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Preparation of hierarchical WO3 dendrites and their applications in NO2 sensing
Ceramics International 43 (2017) 8183–8189
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Preparation of hierarchical WO3 dendrites and their applications in NO2 sensing
MARK
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Bingxin Xiao, Dongxue Wang, Fei Wang, Qi Zhao, Chengbo Zhai, Mingzhe Zhang State Key Laboratory of Superhard Materials, Jilin University, 2699 Qianjin Street, Changchun 130012, People's Republic of China
A R T I C L E I N F O
A BS T RAC T
Keywords: Hierarchical nanostructure WO3 dendrites Formation mechanism NO2-sensing performance
Hierarchical WO3 dendrites were synthesized via low-cost and environmental-friendly solvothermal strategy. Characterization results indicated that WO3 dendrites were composed of several multi-directional dendritic nanosheets. To further understand the formation of WO3 dendrites, time-dependent experiments were carried out and formation mechanism was investigated. Since such dendritic structures rarely occurred in the field of gas sensing, the synthesized WO3 dendrites were subjected to detailed NO2 sensing tests. Results demonstrated that WO3 dendrites based sensors had low detection limit (200 ppb) and fast response and recovery (7 s, 12 s to 5 ppm NO2). Moreover, the sensor was also highly sensitive, selective and stable at low optimal operating temperature of 140 °C.
1. Introduction As a trace component of the atmospheric gases, while nitrogen dioxide (NO2) plays an important role in atmospheric chemistry, it is one of the major air pollutants (i.e. it is one of the important products of O3 and aerosol) [1–3]. Under irradiation of light, NO2 could trigger photochemical reactions that lead to photochemical smog and elevated tropospheric O3 concentration [4]. NO2 is not only one of the causes of acid rain formation, its toxic effect on human can cause irritation and respiratory system, and may lead to more severe respiratory diseases such as lung lesion and pulmonary edema; and at high concentration, it may cause death [5,6]. To protect the public health and environment, development of efficient strategies is urgently needed to monitor and control the level of NO2 gases. The NO2 sensors, therefore, appear exceedingly essential [7]. Over the past years, gas sensors have contributed in the fields of security, medical service and environment [8,9]. In order to extend its practical application, the 4S performance, which includes sensitivity, speed, selectivity and stability, should be increased, and this requires deep exploration in terms of sensing mechanism, materials and device structures [10]. Properties of sensors highly rely on surface area, size and morphology as well as surface and interface structures [11,12]. Hence, these are the key issues remain to be resolved, as well as controllable synthesis and functional regulation need to be determined in order to design the nanostructure principles in light of relationships between the structures and sensing performances. Dendrites, the branch-like growing crystals, can often be found in
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daily life. Their properties and applications are significantly affected by their size and morphology, such that their distributions to the dendritic structure endow the materials with special physical and chemical properties [13,14]. These distributions are, however, rarely reported in the area of gas sensing. As one of the most abundant resources on earth, the non-toxic tungsten trioxide (WO3) has received an intense interest for its adjustable structure, as well as its unique physical and chemical properties. It has been widely used in many aspects, such as electrochromic application [15], photochromism [16], gas sensing [17,18], photocatalysis [19], soar cells [20], and lithium-ion batteries [21]. WO3 is an n-type metal oxide semiconductor material with a wide band gap (Eg) of 2.5–2.8 eV [22]. Up to now, various morphologies of WO3 nanostructures have been extensively fabricated and used as gas sensors [23–27]. While the performances are continually improved, some inevitable disadvantages nevertheless exist. For instance, the slow response, this is rather common especially when use as NO2 sensors. On the basis of above considerations, hierarchical WO3 dendrites have been successfully synthesized via a facile, economical, and lowtemperature (100 °C) solvothermal synthetic route, and introduced to serve as NO2 sensors. When exposed to NO2 environment, the dendritic sensor showed relatively low detection limit，high sensitivity，selectivity, and stability, as well as it is highly responsive toward NO2 gas.
Corresponding author. E-mail address: [email protected] (M. Zhang).
http://dx.doi.org/10.1016/j.ceramint.2017.03.144 Received 15 March 2017; Received in revised form 21 March 2017; Accepted 22 March 2017 Available online 23 March 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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2. Experimental 2.1. Materials and methods All analytical-grade reagents were used without further purification. Deionised water was used throughout all experiments. In a typical procedure, 0.25g of sodium tungstate (Na2WO4) and 0.5g citric acid (C6H8O7) were dissolved in 5 ml H2O, was then acidified by hydrochloric acid (HCl) solution to a pH~1.5, stirried for 10 min at room temperature. Fifteen millilitre of ethanol (C2H5OH) was then slowly added into the solution, and stirred for another 10 min until the ivorywhite suspension was gradually formed. The white precipitate was transferred to a 20 ml vial, sealed and heated at 100 °C for 24 h. The resulting blue precipitates were then centrifuged, washed with water and ethanol, and air-dried at 60 °C for 2 h before subjecting to calcination at 500 °C for 2 h. Fig. 2. XRD pattern of the synthesized WO3 dendrites.
2.2. Characterization The WO3 samples were characterized by power X-ray diffraction (Rigaku D/max-Ra) that was equipped with a Cu Kα1 radiation source (λ=1.54056 Å), and scanned at a rate of 6°/min from 20° to 80°. The characterization was also done by transmission electron microscope (TEM; JEOL JEM-2200FS) and scanning electron microscopy (SEM) that were equipped with Magellan 400, FEI microscope and operated at 20 kV. The TG was carried out on a TGA 209 F1 Netzsch apparatus with a 10 °C/min heating rate.
2.3. Fabrication of gas sensor To fabricate gas sensors, the synthesized materials were mixed with ethanol to form a paste, which were then deposited on the alumina tube-like substrates (area =4.0 mm×1.5 mm, thickness =0.4 mm), attached with Pt wires on a pair of Au electrode. The resulting products were air-dried at room temperature for 24 h. A Ni–Cr alloy coil was subsequently inserted into the alumina tube as a heater to adjust the operating temperature. The schematic illustration of the device is shown in Fig. 1. The gas sensing properties were measured with a CGS-8 characterization system under laboratory conditions (30 ± 3% RH, 25 ± 1 °C). The electrical properties of the sensor were measured by static process. Typically, target gases with calculated amount were injected into the testing chamber by a micro-syringe. The response of the sensor was calculated as Rg/Ra for NO2 and while Ra/Rg ratios were used for reducing test gas, Ra and Rg refer to the resistance of the sensor in air and in the target gases, respectively. In addition, the response and recovery are defined as the ratio of time taken by the sensor in absorption and desorption, respectively, to achieve 90% of the total resistance change.
Fig. 3. (a, b) SEM images of the as-prepared hierarchical WO3 dendrites with different magnification.
3. Results and discussion 3.1. Characterization of WO3 dendrites Powder X-ray diffraction (XRD) was used first to characterize the Fig. 1. Schematic of the gas sensor.
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Fig. 4. (a) and (b) TEM images of WO3 dendrites; (c) SAED and (d) HRTEM image of the WO3 dendrites.
indicated that the WO3 nanostructures adopted such morphology that while some large dendritic nanosheets lied at the bottom and others lied perpendicularly and assembled in random directions. TEM was employed to gain further insight into the WO3 dendrites as shown in Fig. 4a and b. The results demonstrated that the dendrites were composed of a number of dendrites, linked to each other, leading to the structure that is highly favourable for electron transport and gas adsorption. In addition, the secondary dendrites were also observed and found to be parallel and uniformly angled with main branches. Fig. 4c shows a selected area of electron diffraction (SAED) pattern. The diffraction spots showed a hexagon arrangement, which confirmed the hexagonal and single-crystal phase of the WO3 dendrites, as well as it is in conformance with the XRD results. Spots at the surface could be indexed into (100), (200), (300), (110), (210) and (310) planes. The high-resolution transmission electron microscopy (HRTEM) image (Fig. 4d) shows 60° angle between main and secondary branches. The clear and parallel lattice fringes also indicated the dendrites were in single phase; the lattice distances of 0.632 nm and 0.362 nm were correspondent, respectively to the (100) plane of the main branch and the (110) plane of the hexagonal phase of WO3. The thermo-gravimetric (TG) and differential thermal analysis (DTA) curves of the precursor of the as-prepared WO3 product are investigated, as depicted in Fig. 5. It can be realized that the first weight-loss from room temperature to 600 °C is considered to be the release of the water. Then the weight tends to stabilize after 600 °C which indicates the good stability of the WO3 dendrites.
Fig. 5. TG and DTA curves of the WO3 dendrites.
prepared WO3 dendrites. As shown in Fig. 2, the pattern and intensity ratios of the peaks fitted well with standard (JCPDS No. 33-1387), an indication of weak orientation and intrinsic crystallization of the samples. These diffraction peaks were indexed into crystal face of (100), (001), (110), (101), (200), (111), (201), (300), (211), (002), (102), (301), (310), (112) and (401), which corresponds to hexagonal phase of the synthesized WO3 dendrites. Moreover, there were no other peaks found in the pattern, indicated that the WO3 sample were of high purity. Fig. 3a shows SEM images of WO3 nanosheets that are packed closely to form film-like nanostructures. Furthermore, Fig. 3b represents a higher magnification of SEM image in Fig. 3a that showed a clear dendritic nanostructure of each nanosheet. The inset of Fig. 3b shows a close-up view of the edge of the film like nanostructure. These
3.2. Formation mechanism To substantially understand the morphology evolution of the WO3 dendrites, time dependent experiments were carried out to investigate the formation process, as illustrated in Fig. 6. 8185
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Fig. 6. WO3 sample under different reaction time: (a) 2 h; (b) 3 h; (c) 4 h; (d) 5 h and (e) 6 h.
Fig. 7. The hexagonal structure of WO3.
To start with, small nanosheets were observed after 2 h heat treatment (Fig. 6a). These small nanosheets subsequently grew new limbs after an additional 1 h of solvothermal treatment (Fig. 6b). With additional time, similar process repeated so that additional limbs were grown and formed net like nanostructures, which finally leaded formation of the dendrites (Fig. 6c to e). It is known that crystals grow in orientations that favour the reduction of Gibbs free energy: the lower the Gibbs free energy, the larger the driving force and the higher growth rate. When a growth system is under a non-equilibrium state, in other words, when the electric field, magnetic field, temperature field and diffusion field are in disequilibrium, driving force in different direction and position of the crystals is different, and that difference increases with increasing crystal dimension. This is the condition that favours the dendritic structures formation. As for the 60° angle, which is believed to originate from the crystal structure. The corresponding crystal structure of WO3 is illustrated in Fig. 7, in which the hexagonal structure can be distinguished. The basis vector is determined by the hexagonal structure symmetrical characteristics, which reveals that cell parameters are a=b≠c，α=β=90°，γ=120°. Thus the secondary dendrites tends to grow alone the surface with 120° angle (60°). Unlike the commonly seen two-dimensional dendritic nanostructures of snowflake, morphology of the synthesized WO3 dendritic nanostructures is three-dimensional that are assembled with na-
Fig. 8. (a) Responses to 500 ppb of NO2 at different temperature of the WO3 product. (b) Responses to different gases of WO3 dendrites at 140 °C. The inset is the corresponding response curves to reducing gases of the sensor.
nosheets in random directions. This could be attributed to the chelated and mixed alcohol-ester surfactant, which is generated from the citric acid and ethanol reaction. The surfactant promotes the formation of three-dimensional dendrites in water phase [28].
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Fig. 10. (a) The typical response curve cycling to 500 ppb of NO2 at 140 °C. (b) The long-term stability of the WO3 sensor to 500 ppb of NO2 at 140 °C.
Fig. 9. (a) Dynamical response curves of WO3 sensor to 0.25, 0.5 and 1 ppm of NO2. (b) Variation of response and recovery times with different concentration of NO2 at 140 °C.
While competing with each other to grow, some of the secondary dendrites would be eliminated. (5) Final growth. The growth process in stages (3) and (4) further continues so that tertiary, quaternary, and higher classes of dendrites are generated, and thus, the dense branch morphology is finally formed.
Additionally, crystals growth is strongly governed by solute diffusion and temperature field, whereas dendrites formation needs a nonequilibrium system. The introduction of ethanol is thus believed to be the key of the formation of WO3 dendrites: after the ethanol was added, the total concentration of the system decreased, which instantly upset the diffusion field leading to forming defects during the growth of WO3 crystals under different driving force and in different positions; and secondary dendrites then generated on the position of the defects so that the WO3 dendrites were formed [29]. The whole growth process could be inferred as the following five stages: (1) Polymerization. WO42− anions were first protonized and polymerized to form polytungstate anions after being acidified by HCl solution, Eq. (1); (2) Nucleation. With increasing temperature, the saturated polytungstate anions subsequently decomposed into WO3 nucleus, Eq. (2) [30,31]. (3) Primary growth. The nucleus then preferentially grew along the [100] direction, forming main branches, so called primary dendrite. (4) Secondary growth. The secondary dendrites grew on defective areas of the existing primary dendrites.
10WO42− + 16H+ = W10O324− + 8H2O
(1)
W10O324− + 4H+ = 10WO3 + 2H2O
(2)
3.3. Gas-sensing properties To investigate the optimal operating temperature of the WO3 dendrite based sensor, a temperature-dependent experiment, with temperatures ranged from 120 °C to 170 °C was carried out toward 500 ppb of NO2 gases. As shown in Fig. 8a, it is apparent that sensitivity is strongly dependent on the operating temperature. While the response value increased from 1.7, 3.4 to 4.3 when temperature was increased from 120 °C to 140 °C, it decreased to 3.9, 3.3 and 2.3
Table 1 Comparison of NO2 sensing properties of different sensing material. Material
Working temperature
NO2 Concentration (ppm)
Response
Response time
Recovery time
Ref.
Porous rh- In2O3 nanosheets Nonporous rh-In2O3 nanosheets Lotus root slice-like In2O3 microspheres W18O49 nanowire arrays WO3 nanowires Ag loaded WO3 WO3 dendrites
250 °C 250 °C 250 °C 150 °C 250 °C 75 °C 140 °C
50 50 20 1 0.5 1 5
164 57 36 4.4 2.3 44 32.9
5s 5s 5s 78 s 102 s 5.05 min 7s
14 s 23 s 20 s 32 s 108 s 2.46 min 12 s
[38] [38] [39] [40] [41] [42] This work
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The mechanism of the NO2 sensing is, however, of concern. The WO3 dendrites adsorb oxygen from air and capture free electrons from the conduction band, which then cause the chemisorbed-negatively charged oxygen ions (O2−, O− and O2−), as well as generated electrondepleted region on the surface. However, only O2−, O− could be formed when the temperature is lower than 300 °C, which will then it would result in the increase of resistance [33]. These adsorption processes are expressed in Eqs. (3), (4) and (5) [34].
when temperature was further increased to 170 °C. Temperature of 140 °C was believed to be the optimal operating temperature and was applied in subsequent gas-sensing property tests. The cross sensing (selectivity) between NO2 and other gases is still one of the major problem in practical NO2 sensor. In order to evaluate responses of the sensor to mixed gases, two experiments, in which 50 ppm organic testing gases or 500 ppb of NO2 gases was added into the system, were carried out at 140 °C. As shown in Fig. 8b, a bar graph representation showings the sensor response with respect to a variety of gases, the sensor has significantly higher selectivity to NO2 gases than that of other organic gases even the concentration of NO2 is much lower than that of other gases. And the responses were calculated to be 1.5, 2.8, 3.5, 1.2, 1.3, 2.4, and 32.9 to 50 ppm of benzene, methanol, ethanol, toluene, ammonia, acetone, and 5 ppm of NO2. This indicated that structure of WO3 sensor possessed fairly good selectivity to NO2, in which it could be an ideal detector for the NO2 gases in the atmosphere. To further investigate sensing properties of the WO3 dendrites, the dynamic sensing response measurements were carried out by exposing to different concentrations of NO2 gas at 140 °C (Fig. 9a). The results showed that the response increased with increasing NO2 concentration. Although the resistance increased when exposed in NO2 gas, it was completely recovered to the initial value after NO2 gas was removed in the curve. This indicated the good reusability of the sensor. The responses of the sensors were approximately 1.7, 2.3, 4.3, 6.1, 7.5, 18.8 and 32.9 to 200 ppb, 300 ppb, 500 ppb, 800 ppb, 1 ppm, 3 ppm and 5 ppm of NO2 gases, and this demonstrated that the sensor has good response and low detection limit, while being able to operate at low temperature. Selectivity and sensitivity, as well as response/recovery time, which depends upon the gas adsorption/desorption process, are also the key parameters in practical sensing applications. The response and recovery times of the WO3 sensor with respects to different concentration of NO2 are investigated at 140 °C in Fig. 9b, the response and recovery times to 0.2, 0.3, 0.5, 0.8, 1, 3 and 5 ppm of NO2 are 27 s and 32 s, 25 s and 30 s, 23 s and 27 s, 15 s and 23 s, 13 s and 18 s, 11 s and 14 s, 7 s and 12 s, respectively. It is evident that the response and recovery times are shorter than that of many other NO2 sensors, indicating a great potential use of the synthesized WO3 dendrites in fabricating NO2 sensors. For comparison, Table 1 shows NO2 sensing performances of different structures. Although these structures present good results in detecting NO2 gas, they demonstrate inevitable disadvantages, such as low sensitivity, high operating temperature, and long response time. In contrast, the overall properties of the hierarchical WO3 dendrites, which include moderate operating temperature, high sensitivity, and fast response, confirmed its potential use for practical NO2 detection of the hierarchical WO3 dendrites. The long-term performance of the sensor is also an important property of a gas sensor in determining its practical applications. Fig. 10a shows five periods of response curve, it indicated the excellent reversibility and stability. Fig. 10b depicts the responses of the prepared WO3 sensor to 500 ppb NO2 at 140 °C, detected every 3 days. The results demonstrated slight changes of the response with the maximum deviations of less than 2% after 15 days. This demonstrated a good long-term stability of the WO3 sensor in NO2 detection.
O2 (gas) ⇔O2 (ads)
(3)
O2 (ads) + e−⇔ O2−
(4)
O2− + e−⇔ 2O−
(5)
When NO2 was introduced into the system, as a highly reactive oxidizing gas, it intensely captured electrons from the conduction band, and further thickened electron-depleted region and as a result, the resistance of the WO3 sensor was greatly increased. These are described in Eqs. (6), (7) and (8) [35–37]. NO2 (gas) + e−→ NO2− (ads)
(6)
NO2 (gas) + O2− + 2e−→ NO2− (ads) + 2O−
(7)
NO2− (ads) + O− (ads) + 2e−→ NO (gas) + 2O2− (ads)
(8)
4. Conclusion In summary, hierarchical WO3 dendrites were synthesized via a low-cost and environmental friendly solvothermal strategy. Characterization of the WO3 showed that they are assembled by several multi-directional dendritic nanosheets. The time-dependent experiment revealed the five-stage growth process from single nanosheets to the final dendritic nanostructures. Considering the required growth conditions of the dendrites, ethanol and citric acid were considered to be the key parts in generating special dendrite nanostructures. When used as NO2 sensor, in contrast to other reported sensors, the WO3 dendritic sensor presented fast, highly sensitive, selective and stable sensing properties. Acknowledgements This work was funded by the National Natural Science Foundation of China, no. 11174103, 11474124 and 61674065. References [1] P.J. Crutzen, L.E. Heidt, J.P. Krasnec, W.H. Pollock, W. Seiler, Biomass burning as a source of atmospheric gases CO, H2, N2O, NO, CH3Cl and COS, Nature 282 (1979) 253–256. [2] R. Atkinson, Atmospheric chemistry of VOCs and NOx, Atmos. Environ. 34 (2000) 2063–2101. [3] A. Richter, J.P. Burrows, H. Nusz, C. Granier, U. Niemeier, Increase in tropospheric nitrogen dioxide over China observed from space, Nature 437 (2005) 129–132. [4] C. Kirby, M. Fox, J. Waterhouse, T. Drye, Influence of environmental parameters on the accuracy of nitrogen dioxide passive diffusion tubes for ambient measurement, J. Environ. Monit. 3 (2001) 150–158. [5] H. Zhang, J. Feng, T. Fei, S. Liu, T. Zhang, SnO2 nanoparticles-reduced graphene oxide nanocomposites for NO2 sensing at low operating temperature, Sens. Actuators B: Chem. 190 (2014) 472–478. [6] B. Xiao, S. Song, P. Wang, Q. Zhao, M. Chuai, M. Zhang, Promoting effects of Ag on In2O3 nanospheres of sub-ppb NO2 detection, Sens. Actuators B: Chem. 241 (2017) 489–497. [7] H. Cai, R.Z. Sun, X. Yang, X.S. Liang, C. Wang, P. Sun, F.M. Liu, C. Zhao, Y.F. Sun, G.Y. Lu, Mixed-potential type NOx sensor using stabilized zirconia and MoO3– In2O3 nanocomposites, Ceram. Int. 42 (2016) 12503–12507. [8] Q. Yang, Y. Wang, J. Liu, J. Liu, Y. Gao, P. Sun, J. Zheng, T. Zhang, Y. Wang, G. Lu, Enhanced sensing response towards NO2 based on ordered mesoporous Zr-doped In2O3 with low operating temperature, Sens. Actuators B: Chem. 241 (2017) 806–813. [9] Y. Yang, J. Hu, Y. Liang, J. Zou, K. Xu, R. Hu, Z. Zou, Q. Yuana, Q. Chen, Y. Lu,
3.4. Mechanism of the sensing properties Based on the above results, excellent NO2 sensing performance was presented by the synthesized WO3 sample, such performance may be mainly attributed to the intrinsic oxygen deficiency of WO3 and the dendritic nanostructure [32]. While the oxygen deficiency is highly favourable in order to absorb and detect the test gases, the dendrites, as transmission route, endow the net-like structure to facilitate the transmissions of the electrons, leading to facilitate the sensing reaction. Therefore, the hierarchical architecture is the key factor to improve the sensing performance. 8188
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Preparation of hierarchical WO3 dendrites and their applications in NO2 sensing
MARK
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Bingxin Xiao, Dongxue Wang, Fei Wang, Qi Zhao, Chengbo Zhai, Mingzhe Zhang State Key Laboratory of Superhard Materials, Jilin University, 2699 Qianjin Street, Changchun 130012, People's Republic of China
A R T I C L E I N F O
A BS T RAC T
Keywords: Hierarchical nanostructure WO3 dendrites Formation mechanism NO2-sensing performance
Hierarchical WO3 dendrites were synthesized via low-cost and environmental-friendly solvothermal strategy. Characterization results indicated that WO3 dendrites were composed of several multi-directional dendritic nanosheets. To further understand the formation of WO3 dendrites, time-dependent experiments were carried out and formation mechanism was investigated. Since such dendritic structures rarely occurred in the field of gas sensing, the synthesized WO3 dendrites were subjected to detailed NO2 sensing tests. Results demonstrated that WO3 dendrites based sensors had low detection limit (200 ppb) and fast response and recovery (7 s, 12 s to 5 ppm NO2). Moreover, the sensor was also highly sensitive, selective and stable at low optimal operating temperature of 140 °C.
1. Introduction As a trace component of the atmospheric gases, while nitrogen dioxide (NO2) plays an important role in atmospheric chemistry, it is one of the major air pollutants (i.e. it is one of the important products of O3 and aerosol) [1–3]. Under irradiation of light, NO2 could trigger photochemical reactions that lead to photochemical smog and elevated tropospheric O3 concentration [4]. NO2 is not only one of the causes of acid rain formation, its toxic effect on human can cause irritation and respiratory system, and may lead to more severe respiratory diseases such as lung lesion and pulmonary edema; and at high concentration, it may cause death [5,6]. To protect the public health and environment, development of efficient strategies is urgently needed to monitor and control the level of NO2 gases. The NO2 sensors, therefore, appear exceedingly essential [7]. Over the past years, gas sensors have contributed in the fields of security, medical service and environment [8,9]. In order to extend its practical application, the 4S performance, which includes sensitivity, speed, selectivity and stability, should be increased, and this requires deep exploration in terms of sensing mechanism, materials and device structures [10]. Properties of sensors highly rely on surface area, size and morphology as well as surface and interface structures [11,12]. Hence, these are the key issues remain to be resolved, as well as controllable synthesis and functional regulation need to be determined in order to design the nanostructure principles in light of relationships between the structures and sensing performances. Dendrites, the branch-like growing crystals, can often be found in
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daily life. Their properties and applications are significantly affected by their size and morphology, such that their distributions to the dendritic structure endow the materials with special physical and chemical properties [13,14]. These distributions are, however, rarely reported in the area of gas sensing. As one of the most abundant resources on earth, the non-toxic tungsten trioxide (WO3) has received an intense interest for its adjustable structure, as well as its unique physical and chemical properties. It has been widely used in many aspects, such as electrochromic application [15], photochromism [16], gas sensing [17,18], photocatalysis [19], soar cells [20], and lithium-ion batteries [21]. WO3 is an n-type metal oxide semiconductor material with a wide band gap (Eg) of 2.5–2.8 eV [22]. Up to now, various morphologies of WO3 nanostructures have been extensively fabricated and used as gas sensors [23–27]. While the performances are continually improved, some inevitable disadvantages nevertheless exist. For instance, the slow response, this is rather common especially when use as NO2 sensors. On the basis of above considerations, hierarchical WO3 dendrites have been successfully synthesized via a facile, economical, and lowtemperature (100 °C) solvothermal synthetic route, and introduced to serve as NO2 sensors. When exposed to NO2 environment, the dendritic sensor showed relatively low detection limit，high sensitivity，selectivity, and stability, as well as it is highly responsive toward NO2 gas.
Corresponding author. E-mail address: [email protected] (M. Zhang).
http://dx.doi.org/10.1016/j.ceramint.2017.03.144 Received 15 March 2017; Received in revised form 21 March 2017; Accepted 22 March 2017 Available online 23 March 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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2. Experimental 2.1. Materials and methods All analytical-grade reagents were used without further purification. Deionised water was used throughout all experiments. In a typical procedure, 0.25g of sodium tungstate (Na2WO4) and 0.5g citric acid (C6H8O7) were dissolved in 5 ml H2O, was then acidified by hydrochloric acid (HCl) solution to a pH~1.5, stirried for 10 min at room temperature. Fifteen millilitre of ethanol (C2H5OH) was then slowly added into the solution, and stirred for another 10 min until the ivorywhite suspension was gradually formed. The white precipitate was transferred to a 20 ml vial, sealed and heated at 100 °C for 24 h. The resulting blue precipitates were then centrifuged, washed with water and ethanol, and air-dried at 60 °C for 2 h before subjecting to calcination at 500 °C for 2 h. Fig. 2. XRD pattern of the synthesized WO3 dendrites.
2.2. Characterization The WO3 samples were characterized by power X-ray diffraction (Rigaku D/max-Ra) that was equipped with a Cu Kα1 radiation source (λ=1.54056 Å), and scanned at a rate of 6°/min from 20° to 80°. The characterization was also done by transmission electron microscope (TEM; JEOL JEM-2200FS) and scanning electron microscopy (SEM) that were equipped with Magellan 400, FEI microscope and operated at 20 kV. The TG was carried out on a TGA 209 F1 Netzsch apparatus with a 10 °C/min heating rate.
2.3. Fabrication of gas sensor To fabricate gas sensors, the synthesized materials were mixed with ethanol to form a paste, which were then deposited on the alumina tube-like substrates (area =4.0 mm×1.5 mm, thickness =0.4 mm), attached with Pt wires on a pair of Au electrode. The resulting products were air-dried at room temperature for 24 h. A Ni–Cr alloy coil was subsequently inserted into the alumina tube as a heater to adjust the operating temperature. The schematic illustration of the device is shown in Fig. 1. The gas sensing properties were measured with a CGS-8 characterization system under laboratory conditions (30 ± 3% RH, 25 ± 1 °C). The electrical properties of the sensor were measured by static process. Typically, target gases with calculated amount were injected into the testing chamber by a micro-syringe. The response of the sensor was calculated as Rg/Ra for NO2 and while Ra/Rg ratios were used for reducing test gas, Ra and Rg refer to the resistance of the sensor in air and in the target gases, respectively. In addition, the response and recovery are defined as the ratio of time taken by the sensor in absorption and desorption, respectively, to achieve 90% of the total resistance change.
Fig. 3. (a, b) SEM images of the as-prepared hierarchical WO3 dendrites with different magnification.
3. Results and discussion 3.1. Characterization of WO3 dendrites Powder X-ray diffraction (XRD) was used first to characterize the Fig. 1. Schematic of the gas sensor.
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Fig. 4. (a) and (b) TEM images of WO3 dendrites; (c) SAED and (d) HRTEM image of the WO3 dendrites.
indicated that the WO3 nanostructures adopted such morphology that while some large dendritic nanosheets lied at the bottom and others lied perpendicularly and assembled in random directions. TEM was employed to gain further insight into the WO3 dendrites as shown in Fig. 4a and b. The results demonstrated that the dendrites were composed of a number of dendrites, linked to each other, leading to the structure that is highly favourable for electron transport and gas adsorption. In addition, the secondary dendrites were also observed and found to be parallel and uniformly angled with main branches. Fig. 4c shows a selected area of electron diffraction (SAED) pattern. The diffraction spots showed a hexagon arrangement, which confirmed the hexagonal and single-crystal phase of the WO3 dendrites, as well as it is in conformance with the XRD results. Spots at the surface could be indexed into (100), (200), (300), (110), (210) and (310) planes. The high-resolution transmission electron microscopy (HRTEM) image (Fig. 4d) shows 60° angle between main and secondary branches. The clear and parallel lattice fringes also indicated the dendrites were in single phase; the lattice distances of 0.632 nm and 0.362 nm were correspondent, respectively to the (100) plane of the main branch and the (110) plane of the hexagonal phase of WO3. The thermo-gravimetric (TG) and differential thermal analysis (DTA) curves of the precursor of the as-prepared WO3 product are investigated, as depicted in Fig. 5. It can be realized that the first weight-loss from room temperature to 600 °C is considered to be the release of the water. Then the weight tends to stabilize after 600 °C which indicates the good stability of the WO3 dendrites.
Fig. 5. TG and DTA curves of the WO3 dendrites.
prepared WO3 dendrites. As shown in Fig. 2, the pattern and intensity ratios of the peaks fitted well with standard (JCPDS No. 33-1387), an indication of weak orientation and intrinsic crystallization of the samples. These diffraction peaks were indexed into crystal face of (100), (001), (110), (101), (200), (111), (201), (300), (211), (002), (102), (301), (310), (112) and (401), which corresponds to hexagonal phase of the synthesized WO3 dendrites. Moreover, there were no other peaks found in the pattern, indicated that the WO3 sample were of high purity. Fig. 3a shows SEM images of WO3 nanosheets that are packed closely to form film-like nanostructures. Furthermore, Fig. 3b represents a higher magnification of SEM image in Fig. 3a that showed a clear dendritic nanostructure of each nanosheet. The inset of Fig. 3b shows a close-up view of the edge of the film like nanostructure. These
3.2. Formation mechanism To substantially understand the morphology evolution of the WO3 dendrites, time dependent experiments were carried out to investigate the formation process, as illustrated in Fig. 6. 8185
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Fig. 6. WO3 sample under different reaction time: (a) 2 h; (b) 3 h; (c) 4 h; (d) 5 h and (e) 6 h.
Fig. 7. The hexagonal structure of WO3.
To start with, small nanosheets were observed after 2 h heat treatment (Fig. 6a). These small nanosheets subsequently grew new limbs after an additional 1 h of solvothermal treatment (Fig. 6b). With additional time, similar process repeated so that additional limbs were grown and formed net like nanostructures, which finally leaded formation of the dendrites (Fig. 6c to e). It is known that crystals grow in orientations that favour the reduction of Gibbs free energy: the lower the Gibbs free energy, the larger the driving force and the higher growth rate. When a growth system is under a non-equilibrium state, in other words, when the electric field, magnetic field, temperature field and diffusion field are in disequilibrium, driving force in different direction and position of the crystals is different, and that difference increases with increasing crystal dimension. This is the condition that favours the dendritic structures formation. As for the 60° angle, which is believed to originate from the crystal structure. The corresponding crystal structure of WO3 is illustrated in Fig. 7, in which the hexagonal structure can be distinguished. The basis vector is determined by the hexagonal structure symmetrical characteristics, which reveals that cell parameters are a=b≠c，α=β=90°，γ=120°. Thus the secondary dendrites tends to grow alone the surface with 120° angle (60°). Unlike the commonly seen two-dimensional dendritic nanostructures of snowflake, morphology of the synthesized WO3 dendritic nanostructures is three-dimensional that are assembled with na-
Fig. 8. (a) Responses to 500 ppb of NO2 at different temperature of the WO3 product. (b) Responses to different gases of WO3 dendrites at 140 °C. The inset is the corresponding response curves to reducing gases of the sensor.
nosheets in random directions. This could be attributed to the chelated and mixed alcohol-ester surfactant, which is generated from the citric acid and ethanol reaction. The surfactant promotes the formation of three-dimensional dendrites in water phase [28].
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Fig. 10. (a) The typical response curve cycling to 500 ppb of NO2 at 140 °C. (b) The long-term stability of the WO3 sensor to 500 ppb of NO2 at 140 °C.
Fig. 9. (a) Dynamical response curves of WO3 sensor to 0.25, 0.5 and 1 ppm of NO2. (b) Variation of response and recovery times with different concentration of NO2 at 140 °C.
While competing with each other to grow, some of the secondary dendrites would be eliminated. (5) Final growth. The growth process in stages (3) and (4) further continues so that tertiary, quaternary, and higher classes of dendrites are generated, and thus, the dense branch morphology is finally formed.
Additionally, crystals growth is strongly governed by solute diffusion and temperature field, whereas dendrites formation needs a nonequilibrium system. The introduction of ethanol is thus believed to be the key of the formation of WO3 dendrites: after the ethanol was added, the total concentration of the system decreased, which instantly upset the diffusion field leading to forming defects during the growth of WO3 crystals under different driving force and in different positions; and secondary dendrites then generated on the position of the defects so that the WO3 dendrites were formed [29]. The whole growth process could be inferred as the following five stages: (1) Polymerization. WO42− anions were first protonized and polymerized to form polytungstate anions after being acidified by HCl solution, Eq. (1); (2) Nucleation. With increasing temperature, the saturated polytungstate anions subsequently decomposed into WO3 nucleus, Eq. (2) [30,31]. (3) Primary growth. The nucleus then preferentially grew along the [100] direction, forming main branches, so called primary dendrite. (4) Secondary growth. The secondary dendrites grew on defective areas of the existing primary dendrites.
10WO42− + 16H+ = W10O324− + 8H2O
(1)
W10O324− + 4H+ = 10WO3 + 2H2O
(2)
3.3. Gas-sensing properties To investigate the optimal operating temperature of the WO3 dendrite based sensor, a temperature-dependent experiment, with temperatures ranged from 120 °C to 170 °C was carried out toward 500 ppb of NO2 gases. As shown in Fig. 8a, it is apparent that sensitivity is strongly dependent on the operating temperature. While the response value increased from 1.7, 3.4 to 4.3 when temperature was increased from 120 °C to 140 °C, it decreased to 3.9, 3.3 and 2.3
Table 1 Comparison of NO2 sensing properties of different sensing material. Material
Working temperature
NO2 Concentration (ppm)
Response
Response time
Recovery time
Ref.
Porous rh- In2O3 nanosheets Nonporous rh-In2O3 nanosheets Lotus root slice-like In2O3 microspheres W18O49 nanowire arrays WO3 nanowires Ag loaded WO3 WO3 dendrites
250 °C 250 °C 250 °C 150 °C 250 °C 75 °C 140 °C
50 50 20 1 0.5 1 5
164 57 36 4.4 2.3 44 32.9
5s 5s 5s 78 s 102 s 5.05 min 7s
14 s 23 s 20 s 32 s 108 s 2.46 min 12 s
[38] [38] [39] [40] [41] [42] This work
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The mechanism of the NO2 sensing is, however, of concern. The WO3 dendrites adsorb oxygen from air and capture free electrons from the conduction band, which then cause the chemisorbed-negatively charged oxygen ions (O2−, O− and O2−), as well as generated electrondepleted region on the surface. However, only O2−, O− could be formed when the temperature is lower than 300 °C, which will then it would result in the increase of resistance [33]. These adsorption processes are expressed in Eqs. (3), (4) and (5) [34].
when temperature was further increased to 170 °C. Temperature of 140 °C was believed to be the optimal operating temperature and was applied in subsequent gas-sensing property tests. The cross sensing (selectivity) between NO2 and other gases is still one of the major problem in practical NO2 sensor. In order to evaluate responses of the sensor to mixed gases, two experiments, in which 50 ppm organic testing gases or 500 ppb of NO2 gases was added into the system, were carried out at 140 °C. As shown in Fig. 8b, a bar graph representation showings the sensor response with respect to a variety of gases, the sensor has significantly higher selectivity to NO2 gases than that of other organic gases even the concentration of NO2 is much lower than that of other gases. And the responses were calculated to be 1.5, 2.8, 3.5, 1.2, 1.3, 2.4, and 32.9 to 50 ppm of benzene, methanol, ethanol, toluene, ammonia, acetone, and 5 ppm of NO2. This indicated that structure of WO3 sensor possessed fairly good selectivity to NO2, in which it could be an ideal detector for the NO2 gases in the atmosphere. To further investigate sensing properties of the WO3 dendrites, the dynamic sensing response measurements were carried out by exposing to different concentrations of NO2 gas at 140 °C (Fig. 9a). The results showed that the response increased with increasing NO2 concentration. Although the resistance increased when exposed in NO2 gas, it was completely recovered to the initial value after NO2 gas was removed in the curve. This indicated the good reusability of the sensor. The responses of the sensors were approximately 1.7, 2.3, 4.3, 6.1, 7.5, 18.8 and 32.9 to 200 ppb, 300 ppb, 500 ppb, 800 ppb, 1 ppm, 3 ppm and 5 ppm of NO2 gases, and this demonstrated that the sensor has good response and low detection limit, while being able to operate at low temperature. Selectivity and sensitivity, as well as response/recovery time, which depends upon the gas adsorption/desorption process, are also the key parameters in practical sensing applications. The response and recovery times of the WO3 sensor with respects to different concentration of NO2 are investigated at 140 °C in Fig. 9b, the response and recovery times to 0.2, 0.3, 0.5, 0.8, 1, 3 and 5 ppm of NO2 are 27 s and 32 s, 25 s and 30 s, 23 s and 27 s, 15 s and 23 s, 13 s and 18 s, 11 s and 14 s, 7 s and 12 s, respectively. It is evident that the response and recovery times are shorter than that of many other NO2 sensors, indicating a great potential use of the synthesized WO3 dendrites in fabricating NO2 sensors. For comparison, Table 1 shows NO2 sensing performances of different structures. Although these structures present good results in detecting NO2 gas, they demonstrate inevitable disadvantages, such as low sensitivity, high operating temperature, and long response time. In contrast, the overall properties of the hierarchical WO3 dendrites, which include moderate operating temperature, high sensitivity, and fast response, confirmed its potential use for practical NO2 detection of the hierarchical WO3 dendrites. The long-term performance of the sensor is also an important property of a gas sensor in determining its practical applications. Fig. 10a shows five periods of response curve, it indicated the excellent reversibility and stability. Fig. 10b depicts the responses of the prepared WO3 sensor to 500 ppb NO2 at 140 °C, detected every 3 days. The results demonstrated slight changes of the response with the maximum deviations of less than 2% after 15 days. This demonstrated a good long-term stability of the WO3 sensor in NO2 detection.
O2 (gas) ⇔O2 (ads)
(3)
O2 (ads) + e−⇔ O2−
(4)
O2− + e−⇔ 2O−
(5)
When NO2 was introduced into the system, as a highly reactive oxidizing gas, it intensely captured electrons from the conduction band, and further thickened electron-depleted region and as a result, the resistance of the WO3 sensor was greatly increased. These are described in Eqs. (6), (7) and (8) [35–37]. NO2 (gas) + e−→ NO2− (ads)
(6)
NO2 (gas) + O2− + 2e−→ NO2− (ads) + 2O−
(7)
NO2− (ads) + O− (ads) + 2e−→ NO (gas) + 2O2− (ads)
(8)
4. Conclusion In summary, hierarchical WO3 dendrites were synthesized via a low-cost and environmental friendly solvothermal strategy. Characterization of the WO3 showed that they are assembled by several multi-directional dendritic nanosheets. The time-dependent experiment revealed the five-stage growth process from single nanosheets to the final dendritic nanostructures. Considering the required growth conditions of the dendrites, ethanol and citric acid were considered to be the key parts in generating special dendrite nanostructures. When used as NO2 sensor, in contrast to other reported sensors, the WO3 dendritic sensor presented fast, highly sensitive, selective and stable sensing properties. Acknowledgements This work was funded by the National Natural Science Foundation of China, no. 11174103, 11474124 and 61674065. References [1] P.J. Crutzen, L.E. Heidt, J.P. Krasnec, W.H. Pollock, W. Seiler, Biomass burning as a source of atmospheric gases CO, H2, N2O, NO, CH3Cl and COS, Nature 282 (1979) 253–256. [2] R. Atkinson, Atmospheric chemistry of VOCs and NOx, Atmos. Environ. 34 (2000) 2063–2101. [3] A. Richter, J.P. Burrows, H. Nusz, C. Granier, U. Niemeier, Increase in tropospheric nitrogen dioxide over China observed from space, Nature 437 (2005) 129–132. [4] C. Kirby, M. Fox, J. Waterhouse, T. Drye, Influence of environmental parameters on the accuracy of nitrogen dioxide passive diffusion tubes for ambient measurement, J. Environ. Monit. 3 (2001) 150–158. [5] H. Zhang, J. Feng, T. Fei, S. Liu, T. Zhang, SnO2 nanoparticles-reduced graphene oxide nanocomposites for NO2 sensing at low operating temperature, Sens. Actuators B: Chem. 190 (2014) 472–478. [6] B. Xiao, S. Song, P. Wang, Q. Zhao, M. Chuai, M. Zhang, Promoting effects of Ag on In2O3 nanospheres of sub-ppb NO2 detection, Sens. Actuators B: Chem. 241 (2017) 489–497. [7] H. Cai, R.Z. Sun, X. Yang, X.S. Liang, C. Wang, P. Sun, F.M. Liu, C. Zhao, Y.F. Sun, G.Y. Lu, Mixed-potential type NOx sensor using stabilized zirconia and MoO3– In2O3 nanocomposites, Ceram. Int. 42 (2016) 12503–12507. [8] Q. Yang, Y. Wang, J. Liu, J. Liu, Y. Gao, P. Sun, J. Zheng, T. Zhang, Y. Wang, G. Lu, Enhanced sensing response towards NO2 based on ordered mesoporous Zr-doped In2O3 with low operating temperature, Sens. Actuators B: Chem. 241 (2017) 806–813. [9] Y. Yang, J. Hu, Y. Liang, J. Zou, K. Xu, R. Hu, Z. Zou, Q. Yuana, Q. Chen, Y. Lu,
3.4. Mechanism of the sensing properties Based on the above results, excellent NO2 sensing performance was presented by the synthesized WO3 sample, such performance may be mainly attributed to the intrinsic oxygen deficiency of WO3 and the dendritic nanostructure [32]. While the oxygen deficiency is highly favourable in order to absorb and detect the test gases, the dendrites, as transmission route, endow the net-like structure to facilitate the transmissions of the electrons, leading to facilitate the sensing reaction. Therefore, the hierarchical architecture is the key factor to improve the sensing performance. 8188
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