Elsevier

Electrochimica Acta

Volume 50, Issue 20, 25 July 2005, Pages 4146-4154
Electrochimica Acta

Electrochemical and spectroscopic characterization of a tungsten electrode as a sensitive amperometric sensor of small inorganic ions

https://doi.org/10.1016/j.electacta.2005.01.031Get rights and content

Abstract

Cyclic voltammetry was used to investigate the electrochemical behaviour of the tungsten oxide films toward the electroreduction of BrO3, ClO2 and NO2 ions in acidic medium. The effects of the temperature, scan rate, pH, chemical composition of the electrolytic solutions, were investigated and the reduction mechanism was critically discussed.

The reduction currents, evaluated in cyclic voltammetry and measured at −0.250 V versus SCE, increased linearly on increasing analyte concentration up to 20, 55 and 45 mM for nitrite, chlorite and bromate ions, respectively. The detection limits, evaluated in cyclic voltammetry, were 0.1, 0.4 and 0.7 mM for BrO3, ClO2 and NO2, respectively.

The tungsten oxide film was successfully characterized as an amperometric sensor for the analytical determination of BrO3, ClO2 and NO2 ions in flowing stream. Operating under constant applied potential of −0.3 V versus Ag/AgCl the good reproducibility of the peak height and background current level during consecutive injections, indicates the absence of fouling effects and the potential applicability of the amperometric sensor for the routine analytical determination of the investigated inorganic ions. Considering the low values of the background currents (ca. 1.1 ± 0.1 μA) obtained in acidic and not deoxygenated carrier electrolyte, the tungsten sensing electrode seems to compete favourably with other common sensors for the amperometric determination of electroactive molecules under cathodic conditions.

The X-ray photoelectron spectroscopy technique (XPS) was used in order to evaluate the chemical composition of the tungsten film upon electrochemical treatment in 0.1 M H2SO4 solution. Independently of the electrochemical treatment in acid solution, the tungsten surface electrode is generally composed by 50–60% of W0, 35–40% of W6+ and traces of W2+ oxide species.

Introduction

Transition metal oxides and their alloys are extensively characterized and proposed in various fields as corrosion protective coatings, semiconductors, insulators, supercapacitors, energy conversion systems, energy storage, electrochromic and electrocatalytic devices. Thus, these materials are extremely interesting from both the fundamental and the technological point of view. Bulk resistive oxides, like TiO2, ZnO, WO3, etc. have received considerable attention for their use in the fabrication of chemiresistive gas sensors [1]. The sensing principle is based on the change in the conductance undergone by the oxide film when gases are adsorbed and react on its surface. In recent years, some gas-sensitivity devices with tungsten trioxide (WO3) based semiconductors or doped tungsten oxides are considered promising electrode materials for the detection of nitrogen oxides [2], [3], sulfur dioxide [4], [5] and ammonia specie [6] with high sensitivity and selectivity. In addition, tungsten oxides have attracted much attention due to their potential use as sensors for pH measurements [7], [8].

Alternatively, numerous transition metals or their oxides species show interesting electrochemical catalytic activity in relation to the oxidation or reduction of several scarcely electroactive molecules. Thus, direct electroanalysis and/or electrosynthesis processes based on the heterogeneous catalysis between the active oxide species dispersed on the electrode surface and analytes were successfully proposed and used for the synthesis and analytical detection of several important classes of molecules [9], [10], [11]. In this respect, electrodeposited composite WOx based films have been investigated towards the electroreduction of several electroactive molecules such as: bromate [12], methanol [13], [14] and small saturated organic acids [15]. Generally, these active WOx based films incorporated microcenters of foreign transition metals (i.e., Pt, Fe, Ni, Ru, etc.) via electrochemical deposition act as very effective bi-functional electrocatalysts for the oxidation or reduction of low molecular weight compounds at low overpotentials. In addition, these composite active films are less susceptible to poisoning by carbon monoxide and/or other oxidation intermediates [13], [14], [15]. Nevertheless, studies regarding the electrochemical behaviour, and in particular the electrocatalytic activity of the tungsten oxides in acidic solutions have not been systematically performed.

Our interest in this work is focused on the electrochemical and spectroscopic characterization of tungsten oxide electrode towards the electroreduction of small electroactive inorganic anions such as nitrite, chlorite and bromate in acidic solution. The tungsten oxide material was successful characterized as amperometric probe for the analytical determination of the small electroactive inorganic anions in flow injection analysis at constant applied potentials. The relevant results in flowing streams conditions presented here are of interest for their relevance of the analytical prospects for monitoring bromate, nitrite and chlorite in environmental and biological contexts.

Section snippets

Reagents

Solutions were prepared from analytical-reagent grade chemicals without further purification and by using pure water supplied by Milli-Q RG purification system from Millipore (BedFord, MA). Perchloric, sulphuric, nitric acids, sodium chlorite, potassium bromate and sodium nitrite were purchased from Sigma-Aldrich (Steinheim, Germany). Tungsten foil 99.9% 1.0 mm thick, used for XPS measurements was also purchased from Sigma-Aldrich. Unless otherwise specified, experiments were performed by using

Voltammetry

Fig. 1 shows the cyclic voltammogram of a tungsten wire obtained in 0.1 M H2SO4 solution. The presence of two well-defined redox couples Ia1/Ic1 and Ia2/Ic2 centred at about 0.16 and −0.22 V versus SCE, are attributed to a hydrogen intercalation/deintercalation and reduction of W(VI) to lower tungsten oxides. The relevant redox transitions can be written as [12], [16], [17]:WO3 + xH+ + xe  HxWO3WO3 + 2yH+ + 2ye  WO3−y + yH2OThe first couple Ia1/Ic1 has been attributed to the insertion of hydrated H+

Conclusions

In this study, a tungsten oxide film was characterized by electrochemical and XPS techniques towards the electrocatalytic reduction, in acidic solutions, of BrO3, ClO2 and NO2 ions. The dependence of the electroreduction rate of BrO3, ClO2 and NO2 on the temperature, scan rate, pH, chemical composition of the electrolytic solutions, was investigated. Thus, under acidic conditions, the electroreduction mechanism of BrO3, ClO2 species is probably controlled by diffusion processes, while a

Acknowledgments

This work was supported by Ministero dell’Universita’ e della Ricerca Scientifica e Tecnologica (MURST, COFIN 2002). Mrs. M. Contursi acknowledges the University of Basilicata for the financial support provided by a research grant.

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