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Highly Efficient, Near-Infrared and Visible-Light Modulated Electrochromic Devices Based on Polyoxometalates and W18O49 Nanowires Hongxi Gu, Chongshen Guo, Shouhao Zhang, Lihua Bi, Tianchan Li, Tiedong Sun, and Shaoqin Liu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07360 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Nano

Highly Efficient, Near-Infrared and Visible-Light Modulated

Electrochromic

Devices

Based

on

Polyoxometalates and W18O49 Nanowires ‡



Hongxi Gu,1,2 Chongshen Guo,3 Shouhao Zhang,1 Lihua Bi,4 Tianchan Li,1 Tiedong Sun,1 Shaoqin Liu1,3* 1

State Key Laboratory of Urban Water Resource and Environment, School of Life Science and

Technology, Harbin Institute of Technology, Harbin 150080, China 2

Shaanxi Key Laboratory of Phytochemistry, College of Chemistry and Chemical Engineering,

Baoji University of Arts and Sciences, Baoji, Shaanxi 721013, P. R. China 3

Micro- and Nanotechnology Research Center, Harbin Institute of Technology, Harbin 150080,

China 4

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin

University, Changchun, 130012, P. R. China ‡ These authors contributed equally.

Corresponding authors: Prof. Dr. S. Q. Liu (E-mail address: [email protected] )

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KEYWORDS electrochromic device, polyoxometalates, W18O49 nanowires, near infrared, visible light

ABSTRACT

Over the past years the performance of electrochromic smart windows with the promising potential for significant energy savings has been progressively improved, however, the electrochromic windows are not yet to come into use at scale mainly because the electrochromic materials suffer from some significant drawbacks such as low coloration efficiency, slow switching time, bad durability and poor functionality. Herein, we fabricate the optically modulated electrochromic smart devices through sequential deposition of the crown-type polyoxometalates, K28Li5H7P8W48O184·92H2O (P8W48), and W18O49 nanowires. Unlike most reported electrochromic smart devices, the resulting P8W48 and W18O49 nanocomposites allow active and selective manipulation of the transmittance of near infrared (750-1360 nm) and visible light (400-750 nm) by varying the applied potential. Furthermore, thanks to stable nature of both P8W48 and W18O49 and precise structural control over the nanocomposites, the prepared electrochromic smart devices exhibit high efficiency, quick response and excellent stability.

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Currently, almost 41% energy consumption in the world is being used to maintain comfortable light and temperature in buildings and indoor spaces.1 Electrochromic devices, which exhibit reversible colour or transmittance change upon application of a modest driving potential, have shown a wide range of applications like information displays, self-dimming rear mirrors for automobiles, electronic papers, electrochromic e-skins, and so on.2-7 More interestingly, the smart windows based on such electrochromic devices are broadly believed to be excellent candidates for energy saving application because they have the excellent capability to reduce cooling and heating loads through actively controlling the amount of sunlight and solar heat entering buildings, automobiles and aircrafts.8-19 Thus, many inorganic and organic materials including transition metal oxides,4, 11, 13, 14, 20-26 viologen,27 conducting polymers28-32 etc. have been examined as the electrochromic materials, and considerable progress has been made. Unfortunately, the commercial applications of electrochromic smart windows are hindered by its high manufacturing costs impacted by the use of relatively costly transparent conductive oxide substrates, unsatisfactory charge capacity, low coloration efficiency and short-term durability etc., because existing materials suffer from the limitations related to functionality, cost and robustness. 6, 33-36 For example, organic electrochromic materials enable the devices being highly flexible and wearable, while their poor durability under light exposure prevents their real application. Inorganic electrochromic devices composed of transition metal oxides show improved device performance through structural doping and chemical modification, but they encounter the problem of material degradation associated with ion insertion.21, 22 In addition, advanced electrochromic materials, which may offer independent control over the transmittance of visible and near-infrared (NIR) light from the sun, are still being sought to meet the escalating demand of energy-efficient building.37, 38 These electrochromic smart windows are required to

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change their colour to dynamically adjust the daylight and solar heat input to the building or protect the user’s privacy during the daytime, but unfortunately most reported electrochromic materials can only control transmittance of either visible or NIR light. 4, 5, 8, 14, 39-42 To build the ideal electrochromic smart windows with multifunction, several strategies have been adopted to design the dual-band nanocomposites by integrating visible absorption materials and NIR absorption materials into one electrochromic device. 4, 43, 44As a representative example, “nanocrystal-in-glass” nanocomposites are fabricated via introducing tin-doped indium oxide nanocrystals into niobium oxide glass.4 An appealing feature of the “nanocrystal-in-glass” nanocomposites is that they can block NIR and visible light selectively and independently by varying the applied voltage, which provides the opportunity for acquiring smart electrochromic devices. However, the preparation method of “nanocrystal-in-glass” nanocomposites is complex, costly used and needs to carefully control both surface chemistry of nanocrystals and pore size between nanocrystals, because nanocrystal-electrolyte interfacial contact or ion motion within micropores would significantly affect the electrochromic performance.4, 13, 14, 45 In this work, we suggest a simply and universal method to conveniently integrate visible absorption materials and NIR absorption materials into an electrochromic nanocomposite using layer-by-layer (LbL) selfassembly technique. Over the past three decades, LbL self-assembly has been proved to be a facile and powerful method to precisely control over the nanocomposite structure and composition at molecule-/nano-scale. 46, 47 Moreover, this technique allows fabricating large area thin film without requirement of complex set-up. 31, 48-50 In detail, using the LbL approach, we combine the crown-type K28Li5H7P8W48O184·92H2O (P8W48) polyoxometalates (POMs) and ultrathin tungsten suboxide W18O49 nanowires into the nanostructured electrodes. It is wellknown that POMs clusters and W18O49 can undergo reversible and stepwise multi-electron

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transfer reactions and exhibit different colors after accepting electrons, and thus they have been used for preparing electrochromic film.6, 51-58 Moreover, both POMs and the oxygen-deficient stoichiometry of tungsten trioxide have good electrical conductivity and are easy to synthesis. Therefore, the materials and techniques proposed in this study should also yield significant cost reductions. Impressively, the resulting nanostructured electrodes exhibit fast and reversible dynamic control over solar radiation transmittance at near-infrared (NIR) (750-1360 nm) and visible wavelength (400-750 nm) by varying the applied potential. RESULTS AND DISCUSSION In the nanostructured electrodes, the electrochromic components are K28Li5H7P8W48O184·92H2O (abbreviated as P8W48) clusters and ultrathin W18O49 nanowires with an oxygen-deficient stoichiometry of tungsten trioxide (abbreviated as W18O49 NWs). The P8W48 anion is used as visible absorption materials because it is stable under a wide range of conditions and its reduced form exhibits strong absorption of visible light (the extinction coefficient of the eight-electron reduced form of P8W48 POM is up to 1.42× 105 dm3 mol-1 cm-1, comparable to that of organic dyes),59, 60 while the W18O49 NWs are excellent absorbers of NIR light. Figure 1a illustrates the structure of P8W48/W18O49 LbL nanocomposites. P8W48 clusters were synthesized according to the previously reported procedure.61, 62 The structure of the spherical P8W48 anion of ca. 2.0 nm in size was determined by 31P nuclear magnetic resonance (NMR) and Fourier transform infrared (FT-IR) spectroscopy (Figure S1 in the Supporting Information). The W18O49 NWs are chosen as NIR absorption materials. Ultrathin W18O49 NWs were synthesized using n-propanol as solvent and WCl6 as a tungsten source via a solvothermal process.63 The product displays a strong blue colour. Morphology characterization by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) shows that the

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obtained W18O49 NWs are comprised of entangled nanowires with an average length of several micrometers and a rather uniform width of 10-20 nm (Figure S2a and Figure 1b). Highresolution TEM (HRTEM) image indicates that the W18O49 NW is single-crystalline and the lattice fringes with the spacing of 0.38 nm are clearly discerned, agreeing with the (010) planes of monoclinic W18O49 crystal (inset in Figure 1b). Energy-dispersive X-ray spectroscopy (EDX) further confirms that there are only W and O elements in the sample (Figure 1c). The X-ray diffraction (XRD) pattern of W18O49 NWs well matches the monoclinic W18O49 (P2m, JCPDS card number: 712450) (Figure S2b), in which the lattice constants are α = 18.334, β = 3.786, and γ = 14.044 Å, respectively. The relatively higher intensity and narrowing of (010) peak imply that the W18O49 NWs preferentially grow along the [010] direction, consistent with HRTEM observation. In the Raman spectra of W18O49 NWs, the O-W-O bending modes and W-O stretching modes of W18O49 are distinguished at 100-400 nm and 600-900 nm, respectively (Figure S2c).64 Notably, the optical absorption spectrum of W18O49 NWs exhibits a strong absorption from 700 nm to 1360 nm (black curve in Figure 1d); after oxidized with oxygen, the colour of the W18O49 nanowire dispersion changes from deep blue to ivory (inset in Figure 1d), and the absorbance in the range of 400-750 nm and 750-1360 nm dramatically decreases from 39.87% and 77.57% to 19.75% and 12.32% (red curve in Figure 1d), respectively. The sharp colour change and the significantly reduced absorbance in the range of 700-1200 nm are attributed to incorporation of oxygen into W18O49 nanowire lattice, which gives rise to decrease in the carrier concentration. Hence, analogously to the previous reports, the strong absorption of W18O49 NWs in the NIR region is attributed to their localized surface plasmon resonance (LSPR) absorption, which is caused by the oxygen vacancies within W18O49 crystal lattice and high concentration of free electrons.63, 65-68

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To investigate whether combining the W18O49 NWs and the P8W48 anion can selectively modulate NIR and visible light, we first deposited the W18O49 NWs and the P8W48 anion, separately, onto the ITO-coated glass substrates using LbL self-assembly technique (Figure S3), and investigated the electrochromic behaviour of single active components. Since either W18O49 NWs

or

P8W48

anion

are

negatively

charged,

positively

charged

polyelectrolye,

poly(ethyleneimine) (PEI), is used as the electrostatic interaction partner for alternating deposition to form LbL multilayered film (abbreviated as (PEI/W18O49)n or (PEI/P8W48)n, where the subscript n denotes the deposition number). Furthermore, prior to deposition of (PEI/W18O49)n or (PEI/P8W48)n, polyelectrolyte multilayer of poly(sodium 4-styrenesulfonate) (PSS) and PEI is preformed on the ITO surface via LbL method in order to increase surface charge and affinity of ITO substrate.

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Figure 1. (a) Scheme of structure of P8W48/W18O49 LbL nanocomposites. (b) TEM and HRTEM images of W18O49 NWs. (c) EDX spectrum of W18O49 NWs. (d) Vis-NIR absorption spectra of an aqueous dispersion containing 1 mg/mL W18O49 NWs before and after oxidation with oxygen. Inset: photo of the aqueous dispersion before and after oxidation with oxygen. (e) Transmission spectra of [PSS(PEI/PSS)3(PEI/W18O49)30] under varying potentials. (f) Relative transmittance change for PSS(PEI/PSS)3(PEI/W18O49)30 film under different applied potentials (blue curve: NIR+Vis region (400-1360 nm), red curve: NIR region (750-1360 nm), black curve: visible region (400-750 nm) and photo images of [PSS(PEI/PSS)3(PEI/W18O49)30] film at 0.4 V, -0.6 V and

-1.0

V,

respectively

(versus

Ag/AgCl).

(g)

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Transmission

spectra

of 8

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[PSS(PEI/PSS)3(PEI/P8W48)20] film under varying potentials. (h) Relative transmittance change for PSS(PEI/PSS)3(PEI/P8W48)20 film under different applied potentials (blue curve: NIR+Vis region (400-1360 nm), red curve: NIR region (750-1360 nm), black curve: visible region (400750 nm) and photo images of [PSS(PEI/PSS)3(PEI/P8W48)20] film at 0.4 V, -0.6 V and -1.0 V, respectively (versus Ag/AgCl). The typical cyclic voltammetry of as-prepared [PSS(PEI/PSS)3(PEI/W18O49)30] film shows an excellent pseudocapacitive behaviour (Figure S4). A well-resolved set of peaks is discerned in the region of -0.185 V to -0.650 V versus Ag/AgCl, which is ascribed to proton and ion insertion/extraction during the charge/discharge process, the Li+ ion diffusion coefficients are calculated as 4.89 × 10-9 cm2 s-1. The intercalation of protons and electrons into W18O49 lattice leads to a colour change of the [PSS(PEI/PSS)3(PEI/W18O49)30] film, and thus a dark blue colour is observed when a negative bias potential is applied to the film (Figure 1f), while the ITOcoated glass substrates do not show any obvious change (Figure S5). Transmission spectra at a series of potentials for the [PSS(PEI/PSS)3(PEI/W18O49)30] film clearly indicate that the [PSS(PEI/PSS)3(PEI/W18O49)30] film can selectively block NIR light through a plasmonic electrochromic effect (Figure 1e).67-69 As displayed in Figure 1e, at a potential of 0.4 V (black curve), the maximum light transmission in the whole NIR region (750-1360 nm) and the visible range (400-750 nm) for the [PSS(PEI/PSS)3(PEI/W18O49)30] film is approximately 87.38% and 91.68%, respectively, implying good transparency in the bleached state. Applying a negative potential results in significant decrease of the maximum light transmission in the whole NIR region for the [PSS(PEI/PSS)3(PEI/W18O49)30] film. Moreover, the modulation of NIR light might be enhanced by increasing the applied negative potential. For instance, the maximum light transmission in the whole NIR region decreases to 60.01% at a potential of -0.2 V and 35.13% at

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a potential of -0.6 V, respectively (red curve in Figure 1f). In contrast, the optical transmission of the [PSS(PEI/PSS)3(PEI/W18O49)30] film in the visible range decreases quite slowly. The maximum light transmission in the visible range (400-750 nm) still keeps 83.43% at -0.2 V and 69.17% at -0.6 V (black curve in Figure 1f). Above results clearly indicate that the W18O49 NWs facilitate greater modulation of NIR transmittance. As for the [PSS(PEI/PSS)3(PEI/P8W48)20] film, its electrochemical behaviour was also recorded (Figure S6), and the corresponding Vis-NIR absorption spectra were summarized in Figure 1g. As shown in Figure S6, the electrochemistry of P8W48 proceeds through three successive, reversible eight-electron reduction waves in the potential range of 0 V to -1.0 V to form dark blue mixed-valence state species.59, 60 The reduced form of the P8W48 anion (Figure 1h) has a broad absorption feature in the visible range of 500-750 nm, which increases with the reduction degree. It is deduced from Figure 1g that for [PSS(PEI/PSS)3(PEI/P8W48)20] film, the maximum light transmission in the visible range of 500-700 nm decreases to 80.66% at -0.6 V and 52.65% at -1.0 V, respectively (black curve in Figure 1h); whereas the maximum light transmission in the NIR range of 700-1360 nm remains similar of 77.68% at -0.6 V and 70.21% at -1.0 V (red curve in Figure 1h), respectively. Obviously, P8W48 anions favour modulating the visible rather NIR light. Altogether, under the negative electrochemical bias, the W18O49 NWs could selectively block NIR light through a plasmonic electrochromic effect while the P8W48 anion might prevent most visible light. More intriguingly, comparing Figure S4 and Figure S6, one can see that the change in colouration of [PSS(PEI/PSS)3(PEI/W18O49)30] film commences at -0.185 V and finishes at -0.650 V, while the response for the [PSS(PEI/PSS)3/(PEI/P8W48)20] film starts at 0.600 V and ends at -0.950 V. In other words, the alternation in colouration of the P8W48 anion

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and W18O49 NWs occurs in complementary potential window. Therefore, combining these two materials and controlling the applied voltage will be expected to modulate NIR and visible light, respectively. As illustrated in Figure S7a, the P8W48 anion and W18O49 NWs are controllably integrated into the same thin film by LbL self-assembly technique. Figure S7b shows the UV– vis spectra of [PSS(PEI/PSS)3/(PEI/W18O49)30(PEI/P8W48)20]. With the number of W18O49 NWs layer increasing, the absorbance in the range of 750-1300 nm and 200-275 nm corresponding to W18O49 NWs is increasing. Further deposition of P8W48 POMs layers results in the increase in the absorbance in the range of 200-300 nm. The Vis-NIR spectra confirm the successful deposition of the P8W48 anion and W18O49 NWs. In three-electrode system, the electrochemistry of

[PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20]

nanocomposites-coated

ITO

electrode

proceeds through successive, reversible oxidation-reduction waves in the potential range of 0 V to -1.0 V (Figure S8). Impressively, Figure 2a & 2b manifest that the prepared [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film switches progressively between three optical states: transparent (0.4 V to 0 V), NIR-blocking (-0.2 V to -0.6 V) and broadband-blocking of visible and NIR (-0.6 V to -1.0 V). When the applied potential is positive than -0.2 V, the total transmittance of the [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film in the visible (400-750 nm) and NIR range (750-1360 nm) is almost same to be 73.95% and 74.45% (black curve in Figure 2a and blue curve in Figure 2b), respectively, implying a good transparency (the inset of Figure

2b).

After

applying

a

negative

bias

potential

of

-0.6

V,

the

[PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film presents a significant modulation of the NIR portion of the solar spectrum (~44.37% decrease in transmittance, red curve in Figure 2b), while the transmittance in the visible range only decreases by 21.30% (black curve in Figure 2b), suggesting that difference in the transmittance between visible and NIR light reaches 23.07%.

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This result demonstrates that the composite film facilitates favourable modulation of NIR transmittance in the potential range of -0.2 V to -0.6 V. When the applied potential is further switched to be negative than -0.6 V, both NIR and visible light are effectively blocked. For example, at -1.0 V, the transmittance of the film is only 8.68% in the NIR range and 22.67% in the visible range (Figure 2b), respectively, and the film appears a deep blue. Clearly, in the potential range of -0.6 V to -1.0 V, the modulation of the visible portion is slightly larger than that in the NIR range, because the transmittance in the visible and NIR light reduces 30.00% and 21.40%,

respectively.

These

results

highlight

that

the

[PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film could dynamically block the NIR light and visible light by varying the applied electrochemical voltage over a small range of 1.0 V.

Figure 2. (a) Transmittance spectra of [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film at different potentials. The applied potentials are 0.4V, 0 V, -0.2 V, -0.6 V, -0.8 V and -1.0 V (from up to down). Each spectrum is obtained after the potential is held for 4 min. (b) Relative transmittance change for [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film under different

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applied potentials (blue curve: NIR+Vis region (400-1360 nm), red curve: NIR region (750-1360 nm), black curve: visible region (400-750 nm) and the digital photographs of the film at 0.4 V, 0.6

V

and

-1.0

V,

respectively(c,

d).

In

situ

optical

response

of

[PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] multilayer when switching between -0.60 V (100 s), -1.0 V (100 s) to 0.4 V (200 s), whereby each step is measured at 1060 nm (c) and 500 nm (d). (e, f) Optical density variation with respect to the charge density at 1060 nm (red curve) and 500 nm (black curve). The applied potential is -0.6 V (e) and -1.0 V (f) for 100 s. In

order

to

investigate

the

optical

modulating

performance

of

[PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film from one state to another state, we further carried out chronoamperometry on the films in 0.10 M LiClO4 buffer. Here, the [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] multilayer coated ITO electrodes were held at a constant potential for certain time until equilibrium was reached. Figure 2c and 2d display the corresponding in situ transmittance of [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film at 1060 and 500 nm, respectively. When the applying potential is stepped from 0.4 V to -0.6 V, the [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film shows a 45.40% reduction in transmittance at 1060 nm while the transmittance at 500 nm only decreases by 11.80%. With continuous increase

of

the

applying

potential

from

-0.6

V

to

-1.0

V,

the

[PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film changes from NIR-blocking state to broadband-blocking state, showing further 16.00% and 18.30% decrease in transmittance at 1064 nm and at 500 nm, respectively. The switching speed and coloration efficiency (CE) can be determined quantitatively from Figure 2c and 2d. The switching speed is defined as the time required for the film to reach 90% of the maximum transmittance change between one state to another state. The switching time is estimated to be 86 s, 52 s and 26 s for transparent, NIR-

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blocking and broadband-blocking state, respectively, which is faster than that of vacancy-doped tungsten oxide and amorphous niobium oxide (180 s for NIR), WO3 nanoparticle (90 s for 670 nm) and WO3 nanorods (272 s for 632.8 nm).41, 70, 71 Meanwhile, the CE is defined as the change in optical density for a given charge-per-unit-area, which is calculated by the following equation: CE = ∆OD/∆Q = log(Tt/Tc)/ ∆Q Where Tt and Tc are the transmittance in the transparent and coloured states, respectively, and ∆Q is the charge per unit area. It deserves noting that as indicated in Figure 2e and 2f, the [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film exhibits a high CE of 287.49 cm2/C (-0.6 V) and 121.03 cm2/C (-1.0 V) at 1060 nm, and 42.31 cm2/C (-0.6 V) and 21.38 cm2/C (-1.0 V) at 500 nm, respectively, larger than previously reported values.41, 70-73 Apart from fast switching speed and high CE, the [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film also exhibits a very good cycling stability. The [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film sustains good three-band optical modulation characteristics, and the transmittance modulation at 1064 nm and 500 nm only reduces by 3.20% (Figure 3a) and 2.40% (Figure 3b) after being subjected to 500 cycles, respectively. Significantly, the excellent electrochromic performance of the [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film exceeds that of the previously reported materials (Table S1 in the Supporting Information). We suppose that the enhancements in optical contrast, switching speed and stability are related to stable nature of two electrochromic components

as

well

as

open

network

structure

of

the

[PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film, which facilitates ion insertion and extraction. SEM imaging on the [PSS(PEI/PSS)3 (PEI/W18O49)30/(PEI/P8W48)20] film confirms this hypothesis (Figure S9).

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Figure 3. Cycle performance of [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film at 1060 nm (a) and 500 nm (b) measured in 0.10 M LiClO4 buffer (pH 5.0) for 500 cycles. Armed

with

superior

electrochromic

[PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20]

film,

feature

we

attempted

to

of use

the

such

LbL

nanocomposites to develop a smart window that is capable of controlling the amount of sunlight and

solar

heat.

Thereafter,

a

sandwich-type

electrochromic

device,

comprising

a

[PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] multilayer-coated ITO working electrode plus 0.1 M LiClO4 electrolyte, was constructed (Figure 4a). As shown in Figure S10, the [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] nanocomposites-based smart windows show an irregular

shape

of

voltammetry

characteristic,

much

different

from

that

of

[PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] nanocomposites-coated ITO electrode in a threeelectrode

system

(Figure

S8).

Different

electrochemical

behavior

of

[PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] nanocomposites between three-electrode system and smart device may be attributed to their varied cell configuration, and diffusion path length of protons and Li+ in the electrolyte.74,

75

Therefore, according to cyclic voltammetry of

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smart

windows,

a

practical voltage of 2.0 V, -2.5 V and -2.8 V has been selected for investigating the ability of P8W48/W18O49 nanocomposite for controlling the amount of sunlight and solar heat. A 150 W Xenon lamp was used as solar simulator, and the real-time power density change of the transmitted light was measured with optical power meter at different bias potentials (Figure 4b). Figure 4c displays that the original power and power density are 425.8 mW and 136.5 mW/cm2, respectively. As the voltage across the electrochromic device reaches -2.5 V, the power and power density decrease to 222.9 mW and 72.6 mW/cm2, respectively. As the voltage further decreases to -2.8 V, a lower power density 52.8 mW/cm2 is obtained. Once the voltage increases to 2.0 V, the output power density recovers to the original value (136.7 mW/cm2). Power density change of the transmitted light through [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] smart window by varying the applied potential can be repeated for more than 100 times without any change. Simultaneously, the transmittance change is observed by visualization, indicating that the appearance of electrochromic device changes from transparent to light blue to deep blue when the applied voltage is changed from open circuit to -2.5 V to -2.8 V (Movie S1 in Supporting Information). To understand the origin of change of the optical property, an optical filter of 800 nm was covered on the Xeon lamp to only allow the NIR light passing through the device. It can be found that the power density decreases from 65.8 mW/cm2 (open circuit) to 30.9 mW/cm2 (-2.5 V) to 26.8 mW/cm2 (-2.8 V) (Figure S11 & Movie S2), revealing that the [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film mainly modulates NIR light in the potential range between 0 V to -2.5 V, and visible light is largely controlled in the potential range between -2.5 V to -2.8 V. As comparison, the mono-component film based smart window,

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[PSS(PEI/PSS)3(PEI/W18O49)30] or [PSS(PEI/PSS)3(PEI/P8W48)20] film, may only modulate NIR or visible light (Figure S12 and Figure S13).

Figure 4. Scheme (a) and operation (b) of a [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] film based smart window. (c) Power density change of the transmitted light through [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] smart window by varying the applied potential. The applied potential is open circuit, -2.5 V for NIR-blocking state, -2.8 V for broadbandblocking state and 2.0 V for transparent state. CONCLUSIONS In summary, we propose a strategy to design and fabricate high efficient, optically modulated electrochromic smart devices. In order to block NIR and visible light selectively and independently, the ultrathin films integrated of the crown-type POMs with visible light modulation and ultrathin W18O49 NWs with NIR modulation are prepared on ITO substrates by LbL technique. The prepared films can progressively switch between three optical states by varying a small electrochemical voltage: transparent (1.0 V - 0 V versus Ag/AgCl), NIRblocking (-0.2 V to -0.6 V) and broadband-blocking of visible and NIR (-0.6 V to -1.0 V). Remarkably, quantitative analysis discloses that the films possess high colouration efficiency of 287.49 cm2/C at 1060 nm and 42.31 cm2/C at 500 nm, switching rate of 86 s, 52 s and 26 s for transparent, NIR-blocking and broadband-blocking state, as well as excellent electrochemical

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stability of only 3.20% and 2.40% decay at 1060 nm and 500 nm after 500 cycles. It is worth mentioning that such excellent electrochromic performance is obviously superior to the previously reported results (Table S1 in the Supporting Information). Furthermore, we demonstrate that an electrochromic smart device using the W18O49/P8W48 nanocomposite can effectively control the amount of sunlight and solar heat. Although the visible light is also blocked in the potential range of -0.2 V to -0.6 V, the difference in the transmittance between visible and NIR light researches 23.07%. It might be possible to improve their performance by optimizing the type of electrochromic device (such as depositing W18O49 and P8W48 on separate ITO substrate), the concentration and the arrangement of W18O49 and P8W48 in the ordered films. Nevertheless, we believe that this work provides an effective approach toward development of next-generation multifunctional smart glass, which is able to efficiently and dynamically adjust the daylight and solar heat input to the buildings or automobiles. METHODS SECTION Materials. Tungsten hexachloride (WCl6), (3-aminopropyl)trimethoxysilane, polyethyleneimine (PEI, MW = 25 000), and poly(sodium 4-styrenesulfonate) (PSS, MW = 70 000) were purchased from Alfa Aesar. Tin-doped indium oxide coated glass (about 15 Ω/sq) was obtained from Leaguer Film Technology (Shenzhen). Other chemicals were commercially available with the highest purity, and Milli-Q-deionized water was used for all the experiments. Methods. Field-emission scanning electron microscope (FESEM, FEI Quanta 200F), highresolution transmission electron microscopy (HRTEM, FEI TECNAI), and X-ray diffraction (XRD, D/max-rb) were used to characterize the morphology and the crystallographic structures of W18O49 NWs. The transmittance and absorption of materials and films were measured using

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U-4100 UV/Vis spectrophotometer (Hitachi, Japan). The electrochromic and electrochemical measurements were performed with CHI860 electrochemical workstation (Shanghai Chenhua Instrument Corporation, China) and conducted in a three-electrode electrochemical cell containing 0.1 M LiClO4 aqueous solution, where the P8W48/W18O49 nanocomposites coated ITO electrode (with a geometric area of ~1×5 cm) was used as working electrode, an Ag/AgCl electrode (saturated KCl) was used as reference electrode and a Pt wire was used as counter electrode. Transmittance spectra were acquired in situ, under the applied voltage. A 150 W xenon lamp (CROWNTECH, USA) with a filter was utilized as the irradiation source. Optical

Switching

and

Electrochemical

Measurements

of

[PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] Nanocomposites Coated ITO Electrode. Optical switching properties at different voltage were studied in a quartz cuvettes of 10 mm path length, in which the P8W48/W18O49 nanocomposites coated ITO electrode (with a geometric area of ~1×5 cm) was used as working electrode, an Ag/AgCl electrode (saturated KCl) was adopted as reference electrode, a Pt wire was utilized as counter electrode, and 0.10 M LiClO4 aqueous solution (pH 5.0) was used as the electrolyte. The in situ transmission Vis-NIR spectra were recorded as function of the applied potential (0 V to -1.0 V versus Ag/AgCl (sat. KCl). As shown in Figure S5, it can be found that compared with glass, ITO-coated glass substrate exhibited a weak adsorption in the investigated wavelength range (400-1360 nm). However, its adsorption in the range of 400-1360 nm did not increase with increasing the applied potential. Based on above experiments, we think that the optical properties of ITO-coated glass substrates would not affect the

optical

switching

measurements

of

[PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20]

nanocomposites coated ITO electrode and smart windows when the pure ITO-coated glass substrates was used as control. Therefore, all the optical spectra provided in this study were

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obtained by substrating the spectrum of pure ITO electrode from the that of [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] nanocomposites coated ITO electrode at different applied potentials. Stability of [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] Nanocomposites Coated ITO Electrode. The electrochemical stability was evaluated by applying step potential between -0.6, 1.0 and 1.0 V versus Ag/AgCl (sat. KCl) for 500 times. Fabrication of [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] Nanocomposite-Based Smart Windows. To investigate the ability of P8W48/W18O49 nanocomposite for controlling the amount of sunlight and solar heat, the [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] nanocompositesbased smart windows were constructed as follows (Figure 3a): (1) The ITO glass used for smart windows was cleaned and functionalized with (3-aminopropyl)trimethoxysilane. The [PSS(PEI/PSS)3(PEI/W18O49)30/(PEI/P8W48)20] multilayer was then deposited on the ITO surface (the diameter of ITO substrate was 4.95 cm). (2) 0.1 M LiClO4 electrolyte was then spread on the multilayer-coated side of ITO electrode, subsequently a gasket with thickness of 500 µm and a bare ITO electrode was cleaned and put on the top of LiClO4 electrolyte. (3) Finally, a silicone sealant was used to seal the device. For comparison, [PSS(PEI/PSS)3(PEI/W18O49)30] and [PSS(PEI/PSS)3(PEI/P8W48)20] nanocomposite-based smart windows were also fabricated using above procedures. The ability of P8W48/W18O49 nanocomposites for controlling the amount of sunlight and solar heat was measured in the following procedures: A 150 W xenon lamp was used as a solar simulator, and an optical filter of 800 nm was covered on the Xeon lamp to lamp to only allow the NIR light passing through the device. After applying different potentials, the power density

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change of the transmitted light was measured by an optical power meter (PM100D, THORLABS). ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51372054 & 51572059), and HIT Environment and Ecology Innovation Special Funds (Grant No. HSCJ201618). SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website. Experimental details and Supporting Figures/Tables (Word) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions ‡These authors contributed equally. REFERENCES 1. Richter, B.; Goldston, D.; Crabtree, G.; Glicksman, L.; Goldstein, D.; Greene, D.; Kammen, D.; Levine, M.; Lubell, M.; Savitz, M.; Sperling, D.; Schlachter, F.; Scofield, J.; Dawson, J., How America Can Look within to Achieve Energy Security and Reduce Global Warming. Rev. Mod. Phys. 2008, 80, S1-S109. 2. Granqvist, C.-G., Electrochromic Materials: Out of a Niche. Nat. Mater. 2006, 5, 89-90.

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31. Cui, M.; Ng, W. S.; Wang, X.; Darmawan, P.; Lee, P. S., Enhanced Electrochromism with Rapid Growth Layer-by-Layer Assembly of Polyelectrolyte Complexes. Adv. Funct. Mater. 2015, 25, 401-408. 32. Jensen, J.; Hösel, M.; Dyer, A. L.; Krebs, F. C., Development and Manufacture of PolymerBased Electrochromic Devices. Adv. Funct. Mater. 2015, 25, 2073-2090. 33. Cannavale, A.; Cossari, P.; Eperon, G. E.; Colella, S.; Fiorito, F.; Gigli, G.; Snaith, H. J.; Listorti, A., Forthcoming Perspectives of Photoelectrochromic Devices: A Critical Review. Energ. Environ. Sci. 2016, 9, 2682-2719. 34. Granqvist, C.-G., Electrochromic Materials and Devices for Energy Efficient Buildings. In Nanotechnology for the Energy Challenge, Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2010; pp 435-458. 35. Jelle, B. P., Electrochromic Smart Windows for Dynamic Daylightand Solar Energy Control in Buildings. In Electrochromic Materials and Devices, Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2013; pp 419-502. 36. Khandelwal, H.; Schenning, A. P. H. J.; Debije, M. G., Infrared Regulating Smart Window Based on Organic Materials. Adv. Energy Mater. 2017, 7, 1602209. 37. Kraft, A.; Rottmann, M., Properties, Performance and Current Status of the Laminated Electrochromic Glass of Gesimat. Sol. Energ. Mat. Sol. C. 2009, 93, 2088-2092. 38. Lim, S. H. N.; Isidorsson, J.; Sun, L.; Kwak, B. L.; Anders, A., Modeling of Optical and Energy Performance of Tungsten-Oxide-Based Electrochromic Windows Including Their Intermediate States. Sol. Energ. Mat. Sol. C. 2013, 108, 129-135.

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