- Email: [email protected]
shell nanorods: Synthesis, structural and luminescence properties
Accepted Manuscript Designing of luminescent GdPO4:Eu@LaPO4@SiO2 core/shell nanorods: Synthesis, structural and luminescence properties Anees A. Ansari, Joselito P. Labis, M. Aslam Manthrammel PII:
S1293-2558(17)30409-0
DOI:
10.1016/j.solidstatesciences.2017.07.012
Reference:
SSSCIE 5535
To appear in:
Solid State Sciences
Received Date: 24 April 2017 Revised Date:
9 July 2017
Accepted Date: 15 July 2017
Please cite this article as: A.A. Ansari, J.P. Labis, M. Aslam Manthrammel, Designing of luminescent GdPO4:Eu@LaPO4@SiO2 core/shell nanorods: Synthesis, structural and luminescence properties, Solid State Sciences (2017), doi: 10.1016/j.solidstatesciences.2017.07.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Graphical Abstract
ACCEPTED MANUSCRIPT
Designing of luminescent GdPO4:Eu@LaPO4@SiO2 core/shell nanorods: Synthesis, structural and luminescence properties Anees A. Ansari*1 Joselito P. Labis1, M. Aslam Manthrammel2 King Abdullah Institute for Nanotechnology, King Saud University,Riyadh-11451, Saudi Arabia 2 Department of Physics, King Saud University, Riyadh-11451, Saudi Arabia
RI PT
1
Abstract
SC
GdPO4:Eu3+ (core) and GdPO4:Eu@LaPO4 (core/shell) nanorods (NRs) were successfully prepared by urea based co-precipitation process at ambient conditions which was followed by coating with amorphous silica shell via the sol-gel chemical route. The
M AN U
role of surface coating on the crystal structure, crystallinity, morphology, solubility, surface chemistry and luminescence properties were well investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, Fourier Transform Infrared (FTIR), UV-Vis, and photoluminescence spectroscopy. XRD pattern revealed highly purified, well-crystalline, single phasehexagonal-rhabdophane structure of GdPO4 crystal. The TEM micrographs exhibited
TE D
highly crystalline and narrow size distributed rod-shaped GdPO4:Eu3+ nanostructures with average width 14-16 nm and typical length 190-220 nm. FTIR spectra revealed characteristic infrared absorption bands of amorphous silica. High absorbance in a visible
EP
region of silica modified core/shell/Si NRs in aqueous environment suggests the high solubility along with colloidal stability. The photoluminescence properties were remarkably enhanced after growth of undoped LaPO4 layers due to the reduction of
AC C
nonradiative transition rate. The advantages of presented high emission intensity and high solubility of core/shell and core/shell/Si NRs indicated the potential applications in monitoring biological events.
Keywords: gadolinium phosphate, europium, solubility, surface chemistry, luminescence properties. Ph.:+966-11-46768383 Email: [email protected]
Fax: +966-11-4670662
1
ACCEPTED MANUSCRIPT
I. Introduction A wide variety of nanophosphor materials has been fabricated because of their potential application in the light emitting diode, solid laser, panel display and other
RI PT
optoelectronic devices etc. [1-5]. In recent years, lanthanide phosphate based nanophosphors have attracted research community as they are very interesting class of host matrices for activators owing to their variety of favorable properties such as
SC
maximum transparency in the visible spectral region, high thermal stability, good photochemical stability, high refractive index and high quantum yield [6-16]. Lanthanide
M AN U
phosphates exhibit interesting electronic, magnetic and optical properties arising from unfilled 4f-orbital which are shielded by the 5s2p6 orbitals. Generally, these physiochemical properties of lanthanide nanophosphors are strongly dependent on host matrices [7, 12-14, 17-21]. In this context, gadolinium orthophosphate is a good
TE D
luminescent host matrix [11, 22-25]. Gadolinium-based nano-phosphates are special category of multifunctional agents for multimodal bioimaging because the optically active lanthanide ions doped in the phosphate matrix can contribute to luminescence
EP
(optical modality) and Gd3+ ion as a paramagnetic relaxation agent can introduce magnetic resonance imaging (MRI) modality [23, 25, 26]. The unpaired electrons of Gd3+
AC C
on the surface of GdPO4 NRs can be exploited to enhance the contrast for MR imaging so that it can bridge gaps that combine MRI and optical imaging applications [22, 27-29]. Different trivalent lanthanide ions such as neodymium (near infrared (NIR) emitting), europium (red emitting), terbium (green emitting), holmium, erbium, and ytterbium can serve as highly efficient luminescent centers which cover a broad range of the visible and NIR spectral region. Amongst lanthanide activators, Eu gives high emission lines at 595,
2
ACCEPTED MANUSCRIPT
614 and 695 nm and is sensitive to the local environment because Eu3+ ions occupy Gd3+ sites, where EuO8 is highly asymmetric (D2d). For example, Zhang et al. fabricated multifunctional Eu3+ doped gadolinium phosphate hollow spheres via one-pot
RI PT
hydrothermal process [30]. Liviano et al. prepared highly crystalline GdPO4:Eu nanocubes by a simple and fast microwave-assisted complexation process at low temperature [23]. Du et al. proposed simple and effective synthesis process for the
SC
preparation of well-crystalline hexagonal GdPO4:Eu nanorods via template-based silkfibroin peptides [31]. Runowski et al. and Sahu et al. used co-precipitation method for
M AN U
preparation of GdPO4:Eu nanostructured materials and investigated their luminescence properties [32, 33]. Ren et al. applied hydrothermal process for the preparation of different luminescent ion doped gadolinium orthophosphate and simultaneously investigated their applications in optical and MRI [11]. In a similar report, the
TE D
hydrothermal process was utilized for the preparation of GdPO4:Eu nanocrystals and explored their optical properties [21]. The sonochemical process was applied for the preparation of GdPO4:Eu via irradiation of inorganic salt in aqueous environment under
EP
ambient conditions without any surfactant [34]. In these previously adopted conventional methods, low-dimensional GdPO4 including zero-dimensional (0D particle clusters), 1D
AC C
(rod-like) and 2D (disk-like) nanostructures are obtained by high-temperature, hydrothermal process or using special methods involving complex procedures or addition of organic additives that are usually adopted for the synthesis of well-crystalline GdPO4:Eu nanocrystals. Therefore, it is imperative to seek a facile, low temperature, highly efficient and large-scale productive approach to fabricate gadolinium phosphate nanorods which doesn’t require very high temperature or complex procedures.
3
ACCEPTED MANUSCRIPT
Herein, we demonstrate a general strategy for the synthesis of GdPO4:Eu (core) nanorods (NRs) through urea based co-precipitation process at ambient temperature conditions. It is well known that urea is a weak base and decomposed homogeneously at
RI PT
low temperature under the appropriate conditions to release hydrogen carbonate (H(CO3)). Subsequently, an insulating LaPO4 and amorphous silica layers were gradually coated around the luminescent seed core-nanorods. An insulating LaPO4 layer not only reduces
SC
the surface defects but also remarkably improves the physiochemical properties such as crystallinity and photoluminescence properties. The advantages of this method include
M AN U
rapid reaction rate, controllable reaction conditions, low-cost, eco-friendly, large-scale production of well-crystalline, single phased, non-aggregated and highly purified nanocrystals. We observed a substantial reduction in reaction time, reaction temperature as well as product yield with respect to hydrothermal or other chemical route based
TE D
synthesis procedures. The structural, morphology, surface chemistry, optical absorption, and luminescence properties were systematically investigated by XRD pattern, TEM, EDX, FTIR, UV/Vis absorption, excitation and emission spectral techniques. XRD and
EP
TEM results show the well-crystalline, single phased, rod-shaped hexagonal-rhabdophane nano structures with irregular sizes. The silica covered core/shell/Si NRs show high
AC C
absorbance in visible region because of optically active silica surface modification. Comparative structural, optical absorption and photoluminescence properties were discussed in detail to investigate the impact of surface coating on their physiochemical properties. It provides a useful way to improve the luminescence properties and their colloidal stability in most aqueous environment for their further use in biological sciences.
4
ACCEPTED MANUSCRIPT
II. Experimental Procedure Gadolinium oxide (BDH, England), lanthanum oxide (BDH, England), europium
RI PT
oxide (Alfa Aesar, Germany), urea, NaH2PO4, Tetraethylorthosilicate (TEOS), ethanol, ammonia were analytical grade reagents and used directly without further purification. Gd(NO3)36H2O, La(NO3)37H2O and Eu(NO3)36H2O were prepared by dissolving the
SC
corresponding metal oxides into diluted nitric acid. Milli-Q (Millipore, Bedford, USA) H2O was used for synthesis and characterization of the samples.
M AN U
For the synthesis of GdPO4:Eu NRs, 1.7 mg (9.5 ml of 2M) gadolinium nitrate, 0.09 mg (0.5 ml of 2M) europium nitrate and 5 mg urea were dissolved in 50 ml of H2O. Urea was used for homogeneous hydrolysis of the metal nitrate. The solution was vigorously stirred for proper mixing on a hot plate for 2 h and later heated under refluxed
TE D
condition for 2-3 h until complete precipitation occurred. The obtained precipitate was redispersed in dist. water by ultrasonication and mechanical stirring. After that, an aqueous solution of NaH2PO4 (0.707 mg) was added slowly to the above mixture. Then
EP
this solution was transferred to a round bottle 250 ml flask fitted with reflux condenser and heated at 100 oC until complete precipitation. The obtained precipitate was
AC C
segregated by centrifugation and washed with dist. H2O and ethanol, and dried in an oven at 60 oC for 6 h.
In a typical synthesis of GdPO4:Eu@LaPO4 NRs, 100 mg GdPO4:Eu NRs were
dispersed in a minimum amount of dist. water under ultrasonication for homogenization. Then an aqueous dissolved 100 mg La(NO3)37H2O was introduced into the above solution under mechanical stirring. 0.763 mg NaH2PO4 was introduced slowly into the
5
ACCEPTED MANUSCRIPT
above suspension. The suspension was transferred to round bottle flask and kept under refluxed condition for 5-6 h for proper precipitation. Occurred precipitate was isolated by centrifugation and washed dist. H2O and ethyl alcohol and dried in an oven at 60 oC.
RI PT
Stober method was used for silica surface coating on GdPO4:Eu@LaPO4 core/shell NRs. 220 mg GdPO4:Eu@LaPO4 (core/shell) nanoparticle were dispersed in an aqueous solution containing 40 ml H2O, 150 ml ethanol and ~2 ml NH4OH under
SC
ultrasonication and later vigorously stirred on a hot plate at room temperature [35, 36]. 1 ml TEOS was added dropwise into the above solution and solution was kept under
M AN U
constant mechanical stirring for 5-6 h. The obtained precipitate was isolated by centrifugation, washed with H2O and ethyl alcohol and dried in an oven at 60oC. X-ray diffraction (XRD) pattern was performed on a PANalytical X’PERT x-ray diffractometer equipped with a Ni filter and using Cu Kα (λ = 1.5406Å) radiation. TEM
TE D
micrographs were obtained using a Field emission transmission electron microscope (FETEM, JEM-2100F, JEOL, Japan) equipped with energy dispersive X-ray (EDX) analysis operated at an accelerating voltage of 200 kV. FTIR spectra were recorded on Vertex 80
EP
(Bruker, USA) spectrophotometer using KBr pellet technique. Optical absorption spectra were measured by Cary 60 UV-Vis (Agilent Technologies, USA) spectrophotometer
AC C
under UV-Vis range. Photoluminescence spectra were recorded with a Fluorolog-3 (Model: FL 3-11, Horiba JobinYvon, MJ, USA) spectrophotometer. All measurements were performed at room temperature.
III. Results and Discussion
6
ACCEPTED MANUSCRIPT
X-ray diffraction pattern was utilized to determine the crystal structure, phase purity and crystallinity of the as-synthesized nano-products. It is observed in Fig.1 that the XRD pattern of core NRs exhibits broad diffraction band along with some broad with
RI PT
weak intensity diffraction peaks, which implies the semi-amorphous nature of the materials due to the formation of Gd(OH)CO3 nano-structures [30]. However, the XRD pattern of core/shell and core/shell/Si NRs in Fig.1 shows all diffraction peaks which are
SC
well indexed to the hexagonal rhabdophane-type GdPO4 (JCPDS card No. 39-0232) [11, 24, 30, 33, 37, 38]. The intensities and peak positions are closely matched with the
M AN U
literature data [11, 24, 30, 33]. No additional peaks related to Gd(OH)3, Gd2O3 and Eu2O3 are observed by XRD over the entire range revealing the high purity of the as-synthesized nano-products. The absence of impurity peaks indicates the homogeneous distribution of Eu3+ in the GdPO4 crystal lattice or the amount is too small to be detected by XRD
TE D
pattern. It is worth noticing that after silica layer surface coating, no change in peak position is observed. It means high purity of the samples which demonstrate the formation of homogeneous Gd-P-O-Eu and Gd-P-O-Eu-La-P-O solid solutions. The
EP
broadening of the reflection planes manifests that core/shell NRs are made up of primary nanocrystalline size.
AC C
TEM images were used to analyze the morphological structure and silica surface
coating around the luminescent seed core NRs. TEM micrographs demonstrate narrow and irregular size distribution of well crystalline, rod-shaped, nanostructures with the average width 14-16 nm and typical average length around 190-220 nm. TEM image in Fig.2c&d revealed that the thin, uniform and porous silica layer with thickness around 810 nm is effectively grown over the surface of core/shell NRs. The silica shell coating
7
ACCEPTED MANUSCRIPT
can be clearly distinguished between core and silica shell through different electron penetrabilities, in which core is dark black and silica shell is light gray in color. It is observed that after grafting of an insulating LaPO4 and amorphous silica layer, the
RI PT
morphological structure of the NRs is still well-maintained. It advocates that the surface modification has no side effect on the homogeneity of the core/shell/Si nanostructures. As seen in Fig. 2c, after silica surface modification the luminescent NRs are highly
SC
aggregated due to the formation of surface silanol (Si-OH) molecules which connect each particle by Van der Waals forces via hydrogen bonding resulting in the increased
M AN U
agglomeration. Notably, the core/shell/Si NRs display high dispersibility or solubility in an aqueous environment compared to core-NRs which suggests the effect of optically active silica surface modification. Fig.2e shows the EDX analysis of the core/shell/Si sample. It reveals the existence of all doped and surface coated metal contents and is a
TE D
strong evidence of Eu ion doping in the GdPO4 crystal lattice and subsequently surface coating of LaPO4 and amorphous silica layers around the luminescent seed core-NRs. Surface chemistry of the core, core/shell and core/shell/Si NRs was examined by
EP
FTIR spectroscopy. The infrared absorption spectra revealed most prominent bands located at 1090, 627 and 548 cm-1 ascribed to the stretching and bending vibrational
AC C
modes of phosphate (PO4)3- groups [11, 30, 39, 40]. The observed peaks at 1657 and 1535 cm-1 are attributed to the asymmetrical and symmetrical stretching vibrations of COO- group of the formed Gd(OH)CO3. A broadband in between 3000-3700 cm-1 is assigned to the asymmetrical and symmetrical stretching vibrational mode of –OH, causing the chemically attached or physically adsorbed water molecules or Si-OH groups of amorphous silica on the surface of nano-products [41, 42]. Notably, the peak position
8
ACCEPTED MANUSCRIPT
and intensities are greatly diminished in core/shell NRs and it could be due to the formation of an undoped crystalline LaPO4 shell which eliminates the surface bound organic moieties as well as water molecules. However, in the case of core/shell/Si NRs
RI PT
some additional peaks with great shifting in peak positions are observed. These characteristic peaks at 1097, 820 and 480 cm-1 are attributed to the Si-O-Si, Si-OH and Si-O stretching and bending vibrational modes of amorphous silica [43, 44]. It is a strong
SC
evidence for the amorphous silica layer that has been effectively grafted around the core/shell NRs.
M AN U
Optical absorption spectra of all three samples were measured in dist. water over 200-600 nm UV-Vis spectral range. As seen in Fig.4, the spectra show strong absorption in ultraviolet region at around 210 nm, likely originating from oxygen-to-europium (EuO) ion charge transfer transition, which is in accordance with previous observations. A
TE D
slight change in the band shape and absorption edge of core/shell/Si NRs is observed, which could be due to optically active silica surface modification. Photoluminescence spectra of the core, core/shell and core/shell/Si NRs were recorded to investigate the
EP
successful doping of Eu3+ ion in GdPO4 matrix and impact of surface coating on luminescent properties of the as-prepared nano-products. Fig.5 demonstrates the
AC C
excitation spectra of all three samples under monitoring at 595 nm (5D0→7F1) emission wavelength. The excitation spectra shows several sharp excitation transitions in the center of UV/Vis region consisting at 312 (7F0→5I6), 318 (7F0,1→5H3,6), 363 (7F0,1→5D4), 380 (7F0,1→5G3), 393 (7F0,1→5L6), 437 (7F0→5D3) and 463 (7F0,1→5D2) nm, which are ascribed to intra-configuration parity forbidden 4f6→4f6 excitation electronic lines originating from 7F0 ground state to the labelled excited states of Eu3+ ion [45]. A strong
9
ACCEPTED MANUSCRIPT
excitation transition centered at 393 nm (7F0,1→5L6), which is observed for most of the Eu-doped matrices, is attributed to the 4f-4f electronic transition of Eu ion [46, 47]. These observed excitons are well- matched with the previously published observed
RI PT
results [47, 48].
The emission spectra of all three samples were recorded under monitoring at 393 nm excitation wavelength, as shown in Fig.6. The emission spectra display several sharp
SC
peaks ranging from 500-700 nm, which are associated with the transitions from the excited state 5D0,1 to the excited 7FJ(J = 1,2,3 and 4) state of Eu ion [47, 48]. The
M AN U
emission spectra of all three samples demonstrate most prominent transitions at 595 nm and 612 nm assigned to (5D0→7F1) magnetic dipole and (5D0→7F2) electrical dipole transitions of Eu3+ ion, respectively [46]. The most sensitive and strong emission is the observed 5D0→7F2 transition centered at 600-620 nm, corresponding to the red emission,
TE D
in accordance with the Judd-Oflet theory [49, 50]. It is being electrical dipole allowed transition, very sensitive to the local chemical environment and is called hypersensitive transition. Among the observed emission transitions, electrical dipole (5D0→7F2)
EP
transition is most prominent, which may be due to low local symmetry without inversion center for the sites of Eu3+ ion in the GdPO4 host lattice resulting in the highest
AC C
luminescence intensity of 5D0→7F2 transition [46-48]. Moreover, the 5D0→7F1 transition is magnetically-dipole allowed and relatively insensitive to the local chemical environment. As observed in Fig. 6, the magnetic-dipole (5D0→7F1) transition in core/shell NRs is most sensitive in comparison with electric-dipole (5D0→7F2) transition and could be due to the surface growth of monoclinic monazite LaPO4 shell. Lanthanum phosphate in bulk form adopted monoclinic monazite phase with a space group of
10
ACCEPTED MANUSCRIPT
P121/n1, where La3+ ion is coordinated by nine oxygen atoms with site symmetry of C1. It reflects that surface growth of a LaPO4 shell altered the crystal structure of the GdPO4:Eu (core) NRs, as supported by XRD observations. However, in phosphates such
RI PT
as LaPO4/YPO4, the magnetic-dipole (5D0→7F1) transition is dominant with respect to electric-dipole (5D0→7F2) transition of Eu3+ due to the effect of the phosphate group in the host matrices [30]. Generally, the structure of the host matrices and lattice sites of
SC
Eu3+ ions are very important factors that affect luminescence intensity. The surface growth of LaPO4 shell has a monoclinic crystal structure and offers a crystal site with a
M AN U
C1 space group which has very low inversion symmetry resulting in a higher intensity of the electric dipole transitions than those of the hexagonal one. Besides that, a remarkable enhancement in emission intensity is observed in core/shell NRs causing the elimination of non-radiative transition centers by shielding effect of passivated crystalline LaPO4
TE D
layer [51, 52]. Additionally, since the NRs are prepared in an aqueous environment, abundant organic moiety and chemically bonded or physically adsorbed water molecules are attached to the surface of core-NRs. These surface attached high vibrational energy (-
EP
OH, C-H, NO3-) impurities (molecules) scattered the incident and emission light, and suppressed the luminescence intensity as evident from the FTIR spectral results. The
AC C
passivated crystalline LaPO4 layer abolishes these impurities, resulting in increased luminescence intensity. However, in the case of core/shell/Si NRs, the emission intensity is greatly quenched with respect to core and core/shell NRs, due to the silica surface modification. The amorphous silica exists with lot of hydroxyl groups (Si-OH called silanol) and these surface silanol molecules play as efficient luminescence quenchers which enhance the non-radiative transition rate effectively resulting in quenched emission
11
ACCEPTED MANUSCRIPT
intensity. Previously, it has been reported that quenching of emission intensity increases with increase in the surface attached non-radiative centers [42, 53]. Additionally, silica surface encapsulation decreased the total amount of luminescent phases in the final
RI PT
product which is the direct reason for suppression of luminescence intensity in core/shell/Si NRs.
SC
IV. Conclusions
M AN U
In summary, highly purified, well-crystalline hexagonal-rhabdophane phase GdPO4:Eu and GdPO4:Eu@LaPO4 NRs were successfully prepared by urea based coprecipitation process and their surfaces were coated with silica shells via sol-gel process. The prepared nanoproducts were easily dispersed in the aqueous environment whereas
TE D
the silica surface modified NRs revealed high solubility as well as colloidal stability. The development of core/shell/Si type nanostructures, their structure and surface modification by silica layer surrounding the core/shell NRs were verified from TEM, EDX, FTIR,
EP
UV/Vis, excitation and emission spectral measurements. We observed highest luminescent intensity in core/shell NRs with respect to core and core/shell/Si NRs under
AC C
the same experimental conditions, because the passivated LaPO4 shell protects the luminescent centers from nonradiative transition decay. The Eu3+ revealed strong emission of magnetic dipole (5D0→7F2) transition observed around 614 nm, which is shifted from red to orange in core/shell and core/shell/Si NRs. This simple and quick synthesis method of NRs without any surfactant and template can be extended to the preparation of aqueous soluble, biocompatible, highly fluorescent luminescent lanthanide
12
ACCEPTED MANUSCRIPT
NRs. The obtained luminescent spectral results increase the impact of luminescent lanthanide phosphate based nanophosphors on fundamental biomedical researches such
RI PT
as optical bio-probe, biomarker and contrast agent in magnetic resonance imaging.
AC C
EP
TE D
M AN U
SC
Acknowledgment: Author is thankful to the King Abdullah Institute for Nanotechnology, Deanship of Scientific Research, King Saud University, Riyadh, Saudi Arabia.
13
ACCEPTED MANUSCRIPT
References
AC C
EP
TE D
M AN U
SC
RI PT
[1] H.L. Zhu, E. Zhu, H. Yang, L. Wang, D.L. Jin, K.H. Yao, J Am Ceram Soc, 91 (2008) 1682-1685. [2] H.L. Zhu, G.F. Ou, L.H. Gao, Mater Chem Phys, 121 (2010) 414-418. [3] Z.J. Zhang, O.M. ten Kate, A. Delsing, E. van der Kolk, P.H.L. Notten, P. Dorenbos, J.T. Zhao, H.T. Hintzen, J Mater Chem, 22 (2012) 9813-9820. [4] Y. Zhang, X.J. Kang, D.L. Geng, M.M. Shang, Y. Wu, X.J. Li, H.Z. Lian, Z.Y. Cheng, J. Lin, Dalton T, 42 (2013) 14140-14148. [5] Y. Zhang, D.L. Geng, X.J. Kang, M.M. Shang, Y. Wu, X.J. Li, H.Z. Lian, Z.Y. Cheng, J. Lin, Inorg Chem, 52 (2013) 12986-12994. [6] B. Zhou, L.L. Tao, Y. Chai, S.P. Lau, Q.Y. Zhang, Y.H. Tsang, Angew Chem Int Edit, 55 (2016) 12356-12360. [7] D.M. Yang, X.J. Kang, M.M. Shang, G.G. Li, C. Peng, C.X. Li, J. Lin, Nanoscale, 3 (2011) 25892595. [8] F. Vetrone, R. Naccache, V. Mahalingam, C.G. Morgan, J.A. Capobianco, Adv Funct Mater, 19 (2009) 2924-2929. [9] F. Vetrone, V. Mahalingam, J.A. Capobianco, Chem Mater, 21 (2009) 1847-1851. [10] A. Szczeszak, K. Kubasiewicz, T. Grzyb, S. Lis, J Lumin, 155 (2014) 374-383. [11] W.L. Ren, G. Tian, L.J. Zhou, W.Y. Yin, L. Yan, S. Jin, Y. Zu, S.J. Li, Z.J. Gu, Y.L. Zhao, Nanoscale, 4 (2012) 3754-3760. [12] M.N. Luwang, R.S. Ningthoujam, K. Srivastava, R.K. Vatsa, J Mater Chem, 21 (2011) 53265337. [13] K. Kubasiewicz, M. Runowski, S. Lis, A. Szczeszak, J Rare Earth, 33 (2015) 1148-1154. [14] T. Grzyb, M. Runowski, S. Lis, J Lumin, 154 (2014) 479-486. [15] G.Y. Chen, J. Shen, T.Y. Ohulchanskyy, N.J. Patel, A. Kutikov, Z.P. Li, J. Song, R.K. Pandey, H. Agren, P.N. Prasad, G. Han, Acs Nano, 6 (2012) 8280-8287. [16] G.Y. Chen, T.Y. Ohulchanskyy, A. Kachynski, H. Agren, P.N. Prasad, Acs Nano, 5 (2011) 49814986. [17] A. Szczeszak, A. Ekner-Grzyb, M. Runowski, K. Szutkowski, L. Mrowczynska, Z. Kazmierczak, T. Grzyb, K. Dabrowska, M. Giersig, S. Lis, J Colloid Interf Sci, 481 (2016) 245-255. [18] Y.Q. Qu, X.G. Kong, Y.J. Sun, Q.H. Zeng, H. Zhang, J Alloy Compd, 485 (2009) 493-496. [19] V. Mahalingam, C. Hazra, R. Naccache, F. Vetrone, J.A. Capobianco, J Mater Chem C, 1 (2013) 6536-6540. [20] Z.Q. Li, L.M. Wang, Z.Y. Wang, X.H. Liu, Y.J. Xiong, J Phys Chem C, 115 (2011) 3291-3296. [21] H. Lai, H. Yang, Solid State Sci, 13 (2011) 1654-1657. [22] N. Yaiphaba, R.S. Ningthoujam, N.S. Singh, R.K. Vatsa, N.R. Singh, J Lumin, 130 (2010) 174180. [23] S. Rodriguez-Liviano, A.I. Becerro, D. Alcantara, V. Grazu, J.M. de la Fuente, M. Ocana, Inorg Chem, 52 (2013) 647-654. [24] H. Khajuria, J. Ladol, S. Khajuria, M.S. Shah, H.N. Sheikh, Mater Res Bull, 80 (2016) 150-158. [25] M.F. Dumont, C. Baligand, Y.C. Li, E.S. Knowles, M.W. Meisel, G.A. Walter, D.R. Talham, Bioconjugate Chem, 23 (2012) 951-957. [26] J.A. Damasco, G.Y. Chen, W. Shao, H. Agren, H.Y. Huang, W.T. Song, J.F. Lovell, P.N. Prasad, Acs Appl Mater Inter, 6 (2014) 13884-13893. [27] Y. Tian, H.-Y. Yang, K. Li, X. Jin, J Mater Chem, 22 (2012) 22510-22516. [28] N. Yaiphaba, R.S. Ningthoujam, N.R. Singh, R.K. Vatsa, Eur J Inorg Chem, (2010) 2682-2687. [29] B.P. Singh, A.K. Parchur, R.S. Ningthoujam, A.A. Ansari, P. Singh, S.B. Rai, Dalton T, 43 (2014) 4779-4789.
14
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[30] L.H. Zhang, M.L. Yin, H.P. You, M. Yang, Y.H. Song, Y.J. Huang, Inorg Chem, 50 (2011) 1060810613. [31] Q.J. Du, Z.B. Huang, Z. Wu, X.W. Meng, G.F. Yin, F.B. Gao, L. Wang, Dalton T, 44 (2015) 3934-3940. [32] M. Runowski, T. Grzyb, S. Lis, J Nanopart Res, 14 (2012). [33] N.K. Sahu, R.S. Ningthoujam, D. Bahadur, J Appl Phys, 112 (2012). [34] C.C. Yu, M. Yu, C.X. Li, X.M. Liu, J. Yang, P.P. Yang, J. Lin, J Solid State Chem, 182 (2009) 339347. [35] A.A. Ansari, M. Alam, J.P. Labis, S.A. Alrokayan, G. Shafi, T.N. Hasan, N.A. Syed, A.A. Alshatwi, J Mater Chem, 21 (2011) 19310-19316. [36] A.A. Ansari, S.P. Singh, N. Singh, B.D. Malhotra, Spectrochim Acta A, 86 (2012) 432-436. [37] X.Z. Xu, X.Y. Zhang, Y.L. Wu, J Nanopart Res, 18 (2016). [38] L.H. Zhang, G. Jia, H.P. You, K. Liu, M. Yang, Y.H. Song, Y.H. Zheng, Y.J. Huang, N. Guo, H.J. Zhang, Inorg Chem, 49 (2010) 3305-3309. [39] N.K. Sahu, N.S. Singh, R.S. Ningthoujam, D. Bahadur, Acs Photonics, 1 (2014) 337-346. [40] M.N. Luwang, S. Chandra, D. Bahadur, K. Srivastava, J Mater Chem, 22 (2012) 3395-3403. [41] A.A. Ansari, T.N. Hasan, N.A. Syed, J.P. Labis, A.K. Parchur, G. Shafi, A.A. Alshatwi, NanomedNanotechnol, 9 (2013) 1328-1335. [42] A.A. Ansari, R. Yadav, S.B. Rai, Rsc Adv, 6 (2016) 22074-22082. [43] A.A. Ansari, R. Yadav, S.B. Rai, Appl Phys a-Mater, 122 (2016). [44] A.A. Ansari, A.K. Parchur, B. Kumar, S.B. Rai, J Mater Sci-Mater M, 27 (2016). [45] A.I. Becerro, D. Gonzalez-Mancebo, M. Ocana, J Nanopart Res, 17 (2015). [46] A.I. Prasad, A.K. Parchur, R.R. Juluri, N. Jadhav, B.N. Pandey, R.S. Ningthoujam, R.K. Vatsa, Dalton T, 42 (2013) 4885-4896. [47] M.N. Luwang, R.S. Ningthoujam, Jagannath, S.K. Srivastava, R.K. Vatsa, J Am Chem Soc, 132 (2010) 2759-2768. [48] H. Meyssamy, K. Riwotzki, A. Kornowski, S. Naused, M. Haase, Adv Mater, 11 (1999) 840-+. [49] B.R. Judd, Phys Rev, 127 (1962) 750-&. [50] G.S. Ofelt, J Chem Phys, 37 (1962) 511-&. [51] A.K. Parchur, A.I. Prasad, A.A. Ansari, S.B. Rai, R.S. Ningthoujam, Dalton T, 41 (2012) 1103211045. [52] A. Kar, A. Patra, Nanoscale, 4 (2012) 3608-3619. [53] B. Liu, C.X. Li, D.M. Yang, Z.Y. Hou, P.A. Ma, Z.Y. Cheng, H.Z. Lian, S.S. Huang, J. Lin, Eur J Inorg Chem, 2014 (2014) 1906-1913.
15
ACCEPTED MANUSCRIPT
Figure captions
RI PT
Fig.1. X-ray diffraction pattern of core, core/shell and core/shell/Si NRs.
Fig.2. TEM images of (a&b) core-NRs (c&d) core/shell/Si NRs (e) Energy dispersive Xray analysis of core/shell/Si NRs. Fig.3. FTIR spectra of core, core/shell and core/shell/Si NRs.
SC
Fig.4. UV-Vis absorption spectra of core, core/shell and core/shell/Si NRs. Fig.5. Excitation spectra of the core, core/shell and core/shell/Si NRs.
AC C
EP
TE D
M AN U
Fig.6. Emission spectra of the core, core/shell and core/shell/Si NRs.
16
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Figure 1.
17
ACCEPTED MANUSCRIPT
TE D
M AN U
SC
RI PT
Figure 2.
AC C
EP
e
18
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Figure 3.
19
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Figure 4.
20
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Figure 5.
21
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Figure 6.
22
ACCEPTED MANUSCRIPT
Research Highlights Designing of highly luminescent GdPO4:Eu@LaPO4@SiO2 nanorods via urea based co-precipitation process.
•
Simple, cost effective, highly productive, eco-friendly method.
•
High luminescence intensity was observed in GdPO4:Eu@LaPO4 nanorods.
•
Role of surface coating on structural, optical and luminescence properties was investigated
AC C
EP
TE D
M AN U
SC
RI PT
•
S1293-2558(17)30409-0
DOI:
10.1016/j.solidstatesciences.2017.07.012
Reference:
SSSCIE 5535
To appear in:
Solid State Sciences
Received Date: 24 April 2017 Revised Date:
9 July 2017
Accepted Date: 15 July 2017
Please cite this article as: A.A. Ansari, J.P. Labis, M. Aslam Manthrammel, Designing of luminescent GdPO4:Eu@LaPO4@SiO2 core/shell nanorods: Synthesis, structural and luminescence properties, Solid State Sciences (2017), doi: 10.1016/j.solidstatesciences.2017.07.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Graphical Abstract
ACCEPTED MANUSCRIPT
Designing of luminescent GdPO4:Eu@LaPO4@SiO2 core/shell nanorods: Synthesis, structural and luminescence properties Anees A. Ansari*1 Joselito P. Labis1, M. Aslam Manthrammel2 King Abdullah Institute for Nanotechnology, King Saud University,Riyadh-11451, Saudi Arabia 2 Department of Physics, King Saud University, Riyadh-11451, Saudi Arabia
RI PT
1
Abstract
SC
GdPO4:Eu3+ (core) and GdPO4:Eu@LaPO4 (core/shell) nanorods (NRs) were successfully prepared by urea based co-precipitation process at ambient conditions which was followed by coating with amorphous silica shell via the sol-gel chemical route. The
M AN U
role of surface coating on the crystal structure, crystallinity, morphology, solubility, surface chemistry and luminescence properties were well investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, Fourier Transform Infrared (FTIR), UV-Vis, and photoluminescence spectroscopy. XRD pattern revealed highly purified, well-crystalline, single phasehexagonal-rhabdophane structure of GdPO4 crystal. The TEM micrographs exhibited
TE D
highly crystalline and narrow size distributed rod-shaped GdPO4:Eu3+ nanostructures with average width 14-16 nm and typical length 190-220 nm. FTIR spectra revealed characteristic infrared absorption bands of amorphous silica. High absorbance in a visible
EP
region of silica modified core/shell/Si NRs in aqueous environment suggests the high solubility along with colloidal stability. The photoluminescence properties were remarkably enhanced after growth of undoped LaPO4 layers due to the reduction of
AC C
nonradiative transition rate. The advantages of presented high emission intensity and high solubility of core/shell and core/shell/Si NRs indicated the potential applications in monitoring biological events.
Keywords: gadolinium phosphate, europium, solubility, surface chemistry, luminescence properties. Ph.:+966-11-46768383 Email: [email protected]
Fax: +966-11-4670662
1
ACCEPTED MANUSCRIPT
I. Introduction A wide variety of nanophosphor materials has been fabricated because of their potential application in the light emitting diode, solid laser, panel display and other
RI PT
optoelectronic devices etc. [1-5]. In recent years, lanthanide phosphate based nanophosphors have attracted research community as they are very interesting class of host matrices for activators owing to their variety of favorable properties such as
SC
maximum transparency in the visible spectral region, high thermal stability, good photochemical stability, high refractive index and high quantum yield [6-16]. Lanthanide
M AN U
phosphates exhibit interesting electronic, magnetic and optical properties arising from unfilled 4f-orbital which are shielded by the 5s2p6 orbitals. Generally, these physiochemical properties of lanthanide nanophosphors are strongly dependent on host matrices [7, 12-14, 17-21]. In this context, gadolinium orthophosphate is a good
TE D
luminescent host matrix [11, 22-25]. Gadolinium-based nano-phosphates are special category of multifunctional agents for multimodal bioimaging because the optically active lanthanide ions doped in the phosphate matrix can contribute to luminescence
EP
(optical modality) and Gd3+ ion as a paramagnetic relaxation agent can introduce magnetic resonance imaging (MRI) modality [23, 25, 26]. The unpaired electrons of Gd3+
AC C
on the surface of GdPO4 NRs can be exploited to enhance the contrast for MR imaging so that it can bridge gaps that combine MRI and optical imaging applications [22, 27-29]. Different trivalent lanthanide ions such as neodymium (near infrared (NIR) emitting), europium (red emitting), terbium (green emitting), holmium, erbium, and ytterbium can serve as highly efficient luminescent centers which cover a broad range of the visible and NIR spectral region. Amongst lanthanide activators, Eu gives high emission lines at 595,
2
ACCEPTED MANUSCRIPT
614 and 695 nm and is sensitive to the local environment because Eu3+ ions occupy Gd3+ sites, where EuO8 is highly asymmetric (D2d). For example, Zhang et al. fabricated multifunctional Eu3+ doped gadolinium phosphate hollow spheres via one-pot
RI PT
hydrothermal process [30]. Liviano et al. prepared highly crystalline GdPO4:Eu nanocubes by a simple and fast microwave-assisted complexation process at low temperature [23]. Du et al. proposed simple and effective synthesis process for the
SC
preparation of well-crystalline hexagonal GdPO4:Eu nanorods via template-based silkfibroin peptides [31]. Runowski et al. and Sahu et al. used co-precipitation method for
M AN U
preparation of GdPO4:Eu nanostructured materials and investigated their luminescence properties [32, 33]. Ren et al. applied hydrothermal process for the preparation of different luminescent ion doped gadolinium orthophosphate and simultaneously investigated their applications in optical and MRI [11]. In a similar report, the
TE D
hydrothermal process was utilized for the preparation of GdPO4:Eu nanocrystals and explored their optical properties [21]. The sonochemical process was applied for the preparation of GdPO4:Eu via irradiation of inorganic salt in aqueous environment under
EP
ambient conditions without any surfactant [34]. In these previously adopted conventional methods, low-dimensional GdPO4 including zero-dimensional (0D particle clusters), 1D
AC C
(rod-like) and 2D (disk-like) nanostructures are obtained by high-temperature, hydrothermal process or using special methods involving complex procedures or addition of organic additives that are usually adopted for the synthesis of well-crystalline GdPO4:Eu nanocrystals. Therefore, it is imperative to seek a facile, low temperature, highly efficient and large-scale productive approach to fabricate gadolinium phosphate nanorods which doesn’t require very high temperature or complex procedures.
3
ACCEPTED MANUSCRIPT
Herein, we demonstrate a general strategy for the synthesis of GdPO4:Eu (core) nanorods (NRs) through urea based co-precipitation process at ambient temperature conditions. It is well known that urea is a weak base and decomposed homogeneously at
RI PT
low temperature under the appropriate conditions to release hydrogen carbonate (H(CO3)). Subsequently, an insulating LaPO4 and amorphous silica layers were gradually coated around the luminescent seed core-nanorods. An insulating LaPO4 layer not only reduces
SC
the surface defects but also remarkably improves the physiochemical properties such as crystallinity and photoluminescence properties. The advantages of this method include
M AN U
rapid reaction rate, controllable reaction conditions, low-cost, eco-friendly, large-scale production of well-crystalline, single phased, non-aggregated and highly purified nanocrystals. We observed a substantial reduction in reaction time, reaction temperature as well as product yield with respect to hydrothermal or other chemical route based
TE D
synthesis procedures. The structural, morphology, surface chemistry, optical absorption, and luminescence properties were systematically investigated by XRD pattern, TEM, EDX, FTIR, UV/Vis absorption, excitation and emission spectral techniques. XRD and
EP
TEM results show the well-crystalline, single phased, rod-shaped hexagonal-rhabdophane nano structures with irregular sizes. The silica covered core/shell/Si NRs show high
AC C
absorbance in visible region because of optically active silica surface modification. Comparative structural, optical absorption and photoluminescence properties were discussed in detail to investigate the impact of surface coating on their physiochemical properties. It provides a useful way to improve the luminescence properties and their colloidal stability in most aqueous environment for their further use in biological sciences.
4
ACCEPTED MANUSCRIPT
II. Experimental Procedure Gadolinium oxide (BDH, England), lanthanum oxide (BDH, England), europium
RI PT
oxide (Alfa Aesar, Germany), urea, NaH2PO4, Tetraethylorthosilicate (TEOS), ethanol, ammonia were analytical grade reagents and used directly without further purification. Gd(NO3)36H2O, La(NO3)37H2O and Eu(NO3)36H2O were prepared by dissolving the
SC
corresponding metal oxides into diluted nitric acid. Milli-Q (Millipore, Bedford, USA) H2O was used for synthesis and characterization of the samples.
M AN U
For the synthesis of GdPO4:Eu NRs, 1.7 mg (9.5 ml of 2M) gadolinium nitrate, 0.09 mg (0.5 ml of 2M) europium nitrate and 5 mg urea were dissolved in 50 ml of H2O. Urea was used for homogeneous hydrolysis of the metal nitrate. The solution was vigorously stirred for proper mixing on a hot plate for 2 h and later heated under refluxed
TE D
condition for 2-3 h until complete precipitation occurred. The obtained precipitate was redispersed in dist. water by ultrasonication and mechanical stirring. After that, an aqueous solution of NaH2PO4 (0.707 mg) was added slowly to the above mixture. Then
EP
this solution was transferred to a round bottle 250 ml flask fitted with reflux condenser and heated at 100 oC until complete precipitation. The obtained precipitate was
AC C
segregated by centrifugation and washed with dist. H2O and ethanol, and dried in an oven at 60 oC for 6 h.
In a typical synthesis of GdPO4:Eu@LaPO4 NRs, 100 mg GdPO4:Eu NRs were
dispersed in a minimum amount of dist. water under ultrasonication for homogenization. Then an aqueous dissolved 100 mg La(NO3)37H2O was introduced into the above solution under mechanical stirring. 0.763 mg NaH2PO4 was introduced slowly into the
5
ACCEPTED MANUSCRIPT
above suspension. The suspension was transferred to round bottle flask and kept under refluxed condition for 5-6 h for proper precipitation. Occurred precipitate was isolated by centrifugation and washed dist. H2O and ethyl alcohol and dried in an oven at 60 oC.
RI PT
Stober method was used for silica surface coating on GdPO4:Eu@LaPO4 core/shell NRs. 220 mg GdPO4:Eu@LaPO4 (core/shell) nanoparticle were dispersed in an aqueous solution containing 40 ml H2O, 150 ml ethanol and ~2 ml NH4OH under
SC
ultrasonication and later vigorously stirred on a hot plate at room temperature [35, 36]. 1 ml TEOS was added dropwise into the above solution and solution was kept under
M AN U
constant mechanical stirring for 5-6 h. The obtained precipitate was isolated by centrifugation, washed with H2O and ethyl alcohol and dried in an oven at 60oC. X-ray diffraction (XRD) pattern was performed on a PANalytical X’PERT x-ray diffractometer equipped with a Ni filter and using Cu Kα (λ = 1.5406Å) radiation. TEM
TE D
micrographs were obtained using a Field emission transmission electron microscope (FETEM, JEM-2100F, JEOL, Japan) equipped with energy dispersive X-ray (EDX) analysis operated at an accelerating voltage of 200 kV. FTIR spectra were recorded on Vertex 80
EP
(Bruker, USA) spectrophotometer using KBr pellet technique. Optical absorption spectra were measured by Cary 60 UV-Vis (Agilent Technologies, USA) spectrophotometer
AC C
under UV-Vis range. Photoluminescence spectra were recorded with a Fluorolog-3 (Model: FL 3-11, Horiba JobinYvon, MJ, USA) spectrophotometer. All measurements were performed at room temperature.
III. Results and Discussion
6
ACCEPTED MANUSCRIPT
X-ray diffraction pattern was utilized to determine the crystal structure, phase purity and crystallinity of the as-synthesized nano-products. It is observed in Fig.1 that the XRD pattern of core NRs exhibits broad diffraction band along with some broad with
RI PT
weak intensity diffraction peaks, which implies the semi-amorphous nature of the materials due to the formation of Gd(OH)CO3 nano-structures [30]. However, the XRD pattern of core/shell and core/shell/Si NRs in Fig.1 shows all diffraction peaks which are
SC
well indexed to the hexagonal rhabdophane-type GdPO4 (JCPDS card No. 39-0232) [11, 24, 30, 33, 37, 38]. The intensities and peak positions are closely matched with the
M AN U
literature data [11, 24, 30, 33]. No additional peaks related to Gd(OH)3, Gd2O3 and Eu2O3 are observed by XRD over the entire range revealing the high purity of the as-synthesized nano-products. The absence of impurity peaks indicates the homogeneous distribution of Eu3+ in the GdPO4 crystal lattice or the amount is too small to be detected by XRD
TE D
pattern. It is worth noticing that after silica layer surface coating, no change in peak position is observed. It means high purity of the samples which demonstrate the formation of homogeneous Gd-P-O-Eu and Gd-P-O-Eu-La-P-O solid solutions. The
EP
broadening of the reflection planes manifests that core/shell NRs are made up of primary nanocrystalline size.
AC C
TEM images were used to analyze the morphological structure and silica surface
coating around the luminescent seed core NRs. TEM micrographs demonstrate narrow and irregular size distribution of well crystalline, rod-shaped, nanostructures with the average width 14-16 nm and typical average length around 190-220 nm. TEM image in Fig.2c&d revealed that the thin, uniform and porous silica layer with thickness around 810 nm is effectively grown over the surface of core/shell NRs. The silica shell coating
7
ACCEPTED MANUSCRIPT
can be clearly distinguished between core and silica shell through different electron penetrabilities, in which core is dark black and silica shell is light gray in color. It is observed that after grafting of an insulating LaPO4 and amorphous silica layer, the
RI PT
morphological structure of the NRs is still well-maintained. It advocates that the surface modification has no side effect on the homogeneity of the core/shell/Si nanostructures. As seen in Fig. 2c, after silica surface modification the luminescent NRs are highly
SC
aggregated due to the formation of surface silanol (Si-OH) molecules which connect each particle by Van der Waals forces via hydrogen bonding resulting in the increased
M AN U
agglomeration. Notably, the core/shell/Si NRs display high dispersibility or solubility in an aqueous environment compared to core-NRs which suggests the effect of optically active silica surface modification. Fig.2e shows the EDX analysis of the core/shell/Si sample. It reveals the existence of all doped and surface coated metal contents and is a
TE D
strong evidence of Eu ion doping in the GdPO4 crystal lattice and subsequently surface coating of LaPO4 and amorphous silica layers around the luminescent seed core-NRs. Surface chemistry of the core, core/shell and core/shell/Si NRs was examined by
EP
FTIR spectroscopy. The infrared absorption spectra revealed most prominent bands located at 1090, 627 and 548 cm-1 ascribed to the stretching and bending vibrational
AC C
modes of phosphate (PO4)3- groups [11, 30, 39, 40]. The observed peaks at 1657 and 1535 cm-1 are attributed to the asymmetrical and symmetrical stretching vibrations of COO- group of the formed Gd(OH)CO3. A broadband in between 3000-3700 cm-1 is assigned to the asymmetrical and symmetrical stretching vibrational mode of –OH, causing the chemically attached or physically adsorbed water molecules or Si-OH groups of amorphous silica on the surface of nano-products [41, 42]. Notably, the peak position
8
ACCEPTED MANUSCRIPT
and intensities are greatly diminished in core/shell NRs and it could be due to the formation of an undoped crystalline LaPO4 shell which eliminates the surface bound organic moieties as well as water molecules. However, in the case of core/shell/Si NRs
RI PT
some additional peaks with great shifting in peak positions are observed. These characteristic peaks at 1097, 820 and 480 cm-1 are attributed to the Si-O-Si, Si-OH and Si-O stretching and bending vibrational modes of amorphous silica [43, 44]. It is a strong
SC
evidence for the amorphous silica layer that has been effectively grafted around the core/shell NRs.
M AN U
Optical absorption spectra of all three samples were measured in dist. water over 200-600 nm UV-Vis spectral range. As seen in Fig.4, the spectra show strong absorption in ultraviolet region at around 210 nm, likely originating from oxygen-to-europium (EuO) ion charge transfer transition, which is in accordance with previous observations. A
TE D
slight change in the band shape and absorption edge of core/shell/Si NRs is observed, which could be due to optically active silica surface modification. Photoluminescence spectra of the core, core/shell and core/shell/Si NRs were recorded to investigate the
EP
successful doping of Eu3+ ion in GdPO4 matrix and impact of surface coating on luminescent properties of the as-prepared nano-products. Fig.5 demonstrates the
AC C
excitation spectra of all three samples under monitoring at 595 nm (5D0→7F1) emission wavelength. The excitation spectra shows several sharp excitation transitions in the center of UV/Vis region consisting at 312 (7F0→5I6), 318 (7F0,1→5H3,6), 363 (7F0,1→5D4), 380 (7F0,1→5G3), 393 (7F0,1→5L6), 437 (7F0→5D3) and 463 (7F0,1→5D2) nm, which are ascribed to intra-configuration parity forbidden 4f6→4f6 excitation electronic lines originating from 7F0 ground state to the labelled excited states of Eu3+ ion [45]. A strong
9
ACCEPTED MANUSCRIPT
excitation transition centered at 393 nm (7F0,1→5L6), which is observed for most of the Eu-doped matrices, is attributed to the 4f-4f electronic transition of Eu ion [46, 47]. These observed excitons are well- matched with the previously published observed
RI PT
results [47, 48].
The emission spectra of all three samples were recorded under monitoring at 393 nm excitation wavelength, as shown in Fig.6. The emission spectra display several sharp
SC
peaks ranging from 500-700 nm, which are associated with the transitions from the excited state 5D0,1 to the excited 7FJ(J = 1,2,3 and 4) state of Eu ion [47, 48]. The
M AN U
emission spectra of all three samples demonstrate most prominent transitions at 595 nm and 612 nm assigned to (5D0→7F1) magnetic dipole and (5D0→7F2) electrical dipole transitions of Eu3+ ion, respectively [46]. The most sensitive and strong emission is the observed 5D0→7F2 transition centered at 600-620 nm, corresponding to the red emission,
TE D
in accordance with the Judd-Oflet theory [49, 50]. It is being electrical dipole allowed transition, very sensitive to the local chemical environment and is called hypersensitive transition. Among the observed emission transitions, electrical dipole (5D0→7F2)
EP
transition is most prominent, which may be due to low local symmetry without inversion center for the sites of Eu3+ ion in the GdPO4 host lattice resulting in the highest
AC C
luminescence intensity of 5D0→7F2 transition [46-48]. Moreover, the 5D0→7F1 transition is magnetically-dipole allowed and relatively insensitive to the local chemical environment. As observed in Fig. 6, the magnetic-dipole (5D0→7F1) transition in core/shell NRs is most sensitive in comparison with electric-dipole (5D0→7F2) transition and could be due to the surface growth of monoclinic monazite LaPO4 shell. Lanthanum phosphate in bulk form adopted monoclinic monazite phase with a space group of
10
ACCEPTED MANUSCRIPT
P121/n1, where La3+ ion is coordinated by nine oxygen atoms with site symmetry of C1. It reflects that surface growth of a LaPO4 shell altered the crystal structure of the GdPO4:Eu (core) NRs, as supported by XRD observations. However, in phosphates such
RI PT
as LaPO4/YPO4, the magnetic-dipole (5D0→7F1) transition is dominant with respect to electric-dipole (5D0→7F2) transition of Eu3+ due to the effect of the phosphate group in the host matrices [30]. Generally, the structure of the host matrices and lattice sites of
SC
Eu3+ ions are very important factors that affect luminescence intensity. The surface growth of LaPO4 shell has a monoclinic crystal structure and offers a crystal site with a
M AN U
C1 space group which has very low inversion symmetry resulting in a higher intensity of the electric dipole transitions than those of the hexagonal one. Besides that, a remarkable enhancement in emission intensity is observed in core/shell NRs causing the elimination of non-radiative transition centers by shielding effect of passivated crystalline LaPO4
TE D
layer [51, 52]. Additionally, since the NRs are prepared in an aqueous environment, abundant organic moiety and chemically bonded or physically adsorbed water molecules are attached to the surface of core-NRs. These surface attached high vibrational energy (-
EP
OH, C-H, NO3-) impurities (molecules) scattered the incident and emission light, and suppressed the luminescence intensity as evident from the FTIR spectral results. The
AC C
passivated crystalline LaPO4 layer abolishes these impurities, resulting in increased luminescence intensity. However, in the case of core/shell/Si NRs, the emission intensity is greatly quenched with respect to core and core/shell NRs, due to the silica surface modification. The amorphous silica exists with lot of hydroxyl groups (Si-OH called silanol) and these surface silanol molecules play as efficient luminescence quenchers which enhance the non-radiative transition rate effectively resulting in quenched emission
11
ACCEPTED MANUSCRIPT
intensity. Previously, it has been reported that quenching of emission intensity increases with increase in the surface attached non-radiative centers [42, 53]. Additionally, silica surface encapsulation decreased the total amount of luminescent phases in the final
RI PT
product which is the direct reason for suppression of luminescence intensity in core/shell/Si NRs.
SC
IV. Conclusions
M AN U
In summary, highly purified, well-crystalline hexagonal-rhabdophane phase GdPO4:Eu and GdPO4:Eu@LaPO4 NRs were successfully prepared by urea based coprecipitation process and their surfaces were coated with silica shells via sol-gel process. The prepared nanoproducts were easily dispersed in the aqueous environment whereas
TE D
the silica surface modified NRs revealed high solubility as well as colloidal stability. The development of core/shell/Si type nanostructures, their structure and surface modification by silica layer surrounding the core/shell NRs were verified from TEM, EDX, FTIR,
EP
UV/Vis, excitation and emission spectral measurements. We observed highest luminescent intensity in core/shell NRs with respect to core and core/shell/Si NRs under
AC C
the same experimental conditions, because the passivated LaPO4 shell protects the luminescent centers from nonradiative transition decay. The Eu3+ revealed strong emission of magnetic dipole (5D0→7F2) transition observed around 614 nm, which is shifted from red to orange in core/shell and core/shell/Si NRs. This simple and quick synthesis method of NRs without any surfactant and template can be extended to the preparation of aqueous soluble, biocompatible, highly fluorescent luminescent lanthanide
12
ACCEPTED MANUSCRIPT
NRs. The obtained luminescent spectral results increase the impact of luminescent lanthanide phosphate based nanophosphors on fundamental biomedical researches such
RI PT
as optical bio-probe, biomarker and contrast agent in magnetic resonance imaging.
AC C
EP
TE D
M AN U
SC
Acknowledgment: Author is thankful to the King Abdullah Institute for Nanotechnology, Deanship of Scientific Research, King Saud University, Riyadh, Saudi Arabia.
13
ACCEPTED MANUSCRIPT
References
AC C
EP
TE D
M AN U
SC
RI PT
[1] H.L. Zhu, E. Zhu, H. Yang, L. Wang, D.L. Jin, K.H. Yao, J Am Ceram Soc, 91 (2008) 1682-1685. [2] H.L. Zhu, G.F. Ou, L.H. Gao, Mater Chem Phys, 121 (2010) 414-418. [3] Z.J. Zhang, O.M. ten Kate, A. Delsing, E. van der Kolk, P.H.L. Notten, P. Dorenbos, J.T. Zhao, H.T. Hintzen, J Mater Chem, 22 (2012) 9813-9820. [4] Y. Zhang, X.J. Kang, D.L. Geng, M.M. Shang, Y. Wu, X.J. Li, H.Z. Lian, Z.Y. Cheng, J. Lin, Dalton T, 42 (2013) 14140-14148. [5] Y. Zhang, D.L. Geng, X.J. Kang, M.M. Shang, Y. Wu, X.J. Li, H.Z. Lian, Z.Y. Cheng, J. Lin, Inorg Chem, 52 (2013) 12986-12994. [6] B. Zhou, L.L. Tao, Y. Chai, S.P. Lau, Q.Y. Zhang, Y.H. Tsang, Angew Chem Int Edit, 55 (2016) 12356-12360. [7] D.M. Yang, X.J. Kang, M.M. Shang, G.G. Li, C. Peng, C.X. Li, J. Lin, Nanoscale, 3 (2011) 25892595. [8] F. Vetrone, R. Naccache, V. Mahalingam, C.G. Morgan, J.A. Capobianco, Adv Funct Mater, 19 (2009) 2924-2929. [9] F. Vetrone, V. Mahalingam, J.A. Capobianco, Chem Mater, 21 (2009) 1847-1851. [10] A. Szczeszak, K. Kubasiewicz, T. Grzyb, S. Lis, J Lumin, 155 (2014) 374-383. [11] W.L. Ren, G. Tian, L.J. Zhou, W.Y. Yin, L. Yan, S. Jin, Y. Zu, S.J. Li, Z.J. Gu, Y.L. Zhao, Nanoscale, 4 (2012) 3754-3760. [12] M.N. Luwang, R.S. Ningthoujam, K. Srivastava, R.K. Vatsa, J Mater Chem, 21 (2011) 53265337. [13] K. Kubasiewicz, M. Runowski, S. Lis, A. Szczeszak, J Rare Earth, 33 (2015) 1148-1154. [14] T. Grzyb, M. Runowski, S. Lis, J Lumin, 154 (2014) 479-486. [15] G.Y. Chen, J. Shen, T.Y. Ohulchanskyy, N.J. Patel, A. Kutikov, Z.P. Li, J. Song, R.K. Pandey, H. Agren, P.N. Prasad, G. Han, Acs Nano, 6 (2012) 8280-8287. [16] G.Y. Chen, T.Y. Ohulchanskyy, A. Kachynski, H. Agren, P.N. Prasad, Acs Nano, 5 (2011) 49814986. [17] A. Szczeszak, A. Ekner-Grzyb, M. Runowski, K. Szutkowski, L. Mrowczynska, Z. Kazmierczak, T. Grzyb, K. Dabrowska, M. Giersig, S. Lis, J Colloid Interf Sci, 481 (2016) 245-255. [18] Y.Q. Qu, X.G. Kong, Y.J. Sun, Q.H. Zeng, H. Zhang, J Alloy Compd, 485 (2009) 493-496. [19] V. Mahalingam, C. Hazra, R. Naccache, F. Vetrone, J.A. Capobianco, J Mater Chem C, 1 (2013) 6536-6540. [20] Z.Q. Li, L.M. Wang, Z.Y. Wang, X.H. Liu, Y.J. Xiong, J Phys Chem C, 115 (2011) 3291-3296. [21] H. Lai, H. Yang, Solid State Sci, 13 (2011) 1654-1657. [22] N. Yaiphaba, R.S. Ningthoujam, N.S. Singh, R.K. Vatsa, N.R. Singh, J Lumin, 130 (2010) 174180. [23] S. Rodriguez-Liviano, A.I. Becerro, D. Alcantara, V. Grazu, J.M. de la Fuente, M. Ocana, Inorg Chem, 52 (2013) 647-654. [24] H. Khajuria, J. Ladol, S. Khajuria, M.S. Shah, H.N. Sheikh, Mater Res Bull, 80 (2016) 150-158. [25] M.F. Dumont, C. Baligand, Y.C. Li, E.S. Knowles, M.W. Meisel, G.A. Walter, D.R. Talham, Bioconjugate Chem, 23 (2012) 951-957. [26] J.A. Damasco, G.Y. Chen, W. Shao, H. Agren, H.Y. Huang, W.T. Song, J.F. Lovell, P.N. Prasad, Acs Appl Mater Inter, 6 (2014) 13884-13893. [27] Y. Tian, H.-Y. Yang, K. Li, X. Jin, J Mater Chem, 22 (2012) 22510-22516. [28] N. Yaiphaba, R.S. Ningthoujam, N.R. Singh, R.K. Vatsa, Eur J Inorg Chem, (2010) 2682-2687. [29] B.P. Singh, A.K. Parchur, R.S. Ningthoujam, A.A. Ansari, P. Singh, S.B. Rai, Dalton T, 43 (2014) 4779-4789.
14
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[30] L.H. Zhang, M.L. Yin, H.P. You, M. Yang, Y.H. Song, Y.J. Huang, Inorg Chem, 50 (2011) 1060810613. [31] Q.J. Du, Z.B. Huang, Z. Wu, X.W. Meng, G.F. Yin, F.B. Gao, L. Wang, Dalton T, 44 (2015) 3934-3940. [32] M. Runowski, T. Grzyb, S. Lis, J Nanopart Res, 14 (2012). [33] N.K. Sahu, R.S. Ningthoujam, D. Bahadur, J Appl Phys, 112 (2012). [34] C.C. Yu, M. Yu, C.X. Li, X.M. Liu, J. Yang, P.P. Yang, J. Lin, J Solid State Chem, 182 (2009) 339347. [35] A.A. Ansari, M. Alam, J.P. Labis, S.A. Alrokayan, G. Shafi, T.N. Hasan, N.A. Syed, A.A. Alshatwi, J Mater Chem, 21 (2011) 19310-19316. [36] A.A. Ansari, S.P. Singh, N. Singh, B.D. Malhotra, Spectrochim Acta A, 86 (2012) 432-436. [37] X.Z. Xu, X.Y. Zhang, Y.L. Wu, J Nanopart Res, 18 (2016). [38] L.H. Zhang, G. Jia, H.P. You, K. Liu, M. Yang, Y.H. Song, Y.H. Zheng, Y.J. Huang, N. Guo, H.J. Zhang, Inorg Chem, 49 (2010) 3305-3309. [39] N.K. Sahu, N.S. Singh, R.S. Ningthoujam, D. Bahadur, Acs Photonics, 1 (2014) 337-346. [40] M.N. Luwang, S. Chandra, D. Bahadur, K. Srivastava, J Mater Chem, 22 (2012) 3395-3403. [41] A.A. Ansari, T.N. Hasan, N.A. Syed, J.P. Labis, A.K. Parchur, G. Shafi, A.A. Alshatwi, NanomedNanotechnol, 9 (2013) 1328-1335. [42] A.A. Ansari, R. Yadav, S.B. Rai, Rsc Adv, 6 (2016) 22074-22082. [43] A.A. Ansari, R. Yadav, S.B. Rai, Appl Phys a-Mater, 122 (2016). [44] A.A. Ansari, A.K. Parchur, B. Kumar, S.B. Rai, J Mater Sci-Mater M, 27 (2016). [45] A.I. Becerro, D. Gonzalez-Mancebo, M. Ocana, J Nanopart Res, 17 (2015). [46] A.I. Prasad, A.K. Parchur, R.R. Juluri, N. Jadhav, B.N. Pandey, R.S. Ningthoujam, R.K. Vatsa, Dalton T, 42 (2013) 4885-4896. [47] M.N. Luwang, R.S. Ningthoujam, Jagannath, S.K. Srivastava, R.K. Vatsa, J Am Chem Soc, 132 (2010) 2759-2768. [48] H. Meyssamy, K. Riwotzki, A. Kornowski, S. Naused, M. Haase, Adv Mater, 11 (1999) 840-+. [49] B.R. Judd, Phys Rev, 127 (1962) 750-&. [50] G.S. Ofelt, J Chem Phys, 37 (1962) 511-&. [51] A.K. Parchur, A.I. Prasad, A.A. Ansari, S.B. Rai, R.S. Ningthoujam, Dalton T, 41 (2012) 1103211045. [52] A. Kar, A. Patra, Nanoscale, 4 (2012) 3608-3619. [53] B. Liu, C.X. Li, D.M. Yang, Z.Y. Hou, P.A. Ma, Z.Y. Cheng, H.Z. Lian, S.S. Huang, J. Lin, Eur J Inorg Chem, 2014 (2014) 1906-1913.
15
ACCEPTED MANUSCRIPT
Figure captions
RI PT
Fig.1. X-ray diffraction pattern of core, core/shell and core/shell/Si NRs.
Fig.2. TEM images of (a&b) core-NRs (c&d) core/shell/Si NRs (e) Energy dispersive Xray analysis of core/shell/Si NRs. Fig.3. FTIR spectra of core, core/shell and core/shell/Si NRs.
SC
Fig.4. UV-Vis absorption spectra of core, core/shell and core/shell/Si NRs. Fig.5. Excitation spectra of the core, core/shell and core/shell/Si NRs.
AC C
EP
TE D
M AN U
Fig.6. Emission spectra of the core, core/shell and core/shell/Si NRs.
16
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Figure 1.
17
ACCEPTED MANUSCRIPT
TE D
M AN U
SC
RI PT
Figure 2.
AC C
EP
e
18
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Figure 3.
19
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Figure 4.
20
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Figure 5.
21
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Figure 6.
22
ACCEPTED MANUSCRIPT
Research Highlights Designing of highly luminescent GdPO4:Eu@LaPO4@SiO2 nanorods via urea based co-precipitation process.
•
Simple, cost effective, highly productive, eco-friendly method.
•
High luminescence intensity was observed in GdPO4:Eu@LaPO4 nanorods.
•
Role of surface coating on structural, optical and luminescence properties was investigated
AC C
EP
TE D
M AN U
SC
RI PT
•