Effect of annealing on acid-treated WO3·H2O nanoplates and their electrochromic properties

Electrochimica Acta 178 (2015) 673–681

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Effect of annealing on acid-treated WO3
H2O nanoplates and their electrochromic properties Chai Yan Nga , Khairunisak Abdul Razaka,b,* , Zainovia Lockmana,* a

School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia NanoBiotechnology Research and Innovation (NanoBRI), Institute for Research in Molecular Medicine (INFORMM), Universiti Sains Malaysia, 11800 USM, Penang, Malaysia b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 16 June 2015 Received in revised form 4 August 2015 Accepted 14 August 2015 Available online 20 August 2015

The effect of annealing temperature on the electrochromic properties of acid-treated hydrated tungsten oxide (WO3
H2O) nanoplates is reported for the first time. Nanoplates were grown on fluorine-doped tin oxide-coated glass substrate using nitric acid, and annealing temperature was varied from 100–400  C. The as-prepared nanoplates were WO3
H2O, and these nanoplates transformed into tungsten oxide (WO3) after annealing at 200  C. Nanoplate thickness decreased with increasing annealing temperature from 12–55 nm for the as-prepared sample to 12–40 nm for the sample annealed at 400  C. WO3 nanoplates annealed at 200  C showed the best electrochromic properties with coloration efficiency of 28.64 cm2 C1, high optical modulation (58%), good electrochromic cycling stability, as well as short ion insertion and extraction times of 9.6 and 5.1 s, respectively. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Tungsten oxide Sputtered tungsten Nitric acid Acid treatment Hydrothermal

1. Introduction Electrochromic materials can undergo optical coloration with the application of electric potential. Tungsten oxide (WO3) is the most widely studied and used electrochromic material in electrochromic devices. WO3 changes from colorless to blue when electrons and ions, such as H+ or Li+, are added or intercalated in its structure with applied electric potential. WO3 reverts to colorless when electrons and ions are extracted or de-intercalated from the structure. WO3 nanostructures can be obtained in various morphologies, such as nanopores [1], nanorods [2], nanowires [3], nanotrees [4], and nanoplates [5]. Nanoplates generally have a rectangular shape; they are usually grown vertically on the substrate and are less likely to overlap or meet the surrounding nanoplates and aggregate with one another compared with the other morphologies. As the nanoplates are individually standing on the substrate, they could provide more surfaces for the interaction between WO3 and electrolyte, which results in good electrochromic properties. WO3 nanoplates can be grown using acid treatment [6] and seeded growth hydrothermal reaction [7]. Both

* Corresponding authors at: School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia. Fax: +60 4 5941011. E-mail addresses: [email protected] (K. Abdul Razak), [email protected] (Z. Lockman). http://dx.doi.org/10.1016/j.electacta.2015.08.069 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

of these methods are identical, whereby a substrate is immersed inan acid (acid treatment) or hydrothermal solution (seeded growth hydrothermal reaction) to allow direct growth of WO3 nanostructure on the substrate. Acid treatment is preferable for nanoplate formation because it involves only one precursor (acid) for nanoplate growth, whereas hydrothermal reaction usually involves more chemicals including W source precursor, acid, and capping agent. Moreover, acid treatment has several advantages, such as fast synthesis, simple setup, variety of substrate selection, and large-scale production. Nitric acid (HNO3) has been used in acid treatment for nanoplate formation because of its strong oxidizing property [8]. Acid treatment is a liquid-phase synthesis method that produces hydrated WO3 nanoplates, such as WO3
H2O and WO3
2H2O. The hydrated WO3 can be easily transformed to WO3 using annealing (heat treatment). Meanwhile, the heat treatment induces crystallinity, phase transformation, and morphology changes in WO3 [9]. All reported acid treatment works [6,8–16] conducted annealing on the hydrated WO3 nanoplates to obtain the WO3 nanoplates. However, only two works [9,14] reported the effect of annealing on the performance of a WO3 device. Chen et al. [14] showed that annealing (300  C for 1 h) on the WO3 nanoplates improved the response of the nanoplates toward the hydrogen sensing properties. Ou et al. [9] showed that WO3 nanoplates annealed at 480  C for 2 h exhibited improved reflectance response toward hydrogen in comparison with the nanoplates annealed at 300  C for 2 h. These works showed the importance of the annealing on the performance of a WO3 device. To

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the authors' knowledge, reports on the effect of annealing on the electrochromic properties of the acid-treated WO3 nanoplates have not been published. In this work, the effect of annealing (100–400  C) on the acidtreated WO3
H2O nanoplates was studied to observe the morphological and structural changes of the nanoplates. Electrochromic characterizations were then performed on the as-prepared and annealed nanoplates to study the correlation between the electrochromic properties and the morphological and structural properties of the nanoplates. 2. Experimental details W film was deposited on fluorine-doped tin oxide (FTO)-coated glass substrates using a radio frequency sputtering system (BOC Edwards Auto 500) with W being the target. Prior to sputtering, the FTO glass substrates were ultrasonically cleaned with acetone, isopropyl alcohol, ethanol, and deionized water for 10 min each to remove contaminants. In the sputtering process, the sputtering chamber was first pumped to a base pressure of 3.03  103 Pa. Pure argon gas was then introduced into the chamber, and the deposition pressure was kept at 1.95 Pa. A sputtering power of 100 W was applied during deposition. After sputtering process, the W-sputtered FTO glass was acid treated with HNO3 to produce

nanoplates on the glass. Acid treatment was performed by immersing a W-sputtered FTO glass in a screw-capped bottle containing 1.5 M of HNO3 at 80  C for 1 h. The current acid treatment conditions were reported by Widenkvist et al. [8] that capable to produce vertical WO3
H2O nanoplates on the W substrate. After the acid treatment, the substrate was rinsed with deionized water and annealed at 100, 200, 300, and 400  C in air for 30 min at a heating rate of 5  C min1. Morphologies of the samples were observed using fieldemission scanning electron microscopy (FESEM) (Zeiss SUPRA 35) and high-resolution transmission electron microscopy (HRTEM) (FEI Tecnai G2 20 S-TWIN). The phases present in the samples were analyzed using X-ray diffraction (XRD) (Bruker AXS D8 ADVANCE) equipped with Cu Ka radiation and Raman spectroscopy (Renishaw inVia Raman microscope). Cyclic voltammetry (CV) was performed to measure the electrochromic current density of nanoplates using a three-electrode system (mAutolab Type III potentiostat/galvanostat). The nanoplate sample, platinum electrode, and Ag/AgCl/3 M KCl electrode were used as the working, counter, and reference electrodes, respectively. The measurement was conducted between 0.5 and +0.5 V at a scan rate of 100 mV s1 in 0.1 M sulfuric acid (H2SO4) electrolyte. WO3 was observed to show fast response and high charge density in H2SO4 electrolyte owing to small size of H+ [17]. The switching

Fig. 1. FESEM images of (a) FTO glass, (b, c) W-sputtered FTO glass, and (d, e) acid-treated nanoplates on FTO glass. Images (c) and (e) are cross-sectional views.

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spectra of the sample were measured using ultraviolet–visible spectrophotometer (Varian Cary 50 Conc). 3. Results and discussion

Fig. 2. XRD patterns of (a) FTO glass, (b) as-prepared nanoplates, and nanoplates annealed at (c) 100, (d) 200, (e) 300, and (f) 400  C.

times (ion insertion and extraction times) of the sample were measured by chronoamperometry with similar setup as the CV measurement. The chronoamperometry curve was obtained by applying 1 V for 20 s in each state to the film. The transmittance

The surface morphologies of the FTO glass and sputtered W film on FTO glass are shown in Fig. 1a and 1b, respectively. The sputtered W film had a uniform thickness of 350 nm (Fig. 1c). Acid treatment was performed on this W-sputtered FTO glass, and nanoplates with 12–55 nm thickness were formed on the FTO glass after 1 h (Fig. 1d). The nanoplates were vertically grown on the substrate and had good adhesion to the FTO glass. The crosssectional view (Fig. 1e) shows the transformation of W film to nanoplates, and the oxide layer thickened to 570 nm after the acid treatment because of the nanoplate growth. Nanoplate formation initiated with W was oxidized to WO42 when the W-sputtered FTO glass was immersed in HNO3 (strong oxidizing acid). The WO42 reacted with H+ from HNO3 and grew into WO3
H2O nanoparticles (Eq. (1)) [8,16,18]. The WO3
H2O nanoparticles were then gradually grew into nanoplate structure with prolonged reaction period. WO42 + 2H+ ! WO3
H2O

Fig. 3. FESEM images of nanoplates annealed at (a) 100, (b) 200, (c) 200 (cross-section), (d) 300, and (e) 400  C.

(1)

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The nanoplates were annealed at 100, 200, 300, and 400  C for 30 min, and the XRD patterns are shown in Fig. 2. The peaks of the as-prepared nanoplates matched with orthorhombic WO3
H2O (ICDD No. 00-043-0679), indicating the transformation of W film to WO3
H2O nanoplates after the acid treatment. The nanoplates retained the WO3
H2O phase after annealing at 100  C, and they transformed to a more stable monoclinic WO3 (ICDD No. 00-0431035) after annealing at 200–400  C. This transformation happened because of the removal of inter-structural water molecules from WO3
H2O that comprised WO3 octahedron sheets held together by water molecules among them [9,19,20]. The surface morphologies of the annealed nanoplates are shown in Fig. 3. The nanoplate structure retained after the annealing. However, the nanoplate thickness decreased with increasing annealing temperature. The thickness of the as-prepared nanoplates (12–55 nm) (Fig. 1d) decreased to 12–40 nm after annealing at 400  C (Fig. 3e). The thickness distributions of the nanoplates are shown in Fig. 4. The as-prepared nanoplates have a mode thickness of 25–35 nm, whereas the nanoplates annealed at 400  C have a mode thickness of 15–25 nm. The decrease in nanoplate thickness was because of the water molecules removed from the nanoplates as mentioned above. The removal of water molecules among the WO3 octahedron sheets caused the WO3 to bond closer to one another and thus reduced the nanoplate thickness. Ou et al. [9] also observed reduction in WO3 nanoplate thickness on W foil after annealing at 300 and 480  C. Raman spectra of the nanoplates are shown in Fig. 5. The asprepared nanoplates and nanoplates annealed at 100  C had a broad peak at 635 cm1 and a sharp peak at 946 cm1. The broad peak at 635 cm1 is attributed to stretching modes of OWO bridging oxygen, and the sharp peak at 946 cm1 is ascribed to

stretching modes of terminal W¼O bonds [21]. These two peaks corresponded to WO3
H2O peaks, which is in good agreement with the XRD analysis in Fig. 2. New peaks appeared when the annealing temperature was increased to 200–400  C. Two peaks at lowfrequency region (272 and 326 cm1) are assigned to bending modes of O WO bridging oxygen, and two peaks at highfrequency region (713 and 808 cm1) are the corresponding stretching modes [21]. These four peaks are the fingerprints of monoclinic WO3, which is consistent with the XRD observation (Fig. 2). In addition, similar to the XRD results, the intensity of the monoclinic WO3 Raman peaks increased with increasing annealing temperature, showing the increase of nanoplate crystallinity with increasing annealing temperature. HRTEM images of nanoplates annealed at 200  C are shown in Fig. 6a and 6b. Fig. 6a shows a rectangular-shaped nanoplate displaced from the film, whereas Fig. 6b shows a closer view of the nanoplate with 0.36 nm dspacing, representing (2 0 0) plane of monoclinic WO3. Fig. 6c shows the selected area electron diffraction (SAED) of the nanoplate. The regular diffraction spots indicate the single crystalline nanoplate. Photographs of sputtered W films on FTO glasses before and after acid treatment are shown in Fig. 7. The sputtered W film is gray in color (Fig. 7a), and the film transformed to yellowish after acid treatment (Fig. 7b–f). The as-prepared film and film annealed at 100  C are yellowish green (Fig. 7b and 7c) because of the presence of WO3
H2O (WO3
H2O naturally appeared in yellowish green color). The films annealed at 200  C and above exhibited lighter yellowish color (Fig. 7d–f) owing to the transformation of film to WO3 (WO3 appeared naturally in yellow color). The electrochromic behavior of nanoplate film (annealed at 200  C) was investigated, as shown in Fig. 8. The as-made film is colorless (Fig. 8a) and changes to uniform blue when an electric potential was applied to it. Darker blue color was formed with increasing applied electric potentials from 1 to 3 V (Fig. 8b–d). The film was bleached to colorless when a positive electric potential was applied (Fig. 8e). A negative electric potential caused an intercalation of H+ into the WO3, transforming the WO3 to a blue HxWO3, as represented in Eq. (2). By contrast, a positive electric potential de-intercalated the H+ from HxWO3 and transformed back the HxWO3 to colorless WO3. A higher electric potential would result in more H+ intercalated into the WO3, thus causes darker blue color. WO3 þ xHþ þ xe

Fig. 4. Thickness distributions of nanoplates grown on FTO glass: (a) as-prepared nanoplates and (b) nanoplates annealed at 400  C.

intercalationðcoloringÞ

@

deintercalation ðbleachingÞ

Hx WO3

ð2Þ

Fig. 5. Raman spectra of (a) as-prepared nanoplates and nanoplates annealed at (b) 100, (c) 200, (d) 300, and (e) 400  C.

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Fig. 6. (a, b) HRTEM images and (c) SAED pattern of nanoplate annealed at 200  C.

Fig. 7. Photographs of sputtered W films on FTO glasses (a) before and after acid treatment: (b) as-prepared and annealed at (c) 100, (d) 200, (e) 300, and (f) 400  C. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)

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Fig. 8. Photographs of nanoplate film annealed at 200  C: (a) as-made, (b) colored at 1 V for 5 s, (c) colored at 2 V for 5 s, (d) colored at 3 V for 5 s, and (e) bleached at +3 V for 20 s. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)

Fig. 9. Transmittance spectra of nanoplates annealed at 200  C: as-made, colored at 1, 2, and 3 V for 5 s, and bleached at +3 V for 20 s.

Transmittance spectra of the corresponding coloration and bleaching of the nanoplates annealed at 200  C is shown in Fig. 9. The as-made and bleached films had high transmittance, and the colored films had low transmittance. The bleached film showed similar transmittance with the as-made film at 632.8 nm wavelength. This result indicated the capability of nanoplates to revert back to their original color after the de-intercalation process. A wavelength of 632.8 nm was chosen because this value is commonly referred as the maximum absorbance wavelength for the WO3 electrochromic film [22,23]. The films colored at 1, 2, and 3 V for 5 s show maximum transmittance at the wavelength of 515, 451, and 446 nm, corresponding to green blue, blue, and blue-violet, respectively, which is consistent with the electrochromic behavior shown in Fig. 8. The transmittance spectra show a blue shift of maximum transmittance wavelength with

increasing applied negative electric potential. The result also shows the capability of nanoplate film to display a range of bluish colors with various electric potentials. In addition, the transmittance spectra show the capability of the colored films to exhibit low transmittance with only 5 s of applied electric potential. At a wavelength of 632.8 nm, nanoplate film colored with 3 V achieved a low transmittance of 5%, followed by films colored with 2 V (7%) and 1 V (18%). Higher optical modulation was obtained with lower transmittance colored films. The film with 3 V applied had the highest optical modulation of 58%, followed by the films with 2 V (56%) and 1 V (45%) applied. These results are comparable with the results of previous works [3,24,25]. Hung et al. [3] reported an optical modulation of 57% on WO3 nanowires with an electric potential of 2 V for 100 s. Zhang et al. [24] obtained an optical modulation of 58% on WO3 nanowires with the same electric potential of 2 V for 100 s. Ma et al. [25] reported an optical modulation of 66% on cylindrical WO3 nanorod array with an electric potential of 2 V for 60 s. These reported works achieved higher or comparable optical modulation with the current work. However, the current work only applied electric potential on the films for 5 s, which is much shorter time than the applied electric potential period in those reported works. High optical modulation can be achieved within a short period (5 s) because of the large surface area and vertically grown nanoplates on the substrate. The WO3 nanoplates were 17–35 nm thick (i.e., the thickness of nanoplates annealed at 200  C) that resulted in a large WO3 surface area. This large surface area increased the interaction between nanoplates and electrolyte, and thus led to higher charge insertion and extraction density. The vertically grown nanoplates also facilitated the ion movement between the nanoplates and electrolyte. Subsequently, more H+ from electrolyte was intercalated into the nanoplates, resulting in high optical modulation. CV measurement was conducted on the nanoplate films and the cyclic voltammograms (100 cycles) are shown in Fig. 10. The integrated cathodic or anodic current density indicates the amount

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Fig. 10. Cyclic voltammograms of initial and 100th cycles of nanoplates: (a) as-prepared and annealed at (b) 100, (c) 200, (d) 300, and (e) 400  C.

of H+ (from electrolyte) intercalated into or de-intercalated from nanoplates, respectively. The as-prepared nanoplates and nanoplates annealed at 100  C decreased in current density after

100 cycles because of the presence of crystalline-hydrated WO3
H2O phase, where the crystalline-hydrated layer acted as a resistant layer against the electrochemical reaction by depressing

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Fig. 11. Cyclic voltammograms of the as-prepared and annealed nanoplates.

Fig. 12. Chronoamperometry curve of nanoplates annealed at 200  C.

the ion and electron mobility [26]. By contrast, the voltammetric responses for WO3 nanoplates annealed at 200  C were stable and without significant degradation after 100 cycles, illustrating that the nanoplates had good electrochromic cycling stability. Despite the H2SO4 electrolyte could induce degradation of WO3, WO3 nanoplates showed stable voltammetric responses owing to the crystalline behavior of the nanoplates. To compare the current density of the nanoplates, the cyclic voltammograms of the asprepared and annealed nanoplates are compiled in Fig. 11, and the intercalated charge density values were calculated as listed in Table 1. The as-prepared nanoplates and nanoplates annealed at 100  C showed lower charge density (2.65 and 2.71 mC cm2, respectively) because of the hydrated structure described above. Nanoplates annealed at 200  C had the highest charge density (7.66 mC cm2), and the charge density decreased with increasing annealing temperature. The decrease in charge density was because of the increasing crystallinity of the nanoplates with increasing annealing temperature (as seen earlier in the XRD patterns and Raman spectra in Fig. 2 and 5, respectively). Amorphous structure does not possess ordered atomic arrangement that allows rapid movement of H+ through the structure. By contrast, H+ moved slower in higher crystallinity structure and resulted in slower intercalation and de-intercalation processes. A cathodic current (reduction) peak was present on the cyclic voltammograms of the as-prepared nanoplates (crystalline WO3
H2O) and nanoplates annealed at 100  C (crystalline WO3
H2O) and 400  C (crystalline WO3) (Fig. 10a, 10b, and 10e). This peak only visible in the crystalline samples [27–29]; thus, the peak was not present in the lower crystallinity nanoplates annealed at 200 and 300  C (Fig. 10c and 10d). Meanwhile, the anodic current (oxidation) peak of the nanoplates moved to a more positive electric potential value with increasing cathodic current density, as seen in Fig. 11. This relation is because higher electric potential was required to de-intercalate the increasing amount of

intercalated H+ from nanoplates. The coloration efficiency (CE) of the nanoplates, which represents the change in optical density (OD) per unit charge density (Q/A) intercalated into the nanoplate film, can be calculated according to Eq. (3) and (4) [22,23].

Table 1 Intercalated charge density of the as-prepared and annealed nanoplates Nanoplate films

Charge density (mC cm2)

As-prepared 100  C 200  C 300  C 400  C

2.65 2.71 7.66 6.11 4.39

CE = DOD/(Q/A)

(3)

DOD = log (Tb/Tc)

(4)

where Tb and Tc are the transmittances of the nanoplates in its bleached and colored states, respectively. The calculated CE of the nanoplates annealed at 200  C is 28.64 cm2 C1, which is comparable to the reported CE values (21.6–38.2 cm2 C1) of WO3 nanoplates, nanobricks, and microbricks [24,25,30–33]. Chronoamperometry measurement was performed to determine the ion insertion (coloring) and extraction (bleaching) times of the nanoplates (annealed at 200  C), as shown in Fig. 12. The ion insertion and extraction times (extracted for a 90% decrease of current densities) were 9.6 s and 5.1 s, respectively. These switching times are acceptable and comparable to the reported hydrothermal works, where the ion insertion and extraction times (measured from current transient) were reported to be in the range of 4.29–11.05 and 3.18–8.23 s, respectively [7,34–36]. 4. Conclusions The acid-treated WO3
H2O nanoplates were transformed into WO3 after annealing at 200–400  C, and the nanoplate thickness decreased with increasing annealing temperature. WO3 nanoplates showed higher charge density than the WO3
H2O nanoplates. However, the charge density decreased with increasing crystallinity of WO3. WO3 nanoplates annealed at 200  C showed coloration efficiency of 28.64 cm2 C1, high optical modulation (58%), good electrochromic cycling stability, and short ion insertion and extraction times of 9.6 s and 5.1 s, respectively, making them a highly suitable structure for electrochromic device fabrication. Acknowledgements The authors are grateful to the financial support provided by MyBrain15 and Long Term Research Grant Scheme (OneBAJA Project 304/PBAHAN/6050235) from the Ministry of Higher Education Malaysia and PRGS 1001/PBAHAN/8044020 from Universiti Sains Malaysia.

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