Article pubs.acs.org/cm
Soluble Lead and Bismuth Chalcogenidometallates: Versatile Solders for Thermoelectric Materials Hao Zhang,† Jae Sung Son,†,§ Dmitriy S. Dolzhnikov,† Alexander S. Filatov,† Abhijit Hazarika,† Yuanyuan Wang,† Margaret H. Hudson,† Cheng-Jun Sun,∥ Soma Chattopadhyay,⊥ and Dmitri V. Talapin*,†,‡ †
Department of Chemistry and James Franck Institute, University of Chicago, Chicago, Illinois 60637, United States Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States § School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ∥ Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States ⊥ Physical Sciences Department, Elgin Community College, Elgin, Illinois 60123, United States ‡
S Supporting Information *
ABSTRACT: Here we report the syntheses of largely unexplored lead and bismuth chalcogenidometallates in the solution phase. Using N2H4 as the solvent, new compounds such as K6Pb3Te6·7N2H4 were obtained. These soluble molecular compounds underwent cation exchange processes using resin chemistry, replacing Na+ or K+ by decomposable N2H5+ or tetraethylammonium cations. They also transformed into stoichiometric lead and bismuth chalcogenide nanomaterials with the addition of metal salts. Such a versatile chemistry led to a variety of composition-matched solders to join lead and bismuth chalcogenides and tune their charge transport properties at the grain boundaries. Solution-processed thin films composed of Bi0.5Sb1.5Te3 microparticles soldered by (N2H5)6Bi0.5Sb1.5Te6 exhibited thermoelectric power factors (∼28 μW/cm K2) comparable to those in vacuum-deposited Bi0.5Sb1.5Te3 films. The soldering effect can also be integrated with attractive fabrication techniques for thermoelectric modules, such as screen printing, suggesting the potential of these solders in the rational design of printable and moldable thermoelectrics.
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to ∼1.8 for Bi0.5Sb1.5Te3 near room temperature5,6 and to ∼2.5 for PbTe-based materials at ∼900 K.7−9 The enhancement in ZTs was mainly achieved by introducing nanoinclusions and mesoscale grains into the host chalcogenide materials.10−12 Well-designed grain boundaries, ideally with maximum phonon scattering and minimal retardation in charge carrier transport, are critical to achieve a high ZT.6 However, conventional methods toward nano- or microstructured lead or bismuth chalcogenide TE modules usually involve spinal decomposition of melts or powder sintering. The high-temperature solid-state approaches are rather delicate and limited in terms of rational design of the grain boundaries. Such approaches are also difficult to use in the fabrication of thin film TE devices. On the other hand, the attributes of the newly developed semiconductor solders allow an alternative route toward the rational design of solution-processable TE modules.13 In this work, we discuss the syntheses and characterization of a new class of lead and bismuth chalcogenidometallates, such as K6Pb3Te6·7N2H4, highly soluble in various solvents. Such solubility facilitates chemical transformations including the
INTRODUCTION Soldering is widely used in the electronics industry as well as our daily life.1 A large variety of metal solders has been developed for different metals and target applications. In contrast, there are few established solders for semiconductors, primarily due to the much higher sensitivity of semiconductor interfaces to impurities and structural defects compared to metals. In a recent work, we proposed the concept of composition-matched semiconductor solders, which were composed of cadmium, lead, and bismuth chalcogenidometallates.2,3 These molecular compounds were soluble in solvents such as hydrazine (N2H4) and, under relatively mild conditions (below 300 °C), decomposed into semiconductors compositionally and structurally matched to the bonded parts. As solders, they facilitated charge transport across grain boundaries between joint semiconductors, leading to exceptionally high carrier mobilities (e.g., over 400 cm2/(V s) in thin films composed of sintered CdSe nanocrystals and Na2Cd2Se3 solders).4 The control over transport properties at the grain boundaries induced by these solders can benefit a broad range of semiconductor-based devices, such as thermoelectric (TE) materials. The past decade has witnessed the boost of the figure of merit (ZT) of TE materials, for instance, from around unity © 2017 American Chemical Society
Received: May 2, 2017 Revised: June 29, 2017 Published: June 29, 2017 6396
DOI: 10.1021/acs.chemmater.7b01797 Chem. Mater. 2017, 29, 6396−6404
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This process was repeated several times until the supernatant became colorless and no heat was released upon the mixing of fresh N2H4 with resin. N2H5+-loaded resin was recovered by centrifugation and separated from the supernatant. Similarly, NR4+-loaded resin (R: CH3 (Me), C2H5 (Et)) was prepared by the substitution of terminal H+ with NR4+ in a saturated NR4OH solution in methanol. The NR4+loaded resin was dried under vacuum at ∼40 °C to remove residual solvents and stored in a N2-filled glovebox. Cation exchanges for Pb- and Bi chalcogenidometallates with N2H5+ or NR4+ were carried out in a N2-filled glovebox. Excess N2H5+- or NR4+-loaded resin (∼10 mol equiv to alkali cations) was introduced to chalcogenidometallates in N2H4, followed by vigorous vortex for ∼10 min. The solution containing chalcogenidometallates with N2H5+ or NR4+ cations was then separated from resin beads by centrifugation. Cation-exchanged chalcogenidometallates can dissolve in N2H4 (for N2H5+ or NR4+) or acetonitrile and N,N-dimethylformamide (DMF, for NR4+), as listed in Table S6. Formation of PbTe and Bi2Te3 Nanograins by Introducing Pb2+ or Bi3+ to Chalcogenidometallates. The syntheses of PbTe and Bi2Te3 nanograins were carried out under an inert atmosphere. A (NEt4)6Pb3Te6 solution in N2H4 was dried under vacuum to remove most of the solvent. The obtained solid was redispersed in DMF with a concentration of ∼30 mg/mL. To this solution, excess PbI2 powder (2 mol equiv to Pb in (NEt4)6Pb3Te6) was added under stirring, resulting in a stable suspension after ∼15 min. After centrifugation, the recovered solid was washed several times with fresh DMF to remove the unreacted PbI2. Finally, PbTe nanograins suspended in DMF and formed an ink. Bi2Te3 nanograins were synthesized by introducing excess BiCl3 powder to a solution of Na4Bi2Te5 in DMSO (40 mg/ mL). Similar to the synthesis of PbTe nanograins, a suspension of Bi2Te3 nanograins with no noticeable agglomerates was obtained after stirring for 30 min. Bi2Te3 nanograins were purified by rinsing with fresh DMSO to remove unreacted BiCl3 and suspended well in DMSO or N2H4. Fabrication of Thin Films from an Ink of Microparticles with Chalcogenidometallate or Nanograin Solders. All procedures were carried out in an N2-filled glovebox. Lead and bismuth chalcogenide microparticles were obtained from corresponding bulk materials via ball milling for 3 h (SPEX 8000D Mixer/Mill). For PbTe thin films, 100 mg of PbTe microparticles was mixed with 0.3 mL of Na6Pb3Te6 in N2H4 or PbTe nanograins in DMF (solder/microparticles < 10 wt %) and sonicated or votexed at room temperature for 15−60 min. The resulting ink was drop-cast on a hydrophilized glass substrate. The as-deposited thin films were dried at room temperature, followed by annealing at 500 °C for 30 min. Na concentration in the annealed films can be controlled by the choice of solders (Na6Pb3Te6 vs PbTe nanograins) and the amount of solders. A similar procedure was applied to the fabrication of thin films from inks containing Bi2Te3 or Bi0.5Sb1.5Te3 microparticles (∼30 mg/mL in N2H4) with corresponding solders ((N2H5)4Bi2Te5, Bi2Te3 nanograins, or (N2H5)6Bi0.5Sb1.5Te6), followed by annealing at 400 °C for 30 min. The screen-printed pattern was obtained using a concentrated ink (∼100 mg/mL) containing Bi 0.5 Sb 1.5 Te 3 microparticles with (N2H5)6Bi0.5Sb1.5Te6 solder (solder/microparticle ∼ 10 wt %). The patterned module of interconnected Bi0.5Sb1.5Te3 p-legs was fabricated by drop-casting an ink of Bi0.5Sb1.5Te3 microparticles (∼30 mg/mL in N2H4) with 10 wt % (N2H5)6Bi0.5Sb1.5Te6 solder on a prepatterned, hydrophilized glass substrate. Both the screen-printed sample and the patterned p-legs were dried at 25 °C, followed by annealing at 400 °C for 30 min. The patterned Bi0.5Sb1.5Te3 p-legs were interconnected with thermally evaporated Au electrodes (∼50 nm-thick) using a mask. The consolidated 3D blocks were made in a graphite mold using the reported procedure.2 Charge Transport Measurements of Thin Films of Soldered Microparticles. Hall effect measurements were conducted on thin films and consolidated disks with a van der Pauw configuration in a physical property measurement system (PPMS, Quantum Design) under a helium atmosphere. Electrical conductivity measurements (van der Pauw configuration) were carried out using a Keithley 2400 source meter (Keithley Instrument, Inc.) through a Labview interface. The
cation exchange reactions and the formation of stoichiometric ligand-free metal chalcogenide nanograins. This chemistry yields a toolbox of solders, with various compositions and in different size regimes, for lead and bismuth chalcogenides. Charge transport studies on soldered PbTe and Bi2Te3 mesoscale grains reveal tunable transport properties at the grain boundaries depending on the choice of solders. These solders also allow solution-processed thin film TE devices. For instance, preliminary studies show that thin films made from an ink containing (N2H5)6Bi0.5Sb1.5Te6 as a solder for Bi0.5Sb1.5Te3 microparticles exhibit power factors of ∼28 μW/cm K2 at 300 K, which are comparable to those in vacuum-deposited Bi0.5Sb1.5Te3 films. Additionally, we demonstrate the use of solders in screen-printed and patterned modules as well as cast shapes, which would be applicable to printable and moldable TE materials.
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EXPERIMENTAL SECTION
Syntheses of Lead and Bismuth Chalcogenidometallates. All the syntheses and purification procedures were performed in a N2filled glovebox with anhydrous solvents at room temperature. Na6Pb3Te6 was synthesized by mixing PbTe powder (1.25 mmol) and Na2Te (1.25 mmol) in 5 mL of N2H4 and stirred for up to 5 days at 25 °C. The dark red Na6Pb3Te6 solution was then separated from a small amount of unreacted PbTe by centrifugation. To 0.1 mL of this stable solution, 0.2−0.3 mL of anhydrous acetonitrile was added, leading to partial flocculation. After centrifugation, Na2Te-rich impurities mostly remained in the red supernatant (confirmed by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis) while Na6Pb3Te6 (remaining in a small amount of N2H4 at the bottom of the centrifuge tube) was collected as a dense, dark red liquid. Using N2H4 and acetonitrile as the solvent/nonsolvent, this purification process can be repeated several times to thoroughly remove impurities. The purified Na6Pb3Te6 can be dissolved in N2H4 at a high concentration (>300 mg/mL). Similar synthetic and purification procedures can be applied to PbTe based chalcogenidometallates with other alkali cations (K+ or Cs+). Their compositions were determined by the single crystal data of K6Pb3Te6 and confirmed by ICP-OES analysis (A: Pb: Te = 2:1:2, A = Na, K, Cs). K6Pb3Te6· 7N2H4 crystals were grown by slow solvent evaporation method. In detail, 75 mg of K6Pb3Te6 was dissolved in 1.2 mL of a mixture of N2H4 and freshly distilled ethylenediamine (N2H4: ethylenediamine (en) = 1:5 by volume) under vigorous stirring. After several days, the solution was centrifuged to remove any insoluble species. ICP-OES analysis (K, Pb, Te) on this solution showed no change in the composition. For crystallization, 0.3 mL of this solution was flamesealed in a V-shaped glass ampule under inert atmosphere and located at one end of the V-shaped ampule. The end containing K6Pb3Te6 solution was gently heated (5−10 °C higher in temperature than the other end) to allow the solvents to slowly evaporate to the colder end of the ampule and thus to initiate the crystal growth of K6Pb3Te6 at the hotter end. After several days, dark red crystals were observed at the hot end. Note that, for successful crystallization, all solvents need to be extensively purified and glass containers were thoroughly dried prior to use. These crystals were extremely air- and moisture-sensitive, and all further manipulations including crystal mounting were carried out under a precooled (15 °C) N2 atmosphere in a glovebag. A dark red plate-like crystal (0.02 × 0.12 × 0.22 mm3) was used for the X-ray diffraction studies. The composition of crystals was determined as K6Pb3Te6·7N2H4. The detailed syntheses of other lead and bismuth chalcogenidometallates are described in Supporting Information. Cation Exchanges for Lead and Bismuth Chalcogenidometallates. Amberlyst-15 hydrogen form resin was purchased from Fluka. N2H5+-loaded resin was obtained by mixing resin beads with purified N2H4 and vortexing for several minutes in a N2-filled glovebox. The loading occurred through an exothermic reaction between terminal H+ on resin and N2H4. After vortexing, the supernatant turned slightly colored and was replaced by fresh N2H4. 6397
DOI: 10.1021/acs.chemmater.7b01797 Chem. Mater. 2017, 29, 6396−6404
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Chemistry of Materials measured electrical conductivities at 300 K were calibrated by a standard BiSbTe sample (Marlow Industries) with ±5% uncertainty. The thickness of thin film samples was determined by cross-sectional scanning electron microscopy images. The Seebeck coefficient of Bi0.5Sb1.5Te3 thin films at 300 K was measured using a homemade setup described in ref 14 and calibrated with a standard BiSbTe sample provided by Marlow Industries. Seebeck measurements on the patterned Bi0.5Sb1.5Te3 p-legs were conducted on a home-built system shown in Figure S20. Representative values of the electrical conductivity and Seebeck coefficient of drop-cast Bi0.5Sb1.5Te1.5 thin films and the generated voltage of TE modules were reported from 5− 10 devices. Structural and Optical Characterization. Detailed characterization techniques are provided in Supporting Information.
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RESULTS AND DISCUSSION Using the chemistry described by eq 1 in Figure 1, a series of lead and bismuth chalcogenidometallates (A2mMnxCh(m+ny))
Figure 2. (a) Crystal structure of K6Pb3Te6·7N2H4. (b) A K6Pb3Te6 subunit. (c) Linear arrangement of K6Pb3Te6 subunits packed along the [010] direction. (d) 2D slice formed by additional K−Te interactions (viewed along the [010] direction). (e) Simplified configuration of a BTAP [Pb3Te66−] moiety, showing the arrangement of Pb atoms and triangular planes of Te atoms. Colors of atoms: Pb (gray), Te (dark yellow or yellow), K (purple). N (cyan) and H (white).
Figure 1. Photograph of dilute solutions (1−5 mg/mL) of lead and bismuth chalcogenidometallates in N2H4.
comparison, the Pb−Te distance in bulk PbTe with a halite (cubic) structure is about 3.2 Å.16 The K6Pb3Te6 subunits were further arranged into a complex structure via peripheral Pb−Pb interactions along the b axis (Figure 2c, Pb−Pb contact: 3.52 Å) and additional K−Te interactions along a and c axes (Figure 2d, K−Te contact: 3.59−3.69 Å). Details of crystal data and structure refinement are shown in Tables S1 and S2. As described below, the anionic [Pb3Te66−] moiety with a bicapped trigonal antiprism (BTAP) structure (Figure 2e) remained intact in the solution phase, regardless of the alkali cations. To the best of our knowledge, the BTAP [Pb3Te66−] structure has not been reported prior to this work. In previous studies,17−21 chalcogenidoplumbate crystals were generally obtained through a multistep process, including (i) high temperature fusion of K, Pb, and Te (or Se) to form ternary compounds (e.g., nominal KPb0.65Te18), (ii) dispersion of these ternary compounds in en or liquid NH3, typically in the presence of [2,2,2]-crypt18 or crown ether,19 and (iii) crystallization by slow solvent evaporation20 or a solvothermal reaction.21 After this multistep process, seleno- or telluroplumbate crystals with trigonal bipyramidal,17−19 trigonal pyramidal,20 or trihedral geometries21 were extracted. In our approach, no ternary precursors, chelating molecules, or high temperature was required. The formation of the BTAP [Pb3Te66−] suggests the critical role of N2H4; it serves as the solvent during the synthesis as well as a small bidentate, nonchelating ligand, filling the voids between K6Pb3Te6 units and stabilizing the crystals (Figure 2a).
were synthesized using N2H4 as a solvent (Figure 1). In previous attempts, lead or bismuth chalcogenides (MxChy) were considered unreactive in N2H4 in the presence of elemental chalcogens which dissolved in hydrazine as polychalcogenide ions Chn2−, where n ≫ 1.15 The key of the proposed syntheses (eq 1) was the use of alkali chalcogenides (A + 2 Ch 2− ), where Ch 2− are more reactive than Ch n 2− (especially for the elemental chalcogens, which are equivalent to n ≫ 1) toward the electrophilic metal center in MxChy, resulting in the soluble [MnxCh(m+ny)2m−].2 In the following sections, we will show detailed characterization for these compounds, their chemical transformations, and their use as composition-matched solders for the corresponding chalcogenides. Syntheses and Characterization of Lead and Bismuth Chalcogenidometallates. Figure 2 shows the single crystal structure of K6Pb3Te6·7N2H4 (or K6Pb3Te6 below for abbreviation), which was grown from its solution in a mixture of N2H4 and ethylenediamine (en) (Experimental Section). This crystal (triclinic, space group P1̅) was featured by a subunit with one lead atom (Pb1) sitting at the symmetric center, surrounded by six tellurium (Te) atoms. Eight faces of the [PbTe610−] distorted octahedron were occupied by six K+ and two additional Pb2+ (Pb2), compensating the negative charges of [PbTe610−] and resulting in the K6Pb3Te6 moiety (Figure 2b). The bond distances between the center Pb1 atom and six Te vertices were about 3.30 Å, whereas those between peripheral Pb2 atoms and Te vertices were about 2.97 Å. In 6398
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monitor the formation of these new telluroplumbates. The Raman spectrum of Na6Pb3Te6 is shown in Figure S3b. As discussed earlier, N2H4 plays an important role in the stabilization of the [Pb3Te66−] structure. A complete removal of N2H4 solvent under a N2 flow led to the decomposition of A6Pb3Te6 into PbTe (with grain sizes over 200 nm) and A2Te. For example, this decomposition process was detected by the powder X-ray diffraction pattern (XRD) of a dried thin film of Na6Pb3Te6 (Figure 3d). Similar to the synthesis of A6Pb3Te6, mixing Na2Se and PbSe in N2H4 resulted in a new compound, Na2PbSe2 (composition determined by ICP-OES). It exhibited characteristic optical features different from Se or Na2Se (Figure S4). According to previous reports on potassium chalcogenidoplumbates,17,18 it is reasonable to speculate that Na2PbSe2 is isostructural with A6Pb3Te6, consisting of a framework of BTAP [Pb3Se66−] with Na+ as counterions and N2H4 filling the space. EXAFS analysis suggested, on average, each Pb atom was surrounded by 4 Se atoms, whereas each Se had 2 Pb neighbors (Figure S5 and Table S5). This result matched well with the structure of a BTAP [Pb3Se66−]. The electrospray ionization mass spectrometry (ESI-MS) analysis (Figure S6) also showed fragments with the molar ratio of Pb:Se = 1:2. Similar to the case ofimilar to the case of Na6Pb3Te6, upon the removal of N2H4, Na2PbSe2 decomposed into PbSe and Na2Se, as shown by the XRD patterns (Figure S7). Using the general route described by eq 1, various bismuth chalcogenidometallates (Na 4 Bi 2 Te 5 , Na 4 Bi 2 Se 5 , and Na6Bi0.5Sb1.5Te6) were synthesized in N2H4 and soluble in various solvents (dimethyl sulfoxide or DMSO, DMF, en, formamide, Table S6). The versatility in solvents would allow facile solution processing and soldering of bismuth chalcogenides. However, the attempts to obtain single crystals of these compounds led to little success. Their compositions were estimated based on ICP-OES analysis. Similar to the case of chalcogenidometallates, the optical absorption features of Na4Bi2Te5 in N2H4 were determined by the anionic framework and are insensitive to the cations or solvents (Figure S8b,c). In light of the rich structures and compositions of pnictogen metal chalcogenidometallates synthesized by solid-state chemistry,22−27 the exact structure of Na4Bi2Te5 is hard to determine without single crystal data. The coordination numbers of Na4Bi2Te5 derived from EXAFS analysis (Figure S9 and Table S7) were significantly lower than those of bulk Bi2Te3, suggesting that it is composed of small molecular clusters. Chemical Transformation of Lead and Bismuth Chalcogenidometallates. A unique feature of molecular chalcogenidometallates compared to other soluble forms of lead and bismuth chalcogenides, such as colloidal nanocrystals13,28,29 and bulk materials dissolved in mixed solvents,30−32 is their versatile chemistry. In this section, we show two typical reactions of these compounds, cation exchange and the formation of stoichiometric, ligand-free nanograins, which create a new variety of compounds and materials. These chemical transformations also address issues including solvent versatility, dopant control, and morphology, which are important variables in processing TE modules. Alkali metal cations (Na+, K+) have been considered as convenient p-type dopants for lead chalcogenide based TE materials.8,33−35 Thus, Na+ in Na6Pb3Te6 or Na2PbSe2 solders is desirable for doping lead chalcogenides. On the other hand, the same cations are not necessarily favorable for bismuth chalcogenide based TE or materials in other applications,
The structural information on K6Pb3Te6 in solution phase (N2H4) was assessed by extended X-ray absorption fine structure (EXAFS) studies (Figure 3a,b and Tables S3 and
Figure 3. (a, b) Magnitudes of the Fourier transformed EXAFS and fits for (a) Pb and (b) Te edges for K6Pb3Te6 in N2H4. (c) UV−visible absorption spectra of solutions of Na6Pb3Te6, K6Pb3Te6, and Cs6Pb3Te6 in N2H4. (d) XRD patterns of a thin film of Na6Pb3Te6 thoroughly dried at 25 °C and annealed at 500 °C. The vertical lines correspond to the diffractions of bulk Na2Te (red) and PbTe (black). (*) in (d) indicates diffractions from Na2Te.
S4). It revealed an averaged Pb−Te distance of 2.97 Å, in accordance with the Pb−Te bond length from single crystal data. In addition, EXAFS provided the information on the coordination environment for each atom, i.e., the average number of neighboring atoms around Pb and Te atoms, respectively. In K6Pb3Te6, or more specifically the anionic [Pb3Te66−], each Pb had on average 4.0 neighboring Te atoms, while each Te had 2.6 ± 1.4 Pb atoms in the first coordinating shell (Table S3). These values matched well with the coordination environment derived from single crystal structure (for Pb: 4 neighboring Te; for Te: 2 neighboring Pb). The slight deviation and uncertainty may come from the dynamic nature of [Pb3Te66−] in solution. The largely preserved K6Pb3Te6 structure in solid and solution forms was confirmed by the unchanged K:Pb:Te ratio in single crystal and solution samples measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. EXAFS analysis of Na6Pb3Te6 exhibited an identical coordination environment of Pb and Te as well as a similar Pb−Te bond length, suggesting the exchange from K+ to Na+ resulted in no change in the [Pb3Te66−] structure (Figure S2 and Table S3). The strong solvation of [Pb 3Te 6 ]6− units with N 2 H4 also contributed to the high solubility of these compounds (e.g., >300 mg/mL for K6Pb3Te6). The isostructural A6Pb3Te6 (A = Na+, K+, or Cs+) compounds in N2H4 showed identical features in the optical absorption spectra (Figure 3c), which were drastically different from those of A2Te or Te solutions in N2H4 (e.g., Na6Pb3Te6 vs Na2Te or Te, Figure S3). We speculate the absorption features in Figure 3c should arise solely from the transitions between molecular orbitals in [Pb3Te66−] clusters. Although the correlation between spectral features and specific transitions is unclear at this stage, optical absorption spectra can be used to 6399
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case of (N2H5)4Bi2Te5, the evaporation of N2H4 solvent was accompanied by the decomposition of N2H5+ and resulted in the formation of pure Bi2Te3 with traces of Te (Figure 4c), which can be mostly removed by annealing at high temperatures (e.g., ∼450 °C 14,37 ). The decomposition of (N2H5)4Bi2Te5 may occur through steps, including the formation of Bi2Te3 and (N2H5)2Te, which subsequently breaks to N2, H2, and Te. Similarly, the decomposition of other chalcogenidometallates with N2H5+ or NR4+ cations resulted in various stoichiometric bismuth and lead chalcogenides and their alloys (Figures 4d and S14). After being annealed at high temperatures (e.g., 350 °C for (N2H5)4Bi2Te5 and 500 °C for (NEt4)6Pb3Te6), pure chalcogenide phases with large grain sizes (over 200 nm) were obtained. Prior to our work, there have been several efforts toward development of soluble precursors for bismuth- and lead chalcogenides. For example, Webber et al. utilized a diamine−dithiol mixture to dissolve bulk V2VI3 chalcogenides.30,32 This strategy has achieved concentrated solutions of bismuth sulfides and Sb or As chalcogenides but suffered from the low concentration for Bi2Se3 (0.75 wt %) and Bi2Te3 (1.5 wt %) and, more seriously, the formation of undesired phases such as metallic Bi or impurity phases such as Bi2Te2S from side reactions. An earlier work by Wang et al. developed a synthetic route to the family of soluble bismuth chalcogenides by reacting Se or Te with a black viscous liquid containing Bi2S3 and Bi.37 It was also challenging to obtain homogeneous Bi2Te3−xSex alloys using this method. In comparison, chalcogenidobismutates described here were highly soluble and could transform into neat phases or alloys. For instance, (N2H5)4Bi2Te5 held a high solubility in N2H4 (∼150 mg/mL or 10 wt % Bi2Te3). This allows their use as composition-matched solders for bismuth chalcogenides, especially the selenides and tellurides. Moreover, to the best of our knowledge, (NEt4)6Pb3Te6 and (NEt4)2PbSe2 are the first soluble molecular solders or precursors for PbTe and PbSe, respectively. An additional benefit from the cation exchange process is the expanded scope of solvents for NR4+-chalcogenidometallates. The alkyl groups in NR4+ facilitate solubility of these compounds in solvents other than N2H4, such as acetonitrile, DMF, and DMSO. Changing solvents led to negligible changes in the optical absorption spectra, except for slight peak shifts (Figure S15), which may be attributed to the changes in the dielectric environments. The unaltered absorption patterns suggested the preservation of the anionic framework of chalcogenidometallates in various solvents. Molecular chalcogenidometallates can also act as the building blocks for metal chalcogenides in other forms. Adding Pb2+ or Bi3+ to them resulted in stoichiometric metal chalcogenides (Experimental Section), including PbTe ( 2 in Phase-Separated PbTe0.7S0.3. Nat. Commun. 2014, 5, 4515. (35) Pei, Y.; Shi, X.; LaLonde, A.; Wang, H.; Chen, L.; Snyder, J. G. Convergence of Electronic Bands for High Performance Bulk Thermoelectrics. Nature 2011, 473, 66−69. (36) Park, K.; Ahn, K.; Cha, J.; Lee, S.; Chae, S. I.; Cho, S.-P.; Ryee, S.; Im, J.; Lee, J.; Park, S.-D.; Han, M. J.; Chung, I.; Hyeon, T. Extrodinary Off-Stoichiometric Bismuth Telluride for Enhanced nType Thermoelectric Power Factor. J. Am. Chem. Soc. 2016, 138, 14458−14468. (37) Wang, R. Y.; Feser, J. P.; Gu, X.; Yu, K. M.; Segalman, R. A.; Majumdar, A.; Milliron, D. J.; Urban, J. J. Universal and SolutionProcessable Precursor to Bismuth Chalcogenide Thermoelectrics. Chem. Mater. 2010, 22, 1943−1945. (38) Fleurial, J.-P.; Gailliard, L.; Triboulet, R.; Scherrer, H.; Scherrer, S. Thermal Properties of High Quality Single Crystals of Bismuth Telluride−Part I: Experimental Characterization. J. Phys. Chem. Solids 1988, 49, 1237−1247. (39) Fleurial, J.-P.; Gailliard, L.; Triboulet, R.; Scherrer, H.; Scherrer, S. Thermal Properties of High Quality Single Crystals of Bismuth Telluride−Part II: Mixed-Scattering Model. J. Phys. Chem. Solids 1988, 49, 1249−1257. (40) Chi, H.; Liu, W.; Sun, K.; Su, X.; Wang, G.; Lošt'ák, P.; Kucek, V.; Drasar, C.; Uher, C. Low-Temperature Transport Properties of Tl6403
DOI: 10.1021/acs.chemmater.7b01797 Chem. Mater. 2017, 29, 6396−6404
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DOI: 10.1021/acs.chemmater.7b01797 Chem. Mater. 2017, 29, 6396−6404
