Elsevier

Sensors and Actuators B: Chemical

Volume 217, 1 October 2015, Pages 41-50
Sensors and Actuators B: Chemical

Copper oxide based H2S dosimeters – Modeling of percolation and diffusion processes

https://doi.org/10.1016/j.snb.2015.02.001Get rights and content

Abstract

Semiconducting copper oxide (CuO) gas sensing layers show a remarkable conductance behavior if exposed to hydrogen sulfide (H2S) gas at low operating temperature (180 °C). At first conductance decreases as expected for a p-type semiconducting metal oxide offering reducing test gas. After a certain exposure time, however, a sudden steep increase in conductance can be observed.

In a first approach this behavior is explained by the formation of metallic conducting copper sulfide (CuS, degenerate p-type semiconductor) clusters which eventually form conducting pathways across the sensing layers short-circuiting the remaining CuO phase. In the field of statistical physics such behavior can be described by the so-called percolation theory.

Here we present a detailed experimental and theoretical analysis of the observed effect utilizing RF-magnetron-sputtered copper oxide films with different stoichiometry (CuO, Cu4O3 and Cu2O) as model systems. The layers are exposed to H2S for different time spans and analyzed with respect to their morphology (SEM, XRD) and chemical composition (XPS, ToF-SIMS). Analysis of the transient behavior of the conductance by means of a percolation model and comparison of the results to the experimental data allow the identification of different processes. For CuO samples first the formation of different non-CuS copper–sulfur–oxygen phases is observed followed by the percolation regime with the steep conductance increase. Afterwards diffusion processes superimposing the percolation leading to a slower conductance increase and eventually the process is dominated by diffusion of copper ions from the bulk. For oxides with other stoichiometry (Cu2O, Cu4O3) no percolation regime is observed which is attributed to higher diffusion rate of copper ions weakening the percolation effect in these samples.

Based on these observations a model for the electronic conductance behavior of copper oxide gas sensors under exposure to hydrogen sulfide (H2S) at temperatures below 200 °C is proposed. A better understanding of these systems will enable the preparation of reliable sensors with inherent thresholds.

Introduction

Hydrogen sulfide (H2S) is a toxic gas, which occurs e.g. in crude petroleum, natural gas or food [1]. In addition to its toxicity for humans and its impact on the environment H2S leads to corrosion of e.g. gas storage tanks or engines and poisoning of catalysts. Therefore it must be monitored to avoid damages on technical facilities [2], [3], [4]. Among other sensor concepts semiconductor gas sensors are used for the detection of H2S due to their small size, low weight and cost-effectiveness [1].

As for many other sensing applications a main objective is the increase in sensitivity of the sensors. In the early nineties of the last century Tamaki et al. increased the sensitivity for H2S of polycrystalline semiconducting n-type tin oxide (SnO2) sensing layers over some orders of magnitude by doping with semiconducting p-type copper(II) oxide (CuO) [5]. This effect was explained by the authors by the annihilation of p–n junctions at the SnO2–CuO interface. CuO reacts with H2S to the degenerated semiconductor copper(II) sulfide (CuS) exhibiting metallic conductance behavior. Since then several investigations regarding the sensitivity increase for H2S by adding CuO to SnO2 have been published [6], [7].

In recent years also pure CuO structures are used as transducer for the detection of carbon monoxide, nitrogen dioxide, hydrogen or hydrogen peroxide [8], [9], [10], [11]. Especially for H2S sensing copper oxide layers are of broad interest due to their high selectivity [12], [13].

Different groups report on unexpected conductance behavior for p-type conducting CuO under H2S exposure [14], [15], [16]. Below an operating temperature of 200 °C the conductance first decreases as expected for a reducing gas on a p-type semiconductor. This behavior is attributed to a surface reaction with H2S. The amount of adsorbed oxygen decreases and trapped electrons recombine with the holes of the CuO. At higher H2S concentrations or at prolonged exposure times the conductance starts to increase steeply. This is interpreted by the formation of CuS conduction paths. In a former work some of us observed this effect for electrospun CuO nanofibers under H2S exposure at 160 °C [17]. The reciprocal time (1/t) from the beginning of the gas exposure until the steep conductance increase is linearly depending on the offered H2S concentration in this system making it a dose dependent sensor with a so-called switching behavior [17], [18]. This unique property could be explained in the framework of percolation theory and offers interesting possibilities for the design of gas sensors and sensor systems.

The percolation effect allows e.g. the design of sensors with built-in concentration thresholds. A similar effect was previously observed for gallium oxide (Ga2O3) under exposure to hydrogen (H2). At concentrations higher than 26 ppm the sensitivity of the sensor increases clearly [19]. Another material for the detection of H2 utilizing percolation effects was presented by Dankert and Pundt [20]. They showed that the formation of conducting paths in a layer of Palladium (Pd) clusters caused by the expansion of the clusters under H2 exposure is also concentration dependent. Volume expansion was also identified as being able to show responsible for the sensing effect discovered by monitoring fixed bed catalysts under exposure to H2S by Fremerey et al. [21]. Dispersed nickel particles react to nickel sulfide which possesses a larger molar volume. On reaching a certain H2S dose the expanding particles connect and build up a conducting paths.

The number of investigations in the field of gas dosimeters is still growing. The possibility to detect low gas concentrations by accumulation in the sensor or the direct detection of a gas dose are only two of their advantages [22].

The presented work focuses on the underlying mechanism of copper oxide gas dosimeters for the detection of H2S. As model systems films with three different copper oxide stoichiometries (Cu2O, Cu4O3 and CuO) were prepared and exposed to 5 ppm H2S at 180 °C. Besides characterization of the layers by X-ray diffraction (XRD), electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) detailed analysis of the resulting conductance data was performed. Based on theoretical simulations of the gas-exposed copper oxide surface a model describing the conductance behavior in terms of percolation effects and diffusion processes is proposed.

Section snippets

Copper oxide based H2S dosimeters

In previous articles semiconducting p-type CuO nanofibers were investigated as a material for the detection of H2S and a dosimeter-type behavior was observed [17], [23]. A steep increase in the sensor signals over a few orders of magnitude, also referred to as switching behavior, was observed after reaching a certain gas dose (gas concentration * time). The effect was independent of the loading type (continuous or pulsed H2S exposure) [17]. The switching behavior of the sensor was interpreted by

Sample preparation and characterization

The copper oxide films were deposited on float glass substrates by a RF-magnetron sputtering setup SLS/Twin 400/1000 from Pfeiffer Vacuum. A copper target (99.99% purity) and a mixture of argon and oxygen as reactive gas (both with a purity of 99.999%) were used. By varying the oxygen flux films with different stoichiometries were produced. The base pressure of the chamber was 3.7 × 10−7 mbar, the pressure during deposition was around 5.0 × 10−3 mbar. All layers were deposited with a RF-power of 75 

Sample characterization

Fig. 3 shows the diffractograms of the different phases. The lines within the graphs denote the literature positions of the reflexes. The data were taken from the ICDD data base PDFs: 00-005-0667 (Cu2O), 01-083-1665 (Cu4O3) and 00-048-1548 (CuO). As can be seen the reflexes of Cu2O are shifted to smaller diffraction angles which are also commonly observed in our sputtered layers without thermal treatment during the deposition. These shifts can probably due to small deviations in the

Discussion

The XRD measurements show no stoichiometry changes of the samples after thermal treatment (Fig. 3). Cu2O shows small grains, while the surface becomes smoother for the CuO stoichiometry (Fig. 4.). The sample topologies are homogeneous over the whole SEM image. Among the preferred orientation of the samples other orientations were also observed, but only at lower intensities. However, due to these results a comparison of the sample surface with the ideal lattices in the computer simulations is

Conclusion

The electric conductance behavior of copper oxide gas sensors under exposure of H2S at 180 °C operating temperature was investigated. Copper oxide samples with different stoichiometries (CuO, Cu4O3 and Cu2O) were exposed to 5 ppm H2S, undergoing a chemical reaction of semiconducting p-type copper oxide to metallic conducting CuS (degenerate p-type semiconductor). This was analyzed by conductance measurements, data analysis, SEM, XRD, XPS, ToF-SIMS and compared to theoretical simulations. Based on

Acknowledgements

We would like to thank the Laboratory of Materials Research (LaMa) at the Justus-Liebig-University Giessen for the support of this project, Benedikt Kramm for the help with the data analysis of the XPS measurements, Philipp Hering for the fruitful conversations and the DFG for the financial support of our research (KO 719/13-1 and WA 2977/3-1).

Jörg Hennemann received his diploma in physics at the Justus-Liebig-University Giessen in 2010. Since this time he has been working on his doctoral thesis on percolation effects in semiconducting gas sensitive materials. Thereby his main interest is in copper oxide gas dosimeters and their interactions with hydrogen sulfide. Besides he is working on a fine dust detection system for small-scale furnaces.

References (38)

  • M. Ulrich et al.

    Percolation model of a nanocrystalline gas sensitive layer

    Thin Solid Films

    (2001)
  • S.W. Goh et al.

    Copper(II) sulfide?

    Miner. Eng.

    (2006)
  • I. Grozdanov et al.

    Optical and electrical properties of copper sulfide films of variable composition

    J. Solid State Chem.

    (1995)
  • Y.B. He et al.

    Hall effect and surface characterization of Cu2S and CuS films deposited by RF reactive sputtering

    Physica B

    (2001)
  • A. Wellinger et al.

    Biogas upgrading and utilisation

    IEA Bioenergy Task

    (2000)
  • IFA

    GESTIS

    (2014)
  • A. Chowdhuri et al.

    H2S gas sensing mechanism of SnO2 films with ultrathin CuO dotted islands

    J. Appl. Phys.

    (2002)
  • J. Hennemann et al.

    Copper oxide nanofibers for detection of hydrogen peroxide vapor at high concentrations

    Phys. Status Solidi A

    (2013)
  • F. Zhang et al.

    CuO nanosheets for sensitive and selective determination of H2S with high recovery ability

    J. Phys. Chem. C

    (2010)
  • Cited by (0)

    Jörg Hennemann received his diploma in physics at the Justus-Liebig-University Giessen in 2010. Since this time he has been working on his doctoral thesis on percolation effects in semiconducting gas sensitive materials. Thereby his main interest is in copper oxide gas dosimeters and their interactions with hydrogen sulfide. Besides he is working on a fine dust detection system for small-scale furnaces.

    Claus-Dieter Kohl studied physics at the RWTH Aachen, Germany from 1964 to 1968. Since 1991 he has been working as a professor for solid state physics in the Institute of Applied Physics at the University Giessen, Germany. He is an editorial board member of Sensors and Actuators (since 1994) and of the board of EUSAS (automatic fire detection). He is also active as an advisor of GTE Company (since 1992) and of Paragon Company (since 1997). His research interests are gas sensors, semiconductor surface reactions and heat conduction.

    Bernd M. Smarsly received a MSc degree in chemistry, physics and mathematics in 1998 and 1999 at the University of Marburg (Germany). In 2001 he received his Ph.D. at the University of Potsdam based on work performed at Max-Planck Institute of Colloids and Interfaces, where he was working on the quantification of nanoscaled porosity by small-angle scattering techniques. After a postdoctoral stay at the Univ. of Albuquerque (New Mexico, USA) he worked as project leader at Max-Planck Institute of Colloids and Interfaces. Since 2007 he has been working as a full professor for Physical Chemistry, focusing on the synthesis and characterization of nanoscaled oxides and carbons.

    Hauke Metelmann received his diploma in physics at the Justus-Liebig-University Giessen in 2011. His current Ph.D. project in the physical chemistry department concerns the physical analysis of the electrode electrolyte reactions in lithium sulfur batteries by XPS and ToF-SIMS.

    Marcus Rohnke received his diploma in Chemistry at the Leibniz University of Hanover, Germany in 1999 and his doctoral degree from JLU Giessen, Germany in 2003. In 2005 he took up a permanent position as researcher at the Institute for Physical Chemistry at JLU Giessen. Currently he is the main operator of the ToF-SIMS machine and head of the “biomaterials and plasmas” group at the chair for Physical Chemistry. His research interests include plasma electrochemistry, bioanalytics and solid oxide fuel cells.

    Jürgen Janek received the Diploma degree in chemistry in 1989 and the PhD degree from the University of Hannover, Germany, where he worked on transport phenomena in high temperature materials. After finishing his habilitation in Physical Chemistry he took over the chair for Physical Chemistry at Justus Liebig University, Germany, and became director of the Institute of Physical Chemistry. He was visiting professor at Seoul National University, Tohoku University in Sendai and Université d́Aix-Marseille in France and was invited by WCU program of Seoul National University. His main research interests are solid state ionics, electrode and interface kinetics, materials for electrochemical energy technologies.

    Daniel Reppin received his master degree in material sciences at the Justus-Liebig-University Giessen in 2011. He started his work in the field of semiconducting copper oxide thin films and their optical and electrical properties in 2008. In 2012 he started his doctoral thesis in the field of oxygen ion conductors.

    Bruno. K. Meyer received his Ph.D. in 1983 at the University of Paderborn. He continued his work with optically detected EPR and ENDOR intrinsic defects in III–V semiconductors, especially in GaAs and GaP, which was summarized in the habilitation in 1987. In 1990, he went as a Professor to the Technical University Munich. Since 1996 he has been a Professor at the Justus-Liebig-University Gießen and Managing Director of the 1st Physics Institute. His current research interests are wide-bandgap oxides with applications in electro- and thermochromics (VO2, TiV-oxides) and electronics (ZnO, ZnOS).

    Stefanie Russ is a theoretical physicist who received her Ph.D. in 1992 at the University of Hamburg for her work on vibrations in disordered systems. After several years of research on percolation, fractal drums and the Anderson model at the Ecole Polytechnique in France and the Bar-Ilan University in Israel, she went to the University of Gießen, where she extended her research to further subjects of disordered solid state physics, among them percolation effects in semiconducting gas sensors, and habilitated in 2003. In 2006 she went to the Freie Universität of Berlin, where she has been working since then as lecturer and researcher.

    Thorsten Wagner is leader of a Junior Research group at the University of Paderborn. He received his Diploma degree in “Physics” (2005) and his PhD degree (2010) from the University of Giessen, Germany and carried out post-doctoral research at the University of Paderborn. His research is dedicated to the application of nanostructured metal oxides and the synthesis and characterization of photonic crystals in the field of gas sensing.

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