.

3. Perspectives

Flexible VO₂(M) films offer significant potential for the integration of energy-saving smart windows in existing buildings, as well as for application in novel flexile devices, such as sensors and actuators. Various methods for fabricating flexible thermochromic thin films based on the vacuum deposition and solution-based process have been reported; these methods are potentially suitable for commercialization. However, certain issues still remain to be resolved before VO₂-based smart windows can be utilized in practice. For example, flexible VO₂ films fabricated using vacuum deposition and film-transfer techniques show high T_lum and ∆T_sol values; however, these methods are still limited in terms of large-area and mass-production capabilities. In addition, deposition methods with uniform doping should be developed further to systematically reduce T_c while maintaining favorable phase-change optical properties. Conversely, the annealing-free, solution-based process offers advantages such as convenient, low-cost, large-area deposition of phase-change VO₂(M) on flexible substrates. Particularly, hydrothermal synthesis yields highly crystalline VO₂(M) NPs with colloidal stability and moderately useful phase-change behaviors. However, it is still challenging to prepare flexible VO₂(M) films with high T_lum and ∆T_sol values as well as a reduced T_c. The optical properties of the representative flexible VO₂ films fabricated using deposition and solution-based processes are summarized in Figure 13, which displays the opportunities for utilizing flexible VO₂(M) films in energy-saving smart windows. Therefore, large-scale, high-throughput, mass-production capabilities for the fabrication and commercialization of high-performance VO₂(M) films should be realized. Finally, certain limitations in terms of the intrinsic properties of VO₂(M) should be overcome to utilize flexible VO₂ films. First, phase-change VO₂ films show an inherent brown color, which is not desirable for window applications. Therefore, it is highly recommended to develop fabrication methods that can enable control of the apparent colors of VO₂(M) while ensuring a high T_lum and ∆T_sol and low T_c. Moreover, vanadium oxide has various stable phases and a stable stoichiometry; consequently, VO₂(M) films are easily oxidized into other phases under exposure in ambient conditions. Therefore, processes to prevent VO₂(M) from being oxidized, for example, overcoating of VO₂(M) films or using NPs with protective layers, should be developed to enable long-term usage of the films.