decoration of tio2 nanotube layers with wo3 nanocrystals for high - electrochromic activity

Decoration of TiO 2 nanotube layers with WO 3 nanocrystals for high-electrochromic activity A. Benoit 1 , I. Paramasivam, Y C. Nah, P. Roy, P. Schmuki * Department of Materials Science, WW4-LKO, University of Erlangen-Nuremberg, Martensstrasse 7, D-91058 Erlangen, Germany article info Article history: Received 14 January 2009 Accepted 20 January 2009 Available online 24 January 2009 Keywords: TiO 2 nanotubes WO 3 nanoparticle Electrochromism abstract We report a simple approach to decorate ordered TiO 2 nanotube (TiNT) layers with tungsten trioxide nanocrystallites by the controlled hydrolysis of a WCl 6 precursor. These WO 3 nanocrystallites, when formed, are amorphous, but can be annealed to a monoclinic crystal structure. The WO 3 crystallites on the TiO 2 nanotube skeleton are electrochemically active, and hence ion insertion reactions are possible. As a result, the decorated nanotube layers show remarkable enhancement of the electrochromic proper- ties. In particular, a significantly lower threshold voltage and an increased electrochromic contrast can be achieved compared with unloaded (neat) TiO 2 nanotube layers. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Over the last years the anodic formation of ordered TiO 2 nano- tube (TiNT) layers has created significant scientific interest (see Refs. [1,2] for an overview). The application of these nanotube lay- ers has been explored for example in photocatalysis [3], photo- chromism [4], or biomedicine [5]. Due to the tubular nature and large surface area, they were used as hosts for insertion of H + or Li + ions [6,7], and considerably high-electrochromic contrast can be obtained due to the nanotubular architecture. This electrochro- mic effect is based on the fact that when electrons and ions are in- jected under cathodic polarization, the electronic structure of TiO 2 is modified. It was reported [8,9] that the reduced form of Ti 3+ by electron injection leads to local electronic states 0.7 eV under the conduction band, which results in an absorption in the visible range. TiO 2 based devices are mostly built with nanoparticulate systems to shorten the solid-state diffusion path and time [10]. Re- cently, we reported that this solid-state diffusion step can be dras- tically accelerated by widening of the host TiO 2 lattice by doping with Nb 2 O 5 [11] which also allows for the insertion of Na + ions. However, other transition metal oxides, in particular WO 3 , are typically more efficient electrochromic materials than TiO 2 , i.e. typical electrochromic criteria such as coloration efficiency and threshold voltage are reported to be better [12–14]. Recently our group investigated WO 3 nanoporous structures that show a drasti- cally enhanced electrochromic contrast, and a faster switching re- sponse than the compact anodic WO 3 layers [15]. However, a key drawback of pure W is that up to now no highly defined tubular structures could be grown [15,16], and thus the full electrochromic potential of WO 3 based nanotubular systems could not be exploited. We showed that one strategy to overcome this problem is anodizing Ti–W alloys [16]. By suitable substrate alloying, highly defined mixed oxide TiO 2 –WO 3 nanotubes with strongly enhanced electrochromic properties could be grown. In this work, we explore another approach to combine the out- standing WO 3 electrochromic properties with the defined mor- phology of TiO 2 nanotubes. We show a facile route to decorate the TiO 2 nanotubes with WO 3 nanocrystallites and demonstrate that these decorated tubes have significantly enhanced electro- chromic characteristics. 2. Experimental TiO 2 nanotube layers were grown by anodic oxidation of tita- nium foils with 99.6% purity (from Goodfellow, England) of 0.1 mm thickness. Prior to the experiments the titanium foils were sonicated in acetone, isopropanol and methanol (for 3 min each) followed by rinsing with deionized water and then dried in a nitro- gen stream. Anodization was carried out using a high-voltage potentiostat Jaissle IMP 88 using an electrolytic mixture of glycerol (1, 2, 3-propanetriol) and water (60:40 vol%) + 0.27 M NH 4 F [17] at 30 V for 3 h. Ti samples were pressed against an O-ring in an elec- trochemical cell where 1 cm 2 was open to the electrolyte. The set- up [18] consisted of a three electrode configuration with a Pt gauze as counter electrode and a Haber-Luggin capillary with Ag/AgCl as reference electrode. The anodization process forms nanotube lay- ers with a tube length of $1.4 l m and a diameter of $100 nm 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.01.024 * Corresponding author. Tel.: +49 9131 852 7575; fax: +49 9131 852 7582. E-mail address: schmuki@ww.uni-erlangen.de (P. Schmuki). 1 On leave from: Université de Nantes, Nantes Atlantique Universités, Poly- tech’Nantes, Materials Science Department, Rue Christian Pauc, BP 50609, F-44306 Nantes Cedex 3, France. Electrochemistry Communications 11 (2009) 728–732 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom [17]. The TiO 2 nanotube layers were then annealed in air using a thermal treatment in a Rapid Thermal Annealer – Jipelec JetFirst, at 450 °C for 1 h with heating and cooling rate of 30 °C min À1 to form an anatase structure [17]. For WO 3 nanocrystallite deposition, WCl 6 (Aldrich 99.9%) in powder form was dissolved in ethanol (>99.9% Purity, Sigma–Al- drich) to obtain 0.1 M of stock solution and then further diluted to obtain a 0.001 M solution. In this solution tungsten is very sen- sitive to moisture (hydrolysis) and oxygen present in atmosphere. Therefore, a fresh solution was used for each new experiment. In order to preserve the solutions and minimize contact with air, vials with septa and syringe are used for storage handling and dilution of the solutions. For tube decoration, annealed TiNT layers are placed in a beaker containing 10 mL of the fresh 0.001 M WCl 6 solution. The color of the solution is at this moment light green to yellow (if the solution is fresh). Five milliliters of a mixed solution of water and ethanol (50:50) is added to the stirred 0.001 M WCl 6 solution using a syr- inge and the color turns to dark blue. Finally, the closed beaker is placed in water bath. The temperature is raised slowly from 20 °Cto70°C and afterwards it is kept for 1 h at 70 °C. Now the color turns from dark to light blue. The sample is slightly rinsed in ethanol and dried in a nitrogen stream. The WO 3 decorated tubes are then re-annealed at 450 °C, 1 h in the rapid thermal annealer. A Scanning Electron Microscopy, HITACHI SEM FE S4800, was used to acquire micrographs of the tube morphology. The chemical composition of the deposited materiel was analyzed using X-ray photoelectron spectroscopy (PHI 5600 XPS Spectrometer) with Al K a radiation at an incident angle of 45° to the surface normal. XRD measurements were performed using a Philips X’Pert PRO dif- fractometer with monochromatic Cu K a radiation. To characterize the electrochemical and electrochromic behaviour of the TiO 2 nanostructures with and without WO 3 decoration, a conventional three electrode system was used. Samples were pressed against an O-ring with a Cu plate in an electrochemical cell. A platinum plate and a Haber-Luggin capillary with Ag/AgCl (1 M KCl) were used as a counter and a reference electrode, respectively. The elec- trolyte was 0.1 M HClO 4 . The wall of the cell opposite to the sample surface consists of a quartz glass window to allow optical measure- ments during electrochemical cycling. Reflectance measurements were carried out using USB 2000 Fiber Ocean Optics Spectrometer. The cyclic voltammograms and chronoamperometric measure- ments were performed using an Autolab PGSTAT30 Potentiostat/ Galvanostat. Optical images were recorded using a CCD camera. 3. Results and discussion Fig. 1 shows the top view of the TiO 2 nanotube layers used in this work before (Fig. 1a) and after the decoration with WO 3 nano- crystallites (Fig. 1b–d). From the images after the WCl 6 treatment it can be seen that some of the tubes are covered with a hazy very thin layer (Fig. 1b) but most of the surface area shows open and ni- cely decorated tubes (Fig. 1c and d) where individual WO 3 nano- particles have a diameter of $5 nm. XRD investigations were carried out with the decorated tubes before and after additional annealing at 450 °C. Before annealing the XRD spectra only reveal TiO 2 anatase peaks, after annealing clearly peaks of monoclinic WO 3 could be detected (most characteristic at h =23°). This indi- cates that the as deposited WO 3 crystallites are amorphous and only the annealing treatment converts them to the crystalline material. In order to investigate the chemical composition and oxidation state of WO 3 on the decorated and annealed nanotubes, XPS mea- Fig. 1. SEM images showing top views of TiO 2 nanotubes (TiNT): (a) as grown by anodization in a mixture of glycerol and water (60:40 vol%) + 0.2 M NH 4 F at 30 V for 3 h, (b–d) after decoration with WO 3 (WO 3 /TiNT). A. Benoit et al. /Electrochemistry Communications 11 (2009) 728–732 729 surements were performed. Fig. 2b shows an XPS survey spectrum that indicates that the WO 3 /TiNT composite material contains Ti, W, O and some traces of carbon. Fig. 2c shows the high resolution XPS spectra of the W4f peak with W4f 7/2 and W4f 5/2 at 35.3 eV and 37.4 eV, respectively. Even though the determination of the exact position of W4f was difficult because of a partial overlap with Ti3p peak, their positions are in line with the peak positions of pure WO 3 [19]. In order to explore ion insertion properties of the samples, elec- trochemical and optical characterization was performed. Fig. 3a displays the cyclic voltamograms (CVs) of TiNT, WO 3 /TiNT as- formed and annealed at 450 °C carried out in a 0.1 M HClO 4 solu- tion. Peaks I and II in the CVs can be ascribed to proton insertion and extraction into and out from the host lattice [20,15]. This insertion process can either take place into the TiO 2 or the WO 3 and may be described as TiO 2 +xe À +xH + ? TiOOH or WO 3 +xe À + xH + ? H x WO 3 , respectively. In both cases it is associated with a change in color of the material. Compared with neat TiNT, the WO 3 decorated nanotubes show significantly larger current densi- ties, which reflect the fact that proton insertion/extraction is much more favorable in the decorated structures than in the neat tubes. Insertion into neat nanotubes occurs under the same experimental conditions only at potentials negative to À1.0 V [6]. For annealed WO 3 decorated samples, the current densities are smaller than for the ones where the WO 3 is present as amorphous material. This indicates that the crystalline phase formed after annealing at 450 °C, shows a lower ion uptake efficiency – which is in line with literature reports for pure WO 3 [16]. Furthermore, the onset poten- tial for the cathodic reaction for WO 3 /TiNT (with amorphous WO 3 ) is located at $0.3 V while for annealed WO 3 /TiNT (crystalline WO 3 ) it is at $0 V. This means that insertion can be achieved at signifi- cantly lower applied voltage for amorphous sample. It also means that the underneath anatase skeleton of TiO 2 is sufficiently conduc- tive to allow electrochemical switching of the WO 3 . Fig. 3b shows the current density response with time when a cycling pulse potential is applied between À0.5 V and 1.0 V. The integrated current density with time (charge density) is indicative of the amount of protons incorporated during the reactions. When comparing the charge exchanged during cathodic and anodic reac- tions for TiNT and WO 3 /TiNT (as-formed) – compiled in Table (in- set in Fig. 3b) – it is clear that the WO 3 /TiNT show much higher values in charge density. Again, after annealing at 450 °C, the charge density is slightly decreased due to the crystallinity of the material. Fig. 3c shows the electrochromic effects for TiNT, WO 3 /TiNT as-formed, and WO 3 /TiNT annealed (450 °C) during potential switching between À0.5 V and 1.0 V. To quantify the electrochro- mic effects, reflectance spectra were acquired. Compared with Fig. 2. XRD patterns of annealed TiNT and WO 3 /TiNT annealed at 450 °C (a); XPS survey spectra of as-formed WO 3 /TiNT (b); detail of the W4d peak for WO 3 /TiNT (c). 730 A. Benoit et al. /Electrochemistry Communications 11 (2009) 728–732 TiNT, the decorated WO 3 /TiNT shows a strong effect as apparent from the reflectance difference ( D R). At a wavelength of 600 nm, for neat TiO 2 nanotube structures only a 3% change could be ob- tained whereas for the WO 3 loaded systems 45% for the as-formed, and 21% for the annealed structure can be achieved. It is interesting to note that the bleached state of annealed sample does not abso- lutely recover to the original state after the first potential pulsing. These findings indicate that a higher crystallinity not only affects the insertion amount but also the electrochemical reversibility. The response time for the as-formed WO 3 /TiNT is 3.6 s and 2.8 s for the coloration and the bleaching, respectively, while for an- nealed WO 3 /TiNT the values are 11.4 s and 10.1 s for coloration and bleaching, respectively. This again is in accord with literature that proton movement is faster in amorphous than in crystalline WO 3 [16]. It may be noteworthy that the switching threshold voltage of the WO 3 is in the range of À0.3 V Ag/AgCl . This is very close to the flatband potential for the underneath TiO 2 (anatase) nanotubes [21]. This means that the threshold voltage for WO 3 to a certain ex- tent may be dominated by the switching of the underneath (n- type) material from depletion to accumulation conditions; in other words, electron supply (conductivity) over the TiO 2 nanotube skel- eton may determine the switching threshold voltage. However, the results in Fig. 3a show that a significant onset of the electrochem- ical reaction in the WO 3 occur even at a potentials of $0V Ag/AgCl which shows that the nanotubes at this voltage are not entirely in a current blocking state; i.e. are still sufficiently conducting to allow switching of the WO 3 crystallites. In summary, this work demonstrates how TiO 2 nanotubes can be decorated with WO 3 nanocrystallites. The decoration signifi- cantly enhances the contrast and insertion capacity of a TiO 2 nano- tube based electrochromic system. Decoration of the nanotubes with WO 3 may also have a significant impact on other TiO 2 nano- tube applications. Acknowledgements The authors would like to greatly acknowledge DFG for financial support. We extend our sincere thanks to Helga Hildebrand and Ullrike Marten-Jahns for XPS and XRD measurements and also to Hans Rollig and Martin Kolacyak for their valuable technical help. References [1] J.M. Macak, H. Tsuchiya, A. Ghicov, K. 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