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Enhanced Electrochromic Properties of WO3 Nanotree-like Structures Synthesized via a Two-Step Solvothermal Process Showing Promise for Electrochromic Window Application Yiqun Li,† William A. McMaster,†,‡ Hao Wei,† Dehong Chen,*,†,‡ and Rachel A. Caruso*,†,‡ †
Particulate Fluids Processing Centre, School of Chemistry, The University of Melbourne, Melbourne, VIC 3010, Australia Applied Chemistry and Environmental Science, RMIT University, Melbourne, VIC 3000, Australia
‡
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S Supporting Information *
ABSTRACT: Hierarchical WO3 nanotree-like structures grown from nanowires were prepared by a two-step nonseeded solvothermal approach using a fluorine-doped tin oxide coated glass substrate. The nanotrunks formed in the first step before the nanobranches grew in the second step. The trunks were orientated along the (002) plane of hexagonal phase WO3, while the branches were orientated along the (100) and (200) planes. An electrochromic film prepared with WO3 nanotrees had a large active surface area, which enabled a large optical modulation of 74.7% at 630 nm at a low potential of −0.2 V versus Ag/ AgCl, shorter response times of 2.64 s for bleaching and 7.28 s for coloration, and a high coloration efficiency of 75.35 cm2 C−1. Thus, such structures are of interest for electrochromic window application. KEYWORDS: electrochromic, WO3, nanostructure, morphology, solvothermal process, thin film
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INTRODUCTION The optical properties of electrochromic (EC) materials change reversibly and continuously with a switch in small on/off voltage.1−4 With such distinct properties, EC materials can be used in many areas including smart windows, glareless automobile rear-view mirrors, large area high-contrast displays, and for thermal protection and blocking infrared radiation.5−9 Tungsten oxide (WO3) is a promising EC material because of its physical and chemical properties, which has also been used in photocatalysis, photoluminescence, photovoltaics, and gas sensing.10−12 WO3 has a high coloration efficiency (CE) and cycling stability. Furthermore, it is colorless at low positive potentials but displays a dark blue color under a negative electrical bias. Amorphous WO3 EC films are known for their high CE and fast response times; however, due to the low density in structure and chemical instability, especially in a proton electrolyte, amorphous WO3 EC films suffer from a poor cycling life.13−15 In contrast, crystalline WO3 is much more stable and durable in proton electrolytes because of its compact structure, therefore having higher potential for practical long-term use. Yet, bulk crystalline WO3 EC materials usually have longer response times and CEs. Consequently, nanostructured WO3 films have improved EC performance and durability than either amorphous or bulk crystalline films; the nanostructure enables an increase in active surface area for reaction in the electrochemical process and better contact with the electrolyte.16−19 Many groups have reported that the use of various nanostructured WO3 materials in EC devices improve the EC © XXXX American Chemical Society
performance. Among these materials, nanowires and nanorods that are one-dimensional (1D) in structure have attracted attention, because their morphology is important for achieving high EC performance.20−22 However, 1D nanostructures usually grow randomly on substrates, which, along with compact stacking, limits the active surface area. Therefore, growing nanotree-like WO3 structures from nanowires aligned vertically on the substrate would more efficiently use the substrate surface, thus increasing the WO3 active surface area and improving the EC performance.23−26 Solvothermal processing is a facile method for the synthesis of uniquely structured, high surface area, crystalline WO3 materials, including nanorods,27−29 nanowires,30−32 nanosheets,33,34 nanohoneycombs,35 nanothorns,36 and nanoflowers.37 Most reported synthesis approaches to form WO3 nanostructures require a seed layer. Yet, the preparation of WO3 nanostructures with favorable electron transport can be achieved when there is no seed layer, allowing the WO3 film to be in direct contact with the conductive, fluorine-doped tin oxide (FTO) coated substrate.38 In this work, a WO3 nanotree-like structure with hexagonal crystal phase was grown from nanowires synthesized on an FTO substrate via a two-step solvothermal method. The initial transmittance (∼88%) was high for the films, with an optical modulation up to 74.7% under a potential of only −0.2 V Received: February 4, 2018 Accepted: May 29, 2018
A
DOI: 10.1021/acsanm.8b00190 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials versus Ag/AgCl, and good CE of 75.35 cm2 C−1. Additionally, the two-step solvothermal process produced a material with increased cycling stability when compared with the one-step material at large optical modulation.
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was recorded across the frequency range of 0.1 Hz−100 kHz at an open-circuit potential.
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RESULTS AND DISCUSSION Films of WO3 nanostructure were grown on an FTO surface by means of a facile solvothermal process without the need for a presynthesis WO3 seed layer or annealing. XRD patterns of the WO3 nanostructures fabricated in the one- and two-step solvothermal procedures are shown in Figure 1.
EXPERIMENTAL SECTION
Chemicals. The reagents for this work were used as received. To synthesize the WO3 films the following chemicals were used: tungstic acid (H2WO4, 99%, Sigma-Aldrich), thiourea (SC(NH2)2, 99%, SigmaAldrich), hydrogen peroxide (H2O2, 30 wt %, analytical reagent (AR), Chem-Supply), hydrochloric acid (HCl, 32%, Merck), acetonitrile (CH3CN, AR, Ajax Finechem), and maleic acid (C4H4O4, LR, Ajax Finechem). FTO glass (TEC8, Dyesol) was used as the conductive substrate and was washed using detergent (20% Chem-Det Laboratory Detergent in water, Chem-Supply), ethanol (C2H5OH, AR, ChemSupply), and acetone (CH3COCH3, AR, Merck). For electrochemical measurements, the electrolyte was sulfuric acid (H2SO4, 98%, AR, BDH). A Millipore Milli-Q ultrapure water purification system was used as the water supply, having a resistivity higher than 18.2 MΩ·cm. WO3 Nanowire Fabrication. H2WO4 (1.250 g) was added to a water (30 mL)/H2O2 (10 mL) solution and stirred at 95 °C. The clear solution was then diluted to 0.05 M with water. To grow the nanowires the following aqueous solution (3 mL) was prepared: SC(NH2)2 (0.035 g) and C4H4O4 (0.040 g), to which aqueous HCl (3 M, 2 mL), CH3CN (18 mL), and the H2WO4 solution (0.05 M, 1 mL) were added and stirred for 10 min, then transferred to a 28 mL glass vial (Ø 25, height 75 mm, Labtek) containing a clean glass substrate (20 × 25 mm2) with the FTO side facing down. This was placed in a Teflon-lined stainless steel autoclave (50 mL). The FTO glass had been ultrasonically washed prior to use in consecutive baths of detergent, ethanol, and acetone for 10 min in a sonicator (FXP10DH, Unisonics Australia). The autoclave was heated in an oven (ODWFH18, Labec, 180 °C, 3 h). Once cooled and rinsed with water the substrate was oven-dried (50 °C, 1 h). WO3 Nanotree Fabrication. A second solvothermal step was used to grow WO3 nanotree-like structures from the nanowires. A SC(NH2)2 (0.035 g) and C4H4O4 (0.040 g) aqueous solution (3 mL) was prepared, to which aqueous HCl (9 M, 2 mL), CH3CN (18 mL), and H2WO4 solution (0.05 M, 0.5 mL) were added before being transferred to a 28 mL glass vial holding a WO3 nanowire-coated FTO glass substrate from the preceding step. The glass vial containing the substrate and solution was autoclaved (50 mL Teflon-lined stainless steel autoclave, 180 °C, 2 h). The substrate was cooled, water-rinsed, and oven-dried (50 °C, 1 h). Characterization. Scanning electron microscopy (SEM, Quanta 200F FEI environmental scanning electron microscope with a 12.5 kV accelerating voltage) and transmission electron microscopy (TEM, FEI Tecnai F20 transmission electron microscope, operating at 200 kV) were used to characterize the morphology of the (nonsputtercoated) samples. Powder X-ray diffraction (XRD) patterns (Bruker D8 Advance Diffractometer using Cu Kα radiation) were used to study the crystal phase of the samples. A VG ESCALAB 220i-XL spectrometer (equipped with a twin crystal monochromated Al Kα X-ray source, which emitted a photon energy of 1486.6 eV at 10 kV and 22 mA) was used for X-ray photoelectron spectroscopy (XPS). To calibrate the binding energy scale the 284.6 eV C 1s peak was used. UV−Visible absorption spectroscopy was obtained using a PerkinElmer Lambda 1050 UV−vis−NIR spectrophotometer (NIR = near-infrared; 300 to 800 nm, 2 nm s−1scanning rate). To conduct the electrochemical measurements a CH Instruments 760E electrochemical workstation was used with a three-electrode test device. The WO3 film on FTO glass, silver/silver chloride electrode, and platinum wire were the working, reference, and counter electrodes, respectively. Cyclic voltammetry (CV) measurements were performed in 1 M H2SO4 at a scanning rate of 50 mV s−1 from −0.50 to 0.50 V versus Ag/AgCl, with the final data normalized to the film area. Chronoamperometry (CA) tests were conducted in 1 M H2SO4 using a square-wave voltage from −0.20 to 0.50 V versus Ag/AgCl (30 s pulse width, transmittance measured at 630 nm). Electrochemical impedance spectroscopy (EIS)
Figure 1. XRD patterns of the WO3 nanostructures after the initial one- (nanowire) and then two-step (nanotree) solvothermal procedures.
Hexagonal phase WO3 was identified in both patterns corresponding to JCPDS card No. 85−2460. Compared to the cubic and tetragonal phases, hexagonal phase WO3 has larger tunnels that better accommodate the insertion and extraction of charged particles within thin films. Tunnel size influences the rapidity of ion diffusion and, hence, is a key property for EC application.4,9 It was observed that both XRD patterns featured sharp peaks, suggesting that the WO3 nanostructured films were well-crystallized. Moreover, after the second solvothermal step, the hexagonal WO3 phase was retained, while the peak intensity had increased. From this result it was inferred that the two-step solvothermal WO3 sample may have better crystallinity and an increased amount of active material, thus having greater stability in an acid electrolyte and a larger optical modulation.16,19 The roles of C4H4O4 and SC(NH2)2 in the crystal growth mechanism have been discussed before.39 In brief, C4H4O4 both promotes the formation of WO3 crystal nuclei and their adhesion to the FTO surface, enabling further crystal growth. SC(NH2)2 serves as a capping agent that selectively adsorbs to the c-axis facets of the WO3 nanocrystal, resulting in the formation of c-axis oriented nanowires. Therefore, in the present work, after the first solvothermal process involving the addition of 3 M HCl, uniform WO3 nanowires were observed by SEM to have grown on bare (i.e., unseeded) FTO glass (Figure 2a,b). In the second solvothermal process, different synthesis conditions were used, with only half the amount of H2WO4, a higher concentration of HCl (9 M), and a shorter reaction time. After 2 h of solvothermal treatment, small branches had grown on the previously prepared nanowires to form WO3 nanotree-like structures (Figure 2c). The nanostructures obtained through this two-step solvothermal process would have a larger active surface area for electrolyte contact; therefore, a superior EC performance was expected. SEM images at higher magnification and in cross B
DOI: 10.1021/acsanm.8b00190 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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branches (Figure 3b). The nanotrunks were 10−15 nm in diameter, agreeing with the SEM measurements (Figure S1e). Figure 3c revealed that both the WO3 nanotrunks and nanobranches exhibited distinct lattice fringes: nanotrunks with d-spacings of ∼0.38 nm consistent with the (002) plane of the hexagonal WO3 crystal structure, and nanobranches with dspacings of ∼0.32 and 0.63 nm that matched the (200) and (100) planes of hexagonal phase WO3, respectively. Furthermore, growth of the WO3 nanotrunks occurred along the [002] direction of the hexagonal WO3, while the nanobranches grew along [100] and especially [200], resulting in the nanotree morphology. These findings correlate to the increased intensity of the (100) and (200) peaks seen in the nanotree XRD pattern (Figure 1), as well as SEM observations (Figure 2c). The second solvothermal process also intensified the (002) plane, while there was relatively little change in the (202) plane. This two-step process produced nanotrunks first, then the nanobranches, with both nanostructures having the same crystal phase and being the products of similar precursor solutions; however, the direction of the growth in the second step was different than the first. Ion insertion and the EC properties of the WO3 nanostructured films were assessed by CV in a 1 M H2SO4 solution. In Figure 4a are depicted typical cyclic voltammograms that
Figure 2. SEM images and schematic illustrations: (a, d) bare FTO; (b, e) WO3 nanowires prepared after the first solvothermal process, and (c, f) WO3 nanotrees prepared after the second solvothermal process.
section are shown in Figure S1. The diameters of the 1D nanowires and the trunks of the nanotrees were 9.3 ± 0.4 and 13.3 ± 0.8 nm, respectively, while the thicknesses of the nanowire and nanotree thin films were similar at ∼465 nm. To further study the morphological and crystalline structural features of the two-step solvothermal nanotree-like WO3 films, TEM images were obtained (Figure 3). The sample for TEM
Figure 3. (a−c) TEM images of the nanotree-like WO3 sample increasing in magnification.
Figure 4. (a) CV and (b) in situ optical responses of the WO3 nanowire and WO3 nanotree films.
was prepared by scratching the nanostructures off the FTO substrates and sonicating for 10 min. Although most of the nanostructures were crushed and aggregated, low-magnification TEM (Figure 3a) revealed nanotree-like structures, in good agreement with SEM observations (Figure 2c). At higher magnification the TEM image showed that each WO3 nanotree was composed of a single nanotrunk with numerous nano-
represent the proton storage capacity and the active electrochemical surface area. The proton storage capacity of the WO3 nanotree electrode was higher than that of the nanowire electrode, on account of the larger enclosed area of the nanotree voltammogram (Figure 4a). This implied an increase in the active surface area due to an increase in the amount of C
DOI: 10.1021/acsanm.8b00190 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials active material for ion insertion, which resulted from a denser nanotree film (Figure 2c, Figure S1e,f), thus enabling higher proton storage.40 The enclosed CV area further indicated that the nanotree film should have a larger optical modulation than the nanowire film. The optical modulations and response times of the WO3 nanowire and nanotree films were determined from CA tests, and the corresponding in situ transmittance was measured (Figure 4b). Optical modulation is a significant determiner of the potential applications of an EC film. Other important parameters are the coloration (tc) and bleaching (tb) switching response times, which relate to the amount of charge, and the rate of charge transfer occurring at the EC material-electrolyte interface.4,6 The optical modulation (presented as the mean and standard deviation of three tests) and tc and tb values of the WO3 nanostructured films are given in Table 1. Switching time is defined as the time required for the system to reach 90% of total modulation.
Figure 5. Change of in situ optical density (ΔOD) at 630 nm of the nanowire and nanotree films with respect to charge density.
insertion. Furthermore, the CE of the nanotree film was superior to those reported for WO3 films with other morphologies (Table 2), while the CE of the nanowire film was comparable. The CEs from this work are presented as the mean and standard deviation of three tests.
Table 1. Optical Modulations and Switching Times of the WO3 Nanowire and Nanotree Films (−0.2 V vs Ag/AgCl) switching time (s) film
optical modulation (%) at 630 nm
tc
tb
nanowire nanotree
34.3 ± 0.8 74.7 ± 1.6
1.92 7.28
0.44 2.64
Table 2. CEs of WO3 Films of Different Nanostructured Morphologies
The nanotree film had an excellent optical modulation of 74.7% (Table 1), achieved under a small bias of just −0.2 V versus Ag/AgCl. By comparison, annealed WO3 nanowires were reported to have an optical modulation of 58.7% under a −0.35 V bias.39 Hence, the nanotree film fabricated in this work exhibited a 27.3% higher optical modulation while using only 57.1% of the bias. In addition, the optical modulation of this film was more than twice as large as that of the nanowire sample under the same potential (Figure 4b). This result can be attributed to the nanotree structure: nanobranches both increased the contact with the electrolyte and increased the active sites available, in turn improving the insertion of ions inside the nanowires. Therefore, the optical modulation was greatly increased. Figure S2 shows the transmittance of the nanowire and nanotree films. Because of the increased amount of active material on the FTO substrate of the nanotree film, the bleached transmittance of the nanotree film in the regions from 300 to 500 nm and from 700 to 800 nm was a little lower than that of the nanowire film, whereas in the range from 500 to 700 nm the transmittance of the nanowire and nanotree films was similar. The response times (Table 1) of the nanotree sample were longer than the nanowire sample, mainly because the former has a larger transmittance variation.41 Highresolution W 4f XPS spectra for the bleached and colored states of the nanotree sample were obtained (Figure S3) and are discussed in the Supporting Information. The WO3 nanowire and nanotree samples were investigated for CE and stability. Figure 5 displays the change of in situ optical density at 630 nm that was obtained for the inserted charge density under a bias of −0.20 V versus Ag/AgCl. The CEs determined from the slopes were 53.2 and 75.4 cm2 C−1 for the WO3 nanowire and nanotree films, respectively. The difference in CE between the nanowire and nanotree films was due to the branched structure of the latter, which had a higher active surface area and better contact with the electrolyte, so a larger optical modulation was achieved under the same charge
nanostructure
CE (cm2 C−1)
wavelength (nm)
reference
nanotrees nanowires nanoparticles nanorods nanowires nanotrees nanosheets nanowires annealed nanowires
75.4 ± 3.0 53.2 ± 2.0 42.0 60.5 47.8 43.6 55.6 63.0 61.9
630 630 670 600 750 500 700 630 630
this work this work 16 29 32 42 26 39 32
The electrochemical behavior of the WO3 film electrodes was examined by EIS, with Nyquist plots of the nanowire and nanotree thin films shown in Figure 6. Both plots exhibit a semicircle and straight line representing high- and lowfrequency regions, respectively. The semicircle is associated with series resistance (Rs) and charge-transfer resistance (Rct), while the straight line arises from the Warburg diffusion process (WR). The smaller semicircle of the nanotree sample implied
Figure 6. EIS of the WO3 nanowire and nanotree films. D
DOI: 10.1021/acsanm.8b00190 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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For the nanowire film to reach an optical modulation similar to the nanotree film, it would need to operate at a much higher voltage, and its performance would quickly decrease. When a high potential of −0.65 V versus Ag/AgCl was applied to the nanowire film, the initial optical modulation was 70% (Figure S5) but dropped dramatically to 27% after 1200 s (20 cycles) and stabilized at only 5% modulation after 3000 s (50 cycles).
that this film had a smaller charge-transfer resistance than the nanowire sample.29,43 The equivalent circuit for the WO3 samples is shown in Figure S4. The data from the Nyquist plots were fitted to the equivalent circuit, and the values of Rs, Rct, and WR were calculated (Table S1). Of the two films, the nanotree sample had lower Rct and WR values, which indicated that it had better charge transfer and ion diffusion. These results were also attributed to the nanotree structure: nanobranches increased contact with the electrolyte, thus making more active sites available and consequently improving ion insertion within the nanostructures. The UV−visible spectra of the WO3 nanotree film and the corresponding color changes are shown in Figure 7. The blue
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CONCLUSION Without employing a seed layer or annealing, a WO3 nanotree film was grown on an FTO glass substrate, and its properties were determined for a potential electrochromic window application. The nanotree growth was realized by a two-step solvothermal process: preparation of a nanowire layer in the first step and growth of the nanobranches in the second. Compared to the nanowire film, the hierarchical nanotree film exhibited improved EC properties, including enhanced electron and cation transport, a large optical modulation of 74.7%, a high CE of 75.35 cm2 C−1, and a long and stable cycling life. This improvement was credited to the nanotree structure providing more active surface area, hexagonal WO3 crystal phase, and enhanced crystallinity that together supported more charged species to move deeper and faster through the nanotree film, while also increasing the stability of WO3 in an acidic electrolyte.
Figure 7. UV−Visible spectra of the nanotree sample under different potentials and corresponding photographs showing the color changes.
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coloration of the film, and the intensity of the color, are controlled by the amount of potential applied. Thus, when no bias was applied, the film existed in the bleached state and was highly transparent across the visible light range. However, at a small negative potential of −0.10 V versus Ag/AgCl the film was pale blue, becoming distinctly blue at −0.15 V, and finally dark blue at −0.20 V. The results of stability tests over 15 000 s (250 cycles) are shown in Figure 8. The nanowire sample decreased 4.8%
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00190. SEM images of the nanostructures at different magnifications and cross sections of the thin films, allwavelength visible spectra of the nanowire and nanotree films, high-resolution W 4f XPS spectra of the WO3 nanotree film, equivalent circuit used for fitting the EIS spectra, resistance and Warburg diffusion values derived from the EIS, and stability of the WO3 nanowire film under −0.65 V versus Ag/AgCl (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. (R.A.C.) *E-mail:
[email protected]. (D.C.) ORCID
William A. McMaster: 0000-0003-2186-2892 Dehong Chen: 0000-0003-2867-7155 Rachel A. Caruso: 0000-0003-4922-2256 Figure 8. Stability test of the WO3 nanowire and nanotree films under −0.2 V vs Ag/AgCl.
Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.
compared to its initial state, whereas for the nanotree sample the optical modulation began to only slowly decrease after ∼100 cycles, although the decrease in optical modulation by the end of the test was 6.8% of the initial state. Significantly, the stability test results were based on the same bias of −0.2 V versus Ag/AgCl being applied to both samples, not on the same initial optical modulation being achieved. Therefore, because of the design of the WO3 nanotree layer, during every cycle the nanotrees underwent more and deeper insertion and extraction of ions and electrons than the nanowires under the same bias.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Y.L. acknowledges the China Scholarship Council (CSC) for support. Electron microscopy access was provided by the Melbourne Advanced Microscopy Facility at the University of Melbourne. E
DOI: 10.1021/acsanm.8b00190 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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