Facile Surfactant‐Free Synthesis of p‐Type SnSe Nanoplates with Exceptional Thermoelectric Power Factors

Abstract A surfactant‐free solution methodology, simply using water as a solvent, has been developed for the straightforward synthesis of single‐phase orthorhombic SnSe nanoplates in gram quantities. Individual nanoplates are composed of {100} surfaces with {011} edge facets. Hot‐pressed nanostructured compacts (E g≈0.85 eV) exhibit excellent electrical conductivity and thermoelectric power factors (S 2 σ) at 550 K. S 2 σ values are 8‐fold higher than equivalent materials prepared using citric acid as a structure‐directing agent, and electrical properties are comparable to the best‐performing, extrinsically doped p‐type polycrystalline tin selenides. The method offers an energy‐efficient, rapid route to p‐type SnSe nanostructures.

Growing global energy demands,together with the negative impacts resulting from combustion of fossil fuels,h ave diverted attention to technologies for sustainable energy generation and conversion. [1] Thermoelectrics realize direct inter-conversion between thermal and electrical energy and provide opportunities to harvest useful electricity from waste heat (and conversely to perform refrigeration). Thet hermoelectric conversion efficiencyo famaterial is determined by its dimensionless figure of merit, ZT= S 2 sT/k,where S, s, T, and k represent the Seebeck coefficient, electrical conductivity,a bsolute temperature,a nd thermal conductivity,r espectively. [2] Extensive efforts have been devoted to the improvement of the thermoelectric performance of state-of-the-art materials, [3] and to the discovery of new thermoelectrics [4] with ZT values > 2. Single-crystalline SnSe combines ah igh ZTwith arelatively low toxicity and high Earth-abundance of the component elements. [4] SnSe crystals possess very low thermal conductivity owing to lattice anharmocity,y ielding record high ZTvalues of 2.6 and 2.3 at 923 Kalong the b and c crystallographic directions,r espectively. [4] Polycrystalline SnSe materials have been fabricated to improve mechanical properties, [5] but ZT has been limited to 1, owing to both increased electrical resistivity and thermal conductivity. [5] Unfortunately,t he synthesis of SnSe is protracted and energy-intensive,i nvolving heating, melting, and annealing at high temperatures ( % 800-1223 K). [4][5] Before the potential of SnSe can be realized, afast, cost-effective,and large-scale synthesis route to the pure selenide that does not sacrifice performance is essential.
Nanostructuring very effectively enhances ZT. Theh igh density of interfaces improves phonon scattering, decreasing the lattice thermal conductivity. [2,3] Bottom-up solution synthesis methods facilitate control of size,m orphology,c rystal structure,and defects. [6] However,the organic surfactants that can control morphology through surface modification are commonly electrically insulating, which can drastically reduce the electrical conductivity of the materials. [7] Ligand replacement methods switch smaller species for long chain surfactant molecules, [7] but sometimes involve using high toxicity chemicals, [8] and introduce impurities, [7b] which again can adversely influence the transport behavior of the materials. [7b] Organic contamination can be prevented only if as uitable surfactant-free synthesis strategy can be found, [9] and to date solution syntheses of SnSe nanostructures have required organic surfactants and/or solvents,f or example,o leyamine, trioctylphosphine selenide,a nd bis[bis(trimethylsilyl) amino]tin(II), while only yielding milligram quantities of materials. [10] In this study,w ed emonstrate as urfactant-free aqueous solution approach towards the preparation of > 10 g of SnSe nanoplates,b yb oiling am ixture of NaHSe and Na 2 SnO 2 solutions for 2h.The phase-pure nanoplates can be hot pressed into dense pellets with outstanding thermoelectric power factors (Scheme 1).
Tr ansmission electron microscopy (TEM;F igure 1c) showed that the SnSe nanoplates were almost uniformly rectangular,a nd selected area electron diffraction (SAED) patterns obtained with the incident electron beam normal to the face of the nanoplate could be indexed along the [100] SnSe zone axis.Aset of lattice spacings of % 3.0 intersecting with an angle of 93(1)8 8 could be measured from high resolution TEM (HRTEM;F igure 1d)c orresponding to the {011} plane spacings.C ombined with SAED data, the nanoplate face can thus be identified as the bc plane of SnSe and the side facets are defined by {011} planes (Figures 1c,S 4). Theobs erved splitting in diffraction spots suggested twin defects induced by orthorhombic distortion. [12] Images and SAED patterns along the [001] zone axis (beam direction parallel to the nanoplate face;F igure 1e)v erified that:i )the plates are approximately an order of magnitude thinner in the third dimension, and ii)the bc plane forms the nanoplate faces. Further,d iffraction spots are elongated along [100],i ndicating planar defects along the a axis. [13] Lattice spacings of % 5.7 ( d (200) )a nd 4.2 ( d (010) )w ere observed in the corresponding HRTEM image (Figure 1f).
Intermediate products synthesized after only 1min of heating were investigated to understand the morphological evolution. Thep roduct is single-phase orthorhombic SnSe ( Figure S5a) composed principally of irregular, near-rectan-   Figure S4) the facets of the SnSe truncated nanoplate can be depicted as shown in Figure 2a. Hence,the SnSe truncated nanoplate is enclosed by {100} and {011}, together with {001} facets.G iven that no surfactant is used, the nanoplate shape is determined primarily by the intrinsic features of the anisotropic selenide crystal structure. Atomic planes with high surface energies usually exhibit fast growth rates,a nd in SnSe the {001} and {010} planes possess much higher surface energies than the {011} planes. [14] The former planes would thus experience faster initial growth than the {011} planes.T om aintain the minimum surface energy as growth progresses,t he {001} and {010} planes diminish, while the {011} planes feature increasingly in the side facets (Figures 2a,c) until they dominate completely (Figures 1c,2d). TheN aOH concentration is also important in regulating growth, and by decreasing the molar ratio from 15:1 to 15:2 the mean length/width of the SnSe nanoplates is reduced from % 150 nm to % 80 nm (Figures S6, S7). Decreasing the hydroxide concentration further has more profound effects on the reaction chemistry (see the Supporting Information).
Thea bility to prepare > 10 gs urfactant-free SnSe nanomaterials allowed the fabrication of high-density pellets through hot pressing without the necessity of high temperature annealing. Pellets of % 95 %theoretical density,retaining the orthorhombic SnSe structure were obtained (denoted 1; Figure 3a;T ables S3, S4). Strong orientation of the plates in the bc plane is evidenced by the increased intensity of the (h00) PXD reflections,a nd the decrease in peak half-widths indicates al arger crystallite size after hot pressing.T he indirect (direct) optical band gap from diffuse reflectance (DR) UV/Vis spectra [10c] narrows slightly from % 0.89 ( % 1.1) eV to % 0.85 ( % 1.0) eV ( Figure S10) when the nanoplates are consolidated into dense pellets,w hich could be related to sintering effects.T he values are very similar to the indirect band gaps reported for both single crystalline and polycrystalline SnSe. [4, 5c, d] 1 is composed of densely packed particles,t ypically % 200 nm across with flat surfaces (Figures 3b,S11a). TheSn:Se ratio remains at 49(1):51(1) atom % ( Figure S11b). An SAED pattern (Figure 3c), with the beam normal to the face of an anoplate taken from 1 was indexed along the SnSe [100] zone axis.T he single-crystal structure was confirmed by the HRTEM image (Figure 3d). TEM also showed that the nanoplate from 1 consisted of compacted smaller platelets ( Figure S11c). Thermogravimetric analysis (TGA) of 1 under both argon and air revealed negligible weight changes below 500 8 8C, but suggested that thermal decomposition and oxidation, respectively,b egin above this temperature ( Figure S12).
Forc omparison, as econd sample of SnSe nanoparticles ( % 40-60 nm) were synthesized by ac itric-acid-assisted solution synthesis,w hich were also consolidated into dense pellets ( % 92 %o ft he theoretical density) by hot pressing (denoted 2;Figures S13, S14). Compared to 1, 2 possesses the same orthorhombic structure,as imilar optical band gap and forms comparable nanostructures ( % 200 nm across oriented in the bc plane). Importantly,h owever, Cl is detected in 2 (Sn:Se:Cl ratios of 51(1):48(1):1(1) atom %) that likely originates from the replacement of ligated citric acid by Cl during processing. [7b] Thes imilar densities and constituent particle sizes of 1 and 2 allowed for agood comparison of their relative electrical performance.The electrical conductivity of 1 (Figure 4a)increases four-fold from % 840 Sm À1 at 300 Kto % 3500 Sm À1 at 550 K. Themagnitude of the values for 1 can be attributed to the high crystallinity,s mall band gap, surfactant-free particle surface,m icrostructural orientation, and the high level of sintering and densification achieved. By

Angewandte Chemie
Communications contrast, 2 exhibited electrical conductivity increasing from % 55 Sm À1 at 300 Kt oo nly % 250 Sm À1 at 550 K; more than an order of magnitude lower than 1.
Thec ontrast in the variation in the Seebeck coefficient with temperature for 1 and 2 is striking (Figure 4b). S for 1 increases almost linearly with temperature (250 mVK À1 at room temperature to 340 mVK À1 at 550 K). By comparison, 2 shows n-type behavior at room temperature (S % À150 mVK À1 ), with the value of S becoming positive (p-type behaviour) at % 490 Kand rising to % 80 mVK À1 at 550 K. It is possible that the n-type conducting behavior correlates to the presence of Cl and/or aslight excess of Sn, as noted above.We are currently investigating this behavior further in systematic doping experiments.A nn /p or p/n inversion with increasing temperature has also been observed in pellets consolidated from PbTe, Ag 2 Te,a nd PbTe 0.1 Se 0.4 S 0.5 synthesized through surfactant-assisted solution methods, [7,15] and should be related to the thermal activation of higher concentrations of positive or negative charge carriers,r espectively. [7b] It is also notable that both s and S increase with temperature for 1. This phenomenon has been observed in both un-doped and iodine-doped polycrystalline SnSe. [5c,d, 16] Although the origins of the behavior for 1 require further investigation, the combination of superior s values coupled with high values of S leads to exceptional power factors ( % 0.05 mW m À1 K À2 at 300 Kto% 0.40 mW m À1 K À2 at 550 K; Figure 4c). In contrast, the power factors for 2 are much lower, (0.001 mW m À1 K À2 at 300 Kand reaching only 0.05 mW m À1 K À2 at 550 K). Thehuge differences in performance between 1 and 2 further emphasize the importance of the surfactant-free synthesis route,not just in the context of as impler, more sustainable synthesis method, but also in delivering significantly improved electrical properties consistently ( Figure S15). Notably,the power factors for 1 far exceed those for un-doped polycrystalline SnSe across as imilar temperature range (0.028-0.04 mW m À1 K À2 ), [5c-e] and are comparable to those for holedoped materials with high carrier concentrations. [5d, 17] Recent Na-and Ag-doping studies have elegantly demonstrated how the electrical performance and ZT values of SnSe single crystals can be dramatically improved. [18] Given that the samples in our studies were non-optimized, strategies involving systematic hole doping,i nc onjunction with surfactantfree nanostructuring approaches,s hould yield even higher performing p-type SnSe materials and pave the way for onepot synthesis of p-and n-type SnSe nanomaterials.
In summary,asimple,q uick, surfactant-free,a nd energyefficient solution synthesis yielded SnSe nanoplates in gram quantities.T he ensuing nanostructured pellets exhibited exceptional electrical conductivity coupled with high Seebeck coefficients,l eading to power factors surpassing those of polycrystalline and surfactant-coated counterparts.T he technique should be readily adaptable to include dopants and amenable to the discovery of further materials,both p-and ntype,with enhanced thermoelectric properties.

Experimental Section
Full experimentaldetails are provided in the SupportingInformation.
Materials Synthesis.1 00 mmol NaOH and 10 mmol SnCl 2 ·2 H 2 O were added into 50 mL deionizedw ater to yield at ransparent Na 2 SnO 2 solution. 50 mL of NaHSe (aq) preparedfrom Se and NaBH 4 was injected into the boiling solution, leading to the immediate formationofablack precipitate.T he mixture was boiled for 2h,and cooled to room temperature under Ar (g) on aS chlenk line.T he products were washed with deionized water and ethanol and dried at 50 8 8Cf or 12 h. Scaled-up syntheses were performed with six-fold precursor concentrations (94(1)% yield). Fort he surfactant-assisted synthesis,5 0gcitric acid was introduced into SnCl 2 solution with no addition of NaOH and the reaction duration was increased to 24 h.