Complex techniques for the analysis of sensing properties for sensors based on semiconducting oxide materials

Project Director: Dr. Cristian Eugen SIMION

The operation of chemorezistive sensors based on semiconductor oxide materials (MOX) is based on the surface chemical interaction that leads to the change in the concentration of load carriers, translated by the change of an electrical parameter, generally the electrical resistance (conductivity) [1]. The unitary approach of the quasi-chemical formalism (surface chemical processes) and the electronic formalizm (electronic exchange between solid and surface), allows the elaboration of the chemical-physical mechanism underlying the functioning of sensors. From an experimental point of view, this objective involves complex phenomenological investigations, i.e. simultaneous measurements of electrical conductivity, extraction work and catalytic conversion. If the first two allow the evaluation of the translator function of the MOX, the third technique allows the assessment of the receiver function [2]. Both the correlation of measurement techniques and the interpretation of experimental results, constitutes a complex activity without local expertise, despite the existence of the experimental base (Fig. 1). An internship at the Institut für Physikalische und Theoretische Chemie – Eberhard Karls Universität Tübingen allows both the learning of this complex experimental technique and the fundamental understanding of how to correlate the results obtained. The know-how obtained will allow me in the future to identify the elements necessary for the functionalization of this class of materials for the development of punctual applications starting from the specific requirements of environmental monitoring.
The host institution (National Institute of Research Development for Physics of Materials) possesses the techniques mentioned above ( but so far the emphasis has been placed on the development of the translator function of various gas-sensitive materials [3].

Fig. 1 The dynamic gas mixing system and how to interconnect the techniques of catalytic conversion, extraction and electrical conductivity.
Specifically, through the use of the catalytic conversion technique I will gain a deep understanding of the chemical component involved (reaction speed, order of reaction, influence of the operating temperature of the material) which will be modeled using the formalism of the quasi-chemical equations using an adapted notation of the type Kroeger-Vink [4]. Evaluation of extraction work and electrical conductivity will allow to obtain information about the electrical nature of reaction species, and modeling using Poisson equations and electro-neutrality [5]. The advantage of complex phenomenological investigations performed using the dynamic gas mixing system consists in the accurate reproduction and long-term maintenance of a calibrated concentration of test gas (CO, CH4, NO2, H2S, NH3, SO2) in the ppb-ppm field. From an application point of view, the proposed phenomenological approach requires the introduction of a permanent key element present in field operating conditions [6], namely the existence of atmospheric oxygen and relative humidity (RH %).
The expected results based on the techniques involved can be summarized as follows:
Catalytic conversion (ΔX) – the variation of the amount of water vapour (in %@ constant temperature or ppm) before and after exposure of the sensor.
Electrical conductivity (ΔG) – the variation in the concentration of load-free carriers before (G_i) and after (G _f) exposure to oxygen and moisture. The sensor signal can be evaluated using the ratio: S=G_f/G_i ≥1 where: G_f and G_i represent conductivity after and before RH exposure.
Extraction work (ΔΦ) – simultaneously with ΔG monitoring, allows to explain the contribution made by the variation of electronic affinity (Δχ) using the relationship: ∆Φ=k_B Tln(G_f/G_i )+Δχ. The importance of quantitative evaluation (d.p.d.v. energy) of electron affinities is that it brings information about dipolar species in the form of hydroxyl groups related to the metal atom (M^(δ+)- |OH |(δ-) ) which does not involve load transfer (ΔG=const.) but prevents further interactions with the test gases [7].
Thus, through the unitary approach of quasi-chemical formalism together with electronic formalism, we can elaborate the chemical-physical, realistic mechanism involved in the sensing process resulting from the involvement of oxygen and water vapours:
H_2 O^gas+M_Net+O_Net^-(M_net^(δ+)- |OH |(δ-) )+Net H+e^- where the link between electrical conductivity (G) and the partial concentration of water vapour (p_H2O) can be written: G=G_0 |(1+k_ H2O p_H2/O) |^β where: G_0=const., k_H2-O-reaction constant, power-type β-factor Similarly, in the case of a polycrystalline MOX material, the previous approach will be extended depending on the intrinsic properties of the material, type of test gas and operating temperature (Fig. 2).
Based on previously acquired knowledge, involved

Based on previously acquired knowledge involved in the interaction with atmospheric oxygen and water vapour, we can realistically a complete picture of the receptor/translator functions of the test material.


Fig. 2 Polycrystalline material in interaction with O2, RH and various test gases in order to develop the physico-chemical mechanism of interaction.

The impact of the training stage on my research activity will be reflected both in the quality of subsequent publications and in the development of methods for optimizing the most important gas-sensitive parameters: sensitivity, selectivity, stability, response time and return time.

Bibliographic references: [1] N. Barsan, M. Huebner, U. Weimar, Conduction mechanism in semiconducting metal oxide sensing films: impact on transduction, Semiconductor Gas Sensors, Woodhead Publishing Limited (2013) 35-63.
[2] N. Yamazoe, K. Suematsu, K. Shimanoe, Gas reception and signal transduction of neat tin oxide semiconductor sensor for response to oxygen, Thin Solid Films 548 (2013) 695-702.
[3] A. Stanoiu, S. Somacescu, J.M. Calderon-Moreno, V.S. Teodorescu, O.G. Florea, A. Sackmann, C.E. Simion, Low level NO2 detection under humid background and associated sensing mechanism for mesoporous SnO2, Actuators B 231 (2016) 166-174.
[4] S. Wicker, K. Grossmann, N. Barsan, U. Weimar, Co3O4 – A systematic investigation of catalytic and gas sensing performance under variation of temperature, humidity, test gas and test gas concentration, Actuators B 185 (2013) 644-650.
[5] A. Oprea, N. Barsan, U. Weimar, Work function changes in gas sensitive materials: Fundamentals and applications, Actuators B 142 (2009) 470-493.
[6] A. Stanoiu, C.E. Simion, S. Somacescu, NO2 sensing mechanism of ZnO-Eu2O3 binary oxide under humid air conditions, Sens. Actuators B 186 (2013) 687-694.
[7] N. Barsan, U. Weimar, Conduction Model of Metal Oxide Gas Sensors, Journal of Electroceramics 7 (2001) 143-167.


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