Electrochromic-Tuned Plasmonics for Photothermal Sterile Window

Jul 2, 2018 - Electrochromic materials are widely used in smart windows. An ideal future electrochromic window would be able to control visible light ...
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Electrochromic-Tuned Plasmonics for Photothermal Sterile Window Jingwen Xu,†,∥ Yong Zhang,‡,∥ Ting-Ting Zhai,§ Zeyu Kuang,‡ Jian Li,§ Yongmei Wang,‡ Zhida Gao,† Yan-Yan Song,*,† and Xing-Hua Xia*,§ †

College of Sciences, Northeastern University, 110004 Shenyang, China National Laboratory of Solid State Microstructures and College of Engineering and Applied Sciences and §State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, 210093 Nanjing, China

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S Supporting Information *

ABSTRACT: Electrochromic materials are widely used in smart windows. An ideal future electrochromic window would be able to control visible light transmission, tune building’s heat conversion of near-infrared (NIR) solar radiation, and reduce attacks by microorganisms. To date, most of the reports have primarily focused on visible-light transmission modulation using electrochromic materials. Herein, we report the fabrication of an electrochromic-photothermal film by integrating electrochromic WO3 with plasmonic Au nanostructures and demonstrate its adjustability during optical transmission and photothermal conversion of visible and NIR lights. The localized surface plasmon resonance (LSPR) of Au nanostructures and the broadband nonradiative plasmon decay are proposed to be tunable using both the electric field and the WO3 substrate. Further enhanced photothermal conversion is achieved in colored state, which is attributed to coupling of traditional visible-band optical switching with NIR-LSPR extinction. The resulted electrochromic-photothermal film can also effectively reduce the numbers of attacking microorganisms, thus promising for use as a sterile smart window for advanced applications. KEYWORDS: electrochromic, photothermal conversion, near-infrared light, localized surface plasmon resonance, sterile film

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However, the random mixing of the components in the active material blends resulted in complex transport pathways for the charges. The slow kinetics that was especially apparent for NbOx during discharge is undesirable for smart window applications. Moreover, few studies to date have focused on the potential of smart windows to provide highly effective and controllable photothermal conversion for energy savings.8 Motivated by these challenges, the present work proposes the coupling of traditional cation intercalation-induced Visband optical modulation of WO39−11 with NIR localized surface plasmon resonance (LSPR) extinction of metal nanostructure12 to gain a strong broadband nonradiative plasmon decay in the Vis-NIR range, thus achieving a smart window that offers satisfactory photothermal conversion. Metal-oxide semiconductors such as TiO2, WO3, and NbOx are capable of dynamically modulating LSPR via capacitive charging processes.5 This allows the metal oxide layer to support a tunable LSPR that interacts with the NIR light.3

nergy conservation is becoming an increasingly important tissue. Electrochromic materials, which change color dynamically upon application of electrochemical potential stimuli, are widely used in smart windows.1−4 An ideal smart window, which would be particularly useful in aircraft and in high latitude zones, should therefore be able to control both the transmittance of sunlight and the conversion of sunlight into localized heat to save energy. Near-infrared (NIR) light accounts for nearly half of the solar energy that usually propagates through windows, but it does not contribute effectively to energy savings in aircraft and buildings. The problem to be solved is how to integrate these two functions efficiently into a single smart window. To date, most reports on electrochromic devices continue to focus on improvement of the smart window’s transmission performance in the visible (Vis) light region, including the switching speed between the “on” and “off” states and the controllable transmission range. Comparatively recently, the studies of smart windows have extended to the NIR region.5−7 For example, Milliron found that charged NbOx-indium tin oxide nanocrystals could tune visible and NIR light transmission independently by varying the nanoparticle percentage. © 2018 American Chemical Society

Received: March 27, 2018 Accepted: July 2, 2018 Published: July 2, 2018 6895

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Figure 1. Morphology characterizations. Scanning electron microscopy (SEM) images showing (a) the top view and (b) the magnified view of a blank WO3 film (where the inset in part (a) shows a cross-sectional view of a bare blank WO3 film); (c) top view of Au nanoparticledecorated WO3 film; and (d) top view of Au nanoparticle- and nanorod-decorated WO3 film.

prepare plasmonic-electrochromic films on fluorine-doped tin oxide (FTO) glass, a 3D honeycomb-like oxygen-deficient tungsten oxide (WO3−x) film was first grown on FTO glass with a thickness of ∼5.2 μm (Figure 1a and b; for experimental details, please refer to the Supporting Information). The freshly prepared WO3−x film shows a strong reducing ability because of its oxygen deficiency.30 Au nanoparticles (AuPs, Figure 1c) can thus be synthesized directly on the tungsten oxide film via an in situ redox reaction between oxidative AuCl4− and weakly reductive WO3−x without additional reducing agents.31 The AuP size and density can be modulated via the metal salt concentration (Figure S1). X-ray photoelectron spectroscopy (XPS) analysis confirms the formation of Au metal on WO3, which is evident from the Au 4f signals at 83.9 and 87.4 eV (Figure S2a−c). X-ray diffraction (XRD, Figure S2d) pattern shows the monoclinic phase of WO3 (JCPDS 83-0950) and Au (most characteristically at 2θ = 23° and 45°). In the Raman analysis, the signals originating from WO3 substrate are enhanced significantly after Au grafting (Figure S3). Notably, the red shift of Raman peaks is also observed, which can be attributed to the more lattice defects in the resulted AuP/WO3 film.32 Our electrochromic-photothermal nanoarchitecture design is expected to present electrically controlled broadband absorption in the Vis-NIR range and thus provide effective photothermal conversion. The adsorption and scattering cross sections of noble metal nanoparticles are highly dependent on the nanoparticle shape, and Au nanorods have exhibited excellent surface plasmon field enhancement of absorption in the NIR band.33 To obtain an obvious plasmoninduced NIR light harvesting efficiency, we prepared the Au nanorods (Figure S4) and then anchored them to Au nanoparticles using a dithiol linker. The AuPs act as anchor points for grafting the Au nanorods. As shown in Figure 1d, the

Amorphous WO3, as one of the capacitive metal oxides, has been widely used in smart windows and devices because of its outstanding optical tunability in the visible range, fast switching speed, and excellent cycling lifetime.13−15 Gold nanostructures, when compared with other noble metals, exhibit strong, stable, and tunable LSPR absorption in both the visible and NIR bands that is dependent on the shape and size of the structures.16−20 In particular, the strongly absorbed NIR radiation (nonradiative plasmon decay)21 can subsequently be converted into heat because of the related electron−phonon and phonon−phonon interaction processes.17 To date, plasmons in gold nanostructures have been applied in photothermal ablation therapy22−24 and biological sensors25−27 and used as contrast agents for optical detection.28,29 In this work, we propose a hybrid structure by anchoring gold nanostructures into a three-dimensional (3D) porous WO3 film to realize the controllable photothermal conversion in a smart window. Based on both the experimental and simulation results, we demonstrate that there are two mechanisms in the electrochromic-photothermal materials that are keys for the highly enhanced photothermal conversion: (1) the cationinteraction process enhanced NIR LSPR extinction and (2) the strong dependence of LSPR performance on the states of the WO3 film. The as-prepared film can also effectively reduce the numbers of attacking microorganisms, thus promising for use as a sterile window for advanced applications.

RESULTS AND DISCUSSION Integrating 3D Honeycomb-like WO3 with Plasmonic Au Nanostructures. The proposed plasmonic-electrochromic film, as shown in Figure 1, consists of two components: a 3D honeycomb-like WO3 film acting as the electrochromic substrate and gold nanostructures (nanoparticles acting as anchors and nanorods acting as the NIR SPR amplifiers). To 6896

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Figure 2. Electrochromic performance. (a) Transmittance variations in the samples between their colored and bleached states when measured at ±0.8 V at a wavelength of 630 nm. (b) Digital photographs of the samples taken under different applied voltages in the bleached (+0.8 V), open circuit potential (OCP), and colored (−0.8 V) states. (c) Transmission spectra of the WO3, AuP/WO3, and AuPR/ WO3 film-coated FTO slides at OCP and colored states. (d) Relationship of transmittance at 915 nm with the applied electrochemical potential at a scan rate of 10 mV/s.

the electrochemical impedance spectroscopy (EIS, Figure S8) and resistivity measurements (Table S2) of the WO3, AuP/ WO3, and AuPR/WO3 films, the faster switching speeds found on the AuP/WO3 and AuPR/WO3 can be attributed to their better conductivity and faster ion diffusion in these films.35 The corresponding photographs of the WO3, AuP/WO3, and AuPR/WO3 film-coated FTO glasses during potential switching are profiled further in Figure 2b. In line with the in situ transmission curves shown in Figure 2a, the samples change to a deeper blue color when the applied voltage is switched from open current potential (OCP) to colored state. The colored efficiency (CE) of WO3, AuP/WO3, and AuPR/WO3 were calculated as 31.1, 86.9, and 88.1 cm2 C−1, respectively (please see the Experimental Section for the calculating details). Figure 2c shows the transmittance spectra of the WO3, AuP/WO3, and AuPR/WO3 film-coated FTO surfaces in OCP and colored states. When compared with WO3 and AuP/WO3, another enhanced block of NIR light is observed for the AuPR/WO3 film-coated FTO glass in the OCP state because of the SPR properties of the Au nanorods in the NIR region. Amorphous WO3 is well-known as an efficient light absorber in the colored state.36 As a result, definite light extinction over a wide wavelength range (in both the visible and NIR regimes) is observed for these WO3-based electrochromic samples in their colored state (−0.8 V). The visible light transmittance can be modulated continuously via the electrochemical bias (Figure

Au nanoparticle-Au nanorod (AuPR) junctions are located on the 3D porous WO3 film (the sample is denoted by AuPR/ WO3). Successful grafting of the Au nanorods was also confirmed by the enhanced absorption in the NIR range (Figure S5). Electrochromic Performance. As one of the most widely studied electrochromic materials, WO3 has demonstrated good optical tunability in the visible range.9 The tuning mechanism is based on electrochromism, which is a reversible change in optical transmittance that corresponds to electrochemical charging and discharging processes. The switching kinetics of smart windows during electrochemical modulation is thus an important concern. By considering the switching speed, environmental friendliness, operational features, and visiblelight transmittance differences at 630 nm, phosphate-buffered solutions (PBS, 0.1 M, pH 6.0) were chosen as the electrolyte for the subsequent investigation (Figure S6, Table S1). Additionally, the transmittance difference does not show obvious enhancement while applying a voltage more negative than −0.8 V (Figure S7). In this work, the −0.8 V was chosen as the voltage for switching to the colored state. Figure 2a shows the in situ visible (630 nm) transmittance of these films during pulse potential switching between the bleached state (+0.8 V vs Ag/AgCl) and the colored state (−0.8 V vs Ag/ AgCl). Obviously, both the AuP/WO3 and AuPR/WO3 films have faster switching speeds than the WO3 film.34 According to 6897

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Figure 3. NIR-SPR photothermal conversion. (a) Temperature elevations induced in WO3, AuP/WO3, and AuPR/WO3 film-coated FTO surfaces after irradiation by 915 nm laser irradiation in a 3 mL PBS solution for 300 s. (b) Temperature changes in the samples during 915 nm laser irradiation for 500 s in the colored states and when the laser was subsequently turned off. (c) Scattering spectra from single Au nanorod on WO3/FTO surface at different electrochemical bias levels in PBS. (d) Temperature elevations induced in WO3, AuPR/WO3 film-coated FTO substrate, bare FTO substrate, and AuPR nanojunctions after NIR laser irradiation at OCP for 300 s.

Figure 4. Simulation results. (a) Simulation of the LSPR field in solution at the interface between AuPR and WO3 at the OCP. (b) Simulated absorbance of AuPR on WO3 surface at OCP and at a bias of −0.8 V.

chromic coating-based tunable absorber for broadband light through application of different bias levels. NIR-SPR Photothermal Conversion. Given that this broadband light absorption range is a feature of the plasmonicelectrochromic effect, these coatings are promising for a next generation of smart windows that can convert solar light into thermal energy for heating of building interiors. To demonstrate this hypothesis, a 915 nm laser with energy

S9) because the electrochromic properties of the WO3 substrate are dependent on the electrochemical charging process. In addition, as shown in Figure 2d, a better-defined bias-dependent NIR transmission (at a scan rate of 10 mV/s and recorded at 915 nm) is observed for AuPR/WO3 films when compared with the results recorded for WO3 and AuPR/ WO3. These results indicate that the combination of electrochromism with NIR LSPR forms a plasmonic-electro6898

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ACS Nano density of 0.78 W cm−2 was used as an NIR light source to irradiate the samples. Our proposed AuPR/WO3 film, as expected, shows significantly improved photothermal conversion when compared with AuP/WO3 and WO3 film-coated FTO samples at OCP (Figure S10, Figure 3a). The solution temperature change induced by AuPR/WO3 reaches 24.5 °C after 300 s. This value actually exceeds the sum of the individual temperature changes produced by AuP/WO3 and WO3, which are 10.2 and 7.6 °C, respectively. This improved photothermal conversion can be attributed to the coupling effect between absorption from the plasmonic resonance and the WO3 substrate. First, strong absorption of the plasmon resonance in the NIR band occurs at the interface between the AuPR and WO3 substrate (see the simulation results in Figure 4a). Additionally, the absorption of WO3 is enhanced by the greatly increased localized field due to the localized plasmon resonance. In particular, as shown in Figure 3a, the photothermal conversion results exhibit further improvements in the colored state. For example, the solution heating induced by the photothermal effect of the AuPR/WO3 film reaches 37.8 °C in its colored state after 300 s, but the temperature change is only 24.4 °C in the bleached state and 24.5 °C at OCP. The same tendency is also noted for the WO3 and AuP/WO3 films. Obviously, a negative electrochemical voltage can improve the photothermal conversion efficiency. This temperature elevation increases with time (Figure 3b). For example, the temperature elevation induced by the NIR photothermal effect in AuPR/WO3 reaches 45.1 °C in the colored state after 500 s. Moreover, the temperature elevation is also related to the energy density of NIR laser. A lower photothermal conversion is obtained when the energy density of NIR laser is reduced (Figure S11). In the current case, we hypothesize that coupling of the electrochromic effect with the plasmonic effect enables efficient photothermal conversion. To investigate the enhancement mechanism of photothermal conversion in the colored state, we collected the scattering spectra of a single gold nanorod on a WO3/FTO substrate (WO3 coated on a FTO surface by tungsten sputtering followed by annealing at 300 °C in air) under application of various electrochemical bias levels (because the detection wavelength of the spectrometer is limited to 800 nm, we used a gold nanorod with average size of 44 × 100 nm and a ∼697 nm LSPR peak in these measurements; see Figure S12). As shown in Figure 3c, the LSPR peak shifts to a shorter wavelength (∼12 nm shift) and shows increased intensity under application of a negative bias (−0.8 V). The LSPR frequency (ωLSPR) is directly proportional to the bulk plasmon frequency (ωB), which is affected by the free carrier concentration (ωLSPR ∝ n ).17,18,37 When a negative potential is applied, the enhanced free carrier concentration will shift LSPR to a higher energy. For reference, the same bias is also applied to a gold nanorod on a bare FTO substrate without the WO3 layer. In this case, the LSPR peak exhibits a shift of only ∼6 nm to the higher energy. These results imply that coupling of the electrochromic substrate with a negative bias could lead to an improved LSPR at higher energies, which coincides with the earlier conclusions of Milliron and co-workers.3 In our proposed structure, the LSPR frequency modulation and increased peak intensity are both found to depend on the WO3 dielectric substrate, which is a well-known electrochromic material. To determine the attribution of the WO3 substrate in the photothermal conversion of AuPR/WO3, we peeled off all AuPR nanojunctions (i.e., junctions of Au nanoparticles with

Au nanorods) from one AuPR/WO3 sample (for experimental details, please refer to the Supporting Information). These AuPR nanojunctions show two absorption peaks at ∼520 nm and ∼810 nm (Figure S13). Figure 3d shows the temperature change curves induced by photothermal effects of AuPR/WO3, AuPR, WO3, and FTO substrates at OCP. Without the dielectric substrate, the AuPR nanojunction only shows a ∼9.6 °C elevation of the solution after NIR irradiation for 300 s. However, the temperature elevation induced by the AuPR/ WO3 film reaches ∼24.5 °C under the same experimental conditions. Additionally, the photothermal conversion of AuPR/WO3 can be further improved in the colored state with a temperature rise of 45.1 °C (Figure 3a). To confirm that most of the heat conversion comes from the photothermal effect and not resistance heating, we measured the solution temperature changes induced by AuPR/WO3 in the dark under application of a negative bias of −0.8 V (Figure S14a). The results show that the solution temperature is only increased by 0.2−0.4 °C, even after 500 s. These results suggest that (1) the combination of Au nanostructure with WO3 film makes the WO3-Au plasmonic absorbers suitable for NIR-induced photothermal conversion; and (2) capacitive electron accumulation in WO3 film could lead to blue shifts in the NIR extinction peaks while also increasing the NIR plasmonic response. Simulation LSPR Modes. To confirm these mechanisms, we numerically simulated the LSPR modes at various states of AuPR/WO3. Here, we focused on the LSPR peak in the NIR region and the AuPR was set on a WO3 rod. Apart from the plasmonic enhancement in the nanogap between Au nanoparticle and nanorod (Figure S15a), the LSPR mode shows an obvious increase at the interface between the AuPR nanojunctions and WO3 substrate (Figure 4a). The peeled AuPR nanojunctions (Figure S15) exhibit an absorbance peak at ∼840 nm. Our simulated spectra (Figure S15b) confirm that when these AuPR nanojunctions are in contact with the WO3 substrate, the absorbance peak shows an obvious red shift and increased NIR SPR resonance with a broader LSPR extinction peak at ∼943 nm at OCP. Additionally, the LSPR extinction peak is sensitive to changes in the free electron concentration in WO3 substrate. Compared with the LSPR extinction peak of AuPR/WO3 at OCP, the LSPR extinction peak subsequently shifts to a higher energy (∼917 nm, Figure 4b) under application of a negative bias of −0.8 V to the WO3 substrate. A similar wavelength shifting tendency is also observed experimentally using a single Au nanorod (Figure 3c). Because the LSPR peak wavelength is close to that of the NIR source (λ = 915 nm), a stronger NIR LSPR is then obtained under application of a negative bias, resulting in the significant photothermal conversion enhancement. Simultaneously, the electrochromic-plasmonic coatings show a dark color in the colored state (Figure 2a) that is a direct indication of broadband light absorption (Figure 2c). The strong nonradiative plasmon decay could concentrate light in such a nanoscale honeycomb structure (Figure 1a), thus leading to effectively localized window heating. As this dark color does not darken under higher negative potentials, the water temperature does not show additional elevations when the negative bias exceeds −0.8 V (Figure S14b). Antimicrobial Activity and Promising Applications of Plasmonic-Electrochromic Film. Antimicrobial surfaces are a potentially attractive function for smart windows and are of particular interest for use in health care-related fields and 6899

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Figure 5. (a) Antibacterial efficiencies of AuPR/WO3, AuP/WO3 and WO3 films for E. coli after NIR laser irradiation for 300 s. (b) Photographs of reproduction of E. coli bacteria colonies at 0, 24, and 48 h after bactericidal treatment (irradiated by NIR laser for 600 s) with a bare FTO slide (I) and the WO3 (II), AuP/WO3 (III), and AuPR/WO3 (IV) film-decorated FTO slides. (c) Digital photographs of the AuPR/WO3 film on FTO at the bleached state and the colored state.

public transport. To investigate this possibility, Escherichia coli (E. coli) bacterium, which is typically responsible for many infections in daily life,38−40 is used as a model system. Figure 5a shows the survival rates of bacteria after NIR light irradiation (λ = 915 nm, 0.78 W cm−2) for 300 s with the presence of WO3, AuP/WO3, and AuPR/WO3 films on FTO slides (the initial bacteria concentration was 105 colon mL−1, and bare FTO was used as the reference). All samples exhibit better bactericidal abilities in the colored state. The survival rates of E. coli were studied by the irradiation of 915 nm laser for different times in OCP and colored state with the presence of AuPR/WO3, AuP/WO3, and WO3 films (Figure S16). In particular, almost no cells at all survive when the AuPR/WO3 film is employed in colored state for 300 s. For comparison, survival rates of E. coli were also investigated by only using AuP or AuPR nanojunctions (peeled from the one piece of AuP/ WO3 or AuPR/WO3 sample) decorated FTO in OCP and colored state under NIR laser irradiation for the same time. As shown in Figure S17a, the survival rate of E. coli at OCP is ∼88.5−91.4%, which indicates that the photothermal effects from AuP and AuPR on FTO cannot effectively inhibit the growth of bacteria. Furthermore, as plotted in Figure S17b, the survival rate of E. coli is ∼88.7−91.7% at +0.8 V and ∼89.7− 93.8% at −0.8 V in dark, suggesting that in our system the voltage cannot provide the obvious bactericidal effect toward E. coli. These results demonstrate that most of the bactericidal effects of AuP/WO3 and AuPR/WO3 at OCP and colored state originate from their outstanding photothermal effects. We withdrew the bacterial bodies from AuPR/WO3, AuP/WO3, and WO3 films after 600 s of irradiation and subsequently cultured them for another 24 and 48 h for visual inspection of the antibacterial activity of the materials. As shown in Figure 5b, the AuPR/WO3 film shows the lowest long-term bacteria survival rate. These results indicate that the proposed AuPR/ WO3 film is promising as an effective antimicrobial coating. In Figure 5c, a large-scale homogeneous AuPR/WO3 film-coated FTO (30 mm × 40 mm) structure was successfully prepared using a larger reactor. The transparency of the resulting glass can be fully tuned to be completely opaque and dark via potential switching, which indicates that the homogeneous electrochromic-photothermal films can be produced on a large scale for use as multifunctional coatings in advanced smart window applications. Switching stability and large-scale producibility are also important concerns for smart windows. For the AuPR/WO3 film-coated FTO, the transmittance difference after 200

switching cycles is 18 MΩ). Preparation of Au Nanoparticle Decorated WO3 Films. FTOcoated glass substrates (15 mm × 30 mm) were cleaned by sonication for 10 min each individually, in toluene, acetone, and ethanol, and then dried in a stream of N2. The FTO slides were treated inside a plasma cleaning system for 20 min prior to use. To prepare 3D WO3 nanostructures, first, 0.5 g Na2WO4 was dissolved in 20 mL DI water, and the solution was adjusted by 1.0 M HCl until the precipitate appeared.41 Then, 1.0 mL of this resulting solution was dispensed on the surface of an FTO slide by spin coating (500 rpm), followed by annealing at 350 °C for 10 min. The above procedure was performed three times to construct a uniform seed layer. The WO2.72 nanostructures having a straight alignment were prepared according to a previously reported procedure.3 Briefly, 0.4 g of WCl6 was added to 20 mL of nonaqueous ethanol. The resulting 6900

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diameter spot on the sample) in the presence of stirring. The variation in temperature as a function of the irradiation time was monitored using a thermocouple thermometer with ±0.1 °C precision, and the temperature was recorded automatically for 5 s each. Antibacterial Tests. To evaluate the photothermal antibacterial performance of Au/WO3 nanofilms, E. coli (China Center of Industrial Culture Collection-CICC, Beijing) was chosen as the model bacteria. Before performing microbiological experiments, all glassware was sterilized by autoclaving at 121 °C for 20 min. For each antibacterial experiment, 100 μL bacterial liquid containing 105 colony-forming units per milliliter (CFU mL−1) of E. coli was dispersed in 3 mL PBS buffer in a quartz cuvette for antibacterial experiments. Samples were then inserted into the bacteria solution and subjected to irradiation with an NIR laser (915 nm, 0.78 mW cm−2) under continuous stirring for the desired/required time. Subsequently, the glass slides were used as control experiments. After irradiation, 100 μL of the bacteria solution was withdrawn and diluted serially with sterilized water to adjust the bacterial concentration thereby ensuring that the as grown bacterial colonies were legible. At this concentration, 100 μL of the treated solution was spread on a solid LB medium, and the colonies were counted to determine the survival bacterial numbers after being incubated at a constant temperature of 37 °C for 24 h. Meanwhile, 100 μL bacteria solution was incubated inside a liquid medium with continuously vibration (200 rpm) at 37 °C for 0, 12, and 48 h to observe the turbidity of the liquid to study the situation of bacteria reproduction. To present a case of better comparison, bactericidal efficiency of FTO film was also treated using the same procedure. The bactericidal efficiency was determined by comparing with the corresponding colony counts on the control sample (sterilized glass sheet). All the antibacterial experiments were repeated three times to obtain an average value. LSPR Scattering Spectra and Electrochemical Characterization. To study the LSPR spectra of Au on WO3 substrate, the LSPR scattering spectra of single Au nanorod were collected using a SP2556 spectrograph mounted on the microscope, and a 512B_excelon EMCCD was used as the detector (Princeton Instruments). The electrochemical system used in the present case is comprised of three electrodes. In this system, 0.1 M PBS solution was used as the electrolyte. A Pt wire and an Ag/AgCl wire were used as the counter and reference electrodes, respectively. The gold nanorod decorated WO3 films used as the working electrodes in electrochemical measurements were prepared by the following steps: First, a layer of ∼20 nm of tungsten was sputtered onto a clean FTO surface, followed by annealing at 300 °C for 60 min to form a layer of amorphous WO3. Afterward, 10 mL of Au nanorod probes was dropped onto the prepared WO3 film-coated FTO slide. After incubating for 10 min, the FTO slide was washed with deionized water and dried in a stream of N2. Numerical Simulation. The numerical simulation was performed by using the commercial software Lumerical FDTD Solutions. For studying the WO3 and gold nanostructures, a perfectly matched layer is employed as the boundary condition for the simulation studies. The permittivity of gold in the NIR regime is described by the Lorentz− Drude model. The geometrical parameters are set as follows. The radius of Au sphere is 25 nm, and the dimensions of Au rods and WO3 rods are obtained as 20 × 80 nm and 50 × 250 nm, respectively. Instrumentation. The morphologies of the fabricated inorganic layers were characterized using a field-emission scanning electron microscope (Hitachi FE-SEM S4800). The XRD patterns were acquired on an X’Pert X-ray diffraction spectrometer (Philips, USA) using an Mg Kα X-ray source. X-ray photoelectron spectra (XPS) were recorded on a Perkin−Elmer Physical Electronics 5600 spectrometer using Al K radiation at 13 kV as excitation source. The takeoff angle of the emitted photoelectrons was 45°, with a resolution of 0.1 eV, using the binding energy of C 1s signal (284.6 eV) as the reference. The binding energy of the target elements was determined at a pass energy of 23.5 eV with a resolution of 0.1 eV, and the binding energy of C 1s signal (284.6 eV) was used as the reference. The transmittance variation-time curves were measured on a UV−vis-NIR spectrometer (U-3900 Hitachi, Japan), and the in situ

bright yellow solution was transferred into a Teflon-lined stainless autoclave, and the FTO substrate covered with WO3 seeds were then dipped in the solution. Following this, the autoclave was sealed and treated in a muffle furnace at 180 °C for 2 h. After being cooled to room temperature, the WO2.72 film-coated FTO substrates were taken out and washed by DI water for three times. Subsequently, the samples were immersed in an aqueous solution of HAuCl4 for 10 min. The samples were then taken out, washed with DI water, dried in a stream of N2, and then kept inside an oven at 60 °C for 1 day for the complete conversion of AuP/WO3−x to AuP/WO3 before carrying out any further electrochromic and photothermal investigations. Preparation of Au Nanorods. Au nanorods were prepared by the seed-growth method.42 To prepare Au nanorods, first, the seed solution was prepared by the following method: 0.25 mM HAuCl4 and 0.1 M CTAB were dissolved in a 10 mL aqueous solution. Subsequently, 0.6 mL of NaBH4 (0.1 M) solution was rapidly added to the above solution under vigorous stirring (1500 rpm). The above solution mixture was continuously stirred until its color changed from yellow to light brown. Afterward, the solution was aged in dark for 30 min. To prepare the growth solution, 1.80 g CTAB and 0.308 g sodium oleate were dissolved in 50 mL DI water at 60 °C. The solution was cooled down to room temperature, and then 4.8 mL AgNO3 (4 mM) solution was added. The mixture was kept undisturbed for 15 min. In the next step, 50 mL of HAuCl4 solution (1 mM) was added and stirred (700 rpm) for 60 min until the solution became colorless. This was followed by an adjustment in solution pH to 2.0, using a solution of HCl (37 wt % in water, 12.1 M). After another 15 min of slow stirring at 400 rpm, 0.312 mL of 0.064 M fresh ascorbic acid (AA) was added, and the solution was vigorously stirred for 30 s. Finally, 0.16 mL of the seed solution was injected into the growth solution. The mixture was stirred for 30 s and kept as it was at 30 °C for 12 h. The Au nanorods were collected by performing centrifugation at 13,000 rpm for 20 min and followed by three rounds of washing with DI water. The obtained solution containing Au nanorods was diluted by DI water to a certain concentration for further use (the intensity of UV absorbance at 910 nm is ∼0.8). Preparation of Au Nanoparticle-Nanorod Decorated WO3 Films (AuPR/WO3). To graft Au nanorods (AuR), the AuP/WO3 films were first immersed in 10 mL of 2 mM dithioglycol-ethanol solution at room temperature for 12 h. The dithioglycol could directly conjugate to AuP on WO3 films through the formation of Au−S bonds. Followed by three rounds of cleaning with ethanol, the films were then immersed into the as-prepared AuR solution for 6 h. The AuR were grafted via the Au−S bonds on the other side of dithioglycol. Electrochromic Measurements. For electrochemical and electrochromic measurements, a conventional three electrode system was used. The FTO slides were used as the working electrodes, a platinum plate served as the counter electrode, and an Ag/AgCl electrode (saturated) was used as the reference. An electrolyte of 0.1 M PBS was used. To evaluate electrochemical switching of light transmission, transparency difference to visible-light at 630 nm was utilized to study the switching dynamics of the WO3-based electrochromic films. The coloration efficiency (CE) is the change of the optical density (OD) per unit charge density (Q/A, where Q is the charge and A is the area) during switching. The value of CE can be calculated according to the formula CE = ΔOD/(Q/A), where ΔOD = log (Tbleach/Tcolor).43 Photothermal Conversion Tests. Considering the penetration depth of light and the relatively low absorption/scattering by water in this so-called transparent window, a 915 nm laser with an energy density of 0.78 W cm−2 was employed as the near-infrared (NIR) light source to investigate the efficiency of photothermal conversion. The photothermal conversion behavior was monitored by the change in temperature of 3.0 mL aqueous PBS solutions. The electrochromic film (WO3, AuP/WO3, and AuPR/WO3) coated FTO slides were placed in a quartz cuvette with 3 mL PBS solution and irradiated by a 915 nm laser beam (0.78 W cm−2, the laser was focused to a 5 mm 6901

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transmittance spectra were recorded using a UV−vis-NIR spectrometer (PerkinElmer Lambda 35, America). The temperature variation as a function of the irradiation time was monitored using a thermocouple thermometer with ±0.1 °C precision (DT-8891E Shenzhen Everbest Machinery Industry Co., Ltd., China). A layer of tungsten was coated onto the FTO surface by Gatan model 682 precision etching and coating system using a tungsten target in an atmosphere of Ar gas. The cyclic voltammograms and chronoamperometric measurements were performed using a potentiostat (CHI660, USA). EIS was carried out in a solution of PBS (0.1 M) in the frequency range of 0.01 Hz to 100 kHz with an amplitude of 50 mV. The photothermal conversion properties of the materials were assessed by irradiating with a 915 nm laser (MW-IR-915/2500 mW, Changchun Laser Optoelectronics Technology Co., Ltd., Changchun, China). The localized surface plasmon resonance scattering spectra were detected by 512B_excelon EMCCD (Princeton Instruments, USA). Glass slides were cleaned under the ozone system (PSD-UV4, Novascan Technologies, USA). Four point probe measurements (RTS-9, China) were carried out at room temperature under ambient pressure with a standard of Si. The resistivity (ρ) obtained reflects the conductive property of the whole layer.

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research is supported by the National Natural Science Foundation of China (nos. 21775016, 21635004), the National Key R&D Program of China (no. 2016YFA0302500), and the Fundamental Research Funds for the Central Universities (nos. N160502001, N170502003, N170908001). REFERENCES (1) Wang, C.; Shim, M.; Guyot-Sionnest, P. Electrochromic Nanocrystal Quantum Dots. Science 2001, 291, 2390−2392. (2) Llordés, A.; Garcia, G.; Gazquez, J.; Milliron, D. J. Tunable Near-Infrared and Visible-Light Transmittance in Nanocrystal-inGlass Composites. Nature 2013, 500, 323−326. (3) Dahlman, C. J.; Tan, Y. Z.; Marcus, M. A.; Milliron, D. J. Spectroelectrochemical Signatures of Capacitive Charging and Ion Insertion in Doped Anatase Titania Nanocrystals. J. Am. Chem. Soc. 2015, 137, 9160−9166. (4) Garcia, G.; Buonsanti, R.; Llordes, A.; Runnerstrom, E. L.; Bergerud, A.; Milliron, D. J. Near-Infrared Spectrally Selective Plasmonic Electrochromic Thin Films. Adv. Opt. Mater. 2013, 1, 215−220. (5) Runnerstrom, E. L.; Llordes, A.; Lounis, S. D.; Milliron, D. J. Nanostructured Electrochromic Smart Windows: Traditional Materials and NIR-Selective Plasmonic Nanocrystals. Chem. Commun. 2014, 50, 10555−10572. (6) Kim, J.; Ong, G. K.; Wang, Y.; LeBlanc, G.; Williams, T. E.; Mattox, T. M.; Helms, B. A.; Milliron, D. J. Nanocomposite Architecture for Rapid, Spectrally-Selective Electrochromic Modulation of Solar Transmittance. Nano Lett. 2015, 15, 5574−5579. (7) Lu, N. P.; Zhang, P.; Zhang, Q.; Qiao, R.; He, Q.; Li, H. B.; Wang, Y.; Guo, J.; Zhang, D.; Duan, Z.; Li, Z.; Wang, M.; Yang, S.; Yan, M.; Arenholz, E.; Zhou, S.; Yang, W.; Gu, L.; Nan, C. W.; Wu, J.; et al. Electric-Field Control of Tri-State Phase Transformation with a Selective Dual-Ion Switch. Nature 2017, 546, 124−128. (8) Yun, T. G.; Kim, D.; Kim, Y. H.; Park, M.; Hyun, S.; Han, S. M. Photoresponsive Smart Coloration Electrochromic Supercapacitor. Adv. Mater. 2017, 29, 1606728−1606728. (9) Granqvist, C. G. Oxide Electrochromics: an Introduction to Devices and Materials. Sol. Energy Mater. Sol. Cells 2012, 99, 1−13. (10) Santato, C.; Odziemkowski, M.; Ulmann, M.; Augustynski, J. J. Crystallographically Oriented Mesoporous WO3 Films: Synthesis, Characterization, and Applications. J. Am. Chem. Soc. 2001, 123, 10639−10649. (11) Xi, J. Q.; Schubert, M. F.; Kim, J. K.; Schubert, E. F.; Chen, M.; Lin, S. Y.; Liu, W.; Smart, J. A. Optical Thin-Film Materials with Low Refractive Index for Broadband Elimination of Fresnel Reflection. Nat. Photonics 2007, 1, 176−179. (12) Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. A Hybridization Model for the Plasmon Response of Complex Nanostructures. Science 2003, 302, 419−422. (13) Song, Y. Y.; Gao, Z. D.; Wang, J. H.; Xia, X. H.; Lynch, R. Multistage Coloring Electrochromic Device Based on TiO2 Nanotube Arrays Modified with WO3 Nanoparticles. Adv. Funct. Mater. 2011, 21, 1941−1946. (14) Lee, S. H.; Lee, S. H.; Cheong, H. M.; Zhang, J. G.; Mascarenhas, A.; Benson, D. K.; Deb, S. K. Electrochromic Mechanism in a-WO3‑y Thin Films. Appl. Phys. Lett. 1999, 74, 242−244. (15) Choy, J. H.; Kim, Y. I.; Kim, B. W.; Park, N. G.; Campet, G.; Grenier, J. C. New Solution Route to Electrochromic Poly(acrylic acid)/WO3 Hybrid Film. Chem. Mater. 2000, 12, 2950−2956. (16) Atwater, H. A.; Polman, A. Plasmonics for Improved Photovoltaic Devices. Nat. Mater. 2010, 9, 205−213.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b02292. Figure S1. SEM images of AuP/WO3 prepared by different concentrations of HAuCl4. Figure S2. XPS and XRD of AuP/WO3. Figure S3. Raman spectra of WO3 and AuP/WO3. Figure S4. TEM image and UV−vis spectrum of Au nanorods used in photothermal study. Figure S5. Absorbance of the electrochromic films. Figure S6. Electrochromic-transmittance curves of the WO3 film in different electrolyte. Table S1. The time duration required for coloring and bleaching the samples in different aqueous electrolytes. Figure S7. Transmittance variations of AuPR/WO3 film at different bias. Figure S8. EIS of the samples. Table S2. Resistivity test. Figure S9. Change of film transmittance with potential scanning. Figure S10. Photothermal conversion at OCP. Figure S11. Photothermal conversion induced by NIR laser with different energy density. Figure S12. TEM image for Au nanorods used in scattering spectra measurement. Figure S13. UV−vis spectra of AuPR nanojunctions. Figure S14. Influence of light source and potentials. Figure S15. Simulation results at OCP. Figure S16. The survival rates of E. coli with irradiation time. Figure S17. The survival rates of E. coli in the absence of WO3 on FTO. Figure S18. Electrochromic switching stability. Figure S19. Photothermal conversion stability (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yong Zhang: 0000-0003-1158-2248 Yan-Yan Song: 0000-0001-5150-4784 Xing-Hua Xia: 0000-0001-9831-4048 Author Contributions ∥

These authors contributed equally. 6902

DOI: 10.1021/acsnano.8b02292 ACS Nano 2018, 12, 6895−6903

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DOI: 10.1021/acsnano.8b02292 ACS Nano 2018, 12, 6895−6903