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Features of Crystalline and Electronic Structures of Sm2mtao7 (M=Y, In, Fe) and Their Hydrogen Production Via Photocatalysis

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Features of Crystalline and Electronic Structures of Sm2mtao7 (M=Y, In, Fe) and Their Hydrogen Production Via Photocatalysis Ceramics International 43 (2017) 3981–3992 Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Features of crystalline and electronic structures of Sm2MTaO7 (M=Y, In, Fe) MARK and their hydrogen production via photocatalysis ⁎ Leticia M. Torres-Martíneza, , M.A. Ruíz-Gómezb, E. Moctezumac a Departamento de Ecomateriales y Energía, Facultad de Ingeniería Civil, Universidad Autónoma de Nuevo León UANL, Av. Universidad S/N Ciudad Universitaria, San Nicolás de los Garza, Nuevo León C.P. 64455, México b Centro de Investigación y de Estudios Avanzados del IPN (CINVESTAV), Unidad Mérida, Antigua carretera a Progreso, km 6, Cordemex, Mérida, Yucatán C.P. 97310, México c Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí, Av. Manuel Nava #6, San Luis Potosí, S.L.P. C.P. 78290, México ARTICLE INFO ABSTRACT Keywords: This paper reports on the crystal structure determination of a new phase of Sm2YTaO7 synthesized by a solid- Pyrochlore state reaction. Rietveld refinement using X-ray powder diffraction (XRD) data and electron diffraction using Rietveld analysis transmission electron microscopy (TEM) revealed that Sm2YTaO7 crystallized into an orthorhombic system Crystal structure with space group C2221, and according to the crystalline arrangement, it can be considered as a weberite-type Photocatalysis phase. A detailed analysis of the crystal chemistry of the family with formula Sm MTaO (M=Y, In, Fe, Ga) was Hydrogen production 2 7 performed, which indicated that all of these complex oxides are composed of corner-sharing octahedral layers of TaO6 units within a three-, two- or one-dimensional array. In addition, for comparison, the crystal structure, 3+ 3+ 5+ space group and lattice parameters of approximately 100 previously synthesized oxides in the A2 B B O7 family were collected and analyzed, and a structural map based on the radius ratio rA/rB is reported. According to the photocatalytic results, all oxides in the Sm2MTaO7 (M=Y, In, Fe, Ga) family showed hydrogen production from pure water without any cocatalyst. The highest (62 μmol/g h) and lowest (24 μmol/g h) hydrogen production rates were observed for Sm2YTaO7 and Sm2FeTaO7, respectively, which reveals that the photoactivity is strongly dependent on the negative potential of the conduction band. 3+ 4+ 1. Introduction different combinations, the A2 B2 O7-type being the most reported [17–20]. Due to these crystalline features, the chemical stability and The development of new renewable energy sources is currently one the wide variety of cation substitution, A2B2O7 pyrochlore oxides have of the biggest challenges in science. The decomposition of water into H2 attracted much attention for their interesting potential applications in and O2 using solar light and an appropriate photocatalyst is one of the many technological fields related to solid oxide fuel cells [21], nuclear most promising strategies for sustaining the world’s energy supply in waste host materials [22], photocatalysis [23–25], geometrically fru- the future. Photocatalysts derived from several families of compounds, strated magnetism [18,26], luminescence [27], ionic conductivity such as tantalates, niobates and titanium dioxide, have shown inter- [28,29], light emitters [8], catalysis [30–32], pigments [33,34], semi- esting activities for this reaction. Complex compounds with the general conductors [35,36], superconductors [37,38], ferroelectrics [39], and formula A2B2O7 are superstructures that are closely related to fluorite transmutation targets [2]. In addition, the attractive crystal chemistry (space group Fm-3 m) and can be considered to be ordered defect- has also generated interest in extensive crystallographic studies of fluorite phases with systematic oxygen vacancies [1–10]. It should be pyrochlore-type materials [8]. mentioned that oxides in the pyrochlore, weberite and layered per- To maintain charge neutrality, the pyrochlore family can be ovskite families are compounds that can possess the same stoichio- extended by replacing the two B4+ cations by a pair of B3+ and B5+ 3+ 3+ 5+ metry as A2B2O7 [11–16]. cations to give A2 B B O7 phases [17,40,41]. These types of phases The pyrochlore structure is cubic with the space group Fd-3 m, and have received little attention, with the magnetic [40–47] and photo- there are eight molecules per unit cell (Z=8). The A cation (~1 Å) and B catalytic [48–76] properties having been investigated the most. In 3+ 3+ 5+ cation (~0.6 Å) are 8-fold and 6-fold coordinated to oxygen, respec- particular, A2 B B O7-type photocatalysts have been evaluated for tively. These oxides present a wide range of compositions with over 500 the water splitting reaction [48,50,51,53,55–57,59,61– ⁎ Corresponding author. E-mail address: [email protected] (L.M. Torres-Martínez). http://dx.doi.org/10.1016/j.ceramint.2016.11.098 Received 9 August 2016; Received in revised form 5 November 2016; Accepted 14 November 2016 Available online 15 November 2016 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved. L.M. Torres-Martínez et al. Ceramics International 43 (2017) 3981–3992 65,67,69,71,74], as well as for dye degradation under UV [58,68,77], tification of the synthesized oxides was determined by energy disper- visible [59–62,70,72,73,75,76] and solar light illumination [66]. sive X-ray spectroscopy (EDS) by analyzing five random zones. The The presence of cations with different oxidation numbers in the specific surface area (SBET) of each catalyst was determined by physical same crystallographic site can induce distortion in the structure, which adsorption of nitrogen at −196 °C using a Belsorp II mini (Bel Japan modifies the thermal, electrical and photocatalytic behavior of the Inc.) apparatus. Prior to analysis, samples were degassed at 300 °C for compounds [6,56,65,67,69,78,79]. As photocatalysts, A2BB’O7 oxides 1 h. The optical properties were analyzed in the range of 200–700 nm are attractive, as they offer the possibility to manipulate the electron/ at room temperature with a UV–vis spectrophotometer (Lambda 35 hole mobility by introducing different elements into their structure Perkin Elmer Corporation) equipped with an integrating sphere [56,67,69,80]. attachment. The energy bandgap (Eg) was determined using the Structure-property correlation studies have provided a basic under- Kubelka-Munk function. standing for the design and synthesis of new oxides for use in photocatalytic processes. In this regard, few previous works have 2.3. Photocatalytic evaluation reported on the relationship between photocatalytic activity and crystal structure or the electronic structure of complex oxides such as The photocatalytic water splitting reaction was carried out in a pyrochlores and their related phases [55,56,81,82]. reactor with an inner quartz cell equipped with a 400 W high-pressure This paper focuses on the synthesis and crystal structure determi- mercury lamp as the irradiation source [67]. In all cases, 0.3 g of nation of the new oxide Sm2YTaO7. A detailed analysis of the crystal photocatalyst was dispersed into 300 mL of pure water under vigorous chemistry was made and compared to other phases with the formula stirring. Prior to reaction, argon was bubbled through the sample to Sm2MTaO7 (M=In, Fe, Ga) that were previously reported by our deaerate the slurry. The pressure was set to 100 Torr, and the research group. These tantalates were evaluated for hydrogen produc- temperature was kept constant at 20 °C. The evolved gases were tion via photocatalysis, and the results are explained in terms of band analyzed every 30 min with a Varian CP 3380 gas chromatographer structure and crystal structure. In addition, the crystal structures of equipped with a TCD detector and a Hayesep D 100/120 capillary 3+ 3+ 5+ approximately 100 previously synthesized A2 B B O7 oxides were column using argon as the carrier gas. 3+ analyzed, and a stability map for the A2B TaO7 phases was made. 2. Experimental 3. Results and discussion 2.1. Synthesis by solid-state reaction 3.1. Powder XRD studies XRD patterns of the Sm2YTaO7 samples thermally treated at The Sm2YTaO7 complex oxide was synthesized by a solid-state different temperatures are shown in Fig. 1. According to the XRD reaction using Sm2O3,Ta2O5, and Y2O3 (Aldrich, purity > 99.9%) as precursors according to the methodology reported in our previous results, the solid calcined at 1100 °C consists of a mixture of phases works [66–68,83]. Briefly, stoichiometric amounts of the reactants corresponding to the precursor oxides Sm2O3 (PDF 01-070-2642) and were homogeneously mixed in an agate mortar using acetone as the Y2O3 (PDF 01-071-5970), as well as an intermediate oxide Y3TaO7 dispersion media. This mixture was thermally treated at different (PDF 01-083-0308). When the temperature was increased to 1200 °C fi fl temperatures (1100–1400 °C) over 24 h under air atmosphere. The and 1300 °C, a set of well-de ned re ections that may be associated heating rate was 1 °C/min with intermediate regrinding until the with the Sm2YTaO7 phase were observed in the XRD pattern; in addition, small peaks corresponding to the precursor oxides were reaction was complete. Pure phases of Sm2MTaO7 (M=In, Fe, Ga) were also prepared at 1400 °C over 24 h in the same manner using detected. Finally, at 1400 °C, it seems that Sm2YTaO7 was obtained as a single phase. As can be seen, reflections are sharp and well defined, In2O3,Fe2O3,orGa2O3 in substitution of Y2O3 [66–68,83] to compare the physicochemical properties of these complex compounds. indicating good crystallization and large crystal size. This single phase fi θ ≈ – Predetermined amounts of RuO were deposited on the Sm MTaO pattern shows ve characteristic peaks at 2 29 70°, with the most 2 2 7 fl θ ≈ (M=Y, In, Fe, Ga) oxides using the wet impregnation method [67].
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