Synthesis, Characterization and High Pressure Polymorph of RVO4 (R=La, Ce) Nanoparticles Alka B. Garg*1 and Sipra Choudhury2 1
High Pressure and Synchrotron Radiation Physics Division Bhabha Atomic Research Centre, Mumbai 400 085
2
Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085 *Email:
[email protected] Abstract
Synthesis of single phase nanoparticles of CeVO4 and LaVO4 via soft route using microwave assisted heating is reported. The prepared compounds are characterized by angle dispersive xray diffraction technique, scanning electron microscopy and UV-visible absorption spectroscopy. Detailed structural characterization by Rietveld analysis for both compounds show that CeVO4 crystallizes in tetragonal zircon structure with an average particle size of 11.0 nm whereas LaVO4 adopts monoclinic monazite structure with an average particle size of 16.5 nm. Expansion in space lattice is observed for LaVO4 whereas CeVO4 shows marginal contraction. The measured value of band gap for nanoparticles is found to be more than in corresponding bulk compounds. Pressure quenching from non-hydrostatic compression results in an irreversible first order phase transition (from zircon to scheelite) in CeVO4 with large volume discontinuity. LaVO4 does not show any irreversible structural change on pressure quenching. Keywords: Nanoparticles, RVO4, X-ray diffraction, Synthesis, Pressure *
Corresponding Author
Dr. Alka B. Garg High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Center, Mumbai 400 085, India Tel 91-22-25591308 FAX +9122 25505296 Email
[email protected] 1
1.
INTRODUCTION Multifunctional rare earth orthovanadates (R3+V5+O2-4; where R is the rare earth
element including Y and Sc) belong to the large family of RBO4 (where B is a pentavalent cation) type ternary metal oxides. Recently these compounds have caught the attention of both experimental as well as computational researchers because of their varied applications. For example, due to their high radiation threshold and mechanical stability, rare earth orthoarsenates (RAsO4) and rare earth orthophosphates (RPO4) are being used as chemically durable nuclear waste matrices [1,2]. GdPO4 is an excellent candidate for chemically stable, water insoluble neutron absorber for inclusion in spent nuclear fuel canisters [3]. Because of their unique optical and luminescent properties, rare earth orthovanadates (RVO4) and orthophosphates have emerged as efficient thermo-phosphors [4], γ-ray scintillators [5] and host material for solid state laser [6,7]. Most of these compounds are found to be ferroelectric [8]. RVO4 materials are efficient catalyst in treatment of propane [9] and hydrogen sulphide [10]. More recently these compounds have been examined as potential photocatalyst for the degradation of dyes and organics [11]. Eu3+ doped YVO4 nanoparticles have been used for the detection of hydrogen peroxide with temporal resolution. Due to their exceptional optical properties, the rare-earth orthvanadates are potential candidates for optical isolators, circulator beam displacers and components for polarizing optics [12]. Nanoparticles of rare earth orthovanadates have great potential for biological imaging and therapy too [13]. Most of the RVO4 compounds crystallize in tetragonal zircon structure with space group I41/amd. In this structure the isolated VO4 tetrahedra surrounds the R atom to form RO8 dodecahedron. These VO4 tetrahedra and RO8 dodecahedron extend parallel to the c-axis and are chain joined laterally by edge sharing RO8 units. The V-O bond distance remains nearly the same for the entire lanthanide series and R-VO4 interaction is predominantly ionic. As a result these nonmagnetic crystals are optically transparent, chemically stable and 2
mechanically robust which qualify them to be an effective host lattice for rare earth dopant activated luminescence as described earlier. Generally, with increasing ionic radius, R3+ ions show a strong tendency towards monazite-structured orthovanadate due to its higher oxygen coordination number of 9 as compared with 8 of the zircon structure [14] (LaVO4 is bimorphic and can be stabilized in both tetragonal zircon or monoclinic monazite structure [15]). CeVO4 can be stabilized to monazite structure under pressure [16,17]. Most of these orthovanadates show insulating behavior at ambient pressure and temperature with values of direct band gap ranging from 3.5 eV to 3.8 eV except for CeVO4 for which two very different values of band gaps [3.2 eV( absorption); 1.8 eV (reflectance)] have been reported depending upon the technique used for measurements [18]. Since various thermodynamic parameters, such as pressure and temperature can change the phase stability of materials, many of these compounds have been recently investigated against pressure and low/high temperature [1929]. Another parameter which can alter the physical properties of any material is the particle size. Synthesis of RVO4 compounds have been reported in bulk and nanostructure through solid state reaction, hydrothermal and solution based techniques [30-32]. Though the microwaves are extensively used in organic synthesis, its use for inorganic compounds is still a developing field. In particular, the synthesis of nanoparticles and nanostructures, whose growth is highly sensitive to the reaction conditions, could benefit from efficient and controlled heating provided by microwave irradiation. The energy transfer from microwave to the material is believed to occur either through resonance or relaxation which results in rapid heating. This technique is simpler and less time consuming (~15 minutes of heating compared to a few hours of heating in solid state preparation). Earlier we have reported the synthesis of various rare earth orthovanadates in bulk through solid state reaction method and their phase stability against pressure [17, 19-21]. In this work we describe the synthesis of 3
nanoparticles of RVO4 (R = Ce, La) compounds via soft chemical route with microwave assisted heating. Recently there have been a few reports describing microwave assisted route for the synthesis of CeVO4 [33, 34] but no detailed crystallographic data has been given. Also there are no reports on the synthesis of LaVO4 nano particle via microwave assisted heating. The synthesized compounds have been characterized by angle dispersive powder xray diffraction, scanning electron microscopy and their band gaps have been deduced from optical absorption spectroscopy. The phase stability of these compounds against nonhydrostatic compression has also been studied. 2.
EXPERIMENTAL 2.1.
Synthesis La(NO3)3.6H2O (Sigma Aldrich, 99.9%), Ce(NO3)3.6H2O (Sigma
Aldrich, 99.9%), NH4VO3, polyethylene glycol (PEG) are used as such without treating further for purification. For synthesis of Ce/LaVO4 via microwave irradiation, 1mmol of NH4VO3 is taken in a 250 mL of nano pure water and mixed thoroughly by stirring for half an hour. One mmol Ce/La(NO3)3.6H2O is added to the resulting solution which results in a yellow precipitate. After 5 minutes of stirring 100 mg of polyethylene glycol (6000) is added and further stirred for 10 minutes. This product is then subjected to microwave irradiation for 15 minutes in a domestic microwave oven. Microwave power is set at 750 W. The pH of the solution is adjusted in the range of 6-7. The synthesized products are washed thoroughly by nano pure water and acetone and further heated to 300 0C in a furnace for 4 hours. 2.2.
Characterizations
2.2.1. Angle dispersive x-ray diffraction (ADXRD) As prepared samples are characterized for its single phase formation by powder x-ray diffraction (XRD) using rotating anode generator (RAG Rigaku-make) operating at 50 kV and 50 mA with Mo Kα (λ=0.7107 Å) radiation. Single crystal graphite monochromator is used for selecting the Kα radiation of molybdenum. MAR345 image plate is used as an area 4
detector for recording two dimensional powder diffraction rings. Sample to detector distance is calibrated using XRD pattern of CeO2. Two dimensional rings are converted to standard one dimensional intensity versus two theta plots using software FIT2D [35]. One dimensional patterns are used for Rietveld analysis to get structural details using versatile software GSAS [36]. 2.2.2. Scanning Electron Microscopy Surface morphology of the synthesized orthovanadates has been studied by Scanning Electron Microscopy (ATS2100, SERON INC) on the solvent casted thin films on silicon wafer. 2.2.3. Optical absorption Optical absorption measurements are carried out using a double beam UV-visible spectrophotometer (JASCO,V650). A thin layer of material is prepared by first dispersing it in acetone by sonication and then spreading on a quartz plate. 2.3.
High pressure quench experiment
For pressurizing the materials an opposed-anvil Bridgman apparatus with W-C anvils having face diameter as 12.2 mm are employed. A load of 40 ton is applied which corresponds to a pressure of around 10 GPa. CeVO4/ LaVO4 sample are left under a load of 40 ton for 24 hrs in two independent runs. The recovered samples are examined by angle dispersive XRD technique described earlier. 3.
RESULTS AND DISCUSSION The particle size of as synthesized compounds are estimated from the full width at
half maxima (FWHM) of various XRD peaks employing the well known Scherer formula [37] as described below. d=λ/βcosθ
5
Where d is the crystallite size in Å, λ is the wavelength of the x-rays in Å, θ is the diffraction angle and β is the full width at half maximum (FWHM) of diffraction peaks centered at 2θ in radian. The effect of instrumental broadening on the diffraction peaks from the samples has been corrected by using a diffraction pattern of gold particles of micrometer size. The estimated average particle size obtained by using various diffraction peaks is 16.5 nm and 11.0 for LaVO4 and CeVO4 respectively. Well separated peaks have been used for determining the particle size. The one dimensional x-ray diffraction patterns for as synthesized LaVO4 and CeVO4 compounds are shown in figure 1a & b respectively along with Reitveld refined data. The data confirms single phase formation of LaVO4 in monazite structure (monoclinic symmetry, space group P21/n, Z=4) with cell parameters as a=7.0673(6) Å, b=7.3121(6) Å, c=6.7441(5) Å, α=90°, β=105.068(7)°, γ=90°, cell volume V=336.53(5) Å3. The corresponding lattice parameters for bulk LaVO4 are a=7.047(1) Å b=7.289(1) Å c=6.725(1) Å, α=90°, β=104.85(1)°, γ=90°, cell volume V=333.89 (5) Å3 [38]. CeVO4 adopts the zircon structure (tetragonal symmetry, space group I41/amd, Z=4) with cell parameter as a=b=7.3819(5) Å, c=6.4989(6) Å, α=β=γ=90°, cell volume V=354.14(5) Å3. The corresponding lattice parameters for bulk CeVO4 are a=b=7.4013 (4) Å, c=6.4980(3) Å, cell volume V=355.96(5) Å3 [30]. This shows that for LaVO4 lattice is expanded whereas for CeVO4 there is slight reduction in the cell volume indicating the space lattice contraction with reduction in particle size. The probable reason for two compounds showing different lattice behavior could be due to the fact they adopt different crystal structure. In general for oxide nanoparticles lattice is expanded [39,40] whereas metal nanoparticles show space lattice contraction with reduction in particle size [41] due to phenomena of surface tension. Interestingly recent report on synthesis of GdVO4 nanoparticles shows slight space lattice contraction with reduction in particle size [42]. It is to be noted here that the GdVO4 adopts 6
the same zircon structure as observed for CeVO4 in present studies. Refined crystallographic details along with various residuals [43] for both the materials are summarized in table 1. Small values of residuals indicate the goodness of the fit. The recorded scanning electron micrographs showing the morphology for nanoparticles of LaVO4 and CeVO4 are depicted in figure 2a & b respectively. A nonuniform film like porous surface morphology (black region corresponding to bare silicon surface) has been observed in LaVO4. However, the higher magnified image as shown in inset of figure 2a represents the interconnected grains of the aggregated LaVO4 nanoparticles. Formation of spherical grains and their aggregation are clearly observed from the CeVO4 nanoparticle aggregates as shown in figure 2b. The optical absorption data are analyzed using the following classical relation of optical absorption given by Tauc [44] in semiconductor near band edge. α = α0( hν− Eg)n/hν where Eg is the separation between bottom of the conduction band and top of the valence band, hν is the photon energy and n is a constant. The value of n depends on the nature of the transition. In this case n=1/2 for the direct allowed transition. In figure 3a & b, (αhν)2 is plotted as function of photon energy hν for determining the band gap. Extrapolation of the straight-line section to zero absorption coefficient (α=0) leads to estimation of the band gap energy. The estimated band gaps for LaVO4 and CeVO4 using the data of figure 3a & b is 4.8 eV and 4.0 eV respectively. The reported value of band gap for bulk LaVO4 is 3.2 eV whereas for CeVO4 two very different values 3.5 eV and 1.8 eV have been reported. An increase in band gap is observed in both these nano materials. This increase in band gap could be due to the well known phenomena of quantum confinement which has been earlier reported in nanoparticles of a few other materials [45, 46].
7
The observed diffraction patterns of pressure quenched LaVO4 and CeVO4 along with Rietveld refined data is shown in figure 4a & b respectively. The CeVO4 data has a few extra peaks other than the peaks from ambient zircon structure indicating irreversible phase changes in the compound. However, the diffraction pattern for LaVO4 does not show any extra peaks indicating the absence of any pressure induced irreversible phase change. It is to be noted here that most of the bulk rare earth orthovanadates except CeVO4 (bulk CeVO4 transform to irreversible monazite phase under hydrostatic/quasi-hydrostatic compression) shows irreversible zircon to scheelite phase transition. This scheelite phase is nearly 10% denser than zircon phase due to efficient packing of polyhedra. The extra diffraction peaks observed in our XRD pattern of pressure quenched nano CeVO4 could not be fitted to monazite structure. However, we could fit the extra peaks to scheelite structure with tetragonal symmetry having space group I41/a. Two phase Rietveld refinement is carried out to extract the crystallographic details of pressure quenched daughter and ambient zircon phase in case of CeVO4, whereas for LaVO4 single phase refinement was carried out for extracting the crystallographic details of pressure quenched ambient phase. The crystallographic details along with various residuals for pressure recovered daughter phase and pressure quenched ambient phase for CeVO4 and the pressure quenched ambient phase of LaVO4 are tabulated in table 2. The unit cell volume after pressure cycling for LaVO4 remains almost same as for without pressure cycled compound. In figure 5(a)-5(c) we show the schematics of different polyhedral arrangements in zircon, scheelite and monazite structure. It is clear from the figure that for zircon and scheelite the coordination of V and R remains 4 and 8 respectively however in monazite structure, coordination around R changes from 8 to 9. Quantitative phase analysis of CeVO4 data shows the presence of scheelite phase to be around 18% and zircon phase is around 82%. Due to small fraction of high pressure 8
scheelite phase no attempts were made to refine the atom position. The scheelite phase of pressure quenched CeVO4 is found to be 11.4 % denser than the ambient zircon phase due to efficient packing of polyhedral units. 4.
CONCLUSION To summarize nanoparticles of CeVO4 and LaVO4 have been prepared via chemical
route using microwave assisted heating. XRD data shows the single phase formation of both the vanadates with nanometer sized particle. Space lattice is expanded for nano LaVO4 whereas for CeVO4 marginal reduction in space lattice is observed. UV-visible absorption spectra for both the compounds are used to deduce the band gaps of the synthesized compounds. The pressure quench experiment shows the presence of irreversible scheelite phase for CeVO4 whereas LaVO4 do not show any irreversible phase change. REFERENCES [1]
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Acknowledgement One of the authors (ABG) acknowledges the fruitful discussions with Dr. Surinder M. Sharma and Shri K. K. Pandey.
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Figure Captions Figure 1 Rietveld refined x-ray diffraction pattern of as synthesized (a) LaVO4 and (b) CeVO4. Experimentally observed data is represented by cross whereas the calculated profile is presented by solid line. Background has also been refined and plotted along with difference plot. The allowed reflections are shown by vertical bar. Figure 2 Scanning electron micrographs of (a) LaVO4 (b) CeVO4. Figure 3 Plot of (αhν)2 vs. hν (a) LaVO4 (b) CeVO4. The band gaps have been deduced by extrapolating the straight line section of the graph to zero. Figure 4 Rietveld refined x-ray diffraction pattern for pressure quenched (a) LaVO4 and (b) CeVO4. Experimentally observed data is represented by cross whereas the calculated profile is presented by solid line. Background has also been refined and plotted along with difference plot. The allowed reflections are shown by vertical bar. In case of CeVO4, the upper vertical tick marks correspond to the zircon phase and lower tick marks are for the pressure quenched scheelite phase. Figure 5 (colour online) Schematic view of VO4 and RO8/RO9 polyhedral arrangement in different crystal structures (a) zircon (b) scheelite (c) monazite. Green denotes RO8/RO9 polyhedra whereas blue colour is for VO4 tetrahedra.
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Figure 1
Garg et al
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Figure 2
Garg et al
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Figure 3 Garg et al
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Figure 4 Garg et al
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Figure 5
Garg ett al
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Table 1 Structural parameters of as prepared nanoparticles (a) LaVO4 and (b) CeVO4 (a) LaVO4 adopts the monazite phase (monoclinic symmetry, space group P21/n, Z=4) with Rietveld refined cell parameters as a=7.0673(6) Å, b=7.3121(6) Å c=6.7441(5) Å, α=90°,
β=105.068(7)° γ=90°, cell volume = 336.53(5) Å3. Residuals [43] wRp=3.56%, Rp=2.65%, R(F2)=6.19%. Number in the parenthesis denotes the error. Atom x y z La 0.27482(29) 0.15670(28) 0.10362(4) V 0.2987(8) 0.1636(9) 0.6121(9) O1 0.2294(31) -0.0078(15) 0.4256(31) O2 0.3816(19) 0.3470(26) 0.5127(22) O3 0.5012(20) 0.1083(16) 0.8129(19) O4 0.1235(17) 0.2195(21) 0.7232(18) (b) CeVO4 in zircon phase (tetragonal symmetry, space group I41/amd, Z=4). Rietveld refined cell parameter as a=b=7.3819(5) Å, c=6.4989(6) Å α=β=γ= 90°, cell volume =354.14(5)Å3. Residuals [43] wRp=5.9 %, Rp=4.67 %, R(F2)=8.5%. Number in the parenthesis denotes the error. Atom Ce V O
x 0.0000 0.0000 0.0000
y 0.7500 0.2500 0.4340(7)
19
z 0.1250 0.3750 0.2089(8)
Table 2 Crystallographic details of pressure quenched (a) CeVO4 in ambient zircon phase, (b) high pressure scheelite phase and (c) pressure quenched LaVO4 in monazite phase. (a) CeVO4 in zircon phase (tetragonal symmetry, space group I41/amd, Z=4) a=b=7.3911(13) Å, c=6.5142(18) Å, α=β=γ=90°, cell volume=355.86(13)Å3. Residuals [43] wRp=9.91%, Rp=7.48%, R(F2)=7.48%. Number in the parenthesis denotes the error. Atom
x
y
z
Ce
0.0
0.7500
0.1250
V
0.0
0.2500
0.3750
O
0.0
0.4434(18)
0.2142(18)
(b) CeVO4 in Scheelite phase (tetragonal symmetry, space group I41/a, Z=4) a=b=5.176(6) Å, c=11.767(29)Å,α=β=γ=90°, cell volume (V)=315.3(9) Å3. Residuals [42] wRp=9.91 %, Rp=7.48 %, R(F2)=7.48 %. Number in the parenthesis denotes the error. Atom
x
y
z
Ce
0.0
0.2500
0.6250
V
0.0
0.2500
0.1250
O
0.2215
0.1633
0.756
(c)
LaVO4 adopts the monazite phase (monoclinic symmetry, space group P21/n,
Z=4) with cell parameters as a=7.0670(6) Å, b=7.3117(6) Å, c=6.7425(6) Å, α=γ=90°, β=105.067(8)°,cell
volume
(V)=336.42(5)
Å3 .
Residuals
[42]
wRp=3.20%,
Rp=2.47%, R(F2)=3.69%. Number in the parenthesis denotes the error. Atom
x
y
z
La
0.27489(26)
0.15708(25)
0.10296(21)
V
0.3005(7)
0.1659(8)
0.6115(8)
O1
0.2314(27)
-0.0088(13)
0.4246928)
O2
0.3812(17)
0.3468(22)
0.5110(19)
O3
0.4999(17)
0.1910(14)
0.8166(17)
O4
0.1215(14)
0.2227(18)
0.7281(16)
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