Investigation of Ti doping on the structural, optical, and magnetic properties of ZnO nanoparticles

TL;DR: In this article, a co-precipitation approach was used to synthesize Ti-doped ZnO nanoparticles through X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL), UV-Visible spectroscopy, and Vibrating Sample Magnetometer (VSM).

Abstract: Ti-doped ZnO (TixZn1-xO x = 0.00, 0.05, 0.10, 0.15) nanoparticles have been synthesized through co-precipitation approach. X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL), UV–Visible spectroscopy, and Vibrating Sample Magnetometer (VSM) have been used to characterize the samples. X-Ray Diffraction (XRD) analysis manifested the hexagonal wurtzite structure. The crystallite size decreased from 37 to 29 nm as dopant concentration is increased. Fourier transform infrared analysis showed the absorption bands of ZnO, with few within the intensities. SEM investigation showed the irregular shape and agglomeration of the particles. Ti, Zn, and O composition were determined from EDX analysis and confirmed the purity of the samples. PL spectra showed a near-band edge emission and visible emission. Vibrating sample magnetometer (VSM) demonstrated pure and doped samples exhibited ferromagnetism behavior at room temperature.

Summary

1. Introduction:

• In recent years, semimagnetic semiconductors (SMSs) have attracted significant interest because of their versatile capability packages in spintronics devices [1–4].
• They had been generally acquired by doping a small amount of Transition metallic (TM: Fe, Co,Ni, Mn, Cr, and so on.) into semiconductors.
• In Diluted Magnetic oxides, ferromagnetism at room temperature (RTFM) observed may be due to the intrinsic defects ( oxygen vacancies) or presence of secondary phases and it depends on the methods of preparation.
• Still, there are a lot of requirements to enlighten the origin of the RTFM in Transition metal doped ZnO.

2.1 Synthesis of undoped and Ti doped ZnO nanoparticles:

• Analytical reagent chemicals Zinc chloride(ZnCl22H2O) ,Titanium tetraChloride (TiCl4 ) and Sodium hydroxide (NaOH) were purchased from Merck used as the starting materials for Zn, Ti and OH and used as received.
• Pure ZnO and Ti doped ZnO nanoparticles were synthesized by simple precipitation method.
• Initially, Zinc chloride and Titanium tetrachloride were dissolved separately in 100 mL of pure distilled water to make 0.2 M of solution .
• Then, sodium hydroxide solution was added to the above mixture drop by drop and stirred continuously.
• The white precipitate was washed repeatedly with deionized water and absolute ethanol to remove impurities.

2.2. Characterization:

• The phase structure and crystalline size of the samples were determined by X-Ray diffraction (XRD) using a PANalytical X'pertPro diffractometer with Cu-Kα radiation (wavelength of 1.5406 Å ) .
• The surface morphology of the samples were studied by SEM (Carl Zeiss SUPRA-55).
• UV–vis absorption spectra of all the samples were recorded by Shimadzu-UV 2450 spectrophotometer.
• The photoluminescence (PL) measurements were carried out by Perkin Elmer-LS 45 spectrofluorometer with an excitation wavelength 325 nm.
• Fourier transformed infrared (FTIR) spectra of the samples were recorded using a Shimadzu-FTIR spectrometer and room temperature magnetic measurements were obtained by LAKESHORE-7410 vibrating sample magnetometer (VSM).

3.1. Structural characterization

• From the d spacing values, the lattice constant ‘a’ and ‘c’ can be calculated [21] and their values are given in Table 1.
• The decrease in the crystallite size is mainly due to the doped Ti4+ ions that reduce nucleation and rate of growth of ZnO NPs.
• The dislocation density indicates the crystallinity of a crystal, and it will increase with increasing Ti concentration.

3.3 MORPHOLOGICAL ANALYSIS

• Figure 2 shows SEM images of pure and Ti doped ZnO nanoparticles.
• Random agglomeration with cluster shape were observed in all SEM images .
• (FTIR) Figure 3 represents the FTIR spectra for undoped and Ti doped ZnO nanoparticles.
• The band observed at 1400cm-1 is owing to OCO group asymmetric (C=O) and symmetric (C-O) stretching vibrations. [28].

3.5 UV -Vis spectroscopy

• The optical absorption properties of undoped ZnO and Ti doped samples were analysed using UV-VIS spectrometer.
• The redshift in the bandgap is due to the BursteinMoss effect [30].
• The conduction band lower levels are filled with free electrons generated When Ti 4+ ions replace the Zn2+ ions .
• Subsequently, it increase the Fermi Level and also widening the band gap , [31].

3.6. Photoluminescence (PL)

• Room temperature PL emission spectra of undoped ZnO, 5% , 10%,15% Ti doped ZnO samples.
• Technically, all the samples were excited at 325 nm.
• It is noticed that undoped and doped ZnO samples exhibits two peaks (i) 390nm (UV range) originates from the exciton recombination corresponding to the near-band edge (NBE) and (ii) 412nm (violet range), to be the recombination from the defect centerssuch as O and Zn interstitials.
• Defects such as structural defects or vacancies is the main reason for the emission of different deep-trap or colors [34] .
• Interstital zinc ond oxygen are ascribed to structural defects while vacancies are produced due to the presence of Zinc and oxygen vacancy [35].

3.7. Magnetic property:

• Pure and Ti doped ZnO samples magnetic properties were studied at room temperature shown in Figure 6.
• The plots of coercivity and retentivity as a function of dopant concentration are shown for all the samples.
• The room temperature ferromagnetism of the nanoparticles could arise due to extrinsic magnetism or intrinsic magnetism.
• The bound magnetic polarons (BMPs) theoretical model [37] is a defect-induce ferromagnetism model.
• The EDX spectrum confirmed the presence of Ti in ZnO nanoparticles.