Surface Leaching of Pd-Containing SnO2 Gas Sensors Enhances Sensitivity

Introduction Palladium is commonly used to enhance the performance of chemoresistive metal-oxide gas sensors [1]. Typically, this enhancement is attributed to the presence of Pd clusters on the surface of their metal oxide support (e.g. SnO 2 ). More specific, small Pd clusters on SnO 2 were associated to enhanced sensitivity to H 2 due to fermi-level control [2] or spillover effects [1]. However, Pd may not be present in metallic but oxidic form, that is catalytically active as well [3]. E.g. PdO can form during fabrication or upon the typical annealing [4] at 400 - 700 °C [3] for a few hours prior to gas sensing. Furthermore, possible Pd incorporation or embedding into the support rarely has been considered [5]. Here, SnO 2 particles with different Pd contents were prepared by flame spray pyrolysis (FSP). Their surface Pd was removed through leaching with HNO 3 to guaranty the surface and embedded Pd content was measured. Lastly, the influence of surface and embedded Pd on SnO 2 gas sensors was evaluated with acetone, ethanol and CO at 350 °C and 50% relative humidity. Method Pristine and Pd-containing (0 - 3 mol%) SnO 2 nanoparticles were produced by FSP and annealed for 5 h at 500 °C. To remove all Pd from the surface, the particles were reduced in 5% H 2 in Ar at 150 °C for 30 min and then leached in 10% HNO 3 in water at 60 °C for 4 h (Figure 1a). Inductively coupled plasma-optical emission spectrometry was used to determine the Pd content in the leached solution, that corresponds to the surface amount. Sensing films were prepared by doctor-blading the particles onto 15 × 13 × 0.8 mm Al 2 O 3 sensor substrates followed by annealing at 300 °C for 30 min. These sensors were then heated to 350 °C and tested with different concentrations (5 - 1000 ppb) of acetone, ethanol and CO in a gas sensing setup described elsewhere [6]. Results and Conclusions Figure 1b displays the fraction of surface (or leachable) Pd over the nominal content of flame-made and annealed particles. Only a fraction of Pd content is on the surface whereas the rest is embedded in the bulk SnO 2 . For instance, at nominal 0.5 mol% Pd, only 30% of it is leached while at 3 mol% Pd it increased to 45%. Apparently, most Pd is embedded and thereby not exposed to the analytes during sensing. Figure 1c shows the sensor responses at 350 °C to 1 ppm of acetone at 50% RH of annealed Pd-containing SnO 2 as a function of the nominal Pd content. Before leaching (triangles), the addition of small amounts of Pd (up to 0.2 mol%) slightly increases the responses to acetone (within the error bar of reproducibility). At higher Pd contents, the responses continuously deteriorate until they are hardly detectable anymore at 3 mol% Pd. The circles in Figure 1c show the analyte responses after leaching the surface Pd as a function of the nominal Pd content. While the responses of pristine SnO 2 were hardly affected by leaching, surprisingly, adding only 0.2 mol% Pd almost doubled the acetone response from 4 to 7. Overall, SnO 2 containing both surface & embedded Pd (i.e. prior to leaching, triangles) results in lower responses than after leaching (circles). We reveal that flame-made Pd-containing SnO 2 contains a significant fraction of Pd embedded in the bulk of (and/or strongly surface-bonded to) SnO 2 . Furthermore, small amounts of embedded Pd nearly double the responses of SnO 2 to acetone at 350 °C. In contrast, the presence of surface Pd deteriorates the sensor performance below that of pure SnO 2 . This might point out, that small amounts of noble metals embedded in metal oxides can be more effective than on their surface. References [1] Yamazoe, N.; Kurokawa, Y.; Seiyama, T., Effects of additives on semiconductor gas sensors. Sensor Actuator 4 (1983) , 283-289. [2] Matsushima, S.; Teraoka, Y.; Miura, N.; Yamazoe, N., Electronic interaction between metal additives and tin dioxide in tin dioxide-based gas sensors. Jpn J Appl Phys 1 27 (1988) , 1798-1802. [3] Kappler, J.; Barsan, N.; Weimar, U.; Dieguez, A.; Alay, J. L.; Romano-Rodriguez, A.; Morante, J. R.; Gopel, W., Correlation between XPS, Raman and TEM measurements and the gas sensitivity of Pt and Pd doped SnO 2 based gas sensors. Fresen J Anal Chem 361 (1998) , 110-114. [4] Liewhiran, C.; Tamaekong, N.; Wisitsoraat, A.; Tuantranont, A.; Phanichphant, S., Ultra-sensitive H 2 sensors based on flame-spray-made Pd-loaded SnO 2 sensing films. Sensor Actuat B-Chem 176 (2013) , 893-905. [5] Degler, D.; de Carvalho, H. W. P.; Weimar, U.; Barsan, N.; Pham, D.; Mädler, L.; Grunwaldt, J. D., Structure-function relationships of conventionally and flame made Pd-doped sensors studied by X-ray absorption spectroscopy and DC-resistance. Sensor Actuat B-Chem 219 (2015) , 315-323. [6] Pineau, N. J.; Kompalla, J. F.; Güntner, A. T.; Pratsinis, S. E., Orthogonal gas sensor arrays by chemoresistive material design. Microchim Acta 185 (2018). Figure 1