In an article published in the open-access journal Crystals, researchers successfully prepared pure and chromium (Cr)-doped molybdenum trioxide (MoO3) microrods with the help of a novel sol-gel auto combustion technique to study the microstructure and dielectric properties of the synthesized samples. Scanning electron microscopy (SEM), an impedance analyzer of 1 MHz - 3 GHz, energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) were used to investigate the microstructure and optical and dielectric properties of the synthesized Cr-doped MoO3 samples.
The phase evaluation of the manufactured Cr-doped MoO3 samples was also conducted. The findings of the experiments revealed a hexagonal microstructure with a space group (P63) in all the synthesized samples.
A Williamson-Hall (W-H) plot was also prepared to evaluate the synthesized samples' micro-strain and crystallite size variables. The microstructure assessment revealed a proportional increase in the size of the Cr-doped MoO3 microrods with the Cr3+ contents. Contrarily, increasing Cr3+ levels were associated with a decreasing band gap energy which reduced to 2.71 eV from 2.98 eV. Finally, the study also investigated the frequency dependence of the tangent loss and dielectric constant of the Cr-doped MoO3 microrods.
Significance of MoO3 in the Modern Industrial Setting
Recent research innovations have used transition metal oxides for their excellent characteristics. They generate distinct phase microstructure as a result of their different metal-oxygen ratios. Among the other transition metal oxides, MoO3 shows excellent optical, electrical, and dielectric properties due to its unique atomic microstructure orientation.
MoO3 has an n-type semiconductor conductivity with an immense band gap energy of 2.8-3.6 eV. Primarily, MoO3 comes in three polymorphous forms, including- (i) MoO3 (monoclinic), (ii) MoO3 (orthorhombic), and (iii) h-MoO3 (hexagonal).
MoO3 has attracted much attention over the past few years due to its numerous applications in various industries. This interest results from molybdenum trioxide's many noteworthy properties, including its potent photocatalytic activity, ease of battery device assembly, and excellent performance with lithium storage. As a result, MoO3 is frequently used in the industrial sector in various applications, such as catalysts, field effect transistors, gas sensors, and battery electrodes.
Several studies have revealed that MoO3 has exceptional qualities and a wide range of applications in several other fields, including supercapacitors, organic light-emitting diodes (OLEDs), memory devices, solar cell equipment, and dielectric resonator devices.
The current study used the sol-gel auto combustion method to create Cr-doped MoO3 microrods. The impact of the (Cr+3) cation substitution on the microstructure and the band gap energy of MoO3 microrods was simultaneously assessed. Finally, X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and diffuse reflectance spectroscopy were used to evaluate the synthesized pure and Cr-doped MoO3 microrods.
Synthesizing Unique Cr-Doped MoO3 Microrods
Sol-gel auto combustion created the 3 wt% and 6 wt% Cr-substituted MoO3 microrods. As a chelating agent, nitric acid (HNO3) was utilized to create a homogeneous, simple solution. The solution was heated to 90 °C and agitated for one hour using a hot plate and magnetic stirrer to dissolve reactants.
The X-ray diffraction method examined the crystal microstructure of the produced pure and Cr-doped MoO3 microrods. With Cr-doped MoO3 microrods, it was found using the WinXpow software that the lattice parameters and lattice volume increased. The reflection of the 2q value of the X-ray diffraction was used to compute the average crystalline size of all samples using the Debye Scherer equation.
The findings demonstrated that the dielectric properties, including the dielectric constant (r) and dielectric loss (Tan) of un-doped and Cr-doped MoO3 microrods were frequency-dependent. Values of the MoO3 dielectric properties diminished as frequency and Cr3+ levels increased.
The dielectric properties (loss) also worsened as frequency and Cr3+ concentrations increased. It grew to its highest point before declining to a lesser value. The peak was seen between the frequency bands of 1.6 and 1.8 GHz. Dielectric properties tangent loss (tan) frequency graphs showed that the tangent loss (tan) resulting from the dipole relaxation phenomena diminished with frequency.
For all samples in this study, only indirect transition was considered. The plot of F(R)2 and energy revealed a predictable trend in the band gap energy of the samples. The study thoroughly analyzed the band gap energy of MoO3 microrods with and without Cr doping. Pure MoO3 had a more considerable band gap energy of 2.98 eV than MoO3, which was doped with Cr with a band gap energy of 2.71 eV.
A significant increase in the band gap energy was observed as the synthesized sample's particle size decreased, which declined as the concentration of dopants, such as Cr or Ni, increased. Band gap energy often decreased as doping mixtures were increased. Another option was the formation of oxygen vacancies, which could similarly lower the band gap energy of MoO3.
New Sol-Gel Technique Successfully Creates Cr-Doped MoO3 Microrods
This study successfully implemented the sol-gel auto combustion approach to create pure and Cr-doped MoO3 microrods. At the same time, the X-ray diffraction method was used to examine the crystal microstructure of the pure and Cr-doped MoO3 microrods. The results revealed that doping MoO3 with Cr3+ ions significantly changed the microstructure lattice parameter values, average crystallite size, and the average microstrain.
The phase evaluation proved that a hexagonal microstructure with P63 symmetry was formed. The optical band gap energy of 2.71, 2.80, and 2.98 eV was recorded for six wt%, three wt%, and zero wt% Cr-doped MoO3 microrods, respectively.
Studies on the dielectric properties demonstrated that the dielectric constant and tangent loss were both frequency and concentration-dependent. The findings also revealed that MoO3 was a suitable host material for all transition metals or minerals applied to optoelectronic devices.
Zaman, A., et al. (2022). Effect of Cr Doping on the Structural, Optical and Dielectric Properties of MoO3 Microrods Synthesized by Sol-Gel Auto Combustion Method. Crystals, 12(9), 1259. https://www.mdpi.com/2073-4352/12/9/1259/htm