Core–shell Co₃O₄/α-Fe₂O₃ heterostructure nanofibers with enhanced gas sensing properties

Abstract

1D Co₃O₄/α-Fe₂O₃ core–shell nanofibers were successfully fabricated by a facile and template-free coaxial electrospinning method, and developed for acetone gas detection. Morphology and component characterizations confirm that the as-prepared 1D heterostructures possess a well-defined core–shell structure with Co₃O₄ in the core and α-Fe₂O₃ in the shell. The unique 1D core–shell nanostructures, heterojunction effect at the Co₃O₄/α-Fe₂O₃ interfaces and the catalysis of Co₃O₄ endow the Co₃O₄/α-Fe₂O₃ core–shell nanofiber-based sensor with an enhanced gas sensing performance in terms of good sensing selectivity, high sensitivity (11.7), rapid response/recovery times (2 s/20 s) and better reproducibility for acetone gas. The gas sensing mechanism is proposed in detail. Overall, the 1D Co₃O₄/α-Fe₂O₃ core–shell heterostructure nanofibers synthesized through coaxial electrospinning make a promising and effective acetone sensor.

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1. Introduction

With increasing concerns about the effect of air pollution and noxious gas on human health and safety, highly effective oxide semiconductor gas sensors have attracted more and more attention due to their significant resistance change upon exposure to trace concentrations of reducing or oxidizing gases.^1,2 In order to improve gas sensor performances, great efforts have been made in the design and engineering of novel materials.^3–5 Among them, core–shell structured nanomaterials, which consist of chemically distinct components, have been widely investigated as one of the most promising materials for gas detection and have exhibited enhanced gas sensing properties compared to single oxides.^6,7 Up to now, many fabrication techniques have been reported for the preparation of a variety of oxide semiconductor core–shell structures.^8–12 Despite their success, many of these approaches usually involve high temperatures, complex operation steps or especial solvents and additives, which possibly result in increased expense and hence limit their potential applications. Therefore, it is of great necessity to find simpler and more dependable synthetic methods for core–shell nanostructures with designed chemical components and controlled morphologies.

Co₃O₄, a typical p-type metal oxide semiconductor, has been widely studied as a promising gas sensing material.^13,14 On the other hand, α-Fe₂O₃ is an important n-type semiconductor oxide and has been extensively investigated as a gas sensor.^15,16 Furthermore, it has been reported that the combination of p- and n-type nanomaterials can provide higher and faster responses.¹⁷ Thus, the core–shell nanostructures between p-type Co₃O₄ and n-type α-Fe₂O₃ can provide a valuable material platform for high performance gas sensors. Although Co₃O₄ has been made into composites with various n-type semiconductors, very little work is directed toward fabricating Co₃O₄/α-Fe₂O₃ core–shell nanostructures and researching their gas sensing properties. Therefore, the combination of Co₃O₄ and α-Fe₂O₃ as a core–shell structure is of vital scientific and practical significance, and is expected to exhibit enhanced gas sensing properties.

In this work, 1D Co₃O₄/α-Fe₂O₃ core–shell heterostructure nanofibers were successfully fabricated by a simple coaxial electrospinning route combined with a subsequent calcination process. The present synthesis method for the core–shell structure is facile, economical and environmentally friendly. The sensors based on the as-prepared 1D Co₃O₄/α-Fe₂O₃ core–shell nanostructures exhibit significantly enhanced acetone gas sensing performance in terms of good sensing selectivity, high sensitivity, a rapid response/recovery time and better reproducibility, indicating that the as-prepared Co₃O₄/α-Fe₂O₃ core–shell nanostructures can be developed for acetone gas detection.

2. Experimental section

2.1 Setup for coaxial electrospinning and preparation of Co₃O₄/α-Fe₂O₃ core–shell nanofibers

All of the reagents involved in the experiments were of analytical reagent grade and were directly used without further purification. As shown in Fig. 1, the Co₃O₄/α-Fe₂O₃ core–shell nanofibers were fabricated by a coaxial electrospinning method. Four main parts including a high voltage DC power supply, the fluid supply system, composite nozzles and the collecting board formed the setup for coaxial electrospinning. In particular, the fluid supply system was composed of a composite nozzle and thrust unit. The composite nozzle was assembled with two stainless pins in coaxial alignment and the diameters of the two stainless pins were different. In a typical procedure, an outer fluid was prepared by adding 0.6 g FeCl₃ and 1.8 g PVP (polyvinyl pyrrolidone) in 10 mL DMF (N,N-dimethylformamide) to obtain aqueous solutions. A pale green solution was immediately produced under magnetic stirring for 2 minutes. An inner fluid was prepared by dissolving 1 g Co(NO₃)₂·6H₂O in 15 mL PVA (polyvinyl alcohol) solution, which contained 15 mL of deionized water and 1 g PVA. The as-prepared precursor solutions were loaded into the syringes in the electrospinning setup. The fluid speeds of the inner and outer fluids were 0.7 and 0.8 mL h⁻¹, respectively, and could be precisely controlled by the thrust unit. In the experiment, a voltage of 15 kV and a collection distance of 20 cm were employed for electrospinning. An ambient temperature and humidity were maintained at 24–27 °C and 30–40%, respectively, during the whole coaxial electrospinning procedure. The as-electrospun PVA/Co(NO₃)₂·9H₂O core–PVP/FeCl₃ shell composites were calcined at 600 °C for 1 h in an oxygen environment using a tube-type furnace to obtain the desired Co₃O₄/α-Fe₂O₃ core–shell nanofibers. The heating rate was controlled at 1 °C min⁻¹.

Fig. 1 Schematic diagram of the coaxial electrospinning setup.

2.2 Fabrication and measurement of gas sensors

Fabrication of the sensors was similar to our previous work. Briefly, 0.01 g as-prepared Co₃O₄ nanofibers, α-Fe₂O₃ hollow nanofibers and Co₃O₄/α-Fe₂O₃ core–shell nanofibers were uniformly dispersed into 0.1 mL distilled water to form a paste. Then the paste was painted on the surface of a ceramic tube (outer diameter = 1.35 mm, length = 4 mm) on which a pair of gold electrodes was previously printed. A Ni–Cr heating wire was inserted in the tube to form a side-heated gas sensor. The prepared sensors were dried at 100 °C for 3 h and then welded on a socket for testing. Gas sensing properties of the sensor were measured by a RQ-2 series Intelligent Test Meter (China). The response (R) was defined using the formula R = R_a/R_g or R = R_g/R_a for the n-type and p-type oxide semiconductor, respectively. R_a and R_g were the resistance in the absence and presence of acetone gas, respectively. The response and recovery times (t_res, t_rec) were defined as the time taken by the sensor to achieve 90% of the total resistance change upon exposure to the acetone gas and air, respectively.

2.3 Characterization

Field emission scanning electron microscopy (FESEM) images were obtained using a JEOL JSM-6700F microscope. Transmission electron microscopy (TEM) and mapping images were obtained on a JEOL JEM-2200FS microscope operated at 200 kV. The crystal phases of the synthesized samples were characterized by X-ray powder diffraction (XRD, Rigaku D/max-Ra).

3. Results and discussion

3.1 Morphology and composition analysis

The morphologies of the as-prepared Co₃O₄/α-Fe₂O₃ CSNFs, α-Fe₂O₃ HNFs and Co₃O₄ NFs were observed by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The low magnification FESEM image in Fig. 2a shows that the obtained Co₃O₄/α-Fe₂O₃ CSNFs are randomly oriented forming non-woven mats, and are uniform in the long axial direction with diameters of about 70–130 nm. The high magnification FESEM image in Fig. 2b clearly displays the cross section of the as-prepared core–shell structure and further confirms the distinctive bilayer structure consisting of the Co₃O₄ core and the α-Fe₂O₃ shell. Meanwhile, it can be found from Fig. 2b that the surfaces of the Co₃O₄/α-Fe₂O₃ CSNFs are rough due to the decomposition of the PVP polymer. Furthermore, α-Fe₂O₃ HNFs and Co₃O₄ NFs were also fabricated for comparison purposes. Fig. 2c and e show the low-magnification FESEM image of the α-Fe₂O₃ HNFs and the Co₃O₄ NFs respectively, in which both samples present long and continuous surfaces. The hollow structure of the α-Fe₂O₃ nanofibers can be easily observed by the cross-sectional image in Fig. 2d, while the enlarged FESEM image in Fig. 2f shows that the Co₃O₄ NFs are assembled by many small nanoparticles.

Fig. 2 Low and high magnification FESEM images of (a and b) the Co₃O₄/α-Fe₂O₃ CSNFs (core–shell nanofibers), (c and d) the α-Fe₂O₃ HNFs (hollow nanofibers) and (e and f) the Co₃O₄ NFs (nanofibers), respectively.

Further morphology characterization of the Co₃O₄/α-Fe₂O₃ CSNFs, α-Fe₂O₃ HNFs and Co₃O₄ NFs was examined by TEM. Fig. 3a shows a typical TEM image of the Co₃O₄/α-Fe₂O₃ CSNFs. The core in shell structure can be obviously observed and the thickness of the shell layer in the core–shell nanofibers exhibits excellent uniformity. The TEM image is in good agreement with the FESEM result (Fig. 2b). As shown in Fig. 3b, the typical hollow morphology of the α-Fe₂O₃ HNFs is discerned by the TEM image, where the strong contrast difference between the darker marginal region and the brighter central region can be seen. In contrast, the TEM image from Fig. 3c demonstrates that the Co₃O₄ NFs are solid. Moreover, the Co₃O₄/α-Fe₂O₃ core–shell nanostructures were further characterized by an EDX element mapping test. Fig. 3d–f show the element mapping for Fe, Co and O respectively, in a single Co₃O₄/α-Fe₂O₃ core–shell nanofiber. The results demonstrate that the Fe and O are distributed throughout the core–shell nanofiber, whereas Co is concentrated at the core region, which again confirms the Co₃O₄-core/α-Fe₂O₃-shell structure. A similar phenomenon is also reported in the earlier literature.^18,19 In addition, there is a small distribution of Co in the shell resin, because the boundary of PVP and PVA is not so distinct during the process of electrospinning, and a small quantity of Co²⁺ and Co³⁺ which dissolve in the PVA may diffuse to the shell layer during the heating process.

Fig. 3 TEM images of (a) a Co₃O₄/α-Fe₂O₃ CSNF, (b) a α-Fe₂O₃ HNF and (c) a Co₃O₄ NF. (d)–(f) are the elemental mapping images of Fe, Co and O element, respectively, of an individual Co₃O₄/α-Fe₂O₃ CSNF (from (a)).

The structures of the as-prepared products were also characterized using XRD, which provided further insight into the crystallinity of the products. Fig. 4a shows the XRD pattern of the Co₃O₄/α-Fe₂O₃ CSNFs. All the diffraction peaks can be indexed to a mixture of α-Fe₂O₃ and Co₃O₄, which is consistent with the JCPDS file no. 87-1166 and 78-1970, respectively. No impurity phases were detected during the XRD measurements, which confirms the excellent purity of the products. Fig. 4b and c depict the XRD patterns of the as-prepared α-Fe₂O₃ (JCPDS no. 87-1166) HNFs and Co₃O₄ NFs (JCPDS no. 78-1970), respectively. No diffraction peaks from any other impurities were observed.

Fig. 4 XRD patterns of (a) the Co₃O₄/α-Fe₂O₃ CSNFs, (b) the α-Fe₂O₃ HNFs and (c) the Co₃O₄ NFs.

3.2 Acetone sensing properties

Acetone, as a common reagent, is widely used in industries and laboratories, and is harmful to human health and safety. Inhalation of acetone may cause headaches, fatigue, nausea and even narcosis and is harmful to the nervous system.²⁰ On the other hand, acetone is also an important breath biomarker for noninvasive diagnosis of human diabetes.²¹ Hence, the detection of acetone is of great importance and much needed for both environmental protection and human health. Thus, a gas sensor based on the as-prepared 1D Co₃O₄/α-Fe₂O₃ core–shell nanostructures was fabricated and its acetone gas sensing performance were investigated.

In gas sensing studies, the operating temperature is important for the investigation of gas sensing properties due to its great influence on the surface state of sensing materials, as well as the contact reactions during the gas sensing process.²² To investigate the influence of the operating temperature and to obtain an optimum operating temperature for the sensors, the responses of the Co₃O₄/α-Fe₂O₃ CSNF, Co₃O₄ NF and α-Fe₂O₃ HNF sensors to 50 ppm acetone gas, as a function of the operating temperature, were tested and the results are depicted in Fig. 5a. Obviously, the responses of each sensor are strongly dependent on the operating temperature. The responses of three sensors first increase with temperature, reach their maxima, and then decrease with a further increase in temperature. This decrease in responses at higher temperatures may be ascribed to the decrease in the number of active sites for acetone adsorption. The other possibility is that the rate of adsorption is lower than that of desorption at such high temperatures.²³ Moreover, it is obvious that each sensor has an optimal operating temperature, at which the sensor exhibits the highest response to acetone. It is worth mentioning that the highest response of the Co₃O₄/α-Fe₂O₃ CSNF sensor to 50 ppm acetone is 11.7 (at 240 °C), which is almost 2.5 times higher than that of the Co₃O₄ NF and α-Fe₂O₃ HNF sensors. The results also demonstrate that the Co₃O₄/α-Fe₂O₃ core–shell nanostructures have an improved response in comparison to the single Co₃O₄ NFs and α-Fe₂O₃ HNFs. Thus, to obtain a deeper insight into the gas sensing behavior, attention is focused on the Co₃O₄/α-Fe₂O₃ CSNFs, which are the most efficient for acetone gas detection. 240 °C was chosen as the optimal operating temperature, at which the Co₃O₄/α-Fe₂O₃ CSNF sensor exhibited the highest response (11.7) for 50 ppm acetone.

Fig. 5 (a) Responses of different gas sensors based on the Co₃O₄/α-Fe₂O₃ CSNFs, Co₃O₄ NFs and α-Fe₂O₃ HNFs to 50 ppm acetone as a function of operating temperature, and (b) the responses of the Co₃O₄/α-Fe₂O₃ CSNF, Co₃O₄ NF and α-Fe₂O₃ HNF sensors to different concentrations of acetone at 240 °C.

Fig. 5b shows the responses of the Co₃O₄/α-Fe₂O₃ CSNF, Co₃O₄ NF and α-Fe₂O₃ HNF sensors to different concentrations of acetone at 240 °C. It can be easily seen that the responses of three sensors rapidly increase with increasing acetone concentration until 300 ppm. Above 300 ppm, the responses slowly change with the variations of acetone concentration, which indicate that the three sensors become more or less saturated. Furthermore, the sensor based on the Co₃O₄/α-Fe₂O₃ CSNFs exhibits higher responses to acetone at various concentrations compared with those based on the Co₃O₄ NFs and α-Fe₂O₃ HNFs.

For a highly effective gas sensor, the response time and recovery time are very important parameters for practical applications in addition to a superior sensitivity value. Fig. 6a shows typical response transients of the Co₃O₄/α-Fe₂O₃ CSNF sensor when it is exposed to 50 ppm acetone gas. The result indicates that the sensor immediately responds when acetone gas is introduced and then rapidly recovers to its initial value after the acetone is released. The response and recovery times were calculated to be 2 s and 20 s, respectively. The inset of Fig. 6a shows the dynamic response of the Co₃O₄/α-Fe₂O₃ CSNF sensor to different concentrations (1–50 ppm) of acetone gas at 240 °C. It is obvious that the response of the sensor changes rapidly on being exposed to acetone and to air, indicating the excellent reproducibility. In addition, the Co₃O₄/α-Fe₂O₃ CSNF sensor shows a clear and fast response change when exposed to a concentration of acetone as low as 1 ppm, with a response value of about 2.6.

Fig. 6 (a) Response transients of the Co₃O₄/α-Fe₂O₃ CSNF sensor to 50 ppm acetone at 240 °C, and (b) the responses of the Co₃O₄/α-Fe₂O₃ CSNF, Co₃O₄ NF and α-Fe₂O₃ HNF sensors to 50 ppm of various testing gases ((A) trichloromethane, (B) ammonia, (C) methylbenzene, (D) formaldehyde, (E) ethanol, and (F) acetone) at 240 °C. The inset of (a) shows the dynamic response of the Co₃O₄/α-Fe₂O₃ CSNF sensor to different acetone concentrations (1, 5, 10, 30, 50 ppm) at 240 °C.

Gas sensors for practical applications are required not only to possess high responses but also to present great selectivity to the target gases. Hence, a selectivity study of the Co₃O₄/α-Fe₂O₃ CSNF, Co₃O₄ NF and α-Fe₂O₃ HNF sensors was carried out by monitoring the sensors’ resistance changes toward 50 ppm of different gases at 240 °C. As shown in Fig. 6b, the Co₃O₄/α-Fe₂O₃ CSNF sensor shows an obviously high sensing response to acetone and is less sensitive to other gases, indicating that the Co₃O₄/α-Fe₂O₃ CSNF sensor has a very good selectivity for acetone gas. The excellent selectivity for acetone is mainly attributed to the enhanced reaction between the acetone and the absorbed oxygen at the optimal operating temperature.

The enhanced gas sensing performance of the Co₃O₄/α-Fe₂O₃ CSNF gas sensor should be ascribed to three aspects. On the one hand, the 1D structure of the nanofibers can easily transfer the gas molecules to and from the interaction region and improve the rate for charge carriers to transverse the barriers, which is induced by molecular recognition along the nanofibers.^24,25 Moreover, the effective and rapid gas diffusion toward the surface regions of the nanofibers can be easily achieved by the large pores constituted by the space between nanofibers (as shown in Fig. 7a). Therefore, a short response time was obtained. The fast recovery behaviour can be explained as follows. When the sample is exposed to air again, the superficial layer of the Co₃O₄/α-Fe₂O₃ CSNF gas sensor contacts with oxygen molecules. Due to the existence of abundant electrons in the α-Fe₂O₃ shell, the oxygen easily captures the electrons from the sensing materials, which makes the resistance of the Co₃O₄/α-Fe₂O₃ CSNF gas sensor increase rapidly. Then, due to the rapid gas diffusion obtained by the large spaces between the nanofibers, more and more oxygen and sensing materials are involved in the adsorption process. In this way, the fast recovery is eventually achieved. On the other hand, the enhanced gas sensing performance is attributed to the extension of the electron depletion layer by the formation of a heterojunction between the p-type Co₃O₄ and the n-type α-Fe₂O₃.^26,27 The heterojunction region is believed to easily attract reductive and oxidative gases (O₂), thus forming a deeper electron depletion layer and causing a better sensing performance than pristine oxides.²⁸ Specifically, Co₃O₄ and α-Fe₂O₃ are p-type and n-type semiconductor oxides, respectively. The heterojunction is formed at the interface between Co₃O₄ and α-Fe₂O₃ when they have an intimate contact between them (shown in Fig. 3a). Electrons will transfer from α-Fe₂O₃ to Co₃O₄ while holes will flow along the contrary direction of the electrons until the system obtains equalization of the Fermi levels. In this process, the built-in electric field in the interface of Co₃O₄ and α-Fe₂O₃ and the depletion layer can be formed (as shown in Fig. 7b). Due to the formation of a built-in electric field, the electrons and holes mainly flow in the α-Fe₂O₃ shell and the Co₃O₄ core, respectively. Moreover, Co₃O₄ is completely covered by α-Fe₂O₃ and the change of resistance mainly depends on the electron concentration and thickness of depletion layer. Therefore, the gas sensing performance of the Co₃O₄/α-Fe₂O₃ sensor results from the electron transport through the α-Fe₂O₃ shell. In addition, the depletion layer was formed in the interface of Co₃O₄ and α-Fe₂O₃, leading to the increase of resistance of the Co₃O₄ and α-Fe₂O₃ nanofibers. In other words, the main effect of the Co₃O₄ core (or the core–shell heterojunction) is that it decreases the carrier density in the α-Fe₂O₃ shell. That is to say, the Co₃O₄ core (or the core–shell heterojunction) increases the initial resistance of the Co₃O₄/α-Fe₂O₃ composite. When the electron concentration at the α-Fe₂O₃ shell becomes very low owing to the heterojunction, the injection of the same concentration of electrons by the gas sensing reaction leads to relatively higher variations in sensor resistance, which enhances the gas response. Accordingly, when the Co₃O₄/α-Fe₂O₃ sensor is exposed to acetone, the acetone molecules will react with the pre-adsorbed oxygen and release free electrons to the α-Fe₂O₃ shell, leading to an enhancement of the sensing response of the core–shell Co₃O₄/α-Fe₂O₃ sensor compared to the Co₃O₄ NF and α-Fe₂O₃ HNF sensors. Finally, according to the result of the elemental mapping of Fe, Co and O element of an individual Co₃O₄/α-Fe₂O₃ CSNF, a little Co is transferred to the α-Fe₂O₃ shell during the heating procedure. Thus, the well known Co₃O₄ catalytic activity will promote the reactions taking place during the sensing process (as shown in Fig. 7c). To evaluate the catalytic activity of Co, 1 wt% and 2 wt% Co-loaded α-Fe₂O₃ HNFs were prepared and the responses to different gases were tested. The results are shown in Fig. S1.† It can be observed that the loading of Co improves the selectivity of the samples. The 2 wt% Co-loaded α-Fe₂O₃ HNFs present a larger sensitivity enhancement to acetone in comparison with the α-Fe₂O₃ HNFs. However, for other gases, the responses of the 2 wt% Co-loaded α-Fe₂O₃ HNFs are slightly increased compared to the α-Fe₂O₃ HNFs.

Fig. 7 (a) The schematic diagram of gas diffusion through the Co₃O₄@α-Fe₂O₃ CSNFs, (b) a conductivity schematic diagram of the Co₃O₄@α-Fe₂O₃ CSNFs in air and acetone, and (c) an illustration of the catalytic action of Co₃O₄.

A comparison between the sensing performances of the Co₃O₄/α-Fe₂O₃ CSNF based sensor and literature reports is summarized in Table 1. It can be easily seen that although the response value of the Co₃O₄/α-Fe₂O₃ CSNF sensor is not the highest, the optimal operating temperature (240 °C) is lower than some literature reports. Furthermore, the rapid response time (2 s) and recovery time (20 s) of the Co₃O₄/α-Fe₂O₃ CSNFs are also advantageous. Thus, taking the simple synthesis method and better acetone gas sensing performance into account, the Co₃O₄/α-Fe₂O₃ CSNFs can be developed to be highly effective acetone gas sensing sensors.

Table 1 The performance of acetone gas sensors based on various nanostructures in the literature and the Co₃O₄/α-Fe₂O₃ CSNFs in this study

Sample	Acetone (ppm)	S	tres/trec (s)	T (°C)	Ref.
In2O3 nanowire	25	∼7	—/—	400	29
Co-doped ZnO fiber	100	16	6/4	360	30
ZnO hollow sphere	500	∼10	<10/<10	300	31
Cu doped WO3 hollow fiber	20	6.4	5/20	300	32
Y-doped SnO2 nanofibers	50	12	—/—	300	33
Nanoscale Fe2O3 structure	100	15.7	0.8/27	240	34
Co3O4/α-Fe2O3 CSNF	50	11.7	2/20	240	This work

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4. Conclusions

In summary, Co₃O₄/α-Fe₂O₃ core–shell nanostructures have been successfully fabricated through a facile coaxial electrospinning technique at an ambient temperature, followed by a subsequent thermal treatment. The sensor based on the as-prepared Co₃O₄/α-Fe₂O₃ core–shell nanostructures shows an excellent acetone gas sensing performance in terms of superior sensitivity, good selectivity, a low concentration detection limit, and rapid response and recovery times. The facile synthesis method and excellent gas sensing properties indicate that the as-prepared Co₃O₄/α-Fe₂O₃ core–shell nanostructures can be developed for acetone gas detection in the future.

Acknowledgements

This research work was financially supported by the Program for Changjiang Scholars and Innovative Research Team in University (no. IRT1017), the Graduate Innovation Fund of Jilin University (grant no. 20121108), the National Natural Science Foundation of China (grant no. 51102109), the Postdoctoral Science Foundation of China (grant no. 2012M510878), the Program for Science and Technology Development (no. 20110725) and the Postdoctoral Science Foundation of China (no. 2014T70289).

Notes and references

1. Y. Q. Liang, Z. D. Cui, S. L. Zhu, Z. Y. Li, X. J. Yang, Y. J. Chen and J. M. Ma, Nanoscale, 2013, 5, 10916.
2. S. M. Wang, B. X. Xiao, T. Y. Yang, P. Wang, C. H. Xiao, Z. F. Li, R. Zhao and M. Z. Zhang, J. Mater. Chem. A, 2014, 2, 6598.
3. A. Katoch, J. H. Byun, S. W. Choi and S. S. Kim, Sens. Actuators, B, 2014, 202, 38.
4. Y. V. Kaneti, Q. M. D. Zakaria, Z. J. Zhang, C. Y. Chen, J. Yue, M. S. Liu, X. C. Jiang and A. B. Yu, J. Mater. Chem. A, 2014, 2, 13283.
5. X. Z. Wang, S. Qiu, C. Z. He, G. X. Lu, W. Liu and J. R. Liu, RSC Adv., 2013, 3, 19002.
6. F. D. Qu, J. Liu, Y. Wang, S. P. Wen, Y. Chen, X. Li and S. P. Ruan, Sens. Actuators, B, 2014, 199, 346.
7. S. Park, H. Ko, S. Kim and C. Lee, ACS Appl. Mater. Interfaces, 2014, 6, 9595.
8. K. W. Liu, M. Sakurai and M. Aono, J. Mater. Chem., 2012, 22, 12882.
9. J. Cao, Z. Y. Wang, R. Wang and T. Zhang, CrystEngComm, 2014, 16, 7731.
10. Y. F. Li, Y. J. Hu, H. Jiang, X. Y. Hou and C. Z. Li, CrystEngComm, 2013, 15, 6715.
11. F. D. Qu, Y. F. Wang, Y. Wang, J. R. Zhou and S. P. Ruan, RSC Adv., 2014, 4, 24211.
12. D. L. Shao, H. T. Sun, G. Q. Xin, J. Lian and S. Sawyer, Appl. Surf. Sci., 2014, 314, 872.
13. J. N. Deng, L. L. Wang, Z. Lou and T. Zhang, RSC Adv., 2014, 4, 21115.
14. Y. Y. Lu, W. W. Zhan, Y. He, Y. T. Wang, X. J. Kong, Q. Kuang, Z. X. Xie and L. S. Zheng, ACS Appl. Mater. Interfaces, 2014, 6, 4186.
15. X. H. Rao, X. T. Su, C. Yang, J. D. Wang, X. P. Zhen and D. S. Ling, CrystEngComm, 2013, 15, 7250.
16. P. Sun, Y. W. Liu, X. W. Li, Y. F. Sun, X. S. Liang, F. M. Liu and G. Y. Lu, RSC Adv., 2012, 2, 9824.
17. D. Bekermann, A. Gasparotto, D. Barreca, C. Maccato, E. Comini, C. Sada, G. Sberveglieri, A. Devi and R. A. Fischer, ACS Appl. Mater. Interfaces, 2012, 4, 928.
18. F. D. Qu, J. Liu, Y. Wang, S. P. Wen, Y. Chen, X. Li and S. P. Ruan, Sens. Actuators, B, 2014, 199, 346.
19. A. Katoch, S. W. Choi, G. J. Sun, H. W. Kim and S. S. Kim, Nanotechnology, 2014, 25, 175501.
20. S. M. Wang, P. Wang, Z. F. Li, C. H. Xiao, B. X. Xiao, R. Zhao, T. Y. Yang and M. Z. Zhang, New J. Chem., 2014, 38, 4879.
21. M. Righettoni, A. Tricoli and S. E. Pratsinis, Anal. Chem., 2010, 82, 3581.
22. J. Cao, H. M. Dou, H. Zhang, H. X. Mei, S. Liu, T. Fei, R. Wang, L. J. Wang and T. Zhang, Sens. Actuators, B, 2014, 198, 180.
23. X. M. Xu, X. Li, W. B. Wang, B. Wang, P. Sun, Y. F. Sun and G. Y. Lu, RSC Adv., 2014, 4, 4831.
24. Z. J. Wang, Z. Y. Li, J. H. Sun, H. N. Zhang, W. Wang, W. Zheng and C. Wang, J. Phys. Chem. C, 2010, 114, 6100.
25. J. Cao, T. Zhang, F. Li, H. Yang and S. Liu, New J. Chem., 2013, 37, 2031.
26. C. W. Na, H. S. Woo, I. D. Kim and J. H. Lee, Chem. Commun., 2011, 47, 5148.
27. Y. Q. Liang, Z. D. Cui, S. L. Zhu, Z. Y. Li, X. J. Yang, Y. J. Chen and J. M. Ma, Nanoscale, 2013, 5, 10916.
28. Y. J. Liu, G. X. Zhu, J. Z. Chen, H. Xu, X. P. Shen and A. H. Yuan, Appl. Surf. Sci., 2013, 265, 379.
29. A. Vomiero, S. Bianchi, E. Comini, G. Faglia, M. Ferroni, N. Poli and G. Sberveglieri, Thin Solid Films, 2007, 515, 8356.
30. L. Liu, S. C. Li, J. Zhuang, L. Y. Wang, J. B. Zhang, H. Y. Li, Z. Liu, Y. Han, X. X. Jiang and P. Zhang, Sens. Actuators, B, 2011, 155, 782.
31. P. Song, Q. Wang and Z. X. Yang, Mater. Lett., 2012, 86, 168.
32. X. Bai, H. M. Ji, P. Gao, Y. Zhang and X. H. Sun, Sens. Actuators, B, 2014, 193, 100.
33. L. Cheng, S. Y. Ma, X. B. Li, J. Luo, W. Q. Li, F. M. Li, Y. Z. Mao, T. T. Wang and Y. F. Li, Sens. Actuators, B, 2014, 200, 181.
34. X. H. Sun, H. M. Ji, X. L. Li, S. Cai and C. M. Zheng, Sens. Actuators, B, 2014, 600, 111.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra03675e

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