Inorganic Chemistry
Article
a preformed network of metal−sulfur bonds that enables low
temperature decomposition to the corresponding metal
sulfides, which a is relatively clean process due to the
generation of gaseous side products. Bakly et al.,1 demon-
strated a new way to produce CdS, ZnS, and CdZnS thin films
by an inexpensive synthetic technique based on spin-coating
followed by annealing. Thomas et al.,13 reported high quality
nanocrystalline thin films of CdS, ZnS, CuS, and PbS
nanoparticles from thiobiuret complexes [M(SON-
(CNiPr2)2)2] as a single source precursor. Lewis et al.,14
reviewed the synthesis of metal chalcogenide materials using
‘reactive melts’ of precursors and concluded that self-capping
reactions are exceedingly simple and potentially scalable for
nanoparticle synthesis. Afzaal et al.,15 reviewed synthetic
methods for the production of binary metal chalcogenides
from single molecule precursors. Ramasamy et al.,16 reviewed
developments in synthetic routes to metal sulfide materials that
have applications in solar cells: emerging materials include
bismuth sulfide, ternary materials such as copper indium
sulfide, and quaternary materials such as copper zinc tin sulfide
all provide promising sustainable alternatives for solar energy
generation via the photovoltaic effect.
Metal chalcogenide compounds with thiolate ligands may
possess polymeric structures in solution and the solid state but
are often involatile. The lack of volatility in general means
these chalcogenide compounds are not useful as precursors for
thin film deposition techniques such as LP-MOCVD.15,17
However, these single source molecular precursors of precisely
defined composition can potentially provide a high degree of
control during synthesis. Indeed, many studies have demon-
strated the growth of binary and ternary nanostructures of
transition metal chalcogenides as thin films, nanocrystals,
nanosheets, nanoplates, nanowires, nanoribbons, and others
for energy applications.14,18−20
displays a schematic process of formation of inorganic films of
ZnS and CdS by thermal decomposition of the corresponding
zinc and cadmium alkylxanthato precursors. The films were
prepared by the spin-coating of the metal xanthate complexes
followed by annealing at relatively low temperatures in a
nitrogen atmosphere. The study we present demonstrates that
the structure of the ligand is the predominant factor in
controlling the thermal decomposition of metal xanthate
complexes.
EXPERIMENTAL SECTION
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Solvents and Reagents. Potassium hydroxide (≥85%), propa-
nol (≥99%), carbon disulfide (≥98.99%), petroleum ether
(≥98.99%), hexane (≥97%), acetone (≥99%), nbutanol (≥99%),
npentanol (≥99%), nhexanol (≥98%), nheptanol (≥98%), noctanol
(≥99%), potassium ethylxanthate (≥98%), cadmium chloride
(≥99%), zinc chloride (≥97%), acetonitrile (≥99.99%), methanol
(≥99.8%), ethyl acetate (≥99.5%), tetrahydrofuran (≥99.99%), and
N-methyl-2-pyrrolidone (≥99. %) were purchased from Sigma-
Aldrich or Fisher and used without further purification. Deuterated
solvents for NMR were purchased from Sigma-Aldrich.
Materials Characterization. Elemental analysis was conducted in
the microanalytical laboratory at the University of Manchester with a
Carlo Erba EA 1108/Flash 2000 Thermo Scientific elemental
analyzer. Thermogravimetric Analysis (TGA) and Differential
Scanning Calorimetry (DSC) profiles were recorded from 30 to
600 °C with a 10 °C min−1 heating rate under N2, using a Mettler
Toledo TGA-DSC1. Fourier transform infrared (FTIR) spectra were
obtained using a Nicolet iS5 IR Spectrometer-Thermo Fisher
(NMR) spectra were recorded using a 400 MHz Bruker instrument;
a capillary tube using a Stuart SMP10 melting point apparatus.
Powder X-ray diffraction measurements were carried out at room
temperature by using a Bruker D8 Advance diffractometer, using Cu−
Kα radiation, (1.5418 Å), 40 kV, 40 mA. The scanning range was
between 20° and 80° with a step size of 0.050° and a dwell time of 8 s.
The diffraction patterns were processed using X’Pert High Score Plus
software. High resolution transmission electron microscopy
(HRTEM) images and energy dispersive X-ray spectra were obtained
using an FEI Talos F200A microscope equipped with X-FEG electron
source and Super-X SDD EDS detectors. The experiment was
performed using an acceleration voltage of 200 kV and a beam current
of approximately 1 nA. TEM images were recorded with an FEI
CETA 4K CMOS camera. STEM images were acquired with a high
angle annular dark field (HAADF) detector. Photoluminescence (PL)
decays were recorded using a Time-Correlated Single-Photon
Counting (TCSPC) system, equipped with a mode-locked Ti:
sapphire laser (Spectra Physics Mai-Tai HP). The photoluminescence
from the samples was collected and focused into a monochromator
(Spex 1870c). Emission was detected using a multichannel detector
(Hamamatsu R3809U-50). The time correlation of detected photons
was performed with the use of a PC electronic card from Edinburgh
Instruments (TCC900). Absorption spectra were recorded with a
Shimadzu double beam spectrophotometer, model UV-1800, wave-
length range between 800 and 300 at 1 nm resolution. Single crystal
X-ray diffraction data for the compounds were collected on a dual
source Rigaku FR-X rotating anode diffractometer, using a Mo Kα
wavelength at 150 K, and reduced using CrysAlisPro version
171.39.21a. The structures were solved and refined using Shelx-
2016 implemented through Olex2 v1.2.8. CIF files were deposited
Synthesis of Molecular Precursors. The preparation of fourteen
different metal xanthato complexes (7−20) are described in this
section. All syntheses of the ligands (1−6) and precursors (7−20)
were performed under dry N2, using standard Schlenk techniques.
The ligands (1−6) and precursors (7−20) were prepared as
In this study, a series of metal-xanthato precursors (7−20)
were synthesized and their thermal decomposition to metal
chalcogenide materials is studied. The work is grouped into
three main sections; an X-ray crystallographic study of
precursor structure, thermal analyses of precursors, and film
deposition using spin-coating methods and subsequent
structural and optical characterization of the resulting solid
state materials. New single crystal structures of metal
alkylxanthato precursors are presented. We then focus on the
decomposition conditions and the thermal behavior of the
metal xanthate complexes. We find that the ligand structure
plays a leading role in dictating the energy required for thermal
decomposition of metal alkylxanthato precursors. Figure 1
Figure 1. Schematic showing the process of the formation of metal
sulfide films from bis(O-alkylxanthato)cadmium(II) or zinc(II)
compounds by spin-coating followed by annealing at low temper-
atures (T < 300 °C).
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Inorg. Chem. 2021, 60, 7573−7583