Highly enhanced methanol gas sensing properties by Pd_0.5Pd₃O₄ nanoparticle loaded ZnO hierarchical structures

Abstract

Pd_0.5Pd₃O₄ nanoparticle loaded ZnO hierarchical architectures have been successfully synthesized via a facile solution route at room temperature followed by a subsequent thermal treatment. Morphology and component characterizations reveal that Pd_0.5Pd₃O₄ nanoparticles are uniformly deposited on the surface of the ZnO hierarchical architecture. A gas sensor based on the as-prepared Pd_0.5Pd₃O₄ loaded ZnO hierarchical architecture shows excellent gas sensing performances in terms of fast response time (1 s), recovery time (5 s) and a high sensitivity for 50 ppm methanol at a relatively low temperature, which are evidently modified by the appropriate decoration of Pd_0.5Pd₃O₄ nanoparticles in comparison with pure ZnO. The enhanced gas sensing performances are attributed to the appropriate sensitization effect of Pd_0.5Pd₃O₄ nanoparticles. A multistage reaction formation mechanism of such flowerlike hierarchical architecture, and the morphology-dependent gas sensing mechanism are proposed.

---

1. Introduction

Nowadays, highly sensitive gas sensors have attracted tremendous attention because of their prominent roles in the areas of public safety, environmental monitoring and biological detection.^1,2 As important candidates for gas sensors, nanostructured metal oxide semiconductors, especially three dimensional hierarchical nanostructures assembled by 1D and 2D nanoscale building blocks, have been intensively investigated and open a new horizon for high performance sensors due to their extraordinary properties, including high sensitivity and good selectivity and simplicity in the material synthesis and sensing device fabrication.^3,4 This is because these hierarchical structures can increase the interactive surface area of the sensor materials to absorb more target gases and provide more active sites to facilitate the gas diffusion, which can enhance the sensitivities of the sensor.^5,6 Up to now, various hierarchical nanostructures have been prepared by multifarious routes. Among these methods, solution based synthesis has been proven to be an environmentally benign method and offers the potential for low cost and large scale manufacture.^7–9 However, regardless of all the advantages of the solution phase synthetic strategies, it is still a challenge to use these strategies in production of hierarchical architectures.

Moreover, it has been widely reported that in addition to the structure effect on gas sensing performances, the dopant effect also enhances responses of the sensor with the aid of catalytic activity of the noble metal/its oxide (such as Au, Ag, Pd, Pt or Ag₂O), which has been well investigated in a large number of studies.^10–16 Hence, the excellent gas sensing properties of noble metal-loaded hierarchical nanostructures can be eagerly expected.

Herein, the unique Pd_0.5Pd₃O₄ nanoparticles loaded ZnO hierarchical architectures are successfully fabricated by a facile solution route at ambient temperature followed by a subsequent thermal treatment, and are developed for the methanol gas sensing detection. To the best of our knowledge, such distinctive hierarchical architectures obtained by a simple method and used for gas sensors have been rarely reported.^17,18 And the gas sensor based on the appropriate Pd_0.5Pd₃O₄ nanoparticles loaded ZnO hierarchical architecture shows a decreased operating temperature, a highly improved sensitivity and shorten response/recovery dynamics in comparison with pristine ZnO nanomaterials. In addition, the possible formation mechanism of such flowerlike structure and gas sensing mechanism are also discussed.

2. Experimental section

2.1 Fabrication of 3D flowerlike ZnO hierarchical architecture

All the reagents (analytical-grade purity) were used as purchased without further purification. Deionized water was used in the experiments. ZnO flowerlike hierarchical architectures were synthesized by a simple and fast solution route at room temperature. In a typical procedure, 0.32 g zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), 0.732 g CTAB and 0.36 g sodium hydroxide (NaOH) was dissolved to 20 mL deionized water, 40 mL absolute ethanol and 9 mL deionized water under stirring to form a uniform transparent solution, respectively. Subsequently, 20 mL Zn(NO₃)₂ solution was added into 40 mL CTAB ethanol solution under magnetic stirring. After magnetic stirring for 5 min at room temperature, 1 mL NH₃·H₂O (25–28%) was injected quickly. Then, 7 mL NaOH solution was slowly added into the mixture. After being stirred for further 1.5 h, the white precipitates were collected by centrifugation, washed several times with deionized water, dried in air at 60 °C for 12 h and then calcined at 500 °C for 2 h.

2.2 Synthesis of Pd_0.5Pd₃O₄ nanoparticles loaded ZnO hierarchical architectures

In a typical procedure, 0.1 g newly prepared ZnO products were dispersed in 0.5 mL methanol with the help of ultrasonication. Then, 3 mg PdCl₂ was added. After the ultrasonic dispersion for 30 min, the resulting suspension was dried in air at 60 °C for 20 min. Finally, the samples were heated to 500 °C at a rate of 2 °C min⁻¹, and then maintained at this temperature for 2 h in air. The as-prepared product was named as the Pd_0.5Pd₃O₄ loaded ZnO hierarchical architecture.

Meanwhile, in order to study the effect of different Pd_0.5Pd₃O₄ decoration amounts on gas sensing performances, two contrast samples (CS1 and CS2) were also synthesized by using the aforementioned identical procedure. The only change was the quality of PdCl₂ while other conditions were unchanged. The added quality of PdCl₂ for CS1 and CS2 was 1.5 mg and 4.5 mg, respectively.

2.3 Fabrication of gas sensors

0.05 g as-prepared pure ZnO, CS1, CS2 and Pd_0.5Pd₃O₄-loaded ZnO nanomaterial was uniformly dispersed into distilled water (about 0.2 mL) to form a paste, respectively. 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 (diameter = 0.5 mm, resistance = 35 Ω) was inserted into the tube to heat the gas sensor directly. In order to improve their stability and repeatability, the gas sensors were sintered at 260 °C for 10 h in air.

The sensor was welded on a socket and the electrical properties of the sensor were measured by a CGS-8 intelligent gas sensing analysis system. The sensor response was defined as S = R_a/R_g. Here, R_a and R_g were the resistances of the sensors in the air and target gas, respectively. The response and recovery time was defined as the time taken by the sensors to achieve 90% of the total resistance change in the case of adsorption and desorption, respectively.

2.4 Characterization

FESEM images were obtained using a JEOL JSM-6700F microscope. TEM, HRTEM, STEM and mapping images were obtained on a JEOL JEM-2200FS microscope. 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 morphology and structure of the as-prepared products are observed by field emission scanning electron microscopy (FESEM) and transmission electron microscope (TEM). The low magnification FESEM image in Fig. 1a shows that pure ZnO products consist of monodisperse and uniform ZnO hierarchical architecture with diameters of 2–4 μm. The enlarged FESEM image in Fig. 1b confirms that individual flowerlike hierarchical architectures are assembled from large quantities of 2D nanosheets (about 30–40 nm in thickness) emanating from the centers in a divergent way. The internal cross-linked structure will effectively prevent nanosheets aggregation and maintains the long-standing existence of the hierarchical structure. TEM images are used to further observe the detailed morphologies of the flowerlike nanostructures. As shown the Fig. 1c and d, there are a large number of pores on the surface of nanosheets. As expected, the central portion of the superstructure is darker than that of the edge, further confirming the flowerlike interior of the unique intersecting nanosheets.

Fig. 1 (a) Low magnification FESEM, (b) enlarged FESEM, (c) enlarged TEM and (d) high resolution TEM image of pure ZnO, respectively.

Fig. 2 depicts the morphology of the as-obtained Pd_0.5Pd₃O₄ loaded ZnO hierarchical architectures. As shown in Fig. 2a and c, the decoration of Pd_0.5Pd₃O₄ nanoparticles does not alter the shape of flowerlike ZnO. And it can be clearly observed from Fig. 2b that the surfaces of the hierarchical architecture become rough and are covered with homodisperse nanoparticles in comparison with those of pure ZnO. In addition, the enlarged TEM image (Fig. 2d) also indicates that the Pd_0.5Pd₃O₄ with diameters of 10–20 nm is highly dispersed on the surfaces of the nanosheets in the form of small grains.

Fig. 2 (a) FESEM image and (c) TEM image of a single Pd_0.5Pd₃O₄-loaded ZnO nanostructure; (b) and (d) are the corresponding enlarged pattern, respectively.

The locations of the Pd_0.5Pd₃O₄ nanoparticles can be more clearly observed by the bright field image of the STEM pattern (Fig. 3a). To further determine the specific distribution of the Pd element, the EDX mapping of Pd_0.5Pd₃O₄-loaded ZnO nanostructure is conducted. As shown in Fig. 3b, it can be easily seen that the distribution of Pd element is uniform and exhibits a flowerlike structure. Fig. 3c is a typical HRTEM image of the ZnO–Pd_0.5Pd₃O₄ interface region, which clearly shows the lattice fringes of ZnO nanosheets and Pd_0.5Pd₃O₄ nanoparticles, indicating good crystallinity of the composites. The interplanar distances of fringes are measured to be 0.204 and 0.288 nm, which correspond to the spacing of the (220) plane and (200) plane of Pd_0.5Pd₃O₄ nanoparticles, respectively. Furthermore, the interplanar distance of fringes (0.282 nm) corresponds to the spacing of the (100) plane of ZnO. Thus, all above-mentioned results effectively demonstrate that Pd_0.5Pd₃O₄ nanoparticles have well attached onto the surface of the ZnO and the original flowerlike morphology of ZnO are not altered, which is favorable to the enhancement of the gas sensing performances.¹⁹

Fig. 3 (a) Bright field image (from STEM pattern) and (b) the elemental mapping image of Pd element of an individual Pd_0.5Pd₃O₄-loaded ZnO nanostructure, (c) the high resolution image of a juncture of two materials.

The structures of the as-prepared products are also characterized using XRD, which provides further insight into the crystallinity of the products. As shown in Fig. 4a, all diffraction peaks can be indexed to the standard ZnO with the hexagonal structure (JCPDS no. 80-0075). No diffraction peaks from any other impurities are found, which confirms excellent purity of the pure ZnO. Furthermore, due to small decoration amount of Pd_0.5Pd₃O₄ nanoparticles, the XRD patterns of the CS1 (Fig. 4b) and the Pd_0.5Pd₃O₄-loaded ZnO (Fig. 4c) sample show almost no change compared with that of the pure ZnO. However, as shown in Fig. 4d, with further increasing decoration amount of Pd_0.5Pd₃O₄ nanoparticles, the (211), (220), (400) and (332) faces of Pd_0.5Pd₃O₄ (JCPDS no. 71-1866) can be clearly distinguished in the CS2 samples, further definitely indicating the Pd_0.5Pd₃O₄ nanoparticles have been successfully deposited on the surface of the ZnO hierarchical architecture.

Fig. 4 XRD patterns of (a) pure ZnO, (b) CS1, (c) Pd_0.5Pd₃O₄-loaded ZnO hierarchical architectures and (d) CS2.

To understand the growth mechanism of flower architectures, the morphology evolution of the hierarchical structures with different volumes of NaOH aqueous solution is investigated while other conditions are kept constant. As shown in Fig. 5a, when 1 mL NaOH solution is added, it can be observed that the sample is mainly composed of a large number of polyhedrons (6–9 μm) with laminar coarse surfaces, accompanied by few small polyhedrons. The corresponding XRD pattern (Fig. 6a) indicates that all the diffraction peaks can be indexed to the Zn(OH)₂ (JCPDS no. 76-1778). As the volume of NaOH solution is increased to 2 mL, a few embryonic forms of flowerlike structures can be found in addition to large polyhedrons. The corresponding XRD pattern (Fig. 6b) demonstrates that the peaks of ZnO (JCPDS no. 80-0075) have emerged at this stage, and the intensity of diffraction peaks indexed to Zn(OH)₂ phase becomes weak. With the volume of NaOH solution further increasing to 3 mL, two kinds of distinct products, i.e. complex flowerlike ZnO and Zn(OH)₂ polyhedrons are formed simultaneously. But the Zn(OH)₂ polyhedrons decrease in size and quantity. As depicted in XRD pattern (Fig. 6c), most distinguishable peaks are indexed to ZnO. Finally, when the volume of NaOH solution is added to 7 mL, polyhedrons thoroughly evolve into the present flowerlike hierarchical architecture. The corresponding XRD pattern (Fig. 6d) confirms that the pure ZnO is formed and the diffraction peaks of Zn(OH)₂ thoroughly disappear.

Fig. 5 FESEM images of morphology evolution of the product with different volumes of NaOH aqueous solution: (a) 1 mL, (b) 2 mL, (c) 3 mL and (d) 7 mL.

Fig. 6 The XRD patterns of the samples with different volumes of NaOH solution: (a) 1 mL, (b) 2 mL, (c) 3 mL and (d) 7 mL.

On the basis of the experimental results and the investigations, a possible growth mechanism is proposed. The overall reaction in the experiment can be simply formulated as follows.^20,21

Zn²⁺ + 2OH⁻ → Zn(OH)₂↓ (1)

Zn(OH)₂ + 2OH⁻ → Zn(OH)₄²⁻ (2)

Zn(OH)₄²⁻ → ZnO↓ + H₂O + 2OH⁻ (3)

In the synthetic process, the volume of NaOH aqueous solution is found to play a vital role in the formation of ZnO products. At the beginning of the reaction, few NaOH solution is added, and Zn²⁺ will react with available OH⁻ to obtain white Zn(OH)₂ precipitates, according to eqn (1). With a prolonged addition of NaOH solution, Zn(OH)₂ will be successively dissolved by excessive OH⁻ ions to form a homogenous solution containing Zn(OH)₄²⁻ ions, as depicted in the eqn (2), which serve as growth units for the following nucleation and crystal growth of ZnO.²² As the reaction proceeds, the ZnO nuclei is formed by the dehydration of Zn(OH)₄²⁻ ions and further generate oriented ZnO nanoclusters. Furthermore, the surplus Zn(OH)₄²⁻ ions are directly incorporated into each ZnO nuclei in the nanoclusters.²³ After that, because of the intrinsic anisotropic character of hexagonal ZnO and the suitable solution environment, the ZnO nanoclusters will rearrange themselves and evolve into flowerlike hierarchical architecture through oriented attachment and self-assembly to reduce the surface energy.^3,24 Similar results have been reported in McCormack et al.²⁵ and Lu et al. works.²⁶ because of the existence of OH⁻ groups, the electrostatic and hydrogen bonds-induced orientation may be the main driving force for the self-assembly process.^27,28 Thus, it can be concluded that the final flowerlike hierarchical structures are highly dependent on the volume of NaOH solution, the intrinsic anisotropic character of ZnO and suitable solution environments. Moreover, due to the complexity of the solution-based nucleation and growth process, the more detailed growth mechanism for the formation of such hierarchical structure is still under investigation. And here is a working hypothesis that agrees well with the observation of electron microscopy.

3.2 Gas sensing properties

Taking the flowerlike hierarchical structure and the appropriate decoration of Pd_0.5Pd₃O₄ nanoparticles into consideration, we expect that the as-prepared Pd_0.5Pd₃O₄ loaded ZnO nanostructure will be a promising candidate for the high performance gas sensor to detect some chemical contaminants and dangerous gases. Methanol, as an important chemical raw material and clean liquid fuels, has been widely employed. But, methanol is potentially harmful for human nervous and blood system. Therefore, the requirement for effective methanol gas sensor with fast response/recovery time and high sensitivity is of practical significance. For this reason we fabricate Pd_0.5Pd₃O₄-loaded ZnO nanomaterials based gas sensors and investigate their gas sensing performances toward methanol gas.

To investigate the influence of the operating temperature and to obtain an optimum operating temperature of the sensor, the sensing sensitivities of the ZnO sensor to 50 ppm methanol gases as a function of the operating temperature are tested and the results are depicted in Fig. 7. As shown in Fig. 7a, the sensitivities of four sensors first increase with temperature, reach their maxima, and then decrease with a further increase in temperature. This decrease in the response at higher temperature may be attributed to the decrease in the number of active sites for the adsorption of methanol.²⁹ Another possibility is that the rate of adsorption is lower than that of desorption at such high temperature.³⁰ The optimal operating temperature for the CS1, CS2 and Pd_0.5Pd₃O₄-loaded ZnO are all 260 °C, which is probably due to tiny difference in the quality of PdCl₂. Compared to the pure ZnO nanomaterial, the optimal operating temperature presents a slight decrease from 280 °C to 260 °C. In addition, it is also found that the decoration amount of Pd_0.5Pd₃O₄ nanoparticles has a significant effect on the gas sensing performances. Obviously, the three Pd_0.5Pd₃O₄ nanoparticles decorated ZnO sensors show higher sensitivities than the pure ZnO in the temperature range from 200 °C to 300 °C. The Pd_0.5Pd₃O₄-loaded ZnO sensor shows the largest response (10.5 at 260 °C). However, further increasing the Pd contents (the CS2 sample in this case) by adding more PdCl₂ during the fabrication process decreases the gas sensitivity. To further confirm the sensing properties of the as-synthesized Pd_0.5Pd₃O₄-loaded ZnO sensor, a selectivity test towards 50 ppm different gases is conducted at 260 °C. As shown in Fig. S1,† the sensor exhibits an obvious response to methanol (CH₃OH) and less sensitive to formaldehyde (HCHO), toluene (C₇H₈), hydrogen (H₂), carbonic oxide (CO) and ammonia (NH₃), which indicates that the Pd_0.5Pd₃O₄-loaded ZnO sensor has good selectivity towards methanol. Thus, to obtain a deeper insight into the gas sensing behavior, attention is focused on the Pd_0.5Pd₃O₄-loaded ZnO sensor, which is the most efficient for methanol gas detection.

Fig. 7 (a) Responses of different gas sensors based on pure ZnO, CS1, CS2 and Pd_0.5Pd₃O₄-loaded ZnO to 50 ppm methanol as a function of the operating temperatures, (b) temperature-dependent response and recovery time of the sensor based on Pd_0.5Pd₃O₄-loaded ZnO hierarchical architectures to 50 ppm methanol.

Moreover, the correlation between the response and recovery time of the sensor based on the Pd_0.5Pd₃O₄-loaded ZnO toward 50 ppm methanol at different operating temperatures are measured and shown in Fig. 7b. It is found that the response time and recovery time decreases with increasing operating temperature. It is noteworthy that the response time presents very small change while the recovery time exhibits an obvious change from 2.5 s at 300 °C to 37 s at 200 °C. Such recovery behaviors are attributed to the fact that desorption process of methanol gas needs a longer time at a relatively low temperature.

Fig. 8 shows the response transients of the Pd_0.5Pd₃O₄-loaded ZnO sensor exposed to different concentrations of methanol gas at 260 °C. It is apparent that the responses of the sensor change rapidly on being exposed to air and to methanol, indicating the excellent reproducibility of the sensor. And the as-fabricated sensor shows a clear and fast response/recovery change when exposed to a concentration of methanol as low as 1 ppm. For a high sensitive gas sensor, the response time and recovery time are very important parameters for the practical application in addition to a superior sensitivity value. Fig. 9 shows the typical response transients of the Pd_0.5Pd₃O₄-loaded ZnO and pure ZnO sensor exposed to 50 ppm methanol gas at 260 °C. The response time and recovery time of the Pd_0.5Pd₃O₄-loaded ZnO and pure ZnO sensor are calculated to be 1 s and 5 s, and 7 s and 30 s, respectively. It can be easily found that for the Pd_0.5Pd₃O₄-loaded ZnO sensor, the response time and recovery time are 7 and 6 times shorter than that of the pure ZnO gas sensor. The fast response/recovery performance of the sensor can be attributed to the appropriate decoration of the Pd_0.5Pd₃O₄ nanoparticles.

Fig. 8 The dynamic responses of Pd_0.5Pd₃O₄-loaded ZnO hierarchical architecture based sensor to different methanol concentrations (1–100 ppm) at 260 °C.

Fig. 9 The transient response of (a) Pd_0.5Pd₃O₄-loaded ZnO and (b) pure ZnO hierarchical architectures based sensor to 50 ppm methanol at 260 °C.

The gas sensing mechanism of ZnO based sensor belongs to the surface-controlled type, which is based on the change in conductance of the semiconductor that is mainly caused by the adsorption and desorption of gas molecules on the surface of the sensing materials. When pure ZnO is exposed to air, oxygen molecules can be adsorbed on the surface of ZnO and extracts electrons from the conduction band of ZnO to form adsorbed oxygen species, which can produce a depletion layer on the surface of ZnO, resulting in an increase in the resistance. When pure ZnO is exposed to methanol gas, methanol molecules will react with the adsorbed oxygen species, release the trapped electrons back to the conduction band and increase the conductance of the ZnO.

The enhanced gas sensing performances of the Pd_0.5Pd₃O₄-loaded ZnO gas sensor for methanol than that of pure ZnO can be explained by two aspects. The first reason for such excellent gas sensing properties is attributed to the incorporation of the interfaces between the ZnO hierarchical architecture and the Pd_0.5Pd₃O₄ nanoparticle.³¹ This is because that an accumulation of electrons is formed around this region due to the presence of the Pd_0.5Pd₃O₄ nanoparticles on the surface of ZnO hierarchical nanostructure, resulting in the formation of a deeper electron depleted layer to increase the adsorption of oxygen species.^32,33 The other aspect is that the appropriate decoration of Pd_0.5Pd₃O₄ nanoparticles provides more active sites, improves the adsorption of gas molecules and accelerates the electron exchange between the sensor and the methanol gas.^34,35 However, when the loading of the Pd_0.5Pd₃O₄ is superabundant (the CS2 sample in our case), the heavy coating will cause serious agglomeration, which leads to a decrease in the number of the active sites on the surface of the hierarchical architecture, deteriorating the gas response.³⁶ Thus, the appropriate Pd_0.5Pd₃O₄ nanoparticles loaded ZnO hierarchical architecture endow the Pd_0.5Pd₃O₄-loaded ZnO based sensor with a excellent methanol gas sensing performance in terms of the superior sensitivity, the fast response/recovery time and the low concentration detection limit.

A gas sensing performance comparison between the Pd_0.5Pd₃O₄-loaded ZnO based sensor and literature reports is summarized in Table 1. In view of the simple synthesis method and better methanol gas sensing performances, the Pd_0.5Pd₃O₄-loaded ZnO hierarchical architecture presents more obvious advantages compared with previous reports, and can be developed to be high effective methanol gas sensing sensor in practical application.

Table 1 The performance of methanol gas sensors based on various nanostructures in the literatures and Pd_0.5Pd₃O₄-loaded ZnO in this study

Sample	Methanol (ppm)	S	tres/trec (s)	T (°C)	Ref.
ZnO hollow microsphere	200	9.6	−/−	400	16
Porous In2O3 nanobelts	20	9.5	∼10/∼10	370	37
SnO2–ZnO nanofiber	10	8.5	20/40	350	38
Fe2O3 polyhedral	50	2.5	−/−	340	39
Au-decorated ZnO	50	7	−/−	300	40
ZnO/SnO2 nanostructure	100	9.6	−/−	300	41
WO3 particles	100	24	19/8	260	42
Fe2O3 discoid crystal	100	6.4	−/−	250	43
Pd0.5Pd3O4-loaded ZnO	50	10.5	1/5	260	This work

---

4. Conclusions

In summary, the Pd_0.5Pd₃O₄-loaded ZnO hierarchical architecture has been successfully synthesized through a facile solution route at ambient temperature followed by a subsequent thermal treatment. The sensor based on the as-prepared Pd_0.5Pd₃O₄-loaded ZnO shows an excellent methanol gas sensing performance in terms of a superior sensitivity, low concentration detection limit, fast response and recovery time. The facile synthesis route and excellent gas sensing properties make the as-prepared composites developed for methanol gas detection in practice.

Acknowledgements

This work was funded by the National Science Foundation of China, nos 11174103 and 90923032, and Specialized Research Fund for the Doctoral Program of Higher Education of China, no. 20130061110012.

References

1. M. R. Alenezi, S. J. Henley, N. G. Emerson and S. R. P. Silva, Nanoscale, 2014, 6, 235.
2. P. Nag, S. Majumdar, A. Bumajdad and P. S. Devi, RSC Adv., 2014, 4, 18512.
3. D. W. Wang, S. S. Du, X. Zhou, B. Wang, J. Ma, P. Sun, Y. F. Sun and G. Y. Lu, CrystEngComm, 2013, 15, 7438.
4. L. W. Wang, S. R. Wang, Y. S. Wang, H. X. Zhang, Y. F. Kang and W. P. Huang, Sens. Actuators, B, 2013, 188, 85.
5. Q. J. Yu, C. L. Yu, J. Z. Wang, F. Y. Guo, S. Y. Gao, S. J. Jiao, H. T. Li, X. T. Zhang, X. Z. Wang, H. Gao, H. B. Yang and L. C. Zhao, RSC Adv., 2013, 3, 16619.
6. H. J. Zhang, R. F. Wu, Z. W. Chen, G. Liu, Z. N. Zhang and Z. Jiao, CrystEngComm, 2012, 14, 1775.
7. F. Z. Sun, F. T. Tan, W. Wang, X. L. Qiao and X. L. Qiu, Mater. Res. Bull., 2012, 47, 3357.
8. P. P. Wang, Q. Qi, X. X. Zou, J. Zhao, R. F. Xuan and G. D. Li, RSC Adv., 2013, 3, 23980.
9. J. Cao, W. Y. Fu, H. B. Yang, Q. J. Yu, Y. Y. Zhang, S. K. Liu, P. Sun, X. M. Zhou, Y. Leng, S. M. Wang, B. B. Liu and G. T. Zou, J. Phys. Chem. B, 2009, 113, 4642.
10. X. Z. Wang, S. Qiu, C. Z. He, G. X. Lu, W. Liu and J. R. Liu, RSC Adv., 2013, 3, 19002.
11. 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.
12. H. Kim, C. Jin, S. Park, S. Kim and C. Lee, Sens. Actuators, B, 2012, 161, 594.
13. V. Romanovskaya, M. Ivanovskaya and P. Bogdanov, Sens. Actuators, B, 1999, 56, 31.
14. X. Chen, Z. Guo, W. H. Xu, H. B. Yao, M. Q. Li, J. H. Liu, X. J. Huang and S. H. Yu, Adv. Funct. Mater., 2011, 21, 2049.
15. A. S. Zuruzi, M. H. Nurmawati, Y. H. Yeo, S. X. Wu, P. C. H. Lai and Z. Chen, RSC Adv., 2013, 3, 19971.
16. Y. S. Shim, H. G. Moon, D. H. Kim, L. H. Zhang, S. J. Yoon, Y. S. Yoon, C. Y. Kang and H. W. Jang, RSC Adv., 2013, 3, 10452.
17. F. D. Qu, J. Liu, Y. Wang, S. P. Wen, Y. Chen, X. Li and S. P. Ruan, Sens. Actuators, B, 2014, 199, 346.
18. X. Z. Wang, S. Qiu, C. Z. He, G. X. Lu, W. Liu and J. R. Liu, RSC Adv., 2013, 3, 19002.
19. Q. Xiang, G. F. Meng, Y. Zhang, J. Q. Xu, P. C. Xu, Q. Y. Pan and W. J. Yu, Sens. Actuators, B, 2010, 143, 635.
20. X. W. Li, P. Sun, T. L. Yang, J. Zhao, Z. Y. Wang, W. N. Wang, Y. P. Liu, G. Y. Lu and Y. Du, CrystEngComm, 2013, 15, 2949.
21. S. W. Bian, I. A. Mudunkotuwa, T. Rupasinghe and V. H. Grassian, Langmuir, 2011, 27, 6059.
22. Y. J. Sun, L. Wang, X. G. Yu and K. Z. Chen, CrystEngComm, 2012, 14, 3199.
23. J. Xie, Y. J. Hao, Z. Zhou, X. C. Meng, L. Yao, L. Bian and Y. Wei, Res. Chem. Intermed., 2014, 40, 1937.
24. Q. J. Yu, W. Y. Fu, C. L. Yu, H. B. Yang, R. H. Wei, M. H. Li, S. K. Liu, Y. M. Sui, Z. L. Liu, M. X. Yuan and G. T. Zou, J. Phys. Chem. C, 2007, 111, 17521.
25. R. A. McBride, J. M. Kelly and D. E. McCormack, J. Mater. Chem., 2003, 13, 1196.
26. P. Sun, L. You, Y. F. Sun, N. K. Chen, X. B. Li, H. B. Sun, J. Ma and G. Y. Lu, CrystEngComm, 2012, 14, 1701.
27. L. P. Xu, Y. S. Ding, C. H. Chen, L. L. Zhao, C. Rimkus, R. Joesten and S. L. Suib, Chem. Mater., 2008, 20, 308.
28. S. H. Ran, Y. G. Zhu, H. T. Huang, B. Liang, J. Xu, B. Liu, J. Zhang, Z. Xie, Z. R. Wang, J. H. Ye, D. Chen and G. C. Shen, CrystEngComm, 2012, 14, 3063.
29. X. M. Xu, X. Li, W. B. Wang, B. Wang, P. Sun, Y. F. Sun and G. Y. Lu, RSC Adv., 2014, 4, 4831.
30. T. Hyodo, H. Inoue, H. Motomur, K. Matsuo, T. Hashishin, J. Tamaki, Y. Shimizu and M. Egashira, Sens. Actuators, B, 2010, 151, 265.
31. L. L. Wang, Z. Lou, R. Wang, T. Fei and T. Zhang, J. Mater. Chem., 2012, 22, 12453.
32. S. M. Wang, Z. F. Li, P. Wang, C. H. Xiao, R. Zhao, B. X. Xiao, T. Y. Yang and M. Z. Zhang, CrystEngComm, 2014, 16, 5716.
33. W. Zeng, T. M. Liu and Z. C. Wang, J. Mater. Chem., 2012, 22, 3544.
34. X. J. Liu, Z. Chang, L. Luo, X. D. Lei, J. F. Liu and X. M. Sun, J. Mater. Chem., 2012, 22, 7232.
35. G. X. Zhu, Y. J. Liu, H. Xu, Y. Chen, X. P. Shen and Z. Xu, CrystEngComm, 2012, 14, 719.
36. L. Xu, R. Q. Xing, J. Song, W. Xu and H. W. Song, J. Mater. Chem. C, 2013, 1, 2174.
37. Y. S. Li, J. Xu, J. F. Chao, D. Chen, S. X. Ouyang, J. H. Ye and G. Z. Shen, J. Mater. Chem., 2011, 21, 12852.
38. W. Tang, J. Wang, P. J. Yao and X. G. Li, Sens. Actuators, B, 2014, 192, 543.
39. X. H. Rao, X. T. Su, C. Yang, J. D. Wang, X. P. Zhen and D. S. Ling, CrystEngComm, 2013, 15, 7250.
40. X. H. Liu, J. Zhang, L. W. Wang, T. L. Yang, X. Z. Guo, S. H. Wu and S. R. Wang, J. Mater. Chem., 2011, 21, 349.
41. C. C. Li, X. M. Yin, Q. H. Li and T. H. Wang, CrystEngComm, 2011, 13, 1557.
42. Z. H. Li, J. C. Li, L. L. Song, H. Q. Gong and Q. Niu, J. Mater. Chem. A, 2013, 1, 15377.
43. 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.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra05462h

---