3.1. Structural Analysis
Figure 2 (a) shows the room-temperature XRD pattern of the GFOnps calcined at 900°C along with the theoretical spectra obtained from JCPDS file no. 26–0673. It was observed that the GFOnps has orthorhombic structure with Pc21n space group, similar to the studies reported in previously published papers. (Dey et al. 2023 and Kujur and Singh 2020) The prominent peak for GFOnps was observed at 33°, which is a hallmark and fingerprint for the formation of orthorhombic GaFeO3. Rest of the peaks were indexed according to the orthorhombic structure with Pc21n space group and JCPDS file no. 26–0673. Few small peaks at 24.2°, 43.5°, 58.3° and 62.5° have been obtained which are corresponding to GFO with rhombohedral phase (R2c space group). (Mukherjee et al. 2022) These have been marked with asterisk (*) in the Fig. 2 (a). The average of the crystallite size was computed using the Scherrer’s formulae, as given below and was obtained to be 29 nm
$$D=k\lambda /\beta cos\theta$$
1
Where, D = Average crystallite size, k = Scherrer constant (0.94), λ = Wavelength of x-ray, β = Full width half maxima of peak and θ = Braggs angle.
Raman spectra was obtained for structural study and the peaks are shown in the Fig. 2 (b). The peaks of Raman spectra for GFOnps can be ascribed to orthorhombic structure with Pc21n space group, which demonstrate that GFOnps is free of contaminants. The Raman modes were observed at the 221.35, 260.58, 317.95, and 375.08 cm-1, which are due to the Tetrahedra (Ga,Fe)O4 and Octahedra (Ga,Fe)O6 rotational and translational vibrations modes. (Hatnean et al. 2012, Mall et al. 2020, Golosova et al. 2016, Dugu et al. 2019 and Thomasson et al. 2012) The Raman peaks were fitted using Gaussian function to obtain the peaks without baseline correction.
3.2. Shape and Morphology analysis
The shape and morphology analysis of GFOnps was studied using the transmission electron microscopy (TEM) as shown in Fig. 3 (a). TEM image revealed particles uniformly distributed over the surface, with particle sizes between 20 and 40 nm. Inset in Fig. 3 (a) shows the high-resolution TEM (HRTEM) image and the interplanar spacing (d) was 3.4 Å corresponding to (100) plane. This is in-correlation with data obtained from XRD for the same hkl plane. The selected area electron diffraction (SAED) pattern in Fig. 3(b) shows that the GFOnps are highly crystalline. The hkl planes of the GFOnps could be distinguished by the different circular diffraction pattern corresponding to the respective d spacing. The circular spots were concentric in nature which suggests that GFOnps are polycrystalline in nature. In the image each circle was assigned by hkl plane using the equation as given below: (Lakshmi et al. 2022, Verma et al. 2021, Dhiman et al. 2021b, Dalal et al. 2022 and Jalandra et al. 2023)
$$R=\frac{L\lambda }{d}$$
2
Here, R is the radius of circle, d is the interplanar spacing and Lλ = k, where k is the camera constant. All the diffraction patterns were matched with the XRD pattern, and they were found to be analogues to each other confirming the orthorhombic structure of GFO.
3.3. Phtotabsorption study and Elemental and structure analysis
UV-Vis analysis was performed in the range of 250 nm to 900 nm. A broad absorption spectrum was obtained between 500nm to 800nm. The reason for broad visible spectra can be explained based on discrete energy levels which arises due to size-dependent quantum confinement. Surface states and defects distributed due to the high surface-to-volume ratio contribute to additional energy levels, further broadening the spectrum. (Kumar et al. 2023a and Kumar et al. 2023b) Also, multiple electronic transitions, crystal field effects, and charge transfer processes in GFOnps contribute to the overall spectrum broadening. Thus, a broad absorption spectrum is obtained in UV-Vis study. Tauc plot was used to measure the energy band gap of GFOnps and was calculated using the following relation:
$${\left(\alpha h\upsilon \right)}^{2}=A(h\upsilon -Eg)$$
3
Tauc analysis gave energy band gap of Eg = 2.08 eV, similar in values to previously reported literature. (Dhanasekaran and Gupta 2012 and Kujur and Singh. 2020) The band gap of GFOnps was low, which may be due to the discrete energy levels of the nano sized particles. This could also be due to increase in the grain size caused by high calcination temperature. (Kujur et al. 2022, Rana et al. 2023a, Rana et al. 2023b, Reddy et al. 2018 and Singh et al. 2021) Fig. 4 (b) displays the EDS spectrum of GFO, and it is found that percentage of oxygen, gallium and iron are in the correct proportion. Inset of Fig. 4 (b) shows the percentage of each element present in the synthesised GFOnps. The atomic weight percentage confirm presence of Ga, Fe and O elements in the GaFeO3 lattice in proper ratio with slight error. The less percentage of oxygen is due to small X-ray yield of light element such as Oxygen, which gets absorbed by the sample itself, thus limiting its counts in EDS.
3.4. Multiferroic Studies
The P-E hysteresis of the GFOnps was measured from the in-house designed P-E hysteresis loop tracer as shown in Fig. 5 (a). Figure 5 (b) showcases the enlarged portion of PE hysteresis loop between − 1 to 1 kV/cm of electric field (E). The GFO powder was pelletized using the hydraulic press with dimension of 1mm thickness and 10mm diameter. The P-E was measured at room temperature with increasing polarisation. In this study, we have increased the field (from 1.5 kV to 2 kV) to try and attain the saturation and minimize the leakage current of the sample. But even then, saturation was not achieved. This might be due since the ceramic sample has lower density than that of their single crystal counterpart, higher electric field is required to attain the proper saturation polarization therefore proper hysteresis loop could not be obtained. This non-saturation could also be related to the studies that suggest the probable reason to be its leaky nature typically for the multiferroic materials. (Dugu et al. 2020, Dhanalakshmi et al. 2017, Wang et al. 2017, Zhong et al. 2018) This result shows that GFO may have vacancy defects or interstitial defect sites which lead to this leaky behaviour. (Dhanalakshmi et al. 2017, Han et al. 2013, Han et al. 2017, Han et al. 2020, Kumar et al. 2021)
Room temperature magnetic measurement of the GFOnps were performed for verifying the magnetic behaviour. Figure 5 (c) shows the magnetic hysteresis graph of GFO at room temperature while Fig. 5 (d) showcases enlarged portion of the MH hysteresis loop between − 0.3 to 0.3 T of magnetic field. It could be easily deduced that the GFO sample has not saturated even at 6T, the coercive field is low, at around 0.1T which is in alignment with other studies that Ch and Mr of GFO reduces with increase in temperature. (Mukherjee et al. 2023 and Kujur and Singh 2020)The value of Tc changes in different conditions because of the redistribution, stoichiometry and dislocation of the Ga and Fe atoms from their actual lattice sites which depends on the synthesis technique. (Kumar and Singh 2021) The high Mr and low Ch could be an ideal candidate for the use in spintronic.
3.5. Photocatalytic degradation of Methyl Violet (MV) and Methylene Blue (MB)
3.5.1. Control Study
Figure 6 shows the UV-visible absorption spectra of degradation of MV and MB dye in presence of H2O2 and in presence of GFOnps. Figure 6 (a) showcases the study of MV dye with H2O2 only, Fig. 6 (b) showcases the study of MV dye with GFOnps only, Fig. 6 (c) showcases the study of MB dye with H2O2 only, and Fig. 6 (d) showcases the study of MB dye with GFOnps only. As can be seen from the Fig. 6, there is almost no reduction in the absorption intensity when H2O2 and GFOnps interact separately with MV and MV dyes. This study shows that for Fenton photocatalysis both hydrogen peroxide (H2O2) and the GFOnps are required to achieve the maximum degradation yield in both the dyes. (Naik et al. 2009)
3.5.2. Dye Degradation Studies
The GFOnps were used to degrade the two organic dye’s i.e., Methyl violet and Methylene blue. As explained in the experimental section, the dyes were separately mixed with the GFOnps in beakers. Initially the dye particles were allowed to adsorb in the surface of nanoparticles in dark through sonication then only the solution was irradiated with visible light of 400nm wavelength. (Kujur and Singh 2020, Kujur et al. 2022 and Nehra et al. 2023) Fig. 7 (a) and (d) exhibits UV absorption spectrum of all collected samples at different time intervals for MV and MB, respectively. For MV dye sample were collect for 120 minutes whereas for MB dye the time was increased to 150 min to gain the maximum degradation result. The degradation percentage was computed using the equation as given below.
$$Degradation Percentage= \frac{\left({C}_{0}-C\right)}{C}\text{*}100$$
4
Where, Co is the Initial UV absorption intensity of dye molecule and C is the final intensity. The degradation percentage of MV dye was 97% and that of MB dye was 56.6%. It is heterogeneous photocatalysis as the catalyst (GFOnps) and the reactants are in different phase. In Fenton-type reactions, GFOnps are introduced as a solid catalyst into a reaction system, where they interact with the liquid-phase reactants. These nanoparticles facilitate the formation of reactive species, such as hydroxyl radicals (OH•), when exposed to hydrogen peroxide (H2O2) and other reactants. The catalytic activity occurs primarily at the surface of the solid catalyst.
3.5.3. Kinetics Study
The C/Co plot has been represented in the Fig. 7 (b) and (e) for the absorption spectrum of Methyl Violet and Methylene Blue dyes respectively, Subsequently, pseudo First order kinetics was studied using Langmuir-Hinshelwood model.( Kujur and Singh 2020 and Kujur et al. 2022)
$$\frac{{d}_{c}}{{d}_{t}}=KC$$
5
Here, K is rate constant of pseudo first order kinetics, C represents the concentration of the dye molecules at a given time t. When, the above equation is integrated for t = 0 min then the equation is
$$ln\frac{{C}_{0}}{C}=Kt$$
6
log Co/C Vs Time plot was plotted in Fig. 7 (c) And (f) and the rate constant (k) which was found to be 0.023 s− 1 and 0.006 s− 1 for MV and MB dye, respectively.
3.5.4. Cyclic Study
For testing the reusability of GFOnps as photocatalyst, cyclic study was performed for three cycles. Figure 8 (a) and (b) show cyclic study of MV and MB dye respectively. From the plot it is evident that after 3 cycles the dye degradation percentage of GFOnps was more than 80% and 45% for MV and MB dye respectively, which confirms its reusability as photocatalyst.
3.5.5. Chemical Reactions in Fenton Process
The adsorption process is reversible step which involve adsorption of dye molecule in surface of nanoparticles and involves generation of excited electrons due to light which reaches conduction band and the generation of holes at the valence band as given in Eq. (7). The mechanism involved in the dye degradation process further are as follows,
GaFeO3 + hυ → e− + h+ (7)
e− + H2O2 → OH• + OH− (8)
h+ + H2O → OH• + H+ (9)
OH• + dye → intermediate product → degraded product (10)
Equations 8, and 9 are involved in the formation of OH• free radicles, OH- ions and H+ ions, respectively. These OH• free radicles, OH- and H+ ions, react with the adsorbed dye and undergoes mineralisation process by breaking the dye into intermediates and then to harmless products, which is represented in Eq. 10. (Phan et al. 2018 and Haruna et al. 2018) Fenton reaction are given below in Eqs. 12, 13 and 14,
Fe2++H2O2 → Fe3+ + OH• + OH− (12)
Fe3++H2O2 → Fe2+ + OOH• + OH+ (13)
H2O2 + OH• → H2O + OOH• (14)
These OH radicals simultaneously react with the dye molecules to break them down into smaller compounds. So, the nanoparticle itself is acting as the catalyst to provide the iron and initiate the Fenton process with the help of H2O2, which in turn create the OH radicals which drive the first reaction further. (Kanhere and Chen 2014, Ahlawat et al. 2023, Cheng et al. 2023, Kumar et al. 2023, Dhiman et al. 2021c, Ahlawat et al. 2021 and Ahlawat et al. 2024)
Table 1 and Table 2 showcases the comparison of our work with the previously published literature on perovskite structured visible light driven photocatalytic degradation of methyl violet and methylene Blue. As is evident from Table 1, GFOnps have shown excellent photocatalytic performance where more than 97% of dye was degraded in 120 minutes. Compared to previous works our material is lead free and does not require any doping.
Table 1
Comparison of current work with the previously published literature on degradation of MV by perovskite materials based visible light driven photocatalytic procedure.
Material | Degradation Percentage (%) | Time (min) | k (min− 1) | Reference |
SmMnO3 | 93% | 90 | - | 68 |
CsPbI3 | 81.7% | 120 | - | 69 |
Copper-Doped SrTiO3 | 99% | 120 | | 70 |
GaFeO3 | 97 | 120 | 0.023 | Current Work |
Similarly, Table 2 shows, GFOnps have shown excellent photocatalytic performance where more than 56% of MB dye was degraded in 150 minutes under visible light irradiation. It is a known fact that the methylene blue is tough dye to degrade under visible light conditions. Thus, either the time of irradiation has to be very large, or the intensity of the light needs to be increased. Compared to previous works our material has degraded more amount or equal amount of dye in less time period. Though there is still scope of improvement which needs to be achieved in order to make perovskite-based nanomaterials as viable option for photocatalytic degradation of methylene blue.
Table 2
Comparison of current work with the previously published literature on degradation of MB by perovskite materials based visible light driven photocatalytic procedure.
Material | Degradation Percentage (%) | Time (min) | k (min− 1) | Reference |
MnTiO3 | 62.5 | 300 | 0.00525 | 71 |
BaTiO3 | 28 | 200 | 0.124 | 72 |
Bi4Ti3O12 | 36 | 200 | 0.0844 | 72 |
ZnTiO3 | 78 | 200 | 0.0472 | 72 |
GaFeO3 | 56.6 | 150 | 0.006 | Current Work |
3.5.6. Mechanism of degradation of Methyl Violet (MV) and Methylene Blue (MB)
The Fenton process, coupled with photocatalysis, is an effective method for the degradation of cationic dyes such as Methyl Violet and Methylene Blue as shown in Fig. 5. The process involves the generation of hydroxyl radicals (•OH) using a combination of hydrogen peroxide (H2O2) and a catalyst, typically iron ions (Fe2+/Fe3+). In the presence of visible light, the GFOnps absorbs photons and creates electron-hole pairs. The electrons (e-) reacts with H2O2 and the holes (h+) react with water (H2O) adsorbed on the catalyst surface, producing hydroxyl radicals (OH•). The hydroxyl radicals are highly reactive and can degrade organic pollutants. To enhance the degradation process, Fenton reagents, typically comprising Fe2+ (ferrous ions) and H2O2, are introduced into the system. Fe2+ ions act as a catalyst, while H2O2 serves as a source of hydroxyl radicals. The Fe2+ ions react with H2O2 to form hydroxyl radicals through the Fenton reaction. (Kujur and Singh 2020 and Phan et al. 2018)
The hydroxyl radicals (OH•) produced in the Fenton reaction are highly reactive and contribute to the degradation of the cationic dyes. The generated hydroxyl radicals (OH•) attack the cationic dye molecules, leading to the breakdown of their chemical structure. The degradation involves various oxidative processes, including the abstraction of hydrogen atoms, cleavage of aromatic rings, and oxidation of functional groups present in the dye molecules. As the degradation proceeds, the dye molecules are converted into smaller organic fragments and eventually mineralized into carbon dioxide (CO2) and water (H2O). However, intermediate by products may be formed during the degradation process, which should be monitored for potential environmental impacts. By combining photocatalysis with the Fenton process, the degradation efficiency of cationic dyes like Methyl Violet and Methylene Blue can be significantly enhanced, leading to their eventual mineralization and removal from the environment. (Cheng et al. 2023)
Figure 9 is a representation of photocatalysis process as the visible light is irradiated onto the dyes. The Fig. 9a) showcases the MV dye degradation process where, i) shows the interaction of MV dye with GFOnps, ii) represents the adsorption of MV dye over the surface of the GFOnps and iii) shows the photocatalytic degradation of MV using visible light irradiation. This happens due to the production of electron and hole pairs in the conduction and valence band, respectively, with the irradiation of visible light. After this generation of free radicles takes place which then initiates the degradation of the MV dye into CO2 and H2O. Similarly, Fig. 9b) showcases the similar process for the degradation of MB dye. Figure 9b) i) shows interaction of MB dye with GFOnps, ii) represents the adsorption of MB dye over the surface of the GFOnps and iii) shows the photocatalytic degradation of MB using visible light irradiation. Similar to MV degradation process, OH radicals are produced which degrade the MB dye into CO2 and H2O. These degradation process of MV and MB have been explained in detail in subsequent sections and shown in Fig. 10 (MV) and Fig. 11 (MB).
Fenton based photocatalytic degradation process of methyl violet dye has been described in Fig. 10 showcases 5- step degradation processes of methyl violet dye where a series of steps culminate into the end of CO2 and H2O. The intermediates formed include iii) o-Leucoaniline, iv) 1,3-Diphenylurea, 2-Hydroxy benzoic acid, 4-amino phenol, Phenol.73
Fenton based photocatalytic degradation process of Methylene Blue (MB) dye using GFOnps as catalyst has been described in the Fig. 11 with water and CO2 as end product. It describes a mechanism for the Fenton based photocatalytic degradation of Methyl Blue (MB), showcasing three major reaction pathways. First pathway (ii) is reaction initiation due to combination of OH. Second pathway (iii) is due to the initiation due to breaking of bonds which can be further segregated into a sub-pathway (iv) where reaction is occurring due to combination of O. In the end of all three pathways the end products are CO2 and H2O. (Wang et al. 2014 and Dhiman and Singh 2019)