Variation in Structural and Optical Properties of Al Doped ZnO Nanoparticles Synthesized

Mechanics, Materials Science & Engineering, May 2017

ISSN 2412-5954

Variation in Structural and Optical Properties of Al Doped ZnO Nanoparticles Synthesized by Sol-gel Process7 Vanaja Aravapalli1, Seshu Kalakatla1, G. K. S. Prakash Raaju2, Srinivasa Karumuri3 1

Asst. Professor, Dept. of AS&H, Tirumala Engineering College, Jonnalagadda, India

2

Research fellow, CMET (Govt. of India), Hyderbad, India

3

Professor, Dept of ECE, KL University, Vaddeswaram, India DOI 10.2412/mmse.23.89.710 provided by Seo4U.link

Keywords: aluminum nanoparticles, ZnO, XRD, SEM, UV-Vis, PL.

ABSTRACT. This article focuses on analyzing structural and optical properties of Al doped ZnO (AZO) synthesized with two different precursors aluminum chloride and aluminum nitrate. The nanoparticles were successfully fabricated and characterized at room temperature by sol-gel process. The objective of improving properties of ZnO nanoparticles by introducing dopants was successful with formation of nanoparticles having different crystalline sizes, optical absorption and luminescence properties. The two different sources influenced properties of ZnO. The particles with less crystalline size obtained from aluminum nitrate. Change in morphology from spherical to bar like morphology proved from SEM spectra. Presence of functional groups predicted from FTIR spectra. PL spectra proved UV emission and visible emission for AZO nanoparticles synthesized using dopant sources aluminum chloride and aluminum nitrate respectively. The obtained properties prove successful utilization of AZO nanoparticles as building materials in fabrication of optoelectronic devices.

Introduction: Recent days ZnO nanoparticles widely utilized in fabrication of various optoelectronic ics of large and direct band gap, high melting point, thermal conductivity, mobility, inexpensive, luminescence etc. Introducing dopants in ZnO lattice helps in tuning the properties of ZnO nanoparticles [1]. Transition metal ions can be easily penetrated into ZnO lattice because they have ionic radius which is very close to ZnO. In present work, AZO nanoparticles are synthesized by choosing different dopant sources aluminum chloride and aluminum nitrate. AZO is also a promising material for high temperature thermo electronic material. ZnO nanoparticles obtained by replacing Zn2+ with high valence electrons like Al, Ga, In etc regarded as promising materials for optoelectronic device fabrication [2]. AZO nanoparticles is also highly promising as a potential high-temperature thermoelectric material. Al ion can be easily incorporated into ZnO lattice due to its ionic radius near to that of ZnO [3]. AZO nanopowders can be utilized as transparent conductive pastes because of their properties of transparency and conductivity in visible region. ZnAlO is a wide band gap material (Eg~3.8eV). Top down and bottom up approaches were reported in literature for synthesizing nanoparticles. Bottom up methods were more preferred because of their less wastage, high purity etc. [4]. With the development in field of nanoresearch various bottom up techniques like co precipitation, hydrothermal method, laser ablation etc were developed for nanoparticle synthesis [5]. In this work sol-gel method is used for synthesis of AZO nanoparticles. This method is simple, inexpensive, environment friendly. By this method it is possible to control particle size, morphology, high degree of crystallinity, higher nucletion rates etc. The sol-gel process is carried out at room temperature [6].

In this work, AZO nanoparticles were prepared by sol-gel process employing ZnCl2, NaOH, Aluminium nitrate, Aluminium chloride and ethanol as starting materials. The particles were 7

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Mechanics, Materials Science & Engineering, May 2017

ISSN 2412-5954

synthesized without involving any external parameters like pH, temperature etc. The crystal structure, surface morphology, chemical composition, particle size, optical absorption and luminescence properties were characterized byXRD, SEM, FTIR, UV-Vis and PL. The results were compared to determine the influence of Al precursor on structural and optical properties of AZO nanoparticles.

Experimental Work Materials Required. Zinc Chloride (ZnCl2), Sodium hydroxide (NaOH), Aluminium chloride (Al (Cl)2), Aluminium nitrate (Al (NO3)2, Ethanol. All the chemicals are A.R grade purity and they are utilized without further purification. (Al (Cl)2) and (Al (NO3)2 served as dopant sources for synthesis process.

Apparatus Used. Magnetic stirrer (REMI, MLH), Glass jars, Centrifuge Mission (REMI), Micro oven (VSE 230 A/C), High Precision Balance (INFRA), Measuring jars, Centrifuge test tubes.

Synthesis Procedure: For synthesizing AZO nanoparticles using aluminium chloride precursor, At first the glass beakers, test tubes were washed and dried. The aqueous ethanol solution of ZnCl2 is prepared by stirring the chemical ZnCl2 (0.2 M) in ethanol (60 ml) solution using magnetic stirrer for 2 hours. Aqueous ethanol solution of NaOH (0.2 M) is also prepared in same way by dissolving NaOH (0.2 M) in ethanol (60 ml) using magnetic stirrer. After complete dissolution, aqueous solution of NaOH added to ZnCl2 solution. The resulting mixture constantly stirred for one hour till the solution turns milky. The aqueous ethanol solution of (Al (Cl)2) (0.2 M) is prepared and added to mixed solution of (ZnCl2) and NaOH. The transparent sol was sealed and allowed to stay overnight at room temperature to complete the gelation and hydrolysis process. During this process white precipitates of ZnO crystallized and settled down. The resultant white precipitate so formed was taken into test tubes and centrifuged. The resultant solution washed with ethanol to remove the unwanted obtain fine powder of AZO. The powder collected and preserved in an air tight container. In similar manner AZO nanoparticles with Al nitrate as dopant source was synthesized. The obtained nanopowders were further characterized.

The samples synthesized are: 1. AZO nanoparticles using Aluminum chloride 2. AZO nanoparticles using Aluminum nitrate.

Characterization: Structural properties including crystalline size, lattice strain were obtained from XRD spectra. The XRD patterns of the powdered samples were recorded using XRD (Rigaku) =1.5406 Ao). The crystallite size estimated using DebyeScherer equation of the high intense XRD peak. Morphological properties obtained from SEM micrographs recorded from SEM (Carl Zeiss SUPRA-55) spectroscopy. UV- Vis spectra of samples were recorded using UV-VIS Spectrophotometer (Perkin Elmer Spectrum). The powdered sample were dispersed in ethyl alcohol and mounted in the sample chamber while pure ethyl alcohol was taken in the reference beam position. The essential information about the physical properties of materials at molecular levels, including shallow and deep level defects and band gap state for energy level were determined using PL spectroscopy (Bruker S4 Pioneer).

Results and Discussions.

XRD Analysis: The XRD spectra of AZO nanoparticles synthesized with precursors (Al (Cl)2) and Al (NO3)2 are shown in Fig. 1 (a) and Fig. 1 (b).The XRD peaks clearly indexed to hexagonal wurtzite structure (JCPDS 89-0510) with high purity. Wurtzite structure of ZnO is not influenced due to variation in dopant sources. Shift in position of diffraction peaks is noticed due to variation in Al nm respectively. AZO nanoparticles with different crystalline sizes influence optical absorption and luminescence properties.

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Mechanics, Materials Science & Engineering, May 2017

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(a)

(b)

Fig. 1. XRD spectra of AZO nanoparticles synthesized using (a) Al (NO3)2 (b) (Al (Cl)2).

SEM Analysis

(a)

(b) Fig. 2. SEM spectra of AZO nanoparticles synthesized using (a) Al (NO3)2, (b) (Al (Cl)2).

Fig. 2 (a) and 2 (b) represent SEM micrographs of AZO nanoparticles synthesized using Al (NO3 )2 and (Al (Cl)2) respectively. The micrographs reveal the formation of nanoparticles. Fig. 2 (a) confirms the formation of approximate round shape morphology. Fig. 2 (b) shows the prepared nanoparticles have nanoflake like morphology converted to bar like when observed at different magnifications. The dopant source has proved nanoparticle synthesis with different suface areas.

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Mechanics, Materials Science & Engineering, May 2017

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FTIR Analysis. The structural properties of nanoparticles further confirmed from FTIR spectroscopy. Fig. 3 (a) and Fig. 3 (b) represent FTIR spectra of AZO nanoparticles obtained from different precursors Al nitrate and Al chloride respectively. The position and number of absorption bands varies with chemical composition.

In Fig. 3 (a) the absorption bands around 3500 cm-1, 1400 cm-1 attributed to presence of O-H stretching and carbonate molecules. In Fig. 3 (b) the broad and weak absorption peaks centered at 3642 cm-1, 1655 cm-1 attributed to O-H bending and stretching. The week absorption bands found near 2815 cm-1, 1360 cm -1 indicate the presence of C-H and C-O functional groups. Below 700 cm- 1 in Fig. 3 (a) two peaks were observed where as in Fig. 3 (b) the spinels display stretching bands in the 500 oxygen, aluminium oxygen and metal oxygen aluminium from spectra it is clear that the doped samples show minimum two peaks for all the nanomaterials that is assigned to formation of metal aluminates.

(a) (b) Fig. 3. FTIR spectra of AZO nanoparticles synthesized using (a) Al (NO3)2 (b) (Al (Cl)2).

UV-Vis Analysis

(a)

(b)

Fig. 4. UV-Vis spectra of AZO nanoparticles synthesized using (a) Al (NO3)2 (b) (Al (Cl)2).

The room temperature UV-Vis spectra of AZO nanoparticles were shown in Fig. 4 (a) and Fig. 4 (b) respectively. An excitonic absorption peak around at 217 nm is noticed in AZO nanoparticles synthesized using precursor Al nitrate. The peak lies much below band gap wavelength of 388 nm corresponding to bulk ZnO. The peak at ~214 nm is due to interband transition of electron from deep level of valence band. In Fig. 4 (b) two peaks at 238 nm and 303 nm are observed in UV region. This shown AZO nanoparticles obtained using Al chloride as dopant have enhanced UV absorption characteristics. The shift in absorption peak may be due to transition of electrons from inner shell to MMSE Journal. Open Access www.mmse.xyz

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Mechanics, Materials Science & Engineering, May 2017

ISSN 2412-5954

outer shell.

PL Analysis: PL spectra of AZO nanoparticles synthesized with different aluminum nitrate and aluminum chloride recorded at room temperature on irradiating at wavelength 320 nm by a xenon lamp are shown in the Fig. 5 (a) and Fig. 5 (b) respectively. In Fig. 5 (a), The PL spectra show broad luminescence in UV region ranging from 300 - 400 nm. The effect of Al chloride doping into ZnO lattice shows visible emission. This visible PL emission can be attributed to different intrinsic defects. The visible luminescence centered around 451 nm corresponding to green luminescence. Broadening of the band gap due to quantum confinement effect was observed upon Al doping. It was reported that the oxygen vacancies responsible for the green emission are mainly located at the surface.

(a)

(b)

Fig. 5. PL spectra of AZO nanoparticles synthesized using (a) Al (NO3)2 (b) (Al (Cl)2).

Summary. The comparative study on AZO nanoparticles synthesized using different sources is very useful to identify the potentiality of dopant source in enhancing the properties of ZnO nanoparticles.

AZO nanoparticles were successfully synthesized by sol-gel process. The AZO nanoparticles of crystalline sizes are formed with the precursors Aluminium chloride and Aluminium nitrate respectively. Morphology determined from SEM spectra proved the variation in morphology from nanorod to nanobar like morphology. The variation in the crystalline size and lattice parameters proved significant variation in presence of absorption peaks in FTIR spectra. The powders show optical absorption in UV region. The PL spectra with significant UV luminescence noticed in spectra of AZO nanoparticles obtained from Al nitrate. The ZnO nanoparticles prepared from (Al (Cl)2) show intense green luminescence. The UV luminescence and green luminescence properties can be further nanoparticles.

References [1] Soosen Samuel M, Lekshmi Bose, George KC, Optical Properties of ZnO Nanoparticles, Academic Review 2009, P.No 57-65.

[2] Ziaul Raza Khan, Mohd Arif, Arun Singh, Development and study of the structural and optical properties of hexagonal ZnO nanocrystals, International Nano Letters 2012, 2:22, DOI 10.1186/22285326-2-22 [3] P. M. Aneesh, K. A.Vanaja, M. K. Jayaraj, Synthesis of ZnO nanoparticles by hydrothermal method Proc. of SPIE 2007, Vol. 6639, 66390J-1, DOI 10.1117/12.730364 [4] Different Precursors in the Chemical Synthesis of ZnO Nanocrystals, Materials Research. 2011, 14 (2): 264-267, DOI 10.1590/S1516-14392011005000035 MMSE Journal. Open Access www.mmse.xyz

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[5] A.Alkahlout, N.AlDahoudi, I.Grobelsek, M.Jilavi, P.W.deOliveira, Synthesis and Characterization of Aluminum Doped Zinc Oxide Nanostructures via Hydrothermal Route, Hindawi Publishing Corporation Journal of Materials 2014, Article ID 235638.

[6] Anne Aimable, Tomasz Strachowski, Ewelina Wolska, Witold Lojkowski, Paul Bowen Comparison of two innovative precipitation systems for ZnO and Al-doped ZnO nanoparticle synthesis, Processing and Application of Ceramics 4 2010, 107 114, DOI 10.2298/PAC1003107A

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