Acetone sensing of Au and Pd-decorated WO₃ nanorod sensors

Abstract

The sensing properties of WO₃ nanorods decorated with both Pd and Au nanoparticles toward acetone were examined. AuPd-decorated WO₃ nanorods were prepared by immersing the WO₃ nanorods in an acetone/(25 mM) HAuCl₄/(25 mM) PdCl₂ solution followed by UV irradiation and annealing. The WO₃ nanorods decorated with Au and Pd nanoparticles exhibited a far stronger response to acetone gas that increased with increasing acetone concentration than the monometal-decorated counterparts. XPS analysis showed that considerable alloy formation has occurred during the annealing process of the Au and Pd nanoparticle-decorated WO₃ nanorods. Overall, Au and Pd particle-decoration had a synergistic effect on enhancing the sensitivity of the WO₃ nanorod sensor to acetone gas. The underlying mechanism of the enhanced response of the Au/Pd bimetallic particle-decorated WO₃ nanorods is also discussed.

Introduction

Semiconducting metal-oxides have many merits, such as simple fabrication methods, low cost and high compatibility with other processes [1], [2], [3], [4], [5], [6], but their sensing performance at room temperature is unsatisfactory. Further improvements in their sensing performance are still a challenge. In many gas sensors, the electrical response is determined by the efficiency of the catalytic reactions of the gas-sensing material with the target gas. Therefore, techniques, such as decoration or doping with metal catalysts, are commonly used to enhance the catalytic activity of gas sensor materials.

Bimetallic catalyst nanoparticles, which are composed of two catalyst metal elements, often exhibit enhanced catalytic, electrocatalytic, electronic, magnetic, and optical properties compared to their parent metals due to synergistic effects. Over the past decade, a range of bimetallic catalysts have been studied including Pt-Pd, Pt-Au, Pt-Ni, Pt-Rh, Pt-Cu, Pt-Ag, Pd-Au, Pd-Cu, and Au-Ag bimetallic catalysts [7], [8], [9], [10]. Of these bimetallic catalysts, Au-Pd bimetallic catalysts might have been used most widely because of their excellent catalytic activities for a range of reactions, such as CO oxidation, gas-phase hydrodehalogenation of fluorinated compounds, hydrogenation of 4-pentenoic acid, direct oxidation of hydrogen to form hydrogen peroxide, n-heptane isomerization, hydrodesulfurization of thiophenes, acetoxylation of ethene to vinyl acetate [11], and ethanol oxidation in alkaline media [12], [13]. A variety of techniques have been employed to synthesize Pd/Au bimetallic nanoparticles, such as coprecipitation, ultrasound irradiation, and sequential reduction [14], [15]. Most of these techniques are quite complicated. The enhanced catalytic activity of Au-Pd bimetallic catalysts is not completely understood. In the literature, the origin of the enhanced catalytic activity of bimetallic catalysts was attributed to geometric effects, electronic effects, participation of Au-Pd active sites, or reduction of a palladium hydride phase [16].

This study shows the enhanced sensing properties of tungsten trioxide (WO₃) nanorod sensors decorated with both Pd and Au nanoparticles toward acetone gas. In this study, Pd/Au bimetallic nanoparticles were synthesized by immersing tungsten trioxide (WO₃) nanorods in an acetone/HAuCl₄/PdCl₂ solution followed by UV irradiation and annealing. Acetone (CH₃COCH₃) is a good breath marker for the non-invasive diagnosis of diabetes [17], [18]. Metal-oxide semiconductor gas sensors offer advantages in the detection of acetone gas, such as higher sensitivity, smaller size, lower cost, simpler operation, and better reversibility [19], [20], [21] compared to the techniques commonly used for detecting acetone, such as gas chromatography and mass spectrometry [22], [23].