Synthesis of porous In₂O₃ microspheres as a sensitive material for early warning of hydrocarbon explosions

Abstract

Efficient detection/monitoring of low-concentration C1–C3 aliphatic hydrocarbons (e.g., methane) is a challenging task, mainly due to their intrinsically low chemical reactivity and thereby weak sensing response. Herein we report the template-free synthesis of porous nanoparticle-assembled In₂O₃ microspheres that can serve as a highly sensitive material for C1–C3 detection. In particular, porous In₂O₃ microspheres with a BET surface area of 57 m² g⁻¹ are prepared through simple thermal treatment of an indium glycerolate precursor. The gas-sensing properties of the porous In₂O₃ material are evaluated by a series of C1–C3 hydrocarbons including methane (CH₄), ethane (C₂H₆), propane (C₃H₈), ethylene (C₂H₄) and acetylene (C₂H₂). The porous In₂O₃ material has the ability to detect these gases with a rapid response (<10 s) in a wide concentration range from 200 ppm to 50 000 ppm (the lower explosion limit of methane). In the testing range, the logarithm of response shows a good linear dependency on the logarithm of gas concentration, demonstrating that the porous In₂O₃ material may be used for quantitative detection of C1–C3 hydrocarbons. Given the rapid response and high sensitivity below the explosion limit, this porous In₂O₃ material is promising to provide earlier warning against the explosion risk of hydrocarbon compounds.

---

Introduction

Explosive atmospheres can be created by flammable gases. When the concentration of a flammable gas in air is above its explosion limit, all it needs is a source of ignition to cause an explosion. To reduce such a risk, a practicable method is efficient detection/monitoring of dangerous gases for the purpose of providing earlier warning. Thus, semiconductor-type gas sensors are now in widespread use because of their low cost, safety and portability.^1–3 The fundamental principle of semiconductor sensors is based on the measureable variation of sensing material's resistance, which is induced by the interaction between the sensor surface and the gas molecules to be detected.¹ Oxide semiconductor as the sensing material is generally the core component in a sensing device, and its structure determines the sensor's performances (e.g., sensitivity, selectivity, stability and response speed). Thus, it is highly desirable to judiciously tune the (micro)structure of oxide semiconductor to optimize the sensor's performances. In view of the fact that the sensing event generally occurs on the sensor surfaces, creating porous structure in the sensing material is a convenient strategy to improve the sensing properties by increasing the surface area.^4–16 This correspondingly provides the sensing material with high density of surface reactive sites and large contact area with target gases, and thereby enhanced sensing properties. Furthermore, the introduction of a porous structure in the sensing material can also accelerate the diffusion of gas molecules onto the sensing surface, thus leading to the enhancement of sensing performance.⁴

For the reasons described above, our particular interest is exploring facile synthetic methods that can result in porous sensing materials with better structures for sensing application. Herein, we report a facile precursor-mediated synthetic route to prepare a porous In₂O₃ material (hereafter denoted as p-In₂O₃) that possesses a large surface area and excellent sensing responses towards C1–C3 aliphatic hydrocarbons, including methane (CH₄), ethane (C₂H₆), propane (C₃H₈), ethylene (C₂H₄) and acetylene (C₂H₂). This method involves the synthesis of indium alkoxide microspheres (denoted as In-gly) by the solvothermal reaction between In³⁺ ions and glycerol using isopropanol as the solvent, followed by a simple thermal treatment (350 °C) of the resulting In-gly in air to remove organic component in it as well as to form porous In₂O₃ directly (Fig. 1, see details in Experimental section). In comparison with the widely-used template methods,^17–22 our method represents a new template-free synthetic route for the preparation of porous In₂O₃ materials with large surface area and advanced functions.

Fig. 1 Schematic representation of synthesis of porous In₂O₃ microspheres (p-In₂O₃). Step I: solvothermal reaction between In(NO₃)₃·4.5H₂O and glycerol in isopropanol at 180 °C. Step II: thermal treatment of the resulting indium glycerolate precursor in air at 350 °C.

As one of important n-type wide-gap semiconductors, indium oxide (In₂O₃) is a promising candidate for sensing applications. It has been shown that In₂O₃ is sensitive to a wide range of gases (such as H₂S,¹⁹ NH₃,^23,24 O₃ (ref. 25 and 26) and NO₂ (ref. 18, 20, 22, 27 and 28)) and organic vapors (such as methanol, ethanol, acetone and formaldehyde).^29–33 By contrast, In₂O₃ is less sensitive to hydrocarbons (e.g., methane) mainly because of their intrinsically low chemical reactivity.¹⁷ This is also possibly the reason why the researches on In₂O₃'s hydrocarbon sensing are seldom discussed. However, efficient detection/monitoring hydrocarbons, especially those low-carbon (C1–C3) hydrocarbons (e.g., methane and ethylene), is highly significant. This is because: (i) they are colorless and odorless gases; (ii) they have low explosion limits in air (Table 1);^34–37 and (iii) they are the important chemicals for industrial applications. For example, methane, the principal component (>90%) of natural gas, is used for power generation, syngas production, transportation and chemical industry. Ethylene is used to produce ethylene oxide, ethylene dichloride, ethylbenzene and polyethylene at industrial scale.

Table 1 Explosion limits of several C1–C3 hydrocarbons that are involved in this manuscript

Name	Formula	Structure	Explosion limit (in air, r.t.)
Methane	CH4	CH4	5% (50 000 ppm)
Ethane	C2H6	CH3–CH3	3% (30 000 ppm)
Propane	C3H8	CH3–CH2–CH3	2% (20 000 ppm)
Ethylene	C2H4	H2C=CH2	3.5% (35 000 ppm)
Acetylene	C2H2	HC≡CH	2.5% (25 000 ppm)

---

Experimental

Chemicals and reagents

Indium(III) nitrate hydrate and glycerol were purchased from Sinopharm Chemical Reagent Co. Ltd. Isopropanol and ethanol were purchased from Beijing Chemical Works. All the above chemicals were used without further purification and deionized water was used in all experiments.

Synthesis of indium glycerolate microspheres (In-gly)

In(NO₃)₃·4.5H₂O (0.30 g) was dissolved in a mixed solution of glycerol (8 mL) and isopropanol (30 mL). The resulting mixture was then transferred into a 60 mL Teflon-Lined autoclave, which was treated at 180 °C for 1 h. After cooled to room temperature naturally, the white precipitate (i.e., the In-gly precursor) was washed several times with ethanol, and dried in an oven overnight at 60 °C in air.

Synthesis of porous In₂O₃ microspheres from In-gly precursor

The porous In₂O₃ microspheres were prepared by calcining In-gly in air at 350 °C for 3 h. The resulting In₂O₃ material was labeled as p-In₂O₃.

General characterizations

The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2550 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). The transmission electron microscope (TEM) images were obtained with a Philips-FEI Tecnai G2S-Twin microscope equipped with a field emission gun operating at 200 kV. The scanning electron microscope (SEM) images were obtained with a JEOL JSM 6700F electron microscope. The thermogravimetric analysis (TGA) was performed using with a NETZSCH STA 449C TG thermal analyzer by heating the samples from 50 to 800 °C at a heating rate of 10 °C min⁻¹ in air. The nitrogen adsorption and desorption isotherms were measured by using a Micromeritics ASAP 2020M system. The surface areas of the materials were obtained by using the Brunauer–Emmett–Teller (BET) method. The FT-IR spectra were acquired on a Brüker IFS 66v/S FTIR spectrometer.

Sensor fabrication and testing

The gas sensor was fabricated by pasting viscous slurry of the obtained sample (which was made by mixing the sample with a small amount of ethanol) onto a ceramic tube with a diameter of 1 mm and a length of 4 mm, which was positioned with a pair of Au electrodes and four Pt wires on both ends of the tube (Fig. 2). A Ni–Cr alloy coil passing through the tube was employed as a heater to control the operating temperature. The operation temperature was measured on the surface of the sensor. Pt wires were used as conducting wires because of their excellent conductivity and stability. To ensure that the sensing material had an ohmic metallic contacts with the Au electrodes, an aging treatment in air (200 °C, 12 h) for the as-fabricated sensor was necessary before sensing testing. In the testing system (Fig. 2B), a load resistor with a known resistance (R_L) was connected in series with the sensor. The resistance of a sensor in air or a target gas was measured by monitoring the terminal voltage of the load resistor at a test circuit voltage of 5 V (V_s). For comparison, all of the sensors were fabricated using the same method, and only the difference may be the In₂O₃ sensing material. Gas sensing tests were performed on a commercial CGS-8 Gas Sensing Measurement System (Beijing Elite Tech Company Limited). Gas sensing properties were measured using a static test system which included a test chamber (∼1 L in volume). Environmental air with a relative humidity of ∼30% was used as both a reference gas and a diluting gas to obtain the desired concentrations of target gases. A typical testing procedure was as follows. After the target gas was injected into the test chamber for about 5 min by a microsyringe, the sensor was put into the test chamber. When the response reached a constant value, the sensor was taken out to recover in fresh air. The sensor response is defined as the ratio of R_a to R_g, where R_a and R_g are the electrical resistance of the sensor in atmospheric air and in the testing gas, respectively.

Fig. 2 (A) A photograph showing the structure of a typical gas sensor; (B) a schematic diagram of the measurement circuit.

Results and discussion

Synthesis of porous In₂O₃ microspheres from the indium glycerolate precursor

The In-gly precursor used herein was prepared from a mild solvothermal reaction system, in which In(NO₃)₃·4.5H₂O and glycerol were used as reactants, and isopropanol functioned as a solvent. The structure of the resulting In-gly precursor was characterized by powder X-ray diffraction (XRD), FT-IR spectroscopy, and thermogravimetric analysis (TG) (see Fig. S1–S3 in ESI†). All the results suggested that the In-gly precursor was an amorphous indium(III) alkoxide with an indium content of ∼46.6 wt% (see ESI† section for detailed discussion of the characterization results). Due to the amorphous feature of In-gly, its accurate structure has not been resolved yet. The morphology of the In-gly precursor was examined by scanning electron microscopy (SEM). The SEM images (Fig. 3) show that the In-gly precursor exhibits a sphere-like morphology and these microspheres are uniform in shape and size with an average diameter of ∼400 nm. The TEM image of the In-gly precursor (Fig. 3B, inset) further confirms that In-gly possesses a sphere-like morphology and an amorphous non-porous structure.

Fig. 3 (A) and (B) SEM images of In-gly with a TEM image of In-gly in the inset.

Based on the above TG result (Fig. S3 in ESI†), a calcination temperature of 350 °C was selected to remove organic component in In-gly as well as to form In₂O₃ material (i.e., p-In₂O₃) directly. As expected, pure In₂O₃ sample was obtained after the thermal treatment at 350 °C in air. Fig. 4A shows the XRD pattern of the resulting p-In₂O₃ material. The XRD pattern of p-In₂O₃ is well indexed as pure In₂O₃ with a cubic structure (PDF#65-3170). The obvious broadening of diffraction peaks indicates that the obtained p-In₂O₃ are composed of nanocrystals with a small size.

Fig. 4 (A) XRD pattern of p-In₂O₃; (B) SEM, (C) TEM and (D) HRTEM images of p-In₂O₃.

Fig. 4B shows typical SEM image of p-In₂O₃, revealing that p-In₂O₃ maintains the morphology and size of the In-gly precursor. In contrast to the In-gly precursor with a smooth surface (Fig. 3), the surface of p-In₂O₃ is quite rough (Fig. 4B). This indicates that p-In₂O₃ might contain nanoparticle-assembled In₂O₃ microspheres. The TEM image (Fig. 4C) demonstrates that the uniform microspheres are composed of In₂O₃ nanoparticles, and a porous structure might exist in the p-In₂O₃ sample. The HRTEM image (Fig. 4D) clearly shows that In₂O₃ nanoparticles with a size of 15 nm are randomly arranged and interconnected in the whole visible area. The observed lattice spacing is about 0.295 nm, which corresponds to the distance between the (222) crystal planes of cubic In₂O₃ phase.

The porous structure and surface area of p-In₂O₃ were studied by N₂-adsorption measurement. The N₂ adsorption–desorption isotherms of p-In₂O₃ (Fig. 5A) exhibit a characteristic type-IV isotherm with an H3 hysteresis loop, indicating the presence of a mesoporous/macroporous structure in the material. The corresponding BJH pore size distribution (Fig. 5B) derived from the desorption branch of the isotherm shows a wide pore-size distribution ranging from 2 to 120 nm, further confirming the existence of mesopores/macropores in the material. This result is in agreement with the TEM observation. The corresponding BET surface area of this material is about 57 m² g⁻¹, demonstrating that p-In₂O₃ is a porous nanoparticle-assembled material with a high surface area. It is presumable that the formation of porous structure in p-In₂O₃ might be originated from the thermal-driven removal of organic components in In-gly and the simultaneous growth of In₂O₃ nanocrystals.^5,38,39

Fig. 5 (A) N₂ adsorption–desorption isotherms of p-In₂O₃; and (B) the corresponding pore size distribution.

Sensing performance of porous In₂O₃ microspheres

The sensing performance of the as-prepared In₂O₃ microspheres (i.e., p-In₂O₃) was first evaluated using methane as the testing gas because methane sensors are of importance for many industrial applications, as mentioned above. In order to achieve earlier warning against the explosion risk of methane, it is desirable to design effective sensor that can accurately monitor the methane concentration lower than its explosion limit (50 000 ppm). Thus, we studied the sensing performance of p-In₂O₃ in a technically relevant range of methane from 200 ppm to 50 000 ppm. The optimal operating temperature of the p-In₂O₃ sensor was determined by testing 5000 ppm methane. As shown in Fig. 6A, the p-In₂O₃ sensor exhibits the highest response toward methane at 350 °C, and thereby this optimal operating temperature was applied in all the sensing tests hereinafter.

Fig. 6 (A) Response of the sensors based on p-In₂O₃ as a function of the operating temperature for the detection of methane with a concentration of 5000 ppm; (B) dynamic response–recovery curve of the sensor based on p-In₂O₃ for methane detection; (C) variation of the response (R) of the p-In₂O₃ sensor with methane concentration (ppm); (D) response time and recovery time of the p-In₂O₃ sensor as a function of the methane concentration.

Fig. 6B shows the typical dynamic response–recovery curve of the p-In₂O₃ sensor with increasing methane concentrations. It is seen that the sensor has a wide response range of 200–50 000 ppm for methane detection, and the response increases significantly with the increase of methane concentration. For the concentrations of 200, 500, 1000, 2000, 5000, 10 000, 30 000, and 50 000 ppm, the responses of p-In₂O₃ are about 1.9, 2.6, 3.3, 4.4, 6.5, 8.8, 13.7 and 16.6, respectively. In addition, in the testing range from 200 to 50 000 ppm, the logarithm of the sensor response shows a good linear dependency on the logarithm of methane concentration (Fig. 6C), indicating that this p-In₂O₃ sensor may be used as quantitative detection of methane. Furthermore, the response and recovery times are also critical for a gas sensor. We, thus, measured the concentration-dependent response and recovery times for the p-In₂O₃ sensor. The results (Fig. 6D) show that the response time is less than 10 s, and the recovery time is less than 35 s in the whole testing range. The above sensing data demonstrate that the p-In₂O₃ sensor is very promising for early warnings of methane explosion because of its concentration-dependent response and fast response–recovery below explosion limit.

For comparative purpose, we measured the sensing properties of the commercial In₂O₃ nanoparticles (com-In₂O₃) towards methane. The particle size of the com-In₂O₃ is 20–50 nm (see the SEM image in Fig. S4, ESI†) and its BET surface area is around 27 m² g⁻¹. Fig. 7 shows the gas concentration-dependent responses of the sensors based on the p-In₂O₃ and com-In₂O₃ materials. It is obvious that the p-In₂O₃ sensor exhibits responses distinctly higher than those of the com-In₂O₃ sensor, demonstrating the importance of large surface area and porous structure for In₂O₃ sensing materials (57 m² g⁻¹ for p-In₂O₃ versus 27 m² g⁻¹ for com-In₂O₃). Furthermore, the p-In₂O₃ material we obtained exhibits a better sensing performance for methane detection than a previously reported mesoporous In₂O₃ material that was prepared by a hard-template method.¹⁷ For example, this reference material gave a response value of 3.5 in the presence of 6600 ppm methane, whereas our p-In₂O₃ material presented a response value of 6.5 in the presence of 5000 ppm methane.

Fig. 7 Comparison of the methane concentration-dependent responses of the p-In₂O₃ sensor and the com-In₂O₃ sensor.

We next studied the sensing properties of p-In₂O₃ for four more low-weight hydrocarbons including ethane, propane, ethylene and acetylene. Fig. 8 shows the dynamic response–recovery curves of the p-In₂O₃ sensor for detection of ethane, propane, ethylene and acetylene. When the sensor is exposed to the target gas, its resistance rapidly decreases, and the resistance returns to the original value when the sensor is exposed to air again. A higher gas concentration results in a lower resistance, and thereby a higher response. This reveals that the p-In₂O₃ sensor can give a concentration-dependent response behavior for all the four gases. In addition, these four gases, similar to methane, can also be detected quantitatively because the logarithm of response shows a good linear dependency on the logarithm of gas concentration in the testing range (Fig. 9). Furthermore, the p-In₂O₃ sensor has the ability to rapid response–recovery towards ethane, propane, ethylene and acetylene (Fig. 10). Generally, the response time (2–10 s) at the low gas concentration range (<500 ppm) is a little larger than that (<2 s) at the high gas concentration range (>500 ppm). Combined with the explosion limits of the relevant gases (Table 1), the above results demonstrate that the p-In₂O₃ sensor is suitable for explosion prevention of hydrocarbons.

Fig. 8 Dynamic response–recovery curves of the sensor based on p-In₂O₃ for detection of (A) ethane, (B) propane, (C) ethylene and (D) acetylene.

Fig. 9 Variation of the response (R) of the p-In₂O₃ sensor with (A) ethane, (B) propane, (C) ethylene and (D) acetylene concentrations (ppm).

Fig. 10 Response time and recovery time of the p-In₂O₃ sensor as a function of (A) ethane, (B) propane, (C) ethylene and (D) acetylene concentrations (ppm).

We further carefully compared the responses of the p-In₂O₃ sensor to C1–C3 hydrocarbons (methane, ethane, propane, ethylene and acetylene). We found that: (trend 1) for light alkanes, the response follows the order of propane > ethane > methane (Fig. 11A); and (trend 2) for C2 hydrocarbons, the response follows the order of ethylene > acetylene > ethane (Fig. 11B). The sensing responses originate from the chemical interaction between the surface of sensing material and the gas molecules to be detected.¹ For a given sensor, the different response values towards the different target gases is mainly attributed to the intrinsically chemical properties and reactivities of the tested gases. For C1–C3 alkanes, the homolytic C–H bond dissociation energy (i.e., the reactivity of the C–H bond) of propane, ethane and methane follows this same trend in the observed responses (i.e., trend 1).^36,40–42 Moreover, the trend 2 also correlates well with the chemical reactivity of alkanes, alkenes and alkynes.^43–47

Fig. 11 (A) Comparison of the responses of the p-In₂O₃ sensor towards 30 000 ppm methane, ethane and propane; (B) comparison of the responses of the p-In₂O₃ sensor towards 30 000 ppm ethane, ethylene and acetylene.

Conclusions

A template-free route is presented for the preparation of uniform porous In₂O₃ microspheres using indium glycerolate as the precursor for the first time. This porous material has been proven to be highly sensitive for the detection of C1–C3 hydrocarbons, and it is very promising to provide earlier warning against the explosion risk of hydrocarbon compounds. Our experimental results reveal that the porous structure (or large surface area) of In₂O₃ plays an important role in its gas-sensing performance, and the sensing response is also strongly dependent on the chemical reactivities of the gases to be detected. Future work will mainly include the extension of the synthetic method into making other inorganic nanomaterials with advanced functions or improved properties.

Acknowledgements

This work was supported by the NSFC (21371070, 21401066); the National Basic Research Program of China (2013CB632403); Jilin province science and technology development projects (20140101041JC, 20130204001GX); Graduate Innovation Fund of Jilin University (2014052).

Notes and references

1. M. E. Franke, T. J. Koplin and U. Simon, Small, 2006, 2, 36–50.
2. J. H. Lee, Sens. Actuators, B, 2009, 140, 319–336.
3. J. Su, X. X. Zou, Y. C. Zou, G. D. Li, P. P. Wang and J. S. Chen, Inorg. Chem., 2013, 52, 5924–5930.
4. M. Tiemann, Chem.–Eur. J., 2007, 13, 8376–8388.
5. J. Zhao, X. X. Zou, J. Su, P. P. Wang, L. J. Zhou and G. D. Li, Dalton Trans., 2013, 42, 4365–4368.
6. J. Zhang, S. Wang, M. Xu, Y. Wang, B. Zhu, S. Zhang, W. Huang and S. Wu, Cryst. Growth Des., 2009, 9, 3532–3537.
7. Z. Jing and J. Zhan, Adv. Mater., 2008, 20, 3761–3766.
8. M. Chen, Z. Wang, D. Han, F. Gu and G. Guo, J. Phys. Chem. C, 2011, 115, 12763–12773.
9. L. J. Zhou, Y. C. Zou, J. Zhao, P. P. Wang, L. L. Feng, L. W. Sun, D. J. Wang and G. D. Li, Sens. Actuators, B, 2013, 188, 533–539.
10. X. Liu, J. Zhang, L. Wang, T. Yang, X. Guo, S. Wu and S. Wang, J. Mater. Chem., 2011, 21, 349–356.
11. Z. Li, Q. Zhao, W. Zhao, W. Fan and J. Zhan, Nanoscale, 2011, 3, 1646–1652.
12. L. J. Zhou, C. Li, X. X. Zou, J. Zhao, P. P. Jin, L. L. Feng, M. H. Fan and G. D. Li, Sens. Actuators, B, 2014, 197, 370–375.
13. J. Li, H. Fan and X. Jia, J. Phys. Chem. C, 2010, 114, 14684–14691.
14. X. Wang, W. Liu, J. Liu, F. Wang, J. Kong, S. Qiu, C. He and L. Luan, ACS Appl. Mater. Interfaces, 2012, 4, 817–825.
15. Y. Qin, F. Zhang, Y. Chen, Y. Zhou, J. Li, A. Zhu, Y. Luo, Y. Tian and J. Yang, J. Phys. Chem. C, 2012, 116, 11994–12000.
16. J. Ma, J. Teo, L. Mei, Z. Zhong, Q. Li, T. Wang, X. Duan, J. Zhong and W. Zheng, J. Mater. Chem., 2012, 22, 11694–11700.
17. W. Thomas, W. Thorsten, S. Tilman, K. Claus-Dieter and T. Michael, Adv. Funct. Mater., 2009, 19, 653–661.
18. W. Thorsten, K. Claus-Dieter, M. Sara, M. Cesare, D. Nicola, L. Mariangela, N. Giovanni and T. Michael, Chem.–Eur. J., 2012, 18, 8216–8223.
19. J. Tu, N. Li, X. Lai, Y. Chi, Y. Zhang, W. Wang, X. Li, J. Xue and S. Qiu, Appl. Surf. Sci., 2010, 256, 5051–5055.
20. J. Zhao, T. Yang, Y. Liu, Z. Wang, X. Li, Y. Sun, Y. Du, Y. Li and G. Lu, Sens. Actuators, B, 2014, 191, 806–812.
21. X. Liu, R. Wang, T. Zhang, Y. He, J. Tu and X. Li, Sens. Actuators, B, 2010, 150, 442–448.
22. T. Hyodo, S. Furuno, E. Fujii, K. Matsuo, S. Motokucho, K. Kojio and Y. Shimizu, Sens. Actuators, B, 2013, 187, 495–502.
23. N. Du, H. Zhang, B. D. Chen, X. Y. Ma, Z. H. Liu, J. B. Wu and D. R. Yang, Adv. Mater., 2007, 19, 1641–1645.
24. H. Jiang, L. Zhao, L. Gai, L. Ma, Y. Ma and M. Li, CrystEngComm, 2013, 15, 7003–7009.
25. T. Takada, K. Suzuki and M. Nakane, Sens. Actuators, B, 1993, 13, 404–407.
26. M. Epifani, E. Comini, J. Arbiol, E. Pellicer, P. Siciliano, G. Faglia and J. R. Morante, J. Phys. Chem. C, 2007, 111, 13967–13971.
27. X. Xu, X. Li, W. Wang, B. Wang, P. Sun, Y. Sun and G. Lu, RSC. Adv., 2014, 4, 4831–4835.
28. P. S. khiabani, E. Marzbanrad, H. Hassani and B. Raissi, J. Am. Ceram. Soc., 2013, 96, 2493–2498.
29. Y. Li, J. Xu, J. Chao, D. Chen, S. Ouyang, J. Ye and G. Shen, J. Mater. Chem., 2011, 21, 12852–12857.
30. Z. Li, Y. Li, Y. Luan, J. Li and A. Song, CrystEngComm, 2013, 15, 1706–1714.
31. S. Elouali, L. G. Bloor, R. Binions, I. P. Parkin, C. J. Carmalt and J. A. Darr, Langmuir, 2012, 28, 1879–1885.
32. S. Wang, P. Wang, Z. Li, C. Xiao, B. Xiao, R. Zhao, T. Yang and M. Zhang, New J. Chem., 2014, 38, 4879–4884.
33. S. Wang, B. Xiao, T. Yang, P. Wang, C. Xiao, Z. Li, R. Zhao and M. Zhang, J. Mater. Chem. A, 2014, 2, 6598–6604.
34. I. Trotuş, T. Zimmermann and F. Schüth, Chem. Rev., 2014, 114, 1761–1782.
35. M. Vidal, W. J. Rogers, J. C. Holste and M. S. Mannan, Process Saf. Prog., 2004, 23, 47–55.
36. M. C. Carotta, A. Cervia, A. Giberti, V. Guidia, C. Malagu, G. Martinellia and D. Puzzovio, Sens. Actuators, B, 2008, 133, 516–520.
37. M. M. Tong, G. Q. Wu, J. F. Hao and X. L. Dai, Min. Sci. Technol., 2009, 19, 182–184; L. Feng, H. Vrubel, M. Bensimon and X. Hu, Phys. Chem. Chem. Phys., 2014, 16, 5917–5921.
38. J. Zhao, X. X. Zou, L. J. Zhou, L. L. Feng, P. P. Jin, Y. P. Liu and G. D. Li, Dalton Trans., 2013, 42, 14357–14360.
39. J. Zhao, Y. Zou, X. X. Zou, T. Bai, Y. P. Liu, R. Q. Gao, D. Wang and G. D. Li, Nanoscale, 2014, 6, 7255–7262.
40. R. H. Crabtree, Chem. Rev., 1995, 95, 987–1007.
41. S. Chakraborty, A. Sen and H. S. Maiti, Sens. Actuators, B, 2006, 115, 610–613.
42. B. K. Min and S. D. Choi, Sens. Actuators, B, 2005, 108, 119–124.
43. A. Páramo, A. Canosa, S. D. Le Picard and I. R. Sims, J. Phys. Chem. A, 2008, 112, 9591–9600.
44. S. G. Ard, J. J. Melko, J. A. Fournier, N. S. Shuman and A. A. Viggiano, J. Phys. Chem. A, 2013, 117, 10178–10185.
45. K. M. Ervin, S. Gronert, S. E. Barlow, M. K. Gilles, A. G. Harrison, V. M. Bierbaum, C. H. DePuy, W. C. Lineberger and G. B. Ellison, J. Am. Chem. Soc., 1990, 112, 5750–5759.
46. A. Holme, L. J. Sæthre, K. J. Børve and T. D. Thomas, J. Org. Chem., 2012, 77, 10105–10117.
47. R. Gómez-Balderas, M. L. Coote, D. J. Henry, H. Fischer and L. Radom, J. Phys. Chem. A, 2003, 107, 6082–6090.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra13763a

‡ The authors contributed equally to this work.

---