Elsevier

Sensors and Actuators B: Chemical

Volume 201, 1 October 2014, Pages 369-377
Sensors and Actuators B: Chemical

Growth of thermally evaporated SnO2 nanostructures for optical and humidity sensing application

https://doi.org/10.1016/j.snb.2014.04.099Get rights and content

Abstract

Nanoscaled spherical, facetted-spherical and octahedral shapes of high yield and economically viable tin oxide (SnO2) was synthesized using direct sublimation process employing thermal evaporation at 1350 °C without any catalyst. The crystalline structure and morphology of SnO2 nanostructures were characterized using X-ray diffraction and electron microscopy. The morphology reveals that the reaction time, that is evaporation time, is important for successful synthesis of different shape and size of SnO2 nanostructures. High resolution transmission electron microscopy exhibited the particle size of SnO2 between 5 and 6 nm. Nanoscale morphological tuning in size and shape facilitates alteration in optical band gap, absorption and photolumiscence properties. The humidity sensor based on spherical SnO2 nanostructures demonstrates an excellent sensitivity with an increase in relative humidity (RH) at room temperature.

Introduction

Nanoscale synthesis of multifunctional semiconductors with varied morphology like nanocrystal, nanowires, zigzag nanobelt, tetrapods, octahedral etc., with reduced dimensions <10 nm at large scale by physical route is an active area of research leading to potential devices [1], [2], [3], [4], [5], [6], [7], [8]. The nanostructures are mostly functional in sensing and optical devices due to their high surface to volume ratio, chemical stability, remarkable resistivity variation in gaseous environment and high exciton binding energy [9]. At nanoscale, physical, chemical and electrical properties depend on mainly composition, size and shape of materials [10], so morphological control in respect of size and shape of these oxides at nanoscale is important, challenging for the generation of new functional devices.

Tin oxide (SnO2) is important wide band gap n-type semiconductor (Eg = 3.6–3.97 eV) material, which has been extensively used as a transparent conductor in gas sensor, solar cell and light emitting diodes [11], [12], [13]. It is large exciton binding energy (130 eV), is beneficial for direct observation of exciton related phenomenon at room temperature [14]. Direct band gap materials like, SnO2 are more suitable for large number of applications because of their low efficiency in excitation and recombination process and allows minimum energy transition from valence to conduction band. This electron transition results electrical, optical and sensing properties. The sensors based on individual nanostructures will have large surface to volume ratio and channel or network of these single crystalline structures offering enhanced sensitivity. We are aware that the humidity control is well known and important factor in day to day life. Fabrication of humidity sensor based on SnO2 nanostructures will be useful for many fields of research and technology. There are very few reports on SnO2 structure based humidity sensor in literature prepared by physical route. Therefore a further detail investigation in humidity sensing properties of SnO2 nanostructures is needed, as absorption of oxygen atom on the surface of oxide nanostructure plays vital role in sensing mechanism. However an impact of water vapour in air on the conductivity of SnO2 chemical sensor has been attracting attention because water vapour in air sometimes disturbs the response of a sensor to the detected gas [15], [16], [17]. Sensing properties of metal oxides (sensitivity, selectivity and reproductivity) critically depend on structural parameter, particle size and specific surface area. Therefore, preparation of primary product as nano-particles with increasing specific surface area is important [18], [19], [20], [21]. However, preparation of high quality nanocrystals of SnO2 at low cost in large scale by a simple physical technique at high temperature is still a challenge in material production. Several literature reports are available on chemical methods viz., solution route [22], [23], sol–gel [24], sonochemical, [25] organometallic precursor [26], microwave [27], [28], [29], to prepare SnO2 nanoparticles, whereas physical routes are still limited. So we are focussing on controlled synthesis of SnO2 nanostructures by thermal evaporation by varying evaporation time and its influence on size, shape, band gap and sensing properties of SnO2.

In the present investigation, SnO2 nanostructures were prepared by a simple sublimation process employing thermal evaporation at high temperature 1350 °C. Morphological variation in tin oxide with different evaporation time has been studied using electron microscopy. SnO2 nanostructures with spherical and octahedral geometry by sublimation process have been observed with change in evaporation time. The particle size between 5 and 6 nm (<10 nm) of SnO2 spherical nanostructures were obtained at large scale. Moreover the octahedral morphology of SnO2 nanostructure prepared by thermal evaporation is being reported first time. A structure–property correlation of tin oxide nanostructures has been studied by postulating a possible mechanism. Further a large scale synthesized spherical-nanostructures of SnO2 are investigated for humidity sensors.

Section snippets

Experimental

High purity Sn granules (99.999) were evaporated using a simple sublimation method at high temperature of 1350 °C at different evaporation time 0.5, 2, 5 and 8 h to obtain fine nanostructured powder samples. The crystallographic phase identification of synthesized powder samples was carried out by X-ray diffraction (XRD) using CuKα radiation (λ = 1.54059 Å). The surface morphology of synthesized SnO2 was studied by scanning electron microscopy (SEM, model: Zeiss EVO MA 10) and elements present in

Results and discussion

X-ray diffraction patterns of SnO2 nanostructures synthesized at 1350 °C at different evaporation time 0.5, 2, 5 and 8 h is depicted in Fig. 1(a). XRD patterns (Fig. 1a) show that all intense diffraction peaks correspond to the crystallographic planes (1 1 0), (1 0 1), (2 0 0), (1 1 1), (2 1 1), (2 2 0), (0 0 2), (3 1 0), (1 1 2), (3 0 1), (2 0 2) and (3 2 1) are in good agreements with rutile-like-tetragonal structure of SnO2 with cell constant of a = b = 4.738 and c = 3.187 A° (reference: JCPDS, card no. 41-1445). There is

Conclusion

SnO2 nanostructures with different evaporation time were successfully synthesized by simple sublimation process employing thermal evaporation at 1350 °C. Considerably nanoscale structural transformations with spherical, facetted spherical and octahedral shape structures were observed with change in reaction time without any catalyst at atmospheric pressure. HRTEM micrographs reveals particle size of spherical-facetted SnO2 nanostructure in quantum scale 3–6 nm with lattice spacing 0.17 nm along (2 1

Acknowledgements

The authors are grateful to Director, National Physical laboratory, New Delhi for encouragement and motivation to carry out this work. We would like to also thank Mr. Bhikam singh, Dr. Mohit Saxena and Mrs Shweta for Humidity, UV and Raman studies. CSIR–NANOSHE (BSC-0112) is gratefully acknowledged.

Jai S. Tawale obtained his M.Sc. in 2006 from Nagpur University in Physics. Presently he is working as technical assistant at National Physical Laboratory, New Delhi. His current research interests are nanostructure materials and characterization.

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    Jai S. Tawale obtained his M.Sc. in 2006 from Nagpur University in Physics. Presently he is working as technical assistant at National Physical Laboratory, New Delhi. His current research interests are nanostructure materials and characterization.

    Gaurav Gupta obtained his A.M.I.E. in 2010 from Institutions of Engineers (INDIA) in Mechanical Engineering. Presently he is working as technical assistant at National Physical Laboratory, New Delhi. His current research interest is on humidity studies.

    Anand Mohan obtained his Ph.D. and PG degrees in Electronics Engineering from Banaras Hindu University in 1994 and 1977. Presently he is director at NIT, Kurukshetra. He is involved in high quality research in the emerging areas like fault tolerant/survivable system design, information security, and embedded systems.

    Ashavani Kumar obtained his Ph.D. and M.Phil. in physics from A.M.U. Aligarh in 1994 and 1991. Presently he is working as a professor in physics department at NIT Kurukshetra for the past 18 years on the diverse areas of research such as nanostructural materials and high energy physics.

    Avanish K. Srivastava has obtained M.Tech. from IIT, Kanpur and Ph.D. from Indian Institute of Science, Bangalore in metallurgical engineering. He is working as a scientist at National Physical Laboratory, New Delhi.

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