Journal of Chemistry
Volume 2015 (2015), Article ID 382087, 7 pages
http://dx.doi.org/10.1155/2015/382087

Research Article

Preparation of Flower-Like Cu-WO₃ Nanostructures and Their Acetone Gas Sensing Performance

Hao Zhou, Dong-Yao Xu, Hai-Qing Zuo, Wei Liu, and Shuang Lin

School of Chemical & Environmental Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China

Received 7 September 2015; Revised 9 December 2015; Accepted 13 December 2015

Academic Editor: Amel Boudjemaa

Copyright © 2015 Hao Zhou et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Urchin-like Cu-W₁₈O₄₉ and flower-like Cu-WO₃ structures were successfully synthesized using a hydrothermal process followed by calcination. The synthesized products were characterized using XRD, SEM, and TEM. The results revealed that the as-prepared urchin-like and flower-like samples with monoclinic structures, which were approximately 1 μm and 1-2 μm, respectively, possessed microflower architecture assembled by the nanosheet. In addition, the gas sensing properties of monoclinic-structured Cu-WO₃ to acetone were measured using a static state gas sensing test system. The sensor based on the flower-like Cu-WO₃ nanostructures, which were calcined at 600°C, exhibited high sensitivity toward 10 ppm acetone at an optimum temperature of 110°C, and the maximum sensitivity reached 40, which was approximately four times higher than that of urchin-like WO₃ that was annealed at 300°C. The sensitivity was improved by increasing the acetone concentration. The detection limit was as low as 1 ppm. Using linear fit, the sensor was determined to be sufficiently sensitive to detect acetone in a detection range of 1 to 10 ppm even in the presence of interfering gases, which suggests that this type of sensor has excellent selectivity and has the potential for use in acetone gas sensors in the future.

1. Introduction

Tungsten oxide (WO₃) is a wide-band-gap ( = 2.8 eV) -type semiconductor that has been widely used in gas detection sensors. In recent years, WO₃ has become the most promising semiconductor gas sensing material that has been used to detect NOx, NH₃, CH₄, and H₂S [1–4]. This sensitive response toward gas, which is primarily due to variation in the resistance or optical properties caused by inner electrons, is highly dependent on the operating temperature. An increased operating temperature helps to improve the sensitivity of the WO₃ gas sensor to decrease the response and recovery time [5]. However, for single-component WO₃, some limitations, such as a high operating temperature and poor sensitivity, still exist which can prevent the extensive use of WO₃ based semiconductor gas sensors. Therefore, improvement in the sensor properties of these materials can be achieved by adding multicomponent materials (dopants), such as noble metals, transition metal oxides, and rare earth oxides. The introduction of dopants can be achieved in two primary ways. The first approach involves the introduction of impurity atoms modified on oxide surface (surface modification) to produce active centers and interactions with test gas molecules, such as Ag, Au, and Pt [6]. The other approach involves the addition of foreign atoms into the crystal lattice of a semiconductor oxide (lattice doping), which results in generation of impurities within the semiconductor that alter in their electrical properties [7–9]. Currently, many methods have been employed for doping modifications including the impregnation method, the chemical vapor deposition method, the sputtering method, and the plasma method [10–12]. These methods have a high requirement for equipment and a high cost, and most of the doped metal elements can be washed away. Therefore, further improvement is necessary.

In this study, a hydrothermal method and subsequent calcination treatment had been utilized to synthesize urchin-like W₁₈O₄₉ and flower-like WO₃ structures with the addition of Cu(NO₃)₂ as a doping agent in the ethanol system. To simplify the reaction process, WC₁₆ was chosen as the tungsten source. The as-prepared samples were calcined at various temperatures, and their phase and morphology changes were characterized by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Moreover, the gas sensing properties of monoclinic-structured Cu-WO₃ to acetone were measured using a static state system. In addition, the variation in the sensitivity as a function of the operating temperature and acetone concentration are discussed based on a gas-sensitive mechanism toward acetone. Finally, the selectivity to interfering gases was investigated. The purpose of this study was to extend the use of WO₃ based gas sensors for practical applications.

2. Materials and Methods

2.1. Chemicals

All of the chemical reagents were of analytic purity and used directly without further purification. WCl₆ was used as the tungsten source and purchased from the Aladdin Company. Ethanol (C₂H₅OH) and copper nitrate (Cu(NO₃)₂3H₂O) were purchased from the Beijing Chemical Plant. Deionized water with a resistivity greater than approximately 7 Mohm was used for all of the experiments.

2.2. Experimental Process

2.2.1. Preparation of Flower-Like Cu-Doped WO₃

Flower-like copper-doped WO₃ was fabricated using a hydrothermal method. In a typical synthesis, 0.25 g of WCl₆ (as tungsten source) was dissolved in 60 mL of ethanol (C₂H₅OH, as solvent) followed by stirring until the solution was transparent and yellow. Subsequently, a suitable amount of copper nitrate (Cu(NO₃)₂3H₂O, 3.0 wt%) was added to this solution under stirring. Then, the resulting solution was transferred to a 100 mL Teflon-lined stainless autoclave and heated at 180°C for 20 h in an electric oven. After cooling to room temperature, blue products were obtained by centrifugation and washed several times with deionized water and ethanol followed by drying at 60°C. Finally, the products were calcined in a muffle furnace with an air atmosphere at 300°C and 600°C for 2 h.

2.2.2. Gas Sensor Fabrication and Gas Sensing Performance Test

An appropriate amount of the as-prepared sample was mixed with a small amount of ethanol to form a paste. The paste was homogeneously coated onto a small alumina ceramic tube (obtained from Hui Sheng Electronic Technology Co., Ltd., Zhengzhou), and then, a Ni-Cr heating wire was inserted into the tube to form a gas sensor. The coated alumina ceramic tube was welded to a special hexagon pedestal with soldering to form the final sensor unit. To achieve stability, the fabricated sensor elements were aged in air for several days, and then, the gas sensing tests were performed. The corresponding transient response curve was developed [8, 13]. In this study, the senor response value () was defined as follows: , where stands for the resistance of the gas sensors in air and was the resistance of the gas sensors in the target gas.

2.3. Instrumentation

The morphology of the flower-like Cu-WO₃ samples was investigated using field-emission scanning electron microscopy (SEM, Hitachi S-4500). The transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2011. The crystal phase composition was determined using a RIGAKUD/MAX-Ra X-ray diffractometer with Cu Kα radiation ( = 0.15418 nm). The XRD data were collected in a 2θ range of 10° to 80°. The gas sensing properties of the as-prepared flower-like structures were measured by a computer controlled WS-30A gas sensing measurement system (Hui Sheng Electronic Technology Co., Ltd., Zhengzhou).

3. Results and Discussion

3.1. Morphological Characterization of Flower-Like Cu-Doped WO₃

Figure 1 shows the XRD patterns of the Cu-WO₃ samples calcined at 300°C (a) and 600°C (b). As shown in Figure 1, the calcination temperatures have a significant influence on the crystal structure and growth. In the XRD pattern of the 3 wt% Cu-WO₃ sample calcined at 300°C  (Figure 1(a)), all of the diffraction peaks can be easily indexed to the monoclinic W₁₈O₄₉ structure based on the standard diffraction peaks from JCPDS number 36-101 [6]. The two dominant peaks correspond to (010) and (020) lattice planes of Cu-WO₃. The diffraction peaks of the sample calcined at 300°C are consistent with those reported in the literature [14]. However, in the pattern for the sample annealed at 600°C, a monoclinic WO₃ structure formed due to recrystallization during heating. The main diffraction peaks are in good agreement with JCPDS number 72-0677 (lattice parameters: = 7.306 Å, = 7.540 Å, and = 7.692 Å), which indicates that the monoclinic W₁₈O₄₉ structure was transferred to a monoclinic WO₃ structure during heating. To further investigate the morphology changes during the sintering process, the as-prepared samples have been studied using SEM and TEM, and the results are shown in Figure 2.

(a)

(b)

Figure 1: XRD pattern of the Cu-WO₃ samples calcined at different temperatures: (a) urchin-like Cu-doped W₁₈O₄₉ calcined at 300°C and (b) flower-like Cu-doped WO₃ calcined at 600°C.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 2: SEM images of Cu-doped tungsten oxide samples calcined at different temperatures. (a) SEM image of urchin-like Cu-doped W₁₈O₄₉ at 300°C. (b) SEM image of flower-like Cu-doped WO₃ at 600°C. (c) High-magnification image of (b). (d) TEM image of flower-like Cu-doped WO₃ at 600°C. (e) High-resolution TEM (HRTEM) image of the flower-like sample. (f) Selected area electron diffraction (SAED) pattern of the sample in (e).

The SEM image in Figure 2(a) indicates that the Cu-WO₃ sample sintered at 300°C is approximately 1 μm in size and consists of an urchin-like structure. Similar morphologies have been reported by Xi et al. [6]. After calcination at 600°C, this structure is converted to a flower-like structure that is 1~2 μm in size and composed of many interconnected nanosheets, as shown in Figures 2(b) and 2(c). A coarse surface and a flower-like structure that was assembled from many approximately 100 nm thick nanosheets or nanosquares were observed. To elucidate the crystal structure changes, the as-synthesized samples were characterized by TEM. The TEM image indicates that the urchin-like morphology in Figure 2(a) becomes a nanosheet/nanosquare assembled flower-like morphology in Figure 2(d), which is consistent with the SEM result in Figure 2(c). Figure 2(e) shows a high-resolution transmission electron microscopy (HRTEM) image, and lattice fringes were observed with d-spacing of 0.389 nm, which is in agreement with the interplanar distances of (002) lattice planes of monoclinic WO₃ in the XRD data (Figure 1(b)) [15]. The corresponding selected area electron diffraction (SAED) pattern (Figure 2(f)) was recorded, and the results provided additional evidence of the single crystalline nature. After XRD analysis of the WO₃ sample doped with a small amount of copper, no characteristic peaks were observed. The Cu-WO₃ sample after calcination at 300°C, which has a monoclinic W₁₈O₄₉ structure in an urchin shape, has a strong surface effect due to a high specific surface area, oxygen vacancies, stacking faults, and other surface defects in the crystal structure [14]. In comparison to low-dimensional materials, the hierarchical micro/nanostructures that were self-assembled from low-dimensional nanomaterials provided a higher specific surface area [16, 17], which is conducive to gas absorption and diffusion.

3.2. Gas Sensing Properties

The gas sensing performance of WO₃, which is a semiconductor metal oxide, is measured by the ratio of the resistance change of the sensor in air to that in the target gas. The sensitivity of the gas sensor is primarily affected by the operating temperature. Therefore, the gas sensing properties were investigated at different temperature to determine the optimum operating conditions. We can also determine a suitable operational temperature range to improve the selectivity toward different target gases [18]. Figure 3(a) shows the sensitivity of the Cu-doped W₁₈O₄₉ and WO₃ gas sensors to 10 ppm acetone under different operating temperatures. The responses of the gas sensor increased continuously as the operating temperature increased, and a maximum was observed at 110°C followed by a decrease as the operating temperature further increased. A similar phenomenon was found in the urchin-like W₁₈O₄₉ sample. The maximum sensitivity reached 40 at an optimum working temperature of 110°C. Therefore, the operating temperature has an important influence on the sensitivity of the sensor, which is consistent with previous reports [5]. In general, when the operating temperature changes, the kinetics of adsorption and the chemical reactions occurring at the sensor surface are altered, leading to changes in the sensor sensitivity. However, if the temperature is further increased, the decrease in sensitivity after the maximum is considered to be primarily due to a decrease in the actual acetone concentration in the gas sensing region (vicinity of electrodes). The acetone molecules must migrate from the surface of the gas sensor to the gas sensing region inside. When the temperature is exceedingly high, the molecules tend to be consumed by oxidation during migration, and this tendency increased with increasing temperature. As shown in Figure 3(a), the sensor based on the flower-like Cu-WO₃ nanostructure (blue line) that was calcined at 600°C exhibited the highest sensitivity toward 10 ppm acetone at an optimum temperature of 110°C, and the sensitivity was 40, which is approximately four times higher than that of the urchin-like WO₃ (black) sample annealed at 300°C. This result is due to the sample after calcination at 600°C being more crystalline with suitable grain sizes during heating, which accounts for the excellent sensitivity of the flower-like WO₃ sensor to acetone. In Figure 3(b), the different doping concentrations of copper in flower-like WO₃ are shown to determine the optimum doping concentration. The sensitivities of the 0 wt%, 1 wt%, and 5 wt% Cu-doped WO₃ samples were lower than that of the 3 wt% doped sample, suggesting that the optimum copper doping amount is 3 wt%.

(a)

(b)

Figure 3: (a) Sensitivity of the Cu-doped W₁₈O₄₉ (black line) and WO₃ (blue line) sensor towards 10 ppm acetone under different operating conditions. (b) Different doping concentrations of copper in flower-like WO₃.

In comparison to the WO₃ sensor without a doping agent [19], the acetone gas sensing properties of the Cu-doped WO₃ sensor were significantly improved. In general, the electrical conductivity can be altered by doping in an equivalent substance. This doped impurity, which is an electron receptor, provides electrons to the WO₃ conduction band and modifies the electronic structure of the metal oxides and carrier concentrations. In addition, the impurities have an important effect on the surface barrier of the materials [9]. As a result, the gas sensing properties of the materials can be enhanced. Moreover, a new phase of CuWO4−x may be formed because Cu²⁺ (0.73 Å) can substitute for W⁶⁺ (0.62 Å) in the lattice cell of WO₃, which leads to a reduced electron concentration [20]. The direct exchange of electrons between the doped impurity and the gas sensing material can occur in the context of an electronic sensitization mechanism, resulting in changes in the material resistance. Therefore, the gas sensing performance will be improved. At the same time, doping can also change the defect equilibrium in the flower-like WO₃ structure, and crystal defects with high energy play an important role in oxygen and acetone adsorption on the surface of sensing materials as well as in the reactions of the gas molecules adsorbed on the material surface. Therefore, the obtained sample has a hierarchical structure consisting of a flower-like structure assembled from nanosheets or nanosquares (Figure 2(c) or Figure 2(d)) or an urchin-like structure with characteristics due to copper doping. All of these factors are important for achieving a high response to the target gas. More detailed factors should be further investigated.

Figure 4 shows the sensitivity of the flower-like WO₃ based gas sensor to different acetone concentrations at 110°C. The sensitivity increased as the acetone concentration increased. According to previous studies [21], the sensitivity () of a semiconducting oxide gas-sensitive sensor can be empirically represented as , where denotes the prefactor, is the target gas partial pressure, which is directly proportional to its gas concentration, and is the exponent on . Accordingly, the logarithm of can be linear as a function of the gas concentration (). may have some rational fraction value (typically 1 or 0.5) depending on the charge of the surface species and the stoichiometry of the elementary reactions on the surface [13, 22]. Based on this theory, a logarithm was used to fit the experiment data (i.e., the sensitivity values corresponding to different acetone concentrations at an optimum working temperature of 110°C). Then, a linear relationship between Lg and Lg was established by linear fitting. The fit curve of Lg() as a function of Lg() is shown in the inset of Figure 4. The correlation coefficient () of the acetone sensor was 0.95 in the range of 1 to 10 ppm at an operating temperature of 110°C. The value of towards acetone is approximately 0.456, which approaches the ideal value of 0.5. Therefore, little difference was observed between the measured value and the theoretical value. As shown in Figure 4(c), the response () and recovery () times were approximately 40 s and 27 s, respectively, which was measured from the response-time data to 8 ppm acetone at an operating temperature of 110°C. These data demonstrate that the gas sensors based on flower-like Cu-WO₃ are sufficiently sensitive to detect acetone in a range from 1 ppm to 10 ppm.

(a)

(b)

(c)

Figure 4: (a) Sensitivity of flower-shaped WO₃ to different concentrations of acetone at 110°C. (b) Logarithmic linear fitting of sensibility and concentration. (c) Response () and recovery () time measured from the response-time data to 8 ppm acetone at an operating temperature of 110°C.

Practical applications require that gas sensing materials have a high sensitivity and quick response to the target gas. Therefore, the selectivity is regarded as the main factor for evaluating the gas sensing characteristics. Figure 5 shows the responses of a flower-like Cu-WO₃ gas sensor to 10 ppm acetone, methanol, ether, NH₃, ethanol, isopropanol, NO₂, and H₂S at 110°C, respectively. The response of the Cu-WO₃ gas sensor to 10 ppm acetone was 40, which is much larger than that to other interference gases. The response to the other seven target gases varies from 1.08 to 6. In addition, the ratio of the Cu-WO₃ sensor response of acetone to that of the other five target gases ranged from 6.6 to 37. Therefore, the flower-like Cu-WO₃ gas sensor has good selectivity to acetone at 110°C in the presence of the previously mentioned interfering gases.

Figure 5: Responses of the flower-like Cu-WO₃ gas sensor to different gases at 110°C.

4. Conclusions

In summary, monoclinic urchin-like and flower-like Cu-WO₃ structures were synthesized using a hydrothermal process followed by calcination. The crystallography and microstructure of the synthesized samples were characterized using XRD, SEM, and TEM. Then, the acetone sensing properties were investigated. The results indicate that the optimum operating temperature for the detection of acetone was 110°C. At this temperature, the response of the sensor based on flower-like Cu-WO₃ nanostructures that were calcined at 600°C was 40 when exposed to 10 ppm acetone, which is approximately four times higher than that of the urchin-like WO₃ sample annealed at 300°C. The sensitivity increased as the acetone concentration increased. The lowest detection limit was 1 ppm. The sensor was determined to be sufficiently sensitive to detect acetone in a range from 1 to 10 ppm even in the presence of interfering gases. The flower-like Cu-WO₃ material has the potential for use in a practical acetone gas sensor in the future.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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