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 (Eg ≈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.

Full text

German Edition:DOI:10.1002/ange.201601420
Thermoelectrics Very Important Paper International Edition:DOI:10.1002/anie.201601420
Facile Surfactant-Free Synthesis of p-Type SnSe Nanoplates with
ExceptionalThermoelectric PowerFactors
Guang Han, Srinivas R. Popuri, Heather F. Greer,Jan-Willem G. Bos,Wuzong Zhou,
Andrew R. Knox, Andrea Montecucco,Jonathan Siviter,Elena A. Man, Martin Macauley,
Douglas J. Paul, Wen-guang Li, Manosh C. Paul, Min Gao,Tracy Sweet, Robert Freer,
Feridoon Azough, Hasan Baig,Nazmi Sellami, Tapas K. Mallick, and Duncan H. Gregory*
Abstract: Asurfactant-free solution methodology,simply
using water as asolvent, has been developed for the straight-
forwardsynthesis of single-phase orthorhombic SnSe nano-
plates in gram quantities.Individual nanoplates are composed
of {100} surfaces with {011} edge facets.Hot-pressed nano-
structured compacts (Eg0.85 eV) exhibit excellent electrical
conductivity and thermoelectric power factors (S2s)at550 K.
S2svalues are 8-fold higher than equivalent materials prepared
using citric acid as astructure-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,have
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). Thethermo-
electric conversion efficiencyofamaterial is determined by
its dimensionless figure of merit, ZT=S2sT/k,where S,s,T,
and krepresent the Seebeck coefficient, electrical conductiv-
ity,absolute temperature,and thermal conductivity,respec-
tively.[2] Extensive efforts have been devoted to the improve-
ment 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 ahigh
ZTwith arelatively low toxicity and high Earth-abundance of
the component elements.[4] SnSe crystals possess very low
thermal conductivity owing to lattice anharmocity,yielding
record high ZTvalues of 2.6 and 2.3 at 923 Kalong the band c
crystallographic directions,respectively.[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,the synthesis of SnSe is protracted and
energy-intensive,involving 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. Thehigh
density of interfaces improves phonon scattering, decreasing
the lattice thermal conductivity.[2, 3] Bottom-up solution syn-
thesis methods facilitate control of size,morphology,crystal
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 replace-
ment 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 asuitable
surfactant-free synthesis strategy can be found,[9] and to date
solution syntheses of SnSe nanostructures have required
organic surfactants and/or solvents,for example,oleyamine,
[*] Dr.G.Han, Prof. D. H. Gregory
WestCHEM, SchoolofChemistry, University of Glasgow
Glasgow,G12 8QQ (UK)
E-mail:Duncan.Gregory@glasgow.ac.uk
Dr.S.R.Popuri, Dr.J.-W.G.Bos
Institute of Chemical Sciences and Centre for Advanced Energy
Storage &Recovery
School of Engineering &Physical Sciences
Heriot-Watt University
Edinburgh, EH14 4AS (UK)
Dr.H.F.Greer,Prof. W. Z. Zhou
EaStCHEM,School of Chemistry, University of St Andrews
St Andrews, Fife KY16 9ST (UK)
Prof. A. R. Knox, Dr.A.Montecucco, Dr.J.Siviter,Dr. E. A. Man,
Dr.M.Macauley,Prof. D. J. Paul, Prof. W.-g. Li, Dr.M.C.Paul
School of Engineering, University of Glasgow
Glasgow,G12 8QQ (UK)
Dr.M.Gao, Dr.T.Sweet
School of Engineering, Cardiff University
Cardiff, CF24 3AA (UK)
Prof. R. Freer,Dr. F. Azough
Materials Science Centre, School of Materials
UniversityofManchester
Manchester,M13 9PL (UK)
Dr.H.Baig, Dr.N.Sellami, Prof. T. K. Mallick
Environmentand Sustainability Institute, University of Exeter
Penryn Campus, Penryn TR10 9FE (UK)
Supportinginformation and the ORCID identification number(s) for
the author(s) of this article can be found under
http://dx.doi.org/10.1002/anie.201601420.
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trioctylphosphine selenide,and bis[bis(trimethylsilyl)
amino]tin(II), while only yielding milligram quantities of
materials.[10] In this study,wedemonstrate asurfactant-free
aqueous solution approach towards the preparation of >10 g
of SnSe nanoplates,byboiling amixture of NaHSe and
Na2SnO2solutions for 2h.The phase-pure nanoplates can be
hot pressed into dense pellets with outstanding thermoelectric
power factors (Scheme 1).
Injecting NaHSe into aNa
2SnO2solution leads to the
instant precipitation of SnSe nanoparticles [Supporting Infor-
mation, Figure S1;Eq. (1)]:
NaHSe þNa2SnO2þH2O!SnSe þ3NaOH ð1Þ
Boiling the suspension for 2hleads to the formation of
crystalline,phase-pure nanoplates of orthorhombic SnSe
(ICDD card No.48-1224).[11] Rietveld refinement against
powder X-ray diffraction (PXD) data (Figure 1a;
Tables S1,S2) confirmed the orthorhombic structure (space
group Pnma,a=11.5156(5), b=4.1571(2), c=4.4302(3) è).
Scanning electron microscopy (SEM;Figures 1b,S2a)
revealed that the product is comprised of rectangular nano-
plates,each with lateral dimensions of 80–200 nm and athick-
ness of 10–60 nm. Energy dispersive X-ray spectroscopy
(EDS;Figure S2b) consistently generated Sn:Se ratios of
49(1):51(1) atom%, while Fourier transform infrared (FTIR)
spectra (Figure S3) provided no evidence for organic func-
tional groups (as might be expected from equivalent surfac-
tant-assisted syntheses).
Transmission electron microscopy (TEM;Figure 1c)
showed that the SnSe nanoplates were almost uniformly
rectangular,and 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 èintersect-
ing with an angle of 93(1)88could be measured from high
resolution TEM (HRTEM;Figure 1d)corresponding to the
{011} plane spacings.Combined with SAED data, the nano-
plate face can thus be identified as the bc plane of SnSe and
the side facets are defined by {011} planes (Figures 1c,S4).
Theobserved 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;Figure 1e)verified 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,diffraction spots are elongated along [100],
indicating planar defects along the aaxis.[13] Lattice spacings
of 5.7 è(d(200))and 4.2 è(d(010))were observed in the
corresponding HRTEM image (Figure 1f).
Intermediate products synthesized after only 1min of
heating were investigated to understand the morphological
evolution. Theproduct is single-phase orthorhombic SnSe
(Figure S5a) composed principally of irregular, near-rectan-
gular nanoplates,many of which are truncated (Figure S5b).
TEM revealed internal angles of 133(1)88and 94(1)88at the
truncated and regular corners,respectively (Figure 2a). When
Scheme 1. The solution synthesis and hot pressing of SnSe nano-
plates:a)injection of NaHSe(aq) into Na2SnO2(aq) to trigger the
reaction;b)formation of SnSe nanoplates;c)orientation of nanoplates
induced by hot pressing;d)structure model of fabricated bulk pellets.
The insets in (b) show the nanoplate solution and the yield ( 94 %)
from a2hsynthesis.
Figure 1. Characterization of SnSe nanoplates synthesized after 2h:
a) profile plot from Rietveld refinement; b) SEM image ;c)TEM image
of aSnSe nanoplate and its corresponding SAED pattern along the
[100] zone axis;d)HRTEM image of part of the plate shown in (c)
with d-spacings indicated;e)profile HRTEM image of aSnSe nano-
plate and its corresponding SAED pattern along the [001] zone axis;
and f) HRTEM image of part of the plate shown in (e) with d-spacings
indicated.
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correlating the SAED pattern (along the [100] zone axis;
Figure 2b), the TEM image and the crystal plane intersection
angle along the bc plane (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.Given 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,and 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.Tomaintain the minimum surface
energy as growth progresses,the {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). TheNaOH 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). Decreas-
ing the hydroxide concentration further has more profound
effects on the reaction chemistry (see the Supporting
Information).
Theability to prepare >10 gsurfactant-free SnSe nano-
materials allowed the fabrication of high-density pellets
through hot pressing without the necessity of high temper-
ature annealing. Pellets of 95%theoretical density,retain-
ing the orthorhombic SnSe structure were obtained (denoted
1;Figure 3a;Tables S3, S4). Strong orientation of the plates in
the bc plane is evidenced by the increased intensity of the
(h00) PXD reflections,and the decrease in peak half-widths
indicates alarger crystallite size after hot pressing.The
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 nano-
plates are consolidated into dense pellets,which could be
related to sintering effects.The values are very similar to the
indirect band gaps reported for both single crystalline and
polycrystalline SnSe.[4,5c, d] 1is composed of densely packed
particles,typically 200 nm across with flat surfaces (Fig-
ures 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 ananoplate taken from 1was indexed
along the SnSe [100] zone axis.The single-crystal structure
was confirmed by the HRTEM image (Figure 3d). TEM also
showed that the nanoplate from 1consisted of compacted
smaller platelets (Figure S11c). Thermogravimetric analysis
(TGA) of 1under both argon and air revealed negligible
weight changes below 50088C, but suggested that thermal
decomposition and oxidation, respectively,begin above this
temperature (Figure S12).
Forcomparison, asecond sample of SnSe nanoparticles
(40–60 nm) were synthesized by acitric-acid-assisted
solution synthesis,which were also consolidated into dense
pellets ( 92%ofthe theoretical density) by hot pressing
(denoted 2;Figures S13, S14). Compared to 1,2possesses the
same orthorhombic structure,asimilar optical band gap and
forms comparable nanostructures ( 200 nm across oriented
in the bc plane). Importantly,however, 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] Thesimilar densities and constituent
particle sizes of 1and 2allowed for agood comparison of their
relative electrical performance.The electrical conductivity of
1(Figure 4a)increases four-fold from 840 Sm¢1at 300 Kto
3500 Sm¢1at 550 K. Themagnitude of the values for 1can
be attributed to the high crystallinity,small band gap,
surfactant-free particle surface,microstructural orientation,
and the high level of sintering and densification achieved. By
contrast, 2exhibited electrical conductivity increasing from
55 Sm¢1at 300 Ktoonly 250 Sm¢1at 550 K; more than
an order of magnitude lower than 1.
Figure 2. Characterization of SnSe nanostructuressynthesized after
1min:a,b)TEM image and corresponding SAED pattern along the
[100] zone axis of aSnSe truncatednanoplate; and c, d) structure
models of individual SnSe nanoplates with and without truncation,
respectively,established on the basis of the detailed TEM character-
ization.
Figure 3. Characterization of SnSe pellet 1:a)profile plot for 1from
Rietveld refinement against PXD data;b)SEM image of the surface of
1;c)TEM image of aSnSe peeled nanoplate and its corresponding
SAED pattern along the [100] zone axis from the circled area;
d) HRTEM image of part of the plate shown in (c) with d-spacings
indicated.
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Thecontrast in the variation in the Seebeck coefficient
with temperature for 1and 2is striking (Figure 4b). Sfor
1increases almost linearly with temperature (250 mVK¢1at
room temperature to 340 mVK¢1at 550 K). By comparison, 2
shows n-type behavior at room temperature (S
¢150 mVK¢1), with the value of Sbecoming positive (p-type
behaviour) at 490 Kand rising to 80 mVK¢1at 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.Ann/p or p/n inversion with increasing
temperature has also been observed in pellets consolidated
from PbTe, Ag2Te,and PbTe0.1Se0.4S0.5 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,respectively.[7b] It is also
notable that both sand Sincrease 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 1require further investigation, the
combination of superior svalues coupled with high values
of Sleads to exceptional power factors ( 0.05 mW m¢1K¢2at
300 Kto0.40 mWm¢1K¢2at 550 K; Figure 4c). In contrast,
the power factors for 2are much lower, (0.001 mW m¢1K¢2at
300 Kand reaching only 0.05 mW m¢1K¢2at 550 K). Thehuge
differences in performance between 1and 2further empha-
size the importance of the surfactant-free synthesis route,not
just in the context of asimpler, more sustainable synthesis
method, but also in delivering significantly improved elec-
trical properties consistently (Figure S15). Notably,the power
factors for 1far exceed those for un-doped polycrystalline
SnSe across asimilar temperature range (0.028–
0.04 mWm¢1K¢2),[5c–e] and are comparable to those for hole-
doped 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 involv-
ing systematic hole doping,inconjunction with surfactant-
free nanostructuring approaches,should yield even higher
performing p-type SnSe materials and pave the way for one-
pot synthesis of p- and n-type SnSe nanomaterials.
In summary,asimple,quick, surfactant-free,and energy-
efficient solution synthesis yielded SnSe nanoplates in gram
quantities.The ensuing nanostructured pellets exhibited
exceptional electrical conductivity coupled with high Seebeck
coefficients,leading to power factors surpassing those of
polycrystalline and surfactant-coated counterparts.The tech-
nique should be readily adaptable to include dopants and
amenable to the discovery of further materials,both p- and n-
type,with enhanced thermoelectric properties.
Experimental Section
Full experimentaldetails are provided in the SupportingInformation.
Materials Synthesis.100 mmol NaOH and 10 mmol SnCl2·2H2O
were added into 50 mL deionizedwater to yield atransparent
Na2SnO2solution. 50 mL of NaHSe(aq) preparedfrom Se and NaBH4
was injected into the boiling solution, leading to the immediate
formationofablack precipitate.The mixture was boiled for 2h,and
cooled to room temperature under Ar(g) on aSchlenk line.The
products were washed with deionized water and ethanol and dried at
5088Cfor 12 h. Scaled-up syntheses were performed with six-fold
precursor concentrations (94(1)% yield). Forthe surfactant-assisted
synthesis,50gcitric acid was introduced into SnCl2solution with no
addition of NaOH and the reaction duration was increased to 24 h.
Materials Characterization and Testing.PXD was performed
using aPANalyticalX’pert Pro MPD diffractometerinBragg–
Brentano geometry (Cu Ka1radiation, l=1.5406 è). Rietveld
refinementwas performedusing the GSAS and EXPGUI software
packages,[19] with the previously published SnSe structureasarefer-
ence.[20] Imaging and elemental analysis were performedbySEM
(Carl Zeiss Sigma, at 5and 20 kV respectively)equippedwith EDS
(Oxford Instruments X-Max 80). Further imaging and SAED was
conducted by TEM (JEOL 2011, operated at 200 kV). TheSeebeck
coefficient and electrical conductivityof1and 2were measured using
aLinseis LSR-3 instrument from 300–550 K. Pellets were pressed in
agraphite die under Ar (uniaxial pressure of 60 MPa;50088C;
20 min).
Acknowledgements
This work was financially supported by the EPSRC (EP/
K022156/1). Theauthors thank Peter Chung for assistance
with SEM. SRP and JWGB acknowledge the Leverhulme
Trust RPG 2012-576.
Keywords: nanomaterials ·structures ·synthesis ·
thermoelectrics ·tin selenide
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Received:February 9, 2016
Revised: March 23, 2016
Published online: April 20, 2016
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6437Angew.Chem. Int.Ed. 2016,55,6433 –6437 Ó2016 Wiley-VCH Verlag GmbH &Co. KGaA, We inheim www.angewandte.org