Influence of synthesis and calcination temperatures on particle size and ethanol sensing behaviour of chemically synthesized SnO2 nanostructures

Sensors and Actuators B 143 (2009) 226–232

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Influence of synthesis and calcination temperatures on particle size and ethanol sensing behaviour of chemically synthesized SnO2 nanostructures Ravi Chand Singh a,∗ , Manmeet Pal Singh b , Onkar Singh a , Paramdeep Singh Chandi c a b c

Department of Physics, Guru Nanak Dev University, Amritsar 143005, Punjab, India Department of Applied Sciences, BHSB Institute of Engineering & Technology, Lehragaga 148031, Punjab, India Indian Institute of Science Education & Research, Mohali, Chandigarh, India

a r t i c l e

i n f o

Article history: Received 16 May 2009 Received in revised form 15 September 2009 Accepted 17 September 2009 Available online 24 September 2009 Keywords: Nanoparticles Tin oxide Sensors Ethanol

a b s t r a c t Nanoparticles of SnO2 have been synthesized through chemical route at 5, 25 and 50 ◦ C. In this work the synthesized particles were calcined at 400, 600 and 800 ◦ C and their structural and morphological analysis was carried out using X-ray diffraction and transmission electron microscopy. The reaction temperature has been found to be playing a critical role in controlling nanostructure sizes as well as agglomeration. It has been observed that particles synthesized at 5 and 50 ◦ C were smaller and less agglomerated as compared to the particles prepared at 25 ◦ C. The study also reveals that particle size and agglomeration increases with increase in calcination temperature. Thick film gas sensors were fabricated using synthesized tin dioxide powder, and sensing response of all the sensors to ethanol vapours was investigated at different temperatures. The investigations reveal that sensing response of SnO2 nanoparticles is size dependent and smaller particles are highly sensitive. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Metal oxide semiconductors are extensively used in the production of commercial gas sensors. SnO2 sensors have been dominant among these due to their high sensitivity, quick response, resistance to corrosion, etc. [1–3]. The research is still on to make improvement in these qualities. The reduction in particle size is one of the methods to improve the sensitivity of polycrystalline material based sensors [4,5]. SnO2 nanostructure based sensors have given a new dimension to investigations, and these devices show excellent sensing properties [6–9]. Nanoparticles of SnO2 can be produced by using a variety of techniques. The most common among these include sol–gel [10], spray pyrolysis [11], solvothermal [12] and microwave methods [13]. In the present work we have employed chemical method to synthesize nanoparticles. Although the technique is simple and inexpensive, yet the nanostructure morphology can be modified by varying reaction conditions to achieve the desired final product. One of the aims of the present study is to control the size of SnO2 particles by changing the reaction temperature. A high surface energy is associated with nanostructures. The system tends to reduce overall surface energy. This can be achieved

∗ Corresponding author. Tel.: +91 991 412 9939. E-mail address: [email protected] (R.C. Singh). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.09.032

either by combining individual nanostructures together to form large structures so as to reduce the overall surface area or by agglomeration of individual nanostructures without altering the individual nanostructures. Calcination and Ostwald ripening are the specific mechanisms which combine individual nanostructures into large structures. Ostwald ripening, where large particles grow at the expense of smaller ones, proceeds in solution during synthesis provided enough aging time is given. We tried to lower Ostwald ripening by minimizing the aging after reaction. In addition to reaction temperature, calcination was another process which altered the morphology of nanostructures in the present study. Calcination is the consolidation of a powder by means of prolonged use of elevated temperatures, which are, however, below the melting point of any major phase of the material. It facilitates the movement of atoms or molecules through the mechanism of mass transport that may be lattice diffusion, surface diffusion or evaporation–condensation and results in the grain growth which has detrimental effect on the properties of the material. The investigation on the effect of calcination temperature on morphology and size of synthesized nanoparticles has been carried out in the present study. The thin or thick film of active gas-sensing material is deposited using a number of sophisticated techniques such as sputtering [14], screen printing [15], etc. [16,17]. However, thick film gas sensors based upon semiconductor oxides have certain advantages over other type of gas sensors, such as low cost, simple construction,

Table 1 Overview of different techniques for SnO2 nanostructured synthesis and their sensing response to ethanol. Synthesis technique

Size variation method

[4] [6] [7] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]

Sol–gel Gas-phase condensation Evaporation Sol–gel Sol–gel Gel-combustion Electrospinning Commercial Hydrothermal Sol–gel RF sputtering Thermal evaporation Electrospinning Gas-phase condensation CVD

Calcination, doping Oxygen pressure Annealing By additives

Crystallite size (nm) 5–32 10–35 50–200 3–15 6–100 100 35–120

Seed layer

200 90 20 20–121

Sensing response

Operating temperature (◦ C)

Test gas volume (ppm)

Dopant/catalyst

10–360 1.9–2.7 23 1–100 80 9–27 4.5 5–560 19.6 50–200 95 4160 28.7 45 2–3.1

300–400 200–300 300 400 300 200 330 250–350 220 300 350 400 300 400 400

800–1000 100–1000 50 100 500 50–200 10–1000 10–1000 10 200 2000 250 200 1000 90

Al, Sb

Mo Pt Pt, Ag, Ti Fe2 O3 Ceria

Fe Ag Co, Ni, Fe, Ag, Cu

Table 2 Comparison between sensing response and crystallite sizes of SnO2 at different synthesis parameters. Sintering temperature (◦ C)

Reaction temperature (◦ C)

Average particle size from TEM (nm)

Optimum operating temperature (◦ C)

Sensing response (200 ppm ethanol)

400

05 25 50

∼02 ∼10 ∼05

3.78 6.67 4.42

250 350 250

15.29 11.50 14.00

600

05 25 50

∼24 ∼15 ∼20

22.14 13.25 19.52

250 350 300

6.33 8.50 7.34

800

05 25 50

∼28 ∼40 ∼32

30.16 38.96 34.56

300 350 300

5.80 3.00 5.50

Crystallite size from Scherrer’s formula (nm)

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Fig. 1. Schematic of fabricated gas sensor.

small size and good sensing properties [18]. In the present study we have used thick slurry of oxide powder to deposit a thick film. Finally the sensing response of sensor based on SnO2 nanoparticles to ethanol as function of temperature and particle size has been studied. An overview of work on SnO2 ethanol sensor of various researchers [4,6,7,19–30] is summarized in Table 1. The survey reveals that different sophisticated techniques have been used to synthesize and vary nanostructure sizes. We have varied nanoparticle sizes by varying reaction temperature of starting chemical species, which is quite novel and relatively simple technique.

Fig. 2. XRD of SnO2 powder synthesized at 5, 25 and 50 ◦ C and calcined at 400, 600 and 800 ◦ C.

Fig. 3. TEM micrograph of SnO2 nanoparticles synthesized at 5 ◦ C (a), 25 ◦ C (b) and 50 ◦ C (c) and calcined at 400 ◦ C.

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Fig. 4. TEM micrograph of SnO2 nanoparticles synthesized at 5 ◦ C (a), 25 ◦ C (b) and 50 ◦ C (c) and calcined at 600 ◦ C.

2. Experimental details

material particles were analyzed by transmission electron microscope (TEM) with Morgagni 268, operating at 80 kV.

2.1. Preparation of tin dioxide powder Nanoparticles of tin dioxide powder were prepared through fine crystallization in liquid phase. The detailed preparation procedure is as follows. Ammonia water at 25 ◦ C (room temperature) was added slowly to the 0.2 M solution of SnCl4 and precipitate of tin hydroxide was produced. The material produced was separated from rest of the liquid by filtering which follows drying at 120 ◦ C. To investigate the effect of heat treatment on the morphology and sensing characteristics of tin dioxide powder, the material produced was divided into three parts and each part was calcined for 3 h in air at 400, 600 and 800 ◦ C, respectively. In the present study we wanted to device some method to control the particle size by simple technique. To put forward this idea, in the other experiments the reactions during crystallization in liquid phase were carried out at 5 and 50 ◦ C instead of room temperature. The powder samples synthesized at these temperatures were also divided into three parts and subjected to similar calcination procedure as mentioned in the first experiment. In this way nine different samples of powder produced by different procedures were obtained. The crystal structure of the materials produced was characterized by powder X-ray diffraction (XRD) using Cu K␣ radiation with XPERT-PRO Diffractometer system. Morphologies and sizes of

2.2. Sensor fabrication and testing method The synthesized SnO2 powder samples were processed into water based pastes. The paste was coated onto an alumina substrate (12 mm × 5 mm size) between gold electrical contacts (2 mm apart) to obtain a thick film (∼20 ␮m thickness). These samples were sintered at 350 ◦ C for 30 min to give them final shape ready for sensing. The fabricated gas-sensor design is shown in Fig. 1. The sensing characteristics of SnO2 sensors were obtained with a home built apparatus consisting of a simple potentiometric arrangement and a test chamber of known volume in which a sample holder, a small temperature controlled oven and a mixing fan were installed. The fabricated sensor was placed in the oven kept at suitable temperature and a measured quantity of ethanol was injected into the test chamber. The variation of voltage signal across a resistance connected in series with sensor was monitored and recorded with a computer controlled data handling system. Same procedure was repeated for ethanol sensing with all the samples at temperatures from 200 to 400 ◦ C. Ethanol vapour sensing response of the sensor was determined as the (G − Go )/Go , where Go is the conductance of thick film sensor in an air ambience and G is the conductance in a mixture of air and ethanol vapours.

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Fig. 5. TEM micrograph of SnO2 nanoparticles synthesized at 5 ◦ C (a), 25 ◦ C (b) and 50 ◦ C (c) and calcined at 800 ◦ C.

3. Results and discussion 3.1. Structural analysis Plots of X-ray diffraction data of tin dioxide samples prepared at 5, 25 and 50 ◦ C and calcined at 400, 600 and 800 ◦ C are shown in Fig. 2. The diffraction pattern in all the plots are in agreement with the standard X-ray diffraction peaks, which confirmed that the synthesized materials were SnO2 of the tetragonal geometry. The crystallite size ‘D’ was estimated from the peak width with Scherrer’s formula, D = K/ˇ cos , where  is the X-ray wavelength, ˇ is the full width at half maximum (FWHM) of a diffraction peak,  is the diffraction angle, and K is the Scherrer’s constant. The calculated crystallites sizes are comparable with the TEM results given in Table 2 which confirmed the particle growth with calcination temperature. 3.2. Effect of reaction temperature The images represented in Figs. 3–5 are TEM micrographs of SnO2 synthesized at 5, 25 and 50 ◦ C and calcined at 400, 600 and 800 ◦ C, respectively. Tin dioxide particles synthesized at 5, 25 and 50 ◦ C, followed by calcination at 400 ◦ C yielded particles of average sizes of 2, 10 and 5 nm, respectively. It is evident that particles of SnO2 synthesized at 5 and 50 ◦ C are smaller than those prepared at 25 ◦ C. These results may be explained by the nucleation and growth of particles in a solution. Particle morphology is

influenced by the factors such as supersaturation, nucleation and growth rates, colloidal stability, recrystallization and aging process. Generally, supersaturation, which is highly dependent on solution temperature, has a predominant influence on the morphology of the precipitates. A highly supersaturated solution possesses high Gibbs free energy. The tendency of a system to lower its Gibbs free energy is the driving force in the processes of nucleation and growth of particles. The relation between Gibbs free energy change per unit volume, Gv , and supersaturation is given by the following equation: Gv = −

 kT  ˝

ln

C Co

=−

 kT  ˝

ln(1 + )

(1)

where C is the concentration of the solute, Co is the equilibrium concentration or solubility, k is the Boltzmann constant, T is the temperature, ˝ is the atomic volume and  is the supersaturation defined by (C − Co )/Co [31]. Without supersaturation (i.e.  = 0), Gv is zero, and no nucleation would occur. It is clear from the relation that Gv can be significantly increased by increasing the supersaturation for a system. Now the supersaturation is temperature as well as rate of reaction dependent. At low temperature (5 ◦ C in present study) a higher supersaturation leads to large reduction in Gibbs free energy. This energy reduction appears as an increased surface energy favouring continued nucleation with smaller sizes. An increased temperature to 25 ◦ C leads to the increased solubility and hence reduced supersaturation of the solution and as a consequence large size particles were obtained. The high tempera-

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Fig. 6. Comparative sensing response of SnO2 nanoparticles synthesized at different temperatures to 200 ppm of ethanol.

ture (50 ◦ C in present study) favours a fast hydrolysis reaction and results in the high supersaturation, a large Gv , which in turn leads to the formation of a large number of small nuclei. 3.3. Effect of calcination temperature The effect of calcination on particle size growth is represented in the TEM micrograph (Figs. 3–5). Some agglomeration along with individual particles is also seen in the micrographs. The average particle sizes observed at different calcination conditions are given in Table 2. Evidently the calcination temperature promotes enlargement of grain boundaries and consequently particle size increases as a function of calcination temperature. Macroscopically, the reduction of total surface energy is the driving force for calcination and microscopically, the differential surface energy of surfaces with different surface curvatures is the true driving force for mass transport during calcination. Therefore the results obtained were in accordance with the works of other authors [4,6]. 3.4. Sensing characteristics Sensors fabricated with synthesized materials were exposed to 200 ppm of ethanol vapours at different temperature to find out optimum operable temperature which was found for all the samples and is listed in Table 2. In general, observations reveal that smaller particles have optimum temperature around 250 ◦ C and as the particles grow in size and/or agglomerate, the optimum temperature shifts around 350 ◦ C. To study the effect of calcination on the sensing response of the synthesized SnO2 nanoparticles, the fabricated sensors prepared with materials calcined at temperatures mentioned above were tested with fixed volume of the ethanol vapours at optimum operable temperature. The results of sensor response versus time for nanoparticles prepared at 5, 25 and 50 ◦ C and calcined at 400, 600 and 800 ◦ C are shown in Fig. 6. The sensing response of samples prepared by calcination at 400 ◦ C is exceptionally better than the samples treated at 600 and 800 ◦ C. Important point to note is that no catalyst has been incorporated to improve the response of the material. Three samples in each case were prepared and it was found that their performance was almost identical which ensured that the changes in sensing response were due to particle size variation only and not due to the variability in the fabrication process. The results were found to be reproducible as fabricated sensors

231

Fig. 7. Sensor response as a function of crystallite sizes (200 ppm ethanol).

were tested at regular intervals for period of 1 year. The comparison between the sensing response and crystallite size of SnO2 at different preparation parameters is given in Table 2. Fig. 7 represents three sets of sensing response versus crystallite sizes for particles calcined at 400, 600 and 800 ◦ C, respectively. Three different curves have been drawn to compare the sensing response of particles synthesized at different temperatures but calcined at a fixed temperature. Within each set, the sensing response decreases as the particle size increases. The gradual decrease in sensing response is because of the growth of particle size with calcination and reaction temperature. A high surface to volume ratio of nanostructures is believed to be one of the important parameters responsible for enhanced sensing response. As the particle size increases with calcination temperature, the surface to volume ratio decreases and consequently sensor response decreases. Nevertheless nanoparticles of smaller size have larger effective surface area which leads to enhancement in their surface activity. Moreover a large number of small particles can be accommodated on a unit surface area. These contribute to a large number of particle–particle contacts or active sites onto which gaseous species adsorb to initiate sensing process. Large particles offer less number of such sites because of obvious reasons and consequently are less sensitive. It may be mentioned here that we were able to synthesize SnO2 nanoparticles, which were highly sensitive to ethanol vapours without promoters/additives, and results of sensing response of our sensor given in Table 2 are comparable with results of others presented in Table 1. 4. Conclusions It may be concluded from the present study that reaction temperature during synthesis of nanostructures of SnO2 plays significant role in modification of the particle size. Because of moderate supersaturation at room temperature the particles grow larger as compare to those synthesized at low and high temperature due to high supersaturation. The nanoparticles size grows with the calcination temperature and consequently sensing response decreases. It is also found that operable temperature of sensors fabricated with small particles is lower than that of large particles. Acknowledgments Authors would like to thank the following institutions for providing technical support: CIL Punjab University, Chandigarh for

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Biographies Ravi Chand Singh received his Ph.D. in physics from Guru Nanak Dev University, Amritsar, India in 1989. Since then he has had an appointment at the same institute for 1 year, and moved to post-doctoral position at Simon Fraser University, Canada in 1990. He moved to Guru Nanak Dev University Amritsar in 1993, where he is presently a reader of physics. His recent interests are material research for gas sensing and development of new experiments for physics education. Manmeet Pal Singh received his M.Sc. (Hon. School) physics degree from Guru Nanak Dev University, Amritsar, India in 2004. Since then he has been working as a lecturer in the B.H.S.B. Institute of Engineering & Technology, Lehragaga, India. He is also pursuing for Ph.D. in field of nanostructured materials and their application as gas sensors at Guru Nanak Dev University, India. Onkar Singh received his M.Sc. physics degree from Guru Nanak Dev University, Amritsar, India in 2006. Presently he is pursuing for Ph.D. in the field of nano-sized metal oxide materials and gas sensors at the same institute Paramdeep Singh Chandi received his B.Sc. (Hon. School) and M.Sc. (Hon. School) physics degrees from Guru Nanak Dev University, Amritsar, India. He taught physics at Khalsa College Amritsar before joining Indian Institute of Science Education & Research, Mohali, India. His recent interests include study of sensors, development of new experiments and simulations for physics education. At present he is pursuing for Ph.D. in the field of physics education.