Synthesis and spectral characterizations of Fe 3+ doped b-BaB 2 O 4 nano crystallite powder

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  (This is a sample cover image for this issue. The actual cover is not yet available at this time.) This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:http://www.elsevier.com/copyright  Author's personal copy Synthesis and spectral characterizations of Fe 3+ doped  b -BaB 2 O 4  nanocrystallite powder Ch. Venkata Reddy a , Ch. Rama Krishna a , T. Raghavendra Rao a , U.S. Udayachandran Thampy a ,Y.P. Reddy b , P.S. Rao c , R.V.S.S.N. Ravikumar a, ⇑ a Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar 522 510, India b Physical Sciences, Sri Padmavathi Mahila Viswavidyalayam, Tirupati 517 502, India c Department of Chemistry, Pondicherry University, Puducherry 605 014, India a r t i c l e i n f o  Article history: Received 31 October 2011Received in revised form 8 December 2011Accepted 8 December 2011Available online 4 January 2012 Keywords: b -BaB 2 O 4  nano crystallite powderXRD studiesOptical absorptionPL and EPR studies a b s t r a c t Fe 3+ doped  b -BaB 2 O 4  nano crystallite powder is synthesized and characterized by spectroscopic tech-niques. From the Powder X-ray diffraction data, Fe 3+ doped  b -BBO material is observed to be monoclinic.Its average crystallite size is evaluated about 75nm. The particle-like morphology has been observedfrom SEM images. The EPR spectrum shows two resonance signals at  g   =4.23 and  g   =2.07, respectivelywhich are indicative of Fe 3+ ions in tetragonally distorted octahedral site symmetry. Crystal field param-eter (Dq) and Racah parameters (B and C) are evaluated. Fe 3+ doped  b -BaB 2 O 4  nano crystallite powderluminescence properties have been studied. The emissionof   b -BaB 2 O 4 :Fe 3+ sample is exhibitedtwo mainpeaks at 419 and 443nm. FT-IR spectrum shows the characteristic vibrations of host lattice.   2011 Elsevier B.V. All rights reserved. 1. Introduction Nanostructured materials have received much attention due totheir distinguished performance in electronics, optics and photon-ics. The morphology of these materials plays a key role especiallyon the optoelectronic properties of the materials. For many indus-trial and scientific applications metal oxide nanopowders are veryimportant [1]. The nanostructure materials have attracted consid-erable interest in synthetic methodology, due to its sensor devicetechnology. Borate materials with unique properties have madethe research more attractive towards numerous practical applica-tions. Among them, Barium Borate (BBO) is an important opticalmaterial and it exists generally in two modifications namely  a -BBOand b -BBO. The reversible phase transitionfromhigh-temper-ature  a -BBO to low temperature  b -BBO is known [2] with phasetransition temperature of 925  C [3]. The studies of low-tempera-ture BariumBorate ( b -BBO) is gained an importance due to its sig-nificant properties [4,5] in the fields of non-linear optics andelectro-optics. The  b -BBO is built up by Ba 2+ cations and [B 3 O 6 ] 3  anion rings by turns and also exhibits 2nd- and 3rd-order non-lin-ear properties simultaneously, which make it more attractive be-cause it provides the possibility to create a new type of laserfrequency converters. By considering all these enormous applica-tions of   b -BBOin photoelectric industry, a large scale and inexpen-sive preparationmethod is required. Co- precipitation is one of themost promising techniques owing to the large-scale preparationability without special apparatus.Actually the reactivity of nanomaterials depends on dopanttypes andconcentrations. Limbachet al. [6] illustratedthat dopantconcentration is the most important factor that determines reac-tive oxygen species (ROS) (OH, O  2 and H 2 O 2 ) generation by nano-particles. Basedonthe compositionof the nanoparticles, the activesites may vary and generate different amounts of reactive species.The reactivity of nanoparticles is a function of their physicochem-ical properties, such as size, surface characteristics, crystal phase,dopant types and concentrations, agglomerationbehavior andsus-pension stability [7–9]. Various studies have reported that particlesize is an important parameter that affects the catalytic activity of the materials; the size and mobility of the nanoparticles will influ-ence the microbial inactivation. Doping with specific ions is a nor-mal way of obtaining new physical properties or improvingproperties of a given undoped matrix. EPR spectroscopy enablesto identify the oxidation and spin states of the dopant metal ionsaswellasitsbindingsiteandsymmetry.Theenergylevelordering,the structure of the complexes and the site symmetry can be ob-tained from optical absorption studies. Thus, optical absorptionstudy is complementary to EPR technique. Therefore, site symme-try and the dynamic behavior of the metal ion in the host latticecan be investigated using EPR and optical absorption studies. Forstudying the optical and electronic properties of solid state 0022-2860/$ - see front matter    2011 Elsevier B.V. All rights reserved.doi:10.1016/j.molstruc.2011.12.012 ⇑ Corresponding author. Tel.: +91 863 2346381 (O), +91 9490114276 (M);fax: +91 863 2293378. E-mail address:  rvssn@yahoo.co.in (R.V.S.S.N. Ravikumar). Journal of Molecular Structure 1012 (2012) 17–21 Contents lists available at SciVerse ScienceDirect  Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc  Author's personal copy materials suited for optoelectronic applications photolumines-cence(PL)spectroscopyisasanimportanttool.PLdataproviderel-atively direct information about recombination and relaxationprocesses. In general, these are useful for the investigation of theelectronicpropertiesof theexcitedstateandofferseveral advanta-ges over other optical techniques.Recently we have been carried out the synthesis and spectro-scopic characterization of Cu 2+ ions doped  b -BaB 2 O 4  nanopowdersin which the doped Cu 2+ ions entered the host lattice as tetrago-nally distorted octahedral site symmetry and there exists a partialcovalent bonding nature between the doped ions and with its li-gands[10]. Theeffect of thespecificimpurityontheelectrical con-ductivity depends on the substitution site so that the trivalent ionlike Fe 3+ behaves as an acceptor when substitution occurs at thesite or as a donor when it substitutes for the site. In view of this,the present investigations are carried out to determine the oxida-tion state and site symmetry of the dopant ion in Fe 3+ doped BBOnanopowder by co-precipitation method and to ascertain the localstructure of the dopant ions various spectroscopic measurementsare used. The crystal structure and morphological studies are car-ried out by using Powder X-ray diffraction studies and ScanningElectron Microscopy (SEM) with Energy Dispersive Spectropho-tometer (EDS) techniques respectively. 2. Experimental  2.1. Synthesis of nanopowder  Analar grade of 0.02mol% Ba(NO 3 ) 2  6H 2 O and 0.01mol% Na 4- B 4 O 7  10H 2 O is dissolved in de-ionized water separately. Later on0.001mol% of Fe(NO 3 ) 2  solutions is added into the barium nitratesolution and mixed completely with continuous stirring. The mix-tureisthendroppedintotheNa 4 B 4 O 7  solutionat50  Cundermag-netic stirring. After filtering, the precipitate is washed three timeswith 30ml de-ionized water and subsequently dried in an oven at85  C for 24h. The obtained powder is calcined at 800  C at a rateof 10  C/min for 1h.  2.2. Characterizations XRD patterns of the Fe 3+ doped  b -BaB 2 O 4  nano-powder is re-corded on PANalytical Xpert Pro-diffractometer. Scanning electronmicroscopy images are recorded on Carl ZEISS SEM EVO. EDX pat-ternisrecordedonOxfordPentaFET.Thesampleismixedwithnu- jol (liquid paraffin) for optical absorption studies recorded on JASCO V-670 spectrophotometer in the range of 350–800nm. Thephoto luminescence spectrum is recorded on Horiba Jobin-YvonFluorolog-3 spectrofluorimeter with Xe continuous (450W) andpulsed (35W) lamps as excitation sources. The EPR spectrum of the powder sample is recorded on JEOL JES-TE100 ESR spectrome-ter operating at X-band frequencies with a field modulation of 100kHz at room temperature. The FT-IR spectrum is recordedusing KBr pellets on Thermo Nicolet 6700 ranging from 400 to2000cm  1 . 3. Theory  The electronic configuration of the ferric ion is [Ar] 3d 5 . It givesriseto 2 S,  2 P,  2 D,  2 F,  2 H,  4 P,  4 D,  4 F,  4 Gand 6 Sterms. Inaweakcrystalfield, the lower quartet terms transform as follows:  6 S ? 6 A 1g , 4 G ? 4 T 1g  + 4 T 2g  + 4 E g  + 4 A 1g ,  4 D ? 4 T 2g  + 4 Eg,  4 P ? 4 T 1g . Of theabove terms  6 A 1g  lies lowest according to Hund’s rule. It corre-sponds to the strong field configuration t 32g  e 2g . The crystal field(CF) is termed weak when its influence is lesser than that of spinorbit interaction and electronic repulsion, whereas it is strongwhen its influence is greater. If the CF influence is in between thatof spin orbit and electronic repulsion. For a d 5 configuration thetransitions are thus spin forbidden and they appear with lessintensity. The transitions  6 A 1g (S) ? 4 T 1g (G) and  6 A 1g (S) ? 4 T 2g (G)depend on the crystal field strength Dq and give rise to broadbands.Forthetransitionslike 6 A 1g (S) ? 4 E g (G), whichareindepen-dent of Dq the bands would be less broadened [11]. 4. Results and discussion 4.1. XRD, SEM with EDX analysis ThepowderXRDpatternofFe 3+ doped b -BaB 2 O 4  nanopowderisshowninFig. 1. It isclearthat theintroductionof thedopantsdoesnotmuchchangethephasecompositionofthesample.Thecharac-teristic diffraction peaks of monoclinic  b -BaB 2 O 4  can be noticed.The pattern can be assigned well to the reference pattern of   b -BaB 2 O 4  (powder diffraction file No. 38–722). The average crystal-lite size of the particle is calculated using Scherrer’s equation, d  =0.9 k / B  Cos h , where  d  is the crystallite size (nm),  k  is the X-raywavelength,  B  is the full width at half maximum intensity and  h is the diffraction angle. The average crystallite size is found to beabout 75nm. SEM and EDX images of Fe 3+ doped  b -BaB 2 O 4  nanocrystallite powder are shown in Fig 2. SEMimage exhibits the par-ticleslikemorphology.EDXdataindicatesthedistributionofBa,Feandoxygenspeciesalongwiththechemical compositionmapping.It also indicates the presence of doped element. 4.2. Optical absorption studies The optical absorption spectrum of   b -BaB 2 O 4  nano crystallitepowderisshowninFig.3.ThebandsobservedintheUV–VISregionareattributedtodistortedoctahedralsitesymmetryofFe 3+ ions.Thebroad bands observed at 680 and 525nm are attributed to the Dqdependent transitions  6 T 1g (S) ? 4 T 1g (G) and  6 T 1g (S) ? 4 T 2g (G)respectively. With the help of the Tanabe-Sugano diagram, assign-ments for the other bands are identified and are given in Table 1.Based on these assignments, the energy matrices for d 5 configura-tion are solved with Tree’s correction factor,  a  =90cm  1 [12,13]fordifferentcombinationsofcrystalfieldparameterDq,inter-elec-tronic repulsion parameters  B  and  C  . The evaluated parameters aregoodfitwiththeobservedabsorptiondata. Dq ¼ 915 cm  1 ;  B ¼ 680 cm  1 and C ¼ 2800 cm  1 : 4.3. Photo luminescence studies TheemissionandexcitationspectraofFe 3+ doped b -BaB 2 O 4 nanocrystallitepowder at roomtemperature are shownin Figs. 4 and 5, Fig. 1.  XRD pattern of Fe 3+ doped  b -BaB 2 O 4  nano crystallite powder.18  Ch. Venkata Reddy et al./Journal of Molecular Structure 1012 (2012) 17–21  Author's personal copy respectively. The excitation peak is observed at 370nm, theemissionpeaksareobservedat419and443nmrespectively,whichindicate the strong broad UV emission at room temperature. Thebroad band observed at 370nmis certainly not a transition withinthe d shell because of its intensity. Furthermore, this band is muchtoo broad to represent a transition on the Fe 3+ ion itself. Therefore,this band is ascribed to a Fe 3+ ? O 2  charge-transfer transition.The forbidden character of this transition is reflected by the ratherlong decay times. 4.4. Electron paramagnetic resonance studies Fe 3+ has five unpaired electrons in weak crystal fields, with ahigh-spin state ( 6 S 5  /  2 ). In perfect tetrahedral (T d ) or octahedral(O h ), i.e .,  cubic ligand fields, only one signal appears at  g =  2 . 0 inthe X-band EPR spectrum. If the symmetry of the field is lower,sayaxial, thepowder spectrumbecomesanisotropicandmayhavesignals in the 2<  g   <6 interval. Since Fe 3+ belongs to d 5 configura-tion with  6 S ground state in the free ion and there is no spin–orbitinteraction [14], the g-value is expected to lie very near to the freeion value of 2.0023. However g-value very much greater than 2.0often occurs, in particular an isotropic  g  -value at 4.2, due to thepresence of certain symmetry elements. EPR spectrum of Fe 3+ doped  b -BBO nano crystallite powder is shown in Fig. 6. The X-band EPR spectrum of Fe 3+ ion in the present sample indicatestwo resonance signals at  g   =4.23 and  g   =2.07, respectively. Thefeatures of the EPR spectrum can be qualitatively explained as fol-lows: the resonance peak for Fe 3+ at  g   =2.07 can only occur if Fe 3+ is located in a site where the crystal field interaction energy is lessthan the magnetic Zeeman energy and arises due to isolated Fe 3+ ions. The resonance signal at  g   =4.23 is attributed to the strongtetragonally distorted octahedral symmetry in the presence of  Fig. 2.  SEM (a) and EDX (b) images of Fe 3+ doped  b -BaB 2 O 4  nano crystallite powder. Ch. Venkata Reddy et al./Journal of Molecular Structure 1012 (2012) 17–21  19  Author's personal copy oxygen ligands and represent the population of one of the Kra-mer’s’ doublets [15,16]. The bonding between metal ion-ligand isionic nature. 4.5. FT-IR studies The FT-IR spectrum of Fe 3+ doped  b -BaB 2 O 4  nanopowder isshown in Fig. 7. In general, the FT-IR analysis of the studied boratematerial shows four distinct frequency regions. Two regions, i.e.,from 1200 to 1600cm  1 and from 800 to 1200cm  1 , are assignedto the stretching vibrations of both triangular BO 3  and tetrahedralBO 4  units, respectively. Deformation modes of both types of unitsare active between 600 and 800cm  1 [17]. The bands at 460 and570cm  1 are assigned to specific vibrations of Fe–O bonds over  Table 1 Observed and calculated band head data of Fe 3+ doped  b -BaB 2 O 4  nano crystallitepowder. Transition Wavenumber (cm  1 )From  6 A 1g (S) ?  Wavelength (nm) Observed Calculated 4 T 1g (G) 680 14,702 14,868 4 T 2g (G) 525 19,042 18,986 4 E g (G) 446 22,415 22,565422 23,690 4 T 2g (D) 408 24,503 24,504 400 450 500 550 600 650 700020000400006000080000100000120000140000160000180000 443419    I  n   t  e  n  s   i   t  y   (  a .  u   ) Wavelength (nm) Fig. 4.  Emission spectrum of Fe 3+ doped  b -BaB 2 O 4  nano crystallite powder. 300 320 340 360 380 400 420 4400100000200000300000400000500000600000 370    I  n   t  e  n  s   i   t  y   (  a .  u   ) Wavelength (nm) Fig. 5.  Excitation spectrum of Fe 3+ doped  b -BaB 2 O 4  nano crystallite powder. 0 200 400 600 800 Field (mT) Fig. 6.  EPR spectrum of Fe 3+ doped  b -BaB 2 O 4  nano crystallite powder at roomtemperature ( m  =9.154845GHz). Fig. 7.  FT-IR spectrum of Fe 3+ doped  b -BaB 2 O 4  nano crystallite powder. Fig. 3.  Optical absorption spectrum of Fe 3+ doped  b -BaB 2 O 4  nano crystallitepowder.20  Ch. Venkata Reddy et al./Journal of Molecular Structure 1012 (2012) 17–21
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