Application of Au/TiO2–WO3 material in visible light photoreductive ozone sensors
Introduction
Ozone (O3) is a useful oxidation agent that has numerous practical applications. The widespread use of ozone has created a strong demand for accurate sensors capable of measuring ozone gas concentrations under various conditions. For example, ozone measurements are used in medical applications, research laboratories, biotechnological, chemical, and pharmaceutical processes, food processing and storage, and water purification. There are various approaches to detecting ozone concentrations, including electrochemical [1], optical [2], resistive [3], [4], and work functional [5] methods, as well as technologies such as impedance spectroscopy [6] and UV light photoreduction [7], [8], [9], [10], [11], [12].
Photoreduction has several advantages for detecting ozone concentrations, such as high sensitivity and low working temperature [7], [8], [9], [10], [11], [12]. Recently, three metal oxide materials have been used as a gas-sensing material in photoreduction ozone sensors [7], [8], [9], [10], [11], [12]. First, In2O3 and InOx thin films have shown sensor responses (i.e., the ratio of the electrical resistance in an ozone gas without UV to the electrical resistance in air with UV; S = Rozone/Rair) of 105 to 106, respectively, in 2 ppm ozone [7], [8]. Second, ZnO thin film has been employed as a sensing material to measure ozone concentrations [9], [10], [11], [12] and exhibited a sensor response of 1.2 × 108 [9]. Furthermore, 150 to 1000 nm thick polycrystalline ZnO films exhibited sensor responses of 106 and 103, respectively, in 2 ppm ozone [10]. The response and recovery time of these two sensors in 2 ppm ozone exceeded 15 and 10 min, respectively [11], [12]. Third, the wet impregnation method was employed to fabricate Pt/TiO2–SnO2 for use as an ozone sensor [13]. Our research team combined Pt and the coupled semiconductor photocatalyst TiO2–SnO2 as an ozone-sensing material, and obtained a sensor response of 1082. Platinum was crucial, and the Pt/TiO2–SnO2 and Pt/SnO2 exhibited short response and recovery times (160 and 50 s, respectively) in 2.5 ppm ozone at room temperature [13].
Previous studies have shown that Au has a crucial role in various gas-sensing reactions [14], [15], [16], [17]. A previous study used Au–SnO2 to sense 2000 ppm CO and H2 at 450 °C and obtained response times of approximately 30 and 35 s, respectively [14]. The sensor response of 0.1 wt.% Au-loaded CoOOH-WO3 films in 1000 ppm CO was improved by approximately three times that of CoOOH-WO3 alone [15]. The promotion effect of adding Au to WO3 in 0.8 ppm ozone was significant [16]. At 300 °C, the sensor response of WO3 to ozone was 7.6, which was subsequently improved to 123 by adding Au to the WO3 [16]. Au–SnO2 films modified by using Au–SnO2 nanocomposites exhibited a sensor response of approximately 106 in 0.1 ppm ozone at 160 °C [17]. Previous studies have attributed the enhancement mechanism to Au loading, which increased the sensor's ability to adsorb oxygen at the SnO2 surface [15], [17].
The purpose of this research is to identify an ozone sensor that uses a visible blue light source, is simple and cost-efficient to fabricate, and has a high sensor response. After selecting Au/TiO2–WO3 as the sensing material in this study, our research group studied its sensing properties.
Section snippets
Sensing material
TiO2 (ST-21), tungstic acid (H2WO4), and HAuCl4·3H2O were sourced from Ishihara, Fluka, and Alfa Aesar, respectively. The coupled metal oxides were fabricated by mixing a certain weight ratio of TiO2 and H2WO4 before calcining the mixture at 400 °C for 2 h to obtain various weight ratios of TiO2–WO3. To enhance the sensor response, various ratios of the sensing material TiO2–WO3 were tested. Appropriate amounts of HAuCl4 were diluted with distilled water in beakers to form an aqueous solution.
XRD, TEM and EDX characterization
Fig. 2a–d shows the XRD patterns for TiO2, WO3, TiO2–WO3 (3:1) and 5 wt.% Au/TiO2–WO3 (3:1). Based on the pattern of anatase TiO2 shown in Fig. 2a, all peaks were assigned to lattice planes (101), (103), (200), (105), (211), (204), (116), (220), and (215) [18]. In Fig. 2a, plane (101) exhibited a strong refraction peak at 2θ = 25.3°. The particle sizes were estimated by applying the Scherrer formula D = Rλ / βcosθ, where D is the particle diameter of a crystal plane, R is the Scherrer constant (0.89),
Conclusion
The sensing materials TiO2–WO3 and Au/TiO2–WO3 were characterized using TPR, UV/VIS reflective spectra, TEM, and XRD. Among the various ratios of TiO2–WO3 the material TiO2–WO3 (3:1) showed the highest response (23.8) to 2.5 ppm ozone. The addition of Au on the TiO2–WO3 (3:1) improved the sensor response, which was enhanced to 64.0 on the 0.1 wt.% Au/TiO2–WO3 (3:1). The photoreduction mechanism on the TiO2–WO3 and on the Au/TiO2–WO3 was attributed to ozone adsorption, sensing material
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