Thermal-Induced Effects on the Structural and Photocatalytic Properties of Nickel Oxide Nanoparticles for Indigo Carmine Dye Removal

Nickel oxide (NiO) nanoparticles were formed using the chemical precipitation method. The effect of the calcination process on the structural parameters, optical bandgap, and photocatalytic performance was investigated. The structural characteristics were carried out using X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), and scanning electron microscope (SEM). The XRD analysis reveals that the formed NiO crystallized in an fcc crystal structure and the calcination process influences the crystallite size, microstrain, dislocation density, and average surface area. For example, the smallest and largest particle sizes (19.13 nm and 27.63 nm) were achieved for the samples prepared at 800 °C for 4 h and 900 °C for 2 h, respectively. Based on the diffuse reflectance spectroscopy analysis, the energy bandgap has the lowest values (3.33 eV) for the prepared NiO that calcinated at 800 °C for 2 h compared with other samples. The formation of a Ni–O stretching vibration mode is revealed by FTIR, and the broadness of the absorption band confirms that the NiO samples are nanocrystals. The morphology of the prepared NiO reveals the formation of spherical nanoparticles for NiO calcinated at 700 °C, while dodecahedron-like shapes were observed for NiO calcinated at 800 and 900 °C. The photocatalytic performance of NiO nanoparticles as catalysts for the degradation of indigo carmine dye was investigated under ultraviolet–visible irradiation up to 3 h. The best degradation efficiency was found to be 76% for NiO calcinated at 800 °C for 4 h, which belonged to the smallest crystallite size of 19.13 nm, and the highest surface area of 47.02 m2 g−1. The superior and excellent performance of this sample compared to other samples was confirmed by achieving the highest reaction rate constant (4.51 × 10−3 min−1). The proposed photodegradation mechanism shows the importance of increasing the time required for the recombination process between the positive holes and the excited electrons, which is the best possible when using the optimum photocatalyst sample that was prepared at 800 °C for 4 h.

Nickel oxide (NiO) nanoparticles are mostly utilized in nanotechnology applications such as ultraviolet (UV) photodetector, photoluminescence, solar cells, gas sensors, light-emitting diodes, photocatalysts, and so on [8][9][10][11][12][13][14]. NiO is a p-type semiconductor with a bandgap of around 3.6 eV. NiO has been identified as an excellent UV-protective material at wavelengths lower than 375 nm. Also, its bandgap and associated energy are quite similar to those of TiO 2 [10]. Although colloidal deferral's photocatalytic performance has been researched in various studies, its utility as an ecological solution has not been explored or altered [4,5]. The literature's conclusions compare NiO's photocatalytic efficiency to TiO 2 's photocatalytic efficiency [19][20][21]. NiO has the advantage of radiating exceptionally clearly in the visible region. Earlier photochemical experiments looked at the emission properties of NiO nanoparticles based on their size [22][23][24][25]. A visible emission of NiO occurs as a result of anionic vacancies, which are sensitive to hole hunters. Quantitatively, the hole foragers, such as iodide ions, quench the emission [26]. The synthesis of a novel catalyst process that can simultaneously sense and destroy harmful pigments is a desirable aspect for decontaminating air and water has been described [27][28][29][30][31][32]. The photocatalysis process is activated when the system detects an increase in the presence of aromatic compounds in the environment. The sensing properties and biological applications of NiO have been the focus of a lot of research in recent decades [33][34][35]. Whereas NiO is a photocatalyst with restricted efficiency due to the wedding band gap, its photocatalytic efficiency in the visible part of the solar spectrum is ineffective [36]. The calcination of the material at various temperatures and/or times can improve photocatalytic activity and boost sensing applications of calcinated material due to the increased uniformity.
Researchers have been exploring the optical, magnetic, and photocatalytic properties of pure and doped NiO with transition metals such as Co, Cd, Mn, Cu, Cr, V, and Fe, as well as Re elements, for decades. Abo Zeid et al. found that Cd-doped NiO nanoparticles improved the discoloration of methyl orange [37], and Junaid et al. presented a ferromagnetic material made of Fe-doped NiO at ambient temperature [38]. Al Boukhar et al., found a modest blue shift in the optical spectra of Re-doped NiO samples when compared to pure NiO, implying that the optical bandgap has increased [39]. They are investigating the magnetic characteristics of NiO nanoparticles at room temperature and have shown that antiferromagnetic and weak ferromagnetic ordering coexists in both pure and RE 3+ -doped NiO nanoparticles [39]. The addition of Mn to the NiO lattice increases the magnetic characteristics substantially, according to Layek and Verma [40]. Cuong et al., also, improved the catalytic activity of Codoped highly porous NiO nanorods [41]. The production of NiO nanoparticles and their structural characteristics were shown to be highly dependent on the calcination temperature by Teoh et al., [42]. Varunkumar et al., examine the effects of calcination temperatures of 350 °C, 450 °C, and 550 °C for 3 h on phase formation, particle size, and bandgap evolution on as-prepared Cu-doped NiO powder samples [42]. The particle size of the nanoparticles increased from 4 to 9 nm as the calcination temperature was increased from 350 to 550 °C for the Cu doped NiO samples [43]. Patterson et al. showed that increasing the calcination temperature improves the morphological and optical features of Fe doped NiO samples and causes them to crystallize. It has also been demonstrated that the volume of the samples has increased, resulting in a modest bandgap [44].
The indigo carmine (IC) dye is often employed as a textile coloring agent as well as an additive in pharmaceutical tablets and capsules, as well as for medical diagnostic reasons [45]. Indigo carmine is not easily digested but is easily filtered by the kidneys [46]. The IC dye is an extremely poisonous indigoid dye and contact with it can cause skin and eye irritation as well as irreversible cornea and conjunctival harm [47]. Various published studies on the removal of IC dye from wastewater utilizing metal oxides such as TiO 2 , Nb 2 O 5 , and ZnO have been conducted for this purpose [44][45][46][47][48].
In this work, the effects of calcination temperatures of 700, 800, and 900 °C for 2, 3, and 4 h on the structural characteristics of NiO nanoparticles and photocatalytic efficiency in eliminating IC dye from wastewater were investigated. For enhanced photocatalytic performance, the optical band gap was regulated by adjusting the calcination temperature and time. After the structural parameters have been researched and the optical band gap has been determined, the photocatalytic degradation mechanisms, for eliminating IC dye from the wastewater, were investigated based on Langmuir-Hinshelwood kinetics.

3 2 Experimental Details
The pure NiO nanoparticles were synthesized via the chemical precipitation method. In this process, 0.5 M nickel nitrate, Ni(NO 3 ) 2 ·6H 2 O, was dissolved in 100 ml double distilled water for 30 min while stirring. A fresh solution of potassium hydroxide, KOH, (1.5 M diluted in 100 ml deionized water) was supplied dropwise to a nickel solution under continuous stirring at 60 °C for 2 h as a precipitating agent. The Ni(OH) 2 green precipitate was filtered and washed with distilled water and 98% ethanol many times. The produced sample was dried overnight in an air oven at 100 °C before being pulverized several times. To develop the morphology, and structural parameters of NiO and hence enhance their catalytic performance, the calcination process was carried out in a muffle furnace at 700, 800, and 900 °C for 2, 3, and 4 h.
The structural characteristics of NiO nanoparticles were performed by the XRD measurements using a Shimadzu X-600 (Cu-K α , λ = 1.54 Å) from Japan. The spectral data were collected in the 2θ = 30°-80° range at a scanning step and rate of 0.02° and 0.06° s −1 , respectively. A scanning electron microscope model SEM-JSM 6360 LA, Japan, was used to examine the morphology of the samples. Fourier-transform infrared spectroscopy (FTIR) spectra were obtained in the 400-4000 cm −1 range using Thermo Scientific Nicolet iS50 FTIR equipment to analyze the formed bond in NiO nanoparticles. The absorption spectra of ultraviolet-visible light through suspension solution of NiO were recorded using a UV-vis-NIR spectrophotometer model UV-3600.
Due to the high environmental importance of the treatment process of water pollutants, the photochemical decomposition of the IC was chosen to evaluate the photodegradation efficiency using the prepared NiO, and then select the most active sample that achieves the highest degradation rate in a shorter time. Xenon lamp, purchased from Engineering company, limited-ARE, was used as a light source in the procedures of the degradation experiments. The measured received intensity on the catalyst samples was 70 W cm −2 . The first step in the process is the formation of a suspended solution by addition of 100 mg of the prepared NiO catalyst into 100 ml of IC dye with selected concentration 8 × 10 −6 M, then it is stirred for 45 min to achieve complete adsorption equilibrium between NiO nanoparticles sample and the IC dye. The second stage of the work includes the process of irradiating the previously prepared suspension solution for 3 h with continuous stirring to avoid the accumulation of the photocatalyst and thus reduce the chances of any side effects of the destruction process. During the process of exposure to light, 5 ml of the mixture was taken at fixed interval times of 30 min. The efficiency of the photocatalytic activity of the first sample was carried out by following the decrease in the absorption value of the organic dye using spectrophotometer model Shimadzu-UV260 Japan, and its percentage was calculated from the following formula: η(%) = × 100 , where η is the degradation rate percent while C o and C t are the concentration or absorption values of IC dye before and after the irradiation process, respectively. For the remaining eight NiO samples, the same steps are repeated, and then we compared the obtained results to reach the ideal sample to be used as a catalyst. Figure 1 shows the XRD charts for NiO nanoparticles heattreated at temperatures of 700, 800, and 900 °C for 2, 3, and 4 h, respectively, as indicated in Fig. 1a-c. In our prior research, we investigated the structural parameters of NiO as it was prepared [37]. The NiO nanoparticles are normally crystalline, and their crystallinity varies depending on the calcination temperature and/or duration. For all calcination temperatures or times, the observed diffraction peaks belong to the face-centered cubic (fcc) crystal structure of NiO nanoparticles, which is well-matched with the standard JCPDS Card No: 04-0835 [41]. The diffraction peaks are found at 2θ = 37.22°, 43.33°, 63.06°, 75.37°, and 79.38°, respectively, corresponding to the Miller indices of (111), (200), (220), (311), and (222). The existence of the typical diffraction peaks of NiO in the fcc crystal structure form may be seen in XRD charts of calcinated NiO. The well-known Debye Scherrer's equation [43,49]: D = 0.9 ∕ cos , was used to calculate the average crystallite size (D) of the produced phase, where λ is the wavelength of radiation employed in Cu K α (1.5406 Å), β is the full width at half-maximum (in rad), and θ is the angle at the maximum peak's position (in rad). The active surface area (S) was calculated using the Sauter formula: S = 6000∕D , and where ρ is the density of NiO nanoparticles. This equation is better suited to spherical particles. The microstrain ( ) was also calculated using the Williamson-Hall method [50]: cos = 0.9 D + 4 sin . The plot of cosθ versus 4sinθ yields straight lines and the value of the microstrain for the NiO phase in the analyzed samples may be calculated from the slope value of the straight line. Furthermore, the presence of dislocation density has a significant impact on crystallographic characteristics. The relationship between crystallite size and dislocation density ( ) is: Table 1 summarized the results for crystallite size, microstrain, and dislocation density for the investigated NiO nanoparticles.

Structural and Electronic Parameters
Calcination at the proposed temperatures and periods reveals a variety of patterns. For example, increasing the calcination temperature from 700 to 800 °C for 2 and 3 h results in a minor drop in the crystallite size, which is subsequently marginally increased for a further rise in the   47.02 m 2 g −1 , has reached its maximum value. The grain size was increased by agglomeration between neighboring NiO nanoparticles, while the crystallite size was reduced by the particle divider to smaller particles. The change in micro-strain is inversely proportional to the change in the crystallite size. As seen in Table 1, the number of dislocation lines per unit area inside nanoparticles reduces as particle size increases. The estimated values of the equivalent surface area for various NiO nanoparticles are tabulated in Table 1.
The effects of broadening on the particle size distribution of NiO nanoparticles grow in direct proportion to the calcination temperature. As a result of inferring the function of the calcination temperature, less uniform NiO nanoparticles are produced, influencing the broadness of the particle size distribution [51]. The nucleation rate of the particles grows more rapidly as the temperature rises from 800 to 900 °C. This can be explained by an increase in reaction product supersaturation, which speeds up the crystal core formation process and increases crystallite size [52].
The FTIR for various NiO nanoparticles calcinated at 700, 800, and 900 °C for 2, 3, and 4 h is shown in Fig. 2. The goal of the study is to figure out what functional group is present in pure NiO nanoparticles. The band O-H stretching vibrations are responsible for a large absorption band centered at 3440 cm −1 is reduced and became broader which is because calcinated nanomaterials tend to physically absorb water [53]. The disappearance of the characteristic peak at 2330 cm −1 corresponds to the presence of CO 2 at raising the calcination temperature from 700 to 800 °C and then to 900 °C. This result is consistent with the literature reports [41]. In other words, when FTIR sample discs were made in the open air The intensity of a faint band about 1647 cm −1 ascribed to the mode H-O-H bending vibrations was also found to decrease with increased calcination temperature [53]. Other detected peaks at 3133-3138 cm −1 , 1381 cm −1 , 1015 cm −1 , 882 cm −1 , and 829 cm −1 are related to the stretching vibration of the OH group in all NiO samples [54,55]. The serrated absorption bands in the region of 1000-1500 cm −1 are assigned to the O-C=O symmetric and asymmetric stretching vibrations and the C-O stretching vibration, as well as the C-O stretching vibration. Furthermore, the low intensity of this band suggests that the ultrafine powers have a high physical absorption to H 2 O and CO 2 . Furthermore, these observed peaks might be generated by water content, asymmetric stretching of C=O, the vibration of adsorbed CO 2 , C-H stretching, and N-H bending mode. The intensity of the stretching vibrations of Ni-O has appeared in the range 400-600 cm −1 increases with the raising in the calcination temperature [56]. These two peaks are due to the Ni-O stretching vibration mode, and the broadness of the absorption band suggests that the NiO particles are nanocrystals. Because the crystallite size of NiO in nanoparticles was significantly smaller than the  crystallite size of NiO in bulk form, the IR peak of Ni-O stretching vibration in NiO nanoparticles was moved to the blue direction. Furthermore, due to the quantum size effect and spherical nanostructures, the FTIR absorption of NiO nanoparticles is blue-shifted relative to that of the bulk form.
Optical absorption qualities are known to be related to the optical energy gap (E g ). As a result, we drew the Kubelka-Munk function F(R) using UV-visible diffuse reflectance [57]: F(R) = (1−R) 2 2R , here F(R) is the Kubelka-Munk function or the absorption coefficient, and R is reflectivity. The function F(R) can be rewritten in the following expression: , where A is an arbitrary constant, hv is photon energy (E), and n is the probability of transition and equals 2, and ½ for a direct bandgap, and an indirect bandgap, respectively [58]. Figure 3 shows selected plots of the relation between [F(R)E] 2 and E for NiO nanoparticles calcinated at 700, 800, and 900 °C for 4 h. The tangent lines of the [F(R)E] 2 against photon energy (E), and can be used to calculate E g , as illustrated in Fig. 3. Table 1 summarizes the values of E g for various NiO nanoparticles that were calcinated at various temperatures and periods. NiO nanoparticles calcinated at 800 °C for 2 h and NiO calcinated at 700 °C for 3 h had the lowest (3.33 eV) and highest (3.71 eV) optical bandgap values, respectively. The optical band gap values calculated are agreed with those published by Karthik et al. [59]. As a result, the bandgap adjusts the volume fractions for a variety of applications, including photovoltaics, photocatalysis, and thermoelectric. The decreasing bandgap values of NiO nanocrystalline are linked to grain size, shape, and structural faults that can be regulated by the calcination process, it may be concluded.
A reduction in the energy band gap value is related to increasing calcination temperature, most likely owing to a quantum size impact. It has been proposed that the reduction in band gap is caused by transitions between the valance and conduction bands of Ni 2+ ion d-shell electrons [60]. The bandgap values increase as the calcination temperature rises, which can be attributed to the high crystallinity of the samples prepared at 900 °C, which eliminates the possibility of creating additional sub-bandgap energy levels caused by surface defects and abundant interfaces in the agglomerated particles [61]. The bandgap value is determined by several elements, including crystallite size, structural parameters, carrier concentrations, impurity presence, and lattice strain [62]. Figure 4 depicts the surface morphology of the NiO nanoparticles were examined by the SEM. The morphology shape of NiO nanoparticles calcinated at 700 °C for 4 h revealed sphere-shaped nanoparticles composed of unevenly shaped pieces clustered together, as shown in Fig. 4a. Micrographs of NiO samples calcinated at higher temperatures, such as 800 °C and 900 °C for 4 h, show distinct grains, which differ from the spherical nanoparticles, with increased particle size, as shown in Fig. 4b and c, respectively. In the case of 800 °C for a 4 h sample, a dodecahedron-like shape appeared, which was attributed to the optimal temperature and time for good dispersion and agglomeration of the prepared nanostructures. These dodecahedron-like shapes, as well as some nanoparticles, are concentrated at the surface. As the number of nanoparticles increases, so does agglomeration, and as the temperature rises, so does the accumulation of grains, which reduces the dispersity of the particles at the surface. The average particle size estimated using ImageJ equal 0.25 ± 0.05 μm, 0.75 ± 0.1 μm, and 0.25 ± 0.02 μm for NiO nanoparticles calcinated at 700, 800, and 900 °C for 4 h, respectively.

The Photocatalytic Performance of NiO Nanoparticles
Another goal of this research is to use it as a catalyst for treating wastewater containing IC dye. The IC dye is considered one of the most famous dyes of the indigo family, which is a source of water pollution and consequently leads to many diseases such as diarrhea, nausea, and vomiting. at calcination temperatures of 700, 800, and 900 °C, and at calcination times 2, 3, and 4 h, respectively, by photodestruction process of IC dye. It is commonly observed that a peak at λ = 609 nm characterizes the absorbance in IC dye, and the intensity of this peak decreases as the UV-visible irradiation time increases up to 3 h. Or by other words, by fixing the light source, intensity, and UV-visible irradiation time (Xenon lamp and for 3 h), as well as the concentration of the selected dye solution, the results clearly show that the degradation rate in the presence of NiO was prepared at 700 °C equal 56%, 60%, and 66% for 2 h, 3 h, and 4 h, respectively. In the next stage of the work, when the calcination temperature was increased to 800 °C, a rise in the decomposition rate was observed, where the calculated values increased to 68%, 71%, and 76% for 2 h, 3 h, and 4 h, respectively. By using NiO nanoparticles with the highest calcination temperature of 900 °C in the last step of the process, a significant decrease was found in the degradation rate of the dye solution, where the values are decreased to 47%, 50%, and 53% for 2 h, 3 h, and 4 h, at UV-visible irradiation time of 3 h, respectively. The NiO samples with 800 °C have a superior efficiency compared to samples with 700 °C due to the distinct improvement that occurs in the crystalline properties of the catalyst when the calcination temperature and time increase. For example, the crystallite size was reduced, while the active surface area was increased. This observation was confirmed by the specific energy gap values, which are given the lowest values for the photocatalyst samples that were prepared at 800 °C. Also, at the excellent calcination temperature, the catalytic behavior of the catalyst develops, and its efficiency increases due to the improvement of the charge separation process, which leads to an increase in the time required to electron-hole recombination, which subsequently leads to the increased possibility of formation of effective oxidizing radicals (OH· and O 2 · − ), which is a key species in the decomposition process of environmental organic pollutants [63]. The deterioration of the catalytic efficiency of the prepared samples with 900 °C compared to the samples with 800 °C can be attributed to the phenomenon of the reverse effect, which occurs when the calcining temperature is raised to a certain degree, where the increase in the temperature and time of the calcination processes leads to the accumulation of nanoparticles and a decrease in the effective surface area and photocatalytic activity [64,65].  Figure 8 depicts capture experiments of active species in the photocatalytic degradation of IC dye using NiO catalysts after 3 h of UV-visible irradiation. The sample designated F or calcinated at 800 °C for 4 h showed the most deterioration. To examine the catalytic activity of NiO nanoparticles further, the degradation rate constants of IC dye were obtained using Langmuir-Hinshelwood kinetics. The formula that can be used to compute the disintegration rate of a pseudo-first-order response is: ln [66], where k denotes the degradation rate constant, K is the adsorption equilibrium constant, and k app is the apparent rate kinetic constant, also known as the pseudo-first-order reaction constant. The response rate was calculated using k app = −ln C t C 0 ∕t . Table 2 summarizes the k app values determined for various NiO nanoparticles. The greater catalytic performance of the optimal sample of NiO (Sample F) was explained by achieving the highest values of reaction rate constants of 4.51 × 10 −3 min −1 , and the lowest values for the half-life compared with other prepared catalysts as presented in Table 2 and these values were calculated from Fig. 9. Figure 10 presents the mechanism of NiO nanoparticles' action as a photocatalyst and its effect on the photodecomposition process of organic dye. Crystal properties, surface area, energy bandgap, and particle size are the basic components of the photodegradation process that can effectively contribute to the increase in photocatalytic efficiency [67]. High crystallinity and large surface area are two main requirements to raise the rate of photocatalytic performance, as they contribute to increasing the time required for the process of recombination of the excited electron-hole and the absorption of the substrates, respectively [68]. We can present the degradation process in the presence of the prepared NiO samples in the form of a set of stages. The first step is to use the light source to irradiate the surface of the catalyst. As a result of the previous step, the electrons in the valence band are excited and then transferred to the conduction band. The process of electron transfer is accompanied by the formation of positive holes equal to the number of excited electrons, which gives an ideal opportunity to form a pair of positive holes-negative exciting electrons [69]. This step shows the significant effect of the energy gap value on increasing or decreasing the efficiency of the catalyst, as it is noted that the most active samples are the least in the energy gap value and thus it is the one that achieves more time for the process of re-formation of the resulting free radicals. At  this stage, the electrons at the conduction band and holes at the valance band generated by irradiation diffuse to the surface and caused further surface reactions. The generated electrons react with oxygen to give superoxide radical anions. As well as the separated holes react with water to form the OH radicals. Further, the generated radicals are highly reactive and cause the degradation of the selected organic dye. These created radicals represent the basic elements in the process of photo-destruction of the organic dye indigo cochineal through their superior ability to convert the organic pollutant into environmentally friendly compounds, namely carbon dioxide and non-polluting water [63,70]. We can express reactions and processes that occur during the photodegradation process of IC dye as follows:

Conclusion
The chemical precipitation method was successfully used to synthesize NiO nanoparticles in this study, and then their structural parameters and optical bandgap are controlled by the calcination process. The XRD analysis reveals that the formed NiO crystallized in an fcc crystal. The smallest and largest particle size (19.13  (4) Organic dye(IC) + two produced active radicals → environmentally friendly compounds CO 2 and non-polluting water irradiation for up to 3 h, the photocatalytic performance of NiO nanoparticles as catalysts for the degradation of IC dye was investigated. The best degradation efficiency was determined to be 76% for NiO nanoparticles that were calcinated at 800 °C for 4 h, because of its smallest crystallite size (19.13 nm) and the highest surface area (47.02 m 2 g −1 ). The superior and excellent performance of this sample compared to other samples was confirmed by achieving the highest reaction rate constant (4.51 × 10 −3 min −1 ). A mechanism for the degradation process was proposed, and the rate of degradation for various catalysts was estimated.