CePO4:Ln (Ln = Tb3+ and Dy3+) Nanoleaves ... - ACS Publications

In this paper, we demonstrate an efficient method for making luminescent sols based on the sol−gel technique using water/methanol dispersible nanole...
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CePO4:Ln (Ln ) Tb3+ and Dy3+) Nanoleaves Incorporated in Silica Sols A. K. Gulnar,*,† V. Sudarsan,‡ R. K. Vatsa,‡ R. C. Hubli,† U. K. Gautam,§ A. Vinu,| and A. K. Tyagi*,‡

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 5 2451–2456

Material Processing DiVision, Chemistry DiVision, Bhabha Atomic Research Centre, Mumbai 400085, India, and Nanoscale Materials Centre, International World Center for Nano-Architectonics, National Institute for Materials Science, 1-1, Namiki Tsukuba, Ibaraki 3050044, Japan ReceiVed December 13, 2008; ReVised Manuscript ReceiVed February 9, 2009

ABSTRACT: Highly crystalline CePO4, CePO4:Tb3+, and CePO4:Dy3+ nanoleaves with monoclinic structure and dispersible in solvents such as water and methanol were prepared by a low temperature synthesis. Subsequently, these nanoleaves were incorporated into silica sols by the sol-gel method. Such nanoleaves incorporated into silica sols exhibit improved luminescence properties compared to silica sols directly doped with lanthanide ions. The observed difference in the luminescent properties of nanoleaves incorporated into silica sols and silica sols directly incorporated with lanthanide ions has been explained based on the different extent of energy transfer from Ce3+ to Tb3+/Dy3+ ions. The different extent of quenching of the excited-state of lanthanide ions due to OH groups from Si-OH linkages and water molecules, taking place in nanoleaves incorporated into silica sols compared to silica sols directly doped with lanthanide ions, is also responsible for the improvement in the luminescence properties. The present study will be quite relevant for developing biosensors for studying enzyme and protein activities based on luminescent silica sols.

1. Introduction Lanthanide ions doped in nanoparticles/nanorods of different inorganic hosts have potential applications as phosphor materials, biolabels, and up-conversion materials.1-4 It is known that the luminescent properties of nanomaterials are highly sensitive to their shape and dimensionality.5-7 Hence, synthesizing nanomaterials in different shapes is a major challenge before employing these materials for developing functional structures for various applications. Lanthanide phosphates (LnPO4) are robust materials for various technological applications such as laser hosts and up-conversion materials due to their high thermal and photostability, extremely low solubility in water, and wide range of solubility of rare earth ions such as Nd3+.8-10 CePO4 belongs to the lanthanide phosphate category and is found to exist mainly in two forms. One has a hexagonal structure with molecular formula CePO4 · xH2O (x ) 0.3-0.5) (also known as Rhabdophane) and the other is monoclinic CePO4.11,12 Hexagonal undergoes an irreversible phase transition to monoclinic form at 800 °C.12 A number of hexagonal structured lanthanide orthophosphates LnPO4 (Ln ) La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) having wire-like morphology were synthesized and characterized by the hydrothermal method.13-15 For example, hexagonal CePO4/ LaPO4 nanorods as well as CePO4:Tb3+-LaPO4 core-shell nanorods were synthesized and characterized by Cao et al.14 and Zhu et al.15 under hydrothermal conditions in the presence of surfactants and ultrasonication, respectively. Li et al.16 have prepared CePO4:Tb3+ nanowires in aqueous medium at room temperature by the reaction between Ce3+/Tb3+ ions and NH4H2PO4 in the presence of cyclodextrin. Kompe et al.17 employing coprecipitation in the presence of coordinating ligands such as tributyl phosphate (TBP) and trihexyl amine, * Corresponding authors: E-mail: [email protected] (A.K.G.), aktyagi@ barc.gov.in (A.K.T.). † Material Processing Division, Bhabha Atomic Research Centre. ‡ Chemistry Division, Bhabha Atomic Research Centre. § Nanoscale Materials Centre, National Institute for Materials Science. | International World Center for Nano-Architectonics, National Institute for Materials Science.

prepared CePO4:Tb3+ core and CePO4:Tb3+-LaPO4 core-shell nanoparticles in water-free environments at 200 °C. On the basis of these studies, it was observed that the luminescence properties of CePO4:Tb3+ nanorods/nanoparticles are significantly improved when covered by a shell of LaPO4. In all these studies bulky organic ligands are used to stabilize these nanoparticles/ wires. Such bulky organic ligands will be problematic when the nanoparticles/wires are dispersed in sol gel films of silica, TiO2, etc., which is attributed to their poor dispersability in water or alcohols. The improved dispersability of nanoparticles/wires in alcohols and water is a primary requirement for making good quality silica sols and xero-gels incorporated with these nanoparticles. Such sols and xero-gels, when immobilized with proteins and enzymes, combine the advantages of biological inertness, biodegradability, and compatibility with biomolecules. They can also act as efficient luminescence-based biosensors for monitoring the protein and enzyme activities in biological systems. In the present communication, we demonstrate a simple synthetic method to prepare CePO4 and lanthanide ion doped CePO4 nanoleaves (thin rods with very small aspect ratio) at a relatively low temperature of 140 °C in ethylene glycol medium. This method does not involve any moisture- or air-sensitive reagents or hydrothermal conditions. Ethylene glycol acts both as a solvent and a ligand. The leaves are dispersible in solvents such as water, methanol, etc. Such water dispersible nanoleaves were incorporated into silica sol by the sol-gel method and their luminescence properties are compared with that of silica sol which is directly incorporated with lanthanide ions. Bright lanthanide ion luminescence has been observed from these nanoleaves and silica sols incorporated with nanoleaves. To the best of our knowledge, this is the first time that nanoleaves stabilized with such short chain organic ligands are employed for making luminescent silica sols.

2. Experimental Details Synthesis of CePO4, CePO4:Tb3+(5%), and CePO4:Dy3+(5%) Nanoleaves. Chemicals used for the synthesis of CePO4, CePO4: Tb3+(5%), and CePO4:Dy3+(5%) nanoleaves were AR grade

10.1021/cg801349y CCC: $40.75  2009 American Chemical Society Published on Web 03/20/2009

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Figure 1. Schematic representation for the preparation of CePO4:Tb3+ nanoleaves incorporated in silica sols.

Figure 2. XRD patterns for (a) CePO4 and (b) CePO4:Tb3+ and (c) CePO4:Dy3+ nanoleaves. Ce2(CO3)3 · 5H2O (99.9%), Tb4O7 (99.9%), and Dy2O3 (99.9%). All chemicals were used as obtained, without further purification. Around 0.5 g of Ce2(CO3)3 · 5H2O and 0.017 g of Tb4O7/ Dy2O3 were dissolved in dilute HCl and the excess of acid was removed by repeated evaporation by adding water. This was then mixed with 25 mL of ethylene glycol and heated to 100 °C to get a clear solution. At this temperature around 0.2 g of ammonium dihydrogen phosphate (NH4H2PO4) dissolved in 10 mL of ethylene glycol was dropwise added, and then the temperature was raised to 140 °C. At this stage the clear solution became turbid and the temperature was maintained at 140 °C for a period of 2 h. After the reaction, the precipitate was washed several times with acetone and ethanol and dried under ambient conditions. The powder samples were found to be readily dispersible in solvents such as methanol and water. Preparation of Nanoleaves Incorporated in Silica Sols. For preparing nanoleaves incorporated in silica sols, around 50 mg of CePO4:Tb3+ sample was dispersed in 1.5 mL of water, which was then mixed with 3 mL of tetraethyl orthosilicate (TEOS) and 7.8 mL of ethanol.18 Because o fthe presence of ethylene glycol moiety on the surface of the nanoleaves, they form stable dispersion in solvents such as water and methanol. The pH of the solution was adjusted to 2 by adding a few drops of 0.1 N HCl and the resultant solution was stirred for 48 h. Under this condition, TEOS undergoes acid hydrolysis to generate silica which covers CePO4:Tb3+ nanoleaves. A schematic diagram for the preparation of the nanoleaves and nanoleaves incorporated in silica sol is shown in Figure 1. For preparing silica sol directly doped with lanthanide ions, around 0.5 g of Ce2(CO3)35H2O, and 0.017 g of Tb4O7/Dy2O3 were dissolved in 1.5 mL of 0.001 N HCl. A few drops of 0.1 N HCl were added to this to adjust the pH to around 2. It was then mixed with 3 mL of tetraethyl orthosilicate (TEOS) and 7.8 mL of ethanol. The resulting solution was stirred for 48 h to get a clear sol.

Figure 3. TEM image of as-prepared CePO4:Tb3+ nanoleaves. The inset on left-hand side shows selected area diffraction pattern. Characterization. X-ray powder diffraction patterns were recorded using Ni filtered Cu KR radiation (λ ) 1.54178 Å) from a Diano Worburn, X-ray diffractometer attached with a graphite monochromator (operating voltage and current are at 35 kV and 20 mA, respectively). The lattice parameters were calculated from the diffraction peaks using the least-squares fit method. Crystallite size was estimated by X-ray line broadening method based on the Sherer relation d ) 0.9λ/B cos θ where λ is the wavelength of X-ray, θ is the angle of diffraction, and B is the half-maximum line width. The instrumental line broadening was corrected by using standard Si. All luminescence measurements were carried out at room temperature with a resolution of 3 nm using a Hitachi instrument (F-4500) having a 150W Xe lamp as the excitation source. Around 20 mg of sample was dispersed in 5 mL of methanol/ water prior to luminescence measurements. To see the stability of dispersion, the emission measurements were also carried out after 48 h of dispersion of the nanoleaves in methanol. The counts were found to be comparable with the freshly dispersed solution suggesting that the methanol/water dispersions are quite stable. TEM investigations were carried out using a JEOL JEM 3000F microscope. Samples for TEM investigations were prepared by dispersing them in methanol and a drop of the above solution was put on a carbon coated holey TEM grids which were allowed to dry.

3. Results and Discussion Figure 1 illustrates the steps involved in the synthesis of CePO4:Tb3+ nanoleaves incorporated in silica sol. Initially

CePO4:Ln (Ln ) Tb3+ and Dy3+) Nanoleaves

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Figure 4. High resolution TEM images for CePO4:Tb3+ nanoleaves depicting (a) the presence of different domains in the sample and (b) lattice fringes present in a single nanoleaf.

Figure 6. Emission spectrum from CePO4:Tb3+ nanoleaves dispersed in methanol obtained after exciting the sample at 300 nm. The inset shows the excitation spectrum corresponding to 545 nm emission.

Figure 5. Representative TEM image showing orientation of planes in two nanoleaves which are overlapped.

CePO4:Tb3+ nanoleaves were prepared by the coprecipitation method in ethylene glycol solvent. These nanoleaves were isolated and subjected to acid hydrolysis for 48 h to get a clear transparent sol. CePO4:Dy3+ nanoleaves incorporated into silica sol were also prepared by a similar procedure. Figure 2 shows the X-ray diffraction (XRD) patterns for undoped and 5 at % Tb3+ doped CePO4 samples. The patterns are characteristic of the monoclinic structure of CePO4 and match well with that reported.19 Lattice parameters are found

to be a ) 6.779(2) Å, b ) 7.006(3) Å, and c ) 6.470(2) Å, with β ) 103.6° for CePO4 nanoleaves. The values are comparable with that of bulk CePO4 phase.19 For CePO4: Tb3+(5%) nanoleaves the cell parameters are a ) 6.805(1) Å, b ) 6.980(1) Å, and c ) 6.463(3) Å with β ) 103.5°. The b and c parameters are found to decrease, while a is found to increase with Tb3+ doping in CePO4. Such anisotropy in the variation of lattice parameters is possibly due to the monoclinic structure of the CePO4 lattice as well as the selective orientation of (012) plane in these nanomaterials. This is further supported by the intensity changes in the peaks corresponding to (012) plane of CePO4/CePO4:Tb3+ nanomaterials of present study compared to bulk CePO4 phase. The average crystallite size calculated from the diffraction line width is found to be ∼91

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Figure 7. Emission spectrum from CePO4:Dy3+ nanoleaves dispersed in methanol obtained after exciting the samples at 300 nm. The inset shows the excitation spectrum corresponding to 575 nm emission.

nm. The presence of ethylene glycol moieties on the surface of the samples has been confirmed by recording the IR pattern (Figure 1 of the Supporting Information) of the as-prepared CePO4 sample. Broad peaks around 3400, 1640, 1450, and 1050 cm-1 are characteristic of O-H (stretching and bending modes), C-O (stretching mode), and C-H (bending mode) linkages of ethylene glycol moiety. Asymmetric stretching vibration of P-O linkages of CePO4 is characterized by the presence of a peak around 1067 cm-1. TEM image of a representative CePO4:Tb3+ sample is shown in Figure 3. A careful examination of image revealed a thoroughly homogeneous distribution of particles with a morphology resembling the leaves. Widths of the leaves are 20-30 nm, while their lengths range from 50 to 100 nm, thus having an aspect ratio of ∼2.5-3. The thickness of the leaves could

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not be measured as all leaves lie with their flat surface touching the holey film. Selected area electron diffraction (SAED) patterns (inset of Figure 3) showed a ring-like structure along with spots within each ring indicating that the crystallinity has developed within the particles. Such patterns acquired at different locations suggested the highly crystalline nature of the sample. Since CePO4 can have hexagonal or monoclinic structure, an attempt was made to index all the rings to either of these structures. All the rings could be indexed only to the monoclinic structure asserting the fact that the structure of these nanoleaves is monoclinic and not hexagonal. The clear lattice fringes obtained in the HRTEM images (Figure 4) also confirmed the highly crystalline nature of the leaves. Interestingly, many of the leaves have domains which are much smaller than the actual nanoleaves, and many of them are crystallographically aligned to each other.The domains are clearly seen from the different contrasts within a leaf as can be seen from Figure 4a. The spacing 2.4 and 2.9 Å between the lattice fringes shown in Figure 4b matches well with the distance between the (220) and (012) planes of monoclinic structure of CePO4. The lattice images of two nanoleaves which are overlapping each other are shown in Figure 5. It is interesting to see the alignment in these nanoleaves. The spacing 2.50 Å between the lattice fringes is characteristic of the distance between the (211) planes of monoclinic CePO4. It will be of interest to know why under these experimental conditions CePO4 phase forms in the shape of nanoleaves. This is because of the growth mechanism involved in the synthesis the material.20 During coprecipitation in the presence of coordinating ligands, nucleation followed by growth result in nanoparticles formation. At the growth stage if the monomer concentration is in the medium range (i.e., not very high or not very low), it can support the growth of less extended structures such as nanoleaves or spindles. Under the present experimental conditions, the monomer concentration is neither very high nor very low (i.e., a medium monomer concentration). Under such monomer concentrations, formation of particles with geometry similar to nanoleaves is favorable. If the concentration of the nuclei is

Figure 8. Schematic diagram showing the energy transfer between the host CePO4 and Tb3+/Dy3+ ions. Only relevant levels of Dy3+ are shown. Dotted lines are nonradiative transitions.

CePO4:Ln (Ln ) Tb3+ and Dy3+) Nanoleaves

Figure 9. Emission spectrum from silica sols incorporated with CePO4: Tb3+ nanoleaves obtained after exciting the sample at 283 nm. Corresponding excitation spectrum monitored at 545 nm emission is shown as an inset.

Figure 10. Emission spectrum from silica sols incorporated with CePO4: Dy3+ nanoleaves obtained after exciting the sample at 283 nm. Corresponding excitation spectrum monitored at 480 nm emission is shown as an inset.

Figure 11. Emission spectrum from silica sols incorporated with (a) Ce3+ and Tb3+ ions and (b) Ce3+ and Dy3+ ions obtained after exciting the samples at 283 nm. Peaks marked * are artifacts.

lower, normally the nanodots rather than more extended structures like nanoleaves are formed. The emission spectrum of CePO4:Tb3+ nanoleaves obtained after 300 nm excitation is shown in Figure 6. Strong Tb3+ emission along with Ce3+ emission has been observed from these nanoleaves. Presence of both Ce3+ and Tb3+ emission from these samples suggests that the energy transfer between the host CePO4 and Tb3+ is incomplete. This is understandable, as the energy transfer takes place through dipole-dipole interaction, Ce3+ ions which are near to the Tb3+ ions, can only transfer the excited energy to Tb3+ ions. Excitation spectrum corresponding to 545 nm emission (5D4 f 7F5 transition of Tb3+) is shown as an inset in the same figure. The broad peak over the range of 240-350 nm and centered around 300 nm is

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characteristic of the 4f f 5d transition of Ce3+ ions. In crystalline monoclinic CePO4 each Ce3+ ion can have only C1 symmetry, as it is coordinated to nine oxygen atoms forming an irregular polyhedron.21 This leads to splitting of 5d levels into five non-degenerate levels and a minimum of five excitation peaks should be seen corresponding to 4f f 5d transition. However, due to overlapping of the peaks, asymmetry in the line shape is only observed as can be seen from the inset of Figure 6. (There can also be contribution from the 4f f 5d transition of Tb3+ ions with this peak.) Unlike this the direct excitation of Tb3+ ion resulted in very weak luminescence. In order to further substantiate the energy transfer between the CePO4 host and lanthanide ions, as well as the bright lanthanide ion luminescence from these nanoleaves, CePO4:Dy3+ nanoleaves were also prepared and their luminescence properties were investigated. Dy3+ ion is selected because its 4f f 5d transition of Dy3+ and Dy-O charge transfer transitions are well below 200 nm.22 Further it is a less sensitive luminescent species compared to Tb3+ and Eu3+ ions. Figure 7 shows the emission spectrum corresponding to as prepared CePO4:Dy3+ nanoleaves dispersed in methanol and excited at 300 nm. Strong Ce3+ emission along with Dy3+ emission in the blue and yellow regions can be clearly seen. Excitation spectrum corresponding to Dy3+ emission is shown as an inset in the same figure. Broad asymmetric peak centered around 300 nm is characteristic of the 4f f 5d transition of Ce3+ ions. On direct excitation of Dy3+ line at 354 nm emission peaks could not be observed. These results again support the previous inference of energy transfer taking place between the CePO4 host and lanthanide ions as well as the bright luminescence of lanthanide ions from the nanoleaves. Schematic representation of energy transfer between the host and Tb3+ and Dy3+ is shown in Figure 8. Figures 9 and 10 show the emission spectrum from CePO4: Tb3+ and CePO4:Dy3+ nanoleaves dispersed in silica sols. Emission spectra are similar to that observed for CePO4:Tb3+ and CePO4:Dy3+ nanoleaves shown in Figures 6 and 7 suggesting that the nanoleaves are quite stable in the silica sol. Unlike this, silica sols incorporated directly with Ce3+ and Tb3+/ Dy3+ ions (having the same Ce3+:Tb3+ and Ce3+:Dy3+ ratios) showed only Ce3+ emission as can be seen from Figure 11a,b. (Peaks marked * in Figure 11a,b are artifacts. Artifact arises due to the scattering of the excitation light by the sample. Scattering is a wavelength and particles size dependent phenomenon and will be different for silica sol containing Ce3+ and Tb3+ ions compared to the one incorporated with CePO4: Tb/Dy nanoleaves. As the peak is appearing ∼566 nm, which is double the excitation wavelength, it is confirmed that it is not arising from the sample.) Lack of Tb3+/Dy3+ emission from these samples has been attributed the absence of energy transfer between Ce3+ and Tb3+/Dy3+ ions and quenching brought about by vibrations of the solvent molecules and Si-OH linkages. Because of the larger distance between Ce3+ and Tb3+/Dy3+ ions in silica sol directly incorporated with Ce3+ and Tb3+/Dy3+ ions energy transfer from Ce3+ to Tb3+/Dy3+ ions is quite weak. Hence, the present study very clearly establishes that the only way to get luminescent silica sol is to incorporate lanthanide ions doped nanoleaves/nanoparticles rather than directly doping lanthanide ions in the sol.

4. Conclusions Highly crystalline, uniformly distributed nanoleaves of CePO4, CePO4:Tb3+, and CePO4:Dy3+ which are dispersible in solvents such as methanol and water were prepared at a relatively low temperature of 140 °C in ethylene glycol medium.

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These nanoleaves have been incorporated into silica sols, and such nanoleaves incorporated into silica sols have significantly improved luminescence properties compared to silica sols directly incorporated with lanthanide ions. Existence of energy transfer from Ce3+ ions of the host to Tb3+ and Dy3+ ions and a lower extent of quenching of lanthanide ion excited-state by Si-OH vibrations and water molecules in nanoleaves incorporated in silica sol are the reason for its improved luminescence properties compared to silica sols directly doped with lanthanide ions. Supporting Information Available: IR spectrum for confirming the ligand stabilization on nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.

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