ReviewHighly sensitive and selective gas sensors using p-type oxide semiconductors: Overview
Introduction
Various oxide semiconductor based gas sensors have been used to detect harmful and toxic gases [1], [2], [3], [4], [5], [6], [7], [8], [9]. The most representative sensor materials are SnO2 [10], [11] and ZnO [12], [13], which exhibit n-type oxide semiconductivity, and other n-type oxide semiconductors such as TiO2 [14], [15], WO3 [16], [17], In2O3 [18], [19], and Fe2O3 [20], [21] are being widely explored to find new functionalities of chemiresistivity. In contrast, the chemiresistors fabricated using p-type oxide semiconductors such as NiO, CuO, Co3O4, Cr2O3, and Mn3O4 to date have received relatively little attention, and the related research to fabricate such chemiresistors is still in the early stages of development. According to the results of a search of web of knowledge on July 15, 2013 (the keywords used for the search were the chemical formula of the sensor material and “gas sensor*”; e.g., “SnO2” and “gas sensor*” were used to search for SnO2 gas sensors), the number of articles found on gas sensors using p-type oxide semiconductors (i.e., NiO, CuO, Co3O4, Cr2O3, and Mn3O4) was only 9.41% of the 8504 articles available on the oxide-semiconductor-based gas sensors (the search results included the aforementioned p-type oxide semiconductor gas sensors and n-type oxide semiconductor gas sensors fabricated using SnO2, ZnO, TiO2, WO3, In2O3, and Fe2O3) (Fig. 1). Table 1 summarizes the properties of the gas sensors fabricated using the various p-type oxide semiconductors such as NiO, CuO, Co3O4, Cr2O3, Mn3O4, and LaOCl–NiO, as surveyed in the literature [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54]. The gas sensors were used to detect C2H5OH, HCHO, CO, NH3, (CH3)3N (trimethylamine), H2, H2S, C6H4(CH3)2 (xylene), and C6H5(CH3) (toluene) whose concentrations were in the range 1–1000 ppm.
The markedly different gas-sensing characteristics of n- and p-type oxide semiconductors should be understood in the context of the receptor functions, conduction paths, and gas-sensing mechanisms of these two types of materials with different majority charge carriers: (1) the formation of an electron-depletion layer (EDL) in n-type oxide semiconductors or a hole-accumulation layer (HAL) in p-type oxide semiconductors by the adsorption of oxygen with negative charge; (2) conduction through the serial paths (i.e., the semiconducting particle cores and resistive interparticle contacts) in n-type oxide semiconductors or conduction through the parallel paths (i.e., the resistive particle cores and semiconducting near-surface regions) in p-type oxide semiconductors; and (3) chemoresistive variation at the interparticle contacts in n-type oxide semiconductors or at the near-surface regions in p-type oxide semiconductors.
Compared to n-type oxide semiconductor gas sensors, p-type oxide semiconductor gas sensors exhibit not only shortcomings but also promising potentials for practical applications. Hübner et al. [55] suggested that the response of a p-type oxide semiconductor gas sensor to a given gas was equal to the square root of that of an n-type oxide semiconductor gas sensor to the same gas when the morphological configurations of both sensor materials were identical. This finding indicates that the responses of p-type oxide semiconductor sensors to gases should be enhanced in order to more accurately detect trace concentrations of various analyte gases. Nevertheless, the crucial importance of p-type oxide semiconductors as chemiresistive materials should not be underestimated considering that most p-type oxide semiconductors such as NiO, CuO, Cr2O3, Co3O4, and Mn3O4 have been extensively used as good catalysts [56], [57], [58], [59] to promote selective oxidation of various volatile organic compounds (VOCs). From this perspective, p-type oxide semiconductors are promising material platforms for developing the new functionalities of chemiresistors. Moreover, the p–n junction between oxide semiconductor materials can also be used to alter the gas-sensing characteristics of gas sensors by varying the electrical properties near heterointerfaces [60]. Further, the distinctive oxygen adsorption of p-type oxide semiconductors may be used to design high-performance gas sensors that show low humidity dependence and rapid recovery kinetics [61], [62].
Accordingly, p-type oxide semiconductors can provide a variety of new functionalities in oxide chemiresistors. However, to the best of our knowledge, there have been no comprehensive reviews previously published on gas sensors using p-type oxide semiconductors as sensing or additive materials. In this study, we review various high-performance gas sensors using p-type oxide semiconductors. As shown in Fig. 2, our review places special focus on (1) the gas-sensing mechanism of p-type oxide semiconductors, (2) new strategies to enhance the response of p-type oxide semiconductor sensors to gases through electronic and chemical sensitization, (3) highly selective gas sensors fabricated using p-type oxide semiconductors, (4) various gas sensors fabricated using oxide p–n junctions, and (5) designing reliable n-type oxide semiconductor gas sensors by loading p-type oxide additives.
Section snippets
Origin of n- and p-type semiconductivity in oxides
The major charge carriers in Si-based semiconductors can be manipulated by proper doping of donor or acceptors [63]. In contrast, the major charge carriers in wide-bandgap oxide semiconductors are determined either by doping aliovalent cations [64] or by oxygen nonstoichiometry [65]. For example, undoped oxygen-deficient SnO2 shows n-type semiconductivity since the formation of oxygen vacancies accompanies the generation of electrons [66]. The p-type semiconductivity of undoped NiO can be
Highly sensitive gas sensors using p-type oxide semiconductors
It is essential to enhance the gas response of p-type oxide semiconductors in order to use them in practical sensor applications. The gas response of p-type oxide semiconductor gas sensors can be enhanced by (1) tuning the morphology of the nanostructures in the oxide semiconductors, (2) doping with additives to electronically sensitize the oxide semiconductor, or (3) loading noble metals or metal oxide catalysts to chemically sensitize the oxide semiconductors.
Highly selective gas sensors using p-type oxide semiconductors
A range of reducing gases can similarly react with oxygen anions on the surface of oxide semiconductors, which often hinders distinguishing among various gases. The gas selectivities of various n- and p-type oxide semiconductor sensors have been reported during the last 5 years [22], [23], [24], [25], [30], [35], [36], [45], [48], [81], [82], [83], [84], [85], [125], [126], [127], [128], [129], [130], [131]. The ratio between the response to a target gas and the highest response to an
High-performance gas sensors using oxide p–n junctions
The junctions between p- and n-type semiconductors provide a range of valuable applications such as diodes, transistors, solar cells, photo sensors, and light-emitting diodes [60], [150], [151], [152]. However, most studies to date have concentrated on devices fabricated using doped-Si or compound semiconductors, and devices fabricated using oxide p–n junctions have been barely investigated. To date, there has been research on the design of high-performance gas sensors fabricated using (1)
Enhancement of recovery speed by loading NiO onto SnO2 hollow spheres
The rapid response and recovery of gas sensors are essential for quickly detecting and monitoring levels of toxic, explosive, and dangerous gases in real time [7], [11]. The kinetics of gas-sensing and recovery should be understood in the context of a gas-sensing mechanism. Using n-type oxide semiconductors to detect reducing gases consists of a series of steps including the in-diffusion of an analyte gas toward the oxide surface, the oxidation of the reducing gas by reacting with oxygen anions
Conclusions
The oxygen adsorptions, the formation of electrical core–shell structures, conduction mechanisms, catalytic activities, and the interactions with humidity of p-type oxide semiconductors are significantly different from those of n-type oxide semiconductors, which can be used to fabricate gas sensors that exhibit various novel functionalities. Conduction occurs along the hole-accumulation layer near the surface where oxygen anions are adsorbed in p-type oxide semiconductor gas sensors; thus, the
Acknowledgment
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MEST) (No. 2013R1A2A1A01006545).
Hyo-Joong Kim studied Materials Science and Engineering and received his B.S. and M.S. degrees in 2009 and 2011, at Korea University in Korea. He is currently doing his Ph.D. at Korea University. His research interest is the design of high-performance p-type oxide semiconductor gas sensors.
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Hyo-Joong Kim studied Materials Science and Engineering and received his B.S. and M.S. degrees in 2009 and 2011, at Korea University in Korea. He is currently doing his Ph.D. at Korea University. His research interest is the design of high-performance p-type oxide semiconductor gas sensors.
Jong-Heun Lee joined the Department of Materials Science and Engineering at Korea University as an associate professor in 2003 and is currently a professor there. He received his B.S., M.S., and Ph.D. degrees from Seoul National University in 1987, 1989, and 1993. Between 1993 and 1999, he developed automotive air-fuel-ratio sensors at the Samsung Advanced Institute of Technology. He was a fellow of the Science and Technology Agency (STA) of Japan at the National Institute for Research in Inorganic Materials (currently the National Institute for Materials Science (NIMS), Japan) from 1999 to 2000 and was a research professor at Seoul National University from 2000 to 2003. His current research interests include chemical sensors, functional nanostructures, and solid oxide electrolytes.