A simple grinding-calcination approach to prepare the Co₃O₄–In₂O₃ heterojunction structure with high-performance gas-sensing property toward ethanol

Abstract

The development of gas sensing devices with high sensitivity, good selectivity and excellent stability is becoming increasingly important since toxic or harmful gases are a threat to human health. Herein, we report a simple grinding-calcination method to prepare high-performance gas sensing materials based on a novel Co₃O₄–In₂O₃ heterojunction structure. Particularly, morphological and structural analyses indicate that the n-type In₂O₃ and p-type Co₃O₄ semiconductors are successfully combined and form a stable heterojunction structure through only a simple grinding-calcination process, in which electrons transfer from n-type In₂O₃ to p-type Co₃O₄ and then combine with holes belonging to Co₃O₄ nanoparticles, which can explain the formation mechanism of the electron depletion layer or the unique heterojunction structure. Interestingly, the sensing materials based on the Co₃O₄–In₂O₃ heterojunction exhibit excellent sensing properties to ethanol. This enhanced sensing performance can be attributed to the electron depletion layer formed at the interface between n-type In₂O₃ and p-type Co₃O₄. Particularly, the sensing device based on the Co₃O₄–In₂O₃ heterojunction structure with 1 wt% Co₃O₄ (labeled Co₃O₄–In₂O₃ (1%)) in the composite system shows a very high gas response to ethanol (approximately 62.13 at 240 °C, which is 1.36 times higher than that of pure In₂O₃ and 11.4 times higher than that of pure Co₃O₄). Moreover, the Co₃O₄–In₂O₃ (1%) sensing device shows an extremely low detection limit to ethanol (the gas response value can reach up to 4.4 to 5 ppm of ethanol, which is 1.43 times higher than that of pure In₂O₃). Furthermore, the fast response and recovery time (42 and 92 s, respectively), high selectivity and high stability presented by the Co₃O₄–In₂O₃ heterojunction display its great potential in the design of excellent gas sensing materials for practical gas detection.

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Introduction

As is known, environmental pollution is a threat to human health. With the increasing concerns of controlling air pollution and detection of noxious gas, gas sensing devices have been extensively studied in recent years. To date, various materials have been used for the fabrication of sensing devices, such as metal–organic complexes,¹ graphene oxide,² and metal oxides.³ Among these, metal oxide semiconductors, such as SnO₂,^4–6 ZnO,^7–9 In₂O₃,^10,11 and Co₃O₄,^12,13 have attracted significant interest due to their high sensitivity, low detection limits, high stability and low cost.^14,15

Indium oxide (In₂O₃) is an important n-type semiconductor in the abovementioned metal oxide semiconductors. It has been reported to exhibit high sensitivity, selectivity, and stability to many different types of volatile organic compounds (VOCs) or noxious gases.^16–20 In particular, sensing devices based on the In₂O₃ semiconductor have great advantages in detecting ethanol owing to its wide bandgap (E_g = 3.55–3.75 eV).^21–23 In order to further improve the sensing performance of traditional materials, a variety of methods have been employed to improve the sensing properties of gas sensing devices based on the In₂O₃ semiconductor in the last few years. For example, Gai et al. prepared nitrogen-doped indium oxide nanocrystals, which showed high sensing properties toward ethanol.²⁴ Wang et al. synthesized novel Ag-loaded In₂O₃ hierarchical nanostructures, which presented greatly enhanced sensing performances toward methanal.²⁵ Moreover, Singh et al. found that the sensitivity towards different reducing gases could be improved by combining In₂O₃ nanowires and ZnO shell layers.²⁶ However, when it comes to practical application, there are still some deficiencies. For example, much more factors that refer to low detection limits, simplicity in the manufacture process, and low cost should be taken into consideration in the development of practical gas sensing devices. Therefore, it is extremely essential to develop more efficient systems to help meet the growing demands of fabricating high-performance sensing devices based on the In₂O₃ semiconductor.

In fact, the concept of combining n- and p-type semiconductors to fabricate gas sensing materials has been widely accepted. Gas sensing properties would be improved because of the synergistic effect of n- and p-type semiconductors.^27–29 As an important p-type semiconductor, cobaltosic oxide (Co₃O₄), which has a bandgap of 2.2 eV,¹⁵ can form a stable heterojunction with n-type In₂O₃. Essentially, the Co₃O₄–In₂O₃ heterojunction structure has been reported in the past few years.^30–33 However, to date, to the best of our knowledge, few studies about gas sensing devices based on Co₃O₄–In₂O₃ heterojunction structure have been reported. Furthermore, many of the reported synthetic processes of the heterojunction structure are complicated. Therefore, the development of a convenient method for the preparation of the Co₃O₄–In₂O₃ heterojunction structure with excellent gas-sensing properties is highly desirable.

Herein, we report a simple method to synthesize Co₃O₄–In₂O₃ nanoparticles with a heterojunction structure. This novel gas sensing material has been confirmed to have superior sensing properties for ethanol. Moreover, the high response values of the sensing devices assembled in this study can better satisfy modern detection processes than the sensing devices based on the pure In₂O₃ semiconductor. Furthermore, some other superior performances such as low detection limits, short response and recover times, and high selectivity have been discovered. In general, sensing devices based on the Co₃O₄–In₂O₃ heterojunction structure are more accordant with practical application compared with similar materials. We believe that the unique heterojunction structure has great advantages in gas detection.

Experimental

Synthesis of Co₃O₄–In₂O₃ heterojunction structure

All chemicals used in the experiments were of analytic grade and used without further purification. Co₃O₄–In₂O₃ nanoparticles with a heterojunction structure were fabricated via a grinding-calcination process using In(NO₃)₃·4.5H₂O and Co(NO₃)₂·6H₂O as the starting materials. In a typical procedure, 2 mmol In(NO₃)₃·4.5H₂O was transferred to an agate mortar with different contents of Co(NO₃)₂·6H₂O (0.5, 1, 2, 5 and 10 wt% mass of In(NO₃)₃·4.5H₂O). The mixture was ground for 30 min to mix the two reagents evenly. Next, the obtained mixture was calcined at 450 °C for 2 h in air at a heating rate of 1 °C min⁻¹ to obtain the Co₃O₄–In₂O₃ heterojunction structure. Similarly, pure In₂O₃ and Co₃O₄ nanoparticles were each fabricated through a similar procedure for further analyses.

Characterization

Powder X-ray diffraction (XRD) analyses were performed on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.54 Å). Raman measurements were conducted on a Renishaw Raman microscope with a 514.5 nm wavelength incident laser. The elemental compositions of the samples were analyzed on a PHI QUANTERA II X-ray photoelectron spectrometer (XPS), using monochromatic Al Kα radiation as the excitation source (energy resolution < 0.60 eV). The morphologies of the as-obtained products were observed on a transmission electron microscope (TEM, JEOL JEM-2100) and scanning electron microscope (SEM, JEOL JSM-7001F). Thermogravimetric analysis (TGA) was carried out using a DTG-60 (Shimazdu Corporation, Japan) in air, where the samples were heated from 25 °C to 450 °C at a heating rate of 20 °C min⁻¹.

Fabrication and measurement of gas sensors

The gas-sensing properties of the as-prepared Co₃O₄–In₂O₃ heterojunction structure were determined on a WS-30A measuring system (Winsen Electronics Co. Ltd.) under static testing conditions. The detailed fabrication and testing processes of the sensing devices are as follows. Firstly, the as-prepared powder was mixed with several drops of deionized water to form a uniform slurry through slight milling. The resulting paste was then coated onto a ceramic tube (1 mm in diameter and 4 mm in length), which was positioned with gold electrodes and platinum conducting wires. A Ni–Cr heating wire was put through the ceramic tube to supply different operating temperatures, which could be controlled in the range of 100 °C to 400 °C by tuning the heating voltage. Finally, the ceramic tube was welded onto a six-probe pedestal, which was plugged into the measurement board of the sensor measuring system. A photograph and schematic of the sensing device are presented in Fig. 1. Before testing, the sensing devices were aged in air for 12 h at 300 °C for the purpose of eliminating deionized water and improving stability. After that, the obtained sensing devices were placed in a gas chamber, where different concentrations of detected gas could be introduced with a microsyringe. The gas-sensing sensitivity of the sensing devices is defined as S = R_a/R_g, where R_a and R_g are the electrical resistance of the sensor in air and in detected gas at the operating temperature, respectively.³⁴ The response time is defined as the time for the sensor resistance to reach 90% of the equilibrium value following a step increase in the target gas concentration, whereas the recovery time is defined as the time necessary for the sample to return to 10% above the original sensor resistance in air after removing the target gas.³⁵

Fig. 1 Photograph and schematic of the sensing device.

Results and discussion

Structure and morphology of Co₃O₄–In₂O₃ heterojunction structure

The crystal structure of the prepared samples was characterized by XRD. In Fig. 2a, the sharp diffraction peaks indicate highly crystalline Co₃O₄–In₂O₃ (1%) nanoparticles. The characteristic peaks at 2θ = 21.50°, 30.58°, 35.47°, 45.69°, 51.04° and 60.68° can be explicitly indexed to the (211), (222), (400), (431), (440) and (622) crystal planes of the cubic phase of In₂O₃ (JCPDS no. 04-0614), respectively. In addition, no characteristic peaks of impurities such as In(NO₃)₃ or Co(NO₃)₂ are observed. Nevertheless, it cannot be neglected that no typical diffraction peaks of Co₃O₄ are observed in the XRD pattern of Co₃O₄–In₂O₃ (1%), which could be attributed to the lower content and crystallinity of Co₃O₄ compared with In₂O₃. Therefore, XRD analysis of the samples prepared by calcining pure Co(NO₃)₂·6H₂O was also performed to prove the existence of Co₃O₄. Evidently, as shown in Fig. 2a, the characteristic peaks at 2θ = 31.27°, 36.85°, 44.81°, 59.35° and 65.23° can be indexed to the (220), (311), (400), (511) and (440) crystal planes of the cubic phase of Co₃O₄ (JCPDS no. 43-1003), respectively. Furthermore, XRD patterns of Co₃O₄–In₂O₃ nanoparticles with different contents of Co₃O₄ (0, 0.5, 2, 5 and 10 wt%) were also obtained (Fig. S1, ESI†), in which, similarly, it is still hard to distinguish the typical diffraction peaks of Co₃O₄. Nevertheless, the intensity of the In₂O₃ diffraction peaks decreases and the diffraction peaks become wider with an increase in the content of Co₃O₄ from 0 to 10 wt%, which further confirms the existence of Co₃O₄.

Fig. 2 (a) XRD patterns of Co₃O₄–In₂O₃ (1%) and pure Co₃O₄; (b) Raman patterns of pure In₂O₃, Co₃O₄–In₂O₃ (1%) and Co₃O₄–In₂O₃ (10%); (c) XPS survey of whole scan spectrum; (d) In 3d of Co₃O₄–In₂O₃ (1%); (e) Co 2p of Co₃O₄–In₂O₃ (1%); and (f) TGA patterns of In(NO₃)₃·4.5H₂O and Co(NO₃)₂·6H₂O.

Raman spectrum analysis was also performed. Fig. 2b shows the Raman curves of pure In₂O₃, Co₃O₄–In₂O₃ (1%) and Co₃O₄–In₂O₃ (10%). According to previous reports, the peaks observed at 131.7, 309.1, 498.5 and 630.3 cm⁻¹ are in agreement with the In₂O₃ characteristic peaks, whereas the peaks observed at 479.1, 516.6, 687.2 cm⁻¹ are in agreement with the Co₃O₄ characteristic peaks.^36–38 From the Raman pattern of Co₃O₄–In₂O₃ (1%), the characteristic peaks of both Co₃O₄ and In₂O₃ can be observed. However, the intensity of the Co₃O₄ characteristic peaks is still weak. Thus, Raman measurement of Co₃O₄–In₂O₃ (10%) was also performed to further confirm the existence of Co₃O₄. It is noteworthy that the intensity of the Co₃O₄ characteristic peaks is greatly enhanced when the content of Co₃O₄ increases from 1% to 10%, whereas the intensity of the In₂O₃ characteristic peaks becomes weaker. As a result, the Raman patterns also demonstrate the existence of Co₃O₄.

To further confirm the element composition of the Co₃O₄–In₂O₃ (1%) nanoparticles, XPS analysis was also performed. In Fig. 2c, the characteristic peaks of In, O, Co, C can be observed without any other impurity elements. Similarly, the peak intensity of Co is prominently weaker than that of the other elements because of the low content of Co in Co₃O₄–In₂O₃ (1%), which agrees well with the XRD results. The In 3d spectrum (Fig. 2d) exhibits a 3d_5/2 peak at 443.5 eV and 3d_3/2 peak at 451.0 eV,³⁹ which confirms the presence of In₂O₃. The presence of Co 2p in Fig. 2e further demonstrates the successful introduction of a small quantity of Co₃O₄. The characteristic peaks at 794.2 and 779.0 eV with a spin-energy separation of 15.2 eV correspond to Co 2p_1/2 and Co 2p_3/2, which are characteristic of the Co₃O₄ phase.⁴⁰

To simulate the thermal decomposition process of the reactants, TGA of In(NO₃)₃·4.5H₂O and Co(NO₃)₂·6H₂O was also performed. As can be seen in Fig. 2f, In(NO₃)₃·4.5H₂O and Co(NO₃)₂·6H₂O start to lose crystal water at 56 °C and 47 °C, respectively. Shortly afterwards, In(NO₃)₃ and Co(NO₃)₂ start to decompose at 161 °C and 196 °C, respectively. Finally, In(NO₃)₃ and Co(NO₃)₂ are fully decomposed to In₂O₃ and Co₃O₄ at 247 °C and 252 °C, respectively. The decomposition reaction in air can be inferred as the following equations:

4In(NO₃)₃·4.5H₂O → 2In₂O₃ + 12NO₂ + 3O₂ + 9H₂O

3Co(NO₃)₂·6H₂O → Co₃O₄ + 6NO₂ + O₂ + 18H₂O

Combining the equations and TGA patterns, it is found that each phase of the two thermal decomposition curves matches well with the theoretical calculation of the weight change. Moreover, no other changes can be observed when the temperature increases to 450 °C. In general, the TGA patterns further confirm the type of products obtained after the calcination process and demonstrate the reaction process as well.

The morphologies of the as prepared Co₃O₄–In₂O₃ nanoparticles were investigated by TEM and SEM. As shown in Fig. 3a, it is clear that the Co₃O₄–In₂O₃ (1%) nanoparticles are composed of irregular secondary particles after the calcination process. The inset of Fig. 3a reports a size distribution of the obtained Co₃O₄–In₂O₃ (1%) nanoparticles, which indicates that the diameters of the nanoparticles range from a few tens of nanometers to about 500 nanometers. In Fig. 3b, we clearly see that the irregular secondary particles consist of primary particles with sizes of dozens of nanometers. Moreover, we believe that the tiny holes formed between the primary particles have great advantages in gas transmission. Nevertheless, it is still hard to distinguish the Co₃O₄ nanoparticles from the composite system. This can be attributed to the low content of Co₃O₄ and the tight combination of Co₃O₄ and In₂O₃ nanoparticles after high temperature calcination. TEM images of pure In₂O₃ and Co₃O₄ can be seen in Fig. S2.† It can be seen that the diameters of the pure Co₃O₄ particles are about 30 to 50 nm. The small size and low content of pure Co₃O₄ nanoparticles make it more difficult to distinguish the Co₃O₄ nanoparticles from the heterojunction structure. Moreover, from the HRTEM image of the Co₃O₄–In₂O₃ (1%) nanoparticles (Fig. 3d) (taken from the single particle shown in Fig. 3c), the lattice spacing of 0.413 nm can be assigned to the (211) plane of the cubic phase of In₂O₃. This distinct lattice spacing indicates a high degree of crystallinity, which matches well with the XRD analysis. The SEM images of the as-synthesized Co₃O₄–In₂O₃ (1%) nanoparticles can be seen in Fig. 3e and f. It can be visually observed that the composite consists of nanoparticles with irregular diameters. Moreover, the spatial distribution of different compositional elements is clarified by the elemental mapping of In, O, and Co, respectively (Fig. 3g–i). As expected, the distribution of these three elements is homogeneous, which indicates the successful introduction of Co₃O₄ nanoparticles in the composite systems.

Fig. 3 TEM (a–d) and SEM (e and f) images of Co₃O₄–In₂O₃ (1%) nanoparticles, the inset of (a) is histogram of particle size distribution of Co₃O₄–In₂O₃ (1%). (g–i) Elemental mapping images of In, O, and Co respectively of the Co₃O₄–In₂O₃ (1%).

Gas sensing performances

Ethanol is a widely used reagent in industries and labs, and it is necessary to fabricate efficient, inexpensive gas sensing devices for ethanol detection. Accordingly, the obtained Co₃O₄–In₂O₃ heterojunction structure was fabricated as a gas sensing device, and its gas sensing performance was explored. As is known, the operating temperature can greatly influence sensing properties. Therefore, the gas sensing devices fabricated in this study were investigated towards 100 ppm of ethanol at different temperatures ranging from 160 °C to 400 °C (Fig. 4a). It is evident that the responses of pure In₂O₃, Co₃O₄–In₂O₃ (1%), and Co₃O₄–In₂O₃ (10%) toward 100 ppm ethanol first increase with an increase in operating temperature and reach maximum at 240 °C, and then sharply decrease with a further increase in temperature. This result can be ascribable to the kinetics and thermodynamics of gas adsorption and desorption on the surface of the Co₃O₄–In₂O₃ heterojunction structure or other similar semiconducting metal oxides.^18,41 Essentially, when the sensing devices are exposed to ethanol vapor, ethanol molecules are adsorbed on the active sites on the surface of the sensing material. When the operating temperature increases from 160 °C to 240 °C, it provides more energy for thermal reactions between ethanol and oxygen species, which result in improved sensing properties. However, when the operating temperature keeps increasing, the desorption rate of gas molecules exceeds the adsorption rate, which decreases the amount of ethanol on the surface of the sensing materials. A lower concentration of ethanol molecules leads to a decrease in sensitivity of the sensing materials. Therefore, 240 °C is the optimal operating temperature for gas sensing devices based on pure In₂O₃ and the Co₃O₄–In₂O₃ heterojunction structure. Moreover, the response of pure Co₃O₄ is a little different. The response reaches the maximum at 260 °C.

Fig. 4 (a) Response of different samples towards 100 ppm of ethanol at different temperatures; (b) response of samples with different contents of Co₃O₄ towards 100 ppm of ethanol; (c) resistance curves of samples with different contents of Co₃O₄ toward 100 ppm of ethanol; (d) response transients of pure In₂O₃ and Co₃O₄–In₂O₃ (1%) gas sensing devices toward 100 ppm of ethanol.

The Co₃O₄ content in the composite system is another significant element, which should be taken into consideration in the gas sensing study. As can be seen from Fig. 4b, the gas response values of pure In₂O₃, Co₃O₄–In₂O₃ (1%), Co₃O₄–In₂O₃ (10%) and pure Co₃O₄ are 45.82, 62.13, 17.54 and 5.36, respectively. As expected, the response value based on Co₃O₄–In₂O₃ (1%) is 1.36 times higher than that of pure In₂O₃. Remarkably, the response value of Co₃O₄–In₂O₃ (1%) is 11.6 times higher than that of pure Co₃O₄. This result shows that the synergistic effects make the sensing properties of the heterojunction surpass both pure In₂O₃ and Co₃O₄.

Moreover, the gas response of Co₃O₄–In₂O₃ with other contents of Co₃O₄ is exhibited in Fig. S3.† The response values decrease evidently with an increase in Co₃O₄. This phenomenon can be explained in Fig. 4c, in which the resistance curves of pure In₂O₃, Co₃O₄–In₂O₃ (1%), Co₃O₄–In₂O₃ (10%) and pure Co₃O₄ toward 100 ppm of ethanol are presented. At first, all the sensing devices were exposed to air for 15 s. In air atmosphere, the sensing devices based on pure Co₃O₄ evidently exhibit extremely low resistances. In addition, the resistances of the Co₃O₄–In₂O₃ composite system increase with elevated contents of Co₃O₄. The sharp rise in resistance can be ascribed to the p–n junctions formed between n-type In₂O₃ and p-type Co₃O₄, and these results are in accordance with the theory in previous literature.⁴² Subsequently, when ethanol vapor was introduced, the resistances of the sensing devices showed a decreasing trend except for the sensing devices based on Co₃O₄, which indicates the characteristic of n-type In₂O₃ and p-type Co₃O₄ semiconductors.^24,43 Moreover, all the types of heterojunction structures show an n-type response, which can be attributed to the high content of n-type In₂O₃.

For practical application, the gas sensing response and recovery time are also important parameters to evaluate the performance of gas sensors. To simulate the practical gas sensing procedure, the response transients of gas sensing devices based on pure In₂O₃ and Co₃O₄–In₂O₃ (1%) toward 100 ppm of ethanol were also studied (Fig. 4d). From the curves, we find that the Co₃O₄–In₂O₃ (1%) heterojunction structure has a superior ethanol response value (1.41 times higher than pure In₂O₃, which is consistent with the previous test). Nevertheless, these two types of sensing materials show similar responses and recovery times. More details can be discovered in the resistance variation curves obtained by calculating the values of response transients. Apparently, the response and recovery time of Co₃O₄–In₂O₃ (1%) are 42 and 92 s, respectively, which are relatively high. Therefore, some further studies were conducted in order to reduce the response and recovery times.

For investigating the gas sensing limitations, the sensing properties of the pure In₂O₃ and Co₃O₄–In₂O₃ (1%) gas sensing devices toward 5 to 1000 ppm of ethanol were also determined (Fig. 5a). It is evident that the response values increase rapidly with an increase in ethanol concentration. Moreover, the response values of Co₃O₄–In₂O₃ (1%) are significantly higher than that of pure In₂O₃. Notably, the response value of Co₃O₄–In₂O₃ (1%) to 5 ppm of ethanol is 4.4 (1.43 times higher than that of pure In₂O₃), which indicates the high sensitivity and wide detection range of Co₃O₄–In₂O₃ (1%). To further investigate the sensitivity tendency of the obtained samples towards different concentrations of ethanol, the linear fitting method was also performed (Fig. 5b). As can be seen, the responses exhibit a linear growth process when the ethanol vapor concentration increases from 5 to 100 ppm. Moreover, the linear calibration curve (insert of Fig. 5b) further demonstrates that the sensing devices fabricated in this study have good accuracy and reliability.

Fig. 5 (a) Response of pure In₂O₃ and Co₃O₄–In₂O₃ (1%) towards ethanol with different concentrations; (b) sensitivity tendency of pure In₂O₃ and Co₃O₄–In₂O₃ (1%) vs. different gas concentrations, and the inset shows the corresponding calibration curves (5 to 100 ppm); (c) response of pure In₂O₃ and Co₃O₄–In₂O₃ (1%) towards 100 ppm of different target gases; and (d) stability test of pure In₂O₃ and Co₃O₄–In₂O₃ (1%) towards 100 ppm of ethanol.

Fig. 5c presents the selectivity of pure In₂O₃ and Co₃O₄–In₂O₃ (1%) towards ethanol over other VOCs, such as ammonia, acetone, toluene, methanal, and acetic acid, since these six types of gases are very common in daily life or factories, and selectivity is important to an ethanol sensing devices.^44,45 Apparently, these two gas sensing devices show the strongest response to ethanol among different gases with the same concentration (100 ppm), which indicates their superior selective ability in the gas detection process. Notably, the Co₃O₄–In₂O₃ (1%) device exhibits higher response values than that of pure In₂O₃ to all types of VOCs, which confirms the universality of Co₃O₄–In₂O₃ (1%) in different gas atmospheres.

When it comes to practical application, good stability is another important factor that should be taken into consideration. Fig. 5d presents the stability tests of pure In₂O₃ and Co₃O₄–In₂O₃ (1%) toward 100 ppm of ethanol. After these two gas sensing devices worked continuously for 24 h, no visible decrease in gas response was observed. This phenomenon indicates the outstanding stability and repeatability of the Co₃O₄–In₂O₃ heterojunction. High stability and repeatability indicate a superior lifetime for gas sensing devices, which is another significant advantage for practical application. Furthermore, humidity can also affect the response of sensing materials. Thus, we tested our samples in different humidities to simulate the real environment (Fig. S4†). The result shows that the responses decrease by 21% when the humidity changes from 50% to 90%.

Table 1 compares the response of Co₃O₄–In₂O₃ (1%) to ethanol with other metal oxide semiconductors. It is evident that Co₃O₄–In₂O₃ (1%) exhibits higher gas response values and a lower optimal operating temperature than that of other different types of sensing materials. For example, compared with In₂O₃ hierarchical nanostructures, the response of the samples fabricated in this study is 2.25 times higher and their operating temperature is also lower.

Table 1 Comparative analysis of the gas sensing responses of different materials

Material	Ethanol (ppm)	Operating temperature (°C)	Gas response	Reference
Co3O4 nanorod arrays	500	160	70.7	13
In2O3 hierarchical nanostructures	100	320	27.6	17
Bi2O3 nanoparticle-decorated In2O3 nanorods	200	200	17.7	48
ZnO nanorods	100	320	25.4	49
SnO2/ZnO hierarchical nanostructures	100	400	6.2	50
Co3O4–In2O3 heterojunction	100	240	62.13	This work
Co3O4–In2O3 heterojunction	200	240	82.99	This work

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The gas sensing mechanism was also investigated to help explain the enhanced sensing performance of Co₃O₄–In₂O₃ (1%). For n-type metal oxide semiconductors, just like pure In₂O₃ fabricated in this work, the sensing mechanism has been reported in previous studies.^46,47 Firstly, when n-type In₂O₃ semiconductors are exposed in air at an optimal operating temperature, oxygen molecules are adsorbed on the surfaces of the sensing materials, which then trap electrons from the conduction band of In₂O₃ to form ionized oxygen species (O_ads⁻, O²⁻, and O₂⁻), leading to the formation of an electron depletion layer in the surfaces of the sensing materials. Thus, the resistance of the sensing materials greatly increases. When n-type In₂O₃ semiconductors are exposed in the target gas, the resistance of the sensing materials (R_g) is reduced rapidly, because the reductive molecules can react with ionized oxygen species and yield trapped electrons. The response value of n-type sensing materials can be simply summarized as the change in their resistances in different atmospheres.

The sensing mechanism of the Co₃O₄–In₂O₃ heterojunction structure is somewhat different with n-type In₂O₃. It is known that an electron depletion layer would form at the contact interface between n-type and p-type semiconductors.^42,51,52 Fig. 6b exhibits the dynamic process of the formation of the electron depletion layer in this study. Firstly, n-type In₂O₃ and p-type Co₃O₄ nanoparticles are combined together tightly through the calcination process. As is known, in n-type semiconductors, electrons are the majority carriers (labeled as n_n in Fig. 6b). However, in p-type semiconductors, holes are the majority carriers (labeled as p_p in Fig. 6b). Thus, after these two types of semiconductors come into contact with each other, electron transfer from n-type In₂O₃ to p-type Co₃O₄ is easy. Finally, electrons and holes that belong to p-type Co₃O₄ will recombine at the contact interface, which leads to a rapidly reduced concentration of charge carrier and sharply increased resistance compared with pure n-type In₂O₃. This theory is also in agreement with the previous results in this study (Fig. 4c). Besides the electron depletion layer shown in step 1 in Fig. 6a, another type of electron depletion layer would be formed on the surfaces of sensing materials operated at optimal temperature (step 2 in Fig. 6a), and this mechanism is similar to pure n-type metal oxide semiconductors. The two electron depletion layers contribute to the great improvement in resistance. When ethanol molecules are introduced, they react with the ionized oxygen species. At this time, the electrons are released back to the In₂O₃ semiconductor and p–n heterojunction through surface interactions (step 3 in Fig. 6a). Therefore, these two types of electron depletion layers disappear gradually. This phenomenon causes the resistance of sensing devices to decrease sharply, which also indicates higher response values. The reactions in the sensing process can be seen in Fig. 6c.⁵³ These reactions can further describe the details of the gas sensing procedure. Generally, the gas sensing devices based on the Co₃O₄–In₂O₃ (1%) heterojunction structure have superior sensing performance compared with pure In₂O₃ sensing devices.

Fig. 6 (a) Schematic for the gas-sensing mechanism of Co₃O₄–In₂O₃ (1%); (b) formation mechanism of the electron depletion layer formed at the p–n heterojunction structure; and (c) reaction equations of the sensing process.

Conclusions

In summary, the Co₃O₄–In₂O₃ heterojunction structure was successfully prepared through a simple grinding-calcination process without any harmful reagents. The gas sensing devices based on the Co₃O₄–In₂O₃ (1%) heterojunction structure exhibit higher responses and selectivity to ethanol compared with pure In₂O₃. Moreover, Co₃O₄–In₂O₃ (1%) shows a low operating temperature, low detection limit and high stability, which are significantly important in practical application. This work provides a new idea for the fabrication of high sensitivity gas sensing devices via a simple, cheap and environmentally friendly method.

Acknowledgements

Kai Song and Xiaoqian Meng contributed equally to this study. This investigation was supported by the Natural Science Foundation of China (No. 51322212, 51472122), the Fundamental Research Funds for the Central Universities (No. 30915011201), the PAPD and “333 project” of Jiangsu.

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Footnote

† Electronic supplementary information (ESI) available: The other XRD, TEM and sensing response data. See DOI: 10.1039/c6ra23196a

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