Topotactic anion-exchange in thermoelectric nanostructured layered tin chalcogenides with reduced selenium content.

Chemical Science View Article Online Open Access Article. Published on 23 March 2018. Downloaded on 08/05/2018 14:55:21. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. EDGE ARTICLE Cite this: Chem. Sci., 2018, 9, 3828 View Journal | View Issue Topotactic anion-exchange in thermoelectric nanostructured layered tin chalcogenides with reduced selenium content† Guang Han,a Srinivas R. Popuri,b Heather F. Greer,c Ruizhi Zhang,d Lourdes Ferre-Llin,e Jan-Willem G. Bos, b Wuzong Zhou, c Michael J. Reece,d Douglas J. Paul,e Andrew R. Knoxe and Duncan H. Gregory *a Anion exchange has been performed with nanoplates of tin sulfide (SnS) via “soft chemical” organic-free solution syntheses to yield layered pseudo-ternary tin chalcogenides on a 10 g-scale. SnS undergoes a topotactic transformation to form a series of S-substituted tin selenide (SnSe) nano/micro-plates with tuneable chalcogenide composition. SnS0.1Se0.9 nanoplates were spark plasma sintered into phase-pure, Received 6th December 2017 Accepted 23rd March 2018 textured, dense pellets, the ZT of which has been significantly enhanced to z1.16 from z0.74 at 923 K via microstructure texturing control. These approaches provide versatile, scalable and low-cost routes to DOI: 10.1039/c7sc05190e p-type layered tin chalcogenides with controllable composition and competitive thermoelectric rsc.li/chemical-science performance. Introduction Main group metal chalcogenides (MCs) are excellent candidates for thermoelectrics,1–4 (opto)electronics5 and photovoltaics6 due to their outstanding electronic, optical and thermal properties. Bottom-up solution syntheses afford energy-saving means of preparing MC nano/micro-structures with controllable morphology and size. Surface modication using organic surfactants can limit particle growth but such coatings can typically introduce impurities.7–10 By contrast, synthesis without organic surfactants, solvents or precursors can produce nanostructured MCs with impurity-free surfaces and enhanced electrical performance11–14 but require careful experiment design with appropriate synthesis parameters and reagents. Chemical transformations, including ion exchange, topotactic and pseudomorphic reactions, represent a versatile and effective means to produce new materials with control over crystal structure, composition and morphological complexity.15 Such transformations can realise prescribed materials that a WestCHEM, School of Chemistry, University of Glasgow, Glasgow, G12 8QQ, UK. E-mail: b Institute of Chemical Sciences, Centre for Advanced Energy Storage and Recovery, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK c EaStCHEM, School of Chemistry, University of St Andrews, St Andrews, Fife KY16 9ST, UK d School of Engineering & Materials Science, Queen Mary University of London, London, E1 4NS, UK e School of Engineering, University of Glasgow, Glasgow, G12 8LT, UK † Electronic supplementary 10.1039/c7sc05190e information 3828 | Chem. Sci., 2018, 9, 3828–3836 (ESI) available. See DOI: cannot be otherwise prepared.15–17 Indeed, doped compounds, multi-component composites or hetero-structures can be crafted by regulating the progress of transformations, leading to materials with engineered functional properties.18–21 When performed in solution, ion exchange can exploit the solubility difference between precursors and products to enable the rapid synthesis of nano/micro-structures (for example, MCs) with predetermined cation and/or anion compositions.15,18 Combining organic-free synthesis with ion exchange raises the prospect of producing MCs with compositions, crystal structures, morphologies and particle sizes that can be tailored towards delivering high electronic performance. Thermoelectric materials can be utilised to convert thermal energy directly into electricity and vice versa, thus offering opportunities to refrigerate and to harvest electricity from waste heat via the Peltier and Seebeck effects, respectively.22,23 Layered tin chalcogenides (LTCs), including SnSe and SnS, have drawn much attention given a formidable combination of excellent thermoelectric conversion efficiency, relatively low toxicity and the Earth-abundance of their component elements.1,2,24–26 Notably, when p-type SnSe can be grown as a single crystal, it has demonstrated record high ZT values of 2.6 and 2.3 along the b and c crystallographic directions, respectively at 923 K.1 Polycrystalline SnSe and related doped materials have been prepared in an effort to improve mechanical properties, but ZT values cannot yet emulate those in the single crystalline material.27 The capacity to synthesise polycrystalline LTCs of premeditated composition to optimise thermoelectric performance is becoming gradually less elusive.28–50 For example, Agdoped SnSe,28 alkali metal-doped SnSe,29–34 I-doped SnSe1
xSx (0 # x # 1),35 and Sn1
xPbxSe36 have demonstrated improvements This journal is © The Royal Society of Chemistry 2018 View Article Online Open Access Article. Published on 23 March 2018. Downloaded on 08/05/2018 14:55:21. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Edge Article Chemical Science in thermoelectric performance compared to undoped SnSe, pushing ZT to 1.2 at 773 K (Na, K co-doped SnSe)33 and 1.7 at 873 K (phase-separated Sn1
xPbxSe).36 Yet, polycrystalline LTCs are primarily fabricated by high-temperature, energy-intensive processes.27–29,31,32,35 Solution syntheses are an attractive alternative but generally involve using organics (solvents and/or surfactants, for example), can produce small sample yields and have offered little opportunity as yet to exert control over composition.51–60 For LTCs to be a practicable component of thermoelectric devices, a scalable and cost-effective organic-free synthesis approach to materials with tuneable composition and consistently excellent performance is essential. In this study, we demonstrate how the combination of two organic-free aqueous solution strategies (anion exchange following direct precipitation) can be utilised to synthesise LTC nano/micro-plates with tuneable chalcogenide composition (e.g. >10 g SnS and SnS0.1Se0.9, respectively; Fig. S1 and S2†). The plates can be sintered into textured, dense pellets with competitive thermoelectric performance while partly replacing selenium with less toxic and more Earth-abundant sulfur. Results and discussion Characterisation and formation mechanism of anionexchanged SnS1
xSex The synthesis of phase-pure SnS (Fig. 1) involves injection of a Na2S aqueous solution into a Na2SnO2 solution that is subsequently boiled for 2 h (Fig. S1†). Powder X-ray diffraction (PXD) patterns (Fig. 1a) can be indexed exclusively to orthorhombic SnS (ICDD card no. 75-2115).61 Rietveld renement against PXD data (Fig. S3; Tables S1 and S2†) conrms that the SnS product crystallises with orthorhombic space (...truncated)