Pairing of Luminescent Switch with Electrochromism for Quasi-Solid

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Applications of Polymer, Composite, and Coating Materials

Pairing of Luminescent Switch with Electrochromism for Quasi-Solid-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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10790 • Publication Date (Web): 23 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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ACS Applied Materials & Interfaces

Pairing of Luminescent Switch with Electrochromism for Quasi-Solid-State Dual Function Smart Windows Zhenguang Wang,a Minshen Zhu,*,b Siyu Gou,a Zhou Pang,a Yue Wang,a Yibo Su,a Yang Huang,c Qunhong Weng,b Oliver G. Schmidt,b,d and Jianzhong Xu*,a

a

College of Chemistry and Environmental Science, Hebei University, Baoding, 071002, P. R.

China. b

Institute for Integrative Nanosciences, IFW Dresden, 01069, Dresden, Germany

c

College of Materials Science and Engineering, Shenzhen University, Shenzhen, 518000, P.R.

China. d

Material Systems for Nanoelectronics, Technische Universität Chemnitz, 09107, Chemnitz,

Germany.

KEYWORDS: smart windows, electrochromism, luminescent switch, tungsten oxide, copper nanoclusters

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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 electrochromic material and copper nanocluster provides photo-induced 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 the developing multifunctional smart windows by integrating different functional materials into one smart window based on the rational management of solar spectrum. 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,4,5,6-8 thermochromic,9,10 and photochromic windows,11,12 which reversibly change their transmittance under the altered electric field, temperature and light intensity, respectively. Among these three smart windows, electrochromic windows have attracted most of the attention due to their advantages in broad working spectral range, energy


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saving and high controllability and durability.13,14 In specific, the smart concept for electrochromic windows is based on their unique property in 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-solid-state 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 controller, electricity generator or lighting) by artful management of solar spectrum and conversion of the solar energy.20-25 In addition to the visible and near-infrared light, ultraviolet (UV) radiation is also an important component of the solar radiation, which accounts 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 of 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 solar spectrum. In addition to wide application based on the conversion of UV 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 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 features of electrochromic windows.


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Therefore, the luminescent switch and electrochromic window can be blended together, which will greatly widen the application scenario of smart windows. Moreover, using 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 photo-induced 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 emitted orange light under UV is also observed due to the perfect match in optical spectrum between the modulation of electrochromic WO3 and photoluminescence 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% while the response time is as short as 12.6 s, which is very competitive as 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. EXPERIMENTAL SECTION


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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 10000 were purchased from Sigma-Aldrich. 2.2. Characterizations. Absorption/transmittance and photoluminescence (PL) spectra of devices are recorded by a Cary 50 UV-visible 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, USA). 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 FTO. Typically, 0.825 g of Na2WO4 powder were dissolved into 100 mL deionized water, followed by adding 0.6 mL of H2O2 under stirring. Then, the pH of above mixture was adjusted to 1.5 by using 3 M HNO3. Finally, the WO3 coated window was obtained after electro-deposition process under constant voltage (-0.65 v vs. SCE) 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 adding of 2.5 mL of GSH (0.1 M) aqueous solution.


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Subsequently, 0.5 mL of Cu(NO3)2 (0.1 M) was injected into above polymer – GSH mixture, under strong stirring. Above mixture (3 mL) was dropped onto the surface of cleaned window slides, which was 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 the 10 wt% of PVA powder and 0.5 M H2SO4 at 80 ˚C. After the solution cooling to room temperature, the PVA electrolyte is sandwiched by a bare FTO glass and the one with WO3 and Cu NC films. 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 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). TEM image in Figure S1a confirms the particle structure of WO3 and suggests the WO3 nanocrystals are embedded in an amorphous matrix. This is further confirmed by the powder 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 the efficient light modulation in visible and infrared region.6 In order 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


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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 color 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 nm is observed for the WO3 based smart window and higher than 65% of transmittance can still be achieved in the range of 400 to 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 (∆OD) is determined by the equation ∆OD = log(Tbleached/Tcharged), in which Tbleached and Tcharged are the transmittance under the bleached and charged state, respectively. It can be clearly seen that a high ∆OD value (around 0.8) in the range of 400 to 800 nm can be calculated. Figure 1c shows the cyclic voltammetry (CV) curves of the as-prepared 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-solid-state electrolyte, the aqueous electrolyte is utilized here. In general, the WO3 film will turn into blue color when it is catholically polarized. Upon the WO3 film is annodically polarized, the blue color will be bleached, thus returning to the transparent state.17 The total cathodic charge for the WO3 film reaches to 0.45 mC cm-2, indicating a high cation insertion ability that will ensure the high performance of films acting as the electrochromic window. Another important parameter for the electrochromic window is the color efficiency, which represents the change in optical density per


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unit of inserted charge. As shown in Figure 1d, the color efficiency is 16 cm2 C-1, 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. In order 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, containing Cu2+, GSH and PVA, was drop casted onto the window substrate, followed by the evaporation of water solvent until transparent films are 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 film (Figure S2a), the transparency of the as-prepared Cu NC film in most of visible light range is larger than 80% (Figure S2b), indicating that the incorporation of this film into the smart window will not lead to significant compromise in the transmission of the smart window. The morphology structure of 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 previously the reported literatures (Figure 2c).31-33 Bright orange emission (centered at 600 nm) is observed on the PL spectra of Cu NC film, with PL excitation


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(PLE) peaked at 330 nm, achieving a PL quantum yield 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 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 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 near zero, while the absorption dramatically increases under the 3 V potential bias. In specific, 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 Cu NC film (centered at 600nm), thereby switching off the luminescence. Therefore, the function of luminescent switch is incorporated into the WO3 based electrochromic window. Figure 3b shows the excellent light modulation ability of WO3 based electrochromic window. Obviously, more than 88% of 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 the light in the range of 550 to 800 nm, which strengthens the light intensity in blue-green region. One step further, the intensity of the light can be smartly controlled by tuning the voltage added, as shown on Figure S4, further displaying the smart feature of WO3 based electrochromic window in controllable modulating the emission light from photoluminescent material.


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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 demonstrates 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 FTO glass. The Cu cluster film serves as the photoluminescent layer, while the WO3 film serves as the light modulation layer. To electrochemically regulate the light, the electrolyte layer (0.5 M H2SO4 in PVA) and counter FTO electrode is hierarchically assembled. The PVA based quasi-solid-state electrolyte with high thermal stability at normal working temperatures and ionic conductivity (Figure S5) ensures the 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 at “on” state. Under the external bias, the WO3 film changes to intensive blue color, with broad absorption spectrum covering the visible range. This can block the transmission of the visible light, presenting an ultramarine colored window. At this circumstance, either visible light from solar light or the orange light emitted by the Cu NCs film is absorbed. Therefore, this state is called “off” state. Moreover, owing to the highly reversible feature of electrochromic window, the repeatable transition between these two states can be easily and finely controlled by tuning the applied bias. 3.4. Working performances of DFSW. Figure 4a displays electrochromic function of DFSW. Excellent transparency is observed under non-bias status. After applying 3.0 V bias, the smart window changes to dark blue color, showing the ability in regulating the solar light. Furthermore, a bright orange light excited by the UV light is observed (Figure 4b). The bright


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orange light then is blocked when we apply an external bias on the DFSW, unraveling its ability as the luminescent switch. The most important parameter for both functions (electrochromism and luminescent switch) in smart window is the response time between two states. Therefore, the response time of as-fabricated smart window is studied through recording the transmission and PL intensity of the device at 510 nm and 600 nm, respectively, every 0.2 s intervals during charge and discharge process (Figure 4c and d). Significant decrease in both of the transmission and PL intensity is promptly identified under the external bias. It only takes 5.0 s to achieve 50% PL intensity dimming, and totally luminescent switch off is observed after 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 response time for change between on/off states is attributed to superior ability of WO3 in regulating the visible light. For practical applications, the reversible response of smart window towards 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 and 4f. Both the absorption and PL intensity show a highly reversible behavior. In each cycle, the intensity of absorption and PL on device drop sharply and fully recovered under on and off status, respectively. Only minor fluctuation (

Pairing of Luminescent Switch with Electrochromism for Quasi-Solid

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