Hydrothermal One-Step Synthesis of Highly-Dispersed M-Phase VO2

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Energy, Environmental, and Catalysis Applications

Hydrothermal One-Step Synthesis of Highly-Dispersed M-Phase VO Nanocrystals and Application to Flexible Thermochromic Film 2

Deyu Guo, Chen Ling, Chengzhi Wang, Dan Wang, JingBo Li, Zhengjing Zhao, Zehao Wang, Yongjie Zhao, Jiatao Zhang, and Haibo Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08908 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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ACS Applied Materials & Interfaces

Hydrothermal One-Step Synthesis of Highly-Dispersed M-Phase VO2 Nanocrystals and Application to Flexible Thermochromic Film Deyu Guo,1 Chen Ling,1 Chengzhi Wang,1 Dan Wang,1 Jingbo Li,1* Zhengjing Zhao,1 Zehao Wang,2 Yongjie Zhao,1Jiatao Zhang,1 Haibo Jin1*

1 Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China. 2 Department of Chemistry, National University of Singapore, Singapore 119077, Singapore

AUTHOR INFORMATION Corresponding Author *E-mail: (J.L.) [email protected]; (H. J.)[email protected].

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ABSTRACT Preparation of ultrafine highly-dispersed VO2(M) nanoparticles which are essential materials to fabricate thermochromic flexible films remains a challenge preventing effective use of their promising properties. Here, we report an original hydrothermal approach by controlling oxidizing atmosphere of reaction with hydrogen peroxide to prepare ultrafine VO2(M) nanoparticles free from annealing. Hydrogen peroxide is separated from precursor solution in reactor which creates a moderate oxygenation environment, enabling the formation of stoichiometric VO2(M) nanoparticles. The obtained VO2(M) nanoparticles are well-dispersed, highly uniform and single-phase with average particle size ~30 nm. The flexible thermochromic films fabricated with the VO2(M) nanoparticles exhibit excellent thermochromic performance with the solar modulation efficiency of 12.34% and luminous transmittance of 54.26%. While the films prepared with annealed nanoparticles show reduced transmittance due to light scattering of the large size particles resulting from agglomeration and growth during annealing. This work demonstrates a promising technique to realize moderate oxidizing atmosphere in hydrothermal process for preparing well-dispersed stoichiometric nano-oxides.

KEYWORDS ultrafine VO2(M) nanocrystals, one-step hydrothermal reaction, controlled atmosphere, high-thermochromic performance, flexible thermochromic film


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INTRODUCTION

Keeping livable temperature in buildings consumes considerable proportion of the global energy supply.1 Controlling the thermal transfer through the architectural glass is one effective avenue to reduce the energy consumption.2 Vanadium dioxide (VO2) undergoes a reversible metal-to-insulator (MIT) transition at critical temperature (Tc) of 68 oC, accompanied with dramatic changes in near infrared (NIR) optical properties.3 The low-temperature insulator monoclinic (M) phase is transparent to the NIR light and the high-temperature metal rutile (R) phase is strongly reflective to the NIR light, which provides opportunities in thermochromic smart windows for energy conservation.4-5 There are two routes to produce VO2 based smart windows, one is to deposit VO2 films on rigid substrate, the other is to prepare flexible films using VO2 nanoparticles. Considerable efforts have been devoted to prepare VO2 films and nanostructures on rigid substrate for achieving VO2 smart windows. The approaches contain magnetic sputter,6 molecular beam epitaxy,7 pulsed laser deposition,8 chemical vapor deposition,9 wet-chemical deposition,10-11 etc. However, the vapor deposition techniques require expensive equipments and the wet-chemical approaches are generally complicated. Besides, a post-heat treatment process is usually necessary to obtain high quality VO2(M) for good

thermochromic

performances,

which

requires

selected

substrates

with

similar thermal expansion coefficients with VO2 to avoid the damage of VO2 films during heat treatment.12 These disadvantages have restricted industrialization of VO2 in the field of energy conservation. Compared to VO2 films on rigid substrates, flexible films show great advantages in low cost, large scale production, versatility, etc.13 The flexible VO2-based thermochromic films are fabricated by dispersing the VO2 powders in polymer matrix. To achieve satisfactory thermochromic performance of VO2 flexible



films, the particle size of VO2(M) used in smart windows is required to be controlled within 50 nm to reduce the transparent-loss caused by light scattering.14 Up to date, the preparation of the ultrafine high quality VO2(M) nanopowders remains one challenge hindering effective use of their promising properties. Hydrothermal reaction is an efficient method to synthesis VO2 nanoparticles with controllable morphology and good crystallinity. However, the nanoparticles synthesized by solution strategies at low temperature are usually metastable monoclinic VO2(B) nanomaterials or oxygen-deficient VO2.15-16 To obtain M-phase VO2, it is necessary to anneal the as-synthesized nanoparticles at elevated temperature. Unfortunately, the heat treatment will result in the welding joint of nanoparticles to recrystallize into large size particles due to the high-energy surface of nanoparticles.17 One-step hydrothermal synthesis of monoclinic VO2(M) powders has become a research focus to avoid aggregation and growth of nanoparticles during heat treatment. The researches on one-step hydrothermal synthesis of VO2 (M) powders have been reviewed in Ref.18.18 One-dimensional VO2(M) nanorods or nanobelts are the general products.15,19-21 Xie et al. reported the synthesis of M-phase VO2 nanorods at 200 oC by controlling oxidation reaction with the addition of nitric acid.15 Son et al. obtained asterisk or rod shaped M-phase VO2 microcrystals at 220 oC.21 The large size VO2 nanocrystals inevitably degrades the optical performance.22 VO2(M) nanoparticles with sizes ranging from 24 nm to 65.7 nm were produced by one-step hydrothermal synthesis with vanadium pentoxide and diamide hydrochloride at 270~390 o

C.23-24 Ji et al. prepared high quality nano-sized VO2 nanopowders by combining sol-gel and hydrothermal

treatment processes at elevated temperature of 260 oC.25 Liu et.al obtained sphere like VO2(M) nanoparticles with average size of 91.5 nm by hydrothermal ammonium metavanadate and malic acid at 260 oC for 24 h.26 Gao et al. reported the one-pot hydrothermal synthesis of VO2 nanoparticles with doped methodology to control the particle morphology, such as Sb3+, Ti4+, F¯, and the hydrothermal temperature was maintained at


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240 oC or higher.27-29 It is found that relatively high temperatures are required to obtain M-phase VO2 in one-step hydrothermal reaction processes. The high temperature will enhance chemical reaction kinetics according to the Arrhenius relation, meanwhile decrease the thermodynamic driving force.30 Thus the high reaction temperature would make it difficult to control the size of nanoparticles and result in large-size and non-uniform particles. Herein, we propose a novel synthesis route to achieve the one-step preparation of ultrafine VO2(M) nanoparticles by controlling hydrothermal reaction in a self-released oxidizing atmosphere at relative low temperature. Well-dispersed, highly uniform and single-phase VO2(M) nanoparticles with average particle size ~30 nm are obtained. The products-based flexible films display preferable thermochromic performance for the further application.

EXPERIMENTAL METHODS

Preparation of ultrafine VO2 nanoparticles. All the reagents in experiments are analytical grade without further purification, purchased from Sinopharm Chemical Reagent Co. Ltd. In typical procedure, 0.815 g commercial VOSO4 powders were added into 20 ml of deionized water at room temperature. The suspension was continuously stirred until the clear blue solution was formed. The 0.25 ml of hydrazine hydrate (N2H4·H2O, 80 wt% aqueous solution) was added dropwise to the solution resulting in azury suspension while keeping the solution at 65 oC. The pH of the suspension was adjusted to 10 by the addition of 1 M NaOH solution. The color of suspension turned to gray, pink, and brown successively. Heat should be sustained during the dropwise of NaOH solution. The precipitate was collected by



filtering, and washed with deionized water for three times, finally ready for the hydrothermal reaction. In the hydrolytic precipitation process, the addition of hydrazine hydrate is necessary for the final formation of ultrafine VO2 nanoparticles. On the one hand, the hydrazine as a strong reducing agent inhibits the occurrence of V5+ ions and makes hydrolytic precipitates (VO(OH)2-x) formed. On the other hand, the hydrazine working as a structure-directing agent coordinates with VO2+ ions to prevent the hydrolytic precipitates from reassembling into VO2(B) nanobelts during the following hydrothermal process.21,31 However, the addition of hydrazine hydrate leads to the formation of oxygen-deficient VO2 nanoparticles at experimental conditions as introduced below. In the conventional hydrothermal process, the hydrolytic precipitates without drying were dispersed in 20 ml of deionized water in 50 ml Teflon-lined autoclaves. The autoclaves were heated at different temperatures from 180 oC to 260 oC for 48 h. Fig. S1 shows the XRD results and SEM images of hydrothermal products at different temperatures. As hydrothermal temperature is lower than 240 oC, the products are the so-called M* VO2 (an oxygen-deficient VO2 phase as discussed below). For the products synthesized at 180 oC and 200 oC the nanobelts were obtained due to incomplete decomposition of hydrolytic precipitates.31 In the 260 oC sample, the M-phase VO2 exists together with the M* VO2, indicating the high reaction temperature is advantageous to obtain M-phase VO2. However, large size nanorods were grown in the 260 oC sample as shown in Fig. S1e. Apparently, to obtain ultrafine stoichiometric VO2 nanoparticles the hydrothermal process should be carried out at appropriate temperature and a replenishing oxygen condition. H2O2 is a strong oxidizer which can slowly decompose into water and oxygen and boil at ~150 oC. Inspired by the decomposition and evaporation characteristics of H2O2, we designed an original process to produce the VO2(M) nanoparticles in one hydrothermal step by using H2O2 as oxidizer. To make the process easily


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controlled, a quartz vessel was put into Teflon-lined autoclave to contain the hydrolytic precipitation suspension, and the H2O2 aqueous solution was filled into the gap between the quartz vessel and Teflon-lined autoclave, as shown in Scheme 1. In the hydrothermal reaction process, the H2O2 evaporates and partially decomposes into oxygen at elevated temperature, which diffuses into the quartz vessel providing an oxidizing atmosphere. The evolution of synthetic route is shown as follows: N2 H4 +OH-

H2 O2 /O2

VO2+(l) VO(OH)2-x(s) → VO2-x(s) VO2(M) (s)

Scheme 1. Schematic of one-step hydrothermal synthesis process assisted with self-released oxidizing atmosphere.

In the practical operation, the hydrolytic precipitates without drying were dispersed in 10 ml of deionized water in a quartz vessel to form the precursor for hydrothermal reaction. The quartz vessel was transferred into 50 ml Teflon-lined autoclave, which contained 6 ml water with different amount of hydrogen peroxide (H2O2, 30 wt% aqueous solution). As shown in Fig. S1, the nanobelts were obtained at 180 oC and 200 oC due to incomplete decomposition of hydrolytic precipitates.31 According to this result, 220 oC, 240 oC and 260 oC were selected to synthesize the VO2 nanoparticles with the addition of H2O2. The Teflon-lined autoclaves were maintained at 220 oC for 48 h, 240 oC for 36 h and 260 oC for 24 h, respectively. The final black powders were



collected by centrifugation, washed with water, ethanol, and isopropanol successively and dried at 60 oC in air atmosphere for 12 h. The M-phase VO2 nanoparticles were synthesized at 220 oC/48 h (will be depicted below), 240 oC/36 h and 260 oC/24 h (shown in Fig. S2). The particle size increases with increasing temperature and elongating annealing time. The nanoparticles synthesized without H2O2 were annealed at 400 oC for 500 min in an air atmosphere with a pressure of 0.1 Pa and 5 oC/min heat rate as a contrast group. Preparation of VO2-Based thermochromic flexible film. The 50 mg as-prepared products were dispersed ultrasonically in 500 µl N,N-Dimethylformamide (DMF) for 2 h to form uniform suspension, while the 0.6 g polyacrylonitrile (PAN, average Mw 150,000) was added into 5 ml DMF and stirred for 24 h to obtain transparent slurry. The suspension and slurry were mixed by stirred for 2 h. Finally, the mixture was uniformly cast on the PET substrate and dried at 60 oC for 10 min. The different thicknesses of flexible films were controlled by scraper size with 30 µm, 60 µm, 90 µm separately. Characterization. The morphology of the nanoparticles and flexible film were examined by field emission scanning electron microscope (FESEM, Hitachi S-4800) and high-resolution transmission electron microscopy (TEM/HRTEM, FEI Tecnai G2 F20 S-TWIN). The X-ray diffraction (XRD, D/max 2500, X’pert Pro PANalytical in 2θ-θ model, Cu-Kα radiation source) was applied to identify the phase constituent of the samples and crystal structure of crystalline phases. X-ray photoelectron spectroscopy (XPS, Thermoescalab 250Xi, Al Kα radiation source) was employed to measure the chemical valence of vanadium ions. The composite film was characterized by fourier transform infrared spectroscopy (FT-IR) with Thermo Fisher Scientific Nicolet iS 50, and the scanning range of the wavenumber was 400-1200 cm-1. The phase transition properties of the powder samples were detected by differential scanning calorimetry (DSC, DSC200F3,


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NETZSCH) over the temperature range from -10 to 100 °C with a heating/cooling rate of 10 °C/min. The optical transmittance of samples was characterized by Shimadzu UV-3600 UV-VIS-NIR spectrophotometer with a heating accessory. RESULTS AND DISCUSSIONS

Figure 1. (a) XRD patterns of as-prepared nanoparticles with increasing amount of H2O2. (b) XPS V2p3/2 spectra of as-prepared nanoparticles with different amount of H2O2.

Fig. 1a shows the XRD patterns of the as-prepared nanoparticles with different additions of H2O2 at temperature of 220 oC. The sample without addition of H2O2 (the 0 ml one) shows the XRD pattern of the ‘new-phase’ monoclinic VO2 (here, named as M* phase) as reported by Liu et al.32 The XRD pattern of the M* phase is similar to the monoclinic VO2(M) (PDF# 43-1051, Space group: P 21/c) except for the peaks at ~24.3o, ~30.4o, ~55o, and ~70o.32 The M* phase VO2 can transform into the M phase VO2 through low temperature oxidation annealing.33 With increasing the addition of H2O2, the XRD peaks of the M phase VO2 gradually become stronger as indicated by the peaks at ~27.8o and around 55o and 70o, and the two XRD peaks of the M* phase at ~24.3o and ~30.4o also gradually draw close to each other. The 0.3 ml sample (denotes the sample prepared with the 0.3 ml addition of H2O2, similarly hereinafter) is almost the pure M phase VO2. Lattice parameters of the 0.3 ml VO2(M) are calculated to be a=5.760(8) Å, b=4.543(2) Å and c=5.378(6) Å. As H2O2



was added to 0.5 ml, additional peaks at ~25.0o, ~33.9o and ~49.5o are observed, indicating the formation of the second phase. No PDF card can be found to match these peaks, however, the second phase would contain high valence vanadium ions (V5+) due to peroxidation according to the XPS results as shown in Fig. 1b. To study the effect of H2O2 on the oxidation state of vanadium, XPS analysis is carried out. Fig. 1b shows the V2p3/2 core-level photoemission peaks. The V2p3/2 peak of the as-prepared sample without H2O2 is well fitted with V3+ (2p3/2 ~515 eV).34 The V2p3/2 peak shifts to higher binding energies of the V4+ valence state (2p3/2 ~515.96 eV) and V5+ (2p3/2 ~516.9 eV) sequentially with increasing the amount of H2O2,35-36 which confirms the oxidation of VO2 with the addition of H2O2. Stoichiometric VO2 nanoparticles are obtained with 0.3 ml H2O2, in consistence with XRD results. The results demonstrate the oxygen vacancies in the VO2 nanoparticles can be regulated by the hydrothermal atmosphere.

Figure 2. SEM images of the as-prepared nanoparticles with increasing amount of H2O2. (a) 0 ml, (b) 0.1 ml, (c) 0.2 ml, (d) 0.3 ml, (e) 0.5 ml, (f) nanoparticles without H2O2 annealed at 400 oC for 500 min.


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The morphology of products prepared at temperature of 220 oC is characterized by SEM, as shown in Fig. 2. The addition of H2O2 solution does not increase the size of nanoparticles. The as-prepared powders display desirable dispersity, and the average particle size is ~30 nm in Fig. 2 and Fig. 3a. The small size of particles is propitious to weaken the light scattering as they are used in VO2-based thermochromic flexible membranes.14 When the addition of H2O2 is increased to 0.5 ml, the nanorods/nanobelts are observed, corresponding to the impurity phase evidenced by XRD. Fig. 2f shows the VO2(M) nanoparticles obtained by annealing the as-prepared powder without H2O2 at 400 oC for 500 min. The annealed nanoparticles apparently agglomerate and grow to large size (~79 nm). The crystal structure and morphology of the products are further characterized by high-resolution transmission electron microscopy (HRTEM), as shown in Fig. 3b. The clear crystal fringes in HRTEM image indicate that the nanoparticles are single-phase particles and well crystallized. The interplanar spacing and angle of crystal planes in Fig. 3c are consistent with the (011)M and (212 )M planes of the monoclinic VO2(M) phase. Fig. 3d is the fast Fourier transform pattern (FFT) from the square section in Fig. 3b, which is well indexed in the zone axis of monoclinic VO2(M). The HRTEM and corresponding FFT results confirm the information of well crystallized single-phase VO2(M) nanoparticles.



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Figure 3. (a) SEM image of VO2(M) nanoparticles with 0.3 ml H2O2. (b) HRTEM image of the VO2(M) nanocrystals. (c, d) HRTEM image and corresponding FFT image of the square section in (c).

As described in the introduction section, several works have successfully fabricated M phase VO2 nanoparticles in one-step hydrothermal process at 240~390 oC.23-29 Compared to the reports, our proposed route has prepared ultrafine M-phase VO2 nanopowders at a lower hydrothermal temperature (220 oC). The low temperature and moderate reaction environment achieve the formation of stoichiometric VO2(M) nanoparticles, without introducing doped ions or complex procedures. The obtained VO2(M) nanoparticles are well-dispersed, highly uniform and single-phase with average particle size ~30 nm.

Figure 4. (a) DSC curves obtained for VO2 nanoparticles with different additions of H2O2. Arrows indicate the heating and cooling processes. (b) FT-IR spectra of VO2 nanoparticles, PAN and composite film.

Fig. 4a shows DSC curves of VO2 nanoparticles synthesized with different additions of H2O2 at temperature of 220 oC, where the baselines of DSC curves are subtracted (the raw data of DSC measurement are shown in Fig. S3). The DSC results indicate a reversible first-order phase transition of VO2 with endothermic peaks on the heating ramp, exothermal peaks on the cooling ramp and apparent thermal hysteresis. Here, the peak temperature is taken as the transition temperature (Tc) to match the general determination of Tc of VO2 by using

thermal

resistance

measurement,

where

the

peak

temperature


of

derivative

peaks

of

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resistance-temperature curves is determined as Tc.37 The as-grown nanoparticles without H2O2 do not show thermic peaks. The 0.1 ml sample shows the endothermic peak at ~49 oC, and the exothermic/endothermic peaks of nanoparticles shift gradually to high temperature with increasing the amount of H2O2. The 0.3 ml and 0.5 ml samples have the nearly equal Tc of ~62 oC and ~63 oC somewhat lower than the Tc (~68 oC) of bulk VO2(M).38 According to the XPS results, the increase of Tc with the H2O2 addition can be attributed to the reduction in the oxygen vacancy content resulting from the oxidation of H2O2, which is similar to the oxygenation behavior of oxygen-deficient VO2 treated by low-temperature oxygenation annealing.33,39 The suppression of Tc induce by oxygen vacancies was attributed to the local structural distortion and the electron doping caused by oxygen vacancies.33-34,40 The 0.1 ml sample exhibits a thermal hysteresis width (∆T) of ~25 o

C, and the other samples show an approximate constant ∆T of ~21 oC. For single-phase VO2(M)

nanoparticles, the ∆T was reported to be ranging from 20 oC to 50 oC opposite to nanoparticle size.23,41-42 The MIT transition of VO2 is believed to be analogous to the martensitic transformation, in which the structural defects, including grain boundaries, dislocations, oxygen vacancies, doped atoms, etc, act as nucleation sites for the phase transition.43-44 The smaller the nanoparticles become, the wider the thermal hysteresis width appears because of the less amount of defects in smaller particles.42,45 The nanoparticles prepared with less addition of H2O2 contain more amount of oxygen vacancies. Accordingly, the nanoparticles with less H2O2 are supposed to have narrower thermal hysteresis width if oxygen vacancies are the nucleation sites, but, which is contrary to the fact that the samples have a constant ∆T. Considering the similar size of the nanoparticles with different additions of H2O2, it is more possible that the particle surface is the favorable nucleation site for the MIT transition.



The VO2 flexible films are prepared with prepared VO2(M) nanoparticles to investigate the thermochromic performance by casting the VO2- PAN slurry. Fig. 4b shows the FT-IR spectra of VO2 nanoparticles, PAN and composite films. The typical infrared vibrational bands for VO2 mainly involve the coupled vibration of V=O at 990 cm-1 and 715 cm-1, and V–O–V octahedral bending modes at 530 cm-1 and 422 cm-1.46 The composite film contains both the vibrational peaks of VO2 and PAN. By comparing the FT-IR data, no obvious red or blue shifts of vibrational peaks of PAN are observed, indicating the weak impact of VO2 nanoparticles to the chain of PAN.47 It means the VO2 nanoparticles do not bear the chemical force from the PAN matrix.

Figure 5. (a, b) Optical image and scheme image of the VO2 nanoparticles based flexible film. (c) Transmittance spectra of VO2 flexible films with VO2 nanoparticles synthesized with 0.3 ml H2O2 (1#: thickness of ~1.1 µm, 2#: thickness of ~2.3 µm, 3#: thickness of ~4.0 µm) and large size particles obtained by annealing treatment (4#, thickness of ~2.7 µm). The real lines denote transmittance spectra measured at 30 oC, and the dashed lines are measured at 90 oC. (d) Comparison of thermochromic properties in this work with those reported.6,17,48-56


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Fig. 5a shows the excellent flexibility and good visible transmission of flexible films which are cast on PET substrates as illustrated in Fig. 5b. The transmittance spectra of flexible films are measured at 30 oC and 90 oC to examine the thermochromic properties and shown in Fig. 5c. The cross sections of flexible films in Fig. S4 show the thickness of the films. The solar modulations of the flexible films are calculated according to the equation Tlum, sol =

φlum, sol λTλdλ φlum, sol λdλ

∆Tlum, sol =Tlum, sol(30°C) -Tlum, sol(90°C)

(1) (2)

where T(λ) denotes the transmittance at wavelength λ, φ is the solar irradiance spectrum for air mass 1.5 corresponding to the sun standing 37° above the horizon. The thermochromic properties of flexible films are summarized in Table S1. The integrated luminous transmittance (Tlum, 380~780 nm) of the one-step VO2 film (1#, ~1.1 µm) is calculated to be 54.26% and the solar modulating ability (∆Tsol, 300~2500 nm) is 12.34% (Tsol ~63.42% at 30 oC and ~51.08% at 90 oC respectively), indicating a desirable thermochromic performance of the one-step VO2(M) flexible film. The thicker one-step VO2 films display a better solar modulating ability (2#, ~2.3 µm, ∆Tsol, ~14.34%; 3#, ~4.0 µm, ∆Tsol, ~13.25%) with the degradation of integrated luminous transmittance (Tlum, ~37.19%; ~20.96%). The flexible film is fabricated with the annealed VO2 nanoparticles to underline the effect of particle size on thermochromic properties. While, the annealed VO2 film shows a weakened thermochromic performance (Tlum, ~31.49%; ∆Tsol, ~9.38%) due to its large size particles (4#, ~79 nm, the corresponding SEM image in Fig. 2f) scattering solar light. The annealed VO2 flexible film with thickness of ~2.7 µm displays degraded transmittance compared to one-step VO2 films with thickness of ~2.3 µm. According to the Lopez’s reports, the scattering efficiency of ~80 nm particle was 8.9 times in insulating state and 7.1 times in metal state higher than that of ~30 nm particle.57 The degraded transmittance of annealed



film are believed to be caused by light scattering of large size VO2 nanoparticles as suggested by Laaksonen et al.14 There is one other interesting phenomenon which could be also ascribed to the size effect of nanoparticles. The high-temperature transmittance spectra of one-step VO2 films exhibit a pronounced minimum at ~1278 nm, which shows no change with varying the thickness of flexible films, but shifts to ~1350 nm for the annealed VO2 film. The transmittance minimum was thought to originate from surface plasmon absorption of metallic VO2(R) nanoparticles.58 The red shift of the transmittance minimum for the annealed VO2 film would be attributed to the larger size of annealed VO2 particles, which is consistent with previous reports.14,52 Fig. 5d shows the ∆Tsol and Tlum of the thermochromic flexible films fabricated using one-step VO2 nanoparticles compared with reported VO2 films. The flexible films prepared in this work show the top level thermochromic performance, demonstrating the proposed synthesis route has the potential value in preparing high quality ultrafine VO2(M) nanoparticles for flexible thermochromic films. CONCLUSION In conclusion, ultrafine monoclinic VO2(M) single-phase nanoparticles with average size ~30 nm were synthesized by a one-step hydrothermal route with a nested structure of reactor to separate the precursor and the oxidant of H2O2 for the first time. The whole process was carried out under low reaction temperature without complex equipment, and easily controlled. Importantly, the nanoparticles showed good dispersity due to the moderate reaction conditions, overcoming VO2(M) nanoparticles agglomeration in the post-annealing process which is the technical bottleneck limiting the VO2 nanoparticles in practical applications. Using the as-prepared nanoparticles the VO2 flexible films were fabricated. The films exhibited excellent thermochromic performance which is at the top level compared to the reported results. The fantastic thermochromic performance is beneficial from the fine particle size and good dispersity of the one step nanoparticles because


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the ultrafine nanoparticles effectively supress the light scattering which obviously deteriorates the optical properties of the annealed VO2 flexible films due to the larger size of nanoparticles. Our work demonstrates a promising technique of producing well dispersed ultrafine VO2(M) single-phase nanoparticles which are required for the preparation of the flexible smart windows. The proposed growth technique could be extended to other hydrothermal or solvothermal systems requiring specific atmosphere, which offers promising opportunities for exploiting potential nanomaterials.

ASSOCIATED CONTENT

Supporting Information Available: This material is available free of charge on the ACS Publications website at DOI: Morphology and XRD patterns of nanoparticles synthesized by different hydrothermal temperature and content of H2O2, raw DSC dates, cross section images of flexible films, the thermochromic properties of flexible films.

AUTHOR INFORMATION

Corresponding Author *E-mail: (J.L.) [email protected]; (H. J.)[email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTs



We gratefully acknowledge the contribution of Prof. Haizheng Zhong and Dr. Qingchao Zhou to the preparation of the flexible films. This work is supported by the National Science Foundation of China (Grant No. 51572027) and Key Project of Chinese Ministry of Education (Grant No. 313007). REFERENCES

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TOC GRAPHICS


Hydrothermal One-Step Synthesis of Highly-Dispersed M-Phase VO2

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