Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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K2Hg2Te3: Straightforward and Large-Scale Mercury-Flux Synthesis of a Small-Band-Gap Photoconducting Material Günther Thiele,† Philipp Bron,‡ Sina Lippert,§ Frederik Nietschke,⊥ Oliver Oeckler,⊥ Martin Koch,§ Bernhard Roling,‡ and Stefanie Dehnen*,‡ †
Fachbereich Biologie, Chemie und Pharmazie, Freie Universität Berlin, Fabeckstraße 34-36, 14195 Berlin, Germany Fachbereich Chemie und Wissenschaftliches Zentrum für Materialwissenschaften, Philipps-Universität Marburg, Hans-Meerwein-Straße 4, 35043 Marburg, Germany § Fachbereich Physik und Wissenschaftliches Zentrum für Materialwissenschaften, Philipps-Universität Marburg, Renthof 5, 35032 Marburg, Germany ⊥ Fakultät für Chemie und Mineralogie, Institut für Mineralogie, Kristallographie und Materialwissenschaften, Universität Leipzig, Scharnhorststraße 20, 04275 Leipzig, Germany
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‡
S Supporting Information *
heated 3 days to 350 °C, whereupon the reaction product was pestled and subsequently filtered to remove the excess mercury. 1 was obtained in quantitative yield as a black, highly air- and moisture-sensitive powder. Residual mercury on the surface was removed by evaporation in vacuo. In contrast to K2Hg2Se3, no further purification by subsequent solvothermal reaction and excessive washing with ethane-1,2-diamine (en) was required to obtain a pure compound [see the powder X-ray diffraction (PXRD) diagram shown in Figures S5 and S6]; the synthesis can be scaled-up from 5 to 150 g (only limited by the size of the ampule). A considerable amount of elemental mercury evaporates during the initial synthesis and finally condenses at the walls of the silica glass tube upon cooling. If no additional mercury is added to the reaction mixture, mercury vapor is emitted from the reaction mixture, which leads to the formation of polytelluride side products. Although these can be removed by washing with cold en, which also readily dissolves 1, the yield or purity will be reduced significantly upon omission of excess mercury or upon deviation from the desired stoichiometry. 1 does not undergo decomposition upon exposure to light within weeks, which was observed for other metalate compounds with heavy elemental compositions. Nevertheless, finely ground samples of 1 are pyrophoric upon contact with air. To obtain single crystals suitable for X-ray analysis, the powder of 1 was recrystallized from en in a Teflon-lined autoclave at 150 °C over a period of 2 days. An X-ray diffraction experiment revealed the space group P42/ncm with a = 16.0283(4) Å, c = 7.4935(3) Å, and V = 1925.13(12) Å3, which confirmed 1 to be isostructural to its lighter homologue K2Hg2Se3 [a = 15.0690(4) Å, c = 7.1060(3) Å, and V = 1613.59(11) Å3]. As would be expected for a larger ionic radius of tellurium versus selenium, the bond length and thus bond angles and lattice parameters are slightly increased. As in K2Hg2Se3, 1 exhibits an anionic network topology with columnar cavities along [001] that are occupied by K+ cations (see Figure 1).
ABSTRACT: K2Hg2Te3 was synthesized via a mercuryflux synthesis pathway. Single-crystal and powder X-ray diffraction reveal the compound to be isostructural to its lighter congener K2Hg2Se3, yet exhibiting enhanced photoconductivity and electrical conductivity of (several) orders of magnitude and a decreased thermal conductivity and band gap. In this report, we elaborate on the synthesis and properties of the novel ternary compound.
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halcogenide-based materials are extensively investigated for both fundamental research and applications.1−6 As one of the most distinguished compounds, CuInxGa1−xSe2 (CIGS) is nowadays known from textbooks for its tunability of the band gap, depending on the respective gallium content.7−9 CIGS compounds are established materials within the photovoltaic industry, with alterations for improved efficiency exceeding 20%.10 While the metal composition in CIGS can be adjusted to yield band gaps from 1.0 to 1.7 eV, an even larger change of the electronic properties can be achieved by exchange of the respective chalcogens or the incorporation of different types of chalcogens. The replacement of sulfur with selenium or tellurium generally leads to a decrease of the band gap. We have previously reported the large-scale synthesis of K2Hg2Se3 as a prototype for photoconducting materials.11 Its current−voltage (I−V) characteristics are comparable with those of GaAs, and the band gap was determined to be 1.6 eV. The thermal conductivity of K2Hg2Se3 was found to be lower than that of typical thermoelectric selenides by a factor of 2−3; however, low electrical conductivity impeded reliable measurements of the Seebeck coefficient and thus the thermoelectric figure of merit. We were interested in the synthesis of the heavier tellurium-containing homologue, which would be expected to have an enhanced carrier concentration because of a decreased band gap, besides being less toxic, as a welcome side effect. K2Hg2Te3 (1) was formed by the reaction of thoroughly mixed HgTe and K2Te in a 2:1 ratio in a sealed silica glass ampule, in the presence of an excess of elemental mercury (1 mL of mercury per 20 mL of ampule volume). The ampule was © XXXX American Chemical Society
Received: August 15, 2018
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DOI: 10.1021/acs.inorgchem.8b02310 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
(red). The dark current of 1 (140 nA) is higher than that of K2Hg2Se3 (0.1 nA) by a factor of 1000. Light exposure increases the current in both cases (2 vs 200 nA). The curves for both materials show the Schottky behavior for positive and negative bias voltages. The small asymmetry observed in the I−V curves can be attributed to the individual contact quality between the contact needles and the sample. A small difference in the contact quality leads to a change in the current with respect to its direction because the contacts are of the Schottky type. Measurements of the electronic and thermal conductivities as a function of the temperature were performed on the said pellets. The thermal conductivity (Figure 3, left-hand side) is well
Figure 1. Cut-off from the crystal structure of 1 with a polyhedral representation of the K+ coordination environments (blue and red) and emphasis of the columnar extension of the Hg−Te network (green) along [001] (left) and [220] (right). Selected bond length (Å) and angles (deg): Hg−Te 2.7341(8)−2.7914(7), 3.2609(7); K···Te 3.4424(6)−3.779(3); Te−Hg−Te 87.76(2)−128.26(3); Hg−Te− Hg 90.88(2), 98.91(2), 113.16(4), 162.38(3). Detailed views are provided in Figure S1.
In order to investigate the influence of light irradiation on the electrical conductivity of the samples, photocurrent spectroscopy measurements were performed on uniform pellets of 0.95 mm thickness and 5.95 mm diameter (Figure 2; see the
Figure 3. Results for measurements on 1 for the thermal conductivity for multiple successive temperature cycles (left) and the electronic conductivity (right): circles, measured while heating; triangles, measured while cooling. Color code: red, K2Hg2Te3; black, K2Hg2Se3.
reproducible for multiple successive cycles, with the overall values significantly below those of the selenium homologue, as expected for the heavier homologue. The low values of the thermal conductivity are due to enhanced phonon scattering by heavier element atoms and altered phonon dispersion and not a result of a hypothetically diminished contribution of the electronic part because the electronic conductivity of 1 is about 1 order of magnitude higher than that of K2Hg2Se3 (Figure 3, right-hand side). For the telluride, the Seebeck coefficient could be determined. However, they drop below 400 μV/K above 100 °C and can therefore not compensate for the still relatively low electronic conductivity. Consequently, the thermoelectric figure of merit is significantly smaller than 0.01 and thus cannot be determined with certainty. The sample obviously showed some porosity (with a measured density of 73% of the crystallographically determined value), and we furthermore assume a change of the porosity upon quick cooling and heating, as indicated by the hysteresis-like behavior shown in Figure 3. This might have led to an underestimation of the electrical conductivity. However, because the porosity will also lower the thermal conductivity, the overall effect on ZT is supposed to be small. As a result, the compound is most probably not a suitable thermoelectric material without further modification of the composition, e.g., by doping or nanostructuring. In conclusion, we presented the large-scale synthesis of 1, for which a mercury-flux synthesis proved to be essential regarding both the yield and purity. In this way, the series of known solids of the general type K2M2E3 (M = Cd, Hg; E = S, Se, Te)11,12 was complemented by the heaviest congener. The physical properties were determined and compared in detail with those of the isostructural homologue K2Hg2Se3. The telluride exhibits a redshifted band gap of 1.29 eV and a surprisingly increased dark current and photocurrent of 100 and 200 nA, respectively. While its electrical conductivity is enhanced by an order of magnitude at 125 °C, its thermal conductivity is only small, approximately
Figure 2. Left: Absorption spectrum and photocurrent of 1. The photocurrent shows a dominant peak at 1.29 eV. Right: I−V characteristics of 1 (solid circles) in comparison with K2Hg2Se3 (open triangles).
Supporting Information for further details). As shown in Figure 2 (left), the photocurrent shows a sharp maximum at 1.29 eV (961 nm). The corresponding absorption spectrum (Figure 2, left, right axis) shows an edge at 1.29 eV with a shoulder at lower energies. The edge corresponds to the band-gap energy, and the shoulder is attributed to crystal defects. The facts that the absorption spectrum is rather broad and that the photocurrent shows a pronounced peak can be explained as follows: For photon energies above the band gap, the absorption is strong. Therefore, most of the photons are absorbed near the sample’s surface. At the surface, optically excited states recombine faster because of electronic surface states. This results in a small photocurrent. At photon energies below the band gap, the absorption is lower. Thus, photons can penetrate deeper into the sample. Here the lifetime of the optical excited carriers is longer, which results in an increased photocurrent. It was previously shown that the band gap of K2Hg2Se3 is 1.39 eV.11 The presence of heavy-element atoms (e.g., tellurium) shifts the band gap to lower energies, as expected. The I−V characteristics of 1 (Figure 2, right, closed circles) in comparison with those of K2Hg2Se3 (open triangles) show the influence of elements with large atomic numbers on the conductivity. The conductivity for both materials was measured in a dark environment (blue) and under white-light exposure B
DOI: 10.1021/acs.inorgchem.8b02310 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry 0.26 W/(mK) at 100 °C. In principle, the next step of the elemental substitutions to reduce the thermal conductivity and narrow the band gap would include the replacement of K+ with Tl+ to produce K2−xTlxHg2Te3. However, such compounds would be extremely toxic, comparable with HgBe2.13 We refer to an early mentioning of the compound with the nominal composition K2Hg2Te3 in the literature yet as the precursor to various ammonium telluridomercurate salts, without the provision of any chemical or physical characterization.14
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(5) Liu, D.; Liu, Y.; Huang, P.; Zhu, C.; Kang, Z.; Shu, J.; Chen, M.; Zhu, X.; Guo, J.; Zhuge, L.; Bu, X.; Feng, P.; Wu, T. Highly Tunable Heterojunctions from Multimetallic Sulfide Nanoparticles and Silver Nanowires. Angew. Chem., Int. Ed. 2018, 57, 5374−5378. (6) Donsbach, C.; Reiter, K.; Sundholm, D.; Weigend, F.; Dehnen, S. [Hg4Te8(Te2)4]8−: A Heavy Metal Porphyrinoid Embedded in a Lamellar Structure. Angew. Chem., Int. Ed. 2018, 57, 8770−8774. (7) Tinoco, T.; Rincón, C.; Quintero, M.; Pérez, G. S. Phase Diagram and Optical Energy Gaps for CuInyGa1‑ySe2 Alloys. Phys. Status Solidi 1991, 124, 427−434. (8) Feurer, T.; Bissig, B.; Weiss, T. P.; Carron, R.; Avancini, E.; Löckinger, J.; Buecheler, S.; Tiwari, A. N. Single-graded CIGS with narrow bandgap for tandam solar cells. Sci. Technol. Adv. Mater. 2018, 19, 263−270. (9) Mantilla-Perez, P.; Feurer, T.; Correa-Baena, J.-P.; Liu, Q.; Colodrero, S.; Toudert, J.; Saliba, M.; Buecheler, S.; Hagfeldt, A.; Tiwari, A. N.; Martorell, J. Monolithic CIGS-Perovskite Tandem Cell for Optimal Light Harvesting without Current Matching. ACS Photonics 2017, 4, 861−867. (10) Chirilă, A.; Reinhard, P.; Pianezzi, F.; Bloesch, P.; Uhl, A. R.; Fella, C.; Kranz, L.; Keller, D.; Gretener, C.; Hagendorfer, H.; Jaeger, D.; Erni, R.; Nishiwaki, S.; Buecheler, S.; Tiwari, A. N. Potassiuminduced surface modification of Cu(In,Ga)Se2 thin films for highefficiency solar cells. Nat. Mater. 2013, 12, 1107−1111. (11) Thiele, G.; Lippert, S.; Fahrnbauer, F.; Bron, P.; Oeckler, O.; Rahimi-Iman, A.; Koch, M.; Roling, B.; Dehnen, S. K2Hg2Se3: LargeScale Synthesis of a Photoconductor Material Prototype with a Columnar Polyanionic Substructure. Chem. Mater. 2015, 27, 4114− 4118. (12) Axtell, E. A., III; Liao, J.-H.; Pikramenou, Z.; Park, Y.; Kanatzidis, M. G. K2Cd2S3 vs CdS: Can the Properties Of Quantum-Sized CdQ Semiconducotrs Be Emulated by Bulk Alkali Metal Ternary A/Cd/Q Phases (Q = Chalcogen)? J. Am. Chem. Soc. 1993, 115, 12191−12192. (13) Kells, M. C.; Holden, R. B.; Whitman, C. I. The preparation of Beryllium Amalgam. J. Am. Chem. Soc. 1957, 79, 3925−3925. (14) Dhingra, S. S.; Warren, C. J.; Haushalter, R. C.; Bocarsly, A. C. One-Dimensional Mercury Telluride Polymers: Synthesis and Structure of (Et4N)2Hg2Te4 and (Me4N)4Hg3Te7·0.5en. Chem. Mater. 1994, 6, 2382−2385.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02310. Details on the synthesis, crystallographic data (singlecrystal X-ray diffraction and PXRD), thermal analysis, Xray fluorescence spectroscopy, and photoelectric and thermoelectric measurements (PDF) Accession Codes
CCDC 1862365 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Bernhard Roling: 0000-0001-7383-1495 Stefanie Dehnen: 0000-0002-1325-9228 Author Contributions
G.T. and P.B. contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Data-handling support by Dr. Bastian Weinert is greatly appreciated. G.T. thanks the Verband der Chemischen Industrie for a Liebig Scholarship. Financial support from the DFG within the framework of SPP 1708 is gratefully acknowledged.
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REFERENCES
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DOI: 10.1021/acs.inorgchem.8b02310 Inorg. Chem. XXXX, XXX, XXX−XXX
