Effect of Cr Doping on the Structural, Optical and Dielectric Properties of MoO 3 Microrods Synthesized by Sol-Gel Auto Combustion Method

: In the present work, pure and Cr-doped MoO 3 microrods were successfully prepared through the sol gel auto combustion method. The phase evaluation and microstructural, dielectric, and optical properties of synthesized samples were investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and an impedance analyzer (1 MHz–3 GHz). All the samples showed hexagonal structure with space group (P63). According to Vegard’s law, lattice parameters increase with the increase in chromium (Cr 3+ ) contents. In addition, the Williamson–Hall (W–H) plot was drawn for evaluating the micro-strain ( ε W-H ) and crystallite size (D W-H ) parameters. From microstructural analysis it was found that the size of microrods increased along with Cr 3+ contents. Decreasing band gap energy was observed (from 2.98 to 2.71 eV) with increasing Cr 3+ contents. The variation of the dielectric constant and tangent loss of MoO 3 microrods with respect to frequency were analyzed.


Introduction
Transition metal oxides have been used in recent research innovations due to their excellent properties. They produce different phase structures due to their different metaloxygen ratios. Molybdenum trioxide (MoO 3 ) has good electrical, optical, and microwave dielectric properties due to its structural orientation [1]. MoO 3 has wide band gap energy (2.8-3.6 eV) with n-type semiconductor conductivity [2]. Basically, there exist three polymorphous structures of MoO 3 : β-MoO 3 (monoclinic), α-MoO 3 (orthorhombic), and h-MoO 3 (hexagonal). Thermodynamically, the structure of α-MoO 3 is very stable and attractive for practical uses because of its anisotropic compositions along the (010) direction [3]. This anisotropic chemistry is created by stacking a bi-layer sheet of octahedral Mo 2 O 6 bonded through Van der Waal forces. Due to intrinsic structural anisotropy and the ability of transformation of oxidation states between molybdenum-IV and V ions, α-MoO 3 (orthorhombic) has been verified to have exceptional applications in the fields of catalysis, field emissions, lithium-ion batteries, energy storage devices, gas sensing, and electrochromic and photochromic devices [4,5].
Over the last couple of years, molybdenum trioxide has received extensive attention because of its many applications in various fields. The various significant aspects of molybdenum trioxide have led to this interest, including strong photo catalytic ability, battery device assembly, and Li storage performance [6]. As a result, MoO3 is widely employed in industry, including catalysts [7], field effect transistors [8], gas sensors, and battery electrodes [9]. Many researchers reported that MoO 3 has outstanding properties and vast uses in the field of super capacitors, memory devices, OLEDs (organic lightemitting diodes), dielectric resonator devices, and solar cell equipment [10][11][12][13][14][15][16][17]. MoO 3 has three polymorphic phases: (a) orthorhombic, (b) hexagonal, and (c) monoclinic. Of these phases, the orthorhombic phase is more stable, and along with bi-layered octahedral distortion it has good electrical, optical, and magnetic properties [18]. Many researchers study it and found it in the form of nanowires, nanorods, thin film, microrods, quantum dots, and nanobelts [19][20][21][22][23].
In the current research work, we focused on synthesizing chromium Cr-doped MoO 3 microrods and use the sol gel auto combustion route. The effect of (Cr +3 ) cation substitution is simultaneously evaluated on structural and microstructural development and band gap energy of MoO 3 microrods. Correspondingly, the prepared pure and Cr-doped MoO 3 microrods were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and diffuse reflectance spectroscopy (DRS).

Experimental Method
The Cr-substituted MoO 3 microrods with 3 wt% and 6 wt% were prepared by the sol-gel auto combustion technique. The stoichiometric amounts of MoO 3 (purity 99.97%, Sigma-Aldrich chemicals, St. Louis, MI, USA) and chromium chloride (purity 99.5%, Sigma-Aldrich chemicals, St. Louis, MI, USA) including those with with nitric acid were used to prepare the product powders. We used the nitric acid (HNO 3 ) as a chelating agent to obtain a homogenous and straightforward solution. The solution was stirred with the help of a hot plate and magnetic stirrer at 90 • C for one hour to dissolve reactants in distilled water. Then, the concentration of chromium chloride (CrCl 3 ) (3 wt%, and 6 wt%) was doped into the solution. At that point, the solution was stirred magnetically at 90 • C for 6 h, adding ammonia drop by drop to maintain the pH value at 6-7. At the end, the solution changed into a viscous brown gel and then self-combustion happened, as shown in the graphical abstract. After complete crushing and grinding, the fine powder was then used in vacuum furnace sintering at 500 • C for 2 h in the nitrogen and hydrogen atmosphere. X-ray diffractometer (XRD) (JDX-3532, JEOL, Japan) was used for studying the crystal structure of all fabricated samples functioned with Cu-K radiations of wavelength λ = 0.1540598 nm, at 45 × 10 3 V and 40 × 10 −3 A in the 2θ range of 5-60 • . The microstructures functional at 20 KV were analyzed using a scanning electron microscope (SEM) (JSM-5910, JEOL).

Phase Analysis
The crystal structure of the synthesized pure and Cr-doped MoO 3 microrods was investigated by XRD method, as shown in Figure 1a,b. The observed diffraction peaks revealed the formation of a single-phase base composition MoO 3 that corresponded to PDF card no. 00-021-0569, which shown the hexagonal crystal structure with space group (P63) [24,25]. The sharp and intense peaks depict the crystalline behavior of all the samples. Among these planes, the (2 1 0) plane at 26.07 • revealed the maximum intensity. The base Crystals 2022, 12, 1259 3 of 12 sample was indexed properly by using WinXpow software. It can also be observed that the lattice parameters as well as lattice volume increased with Cr-doped MoO 3 microrods. Compared with the pure MoO 3 sample, the corresponding diffraction peaks shifted to lower angles, the intensity of peaks increased, and the width of peaks also increased following the doping of Cr 3+ contents. Moreover, the change of the peaks' positions could be ascribed to the inhomogeneity and micro-strain in the samples or may be due to the substitution of the relatively larger ionic radius of Cr 3+ (R Cr = 0.61 Å) than Mo (R Mo = 0.59 Å), which is consistent with the deduction of Bragg's diffraction law (2dSinθ = mλ), as shown in Figure 1b [26,27].

Phase Analysis
The crystal structure of the synthesized pure and Cr-doped MoO3 microrods was investigated by XRD method, as shown in Figure 1a,b. The observed diffraction peaks revealed the formation of a single-phase base composition MoO3 that corresponded to PDF card no. 00-021-0569, which shown the hexagonal crystal structure with space group (P63) [24,25]. The sharp and intense peaks depict the crystalline behavior of all the samples. Among these planes, the (2 1 0) plane at 26.07° revealed the maximum intensity. The base sample was indexed properly by using WinXpow software. It can also be observed that the lattice parameters as well as lattice volume increased with Cr-doped MoO3 microrods. Compared with the pure MoO3 sample, the corresponding diffraction peaks shifted to lower angles, the intensity of peaks increased, and the width of peaks also increased following the doping of Cr 3+ contents. Moreover, the change of the peaks' positions could be ascribed to the inhomogeneity and micro-strain in the samples or may be due to the substitution of the relatively larger ionic radius of Cr 3+ (RCr = 0.61 Å) than Mo (RMo = 0.59 Å), which is consistent with the deduction of Bragg's diffraction law (2dSinθ = mλ), as shown in Figure 1b [26,27].  (1) where 'D' is the average crystallite size, the value of Scherer constant 'k' is 0.94, λ = 0.15406 nm is the wavelength of the X-ray beam, and is the full-width half. This technique is used with XRD data, where the crystallite size (D) is associated with the expansion of The Debye Scherer equation is used to calculate the average crystalline size of all samples using the reflection of the 2θ value of the XRD data [28], where 'D' is the average crystallite size, the value of Scherer constant 'k' is 0.94, λ = 0.15406 nm is the wavelength of the X-ray beam, and β is the full-width half. This technique is used with XRD data, where the crystallite size (D) is associated with the expansion of intensive peaks. The instrumental contribution of β was removed before the analysis of the crystallite size and micro-strain. Figure 2 presents the behavior of crystallite size. The crystallite size depended on the lattice micro-strain and radius of the substituted ions. It has been observed that generally the average crystallite size decreases with the increasing Cr 3+ content (x), and this may be attributed to the growth of the crystal structure, which may be due to that the ionic radius of the substitution element, Cr 3+ , is larger than molybdenum [26]. intensive peaks. The instrumental contribution of was removed before the analysis of the crystallite size and micro-strain. Figure 2 presents the behavior of crystallite size. The crystallite size depended on the lattice micro-strain and radius of the substituted ions. It has been observed that generally the average crystallite size decreases with the increasing Cr 3+ content (x), and this may be attributed to the growth of the crystal structure, which may be due to that the ionic radius of the substitution element, Cr 3+ , is larger than molybdenum [26]. = + 4 (2) where ′ ′ is the full width at half maximum (FWHM), ′ ′ is the Braggs angle, ′ ′ is the size factor, ′ ′ is the wavelength of X-rays, and 'D' is the average crystallite size. Finding the slope of a linear plotted graph against 4 gives the information about the lattice strain and crystalline size (DW-H) for all the samples (un-doped and Cr-doped), as shown in Figure 3 Table 1. Around the fitted line, the points are noticed to be widely scattered. It was observed that certain additional parameters of the characterized sample were not taken into consideration or some alternative method should have been adopted.  [29].
where 'β' is the full width at half maximum (FWHM), 'θ' is the Braggs angle, 'k' is the size factor, 'λ' is the wavelength of X-rays, and 'D' is the average crystallite size. Finding the slope of a linear plotted graph against 4sinθ gives the information about the lattice strain and crystalline size (D W-H ) for all the samples (un-doped and Cr-doped), as shown in Figure 3 Table 1. Around the fitted line, the points are noticed to be widely scattered. It was observed that certain additional parameters of the characterized sample were not taken into consideration or some alternative method should have been adopted.    Figure 4a,b it can be clearly observed from SEM images that the size of microrods grow with the increase in Cr content. The surfaces of the synthesized rods are clean, but their crystal structures are found in spherical shape whose sizes are in the range of microns, which indicates its one-dimensional hexagonal rod geometry [6,31]. Figure 4c shows irregular surface structure microrods. Basically, these are the small microrods that are stacked together in a cluster shape, and their appearance shows to develop on the microrods' surface [32]. It is clear from the image that the nanorods have poor morphology because the lengths and diameters of the nanorods are not uniform. The diameter of the microrods varies from 0.07 µm to 0.21 µm, seen by

Surface Morphology
Figure 4a-c show the surface micrographs of un-doped and Cr-doped MoO 3 microrods with (×20,000) magnification, respectively. In Figure 4a,b it can be clearly observed from SEM images that the size of microrods grow with the increase in Cr content. The surfaces of the synthesized rods are clean, but their crystal structures are found in spherical shape whose sizes are in the range of microns, which indicates its one-dimensional hexagonal rod geometry [6,31]. Figure 4c shows irregular surface structure microrods. Basically, these are the small microrods that are stacked together in a cluster shape, and their appearance shows to develop on the microrods' surface [32]. It is clear from the image that the nanorods have poor morphology because the lengths and diameters of the nanorods are not uniform. The diameter of the microrods varies from 0.07 µm to 0.21 µm, seen by using ImageJ software. In addition, they are not well aligned in the direction perpendicular to the substrate. The reason for that may be that the surface of the substrate may not be smooth at the microscale or due to the miss-matching lattice structure, which greatly affects the morphology of microrods [33].
using ImageJ software. In addition, they are not well aligned in the direction perpendicular to the substrate. The reason for that may be that the surface of the substrate may not be smooth at the microscale or due to the miss-matching lattice structure, which greatly affects the morphology of microrods [33].     Figure 4b,c confirm the presence of Cr elements. The 3 wt% and 6 wt% of Cr was doped in the solid solution of MoO 3 microrods. These compositions of Cr by atom % were calculated by using the following formulae: Crystals 2022, 12, 1259 7 of 12 where C 1 is the atom %, C 1 is weight %, and A 1 is the atomic number of the un-doped elements, while C 2 is the atom %, C 2 is weight %, and A 2 is the atomic number of the doped elements. Equations (3) and (4) represent the conversion of wt% into atom % of two elements. The values of all the samples by weight % and atom % are shown in Table 2.

Diffuse Reflectance Spectroscopy
The optical absorption characterization of pure and Cr-doped MoO 3 microrods was performed by diffuse reflectance spectra (DRS) in the range of 200 to 800 nm, as depicted in Figure 5. It was observed that there was a strong reflectance behavior between 450 nm and 500 nm that revealed the high absorption behavior within the visible region [34]. The sharp characteristic absorption band edge was found to be around 325 to 475 nm for each rod and the transition of the band gap was attributed to it by absorbing light from the visible range. The reflectance spectra demonstrated that the reflection percentage increased from 350 nm to 800 nm for upper absorption. The 340 nm spectrum indicated that the reflectance percentage was decreased due to the absorption behavior of the sample [35].

Optical Properties
Figure 6a-c show that the defect-free band structure was identified and the optical band gap was derived from the Kubeka-Munk function, which is [36]; where 'F(R)' is the re-emission function, 's(λ)' is the scattering coefficient, 'K(λ)' is the absorption coefficient, 'hν' is the photon energy, 'Eg' is the band gap energy, 'R' is the

Optical Properties
Figure 6a-c show that the defect-free band structure was identified and the optical band gap was derived from the Kubeka-Munk function, which is [36]; where 'F(R)' is the re-emission function, 's(λ)' is the scattering coefficient, 'K(λ)' is the absorption coefficient, 'hν' is the photon energy, 'Eg' is the band gap energy, 'R' is the diffuse reflectance, and 'n' is the exponent term that identifies the transition types, i.e., indirect transition (n = 1/2), direct transition (n = 2), indirect forbidden energy gap transitions (n = 1/3), and direct forbidden energy gap transition (n = 2/3). In this work, only indirect transition was considered for all samples. The band gap energy was predictable from the plot of F(R) 2 and energy. Figure 6d shows the band gap energy of the un-doped and Cr-doped MoO 3 microrods. The energy band gap of the pure MoO 3 (2.98 eV) is greater than that of the Cr-doped MoO 3 (2.71 eV). The band gap energy increases due to decreasing the particle size of the synthesized sample and reduces with increasing the dopants' concentration, i.e., Cr or Ni [37][38][39]. Generally, band gap energy decreases with increasing doping compositions. There is also the possibility of creating oxygen vacancy, which can also reduce the band gap energy of MoO3. The band gap energy values of pure and doping samples are shown in the Table 3. Table 3. Data on structural and physical parameters of un-doped and Cr-doped MoO3 microrods.  There is also the possibility of creating oxygen vacancy, which can also reduce the band gap energy of MoO 3 . The band gap energy values of pure and doping samples are shown in the Table 3.  Figures 7 and 8 show the frequency dependence of the dielectric constant (ε r ) and dielectric loss (Tanδ) of un-doped and Cr-doped MoO 3 microrods. The value of the dielectric constant of MoO 3 decreases with the increases in frequency as well as Cr 3+ (Figure 7). This might be due to the alignment of permanent dipoles with the direction of the electric field at lower frequencies, which contributes to the dielectric material's total polarization. On the other hand, the dipole can no longer follow the field at higher frequencies since the field rapidly varies [40]. The dielectric loss of MoO 3 increases with the increases in frequency as well as Cr 3+ contents ( Figure 8). It increases to a maximum value, after which it goes to a lower value. The peak is observed between 1.6-1.8 GHz frequency ranges. This may be due to the dielectric relaxation phenomena occurring in the compound [41]. The tangent loss (tanδ) caused by the dipole relaxation phenomena decreases with frequency, as seen in the tanδ frequency plots [42]. which it goes to a lower value. The peak is observed between 1.6-1.8 GHz frequency ranges. This may be due to the dielectric relaxation phenomena occurring in the compound [41]. The tangent loss (tanδ) caused by the dipole relaxation phenomena decreases with frequency, as seen in the tanδ frequency plots [42].

Conclusions
In the current study, pure and Cr-doped MoO 3 microrods were successfully synthesized via the sol gel auto combustion method. The average crystallite size, lattice parameter, and average micro-strain value change when Cr 3+ ions are doped in MoO 3 were found. The formation of a hexagonal structure with P63 symmetry was confirmed by phase analysis. The average crystallite size (D) also decreased from 76.69 nm to 70.57 nm. The optical band gap energies of 2.98, 2.80, and 2.71 eV were recorded for 0 wt%, 3 wt%, and 6 wt% Crdoped MoO 3 microrods, respectively. The surface morphology of all the samples revealed the formation of microrods with different sizes (small and large). The micropores were observed in the group of hexagonal microrods. Dielectric studies showed that both the dielectric constant and tangent loss are frequency-as well as concentration-dependent. The obtained results declared that MoO 3 is an appropriate host material for all the transition metals or minerals which are used for the application of optoelectronic devices.