General Syntheses of High-Performance Thermoelectric Nanostructured Solids without Post-Synthetic Ligand Stripping

: Ligand-assisted wet chemical synthesis is a versatile methodology to produce controllable nanocrystals (NCs). The post-treatment of ligands is significant for the performance of functional devices. Herein, a method that retains ligands of colloidal-synthesized nanomaterials to produce thermoelectric nanomaterials is proposed, which differs from the conventional methods that strip ligands using multistep cumbersome processes. The ligand-retention method can control the size and dispersity of nanocrystals during the consolidation of the NCs into dense pellets, in which retained ligands are transformed into organic carbon within the inorganic matrices, establishing clear organic − inorganic interfaces. Characterizations of the nonstripped and stripped samples confirm that this strategy can affect electric transport slightly but reduce the thermal conductivity largely. As a result, the materials (e.g., SnSe, Cu 2 − x S, AgBiSe 2 , and Cu 2 ZnSnSe 4 ) with ligands retained achieve higher peak zT and better mechanical properties. This method can be applied to other colloidal thermoelectric NCs and functional materials.

−6 The synthesized colloidal nanocrystals usually consist of inorganic crystalline cores with layers of organic molecule ligands (e.g., oleic acid, trioctylphosphine oxide, alkanethiols, etc.) attached to their surfaces.The choice of ligands is of utmost importance for the colloidal stability NCs and the function of NC-based devices.The size, chemical composition, structure, and morphology of the nanocrystal cores are regulated by ligands during the nucleation and growth process, determining the physiochemical properties. 7,8The interparticle distance, colloidal dispersion, and packing density of an NCs assembly determined by the ligand nature/coverage strongly affect the charge carrier transport property, thermal conductivity, and chemical reactivity. 9−13 For example, Wang et al. proposed a novel ligand-cross-linking process to increase the thermal conductivity of NC solids by overcoming the thermal-transport bottleneck between adjacent colloidal NCs. 14 Zheng et al. demonstrated that the unique ligand system could modify the active sites and improve the catalytic capability of PdAu bimetallic nanocluster catalysts. 15or TE materials, the performance is evaluated by the dimensionless figure of merit zT (zT = S 2 σT/κ, where S, σ, T, and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively). 10,16,17A good TE material requires a high power factor (PF = S 2 σ) and low κ.An effective strategy to achieve depressed κ is to sinter ligand-assisted wet-chemistrysynthesized nanoparticles into dense nanostructured blocks, which enhance phonon scattering at numerous grain boundaries. 18−21 For instance, Yu et al. 22 removed organic ligands of large-scale colloidal Cu 2 ZnSnSe 4 NCs by dealing with hydrazine before NCs consolidation.With this strategy, various materials such as Cu 2 SnSe 3 , 23 Bi 2 Te 3 , 24 Cu 2 CdSnSe 4 , 25 and Cu 5 FeS 4 26 were sintered into pellets by spark plasma for TE applications.Maria Ibańẽz et al. 27 removed ligands by heating the nanoparticle powders under inert gas before the hot press procedure.−36 However, all these postligand treatments are multistep processes, which are cumbersome and costly. 4,9,37Also, they often need expensive hydrazine or thiols, which are toxic or flammable.On the contrary, without stripping the organic ligands, the structure of the ligands can influence the assembled semiconductor nanocrystals and achieve their interfacial modification at the surfaces of NCs, which has rarely been studied and remains a challenge.This motivated us to delve into how ligand retaining can regulate the TE properties of colloidal NCs.
Herein, a new strategy to retain the ligands of TE nanomaterials is demonstrated in SnSe, Cu 2−x S, AgBiSe 2 , and Cu 2 ZnSnSe 4 NCs, in contrast to the traditional methods that strip the ligands.Ligand coatings can be partially retained and used as controllable molecular solders, simultaneously acting as precursors for forming secondary carbon phases during NCs consolidation.Calculations indicate that the introduced highdensity organic−inorganic semiconductor interface suppresses the κ of the materials.As a result, compared with control samples (with ligand stripping), the samples without ligand stripping show higher peak and average zT values.In addition to excellent TE performance, Cu 2−x S without ligand stripping shows improved mechanical properties, which provide the potential for device applications.The preparation method without ligand stripping reduces solvents and toxic reagents, enabling the cost-effective fabrication of efficient TE materials, a practical example of atom economy and green chemistry.
Figure 1 shows the synthesis processes, comparing the ligand-stripping and ligand-retaining methods.First, the nanoparticles are synthesized by a colloidal synthesis approach, which forms a layer of ligand molecules on the nanoparticle surfaces.Hydrazine, as a Bronsted base, traditionally removes the original capping ligands from the nanocrystal surfaces.Whether these nanoparticles are treated with hydrazine to remove ligand molecules or not, the process is named the ligand-stripping or ligand-retaining process.The nanoparticles are sintered into dense nanostructured blocks using Spark Plasma Sintering (SPS).As a proof of concept, we select SnSe (the record-high zT TE material), phonon-liquid electroncrystal Cu 2−x S, diamond-like structured AgBiSe 2 , and multinary Cu 2 ZnSnSe 4 with the commonly used long-alkyl-chain organic ligands (oleylamine (OAm), oleic acid (OA), and dodecanethiol (DDT)) as examples.The sintering and molding of as-synthesized nanocrystals lasted for 30 min in a closed die during SPS, preventing the volatilization of ligands.Therefore, the remaining organic ligands thermally decompose and transform into organic carbon as a secondary phase within the matrices, forming the inorganic NCs-organic carbon interface (Figures 1 and S1).Reduced κ in the non-L-S samples is attributed to residual carbon at the grain boundaries and the slightly smaller crystal domains than in the L-S samples.The well-developed surface chemistry of colloidal NCs by retained ligands offers the opportunity to engineer the interfaces and microstructures of the assembled solids, resulting in a high average zT and excellent mechanical properties.In this study, NCs treated with hydrazine hydrate for 0, 1, and 3 time(s) are referred to as non-L-S sample, L-S sample-1, and L-S sample-3, respectively.
The atomic structures of the samples are characterized through several techniques.The representative X-ray diffraction (XRD) patterns (Figure S2) demonstrate the crystalline structure of the SnSe, Cu 2−x S, AgBiSe 2 , and Cu 2 ZnSnSe 4 NCs, which are indexed to the orthorhombic Pnma phase, tetragonal P4 3 2 1 2 and monoclinic P2 1 /c phase, hexagonal P3̅ m1 phase, and cubic F4̅ 3m phase, respectively.The hydrazine treatment does not change the crystal structure of the materials.Table S4 in the Supporting Information shows the cell parameters of the above materials.The transmission electron microscopy (TEM) images of the SnSe, Cu 2−x S, AgBiSe 2 , and Cu 2 ZnSnSe 4 NCs characterize the size distribution and morphology of as- synthesized NCs (Figures 3a and S4).Disk-like, small spherelike, irregular, and cubic shapes with average sizes of around 336 (±65) nm, 18 (±3) nm, 19 (±4) nm, and 13 (±2) nm are obtained for these four materials, respectively.The highresolution (HR) TEM of SnSe, Cu 2−x S, AgBiSe 2 , and Cu 2 ZnSnSe 4 are shown in Figures 3b and S4.
To identify the chemical environment of the organic surfactant shell surrounding the TE materials before and after ligand stripping, we perform Fourier transform infrared (FTIR) spectroscopy for dried samples.As shown in Figure 2c, the presence of the OAm molecules is identified by the symmetric and asymmetric stretching vibrations of −NH 2 at 3430.74 cm −1 , the bending vibrations of C�C bonds at 1626.72 cm −1 , and the bending vibration of the C−N bond at 1109.88 cm −1 .These features are weaker in the L-S SnSe-3 sample compared with the nonstripped samples.The L-S SnSe-3 sample also shows the removal of ligands from the surface of colloidal nanoparticles by exchange with hydrazine.This result is in good accordance with Raman spectra (Figures 2d and S3), in which the carbon peaks of the samples are calibrated relative to the characteristic B 3g and A g Raman modes of SnSe at 109, 128.5, and 150 cm −1 . 38,39Two dominant carbon peaks of SPSed SnSe samples without ligand stripping at around 1334 and 1526 cm −1 correspond to the D and G bands, respectively (Figure 2d).The D band is assigned to the breathing mode of A 1g symmetry or sp 3 -rich phase for the disordered graphite.The G band corresponds to the E 2g mode of 2D graphite, which is related to the vibration of sp 2hybridized carbon.The D and G bands indicate that the surface ligands are turned into organic carbons during sintering.−42 For SPSed L-S SnSe-3, the intensity of the D and G bands are significantly decreased due to the ligand stripping and the elimination of carbon during the annealing process. 41,42This is also proved by carbon composition analysis.The carbon content (1.23 wt %) for SPSed non-L-S SnSe pellets was much higher than that (0.264 wt %) of L-S SnSe-3 pellets.In addition, the N atom in oleylamine ligands will incorporate into the nanostructure solids during the bulk consolidation step to provide a large density of point defects, facilitating the reduction of thermal conductivities (Figure 2b).To test the formation of the carbon-heteroatom bond, soft X-ray absorption near-edge structure (XANES) measurements are used to detect the Kedge excitation of elemental C (Figure 2a) to investigate the electronic structure and the nature of the carbon defect of the SnSe-Carbon sample.Four peaks at ∼285.5, 287.8, 288.7, and 292.1 eV are denoted as A, B, D, and E, respectively.The absorption peak A corresponds to π* excitations of C�C bonds at the defect sites, 43 and the peak E corresponds to the σ* structure of C−C bonds. 44The strong π* and σ* bands structures show that the carbon is graphitized. 45−45 X-ray photoelectron spectroscopy (XPS) analysis further proves this phenomenon (Figure 2e−h).The Sn peaks of non-L-S SnSe are slightly shifted to lower binding energies (BEs) compared to the reported SnSe (∼0.33 eV), 46 demonstrating the bonding of C−N−Sn.The same phenomenon has also occurred in the Se 3d spectra, shifting about 0.12 eV to lower BE.The fitting result of the N 1s peak shows that two peaks at 400.5 and 404.2 eV correspond to pyridinic-N and graphitic-N, and pyridinic-N is the dominant species. 44,45,47The C 1s spectrum can be divided into two peaks at 284.8 eV and a higher BE of 286.6 eV, corresponding to the sp 2 hybridized graphitic carbon and C−N species. 48EM images (Figure 3c,d) demonstrate that the Cu 2−x S NCs are embedded in the nanostructured solids, and the ligands are carbonized as a carbon coating on the grain of Cu 2−x S. Figure 3d shows the d-spacing of carbon is approximately 0.41 nm. 49The carbon is mainly distributed along with the marginal area of Cu and S elements, as depicted in the EDS elemental mapping in Figure 3g−j.The carbon coating of Cu 2−x S is rarely observed in the ligand-stripped samples (Figure 3e), consistent with the FTIR, Raman, and carbon-content analysis results as discussed above.In addition, elemental mappings are performed to identify Sn, Se, Ag, Bi, Cu, and Zn elements in nanoparticles for SPSed SnSe, SPSed AgBiSe 2 , and SPSed Cu 2 ZnSnSe 4 (Figures S6−S8).
The TE properties of Cu 2−x S are measured in the temperature range from 300 to 937 K (Figure 4).The dependence of the σ and the S on the temperature of non-L-S samples is similar to that of L-S samples.The electrical conductivity shows two peaks because of the phase transition, which is similar to the reports in the literature. 50,57For non-L-S Cu 2−x S, the two peaks are 21.5 × 10 3 and 32.2 × 10 3 S m −1 at 323 and 650 K, respectively.Relatively high electrical conductivities were obtained for the ligand-stripped samples, and the carbon capping layer slightly affects the electric transport of the nonligand-stripped sample.The value of the S obtained for the Cu 2−x S samples increases with temperature and decreases with increasing ligand stripping throughout the entire temperature.For example, as shown in Figure 4d, the room-temperature S for the non-L-S Cu 2−x S, L-S Cu 2−x S-1, and L-S Cu 2−x S-3 samples are 79.1, 74.5, and 58.3 μV•K −1 , respectively.Such difference is largely related to the n H (Figure 4b), derived from the copper vacancy and the residual carbon at the grain boundaries.The non-L-S Cu 2−x S with more carbon components have lower n H , and alkaline hydrazine hydrate treatment can lead to the etching of the surface of NCs, in which the generation of metal vacancies will increase their n H . Finally, the PF of all the samples is shown in Figure 4e.At 937 K, the PF of non-L-S Cu 2−x S (0.97 mW m −1 K −2 ) is higher than that of the L-S samples (0.90 mW m −1 K −2 ) due to its higher S.
The κ tot (Figure 4g) of the non-L-S Cu 2−x S pellet is 0.63 W m −1 K −1 at 323 K, which is lower than those of L-S Cu 2−x S-1 (0.73 W m −1 K −1 ) and L-S Cu 2−x S-3 (0.83 W m −1 K −1 ).According to the Wiedemann−Franz law, the electronic thermal conductivity can be calculated via κ ele = LσT, where L = (1.5 + exp(−|S|/116)) × 10 −8 ) is the Lorenz number.After subtracting κ ele from κ tot , we obtain the lattice thermal conductivity κ lat , as shown in Figure 4f. 16,58It is found that the κ lat decreased by as much as 43% (from 0.35 to 0.20 W m −1 K −1 ) upon retaining the ligands.Due to the slightly enhanced PF and much-reduced κ tot , a high zT max of 1.9 (at 937 K) is achieved for non-L-S Cu 2−x S (Figure 4h). Figure 4i compares the zT values achieved in this work with the state-of-the-art peak zT in Cu 2−x S compounds, indicating that the method of retaining ligands in this work positively impacts improving the TE properties.In addition, the theoretical maximum efficiency of 10% for the non-L-S Cu 2−x S and a theoretical maximum figure of merit ZT of a TE device of the 0.5 for 320−930 K temperature range was calculated using the method provided by Snyder et al. (Figure S10). 59o understand the decreased lattice thermal conductivity with retaining ligands (or increasing amorphous carbon content), we conducted theoretical analyses using Cu 2−x S as an example.To estimate the interfacial thermal resistance (R K ) between Cu 2 S and amorphous carbon, we calculated the phonon density of states (DOS) of Cu 2 S by density functional theory (DFT).We collected the phonon DOS of different forms of carbon, 60,61 as shown in Figure 5b.It is found that they exhibit a great mismatch: Cu 2 S has extremely high DOS below 10 THz, where the DOS of carbon is minimal, indicating that R K should be very large.To estimate whether R K can play a role in reducing the thermal conductivity of Cu 2 S, we estimated R K using the diffusive mismatch model (DMM) 62 (See page 7 of Supporting Information for detailed calculations).
Owing to the reduced κ and the increased PF, higher zT values of non-L-S Cu 2−x S are achieved as 1.9 at 937 K, 1.6-fold of the value obtained by L-S Cu 2−x S-3 (zT = 1.2), indicating our strategy of retaining ligands affects improving TE performance (Figure 4f).To validate the potential of the described procedure to prepare nanomaterials with enhanced performance, in addition to Cu  of phonons at interfaces in these nanocrystalline alloy composites.
Finally, the zT values of the above non-L-S samples prepared by the retained ligands route are all higher than those of L-S sample-3 due to reduced κ tot without deterioration of the electrical transport performance.As shown in Figure 1, the highest zT for the L-S SnSe-3, L-S Cu 2−x S-3, L-S AgBiSe 2 -3, and L-S Cu 2 ZnSnSe 4 -3 are 0.9, 1.2, 0.4, and 0.6, respectively, while the highest zT for non-L-S samples are 1.3, 1.9, 0.7, and 0.7, respectively, and the average zT has a similar trend.We believe that the surface-ligand influence on properties is related to the effect of surface treatment on the final surface composition and structure.The inorganic NCs-organic carbon interfaces effectively scatter long mean-free-path electrons and phonons, which is beneficial to reducing the κ.Although the carbon-nano grain interfaces slightly, there is a decrease of the σ due to the insulating capping layer of carbon.Under certain conditions, they also play an important role in enhancing the S. Therefore, the zT can be improved by balancing the electronic and thermal properties by regulating the amount of retained organic ligands.
After we achieved an improved zT value, we also evaluated the fracture toughness to understand the effect of retained ligands on the mechanical properties (take the Cu 2−x S system, for example).The Vickers hardness (H v ) and fracture toughness (K IC ) are measured on the polished surface according to the equations H v = 1854.4•F/D 2 and K IC = A• (E/H v ) 0.5 •(F/c 1.5 ), where F is the applied load, D is the length of the diagonal of the resultant impression, the calibration constant is a value of 0.01, 63,64 c is the radial crack length of Vickers indentation, and E is Young's modulus of the Cu 2−x S samples. 65We use nanoindentation to measure Young's modulus.Figure 5f,g shows that the hardness and the fracture toughness of L-S Cu 2−x S-3 are about 0.62 GPa and 1.9 MPa• m 1/2 , respectively.In contrast, the hardness and fracture toughness of the non-L-S Cu 2−x S can reach as high as 0.86 GPa and 4.2 MPa•m 1/2 .Ligands used as controllable molecular solders can generate a bridging effect between Cu 2−x S grains and improve their stronger interaction, resulting in optimized mechanical durability.
In conclusion, we develop a general synthesis method to avoid complex ligand stripping procedures and introduce controlled amounts of secondary carbon phases.We have demonstrated that residual carbon between grain boundaries suppresses thermal conductivity and enhances the mechanical properties of NC solids.Furthermore, the convenience and potential of the strategy are demonstrated with SnSe, Cu 2−x S, AgBiSe 2 , and Cu 2 ZnSnSe 4 NCs, which possess higher zTs as compared to control samples with ligand stripping.Our result provides a general route for suppressing thermal transport in organic/inorganic hybrid materials.Beyond the above metallic chalcogenides in this work, the strategy will likely be broadly applicable to other colloidal nanomaterials.It should help to expand interface engineering to other device applications, such as photovoltaics, electronics, and light-emitting diodes.

Figure 3 .
Figure 3. Morphology characterizations of Cu 2−x S samples.(a) The TEM images with the grain size distribution histograms of the assynthesized sphere-like Cu 2−x S nanoparticles.(b) The HRTEM image of Cu 2−x S nanoparticles.(inset) The corresponding FFT pattern.(c, d) TEM and HRTEM images of the non-L-S Cu 2−x S sample, the carbon is marked in the black dashed box.(e, f) TEM and HRTEM images of the L-S Cu 2−x S-3.(g−j) The energy-dispersive spectroscopy (EDS) elemental mapping of C (yellow), Cu (blue), and S (orange) for non-L-S Cu 2−x S sample.

Figure 5 .
Figure 5. (a) Lattice thermal conductivity (κ lat ) at 323 K with different volume fractions of carbon.The thermal conductivities are predicted by the Hashin-Shtrikman bound model (H−S ub and H−S lb), the rule of mixture (ROM) model in parallel and series patterns, and the Mori-Tanaka (M-T) model for fiber-like particles.(b) The mismatch in phonon density of states between Cu 2 S and different forms of carbon. 60,61Reprinted with permission from ref 60.Copyright [2004] [Elsevier Ltd.].Reprinted with permission from ref 61.Copyright [2018] [Informa UK Limited].The microscopic appearance of indentation of the (c) non-L-S Cu 2−x S, (d) L-S Cu 2−x S-1, and (e) L-S Cu 2−x S-3.(f) Micro Vickers hardness and (g) fracture toughness of the non-L-S Cu 2−x S, L-S Cu 2−x S-1, and L-S Cu 2−x S-3.