Experimental and First-Principles Investigations of Lattice Strain Effect

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Experimental and First Principles Investigations of Lattice Strain Effect On Electronic and Optical Properties of Biotemplated BiFeO Nanoparticles 3

Nurul Syamimi Abdul Satar, Azia Wahida Aziz, Muhamad Kamil Yaakob, Muhd Zu Azhan Yahya, Oskar Hasdinor Hassan, T.I.T. Kudin, and Noor Haida Mohd Kaus J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08548 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 29, 2016

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The Journal of Physical Chemistry

Experimental and First-Principles Investigations of Lattice Strain Effect on Electronic and Optical Properties of Biotemplated BiFeO3 Nanoparticles N.S.A. Satar1, A.W. Aziz1, M.K. Yaakob2, 5, *, M.Z.A. Yahya3, 5, O.H. Hassan4, 5, T.I.T. Kudin2, 5, N.H.M. Kaus1, * 1

Nano | Hybrid | Materials Research Group, School of Chemical Sciences, Universiti Sains Malaysia,

11800, Penang, Malaysia 2

Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Malaysia

3

Faculty of Defence Science and Technology, Universiti Pertahanan Nasional Malaysia, 57000 Kuala

Lumpur, Malaysia 4

Faculty of Art and Design, Universiti Teknologi MARA, 40450 Shah Alam, Malaysia

5

Ionics Materials and Devices (iMADE) Research Laboratory, Universiti Teknologi MARA, 40450 Shah

Alam, Malaysia

ABSTRACT: High purity BiFeO3 nanoparticles were successfully prepared by employing green and facile biotemplated method which utilised polysaccharides of κ-carrageenan. The particles’ size of BiFeO3 nanoparticles exhibited significant correlations between structural, electronic and optical properties that have been investigated by both experimental and first-principles methods. First-principles calculations were performed by means of density functional theory (DFT). Structural analysis revealed that the all prepared BiFeO3 was crystallized in a rhombohedrally distorted structure, along with an average crystallite size of 14.59 nm. The refined lattice parameter and crystal volume obtained by means of Rietveld refinement method indicated a good agreement with the calculated data; the lattice strain is found in BiFeO3 nanoparticles. From the linear fit using Kulbeka-Munk method, small direct and indirect optical energy gap was

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found to be 2.17 eV and 1.84 eV respectively. In addition, first-principles study in relation to lattice strain effect on electronic properties of BiFeO3 have thus far revealed that the calculated energy band gap decrease with the increasing and decreasing of lattice parameters. Our results which were obtained by means of facile green route and first principles approaches could shed new insights on the lattice strain effect on physical properties of BiFeO3 nanoparticles. Keywords: Multiferroic, κ-carrageenan, density functional theory (DFT), Kulbeka-Munk, Rietveld refinement method

1.

INTRODUCTION

Bismuth ferrite (BiFeO3) is a multiferroic material which is capable of showing simultaneously the ferromagnetic, ferroelectric and/or ferroelastic ordering with high Curie temperature (Tc = 810-830°C) and anti-ferromagnetic order below Neel temperature (TN = 370°C)1-3. There has recently been a renewed interest among scholars in BiFeO3 due to its remarkable features for many potential device applications it has to offer in exploiting its photovoltaic4-6, piezoelectric or magnetoelectric7-8 capabilities as well as photocatalyst9. Being a semiconductor with an absorption edge in the visible region, BiFeO3 (Eg = 1.3eV – 3.0eV) promise significant benefits in relation to the efficiency of the photo-generated electron transfer over other materials with wider energy gap such as TiO2 and ZnO. Therefore, the formation of the high purity BiFeO3 is considered very essential as the reaction of the byproducts (Bi25Fe2O39, Bi25FeO4, Bi2Fe4O9, Bi46Fe2O72, Bi36Fe24O57) obtained from the conventional methods

10-12

could affect and modify

the resulting properties. Nevertheless, a high temperature treatment may require ensuring first of whether the formation of high purity of BiFeO3 will cause drawbacks due to bismuth volatilization13-14. 2 ACS Paragon Plus Environment

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Owing to the recent growing interest in environmental issues, there is now a greater emphasis on greener routes to functional materials. In this regard, biotemplates are considered good candidates as they possess preferential chelation sites from the functional group of polysaccharides (amine, carboxylate, hydroxyl or amide groups) within molecular structures and thus may enable new macromorphological and crystallochemical control of the BiFeO3 nanoparticles. Carrageenan is one of the most abundant and naturally occurring polysaccharides, which are reportedly extracted from brown seaweed. The anionic characteristics of carrageenan (SO42-) are believed to help bring different properties and chemical functionalities as a carrier or template for the construction of metal nanoparticles. The polymer-metal ion complex can be synthesized under mild conditions, and the polysaccharides chains in turn may impair the aggregation, hence it controls the growth and purity of the synthesis BiFeO3 nanoparticles. The reduction of particle size from a micro-size to a nano-size has widely been reported to change its magnetic and optical properties, which indicates the significant particle size effect on physical properties of BiFeO3 nanoparticles and nanofilms15-18. Related studies have been recently reported the particle size effect on optical, ferroelectric, magnetic and photocatalytic properties of BiFeO3 nanoparticles19-22. For instance, McDonnell et al.23 carried out an investigation into photo-active and optical properties of BiFeO3 by means of both experimental and DFT studies. Notwithstanding the wealth of literature on BiFeO3 nanoparticles and nanofilms, studies zooming in on the lattice size effect on structural, electronic and optical properties are relatively scant, particularly in relation to the BiFeO3 nanoparticles synthesized via biotemplated technique. Considering the paucity of studies involving both experiments and first-principles, the present study, to the researchers’ knowledge, can be considered among the pioneer studies 3 ACS Paragon Plus Environment


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carried out to investigate the lattice strain effect on electronic and optical properties of biotemplated BiFeO3 nanoparticles using both experimental and first principles studies. The facile route utilizing anionic polysaccharides of κ-carrageenan as biopolymer-mediated for the synthesis of high purity BiFeO3 nanoparticles were demonstrated. Both the structural and optical properties of BiFeO3 nanoparticles were studied by powder X-Ray Diffraction, Scanning Electron Microscopy and UV-Vis spectroscopy. Further investigation of lattice strain effect on electronic and optical properties of BiFeO3 nanoparticles were carried out using GSAS rietveld refinement and first-principles based on corrected GGA-PBEsol+U functional methods. 2.

EXPERIMENTAL AND CALCULATIONS SECTIONS 2.1.

Synthesis of BiFeO3 Nanoparticles via Polysaccharides Biotemplated

Method. Bismuth nitrate, Bi(NO3)3.5H2O (1.42 g, 3.0 mmol) and iron nitrate, Fe(NO3)3.9H2O (2.02 g, 0.5 mmol) were weighed and dissolved in 25 mL deionised water to form 0.2 M of precursor solution. This was followed by 5 ml of precursor solution being dissolved in 0.5 wt% of carrageenan solution. Sodium hydroxide, NaOH (1.0 M) was titrated into the solution until the pH reached the level of pH 8. The light yellow mixture was heated at 80 °C under vigorous stirring for 1 hour to ensure that all the solvents were evaporated. The mixtures were dried in an oven overnight. The dried brown powders obtained were calcined in a furnace at 550 °C for 2 hr. 2.2.

Characterization of BiFeO3 Nanoparticles. The measurement of electronic

absorption spectra by means of diffuse reflectance technique was made in a UV-Vis spectrophotometer (LAMBDA 25). The powder XRD patterns were collected in a PANalytical X’Pert PRO θ - 2θ using a Cu source (K α,λ = 0.15406 Å). Later on, the rietveld structure refinement was performed by means of the General Structure Analysis System (GSAS) program 24-25

. A FESEM observation was carried out by means of FEI-QUANTA FEG 650 at an 4 ACS Paragon Plus Environment

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accelerating voltage of 10 kV equipped with an X-ray system Oxford Instrument X-MAX for elemental and composition analysis purposes. 2.3.

First-Principles Details. The first-principles calculation in this study was

performed by means of density functional theory as implemented in total energy PWPP CASTEP code26. The electron–ion interaction was described in a softened ultrasoft pseudopotential27 within spin-polarized28 generalized gradient approximation for an exchange in solids and surfaces (GGA-PBEsol) functional

29

. In an attempt to ensure an accurate prediction of the

multiferroic materials which normally contain strong electronic localized transition metals of 3d electrons, the self-interaction corrected GGA-PBEsol+U exchange correlation functional was applied. In this regard, the details of the on-site Coulomb repulsion of U which were included in GGA-PBEsol+U method is described in reference30. In CASTEP computer code, the Coulomb energy U and the exchange energy J were combined into a single effective on-site Coulomb repulsion of U. The selection of a suitable or proper U value used in DFT+U method is of utmost important in studies looking into the structural, electronic, elastic and optical properties of strain-free and strained BiFeO3 material. For this calculation involved in this study, the U values of 3 eV which was proven to have accurately been used for BiFeO3 was chosen for the treatment of the strong electronic localized Fe 3d electrons31-33. In pseuedopotential method, the electrons orbital of Bi (6s26p3), Fe (3d64s2) and O (2s22p4) were treated as valence electrons. The geometry optimization convergence thresholds were set to 1.0x10-5 eV/atom for a maximum of energy change, 0.03 eV/Å for a maximum force, 0.05 GPa for a maximum stress and 0.001Å for a maximum displacement. The energy cutoff of 340 eV and k-point grid of 4x4x4 was used for all the calculations throughout. 5 ACS Paragon Plus Environment


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RESULTS AND DISCUSSIONS 3.1.

Structural and Particle Size Analysis of BiFeO3 Nanoparticles. The refined

XRD patterns for BiFeO3 nanoparticles synthesized from biotemplate method utilising polysaccharides of κ-carrageenan are shown in Figure 1. In Rietveld refinement studies, the hexagonal structure phase (R3c space group) with lattice parameter of a = 5.58132 Å and c = 13.87698 Å and atomic positions Bi (0.000, 0.000, 0.000), Fe (0.000, 0.000, 0.221) and O (0.446, 0.017, 0.952) have been used as starting structure34. The XRD patterns revealed that BiFeO3 nanoparticles may crystallize in rhombohedral distorted structure with the R3c space group. Moreover, the XRD pattern showed high purity nanoparticles sample with single phase of BiFeO3, without any impurities (Bi25Fe2O39, Bi25FeO4, Bi2Fe4O9, Bi46Fe2O72, Bi36Fe24O57 phases) presents. Therefore, the environmentally friendly process by means of employing a green and facile biotemplate used in this work may have significant implications and serve as an important method in future for producing high purity BiFeO3 nanoparticles.

Figure 1. Rietveld refined XRD pattern of aggregated BiFeO3 nanoparticles synthesized with carrageenan as biotemplate. 6 ACS Paragon Plus Environment

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Besides, the refined XRD results were in good agreement with the hexagonal R3c phase, thus culminating in the refined lattice parameter a=b= 5.62528 Å and c= 13.88216 Å with the goodness of fit index, χ2= 1.234 and reliability parameters, Rwp= 6.79 %, Rp= 5.21 % and RF= 4.99 %. Goodness of fit which is of less than 2 and all reliability parameters being reported less than 10 % may indicate that an excellent Rietveld fit to the XRD patterns. The average crystallite size determined as per the Scherrer equation was found to be 14.59 nm. It was discovered that the refined lattice parameter of BiFeO3 nanoparticles as synthesized using κ-carrageenan was relatively larger than that of BiFeO3 microparticles or nanoparticles reported in literatures34-39. In Table 1 provides a summary of the experimental and calculation data of BiFeO3 nanoparticles with rhombohedral distorted structure phase (R3c space group) obtained using Rietveld refinement method and first-principles calculation. In the present work, the atomic positions, lattice parameter, unit cell angle, crystal volume, volumetric strain and local magnetic moment of BiFeO3 were considered at 0 K for first principles calculation and 300 K for the experimental study respectively. The experimental data from other works34, 39-40 were also taken into consideration for comparison purposes. It is noteworthy that the atomic positions of Bi, Fe and O are reportedly fixed to the starting structure in Rietveld refinement studies. From the results, it can be clearly seen that the calculated data determined from GGAPBEsol+U with an effective U value of 3 eV was in reasonable agreement with experimental data. The present refined lattice parameter of BiFeO3 nanoparticles were observed to be 5.653 Å, which was larger than that of the other measured data of 5.637 Å obtained in BiFeO3 microparticles34. As a rule of thumb, it has been recommended that the lattice strain effect presence in the BiFeO3 nanoparticles with the percentage of volumetric strain calculated to be +1.62 %. It was found that the BiFeO3 nanoparticles produced in this work experienced a small



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lattice expansion. This lattice expansion was due to the nanoscale sized particles; which also have been reported for other metal oxide nanoparticles41-42; suggests that this could be from the decrease of electrostatic force and surface stress effect

43-44

. Additionally, the calculated atomic

positions and magnetic moment were also in good agreement with the experimental data. Table 1. Summary of the calculated and measured atomic positions, lattice parameter, unit cell angle, crystal volume, volumetric strain and local magnetic moment of rhombohedral phase of BiFeO3. Calculation

Experiment Ref. 34 T= 298 K

Present Work T= 0 K GGA-PBEsol+U Bi (2a) x 0.000 Fe (2a) x 0.221 O (6b) x 0.524 y 0.934 z 0.397 ar (Å) 5.625 αr (o) 59.39 3 Ω (Å ) 124.12 e (%) M (µB/Fe) 3.98 a Neutron diffraction (Ref. 39) b Neutron diffraction (Ref. 40) 3.2.

T= 300 K Carrageenan 0.000 0.221 0.523 0.935 0.398 5.653 59.67 126.81 +1.62

0.000 0.221 0.523 0.935 0.398 5.637 59.34 124.79 0 3.75a, 4.00b

Morphology and FTIR Studies. The morphology of BiFeO3 nanoparticles

synthesized by means of sulphated (SO42-) group of carrageenan biopolymer was observed to have aggregated the rhombohedral structure with a particle size of 84 nm as shown in Figure 2a. By means of the results, it was learnt that the average particle size of BiFeO3 nanoparticles prepared utilizing carrageenan biotemplate produced were more uniform, dense and homogeneous in shape. Besides, it was also discovered that the sequestration of discrete metal cations that bind with sulfate (SO42-) groups of the biopolymer produced much stronger, contracted gel and thus preventing an uncontrolled growth via cation-mediated interaction prior 8 ACS Paragon Plus Environment

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calcination. In addition, the morphological regularities of metal-sulfate binding in carrageenan may provide spatially-defined sites of nucleation and growth, thereby imbuing the inorganic material with a complex morphology matching that of the underlying matrix45. The experimental values for the atomic percentage of Bi, Fe and O in both samples of BiFeO3 nanoparticles determined by FESEM-EDX (Figure 2b) were 20.79%, 19.98% and 58.04% and these values were close to those obtained theoretically (20, 20, and 60).

Figure 2. FESEM (a) and EDAX (b) of the aggregated BiFeO3 nanoparticles synthesized with carrageenan as biotemplate. Figure 3 illustrates the FTIR spectra obtained from the precursor and BiFeO3 nanoparticles calcined at varying temperatures. This approach utilizes the functionality of polysaccharides to control the hydrolysis/polycondensation and the nucleation growth processes.



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The carrageenan containing functional groups of SO4 that can bind with metal ion is required to undergo a hydrolysis process first. The sol-gel transition can be triggered by an addition of metal ions to form complex and organized gels in water by means of binding with metal ions. The organized gel structures which are formed by means of biopolymers can be exploited to form metal-oxo-metal bond via poly condensation. The broad absorption band in the range of 3500– 3000 cm-1 can be assigned to O–H stretching, which may decrease in intensity as the thermaltreatment temperature increases. This can be attributed to the synerisis within precursor gel network before the removal of metal-OH through calcination. On the other hand, the intense bands located at 1384, 1015 and 800 cm-1 can be attributed to the presence of the nitrate ions. In the IR spectrum of sample heated at 550°C for 2 hours, all the organic and nitrate peaks were seen to have disappeared and a new absorption peaks appeared at ~548 and ~620 cm-1, which are the characteristics of Fe–O stretching and bending vibrations of octahedral FeO6 groups in the perovskite compounds.

Figure 3. FTIR spectra of BiFeO3 sample prepared with carrageenan biotemplate before and after calcined by different hours. 10 ACS Paragon Plus Environment

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3.3.

Electronic Properties of BiFeO3. Detailed investigations on electronic properties

may provide more comprehensive basic information on electrical, optical, elastic and magnetic properties of materials. The electronic properties such as electronic band structure and density of states for rhombohedral R3c phase of BiFeO3 were carried out using first-principles calculation and they are presented in Figure 4. In such a calculation, the electronic band structure of rhombohedral R3c phase of BiFeO3 was determined in the electron’s wave propagating through F-G-Z-L symmetry points of the Brillouin zone (see Figure 4a and 4b). The calculated results showed that the GGA-PBEsol+U was capable of predicting an indirect energy band gap of 1.73 eV with the lowest of conduction band located at L point and the highest valence band located in between F and Γ points. This calculated indirect energy band gap was notably in reasonable agreement with the other calculated values as reported in the literature33, 46. A comprehensive discussion on the effect of on-site Coulomb repulsion U in LDA+U calculation (CASTEP code) of structural, electronic, elastic and optical properties of BiFeO3 is available in reference 33. From Figure 4c, it clearly shows that the top of valence band is mainly occupied by O 2p states with a small contribution from Fe 3d states and Bi 6s states. On the other hand, the bottom of conduction band was mainly occupied by Fe 3d states with a small contribution from Bi 6p states and O 2p states. Moreover, the calculated DOS indicated that the hybridization between O 2p, Bi 6s, Bi 6p and Fe 3d states basically provide essential information for chemical bonding, electronic excitation, magnetic and electrical properties of BiFeO3.



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Figure 4. Crystal symmetry within Brillouin zone of F-Γ-Z-L points (a), calculated energy band structure of rhombohedral BiFeO3 with alpha (spin-up) and beta (spin-down) states (b) and calculated total and partial DOS of rhombohedral BiFeO3 (c). 3.4.

Optical Properties of BiFeO3 Nanoparticles. The UV-Vis absorption spectra

measurement of BiFeO3 nanoparticles synthesized from carrageenan biotemplate which is commonly carried out for optical properties investigation purposes. The corresponding optical energy gap was determined as per to the Kubelka-Munk theory47. In Figure 5, the optical energy gap of BiFeO3 nanoparticles estimated from the tangent line in the plot of Kubelka-Munk functions F(R) against photon energy are delineated. From a linear extrapolation [F(R)]2 and [F(R)]1/2 to 0, it suggests both direct and indirect energy gap of 2.17 eV and 1.84 eV respectively for BiFeO3 nanoparticles synthesized from biotemplate method utilising polysaccharides of κcarrageenan. 12 ACS Paragon Plus Environment

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Figure 5. Direct (a) and indirect (b) optical energy gap of the carrageenan biotemplated BiFeO3 nanoparticles Both the direct and indirect energy gap of BiFeO3 obtained from various experimental and first-principles studies are summarized in Table 2. The GGA-PBEsol+U predicted both direct and indirect energy band gap of 1.82 eV and 1.73 eV respectively, these values were considered larger than the calculated data determined by means of LDA+U functional33. Thus, it can be implied that the calculated energy band gap was close to the experimental optical energy gap obtained in BiFeO3 nanoparticles. Besides, as in the case of the present work, both the direct and indirect optical energy gap of BiFeO3 nanoparticles obtained showed smaller value than that of the optical energy gap obtained in previously reported studies in the literature for single crystal48-49, thin film50 and nanoparticles51. A decreased optical energy gap in BiFeO3 nanoparticles compared with that of the rhombohedral thin film was also reported in previous



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studies in the literature51. Therefore, it can be inferred that the reduction in optical energy gap could be due to the presence of lattice strain effect in BiFeO3 nanoparticles. This indicates that the reduction of particle size; which reportedly account for the presence of lattice strain effect; could be used as an effective way to control the energy gap of BiFeO3. On the other side, the incorporation of oxygen vacancies that exist in BiFeO3 as point defect may be another plausible that give significant effect on optical energy gap15, 52 as well as its magnetization properties. Table 2. Summary of direct and indirect energy gap of rhombohedral BiFeO3 nanoparticles from various experimental and first principles studies.

Direct Eg (eV) Indirect Eg (eV)

Present work Calculation Experiment a 1.82 2.17b 1.73a

1.84b

Other work 1.48c, 2.74e, 2.40f, 3.00g 1.43c, 1.30d, 1.30f, 2.18h

a

CASTEP (GGA-PBEsol+U, U= 3 eV) UV-Vis (Nanoparticles-carrageenan) c CASTEP (LDA+U, U= 3 eV) d VASP (LDA+U, U= 2 eV) e UV-Vis (Epitaxial thin film- Ref. 49) f UV-Vis (Nanocrystalline film- Ref. 56) g UV-Vis (Single crystal- Ref48) h UV-Vis (Nanoparticles- Ref.54) b

Besides, the calculated optical dielectric function, refractive index and extinction coefficient of BiFeO3 in comparison with the experimental data collected from reference53 is illustrated in Figure 6. It can be observed that the calculated data for optical dielectric function, refractive index and extinction coefficient of BiFeO3 was very close to the experimental data. This also demonstrated that the corrected GGA-PBEsol+U (U= 3 eV) exchange correlation functional was accurate enough for investigation of BiFeO3 materials system, where various experimental data for physical properties could be well reproduced.


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Figure 6. Calculated optical dielectric function, refractive index and extinction coefficient of BiFeO3 using GGA-PBEsol+U (U= 3 eV) exchange correlation functional. The experimental data 53 is assigned with solid black line. 3.5.

Lattice Strain Effects on Electronic Properties of BiFeO3. In order to further

investigate the lattice strain effect on electronic properties of BiFeO3 nanoparticles, the calculation of electronic band structure and density of states as a function of volumetric strain were performed as shown in Figure 7. The energy band gap without lattice strain effect (at a volumetric strain 0 %) is was about 1.727 eV. It was discovered from the calculated energy band gap (Figure 7a), that the electronic energy band gap of BiFeO3 is predicted to decrease with the decreasing and increasing of lattice parameters (compression and expansion of volumetric strain). However, the change of calculated energy gap was found to be more sensitive than 15 ACS Paragon Plus Environment


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originally envisage, at the compression volumetric strain than expansion volumetric strain. Then, our calculated results demonstrated that the energy gap of BiFeO3 could be controlled by lattice strain effect, as has widely been reported about BiFeO3 nanoparticles51, 54 and epitaxial BiFeO3 thin films49-50, 55. Interestingly, the calculated energy band gap at the experimental volumetric strain, as indicated by the dotted red lines, shows that the energy band gap at the volumetric strain of +1.6 % was predicted to be at ~1.720 eV. Notably, this smaller predicted energy band gap at the volumetric strain of +1.6 % as compared to the energy band gap of BiFeO3 without lattice strain effect was proven to be in strong agreement with smaller measured optical energy gap for BiFeO3 nanoparticles. Our experimental and theoretical findings strongly indicate that the smaller optical energy gap which has been observed in BiFeO3 nanoparticles could be attributed to the lattice strain effect. In addition, it can be inferred that the presence of lattice strain effect (compression and expansion volumetric strain) may in turn contribute to the change of electrical and magnetic properties of BiFeO3 nanoparticles.


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Figure 7. Calculated energy band gap (a) and partial density of states (b) of BiFeO3 at volumetric strain of 0 %, +3 % and -3%. The origin of lattice strain effect on electronic properties of BiFeO3 has also been considered in this work. Figure 7 (b) delineates the details of partial electronic density of states of BiFeO3 at both with (at volumetric strain of +3 % and -3%) and without (at volumetric strain of 0 %) lattice strain effect. It clearly showed that all Bi, Fe and O states were shifted for both volumetric strain of +3 % and -3%. Furthermore, all electronic states were shifted to lower energy level at the bottom of conduction band and to higher energy level at the top of valence band which could be because the narrower energy band gap observed. At the valence band, all electronic states were shifted to the lower energy level for BiFeO3 with volumetric strain of -3% and to higher energy level for BiFeO3 with volumetric strain of +3%. On the other hand, at the 17 ACS Paragon Plus Environment


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conduction band, all electronic states were shifted to the higher energy level for BiFeO3 with a volumetric strain of -3% and to lower energy level for BiFeO3 with a volumetric strain of +3%. Besides its fundamental significant, this calculated DOS may also serve as a reference for future experimental works on electronic properties of BiFeO3 under strain effect. 4.

CONCLUSIONS

BiFeO3 nanoparticles in this study were synthesized via the facile biotemplated method employing the carrageenan for a controlled growth of the material. The average crystallite size of rhombohedral structure obtained under this condition was 14.5 nm. The XRD confirmed the synthesized BiFeO3 via biotemplated technique may have a high potential for producing homogeneous single phase nanoparticles. Refined XRD patterns suggested that the lattice strain effect’s presence in our sample of BiFeO3 nanoparticles with a small expansion volumetric strain of +1.6 %. The calculated lattice parameter, crystal volume and magnetic moment of rhombohedrally distorted structure (R3c space group) of BiFeO3 obtained from corrected GGAPBEsol+U functional were shown to be in reasonable agreement with the experimental data. The formation of perovskite structure can be confirmed by the presence of metal-oxygen bond in FTIR spectrum. UV-vis measurement showed small direct and indirect optical energy gap found to be at 2.17 eV and 1.84 eV respectively. Furthermore, first-principles study of lattice strain effect on electronic properties of BiFeO3 has revealed that the calculated electronic energy band gap may decrease with the increasing and decreasing of lattice parameters. Then, it was also found that from the experimental and first-principles studies that the presence of lattice strain effect (expansion volumetric strain) plays significant role for the decrease of optical energy gap observed in BiFeO3 nanoparticles.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Tel: +6 04 653 3598 *E-mail: [email protected] Tel: +6 03 5543 7833 Notes: The authors declare no competing financial interest. ACKNOWLEDGEMENT This work is supported by the Research University Grant (RUI 1001/PKIMIA/811249). The authors also wish to thanks Prof Dr Rohana Adnan and Dr Lee Hooi Ling from School of Chemical Sciences, USM for their continuous support in this project.

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Experimental and First-Principles Investigations of Lattice Strain Effect

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