Influence of Mono- and Bimetallic PtOx, PdOx, PtPdOx Clusters on CO Sensing by SnO₂ Based Gas Sensors.

Pavel Kutukov,Anatoly Aksenenko,Valeriy Krivetskiy,Marina Rumyantseva,Maria Batuk,Nikolay Khmelevsky,Alexander Gaskov,Darya Filatova,Joke Hadermann

doi:10.3390/nano8110917

Abstract

To obtain a nanocrystalline SnO2 matrix and mono- and bimetallic nanocomposites SnO2/Pd, SnO2/Pt, and SnO2/PtPd, a flame spray pyrolysis with subsequent impregnation was used. The materials were characterized using X-ray diffraction (XRD), a single-point BET method, transmission electron microscopy (TEM), and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) with energy dispersive X-ray (EDX) mapping. The electronic state of the metals in mono- and bimetallic clusters was determined using X-ray photoelectron spectroscopy (XPS). The active surface sites were investigated using the Fourier Transform infrared spectroscopy (FTIR) and thermo-programmed reduction with hydrogen (TPR-H2) methods. The sensor response of blank SnO2 and nanocomposites had a carbon monoxide (CO) level of 6.7 ppm and was determined in the temperature range 60–300 °C in dry (Relative Humidity (RH) = 0%) and humid (RH = 20%) air. The sensor properties of the mono- and bimetallic nanocomposites were analyzed on the basis of information on the electronic state, the distribution of modifiers in SnO2 matrix, and active surface centers. For SnO2/PtPd, the combined effect of the modifiers on the electrophysical properties of SnO2 explained the inversion of sensor response from n- to p-types observed in dry conditions.

Highlights

Because SnO2 is a wide-bandgap oxygen-deficient n-type semiconductor with optical transparency, electron conductivity, and a high specific surface area, it is suitable for a large range of applications, including in solar cells, as catalytic support, and as solid state gas sensors [1]
In bimetallic nanocomposite SnO2/PtPd, platinum was located in large bimetallic PtPdOx particles with a different Pt/Pd ratio, but palladium was present in the form of small nanoparticles PdOx
Active surface sites with oxidizing properties were determined by the presence of Pt-containing particles, while PdOx nanoparticles were responsible for the increase in the surface hydroxyl concentration

Summary

Introduction

Because SnO2 is a wide-bandgap oxygen-deficient n-type semiconductor with optical transparency, electron conductivity, and a high specific surface area, it is suitable for a large range of applications, including in solar cells, as catalytic support, and as solid state gas sensors [1]. The authors assumed that if platinum is introduced by impregnation after the calcination of the SnO2 matrix, it forms a separate oxide phase, which creates additional reaction sites not electronically coupled to the SnO2 In our investigation, this assumption could not be used since the introduction of PtOx clusters led to the important (~103 times) increase in SnO2 resistance in dry air (Figure 1a). In humid air, the surface of tin dioxide became inactive due to the partial replacement of chemisorbed oxygen by hydroxyl groups according to the following reaction [80] The combination of these factors suggests that CO oxidation can take place at the Pt–O–Sn sites at the three-phase boundary between PtOx and SnO2 [76], providing a very slight increase in sensor response as compared with detection in dry air. In a humid atmosphere, electrons remained the main charge carriers in the SnO2/PtPd nanocomposite and no inversion of sensor response from n- to p-type was observed

Materials and Methods

Conclusions