Sol-gel Synthesis and Properties of Zinc Doped Tin Oxide (Zn-SnO2) Nanostructures

Shivani Sharma1a, Ritika2b, Virender Singh3c, Suresh Kumar4d and Anil Vohra4e

1Department of Electronic Science, Kurukshetra University, Kurukshetra, India-136119 2 Department of Electronic Science, Kurukshetra University, Kurukshetra, India-136119 3 Department of Electronic Science, Kurukshetra University, Kurukshetra, India-136119 4Department of Electronic Science, Kurukshetra University, Kurukshetra, India-136119

a)shrmshivani@gmail.com; b) rbhutani786@gmail.com; c)vsingh@kuk.ac.in; d)kumars@kuk.ac.in; e)vohra64@gmail.com

Abstract. In the present work Zn-doped SnO2nanoparticles (NPs) were successfully synthesized by a low cost sol-gel method. Tin chloride pentahydrate, zinc chloride and sodium hydroxide were used as the precursor materials for the synthesis of Zn-doped SnO2 NPs. The prepared SnO2 and doped SnO2 NPs obtained via sol gel method and further calcinated at 400˚C and characterized. The SEM analysis confirms the formation of SnO2 and Zn-doped SnO2 with tetragonal morphologies. The EDX spectrum confirms the presence of elements Sn, O and Zn with stoichiometric ratio in the prepared samples. The X-ray diffraction analysis confirmed the crystalline structural growth of SnO2 and Zn-doped SnO2 with tetragonal phase and size of nanoparticles are found to be in the range from 10-22nm. The band gap energy is found to be decreased with increasing Zn content in SnO2. The photoluminescence spectroscopy reveals the strong peak at 482nm with an emission peak in near ultra violet region.

Keywords:Sol-gel, Doped SnO2, nanoparticles, XRD, SEM and PL

1. INTRODUCTION

Metal oxide semiconductors are very important class of materials for their potential applications in various electronic devices and studied extensively during the past decade. SnO2 is a very promising candidate for a number of electronic applications like gas sensors [1-2], optoelectronic devices [3], transparent conducting electrodes [4) spintronics [5], solar cell (5), lithium-ion battery (6) and in many other applications. It has been found that the surface, electrical, magnetic and sensing properties of metal oxides semiconductors can be modified by the addition of adequate doping with other materials.

Chetri et al. [7] presented structural and optical changes in Cu-doped SnO2 system via both theortically and experimentally. Salah et al. [8] synthesized Mn-doped SnO2 nanoparticles (NPs) via the microwave method and observed increase in resistivity with Mn content. Khan et al. [9] prepared (Zn, Co) co-doped SnO2 NPs using a co-precipitation method and reported excellent dielectric, magnetic properties and high electrical conductivity. Li et al. [10] demonstrated Zn-doped SnO2 nanocrystals as good photoanode materials for highly efficient dye-sensitized solar cell. Recently, it is reported in literature that Zn-doped SnO2 NPs improves the sensitivity of SnO2 based gas sensor [11].

Many synthesis techniques such as co-precipitation, hydrothermal, sol-gel, microwave, polyol, surfactant-mediated etc. have been employed by many researchers and found in literature. In this research work, a low cost sol-gel technique has been employed to synthesize SnO2 and Zn-doped SnO2 NPs. The prepared NPs has been characterized by various physical material characterization techniques like X-ray diffraction (XRD), field emission

scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), UV-vis Spectroscopy, photoluminescence (PL) Spectroscopy etc. to study their physical, optical and chemical properties and presented in this paper.

2. EXPERIMENTAL DETAILS

In this work, pure SnO2 and Zn-doped SnO2 NPs were synthesized via sol gel process and characterised using various physical characterization techniques. Analytical grade chemicals i.e. SnCl2.5H2O, ZnCl2 and NH4OH were purchased from Himedia Labs and used without any further purification.

2.1 Preparation of SnO2 NPs

For the preparation of SnO2 NPs, 1M concentration of SnCl2.5H2O (17.16g) was dissolved into 50ml of de-ionized water (18Ω resistivity) in a beaker and stirred for 30 min to get a homogeneous solution. After stirring a NH4OH solution is added drop wise into tin chloride solution with continuous stirring and the solution become white. The NH4OH solution is added until the solution becomes stable for sample A.

2.2 Preparation of Zn-doped SnO2 NPs

For the preparation of Zn-doped SnO2 NPs, 1M concentration of SnCl2.5H2O (17.17g) was dissolved in 50ml of DI water in a beaker (sol 1) and placed on magnetic stirrer for 30 min to get homogeneous solution. In another beaker, 2wt.% of ZnCl2 was dissolved in 50 ml of DI water and stirred to get a transparent solution (sol 2). In the next step, pour solution 2 into solution 1 drop wise to male mixed solution of both tin chloride and zinc chloride. Under constant stirring NH4OH solution was added drop wise in sol 2 and the solution turns into a stable white sol for sample B. In the similar way, sample C and sample D were prepared with 4wt.% and 6wt.% of ZnCl2 respectively for preparation of zinc doped tin oxide NPs. During the preparation of the samples, it was observed that SnO2 sample was in milky white colour while the colour of Zn-doped SnO2 became light yellow. The colour of Zn-doped SnO2 became dark as concentration of zinc increased. Finally, sample A, B, C and D were filtered & washed 4-5 times by ethanol and all the samples were dried at 80˚C for 24 hours. Then all the samples were calcinated at 400˚C for 3 to 4 hours to prepare the final samples. All the samples were characterized using different techniques such as FE-SEM, XRD, UV-vis spectroscopy and PL were employed to study their physical and optical properties.

The morphologies and microstructures of these nano particles were studied using jeol field emission scanning electron microscope (FE-SEM), The sample phases, crystal structure of the Zn doped SnO2samples were detected by x-rays diffractometer (XRD). The optical properties of nanoparticles were studied by lambda 650 UV-vis absorption spectrometer (UV-vis) perkin elemer and photoluminescence (PL) carried by. The moleculer structure of the synthesized nanoparticles was studied using FTIR spectrometer.

3. RESULTS AND DISCUSSION

3.1 Morphological Studies

The morphological studies of SnO2 and Zn- doped SnO2 NPs were conducted by Ziess Sigma FE-SEM under high vacuum. Figure 1 show thedetailed morphologies of the SnO2 and Zn- doped SnO2 NPs. Figure 1(a) shows SEM image of SnO2 NPs which depicts that the small nanoparticles are agglomerated to form clusters. The shapes of the nanoparticles are not clear in this image due to agglomerations of small nanoparticles. Figures 1 (b-d) shows the morphologies of Zn- doped SnO2 NPs with different percentage of Zn i.e. 2%, 4% and 6% in SnO2. In figure 1(b-d), the SEM image shows the small nanorods of tetragonal shapes with different size from 9-20nm. The SEM study reveal the formation of nanostructures of SnO2 and Zn- doped SnO2 via low cost sol-gel process.

FIGURE. 1 SEM micrographs of (a) SnO2 NPs (b) SnO2: Zn (2%) NPs (c) SnO2: Zn (4%) NPs and (d) SnO2: Zn (6%) NPs

FIGURE. 2 EDX spectrum of (a) SnO2 NPs (b) SnO2: Zn (2%) NPs (c) SnO2: Zn (4%) NPs and (d) SnO2: Zn (6%) NPs

3.2 CHEMICAL STUDIES

The chemical studies were carried out to study the chemical composition of SnO2 and Zn-doped SnO2 NPs by a BRUKER EDX system in conjunction with a Zeiss scanning electron microscope. The Fig. 2 shows the EDX spectrum of SnO2 and Zn- doped SnO2 NPs that reveal the synthesized NPs is constituted of elements Sn, O and Zn with stoichiometric ratio. Other elements such as O, C are also present in the system but their percentage is very less and their presence can be attributed to impurities present in the sample and during the characterization that arises from chemicals and substrate material. The stoichiometric atomic percentage is Sn=21.19%, & O=78.81% in fig. A, Sn=25.77%, O=72.38% & Zn=1.85% in fig. B, Sn=24.95%, O=71.33% & Zn=3.72% in fig. C and Sn=25.32%, O=69.05% & Zn=5.63%.

3.3 Structural Studies

The structural studies of as-prepared SnO2 and Zn-doped SnO2 NPs was performed with X-ray Diffraction system (PANalytical X’PERT-PRO X-ray powder diffractometer) using wavelength (Cu-Kα) 1.54060Å at 40 mA, 45 keV. The X-ray diffraction pattern was recorded using a step size of 0.0170 from 15-80° of 2θ angle. Fig. 5 shows the diffraction pattern of SnO2 and Zn-doped SnO2 NPs. The diffraction peaks at 2θ values of 26.6°, 33.8°, 37.9°, 51.8°, 58.05°, 61.9°, 64.6°, 65.7° 71.3° and 78.6° can be associated with (110), (101), (200), (211), (220), (002), (310), (112), (301), (202) and (321) respectively andcan be readily indexed to the tetragonal rutile phase of SnO2 (JCPDS No. 41-1445). The XRD study reveals that the formed nanostructures are of tetragonal phase as analyzed in SEM micrographs. The average crystal size as calculated by Scherrer formula from XRD measurements was estimated in the range of 10-22nm.

FIGURE. 3 XRD patterns of SnO2 and Zn-doped SnO2 NPs calcined at 400˚C(a) SnO2 NPs (b) SnO2: Zn (2%) NPs

(c) SnO2: Zn (4%) NPs and (d) SnO2: Zn (6%) NPs

3.4 Optical Studies

The optical absorption studies of SnO2 amd Zn-doped SnO2 NPs were performed using Perkin Elmer 2550 UV-Vis spectrometer. Fig.8 shows the Tauc’s plot for SnO2 and Zn-doped SnO2 NPs for determination of their optical energy band gaps. The energy band gap of SnO2 and Zn-doped SnO2 NPs is calculated from absorption coefficient data as a function of wavelength using Tauc relationship [12] given by

ngh B h E

………………………….. (1) where ‘α’ the absorption coefficient, ‘hν’ photon energy, ‘Eg’ the optical energy band gap, ‘B’ (a constant term)the band tailing parameter and ‘n’ an index and can have different values i.e. 2, 3, 1/2 and 1/3 depending on band transitions.

FIGURE 4 Tauc,s Plot for energy band gap calculation for SnO2 and Zn-doped SnO2 NPs

The energy band gaps for direct transition are found to be 2.94eV for SnO2, 2.86 for SnO2: Zn (2%), 2.80 SnO2:

Zn (4%) and 2.73ev forSnO2: Zn (6%). It is observed that the band gap energy of the SnO2 is decreased for increasing the content of Zn which makes Zn-doped SnO2 NPs useful for electronic and optoelectronic applications.

3.4 PHOTOLUMINESCENCE STUDIES

Photoluminescence (PL) measurement is an important technique to study the optical properties of semiconducting nanostructures. PL studies reveals the information of energy states of impurities and defects to understand structural defects, crystallinity, surface defects, energy bands and excitations in the materials. The oxygen vacancies in grain boundaries and the presence of doping atoms in the surface and inter-granular layers affect the properties of metal oxide nanomaterials. The room temperature PL spectra of SnO2 and Zn-doped SnO2 NPs in the wavelength range 400-700nm is shown in fig 5. All the spectra contain one very prominent strong peak occurs at around 482nm for 2% 4%, 6% Zn doping. The PL spectrum also shows some peaks at 410, 449 and 466nm. The peak at 410nm is found in near UV region and also reported by many researchers in the literature. The

emission peak at around 466 nm is generally found in SnO2 structures because of the near band edge emission as the energy corresponding to this peak is almost equal to the band gap energy of SnO2 structure. Other minor peaks at 601nm and 644nm in visible region also exists in the spectrum due to impurities present in the system.

FIGURE. 5 PL spectra of SnO2 and Zn-doped SnO2NPs calcined at 400˚C

4. CONCLUSIONS

SnO2 and Zn-doped SnO2 nanoparticles were successfully synthesized by a low cost sol-gel method. The XRD pattern of the prepared SnO2 and Zn-doped SnO2 have been indexed to the tetragonal rutile structure of SnO2 and the calculated particle size in the range 10-22nm. The tetragonal phase is well matched with both SEM and XRD studies. The optical bandgap of SnO2 and Zn-doped SnO2 were found to be 2.94eV (SnO2), 2.86 (Zn=2%), 2.80 (Zn=4%) and 2.73ev (Zn=6%). The photoluminescence emission spectra of SnO2 nanoparticles exhibit emission at 482nm and other emission peak at 410nm in near UV region. The Zn-doped may have applications in electronic and optoelectronics devices.

ACKNOWLEDGEMENTS

The authors are highly thankful to Science and Engineering Research Board, Department of Science & Technology (DST), Govt. of India (Grant No. SERB/F/2139/2013-14) for providing financial support for this research work.

REFERENCES

1. Z. Ying, Q. Wan, Z.T. Song and S.L. Feng, Nanotechnology 15, 1682 (2004). 2. A. Aoki and H. Sasakura, Japan J. Appl. Phys. 9, 582 (1970). 3. L.N. Moghadam, A.E.B. Karimabad, M. Salavati-Niasari and H. Safardoust, JNS 5, 47-53 (2015). 4. Y. Wang and J.Y. Lee, J. Phys.Chem. B 108, 17832-17837 (2004). 5. S. Mehraj, M.S. Ansari and Alimuddin, Thin Solid Films 589, 57-65 (2015).

6. T. Minami, MRS Bull 38, 25 (2000). 7. P. Chetri, B. Saikia and A.Choudhury, Journal of Applied Physics 113, 233514 (2013). 8. N. Salah, S. Habib, A. Azam, M. Shahnawaze Ansari and W.M. AL-Shawafi, Nanomater Nanotechnol 6, 17

(2016) 9. R. Khan and F.M. Hu, Chin. Phys. B 24, 12 (2015) 10. X. Li, Q.Yu, C. Yu, Y. Huang, R. Li, J. Wang F. Guo, Y. Zhang, S.Gao and L. Zhao,J. Mater. Chem. A

3, 8076-8082 (2015) 11. R.K. Mishra and P.P. Sahay, Materials Research Bulletin 47, 4112-4118 (2012) 12. E.A. Davis and N.F. Mott, Phil. Magn. 22, 903 (1970)