Influence of Doping Concentration on the Zinc Doped Nickel Oxide Nanostructures: Morphological, Structural, and Optical Properties

The zinc doped nickel oxide (Zn:NiO) nanostructures with 0 at.% (UD), 1 at.% (1ZNO), and 2 at.% (2ZNO) of Zn concentrations were successfully deposited on the NiO seed-coated glass substrates. The films were successfully produced from two synthesis techniques: the sol-gel spin coating for NiO seed-coated and the solution immersion for the Zn:NiO nanostructures. The films were then pre-baked at 150 °C and subsequently annealed at 500 °C. The Zn doping concentrations affecting NiO in terms of morphological, structural, and optical properties were investigated. The surface morphologies and cross-sectional images of the Zn:NiO nanostructures were observed by field emission scanning microscopy. The observation showed that the groups of nanoflower (NF), which were grown above the nanosheet (NS) array were gradually decreased with the increasing percentage of the Zn doping. The thickness of the NS also drastically decreases as the Zn dopant is increased. The thicknesses measured are 910 nm, 410 nm, and 100 nm for UD, 1ZNO, and 2ZNO, respectively. The X-ray diffraction analysis showed the Zn:NiO peak intensities were affected and decreased as the Zn doping is increased. Take into account the significant differences in the (200) plane, the crystal parameters were calculated. The dislocation density, interplanar spacing, lattice parameter, and unit cell volume increased as the Zn doping increased. Meanwhile, the average crystallite size reduces when the percentage of Zn doping is increased. This condition makes the Zn:NiO nanostructures are promising for sensing applications due to enhanced surface area. The strain and stress values of the Zn:NiO showed the tensile strain and compressive stress, respectively. The transmittance spectra showed the transparencies in the visible region within 400 to 800 nm wavelength increases when the percentage of Zn doping is increased. The average transmittance percentages are 32.1 %, 62.8 %, and 67.0 % for UD, 1ZNO, and 2ZNO, respectively. The optical bandgap increases sharply as the Zn doping increased from 3.55 to 4.75 eV.


Introduction
Nickel oxide (NiO) exhibits p-type semiconducting properties and has a wide bandgap within the range of 3.6 to 4.0 eV with cation vacancies [1][2][3]. In the past few years, NiO nanostructures have demonstrated outstanding features in their structural, electrical, optical, magnetic, catalytic, and electrochemical properties [2,[4][5][6][7]. It is widely used in many electronic sections and devices such as field-effect transistors [8], superconductors [9], solar cells [10], and sensors [11][12][13]. Many efforts have been employed to customize the properties of NiO nanostructures by doping with various elements. Among the transition metal elements that can be engaged for doping with NiO are ferrum (Fe), aluminium (Al), chromium (Cr), manganese (Mn), and zinc (Zn). Zn is identified to be an attractive element for NiO because the properties of NiO nanostructures can be further improved to be used in certain applications. For instance, Fahmi et al. reported on Zn doped NiO for thin films prepared by sol-gel spin coating [14]. They investigated the effect of Zn on the structural and optical properties for the application of humidity sensors. Fomekong et al. investigated the Zn doped NiO to improve the gas sensors for formaldehyde detection [15]. Rani et al. studied the Zn doped NiO nanocluster electrocatalysts for electrochemical water splitting applications [16]. In this work, the morphological, structural, and optical properties were considered to view the influence of the Zn dopant on NiO nanostructures. The Zn doped NiO (Zn:NiO) nanostructures are limited to the Zn concentration of 0 at.% (undoped named UD), 1 at.% (1ZNO), and 2 at.% (2ZNO). The nanostructures were synthesized by using two techniques, which are the sol-gel spin coating and immersion techniques. They were then annealed at 500 °Ϲ in an ambient environment. It can be suggested that these nanostructures have the potential of being employed in several types of sensors.

Experimental Methods
At first, NiO seed-coated was deposited onto the glass substrates by the sol-gel using the spin coating technique. The sol-gel preparation involved nickel acetate as the precursor, ethylene glycol monoethyl ether as the solvent, and diethanolamine as the stabilizer, where they were mixed to produce a solution. The solution was stirred for 2 hours at 400 rpm before depositing onto the glass substrates using a spin coater at 4000 rpm. Before finally annealed at 400 °Ϲ for 2 hours, the resulting NiO seedcoated layer was pre-heated at 250 °Ϲ for 5 minutes. The spin-coating and pre-heating processes were repeated five times to improve and increase the seed-coated thickness. For the Zn:NiO nanostructures, the immersion technique was used. Each solution of NiO consists of nickel (II) nitrate hexahydrate, hexamethylenetetramine, and deionized (DI) water with different concentrations of Zn dopant. The dopant of Zn used is from zinc nitrate hexahydrate from Friendemann Schmidt. The solutions went through a sonication process using the ultrasonic bath and were stirred at 300 rpm using a magnetic stirrer at room temperature. Once both processes were completed, each solution containing UD, 1ZNO, and 2ZNO was transferred to the respective Schott bottles with NiO seed-coated glass substrates were inside. The Schott bottles were immersed in a water bath instrument for 2 hours at a temperature of 95 ºC. After that, the Zn:NiO nanostructure samples were removed from the Schott bottles and rinsed with DI water. Then, the Zn:NiO samples were pre-baked for 15 minutes at 150 ºC before annealed in a furnace at 500 ºC for 1 hour. The surface morphological studies of the Zn:NiO nanostructures were characterized using field emission scanning electron microscopy (FESEM) (model: Hitachi SU-8030). The structural properties were analyzed by X-ray diffraction (XRD) (model: PANalytical X'Pert PRO). The optical studies were characterized using the ultraviolet-visible (UV-vis) spectrophotometer (model: Jasco/V-670 EX).

Morphological Properties
As shown in Figure 1 (a) to (c), the FESEM images show the surface morphologies of the Zn:NiO nanostructures, which were grown on the NiO seed-coated glass substrates. As can be observed in Figure 1 (a), the surface morphology of the UD NiO shows the group of nanoflowers (NFs) was grown abundantly on the nanosheet (NS) array irregularly. The NF and NS structures of UD NiO at higher magnification are shown in Figure 1 17,18]. With the increase of Zn dopant percentage by 1 at.%, the number of NF began to decrease, as shown in Figure 1 (b). The condition becomes more pronounced with an increase of 2 at.% Zn as shown in Figure 1 (c), where the number of NF decreased drastically, and the NS array is increasingly visible. It can be said that, with the increasing of Zn doping percentage, the number of NF on the NS arrays decreased, and the morphology structures changed. The changes in the morphology of the synthesized nanostructures will also affect the optical properties [19][20][21]. The NS structure from a cross-sectional image of UD NiO is shown in Figure 1 (f). It can be seen that the NS has successfully grown above the NiO seed-coated substrate uniformly. The thicknesses of the Zn:NiO nanostructures were measured, and the values are 910 nm, 410 nm, and 100 nm for UD, 1ZNO, and 2ZNO, respectively. These values indicate the thickness decreases crucially as the Zn dopant concentration is increased. As mentioned by Chen et al., the thicker NiO films can develop a precise microstructure, and the crystallite size becomes larger [22].  Figure 2 shows the XRD patterns of the Zn:NiO nanostructures, which were annealed at 500 °C for 1 hour in the ambient environment. The observed peaks for all Zn:NiO nanostructures match with the polycrystalline NiO structure, which is indexed to the cubic β-NiO (JCPDS 47-1049). For the UD NiO, three peaks clearly observed at 2θ = 36.7 º, 42.7 º, and 62.5 º, which correspond to the (111), (200), and (220) crystal planes, respectively. It can be seen, the nanostructure of the UD NiO shows a good crystallization degree, which is represented by the sharpness and higher intensities of the peaks. Furthermore, the intensities of the peaks were gradually decreased as the Zn doping is increased. Only two weaker peaks were observed in both 1ZNO and 2ZNO samples that correspond to (111) and (200) crystal planes. Those peaks show lower intensities and broader compared to the peaks of UD NiO. The broad peak between 20 º to 40 º corresponds to the glass substrate [23]. The structural properties obtained from the XRD results are in parallel with the research reported by Manouchehri et al., where NiO crystallization is strongly influenced by zinc dopant concentration percentage [1]. More precisely, they said that when the Zn doping percentage increases, the crystalline structure of NiO gradually starts to vitiate. This behavior may be related to the crystal defects induced by the Zn doping increased. Besides, no other phases were detected on the pattern, which confirmed that the Zn:NiO nanostructures have a pure phase of the cubic NiO [24]. Manouchehri  crystallographic structure of NiO due to the ionic radii of Ni and Zn are more or less the same [1]. The ionic radii of Ni and Zn are 0.078 nm and 0.074 nm, respectively [25]. It causes Ni ions to be easily replaced with Zn ions without significant lattice distortion.  Table 1. The crystallite size (D) can be calculated using the Scherrer formula, as stated in Eq. (1).

=
(1) where k is a constant which is taken to be 0.94,  is the X-rays wavelength (1.542 Å), β is the full width at half maximum (FWHM), and θ is the angle of diffraction. From the calculations, the largest of D (21.4 nm) is owned by UD NiO, while 2ZNO owns the smallest of D (12.2 nm). The result shows that D decreases when the Zn doping is increased. According to Rani et al., the piecemeal decrease of D is because of the excess amount of dopant that can drag on the boundary motion and further slow down the kinetic system for the growth [16]. The decrease in D as the dopant concentration increased may also be attributed to the slightly smaller Zn 2+ ion atomic radius than the NiO host. The dislocation density (δ), interplanar spacing (d), lattice parameter (a), and unit cell volume (V) increase as the Zn doping is increased. Besides, the positive strain and negative stress values of these Zn:NiO nanostructures are considered as tensile strain and compressive stress, respectively [26,27].

Optical Properties
The optical properties of the Zn:NiO nanostructures are presented in Figure 3. As observed in the transmittance percentage over wavelength (300 -800 nm) in Figure 3 (a), the transmittance in the  [14]. In our work, the average transmittance values in the wavelength range between 400 to 800 nm were calculated where the values are 32.1 %, 62.8 %, and 67.0 % for the UD, 1ZNO, and 2ZNO, respectively. These values were then plotted in Figure 3 (b). The average transmittance plot clearly shows an upward trend along with an increase in Zn doping. The results show that the UD has the lowest transparency, while the 2ZNO has the highest transparency in the visible region. These results show the transparency is closely related to the thickness of the respective films, where the thinnest has the highest transmittance percentage. In contrast, the thickest has the lowest transmittance percentage. Figure 3 (c) shows the optical bandgap (E g ) of the Zn:NiO nanostructures, which were estimated by Tauc's plot. The E g values obtained were 3.55 eV, 4.50 eV, and 4.75 eV for UD, 1ZNO, and 2ZNO, respectively. The E g values revealed the abrupt increases as the percentage of the Zn doping increased. It shows that the optical properties of the NiO are significantly affected by the Zn doping concentration. According to Goel et al., the crystallite size changes may change the bandgap [2]. It can be seen through the bandgap values of 1ZNO and 2ZNO is outside the range of 4.0 eV. The obtained results can be due to many factors, such as the sharply reduced film thicknesses from 910 to 100 nm. In addition, the changes in the crystal structure as the Zn doping increased also caused the bandgap to increase [28].

Conclusion
The Zn:NiO nanostructures were grown on the NiO seed-coated glass substrates at different Zn concentrations by the sol-gel spin coating and immersion techniques. We have studied the changes in the NiO characteristics in terms of morphological, structural, and optical properties when doped with different concentration percentages of the Zn dopant. The growth of nanoflowers on the nanosheet arrays was gradually reduced with the increases of the Zn dopant percentage. The XRD patterns of the Zn:NiO showed significant influence on the crystallinity such as dislocation density, interplanar spacing, lattice parameter, and unit cell volume that increased as the Zn doping is increased. The crystallite size reduced as the Zn dopant increased. The strain and stress values showed the tensile strain and compressive stress due to the positive and negative signs, respectively. The optical properties revealed that the average transmittance value increased with the increase in the Zn dopant percentage. The bandgap energies were estimated at 3.55 eV, 4.50 eV, and 4.75 eV for UD, 1ZNO, and 2ZNO, respectively. In conclusion, these Zn:NiO nanostructures may have potential in applications of sensors such as gases and humidity due to their impressive properties in the morphologies. The highly porous Zn:NiO nanostructures enable a faster response during the adsorption and desorption processes in sensing activity. The crystallite size that becomes smaller when Zn doping is increased can provide a larger surface area and suitable for sensing activity.