3.4.1. UV-Visible absorption and optical energy gap
For the present Co0.5Cu0.5LaxFe2−xO4; (x = 0.0, 0.03, 0.06, 0.09, 0.12, 0.15) ferrite nanoparticles, one can notice a broad absorption band positioned nearly at 500 nm; as shown in Fig. 9. Generally, ferrite materials are opaque at wavelengths ≤ 200 nm; at which the photon energy is ≥ optical band gap (Eg), which is required for transition from valence to conduction bands [43]. The Eg values of Co0.5Cu0.5LaxFe2−xO4 nanoferrites can be calculated by extrapolating the linear portion curve of (αhν)2 vs. photon energy (hν) to (αhν)2= 0; for direct allowed transition (Tauc's plot) (Fig. 10(a-f)) [44]. From Tauc's plot, Eg values of CCL nanoferrites are determined. Basically, the band gap can be tuned based on several factors: e.g., crystallite size, structural parameter and impurities. The calculated Eg values of CCL nanoferrites are displayed in Fig. 11. In fact our energy gap possesses a peculiar demeanor; a red shift from 3.04 eV to 2.46 eV (for 0.0 ≤ x ≤ 0.09) and a blue shift from 2.46 eV to 2.98 eV for (0.09 ≤ x ≤ 0.15). A similar Eg behavior was observed in an earlier work for Zn-Mg nanoferrites [13]. This behavior of Eg can be explained trough two scenarios. Firstly, further La content generates a lot of donor levels in the forbidden band, producing Eg decrement. Secondly, this tendency of Eg may be accredited to the decrement attitude of CCL nanoferrites crystallite size in the range 0.09 ≤ x ≤ 0.15 (see Fig. 2); where the relationship between the band gap and particle size is an inversely one [15]. Also the augmentation of conductivity of Co-Cu nanoferrites with further La3+ substituting confirms the Eg behavior in that range.
3.4.2. Photocatalytic activity
The influence of Co0.5Cu0.5LaxFe2−xO4 nanoferrites on photo-catalytic dye degradation of Rhodamine B (RhB) was investigated using absorption spectra. However, the Eg of photocatalyst governs the absorbed wavelength and produces electron-hole pairs. The nanoferrite specimens with x = 0.0 and 0.09 La3+ were chosen for this investigation because x = 0.0 represent the pristine ferrite and x = 0.09 has the least Eg value. This choice based on the opposite relation between Eg and photodegradation behavior [13]. Moreover, the replacement of rare earth cations in Co-Cu ferrite gives good optical absorption in visible range point toward enhanced photodegradation efficiency. As a consequence of metastable La-4f energy levels creation near the lower edge of the conduction band of Co-Cu ferrite, this indicates the decrease in the band gap. A further factor is the defects resulting from La doping which act as trapping centers and simplify the split-up of photogenerated electron-hole pairs and increase the life time of charge carrier [45].
The photo-catalytic degradation efficiency is obtained from the variation in Absorbance for the reason of direct relation between concentration (c) and absorbance (A) [46]. It is well known that, the self-degradation efficiency of RhB (without catalyst) is very small value for dye disposal [13]. Figure 12(a-c) manifests photocatalytic degradation of RhB and RhB + Co0.5Cu0.5LaxFe2−xO4 (x = 0.0, and 0.09) irradiated under solar light at variable times (from 0-180 min). The reduction in absorbance intensity designates enhancement in dye degradation. The mechanism for RhB photodegradation of the nanoferrite in which x = 0.9 La (as an example) is understood with aid of some of free radicals. All the spectra evident the characteristic absorption curves of RhB with a peak, at ~ 552 nm and a shoulder at ~ 512 nm [47].
The % degradation of samples is determined via the equation listed in Ref. [48]. Figure 13 illustrates % degradation for pure RhB and RhB over CCL nanoferrites with (x = 0.0 and 0.09). The % degradation, after irradiation for 180 min, of pure RhB dye is just 3.31%; which is an unsatisfactory impact. As for the % degradation for RhB over CCL nanoferrites with (x = 0.0 and 0.09) photocatalysts is enlarged; (22.37% and 94.50%, respectively). Hence, the catalytic recital was enhanced with La/Fe substitution process via the next most probable discussion. When the electrons were excited from valence band (VB) to created energy level CCL conduction band (CB) in the sample under sunlight irradiation, the photogenerated holes in VB react with surface water or hydroxyl ion to yield \({\text{O}\text{H}}^{\bullet }\) radical, which is a good oxidant in the degradation of RhB and instantaneously, electrons in the CB reacts with adsorbed oxygen molecule to yield \({\text{O}}_{2}^{.-}\). Moreover, it combines with H+ to yield \({\text{H}\text{O}}_{2}\). [49], which react with trapped electrons to yield \({\text{O}\text{H}}^{\bullet }\) [50]. It is obvious that, \({\text{O}\text{H}}^{\bullet }\), \({\text{H}\text{O}}_{2}\)., \({\text{O}}_{2}^{.-}\) and \({h}_{VB}^{+}\) are active species included in RhB photodegredation. Regarding the previous argument, the photochemical reaction for the degradation of RhB under sunlight irradiation of Co-Cu-La ferrite photocatalyst was summarized as follows [13].
Co 0.5 Cu 0.5 La 0.09 Fe 1.91 -O 4 + hν\(\to\) Co0.5Cu0.5La0.09-Fe1.91O4 + (e− and h+)
e − +\({O}_{2}\)\({\to O}_{2}^{.-}\)
h + +\({HO}_{2}\) \(\to {H}^{+}\)+\({OH}^{\bullet }\)
\({{O}_{2}^{.-}+H}^{+}{\to HO}_{2}\) ., 2e−\({+HO}_{2}\).\(+\) \({H}^{+}{\to OH}^{\bullet }+{OH}^{-}\) and h+ \({+ OH}^{-}\).\({\to OH}^{\bullet }\)
\({OH}^{\bullet }\) , \({HO}_{2}\)., \({O}_{2}^{.-}\), \({h}_{VB}^{+}\)+RhB\(\to Degraded products\)
These radicals, formed from the previous steps, can interact with the toxic RhB dyes, converting its complex molecules to simple and non-toxic ones. Dhiman et al. [51] in previous work obtain comparable mechanisms for photocatalytic behaviors for CoFe2O4 ferrite doped with various rare earths.
To distinguish the photocatalytic activity protocol of CCL photocatalyst, the three kinetic models (0th, 1st and 2nd orders) are determined using the following equations [52].
\({A}_{t}={A}_{o}-{k}_{o}t\) , \({A}_{t}={A}_{o}{e}^{-{k}_{1}t}\),\(\frac{1}{{A}_{t}}=\frac{1}{{A}_{o}}+{k}_{2}t\)
where At and Ao are absorbance of RhB dye after and before irradiation time (t), respectively. For this mission, the plots of At, ln(Ao/A) and 1/At versus time are determined with their linear fitting; see Fig. 14(a-c). The three kinetic constants (k0, k1 and k2) of zeroth, first and second order reaction kinetics, respectively of RhB and (RhB + CCL samples (x = 0.0 and 0.09) are calculated and tabulated in Table 2. Also, correlation coefficient (R2) of each order kinetic is calculated and inserted in Table 2. The second order is the most favorable model for pure RhB, where its R2 value is 0.980 (the highest value compared with other orders). As for RhB over CCL sample with (x = 0.0) has R2 value 0.998 for both zeroth and first-orders; declaring that these models are favorable models for degradation. Meanwhile for RhB over CCL sample with (x = 0.09) has R2 value 0.995 for zeroth; demonstrating this model is the most suitable model for degradation of this sample. Finally, the k0, k1 and k2 values for RhB dye degradations in presence of CCL nanoferrite powders are higher than those of pristine RhB dye; see Table 2. These outcomes confirm the CCL nanoferrites are capable of enhancing the RhB dye degradation efficiency in industrial community.