Direct growth of NiO films on Al2O3 ceramics by electrochemical deposition and its excellent H2S sensing properties

Sensors & Actuators: B. Chemical 296 (2019) 126619

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Direct growth of NiO films on Al2O3 ceramics by electrochemical deposition and its excellent H2S sensing properties

T

⁎

Yueying Liu, Fengmin Liu , Jihao Bai, Tianyu Liu, Ziyang Yu, Meng Dai, Linsheng Zhou, ⁎ Hongtao Wang, Yiqun Zhang, Hui Suo , Geyu Lu State Key Laboratory on Integrated Optoelectronics, Key Laboratory of Gas Sensors, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, Jilin Province, 130012, China

A R T I C LE I N FO

A B S T R A C T

Keywords: NiO film Cu doping Electrochemical deposition H2S sensing

NiO thin films were grown directly on the Al2O3 substrates by electrochemical deposition. The H2S gas sensing properties of the thin films were enhanced by Cu doping. The X-ray diffraction revealed the as-prepared films were NiO crystalline phase. The scanning electron microscope showed the film has 1.87 μm thickness and pores of different sizes, more porous has a higher exposed surface, which can provide more active sites for reaction with the gas. The atomic force microscopy characterization further confirmed the high roughness and porosity of the thin films. Gas sensitivity test displayed that the thin film sensor based on Ni: Cu molar ratio of 9:1 exhibited the highest response and good selectivity to H2S at 140 °C. Furthermore, the NiO doped by Cu thin film gas sensor also possesses good stability and low detection limit of 100 ppb. The extraordinary gas sensing properties can be attributed to the lamellar structures with the porous channel and substitution doping of Cu.

1. Introduction Metal oxide semiconductor (MOS) gas sensors have received extensive attention because of their advantages such as high sensitivity, low price, and adjustable performance [1–3]. In order to develop highperformance gas sensors, the performance of gas sensors can be improved by adjusting the microscopic morphology of sensitive materials [4–7], loading noble metal catalysts [8–11] and compounding various oxides [12–15], etc. The fabricating process of the most reported sensors includes the preparation of powders and then coating the powders to the substrate. This stepwise preparation is complex and time-consuming. Moreover, the porosity of the above mentioned MOS sensing film is difficult to control and the uniformity is poor. Instead, the method of directly growing the sensitive films on the substrate has no intermediate steps and has good consistency. To date, many preparation methods can directly grow metal oxide sensitive films on the substrates, such as sol-gel technique [16–21], radio frequency sputtering [22–26], chemical vapor deposition [27–31] and electrochemical deposition method [32–36], etc. For example, Nilam B. Patil et al. [37] prepared ZnO thin film successfully by a solgel spin coating technique on a glass substrate, and the ZnO thin film gas sensor exhibits high response to NO2 gas. Tai et al. [38] prepared polyaniline/titanium dioxide nanocomposite thin films by an in-situ

⁎

self-assembly method on a silicon substrate with gold interdigital electrodes, and the study found that the sensor has a good response to NH3. Liang et al. [39] used rf sputtering to fabricate a p-type ternary ZnCr2O4 thin film on the sapphire substrate and detected reducing gases. It could be found that the sensitive films grown directly on the substrate have good sensing properties and control of thickness. Since the alumina ceramic substrate has advantages such as low cost, high-temperature resistance, good mechanical properties [40–42], etc., in the field of gas sensors, the substrate materials generally used are ceramic substrates. However, the direct growth of metal oxide sensitive films on ceramic substrates has not been reported in the field of gas sensors. In this study, the NiO thin film was prepared by an electrochemical deposition method on the alumina ceramic substrates and used as gas sensors for detection of H2S. The effect of the molar amount of Cu doped on NiO film microstructure and sensing properties was determined. A comparative investigation into sensing properties revealed that the sensing performance of NiO doped by Cu thin films exhibited superior performance than the pure NiO thin film. In addition, the gas-sensitive mechanism of NiO thin films has also been explored.

Corresponding authors. E-mail addresses: [email protected] (F. Liu), [email protected] (H. Suo).

https://doi.org/10.1016/j.snb.2019.05.096 Received 29 January 2019; Received in revised form 1 May 2019; Accepted 26 May 2019 Available online 08 June 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.

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2. Experimental

the time taken for the sensor resistance value to change from the initial value Ra to 90% of the total resistance change in the stable value Rg. The recovery time((t90%-recovery) is defined as the time taken for the resistance value to recover from the Rg to 90% of the total resistance change after the sensor is removed from the gas to be measured.

2.1. Growth of NiO thin films In the experiment, all the chemical reagents were analytical grade and used without any further purification. Acetone and hydrochloric acid reagents were produced from Beijing Chemical Works of China, Ni (NO3)2·6H2O was produced from Sinopharm Chemical Reagent Co., Ltd., and Cu(NO3)2·3H2O was produced from Xiqiao Chemical Co., Ltd. Pure NiO and NiO doped by Cu thin films were prepared by electrochemical deposition method on the alumina ceramic substrates (8 mm × 8 mm) with a channel width of 35 μm of Au interdigitated electrodes. The synthesis process was described as follows: Firstly, the alumina ceramic substrates with Au interdigitated electrodes were cleaned with acetone and hydrochloric acid for 10 min. Secondly, a seed layer was prepared on the substrate, in detail, 0.4362 g Ni (NO3)2·6H2O were dissolved in 20 mL of deionized water under vigorous stirring for 30 min to form a uniform green solution. The above solution was spin-coated on the substrate and then heated at 300 °C for 30 min in air. Next, electrochemical deposition was carried out. Electrodeposition solution was formed by the total amount of 4 mmol Ni(NO3)2·6H2O and Cu(NO3)2·3H2O according to different molar ratios of 100:0, 97:3, 95:5, 93:7, 90:10. Electrochemical deposition was carried out using an electrochemical working station (CHI660D, Shanghai, China) by the three-electrode system at room temperature. The preparation process of sensitive films was given in Fig. 1(a). The reference electrode used a saturated calomel electrode (SCE) while the counter electrode was a platinum electrode. The substrate with seed layer served as the cathodic, experiments were performed under a constant potential for -1 v and deposition time of 300 s. The resulting films were rinsed with deionized water for 3–5 times to remove the deposition fluid on the surface and then dried at 60 °C for 3 h. Finally, the sensitive films were obtained at 500 °C for 2 h in the air atmosphere.

2.3. Characterization X-Ray diffractometer (XRD, Rigaku D/Max 2550, operated at 40 KV/200 mA with Cu-Kα radiation (λ =1.5406 Å)) were used to determine the composition and crystal structure of the thin films. The microstructure, morphology and roughness of the synthesized films were characterized by field emission electron microscopy (FESEM, JEOL JSM-7500 F) and atomic force microscopy (AFM, Being NanoInstrument, Ltd., CSPM5500, China). The surface elements and components of NiO films were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi, Thermo Fisher Scientific co., America); NiO films were prepared by electrochemical workstation (CHI660D, Shanghai Chenhua co., China). 3. Results and discussion 3.1. Structural and morphological characteristics X-ray diffraction (XRD) is used to analyze the crystallographic information and crystallinity of products. Fig. 2 depicts the XRD patterns of pure NiO and NiO doped by Cu sensing thin films. Clearly, the diffraction peaks of the substrate are easily indexed to the rhombohedral structure of Al2O3, which matched well with the standard value of JCPDS file No. 73-1512. Al2O3 is the main component of the substrate material of the Au interdigitated electrodes. However, as for the electrodeposited films, the diffraction peaks are still shown as Al2O3 (JCPDS file No. 73-1512), which might be ascribed to the short deposition time and insufficient film thickness. Except for the peak of Al2O3, the other diffraction peaks of XRD indexed to the NiO (JCPDS: 65–2901), Ni2O2(OH) (JCPDS: 84–1459) and NiOOH (JCPDS: 6–75). Fig. 2(b) displays the XRD patterns of the pure NiO and Cu doped NiO sensing thin films at the range of 35° to 55°. The diffraction peaks different from the Al2O3 substrate at 2θ of around 37.2°, 41.1°, 42.8°, 43.0°,44.36°, 48.4°, 49.44° and 51.9° could be observed, which are attributed to the samples. The diffraction peaks appearing at 37.2° and 43.0° can be indexed to the cubic structure of NiO (JPCDS: 65–2901), the diffraction appearing peaks at 41.1°, 48.4° and 51.9° point to the common peaks of NiOOH (JPCDS: 6–75) and Ni2O2(OH) (JPCDS: 84–1459), and peaks appearing at 42.8°, 44.36° and 49.44° can be indexed to Ni2O2(OH) (JPCDS: 84–1459). Thus, the main component of the films is NiO containing a small amount of hydroxyl, and the attribution of the product components was also confirmed by the XPS (Fig. 5) analysis. The existence of hydroxide may be due to insufficient sintering of the materials. However, with the variation of Cu2+ content in the precursor solutions, no additional phases about Cu were observed from the XRD patterns and the peak position of nickel slightly shifted to the right,

2.2. Measurement of the gas sensor The operating temperatures of the sensing devices were controlled by a ceramic heater. Diagram of sensor device was given in Fig. 1(b). The gas sensor performances were tested in a static working system in this work. In the actual test, the sensors were placed in the closed container, and the pure air (20 ± 2 °C, 25 ± 5%RH) and target gas to be tested were filled into the airtight container through an additional air valve to achieve the sensing detection of sensors. The data collection instrument in the test was the Fluke 8846A digital multimeter, which was used in conjunction with the Fluke 8846A software to test the resistance range from 1 Ω to 1 GΩ. During testing, the multimeter read a resistor value per second to achieve real-time detection, and the data were recorded for performance analysis of sensors. NiO is a p-type semiconductor and its response to reducing gas can be defined as S = Rg/Ra. Ra is the resistance value of the sensor in the pure air, and Rg is the resistance value of the sensor in a certain concentration of the gas to be tested. In addition, we define the response time(t90%-response) as

Fig. 1. Schematic diagram of (a) preparation process and (b) device structure. 2

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Fig. 2. XRD patterns of the as-prepared NiO films with different molar ratios of Ni and Cu, (b) is the form of the logarithmic scale.

hinges on the morphology of the substrates. It is also apparent that all NiO films prepared by electrochemical deposition are porous structure in nature. The porous structure enables the materials to have a larger contact area with the gas thus has more active sites for gas adsorption. Meanwhile, as the copper content increases, the pores gradually become larger and become more continuous. This increases the roughness of the film surface, and it is also more conducive to the adsorption of gases. Fig. 3(g) and (h) display the detailed information of the film with Ni: Cu molar ratio of 9:1 in the raw material. It can be seen from Fig. 3(h) that the prepared film material is formed by the aggregation of NiO grains to form the lamellar structure. Also, it can be found in Fig. 3(g) that the lamellar structure is uniformly distributed, the porous structure is clearly observable and the film has a thickness of 1–2 μm as shown in the illustration. In order to investigate the surface morphology and roughness of the films, AFM was used to illustrate the morphology of the films, as

which might be ascribed to the incorporation of Cu into the crystal lattice of NiO. The doped Cu2+ replaced a small amount of Ni2+ in NiO, and the atomic radius of Cu is slightly smaller than the atomic radius of Ni, so the lattice size is reduced and the peak position shifted slightly to the right. And with the increase of Cu content, the peak intensity of Ni2O2(OH) increases, indicating that the addition of Cu increases the content of Ni2O2(OH). The surface morphology of pure NiO and NiO doped by Cu sensing thin films of different molar ratios (100:0, 97:3, 95:5, 93:7, 90:10) were studied by FESEM and the results were presented in Fig. 3. The deposited films were relatively uniform from the surface of the electrode. After annealing, the material adhered strongly to the interdigitated electrodes. As can be seen in Fig. 3(a–f), the SEM patterns of the materials obtained under the different deposition conditions are similar and the NiO films exhibit almost the same morphology as the substrate underneath, indicating that the morphology of the NiO films highly

Fig. 3. FESEM images of different molar ratios of Ni and Cu composite films. (a) pure NiO film; (b) Ni: Cu = 100:0; (c) Ni: Cu = 97:3; (d) Ni: Cu = 95:5; (e) Ni: Cu = 93:7; (f) Ni: Cu = 90:10; (g–h) high-magnification SEM images of Ni: Cu molar ratio of 9:1 composite film and thickness of composite film (insert). 3

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Fig. 4. (a) AFM images of pure NiO film and (b–f) different molar ratios (100:0, 97:3, 95:5, 93:7, 90:10) of Ni and Cu composite films, all for a scanning area of 20 μm × 20 μm.

Fig. 5. XPS spectra of Ni: Cu molar ratio of 9:1 composite film (a) survey; (b) Ni 2p; (c) Cu 2p; (d) O 1s and the O 1s spectra of pure NiO film (insert).

4

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adsorbed oxygen on its surface and the gas molecules to be tested. At relatively low temperatures, the H2S gas molecules to be tested without sufficient thermal energy to combine with the adsorbed oxygen on the surface of the NiO thin films, so the response is relatively low. As the operating temperature gradually increases, the H2S gas molecules become active enough to rise above the activation energy barrier of the reaction on the surface of the NiO thin films, which results in a significant increase in the response. However, with the operating temperature further increase, the desorption process of H2S begins to dominate. The response of the gas sensors begins to decrease because of the difficulty in capturing H2S gas molecules. Therefore, the response of the gas sensor tends to “increase-maximum-decrease”. According to the results obtained in Fig. 6, when the molar ratio of nickel to copper in the raw material is 90:10, the sensor presents the highest response value at 140 °C. Since selectivity is an important indicator for evaluating sensing performance, the selectivity of NiO film to gas was tested. Fig. 7(a) exhibits the responses of the thin film having a Ni: Cu molar ratio of 9:1 in the raw material to gases of NO2, SO2, NH3, Cl2, CO and acetylene. It can be obviously seen that the sensor presented the highest response to H2S and the response value was much stronger than other gases. Therefore, it could be concluded that the NiO doped by Cu thin film sensor has good selectivity for H2S gas. Fig. 7(b) and (c) exhibits the transient response curves of the NiO doped by Cu thin film sensor for different concentrations of H2S at 140 °C, and the gas concentration ranges from 0.1 to 10 ppm. It could be seen that the resistance value of the sensor in the gas increases gradually with the increasing of H2S concentration. After several measurements, the resistance of the sensor can be recovered to the initial value basically, which indicates the good recovery ability of the sensor. The relationship between the sensor response and the concentration of H2S is given in the illustration in Fig. 7(b) and (c). The sensor exhibits a proportional increase as the H2S concentration is increased. This is mainly because the more H2S will react with adsorbed oxygen on the surface of the film with the increase of H2S concentration, thereby enhancing the sensing response. The response value of the sensor in a low concentration of H2S gas at 0.1-0.5 ppm is displayed in Fig. 7(b). It could be observed that the response value of the sensor to 0.1 ppm H2S is 1.28, with a relatively low detection limit. Meanwhile, the response and recovery time are also the important factors in evaluating the gas sensor. Fig. 8(a) is the instant response curve of NiO doped by Cu film to 10 ppm H2S gas at the optimum operating temperature. On the basis of the definitions of the sensor response and recovery time, as depicted in the Fig. 8(a), the response time is about 89 s and the recovery time is about 16 s. In Fig. 8(b), the thin film sensor performs five consecutive repeat tests to 10 ppm H2S. The response and recovery curves of the sensor basically show a similar shape and transient characteristics, and the resistance can be recovered to the initial value, which demonstrates that the sensor has good repeatability. Moreover, the result of the sensor to multiple tests in 10 ppm H2S gas is displayed in Fig. 8(c). It could be seen that the sensor responses to H2S are relatively constant with a few small fluctuations, which are basically stable at around 900. These results indicate that The NiO doped by Cu thin film sensor has good stability. Here, based on the above results, the gas sensing performances comparison of H2S sensors between the as-prepared NiO thin film and other sensors in reported literature in Table 1. It is obvious that the present sensor exhibited a high response at a relative low working-temperature and showed a low detection limit, which indicated that the fabricated sensor utilizing NiO doped by Cu thin film has a promising prospect and a large improvement space (response and recovery time) in H2S detection at low concentrations.

depicted in Fig. 4. The surface root mean square (Ra) roughness of the seed layer and Ni: Cu molar ratio of 100:0, 97:3, 95:5, 93:7, 90:10 thin films were 228, 196, 201, 178, 191, 220 nm, respectively. The roughness of the films under the different deposition conditions are about 200 nm, and the film of Ni: Cu molar ratio of 9:1 has larger roughness, which indicates that more active sites can be provided for the gas. And it can be seen from the AFM images that the morphology of the materials is composed of nanoparticles and has the porous structures. As the copper content increases, the porous structures gradually increase and become larger, which is consistent with the results found with SEM. XPS analyses were conducted to further characterize the compositions and chemical states of the NiO sensing thin film with Ni: Cu molar ratio of 9:1. It can be seen from the survey spectrum (Fig. 5(a)) that the presence of Ni, Cu and O elements in the prepared sensing material. As depicted in the XPS spectrum of Ni 2p (Fig. 5(b)), five peaks could be clearly observed from the high resolution scan. the two major peaks centered at 854.4 eV and 873.3 eV are assigned to the Ni 2p3/2 and Ni 2p1/2 [43], respectively. And the shakeup-type satellite peaks centered at 861.6 eV and 880.1 eV are observed, corresponding to Ni2+. Besides, the binding energy peak positions at around 856.1 eV may be attributed to the NiO(OH)x species on the surface. As displayed in Fig. 5(c), two obvious peaks centered at 934.5 eV and 942.5 eV are ascribed to the Cu 2p3/2 and the peaks located at 954.5 eV and 962.4 eV are characteristic of Cu 2p1/2 [43], indicating the existence of Cu2+. In the O 1s spectrum (Fig. 5(d)), the binding energy peak position at 529.9 eV was ascribed to lattice oxygen and the peak located at 531.5 eV ascribed to chemisorbed oxygen species in the surface of Cu doped NiO thin film [44]. 3.2. Gas sensing characteristics In order to prove the potential application of this work, gas sensors based on pure NiO and NiO doped by Cu thin films were fabricated and the gas sensing properties were systematically investigated. The gas sensing performances of the fabricated NiO sensing thin films of different molar ratios (100:0, 97:3, 95:5, 93:7, 90:10, 80:20) at different temperatures to 10 ppm H2S were investigated as exhibited in Fig. 6. It could be seen that the gas sensors exhibited the volcano shape to 10 ppm H2S for all the sensitive films at the operating temperature ranged from 110 °C to 195 °C. For the pure NiO thin film device, its response to 10 ppm H2S reached a maximum of 27.7 at 155 °C. Meanwhile, for the NiO doped by Cu film sensors, the response at the optimum operating temperature to 10 ppm H2S can reach the values of 323.5 (Ni: Cu = 97:3), 407.7 (Ni: Cu = 95:5), 387.5 (Ni: Cu = 93:7), 886.9 (Ni: Cu = 90:10), 120.7 (Ni: Cu = 80:20), respectively. The response of the gas sensor depends on the chemical reaction between the

3.3. Gas sensing mechanism Fig. 6. Response values of sensors with different molar ratios of Ni and Cu to 10 ppm H2S at different operating temperatures.

As known, gas sensing mechanism of the p-type metal oxide 5

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Fig. 7. (a) The selectivity of the sensor with Ni: Cu molar ratio of 9:1 to various gases at 140 °C; (b–c) Dynamic response-recovery curves to different concentrations of H2S at the operating temperature and gas responses of the film to different H2S concentrations at 140 °C (insert).

materials and capture free electrons from the conduction band of NiO material to form chemisorbed oxygen species. This leads to the increase in hole concentration and the decrease of sensor resistance. According to the experimental conditions (140 °C) of this work, the main component of chemisorbed oxygen on the surface of NiO materials is O−,

chemical sensor is mainly based on the change of the sensor resistance, and the change is mainly attribute to the adsorption and desorption of the gas on the surface of the sensing materials. As for the NiO thin films prepared by this work, when the sensing material is exposed to air, oxygen molecules could easily adsorb on the surfaces of NiO thin film

Fig. 8. (a) Dynamical responses of Ni: Cu molar ratio of 9:1 composite film to 10 ppm H2S at 140 °C; (b) Continuous response and recovery curves of the present sensor to 10 ppm H2S; (c) Long-term stability of the sensor to 10 ppm H2S at 140 °C. 6

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Table 1 Comparison of H2S sensing performance of various sensors fabricated by current work and previous reports. Sensing material

Temp. (°C)

H2S con. (ppm)

Response

Res. & Recov. Time (s)

LOD

Ref.

NiO microflowers 5 wt% PdO-NiO Nanoscale Heteromixture

RT 60

97 20, 100

8.8 43%, 95%

0.485 ppm –

[45] [46]

CdO-decorated NiO nanofilm NiO/ZnO nanowire CuO-NiO core-shell microspheres NiO porous nanowall arrays Au@NiO yolk–shell Fe2O3-loaded NiO nanoplates NiO@ZnO NTs NiO film doped by Cu

92 RT 260 92 300 200 215 140

50 100 100 10 5 50 50 10

47.8 47.6 47.6 12.9 108.92 26.48 474 886.9

3.5 & 19-25 & 6-10 to 100 ppm – 15 & 18 & 29 49 & 123 – – – 89 & 16

0.5 ppb – – 1 ppb 1.25 ppm 1 ppm – 0.1 ppm

[47] [48] [49] [50] [51] [52] [53] This work

that the gas sensitivity of NiO to H2S increases gradually with the addition of Cu. When the copper content increases to a higher degree, the gas-sensitive reaction can occur at a lower temperature. This is because the activity of the entire material to H2S increases as the Cu content increases, thereby increasing the gas sensitivity.

and there may be a small amount of O2- [46,54–56]. When the NiO material is exposed to H2S (reducing gas), the H2S molecules to be tested could combine with the adsorbed oxygen ions on the surface of the NiO material. The captured electrons are released after the reaction, resulting in the increase of the resistance under the H2S atmosphere. The reaction of this process is as follows:

4. Conclusion

O2(gas) → O2(ads)

In summary, the NiO thin film materials were successfully prepared via a simple electrochemical deposition method. The porosity structure of the thin films is beneficial to gas adsorbtion. The gas sensing performance demonstrated that the NiO doped by Cu thin films showed higher sensitivity to H2S gas than the pure NiO thin film. The NiO thin film gas sensor with Ni: Cu molar ratio of 9:1 in the raw material achieved the highest sensitivity. The response of the sensor to 10 ppm H2S was about 900 at 140 °C and the low detection limit was 0.1 ppm at the optimum operating temperature. In addition, the applicable gas sensing mechanism of NiO film to H2S was discussed in detail. The NiO doped by Cu thin film can be a promising material for fabricating the high performance H2S sensor, notwithstanding the response and recovery times of the sensor needs to be improved.

O2(ads) + e− → O2−(ads) −

O2 (ads) + e 2H2S +

−

−

→ 2O (ads)

3O2−(ads)

→ 2H2O + 2SO2 + 3e−

H2S + 3O−(ads) → H2O+SO2 + 3e− Compared with the NiO thin film, the gas sensing property of NiO doped by Cu materials is significantly increased. The excellent sensing property based on NiO doped by Cu thin film sensors possibly due to the following factors: Firstly, Cu doping NiO thin films can provide plenty of active sites for the reaction between H2S gases and adsorbed oxygen. Meanwhile, the porous structure of the thin films can accelerate the gas transmission rate, so that the sensing performance of H2S gas is greatly enhanced. Secondly, according to the analysis results of XPS O1 s (Fig. 5d), the adsorbed oxygen content decreased slightly after doping, which indicates that the change of adsorbed oxygen content is not the main reason for the increase of the response after doping. Besides, the addition of copper significantly improves the sensitivity to H2S gas, the possible reason is as follows. According to the results of XRD, Cu entered the lattice of NiO in the form of substitution doping, so the existence form of Cu is CuO. Due to the presence of CuO, when the materials place in H2S gas, CuO will rapidly start to the following reactions:

Acknowledgements This work was supported by National Nature Science Foundation of China (Nos. 61871198 and 61474057), National key Research and Development Program of China (No. 2016YFC0201002), National High-Tech Research and Development Program of China (863 Program, No. 2014AA06A505). References

CuO + H2S → CuS + H2O

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The rapid reaction of H2S molecules with CuO results in the formation of a CuS layer on the surface of CuO. The presence of CuS layer passivates the CuO against oxygen adsorption and that the electrons are free from CuO. This hinders the transport of carriers in the material and increases the resistance of the sensor. At the same time, CuO is in contact with CuS to form a P-P junction, which increases the interface in the whole materials and decreases the free charge, so the resistance of the sensors also has increased. At the same time, during the recovery process, CuS will be oxidized in the air and returned to CuO by the following reaction: 2CuS + 3O2 → 2CuO + 2SO2 Therefore, the hole carrier concentration of the NiO is recovered and the resistance is restored to the initial value. According to the definition (S = Rg/Ra) of the response of p-type semiconductor to reducing gas, gas sensitivity is greatly improved. As we can see from Fig. 6 7

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Yueying Liu received the BS degree in Department of Micro-Electronics in 2017. She is currently studying for her M.E Sci. degree in College of Electronic Science and Engineering, Jilin University, China. Fengmin Liu received the B. Eng. degree in Department of Electronic Science and Technology in 2000. She received her Doctor’s degree in College of Electronic Science and Engineering at Jilin University in 2005. Now she is a professor in Jilin University, China. Her current research is preparation and application of semiconductor oxide, especial in gas sensor and solar cell. Jihao Bai received the BS degree in Department of Micro-Electronics in 2018. He is currently studying for his M.E Sci. degree in College of Electronic Science and Engineering, Jilin University, China. Tianyu Liu received the BS degree in Department of Micro-Electronics in 2017. He is currently studying for his M.E Sci. degree in College of Electronic Science and Engineering, Jilin University, China. Ziyang Yu received the BS degree in Department of Chemistry in 2013. He is currently studying for his pH.D. degree in College of Electronic Science and Engineering, Jilin University, China. Meng Dai received the BS degree in Department of Micro-Electronics in 2018. She is currently studying for her M.E Sci. degree in College of Electronic Science and Engineering, Jilin University, China.

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Sensors & Actuators: B. Chemical 296 (2019) 126619

Y. Liu, et al.

Linsheng Zhou received the BS degree in Department of Electronic Science and Technology in 2017. He is currently studying for his M.E Sci degree in College of Electronic Science and Engineering, Jilin University, China.

Hui Suo received her PhD degree in 1999 from Department of Chemistry, Jilin University, China. Now she is a professor in Jilin University, China. Her current research is preparation and application of supercapacitors and gas sensors.

Hongtao Wang received his bachelor’s degree in 2015 in chemical engineering and technology from Changchun University of Technology, China. He is currently studying for his pH.D. degree in College of Electronic Science and Engineering, Jilin University, China.

Geyu Lu received the B. Sci. degree in electronic sciences in 1985 and the M.S. degree in 1988 from Jilin University in China and the Dr. Eng. degree in 1998 from Kyushu University in Japan. Now he is a professor of Jilin University, China. His current research interests include the development of chemical sensors and the application of the function materials.

Yiqun Zhang received the BE degree in Department of Electronic Sciences and Technology in 2009. She is currently studying for her Dr Sci degree in College of Electronic Science and Engineering, Jilin University, China.

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