An effective H₂S sensor based on SnO₂ nanowires decorated with NiO nanoparticles by electron beam evaporation

Abstract

The highly toxic hydrogen sulphide (H₂S) present in air can cause negative effects on human health. Thus, monitoring of this gas is vital in gas leak alarms and security. Efforts have been devoted to the fabrication and enhancement of the H₂S-sensing performance of gas sensors. Herein, we used electron beam evaporation to decorate nickel oxide (NiO) nanoparticles on the surface of tin oxide (SnO₂) nanowires to enhance their H₂S gas-sensing performance. The synthesised NiO–SnO₂ materials were characterised by field-emission scanning electron microscopy, transmission electron microscopy and energy dispersive spectroscopy analysis. H₂S gas-sensing characteristics were measured at various concentrations (1–10 ppm) at 200–350 °C. The results show that with effective decoration of NiO nanoparticles, the H₂S gas-sensing characteristics of SnO₂ nanowires are significantly enhanced by one or two orders compared with those of the bare material. The sensors showed an effective response to low-level concentrations of H₂S in the range of 1–10 ppm, suitable for application in monitoring of H₂S in biogas and in industrial controls. We also clarified the sensing mechanism of the sensor based on band structure and sulphurisation process.

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1. Introduction

Vietnam is a developing country characterised by the growth of industrial zones and farm houses, thereby contributing to the environmental and air pollution problems.¹ The World Health Organisation reported in 2018 that approximately 60 000 annual deaths in Vietnam are linked to air pollution. Among pollutant gases, hydrogen sulphide (H₂S), a colourless, poisonous, corrosive and flammable gas with a rotten eggs smell that is mainly produced by microbial degradation of organic substances always exists in biogas.² However, most biogas produced in Vietnam is used without any monitoring or desulphurisation,³ although the permissible exposure limit for 8 h time-weighted average concentration of this gas is 10 ppm as announced by the Occupational Safety and Health Administration.⁴ Monitoring H₂S gas⁵ at low concentrations (ppm level) is important and the key issue in the safe usage of biogas and industrial processes.⁶ Different nanomaterials and/or nanostructures have been used for H₂S monitoring,^7,8 where metal oxide-based sensors are the most popular due to their low cost, high sensitivity, compact size, real-time detection, ease of use, portability and low power consumption.^9,10,11 Tin dioxide (SnO₂) is a well-known n-type semiconductor used as a sensing material in resistive gas sensors owing to its high sensitivity for different gas species.¹² However, this material shows relatively low sensitivity to H₂S gas; thus, numerous attempts have been made to enhance its performance.^13–16 Enhancement of gas-sensing performance of metal oxides by doping¹⁷ or surface decoration is an effective method because it can utilise the advantages of surface modulation and high catalytic activity of decorated materials.^18,19 Noble metals are generally used as decorative materials to enhance gas sensing performance, but they are expensive, leading to high cost of products.^14,20 Using other abundant materials to functionalise the surface of SnO₂ to enhance its sensing performance has becomes one of the priorities in recent years.^21,22 The common materials used to functionalise the surface of n-type SnO₂ to enhance H₂S sensing performance include p-type semiconductors, such as CuO and nickel oxide (NiO); these materials are applied to utilise the synergic effects of p–n heterojunction and catalytic activity of decorated materials.^22,23 The p-type NiO is highly reactive with H₂S, that is, H₂S can convert NiO into NiS;²⁴ thus, this material is currently used to decorate or functionalise SnO₂ to enhance its H₂S-sensing performance.²⁵ Several researchers have studied the decoration of SnO₂ nanomaterials by NiO nanoparticles to enhance their gas sensing performance.²⁶ Lee et al.²⁷ reported the improvement of H₂S sensing properties of thick-film SnO₂-based gas sensors by surface decoration with NiO and MoO₃ nanoparticles. Wet chemical pathways, such as electrospinning²⁸ drop casting,²⁶ sol–gel²⁹ and hydrothermal^30,31 methods, have been used to decorate NiO on the surface of SnO₂ to enhance its gas-sensing performance. Wet chemical methods feature advantages, such as simple and low-cost fabrication, but face limitations in exact control of decoration density.³² By contrast, electron beam evaporation is highly effective method for deposition of extremely thin NiO film over the SnO₂ layer to enhance its gas-sensing properties.³³ However, no report was conducted on the surface decoration of SnO₂ nanowires with NiO nanoparticles by electron beam evaporation for enhancement of H₂S-sensing performance, although this method is an effective technique to synthesise high-quality NiO thin films of different thicknesses.³⁴

In this work, we report our study on the electron beam evaporation-based decoration of NiO nanoparticles on the surface of on-chip grown SnO₂ nanowires to enhance their H₂S gas-sensing performance. The effects of NiO thickness or density on the H₂S gas-sensing performance of SnO₂ nanowire sensors were studied. The results demonstrate that by surface decoration with NiO nanoparticles, the SnO₂ nanowire sensor showed excellent performance for monitoring low H₂S concentrations. The H₂S gas-sensing mechanism of SnO₂ nanowires decorated with NiO nanoparticles was discussed under the light of band structure and sulphurisation process.

2. Experimental

Fig. 1(A) shows the design of the gas sensor based on SnO₂ nanowires decorated with NiO nanoparticles. The sensor includes SnO₂ nanowires grown on-chip on thermally oxidised silicon substrate deposited with a pair of interdigital Pt electrodes and a thin layer of Au on top as catalyst. On the surface of SnO₂ nanowires, tiny NiO nanoparticles were decorated to modulate the conductive channel of the nanowires by electron beam evaporation followed by thermal oxidation in air. NiO is a p-type semiconductor, thus, this compound forms a p–n heterojunction after deposition on the surface of n-type SnO₂, as depicted in Fig. 1(B). H₂S gas sensor based on SnO₂ nanowires decorated with NiO nanoparticles was fabricated by a two-step process. Firstly, SnO₂ nanowire sensor was fabricated by on-chip growth vapor–liquid–solid techniques as reported elsewhere.³⁵ Briefly, 0.3 g of tin (purity of 99.9%) was loaded in an alumina boat and placed at the central zone of a horizontal quartz tube furnace. Silicon substrate containing arrays of electrodes was placed 2 cm from the alumina boat. The entire system was purged with Ar (99.99%) with a flow rate of 300 sccm for 5 min, and temperature was increased at a rate of 36 °C min⁻¹ from room temperature to 750 °C. The chemical vapor deposition process was subsequently carried out at a temperature of 750 °C with an oxygen gas flow of 0.5 sccm and pressure of 1.8 × 10⁻¹ Torr. Growth time was maintained at 750 °C for 20 min before turning off the furnace and cooling naturally to room temperature. The SnO₂ nanowires grown by this method presented a clean surface (Fig. S1, ESI†).

Fig. 1 (A) Design of sensor based on SnO₂ nanowires decorated with NiO nanoparticles; (B) band structure and current flow of SnO₂–NiO heterojunction.

After NiO nanoparticle decoration on the surface of SnO₂, nanowires were prepared by electron beam evaporation. Ni with three different thicknesses (thickness estimated from the deposition rate of the system) of 3, 5 and 10 nm were deposited to investigate the effect of Ni thickness on the sensing performance. The decorated samples were annealed in air at 600 °C for 3 h to convert Ni into NiO by increasing the temperature to 600 °C at a rate of 5 °C min⁻¹. The synthesised materials were studied by field-emission scanning microscopy (JEOL JSM-7600F) and transmission electron microscopy (TEM, JEM, 2100F). The gas sensing characteristics of the fabricated sensors were measured at temperatures of 200 °C, 250 °C and 300 °C by a dynamic technique as reported elsewhere.⁸ During measurement, sensor resistance was continuously measured by a current-source meter (Keithley model 2602B) interfaced with a computer, whereas the gas was switched on/off from air to H₂S gas. The total flow rate of analytic gas was 400 sccm, whereas H₂S concentration varied from 1 ppm to 10 ppm.³⁶ Sensor response is defined as S = R_a/R_g, where R_a and R_g denote the resistances of the sensor in dry air and tested gas, respectively.

3. Results and discussion

3.1 Materials and gas sensing characteristics

Fig. 2 shows the low- and high-magnification SEM images of the synthesised materials. The inset in Fig. 2(A) displays the SEM image of the sensor chip, which includes two electrodes with numerous fingers of average size of is 20 μm; the gap between two fingers is 20 μm. The SnO₂ nanowires were grown on entirely Pt electrode fingers. The SnO₂ nanowire film was considerably thick. Thus, bare silicon substrate cannot be observed, ensuring that Ni deposition could not form a continuous layer. The SnO₂ nanowires exhibited an average size of about 100 nm. Herein, as the Au catalyst was deposited on Pt electrode, the SnO₂ nanowires featured a needle-like morphology resulting from the poor wettability of Au on Pt. After decoration with NiO, the nanoparticles were distributed homogenously on the surface of SnO₂ nanowires [Fig. 2(C and D)]. The NiO nanoparticles exhibited a very small size of approximately 5 nm at a Ni sample with 3 nm thickness. The size and density of NiO nanoparticles are very low on the surface of nanowires decorated with a 3 nm thick Ni decoration. However, with increasing Ni thickness from 3 to 5 nm, the density and size of NiO nanoparticles increased. The NiO nanoparticles showed not a spherical but an irregular shape. This finding can be explained by the strong adhesion between deposited Ni and SnO₂. Thus, upon heat treatment, the Ni layer was partially melted and oxidised to form irregular shapes. Ni deposition formed no continuous layer covering the SnO₂ nanowires but incoherent nanoparticles. SEM observation showed the formation of NiO nanoparticles, whereas energy-dispersive X-ray spectroscopy analysis confirmed the composition of Si, Ni, O and Sn in the sample (Fig. S2, ESI†).

Fig. 2 SEM images of SnO₂ nanowires decorated with NiO of different thicknesses: (A and B) 3 nm; (C and D) 5 nm; (E and F) 10 nm. Inset of (A) is a SEM image of sensor device.

Further characteristics of the SnO₂ nanowires and NiO–SnO₂ nanowires were studied by TEM images (Fig. 3). Fig. 3(A) reveals low-magnification TEM image of bare SnO₂ nanowires featuring a smooth surface. The SnO₂ nanowires showed a single crystallinity nature where clear lattice fringes are present [Fig. 3(B)]. The gap between adjacent lattice fringes is 0.33 nm, corresponding to the interspace of (110) plane of SnO₂.³⁷ Fig. 3(C) shows the surface of a SnO₂ nanowire after decoration with NiO nanoparticles. The NiO nanoparticles of approximately 10 nm were decorating homogenously the SnO₂ nanowires. Fig. 3(D) displays the high-magnification TEM image of SnO₂ nanowire decorated with NiO nanoparticles. The lattice fringes of SnO₂ is 0.33 nm. The NiO nanoparticles are highly crystalline, in which lattice fringes can be observed in with an interspace of 0.24 nm, corresponding to the distance between (111) atomic layers.³⁸

Fig. 3 Low and high magnification TEM images of (A and B) bare SnO₂ nanowires and (C and D) SnO₂ nanowires decorated with NiO nanoparticles.

H₂S is a toxic gas that is mainly present in biogas at relatively high concentrations at the ppm level. Thus, the sensing characteristics of fabricated sensors were measured at H₂S gas concentrations in the range of 1–10 ppm at different temperatures [Fig. 4–6]. Fig. 4 illustrates the transient resistance versus time upon exposure to different concentrations of H₂S measured at temperatures ranging from 200 °C to 350 °C of SnO₂ nanowire sensor decorated with 3 nm NiO. The base resistance of sensor at 200 °C in air was approximately 45 kΩ. Upon exposure to 10 ppm H₂S, the sensor resistance decreased to 13 kΩ with a response time of approximately 15 s [Fig. 4(A)]. When H₂S gas stopped flowing, the sensor resistance recovered to the initial value in a few minutes. With variation of H₂S concentrations from 10 ppm to 1 ppm, the sensor still exhibited good response characteristics, indicating the possibility of detection limit at the sub-ppm level. At a relatively high working temperature of approximately 250 °C, the sensor showed similar trend in response, that is, the resistance decreased significantly upon exposure to different H₂S gas. The response and recovery time of the sensor measured at 250 °C were 12 and 58 s, respectively. The response and recovery speeds of sensor improved with increased working temperature. At a high working temperature of 300 °C, the response and recovery time of the sensor approximated 6 and 35 s, respectively. Fig. 4(D) shows the sensor response as a function of H₂S concentration at different working temperatures. Sensor response increased linearly with the increase in H₂S concentrations from 1 ppm to 10 ppm. At a given H₂S concentration, the sensor response increased with increase in working temperature from 200 °C to 300 °C. The sensor response to 1 ppm H₂S was approximately 1.7 at 200 °C and increased to 2.1 and 2.7 with increasing working temperature to 250 °C and 300 °C, respectively. The sensor response improved, but it was still very low when considering that the response of bare SnO₂ nanowire-based sensor to 10 ppm H₂S at 300 °C approximated 3.2 (Fig. S3, ESI†).

Fig. 4 H₂S sensing characteristics of SnO₂ nanowires decorated with NiO (3 nm) measured at different temperatures: (A) 200 °C; (B) 250 °C; (C) 300 °C; (D) sensor response as a function of H₂S concentration.

Fig. 5 H₂S sensing characteristics of SnO₂ nanowires decorated with NiO (5 nm) measured at different temperatures: (A) 200 °C; (B) 250 °C; (C) 300 °C; (D) sensor response as a function of H₂S concentration.

Fig. 6 H₂S sensing characteristics of SnO₂ nanowires decorated with NiO (10 nm) measured at different temperatures: (A) 200 °C; (B) 250 °C; (C) 300 °C; (D) sensor response as a function of H₂S concentration.

Fig. 5(A–D) display the H₂S sensing characteristics of SnO₂ nanowires decorated with 5 nm NiO. The base resistance of the sensors at 200 °C in air was approximately 1.1 MΩ. This value is much higher than that of SnO₂ nanowires decorated with 3 nm NiO, indicating the enhanced depletion region at the interface between NiO and SnO₂. Upon exposure to H₂S gas, sensor resistance decreased, similar to that of the sample decorated with 3 nm NiO. However, the sensor showed the highest response at a working temperature of 200 °C but not at 300 °C. This result implies that we can reduce the working temperature of SnO₂ nanowire sensors by increasing NiO thickness. In addition, the 5 nm NiO sensor response was relatively high, and the response to 1 ppm H₂S was approximately 56 at 200 °C. The sensor response increased from 56 to 100 with increasing H₂S concentration from 1 ppm to 10 ppm, indicating that the sensor was still unsaturated in the measured range.

Fig. 6(A–D) shows the H₂S sensing characteristics of SnO₂ nanowires decorated with 10 nm NiO. Further increase in the NiO thickness to 10 nm improved the sensor response, but a notably poor recovery characteristic was observed. Sensor resistance could not recover to the initial value after H₂S gas stopped flowing for 1000 s, thus limiting its reusability. The result is similar to that of 5 nm NiO sensor, possibly due to the strong reduction of NiO into NiS. In this case, we tested the H₂S response of the sensor to various H₂S concentrations at different temperatures and observed that the responses at 200 °C were the highest. The sensor response increases from 77 to 219 with increase of H₂S concentration from 1 to 10 ppm [Fig. 6(D)]. However, poor recovery characteristics can limit the practical applications of sensor.

Selectivity and stability of the sensor were studied for the 10 nm NiO sample, and the data are shown in Fig. 7. The data demonstrates that the NiO–SnO₂ nanowire sensor has good selective response to H₂S among other gases including of CO, H₂, NH₃, CH₄, and SO₂ at measured temperature of 200 °C [Fig. 7(A)]. In addition, the sensor also showed good stability upon five cycles exposure to H₂S gas and back to air, as shown in Fig. 7(B). Such those results confirm that the NiO–SnO₂ nanowire sensor is effective for application in monitoring H₂S gas at low concentrations with high sensitivity and good selectivity.

Fig. 7 (A) Selectivity and (B) stability of NiO–SnO₂ nanowire sensor measured at 200 °C.

Table 1 summarizes recent studies on sensing performance of H₂S gas sensors based on different materials. As can be seen, the pristine metal oxides showed low sensitivity to H₂S. However, by surface decoration, the sensor response was enhanced. Our sensor showed much higher response compared to others.

Table 1 A comparison data of our results and recent studies

No.	Materials	Working condition	S = Ra/Rg	Ref.
1	α-Fe2O3 nanochains	5 ppm H2S @ 285 °C	5	39
2	α-Fe2O3 nanosheets	5 ppm H2S at 135 °C	5.8	40
3	ZnFe2O4 nanoparticles	5 ppm H2S at 135 °C	15.1	41
4	Porous ZnFe2O4 nanosheets	1 ppm H2S at 85 °C	39.8	42
5	Fe2O3/ZnO nanoplates	100 ppm H2S at 250 °C	130	43
6	NiO/ZnO nanowires	100 ppm H2S at room temperature	∼0.31	44
7	Au:Fe2O3 thin film	10 ppm H2S at 250 °C	6.38	45
8	Ag-doped CaCu3Ti4O12 films	10 ppm H2S at 250 °C	100	46
9	Ni-doped CaCu3Ti4O12 films	10 ppm H2S at 250 °C	120	47
10	rGO/ZnFe2O4 nanofibers	1 ppm H2S at 350 °C	147	8
11	Olive-shaped SnO2 nanocrystals	10 ppm H2S at 240 °C	120	48
12	NiO–SnO2 nanowires	1 ppm H2S at 200 °C	150	This study

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3.2 Gas sensing mechanism

Herein, sensor resistance decreased upon exposure to H₂S gas, indicating the increase in free electron in SnO₂. H₂S response is mainly based on the following reaction with pre-adsorbed oxygen species.⁸

2H₂S_(gas) + 6O_(ads)⁻ = 2H₂O_(gas) + 2SO_2(gas) + 6e⁻ (1)

2H₂S_(gas) + 3O_2(ads)⁻ = 2H₂O_(gas) + 2SO_2(gas) + 3e⁻ (2)

The released electrons contribute to increasing the main carriers in n-type SnO₂ semiconductor, thus decreasing sensor resistance. In addition, H₂S molecules can react with decorated NiO nanoparticles to form NiS upon exposure to H₂S gas. This reaction can occur easily at low temperature. Reaction with NiO results in the following:

H₂S_(gas) + NiO = H₂O_(gas) + NiS (3)

NiS shows a metallic behaviour⁴⁹ with a work function of 5.5 eV.⁵⁰ The conductivity of NiS can be expressed by the following equation:

where m* refers to the effective mass of electrons in the material, n denotes electron density, and τ represents the average time between two collisions of electrons in the material crystal. The charge carrier density of NiS is approximately 3.67 × 10²² cm³, and electron mobility is approximately 0.6 cm² V⁻¹ s⁻¹.⁴⁹ The carrier density and hole mobility of NiO are 4.30 × 10¹⁰ cm³ and 9.67 × 10² cm² V⁻¹ s⁻¹, respectively.⁵¹ The conductivity of NiO is much lower than that of NiS. Therefore, when exposed to H₂S gas, sulfidation of NiO to NiS reduced the sensor resistance. The SnO₂ nanowires feature a carrier density of the order of n ≈ 10¹⁶ cm⁻³ and mobility of μ ≈ 70 cm² V⁻¹ s⁻¹.^52,53 Furthermore, the work function of SnO₂ nanowires is approximately 4.9 eV.⁵⁴ The work function of NiO varies from 5.23 eV to 6.43 eV (ref. 55) depending on the adsorbate gases, such as oxygen or carbon dioxide. The decoration of NiO on the surface of SnO₂ nanowire forms the p–n junction, modulating the conducting channel of nanowires.⁵⁶ The depletion region of p–n junction is calculated by the following equation:where ε_r refers to the relative dielectric permittivity of the semiconductor, V_bi specifies the built-in voltage, and V denotes the applied bias. N_A and N_D represent the densities of ionised donors and acceptors, respectively. Upon exposure to H₂S, the NiO is sulphurised into NiS. As the work function of NiS (5.5 eV) is larger than that of n-type SnO₂ (4.9 eV), Schottky contact forms at the interface of SnO₂–NiS. The depletion width of the Schottky contact is calculated by the following equation:

The barrier value of V_{bi p–n} is approximately 1.2 eV, which is larger than the V_{bi Schotkky} value of approximately 0.6 eV. N_A ≈ 4.30 × 10¹⁰ cm³, N_D ≈ 10¹⁶ cm⁻³. The width Schottky contact is smaller than that of the p–n junction, reducing the sensor resistance. Therefore, decoration of NiO on the surface of SnO₂ significantly increased the H₂S sensing performance of SnO₂ nanowires.

4. Conclusion

We successfully decorated the surface of SnO₂ nanowires with NiO nanoparticles by electron beam evaporation. The density of NiO nanoparticles was controlled by varying the Ni thickness at 3, 5 and 10 nm. With effective decoration of NiO, the H₂S sensing characteristics of SnO₂ nanowires significantly improved by one or two orders. We also clarified the sensing mechanism of the sensor based on band structure, where the decoration of NiO nanoparticles on the surface of SnO₂ nanowires formed the p–n heterojunction and modulated the conducting channel of the nanowires. Such p–n heterojunctions are strongly sensitive to environmental H₂S gas, thereby improving the sensing performance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by the Hanoi University of Science and Technology (Grant No. T2017-PC-170). We thank Dr N. D. Dung (BKEMMA, HUST) for SEM and EDS measurements.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra01105f

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