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

A series of La1−xCexNi1−yFeyO3 (x, y = 0.00–0.25) NPs was fabricated via micro-emulsion route and effect of doping was investigated on the basis of optical, photocatalytic and structural properties. The as-synthesized NPs were characterized via XRD, Raman analysis, SEM and UV–visible techniques. The XRD results confirmed the rhombohedral perovskite phase particles with particles of 60–80 nm range. UV–vis absorption edge showed significant red shift thereby tuning the band gap from 2.77 to 2.64 eV. The photocatalytic effectiveness of LaNiO3 and La0.80Ce0.20Ni0.80Fe0.20O3 catalysts was performed by degrading Congo red (CR) dye under visible light exposure. Substituted catalyst exhibited superior photodegradation by showing 97% degradation in comparison to pristine LaNiO3 (63% only) in 120 min. Degradation of CR followed the pseudo fist order kinetics. In addition, the catalyst dose effect, dye concentration and pH variation was studied for Cr dye degradation. Enhanced photocatalytic activity and narrow bandgap of Ce and Fe doped LaNiO3 introduce such materials as efficient visible active photocatalysts to be utilized in dye removal application from waste water and in photovoltaic applications, respectively.


Introduction
In recent years, a great concern for health and environmental remediation has driven an inclusive interest to design and implement different materials and routes in removing the harmful and toxic pollutants from industrial waste water. Degradation of organic particulates in wastewater by oxidative photocatalysis has gained much attention [1][2][3]. Effluents from industrial units, i.e., textile, tanneries, leather, printing industries containing water soluble organic dyes are discharged into water bodies. These water-soluble organic dyes and other pollutants are thought to be the major source of water pollution as they are non-biodegradable and toxic, which impose a series health issues to humans, animals, plants and marine organisms [4][5][6][7][8]. Regrettably, only a few industrial units treat their wastes by different techniques including chemical, physical and biological approaches. But these methods are inadequate to remove pollutants effectively, which need advanced approach for the mitigation of pollutants [9][10][11][12][13][14][15]. Actually, these dye eradicating techniques have their own limitations, i.e., selective for specific dyes only, low removal efficiency, causing secondary pollution issue, and high operational cost. Hence, there is demand of efficient, novel, economic, ecofriendly approach for complete removal of these 2. Material and methods

Synthesis of La 1−x Ce x Ni 1−y Fe y O 3
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, NH 4 OH 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 LaNiO 3 [32]. (Equations (1)-(2)).

Characterization
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.  (3))

Dye degradation evaluation
Where, C o and C t represent the dye concentration at zero time and specific time 't', respectively.

Results and discussion
3.1. XRD analysis Figure 1 depicts XRD analysis of La 1−x Ce x Ni 1−y Fe x O 3 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 LaNiO 3 (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 Ce 3+ and Fe 3+ 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 Ce 3+ (115 pm) and La 3+ (117 pm) at A-site compared with Ni 3+ (0.70 pm) and Fe 3+ (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 [38,39]. Moreover, the segregation of CeO 2 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 CeO 2 and NiO cannot react mutually to form perovskite structure [40]. Similar effects were also observed by Lima et al who synthesized La 1−x Ce x NiO 3 NPs with different concentration of Cerium ion (x=0.00, 0.05, 0.4 and 0.7) via precipitation technique [38].
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 (Ni 3+ , Fe 3+ ) cations than A-site (La 3+ , Ce 3+ ) 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]. LaNiO 3 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.

Raman spectra
To get more insight in to the structure, La 1−x Ce x Ni 1−y Fe x O 3 (x, y=0.00, 0.10 and 0.20) NPs Raman analysis was performed (figure 3(A)). Group theory proposes that LaNiO 3 having rhombohedral structure with R3‾c space group shows five modes active in Raman spectrum: Γ Raman =A 1 g+4Eg modes [42,43]. Five main Raman peaks: 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 NiO 6 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][46][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 LaNiO 3 might be scaled by almost ∼23 cm −1 /°with tilting angle [42][43][44].

Optical analysis
3.2.1. Photoluminescence (PL) property PL spectroscopy, being a reliable tool has been employed to analyze the electronic structure, migration and recombination of photoinduced electron-hole pair phenomenon in a material [48,49]. The charge transferring rate of these light induced carriers is associated to intensity of PL peak, and the peak height specifies the rate at which holes and electron combine [50]. Figure 3(B) depicts the induced fluorescence intensity of all compositions of La 1−x Ce x Ni 1−y Fe y O 3 (x, y=0.00-0.25) recorded at λ exc =480 nm. As the graph shows, the similar and symmetrical PL emission peaks were appeared at 538 nm for all the samples and their intensities were reduced onwards with increase in doping content in LNO crystal structure. This decline in intensity specifies the inhibitory character of doped elements in the recombination of e − , h + thereby enhancing the charge separation and ultimately improving the photocatalytic performance of substituted LNO samples [27].
Probably, on substitution of host cations by Ce 3+ and Fe 3+ in perovskite structure, more electronic states are generated which inhibits the recombination of e − -h + and efficiently stabilize the charge carriers in doped martials [51]. Among all the synthesized materials, the La 0.80 Ce 0.20 Ni 0.80 Fe 0.20 O 3 composition showed the weakest intensity of the PL emission peak. The decline in PL intensity was in consistent with the increase in charge separation or suppression in e − , h + recombination, which suggested that the La 0.80 Ce 0.20 Ni 0.80 Fe 0.20 O 3 composition should impart improved photocatalytic performance by taking the advantage of its highest charge separation and lower recombination possibility of the photoinduced electron-hole pair.

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 redshifting 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 Ni 3+ and 3d orbital of Fe 3+ which introduces intraenergy band gap states [27]. Further, the other possible reason for this shifting could be ascribed to chargetransferring transition between the valance and conduction bands of doping elements and LaNiO 3 [51].
The UV-visible absorption spectra of pristine LNO and La 1−x Ce x Ni 1−y Fe y O 3 (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)).
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.  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 Ce 4+ ions substituting large sized La 3+ 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][54][55].  (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).

Effect of reaction parameters on dye degradation 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 0.80 Ce 0.20 Ni 0.80 Fe 0.20 O 3 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].

Dye initial concentration
The dye concentration impact on the removal of dye was considered in 10-40 mg l −1 range using the optimum dose of catalyst, i.e., 30 mg l −1 . The values of rate constants for aforementioned concentrations of dye were 0.0130, 0.0136, 0.0119, 0.012 min −1 respectively. Outcome displayed that rate of degradation was enhanced with the dye concentration up to 20 mg L −1 and then, it was declined ( figure 7(B)). Infact, the generation of radicals (hydroxyl · ) on the surface of catalyst and reaction between OH • radicals and dye molecules was higher, which enhanced the dye removal. Initially, on increasing the concentration of dye more dye ions are available for excitation as well as for energy transfer [51]. This is due to the adsorption of dye on the surface (catalyst), which is favored at higher dye concentration until on reaching the critical level, the catalyst surface becomes fully covered leading to constant rate of reaction [27]. Decline in degradation efficacy at higher concentration of dye, on the other hand happens due to several reasons. On increasing the conc. of dye, more dye ions are getting adsorbed on catalyst surface and major extent of visible light is absorbed by the dye molecules, which may decrease the light penetration and resultantly, dye removal rate was diminished [51]. The active sites was occupied dye ions, generation of hydroxyl radicals may also be decreased. The intermediates adsorbed on the catalyst surface also impedes the hydroxyl radical generation, which declined the CR dye removal [27].  decreased for pH 10. This trend in variation of photoreaction might be due to the formation of oxygen anion O 2− radical due to the reaction between photoinduced e − and O 2 molecules, which facilitates the rate of photocatalytic degradation process. Decline in degradation rate with increase in pH might be ascribed to repulsive forces between the negatively charged surface of La 0.80 Ce 0.20 Ni 0.80 Fe 0.20 O 3 and anionic species. In acidic pH range, it is probable that a large amount of conjugate base (Cl − ions) may be increased in the solution.

Effect of pH
A possible reaction might be occurred between Cl − ions and OH • radicals thus producing ClO −• radicals, which being less reactive in comparison to OH • radicals may result a decrease in rate of photodegradation reaction [27].

Photodegradation mechanism
Photocatalytic degradation of CR due depends upon the electron/hole pairs, which is related to band gap energy [24].  (13)). The effect of different scavengers on the degradation of CR dye over La 0.80 Ce 0.20 Ni 0.80 Fe 0.20 O 3 was studied using KI, Ascorbic acid and n-butanol as scavengers and response is presented in supporting information.

Conclusion
Herein we report the Ce and Fe substituted LaNiO 3 NPs synthesized successfully by micro-emulsion approach and the influence of doping (Ce and Fe) on structural and phtotocatalytic properties of LaNiO 3 in detail. The rhombohedral crystalline phase of as-prepared NPs was established from XRD results. SEM results showed relatively clustering and agglomeration in particles with average crystallite size in 60-80 nm range.