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Synthesis of optically active silica-coated NdF3 core–shell nanoparticles
Spectrochimica Acta Part A 86 (2012) 432–436
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis of optically active silica-coated NdF3 core–shell nanoparticles Anees A. Ansari ∗,1 , S.P. Singh 2 , N. Singh 2 , B.D. Malhotra ∗,2 Biomedical Instrumentation Section, Materials Physics & Engineering Division, National Physical Laboratory, K. S. Krishnan Marg, New Delhi 110012, India
a r t i c l e
i n f o
Article history: Received 19 September 2011 Received in revised form 21 October 2011 Accepted 28 October 2011 Keywords: Silica-coated NdF3 core–shell nanoparticles HR-TEM Optical absorption spectra Photoluminescence spectra
a b s t r a c t Silica surface-modified NdF3 core–shell nanoparticles were prepared by sol–gel route. The prepared core–shell nanoparticles were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), UV–vis absorption and photoluminescence (PL) spectroscopy studies. Phase identification of the NdF3 and silica-coated NdF3 core–shell nanoparticles which was carried-out by XRD, confirms the formation of a well-crystallized hexagonal phase structure. Due to the silica-surface modification, the nanoparticles were not found to be wellseparated (agglomerated) in ethanol solvent as scanned by TEM. The results of the FTIR studies conducted on these core–shell reveal the binding of silica with the NdF3 nanoparticles. The largest intensity and shape variation were observed in all transitions as compared to non-silica modified NdF3 nanoparticle spectra, and were attributed to the environment around the Nd(III) ion due to coordination of silica molecule(s). A significant enhancement in the emission intensity was measured in silica surface modified NdF3 core–shell nanoparticles due to the successful silica coating on the surface of nanoparticles. The results of these studies suggest that these nanoparticles may find potential applications in the areas of bioimaging, protein-labeling, optical biosensors and drug delivery, etc. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Silica-coated core–shell nanoparticles have recently attracted much attention due to their rich surface chemistry, high biocompatibility, controllable porosity, good optical transparency, cheapness and photochemical stability observed under laser photolysis [1,2]. These have been attributed to the silanol groups on the surface of silica coatings that can be utilized to introduce specific functional groups [3–5]. These specific functional groups including amines, thiols and carboxyls on the surface of silica nanoparticles made them ideal for bioanalysis applications [3–10]. Among the various approaches to the functionalization or core–shell formation on the surfaces of metal nanoparticles, the luminescent metal nanoparticles coated with silica core–shells have been considered very important [10–15]. And recently, several kinds of silica-coated material nanoparticles including Ag/SiO2 [16,17], Au/SiO2 [18,19], Fe3 O4 /SiO2 [5,20–23], QDs/SiO2 , and
∗ Corresponding authors. Tel.: +96 61 4676838; fax: +96 61 4670662. E-mail addresses: [email protected] (A.A. Ansari), [email protected] (B.D. Malhotra). 1 King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia. 2 Biomedical Instrumentation Section, Materials Physics & Engineering Division, National Physical Laboratory, K.S.Krishnan Marg, New Delhi110012, India. 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.10.063
luminescent lanthanides/SiO2 nanoparticles [3,5,10,11,15,23–26] have been reported [4,12,14,24]. The fluorescent labeling of bio-macromolecules (proteins and enzymes) is known to provide useful information for clinical diagnostics. The labels often used are organic dyes that have many limitations including broad spectral features, low signal intensities, short lifetimes (the natural fluorescence of proteins occurs within 1–10 ns) [27], and potential toxicity to cells [28]. The alternative labels can perhaps be based on lanthanide-derived phosphors. The LnF3 -based nanoparticles have a number of advantages including long luminescent lifetimes and high-quantum yields for use as probes in bio-conjugation applications [3,29–32]. The luminescent lanthanides nanoparticles, due to their unique luminescence properties, are finding increasing applications in a wide variety of bioanalytical assays, diagnostics research and drug discovery, etc. [31]. Several groups, including those of Depu Chen, M. Haase, Frank C.J.M. van Veggel, and Thomas Nann, have revealed that Ln3+ -doped LnF3 can perhaps be used as biolabels with specific binding to certain protein [29–37]. In spite of these interesting developments, the practical applications of lanthanide fluoride nanoparticles are currently limited due to the problems of surface modification and safety issues. The silica coating on surface of the fluorescent lanthanide fluoride nanoparticles may provide useful functional groups for conjugation of desired molecules. Among the lanthanides, neodymium (Nd3+ ) ions have been found to exhibit interesting optical properties, such as sharp spectral lines (4f–4f) in the UV–vis region [38].
A.A. Ansari et al. / Spectrochimica Acta Part A 86 (2012) 432–436
In this paper, we will report the results of our studies on the preparation of silica surface modified NdF3 core–shell nanoparticles using sol–gel chemical route. Further, the changes in optical performance of the silica-coated NdF3 core–shell nanoparticles at room temperature which have also been investigated will be presented. The silica-coated NdF3 core–shell nanoparticles were found to be transferrable to water which in turn can then be utilized for biological labeling. 2. Experimental details 2.1. Chemicals and reagents Neodymium oxide (99.9%, Lieco Chemicals, USA) is used for the preparation of neodymium nitrate hexahydrate. Sodium fluoride (NaF), NH4 OH, ethanol (E-Merck, UK) and tetraethyl-orthosilicate (TEOS, BDH, England) are of spectroscopic grade. The deionized water obtained from Millipore water purification system (Milli Q 10 TS) was used for the preparation of the NdF3 nanoparticles. 2.2. Preparation of NdF3 nanoparticles In a typical synthesis, 2 g of Nd(NO3 )3 ·6H2 O (1 mol) is added in a beaker containing 50 ml of distilled water. After being stirred magnetically for about 10 min at 70 ◦ C, the mixture becomes light violet in color. Then slight excess sodium fluoride (NaF), 0.71 g (3 mmol) dissolved in deionized water (50 ml) is mixed in Nd(NO3 )3 ·6H2 O solution. The mixture is subsequently heated up to 70 ◦ C for 5 h under constant stirring and the reaction is continuously monitored until a light-violet suspension appears indicating the formation of NdF3 nanoparticles. The obtained suspension is centrifuged and washed sequentially with ethanol and deionized water for several times to remove the un-reacted soluble byproduct (NaNO3 ) and possible residual reactants, such as Nd(NO3 )3 ·6H2 O and NaF. The NdF3 nanoparticles are dried at 80 ◦ C in a vacuum oven and then collected for characterization. 2.2.1. Silica-coated NdF3 core–shell nanoparticles The NdF3 microspheres are prepared through a versatile solution sol–gel process. For this purpose, an aqueous dispersion of the optically active NdF3 nanoparticles (40 ml, 2 mg/ml) is added to a three-neck round-bottomed flask charged with absolute ethanol (200 ml) and concentrated ammonia solution (4 ml) under mechanically stirring for about 15 min at ambient temperature. A 3 ml TEOS is the added drop-wise for about 2 min, and the reaction is allowed to proceed over night under constant stirring. The silica-coated NdF3 nanoparticles are collected by centrifugation and washed with absolute ethanol and deionized water to remove the un-reacted reactants. The white-violet colored product is then obtained and dried for about 4 h at 60 ◦ C. Scheme 1 shows the preparation of silica-coated NdF3 nanoparticles.
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2.3. Characterization The phase identification of the synthesized NdF3 nanoparticles and silica-coated NdF3 core–shell nanoparticles has been performed by powder X-ray diffraction (Rigaku) using Cu K␣ radiation. The TEM micrographs were obtained using the JEM-2010 (JEOL, Japan) operated at an acceleration voltage of 200 kV. The FTIR spectra were recorded on the Perkin-Elmer FTIR spectrophotometer using KBr method disc in the range 4000–400 cm−1 . For the FTIR studies, a small amount of powder sample was mixed with KBr and then pressed to make a thin pellet. The optical absorption spectra of NdF3 nanoparticles and silica-coated NdF3 core–shell nanoparticles were recorded on HR 4000 CE high-resolution Ocean optics spectrophotometer in the wavelength range, 200–900 nm at ambient temperature. The excitation and emission spectra were recorded on Perkin-Elmer LS-55 photoluminescence spectrophotometer.
3. Results and discussion Fig. 1 shows the results of the X-ray diffraction studies of NdF3 nanoparticles and silica-coated NdF3 core–shell nanoparticles, recorded using the Rigaku (XRD-6000) diffractometer equipped ˚ X-ray source. The results of XRD pattern with Cu K␣ (1.5406 A) indicate that NdF3 core–shell nanoparticles have well-crystalline structure revealing all diffraction peaks corresponding to (0 0 2), (1 1 0), (1 1 1), (3 0 0), (1 1 3), (3 0 2) and (2 2 1) of NdF3 nanoparticles (Fig. 1a and b) which closely resemble with the reported values for NdF3 nanoparticles [39]. As shown in Fig. 1a, the reflection peaks of the XRD pattern can be readily indexed to a pure hexagonal phase (space group: P63cm (JCPDS 32-0483)) of NdF3 with ˚ b = 9.333 A˚ and c = 13.7827 A. ˚ The lattice parameters a = 4.0777 A, average crystalline size of NdF3 nanocrystals is found to be 30 nm as estimated using the Debye–Scherer equation (D = (˛)/(ˇ cos ), where D mean particle size, ˛ geometric factor is a constant (0.89), ˚ and ˇ is half-width full maxima of is x-ray wavelength (1.541 A) diffraction peak. The broadening of the diffraction peaks reveals a decrease in the sizes of the core–shell nanoparticles [29,39]. The TEM micrograph provides insight into the morphology and microstructural details of NdF3 nanoparticles is shown in Fig. 2a. As shown the particles are spherical in shape with diameter/thickness of the NdF3 nanoparticles are estimated to be in the range from 30 to 80 nm. As shown in Fig. 2b, thick silica-shell is coated over the NdF3 nanoparticles using the sol–gel Stober method. The TEM micrograph of the NdF3 nanoparticles further confirms the silica coating on nanoparticles surface, which is nearly spherical in shape with rough surface. Due to the silica-surface modification, nanoparticles are not well separated (aggregated) in organic solution (ethanol) media as evident from the TEM micrographs. These silica-coated core–shell nanoparticles exhibit excellent dispersibility in polar solvents such as water, ethanol as shown in the inset of Fig. 2. Fig. 3a and b shows the FTIR spectra of NdF3 and silicacoated NdF3 core–shell nanoparticles. A broad absorption band seen between 3100 and 3450 cm−1 and a sharp band at 1645 cm−1
Scheme 1. Preparation of silica-coated NdF3 core–shell nanoparticles.
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Fig. 1. X-ray diffraction pattern of (a) NdF3 nanoparticles and (b) silica-coated NdF3 core–shell nanoparticles.
correspond to O–H stretching and bending vibrations of the physically absorbed water molecules on the nanoparticles surface, respectively [40]. The bands at 484, 592, 734, 836, 1054, 1127, 1380, 1645, and 3456 cm−1 in FTIR spectra of silica-coated NdF3 core–shell nanoparticles are observed, which correspond to the stretching and bonding vibration modes of Si–O, Si–O–Si, O–Si–O, Si–O–H, and OH− , asymmetric, symmetric stretching vibrations and bending vibrations, respectively, (Fig. 3b). These observed results suggest that the silica is successfully coated onto the NdF3 nanoparticles [41–44]. UV–vis absorption spectrum of NdF3 nanoparticles, recorded using HR 4000 CE high resolution Ocean optics model spectrophotometer, shows several multiplet-to-multiplet 4f–4f absorption
transitions in UV–vis region from 4 I9/2 ground state to the excited state, with the special attention to absorption bands, such as 4 F5/2 ; 2 H9/2 , 4 F5/2 ; 4 S3/2 ,4 F7/2 ; 4 G5/2 ,2 G7/2 ; 2 K13/2 ,4 G7/2 ,4 G9/2 ; 2K 2 2 4 15/2 , G9/2 , (D, P) 3/2 , G11/2. Among the various transitions of Nd3+ , the intensity of the 2 G7/2 ,4 G5/2 ← 4 19/2 pair, near the middle of the visible region (587 nm) exhibits the maximum sensitivity (hypersensitive transition) [38,45]. These hypersensitive transitions are known to be sensitive to the change in the local environment and have small splitting when the symmetry of the ion changes [38,45]. The optical absorption spectrum of silicacoated NdF3 core–shell nanoparticles is quite different from NdF3 nanoparticles (Fig. 4b). As shown in Fig. 4b, the intensities of the spectral transitions and the band shapes are greatly suppressed
Fig. 2. Transmission electron micrographs (a) NdF3 nanoparticles and (b) silica-coated NdF3 core–shell nanoparticles.
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435
463
b
400
482
Relative intensity (a.u.)
%T
a
700 721
507 535
b a
583 608
800 1200 1600 2000 2400 2800 3200 3600 4000 -1
Wavenumber (cm ) Fig. 3. FTIR spectra of (a) NdF3 nanoparticles and (b) silica-coated NdF3 core–shell nanoparticles.
in silica-coated NdF3 core–shell nanoparticles. The band shapes of silica-coated NdF3 core–shell nanoparticles did not resemble to those of the bands in the NdF3 nanoparticles. The occurrence of different band shapes for the two materials is indicative of different symmetries, i.e. different geometrical arrangement of silica with neodymium metal ion [45]. The UV–vis absorption spectrum of silica-coated NdF3 nanoparticles yields distinctively different shape of bands in the solid phase in the UV-region. Moreover, significant changes in the absorption bands seen at 240 and 290 nm are observed. These observations are in agreement with the reported absorptions of silica [3,34] indicating silica coating on the NdF3 nanoparticles surface. Three effects are observed in absorption spectrum of silicacoated NdF3 nanoparticles comparison with NdF3 nanoparticles (i) a small shift in the bands toward the longer wavelength (ii) bands undergo with an additional splitting of j-levels (or at least show a different splitting), and (iii) there is a significant change in the molar absorptivity of the individual bands. These effects are
450
500
550
600
650
700
750
800
Wavelength (nm) Fig. 5. Photoluminescence spectra (a) NdF3 nanoparticles and (b) silica-coated NdF3 core–shell nanoparticles.
assigned to the changes in the strength and symmetry of the crystal field produced by silica coating on NdF3 nanoparticle surface. The absorption spectrum of NdF3 nanoparticles shows the crystalfield effects on the 2s+1 Lj electronic levels, which consist of several groups of lines corresponding to the transitions between the 4 I9/2 ground state and higher energy states of the 4f3 electronic configuration of the Nd3+ ion. An important parameter relates to the process of silica encapsulation on NdF3 nanoparticles surface without altering optical properties of NdF3 nanoparticles. The emission spectra of NdF3 and silica-coated NdF3 core–shell nanoparticles were recorded (excitation at 210 nm) at room temperature. Fig. 5 shows photoluminescence (PL) spectra of NdF3 nanoparticles and three strong emission transitions includes blue, green and orange located at 463, 507 and 700 nm are observed. In addition, two blue emissions at 463 and 507 nm corresponding to 4 G7/2 → 4 I9/2 , 4 I11/2 , 4 I13/2 and 4 D3/2 → 4 I15/2 , 2 P3/2 → 4 I13/2 of Nd3+ ion were also observed. These emission transitions in the PL spectrum are reported in other neodymium materials attributed to the transitions from the 4 G7/2 and 4 G5/2 levels [45–48]. The principle emission transitions show a qualitative difference between the NdF3 and silica-coated NdF3 core–shell nanoparticles in the emission spectrum range from 400 to 800 nm. The observed enhancement in the intensity with red shift in the emission transitions of silica-coated NdF3 nanoparticles is due to formation of new bond between NdF3 and SiO2 matrix (Fig. 5b). The observed splitting in the emission transitions may be due to the splitting in the energy levels of Nd3+ ions due to crystalfield effect. The observed red shift could also be assigned to the radioactive recombination of self-trapped excitation. For this could also be due to the effect of SiO2 on the host material. Furthermore, these emission transitions may also be influenced by the coating, excitation power, temperature of preparation and impurities. 4. Conclusion
Fig. 4. UV–vis absorption spectra (a) NdF3 nanoparticles and (b) silica-coated NdF3 core–shell nanoparticles inset show the photograph of silica-coated NdF3 core–shell nanoparticles suspended in de-ionized water.
We prepared the luminescent silica surface modified NdF3 core–shell nanoparticles using the sol–gel process. The results of X-ray diffraction, TEM and FTIR spectrum confirm the phase, morphological structure, crystalline nature (as well as coating of silica core on the surface) of NdF3 nanoparticles. The resulting silica surface modified NdF3 core–shell nanoparticles were not found to be well separated as observed by TEM. The optical absorption spectra of silica-coated NdF3 core–shell nanoparticles exhibit a significant
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difference in bands intensity and shape of hypersensitive transitions (2 G7/2 ,4 G5/2 ← 4 19/2 ) indicating a modification of coordination environment (geometrical structure) around the Nd3+ ion after formation of the silica core–shell. These effects are assigned to the changes in the strength and the symmetry of the crystal field produced by the silica encapsulation on NdF3 nanoparticles. The PL spectra of silica surface modified NdF3 core–shell nanoparticles show emission bands in the visible region located at 482, 535, 608 and 700 nm. These blue, green yellow and red emissions are attributed to transitions from 4 G7/2 excited state. The results of photoluminescence studies further suggest that silica-coating results in enhanced intensity of Nd3+ ions emission transitions. It should be interesting then to conduct studies to delineate the mechanism of growth and potential applications of these luminescent silicacoated lanthanides core–shell nanoparticles. Acknowledgements We thank Director, NPL, New Delhi, India for providing experimental facilities. AAA is thankful CSIR for financial support. Authors also thank Prof. Shamim Ahmad Ex. Vice Chancellor of the Jamia Hamdard University, Delhi, India for fruitful discussions. References
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Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis of optically active silica-coated NdF3 core–shell nanoparticles Anees A. Ansari ∗,1 , S.P. Singh 2 , N. Singh 2 , B.D. Malhotra ∗,2 Biomedical Instrumentation Section, Materials Physics & Engineering Division, National Physical Laboratory, K. S. Krishnan Marg, New Delhi 110012, India
a r t i c l e
i n f o
Article history: Received 19 September 2011 Received in revised form 21 October 2011 Accepted 28 October 2011 Keywords: Silica-coated NdF3 core–shell nanoparticles HR-TEM Optical absorption spectra Photoluminescence spectra
a b s t r a c t Silica surface-modified NdF3 core–shell nanoparticles were prepared by sol–gel route. The prepared core–shell nanoparticles were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), UV–vis absorption and photoluminescence (PL) spectroscopy studies. Phase identification of the NdF3 and silica-coated NdF3 core–shell nanoparticles which was carried-out by XRD, confirms the formation of a well-crystallized hexagonal phase structure. Due to the silica-surface modification, the nanoparticles were not found to be wellseparated (agglomerated) in ethanol solvent as scanned by TEM. The results of the FTIR studies conducted on these core–shell reveal the binding of silica with the NdF3 nanoparticles. The largest intensity and shape variation were observed in all transitions as compared to non-silica modified NdF3 nanoparticle spectra, and were attributed to the environment around the Nd(III) ion due to coordination of silica molecule(s). A significant enhancement in the emission intensity was measured in silica surface modified NdF3 core–shell nanoparticles due to the successful silica coating on the surface of nanoparticles. The results of these studies suggest that these nanoparticles may find potential applications in the areas of bioimaging, protein-labeling, optical biosensors and drug delivery, etc. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Silica-coated core–shell nanoparticles have recently attracted much attention due to their rich surface chemistry, high biocompatibility, controllable porosity, good optical transparency, cheapness and photochemical stability observed under laser photolysis [1,2]. These have been attributed to the silanol groups on the surface of silica coatings that can be utilized to introduce specific functional groups [3–5]. These specific functional groups including amines, thiols and carboxyls on the surface of silica nanoparticles made them ideal for bioanalysis applications [3–10]. Among the various approaches to the functionalization or core–shell formation on the surfaces of metal nanoparticles, the luminescent metal nanoparticles coated with silica core–shells have been considered very important [10–15]. And recently, several kinds of silica-coated material nanoparticles including Ag/SiO2 [16,17], Au/SiO2 [18,19], Fe3 O4 /SiO2 [5,20–23], QDs/SiO2 , and
∗ Corresponding authors. Tel.: +96 61 4676838; fax: +96 61 4670662. E-mail addresses: [email protected] (A.A. Ansari), [email protected] (B.D. Malhotra). 1 King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia. 2 Biomedical Instrumentation Section, Materials Physics & Engineering Division, National Physical Laboratory, K.S.Krishnan Marg, New Delhi110012, India. 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.10.063
luminescent lanthanides/SiO2 nanoparticles [3,5,10,11,15,23–26] have been reported [4,12,14,24]. The fluorescent labeling of bio-macromolecules (proteins and enzymes) is known to provide useful information for clinical diagnostics. The labels often used are organic dyes that have many limitations including broad spectral features, low signal intensities, short lifetimes (the natural fluorescence of proteins occurs within 1–10 ns) [27], and potential toxicity to cells [28]. The alternative labels can perhaps be based on lanthanide-derived phosphors. The LnF3 -based nanoparticles have a number of advantages including long luminescent lifetimes and high-quantum yields for use as probes in bio-conjugation applications [3,29–32]. The luminescent lanthanides nanoparticles, due to their unique luminescence properties, are finding increasing applications in a wide variety of bioanalytical assays, diagnostics research and drug discovery, etc. [31]. Several groups, including those of Depu Chen, M. Haase, Frank C.J.M. van Veggel, and Thomas Nann, have revealed that Ln3+ -doped LnF3 can perhaps be used as biolabels with specific binding to certain protein [29–37]. In spite of these interesting developments, the practical applications of lanthanide fluoride nanoparticles are currently limited due to the problems of surface modification and safety issues. The silica coating on surface of the fluorescent lanthanide fluoride nanoparticles may provide useful functional groups for conjugation of desired molecules. Among the lanthanides, neodymium (Nd3+ ) ions have been found to exhibit interesting optical properties, such as sharp spectral lines (4f–4f) in the UV–vis region [38].
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In this paper, we will report the results of our studies on the preparation of silica surface modified NdF3 core–shell nanoparticles using sol–gel chemical route. Further, the changes in optical performance of the silica-coated NdF3 core–shell nanoparticles at room temperature which have also been investigated will be presented. The silica-coated NdF3 core–shell nanoparticles were found to be transferrable to water which in turn can then be utilized for biological labeling. 2. Experimental details 2.1. Chemicals and reagents Neodymium oxide (99.9%, Lieco Chemicals, USA) is used for the preparation of neodymium nitrate hexahydrate. Sodium fluoride (NaF), NH4 OH, ethanol (E-Merck, UK) and tetraethyl-orthosilicate (TEOS, BDH, England) are of spectroscopic grade. The deionized water obtained from Millipore water purification system (Milli Q 10 TS) was used for the preparation of the NdF3 nanoparticles. 2.2. Preparation of NdF3 nanoparticles In a typical synthesis, 2 g of Nd(NO3 )3 ·6H2 O (1 mol) is added in a beaker containing 50 ml of distilled water. After being stirred magnetically for about 10 min at 70 ◦ C, the mixture becomes light violet in color. Then slight excess sodium fluoride (NaF), 0.71 g (3 mmol) dissolved in deionized water (50 ml) is mixed in Nd(NO3 )3 ·6H2 O solution. The mixture is subsequently heated up to 70 ◦ C for 5 h under constant stirring and the reaction is continuously monitored until a light-violet suspension appears indicating the formation of NdF3 nanoparticles. The obtained suspension is centrifuged and washed sequentially with ethanol and deionized water for several times to remove the un-reacted soluble byproduct (NaNO3 ) and possible residual reactants, such as Nd(NO3 )3 ·6H2 O and NaF. The NdF3 nanoparticles are dried at 80 ◦ C in a vacuum oven and then collected for characterization. 2.2.1. Silica-coated NdF3 core–shell nanoparticles The NdF3 microspheres are prepared through a versatile solution sol–gel process. For this purpose, an aqueous dispersion of the optically active NdF3 nanoparticles (40 ml, 2 mg/ml) is added to a three-neck round-bottomed flask charged with absolute ethanol (200 ml) and concentrated ammonia solution (4 ml) under mechanically stirring for about 15 min at ambient temperature. A 3 ml TEOS is the added drop-wise for about 2 min, and the reaction is allowed to proceed over night under constant stirring. The silica-coated NdF3 nanoparticles are collected by centrifugation and washed with absolute ethanol and deionized water to remove the un-reacted reactants. The white-violet colored product is then obtained and dried for about 4 h at 60 ◦ C. Scheme 1 shows the preparation of silica-coated NdF3 nanoparticles.
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2.3. Characterization The phase identification of the synthesized NdF3 nanoparticles and silica-coated NdF3 core–shell nanoparticles has been performed by powder X-ray diffraction (Rigaku) using Cu K␣ radiation. The TEM micrographs were obtained using the JEM-2010 (JEOL, Japan) operated at an acceleration voltage of 200 kV. The FTIR spectra were recorded on the Perkin-Elmer FTIR spectrophotometer using KBr method disc in the range 4000–400 cm−1 . For the FTIR studies, a small amount of powder sample was mixed with KBr and then pressed to make a thin pellet. The optical absorption spectra of NdF3 nanoparticles and silica-coated NdF3 core–shell nanoparticles were recorded on HR 4000 CE high-resolution Ocean optics spectrophotometer in the wavelength range, 200–900 nm at ambient temperature. The excitation and emission spectra were recorded on Perkin-Elmer LS-55 photoluminescence spectrophotometer.
3. Results and discussion Fig. 1 shows the results of the X-ray diffraction studies of NdF3 nanoparticles and silica-coated NdF3 core–shell nanoparticles, recorded using the Rigaku (XRD-6000) diffractometer equipped ˚ X-ray source. The results of XRD pattern with Cu K␣ (1.5406 A) indicate that NdF3 core–shell nanoparticles have well-crystalline structure revealing all diffraction peaks corresponding to (0 0 2), (1 1 0), (1 1 1), (3 0 0), (1 1 3), (3 0 2) and (2 2 1) of NdF3 nanoparticles (Fig. 1a and b) which closely resemble with the reported values for NdF3 nanoparticles [39]. As shown in Fig. 1a, the reflection peaks of the XRD pattern can be readily indexed to a pure hexagonal phase (space group: P63cm (JCPDS 32-0483)) of NdF3 with ˚ b = 9.333 A˚ and c = 13.7827 A. ˚ The lattice parameters a = 4.0777 A, average crystalline size of NdF3 nanocrystals is found to be 30 nm as estimated using the Debye–Scherer equation (D = (˛)/(ˇ cos ), where D mean particle size, ˛ geometric factor is a constant (0.89), ˚ and ˇ is half-width full maxima of is x-ray wavelength (1.541 A) diffraction peak. The broadening of the diffraction peaks reveals a decrease in the sizes of the core–shell nanoparticles [29,39]. The TEM micrograph provides insight into the morphology and microstructural details of NdF3 nanoparticles is shown in Fig. 2a. As shown the particles are spherical in shape with diameter/thickness of the NdF3 nanoparticles are estimated to be in the range from 30 to 80 nm. As shown in Fig. 2b, thick silica-shell is coated over the NdF3 nanoparticles using the sol–gel Stober method. The TEM micrograph of the NdF3 nanoparticles further confirms the silica coating on nanoparticles surface, which is nearly spherical in shape with rough surface. Due to the silica-surface modification, nanoparticles are not well separated (aggregated) in organic solution (ethanol) media as evident from the TEM micrographs. These silica-coated core–shell nanoparticles exhibit excellent dispersibility in polar solvents such as water, ethanol as shown in the inset of Fig. 2. Fig. 3a and b shows the FTIR spectra of NdF3 and silicacoated NdF3 core–shell nanoparticles. A broad absorption band seen between 3100 and 3450 cm−1 and a sharp band at 1645 cm−1
Scheme 1. Preparation of silica-coated NdF3 core–shell nanoparticles.
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Fig. 1. X-ray diffraction pattern of (a) NdF3 nanoparticles and (b) silica-coated NdF3 core–shell nanoparticles.
correspond to O–H stretching and bending vibrations of the physically absorbed water molecules on the nanoparticles surface, respectively [40]. The bands at 484, 592, 734, 836, 1054, 1127, 1380, 1645, and 3456 cm−1 in FTIR spectra of silica-coated NdF3 core–shell nanoparticles are observed, which correspond to the stretching and bonding vibration modes of Si–O, Si–O–Si, O–Si–O, Si–O–H, and OH− , asymmetric, symmetric stretching vibrations and bending vibrations, respectively, (Fig. 3b). These observed results suggest that the silica is successfully coated onto the NdF3 nanoparticles [41–44]. UV–vis absorption spectrum of NdF3 nanoparticles, recorded using HR 4000 CE high resolution Ocean optics model spectrophotometer, shows several multiplet-to-multiplet 4f–4f absorption
transitions in UV–vis region from 4 I9/2 ground state to the excited state, with the special attention to absorption bands, such as 4 F5/2 ; 2 H9/2 , 4 F5/2 ; 4 S3/2 ,4 F7/2 ; 4 G5/2 ,2 G7/2 ; 2 K13/2 ,4 G7/2 ,4 G9/2 ; 2K 2 2 4 15/2 , G9/2 , (D, P) 3/2 , G11/2. Among the various transitions of Nd3+ , the intensity of the 2 G7/2 ,4 G5/2 ← 4 19/2 pair, near the middle of the visible region (587 nm) exhibits the maximum sensitivity (hypersensitive transition) [38,45]. These hypersensitive transitions are known to be sensitive to the change in the local environment and have small splitting when the symmetry of the ion changes [38,45]. The optical absorption spectrum of silicacoated NdF3 core–shell nanoparticles is quite different from NdF3 nanoparticles (Fig. 4b). As shown in Fig. 4b, the intensities of the spectral transitions and the band shapes are greatly suppressed
Fig. 2. Transmission electron micrographs (a) NdF3 nanoparticles and (b) silica-coated NdF3 core–shell nanoparticles.
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435
463
b
400
482
Relative intensity (a.u.)
%T
a
700 721
507 535
b a
583 608
800 1200 1600 2000 2400 2800 3200 3600 4000 -1
Wavenumber (cm ) Fig. 3. FTIR spectra of (a) NdF3 nanoparticles and (b) silica-coated NdF3 core–shell nanoparticles.
in silica-coated NdF3 core–shell nanoparticles. The band shapes of silica-coated NdF3 core–shell nanoparticles did not resemble to those of the bands in the NdF3 nanoparticles. The occurrence of different band shapes for the two materials is indicative of different symmetries, i.e. different geometrical arrangement of silica with neodymium metal ion [45]. The UV–vis absorption spectrum of silica-coated NdF3 nanoparticles yields distinctively different shape of bands in the solid phase in the UV-region. Moreover, significant changes in the absorption bands seen at 240 and 290 nm are observed. These observations are in agreement with the reported absorptions of silica [3,34] indicating silica coating on the NdF3 nanoparticles surface. Three effects are observed in absorption spectrum of silicacoated NdF3 nanoparticles comparison with NdF3 nanoparticles (i) a small shift in the bands toward the longer wavelength (ii) bands undergo with an additional splitting of j-levels (or at least show a different splitting), and (iii) there is a significant change in the molar absorptivity of the individual bands. These effects are
450
500
550
600
650
700
750
800
Wavelength (nm) Fig. 5. Photoluminescence spectra (a) NdF3 nanoparticles and (b) silica-coated NdF3 core–shell nanoparticles.
assigned to the changes in the strength and symmetry of the crystal field produced by silica coating on NdF3 nanoparticle surface. The absorption spectrum of NdF3 nanoparticles shows the crystalfield effects on the 2s+1 Lj electronic levels, which consist of several groups of lines corresponding to the transitions between the 4 I9/2 ground state and higher energy states of the 4f3 electronic configuration of the Nd3+ ion. An important parameter relates to the process of silica encapsulation on NdF3 nanoparticles surface without altering optical properties of NdF3 nanoparticles. The emission spectra of NdF3 and silica-coated NdF3 core–shell nanoparticles were recorded (excitation at 210 nm) at room temperature. Fig. 5 shows photoluminescence (PL) spectra of NdF3 nanoparticles and three strong emission transitions includes blue, green and orange located at 463, 507 and 700 nm are observed. In addition, two blue emissions at 463 and 507 nm corresponding to 4 G7/2 → 4 I9/2 , 4 I11/2 , 4 I13/2 and 4 D3/2 → 4 I15/2 , 2 P3/2 → 4 I13/2 of Nd3+ ion were also observed. These emission transitions in the PL spectrum are reported in other neodymium materials attributed to the transitions from the 4 G7/2 and 4 G5/2 levels [45–48]. The principle emission transitions show a qualitative difference between the NdF3 and silica-coated NdF3 core–shell nanoparticles in the emission spectrum range from 400 to 800 nm. The observed enhancement in the intensity with red shift in the emission transitions of silica-coated NdF3 nanoparticles is due to formation of new bond between NdF3 and SiO2 matrix (Fig. 5b). The observed splitting in the emission transitions may be due to the splitting in the energy levels of Nd3+ ions due to crystalfield effect. The observed red shift could also be assigned to the radioactive recombination of self-trapped excitation. For this could also be due to the effect of SiO2 on the host material. Furthermore, these emission transitions may also be influenced by the coating, excitation power, temperature of preparation and impurities. 4. Conclusion
Fig. 4. UV–vis absorption spectra (a) NdF3 nanoparticles and (b) silica-coated NdF3 core–shell nanoparticles inset show the photograph of silica-coated NdF3 core–shell nanoparticles suspended in de-ionized water.
We prepared the luminescent silica surface modified NdF3 core–shell nanoparticles using the sol–gel process. The results of X-ray diffraction, TEM and FTIR spectrum confirm the phase, morphological structure, crystalline nature (as well as coating of silica core on the surface) of NdF3 nanoparticles. The resulting silica surface modified NdF3 core–shell nanoparticles were not found to be well separated as observed by TEM. The optical absorption spectra of silica-coated NdF3 core–shell nanoparticles exhibit a significant
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difference in bands intensity and shape of hypersensitive transitions (2 G7/2 ,4 G5/2 ← 4 19/2 ) indicating a modification of coordination environment (geometrical structure) around the Nd3+ ion after formation of the silica core–shell. These effects are assigned to the changes in the strength and the symmetry of the crystal field produced by the silica encapsulation on NdF3 nanoparticles. The PL spectra of silica surface modified NdF3 core–shell nanoparticles show emission bands in the visible region located at 482, 535, 608 and 700 nm. These blue, green yellow and red emissions are attributed to transitions from 4 G7/2 excited state. The results of photoluminescence studies further suggest that silica-coating results in enhanced intensity of Nd3+ ions emission transitions. It should be interesting then to conduct studies to delineate the mechanism of growth and potential applications of these luminescent silicacoated lanthanides core–shell nanoparticles. Acknowledgements We thank Director, NPL, New Delhi, India for providing experimental facilities. AAA is thankful CSIR for financial support. Authors also thank Prof. Shamim Ahmad Ex. Vice Chancellor of the Jamia Hamdard University, Delhi, India for fruitful discussions. References
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