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1. Introduction

Photoluminescent (PL) materials with high quantum efficiency present practical applications in many research areas, such as optoelectronics, medicine, biolabels, physics, among others (Ferreira et al., 2018; Panayiotakis et al., 1996; Shen et al., 2010). Consequently, several inorganic matrices are studied, in which its PL property was deeply explored due to its host lattice composition, structure, morphology as well as doping and others crystal modifications (Li et al., 2021). Ideally, these materials may present well-defined characteristics such as size, optical properties, and a wide range of emission colors (Liu et al., 2016).

Moreover, visible-emitting phosphors can be achieved by doping different kinds of rare earth (RE) ions into lanthanide orthovanadates. The orthovanadate matrix absorbs in the ultraviolet region of electromagnetic spectrum due to ligand-metal charge transfer (LMCT) from the 2p orbital in O^2– to the 3d orbital in vanadate. The YVO₄ nanoparticles, as an example, are an ideal transparent host lattice for PL activators and present low toxicity in biological medium (Rivera-Enríquez and Fernández-Osorio, 2021). YVO₄ also presents relative low phonon energy, excellent thermal, mechanical, and chemical stability and high optical performance. Furthermore, the D_2d local point symmetry of the eight-coordinated Y³⁺ ion in the tetragonal crystal structure (space group D_4h) is an ideal doping site for RE³⁺ ions (Liu et al., 2015). For instance, controlled fabrication of YVO₄:Eu³⁺ nanoparticles and nanowires were achieved by microwave assisted chemical synthesis (Huong et al., 2016).

Several works related the doping of RE³⁺ ions into different types of inorganic matrices (Pinatti et al., 2015; 2016; 2019a; 2019b; Yang et al., 2018). The RE emissions arise from the 4f–4f or 5d–4f transitions from the UV to near-IR range of electromagnetic spectrum. Also, upconverting (UC) materials are an unprecedented technology which consists of absorption of two or more lower-energy photons and subsequently emission of one higher-energy photon. This strategy is specially used for solar energy materials, bioimaging, among other applications. Materials composed of Yb³⁺/Er³⁺ as activator ions can be efficiently excited using NIR (near-infrared) laser radiation to generate visible emission. For example, photostable and small YVO₄:Yb/Er upconversion nanoparticles in water were obtained and presented intense upconversion emission (Alkahtani et al., 2021). However, many of these materials present poor luminescence efficiency and/or complicated synthesis procedure, which results in no defined or irregular sizes particles (Ji et al., 2021; Kshetri et al., 2018; Sousa Filho et al., 2019; Woźny et al., 2019).

Accordingly, in this work, we report the synthesis of YVO₄:RE (RE = Eu, Tm, and Yb/Er) nanoparticles by the microwave-assisted hydrothermal (MAH) method. These nanoparticles were structurally characterized and potentially studied in terms of its PL properties. In addition, the structure, vibrational frequency and morphology are compared to rationalize the structure, morphology, and PL emissions.

2. Experimental

2.1 Synthesis

One mmol of NH₄VO₃ (99%, Sigma-Aldrich) was dissolved in 40 mL of distilled water at room temperature under magnetic stirring until the reagent was completely dissolved. Additionally, 2 mmol of Y(NO₃)₃∙4H₂O (99.999%, Sigma-Aldrich) was dissolved in 40 mL of distilled water at room temperature. RE(NO₃)₃ (RE = Eu, Tm, Yb, and Er) solutions were prepared by dissolving RE₂O₃ in aqueous hot solution of HNO₃ and evaporating the excess of acid. Stoichiometric volume of RE solutions were mixed together with the Y solution. The amount of 5 mol% of Eu³⁺, and Tm³⁺; and 5 mol%Yb³⁺/2 mol%Er³⁺ were chosen due to previous works related to maximum PL emission intensity achieved. After complete dissolution of the reactants, the V solution was mixed with the Y solution to obtain YVO₄ and with the Y/RE solution to obtain YVO₄:RE nanoparticles. Subsequently, the mixture was stirred for 10 min, and, thereafter, it was transferred to the MAH system at 160 °C for 32 min, as it was the ideal conditions for many materials obtained by this methodology. The precipitates formed were collected at room temperature, washed with distilled water until the pH was neutralized, and dried in a conventional furnace at 60 °C for 12 h. Figure 1 shows a representation of the synthesis procedure herein described.

2.2 Characterizations

The nanoparticles were structurally characterized by X-ray diffraction (XRD) patterns using a D/Max-2000PC diffractometer Rigaku (Japan) with Cu Kα radiation (λ = 1.5406 Å) in the 2θ range from 10° to 80° in the normal routine, with a scanning velocity of 2° min^–1. This unit cell was modelled using the visualization for electronic and structural analysis (VESTA) program (Momma and Izumi, 2008; 2011), version 3. Micro-Raman spectroscopy was conducted on a Horiba Jobin-Yvon (Japan) spectrometer charge-coupled device detector and argon-ion laser (Melles Griot, United States) operating at 532 nm with a maximum power of 200 mW. The ultraviolet-visible spectrophotometry (UV-vis) spectra were taken using a spectrophotometer (model Cary 5G) (Varian, USA) in diffuse-reflectance mode. Morphological analysis of the particles was recorded via field-emission scanning electron microscopy (FE-SEM) using a Carl Zeiss microscope (model Supra 35) operated at an accelerating voltage of 30 kV and a working distance of 3.7 mm. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) analysis was performed using a Jeol JEM-2100F with a field-emission gun (FEG) operating at 200 kV. For the micrographs, the samples (approximately 1 mg) were dispersed in 3 mL of distilled water and kept 15 min in the ultrasound bath. Then, one drop of the suspension was deposited on a silicon wafer, dried at room temperature and finally attached to a sample stub using carbon tape for FE-SEM analysis; and one drop of the suspension was deposited on the cupper grid and dried at room temperature for TEM analysis. Photoluminescence (PL) measurements were performed by two distinct equipment. In the first one, the samples were excited by a 355 nm laser (Cobolt/Zouk) focused on a 20 µm spot, 50 µW of power. The backscattered luminescence was dispersed by a 20 cm spectrometer with the signal detected by a charged coupled device detector (Andor technologies). In the second one, the PL spectra were carried out with 325 nm excitation source of a krypton ion laser (Coherent Innova) and 200 mW laser output, at monochromator Thermal Jarrel-Ash Monospec and a Hamamatsu R446 photomultiplier. All measurements were performed at room temperature.

3. Results and discussion

3.1 X-ray diffraction patterns

Figure 2 shows the XRD patterns of YVO₄:RE, and all the diffraction peaks can be readily indexed to the pure tetragonal YVO₄ phase (PDF No. 17-0341) (Rivera-Enríquez and Fernández-Osorio, 2021; Yu et al., 2002). The intense and sharp peaks confirm the samples are pure and present high crystallinity, as well as structural long-range order. Also, Y³⁺ site is an ideal environment with a D_2d point symmetry for RE emitter. So, effectively Y-by-RE substitution occurs in the host lattice because RE³⁺ and Y³⁺ have similar ionic radius, as widely reported by many works (Matos et al., 2016; Rivera-Enríquez and Fernández-Osorio, 2021). This substitution was not perceived on the XRD patterns due to the limitation of detection of the XRD instrument.

A representation of the unit cell for the orthorhombic YVO₄:RE nanoparticles are presented in Fig. 3. This unit cell was modelled using the lattice parameters and atomic positions, as well as the possible RE-by-Y substitution. The Y/RE coordination environment is a distorted dodecahedral [YO₈]/[REO₈] clusters, while V is a distorted tetrahedral [VO₄] cluster.

3.2 Raman spectroscopy

Figure 4 shows the room temperature Raman spectra of YVO4:RE nanoparticles excited by a green laser. Experimentally, seven active Raman modes were observed at 155, 260, 367, 482, 796, 820, and 874 cm^–1 for the YVO₄, YVO₄:5Eu, and YVO₄:5Tm samples. For the YVO₄:5Yb/2Er, six active Raman modes were observed at 328, 402, 638, 796, 818, and 871 cm^–1. Also, the YVO₄:5Yb/2Er nanoparticle present a broad PL emission, as observed in the Raman spectra, due to the Yb/Er ions. These results confirm the structural short-range order of all samples (Jayaraman et al., 1987).

3.3 UV-vis spectroscopy

Figure 5 illustrates the UV-vis diffuse reflectance spectra of the YVO₄:RE nanoparticles in the range of 275–750 nm. The samples showed absorption in the ultraviolet region at approximately 450 nm. The absorption is a result of electronic transition between the valence band (VB) formed predominantly by O 2p state, and the conduction band (CB) composed mainly by V 3d states (Yang et al., 2018). Also, the YVO₄:5Tm sample present the ³H₆→³F₃ transition, and the YVO₄:5Yb/2Er present the ⁴I_15/2→²H_J (J = 11/2 and 9/2) transitions.

The band gap energy (E_gap) values were calculated using the relation of the Kubelka–Munk and Wood Tauc function, as previously reported (Pinatti et al., 2019a), and it was obtained by linear extrapolation of the UV-vis curve in the [F(R infinity symbol)hʋ]ⁿ. versus hʋ graph. F(Rinfinity symbol) is the Kubelka–Munk function, hʋ is the photon energy, and n is a constant related to the type of electronic transition of a semiconductor (n = 0.5 for direct allowed, n = 2 for indirect allowed, n = 1.5 for direct forbidden, and n = 3 for indirect forbidden). The theoretical calculation predicts a direct allowed transition for YVO₄. Thus, the E_gap values obtained were 3.54, 3.41, 3.46, and 3.39 eV for the YVO₄, YVO₄:5Eu, YVO₄:5Tm, and YVO₄:5Yb/2Er samples, respectively (Fig. 6). These results show that the E_gap values decrease due to insertion of the RE ions, indicating that the degree of order-disorder at electronic level were affected due to Y-by-RE substitution. This behavior was previously observed in other RE doped materials and is attributed mainly by the contribution of 4f. electrons of RE³⁺ ions either to the VB or CB, which can increase the covalent bonding of V−O and reduce the E_gap. This happens because the energy level of RE³⁺ ions matches the energy level of VO₄^3-, contributing to an effective energy transfer from the VO₄^3– to the excited states of RE³⁺ ions (Yang et al., 2018).

3.4 Field-emission scanning electron microscopy and TEM

The detailed morphology and particle size of YVO₄ nanoparticles were assessed by FE-SEM, and the nanostructures were further characterized by TEM and HRTEM. Field emission scanning electron microscopy micrographs of the YVO₄ nanoparticles are shown in Fig. 7a and b. It is clearly seen spherical nanoparticles which exhibit a high degree of homogeneity in the shape and size. As shown in Fig. 7a and b, the particles present smooth surface, well-defined shape, and are mainly aggregated with a monodisperse size distribution.

Figure 7c shows the TEM image of YVO₄ nanoparticles. It was mainly observed spherical-like particles of sizes ranging from 20 to 50 nm. Most of them have perfect circular morphology, while other present small deformations. Figure 7d shows the HRTEM image of YVO₄ nanoparticles. The YVO₄ nanoparticles presented a single crystalline nature and the lattice spacing was calculated to be 0.363 nm between two adjacent lattice fringes, which could be indexed to 200 planes of zircon-type YVO₄. This is in agreement with the XRD results (Shen et al., 2010). Moreover, the other YVO₄:RE samples also showed similar morphology and as single-crystalline and this can be attributed to the similar preparative conditions and the low dopant concentration of RE³⁺ ions (data not shown).

These results confirm that nanosized YVO₄ of spherical morphology can be obtained by the MAH method at short reactional time and low temperature. Moreover, this morphology, as well as the size, are effectively acquired without the use of surfactants, templates, organic solvents, or adjustment of pH value of the medium, which is usually required to obtain homogeneous and nanosized particles.

3.5 Photoluminescence spectroscopy

Figure 8 shows the PL emission spectra at room temperature of YVO₄:RE nanoparticles under the excitation wavelength of 355 nm. Figure 8a shows the PL emission spectra of YVO₄:RE (RE = Tm, and Yb/Er) nanoparticles, presenting an intense band at 540 nm due to VO₄^3– clusters (Jin et al., 2011), and the YVO₄:5Tm nanoparticles also present the ³H₄→³H₆ transition at 806 nm. Particularly, the YVO₄:Eu nanoparticles present intense ⁵D₁→⁷F_J (J = 1 and 2) and ⁵D₀→⁷F_J (J = 1–4) transitions, which arises due to the efficient energy transfer from VO₄^3– clusters to the Eu³⁺ ions (see Fig. 8b) (Matos et al., 2016; Pinatti et al., 2019a; Saltarelli et al., 2014).

Figure 9 shows the PL emission spectra at room temperature of YVO₄:RE nanoparticles under the excitation wavelength of 325 nm, as well as the CIE chromatic diagram. Figure 9a shows the PL emission spectra of the YVO₄:5Eu nanoparticles, which presents characteristic Eu³⁺ peaks at 543, 564, 595, 622, 655, and 705 nm ascribed to the ⁵D₁→⁷F_J (J = 1 and 2), and ⁵D₀→⁷F_J (J = 1–4) transitions, respectively (Almeida et al., 2021; Pinatti et al., 2015). Figure 9b shows the PL emission spectra of the YVO₄:5Tm nanoparticles, which present characteristic Tm³⁺ peaks at 480, 548, 650, and 795 nm related to the ¹D₂→³F_J (J = 4 and 5), ¹G₄→³F₄, and ³H₄→³H₆ transitions, respectively (Pinatti et al., 2019a). Figure 9c shows the PL emission spectra of the YVO₄:5Yb/2Er nanoparticles, which present characteristic Er³⁺ peaks at 530, 555, and 671 nm attributed to the ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2 transitions, respectively (Alkahtani et al., 2021; Mahata et al., 2015; Sun et al., 2006; Woźny et al., 2018; Zhang et al., 2010). Figure 9d shows the CIE chromatic diagram and the respective positions of x, and y coordinates of the YVO₄:RE (RE = Eu, Tm, and Yb/Er) nanoparticles obtained through the PL emission spectra. The (x;y) chromatic coordinates positions are listed in Tab. 1. The YVO₄:5Eu, YVO₄:5Tm, and YVO₄:5Yb/2Er nanoparticles present intense emitting color in the red, blue, and green region of the diagram, respectively. These results confirm the pureness and brightness of the samples and can be considered as optimum materials for optical devices.

Figure 10 shows a schematic energy level diagram and a proposed energy transfer mechanism for the YVO₄:RE nanoparticles. For the YVO₄:5Eu nanoparticles, it is observed that, under excitation at 325 nm, electrons are excited from VB into the charge transfer state (CTS) of the VO43− clusters. Then, the excitation energy is transferred from the VO43− group to the 5D4 level of Eu3+ cations. Afterwards, Eu3+ cations in the populated 5D4 level undergo multiphonon relaxation to the 5D1 level that radiatively decay to the 7FJ (J = 1 and 2) levels; and to the 5D0 level that radiatively decay to the 7FJ (J = 1–4) levels. For the YVO4:5Tm nanoparticles, the excitation energy is transferred from the VO43− group to the 3P2 level of Tm3+ cations. Then, Tm3+ cations in the populated 3P2 level undergo multiphonon relaxation to the 1D2 level that radiatively decay to the 7F4 and 3H4 levels; and to the 1G4 level that radiatively decay to the 7F4 and 3H6 levels. Finally, for the YVO4:5Yb/2Er nanoparticles, the excitation energy is transferred from the VO43− group to the 4F7/2 level of Er3+ cations. Then, Er3+ cations in the populated 4F7/2 level undergo multiphonon relaxation to the 2H11/2 and 4S3/2 levels that radiatively decay to the 4I15/2 level; and to the 4F9/2 level that radiatively decay to the 4I15/2 level. Alternatively, according to the energy conservation law, a two-photon process can occur and populate the green and red UC emissions of Er3+ ions. The successive energy transfers are: 4I15/2 (Er3+) + 2F5/2 (Yb3+) → 4I11/2 (Er3+) + 2F7/2 (Yb3+) and 4I11/2 (Er3+) + 2F5/2 (Yb3+) → 4F7/2 (Er3+) + 2F7/2 (Yb3+) excite Er3+ ions to the 4F7/2 state. Er3+ ions at the 2H11/2/4S3/2 states, arising from the nonradiative relaxation (NR) process of the 4F7/2 state, radiatively decay to the 4I15/2 state, resulting the green UC emissions. The 4F9/2 red emitting state is populated by the process: 4I13/2 (Er3+) + 2F5/2 (Yb3+) → 4F9/2 (Er3+) + 2F7/2 (Yb3+), where the 4I13/2 state is populated by NR process of the 4I11/2 state (Ji et al., 2021).

4. Conclusions

In summary, we reported the efficient synthesis of YVO₄:RE nanoparticles by the microwave-assisted hydrothermal method. Long-range order was confirmed by XRD patterns, which showed sharp and well-defined peaks with no segregated materials. Vibrational Raman modes observed represent a signature of the structural organization in the short-range. The UV-vis spectra indicate that the band gap value decreases due to RE doping attesting structural order-disorder of the materials. The FE-SEM, TEM, and HRTEM images prove that the materials are spherical and in the nanoscale size. Photoluminescent emission spectra present transitions in the red, blue, and green regions, attesting these materials as good phosphors in the visible region. Also, the YVO₄:5Yb/2Er is a good candidate as promising material for UC phosphor.

Authors’ contribution

Conceptualization: Pinatti, I. M.; Foggi, C. C.; Teodoro, M. D.; Longo, E.; Simões, A. Z.; Rosa, I. L. V.

Data curation: Pinatti, I. M.; Foggi, C. C.; Teodoro, M. D.; Longo, E.; Simões, A. Z.; Rosa, I. L. V.

Formal Analysis: Pinatti, I. M.; Foggi, C. C.; Teodoro, M. D.; Longo, E.; Simões, A. Z.; Rosa, I. L. V.

Funding acquisition: Pinatti, I. M.; Foggi, C. C.; Teodoro, M. D.; Longo, E.; Simões, A. Z.; Rosa, I. L. V.

Investigation: Pinatti, I. M.; Foggi, C. C.; Teodoro, M. D.; Longo, E.; Simões, A. Z.; Rosa, I. L. V.

Methodology: Pinatti, I. M.; Foggi, C. C.; Teodoro, M. D.; Longo, E.; Simões, A. Z.; Rosa, I. L. V.

Project administration: Pinatti, I. M.; Foggi, C. C.; Teodoro, M. D.; Longo, E.; Simões, A. Z.; Rosa, I. L. V.

Resources: Pinatti, I. M.; Foggi, C. C.; Teodoro, M. D.; Longo, E.; Simões, A. Z.; Rosa, I. L. V.

Software: Not applicable.

Supervision: Longo, E.; Simões, A. Z.; Rosa, I. L. V.

Validation: Pinatti, I. M.; Foggi, C. C.; Teodoro, M. D.; Longo, E.; Simões, A. Z.; Rosa, I. L. V.

Visualization: Pinatti, I. M.; Foggi, C. C.; Teodoro, M. D.; Longo, E.; Simões, A. Z.; Rosa, I. L. V.

Writing – original draft: Pinatti, I. M.; Foggi, C. C.; Teodoro, M. D.; Longo, E.; Simões, A. Z.; Rosa, I. L. V.

Writing – review & editing: Pinatti, I. M.; Foggi, C. C.; Teodoro, M. D.; Longo, E.; Simões, A. Z.; Rosa, I. L. V.

Data availability statement

The data will be available upon request.

Funding

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP). Grant No.: 13/07296-2; 17/12594-3; 19/03722-3; 19/25944-8.

Acknowledgments

The authors would like to acknowledge Maximo Siu Li (IFSC-USP), Rorivaldo Camargo (CDMF-UFSCar), and Sandra Maria Terenzi Bellini (CDMF-UFSCar) for technical and scientific contributions.

This manuscript is dedicated to the memory of Professor Dr. José Arana Varela, pioneer with notable achievements in the fields of materials science, physics and chemistry. He was one of the founders of the Interdisciplinary Laboratory of Electrochemistry and Ceramics (LIEC), Brazil. Dr. Varela contributed significantly to the development of Brazilian science, and had an outstanding international reputation as an educator and researcher.

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