Eu-Doped BaF2 Nanoparticles for Bioimaging Applications - ACS

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Eu-Doped BaF2 Nanoparticles for Bioimaging Applications Rahul Kumar Sharma, Sandeep Nigam, Yogendra Nath Chouryal, Shubham Nema, Siba Prasad Bera, Yogesh Bhargava, and Pushpal Ghosh ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02180 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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ACS Applied Nano Materials

Eu-Doped BaF2 Nanoparticles for Bioimaging Applications Rahul Kumar Sharma,a Sandeep Nigam,b Yogendra Nath Chouryal,a Shubham Nema,c Siba Prasad Bera,d Yogesh Bhargava,c and Pushpal Ghosh a* aSchool

of Chemical Sciences and Technology, Department of Chemistry, Dr. H. S. Gour University (A Central University), Sagar-470003, Madhya Pradesh, India. [email protected].

bChemistry

Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085, India

cDepartment

of Microbiology, Dr. H. S. Gour University (A Central University), Sagar-470003, Madhya Pradesh, India.

dDepartment

of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal Bypass Road, Bhauri, Bhopal-462066, Madhya Pradesh, India

Abstract Pure Eu3+ ion doped BaF2 nanoparticles with tunable properties or property combinations are accessible via an ionic liquid-assisted solvothermal method. Structural parameters such as cell parameters, lattice strain and especially morphology are judiciously tuned with calcination temperatures. For example, tensile strain is observed in samples calcined up to 400oC, however compressive strain appears in samples calcined at 600oC and beyond. Larger surface area up to the sample calcined at 400oC and observation of layer structure at higher calcinations temperature (650oC and beyond) have been rationalized based on secondary nucleation. 3dimensional island-like morphology with step-like layer structure caused by secondary nucleation and self-assembly are visualized and explained by Scanning Electron Microscope analysis. Moreover, emission intensity, decay time, quantum yield and Judd-Ofelt parameter of the Eu3+ ions increase systematically with calcinations temperature to a maximum at 400oC, above which they decrease and finally vanish at 800oC. Our results suggest that, smaller sized nanoparticles with 3-dimensional island-like structures, generated due to secondary nucleation at 1 ACS Paragon Plus Environment

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higher calcinations temperature may cause the increase of surface defects and subsequent luminescence quenching. To the best of our knowledge, the interplay between calcinations and secondary nucleation followed by drastic changes in the luminescence properties is new and previously unreported for the nanopowders. In addition, to improve the dispersibility, asprepared nanoparticles are coated with silica and solubility of nanoparticles is measured in different solvents so that it can be useful for bio-imaging applications also. KEYWORDS: rare-earth ion, nanoparticles, calcination, photoluminescence, secondary nucleation. 1. Introduction Recently rare-earth ion (RE3+) doped nanomaterials have gained ample attention in photonic and biophotonic applications compared to conventional semiconducting nanoparticles and organic fluorophores.1-9 This is fundamentally due to longer lifetime, unprecedented photostability, sharp emission spectrum, low toxicity and reduced photobleaching.9 In addition, RE3+ ions doped nanoparticles are being extensively used for the bio-imaging applications as they cover the wide range of solar spectrum (ultra violet to near infra-red region) for their excitation and then emission in visible region.1-9 Normally, optical properties of RE3+ ion(s) doped nanoparticles are size independent, however they may still be tuned by varying the crystal phase, morphology, lattice strain, core-shell structure, etc.8,10-12 For instances, Patra et al. studied the effect of calcination on the crystal phase and particle size of Er3+ doped ZrO2 nanoparticles. In addition, it is also noticed that on increasing calcination temperature, the overall intensity of Er3+ emission increases due to change in crystal phase (tetragonal to monoclinic) and particle size.13 Furthermore, Ghosh et al. illustrated the effect of temperature on the crystal phase of Eu3+ doped NaYF4.14 When as-prepared cubic nanoparticles are calcined at 400oC, a mixed phase consisting of cubic and hexagonal results. However, calcinations at 600oC and 720oC leads to mainly hexagonal and cubic phases respectively.14 Furthermore, judicious selection of host materials is also a prerequisite for doping of RE3+ ions. Now-a-days, various inorganic compounds such as oxides, phosphates, vanadates and fluorides have been explored as host materials.7-18 Among 2 ACS Paragon Plus Environment

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them, binary and ternary alkali/alkaline or RE based fluoride nanoparticles have drawn significant attention due to the fulfillment of desired prerequisites such as low phonon energy, high refractive index, tunable crystal phase, low cost of production and large band gap.19-29 In nanoparticle synthesis, nucleation process plays a pivotal role in controlling crystal phase, shape and size of the nanoparticles. Normally capping/templating agents such as long chain amines, surfactants, ionic liquids (ILs) etc. are used for desired nucleation and growth of nanoparticles.7,20,30-33 Recently, ILs, which are often called “green” and “designer” solvents, have drawn huge attention for several applications especially in nanomaterial synthesis. ILs can be employed as solvent, reaction partner as well as capping/templating agents.30-33 In addition, by changing the alkyl chain length and cation-anion combinations of ILs, physical and chemical properties of the host materials can be effectively tuned.30-33 Usually, such additives as ILs bind at a nucleation site to stabilize nanoparticles by lowering the interfacial energy of nuclei, leading to the controlled growth of nanoparticles. Starting from the nucleation to nanomaterial growth, Ostwald ripening process plays a major role.31 Similarly, when two lattice planes have same or similar energy, nanomaterials can grow via an oriented attachment mechanism.31 An unusual phenomenon in nucleation often called “secondary” nucleation is noticed for thin films and materials prepared on glass substrates or by chemical or physical vapor deposition techniques. McKenzie and co-workers studied the effect of concentration and chain length of organic diamine additives on the secondary growth of ZnO crystals.34 On the contrary, Xia et al. studied the growth of Au nanoparticles in presence of citrate and Ag+ ions. In presence of citrate and Ag+ ions, secondary growth is suppressed leading to the formation of quasi-small and spherical shaped Au nanoparticles.35 In addition, deposition of Pt as electrocatalyst on the Glassy Carbon (GC) electrode has also shown the appearance of dendrite shaped Pt on GC, which is due to secondary nucleation.36 Such type of growth is dependent on the presence of active centers on the particles. Guo et al. have also revealed that pH and reaction temperature substantially influence the re-growth of silica nanoparticles via secondary nucleation.37 However, literatures related to secondary nucleation and subsequent growth is very limited and almost non-existent for nanopowders. Thereby, a focused and cohesive effort is needed to unravel the different aspects of the secondary nucleation mediated growth mechanism. Normally in the case of rare-earth doped nanopowders, it is observed that with calcination the nanoparticle’s size increases due to Ostwald ripening resulting increase of emission intensity. To 3 ACS Paragon Plus Environment


the best of our knowledge, the formation of island-like structures due to secondary nucleation and their effect on luminescence is not yet clear. A very broad range of unprecedented structural and optical properties can be observed by changing the calcinations temperature of Eu3+ doped BaF2 nanoparticles. In the present study we are addressing the following issues: how the 3dimensional (3D) island-like structures are developed due to secondary nucleation at higher calcination temperatures; and how they influence the intensity, quantum efficiency and decay time of Eu3+ ions in BaF2 nanocrystals? Of particular interest to our research program is how the structural and physical properties of the nanocrystals can be correlated with the optical properties of the dopant ions, mainly in the context of secondary nucleation and formation of 3D island-like structures. Furthermore, solubility of as-prepared nanoparticles are studied in different solvents like DMSO, water and water with polyvinyl pyrrolidone (PVP) and emission spectra of Eu3+ ions are measured. 2. Experimental Section 2.1 Chemicals: Details of the chemicals used and their purity are mentioned in SI. 2.2 Synthesis of [C2mim]Br- IL: Modifying a literature procedure, 10 ml (0.126 mole) of 1methylimidazole was taken in a three-necked round bottom flask on a cold water bath under inert environment (Ar gas) followed by drop wise addition of 12.40 ml (0.166 mole) C2H5Br. The reaction mixture was refluxed for 3-4 hours and the obtained white solid crystal was crushed and washed for 2-3 times with ethyl acetate. Final product was dried under vacuum for 24 hours.38 2.3 Synthesis of BaF2:Eu3+ (1%) nanoparticles: In a typical synthesis, 0.728 g (2.8 mmol) barium acetate, 0.01298 g (2.909 x 10-5 mole) Eu(NO3)3.6H2O and 0.2110 g (5.7 mmol) NH4F were dissolved in 5ml of DI water in separate beakers. Then barium acetate and Eu(NO3)3.6H2O solutions were mixed with 40 ml of ethanol solution which also contains 10 ml aqueous 5 wt% [C2mim]Br solution and stirred for 30 minutes. After that, NH4F solution was added to the same solution. This reaction mixture was transferred to 100 ml TeflonTM lined autoclave with stainless steel jacket and then put inside the hot air oven for 3 hours at 150oC. The obtained product was washed with acetone, ethanol and methanol and was dried at 80oC for 12 hours. Similar procedure was adopted for the synthesis of pure BaF2 nanoparticles. Furthermore, as-prepared

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Eu3+ doped BaF2 nanoparticles are coated with silica to increase their dispersity and biocompatibility (detail is provided in Supporting Information file). 2.4 Characterizations: PXRD was carried out on a D8 Advance BRUKER, equipped with Cu Kα (1.54060 Å) as the incident radiation. The crystallite size was calculated using Scherer equation D = K/cos , where K= 0.9, D represents crystallite size (Å),  is the wavelength of Cu-K radiation and  is the corrected half-width of the diffraction peak. TEM (FEI Tecnai STWIN-T30 using 300 kV electron beam source) was used to map the shape, size and lattice structure of the nanocrystals dispersed on a carbon-coated copper grid from acetone solution. Morphological characterization was also carried out by SEM using a NOVA NANO SEM-450, FEI. FT-IR results were measured using FTIR-8400S SHIMADZU. TG/DTA was measured using the Mettler Toledo Thermal Analyzer. Excitation, emission spectra and decay time of all samples were recorded on Edinburgh Instruments FLSP 920 spectrofluorometer attached with 450W Xe lamp as the excitation source for steady state measurement. Life time measurements were carried out using microsecond flash lamp. The surface areas of as-prepared and calcined BaF2 nanocrystals were investigated through BET (Bellsorp MR6, Japan). 3. Results and Discussion 3.1 Structural characterization by powder X-ray diffraction (PXRD) and phase identification The PXRD patterns (Figure S1) of as-prepared barium fluoride nanomaterials were analyzed and well-defined diffraction peaks can be assigned exactly to the cubic BaF2 (Fm3m) without any impurity. The lattice planes also match the JCPDS card no. C4-452. It is seen that the crystallite size of the as-prepared nanoparticles prepared in the presence of IL ([C2mim]+Br-) are smaller compared to that of the samples prepared without IL (Table 1). For instance, cal. ~22.92 nm and cal. ~25.07 nm size of the particles are obtained for BaF2 prepared with (B1) and without IL (B2) respectively. Results indicate that IL cation can act as an effective templating or capping agent.



Figure 1. (a) Scheme representing the growth mechanism of nanoparticles, (b) PXRD patterns of as-prepared BaF2:Eu3+ nanoparticles and calcined at different temperatures: (i) standard, (ii) as-prepared (dried at 80oC),(iii) calcined at 200oC, (iv) calcined at 400oC, (v) calcined at 600oC, (vi) calcined at 650oC and (vii) calcined at 800oC, (IL=[C2mim]Br) and (c) Lattice strain of asprepared and calcined BaF2:Eu3+ nanoparticles at different calcinations temperature: (i) asprepared, (ii) 200oC, (iii) 400oC, (iv) 600oC, (v) 650oC and (vi) 800oC. 3.1.1 Effect of calcination temperature Furthermore, when the as-prepared BaF2:Eu3+ nanoparticles (B3) are calcined at 200oC (B5), 400oC (B6), 600oC (B7), 650oC (B8) and 800oC (B9); no change in the crystal phase is found, indicating high thermal phase stability of the nanoparticles up to 800oC. However, crystallinity and crystallite size of the nanoparticles are increased [Figure 1b(i-vii)] significantly. For instances, cal. 23.39 nm, 25.63 nm, 33.79 nm, 72.42 nm, 74.23 nm and 79.07 nm are obtained for as-prepared BaF2:Eu3+ nanoparticles (at 80oC) and calcined at 200oC, 400oC, 600oC, 650oC 6 ACS Paragon Plus Environment

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and 800oC respectively (Table 1). Analogous results are also obtained for pure BaF2 (B2) and BaF2:Eu3+ (B4) nanoparticles prepared in the absence of IL (Table 1, Figure S2-S3). However, significant changes in cell parameters are noticed in case of calcining the as-prepared samples (B2, B3 and B4) at different temperatures (Table 1, Table S1). Initially, cell volume and cell length of crystallites increase from as-prepared sample (80oC) to 200oC and then decrease at 400oC. They increase on further calcining the sample at 600oC. This trend is observed for all three sets of samples (B2, B3, B4 and their calcined counterparts). Analysis confirms that cell parameters of nanoparticles can be significantly influenced by calcination temperature. From PXRD pattern it is also revealed that there is no change in the crystal phase due to silica coating (shown in Figure S4b). Table 1. Crystal phase, cell parameters, lattice strain and crystallite size of the as-prepared and calcined nanoparticles. Sample Code

Sample name

IL (Y/N)a

Phase

Crystallite size (nm) (±2)

Average lattice strain (%)

A (Ao)

Volume (Ao)3

B1

BaF2

Yes

As-prepared

6.203(12)

238.67(8)

Cubic

22.92

0.878

Tensile

B2

BaF2

No

As-prepared

B3

BaF2: (1%)Eu3+

Yes

As-prepared

6.195(4)

237.749(24)

Cubic

25.07

0.836

Tensile

6.199(4)

238.315(24)

Cubic

23.39

0.656

Tensile

B4

BaF2: (1%)Eu3+

No

B5

BaF2

:(1%)Eu3+

As-prepared

6.184(22)

236.52(14)

Cubic

24.11

0.457

Tensile

Yes

200oC

B6

BaF2

:(1%)Eu3+

6.2014(10)

238.49(7)

Cubic

25.63

1.12

Tensile

Yes

400oC

B7

BaF2:(1%)Eu3+

6.1943(6)

237.67(4)

Cubic

33.79

0.688

Tensile

Yes

600oC

6.199(22)

238.316(15)

Cubic

72.42

-2.509

Compressive

B8

BaF2

:(1%)Eu3+

Yes

650oC

6.2027(7)

238.64(4)

Cubic

74.23

-0.299

Compressive

B9

BaF2:(1%)Eu3+

Yes

800oC

6.1986(4)

238.17(3)

Cubic

79.07

-0.0648

Compressive

aIL=[C mim]+Br-, 2

Asprepared / calcined

Cell parameter

Lattice strain

Y=Yes and N=No.

3.2 Lattice strain and effect of calcination Generally, the broadening of the diffraction peaks depends upon crystallite size and lattice strain. The lattice strain can be calculated by using Williamson and Hall method:8 𝛽𝑐𝑜𝑠𝜃 𝜆

1

=𝐷+𝜂

𝑠𝑖𝑛𝜃

(1)

𝜆

When βcosθ/λ vs sinθ/λ is plotted, lattice strain (η) can be quantitatively determined from the slope of the graph and crystallite size from the intercept. The compressive and tensile strains 7 ACS Paragon Plus Environment


correspond to the negative and positive magnitude of slope of the graph respectively. Figure 1c (i-iii), S5 and S6 show that tensile strain is obtained for all the as-prepared and calcined nanoparticles (upto 400oC). It is noteworthy that the lattice strain of pure BaF2 has greater magnitude compared to the doped under the similar reaction conditions (Figure S5, S6). For example, 0.878% and 0.656% lattice strains are observed for the BaF2 (B1) and BaF2:Eu3+ (B3), respectively under identical experimental conditions. However, on calcining the as-prepared samples, tensile strain is increased from 0.656% at 80oC to 1.12% at 200oC, whereafter the tensile strain is gradually decreased until 400oC calcination temperature [0.688%]. Again, for the samples calcined at 600oC [-2.509%], 650oC [-0.299%] and 800oC [-0.0648%] completely compressive strains are obtained even though the crystallite size increases with the calcination temperature (Figure 1c, Table 1). Furthermore, analogous trends of lattice strain are also found in the case of pure BaF2 (B2) and BaF2:Eu3+ (B4) prepared without IL (Table 1, Table S1and Figure S7-S8). 3.3 Understanding the melting behavior of nanoparticles through TGA/ DTA

DTA

TGA 100

0.00010 0.00005 0.00000

99

-0.00005 98

-0.00010 -0.00015

97

-0.00020 -0.00025

96

Heat flow

Weight loss (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-0.00030 95

-0.00035 100

200

300

400

500

600

Temperature (o C)

700

800

900

Figure 2. TGA/DTA of as-prepared BaF2:Eu3+ nanoparticles prepared using [C2mim]+Br- IL. Size-dependent melting point depression was previously noticed for nanoscale particles.39 This melting point depression varies inversely with the crystallite size (D) of the particles and can be represented as:40,41 TM ( D )  TM () 



(2)

D


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Where, TM(D) and TM(∞) are the melting points of the nanoparticles and the bulk materials, respectively. Using TGA/DTA techniques, Zhao et al. determined the melting temperature of bismuth nanoparticles.42 Similarly, the size-dependent melting point of gold nanoparticles, which were encapsulated in silica matrix, was determined using TGA/DTA.43 Herein, since as-prepared BaF2:Eu3+ (B3) nanoparticles are on the nanoscale, it can be anticipated that melting point will be lower than that of its bulk analogue. TGA results (Figure 2) show that a 1-3% mass loss occurs in the range of 170o to 450oC, which may be due to the decomposition of unwashed ILs from the nanoparticles. A broad endothermic event is observed at 400oC, which may be due to the surface melting or full melting of BaF2 nanoparticles. Melting point of bulk BaF2 is 1386oC. 24,44

In the present case, it may be indicated that a melting like situation arises at 400oC.45

Normally, broadness of the endotherm in DTA curve directly indicates that the size of the particles is drastically decreased at this particular temperature.43 Interestingly, in the entire range of DTA curve, weak endothermic peaks appear, indicating that assembly of nanoparticles is continuously taking place in the whole range of the measurement. Congruency of TGA and DTA results clearly indicate that sizes of the particles decrease due to the melting which is further supported by the FESEM images. 3.4 Structural characterizations by Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) 3.4.1 Morphological evolution evidenced through SEM



Figure 3. FESEM images of calcined BaF2:Eu3+ [prepared with ([C2mim]+Br-) IL] at different calcinations temperature: (a) calcined at 600oC and (b) calcined at 800oC. Low magnification SEM images of as-prepared materials using IL show that cuboid or cube like morphologies in the range of micrometer are obtained (inset of Figure S9c); however, distinct, larger size BaF2 nanoparticles (B2) are obtained when no IL is used (Figure S9b). Analogous morphology is also noticed for BaF2:Eu3+ nanoparticles (Figure S9c). The details of the analysis are given in the Supplementary information. A representative sample BaF2:Eu3+ (B3) is chosen to understand the morphology change of the nanoparticles upon calcinations at different temperatures. A significant change in the morphology of the calcined nanoparticles is found. In beginning, Ostwald ripening and coalescence of nanoparticles occurs until 600oC (Figure 3a). On analyzing the FESEM images carefully, it is noticed that the surface of nanoparticles become smoother at 400oC (Figure S10b) than the as-prepared (80oC) and calcined at 200oC. However, drastic changes in the morphology are seen on calcining the as-prepared sample at 800oC (Figure 3b). In an attempt to understand the intermediate morphology states more clearly, as-prepared samples were also calcined at 550oC, 625oC, 650oC and 700oC (Figure 4). From careful analysis of the sample (B3) calcined at 650oC, 700oC and 800oC (Figure 3b, 4c and 4d), step-like structures are observed whereby the steps are arranged in definite patterns in all possible orientations. A significant change in surface area upon calcination is also observed. While as-prepared and 400oC-calcined samples show very similar specific surface areas (11.61 m2/g and 11.46 m2/g, respectively), a significant decrease in specific surface area is noticed for 600oC (3.59 m2/g) and 800oC (2.05 m2/g) samples, confirming coalescence of nanoparticles after a certain annealing temperature.


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Figure 4. FESEM images of BaF2:Eu3+(1%) nanoparticles calcined at different temperatures: (a) 550oC, (b) 625oC, (c) 650oC and (d) 700oC. In addition, EDX and elemental mapping of 1% Eu3+ ions doped BaF2 nanocrystals which are calcined at 400oC, 650oC and 800oC are carried out (Figure S11-15). EDX results show that elemental composition of Ba2+, F- and Eu3+ ions approximately matches the theoretical value of calcined samples (Figure S11-S15). However, elemental mapping results of barium, fluoride and europium ions depict that the Eu3+ ion distribution over the surface of BaF2 nanocrystals is completely different in all the calcined (at 400oC, 650oC and 800oC) samples (Figure S11a-b, Figure S14). Eu3+ ions are well distributed over the surface of calcined BaF2 nanocrystals at 400oC and 650oC; compared to which Eu3+ ions appear more densely on the surface of sample calcined at 800oC (Figure S15) 3.4.2 Evolution of 3D island morphology FESEM images have shown that island-like structures begin to appear at 650oC (Figure 4c). Previously, this type of morphology has been observed for materials prepared by molecular beam 11 ACS Paragon Plus Environment


epitaxy (MBE), spin-coating methods, etc., in which the precursors are deposited onto a substrate such as BaF2 and mTiO2 and subsequently annealed at different temperatures.46,47 In the early development of 3D islands, BaF2:Eu3+ (B3) nanoparticles are agglomerated to form a uniform surface. During the entire formation of 3D island, two significant changes have been seen: a) formation of smaller nanoparticles which is substantially controlled by secondary nucleation and b) formation of 3D island morphology by reassembly of newly formed nanoparticles on cooling to room temperature.48 Earlier, cell parameters, lattice strain, surface area analysis and TGA/DTA results reflect that at 400oC (B6), there are some significant internal changes happened. For example, TGA/DTA results clearly indicate that the melting of nanoparticles takes place around 400oC (B6) (Figure 2). Similarly, the 400oC calcined sample shows a larger surface area (11.46 m2/g) which significantly decreases for the 600oC calcined sample (3.5927 m2/g) confirming the occurrence of melting. At the time of melting, nanoparticles segregate and fuse leading to the appearance of a smooth surface, a process which is continued until 650oC (B8) (Figure 4c). The PXRD pattern of calcined sample upto 800oC (B9) reveals that the cubic phase of BaF2 is still intact and assembly of nanoparticles at such high temperature is completely different than that of as-prepared nanoparticles (Figure 1a). Here, it can be assumed that the cooling process of calcined sample is very much similar to the film formation of nanoparticles using the thin film deposition of the precursor onto a particular substrate.48 Therefore in the present case, the growth mechanism of nanoparticles and 3D island formation can be illustrated as growth of a thin-film, which is already proposed by Zhang et al.48 On carefully analyzing the FESEM images, it is found that on cooling to room temperature the melted as-prepared nanoparticles appear to smaller nanoparticles (Figure S16). This is attributed to kinetically controlled nucleation in which rate of nucleation is much faster than the growth of nanoparticles and that is considered as a secondary nucleation (Figure 1a).48 On the other hand, formation of 3D island structure is dependent on the reassembly and deposition of newly formed nanoparticles layer-by-layer in the course of cooling. FESEM images (Figure 3b, 4c-d and S16) of calcined sample at high temperatures (above 650oC) indicate that under thermal stress condition, sliding of one nanoparticle’s layer on another takes place leading to the arrangement of step-like structures of the BaF2 nanoparticles. Likewise, on calcining the other samples e.g., 12 ACS Paragon Plus Environment

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Ce3+- and Tb3+-doped BaF2 nanoparticles at 800oC, similar morphology changes are observed (Figure S17 and S21-22). To the best of our knowledge, this is the first observation of such morphological change and arrangement as a function of calcination temperature for nanopowders. 3.4.3 Morphological evolution of BaF2:RE3+ evidenced via TEM analysis

Figure 5. (a)TEM, (b) HRTEM and (c) SAED images of BaF2 (B1) nanoparticles. Figure 5a depicts low magnification TEM image of as-prepared BaF2 nanoparticles. Nanoparticles have mixed morphology i.e. cubic and cuboidal, though the majority of the particles have a cuboidal structure. To get deep insight about the growth mechanism, HRTEM images are examined (Figure 5b) carefully. For instance, spacing of the lattice plane of BaF2 nanoparticles (B1) is approximately ca. 0.353 nm which corresponds to the (111) plane and matches nicely with most intense peak of PXRD patterns. Herein, the attachment of the particles 13 ACS Paragon Plus Environment


is also occurring by the overlapping of the alike (111) plane due to orientation attachment of analogous surface with same energy and dislocation along the arrow head (Figure 5b and S18). Stacking faults can be evidently seen in the Figure S18, where faults up to three atoms thick ca. ~1.55 nm are revealed due to internal stress. Figure S19 shows the schematic representation of BaF2 nanocrystals along (111) direction. Previously it was shown that (111) is the thermodynamically stable surface, so it is expected that samples, where huge surface area is available will exhibit (111) plane surface. The (111) surface has hexagonal layer by layer structure and growth in this direction can easily be facilitated by the layer by layer mode (Figure S19). Indeed samples annealed at higher temperatures show layer-by-layer growth evident in SEM analysis. To confirm the growth mechanism of as-prepared nanoparticles, SAED (Selected area electron diffraction) studies were also carried out. SAED images (Figure 5c) reveal the brightest spots belong to (111), (200) and (220) planes. Morphology of the silica coated Eu3+ doped BaF2 nanoparticles is confirmed through the low magnification TEM image (Figure S23). 3.4.4 Morphology evolution by calcination Figure 6 depicts that a drastic change in morphology occurs when as-prepared BaF2:Eu3+ nanoparticles are calcined from 80oC to 400oC (Figure 6a and 6b). Cube-/cuboid-shaped (indicated by rectangular box) particles appear in the case of as-prepared BaF2 and BaF2:Eu3+ nanoparticles (Figure 5a and 6a), which are completely transformed to oval or spherical shaped nanoparticles (marked by the dashed rings in Figure 6b) when calcined at 400oC. Previously, morphology transformation has been observed due to several factors. For instances, due to ultrasonic irradiation, nanowires of Cu(OH)2 are transformed to 3D CuO microstructures; Na(Y1.5Na 0.5)F6:Ce3+ nanorods are converted to nanoparticles etc..49,50


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Figure 6. Low magnification TEM images of Eu3+ doped BaF2 nanoparticles at different temperature: (a) as-prepared (80oC), (b) calcined at 400oC and (c) calcined at 800oC. In addition, Son et al. reported the influence of diffusion controlled cation exchange on the morphology of CdSe nanorods, in which Cd2+ ions are replaced by Ag+ ions leading to the formation of spherical shaped Ag2Se nanoparticles.51 On the other hand, highly agglomerated and larger sized particles are observed when the as-prepared sample is calcined at 800oC compared to 400oC (Figure 6c). HRTEM images show that in both cases, calcined particles exhibit (111) as the dominant plane (Figure S20). 3.5 Optical Properties 3.5.1 Effect of calcination temperature on photoluminescence spectra and decay time Figure 7 depicts the emission spectra of as-prepared (B3) and calcined BaF2:Eu3+ (1%) nanoparticles (ex=391 nm). It is seen that excitation and emission intensities are gradually increasing with increasing the calcination temperature until 400oC (B6), whereafter said intensities decrease up to 650oC calcinations (B8) and are negligible at 800oC (Figure 7 and S25). The intensity ratio (Ie/Im) of electric dipole (appearing at 611 nm) and magnetic dipole (appearing at 591 nm) transitions which correspond to the 5D0-7F2 and 5D0-7F1 electronic transitions, respectively, also follow the same trend (Table 2). The decrease in the PL emission intensity at high calcination temperatures can be rationalized on the basis of FESEM and elemental mapping results. FESEM results illustrate that smaller sized nanoparticles are generated due to secondary nucleation which leads to the increase of surface defects and decrease of luminescence intensity.



(f)

o

800 C

ex-391nm

o

(e)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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650 C

(d)

o

600 C

(c) o

400 C 5

(b)

7

D0- FJ J=1 J=2

o

200 C

J=4

(a)

o

80 C

500

550

600

650

Wavelength (nm)

700

750

Figure 7. Emission spectra of as-prepared BaF2:Eu3+ (B3) nanoparticles and calcined at different temperatures: (a) as-prepared, (b) calcined at 200oC, (c) calcined at 400oC, (d) calcined at 600oC, (e) calcined at 650oC and (f) calcined at 800oC (λex =391 nm). Elemental mapping results (Figure S11) of Eu3+ ions clearly indicate that concentration of Eu3+ ions on the surface of BaF2 nanoparticles increases with higher calcinations temperature. Density of Eu3+ ions at the surface of calcined BaF2 nanoparticles at 800oC is much higher than that of 400oC and 650oC calcined samples. This means that, on calcining, europium ions are more likely to appear on the surface of BaF2 nanoparticles rather than within the asymmetric lattice. It follows that concentration dependent quenching (via non-radiative crossing over relaxation) of both the PL excitation and emission would be increased for a 800oC calcined sample. In addition, due to the presence of more vacant coordination sites at the surface of nanoparticles, probability of attachment of Eu3+ ions with water and O2 molecules increases, which causes the decrease of excitation and emission intensity at higher calcinations temperature. To obtain more information regarding the local structure of the Eu3+ ions, Judd-Ofelt parameters (Ω2) of as-prepared and calcined BaF2:Eu3+ nanoparticles at different temperatures are calculated (See Supporting information for details).8 Magnitude of Ω2 parameter is observed at a maximum for the 400oC calcined sample (Ω2=3.42x10-20 cm2) while a minimum value (Ω2=2.03x10-20 cm2) is observed for the 650oC calcined sample (B8) (Table 2), trend also the (Ie/Im) ratio. This clearly 16 ACS Paragon Plus Environment

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indicates that Eu3+ ions are in a more asymmetric environment at 400oC compared to the other samples. Table 2: Decay time, Judd-Ofelt parameter (Ω2) and quantum efficiency (η %) of as-prepared BaF2:Eu3+(1%) and calcined nanoparticles at different temperatures. Sample

τav(ms)

χ2

a1 (%)

a2(%)

Ie/Im

Ω2 (cm2)

η (%)

B3

2.64

0.993

61.12

38.87

0.55

2.78X10

18.67

B5

4.86

0.985

38.08

61.91

0.47

2.4 X10-20

32.81

B6

5.61

0.995

53.70

46.29

0.68

3.42 X10-20

42.84

B7

1.54

0.981

46.76

53.23

0.46

2.36 X10

10.33

B8

1.15

0.926

25.59

74.40

0.40

2.03 X10-20

-20

-20

9.69

It is also seen that quantum efficiency increases (Table 2) with calcination temperature from 80oC (B3) (η=18%) to 400oC (B6) (η=42.84%) and then decreases until 650oC (B8) (η= 9.96%) (See SI for details). Results indicate that the contribution of non-radiative (defects) components to quantum efficiency increases on further increasing the calcination temperature beyond 400oC. Figure S26 shows the PL decays of samples calcined at different temperatures. Results indicate that the decay time increases with calcination temperature up to 400oC and then decreases till 650oC (Figure S26).8 Obviously, no decay time is noticed for 800oC calcined samples. Biexponential decay is observed in all other cases. A maximum value of decay time (τav) among the calcined samples is observed for the 400oC calcined sample (cal. τav ~5.61 ms) while a minimum value is noticed for the sample calcined at 650oC (cal. τav ~1.15 ms). Additionally, solubility of as-prepared nanoparticles are also checked in different solvents like DMSO, water and water mixed with PVP. It is observed that nanoparticles are completely soluble in DMSO while these are not fully soluble in water. However, PVP (100 mg polyvinyl pyrrolidone in 5ml ethanol) solution is added to disperse and stabilized the nanoparticles in water. 52-53 Then photoluminescence (PL) excitation (λex) and emission (λem) of aforementioned solutions of nanoparticles are monitored. Interestingly, PL results show that emission spectra of Eu3+ ions are occurring in all three solutions (shown in Figure S27), indicating these nanomaterials have potential of bio-imaging applications. 4. Conclusions



In summary, we have succeeded in preparing small sized (~20 nm), phase pure RE3+ doped BaF2 nanocrystals using ILs as “nanosynthetic template”. It is observed that calcination has a significant effect not only on the structural parameters like cell parameters, lattice strain, etc., but also on the morphology, which again controls the luminescence properties. Cuboidal shaped asprepared nanoparticles are converted to the spherical/oval shaped nanoparticles upon calcination at 400oC. A significant decrease in the surface area for the samples calcined at 600oC compared to its 400oC analogue, sudden changes in lattice strain (at 600oC) and appearance of layer structure indicate secondary nucleation. Analysis reveals that at higher calcination temperatures (650oC), secondary nucleation, which controls the formation of smaller nanoparticles and their subsequent re-assembly, causes island-like morphology with step-like structures. These 3D island-like structures have a significant effect on photoluminescence of Eu3+ ions. The larger 2 value for 400oC (B6) calcined sample indicates that Eu3+ ions reside in more asymmetric sites compared to the other calcined samples, which is consistent with the emission spectra and lifetime results. Maximum quantum yield (42.8%) is obtained for the 400oC calcined sample, which then decreases at higher calcination temperatures revealing a major role played by the defects due to the formation of smaller nanoparticles. To the best of our knowledge, the role of secondary nucleation in forming 3D island-like structures at higher calcinations temperatures and its effect on photoluminescence have not yet been reported. Photoluminescence emission spectra of Eu3+ doped BaF2 nanoparticles in different solvents like DMSO, water and water with PVP solution confirm that these nanoparticles can be effectively used for the bio-imaging applications. Acknowledgements PG and YB acknowledge support from the Science and Engineering Research Board (Grant No: SB/FT/CS-014/2014), Govt. of India. PG acknowledges Board of Research in Nuclear Sciences (Grant No: 34/14/27/2016-BRNS/34368) and UGC Start-Up Grant [Grant No: F.3056/2014(BSR)], Govt. of India. R.K.S. and S.N. acknowledge Dr. H. S. Gour University for providing graduate fellowships. S. Nigam acknowledges Dr. V. Sudarsan for fruitful discussion. The authors acknowledge the support from Sophisticated Instrumentation Centre (SIC) and Department of Chemistry of Dr. H. S. Gour University for characterizations. Authors acknowledge Dr. P. S. Campbell for his valuable suggestions. 18 ACS Paragon Plus Environment

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Supporting Information The Supporting Information is available free of charge on the ACS publication website. Chemical details, Silica coating of BaF2:Eu3+ nanoparticles, PXRD patterns, lattice strain, EDX and elemental mapping images, electron microscopy images, FTIR images, Photoluminescence spectra, Judd-Ofelt parameter (Ω2) and quantum efficiency (η%) calculation in detail. References 1

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