Facile Hydrogen Evolution Reaction on WO3Nanorods
© to the authors 2007
Received: 13 April 2007
Accepted: 10 August 2007
Published: 1 September 2007
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© to the authors 2007
Received: 13 April 2007
Accepted: 10 August 2007
Published: 1 September 2007
Tungsten trioxide nanorods have been generated by the thermal decomposition (450 °C) of tetrabutylammonium decatungstate. The synthesized tungsten trioxide (WO3) nanorods have been characterized by XRD, Raman, SEM, TEM, HRTEM and cyclic voltammetry. High resolution transmission electron microscopy and X-ray diffraction analysis showed that the synthesized WO3nanorods are crystalline in nature with monoclinic structure. The electrochemical experiments showed that they constitute a better electrocatalytic system for hydrogen evolution reaction in acid medium compared to their bulk counterpart.
One-dimensional (1D) nanostructures such as nanorods, nanowires and nanotubes have attracted attention due to their novel physical and chemical properties as well as their potential use in a wide range of advanced applications in the past decade [1, 2]. As a consequence, many synthetic methods have been developed to prepare various 1D nanostructures [3–6]. Of particular interest is the preparation of 1D nanostructure of tungsten trioxides (WO3) and its suboxides (WO3 − x ). WO3 is used extensively as materials for electrochromic devices [7–10], gas sensors [11, 12], catalysts [13, 14] and secondary batteries . Several synthetic approaches including electrochemical techniques, sonochemical approach, template mediated synthesis, bioligation, hydrothermal, wet organic and inorganic routes and thermal methods have been reported to fabricate WO3 nanorods [16–51]. A method for the synthesis of tungsten oxide nanorods with planar defects or textured structure has been introduced by Zhang et al. and this method involves growth of WO3 − x nanorods on the tips of electrochemically etched tungsten filament . However, synthesis of such structure has been possible only in the presence of H2 atmosphere. The nanorods of WO3 have also been obtained by sonochemical method wherein Koltypin et al. have synthesized a mixture of WO2–WO3 nanorods by ultrasound irradiation of W(CO)6 in diphenylmethane . Tungsten oxide nanorods could also be obtained from templated route by using CNTs and colloidal gas aphrons as templates [18–20]. Therese et al. have adopted an organic amine assisted low temperature hydrothermal route for the synthesis of hexagonal WO3 nanorods . Inorganic compounds such as Na2SO4, Rb2SO4 and K2SO4have been demonstrated as structure directing agents for the hydrothermal synthesis of 1-D WO3 nanorods by Gu et al., Lu et al. and Xiao et al. [24–26]. Srivastava et al. have reported sol–gel followed by dip coating to produce WO3 nanorods . By altering the composition and concentration of solvent, it was shown that different morphologies and phases of WO3 nanorods can be achieved . However, all of these reported efforts involve multistep processes and limited to the use of directing agents such as CNTs and M2SO4 (M = Na, Rb and K). Hence, the synthesis would be tedious and requires careful removal of the structure directing agents to avoid contaminants. Moreover, sol–gel and hydrothermal methods proceed with a low yield. In order to overcome the difficulties mentioned, thermal method has been employed widely for the large scale synthesis of tungsten oxide nanorods/nanowires as they are simple, easy and free from catalysts and contaminants. Heat treatment of tungsten metal, such as, tungsten foil or tungsten filament heated at 800–1,600 °C [40–42], tungsten powder heated at 950 °C under the Ar gas flow on ITO glass/tungsten substrate at 900–1,100 °C , thermal evaporation of tungsten powder  and tungsten hexacarbonyl heated at 700 °C  have yielded 1-D WO3 nanorods. All the above thermal methods related to 1-D tungsten oxide formation from the gaseous phase (vapor deposition) or thermal evaporation techniques are technically complex, require high temperature, harsh growth conditions, expensive experimental setup and complicated control processes. Recently, the single source molecular precursor route has opened a useful way for the synthesis of WO3 nanorods/nanowires by thermal decomposition method [49–51]. It offers the distinct advantage of simplified fabrication procedure and equipment as compared with the thermal evaporation or vapor deposition methods. However, multiple steps for the synthesis of both precursor and 1-D WO3, longer reaction time for precursor (6 days or 10 h) and relatively higher pyrolysis temperature (750 °C) were required. In this report, a facile synthesis of WO3 nanorods based on the thermal decomposition of tetrabutylammonium decatungstate has been described. The method has several unique advantages. Firstly, it has been possible to obtain high yields of WO3 nanorods at a relatively low temperature (450 °C) and short reaction time as compared to previously reported methods involving high temperature (≥700 °C). Secondly, different morphologies of the material (nanosheets and nanorods) can be achieved by altering the pyrolysis time. Moreover, the method followed for the synthesis of precursor is a simple precipitation which does not require any tedious experimental set up or does not consume much time when compared to the other methods. Finally, it is a generic method which can be applied for synthesis of other metal oxides such as MoO x and V2O5 by suitably altering the metal in the precursor.
There has been a continuous interest in studying the dimensionality-dependent properties of WO3 and ultimately to fabricate nanodevices. Wang et al. have shown high Li intercalation capacity (1.12 Li per formula unit) for WO3 nanorods than its bulk counterpart (0.78 Li per formula unit) . The enhanced electrochemical performance has been attributed to the unique rod-like structure combined with increased edge and corner effects. Non stoichiometric WO2.72 nanorods are found to function as sensors with extraordinary sensing ability and the activity has been attributed to the very small grain size and high surface to volume ratios associated with the nanorods . Liu et al. have exhibited low turn on field for electronic emission by WO2.9 nanorods . Photoluminescent emission spectrum of tungsten oxide nanorods was found to show an additional blue emission peak at 437 nm than its bulk system . All these results show the unique properties of WO3 nanorods in comparison to their bulk counterpart. Similar enhanced activity can be expected for WO3 nanorods in hydrogen evolution reaction. The aim of the current study is to verify such a supposition experimentally which is significant in the development of electrodes for electrochemical hydrogen production.
In recent years, hydrogen, in combination with fuel cells, has been proposed as a major alternative energy source. It provides energy at less environmental damage, with greater efficiency and acceptable cost compared to the conventional fossil fuels. Electrochemical production is one of the possible methods for hydrogen production. Materials such as Raney Ni, Ni–Mo and noble metals such as Pt, Pd and Ru have been employed for this purpose [52–55]. Inspite of their high catalytic activity for hydrogen evolution reaction, the process involving Pt and Pd are not commercialized due to their high cost and low abundance. This has lead to the investigation of newer materials or reduction of the loading of noble metals. Savadogo et al. have shown the HER using nickel electrodes with phosphotungstic acid. It has been demonstrated that the presence of W in the form of WO3 in the polyoxometalates enhanced the electrocatalytic activity for hydrogen evolution [56, 57]. Baruffladi et al. have shown electrodeposited composites of non-stoichiometric tungsten oxides and either RuO2 or IrO2 to catalyze the hydrogen evolution in acid medium . Platinum when supported on tungsten trioxide showed electrocatalytic activity for HER due to the synergism towards reactions in acid involving hydrogen atoms . All these results show the significance of WO3 in hydrogen evolution reaction.
In this article, we report a surfactant directed large scale synthesis of monoclinic WO3nanorods. This has been achieved by a simple pyrolysis of a single source precursor which consists of surfactant encapsulated tungsten oxide clusters. The employed route is template free, contaminant free, easy, economical and requires a low temperature for the fabrication of WO3nanorods. The morphology, chemical composition and structure were characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), High resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The as-synthesized tungsten trioxide nanorods have been employed as an electrocatalyst for hydrogen evolution reaction (HER). The electrocatalytic activity of the material for HER was investigated by cyclic voltammetry, linear sweep voltammetry and Tafel plots. The activity was compared with that of commercially obtained bulk tungsten trioxide. Enhanced catalytic activity has been observed for WO3nanorods compared to bulk WO3as an electrocatalyst for HER.
The commercial tungsten trioxide was purchased from Alfa Aesar. All other chemicals were purchased from Sisco Research Laboratories Pvt. Ltd and used as received.
Tungsten trioxide nanorods have been synthesized by using tetrabutylammonium decatungstate as the precursor material. The starting material was prepared according to the work described elsewhere . The typical procedure involved the precipitation of tetrabutylammonium decatungstate by adding an aqueous tetrabutyl ammonium bromide solution to a clear yellow solution of tungstic acid preformed using sodium tungstate and concentrated hydrochloric acid. The white precipitate was washed with boiling water and ethanol, filtered, dried and then recrystallized in hot dimethyl formamide to give yellow crystals. The thermogravimetric analysis revealed that the tetrabutylammonium cation content in the compound is 29.0% (theoretical value: 29.2%) and the decomposition temperature is around 450 °C as reported . The synthesis of tungsten trioxide (WO3) nanorods from tetrabutylammonium decatungstate ((C4H9)4N)4W10O32) is carried out as follows: The precursor compound (1 g) was taken in an alumina or quartz boat and loaded inside a tubular furnace and heated at 450 °C at a heating rate of 25 °C per min under Ar atmosphere for 3 h. This was followed by gradual cooling to room temperature to obtain a blue powder of WO3 nanorods. The total yield of the obtained material was 71% by weight (relative to the starting material). To further investigate the role of tetrabutylammonium (TBA) group on the morphology of WO3, an experiment has been carried out in the absence of tetrabutylammonium ion. To achieve this, (NH4)10H2W12O42 · X H2O has been taken as the precursor and pyrolysed under similar experimental conditions that were employed for the formation of WO3 nanorods.
X-Ray Diffraction (XRD) patterns were obtained by a powder diffractometer (XRD - SHIMADZU XD-D1) using a Ni-filtered CuKα X-ray radiation source. CRM 200 Raman spectrometer was employed, using the 514.5 nm line of an Ar ion laser as the excitation source. The morphology of the WO3nanorods was investigated by a scanning electron microscopy (SEM) (FEI, Model: Quanta 200). Transmission electron microscopy (TEM), Electron diffraction and Energy Dispersive X-ray Analysis (EDAX) were performed on a Philips CM12/STEM instrument. High-resolution Transmission Electron Microscopy (HRTEM) was carried out on a JEOL 3010.
A three electrode cell consisting of the glassy carbon as working electrode (0.07 cm2), Pt wire and Ag/AgCl (satd. KCl) electrodes as counter and reference electrodes respectively were used. All electrochemical measurements were performed using a CHI660A potentiostat/galvanostat. The working electrodes for electrochemical measurements were fabricated by dispersing 5 mg of the catalyst in 100 μL of deionized water by ultrasonication for 20 min. From this dispersion 10 μL has been taken and placed on a glassy carbon electrode. The solvent was slowly evaporated by placing the electrode in an oven at 70 °C. Five microliter of nafion solution has been coated on the electrode as a binder and dried at room temperature. One molar H2SO4was used as the electrolyte. The electrolyte solution was deaerated with high purity N2(99.99%) for 30 min before the electrochemical measurements.
Electrochemical parameters of the HER obtained from the Tafel plots for different electrode materials
Slope, b (V A−1 cm2)
i o(A cm−2)
8.57 × 10−7
2.75 × 10−6
In summary, we demonstrate a thermal decomposition method for the synthesis of 1-D WO3nanorods in high yield using a single source precursor. One of the important aspects of this method is the in situ formation of nanostructures due to the surfactant encapsulated metal oxide clusters. The advantage of this method is the tunability of the metal precursor and the surfactant group. This aspect of the method can be exploited for the synthesis of several other transition metal oxide nanorods. We have also synthesized nanorods of molybdenum oxide and mixed oxides of molybdenum and vanadium using similar strategy. The as synthesized WO3nanorods perform well as an electrocatalyst with enhanced electrocatalytic activity for HER when compared to its bulk counterpart. The results show the possibility of minimizing the loading of noble metal electrocatalyst for HER by using WO3nanorods as catalyst support.