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

Urchin-like Cu-W 18 O 49 and flower-like Cu-WO 3 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 withmonoclinic structures, which were approximately 1 μmand 1-2μm, respectively, possessed microflower architecture assembled by the nanosheet. In addition, the gas sensing properties of monoclinic-structured Cu-WO 3 to acetone were measured using a static state gas sensing test system.The sensor based on the flower-like Cu-WO 3 nanostructures, which were calcined at 600C, exhibited high sensitivity toward 10 ppm acetone at an optimum temperature of 110C, and the maximum sensitivity reached 40, which was approximately four times higher than that of urchin-like WO 3 that was annealed at 300C.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.


Introduction
Tungsten oxide (WO 3 ) is a wide-band-gap ( = 2.8 eV) -type semiconductor that has been widely used in gas detection sensors.In recent years, WO 3 has become the most promising semiconductor gas sensing material that has been used to detect NO x , NH 3 , CH 4 , and H 2 S [1][2][3][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 WO 3 gas sensor to decrease the response and recovery time [5].However, for single-component WO 3 , some limitations, such as a high operating temperature and poor sensitivity, still exist which can prevent the extensive use of WO 3 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][8][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][11][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 urchinlike W 18 O 49 and flower-like WO 3 structures with the addition of Cu(NO 3 ) 2 as a doping agent in the ethanol system.To simplify the reaction process, WC 16 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-WO 3 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 WO 3 based gas sensors for practical applications.3 .Flowerlike copper-doped WO 3 was fabricated using a hydrothermal method.In a typical synthesis, 0.25 g of WCl 6 (as tungsten source) was dissolved in 60 mL of ethanol (C 2 H 5 OH, as solvent) followed by stirring until the solution was transparent and yellow.Subsequently, a suitable amount of copper nitrate (Cu(NO 3 ) 2 ⋅3H 2 O, 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.

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.

Instrumentation.
The morphology of the flower-like Cu-WO 3 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 K radiation ( = 0.15418 nm).The XRD data were collected in a 2 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).

Morphological Characterization of Flower-Like
Cu-Doped WO 3 .Figure 1 shows the XRD patterns of the Cu-WO 3 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-WO 3 sample calcined at 300 ∘ C (Figure 1(a)), all of the diffraction peaks can be easily indexed to the monoclinic W 18 O 49 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-WO 3 .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 WO 3 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 W 18 O 49 structure was transferred to a monoclinic WO 3 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.
The SEM image in Figure 2(a) indicates that the Cu-WO 3 sample sintered at 300 ∘ C is approximately 1 m 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 m 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 WO 3 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 WO 3 sample doped with a small amount of copper, no characteristic peaks were observed.The Cu-WO 3 sample after calcination at 300 ∘ C, which has a monoclinic W 18 O 49 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 lowdimensional nanomaterials provided a higher specific surface area [16,17], which is conducive to gas absorption and diffusion.

Gas Sensing Properties.
The gas sensing performance of WO 3 , 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 W 18 O 49 and WO 3 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 W 18 O 49 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-WO 3 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 WO 3 (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 WO 3 sensor to acetone.In Figure 3(b), the different doping concentrations of copper in flower-like WO 3 are shown to determine the optimum doping concentration.The sensitivities of the 0 wt%, 1 wt%, and 5 wt% Cu-doped WO 3 samples were lower than that of the 3 wt% doped sample, suggesting that the optimum copper doping amount is 3 wt%.
In comparison to the WO 3 sensor without a doping agent [19], the acetone gas sensing properties of the Cudoped WO 3 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 WO 3 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 CuWO 4−x may be formed because Cu 2+ (0.73 Å) can substitute for W 6+ (0.62 Å) in the lattice cell of WO 3 , 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 flowerlike WO 3 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 WO 3 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  = 1 +   (  )  , where   denotes the prefactor,   is the target gas partial pressure, which is directly proportional to its gas concentration, and b 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 ( res ) and recovery ( rec ) 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 flowerlike Cu-WO 3 are sufficiently sensitive to detect acetone in a range from 1 ppm to 10 ppm.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-WO 3 gas sensor to 10 ppm acetone, methanol, ether, NH 3 , ethanol, isopropanol, NO 2 , and H 2 S at 110 ∘ C, respectively.The response of the Cu-WO 3 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-WO 3 sensor response of acetone to that of the other five target gases ranged from 6.6 to 37. Therefore, the flower-like Cu-WO 3 gas sensor has good selectivity to acetone at 110 ∘ C in the presence of the previously mentioned interfering gases.

Conclusions
In summary, monoclinic urchin-like and flower-like Cu-WO 3 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-WO 3 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 WO 3 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-WO 3 material has the potential for use in a practical acetone gas sensor in the future.

Figure 3 :
Figure 3: (a) Sensitivity of the Cu-doped W 18 O 49 (black line) and WO 3 (blue line) sensor towards 10 ppm acetone under different operating conditions.(b) Different doping concentrations of copper in flower-like WO 3 .

Figure 4 :
Figure 4: (a) Sensitivity of flower-shaped WO 3 to different concentrations of acetone at 110 ∘ C. (b) Logarithmic linear fitting of sensibility and concentration.(c) Response ( res ) and recovery ( rec ) time measured from the response-time data to 8 ppm acetone at an operating temperature of 110 ∘ C.

Figure 5 :
Figure 5: Responses of the flower-like Cu-WO 3 gas sensor to different gases at 110 ∘ C.
2.1.Chemicals.All of the chemical reagents were of analytic purity and used directly without further purification.WCl 6 was used as the tungsten source and purchased from the Aladdin Company.Ethanol (C 2 H 5 OH) and copper nitrate (Cu(NO 3 ) 2 ⋅3H 2 O) were purchased from the Beijing Chemical Plant.Deionized water with a resistivity greater than approximately 7 Mohm was used for all of the experiments.