Synthesis of Y₂O₂S:Eu³⁺ luminescent nanobelts via electrospinning combined with sulfurization technique

Abstract

Y₂O₂S:Eu³⁺ nanobelts were successfully prepared via electrospinning method and sulfurization process using the as-prepared Y₂O₃:Eu³⁺ nanobelts and sulfur powders as sulfur source by a double-crucible method for the first time. X-ray diffraction analysis indicated that the Y₂O₂S:Eu³⁺ nanobelts were pure hexagonal in structure with space group P $\bar{3}$ m1. Scanning electron microscope images showed that the width and thickness of the Y₂O₂S:Eu³⁺ nanobelts were ca. 6.7 μm and 125 nm, respectively. Under the excitation of 325-nm ultraviolet light, Y₂O₂S:Eu³⁺ nanobelts exhibited red emissions of predominant peaks at 628 and 618 nm, which are attributed to the ⁵D₀ → ⁷F₂ transition of the Eu³⁺ ions. It was found that the optimum doping concentration of Eu³⁺ ions in the Y₂O₂S: Eu³⁺ nanobelts was 3 %. Compared with bulk particle, Eu³⁺–O²⁻/S²⁻ charge transfer bands (260 and 325 nm) of the Y₂O₂S:Eu³⁺ nanobelts showed a blue-shift significantly. The formation mechanism of the Y₂O₂S: Eu³⁺ nanobelts was also proposed. This new sulfurization technique is of great importance, not only to inherit the morphology of rare earth oxides but also to fabricate pure-phase rare earth oxysulfides at low temperature compared with conventional sulfurization method.

Introduction

As a group of important wide-gap (4.6–4.8 eV) semiconductor materials, rare earth oxysulfides with high chemical and thermal stability have been extensively used as optical functional materials [1, 2], owing to their high absorption of light and efficient energy transfer. Among these materials, rare earth ions-activated lanthanide oxysulfides have become a very important family of inorganic phosphor materials [3], especially for Y₂O₂S:Eu³⁺, one of the most efficient phosphors [4] that have wide applications in the fields of color television tubes, field emission display, and long-lasting phosphorescence. Recently, a large number of fabrication methods have already been employed to prepare micro- and nanosized Y₂O₂S:Eu³⁺, such as hydrothermal and solvothermal method [5], solid-sate reaction method [6, 7], combustion method [8, 9], microwave-heating method [10, 11], etc. These methods led to produce Y₂O₂S:Eu³⁺ nanoparticles [12], nanotubes, nanowires, nanorods [13], and nanoflowers [14]. However, rare earth oxysulfides nanobelts were limited. Therefore, fabrication of rare earth oxysulfides nanobelts is a meaningful subject of study.

Conventionally, RE₂O₂S:Ln³⁺ (RE = Y, La, Gd; Ln = Eu, Tb) luminescent bulk materials were prepared by the calcination of the mixture of rare earth oxides [10, 11] or oxalate compounds [15, 16] or carbonates [17, 18], sulfur powders, and flux (Na₂CO₃, Mg₂CO₃·4 Mg(OH)₂·5H₂O, TiO₂) at above 1,100 °C for 2 h in a reduced or protective atmosphere. In this way, the as-prepared bulk materials often have irregular morphology and cannot inherit the peculiar morphologies of rare earth oxide precursors because sulfur powders and flux will melt and destruct the morphologies of rare earth oxides. Therefore, it is difficult to obtain rare earth oxysulfide nanobelts via the conventional sulfurization method using rare earth oxide nanobelt as a precursor.

Electrospinning is a simple, convenient, and versatile technique to prepare long fibers with diameters ranging from tens of nanometers up to micrometers, including rare earth oxyfluoride nanofibers [19], rare earth oxide, and composite oxide nanofibers and nanobelts [20–23]. However, to the best of our knowledge, there have been no reports on the preparation of rare earth oxysulfides nanobelts by electrospinning combined with sulfurization technique.

In this paper, Y₂O₂S:Eu³⁺ nanobelts were fabricated through the sulfurization of the relevant Y₂O₃:Eu³⁺ nanobelts which were prepared by the calcination of the electrospun belts of PVP/[Y(NO₃)₃ + Eu(NO₃)₃] composites. A sulfurization mechanism for the Y₂O₂S:Eu³⁺ nanobelts was proposed. Finally, we investigated the photoluminescence properties of the Y₂O₂S:Eu³⁺ nanobelts. This synthetic route is also suitable for fabricating other rare earth oxysulfide nanomaterials.

Experimental section

Chemicals

Polyvinyl pyrrolidone (PVP) (Mw = 90,000, AR), yttrium oxide (Y₂O₃) (99.99 %), and europium oxide (Eu₂O₃) (99.99 %) were purchased from Kemiou Chemical Co. Ltd. N,N-Dimethylformamide (DMF, AR) was bought from Tiantai Chemical Co. Ltd. Nitric acid (AR) and sulfur(AR) were purchased from Beihua Fine Chemical Co. Ltd. All chemicals were directly used as received without further purification.

Preparation of PVP/[Y(NO₃)₃ + Eu(NO₃)₃] composite nanobelts via electrospinning

Y₂O₂S:x % Eu³⁺ [x = 1,3,5,7, x stands for molar ratio of Eu³⁺ to (Eu³⁺ + Y³⁺)] were prepared by an electrospinning method combined with sulfurization technique. In a typical procedure of preparing Y₂O₂S:3 %Eu³⁺, rare earth nitrates were first prepared by dissolving 0.0382 g of Eu₂O₃ and 0.7913 g of Y₂O₃ in dilute HNO₃ (1:1, volume ratio) at elevated temperatures. Then, 13.9128 g of DMF was added into the rare earth nitrates, and a clear DMF solution was formed. Finally, 4 g of PVP was added into the above solution under stirring for 4 h to form homogeneous transparent precursor solution. In the precursor solution, the mass ratios of rare earth nitrates, DMF, and PVP were equal to 10:70:20. Subsequently, the precursor solution was electrospun at room temperature under a positive high voltage of 8 kV, the distance between the capillary tip and the collector (Al foil) was fixed to 15 cm, and relative humidity was 60 ~ 80 %. PVP/[Y(NO₃)₃ + Eu(NO₃)₃] composite nanobelts were obtained on the collector with the evaporation of DMF.

Preparation of Y₂O₃:Eu³⁺ nanobelts

The as-prepared PVP/[Y(NO₃)₃ + Eu(NO₃)₃] composite nanobelts were annealed at 700 °C for 8 h with a heating rate of 1 °C/min. Then, the calcination temperature was decreased to 200 °C at a rate of 1 °C/min. Finally, samples were down to room temperature naturally and Y₂O₃:Eu³⁺ nanobelts were obtained.

Fabrication of Y₂O₂S:Eu³⁺ nanobelts

Y₂O₂S:Eu³⁺ nanobelts were prepared by an Ar gas-aided sulfur treatment through calcining Y₂O₃:Eu³⁺ nanobelts precursor using sulfur powder as a sulfurization agent. Some sulfur powders were put into a small crucible, the sulfur powders were covered by some carbon rods, and the Y₂O₃:Eu³⁺ nanobelts were placed on the carbon rods. Next, this small crucible was put into a big crucible, some sulfur powders were loaded into the space between the two crucibles, and the big crucible was covered with the lid. We call this process as a double-crucible method. The crucibles were annealed at 800 °C for 4 h under Ar gas atmosphere at the heating rate of 5 °C/min. Then, the calcination temperature was decreased to 200 °C at a rate of 5 °C/min followed by decreasing it down to room temperature naturally. Thus, Y₂O₂S:Eu³⁺ nanobelts were successfully acquired.

Synthesis of Y₂O₂S:Eu³⁺ particles

For comparison, we prepared Y₂O₂S:Eu³⁺ bulk particles through a conventional solid-state reaction method according to the Ref. [24]. The mean size of particles is ca. 6 μm.

Characterization methods

X-ray diffraction (XRD) measurements were carried out using a Rigaku D/max-RA XRD diffractometer with Cu Kα radiation of 0.15418 nm. The size and morphology of the products were observed by a field emission scanning electron microscope (FESEM, XL-30, FEI Company). The purity of the products was examined using OXFORD ISIS-300 energy dispersive spectrometer (EDS). Transmission electron microscope (TEM) analysis was performed using a JEM-2010 transmission electron microscope under a working voltage of 200 kV. The excitation and emission spectra of samples were recorded using a HITACHI F-7000 Fluorescence Spectrophotometer using a Xe lamp as the excitation source.

Results and discussion

XRD analysis

Figure 1 shows the XRD patterns of the as-prepared Y₂O₃:3 %Eu³⁺ nanobelts and Y₂O₂S:3 %Eu³⁺ nanobelts with different doping concentration of Eu³⁺ ions. All the diffraction peaks of the Y₂O₃:3 %Eu³⁺ (Fig. 1a) can be readily indexed to those of the pure cubic phase with primitive structure of Y₂O₃ (PDF#43-0661), space group is Fm $\bar{3}$ m. No other phases are identified. The reflection peaks of the Y₂O₂S:Eu³⁺ (Fig. 1b–e) can be perfectly indexed as the pure hexagonal phase of Y₂O₂S (PDF#24-1424), space group is Pm $\bar{3}$1. No diffraction peaks of any other phases or impurities are also detected. These results indicate that the pure-phase Y₂O₃:Eu³⁺ could be obtained after the PVP/[Y(NO₃)₃ + Eu(NO₃)₃] composite were calcined at 700 °C for 8 h and the hexagonal Y₂O₂S:Eu³⁺ were acquired via the sulfurization of the cubic Y₂O₃:Eu³⁺ at 800 °C for 4 h. The doping of Eu³⁺ did not change the Y₂O₂S host lattice structure.

Fig. 1

XRD patterns of Y₂O₃:3 %Eu³⁺ nanobelts (a) and Y₂O₂S: x %Eu³⁺ [x = 1(b), 3(c), 5(d), 7(e)] nanobelts with PDF standard cards of Y₂O₃ (43-0661) and Y₂O₂S(24-1424)

SEM, TEM, and EDS analysis

Figure 2a shows the typical SEM image of PVP/[Y(NO₃)₃ + Eu(NO₃)₃] composite before calcinations. One can see that the composite is composed of nanobelts with smooth surface and uniform width, indicating the formation of nanobelt. After annealing at 700 °C, these nanobelts experience about 70 % reduction in width due to loss of the PVP and associated organic components [22], as shown in Fig. 2b. The obtained Y₂O₃:3 %Eu³⁺ nanobelts still have smooth surface. Figure 2c shows the SEM image of the Y₂O₂S:3 %Eu³⁺ nanobelts. It reveals that morphology and the widths of Y₂O₂S:3 %Eu³⁺ nanobelts are nearly similar to those of Y₂O₃:3 %Eu³⁺ nanobelts. Preliminarily, we can conclude that the sulfur atmosphere plays an important role in keeping the morphology of the nanobelts. The thickness of Y₂O₂S:3 %Eu³⁺ nanobelts becomes thinner due to the decomposition of the organic species and the formation of inorganic phase. The thickness of PVP/[Y(NO₃)₃ + Eu(NO₃)₃] composite nanobelts, Y₂O₃:3 %Eu³⁺ nanobelts, and Y₂O₂S:3 %Eu³⁺ nanobelts are 494, 172, and 125 nm, respectively. Under the 95 % confidence level, the widths of those nanobelts analyzed by Shapiro–Wilk method are normally distributed. Histograms of widths of these belts are shown in Fig. 3. The widths of PVP/[Y(NO₃)₃ + Eu(NO₃)₃] composite nanobelts, Y₂O₃:3 %Eu³⁺ nanobelts, and Y₂O₂S:3 %Eu³⁺ nanobelts are 7.2 ± 0.6, 6.6 ± 0.5, and 6.7 ± 0.6 μm, respectively.

Fig. 2

SEM images of PVP/[Y(NO₃)₃ + Eu(NO₃)₃] a Composite nanobelts, b Y₂O₃:3 %Eu³⁺ nanobelts, and c Y₂O₂S:3 %Eu³⁺ nanobelts

Fig. 3

Histograms of widths of PVP/[Y(NO₃)₃ + Eu(NO₃)₃] a Composite nanobelts, b Y₂O₃:3 %Eu³⁺ nanobelts, and c Y₂O₂S:3 %Eu³⁺ nanobelts

Figure 4a illustrates TEM images of the Y₂O₂S:3 %Eu³⁺ nanobelts. It indicates that the nanobelts composed of nanoparticles with size of 10–30 nm. The corresponding selected area electron diffraction (SAED) patterns in Fig. 4b exhibit typical polycrystal diffraction patterns and are in good agreement with those of pure Y₂O₂S phase. These results further confirm the formation of the Y₂O₂S:Eu³⁺ nanobelts.

Fig. 4

a TEM image and b SAED pattern of Y₂O₂S:3 %Eu³⁺ nanobelts

EDS spectrum of the Y₂O₂S:3 %Eu³⁺ nanobelts (Fig. 5) reveals the presence of Y, O, S, and Eu elements, and no other impurity elements including carbon, indicating that the pure Y₂O₂S:3 %Eu³⁺ nanobelts are obtained. Au peak is from the conductive film of Au plated on the sample for SEM observation.

Fig. 5

EDS spectrum of Y₂O₂S:3 %Eu³⁺ nanobelts

Photoluminescence properties

Figure 6 illustrates the excitation (monitored by 628 nm) and emission (excited by 325 nm) spectra of the different Y₂O₂S:Eu³⁺ nanobelts. It is found from Fig. 6a that there is a wide band with two peaks at 260 and 325 nm, which are attributed to Eu³⁺–O²⁻ CTB (charge transfer band) and Eu³⁺–S²⁻ CTB, respectively. In the longer wavelength region (400–500 nm), the f–f transition peaks of the Eu³⁺ ions can be observed with very weak intensity compared with those of the CTB. The strongest intensity of the excitation spectrum (CTB) is obtained when the doping concentration of Eu³⁺ ion is 3 %. The matrix absorption band at 247 nm is also observed.

Fig. 6

a Excitation and b emission spectra of Y₂O₂S:x Eu³⁺ (x = 1, 3, 5, 7 %) nanobelts

Figure 6b demonstrates the emission spectrum of the Y₂O₂S nanobelts doped with various concentration of Eu³⁺. When the Eu³⁺ content is greater than 3 %, the concentration quenching effect occurs, as shown in the Fig. 7. At this time, the bond length between Eu³⁺ and O²⁻ becomes shorter and the mixture of wave function of Eu³⁺ and O²⁻ is enhanced [25], therefore, shows the strongest luminescence at 3 %. It is remarkably seen that the main emission peaks between 550 and 650 nm are observed from Fig. 6b, which is in agreement with the results reported by Kader and Elkholy [26]. The emission spectrum is associated with the transitions from the excited level of the ⁵D_J (J = 0, 1) to the level of ⁷F_J (J = 1, 2) of the Eu³⁺ ions. The strongest red emission split into two peaks at 628 nm and 618 nm are ascribed to the ⁵D₀ → ⁷F₂ transition of the Eu³⁺ ions, indicating that non-Y₂O₃ phase exists and the oxysulfide host has been formed [26]. Among several luminescence transitions of Eu³⁺ ion, the ⁵D₀ → ⁷F₁ (~ 592 nm) is mainly a magnetic-dipole transition, while the ⁵D₀ → ⁷F₂ (~628 nm) is an electric-dipole transition which is closely related to the coordination environment around Eu³⁺. The stronger ⁵D₀ → ⁷F₂ transition suggests that the Eu³⁺ ions occupy the sites at low symmetries without inversion center since this transition is hypersensitive to crystal structure and chemical surroundings [11, 27, 28]. The other emission peaks at 558 and 566 nm are attributed to the electron transition from the ⁵D₁ to ⁷F₂ level of the Eu³⁺ ions. It is clearly indicated from these emissions that the Eu³⁺ ions have been effectively distributed into the Y₂O₂S matrix.

Fig. 7

Change of the emission intensity of peak at 628 nm (⁵D₀ → ⁷F₂) with Eu³⁺ doping concentrations

Compared with the Y₂O₂S:Eu³⁺ bulk particles, the excitation bands (Eu³⁺–O²⁻/S²⁻CTB) in the Y₂O₂S:Eu³⁺ nanobelts show apparent blue-shift, as shown in Fig. 8a. This result can be possibly explained by size-dependent changes associated with quantum confinement effect in this wide-gap semiconductor materials [18]. Figure 8b shows emission spectra of the Y₂O₂S:Eu³⁺ bulk particles and the Y₂O₂S:Eu³⁺ nanobelts; it is clearly seen that the emission intensity of bulk particles is obviously stronger than that of nanobelts due to more surface defects of the Y₂O₂S:Eu³⁺ nanobelts. All results are in good agreement with that of the Ref. [29, 30].

Fig. 8

a Excitation and b emission spectra of Y₂O₂S:3 %Eu³⁺ bulk particles and nanobelts

Formation mechanism for the Y₂O₂S:Eu³⁺nanobelts

Formation mechanism of Y₂O₂S:Eu³⁺ nanobelt is shown in Fig. 9. PVP, Y(NO₃)₃, and Eu(NO₃)₃ were mixed with DMF to form precursor solution. Y³⁺, Eu³⁺, and NO₃ ⁻ were mixed or absorbed onto PVP to form sol with certain viscosity. Then, PVP/[Y(NO₃)₃ + Eu(NO₃)₃] composite nanobelts were fabricated via electrospinning. PVP acted as template during the formation of Y₂O₃:Eu³⁺ nanobelts. In the process of calcination, PVP was oxidized to break the chain and then to volatilize. Nitrates were decomposed and oxidized to produce NO₂, and Y³⁺/Eu³⁺ was oxidized to form Y₂O₃:Eu³⁺ crystallites; many crystallites were combined into nanobelt. In the sulfurization process, Y₂O₃:Eu³⁺ nanobelt was sulfurized using S as a sulfurization agent and S was gasified at about 350 °C. With the increase of calcination temperature, gasified sulfur reacts with Y₂O₃:Eu³⁺ nanobelts to produce Y₂O₂S:Eu³⁺ nanobelt. During the reaction process, sulfur powders and Y₂O₃:Eu³⁺ nanobelts were separated by carbon rods which prevented Y₂O₃:Eu³⁺ nanobelts from the morphology damage and also played a key role in reduction through reacting with oxygen species of Y₂O₃:Eu³⁺ in the heating process. The double-crucible method we proposed here is actually a solid–gas reaction, which has been proved to be an important method, not only can retain the morphology of the Y₂O₃:Eu³⁺ nanobelts but also can fabricate the Y₂O₂S:Eu³⁺ nanobelts with pure phase at relatively low temperature. Reaction schemes for the formation of the Y₂O₂S:Eu³⁺ nanobelts proceed as follows:

Fig. 9

Formation mechanism of Y₂O₂S:Eu³⁺ nanobelt

$$ {\text{PVP}}/\left[ {{\text{Y}}{\left( {{\text{NO}}_{3}}\right)}_{3}+{\text{Eu}}{\left( {{\text{NO}}_{3}}\right)}_{3}} \right]+{\text{O}}_{2} \to {\text{CO}}_{2} +{\text{H}}_{2}\text{O}+ {{\rm NO}}_{2} +{\text{Y}}_{2} {\text{O}}_{3}{:}{\text{Eu}}^{3+}$$

(1)

$$ {\text{Y}}_{ 2} {\text{O}}_{ 3} {:} {\text{Eu}}^{{ 3 { + }}} +{\text{S}} + {\text{C}}\mathop{\longrightarrow}\limits^{{800\,^{\circ}{\text{C}}}}{\text{Y}}_{ 2} {\text{O}}_{ 2} {\text{S:Eu}}^{{ 3 { + }}} +{\text{ CO}} $$

(2)

$$ {\text{Y}}_{ 2} {\text{O}}_{ 3} {:} {\text{Eu}}^{{ 3 { + }}} +{\text{S}} + {\text{CO}}\mathop{\longrightarrow}\limits^{{800\,^{\circ} {\text{C}}}}{\text{Y}}_{ 2} {\text{O}}_{ 2} {\text{S:Eu}}^{{ 3 { + }}} +{\text{ CO}}_{ 2} $$

(3)

Conclusions

In summary, pure hexagonal phase Y₂O₂S:Eu³⁺ nanobelts with space group P $\bar{3}$ m1 were fabricated via the sulfurization of the cubic Y₂O₃:Eu³⁺ nanobelts. The morphology of the Y₂O₃:Eu³⁺ nanobelts precursor can be inherited to Y₂O₂S:Eu³⁺ nanobelts using sulfur powders as sulfurization reagent via a double-crucible method. The width and thickness of the Y₂O₂S:Eu³⁺ nanobelts are 6.7 ± 0.6 μm and 125 nm, respectively. PL measurements demonstrate the strongest emission peak is at 628 nm, originating from the ⁵D₀ → ⁷F₂ transition of the Eu³⁺ ions, indicating that the Eu³⁺ ions occupy a site without inversion center in the Y₂O₂S:Eu³⁺ nanobelts. The strongest luminescence is obtained when doping concentration of Eu³⁺ is 3 %. Compared with Y₂O₂S:Eu³⁺ bulk particles, the Y₂O₂S:Eu³⁺ nanobelts show a significant blue-shift in Eu³⁺–O²⁻/S²⁻ CTB. The double-crucible method proposed here is of great importance. This technique can be employed to fabricate rare earth oxysulfide nanomaterials with various morphologies.