Synthesis, growth and photoluminescence behaviour of Gd₂O₂SO₄:Eu³⁺ nanophosphors: the effect of temperature on the structural, morphological and optical properties

Abstract

Gd₂(SO₄)₃·8H₂O, Gd₂O₂SO₄, and Gd₂O₂SO₄:Eu³⁺ nanoparticles have been synthesized in the presence of Gd³⁺ ions and sodium dodecyl sulphate (SDS) by the simple complexation-thermal decomposition (CTD) method. The structural analysis, growth mechanism and optical properties of the Gd₂(SO₄)₃·8H₂O, and Gd₂O₂SO₄ are described by the diffraction pattern, functional group analysis, Raman, morphology, elemental analysis, and absorbance spectra. The most intriguing factor was that the Gd₂O₂SO₄ nanoplates are in the range of 42–50 nm without adding any external stabilizer. The results revealed that the Gd₂O₂SO₄ nanoparticles with an orthorhombic structure have a band gap of 3.12 eV. Furthermore, Gd₂O₂SO₄ shows an intense red photoluminescence associated with the ⁵D₀ → ⁷F₂ transition in the presence of Eu³⁺. The results suggest that the Gd₂O₂SO₄:Eu³⁺ nanophosphors, may have a beneficial approach in the field of biomedical application as luminescent probe/labels.

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1. Introduction

Phosphors have stimulated intensive interest within research in the last few years, due to their technological importance and scientific application. Rare earth (RE) phosphors with unique physico-chemical properties (magnetic, electrical, electronic, optical and so forth) makes them critically important in magnets, superconducting and electronic materials, medical imaging and cancer therapy, pigments, phosphor screens and fluorescent materials.^1–6 It is important to systematically research the rare earth host in different kinds of crystals with good structural, morphology, and optical properties as efficient emitting materials. The optical activation of RE ions doped crystals can be enhanced significantly because of energy transfer from the host and charge transfer to RE.⁷ Especially, Eu³⁺ ion (4f⁶) results very intense luminescence in the red region (610–620 and 690–710 nm). The red emission is included to the biological window, which can be used in many display applications. These red emitter nanoparticles (RENPs) are used in biomedical applications as luminescence probes/labels. In particular, the nanostructured rare-earth compounds with high unpaired f-electron spin values have been applied to the biomedical field of therapeutic agent, MRI contrast agent, single phase multifunctional bio-probes and drug delivery host matrix for upconversion fluorescence.^8–11

Rare earth oxysulfate (RE₂O₂SO₄) has considered as a host matrix for activates the electronic transition with the Eu³⁺ ions to reveals luminescence. Not only RE₂O₂SO₄, but also several fluorides,¹² molybdates,¹³ phosphates,¹⁴ and sulfites¹⁵ have also been reported as the host crystals of phosphor. Amongst, RE₂O₂SO₄ has some unique chemical and optical features, such as low toxicity, sharp emission lines, high magnetic moment due to the seven unpaired electrons (⁸S_7/2), and long lifetimes.^16–18 It was well known that, the behaviour of nanostructured materials purely depends on their particle size and shape.¹⁹ In addition, it has been widely reported that the bulk to nanoscale size reduction play the major role on the physico-chemical properties of various nanomaterials.²⁰ synthesis of advanced materials with controlled size and/or polydispersity to obtain improved or novel properties by using colloidal assemblies as templates.²¹ Anionic surfactant like sodium dodecyl sulphate (SDS) was good candidate for controlling the size and shape of nanoparticles in various methods including thermal and nonthermal method.^22,23 Gadolinium, the most abundant element in the rare-earth metal family has notable importance, because of its high magnetic moment due to the seven unpaired electrons (⁸S_7/2). However, relatively few publications about the synthesis, controlled size and growth mechanism of Gd₂O₂SO₄ have been reported and the resulting products are limited to needle-like hierarchical and plate morphology. The focus of this work has been to explore the structural, growth, and optical property of size-reduced Gd₂O₂SO₄ obtained by the absence of any external capping agents. Interestingly, dodecyl sulphate itself acts as a complexing agent as well as size controller. To the best of our knowledge, the investigation on growth mechanism of gadolinium oxysulfate from gadolinium dodecyl sulphate by the simple complexation-thermal decomposition (CTD) method has not been reported so far. In this present work, Gd₂(SO₄)₃·8H₂O, Gd₂O₂SO₄ nanoparticles and Gd₂O₂SO₄:Eu³⁺ nanophosphors were synthesized by the simple CTD method. The synthetic experiments indicate that the formation of gadolinium dodecyl sulphate needle-like hierarchical nanostructure is through the reaction between Gd³⁺ and DS⁻ ions by an orientation mechanism. The oriented nanoneedles deformed to plate-like nanostructure by the Ostwald ripening method. Growth mechanism, crystal structure, optical band gap, morphology, and photoluminescence property of Gd₂O₂SO₄ have been investigated.

2. Experimental section

2.1. Materials

Gadolinium(III) nitrate hexahydrate (Gd(NO₃)₃·6H₂O, 99.9%), and europium(III) acetate hydrate (Eu(CH₃CO₂)₃·H₂O, 99.9%) was purchased from Sigma-Aldrich. Sodium dodecyl sulphate (C₁₂H₂₅NaO₄S, 99%) and ethanol were purchased from SRL India Ltd., and used as received. Double distilled water used as solvent throughout the experiment.

2.2. Synthesis of the gadolinium dodecyl sulphate (GdDS), gadolinium sulphate and gadolinium oxysulfate (GdOS)

Gadolinium dodecyl sulphate was synthesized by reacting stoichiometric amount of gadolinium nitrate and SDS. 0.45 g of gadolinium nitrate was dissolved in 150 mL distilled water and stirred for 20 min. After 20 min, 0.83 g of SDS in 50 mL distilled water was added slowly into the gadolinium nitrate solution. After few seconds the colourless-cloudy colloidal particles were formed. The resulting colourless foam-like product was filtered and washed with de-ionized water. The obtained gadolinium dodecyl sulphate was redispersed in ethanol and evaporated. Further, gadolinium sulphate (Gd₂(SO₄)₃), and gadolinium oxysulfate (Gd₂O₂SO₄) were obtained by decomposing the gadolinium dodecyl sulphate complex in air atmosphere at 400 °C and 900 °C for 2 h, respectively. The obtained samples were collected for further characterization.

2.3. Synthesis of the europium-doped gadolinium oxysulfate (EuGdOS)

5% (w/w) europium-doped gadolinium oxysulfate (Gd_0.95Eu_0.05)₂O₂SO₄ was synthesized by the above mentioned CTD method. 0.43 g of gadolinium nitrate and 0.02 g of europium acetate was dissolved in 150 mL distilled water, which was homogenized for 20 min with stirring. After 20 min, 0.83 g of SDS in 50 mL distilled water was added slowly into the europium–gadolinium nitrate solution. The obtained gadolinium oxysulfate (EuGdOS) by the above mentioned procedure was collected for further analysis.

2.4. Characterization techniques

Crystal structure, crystallite size and lattice parameter of the products were determined by Rich Siefert 3000 diffractometer with Cu Kα₁ radiation (λ = 1.5406 Å). Fourier Transform Infrared spectra (FTIR) were recorded on a Perkin Elmer FTIR spectrophotometer in the region of 4000 to 400 cm⁻¹. The morphology was analyzed by HITACHI SU6600 (FE-SEM) Field Emission Scanning Electron Microscopy coupled with Energy Dispersive X-ray analysis (EDAX) and High Resolution Transmission Electron Microscopy (HRTEM) using a FEI TECNAI G² model T-30 at accelerating voltage of 250 kV. Raman spectrum was recorded using laser confocal microscope, Raman-11 Nanophoton Corporation, Japan and Ultraviolet-Visible Diffuse Reflectance spectroscopy (DRS-UV-Vis) was recorded using Perkin Elmer lambda650 spectrophotometer. Photoluminescence spectra were recorded using a Perkin-Elmer LS45 luminescence spectrophotometer.

3. Results and discussion

Gadolinium oxysulfate nanoparticles were synthesized by a simple approach involving calcination of gadolinium dodecyl sulphate complex. The dodecyl sulphate (DS⁻) self-assembled with gadolinium nitrate by complexation method was re-dissolved in ethanol to form a transparent colloidal solution under ambient condition. GdDS complex results numerous products including gadolinium sulphate and gadolinium oxysulfate at various condition. The graphical representation of possible growth mechanism was shown in Scheme 1. The X-ray diffraction (XRD) pattern of the prepared sample calcined at (a) 400 °C, (b) 700 °C, and (c) 900 °C were given in Fig. 1. The powder XRD pattern in Fig. 1a, all the diffraction peaks could be indexed to the end centered monoclinic phase matched with standard value (JCPDS no. 081-1794) of Gd₂(SO₄)₃·8H₂O, with C2/c (15) space group. The lattice constants were calculated using the XRD pattern, a = 11.97 Å, b = 7.09 Å, c = 17.95 Å. In Fig. 1b, the mixed phase of (a) and (c) occur in the calcination at 700 °C, which gives clue about the phase transformation from Gd₂(SO₄)₃ to GdO₂SO₄, which has grown on (013) plane. When the temperature was raised to 900 °C, well defined diffraction peaks appeared and the lattice parameters (a = 4.04 Å, b = 4.17 Å, c = 12.97 Å) could be indexed to the orthorhombic phase of Gd₂O₂SO₄ matched with the standard value (JCPDS no. 029-0613) of Gd₂O₂SO₄, suggesting that the Gd₂(SO₄)₃ crystal structure has been converted into Gd₂O₂SO₄. The similarity of peak broadening in the entire diffraction patterns indicates there were no vast changes in crystallite size. The FTIR spectra are showed in Fig. 2 depicts the changes occurring in the GdDS under the various heat treatment conditions (100 °C to 900 °C). The obtained peaks were assigned to S–O asymmetric stretching vibration at 1236 cm⁻¹, and symmetric stretching vibration at 1060, and 1004 cm⁻¹. The peaks at 2976, 2914, 2854, and 823 cm⁻¹ were assigned as C–H stretching and bending modes.²⁴ Fig. 2 demonstrates characteristic broad intense bands in the range of 3000–3750 cm⁻¹, and 1620 cm⁻¹ were corresponds to the H–OH vibrations. The peaks near 1384 and 1523 cm⁻¹ were assigned to the impurities like C–O absorbed on the sample surface at the decomposition stages. The peaks in the region of 570 to 670 cm⁻¹ corresponds to the vibration of Gd–S bond and 526 cm⁻¹ corresponds to the vibration of Gd–O bond.²⁵ In FTIR spectrum, the deformation of H–OH and C–H and the appearance of Gd–O bond at corresponding decomposition stages clearly explain the growth mechanism of Gd₂(SO₄)₃, and Gd₂O₂SO₄ from gadolinium dodecyl sulphate.

Scheme 1 Schematic representation of the processes occurring at different stages leads to the growth of GdOS nanoplates.

Fig. 1 XRD pattern of GdDS decomposed at various temperatures (a) 400, (b) 700, and (c) 900 °C.

Fig. 2 FT-IR spectra of GdDS decomposed at various temperatures 100, 200, 300, 400, 500, 700, and 900 °C.

Fig. 3a and b shows the FESEM-EDAX images of the as prepared GdDS, which clearly indicates that the product consists of cactus needle-like nanorices assembled by orientation mechanism. The high magnification FESEM image (Fig. 3b) exhibits the formation of needle shaped particle with sharp headed tips. The inspection of images suggests that the coalescence between the particles may contribute to the growth by the orientation mechanism in the complexation method. In addition, the elemental analysis was obtained by EDAX analysis, shown in Fig. 3c, which clearly demonstrates a homogeneous distribution of gadolinium, carbon, oxygen, and sulphur elements. HRTEM images of GdDS shown in Fig. 4a and b reveals that the observed results well-matched with the FESEM results. In agreement with the FE-SEM results, the average breadth in the middle part of needles was measured in the range of 50–55 nm. Selected area energy diffraction (SAED) pattern shown in Fig. 4c reveals that the nanoneedles were highly crystalline in nature. Fig. 5a represents FESEM-EDAX images of hydrated Gd₂(SO₄)₃ at 400 °C, reveals that the particles were close to elongated spheroids with aggregation due to the temperature. Ripening at this stage leads to the formation of elongated spheroid shaped particle from melted needles. FESEM-EDAX images of sample at 700 °C shown in Fig. 5b reveals that the shape transformation from spheroid into plate shape has taken place. According to the results the possible formation mechanism of the cactus needle like nanorices, the experimental results could be explained as follows. Self-assembly of surfactant (SDS) will results the formation of oil-in-water aggregates, commonly called as micelles. To make particles, the metal ion of the surfactant (sodium) is replaced by the reactive ion (gadolinium) in the chemical reaction. The increasing concentration of SDS concentration acts as one of the reactant, which reduces flocculation and stimulates the formation of well-defined particles due to the inherent van der Waals attraction between particle and dispersion forces.^21,26 While increasing temperature at the thermal decomposition state the ripening process takes place due to the thermal diffusion of ions, which induces the structural and shape deformation. The following inspection of the results reveals that the formation of GdDS nanorices obtained by the orientation mechanism and further growth process begun to nanoplates from nanorices by the Ostwald ripening mechanism. The growth of the plate like structure was further confirmed by the FESEM-EDAX images of sample calcined at 900 °C shown in Fig. 5c, which evidences the presence of gadolinium, oxygen, sulphur and absence of carbon elements, and it was in good agreement with the results shown above.

Fig. 3 (a) FESEM image of GdDS, (b) high magnified view of GdDS, and (c) EDAX spectrum of GdDS nanoneedles.

Fig. 4 (a) HRTEM image of GdDS, (b) high magnified view of GdDS, and (c) SAED pattern of GdDS nanoneedles.

Fig. 5 FESEM-EDAX images of GdDS decomposed at various temperatures (a) 400, (b) 700, and (c) 900 °C.

Fig. 6 shows the absorption spectra of the GdDS nanoparticles calcined at different temperatures 400, 700, and 900 °C, respectively, carried out using DRS UV-Vis spectrometer, in order to characterize the optical absorbance properties. It can be seen from spectra for samples calcined at 700, and 900 °C temperatures, the UV absorption in the region of short wavelength consist of a significant broad band at 240 nm, small hump at 274 nm and the predominant peak at 344 nm. While increasing the calcination temperature of GdOS nanoparticles from 700 to 900 °C, the relative intensity of the two broad band's increases significantly, this elucidates that the intensity of the composed bands strongly depends on the calcination temperature. The absorption band near short wavelength region at 344 nm in the spectra can be related to the transition from the ground state ⁸S_7/2 to emitting state ⁶P_7/2 of Gd³⁺ ion.^27,28 The weaker absorption band located at 240 nm was due to the transition from the 2p orbitals of O²⁻ to the 4f orbitals Gd³⁺ ion, the band ascribed to the absorption of Gd₂O₂SO₄ host crystal. Another peak with weak absorption at 274 nm was attributed to the ⁸S_7/2 to ⁶I_7/2 internal f–f transition.²⁹ The band gap (E_g) of the Gd₂O₂SO₄ can be calculated from Tauc equation.

αhν = A(hν − E_g)^1/n

where α – absorption coefficient, h – Planck constant, ν – frequency, A – constant, and n = 1/2, 2, 3/2, or 3 for direct allowed transition, indirect allowed transition, direct forbidden or indirect forbidden transitions, respectively. The optical band gap of the GdDS calcined at 900 °C inferred from the Tauc plot was shown in Fig. 6. The value of hν extrapolated to α = 0 gives the band gap energy of GdOS is approximately 3.12 eV, which exhibits the nature of semiconductivity. The Raman spectra of GdDS under various heat treatment conditions (100 to 900 °C) are shown in Fig. 7, recorded at ambient condition. The bands were located in between the region of 60 to 2400 cm⁻¹ was listed in Table 1. The D and G bands in the region of 1330–1555 cm⁻¹ have disappeared after 400 °C. The disappearance of D and G bands confirms the removal of hydrocarbon from the GdDS and the formation of Gd₂(SO₄)₃ begun at 400 °C. Strong band located in the region of 980–1010 cm⁻¹ corresponds to the symmetric stretching vibration of SO₄²⁻ ions. However, little signal intensity was observed for the triply degenerate (ν_as-SO₄²⁻) modes in the region of 1080–1170 cm⁻¹ in accordance with the weak Raman transition for asymmetric stretching vibrations.^30,31 After calcination at 700 °C, appearance of new bands in the low frequency region evidences the formation of Gd₂O₂SO₄ from gadolinium sulphate. Raman spectrum obtained for GdDS decomposed at 900 °C was corresponding to the gadolinium oxysulfate, is in good agreement with the results reported.⁷ The obtained Raman results have good relationship with the discussed FTIR results. Accordingly, temperature consequences the phase transition and the phase was identified to be orthorhombic Gd₂O₂SO₄.

Fig. 6 Tauc plot (inset: DRS-UV-Vis absorption spectra) of GdDS with optimum conditions.

Fig. 7 Confocal Raman spectra of GdDS decomposed at various temperatures 100, 200, 300, 400, 500, 700, and 900 °C.

Table 1 Raman bands listed for GdDS samples decomposed at various temperatures in the region of 60 to 2400 cm⁻¹

Temperature °C	Raman bands (cm^−1)
100	339, 464, 540, 598, 998, 1097, 1357, 1531, 1761, 2030
200	566, 790, 941, 999, 1082, 1334, 1554, 2026, 2323
300	423, 564, 790, 1096, 1331, 1551, 2022, 2389
400	254, 414, 558, 987, 1097, 2325
500	149, 329, 469, 613, 998, 2330
700	618, 1002, 1072, 1137, 1557, 1756, 1975, 2030
900	125, 162, 191, 343, 473, 602, 646, 739, 794, 852, 1005, 1087, 1135, 1167, 1531, 1697, 1753, 1974, 2030, 2326

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FESEM and HRTEM images of (Gd_0.95Eu_0.05)₂O₂SO₄ nanoplates were shown in Fig. 8b and c, respectively. The HRTEM-EDX spectrum of Gd₂O₂SO₄:Eu³⁺ (Fig. 8a) confirms the presence of europium in the Gd₂O₂SO₄ host. It confirms that successful doping could be attained through current synthetic approach. The HRTEM higher magnification image was shown in Fig. 8d, which elucidate that the particles have nanoscale with plate like morphology. The average breadth of the plates was measured in the range of 42–50 nm and the length was estimated to be 520–560 nm. SAED spectrum shown in Fig. 8e explains the crystalline nature of the orthorhombic (Gd_(0.95)Eu_0.05)₂O₂SO₄ nanophosphors. Europium-doped gadolinium oxysulfate nanophosphors have been calcined for 2 h at 900 °C in order to study the Eu³⁺ emission spectra evolution in the hosts. Eu³⁺ (4f⁶) were inside orthorhombic Gd₂O₂SO₄ nanoparticles, it tends to capture an oxygen 2p electron to move towards the highly stable 4f⁷ configuration.^31,32 Therefore a low-energy state was created in the triplet state, which was well suited for energy transfer by trivalent europium f–f transition leading to the emission lines for the (⁵D₀ → ⁷F_J (J = 0 to 4) transitions. The photoluminescence spectrum of the Gd₂O₂SO₄:Eu³⁺ were shown in Fig. 9. The broad dominant peaks splits as 614, 618 nm arising from the ⁵D₀ → ⁷F₂ transition usually associated with Eu³⁺ cations occupying crystallographic sites,³³ which is exist with the displacement of Gd³⁺ by Eu³⁺ ions. The remaining bands have been assigned as ⁵D₀ → ⁷F₀ at 578.9 nm, ⁵D₀ → ⁷F₁ the group of peaks at 588, 597 nm, ⁵D₀ → ⁷F₃ at 657 nm, and ⁵D₀ → ⁷F₄ at 701 nm. The ⁵D₀ → ⁷F₀ of Eu³⁺ was a transition between two non-degenerate levels. Some weak bands were appeared around its expected location due to the presents of Eu³⁺ ions at defect sites. The 0–0 (zero phonon) line transitions between ⁵D₀ → ⁷F₀ gives information on the environment around the doped Eu³⁺ ion at low-symmetry sites in crystal.³⁴ Moreover, the presence of oxygen in the Gd₂O₂SO₄ particles gave rise to a charge transfer (at 400 nm) which could be used to excite the 4f orbital of Eu³⁺ ions more efficiently and the energy position of the lines from trivalent europium ions is basically independent on the host. Orthorhombic Gd₂O₂SO₄ nanoplates synthesized by CTD method have a long-term stability and further lanthanide sensitization indispensible and responsible for the optical properties and life science application.

Fig. 8 (a) EDX spectrum (b) FESEM image (c) HRTEM image (d) high magnified view and (e) SAED pattern of Gd₂O₂SO₄:Eu³⁺ nanoplates.

Fig. 9 Photoluminescence spectra of Gd₂O₂SO₄:Eu³⁺, inset: excitation spectrum of Gd₂O₂SO₄:Eu³⁺.

4. Conclusions

A simple method has been developed to synthesize Gd₂(SO₄)₃, and Gd₂O₂SO₄ by CTD method. The possible growth mechanism is discussed based on the FE-SEM, HRTEM, XRD, FTIR, DRS-UV-Vis, and Raman results. CTD method has a number of advantages including the simple reaction conditions, environmentally friendly, and high reproducibility. The simple CTD method offers a convenient method to synthesize nanostructured materials. This result suggests that the Gd₂O₂SO₄ nanophosphors with tailorable nanostructures were hopeful for life science applications as luminescent probe/labels. These finding are important not only for enabling life science application but also for providing sensor platform as well as optoelectronics due to its band gap and emission properties.

Acknowledgements

The authors acknowledge National Centre for Nanoscience and Nanotechnology, University of Madras for providing financial support and FE-SEM, HRTEM, DRS-UV-Vis and Raman facilities.

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