Low temperature CO sensing under infield conditions with in doped Pd/SnO2
Graphical abstract
Introduction
Colorless, odorless but toxic, carbon monoxide (CO) is generated by the incomplete oxidation during combustion from furnaces, stoves, heaters, and automobiles. CO exposure causes mild to severe symptoms [1]. Environmental Protection Agency (EPA) lists two primary standards, for public health and welfare protection with the time-weighted average (TWA) of 9 ppm (ppm) measured over 8 h, and 35 ppm measured over 1 h respectively. World Health Organization (WHO) sets TWA exposure limits of 10 ppm for 8 h, 25 ppm for 1 h, 50 ppm for 30 min, and 90 ppm for 15 min [2,3].
For health reasons, CO detection sparked scientific interest since the early 60′s, when Seiyama and Taguchi [4,5] have reported the sensing properties of semiconducting metal oxides (MOS). Even now, the nature of surface chemisorbed gas species for the case of doped/decorated SnO2 is still a matter of debate. The challenge goes from fundamental to applicative issues such as: energetic position of surface oxygen levels, mechanism of water vapor influence over the gas sensing performance, selective sensitivity, detection limit, transients, drift, reliability, power consumption. For MOS sensors, CO detection is based on reducing the pre-adsorbed oxygen species on the surface, translated by variation of conductance due to the injection of electrons into the semiconducting material [6]. The gas-sensing phenomena can occur within seconds when the sensor materials exhibit high surface-to-volume ratio, ensuring fast diffusion of gas and when the sensing temperature is sufficiently high to induce a rapid reaction between the gas and the oxygen species. However, such rapidly responding, porous n-type sensor materials often show prolonged recovery times at the same temperature [7]. As such, low operating temperature CO detection is one of the nowadays issue, aiming to decrease the power consumption of the sensor [8]. In addition, it is known that with the decrease in the operating temperature, less cross-sensitivity issues are encountered [9]. This is related to the need of a certain activation energy in order to promote the subsequent gas-surface interactions. It has been demonstrated that by using different material design strategies together with appropriate surface catalyst approach, the overall gas sensing performances can be optimized up to the desire level [10,11].
One should take into account the balance between two factors when decide to call one of the most important catalysts, such as: Pt, Pd, or Au. The first is related to the oxygen splitting as main reaction partner for CO, while the second refers to the “burning” process of CO itself prior to the oxidation cycle, resulting in a low sensing response.
Another aspect of key importance in CO detection is played by the role of water vapors, which represents the main interfering agent within the in-field working conditions [12]. Moreover, low-temperature oxidation process of CO represents one of the most studied chemical reaction within the field of heterogeneous catalysis [13]. Accordingly, X. Xie et al. have reported not only the simple CO oxidation process at −77 °C but also the fact that Co3O4 was able to remain stable in the presence of relative humidity and gas flow [14]. Wang et al. [15] obtained best sensor’s performance at 100 °C and 100 ppm CO, by 3.0 wt% Pd-loaded SnO2. Ma et al. [16] studied the effect of the different humidity conditions on the sensing properties toward hydrogen and CO on Pd/SnO2. According to K.C Lee et al. [17] the decoration with oxidized Pd surface sites, enhances the sensing response of SnO2. Such behavior is explained through the formation of p-n junctions (PdO-SnO2) which induces a subsequent modification of the charge density distribution in the depletion region of SnO2. It was found that the loading of Pd enhanced the signal sensor as well as reduces the water vapor poisoning effect on electric resistance. Zhu et al. [18] investigated the CO sensing behavior at room temperature for Pd-SnO2 composite nanoceramics with 0.125–10 wt% Pd. The formation of the Pd4+, detected by XPS, only for the sensor containing 2 wt% Pd, explain the increased signal, possibly due the formation of the [Pd(CO)4]O4 as intermediate product. Although the sensor obtained works very well at room temperature, the very high temperature required to obtain nanocomposites is still a disadvantage. Che et al. [19] prepared by co-precipitation method 0–2.5 wt% Pd doped SnO2. The optimum Pd amount was found to 1.5 wt% and the best response of 6.59 was obtained for 400 ppm CO, at 260 °C. Hsu et al. [20] developed sensors based heterojunction structures such as La0.8Sr0.2Co0.5Ni0.5O3/TiO2 nanotube/Ti combining an electrochemical method and a sol-gel synthesis route. A moderate response (38.41 %) was achieved for 400 ppm CO at 200 °C. D. Naberezhnyi highlighted high sensitivity to CO of In2O3/Au-UV nanocomposite at room temperature [21]. He explained the effect of proposing by a new mechanism taking into account the participation of hydroxyl groups on the In2O3 surface. On the other hand, Pd loading induces the decrease in the operating temperature besides improved selectivity.
The present study presents the sensing properties of Pd/SnO2 acquired by fine tuning of In doping. Advantages such as low operating temperature, reduced moisture interferences and high selective sensitivity to CO detection was highlighted. The associated sensing mechanism is proposed, based on phenomenological investigations that provide insight about surface reactions.
Section snippets
Powder synthesis and sensors fabrication
x mol. % In doped 2 mol.% Pd/SnO2 (x = 1, 10) powders were prepared following a synthesis protocol based on hydrothermal treatment using a non-ionic surfactant - Brij 35 and Polyethylene glycol 6000 (PEG) as templates. Water and 1 Propanol were used as co-solvents. In the first step 1 g Brij 35 and 2 g PEG were very well dispersed in 40 ml deionized water and 25 ml 1-Propanol leading to the formation of micellar solutions. Tin(IV) chloride pentahydrate (SnCl4·5H2O), Indium(III) nitrate hydrate
Structural and morphological investigations
The crystalline structure of the samples was investigated by X-ray diffraction (XRD) using the Bragg-Brentano configuration of a Bruker D8 Advance powder diffractometer equipped with a Cu anti-cathode X-ray generator; Rietveld analysis method was employed for structure refinement. Structural, morphological and compositional information has been collected from micrometric to nanometric scale by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For the analytical SEM
Structural and morphological properties
The X-ray diffraction patterns of Pd1InSn and Pd10InSn samples (Fig. 1a, b) show a broadening of the diffraction peaks for the Pd10InSn samples compared to the Pd1InSn samples, which means that the average spherical diffraction crystallite size (dcr) for SnO2 decreases while increasing the doping/decoration level, namely, dcr for the Pd1InSn samples is 12 nm while for Pd10InSn samples decreases to 4 nm (see Table 1S). The effect of decreasing the average spherical diffraction crystallite size
Conclusions
In summary, x mol. % In doped 2 mol.% Pd/SnO2 (x = 1, 10) powders were prepared by one-step method based on hydrothermal synthesis route using a non-ionic surfactant - Brij 35 and Polyethylene glycol 6000 (PEG) as templates. The SEM investigations reveal that the powders consist in nanosized crystallites packed into micrometric grains with fine porosity. Final sensor structures consist by thick porous layers labeled as Pd1InSn and Pd10InSn, depending on In content. Both X-ray diffraction
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was funded by the CNCS-UEFISCDI through the project PN-III-P4-ID-PCE-2016-0529 and by the Romanian National Authority for Scientific Research through the Core Program PN19-03 (contract no. 21 N/08.02.2019).
Adelina Stanoiu received her PhD (2007) in Condensed Matter Physics from University of Bucharest-Faculty of Physics. Presently she is the leader of the Gas Sensors Group as senior researcher at the National Institute of Materials Physics, Bucharest, Romania. She has more than 20 years of experience in the field of chemical gas sensing. Her scientific activity is mainly focused on fundamental and experimental research in the field of gas sensors based on metal oxide semiconductors.
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Adelina Stanoiu received her PhD (2007) in Condensed Matter Physics from University of Bucharest-Faculty of Physics. Presently she is the leader of the Gas Sensors Group as senior researcher at the National Institute of Materials Physics, Bucharest, Romania. She has more than 20 years of experience in the field of chemical gas sensing. Her scientific activity is mainly focused on fundamental and experimental research in the field of gas sensors based on metal oxide semiconductors.
Corneliu Ghica has got his PhD degree in 2001 in Physics from the University Louis Pasteur Strasbourg I and the University of Bucharest within a co-tutorial PhD system. He specialized in microstructural characterization of materials using advanced techniques of analytical electron microscopy. At present he is senior researcher at the Institute of Materials Physics in Bucharest-Magurele, Romania.
Simona Somacescu received her PhD (2009) in Chemistry from “Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Bucharest. Presently she is senior researcher at the same institute and member of the X-Ray Photoelectron Spectroscopy (XPS) Group. Her activity is focused on the synthesis of mesoporous materials with nanocrystalline framework using preparative chemistry assisted by surfactants and surface chemistry investigations of the semiconductor metal oxide by XPS for SOFCs and sensor applications.
Andrei Cristian Kuncser received his PhD in Condnsed Matter Physics from Faculty of Physics, University of Bucharest (2018). He is a scientific researcher in the Laboratory of Atomic Structures and Defects in Advanced Materials, National Institute for Materials Physics, Bucharest, Romania. His work is focused on micro-structural analysis of advanced materials by Transmission Electron Microscopy.
Aurel Mihai Vlaicu received his Ph.D. (2000) in Science at Kyoto University, Japan, and continued his post-doctoral studies at National Institute for Material Science - Japan in the field of XRF, XPS, and XRD at beamline BL15XU at SPring-8 synchrotron facility. Since 2007 is involved with structural and chemical characterization of functional ceramics at National Institute of Materials Physics in Magurele, Romania, using XRD, XPS, SEM-EDS.
Ionel Florinel Mercioniu received his PhD (2011) in Electrical Engineering from University POLITEHNICA of Bucharest. In present is employed in National Institute of Materials Physics – group of the Electron Microscopy. His work is focused on Scanning Electron Microscopy and Focused Ion Beam from different materials.
Ovidiu Gabriel Florea studied Technological Physics at the University of Bucharest—Faculty of Physics and received his MSc in 2015. He is currently employed as engineer in the Gas Sensors Group at the National Institute of Materials Physics, Bucharest, Romania. His work is focused on experimental Physics in the field of chemical gas sensors.
Cristian Eugen Simion received his PhD (2011) in Condensed Matter Physics from University of Bucharest-Faculty of Physics. Presently he is scientific researcher in the Gas Sensors Group at the National Institute of Materials Physics, Bucharest, Romania. His field of interest is metal oxides solid state gas sensors.