Structural Expansion and Enhanced Photocurrent Conversion of Selenido Stannates with Cu+ Ions

Abstract

As a means of tuning the electronic properties of tin-chalcogenide-based compounds, we present a strategy for the compositional and structural expansion of selenido stannate frameworks under mild conditions by introducing Cu⁺ ions into binary anionic Sn/Se aggregates in ionothermal reactions. The variable coordination modes of Cu⁺─contrasting with tetrahedral {SnSe₄} or trigonal bipyramidal {SnSe₅} units─and corresponding expansion toward ternary Cu/Sn/Se substructures helped to add another degree of freedom to the nanoarchitectures. As desired, the variation of the structural features was accompanied by concomitant changes of the physical properties. Upon treatment of alkali metal salts of the [SnSe₄]^4– anion at slightly elevated temperatures (120 or 150 °C) in ionic liquids, we isolated a series of compounds comprising ternary or quaternary cluster molecules or networks of cluster units, (C₂C₂Im)₉Li[Cu₁₀Sn₆Se₂₂] (1), (C₂C₂Im)₄[Cu₈Sn₆Se₁₈] (2), (C₂C₁Im)₃[Cu₅Sn₃Se₁₀] (3), and (C₂C₂Im)₅[Cu₈Sn₆Se₁₈F]·(C₂C₂Im)[BF₄] (4; C₂C₂Im = 1,3-diethyl-imidazolium, C₂C₁Im = 1-ethyl-3-methyl-imidazolium), which were investigated in terms of their optical gaps and photocurrent conversion properties. As illustrated by the synthesis and characterization of an additional salt that does not include Cu⁺, {(C₂C₂Im)₂[Sn₃Se₇]}₄·{(C₂C₂Im)[BF₄]}₂ (5), the significant role of Cu⁺ in this system was shown to be 3-fold: (a) structural expansion, (b) narrowing of the optical gap, and (c) photocurrent enhancement. By this three-in-one effect, the work offers an in-depth understanding of chalcogenido metalate chemistry with atomic precision.