Pairing of Luminescent Switch with Electrochromism for Quasi-Solid

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Pairing of Luminescent Switch with Electrochromism for QuasiSolid-State Dual-Function Smart Windows Zhenguang Wang,† Minshen Zhu,*,‡ Siyu Gou,† Zhou Pang,† Yue Wang,† Yibo Su,† Yang Huang,§ Qunhong Weng,‡ Oliver G. Schmidt,‡,∥ and Jianzhong Xu*,† †

College of Chemistry and Environmental Science, Hebei University, Baoding 071002, P. R. China Institute for Integrative Nanosciences, IFW Dresden, Dresden 01069, Germany § College of Materials Science and Engineering, Shenzhen University, Shenzhen 518000, P. R. China ∥ Material Systems for Nanoelectronics, Technische Universität Chemnitz, Chemnitz 09107, Germany

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ABSTRACT: Smart window is a promising green technology with feature of tunable transparency under external stimuli to manage light transmission and solar energy. However, more functions based on the intelligent management of the solar spectrum need to be integrated into present smart windows. In this work, a dual-function smart window is fabricated by pairing the luminescent switch with the electrochromic window. The dual function is based on a single fluorine doped tin oxide coated glass functionalized with tungsten oxide and copper nanocluster, among which tungsten oxide serves as an electrochromic material and copper nanocluster provides photoinduced luminescence. Along with the regulation of the visible light based on the electrochromism of the window, the luminescence can be finely switched on and off, which establishes a pair of reversible states (“on” and “off”) for the dual-function smart window. The contrast between two states reaches 88%. Furthermore, the manipulation of dual-function smart window is highly reversible with a short response time of 12.6 s. This prototype of dual-function smart window paves the way for developing multifunctional smart windows by integrating different functional materials into one smart window based on the rational management of the solar spectrum. KEYWORDS: smart windows, electrochromism, luminescent switch, tungsten oxide, copper nanoclusters

1. INTRODUCTION Smart windows, with feature of reversible switching between transparent and opaque state under external stimuli, promise to work in energy-efficient buildings, self-dimming mirrors, and information displays.1−3 Three types of smart windows are categorized by their operating mechanisms: electrochromic,4−8 thermochromic,9,10 and photochromic windows,11,12 which reversibly change their transmittance under an altered electric field, temperature, and light intensity, respectively. Among these three smart windows, electrochromic windows have attracted most attention due to their advantages in broad working spectral range, energy saving, and high controllability and durability.13,14 Specifically, the smart concept for electrochromic windows is based on their unique property of dynamically regulating the visible and near-infrared light, thereby modulating the light transmittance and solar heat. In addition, the significant progress achieved in the quasi-solidstate electrolyte with high ionic conductivity and stability further enhances the practicability of electrochromic windows in real applications.8,15−19 With this single artificial functionality, however, the electrochromic windows cannot perfectly interpret the “smart” concept. Therefore, it is highly desirable to integrate additional functionalities (i.e., temperature © XXXX American Chemical Society

controller, electricity generator, or lighting) by artful management of solar spectrum and conversion of solar energy.20−25 In addition to the visible and near-infrared light, ultraviolet (UV) radiation is also an important component of solar radiation, accounting for around 5% of the total solar radiation.26 Moreover, UV light has the ability to trigger chemical reactions and excite fluorescence of materials, resulting in its extensive application scenarios.27 For example, the self-powered electrochromic window was constructed by harvesting near-UV light to produce electricity and power the electrochromic window, which avoided the competition for the same spectral range.28 This prototype research sheds light on the promising future in fabricating real smart windows by innovating the configuration of smart windows based on the intelligent management of the UV light in the solar spectrum. In addition to wide application based on the conversion of UV light to electricity, the luminescent switch, converting the UV light to visible light by luminescent materials and then regulating the emitted visible light under external stimuli, also Received: June 28, 2018 Accepted: August 23, 2018 Published: August 23, 2018 A

DOI: 10.1021/acsami.8b10790 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) SEM image of as-deposited WO3, (b) transmittance and optical density (ΔOD) of WO3 films under bleached and charged state, (c) cyclic voltammetry (CV) curves under the scan rate of 10 mV/s in 0.5 M H2SO4, and (d) change in optical density with increasing charge density.

2. EXPERIMENTAL SECTION

holds much promise in sensors, optical data storage devices, and displays.29,30 The unique requirement of luminescent switch in modulation of the visible light by external stimuli perfectly matches the features of electrochromic windows. Therefore, the luminescent switch and electrochromic window can be blended together, which will greatly widen the application scenario of smart windows. Moreover, using an electrochromic window to modulate the emitted light from luminescent switches delicately resolves the intrinsic problems, such as irreversibility, low on/off contrast, and slow response, encountered by traditional luminescent switches. Herein, we proposed the first prototype of a quasi-solid-state dual-function smart window (DFSW) by integrating the electrochromic tungsten oxide (WO3) film with a thin photoluminescent copper nanocluster (Cu NC) film. Two states are established: the DFSW is highly transparent under “on” status (without external bias), at which the bright orange light can be observed under UV light due to the photoinduced luminescence of Cu NC films. Oppositely, under “off” status (with external bias), the DFSW turns to opaque owing to the electrochromism of the WO3 film. Dimming or even blocking of the emitted orange light under UV light is also observed due to the perfect match in the optical spectrum between the modulation of electrochromic WO3 and the photoluminescence (PL) of Cu NC. The switching between on and off status is highly reversible based on the reversible feature of the electrochromic window. This unique feature advances the development of the reversible photoluminescent switches. Moreover, the on/off contrast can reach 88%, whereas the response time is as short as 12.6 s, which is very competitive for both electrochromic window and luminescent switch. Therefore, the integration of electrochromic film and photoluminescent film into one smart window simultaneously realizes a DFSW, which expands the application scenario for the smart windows.

2.1. Materials. All chemicals including sodium tungstate dihydrate (Na2WO4), 30% aqueous solution of hydrogen peroxide (H2O2), nitric acid (HNO3), copper(II) nitrate [Cu(NO3)2], glutathione (GSH), and poly(vinyl alcohol) (PVA) with a molecular weight of 10 000 were purchased from Sigma-Aldrich. 2.2. Characterizations. Absorption/transmittance and photoluminescence (PL) spectra of the devices are recorded by a Cary 50 UV−vis spectrophotometer and Varian Cary Eclipse fluorescence spectrometer, respectively. Electrochemical measurements were performed on an electrochemical workstation (Multi Autolab M204, Metrohm Autolab B. V.). The morphologies of WO3 films and Cu cluster films were studied using an environmental scanning electron microscope (ESEM, FEI/Philips XL30), and a Philips CM 20 microscope operating at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ESCALAB-MKII 250 instrument (Thermo). The crystallographic measurements of WO3 films were performed on a Bruker D2 PHASER diffractometer equipped with Cu Kα irradiation. The time-resolved PL decays curves were recorded on a time-correlated single-photon counting setup. 2.3. Fabrication of WO3 Coating. WO3 was electrodeposited onto the surface of window substrate coated with fluorine tin oxide (FTO). Typically, 0.825 g of Na2WO4 powder was dissolved to 100 mL deionized water, followed by addition of 0.6 mL of H2O2 under stirring. Then, the pH of the above mixture was adjusted to 1.5 by using 3 M HNO3. Finally, the WO3-coated window was obtained after electrodeposition process under constant voltage (−0.65 V vs saturated calomel electrode) for 300 s. 2.4. Fabrication of Cu NC Film Coating. PVA was dissolved in deionized water at 80 °C with a concentration of 60 mg/mL, followed by the addition of 2.5 mL of GSH (0.1 M) aqueous solution. Subsequently, 0.5 mL of Cu(NO3)2 (0.1 M) was injected into the above polymer−GSH mixture under strong stirring. The above mixture (3 mL) was dropped onto the surface of cleaned window slides, which were kept in vacuum oven at 40 °C for 10 h, until transparent and bright emission films were obtained. 2.5. Fabrication of DFSW. To enhance the practicality of the DFSW, the quasi-solid electrolyte is synthesized. The PVA electrolyte is synthesized by mixing 10 wt % of PVA powder and 0.5 M H2SO4 at B


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Figure 2. (a) Schematic illustration for the fabrication process of Cu nanocluster film. (b) TEM image (the inset shows size distribution of Cu nanocluster), (c) high-resolution XPS spectrum of Cu 2p electrons, and (d) PL (orange) and PLE (blue) spectra of Cu nanocluster film. 80 °C. After cooling the solution to room temperature, the PVA electrolyte is sandwiched by a bare FTO glass and the one with WO3 and Cu NC films.

nm is observed for WO3-based smart window and higher than 65% of transmittance can still be achieved in the range of 400− 500 nm. After applying a small bias of 3.0 V, the device immediately changes to opaque. The transmission dramatically decreases to lower than 10% in the whole range from 400 to 800 nm. The optical density (ΔOD) is determined by the equation ΔOD = log(Tbleached/Tcharged), in which Tbleached and Tcharged are the transmittance under bleached and charged state, respectively. It can be clearly seen that a high ΔOD value (around 0.8) in the range of 400−800 nm can be calculated. Figure 1c shows the cyclic voltammetry (CV) curves of the asprepared WO3 film measured in aqueous electrolyte (0.5 M H2SO4). To better characterize the electrochemical process of the as-prepared WO3 film and avoid the effect of quasi-solidstate electrolyte, the aqueous electrolyte is utilized here. In general, the WO3 film will turn blue when it is catholically polarized. The color of the WO3 film will be bleached upon anodic polarization, thus returning to the transparent state.17 The total cathodic charge of the WO3 film reaches 0.45 mC/ cm2, indicating a high cation insertion ability that will ensure the high performance of the films acting as the electrochromic window. Another important parameter for the electrochromic window is color efficiency, which represents the change in optical density per unit of inserted charge. As shown in Figure 1d, the color efficiency is 16 cm2/C, further revealing the high performance of WO3 in modulation of the visible light. 3.2. Fabrication and Characterization of the Cu NC Film on DFSW. Further utilization and regulation of the light can be realized by integrating the transparent and functional film into the window. To utilize the UV light, the UV-excited photoluminescent film was fabricated by casting the precursor solution containing Cu NCs and poly(vinyl alcohol) (PVA) onto the backside of the FTO-coated glass. Figure 2a schematically illustrates the fabrication process of the photoluminescent layer. Specifically, the precursor solution, contain-

3. RESULTS AND DISCUSSION 3.1. Fabrication and Characterization of the WO3 Film on DFSW. Regulation of the solar light by a smart window is based on the electrochromism properties of WO3. Therefore, the electrochromic layer, WO3, was electrochemically deposited onto the surface of fluorine tin oxide (FTO) side of the FTO-coated glass. After electrochemical deposition for 300 s, a thin WO3 film is coated onto the FTO. The SEM image shows the disordered distribution of WO3 particles (Figure 1a). The transmission electron microscopy (TEM) image in Figure S1a confirms the particle structure of WO3 and suggests that WO3 nanocrystals are embedded in an amorphous matrix. This is further confirmed by the powder X-ray diffraction (XRD) patterns shown in Figure S1b. Two broad peaks can be identified in the XRD patterns, further elucidating the unique structure of WO3 with nanocrystals embedded in the amorphous matrix. This unique structure ensures efficient light modulation in the visible and infrared regions.6 To further investigate the chemical composition and oxidation state of the WO3 film, the XPS spectrum is recorded as shown in Figure S1c. A pair of doublet peaks located at 35.9 and 38.0 eV is observed (Figure S1d), which corresponds to W 4f7/2 and W 4f5/2 core levels of W atoms in the 6+ oxidation state, respectively.16 As an electrochromic material, the WO3 film can change from transparent to ultramarine due to the formation of blue center W5+ polarons and recover to transparent state under reversibly applied potential.15 Accordingly, the transmittance of solar light can be effectively modulated by tuning the voltage on the WO3-based smart window. As shown in Figure 1b, a transmittance of higher than 80% in the range from 500 to 700 C


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Figure 3. (a) Absorption of DFSW under bleached and charged states. (b) PL spectra of DFSW under on and off states. (c) Schematic illustration and the working principles of DFSW.

ing Cu2+, GSH, and PVA, was drop casted onto the window substrate, followed by the evaporation of water solvent until transparent films were uniformly coated on the window. During the film-formation process, clusters of Cu NCs were assembled with glutathione, which serves as both reduction agent to reduce Cu2+ into lower valence states (Cu0/Cu+) and stabilizer to protect the clustered Cu0/Cu+ from aggregation. In addition to the high uniformity and flatness of the Cu NC film (Figure S2a), the transparency of the as-prepared film in most of the visible-light range is larger than 80% (Figure S2b), indicating that the incorporation of this film into a smart window will not lead to significant compromise in the transmission of the smart window. The morphology structure of the Cu NC films is further evaluated by TEM measurement, which shows small sized copper clusters incorporated in the film (black dots shown in Figure 2b). The size distribution of the Cu NCs is revealed in the inset of Figure 2b. It is obvious that the size of Cu NCs is around 1 nm. The high-resolution XPS spectrum of Cu on the film further confirmed the presence of Cu NCs, where obvious peaks at 932 and 952 eV were assigned to Cu 2p3/2 and Cu 2p1/2 states of Cu(0)/Cu(I), consistent with the previously reported literatures (Figure 2c).31−33 A bright orange emission (centered at 600 nm) is observed on the PL spectra of the Cu NC film, with PL excitation (PLE) peaked at 330 nm, achieving a PL quantum yield of as high as 24% (Figure 2d). This is consistent with previous PL properties of Cu NCs with aggregation-induced emission, which is further confirmed by the PL lifetime decay results, with a long average lifetime of 18.5 μs (Figure S3).34−36 The huge Stokes shift (>250 nm) and the long PL lifetime (microseconds) indicate that the PL takes place through a phosphorescence mechanism, which is attributed to a ligand-to-metal charge transfer from GSH to the Cu atoms.36 3.3. Design and the Working Mechanism of DFSW. The critical requirement for blending the luminescent switch and electrochromic window together is the maximum spectral

overlap between the emission spectrum of the photoluminescent materials and the tunable absorption spectrum of the electrochromic window. As shown in Figure 3a, the absorption of visible light under bleached state of the WO3 electrochromic window is nearly zero, but it increases dramatically under a 3 V potential bias. Specifically, the absorption of light with the wavelength larger than 500 nm is very significant. As the result, the charged electrochromic window absorbs the emitted light from the Cu NC film (centered at 600 nm), thereby switching off the luminescence. Therefore, the function of the luminescent switch is incorporated into the WO3-based electrochromic window. Figure 3b shows the excellent light modulation ability of the WO3-based electrochromic window. Obviously, more than 88% of the light emitted from the Cu NCs film is absorbed, indicating an on/off contrast of 88% as the luminescent switch. In addition, the blue shift under the off state is observed. This is attributed to the absorption of light in the range of 550−800 nm, which strengthens the light intensity in the blue-green region. One step further, the intensity of the light can be smartly controlled by tuning the voltage applied, as shown on Figure S4, further displaying the smart feature of the WO3based electrochromic window in controllable modulation of the light emitted from the photoluminescent material. Based on the perfect overlap between the spectrum of photoluminescent emission and absorption through electrochromism, the pairing of luminescent switch and electrochromism can be achieved, which is demonstrated as a DFSW. Figure 3c schematically illustrates the structure and the working principle of the DFSW. Briefly, the device is fabricated by integrating WO3 film and copper cluster films on a FTO glass. The Cu cluster film serves as the photoluminescent layer, whereas the WO3 film serves as the light-modulation layer. To electrochemically regulate the light, the electrolyte layer (0.5 M H2SO4 in PVA) and the counter FTO electrode are hierarchically assembled. The PVA-based quasi-solid-state D


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Figure 4. Photographs of on and off states of smart windows under solar light (a) and UV light (b) with (top) and without (bottom) external bias, change in (c) transmittance (at 510 nm) and (d) PL intensities (at 600 nm) of two reversible transitions of DFSW, and reversibility of (e) transmittance (at 510 nm) and (f) PL intensities of DFSW.

3.4. Working Performances of DFSW. Figure 4a displays the electrochromic function of DFSW. Excellent transparency is observed under a nonbias status. After applying 3.0 V bias, the smart window changes to dark blue, showing the ability in regulating the solar light. Furthermore, a bright orange light excited by the UV light is observed (Figure 4b). The bright orange light then is blocked when we apply an external bias on the DFSW, unraveling its ability as a luminescent switch. The most important parameter for both functions (electrochromism and luminescent switch) in a smart window is the response time between the two states. Therefore, the response time of the as-fabricated smart window is studied by recording the transmission and the PL intensity of the device at 510 and 600 nm, respectively, at 0.2 s intervals during the charge and discharge processes (Figure 4c,d). Significant decrease in both transmission and PL intensity is promptly identified under an external bias. It only takes 5.0 s to achieve 50% PL intensity dimming, and total luminescent switch off is observed after

electrolyte with a high thermal stability at normal working temperatures and ionic conductivity (Figure S5) ensures high performance in regulating the light under external bias in practical applications. Before applying the external bias, the window is transparent. Visible light can pass through the window. Meanwhile, the orange light excited by the UV light will also penetrate the window. Therefore, the window is in on state. Under the external bias, the WO3 film changes to intensive blue, with a broad absorption spectrum covering the visible range. This can block the transmission of the visible light, presenting an ultramarine window. In this circumstance, either the visible light from the solar light or the orange light emitted by the Cu NCs film is absorbed. Therefore, this state is called the off state. Moreover, owing to the highly reversible feature of the electrochromic window, the repeatable transition between these two states can be easily and finely controlled by tuning the applied bias. E



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ACKNOWLEDGMENTS This work is supported by the research project of Hebei Province, Hebei University (801260201235), and Science Technology and Innovation Committee of Shenzhen Municipality (JCYJ20170818104224667).

charging for 12.6 s, which is comparable to most of the reported switches.1,9 The inverse process is also very fast, as depicted by the small response time of 3 s. This superlative performance in terms of response time for a change between on/off states is attributed to the superior ability of WO3 in regulating the visible light. For practical applications, the reversible response of smart window toward stimuli becomes a critical parameter. Statistical analysis of the change in transmission and PL intensity under charged and bleached cycles is presented in Figure 4e,f. Both the absorption and PL intensity show a highly reversible behavior. In each cycle, the intensity of the absorption and PL on the device drop sharply and are fully recovered under on and off status, respectively. Only minor fluctuation (

Pairing of Luminescent Switch with Electrochromism for Quasi-Solid

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