Impact of the annealing temperature on perovskite strontium doped neodymium manganites nanocomposites and their photocatalytic performances

Impact of the annealing temperature on perovskite strontium doped neodymium manganites nanocomposites and their photocatalytic performances

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Impact of the annealing temperature on perovskite strontium doped neodymium manganites nanocomposites and their photocatalytic performances I.A. Abdel-Latif a,c,d,∗, Adel A. Ismail b,∗, M. Faisal c, Atif M. Ali e,f, A.E. Al-Salmi e, A. Al-Hajry a,c a

Physics Department, College of Science, Najran University, Najran, P.O. Box 1988, Najran 11001, Saudi Arabia Nanostructured Materials and Nanotechnology Division, Central Metallurgical Research and Development Institute (CMRDI), P.O. Box: 87 Helwan, Cairo 11421, Egypt c Advanced Materials and Nano-Research Centre, Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia d Reactor Physics Department, NRC, Atomic Energy Authority, Abou Zabaal P.O. Box 13759, Cairo, Egypt e Research Center for Advanced Materials Science (RCAMS), King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia f Department of Physics, Faculty of Science, Assiut University, Assiut 71516, Egypt b

a r t i c l e

i n f o

Article history: Received 25 October 2016 Revised 19 March 2017 Accepted 22 March 2017 Available online xxx Keywords: Perovskite Nd0.6sr0.4mno3 Nanocomposites Photocatalysts Visible light

a b s t r a c t In the present work, Nd0.6 Sr0.4 MnO3 nanocomposite perovskite has been synthesized using sol-gel method in the presence of polyethylene glycol and citric acid. As-synthesized Nd0.6 Sr0.4 MnO3 perovskite was annealed at different temperatures (500–1150 °C). The obtained perovskite Nd0.6 Sr0.4 MnO3 nanocomposites exhibit wide optical absorption in the UV–visible range with low bandgap values ∼ 2–2.98 eV relied on annealing temperatures. The crystalline size of the annealed sample at 500 °C was 55 nm increased with boosting temperature to 168 nm at 1150 °C. The newly prepared perovskite has been assessed by MB photodegradation under visible light. Our findings reveal that, Nd0.6 Sr0.4 MnO3 perovskite with 26.18% orthorhombic and 73.82% monoclinic phases annealed at 500 °C is a superior photocatalyst than that of Nd0.6 Sr0.4 MnO3 perovskite containing 82.22% cubic and 17.78% orthorhombic phases annealed at 1150 °C. 100% of MB could be photodegraded by the Nd0.6 Sr0.4 MnO3 perovskite annealed at 500 °C. However, with the increase annealing temperature to 1150 °C, the photocatalytic efficiency was reduced to 60%. The overall photodegradation rate of Nd0.6 Sr0.4 MnO3 perovskite annealed at 500 °C is significantly 3-times higher than that of Nd0.6 Sr0.4 MnO3 perovskite annealed at 1150 °C. The superiority of the perovskite Nd0.6 Sr0.4 MnO3 annealed at 500 °C is attributed to the double phases formed (monoclinic/orthorhombic) framework, high crystallinity, and the high distortion of the Mn-O polyhedron. It is believed that the high visible-light absorption, lattice distortion and narrow band gap are considered to be the key factors for the high photocatalytic activity of the obtained Nd0.6 Sr0.4 MnO3 perovskite annealed at 500 °C. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction ABO3 perovskites are very essential family of oxide materials and possess very interesting physical and chemical properties that may lead to potential applications. They include the corner-shared octahedral BO6 lattice, which could be contributed to transfer of oxygen and electrons easily and may lead to nonstoichiometry of oxygen [1–16]. Moreover, the valence state of the transition metal at B-site is also critical parameter in perovskite-type oxides activity. Nevertheless A and B cations species, the ABO3 perovskites ∗ Corresponding author at: Physics Department, College of Science, Najran University, Najran, P.O. Box 1988, Najran 11001, Saudi Arabia. E-mail addresses: [email protected] (I.A. Abdel-Latif), [email protected] (A.A. Ismail).

exhibit a lot of applications, such as electrocatalysts for O2 evolution [1–3], catalysts [4,5], photo/electro- catalysts for hydrogen production and pollutants degradation [6–12] and electrode material in fuel cells [13]. Perovskite oxides have been synthesized such as tantalate [14–18], titanate [19–26], ferrite, [27–28] vanadium-and niobium-based perovskites [29–32], and manganites [33–35] and they have shown visible light photocatalytic activity as a result of their unique electronic properties and crystal structures [36]. The new doped alkaline rare-earth transition metal perovskite oxides are received predominately attention owing to reduce band-gap energy values and enhances the separation of charge carriers (photogenerated electrons and holes) [37]. A lot of studies have been done on these materials due to the capability of tuning

http://dx.doi.org/10.1016/j.jtice.2017.03.030 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: I.A. Abdel-Latif et al., Impact of the annealing temperature on perovskite strontium doped neodymium manganites nanocomposites and their photocatalytic performances, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.03.030

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their electrical and optical properties, indicating a wide range of potential applications and also due to the potential in governing their rational design structure by substitutions of cationic in ABO3 pervoskite [38,39]. The perovskite compounds exhibit well structure with adapting the bandgap values to harvest visible-light absorption and the potentials of band edge to tailor the needs of particular photocatalysis. Furthermore, the lattice distortion of perovskite compounds strongly affects the separation of photogenerated charge carriers [36,40,41]. The distortion bond angles of either metal–ligand or the metal–ligand–metal into perovskite framework are significantly related to their charge carriers and band gap values [42–44]. The efficiency of photocatalysts relies on their crystallinity, phase structure, size, and surface area. Consequently, control of shape of perovskites and the size and crystal phase is critically significant for assessing their phase-dependent photoactivity and promoting pervoskites-based driven visible light photocatalysts. To promote the perovskite Nd0.6 Sr0.4 MnO3 as superior photocatalyst under visible light, different modifications of perovskite Nd0.6 Sr0.4 MnO3 have been carried out to obtain high harvesting of photons and enhancing the migration and separation of the photogenerated charge carriers through the photocatalytic reaction [40–44]. In this contribution, for the first time, Nd0.6 Sr0.4 MnO3 perovskite has been prepared by sol–gel method in the presence of polyethylene glycol and citric acid. The obtained perovskite Nd0.6 Sr0.4 MnO3 was annealed at different temperatures to investigate their impact on phase structures and photocatalytic efficiencies under visible light. Nd0.6 Sr0.4 MnO3 perovskite annealed at 500 °C is a superior photocatalyst than that annealed at 800, 10 0 0 and 1150 °C.

man Station 400 was employed to record Raman spectra for all prepared samples. The reflectance spectra of all obtained samples were performed using UV–visible spectrophotometer (lambda 950 PerkinElmer) connected with universal reflectance accessory in the range from 200 to 800 nm. When the UV–vis diffuse reflectance spectra (R) measurements were conducted, the results were converted to the Kubelka–Munk function F(R) to subtract the light absorption extent from scattering one [39,40]. 3. Photocatalysis experiments In the photocatalytic reaction, methylene blue (MB) was employed as a model pollutant to assess the photocatalytic efficiency of perovskites NSMO-50 0, NSMO-80 0, NSMO-10 0 0 and NSMO1150 nanocomposites. Photocatalytic reactions were performed in 100 ml reactor by stirring with a magnetic bar and 250 W visible lamp (Osram, Germany) was horizontally maintained above the photoreactor. Before the illumination, a suspension containing NSMO samples and MB (0.02 mM) was continuously stirred for 2 h without light source, to acquire adsorption equilibrium. Thus, the adsorbed MB through adsorption reactions was taken into consideration in our calculations. The suspension was continuously stirred and purged with oxygen bubbling throughout the experiment. The photocatalytic efficiency of the prepared photocatalysts for photodegradation of MB dye was monitored by acquiring the absorption spectra at regular interval. The MB concentration was recorded using UV–visible spectrophotometer at λ = 663 nm absorbance matching to the maximum absorption wavelength of MB. 4. Results and discussions

2. Experimental details 4.1. Nd0.6 Sr0.4 MnO3 structure investigations 2.1. Materials Sr(NO3 )2, Nd(NO3 )3 ·6H2 O, and Mn(NO3 )2 , citric acid, polyethylene glycol (average M.W. = 190 0–220 0), and NH4 OH (28–30% NH3 ) were purchased from Sigma–Aldrich Chemical Company. All the used chemicals in the present work were of analytical grade and used without further purification. 2.2. Preparation of Nd0.6 Sr0.4 MnO3 perovskites (NSMO) In our previous work, R0.6 Sr0.4 MnO3 nanocomposite perovskites were synthesized and annealed at 800 °C [45]. In the current contribution, we have adapted our previous work as follow: In a typical experiment, appropriate amounts of Nd(NO3 )3 ·6H2 O, Sr(NO3 )2 and Mn(NO3 )2 ·6H2 O were added and dissolved in 100 ml H2 O, and subsequently citric acid as chelating agent was mixed with the above solution with continuous stirring for 1 h at 80 °C (the molar ratio of nitrate salts to citric acid is 1‫׃‬2). Then, 1 g of polyethylene glycol as structure-directing agent was gradually added to mixture with continues stirring for more 2 h. The pH of the mixture solution was adjusted by adding few drops of NH4 OH to maintain the value at 8 to produce pure solution. The obtained sol was aged for 24 h at 80 °C to evaporate H2 O and polymerization organic compounds including inorganic oxides until the gels formed. The obtained as-prepared powder was annealed at 50 0, 80 0, 10 0 0 and 1150 °C for 6 h to produce NSMO-50 0, NSMO-80 0, NSMO-10 0 0 and NSMO-1150. 2.3. Characterizations Field emission-secondary electron microscope (FE-SEM) images were acquired with a FE scanning electron microanalyzer (JEOL6300F, 5 kV). PANalytical diffractometer was used for recording Xray diffraction (XRD) data using Cu X ray tube. Perkin Elmer Ra-

Crystal structures of rare earth manganites are in different crystalline forms and range from orthorhombic [46–49], rhombohedral [50–52], hexagonal [53–56], and monoclinic [57–59] to the cubic [60–63] perovskites crystal structure. The perovskites phase structure relies on the procedure of synthesis and the annealing temperatures. The crystallization mechanism of NSMO perovskite is shown in Scheme 1 where in the beginning the oxygen atoms are not in the stoichiometric state (Scheme 1A and B) at 500 and 800 °C. However, at 1150 °C, oxygens occupy their position (Scheme 1C) and the cubic shape crystalline was formed. The XRD measurements of Nd0.6 Sr0.4 MnO3 acquired after at different annealing temperatures are depicted in Fig. 1. XRD patterns were refined using Rietveld method (Fullprof software [64]) indicating the orthorhombic and the monoclinic crystal symmetry with different percentage ratios relying on annealing temperatures 50 0, 80 0, and 10 0 0 °C were formed (Table 1), while the crystal structure transformation was observed at 1150 °C within crystal symmetry of cubic and small orthorhombic. Both the orthorhombic, with the Pbnm symmetry, and the monoclinic, of C2/c space group, phases existed as a result of the annealing at temperatures of 50 0, 80 0 and 10 0 0 °C. At high annealing temperature 1150 °C, the cubic crystal structure was observed as orthorhombic crystal form with space groups; Pm −3 m and Immm, respectively. The calculated crystalline size using the well-known Scherrer formula and lattice parameters for all prepared samples are listed in Table 1.

Crystalline size = kλ/(B sinθ ), where B is equal to Bobs .–Bstd . (Bobs . is FWHM of observed sample and Bstd is FWHM of standard sample). The calculated crystalline size and lattice parameters for all prepared samples are listed in Table 1. The crystalline size is in the range 55–168 nm for Nd0.6 Sr0.4 MnO3 depending on the annealing temperatures. The Nd/Sr atoms, in the orthorhombic symmetry Pbnm of NSMO-500,

Please cite this article as: I.A. Abdel-Latif et al., Impact of the annealing temperature on perovskite strontium doped neodymium manganites nanocomposites and their photocatalytic performances, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.03.030

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Scheme 1. The different stages of oxygen formation in the unit cell. Table 1 Lattice parameters and phases structure for all prepared samples.

occupy (x, y, 1/4) positions, where x = 0.01082 and y = −0.02568, while the Mn atoms occupy (0, 0, 1/2) positions. Oxygen atom positions in our structure are very important where four O(1) atoms occupy the (x, y,1/4) coordinates and eight O(2) atoms have (x, y, z) coordinates. The position of O(1) atoms in the unit cell situated in (0.71870, 0.10311, 0.250 0 0) and the O(2) atoms po-

sition is in (0.29298, −0.05321, −0.05697). In the C2/c monoclinic phase of NSMO-500 Mn atoms occupy two positions; Mn1 at (0.0 0 0 0 0, 0.50 0 0 0, 0.0 0 0 0 0) coordinates and Mn2 at (0.0 0 0 0 0, 0.0 0 0 0 0, 0.0 0 0 0 0) coordinates, respectively. Nd/Sr atoms are located in (0.23864, 0.27431, and 0.250 0 0) position. Oxygen atoms O1, O2, O3 and O4 exist in (0.08431, 0.07051, 0.00281), (−0.02365,

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Raman Intensity (a.u.)

1150 oC

1000 oC

800 oC Fig. 1. XRD patterns of nanocomposites Nd0.6 Sr0.4 MnO3 pervoskite at different annealing temperatures.

500 oC Ag

0.24076, 0.05183), (0.0 0 0 0 0, 0.01747, 0.250 0 0) and (0.0 0 0 0 0, 0.57923, 0.250 0 0) positions, respectively. There are slightly differences in the positions of Nd-Sr-Mn-O atoms in NSMO-800 and NSMO-10 0 0, which still possess the same phases. This difference leads to change in distortion. Jahn–Teller distortion is given by the octahedral distortion parameter d which defined as [65].



6 1  ( dn − d ) d = 6 d

2

Ag 200

400

Ag

Ag&B2g

600

800

1000

Raman shift (cm-1) Fig. 2. Raman spectra of nanocomposites Nd0.6Sr0.4MnO3 pervoskite at different annealing temperatures. Shifted for sake of clarity. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

n=1

where d is the mean manganese-oxygen bond distance (Mn–O) and dn are the individual manganese-oxygen (Mn–O) bond distances. The distortion calculated for NSMO-500 showed maximum distortion of the orthorhombic unit cells within d∼1.37 × 10−10 and the distortion decreased to d∼1.19 × 10−5 for NSMO-10 0 0. The distortion is very important parameter in understanding properties of materials and photocatalysis application as shown below [65–66]. Raman spectroscopy was employed to confirm the orthorhombic pbnm structure (long axis c) and the chemical phase. For the NSMO orthorhombic compound, there are sixty phonon modes associated with the  -point. 24 of them are Raman active modes (7 Ag + 7 B1g + 5 B2g + 5 B3g ), where Ag + B1g , 2 Ag + 2 B2g + B1g + B3g , Ag + 2 B1g + B3g , 3 Ag + B2g + 3B1g + B2g , and 2B2g + 2B3g are symmetric modes, rotation and tilt modes of the octahedral, bending modes, modes related locally to the rareearth ion movements and antisymmetric stretching modes, respectively [67]. The remaining modes are: eight inactive modes (8 Au ), three acoustic translational modes (1 B1u + 1 B2u + 1 B3u ), and twenty five infrared-active modes (7 B1u + 9 B2u + 9 B3u ) [67,68]. In the present study, Raman spectra show many Raman peaks located at frequencies of ∼230, ∼313, ∼380, ∼430, ∼460, ∼530, ∼560, ∼610, ∼630, ∼710, ∼725, ∼830 and ∼875 cm−1 , respectively (Fig. 2). Most of these peaks exhibit a red shift with annealing temperatures, as shown in Fig. 2, due to a structural phase transformation and/or NSMO nanocomposite is under strain. It is well known that there are two kind of stress; one is a compressive stress results in the shift of the Raman peak frequency of towards higher wavenumber region (blue shift), and the second one is a tensile stress resulting in a shift towards lower wave number of Raman peak (red shift) [69,70]. Therefore, the red Raman peak shift indicates a decrease in the compressive stress or an increase in the tensile stress [71]. Jandl et al. [68] studied of Raman active Ag and B2g phonon for NdMnO3 perovskite at different annealing temperatures. They observed Raman peaks for B2g symmetry at wavenum-

ber of 314, 453, 482, 500, and for Ag symmetry at 601 cm−1 and 205, 245, 335, 468, and 495 cm−1 [68] in good agreement with the present work at peak frequency <600 cm−1 (see Fig. 2). In addition, both Ag and B2g symmetry was assigned an excitation at 645 cm−1 [68,72]. Martin–Carron et al. [73] observed few Raman peaks at 280, 480, and 610 cm−1 . These peaks related to the tilt of the octahedral (Ag symmetry), the symmetric stretching (Ag symmetry) accompanied with distortion of Jahn–Teller (JT), and the symmetric stretching of the basic oxygen of the octahedral (B1g symmetry), respectively [73]. In addition, four Raman peaks observed by Martin–Carron et al. in another work [74] at peak frequencies of 242, 323, 490, and 606 cm−1 . The observed peaks are related to octahedral z-rotation, octahedral tilt, asymmetric stretching (JT mode), and symmetric stretching, respectively. [74] The main features of the NSMO Raman spectra correspond to three high frequency peaks at ∼725, ∼830, and ∼875 cm−1 (see Fig. 2). We suggest that the highest frequencies peaks of 700–950 cm−1 assign to an in phase stretching mode of oxygen in a close vicinity of the rare earth ion. As the annealing temperatures increase up to 10 0 0 °C, the intensity of the first peak (∼725 cm−1 ) increases then decreases in good agreement with the structure transition from orthorhombic phase to cubic perovskite structure (see Table 1). On the other hand, the second two peaks (shoulder at ∼830 cm−1 and peak at ∼875 cm−1 ) modified to one peak and then divided into two peaks with the annealing temperatures corresponding well with the change in the monoclinic structure as seen in Table 1. The crystallite morphology upon annealing temperatures changing was investigated using FE-SEM images. As shown in Fig. 3a, NSMO-500 exhibited homogeneous spherical particles and disarrayed nanosheets. The particle size of NSMO-500 is likely 50 nm. With increasing annealing temperature as shown in NSMO-800 sample, the SEM shows a spherical particles and rods (Fig. 3a and b). However, at NSMO-10 0 0 sample, the observed particles were aggregated with few rods and the pores between the particles

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Fig. 3. FE-SEM of nanocomposites Nd0.6 Sr0.4 MnO3 pervoskite annealing at 50 0 °C (a), 80 0 °C (b), 10 0 0 °C (c), and 1150 °C (d), the wide area EDX pattern obtained from nanocomposites Nd0.6 Sr0.4 MnO3 pervoskite at 500 °C (e).

was blocked. NSMO-1150 reveals polygon-shaped crystallites with the particle size of 0.1–0.3 μm as a result of phase transformation from monoclinic/orthorhombic to cubic/monoclinic. The FESEM images are in consistent with XRD and Raman results. The analytical atomic ratio of NSMO using EDS shown in Fig. 3e has a good match with stoichiometric ratio of the obtained NSMO perovskite. It is clearly indicated the presence of Sr, Mn, Nd and O ions.

Fig. 4 shows the values of (α hυ )2 versus photon energy for NSMO-500 nanocomposite at different annealing temperature. From these curves, the optical band gaps (Eg ) can be obtained from the Tauc equation given by [75]:

(α hυ )2 = A(hυ − Eg ) where hυ is the incident photon energy, A is a feature parameter independent of the photon energy and α is the absorption coef-

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a

b

Fig. 4. Optical bandgap energy for Nd0.6 Sr0.4 MnO3 nanocomposites annealing at 500 °C(a), relation between the Eg values and the ratio of monoclinic structure (b).

ficient. The extrapolation edge part of the linear absorption spectrum with a straight line to (α hυ )2 = 0 axis provides the value of the Eg . We find a close relation between the Eg values and the ratio of monoclinic structure as shown in Fig. 4b and Table 1. The expected characteristic could be elucidated by the variation in the particles size and phase transformation with annealing temperatures. Both two factors can take part to adjust the band structure of d level of the transition metal and the O 2p level [76]. So, the intense absorption edge on the spectra in the visible light zone can be principally ascribed to the excitation of electrons from O 2p valence band to the Mn 3d conduction band [34]. Thus, the substitution of Mn in perovskites is considered a useful method to tune electronic structures and the band gap values of NSMO perovskites, and their crystal structures can impact on their optical properties [77]. 4.2. Photocatalytic performance of Nd0.6 Sr0.4 MnO3 nanocomposites Nd0.6 Sr0.4 MnO3 (NSMO) is a semiconductor with a narrow band gap energy values ranged from 2 to 2.98 eV that vary by change in annealing temperatures. Upon excitation by a visible light illumination, each of the absorbed photons can create charge carriers (photogenerated electrons and holes) into the bulk or onto the surface of the NSMO nanocomposites. The photogenerated hole pro-

Fig. 5. Change in concentration vs. illumination time in the presence of Nd0.6 Sr0.4 MnO3 nanocomposites annealing at 500 °C (a) and 1150 °C (b) for MB photodegradation.

duced in the valence band reacts with the adsorbed − OH ions or H2 O onto the surface of NSMO to generate OH· ; meanwhile photogenerated electron in the conduction band reduces O2 to get O2 · ¯ and therefore other oxidative O2 species (i.e., OH· and H2 O2 ). Oxidative O2 species are exceedingly reactive for the sake of photodegradation of toxic organic compounds [78]. The photocatalytic efficiencies of the NSMO nanocomposites were evaluated for the MB photodegradation. The MB photodecomposition under visible light illumination was calculated by recording absorption spectra. The findings indicated that the photolysis MB is negligible and it is stable after visible light illumination for 3 h Furthermore, when the NSMO photocatalysts are suspended with MB solution in dark, there is a slight decrease in MB concentration as a result of adsorption onto NSMO surface. The experimental findings are displayed in Fig. 5. The intense MB absorption bands assigned at λ = 663 nm and λ = 291 nm gradually decrease upon boosting illumination times. As obviously in Fig. 6, the photocatalytic activity was significantly promoted when NSMO-500 was served as a photocatalyst and it exhibits the highest photocatalytic efficiency for MB photodegradation. For NSMO-500, the absorbance at 663 nm is drastically decreased from 1.2 to 0.07 after 3 h of illumination

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a

b

Fig. 6. (a) The variation of (C/C0 ) versus illumination time for MB photodegradation using nanocomposites Nd0.6 Sr0.4 MnO3 (NSMO) pervoskite at different annealing temperatures 50 0, 80 0, 10 0 0 and 1150 °C. (b) MB photodegradation rate over the nanocomposites Nd0.6 Sr0.4 MnO3 (NSMO) pervoskite at different annealing temperatures 500, 800, 1000 and 1150 °C.

(Fig. 5a). Whereas, NSMO-1150 was able to degrade MB only to an absorbance value of 0.41 at 663 nm (Fig. 5b). The concentration of MB gradually decreases with the boosting illumination time and the photocatalytic efficiency reaches to 100%, suggesting the complete of MB decomposition (Figs. 5 and 6). These results obviously reveal that the MB decoloration can be accomplished under visible light illumination. MB decolorizes either by oxidizing MB or by reducing via two-electrons to its colorless form [79,80]. We could determine a tiny peak at 256 nm of the distinctive leuco-MB absorption band. As a consequence, the MB decoloration is explained by oxidizing MB degradation. Fig. 6a reveals the C/Co against illumination time. It is distinctly demonstrated that the photocatalytic efficiency of NSMO500 photocatalyst was significantly enhanced for the MB degradation as compared to the all obtained samples. 100% of MB could be photodegraded by the NSMO-500 photocatalyst under visible light for 3 h illumination, however, with the increase in annealing temperature to 1150 °C, the photocatalytic efficiency reduced to 60% which can be ascribed to its fast recombination rate of

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charge carriers. This enhancement of the photocatalytic performance is explained by the synergistic effect between orthorhombic (26.18%) and monoclinic (73.82%) structures, which has a necessary role in the charge carriers’ separation. On the other hand, the MB photodegradation rates in the presence of NSMO at various annealing temperatures shown in Fig. 6b. MB photodegradation rate was calculated by measuring the slope of linearly decrease of MB absorbance during illumination time. The MB photodegradation rate moves quicker in the presence of NSMO-500 photocatalyst (3 × 10−7 molL−1 min−1 ) as compared to NSMO-1150 photocatalyst (1.16 × 10−7 molL−1 min−1 ). It was demonstrated that the photodegradation rate was linearly decreased with boosting annealing temperature from 500 to 1150 °C. Interestingly, the NSMO500 photocatalyst has the highest MB photodegradation rate three times faster than NSMO-1150 photocatalyst. The possible explanation is given by three reasons as follows: As obvious in Fig. 6 and Table 1, NSMO-500 which consists of 26.18% orthorhombic and 73.82% monoclinic displays a higher photocatalytic efficiency than other prepared samples annealed at higher temperatures. This can be attributed to the variation in band gaps Eg values of orthorhombic and monoclinic [81,82]. It is believed to promote electron transfer interfacial and the energy barrier would inhibit electron reverse transfer. Consequently, the holes behind in the valence band effectively oxidize MB. However, the photogenerated electrons in conduction band were reduced to adsorbed O2 . This leads to better charge carrier separation by moving the excited electrons from one phase to the other one [81]. Moreover, by comparing the photodegradation rate per monoclinic phase content, one can say that cubic phase is hindered by the photocatalytic reaction as in the case of the NSMO-1150 sample, indicating that the proper amount of double phases monoclinic/orthorhombic are crucial as clear as in the NSMO-500 sample [81]. It is also documented that NaTaO3 orthorhombic with TaO6 octahedra is proper to harvest light as a desired photocatalyst compared to its cubic phase [83,84]. Furthermore, it seems that the monoclinic NSMO-500 would be a better candidate than the cubic NSMO-1150 for light harvesting when the energy level matching is compared [77]. At higher annealing temperatures, the particles would agglomerate to form large particles, which would decrease the photocatalytic activity [85]. It is clearly seen that the photocatalytic efficiency was also altered by the difference between double phase contents of orthorhombic and monoclinic frameworks, even if they had the same structure as seen in the case of the NSMO-50 0, NSMO-80 0 and NSMO-10 0 0 samples. Moreover, lattice distortion is likely another cause for the superior photocatalytic efficiency of all these NSMO perovskites. As shown by the XRD calculation, the distortion calculated for NSMO-500 is minimum. This references that the metal–oxygen polyhedron distortion is the key factor affecting the enhancement of photocatalytic efficiencies [82]. In general, the perovskites which exhibit larger lattice distortion (octahedral tilting distortion) possess greater photocatalytic efficiencies by boosting the extra baths of holes trapping and reducing the electron-hole pairs recombination rate [34,41,86]. One plausible explanation is that the metal cations vacancy and O2 − anions in perovskite frameworks increase the adsorbed oxygen onto their surface [87], which enhances the photocatalytic efficiency. However, the amount of adsorbed O2 is influenced by O vacancy which is beneficial to some reactions and the appropriate number of O vacancy can efficiently inhibit the recombination of charge carriers [88,89]. 5. Conclusions In summary, Nd0.6 Sr0.4 MnO3 perovskite has been synthesized via sol–gel technique in the presence of polyethylene glycol as directing structure agent and citric acid as chelating agent. The obtained Nd0.6 Sr0.4 MnO3 gel was annealed at different

Please cite this article as: I.A. Abdel-Latif et al., Impact of the annealing temperature on perovskite strontium doped neodymium manganites nanocomposites and their photocatalytic performances, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.03.030

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temperatures. The XRD results confirmed that both the orthorhombic and the monoclinic phases exist with different ratios as a result of annealing at temperatures of 500 800 and 1000 °C. However, at 1150 °C, the cubic crystal structure appeared with orthorhombic crystal form. The monoclinic NSMO-500 would be a better candidate than the cubic NSMO-1150 for light harvesting. The distortion of NSMO perovskites likely influences the separation of photo generated holes and electrons. In this way, photocatalytic efficiency is improved, where the NSMO-50 0 0 has higher distortion than NSMO-10 0 0 of the same crystal structure.

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