Solution-Processed VO₂-SiO₂ Composite Films with Simultaneously Enhanced Luminous Transmittance, Solar Modulation Ability and Anti-Oxidation property

Abstract

Recently, researchers spare no efforts to fabricate desirable vanadium dioxide (VO₂) film which provides simultaneously high luminous transmittance and outstanding solar modulation ability, yet progress towards the optimization of one aspect always comes at the expense of the other. Our research devotes to finding a reproducible economic solution-processed strategy for fabricating VO₂-SiO₂ composite films, with the aim of boosting the performance of both aspects. Compare to VO₂ film, an improvement of 18.9% (from 29.6% to 48.5%) in the luminous transmittance as well as an increase of 6.0% (from 9.7% to 15.7%) in solar modulation efficiency is achieved when the molar ratio of Si/V attains 0.8. Based on the effective medium theory, we simulate the optical spectra of the composite films and the best thermochromic property is obtained when the filling factor attains 0.5, which is consistent with the experimental results. Meanwhile, the improvement of chemical stability for the composite film against oxidation has been confirmed. Tungsten is introduced to reduce the phase transition temperature to the ambient temperature, while maintain the thermochromism required for application as smart window. Our research set forth a new avenue in promoting practical applications of VO₂-based thermochromic fenestration.

Introduction

With the aim of reducing energy consumption in architectures, energy-efficient windows, especially “smart windows” based on chromogenic materials are considered to be good candidates for energy conservation and have attracted considerable concerns¹. Vanadium dioxide (VO₂), a representative thermochromic material, can initiate an automatic reversible semiconductor-metal phase transition (SMT) from the monoclinic structure (P2₁/c, M1 phase: the most stable monoclinic phase in room temperature) to the tetragonal structure (P4₂/mnm, R phase) at ca. 68°C, giving rise to a dramatic modification of the optical properties from infrared (IR) transmitting to IR reflecting and maintains the visible transparency, which is suitable for the application of energy-efficient windows^2,3,4. To meet the requirement for large-scale implementation on architectural buildings, VO₂-based materials should satisfy the following criteria: the phase transition temperature (T_t) closes to the ambient temperature (≈25°C), the integrated luminous transparency (T_lum) ≥ 40% as well as the solar modulation ability (ΔT_sol) ≥ 10%⁵.

Generally, doping metal ions with high valence (such as W⁶⁺, Mo⁶⁺, Nb⁵⁺) is confirmed to be an effective way to reduce T_t to room temperature^6,7,8. Among them, tungsten is proved to be the most effective dopant⁹. However, T_lum is inevitably limited to <40% if a relative high ΔT_sol is maintained, which cannot balance the requirements for T_lum and ΔT_sol¹⁰. Therefore, a tougher challenge is to simultaneously and substantially improve both of T_lum and ΔT_solvia an economic method. It is believed that the low visible transparency of VO₂ film originates from the strong inner-band and inter-band absorptions in the short-wavelength range^11,12. Experiments and calculations have demonstrated that some special dopants (such as F⁻ and Mg²⁺) can serve to widen the band gap of VO₂ and induce a blue-shift of the absorption edge, leading to an increase of the visible transparency^13,14. However, the improvement of T_lum is usually accompanied by a deteriorated switching behavior. Deposition of single/multi-layer antireflection (AR) layers with high reflective index dielectric materials (i.e., SiO₂, ZrO₂, TiO₂) is a traditional strategy to enhance T_lum^15,16. Nonetheless, it requires additional heating process and the presence of AR layer usually jeopardizes the near infrared (NIR) switching ability. Moreover, film thickness and microstructures markedly affect the optical properties and switching behaviors of VO₂-based films. The transmittance spectra for VO₂ films with various thicknesses illustrate that the improvement of T_lum is inevitably at the expense of ΔT_sol¹⁷. Zhou et al. have proposed a novel periodic porous structure with an unprecedented high visible light transmittance (81%). However, the switching efficiency is only ca. 23% at 2000 nm¹⁸. To our knowledge, the optical properties of VO₂ films obtained by the above approaches rarely can increase both T_lum and ΔT_sol, which otherwise tends to be undesirably low and constitutes a major obstacle for practical implementation of thermochromic fenestration.

A thrilling concept that nanocomposites materials comprising VO₂ nanoparticles and highly transparent materials is providing an intriguing interest due to the unique properties. Optical calculations have confirmed that VO₂ nanoparticles distributed in a dielectric matrix exhibit higher T_lum and better ΔT_sol than pure VO₂ films, which offers a novel perspective to achieve a favorable optical performance¹⁹. Inspired by this concept, some pioneering researches have devoted to fabricate VO₂-based composite particles and/or to cast them on glass or flexible substrate to obtain films with excellent thermochromic properties. Li et al. fabricated core-shell VO₂@TiO₂ composite nanopowders with both thermochromic and photocatalytic properties²⁰. Gao et al. prepared flexible VO₂ nanocomposite foils based on SiO₂/VO₂ core/shell structures with an improved T_lum from 29.2% to 55.3%, while ΔT_sol exhibited a decrease from 13.6% to 7.5%²¹. Core-shell VO₂(M)@SiO₂ films on a transparent Teflon film were obtained by casting the hydrothermal synthesized composite nanoparticles to gain an increased visible transmittance from 31.72% to 35.96%, accompanied by a significant increase in ΔT_sol of 8.4%²². VO₂ nanocrystals dispersed in SiO₂ matrix are highlighted to be a new pathway to exhibit superior visible transmittance without compromising the NIR modulation ability as compared to conventional VO₂ thin film²³, offering a multitude of possibility for energy conservation.

To our knowledge, the V-O system is a complex binary system which contains a variety of compounds including V₂O₃, VO₂, V₂O₅ and so on. Moreover, the various polymorphs of VO₂, including VO₂ (R), VO₂ M1, VO₂ (B) and VO₂ (A), introduce a series of difficulties to fabricate pure VO₂ films with thermochromic properties²⁴. As a result, an accurate control of annealing atmosphere is vital to the formation of pure VO₂ (M1). The majority of the fabrications of pure VO₂ (M1) usually demand on high-vacuum equipment and it inevitably increases the fabrication cost and raises the complexity of the operation, which is unsuitable for the large-scale fabrication of VO₂-based intelligent window. Additionally, as we can see, previous studies on VO₂-based composite structure mainly focuses on a prior preparation of pure VO₂ (M1) particles followed by the formation of a composite structure due to the difficulties of direct fabrication of VO₂-based composite structure, which are closely related to the hard-controllable annealing atmosphere. Due to the above reasons, it is really significant and challengeable to develop an available pathway for directly fabricating VO₂-based composite films without the complex vacuum process, thus offering a unique opportunity to the practicable application of thermochromic coatings as “smart windows”.

Differ from the composite structure usually obtained via a two-step process, here we present a direct synthesis of VO₂-SiO₂ composite films on glass substrate through an economic solution-based method and a relationship is drawn between the film products and the annealing conditions. With no need of a laborious vacuum processing, pure VO₂-SiO₂ composite films (M1 phase) with different molar ratios of Si/V can be easily obtained by simply varying the gas flow of nitrogen. The influence of SiO₂ on the structure, morphology as well as optical properties are detailed evaluated. We simulate the optical spectra of the composite films based on the effective medium theory and obtain a relationship between the volume filling factor (f) and ΔT_sol. Next, we focus on the thermochromic properties of VO₂-SiO₂ films in the function of SiO₂ contents. Our results provide a series of well-formed composite films with simultaneously enhanced T_lum combined with ΔT_sol. The obtained results are in good agreement with our calculations. The anti-oxidation property of the composite film has also been investigated. Tungsten is introduced to reduce T_t of the composite film to the vicinity of room temperature, while maintain an excellent thermochromic property. Our work hence shortens the distance of VO₂-based thermochromic films towards smart window coatings for improved building energy conservation.

Results

Optimization of annealing parameters

To obtain the pure M phase (M1) of VO₂ thin films, the appropriate annealing parameters are investigated. Fig. 1 shows the XRD patterns of films prepared at various annealing temperatures and times. As shown in Fig. 1 (a), for film annealed at 400°C, it exhibits apparent diffraction peaks which can be indexed to the pure monoclinic structure. The intensities of the diffraction peaks significantly increase as enhancing the annealing temperature to 450°C, indicating an improved crystallinity of the film. Some additional diffraction peaks which can be attributed to NaV₆O₁₅ appear when the annealing temperature further increases to 500°C. The emergence of NaV₆O₁₅ could be particularly originated from the diffusion of Na⁺ from glass substrates as increasing the annealing temperature²⁵. Fig. 1 (b) shows the XRD patterns of films annealed at 450°C for different annealing times. The nitrogen atmosphere, which contains ca. 0.01% O₂, greatly affects the oxidation state of the films. The main products with different annealing times are V₅O₉, VO₂ and V₆O₁₃, respectively. Additionally, NaV₆O₁₅ appears when the annealing time is long. Based on the above discussion, the optimized annealing temperature and time to obtain pure M1 phase VO₂ thin film are 450°C and 1 hour, respectively.

Figure 1

XRD patterns of films prepared at different annealing conditions.

(a) The annealing time is kept at 1 hour and the annealing temperature varies from 400°C to 500°C; (b) The annealing temperature is kept at 450°C and the annealing time varies from 0 hour to 2 hours.

In the above discussion for the fabrication of pristine VO₂ film, we choose the nitrogen gas as the annealing atmosphere with a gas flow of 0.200 L·min⁻¹. However, this annealing parameter suitable for pristine VO₂ film is inapplicable for VO₂-SiO₂ composite films. For example, when Si/V = 1.0, the composite film annealed under this gas flow is characterized as an amorphous carbon film with gray color. The formation of this amorphous film can be possibly attributed to the carbon residues in the films due to the incomplete degradation of the organic matter originating from the addition of SiO₂ sols. The production of carbon residues possibly results in a reduction effect on the resulting amorphous films. To consume the carbon residues, a certain amount of oxygen is required to fabricate a composite film with pure M1 phase. As we have referred, the annealing treatment is operated without a vacuum supply process. Additionally, the nitrogen gas also contains a certain amount of O₂. Consequently, in the following experimental, we can adjust the content of oxygen acted on the residual carbon substance by simply varying the gas flow of the nitrogen and the crystallization to VO₂-SiO₂ composite film (Si/V molar ratio = 1.0) occurs when the gas flow is 0.525 L·min⁻¹. As such, we discuss the appropriate atmosphere for other composite films and the detailed gas flow of nitrogen for each film is listed in Table 1.

Table 1 The molar ratios of Si/V in the films prepared on aluminium foils and the thicknesses of the composite films on glass substrates with different Si/V molar ratios as well as the calculated filling factor according to the molar ratios of Si/V based on the EDX results. The area selected for investigation of EDX spectra is 60 × 60 μm²

Film structure

Fig. 2 depicts the typical XRD patterns of VO₂-SiO₂ composite films with different Si/V molar ratios, which can be used to further evaluate the evolution of the phase composition, grain size and crystallinity of the films. It is revealed that all films exhibit the typical diffraction peaks of M1 phase VO₂ (JCPDS card no. 72-0514, P2₁/c, a = 0.5743 nm, b = 0.4517 nm, c = 0.5375 nm and β = 122.61°). Since the annealing temperature is far below the crystallization point of SiO₂, it is most likely that the SiO₂ in the composite films is amorphous, which cannot be detected by XRD. Additionally, no evidence can be seen for other crystalline phases, such as V₂O₅, V₂O₃ or VSi_x, indicating the purity of the composite films. None of the diffraction peaks exhibit significant migration with increasing the molar ratio of Si/V. However, a broadening and weakening tendency of the diffraction peak of (011) crystalline plane is observed, which is largely due to the grain refinement as increasing the molar ratio of Si/V.

Figure 2

XRD patterns of VO₂-SiO₂ composite films upon increasing the Si/V molar ratios in the deposition solution.

The annealing temperature and time were kept at 450°C and 1 hour, respectively.

Film morphology

The morphology of the prepared films is of paramount importance to obtain the desired optical properties and the changes in the particles size and morphology of the films induced by the addition of SiO₂ can be observed via SEM images. Fig. 3 exhibits the morphology evolution of the composite films upon enhancing the Si/V molar ratio. For pristine VO₂ film, the film is composed with uniform and well-formed grains. Clear grain boundaries appear between the particles. Upon the addition of SiO₂, the particle sizes are apparently depressed in Fig. 3 (b), (c) and (d), which is consistent with the earlier report²². The size reduction can also be confirmed in the cross-sectional SEM observation of the VO₂ film and VO₂-SiO₂ composite film (Si/V = 0.8), as illustrated in Fig. 3 (e) and (f). Compared to the pristine VO₂, the VO₂-SiO₂ composite film shows relative homogenous and narrow particle size distributions, indicating that the SiO₂ in the composite film play a crucial role to reduce the degree of growth and agglomeration of VO₂ particles. According to these observations, it may be concluded that SiO₂ particles are well formed and a composite structure is obtained in the film. With the aim to determine the effective molar ratios of Si/V in the composite films, we fabricate the films on the aluminum foils (to exclude the influence of the Si in the glass substrate) under the same procedure discussed above and the followed EDX analysis results are demonstrated in Table 1. The atomic percent ratios of Si/V are respectively ca. 0, 0.23, 0.45, 0.62, 0.83, 1.12, which are in close proximity to the initial ratios in the deposition solutions.

Figure 3

The surface morphology of films with various Si/V molar ratios.

Si/V = (a) 0; (b) 0.4; (c) 0.6; (d) 0.8. The cross sectional of pristine VO₂ film (e) and VO₂-SiO₂ composite film with Si/V = 0.8 (f).

To further detect the microstructure of the composite film, high-resolution transmission electron microscopy (HRTEM) images and corresponding selected area electron diffraction (SAED) patterns are collected for the VO₂-SiO₂ composite film (Si/V molar ratio = 0.8). As shown in Fig. 4 (a), the film is composed of well-fined particles, which is in agreement with the SEM images observed previously. The crystallized VO₂ and the amorphous SiO₂ particles are distributed as separate phases as shown in Fig. 4 (b). A more detailed microstructural analysis further confirms that the sample is crystallized into monoclinic VO₂. As is found in the SAED patterns (Fig. 4 (d)), bright diffraction rings evident the polycrystallinity nature of the sample, which is consistent with the XRD results. In Fig. 4 (c), clear and ordered lattice fringes are observed in the HRTEM images, corresponding to the (011) crystalline plane. The distributions of VO₂ and SiO₂ are examined by scanning transmission electron microscopy (STEM) and elemental mapping measurements. The annular dark-field (ADF) image of VO₂-SiO₂ (Fig. 4 (e), (f)) and the elemental maps respectively corresponding to V, Si and O (Fig. 4 (g), (h), (i)) further illustrate the inhomogeneous distribution of VO₂ and SiO₂ nanoparticles in the composite film. The particle sizes of VO₂ range from 30 to 100 nm, which is in good agreement with the SEM images.

Figure 4

The TEM image (a), HRTEM (b, c), SAED patterns (d), ADF (e, f) images of the VO₂-SiO₂ composite film (Si/V = 0.8) and the elemental maps respectively corresponding to V, Si and O ((g), (h), (i)).

Optical properties

Fig. 5 depicts the variation of visible transmittance of VO₂-SiO₂ composite films in response to the variation of Si/V molar ratios. It is clearly that the maximum visible transparency greatly enhances from 41.9% to 64.7% at 630 nm as increasing the proportion of SiO₂ in the film. Compare the insert images which represent the photographs of VO₂ and VO₂-SiO₂ films (Si/V molar ratio = 0.8), a significant enhancement of the visible transparency is observed. Lopez et al. have previously revealed that the scattering effect of VO₂ gradually decreases with the reduction of the particle size, indicating that the transmittance would reasonably increase²⁶. The addition of SiO₂ restricts the growth and aggregation of VO₂ particles and further reduces the initial reflectance of particles and thus inherits a better high luminous optical property, which is in consistent with the previous report²³. In addition, the visible transmittance is enhanced possibly through an AR effect²⁷. Upon the addition of SiO₂ in the composite films, SiO₂ stack on the surface of VO₂ particles and serve as the AR layer, which can effectively affect the reflected light from the air-coating and coating-substrate interfaces, leading to a higher transmittance over the visible region spectrum²⁸. Wavelength dependent refractive indices (n) and extinction coefficient (k) of VO₂-SiO₂ composite films with various Si/V molar ratios are plotted in Fig. 6. Expectedly, both the n and k of the composite thin films exhibit a significant depression in whole measured wavelength upon the increase of SiO₂ addition. Fig. 7 illustrates the reflectance spectra of VO₂-SiO₂ composite films of the semiconductor state. Upon the addition of SiO₂ in the composite film, the reflectance shows a significant depression in the whole wavelength. According to the Fresnel's equation²⁹, the reduced n (2.58 for VO₂ film and 2.30 for VO₂-SiO₂ film with Si/V = 1.0) contributes to a decreased reflectance of the film.

Figure 5

The luminous transmittance of films with different Si/V molar ratios ranges from 380–780 nm measured at 30°C.

The insert images correspond respectively to photographs of the pristine VO₂ film (below) and VO₂-SiO₂ composite film (Si/V = 0.8) (upper).

Figure 6

The wavelength dependent refractive index (a) and extinction coefficient (b) of films with different Si/V molar ratios.

All of the optical constants are measured at 30°C.

Figure 7

The reflectance spectra measured at 30°C of VO₂-SiO₂ composite films with different Si/V molar ratios range from 250 to 2500 nm.

The spectral transmittance of VO₂-SiO₂ composite films of the semiconductor state ranges from 250 nm to 2500 nm are characterized as shown in Fig. 8. VO₂ thin film exhibits salient switching of the NIR transmittance, indicating a good thermochromic property. Especially, the composite films show better optical performance with a larger transmittance change in 2000 nm (ΔT_200nm) upon the increase in the addition of SiO₂, as shown in Table 2. The phenomenon is consistent with the previously studies that not only the luminous transmittance but also the transmittance change upon the phase transition greatly increase with the decrease of VO₂ particle size³⁰. To further quantify the potential application of the composite films as coatings on smart windows, we calculate the integral luminous transmittance (T_lum, 380–780 nm) and solar transmittance (T_sol, 250–2500 nm) of all the composite films based on the measured spectra using the following expression:

where T(λ)denotes the transmittance at wavelength λ, φ_lum is the standard luminous efficiency function for the photopic vision of human eyes and φ_sol is the solar spectral irradiance for the air mass 1.5 (The receiving surface is defined as an inclined plane at 37° tilt toward the equator)³¹. The luminous transmittance modulation (ΔT_lum) and solar energy modulation (ΔT_sol) are defined as:

T_lum,s, T_lum,m, T_sol,s and T_sol,m denote as the luminous transmittance and solar transmittance at the semiconducting state and the metallic state, respectively. The obtained results are elaborated in Table 2. It is clear that the ΔT_sol greatly increases from 9.7% for VO₂ thin film to a maximum of 15.7% when Si/V molar ratio attains 0.8, accompanied by a huge increment of T_lum,s from 29.6% to 48.5%. The obtained values are the best among the reported values of T_lum and ΔT_sol for a typical single-layered VO₂ film (41.0%, 6.7%)²⁷, TiO₂/VO₂ double-layered films (46.0%, 7.0%)³², TiO₂/VO₂/TiO₂/VO₂/TiO₂ five-layered films (44%, 12.1%)³³, VO₂-Sb:SnO₂ composite films (49.8%, 11.7%)³⁴ and core-shell VO₂(M)@SiO₂ casted foils (35.9%, 8.4%)²².

Table 2 Optical parameters during the phase transition of VO₂-SiO₂ films with different Si/V molar ratios

Figure 8

Transmittance spectra of VO₂-SiO₂ composite films upon increasing Si/V molar ratios in the wavelength ranges from 250 nm to 2500 nm.

Solid line: 30°C; Dash line: 100°C. The gray area indicates the normalized values of the solar spectral irradiance (φ_sol).

Phase transition temperature

The switching characteristics of the composite films are investigated by recording the temperature-dependent optical transmittance during the heating and cooling process at 2000 nm as illustrated in Fig. 9. Detailed parameters of the hysteresis loops are obtained as follows. Briefly, a plot of d(Tr)/d(T) &T is obtained from each hysteresis loop, yielding two peaks with a well-defined maximum. Tr and T denote as the transmittance and temperature, respectively. Each d(Tr)/d(T) &T curve has been fitted with a Gaussian function using the Origin pro 8.0 software, as shown in Fig. 10. The temperature corresponding to the maximum of each d(Tr)/d(T)&T curve is defined as the phase transition temperature of heating and cooling branch, denote as T_h and T_c, respectively. The phase transition temperature (T_t) of the film is defined as:

The hysteresis loop width (ΔT_t) is defined as the difference between T_h and T_c. The sharpness of the phase transition for heating or cooling run is characterized by the full width at half maximum (FWHM) of the d(Tr)/d(T) &T curve, smaller value means sharper change of the slope. The detailed parameters are summarized in Table 3.

Table 3 The detail parameters derived from the hysteresis loops at 2000 nm of VO₂-SiO₂ composite films with different Si/V molar ratios

Figure 9

(a) The hysteresis loops at 2000 nm of VO₂-SiO₂ composite films with different Si/V molar ratios. (b) The relationship between the transition temperature and the Si/V molar ratio during the heating and cooling processes.

Figure 10

The d(Tr)/d(T)&T plots of VO₂-SiO₂ composite films with different Si/V molar ratios to describe the transition parameters of heating and cooling branches.

The molar ratio of Si/V = (a) 0; (b) 0.2; (c) 0.4; (d) 0.6; (e) 0.8; (f) 1.0. Tr and T represent transmittance and temperature at 2000 nm. T_h and T_c express the phase transition temperature of heating and cooling branches, respectively.

As we can see, a large hysteresis of the pristine VO₂ thin film, of around 9.9°C, is directly related to the nanoparticle microstructure³¹. Broader hysteresis loops are obtained for samples with the addition of SiO₂. It is striking to notice that by continuously increasing the Si/V molar ratios, T_c of the film shows no obvious variation, while T_h exhibits a progressive increase. Meanwhile, the transition sharpness becomes smoother (expressed as the apparent enhancement of FWHM) upon enhancing the Si/V molar ratio. We ascribe this phenomenon to the surrounding SiO₂, which helps to absorb the locally induced stresses among VO₂ lattices during the phase transition²³.

Anti-oxidation properties

The poor long-term oxidation resistance is another obstacle to the application of smart windows due to the thermodynamically instability of VO₂. It has been previously reported that VO₂ is gradually oxidized into toxic V₂O₅ when it is exposed to the air for a long period or abused at high temperature above 300°C^35,36,37, which leads to a considerable deterioration of the thermochromic properties. In this case, improving the chemical stability of VO₂ against oxidation is urgently required. The anti-oxidation properties of the film after the addition of inert SiO₂ are investigated compared with the pristine VO₂ thin film. As illustrated in Fig. 11, after expositing the films in air at room temperature for 6 months, the pristine VO₂ film cannot maintain the original pure monoclinic phase with the (011) peak weakens while some additional peaks assigned to V₂O₅ appear. For the composite film (Si/V molar ratio = 0.8), the XRD pattern shows no obvious change and all peaks can be assigned to the monoclinic VO₂. The results firmly confirmed that the addition of SiO₂ in the composite structure can effectively protect VO₂ from being oxidized, offering a similar anti-oxidation protection comparable to the anti-reflection layer²⁷.

Figure 11

The XRD patterns of pristine VO₂ film and VO₂-SiO₂ composite film (Si/V molar ratio = 0.8) after depositing in air for 6 months.

(a) VO₂ film; (b) VO₂-SiO₂ composite film (Si/V molar ratio = 0.8).

Tungsten doped VO₂-SiO₂ composite film

For our VO₂-SiO₂ composite film, the simultaneously enhanced luminous transparency and the solar energy modulation have met the requirements (T_lum ≥ 40%, ΔT_sol ≥ 10%). Although, the T_t of the composite film increase upon the addition of SiO₂, it can be readily reduced to the ambient temperature by replacing some vanadium atoms with tungsten. Fig. 12 shows the XRD pattern, transmittance spectra as well as the hysteresis loop of W-doped VO₂-SiO₂ composite film (molar ratio: W/V = 0.015; Si/V = 0.8). The composite film exhibits pure monoclinic structure as well as significant transmittance change in NIR spectrum across the phase transition. The phase transition temperature is successfully reduced to 34.2°C and the hysteresis loop width is also narrowed down by 3.8°C. The NIR switching efficiency is diminished compared with undoped film due to the incorporation of W dopant. Nonetheless, T_lum and ΔT_sol of the W-doped composite film are calculated to be respectively 43.3% and 11.3%, satisfying the requirements for practical thermochromic fenestration.

Figure 12

JCPDS card No. 72-0514 (a), transmittance spectra at 25°C and 100°C (b), hysteresis loop at 2000 nm (c) as well as corresponding d(Tr)/d(T) &T plot of W-doped VO₂-SiO₂ composite film (molar ratio: W/V = 0.015; Si/V = 0.8).

Tr and T represent transmittance and temperature at 2000 nm, respectively.

Discussion

In order to give a deeper analysis for the change of optical properties upon the addition of SiO₂, we simulate the optical spectra of the composite films as shown in Fig. 13. Clear evidence for thermochromism is seen in reflectance (R) and transmittance (T) regardless of the volume filling factor (f). The R (T) of the semiconductor state exhibits a significant decrease (increase) with enhancing the f and vice versa. T_lum, T_sol, ΔT_sol as well as the ΔT_lum of the films corresponding to each f are calculated and plotted as shown in Fig. 14. Both T_lum and T_sol demonstrate a progressive increase when f is enhanced. Meantime, not only the T_sol but also the T_lum of the semiconductor state is higher than that of the metallic state, indicating that the thermochromism is not confined to the NIR region but takes place in the whole wavelength. Moreover, it can be referred from Fig. 14 (c) that ΔT_sol exhibits a broad maximum of 16.07% when f = 0.5 and is compatible with T_lum,s = 57.85% and T_lum,m = 51.22%. The calculation for VO₂ particles dispersed in In₂O₃: Sn performed by Li et al. exhibits a same tendency and ΔT_sol displays a broad maximum of ca. 12.8% for f = 0.35, which is close to the value where the VO₂ units start to percolate in the Bruggeman model³⁸.

Figure 13

The computed reflectance (a) and transmittance (b) spectra of VO₂-SiO₂ composite film with different SiO₂ volume fractions.

The volume fraction of SiO₂ is denoted as f. Solid line: semiconducting state; Dash line: metallic state.

Figure 14

Wavelength-integrated luminous (a), solar (b) transmittance and thermochromic modulation of both the luminous transmittance and solar energy (c) based on the computed transmittance spectra as a function of f for VO₂-SiO₂ composite film.

T_lum, T_sol, ΔT_lum and ΔT_sol refer to the integral luminous transmittance, solar transmittance, luminous transmittance modulation and solar modulation ability, respectively.

As we have noted, the ΔT_sol greatly increases from 9.7% for VO₂ thin film to a maximum of 15.7% when Si/V molar ratio attains 0.8 (corresponds to f = 0.56), accompanied by a huge increment of T_lum,s from 29.6% to 48.5%. The experimental results are in good agreement with the calculations. The synergistic performances suggest that high T_lum and excellent ΔT_sol can be realized simultaneously via optimizing the f of SiO₂ in the composite film, which paves a new way for the application of VO₂-based smart windows.

Our calculations mentioned above have confirmed that the thermochromism of VO₂ occurs in the entire solar range. As a result, ΔT_sol can be described as:

ΔT_lum and ΔT_nir denote as the integral transmittance modulation of the visible and NIR spectra, respectively. It is worthy noted that solar energy between 250–2500 nm constitutes the majorities of the total heat energy in the whole solar irradiance spectra, among which, the heat energy in the visible light and NIR light accounts for ca. 43% and 48%, respectively³⁹. Expectedly, not only the transmittance change in the NIR region but also the changes in visible light region of VO₂ thin film to undergo the phase transition play an important role in determining the ΔT_sol. Previous researches have demonstrated that the change in the luminous transmittance (ΔT_lum, the difference between T_lum,s and T_lum,m) of the film is thickness-dependent⁴⁰. For film with less than 50 nm thick, T_lum,s of the film is generally lower than T_lum,m due to the interference effect, and, vice versa¹⁷. In our experiment, the thicknesses of obtained films are determined to be approximately 102, 109, 117, 128, 136 and 144 nm respectively by the step profiler (listed in Table 1), increases with the enhancement of the molar ratio of Si/V (Higher viscosity of the precursor solution is obtained when increasing the proportion of SiO₂ sols, contributing to a slight increase in film thickness due to the same spin-coating parameters.). The ΔT_lum value of the composite films increases as enhancing the molar ratio of Si/V (from 2.8% for Si/V = 0 to 5.3% for Si/V = 0.8), which is in line with the calculated results. Of note, with enhancing the molar ratio of Si/V in the film, the value of ΔT_{2000 nm} for the composite film exhibits an obvious improvement compared to the VO₂ film, which, to a large extent, determines the ΔT_nir, giving rise to the increase of ΔT_sol. The improvement of ΔT_lum, together with the enhancement of ΔT_nir, jointly contribute to the larger ΔT_sol for films with larger Si/V molar ratio.

For our VO₂-SiO₂ composite film, T_h shows a progressive increase by continuously increasing the Si/V molar ratios, while leaving T_c unchanged. A similar phenomenon has also been observed in VO₂/Pt multi-layers due to the replacement of VO₂/air interfaces with VO₂/Pt interfaces by Pt deposition, which confirms that the VO₂ interact with the surrounding media play a role in modifying the SMT path^41,42. To our knowledge, the occurrence of the two transitions is dominated by totally different defects. For the transition from semiconductor phase to metal phase, the vast majority of the newly formed metal-phase VO₂ nuclei are located extremely close to grain boundaries due to the surface-localized defects. While for the reversal transition, it is dominated by bulk defects in VO₂ particles⁴³. The shift of T_h towards higher temperatures indicates that the SMT is retarded by nucleation of the metal phase after the change of surrounding medium. The diminished availability of defects which can serve as nucleation sites lowers the possibility to trigger the phase transition, contributing to the shift of T_h towards higher temperatures⁴⁴. Furthermore, the composite films are annealed in a relative oxygen-rich atmosphere, the excess oxygen can act as an acceptor and stabilize the semiconductor state, leading to a higher T_h⁴⁵. However, the influences of these defects on the reversal transition are much less effectively⁴⁶. As such, the broadening of the hysteresis loops can also be ascribed to it⁴⁷. The shift of T_h towards higher temperature gives a clue that the chemical stabilization of the semiconductor state is enhanced.

Our research provides a simple and economic solution-based method to fabricate VO₂-SiO₂ composite films with the aim to simultaneously improve the luminous transmittance and the solar energy modulation. Differ from the common method to fabricate pure VO₂ (M1), we provide an easy-controllable method by simply varying the gas flow of nitrogen and have no requirement of high-vacuum equipment. All of the obtained composite films exhibit pure monoclinic phase structure with excellent crystallinity. According to the TEM results, VO₂ and SiO₂ are distributed as separate phases and the addition of SiO₂ in the films plays an important role in hindering the growth and aggregation of VO₂.

Based on the effective medium theory, we calculate the optical constants and simulate the corresponding optical spectra of the composite films of various filling factors. The calculations illustrate that the composite film exhibits the best thermochromic property when the filling factor attains 0.5, which is consistent with the experimental result. With the exception of the high luminous transmittance and the superior modulation of solar modulation (T_lum = 48.5%, ΔT_sol = 15.7%), the anti-oxidation property of the composite film is discussed. VO₂-SiO₂ composite film maintains the original monoclinic structure after depositing in air for six months. Tungsten is introduced to effectively reduce the phase transition temperature to the ambient temperature, while retains the thermochromism required for application as smart window. Our VO₂-SiO₂ composite films enable effective tailoring of the optical performance, which open a new horizon of multifunctional coating as the architectural fenestration.

Methods

Materials

All the chemicals of analytical grade were commercially available and used without further purification. Vanadium pentoxide (V₂O₅) was chosen as the source of vanadium and oxalic acid (H₂C₂O₄·2H₂O) was selected as the reducing agent. Tetraethyloxysilane (TEOS) and hydrochloric acid (HCl, 37%) were used as the silica precursor and hydrolyzing catalyst, respectively. Pluronic F127 (EO₁₀₀-PO₆₅-EO₁₀₀, M_w = 12,600 g/mol) was added as a pore-forming agent. Dehydrates ethanol (EtOH, 95%) was used as solvent in both systems. Ammonium tungstate hydrate ((NH₄)₅H₅[H₂(WO₄)₆]·H₂O) was chosen as dopant to lower the phase transition temperature.

Synthesis of VO₂-SiO₂ composite thin films

The preparation process of the VO₂-SiO₂ composite films is shown schematically in Fig. 15. In a typical synthesis, certain amount of V₂O₅ was reduced by H₂C₂O₄ (molar ratio is 1:3) in EtOH solution. After refluxing at 120°C for 10 hours (hrs), a transparent blue solution of VOC₂O₄·xH₂O (1.0 M) was obtained. For preparing a VO₂-SiO₂ composite solution, a SiO₂ sol was prepared by mixing TEOS, EtOH and H₂O (The pH value was adjusted to 2 with HCl) at a given volume ratio as = 1:10:0.2. F127 was added as a pore-forming agent to increase the porosity of the film. The solution was stirred for 1 hour followed by aging for at least one day before merging with the VOC₂O₄·xH₂O solution. After blending thoroughly for 1 hour, a homogeneous, clear and blue solution was obtained. The final concentration of VOC₂O₄·xH₂O was diluted to a constant of 0.5 M by EtOH and the nominal Si/V atomic ratio varied from 0 to 1.0. For tungsten doped sample, the amount of the additive was carefully controlled for the realization of a fixed W doping level up to 1.5 at.%. The precursor solutions were aged for 2 hrs at room temperature before spin coating.

Figure 15

(a) The phase diagrams of V-O system⁵²; (b) The flow chart of the preparation procedure of the VO₂-SiO₂ composite films; (c) A relationship drawn between the annealing products and the gas flow of N₂; (d) The TEM image of the VO₂-SiO₂ composite film (Si/V molar ratio = 0.8) and the insert image is the corresponding spectrum of this sample.

Composite films were deposited on borosilicate glasses (20 × 20 mm²) by a spin-coating technology with the speed of 3000 rpm for 20 s. To analyze the elemental composition and the molar ratio of Si/V in the composited films, films were prepared on aluminum foils under the same procedure. All the substrates were sequentially cleaned by acetone, ethanol and deionized water to remove surface contaminations. After drying at 60°C for 10 min to remove the excess solvent, the as-prepared films were annealed by the step of 5°C/min in tube furnace under an appropriate atmosphere.

Characterization

The crystal structure of the VO₂-SiO₂ composite films was collected by Glancing Angle X-ray diffractometer (GAXRD, PANalytical X'pert Pro MPD) using Cu-Kα radiation (λ = 1.54 Å). The operating voltage and current were kept at 40 kV and 40 mA, respectively. The surface morphology and element distributions were characterized by field emission scanning electron microscopy (FESEM, S-4800, Hitachi Japan) and transmission electron microscopy (TEM, 2010F, JEOL and TF20, FEI) equipped with an energy-dispersive spectrometer (EDS, Oxford) attachment. The thicknesses of the composite films were determined by the step profiler (Alpha-Step D-100, KLA-Tencor USA). Specimens for TEM analysis were prepared by cutting, grinding, dimpling and ion-milling (Gatan 691, operated at 3.0 to 0.5 kV and cooled with liquid nitrogen), followed by surface cleaning using an ion cleaner (JIC-410, JEOL Japan) to remove completely any residual amorphous film. The thermochromic properties of the films were monitored by ultraviolet-visible-near infrared spectrophotometer (UV-Vis-NIR, Lambda 750), from 250 to 2500 nm at temperature 30°C and 100°C, respectively. Temperature was measured with the assistance of a temperature sensor in contact with the films which was connected with a temperature controlling unit (FP23, SHIMADEN Japan). The refractive index and extinction coefficient of the films were performed on spectroscopic ellipsometry (SE) (M-2000UI, J.A. Woollam USA) in the spectral range of 245–1680 nm. Hysteresis loops were measured by collecting the transmittance of the films at a fixed wavelength (2000 nm) under variable temperatures at an approximate interval of 2°C. The temperature errors were less than ±1.0°C based on repeated measurements.

Calculation based on the effective medium theory

Based on the effective medium theory, we calculate the optical constants of the composite films. It is considered that there is a random mixture of VO₂ and SiO₂ nanoparticles dispersed in our composite films. Both of the nanoparticles are taken to be spherical with size far less than λ, so that the composite film can be regarded as an “effective medium” model with properties intermediate between those nanoparticles. Assuming that the VO₂ and SiO₂ units are topologically equivalent, it is appropriate to get the effective dielectric function (ε) based on the Bruggeman theory^4,48,49,50.

Among which,

and are the effective dielectric functions of the two nanoparticles, respectively. f denotes as the “filling factor” (i.e., volume fraction) of SiO₂. ε₁ and ε₂ are the real part and the imaginary part of the dielectric function, respectively. The optical constants of VO₂ and SiO₂ are adopted from the literatures^19,51. Based on the calculated optical constants of composite films with different f, we simulate the optical spectra of a single-layer composite film on borosilicate glass substrate by the Essential Macleod software.