Development of molecular precursors for deposition of indium sulphide thin film electrodes for photoelectrochemical applications

Article abstract of DOI:10.1039/c3dt50781e

Symmetrical and unsymmetrical dithiocarbamato pyridine solvated and non-solvated complexes of indium(iii) with the general formula [In(S ₂CNRR′)₃]·n(py) [where py = pyridine; R,R′ = Cy, n = 2 (1); R,R′ = ⁱPr, n = 1.5 (2);

Full text of DOI:10.1039/c3dt50781e

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Development of molecular precursors for deposition of
indium sulphide thin film electrodes for
photoelectrochemical applications†
Cite this: Dalton Trans., 2013, 42, 10919
Muhammad Ali Ehsan,a T. A. Nirmal Peiris,b K. G. Upul Wijayantha,b
Marilyn M. Olmstead,c Zainudin Arifin,a Muhammad Mazhar,*a K. M. Loa and
Vickie McKeeb
Symmetrical and unsymmetrical dithiocarbamato pyridine solvated and non-solvated complexes of
indium(III) with the general formula [In(S2CNRR’)3]·n(py) [where py = pyridine; R,R’ = Cy, n = 2 (1); R,R’ =
iPr, n = 1.5 (2); NRR’ = Pip, n = 0.5 (3) and R = Bz, R’ = Me, n = 0 (4)] have been synthesized. The compo-
sitions, structures and properties of these complexes have been studied by means of microanalysis, IR
and 1H-NMR spectroscopy, X-ray single crystal and thermogravimetric (TG/DTG) analyses. The applicability
of these complexes as single source precursors (SSPs) for the deposition of β-In2S3 thin films on fluorine-
doped SnO2 (FTO) coated conducting glass substrates by aerosol-assisted chemical vapour deposition
(AACVD) at temperatures of 300, 350 and 400 °C is studied. All films have been characterized by powder
X-ray diffraction (PXRD) and energy dispersive X-ray analysis (EDX) for the detection of phase and stoichio-
metry of the deposit. Scanning electron microscopy (SEM) studies reveal that precursors (1)–(4), irrespective
of different metal ligand design, generate comparable morphologies of β-In2S3 thin films at different
temperatures. Direct band gap energies of 2.2 eV have been estimated from the UV-vis spectroscopy for
the β-In2S3 films fabricated from precursors (1) and (4). The photoelectrochemical (PEC) properties of
β-In2S3 were confirmed by recording the current–voltage plots under light and dark conditions. The plots
showed anodic photocurrent densities of 1.25 and 0.65 mA cm−2 at 0.23 V vs. Ag/AgCl for the β-In2S3
films made at 400 and 350 °C from the precursors (1) and (4), respectively. The photoelectrochemical
performance indicates that the newly synthesised precursors are highly useful in fabricating β-In2S3
electrodes for solar energy harvesting and optoelectronic application.
Received 23rd March 2013,
Accepted 22nd May 2013
DOI: 10.1039/c3dt50781e
www.rsc.org/dalton
optoelectronic and photoelectrochemical properties and is a
promising candidate for many useful applications due to its
Introduction
Semiconductors with suitable band gaps and band edges are stability, moderate band gap and photoconductive behaviour.3
key to the development of efficient light harvesting appli- β-Indium sulphide (Eg = 2.0–2.2 eV),4 having a defect spinel
cations such as solar cells and solar water splitting.1,2 The structure, displays excellent properties of high photosensitivity
β-polymorph of indium sulphide (β-In2S3) is known for its and photoconductivity,5 stable chemical composition and low
toxicity.6 Thus, it is widely applied for displays,7 as a photo-
catalyst for dye degradation,8 for water splitting6b and for solar
cells.9,10 Moreover, β-In2S3 solar cell devices exhibit a high
aDepartment of Chemistry, Faculty of Science, University of Malaya, Lembah Pantai,
16.4% power conversion efficiency,11,12 which is comparable to
50603 Kuala Lumpur, Malaysia. E-mail: maliqau@ymail.com,
zainudin@um.edu.my, kmlo@um.edu.my, mazhar42pk@yahoo.com;
CdS because of their equivalent band gaps and has been con-
Tel: +60 (03) 79674269
bDepartment of Chemistry, Loughborough University, Loughborough, LE11 3TU, UK.
sidered as a substitute for the highly toxic CdS.13
Much effort has been expended to produce β-In2S3 films
E-mail: T.A.N.Peiris@lboro.ac.uk, u.wijayantha@lboro.ac.uk, V.McKee@lboro.ac.uk;
and powders with a variety of structures and morphologies.
These methods include chemical bath deposition,14,15 sono-
chemical,16 hydrothermal,17,18 solvothermal,19 solution phase
synthesis,20 thermal evaporation,21 chemical spray,22 solvent
reduction route,23 electrodeposition24 and metal–organic
chemical vapour deposition (MOCVD).25 However, increasing
Tel: +44 (0) 1509222574
cDepartment of Chemistry, University of California, Davis, CA 95616, USA.
E-mail: marilyn_olmstead@hotmail.com
†Electronic supplementary information (ESI) available: Microanalysis results,
figures for NMR, XRD, TGA and EDX analyses. CCDC 924187–924190. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt50781e
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applications of semiconducting materials in optoelectronic Analyzer at a heating rate of 10 °C min−1 under a constant flow
and photovoltaic devices in the form of thin films have driven of nitrogen at a rate of 50 mL min−1
researcher’s interest in CVD routes. The demand for CVD
.
routes to a variety of materials has, in turn, generated interest
General procedure for synthesis of [In(S2CNRR′)3]·n(py)
complexes, (1)–(4)
in the design of single source precursors (SSP).26 Historically,
a variety of SSP have been investigated including dialkylindium
alkanethiolate,27 alkylindium bis-thiolate, tris(dialkyldithiocar- A sample of sodium dicyclohexyldithiocarbamate (0.50 g,
bamates),28,29 dialkylmonodithiocarbamate,30 tris(dialkylmo- 1.80 mmol) was dissolved in methanol (20 mL) and placed in a
nothiocarbamte),31 and alkyl xanthate32 for the deposition of three-necked round bottom flask (100 mL) fitted with a drop-
different phases of indium sulphide on a variety of substrates ping funnel, reflux condenser and inert gas line followed by
through CVD based techniques. However, there are restrictions addition of indium trichloride (0.13 g, 0.60 mmol). The resul-
imposed by the precursors on the use of this method. These tant milky white solution was stirred for 30 min. At this point
issues directly affect thin film properties such as deposited pyridine (30 mL) was added to give a clear and colourless solu-
phase, degree of crystallinity, crystallographic orientation, tion and stirring was continued for another hour. Filtration
composition, morphology and possibly the structure of the de- and slow evaporation of the reaction mixture afforded
posited film. These properties are highly desirable for the fab- [In(S2CNCy2)3]·2py (1) as colourless crystals; yield (0.49 g,
rication of device quality films.33 By developing precursors 78%), mp. 255–260 °C (decomposition). Elemental analysis
specifically for aerosol assisted chemical vapour deposition found: C, 56.73; H, 7.83; N, 6.32%; C49H76InN5S6 requires: C,
(AACVD),34 the volatility and thermal sensitivity restrictions 56.46; H, 7.34; N 6.71%. IR (νmax/cm−1): 2925s, 2853s, 1579m,
can be reduced and as a result a larger range of precursors and 1471w, 1440s, 1378w, 1361s, 1346m, 1332w, 1300s, 1266m,
films can be investigated.35 By carefully monitoring AACVD 1241s, 1166s, 1148s, 1105s, 1020w, 997s, 949s, 922s, 895s,
deposition conditions such as solution concentration, solvent, 882w, 748w, 746m, 705s, 658s, 612s, 602m, 503w, 492w, 473s.
nature of the carrier gas, carrier gas flow rate, deposition time 1H-NMR δH (400 MHz, CDCl3): 1.13–2.17 (60H, m, 3(C10H20)),
and substrate temperature, high quality nanostructured thin 3.12 (2 H, s, (NCH)), 3.40 (2 H, s, (NCH)), 4.82 (2 H, s, (NCH))
films can be produced that can be employed in a range of and 7.30–8.63 ppm (10H, m, 2(NC5H5)). TGA: 55–110 °C
applications such as photovoltaic, gas sensors, thermoelectric, (11.60% wt. loss); 230–400 °C (73.21% wt. loss); residual mass
fuel cells and catalysts.36
of 15.19%, calcd for β-In2S3: 15.64%.
In continuation of our efforts on the exploration of metal
Complexes (2), (3) and (4) were prepared as colourless crys-
dithiocarbamate/dithiocarbonate pyridine adducts,37,38 we tals adopting the method used for the synthesis of (1).
have devoted our attention to the syntheses of SSPs tris(dialkyl-
dithiocarbamato)indium(III), [In(S2CNRR′)3]·n(py) [where py = (3) and (4) and their analytical data are as follows.
pyridine; R,R′ = Cy, n = 2 (1); R,R′ = iPr, n = 1.5 (2); NRR′ = Pip,
Complex (2): Sodium diisopropyldithiocarbamate (0.50 g,
The quantities of the reactants used for the synthesis of (2),
n = 0.5 (3) and R = Me, R′ = Bz, n = 0 (4)] for the deposition of 2.50 mmol) and indium trichloride (0.19 g, 0.83 mmol); yield
β-In2S3 films by AACVD technique. The thin films properties (0.51 g, 80%), mp. 265–270 °C (decomposition). Elemental
have been examined by PXRD, SEM, EDX and UV-vis spec- analysis found: C, 43.72; H, 6.69; N, 7.94%; C28.5H49.5InN4.5S6
troscopy, for their crystallinity, phase identification, surface requires: C, 44.90; H, 6.54; N, 8.26%. IR (νmax/cm−1): 2971m,
morphology, stoichiometry and optical band energies, respect- 2931w, 1581w, 1480s, 1460w, 1444s, 1369s, 1321s, 1192s,
ively. Furthermore, the photoactivities of the deposited β-In2S3 1142s, 1117w, 1036s, 991w, 942s, 906s, 848s, 789s, 748w, 705s,
films were observed by means of photoelectrochemical (PEC) 614m, 582s, 529s, 476s. 1H-NMR δH (400 MHz, CDCl3):
measurements and the results confirmed that these SSPs are 1.48–1.68 (36 H, br d, 6CH(CH3)2), 3.95 (2 H, br s, CH(CH3)2),
suited to obtain better performing thin films of β-In2S3 for 5.16 (4H, br s, CH(CH3)2) and 7.27–8.60 (7.5 H, m, 1.5(NC5H5))
various optoelectronic applications.
ppm. TGA: 55–115 °C (15.50% wt. loss); 230–400 °C (64.25%
wt. loss); residual mass of 20.25%, Calcd for β-In2S3 21.36%).
Complex (3): Sodium piperidinedithiocarbamate (0.50 g,
2.73 mmol) and indium trichloride (0.20 g, 0.90 mmol); yield
(0.48 g, 83%), mp. 280–283 °C (decomposition). Elemental
analysis found: C, 37.70; H, 5.05; N, 7.40%; C20.5H32.5InN3.5S6
Experimental
Preparation and characterization of complexes
All preparations were carried out at room temperature. All requires: C, 38.76; H, 5.15; N, 7.71%. IR (νmax/cm−1): 2937s,
materials were obtained commercially and used as received. 2853m, 1581w, 1484s, 1456w, 1436s, 1359w, 1346w, 1278m,
The dithiocarbamate ligands were prepared according to pub- 1258m, 1232w, 1132m, 1109s, 1069w, 1022m, 1001m, 970s,
lished procedures.39
948m, 883s, 855m, 743w, 703m, 613m, 559m, 514s, 464m.
The elemental analysis was performed using a Perkin Elmer 1H-NMR δH (400 MHz, CDCl3): δ = 1.53–1.86 (18H, m, 3(CH2)),
CHNS/O Analyzer series II 2400. Infrared spectra were recorded 3.78–4.15 (12H, m, 3(NCH2)) and 7.27–8.61 (2.5H, m, 0.5
with a Perkin Elmer Spectrum 400 FT-IR/FT-FIR spectrometer (NC5H5)) ppm. TGA: 45–110 °C (6.84% wt. loss); 230–400 °C
(4000–400 cm−1). The TGA measurements were carried out on (69.31% wt. loss); residual mass of 23.85%, calcd for β-In2S3
a METTLER TOLEDO TGA/SDTA 851e Thermogravimetric 25.64%).
10920 | Dalton Trans., 2013, 42, 10919–10928
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Complex (4): Sodium benzyl(methyl)dithiocarbamate (0.5 g, then heated up to the desired temperature before deposition
2.28 mmol) and indium trichloride (0.17 g, 0.76 mmol); yield from (1)–(4). The aerosols of the precursor solution were
(0.46 g, 85%), mp. 180 °C. MW 703.69. Elemental analysis formed by keeping the round-bottomed flask in a water bath
found: C, 45.70; H, 4.33; N, 6.05%; C27H30InN3S6 requires: C, above the piezoelectric modulator of an ultrasonic humidifier.
46.08; H, 4.29; N, 5.96%. IR (νmax/cm−1): 3027w, 2927w, 1491s, The generated aerosol droplets of the precursor were trans-
1453s, 1433m, 1392s, 1357w, 1348w, 1301w, 1267w, 1242s, ferred into the hot wall zone of the reactor by the carrier gas.
1196s, 1157w, 1088s, 1070w, 1029m 983m, 955s, 817w, 745s, The reactor was placed in a tube furnace. The exhaust from the
732s, 694s, 624s, 569s, 549s, 485s. 1H-NMR δH (400 MHz, reactor was vented directly into the extraction system of the
CDCl3) 3.35 (9H, s, 3(CH3)), 5.10 (6H, s, 3(CH2)) and 7.26–7.42 fume hood where the deposition had taken place. At the end
(15H, m, aromatic 3(C6H5)) ppm. TGA: 230–400 °C (wt loss of the deposition, the aerosol line was closed and pure argon
74.60%); residual mass of 25.40%, calcd for β-In2S3 23.15%.
was allowed to flow over the thin films to cool them to 40 °C
before the characterization.
X-Ray crystallography
Surface characterization of thin films
Single crystal X-ray diffraction data were collected on a Bruker
D8 Apex II diffractometer for (1), (3) and (4) and on an Agilent Film morphology and composition were determined by a field-
SuperNova Dual for (2) with use of an Oxford low temperature emission gun scanning electron microscope (FE-SEM, FEI
apparatus. Multi-scan methods for absorption correction were Quanta 400) equipped with an energy dispersive X-ray spectro-
applied.40 The structures were solved by direct methods and meter EDX (INCA Energy 200 (Oxford Inst.)) operated at an
difference Fourier synthesis and refined by full-matrix least- accelerating voltage of 20 kV and a working distance of
squares based on all data.41 Crystal data are summarized in 9.2 mm. The type of phase and the crystallinity of the de-
Table 1.
posited films were determined using a PANanalytical, X’Pert
HighScore diffractometer with primary monochromatic high
intensity Cu-Kα(λ = 1.54184 Å) radiation. The data were col-
lected by scanning from 5° to 90° in a step size of 0.026° oper-
ated at 40 kV and 40 mA to cover all possible diffraction peaks
of the deposited material.
Deposition of thin films by AACVD
β-In2S3 thin films were synthesised from the precursors
[In(S2CNCy2)3]·2py (1), [In(S2CN(iPr)2)3]·1.5py (2), [In(S2CPip)3]·
0.5py (3) and [In(S2CNBzMe)3] (4) were deposited on FTO
(1 × 2 cm2) glass substrates (TEC 15, 12–14 Ω per square) by
using a self-designed ultrasonic AACVD method. The glass
Optical and photoelectrochemical characterization
substrate was washed ultrasonically with distilled water, The optical absorbance of thin films was measured with a
acetone and finally with ethyl alcohol before use. In a typical Lambda 35 Perkin-Elmer UV-vis spectrophotometer. The data
deposition experiment, 0.1 g of each of the precursors was dis- were registered from 350 to 1000 nm using the FTO glass sub-
solved in 12 mL of tetrahydrofuran and the mixture was added strate as a reference. The photoelectrochemical (PEC) pro-
to a round-bottomed flask connected to a deposition assembly. perties of β-In2S3 films were measured using the standard
Argon at a flow rate of 120 mL min−1 was used as the carrier three electrodes electrochemical cell fitted with a quartz
gas and the flow rate was controlled by an L1X linear flow window. An Ag/AgCl electrode and Pt were employed as the
meter. Substrate slides were placed inside the reactor tube and reference and counter electrodes, respectively. An aqueous
Table 1 Crystal data and refinement parameters for the complexes [In(S2CNCy2)3]·2py (1), [In(S2CN(iPr)2)3]·1.5py (2), [In(S2CPip)3]·0.5py (3) and [In(S2CNBzMe)3] (4)
(1)
(2)
(3)
(4)
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C
49H76InN5S6
C28.5H49.5InN4.5S6
762.40
100(2)
C20.5H32.5InN3.5S6
635.18
100(2)
C27H30InN3S6
703.72
100(2)
1042.33
100(2)
0.71073
Monoclinic
P21/c (No. 14)
10.5733(2)
22.1491(6)
22.3726(6)
90
91.804(2)
90
5236.8(2)
4
0.71073
0.71073
0.71073
Triclinic
Triclinic
Triclinic
ˉ
ˉ
ˉ
P1 (No. 2)
P1 (No. 2)
P1 (No. 2)
11.6384(4)
11.9426(4)
14.4945(3)
92.781(2)
102.078(2)
111.637(3)
1813.63(9)
2
9.4396(2)
10.7753
14.0452(3)
93.148(2)
91.655(2)
107.443(3)
1359.31(6)
2
9.7966(4)
12.2791(8)
12.7790(8)
96.142(4)
92.353(4)
100.383(5)
1500.51(15)
2
γ (°)
Volume (Å3)
Z
F (000)
2200
49 516
12 019 (Rint = 0.0679)
0.0476
0.1055
794
16 509
9388 (Rint = 0.0209)
0.0219
0.0515
650
13 467
6491 (Rint = 0.0114)
0.0162
0.0403
716
13 669
Reflections collected
Independent reflections
R1 (I > 2σ(I))
wR(F2) (all data)
6504 unique (Rint = 0.0177)
0.0228
0.0514
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Na2SO4 (0.01 M) and Na2S (0.10 M) electrolyte was used to
In the IR spectra ν(C–N) stretching vibrations for complexes
measure the PEC properties of β-In2S3 electrodes.42 Steady- (1)–(4) were detected in the range of 1471–1491 cm−1. This fact
state current–voltage measurements of the cells were carried implies that the dialkyldithiocarbamate is coordinated to the
out using a potentiostat (Eco Chemie micro-Autolab type III), metal as a bidentate ligand and indicates the partial double
while the cells were illuminated by an AM 1.5 Class A solar bond character in the CGN bond.43b,44
simulator (Solar Light 16S-300 solar simulator), at 100 mW
cm−2 light intensity, calibrated by a silicon pyranometer (Solar
Crystal structures of (1)–(4)
Light Co., PMA2144 Class II). The effective area of the electro-
The four tris-dithiocarbamate complexes have similar geo-
metric features but the structures differ in the number of pyri-
dine solvate molecules. The In–S distances of the bidentate
dithiocarbamate ligands fall within a narrow range, as do the
C–S distances, indicating full delocalization. Furthermore, the
CvN distances are short, as is typical for these ligands.44 To
illustrate these similarities, the average complex geometry is
given in Table 2, together with the average deviations from the
mean.
des was 1 cm2.
Results and discussion
The indium dithiocarbamates with general formula [In-
(S2CNRR′)3]·n(py) [where py = pyridine; R,R′ = Cy, n = 2 (1); R,R′
= iPr, n = 1.5 (2); NRR′ = Pip, n = 0.5 (3) and R = Bz, R′ = Me, n =
0 (4)] are conveniently prepared via metathetical reaction of
the indium trichloride and sodium dithiocarbamate in metha-
nol–pyridine solutions as shown in chemical reaction (1). Our
approach in developing SSPs differs from that of previously
used methods in the synthesis of indium [In(S2CNR)3] com-
plexes, in that we used a Lewis base (pyridine) as a solvent for
the crystallization of complexes (1)–(4). The dithiocarbamates
with Lewis bases are a class of precursor compounds with ver-
satile properties that include: air stability, ease of synthesis at
room temperature, enhanced volatility and facile decompo-
sition at relatively low temperatures. However, in the present
case, pyridine did not coordinate with the metal centre but
exists as solvate molecules in the crystal lattices (1), (2) and (3).
Structure of [In(S2CNCy2)3]·2py (1)
The crystal structure of [In(S2CNCy2)3]·2py (1) is shown in
Fig. 1. The complex (1) has non-crystallographic three-fold
symmetry as viewed down the plane of C1/C14/C27 and
without inclusion of the cyclohexyl conformations. All the
cyclohexyl rings are connected equatorially to their parent
nitrogen atoms. The two molecules of pyridine were dis-
ordered over two orientations. The two sets of atoms
Table 2 Selected bond distances (Å) and angles (°) for the complexes
[In(S2CNCy2)3]·2py (1), [In(S2CN(iPr)2)3]·1.5py (2), [In(S2CPip)3]·0.5py (3) and
[In(S2CNBzMe)3] (4) with average deviations from the mean given in square
brackets
InCl3 þ 3NaðS2CNRR′Þ Pyꢀri!dine ½InðS2CNRR′Þ3ꢁ ꢂ nðpyÞ þ 3NaCl ð1Þ
MeOH
(1)
(2)
(3)
(4)
The synthesized complexes were characterized by micro-
analysis, IR, NMR, TGA and single crystal X-ray techniques.
These complexes are produced in high yield and are stable in
air and moisture, and highly soluble in common organic sol-
vents such as THF, chloroform and pyridine. This makes
them promising precursors for application in the deposition
of indium sulphide thin films by AACVD. The CHN microana-
lysis found for complexes (1), (2), (3) and (4) agrees with the
stoichiometry of the complex as calculated from the crystal
data.
In–S (Å)
S–In–S bite angle (°)
C–S (Å)
2.590[4]
69.26[3]
1.732[6]
1.335[3]
2.594[9]
69.00[2]
1.730[2]
1.331[2]
2.592[11]
69.70[3]
1.730[2]
1.325[1]
2.594[11]
69.70[13]
1.726[3]
1.332[1]
CvN (Å)
1
The H-NMR studies of complexes (1), (3) and (4) reveal the
usual splitting patterns for the corresponding R groups,
however, an unusual splitting behaviour has been detected in
the 1H-NMR spectra of complex (2) where the isopropyl (CH-
(CH3)2) group displays three broad signals. Two equally
intense broad singlets appearing at 3.95 and 5.10 ppm are
assigned to the methine protons of (NCH(CH3)2), originating
from the different positions (syn or anti) with respect to the
dithiocarbamate group. The doublet signal of unequal intensi-
ties appearing at 1.48 and 1.68 ppm has been assigned to the
methyl protons of the isopropyl group and has been inter-
preted in terms of coordination and barrier rotation about
C–N, which makes the nitrogen substituents magnetically non-
equivalent.43
Fig. 1 The molecular structure of complex [In(S2CNCy2)3]·2py (1). Displacement
ellipsoids are shown at the 50% probability level.
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comprising the pyridine molecules were refined as rigid ideal-
ized hexagonal groups with occupancies that were initially
refined and then fixed at {0.50/0.50} and {0.30/0.70} for the
sets involving N4 and N5, respectively. With two pyridine mole-
cules removed from the refinement, the solvent accessible void
is 1256 Å3. In the structure, one pyridine occupies a channel
perpendicular to the bc plane. The second pyridine is sand-
wiched between cyclohexyl rings in an alternating fashion.
Structure of [In(S2CN(iPr)2)3]·1.5py (2)
In the structure of [In(S2CN(iPr)2)3]·1.5py (2), shown in
Fig. 2, there is one pyridine solvent molecule in a general
position and a second pyridine at half-occupancy disordered
about a centre of inversion. There is also small orientational
disorder (83 : 17) in one of the isopropyl groups (not shown).
With pyridine molecules removed from the refinement,
the accessible solvent volume is 438 Å3. In the structure
the 1.5 pyridine solvate molecules are encaged by isopropyl
groups.
Fig. 3 Structure of [In(S2CPip)3]·0.5py (3). Displacement ellipsoids are shown at
the 50% probability level.
Structure of [In(S2CPip)3]·0.5py (3)
In this complex the dithiocarbamate has a smaller footprint, as
the NRR′ has been replaced by piperidine as shown in Fig. 3.
There is one pyridine site, which is both rotationally disordered
and disordered with respect to a crystallographic center of
symmetry. With pyridine removed from the refinement,
the solvent accessible void is 167 Å3. In the structure the
pyridine molecules occupy a channel perpendicular to the bc
plane.
Structure of [In(S2CNBzMe)3] (4)
Complex (4), shown in Fig. 4, has no solvate molecules in the
structure. There is a non-classical hydrogen bond between S4
and a methyl H atom of a neighbouring molecule. The S⋯H
Fig. 4 Structure of [In(S2CNBzMe)3] (4). Displacement ellipsoids are shown at
the 50% probability level.
distance is 2.81 Å. In this solvate-free structure the packing
index is 68.4%, and there is no solvent accessible void.
The average In–S bond lengths 2.590[4], 2.594[9], 2.592[11]
and 2.594[11] Å observed in complexes (1)–(4), respec-
tively, are similar to those reported for analogues of these
compounds. The average C–S distances are close to 1.72 Å
indicating delocalization in the CS2 skeleton. The average
CvN bond distances, which are approximately 1.33 Å, are sig-
nificantly shorter than the normal single bond, suggesting
considerable double bond character, as expected for metal
dithiocarbamates.44
Fig. 2 Structure of [In(S2CN(iPr)2)3]·1.5py (2). Displacement ellipsoids are
shown at the 50% probability level.
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Thermal decomposition studies of complexes (1)–(4)
essentially due to the loss of pyridine. The TG and DTG data
reveal that the precursors (1)–(4) decompose in the tempera-
ture range 230–400 °C in a single step to produce stable
residue masses of 15.19, 20.09, 23.85 and 25.40%, respectively.
These mass losses agree well with the theoretical values of
15.62, 21.36, 25.64 and 23.15% for the formation of β-In2S3
from (1)–(4). The DTG curves of the pyridine solvated com-
plexes (1)–(3) indicate two maxima of heat intake. The first
heat intake occurs at 94 °C for (1) and (3) and at 84 °C for (2),
indicating the removal of pyridine, while the second maxima
appeared at 340 °C for (1), 290 °C for (2) and 330 °C for (3).
The DTG curve of non-solvated (4) shows maximum heat
intake at 340 °C and confirms its one step decomposition
(S.Fig. 5a–d, ESI†). The observed final residues upon heating
to 600 °C did not undergo any change in their weight
suggesting that complexes (1)–(4) decompose quantitatively to
furnish β-In2S3 as a stable end product.
The efficacy of complexes (1)–(4) as SSPs for the deposition of
indium sulphide thin films was probed by thermogravimetric
experiments and their TG and DTG curves are presented in
Fig. 5 and S.Fig. 5a–d (ESI†). The TG profiles of pyridine sol-
vated (1), (2) and (3) exhibit two-steps while the non-solvated
precursor (4) shows a one-step pyrolysis towards the end
product. The initial weight loss at temperature range of
55–115 °C from (1) and (2) and at 45–110 °C for (3) is attribu-
ted to the loss of pyridine molecules. The observed weight
losses of 11.60 (1), 15.50 (2) and 6.84% (3) show some agree-
ment with the calculated values of 11.37, 15.54 and 6.21% for
the loss of 1.5 and 0.5 molecules of pyridine from (1), (2) and
(3), respectively. The TG experiment on solvent free (4) did not
show any weight loss in the temperature range of 45–120 °C
confirming that the initial weight loss in (1), (2) and (3) is
Phase and crystalline structure identification of indium
sulphide thin films
Indium sulphide thin films were deposited from precursors
(1)–(4) on a FTO substrate at temperatures of 300, 350 and
400 °C by AACVD. The crystalline patterns obtained for the
indium sulphide thin films as a function of substrate tempera-
ture were investigated by powder X-ray diffraction (PXRD) and
are displayed in Fig. 6. Interestingly, similar PXRD patterns
were observed for all indium sulphide thin films prepared
from precursors (1)–(4) that agree well with the standard In-
organic Crystal Structure Database ICSD = [98-002-3844]45 file
(S.Fig. 6, ESI†) in terms of their 2θ, d-spacing and reflection
planes and identify the deposited product as the β-In2S3
Fig. 5 TG curves presenting losses in weight against temperature for pre-
cursors (1)–(4).
Fig. 6 PXRD patterns of β-In2S3 deposited from precursor [In(S2CNCy2)3]·2py (1), [In(S2CN(iPr)2)3]·1.5py (2), [In(S2CPip)3]·0.5py (3) and [In(S2CNBzMe)3] (4) at 350
(black) and 400 °C (red) on a FTO glass substrate. All of the diffraction peaks can be matched with ICSD = [98-002-3844].
10924 | Dalton Trans., 2013, 42, 10919–10928
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crystallizing in the tetragonal crystal system with a = b = 7.632,
SEM images of the thin films prepared using THF solutions
c = 32.36 Å. The peaks indexed by (*) correspond to the SnO2 of the precursors (1)–(4) on FTO glass substrates at tempera-
layer of the FTO substrate. The intensity of the peaks is signifi- tures 300, 350 and 400 °C are presented in Fig. 7. These indi-
cantly strong, which indicates that the product is well-crystal- cate that the shape and design of β-In2S3 films vary with the
lized in all cases. All the spectra are dominated by diffraction rise in substrate temperature. Thin films grown from (1)–(4) at
peak (123) located at 2θ = 27.7° (d = 3.21 Å) and there is no evi- 300 °C were too thin and were not detected by PXRD and SEM
dence of significant grain orientation. However, the diffraction micrographs. Fig. 7(a, d, g and j) show indistinguishable
peak intensities displayed a remarkable increase with the rise designs and crystallites stacked together with poor coverage of
in deposition temperature, which is probably due to improved substrate. These observations suggest that none of the four
crystallinity of the films when the substrate temperature was precursors is completely decomposed and the slow growth rate
raised from 350 to 400 °C. These PXRD peak patterns show no of the films at 300 °C is due to low deposition temperature.
characteristic peaks arising from possible impurities such as This observation is also confirmed by TGA where decompo-
InS, In2SO3 and other phases of In2S3. This clearly indicates sition of all the complexes occurs in the temperature range
that precursors (1)–(4) are capable of producing pure crystal- 340–375 °C. With the increase in deposition temperature to
line tetragonal phase β-In2S3 at 350 and 400 °C.
350 °C, thin film growth rates from all (1)–(4) become better
Previous studies have shown that the deposition tempera- and the substrate surface is fully covered with different mor-
ture, indium sulphur contents and molecular design of precur- phologies of β-In2S3 i.e. (1) granular crystallites with clear
sors are major parameters in determining the nature of the boundaries, Fig. 7b, (2) lumps, Fig. 7e, (3) agglomerates of
deposited indium sulphide phase i.e. InS, In6S7, α-In2S3 and particles along with some buds, Fig. 7h, and (4) multi-
β-In2S3.46 For example the compound [InnBu(SiPr)2] yielded shaped (triangular, square and hexagonal based pyramidal)
tetragonal β-In2S3 at 400 °C, however, the sulphur deficient interconnected crystallites, Fig. 7k. Further increasing the
phase, In6S7, was also grown with the rise in temperature to deposition temperature to 400 °C, (1) showed a similar kind of
450 °C.27b This is because of the dissociation of the thiolate morphology, Fig. 7c, as was observed for (4) at 350 °C.
moiety from the precursor, resulting in deposition of In6S7 at a However, (2), (3) and (4) developed large, small and flake like
high temperature of 450 °C. In a comparative growth study of architectures, Fig. 7f,i,l, respectively.
two different alkyl thiolate precursors [InMe2(StBu)]2 and
All the indium sulphide films produced from precursors
[IntBu2(StBu)]2, the methyl derivative produced a mixture of (1)–(4) adhered well to the FTO substrate and passed the
In6S7 and In2S3 while the butyl analogue generated only the
tetragonal InS phase.27a This behaviour was explained in
terms of the stronger In–C bond in the methyl substituted
complex that results in fragmentation of the precursor upon
decomposition to give a mixture of In6S7 and In2S3 phases,
whereas the tert-butyl derivative exhibits a clean ligand loss to
furnish InS. The bis-thiolate precursor [InMe(StBu)2]2 gener-
ates an amorphous phase at 400 °C that needs further anneal-
ing to yield crystalline β-In2S3.27a
When indium tris-(dialkyldithiocarbamates) complexes
were investigated as SSPs by LPCVD technique, their unsym-
metrical forms successfully yielded cubic α-In2S3 thin films at
500 °C while the symmetrical derivative proved not to be vola-
tile and no deposition occurred.28 A comparison of the films
deposited by LPCVD at 350 °C using two different indium tris
(monothiocarbamates) [In(SOCNEt2)3] and [In(SOCNiPr2)3]
precursors, reveals that the former complex produced thin
films with poor morphology while the diisopropyl derivative
deposited good quality β-In2S3 thin films.31 The foregoing
discussion summarizes that steric demand and coordinative
flexibility of the ligand have a great influence on the aggrega-
tion, stability and volatility of the resulting complexes and
therefore the choice of appropriate deposition technique is
required for the growth of good quality thin films.
Surface characterization of β-In2S3 thin films
Fig. 7 SEM images of β-In2S3 films deposited using precursor [In-
(S2CNCy2)3]·2py (1) at (a) 300 °C, (b) 350 °C and (c) 400 °C; [In(S2CN-
(iPr)2)3]·1.5py (2) at (d) 300 °C, (e) 350 °C and (f) 400 °C; [In(S2CNPip)3]·0.5py
(3) at (g) 300 °C (h), 350 °C and (i) 400 °C and [In(S2CNBzMe)3] (4) at ( j) 300 °C,
(k) 350 °C and (l) 400 °C.
The morphology and architectures of β-In2S3 thin films fabri-
cated from precursors (1)–(4) were investigated by scanning
electron microscopy (SEM).
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Table 3 Description of morphologies (Fig. 7) and In/S ratio in thin films fabricated from precursors (1)–(4) at 300, 350 and 400 °C
Deposition temperature °C, morphology, In/S ratio
Type of
precursor
300
350
400
(1)
(2)
(3)
(4)
(a) Flakes
1 : 1.69
(d) Unstructured deposit
1 : 1.61
(g) Unstructured deposit with poor coverage of substrate
1 : 1.68
(j) Unstructured deposit with poor coverage of substrate
1 : 1.89
(b) Irregular shaped crystallites
1 : 1.49
(e) Lumps
1 : 1.66
(h) Lumps
1 : 1.57
(k) Multi-shaped crystallites
1 : 1.56
(c) Multi-shaped crystallites
1 : 1.48
(f) Agglomerates
1 : 1.64
(i) Deteriorated crystallites
1 : 1.58
(l) Flakes
1 : 1.47
scotch tape test. Visual inspection of the films revealed that
dark yellow films were made at 300 and 350 °C while their
colour turned to orange red at 400 °C.
The film surface composition was determined by energy
dispersive X-ray analysis (EDX). Table 3 and S.Fig. 7, ESI† show
that the In/S atomic ratio existing in all the films prepared
from precursors (1)–(4) are close to the expected ratio of 1 : 1.5,
indicating the formation of β-In2S3.
Optical properties and band gap of thin films
The optical band gap energies of β-In2S3 thin films fabricated
from precursor (1) at 400 °C and (4) at 350 °C were studied by
spectrophotometry and are presented in Fig. 8. The UV-vis
spectra of these films recorded in the wavelength range
between 350–1000 nm indicate significant absorption in the
ultraviolet and the early visible regions, Fig. 8a.
The optical band gap (Eg) values were obtained from eqn (2)
below:
n
ðαhνÞ ¼ Aðhν ꢀ EgÞ …:
ð2Þ
where α is absorption coefficient, A is a constant, hν the
photon energy and n is an exponent which is equal to 1/2 or 2
for direct or indirect transition, respectively. The optical band
gaps were calculated from the absorbance data by plotting
(αhν)2 vs. photonic energy (hν) and extrapolating the linear
part of the curve on the energy axis and were found to be
2.2 eV (Fig. 8b). The thickness of the β-In2S3 films used for the
optical experiments was estimated by profilometer to be 395
and 358 nm, respectively. Our band gap value is in agreement
with the band gap energies of β-In2S3 thin films made through
other synthetic routes and with the bulk material.47 The band
structure indicates that charge transfer upon photo excitation
occurs from the sulphur 3p orbital to the indium 5p empty
orbital.
Fig. 8 (a) UV-vis spectra of β-In2S3 thin films deposited using precursor (1) at
400 °C and (4) at 350 °C on the FTO substrate. (b) Shows the direct band gaps
of 2.2 eV for β-In2S3 films.
concentration, deposition temperature, deposition time etc.
The PEC characteristics of as-prepared β-In2S3 films from pre-
cursors (1) and (4) by AACVD at 400 and 350 °C under the
alternative dark and illumination conditions are shown in
Fig. 9 and 10, respectively. The plots indicated that under illu-
mination a β-In2S3 electrode exhibits an anodic photocurrent,
which increases with increasing anodic bias.48 The rectified I–
Photoelectrochemical properties
Photoelectrochemical (PEC) characteristics obtained from the V plot (Fig. 9) of β-In2S3 from precursor (1) at 400 °C shows
1.25 mA cm−2 of photocurrent density at 0.23 V vs. Ag/AgCl
electrical and optical studies of an electrode–electrolyte inter-
face are an important tool for identifying the stability of the and rises steeply up to 1.5 mA cm−2 as the applied voltage
deposited films for electrochemical photovoltaic applications. further increased without reaching the saturation point. The
β-In2S3 films prepared using precursor (1) at 400 °C displayed
The PEC performance of such deposited electrodes can be
improved by using desirable precursors, composition, higher photocurrent response compared to the other
10926 | Dalton Trans., 2013, 42, 10919–10928
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Conclusions
Pyridine solvated (1)–(3) and non-solvated (4) tris(N,N-dialkyl-
dithiocarbamato)indium(III) complexes, with the general
formula [In(S2CNRR′)3]·n(py), have been prepared, character-
ized and implemented as AACVD precursors for the depo-
sition of β-In2S3 thin films. Despite of the different nature of
the alkyl groups, metal ligand designs and deposition temp-
eratures, each of the complexes exhibits only one phase and
stoichiometry equivalent to β-In2S3 as confirmed by PXRD,
SEM and EDX analyses of the thin films. These studies have
discovered that regardless of the attachment of different
ligands to In, comparable morphologies of β-In2S3 at
different temperatures were obtained. UV-vis measurements
of the β-In2S3 films showed band gap energies (Eg) of 2.2 eV,
making them suitable for photocatalytic activities. The
β-In2S3 films deposited from precursor (1) at 400 °C exhibited
a photocurrent density of 1.25 mA cm−2 vs. Ag/AgCl with an
applied bias of 0.23 V. The results confirmed that the
newly synthesised single source precursors are suitable to obtain
promising photoactive β-In2S3 thin films by AACVD method for
application in solar cells and other optoelectronic devices.
Fig. 9 J–V plot of β-In2S3 films obtained from precursor (1) by AACVD at
400 °C.
Acknowledgements
The authors acknowledge High-Impact Research scheme (grant
numbers UM.C/625/1/HIR/131) and the UMRG scheme (grant
number UM.TNC2/RC/261/1/1/RP007-13AET) for funding this
research. TANP and KGUW acknowledge the support from the
UK EPSRC and Johnson Matthey Plc.
Notes and references
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