32
O. Farinre et al.
MD simulations were also carried out using the LAMMPS software code to provide
additional insight into the vibrational properties of the GnPs and to facilitate comparison
with experiment.
2 Materials and Methods
2.1 Experimental
Tin Dioxide (SnO 2 ): Tin dioxide powder with 325 mesh, 99.9% purity, was commercially purchased from US Research Nanomaterials TM . The Raman instrument used was
a SmartDXR™ spectrometer from Thermo Electron with an excitation wavelength of
780 nm. A Ventacon™H4-200 heated cell was employed to control the temperature of
the SnO 2 powder sample during the Raman measurements. A Thermo Scientific ARL TM
EQUINOX 100 X-Ray diffractometer was used to record the XRD spectra of SnO 2 . The
XRD instrument had a Cu-K α monochromatic radiation source at a wavelength of
λ = 1.5406 Å. The beam size was approximately 5 mm × 300 μ m, along with a spinning stage. A Thermo Scientific Phenom Pure TM Desktop SEM with an electron optical
magnification in the range 80–65,000x was used to study the surface morphology of
both SnO 2 and GnPs.
Graphene Nanoplatelets (GnPs): The pristine GnPs supplied by US Research
Nanomaterials TM , Inc. had a planar size of 4–12 and planar thickness of 2–8 nm.
Each nanoplatelet is composed of ~ 3–6 layers of graphene sheets. The functionalized
GnPs supplied by Graphene Supermarket TM had a planar size of 0.3–5 and planar thickness < 50 nm. The carboxyl (COOH) functionalized GnPs with 35 wt% functionality
purchased from Cheap Tubes Inc. TM had a planar size of 1–2 and planar thickness of
3–10 , with each nanoplatelet composed of approximately 4 layers.
2.2 Computational and Simulation Method
Tin Dioxide (SnO 2 ): The Buckingham interatomic potential was used to demonstrate
the interactions between the Sn and O atoms. This potential can be applied by using the
shell model available in the CORESHELL package in LAMMPS. Each SnO 2 molecule
is split into a core of charge X and a satellite (shell) of charge Y connected by a harmonic
spring, where the total charge is X + Y. The USER-PHONON package in LAMMPS is
added to develop the phonon dispersion curves for SnO 2 and utilized the Verlet algorithm,
an (NPT) ensemble [13], and compiled the core/shell model to compare the lattice
constants SnO 2 with other studies.
Graphene Nanoplatelets (GnPs): The optimized Tersoff and Brenner empirical interatomic potential [12] was used to describe the interactions between the carbon atoms in
GnPs. The Tersoff potential is commonly employed to calculate the phonon properties
of graphene and its derivatives in MD simulations because the optimized parameter sets
yield vibrational frequencies that are in better agreement with experimental data. Trilayer graphene was used initially for our GnP model because the individual platelets in
O. Farinre et al.
MD simulations were also carried out using the LAMMPS software code to provide
additional insight into the vibrational properties of the GnPs and to facilitate comparison
with experiment.
2 Materials and Methods
2.1 Experimental
Tin Dioxide (SnO 2 ): Tin dioxide powder with 325 mesh, 99.9% purity, was commercially purchased from US Research Nanomaterials TM . The Raman instrument used was
a SmartDXR™ spectrometer from Thermo Electron with an excitation wavelength of
780 nm. A Ventacon™H4-200 heated cell was employed to control the temperature of
the SnO 2 powder sample during the Raman measurements. A Thermo Scientific ARL TM
EQUINOX 100 X-Ray diffractometer was used to record the XRD spectra of SnO 2 . The
XRD instrument had a Cu-K α monochromatic radiation source at a wavelength of
λ = 1.5406 Å. The beam size was approximately 5 mm × 300 μ m, along with a spinning stage. A Thermo Scientific Phenom Pure TM Desktop SEM with an electron optical
magnification in the range 80–65,000x was used to study the surface morphology of
both SnO 2 and GnPs.
Graphene Nanoplatelets (GnPs): The pristine GnPs supplied by US Research
Nanomaterials TM , Inc. had a planar size of 4–12 and planar thickness of 2–8 nm.
Each nanoplatelet is composed of ~ 3–6 layers of graphene sheets. The functionalized
GnPs supplied by Graphene Supermarket TM had a planar size of 0.3–5 and planar thickness < 50 nm. The carboxyl (COOH) functionalized GnPs with 35 wt% functionality
purchased from Cheap Tubes Inc. TM had a planar size of 1–2 and planar thickness of
3–10 , with each nanoplatelet composed of approximately 4 layers.
2.2 Computational and Simulation Method
Tin Dioxide (SnO 2 ): The Buckingham interatomic potential was used to demonstrate
the interactions between the Sn and O atoms. This potential can be applied by using the
shell model available in the CORESHELL package in LAMMPS. Each SnO 2 molecule
is split into a core of charge X and a satellite (shell) of charge Y connected by a harmonic
spring, where the total charge is X + Y. The USER-PHONON package in LAMMPS is
added to develop the phonon dispersion curves for SnO 2 and utilized the Verlet algorithm,
an (NPT) ensemble [13], and compiled the core/shell model to compare the lattice
constants SnO 2 with other studies.
Graphene Nanoplatelets (GnPs): The optimized Tersoff and Brenner empirical interatomic potential [12] was used to describe the interactions between the carbon atoms in
GnPs. The Tersoff potential is commonly employed to calculate the phonon properties
of graphene and its derivatives in MD simulations because the optimized parameter sets
yield vibrational frequencies that are in better agreement with experimental data. Trilayer graphene was used initially for our GnP model because the individual platelets in