Hydrothermal synthesis of ceo2 sno2 nanocomposites with highly enhanced gas sensing performance towards n butanol

Journal of Science: Advanced Materials and Devices (2018) 139e144 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Hydrothermal synthesis of CeO2eSnO2 nanocomposites with highly enhanced gas sensing performance towards n-butanol K Rackesh Jawaher a, R Indirajith a, S Krishnan b, *, R Robert c, S.K Khadheer Pasha d, Kalim Deshmukh a, S Jerome Das e a Department of Physics, B S Abdur Rahman Crescent Institute of Science and Technology, Chennai 600048, Tamil Nadu, India Department of Physics, Ramakrishna Mission Vivekananda College (Autonomous), Mylapore, Chennai 600004, Tamil Nadu, India Department of Physics, Government Arts College for Men, Krishnagiri 635001, India d Department of Physics, VIT-AP, Amaravati Campus, Guntur 522501, Andhra Pradesh, India e Department of Physics, Loyola College, Chennai 600 034, Tamil Nadu, India b c a r t i c l e i n f o a b s t r a c t Article history: Received 22 January 2018 Received in revised form 21 March 2018 Accepted 22 March 2018 Available online 30 March 2018 CeO2eSnO2 nanocomposite, a sensing material, was successfully synthesized by a hydrothermal method The structural, optical and morphological properties of as-prepared CeO2eSnO2 nanocomposites were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectroscopy and field emission scanning electron microscopy (FESEM) The results reveal that the highly crystalline nanocomposite heterostructure is formed, and it possesses a spherical-like morphology with an average grain size of 30 nm Furthermore, the gas sensing properties towards several volatile organic compounds (VOCs) such as n-butanol, isopropanol, ethanol and acetone were studied Interestingly, comparing with bare SnO2 and CeO2, the CeO2eSnO2 sensor shows the highest sensitivity towards n-butanol at an operating temperature of 110  C This indicates that the CeO2eSnO2 nanocomposite can be a promising candidate for sensor application towards the detection of n-butanol © 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: TEM FESEM CeO2eSnO2 nanocomposites VOCs Gas sensors Introduction Volatile organic compounds (VOCs) include a category of organic compounds, such as alcohols and aromatic hydrocarbons, which can easily evaporate at room temperature [1] In general, it is difficult to detect VOCs because they are colorless and odorless However, after evaporation, VOCs are toxic, irritating smell and can cause serious adverse effects on human health [2] In VOCs detection, gas sensors play a significant role in the areas of emission control, biological detection, public safety, and environmental protection [3] Recently, there has been substantial interest in the preparation of binary oxide nanocomposite materials such as ZnOeCdO [4], In2O3eSnO2 [5], CeO2eZnO [6], and SnO2eTiO2 [7], which plays a vital role in the detection of different VOCs Hung et al have provided an updated review on semiconductor metal oxide nanowires for VOCs gas sensors [8] SnO2 is an n-type semiconductor, with a wide band gap (Eg) of 3.6 eV It has been devoted * Corresponding author Fax: þ91 44 42169045 E-mail address: skrishnanjp@gmail.com (S Krishnan) Peer review under responsibility of Vietnam National University, Hanoi to extensive applications such as gas sensors [9e11], dye sensitized solar cells [12], and catalytic process [13] Particularly, it has received a high degree of interest in the field of gas sensor It has been well studied as a sensor material for the detection of both reducing and oxidizing gases [14e17] Meanwhile, mixing with rare earth oxides, especially cerium oxide, is particularly attractive as an additive, due to its low cost, rich surface oxygen vacancies and low redox potential between Ce3ỵ and Ce4ỵ [18] Due to this, CeO2 and SnO2 could remarkably enhance the gas sensing mechanism A few literature reports are available on the study of CeO2eSnO2 based sensors for the detection of VOCs Recently, Liu et al prepared CeO2 decorated SnO2 hollow spheres by the two-step hydrothermal technique showing the enhanced gas sensing property to 100 ppm in ethanol [19] Qin et al successfully synthesized porous CeO2eSnO2 composite nanofibers using electrospinning technique, thereby exhibiting the excellent sensing performances towards H2S and ethanol [20] However, to the best of our knowledge, the gassensing performance of CeO2eSnO2 binary oxides towards nbutanol has not been reported yet N-butanol is very largely used as an industrial solvent, an extracting agent and an organic synthesis intermediate Prolonged exposure to the n-butanol environment https://doi.org/10.1016/j.jsamd.2018.03.006 2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 140 K.R Jawaher et al / Journal of Science: Advanced Materials and Devices (2018) 139e144 could cause symptoms such as dizziness, headache, somnolence and dermatitis [21e23] Hence, it is very necessary to detect nbutanol gas for both environmental safety and human healthcare Herein, a CeO2eSnO2 nanocomposite was successfully synthesized through a simple hydrothermal method The obtained product was further used to investigate the structural, optical and the morphological properties The gas-sensing properties such as sensitivity, selectivity and response and recovery time were evaluated, and the possible gas sensing mechanism was also discussed This binary oxide nanocomposite will be very useful for gas sensing applications Experimental 2.1 Synthesis of CeO2 nanoparticles For the synthesis of CeO2 nanoparticles, ammonium ceric sulfate ((NH)4)4 [Ce (SO4)4]$2H2O) and C2H2O4$2H2O were used as precursor materials 0.1 M of ((NH)4)4[Ce(SO4)4]$2H2O) was dissolved in 20 ml of deionized water and then the solution was stirred vigorously Subsequently, 0.3 M of C2H2O4$2H2O was dissolved in 20 ml of deionized water and added drop by drop to the solution of ((NH)4)4[Ce(SO4)4]$2H2O) under continuous stirring Then, 0.75 g of polyvinylpyrrolidine (PVP) was dissolved in 10 ml of deionized water and added drop by drop to the mixture of C2H2O4$2H2O and ((NH)4)4[Ce(SO4)4]$2H2O) The resulting solution was continuously stirred for h Finally, the precipitated solution was transferred to a 100 ml Teflon-lined stainless steel autoclave and heated for 24 h at 180  C Then, the autoclave was cooled down to room temperature The resulting product was collected by centrifugation and washed several times with distilled water and ethanol followed by drying at 100  C for 12 h The dried product was grounded in agate mortar and calcined at 450  C for h to get bare cerium oxide nanopowder 2.2 Synthesis of SnO2 nanoparticles For the synthesis of SnO2 nanoparticles, inorganic compounds such as tin chloride dihydrate (SnCl2$2H2O) and oxalic acid (C2H2O4$2H2O) were used as starting materials In a typical process, 0.2 M of SnCl2$2H2O was dissolved in 20 ml of deionised water and stirred for 30 Subsequently, 0.3 M of oxalic acid was dissolved in 20 ml deionised water The solution of tin chloride dihydrate was added drop by drop into the solution of oxalic acid and then 0.75 g of PVP was added This solution was stirred for few hours until the milky white precipitate was obtained Later, this white solution was transferred into a 100 ml Teflon-lined stainless-steel autoclave and heated for 12 h at 180  C under static conditions Then, the autoclave was cooled down to room temperature The precipitate was centrifuged and washed several times with distilled water and ethanol and dried at 100  C The dried product was grounded in agate mortar Finally, the powder was transferred into alumina crucible and calcined at 450  C for h to get bare tin oxide nanopowder precipitate The precipitated solution was transferred to a 100 ml Teflon-lined stainless steel autoclave and kept at 180  C for 24 h and then cooled down to room temperature The resulting products were washed with deionized water and absolute ethanol for several times by centrifugation and finally dried at 100  C for 24 h The conversion of Ce(OH)4 and Sn(OH)2 to CeO2 and SnO2 nanostructures was carried out in muffle oven at 450  C for h 2.4 Characterizations The synthesized samples were characterized by X-ray diffraction (XRD) using XPERT PRO X-ray diffractometer with Cu-Ka radiation (l ¼ 1.5406 Å) in the 2q range between 20 and 80 at room temperature The optical absorption spectra of the samples were recorded using a UV-VIS spectrophotometer (Shimadzu, UV-1800) in the wavelength range 200e800 nm Surface morphology and elemental compositions of the as-prepared nanocomposites have been determined using field emission scanning electron microscope (FESEM with EDAX, Carl Zeiss, Supra 55) High-resolution transmission electron microscopy (HRTEM) observations were carried out with Tecnai G2 (TF20) microscope operated at an accelerating voltage of 200 kV 2.5 Gas sensing measurement The gas sensor setup was constructed within a glass chamber having provision of inlet and outlet with a semiconducting alumina ceramic tube and heating coil In order to produce and maintain temperature of the surrounding of a sensor, a temperature controller is connected with power and voltage suppliers The dimension of the tube was taken as 5e6 mm in length and 2e3 mm in radius Initially, the CeO2eSnO2 composites were mixed and grounded with the binder polyvinyl alcohol (PVA) which was subsequently coated on a alumina ceramic tube The thickness of coated film was kept around 20e30 mm Results and discussion 3.1 Structural characteristics XRD measurement was performed to identify the phase compositions of the as-prepared SnO2, CeO2 and CeO2eSnO2 nanocomposites and the results are shown in Fig All the diffraction peaks can be perfectly indexed with those of standard patterns for cubic structure of CeO2 (JCPDS.34-0394) and tetragonal rutile structure of SnO2 (JCPDS 41-1445) For bare CeO2, the diffraction peaks at 28.5 , 33.1, 47.4 , 56.3 , 59.1, 69.4 , 76.7 and 79.1 were 2.3 Synthesis of CeO2eSnO2 nanocomposite For synthesis of CeO2eSnO2 nanocomposite, SnCl2$2H2O, ((NH)4)4[Ce(SO4)4]$2H2O) and C2H2O4$2H2O were used as precursors In a typical synthesis process, 0.2 M of SnCl2$2H2O and 0.1 M of ((NH)4)4[Ce(SO4)4]$2H2O were first dissolved individually in de-ionized water and stirred for h and subsequently 0.3 M of C2H2O4$2H2O was dissolved into 20 ml of deionised water This solution was added drop by drop to the mixture of SnCl2$2H2O and 0.1 M of ((NH)4)4[Ce(SO4)4]$2H2O under continuous stirring Later, 0.75 g of PVP was slowly added into the solution resulting in a white Fig XRD patterns of pure SnO2, pure CeO2 and CeO2/SnO2 nanocomposite K.R Jawaher et al / Journal of Science: Advanced Materials and Devices (2018) 139e144 detected which can be assigned to the face-centered cubic CeO2 phase For bare SnO2, the diffraction peaks at 26.6 , 33.8 , 37.8 , 51.7, 52.1, 54.7 and 65.1 are primarily ascribable to the tetragonal rutile SnO2 phase Furthermore, two sets of diffraction peaks exist for CeO2eSnO2 nanocomposites, corresponding to the cubic structure CeO2 and the rutile structure of SnO2 No obvious phase shift and no additional peaks were observed, suggesting that the synthesized materials are composed of nanocrystalline CeO2 and SnO2 The intensity of the CeO2 peaks was relatively low This indicates a relatively small amount of CeO2 present in the composite nanostructure According to the Scherrer equation, the crystallite sizes of CeO2 and SnO2 nanostructures are estimated to be 25.6 nm and 23.19 nm, respectively 3.2 Optical characteristics Fig shows the typical room-temperature UVeVis absorption spectra for synthesized nanocomposites calcined at 450  C The band gap energy (Eg) of CeO2, SnO2 and CeO2eSnO2 are obtained from the wavelength value corresponding to the intersection point of the vertical and horizontal part of the spectrum using the following equation: Eg ¼ hc l eV ¼ 1240 l eV: where, Eg is the band gap energy (eV), l is the wavelength (nm), h is the Planck's constant (6.626 Â 10À34 Js) and C is the velocity of light (3 Â 108 m/s) The absorbance edge wavelengths of CeO2 and SnO2, CeO2eSnO2 nanocomposite were measured to be 280, 336 and 285 nm with the corresponding band gap energies of 4.42, 4.35, 3.6 eV, respectively Furthermore, the absorption edge of the CeO2eSnO2 nanocomposite lies in between that of the bare CeO2 and SnO2 Compared to the pure CeO2, the CeO2eSnO2 nanocomposite showed an obvious absorption edge decrement of about 51 nm Nevertheless, it was close to that of SnO2 because of 2:1 composition of the material This result suggests that the absorption edge of the heterostructure such as CeO2eSnO2 nanocomposite is due to contribution of the each component in the system 3.3 Morphological characteristics The morphology of CeO2eSnO2 nanocomposite was investigated by FE-SEM as shown in Fig (a) The typical FESEM image of the as-prepared products revealed that the CeO2eSnO2 nanocomposites are homogeneously dispersed and composed of Fig UVevis absorption spectra of SnO2, CeO2 and CeO2/SnO2 nanocomposite 141 spherical morphology Moreover, the heterointerface contact between CeO2eSnO2 nanoparticles can be envisaged Further, the elemental analysis of the CeO2eSnO2 nanocomposites was carried out by using energy dispersive X-ray spectrometer (EDS) and the results are shown in Fig (b) It distinctly shows that the synthesized sample contains only Ce, Sn, and O elements which confirm that cerium oxide has been successfully mixed with SnO2 crystal lattice This is further revealed by the respective TEM and HRTEM images of CeO2eSnO2 nanocomposites which are shown in Fig 4a and b From Fig 4a, the TEM image clearly shows an interconnected agglomeration with highly crystalline in nature which is in good agreement with FESEM result The magnified HRTEM image in Fig 4b exhibits a clear lattice fringes with an interplanar distance of 0.314 and 0.336 nm, corresponding to the (111) and (110) planes of cubic CeO2 and tetragonal rutile SnO2, respectively The result suggests that the particle size was about 28 nm and this value is in the close vicinity to value obtained from XRD analysis Further, the selected area electron diffraction (SAED) pattern reveals the diffracted rings of CeO2 and SnO2 which are presented in Fig 4c 3.4 Gas sensing performances The sensor response (Sg) was defined as (Ra À Rg)/Ra or Sg ¼ Ra/ Rg, where Ra and Rg were the electrical resistances of the sensor in air and in test to each organic vapor, respectively [24] The sensor response towards each gas is substantially governed by its operating temperature Fig depicts the sensor response of the CeO2, SnO2 and CeO2eSnO2 nanocomposites towards n-butanol gas were examined over a range of operating temperature The optimal operating temperatures of corresponding sensors were determined to be 110  C The maximum sensor response of CeO2eSnO2, SnO2 and CeO2 gas sensors were found upon exposure toward n-butanol vapor at 100 ppm concentration with the value of 75.6, 59.8 and 44.8, respectively It is clearly evident that CeO2eSnO2 sensor shows the remarkably improved sensor response compared to the bare CeO2 and SnO2 sensor It was manifested that the sensing response of the sensor initially was increased with working temperature, attains the maximum, and then gradually decreased Furthermore, the sensor response also depends on the concentration of the gas Fig illustrates the sensor response of the CeO2, SnO2 and CeO2eSnO2 nanocomposite sensor at the different concentrations of n-butanol ranging from to 400 ppm at 110  C The response of the sensor was found to increase with increasing the gas concentration With further increasing the gas concentration above 100 ppm, the response gets saturated Similarly, Fig shows the selectivity of sensor response towards 100 ppm of various gases such as acetone, ethanol, isopropanol and n-butanol was conducted at 110  C The selectivity of gas sensors are usually defined as the ratio of sensitivity to different test gases atmosphere at the same concentration and temperature The maximum sensor response enhancement toward n-butanol vapor with the highest value is 75.52 It can be clearly seen that n-butanol exhibits high selectivity as compared with other gases recorded Response and recovery times are very important basic parameters to determine the working of a sensor in real time fast changing environment [25] Fig shows the response transients of CeO2eSnO2, CeO2 and SnO2 gas sensors exposed to 100 ppm nbutanol gas at 110  C The sensing measurements were performed by injecting n-butanol gas into the chamber first and sensor's resistance was measured in air and in the presence of n-butanol gas Similarly, the same experimental conditions were used for measuring the sensing behavior of bare sensor The response and recovery time of CeO2eSnO2, CeO2 and SnO2 sensor towards n-butanol gas were determined to be 5/18 s, 10/32 s, and 12/39 s, respectively Manifestly, CeO2eSnO2 sensor exhibits faster response/recovery time 142 K.R Jawaher et al / Journal of Science: Advanced Materials and Devices (2018) 139e144 Fig (a) FE-SEM image, (b) EDS spectra of the as-prepared CeO2/SnO2 nanocomposite Fig (a) TEM image, (b) HRTEM image and (c) SAED pattern of the as-prepared CeO2/SnO2 nanocomposite than the bare sensor As a consequence, n-butanol shows the faster reaction of adsorbed oxygen with enhancement in the electrical conductivity of the sensor and as a result enhances the sensitivity 3.5 Gas sensing mechanism As typical n-type semiconductor oxides, the sensing mechanism can be mainly induced by the oxygen adsorption on the surface of the material leading to the resistance variation of the gas sensor When the CeO2eSnO2 sensor is exposed to air, oxygen molecules can be adsorbed easily onto the surfaces of both the oxides Especially, the presence of CeO2 nanoparticles onto the surface of SnO2 nanoparticles could lead to the formation of heterojunction interface between two oxides This may due to difference in their work functions (4.69 eV for CeO2 and 4.90 eV for SnO2) [26,27] As the work function of SnO2 is higher than that of CeO2, the Fermi energy level of SnO2 is lower than that of CeO2 As a consequence, the lower energy conduction band from CeO2 stimulates the electron transfer to SnO2, equalizing the Fermi energy level between them, leading to the formation of depletion layer at CeO2 surface (due to K.R Jawaher et al / Journal of Science: Advanced Materials and Devices (2018) 139e144 Fig Sensor responses of the SnO2, CeO2 and CeO2/SnO2 composite at different working temperatures 143 Fig Variation of the sensor response with time of the SnO2, CeO2 and CeO2/SnO2 composite towards n-butanol gas response When the CeO2eSnO2 sensor is exposed to reductive VOCs gases, the gas molecules react with adsorbed oxygen species and enormous amount of electrons will transfer into the conduction bands of the CeO2 and SnO2 simultaneously, decreasing the height of the potential barrier and consequently increasing the conductivity which eventually leads to the drastic improvement in the sensitivity of the CeO2eSnO2 sensor Conclusion Fig Sensing response of the SnO2, CeO2 and CeO2/SnO2 composite towards nButanol gas at different concentrations at 110  C A simple hydrothermal route was employed for the synthesis of the CeO2eSnO2 nanocomposite The XRD spectral analysis clearly reveals the existence of both phases in the synthesized material The crystallite sizes of CeO2 and SnO2 nanostructures are estimated to be 25.6 nm and 23.19 nm, respectively The optical properties of the CeO2eSnO2 nanocomposite suggest that the absorption edge of the heterostructure is the contribution of both the components of nanocomposites The FESEM image clearly indicates the formation of the spherical-like morphology with uniform agglomeration The gas sensing studies were recorded towards n-butanol, acetone, ethanol, and isopropanol vapor at an operating temperature of 30e200  C The maximum sensor response of CeO2eSnO2, SnO2 and CeO2 gas sensors were found upon exposure toward n-butanol vapor at a 100 ppm concentration with the values of 75.6, 59.8 and 44.8, respectively, at an operating temperature of 110  C As a result, the CeO2eSnO2 nanocomposite exhibits a higher sensor response and a good selectivity toward n-butanol gas From the above studies, it can be concluded that the CeO2eSnO2 nanocomposite is a promising candidate for gas sensor application for the detection of VOCs gases References Fig Sensing 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were examined over a range of operating temperature The optimal operating temperatures of corresponding sensors