Facile hydrothermal synthesis of nanobricks assembled WO3 microflowers and their ethanol sensing properties

Accepted Manuscript Facile hydrothermal synthesis of nanobricks assembled WO3 microflowers and their ethanol sensing properties Swati S Mehta, Mohaseen S Tamboli, Imtiyaz S Mulla, Sharad S Suryavanshi PII: DOI: Reference:

S0167-577X(17)31098-4 http://dx.doi.org/10.1016/j.matlet.2017.07.061 MLBLUE 22903

To appear in:

Materials Letters

Received Date: Revised Date: Accepted Date:

19 April 2017 2 July 2017 8 July 2017

Please cite this article as: S.S. Mehta, M.S. Tamboli, I.S. Mulla, S.S. Suryavanshi, Facile hydrothermal synthesis of nanobricks assembled WO3 microflowers and their ethanol sensing properties, Materials Letters (2017), doi: http://dx.doi.org/10.1016/j.matlet.2017.07.061

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Facile hydrothermal synthesis of nanobricks assembled WO3 microflowers and their ethanol sensing properties Swati S Mehta,a Mohaseen S Tamboli, b Imtiyaz S Mulla, c Sharad S Suryavanshia* a

School of Physical Sciences, Solapur University, Solapur (M.S.) 413255, India,

b

Centre for Materials for Electronics Technology(C-MET), Ministry of Electronics and

Information Technology (MeitY), Govt. of India. Pune c

Emeritus Scientist (CSIR), India

*Corresponding author:[email protected] Abstract

In this work efforts are made to explore the amount and effect of complexing surfactants to fabricate ethanol sensor using thick films of tungsten oxide (WO3). The hierarchical architectures of WO3 microflowers were synthesized by a facile hydrothermal method using potassium sulfate (K2SO4) and oxalic acid (C2H2O4). The microflower with 3 to 7 µm diameter comprises of nanobricks and nanosheets- like hierarchical morphology with their thickness ranging from 30 100 nm. The sensor achieves its maximum sensitivity of 72% towards 1 ppm of ethanol vapor at the optimal operating temperature of 250○C The sensor shows good selectivity towards ethanol. Keywords Electron microscopy; Nanocrystalline materials; Thick films; Sensors;

Introduction WO3 is

n-type semiconductor has attracted

wide

attention because of its promising

applications in various fields such as, electrochromic device [1], photocatalyst [2] and gas sensor [3-4]. In particular WO3 is widely used material for the recognition of different gases, e.g. Hydrogen [5], acetone [6-7], H2S [8], NOX [9], NH3 [10]. In addition, WO3 has not been extensively studied as an ethanol sensing material and only few reports on it are available in the

literature [11-18]. However, S. Cao et al. reported a response of 62 for 100 ppm of ethanol at 350○C [18]. Ethanol as a vital chemical raw material has been mainly used in medicine industry, chemical factories, paint and cosmetics. It is volatile and combustible and its long time exposure results in the health problems such as headache, liver damage and central nervous system disorders. The quantitative recognition of ethanol vapors is on demand in applications, such as wine quality improvement, breath detector to identify drunk drivers, quality control of products in the food, beverage, and other industries, indicate that ppm level detection of ethanol is required. Therefore, the inexpensive and elevated performance ethanol sensors are highly necessary. In the present work, we report a facile hydrothermal synthesis of WO3 hierarchical microflowers assembled by nanosheets. Hierarchical structure is a kind of high dimensional micro or nanostructure shaped by many low-dimensional, nanostructured building blocks (particles, rods, wires, or sheets). At present many nanostructures such as nanobricks composed of nanosheets with well controlled shapes and sizes forming hierarchical flower-like structures have been reported. The method of synthesis and precursor play an important role in the formation of different hierarchical structures. The surfactants, being very active surface agent are used for modifying the morphology and size of the products in hydrothermal synthesis route. Although, in previous studies based on the hydrothermal synthesis of WO3 the use of a lone surfactant has been generally used to obtain desired morphology and size, however, there are few reports available on the controlled synthesis of WO3 by using a complexing surfactants mediated hydrothermal method [19-21]. Experimental 8.246 gm (0.025M) Na2WO4.2H2O powder was dissolved in 100 ml double distilled water by means of magnetic stirrer. This solution was acidified to pH 1-1.5 using 3M H2SO4 solution to produce white precipitate. 6.3 gm oxalic acid was added to the above mixture and diluted to 250 ml. After that a homogeneous and stable sol was formed. 200 ml volume of WO3 sol was transferred to 250 ml Teflon container. Autoclave was fastened and kept at 180○C for 24 h. The precipitate was filtered, washed with water and dried at 60○C. The obtained powder was labeled as K0. Similar procedure was adopted with the addition of 1gm and 2gm of K2SO4 in WO3 sol before transferring the mixture in to the Teflon container for preparation of samples

which were labeled as K1 and K2, respectively. The dried powders were sintered at 400ºC for 2 hrs. 2.2 Fabrication of gas sensor: In the 0.5g resulting sintered powder butyl carbitol acetate was mixed to form a thixotropic paste. The paste was screen-printed on an alumina substrate (10 x 20 mm) and dried for 15 min. The sensor component was heat treated at 400°C for 2h in air atmosphere to remove the binders in the paste. Results and Discussions The XRD profiles of sintered product are depicted in Fig.1. All the peaks of K0 & K1 samples are indexed to the orthorhombic structure of WO3, with standard spectrum file 20-1314, however, the XRD pattern of K2 exhibits mixture of hexagonal along with orthorhombic pattern. The hexagonal peaks are in accordance with JCPDS card no. 33-1387. Moreover, pure oWO3 is obtained when the mass of K2SO4 was up to 1g, while the product transformed into the mixed h-WO3 after adding a little more K2SO4 (2 g) as represented in Fig.1, signifying that there exists a critical concentration of K2SO4 for phase transition. In short, a high concentration of K2SO4 emanates in the formation of h-WO3 phase [16, 19]. The morphology of the prepared WO3 products with different amount of K2SO4 and without K2SO4 is shown in Fig. 2. It shows that in the absence of K2SO4, the sample K0 exhibits flower-like shape with a diameter of about 3-3.5 µm and is made up of many nanobricks. FESEM image with high magnification demonstrates that these microflowers are constructed by numerous well-defined individual nanobricks. These nanobricks are about 275 nm in length, 100 nm in breadth and 30 nm in thickness which forms a multi-layered architecture. As can be viewed in figure, after addition of K2SO4 the diameter of microflower K1 and K2 increases to 44.45 µm, and 6-7 µm, respectively. Further increase in the amount of K2SO4 increases the size of microflowers but the size of individual building blocks, that is the size of nanosheets decreases. It agrees well with the observation of X.C. Song et al. who speculated that K2SO4 content plays a major role in the dimensions of nanoparticles. As the content of K2SO4 is increased dimensions of the nanoparticles decreased [22].

Thermogravimetric analysis (TGA) is conducted and shown in the supporting information (SI) in Fig. S1 shows that the WO3 precursors have a sharp weight loss between 100 to 400°C.

The

optical properties (UV-VIS) of all the samples have also been provided in SI S2. The morphology and microstructure of synthesized materials was further characterized by high-resolution transmission electron microscopy (TEM). Fig.2b & c (K1 sample) clearly shows that the nanoplate-like morphology have length of around 180 nm and breadth 160 nm, respectively which is consistent with the FESEM results. The 2-D crystal lattice with interplanar spacing of 0.38 nm analogous to (2 0 0) crystal plane is confirmed from Fig.2c. The single electron diffraction pattern (the inset) indicates its polycrystalline nature. Furthermore, EDS mapping and elemental compositions of all the samples are given in SI S3. The correlations of the sensor response to test gas with operating temperature were assessed by measuring the resistivity change of the sensor before and after the test gas were introduced {% Response= (Ra-Rg/Ra)*100}[3]. We first evaluated the response of all samples at various operating temperature for 100 ppm of ethanol in order to find the optimal operating temperature, the sensor was tested for temperature range of 125 °C to 350 °C. Monotonous increase in the response of the sensor with the temperature up to 250 °C and then a decline with further rise in the temperature is seen. Fig.3 shows that 250 °C is optimum temperature for the sensor which is lower than that of reported WO3 ethanol sensors [11-18]. It is meaningful to explore the response of WO3 sensor to gases such as acetone, ethanol, propanol and NOX at optimum temperature. In this study the concentration of each selected gas is 1 ppm. Fig.3 (b) shows response property of sensor K1 to above mentioned gases at given experimental conditions. K1 sensor exhibited higher selectivity towards ethanol than K0 and K1 . The amount of K2SO4 greatly affects the initial resistance of the sensor, resistance is minimum for sensor K1 simultaneously the % response is higher. Fig.3(c) represents graph of initial resistance and % response of sensor with different contents of K2SO4 to 100 ppm of ethanol at 250○C. Fig.3 (d) depicts the relationship of the % response and ethanol concentration in the span of 1-500 ppm at 250○C which clearly shows that the response increases with the concentration. Fig.3 (e) shows the response-recovery features of sensor functioning at their optimum operating temperatures, with gas concentration of 1 ppm. Fast response was observed in the sensor K1 which is 40s. Finally, stability is also an important factor for gas sensor. The response

of K1 sensor to 1 ppm ethanol was repeated after 60 days at 250○C, as illustrated in Fig.3 (f). The response showed slight change after continuous tests for 60 days, indicating that the sensor had good stability for relatively long period. Conclusion: Nanocrystalline tungsten oxide powders were synthesized successfully by complexing surfactant mediated hydrothermal method. The amount of K2SO4 used is found to introduce significant structural and morphological changes in WO3. The thick film sensor K1 exhibited high response of 84% at 250○C towards 100 ppm of ethanol, so it can be used as a promising candidate for fabrication of good performance ethanol sensor at comparatively low temperature. Acknowledgement: One of the author, I .S. Mulla acknowledges CSIR, India for granting him Emeritus Scientist Scheme. References: [1]L.Liu, M.Layani, S.Yellinek, A.Kamyshny,H.Ling, P.See Lee, S.Magdassi, D.Mandler. J. Mater. Chem.A,2014, 2,16224 [2]J. Yu, L. Qi, B. Cheng, X. Zhao Journal of Hazardous Materials 160(2008)621–628. [3]S. S. Mehta, S. S. Suryavanshi, I. S. Mulla. Proceedings of 2nd International Symposium on Physics and Technology of Sensors 978-1-4673-8018-8/15©2015 IEEE. [4]V.B.Patil, P.V.Adhyapak, P.S.Patil, S.S.Suryavanshi, I.S.Mulla Ceramics International 41(2015)3845– 3852. [5]M.Z.Ahmad, A.Z. Sadek, M.H. Yaacob, D.P. Anderson, G.Matthews, V. B. Golovko, W.Wlodarski. Sensors and Actuators B 179(2013)125– 130 [6]Z.Wang, X.Zhou, Z.Li, Y. Zhuo, Y.Gao, Q.Yang, X. Li, Geyu Lu. RSC Adv.,2014,4,23281. [7]S.Wei, G.Zhao, W.Du, Q.Tian. Vacuum 124(2016)32-39. [8] J.Shi, Z.Chenga, L.Gao, Y.Zhang, J.Xua, H.Zhao. Sensors and Actuators B 230(2016)736–745. [9]J.S. Kim, J.W. Yoon, Y.J. Hong, Y. C.Kang, F.A.Hady, A.A.Wazzan, J.H.Lee. Sensors and Actuators B 229(2016)561–569. [10]Y.M. Zhao, Y.Q. Zhu Sensors and Actuators B 137(2009)27–31. [11]J.Li, J.Zhu, X.Liu. NewJ.Chem., 2013,37,424-4249. [12]W. Zeng, B. Miao, T. Li, H. Zhang, S. Hussain, Y. Lia, W. Yu, Thin Solid Films 584(2015)294-299. [13]J.Xiao, C.Song, W.Dong, C. Li, Y.Yin, X.Zhang, M.Song ASM International 1059-9495 [14]C. Song, C. Li, Y. Yin, J. Xiao, X. Zhang, M. Song, W. Dong, Vacuum 114(2015)13-16. [15]W. Zeng, H. Zhang, Z. Wang, Appl. Surf. Sci. 347(2015)73-78. [16]T. Li, W. Zeng, B. Miao, Sh. Zhao, Y. Li, H. Zhang, Materrials Letters. 144(2015)106-109. [17]B.Miao, W.Zeng, Y. Mu, W. Yu, S. Hussaina, S. Xu, H. Zhang, T. Li, Applied Surface Sci. 349(2015)380-386. [18]S. Cao, C. Zhao, T. Han, L. Peng, Materials Letters 169(2016)17-20. [19]Y.Shen, X.Chen, W.Wang, Y.Gong, S.Chen, J.Liu, D. Wei, D. Meng, X. San. Materials Letters 163(2016)150–153 [20]Y. Shen, W.Wang, X.Chen, B. Zhang, D.Wei, Shuling Gao, B.Cui J. Mater. Chem. A, 201,1-3

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Figure captions Fig. 1 XRD profiles of samples K0,K1,K2.

Fig. 2 (a) SEM images of K0, K1 and K2at low and high magnification.(b)TEM and HRTEM images with SAED pattern (inset) of sample K1. Fig. 3(a) Variations in sensor response with operating temperature for different samples of WO3 towards 100 ppm ethanol. (b) Comparison of selective response of K1 to different test gases at 250○C. (c) Effect of sensor on resistance and % response at 250○C. (d) Variations in sensor response towards different concentrations of ethanol at 250○C. (e)Transient response characteristics of sensors exposed to 1 ppm ethanol at 250○C. (f)The ethanol sensing stability studies for K1 an optimum temperature.

Fig. 1

Fig. 2

(a)

(d) Fig. 3

(b)

(e)

(c)

(f)

Highlights: • WO3 microflowers were synthesized by hydrothermal route using K2SO4 and H2C2O4. • There exists a critical concentration of K2SO4 for phase transition from o-WO3 to h-WO3 phase. • The amount of K2SO4 greatly affects the initial resistance of the sensor. • WO3 microspheres exhibited excellent response of to ethanol with decrease in operating temperature.