Nanowire-based electrochromic devices - University of Louisville

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Solar Energy Materials & Solar Cells 91 (2007) 813–820 www.elsevier.com/locate/solmat

Nanowire-based electrochromic devices S. Gubbala, J. Thangala, M.K. Sunkara Department of Chemical Engineering, University of Louisville, Louisville, KY 40292, USA Received 19 December 2006; accepted 20 January 2007 Available online 13 March 2007

Abstract This report presents studies performed on high contrast electrochromic devices based on WO3 nanowires. The devices were made in two formats. The first kind was made with vertically oriented nanowires grown on fluorinated tin oxide (FTO) coated glass substrates using a low pressure hot filament CVD process. The devices made with these substrates showed a highly reversible transmission modulation of over 70% with almost 0% transmission in the colored state, at a wavelength of 700 nm. In the second kind of devices, made using dispersed nanowires in a mat-like format, a transmission modulation of over 50% was observed in the same wavelength regime. The bleaching times of electrodes with high densities of nanowire arrays showed a large dependence of bleaching timescales based on the coloration timescales used. Beyond the observed enhancement in the optical transmission contrast characteristics, the coloration and bleaching timescales can be further improved by optimizing the nanowire array characteristics such as their densities and aspect ratios. r 2007 Elsevier B.V. All rights reserved. Keywords: Electrochromics; Nanowires; CVD; WO3

1. Introduction Nanoporous devices based on nanowires and columnar structures are gaining importance in various electrochemical device applications due to their high surface area and their potential to offer low resistance to charge and mass transport [1]. So, there is a need to synthesize nanowirebased films in a controlled and uniform manner and understand their behavior in a variety of electrochemical energy applications. So far, there have been only a few reports on the use of vertical arrays of nanowires for both dye sensitized solar cells and electrochromic devices [2–5]. But, it is not clear whether the nanowire-based porous films made using nanowire dispersions could be employed for high contrast electrochromic devices. Also, much more needs to be understood in order to optimize the use of vertical arrays for electrochromic devices. In addition, the ease of mat-like thin film preparation using nanowire dispersions is suitable for roll-to-roll manufacturing processes on an industrial scale and onto a wide variety Corresponding author.

E-mail address: [email protected] (M.K. Sunkara). 0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.01.016

of substrates. So, we fabricated electrochromic devices both in the mat-like and nanowire array formats and investigated their electrochromic performance. Electrochromism is the phenomenon of inducing a reversible change in the optical properties of a material by the application of a small electric field [1]. Transmission modulation in these films is achieved by varying the oxidation state of the electrochromic material by an electric field assisted insertion and extraction of small alkali metal ions, like lithium (Li+) [1]. Tungsten trioxide has been a choice for these applications for a long time owing to its high stability and good electrochromic behavior [6]. Development of electrochromic materials has gained increased importance in the last few years due to their potential to reduce air conditioning costs and thus the energy consumption in buildings by cutting off the infrared (IR) part of the solar spectrum, and also as information display devices [7]. Thin films of WO3 deposited using various methods like sol–gel technique, photochemical vapor deposition, vacuum evaporation, anodic oxidation, template assisted methods, etc. [8–12] have been successfully used in the past for making these devices. The performance of electrochromic devices is a function of the

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nature of the material (amorphous or polycrystalline), morphology, composition, as well as their water content [1]. Nanocrystalline forms of these materials offer higher porosity while still maintaining a high intercrystalline contact necessary for electrical conductivity. Such films have higher transmission and are expected to exhibit better electrochromic optical modulation compared to the conventional films due to their open structure [6]. There have been only two reports in literature on the fabrication of electrochromic devices based on nanowires [4,5]. The materials used in these studies were oxygen deficient W18O49 and V2O5 nanowires. In both the reports, the vertically oriented nanowires were directly deposited on the FTO substrates by thermal evaporation using powders as sources. Nanowires in the present study were synthesized using a scalable hot filament CVD process. This method of synthesis allows easy change in processing conditions to vary the density, diameters, and aspect ratio of the nanowires. 2. Experimental Fig. 1 shows a schematic of the hot filament CVD reactor setup used for the synthesis of tungsten oxide nanowires. The setup consists of a 2-inch diameter quartz tube housed in a tube-furnace. The ends of the quartz tube are connected to the necessary accessories for flow and pressure control. The filament used for the experiments is mounted on hollow ceramic rod as shown in Fig. 1. A 0.5 mm diameter tungsten wire was used as the source of tungsten in these experiments. The tungsten filament is heated up using an electrical feed-through to temperatures of about 1950 K. The temperatures within our system were monitored using a dual wavelength pyrometer. Quartz boats were employed to curtail the deposition of tungsten oxide directly onto the tube walls. Typically, the substrates (FTO coated glass slides) were placed on the boat as shown in the schematic. The temperature of the substrate during the nanowire growth was 823 K at 770 mTorr and a flow of 11 sccm of air. The synthesis procedure and the nanowire growth mechanism is explained in more detail elsewhere [13].

In order to make dispersions of the synthesized nanowires, the as-synthesized nanowires were collected as dry powders by scraping the material from the quartz substrates. The as-obtained nanowire powder was dispersed into dimethyl formamide (DMF) with an ultrasonic horn for 2 min followed by sonication in a low energy density bath for about 15 min. The dispersions were allowed to settle for few minutes and the initial sediments were taken out and weighed. The initial sediments always contained nondispersed agglomerates and thicker nanowire bundles. The dispersions were allowed to settle over two days. The sediments were examined using SEM to observe the agglomeration patterns. The upper portions of the solution contained well-dispersed nanowires. This solution was used to make the mat-like electrodes. Electrodes in mat-like format were made by depositing 1 wt% dispersion of WO3 nanowires in DMF solution on FTO substrates, over an area of approximately 0.25 cm2. After depositing the nanowires, the electrodes were heated in ambient atmosphere at a temperature of 773 K, to oxidize the substoicheometric tungsten trioxide to WO3. The electrodes after oxidation became highly transparent forming the WO3 phase, which was confirmed by XRD. The counter electrode for this device was another FTO glass piece. The two electrodes were assembled face to face into a sandwich-like structure with 1 M LiClO4 in propylene carbonate as the electrolyte between them. For this purpose, the electrodes were separated by a plastic spacer of 150 mm thickness. Electrochromic devices based on nanowire arrays were also made in a similar fashion. Schematic representations of these devices are shown in Figs. 2(a) and (b). 3. Results and discussion Fig. 3(a) shows the SEM micrograph of a substrate on which the nanowires were grown for 10 min, indicating a high density of vertically oriented nanowire arrays. The nanowires obtained had diameters ranging from 40 to 60 nm and lengths up to 2 mm. The number density of nanowires on these substrates was found to be 7 1010 nw/cm2 (Fig. 3(b)). From the SEM image, it can be seen that there is a

MFC's O2 Ar Air Quartz To Vacuum Pump To Electrical Feed through Pressure Transducer

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Fig. 1. Schematic representation of the CVD reactor used for the synthesis of WO3 nanowires.

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Fig. 2. Schematic representation of the electrochromic devices made with (a) vertical arrays of WO3 nanowires grown on FTO substrates, and (b) nanowires deposited from dispersions.

Fig. 3. Scanning electron micrographs of (a) cross section of vertical arrays of nanowires grown on FTO substrates, (b) top view of the nanowires, and (c) nanowires deposited from dispersions.

modification in the interface between the nanowires and the FTO substrate, compared to the bare FTO surface before the experiment. This modification could have helped in decreasing the contact resistance by making an alloy at the interface. However, the Li+ intercalation/deintercalation characteristics for such alloys may differ from WO3. Also, the diffusion of sodium atoms into the FTO layer during the nanowire synthesis may have adversely affected the final characteristics of the electrode. The properties of the interface, although interesting with regard to the electrochromic behavior of the electrodes, have not been carried out at this point in time. The as-synthesized nanowires were blue in color. The X-ray diffraction (XRD) spectrum (Fig. 4(a)) indicated that the assynthesized bluish nanowire deposit was composed of an oxygen deficient monoclinic W18O49 phase (JCPDS#05-0392, a ¼ 18.28 A˚, c ¼ 13.98 A˚, and b ¼ 3.775 A˚). Oxidation of the as-synthesized nanowires in ambient atmosphere at a temperature of 500 1C for about 30 min changed the color of the nanowires to yellow, and the corresponding XRD spectrum (Fig. 4(b)) indicated that the phase of the nanowires changed from W18O49 to WO3 (JCPDS #20-1324; a ¼ 7.384 A˚, b ¼ 7.512 A˚, and c ¼ 3.846 A˚). At the same time, no structural damage or change is observed for nanowires after the oxidation. In comparison, the SEM of the electrodes with nanowires deposited on them from dispersions had a number density of 109 nw/cm2 (Fig. 3(c)). The optical transmission measurements on these electrodes were performed using a Perkin-Elmer Lambda 950 UV–vis spectrometer. An EG&G PAR 273A potentiostat was used to apply potentials to the WO3 electrode with respect to the counter electrode (bare FTO glass). All the optical transmission measurements were performed with respect to a blank device which consisted of two FTO coated glass pieces sandwiched and sealed together in the same fashion as the devices and filled with the electrolyte. Thus, the transmission measurements included contributions from


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Fig. 5. Optical photographs of the electrochromic device made with vertical arrays of nanowires in the bleached and colored states.

the WO3 films only. For coloration of the films, a voltage of 3.5 V was applied to the working (WO3) electrode. During the bleaching process, a voltage of 2.5 V was applied to the electrode. Optical photographs of electrodes made with vertical arrays of nanowires in the colored and bleached states are shown in Fig. 5. All the spectra were collected in

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Fig. 6. (a) The optical transmission spectra of the devices made from vertically oriented nanowires with no applied bias, 3.5 V and 2.5 V. (b) The variation in transmission at 700 nm with a step variation in the potential.

the wavelength range of 350–800 nm. Fig. 6(a) shows the variation of optical transmission through an electrochromic device made using a WO3 vertical array of WO3 nanowires on an FTO substrate. The plots show the transmission through the device at potentials of 0.0, 3.5, and 2.5 V. In this case, film electrode was kept at the coloration potential for 5 min. These films show a high change in the transmission before and after applying the coloration and bleaching potentials. From the transmission spectra of these devices, it can be seen that the films are highly electrochromic in the near IR wavelengths, with the transmission values falling to 0% from 76% on applying the potential. Upon applying bleaching potential, the optical transmission through the films go back to 71% transmission. It was found that although the coloration process seems to be fast in these films, the bleaching process takes a long time. In the above case, the coloration took place almost instantaneously, but the bleaching took almost 30 min to reach a value of 71%. The difference in the coloration and bleaching timescales was also observed when the electrodes were subjected to step potential changes at intervals of 5 min (Fig. 6(b)). As shown in Fig. 6(b), a transmission modulation of about 40% at 700 nm is observed. Although the slow


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bleaching is consistent with reports by Maruyama and Kanagawa [9] for CVD grown films at 300 1C, the reason attributed for it was the inclusion of microcrystalline particles, whereas the films prepared in this report have much smaller length scales. It was also observed that the transmission through the electrodes increased continuously with the application of a bleaching potential, reaching a saturation value after about 30 min. The initial steep rise in the bleaching behavior and a slowdown in the bleaching with time indicate that there are two different steps involved in the bleaching process, a fast component and a slow component. The channels in the nanowire array films get narrower in the interior parts as they go toward the substrate. As soon as the coloration potential is applied, all the Li+ ions intercalate into the upper regions of the WO3 nanowire arrays instantaneously. Most of the coloration in the first few seconds is observed mostly due to the intercalation of Li+ ions in the upper parts of the nanowires closer to the bulk electrolyte solution. Further application of the potential makes Li+ ions intercalate into the interior parts of the nanowires as more and more Li+ ions diffuse into interior regions depleted with the Li+ ions. The continued application of coloration potential and subsequent intercalation of Li+ into the interior parts of the WO3 nanowire arrays does not seem to effect the optical contrast significantly. During this period, a significant amount of Li+ intercalation related current is observed. The rapid coloration of these films due to the top layer of the film is further confirmed by the XPS spectra of these films taken after 15 s of coloration. The XPS spectra of the films clearly show the well-resolved spin-orbit split doublet peaks which correspond to the W 4f7/2 and W 4f5/2. Fig. 7(a) shows the W 4f7/2 and W 4f5/2 peaks at 36.6 and 38.6 eV, respectively, which correspond to the W+6 oxidation state. Fig. 7(b) shows the peak positions at 36 and 38 eV, corresponding to the +5 oxidation state of W. The bleaching process seems to be most effected by the longer durations of the coloration. When bleaching potential is applied, the Li+ deintercalates from the WO3 and goes into the electrolyte solution. The Li+ from the upper parts of the nanowires quickly deintercalates but the optical transmission does not change significantly because of intercalation of interior parts of the nanowire arrays. The subsequent deintercalation process from the interior parts of the WO3 nanowire arrays occurs slowly for the following reasons: First, due to the Li+ already present in the electrolyte solution, further deintercalation becomes difficult and due to the slow rate of mass transport of Li+ from the interior parts of the film to the bulk, Li+ ion concentration builds up, which slows down the deintercalation even further. Therefore, the color which remains for a long time during the deintercalation process is due to the Li+ ions in the interior parts of the WO3 film. The above dependence of the performance of these devices on the nanowire array density requires further optimization of these structures.

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Fig. 8. SEM image of nanowires grown on the FTO substrates for 2 min.

The above measurements were also performed on WO3 films grown for 2 min which had very high density of nanowires (Fig. 8). It was observed that the timescales associated with the transmission change through the


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electrodes was a strong function of the time for which they were subjected to a coloration potential. Fig. 9 shows the dependence of transmission modulation of these electrodes with time when a coloration potential of 3.5 V was applied for (a) 10 s, (b) 30 s, and (c) 5 min, followed by a bleaching potential for 5 min. These substrates had an even higher density of nanowires. The variation of bleaching time scales with different coloration times is again due to the high density of nanowires on the films which limits the mass transport of Li+ out of the nanowires after applying long coloration potentials and indicates that the bleaching behavior is more dependent on the nanowire density, rather than the interface between the nanowires and the 60

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FTO. The dependence of coloration and bleaching times on the porosity of tungsten trioxide films was studied recently in ‘‘nanohole-array membranes’’ by Nishio et al. [14] Their study showed that as the hole size in the films increased the coloration and bleaching times decreased significantly. This was attributed to the different rate of diffusion of Li+ ions into the WO3 membranes with different porosities. The dependence of bleaching times on the density of nanowires suggests that the density of these nanowires on the substrate has to be optimized in order to obtain bleaching times to less than 30 min irrespective of the coloration time scales and at the same time maintaining high contrast. Fig. 10 shows the cyclic voltammogram performed on one of these devices. The voltammograms shows cycles after 50 and 200 cycles. The electrochemical cycling performed on these electrodes at 100 mV/s scan rate in the potential range of 3.5 and 2 V does not indicate any degradation with lithium intercalation and deintercalation even after 200 cycles. The electrochromic performance of mat electrodes was also measured in a similar fashion. The optical photographs of these electrodes are shown in Fig. 11. Fig. 12(a) shows the transmission spectra of the electrode from 350 to 800 nm. The scans were taken with 0 bias, 3.5 V, and 2.5 V. From the transmission spectra of these electrodes, it can be seen that these films also show an acceptable electrochromic behavior. The transmission at 700 nm for these films at 0 bias is about 85%, which falls down to 32% upon Li+ intercalation. The transmission goes back to 80% upon bleaching. The cycling behavior of these devices taken in 5 min intervals shows a transmission modulation of about 35% as shown in Fig. 12(b). These devices also show no electrochemical degradation with cycling similar to the data shown in Fig. 10 for devices made using nanowire arrays grown directly on FTO

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state can be decreased further by increasing the loading of nanowires on these substrates. As in the case of the electrodes with WO3 vertical arrays, the bleaching times are again dependent on the time of coloration. Chronoamperometry was performed on both kinds of devices to study the currents during lithium intercalation and deintercalation. The plots (shown in Fig. 13) indicate a faster decay in the currents in the case of mat-like devices when compared to the array-based devices, both during coloration and bleaching. Although, this suggests that bleaching and coloration should be faster in mat-like configurations, it is observed that coloration is faster in the case of array electrodes, while bleaching is faster in the case of mat-like electrodes. This indicates that the change in the transmission of these electrodes is not a simple function of current.

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Fig. 12. (a) The optical transmission spectra of the devices made from mat-like structures of nanowires with no applied bias, 3.5 V and 2.5 V. (b) The variation in transmission at 700 nm with a step variation in the potential.

substrates. The high transmission through these films in the colored state is due to the low density of these wires on the FTO substrate, which allows the direct passage of light through the FTO, without interacting with the nanowires. The transmission through these electrodes in the colored

The results described here demonstrate that WO3 nanowires can be used as high contrast electrochromic devices. The devices can be made from vertically oriented nanowires on FTO substrates or with nanowires deposited on FTO substrates from dispersions. The optical transmission modulation of the two devices was comparable, with the vertical arrays showing a relatively higher performance. The bleaching times of the devices made with nanowires in the mat-like format were found to be shorter than the ones made with vertical arrays. The transmission characteristics of devices made in mat-like format can be further improved by optimizing the nanowire density, diameters, and their aspect ratios. Acknowledgments The authors acknowledge financial support from the US Department of Energy for supporting the Institute for Advanced Materials (DE-FG02-05ER64071), Kentucky

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Renewable Energy Commission/US-DOE (DE-FG3605G085013A), and the University of Louisville for fellowship support to J. Thangala. References [1] C.G. Granquest (Ed.), Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995. [2] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P. Yang, Nat. Mater. 4 (2005) 455. [3] J.B. Baxter, E. Aydil, Appl. Phys. Lett. 86 (2005) 053114. [4] C.C. Liao, F.R. Chen, J.J. Kai, Sol. Energy Mater. Sol. Cells 90 (2006) 1147. [5] K.C. Cheng, F.R. Chen, J.J. Kai, Sol. Energy Mater. Sol. Cells 90 (2006) 1156.

[6] M. Deepa, A.G. Joshi, A.K. Srivastava, S.M. Sivaprasad, S.A. Agnihotry, J. Electrochem. Soc. 153 (2006) C365. [7] I.F. Chang, B.L. Gilbert, T.I. Sun, J. Electrochem. Soc. 122 (1975) 955. [8] E. Ozkan, S.H. Lee, P. Liu, C.E. Tracy, F.Z. Tepehan, J.R. Pitts, S.K. Deb, Solid State Ionics 149 (2002) 139. [9] T. Maruyama, T. Kanagawa, J. Electrochem. Soc. 141 (1994) 2435. [10] M. Deepa, R. Sharma, A. Basu, S.A. Agnihotry, Electrochim. Acta 50 (2005) 3545. [11] B. Reichman, A.J. Bard, J. Electrochem. Soc. 126 (1979) 583. [12] P. Bonhote, E. Gogniat, M. Gratzel, P.V. Ashrit, Thin Solid Films 350 (1999) 269. [13] J. Thangala, S. Vaddiraju, R. Bogale, R. Thurman, T. Powers, B. Deb, M.K. Sunkara, Small, (2007) accepted for publication. [14] K. Nishio, K. Iwate, H. Masuda, Electrochem. Solid-State Lett. 6 (2003) H21.