Microelectronic Engineering 126 (2014) 88–92
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Hydrothermal synthesis of K2W4O13 nanowire with high H2S gas sensitivity Sitthisuntorn Supothina a,⇑, Mantana Suwan a, Anurat Wisitsoraat b a b
National Metal and Materials Technology Center, 114 Thailand Science Park, Paholyothin Rd., Klong Luang, Pathumthani 12120, Thailand National Electronics and Computer Technology Center, 112 Thailand Science Park, Paholyothin Rd., Klong Luang, Pathumthani 12120, Thailand
a r t i c l e
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Article history: Received 21 October 2013 Received in revised form 3 March 2014 Accepted 20 June 2014 Available online 30 June 2014 Keywords: Potassium tungsten oxide Nanowire Hydrothermal Hydrogen sulfide Gas sensor
a b s t r a c t Single crystalline, hexagonal K2W4O13 nanowire was synthesized by hydrothermal treatment of WO30.33H2O precursor which was synthesized by precipitation reaction of the tungstate salt and hydrochloric acid. The hydrothermal reaction was performed under static condition at 180 °C in the presence of K2SO4 employed as the crystal growth-directing agent. XRD and TEM analysis revealed well crystalline K2W4O13 nanowires with the average diameter of 11.7 ± 3.5 nm and the length up to several hundreds nanometers. The K2W4O13 nanowires were stable up to 400 °C and transformed to K2W7O22 after annealing at 500 °C. Without the K2SO4, only irregular platelet particle of WO30.33H2O with the primary size of 20–50 nm was obtained. Crystallization of the nanowires took place after 30 min of hydrothermal treatment. Thick-film K2W4O13 nanowire sensor was tested for sensing performance towards 0.3–10 ppm H2S, 0.2–10 ppm NO2, 20–1000 ppm SO2 and 20–1000 ppm CO. The result revealed that the sensor was most sensitive to H2S followed by NO2 at the optimum operating temperature of 300 and 350 °C, respectively, and insensitive to SO2 and CO, indicating its high selectivity to both H2S and NO2. In addition, the sensor exhibited high linearity. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The discovery of the resistance’s change upon adsorption– desorption of a semiconducting oxide in the early 1960s has triggered the research in this field [1]. Gas-sensing devices have been fabricated from the semiconducting oxides such as tin oxide (SnO2) and tungsten trioxide (WO3) for inflammable and toxic gas detections. However, the need for highly selective, low-concentration detection has driven the research for high-performance materials. During the past decade, nanostructured materials of various morphologies such as nanowire, nanorod, nanobelt, nanoflower and hollow sphere have been investigated for gas-sensing performance [2–6]. The studies revealed that gas-sensing performance can be enhanced when the sensing material is in the form of nanostructure. Potassium tungsten oxides (KxWO3, 0 < x < 1) are one type of non-stoichiometric tungsten bronzes that are formed when potassium atoms partially occupy tunnels in the WO3 host framework formed by corner sharing of WO6 octahedral [7]. The KxWO3 exhibit wide range of electronic properties and thus have been investigated for potential applications as electrodes in electrochemical ⇑ Corresponding author. Tel.: +662 5646500; fax: +662 5646447. E-mail address:
[email protected] (S. Supothina). http://dx.doi.org/10.1016/j.mee.2014.06.015 0167-9317/Ó 2014 Elsevier B.V. All rights reserved.
devices, gas sensor and solar filter [8–11]. For gas-sensing application, it has been reported that the K0.475WO3 nanosheet exhibited good responses to 10–50 ppm H2S (10–40) and 10–60 ppm acetone (20–95) at 300 °C [10]. Nanostructured potassium tungsten oxides have been synthesized in variety of shapes such as nanosheet [10], nanorod [12] and nanowire/nanofiber [11,13]. In this study, K2W4O13 nanowires have been synthesized by a simple hydrothermal method using hydrous tungsten oxide compound as a precursor. Effect of hydrothermal time on the formation of K2W4O13 was investigated. In order to evaluate its gas-sensing performance, the K2W4O13 was calcined at 400 °C to study thermal stability. The gas-sensing performance was investigated towards H2S, NO2, CO and SO2 over concentration range of 0.2–1000 ppm.
2. Experimental 2.1. Synthesis of K2W4O13 nanowires The K2W4O13 nanowires were prepared by hydrothermal treatment of WO30.33H2O precursor which was synthesized by precipitation reaction of the tungstate salt and hydrochloric acid as follow. A 1 g of ammonium tungstate parapentahydrate ((NH4)10
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2.2. Characterizations Crystal structure was determined by using an X-ray diffractometer (JDX 3530, JEOL, Tokyo, Japan). Crystal morphology was observed by using a transmission electron microscope (JEM 2010, JEOL, Tokyo, Japan).
2.3. Gas-sensing performance testing Thick-film sensor was prepared as follow: An ethyl cellulose (Fluka, 30–70 mPa.s) temporary binder was dissolved in terpineol (Aldrich, 90%) solvent under stirring and heating at 80 °C. The solution was then mixed with an appropriate amount of the K2W4O13 powder and thoroughly ground for 30 min to form a paste which was then screen printed on 0.40 cm ( 0.55 cm 0.04 cm Al2O3 substrate interdigitated with Au electrodes (interdigit width, interdigit spacing and electrode area were 100 lm, 100 lm and 0.24 cm 0.4 cm, respectively). The sensor was calcined at 400 °C for 1 h for binder removal prior to the gas-sensing test. The gas-sensing properties of K2W4O13 sensors were tested by the standard flow through technique towards four hazardous gases including H2S (0.3–10 ppm), NO2 (0.2–10 ppm), CO (20–1000 ppm) and SO2 (20–1000 ppm). A constant flux of synthetic air of 2 L/min as gas carrier was accurately mixed with the desired concentration of pollutants dispersed in synthetic air using a computer controlled multi-channel mass flow controller. All the measurements were conducted in a temperature-controlled sealed chamber under controlled humidity at different operating temperatures ranging from 200 °C to 350 °C. The sensor resistance was continuously monitored using a voltage–amperometric technique with 10 V DC bias and current measurement through a picoammeter controlled by a computer. The sensor was exposed to a gas sample for 10 min at each gas concentration and the air flux was then restored for 25 min. The sensor response (S) is defined as the resistance ratio of Ra/Rg for reducing gas (H2S, CO and SO2) and Rg/Ra for oxidizing gas (NO2), where Ra is the resistance in dry air and Rg is the resistance in a reducing or oxidizing gas.
3. Results and discussion 3.1. Synthesis of K2W4O13 nanowires Fig. 1 shows XRD patterns of the precipitating precursor and the products obtained by hydrothermal treatment of the precursor at 180 °C for 6 h in the absence and presence of 5 and 20 g K2SO4. The XRD pattern of the as-precipitate prepared by dropping HCl into the tungstate solution (pattern (a)) has weak reflections and strong background indicating poor crystallinity. Its pattern is identified as orthorhombic tungsten oxide hydrate, WO30.33H2O (JCPDS# 54-1012), and ammonium oxide chlorate hydroxylamine, (NH4O)ClO4NH2OH (JCPDS# 35-0836), as the minor phase. Poor crystallinity of the as-precipitate can be explained based on fast precipitation taking place once the 3 M HCl solution was added into the tungstate solution. Such reactant mixing resulted in local supersaturation at the regime where the acid was in contact with the tungstate solution. The product obtained from hydrothermal treatment of the precursor in the absence of the K2SO4 (pattern (b)) shows a complicated XRD pattern with sharp peaks that can be identified as the WO30.33H2O (JCPDS# 54-1012), and weak peaks of the (NH4O)ClO4NH2OH (JCPDS# 35-0836). Very weak peak at 31.2°. of an unidentified phase is also observed. This result indicates that the WO30.33H2O and the (NH4O)ClO4NH2OH in the precursor underwent more crystallization upon hydrothermal treatment. When the hydrothermal treatment was conducted in the presence of 5 or 20 g K2SO4, the products show the same diffraction patterns which consist of characteristic peaks at 2h = 23.12° and 47.08° that can be well matched with the pattern of a hexagonal K2W4O13 (JCPDS No. 20-0942) and very broad, amorphous-like structure at 25–40°. For the K2W4O13, relative intensity of the (0 0 1) peak is highest, instead of the (3 2 0) for the standard powder diffraction, indicating a preferred orientation along the (0 0 1) direction. The amorphous-like structure may be ascribed to an intermediate phase between the crystalline and the amorphous K2W4O13 due to the freezing of the in-plane rotational degree of freedom of some [WO3]6 units [14]. Increasing
K2W4O13 (NH4O)ClO4 NH2OH WO3 0.33H2O Unidentified
Relative intensity (a.u.)
W12O415H2O, WAKO) was dissolved in 30 mL of deionized water. Then, 5 mL of 3 M hydrochloric acid (HCl, AR grade, 37%, Merck) was added dropwise into the tungstate solution under stirring. The resulting mixture was stirred for 30 min at room temperature. The resulting precipitate was washed thoroughly with deionized water and separated by centrifugation. Then, the precipitate was dispersed in 30 mL of deionized water and stirred for 30 min to obtain complete suspension followed by the addition of 5 or 20 g of potassium sulfate (K2SO4, Univar). After stirring for 30 min, the mixture was transferred into a Teflon-lined stainless steel vessel, and subjected to hydrothermal treatment in an electric furnace at 180 °C for 6 h. After cooling down to room temperature, the final product was washed 3 times by stirring it in large amount of deionized water for at least 30 min. Finally, it was separated by means of vacuum filtration using a 0.1-lm cellulose membrane and finally dried at 105 °C overnight. The control experiment was also conducted by hydrothermal treatment of the precipitate under the same condition but in the absence of the K2SO4. The effect of hydrothermal time on the nanowire formation was studied by performing the hydrothermal treatment for shorter times, i.e. 30 min and 2 h. Moreover, to investigate thermal stability for gas-sensing application some parts of the product prepared in the presence of 20 g K2SO4 were calcined in air at 400, 500, 600 and 700 °C and then analyzed for phase change.
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2-Theta (degree) Fig. 1. XRD patterns of (a) precursor, and the hydrothermal products prepared in the presence of (b) 0 g, (c) 5 g and (d) 20 g K2SO4.
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(400)
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Fig. 2. TEM micrographs of (a) precursor, and the products prepared in the presence of (b) 0 g, (c) 5 g and (d) 20 g K2SO4.
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an amount of the K2SO4 from 5 to 20 g did not affect the crystalline nature of the products because relative intensity of the (0 0 1) peak remained the same. The effect of K2SO4 on promoting the nanowire formation can be explained based on the preferential adsorption of SO24 ions onto interfaces of the growing K2W4O13 crystal [15–16]. In the absence of the K2SO4, the nuclei grew to its typical lamellae structure resulting to square platelet particles. In contrast, in the presence of SO24 ions, the SO24 ions preferentially absorbed onto
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Fig. 3. High-resolution TEM micrographs of K2W4O13 nanowires prepared by hydrothermal treatment in the presence of 20 g K2SO4.
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2 Theta Fig. 4. XRD patterns of the nanorods annealed in air at (a) 400 °C, (b) 500 °C, (c) 600 °C and (d) 700 °C.
the faces parallel to the c-axis of the growing crystal that retarded the growth of these faces, leading to preferential growth along the c-axis.
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Fig. 5. TEM micrographs of the products obtained by hydrothermal treatment for (a) 30 min and (b) 2 h.
Resistance (Ohm)
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Fig. 2 shows TEM micrographs of the precursor and the products prepared in the absence and presence of the K2SO4. A distinct difference in particle morphology is clearly observed. The precursor (Fig. 2(a)) consists of irregular-shaped particle and an amorphous material. A selected-area electron diffraction (SAED) pattern (inset) shows very broad and diffused pattern which is a characteristics of the amorphous material. In the absence of the K2SO4, the hydrothermal product shows an irregular-shaped platelet particle with the primary size of 20–50 nm (Fig. 2(b)). Its SAED pattern consists of many discrete rings and dots with d-spacings corresponding to major planes of the WO30.33H2O and the (NH4O)ClO4NH2OH. In contrast, hydrothermal synthesis carried out in the presence of the K2SO4 resulted in uniform nanowires with an average diameter of 11.7 ± 3.5 nm and the length ranges from about 100 to several hundreds nanometer (Fig. 2(c) and (d)). Their SAED patterns consist of 2 sharp rings with d-spacings = 3.88 and 1.95 Å, corresponding to (0 0 1) and (0 0 2) planes, respectively, of the K2W4O13. An increased amount of the K2SO4 employed as shape-directing agent from 5 to 20 g did not show the difference of the nanowire length suggesting that 5 g K2SO4 was sufficient for this synthesis condition. Fig. 3 shows high-resolution TEM micrograph of a single nanowire. After a thorough observation along the entire nanowire, it is clear that the nanowire is single crystalline as parallel fringes are visible with an average spacing of 0.387 nm which corresponds to the (0 0 1) plane of the hexagonal K2W4O13. This is consistent with the result of the SAED patterns and XRD analysis that the nanowire grew along the c-axis. Fig. 4 shows XRD patterns of the nanowires obtained by hydrothermal treatment in the presence of 20 g K2SO4, and then annealed in air at 400, 500, 600 and 700 °C. It is evident that the prepared K2W4O13 was stable up to 400 °C and completely transformed to K2W7O22 (JCPDS No. 21-0700) and a trace amount of K2W6O19 (JCPDS No. 31-1115) after annealing at 500 °C. Only the two phases were observed at the annealing temperature up to 700 °C indicating their good thermal stability for gas-sensing which is typically operated at 200–400 °C. In order to investigate the effect of time, the hydrothermal treatment was conducted for 30 min and 2 h. TEM images of the hydrothermal products are shown in Fig. 5. By performing 30 min hydrothermal treatment, only nanoparticles, roughly 20– 30 nm in diameter, were obtained. It is the amorphous based on its SAED pattern (inset, Fig. 5(a)) which is very broad and diffused. The significant development of particle morphology was observed within 2 h of hydrothermal treatment; the nanowires having the diameter about 10–20 nm and length of 100 nm to a couple
y = 4.786x + 0.8491 2 R = 0.9983
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H2S concentration (ppm) Fig. 6. (a) Resistance change of K2W4O13 nanowire sensor under exposure to various concentrations of H2S ranging from 0.3 to 10 ppm at 300 °C and (b) corresponding response vs. H2S concentration.
microns were obtained. Its SAED pattern (inset, Fig. 5(b)) consists of discrete rings with d-spacings = 3.88 and 1.95 Å corresponding to (0 0 1) and (0 0 2) planes, respectively, of the K2W4O13. An extension of the hydrothermal treatment to 6 h led to the growth of the nanowire along the (0 0 1) direction while the diameter was about the same as shown in Fig. 2(d). The crystallization of the nanowire can be explained based on the dissolution-re-crystallization mechanism. During the hydrothermal treatment, the very fine, amorphous-like WO30.33H2O particle was uniformly dissolved
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resulting in rapid increase of concentration of the hydrolyzed species of the W6+. Once the critical supersaturation was established, a burst of single nucleation occurred in a short time interval followed by crystal growth via atomic attachment to the growing crystals resulting in longer nanowires with the increased hydrothermal time. 3.2. Gas-sensing performance Fig. 6(a) show a typical resistance change plot of the K2W4O13 nanowire sensing layer under exposure to various concentrations of H2S ranging from 0.3 to 10 ppm at 300 °C. The resistance decreases drastically during the gas exposure with increasing gas concentration, confirming a typical n-type semiconducting behavior towards reducing gas. As the H2S gas concentration increases, the responses to H2S are increased correspondingly while the response and recover times are in the ranges of 1–2 and 5– 6 min, respectively. In addition, a slight downward baseline shift is observed after several H2S exposure. Fig. 6(b) illustrates the corresponding response plot as a function of H2S concentration. It is evident that the H2S response of the sensor is highly linear within this range of concentration and the slope of response gives a good sensitivity of 4.8 ppm 1. The high linearity makes the sensor more practical for real world sensing application. Fig. 7 shows the response plots of K2W4O13 nanowire sensing layer versus operating temperature under exposure to 10 ppm H2S, 10 ppm NO2, 1000 ppm CO and 1000 ppm SO2. It reveals that the K2W4O13 nanowire exhibits high, moderate, low and very low response to H2S, NO2, SO2 and CO respectively. In addition, it has different optimal operating temperatures for different gases. The optimal operating temperature for H2S, SO2 and CO is around 300 °C while that for NO2 is about 350 °C. The optimum response of the sensor to 10 ppm H2S, 10 ppm NO2, 1000 ppm CO and 1000 ppm SO2 are estimated to be 51.3, 9.6, 1.52 and 1.4 respectively. The observed results are comparable and better than the reported K0.475WO3 and some other metal oxide gas sensors fabricated by various synthetic methods. The K0.475WO3 sensor prepared by heating a KOH-treated tungsten foil shows considerably lower response of 10–10 ppm H2S [10]. In addition, In2O3 gas sensors prepared by hydrothermal and carbothermal processes [17–18] exhibit relatively low response of 25–36 at 10 ppm H2S while ZnO sensor produced by electrostatic spray deposition [19] and Cu-loaded SnO2 synthesized by spray pyrolysis [20] give very
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Temperature ( C) Fig. 7. Response of K2W4O13 nanowire sensor versus operating temperature under exposure to 10 ppm H2S, 10 ppm NO2, 1000 ppm CO and 1000 ppm SO2.
low response of 1.8–9 at 10 ppm H2S. Nevertheless, the results are still inferior to some other metal oxide gas sensors mostly loaded with noble metals such as Pt-loaded WO3 and Ag-loaded SnO2 fabricated by RF/DC sputtering (high response of 300–1000 at 10 ppm H2S) [21–22]. Compared with other candidates, the K2W4O13 based sensor presents some key advantages including high linearity, good sensitivity and high stability, which could be attributed to large specific surface area and high crystallinity of the nanowire structure. In addition, its low material cost and simple hydrothermal preparation method makes the K2W4O13 nanowires highly attractive for gas-sensing application. 4. Conclusions Hexagonal K2W4O13 nanowires having average diameter of 11.7 ± 3.5 nm and length of several hundreds nanometers have been synthesized by a facile hydrothermal reaction of the tungsten oxide hydrate and K2SO4 employed as a shaped-directing agent. Calcination study revealed phase stability of the K2W4O13 nanowire up to 400 °C. It completely transformed to K2W7O22 and a trace amount of K2W6O19 at 500 °C, indicating good thermal stability for gas-sensing which is typically operated at 200–400 °C. Thick-film K2W4O13 nanowire sensor was most sensitive to H2S followed by NO2 with fast response and recovery times at the optimum operating temperature of 300 and 350 °C, respectively. The sensor showed very low responses to SO2 and CO, indicating its high selectivity to H2S. In addition, the sensor exhibited high linearity for 0.3–10 ppm H2S making it suitable for H2S sensor. Acknowledgments This work is supported by the National Metal and Materials Technology Center (Thailand) under Grant No. MT-B-52-CER-07230-I and MT-B-55-CER-07-292-I. References [1] T. Seiyama, A. Kato, K. Fujushi, M. Nagatani, Anal. Chem. 34 (1962) 1502. [2] A. Biaggi-Labiosa, F. Solá, M. Lebrón-Colón, L.J. Evans, J.C. Xu, G.W. Hunter, G.M. Berger, J.M. González, Nanotechnology 23 (2012) 455501. [3] L.E. Greene, B.D. Yuhas, M. Law, D. Zitoun, P. Yang, Inorg. Chem. 45 (2006) 7535. [4] I.-S. Hwang, S.-J. Kim, J.-K. Choi, J.-J. Jung, D.J. Yoo, K.-Y. Dong, B.-K. Ju, J.-H. Lee, Sensor Actuator. B: Chem. 165 (2012) 97. [5] A.A. Firooz, A.R. Mahjoub, A.A. Khodadadi, Sensor Actuator. B: Chem. 141 (2009) 89. [6] F. Gyger, M. Hübner, C. Feldmann, N. Barsan, U. Weimar, Chem. Mater. 22 (2010) 4821. [7] P.G. Dickens, M.S. Whittingham, Q. Rev. Chem. Soc. 22 (1968) 30. [8] M.A. Wechter, H.R. Shankes, G. Carter, G.M. Ebert, R. Guglielmio, A.F. Voigt, Anal. Chem. 44 (1972) 850. [9] A.M. Cruz, L.G.C. Torres, Ceram. Int. 34 (2008) 1779. [10] B. Zhang, J. Liu, S. Guan, Y. Wan, Y. Zhang, R. Chen, J. Alloys Comp. 439 (2007) 55. [11] C. Guo, S. Yin, L. Huang, T. Sato, ACS Appl. Mater. Interfaces 3 (2011) 2794. [12] V. Potin, S. Bruyere, M. Gillet, B. Domechini, S. Bourgeois, J. Phys. Chem. C 116 (2012) 1921. [13] S. Supothina, R. Rattanakam, Mater. Chem. Phys. 129 (2011) 439. [14] L. Sangaletti, L.E. Depero, E. Bontempi, R. Salari, G. Sberveglieri, J. Solid State Chem. 131 (1997) 215. [15] Z. Gu, Y. Ma, W. Yang, G. Zhang, J. Yao, Chem. Commun. (2005) 3597. [16] G.R. Patzke, A. Michailovski, F. Krumeich, R. Nesper, J.D. Grunwaldt, A. Baiker, Chem. Mater. 16 (2004) 1126. [17] J. Xu, X. Wang, J. Shen, Sensor Actuator. B: Chem. 115 (2006) 642. [18] M. Kaur, N. Jain, K. Sharma, S. Bhattacharya, M. Roy, A.K. Tyagi, S.K. Gupta, J.V. Yakhmi, Sensor Actuator. B: Chem. 133 (2008) 456. [19] R.S. Niranjan, K.R. Patil, S.R. Sainkar, I.S. Mulla, Mater. Chem. Phys. 80 (2003) 250. [20] C.M. Ghimbeu, J. Schoonman, M. Lumbreras, M. Siadat, Appl. Surf. Sci. 253 (2007) 7483. [21] C. Jin, T. Yamazaki, K. Ito, T. Kikuta, N. Nakatani, Vacuum 80 (2006) 723. [22] W.H. Tao, C.H. Tsai, Sensor Actuator. B: Chem. 81 (2002) 237.