3.1. Structural and electronic parameters
Figure 1 shows the XRD charts for NiO nanoparticles heat-treated at temperatures of 700, 800, and 900°C for 2, 3, and 4 h, respectively, as indicated in Figs. 1a, 1b, and 1c. In our prior research, we investigated the structural parameters of NiO as it was prepared [27]. 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 [31]. 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 equation: \(D=\frac{0.9\lambda }{\beta cos\theta }\), was used to calculate the average crystallite size (D) of the produced phase, where λ is the wavelength of radiation employed in Cu Kα (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 microstrain (\(\epsilon\)) was also calculated using the Williamson-Hall method [35]: \(\beta cos\theta =\frac{0.9\lambda }{D}+4\epsilon sin\theta\). The plot of \(\beta cos{\theta }\) versus \(4sin{\theta }\) 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 (\(\delta\)) is: \(\delta =\frac{1}{{D}^{2}}\). Table 1 summarized the results for crystallite size, microstrain, and dislocation density for the investigated NiO nanoparticles.
Table 1
Summarize the crystallite size (D), micro-strain (ε), dislocation density (δ), equivalent surface area (S), and energy bandgap (Eg) for NiO nanoparticles at various calcination temperatures and times.
Calcination time (h)
|
2
|
3
|
4
|
Temp. (°C)
|
D (nm)
|
ε
×10−3
|
δ
×10−3
|
S
(m2.g−1)
|
Eg
(eV)
|
D (nm)
|
ε
×10−3
|
δ × 10−3
|
S
(m2.g−1)
|
Eg
(eV)
|
D (nm)
|
ε
×10−3
|
δ
×10−3
|
S
(m2.g−1)
|
Eg
(eV)
|
700
|
25.03
|
-2.45
|
1.60
|
35.94
|
3.42
|
19.24
|
-0.03
|
2.70
|
46.75
|
3.71
|
25.79
|
-1.53
|
1.50
|
34.88
|
3.42
|
800
|
24.44
|
-2.45
|
1.67
|
36.81
|
3.33
|
26.94
|
0.37
|
1.38
|
33.40
|
3.42
|
19.13
|
-3.4
|
2.73
|
47.02
|
3.39
|
900
|
27.63
|
-4.44
|
1.31
|
32.56
|
3.43
|
26.64
|
-2.03
|
1.41
|
33.77
|
3.42
|
23.13
|
-2.74
|
1.87
|
38.90
|
3.66
|
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 calcination temperature from 800 to 900°C. The crystallite size of NiO calcined at 800°C for 4 h has the smallest value that equal to 19.13 nm among the evaluated values. The equivalent surface area, 47.02 m2/g, has reached its maximum value. The grain size is increased by agglomeration between neighboring NiO nanoparticles, whilst the crystallite size is 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 FTIR for various NiO nanoparticles calcined at 700, 800, and 900°C for 2, 3, and 4 h is shown in Figure 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 [36]. Furthermore, when FTIR sample discs were made in the open air, a faint band about 1647 cm–1 ascribed to the mode H–O–H bending vibrations was identified owing to water adsorption in the air [36]. 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 [37, 38]. The serrated absorption bands in the 1000–1500 cm–1 range could correspond to the O–symmetric and asymmetric stretching vibrations, 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 H2O and CO2. Furthermore, these observed peaks might be generated by water content, asymmetric stretching of C=O, the vibration of adsorbed CO2, C–H stretching, and N-H bending mode. The Ni–O stretching also has stretching vibrations at 473 cm−1, and 664 cm−1 [39]. 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 (Eg). As a result, we drew the Kubelka-Munk function F(R) using UV-visible diffuse reflectance: \(F\left(R\right)=\frac{{(1-R)}^{2}}{2R}\), 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: \(F\left(R\right)=\frac{A{(h\nu -{E}_{g})}^{n}}{h\nu }\), 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 [40]. Figures 3a-c show selected plots of the relation between [F(R)E]2 and E for NiO nanoparticles calcined at 700, 800, and 900°C for 4 h. The tangent lines of the\({\left[F\left(R\right)E\right]}^{2}\) against photon energy (E), and can be used to calculate Eg, as illustrated in Fig. 3. Table 1 summarizes the values of Eg for various NiO nanoparticles that were calcinated at various temperatures and periods. NiO nanoparticles calcined at 800°C for 2 h and NiO calcined 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 Dhanuskodi et al. [41]. 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.
Figure 4 depicts the surface morphology of the NiO nanoparticles were examined by the SEM. The morphology shape of NiO nanoparticles calcined 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 calcined 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 Figs. 4b and 4c, 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.
3.2. The photocatalytic performance of NiO nanoparticles
Another goal of this research is to use it as a catalyst for treating wastewater from 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. In this study, nine different NiO nanoparticle samples are used to examine the photocatalytic performance of removing the IC dye from wastewater. For convenience, these samples are labeled A, B, C, D, E, F, G, H, and I for NiO calcinated at calcination temperatures and times of 700°C for 2 h, 700°C for 3 h, 700°C for 4 h, 800°C for 2 h, 800°C for 3 h, 800°C for 4 h, 900°C for 2 h, 900°C for 3 h, and 900°C for 4 h, respectively. Figures 5a-c show the relationship between the absorbance spectra and the incident photon wavelength through IC dye that contains NiO nanoparticles for various UV-visible irradiation times. The purpose of this examination is to evaluate the efficiency of the photocatalytic properties of selected NiO samples were prepared 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 O2−2), which is a key species in the decomposition process of environmental organic pollutants [42]. 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 [43, 44].
Figure 6 depicts the photodegradation rate of IC dyes (\(\frac{{C}_{t}}{{C}_{o}}\)) versus UV-visible irradiation time (t) for NiO nanoparticles. The NiO nanoparticles that were calcined at 800°C for 4 h demonstrated a higher degradation rate of the IC dye. Similarly, Figure 7 depicts the removal effectiveness of IC dye as a function of UV-visible irradiation time in the instance of NiO nanoparticles. The effect of the prepared nanocompounds on the absorbance change of organic dye and the values of degradation rates at various time intervals with a time step of 30 min for each run. 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: \(\text{ln}\left(\frac{{C}_{t}}{{C}_{0}}\right)=-kKt=-{k}_{app}t\) [45], where k denotes the degradation rate constant, K is the adsorption equilibrium constant, and kapp is the apparent rate kinetic constant, also known as the pseudo-first-order reaction constant. The response rate was calculated using \({k}_{app}=\) \(-\text{ln}\left(\frac{{C}_{t}}{{C}_{0}}\right)/t\). Table 2 summarizes the kapp 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.
Table 2
The reaction rate constants (kapp) for several NiO catalysts for IC dye removal.
Sample No.
|
A
|
B
|
C
|
D
|
E
|
F
|
G
|
H
|
I
|
Calcination temperature (°C)
|
700
|
800
|
900
|
Calcination time (h)
|
2
|
3
|
4
|
2
|
3
|
4
|
2
|
3
|
4
|
Degradation (%)
|
56
|
60
|
66
|
68
|
71
|
76
|
47
|
50
|
53
|
kapp ×10−3 (min−1)
|
3.01
|
3.40
|
3.52
|
3.55
|
3.93
|
4.51
|
2.53
|
2.55
|
2.77
|
Eg (eV)
|
3.42
|
3.71
|
3.42
|
3.33
|
3.42
|
3.39
|
3.43
|
3.42
|
3.66
|
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 [46]. The photocatalytic activity is directly proportional to the surface area, which leads to more adsorption of the reactant materials [47]. 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 [48]. 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 most important process occurs, which is the formation of hydroxyl and oxygen radicals at the positive holes and the excited electrons, respectively. These 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 [42, 49]. We can express reactions and processes that occur during the photo detoxification process of IC dye as follows:
Prepared sample (NiO nanoparticles) + light source (hv) → photocatalyst with positive holes (h+) and excited electrons (e−) (1)
Excited electrons + Oxygen molecule (O2) → Oxygen radicals (O2−−.) (2)
Positive holes + water molecule (H2O) → hydroxyl radical (OH.) (3)
Organic dye (IC) + two produced active radicals → environmentally friendly compounds (CO2 and non-polluting water) (4)