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.

Scheme 1. Summary of Common Coordination Geometries of Tt⁴⁺ (Sn⁴⁺ and Ge⁴⁺); Tt²⁺ (Sn²⁺ and Ge²⁺); Tr³⁺ (Ga³⁺ and In³⁺); M²⁺ (Zn²⁺, Cd²⁺, Mn²⁺, Fe²⁺, etc.) and Cu⁺ with Ch^2– (S^2– and Se^2–) Observed in Binary or Multinary Chalcogenido Metalate Substructures

Scheme 2. Schematic Illustration of the Syntheses of Compounds 1–5

1 and 3–5 were prepared by the use of [Li₄(H₂O)₁₃][SnSe₄] in ionic liquids; 2 was formed by mixing [K₄(H₂O)₄][SnSe₄] and LiCl salts as starting materials instead. The use of CuI is necessary for the formation of 1–4, and the DMMP auxiliary is needed for all syntheses.

Figure 1

Illustration of the crystal structure of 1. View of the unit cell of 1. Ionic liquid cations are shown in wire mode; H atoms are omitted. For clarity, the {Sn₂Se₇} units of different [Cu₁₀Sn₆Se₂₂]^10– cluster anions are shown in polyhedral mode with different colors (a). Molecular structure of the [Cu₁₀Sn₆Se₂₂]^10– cluster anion in 1; atom numbers are given in the color code indicated for the atom types (b). The {Cu₁₀Se} core of [Cu₁₀Sn₆Se₂₂]^10– (c). Dimeric {Sn₂Se₇} capping motif in [Cu₁₀Sn₆Se₂₂]^10– (d). Thermal ellipsoids are drawn at the 50% probability level in Figure 1b–d. Structures of reported cluster anions [Cu₇As₃Se₁₃]^4– (left) and [Cu₇As₃Te₁₃]^4– (right) for comparison; gray shade of atoms: Cu—black, As—dark gray (70%), Te—gray (40%), and Se—light gray (10%) (e). See the Supporting Information for further structural details.

Figure 2

Asymmetric unit of 2 with the labeling scheme. Thermal ellipsoids are drawn at 50% probability level. A corresponding color code was used for the atom labels (a). View of the two-dimensional anionic substructure of 2. Counterions are omitted for clarity. The simplified hcb net that is obtained when each repeating unit is regarded as a node (displayed as a lime sphere) is highlighted by sticks of rose color. The bridging {Sn₂Se₆} units are highlighted in polyhedral mode, and one of them is shown in bond and stick mode for clarity (b). Illustration of the cyclic substructure in the (distorted) honeycomb-like 2D network including one counterion (c). Side view of the corrugated 2D layers and the counterions in between them (d). See the Supporting Information for further structural details.

Figure 3

Structure of one secondary building unit in 3a and 3b, shown for 3a as an example; a corresponding color code was used for the atom labels (a). Illustration of the enantiomeric linking mode between the second building units in 3a (b) and 3b (c), highlighted with increasing transparency as the units become more distant from the viewpoint of the viewer. Illustration of the simplified chiral qtz net of 3a (d) and 3b (e) upon treatment of each building block as a node. The M-helix is observed along axis c in 3a (f). The P-helix observed along axis c in 3b (g). See the Supporting Information for further structural details.

Figure 4

Structure of the building units in the unit cell in 4. Thermal ellipsoids are drawn at 50% probability level. A corresponding color code was used for the atom labels (a). Illustration of the linkage of building units {FCu₈Se₁₂} and {Sn₂Se₂} in 4 (b). Extension of the anionic substructure in a 3 × 3 × 3 supercell (c). The simplified pcu net of 4 was obtained upon treating each {FCu₈Se₁₂} motif as a node and each {Sn₂Se₂} unit as a linker. The disordered [BF₄]⁻ anions have been omitted for clarity (d). See the Supporting Information for further structural details.

Figure 5

Top view of the two-dimensional, honeycomb-like anionic substructure of 5, highlighting the six-membered-ring motif in the polyhedral mode (a). Side view of stacking of the layers along the b axis in the crystal structure of 5. Organic cations and [BF₄]⁻ anions are omitted for the sake of clarity (b). See the Supporting Information for further structural details.

Figure 6

Solid-state UV–vis diffuse-reflectance spectra of 2, 3, and 4 plus 5 (a–c). Comparison of the differences between the optical band gap energies E_gap of 2–5 (d). The Tauc plots derived from the solid-state UV–vis diffuse-reflectance spectra are shown in Figure S31; they were generated using the Kubelka–Munk function (F(R∞)hν)^1/γ, with γ = 2, indicative of an indirect optical gap. A corresponding measurement of 1 was hampered by its small yield.

Figure 7

Photocurrent measurements of 2 (a), 3 (b), 4 (with small amounts of side product 5; c), and 5 (d), given as cyclic voltammograms (CV) of pulverized crystals deposited on carbon cloth. Red curves represent measurements under white-light irradiation. Black curves were recorded under the exclusion of light. Scan rate: 10 mV/s. An Ar-saturated phosphate buffer (c = 0.1 M) at pH = 7 was used as electrolyte, the electrode area was 0.246 cm². See the Supporting Information for further details.