CRYSTAL GROWTH & DESIGN
Synthesis of Nanostructured Tungsten Oxide Thin Films Erika Widenkvist,*,† Ronald A. Quinlan,‡,§ Brian C. Holloway,§ Helena Grennberg,| and Ulf Jansson† Department of Materials Chemistry, Uppsala UniVersity, Ångstro¨m Laboratory P.O. Box 538, SE-751 21 Uppsala, Sweden, Department of Applied Science, The College of William and Mary, 325 McGlothin Street Hall, Williamsburg, Virginia 23187, Luna InnoVations NanoWorks DiVision, 521 Bridge Street, DanVille, Virgina 24541, and Department of Biochemistry and Organic Chemistry, Uppsala UniVersity, BMC, P.O. Box 576, SE-751 23 Uppsala, Sweden
2008 VOL. 8, NO. 10 3750–3753
ReceiVed April 14, 2008; ReVised Manuscript ReceiVed June 26, 2008
ABSTRACT: A facile and inexpensive method to produce thin films of nanostructured tungsten oxide is described. A nanocrystalline tungstite (WO3 · H2O) film is spontaneously formed when a tungsten substrate is immersed in nitric acid at elevated temperatures. The resulting thin film is composed of plate-like tungstite crystals with edges preferentially directed out from the substrate surface. The tungstite can easily be transformed into WO3 by annealing. Patterned WO3 · H2O/W structures can be obtained by a combination of lithographic techniques and etching. In this study, the effect of exposure time, acid concentration, and temperature on the microstructure of the films has been investigated. The potential of this inexpensive synthesis method to produce large-area coatings of nanostructured tungsten oxide as well as patterned films makes it interesting for several different applications, such as batteries, gas sensors, and photocatalysts. Introduction Tungsten oxide films have several interesting physical and chemical properties suggesting the potential use of this material in a multitude of applications such as photocatalysis,1 gas sensors,2 and batteries.3 Nanostructured tungsten oxide films are already used in a number of applications, for example, in smart windows and displays utilizing the electrochromic properties of the material.4 Several different techniques have been used to deposit tungsten oxide films,4 including reactive sputtering,5 chemicalvapordeposition(CVD),6,7 evaporation,8,9 anodization,1,10 and sol-gel techniques.11,12 Many of these techniques unfortunately require expensive equipment or have other limitations such as a small maximum substrate area that can be coated in each batch. Consequently, there is a need for a fast, inexpensive synthesis process which can be used to produce nanostructured tungsten oxide on large substrate areas. In this paper we demonstrate such a process; a simple immersion of a W substrate in nitric acid solution at elevated temperatures results in the formation of a nanostructured film of WO3 · H2O (tungstite). By adding an annealing step, the tungstite film can be transformed to nanocrystalline WO3. The effects of exposure time, temperature, acid concentration, and stirring on the microstructure of the oxide films have been investigated. The resulting films have been characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy. How this synthesis process can be used to create patterned WO3 · H2O/W structures will also be demonstrated. Experimental Section The tungstite films were prepared by placing pieces of tungsten foil (99.95%) in a reaction flask and adding 50 mL of HNO3 solution. A condenser connected to a gas-trap (with H2O) was attached to the flask, and the flask was heated in an oil-bath. After the acid treatment the * To whom correspondence should be addressed. E-mail: erika.widenkvist@ mkem.uu.se. Phone: +46 18 471 3775. Fax: +46 18 51 3548. † Department of Materials Chemistry. ‡ Department of Applied Science. § Luna Innovations NanoWorks Division. | Department of Biochemistry and Organic Chemistry.
samples were washed for 30 min in ∼50 mL of deionized water (at room temperature, RT) and dried for 15 min in ∼100 °C. WO3 films were prepared by annealing the as-deposited tungstite samples at 350 °C in air for 1 h. Patterned samples were prepared by spin-coating positive photoresist (AZ 4562, 10 µm) on to the tungstite film, soft baking (3 min, 90 °C) on a hotplate, exposing the sample to UV-light through a mask, removing the exposed resists and at the same time etching the tungstite film by dipping in KOH, and finally removing the remaining resist using acetone. All chemicals used in this work were obtained from commercial sources and were of analytical grade. The influence of the deposition temperature was studied by varying the temperature of the oil-bath in the RT to 95 °C range. The effect of acid-concentration was studied in the 0.75-10 M range. Control experiments using HCl solutions or H2SO4 solutions instead of HNO3 were also performed. The influence of stirring was investigated using a mechanical glass/Teflon stirrer placed over the sample during deposition. The films were characterized by SEM using an LEO1550 FEG microscope equipped with an Inlens detector, XRD using a Siemens D5000 diffractometer, and Raman spectroscopy using a Renishaw micro-Raman system 2000 and an excitation wavelength of 514 nm.
Results and Discussion Exposing tungsten foil to nitric acid (aq) at elevated temperature results in oxide formation on the surface. Figure 1 a-c shows micrographs of samples that have been immersed for 3 h in a 1.5 M HNO3 solution at 50 °C. After drying the samples, the W foils were covered by a green/yellow film, clearly visible to the naked eye. As can be seen in Figure 1a-d, the film consists of standing nanocrystallites with a regular plate-like shape. A cross-sectional SEM image of a film (Figure 1c) indicates that the plates nucleate directly on the tungsten surface. In the literature a wide variation in crystal morphology of tungsten oxide films has been reported.12,13 For example, Blackman et al. report very different morphologies for films deposited by CVD using different reactants but otherwise identical conditions.14 Nanocrystalline tungstite films with similar plate-like crystals has been synthesized by sol-gel techniques.11 As will be demonstrated below the shape and size of the plates are dependent on the temperature, acid concentration, and immersion time. Typically, however, the square-shaped plates
10.1021/cg800383c CCC: $40.75 2008 American Chemical Society Published on Web 08/23/2008
Nanostructured Tungsten Oxide Thin Films
Figure 1. SEM-micrographs of tungsten samples immersed in 1.5 M HNO3 at 50 °C for 3 h, (a, b) Top-view images at two different magnifications and (c) cross-sectional view. (d) High magnification image of a crystal plate on a sample refluxed for 6 h at 95 °C in 2.8 M HNO3, showing surface steps forming a growth spiral.
exhibited an edge length in the range of 150 nm up to 2 µm and a thickness of 20-150 nm. The height of the film was difficult to estimate because of the flake-like morphology of the crystallites but as can be seen in Figure 1c, several individual crystals have a length of more than several micrometers. The crystallites seen in Figure 1c can be estimated to have had a growth rate, perpendicular to the surface, of 0.5-1 µm/h. Furthermore, the crystalline quality of the plates was very high and clear surface steps could be observed in the SEM. Some of the crystal surfaces exhibits clear growth spirals possibly because of screw dislocations terminating at the surface (Figure 1d).15 XRD showed that the oxide film formed during the immersion step consists of tungstite (WO3 · H2O) (Figure 2a).16 The diffractogram exhibits sharp and clear peaks with no evidence of a preferential growth direction of the plates or texture of the film. The formation of WO3 · H2O was also confirmed by Raman spectroscopy where the spectra exhibited peaks at, for example, 946, 646, and 192 cm-1 which can attributed to WO3 · H2O (Figure 2b).17 No indication of the presence of neither an anhydrous WO3 phase nor an amorphous WO3 was observed in the diffractograms or Raman spectra of the as-deposited films. The growth behavior of the tungstite film is influenced by the experimental parameters used. The single most important factor was found to be the temperature. At RT, the WO3 · H2O growth was extremely slow and only a few crystals could be observed on the surface after 1 h (Figure 3a). These crystals exhibited a more irregular shape as compared to the plate-like form observed at higher temperatures. As can be seen in Figure 3b, considerably larger crystals were found on the substrate surface after 1 h using a deposition temperature of 50 °C. At this temperature, the crystallites exhibited a clear plate-like shape with the edges preferentially directed out from the tungsten substrate. By comparing Figures 3a and 3b, it can be concluded that the growth rate of the tungstite was dramatically increased when raising the temperature from RT to 50 °C. A further increase of the temperature to 75 °C yielded even larger crystallites, as can be seen in Figure 3c. Figure 3d shows the morphology of the crystallites formed at 95 °C. As can be seen this temperature leads to more but smaller crystallites suggesting an increase in the nucleation rate. Also, the individual crystallites are thicker and seem to exhibit a more layered and uneven edge structure. Compared to the
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effect of temperature, the concentration of the nitric acid was found to play a less important role for defining the final crystallite morphology. However, a decrease in acid concentration resulted in a considerably slower nucleation rate. After a 30 min immersion at 50 °C very few tungstite crystallites were observed using an HNO3 concentration of 0.75 M, but for a concentration of 3 M crystals were clearly visible in the SEM. The effect of immersion time was investigated for films deposited at 50 °C in 1.5 M. On samples exposed to the acid only 30 min, small irregular tungstite crystallites were found. Increasing the deposition time to 1 h resulted in clear platelike crystals on the surface, and after 3 h a continuous film of tungstite crystals was observed. As was already mentioned, measuring the film thickness proved to be difficult because of the morphology of the samples and also because of the substrate consisting of tungsten. However, a rough estimate of the film thicknesses could be obtained from cross-sectional SEM images. A comparison of the micrographs of two samples, one immersed for 12 h and one immersed for 24 h, showed no significant difference in crystal height between the resulting films. This suggests a drastic decrease in growth rate after a continuous film has formed on the tungsten substrate. The crystal structure of the tungstite (WO3 · H2O) phase was determined by Szymanski and Rogers,16 who found that the compound has a layered structure with sheets of distorted W-O octahedral units sharing corners. The sheets are bonded to each other by water molecules hydrogen bonded with oxygen atoms in adjacent W-O layers. As a consequence of the layered structure a mild thermal annealing of the tungstite should remove the water and form WO3. This was also confirmed with XRD, where clear peaks from WO3 (see Figure 2c) were observed after heating the WO3 · H2O films to 350 °C. Because of broadening of the peaks it was difficult to identify which of the many WO3 polymorphs had formed. However, the Raman spectra (Figure 2b) with major peaks at 807 cm-1, 711 cm-1, and 270 cm-1 closely resembles that reported for monoclinic WO3.17,18 SEM analysis of annealed samples showed a change in the surface structure of the plates (Figure 4). A corrugation of the face of the plates was clearly detectable after the heattreatment and was most pronounced on a sample prepared at 95 °C. Considering that water is driven out from in between the WO3 layers in the tungstite structure, this type of change in shape is expected. To further control and improve the deposition technique the mechanism behind the formation of WO3 · H2O needs to be determined. Because nitric acid is a strongly oxidizing acid, it is likely that the tungsten initially is oxidized to WO42-. This is supported by the Pourbaix diagram (potential vs pH) of tungsten.19 The Pourbaix diagram also suggests that the initially formed WO42- should precipitate to tungstite according to eq 1 because of the low pH. + WO24 + 2H f WO3·H2O
(1)
Exposing tungsten substrates to non-oxidizing hydrochloric acid or sulfuric acid under otherwise identical conditions rendered no formation of WO3 · H2O. This supports a reaction path were soluble WO42- ions are formed in an initial oxidation step. The observed influence of growth temperature on the structure of the films (Figure 3) can also be attributed to the formation of soluble WO42- ions. As the temperature is increased, WO42is expected to form at a higher rate bringing about an increase in the nucleation as well as the growth rate. Thus an increase in temperature should lead to an increase in crystal size. However, when the point of supersaturation for WO42- is
3752 Crystal Growth & Design, Vol. 8, No. 10, 2008
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Figure 2. X-ray diffractogram (θ-2θ scan) and Raman spectra of (a) a tungstite (WO3 · H2O) film formed by immersion in 1.5 M nitric acid at 50 °C, and (b) a WO3 film formed after annealing an as-deposited tungstite film 1 h at 350 °C. Peaks originating from the tungsten substrate are marked with W.
Figure 3. Tungstite films deposited for 1 h in 1.5 M (aq) HNO3 at (a) RT, (b) 50 °C, (c) 75 °C, and (d) 95 °C.
Figure 4. Surface morphology of WO3 formed after annealing (350 °C, 1 h) a tungstite film deposited at 95 °C in 1.5 M HNO3.
reached the formation of more but smaller crystallites is expected, in agreement with the observations when the temperature is raised to 95 °C. Another piece of evidence pointing toward the involvement of soluble species in the formation of the films is the fact that the introduction of stirring of the nitric acid solution during the reaction affects the microstructure of the tungstate crystallites. A film deposited with stirring at 95 °C exhibited a similar crystallite size and shape as films deposited at 50 °C without stirring. This is expected under the assumption that stirring reduces the concentration of soluble WO42- above the substrate surface. The decrease in growth rate at long immersion times can also be explained by the proposed formation mechanism. For the initial oxidation step to occur, the tungsten substrate needs to be in contact with the oxidizing acid. When a continuous film of tungstite has formed on the
tungsten surface, the formation of WO42- will be hindered and the growth rate should decrease drastically. From the Pourbaix diagram, it can also be concluded that the tungstite will be etched at high pH-values. This opens the possibility to obtain patterned tungstite films by a combination of standard lithographic techniques and KOH etching, and Figure 5 shows SEM-micrographs of a patterned sample. It should be noted that the annealed WO3 films exhibited a much lower etch rate than the WO3 · H2O films. Conclusions In summary, we describe an inexpensive method to produce nanocrystalline thin films of tungsten oxide. The most important factor influencing the shape and microstructure of the tungstite crystallites in the deposited films proved to be the deposition
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References
Figure 5. Patterned tungstite film produced by standard lithographic techniques, (a) lines of intact tungstite film remaining on the tungsten substrate after KOH etching and (b) high magnification image showing the edge of a tungstite line.
temperature. By adding an annealing step to the synthesis procedure nanocrystalline WO3 films could be fabricated. Patterning of the tungstite films could also be achieved by utilizing standard lithographic techniques. In this work the substrates themselves were used as the tungsten source, but the method should be equally applicable on, for example, sputtered tungsten films. The potential of this inexpensive method to produce uniform large-area coatings as well as patterned films of nanocrystalline tungsten oxide makes it an interesting tool for several possible applications. The use of the thin films in batteries and for photocatalysis is currently under investigation. Acknowledgment. This work was supported by the Swedish Research Council (VR). Dr. Johan Bjurstro¨m, Uppsala University, is acknowledged for assistance with photolithography and patterning. This material is also based in part upon work supported by the Air Force Office of Scientific Research under Contract No. FA9550-06-C-0010. Any opinion, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Air Force Office of Scientific Research.
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