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Preparation of quaternary tungsten bronze nanoparticles by a thermal decomposition of ammonium metatungstate with oleylamine
Chemical Engineering Journal 281 (2015) 236–242
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Preparation of quaternary tungsten bronze nanoparticles by a thermal decomposition of ammonium metatungstate with oleylamine Jaehyuk Choi a, Kyonghwan Moon a, Insung Kang a, Sangbum Kim b, Pil J. Yoo c, Kyung Wha Oh d, Juhyun Park a,⇑ a
School of Chemical Engineering and Materials Science, Chung-Ang University, Seoul 156-756, Republic of Korea Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003, USA School of Chemical Engineering and SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea d Department of Fashion Design, Chung-Ang University, Seoul 156-756, Republic of Korea b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Tungsten bronze nanoparticles
prepared via a thermal decomposition with oleylamine. Quaternary tungsten bronze nanocrystals doped with sodium and cesium were prepared. Sodium and cesium ions were intercalated into the cubic pyrochlore structure of WO3. NIR absorption is broader and stronger than that of the equivalent ternary compounds.
a r t i c l e
i n f o
Article history: Received 9 March 2015 Received in revised form 18 June 2015 Accepted 23 June 2015 Available online 30 June 2015 Keywords: Tungsten bronzes Nanoparticles Near infrared absorption Thermal decomposition Oleylamines
a b s t r a c t We report the synthesis of quaternary tungsten bronze nanocrystals (QTBN) doped with sodium and cesium, NaxCsyWOz, which were prepared via a simple thermal decomposition process involving the combination of ammonium metatungstates with oleylamine as both the surfactant and the solvent. The QTBN capped with oleylamine had an average diameter of about 30 nm and exhibited a shielding property of approximately 97% of near-infrared radiation across a wavelength range of 780–2100 nm, while transmitting 64% of visible light at 432 nm upon dispersion in a non-polar solvent of toluene. Our characterizations showed that both sodium and cesium ions could successfully be intercalated into the framework of the cubic pyrochlore structure of tungsten oxide at a relatively low reaction temperature and within a short time, generating the quaternary compound of tungsten bronze nanoparticles. As a result, our procedure conferred near infrared absorption properties upon the QTBN that are superior to those of ternary tungsten bronze nanoparticles in terms of absorption range and intensity, suggesting a significantly advanced solution process capable of producing useful near infrared absorbents. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Over the last a few decades, the development of nanomaterials capable of effectively absorbing near-infrared radiation (NIR, wavelengths ranging from 780 to 2500 nm) from the sun in a range ⇑ Corresponding author. E-mail address: [email protected] (J. Park). http://dx.doi.org/10.1016/j.cej.2015.06.101 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.
as wide as possible, has been a significant topic for various applications. Smart windows coated with NIR absorbents are able to prevent heat gain or loss, while being transparent for visible light, thereby reducing energy consumption by buildings or automobiles [1]. In addition, NIR absorbents are useful materials for solar collectors and optical filters [2]. Tungsten bronzes (MxWO3), which are tungsten trioxide compounds doped with alkali metals, have non-stoichiometric compositions with various structures, such as
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cubic, hexagonal, tetragonal, and pyrochlore [3]. These compounds are attractive NIR absorbents, due to their distinctive electrical and optical properties, which depend on their stoichiometric composition [4]. When alkali metals are intercalated into the framework formed by corner-sharing WO6, the metal ions are believed to contribute their electrons to the conduction band of WO3, forming surface plasmon polariton of free electrons and introducing empty, higher sub-bands or states in the conduction band [5–8]. In this regard, tungsten bronze nanoparticles are attractive NIR absorbing materials, because of their unique and highly desirable optical properties, namely, strong absorption in the NIR wavelength range based on sub-band energy levels, and transparency in the visible wavelength range [9–13]. Current technologies for synthesizing tungsten bronzes nanoparticles can mainly be classified into solid-state reactions and hydrothermal synthesis. The solid state reaction typically employs tungsten precursors and a heating process in a furnace at high temperatures over 500 °C, while exposed to a gas flow consisting of a mixture of H2 and N2 to obtain products in the reduced state [11]. However, this conventional process is considered to be risky, because of the use of hydrogen gas. Furthermore, the process is also time-consuming because of the mechanical milling procedure required to reduce the particle size from micro- to nanometer scale to increase the availability of free electrons on the surface by increasing the surface area. On the other hand, the hydrothermal synthesis method utilizes chemical reactions at a temperature below 250 °C without requiring mechanical milling. However, the hydrothermal method requires the use of tungstate salts, dissolved in water or ethanol [14], in which case the reaction has to be performed in a high-pressure reactor with a reaction time exceeding 18 h, because the reaction temperature exceeds the boiling points of the solvents. In addition, the resulting products require a reduction process to achieve conversion to the reduced state, because they are readily oxidized by oxygen in polar solvents [15,16]. Thus, the development of an advanced technology that would enable the direct synthesis of nanocrystals of tungsten bronzes in the reduced state via a solution-based process [17] utilizing relatively mild reaction conditions and shorter reaction times would be highly desirable. Furthermore, it would also be necessary to synthesize the nanocrystals of tungsten bronze such that they exhibit a wide NIR absorption range to compensate for the fact that ternary compounds of tungsten bronzes, that is, tungsten trioxides doped with an alkali metal, typically only exhibit strong NIR absorption over a limited range of wavelengths. In this paper, we report a simple process to synthesize quaternary tungsten bronze nanoparticles (QTBN) in the reduced state by using a simple, convenient, reproducible route in which we employ oleylamine to provide nanocrystals of NIR absorbents with a broad working waveband. A previous report of ours described the use of a solid state reaction and a conventional mechanical milling process to produce nanocrystals of quaternary tungsten bronzes doped with sodium and cesium, which displayed remarkably improved NIR shielding properties in comparison to ternary cesium tungsten bronze [18]. This study demonstrates the facile synthesis of the quaternary compound of sodium cesium tungsten bronze nanoparticles, via a one-pot process, to maximize their NIR shielding properties. The synthesis of inorganic nanocrystals solely using oleylamine is well known, but has never been explored for preparing nanocrystals of tungsten bronzes. 2. Experimental 2.1. Materials Ammonium metatungstate hydrate (AMT, (NH4)6H2W12O40)xH2O), cesium hydroxide monohydrate
(CsOHH2O), oleylamine (>70%), and toluene were purchased from Sigma–Aldrich. Sodium hydroxide (NaOH) and acetone were purchased from Samchun Chemical. All reagents used in this study were analytical reagent grade and were used as received. 2.2. Nanoparticle synthesis A slurry was prepared by adding 0.1 mmol (0.2956 g) of AMT to 20 mL oleylamine with magnetic stirring, after which a pre-determined amount of alkali metal precursors was added to produce CsyWOz and NaxCsyWOz, which was stirred for 1 h (Table 1). The suspension was transferred to a three-neck round-bottomed flask connected to a reflux condenser. The atmosphere in the reactor was replaced with nitrogen gas by purging for an hour, following which the flask was heated at 250 °C under stirring while maintaining the nitrogen atmosphere. After 2 h the reaction was observed to be complete and the reaction mixture was allowed to naturally cool down to room temperature. The precipitate was collected by using centrifugation for 15 min at 8000 rpm, after which it was washed twice with acetone to remove the excess oleylamine. The collected precipitate was then magnetically stirred with acetone for 2 h to re-disperse the agglomerated nanoparticles, subsequent to which it was dried at room temperature. After drying, the powder was suspended in solvents such as toluene, hexane, and chloroform for the analysis of its characteristics (see Fig. 1). 2.3. Characterization of tungsten bronze nanoparticles The crystal structures of the tungsten bronze nanoparticles were identified by X-ray diffraction (XRD, Bruker-AXS NEW D8-Advance). The morphology and structure of the samples were studied using field emission transmission electron microscopy (FE-TEM, FEI Tecnai G2 F30 S-Twin). The particle size distribution of the tungsten bronze nanoparticles was determined by Dynamic Light Scattering (Nano partica SZ-100 series, Horiba Scientific) at a temperature of 25 °C with a scattering angle of 90°. The compositions of the particles were estimated by energy dispersive spectroscopy (EDS, Thermo Scientific NORAN System 7) and X-ray photoelectron spectroscopy (XPS, hv = 1486.6 eV Al Ka). The existence and quantity of oleylamine on the surface of the tungsten bronze nanoparticles were investigated using Fourier transform infrared (FT-IR, OTSUKA ELS-Z) spectroscopy and thermogravimetric analysis (TGA, Scinco TGA N-100). Absorption and transmittance spectra were obtained using a spectrophotometer (JASCO V-670) in the range of 300–2100 nm. 3. Results and discussion 3.1. Crystal structure of tungsten bronze nanoparticles (TBN) The QTBN were successfully synthesized by mixing AMT (as the tungsten precursor), and cesium and sodium hydroxide (as the alkali metal precursors) in oleylamine, followed by heating at 250 °C for 2 h; that is, a mild reaction temperature and a short reaction time. The crystal structures of the resulting products were
Table 1 The molar ratios in which the elements were mixed for synthesizing cesium tungsten bronze and sodium cesium tungsten bronze nanoparticles based on 1 mol tungsten. Sample
CsyWOz NaxCsyWOz
Concentration (mmol) Na
Cs
W
0 0.11
0.33 0.22
1 1
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Fig. 1. Schematic diagram of the synthesis of a tungsten bronze nanoparticle capped with oleylamine.
examined by XRD and the XRD patterns are shown in Fig. 2(a). All the peaks are indexed to the cubic (Cs2O)0.44W2O6 (Fig. 2(b), PDF 01-047-0566, space group = Fd-3m, a = 10.304 Å, a = b = c = 90°). It should be noted that the product is a quaternary compound, and not a mixture of two ternary compounds of sodium tungsten bronze and cesium tungsten bronze. As shown in Fig. 2(c), the XRD pattern of a mixture of the ternary compounds at a mixing ratio (Cs0.33WO3:Na0.33WO3 = 2:1 by weight), which is similar to that of the precursors that were used to synthesize QTBN, has a peak characteristic of a cubic Na0.11WO3 (Fig. 2(d), ICDS ID
Fig. 2. XRD patterns of (a) quaternary tungsten bronze nanoparticles, NaxCsyWO3 (b) reference cubic (Cs2O)0.44W2O6 (c) the mixture of ternary compounds of Na0.33WO3 and Cs0.33WO3, (d) reference cubic Na0.11WO3, (e) Na0.33WO3, and (f) Cs0.33WO3.
11855) space group = Pm-3m. The pattern presents a sharp, strong peak for the (0 0 1) plane at 2h = 23.22°, which also appears in the XRD pattern of sodium tungsten bronze nanoparticles (Na0.33WO3, Fig. 2(e)). In comparison, the XRD pattern of QTBN displays a peak at 2h = 24.43°, which is assigned to the (2 2 0) plane, and which also appears in the XRD pattern of the cesium tungsten bronze nanoparticles (Cs0.33WO3, Fig. 2(f)). These results indicate that the QTBN prepared in this study is a compound, as opposed to a mixture, and that the crystal frame of QTBN consists of a cubic pyrochlore structure, similar to that of the cesium tungsten bronze nanoparticle. The reason for synthesizing the quaternary compound in the framework of the cesium tungsten bronze nanoparticle is justified as follows. First, the ammonium metatungstate hydrate decomposes into the thermodynamically stable structure of the metatungstate anion ([a-H2W12O40]6). This ion has a cubic pyrochlore structure that has three tunnel positions: 8b, 16d, and 32e. However, because of the instability of the structure, small alkali metal ions, such as Li+, Na+, and K+, roughly are intercalated into the 16d positions. Thus, ions with larger radii, such as Rb+ and Cs+, can be expected to first be inserted into the 8b and 32e positions to stabilize the structure, following which the small ions would be intercalated into the 16d positions. In this way, sodium cesium tungsten bronze retains the same structure as cesium tungsten bronze, because essentially, the small sodium ions are filling in the space that remains after the large cesium ions have been intercalated into tungsten bronze [19,20]. The morphology and structure of QTBN were further analyzed by using FE-TEM images and selected area electron diffraction (SAED) patterns. Fig. 3(a) shows the uniform structure of NaxCsyWOz particles that ranged in size from 10 to 30 nm, capped with oleylamine: the average particle size is 32.0 nm with a standard deviation of 17.1 nm. Fig. 3(b) reveals that the d-spacing is 0.3106 nm, which is in agreement with the interplanar spacing of the (3 1 1) plane from the XRD pattern. The SAED pattern in Fig. 3(c) can be indexed to the (1 1 1), (2 2 0), and (3 1 1) planes, which is consistent with the XRD data. (See also Fig. S1 FE-TEM images and SAED pattern of cesium tungsten bronze nanoparticles.)
3.2. Nanoparticle compositions The composition of the tungsten bronze nanoparticles was determined using EDS analysis and XPS spectra of the core level
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Fig. 3. (a) FE-TEM image of NaxCsyWOz nanocrystals; (b) high magnification FE-TEM image with a lattice spacing; (c) SAED pattern and (d) particle size distribution.
tungsten (W4f). The EDS analysis shows that cesium tungsten bronze has the formula Cs0.25 WO3.44 and that of sodium cesium tungsten bronze is Na0.09Cs0.17WO3.34. (The calculated data are summarized in Table S1.) The XPS spectra shown in Fig. 5 were fitted as two spin–orbit doublets, W4f5/2 and W4f7/2, with a separation interval of 2.3 eV. The peaks at 33.45–35.75 eV (CsyWOz, Fig. 5(a)) and 33.28–35.58 eV (NaxCsyWOz, Fig. 5(b)) are attributed to W5+, while the peaks at 34.86–37.16 eV (CsyWOz, Fig. 5(a)) and 34.48–36.75 eV (NaxCsyWOz, Fig. 5(b)) are assigned to W6+. Tungsten trioxides have a charge of 6+ at first and obtain free electrons from the alkali metals, thereby generating reduced tungsten (W5+) coexisting with W6+. Thus, the estimation of the amount of doped alkali metal was carried out by integrating the area of the W5+ peak in the XPS spectra using Eq. (1). The intercalated amounts of alkali metals in ternary and quaternary tungsten bronze are 0.259 and 0.266, respectively, which is in good agreement with the EDS results. The binding energies of the metal fractions x and y are summarized in Table 2. It should be noted that the mixing ratios of the precursors in our experiments were
designed to synthesize nanoparticles with formulas of Cs0.33WO3 and Na0.11Cs0.22WO3, but the calculated and experimental values were found to differ from each other. This difference is attributed to the low reaction temperature used in our solution process. Similar to the process using hydrothermal synthesis [21], it is suggested that a reaction temperature of 250 °C cannot generate enough energy to entirely activate alkali metals to enter the tungsten trioxide framework (see Fig. 4).
W6þ O3 ! Mx W1x 6þ Wx5þ O3
ð1Þ
3.3. Characterization of oleylamine capping The confirmation of the existence of the oleylamine coating on the surface of tungsten bronze nanoparticles was conducted by analyzing the FT-IR spectra (Fig. 6) and TGA thermograms (Fig. 7). Detailed assignments of the bands are summarized in Table S2. Among the many bands related to WO3 and oleylamine,
Fig. 4. EDS analysis of (a) CsyWO3, (b) NaxCsyWO3 nanoparticles.
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Fig. 5. W4f core-level XPS spectra of (a) CsyWO3, (b) NaxCsyWO3 nanoparticles.
Table 2 Binding energies of the two spin–orbit doublets, W4f5/2 and W4f7/2, and molar ratio of alkali metal in tungsten bronze as attributed to W5+ and W6+. Sample W5+ W6+ Total alkali metal NaxCsyWO3 fraction W4f5/2(eV) W4f7/2(eV) W4f5/2(eV) W4f7/2(eV) CsyWOz 33.45 NaxCsyWOz 33.28
34.86 34.48
35.75 35.58
37.16 36.78
0.259 0.266
the band at a wavenumber of 1509 cm1 is absent from the FT-IR spectra of pure oleylamine [22] and results from the vibration of the WAN bond. Thus, it is clear that oleylamine is attached to the surface of the tungsten bronze nanoparticles. The amount of oleylamine capping the surface of the tungsten bronze was estimated by recording a TGA thermogram in the temperature range 100–800 °C. The thermogram is shown in Fig. 7 and presents two regions of mass reduction in the temperature ranges 100–300 °C and 500–700 °C. The change in the lower temperature region indicates the removal of the oleylamine hydrocarbon chains, which is followed by the removal of the nitrogen head groups at 500–700 °C [23]. After removal of the capping agent, the residual weight percentages of cesium tungsten bronze and sodium cesium tungsten bronze are 82.25% and 83.20%, respectively. Therefore, those of oleylamine on the tungsten bronze nanoparticles are estimated as 14.75% and 16.80%, respectively.
Fig. 7. TGA thermograms of (a) CsyWOz and (b) NaxCsyWOz nanoparticles.
3.4. Proposed reaction mechanism Considering the results all together, the following reaction mechanism is proposed for the synthesis of the tungsten bronze nanoparticles in oleylamine medium. Eq. (2) represents the overall reaction during the thermal decomposition of AMT. The reaction mixture consisting of oleylamine and the precursors is opaque in the form of a slurry and becomes transparent upon reaching 100 °C because of the dissolution of the precursor as represented by Eq. (3). Between 190 °C and 220 °C, water and ammonia are gradually produced as shown in Eqs. 4–6. At 250 °C, the alkali metal ions start being intercalated into the tungsten trioxide lattice (Eq. (7)) and the translucent solution turns blue after 30 min because of the reduction of tungsten trioxide (Fig 8). The role of oleylamine in our study is to surround the dissolved aqueous mixture of precursors by the hydrophilic amine functionality and to homogeneously disperse the matter as small particles within the oleylamine medium. As a result, the aqueous nanodispersion becomes a collection of nanometer-scale reactors with the ability to synthesize tungsten bronze nanoparticles. Moreover, the presence of the oleylamine ensures the stabilization and prevents aggregation of the tungsten bronze nanoparticles after the reaction [24].
ðNH4 Þ6 H2 W12 O40 xH2 O ! ðx þ 4ÞH2 O þ 6NH3 þ 12WO3 100 C
ðNH4 Þ6 H2 W12 O40 xH2 O ! xH2 O þ ðNH4 Þ6 H2 W12 O40 Fig. 6. FT-IR spectra of (a) CsyWO3 and (b) NaxCsyWO3 nanoparticles.
ð2Þ ð3Þ
J. Choi et al. / Chemical Engineering Journal 281 (2015) 236–242 190220 C
ðNH4 Þ6 H2 W12 O4 ! ðNH4 Þ6 O3 þ H2 O þ 12WO3 190220 C
ðNH4 Þ6 O3 ! 6NH3 þ 3H2 O 190220 C
241
ð4Þ ð5Þ
NHþ4 ! NH3 þ Hþ
ð6Þ
Mx þ WO3 ! Mx WO3
ð7Þ
To gain a deeper understanding of the intercalation of alkali metals into the framework of tungsten trioxide, we monitored the variation in the optical properties of QTBN by measuring its absorption and transmittance spectra as a function of time (Figs. S2 and 8). Initially, when the reaction temperature reaches 250 °C, no absorption was detected in the near-infrared region, with the exception of an absorption band at 300 nm, which indicates a transition due to the band gap. After 0.5 h at 250 °C, weak absorption appears in the NIR region, accompanied by transmittance in the visible light region, which means that the intercalation of alkali metals into the lattice has commenced. The typical optical properties of QTBN, namely, a dark blue color and absorption over a wide NIR region, were observed after 1 h of reaction at 250 °C as shown in the inserted photographs in Figs. 8 and S2, and after another 1.5 h, additional intercalation of alkali metal ions improved the NIR cutoff properties (Fig. 8). These results suggested that the optical properties varied with time, due to the onset of the formation of tungsten bronze, caused by the intercalation of alkali metal. The tungsten oxide particles transforms into a tungsten bronze particle after 2 h at a steady temperature of 250 °C. 3.5. Optical properties of tungsten bronze nanoparticles The difference in the optical properties of the ternary and quaternary tungsten bronze nanoparticles was examined by measuring the UV–vis spectra (Fig. 9(a)). Both the ternary and quaternary tungsten bronze nanoparticles have a narrow absorption band near 300 nm, which is attributed to the interband transition caused by the excitation of an electron from the oxygen 2p to the tungsten 5d orbital [25,26]. Cesium tungsten bronze shows a wide absorption band that starts at 830 nm and has a maximum absorbance at 1602 nm. The absorption at 830 nm is considered to be caused by polarons from localized electrons in the band structure, whereas the plasmon excitations of free electrons donated by the alkali metals are observed at 1602 nm [27–29]. Upon addition of sodium to cubic cesium tungsten bronze, the
Fig. 9. (a) Absorption and (b) transmittance spectra of tungsten bronze nanoparticles. The inserted image shows nanoparticles dispersed at 0.18 wt% in toluene. The spectral disturbances between 1620 nm and 1790 nm are due to the toluene solvent.
Na0.09Cs0.17WO3.34 strengthens the absorption at 1600 nm and broadens that around 1112 nm. The first of these phenomena is due to an increased abundance of free electrons in quaternary tungsten bronze. Even though the two bronzes have the same pychlore crystal structure, unlike cesium tungsten bronze, sodium cesium tungsten bronze contains sodium ions that occupy the free space that remains after the cesium ions have been intercalated. This results in a larger number of free electrons in the conduction band of tungsten bronze, a result that correlates well with the XPS results. The second phenomenon can be explained by the introduction of an additional localized level in the W5+, created by the sodium ions, which influences absorption in the range of 780–1200 nm [30]. Fig. 9(b) shows that the transmittance spectra of the tungsten bronze nanoparticles are closely related to the absorption spectra. The ternary tungsten bronze is responsible for 57% of maximum visible light transmittance at 474 nm and the lowest transmittance in the NIR region at about 10.6%. On the other hand, the quaternary tungsten bronze shows approximately 90% of NIR cutoff with 64% of maximum visible light transmittance at 432 nm. The additional doping of the cesium tungsten bronze highly improves the shielding property in the range of 780–1200 nm.
4. Conclusion Fig. 8. Transmittance spectra and color change (the inserted image) of quaternary tungsten bronze nanoparticles with different reaction time at 250 °C for (i) 0, (ii) 0.5, (iii) 1, (iv) 1.5, (v) 2.0 h.
A quaternary compound of sodium cesium tungsten bronze nanoparticles NaxCsyWOz was successfully synthesized in solution via a simple process by using oleylamine as both the capping agent
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and solvent at a mild reaction temperature for a short reaction time without a mechanical milling process. Our solution-based, one-pot process simply requiring the precursors to be mixed with oleylamine suggests an advanced synthetic route, which is less hazardous and more cost-effective, for preparing nanoparticles of novel tungsten bronze NIR absorbents. Most importantly, as-synthesized nanoparticles are already in the reduced state; hence, no further reduction process is necessary. It was demonstrated that the quaternary tungsten bronze nanoparticles, with an average diameter of about 32 nm, had a cubic pychlore crystal structure. The sodium ions were intercalated into the unfilled spaces of the cesium tungsten bronze nanocrystals, resulting in an NIR absorption that is broader and stronger than that of the equivalent ternary compounds of cesium tungsten bronze nanoparticles. The new materials have significantly improved NIR absorption and shielding properties, resulting from the enrichment of the particle surface with free electrons and the contribution from the reduced localized W5+ ions. It should be mentioned that a 5 times scaled-up synthesis of QTBN required a long reaction time of 60 h to gain optical properties comparable to original results, as shown in Fig. S3, suggesting that careful control over process parameters such as heat transfer might be necessary for mass production. Acknowledgements This research was financially supported by the Chung-Ang University Excellent Freshman Scholarship and by Grants from the Korea Research Foundation (Grant No. 2014R1A1A3049867), Republic of Korea. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2015.06.101. References [1] L.S. Long, H. Ye, Y.F. Gao, R.Q. Zou, Performance demonstration and evaluation of the synergetic application of vanadium dioxide glazing and phase change material in passive buildings, Appl. Energy 136 (2014) 89–97. [2] B. Baloukas, J.M. Lamarre, L. Martinu, Electrochromic interference filters fabricated from dense and porous tungsten oxide films, Sol. Energy Mater. Sol. Cells 95 (3) (2011) 807–815. [3] J.-D. Guo, M.S. Whittingham, Tungsten oxides and bronzes: synthesis, diffusion and reactivity, Int. J. Mod. Phys. B 7 (23–24) (1993) 4145–4164. [4] C. Guo, S. Yin, T. Sato, Effects of crystallization atmospheres on the nearinfrared absorbtion and electroconductive properties of tungsten bronze type MxWO3 (M = Na, K), J. Am. Ceram. Soc. (2012). [5] M. Green, Z. Hussain, Optical properties of dilute hydrogen tungsten bronze thin films, J. Appl. Phys. 69 (11) (1991) 7788–7796. [6] O. Schirmer et al., Dependence of WO3 electrochromic absorption on crystallinity, J. Electrochem. Soc. 124 (5) (1977) 749–753. [7] G. Liu et al., Electrostatic-induced synthesis of tungsten bronze nanostructures with excellent photo-to-thermal conversion behavior, J. Mater. Chem. A 1 (35) (2013) 10120–10129.
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Preparation of quaternary tungsten bronze nanoparticles by a thermal decomposition of ammonium metatungstate with oleylamine Jaehyuk Choi a, Kyonghwan Moon a, Insung Kang a, Sangbum Kim b, Pil J. Yoo c, Kyung Wha Oh d, Juhyun Park a,⇑ a
School of Chemical Engineering and Materials Science, Chung-Ang University, Seoul 156-756, Republic of Korea Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003, USA School of Chemical Engineering and SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea d Department of Fashion Design, Chung-Ang University, Seoul 156-756, Republic of Korea b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Tungsten bronze nanoparticles
prepared via a thermal decomposition with oleylamine. Quaternary tungsten bronze nanocrystals doped with sodium and cesium were prepared. Sodium and cesium ions were intercalated into the cubic pyrochlore structure of WO3. NIR absorption is broader and stronger than that of the equivalent ternary compounds.
a r t i c l e
i n f o
Article history: Received 9 March 2015 Received in revised form 18 June 2015 Accepted 23 June 2015 Available online 30 June 2015 Keywords: Tungsten bronzes Nanoparticles Near infrared absorption Thermal decomposition Oleylamines
a b s t r a c t We report the synthesis of quaternary tungsten bronze nanocrystals (QTBN) doped with sodium and cesium, NaxCsyWOz, which were prepared via a simple thermal decomposition process involving the combination of ammonium metatungstates with oleylamine as both the surfactant and the solvent. The QTBN capped with oleylamine had an average diameter of about 30 nm and exhibited a shielding property of approximately 97% of near-infrared radiation across a wavelength range of 780–2100 nm, while transmitting 64% of visible light at 432 nm upon dispersion in a non-polar solvent of toluene. Our characterizations showed that both sodium and cesium ions could successfully be intercalated into the framework of the cubic pyrochlore structure of tungsten oxide at a relatively low reaction temperature and within a short time, generating the quaternary compound of tungsten bronze nanoparticles. As a result, our procedure conferred near infrared absorption properties upon the QTBN that are superior to those of ternary tungsten bronze nanoparticles in terms of absorption range and intensity, suggesting a significantly advanced solution process capable of producing useful near infrared absorbents. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Over the last a few decades, the development of nanomaterials capable of effectively absorbing near-infrared radiation (NIR, wavelengths ranging from 780 to 2500 nm) from the sun in a range ⇑ Corresponding author. E-mail address: [email protected] (J. Park). http://dx.doi.org/10.1016/j.cej.2015.06.101 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.
as wide as possible, has been a significant topic for various applications. Smart windows coated with NIR absorbents are able to prevent heat gain or loss, while being transparent for visible light, thereby reducing energy consumption by buildings or automobiles [1]. In addition, NIR absorbents are useful materials for solar collectors and optical filters [2]. Tungsten bronzes (MxWO3), which are tungsten trioxide compounds doped with alkali metals, have non-stoichiometric compositions with various structures, such as
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cubic, hexagonal, tetragonal, and pyrochlore [3]. These compounds are attractive NIR absorbents, due to their distinctive electrical and optical properties, which depend on their stoichiometric composition [4]. When alkali metals are intercalated into the framework formed by corner-sharing WO6, the metal ions are believed to contribute their electrons to the conduction band of WO3, forming surface plasmon polariton of free electrons and introducing empty, higher sub-bands or states in the conduction band [5–8]. In this regard, tungsten bronze nanoparticles are attractive NIR absorbing materials, because of their unique and highly desirable optical properties, namely, strong absorption in the NIR wavelength range based on sub-band energy levels, and transparency in the visible wavelength range [9–13]. Current technologies for synthesizing tungsten bronzes nanoparticles can mainly be classified into solid-state reactions and hydrothermal synthesis. The solid state reaction typically employs tungsten precursors and a heating process in a furnace at high temperatures over 500 °C, while exposed to a gas flow consisting of a mixture of H2 and N2 to obtain products in the reduced state [11]. However, this conventional process is considered to be risky, because of the use of hydrogen gas. Furthermore, the process is also time-consuming because of the mechanical milling procedure required to reduce the particle size from micro- to nanometer scale to increase the availability of free electrons on the surface by increasing the surface area. On the other hand, the hydrothermal synthesis method utilizes chemical reactions at a temperature below 250 °C without requiring mechanical milling. However, the hydrothermal method requires the use of tungstate salts, dissolved in water or ethanol [14], in which case the reaction has to be performed in a high-pressure reactor with a reaction time exceeding 18 h, because the reaction temperature exceeds the boiling points of the solvents. In addition, the resulting products require a reduction process to achieve conversion to the reduced state, because they are readily oxidized by oxygen in polar solvents [15,16]. Thus, the development of an advanced technology that would enable the direct synthesis of nanocrystals of tungsten bronzes in the reduced state via a solution-based process [17] utilizing relatively mild reaction conditions and shorter reaction times would be highly desirable. Furthermore, it would also be necessary to synthesize the nanocrystals of tungsten bronze such that they exhibit a wide NIR absorption range to compensate for the fact that ternary compounds of tungsten bronzes, that is, tungsten trioxides doped with an alkali metal, typically only exhibit strong NIR absorption over a limited range of wavelengths. In this paper, we report a simple process to synthesize quaternary tungsten bronze nanoparticles (QTBN) in the reduced state by using a simple, convenient, reproducible route in which we employ oleylamine to provide nanocrystals of NIR absorbents with a broad working waveband. A previous report of ours described the use of a solid state reaction and a conventional mechanical milling process to produce nanocrystals of quaternary tungsten bronzes doped with sodium and cesium, which displayed remarkably improved NIR shielding properties in comparison to ternary cesium tungsten bronze [18]. This study demonstrates the facile synthesis of the quaternary compound of sodium cesium tungsten bronze nanoparticles, via a one-pot process, to maximize their NIR shielding properties. The synthesis of inorganic nanocrystals solely using oleylamine is well known, but has never been explored for preparing nanocrystals of tungsten bronzes. 2. Experimental 2.1. Materials Ammonium metatungstate hydrate (AMT, (NH4)6H2W12O40)xH2O), cesium hydroxide monohydrate
(CsOHH2O), oleylamine (>70%), and toluene were purchased from Sigma–Aldrich. Sodium hydroxide (NaOH) and acetone were purchased from Samchun Chemical. All reagents used in this study were analytical reagent grade and were used as received. 2.2. Nanoparticle synthesis A slurry was prepared by adding 0.1 mmol (0.2956 g) of AMT to 20 mL oleylamine with magnetic stirring, after which a pre-determined amount of alkali metal precursors was added to produce CsyWOz and NaxCsyWOz, which was stirred for 1 h (Table 1). The suspension was transferred to a three-neck round-bottomed flask connected to a reflux condenser. The atmosphere in the reactor was replaced with nitrogen gas by purging for an hour, following which the flask was heated at 250 °C under stirring while maintaining the nitrogen atmosphere. After 2 h the reaction was observed to be complete and the reaction mixture was allowed to naturally cool down to room temperature. The precipitate was collected by using centrifugation for 15 min at 8000 rpm, after which it was washed twice with acetone to remove the excess oleylamine. The collected precipitate was then magnetically stirred with acetone for 2 h to re-disperse the agglomerated nanoparticles, subsequent to which it was dried at room temperature. After drying, the powder was suspended in solvents such as toluene, hexane, and chloroform for the analysis of its characteristics (see Fig. 1). 2.3. Characterization of tungsten bronze nanoparticles The crystal structures of the tungsten bronze nanoparticles were identified by X-ray diffraction (XRD, Bruker-AXS NEW D8-Advance). The morphology and structure of the samples were studied using field emission transmission electron microscopy (FE-TEM, FEI Tecnai G2 F30 S-Twin). The particle size distribution of the tungsten bronze nanoparticles was determined by Dynamic Light Scattering (Nano partica SZ-100 series, Horiba Scientific) at a temperature of 25 °C with a scattering angle of 90°. The compositions of the particles were estimated by energy dispersive spectroscopy (EDS, Thermo Scientific NORAN System 7) and X-ray photoelectron spectroscopy (XPS, hv = 1486.6 eV Al Ka). The existence and quantity of oleylamine on the surface of the tungsten bronze nanoparticles were investigated using Fourier transform infrared (FT-IR, OTSUKA ELS-Z) spectroscopy and thermogravimetric analysis (TGA, Scinco TGA N-100). Absorption and transmittance spectra were obtained using a spectrophotometer (JASCO V-670) in the range of 300–2100 nm. 3. Results and discussion 3.1. Crystal structure of tungsten bronze nanoparticles (TBN) The QTBN were successfully synthesized by mixing AMT (as the tungsten precursor), and cesium and sodium hydroxide (as the alkali metal precursors) in oleylamine, followed by heating at 250 °C for 2 h; that is, a mild reaction temperature and a short reaction time. The crystal structures of the resulting products were
Table 1 The molar ratios in which the elements were mixed for synthesizing cesium tungsten bronze and sodium cesium tungsten bronze nanoparticles based on 1 mol tungsten. Sample
CsyWOz NaxCsyWOz
Concentration (mmol) Na
Cs
W
0 0.11
0.33 0.22
1 1
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Fig. 1. Schematic diagram of the synthesis of a tungsten bronze nanoparticle capped with oleylamine.
examined by XRD and the XRD patterns are shown in Fig. 2(a). All the peaks are indexed to the cubic (Cs2O)0.44W2O6 (Fig. 2(b), PDF 01-047-0566, space group = Fd-3m, a = 10.304 Å, a = b = c = 90°). It should be noted that the product is a quaternary compound, and not a mixture of two ternary compounds of sodium tungsten bronze and cesium tungsten bronze. As shown in Fig. 2(c), the XRD pattern of a mixture of the ternary compounds at a mixing ratio (Cs0.33WO3:Na0.33WO3 = 2:1 by weight), which is similar to that of the precursors that were used to synthesize QTBN, has a peak characteristic of a cubic Na0.11WO3 (Fig. 2(d), ICDS ID
Fig. 2. XRD patterns of (a) quaternary tungsten bronze nanoparticles, NaxCsyWO3 (b) reference cubic (Cs2O)0.44W2O6 (c) the mixture of ternary compounds of Na0.33WO3 and Cs0.33WO3, (d) reference cubic Na0.11WO3, (e) Na0.33WO3, and (f) Cs0.33WO3.
11855) space group = Pm-3m. The pattern presents a sharp, strong peak for the (0 0 1) plane at 2h = 23.22°, which also appears in the XRD pattern of sodium tungsten bronze nanoparticles (Na0.33WO3, Fig. 2(e)). In comparison, the XRD pattern of QTBN displays a peak at 2h = 24.43°, which is assigned to the (2 2 0) plane, and which also appears in the XRD pattern of the cesium tungsten bronze nanoparticles (Cs0.33WO3, Fig. 2(f)). These results indicate that the QTBN prepared in this study is a compound, as opposed to a mixture, and that the crystal frame of QTBN consists of a cubic pyrochlore structure, similar to that of the cesium tungsten bronze nanoparticle. The reason for synthesizing the quaternary compound in the framework of the cesium tungsten bronze nanoparticle is justified as follows. First, the ammonium metatungstate hydrate decomposes into the thermodynamically stable structure of the metatungstate anion ([a-H2W12O40]6). This ion has a cubic pyrochlore structure that has three tunnel positions: 8b, 16d, and 32e. However, because of the instability of the structure, small alkali metal ions, such as Li+, Na+, and K+, roughly are intercalated into the 16d positions. Thus, ions with larger radii, such as Rb+ and Cs+, can be expected to first be inserted into the 8b and 32e positions to stabilize the structure, following which the small ions would be intercalated into the 16d positions. In this way, sodium cesium tungsten bronze retains the same structure as cesium tungsten bronze, because essentially, the small sodium ions are filling in the space that remains after the large cesium ions have been intercalated into tungsten bronze [19,20]. The morphology and structure of QTBN were further analyzed by using FE-TEM images and selected area electron diffraction (SAED) patterns. Fig. 3(a) shows the uniform structure of NaxCsyWOz particles that ranged in size from 10 to 30 nm, capped with oleylamine: the average particle size is 32.0 nm with a standard deviation of 17.1 nm. Fig. 3(b) reveals that the d-spacing is 0.3106 nm, which is in agreement with the interplanar spacing of the (3 1 1) plane from the XRD pattern. The SAED pattern in Fig. 3(c) can be indexed to the (1 1 1), (2 2 0), and (3 1 1) planes, which is consistent with the XRD data. (See also Fig. S1 FE-TEM images and SAED pattern of cesium tungsten bronze nanoparticles.)
3.2. Nanoparticle compositions The composition of the tungsten bronze nanoparticles was determined using EDS analysis and XPS spectra of the core level
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Fig. 3. (a) FE-TEM image of NaxCsyWOz nanocrystals; (b) high magnification FE-TEM image with a lattice spacing; (c) SAED pattern and (d) particle size distribution.
tungsten (W4f). The EDS analysis shows that cesium tungsten bronze has the formula Cs0.25 WO3.44 and that of sodium cesium tungsten bronze is Na0.09Cs0.17WO3.34. (The calculated data are summarized in Table S1.) The XPS spectra shown in Fig. 5 were fitted as two spin–orbit doublets, W4f5/2 and W4f7/2, with a separation interval of 2.3 eV. The peaks at 33.45–35.75 eV (CsyWOz, Fig. 5(a)) and 33.28–35.58 eV (NaxCsyWOz, Fig. 5(b)) are attributed to W5+, while the peaks at 34.86–37.16 eV (CsyWOz, Fig. 5(a)) and 34.48–36.75 eV (NaxCsyWOz, Fig. 5(b)) are assigned to W6+. Tungsten trioxides have a charge of 6+ at first and obtain free electrons from the alkali metals, thereby generating reduced tungsten (W5+) coexisting with W6+. Thus, the estimation of the amount of doped alkali metal was carried out by integrating the area of the W5+ peak in the XPS spectra using Eq. (1). The intercalated amounts of alkali metals in ternary and quaternary tungsten bronze are 0.259 and 0.266, respectively, which is in good agreement with the EDS results. The binding energies of the metal fractions x and y are summarized in Table 2. It should be noted that the mixing ratios of the precursors in our experiments were
designed to synthesize nanoparticles with formulas of Cs0.33WO3 and Na0.11Cs0.22WO3, but the calculated and experimental values were found to differ from each other. This difference is attributed to the low reaction temperature used in our solution process. Similar to the process using hydrothermal synthesis [21], it is suggested that a reaction temperature of 250 °C cannot generate enough energy to entirely activate alkali metals to enter the tungsten trioxide framework (see Fig. 4).
W6þ O3 ! Mx W1x 6þ Wx5þ O3
ð1Þ
3.3. Characterization of oleylamine capping The confirmation of the existence of the oleylamine coating on the surface of tungsten bronze nanoparticles was conducted by analyzing the FT-IR spectra (Fig. 6) and TGA thermograms (Fig. 7). Detailed assignments of the bands are summarized in Table S2. Among the many bands related to WO3 and oleylamine,
Fig. 4. EDS analysis of (a) CsyWO3, (b) NaxCsyWO3 nanoparticles.
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Fig. 5. W4f core-level XPS spectra of (a) CsyWO3, (b) NaxCsyWO3 nanoparticles.
Table 2 Binding energies of the two spin–orbit doublets, W4f5/2 and W4f7/2, and molar ratio of alkali metal in tungsten bronze as attributed to W5+ and W6+. Sample W5+ W6+ Total alkali metal NaxCsyWO3 fraction W4f5/2(eV) W4f7/2(eV) W4f5/2(eV) W4f7/2(eV) CsyWOz 33.45 NaxCsyWOz 33.28
34.86 34.48
35.75 35.58
37.16 36.78
0.259 0.266
the band at a wavenumber of 1509 cm1 is absent from the FT-IR spectra of pure oleylamine [22] and results from the vibration of the WAN bond. Thus, it is clear that oleylamine is attached to the surface of the tungsten bronze nanoparticles. The amount of oleylamine capping the surface of the tungsten bronze was estimated by recording a TGA thermogram in the temperature range 100–800 °C. The thermogram is shown in Fig. 7 and presents two regions of mass reduction in the temperature ranges 100–300 °C and 500–700 °C. The change in the lower temperature region indicates the removal of the oleylamine hydrocarbon chains, which is followed by the removal of the nitrogen head groups at 500–700 °C [23]. After removal of the capping agent, the residual weight percentages of cesium tungsten bronze and sodium cesium tungsten bronze are 82.25% and 83.20%, respectively. Therefore, those of oleylamine on the tungsten bronze nanoparticles are estimated as 14.75% and 16.80%, respectively.
Fig. 7. TGA thermograms of (a) CsyWOz and (b) NaxCsyWOz nanoparticles.
3.4. Proposed reaction mechanism Considering the results all together, the following reaction mechanism is proposed for the synthesis of the tungsten bronze nanoparticles in oleylamine medium. Eq. (2) represents the overall reaction during the thermal decomposition of AMT. The reaction mixture consisting of oleylamine and the precursors is opaque in the form of a slurry and becomes transparent upon reaching 100 °C because of the dissolution of the precursor as represented by Eq. (3). Between 190 °C and 220 °C, water and ammonia are gradually produced as shown in Eqs. 4–6. At 250 °C, the alkali metal ions start being intercalated into the tungsten trioxide lattice (Eq. (7)) and the translucent solution turns blue after 30 min because of the reduction of tungsten trioxide (Fig 8). The role of oleylamine in our study is to surround the dissolved aqueous mixture of precursors by the hydrophilic amine functionality and to homogeneously disperse the matter as small particles within the oleylamine medium. As a result, the aqueous nanodispersion becomes a collection of nanometer-scale reactors with the ability to synthesize tungsten bronze nanoparticles. Moreover, the presence of the oleylamine ensures the stabilization and prevents aggregation of the tungsten bronze nanoparticles after the reaction [24].
ðNH4 Þ6 H2 W12 O40 xH2 O ! ðx þ 4ÞH2 O þ 6NH3 þ 12WO3 100 C
ðNH4 Þ6 H2 W12 O40 xH2 O ! xH2 O þ ðNH4 Þ6 H2 W12 O40 Fig. 6. FT-IR spectra of (a) CsyWO3 and (b) NaxCsyWO3 nanoparticles.
ð2Þ ð3Þ
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ðNH4 Þ6 H2 W12 O4 ! ðNH4 Þ6 O3 þ H2 O þ 12WO3 190220 C
ðNH4 Þ6 O3 ! 6NH3 þ 3H2 O 190220 C
241
ð4Þ ð5Þ
NHþ4 ! NH3 þ Hþ
ð6Þ
Mx þ WO3 ! Mx WO3
ð7Þ
To gain a deeper understanding of the intercalation of alkali metals into the framework of tungsten trioxide, we monitored the variation in the optical properties of QTBN by measuring its absorption and transmittance spectra as a function of time (Figs. S2 and 8). Initially, when the reaction temperature reaches 250 °C, no absorption was detected in the near-infrared region, with the exception of an absorption band at 300 nm, which indicates a transition due to the band gap. After 0.5 h at 250 °C, weak absorption appears in the NIR region, accompanied by transmittance in the visible light region, which means that the intercalation of alkali metals into the lattice has commenced. The typical optical properties of QTBN, namely, a dark blue color and absorption over a wide NIR region, were observed after 1 h of reaction at 250 °C as shown in the inserted photographs in Figs. 8 and S2, and after another 1.5 h, additional intercalation of alkali metal ions improved the NIR cutoff properties (Fig. 8). These results suggested that the optical properties varied with time, due to the onset of the formation of tungsten bronze, caused by the intercalation of alkali metal. The tungsten oxide particles transforms into a tungsten bronze particle after 2 h at a steady temperature of 250 °C. 3.5. Optical properties of tungsten bronze nanoparticles The difference in the optical properties of the ternary and quaternary tungsten bronze nanoparticles was examined by measuring the UV–vis spectra (Fig. 9(a)). Both the ternary and quaternary tungsten bronze nanoparticles have a narrow absorption band near 300 nm, which is attributed to the interband transition caused by the excitation of an electron from the oxygen 2p to the tungsten 5d orbital [25,26]. Cesium tungsten bronze shows a wide absorption band that starts at 830 nm and has a maximum absorbance at 1602 nm. The absorption at 830 nm is considered to be caused by polarons from localized electrons in the band structure, whereas the plasmon excitations of free electrons donated by the alkali metals are observed at 1602 nm [27–29]. Upon addition of sodium to cubic cesium tungsten bronze, the
Fig. 9. (a) Absorption and (b) transmittance spectra of tungsten bronze nanoparticles. The inserted image shows nanoparticles dispersed at 0.18 wt% in toluene. The spectral disturbances between 1620 nm and 1790 nm are due to the toluene solvent.
Na0.09Cs0.17WO3.34 strengthens the absorption at 1600 nm and broadens that around 1112 nm. The first of these phenomena is due to an increased abundance of free electrons in quaternary tungsten bronze. Even though the two bronzes have the same pychlore crystal structure, unlike cesium tungsten bronze, sodium cesium tungsten bronze contains sodium ions that occupy the free space that remains after the cesium ions have been intercalated. This results in a larger number of free electrons in the conduction band of tungsten bronze, a result that correlates well with the XPS results. The second phenomenon can be explained by the introduction of an additional localized level in the W5+, created by the sodium ions, which influences absorption in the range of 780–1200 nm [30]. Fig. 9(b) shows that the transmittance spectra of the tungsten bronze nanoparticles are closely related to the absorption spectra. The ternary tungsten bronze is responsible for 57% of maximum visible light transmittance at 474 nm and the lowest transmittance in the NIR region at about 10.6%. On the other hand, the quaternary tungsten bronze shows approximately 90% of NIR cutoff with 64% of maximum visible light transmittance at 432 nm. The additional doping of the cesium tungsten bronze highly improves the shielding property in the range of 780–1200 nm.
4. Conclusion Fig. 8. Transmittance spectra and color change (the inserted image) of quaternary tungsten bronze nanoparticles with different reaction time at 250 °C for (i) 0, (ii) 0.5, (iii) 1, (iv) 1.5, (v) 2.0 h.
A quaternary compound of sodium cesium tungsten bronze nanoparticles NaxCsyWOz was successfully synthesized in solution via a simple process by using oleylamine as both the capping agent
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and solvent at a mild reaction temperature for a short reaction time without a mechanical milling process. Our solution-based, one-pot process simply requiring the precursors to be mixed with oleylamine suggests an advanced synthetic route, which is less hazardous and more cost-effective, for preparing nanoparticles of novel tungsten bronze NIR absorbents. Most importantly, as-synthesized nanoparticles are already in the reduced state; hence, no further reduction process is necessary. It was demonstrated that the quaternary tungsten bronze nanoparticles, with an average diameter of about 32 nm, had a cubic pychlore crystal structure. The sodium ions were intercalated into the unfilled spaces of the cesium tungsten bronze nanocrystals, resulting in an NIR absorption that is broader and stronger than that of the equivalent ternary compounds of cesium tungsten bronze nanoparticles. The new materials have significantly improved NIR absorption and shielding properties, resulting from the enrichment of the particle surface with free electrons and the contribution from the reduced localized W5+ ions. It should be mentioned that a 5 times scaled-up synthesis of QTBN required a long reaction time of 60 h to gain optical properties comparable to original results, as shown in Fig. S3, suggesting that careful control over process parameters such as heat transfer might be necessary for mass production. Acknowledgements This research was financially supported by the Chung-Ang University Excellent Freshman Scholarship and by Grants from the Korea Research Foundation (Grant No. 2014R1A1A3049867), Republic of Korea. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2015.06.101. References [1] L.S. Long, H. Ye, Y.F. Gao, R.Q. Zou, Performance demonstration and evaluation of the synergetic application of vanadium dioxide glazing and phase change material in passive buildings, Appl. Energy 136 (2014) 89–97. [2] B. Baloukas, J.M. Lamarre, L. Martinu, Electrochromic interference filters fabricated from dense and porous tungsten oxide films, Sol. Energy Mater. Sol. Cells 95 (3) (2011) 807–815. [3] J.-D. Guo, M.S. Whittingham, Tungsten oxides and bronzes: synthesis, diffusion and reactivity, Int. J. Mod. Phys. B 7 (23–24) (1993) 4145–4164. [4] C. Guo, S. Yin, T. Sato, Effects of crystallization atmospheres on the nearinfrared absorbtion and electroconductive properties of tungsten bronze type MxWO3 (M = Na, K), J. Am. Ceram. Soc. (2012). [5] M. Green, Z. Hussain, Optical properties of dilute hydrogen tungsten bronze thin films, J. Appl. Phys. 69 (11) (1991) 7788–7796. [6] O. Schirmer et al., Dependence of WO3 electrochromic absorption on crystallinity, J. Electrochem. Soc. 124 (5) (1977) 749–753. [7] G. Liu et al., Electrostatic-induced synthesis of tungsten bronze nanostructures with excellent photo-to-thermal conversion behavior, J. Mater. Chem. A 1 (35) (2013) 10120–10129.
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