Abstract

1. Introduction

Alkaline earth stannates (MSnO₃, M = Ba, Sr, Ca) with perovskite structure have become alternative materials to the technological sector due to its applications as dielectric components. Its use as photocatalyst has also been reported, especially for water splitting¹1 Omeiri S, Hadjarab B, Bouguelia A, Trari M. Electrical, optical and photoelectrochemical properties of BaSnO3-δ: Applications to hydrogen evolution. Journal of Alloys and Compounds. 2010;50(2):592-597.

2 Lee CW, Kim DW, Cho IS, Park S, Shin SS, Seo SW, et al. Simple synthesis and characterization of SrSnO3 nanoparticles with enhanced photocatalytic activity. International Journal of Hydrogen Energy. 2012;37(14):10557-10563.^-33 Wang W, Bi J, Wu L, Li Z, Fu X. Hydrothermal synthesis and catalytic performances of a new photocalalyst CaSnO3 with microcube morphology. Scripta Materialia. 2009;60(3):186-189. and for the photodegradation of organic dyes, with emphasis in the SrSnO₃⁴4 Junploy P, Thongtem S, Thongtem T. Photoabsorption and photocatalysis of SrSnO3 produced by a cyclic microwave radiation. Superlattices and Microstructures. 2013;57:1-10.^,55 Lobo TM, Lebullenger R, Bouquet V, Guilloux-Viry M, Santos IMG, Weber IT. SrSnO3:N - Nitridation and evaluation of photocatalytic activity. Journal of Alloys and Compounds. 2015;649:491-494.. Previous results of our research group indicated that BaSnO₃ has a higher photocatalytic activity than SrSnO₃ for the degradation of an azo-dye, the golden yellow remazol (RNL)⁶6 Sales HB, Bouquet V, Députier S, Ollivier S, Gouttefangeas F, Guilloux-Viry M, et al. Sr1-xBaxSnO3 system applied in the photocatalytic discoloration of an azo-dye. Solid State Sciences. 2014;28:67-73..

BaSnO₃ has been studied in various applications in recent years but it is not widely explored as photocatalyst. In spite of its band gap of 3.1 eV, a small activity is usually reported, being assigned to a high electron-hole recombination rate⁷7 Mizoguchi H, Woodward PM, Park CH, Keszler DA. Strong near-infrared luminescence in BaSnO3. Journal of the American Chemical Society. 2004;126(31):9796-9800.. This drawback may be overcome by the use of nanostructured materials, as reported by Moshtaghi et al.⁸8 Moshtaghi S, Zinatloo-Ajabshir S, Salavati-Niasari M. Nanocrystalline barium stannate: facile morphology-controlled preparation, characterization and investigation of optical and photocatalytic properties. Journal of Materials Science: Materials in Electronics. 2016; 27(1):834-842.^,99 Moshtaghi S, Zinatloo-Ajabshir S, Salavati-Niasari M. Preparation and characterization of BaSnO3 nanostructures via a new simple surfactant-free route. Journal of Materials Science: Materials in Electronics. 2016;27(1):425-435., who attained a high activity in the photodegradation of organic dyes. A high activity may also be attained, using the perovskite ability to form solid solutions resulting in defects, which can therefore improve its photocatalytic properties¹⁰10 Borse PH, Joshi UA, Ji SM, Jang JS, Lee JS, Jeong ED, et al. Band gap tuning of lead-substituted BaSnO3 for visible light photocatalysis. Applied Physics Letters. 2007;90(3):034103.

11 Borse PH, Lee JS, Kim HG. Theoretical band energetics of Ba(M0.5Sn0.5)O3 for solar photoactive applications. Journal of Applied Physics. 2006;100(12):124915.^-1212 Yuan Y, Lv J, Jiang X, Li Z, Yu T, Zou Z, et al. Large impact of strontium substitution on photocatalytic water splitting activity of BaSnO3. Applied Physics Letters. 2007;91(9):094-107.. For instance, the solid solution BaSn_0.2Pb_0.8O₃ has been evaluated by Borse et al. showing a high activity for the photo-oxidation of water¹⁰10 Borse PH, Joshi UA, Ji SM, Jang JS, Lee JS, Jeong ED, et al. Band gap tuning of lead-substituted BaSnO3 for visible light photocatalysis. Applied Physics Letters. 2007;90(3):034103.. Literature also reports the use of La^3+/2+, Ni^3+/2+, Fe^3+/2+, Cu^2+/+, Co^3+/2+ as dopant into oxides as TiO₂, ZnTiO₃, ZnO¹³13 Weber AS, Grady AM, Koodali RT. Lanthanide modified semiconductor photocatalysts. Catalysis Science & Technology. 2012;2(4):683-693.

14 Mostaghni F, Abed Y. Structural, Optical and Photocatalytic Properties of Co-TiO2 Prepared by Sol-Gel Technique. Materials Research. 2016;19(4):741-745.

15 Surendar T, Kumar S, Shanker V. Influence of La-doping on phase transformation and photocatalytic properties of ZnTiO3 nanoparticles synthesized via modified sol-gel method. Physical Chemistry Chemical Physics. 2014;16(2):728-735.

16 Banić ND, Abramović BF, Šojić DV, Krstić JB, Finčur NL, Bočković IP. Efficiency of neonicotinoids photocatalytic degradation by using annular slurry reactor. Chemical Engineering Journal 2016;286:184-190.^-1717 Pham TD, Lee BK, Lee CH. The advanced removal of benzene from aerosols by photocatalytic oxidation and adsorption of Cu-TiO2/PU under visible light irradiation. Applied Catalysis B: Environmental. 2016;182:172-183..

Fe³⁺ has been used as TiO₂ dopant in several studies¹⁸18 Sood S, Umar A, Mehta SK, Kansal SK. Highly effective Fe-doped TiO2 nanoparticles photocatalysts for visible light driven photocatalytic degradation of toxic organic compounds. Journal of Colloid and Interface Science. 2015;450:213-223.

19 Garza-Arévalo JI, García-Montes I, Hinojosa Reyes M, Guzmán-Mar JL, Rodríguez-González V, Hinojosa Reyes L. Fe doped TiO2 photocatalyst for the removal of As(III) under visible radiation and its potential application on the treatment of As-contaminated groundwater. Materials Research Bulletin. 2016;73:145-152.

20 Ma J, He H, Liu F. Effect of Fe on the photocatalytic removal of NOx over visible light responsive Fe/TiO2 catalysts. Applied Catalysis B: Environmental. 2015;179:21-28.

21 Tian F, Wu Z, Tong Y, Wu Z, Cravotto G. Microwave-Assisted Synthesis of Carbon-Based (N, Fe)-Codoped TiO2 for the Photocatalytic Degradation of Formaldehyde. Nanoscale Research Letters. 2015;10:360.

22 Li Z, Shen W, He W, Zu X. Effect of Fe-doped TiO2 nanoparticle derived from modified hydrothermal process on the photocatalytic degradation performance on methylene blue. Journal of Hazardous Materials. 2008;155(3):590-594.^-2323 Hemmati Borji S, Nasseri S, Mahvi AH, Nabizadeh R, Javadi AH. Investigation of photocatalytic degradation of phenol by Fe(III)-doped TiO2 and TiO2 nanoparticles. Journal of Environmental Health Science & Engineering. 2014;12:101., behaving as electron scavenger which suppress electron-hole recombination improving the photocatalytic efficiency. Fe³⁺ has also been used as BaSnO₃ dopant and leads to formation of multi energy levels below the conduction band edge. Charge balance is obtained by formation of oxygen vacancies, besides oxidation or reduction of Fe³⁺, which contribute to the perovskite stabilization²⁴24 Vieira FTG, Oliveira ALM, Melo DS, Lima SJG, Longo E, Maia AS, et al. Crystallization study of SrSnO3:Fe. Journal of Thermal Analysis and Calorimetry. 2011;106:507-512.

25 Alves MCF, Souza SC, Lima HHS, Nascimento MR, Silva MRS, Espinosa JWM, et al. Influence of the modifier on the short and long-range disorder of stannate perovskites. Journal of Alloys and Compounds. 2009;476(1-2):507-512.^-2626 Lucena GL, Maia AS, Souza AG, Santos IM. Structural changes in Fe-doped SrSnO3 perovskites during thermal analysis. Journal of Thermal Analysis and Calorimetry. 2014;115(1):137-144.. The magnetic properties of this Fe-doped perovskite have been studied, but, up to our knowledge, its use as photocatalyst has not been reported yet. The heterogeneous catalysis offers technical and environmental advantages over homogeneous catalysis, allows the recycling of the solid catalyst over its useful life and minimizes the generation of effluents. Several solids have been proposed as potential catalysts for photodegradation of textile dyes. The performance of these materials as catalysts is naturally related to the nature of the acid or basic sites found in these materials.

In the present work, Fe-doped BaSnO₃ was synthesized by the modified Pechini method and applied in the photoactivity degradation of RNL azo-dye.

2. Experimental

2.1 Synthesis of photocatalysts

Fe³⁺-doped BaSnO₃ (0; 0.05 and 0.1 in mol) was synthesized by the modified Pechini method, similarly to the methodology described Lucena et al.²⁷27 Lucena GL, Souza JJN, Maia AS, Soledade LEB, Longo E, Souza AG, et al. New methodology for a faster synthesis of SrSnO3 by the modified Pechini method. Cerâmica. 2013;59(350):249-253.. After tin dissolution in a 0.1 mol.L^-1 nitric acid aqueous solution cooled with an ice bath, citric acid (C₆H₈O₇.H₂O, Cargill - 99.5 %) was added into the solution. The pH of the solution was adjusted to 3 by adding ammonium hydroxide (NH₄OH, Vetec - 28-30 %). Solutions of iron citrate or barium citrate were prepared from the respective nitrates, Ba(NO₃)₂ (Vetec - 99 %), Fe(NO₃)₃.9H₂O (Vetec - 99.5 %), and added into the tin citrate solution under a slow agitation at 25ºC for 12 h. A molar ratio citric acid:metal of 3:1 was used for all of the citrates. Ethylene glycol (Vetec - 99.5 %) was added into the solution at 90ºC to promote its polymerization. The mass ratio of citric acid to ethylene glycol was 60:40.

The polymeric resins were calcined at 250ºC/2 h, deagglomerated, dry milled in a Spex mill, and sieved (100 mesh) to obtain the powder precursors. A heat treatment under an oxygen atmosphere (O₂) was performed at a temperature of 300ºC /6 h at a heating rate of 1ºC min^-1 under a flow of 1000 cm³ min^-1 to partially eliminate the organic material. The materials were calcined from 300 to 800ºC/4 h under stagnated air atmosphere at a rate of 10ºC min^-1.

The precursors obtained after heat treatment under oxygen atmosphere were characterized by thermogravimetry (TG) and its derivative curve (DTG) using a SDT-2960 thermobalance (TA Instruments) with a heating rate of 10ºC min^-1 up to 1000ºC under synthetic air with a flow of 100 mL min^-1 using alumina crucibles. The samples calcined at 300-800ºC were characterized by X-ray diffraction (XRD) using an XRD-6000 Shimadzu diffractometer with Cu Kα radiation between 10º and 80º, using a step size of 0.02º and a step time of 2 s. The values of the lattice parameters were obtained with the Rede93 software using the least squares method. Micro-Raman spectra were obtained in the region of 100-1000 cm^-1 by an InVia spectrophotometer from Renishaw, using an Ar laser (514 nm) with a power of 20 mW and an objective lens of 50x. UV-vis spectra were obtained by reflectance mode using an UV-2550 Shimadzu spectrophotometer, in the 190-900 nm range.

2.2 Photodegradation reaction

During the photocatalytic test, 15.0 mL of a RNL solution with a concentration of 10 mg L^-1 and pH = 3 and 6 was placed in a Petri plate with 10 mg of the photocatalyst. The suspensions were irradiated for 1, 2 and 4 h with a Super Niko UVC lamp (0.5-1.0mW), model ZG-30T8. After the reaction, the suspensions were centrifuged at 5000 rpm for 30 min at room temperature before being filtered. The solutions were analyzed by UV-Vis spectroscopy using a SHIMADZU UV-2550 spectrometer, in the range of 300 to 700 nm. The dye discoloration percentage was determined by the measured absorbance at λ = 411 nm, which was assigned to the azo group.

3. Results and Discussion

3.1 Synthesis and characterization of Fe-doped BaSnO₃

Figure 1 shows the TG and DTG curves of the precursor. Three thermal decomposition steps were observed in the TG curves. In the first step, water and gases adsorbed on the powder surfaces were eliminated; the second step was assigned to the decomposition of the Ba(NO₃)₂ between 460 and 680ºC; the third step was assigned to the carbonate decomposition²⁸28 Udawatte CP, Kakihana M, Yoshimura M. Preparation of pure perovskite-type BaSnO3 powders by the polymerized complex method at reduced temperature. Solid State Ionics. 1998;108(1-4):23-30.. Similar behaviors were obtained by Udawatte et. al.²⁸28 Udawatte CP, Kakihana M, Yoshimura M. Preparation of pure perovskite-type BaSnO3 powders by the polymerized complex method at reduced temperature. Solid State Ionics. 1998;108(1-4):23-30. and Li et. al.²⁹29 Li B, Tang Y, Luo L, Xiao T, Li D, Hu X, et al. Fabrication of porous BaSnO3 hollow architectures using BaCO3@SnO2 core-shell nanorods as precursors. Applied Surface Science. 2010;257(1):197-202. for the synthesis of BaSnO₃ using BaCO₃, SnO₂ and BaCl₂ as precursors.

The X-ray diffraction (XRD) patterns of the materials obtained after heating between 300 and 800 ºC are shown in Figure 2.

The planes were indexed according to ICDD 01-074-1300 (BaSnO₃), 00-041-1445 (SnO₂), 00-045-1471 (BaCO₃), 00-024-0053 [Ba(NO₃)₂] and 00-0010891 (Sn). For the precursor's heat treated at 300 ºC, peaks assigned to tetragonal SnO₂, Ba(NO₃)₂, Sn and BaCO₃ were observed. After heat treatment at 400 ºC, higher intensity peaks were observed for Ba(NO₃)₂, while the intensities of these peaks decrease at 500 ºC. The formation of the cubic BaSnO₃ (Pm3m) was observed at 600ºC besides a small amount of SnO₂ and BaCO₃, which is in agreement with the TG/DTG analysis shown in figure 1. This crystallization temperature below 600ºC is quite low compared with other synthesis methods as solid state reaction³⁰30 Huang C, Wang X, Liu X, Tian M, Zhang T. Extensive analysis of the formation mechanism of BaSnO3 by solid-state reaction between BaCO3 and SnO2. Journal of the European Ceramic Society. 2016;36(3):583-592.

31 Manju MR, Kumar VP, Dayal V. Investigation of ferromagnetic properties in Fe/Co substituted BaSnO3 perovskite stannates. Physica B: Condensed Matter. 2016;500:14-19.

32 Ochoa YH, Schipani F, Aldao CM, Ponce MA, Savu R, Rodríguez-Páez JE. Electrical behavior of BaSnO3 bulk samples formed by slip casting: Effect of synthesis methods used for obtaining the ceramic powders. Materials Research Bulletin. 2016;78:172-178.^-3333 Kumar AA, Kumar A, Quamara JK, Dillip GR, Joo SW, Kumar J. Fe (III) induced structural, optical, and dielectric behavior of cetyltrimethyl ammonium bromide stabilized strontium stannate nanoparticles synthesized by a facile wet chemistry route. RSC Advances. 2015;5:17202-17209.. No significant change was observed with temperature increase from 600 to 800 ºC.

The XRD patterns of Fe-doped BaSnO₃ are shown in Figure 3. Highly crystalline cubic BaSnO₃ was observed while no peaks assigned to Fe₂O₃ (ICDD 03-065-3107) were found. A slight shift in the diffraction peaks towards higher 2θ values was observed after doping indicating that Fe³⁺ got into BaSnO₃ lattice. The lattice parameters, a, of BaSnO₃ were calculated and indicated that a small lattice decrease took place, which is assigned to the smaller ionic radius of Fe³⁺ (0.64 Å) compared to Sn⁴⁺ (0.69 Å)³⁴34 Balamurugan K, Kumar ES, Ramachandran B, Venkatesh S, Kumar NH, Rao MS, et al. Dielectric resonance and magnetic properties of Fe-3% doped BaSnO3 thin films grown by pulsed laser deposition. Journal of Applied Physics. 2012;11(7)074107..

Figure 4 shows the Raman spectra of the Fe³⁺-doped BaSnO₃ (0; 0.05 and 0.1 in mol). The group theory predicts the absence of active modes in the Raman spectra for a perfect Pm3m perovskite structure. In spite of this, Cerda et. al.³⁵35 Cerdà J, Arbiol J, Diaz R, Dezanneau G, Morante JR. Synthesis of perovskite-type BaSnO3 particles obtained by a new simple wet chemical route based on a sol-gel process. Materials Letters. 2002;56(3):131-136. reported bands at 238, 408, 543 and 724 cm^-1, attributed to distortions of the cubic structure of BaSnO₃ due to defects, which modify the internal symmetry of the perovskite phase, leading to unexpected modes in Raman spectra. These modes were assigned to the six fundamental vibrations of SnO₆ with O_h symmetry. Similar studies on various perovskite compounds show that distortions of these materials are due to the presence of defects (V_O^x, V^•_O, V^••_O, Sn²⁺)³⁶36 Stanislavchuk TN, Sirenko AA, Litvinchuk AP, Luo X, Cheong SW. Electronic band structure and optical phonons of BaSnO3 and Ba0.97La0.03SnO3 single crystals: Theory and experiment. Journal of Applied Physics. 2012;112(4):044108.

37 Zhang W, Tang J, Ye J. Structural, photocatalytic, and photophysical properties of perovskite MSnO3 (M = Ca, Sr, and Ba) photocatalysts. Journal of Materials Research. 2007;22(7):1859-1871.^-3838 Zheng H, de Györgyfalva GDCC, Quimby R, Bagshaw H, Ubic R, Reaney IM, et al. Raman spectroscopy of B-site order-disorder in CaTiO3-based microwave ceramics. Journal of the European Ceramic Society. 2003;23(14):2653-2659..

Balamuragan et al.³⁹39 Balamurugan K, Harishn Kumar N, Arout Chelvane J, Santhosh PN. Room temperature ferromagnetism in Fe-doped BaSnO3. Journal of Alloys and Compounds. 2009;472(1-2):9-12.^,4040 Balamurugan K, Harish Kumar N, Arout Chelvane J, Santhosh PN. Effect of W co-doping on the optical, magnetic and electrical properties of Fe-doped BaSnO3. Physica B: Condensed Matter. 2012;407(13):2519-2523. evaluated the optical and electromagnetic properties of Fe-doped BaSnO₃. According to the authors, when iron is added into the perovskite lattice, a center of extrinsic defects is formed with the formation of oxygen vacancies for charge compensation, as showed in Equation (1).

In the present work, the mode at 150 cm^-1 was attributed to the vibration of carbonate groups. Undoped BaSnO₃ showed bands at similar regions to those reported by Cerda et al.³⁵35 Cerdà J, Arbiol J, Diaz R, Dezanneau G, Morante JR. Synthesis of perovskite-type BaSnO3 particles obtained by a new simple wet chemical route based on a sol-gel process. Materials Letters. 2002;56(3):131-136.⁾ indicating that distortions are present in the structure. After doping, dislocation of the bands to 252, 413, 535 and 663 cm^-1 took place. A higher definition was observed for the bands at 252 and 413 cm^-1, which may be correlated to the oxygen vacancies, which change the symmetry.

Figure 5 shows the absorption spectra of Fe³⁺- doped BaSnO₃, with a strong absorption in the visible region. The optical absorption edge of BaSnO₃ was observed around 477 nm, with a red shift as doping concentration increases. The band gap values of the as-synthesized samples were estimated from diffuse reflectance spectra using the Wood-Tauc method⁴¹41 Wood DL, Tauc J. Weak Absorption Tails in Amorphous Semiconductors. Physical Review B. 1972;5(8):3144-3151.. Incorporation of Fe³⁺ into the lattice resulted in a band gap decrease, indicating that intermediate levels were formed inside the band gap.

3.2 Photocatalytic properties

The photocatalytic decomposition of RNL by Fe-doped BaSnO₃ is presented in Figure 6. The highest degradation efficiency occurred at pH = 3 and the lowest degradation occurred at pH = 6 (aqueous solution of the dye).

Photocatalysis may occur by two different mechanisms: direct or indirect one. For the direct mechanism, dye is adsorbed on the photocatalyst surface and electron transfer takes place without the formation of intermediate compounds. During the indirect mechanism, hydroxyl radicals are formed due to electron/hole transfer between the surface and compounds as O₂, H₂O and OH^-. Then, hydroxyl radicals in solution react with the substrate.

The possibility of a discoloration by a direct mechanism was evaluated by the adsorption analysis, as adsorption of the dye on the material surface is a requested prerequisite step for direct charge transfer⁴²42 Tang WZ, Huang CP. Photocatalyzed oxidation pathways of 2,4-dichlorophenol by CdS in basic and acidic aqueous solutions. Water Research. 1995;29(2):745-756.. Results displayed in Figure 7 indicate that the maximum discoloration due to adsorption process was 7% for the BaSn_0.9Fe_0.1O₃ sample, much smaller than the discoloration percentage under UVC irradiation (93 %). This small adsorption indicates that the indirect mechanism prevails for this system.

The effect of pH on photocatalysis has been evaluated by different researchers, as reported in the review published by Akpan and Hameed⁴³43 Akpan UG, Hameed BH. Parameters affecting the photocatalytic degradation of dyes using TiO2-based photocatalysts: A review. Journal of Hazardous Materials. 2009;170(2-3):520-529.. Much of them report a direct mechanism at low pH when TiO₂ is used as photocatalyst. On the other hand, according to Guo et al.⁴⁴44 Guo Z, Ma R, Li G. Degradation of phenol by nanomaterial TiO2 in wastewater. Chemical Engineering Journal. 2006;119(1):55-59., hydrogen radicals also take part in the photodegradation of phenol using TiO₂ as photocatalyst. These ^•H radicals may be produced from H₂O molecules and also from H₃O⁺ ions especially in acid media, and may react with O₂ forming HO₂^• which finally convert to ^•OH. According to Texeira et. al.⁴⁵45 Teixeira TPF. Avaliação da eficiência do uso de hidrotalcitas calcinadas na remoção de azo corantes aniônicos presentes em efluentes de indústria têxtil. Dissertation. Ouro Preto: Universidade Federal de Ouro Preto; 2011. 93 f.⁾ the RNL azo dye has three pK_a values: the sulphonic group is deprotonated at pH = 3, the sulphate group is deprotonated at pH = 3.5 and the amide group is deprotonated at pH = 6, which results in a large negative charge. Therefore, an attractive force between the positive surface charge of the perovskite and the negative charge of the azo dye occurs at pH 3 favoring the dye attraction and the highest solution discoloration⁴⁶46 Teixeira TPF, Pereira SI, Aquino SF, Dias A. Use of Calcined Layered Double Hydroxides for Decolorization of Azo Dye Solutions: Equilibrium, Kinetics, and Recycling Studies. Environmental Engineering Science. 2012;29(7):685-692.. At pH 6, few molecules are attracted on the BaSnO₃ surface due to the slight positive surface charge, leading to a small discoloration of the solution.

In the present work, the highest efficiency at acid media cannot be assigned to a direct mechanism, as only a small adsorption was detected. It seems clear that RNL photodegradation is promoted by ^•OH radicals whose formation is favored at acidic conditions, probably due to the highest amount of H₃O⁺ ions.

Figure 6c shows that the color was halved after 1 h of photocatalysis with BaSn_1-xFe_xO₃ (x = 0.05 and 0.10), 2.5x higher than undoped BaSnO₃. For longer times, doped samples also presented a higher photoactivity than pure one, increasing with Fe content. After 4 h, a photocatalytic efficiency of about 93% was attained for the sample BaSn_0.90Fe_0.10O₃.

Several papers using Fe-doped TiO₂ nanoparticles¹⁷17 Pham TD, Lee BK, Lee CH. The advanced removal of benzene from aerosols by photocatalytic oxidation and adsorption of Cu-TiO2/PU under visible light irradiation. Applied Catalysis B: Environmental. 2016;182:172-183.

18 Sood S, Umar A, Mehta SK, Kansal SK. Highly effective Fe-doped TiO2 nanoparticles photocatalysts for visible light driven photocatalytic degradation of toxic organic compounds. Journal of Colloid and Interface Science. 2015;450:213-223.

19 Garza-Arévalo JI, García-Montes I, Hinojosa Reyes M, Guzmán-Mar JL, Rodríguez-González V, Hinojosa Reyes L. Fe doped TiO2 photocatalyst for the removal of As(III) under visible radiation and its potential application on the treatment of As-contaminated groundwater. Materials Research Bulletin. 2016;73:145-152.

20 Ma J, He H, Liu F. Effect of Fe on the photocatalytic removal of NOx over visible light responsive Fe/TiO2 catalysts. Applied Catalysis B: Environmental. 2015;179:21-28.

21 Tian F, Wu Z, Tong Y, Wu Z, Cravotto G. Microwave-Assisted Synthesis of Carbon-Based (N, Fe)-Codoped TiO2 for the Photocatalytic Degradation of Formaldehyde. Nanoscale Research Letters. 2015;10:360.^-2222 Li Z, Shen W, He W, Zu X. Effect of Fe-doped TiO2 nanoparticle derived from modified hydrothermal process on the photocatalytic degradation performance on methylene blue. Journal of Hazardous Materials. 2008;155(3):590-594. assumed that a higher photoactivity for Fe-doped samples is possible in comparison with the undoped material, especially because Fe³⁺ can act as both hole and electron traps to enhance lifetimes of electrons and holes.

Fe-doped BaSnO₃ has been studied for magneticelectronics applications, classified as oxide-diluted magnetic semiconductor, displaying ferromagnetism even with small doping amounts. This property is enhanced due to a F-center exchange mechanism, which enables Fe-ions to order ferromagnetically. This F-center is characterized by a Feⁿ⁺-V_O-Feⁿ⁺ configuration which is able to trap electrons³⁹39 Balamurugan K, Harishn Kumar N, Arout Chelvane J, Santhosh PN. Room temperature ferromagnetism in Fe-doped BaSnO3. Journal of Alloys and Compounds. 2009;472(1-2):9-12.^,4040 Balamurugan K, Harish Kumar N, Arout Chelvane J, Santhosh PN. Effect of W co-doping on the optical, magnetic and electrical properties of Fe-doped BaSnO3. Physica B: Condensed Matter. 2012;407(13):2519-2523.^,4747 Swatsitang E, Karaphun A, Phokha S, Putjuso T. Characterization and magnetic properties of BaSn1-xFexO3 nanoparticles prepared by a modified sol-gel method. Journal of Sol-Gel Science and Technology. 2016;77(1):78-84..

In the present work, XRD patterns and Raman spectra indicated that Fe³⁺ was added into the BaSnO₃ lattice leading to a shift of the absorption onset to the visible region due to the formation of intermediate levels inside the band gap. These intermediate levels may trap electrons preventing the electron-hole recombination. As a consequence, higher photocatalytic efficiency is obtained.

4. Conclusions

BaSn_1-xFe_xO₃ (x = 0, 0.05 and 0.10) was successfully synthesized by the modified Pechini method, with crystallization around 600ºC. XRD patterns and Raman spectra indicated that Fe³⁺ got into the perovskite lattice leading to a decrease of the band gap. The samples showed high potential for photodegradation of the RNL azo-dye at pH = 3 with prevalence of indirect mechanism. Efficiency was improved by Fe³⁺ doping probably due to the formation of intermediate levels inside the band gap, which may trap electrons avoiding electron-hole recombination.

5. Acknowledgements

This work was supported by Brazilian Funding Agencies CT-INFRA/FINEP/MCTIC and CAPES.

6. References

Publication Dates

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