Zinc Oxide Nanoparticles-Solution-Based Synthesis and Characterizations

Abstract

Zinc oxide (ZnO) nanoparticles have shown great potential because of their versatile and promising applications in different fields, including solar cells. Various methods of synthesizing ZnO materials have been reported. In this work, controlled synthesis of ZnO nanoparticles was achieved via a simple, cost-effective, and facile synthetic method. Using transmittance spectra and film thickness of ZnO, the optical band gap energies were calculated. For as-synthesized and annealed ZnO films, the bandgap energies were found to be 3.40 eV and 3.30 eV, respectively. The nature of the optical transition indicates that the material is a direct bandgap semiconductor. Spectroscopic ellipsometry (SE) analysis was used to extract dielectric functions where the onset of optical absorption of ZnO was observed at lower photon energy due to annealing of the nanoparticle film. Similarly, X-ray diffraction (XRD) and scanning electron microscopy (SEM) data revealed that the material is pure and crystalline in nature, with the average crystallite size of ~9 nm.

Keywords: absorbance; absorption coefficient; bandgap; crystal structure; ellipsometry; nanoparticle; surface morphology; znc oxide.

Figure 1

XRD patterns of annealed film of ZnO nanoparticles deposited on borosilicate glass substrate (Fisher brand microscope slide). The film was heated at 300 °C for 30 min.

Figure 2

Scanning electron microscopy (SEM) images of ZnO nanoparticles. The nanoparticle thin film was spin-coated and SEM images were taken without the prior heat treatment.

Figure 3

Dielectric function spectra for ZnO nanoparticle thin film measured by SE at room temperature. The red curve represents ε₁ plotted to the left y-axis and is represented by an arrow with a circle and black curve represents ε₂ as shown by a circle with an arrow to the right.

Figure 4

VU/Vis/NIR spectroscopy results: (A) Absorbance spectrum of as-obtained ZnO nanoparticles dispersed in chloroform. (B) Transmission % of ZnO thin film of ~100 nm before and after annealing the films. (C) Temperature-dependent transmission of ZnO thin films annealed for 30 min. When temperature is increased, transmission in infrared and visible regions is increased, whereas it is decreased in the band edge absorption regions. (D) Absorption coefficients (α, cm⁻¹) of thin films of ZnO with respect to wavelength in nm. Small changes in α were observed when the films were annealed for different time intervals.

Figure 5

(A) Plot of ${\left(\alpha h\nu \right)}^2$ vs. $h\nu$ of ZnO nanoparticles’ thin films. The red curve is from the as-synthesized film with y-axis to the right and the rest are from the heated films with y-axis to the left. Yellow circles and arrows represent directions of y-axes for respective curves. (B) Urbach energy calculation of ZnO nanoparticles. Low $E_u$ indicates that annealed films are pure & crystalline. More defect states are present in the as-synthesized film, as depicted by higher $E_u$.