LnPO₄:Eu³⁺ nanoparticles: Role of host lattices on physiochemical and luminescent properties

2.1 Materials

La₂O₃(98 %, BDH chemicals, UK), Gd₂O₃(98.8 %, BDH chemicals, England), Yttrium oxide (BDH, chemicals, UK), urea, HNO₃, C₂H₅OH were purchased AR grade and employed as received without further purification. All metal oxides were converted into metal nitrates by dissolving in diluted HNO₃. Milli-Q (Millipore, Bedford, USA) H₂O was utilized for the preparation and characterization of the metal phosphate NPs.

2.2 Synthesis of the Eu³⁺-ion doped metal phosphate nanoparticles

For the preparation of the LnPO₄:Eu NPs, freshly prepared 9.5 ml of 2 M La(NO₃)₃7H₂O dissolved in aqueous media, and 0.05 ml 2 M Eu(NO₃)₃6H₂O were mixed into 100 ml kept on the hot plate for vigorous mechanical stirring at 70 °C(Phaomei et al., 2010). 5 g urea liquified in an aqueous media was dropwise injected into an ongoing magnetically stirred reaction mixture for slow thermal decomposition of the metal nitrates into metal carbonates. This reaction proceeded on the hot plate until the homogeneous transparent solution occurred. After that, the reaction was transferred into the reflexing condition at an elevated temperature of 150 °C for the complete decay of urea into amine and carbonates. Then an equal volume of NaH₂PO₄ dissolved in an aqueous mixture was dropwise added into the ongoing reaction mixture. This reaction further proceeded for ∼ 5 hrs. Occurred precipitate was separated by centrifugation, washed with H₂O to eliminate the unreacted byproducts, and dried overnight in an oven at 80 °C. Similar reaction conditions were employed for the synthesis of GdPO₄:Eu and YPO₄:Eu NPs.

2.3 Characterization

Powder XRD pattern (PANalytical X’PERT, Netherland) x-ray diffractometer fitted with Ni filter and Cu Kα (λ = 1.5404 Å) radiation. FTIR spectra Vertex 80 (Bruker, USA) spectrometer was employed using by KBr pellet procedure in 400–4000 cm⁻¹ frequency range. Raman spectra were measured from the (IY-Horiba-T64000) Raman spectrometer at ambient temperature under monitoring from 325 nm excitation wavelength. UV/Visible spectra were recorded from the Cary 60 (Agilent Technologies, USA) spectrophotometer in ethanol in the range of 200–650 nm wavelength. Emission and excitation spectra were measured from the Flourolog-3 (Model FL3-11, Horiba Jobin Yvon, Edison, USA) spectrophotometer.

3.1 Crystallographic study

A Powder XRD profile was used to inspect the comparative crystallographic, crystal phase, crystallinity, and phase cleanliness of the as-prepared nano products. As proved in Fig. 1 the diffraction lines' position and intensities in the XRD pattern of the LaPO₄:Eu, GdPO₄:Eu, and YPO₄:Eu sample were completely matched with the hexagonal structure, which is well-indexed to the JCPDS Card No. 046–1439(Runowski et al., 2014), JCPDS card No. 039–0232(Zhang et al., 2011, Ren et al., 2012) and JCPDS Card No. 042–0082, respectively(Luwang et al., 2010, Lai et al., 2014). No additional peaks related to the metal oxide or other additives are detected in the entire XRD profile, it endorsed the synthesis of a highly pure, one-phase nanoproduct. As presented in Fig. 1a the width of the diffraction planes in all three metal phosphate samples was highly broad, it indicated the small crystalline size of the particles. A slight shift in the diffraction peak positions is detected in the YPO₄:Eu NPs than in the LaPO₄:Eu and GdPO₄:Eu NPs. It assumes that the shifting in the diffraction peaks because of the larger ionic radii Eu³⁺=0.95 Å doped into the smaller size ionic radii Y³⁺ crystal lattice Y³⁺=0.89 Å (Fig. 1b). It indicated that the substituted Eu³⁺-ion occupied the Y³⁺ cation positions, therefore the lattice of the crystal was slightly enlarged. The slight enlargement of the host lattice distorts the symmetry because of alterations in the bond angle and bond distances. Causing the symmetry distortion the diffraction plane intensities as well as peak positions were shifted in the XRD pattern (Jia et al., 2010, Yang et al., 2011). Jia et al reported that the ionic radii differences affect the diffraction peak positions in the XRD pattern such as doping of the small size ion into bigger ionic radii host lattice shifts the diffraction peaks at a higher angle, whereas the bigger ionic radii dopant with smaller size ionic radii ions shift the diffraction lines at a lower angle (Y³⁺=1.019 Å, La³⁺=1.16 Å, Eu³⁺=1.06 Å, and Gd³⁺=1.05 Å in eight coordinated)(Jia et al., 2010). An observed lattice constant are a = 6.798 Å, b = 7.115 Å, and c = 6.591 Å for the LaPO₄:Eu; a = 6.708 Å, b = 7.101 Å, and c = 6.482 Å for the GdPO₄:Eu; and a = 6.831 Å, b = 6.175 Å, c = 6.652 Å for the YPO₄:Eu NPs. The observed lattice parameters are similar to the previous reports (Parchur et al., 2010, Yaiphaba et al., 2010). A significant reduction in the calculated lattice parameters was observed it reflects the impact of the variation in the host and doped ions ionic radii (Phaomei et al., 2011). Additionally, the substituted doped ion replaces the host ion causing it to reduce the lattice parameters, which are estimated in the order YPO₄:Eu˃LaPO₄:Eu˃ GdPO₄:Eu, respectively (Fig. 1)(Grzyb et al., 2014). It is anticipated that in luminescent materials higher crystalline phase typically correlates to fewer trapping agents and brighter fluorescence. In the optically active or photosensitive particle crystal structure, particle size plays an important role in enhancing emission efficiency. A better crystalline phase results in fewer bulk and surface imperfections, which act as photo-induced electron quenching points and reduce luminescence intensity. The average crystalline size of the nanoproducts was assessed from the full-width half maxima of the strongest diffraction plane (1 0 2) observed at 31.7° calculated through the Scherrer equation to be 13.9, 14.89, and 18.7 nm for LaPO₄:Eu, GdPO₄:Eu, and YPO₄:Eu NPs, respectively.

Close

Fig. 1

(a&b)X-ray diffraction pattern of the LaPO₄:Eu, GdPO₄:Eu, and YPO₄:Eu NPs.

Export to PPT

3.2 Surface properties

FTIR spectra were performed to inspect the attachment of the organic and water molecules surrounding the exterior of the nanocrystals. Fig. 2 displays the FTIR spectra of the as-synthesized LaPO₄:Eu, GdPO₄:Eu, and YPO₄:Eu NPs in a comparative analysis. All FTIR spectrum shows a diffused band in between 300 and 3600 cm⁻¹ attributed to the asymmetric/symmetric stretching vibrational mode of the (O–H) functional group related to the physically or chemically outward adsorbed H₂O particles (Phaomei et al., 2011, Lai et al., 2014). Three strong intensity bands located at ∼ 1750, 1250, 640, and 550 cm⁻¹ are associated with the asymmetric stretching, bending, and wagging vibrational modes of the phosphate group (PO₄^3-) and hydroxyl groups(Ansari, 2017, Ansari et al., 2017). A sharp with a strong middle-intensity peak at a lower frequency appeared at ∼ 462 cm⁻¹, which is ascribed to the meta-oxygen network (Ansari, 2018, Ansari and Khan, 2018) in the crystal lattice.

Close

Fig. 2

FTIR spectra of the LaPO₄:Eu, GdPO₄:Eu, and YPO₄:Eu NPs.

Export to PPT

3.3 Raman shift

Raman spectra were recorded to estimate the structural dis-order in three Eu³⁺-ion doped metal phosphate NPs. Fig. 3 illustrated the comparative Raman spectra of the LaPO₄:Eu, GdPO₄:Eu, and YPO₄:Eu NPs to distinguish the peak sensitivity and position of the observed Raman vibrational bands(Du et al., 2015, Saraf et al., 2015, Colomer et al., 2017). Raman spectra of the three samples exhibited two strong broadband located at 811, and 880 cm⁻¹, which are credited to the hexagonal phase of the metal phosphates(Colomer et al., 2017). The Raman peak intensity was remarkably higher in the GdPO₄:Eu NPs in contrast to the LaPO₄:Eu and YPO₄:Eu NPs, it may be the impact of the host and guest ion size, which shrinks/enlarges the crystal lattice-based on the doped cation ionic radii, as described in the XRD discussion.

Close

Fig. 3

FT-Raman spectra of the LaPO₄:Eu, GdPO₄:Eu, and YPO₄:Eu NPs.

Export to PPT

3.4 Optical characteristics

Fig. 4 displayed the comparative absorption spectra of the as-synthesized Eu³⁺-ion doped metal phosphate NPs to explore the optical characteristics, aqueous dispersibility, and colloidal solidity in an aqueous solution. The absorption spectra of the LaPO₄:Eu, GdPO₄Eu, and YPO₄:Eu NPs in an aqueous solution revealed good absorbance in the ultraviolet region. A strong absorbance in the UV range of the as-prepared NPs specified good dispersibility and colloidal stability. It assumes that the exterior of the metal phosphate NPs is shielded with abundant hydroxyl molecules which assist in the formation of hydrogen bonding through van der Waal force interaction (Fig. 4). Generally, it is expected that the high colloidal dispersibility of the NPs leads to an increase in the biocompatibility and non-toxicity of the metal phosphate NPs.

Close

Fig. 4

UV/Visible absorption spectra of the LaPO₄:Eu, GdPO₄:Eu, and YPO₄:Eu NPs.

Export to PPT

UV/Visible spectra were exploited to monitor the optical energy bandgap (E_g) of the as-prepared three metal phosphate NPs to understand the optical characteristics and their correlation with the grain size of the ceramic materials. Tauc formulae were employed to regulate the bandgap energy, in which absorption spectra were plotted photon energy (hν) versus (αhν)² as demonstrated in Fig. 5(Tauc and Menth, 1972). According to the curve plotted in Fig. 5, the straight portion of the curve exhibited the indirect E_g values 5.14, 5.03, and 4.90 eV for the LaPO₄:Eu, GdPO₄:Eu, and YPO₄:Eu NPs, respectively.

Close

Fig. 5

Energy bandgap plotting a graph between (αhν)² versus photon energy (hν,eV) of the LaPO₄:Eu, GdPO₄:Eu, and YPO₄:Eu NPs.

Export to PPT

3.5 Photoluminescence properties

Fig. 6 displays the excitation spectrums of the LaPO₄:Eu, GdPO₄:Eu, and YPO₄:Eu NPs on irradiation of all samples from the 595 nm(⁵D₀→⁷F₁) emission wavelength. The excitation spectra of the three-metal phosphate exhibited seven 4f-4f intra-configurational exciton located at 312, 317–324, 363, 375, 393, 437, and 465 nm, which correspond to ⁷F₀→⁵I₆, ⁷F_0,1→⁵H_3,6, ⁷F_0,1→⁵D₄, ⁷F_0,1→⁵G₃, ⁷F_0,1→⁵L₆, ⁷F_0,1→⁵D₃, and ⁷F_0,1→⁵D₂ of the Eu³⁺-ion, respectively (Fig. 6). An observed intensity in the three metal phosphate NPs is in order GdPO₄:Eu˃YPO₄:Eu˃LaPO₄:Eu NPs. As illustrated in Fig. 6 the intensity of the exciton transitions in the case LaPO₄:Eu NPs is very weak in contrast to the GdPO₄:Eu and YPO₄:Eu NPs. It depends on the charge transfer and crystallinity of the host lattice, which assists in promoting the luminescent efficiency of the doped luminescent center ions.

Close

Fig. 6

Excitation spectra of the LaPO₄:Eu, GdPO₄:Eu, and YPO₄:Eu NPs.

Export to PPT

Emission spectra were monitored under excitation from the e_x = 393 nm wavelength in the range from 450 to 750 nm wavelength as shown in Fig. 7a&b. The emission spectra in Fig. 7 demonstrated the most intensive luminescence lines located at 467, 534, 590, 612, 644, and 696 nm, which are labeled as ⁵D₁→⁷F₁, ⁵D₁→⁷F₃, ⁵D₀→⁷F₁, ⁵D₀→⁷F₂, ⁵D₀→⁷F₃, and ⁵D₀→⁷F₄, of the 4f-4f intra-configurational Eu³⁺-ion most emissive transitions, respectively(Ansari, 2017). The emission spectra of the three metal phosphate NPs display the two most intensive emission transitions in the middle of the spectrum, which are assigned as ⁵D₀→⁷F₁ magnetic-dipole, and ⁵D₀→⁷F₂ electric-dipole emission lines. The most noticeable emission peak is the so-called hypersensitive red luminescent transition, which is positioned at 612 nm (⁵D₀→⁷F₂). The electric dipole (⁵D₀→⁷F₂) transition is more robust than the corresponding magnetic-dipole (⁵D₀→⁷F₁) transition in the luminescent spectrum if the Eu³⁺-ion is in a low symmetry location, which indicates it lacks an inversion center(van Hest et al., 2017). However, the position symmetry at which the Eu³⁺-ion is located does not affect the magnetic dipole (⁵D₀→⁷F₁) transition (Yaiphaba et al., 2010). The luminescence spectra of the GdPO₄:Eu and YPO₄:Eu NPs illustrated the most sensitive electric dipole (⁵D₀→⁷F₂) emission transition(Zhang et al., 2011). In which, magnetic-dipole (⁵D₀→⁷F₁) emission transition is less sensitive as exhibited in Fig. 7a&b. It implies that the doped Eu³⁺-ion in the GdPO₄ and YPO₄ crystal matrix is present in the asymmetric position in both host materials(Zhang et al., 2010, Sahu et al., 2012).

Close

Fig. 7

(a&b) Emission spectra of the LaPO₄:Eu, GdPO₄:Eu, and YPO₄:Eu NPs.

Export to PPT

Although in the case of LaPO₄:Eu NPs magnetic-dipole and electric-dipole, both transitions showed equal sensitivity, even though the efficiency of the magnetic-dipole emission transition is slightly greater in comparison to the electric-dipole transition, it endorsed that the Eu³⁺-ion exists in the host lattice in inversion symmetry with C₁ space group. These results are quite similar to the previous reports (Buissette et al., 2004, Lai et al., 2009). Rambabu et al. described that the enhancement in the magnetic-dipole (⁵D₀→⁷F₁) emission peak in contrast to the electric-dipole (⁵D₀→⁷F₂) emission peak caused the easy charge transfer between the host and doped cations(Rambabu and Buddhudu, 2001). Similar observations were reported by Yu et al., an alteration in the emission intensity of the two most sensitive luminescence transitions of Eu³⁺-ion(Yu et al., 2004). Despite low-temperature preparation, the primary Eu³⁺ positions symmetry is similar to that of the macro-size material(Ferhi et al., 2009). As exhibited in Fig. 7 the intensity ratio of the emission transitions indicated that a large number of Eu³⁺ ions inhibit the inversion position in the LaPO₄:Eu NPs(Phaomei et al., 2010). The host lattice diameter, shape, crystal phase, and lattice locations for Eu³⁺ ions are significant variables that significantly distress luminescent efficiency. The La³⁺ site that exhibits C₁ point group symmetry in the monoclinic crystal structure of LaPO₄ has non-inversion symmetry(Lai et al., 2009, Phaomei et al., 2010).

It is a fact that due to small atomic size differences, Eu³⁺ ions (1.06 Å) and La³⁺ ions (1.16 Å) can both occupy the same locations in a crystal. The emission efficacy of the materials is greatly influenced by the excellent crystallinity and crystal shape of the ceramic materials(Phaomei et al., 2010). It is a proven fact that good crystalline structure constantly favors increasing emission and excitation efficiency. Small grains and a large surface area allow for easy H₂O molecule adsorption on their exterior, which also allows for the coordination of organic moieties. Considering that metal phosphate NPs are made in an aqueous solvent and that hydroxyl groups may be present on their surface, these findings are also supported by the observed FTIR spectral results. Surfaces adsorbed high vibrational energy either H₂O or other organic functional group molecules quenched the luminescence intensity owing to the non-radiative recombination trap molecules that are accountable for suppressing the emission efficiency of the materials. As observed in Fig. 7, the emission spectra display a feeble intensity of the emission transition situated at ∼ 534 nm corresponds to the (⁵D₁→⁷F₁) transition, which seems caused by the LnPO₄ host lattice's low vibrational energy (350 cm⁻¹). This finding suggests that the host material protects the Eu³⁺ ion from the nearby H₂O molecules, because high phonon energy (3500 cm⁻¹) molecules, effectively quenched the emission efficiency of the doped luminescent ion.

In a comparative spectral study, the emission transitions in the GdPO₄:Eu NPs are significantly highly sensitive in comparison to the YPO₄:Eu and LaPO₄:Eu NPs(Lai et al., 2014). It is a fact that the Gd³⁺ ion displays strong absorption transition located at ∼ 250, and 270 nm assigned to ⁸S→⁶D and ⁸S_7/2→⁶I_j transitions overlap on the ⁵D₀ level of the Eu³⁺ ion as appeared in the luminescent spectrums, that effectively charge transfer from Gd³⁺-ion to the doped Eu³⁺-ion leading to enhance the emission efficiency, similar observations also reported in a previous report(Fig. 7b) (Buissette et al., 2004, Rodriguez-Liviano et al., 2013). Because yttrium and lanthanum both are diamagnetic because their 4d subshell is filled and the absence of the 4f-subshell. However, the observed emission spectral results suggested that the LaPO₄ is not a suitable host for doping of the Eu³⁺-ion in comparison to the GdPO₄ and YPO₄ host lattice. However, the emission and excitation findings suggested that the GdPO₄:Eu³⁺ is the best host matrix for producing excellent emission efficiency.