The formation of WO₃ nanorods using the surfactant-assisted hydrothermal reaction

Abstract

Monoclinic tungsten oxide (WO₃) nanorods were grown using the hydrothermal method on a seeded W foil. The seed layer was formed by thermal oxidation of W foil at 400°C for 30 min. Cetyltrimethylammonium bromide (CTAB) or hexamethylamine (HMT) was used in the reactive hydrothermal bath, along with sodium tungstate dihydrate (Na₂WO₄.2H₂O) and hydrochloric acid (HCl). The concentration of CTAB was varied from 0.01 M to 0.07 M and the concentration of HMT was varied from 0.01 M and 0.05 M. The result showed that CTAB-assisted hydrothermal reaction produced WO₃ nanorods with 4–7 nm diameter, and provided that CTAB concentration was less than 0.07 M. WO₃ nanorods could not be obtained when CTAB concentration was 0.07 M. Columnar structured WO₃ was produced with the presence of HMT in the hydrothermal bath. This was due to decomposition of HMT to form hydroxyl ions (OH⁻) that inhibited the growth of nanorods. Cyclic voltammetry (CV) analysis showed better electrochromic property of WO₃ nanorods compared to columnar structured WO₃.

Keywords:

• tungsten oxide
• surfactant
• CTAB
• HMT
• seeded growth

1. Introduction

Tungsten oxide (WO₃) has attracted great attention since it has potential applications as electrochromic, photocatalytic and sensors [1]. Out of these applications, electrochromic property is the most interesting as WO₃ film has the ability to undergo optical colouration when voltage is applied. The colouration of WO₃ film is useful in the formation of various optical devices. WO₃ can change colour from yellowish-transparent to opaque-blue when ions like Li⁺, Na⁺ or H⁺ are inserted or intercalated in its structure. The electrochromism of WO₃ has generated interest in the use of WO₃ in energy-saving smart/dynamic windows, antiglare mirrors, high contrast displays and active camouflage [2]. WO₃ nanostructures have been prepared typically by common methods such as sol-gel, electrodeposition, chemical vapour deposition and sputtering. Hydrothermal reaction is a promising method to prepare various WO₃ nanostructures due to its simple manner, low cost and large scale production.

Cetyltrimethylammonium bromide (CTAB) is a cationic surfactant and limited hydrothermal works have been done by using CTAB to produce WO₃ [3–9]. All these research studied on the formation of WO₃ powder instead of directly grow WO₃ film on a substrate. It is believed that by directly growing WO₃ on a substrate, better contact between WO₃ nanostructures and substrate could be obtained. Hexamethylamine (HMT) is a non-ionic surfactant that could be useful in the hydrothermal growth of WO₃. To the authors’ knowledge, no HMT-prepared WO₃ has been reported to date. To allow intercalation of ions, a large surface area of WO₃ is desired. Huge surface area of WO₃ can be achieved by the formation of thin WO₃ with nanostructured grains. In this work, WO₃, in the form of nanorods, was prepared using the seeded growth hydrothermal reaction on W foils. The foil was thermally oxidised to form a native oxide layer that was used as a seed layer. This seed layer acted as a template for the formation of WO₃ nanorods by the hydrothermal reaction in a reactive solution.

2. Experimental details

W foil (1 × 1 cm²) was cleaned ultrasonically with ethanol for 15 min and subjected to thermal oxidation at 400°C for 30 min in air to form WO₃ seeds. The seeded W foil was then placed in a screw capped bottle filled with the hydrothermal bath solution. This solution was prepared by dissolving 0.3 M sodium tungstate dihydrate (Na₂WO₄.2H₂O) and surfactants of cetyltrimethylammonium bromide (C₁₉H₄₂BrN; CTAB) or hexamethylamine (C₆H₁₂N₄; HMT) in deionised water. The amount of surfactants was varied between 0.01–0.07 M in order to investigate the effect of concentration of surfactant on the growth of WO₃ nanorods. Hydrochloric acid (HCl) was added into hydrothermal bath solution with continuous stirring to obtain pH 2. The hydrothermal process was conducted at 80°C for 8 h. The morphology of samples was observed using field emission scanning electron microscope (FESEM, Zeiss SUPRA 35) and transmission electron microscope (TEM, Philips CM 12). The phases present in the samples were analysed using an X-ray diffractometer (XRD, Bruker AXS D8 ADVANCE), equipped with Cu Kα radiation. The electrochromic properties of samples were measured by cyclic voltammetry (CV) in 0.1 M sulphuric acid (H₂SO₄) electrolyte using Autolab (μAutolab Type III). The working electrode, counter electrode and reference electrode were the WO₃ electrode, platinum electrode and Ag/AgCl electrode, respectively.

3. Results and discussion

WO₃ nanorods were grown for 8 h in the hydrothermal reactive bath containing either CTAB or HMT. The morphologies of the samples are compared. Figure 1(a) shows the seed layer on W foil, which comprises of compact grains. After the hydrothermal reaction for 8 h with CTAB, WO₃ nanorods were observed as in Figure 1(c)–(g). The mechanism of nanorods formation under the influence of CTAB can be explained with Equations (1) and (2)(1) (2)

Figure 1. FESEM micrographs of: (a) thermal oxidised W foil, (b) WO₃ prepared without surfactant and WO₃ prepared with: (c) 0.01 M CTAB, (d) 0.03 M CTAB, (e) 0.05 M CTAB, (f) 0.05 M CTAB (cross sectional view) and (g) 0.07 M CTAB.

CTAB is a positively charged cationic surfactant with a long hydrophobic tail. Na₂WO₄.2H₂O dissolved in deionised water and reacted with HCl to form H₂WO₄.nH₂O. During the hydrothermal reaction at 80°C, H₂WO₄.nH₂O decomposed on WO₃ seed layer to form WO₃. WO₃ comprised of [WO₆]⁶⁻ octahedrons and CTA⁺ were then attracted to these [WO₆]⁶⁻ octahedrons and allowed one dimensional growth of WO₃ nanorods.

For the sample prepared without surfactant, WO₃ nanorods could still be obtained as shown in Figure 1(b). This was due to the presence of NaCl (from Equation (1)) that acted as a capping agent and adsorbed on the crystal planes parallel to the c-axes of WO₃ nucleus [10]. When a small amount of CTAB (0.01 M or 0.03 M) was used in the hydrothermal reaction solution, the WO₃ nanorods formed were short as shown in Figure 1(c) and (d). This was due to the insufficient amount of CTAB, and the capping roles of NaCl became more dominant. With the addition of 0.05 M CTAB as in Figure 1(e) and (f), longer nanorods could be obtained. However, with further increase of CTAB to 0.07 M (Figure 1(g)), nanorod was not formed, but the film had flat and compact surface. This was due to the presence of excess CTAB in the hydrothermal reaction solution that covered the whole seeds surface. As a result, diffusion of ions could not take place and the formation of WO₃ nanorods was inhibited. Figure 2 shows the morphology of the samples prepared with 0.05 M CTAB for 10 h hydrothermal process. By comparing to the sample after hydrothermal reaction for 8 h (Figure 1(e) and (f)), 10 h sample showed more nanorods formation with better uniformity. This suggested that the hydrothermal reaction time is crucial for the nanorods formation as longer time allows more diffusion process to take place and forms more and longer nanorods.

Figure 2. Morphologies of WO₃ prepared with 0.05 M CTAB for 10 h: (a) surface morphology and (b) cross sectional view.

Figure 3 shows the WO₃ microstructures prepared using HMT. These HMT-prepared WO₃ comprised of columnar structure as shown in Figure 3(b). It was further determined that HMT experienced thermal degradation during hydrothermal process and released hydroxyl ions (OH⁻). These OH⁻ ions were then reacted with hydrogen ions (H⁺) from the acid and form water (H₂O). The presence of water neutralised the hydrothermal reaction solution and inhibited the growth of WO₃ nanorods. Since OH⁻ reacted with H⁺ from the HCl, plenty of Cl⁻ ions were left in the solution. Therefore, more Cl⁻ ions were reacted with Na⁺ from the Na₂WO₄.2H₂O and resulted in the formation of more NaCl. As NaCl would act as the capping agent, it then capped the WO₃ seed layer and caused a slow diffusion process of WO₄²⁻ into the seed layer. This slower process resulted in the formation of columnar structure WO₃.

Figure 3. Morphologies of WO₃ prepared with: (a) 0.01 M HMT, (b) 0.01 M HMT (cross sectional view) and (c) 0.05 M HMT.

To investigate if indeed nanorods have formed on the surface of WO₃, TEM analysis was performed. Figure 4 shows the TEM image of the sample prepared with 0.05 M CTAB. It can be observed that WO₃ nanorods have a diameter of 4–7 nm. The XRD patterns of the samples prepared with CTAB and HMT are shown in Figure 5. The spectra showed that both CTAB and HMT-prepared WO₃ had a monoclinic phase that corresponded to ICDD number of 43–1035.

Figure 4. TEM micrograph of WO₃ nanorods prepared with 0.05 M CTAB.

Figure 5. XRD spectra of WO₃ prepared with various amount of surfactants.

To evaluate the electrochromic properties of WO₃ nanorods, cyclic voltammetry (CV) measurement was performed on the samples. The CV measurement was taken between −0.5 and 1.0 V with a scan rate of 100 mVs⁻¹ in 0.1 M H₂SO₄ electrolyte. Figure 6 shows the cyclic voltammogram of WO₃ prepared with 0.05 M CTAB and 0.05 M HMT. With decreasing voltage, current moves negatively, corresponding to intercalation of H⁺ into WO₃ and resulting in colouration of the film. With increasing voltage, current moves positively, which corresponds to de-intercalation of H⁺ from H_xWO₃ and resulting in bleaching of the film. This electrochromic process can be represented as:(3)

Figure 6. Cyclic voltammogram of WO₃ prepared with 0.05 M CTAB and 0.05 M HMT.

For the WO₃ prepared with 0.05 M CTAB, a cathodic current peak could be observed at −0.10 V, which revealed that an electrochemical process took place at that voltage. By increasing the voltage, two anodic current peaks appeared at −0.30 V and −0.03 V, which corresponded to the shallow trap H⁺ sites and deep trap H⁺ sites, respectively [11]. The integrated cathodic-current density is equal to the amount of H⁺ intercalated to form H_xWO₃ [2]. By comparing cathodic charge quantities of WO₃ prepared with 0.05 M CTAB (nanorods) and 0.05 M HMT (columnar), WO₃ nanorods showed a higher charge-insertion density over the same period. This better electrochromic property of WO₃ nanorods was due to the higher surface area of WO₃ for faster intercalation and de-intercalation process.

4. Conclusions

WO₃ nanorods were successfully prepared with CTAB surfactant (< 0.07 M) using the hydrothermal reaction at 8 h. Among these samples, sample with 0.05 M CTAB produced the longest nanorods. Moreover, the hydrothermal reaction using 0.05 M CTAB for 10 h produced more uniform and longer WO₃ nanorods. On the other hand, columnar WO₃ was obtained in the HMT-prepared samples as HMT thermally degraded in the hydrothermal solution that neutralised the solution and inhibited the growth of nanorods. CTAB-prepared WO₃ nanorods showed excellent electrochromic property compared to HMT-prepared WO₃ columnar.

Acknowledgements

The authors express their sincere appreciation to the technical support of the School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia. This research was supported by MyBrain15, Ministry of Higher Education, Malaysia, Research University grant 1001/PBAHAN/814086 and PRGS 1001/PBAHAN/8044020.