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

Ceramics International 43 (2017) 3981–3992

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Ceramics International

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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]. The intense re ection located at 2 29°; this means that the new oxide has a crystal structure related to the fluorite superstructure [15].In Sm2MTaO7 samples were immersed into a solution of triruthenium addition, the presence of several weak reflections at 2θ ≈15.6, 23.9, dodecacarbonyl, Ru3(CO)12 (Aldrich, 99%), in tetrahydrofuran and mixed for two hours. The solvent was evaporated by heating the slurry 24.8, 26.4, 27.7 and 28.3° suggests that Sm2YTaO7 crystallizes in a at 80 °C under vacuum. Then, each material was calcined at 400 °C for one hour under air atmosphere.

2.2. Characterization

All Sm2MTaO7 complex oxides were analyzed by X-ray diffraction using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ=1.5406 Å) as the incident X-ray source. XRD data were collected at room temperature from 10 to 100° with a step interval of 0.01° and a counting time of 1 s/step. A detailed analysis of the crystal structure was performed using the Rietveld refinement method by the TOPAS R3 software. The high-resolution electron microscopy and selected area electron diffraction patterns of the sample were taken using a JEOL JEM-2200FS transmission electron microscope. A small amount of solids were ultrasonically dispersed in ethanol, and a drop was deposited onto a standard copper grid with a lacey carbon support film. Image analysis was performed using the DigitalMicrograph software from GatanTM.

The morphology and particle size of the materials were observed Fig. 1. XRD patterns of Sm2YTaO7 prepared by solid-state reaction at diﬀerent using a scanning electron microscope JSM-6490 LV. Elemental quan- temperatures.

3982 L.M. Torres-Martínez et al. Ceramics International 43 (2017) 3981–3992

Table 1

Structural parameters of Sm2YTaO7 obtained from Rietveld reﬁnement.

Parameter Sm2YTaO7

a (Å) 10.6699(6) b (Å) 7.4971(5) c (Å) 7.5569(5)

Space group C2221 Z4 System Orthorhombic

Rwp’ (%) 9.28 χ2 2.78

Table 2

Selected hkl values acquired from Rietveld reﬁnement of Sm2YTaO7.

hkl D-spacing (Å) 2θ (degrees) Relative intensity

1 1 0 6.118 14.56 4.2 Fig. 2. XRD patterns of Sm2MTaO7 (M=In, Fe, Ga, Y) oxides prepared by solid-state 2 0 0 5.315 16.81 2.9 reaction at 1400 °C. 1 1 1 4.737 18.72 1.5 2 0 1 4.331 20.50 0.5 weberite structure; this is because there are several extra reflections in 0 0 2 3.752 23.69 2.8 weberite, which are systematically absent in cubic pyrochlores [15]. 1 1 2 3.202 27.84 1.1 2 2 0 3.059 29.17 100.0 Recently, our group reported the crystal structure of complex oxides 0 2 2 2.654 33.74 36.2 – with the general formula Sm2MTaO7 (M=In, Fe, Ga) [66 68,83]. Now, 0 0 4 1.878 48.44 30.5 the new crystalline phase Sm2YTaO7 is being reported. The XRD 2 2 4 1.602 57.48 26.4 patterns of all structures can be seen in Fig. 2. The differences between 4 0 4 1.533 60.34 5.2 the monoclinic structure compounds (Sm FeTaO and Sm GaTaO ) 0 4 4 1.331 70.74 2.9 2 7 2 7 2 0 6 1.219 78.32 3.5 and the In- and Y-containing phases can be observed. 8 2 2 1.189 80.76 4.4 The presence of very weak reflections at 2θ ≈15.5–28.3° in the 4 2 6 1.085 90.45 3.8 pattern of Sm2YTaO7 are the characteristic peaks of orthorhombic 2 6 4 1.024 97.52 3.1 weberite [15], and these peaks are the main differences from the cubic pyrochlore structure Sm InTaO . Therefore, it is necessary to perform 2 7 ff fl a careful analysis of the XRD results to determine the correct crystal agreement; thus, all di raction re ections were indexed using the structure of these phases. orthorhombic crystal structure and the space group C2221. The cell parameters and acceptable reliability factors obtained from the refinement results are given in Table 1. These parameters are in good 3.2. Crystal structure determination agreement with previously reported values for similar oxides with orthorhombic structures. [75,84,87] Table 2 presents a list of selected In previous works, the space group C2221 was proposed to describe hkl values, positional 2θ (°) values and relative intensities of the – oxides such as Sm3TaO7 [84 86],Y2GdSbO7 [86,87], and Y2YbSbO7 Sm2YTaO7 refined structure. [75]. For this reason, to determinate the crystal structure of the new The atomic positions of Sm2YTaO7, reported in Table 3, were also oxide Sm2YTaO7, a Rietveld refinement was carried out based on a obtained by XRD Rietveld refinement. In the crystal structure arrange- model consisting of an orthorhombic unit cell with the space group ment, the Sm3+,Y3+ and Ta5+ cations occupy the 8c, 4b and 4b Wyckoff C2221 (No. 20) where the occupancies were determined as per the positions, respectively. Additionally, there are five oxygen anions, two stoichiometry of Sm2YTaO7. in the 8c position and three in the 4a position. Table 4 shows the A–O fi The XRD pattern obtained by Rietveld re nement and the experi- and B–O bond distances presented in the orthorhombic Sm2YTaO7 mental XRD pattern are shown in Fig. 3. Both XRD patterns were in structure. The 7-fold coordinated Sm3+ cations and 8-fold coordinated Y3+ cations show average bond lengths of 2.339 Å and 2.415 Å, respectively, whereas Ta5+ cations with 6-fold coordination present an average bond length of 2.145 Å. In all cases, the bond distances in

Sm2YTaO7 are realistic interatomic lengths that are in good agreement with previous reports for similar oxides. [84,87].

Table 3

Atomic positions of Sm2YTaO7 calculated by Rietveld reﬁnement.

Atoms Wyckoﬀ position Occupancy Atomic position

xYz

Sm 8c 1 0.239(1) 0.242(2) 0 Y 4b 1 0 0.496(1) 0.25 Ta 4b 1 0 0.012(2) 0.25 O1 8c 1 0.138(2) 0.222(3) 0.253(6) O2 8c 1 0.106(4) 0.786(2) 0.263(6) O3 4a 1 0.091(5) 0.5 0 Fig. 3. XRD Rietveld patterns for Sm2YTaO7. The experimental, calculated and O4 4a 1 0.157(4) 0.5 0.5 ff fi θ di erence pro les are shown. The vertical marks on the 2 axis are the positions O5 4a 1 0.107(6) 0 0 calculated for the Bragg reflections.

3983 L.M. Torres-Martínez et al. Ceramics International 43 (2017) 3981–3992

Table 4 double that of the (220) plane can also be observed, which corresponds fi Bond lengths of Sm2YTaO7 calculated by Rietveld re nement. to the (110) plane. The interplanar spacings observed by HRTEM are in agreement with the values shown in Table 2, where the (220) plane Bond Å Bond Å presents the highest intensity in the XRD pattern. Sm-O1 2.201(2) Y-O1×2 2.533(6) Experimental and simulated selected area electron diffraction Sm-O1′ 2.297(6) Y-O2×2 2.483(5) patterns obtained for Sm2YTaO7 are shown in Fig. 6. The planes of Sm-O2 2.703(4) Y-O3×2 2.121(2) the weberite-type phase were indexed using the ratio method, and they Sm-O2′ 2.245(3) Y-O4×2 2.525(4) Sm-O3 2.502(5) Average 2.415 were compared with the corresponding XRD results. The experimental Sm-O4 2.128(3) and simulated patterns are in good agreement. These results further Sm-O5 2.303(4) support that Sm2YTaO7 has an orthorhombic crystal structure (space Average 2.339 Ta-O1×2 2.153(5) group C2221), as previously determined by XRD Rietveld refinement. Ta-O2×2 2.076(5) Ta-O5×2 2.207(3) Average 2.145 3.4. Stability map of Sm2MTaO7 (M=Y, In, Fe, Ga) oxides

Pyrochlore oxides with the general formula A2B2O7 have a wide range of applications due to the large variety of possible substitutions at both the A and B sites. [6,8,17–20] It is well known that one of the key parameters influencing structural disorder in pyrochlores is the cation radius ratio, rA/rB. [17,88] Pyrochlores with the general 3+ 4+ formula A2 B2 O7 are the most common because a large number of A3+ and B4+ cations have suitable ionic radii for the formation of this structure. These pyrochlores can be formed in the typical cubic structure (space group Fd-3 m) when the rA/rB ratio is within the range of 1.46–1.78 at atmospheric pressure. When rA/rB < 1.46, an anion-deficient fluorite structure is expected; whereas, a monoclinic layered perovskite-type structure is favored when rA/rB > 1.78. [2,3,8,12–14,16,17] As a part of the family of pyrochlore oxides, it is 3+ 3+ 5+ expected that compounds with the formula A2 B B O7 present a similar behavior. Thus far, it has been useful to establish stability maps and tolerance factors for pyrochlore oxides and related structures to predict the crystal structure, lattice constants and other properties before synthe- sizing these compounds. The literature contains several important works that have contributed significantly to the field, such as those by Subramanian et al., [17] Lopatin et al., [89,90] Sych et al., [91] Cai et al., [15,88] Brik et al., [92] and Mouta et al. [93]. It is important to mention that previous works have focused mainly

on A2B2O7 pyrochlores. In the present work, we summarize in Table 5 and Fig. 7 16 previously reported phases corresponding to tantalate Fig. 4. Polyhedral view of weberite-type Sm2YTaO7. Yellow octahedra and blue cubes 3+ compounds (A2BTaO7). Their crystal structure is compared with the correspond to Ta–O6 and Y–O8, respectively. The Sm atoms are represented as red spheres. (For interpretation of the references to color in this ﬁgure legend, the reader is new oxide Sm2YTaO7 reported in this work. Additionally, 103 oxides 3+ 3+ 5+ referred to the web version of this article.) with the formula A2 B B O7 are analyzed in Table S1 (Supporting Information).

Fig. 4 shows the orthorhombic crystal structure of the Sm2YTaO7 As can be seen in Fig. 7, cubic pyrochlores are formed when the A phase. This structure consists of edge-shared YO8 distorted cubes and site is occupied by larger cations (Bi and Nd). In particular, the cubic 3+ ≤ ≤ vertex-shared TaO6 octahedra, which both form an infinite one- oxides Bi2B TaO7 are obtained when 1.39 rA/rB 1.86. Therefore, 3+ dimensional zig-zag layer parallel to the [001] direction. The TaO6 Bi (1.17 Å) [94] occupancy of the A site can tolerate a wide range of ff octahedra are tilted along the [010] direction. In addition, the YO8 and cation sizes at the B site (the rB di erence is approximately 20%) to TaO6 chains are alternately parallel to the (100) plane, where 7-fold maintain the cubic structure. coordinated Sm cations are positioned between the Y–O and Ta–O Considering the Sm series, only 2 of the 5 oxides show the ideal layers. Because this structural arrangement does not exhibit a three- pyrochlore cubic structure, Sm2InTaO7 and Sm2ScTaO7 with rA/rB of dimensional TaO6 octahedral network, the Sm2YTaO7 phase can be 1.50 and 1.56, respectively. The compounds Sm2FeTaO7 and considered to be a weberite-type oxide rather than a weberite oxide. Sm2GaTaO7 crystallize in the monoclinic structure (zirconolite-type), [15] It is important to mention that the weberite structure is an anion which is favored when rA/rB ≥1.69; whereas, the yttrium-containing deficient fluorite-related structure, as discussed below. phase corresponds to the weberite-type Sm2YTaO7 phase with an rA/rB value of 1.40 and an orthorhombic structure. This means that in the

Sm2BTaO7 phases, substitution of the B site with cations of different 3.3. HRTEM and electron diffraction analyses ionic radii greatly influences the crystal structure. For the structure with the largest Bi3+ cation (1.17 Å), [94] the A TEM analysis was used to explore the crystal phase at the atomic site should not decrease more than 8% in order to maintain the cubic level. Fig. 5 shows two representative lattice-resolved high-resolution crystal structure, and the Sm3+ cation size (1.08 Å) [94] represents a images and the histograms of a few lattice planes of Sm2YTaO7. The limit for cubic crystal formation. Although the ratio rA/rB is within lattice fringes, which are clearly shown, indicate a well-defined crystal 1.39≤ rA/rB ≤1.86 for all Sm-containing phases, only two phases (In structure. and Sc oxides) crystallized in the cubic structure. An interplanar spacing of 0.311 nm is observed in both images, and It can be assumed that the smaller ionic radius of Sm3+ compared 3+ it was assigned to the (220) plane. An interplanar spacing exactly with that of Bi is not large enough to maintain a regular B-O6

3984 L.M. Torres-Martínez et al. Ceramics International 43 (2017) 3981–3992

Fig. 5. HRTEM images of Sm2YTaO7 and the corresponding histogram proﬁles of a few lattice planes.

Fig. 6. Experimental and calculated electron diﬀraction patterns of weberite-type Sm2YTaO7. octahedral [17] in the lowest and largest rA/rB ratios (B=Ga, Fe, Y), the A site 1–6% with respect to Sm3+) is enough to modify the crystal which crystallize in lower symmetry structures (monoclinic and structure, which can also be attributed to the small rB value (Fe–Ta orthorhombic). Alternatively, all tantalate iron oxides, A2FeTaO7, =0.64 Å) [94] of these phases. crystallized in the trigonal structure (R3 symmetry) since the A site As previously mentioned, the crystal structure of A2FeTaO7 oxides is occupied by smaller cations such as Eu3+(1.07 Å), Gd3+(1.05 Å), changed as a function of rA from cubic (for A=Bi) to monoclinic (for Dy3+(1.03 Å) and Y3+(1.02 Å). [94] It seems that a minor variation at A=Sm) and ﬁnally to trigonal (for A=Eu, Gd, Dy, Y). It was observed

3985 L.M. Torres-Martínez et al. Ceramics International 43 (2017) 3981–3992

Table 5 3+ 3+ Relation of phases with the formula A2 B TaO7.

a b f Entry Oxide Year rA rB rA/rB Crystal Space Lattice parameters (Å) Analysis for crystal structure Properties Ref. structurec group determinationd investigatede

1Bi2ScTaO7 1983 1.17 0.69 1.70 C Fd-3m a=10.676 NR [17]

2Bi2CrTaO7 0.63 1.86 a=10.397 2012 NR XRD-Rietveld Magnetic [42]

3Bi2FeTaO7 1983 0.64 1.82 a=10.502 NR [17] 2005 a=10.4478 XRD-Rietveld Photocatalysis [56] 2012 NR Magnetic [42]

4Bi2GaTaO7 2004 0.63 1.86 a=10.4516 Photocatalysis [58] 2004 a=10.4478 [57] 2005 a=10.5041 [56]

5Bi2InTaO7 1983 0.72 1.63 a=10.709 NR [17] 2004 a=10.7612 XRD-Rietveld Photocatalysis [57] 2005 a=10.7612 [56]

6Bi2LaTaO7 2006 0.84 1.39 a=10.9600 [59]

7Bi2YTaO7 0.77 1.52 a=10.7859

8Nd2InTaO7 2009 1.11 0.72 1.54 a=10.5539 XRD-Rietveld, Raman [64]

9Sm2FeTaO7 2012 1.08 0.64 1.69 M C2/c a=13.1307 XRD-Rietveld, ED-TEM [66] b=7.5854 c=11.6425 β=100.971

10 Sm2GaTaO7 2013 0.63 1.71 a=13.1386 XRD-Rietveld [67] b=7.5911 c=11.5495 β=101.087

11 Sm2ScTaO7 1995 0.69 1.56 C Fd-3m a=10.486 NR [95] 2014 a=10.467 XRD E, TM, M [40]

12 Sm2InTaO7 2010 0.72 1.50 a=10.5448 XRD-Rietveld Photocatalysis [69] 2013 a=10.5676 [68]

13 Sm2YTaO7 2016 0.77 1.40 O C2221 a=10.6699 XRD-Rietveld, ED-TEM TW b=7.4971 c=7.5569

14 Eu2FeTaO7 2010 1.07 0.64 1.66 Tr R3 a=7.5428 XRD-Rietveld Magnetic [46] c=17.0864

15 Gd2FeTaO7 1.05 0.64 1.64 a=7.5159 c=17.0609

16 Dy2FeTaO7 1.03 0.64 1.60 a=7.4680 c=17.0063

17 Y2FeTaO7 1.02 0.64 1.59 a=7.4428 c=17.0043

a Ionic radius with coordination number of 8. bAverage of ionic radius with coordination number of 6. cC=cubic, M=monoclinic, O=orthorhombic, Tr=trigonal. dXRD=X-rays powder diﬀraction, ED-TEM=Electron diﬀraction by transmission electron microscopy. eE, TM, M=electrical, thermo-mechanical, magnetic. fTW=This work. NR=Not reported.

trigonal (R3) symmetries using only the ionic radiius of the constituent cations. Therefore, this ﬁeld map could help predict the crystal 3+ 3+ 5+ structure of complex oxides with the general formula A2 B B O7 before synthesis. 3.5 Relationship of cubic, monoclinic and orthorhombic structures

of Sm2MTaO7 (M=In, Fe, Ga, Y) oxides. Oxides such as pyrochlore (cubic), zirconolite (monoclinic) and weberite (orthorhombic) are compounds that can possess the same stoichiometry. All are superstructures closely related to ﬂuorite and can be considered to be ordered defect-ﬂuorite oxides with systematic oxygen vacancies. [11,15,96–98]. 3+ 3+ 5+ In some cases, A2 B B O7 oxides can adopt a zirconolite structure, and this structure can be viewed as a derivative of cubic

pyrochlore. In cubic pyrochlore (Sm2InTaO7), the A and B sites represent large 8-fold coordinated and small 6-fold coordinated

cations, respectively. For monoclinic zirconolite (Sm2FeTaO7 and Sm2GaTaO7), the A site shows 8- and 7-fold coordination in the same Fig. 7. Structural map for A2BTaO7 oxides as a function of rA/rB and rA. The dashed proportion (50%), whereas only 75% of the B cations are 6-fold blue line is the limiting border for the cubic structure. The numbers in the structural map coordinated. In pyrochlore and zirconolite structures, B sites with 6- indicate the oxides listed in Table 6. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) fold coordination form octahedra interconnected in a hexagonal tungsten bronze (HTB)-type network. that only the largest cation Bi3+(1.17 Å) [94] can maintain the ideal The pyrochlore structure is based on a three-dimensional (3D) pyrochlore cubic structure. array of HTB blocks, while the monoclinic structure contains 2D HTB 3+ 3+ 5+ blocks perpendicular to the [001] direction, showing the diﬀerence in From the A2 B B O7 stability map it is easy to distinguish stacking sequence. It is important to mention that the HTB block is a among cubic (Fd-3 m), monoclinic (C2/c), orthorhombic (C2221) and fundamental framework in cubic pyrochlore and related structures.

3986 L.M. Torres-Martínez et al. Ceramics International 43 (2017) 3981–3992

[96–98]. Table 7 In the monoclinic structure, the interstices formed by the six- Analysis of RuO2 content deposited on the photocatalysts. membered rings of the octahedra are occupied by B3+ and B5+ cations, RuO (% Weight) while in the pyrochlore structure, the corresponding sites are occupied 2 3+ by large A cations. Essentially, in the monoclinic structure the B and Material Composition a Composition b Composition c B5+ cations are fully ordered in their sites in contrast to the cubic (0.2%) (1.0%) (1.5%) pyrochlore structure. [97]. Sm2InTaO7 0.2 0.9 1.6 Sm2FeTaO7 0.2 1.0 1.5 As previously mentioned, the Sm2YTaO7 phase shows a weberite- Sm2GaTaO7 0.2 0.9 1.5 type arrangement (rather than an ideal weberite structure), where Sm, Sm2YTaO7 0.3 1.1 1.6 Y and Ta are 7-, 8- and 6-fold coordinated, respectively. In this case, only vertex-shared TaO6 octahedra are observed, and they do not form even a 2D HTB structure; therefore, the weberite-type oxide Sm2YTaO7 is not related to cubic pyrochlore phases such as Sm2InTaO7. However, Sm2YTaO7 oxide is directly related to the fluorite structure; considering 4+ that the fluorite unit cell of oxides has a composition of M4 O8, if the four cations are replaced by three M3+ cations and one M5+ cation, one oxide vacancy per fluorite cell is formed. The cation vacancy occurs at the metal sites due to significant differences in the ionic radii of M3+ and M5+, while the oxide vacancy occurs at the anion sites. [15,85] In consideration of the above, the difference in ionic radii between Sm(VII) (1.02 Å) [94] and Y(VIII) (1.02 Å) [94] with respect to

Ta(VI) (0.64 Å) [94] in the Sm2YTaO7 oxide favors the crystallization of this phase as a weberite-type structure.

It has been well established that the ideal cubic pyrochlore, A2B2O7, tends to change to a defect-fluorite structure when the B cation has a smaller ionic radius than the A cation (low rA/rB ratio); [2,3,8,12– 14,16,17] therefore, a similar behavior was observed in the transition of cubic Sm2InTaO7 to orthorhombic Sm2YTaO7. Despite Sm2MTaO7 (M=Y, In, Fe, Ga) oxides having different crystal structures, all are formed by TaO6 octahedra chains showing – diverse arrangements. This structural feature could be very important Fig. 8. UV vis spectra of Sm2MTaO7 (M=Y, In, Fe, Ga) oxides. in photocatalytic processes, where the mobility of electrons and holes play a significant role. [23,24,55,56,81,82]. incomplete localization of the samarium 4 f electrons and their participation in chemical bonding through hybridization. The absorption bands located in the visible region at λ > 400 nm are attributed to 3.5. Compositional analysis of the photocatalysts internal transitions in the localized and partly filled Sm 4 f shell. [99– 101]. The chemical compositions of the prepared oxides were analyzed by Alternatively, the Sm FeTaO spectrum presents significant differ- energy dispersive X-ray spectroscopy (EDS), and the results are 2 7 ences in comparison to the other oxides; it shows absorption in the presented in Table 6. According to the results, the obtained atomic extended visible region from 400 to 800 nm, and an absorption edge is percentages are in agreement with the theoretical formula of the oxides observed at approximately 630 nm. The visible light sensitivity is (50% Sm, 2% M=In, Fe, Ga, Y; and 25% Ta). After modification with a related to Fe cations in the crystal structure, where the Fe3+ ion has cocatalyst, the actual content of RuO2 deposited was determined and is a3d5 configuration, which has a sextet state in the octahedral crystal shown in Table 7. The measured weight percentages of RuO2 in the field. The intense absorption in the visible region is probably due to samples are in agreement with the theoretical amount of cocatalyst Fe3+ d-d transitions, as well as to metal to metal charge transfer deposited (0.2%, 0.5%, 1.0% and 1.5%). These results demonstrate that transitions between the iron cations. [102–104]. there was no significant weight loss during the deposition process and The bandgap energy of the samarium tantalates calculated by that RuO2 was deposited effectively on the photocatalyst material. means of Kubelka-Munk analysis were 4.3, 3.7, 2.0 and 4.1 eV for the Y, In, Fe and Ga phases, respectively. 3.6. UV–vis spectroscopy and band structure It is well known that the conduction band (CB) and valence band (VB) edges of a semiconductor must be located at appropriate levels to The UV–vis DR spectra of Sm MTaO (M=Y, In, Fe, Ga) powders 2 7 carry out the water splitting process. Specifically, the CB should be are shown in Fig. 8. For the samarium tantalates containing Y, In and − above E (H /H+), and the VB should be below E (OH /O ). The Ga, the spectra exhibit a major absorption band at λ < 350 nm. In red 2 ox 2 theoretical predictions of the band edge positions for Sm MTaO addition, these spectra show several bands located between 350 and 2 7 (M=Y, In, Fe, Ga) oxides were calculated using a mathematical method 400 nm. As previously reported, [66,67] these bands are due to the previously reported in the literature. [67,105, 106]. The proposed band structures, as well as the obtained values for the Table 6 Average elemental composition obtained by EDS. VB and CB levels, are schematically represented in Fig. 9. The VB of all phases is formed through the contribution of Sm 4f and O 2p orbitals, Average atomic content whereas the CB is mainly composed of Ta 5d orbitals and the orbitals of the complementary cation, In (5s5p), Fe (3d), Ga (4s4p), or Y (4d). Material %Sm %M %Ta Upon illumination, the Sm 4f and O 2p orbitals in the VB interact to

Sm2InTaO7 50.2 25.0 24.8 transfer electrons to the CB, and the hole-electron pairs participate in

Sm2FeTaO7 49.9 24.7 25.4 the redox reaction on the semiconductor surface. Sm2GaTaO7 49.8 24.6 25.6 It was observed that Y-, In-, and Ga-containing oxides possess Sm2YTaO7 50.1 25.1 24.8 negative CB levels of −1.1, −0.8, and −1.0 eV, respectively, as seen in

3987 L.M. Torres-Martínez et al. Ceramics International 43 (2017) 3981–3992

Fig. 9. Band structure and potential levels of Sm2MTaO7 (M=Y, In, Fe, Ga) oxides. The potential for the reduction and oxidation of water was estimated at pH =7.

Fig. 9. This indicates that hydrogen production from the water splitting out in the presence of a photocatalyst and illuminated with a 400 W reaction using these photocatalysts is thermodynamically feasible. The high-pressure mercury light lamp. Fig. 11 shows the hydrogen evolu- 3+ Sm2FeTaO7 oxide showed a midband due to the presence of Fe , tion of the samples in μmol/g h. According to the positions of the which is located below (CB=+0.2 eV) the level of proton reduction. calculated conduction and valence bands, all samarium tantalates However, the presence of Ta 5d orbitals, which are generally negative studied in this work are able to perform the oxidation and reduction enough to reduce protons, could promote hydrogen evolution. of water, where electrons reduce water to produce hydrogen, and holes oxidize water to produce oxygen. However, the evaluation of oxygen ffi 3.7. Photocatalytic hydrogen production of Sm2MTaO7 (M=Y, In, Fe, evolution is di cult due to oxygen being adsorbed on the photocatalyst Ga) surface and oxygen having a higher solubility in water (69.93 mg/kg at 273 K and 1 bar) compared to hydrogen (1.92 mg/kg at 273 K and First, the water splitting reaction was carried out in a photoreactor 1 bar) [108], which makes its quantification more difficult. For these under the illumination of a mercury lamp without any added catalyst. A reasons, oxygen was not detected in any of the samples of gas evolved second experiment was carried out with Sm2MTaO7 catalysts under during the water splitting reaction analyzed by GC. dark conditions. Hydrogen was not detected in any of these experi- In the hydrogen evolution reaction, the materials showed activity ments, confirming that the presence of a photocatalyst and light without the use of sacrificial agents. These results are presented in radiation is necessary to decompose the water molecule. Fig. 11 and summarized in Table 8. In this figure, it is observed that the Fig. 10 shows the hydrogen evolution, in μmol, in the water photocatalysts exhibited an increase in the gas production rate with splitting reaction as a function of time. These experiments were carried reaction time. Based on this, it was established that the photocatalysts

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Fig. 12. Photocatalytic stability test of the sample with the highest hydrogen evolution Fig. 10. Photocatalytic hydrogen evolution (μmol) over the Sm2MTaO7 photocatalysts. rate (Sm2YTaO7).

lowest activity for hydrogen production, whereas Sm2YTaO7 exhibited the highest activity. It has been suggested that the formation of a 4f- 10 0 d -d electronic configuration of Sm2MTaO7 (M=Y, In) promotes the mobility of electrons in the conduction band. [67,69] For Sm2FeTaO7, hydrogen production is associated with the energy level of Ta 5d (which is responsible for water reduction) rather than the sub-band associated with Fe 3d. Additionally, concerning the thermodynamic requirements for the water splitting reaction, it has been reported that the photocatalytic activity of complex metallic oxides is influenced by several physicochemical properties such as chemical composition, crystal structure, particle size and electronic configuration. [56,81,82,99–101] In this

work, the Sm2MTaO7 (M=Y, In, Fe, Ga) catalysts were prepared at high temperature under the same reaction conditions. They have large particle sizes and similar crystal structures composed of corner-shared

octahedral layers of TaO6 units within a three-, two- or one-dimensional array [107]. Due to this, as discussed previously, the photo-

catalytic eﬃciency of Sm2MTaO7 (M=Y, In, Fe, Ga) materials was directly associated with the most negative value of their conduction band potential. To enhance the photocatalytic activity of the materials for hydrogen production, a small amount of ruthenium oxide was deposited on the Fig. 11. Photocatalytic hydrogen evolution (μmol/g h) over the photocatalysts synthe- surface of the semiconductors, as it has been previously reported to be sized in this work. a good cocatalyst in the hydrogen evolution reaction. [23,67] It has been suggested that the electrons generated by the semiconductor

Table 8 catalyst are effectively transferred to the RuO2 particles to provide Photocatalytic activity of Sm2MTaO7 (M=Y, In, Fe, Ga) complex oxides for hydrogen active sites for hydrogen production. The most significant effect was production. obtained by Ga and Y tantalates, showing an almost 2.4 times increase

Photocatalyst Hydrogen evolution reaction rate [μmol.g−1 h−1] in activity with respect to the activity of the tantalates without cocatalyst. It was observed that the activity for hydrogen evolution

Sm2YTaO7 62.25 decreased with the amount of RuO2 loading due to the saturation of the Sm2InTaO7 47.43 photocatalyst surface, which obstructs light absorption and limits the Sm FeTaO 24.20 2 7 activation of the photocatalyst. Sm2GaTaO7 58.00 Though 0.2 wt% RuO2 deposited on the surface of the Sm2FeTaO7 catalyst also increases hydrogen production, this effect is not easily are not deactivated after 5 h of reaction. Fig. 12 presents the photo- measurable. The results in Fig. 13 also confirm that an excess of catalytic stability test of the material with the highest evolution rate cocatalyst limits light absorption and reduces hydrogen production - + (Sm2YTaO7) performed over four reaction cycles. As shown in this because the formation of the electronic pair (e -h ) decreases. For figure, the material maintained similar photocatalytic activity after four Sm2GaTaO7, the activity of the photocatalyst was doubled with a reaction cycles. loading of 0.2 wt% RuO2, as reported in our previous work [67]. It is possible to confirm that the photocatalytic activity of the

Sm2MTaO7 materials (Sm2YTaO7 >Sm2GaTaO7 >Sm2InTaO7 > 4. Conclusions Sm2FeTaO7) is associated directly with the negative character of the conduction band of the oxides (Fig. 9) containing Y (−1.00 eV), Ga Structural relationships were established based on the crystal − − ( 0.95 eV), In ( 0.66 eV) and Fe (+0.22 eV). Sm2FeTaO7 showed the chemistry of new non-isostructural Sm2MTaO7 oxides (M=In, Fe, Y),

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Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.ceramint.2016.11.098.

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