Chlorine-Enabled Electron Doping in Solution-Synthesized SnSe Thermoelectric Nanomaterials

Abstract

An aqueous solution method is developed for the facile synthesis of Cl-containing SnSe nanoparticles in 10 g quantities per batch. The particle size and Cl concentration of the nanoparticles can be efficiently tuned as a function of reaction duration. Hot pressing produces n-type Cl-doped SnSe nanostructured compacts with thermoelectric power factors optimized via control of Cl dopant concentration. This approach, combining an energy-efficient solution synthesis with hot pressing, provides a simple, rapid, and low-cost route to high performance n-type SnSe thermoelectric materials.

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Chlorine-Enabled Electron Doping in Solution-Synthesized
SnSe Thermoelectric Nanomaterials
Guang Han, Srinivas R. Popuri, Heather F. Greer, Lourdes F. Llin, Jan-Willem G. Bos,
Wuzong Zhou, Douglas J. Paul, Hervé Ménard, Andrew R. Knox, Andrea Montecucco,
Jonathan Siviter, Elena A. Man, Wen-guang Li, Manosh C. Paul, Min Gao, Tracy Sweet,
Robert Freer, Feridoon Azough, Hasan Baig, Tapas K. Mallick, and Duncan H. Gregory*
DOI: 10.1002/aenm.201602328
precursors,[4] doping proves challenging
for solution-synthesized MC nanostruc-
tures.[5] Recently, post-synthesis halide
treatment of nanocrystals in solution has
been developed which involves switching
halogens for long chain surfactant mol-
ecules absorbed on the surface.[2,6] In fact,
sorption of halogens can be realized as
part of a one-pot synthesis using metal
halide precursors.[7] Although this strategy
was initially developed for passivation of
MC quantum dots against oxidation,[2,6,7]
annealing or hot pressing halogen-coated
nanoparticles allows halides to diffuse into the MC lattice and
substitute for chalcogenide anions.[8] However, controlling
doping levels is not straightforward and such methods can
introduce rather high halide concentrations in small nanocrys-
tals leading to reduced electrical conductivity.[8b] Hence exerting
control over dopant concentration without sacrificing electrical
performance is imperative.
Thermoelectrics realize direct interconversion between
thermal and electric energy, thus providing an important route
An aqueous solution method is developed for the facile synthesis of Cl-con-
taining SnSe nanoparticles in 10 g quantities per batch. The particle size and
Cl concentration of the nanoparticles can be efficiently tuned as a function of
reaction duration. Hot pressing produces n-type Cl-doped SnSe nanostruc-
tured compacts with thermoelectric power factors optimized via control of Cl
dopant concentration. This approach, combining an energy-efficient solution
synthesis with hot pressing, provides a simple, rapid, and low-cost route to
high performance n-type SnSe thermoelectric materials.
Doping plays a vital role in modifying the electronic properties
of semiconductors and is pivotal for (opto)electronics,[1] photo-
voltaics (PV),[2] and thermoelectrics.[3] Metal chalcogenides
(MCs) form a diversity of functional materials well-suited to
such applications. Halogen doping in MCs has proven effec-
tive to realize n-type conducting behavior and tune carrier
concentrations.[2–4] Enhanced thermoelectric and PV perfor-
mance can result.[2–4] While halogens can be readily doped into
bulk MCs by high-temperature synthesis using metal halide
Dr. G. Han, Prof. D. H. Gregory
WestCHEM
School of Chemistry
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 and Recovery
School of Engineering and 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
Dr. L. F. Llin, Prof. D. J. Paul, Prof. A. R Knox, Dr. A. Montecucco,
Dr. J. Siviter, Dr. E. A. Man, Dr. W.-g. Li, Dr. M. C. Paul
School of Engineering
University of Glasgow
Glasgow G12 8QQ, UK
Dr. H. Ménard
Sasol (UK) Ltd
St Andrews, Fife KY16 9ST, UK
Prof. M. Gao, Dr. T. Sweet
School of Engineering
Cardiff University
Cardiff CF24 3AA, UK
Prof. R. Freer, Dr. F. Azough
School of Materials
University of Manchester
Manchester M13 9PL, UK
Dr. H. Baig, Prof. T. K. Mallick
Environment and Sustainability Institute
University of Exeter
Penryn Campus, Penryn TR10 9FE, UK
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to produce useful electricity from waste heat and to perform
refrigeration (via the Seebeck and Peltier effects, respectively).[9]
SnSe is a layer-structured MC semiconductor and potentially
useful thermoelectric material given its excel-
lent energy conversion efficiency, relatively
low toxicity, and the high earth-abundance of
the component elements.[10] Most research to
date has concentrated on p-type SnSe.[10,11]
Conversely n-type SnSe is difficult to achieve;
only I and BiCl3 have been used successfully
to dope bulk SnSe with electrons. Moreover,
high-temperature, energy-intensive processes
are needed to achieve this.[12] There are no
reports of solution-synthesized SnSe nano-
structures with tunable n-type conducting
behavior. Before the potential of SnSe can be
fully realized, it is critical to develop a cost-
effective and large-scale synthesis of high
performing n-type SnSe, to complement
existing p-type materials.
In this study, we demonstrate the intro-
duction of Cl to SnSe nanoparticles by a
one-pot in situ solution approach to prepare
>10 g of doped SnSe nanoparticles on short
timescales (Scheme 1; Figure S1, Supporting
Information). The strategy exploits the nucle-
ophilic nature of the halide anion and the
electrophilicity of coordinatively unsaturated
metal cations at the nanoparticle surface,[6]
coupled with the acidic conditions that pro-
mote the formation of metal–halide bonds by
ligand replacement.[8a] The simple solution
synthesis is achieved using aqueous SnCl2
both as reactant and Cl source and citric
acid both as surfactant to restrict particle
growth and means to control pH. Controlling
the reaction duration allows us to engineer
nanoparticle size and regulate the Cl dopant
level. The nanoparticles can be hot-pressed
into Cl-doped SnSe dense pellets with controllable dopant con-
centration and consistent n-type conducting behavior.
Injection of an NaHSe aqueous solution into an SnCl2 solu-
tion (26:1 molar ratio of citric acid:SnCl2) leads to the imme-
diate formation of an SnSe precipitate (Equation (1))
+→++Na
HSeSnClSnSeNaClHCl
2 (1)
Boiling the suspension for 2 h generates crystalline, phase-
pure SnSe nanoparticles. Powder X-ray diffraction (PXD) pat-
terns can be exclusively indexed to orthorhombic SnSe (ICDD
card No. 48–1224).[13] Rietveld refinement against PXD data
(Figure 1a; Tables S1 and S2, Supporting Information) confirms
that the single phase SnSe product crystallizes in orthorhombic
space group Pnma, with a = 11.5424(8) Å, b = 4.1775(4) Å, and
c = 4.3841(5) Å. Scanning electron microscopy (SEM) images
(Figure S2a,b, Supporting Information) reveal that the product
is an agglomeration of nanoparticles with individual sizes of
15–55 nm. Energy dispersive X-ray spectroscopy (EDS) spectra
(Figure S2c, Supporting Information) taken across the samples
as point and area scans consistently generate Sn:Se:Cl atomic
ratios of 50.6(5):48.8(5):0.6(1). The existence of Cl should result
from the interaction of nucleophilic Cl− and electrophilic Sn2+
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Scheme 1. The strategy to fabricate n-type SnSe nanostructured pellets
utilizes Cl concentration and nanoparticle size from solution-synthesis.
Figure 1. Characterization of SnSe nanoparticles synthesized after 2 h. a) Profile plot from Riet-
veld refinement against PXD data. b) TEM image and corresponding SAED pattern collected
from the nanoparticles. c) HRTEM image of an individual nanoparticle with the (201) d-spacing
indicated. d) EDS mapping of a nanoparticle cluster. The four panels in (d) are (clockwise from
top left) high angle annular dark field (HAADF) image, elemental mapping for Sn (red), Cl
(yellow), and Se (green).

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at the nanoparticle surface[6] together with the replacement
of ligated citric acid by Cl− in the acidic environment during
solution heating.[8a] Transmission electron microscopy (TEM)
images (Figure 1b; Figure S2d, Supporting Information) con-
firm that SnSe nanoparticles assemble into clusters with an
average individual particle size of ≈35 nm. Selected area electron
diffraction (SAED) patterns (Figure 1b) collected from a section
of such a cluster reveal the polycrystalline nature of the SnSe
particles. High-resolution TEM (HRTEM) images (Figure 1c)
nevertheless demonstrate the single crystalline nature of indi-
vidual nanoparticles. Elemental mapping (Figure 1d) confirms
the existence and uniform distribution of Sn, Se, and Cl in the
nanoparticles.
Controlling the synthesis duration can predetermine both
the particle size and Cl content of the SnSe nanoparticles. To
illustrate this, we synthesized materials from 1 min, 5 min, and
24 h of solution heating. PXD (Figures S3 and S4; Table S1,
Supporting Information) reveals each product to be single-
phase with the orthorhombic SnSe structure. The refined cell
volumes increase slightly with reaction time and the Bragg half-
widths decrease gradually as the reaction duration increases,
indicating likely crystallite growth. SEM (Figure S5a–f, Sup-
porting Information) and TEM (Figure 2a,b: Figure S6a,
Supporting Information) images show that the products are
composed of nanoparticles, and the average particle size
increases from ≈25 nm through ≈30 to ≈50 nm, as heating is
extended. EDS (Figure S5g–i, Supporting
Information) confirms the existence of Cl
in all the samples and shows increased
Cl levels for shorter reaction times; spe-
cifically, the Sn:Se:Cl atomic ratios are
48.2(5):50.5(5):1.3(2), 51.6(5):47.7(5):0.7(1),
and 51.5(5):48.2(5):0.3(1) for nanoparticles
synthesized after 1 min, 5 min, and 24 h,
respectively. SAED patterns (Figure 2a,b;
Figure S6b, Supporting Information) con-
firm the polycrystalline nature of the SnSe
nanoparticles. Furthermore, HRTEM
(Figure 2c; Figure S6c,d, Supporting Infor-
mation) reveals that the products synthe-
sized after 1 and 5 min have relatively poor
crystallinity, and nanoparticles with sizes of
2–4 nm are attached on the surface of larger
particles. When the reaction time is increased
from 2 to 24 h (Figure 2d), individual parti-
cles become single crystalline. This suggests
that the aggregation and coalescence of small
nanoparticles leads to the formation of larger
single crystalline nanoparticles.[14]
The ability to prepare SnSe nanomate-
rials in >10 g quantities allowed the facile
fabrication of SnSe pellets via hot pressing
without the variations in sample morphology
that could ensue from repeated sample
preparation. Pellets with ≈95% of the SnSe
theoretical density, consolidated from 2 h
solution-synthesized powder, were obtained
(denoted 1). Rietveld refinement (Figure S7;
Table S3, Supporting Information) shows
that the pellets are composed principally of orthorhombic SnSe
(79.1(1) wt%) but also of two minority phases of trigonal SnSe2
(11.1(3) wt%) and tetragonal SnO2 (9.8(2) wt%). This indicates
that a small proportion of SnSe was oxidized to SnO2 and SnSe2
during the hot pressing process (2SnSe + O2 → SnO2 + SnSe2).
A series of subsequent experiments (see Figures S8–S15;
Tables S4–S6, Supporting Information) confirmed that the oxi-
dation was less in nanomaterials prepared at longer reaction
durations. Both particle size (and by inference surface area) and
the relative amount of surface carboxyl groups could be traced
as contributors to the oxidation process.
Preferred orientation of both SnSe and SnSe2 crystallites is
evidenced by the increased intensity of the (h00) and (00l) reflec-
tions respectively in PXD patterns from the pellets (Figure S16,
Supporting Information). This suggests that the longest crystal-
lographic axes of the respective cells align parallel to the hot
pressing direction. Considering that the magnitude of the elec-
trical conductivity is highest perpendicular to these long axes
in both SnSe and SnSe2,[10a,15] such an orientation should be
beneficial in enhancing electrical conductivity perpendicular to
the hot pressing direction of the pellets.
SEM and TEM images (Figure S17a,b and S18a, Supporting
Information) show that 1 is composed of densely packed plates
with almost uniformly distributed particles. EDS (Figure S17c–g,
Supporting Information) confirms the existence and uniform
distribution of Cl in 1. SAED patterns (Figure S18b, Supporting
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Figure 2. TEM characterization of SnSe nanoparticles synthesized after a,c) 1 min and b,d) 24 h.
a,b) TEM images and corresponding SAED patterns collected from the particles. c,d) HRTEM
images of individual nanoparticles with selected d-spacings indicated.

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Information) taken from a number of plates and nanoparticles
from 1 confirm the presence of the above-mentioned three
phases: SnSe, SnSe2, and SnO2. HRTEM images demonstrate
that the predominant plate-like structures in 1 are crystalline
SnSe (Figure S18c, Supporting Information). High-resolution
images also show that some of the smaller irregular nano-
plates are formed by SnSe2 (Figure S18d, Supporting Informa-
tion) while some nano particles of SnO2 are distributed among
the SnSe plates (Figure S18e, Supporting Information). Ther-
mogravimetric-differential thermal analysis of 1 under argon
(Figure S19, Supporting Information) shows negligible weight
loss below 500 °C, but reveals that decomposition begins above
this temperature and corresponds to an endothermic Se subli-
mation process.
For comparison, the nanomaterials prepared over 1 min,
5 min, and 24 h were also hot pressed into pellets (denoted
2, 3, 4, respectively) using the same processing parame-
ters, achieving ≈85%, ≈90%, and ≈92% of the SnSe theoretical
density, respectively. Rietveld refinement against PXD data
(Figure S20; Tables S7–S9, Supporting Information) shows
that the SnSe phase fraction increases from ≈71 through 72 to
90 wt% for 2, 3, and 4, respectively, again indicating a direct
correlation between synthesis time (and particle size/surface
citric acid amount) and the tendency to oxidation. HRTEM
on 3 confirms that SnO2 nanoparticles are distributed in close
proximity to the SnSe plates (Figure S21, Supporting Infor-
mation). EDS (Figure S22, Supporting Information) confirms
that the pellets contain Cl at levels consistent with the corre-
sponding solution synthesized SnSe nanoparticles, suggesting
no loss during the hot pressing process. X-ray photoelectron
spectroscopy (XPS) was used to verify the presence of chlorine
and analysis of 3 (Figure S23, Supporting Information) shows
that the peaks at 200.5 and 198.9 eV can be assigned to Cl
2p1/2 and Cl 2p3/2 states, respectively, indicating that Cl exists
in the form of Cl−.[7a,16] This implies electron doping indeed
originates from the halide on substitution for Se2−.[16] The
indirect optical bandgap from diffuse reflectance (DR) UV–Vis
spectra (Figure S24, Supporting Information) narrows from
≈0.8 through ≈0.75 to ≈0.7 eV, when the Cl concentration is
increased from ≈0.3% (4) through ≈0.7%–0.6% (3, 1) to ≈1.3%
(2), respectively. This insinuates that the indirect bandgap
of SnSe is reduced slightly but significantly by increased Cl
doping. A similar bandgap narrowing was observed in I-doped
SnSe and is expected to improve the electrical conductivity in
SnSe.[12a]
We selected 1, 3, and 4 for electrical measurements due to the
relatively low percentage of SnO2 and SnSe2 components and
the high density achieved (≥90%). Hall measurements (Table 1)
give a clear correlation between the Cl and carrier concentra-
tions (where the majority carriers are electrons). 1 and 3 have
higher carrier concentrations than 4 due to the twofold increase
in Cl content, while 3 has a lower carrier concentration than
1, which is probably related to the higher level of impurities.
The contrast in the temperature-dependent Seebeck coefficient
(S) for the different pellets is striking (Figure 3a; Table 1). The
variation in the absolute value of S with temperature for 1 and
3 is very similar increasing from 300 K to reach a maximum at
≈410–425 K before decreasing by 540 K. The decreasing value
of S at higher temperature could be due to thermal excitation
of minority carriers (holes) that are related to intrinsic defects
in SnSe (e.g., Sn vacancies)[17] as manifested by the significantly
enhanced electrical conductivity (Figure 3b). The absolute value
of 1 is slightly lower than that of 3 at 300 K, but S for both 1 and
3 are negative within the whole temperature range, indicating
n-type behavior consistent with Cl doping. By comparison, 4,
with the lowest Cl doping level, shows n-type behavior at 300 K
and transforms to p-type behavior at ≈475 K with S ≈ 75 µV K−1
at 540 K. The n- to p-type transition could also be related to the
thermal excitation of holes at high temperature. We also note
that the impurity phases, SnSe2 and SnO2, are both intrinsic
n-type semiconductors.[16,18]
The electrical conductivity (
σ
) of 1 (Figure 3b) increases from
≈255 S m−1 at 300 K to ≈910 S m−1 at 540 K. With a similar
Cl doping concentration and indirect bandgap to 1, the
σ
of 3
is slightly lower (from ≈185 S m−1 at 300 K to ≈685 S m−1 at
540 K), probably due to the increase in the less conductive SnSe2
and SnO2 components,[16,18b] together with the increased car-
rier scattering from SnO2 nanoparticles. This is consistent with
the higher carrier concentration and mobility in 1 (Table 1). By
contrast, with the lowest Cl doping level, 4 exhibits the lowest
carrier concentration and
σ
among the three pellets, although
it is the least oxidized. In fact, 1 and 3 demonstrate higher
σ
values than bulk I-doped SnSe materials with similar doping
concentrations within the same temperature range (e.g.,
σ
for
SnSe0.98I0.02 increased from ≈0.23 S m−1 at 300 K to ≈105 S m−1
at 565 K) and comparable
σ
to Bi and Cl codoped materials
(e.g., SnSe0.95-0.2 mol% BiCl3 yielded
σ
values of ≈1170 S m−1
at 300 K and ≈1815 S m−1 at 560 K).[12]
In an attempt to understand the possible origins of the
n-type conducting behavior more fully, we also measured the
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Table 1. A summary of Cl concentration, phase fraction, electrical properties, n–p transformation temperature, room temperature Hall carrier concen-
tration (nH), and mobility (
µ
H) of the pellets.
Pellet at% Cl S300 K
[µV K−1]
σ
300 K
[S m−1]
Tn–p
[K]
wt% SnO2wt% SnSe2nH
[1018 cm−3]
µ
H
[cm2 V−1 s−1]
1 0.6(1) −265 255 –a) 9.8(2) 11.1(3) 6.43 3.60
3 0.7(1) −295 185 – a) 13.0(2) 15.3(3) 3.47 2.66
4 0.3(1) −145 55 475 3.5(2) 6.1(2) 2.56 0.85
SF1 0.3(1) −175b) 55b) 520 7.9(1) 6.7(2) 1.68 2.14
SF2 0.1(1) −95b) 50b) 400 5.5(1) 6.8(2) 1.24 1.62
a)No transition below 550 K; b)Values obtained at 325 K.

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thermoelectric performance of two pellets synthesized using
hydrochloric acid in place of citric acid (SF1 and SF2; see Sup-
porting Information) for comparison (Figure 3; Table 1). EDS
shows that these surfactant-free samples have larger particle size
and lower Cl concentrations compared to their citric acid “ana-
logues,” 1 and 4. (The Sn:Se:Cl ratios are 51.7(5):48.0(5):0.3(1)
and 51.5(5):48.4(5):0.1(1) for SF1 and SF2, respectively;
Figure S14, Supporting Information). Three notable compari-
sons can be made: (1)SF2 contains slightly more SnO2 and
SnSe2 than 4 but contains less dopant Cl. SF2 has a lower room
temperature carrier concentration and
σ
, slightly lower magni-
tude Seebeck coefficient and transforms from n- to p-type at a
lower temperature than 4; (2) SF1 has a similar Cl concentration
but notably contains more SnO2 than 4. SF1 transforms from
n- to p-type at a higher temperature than 4; (3) 1 and 3 remain
n-type below 550 K with similar Seebeck coefficients, although
the electrical conductivity of 1 is higher than 3. Although there
are likely to be other contributing factors, the results indicate
that as Cl doping levels increase so does the electrical conduc-
tivity and the temperature of the p–n transition, both obser-
vations being consistent with a higher number of negative
charge carriers. Nevertheless, the presence of SnO2 (and SnSe2)
clearly also has an effect on the electrical properties, apparently
reducing the conductivity and increasing the temperature of the
n–p transition. These observations would certainly be consistent
with the presence of SnO2 as a wide band gap, n-type semicon-
ductor (Eg = 3.6 eV; typically
σ
≥ 4 S m−1, S ≈ −200 µV K−1 at
≈300 K, depending on oxygen vacancy concentration).[18] (SnSe2
has a gap of 1.6 eV,[19]
σ
of ≈170 S m−1,[16] S ≈ −238 µV K−1 at
≈300 K.[20]) Moreover, controlled doping of Cl− is clearly very
effective in producing high performance n-type SnSe, but oxide
impurities need to be minimized to optimize this performance.
It is also worth noting that the absence of surfactant in the prep-
aration of SF1 and SF2 ultimately leads to significant improve-
ments in
σ
, especially at higher temperature (cf. 4).[11d]
The combination of better
σ
values coupled with high values
of S leads to higher power factors (S2
σ
) in 1 (≈0.018 mW m−1 K−2
at 300 K to ≈0.068 mW m−1 K−2 at 530 K) (Figure 3c). 3 shows
slightly lower values than 1 (S2
σ
≈ 0.016 mW m−1 K−2 at 300 K
and ≈0.054 mW m−1 K−2 at 525 K) as noted above. In contrast,
the S2
σ
values for 4 are much lower (≈0.001 mW m−1 K−2 at
300 K and reaching only ≈0.002 mW m−1 K−2 at 540 K). SF1
and SF2 achieve similar S2
σ
values at room temperature where
they are both n-type, whereas SF2 has a higher power factor at
525 K (where it is p-type). The contrast in performance between
samples underscores the importance of being able to tune the
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Figure 3. Electrical properties of SnSe pellets 1, 3, 4, SF1, and SF2 measured perpendicular to the hot pressing direction: a) the Seebeck coefficient,
b) the electrical conductivity, and c) the power factor as a function of temperature.

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degree of Cl doping and to control the pellet phase composi-
tion during fabrication. It is especially notable that the power
factor for 1 compares very favorably with those for I-doped
SnSe (e.g., SnSe0.98I0.02, with S2
σ
of ≈0.016 mW m−1 K−2 at
565 K) and co-doped SnSe (e.g., SnSe0.95-0.2 mol% BiCl3, with
an S2
σ
of ≈0.104 mW m−1 K−2 at 515 K) bulk materials with
similar doping levels within the same temperature range.[12]
If oxidation could be reduced, it might conceivable to surpass
such values. Hence, it is achievable to produce high performing
n-type SnSe materials in bulk quantities via energy-efficient,
sustainable methods (Figure S25, Supporting Information). In
principle, it should be possible to produce new co-doped SnSe
nanomaterials controllably (e.g., with both Bi- and Cl-dopants
among others) with only minor adaptations to the present syn-
thesis method.
Preliminary thermal conductivity measurements (
κ
) per-
formed on 1 and 4 along the direction parallel to pressing
(Figure S26, Supporting Information) are also encouraging.
κ
for 1 (4) decreases from ≈0.89 (≈0.72) W m−1 K−1 at 300 K
to ≈0.62 (≈0.40) W m−1 K−1 at 540 K. The higher
κ
for 1 could
be due to its higher percentage of more thermally conductive
SnO2. However,
κ
values for both 1 and 4 are still relatively low
compared to other examples of n-type polycrystalline SnSe and
to p-type single crystals (Figure S26d,e, Supporting Informa-
tion). This could be due to enhanced phonon scattering either
from the SnO2 nanoinclusions in these materials or as a result
of the nanostructuring of SnSe itself.
In summary, a simple, quick, low-cost solution synthesis
produces Cl-containing SnSe nanoparticles in gram quantities
(>10 g per run for a 2 h growth). Such nanoparticles have been
consolidated into n-type Cl-doped SnSe nanostructured pel-
lets, whose thermoelectric power factors can be significantly
improved by optimizing the Cl doping level. This study not
only provides a convenient method for the large-scale synthesis
of SnSe nanostructures, but also demonstrates a facile and reli-
able route to engineer n-type SnSe with well-defined doping
concentration. Considering also that p-type SnSe can be syn-
thesized by a very similar method,[11d] the way is clear toward
a unified, cost-effective processing route to large quantities
of both the constituent materials needed for a thermoelectric
device.
Experimental Section
Full experimental details are provided in the Supporting Information.
Materials Synthesis: 260 mmol citric acid and 10 mmol SnCl2·2H2O
were added into 50 mL deionized water (DIW) to yield a transparent
solution that was heated to boil. 50 mL of freshly prepared NaHSe(aq)
was promptly injected into the boiling solution. The solution was boiled
for 2 h and cooled to room temperature under Ar(g) on a Schlenk line.
The products were washed with DIW and ethanol and dried at 50 °C
for 12 h. Scaled-up syntheses were performed with 5.5-fold precursor
concentrations. The yield was 96(1)% of theoretical production. To tune
the particle size and Cl level, samples synthesized over 1 min, 5 min,
or 24 h durations were also prepared. For the surfactant-free synthesis,
4 mL hydrochloric acid was introduced into SnCl2 solution in place
of citric acid. Pellets were pressed in a graphite die under Ar (uniaxial
pressure of ≈60 MPa; 500 °C; 20 min).
Materials Characterization and Testing: PXD data were recorded
by a PANalytical X'pert Pro MPD diffractometer in Bragg-Brentano
geometry (Cu K
α
1 radiation,
λ
= 1.5406 Å). Rietveld refinement was
performed against PXD data using the GSAS and EXPGUI software
packages.[21] Imaging and elemental analysis were performed by SEM
(Carl Zeiss Sigma, at 5 and 20 kV, respectively) equipped with EDS
(Oxford Instruments X-Max 80). Further imaging, elemental analysis
and SAED were conducted by TEM (FEI Titan Themis 200 and JEOL
JEM-2011, operated at 200 kV). Optical bandgaps were measured by
DR-UV–Vis spectroscopy (Shimadzu, UV-2600). The Seebeck coefficient
and electrical conductivity of pellets were measured using a Linseis
LSR-3 instrument from 300 to 540 K. Thermal diffusivity (D) of pellets
was measured using a Linseis LFA 1000 instrument within the same
temperature range and thermal conductivity (
κ
) was calculated using
κ
=
DCp
ρ
, where Cp and
ρ
are specific heat capacity and density, respectively.
Hall measurements were performed on a nanometrics HL5500 Hall
system using a Van der Pauw configuration. The XPS experiments were
performed using a Kratos Axis Ultra-DLD photoelectron spectrometer
with an Al monochromatic X-ray source.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was financially supported by the EPSRC (EP/K022156/1).
The authors thank Peter Chung for assistance with SEM and Jialu
Chen for assistance with TEM elemental mapping. S.R.P. and J.-W.G.B.
acknowledge the EPSRC for support (EP/N01717X/1). H.F.G. and W.Z.
acknowledge the EPSRC for the Equipment Grant to purchase Titan
Themis 200 microscope (EP/L017008/1).
Received: October 21, 2016
Revised: January 5, 2017
Published online: February 20, 2017
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