Download Structure Identification of Ni-Fe Oxide Mixture

CARTHAGE COLLEGE
Structure Identification of NiFe Oxide Mixture using
Raman Spectroscopy, and
X-ray Absorption Near-Edge
Spectroscopy
Avila Hoffman
“Structure Identification…”
Avila Hoffman
Abstract
Advancement in instrumentation has made identification of metal structures
easier with respect to determining structure and how multiple metals in a structure affect
one another. The metal NiO was examined to see how the addition of Fe changes the
structure of NiO. Raman spectroscopy and the X-ray absorption near-edge
spectroscopy (XANES) was used to find the structure of the Ni-Fe oxide mixture. The
Raman spectrum showed that at 10% mol Fe composition a spinel structure was formed
compared to the face-centered cubic structure of NiO. Since the spectrum showed a
spinel structure, it suggested either the Fe or the Ni from the mixed oxide would have a
tetrahedral coordination or an octahedral coordination. XANES showed that the
structure of the Fe in the mixture had a tetrahedral structure, which meant that Ni had
the octahedral structure. Since Ni and Fe were found to be in the opposite position of
what normally is expected of a spinel structure, the Ni-Fe oxide mixture has an inverse
spinel structure.
Introduction
Metal oxides form many interesting structures, and determining their structure is
important because they can be used as catalysts. Instruments can now help scientists
identify the structure of metal complexes. In this thesis, nickel oxide (NiO) will be
examined as a reference to find the structure of Ni-Fe oxide mixtures. Nickel and iron
will be used because Ni-Fe oxide mixtures have been reported to have superior
reactivities in comparison to the pure metals from which they are made1. The structure
of the Ni-Fe oxide mixture could help in understanding why Ni-Fe oxide mixture
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catalysts have stable activity. Moreover, the Ni-Fe oxide mixtures could take part in
oxygen evolution reactions, which is a reaction being studied as a renewable energy
source1.
Nickel Oxide packs in a NaCl lattice, a face-centered cubic lattice with
octahedral holes. The Fe in Fe2O3 also resides in an octahedral hole. Since NiO and
Fe2O3 are found in an octahedral hole, they have an octahedral coordination. The facecentered cubic lattice has the cation in the edge and body positions of the unit cell,
while the anion will be at the corners and faces of the unit cell (Figure 12). Therefore,
the Ni in NiO is located at the edges and body of the unit cell and the O is located at the
corners and faces of the unit cell. The number of cations and anions in NiO can be
calculated by multiplying the number of atoms in the corners by , in the body by 1, in
the faces by , and in the edges by . For example, the number of Ni atoms in NiO is
1(1)+12( )= 4 and the number of O atoms is 8( +6( )= 4. Therefore, the ratio of Ni to O
atoms is 1:1. The lattice structure of NaCl, NiO, and Fe2O3 can be estimated using the
Radius Ratio rule. The rule compares the radius of the cation to the radius of the anion
to indicate if the metal resides in a tetrahedral, octahedral, or cubic hole. A depiction of
the different types of holes is shown in Figure 2 2. The size of the cation decreases as it
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loses electrons and the size of the anion increases as it gains electrons. As the size of
the cation decreases, the cation goes from residing in a cubic hole, where it has a big
ionic radius, to an octahedral hole and finally to a tetrahedral hole, where it has a small
ionic radius. Metals that reside in a cubic hole have a body center cubic cell lattice. A
metal with face-centered cubic lattice has its atoms residing in a tetrahedral or
octahedral hole. Therefore, the Radius Ratio rule can be used for investigating the
lattice of a metal compound. However, this equation can only be used for AB or AB2
compounds2, which can only be composed of two elements.
For a compound like Ni-Fe oxide mixture, its molecular formula would be
NiFe2O4. Since this compound is an AB2O4 compound, it has a spinel structure3(Figure
34). The AB2O4 compounds are called AB2 because the cations A and B have
coordination numbers that are in a 2 to 1 ratio. A coordination number is the number of
ligands attached to the metal. In a spinel compound, A typically has a +2 charge and B
typically has a +3 charge. The cations, A and B, reside in of the tetrahedral holes and
of the octahedral holes2,3. The A component of the compound resides in the
tetrahedral holes while B resides in the octahedral holes3. However, there are spinels
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that have an inverse spinel structure. When the metal has an inverse spinel, the A
component is in an octahedral hole and the B component is equally divided in both
tetrahedral and octahedral holes3. Therefore, to find whether the Ni-Fe oxide mixture is
a spinel or an inverse spinel, the crystal field stabilization energy (CFSE) of Ni and Fe
has to be analyzed because the spinel structure follows the CFSE to see which metal
prefers to be in which type of hole2.
CFSE comes from crystal field theory, which says that some electron
configurations are stabilized by the distribution of the d-orbital electrons. The distribution
of the d-orbital electrons can be shown using a splitting diagram. The splitting diagram
is composed of five d-orbitals as shown in Figure 4. The five d-orbitals separate
according to which orbitals have their electrons near the x, y and z axes and which ones
don’t have electrons near the axes. The electrons near the axes cause repulsion,
decreasing stabilization and increasing energy. In an octahedral coordination, the dorbitals dz2 and dx2-y2 have their electrons near the axis, so they have a higher energy
than the dxy, dxz, and dyz, which have electrons between the axes. In a tetrahedral
coordination, the arrangement of the d-orbitals is the exact opposite. The dxy, dxz, and
dyz orbitals have their electron near the axis so they have higher energy than dz2 and
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dx2-y2. The d-orbital electrons are added to the splitting diagram so that CFSE can be
calculated. An example of the d-orbital splitting diagrams and the CFSE is shown in
Figure 5, using chromium. The d-orbital splitting diagram of Cr+2, which has an electron
configuration of d4, shows that chromium prefers to be in an octahedral
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coordination. The reason why Cr prefers an octahedral hole is because the CFSE for an
octahedral coordination is more stable than the CFSE for a tetrahedral coordination.
The d-orbital splitting pattern and CFSE can help predict the preferred coordination of Ni
and Fe in the Ni-Fe oxide mixture.
The Ni-Fe oxide mixtures were synthesized using the evaporation-induced selfassembly (EISA) technique. Five grams of deionized water and 5g of ethanol were used
to dissolve one gram of P123 surfactant1. A surfactant is a compound that has both a
hydrophilic and hydrophobic area. Metal nitrate precursor salts with the desired molar
composition were dissolved in the mixture and then transferred into a ceramic boat,
where the solution was heated to 120ᵒC for 12.5 hours to remove any excess ethanol
and water1. The interaction of the precursor and the surfactant lowers the total
interfacial energies of the product 5, meaning the surface tension was lowered. After
12.5 hours, a gel formed and was further heater to 350ᵒC at a rate of 1ᵒC/min and then
held for 4 hours1. The final heating was done to remove any remaining surfactant 1. The
product was then ground using a mortar and pestle and stored until needed 1. This
process was used to make the Ni-Fe oxide mixtures that were used in the instruments.
Raman Spectroscopy can also be used to find the structure of metal compounds.
This technique examines the vibration and rotation of molecules. A Raman
spectrometer is shown in Figure 6 6. The source inside the instrument is a laser with a
narrow bandwidth, providing near monochromatic light. The laser hits the sample and
causes Raman scattering6. Scattering is when small fractions of the signal are
transmitted at all angles. The scattering signals, which have different wavelengths, are
then filtered using a monochromator. The monochromator is used to separate the
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wavelengths and to narrow the range of wavelengths. Inside the monochromator, a
grating is used to separate and diffract the wavelengths at different angles. Since the
grating has many small groves, each wavelength is reflected at different angles. The
separated signals end up in the Raman spectrum and show different vibrational
frequencies. Each vibrational frequency depends on atomic mass and bond strength7.
Therefore, each peak would be a different chemical species. For example, the
frequency for a C-C bond is 600 to 1300 cm-1, but when the bond becomes a C=C bond,
the frequency is 1500 to 1900 cm-1 8. Thus, if the structure of a compound changes, its
peaks would also change. Therefore, Raman spectroscopy can be used to see if the
addition of Fe changes the structure of NiO.
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X-ray absorption near-edge spectroscopy (XANES) can also be used to find the
structure of a compound. XANES compares the light that goes in and out of the sample.
When light hits a sample, scattering and fluorescence emission occurs. Therefore, the
light that hits the sample is not the same as the light that gets transmitted. The photons
from the light can use their energy to move electrons of the sample from the ground
state to a higher energy state in a process called absorption. The electrons undergoing
absorption are now in an excited state. Fluorescence occurs when the excited electron
emits the light it absorbed, returning to the ground state. Since every atom absorbs and
emits light differently, the light that ends up being transmitted will be different for every
sample. Therefore, the difference between the initial light and the transmitted can be
used to identify the atoms in a metal compound. The XANES instrument works by
having X-rays from an x-ray tube, or synchrotron, pass through a monochromator 9. The
monochromator separates the wavelengths in the X-ray and allows only a narrow
wavelength of light to hit the sample. The light that hits the sample transmits a light that
then goes through a detector and reaches the signal processing area, where the signal
is turned into a spectrum. XANES measures the initial energy state and the final energy
state9. The energy between the two states can reveal the structure and can characterize
bond orientation10. Therefore, XANES can be used to find the structure of the sample.
Using all of these techniques, the structure of a metal compound can be found.
The techniques can be used to analyze Ni-Fe oxide mixture and to find which metal, Fe
or Ni, has the octahedral or the tetrahedral coordination. Whether the structure has a
spinel or an inverse spinel structure can be found using the information obtained about
which metal resides in what coordination.
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Results and Discussion
The Raman spectra of NiO, Fe2O3, and several Ni-Fe oxide mixtures is shown in
Figure 71. The main peaks for NiO were present at 1101 and 1480 cm-1. The band at
1101 cm-1 was present in 5% and 10% Fe but disappeared in the 20% Fe spectrum.
This data suggested that the structure of NiO is retained only until a 10% Fe
composition and changed at 20% Fe. At 5% Fe, three new bands appeared at 450 to
700 cm-1. The peaks there showed that Ni-Fe oxide mixture is a spinel structure
because spinel structures normally have three peaks between 350 to 800cm-1
11
.
Therefore, the Ni-Fe oxide mixture’s structure resembles that of a spinel structure. This
data suggested that either Ni or Fe must have a tetrahedral structure in the Ni-Fe oxide
mixture.
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XANES analysis was done to further analyze the structure of the Fe in the 10
mol% oxide mixture. The XANES spectrum for the 10% Fe mixture and the reference
samples of FeO, Fe3O4, and Fe2O3 were shown in Figure 81. Since the 10% Fe
mixture’s spectrum looked similar to the Fe3O4, and Fe2O3, it suggested that the
oxidation state of Fe in the mixture was +3 and had some Fe atoms that were
tetrahedral. The line for Fe2O3 overlapped the line for 10% Fe from 7120 to 7140 eV
and was very close to the line before 7120 eV and after 7140 eV. The line of Fe 3O4 and
FeO crossed the line of 10% Fe but never overlapped with it, so the line of Fe2O3 was
the line most similar to the line of 10% Fe, suggesting that 10% Fe has an oxidation
state of +3. The major peak that was present in all of the compounds at 7135 eV
resulted from the transition 1s→4p. The peak at 7115 eV, which results from the
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transition of 1s→3d, was a signature signal for tetrahedral Fe1. Therefore, since the 10%
Fe has that peak, it suggested that the 10% Fe has some tetrahedral Fe.
Another reason why iron had a tetrahedral geometry was because when looking
at the spinel structure, either Ni or Fe had to be tetrahedral. The CFSE of Ni (Figure 92)
showed that Ni+2, which has an electron configuration of d8, preferred to have an
octahedral coordination, making it prefer an octahedral shape. The CFSE of Fe+3, which
has an electron configuration of d5, showed that it had no preference over an octahedral
or tetrahedral coordination as both geometries provided 0 CFSE. Therefore, since Fe
has no energetic preference, Ni would reside in an octahedral hole and Fe would reside
in a tetrahedral hole. Since the inverse spinel structure has the A component in an
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octahedral hole and the B component in both a tetrahedral and octahedral hole, the NiFe oxide mixture has an inverse spinel structure.
Conclusion
The Ni-Fe oxide mixture was compared to NiO by Raman spectroscopy. The
spectrum showed that the addition of Fe didn’t change the structure of NiO until 10%
mol Fe composition. It also showed that at 5% Fe composition, a spinel structure was
being formed. The 10%mol Fe was compared to FeO, Fe2O3, and Fe3O4 by XANES.
The spectrum from XANES showed that the 10% Fe has an oxidation state of +3 and a
tetrahedral structure. This agreed with the CFSE of Fe because Fe has no energetic
preference between tetrahedral and octahedral coordination. The CFSE of Ni showed
that Ni preferred the octahedral coordination. Since in the Ni-Fe oxide mixture Ni is in
the octahedral coordination and Fe is in both tetrahedral and octahedral coordination,
the structure of Ni-Fe oxide mixture is an inverse spinel structure.
These techniques can be used in many different fields of chemistry. Examples of
fields in which the identification of metal structures can be used are in the medical field
and in environmental science. Knowing what the structures of the metals are can help
scientists understand how stable the metal is and how the metal could react with other
compounds. A future work could be to study the structure of the oxides that are
currently being used for oxygen evolution reaction, and see how their structure and
efficiency differs from the mixture Ni-Fe oxide.
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