Study on the Synthesis, Structural, Optical and
Electrical Properties of ZnO and Lanthanum Doped
ZnO Nano Particles by Sol-Gel Method

V Porkalai, D Benny Anburaj, B Sathya, G Nedunchezhian, R Meenambika

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V Porkalai, D Benny Anburaj, B Sathya, G Nedunchezhian, R Meenambika. Study on the Synthesis, Structural, Optical and Electrical Properties of ZnO and Lanthanum Doped ZnO Nano Particles by Sol-Gel Method. Mechanics, Materials Science & Engineering Journal, 2017, 9, 10.2412/mmse.77.37.393. hal-01499405

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# Study on the Synthesis, Structural, Optical and Electrical Properties of ZnO and Lanthanum Doped ZnO Nano Particles by Sol-Gel Method

V. Porkalai¹, D. Benny Anburaj¹,ᵃ, B. Sathya¹, G. Nedunchezhian¹, R. Meenambika²

1. PG, Research Department of Physics, Thiru. Vi. Ka. Government Arts College, Thiruvarur, Tamil Nadu, India

2. Marthandam College of Engineering & Technology, Kanyakumari District, India

a. bennyanburaj@rediffmail.com

<figure><img src="image_2.png" /></figure>

DOI 10.2412/mmse.77.37.393 provided by [Seo4U.link](https://linkAGE04U.seo)

**Keywords:** lanthanum, photo luminescence, morphology, nanoparticles.

**ABSTRACT.** In this study, pure and lanthanum doped ZnO nano particles have been successfully synthesized by sol-gel method using the mixture of Zinc acetate dihydrate and ethanol solution. The powders were calcined at 600°C for 2h. The effect of lanthanum incorporation on the structure, morphology, optical and electrical conductivity were examined by X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Energy Dispersive X-ray Absorption (EDAX), Fourier transform infrared spectroscopy (FTIR), UV and Photo Luminescence (PL) Characterization. The average particle size of the synthesized ZnO nanoparticles is calculated using the Scherrer formula and is found to be of less than 20 nm. Luminescence as well as conductivity properties were found to be enhanced for the La doped ZnO nanoparticles.

**Introduction.** Synthesize and study of nanostructured materials have become a major attractive interdisciplinary area of research over the past few decades. Recently rare earth ion doped II-IV semiconductor nano particles have received much attention because such doping can modify and improve optical properties of II-VI semiconductor nanoparticles by large amount [1-4]. Zinc Oxide is a transparent electro conductive and piezo electric material. Zinc Oxide is an excellent ultraviolet absorber and antibacterial agent. ZnO is one of the metal oxides which attracts due to its direct band gap energy of 3.37eV and large excitation binding energy of 60 meV at room temperature which provides excitonic emission more efficiently even at high temperature. ZnO is particularly important because of their unique optical/electronic properties and promising applications in various fields such as photonic catalysis [5], light emitting diodes [6], field emission, gas sensors [7], fluorescent materials and solar cells [8].

Doping with rare earth elements leads to many interesting properties of ZnO. Usually, semiconducting nanoparticles are known to exhibit exotic physico-chemical properties due to quantum confinement effect. Especially, doped luminescent nanoparticles are predicted to show improved optical properties, viz., luminescence efficiency and delay time and band edge emission with respect to particle size variation. ZnO nanoparticles at different Lanthanum (La) doping concentration varied from 0.1 to 0.3 mole % have been synthesized via sol-gel route and characterization of the sample by XRD, SEM, EDAX and FTIR analysis. UV and Photo Luminance (PL).

**Experimental Procedure.** Zinc Oxide nanoparticle were synthesized by dissolving Zinc acetate (Zn (CH₃COO)₂ 2H₂O) in distilled water by continuous stirring for half an hour. Lanthanum chloride (LaCl₃) taken at appropriate proportion of 0.1M, 0.2M and 0.3M respectively was added drop by drop and mixed thoroughly. TEA (Triethylamine) was added as surfactant to control the morphology and
size of nanoparticles. Suitable amount of NH4OH solution was added to maintain the pH level at 10. The whole setup is maintained at a temperature of 80°C for 2 hours. The colloidal precipitate obtained was cooled and washed several times with ethanol and acetone to remove the organic impurities present, if any. The solution is then preheated in microwave oven for 30 mins for the evaporation of the solvent. The product was then calcinated to 600 °C for 2 hours.

## Results and Discussions:

**Structural Analysis.** Fig. 1 shows the XRD pattern of pure and La-doped ZnO (0.1, 0.2 and 0.3 mol%) calcined at 600 °C. The strong intensities of diffraction peaks (100), (002), (101), (102), (110), (103), (112) and (201) can be indexed to the hexagonal wurtzite structure of ZnO (JCPDS# 79-0208) [13].

<figure><img src="image_3.png" /><figcaption>Fig. 1. XRD pattern of pure and Lanthanum doped ZnO nanoparticles.</figcaption></figure>

The average crystalline size can be determined through FWHM of X-ray diffraction peak by using Debye-Scherer's equation as

$$D = \frac{0.9\lambda}{\beta \cos\theta}$$

where λ is the wavelength of the X-ray (1.5405Å), D is the particle size, θ is the Bragg diffraction angle and β is the full width half maximum (FWHM) of the diffraction peak respectively.

The particle size of pure and Lanthanum doped ZnO was found. It is observed that as the doping concentration increases, the intensity of the peaks decreases which in turn decreases the size of the nanoparticles.
<table><caption>Table 1. Structural Analysis of La doped ZnO nanoparticles.</caption><thead><tr><th>No</th><th>Samples</th><th>2θ</th><th>FWHM (β) (rad)</th><th>Particle Size (D) nm</th><th>Dislocation Density (δ) m<sup>-2</sup></th><th>Micro strain X10<sup>-3</sup></th><th>Crystal structure</th></tr></thead><tbody><tr><td>1</td><td>Pure ZnO</td><td>42.6</td><td>0.0103</td><td>15.12</td><td>4.37 X10<sup>15</sup></td><td>9.56</td><td rowspan="4">Hexagonal</td></tr><tr><td>2</td><td>0.1M</td><td>41.1</td><td>0.0167</td><td>8.11</td><td>1.51 X10<sup>16</sup></td><td>15.63</td></tr><tr><td>3</td><td>0.2M</td><td>41.13</td><td>0.0118</td><td>11.48</td><td>7.58 X10<sup>15</sup></td><td>11.04</td></tr><tr><td>4</td><td>0.3M</td><td>42.16</td><td>0.0104</td><td>12.98</td><td>5.93 X10<sup>16</sup></td><td>9.70</td></tr></tbody></table>

When lanthanum was doped into ZnO matrix, diffraction peaks of the doped products are almost similar to those of undoped hexagonal ZnO crystal. Their crystalline structure remains unchanged, which indicates that La³⁺ uniformly disperses across the hexagonal ZnO matrix[10]. The sharp diffraction peaks manifests that the pure ZnO and La-doped ZnO nanostructures have crystalline nature

## EDAX Analysis.

<figure><img src="image_4.png" /><figcaption>Fig. 2. (a, b) EDAX spectrum of pure and Lanthanum doped ZnO nanoparticles.</figcaption></figure>

In EDAX spectrum, the peaks were evident related to Zn, O and La, which clearly support that nanoparticles are made of Zn, O and La. No other peaks related to impurities was detected in the spectrum, which further confirms the purity of the compounds.

**Morphological analysis.** The morphological studies were investigated using scanning electron microscopy and displayed in Fig. 3 for pure and La doped ZnO nano particles.

These micrographs exhibited the formation of nanoparticles of doped ZnO. Crystal formation in solution can be divided into two stages: crystal nucleation and growth rates. These two stages are
responsible for the formation of the different morphologies of ZnO particles with different
morphologies.

<figure><img src="image_5.png" /><figcaption>Fig. 3. (a, b, c and, d) SEM photographs of pure and Lanthanum doped ZnO nanoparticles.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. FTIR spectrum of pure and Lanthanum doped ZnO nanoparticles</figcaption></figure>

Fourier Transform Infrared Spectroscopy Study.

Synthesized La doped ZnO material was analysed by FT-IR in the range from 400 to 4000cm⁻¹ at room temperature. The FT-IR spectrum contains several bands with remarkable features. The spectral band at 416 cm⁻¹ and the band at 607 cm⁻¹ clearly show the presence of ZnO and La. Bands at 1065cm⁻¹ correspond to C-O stretching vibrations. Bands at 1394 cm⁻¹ corresponds to C=O, 1580 cm⁻¹ indicates C=O stretching vibration, 2976 cm⁻¹ indicates CH₂ unsymmetrical stretching vibrations and 880 cm⁻¹ corresponds to N-O deformation vibration .Also the bands at 3331cm⁻¹ indicate the presence of N-H axial deformation. It is evident from the FTIR data that the Zn-O vibrational mode was more prominently observed and this clearly concludes a strong doping between La doped ZnO nanoPticles materials.

UV-Vis Spectral Analysis of La doped ZnO Nanoparticles.
<figure><img src="image_7.png" /><figcaption>Fig. 5. UV-Vis spectrum of pure and Lanthanum doped ZnO nanoparticles.</figcaption></figure>

The optical band gap increased from 3.28 ev to 3.44 ev with an increase in the concentration of La doping. There are two reasons that's may contribute to the variations in band gap energies, namely, quantum size effect and electronic structure modifications. In the case of quantum size effect, appropriate interaction between surface oxidic size of ZnO and La³⁺ may also be the cause for shift of wavelength. Instead, La³⁺ doping in ZnO could modify the electronic structures

## Photoluminescence (PL).

<figure><img src="image_8.png" /><figcaption>Fig. 6. PL spectra of pure and Lanthanum doped ZnO nanoparticles.</figcaption></figure>

The PL spectra are useful to disclose the efficiency of charge carrier trapping, immigration and transfer and also to understand the fate of electron hole pairs in semiconductor particles since PL emission results from the recombination of free carriers. Photoluminescence (PL) spectra of the pure and La doped ZnO for different doping concentration is as shown in Fig. 6. The luminescence peak of pure ZnO was observed at 388 nm and for La doped ZnO was observed at 422 nm. There is substantial enhancement of luminescence intensity due to increase of La concentration, which acts as effective luminescent centers. The PL results show that doped rare earth elements are
the major luminescent component and can effectively improve the luminescence of La doped ZnO.
In this study 1 mol% La doped ZnO had the most oxygen vacancies (Fig. 6) [12].

**Summary.** Pure and Lanthanum doped ZnO nanoparticles were successfully synthesized by sol-gel method. The XRD pattern of La doped ZnO it clearly shows that the sharp peak obtained from ZnO planes. The particle size of pure and Lanthanum doped ZnO nanoparticles and the microstrain in lattice has been determined. The dislocation density of grains is determined to be ~4.37×10¹⁵m⁻² and the microstrain in lattice is found to be 9.56×10⁻³. X-ray analysis reveals that La doped ZnO crystallized in Hexagonal structure. The morphology of the ZnO particles was obtained from SEM. Elemental compositions have been estimated by EDAX. Chemical and optical properties are studied FTIR and UV-VIS spectrophotometer. From the UV-Vis spectral analysis we have calculated the band gap of La doped ZnO, which is found to be approximately 3.33eV, at the wavelength 364 nm. From I-V studies, we observed that in the doped mixture, the current conductivity gets increased significantly to 4.06×10⁻⁴ (1/Ωm), due to the incorporation of La. The PL results show that doped rare earth elements are the major luminescent component and can effectively improve the luminescence of La doped ZnO.

**References**

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Cite the paper

V. Porkalai, D. Benny Anburaj, B. Sathya, G. Nedunchezhian, R. Meenambika (2017). *Study on the Synthesis, Structural, Optical and Electrical Properties of ZnO and Lanthanum Doped ZnO Nano Particles by Sol-Gel Method*. *Mechanics, Materials Science & Engineering*, Vol 9. doi:10.2412/mmse.77.37.393