Synthesis and NO₂ sensing properties of indium oxide nanorod clusters via a simple solvothermal route

Abstract

In this work, a low-cost and environmentally friendly solvothermal route to the synthesis of indium oxide nanorod clusters was described in the presence of sodium chlorate and urea. The morphologies and structures of the as-prepared samples were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The results clearly revealed that the as-prepared indium oxide was composed of nanorods with a diameter of about 15 nm. The gas sensing properties of the as-prepared indium oxide samples were tested towards various gases. The indium oxide nanorod cluster based sensor showed high response and good selectivity toward NO₂ at an operating temperature of 150 °C, giving a response of about 41 to 500 ppb.

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1. Introduction

With the development of industry and sharp increase of car ownership, more and more air pollutants are discharged into the environment. These pollutants affect people's daily life and production and endanger human personal and property safety. Increasing requirements for environmental monitoring, medical diagnosis, air quality control, and detection of explosive and toxic gases have led to growing interest in high-performance gas sensors.^1–3 Nitrogen dioxide (NO₂), as a typical air pollutant mainly comes from the exhaust gases of combustion processes, which severely affects the respiratory system of human beings and animals.^4–6 Micro air pollutant NO₂ for fast, accurate monitoring has a very important environmental significance. In recent years, the rapid development of networking technology to build a new environmental monitoring network provides an opportunity, as a new gas sensor research and development of monitoring nodes and therefore by the people concerned.

Over the past decades, the design and development of gas sensors with high sensing performance has been a research hotspot. Thanks to the advances in nanotechnologies and synthetic methods, various kind of sensing materials, such as metal-oxide semiconductors,^7–10 polymers,^11,12 carbon nanotubes,^13–15 active molecular plasmonics¹⁶ and their composite materials,¹⁷ have been developed and applied in gas sensors. Especially various kinds of metal oxides, such as ZnO, α-Fe₂O₃, WO₃, In₂O₃, SnO₂, NiO, etc. have been extensively investigated as sensing materials owing to their superior stability, low cost, and simplicity in preparation.

Metal oxides are the basis of functional materials that have tunable properties and important technological applications. Among them, as a very important wide-band-gap n-type semiconductor, In₂O₃ has been extensively studied for various applications, including window heaters, solar cells, liquid-crystal displays, and gas sensors for detection of CO, O₃, Cl₂, CO₂, H₂S, VOCs and NO₂.¹⁸ Among most of the potential applications, the morphology and structure of the nanomaterials will undoubtedly play the pivotal role in determining their properties. The unique structures may show new properties and promising applications in many fields. Thus, over the past decades, several groups have synthesized In₂O₃ micro- and nanostructures with different morphologies by various preparation routes, including thermal evaporation,^19,20 hydrothermal reaction,²¹ chemical coprecipitation²² and sol–gel method.²³ Therefore, researchers are actively engaged in developing indium oxide nanostructures with different sizes and shapes in various applications, such as hollow microspheres, nanowires, nanobelts, nanotubes, nanocubes, nanofibers, nanosheets and complex hierarchical structures constructed with nanoscale building blocks.

Herein, we report a facile method for the preparation of In₂O₃ nanorod clusters by a simple solvothermal process. The diameter of In₂O₃ nanorod clusters was about 15 nm. Gas-sensing properties of the as-obtained In₂O₃ products are also investigated. The sensor based on the as-obtained In₂O₃ exhibited superior sensing performance to low concentration NO₂ at 150 °C.

2. Experimental

2.1. Synthesis and characterization of In₂O₃ nanorod clusters

All the reagents (analytical-grade purity) were used without any further purification. In a typical synthesis, 0.381 g of In(NO₃)₃·4.5H₂O, some sodium chlorate and urea were added to 36 mL mixture of propyl alcohol and water (1 : 1 v/v). After being stirred and ultrasonically treated alternately, the mixture solution was transferred to a 45 mL Teflon-lined stainless steel autoclave, sealed tightly, and maintained at 160 °C for 12 h. After the autoclave was cooled to room temperature naturally, the precipitates were washed with deionized water and absolute ethanol for several times using centrifuge, and then dried at 80 °C for 1 day. The precipitates were loaded into an alumina boat, which was placed in a furnace. For the heat treatment, the samples were calcined at 500 °C for 2 h with a heating rate of 2 °C min⁻¹. The calcined products were then collected for further analyses.

X-ray power diffraction (XRD) analysis was conducted on a Rigaku D/max-2500 X-ray diffractometer with Cu Kα1 radiation (λ = 1.540 Å) in the range of 20–70°. The mean crystallite size was calculated using the Debye–Scherer formula, D = 0.89λ/(β cos θ), where λ is the X-ray wavelength, θ is the Bragg diffraction angle and β is the peak width at half maximum. The specific surface area was estimated using the Brunauer–Emmett–Teller (BET) equation based on the nitrogen adsorption isotherm obtained with a Micromeritics Gemini VII apparatus (Surface Area and Porosity System). Samples were degassed under vacuum at 200 °C for 4 h prior to the measurements. The pore size distribution was determined with the Barrett–Joyner–Halenda (BJH) method applied to the desorption branch of adsorption–desorption isotherm. Field emission scanning electron microscopy (FESEM) images were recorded on a JEOL JSM-7500F microscope operating at 15 kV. Transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and high-resolution transmission electron microscopy (HRTEM) measurements were obtained on a JEOL JEM-2100 microscope operated at 200 kV.

2.2. Fabrication and measurement of sensor

Gas sensors were fabricated as follows: the as-obtained powder was mixed with the deionized water in order to make a paste, which was coated onto an alumina tube (4 mm in length, 1.2 mm in external diameter, and 0.8 mm in internal diameter, attached with a pair of gold electrodes) by a small brush to form a thick film. After drying at room temperature for 30 min, the sensing devices were sintered at 500 °C for 2 h. A pair of gold electrodes was installed at each end of the ceramic tube before it was coated with the paste, and each electrode was connected with two Pt wires. A Ni–Cr heating wire was inserted into the tube to form an indirect-heated gas sensor. The structure of the sensor is shown in Fig. 1. The electrical properties of the sensor were measured by an RQ-2 series Intelligent Test Meter (China).

Fig. 1 (a) Schematic image of the In₂O₃ sensor and (b) ceramic tube images (without gas sensing materials), (c) structure of the gas sensor.

3. Results and discussion

3.1. Structural and morphological characteristics of the as-obtained In₂O₃

X-ray powder diffraction (XRD) analysis was performed to investigate the crystal phases of the In₂O₃ products. It can be seen from Fig. 2 that all the diffraction peaks could be very well indexed to the standard cubic In₂O₃, which was consistent with the standard data file (JCPDS file no. 06-416), with space group Ia3̄ (no. 206) and lattice parameters of a = 10.118 Å. No diffraction peaks from any other impurities were observed, indicating the high purity of the products. No diffraction peaks from any other impurities were observed, indicating the high purity of the products. Compared with those of the bulk material, the peaks were relatively broadened, which demonstrated that the In₂O₃ had a small crystal size.

Fig. 2 X-ray diffraction patterns of as-prepared samples.

The morphology of the samples was investigated by field emission scanning electron microscopy (FESEM). Fig. 3 shows typical FESEM images of the sample In₂O₃ at different magnification. The low magnification FESEM image (Fig. 3a) clearly displayed that the products are composed of nanorod clusters. No other morphologies can be found in Fig. 3a, which indicated the high uniformity of the as-obtained products. The detailed morphology information about In₂O₃ is presented in an enlarged-magnification FESEM (Fig. 3b–d), it can be observed that the diameter of nanorod clusters was about 15 nm.

Fig. 3 FESEM images of the as-synthesized In₂O₃: (a) a panoramic and (b) an enlarged of a part of samples. (c and d) High-magnification FESEM image of In₂O₃ nanorod clusters.

Further detailed structural analysis of products was carried out using TEM. Fig. 4a shows the TEM image of representative In₂O₃ nanorod clusters. It can be seen that the size and shape of product were similar to those of the FESEM observations. A high-magnification TEM (Fig. 4b) image presents that the diameter of these nanorods was about 15 nm. The selected-area electron diffraction (SAED) pattern of an individual In₂O₃ nanorod (Fig. 4c) confirms that the as-synthesized products were polycrystalline in structure. The high-resolution transmission electron microscopy (HRTEM) image (inset of Fig. 4b) showed fringe distance of 0.292 nm, corresponding to the lattice distances of the (222) plane of cubic In₂O₃. Fig. 4d shows the high angle annular dark field (HAADF) image of the In₂O₃ sample and Fig. 4e and f show the corresponding EDX elemental mapping of In and O, respectively. It was observed that In and O are uniform and homogeneous.

Fig. 4 Typical TEM images (a and b) of In₂O₃ nanorod clusters. The inset of (b) is the HRTEM images. The corresponding SAED (c) pattern. (d) High angle annular dark field STEM images of In₂O₃ nanorod clusters, (e) EDX elemental mapping of In, (f) EDX elemental mapping of O.

3.2. Gas-sensing properties for NO₂

For semiconductor oxide sensors, the working temperature range is an important functional characteristic. Fig. 5 shows the correlations of the response and response time of the sensor based on the In₂O₃ nanorod clusters to 0.5 ppm NO₂ with the operating temperature. It revealed that the response curves (S = R_g/R_a; R_a: resistance in dry air; R_g: resistance in target gas) are obviously different at different operating temperatures and the response time decreases with increasing operating temperature. For the sensor using the In₂O₃ nanorod clusters, the response to 0.5 ppm NO₂ increases when the operating temperatures varies and reaches a maximum of 41 at about at 150 °C, and then gradually decreases.

Fig. 5 Correlations between the gas response and response time to 0.5 ppm NO₂ and the operating temperature for the sensor using the as-obtained In₂O₃.

This tendency indicates that fast response can be obtained through increasing the operating temperature. However, when the operating temperature is too high, the gas response decreases sharply. The decrease of the response at higher temperatures may be attributed to the decrease in number of active sites for the adsorption of NO₂. Since at high temperature larger amount of oxygen molecules dissociate and adsorb on the active sites, the free active sites for the adsorption of NO₂ molecules can be sharply reduced the response to NO₂ becomes smaller. Another possibility may be that at such higher temperature the rate of adsorption is lower as compared to desorption.^23–25 Accordingly, considering two aspects including the larger gas response and faster response speed of the sensor to 0.5 ppm NO₂, the optimum operating temperature for the sensor using the as-obtained In₂O₃ is believed to be 150 °C, which is applied in all the investigations hereinafter. The response transients of the In₂O₃ nanorod clusters sensor to 0.5 ppm NO₂ was measured at 150 °C (Fig. 6), the response time and recovery time were about 50 s and 30 s, respectively. The four reversible cycles of the response curve indicated a stable and repeatable response characteristic, as shown in the inset of Fig. 6. It can be observed that the average response was 41 to 500 ppb NO₂.

Fig. 6 Response transients of the sensor to 0.5 ppm NO₂ at 150 °C, the inset displaying four periods of response curve.

The dynamic response resistances of the sensors based on In₂O₃ nanorod clusters (Fig. 7a) to different NO₂ concentration were investigated at 150 °C. The resistance of the sensor increases upon exposure NO₂, whereas it decreases upon the removal of NO₂. Furthermore, the response of the sensor to NO₂ increased with the increasing of the NO₂ concentration. The sensor can detect 50 ppb NO₂, and the response is about 4. The responses to 0.1, 0.2, 0.3, 0.4 and 0.5 ppm NO₂ were about 7, 14, 20, 29 and 41, respectively. Even in ppb level of NO₂ concentration (e.g. 50 ppb), the In₂O₃ nanorod clusters sensor shows an acceptable response from the view of the practical application. Many models have been proposed to describe the sensing behavior of metal oxides. When the process is mainly controlled by diffusion, the response of semiconductor oxide type gas sensor can be empirically represented as S = 1 + A_g(P_g)^β, where P_g is the target gas partial pressure, which is directly proportional to the gas concentration, A_g is a prefactor, and β is the exponent on P_g.^26,27 Generally, the exponent β has an ideal value of either 0.5 or 1, which is derived from the surface interaction between chemisorbed oxygen and reducing gas to n-type semiconductor.^28,29 Fig. 7b shows a chart of the sensor response versus the NO₂ concentration. The linear fitting was quite good, and the correlation coefficient R of the sensor fit curve was 0.998. A comparison between the sensing performances of the sensor and the literature reports^30–36 is summarized in Table 1. From the table, it can be observed that the sensor based on the nanorod clusters In₂O₃ has a correspondingly higher gas response and lower working temperature.

Fig. 7 (a) Responses to different concentrations of NO₂ for the sensor using the In₂O₃ at 150 °C in the range of 0.05–0.5 ppm. (b) Linear fit curve of the response of the sensor to the concentration of NO₂.

Table 1 Comparison of gas-sensing characteristics of sensing material in present work and those reported in the literatures

Sensor material	NO2 concentration	Working temperature	Response	Reference
WO3 nanosheets	0.1 ppm	150 °C	∼7	30
WO3 thin films	10 ppm	150 °C	∼57	31
In2O3 microspheres	20 ppm	250 °C	∼36	32
Zn-Doped In2O3	50 ppm	300 °C	821.7	33
Porous In2O3	50 ppm	250 °C	164	34
Fe-Doped In2O3	1 ppm	150 °C	71	35
In2O3 nanofibers	100 ppm	300 °C	1.8	36
In2O3 nanorod clusters	500 ppb	150 °C	41	Present work

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Selectivity is an important parameter for gas sensor. Fig. 8 showed the cross-sensitivities of the sensor using the In₂O₃ nanorod clusters to various gases, including NH₃, O₃, H₂S, CO, SO₂, Cl₂, and C₂H₅OH. It is clear that the In₂O₃ sensor exhibits the largest response to NO₂ among the tested gases. Such result indicates that the sensor using the In₂O₃ nanostructure exhibits an excellent selectivity to NO₂ against the other tested gases at the working temperature of 150 °C. For most semiconducting oxide type gas sensors, the change in resistance is primarily caused by the chemical adsorption and reaction of the gas molecules on the surface of the sensing material. The sensitivity of indium oxide towards NO₂ is generally attributed to an adsorption effect. Indium oxide surfaces tend to strongly adsorb NO₂, mostly as nitrato (NO₃) species, which replace the oxygen at the surface, some of the NO₂-related nitrato adsorbates are very stable with high binding energies. By first principles calculations,^37,38 a high net charge transfer from the semiconductor to these adsorbates was determined, thus explaining the pronounced conductivity drop in the presence of NO₂. In this work, this special In₂O₃ nanorod clusters will provide offer abundant active sites for diffusion, chemisorptions and reactions of NO₂, then achieve high sensitivity. The more detailed reason and qualitative explanation need further investigation.

Fig. 8 Cross-responses of the sensor to various test gases at 150 °C.

4. Conclusion

In summary, In₂O₃ nanorod clusters had been successfully synthesized through a simple one-step solution route combined with a subsequent calcining process. Field emission scanning electron microscopic and transmission electron microscopic results demonstrate that the diameter of nanorod clusters was about 15 nm. The gas sensing properties of sensors based on as-synthesized In₂O₃ towards NO₂ were investigated. The sensor fabricated from these hierarchical structures exhibits excellent NO₂ sensing properties. The sensor response is about 41 to 500 ppb NO₂ at 150 °C. These results suggest that our sensor have potential application in NO₂ detection.

Acknowledgements

This work is supported by the National Nature Science Foundation of China (No. 61327804, 61473132, 61304242, 11504188 and 61474057), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (No. C20150029). Natural Science Foundation of Nanyang Normal University (ZX2015003, ZX2015006), Program for Chang Jiang Scholars and Innovative Research Team in University (No. IRT1017) and “863” High Technology Project (2013AA030902).

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