Impact of cobalt doping on structural, electrical, magnetic and optical properties of Zn 1-x Co x O nanocomposites: Experimental and theoretical study

Doping of nanocomposites (NCs) with different metal oxide leads to a significant change in its structural, optical, electrical, mechanical, thermal, catalytic, and magnetic properties. The effect of the addition of CoO nanoparticles (NPs) on the structural, optical, electrical, and magnetic properties of Zn/CoO NCs have been investigated in detail. Zn 1-x Co x O NCs were synthesized by the sol-gel method followed by annealing at 400 0 C. The NCs were characterized by UV-visible, XRD, FTIR, TEM, and vibrating sample magnetometer (VSM) techniques. The structural and surface study was performed by X-ray diffraction and TEM techniques which shows spherical and cubical NCs with an average size of 25-55 nm. Computational study (DMol3, CASTEP, Forcite, and Reflex) was used to study the electronic and optical properties of metal NPs. The Co 2+ ions replace Zn 2+ ions in the ZnO lattice resulting a change in its structure from Wurtzite (ZnO NPs) to cubic Zn 1-x Co x O NCs. The lattice parameters, strain, and dislocation density were found to decrease with an increase in CoO concentration in Zn 1-x Co x O NCs. The saturation magnetization, retentivity, and coercivity were found to be lesser in Zn 1-x Co x O NCs. The synthesized cobalt doped Zn 1-x Co x O NCs can act as an efficient material for spintronic applications. on Zn 1-x Co x O NCs was thoroughly investigated by UV-visible, FTIR, XRD, TEM, W-H plots, and VSM techniques. The experimental study was supported by the theoretical (DFT) study. The optical band gap energy (theoretical) was found lesser in CoO as compared to ZnO NPs. The Co-doped ZnO NCs were cubic phase as compared to the Wurtzite structure of ZnO NPs and CoO NPs has secondary phase. As the CoO NPs concentration was increased, the intensity of the peak increases and the width decreases. The micro strain peak broadening was observed using a W-H plot and the dislocation density was calculated for each NCs. The micro-strain and dislocation density decrease with increase in the CoO concentration. The grain size increases uniformly due to the incorporation of CoO in ZnO matrix. The particle size of the crystallites increases from 14.93 to 28.78 nm due to growth of NCs involving the Co ions. FTIR analysis confirms the formation of defect centers in the structure. TEM images reveal the formation of NPs cluster that may be due to magnetic dipole interaction between ZnO NPs. The magnetic properties like retentivity, coercivity, and saturation magnetization of NCs decreases while the particle size increases. The structural, electronic, magnetic, and optical properties of Co-doped ZnO NCs prove that Zn 1-x Co x O nanocomposite will act as an efficient material for spintronic applications.


Introduction
The nanostructured materials have achieved special attention due to their novel properties. Controlling the dimensionality and morphology of materials have gained interest for designing functional devices having unique magnetic as well as optical properties. "The oxide semiconductors play a vital role in the area of photonic and spintronic. Recently, spintronics has drawn attention to make use of both spin and charge freedom of the carriers for the new devices. The spintronic is the emerging technology based on the concept of spin states (up and down) of electrons to carry the information that can be manipulated by an external applied magnetic field. The concept of spin helps in the designing of devices for high data processing, large integration capacity, reduced power consumption, and better stability. The transition metals doped semiconductors are useful in different research activities because of their usual optical properties and encouraging potential for applications in optoelectronic devices [1,2]. Among these semiconductors, few efforts have been made for the doping of ZnO nanocrystals with cobalt ions to extend the application potential [3,4]. ZnO is one of the hopeful materials for low voltage and short-wavelength optoelectronic applications like UV devices, light-emitting diodes, gas sensors, and laser diodes" [5][6][7][8][9]. The transition metals doped nanostructure is effective to adjust the energy levels and surface states of ZnO, which can introduce changes in structural and magnetic properties. In the present work, we have investigated the structural properties in cobalt doped ZnO nanocomposite prepared by a solgel technique which is a simple and low-temperature method, the possibility of making of finely dispersed powder and it yields a good end product. The synthesized samples are characterized by different techniques i.e., XRD, TEM, and UV-visible spectroscopy.
In continuation to our earlier work [10][11][12][13], here, we have reported the effect of cobalt NPs on the structural and optical properties of Zn1-xCoxO NCs. The Zn1-xCoxO (x = 0.3, 0.5, 0.7) NCs were synthesized by sol-gel technique at 80 C and annealed at temperatures 400 o C for 6 h.
The effect of CoO concentration on the structure of NCs was studied by FTIR, TEM, and XRD.

Computational study
The theoretical study of synthesized metal oxide NPs was carried out by Material Studio 2017, USA software. The electronic and optical properties were studied by geometry optimization, energy, energy density, frequency, and orbital studies by DMol3, CASTEP, Forcite, and Reflex modules. Fig. 11 and 12 shows DMol3 density of states (DOS), Forcite radial distribution function (RDF), Forcite X-ray intensity versus 2 (XRD), CASTEP band structure, CASTEP density of states (DOS), and 3D molecular crystal structure depicting reciprocal lattice and Brillouin zone paths of ZnO and CoO NPs obtained from Materials Studio 2017 software. Fig.   11 shows CASTEP band structure, CASTEP density of states (DOS), Forcite X-ray intensity versus 2 (XRD), Forcite radial distribution function (RDF), XRD, the spatial distribution of atoms, and 3D molecular structure showing reciprocal lattice and Brillouin zone paths of ZnO NPs. Fig. 12 shows CASTEP band structure, DMol3 DOS, Forcite radial distribution function (RDF), XRD, 3D crystal structure, spatial distribution of atoms, and 3D molecular structure showing reciprocal lattice and Brillouin zone paths of CoO NPs.

Results and discussion
The Zn1-xCoxO NCs with different ratio of cobalt as dopant material (x = 0, 0.3, 0.5, 0.7, 1) were synthesized. The characterization of NCs was performed by XRD, FTIR, VSM, and TEM techniques.

XRD study
The lattice parameters of hexagonal lattice were calculated by using Eq. (2).
The lattice parameters of cubic lattice were calculated by using Eq. (3).
The micro strain broadening and crystallite size were calculated from the W-H relationship Eq. For the determination of the crystallite size and strain contribution to the peak broadening, a graph is plotted between and 4 according to W-H eq. (5) shown in Fig. 1 The crystal size corresponding to the highest intensity peak was 26.19 nm and the corresponding lattice strain was 0.00042. Fig. 2(a) shows some extra and faint reflections which may be due to the traces of Co(OH)2 and are marked as impurities [19].
To calculate the size and strain contribution to the peak broadening in the lattice, a graph was plotted between β cos and 4 sin according to W-H Eq. (5) shown in Fig. 2 shows isotropic line broadening (Fig. 4). The diffracting domains were observed to be isotropic and there was a micro strain contribution.  The lattice parameters decrease slightly with an increase in CoO concentration in the NCs. The Co +2 ions (small radii) replaces Zn +2 in the ZnO crystal lattice and interstitial sites with a cubic crystal structure. The lattice parameters were calculated using the X-ray diffraction method for all the NCs [22]. The corresponding lattice parameters were tabulated in Table 1. The lattice parameters of the nanocomposite do not change significantly with an increase in CoO concentration, which shows that Co 2+ (0.58 Å) and Zn 2+ (0.6 Å) have almost identical radii [23].

FTIR spectra
The FTIR spectroscopy was used to find the purity and nature of NPs and NCs. The infrared spectra of Zn1-xCoxO (x = 0, 0.3, 0.5, 0.7, and 1) were used to provide the information of absorption bands to specific vibrational mode.  [24,25]. The Zn-O and Co-O stretching vibrations were seen at 500-1000 cm -1 . Fig. 5(b) shows the FTIR spectra of the CoO powder in the range of 4000-500 cm -1 . The absorption peak at 1756 cm -1 is due to the OH group in the metal alkoxides present in the gel and 1612 cm -1 shows bending vibration of H2O. The absorption peak at 1081, 806, 768 cm -1 stands for C-O stretching in presence of phosphine oxide. The peak at 567 cm -1 [26] was due to Co-O stretching and at 660 cm -1 corresponds to bridging O-Co-O bond [27].  Table 2). An increase in the concentration of CoO, the frequency of absorption swing towards lower wavenumber (redshift). A shift in the ZnO peak reveals that the ZnO NCs network was distressed by the addition of CoO. The FTIR study indicates that Co is replacing Zn atoms from the lattice site in the NCs matrix and the same was supported by XRD.  respectively. A close agreement in particle size was observed from TEM and XRD techniques.

TEM study
TEM image confirms that ZnO and CoO NPs fuse to increase the NCs size and volume.

Vibrating Sample Magnetometer (VSM) study
The magnetic properties of NPs and NCs of ZnO, CoO, and Zn0.3-CoO0.7 was studied VSM technique in the range of -10 to10 k Oe (Fig. 9). Fig. 9 shows the hysteresis curve of different metal NPs and their NCs. All the NCs show hard ferromagnetic behavior at room temperature.
The diamagnetic component was subtracted from the original data to determine the ferromagnetic part. The value of magnetic remanence (Mr) and Coercivity (Hc) of ZnO, CoO, and Zn-CoO NCs were shown in Table 2. The coercivity (Hc) increases and remanence (Mr) decreases with the addition of CoO NPs in ZnO NPs. The presence of ferromagnetic behavior in NCs was due to intrinsic coupling between the magnetic dipoles [30,31]. The possibility of ferromagnetic behavior in the NCs was due to the formation of metallic clusters and secondary phases due to traces of Co(OH)2 impurities.
The VSM shows that the ferromagnetic behavior of synthesized NCs decreases at room temperature. The CoO form clusters rather than substituting the Zn lattice sites. At higher CoO concentration, the possibility of the formation of CoO clusters increases.

UV-visible absorption study
To study the optical and electrical properties of CoO/ZnO NCs, a UV-visible absorption study was carried out for ZnO, CuO NPs, and CoO/ZnO NCs. Fig. 10 [32] given in Eq. (6).
Here, is the frequency of light, B is the band tailing parameter, Eg is the optical band gap, and is the optical index whose value is 2 and ½ for indirect allows and direct allowed, respectively.
The indirect bandgap for ZnO, CuO NPs, and CoO/ZnO (0.3/0.7) NCs was found to be 3.2, 2.7, and 2.1 eV, respectively. It was observed that the electrical conductivity of CoO/ZnO NCs (lesser optical band gap) was found to be higher than ZnO and CuO NPs.

Computational study
The theoretical study was carried out to investigate band structure, density of states (DOS),

XRD (theoretical), Radial Distribution Function (RDF), and Brillouin zone paths for CoO and
ZnO NPs (Figs. 11-12). Band structure shows the dependency of electronic states on the kvector and high symmetry vectors of the Brillouin zone. Band structure of CoO and ZnO NPs helps in qualitative analysis of the electronic structure and optical properties. Fig. 11 atoms for the distance (r). Fig. 11(C) shows RDF between Zn and O atoms. The maximum distance selected was 20 A 0 and the interval was 0.01 A 0 . The g(r) was found to have a maximum up to 12 A 0 after that it was almost constant. Fig. 11(D) shows the Forcite module for X-ray intensity versus 2 . The highest intensity peak in theoretical XRD spectra was observed at 2 = 36.4 for ZnO NPs. Fig. 11 (E, G, and G) shows the 3D molecular plane, 3D molecular crystal structure showing reciprocal lattice, and Brillouin zone paths, respectively for ZnO NPs. Fig. 12(A), shows the electronic states of CoO NPs. All the energies are related to the Fermi level. The maximum band gap observed in the case of CoO NPs was 0.157 eV. Fig. 12(B), shows CASTEP density of states (DOS) and the histogram for different k-points.
The radial distribution function (RDF) was carried out by the Forcite module for finding the probability function, g(r) between Co and O atoms for the distance (r). Fig. 12(C) shows RDF between Co and O atoms. The maximum distance selected was 20 A 0 and the interval was 0.01 A 0 . The g(r) was found to have a maximum up to 14 A 0 after that it was almost constant. Fig.   12(D) shows the Forcite module for X-ray intensity versus 2 . The highest intensity peak in theoretical XRD spectra was observed at 2 = 36.5 and 42.4 for CoO NPs. Fig. 12 (E, G, and G) shows 3D molecular crystal structure showing reciprocal lattice and Brillouin zone paths (E), 3D molecular plane (F), Ball and stick model, respectively for CoO NPs.

Conclusions:
The cobalt doped Zn1-xCoxO NCs were prepared by sol-gel route. The effect of CoO concentration on Zn1-xCoxO NCs was thoroughly investigated by UV-visible, FTIR, XRD, TEM, W-H plots, and VSM techniques. The experimental study was supported by the theoretical (DFT) study. The optical band gap energy (theoretical) was found lesser in CoO as compared to ZnO NPs. The Co-doped ZnO NCs were cubic phase as compared to the Wurtzite structure of ZnO NPs and CoO NPs has secondary phase. As the CoO NPs concentration was increased, the intensity of the peak increases and the width decreases. The micro strain peak broadening was observed using a W-H plot and the dislocation density was calculated for each NCs. The micro-strain and dislocation density decrease with increase in the CoO concentration.
The grain size increases uniformly due to the incorporation of CoO in ZnO matrix. The particle size of the crystallites increases from 14.93 to 28.78 nm due to growth of NCs involving the Co ions. FTIR analysis confirms the formation of defect centers in the structure. TEM images reveal the formation of NPs cluster that may be due to magnetic dipole interaction between ZnO NPs. The magnetic properties like retentivity, coercivity, and saturation magnetization of NCs decreases while the particle size increases. The structural, electronic, magnetic, and optical properties of Co-doped ZnO NCs prove that Zn1-xCoxO nanocomposite will act as an efficient material for spintronic applications.