CARTHAGE COLLEGE Structure Identification of Ni- Fe Oxide Mixture using Raman Spectroscopy, and X-ray Absorption Near-Edge Spectroscopy
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 made 1. The structure of the Ni-Fe oxide mixture could help in understanding why Ni-Fe oxide mixture Page 2 of 15
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 source 1. Nickel Oxide packs in a NaCl lattice, a face-centered cubic lattice with octahedral holes. The Fe in Fe 2 O 3 also resides in an octahedral hole. Since NiO and Fe 2 O 3 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 1 2 ). 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 Fe 2 O 3 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 Page 3 of 15
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 AB 2 compounds 2, which can only be composed of two elements. For a compound like Ni-Fe oxide mixture, its molecular formula would be NiFe 2 O 4. Since this compound is an AB 2 O 4 compound, it has a spinel structure 3 (Figure 3 4 ). The AB 2 O 4 compounds are called AB 2 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 holes 2,3. The A component of the compound resides in the tetrahedral holes while B resides in the octahedral holes 3. However, there are spinels Page 4 of 15
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 holes 3. 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 hole 2. 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 d- orbitals dz 2 and dx 2 -y 2 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 dz 2 and Page 5 of 15
dx 2 -y 2. 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 d 4, shows that chromium prefers to be in an octahedral Page 6 of 15
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 surfactant 1. 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 water 1. 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 hours 1. 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 scattering 6. 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 Page 7 of 15
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 strength 7. 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. Page 8 of 15
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 state 9. The energy between the two states can reveal the structure and can characterize bond orientation 10. 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. Page 9 of 15
Results and Discussion The Raman spectra of NiO, Fe 2 O 3, and several Ni-Fe oxide mixtures is shown in Figure 7 1. 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. Page 10 of 15
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, Fe 3 O 4, and Fe 2 O 3 were shown in Figure 8 1. Since the 10% Fe mixture s spectrum looked similar to the Fe 3 O 4, and Fe 2 O 3, it suggested that the oxidation state of Fe in the mixture was +3 and had some Fe atoms that were tetrahedral. The line for Fe 2 O 3 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 3 O 4 and FeO crossed the line of 10% Fe but never overlapped with it, so the line of Fe 2 O 3 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 Page 11 of 15
transition of 1s 3d, was a signature signal for tetrahedral Fe 1. 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 9 2 ) showed that Ni +2, which has an electron configuration of d 8, preferred to have an octahedral coordination, making it prefer an octahedral shape. The CFSE of Fe +3, which has an electron configuration of d 5, 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 Page 12 of 15
octahedral hole and the B component in both a tetrahedral and octahedral hole, the Ni- Fe 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, Fe 2 O 3, and Fe 3 O 4 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. Page 13 of 15
Bibliography 1. Landon, J.; Demeter, E.; İnoğlu, N.; Keturakis, C.; Wachs, I. E.; Vasić, R.; Frenkel, A. I.; Kitchin, J. R., Spectroscopic Characterization of Mixed Fe Ni Oxide Electrocatalysts for the Oxygen Evolution Reaction in Alkaline Electrolytes. ACS Catalysis 2012, 2 (8), 1793-1801. 2. Rodgers, G. E., Descriptive Inorganic, Coordination, and Solid-State Chemistry. 3rd ed.; Brooks/Cole: Belmont, CA, p 624. 3. Swaddle, T. W., Inorganic Chemistry. Academic Press: San Diego, California, 1997; p 482. 4. Ionic Crystals. H. Foll. 5. Mitzi, D., Solution Processing of Inorganic Materials. Wiley: Hoboken, NJ, USA, 2009. 6. Crouch, S. H., Principles of Instrumental Analysis. 6th ed.; Thomson Book/Cole: p 1039. 7. Ferraro, J. R.; Nakamoto, K.; Brown, C. W., Introductory Raman Spectroscopy. Academic Press: Burlington, MA, USA, 2002. 8. Raman Data and Analysis. http://www.horiba.com/fileadmin/uploads/scientific/documents/raman/bands.pdf. 9. Willmott, P., Introduction to Synchrotron Radiation : Techniques and Applications. Wiley: Hoboken, NJ, USA, 2011. 10. Ade, H.; Stoll, H., Near-edge X-ray absorption fine-structure microscopy of organic and magnetic materials. Nature Materials 2009, 8 (4), 281. Page 14 of 15
11. Hosterman, B. D. Raman Spectroscopic Study of Solid Solition Spinel Oxides. University of Nevada, Las Vegas, 2011. Page 15 of 15