Structural, Optical, and Magnetic Properties of Cobalt-Doped ZnAl2O4 Nanosheets Prepared by Hydrothermal Synthesis

Nanomaterials with collective optical and magnetic properties are called smart or functional materials and have promising applications in many fields of science and technology. Undoped and Codoped ZnAl2O4 were prepared using a co-precipitation-assisted hydrothermal method. A systematic investigation was carried out to understand the effects of the Co concentration on the crystalline phase, morphology, and optical and magnetic properties of Co-doped ZnAl2O4. X-ray diffraction confirmed the cubic spinel structure with the Fd3m space group, and there was no impurity phase. X-ray photoelectron spectroscopy of Co-doped ZnAl2O4 confirmed the existence of Zn, Al and O, and the Co in the optimized sample of ZAO-Co-3 confirmed the oxidation state of cobalt as Co2+. Transmission electron microscopy of pure and Co-doped ZnAl2O4 revealed micro-hexagons and nanosheets, respectively. The optical absorption results showed that the bandgap of ZnAl2O4 decreased with increasing Co concentration. The hysteresis loop of Co-doped ZnAl2O4 revealed clear ferromagnetic behavior at room temperature. The as-prepared materials are suitable for energy storage applications, such as in supercapacitors and fuel cells. This work aims to focus on the effect of cobalt ions in different concentrations on structural, optical and magnetic properties.


Introduction
Over the past few decades, spinel-structured metal oxide nanomaterials have attracted considerable attention because of their unique crystal structure and optical properties [1][2][3][4][5]. Spinel semiconductors are comprised of a group of metal oxides having a chemical complex AB 2 O 4 , where A and B are di-and trivalent cations, respectively. Among the many spinelbased nanomaterials known, zinc aluminate (ZnAl 2 O 4 ) has attracted interest owing to its utility in various fields, such as ceramics, sensors, catalysts, aerospace, lubricants, optical coatings, transparent conducting oxides (TCO), dielectrics, electronics, and optoelectronic devices [6][7][8][9][10][11]. ZnAl 2 O 4 spinel is a natural mineral known as gahnite. ZnAl 2 O 4 belongs to the Fd3m space group, having a cubic structure and wide bandgap of 3.8 eV. ZnAl 2 O 4 has several advantages, such as high mechanical strength, high chemical and thermal stability, high quantum yields, and catalytic activity [12]. These advantages and the wide bandgap of ZnAl 2 O 4 make it a potential candidate for numerous applications in materials science and technology [11]. The huge application potential of spinel ZnAl 2 O 4 has prompted many studies on understanding and enhancing the fundamental properties.
Several investigations have reported that the incorporation of a transition metal or rare-earth element into the ZnAl 2 O 4 spinel lattice enhances many important properties [6,[13][14][15][16]. In particular, the optical properties of ZnAl 2 O 4 spinel nanostructures may be improved by modifying the bandgap. The bandgap of ZnAl 2 O 4 spinel can be tuned by the incorporation of transition metal ions with controllable optical properties, which offers superior potential applicability in optoelectronics [17]. Previous reports have shown that the optical properties of nanostructured materials depend strongly on the particle size, crystal structure, morphology, uniform distribution of activators in the host lattice, and synthesis method [4,[18][19][20]. Several approaches have been attempted for the synthesis of ZnAl 2 O 4 spinel nanostructures, such as co-precipitation [21], solid-state reaction [22], microwave-assisted hydrothermal methods [23], and sol-gel [2,[24][25][26], solvothermal [27], hydrothermal [5,28], solution combustion [4,29], and mechanical alloying methods [30]. Among these, the hydrothermal method is a suitable method for synthesizing ZnAl 2 O 4 spinel nanostructures because of its high crystallinity, purity, and homogeneity. Moreover, the homogeneity of dopants in the spinel matrix improves the optical properties significantly. In this context, this study adopted a co-precipitated-assisted hydrothermal method for the synthesis of undoped and Co 2+ -doped ZnAl 2 O 4 spinel nanostructures. To better understand the effects of the dopant concentration on the size, optical, and magnetic properties of ZnAl 2 O 4 spinel nanostructures, undoped and Co 2+ -doped ZnAl 2 O 4 spinel nanostructures were characterized using a range of spectroscopic techniques, such as X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron microscopy (XPS), ultraviolet-diffuse reflectance spectroscopy (UV-DRS), and vibrating sample magnetometry (VSM).

Materials and Methods
All chemicals used in this experiment (Zn(NO 3 ) 2 , Al(NO 3 ) 3 , Co(NO 3 ) 2 ·6H 2 O, ethanol, ammonia (25%), deionized water) were analytical grade and used without further purification. All the chemicals were purchased from Sigma-Aldrich Co., Ltd., and the purity of all the chemicals was ≥98%. The samples of 0%, 1%, 3% and 5% Co-doped ZnAl 2 O 4 nanostructures were prepared via the hydrothermal method. This involves the mixing of nitrate ions acting as oxidizing agents. Zn(NO 3 ) 2 .6H 2 O (0.2 mol%) was dissolved in 20 mL of deionized water on a hot plate at 40 • C. Al(NO 3 ) 3 .9H 2 O (0.4 mol%) was dissolved in 20 mL deionized water and stirred magnetically. Then, 20 mL of zinc solution was added dropwise to the aluminum solution with continuous stirring; 20 mL of Co(NO 3 ) 2 ·6H 2 O solution (0.01, 0.03 and 0.05 mol%) was then added dropwise into the above mixture solution with continuous stirring for 20 min. After this, ammonia solution (25%) was added to the solution drop by drop by adjusting the pH to 9 and stirring for 60 min till a white precipitate occurred. The obtained solution was transferred into a Teflon-lined stainless steel autoclave and sealed tightly; then, it was introduce into a muffle furnace at 220 • C for 8 h. The white precipitate solution was washed and centrifuge thrice with deionized water, ethanol and water, respectively, then dried at 80 • C for 6 h. After drying, the resultant powder was annealed at 1000°C for 5 h and ground manually in order to get fine particles. The obtained samples were named ZAO, ZAO-Co-1, ZAO-Co-3 and ZAO-Co-5, respectively. An undoped sample was prepared by the same procedure as above without cobalt nitrate, and the sample was named ZAO.
Powder XRD (Shimadzu 6100, Japan) patterns of the prepared samples were recorded using CuKα radiation. The surface morphology of the as-synthesized samples was examined by field emission scanning electron microscopy (FESEM, Hitachi, S-4800, Japan) and transmission electron microscopy (TEM; Hitachi, H-7600, Europe). The optical absorption spectra were obtained using a Cary 5000 UV-vis-NIR spectrophotometer. The elemental distribution and their oxidation states on the surface were examined by X-ray photoelectron spectroscopy (XPS; Thermo Scientific K-alpha, South Korea). The magnetic hysteresis loop was obtained via vibrating sample magnetometer (VSM; Lake Shore 7407, USA). cubic spinel ZnAl 2 O 4 phase (JCPDS file no.: 05-0669). No characteristic XRD peaks of any impurity phase, such as Co clusters or Co oxide, were observed, confirming the successful incorporation of doped Co 2+ ions into the Zn 2+ ion sites without distorting the crystal structure of zinc aluminate. The average crystalline size (D) was calculated using Scherrer's equation, D = Kλ/β cosθ, where λ = wavelength of X-ray used, k = 0.9, and β = width at half maximum. The induced lattice strain was calculated using the expression ε = βcosθ/4. The lattice cell parameters and interplanar spacing were also calculated. Figure 1 shows the XRD patterns of ZnAl2O4 and ZnAl2-xCoxO4 (x = 0.00, 0.01, 0.03 and 0.05) nanoparticles. The XRD pattern of ZnAl2O4 revealed peaks at 31.32°, 36.61°, 44.62°, 48.90°, 55.54°, 59.11°, 64.99°, 74.04°, and 77.17° 2θ, which were indexed to the (2 2 0), (3 1 1), (4 0 0), (3 3 1), (4 2 2), (5 1 1), (4 4 0), (6 2 0), and (5 3 3) crystal planes of the single-phase spinel ZnAl2O4, respectively. The XRD planes could be indexed to the cubic spinel ZnAl2O4 phase (JCPDS file no.: 05-0669). No characteristic XRD peaks of any impurity phase, such as Co clusters or Co oxide, were observed, confirming the successful incorporation of doped Co 2+ ions into the Zn 2+ ion sites without distorting the crystal structure of zinc aluminate. The average crystalline size (D) was calculated using Scherrer's equation, D = Kλ/β cosθ, where λ = wavelength of X-ray used, k = 0.9, and β = width at half maximum. The induced lattice strain was calculated using the expression ε = βcosθ/4. The lattice cell parameters and interplanar spacing were also calculated.  Table 1 lists the average crystallite size, lattice strain, lattice cell parameters, and interplanar spacing of ZnAl2O4 and Co-doped ZnAl2O4. The XRD peaks shifted slightly towards lower angles than the pure ZnAl2O4 ( Figure 1). The interplanar spacing increased with a blue shift of the XRD angles, which may have arisen due to the smaller ionic radius of Co 2+ (0.65 Å) than that of Zn 2+ (0.74 Å). Moreover, a small positive shift in interatomic spacing and expansion in the lattice parameters occurred with increasing doping concentration. The average crystallite size of ZnAl2O4 shifted from 26 to 17 nm. The micro-strain of the ZnAl2O4 lattice increased with increasing Co doping concentration. Therefore, the particle size and lattice parameters can be controlled by the incorporation of foreign elements into the host lattice [26].   Table 1 lists the average crystallite size, lattice strain, lattice cell parameters, and interplanar spacing of ZnAl 2 O 4 and Co-doped ZnAl 2 O 4 . The XRD peaks shifted slightly towards lower angles than the pure ZnAl 2 O 4 ( Figure 1). The interplanar spacing increased with a blue shift of the XRD angles, which may have arisen due to the smaller ionic radius of Co 2+ (0.65 Å) than that of Zn 2+ (0.74 Å). Moreover, a small positive shift in interatomic spacing and expansion in the lattice parameters occurred with increasing doping concentration. The average crystallite size of ZnAl 2 O 4 shifted from 26 to 17 nm. The micro-strain of the ZnAl 2 O 4 lattice increased with increasing Co doping concentration. Therefore, the particle size and lattice parameters can be controlled by the incorporation of foreign elements into the host lattice [26].

TEM Analysis
The morphology of ZAO and Co-doped ZAO nanosheets was examined by TEM, as shown in Figure 2. TEM of the pristine ZAO sample revealed a clear hexagon-shaped structure (Figure 2a). The ZAO hexagons showed slightly different sizes, but they were uniform in shape. Not all the sides of the hexagons were under 100 nm. Therefore, they can be considered micro hexagon structures. The reason behind the uniformity in shape might be the adopted hydrothermal procedure and the precursor concentration ratios.

TEM Analysis
The morphology of ZAO and Co-doped ZAO nanosheets was examined by TEM, as shown in Figure 2. TEM of the pristine ZAO sample revealed a clear hexagon-shaped structure (Figure 2a). The ZAO hexagons showed slightly different sizes, but they were uniform in shape. Not all the sides of the hexagons were under 100 nm. Therefore, they can be considered micro hexagon structures. The reason behind the uniformity in shape might be the adopted hydrothermal procedure and the precursor concentration ratios. Cobalt doping into ZAO resulted in a significant modification of the morphology of ZAO. First, Co-doping changes the morphology from micro-hexagon structures to the nanoscale, as nanosheets. Second, the doping of cobalt disturbed the hexagon structure. This might be due to the successful doping of cobalt ions into the ZAO crystal lattice. All the Co-doped ZAO samples showed clear nanosheet-like morphologies compared to the undoped ZAO sample. Moreover, the increasing concentration of cobalt ions increases the distribution/separation of the nanosheets. The nanosheets of ZAO-Co-1 (Figure 2b) were densely packed to each other, but as the cobalt ion content increased further, the Cobalt doping into ZAO resulted in a significant modification of the morphology of ZAO. First, Co-doping changes the morphology from micro-hexagon structures to the nanoscale, as nanosheets. Second, the doping of cobalt disturbed the hexagon structure. This might be due to the successful doping of cobalt ions into the ZAO crystal lattice. All the Co-doped ZAO samples showed clear nanosheet-like morphologies compared to the undoped ZAO sample. Moreover, the increasing concentration of cobalt ions increases the distribution/separation of the nanosheets. The nanosheets of ZAO-Co-1 (Figure 2b) were densely packed to each other, but as the cobalt ion content increased further, the nanosheets were well separated, as observed in the case of ZAO-Co-3 and ZAO-Co-5 (Figure 2c,d). TEM confirmed that the doping of cobalt reduced the micro-hexagon structures to wellseparated nanosheets.

XPS Analysis
XPS was applied to ZAO-Co-3 to evaluate the oxidation states and confirm the constituent elements of the prepared samples. Figure 3a provides details of the XPS survey spectrum, confirming the existence of Zn, Al, O, and Co in the optimized sample of ZAO-Co-3. Figure 3b presents the high-resolution XPS spectrum of Zn. This spectrum showed two prominent XPS peaks at binding energies of 1021.66 and 1044.70 eV. These two peaks at 1021.66 and 1044.70 eV were assigned to Zn 2p 3/2 and Zn 2p 1/2 , which confirmed the presence of Zn in the +2 oxidation state. The binding energy difference between Zn 2p 3/2 and Zn 2p 1/2 was 23.04 eV, confirming the presence of Zn 2+ [31]. The XPS peak at 74.1 eV in Figure 3c was attributed to Al 2p. The oxidation state of Al was confirmed to be 3+, and these Al 3+ cations were bonded to oxygen as Al-O [32]. Figure 3d shows the core-level XPS spectrum of O 1s with prominent and broad XPS peaks at 530.93 eV and a small intense peak at 532.81 eV. The O 1s peak at 530.93 was assigned to the contribution of oxygen and was attributed to Zn-O and Al-O bonds in the ZnAl 2 O 4 lattice. The low intense XPS peak at 532.81 eV indicates an oxygen deficiency in the ZnAl 2 O 4 crystal lattice [33], showing that some oxygen vacancies were present in the prepared samples. Figure 3e presents the high-resolution XPS spectrum of Co 2p, showing two major peaks at 781.2 and 796.43 eV, which were attributed to Co 2p 3/2 and Co 2p 1/2 eV, respectively [34]. In addition, two more satellite peaks were observed at approximately 786.75 and 802.73 eV. These outcomes confirm the presence of cobalt in mixed oxidation states of 2+ and 3+. The energy difference (∆E) between Co 2p 3/2 and Co 2p 1/2 eV was 15.23 eV, which further confirmed the oxidation state of cobalt as Co 2+ .

DRS Analysis
DRS, as shown in Figure 4, examined the optical properties of undoped ZAO and Codoped ZAO nanosheets. The optical absorption depends on many parameters, such as the bandgap, oxygen deficiency, and impurity centers. The bandgap of pristine ZAO was estimated to be 3.04 eV from its sharp absorption edge [35]. The doping of the ZAO matrix with cobalt ions causes changes in the optical properties of ZAO. The successful doping of Co 2+ ions into the ZAO crystal system decreases the bandgap of ZAO with increasing

DRS Analysis
DRS, as shown in Figure 4, examined the optical properties of undoped ZAO and Co-doped ZAO nanosheets. The optical absorption depends on many parameters, such as the bandgap, oxygen deficiency, and impurity centers. The bandgap of pristine ZAO was estimated to be 3.04 eV from its sharp absorption edge [35]. The doping of the ZAO matrix with cobalt ions causes changes in the optical properties of ZAO. The successful doping of Co 2+ ions into the ZAO crystal system decreases the bandgap of ZAO with increasing cobalt content. The bandgaps of ZAO-Co-1, ZAO-Co-3, and ZAO-Co-5 were estimated to be 2.72, 2.37, and 2.23 eV, respectively. These bandgaps of the Co-doped samples were lower than the undoped ZAO sample, which was also expected due to the quantum limiting effect. The redshift in the bandgap of Co-doped ZAO was attributed to the sp-d interaction between the electrons in the band and local electrons in the d-shell of the substituted Co 2+ . This may lead to a decrease in the conduction band potential and an increase in the valence band potential, leading to a decrease in bandgap [1]. In addition, the intensity of absorption increased with increasing Co content. The absorption edge of the Co-doped ZAO samples extended into the visible region. The new energy bands in the visible region arose due to the formation of new energy levels from Co ion-doping. The addition of energy bands in the visible region was assigned to the dopant Co ions as the inter-atomic d-d charge transfer of Co 2+ cations [36]. The additional energy bands at approximately 406.6, 478.2, 546.5, and 620.7 nm were ascribed to 4T 1g (F)→2A 1g (G), 4T 1g (F)→4T 1g (G), 4T 1g (F)→4A 2g (F), and 4T 1g (F)→2T 1g (G) energy transitions, respectively [37]. These outcomes of bandgap reduction and addition energy bands in the visible region and the increased intensity of additional bands confirmed the successful doping of cobalt ions into the ZnAl 2 O 4 crystal system.

VSM Analysis
The magnetic behaviors of ZnAl2O4 and Co-doped ZnAl2O4 nanocrystals were investigated by VSM, as shown in Figure 5. Pure ZnAl2O4 nanocrystals showed no hysteresis loop, indicating its non-magnetic behavior [6]. On the other hand, the Co-doped ZnAl2O4 nanocrystals exhibited a clear ferromagnetic hysteresis at room temperature. Table 2 lists the saturation magnetization (Ms), retentivity (Mr), and coercive field (Hc) values. All Codoped ZnAl2O4 nanocrystals exhibited a soft ferromagnetic nature with a low coercive field. The saturation magnetization and retentivity values increased gradually with increasing dopant concentration, and reached a maximum at ZAO-Co-3, then decreased with further increases in Co concentration. This indicates that most of the Co ions remain uncoupled in ZAO-Co-5 and do not contribute to the ferromagnetic phase. The ferromagnetic behavior depends mainly on the Co dopant concentration. There are three main reasons for the ferromagnetism in the Co-doped ZnAl2O4 nanocrystals. One is the formation of a secondary phase, such as Co 2+ clusters or oxides. XRD confirmed that Co 2+ ions were incorporated into the ZnAl2O4 nanocrystals at the Zn 2+ sites, and there was no possibility

VSM Analysis
The magnetic behaviors of ZnAl 2 O 4 and Co-doped ZnAl 2 O 4 nanocrystals were investigated by VSM, as shown in Figure 5. Pure ZnAl 2 O 4 nanocrystals showed no hysteresis loop, indicating its non-magnetic behavior [6]. On the other hand, the Co-doped ZnAl 2 O 4 nanocrystals exhibited a clear ferromagnetic hysteresis at room temperature. Table 2 lists the saturation magnetization (Ms), retentivity (Mr), and coercive field (Hc) values. All Co-doped ZnAl 2 O 4 nanocrystals exhibited a soft ferromagnetic nature with a low coercive field. The saturation magnetization and retentivity values increased gradually with increasing dopant concentration, and reached a maximum at ZAO-Co-3, then decreased with further increases in Co concentration. This indicates that most of the Co ions remain un-coupled in ZAO-Co-5 and do not contribute to the ferromagnetic phase. The ferromagnetic behavior depends mainly on the Co dopant concentration. There are three main reasons for the ferromagnetism in the Co-doped ZnAl 2 O 4 nanocrystals. One is the formation of a secondary phase, such as Co 2+ clusters or oxides. XRD confirmed that Co 2+ ions were incorporated into the ZnAl 2 O 4 nanocrystals at the Zn 2+ sites, and there was no possibility of secondary phase formation [38]. The second one is the carrier-induced ferromagnetism, i.e., the increase in free charge carriers due to the doping of Co2+ ions into ZnAl2O4 nanocrystals [39]. Another reason for the ferromagnetism is the presence of defects, such as oxygen and zinc vacancies or interstitials, in the host lattice [40]. The ferromagnetic behavior of Co-doped ZnAl 2 O 4 nanoparticles may be due to the incorporation of Co 2+ ions into the Zn 2+ sites and the exchange-couple interaction between Co 2+ ions and Zn 2+ ions [41]. As the Co concentration increases, the interaction becomes stronger, and the saturation magnetization increases.

Conclusions
Pure and Co-doped ZnAl2O4 were synthesized using a co-precipitation-assisted hydrothermal method at 220 °C. The pure and Co-doped ZnAl2O4 exhibited a single-phase cubic spinel structure without any secondary phases. The average crystallite size decreased with increasing Co concentration, and the lattice strain increased. XPS revealed the existence of constituent elements in the prepared sample. The energy difference (ΔE) between Co 2p3/2 and Co 2p1/2 was 15.23 eV, confirming the Co 2+ oxidation state. TEM showed that the successful incorporation of Co ions converts the micro-hexagon structure of ZnAl2O4 to a nanoscale sheet-like structure. UV-DRS revealed a decrease in the bandgap of ZnAl2O4 with increasing Co content. The exchange couple interaction between Co 2+ and Zn 2+ ions enhanced the ferromagnetic behavior with increasing Co content. The successful doping of cobalt into ZnAl2O4 nanosheets can be useful in many energy-related applications.
Author Contributions: Conceptualization and writing-original draft preparation: T.R.G.; method-

Conclusions
Pure and Co-doped ZnAl 2 O 4 were synthesized using a co-precipitation-assisted hydrothermal method at 220 • C. The pure and Co-doped ZnAl 2 O 4 exhibited a single-phase cubic spinel structure without any secondary phases. The average crystallite size decreased with increasing Co concentration, and the lattice strain increased. XPS revealed the existence of constituent elements in the prepared sample. The energy difference (∆E) between Co 2p 3/2 and Co 2p 1/2 was 15.23 eV, confirming the Co 2+ oxidation state. TEM showed that the successful incorporation of Co ions converts the micro-hexagon structure of ZnAl 2 O 4 to a nanoscale sheet-like structure. UV-DRS revealed a decrease in the bandgap of ZnAl 2 O 4