Structural investigations of SnS1-xSex solid solution synthesized from chalcogeno-carboxylate complexes of organo-tin by colloidal and solvent-less routes

Article abstract of DOI:10.1039/c8dt01266k

Tin chalcogenides are important semiconducting materials due to their non-toxic nature, cost effectiveness and layered structure. In this study, a facile synthetic route has been employed for the synthesis of a bis(selenobenzoato)dibutyltin(iv) complex, and used along with the bis(thiobenzoato)dibutyltin(iv) complex, as single source precursors, to prepare binary tin chalcogenides and their solid solution (SnS_1-xSe_x) in the entire range. The synthesis of the solid solution was carried out by colloidal and melt methods. The comparative analysis of the solid solution obtained from both routes indicates that the colloidal method provides superior control over composition. The UV-Vis-NIR analysis showed a gradual change in the band gap, while moving from SnS to SnSe.

Full text of DOI:10.1039/c8dt01266k

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PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
rsc.li/dalton

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Structural investigations of SnS Se solid solution synthesized
1
-x
x
from chalcogeno-carboxylate complexes of organo-tin by the
colloidal and solvent-less routes
Received 00th January 20xx,
Accepted 00th January 20xx
a,d
a,b
c
d*
DOI: 10.1039/x0xx00000x
Malik Dilshad Khan, Mohammad Aamir, Ghulam Murtaza, Mohammad Azad Malik and
a*
Neerish Revaprasadu
www.rsc.org/
Tin chalcogenides are important semiconducting materials due to their non-toxic nature, cost effectiveness and layered
structure. In this study, a facile synthetic route has been employed for the synthesis of a bis(selenobenzoato)dibutyltin(IV)
complex, and used along with the bis(thiobenzoato)dibutyltin(IV) complex, as single source precursors, to prepare binary
x
tin chalcogenides and their solid solution (SnS1-xSe ) in the entire range. The synthesis of the solid solution was carried out
by the colloidal and melt methods. The comparative analysis of the solid solution obtained from both routes, indicate that
the colloidal method provides superior control over composition. The UV-Vis-NIR analysis showed a gradual change in
band gap, while moving from SnS to SnSe.
1
2
nanocrystal based photovoltaics. Hence, tin chalcogenides
are highly desirable as thin films, nanocrystals and alloys with
controllable morphology, size and composition for tunable
Introduction
Nanostructured layered materials are 2-dimensional
materials with interesting mechanical, catalytic and electronic
1
3, 14
electronic properties.
1
-4
SnS and SnSe are both isostructural and exist in the
orthorhombic structure, and each unit cell consists of eight
properties.
Besides graphene, many metal chalcogenide
layered materials have gained attention due to their flexibility
4
in the nature of metals and their oxidation states.
1
-7 atoms. SnSe has both direct and indirect band gaps of around
5
1
.9 eV and 1.3 eV respectively. SnSe has been used in solar
6
0
Depending upon the kind of metal and its oxidation state, a
layered material can exist in different polymorphic geometries,
such as octahedral, trigonal prismatic, rhombohedral and
orthorhombic (also considered as the distorted rock salt
1
7-19
cells, memory switching and as an anode material.
Similarly, SnS is also a promising solar material with an
4
5
absorption coefficient of the order 10 -10 and its band gap is
20
8
, 9
commensurate to that of silicon. Due to the identical crystal
structure of both the materials, a solid solution over the entire
range is also known.
structure) crystal lattices.
Tin chalcogenides belong to the IV-VI class of
semiconductors and exists in either the MX (+2 oxidation) or
SnS1-xSe
x
alloy thin films were deposited by the
2
MX (+4 oxidation) form. However, tin(II) chalcogenides have
2 2 3
electrodeposition method, using sodium thiosulfate (Na S O ),
attracted considerable interest due to their suitable band gaps
and intense absorption in the electromagnetic spectrum,
which make them suitable candidates for photovoltaic and
1
4, 21, 22
2 2
selenium dioxide (SeO ) and stannous chloride (SnCl ).
The effect of the different parameters, such as, potential,
deposition time and composition on the electronic properties
of the solid solution were studied. Another common method
to prepare solid solutions is solid state synthesis, whereby the
elements are mixed in stoichiometric quantities and heated at
1
0, 11
optoelectronic applications.
Moreover, they have low
toxicity, are relatively earth abundant and cost effective
materials, properties that render them promising materials in
1
3, 23-25
high temperature for a prolonged time.
The resultant
alloy solid solution was used as electrodes for lithium/sodium
based batteries, where significant enhancement was observed
in the thermoelectric performance. The band gap was also
tuned by changing the S:Se ratio. However, the solid state
method has inherent problems, as the process is laborious,
inefficient in terms of energy and also not suitable when one
of the components is more volatile than others, eventually
leading to the formation of a non- stoichiometric product.
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The gas phase laser photolysis method was also used for further 15 min, after which Bu
Journal Name
2
SnCl
2
(0.95 g, 3.0 mmol) in 15.0
2
6
the composition tuned synthesis of the SnS1-xSe
Although, a facile control on phase and composition was after which 10.0 mL of distilled water was added and the
3
achieved, the method requires the use of highly toxic H S and product was separated by solvent extraction using CHCl .
x
alloy.
mL ethanol was added dropwise. The oily product was formed
2
dimethyl selenium. The use of single source precursors is often Magnesium sulphate was added to remove the water
advantageous as it prevents the usage of noxious reagents and molecules from chloroform and the organic layer was
provides better control over phase and stoichiometry of thin evaporated to afford the product (complex (1)). Yield; 81 %,
2
7-33
1
films and nanomaterials.
source precursors for the synthesis of SnS1-xSe
not well explored and the only precursors used until the Hz, CH
Unfortunately, the use of single Melting point 71-73 °C, H-NMR (CDCl
solid solution is J=6.96 Hz, CH ), 1.48 (4H, d, J=6.84 Hz, CH
CH ), 1.94 (4H, d, J=7.32 Hz, CH Sn), 7.49 (4H, d, J=7.14
The Hz, o-ArH), 7.63 (2H, d, J=13.68 Hz, p-ArH), 8.05 (4H, d, J=7.38
3
) δ (ppm): 0.95 (6H, t,
x
3
2
), 1.80 (4H, d, J=7.14
2
3
2
3
4, 35
present are benzyl substituted tin chalcogenides.
precursors were not suitable to prepare a solid solution due to Hz, m-ArH). H-NMR (CDCl
the use of elemental selenium, and traces of elemental tin 28.7, 128.5, 128.7, 133.7, 139.9, 200.5. Elemental analysis cal.
were also observed in the final product obtained by the (%) for C22 SnSe : C 43.96, H 4.69, Sn 19.74; Found: C
44.06, H 4.72, Sn 19.65.
1
3
3
) δ (ppm): 13.5, 22.2, 26.2, 28.6,
H
28
O
2
2
decomposition of these precursors.
The design and use of suitable chalcogeno- single source
precursors for an efficient and composition controlled
Synthesis of bis(thiobenzoato)dibutyltin(IV) complex
synthesis of SnS1-xSe
the use of
Sn(Ph PSe
however, the synthesis requires the use of highly toxic and mmol) while stirring. The stirring was continued for 15 min. at
x
is required. Previously, we have reported The sodium salt of thiobenzoic acid was prepared by adding an
bis(diphenylphosphinediselenoato)tin(II) ethanolic solution (25.0 mL) of KOH (0.85g, 15.0 mmol) in an
[
2
2 2
) ] complex for deposition of SnSe thin films, ethanolic solution (25.0 mL) of thiobenzoic acid (2.1 g, 15.0
3
6
expensive diphenyl phosphine. Recently, we have developed room temperature, after which an ethanolic (20.0 mL) solution
a new and an efficient route for the synthesis of a novel of Bu SnCl (2.3 g, 7.5 mmol) was added dropwise under inert
conditions. The reaction mixture was stirred for further half an
2
2
3
7
phosphine- free bis(selenobenzoato)dibutyltin(IV) complex.
The thio- analogue, bis(thiobenzoato)dibutyltin(IV), was also hour, to ensure completion of the reaction. Distilled water
synthesized and both complexes were used to prepare tin (50.0 mL) was added to the reaction mixture, the solution
x
chalcogenides and/or SnS1-xSe alloy solid solutions by two became turbid with the formation of an oily product. The
different routes. The use of a similar type of single source product was separated by the solvent extraction method using
precursors with different chalcogeno-congeners is highly chloroform, magnesium sulfate was added in chloroform to
feasible, as both may follow similar decomposition behaviour remove water molecules. The chloroform solution was
and/or intermediate steps during the reaction.
The synthesis of a SnS1-xSe solid solution in the entire product (complex (2)). Yield; 83 %, Melting point 96-97 °C, H-
range of composition, from chalcogeno- (thio/seleno) NMR (CDCl ) δ (ppm): 0.93 (6H, t, J=7.32 Hz, CH ), 1.47 (4H, q,
benzoate complexes of organotin, by a colloidal hot injection J=7.26 Hz, CH ), 1.80 (4H, m, J=7.62 Hz, CH CH ), 1.90 (4H, t,
method, using oleylamine (OLA) as capping agent is described. J=7.95 Hz, CH Sn), 7.49 (4H, t, J=7.47 Hz, o-ArH), 7.62 (2H, t,
For comparison, the solid solution was also synthesized by the J=7.35 Hz, p-ArH), 8.14 (4H, d, J=7.74 Hz, m-ArH). C-NMR
solvent-less melt method, to observe the effect of capping (CDCl ) δ (ppm): 13.5, 22.8, 22.9, 24.4, 25.6, 25.9, 26.0, 26.2,
26.3, 26.6, 27.3, 27.9, 28.6, 28.1, 128.3, 128.5, 128.7, 129.0,
evaporated and an off-white precipitate appeared as the
1
x
3
3
2
2
3
2
1
3
3
agents on the synthesized alloyed materials.
1
C
5
33.5, 137.5, 202.1. Elemental analysis cal. (%) for
SnS : C 52.04, H 5.56, Sn 23.39, S 12.62; Found: C
2.08, H 5.54, Sn 23.32, S 12.56.
22
H
28
O
2
2
Experimental
Materials
Colloidal synthesis of SnS1-xSe solid solution
x
The reagents and solvents i.e. oleylamine (OLA),
The solid solution was synthesized by dispersing a mixture of
4
dibutyltindichloride, NaBH , KOH, elemental selenium, benzoyl
complex (1) and (2) in appropriate molar compositions in OLA
chloride, thiobenzoic acid and ethanol were purchased from
Sigma Aldrich and used as received.
(
2.0 mL) and injected into pre-heated OLA (8.0 mL) at 230 °C
with continuous stirring, under a nitrogen environment
Supplementary Table S1). The temperature was maintained
(
Synthesis of bis(selenobenzoato)dibutyltin(IV) complex
for 30 min., after which the reaction mixture was removed
from the heating source for cooling. A mixture of acetone and
methanol (1:1) was used for precipitation, and the synthesized
material was washed and separated by centrifugation.
NaHSe was prepared by slight modification of the reported
38
4
method. An ethanolic solution of NaBH (12.0 mmol in 15.0
mL ethanol) was added to an ethanolic solution of metallic Se
powder (0.5 g, 6.0 mmol) under inert conditions. The resultant
reddish solution became colourless within 5 min of stirring.
Benzoyl chloride (6.0 mmol) was then added into the freshly
prepared NaHSe solution, which resulted in the formation of
the seleno-benzoate ligand. The stirring was continued for a
x
Synthesis of SnS1-xSe solid solution by melt method
In the melt method, the solid solution was prepared by the
thermal decomposition of the complexes (1) and (2), mixed in
required stoichiometric quantities (Supplementary Table S2).
2
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For this purpose, both complexes were mixed together the cell. Generally for an alloy system, Vegard’s law is expected
4
5
homogenously, and placed in a ceramic combustion boat. The to be obeyed. The law emphasizes the fact that the sizes of
boat, in a glass tube, was placed inside the carbolite tube the atoms, independent of the other parameters, have a
furnace. The end of the tube was sealed by a rubber septum marked effect on the crystal structure. A linear relationship
and a vacuum was applied to remove air from the tube between lattice constants-composition can be found in the
followed by filling of nitrogen gas. As complex (1) and (2) both alloys or solid solutions. The extent of solubility of a
have low melting points, the temperature was maintained at substituent component can be determined by Hume-Rothery
4
6
1
00 °C for 10 min., to obtain a homogenized molten mixture of rules, which states that the formation of the solid solution is
both complexes. Afterwards, the temperature was raised facilitated by (a) analogous electronegativity of solute and
slowly to 330 °C and maintained for one hour under an inert solvent (b) the difference in radii of solute and solvent must be
atmosphere. After an hour, the heating was turned off and the less than 15 % (c) similar valency, and (d) identical crystal
combustion boat was allowed to cool naturally to room structures of solute and solvent for complete solubility. The
temperature. The tube was removed from the furnace and chalcogenide atoms (sulfur and selenium), fulfil these
powder residue was collected for analysis.
requirements and favours the formation of alloys of the type
. Hence, the thio/seleno-carboxylate complexes of the
organotin were used to generate the SnS Se solid solution in
MS1-xSe
x
Characterization
1
-x
x
Microanalysis was performed using a Thermo Scientific Flash the entire range.
1
000 Organic Elemental Analyzer. H and C NMR were
13
2
The thermal stability of both complexes was analysed by
recorded on a Bruker 600 MHz instrument with TMS as thermogravimetric analysis (Supplementary Figure S1). The
external reference at room temperature. Thermogravimetric TGA curve for both complexes show decomposition in a single
analyses were performed using a Mettler-Toledo TGA/DSC step, and the complete decomposition occurs around 315 °C.
under nitrogen gas flow rate of 10 mL/min. X-ray diffraction Complex (2) decomposes at a slightly higher temperature as
was performed using a Bruker D8 Discover Diffractometer compared to complex (1). The differential thermogravimetric
using CuKα radiation (λ = 1.54178 Ǻ), in a 2ϴ range from 10ᵒ to analysis indicates that the maximum mass loss for complex (1)
7
0ᵒ. The data collected was used to determine the lattice occurred at 290 °C, and a slight loss was indicated by a
parameters and crystal phase. Scanning electron microscopy shoulder peak at 310 °C (Supplementary Figure S1(a)).
SEM) was carried out using a Philips XL30 FEG-SEM. Energy- Similarly, DTG curve for complex (2), indicates a comparatively
(
dispersive X-ray (EDX) spectroscopy was performed using a higher stability of complex (1) and maximum weight loss was
DX4 detector. All samples were carbon coated using an observed at 313 °C (Supplementary Figure S1(b)). The residual
Edwards coating system E306A prior to SEM analysis. UV-vis- mass for both of the complexes was around 15 %, which is less
NIR spectra were recorded using a PerkinElmer Lambda 1050 than the expected masses required for SnSe (32.8 %) and SnS
instrument, using quartz cuvettes with a path length of 1 cm.
(29.7 &). The residual mass does not correspond to any other
selenide or sulfide phase, elemental tin, sulfur, selenium or
oxide based product. It was observed that both organotin
complexes were volatile and undergo sublimation during the
thermogravimetric analysis. There are some previous reports
Results and discussions
Amongst the metal complexes of chalcogeno carboxylic
acids, considerable work has been done on the thio-
6
, 47
on the sublimating nature of the organotin complexes.
The
3
9-41
mass loss during sublimation can prevent the accurate
determination of the final residue, hence the nature of residue
cannot be determined simply on the basis of percentage mass
left at the end.
analogues,
however very little work has been done on the
4
2
complexes of heavier chalcogen congeners. The primary
reason is the difficulty of preparation, as the resulting acids are
4
3
highly unstable. In this study, we have used a new and
efficient route for the synthesis of a metal selenobenzoate
complex and used it as suitable precursor for the synthesis of
the respective metal selenide and alloy solid solution.
Previously, sodium selenide was used to prepare
The nature of the residue was observed by heating both
complexes separately from room temperature to 500 °C in a
tube furnace, under continuous nitrogen flow for one hour.
After an hour, the heating was turned off and the furnace was
allowed to cool to room temperature. The black residue
obtained after cooling was collected and analysed by p-XRD.
The analysis of residual powder obtained from decomposition
of complex (1) shows the formation of pure SnSe, whereas the
decomposition product of complex (2) yields pure SnS
4
4
selenobenzoate ligand, however, the synthesis of sodium
selenide requires the use of sodium metal, a highly pyrophoric
material. In addition, the completion of the reaction requires ≈
24 hours. Our method excluded the use of sodium metal, and
the reaction can be completed within 30 minutes.
(
Supplementary Figure S2).
A solid solution can be generated by substitution of one or
more components without destroying the crystal structure.
However, slight distortions in the crystal lattice can be
observed in the form of defects, which may arise due to the
vacancies or stacking faults, subject to the reaction
Colloidal synthesis of SnS1-xSe solid solution
x
Oleylamine (OLA) is a strong coordinating agent and often
catalyses the degradation of complexes at temperatures quite
lower than their decomposition temperature, as indicated by
45
conditions. The lattice parameters are affected by the type of
the substituent used, leading to the shrinkage or expansion of
3
3, 48
their TGA profile.
During temperature optimization, it was
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observed that the complexes show no sign of decomposition preferential substitution of selenium in any particular
below 190 °C, whereas at 200 °C, both complexes decompose direction. The volume of the cell changes from 192.8 to 211.9
3
partially giving a small product. However at 230 °C, complete
Å
on replacement of sulfur by selenium (inset Figure 3).
decomposition of the complexes occurred giving products with However, the change in the volume was not very smooth and a
good crystallinity and yields. This indicates high stability of sudden increase in size was observed in the start, at the mole
both complexes as some other thio/seleno-benzoate fraction value of x = 0.2 and x = 0.4 and little increase at later
complexes are reported to decompose immediately in the concentrations.
4
0, 41,
presence of a primary amine, even at room temperature.
9
Nevertheless, the breakdown of both complexes below their synthesized solid solution, was determined by EDX
The percentage of all the elements present in the
4
decomposition temperature, indicated by their TGA profile, spectroscopy (Supplementary Figure S3). Qualitative analysis
shows the role of the OLA as a catalyst and decomposition indicates presence of only desired elements i.e. Sn, S and/or
initiator. Hence, all reactions as part of the colloidal synthesis Se, whereas the quantitative analysis shows a successive
were performed at an optimized temperature of 230 °C.
increase in selenium concentration with successive decrease in
The p-XRD pattern of all the samples prepared by the sulfur content by changing the Se/(Se+S) composition between
colloidal synthesis is shown in Figure 1. The peaks were sharp x = 0 to 1 (Supplementary Figure S4). The analysis was carried
for all the samples which confirm their crystalline nature and it out at three arbitrary points and averaged out to obtain the
is also helpful to observe the shift in the peaks. The mole final results, as listed in Supplementary Table S1. A linear trend
fraction (x = 0) represents the p-XRD pattern of SnS (ICDD # 01- between selenium and sulfur was obtained, whereas the
0
75-2115) and (x = 1) shows diffraction pattern of SnSe (ICDD # percentage of tin almost always remain the same. The
0-014-0159), both match well with the standard pattern. It stoichiometry of the alloy was found to be very close to the
0
was interesting to note that, in the starting precursor, the Sn expected value.
atom is in the +4 oxidation state but both of the complexes
The even distribution of all elements in the alloyed samples
yielded tin monochalcogenide as the main product. It is was analysed by the EDX elemental mapping which showed
probably due to the reductive nature of the OLA that Sn+4 was uniform distribution of Sn, S and Se throughout the samples
5
0
reduced to the Sn+2 state during the reaction.
The diffraction peaks for the SnS1-xSe solid solution lie further confirms the formation of a single phase and existence
intermediate of the diffraction peaks of SnS and SnSe. The of both S and Se within the same crystal structure.
peaks show a gradual shift with the change in the ratio of mole The morphology of the synthesized binary and alloyed
fraction. There was no indication of phase separation or SnS1-xSe system was observed by SEM analysis and images are
(Supplementary Figure S5). The homogeneity of the samples
x
x
multiple sets of peaks, only a single phased diffraction pattern shown in Figure 4. The images are marked with different
was observed, which confirms the formation of the solid compositions of selenium from x = 0 to x = 1, and all presented
solution. The shift in the peaks is in agreement with Vegard’s images are taken at the same magnification for comparison.
law i.e. change in d-spacing with change in the composition.
SnS shows cubic sheet like structures, which is in the order
The peaks shifted towards a lower theta value with increase in of microns (Figure 4(a)). With the introduction of selenium, the
selenium content, due to an expansion of the lattice sheets started transforming from cubic to a hexagonal
parameters with substitution of the smaller sulfur atoms morphology (Figure 4(b,c)). Further increases in selenium
(
1.84Å) by the larger selenium atoms (1.98Å).
The unit cells of both the orthorhombic SnS and SnSe are giving short sheets or plate like structures.
shown in Figure 2, along with their bond distances and Van der In general, all the samples showed sheet-like structure.
content result in a decrease in size of the sheets, eventually
Waals interactions. The crystal structure of both materials is Both SnS and SnSe are isostructural and the growth of sheets
similar and the slightly larger size of the selenium atom is can be explained on the basis of orientation and the type of
responsible for expansion (≈2.5 %) of the SnSe unit cell
interactions present in their building blocks. The preferential
The change in bond lengths accounts for the gradual shift growth crystallization for SnS is an orthorhombic structure
in p-XRD peaks. A change in the intensity of the peaks present with Pnma space group. Virtually, the SnS structure can be
at 2
θ
= 27.47° and 26.03° was also observed, upon selenium viewed as a slightly distorted rock salt structure, due to the
more
with increase in selenium mole fraction. Similarly, the intensity electronegative S atom. Furthermore, the presence of a lone
of peaks at 2 = 30.49°, 31.54° and 31.96° also changes, along pair of electrons on the Sn atom has a significant impact in the
incorporation (Figure 1). The intensity of both peaks decreases withdrawal of electron density from Sn to
a
5
1
θ
with the shift to lower angle, in moving from SnS to SnSe. The distortion of the rock salt structure to the distorted
intensity of the middle peak increases while the other two orthorhombic layered structure. In the layered structure, the
decreases in intensity.
Sn atom is strongly connected to S atoms within the layer, with
The lattice parameters were calculated using p-XRD data short and strong Sn-S bonds (≈ 0.266 nm), whereas the layers
which further confirms the expansion of the cell and the values are interconnected to each other by comparatively long and
5
of the lattice constants a, b and c show a near-linear weak Van der Waals interactions (≈ 0.388 nm). As a result,
2
relationship between composition and the lattice spacing the layers of SnS can be easily separated and cleaved
(Figure 3). The composition dependent variation was observed perpendicular to the c-axis. SnSe has a similar structure and
in all lattice constants, which indicates that there was no
4
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bonding structure, hence all the samples showed a sheet-like less melt method (Supplementary Figure S6). The analysis
structure.
reveals that the binary as well as ternary alloys were
chalcogenide deficient materials. Although, the amount of
both sulfur and selenium was less than the required value, it
x
Melt method for the synthesis of SnS1-xSe solid solution
x
The SnS1-xSe solid solution was also synthesized by the can be seen that the deviation in the case of selenium is even
solvent-less melt method. The melt method was used to more as compared to sulfur. Furthermore, the composition of
observe the influence of the capping agent on the formation of the binary compounds i.e. SnS and SnSe also shows higher
the product. In the melt method, the compounds are heated deficiency of selenium in SnSe as compared to sulfur in SnS
under an inert environment and the reactive melt formed (Supplementary Figure 7). It shows that the sulfur is more
undergoes decomposition, with removal of organic moieties, competitive in the reaction conditions used for the melt
leading to the formation of the pure product. method. The deficiency of the chalcogenides is probably due
Thermogravimetric analysis indicates that, both complexes to the high temperature used, and high partial pressure
decompose completely in the temperature range of 305-315 associated with the chalcogens. It is a common observation
°
C, therefore a slightly higher temperature of 330 °C was used that thin films of metal chalcogenides deposited at high
5
3
to ensure the complete decomposition of the complexes and temperature are often chalcogen deficient. Hence, the
high crystallinity of the product. The decomposition of the stoichiometry of the synthesized alloys by the melt method is
precursors yielded tin monochalcogenides respectively. The p- significantly altered (Supplementary Table S2) as compared to
XRD analysis of SnS, SnS1-xSe
x
and SnSe synthesized by the the alloys prepared by the colloidal method.
The morphology as determined by SEM, indicates the
solvent-less method is shown in Figure 5.
The peaks were sharp and the diffraction patterns for the formation of sheet-like structures for all samples, which shows
binary tin monochalcogenides i.e. for the mole fraction x = 0 that the morphology was not influenced by the difference in
(SnS) and x = 1 (SnSe), match well with the standard pattern the method of preparation (Figure 7). The Van der Waals
and confirm the formation of the pure product. A shift in peak interactions between the sheets are too fragile to sustain
position was observed when the mixture of complexes was under harsh conditions such as presence of strong nucleophilic
used to prepare a solid solution, however, when the diffraction ligand and/or high temperature. The control over size also
peaks were zoomed in, multiple peaks were also observed as becomes challenging in such situations. It can be seen that the
shoulders for the mole fraction value of x = 0.2 and x = 0.4. The size of the SnS sheets (Figure 7(a)) is in the micron range and
splitting of the peaks present at 2θ = 30.5 to 32.5 shows that unlike OLA capped SnS, which shows predominantly cube like
there is a presence of SnS, which exists along with the solid sheets, a mixture of irregular and hexagonal shaped plates
solution and results in the phase segregation of the were observed.
synthesized material. The separation was prominent at lower
After the inclusion of selenium in the lattice, the
concentration of selenium (i.e. x = 0.2) and fades away at morphology of the sheets showed a hexagonal-like structure
higher incorporation of selenium. The intensity profile was also but with some eroded edges. At higher selenium
different from the diffraction pattern observed for the samples concentration i.e. when x = 0.6 and 0.8, the shape became
synthesized by the colloidal method.
more defined and the size of the crystallites was also
It was observed during TGA analysis that the complexes are decreased. The trend of decreasing size in moving from SnS to
volatile and undergo sublimation, making accurate residual SnSe, was similar to the pattern observed for the samples
mass determination difficult. Similarly, the volatile nature may synthesized by the colloidal synthesis.
be responsible for improper mixing, as some of the complex
The homogenized distribution of all the elements in the
system was observed by elemental mapping
might have sublimed and decomposed on the walls of the SnS1-xSe
x
ceramic combustion boat, and appear as an impurity in the (Supplementary Figure S8). The images show uniform
final product. However, the selenium was also merged into the distribution of the elements throughout the material.
crystal lattice, which can be confirmed by the shift in the
The optical behaviour of the solid solution series was
investigated by the UV-Vis-NIR spectroscopy and is shown in
diffraction peaks due to the change in the d-spacing.
The expansion in the cell as a result of the change in lattice Figure 8. The stoichiometry of the solid solution obtained by
parameters was calculated from the data of the intense peaks colloidal method was comparatively better, hence, only those
present in the diffraction pattern (Figure 6). The change in the samples were used for the optical studies. The band gap
lattice parameters with respect to the composition was non- decreases slightly (from 1.48 to 1.1 eV) with addition of
linear, the maximum deviation occurred at the Se/(Se+S) selenium, and it can be seen that there is a little difference in
composition of x = 0.2 and x = 0.4. The irregularity in the lattice band gaps of SnS1-xSe
2
x
system. Similar observations have been
6, 54
parameters also compliments the p-XRD pattern, and the reported earlier,
and it can be assumed that it is due to the
phase segregation can be seen clearly at these compositions. similar nature of their band structures.
The change in volume of cell from SnS to SnSe was plotted
against composition and as expected the alteration in the
Conclusions
volume was also uneven.
The EDX analysis was conducted to have an insight of the
Thio/seleno-benzoate complexes of organotin were
composition of the solid solution synthesized by the solvent-
synthesized and used as single source precursors for the
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synthesis of SnS, SnSe and SnS1-xSe
x
solid solution over the 10.
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entire range, by two different synthetic routes. Both
complexes have a volatile nature and undergo sublimation,
which results in ambiguity in accurate mass determination of
the residue by TGA, but p-XRD indicates clean decomposition
to SnS and/or SnSe. In the colloidal route, OLA was used as the
capping agent which helps in the decomposition of the
complexes at temperatures lower than their decomposition
temperature. Whereas, the melt method requires a high
decomposition temperature of 350 °C. The volatility of
complexes causes some problems, such as, sublimation of the
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a
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and EDX analysis also shows a wider deviation from the
required percentage. A comparison of the composition of the
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was independent of the method used, and a similar trend in
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The authors are grateful to the National Research Foundation
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228-3236.
(NRF) South African Research Chairs Initiative (SARChI) for
H. S. Im, Y. Myung, Y. J. Cho, C. H. Kim, H. S. Kim, S. H.
Back, C. S. Jung, D. M. Jang, Y. R. Lim and J. Park, RSC Adv.,
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(
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(
b)
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Figure 3. Lattice constants a, b, and c derived from diffraction peaks, and inset shows change in
volume of the cell, as a function of Se concentration ‘x’ in the OLA capped SnS1-xSe alloy system.
x
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Figure 4. SEM images of (a) SnS, (b-e) SnS Se and (f) SnSe sheets synthesized by the hot
1
-x
x
injection method.
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(
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(
b)
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Figure 6. Lattice constants a, b, and c derived from the intense p-XRD diffraction peaks, and
inset) change in the volume of the cell, as a function of Se concentration ‘x’ in the SnS1-xSe alloy
(
x
system synthesized by the melt method.
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Figure 7. SEM images of (a) SnS, (b-e) SnS1-xSe
method.
x
and (f) SnSe sheets synthesized by the melt
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Figure 8. (a) The UV-Vis-NIR spectra and (b) band gaps of the solid solution prepared by colloidal
method.
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Table 1. Comparison of the compositions observed by the EDX analysis for samples synthesized
by colloidal and melt method.
Composition
Se]/[Se]+[S]
(SnS)
Target
composition
SnS
Stoichiometry
Stoichiometry
[
from colloidal method
from melt method
0
Sn1.02
S
0.98
Sn1.07S
0.93
0
0
0
0
.2
.4
.6
.8
SnS0.80Se0.20
SnS0.60Se0.40
SnS0.40Se0.60
SnS0.20Se0.80
SnSe
Sn1.03S0.76Se0.21
Sn1.06S0.77Se0.17
Sn1.02
Sn1.03
Sn1.02
S0.61Se0.37
S0.39Se0.58
S0.19Se0.79
Sn1.05
Sn1.11
Sn1.08
S0.58Se0.37
S0.37Se0.52
S0.18Se0.74
1
(SnSe)
Sn1.03Se0.97
Sn1.11Se0.89
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