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JNanopart Res (2021) 23:138
https://doi.org/! 0.1007/s11051-021-05244-y
R E S E A R C H P A P E R
On thermal stability of nanocrystalline Ag-Cu-S powders
Jiří Sopoušek • Dáša Drenčaková • Pavel Brož •
Jiří Buršík • Adéla Zemanová • Pavla Roupcová
Received: 30 December 2020 / Accepted: 23 May 2021 / Published online: 30 June 2021
© The Author(s), under exclusive licence to Springer Nature B.V. 2021
Abstract The nanocrystalline semiconducting compounds
based on the AgCuS system are considered
as low-cost candidates of thermoelectric materials
with improved thermal stability. The nanocrystalline
A g - C u — S powders were prepared from metal
nitrates and sulphur powder in tetraethylene glycol
(TEG) solvent by reductive agent N a B H 4 . The crystallite
sizes of the observed phases were in the range
between 60 and 80 nm. The chemical compositions
of the as-received samples were analysed by the
ICP-AES method and their phase compositions were
evaluated by X R P D . The investigation was supplemented
by D S C and in situ H T X R D thermal analysis.
A more detailed in situ experiment was performed for
J. Sopoušek (El) • D. Drenčaková • P. Brož
Faculty of Science, Department of Chemistry, Masaryk
University, Kotlářská 2, 611 37 Brno, Czech Republic
e-mail: sopousek@mail.muni.cz
J. Sopoušek • P. Brož
Central European Institute of Technology, CEITEC,
Masaryk University, Kamenice 753/5, 625 00 Brno,
Czech Republic
J. Buršík • A. Zemanová • P. Roupcová
Institute of Physics of Materials, Academy of Sciences
of the Czech Republic, Žižkova 22, 616 62 Brno,
Czech Republic
P. Roupcová
Brno University of Technology, Central European
Institute of Technology, Purkyňova 123, 612 00 Brno,
Czech Republic
a sample containing ternary phases. The nucleation
and growth of silver micro-wires were first observed
on the substrate involving ternary phases at isothermal
heat treatment. The formation of silver wires and
semi-conductive ternary thermoelectric phase (stromayerite
x3) is explained by observed phase transformation.
The obtained results are complemented by
microscopy ( L M , S E M , TEM).
Keywords Nanoparticles • Thermoelectrics • D S C •
In situ X R P D • Stromayerite • Silver wires
Introduction
Awareness of the behaviour of minerals based on
the A g - C u - S system has a deep history. The minerals
that make up sulphur with silver and copper have
been of interest since the early days of mining and
metallurgy.
Scientific research in our sense began at the beginning
of the twentieth century, when the pseudo-binary
diagram A g 2 S - C u 2 S (Friedrich 1907) was observed
by K . Friedrich. A thorough examination of the
pseudo-binary diagram and extension to the ternary
diagram of A g - C u - S was performed later by Harlov
(Harlov and Sack 1995).
The ternary phases of the A g - C u - S system are
already described by Djurle et al. in reference (Djurle
et al. 1958) and in particular by Skinner (Skinner 1966;
Skinner et al. 1966). The latest summary information
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138 Page 2 of 14 J Nanopart Res (2021) 23:138
including a crystallographic description of phases,
thermodynamic properties and several isothermal
phase diagrams was given in 2006 by G. Effenberg
(Materials Science International Team MSIT® 2006).
The pseudo-binary diagram of Ag2 S-Cu2 S is shown
in Fig. 1 and an isothermal section of the A g - C u - S
system is shown in Fig. 2. Unfortunately, the complex
assessment of the A g - C u - S system by the usual
C A L P H A D method (Saunders and Miodownik 1998)
has not been carried out yet.
The renaissance of chalcogenide-based materials
has occurred in connection with the search for
thermoelectric materials capable of converting thermal
energy into electrical energy (He and Tritt 2017;
Liang et al. 2017; Zheng et al. 2018). New promising
bulk thermoelectrics: intermetallics, pnictides and
chalcogenides, are described in Goncalves and Godart
(2014). The object of the thermoelectric search is
mainly metal chalcogenides for their cheapness and
nontoxicity (Goncalves and Godart 2014; Lokhande
et al. 2016). Promising thermoelectrics are also Cuchalcogenide-based
materials (Qiu et al. 2016).
The empirical parameter "figure of merit Z T "
was introduced for a general assessment of thermoelectric
properties of studied materials. The value
of Z T had been observed for copper chalcogenide
thermoelectric materials in the range of 0-2.7 (Wei
et al. 2019). The high Z T does not mean that the
material is a practically usable thermoelectric neither
if the colossal Seebeck effect measured for Cu2 Se
(Byeon et al. 2019).
The ternary phases of the A g - C u - S system are
semiconductors. A more detailed study of the structure
and lattice dynamics of copper- and silver-based
superionic conducting chalcogenides and especially
the properties of the jalpaite ternary phase were
investigated by Trots and co-workers (Trots et al.
2008). The ternary phase of T 3 stromayerite has been
shown to be a thermoelectric material although not
having a very high Z T factor.
To study pure ternary phases, syntheses of complex
sulphides of AgCuS and A g 3 C u S 2 from the elements
under hydrothermal conditions were proposed
by Tokuhara (Tokuhara et al. 2009). Attention is
also paid to the synthesis and research of the properties
of bulk materials based on binary systems A g - S
(Zivkovic et al. 2013) and Cu-S (He et al. 2014; Jiang
et al. 2014).
The recent investigations move from bulk material
research to nanomaterials (Alam and Ramakrishna
2013) and nanocomposites (Liu et al. 2012). It is also
a new impulse to improve thermoelectric materials as
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J Nanopart Res (2021) 23:138 Page 3 of 14 138
Fig. 2 The equilibrium
Ag-Cu-S phase diagram
(Materials Science International
Team MSIT® 2006)
at 25 °C complemented
by overall compositions of
synthetized nanocrystalline
samples referred to in
Table 1
acantite (A$2S)
CuS covelite
[1 jalpaite (Ag}CuS2)
f2 Mckinstcyite (Ag(1.2-x)Cu<0,8+x>S)
'J
5
ß-sttomayerite, flg(l-n)Cu(l+x)S(r),0<=x<l
anilite Cul,75S
(S) sulphur
(Ag) silver
CuS-
T,+T2+€uS\
Of
T,+CuS+5/ 3 * "
1,+CuS+a,
. 4 0
a
-a,
80 ,
'*r
_20
M C u ) i ( A g ) - a 2 /
Cu
J \ / \ / \ / \ / ~'"^>&
20 40 60 80
A g
can be seen in the published report (Kanatzidis 2010).
It appears that nanostructuralization significantly
influences the value of Z T [2010], but according to
most researches, it has been rather decreased values
so far.
The latest investigation of the Ag-Cu-S system
again focuses on monitoring the thermoelectric properties
of the T 3 phase (stromayerite) in the bulk form
(Dutta et al. 2018). However, research is also partly
focused on the study of nanocrystalline powders (Guin
et al. 2016). One of the variants under investigation
is the preparation of nanocrystalline thermoelectrics
modified by graphene (Al Alwani Ammar et al. 2019).
Some conductive polymers have been proposed for
the preparation of these nanotermoelectric composites
(Bharti et al. 2018). The pioneering work was presented,
for example, by the study of properties of CuS
nanocomposite (Tang et al. 2018), but more detailed
experimental studies of other nanocrystalline thermoelectric
composites are still rare.
Experimental section
Materials
The following chemicals were all used as obtained:
tetraethylene glycol ( C 8 H 1 8 0 5 , C A S 112-60-7, 99%
chemical grade, AlfaAesar), sulphur (powder, C A S
7704-34-9, chemical grade: purists p. a., Lachner),
polyvinylpyrrolidone (PVP, C A S 9003-39-
8, chemical grade: for synthesis, Merck), sodium
borohydride ( N a B H 4 , powder, 97% chemical grade
for synthesis, C A S 16940-66-2), silver nitrate
( A g N 0 3 , C A S 7761-88-8, chemical grade: purists
p. a., Lachner), copper(II) nitrate trihydrate ( C u N 0 3
• 3 H 2 0 , C A S 10031^13-3, chemical grade: purists
p. a., Penta), regenerated graphene oxide (rGO, prepared
in the laboratory of Department of Chemistry
according to [lit] Dr. Z . Moravec), water (deionized,
< 1 uS/cm), liquid nitrogen (> 4.5 N , S I A D
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138 Page 4 of 14 J Nanopart Res (2021) 23:138
Rajhrad) and ethanol ( C 6 H 5 O H , denatured with
about 1% methyl ethyl ketone).
Nanoparticle synthesis
The method reported by Guin (Guin et al. 2016) was
adapted. Shortly, the samples of the AgCuS nanoparticles
(AgCuS NPs) were prepared by wet synthesis
from silver and copper nitrate precursors in tetraethylene
glycol by use of N a B H 4 at laboratory temperature.
The synthesized product was separated using
a high-speed centrifuge. The product was purified
by repeated dissolving in ethanol and by subsequent
centrifuging. Some samples were prepared with the
addition of P V P and rGO to the reaction mixture to
stabilize and separate the individual nanoparticles of
AgCuS. The samples in the form of black coloured
AgCuS ethanol suspensions were obtained after purification.
The drying under vacuum at laboratory temperature
was used to obtain dry powder samples.
Instrumentations
Dynamic light scattering (DLS) The sonicated
solutions of the prepared AgCuS samples (at 25 °C)
were characterized by the D L S method on a Zetasizer
Nano ZS Z E N 3500 instrument (Malvern U K ) working
at the scattering angle of 173°. The method measures
a hydrodynamic size of NPs, which is given by
both the metal core and the stabilizing organic layer
of the core.
Overall chemical analysis The chemical composition
of the AgCuS samples was obtained by elemental
analysis. The AgCuS powder was weighed and dissolved
in nitric acid at elevated temperatures. The
Ag, C u and S content was analysed by inductively
coupled plasma atomic emission spectroscopy (ICPOES)
calibrated on pure metals and sulphur. The difference
between the mass of the dry sample and the
weight of the Ag, Cu and S elements was presumed to
be the mass of organic matter.
Transmission electron microscopy ( T E M and
HRTEM) The metal cores of the AgCuS samples
were investigated by electron microscopy because
this technique is sensitive to heavier elements and
less sensitive to light elements that form an organic
stabilizing layer of the particles. To prepare samples,
a powder was placed on a holey carbon film-coated
Cu grid. The size and shape of AgCuS particle cores
were investigated using a Philips C M 12 S T E M
microscope with a thermoemission source operated at
120 k V and a J E O L J E M 21 OOF high-resolution T E M
(HRTEM) with an F E G source operated at 200 k V
(point resolution of 2.3 A ) . Both transmission electron
microscopes were equipped with energy-dispersive
X-ray (EDX) detectors.
X-ray powder diffraction (XRPD) Measurements
were carried out on a G N R Europe X R D
600 diffractometer equipped with a Co lamp,
XKal2= 1.7903 A , 40 kV, 15 mA, with theta/2theta
configuration. Samples were measured in a step
scan of 0.2° for 10 s in reflection mode on plastic
or aluminium sample holders. A I D detector D E C TRIS
Mythen2R was used. If noted in the article,
the accuracy of phase fraction is about 5 mol% and
the complement to 100% are phase impurities. The
X R P D method gives a size of crystallite domains
(abbreviation "cryst." used).
The multipoint B E T method The Quantachrome
Autosorb IQ 3 helium porosimeter and at least five
data points method with relative pressures between
0.05 and 0.23 was used for evaluation of the specific
surface area of the samples.
High-temperature X-ray powder diffraction
(HTXRD) Empyrean PANalytical (Netherlands)
diffractometer with CoKotj 2 radiation and H T K
1200 Anton Paar (Austria) was used for H T X R D .
The diffractograms were collected in a helium (5 N)
floating atmosphere at each temperature during
collecting data. The average heating rate including
sample heating and hold at temperature during
measurement was 5 K/min. The phase composition,
lattice constants of phases and crystallinity were
determined by the Rietveld method used HighScore+4.0
equipped by I C D D database which
included structural models.
Mass spectroscopy with direct input probe (DIP
MS) Measurements were performed on mass spectrometer
TSQ Quantum X L S with the temperature
program: stabilization 30 °C for 30 s, heating 50 °CI
min 30^1-50 °C, ionization energy 70 or 22 eV, and
detector temperature was set to 200 °C.
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J Nanopart Res (2021) 23:138 Page 5 of 14 138
Simultaneous thermal analysis (DSC/TGA) Three
instruments were used. The thermal properties (DSC)
of AgCuS samples were examined by rate 10 K m i n on
a Netzsch STA 409 CD/3/403/5/G apparatus under
flowing (70 c m 3
m i n - 1
) pure (6 N) argon from room
temperature to approx. 800 °C. The oxygen-free
atmosphere was maintained by a metallic zirconium
trap located in the hot zone of the calorimeter. The
T G A / D S C analysis was done by Netzsh STA 449
Jupiter under flowing (70 c m 3
m i n - 1
) pure (4.5 N)
nitrogen from room temperature to approx. 800 °C.
The thermal properties (DSC) of AgCuS samples
were examined on a Netzsch Pegasus 404C apparatus
under flowing (50 c m 3
m i n - 1
) pure (5 N) argon from
room temperature to 800 °C. The samples (approx.
50-80 mg) were always measured in Y2 03 -coated
alumina crucibles covered with a lid.
Scanning electron microscopy (SEM) Morphology
and overall composition of the samples were
observed by scanning electron microscopy (SEM)
using a T E S C A N L Y R A 3 X M U F E G / S E M microscope
with an X-Max 80 E D X Oxford Instruments
detector. Field-emission scanning electron microscope
(FESEM) Thermo Fisher Verios 460L working
in energy range (0.7 nm @ 1 keV, 0.6 nm @
2-30 keV) equipped with E D S analyser E D A X S D D
Octane Super with energy resolution (129 eV @
MnKot).
Results and discussion
The samples in the form of the AgCuS ethanol black
suspensions were investigated by the D L S method.
The measurements revealed big particles and great
fluctuations in hydrodynamic particle size that indicates
nanoparticle aggregation. As a consequence, the
synthesis was modified by the addition of the stabilizing
agents: polyvinylpyrrolidone (PVP) and regenerated
graphene oxide (rGO). The additions (2-3 wt%
of the sample weight) of rGO to suspension decreases
the hydrodynamic size of particles; however, clusters
of the AgCuS nanoparticles still remain in colloid
solution (see Fig. 3).
Size, shape and composition of AgCuS samples
were investigated in detail by electron microscopy
on dried drops of sample suspensions. The samples
were black powders, which all reveal the microstructures
nearly the same as given in Fig. 4. At higher
magnification, powders had a coral-like structure consisting
of aggregated AgCuS nanoparticle units. The
nanoparticle units are joined by interconnection necks
(see detail in Fig. 4). The size of the nanoparticle
unit was in reasonable agreement with the crystallite
size obtained by the powder X-ray diffraction method
(see Table 1). The same morphology of samples was
presented by Guin et al. for nanocrystalline AgCuS
(Guin et al. 2016). The authors have investigated the
nanocrystalline phase, which was entitled AgCuS in
your article (i.e., stromayerite phase T 3 in Fig. 2).
200 nm
Fig. 3 Clusters of aggregated AgCuS nanoparticles. Microstructure of nanocrystalline AgCuS powder (sample 1 with rGO). Overview
in left and detail in right (TEM)
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138 Page 6 of 14 J Nanopart Res (2021) 23:138
PM AS CR. Brno
Fig. 4 Aggregated microstructure of nanocrystalline AgCuS powder (sample 4). Overview in left and detail in right (SEM)
The overall compositions of dry AgCuS samples
have been measured by the ICP-AES method.
The results of the analyses are given in Table 1. The
chemical composition of the AgCuS powders was
tested also by an E D X detector inside an electron
microscope. The E D X compositions of the samples
were in agreement with element analysis in Table 1
but with higher uncertainty.
The phase compositions of the dry AgCuS samples
were identified by the X-ray diffraction powder
method (XRPD). A n example of diffraction patterns
is given in Fig. 5. These measurements enabled us
to identify phases in the experimental samples and
to obtain the average crystallite size of the samples
applying the Scherer equation (Patterson 1939) to the
peaks in the X R P D experimental pattern. Moreover,
the specific surface areas of samples 4 and 8 were
analysed by means of B E T porosimeter (see values in
Table 1).
The coexisting phases identified by the X R P D
method in the as-prepared AgCuS powder samples
are in Table 1. The coexisting phases in bulk samples
of the same chemical composition as experimental
samples are given for comparison. The coexisting
Table 1 Chemical (ICP-AES) and phase compositions (XRPD) of AgCuS samples. The names of phases in bold means major phase
and brackets means minor phase
Sample no Composition/at%
(stabilization)
Composition/at% Average crystallite
size according to
Scherer eq./nm (surface
area/m2
g_ 1
)
Coexisting phasesComposition/at%
(stabilization)
Ag Cu S
Average crystallite
size according to
Scherer eq./nm (surface
area/m2
g_ 1
)
Experiment Bulk equilibrium
(Materials Science
International Team
MSIT® 2006)
1 66Ag00Cu34S (rGO) 65.6 0.4 34 106.3H
(11.25) «i «i
2 33Ag28Cu38S 33.3 28.4 38.4 85.62 T J + T J + C U S x1 + x2 + CuS
3 28Ag45Cu27S 28.2 44.6 27.2 70.85 T1 + T2 Ag + T3 +Cu2 S
4 25Ag40Cu34S 25.0 40 34 68.04 T3 + CuS-f-5
5 29Ag41Cu30S (PVP) 28.6 41 30.4 (7.04) Cu2 S + CuS + (CuO) + (Ag) Ag + Cu2 S + (Cu)
6 25Ag37Cu38S (rGO) 25.0 37.2 37.8 - x2 + CuS Ag + Cu2 S + (T3)
Legend: a L acanthite (Ag2S), CuS... covelite, Cu2 S... (Ag,Cu)2S, t l jalpaite (Ag3CuS2), x2 mckinstryite (Ag( 1 2 _ X J C U ( 0 8 + X J S ) ,
x3 p—stromayerite, Ag( 1 _x ) Cu( 1 + x )S (0 < x < 1), 5... anilite Cu1 7 5 S, (S)... sulphur, (Ag)...silver, (Cu)...copper, rGO...reduced graphene
oxide, PVP.. .polyvinylpyrrolidone.H
.. .after heating to 300 °C and cooling during DSC measurement
4J) Springer
J Nanopart Res (2021) 23:138 Page 7 of 14 138
Experimental pattern
Background
196-901 -25991 Ag3 Cu S2 Jalpacto
J96-901-3956| Ag4 925 Co3 075 S4 Mckindryil*
25.00
Co-Ka (1 790300 A)
AO 0 0 .1 . I, • M.I 1.1.1
86.00 • Li :u 66.00 70 00 75.00 80 00
2theta
Fig. 5 The powder X-ray powder diffraction pattern of the AgCuS sample (6)
phases in bulk material were obtained from Fig. 2,
which shows the phase ternary diagram of the
Ag-Cu^S system at 25 °C (Materials Science International
Team MSIT® 2006).
The agreement in phase composition exists for
sample 1 or 2. In the other cases, the phase compositions
differ and show that used nanoparticle
synthesis incline to form more simple phases
(binary sulphides). The ternary semi-conductive
phase x3 (stromayerite, AgCuS) was not prepared
by our way of synthesis. Instead of x3, the ternary
phases xt (jalpaite Ag3 CuS2 ) and x2 (mckinstryite,
A§3.925Cu3.075S2) w e r e
found in yield.
In general, the inorganic nanoparticles are usually
covered by a big portion of organic matter that has
an origin in chemical synthesis and it is difficult to
remove organic matter from the samples without great
particle loss. It was also observed in our experiment.
The mass of organic matter in the nanocrystalline
AgCuS powders was evaluated from the difference
between sample weight and the mass sum of elements
of Ag, C u and S obtained by ICP-AES analysis. The
organic matter content in the nanocrystalline AgCuS
powders was in the range of 22 (sample 4) minima
to 58 (sample 2) maxima wt%. Samples 1 and 6 with
rGO and P V P were overweight by organic matter.
The matter difference was a function of the purification
procedure.
The stability of the nanocrystalline AgCuS powders
without r G O was investigated by DIP M S . The
results are given in Fig. 6. The investigated sample
4 evolves the signals of organic matter origin
(mainly tetraethylene glycol fragments and PVP). The
increased instability occurred at a higher temperature
above approximately 300 °C. Above other the M / z
Eq. 63.68 has occurred. It indicates S 0 2 evolution in
the instability region. The S 0 2 evolution has occurred
for all experimental samples at temperatures about
300 °C.
The nanocrystalline AgCuS powders with rGO
were investigated by the D S C method using three
different instrumentations. The D S C / T G A investigation
was dome by Netzsch STA 449 Jupiter in 5 N
nitrogen carrier gas. The investigated sample 1 (composition
is close to Ag2 S compound) reveals a low
2.6% weight loss with a maximum rate of change at
4J) Springer
138 Page 8 of 14 J Nanopart Res (2021) 23:138
13 3 201» 13 36 54
1.67 3 3 4 3<S6 4 0ft
4.28 S O * S BO
— i — i — i — r -
1
— I — i — i — i — i 1 — i — i — i — i — i —
2 3 4
T i m e [ m n )
v i o - e * 7-RVPW294S RT: 6.51 A V I S B : 246 0.04-0 58 N L 2 . 1 7 E 7
T: » C E I 0 1 U S [15000-400 OCO|
10O
95
85^
TPS
60-=
5 so
40f
25^
20
1*
10^
31.56
118.64
SO G9
l,..l,.4fc.,l^Ľl|H
143.63
1 5 6 78
•L 1
. ? " " . Ĺ " ^ " 2 2 3
^ 5
2 » ™ 3Q2.S7
H,Lh.fc. . 41, h ť —, : • i • • i , -r 1 r- r-300
340 7 1 354 68 386 35
*00
-r
3S0SO 100 ISO 200
mix
Fig. 6 DIP MS measurement of the nanocrystalline AgCuS powder (sample 4). Sample heated from laboratory temperature with
rate 50 °C/min. Upper: mass outflow from the sample. Below: MS spectrum at 6.51 min (i.e., at approx. 305 °C)
196 °C. The weight was then stable up to 700 °C. The
DSC signal reveals an endothermic signal (onset at
168 °C) followed by an exothermic effect with maxima
of 207 °C.
The origin of the endothermic effect seems to
be in agreement with the phase transformation of
the A g 2 S compound a r A g 2 S (acanthite) —>/?-Ag2S
(argentite), which is referred in Sharma and
Chang (1986); Sadovnikov et al. 2018) at 178 °C.
The 10 °C difference can be explained as phase
4J) Springer
transformation depression due to nanocrystallites
or experimental uncertainty. The exothermic effect
at 207 °C seems to be the combustion of organic
matter with traces of oxygen in nitrogen carrier gas
because it was not observed if an extra pure argon
gas by Netzsch S T A 409 (see details in the Instrumentations
section) was used. The particle layer
from r G O (or its carbonization product) seems to be
stable in oxygen-free gas to high temperatures.
J Nanopart Res (2021) 23:138 Page 9 of 14 138
Fig. 7 DSC analysis of
nanocrystalline sample
2 by Netzsch Pegasus
404C (10 K/min, 5 N Ar).
Low-temperature range A
(25-280 °C). Central temperature
range B (approx.
280-500 °C). Temperature
cycling at high-temperature
range C (500-800 °C)
D S C /(mW/mg)
f exo
0.0 -
-0,2
-0,4 •
-0,6
-0,8 •
-1,0 •
-1,2 •
10K/min, 70mlAr6N/min
100 200 300 400 500
Temperature /°C
600 700
The heating effects, liquidus and solidus temperature,
were evaluated also by the Netzsch Pegasus
404C apparatus. The D S C signal of the samples was
similar (see example in Fig. 7) and can be divided
into three parts A , B and C.
The low-temperature range A exists up to approx.
280 °C. At this temperature range, we suppose evaporation
of solvent traces and low-temperature phase
transformations of ternary phases (see Fig. 1). The
onsets of decompositions are given in Table 2.
The most intensive heat endothermic effect can
be found in central temperature range B (approx.
280-450 °C). This intensive heat effect does not
exist if argon carrier gas was free of oxygen traces
(use Netzsch S T A 409, see details in the Instrumentations
section). The endothermic peak occurs
in the presence of at least a trace amount of oxygen
at temperatures where the formation of sulphur
dioxide has been detected (see M S spectrum
in Fig. 6).
At high-temperature range C (450-800 °C), the
samples reveal melt and crystallization. The solidus
and liquidus temperatures were evaluated using special
method published in Boettinger et al. (2006) and
the values are plotted in Cu2 S-Ag2 S pseudo-binary
phase diagram in Fig. 1.
The D S C measurement indicates more events of
phase transformations (T1 ? T2 ) (Materials Science
International Team MSIT® 2006) at low-temperature
region 25-125 °C, but due to the sensitivity limit of
used instrumentation, the signals cannot be evaluated
(see Fig. 7). In the central temperature range, an
unclear endothermic effect was observed. This endothermic
effect was not present if extra pure argon was
used by Netzsch STA 409 instrumentation. The endothermic
effect may be associated with some decomposition
reaction when the sample is heated. The
melting of the samples at high-temperature range C
was occurred. The experimental results are summarized
in Table 2.
Table 2 Temperatures (in Celsius) obtained via DSC measurement of nanocrystalline AgCuS powders (Netzsch Pegasus 404C
apparatus using 5 N argon, impurities: N 2 and 02 )
Sample no Decomposition of
ternary phases
Maxima for organic
matter combustion
Onset of big endothermic
signal
Onset of unidentified
phase change
Solidus Liquidus
l Z r
- - - - 653 695
2 5 N
118 166 303 365 644.6 652.45
3 5 N
101 193 313 417 670 693.95
4 5 N
- - - 379 668.2 692.9
5 5 N
112 187 337 - 671.6 690.95
6 5 N
100 186 301 411 656.95 679.7
.. .measured by Netzsch STA 409 using argon purified by Zr
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138 Page 10 of 14 J Nanopart Res (2021) 23:138
Spontaneous Ag micro-wire growth
Sample 2 was examined in more detail by in situ
H T X R D when it was gradually heated to 400 °C and
cooled to room temperature under pure helium gas.
The X R D measurements were performed at 25 °C
and isothermal delays at 100, 150, 200, 250, 275,
300, 310, 320, 350 and 400 °C. The time of each isothermal
delay was 20 min. Among other things, the
fractions of the identified phases and their crystallinity
were evaluated from the measurements. Experiments
were conducted with the sample 2 as synthesized
with phase composition: (64% Tj (crystallinity
65 nm) + 22% T 2 (cryst. 53 nm)+14%CuS (cryst.
10 nm)).
During the heating of sample 2, phase T 2 disappeared
at a temperature somewhere between
25-100 °C. Phase T 2 disappeared at a temperature
between 100 and 150 °C. The CuS phase disappeared
at a temperature between 300 and 310 °C. At
a temperature between 150 and 200 °C, a new phase
appeared in the X R P D sheet. This phase should be
mixed sulphide phase y-(Ag,Cu)2 S with antifluorite
crystal structure (Skinner 1966; Du et al. 2017) (see
Fig. 1), but our data do not confirm the existence of
that phase. Other possibilities are as well of presence
impurities evaporated from the chamber or chemical
reaction this vapours with the sample. The X R D pattern
was not found in the available crystallographic
databases PDF2 (2017), C O D (2019) and PDF4;
this may be because y-(Ag,Cu)2 S is not quenchable
(Newman et al. 1982). The significant amorphization
of the sample is visible at 200 and 310 °C by loss of
crystalline fraction.
When cooling sample 2 from a temperature of
400 °C, the y-(Ag,Cu)2 S phase disappeared somewhere
between 200 and 100 °C. Phase T 2 reoccurred
between 100 and 25 °C. The new low-temperature
semi-conductive phase: T 3 stromayerite appeared
between 100 and 25 °C. At the same time, the formation
of elemental A g in the form of micro-wires
was observed first at these conditions. A subsequent
experiment was devoted to this phase transformation
effect.
The weight of sample 2 as synthesized was heated
to 600 °C, annealed for 200 min and cooled to room
temperature. The result was the growth of silver
micro-wires on sample 2 to a greater extent, as evidenced
by the photo in Figs. 8 and 9. The diameter of
these wires was about 2 \im and the bundles of these
wires reached a size of up to 0.1 mm. The length of
silver wires reaches about 7 mm. The individual
wires seem to be single crystals (pure Ag) in contrary
to finding in natural silver wires (Boellinghaus
et al. 2018), synthetic silver wires above acanthite
(Anderson et al. 2019) or at silver corrosion (whisker
formation) (Chudnovsky et al. 2002).
Silver nucleation and its growth in wire form is
obviously related to the reduction of sulphur content
in the sample at higher temperatures (compare
the detection of S 0 2 in Fig. 6). In Fig. 9, we see
that the wires grow on nucleation centres, which are
Fig. 8 Nanocrystalline sample 2 after isothermal heating at 600 °C/200 min. The Ag micro-wires growing on (stromayerite
+x2) substrate. Left: light microscopy. Right: SEM detail of joining between Ag wires and substrate
4J) Springer
J Nanopart Res (2021) 23:138 Page 11 of 14 138
250°C
x unidentified 350°C x
unidentified 4 0 0 ° c
300°C
200°C
• T,Jalpaite
n
X2Mckinstryite
• CuS - Covellite
150°C
• •
100°C
350°C
300°C
200°C
a
T2Mckinstryite
o T3Stromeyerite
100°C
25°C
30 40 50 60
Position 20[°] (Co radiation)
30 40 50 60
Position 29[°] (Co radiation)
Fig. 9 X-ray powder diffraction sheet of sample 2 before heating (x: +T2 + CUS), at higher temperatures (unidentified phase), and
after cooling to 25 °C (x2 +x3)
located on the surface of sample 2. The E D X analysis
confirmed that the composition of the wires is
pure silver and the rest of sample 2 is composed of
subsequent phases: stromayerite (64% of ternary
phase x 2 , cryst. 40 nm) and mckinstrite (T2 , 34%,
cryst. 42 nm). The H T X R D measurement during
this experiment is in Fig. 10. The X R D mapping
of chemical elements of the junction between wires
and rest of the sample 2 is shown in Fig. 10. The
growth of wires is influenced also by the temperature
gradient inside the device, because the sample
is heated by contact with the silicon support.
The result of the experiment is a composite material
(see Fig. 8) containing stromayerite (considered
as a thermoelectric) and mckinstryite joined
with pure silver wires that can serve as electrical
contacts.
Conclusions
The synthesized AgCuS nanoparticles were prepared
and characterized. The test of using P V P and r G O for
better stabilization of the samples in colloids. The
differences between as-synthesized and equilibrium
phase compositions were found. It turned out that
the experimental phase composition of nanostructured
samples does not contain ternary phases exactly
according to the equilibrium diagram for a compact
material of the same chemical composition (see
Table 1).
The temperature stability of AgCuS samples was
investigated by D S C and DIP M S . Both endothermic
and exothermic effects were measured within
the D S C experiment (see Table 2). The endothermic
effects correspond to the decomposition of the
4y Springer
138 Page 12 of 14 J Nanopart Res (2021) 23:138
ternary phases and the melting of the samples (see
Fig. 1). In the mean temperature range for all samples,
a strong endothermic signal was observed
(around 300-350 °C), which may be related to a
change in chemical composition due to the release of
S 0 2 . Exothermic signals correspond to organic matter
combustion.
Nucleation and growth of pure silver wires
were observed in sample 2 with a composition of
33% Ag28% CuS (see Fig. 10). In a more detailed
X R P D experiment, it was possible to identify not
only the initial low-temperature phases but also the
phase occurring at a medium temperature range of
2 0 0 ^ 0 0 °C. This phase should be the y-(Ag,Cu)2 S
phase. However, this has not been confirmed. On
heating, on the other hand, the amorphous phase and
a small amount of a new unidentifiable phase were
formed (see Fig. 10). After cooling to room temperature,
not only wire silver was observed, but in the
rest of the sample the mckinstryite and stromayerite
phases. The second of these phases is considered
a potential thermoelectric. In addition, after cooling
of sample 2, the nano-structuring formed at synthesis
was still preserved. The heating experiment was
therefore repeated under conditions of isothermal
annealing at 600 °C/200 min when it was possible to
grow A g wires up to 7 mm in size.
The original experiments monitoring the temperature
transformations of the AgCuS system are,
according to the research performed, of an older date.
The use of methods available today shows that some
experimental information needs to be refined.
Acknowledgements We acknowledge CzechNanoLab
Research Infrastructure supported by MEYS CR (LM2018110)
for FES EM imaging.
Author contributions Not applicable.
Funding Financial support of the Czech Science Foundation
for the project "Thermal and phase stability of advanced
thermoelectric materials" (GA 17-12844S) is gratefully
acknowledged.
Data availability All data can be published publicly.
Code availability Not applicable.
Compliance with ethical standards
Conflict of interest The authors declare that they have no
conflict of interest.
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