Cationic quaternary chalcohalide nanobelts: Hg₄In₂Q₃Cl₈ (Q = S, Se, Te)

Abstract

Novel cationic quaternary chalcohalide nanobelts were found in Hg₄In₂Q₃Cl₈ (Q = S, Se, Te), obtained by solid-state reaction. Due to the effects of dimensional reduction, both theoretical and experimental results demonstrate that their bandgaps are remarkably increased compared to those of the zinc-blende structure HgQ (Q = S, Se, Te).

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Crystalline chalcohalides and chalcogenides have received great interest in advanced materials research because of their diverse structural features and interesting physical properties, which could have potential applications in non-linear optics, phase-transitions, thermoelectricity generation, X-ray detection and so on.^1–8 In particular, low dimensional chalcogenide and chalcohalide materials are very attractive, because these complexes usually display novel and enhanced physical properties.

It is well-known that low-dimensional structures could be obtained via the “dimensional reduction” (DR) method.^9–11 In the metal chalcogenide system, alkali sulfide and metal halides are widely used to act as “scissors” to reduce the dimensionality of the parent metal chalcogenide structures. In addition, theoretical calculations show that orbital overlap is reduced in certain space directions and the bandgaps (the difference between the valence band and the conduction band) are increased. Thus, the bandgaps of the dimension-reduced compounds could be tuned by changing the amounts of metal halides and alkali sulfides.

Some interesting illustrations of dimensional reduction in multiple chalcohalides and chalcogenides have been demonstrated. For example, the family of binary HgQ (Q = S, Se, Te) chalcogenides, which crystallize in the 3D zinc-blende structure, have a zero energy gap owing to the inverted bands near the Fermi level through the electronic band structure calculations.¹² After the introduction of the dimensional reduction agent A₂Q (A = K, Rb, Cs, Tl; Q = S, Se, Te) into the parent compounds HgQ, a series of ternary A/Hg/Q compounds with reduced dimensions were produced. It is found that the choice of countercation has a significant effect on the derived framework.^10c,d,13,14 Although A₂Q compounds were successfully employed to tailor HgQ networks, relatively few of the new dimensional reduction agents have been reported. To the best of our knowledge, HgQ does not breed quaternary Hg–M–Q–X chalcohalides, where M represents main group metals. Four exceptions are two three-dimensional (3D) frameworks: Hg₇InS₆Cl₅,^15a Hg₂PbI₂S₂,^15b and two isostructural one-dimensional (1D) structures: 2HgS·SnBr₂ and 2HgS·PbBr₂.^8d,16 Here, we report a new class of 1D chalcohalides, Hg₄In₂Q₃Cl₈ (Q = S, Se, Te), featuring novel cationic quaternary chalcohalide nanobelts. It is shown for the first time that 1D cationic quaternary chalcohalide nanobelts could be produced in chalcohalide systems.

The new compounds Hg₄In₂Q₃Cl₈ (Q = S, Se, and Te) were prepared via the solid state reaction from a mixture containing Hg₂Cl₂ (1.15 mmol, Warning: due to the toxicity of mercury, more attention should be paid and extreme care must be taken when handling the sample and products), In₂Q₃ (0.30 mmol) and Q (1.15 mmol). A detailed synthesis has been provided in the supporting information.† The resulting products were obtained as rod-shaped crystals, which can be separated manually. The colors of the as-prepared crystals are yellow, grey, and chocolate for the S, Se, and Te analogues, respectively, which are in agreement with the electronic absorption spectra discussed below. Note that the crystalline Hg₄In₂S₃Cl₈ can only be stable in air for about 4 days, and then becomes amorphous, while the other two samples (Q = Se and Te) do not change in air for months. The crystal structures of the as-prepared complexes were determined using a Bruker APEX II CCD diffractometer equipped with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The purities of all samples are confirmed by X-ray diffraction (XRD) patterns (Fig. S1, ESI†). The calculated atomic Hg/In/Q/Cl ratios from the single-crystal structure analysis are in agreement with energy-dispersive X-ray spectroscopy analysis (EDS, see Fig. S2, ESI†), and thermal gravimetric analysis (TGA) shows that all materials are stable up to 300 °C (Fig. S3, ESI†).

The isostructural chalcohalides Hg₄In₂Q₃Cl₈ (Q = S, Se, Te) crystallize in an orthorhombic unit cell with the space group Pnma (No. 62). X-ray crystallography study reveals that the new quaternary phases feature the cationic nanobelts [Hg₄Q₃InCl₄]⁺ with the charge-balanced isolated anions [InCl₄]⁻, which are located in the channels constructed from the nanobelts (Fig. 1a). The nanobelts are made of infinite ribbons of [Hg₄Q₃]²⁺, which are decorated by coordinated [InCl₄]⁻ anions along the b-axis (Fig. 1b). Furthermore, the ribbons of [Hg₄Q₃]²⁺ are composed of 12-membered Hg₆Q₆ cyclohexane rings with a chair-like configuration (Fig. 1c). In the ribbons, every ring shares four edges with the neighboring rings, and the other two edges are forbidden to share due to the steric hindrance effect from coordinated [InCl₄]⁻ anions.

Fig. 1 (a) Ball-and-stick model of the network of Hg₄In₂Q₃Cl₈ with atomic labels along the b direction. (b) Projection of the nanobelts in Hg₄In₂Q₃Cl₈. (c) The chair-like configuration of Hg₆Q₆.

The nanobelts were interconnected with each other to form the pseudo-3D framework through weak Hg⋯Cl bonds, in which the apical Cl atoms coming from the coordinating [InCl₄]⁻ anions are situated at the center of the ring. With increasing chalcogenide radius, the width of the nanobelts also increases from 22.587 to 23.324 Å. In the nanobelts, all of the crystallographically independent Hg sites show a linear shape with two Q atoms. Meanwhile the In site adopts a slightly distorted trigonal bipyramidal geometry, in which the Cl and Q atoms are located on two vertices (Fig. 1b). All Q (Q = S, Se, Te) atoms acting as tridentate metal-linkers are trigonally coordinated by three Hg atoms or one In atom and two Hg atoms, while all Cl atoms coordinated with In atoms are unidentate terminal ligands. The crystallographically independent [InCl₄]⁻ ions adopt a moderately distorted tetrahedral coordination (Fig. 1a). In these structures, the Hg–Q bond lengths fall in the range 2.355(2) to 2.654(4) Å. The In–Q bond distances vary from 2.587(4) to 2.892(8) Å, which is similar to the reported results;¹⁸ the In–Cl bond distances range from 2.479(3) to 2.560(2) Å, which are comparable to the related compounds, such as Tl₂Hg₃Q₄ (Q = S, Se, Te), [Hg₃Te₂][UCl₆], Hg₃ZnS₂Cl₄, [Ru(Te₉)](InCl₄)₂, InSb₂S₄Cl, In₅Se₅Cl, and Hg₅In₂Te₈.^6c,17

The densities-of-state (DOS) and the electronic band structure for Hg₄In₂Se₃Cl₈ calculated by CASTEP,¹⁹ are shown in Fig. 2. The calculation results for Hg₄In₂S₃Cl₈ and Hg₄In₂Te₃Cl₈ are provided in the supporting information (Fig. S4 and Fig. S5, ESI†). As depicted in Fig. 2a, the valence bands (VBs) between the energy level −4.0 eV and the Fermi level (0.0 eV) contain significant contributions from the Hg 6s, Se 4p, and Cl 3p states mixed with a small amount of the In 5p and Se 4s states. The VBs ranging from −4.0 to −10.0 eV are mostly formed by Hg 5d states hybridized with a small portion of the In 5s and Cl 3p states. The VBs spreading from −10.0 to −15.0 eV are primarily the In 4d, Se 4s, and Cl 3s states. The conduction bands (CBs) above Fermi energy (E_F) are mostly made up of the Hg 6s and Se 4p states, with minor contributions from the Se 4s states. Briefly, the non-bonding states of Se 4p and Cl 3p and the antibonding states of Hg 6s and Se 4p mainly influence the energy gaps of Hg₄In₂Se₃Cl₅. Since the CB minimum (2.25 eV) and VB maximum (0.00 eV) are not located at the same k-point, Hg₄In₂Se₃Cl₈ is an indirect semiconductor with a calculated band gap of 2.25 eV (Fig. 2b), which is in good agreement with the experimental value of 2.34 eV deduced from the solid state UV-visible absorption (Fig. 3). The other two S and Te analogues have similar DOS and electronic band structures to those of Hg₄In₂Se₃Cl₈ (Table 1, Fig. S4, and Fig. S5, ESI†). Clearly, these calculated results are much bigger than the theoretical bandgaps of HgQ²⁰ (HgS, 0.30 eV; HgSe, −0.24 eV; HgTe −0.34 eV). Both theoretical studies and experimental measurements show that the band gap of Hg₄In₂Q₃Cl₈ increases remarkably, relative to HgQ, because of the effects of DR.

Fig. 2 (a) Total and partial densities of state (DOS) and (b) band structure of Hg₄In₂Se₃Cl₈.

Fig. 3 UV-visible absorbance of Hg₄In₂Q₃Cl₈.

Table 1 Calculated and experimental values of the band gap for Hg₄In₂Q₃Cl₈ (Q = S, Se, Te)

	Hg4In2S3Cl8	Hg4In2Se3Cl8	Hg4In2Te3Cl8
Calculated	2.30 eV	2.25 eV	2.21 eV
Experimental	2.43 eV	2.34 eV	1.89 eV

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In summary, three isostructural quaternary chalcohalides Hg₄In₂Q₃Cl₈ (Q = S, Se, Te) have been successfully synthesized through the solid state method. These compounds have a novel cationic nanobelt structure. Both experimental and theoretical studies confirm that they are semiconductors with narrow bandgaps. This research could offer us a new strategy to prepare other low-dimensional chalcohalides.

QZ thanks the support from the MOE Tier 1 fund and the Singapore National Research Foundation under CREATE programme: Nanomaterials for Energy and Water Management, and Singapore MPA 23/04.15.03 RDP 009/10/102 grant.

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Footnote

† Electronic Supplementary Information (ESI) available: Experimental methods, XRD patterns, EDS and TGA analysis, additional table for crystal data and structure refinements, and figures for total and partial densities of state (DOS) and band structure of Hg₄In₂S₃Cl₈ and Hg₄In₂Te₃Cl₈. Files CSD-424490, 424491 and 424492 for Hg₄In₂Te₃Cl₈, Hg₄In₂Se₃Cl₈ and Hg₄In₂S₃Cl₈. See DOI: 10.1039/c2ra20781h/

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