Ce and Fe doped LaNiO₃ synthesized by micro-emulsion route: Effect of doping on visible light absorption for photocatalytic application

The salts and solvents like Ce(NO₃)₃.6H₂O, La(NO₃)₃.6H₂O, Ni(NO₃)₂.6H₂O, Fe(NO₃)₂.9H₂O and CTAB were acquired from Sigma Aldrich. The NH₄OH and CR dye were purchased from the BDH.

Various compositions of La1−xCexNi1−yFeyO3 (x, y = 0.00–0.25) NPs were synthesized by micro-emulsion technique at low temperature. For this, the aqueous solutions of all the respective metal nitrates were prepared in stoichiometric amount and were mixed according to composition scheme. All the mixed solutions were placed on hot plates with magnetic stirring and temperature was raised up to 80 °C. After attaining the required temperature, stirring was continued, but heating was switched off until room temperature was reached. Then, NH₄OH was added drop wise to maintain the pH ∼11 and stirring was continued for 6 h. Then, precipitates were washed by distilled water several times to attain neutral pH and were dried overnight in oven at 100 °C. Finally, the calcined was performed at 700 °C for 6 h and subjected to characterization (figure 1S in supporting information available online at stacks.iop.org/MRX/8/085009/mmedia). Before calcination step, on adding aqueous ammonia solution lanthanum and nickel nitrate are converted into corresponding metal hydroxides, which on calcination temperature were decomposed finally into LaNiO₃ [32]. (Equations (1)–(2)).

$$\begin{eqnarray}&&{\rm{La}}{\left({{\rm{NO}}}_{3}\right)}_{3}+{\rm{Ni}}{\left({{\rm{NO}}}_{3}\right)}_{2}\to {\rm{La}}{\left({\rm{OH}}\right)}_{3}+{\rm{Ni}}{\left({\rm{OH}}\right)}_{2}\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{\rm{La}}{\left({\rm{OH}}\right)}_{3}+{\rm{Ni}}{\left({\rm{OH}}\right)}_{2}+\displaystyle \frac{1}{2}{O}_{2}\to {\rm{La}}N{i}_{3}+\displaystyle \frac{5}{2}{{\rm{H}}}_{2}{\rm{O}}\end{eqnarray} \tag{ 2 }$$

The Xray diffraction analysis (X'Pert PRO diffractometer) in 20°–60° range was performed for structural analysis, whereas morphology was evaluated by SEM analysis (JOEL-JSM-6490LASEM). The Raman study was performed at 298 K by T6400 triple JobinYvon-Atago/Bussan spectrometer. Hitachi F-7000 spectrophotometer was used for photoluminescence spectrum (PL) studies. The UV-Vis studies were performed by employing Shimadzu 3101 UV–Vis spectrophotometer.

The PCA of pure LaNiO₃ and La_0.80Ce_0.20Ni_0.80Fe_0.20O₃ NPs was investigated by studying the photodegradation of CR dye (figure 2S in supporting information) under solar light exposure (200 W Argon lamp). A 0.08 g of La_0.80Ce_0.20Ni_0.80Fe_0.20O₃ was added to 200 ml of CR dye (10 mg l⁻¹) along with 10 ml of H₂O₂ (10%). The mixture was kept in dark (stirred) for 0.5 h and placed in reactor for specific time intervals, then, a 5 ml of suspension was taken, filtered and absorbance was checked at λ_max = 498 nm. The CR dye removal (%) was calculated (equation (3))

$$\begin{eqnarray}&&\mathrm{Degradation}\,\left( \% \right)=1-\displaystyle \frac{{{\rm{C}}}_{{\rm{t}}}}{{{\rm{C}}}_{0}}\times 100\end{eqnarray} \tag{ 3 }$$

Where, C_o and C_t represent the dye concentration at zero time and specific time 't', respectively.

Figure 1 depicts XRD analysis of La_1−xCe_xNi_1−yFe_xO₃ compositions sintered at 700 °C in 2θ, i.e., 20°–60° range. The characteristic diffraction peaks appeared at 2theta i.e. 23.20°, 32.80°, 47.28 ° and 58.48° were indexed as (101), (110), (202) and (122) miller planes, which is a rhombohedral crystalline structure of the LaNiO₃ (JCPDS # 034–1028) [33, 34]. For x, y = 0.05 and 1.0 compositions, the most intense peak at 32.80° was shifted slightly toward lower 2θ axis along with minor decline in intensity which indicated the successful substitution of Ce³⁺ and Fe³⁺ contents in perovskite structure thus causing some structural defects and reduction in crystallinity of doped compositions. This shifting might be ascribed to relatively low difference of ionic radii of Ce³⁺ (115 pm) and La³⁺ (117 pm) at A-site compared with Ni³⁺ (0.70 pm) and Fe³⁺ (0.64 pm) at B-site which induces structural defects [35, 36]. For compositions with x, y > 1.0 the most intense peak, i.e., 32.80° was declined significantly with origination of some additional low intensity peaks at 2θ = 27.5°, 43.2° and 45.8° in diffraction pattern which were assigned to La₂O₃ [JCPDS #83-1355), NiO [JCPDS #78-0643] and CeO₂ [JCPDS #320196] phases present along with major perovskite phase. Peak at 27.5° was originated due to overlap of CeO₂ and La₂O₃ phases [37]. It is reported that high doping content of Ce³⁺ ions in La_1−x Ce_x NiO₃, result in the segregation of Ce from perovskite phase in the form of CeO₂ which clarify that some of the Ce ions existed as Ce⁴⁺ (0.98 pm) [38, 39]. Moreover, the segregation of CeO₂ out of the perovskite phase is accompanied with NiO segregation, which is in accordance with the decline in La/Ni ratio, in addition to the fact that segregated CeO₂ and NiO cannot react mutually to form perovskite structure [40]. Similar effects were also observed by Lima et al who synthesized La_1−xCe_xNiO₃ NPs with different concentration of Cerium ion (x = 0.00, 0.05, 0.4 and 0.7) via precipitation technique [38].

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Cell volume (V_cell) was declined from 332.66 nm for pristine LNO to 330.10 nm for highest doping composition. The observed suppression in V_cell might be attributed again to significant ionic radii difference of B-site (Ni³⁺, Fe³⁺) cations than A-site (La³⁺, Ce³⁺) ions which results in decrease of lattice parameters and hence origination in lattice strain. The crystallite size related to most intense diffraction peak i.e. at 32.80° was calculated to be in 56–83 nm range. The increase in x-ray density (ρ_x−ray) values in substituted compositions might be due to doping of cations having greater atomic masses at host cations with lower masses as well as decline in V_cell on substitution onwards. While less values of bulk densities (ρ_m) in comparison to ρ_x−ray values was an indication of presence of some porosity in doped catalysts. The theoretical porosity calculated from ρ_m and ρ_x−ray data was calculated to be increased from 65.71% to 68% with increase in doping amount (table 1). The possible high porosity make such materials as efficient photocatalysts in degradation of dye effluents from industrial wastewater [27].

3.1.2. Surface analysis

Figures 2(a)–(f) shows the FESEM micrographs of Ce and Fe doped LaNiO₃ NPs. Almost all the compositions displayed relatively spherical and elongated shaped particles showing heterogeneous agglomeration having narrow range of grain size distribution. The approximated size analyzed from SEM micro images was calculated to be in 60–80 nm range which was found to be in agreement with XRD results (56–83 nm). The agglomeration of nanocrystallites might be due to fact that the nanoparticles having narrow dimensions and enough surface energy may merge together easily to form large clusters or aggregates [41]. This agglomeration was yet more significant in low doping compositions as demonstrated from SEM micrographs.

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3.1.3 Raman spectra

To get more insight in to the structure, La_1−xCe_xNi_1−yFe_xO₃ (x, y = 0.00, 0.10 and 0.20) NPs Raman analysis was performed (figure 3(A)). Group theory proposes that LaNiO₃ having rhombohedral structure with R3‾c space group shows five modes active in Raman spectrum: Γ_Raman = A₁g + 4Eg modes [42, 43]. Five main Raman peaks: E_g(1), E_g(2), A_g(1), E_g(3) and E_g(4) were observed at 71, 152, 209, 399, and 453 (cm⁻¹), respectively. The band located at 71 cm⁻¹ is out of recorded range of Raman spectra. Rotational–vibrational mode (A_g(1)) appeared at 209 cm⁻¹ relates to an antiferrodistortive type soft mode of R3‾c structure (crystalline) [44]. E_g(3) mode might be attributed to the NiO₆ octahedra vibrations. Broadening in A_g(1) and E_g modes was observed with increase in Ce and Fe concentration in LNO, this effect was however more pronounced in A_g(1) mode. The substitution of overall larger sized host cations by smaller sized substituted cations causes compressive strain in lattice at the both sites (tetrahedral and octahedral), which cause a blue-shift in Eg(2) and Eg(3) bands [45–47]. On increasing the doping content from x, y = 0.00 to 0.25, a shift of about 45 cm⁻¹ in the A_g(1) mode was observed. Earlier reports propose that position of A_1g mode in LaNiO₃ might be scaled by almost ∼23 cm⁻¹/° with tilting angle [42–44].

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3.2.1. Photoluminescence (PL) property

3.2.2. UV-Visible analysis

To study the optical behavior of pure and doped LNO compositions, the UV–vis DRS spectra was recorded in 220–800 cm range (figure 4(A)). The Pure LNO sample displayed an absorption edge at around 378 nm. However, the absorption tail of substituted LNO samples was shifted towards visible region indicating red-shifting in absorption which showed the high visible active absorption of doped materials. The red-shift noticed in DRS might be due to the interaction between 3d orbital of Ni³⁺ and 3d orbital of Fe³⁺ which introduces intra-energy band gap states [27]. Further, the other possible reason for this shifting could be ascribed to charge-transferring transition between the valance and conduction bands of doping elements and LaNiO₃ [51].

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The UV–visible absorption spectra of pristine LNO and La_1−xCe_xNi_1−yFe_yO₃ (x, y = 0.05–0.25) NPs is shown in figure 4(B). Tauc's model was employed to calculate the band gap energy (E_g) [52] (equation (4)).

$$\begin{eqnarray}&&{\left(\alpha {\rm{h}}\nu \right)}^{n}={\rm{k}}\left({\rm{h}}\nu -{E}_{g}\right)\end{eqnarray} \tag{ 4 }$$

Where, α and hν are the coefficient and photon energy of irradiation, while k and n denotes the type of transition, respectively. For pristine LNO, the E_g was found to be 2.77 eV while for x, y = 0.05–0.25 compositions declining trend in band gap i.e. 2.74–2.64 eV was noticed. The following causes might be accounted for this narrowing in band gap: (i) the generation of impurity energy band levels due to Fe³⁺ doping in LNO lattice, which occurred in center of CB and VB leading to reduction in E_g. (ii) charge transferring transition i.e. excitation of 3d orbital electrons of Fe³⁺ into conduction band of LNO (iii) or charge transfer between Fe³⁺ (Fe³⁺ + Fe³⁺ → Fe²⁺ + Fe⁴⁺) ions [27, 51].

The PCA of pristine LaNiO₃ and La_0.80Ce_0.20Ni_0.80 Fe_0.20O₃ NPs particles was calculated for CR dye. Figures 5(A)–(B) shows the spectra of the CR dye treated using pure and substituted LNO for 15 min under visible light exposure (CR dye + photo-catalyst). A 63% degradation of CR was observed in case of pure LNO after 120 min exposure of irradiation, however, the doped material (La_0.80Ce_0.20Ni_0.80Fe_0.20O₃) exhibits improved PCA, i.e., >97%. The enhanced catalytic activity of substituted LNO might be ascribed to structural defects caused by substitution of cations having different ionic radii and electronic charge because oxygen or cation vacancies are generated to maintain the electro neutrality by small sized Ce⁴⁺ ions substituting large sized La³⁺ ions. Infact, dopants not only alter the energy band gap (E_g) of base (LNO), but also promote the e⁻ and h⁺ and preclude them to recombine [53–55]. Table 2 shows the degradation comparison of CR by different photocatalytic reported earlier. The findings revealed that the La_0.80Ce_0.20Ni_0.80 Fe_0.20O₃ NPs is highly efficiency for the removal of dyes. Also, in view of present condition of ecological contamination [56–61], to develop and employ the efficient approaches for the mitigation of toxic pollutants [23, 62–64] is obligatory and La_0.80Ce_0.20Ni_0.80 Fe_0.20O₃ NPs under visible light irradiation is highly promising in this regard.

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In comparison to above mentioned photocatalysts, Ce and Fe substituted LNO photocatalyst in present study showed improved photodegradation efficiency. The Langmuir–Hinshelwood model was used for kinetic study (equation (5)).

$$\begin{eqnarray}&&{\rm{Rate}}=-\displaystyle \frac{{\rm{dc}}}{{\rm{dt}}}=\displaystyle \frac{{{\rm{K}}}_{{\rm{r}}}{{\rm{K}}}_{{\rm{c}}}}{1+{{\rm{K}}}_{{\rm{c}}}}\end{eqnarray} \tag{ 5 }$$

Where, -d_C/d_t is the degradation rate with time (t), whereas k_r and k_c represent the photoreaction rate constants and dye adsorption coefficient, respectively. Equation (5) can be modified to equation (6).

$$\begin{eqnarray}&&{\rm{Rate}}=-\displaystyle \frac{{\rm{dc}}}{{\rm{dt}}}={{\rm{K}}}_{{\rm{r}}}{{\rm{K}}}_{{\rm{c}}}=kc\end{eqnarray} \tag{ 6 }$$

Where, k (min⁻¹) denotes rate constant. At time t = 0 min and C = C_o. Equation (6) changes to equations (7) and (8).

$$\begin{eqnarray}&&{C}_{t}={C}_{0}{e}^{-kt}\end{eqnarray} \tag{ 7 }$$

$$\begin{eqnarray}&&-\,\mathrm{ln}\,\displaystyle \frac{{C}_{t}}{{C}_{0}}=kt\end{eqnarray} \tag{ 8 }$$

The relation −lnC_t/C_o versus time (t) was employed to estimate the rate constant (k) [72–74]. The linear fittingf of −1lnC_t/C_o verses time (t) reveals that CR due followed first order kinetics (figures 6(A)–(C)). Rate constants for pristine LNO and La_0.80Ce_0.20Ni_0.80 Fe_0.20O₃ for CR dye degradation under visible irradiation were 0.0084 and 0.012 (min⁻¹) respectively.

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3.4.1 Catalyst dosage

The rate of photodegradation of dyes is considered to be influenced by the concentration of catalyst dose. The effect of La_0.80Ce_0.20Ni_0.80Fe_0.20O₃ catalyst dose on CR dye removal using catalyst dose 10 to 40 mg/100 ml dye solution and the degradation rates were 0.0116, 0.0126, 0.0130, 0.0120 (min⁻¹) for 10–40 mg/100 ml, respectively (figure 7(A)). Outcome showed that rate of degradation was augmented with the catalyst dose up to 30 mg and declined beyond this dose. Low rate of degradation at low catalyst dose may probably due to fact that more light gets transmitted through the reaction mixture and minor transmitted will only be consumed in photocatalytic reaction. At high concentration of catalyst loading however, the increase in photodegradation is due to active sites on the catalyst surface due to doping [27]. This is essentially due to increase in number of hydroxyl ions generated on illumination of catalyst. After applying the optimum dosage of catalyst, the decline in rate of reaction attribute to opacity of reaction mixture, which causes scattering of light. Also, the agglomeration of photocatalyst particles at higher dose results in decline in active sites accessible for catalytic degradation thereby deactivating the activated molecules by the collision with those of in ground states [51].

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3.4.2. Dye initial concentration

3.4.3. Effect of pH

Photocatalytic degradation of CR due depends upon the electron/hole pairs, which is related to band gap energy [24]. Upon irradiation, the electrons (e⁻) in valance band (VB) of La_0.80Ce_0.20Ni_0.80Fe_0.20O₃ by absorbing light energy ≥ band gap, are excited to conduction band (CB) leaving behind the holes (h⁺) in valance band (VB). Reaction between the photo excited electron and O₂ produce O₂ ^•− radical and finally, this is converted in to HOO^• radical, which changes in to HO^• radical by reacting with water molecule (figure 8). This HO^• destruct dye by oxidation mechanism into pbyroducts [51]. Photoinduced holes in valance band (h_VB ⁺) also yield HO^• radicals by breaking down the water molecule, which directly involve in degradation of dye [27]. There is possibility of recombination of photoinduced e⁻ and h⁺, which may suppress the degradation competence of the catalyst. The chance of this possible recombination can be minimized by creation of structural defects, lattice distortion and oxygen anion (O²⁻) vacancies catalyst. Perovskite based catalysts with lattice distortion provide a pathway for the suppression of e⁻–h⁺ recombination thereby displaying better catalytic competence. Actually, these oxygen anion vacancies in perovskite structure enhance O₂ adsorption onto the catalyst surface, thus improving the degradation efficacy. Doped catalysts generate HO^• species, which degrade the dye into H₂O and CO₂ and inorganic ions [1, 2, 27, 51, 75] (equations (9)–(13)). The effect of different scavengers on the degradation of CR dye over La_0.80Ce_0.20Ni_0.80Fe_0.20O₃ was studied using KI, Ascorbic acid and n-butanol as scavengers and response is presented in supporting information.

$$\begin{eqnarray}&&L{a}_{0.80}C{e}_{0.20}N{i}_{0.80}F{e}_{0.20}{O}_{3}+{\rm{h}}\upsilon \left({\rm{visible}}\,{\rm{light}}\right)\to L{a}_{0.80}C{e}_{0.20}N{i}_{0.80}F{e}_{0.20}{O}_{3}\left({e}_{CB}^{-}+{h}_{VB}^{+}\right)\end{eqnarray} \tag{ 9 }$$

$$\begin{eqnarray}&&{{\rm{e}}}_{{\rm{CB}}}^{-}+{{\rm{O}}}_{2}\to {{\rm{O}}}_{2}^{\bullet -}\end{eqnarray} \tag{ 10 }$$

$$\begin{eqnarray}&&{{\rm{O}}}_{2}^{\bullet -}+{{\rm{H}}}_{2}{\rm{O}}\to \to \to H{O}^{\bullet }\end{eqnarray} \tag{ 11 }$$

$$\begin{eqnarray}&&{h}_{VB}^{+}+{{\rm{H}}}_{2}{\rm{O}}\to H{O}^{\bullet }\end{eqnarray} \tag{ 12 }$$

$$\begin{eqnarray}&&H{O}^{\bullet }+dye\to {{\rm{CO}}}_{2}+{{\rm{H}}}_{2}{\rm{O}}+{\rm{inorganic}}\,{\rm{ions}}\end{eqnarray} \tag{ 13 }$$

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