Electrochemically Deposited Nanostructured ZnO ...

Electrochemically Deposited Nanostructured ZnO Thin Films For Biosensor Applications Hemalata Bhadane1,*, Edmund Samuel㸰 and D. K. Gautam3 1

IACQER, Nanotechnology Laboratory for Optoelectronics and Biosensors, Raghu Engineering College, Visakhapatnam-531162, India 2 Research center for Advanced Photon Technology, Toyota Technological Institute ,Nagoya, Japan 3 Department of Electronic, North Maharashtra University, Jalgaon-425001, India *E-mail:[email protected]

Abstract. Zinc Oxide thin films have been deposited by electrochemical method on stainless steel using Zinc nitrate hexahydrate as precursor and 0.05 M potassium chloride (KCl) as supporting electrolyte. The paper reveals thorough investigation of effect of concentration of Zinc nitrate. Further, morphological, structural and optical analysis has been carried out using the FESEM, XRD and PL spectroscopy respectively. From FESEM hexagonal shape nanorods ZnO films fabricated for 1 hour using 0.05M and 0.1M concentration are clearly observed. The XRD of ZnO thin films shows strong peaks along c-axis with (0 0 2) orientation of ZnO nanorods which implies deposited nanorods are perpendicular to the substrate surface and wurtzite hexagonal phase. The hexagonal ZnO nanorod grown using a 0.05M zinc nitrate concentration exhibited the sharpest and most intense PL peak in at 382 nm near UV band edge, indicates the enhanced crystalline structure of ZnO film. Keywords: Electrochemical deposition, ZnO nanorod. PACS: 82.45.Aa, 77.55.hf, 81.05.Dz, , 78.55.Et, 87.85.fk, 61.05.cp

INTRODUCTION Zinc oxide (ZnO) having wide application in the optoelectronic, microelectronics, sensor and biosensors due to wide direct band-gap (~3.37eV) semiconductor and high exciton binding energy of 60 meV produces UV excitation emission at room temperature [1]. ZnO nanostructure having developed interest in various fields especially sensor and biosensor because of high response time and high surface to volume ratio. ZnO nanostructure thin films deposited using various techniques such as chemical vapour deposition (CVD) [2], magnetron sputtering [3] thermal evaporation technique [4], pulse laser deposition (PLD) [5], and sol-gel techniques [6] and electrochemical deposition technique. However, an electrochemical technique is easy, cost effective, low processing temperature and ease to tailor the dimensions of the nanostructure.

EXPERIMENTAL DETAILS The ZnO films were electrochemically deposited on stainless steel from Zinc nitrate Zn(NO3)2 ,6H2O

used as precursor and 0.05 M potassium chloride (KCl) as supporting electrolyte. The aqueous solutions were prepared by using distilled deionized water. The elecrochemical deposition was carried out using a CH instrument 600D electrochemical analyzer with potentiostat–galvanostat. The electrochemical deposition procedure involves a three electrode setup with Platinum is used as counter electrode and Ag/AgCl(0.1M potassium chloride (KCl) saturated) electrode as the reference electrode. Stainless steel substrate cleaned in acetone, and then washed by water for 30 min. The depositions performed under a constant temperature of 80OC by water bath treatment and at constant voltage of -0.9V. After deposition films were rinsed thoroughly with acetone and deionized water respectively and dried at 150oC in air. Prepared films were further annealed at 300oC in muffle furnace for 1 hour. The cathodic electrochemical deposition of zinc oxide at different electrode potentials was initiated by the reduction of nitrate ions. Nitrate ions at first reduced and form nitrite and hydroxide ions electrochemically. These hydroxide ions react with zinc ions and form zinc hydroxide at the cathode.

Solid State Physics AIP Conf. Proc. 1591, 1012-1014 (2014); doi: 10.1063/1.4872836 © 2014 AIP Publishing LLC 978-0-7354-1225-5/$30.00

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These zinc hydroxides spontaneously dehydrates and form zinc oxide. The effect of concentration of the Zinc nitrate on structural, optical and morphological properties of ZnO thin film have been investigated by different characterizations like X-ray Diffraction (XRD), Photoluminescence (PL) and Field Effect Scanning Electron Microscopy (FESEM).

difference in the c-axis lattice parameter has due to the strain in the thin film. Under the compressive strain, the c-axis lattice parameter decreases, leading to somewhat larger interplanar distance for (002) planes.

RESULTS AND DISCUSSIONS Fig.1(a) and (b) illustrate XRD of ZnO nanorod film for 0.05M and 0.1M concentration of zinc nitrate respectively. XRD pattern for 0.05M concentration peaks at 31.87, 34.54, 36.37, 47.61, 62.93 which can be attributed to the (100), (002), (101), (102) and (103) matches with the standard JCPDS 36-1451. The peak indicated by abbreviation ‘*’attributed the stainless steel substrate. The XRD for the 0.05M and 0.1 M concentration of ZnO thin films with deposition time of 1 hour shows strong peaks along c-axis with (0 0 2) orientation of ZnO nanorods which implies deposited nanorods are perpendicular to the substrate surface. The (002) peak shows stronger and sharper intensity, as well as a small full width at half maximum (FWHM) as compared with the other peaks revealed that a high pure wurtzite hexagonal phase of ZnO film. Whereas, the FWHM of (002) orientation peak is observed to be lowered with increase in Zinc nitrate concentration demonstrating increased crystal size. The lattice parameter ‘a’ and ‘c’ of wurtizite structure for ZnO can be calculated by using the following formula

d hkl =

1 4(h + k + hk ) l 2 + 2 3a 2 c 2

2

FIGURE 1. XRD of the ZnO nanorod films for 0.05M and 0.1M concentration of zinc nitrate.

(1)

Where a, c are the lattice parameters and is the crystalline surface distance for the h k l indices. The strain ( following formula,

ε zz =

) along the c-axis calculated by

c − co 100%. co

(2)

Where co = 0.5207 nm is the unstrained lattice parameter of ZnO, c is the lattice parameter of the deposited ZnO films calculated from X-ray diffraction data. The calculated lattice parameter and strain along c-axis are summarised in the table 1. The lattice parameter close to the parameters ao= 0.3250 nm and co=0.5207 nm (JCPDS 36-1451). The calculated lattice parameters are smaller than the standard valve. The

Figure 2. PL spectrum of the ZnO nanorod films for 0.05M and 0.1M concentration of zinc nitrate. The optical properties of the ZnO nanorods film were measured by PL spectroscopy. The ZnO film was excited with a wavelength of 325nm at room temperature PL spectrum illustrated in fig.2. The PL of the ZnO nanorod film with different zinc nitrate concentration exhibited in fig.2. The hexagonal ZnO nanorod grown using a 0.05M zinc nitrate concentration exhibited the sharpest and most intense PL peak in UV range, indicates the enhanced


crystalline structure of the ZnO film. The PL spectra show strong peak at 382nm near the UV band edge due to free electron transition from conduction band to the valance band and weaker peak at the 364nm peak correspond to the phonon scattering for 0.05M ZnO concentration. However, for 0.1 M concentration the PL spectra show shift in strong and weak peaks at 378nm and 358 nm respectively. Fig. 3 shows the surface morphologies of the ZnO film fabricated for the0.05M and 0.1M concentration at 1 hour deposition time hexagonal shape nanorods were observed. The ZnO nanorods grown for 0.05M concentration had better morphology and higher density of nanorod than for 0.1M concentration. ZnO nanorod size is increased from 417 nm to 479 nm with increase in zinc nitrate concentration which is also observed in XRD.

CONCLUSION Fabrication of ZnO nanorods with various dimensions has been observed with the variation of concentration of ZnO. The strain increases with higher concentration due to change in lattice structure. FESEM reveals hexagonal and cylindrical ZnO nanorods. Thus, the deposited ZnO films with nanorods shape are expected to increase the surface to volume ratio and consequently enhancing the surface to volume ratio of ZnO biosensor which is under investigation.

TABLE 1. Lattice constant and the stain for the ZnO prepared films for 0.05M and 0.1M concentration Concentration of Zinc c-lattice constant a-lattice constant Strain Grain Size(D ) Nitrate (nm) 0.05M 5.1886 3.1730 -0.35 5.25 0.1M 5.1816 X -0.48 16.18

FIGURE 3. FESEM images for the ZnO nanorod film a),b) &c) for 0.05M and d),e)&f) for 0.1Mconcentration of precursor. 2.

ACKNOWLEDGMENTS This project is supported in grant in aid by Department of Science and technology (DST), New Delhi. One of the author HB, express her gratitude towards DST for funding and International Accreditation Council of Quantity Education and Research, New Delhi and Raghu Engineering College, Visakhapatnam for providing the infrastructure.

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