In₂O₃ nanostructures: synthesis and chlorobenzene sensing properties

Abstract

In this work, In₂O₃ nanorods and nanoparticles have been successfully prepared by solvothermal methods followed by annealing of the precursors at 300 °C. Experimental results show that the morphology and phase are strongly influenced by the solvents used. Moreover, we have developed gas sensors based on ultrathin In₂O₃ nanorods, which demonstrate excellent o-dichlorobenzene and chlorobenzene sensing properties, superior to those of nanoparticles with a similar size. It is expected that our as-synthesized In₂O₃ nanorods could potentially be applied in real-time monitoring of polychlorobenzene concentrations in the future.

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Introduction

Gas sensors are thought to be one kind of functional detector which could be used for the real-time detection of gases over a wide temperature range.^1–4 The performance of gas sensors is strongly dependent on the native composition, size and shape of the sensing materials.^5–7 To date, metal oxides such as Fe₂O₃,^5–8 NiO,^9–12 SnO₂,^13–16 CuO,^17–19 TiO₂,^20,21 Co₃O₄,^22,23 WO₃,^24–26 In₂O₃,^27–42 and so on, have been intensively studied as sensing materials. Among them, In₂O₃ is one of the most studied gas sensing materials, which is sensitive to a variety of gases (ethanol, H₂, CO, CH₃OH, NO_x, NH₃ and H₂S) due to its unique characteristics.^27–42 For example, some composites such as ZnO–In₂O₃ nanofibers,³⁰ Bi₂O₃-core/In₂O₃-shell nanorods,³¹ SnO₂/In₂O₃ hetero-nanofibers,³² Pt and Pd nanoparticle decorated In₂O₃ flower-like nanobundles,³³ etc., have shown excellent gas sensing performance for various gases. In addition, different In₂O₃ nanostructures including nanorods,^34,35 nanowires,^36–38 hollow nano- and microspheres,^39–41 other structures,⁴² etc., have been reported to display better gas sensing performance. Among these nanostructures, one-dimensional (1D) nanostructures, nanorods or nanowires, are promising for us to detect toxic gases with satisfactory performance.^6,15 Therefore, obtaining 1D nanostructures, especially with small sizes, is of extreme importance for us to obtain high-performance sensing materials.

In the past decade, the preparation of In₂O₃ nanostructures has attracted a great deal of attention due to their wide range of applications.^{27–29,43–46} With regard to 1D nanostructures, there are also many reports on synthetic techniques, such as laser-ablation chemical vapor deposition (CVD),⁴⁷ CVD,^48–50 Au-catalyzed vapor–liquid–solid (VLS),⁵¹ hydrogen assisted thermal evaporation,⁵² phase transformation from InOOH to In₂O₃ nanowires through lanthanide doping,⁵³ thermal evaporation with an Au catalyst,³⁸ thermal evaporation and condensation,⁵⁴ epitaxial growth,⁵⁵ electrospinning,³⁶ and so on. However, most of these methods have drawbacks such as needing high temperatures, noble metal catalysts, substrates, high amounts of energy, etc. 1D In₂O₃ nanostructures with diameters below 5 nm have not often been obtained by facile methods.⁴⁷ Therefore, it is still a big challenge for us to obtain ultrathin 1D In₂O₃ nanostructures by facile methods.

In this work, we report a facile solvothermal route to prepare ultrathin InOOH nanorods and then convert them into the corresponding In₂O₃ nanorods. We also report a chlorobenzene and o-dichlorobenzene sensor with high sensitivity and a rapid response using single crystalline In₂O₃ nanorods as the active material. The results indicate that the enhanced chlorobenzene and o-dichlorobenzene sensing performance can be attributed to the rod-like morphology due to the advantages of the effective adsorption and rapid diffusion of the chlorobenzene and o-dichlorobenzene molecules.

Experimental

Synthesis of In₂O₃ nanostructures

In a typical synthesis of In₂O₃ nanorods, 2 mmol of In(NO₃)₃·5H₂O was dissolved into 35 ml of diethylenetriamine, and then transferred to a 45 ml Teflon-lined stainless steel autoclave at 150 °C for 20 h, followed by cooling the autoclave down to room temperature naturally. White precipitate was collected by centrifugation, washed with distilled water, absolute alcohol and acetone several times, followed by drying at room temperature for 24 h. Finally, yellow In₂O₃ nanorods were obtained through annealing the above InOOH sample in air at 300 °C for 2 h with a heating rate of 2 °C min⁻¹, and cooling in air.

The process and parameters of synthesizing In₂O₃ nanoparticles were the same as for the InOOH and In₂O₃ nanorods, except the solvent was changed to 5 ml of distilled water and 30 ml of ethanolamine.

Characterization

The morphology and structural characteristics were observed using X-ray diffraction (XRD, Rigaku D/max 2500 diffractometer), scanning electron microscopy (SEM, Hitachi S4800), transmission electron microscope (TEM, JEOL 2010 with an accelerating voltage of 200 kV).

Gas sensing measurements

The fabrication and testing principles of the gas sensor are similar to those described in our previous reports.^5,6 Firstly, the as-synthesized In₂O₃ samples were mixed with terpineol to form a paste and then coated onto the outside surface of an alumina tube with a diameter of 1 mm and a length of 5 mm. A platinum coil through the tube was employed as a heater to control the operating temperature. To improve their stability and repeatability, the gas sensors were aged at 300 °C for 10 h in air. Here, the sensing properties of the sensors were measured by a NS-4003 series gas sensing measurement system (China Zhong-Ke Micro-nano IOT, Internet of Things, Ltd.). The relative humidity (RH) was about 45%. The response and recovery times were defined as the time required for a change of the resistance to reach 90% of the equilibrium value after injecting and removing the detected gas, respectively.

InOOH nanorods were synthesized through the hydrolysis of In(NO₃)₃·5H₂O in diethylenetriamine under solvothermal conditions at 150 °C. The XRD pattern of the as-obtained orthorhombic InOOH nanorods can be well matched with the standard card (JCPDS: 17-0549), as shown in Fig. 1a. The morphology of the as-obtained InOOH nanorods is shown in Fig. 2a and b, which show that the nanorods have a diameter of about 4 nm and a length ranging from 15 to 30 nm. The chemical route to InOOH nanorods in diethylenetriamine can be suggested as eqn (a) and (b):

In³⁺ + 3H₂O → In(OH)₃ + 3H⁺ (a)

In(OH)₃ → InOOH + H₂O (b)

Fig. 1 XRD patterns of the as-obtained nanorods: (a) InOOH, and (b) In₂O₃.

Fig. 2 TEM images of the as-obtained nanorods: (a and b) InOOH, and (c and d) In₂O₃.

The growth of the InOOH nanorods strongly depends on the solvent used, which indicates that diethylenetriamine facilitated the growth of InOOH into 1D nanostructures in our experiment. In addition, InOOH easily grows into 1D nanostructures with 1D orthorhombic crystal growth habits.^56,57

In₂O₃ nanorods could be obtained with the same rodlike shape upon annealing of the corresponding InOOH nanorods at 300 °C, which implies that the In₂O₃ nanorods are stable at this temperature. The crystalline structure and morphology of the corresponding In₂O₃ nanorods are given in Fig. 1b and 2c and d, respectively. In Fig. 1b, the XRD pattern of the In₂O₃ nanorods can be well matched with the standard XRD data file of In₂O₃ (JCPDS: 22-0336). It should be noted that the as-obtained In₂O₃ nanorods are highly crystalline, which can be confirmed from the clear crystalline fringes of the nanorods. Moreover, these lattice distances can be well matched with rhombohedral In₂O₃ (r-In₂O₃); the distance 3.97 Å corresponds to the lattice plane {012}, 2.833 Å to {104}, and 2.753 Å to {110}.

When aqueous ethanolamine was used as a solvent while the other conditions were kept constant, indium oxide nanocubes were produced. As shown in Fig. 3a, one finds that the as-synthesized nanocubes were composed of mixed phases of In₂O₃ (JCPDS: 22-0336) and InOOH (JCPDS: 17-0549). The coexistence of In₂O₃ and InOOH indicates that some of the rhombohedral In₂O₃ (r-In₂O₃) could originate from the as-formed InOOH. A low-magnification TEM image (Fig. 4a) shows that the sample is composed of many uniform nanocubes. In Fig. 4b, one clearly sees that the nanoparticles have a size of 5–8 nm.

Fig. 3 XRD patterns of the as-obtained nanocubes: (a) indium oxide, and (b) In₂O₃.

Fig. 4 TEM images of the as-obtained indium oxide nanomaterials: (a and b) nanocubes, and (c and d) nanoparticles.

Interestingly, after annealing the mixture at 300 °C in air, pure cubic In₂O₃ (c-In₂O₃) was obtained, together with the InOOH and r-In₂O₃ being transformed to c-In₂O₃. The XRD pattern of the as-obtained In₂O₃ (Fig. 3b) could be well matched with the standard JCPDS card (06-0416). In addition, one finds that the regular nanocubes became spherical nanoparticles at higher temperature. Low and high magnification TEM images (Fig. 4c and d) clearly show that our In₂O₃ consists of uniform nanoparticles. The lattice distances (4.13 Å, 2.921 Å, 1.788 Å) can be matched to the crystal planes {211}, {222} and {440}, respectively.

Due to the smaller size of the In₂O₃ nanostructures, it is expected that the as-synthesized In₂O₃ nanostructures will show excellent gas sensing performance in gas sensors. The sensor sensitivity S, which is defined as S = R_a/R_g, where R_a is the resistance of the sensor in air and R_g is the resistance of the sensor in the target gas, is an important factor in gauging the ability of a sensor. Fig. 5 shows the sensitivity–temperature profiles of the two sensors of In₂O₃ nanorods and nanoparticles to 30 ppm of o-dichlorobenzene. The results indicate that the sensitivities of both the nanorods and nanoparticles are enhanced with an increasing temperature. Moreover, it is obvious that the In₂O₃ nanorod sensor has a higher sensitivity than the In₂O₃ nanoparticle sensor over the whole temperature range from 230 to 430 °C. The sensitivities for the In₂O₃ nanorod sensor to 30 ppm o-dichlorobenzene are 1.2 at 230 °C, 1.4 at 265 °C, 1.7 at 300 °C, 2.4 at 350 °C and 3.3 at 430 °C. Therefore, from the above analysis, in the following section the temperature of 430 °C was chosen for further sensing analysis.

Fig. 5 The sensitivity of the In₂O₃ sensors towards 30 ppm o-dichlorobenzene at different working temperatures: (a) nanorods, and (b) nanoparticles.

Fig. 6 shows the dynamic responses of the In₂O₃ nanorods and nanoparticles at an operating temperature of 430 °C for various concentrations of o-dichlorobenzene. In Fig. 6a and b, one can see that the response amplitude of the nanorod and nanoparticle sensors increases with increasing o-dichlorobenzene concentration. In Fig. 6c, the corresponding sensitivities of the In₂O₃ nanorod sensor to o-dichlorobenzene are found to be 2.2 for 10 ppm, 3 for 30 ppm, 5.6 for 50 ppm, 6.5 for 100 ppm, 7.7 for 200 ppm, 11.8 for 500 ppm and 18 for 1000 ppm, which are superior to those of the In₂O₃ nanoparticle sensor. The corresponding sensitivities of the In₂O₃ nanoparticle sensor are 1.6, 2.3, 2.9, 3.2, 3.5, 5 and 6.3, respectively. The difference in o-dichlorobenzene sensing performance between the r-In₂O₃ nanorods and c-In₂O₃ nanoparticles might be attributed to shape, size, or phase. o-Dichlorobenzene is a typical polar molecule, so its adsorption and desorption are a little sluggish on the In₂O₃ surface in our study.^58,59

Fig. 6 The real-time response curve of the In₂O₃ sensor to o-dichlorobenzene with increasing concentration at a working temperature of 430 °C, using nanorods (a) and nanoparticles (b) as sensing materials; (c) a comparison of sensor sensitivities to different o-dichlorobenzene gas concentrations for nanorods and nanoparticles, respectively.

In addition, we also investigated the corresponding sensitivities of the In₂O₃ nanorod and nanoparticle sensors to chlorobenzene. The response curve of the In₂O₃ sensor increased with an increase in chlorobenzene concentration, as shown in Fig. 7a and b. In Fig. 7c, the In₂O₃ nanorod sensor has a sensitivity of 2.8 for 10 ppm, 3.2 for 30 ppm, 5.4 for 50 ppm, 6.6 for 100 ppm, 10.5 for 200 ppm, 14.9 for 500 ppm and 20 for 1000 ppm, which are superior to those of the In₂O₃ nanoparticle sensor. The corresponding sensitivities for the In₂O₃ nanoparticle sensor are 2.4, 2.7, 4.3, 4.4, 4.8, 6.9 and 9. Obviously, the In₂O₃ sensor has better sensitivity to chlorobenzene than o-dichlorobezene.

Fig. 7 The real-time response curve of the In₂O₃ sensor to chlorobenzene with increasing concentration at a working temperature of 430 °C, using nanorods (a) and nanoparticles (b) as sensing materials; (c) a comparison of sensor sensitivities to different chlorobenzene concentrations for nanorods and nanoparticles, respectively.

The mechanism of In₂O₃ sensitivity to o-dichlorobenzene and chlorobenzene is further explained. According to the classical electron depletion theory, the change of resistance is dependent on species of chemisorbed oxygen (O₂⁻, O²⁻ and O⁻) on the surface,⁶ which would cause electron depletion, and consequently increase the resistance of the sensor. When chlorobenzene or o-dichlorobenzene gases are present at a certain temperature, they would be oxidized by these chemisorbed oxygen species on the surface of the sensor. This will then lead to an increase in the number of electrons in the conduction band, and an increase in sensor conductivity. The equations involved in the response process are as follows:

O₂ + e⁻ → O₂⁻ (a)

O₂⁻ + e⁻ → 2O⁻ (b)

O⁻ + e⁻ → O²⁻ (c)

C₆H₄Cl₂ → C₆H₄Cl₂⁺ + e⁻ (d)

O₂⁻ → O₂ + e⁻ (e)

Moreover, owing to higher reaction activity, the In₂O₃ sensor has a higher sensitivity to chlorobenzene. These results also further confirm that our proposed mechanism is reasonable. The electron transfer on the In₂O₃ sensor surface is clearly shown in Scheme 1.

Scheme 1 The electron transfer process on the In₂O₃ sensor surface.

Conclusions

In summary, a new solvothermal route has been developed to synthesize ultrathin InOOH nanorods, which could be further converted into In₂O₃ nanorods through calcination at a high temperature. Furthermore, it was found that the gas sensing performance of the In₂O₃ nanorods for o-dichlorobenzene could be improved in comparison to nanoparticles, as a result of their high sensitivity and rapid response to o-dichlorobenzene due to the advantages of the effective adsorption and rapid diffusion of the o-dichlorobenzene molecules. The as-synthesized ultrathin In₂O₃ nanorods could be applied to the on-line monitoring of dioxins in the future.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 21103046, 21373081, 51302079 and 61376073) and the Young Teachers' Growth Plan of Hunan University (Grant no. 2012-118).

Notes and references

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