Sn2SiS4, synthesis, structure, optical and electronic properties

Sn2SiS4, synthesis, structure, optical and electronic properties

Optical Materials xxx (2015) xxx–xxx Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat S...

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Optical Materials xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Sn2SiS4, synthesis, structure, optical and electronic properties Chao Li a,b,c, Zuohong Lin a,b,c, Lei Kang a,b,c, Zheshuai Lin a,b, Hongwei Huang d,⇑, Jiyong Yao a,b,⇑, Yicheng Wu a,b a

Center for Crystal Research and Development, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China Key Laboratory of Functional Crystals and Laser Technology, Chinese Academy of Sciences, Beijing 100190, China c University of Chinese Academy of Sciences, Beijing 100049, China d Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China b

a r t i c l e

i n f o

Article history: Received 24 April 2015 Received in revised form 5 June 2015 Accepted 5 June 2015 Available online xxxx Keywords: Sn2SiS4 Synthesis Crystal structure Optical properties Electronic properties

a b s t r a c t The new ternary sulfide Sn2SiS4 has been synthesized via high-temperature solid state reaction. It crystallizes in the centrosymmetric space group P21/c of the monoclinic system. In the structure, the Sn2+ cations are coordinated to a heavily-distorted octahedron of six S atoms or a pentagonal pyramid of six S atoms, both geometries clearly demonstrating the effect of the stereo-chemically active electron lone pair on the Sn coordination environment. These SnS6 polyhedra and the SiS4 tetrahedra are connected to each other via corner and edge-sharing to generate a three-dimensional framework. Based on the diffuse reflectance measurement and the electronic structure calculation, Sn2SiS4 has an indirect band gap of 2.00 eV. Interestingly, Sn2SiS4 exhibits an efficient visible-light-driven photocatalytic activity pertaining to Rhodamine B (RhB) degradation, which is superior to the important photocatalyst C3N4. Moreover, the photocatalytic mechanism was also elucidated based on the active species trapping experiments. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Recently, metal chalcogenides containing the group 14 elements, in particular the Sn metal, have received increasing interest [1–11]. Sn atom can be stabilized at both the +2 oxidation state with an electron lone pair and the +4 oxidation states without the lone pair electrons in chalcogenides. And its coordination environment can range from 3-fold trigonal pyramids, to 4-fold tetrahedra, to 5-fold square pyramids, to 6-fold octahedra, etc. [5,7,8] in chalcogenides. Its mixed valence property and the various coordination environments will certainly increase the stoichiometric and structural diversity of resultant compounds, and hence may lead to interesting properties. As a result of extensive exploration, many new Sn-containing metal chalcogenides have been discovered, including Ln2SnS5 (Ln = Pr, Nd, Gd and Tb) [2], Ag6SnS4Br2 [3], Li2CdSnS4 [4], EuCu2SnS [4], Ba7Sn5S15 [5], BaSn2S5 [5], Ba6Sn7S20 [5], In4PbxSnySe3 [6], Ba6Sn6Se13 [7] and SnGa4Q7 (Q = S, Se) [8]. Among them, In4PbxSnySe3 is a promising mid-temperature ⇑ Corresponding authors at: Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China (Jiyong Yao). E-mail addresses: [email protected] (H. Huang), [email protected] (J. Yao).

thermoelectric material candidate [6]; Ba7Sn5S15 demonstrates the coexistence of Sn2S3 trigonal-bipyramids and SnS4 tetrahedra and shows a strong SHG effect [5]; Ba6Sn6Se13 has mixed valent Sn2+ and Sn4+ cations in 3-fold, 4-fold, and 5-fold coordination environments respectively and shows a moderate NLO effect [7]. In the meanwhile, Cu2ZnSnS4, Cu2FeSnS4 and Ag2ZnSnS4 have attracted great interest for their photovoltaic and H2 production performance [11–16]. In this paper, we focus on the ternary Sn/M/Q (M = Si, Ge; Q = S, Se, Te) system, hoping that the interplay of the various SnQx polyhedra and the MQ4 tetrahedra will result in new compounds with interesting structures and properties. Our efforts led to the discovery of the Sn2SiS4 compound, the first member in this system. In the structure, the Sn2+ cations are coordinated to a heavily-distorted octahedron of six S atoms or a pentagonal pyramid of six S atoms as a result of the stereo-chemical activity of electron lone pair. Sn2SiS4 possesses a three-dimensional framework built from these SnS6 polyhedra and the SiS4 tetrahedra and an indirect band gap of 2.00 eV, lying in the visible range. Furthermore, stimulated by the result that many compounds containing cations with electron lone pair (such as Bi3+) demonstrate excellent photocatalytic activities [17–20]. We investigate the photo-catalytic performance of Sn2SiS4. Interestingly, Sn2SiS4 displays a much higher photocatalytic performance for

http://dx.doi.org/10.1016/j.optmat.2015.06.008 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.

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C. Li et al. / Optical Materials xxx (2015) xxx–xxx

decomposition of Rhodamine B (RhB) under visible light irradiation (k > 420 nm) than C3N4 does. The corresponding photocatalytic mechanism was also investigated. 2. Experimental section 2.1. Syntheses The following reagents were used as obtained: Sn (Sinopharm Chemical Reagent Co., Ltd., 99.9%), Si (Sinopharm Chemical Reagent Co., Ltd., 99.99%), and S (Sinopharm Chemical Reagent Co., Ltd, 99.99%). The binary starting materials, SnS and SiS2, were first synthesized by the stoichiometric reactions of elements at high temperatures in sealed silica tubes evacuated to 103 Pa. Reaction mixtures of 2 mmol of SnS and 1 mmol SiS2 were ground and loaded into 12 mm inner-diameter fused-silica tubes under an Ar atmosphere in a glovebox. Then the tubes were flame-sealed under a high vacuum of 103 Pa and then placed in a computer-controlled furnace. The samples were heated to 1173 K within 15 h, kept for 72 h, then slowly cooled to 673 K at a rate of 3 K/h, and finally cooled to room temperature by switching off the furnace. Many block-shaped orange crystals were found in the ampoules and subsequently determined as Sn2SiS4. The percent yield is about 20%. Analyses of the crystals with an EDX-equipped Hitachi S-4800 SEM indicated the presence of Sn, Si, and S in the approximate ratio of 2:1:4, which was in agreement with the stoichiometric proportion from single-crystal X-ray structure analysis result. The crystals are stable in air for months. 2.2. Structure determination Single-crystal X-ray diffraction measurement was performed on a Rigaku AFC10 diffractometer equipped with a graphitemonochromated Mo Ka (k = 0.71073 Å) radiation at 293 K. The collection of intensity data was carried out with Crystalclear software [21], and face-indexed absorption corrections were performed numerically with the use of the program XPREP [22]. The structure was solved with the direct methods SHELXTLS program and refined with the least-squares program SHELXL of the SHELXTL.PC suite of programs [22]. Additional experimental details are given in Table 1 and selected metrical data are given in Table 2. Further information can be found in Supporting Information. X-ray powder diffraction analysis of the ground single crystals was performed at room temperature in the angular range of

Table 1 Crystal data and structure refinement for Sn2SiS4.

Table 2 Selected interatomic distances (Å) for Sn2SiS4. Sn1–S3 Sn1–S2 Sn1–S1 Sn1–S1 Sn1–S3 Sn1–S3 Sn2–S4 Sn2–S1 Sn2–S2 Sn2–S4 Sn2–S2 Sn2–S4 Si–S3 Si–S4 Si–S1 Si–S2

2.6394(17) 2.7000(16) 2.7928(16) 3.0505(16) 3.1985(18) 3.3391(22) 2.6545(19) 2.6976(16) 2.8194(16) 3.1525(18) 3.3085(15) 3.3552(21) 2.104(2) 2.113(2) 2.144(2) 2.151(2)

2h = 10–70° with a scan step width of 0.02° and a fixed counting time of 0.1 s/step using an automated Bruker D8 X-ray diffractometer equipped with a diffracted monochromator set for Cu Ka (k = 1.5418 Å) radiation. The experimental pattern was in agreement with the calculated one on the basis of the single-crystal crystallographic data (Fig. 1), which proves the purity of the bulk sample. The difference in the relative peak intensity may be due to the orientation preference of the Sn2SiS4 powder. 2.3. Diffuse reflectance spectroscopy Single crystals of Sn2SiS4 (about 0.5 g) were hand-picked from reaction products and ground to powder. Then a Cary 5000 UV– vis–NIR spectrophotometer with a diffuse reflectance accessory was used to measure the spectrum of Sn2SiS4 over the range of 350 nm (3.54 eV)–2000 nm (0.62 eV). 2.4. First-principles calculations The first-principles calculations at the atomic level for the Sn2SiS4 crystal were performed by the plane-wave pseudopotential method [23] implemented in the CASTEP program [24] based on density functional theory (DFT) [25,26]. For comparison, we have conducted four calculations of geometry optimization for the present structure, including two typical functionals local density approximation (LDA) [27] and generalized gradient approximation (GGA–PBE) [28] combined with two usual pseudopotential ultrasoft (usp) [29] and Norm-conserving (ncp) [30], i.e., LDAusp, LDAncp, PBEusp and PBEncp. The calculated cell parameters

Sn2SiS4 Fw T (K) a (Å) b (Å) c (Å) a (o) b (o) c (o) Space group V (Å3) Z qc (g/cm3) l (cm–1) R(F)a RW(F2o)b w1 = r2(F2o) + (zP)2, where P = (Max(F2o, 0) + 2 F2c )/3. P P a R(F) = |Fo|  |Fc|/ |Fo| for F2o > 2r (F2o). P P b Rw(F2o) = { [w(F2o  F2c )2]/ wF4o}½ for all data.

393.7 293 8.1831(16) 8.2851(17) 11.174(4) 90.00 116.84(2) 90.00 P21/c 676.0(3) 4 3.869 8.674 0.0478 0.1035

Fig. 1. Powder X-ray diffraction patterns of Sn2SiS4.

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C. Li et al. / Optical Materials xxx (2015) xxx–xxx Table 3 Comparison of experimental and calculated crystallographic data and band gaps of Sn2SiS4 crystal. a (Å)

b (Å)

c (Å)

a (°)

b (°)

c (°)

Volume (Å3)

Band gap (eV)

Exp. results

8.1831

8.2851

10.4520

90

107.470

90

675.937

2.00

Cal. by LDAusp Error

7.9938 2.3%

7.9733 3.8%

10.0991 3.4%

90

106.908 0.5%

90

615.860 8.9%

1.57

Cal. by LDAncp Error

8.0197 2.0%

8.0159 3.2%

9.9828 4.5%

90

105.455 1.9%

90

618.535 8.5%

1.25

Cal. by PBEusp Error

8.4897 3.7%

8.4954 2.5%

10.6754 2.1%

90

108.641 1.1%

90

729.553 7.9%

2.07

Cal. by PBEncp Error

8.4231 2.9%

8.9126 7.6%

10.1667 2.7%

90

106.374 1.0%

90

732.273 8.3%

1.73

As for the set-ups of k-points, we all select the ultra-fine [31] parameter to ensure the accuracy of the present purpose. 2.5. Photocatalytic activity experiment The organic dye molecule Rhodamine B (RhB) was selected as the degradation model to investigate the photocatalytic activity. In a typical photodegradation process, 50 mg of photocatalyst was dispersed into 50 mL of 3 ⁄ 105 mol/L RhB aqueous solution. Before light on, the mixture was vigorously stirred in dark for 1 h to get an adsorption–desorption equilibrium between the RhB and photocatalyst. Afterward, the suspensions were exposed to a 500 W Xe lamp which was coupled with 420 cut off filters (k > 420 nm). At appropriate intervals, 5 mL of the reaction suspension was taken off, and centrifuged at 5000 rpm for 10 min to remove the solid. Finally, the concentration of RhB was determined by a UV-5500PC spectrophotometer on the basis of the absorption band of 554 nm. Fig. 2. Coordination environment of all cations in Sn2SiS4.

2.6. Active species trapping experiments (Table 3) are all in a good agreement with the experimental determinations (the error are less than 5%), which show that these computational methods are closely accurate. Sn 5s25p2, Si 3s23p2, and S 3s23p4 electrons are treated as the valence electrons, respectively.

Ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), benzoquinone (BQ) and isopropanol (IPA) were added as scavengers to detect the active species holes (h+), superoxide radical  (O 2 ) and hydroxyl radicals ( OH) generated during photo oxidation

Fig. 3. Polyhedral views of Sn2SiS4: (a) layers formed by the Sn1S6 pentagonal pyramids parallel to the bc plane; (b) layers formed by the Sn2S6 distorted octahedra parallel to the bc plane; (c) the three-dimensional structure of Sn2SiS4 formed by two kinds of layers (a and b) and the SiS4 tetrahedra.

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Fig. 4. The diffuse reflectance spectrum of Sn2SiS4.

process, respectively [32,33]. The trapping experiments were similar with the above photocatalytic activity experiment only that the additional scavengers were added into the RhB solution prior to addition of the photocatalyst.

3. Results and discussion 3.1. Crystal structure The sulfide Sn2SiS4 was obtained by high-temperature spontaneous nucleation method without seed, representing the first member in the Sn/M/Q (M = Si, Ge; Q = S, Se, Te) system. We also

tried to synthesize the selenide and telluride analogues. Unfortunately, the suspected Sn2SiSe4 crystal was of poor quality and no analogous telluride was found. Thus we only report our study on Sn2SiS4 here. The Sn2SiS4 compound crystallizes in the centrosymmetric space group P21/c of the monoclinic system with unit cell parameters of a = 8.1831(16) Å, b = 8.2851(17) Å, c = 11.174(4) Å, b = 116.84(2) and Z = 4. The asymmetric unit contains two crystallographically independent Sn atoms, one Si atom and four S atoms (all at Wyckoff 4e general positions). Fig. 2 illustrates the coordination environments of cations. Sn1 is coordinated to six S atoms, which may be described as in an arrangement of pentagonal pyramid. Sn2 is also coordinated to six S atoms in a bit different geometry, which may be depicted as a heavily distorted octahedron. The Sn–S distances are from 2.6394(17) to 3.3552(21) Å (Table 2), which are comparable with those of 2.659–3.102 Å in Sn2S3 [34], and the calculated bond valence sum (BVS) [35] is 1.504 and 1.616 for Sn2 and Sn1 respectively. The Si atoms are tetrahedrally coordinated to four S atoms with the Si–S distances ranging from 2.104(2) to 2.151(2) Å (Table 2), which are similar to those of 2.121(2)–2.124(3) Å in Eu2SiS4 [36], 2.081(13)–2.143(15) Å in Ag2SiS3 [37], and 2.131(1)–2.143(3) Å in Cu2ZnSiS4 [38], and the calculated bond valence sum (BVS) [35] for Si is 3.985. Considering the bonding characteristics in the structure, the oxidation states 2+, 4+ and 2 can be attributed to Sn, Si, and S, respectively and the charge balance can be accessed in this way. As shown in Fig. 3, the Sn1S6 pentagonal pyramids are first connected to each other by edge-sharing to form chains along the b direction, which are then linked via edge and corner-sharing to generate layers parallel to the bc plane (Fig. 3a). Similarly, the

Fig. 5. (a) The band structure calculated by the PBEusp method and (b) PDOS calculated by four different methods of the Sn2SiS4 crystal.

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pentagonal bipyramid of seven S atoms, with the distances of Pb–S ranging from 2.81 to 3.37 Å. In the Pb2SiS4 one Pb is in sevenfold coordination and the other Pb is in eightfold coordination. The Pb–S distances range from 2.82 to 3.50 Å. In Pb2SiSe4 both Pb atoms are in sevenfold coordination with Pb–Se distances varying from 2.97 to 3.54 Å. As for Ba2SnS4, without the influence of the stereochemical active electron lone pair, the Ba atoms are in more ‘‘spherical’’ environment of S atoms: two of the four crystallographically unique Ba atoms are bicapped trigonal prismatically coordinated to eight S atoms with the distances of Ba–S ranging from 3.06 to 3.47 Å, and the other two Ba atoms are in sixfold coordination of trigonal prism with the distances of Ba–S ranging from 3.05 to 3.37 Å. The difference in the Ba–S distances is much smaller than that in Pb/Sn–S/Se distances. 3.2. Experimental band gap The UV–visible–NIR diffuse reflectance spectrum of the title compound is shown in Fig. 4. Based on the diffuse reflectance measurement and the electronic structure calculation, the Sn2SiS4 should possess an indirect band gap of about 2.00 eV [41,42]. 3.3. First-principles calculations

Fig. 6. (a) Photocatalytic degradation curves and (b) apparent rate constants of RhB over Sn2SiS4 and C3N4 samples and with no catalyst under visible light irradiation.

corner and edge-sharing Sn2S6 distorted octahedra produce another set of layers parallel to the bc plane (Fig. 3b). These two kinds of layers are then packed alternately along a direction and linked by the SiS4 tetrahedra to produce the three-dimensional structure (Fig. 3c). As a result of such packing, all the S atoms are tetrahedrally coordinated to three Sn atoms and one Si atom. Although Sn2SiS4 possesses the same stoichiometry as the previously reported Pb2GeS4, Pb2SiS4, Pb2SiSe4 and Ba2SnS4 compounds [39,40], they crystallize in different structure types with different cell parameters (a = 8.1831(16) Å, b = 8.2851(17) Å, c = 10.452(2) Å, Z = 4 in space group P21/c for Sn2SiS4 compared with those of a = 7.9742(6) Å, b = 8.9255(8) Å, c = 10.8761(8) Å, Z = 4 in space group P21/c for Pb2GeS4; a = 6.4721(5) Å, b = 6.6344(9) Å, c = 16.832(1) Å, Z = 4 in space group P21/c for PbSi2S4; a = 8.5670(2) Å, b = 7.0745(3) Å, c = 13.6160(3) Å, Z = 4 in space group P21/c for PbSi2Se4; and a = 17.823(3) Å, b = 7.359(1) Å, c = 12.613(2) Å, Z = 8 in space group Pna21 for Ba2SnS4) [39,40]. Such differences may result largely from the different radii and coordination preferences among Sn, Pb and Ba atoms. For Sn2SiS4, each Sn atom is coordinated to six S atoms with the Sn–S distances from 2.6394(17) to 3.3552(21) Å. However, as a result of their larger size, the Pb atoms in the Pb2GeS4 are coordinated to a heavily distorted octahedron of six S atoms or a

For comparison, we simulated the band structures of the four methods. The partial density of states (PDOS) are very close to each other, indicating the similar accuracy of the LDA/GGA and ultrasoft/Norm-conserving methods. The calculated band gap of PBEusp is closest to the experiment value (2.07 eV vs. 2.00 eV). Therefore, the electronic structure obtained via PBEusp will be discussed in detail here. The electronic band structure of the Sn2SiS4 crystal in the unit cell is plotted along the symmetry lines in Fig. 5a. It is shown that Sn2SiS4 is an indirect gap crystal, and its calculated value is 2.07 eV, which is quite close to the experimental value (2.00 eV). The corresponding DOS/PDOS projected on the constitutional atoms is displayed in Fig. 5b. It is found that the energy band is divided into two regions, i.e., valence band (VB) and conduction band (CB). The energy region below 10 eV is mainly composed of the isolated inner s orbitals of Si (3s) and S (3s). The Sn 5s orbitals are located at about 5 eV and strongly hybridized with the S 3p orbitals. The top of VB and the bottom of CB are mainly occupied by the 3p orbitals of Sn, Si and S, which are overlapped with each other to form the covalent SiS4 and SnS6 polyhedra.

Fig. 7. Photocatalytic degradation curves of RhB over in the Sn2SiS4 presence of the scavengers of IPA, EDTA or BQ.

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3.4. Photocatalytic activities and investigation on the mechanism Photodegradation of the dye Rhodamine B (RhB) is usually utilized to evaluate the photocatalytic activity of a photocatalyst. For the sake of comparison purpose, the important photocatalyst C3N4 were used for references. Fig. 6 displays the photocomposition curves of RhB over the obtained Sn2SiS4 and C3N4 under visible light irradiation (k > 420 nm). Obviously, RhB is stable and photolysis can be ignored. Within 2 h illumination, it can be seen that only 72% of RhB can be degraded over C3N4. In contrast, Sn2SiS4 exhibits a highly efficient photocatalytic activity, which can remove almost 100% RhB in the same period. In order to understand the reaction kinetics during the photodegradation process of RhB quantitatively, the pseudo first-order model was fitted according to the Langmuir– Hinshelwood (L–H) kinetics model [43]:

lnðC 0 =CÞ ¼ kapp t

ð1Þ

Herein, C0 represents the initial RhB concentration (mol/L), C is the instantaneous concentration of RhB solution at time t (mol/L) and kapp is the apparent pseudo-first-order rate constant (h1). Fig. 6b shows that the corresponding apparent rate constants are 0.0349 and 0.0110 for Sn2SiS4 and C3N4, respectively. In other words, the photocatalytic activity of Sn2SiS4 is 3.2 times higher than that of C3N4, respectively. The occurrence of organic molecule photodegradation is attributed to the photo oxidation by the active radical species generated during the photoreaction process. Under the irradiation of photons with energy larger than the band gap of the semiconductor photocatalyst, the electron (e)–hole (h+) pairs appear. The electrons can jump to the conduction band (CB) to further react with O2 molecules adsorbed on the surface of photocatalyst to generate superoxide radicals (O 2 ) with powerful oxidization. Meanwhile, holes can oxidize the OH ions in solution to produce reactive hydroxyl +  radicals (OH). The O 2 , h and OH all can act as active species playing very critical roles in the photo oxidation process. In order to elucidate the photocatalytic mechanism and detect which active species dominates the degradation of RhB in the photocatalytic reaction, the trapping experiments on radicals and holes were performed. Benzoquinone (BQ), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), and isopropanol (IPA) were added in the + dye solution to capture the superoxide radicals (O 2 ), holes (h ), and hydroxyl radicals (OH) scavengers, respectively. Fig. 7 shows the degradation curves of RhB with the addition of different scavengers. IPA has almost no effect on the photodegradation of RhB, indicating no OH was generated in photocatalytic process. Nevertheless, by the addition of BQ and EDTA-2Na, it can be observed that the photocatalytic efficiency was greatly depressed. + Thus, both superoxide radicals (O 2 ) and photogenerated holes (h ) serves as the main active species for the photodegradation of RhB over Sn2SiS4 under visible light irradiation. 4. Conclusion In summary, the first chalcogenide in the ternary Sn/M/Q (M = Si, Ge; Q = S, Se, Te) system: namely Sn2SiS4, has been synthesized. The compound Sn2SiS4 crystallizes in the centrosymmetric space group P21/c of the monoclinic system. In the structure, Si atoms are tetrahedrally coordinated with four S atoms, while the Sn atoms are coordinated to six S atoms, demonstrating the stereochemical activity the 5s2 electron lone pair The Sn1S6 pentagonal pyramids and the Sn2S6 distorted octahedra themselves form two kinds of layers parallel to the bc plane respectively, which are then adjoined by the SiS4 tetrahedra to generate a three-dimensional framework. Based on the UV–vis–NIR spectroscopy measurement

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