Ultrathin W18O49 Nanowire Assemblies for Electrochromic Devices

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Ultrathin W18O49 Nanowire Assemblies for Electrochromic Devices Jian-Wei Liu, Jing Zheng, Jing-Long Wang, Jie Xu, Hui-Hui Li, and Shu-Hong Yu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl401304n • Publication Date (Web): 19 Jul 2013 Downloaded from http://pubs.acs.org on July 24, 2013

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Ultrathin W18O49 Nanowire Electrochromic Devices

Assemblies

for

By Jian-Wei Liu, Jing Zheng, Jin-Long Wang, Jie Xu, Hui-Hui Li & Shu-Hong Yu* Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China. *

Corresponding author: Fax: +86 551 63603040, E-mail: [email protected]

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Abstract: Ordered W18O49 nanowire thin films were fabricated by Langmuir Blodgett (LB) technique in the present of poly(vinyl pyrrolidone) coating. The well-organized monolayer of W18O49 nanowires with periodic structures can be readily used as electrochromic sensors, showing reversibly switched electrochromic properties between the negative and positive voltage. Moreover, the electrochromism properties of the W18O49 nanowire films exhibit significant relationship with their thickness. The coloration/bleaching time was around 2 s for the W18O49 nanowire monolayer, which is much faster than the traditional tungsten oxide nanostructures. Moreover, the nanowire devices display excellent stability when color switching continues, which may provide a versatile and promising platform for electrochromism device, smart windows and other applications.

KAYWORDS: W18O49 nanowires, Electrochromic, Poly(vinyl pyrrolidone), Macroscopic-scale assembly

Graphic Figure


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Macroscopic-scale nanofabrication using ordered nanowire nanostructures with new functionalities has become one of the most active research areas of materials science.1-, 2, 3, 4, 5, 6 Semiconductor nanowires have been intensively investigated because they possess distinctive physical and chemical properties, which endow them to be effective candidates in various applications.7-, 8, 9, 10, 11, 12 Strategies to assembly semiconductor nanowires into complex and novel mesostructures have appeared during the past few years, resulting in new properties and functionalities, exhibiting broad application potentials that range from traditional electronic devices (logic and memory) to novel bio-molecular and chemical sensors, thermoelectric materials, and optoelectronic devices.1, 7, 13-, 14, 15, 16 Among the traditional semiconductors, tungsten oxide (WO3-y), as an important n-type semiconductor, has attracted considerable attention due to its applications in electrochromic, photocatalytic, and gas sensing materials.17-, 18, 19, 20, 21, 22 Electrochromism is the phenomenon related to color changes induced in selected materials by a reversible electrochemical process.23, 24 It results from the generation of different electronic absorption bands in the visible region, which is correlated to redox states switching. As a well-known inorganic electrochromic material, WO3-y can display colorless and blue color by alternately applying positive and negative electrical voltages which has been extensively studied since it was discovered by Deb in 1969.25, 26 Poly(vinyl pyrrolidone) (PVP), as a common surfactant, has been extensively used in the solutionphase synthesis of many types of nanomaterials, where it is mainly considered as a steric stabilizer or capping agent with a major role to protect the product from agglomeration.27,

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Besides synthesis,

surfactants, for example PVP, play an important role in assembly.14, 29 Herein, we report the fabrication of ordered W18O49 nanowire thin films by LB technique. The wellorganized monolayer of W18O49 nanowires with periodic structures can be readily used as electrochromic sensors, showing reversibly switched electrochromic properties between the negative ACS Paragon Plus Environment

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and positive voltage. Moreover, the electrochromism properties exhibit significant relationship with the thickness of the nanowire film.

Figure 1. Morphology and structure of the as-obtained W18O49 nanowires. (a, b) SEM images of the W18O49 nanowires with different magnifications. (c) TEM image of disordered nanowires. (d) HRTEM image of the single W18O49 nanowire. Insets of (a, d): Picture of W18O49 nanowires dispersed in water and crystal structure of monoclinic W18O49 nanowires.

Firstly, uniform, hydrophilic W18O49 nanowires with a high aspect ratio, which were sub 5 nm in diameter and tens of µm in length were obtained by modified methods described previously30 (Figure 1a, b and c). The dispersion of ultrathin W18O49 nanowires in water shows an ivory color (insets in Figure 1a). A high-resolution transmission electron microscopy (HRTEM) image taken on a single nanowire showed that it was well crystalline (Figure 1d). The X-ray diffraction (XRD) pattern confirms the products obtained from reduction of WCl6 in the presence of ethanol by adding a small amount of PVP by hydrothermal process (see Supporting Information, Figure S1). All peaks in this pattern can be indexed to the monoclinic structure type (P2m) of W18O49 with a cell constants a = 18.318 Å, b = 3.782 Å, and c = 14.028 Å, which are in good


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agreement with the standard literature data (JCPDF card number: 84-1516). The XPS spectra of the freshly prepared monolayer of W18O49 nanowires, and the peak values at 35.7 and 37.8 eV can be readily assigned to the binding energies of W6+ (see Supporting Information, Figure S2), indicating that the samples are composed of element W.30 The Raman scattering spectrum taken for the synthesized W18O49 nanowire monolayer at room temperature shows the characteristic vibration peaks at 267, 778, and 969 cm-1 (see Supporting Information, Figure S3), which are close to those reported previously.31 An energy dispersive X-ray spectrometer (EDS) spectrum was used to analyze the composition of the nanowires (see Supporting Information, Figure S4). Strong W peaks undoubtedly confirmed that the product contains W. The signals of Cu and C peaks come from the carbon-coated copper grid, which is a normal observation for TEM samples.

Figure 2. Two styles of ordered nanowire nanostructures. (a-d) TEM and SEM images the assembled nanowire films with parallel and cross style. Inset of a and b, Illustrate pictures.


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W18O49 nanowires which are free of surfactant by Xi et al. were hard for us to assembly into ordered assemblies.30 Coated by PVP, however, ordered W18O49 nanowire films could be fabricated by the LB techniques.13, 14 Schematic illustrations of transferring of two layers of aligned nanowires into structures with designed both in parallel and crossing style was illustrated in Figure 2a and 2b. After the compressed nanowire monolayer transferred directly to a planar substrate to yield, holding 300 s, a crossed structure of two layers can be obtained by turning an angle during deposition. While without turning the angle, TEM and SEM images in Figure 2c and Figure 2e show the as-obtained assembled W18O49 nanowire monolayer parallels to each other and aligned side-by-side over a large area (see Supporting Information, Figure S5). TEM and SEM images in Figure 2d and Figure 2f show the W18O49 nanowire film with the cross style, when the turning angles could be also designed.

Figure 3. Electrochromic performance of the as-prepared ordered W18O49 nanowire films. (a) Illustration of reversible electrochromic W18O49 nanowire film device. (b) Picture of ordered W18O49 nanowire films with different thickness on ITO substrate prior (left) and after (right) to application of the external voltage. (c, d) Optical transmittance of ordered W18O49 nanowire films with different thickness on ITO substrate prior (left) and after (right) to application of the external voltage. (e, f) Electrochromic switching of the ordered W18O49 nanowire films. Optical transmittance and electrochromic switching monitored at 632.8 nm for the cycling ability study of W18O49 nanowire films at an applied voltage of -1 V for 30 s and 1 V for 30 s for cycles.

Inorganic electrochromic (EC) devices are capable of reversibly changing their optical properties


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(transmittance, absorbance and reflectance) upon ions insertion/extraction into/out of EC materials induced by the external voltage.32, 33, 34 Compared with traditional EC materials, the nanoscale devices enable new functions, high efficiency, and enhanced performance (Figure 3a). The Li ions and electrons insertion and extraction result in the coloration and bleaching processes of the W18O49 nanowire films. The electrochromic mechanism of W18O49 in 0.1 mol/L LiClO4 aqueous solution as the electrolyte can be expressed as: insertion  → LixW18O49 W18O49 + xe − + Li + ←  extraction

Electrons and Li+ ions are co-inserted (blue, colored state) and extraction (colorless, bleached state) into the W18O49 nanowire films which results in its coloration and bleaching processes. As shown in Figure S6, the current moved negatively with decreasing voltage, corresponding to the co-intercalation of electrons and Li+ into W18O49 nanowire films to form LixW18O49. Well-aligned PVP coated W18O49 nanowire monolayer or multi-layer films with a diameter of sub 5 nm and a length of tens of micrometers can be rapidly obtained by modified LB techniques.14 LB technique is a good candidate for arranging vast numbers of nanostructures, including nanoparticles, nanorods, and nanowires.13 PVP was used in the manuscript as the surfactant to improve the nanowire assembly. Firstly, the interaction between the surfactants (PVP) and the nanowires cause the nanowires to float on the water surface of a LB trough.

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Secondly, the week interactions between PVP of the

nanowires benefit the nanowire assembly, during the solvent (typically chloroform or hexane) evaporation process.36 The electrochromic properties of the nanodevices were investigated with an IM6ex electrochemical workstation (Zahner, Germany) and an UV-2501PC/2550 (Shimadzu Corporation, Japan) at room temperature in air. The observed electrochromic phenomena of the asprepared ordered W18O49 nanowire films on ITO substrate without any treatment were shown in Figure 3b. It was found directly that the as-prepared nanowire film is almost colorless even the thickness was


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approximately from sub 5 nanometer of monolayer to 200 nanometers of 20-layer nanowire films, which indicates the film is transparent enough. When the external voltage is 0 V, from monolayer, twolayer, five-layer, ten-layer, fifteen-layer, to twenty-layer W18O49 nanowire films, the optical transmittance exceeded 95%, 95%, 90%, 87%, 83% to 76%, respectively, in the range 450~700 nm, covering the majority of the visible light region (Figure 3c). As a result, the ultrathin W18O49 nanowire electrode, optimized for a glass substrate by enhancing the total optical transmittance in the visible region, has potential for use in high transparency electrochromic devices. When the external voltage is 1 V, the films turn to blue immediately which is more distinct with the thickness increasing (Figure 3d). The color and the optical transmittance of film differ from the value of the external voltage, which is shown in Figure S7. The external voltage on in situ transmittance spectra of the W18O49 nanowire films is +1, +0.3, +1, 0, -0.1, -0.3, -0.5, -0.7, and -1 V, respectively. The color is deeper, when the voltage is increasing from 0 to -1 V, while the voltage is more than -1.0 V, the color change is not obvious, which is more smaller than many literature.37 Increasing the thickness to 20 layers, the film displayed a deep blue color. The obvious color change indicates that the ordered W18O49 nanowire has potential for further use. Correspondingly, the optical transmittance decreased to 84%, 80%, 38%, 14%, 12% and 8.8%. Thus, decreasing thickness of the nanowire films lead to accelerated ions insertion/extraction into/out of EC materials, benefiting the response speed of the device. However, as the thickness of the nanowire films down to monolayer, the color change between 1 V and -1 V turns inconspicuous. When a voltage of +1.0 V was applied to the deep blue film, its color was bleached quickly and a transparent film was obtained again. Figure 3e and 3f show the in situ coloration/bleaching characteristic of the asprepared nanowire films recorded at the absorbance wavelength of 632.8 nm. The coloration and bleaching time increases gradually as the thickness increased. The coloration/bleaching time of the monolayer nanowire film is less than 2 s (see Supporting Information, Figure S8a), which is much faster than the amorphous and crystalline WO3 structures,37, 38 and TiO2 nanostructures.39 As the thickness increases, the coloration/bleaching time grows gradually. When the thickness reaches 20 layers (see


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Supporting Information, Figure S8), the coloration/bleaching time is closed to 30s which is similar to the previous literature. The fast switching time should be attributed to the large active surface and ultrathin thickness of the as-assembled nanowire films, which makes the nanowire device suitable for electrochromic devices. As we know, tiny poly (vinyl pyrrolidone) (PVP) as the surfactants is benefited for the nanowire assembly process by LB technique.14 To test the property degradation, the PVP of nanowires is fully removed after the device fabrication by plasma cleaner (MYCRO, MODEL PDC002). From the FTIR spectra, we can find the PVP is fully removed after the plasma treatment (see Supporting Information, Figure S9a). And the corresponding electrochromic performance shows that there is no obvious optimization when the PVP is removed (see Supporting Information, Figure S9b). In our opinion, the amount of PVP on the nanowires is too small to affect its property. So, tiny PVP as surfactant is Okay. Compared the electrochromic performance of the randomly and ordered arranged nanowires with the same nanowire density, and the electrochromic performance of the disordered nanowire films shows unstable, and the repeatability is poorer (see Supporting Information, Figure S10a and Figure S10b). Moreover, the periodicity of the nanowires can be cursory tuned by controlling the surface pressure (see Supporting Information, Figure S11a and Figure S11b). It was found that the electrochromic performance turns better when the ordering degree increases.

Figure 4. (a, b) Stability of electrochromic switching of the as-prepared ordered 20-layer W18O49 nanowire films. (a) electrochromic switching for continues 5000 s. (b) Enlarged plots from 1000 s to 1300 s. ACS Paragon Plus Environment

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To estimate the stability of the switch states, we probed them continuously by applying electric pulses for more than 5000 s of cycles monitored at 632.8 nm at an applied voltage of -1 V for 30 s and +1 V for 30 s (Figure 4). The results show that the nanowire device retained a state even after about 1000 number of cycles (see Supporting Information, Figure S12), showing no state disturbance (no overlap of a bleached state and a colored state). In summary, we report a new route for ordering W18O49 nanowires with high aspect ratios by LB technique in the presence of PVP coating. An electrochromism nano-device can be fabricated based on the well-organized W18O49 nanowire films. The electrochromism performance of the ordered nanowire films is strongly dependent on the thickness of the nanowire films. With the thickness increases, the color change is bigger. Decreasing thickness of the nanowire films from 20-layer to monolayer lead to accelerated ions insertion/extraction into/out of EC materials, benefiting the response speed of the device, especially for the ordered W18O49 nanowire monolayer, which the coloration/bleaching time was around 2 s, which is much faster than the traditional tungsten oxide nanostructures. Moreover, the nanowire devices display excellent stability when color switching continues, which may provide a versatile and promising platform for electrochromismdevice, smart windows and other applications.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS S.H.Y. acknowledges the funding support from the National Basic Research Program of China (Grant 2010CB934700), the National Natural Science Foundation of China (Grant 91022032), the Chinese ACS Paragon Plus Environment

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Academy of Sciences (Grant KJZD-EW-M01-1), and the Principal Investigator Award by the National Synchrotron Radiation Laboratory at the University of Science and Technology of China. J.W.L. thanks the Fundamental Research Funds for the Central Universities (WK 2340000033) REFERENCES (1) Liu, J.-W.; Liang, H.-W.; Yu, S.-H., Chem. Rev. 2012, 112 (8), 4770-4799. (2) Ozin, G. A.; Hou, K.; Lotsch, B. V.; Cademartiri, L.; Puzzo, D. P.; Scotognella, F.; Ghadimi, A.; Thomson, J., Mater. Today 2009, 12 (5), 12-23. (3) Goldberger, J.; Hochbaum, A. I.; Fan, R.; Yang, P. D., Nano Lett. 2006, 6 (5), 973-977. (4) Sun, X.; Chen, T.; Yang, Z.; Peng, H., Acc. Chem. Res. 2012, 46 (2), 539–549. (5) Whang, D.; Jin, S.; Wu, Y.; Lieber, C. M., Nano Lett. 2003, 3 (9), 1255-1259. (6) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y.; Yang, P., Nano Lett. 2003, 3 (9), 1229-1233. (7) Yan, H.; Choe, H. S.; Nam, S. W.; Hu, Y. J.; Das, S.; Klemic, J. F.; Ellenbogen, J. C.; Lieber, C. M., Nature 2011, 470 (7333), 240-244. (8) Yang, P. D.; Law, M.; Goldberger, J., Ann. Rev. Mater. Res. 2004, 34, 83-122. (9) Thomson, J. W.; Lawson, G.; O'Brien, P.; Klenkler, R.; Helander, M. G.; Petrov, S.; Lu, Z. H.; Kherani, N. P.; Adronov, A.; Ozin, G., Adv. Mater. 2010, 22 (39), 4395-4400. (10) Hochbaum, A. I.; Yang, P. D., Chem. Rev. 2010, 110 (1), 527-546. (11) Xu, S.; Qin, Y.; Xu, C.; Wei, Y. G.; Yang, R. S.; Wang, Z. L., Nat. Nanotechnol. 2010, 5 (5), 366-373. (12) McDowell, M. T.; Lee, S. W.; Ryu, I.; Wu, H.; Nix, W. D.; Choi, J. W.; Cui, Y., Nano Lett. 2011, 11 (9), 4018-4025. (13) Tao, A. R.; Huang, J. X.; Yang, P. D., Acc. Chem. Res. 2008, 41 (12), 1662-1673. (14) Liu, J. W.; Zhu, J. H.; Zhang, C. L.; Liang, H. W.; Yu, S. H., J. Am. Chem. Soc. 2010, 132 (26), 8945-8952. (15) Bai, C.; Liu, M., Nano Today 2012, 7 (4), 258-281. (16) Liu, J.-W.; Xu, J.; Liang, H.-W.; Wang, K.; Yu, S.-H., Angew. Chem. Int. Ed. 2012, 51 (30), 7420-7425. (17) Moshofsky, B.; Mokari, T., Chem. Mater. 2013, 25 (8), 1384–1391. (18) Zhang, J.; Tu, J. P.; Xia, X. H.; Wang, X. L.; Gu, C. D., J. Mater. Chem. 2011, 21 (14), 54925498. (19) Lu, X.; Zhai, T.; Zhang, X.; Shen, Y.; Yuan, L.; Hu, B.; Gong, L.; Chen, J.; Gao, Y.; Zhou, J.; Tong, Y.; Wang, Z. L., Adv. Mater.s 2012, 24 (7), 938-944. (20) Li, X.-L.; Lou, T.-J.; Sun, X.-M.; Li, Y.-D., Inorg. Chem. 2004, 43 (17), 5442-5449. (21) Li, X. Z.; Li, F. B.; Yang, C. L.; Ge, W. K., J. Photochem. Photobiol. A-Chem. 2001, 141 (2–3), 209-217. (22) Liao, C.-C.; Chen, F.-R.; Kai, J.-J., Sol. Energy Mater. Sol. Cells 2006, 90 (7–8), 1147-1155. (23) Yao, C. J.; Zhong, Y. W.; Nie, H. J.; Abruna, H. D.; Yao, J. N., J. Am. Chem. Soc. 2011, 133 (51), 20720-20723. (24) Granqvist, C.-G., Nat. Mater. 2006, 5 (2), 89-90. (25) Deb, S. K., Appl. Opt. 1969, 8 (S1), 192-195. (26) Niklasson, G. A.; Granqvist, C. G., J. Mater. Chem. 2007, 17 (2), 127-156. (27) Sun, Y., Chem. Soc. Rev. 2013. 42 (7), 2497-2511 (28) Qian, H. S.; Yu, S. H.; Gong, J. Y.; Luo, L. B.; Fei, L. F., Langmuir 2006, 22 (8), 3830-3835. (29) Liu, J. W.; Wang, J. L.; Huang, W. L.; Yu, L.; Ren, X. F.; Wen, W. C.; Yu, S. H., Sci. Rep. 2012, 2, 987. ACS Paragon Plus Environment

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