Synthesis and electrochemical capacitance of long tungsten oxide nanorod arrays grown vertically on substrate

doi:10.1016/j.materresbull.2012.06.053

Materials Research Bulletin

Volume 47, Issue 11, November 2012, Pages 3612-3618

https://doi.org/10.1016/j.materresbull.2012.06.053 Get rights and content

Abstract

Long tungsten oxide nanorods are vertically grown on Al/W/Ti coated silicon substrates using a two-step anodization process. The first anodization of the Al film forms a mesh-like mask of anodic aluminum oxide, and the second anodization of the W film results in the formation of a buffer layer, a bottom nanorod, and a top nanorod of amorphous tungsten oxide. A pore-widening process prior to the second anodization leads to the enhancement of nanorod length above approximately 500 nm. After a heat-treatment, the tungsten oxide nanorods are crystallized to form a single crystalline structure while the buffer layer forms a polycrystalline structure. The crystalline tungsten oxide nanorods show a cyclic voltammogram retaining the quasi-rectangular shape of an electrochemically reversible faradaic redox reaction, i.e., a typical pseudocapacitive behavior. The maximum electrochemical capacitance per apparent surface area reaches approximately 2.8 mF cm⁻² at the voltage scan rate of 20 mV s⁻¹, and the excellent cyclability of charge–discharge process is maintained up to 1000 cycles.

Graphical abstract

Highlights

► Growth of long amorphous tungsten oxide nanorods on a substrate. ► Formation of single-crystalline tungsten oxide nanorods by a heat-treatment. ► High electrochemical pseudocapacitance of 2.8 mF cm⁻². ► Excellent cyclability of psuedocapacitance up to 1000 cycles.

Introduction

Semiconducting tungsten oxides with various stoichiometric crystal structures have attracted much attention as the electrochromic and photochromic properties have potential application in information displays, smart windows, write–read–erase optical devices, solar energy devices, and photocatalysts [1], [2], [3]. Recently, tungsten oxides have also been investigated for application in electrochemical capacitors due to their chemical reactivity [4], [5].

Thin films and nanostructures of tungsten oxides have been fabricated by thermal evaporation [6], vapor–solid growth [7], chemical vapor deposition [8], hydrothermal synthesis [9], sol–gel process [10], solvothermal process [11], electrodeposition [12], and electrochemical anodization [13]. Recently, the anodization process using an anodic aluminum oxide (AAO) mask was used to produce tungsten oxide nanorods with good vertical alignment on a substrate [14]. The anodic process was based on the theoretical idea and experimental fact that the tungsten atom, as a valve metal, with a low ionic resistivity, can diffuse through tungsten oxide and the barrier layer of AAO [15], [16]. The lengths of tungsten oxide nanorods depend on the final anodic voltage, the driving force transporting the tungsten ions along the AAO nanochannel [14]. Thus, the nanorod length increases with an increase in anodic voltage. However, as the anodic voltage exceeds a certain point, e.g. 120 V, the voltage fluctuations occur and Kirkendall voids form beneath the tungsten oxide layer, and then a failure occurs [14], [17].

In the present study, we were motivated to suppress the formation of voids beneath the tungsten oxide nanorods and to grow longer tungsten oxide nanorods in order to utilize the high surface-to-volume ratio of the nanostructure. Since tungsten oxides are electrochemically reactive enough to be intercalated by several ions of H⁺, Li⁺ and Na⁺ and to form tungsten bronze [18], we then investigated the electrochemical capacitive properties of the longer tungsten oxide nanorods.

Section snippets

Experiment

A multilayered thin film structure of Al/W/Ti with thicknesses of 330, 220, and 20 nm was sputter-deposited on 6-in. Si (1 0 0) substrate. The wafer was divided into pieces 2 cm × 2 cm. It was then electrochemically anodized for the area of 1.6 cm × 1.2 cm by the first anodization process. This anodization formed an anodic aluminum oxide (AAO) template in an aqueous electrolyte of 0.3 M oxalic acid under a constant anodic potential of 40 V at 274 K. After the first anodization, a pore-widening process was

Results and discussion

During the first anodization of the Al film at a constant potential of 40 V, a high anodic current initially flowed because the bare Al surface was exposed to the electrolyte, and then the current abruptly decreased with the anodic oxidation of Al surface, as shown in the first stage (I) of Fig. 1a. The anodic current increased again and reached saturation during anodization, while the AAO pores grew toward the W layer. When the barrier layer at the bottom of the AAO pores reached the W layer,

Conclusions

Long tungsten oxide nanorods were vertically grown using a two-step anodization process. In the present study, the limit of the TONs’ lengths was overcome by an intermediary pore-widening process, and the length reached above 500 nm, i.e., an aspect ratio of 9. The pseudocapacitive properties of the crystalline TONs were systematically investigated along the TON length. The TONs showed an electrochemically-reversible faradaic redox reaction, i.e., a typical pseudocapacitive behavior. The

Acknowledgment

This work was supported by the IT R&D program of MKE/KEIT (KI002130, Development of high quality GaN single crystal and wafer for white LED).

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Arrays of self-organized WO₃-based semiconductor nanorods are prepared from a thin W layer, W/Ti bilayer (tungsten-on-titanium), and W-10at.%Ti alloy layer via the porous-anodic-alumina (PAA)-assisted anodization at various conditions to address the radius/length ratio of~13/130 and ~70/700 nm (respectively ‘small’ and ‘big’ nanorods). Doping the WO₃ nanorods with TiO₂ was achieved, for the first time, simply by anodizing the W/Ti and W-10at.%Ti layers through the alumina nanopores. The post-anodizing treatments combined PAA dissolution with annealing in air and vacuum at 500–550 °C to alter the film composition, crystal structure, and electrical properties. The air-annealed big nanorods comprising monoclinic and triclinic WO₃ crystal phases reveal their superior performance in photoelectrochemical (PEC) water splitting, showing a low onset potential (0.5 V_RHE) and a competitive value of photocurrent (15.5 mA cm⁻²) in 0.1 mol dm⁻³ Na₂SO₄ solution (pH 5.0) under chopped illumination at a single wavelength of 405 nm, 1 W cm⁻², with no sign of photocorrosion. Paradoxically, the presence of monoclinic WO_2.9 phase in the vacuum-annealed nanorods worsens the PEC behavior and stimulates the peroxo-assisted dissolution. Unexpectedly, electrochemically doping both the WO₃ and WO_2.9 big nanorods with TiO₂ causes the photocurrent to decrease dramatically. An advanced approach developed for modeling charge transport processes in the PAA-assisted WO_x nanorods predicts a 7-fold further rise in the solar current should the big nanorods grow longer (1.5 µm) and wider (300 nm) to absorb a bigger portion of light and support a thicker depletion layer, without, however, getting fully depleted, which is the case of the small nanorods.
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Synthesis and electrochemical capacitance of long tungsten oxide nanorod arrays grown vertically on substrate

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Experiment

Results and discussion

Conclusions

Acknowledgment

Sol. Energy Mater. Sol. Cells

J. Power Sources

Scripta Mater.

Sol. Energy Mater. Sol. Cells

Electrochem. Commun.

Electrochim. Acta

Electrochim. Acta

Biophys. Chem.

Electrochim. Acta

J. Power Sources

Adv. Mater.

Chem. Commum.