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. 2023 Feb 20;8(8):7595–7601. doi: 10.1021/acsomega.2c07092

Enhanced H2S Gas-Sensing Performance of Ni-Doped ZnO Nanowire Arrays

Shaoyu Liu †,‡, Weiye Yang †,‡, Lei Liu †,‡, Huohuo Chen †,‡, Yingkai Liu †,‡,§,*
PMCID: PMC9979365  PMID: 36873010

Abstract

graphic file with name ao2c07092_0008.jpg

Ni-doped ZnO nanowire arrays (Ni–ZnO NRs) with different Ni concentrations are grown on etched fluorine-doped tin oxide electrodes by the hydrothermal method. The Ni–ZnO NRs with a nickel precursor concentration of 0–12 at. % are adjusted to improve the selectivity and response of the devices. The NRs’ morphology and microstructure are investigated by scanning electron microscopy and high-resolution transmission electron microscopy. The sensitive property of the Ni–ZnO NRs is measured. It is found that the Ni–ZnO NRs with an 8 at. % Ni precursor concentration have high selectivity for H2S and a large response of 68.9 at 250 °C compared to other gases including ethanol, acetone, toluene, and nitrogen dioxide. Their response/recovery time is 75/54 s. The sensing mechanism is discussed in terms of doping concentration, optimum operating temperature, gas type, and gas concentration. The enhanced performance is related to the regularity degree of the array and the doped Ni3+ and Ni2+ ions, which increases the active sites for oxygen and target gas adsorption on the surface.

1. Introduction

Due to the rapid development of industrialization and urbanization, toxic and hazardous gases have posed a great threat to the ecosystem. Among them, the detection and continuous monitoring of volatile organic compounds (VOCs) and toxic gases has become a top priority in society’s efforts to provide a safe and healthy environment.1−3 H2S is a highly toxic gas, and even small amounts inhaled will cause death in a short time.4−6 Even low concentrations of inhalation will have some effect on the eyes, respiratory system, and central nervous system.7−9 However, VOCs are widely presented in the decomposition of organic compounds, the placement of sewage, and the biogas produced by landfill sites.10,11 It is crucial to monitor them in daily life. Among the existing gas sensors, metal oxides are widely used because of their high sensitivity, low cost, flexible production methods, and compatibility with electronic devices.12,13 ZnO, as a typical n-type semiconductor, has received much attention because it owns high electron mobility and photoelectric response, chemical stability, and thermal stability.14 ZnO is employed in various scenarios including solar cells,15 photocatalysis,16 optoelectronic devices, and gas sensors.17−20 More and more researchers have devoted themselves to the research and development of metal oxide sensors since Katoch et al. developed a simple ZnO gas sensor in 1971. M. R. Modaberi prepared a Ni-doped ZnO nanorod as a H2S sensor with a response of 45.3 to 100 ppm H2S at 200 °C.21 Bhati et al. synthesized reduced graphene oxide (rGO) nanosheets (0, 0.04, 0.11, 0.17, and 1.04 wt % rGO-ZnO) to nanofibers to detect NO2 by electrostatic spinning and found that 0.04 wt % rGO-ZnO exhibited the highest performance at 400 °C to 5 ppm NO2.22 The above-mentioned work revealed that doping played an effect on the performance of ZnO gas sensors.23−28 In this work, ZnO NRs are fabricated on etched FTO electrodes by a hydrothermal method and the composition of Ni (0–12 at. %) in ZnO NRs is controlled by means of a bimetallic-organic precursor in a single hybridization. Multitudinous characterization approaches are employed to explore the morphology, microstructure, elemental composition, and valence state of the samples. A series of the Ni-doped ZnO nanowire array (Ni–ZnO NR) sensors are developed to monitor H2S. Compared to the pure counterpart, it revealed that the obtained Ni–ZnO NRs with an 8 at. % nickel precursor concentration (hereafter denoted as the 8 at. % Ni–ZnO NRs) have the strongest responsivity.

2. Experimental Details

2.1. Synthesis of ZnO NP Gas Sensors

The growth mode is as follows.29 The etched FTO electrodes were used as the gas-sensitive electrodes. The etched channel was 1 mm in length with a width of 0.5 mm. Before preparing the ZnO sensors, the etched FTO electrodes were washed with acetone, ethanol, as well as deionized water for 10 min by ultrasonication and then blown dry by a nitrogen gun and placed in a blast drying oven for 20 min. After that, the electrode was put into the plasma cleaning instrument for 10 min.

The sensors of ZnO NRs were prepared in situ on electrodes using a simple drop coating technique and the hydrothermal method. The gas-sensitive electrode was placed on a horizontal surface. Then, 0.02 g of a seed solution was placed on the center of the electrode using a capillary tube with approximately 1 mm diameter. The electrodes were put into a blast dryer for 20 min and then annealed in a tube furnace at 350 °C for 20 min. In this way, we obtained a uniform seed layer growing on the surface of the etched FTO electrodes. The treated electrodes were then placed into the reactor. Second, a 0.25 M aqueous solution of hexamethylenetetramine and a 0.25 M aqueous solution of Zn(NO3)2·6H2O (99.0%) were put into the inner liner of Teflon in the ratio of 1:1. The reactions were carried out in an autoclave at 95 °C for 3 h. We removed the sample and cleaned it with deionized water after the reactor had naturally cooled to room temperature. After that, the cleaned electrodes were placed in a tube furnace and annealed at 400 °C for 60 min. We repeated this proceed three times in order to obtain longer and more homogeneous nanowire arrays. Consequently, the ZnO NRs on the etched FTO electrodes were fabricated and annealed at 650 °C for 1 h to obtain the device.

In addition, we further fabricated Ni–ZnO NRs with different concentrations via a single hybridized bimetallic-organic precursor approach. Different concentrations of Zn(NO3)2·6H2O (99.0%) to Ni(NO3)2·6H2O (99.0%) to replace a 0.25 M aqueous solution of Zn(NO3)2·6H2O (99.0%) were used. The precursor solution produced in the above step was used as the starting precursor to prepare Ni-doped ZnO NRs with different concentrations in the above-mentioned manner.

The production flow chart of Ni-doped ZnO NRs is shown in Figure 1.

Figure 1.

Figure 1

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Preparation of the ZnO NR sensor.

2.2. Material Characterization

X-ray photoelectron spectroscopy (XPS, Kα, America) was conducted in an ion-pumped chamber. The morphologies of the samples were characterized by field-emission scanning electron microscopy (Quanta FEG 250, America) and transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN instrument with a field emission gun at 20 kV).

2.3. Measurement of the Gas Sensors

The sensing property of the device was measured using an intelligent gas-sensitive analysis system (CGS-1TP). The process was as follows: first, the sensor was put on a hot table at 250 °C for 48 h. Then, the sensor was connected to the platform electrodes to form a closed loop. The steps for gas response measurements were as follows: (1) the prepared device was placed on a test bench; (2) after tying the probe to the etched FTO electrode to form a loop, the cap was closed to form a closed system, and the Rg value was recorded; (3) a certain amount of the gas was injected into the evaporation tray to fill the entire cavity. The resistance of the sensor changed. Finally, the cavity cover was removed for desorption.30 All measurements were performed in a well-ventilated laboratory. The dehumidifier retained the relative humidity at 45%. The sensor’s response is defined as the ratio of exposure resistance of the ZnO NP sensor in air (Ra) to the resistance of the target gas in the atmosphere (Rg) and can be expressed as

2.3. 1

3. Results and Discussion

3.1. Structure and Morphology

The morphology and microstructure of the 8 at. % Ni–ZnO NRs are investigated by SEM and TEM. Figure 2a,b shows the top view SEM images of the 8 at. % Ni–ZnO NRs at low and high magnifications. It revealed that the large area and homogeneous structures of NRs were successfully synthesized. Figure 2c shows the side view by SEM of the sample. It can be noted that the nanowires have a good array structure with a length of 3–4 μm. Figure 2d shows the SEM image of the Ni–ZnO NRs on the etched FTO electrodes. It is observed that our Ni–ZnO NRs evenly grow on an intercalated electrode, in which the etched channel is 1 mm in length with an electrode width of 0.5 mm. We can further observe that the NRs were distributed sparsely on the channel. Figure 2e shows the TEM image of the NRs, indicating that every NR has a smooth surface with a uniform diameter. Figure 2f shows the high-resolution TEM (HRTEM) image of a single Ni–ZnO NR at the cylindrical rod junction. The NR consists of small and uniform arrangements of a uniform rod-like structure, and the array structure is more conducive to efficient electron transfer. Figure 2f shows a clear lattice stripe with a 0.2593 nm spacing distance, corresponding to the (002) plane of ZnO. The small and sparse structure of ZnO NRs is more conducive to their making full contact with the gas and improving its sensitivity.

Figure 2.

Figure 2

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SEM, TEM, and HRTEM images of 8 at. % Ni–ZnO NRs. (a,b) Top view SEM images at low and high magnifications. (c) Side view of the SEM images. (d) SEM image of the etched FTO electrodes. (e) TEM image. (f) HRTEM image.

The chemical states of the atoms in the samples are determined by XPS. Figure 3a shows the survey scan XPS spectra of the Ni–ZnO NRs and the undoped counterpart. The constituent elements of the Zn 2p, Ni 2p, and O 1s peaks are detected. The binding energies obtained in the XPS data are standardized for specimen charging using C 1s as the reference at 284.8 eV. Figure 3b shows the Zn 2p peaks for all samples. Compared to the undoped counterpart, the Zn 2p3/2 and Zn 2p1/2 nuclear energy levels are centered around 1021.28 and 1043.68 eV, respectively. However, the Ni-doped ZnO nuclear energy levels undergo a shift of 0–0.7 eV toward a high energy. The peak difference (23.1 eV) in the spin orbit splitting energy between the two nuclear energy levels remains unchanged.

Figure 3.

Figure 3

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XPS spectra of ZnO and Ni–ZnO NRs with Ni precursor concentrations of 4, 8, and 12 at. %. Full spectra of (a) undoped and Ni-doped ZnO NRs and (b) Zn 2p3/2 and Zn 2p1/2.

The binding energy spectra of O 1s for the different samples are shown in Figure 5a–d, in which the O 1s peak was deconvoluted into three sub-peaks. Figure 5a–d shows that these three peaks can be divided into two types: the regular lattice sites31 and hypoxic sites (OV).32 The OL is divided into two types, O–Zn/Ni and O–C. The OV intensity ratio increases with the increase of doping concentration. It is also found that with an increase of doping concentration, the peak area OV to O 1s ratios of the Ni–ZnO NRs to that of the pure counterpart are 5.7, 5.2, 6.8, and 5.0, respectively, revealing that doping effectively increases the amount of anoxia to (OV). Therefore, from the analysis of O 1s, the 8 at. % Ni–ZnO NRs having more abundant OV should have better performance, which is consistent with our test results.

Figure 5.

Figure 5

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(a–c) XPS spectra of ZnO and Ni–ZnO NRs with Ni precursor concentrations of 4, 8, and 12 at. %.

Figure 6a–c shows that the Ni concentrations in the precursor solution are 4, 8, and 12 at. % in ZnO NRs. Figure 6a–c shows the curves with Ni 2p3/2, Ni 2p1/2, and shakeup satellite peaks. The Ni 2p3/2 peak at 855.28 eV in Figure 6b can be divided into two sub-peaks, corresponding to Ni2+ and Ni3+ valence states, respectively. The nickel ions are mainly present in the divalent state in ZnO NRs at lower nickel concentrations. However, the proportion of the trivalent state increases with increasing nickel content. The spin–orbit splitting energy is 17.36 eV, being different from 17.8 eV of metallic Ni, certifying that the nickel has successfully taken the place of zinc, and the reason for this phenomenon is the charge-transfer leap of the O 2p and Ni 3d hybridization in the final state.

Figure 6.

Figure 6

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Sensitive properties of the Ni–ZnO NRs and undoped counterpart. (a) Response versus temperature (T) to 5 ppm H2S; inset: response comparison of four samples at 250 °C. (b) Selectivity of 8 at. % Ni–ZnO NRs to different gases. (c) Dynamic response of 8 at. % Ni–ZnO NRs. (d,e) Cyclic stability of 8 at. % Ni–ZnO NRs and the single-set desorption curve at 250 °C. (f) Long-term stability of 8 at. % Ni–ZnO NRs for 30 days.

According to the references, it can be concluded theoretically that the presence of an appropriate content of Ni2+ in ZnO will reduce the activation energy of H2S gas adsorption, which is also the reason why doping can improve the performance. The formation of a small amount of Ni3+ is caused by the defect of hydrothermal growth. Moreover, the content of Ni3+ is less than that of Ni2+, so it does not play a major role in this process.

We also performed an EDX analysis of the material as shown in the Supporting Information Figure S1. As shown in Figure S1, we can see that Ni element is successfully doped into the ZnO material. It can be further demonstrated that the materials prepared in this paper have a uniform distribution of Zn, Ni, and O elements.

As shown in the Supporting Information Figure S2, we have performed X-ray diffraction spectra of four materials with different Ni doping concentrations in the range of 30–70°. The detection results demonstrate the ZnO (JCPDS card no. 36-1451) (100), (002), (101), (102), (110), and (103) facets corresponding to the hexagonal fibrillated zincite structure. The reason for this phenomenon is that the small amount of Ni element does not constitute the cluster structure of NiO but replaces the lattice position of Zn and does not change the crystal structure of the material. This phenomenon also indicates that the single-source metal precursor-induced growth method used in this paper can effectively dope Ni into ZnO NRs.

3.2. Gas-Sensitive Performance Analysis

Figure 4a shows the sensitive performance of the Ni–ZnO NRs with (0 to 12%) Ni precursor concentration to 5 ppm H2S at 90–350 °C. The response increases with increasing temperature up to 250 °C, which is the optimum work temperature, indicating that the fastest surface chemistry has good carrier mobility. At above 250 °C, oxygen desorption may be faster than oxygen adsorption and thus less adsorbed oxygen molecules are available to react with the target gas, resulting in a decrement in response at higher temperatures.33,34 The increasing temperature accelerates the electron leap between the conductor band and the surface Fermi energy level, which facilitates the desorption reaction.35

Figure 4.

Figure 4

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(a–d) O states of undoped and Ni–doped ZnO NRs.

The responses for undoped and Ni–ZnO NRs at 250 °C for 5 ppm H2S gas are presented in the inset of Figure 6a. This shows that the response is the highest for ZnO NRs grown in a precursor solution with a Ni concentration of 8 at. % Ni–ZnO NRs, and the response to H2S is improved by a factor of 6.24 compared to that of the pure ZnO NRs. It is worthy of attention that the response of the 8 at. % Ni–ZnO NRs is enhanced in a non-linear manner. Moreover, the performance of the gas-sensitive material decreases when the doping concentration continues to increase. As the Ni concentration increases the proportion of Ni3+ ions increases supersaturation can form a stable structure with more oxygen. Therefore, when the Ni concentration is greater than 8%, OV decreases instead. The reason is that the change in OV concentration is caused by Ni doping. The XPS results show a decrease in the OV concentration of the material at a concentration of 12%.

The following tests are conducted at 250 °C. Figure 6b shows the responses of the 8 at. % Ni–ZnO NRs to 5 ppm H2S as well as NO2 and 100 ppm VOCs. It is seen that the device has a good selectivity for H2S, resulting in better adaptability in practical applications.

Figure 6b shows the response of the 8 at. % Ni–ZnO NRs to different VOCs and toxic gases. They exhibit excellent selectivity to H2S. To evaluate their performance with respect to H2S, Figure 6c shows their response in the range of 0.5–10 ppm. Two cycles of testing are carried out for different concentrations of the gas, and we can see a good positive correlation between the response and the gas concentration as the H2S concentration increases from 0.5 to 10 ppm. We also calculated the theoretical limits of H2S using the signal-to-noise ratio, which are expressed as follows

3.2. 2
3.2. 3
3.2. 4

The calculation yields a detection limit of 3.41 ppb for the 8 at. % Ni–ZnO NRs.

Figure 6d shows a five-cycle test of the gas-sensitive performance of the device at 5 ppm. It is found that the device has good cycling stability. Figure 4e shows the response/recovery time to 5 ppm H2S at 250 °C. The response/recovery time is 75/57 s. The response time on attachment and the recovery time on desorption are dependent on the gas concentration, type, working temperature, and doping concentration. The results for all H2S concentrations show that higher temperatures lead to a faster response and recovery time, thus reducing the response time. The rapid response/recovery course at higher temperatures can be attributed to faster surface chemistry, enhanced gas phase diffusion, and higher charge carrier mobility. The long-term stability of the sensor is evaluated over a period of 40 days, and the results are shown in Figure 6f. Its response fluctuated between 62 and 68.9 to 15 ppm H2S over a long period. It is also found that the test environment also had an effect on the performance of the device. The sensor is stored at atmosphere conditions for the duration of the test and is not treated in a special way, suggesting that the device has good stability.

The operating temperature is an important parameter for gas sensors. Different temperatures have significant effects on the surface state of the material. At lower temperatures, the molecules do not have sufficient active energy to adsorb onto the sensing channels of the material, resulting in a lower response.36 As the operating temperature increases, adsorption gradually changes from physical to chemisorption, providing higher energy levels accompanied with a different charge transfer. Chemisorption at relatively high temperatures has two kinds of mechanisms for two gases that work together in this process, one is an oxidizing gas O2 and the other is a reducing gas, such as H2S, which can occur on the surface as follows

3.2. 5
3.2. 6

The property of sensitivity to most metal oxide semiconductors is related to the oxygen ions adsorbed on the surface of the sensor.37 Oxygen molecules are absorbed on the surface, and electrons in the conduction band of the metal oxide are trapped by the absorbed oxygen molecules, forming negatively charged chemisorbed oxygen ions as they are exposed to air. Depending on the operating temperature, oxygen ions may take different forms. It takes less time to reach thermal equilibrium because H2S possesses a lower adsorption energy, but the true reaction rate depends on the concentration of H2S in the gas and the effective surface location where it can be captured. The reaction involved in the adsorption of H2S gas is chemisorption. Its activation energy can be determined from the Arrhenius equation: R = R0 exp(−E/kT). Based on Modaberi’s work,38 it is known that Ni doping can effectively reduce the activation energy of H2S adsorption at all temperatures. The activation energy of adsorption increases with the increase of Ni content when the content of Ni is greater than 8%, which is consistent with our performance test results.

As shown in Table 1, we have analyzed the performance of similar oxide semiconductor substrate H2S detectors. After comparison, it is found that compared with other sensors of the same type, it has a higher response value and a lower detection limit at the best operating temperature. It provides a better choice for H2S gas detection.

Table 1. Comparison of the Reported H2S Sensing Performances Based on Different Nanostructures and This Work.

sensing materials operating temperature (°C) concentration (ppm) response LOD (ppm) references
Ni doping-ZnO nanorods 200 100 45.6   (38)
ZnO-rGO RT 20 14.9 8 (39)
ZnO/CuO hollow 170 50 66.6 0.05 (40)
α Fe2O3 nanoparticles 300 10 5.3 0.05 (41)
ZnO nanorod-bundle 350 50 3   (37)
CuO NP decorated porous ZnO NRs 100 200 4.69   (42)
CdS/CdO 400 5 73.5 0.01 (43)
CuO 150 10 76.5   (44)
ZnO–ZnFe2O4 250 10 84.5   (45)
Cr2O3 170 100 42.81   (46)
Ni-doped ZnO NRs 250 5 68.9 0.003 this study
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4. Conclusions

In this paper, we successfully synthesized Ni-doped ZnO NRs with different concentrations using a precursor solution with bimetallic hexamethyl-hypotetra as a single source. The method can selectively control the atomic ratio of Ni to Zn in double-click metal but also has the advantages of low cost, high yield, short reaction time, and chemical safety. The XPS results indicate that the nickel element effectively replaces the zinc lattice in the material. A series of characterization examinations revealed that the 8 at. % Ni–ZnO NRs had abundant oxygen vacancies, excellent thermal stability, and a large specific surface area. Gas sensitivity indicated that the sensor had a response (68.9) and faster response/recovery (79/54 s) to 5 ppm H2S, whose response is 6.26 times as large as that of the pure sample. In addition, the detection limit of our device is 3.41 ppb, ensuring long-term detection of H2S and ppb-level detection in practical applications. Therefore, the concept of increasing the oxygen vacancies in the material by adding a moderate amount of doping to improve the gas-sensitive properties can be extended to other gas-sensitive materials. This also provides a new option for H2S gas detection.

Glossary

Abbreviations

VOCs

volatile organic compounds

XPS

X-ray photoelectron spectroscopy

FESEM

field-emission scanning electron microscopy

TEM

transmission electron microscopy

NRs

nanowire arrays

LODs

limits of detection

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c07092.

  • Details of materials required for additional experiments; EDX point scan of 8 at. % Ni–ZnO NRs; and XRD patterns of the as-grown un-doped and Ni-doped ZnO NRs (PDF)

Author Contributions

S.Y.L. and Y.K.L. performed the experimental design and analysis and wrote the manuscript, W.Y.Y. contributed to the preparation of the devices and obtaining SEM measurements, and H.Z. and H.H.C. contributed to analyzing data and sensing measurements. All authors have read and approved the final manuscript.

This work is funded by the National Natural Science Foundation of China (grant nos. 11764046, 12264056, and 62164013) and Yunnan Expert Workstation Project (202205AF150008).

The authors declare no competing financial interest.

Notes

Availability of data and materials; all data are fully available without restriction from the corresponding author on reasonable request.

Supplementary Material

ao2c07092_si_001.pdf (200KB, pdf)

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