Highly Selective and Sensitive Detection of Acetone by ZnWO₄-WO₃ Hetero-composite Spheres

1. INTRODUCTION

Humans generally use carbohydrates as their primary energy source under normal conditions. However, when the carbohydrate intake is minimal, the body is forced to consume fat as an alternative energy source. When fat is catabolized during carbohydrate cut-off, acetone molecules are produced in the liver as a by-product of fatty acids. These diffuse into the blood and are finally released via human breath through gas exchange between the alveoli and capillaries [1-4]. Therefore, if breath acetone can be selectively and precisely detected through breath analysis, body fat loss can be effectively monitored. It is crucial for monitoring weight loss of obese patients.

Metal-oxide semiconductor (MOS) gas sensors have enabled the development of portable breath acetone analyzers owing to their superior miniaturization potential, simple gas-sensing mechanism, rapid response, and low cost. The gas-sensing characteristics of MOS can be tuned by modifying the material, such as tailoring the surface modification, doping, and nanostructure designs [5-7].

Different MOS gas sensors have been explored for the detection of acetone, including Si-WO₃ nanoparticles [8], Rh-WO₃ spheres [9], CeO₂-In₂O₃ hollow spheres [10], and Tb-SnO₂ yolk-shell spheres [11]. However, the development of acetone sensors is still in its infancy, and further research is required. Utilizing a heterocomposite as a sensing material is another strategy for designing highly selective and sensitive acetone gas sensors.

In this study, pure WO₃ and 5at% Zn-added WO₃ microspheres were prepared by ultrasonic spray pyrolysis with post-heat treatment. Their gas-sensing characteristics to 1 ppm acetone, ethanol, 20 ppm hydrogen (H₂), 5 ppm carbon monoxide (CO), 25 ppb toluene, and 5 ppm ammonia (NH₃) were evaluated at 325–400^oC. The sensor using the Zn-added WO₃ microspheres demonstrated high selectivity and sensitivity to acetone even under high humidity conditions (80% relative humidity [RH]), and its gas-sensing mechanism was discussed in relation to electronic sensitization by n-n heterojunction formation between WO₃ and ZnWO₄ and the acidic properties of WO₃.

2. EXPERIMENTAL

2.1 Preparation of pure WO₃ and 5Zn-WO₃ microspheres

We prepared a 5at% Zn-added WO₃ microsphere (5Zn-WO₃) via ultrasonic spray pyrolysis, followed by heat treatment (Fig. 1). Next, 2.318 g of WO₃ powders (WO₃, powder, puriss., 99.9% Sigma-Aldrich Co., Ltd, USA), 0.110 g of Zn acetate dihydrate (Zn(CH₃COO)₂·2H₂O, ACS reagent, ≥ 98%, Sigma-Aldrich Co., Ltd, USA), and 2.101 g of citric acid monohydrate (HOC(COOH)(CH₂COOH)₂·H₂O, ACS reagent, ≥ 99%, Sigma-Aldrich Co., Ltd, USA) ([Zn]/[W] = 5at%) were dissolved in 200 mL of diluted ammonia solution (170 mL of D.I. water + 30 mL of 30% NH₃ solution). The solutions were heated at 80^oC with constant stirring until the solution became clear. Pure WO₃ microspheres were prepared using the same procedure as Znadded WO₃, but without adding Zn acetate dihydrate. The precursors of pure WO₃ and Zn-added WO₃ were annealed at 600^oC for 2 h in a box furnace. For simplicity, 5at% Zn-added WO₃ (ZnWO₄-WO₃ hetero-composite microspheres) is referred to as 5Zn-WO₃. The morphology and phase of pure WO₃ and 5Zn-WO₃ microspheres were analyzed by field-emission scanning electron microscopy (FE-SEM; S-4300 Hitachi Co. Ltd., Japan) and X-ray diffraction (XRD; Rigaku Model/MAX-2500, Source: CuKα).

Fig. 1.

Synthesis process of pure WO3 and 5Zn-WO3 microspheres.

3. RESULTS AND DISCUSSION

3.1 SEM images

The morphologies of the pure and 5Zn-WO₃ microspheres were observed using SEM (Fig. 2). Pure WO₃ and 5Zn-WO₃ microspheres exhibited similar size distributions and spherical morphologies with no distinct aggregation. In addition, the two specimens were composed of densely or thickly shelled spheres, which comprised nanosized primary particles.

Fig. 2.

SEM images of (a) pure and (b) 5Zn-WO3 microspheres.

3.2 Phase analysis

The phases of pure and 5Zn-WO₃ microspheres were analyzed by X-ray diffraction (Fig. 3). Pure WO₃ microspheres displayed only the γ-WO₃ phase (JCPDS #43-1035), a stable phase under room and gas-sensing temperatures [12]. 5Zn-WO₃ microspheres exhibited mixed phases of γ-WO₃ and ZnWO₄ phases. It can be attributed to sub-atomic scale uniform mixing of Zn and W cations via ultrasonic spray pyrolysis and relatively high annealing temperature (600^oC) [13]. In addition, no significant shift in the WO₃ peaks was observed, suggesting that almost all Zn cations were used for ZnWO₄ formation without doping the WO₃ lattice.

Fig. 3.

XRD patterns of (a) pure and (b) 5Zn-WO3 microspheres.

3.3 Gas-sensing characteristics and discussion

The gas-sensing characteristics of sensors using pure and 5Zn-WO₃ microspheres to 1 ppm acetone, ethanol, 20 ppm H₂, 5 ppm CO, 25 ppb toluene, and 5 ppm NH₃ were measured at 325–400^oC (Fig. 4). The sensing characteristics at < 325^oC and > 400^oC were not measured because the kinetics of gas sensing and recovery of < 325^oC region were extremely slow, and the high temperature at > 400^oC is unsuitable for long-term sensor operation and MEMS application. All sensors exhibited the typical characteristics of n-type metal oxide semiconductor gas sensors: a decrease in sensor resistance upon exposure to a reducing gas, which returned to its baseline upon exposure to air. Therefore, the gas response (S) was defined as R_aR_g^-1 - 1 (R_a: resistance in the air, R_g: resistance in analyte gas).

Fig. 4.

Gas-sensing characteristics of (a) pure and (b) 5Zn-WO3 microspheres to 1 ppm acetone, ethanol, 20 ppm H2, 5 ppm CO, 25 ppb toluene, and 5 ppm NH3 at 325–400oC and RH 80%

Sensors using pure WO₃ microspheres exhibited elevated gas responses with increased temperature. In addition, pure WO₃ microspheres exhibited the highest response to acetone gas, secondarily high gas responses to ethanol, and a relatively negligible gas response to other analyte gases. However, pure WO₃ microspheres still exhibit relatively low selectivity to acetone (1.67–1.97), which is insufficient for breath acetone analyzer application. In contrast, 5Zn-WO₃ microspheres exhibited a higher gas response to acetone than pure WO₃ microspheres, even under high humidity conditions (80%). In addition, the selectivity values were greatly increased with Zn addition to WO₃ (8.3–9.3) (Fig. 5). Considering gas response, selectivity, and responding speeds, the optimum gas-sensing temperature of 5Zn-WO₃ was 375^oC.

Fig. 5.

Selectivity values of pure and 5Zn-WO3 microspheres.

The increase in gas response and selectivity of 5Zn-WO₃ can be understood in relation to the n-n heterojunction formation between ZnWO₄ and WO₃. The interface between ZnWO₄ and WO₃ extended the electron-depletion layer of WO₃ by transferring electrons from WO₃ to ZnWO₄ and increasing the resistance of WO₃ [14]. The 5Zn-WO₃ microspheres exhibited a higher resistance than pure WO₃ microspheres in all sensing temperature ranges (Fig. 6). This increase in resistance can be considered a major reason for the enhancement of the gas response because a sensor with a low background charge-carrier concentration exhibits a larger chemo-resistive variation if a fixed amount of charge carriers is injected into the gas sensor via a gas-sensing reaction (electronic sensitization) [15]. Several previous studies have reported that heterojunction formation significantly enhances gas responses. For example, NiWO₄-NiO [16], NiMoO4-NiO [17], and Fe₂O₃-ZnFe₂O₄ [18] have demonstrated significant increases in response to analyte gases.

Fig. 6.

Sensor resistance of pure and 5Zn-WO3 microspheres at 325–400oC in air atmosphere.

However, the resistance increase in WO₃ can only explain the increase in the acetone response; high selectivity to acetone cannot be solely explained by the resistance increase. It is attributed to the acidic property of WO₃ as well. WO₃ has been reported to be an acidic metal oxide semiconductor that can convert reactive ethanol to less reactive ethylene via a dehydration reaction [19]. Because of the high stability of ethylene, it cannot undergo gas-sensing reactions with the sensing material surface, and gas responses to ethanol did not increase compared with those to acetone. Therefore, the increase in selectivity can be explained by the acidic properties of WO₃.

Acetone is a key gas used to evaluate the effectiveness of body fat loss in obese patients. For instance, breath acetone concentrations increase to 2–5 ppm when the body fat decomposition rate reaches 2 g h⁻¹. Considering the concentration of acetone released from human breath, the breath analyzer consisting of 5Zn-WO₃ used in this study can be used for monitoring body fat loss.

4. CONCLUSIONS

A highly selective and sensitive gas sensor for detecting breath acetone was designed using 5Zn-WO₃ microspheres. The pure WO₃ sensor exhibited neither high gas response nor selectivity for acetone. In stark contrast, adding 5at% Zn ions to WO₃ formed a ZnWO₄ phase with a major WO₃ conduction path. The heterojunction between WO₃ and ZnWO₄ in 5Zn-WO₃ significantly enhanced the gas response and selectivity to sub-ppm level acetone, even under high-humidity conditions, through electronic sensitization. In addition, the acidic properties of WO₃ are suggested to be the reason for its high selectivity for acetone.

Acknowledgments

This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT, nos. 2021M3H4A3A02086430 and RS-2024-00454367). This work was also supported by the Human Resource Development Program for Industrial Innovation (P0017012), funded by the Ministry of Trade, Industry, and Energy (MOTIE), Korea.

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