Multifunctional Lanthanide-Doped Binary Fluorides and Graphene Oxide Nanocomposites Via a Task-Specific Ionic Liquid.

Graphene oxide-based nanocomposites (NCMs) exhibit diverse photonic and biophotonic applications. Innovative nanoengineering using a task-specific ionic liquid (IL), namely, 1-butyl-3-methyl tetrafluoroborate [C4mim][BF4], allows one to access a unique class of luminescent nanocomposites formed between lanthanide-doped binary fluorides and graphene oxide (GO). Here the IL is used as a solvent, templating agent, and as a reaction partner for the nanocomposite synthesis, that is, "all three in one". Our study shows that GO controls the size of the NCMs; however, it can tune the luminescence properties too. For example, the excitation spectrum of Ce3+ is higher-energy shifted when GO is attached. In addition, magnetic properties of GdF3:Tb3+ nanoparticles (NPs) and GdF3:Tb3+-GO NCMs are also studied at room temperature (300 K) and very low temperature (2 K). High magnetization results for the NPs (e.g., 6.676 emu g-1 at 300 K and 184.449 emu g-1 at 2 K in the applied magnetic field from +50 to -50 kOe) and NCMs promises their uses in many photonic and biphotonic applications including magnetic resonance imaging, etc.

Introduction

Graphene oxide (GO) and inorganic- or organic-based nanocomposite materials (NCMs) have drawn a center of curiosity due to their widespread applications in optoelectronics, batteries, catalysis, photocatalysis, magnetism, bioimaging, solar cells, etc.[1−15] Normally, making a composite between two different materials enables one to obtain a property combination that may not be available from a single material.[11,15−18] Graphene oxide is obtained by the oxidation and subsequent exfoliation of graphite via a chemical route. Graphene is fundamentally one atom thick and two-dimensional (2D) with a honeycomb-like structure that was discovered by Novoselov et al. in 2004.[19] Since then, it has been attracting tremendous attention in nanoscience and nanotechnology. However, rare-earth (RE3+)-doped binary/ternary fluorides and GO-based nanocomposite materials are very limitedly studied. Commonly, nanocomposite materials are lanthanide oxides and GO nanocomposites (Ln = Ce and Gd), GO doped with RE ions, and lanthanide-based metallo-organic frameworks (MOFs) attached with GO, etc.[13,15,20−27] However, RE3+-doped fluorides with GO nanocomposite are still in their infancy. Currently, a very limited number of rare-earth fluorides and graphene oxide nanocomposite materials are explored including upconverting NaYF4 doped with Yb, Er/Tm nanoparticles (NPs).[12−15,28,29] There are numerous approaches for the synthesis of graphene, graphene oxide, and derivatives of GO-based nanocomposite materials. Among them, hydrothermal and solvothermal methods are frequently being used for the synthesis of inorganic nanomaterials and GO-based composite nanomaterials.[15,30,31] In this perspective, inorganic nanomaterials synthesis using task-specific ionic liquids (ILs) are paving a new way due to their interesting and tunable features. Normally ILs are comprised of highly tunable organic cation and organic/inorganic anion combinations with a melting point less than 100 °C in ambient conditions.[32] ILs are often considered as green and designer solvents in various applications including nanomaterials synthesis.[33,34] However, with a judicious selection of the cation and anion combinations, physical and chemical properties of ILs can be tuned, which can influence nanomaterials synthesis.[32,34−41] The intricate properties of ILs allow them to serve as a reaction medium, capping/templating agent, and reaction partner, sometimes “all three in one”.[32,34−41] For example, ILs containing tetrafluoroborate (BF4–), hexafluorophosphate (PF6–), and dihydrogenphospate (H2PO4–) anions as counterions have been used for an in situ synthesis of fluoride- and phosphate-based RE3+-doped nanoparticles, respectively.[39−42] Besides this, tunable properties of ILs have been used to control the crystal phase, size, and morphology to stabilize nanoparticles, particularly metallic nanoparticles, etc.[34−38,40]

Herein, we synthesized RE3+ (Ce or Tb or Ce/Tb)-doped binary fluorides with GO nanocomposites (NCMs) using a 1-butyl-3-methyl tetrafluoroborate [C4mim][BF4] IL-assisted hydrothermal method, where the IL not only acts as a solvent and templating agent but is also employed as a fluoride source, that is, all three in one (Figure and Table ). Here, an environmentally benign and “green” method is adopted in which only water and [C4mim][BF4] IL are used. Water is only employed with an IL to facilitate the hydrolysis of the BF4– anion for readily releasing the F– ions (For details of the synthesis/characterization of pure GO, nanomaterials, nanocomposites, etc., see the Supporting Information). In this way, the IL is used in a one-pot synthesis route in which all the reacting precursors are put simultaneously under the similar reaction conditions ([C4mim][BF4] IL, 6 h and 150 °C) via the hydrothermal method for the preparation of RE3+ (Ce3+, Tb3+, or Ce3+/Tb3+)-doped binary fluoride nanoparticles such as BaF2:Ce3+(M1), BaF2:Ce3+/Tb3+(M2), LaF3:Ce3+/Tb3+(M5), CeF3:Tb3+(M6), and GdF3:Tb3+(M7) and their nanocomposites with GO like BaF2:Ce3+-GO (M3), BaF2:Ce3+/Tb3+-GO (M4), LaF3:Ce3+/Tb3+-GO (M8), CeF3:Tb3+-GO (M9), and GdF3:Tb3+-GO (M10) (Figure ). In addition, the role of GO in the crystallite size, morphology, and excitation and emission spectra of RE3+-doped nanoparticles is also studied. From an application point of view, the magnetization of Gd3+-doped NPs and NCMs is also measured.

Figure 1

Schematic representation of the synthesis method of nanocomposites using the IL that is [C4mim][BF4] to assist via a hydrothermal method.

Table 1

Phase, Crystallite Size, and Lattice Strain of As-Prepared RE3+-Doped Binary Fluorides Nanoparticles and Nanocomposites (with Graphene Oxide, GO)

code	sample name	IL	crystal phase	crystallite size (nm)	avg strain (η%)	lattice strain
M1	BaF2:Ce3+	yes	cubic	92.3	–0.35	compressive
M2	BaF2:Ce3+/Tb3+	yes	cubic	100.7	–0.10	compressive
M3	BaF2:Ce3+-GO	yes	cubic	82.1	–0.24	compressive
M4	BaF2:Ce3+/Tb3+-GO	yes	cubic	81.5	–0.22	compressive
M5	LaF3:Ce3+/Tb3+	yes	hexagonal	32.6	0.49	tensile
M6	CeF3:Tb3+	yes	hexagonal	38.6	0.29	tensile
M7	GdF3:Tb3+	yes	orthorhombic	24.0	1.38	tensile
M8	LaF3:Ce3+/Tb3+-GO	yes	hexagonal	29.2	0.54	tensile
M9	CeF3:Tb3+-GO	yes	hexagonal	20.0	0.62	tensile
M10	GdF3:Tb3+-GO	yes	orthorhombic	22.7	0.23	tensile

Experimental Section

Chemicals

Details and the purity of the chemicals used are given in the Supporting Information.

Synthesis of [C4mim][Br] IL

By modifying a previously reported method, 1-butyl-3-methylimidazolium bromide [C4mim][Br] was synthesized by adding the 0.166 mol of 1-bromobutane dropwise to stirring (0.126 mol) 1-methyl imidazole over 30 min at 0 °C in a 250 mL round-bottom flask.[43] Then the round-bottom flask was covered with aluminum foil, and the reaction proceeded at room temperature for 96 h. After the reaction was completed, the obtained product was washed with ethyl acetate and stirred for 1 h. The obtained product was dried in vacuum for 12 h to get a white solid crystal. This white crystalline product can be further used for the preparation of [C4mim][BF4]IL.

Synthesis of [C4mim][BF4] IL

The room-temperature ionic liquid (RTIL) [C4mim][BF4] was prepared by adding NaBF4 (21 g) and 1-butyl-3-methylimidazolium bromide [C4mim][Br] (42 g) in acetone (100 mL) and allowing this to stir at room temperature (RT) for 3–4 d.[41,44] The obtained product was filtered and then stirred continually for further 16 h with 1 g of activated charcoal. The activated charcoal was filtered, and acetone was finally removed by a rotary evaporator under vacuum. The obtained product was further mixed with ∼60 mL of dichloromethane (at least three to four times) to make it free from impurities. A little amount of silver nitrate solution was added to the washed ionic liquid to confirm the chloride ions. The pale yellowish liquid was isolated and further dried in a vacuum for 12 h. Details of 1H and 13C NMR spectra are given in the Supporting Information.

Synthesis of Graphene Oxide (GO)

Details of the synthesis of graphene oxide and its characterization through powder X-ray diffraction (PXRD) and Raman spectra are given in the Supporting Information (Figure S1 and Figure S2).

Synthesis of BaF2:RE3+ (Ce3+) NPs

In the present work, binary fluoride is used as a material both for nanoparticles and nanocomposites. A good “host” for efficient doping of rare-earth ions and its luminescence must adhere the following criteria: it should have low phonon energy, high refractive index, tunable crystal phase, etc. In the present case, binary fluorides like BaF2, LaF3, CeF3, GdF3, etc. all follow those prerequisites mentioned and are used as host materials.

1% Ce3+-doped BaF2 NPs are typically synthesized by a reported method.[41] Separately, 2.5 mL of aqueous solutions of barium acetate (2.8 mmol) and cerium nitrate were stirred for 5 min in separate beakers. The barium acetate and cerium nitrate solutions were mixed. Then, 4.5 mL of [C4mim][BF4] IL was added to the barium acetate and cerium nitrate solution, and this was stirred for 5 min followed by the addition of 10 mL of DI water. Then the reaction mixture was transferred to a 100 mL Teflon-lined autoclave with a stainless steel jacket and kept inside the hot air oven for 6 h at 150 °C. And then the obtained product was washed several times with methanol, ethanol, and acetone and then dried at 60 °C overnight. Other products such as BaF2:Ce3+/Tb3+, CeF3:Tb3+, LaF3:Ce3+/Tb3+, and GdF3:Tb3+ were also synthesized using a similar procedure.

One Pot Synthesis of BaF2:RE3+(RE = Ce3+)-GO NCMs

50 mg of GO was added in 10 mL of DI water and sonicated for 30 min (Steps 1 and 2 in Figure ). Thereafter, 2.5 mL aqueous solutions of barium acetate (2.8 mmol) and cerium nitrate were stirred for 5 min in separate beakers (Steps 3 and 4). The barium acetate and cerium nitrate solutions were mixed. Then, 4.5 mL of [C4mim][BF4] IL (Step 5) was added to the barium acetate and cerium nitrate solution, which was stirred continually for 5 min leading to form a reaction mixture (Step 6 of Figure ). GO solution (10 mL) was added to the above solution and stirred for 5 min (Step 7 of Figure ). Then the reaction mixture was transferred to a 100 mL Teflon-lined autoclave with a stainless steel jacket and kept inside the hot air oven for 6 h at 150 °C. And then the obtained product was washed several times with methanol, ethanol, and acetone and then dried at 60 °C overnight (Step 8 of Figure ). Using the aforementioned synthesis protocol, other products, such as BaF2:Ce3+/Tb3+-GO and CeF3:Tb3+-GO, LaF3:Ce3+/Tb3+-GO and GdF3:Tb3+-GO, etc., were also synthesized (Figures and 2). In this work, Ce3+ works mainly as a sensitizer and Tb3+ ion as an activator. Reduction of GO during synthesis of NCMs was confirmed using Fourier transform infrared (FTIR) spectra (as shown in Figure S3).

Figure 2

Schematic representation of the synthesis method of BaF2:Ce3+-GO nanocomposites using [C4mim][BF4] IL-assisted via a hydrothermal method.

Characterizations

Powder X-ray diffraction (PXRD) data of the samples were obtained with a D8 Advance Bruker, equipped with Cu Kα (1.540 60 Å) as the incident radiation. The morphology of the as-prepared nanoparticles and nanocomposites was measured using the field-emission scanning electron microscopy (FESEM) of NOVA NanoSEM 450 and transmission electron microscopy (TEM) of FEI Tecnai G2 20 S-Twin, 200 kV. The atomistic level growth of nanoparticles and nanocomposites was confirmed using high-resolution (HR) TEM. The FTIR measurement was performed using a Bruker Tensor 37. Photoluminescence (PL) spectra were recorded using a Horiba Jobin Yvon Fluoromax-4 spectrofluorometer. Raman spectra were measured using a Renishaw via a Raman microscope. Nuclear magnetic resonance spectrometry (JEOL Bruker ECX 500 (500 MHz)) was used for measuring NMR spectra in CDCl3. The magnetization measurement was performed using a Quantum Design SQUID-VSM magnetometer at 2 and 300 K.

Results and Discussion

Structural Characterization by Powder X-ray Diffraction (PXRD)

The PXRD patterns of the as-synthesized phase-pure cubic BaF2 NPs and NCMs doped with Ce3+ and Ce3+/Tb3+ nicely matches with JCPDS card No. C4-452 (Figure S4(i)). Additionally, PXRD patterns of other Ln-based NPs and NCMs match with JCPDS card Nos. C32-483 (LaF3), C8-45 (CeF3), and C12-788 (GdF3), respectively (Figure a and Figure S4(ia)). No extra peak due to any impurity or dopant ion(s) is noticed. The crystallite size of RE3+-doped BaF2 NPs is also measured with and without GO (Table ). Interestingly, the crystallite size of BaF2:RE3+(1%) NPs is found bigger and significantly diverse in the absence of GO than that of with GO. In the absence of GO, crystallite sizes are 92.3 and 100.7 nm for BaF2:Ce3+ and BaF2:Ce3+/Tb3+ NPs, respectively. However, when GO is employed to synthesize the NCMs according to Figures and 2, a smaller size of Ce3+ or Ce3+/Tb3+-doped BaF2 NPs is found. For example, 81.5 and 82.1 nm are obtained for Ce3+/Tb3+ and Ce3+-doped BaF2-GOs NCMs, respectively (Table ). Likewise, for other Ln-based NPs and their NCMs, a significant change in the crystallite size of NPs compared to NCMs is found. For instance, in the absence of GO, the crystallite size of LaF3:Ce3+/Tb3+, CeF3:Tb3+, and GdF3:Tb3+ corresponds to 32.6, 38.6, and 24.0 nm, respectively (Table ). However, when these nanoparticles are prepared in situ with GO as a nanocomposite, the sizes of NPs are noticeably reduced to 29.2, 20.0, and 22.7 nm for LaF3:Ce3+/Tb3+-GO, CeF3:Tb3+-GO, and GdF3:Tb3+-GO, respectively (Table ). Thus, the crystallite size of NPs is substantially reduced in the presence of GO indicating the role of GO itself as a capping agent.

Figure 3

(a) PXRD patterns: (i) LaF3 standard, (ii) LaF3:Ce3+/Tb3+-GO (M8), (iii) CeF3:Tb3+-GO (M9), (iv) GdF3 standard, (v) GdF3: Tb3+-GO (M10). (b) Low-magnification TEM image of BaF2:Ce3+-GO NCMs, (c) HRTEM image BaF2:Ce3+/Tb3+-GO NCMs, and (d) low-magnification TEM image of CeF3:Tb3+-GO NCMs formed by one-pot synthesis using the [C4mim][BF4] IL-assisted hydrothermal method.

Lattice Strain

Commonly, the nature of diffraction peaks of particles can be related to the crystallite size and lattice strain. Therefore, the lattice strain and crystallite size can be determined by the equation of Williamson and Hall[41]where η is the effective strain, D is the crystallite size, and λ is the X-ray wavelength. When we plot β cos θ/λ versus sin θ/λ, the strain can be derived from the slope. The positive and negative magnitudes of the slope indicate the tensile and compressive strains, respectively.

The lattice strain of NPs and NCMs are determined using the Williamson-Hall equation as mentioned above. The compressive strain is observed for BaF2:Ce3+ and BaF2:Ce3+/Tb3+ binary fluorides in the presence and absence of GO (Figures (i)). Interestingly, the magnitude of the compressive strain is found very close for BaF2:Ce3+-GO (−0.24%) and BaF2:Ce3+/Tb3+-GO NCMs (−0.22%) (Figure (i)a,b). However, in the case of bare BaF2:Ce3+ and BaF2:Ce3+/Tb3+ NPs, compressive strain is found to be −0.35% and −0.1%, respectively (Figure (ii)a,b). This can be attributed to having an approximately similar crystallite size of (BaF2:Ce3+ and BaF2:Ce3+/Tb3+) NPs in the presence of GO (Table ). However, in the case of CeF3:Tb3+, LaF3:Ce3+/Tb3+, and GdF3:Tb3+ NPs and their NCMs with GO, only tensile strain is found. The significant influence of GO on the magnitude of tensile strain is noticed. For instance, 0.49%, 0.29%, and 1.38% tensile strains are found for LaF3:Ce3+/Tb3+, CeF3:Tb3+, and GdF3:Tb3+ NPs in the absence of GO, respectively (Figure (ii)c–e). Unlike the other two, on incorporating the GO to form NCMs, tensile strain is noticeably reduced to 0.23% for GdF3:Tb3+-GO NCMs (shown in Figure (i)e).

Figure 4

Lattice strain of NCMs: (i) (a) BaF2:Ce3+-GO, (b) BaF2:Ce3+/Tb3+-GO, (c) CeF3:Tb3+-GO, (d) LaF3:Ce3+/Tb3+-GO, (e) GdF3:Tb3+-GO; (ii) (a) BaF2:Ce3+; (b) BaF2:Ce3+/Tb3+; (c) CeF3:Tb3+; (d) LaF3:Ce3+/Tb3+; (e) GdF3:Tb3+ synthesized using the [C4mim][BF4] IL-assisted hydrothermal method.

Structural Characterizations by Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM)

The morphology of the as-prepared GO, NPs, and NCMs is depicted using the FESEM and TEM images (see Figures b–d, 5, and S5–S7). In the case of RE3+ (Ce3+ and Ce3+/Tb3+)-doped BaF2 NPs and their NCMs counterpart, agglomerated, small spherical or oval-shaped along with a bigger size and cubic/cubical morphology of particles are found (Figures b). LaF3:Ce3+/Tb3+, CeF3:Tb3+, and GdF3:Tb3+ NPs and their respective NCMs are also prepared using GO under the similar reaction condition. The morphology of LaF3:Ce3+/Tb3+ and CeF3:Tb3+ NPs are found to be spherical-shaped (Figures d, 5c, and S6a). However, TEM images reveal that a mixed morphology is obtained for as-prepared LaF3:Ce3+/Tb3+ NPs and CeF3:Tb3+ NPs, in which small NPs are oval-shaped while matured NPs have the elongated hexagonal shape (Figures d, 5c, and S6a). However, GdF3:Tb3+ NPs and its NCMs with GO have completely different morphology, that is, spindle-shaped (in Figures d and S6b), and it seems to be made of small NPs. Thus, an analysis reveals that the morphology is dependent on the nature of the precursor ions used for the synthesis of the host matrix and also illustrated earlier.[45] For understanding the growth mechanism from the atomistic level and to confirm NCMs formation, HRTEM images are taken. HRTEM images of as-prepared BaF2:Ce3+/Tb3+(1%)-GOs NCMs reveal that the growth of NPs is preferentially taking place along the (111) plane, which corresponds to d-spacing of ∼0.357 nm (Figure c). The formation of cubic-shaped NPs is basically due to the isotropic growth of particles along the (111) dominant plane. Besides, Moiré patterns (highlighted by a yellow circle) are also appearing due to the overlapping of two similar planes having comparable energy (Figure c). From TEM images, it is clear that NPs are distributed onto the GO surface almost homogeneously (according to Figure ). The attachment of NPs on the surface of GO is attributed to the electrostatic attraction between GO and NPs, which leads to the formation of NCMs (Figures b,d and S7b).[17] This synthesis strategy has shown that RE3+ ion-doped BaF2/LnF3-GO NCMs can be effortlessly synthesized at moderate temperature using IL.

Figure 5

TEM images of as-prepared RE3+(1%)-doped binary fluoride NPs. (a) BaF2:Ce3+, (b) BaF2:Ce3+/Tb3+, (c) LaF3:Ce3+/Tb3+, and (d) GdF3:Tb3+, synthesized using the [C4mim][BF4] IL-assisted hydrothermal method.

It is worth mentioning that, even though the lanthanide-doped binary fluorides (BaF2, CeF3, LaF3, GdF3) have different crystal structures (Figure ), the ionic liquid 1-butyl-3-methyl tetrafluoroborate [C4mim][BF4] is able to facilitate the synthesis of nanoparticles and nanocomposites with graphene oxide (GO). This may be attributed to the involvement of the ionic liquid at a microscopic level/atomic level in controlling the reaction and facilitating the formation of a metal–fluorine bond under a different crystallographic symmetry.

Figure 6

Representative crystal structures of BaF2 [Fm3m (cubic)], CeF3 [P63/mcm (hexagonal)], LaF3 [P3̅c1 (hexagonal)], and GdF3 [Pnma (orthorhombic)].

It is known that ILs act as a reaction partner, solvent, and capping/templating agent. On the one hand, an anionic part like BF4–, H2PO4–, Br–, I–, etc. of ILs acts as a source of reaction partner, and the anionic (BF4–, H2PO4–) part of the IL is decomposed into F– and PO43– in the presence of water. On the other hand, the cationic part (imidazolium, pyridinium, ammonium, ions, etc.) participates in the capping/templating agent.[46−48] For example, Zheng et al. have shown that the [Emim]+ ions can serve as capping agents based on their strong interactions with the (110) facets of rutile, and the [Emim]Br favors the formation of the rutile structure with a rod-like shape due to the mutual π-stacking interactions of imidazole rings.[48] It has been experimentally as well as theoretically proven that the cationic moiety of an IL is self-assembled about the nucleation site during the growth of nanoparticles. And by binding at the particular facet of nanoparticles, an IL governed the growth of nanoparticles.[48−51] Wu et al. synthesized BaY0.78F5:Yb0.7,Tm0.02 upconverting nanoparticles in which NaBF4 is used as a source of fluoride (F–) ion and ethylenediaminetetraacetic acid (EDTA) as a surface active agent.[52] Thereafter, as-prepared nanoparticles are functionalized with other moieties (tetraethyl orthosilicate (TEOS), (3-aminopropyl)triethoxysilane (APTES), avidin, etc.) in order to bind the nanoparticles with a graphene oxide surface for desired applications. Similarly, Malik et al. discloses the incorporation of graphene quantum dots (GQDs) on the growth of β-NaYF4:Gd3+/Tb3+ phosphor crystals via a hydrothermal route.[53] The GQDs function as a nucleation site, and by changing the concentration of GQDs, the morphology of β-NaYF4:Gd3+/Tb3+ phosphors was changed from a rod to flowerlike to disklike structure, without a phase transformation. Likewise, Wei et al. synthesized GO–NaYF4:Yb/Er nanocomposites in the water/ethanol (1:1, v/v) solution.[14] And then they have studied the optical limiting property of GO–NaYF4:Yb/Er nanocomposites. During synthesis, sodium fluoride (NaF) and ethanol are used as a source of fluoride ion and cosolvent, respectively. They have also reported that the luminescence of nanoparticles is significantly quenched in the presence of GO.

In the present work, the nanocomposites are prepared by a one-pot process using an ionic liquid-assisted solvothermal method. Particularly, ionic liquid is utilized for an in situ synthesis of rare-earth-doped binary fluorides on to the graphene oxide. One of the preferred ionic liquids used is 1-butyl-3-methylimidazolium tetrafluoroborate [C4mim][BF4]. Here ionic liquid [C4mim][BF4] is itself working as a fluoride source, as solvent, and templating agent, that is, all three in one. Certainly, the synthesis methodology described in the present work is new and novel and not reported to the best of our knowledge.

Optical Properties

Figure illustrates the photoluminescence excitation and emission spectra of the as-prepared RE3+-doped NPs and their NCM counterparts, which are measured at RT. In both the NPs and NCMs, Ce3+ and Gd3+ ions are employed as a sensitizer and Tb3+ ion as an activator. In the case of Ce3+ ions, a broad excitation band is noticed for NPs as well as NCMs due to the 4f-5d electronic transition (shown in Figure a,b and Figures a and S8c).[54] Interestingly, significant shifting in the normalized excitation peaks of Ce3+ ion-doped/based NPs and NCMs is found in different host matrices and in the presence of GO. For instance, two excitation peaks of Ce3+ ions appeared at 260 and 287 nm for cubic Ce3+ or Ce3+/Tb3+ codoped BaF2 NPs (Figures a and S8c). The intense peak at 287 nm is not found to be shifted more than the less intense peak that appeared at 260 nm for BaF2:Ce3+/Tb3+ NPs (M2) and BaF2:Ce3+/Tb3+-GO NCMs (M4). However, the excitation peak appearing at 260 nm in NPs (M2) is shifted ∼4 nm toward the lower wavelengths NCMs (M4), and it now appears at 256 nm (Figure a).

Figure 7

PL excitation spectra: (a) BaF2:Ce3+/Tb3+ NPs (M2) and BaF2:Ce3+/Tb3+-GO NCMs (M4), (b) CeF3:Tb3+ NPs (M6) and CeF3:Tb3+-GO NCMs (M9). PL emission spectra: (c) BaF2:Ce3+/Tb3+ NPs (M2) and BaF2:Ce3+/Tb3+-GO NCMs (M4) and (d) CeF3:Tb3+ NPs (M6) and CeF3:Tb3+-GO NCMs (M9), inset shows emission of NCMs (M9) at higher slit width and measured at RT, (e, f) BaF2:Ce3+/Tb3+-GO and CeF3:Tb3+-GO NCMs are illuminated using UV lamp (6 W).

Figure 8

PL excitation spectra: (a) LaF3:Ce3+/Tb3+ NPs (M5) and LaF3:Ce3+/Tb3+-GO NCMs(M8), (b) GdF3:Tb3+ NPs(M7), and GdF3:Tb3+-GO NCMs (M10) synthesized by a hydrothermal method using [C4mim][BF4] IL.

Similar trends of excitation peak shifting in the presence of GO are also found for LaF3:Ce3+/Tb3+-GO NCMs (M8) and CeF3:Tb3+-GO (M9) and (Figures a and 7b). For example, single broad excitation peaks at 250 and 259 nm are noticed for hexagonal LaF3:Ce3+/Tb3+(M5) and CeF3:Tb3+(M6) NPs, respectively. However, in the case of CeF3:Tb3+-GO NCMs (M9), the excitation peak is considerably shifted 6 nm toward the lower wavelength (now appearing at 253 nm) compared to the CeF3:Tb3+ NPs analogue (M6) in which an excitation peak was found at 259 nm (Figure b). Similarly, for LaF3:Ce3+/Tb3+-GO NCMs (M8), an excitation peak appeared at 247 nm, while in LaF3:Ce3+/Tb3+ NPs (M5), it is observed at 250 nm (Figure a). However, there is no change in the excitation (8S7/2-6IJ) peak position of the Gd3+ ion at 272 nm for GdF3:Tb3+ NPs (M7) and GdF3:Tb3+-GO NCMs (M10) due to an f-f transition (Figure b).

In semiconductors, size-dependent photoluminescence properties are found due to the quantum-confinement effect.[55] Consequently, excitation and emission bands are significantly shifted on varying the size of the particles. However, except the Ce3+ ion, other RE3+-doped nanoparticles show size-independent photoluminescence behavior due to their f-f electronic transition. Therefore, narrow excitation and emission bands are observed for the rare-earth ions except the Ce3+ ion. In the case of Ce3+ ions, the 4f-5d transition occurs, which is subject to a crystal field effect. It means that, on varying the crystal field effect, the excitation band may be considerably shifted. It is reported that the Ce3+ ion has 2F5/2 and 2F7/2 as ground states and that the excited level is degenerated 2D. Ghosh et al. have studied the effect of the ligand field on the photoluminescence behavior of Na(Y1.5Na0.5)F6:Ce (1). They found that the excitation pattern of Ce3+ ions is significantly varied in the presence of strongly hydrogen-bonded interlayered/confined water in a layered structure.[56] Since strongly hydrogen-bonded, interlayered water molecules generate a stronger crystal field on the Ce3+ ion, a degenerated 2D excited level is further split, leading to different excited levels. We herein assumed that same situation might also be there in the presence of graphene oxide in the nanocomposite. As the reduced graphene oxide attaches to the surface of Ce3+-doped binary fluorides, it (GO) can enhance the crystal field strength, which leads to the splitting as well as shifting of the excitation peak to a higher energy side. Similarly, Reeves et al. reported the optical properties of CdF2–CaF2 superlattices using synchrotron spectroscopy of confined carriers and measured the exciton emission.[55] Also in their experiment they have mentioned the exciton confinement phenomenon.

To get better insight on the effect of GO on the emission of NPs, NPs are excited at their corresponding excitation wavelength as previously discussed. On the one hand, by exciting the bare NPs and NCMs with either Ce3+, Ce3+/Tb3+-doped, or Ce3+-based samples, a broad emission peak of Ce3+ ions appears due to the 4d-5f transition, which is susceptible to the external environment. On the other hand, narrow emission bands of Tb3+ ions appear at 489, 541, 585, and 620 nm, and these correspond to the transitions from 5D4 to 7F6, 7F5, 7F4, and 7F3, respectively.

The intensity of only Ce3+-doped or Ce3+/Tb3+ codoped BaF2 NPs (M1 and M2) and BaF2:Ce3+-GO or BaF2:Ce3+/Tb3+-GO NCMs (M3 and M4) is significantly governed by the excitation wavelength and presence of GO. For example, the intensity of the broad emission peak appearing in bare and doped BaF2NPs is not only found more intense upon exciting at 287 nm [appearing at 328 nm (M1 and M2)] as compared to the excitation at 260 nm [(341 (M1)/335 nm (M2)] but also the emission peak is shifted toward a lower wavelength on varying the excitation wavelength (Figures S8a and S9a). This might be attributed to the excitation peak at 287 nm being stronger than the excitation peak of 260 nm. Under similar measurement conditions, the emission intensity of Ce3+ or Tb3+ ions in NCMs is considerably quenched in the presence of GO (Figure c). But, on increasing the slit width up to twofold, emission intensities of Ce3+ and Tb3+ are significantly increased (but still less intense than the emission intensity of bare NPs in all NCMs). It is also noticed that the emission peak of Ce3+ ions in the BaF2:Ce3+-GO (M3) and BaF2:Ce3+/Tb3+-GO(M4) NCMs are considerably shifted toward higher energy as compared to their NP analogues (Figures S8b, S9b, and S10b). In another sample, the emission intensity of Tb3+ ions in CeF3:Tb3+-GO (M9) NCMs is significantly more quenched than the bare CeF3:Tb3+ (M6) NPs on using the same slit width (Figure d). However, on increasing the twofold slit width, the emission intensity of CeF3:Tb3+-GO NCMs (M9) is substantially enhanced, and the emission intensity is even close to that of bare NPs, as shown in the inset of Figure d. Likewise, the emission intensity of Tb3+ ions in the LaF3:Ce3+/Tb3+-GO(M8) NCMs is also significantly quenched than that of bare LaF3:Ce3+/Tb3+ (M5) NPs, whereas on increasing the slit width their emission intensity is increased to some extent. When a Gd3+ ion in GdF3:Tb3+(M7) NPs and GdF3:Tb3+-GO (M10) NCMs is excited at 272 nm (8S7/2-6IJ), then energy transfer takes place from excited Gd3+ ions to Tb3+ ions. And then by a nonradiative process, energy is relaxed to the 5D4 excited states leading to a 5D4-7F5 transition.[42] As a result, a highly intense peak is observed in the green region at 542 nm (Figure S9c). The green color luminescence is observed when as-prepared BaF2:Ce3+/Tb3+-GO (M4) and CeF3:Tb3+-GO (M9) NCMs are also illuminated at 254 nm using a 6 W UV lamp as shown in Figure e,f.

Magnetic Properties

Additionally, since the Gd3+ ions have seven unpaired electrons, the paramagnetic characteristic of Gd3+ ions in the GdF3:Tb NPs (M7) and GdF3:Tb-GO NCMs (M10) are studied at room temperature (300 K) and low temperature (2 K), since the magnetization is a function of the applied magnetic field (−50 to +50 kOe), as shown in Figure . At 300 K, the magnetization (emu g–1) of the M7 (NPs) and M10 (NCMs) samples is found to be 6.676 and 6.477 emu g–1 (Figure ). This means that no significant loss of magnetization of nanoparticles in the presence of GO is found. At a very low temperature (2 K), the magnetization is increased to many folds for the samples GdF3:Tb NPs (M7) and GdF3:Tb-GO NCMs (M10) due to the decrease in thermal fluctuation. In this case, the magnetization is found to be 184.449 and 182.620 emu g–1 for as-prepared GdF3:Tb NPs and GdF3:Tb-GO NCMs, respectively (Figure ).

Figure 9

Magnetization vs applied magnetic field plot. (a) GdF3:Tb3+ NPs(M7) and (b) GdF3:Tb3+-GO NCMs (M10) measured at 300 and 2 K.

Besides this, comparing to other existing literature and to the best of our knowledge, we have reported the maximum magnetization for GdF3 nanoparticles and their nanocomposites with GO in the same range of applied magnetic field, as shown in Table .[57−59] For example, Guan et al. reported 3 emu g–1 magnetization at 300 K for the GdF3:2%Dy3+,2%Tb3+, 2% Eu3+ nanoparticles; however, GdF3:Tb3+ NPs and GdF3:Tb3+-GO NCMs reported in the present work show 4.015 and 3.879 emu g–1 magnetization in the applied magnetic field from −30 to +30 kOe (Table ).[56] A similar trend is observed for a low-temperature measurement too at 2 K (Table ). Furthermore, the high magnetization of the GdF3:Tb3+ NPs and GdF3:Tb3+-GO NMCs promise their potential use in a biomedical application like MRI imaging, etc.[60,61] In addition, as NPs and NCMs show the maximum magnetization at the high applied magnetic field at RT as well as at a low temperature, these may also be applied for a cryogenic magnetic coolant.[62]

Table 2

Comparison of the Magnetization and Applied Magnetic Field of GdF3:RE3+ Nanoparticles

		magnetization (emu/g)
S. No.	nanoparticles	300 K	2 K	applied magnetic field	refs
1.	GdF3:2%Dy3+, 2%Tb3+, 2% Eu3+	3	138	–30 to +30 kOe	(56)
2.	GdF3:23%Yb3+,1%Tm3+	2	not measured	–20 to 20 kOe	(54)
3.	GdF3:Eu3+ nanoparticles	2	measured at 77K	–20 to 20 kOe	(55)
4.	GdF3:Tb3+ nanoparticles	6.676	184.449	–50 to 50 kOe	this work
5.	GdF3:Tb3+-GO NCMs	6.477	182.620	–50 to 50 kOe	this work
6.	GdF3:Tb3+ nanoparticles	4.015	182.856	–30 to 30 kOe	this work
7.	GdF3:Tb3+-GO NCMs	3.879	180.517	–30 to 30 kOe	this work
8.	GdF3:Tb3+ nanoparticles	2.674	177.269	–20 to 20 kOe	this work
9.	GdF3:Tb3+-GO NCMs	2.58	171.091	–20 to 20 kOe	this work

Conclusions

In summary, a new class (any lanthanide ion as dopant or host can be used) of novel NCMs of RE3+-doped binary fluorides with GO have been synthesized using the [C4mim][BF4] IL-assisted hydrothermal method where IL is used as a solvent, templating agent, and a reaction partner, that is, all three in one. No other solvent is used except water in the synthesis. It is found that GO also acts as a templating agent along with IL in the synthesis of NCMs. In the presence of GO, for all the cases, not only is the crystallite size reduced but also lattice strain is significantly tuned. As an example, the crystallite size for the CeF3:Tb nanoparticle is found at 38.6 nm; however, CeF3:Tb-GO shows the crystallite size of 20.0 nm. Interestingly, it is observed for the first time that optical properties, especially excitation and emission spectra of Ce3+ ions, are considerably tuned in the presence of GO; especially, excitation spectra of the Ce3+ ion are shifted to higher energy. For example, an excitation peak is observed at 259 nm for CeF3:Tb nanoparticles; however, it is shifted to 6 nm for CeF3:Tb-GO nanocomposites. Though the emission intensity decreases for the nanocomposites, it shows a high luminescence property, which is promising for the application of NCMs in various photonic/biphotonic fields. In addition, the GO part of the nanocomposite can be useful for suitable applications too. GdF3:Tb3+ NPs and their NCM analogues with GO (GdF3:Tb3+-GO) have revealed maximum magnetization; therefore, these can also be employed as a potential candidate for imaging and cryogenic magnetic coolant, etc.