Article pubs.acs.org/cm
(NH4)4Sn2S6·3H2O: Crystal Structure, Thermal Decomposition, and Precursor for Textured Thin Film Peter Nørby,† Jacob Overgaard,† Per S. Christensen,† Bo Richter,† Xin Song,‡ Mingdong Dong,‡ Anpan Han,‡ Jørgen Skibsted,⊥ Bo B. Iversen,*,† and Simon Johnsen*,† †
Center for Materials Crystallography (CMC), Department of Chemistry and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Langelandsgade 140, Aarhus C, DK-8000, Denmark ‡ Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Gustav Wieds Vej 14, 8000 Aarhus C, Denmark ⊥ Instrument Centre for Solid-State NMR Spectroscopy, Department of Chemistry and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark S Supporting Information *
ABSTRACT: Understanding the condensation of the dimeric thiostannate(IV) [Sn2S6]4− to SnS2 is of key importance for the development of solution processing of advanced tin(IV) sulfide based electronic devices such as photovoltaics (e.g., Cu2ZnSnS4, CZTSSe) and thin-film transistors. Here, we report the crystal structure of tetraammonium thiostannate(IV) trihydrate ((NH4)4Sn2S6·3H2O), which can be used as a more environmentally friendly alternative to the hydrazinium analogue in solution processed advanced tin(IV) sulfide based electronic devices, e.g., CZTSSe. Hirshfeld surface analysis shows that crystal bound water molecules play a significant role in the structure and interact strongly with the sulfur atoms in the dimeric thiostannate(IV) complex [Sn2S6]4−. The thermal decomposition and corresponding condensation of ((NH4)4Sn2S6·3H2O) to SnS2 have been studied by TG/DSC-MS and solid-state 119Sn MAS NMR. It involves the formation of the relatively more condensed thiostannate(IV) complex [Sn4S10]4− at 90 °C via evaporation of ammonia, hydrogen sulfide, and water from the structure. With increasing temperature, more tin is transformed from tetrahedral to octahedral coordination, and at 220 °C, crystalline SnS2 is formed. In an aqueous ammonium sulfide based solution, the structure of dimeric [Sn2S6]4− is retained, and aqueous solutions of (NH4)4Sn2S6·3H2O can be spin coated and thermally decomposed to form crystalline SnS2 thin films. X-ray scattering techniques show that the solution processed SnS2 thin film is highly textured with the ab plane parallel to the substrate. Furthermore, AFM and TEM reveal that the thin film is continuous and with an inherent porous surface structure from the gaseous formation byproducts.
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electronic “glue” between nanoparticles or as a building block for multinary phases.22−26 An example of the latter is the CZTSSe solar cells formed by using [Sn2X6]4− (X = S and Se) complexes, which have recently received great scientific and technological interest.8−13 Winkler et al. have solution processed the record holding solar cell (η = 12.0%) from an optimized hydrazinium tin(IV) chalcogenide based hydrazine solution.13 Dimeric [Sn2X6]4− complexes have also been utilized as molecular metal chalcogenide surface ligands, because of their strong affinity to metal chalcogenide nanoparticle surfaces and their consequent ability to ligand exchange.22−27 Despite their widespread use and broad range of applications, little is known about the [Sn2X6]4− dimers and their thermal decomposition. Further knowledge about the solution and solid-state chemistry of [Sn2X6]4− dimers as well as their thermal decomposition and condensation steps toward
INTRODUCTION Solution processing of main-group metal chalcogenide electronic films, such as CuIn 1−x Ga x Se 2 (CIGS), 1−7 Cu2ZnSe4‑ySy (CZTSSe),8−13 and SnSe2‑zSz,14,15 is a promising technique to produce high quality electronic films at lowtemperature and with high throughput. Solutions consisting of fully inorganic molecular metal chalcogenide complexes are particularly interesting as they can lead to very homogeneous films without detrimental carbon contaminants.16,17 However, the covalent nature of the metal chalcogenides resulting in their good electronic properties makes it challenging to break up the metal chalcogenide framework. Yet, selected solvents can dismantle the metal chalcogenides.16,18−21 Hydrazine is a particularly good solvent but difficult to handle since it is highly toxic, carcinogenic, and explosive.5,19 The dimeric thiostannate(IV) complex [Sn2S6]4− present in hydrazine and ammonium sulfide based solutions has been utilized in different technological applications to improve the electronic properties.22 The thermally decomposed thiostannate(IV) complex has been used as the source of SnS2, which either acts as © 2014 American Chemical Society
Received: May 9, 2014 Revised: July 4, 2014 Published: July 7, 2014 4494
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crystalline SnX2 are likely to yield improvements in [Sn2X6]4− based solution processing. In this work, we have used aqueous ammonium sulfide as a safer and more environmentally friendly alternative to hydrazine as solvent for metal chalcogenides. While the ammonium sulfide based dimeric thiostannate(IV) [Sn2S6]4− complex has been used as a ligand, SnS2 source, and building block for CZTSSe,25 the chemical nature and thermal decomposition has not been investigated in detail. Here, we present the synthesis and a detailed crystallographic study of the new structure (NH4) 4Sn2S 6·3H2O (tetraammonium thiostannate(IV) trihydrate) and show how aqueous solutions of (NH4)4Sn2S6·3H2O can be solution processed to yield highly textured SnS2 thin films. Furthermore, we have investigated the thermal decomposition of (NH4)4Sn2S6·3H2O crystals and the condensation steps toward SnS2. Specific focus is given to the decomposition of (NH4)4Sn2S6·3H2O in either the crystalline form or as a solution deposited amorphous thin film.
Low-temperature and room-temperature single-crystal diffraction data and powder X-ray diffraction (PXRD) data were collected on a SuperNova diffractometer from Agilent Technologies (former Oxford Diffraction Inc.) with a microfocus Mo Kα source and an Atlas CCD detector. The source is equipped with a multilayer optics system to focus and monochromate the radiation. The data collections were controlled by the CrysAlisPro software package (Agilent Technologies UK Ltd. (2010), CrysAlisPro, version 171.34.44). Integration and data reduction including an empirical absorption correction were carried out using the program CrysAlisPro. The structures were solved using olex2.solve28 and subsequently refined in SHELXL29 using the OLEX228 program. Symmetric θ/2θ X-ray measurements on thin films were acquired on a Rigaku SmartLab diffractometer with Cu radiation (Kα1) and parallel beam optics equipped with incident and diffracted beam Soller 5° slits. A graphite heating dome was used to heat the samples under a nitrogen atmosphere. Crystallographic Data. Crystal data for (NH4)4Sn2S6· 3H2O, M = 555.95 g mol−1, a = 8.56294(8) Å, c = 22.7703(3) Å, V = 1669.61(4) Å3, T = 100 K, tetragonal space group P41212 (no. 92), Z = 4, dc = 2.212 g cm−3, μ(Mo Kα) = 3.738 mm−1, 21 147 reflections collected, 4315 unique [Rint = 0.0267], which were used in all calculations. Goodness-of-fit on F2 = 1.077. The final R(F) = 0.0185 and Rw(F2) = 0.0322. CSD number 993724. Room-temperature cell a = 8.592(2) Å and c = 22.996(5) Å. Elemental Analysis (TG/DSC-MS and CHNS). Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were measured using a PerkinElmer STA 6000 connected to a mass spectrometer (MS; Hiden Analytical HPR-20 QMS sampling system) with a heated liner between the sampling area and detector in order to avoid condensation of lighter boiling gases. Dry crystals were transferred to an argon filled glovebox where they were packed into an Al2O3 crucible. TG/DSC-MS measurement was done in an argon atmosphere (37−450 °C, 2 °C/min, 20 mL argon/min). CHNS elemental analysis was performed on an Elementar Vario MACRO cube, which has been calibrated by sulfanilamide (eight samples) and blanks (three samples). Three samples of approximately 25 mg of dry crystals were packed in tin capsules with a total volume of 0.1 mL. Helium was used as a carrier gas. 119 Sn MAS NMR Spectroscopy. Solid-state 119Sn MAS NMR spectra were acquired on a Varian INOVA-400 (9.39 T) spectrometer using a home-built CP/MAS NMR probe for 7 mm o.d. zirconia (PSZ) rotors, spinning speeds of νR = 5.0−6.0 kHz, single-pulse excitation with a pulse width of 3.0 μs for an rf field strength of γB1/2π = 65 kHz, a relaxation delay of 120 s, and 512−1560 scans. The 119Sn chemical shifts (δ) are referenced to (CH3)4Sn, using SnO2(s) as a secondary reference material (δiso = −604.3 ppm). Simulations and least-squares fitting of the experimental spectra were performed using the STARS simulation software,30,31 considering only the first-order chemical shift anisotropy (CSA) interaction. The CSA parameters are defined as δiso = (δxx + δyy + δzz)/3, δσ = δiso − δzz, and ησ = (δxx − δyy)/δσ, using the convention |δzz − δiso| ≥ |δxx − δiso| ≥ |δyy − δiso|. The spectra were recorded at ambient temperature for dry crystals annealed at different temperatures (room temperature, 90 °C, 150 °C, 175 °C, and 450 °C). The samples were ground and packed into airtight,
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EXPERIMENTAL SECTION Single-Crystal Synthesis and Thin-Film Fabrication. Caution! H2S is a highly toxic gas and should be handled with care and appropriate safety precautions. Amorphous SnS2 was made by dissolving SnCl4·5H2O (98%) in 0.5 M HCl and bubbled with nitrogen and subsequently H2S (99.5%). A total of 10 mL of 96% ethanol was added to a total volume of 5 mL of 0.25 M ammonium thiostannate(IV) solution (based on the [Sn2S6]4− dimer) made by dissolving amorphous SnS2 in aqueous (NH4)2S. Full crystallization occurred within 1−3 days (yield ∼50%). Note: Aqueous (NH4)2S can be replaced with (NH4)2Sx (ammonium polysulfide). Crystals were dried by three times decantation of ethanol and subsequently two times with diethyl ether, which was evaporated under a flow of nitrogen gas. The 0.3426 g of crystals were redissolved in Milli-Q-water and two drops of (NH4)2S (to compensate for H2S and NH3 losses occurring when crystals are removed from the mother liquor), resulting in a total volume of 3.4 mL with a concentration of 0.18 M. A thin film was spin-coated on a pretreated glass slide, which was cleaned in a 1:4 Micro-90 (International Products Corp.) and Milli-Q-water solution heated nearly to boiling, subsequently washed three times in Milli-Q-water in an ultrasonic bath (10 min). Spin-coating was performed using a Laurell WS-650Mz23NPP spin-coater, with a spin-coating step for 120 s at 1500 rpm (acceleration: 1000 rpm/s) and 1 s deacceleration step. Annealing of the spin-coated thin-film was carried out on a calibrated hot plate at 350 °C for 5 min. The thin films were produced in a nitrogen filled glovebox. X-Ray Diffraction. Crystals in mother liquor were rapidly moved into a drop of Paratone-N oil in order to prevent a loss of solvent and decay of the crystals. The crystals are transparent and colorless, although the surface quickly changes to light yellow in color owing to solvent loss. Upon exposure to ambient conditions, the crystals turned yellow. The crystals were mounted on a thin glass fiber using Paratone-N oil and quickly moved into a cold nitrogen stream. No decomposition was noted while the crystal was in the nitrogen stream at 100(1) K. At room temperature, only the unit cell dimensions were determined for the single crystals. Powder diffraction data were collected on single crystals finely ground under diethyl ether. The powder was transferred to a capillary under diethyl ether. 4495
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end-capped zirconia rotors inside a nitrogen filled glovebox. 119 Sn MAS NMR spectra were also acquired for SnS2 made by a conventional melt reaction (details in Supporting Information) and the reference compound, [(CH3)4N]4Sn4S10, which was synthesized according to literature references.32,33 Synchrotron Radiation X-Ray Total Scattering. Total X-ray scattering measurements on an aqueous 0.5 M solution of ammonium thiostannate(IV) were performed at beamline P02.1 at PetraIII, DESY, Hamburg, Germany. The experimental setup has been described in detail elsewhere.34 The solution was injected into a 0.7 mm i.d. fused silica capillary with a 0.09 mm wall thickness, thus ensuring low X-ray absorption. A PerkinElmer XRD1621 CsI bonded amorphous silicon detector measuring 400 × 400 mm2 with a 189 mm sample to detector distance was used. The wavelength was 0.20726 Å, hence q-max ∼ 23 Å−1. Fit2D35 was used to integrate the raw total scattering data, and the pair distribution function (PDF) was subsequently obtained using PDFgetX3.36 Scattering from a capillary with (NH4)2S was subtracted from the integrated pattern before Fourier transforming data in the Q-range from 0.72 to 20 Å−1. The PDF was modeled in PDFgui.37 Three different models were fitted for the complex in solution: [SnS4]4−, [Sn2S6]4−, and [Sn4S10]4−. Thin-Film X-Ray Reflectivity, AFM, and TEM. The spincoated thin film has been subject to X-ray diffraction and reflectivity measurement on a Rigaku SmartLab diffractometer with Cu radiation (Kα1) and parallel beam optics equipped with a Ge(220)x2 monochromator. The thin film was measured in two different scan modes, θ/2θ and reflectivity. Atomic force microscopy (AFM) data were acquired on a commercial MultiMode VIII atomic force microscope (Bruker Nano Inc., Santa Barbara, CA, USA) under ambient conditions. Topographical images were recorded with tapping mode by using OMCL-AC160TS-R3 (Olympus) cantilevers with a nominal spring constant of 26.1 N/m. The scan rates range from 0.6 to 1 Hz. All data presented have been subject to second-order plane fit to compensate for sample tilt. Transmission electron microscopy (TEM) pictures were acquired on a Phillips CM20 microscope operating at an acceleration voltage of 200 keV. Energy dispersive X-ray spectroscopy (EDX) was performed using an EDAX Genesis detector attached to the TEM column. For low background, a sample holder with a beryllium specimen cup and screw was used to hold the sample mounted carbon-coated copper grid.
Figure 1. (a) The asymmetric unit for (NH4)4Sn2S6·3H2O. There is a 2-fold rotation axis through S2−S3, which generates the [Sn2S6]4− dimer. (b) The unit cell including four of the asymmetric units shown in part a. For clarity, all hydrogen atoms are omitted.
observed in the angles between the terminal sulfur atoms which are all above 108°, but only 87° between the bridging sulfur atoms (see Supporting Information). This geometry is in agreement with literature values for compounds containing the thiostannate(IV) dimer [Sn2S6]4− as shown by the comprehensive bond distance statistics presented in the Supporting Information.38−62 However, variations in the Sn−S bond length also occur among the terminal sulfur atoms. Detailed information about the interactions between sulfur atoms in the anionic dimer and the surrounding water molecules and ammonium cations can be obtained through a Hirshfeld surface analysis, as described by Spackman and co-workers.63,64 Figure 2b shows the Hirshfeld surface for the anionic dimeric [Sn2S6]4− complex with dnorm plotted on the surface, where red indicates distances shorter than the sum of the van der Waals radii and blue, longer distances. It is clearly seen that there is a bridging ammonium cation between S1 and S4, which is located almost in the middle. Furthermore, two additional ammonium cations are in close contact with S4, while S1 is in close contact with three water molecules and one ammonium cation. Surprisingly, the Sn−S4 bond is shorter than the Sn−S1, since S4 is in close contact with one more ammonium cation than S1. Normally, cations withdraw electrons from the bond and thereby weaken and elongate the bond. Hence, the water molecules in close contact with S1 participate in the weakening of the Sn−S1 bond, which is also evident from the strong
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RESULTS Single crystals of (NH 4 ) 4 Sn 2 S 6 ·3H 2 O (tetraammonium thiostannate(IV) trihydrate) with maximum dimensions of several cubic millimeters can be obtained when the aqueous ammonium thiostannate(IV)/ethanol mixture is left for crystallization for 1−3 days. The crystallographic structure of (NH4)4Sn2S6·3H2O contains [Sn2S6]4− anionic dimers and four ammonium counter-cations together with three crystallographic bound water molecules. The asymmetric unit is shown in Figure 1, and the unit cell includes four of these units. There is a 2-fold axis through S2 and S3, which generates the thiostannate(IV) dimer [Sn2S6]4−; hence, there is only one unique tin atom. The tin atom in the Sn−S anionic dimer is coordinated in a distorted tetrahedral environment. The Sn−S bond lengths range from 2.34 to 2.46 Å as shown in Figure 2. As expected from charge distribution, the longest bonds are found for the bridging sulfur atoms, while the shortest bonds are found for the terminal sulfur atoms. This asymmetry is also 4496
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Figure 3. PXRD pattern (Mo Kα source) collected at room temperature on finely ground single crystals of (NH4)4Sn2S6·3H2O. Le Bail fit of the PXRD pattern displays that the powder retains the same crystal structure as found in the single-crystal experiment.
Table 1. CHNS Experimental Data for (NH4)4Sn2S6·3H2O Compared with the Theoretical Content in the SingleCrystal Structure theoretical experimental
Figure 2. (a) The anionic dimeric thiostannate(IV) [Sn2S6]4− complex with bond distances indicated. Furthermore, the nonbonding distances between the two tin atoms and the two bridging sulfur atoms are indicated. (b) Hirshfeld surface with dnorm plotted on the surface, where red indicates distances shorter than the sum of the van der Waals radii and blue longer distances. For clarity, only water molecules and ammonium cations in close contact with either S1 or S4 are shown.
wt % (C)
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