Micromachined Gas Sensors Based on Au-functionalized SnO2 Nanorods Directly Integrated without Catalyst Seeds via AA-CVD

Procedia Engineering 168 ( 2016 ) 1078 – 1081 1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference doi: 10.1016/j.proeng.2016.11.344 ScienceDirect Available online at www.sciencedirect.com 30th Eurosensors Conference, EUROSENSORS 2016 Micromachined gas sensors based on Au-functionalized SnO 2 nanorods directly integrated without catalyst seeds via AA-CVD S. Vallejos, a* S. Selina,b F. E. Annanouch,c, e I. Gràcia,d E. Llobet,c C. Blackmanb a SIX Research Centre, Brno University of Technology, Brno, Czech Republic b Department of Chemistry, University College London, London, UK c MINOS-EMaS, Departament d’Enginyeria Electrònica, Universitat Rovira i Virgili, Tarragona, Spain d Instituto de Microelectrónica de Barcelona (IMB-CNM, CSIC), Barcelona, Spain e Aix Marseille Université, Université de Toulon, IM2NP UMR 7334, Marseille, France Abstract Tin oxide nanorods functionalized with Au nanoparticles are vapour synthesised at relatively lower temperatures than previously reported and without the need of catalyst seeds using co-deposition method via aerosol-assisted chemical vapour deposition. These functionalized structures formed directly, in a single-step process, on silicon micromachined platforms are tested toward H 2 , showing 12-fold greater response, 6-fold faster response time and better selectivity to CO compared to a similar non-functionalized system. Results show the significance of these method to form highly gas sensitive nanostructures compatible with the complementary electronic for the fabrication of gas microsensor devices. © 2016 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Keywords: Nanostructures; tin oxide; functionalization; AACVD; gas sensors 1. Introduction Tin oxide is an intrinsic n-type wide-bandgap semiconducting metal oxide with applications in transparent conducting electrodes, solar cells and gas sensors.[1, 2] In particular, tin oxide is used in most current commercial resistive gas sensors and is the most studied material in the gas sensing literature, with demonstrated sensitivity to * Corresponding author. Tel.: +420 541 146 153; fax: +420 541 146 2198. E-mail address: [email protected] © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference 1079 S. Vallejos et al. / Procedia Engineering 168 ( 2016 ) 1078 – 1081 carbon monoxide, hydrogen, ethanol, and nitrogen dioxide, amongst others.[2-4] Whilst tin oxide nanorods (NRs) have been synthetized (often with catalyst seeds) using various routes, including chemical vapour deposition (CVD), the formation of these structures via aerosol-assisted (AA) CVD without catalyst seeds, i.e., via vapor-solid (VS) mechanism, and their in-situ functionalization with Au nanoparticles (NPs) have not been developed before. AA-CVD works at atmospheric pressure and relies on a solution-based delivery approach, providing advantages over traditional CVD as it allows for a wider range of precursors to be utilised. It also allows for the functionalization of SMOx nanostructures with metal NPs in a single processing step via co-deposition, as demonstrated previously for incorporation of gold or platinum NPs segregated at the surface of tungsten oxide nanostructures.[5] Here we report the AA-CVD of Au-functionalised and non-functionalized tin oxide NRs (Au@SnO2 and SnO2, respectively) directly on silicon micromachined platforms (ȝMP) for the fabrication of chemoresistive gas sensors. 2. Materials and Methods Au@SnO2 and SnO2NRs were deposited directly on ȝMP at 620 °C via AA-CVD of a mixture of tin (IV) chloride pentahydrate (30 mg, SnCl4.5H2O, Sigma-Aldrich, 98%) and tetrachloroauric acid trihydrate (4.2 mg, HAuCl4·3H2O, Sigma-Aldrich, 99.9%) dissolved in acetone (15 ml, Sigma-Aldrich, 99.6%) or only tin (IV) chloride pentahydrate (30 mg, SnCl4.5H2O, Sigma-Aldrich, 98%) dissolved in acetone (15 ml, Sigma-Aldrich, 99.6%), respectively, using the method and system reported previously.[5] The ȝMP consisted of an array of four SiO2/Si3N4/SiO2 membrane, each of them with isolated polysilicon heaters and platinum electrodes (gap: 50 ȝm, thick: 0.2 ȝm).[6] The sensing films were deposited on the ȝMP using a shadow mask in order to protect the contacts and subsequently bonded in a TO-8 package (inset in Fig. 1). The morphology of the samples was examined using SEM (Tescan FE Mira II LMU) and TEM (JEOL JEM-100CX II, 100 kV). The structure using XRD (Rigaku Smartlab 9kW) and the chemical composition using WDX (Philips, XL30ESEM). Gas sensors were tested in a continuous flow (200 sccm) test chamber (280 cm3) comprised of a mass flow system (Bronkhorst hi-tech 7.03.241) and calibrated cylinders of hydrogen (H2, Praxair, 1000 ppm), carbon monoxide (CO, Praxair, 1000 ppm) and synthetic air (Carburos Metálicos, 99.99%) as described previously.[6] The sensor response was defined as R= Ra/Rgas, where Ra is the sensor resistance in air and Rgas the sensor resistance after 10 min of the analyte exposure. The response time (tR) was defined as the time required for the sensor to reach 90% of the sensor response, and the recovery time (trec) as the time required to reach 10% of the initial baseline resistance after the analyte was purged. Fig. 1. XRD of a microsensor based on Au@SnO 2 NRs grown via AA-CVD. Diffraction peaks are indexed to a tetragonal phase (P42/mnm, ICCD card no. 41-1445) of tin oxide and face-centered cubic Au (Fm3m, ICCD card no. 4-0784). Platinum (Pt) and boron nitride (BN) diffraction peaks coming from the microsensor platform are also identified. 1080 S. Vallejos et al. / Procedia Engineering 168 ( 2016 ) 1078 – 1081 3. Results and discussion XRD of the films composed of Au@SnO2 and SnO2NRs indicated the presence of tetragonal SnO2 (P42mnm space group, a=4.7382 Å, c=3.1871 Å; ICCD card no. 411-1445) with a week diffraction at 44.3 degrees in the Au@SnO2 films corresponding to the (200) reflections of face-centered cubic Au (Fm3m space group, a=4.07860 Å; ICCD card no. 04-0784) (Fig. 1). XPS of the films indicated a (0.9 at.%) 3.7 wt.% Au in the films with the characteristics of Au 4f core level spectra being in agreement with that reported for gold metal,[5] which suggests the gold NPs incorporated at the surface of the tin oxide NRs are in the metallic state. SEM of the ȝMP showed films composed of non-aligned NRs, grown uniformly on the electrodes (Fig. 2a and 2b). Similarly, TEM confirmed the formation of prism-like NRs terminated in a pyramidal cap and the incorporation of Au NPs (׽35 nm) with spherical morphologies at the NR surface (Fig. 2c and 2d), proving consistency with our previous observations for the co-deposition of tungsten oxide and gold via AA-CVD.[5] The non-functionalized NRs grown on the ȝMP via AA-CVD showed similar morphological, chemical and crystal structure for SnO2. Fig. 2. Typical low (a) and high (b) magnifications SEM imaging of the SnO 2 and Au@SnO 2 NRs grown via AACVD on the ȝMP. TEM of a single SnO 2 (c) and Au@SnO 2 (d) NR. Test of the samples towards H2 and CO registered stable signal and very low drift of electrical resistance over the testing period, with the Au@SnO2sensors showing enhanced sensing characteristics compared to SnO2sensors, which included higher and faster response and lower-cross response (Table 1 and Fig. 3). A comparison of these results with those recorded for similar systems synthesised via sol-gel in the literature[7] revealed nearly 7 times higher values for our Au@SnO2NRs, suggesting the AA-CVD method presented here allows for not only a direct integration with ȝMP, but also for an effective functionalization of SnO2. Table 1. Sensor response and cross-response (ǻR) towards H 2 and CO for the SnO 2 and Au@SnO 2 sensors operating at 290 °C. Analytes & cross-response Response, R a /R gas SnO 2 Au@SnO 2 250 ppm H 2 3.3 35.4 500 ppm H 2 3.8 41.1 250 ppm CO 1.1 1.6 500 ppm CO 1.2 1.9 ǻR500 ppm= (R H2 -R CO ) 2.6 39.2 1081 S. Vallejos et al. / Procedia Engineering 168 ( 2016 ) 1078 – 1081 Fig. 3. Maximum sensor response recorded with the SnO 2 and Au@SnO 2 NRs towards 500 ppm of either H 2 or CO at 290 °C or 320 °C, respectively. 4. Conclusion Micromachined chemoresistive gas sensors based on SnO2 and Au@SnO2 NRs were developed using AA-CVD of SnCl4·5H2O at 620 °C, a much lower onset temperature compared to other CVD methods based on a VS mechanism, which typically requires temperatures exceeding 850 °C. The gas microsensors were validated towards H2 and CO and show sensing properties that are in agreement with the literature, with notable enhancement of sensing properties for Au@SnO2 NRs which showed 12-fold higher response with 6-fold faster response and improved selectivity to H2 compared to the gas sensors based on intrinsic SnO2 NRs. Acknowledgements S.V. is supported by the SoMoPro II Programme, cofinanced by the European Union and the South-Moravian Region, via Grant 4SGA8678. E.L. is supported by the Catalan Institution for Research and Advanced Studies via the ICREA Academia Award. This work was funded in part by MINECO under grant no TEC2013-48147 and TEC201571663-R, and was carried out using the infrastructures of the SIX Research Centre and the core facilities of CEITEC - Central European Institute of Technology under CEITEC - open access project, ID number LM2011020, by the Ministry of Education, Youth and Sports of the Czech Republic. References [1] M. Batzill, U. Diebold, The surface and materials science of tin oxide, Prog. Surf. Sci. 79 (2005) 47-154. [2] J. Pan, H. Shen, S. Mathur, One-Dimensional SnO 2 Nanostructures: Synthesis and Applications, J. Nanothecnology 2012 (2012) 12. [3] J. F. McAleer, P. T. Moseley, J. O. W. Norris, D. E. Williams, B. C. Tofield, Tin Dioxide Gas Sensors. Part 2.-The Role of Surface Additives, J. Chem. Soc., Faraday Trans. 1 84 (1988) 441-457. [4] S. Vallejos, F. Di Maggio, T. Shujah, C. Blackman, Chemical vapour deposition of gas sensitive metal oxides, Chemosensors 4 (2016) 4. [5] S. Vallejos, P. Umek, T. Stoycheva, F. Annanouch, E. Llobet, X. Correig, P. De Marco, C. Bittencourt, C. Blackman, Single-step deposition of Auand Pt-nanoparticle-functionalized tungsten oxide nanoneedles synthesized via aerosol-assisted CVD, and used for fabrication of selective gas microsensor arrays, Adv. Funct. Mater. 23 (2013) 1313-1322. [6] S. Vallejos, V. Khatko, J. Calderer, I. Gracia, C. Canè, E. Llobet, X. Correig, Micro-machined WO 3 -based sensors selective to oxidizing gases, Sens. Actuators B 132 (2008) 209-215. [7] X.-T. Yin, X.-M. Guo, Sensitivity and selectivity of (Au, Pt, Pd)-loaded and (In, Fe)-doped SnO 2 sensors for H 2 and CO detection, J Mater Sci: Mater Electron 25 (2014) 4960-4966.