Carbon monoxide gas-sensing properties of CeO2–ZnO thin films
Introduction
Carbon monoxide (CO) sensors are essential to the control of emissions from combustion processes. More precise control of the air/fuel ratio in a combustion processes can yield significant gains in efficiency and results in substantial savings in fuel consumption. In addition, the toxic nature of CO necessitates the detection of this gas for household and environmental applications. Metal oxide semiconductors have been employed in the detection of CO. One of the advantages of these materials is that they enable high temperature operation, making them unique for hostile industrial applications. Moreover, many gas reactions are plausible only at such elevated temperatures. The basic property of metal oxides that is of interest in gas-sensing applications is the dependence of their electrical conductivity on the ambient gas. Most metal oxide semiconductors are naturally of n-type conductivity due to the presence of a large number of oxygen vacancies. When such a material is exposed to the atmosphere, oxygen molecules are adsorbed at the grain boundaries and pick up electrons from the conduction band and create a space charge layer between the grains [1]. This leads to the formation of Schottky barriers at the surfaces of the grains, and increases the resistivity of the material [2]. Exposure of the material to reducing gases (such as carbon monoxide) causes a reaction of these gases with the adsorbed oxygen, increasing the electronic conduction and reducing the resistance [1], [2]. The sensing properties are based on surface reactions and are greatly affected by the microstructure of the material [3]. Metal oxide semiconductor sensors have been used both as bulk and thin films. Thin films offer the added advantage of higher surface-to-volume ratio. In addition to the choice of the semiconducting oxide, other film parameters that are widely known to affect the sensing properties of a thin film are surface roughness, stoichiometry, and porosity. Furthermore depending on the preparation technique of the sensing layers, large differences of behavior concerning gas response and selectivity were observed.
Cerium oxide (CeO2) is a wide band gap rare earth metal oxide. It has found various applications in optoelectronics [4], solid oxide fuel cells, heterogeneous catalysis, and corrosion protection [5]. CeO2 thin films have columnar microstructure, with a packing density of about 0.6 [3], [6]. Thus, these films are highly porous and are most suited for sensing applications. Due to its chemical stability and high diffusion coefficient for oxygen vacancies, CeO2 has been well established as an oxygen sensor [7]. Moreover, CeO2 has been employed in the detection of other gases such as NO and acetone [8], H2S [9], and was used as an additive in other semiconductor-based gas sensors [1], [2], [7]. Cerium oxide has been shown to enhance the dissociation of CO on ceramic surfaces [7]. Therefore, CeO2 powders have been used in the detection of CO.
Zinc oxide (ZnO) is another semiconductor characterized by a direct wide band gap, n-type conductivity, high transparency, strong exciton emission, and strong piezoelectric coefficient. Therefore, there has been extensive research on the growth and characterization, and applications of ZnO thin films in the fields of optics, optoelectronics, and sensors [10], [11]. ZnO thin films have been widely used as gas sensors, owing to their high chemical sensitivity and stability, suitability for doping, non-toxicity, and low cost [12]. In particular, several reports have appeared on the use of ZnO thin films as CO gas sensors [13], [14], [15], [16].
Mixed metal oxides have begun to be explored as viable alternatives to pure metal oxides [17]. This is motivated by several reasons: (i) increasing the sensitivity to gaseous species, (ii) tailoring the sensitivity of the binary composite towards specific gas species (selectivity), (iii) stabilization of the crystalline grains forming the films, and (iv) reducing the operating temperature of the sensor, thus shortening the recovery time [17], [18]. We have recently reported the CO gas-sensing properties of a bare CeO2 sensor, which showed high sensitivity for CO detection [19]. For a bare ZnO sensor, we found that the as-deposited films were dark and conductive, indicating the presence of excess zinc. These films were unstable, in that their resistivity changed significantly over time. When the films were annealed in air, they became transparent but their resistivity increased by several orders of magnitude. Such films were very resistive and were not considered for sensor applications.
In the present work, mixed metal oxide thin film gas sensors were deposited by co-evaporation of cerium oxide and zinc oxide, and were tested for their application as carbon monoxide gas sensors.
Section snippets
Experimental
Mixed-oxide thin films were prepared by co-evaporation in a Leybol L560 box coater pumped by a turbomolecular pump. The system was initially pumped to a base pressure of 1 × 10−4 Pa. The starting materials were CeO2 pellets (99.99% purity from Balzers) and ZnO powder (99.9% purity from Alfa Aezar). Before deposition, the materials were slowly outgassed, with a shutter blocking the vapor from the substrates. The two materials were simultaneously evaporated. CeO2 was evaporated using a 4 kW electron
Morphology of the films (AFM)
Fig. 2 shows typical AFM images of mixed CeO2–ZnO thin films before and after annealing in air at 500 °C. The morphology of the as-deposited films shows a mixture of granular and columnar microstructures. The average root-mean-square roughness (Rrms) of these films was 2.58 nm, and the average grain size was 31 nm. The morphology of the annealed film shows a uniform columnar microstructure. The Rrms of these films was 1.53 nm, and the average grain size was 45 nm. Evidently, the grain size grew
Conclusions
Mixed metal oxide thin films were prepared by co-evaporation of cerium oxide and zinc oxide, and were subsequently annealed in air at 500 °C for 2 h. The thickness of the films was 160 nm. The films were amorphous with a uniform columnar microstructure. The films were transparent with an optical band gap of 3.33 eV. XPS measurements revealed that the films consisted of CeO2 and ZnO, with almost equal atomic concentrations of cerium (16%) and zinc (18%). The XPS O 1s peak consisted of components
Acknowledgments
The support provided by the Deanship of Scientific Research and the Physics Department of King Fahd University of Petroleum and Minerals is acknowledged. This work is part of project # PH/CERIUM/355.
References (39)
- et al.
Sens. Actuator B
(2005) - et al.
Sens. Actuator B
(2001) - et al.
Solid State Ionics
(2003) - et al.
J. Alloy Compd.
(2006) - et al.
Sens. Actuator B
(2003) - et al.
Sens. Actuator B
(2000) - et al.
Sens. Actuator B
(2000) - et al.
Mater. Today
(2007) - et al.
Thin Solid Films
(2006) - et al.
Sens. Actuator B
(2003)
Sens. Actuator A
Thin Solid Films
Sens. Actuator B
Sens. Actuator B
Sens. Actuator B
Sens. Actuator B
Sens. Actuator B
Nucl. Instrum. Methods Phys. Res. Sect. B: Beam Interact. Mater. Atoms
J. Solid State Chem.
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2022, Ceramics InternationalCitation Excerpt :However, it is important to recognize that the ideal CeO2 is not found; in reality, the material has a high oxygen storage capacity since it can easily shift between Ce(IV) and Ce(III), forming bulk and surface oxygen vacancies with consequent high reducibility [10], resulting in the formula CeO2-x [11]. The gas sensor properties of CeO2 towards CO have been widely explored [9,12–15]. Izu et al. [13] demonstrated that CeO2 has a faster sensor response to CO gas than SnO2.