Anomalous conductivity-type transition sensing behaviors of n-type porous α-Fe2O3 nanostructures toward H2S

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Abstract

Porous urchin-like α-Fe2O3 nanostructures with n-type semiconducting properties were used as gas sensing materials. Interestingly, it was observed abnormal n–p transition sensing behavior induced by the variation of working temperature and p–n transition sensing behavior related to the increase of H2S concentration. Large density of unstable surface states resulting from high surface-to-volume ratio would be beneficial for the formation of a surface inversion layer and account for the n–p transition. Furthermore, the as-prepared sensor showed good H2S sensing performances with short response/recovery time within 5/10 s, and relatively low detection limit of 1 ppm. These results help us to understand the sensing mechanism of α-Fe2O3 and hint the potential application of the as-prepared sensor in monitoring H2S.

Introduction

In recent years, considerable attention has been focused on the detection and monitoring of flammable and poisonous gases, which are important to environment, public safety and human health [1], [2], [3], [4], [5], [6], [7]. Gas sensors based on metal oxide semiconductors are considered to be the most promising technique, owing to the advantages of low cost, simple operation and good reversibility. Hence, many semiconductor oxides such as SnO2 [8], [9], ZnO [2], [5], [10], [11], Fe2O3 [6], [12], [13], In2O3 [14], Co3O4 [4], [15], and CuO [16] have been widely explored to detect the polluting, toxic and flammable gases. Generally, semiconducting oxides are classified according to the direction of the resistance change owing to their exposure to reducing gases (e.g., CO, ethanol) [17], [18], [19], [20], such as n-type (resistance decreases, e.g., ZnO, SnO2, Fe2O3) or p-type (resistance increases, e.g., Co3O4, CuO), which are determined by the nature of the majority carriers at the surface, i.e., electrons or holes. In general, n-type semiconducting oxides are easily formed due to oxygen deficiencies (vacancies), and exhibit typically n-type sensing behavior in most cases. By contrast, anomalous p-type sensing behavior (n–p transition) has also been observed, which could be induced either by special ambient conditions (under certain kinds of reductive ambient) [19], additives (Pt nanoparticles decorated) [21], or different surface reactions under different conditions (humidity, temperature, composition, etc.) [5], [17], [18], [22], [23]. This transition phenomenon is fascinating, because it is beneficial to understand the sensing mechanism of metal-oxide semiconductor. Nevertheless, in spite of extensive research efforts, the details of the n–p transition are still not fully understood and subject to ongoing investigation.

Hematite (α-Fe2O3), the most stable iron oxide with n-type semiconducting properties (Eg = 2.1 eV) under ambient condition, is extensively used as gas sensors, catalysts, pigments, and electrode materials [12], [24], [25], [26], [27], [28]. Recently, the n–p transition has been demonstrated in In2O3–Fe2O3 mixture and α-Fe2O3 bulk samples [17], [18], [19], for which the main principle was based on the surface adsorption of oxygen that led to the formation of a surface inversion layer. It is well known that the surface of nanostructures with high surface-to-volume ratio is very unstable, and it easily adsorbs foreign molecules for stabilization [9], [19]. We propose that the n–p transition may be much more easily achieved by using nonmaterial compared with bulk counterpart. Nevertheless, to the best of our knowledge, there is rarely report on the study of n–p transition sensing behavior of α-Fe2O3 at nanometer scale. Furthermore, it is of great significance to detect and monitor sulfureted hydrogen (H2S), which is extremely dangerous for human health even at low concentration, just exposure to 250 ppm can seriously injure the human body and even cause death [3].

In this paper, porous urchin-like α-Fe2O3 nanostructures were prepared by a simple solution route, and sequentially calcined in air. The obtained α-Fe2O3 nanostructures were intrinsic n-type semiconductor. Sensor based on these nanostructures showed low detection limit and fast response to H2S. Furthermore, it exhibited interestingly p-type sensing behavior (n–p transition) toward low concentration H2S at high working temperatures and then transferred from p-type to n-type induced by the increase of H2S concentration. The possible reasons for these transitions were also discussed.

Section snippets

Experimental

Porous α-Fe2O3 nanostructures were prepared through a two-step process including the solution-based synthesis of α-FeOOH precursors and sequential calcinations that described in our previous reports [29]. In a typical procedure, 1.39 g of FeSO4·7H2O and 1.36 g of CH3COONa·4H2O were dissolved in 50 mL of deionized water at room temperature. After stirred vigorously for a period at 60 °C, the yellow slurry was centrifuged and washed several times with distilled water and absolute alcohol, and finally

Structure characterizations

Fig. 2a shows the XRD pattern of the final products, all the diffraction peaks can be indexed as the hexagonal phase of α-Fe2O3 (JCPDS 89-596). No characteristic peaks of impurities are observed. The morphology and microstructure of the samples were investigated by SEM and TEM. Typical SEM image of α-Fe2O3 is displayed in Fig. 2b; it can be observed that the samples are uniform urchin-shaped nanostructures with a diameter of about 1 μm. The TEM images (Fig. 2c and inset) clearly demonstrate that

Conclusions

In summary, gas sensing performances of intrinsic n-type porous α-Fe2O3 nanostructure-based sensors were investigated. Sensor based on these nanostructures showed interesting n–p transition sensing behavior induced by increasing the working temperature and p–n transition related to the H2S concentration. The abnormal n–p transition behavior could be ascribed to the strong surface absorption (O or O2−), which resulted in the formation of a surface inversion layer, and thus the inversion of the

Acknowledgments

This work was partly supported by “973” National Key Basic Research Program of China (grant no. 2007CB310500), Hunan Provincial Natural Science Foundation of China (grant no. 10JJ1011), and National Natural Science Foundation of China (grant no. 21003041).

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