3.1 XRD Analysis
The X-ray diffraction (XRD) patterns of nickel-doped manganese aluminum ferrite NiXMn1−XAl0.5Fe1.5O4 (X = 0,0.3) nanoparticles prepared with X = 0 and 0.3 showed distinct diffraction peaks at 2θ = 18.5°, 29.8°, 35.1°, 42.7°, 52.9°, and 62.5°, which are attributed to the (111), (220), (311), (400), (422), and (440) planes of the cubic spinel ferrite structure. The absence of impurity peaks indicates the high phase purity of the synthesized materials. The average crystallite size, calculated using the Scherrer equation, was 30 and 39 nm for Ni0.3Mn0.7Al0.5Fe1.5O4 and Mn1Al0.5Fe1.5O4, respectively.
Table 1
The XRD Structural parameters of the synthesized MixMn1−xAl0.5Fe1.5O4 (X = 0,0.3) ferrite
Parameters | Mn1Al0.5Fe1.5O4 | Ni0.3Mn0.7Al0.5Fe1.5O4 |
---|
FWHM | 0.1987 | 0.2576 |
D-Spacing | 2.5546 | 2.5267 |
Lattice constant (Å) | 8.4726 | 8.3801 |
Cell Volume (Å)3 | 608.194 | 588.503 |
Hoping length La (Å) | 3.668 | 3.628 |
Hoping length Lb (Å) | 2.995 | 2.962 |
Crystalline size (nm) | 39.79 | 30.66 |
X-Ray Density (g/cm3) | 2.77E-18 | 3.67E-18 |
Dislocation density (g/cm3) | 0.000631 | 0.001063 |
Lattice strain | 0.0157 | 0.0206 |
Micro-Strain (lines/m-4) | 0.0473 | 0.0613 |
The lattice parameter was determined to be 8.47 and 8.38 nm for Mn1Al0.5Fe1.5O4 and Ni0.3Mn0.7Al0.5Fe1.5O4, respectively. As the Ni content increased the ferrite peaks became slightly broader and less intense, suggesting a decrease in crystallinity due to the substitution of Ni2+ ions in the ferrite lattice. Based on the data in Table 1, it appears that the following parameters decrease with nickel doping: lattice constant, cell volume, hoping length La, hoping length Lb, crystalline size, and X-ray density. There are a few possible explanations for these observations. One possibility is that the nickel ions are smaller than the manganese ions they are replacing, which would lead to a decrease in the lattice constant and cell volume. Additionally, the nickel ions may be causing distortions in the crystal lattice, which would also lead to a decrease in the hoping length La and hoping length Lb. Finally, the nickel ions may be causing the formation of smaller crystals, which would lead to a decrease in the crystalline size. Another possibility is that the nickel ions are causing an increase in the dislocation density, which would lead to an increase in the micro-strain. The dislocation density is a measure of the number of dislocations in a material, and dislocations are imperfections in the crystal lattice that can cause strain.
3.7 Photodegradation Experiments
3.7.1 Doping effect on photodegradation
The fact that photolysis observed no degradation suggests that the photodegradation observed in the presence of the Ni-doped catalyst is due to a catalytic effect of the catalyst, rather than direct photolysis of the pollutant in illustrated Fig. 7 (a). This is further supported by the observation that the photodegradation efficiency is significantly higher for the Ni-doped catalyst (98.96%) than for the undoped catalyst (83%). Ni-doped catalyst improves photodegradation by absorbing more visible light, preventing electron-hole recombination, and increasing the number of active sites [15].
3.7.2 Effect of pH
The photodegradation of rhodamine B was evaluated at pH 3 to 11 and was found to be 99.80–83.23%, respectively in illustrated Fig. 7 (b). This is because the protonated form of rhodamine B is more susceptible to photodegradation. At low pH, rhodamine B is protonated, which increases its positive charge and makes it more reactive with hydroxyl radicals. At high pH, rhodamine B is deprotonated, which reduces its positive charge and makes it less reactive with hydroxyl radicals in addition, Rhodamine B aggregates at high pH, making them more difficult to photodegrade.
3.7.3 Effect of Dye Concentration
The degradation of rhodamine B decreased from 99.61% at 5 ppm to 62% at 50 ppm. This is a significant decrease, and the dye concentration has a strong influence on the photodegradation rate illustrated in Fig. 7 (c). The decrease in photodegradation of rhodamine B with increasing dye concentration can be attributed to Limited active sites: Competition for active sites between rhodamine B molecules and hydroxyl radicals decreases the number of rhodamine B molecules that are degraded. Self-shielding: Dye molecules absorb photons of light before they reach the photocatalyst surface, preventing photocatalyst activation and hydroxyl radical generation. Reaction intermediates: The formation of reaction intermediates competes with rhodamine B molecules for active sites and hydroxyl radicals, decreasing the degradation efficiency[16].
3.7.4 Effect of Catalyst Dosage
The photodegradation of rhodamine B increased from 98.96–99.8% as the catalyst dosage was increased from 100 mg to 250 mg but decreased to 76% when the dosage was increased to 300 mg. The photodegradation of rhodamine B increased as the catalyst dosage increased up to an optimal value, and then decreased beyond that value can be seen in Fig. 7 (d). This is because increasing the catalyst dosage increases the number of active sites available to generate reactive oxygen species (ROS) which degrade the dye molecules. However, too much catalyst can lead to self-shading, where the catalyst particles block each other from accessing light, and agglomeration, where the catalyst particles clump together and reduce their surface area [17].
3.7.5 Effect of Temperature
The photodegradation of rhodamine B increased from 98.96% at 20\(℃\) to 99.07% at 40 \(℃\) and 99.80% at 60 \(℃\) illustrated in Fig. 8 (a). This increase in photodegradation with increasing temperature can be attributed to the following factors. At higher temperatures, the mobility of rhodamine B molecules increases, making it easier for them to encounter the photocatalyst and absorb photons of light. Additionally, the adsorption capacity of the photocatalyst increases at higher temperatures, leading to more rhodamine B molecules being available for photodegradation [18]. Finally, the generation of hydroxyl radicals, which are the primary reactive species responsible for the photodegradation of rhodamine B, also increases at higher temperatures.
3.7.6 Effect of Light Intensity
The degradation efficiency of RhB under 100 W, 200 W, and sunlight illumination was 98.96%, 99.8%, and 99.8%, respectively illustrated in Fig. 8 (b). The photodegradation rate of Rhodamine B is directly proportional to light intensity. This is because higher light intensity generates more electron-hole pairs on the photocatalyst surface, which enhances the production of reactive oxygen species (ROS), such as hydroxyl radicals (OH˙), superoxide radicals (O2⁻˙), and hydrogen peroxide (H2O2). These ROS are the primary oxidants responsible for the degradation of Rhodamine B. Sunlight is the most abundant and renewable source of light energy for photocatalytic degradation, as it contains both UV and visible light, which can excite a wide range of photocatalysts.
3.7.7 Effect of H2O2 Addition
Adding hydrogen peroxide (H2O2) increased the degradation of rhodamine B dye up to a concentration of 6 mM H2O2. Degradation increased from 83.96–98.61% as the H2O2 concentration was raised from 0 to 6 mM. However, at 8 mM H2O2, degradation decreased to 77.61% illustrated in Fig. 8 (c). The increase in degradation is attributed to higher production of hydroxyl radicals (HO•) at higher H2O2 levels, which degrade the dye. The subsequent drop in degradation at 8 mM H2O2 is due to (a) scavenging of HO• by excess H2O2, (b) formation of less reactive per hydroxyl radicals (HO2•) from HO• at high H2O2 levels, and (c) inhibition of the Fenton reaction by high H2O2 competing with dye for HO• and complexing the catalyst [19].
3.7.8 Scavenger Analysis
To pinpoint the primary reactive species, the impacts of scavengers such as benzoquinone (a scavenger for O2̇−•), tert-butyl alcohol (a scavenger for OH•), and ethylenediaminetetraacetic acid (a scavenger for h+) were investigated [20]. It was found that hydroxyl radicals are the key species driving the degradation of Rhodamine B (degraded 48% with TBA, a scavenger of hydroxyl radicals, compared to 98.96% with no scavenger) illustrated in Fig. 8 (d). Scavenging experiments with benzoquinone (BQ) and ethylenediaminetetraacetic acid (EDTA), scavengers of superoxide radicals (O2−•) and metal ions, respectively, have little effect on the degradation of Rhodamine B (95% and 92% degradation, respectively).
3.7.9 Photodegradation Mechanism
A potential mechanism for the photodegradation of Rhodamine B over the ferrite photocatalyst was suggested based on the trapping experiments. When exposed to visible light, electron-hole pairs are formed. These holes oxidize water that is adsorbed on the surface, resulting in the creation of highly reactive OH• radicals. Electrons are trapped by surface defects or react with dissolved O2 to give O2̇− radicals. The Rhodamine B molecules are then degraded by these reactive species through oxidation, decomposition, and mineralization reactions. When ferrite undergoes irradiation, the process of electron excitation from the valence band to the conduction band occurs[21]. This generates holes that lead to the production of hydroxyl radicals, as described below:
Step 1
Generation of electron-hole pairs
$${\text{N}\text{i}}_{0.3}{\text{M}\text{n}}_{0.7}{\text{A}\text{l}}_{0.5}{\text{F}\text{e}}_{1.5}{\text{O}}_{4}+hv{\to {\text{N}\text{i}}_{0.3}{\text{M}\text{n}}_{0.7}{\text{A}\text{l}}_{0.5}{\text{F}\text{e}}_{1.5}{O}_{4}^{*}+\text{e}}^{-}+ {\text{h}}^{+} \left(2\right)$$
where: Ni0.3Mn0.7Al0.5Fe1.5O4is the ferrite photocatalyst, hν is a photon of visible light, \({\text{N}\text{i}}_{0.3}{\text{M}\text{n}}_{0.7}{\text{A}\text{l}}_{0.5}{\text{F}\text{e}}_{1.5}{O}_{4}^{*}\)is the excited ferrite photocatalyst, e⁻ is an electron and h⁺ is a hole.
Step 2
Oxidation of surface adsorbed water
$${h}^{+}+{H}_{2}O\to {OH}^{*}+{H}^{+} \left(3\right)$$
where: \({OH}^{*}\) is a hydroxyl radical
Step 3
Trapping of electrons
$${e}^{-}+{O}_{2}\to {O}_{2}^{*-} \left(4\right)$$
where: \({O}_{2}^{*-}\) is a superoxide radical
Step 4
Degradation of Rhodamine B
$$RhB+{OH}^{*}\to intermediates\to {CO}_{2}+{H}_{2}O+Other inorganic products$$
5
Or
$$RhB+{O}_{2}^{*-} \to intermediates\to {CO}_{2}+{H}_{2}O+Other inorganic products \left(6\right)$$
The overall reaction for the photodegradation of Rhodamine B can be written as:
$$RhB+\text{F}\text{e}\text{r}\text{r}\text{i}\text{t}\text{e}+hv\to {CO}_{2}+{H}_{2}O+Other inorganic products+Ferrite \left(7\right)$$
The dye photodegradation by visible light irradiation is an effective and environmentally friendly technique that produces only CO2 and H2O as end products [22].
3.7.10 Optical Properties
The Tauc equation, which is used to determine the band gap of both direct and indirect semiconductors, can be expressed as follows:
$${{\alpha }\text{h}\text{v}=\text{A} \left(\text{h}\text{v}-\text{E}\text{g}\right)}^{n} \left(8\right)$$
In this context: α represents the absorption coefficient at a given photon energy hν, A is a constant value, Eg refers to the band gap, and n is a constant that varies based on the specific type of semiconductor. Nickel (Ni) doping in NiXMn1−XAl0.5Fe1.5O4 can decrease the band gap from 2.4 to 2.2 eV can be seen in Fig. 9. Nickel doping enhances photodegradation by introducing new energy levels within the band gap, leading to increased absorption of visible light and the generation of more electron-hole pairs. Additionally, Ni atoms act as electron traps, preventing recombination and extending the lifetimes of these charge carriers, further enhancing photocatalytic activity.
2.7.12 Kinetics
A kinetic study was undertaken to elucidate the reaction order of the decomposition of rhodamine B. The concentration of RhB was monitored over time. The concentration data was fitted to equations for first order and second-order reaction kinetics, given below:
First-order rate equation:
$$\text{ln}\left(\frac{{C}_{t}}{{C}_{0}}\right) \left(9\right)$$
Second-order rate equation:
$$\frac{1}{{C}_{t}}=\frac{1}{{C}_{0}}+{K}_{t } \left(10\right)$$
In this context: Ct represents the concentration at a specific time ‘t’, C0 denotes the initial concentration, ‘k’ is the rate constant, and ‘t’ is the time elapsed. The evaluation of the kinetic data indicates that the decomposition reaction aligns more accurately with first-order kinetics, as evidenced by a high R2 value of 0.998 for the first-order plot shown in Fig. 10 (a). The second-order plot Fig. 11 (b) gave a slightly lower R2 value of 0.917. These results provide clear evidence that the decomposition of RhB at this concentration follows first-order kinetics. The reaction rate is dependent on the concentration of RhB, indicating a mechanism involving one molecule of dye reacting in the rate-determining step. This kinetic study elucidates the rate law and reveals insights into the reaction mechanism.
3.7.13 Comparison Study
Table 3 presents a comparison of various photocatalytic materials and their effectiveness in breaking down the Rhodamine B dye under the influence of visible or UV light. It compares the photocatalytic activity of different composite materials containing iron oxide with additions like ZnO, CuS, graphene oxide, etc. The key metric assessed is the removal efficiency of Rhodamine B dye after irradiation for different periods ranging from 90 minutes to 230 minutes. The catalyst dosage also varies from 0.1 g/L to 2.0 g/L for the different composites. Different light sources are tested - visible light vs UV light. The current study of nickel-doped manganese aluminum ferrite, which achieved 98.96% dye removal in 120 minutes using 0.1 g/L catalyst dosage - was one of the best results.
Table 3
Photocatalytic degradation of Rhodamine B results as compared to previous studies
Compositions | Light source | Time (minutes) | Catalyst dosage(g/L) | | Removal efficiency (%) | Ref. |
---|
CoNd0.05Fe1.95O4 | Visible light | 120 | 0.15 | | 94.7 | [23] |
NiFe2O4 | UV light | 105 | 1.5 | | 84 | [24] |
NiFe2O4@HAp-Sn2+ | Visible light | 230 | 0.65 | | 84.4 | [25] |
NiFe2O4 | Visible light | 120 | 0.35 | | 90 | [26] |
ZnFe@CuS | Visible light | 90 | 1.5 | | 93 | [27] |
ZnFe2O4/graphene oxide | Visible light | 180 | 1.3 | | 94 | [28] |
Zn-doped Fe3O4 | UV light | 90 | 2.0 | | 97 | [29] |
ZnFe2O4-50%@ ZnO | Visible light | 160 | 0.5 | | 79 | [5] |
Ni0.3Mn0.7Al0.5Fe1.5O4 | Visible light | 120 | 0.1 | | 98.96 | Current study |
3.7.14 Catalyst Recyclability
The recyclability of the catalyst decreases with repeating cycles, with the degradation of rhodamine B decreasing from 98.96% in the first cycle to 95% in the fifth cycle shown in Fig. 11. This is likely due to several factors, including Loss of catalyst: The catalyst is lost during the degradation process, reducing the amount of catalyst available for the next cycle. Deactivation of the catalyst: The catalyst becomes deactivated over time, reducing its activity and ability to degrade rhodamine Fouling of the catalyst: The catalyst is blocked by the reaction products or impurities, reducing its ability to degrade rhodamine B. The decrease in recyclability of the catalyst with repeating cycles is a common problem in catalysis.