electrochromics of single crystalline wo3 · h2o nanorods

Electrochromics of single crystalline WO 3 Æ H 2 O nanorods Xiaolan Wei 1 , Pei Kang Shen * State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen University, 135 Xingang Road, Guangzhou, Guangdong 510275, China Received 20 November 2005; received in revised form 9 December 2005; accepted 9 December 2005 Available online 18 January 2006 Abstract Single crystalline WO 3 Æ H 2 O nanorods have been prepared using a facile solution route for the first time, in which PVA-124 is used as a structure-directing agent and glacial acetic acid as a stabilizer. The results prove that the x value in the fully reduced form of M x W 6+ (1Àx) W 5+ x O 3 is close to 1 for the rod-type WO 3 Æ H 2 O nanocrystals instead of 0.7 for the platelet-type WO 3 Æ H 2 O nanocrystals. The electrochromic performance of the rod-type WO 3 Æ H 2 O nanocrystals is significantly improved in terms of fast response time and high contrast due to the plane with wider lattice spacing of d = 5.36 A ˚ is parallel to the growth direction and faces towards the electrolyte. Ó 2005 Elsevier B.V. All rights reserved. Keywords: WO 3 Æ H 2 O nanorods; Single crystals; Electrochromics; One-dimensional oxides 1. Introduction It is well known that tungsten oxide (WO 3 ) has promis- ing properties for applications in electrochromic devices [1–6], gas sensors [7,8], photoelectrochemistry [9] and elect- rocatalysis [10,11]. As an electrochromic material, WO 3 is the first and most extensivel y studied compound and remains the most promising candidate for electrochromic devices, such as antidazzle car rear-view mirrors and smart windows [2–6]. A great deal of effort has been placed on improving the performance of WO 3 films [2–6,12,13], including the efforts to obtain nanoscale porous tungsten oxide films [14,26]. However, there has been no momentous breakthrough for decades. In the latest reports, new and interesting applications of existing or prospective tungsten oxide monohydrate based devices have come to light [15]. An all-plastic WO 3 Æ H 2 O/polyaniline flexible electrochro- mic device was reported recently [16]. Both amorphous and crystalline electrochromic WO 3 films were prepared and compared [17]. Crystalline WO 3 showed pronounced coloring persistence relative to amor- phous. However, intercalation and deintercalation appeared to be faster for a-WO 3 due to the slower kinetics for c-WO 3 which possesses denser structure and smaller diffusion pathway. Livage and Guzman [18] synthesized hydrous WO 3 Æ H 2 O crystals aimed to improve the response time. WO 3 Æ H 2 O crystals belong to orthorhombic crystal system. WO 3 Æ H 2 O is formed of layers built up by corner sharing [WO 6 ] octahedra, with water molecules between these planes. Intercalated water molecules lead to WO 3 Æ H 2 O layers with a basal spacing d = 5.36 A ˚ , exhibit- ing enhanced intercalation properties towards cationic spe- cies such as H + ,Li + , even long-chain alkylammonium ions comparing with anhydrous WO 3 [18]. If a surface with wide spacing, d = 5.36 A ˚ in WO 3 Æ H 2 O is exposed to electrolyte, the efficiency of intercalation towards cationic species should be enhanced, and the redox performance would be improved. One-dimensional WO 3 Æ H 2 O nanocrystals would meet this expectation. One-dimensional nanostruc- tures may offer opportunities for investigating the effect of size and dimensionality on their comprehensive optical, magnetic, and electronic properties [19] . 1388-2481/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2005.12.008 * Corresponding author. Tel.: +86 20 84036736; fax: +86 20 84113369. E-mail address: stdp32@zsu.edu.cn (P.K. Shen). 1 Present address: Department of Applied Chemistry, South China University of Technology, Guangzhou, China. www.elsevier.com/locate/elecom Electrochemistry Communications 8 (2006) 293–298 One-dimensional oxides can be prepared by different methods [20–23]. However, the controllable growth of the selective crystal plane is extremely difficult. In this paper, we report a facile solution route to single crystalline WO 3 Æ H 2 O shaped with rod and platelet. Poly(vinyl alco- hol) (PVA-124) was used as a structure-directing agent and glacial acetic acid as a stabilizer. The effect of the mor- phologies of WO 3 Æ H 2 O for the same crystalline phase and the different intercalation degrees of cations into the WO 3 films on the electrochromic performance was investigated. 2. Experimental 2.1. WO 3 Æ H 2 O nanocrystal preparation The rod-type WO 3 Æ H 2 O was prepared by adding 0.4 g PVA-124 (average polymerization degree 2400–2500, Kura- ray, Japan) in 40 ml distilled–deionized water and stirring at 100 °C for 10 min. 5 ml HAc and 4 g Na 2 WO 4 Æ 2H 2 O were added after the solution had been cooled to 60 °C. The mix- ture was stirred at 60 °C for 30 min to form a transparent solution. Afterwards , 15 ml 5 mol L À1 HNO 3 was added into the solution very slowly (0.4 mL per minute) at the same temperature, resulting in a pale yellow solution. Keep- ing the solution at 60 °C for 4 h, the yellow precipitates formed increasingly and then were aged at room tempera- ture for another 4 days. The precipi tates were separated and washed carefully using distilled-deionized water until the sodium ions are undetectable. Finally, the precipitates were washed by acetone and dried in air. The platelet-type WO 3 Æ H 2 O was prepared by the same procedure. The differ- ence was that the 5 ml concentrated HNO 3 solution instead of 15 mL 5 mol L À1 HNO 3 was added at fast speed and without aging. The precipitates were formed during the addition of the concentrated HNO 3 . The purified rod-type and platelet-type WO 3 Æ H 2 O was, respectively, suspended into distilled-deionized water under an ultrasonic stirring for 1 h to form a colloidal solution (5 g L À1 WO 3 Æ H 2 O). WO 3 Æ H 2 O films were formed by dropping 20 lL of the col- loidal solution onto pre-cleaned 1 cm 2 ITO glass substrates (15 X/h) and then drying in air at room temperature. 2.2. WO 3 Æ H 2 O nanocrystal characterization The microstructure of WO 3 Æ H 2 O was examined with a high-resolution transmission electron microscope (HRTEM, JEOL JEM–2010, operated at 200 kV). An X- ray diffractometer D/Max-IIIA (Rigaku Co., Japan) using Cu K a1(k = 1.540056 A ˚ ) as the radiation source, which was used to examine the crystalline structure. The measure- ment of the electrochromic properties of WO 3 Æ H 2 O films was carried out in situ on VoltaLab 80 Universal Electro- chemical Laboratory (Radiometer Analytical Laboratory, France) and a UV-2102PC Spectrometer (UNICO Instru- ment Co., USA) in a standard three-electrode cell. A plat- inum wire was used as the counter electrode and Ag|AgCl as the reference electrode, respectively. Cyclic voltammetric experiments were carried out in the potential range of À0.6 V to 0.6 V (vs. Ag|AgCl) at different scan rates. 3. Results and discussion Either rod-type or platelet-type WO 3 Æ H 2 O nanocrystals could be prepared, depending on the preparation condi- tions and the aging time. Fig. 1 shows the TEM images of the prepared rod-type WO 3 Æ H 2 O nanocrystals (Fig. 1a and c) and platelet -type WO 3 Æ H 2 O nanocrystals (Fig. 1 b). The corresponding selected area electron diffrac- tion (SAED) patterns shown in the insets of Fig. 1a and b indicate that both of the two nanocrystals are single crys- tals. On the top of rod-type WO 3 Æ H 2 O nanocrystals, the lattice spacing of d = 5.36 A ˚ is parallel to the growth direc- tion and is faced towards the electrolyte. The wider spacing (5.36 A ˚ ) towards the surface would be of benefit to the H + intercalation and deintercalation, resulting in an increased electrochromic speed. While for the platelet-type WO 3 Æ H 2 O nanocrystals, the plane faced towards the electrolyte is the plane with the lattice spacing of d = 3.47 A ˚ in two- dimensional directions. The crystalline phase of the prepared WO 3 was identi- fied by X-ray powder diffraction (Fig. 2). XRD patterns of the products were exclusively orthorhombic WO 3 Æ H 2 O (Pmnb, a = 5.249 A ˚ , b = 10.71 A ˚ , c = 5.133 A ˚ , JCPDS- ICDD 84-0886). Three strongest lines in the Fig. 2a (rod- type sample) are 3.4795 A ˚ , 5.3875 A ˚ and 2.5742 A ˚ and in Fig. 2b (platelet-type sample) are 3.4793 A ˚ , 5.3833 A ˚ and 2.5714 A ˚ . These values are consistent with the standard values of 3.47 A ˚ , 5.36 A ˚ and 2.56 A ˚ in JCPDS-ICDD 84- 0886, corresponding to (111), (020) and (1 31) planes. The results suggest that the rod-type and the platelet-type nanocrystals are the same in c rystalline phase. The inten- sity ratio of the peak (020) to peak (111) for rod-type sam- ples is larger than that of for platelet-type samples, suggesting that the abundant planes in rod-type WO 3 Æ H 2 O are (0 2 0) planes, and vice versa. No diffraction peaks due to WO 3 Æ 2H 2 O or other tungsten oxides were discerned, which indicates the high purities of the nanorods and nanoplatelets. The confirmation of the WO 3 Æ H 2 O structure was further conducted by the thermogravimetric (TG) analyses (Fig. 3). It is obv ious that the weight loss before 60 ° C is the evaporation of water moisture. After- wards, the WO 3 Æ H 2 O started to decompose and lose the coordinated water at higher temperature. The weight loss measured was 6.5% for sample (a) and 6.7% for sample (b), respectively, which are in keeping with the theoretical calculation (7.2%) for the WO 3 Æ H 2 O structure. WO 3 Æ H 2 O is formed of layers built up by corner shar- ing [WO 6 ] octahedral, with water molecules between these planes. Hydrous oxides are precipitated upon the acidifi- cation of tungsten [WO 4 ] 2À (Na 2 WO 4 as precursors in this study). Coordi nation expansion leads to the formation of sixfold coordinated W VI via the nucleophilic addition of two water molecules [18]. One water molecule is bonded along the z-axis opposite to the W@O bond while the four 294 X. Wei, P.K. Shen / Electrochemistry Communications 8 (2006) 293–298 OH groups are in the equatorial xy plane. Oxolation along equivalent x and y directions leads to the formation of the layered WO 3 Æ H 2 O nanocrystals (Fig. 4a). The structure of the oxide network depends on the acidification rate and the aging time. The preparation of platelet WO 3 Æ H 2 O has been reported by different methods. Bala ´ zsi and Pfeifer [24] prepared microscaled platelet WO 3 Æ H 2 O under the quick addition of strong acid without the control. Stucky and co-workers [25] used benzyl alcohol and WCl 6 synthe- sized platelet WO 3 Æ H 2 O. In our case, nanoscaled platelet WO 3 Æ H 2 O crystals were prepared in the presence of glacial acetic acid and PVA-124 under the quick addition of con- centrated HNO 3 . However, by controlling the adding rate of the dilute HNO 3 and the aging time we synthesized the nanorod-type WO 3 Æ H 2 O for the first time. PVA-124 is a structure-directing agent in this study. It is estimated that the distance between two hydroxyl oxygen is 0.251 nm since the C–C bond is 0.154 and \CCC is 109°. The hydrogen of hydroxyl group will tilt due to the influ- ence of lone pare electrons to fit the narrow spacing of W–O–W (3.47 A ˚ ) by the formation of hydrogen bonds (Fig. 4b). The crystals growth substantially favors the direction of (0 20) plane instead of (111) plane. However, Fig. 1. TEM images of the rod-type (a) and platelet-type (b) WO 3 Æ H 2 O nanocrystals. (c) HRTEM image of rod-type WO 3 Æ H 2 O nanocrystals. The selected area electron diffraction patterns of the rod-type (inset in a) and platelet-type (inset in b) WO 3 Æ H 2 O nanocrystals are shown as well. 10 20 30 40 50 60 70 80 0 200 400 600 0 200 400 600 131 111 020 b 2theta(deg.) 131 111 020 a Intensity/cps Fig. 2. XRD patterns of WO 3 Æ H 2 O nanorod (a) and nanoplatelet (b). X. Wei, P.K. Shen / Electrochemistry Communications 8 (2006) 293–298 295 the PVA-124 might lose the effect if the crystal growth is too fast. The electrochromic properties of the nanorod and nano- platelet WO 3 Æ H 2 O were characterized as a typical applica- tion. Fig. 5a shows the cyclic voltammograms of WO 3 Æ H 2 O films on indium-doped tin oxide (ITO) glass substrates. The CV curves shaped as previous reported [26–28]. It is worth noting that the second anodic peak is remarkable and shows evidence of the existence of meso- porous structure [26,27]. Comparing the current density of the platelet WO 3 Æ H 2 O and the rod-type WO 3 Æ H 2 Oat the same loading (in mg cm À2 of WO 3 Æ H 2 O), we found that more than double the current density was observed on the latter sample. For example, the reduction current 0 50 100 150 200 250 300 b -24 -20 -16 -12 -8 -4 Heat flow/mW 0 50 100 150 200 250 300 80 84 88 92 96 100 104 Temperature/ o C a Weight/% Fig. 3. Thermogravimetric (TG) analyses of (a) rod-type WO 3 Æ H 2 O and (b) platelet-type WO 3 Æ H 2 O. W O O H 2 O W O O H 2 O W O O W O H 2 O O H 3 O £« OH 2 C C CH 2 C CH 2 HO O O H H O O H O O H W W O O W W O O ~0.35nm ~109 o ¡« 0.2 5 0 . 1 5 4 O O ab Fig. 4. Structures of (a) layered WO 3 Æ H 2 O and (b) hydrogen bond between WO 3 Æ H 2 O and PVA-124. -600 -400 -200 0 200 400 600 -40 -30 -20 -10 0 10 20 30 2 1 a Potential/mV uCrrneedtsntiy/mA mc 2- gm 1- 050250 75 80 85 90 95 b 2 1 Trans/% Time/s Fig. 5. Cyclic voltammograms of platelet-type (1) and rod-type WO 3 Æ H 2 O (2) in 0.5 mol L À1 H 2 SO 4 with the scan rate of 100 mV s À1 (a) and the in situ transmittance changes (b). 296 X. Wei, P.K. Shen / Electrochemistry Communications 8 (2006) 293–298 density of rod-type WO 3 Æ H 2 O was 40 mA cm À2 mg À1 and only 6 mA cm À2 mg À1 for the platelet-type sample at À0.6 V. The in situ UV–Vis transmittance responses at 650 nm during the potential cycling were recorded and shown in Fig. 5b. When the WO 3 Æ H 2 O films were cathod- ically polarized, the films turned blue in color and the color intensified with the increase in the cathodic potential. The blue films were bleached under the anodic polarization. The optical responses were symmetrical upon the symmet- ric potential cycling, indicating electrochromic reversibility. Also, the change in transmittance was more than doubled for rod-type compared with that of platelet-type WO 3 Æ H 2 O. The possible explanation of why the current density or the contrast of the two samples is different at the same potential with the same loading is that the amount of inserted cations is different. The color of WO 3 film switches reversibly from transparent to blue upon electrochemical redox reactions WO 3 þ xðM þ þ e À Þ!M x W 6þ ð1ÀxÞ W 5þ x O 3 M ¼ H; Li; Na; When a negative potential is suppli ed to the WO 3 Æ H 2 O, there is an influx of conducting electrons into the tungsten oxide film. To keep the system electrically neutral, cations (H + ,Li + ) should move into the film from the surrounding electrolyte. Without this charge compensation, the elec- trons could not be injected into the film. The diffusion of the cations into the oxide layer is slow which determines the response time. We presum ed that the nano WO 3 Æ H 2 O with the larger lattice spacing toward the electrolyte would be faster in coloration/bleaching rate. The cathodic charges were integrated and compared at different scan rates. These charges corresponded to the for- mation of a H x WO 3 compound with different x values, assuming all the charge intercalated contributed to interca- lation of H ions. The stoichiometric charge for one electron reduction of WO 3 is 387 mC mg À1 . Therefore, the x values can be calculated. Table 1 compares the electrochromic effi- ciencies of two nano WO 3 Æ H 2 O. Fig. 6 shows the plots of x against scan rate. In the case of rod-type WO 3 Æ H 2 O, the x value closes 1 at fully reduced state and is still over 0.9 at scan rate as high as 20 mV s À1 . While, platelet-type WO 3 Æ H 2 O could not be fully reduced and the x value is only less than 0.3 at scan rate of 5 mV s À1 . The results indicate that the rod-type WO 3 Æ H 2 O can be deeply colored by the insertion of more cations. Moreover, the coloration/b leaching rate is very fast as a result of the fast mass transport due to the direct connection of electrolyte with larger lattice spacing. The be tter redox performance of the rod-type WO 3 Æ H 2 O nanocrystals is related to its structure. The hydrogen ions in the solution are normally hydrated and intercalate into the oxide matrix as hydrous ions. The inser- tion of the hydrous ions is easier along with the larger lat- tice spacing. The (020) plane of the rod-type WO 3 Æ H 2 O faced to the solution and the hydrous hydrogen ions inserted into this plane by relaying water molecules in the layer spacing one by one as shown in Fig. 4b. The short depth and larger width of the rod-type WO 3 Æ H 2 O struc- ture resulted in a fast coloration/bleaching speed and fully intercalation. The hydrous hydrogen ions inserted into the oxide matrix along with the (1 1 1) plane that is perpendic- ular against the (0 20) plane in the case of platelet-type WO 3 Æ H 2 O. The intercalation and deintercalation speed of the cations are slower due to the small lattice spacing and without the relay by the bonded water. 4. Conclusion A facile solution route prepared single crystalline WO 3 Æ H 2 O nanorods for the first time, in which PVA- 124 was used as a structure-directing agent and glacial acetic acid as a stabilizer. For the rod-type WO 3 Æ H 2 O nanocrystals, the lattice spacing of d = 5.36 A ˚ is parallel Table 1 Comparison of the electrochromic efficiencies of two nano WO 3 Æ H 2 O at different scan rates Rod-type WO 3 Æ H 2 O (mV s À1 ) 100 50 20 5 1 Charge, mC 9.1 9.5 18.7 19.3 19.5 Charge, mC mg À1 172 179 352 364 368 x value 0.44 0.46 0.91 0.94 0.95 Platelet-type WO 3 Æ H 2 O (mV s À1 ) 100 50 20 5 1 Charge, mC 6.9 7.1 9.4 9.6 24.9 Charge, mC mg À1 77 79 104 107 276 x value 0.198 0.204 0.27 0.28 0.71 0 20 40 60 80 100 0.0 0.2 0.4 0.6 0.8 1.0 x Scan rate / mV s -1 rod platelet Fig. 6. Plots of the normalized amount of inserted ions against the scan rate. X. Wei, P.K. Shen / Electrochemistry Communications 8 (2006) 293–298 297 to the growth direction and is faced toward the electrolyte. The wider spacing (5.36 A ˚ ) toward the surface would be of benefit to the H + intercalation and deintercalation, result- ing in an increased electrochromic speed, whereas for the platelet-type WO 3 Æ H 2 O nanocrystals, the plane faced toward the electrolyte is the plane with the lattice spacing of d = 3.47 A ˚ in two-dimensional directions. The results prove that the x value in the fully reduced form of M x W 6+ (1Àx) W 5+ x O 3 is close to 1 for the rod-type WO 3 Æ H 2 O nanocrystals, while it is 0.7 for the platelet-type WO 3 Æ H 2 O nanocrystals. The fast response time and high x value of the rod-type WO 3 Æ H 2 O nanocrystals improved significantly the redox performance. Our results also point to the fundamentally impor tant possibility of fine-tuning the electrochromic performance and catalytic activity of tungsten oxide and other transition metal oxides. 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