Surface Defect Generation on SnO2 Nanoparticles Using High-Energy Ball Milling for H2S Gas Sensor Applications
Article information
Abstract
Hydrogen sulfide (H2S) is a highly toxic and dangerous gas with a flammable and corrosive nature, making the development of reliable gas sensors for its detection vital. This study investigated the enhancement in H2S gas sensing performance of commercial SnO2 powders after high-energy milling. SnO2 powders were subjected to high-energy milling for 30, 60, and 90 min and then were characterized using advanced techniques to evaluate their morphology, chemical composition, and crystallinity. The response of a pristine SnO2 gas sensor, and ones where the SnO2 was milled for 30, 60 and 90 min, were 2.46, 2.27, 3.01, and 1.98, respectively, to 10 ppm H2S at 300°C. Thus, the H2S gas sensing results revealed that the SnO2 powders milled for 60 min exhibited the highest sensing performance. This improvement in H2S sensing performance was attributable to the reduced particle sizes achieved through the high-energy milling process, which increased the surface area and created defects on the surface of the SnO2 particles, thereby enhancing the interaction between the gas molecules and sensor material. The smaller morphological size of the particles and surface defects subsequently promoted the resistance modulation crucial for H2S gas detection. This study demonstrates that high-energy ball milling can effectively boost the gas-sensing features of SnO2 powders. The findings can provide guidance for enhancing the gas-sensing capabilities of other resistive sensors.
1. INTRODUCTION
Hydrogen sulfide (H2S) is a highly flammable and dangerous gas, known for its strong acidic nature. It is commonly emitted from industrial processes such as oil refining and metal smelting, as well as from crude oil and natural gas fields [1,2]. The corrosive nature of H2S leads to the degradation of metal parts in equipment [3]. Furthermore, exposure to low concentrations of H2S can result in nausea, headaches, dizziness, and irritation to the eyes, skin, throat, and nose [4]. At higher concentrations, it can cause severe damage to the cardiovascular and neuromodulatory systems [5]. H2S is also considered as a biomarker for various health conditions, including halitosis [6,7]. Thus, the reliable detection of H2S gas is important from multiple perspectives. Various sensors have been developed for the detection of H2S, including optical [8], surface acoustic wave [9], electrochemical [10], and resistive [11] gas sensors. Among these, resistive sensors, which are mostly based on semiconducting metal oxides, are favored because of their low cost, high sensitivity and stability, as well as fast dynamics [12-14].
SnO2, an n-type semiconductor (Eg= 3.7 eV [15)], is one of the materials most used for gas sensors because of its high electron mobility, non-toxicity, availability, low synthesis costs, and good sensing properties [16,17]. SnO2 has been used in the detection of various toxic gases, including H2S [18,19].
SnO2 powders or films can be produced by several methods, including chemical precipitation [20], vapor– liquid–solid (VLS) [21], combustion [22], evaporation [23], pulsed laser [24], hydrothermal/solvothermal [25,26], and sol-gel [15] methods. While chemical methods produce fine powders with high purity, they are time-consuming and need highly pure precursors. In addition, certain physical routes require specialized and expensive facilities. In comparison, milling is among the less expensive and simpler methods for preparing small-sized SnO2 powders.
High-energy ball milling is a mechanical process in which the continuous collisions of milling balls cause the powders to deform and fracture, resulting in crushing and mixing. The process can also reduce the activation energy of reactions, and decrease the particle size to the nanometer scale, as well as enhance powder reactivity, and induce chemical reactions at low temperatures. High-energy ball milling involves several stages, including particle size reduction, local hotspot formation, lattice relaxation, and structural decomposition [27]. Nanoparticles (NPs) can be produced using high-energy ball milling, significantly enhancing the physicochemical properties of the milled materials. NPs offer a larger surface area, thereby increasing the rate of chemical reactions and improving mechanical strength. Furthermore, this method ensures uniform mixing of different powder materials, leading to consistent properties in the final product [28]. Smaller particles lead to more contact points in the powders, resulting in higher resistance modulation when exposed to gases. Kersen et al. [29] used ball milling to prepare SnO2 powders for H2S gas sensing. Later, this group leveraged mechanochemical synthesis to prepare contamination-free SnO2 powders for H2S gas sensing [30]. Sapkota et al. [31], prepared ZnO particles using planetary ball-milling to detect H2 gas. CaO et al. [32], used ball milling to prepare a Fe2O3-ZrO2 composite for low temperature sensing of oxygen gas. Shin et al. [33], synthesized a WO3-In2O3 composite for CH4 gas sensing. Yadav et al. [34], synthesized TiO2 particles via a ball milling process for liquefied petroleum gas sensing. However, despite these advances, only a limited amount of research has focused on the H2S gas-sensing features of SnO2 powders milled for varying durations.
Effective high-energy ball milling can minimize powder contamination, which is crucial for applications in the electronics and biomedical fields. High-energy ball milling can be scaled up to industrial production, facilitating the rapid introduction of new materials into industry. Additionally, it produces high-quality NPs at relatively low cost, offering substantial economic benefits. As a result, the technique is valuable in materials science and engineering and is extensively employed to synthesize NPs, microwave dielectric materials, and composites.
In this study, commercial SnO2 powders were subjected to high-energy milling for 30, 60, and 90 min. After milling, the powders were characterized using advanced techniques to evaluate their morphology, chemical composition, and crystallinity. The results indicated that milling for 60 min produced SnO2 powder with an optimal combination of crystallinity and a nearly round morphology.
Subsequently, a gas sensor was fabricated, and its H2S detection performance was assessed. Gas sensors fabricated from these powders demonstrated the highest H2S sensing performance, which was related to the generation of fine particles and the formation of double Schottky contacts among the SnO2 powders. Our findings indicated that SnO2 powders milled for 60 min exhibited superior sensing performance, which could be attributed to the reduced particle size and increased surface defects, which enhance the interaction between the gas molecules and sensor material.
Additionally, we comprehensively discuss the basic sensing mechanism of the as-fabricated SnO2 powder, providing insights into the role of milling in improving the sensing properties of SnO2 powders. The findings highlight the potential of milling as a simple yet effective technique to improve the sensing capabilities of metal-oxide-based sensors, with applications extending beyond SnO2 to other metal oxides.
2. EXPERIMENT
2.1. High-energy ball milling of SnO2 powders
Initially, 1 g of commercial SnO2 powders were placed in a mill with four stainless-steel balls, each with a diameter of 0.5 mm. The mill chamber was placed inside a glove box under vacuum (Fig 1 (a)). The milling time was set to be 30, 60, and 90 min.
Schematics of (a) synthesis procedure (b) gas sensor (c) sensing measurement system.
2.2. Material characterization
Morphology was explored via scanning electron microscopy (SEM, HITACHI-Regulus 8220, JPN). Transmission electron microscopy was used to check the crystallinity of samples. X-ray diffraction (XRD, Panalytical (EMPYREAN), GB) was used to analyze the phase and crystalline nature of the samples. X-ray photoelectron spectroscopy (XPS, ThermoFisher (NEXSA), USA) was used to analyze the surface composition.
2.3. Sensor fabrication
SnO2 powders were dispersed in ethanol and then coated on an alumina substrate equipped with gold electrodes (50 μm) using a micropipette to fabricate the sensing device. The device was then dried at 60 °C (Fig 1 (b)).
2.4. Gas sensing measurement
Figure 1 (c) presents the gas-sensing measurement system. The fabricated sensors were put inside a temperaturecontrolled gas chamber. To dilute the gases to desired concentrations, they were mixed with dry air, then using mass-flow controllers they were introduced into the gas chamber. In all experiments, the total flow rate was fixed to 500 sccm. The resistance of the sensors was continuously recorded using a source meter (Keithley 2450). Then, the resistance in air (Ra) and the resistance in the presence of the target gas (Rg) were used to calculate Response, as R = Ra/Rg for reducing gases and R=Rg/Ra for NO2 gas.
3. RESULTS AND DISCUSSION
3.1. Characterization studies
Figures 2 (a)–(b) provide SEM views of commercial neat SnO2 powders without milling, and Figs 2 (c)–(d) show SnO2 powders after being milled for 60 min.
SEM views of (a–b) commercial SnO2 powders and (c–d) SnO2 powders after milling for 60 min.
A comparison of the two samples revealed that while the overall morphology remained largely unchanged after 60 min of milling, the number of particles with an initial diameter of 400 nm significantly decreased. This outcome indicated that the milling process reduced the particle size. In other words, simple milling for 60 min can reliably reduce the size of commercial SnO2 powders (sub-nano level) to the nanoscale.
Figures 3 (a)–(d) show the high-angle annular dark field (HAADF) elemental mapping analysis results of the SnO2 powders after milling for 60 min, clearly demonstrating the uniform distribution of both Sn and O elements.
(a)–(d) HAADF elemental mapping analysis of SnO2 powders after milling for 60 min.
Figure 3 (e) presents the results of the HAADF chemical analysis of the SnO2 powders after milling for 60 min. Both Sn and O elements were detected, and their amounts were 14.08 and 85.92 wt.%, corresponding to 54.87 and 45.13 at.%, respectively, closely matching the chemical formula of SnO2. Figure 4 (a) depicts the XRD patterns of commercial SnO2 powers as well as those milled for different durations.
(a) XRD patterns of commercial SnO2 powders and powders after milling for different times. (b)–(c) Enlarged XRD patterns.
All of the samples exhibited the same peaks, which wellmatched JCPDS Card No. 41-1446 for SnO2 with a tetragonal rutile crystal structure. The absence of other peaks is indicative of high sample purity and a clean milling procedure, free from unwanted contamination.
To further examine the peak positions, Figs 4 (b)–(c) show enlarged XRD patterns within the Bragg angle ranges of 26.2°–27° and 33.4°–34.4°, respectively. In both cases, the peak positions were slightly shifted toward larger Bragg angles, indicating a decrease in the distance of crystalline planes according to Bragg’s law (d = λ/2sinθ). Specifically, the 2θ value of the peak corresponding to the (110) plane shifted from 26.63° to 26.76°, and the 2θ value of the peak corresponding to the (101) plane shifted from 33.91° to 34.05°. Bragg’s law was used to calculate the change in interplanar distance, indicating that the interplanar distance for the (110) plane decreased from 0.334 nm to 0.333 nm and that for the (101) plane decreased from 0.264 nm to 0.263 nm. This decrease in interplanar distance can be partially related to the formation of oxygen vacancies. However, in milled samples, lattice strain can also contribute to a shift in peaks [35], therefore, the entire amount of peak shifting cannot be related to the formation of oxygen vacancies.
Figure 5 (a) presents a high-resolution TEM (HRTEM) image of a single SnO2 particle after milling for 60 min.
(a)–(e) HRTEM images of SnO2 powders after milling for 60 min.
The particle exhibited a round shape with a diameter of approximately 40 nm. Figures 5 (b)–(e) present higher magnification HRTEM images, with parallel spacings between planes of 0.243 nm and 0.328 nm, corresponding well to the XRD results for the (101) and (110) crystalline planes of rutile tetragonal SnO2, respectively [36,37].
The extent of crystallinity can be established using fast Fourier transform (FFT) patterns. Figures 6 (a) and (c) show the FFT patterns of the (110) and (101) planes of commercial SnO2, and SnO2 powders after milling for 60 min, respectively.
(a) and (c) FFT patterns of commercial SnO2 and SnO2 powders after milling for 60 min, respectively. (b) and (d) Corresponding image intensity contrast distributions in two vertical directions.
The sharp patterns in both cases demonstrate the high crystallinity of both analyzed powders [38]. Figures 6 (b) and (d) display the corresponding image intensity contrast distributions in two vertical directions. The results confirm the consistency of the interplanar distances calculated using Bragg’s law from the XRD results, the visually observed interplanar distances from HRTEM images, and distances determined through FFT. After milling, the SnO2 particles were fractured, which led to the formation of oxygen vacancies and a reduction in interplanar distances.
Figures 7 (a)–(d) illustrate the O1s XPS core-level regions of commercial SnO2 powders and SnO2 powders after milling for 30, 60 and 90 min, respectively.
O1s XPS core-level regions of (a) commercial SnO2 powders and SnO2 powders milled for (b) 30 (c) 60 and (d) 90 min.
In all cases, O1s is fitted into three peaks located at 530.4, 531.6, and 532.6 eV, corresponding to lattice oxygen, oxygen vacancy, and adsorbed oxygen species, respectively. As both oxygen vacancies and adsorbed oxygen species are important for gas sensing, the amounts of these oxygen species were also calculated (Fig 7). For commercial SnO2 powders, the areas of the peaks related to oxygen vacancies and adsorbed oxygen species were 15.24% and 6.31%, respectively. For SnO2 powders milled for 30, 60 and 90 min, these values were 14.99% and 7.35%; 17.30% and 5.65%; and 16.61% and 5.50%, respectively. Accordingly, milling for 60 min resulted in the highest amounts of oxygen vacancies and adsorbed oxygen ions, which are expected to be beneficial for H2S detection.
3.2. Gas sensing studies
Figures 8 (a)–(d) present the response curves of both commercial and milled (30, 60, and 90 min) SnO2 gas sensors at 300 °C toward 2–10 ppm H2S gas, respectively.
Dynamic response curves of (a) commercial SnO2 and SnO2 powders milled for (b) 30, (c) 60 and (d) 90 min to 2–10 ppm H2S gas at 300 °C.
Figure 9 presents the corresponding calibration curves, and Table 1 compares the sensing results.
H2S calibration curves of commercial SnO2 and SnO2 powder gas sensors milled for different times at 300 °C.
H2S gas sensing performance of SnO2 gas sensors at 300 ℃.
The response of commercial SnO2 powders to 2, 4, 6, 8, and 10 ppm H2S was 1.02, 1.40, 1.64, 2.10, and 2.46, respectively. However, after milling for 60 min, the response to these concentrations improved to 1.44, 1.84, 2.26, 2.75, and 3.01, respectively. This indicates that sufficient milling time enhances the sensing performance. Notably, after 90 min of milling, the response to most H2S gas concentrations decreased. Additionally, the sensor fabricated from SnO2 powders milled for 30 min showed poorer performance compared with that milled for 60 min.
The selectivity of the optimal sensor was further explored by exposing it to 10 ppm of various gases (C2H5OH, NH3, C6H6, and NO2) at 300 °C (Fig 10 (a)-(d)).
(a)-(d) Dynamic response graphs of SnO2 powders milled for 60 min to 10 ppm toward various gases at 300 °C. (e) Corresponding selectivity histogram.
The sensor exhibited no meaningful response to these gases, demonstrating its selectivity to H2S gas (Fig 10 (e)). Moreover, we studied the sensing behavior in the presence of 60% relative humidity (RH) and 10 ppm H2S gas at 300 °C (Fig 11).
Dynamic response graphs of SnO2 powders milled for 60 min to 10 ppm toward H2S gas at 300 °C and 0 and 60% RH.
The sensor response decreased by more than 30% in the humid environment (60% RH). This decrease is expected as water molecules occupy the available adsorption sites on the sensor surface, reducing the available sites for adsorption of H2S gas. Consequently, the amount of adsorbed H2S gas is lower, resulting in a diminished response in a humid atmosphere [39].
3.3. Gas sensing mechanism
Initially, oxygen molecules are adsorbed on the sensor, and due to their high electrophilic nature, they extract the electrons from the conduction band of the SnO2 [40]:
Hence, an electron depletion layer (EDL) forms on SnO2 in air. Upon exposure to H2S gas, it reacts with adsorbed oxygen species, releasing electrons to the surface of the gas sensor. The potential reaction is [41]
This reaction narrows the EDL, resulting in a decrease in the sensor resistance. Additionally, in the contact areas between the SnO2 NPs, double Schottky barriers form in air. The effect of these barriers on the resistance of the sensor can be expressed as follows [42]:
where R represents the sensor resistance; Ra is the initial resistance; qv represents the barrier height; and k and T denote the Boltzmann constant and temperature, respectively. Upon exposure to H2S gas, the released electrons return to the sensor surface, reducing the height of the double Schottky barriers, contributing to the sensing signal.
The selectivity to H2S gas is attributable to the working temperature, the higher reactivity of H2S relative to other gases, and the small bond energy of H–SH. The bond energy of 381 kJ/mol for H–SH in H2S can be easily broken to react with the oxygen ions [43,44].
The enhanced response of the sensor fabricated from SnO2 particles milled for 60 min compared with other gas sensors is attributable to both smaller particle sizes and a higher total amount of adsorbed oxygen and oxygen vacancies on these powders. From a particle size point of view, we know that increasing milling time transfers more energy to the particles, and more grinding and wearing of particles occurs, leading to the formation of finer particles. However, when the particles become very fine their surface reactivity significantly increases, leading to the agglomeration of particles and the formation of larger particles.
Therefore, it seems that in the present study, the optimal milling time is 60 min, which results in the formation of finer SnO2 particles. However, by further increasing milling time to 90 min, the agglomeration of fine particles is likely, contributing to the generation of larger particles. Smaller particles increase the contact areas among the powders, leading to higher resistance modulation for the sensor fabricated from SnO2 particles milled for 60 min.
Additionally, based on the total amount of adsorbed oxygen and oxygen vacancies, the presence of more adsorbed oxygen species leads to greater sensing reactions with H2S gas, thereby producing a higher response. Oxygen vacancies serve as favorable sites for the adsorption of oxygen gas at the sensing temperature [45, 46], which increases the amount of adsorbed oxygen ions and enhances the sensor response. Therefore, it seems that for the optimal sensor, the presence of high amounts of adsorbed oxygen species and oxygen vacancies is effective for sensing performance towards H2S gas.
4. CONCULUSIONS
Commercial SnO2 powders were subjected to high-energy milling for 30, 60, and 90 min for H2S gas sensing studies. XRD, SEM/TEM, and HAADF characterizations revealed that crystalline SnO2 powers with the desired composition and nearly round morphology were obtained after milling for 60 min. The results of the H2S gas sensing studies indicated that the sensor fabricated from SnO2 powders milled for 60 min had the highest sensing performance. The improved performance was attributable to the formation of fine SnO2 powders and double Schottky contacts among SnO2 particles. This study demonstrates the promising effects of milling to achieve higher sensing performance in metal oxide gas sensors.
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT (MSIT) (NRF-2021R1A5A8033165). This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20224000000150).