High response and selectivity of platinum modified tin oxide porous spheres for nitrogen dioxide gas sensing at low temperature

Highlights

• The SnO₂ porous spheres with different Pt concentrations were synthesized.

• The gas sensing property of Pt-SnO₂ towards NO₂ gas was studied.

• The 0.25 wt% Pt-SnO₂ displays high sensitivity (S = 5770) to 5 ppm NO₂ at 80 °C.

• The 0.25 wt% Pt-SnO₂ exhibits rapid response and recovery times (30/90 s).

• The 0.25 wt% Pt-SnO₂ shows superior selectivity for trace of NO₂ detection in air.

Abstract

The Pt modified SnO₂ porous spheres for NO₂ gas sensor have been synthesized through facile hydrothermal method followed by a chemically reducing process. The porous Pt-SnO₂ spheres with the size of 400–700 nm in diameter exhibit a dominant pore size of ca. 15 nm and specific surface area of 36.6 m² g⁻¹, which can provide substantial chemical adsorption sites and abundant channels for the diffusion of NO₂ molecules. The SnO₂ porous spheres with an optimized loading amount of Pt nanoparticles (0.25 wt%) display superior gas sensing performances including higher response (5770) and selectivity to 5 ppm NO₂ gas, as well as shorter response and recovery times (30 s/90 s) than other reported NO₂ sensing materials at a lower operating temperature (80 °C). The comparison experiments indicate that Pt catalyst loading not only improves the response to NO₂ gas and reduces the operating temperature, but also exhibits high selectivity to NO₂ gas.

Introduction

With the development of industry and extensively application of motor vehicles, we are facing serious atmosphere pollution problems. As is known, nitrogen dioxide (NO₂) mainly caused by the industrial waste gas and automobile exhaust is one of primary pollution in air, which is a major inducing factor of acid rain as well as photochemical smog [1], [2], [3]. Therefore, the detection of nitrogen dioxide in air has attracted increasing attention due to its toxicity and causticity. Recently, a lot of semiconductor oxides have been investigated as sensing materials for NO₂ gas detection such as SnO₂ [4], [5], ZnO [6], [7], In₂O₃ [8], [9], WO₃ [10], [11] and NiO [12], [13]. Among them, tin oxide (SnO₂) with a wide band gap (E_g = 3.6 eV), a well-known n-type semiconductor, has been extensively used for gas sensing [14], [15], [16]. However, SnO₂ as a gas sensing material is still facing some challenges for NO₂ detection, e.g., low sensitivity and selectivity, high operating temperature [5], [17], [18].

As regards the aforementioned issues, many efforts have been tried to improve the gas sensing performances of SnO₂. For example, SnO₂ thin film [19], networked SnO₂ nanowires [20], and discoid tin oxide for NO₂ detection [21] have been synthesized and the corresponding sensing performances have been investigated. The previous studies proved that SnO₂ nanostructures with higher specific surface and porosity could improve sensitivity, and reduce response and recovery times. It can be explained that high specific surface and porosity not only increase the contacting area between tested gas and SnO₂ surface, but also provide abundant diffusion channels for gas molecules. In addition, some reports concerned with catalyst hybridization such as Au-SnO₂ spheres for H₂ and CO [22], Pt-SnO₂ films for methanol [23], Pt-SnO₂ for H₂ and CO [24], Pt-SnO₂ films for toluene and HCHO [25], Pd-SnO₂ hollow microcubes for ethanol [26], Pd/Sb-SnO₂ nanoparticles for H₂O [27], and Co₃O₄-PdO-SnO₂ hollow nanocubes for acetone [28], demonstrated that the introduction of catalysts improved the gas sensing performances of metal oxide. After modified by catalysts, such as Au, Pt and Pd etc., oxygen molecules become more easily adsorbed and transformed into oxygen ions on the surface of SnO₂. The catalysts increase the oxygen molecule to ion conversion rate, resulting in faster electron depletion and a higher reaction rate with tested gas [29], [30], [31], [32]. Previous studies indicated that SnO₂ nanostructures with high specific surface and porosity hybridized by catalysts exhibit enhancing sensing performances to some types of reduction gases. However, to the primary air pollution NO₂ oxidizing gas, no research has presented on utilizing SnO₂ hybridized with Au, Pt or Pd catalysts as the sensing material.

In this work, SnO₂ porous spheres modified with different concentration of Pt nanoparticles (0.1 wt%, 0.25 wt% and 0.5 wt%) have been fabricated via the chemical reduction of H₂PtCl₆. The gas sensing measurements confirmed that, comparing with pure SnO₂ porous spheres and some other previous reports, the 0.25 wt% Pt modified SnO₂ porous spheres displayed superior sensing performances to NO₂ gas, including high sensitivity and selectivity, low operating temperature, as well as fast response and recovery speeds.