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Reversal in the Lattice Contraction of #-FeO Nanoparticles Monika Sharma, Sevi Murugavel, Dinesh Kumar Shukla, and Frank M.F. de Groot J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00550 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018
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Reversal in the Lattice Contraction of α-Fe2O3 Nanoparticles Monika Sharma,1 Sevi Murugavel,1* Dinesh Kumar Shukla,2 and Frank M. F. De Groot3 1
Department of Physics and Astrophysics, University of Delhi, Delhi–110 007, India UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore - 452 001, India 3 Inorganic Chemistry & Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG, The Netherlands 2
ABSTRACT: We report the discovery of size dependent lattice contractions and elongations in nanoscale hematite (α-Fe2O3) structure revealed by high resolution x-ray diffraction analysis and spectroscopic measurements. The observed lattice modification effects are classified into two different regions with crystallite size with a threshold anomaly at ≈30 nm. In addition to the commonly observed lattice expansion in ionic solids, we report the discovery of lattice contraction at the nanoscale level (< 30 nm crystallite size). The consequences of anomalous structural behaviour with crystallite size are reflected in the electronic and vibrational properties of hematite structure. The characteristic behaviour in structural and electronic properties of the hematite structure is closely linked with changes in the bonding character which show strong dependence on the crystallite size. We suggest that the lattice expansion is caused by weaker hybridization of eg states, whereas at the nano scale regime the increased level of Fe 4sp-O 2p hybridization leads to the less oxygen 2p to hybridize with the 3d-states implying an increased level of covalency and reduction in the unit cell parameters. Further, the change in the bonding characteristics lead to the enhanced polaronic conductivity of four orders of magnitude at the nanoscale level, which is highly beneficial for the unique structural advantage of the iron oxide and its derived compounds.
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INTRODUCTION Iron oxides constitute an important class of mineral that is pervasive in nature. The most important geo- and biochemical redox reactions are lined either with formation or transformation of iron oxides from one state to another by charge transfer processes which cycle the iron between its two oxidation states1-5. Recently, much attention has been devoted to synthesis and characterization of hematite nanostructures towards their applications in photo electrochemical cell for water splitting, gas sensors, magnetic recording media, and as anode material for lithium ion batteries6-9. Among the various form of iron oxides, bulk hematite structure (α-Fe2O3) is an n-type semiconductor with an energy gap of ≈1.9 eV at room temperature and is the most thermodynamically stable phase. Due to its semiconducting nature, the nanosized hematite particles have great advantage of tuning its physical and chemical properties. Its applicability as a better anode material for lithium ion batteries merits in terms of its non-toxicity, low cost and high capacity. In comparison with conventional graphite anode material for lithium ion batteries, α-Fe2O3 exhibits high capacity (1005 mAhg1
), which is 2.7 times higher than that of graphite. In recent years, various morphologies of
Fe2O3 such as monodisperse nanocrystals, nanotubes, hollow structure, nanowire and freestanding nano membranes are demonstrated to be highly advantageous to improve the performance of Li-ion batteries9. However, the commercialization of the α-Fe2O3 as anode material for energy storage devices is still limited by the poor capacity retention with short life due to the volume expansion/contraction upon repeated cycling and low polaronic conductivity. Thus, there are still great challenges to achieve extended cycle life and high rate capability with adequate reversible capacity for high power energy storage devices. In spite of the growing importance of nanosized α-Fe2O3 structure in practical applications, the fundamental understanding of the intrinsic defects in hematite structure is 2 ACS Paragon Plus Environment
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not clear yet. The electronic properties are controlled by the defects present in the hematite structure since the concentration and type of the defect decides the charge transport mechanism. In particularly, the oxygen vacancies are expected to be dominant one and act as shallow donor dopants in hematite, i.e. energy levels close to the conduction band minimum 10
. The creation of each oxygen vacancy leads to two excess electrons and forms polaronic
sites (Fe2+) and this would increase the carrier (polaron) concentration and therefore the overall electrical conductivity enhancement. An early experimental investigation on nonstoichiometric Fe2O3-δ revealed its partial pressure dependency on the electrical properties 11. There have been few computational investigations on the defect properties of α-Fe2O3 using classical approaches 12. More importantly, these studies reveal that the changes in the oxygen vacancy concentrations can lead to structural phase transition from hematite to magnetite (γFe2O3) 13. Therefore, by considering the complex nature of multivalent oxides like α-Fe2O3, a detailed investigation at the ab initio level is highly desirable. In this context, the importance of defect states in hematite structure become more relevant in nano sized α-Fe2O3 than the bulk one due to the surface energy kinetics. Another interesting aspect of transition metal oxides (TMO) are the size dependent lattice anomaly with the reduction of particle size below a micron meter scale. More commonly, lattice parameters are the basic quantity which characterizes any crystalline solid and thus their variation directly affects the physical properties. The size dependent lattice expansion has been observed in various TMO nanoparticles prevails for pure metal nanoparticles
14, 15
, while lattice contraction
16, 17
. Though the size induced lattice changes are
observed in wide range of oxides, the fundamental origin has been the subject of debate 18. In order to understand the various intriguing properties of α-Fe2O3 structure with crystallite size (CS), we have carried out detailed and systematic investigations at the atomic level.
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In this contribution, we report for the first time, combined physical and structural properties of α-Fe2O3 structure with different crystallite sizes. In this context, we have synthesized the various crystallite sizes of α-Fe2O3 ranging from bulk (75 nm) to 15 nm by adopting the typical sol-gel technique. The phase confirmation of different crystallite sizes of α-Fe2O3 was established by using the high resolution x-ray diffraction analysis along with the vibrational spectroscopic techniques like Raman and FTIR. More quantitatively, the analysis of the x-ray diffraction pattern by Rietveld method yielded systematic changes in the unit cell parameters with crystallite size. It is worthwhile to mention that the lower limit of crystallite size has been restricted to 15 nm, since below this size the hematite structure has transformed into magnetite (γ-Fe2O3)
13
. Based on the obtained results, we classify α-Fe2O3 into two
different regions with respect to crystallite size, where it shows the threshold anomaly in different physical, electronic and structural properties. Size dependent lattice expansion of hematite structure is observed between bulk (75 nm) – 30 nm crystallite size (named as Region-II). On the other hand, below 30 nm we observe lattice contraction continuously up to 15 nm crystallite size (named as Region-I), which brings many beneficial properties for different applications. The observed anomalous behaviour of α-Fe2O3 with crystallite size is closely related to changes in the hybridization between metal and ligand orbital monitored by the x-ray absorption spectroscopy (XAS) technique. The anomalous changes in the structural properties are reflected in electronic properties of hematite structure where we find polaronic conductivity enhancement of four orders of magnitude at the nanoscale level.
EXPERIMENTAL SECTION Different crystallite sized α-Fe2O3 has been synthesized by using sol-gel procedure. We used hydrated iron nitrate Fe (NO3)3. 9H2O as precursor solution and it was gelated by using mono hydrated citric acid (Aldrich 98%) solution (0.05 M) as ligand molecules, and
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singly distilled water as the solvent. The iron precursor solution was added into the citric acid solution drop wise with vigorous stirring at 70˚C until the gel was formed. In order to obtain monodispersed crystallites of different sizes, we have varied molar concentration of citric acid and iron nitrate. Subsequently, the dried gel was annealed at 550 ˚C yielded various crystallite sized α-Fe2O3 structures. We attempted to prepare below crystallite size of below 15 nm but ended up in magnetite (γ-Fe2O3) phase. Therefore, the present work deals only on hematite (α-Fe2O3) structure with crystallite size ranging between 75 and 15nm. XRD measurements were performed on a Bruker D8 diffractometer using Cu Kα source, (λ = 1.5441 Ả) arrayed with scintillation detector operating at 40 kV-40 mA with the 2θ scan range of 15˚ to 70˚ providing the step size 0.02 ˚. The crystal structures were then refined with the help of Rietveld refinements method by using TOPAS software, starting the refinement with the initial cell parameters reported in the JCPDS data (File No.-01-079-007) and literature. The profiles were refined to Rwp
