Y₂O₂S:Yb³⁺, Er³⁺ nanofibers: novel fabrication technique, structure and up-conversion luminescent characteristics

Abstract

Y₂O₃:Yb³⁺, Er³⁺ nanofibers were prepared by calcination of the electrospun polyvinyl pyrrolidone (PVP)/[Y(NO₃)₃ + Yb(NO₃)₃ + Er(NO₃)₃] composite nanofibers. For the first time, Y₂O₂S:Yb³⁺, Er³⁺ up-conversion luminescent nanofibers were successfully synthesized via inheritance of the morphology and sulfurization of the above electrospinning-derived Y₂O₃:Yb³⁺, Er³⁺ nanofibers using sulfur powders as sulfur source by a double-crucible method we newly proposed. XRD analysis indicates that Y₂O₂S:Yb³⁺, Er³⁺ nanofibers are pure hexagonal in structure with space group ${\text{P}}\overline{ 3} m 1$. Observation results of FESEM and TEM reveal that the diameters of Y₂O₂S:Yb³⁺, Er³⁺ nanofibers are 105 ± 13 nm, and Y₂O₂S:Yb³⁺, Er³⁺ nanofibers are composed of nanoparticles with the diameter ranging from 40 to 70 nm. Up-conversion emission spectrum analysis manifests that Y₂O₂S:Yb³⁺, Er³⁺ nanofibers exhibit strong green and red upconversion emission centering at 526, 548 and 668 nm, respectively. The green emissions and the red emission are respectively assigned to ²H_11/2/⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_l5/2 energy levels transitions of Er³⁺ ions. The formation mechanism of Y₂O₂S:Yb³⁺, Er³⁺ upconversion luminescence nanofibers is also proposed. More importantly, this new strategy and fabrication technique are of universal significance to prepare other rare earth oxysulfides with various morphologies.

1 Introduction

As a group of important wide-gap (4.6–4.8 eV) semiconductors, rare earth oxysulfides with high chemical and thermal stability have been extensively used as luminescent matrix owing to their high absorption of light and energy transfer efficiency, and have become one of the major optical functional materials [1, 2]. Especially, rare earth ions activated rare earth oxysulfides have become a very important family of inorganic phosphor materials [3]. Upconversion (UC) luminescence nanophosphors doped with trivalent rare earth ions have attracted considerable interests [4, 5]. As the candidate of upconversion nanomaterial, Y₂O₂S nanocrystals doped with Er³⁺ ions and sensitized by Yb³⁺ ions have been extensively studied, the sensitizer is excited by an infrared radiation (IR) source and the energy is transfered to the activator that emits a visible photon. Generally, Er³⁺ ion serves as luminescence center owing to its abundant energy levels and narrow emission spectral lines [6, 7], and it is in perfect agreement with the energy levels of Yb³⁺ ion absorption [8]. Recently, a large number of fabrication methods have already been employed to fabricate nano- and microsized Y₂O₂S:Yb³⁺, Er³⁺, such as hydrothermal and solvothermal methods [9], solid sate reaction method [10, 11], combustion method [12, 13], microwave-heating method [14, 15], etc. These methods are usually used to produce nanoparticles, nanotubes, nanowires, nanorods and nanoflowers [16–18]. In order to obtain a new morphology of Y₂O₂S:Yb³⁺, Er³⁺ nanomaterial, fabrication of Y₂O₂S:Yb³⁺, Er³⁺ upconversion luminescent nanofibers is a meaningful subject of study. To our knowledge, no reports on Y₂O₂S:Yb³⁺, Er³⁺ upconversion luminescent nanofibers are found in the literatures.

Conventionally, RE₂O₂S:Ln³⁺ (RE = Y, La, Gd; Ln = Eu, Tb) luminescent bulk materials were prepared by calcination of the mixture of respective rare-earth oxides [10, 11] or oxalate compounds [19, 20] or carbonates [21, 22], sulfur powders and flux (Na₂CO₃, Mg₂CO₃·4 Mg(OH)₂·5H₂O, TiO₂) at above 1100 °C for 2 h in a reduced or protective atmosphere. In this way, the morphology of the as-prepared bulk materials is often irregular particles. Even if rare earth oxides nanofibers are applied as precursor, the obtained rare earth oxysulfides do not inherit the peculiar morphologies of rare earth oxides using the above conventional sulfurization method because sulfur powders and flux will melt and destruct the morphologies of rare earth oxides. Therefore, rare earth oxysulfides nanofibers can not be prepared via the above conventional sulfurization method using rare earth oxides nanofibers as precursor.

Electrospinning is a simple, convenient and versatile technique to prepare long fibers with diameters ranging from tens of nanometers up to micrometers, including multifunctional composite nanofibers [23–25], rare earth oxyhalides nanofibers [26–28], metallic oxide and composite oxide nanofibers [29–31], coaxial nanofibers [32, 33], and Janus nanofibers [34–36]. However, to the best of our knowledge, there have been no reports on the preparation of rare earth oxysulfides upconversion luminescence nanofibers by electrospinning combined with sulfurization technique.

In this paper, polyvinyl pyrrolidone (PVP) was selected as template for electrospinning owing to its good solubility, high flexibility, non-toxicity, excellent spinability, easy obtainment, and low costs, etc. Y₂O₂S:Yb³⁺, Er³⁺ nanofibers were fabricated by sulfurization of the relevant Y₂O₃:Yb³⁺, Er³⁺ nanofibers which were prepared by calcination of the electrospun fibers of PVP/[Y(NO₃)₃ + Yb(NO₃)₃ + Er(NO₃)₃] composites. The morphology, structure, upconversion luminescent performance and formation mechanism of Y₂O₂S:Yb³⁺, Er³⁺ nanofibers were systematically studied, and some meaningful results were obtained.

2 Experimental sections

2.1 Chemicals

Polyvinyl pyrrolidone (PVP, Mw = 1300000, AR), yttrium oxide (Y₂O₃, 99.99 %), erbium oxide (Er₂O₃, 99.99 %) and ytterbium oxide (Yb₂O₃, 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.

2.2 Preparation of PVP/[Y(NO₃)₃ + Yb(NO₃)₃ + Er(NO₃)₃] composite nanofibers via electrospinning

Y₂O₂S:10 %Yb³⁺, x %Er³⁺ [x = 0.5, 1, 2, 3, x stands for molar ratio of Er³⁺ to (Er³⁺ + Yb³⁺ + Y³⁺)] were prepared by a method of electronspinning combined with sulfurization technique. Typical procedure of preparing Y₂O₂S:10 %Yb³⁺, 2 %Er³⁺ was indicated below. Firstly, rare earth nitrates were prepared by dissolving 0.0241 g of Er₂O₃, 0.1245 g of Yb₂O₃ and 0.6277 g of Y₂O₃ in dilute HNO₃ (1:1, volume ratio) at elevated temperatures, then dissolved in 16.1153 g of DMF, and then 1.8 g of PVP was added into the above solution under stirring for 4 h to form homogeneous transparent spinning solution. In the spinning solution, the mass ratios of rare earth nitrates, DMF and PVP were equal to 9:82:9. Subsequently, the spinning solution was electrospun at room temperature under a positive high voltage of 13 kV, the distance between the spinneret tip and the collector (Al foil) was fixed to 16 cm, and relative humidity was 60–70 %. Thus, PVP/[Y(NO₃)₃ + Yb(NO₃)₃ + Er(NO₃)₃] composite nanofibers were obtained on the collector with the evaporation of DMF.

2.3 Synthesis of Y₂O₃:Yb³⁺, Er³⁺ nanofibers

The as-prepared PVP/[Y(NO₃)₃ + Yb(NO₃)₃ + Er(NO₃)₃] composite nanofibers were annealed at a heating rate of 1 °C/min and remained for 8 h at 700 °C, then the calcination temperature was decreased to 200 °C at a rate of 1 °C/min, and then naturally down to room temperature. Thus, Y₂O₃:Yb³⁺, Er³⁺ nanofibers were obtained.

2.4 Fabrication of Y₂O₂S:Yb³⁺, Er³⁺ nanofibers

Fabrication of Y₂O₂S:Yb³⁺, Er³⁺ nanofibers were performed by Ar gas aided sulfur treatment through calcining Y₂O₃:Yb³⁺, Er³⁺ nanofibers used precursor with sulfur powder as sulfurization agent. Some sulfur powders were put into a small crucible, and carbon rods were loaded on the sulfur powders, Y₂O₃:Yb³⁺, Er³⁺ nanofibers were placed on the carbon rods, then the small crucible was placed into a big crucible, and excess sulfur powders were added into the space between the two crucibles, then the big crucible was covered with its 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 a heating rate of 5 °C/min, and then the calcination temperature was decreased to 200 °C at a rate of 5 °C/min, followed by down to room temperature naturally, and Y₂O₂S:Yb³⁺, Er³⁺ nanofibers were successfully acquired.

2.5 Characterization methods

X-ray diffraction (XRD) measurements were performed using a Rigaku D/max-RA XRD diffractometer with Cu Kα radiation of 0.15418 nm, and the operation voltage and current were respectively kept at 40 kV and 20 mA. The size and morphology of the samples were observed by a field emission scanning electron microscope (FESEM, XL-30, FEI Company). The purity of the samples was examined by OXFORD ISIS-300 energy dispersive X-ray spectrometer (EDX). Transmission electron microscopy (TEM) analysis was carried out using a JEM-2010 transmission electron microscope under a working voltage of 200 kV. The up-conversion luminescent spectra of the samples were recorded with a HITACHI F-7000 fluorescence spectrophotometer using a power-tunable 980-nm diode laser as the excitation source.

3 Results and discussion

3.1 XRD analysis

Figure 1 shows the XRD patterns of Y₂O₂S:10 %Yb³⁺ nanofibers doped with different doping concentration of Er³⁺ ions. There are some obvious diffraction peaks near 2θ = 27.2° (100), 30.6° (101), 38.7° (102), 41.1° (003), 48.2° (110), 50.3° (103), 56.1° (112), 58.0° (201), etc. All the reflection peaks can be perfectly indexed to those of the pure hexagonal phase with primitive structure of Y₂O₂S (PDF#24-1424), and the space group is ${\text{P}}\overline{ 3} {\text{m1}}$. No diffraction peaks of any other phases or impurities are detected, indicating that the Yb³⁺ and Er³⁺ ions are effectively built into the host lattice via replacing Y³⁺ ions, and the substitution of Y³⁺ with Yb³⁺ and Er³⁺ does not remarkably change the Y₂O₂S host lattice structure.

Fig. 1

XRD patters of Y₂O₂S:10 %Yb³⁺, x %Er³⁺ nanofibers with PDF standard card of Y₂O₂S

3.2 SEM, TEM and EDS analysis

Figure 2a shows the typical FESEM image of PVP/[Y(NO₃)₃ + Yb(NO₃)₃ + Er(NO₃)₃] composite nanofibers before calcination, fibers have smooth surface and uniform diameter. After annealing at 700 °C, the diameters of these nanofibers greatly decrease due to loss of the PVP and associated organic components [31], as shown in Fig. 2b, the surface morphology of Y₂O₃:10 %Yb³⁺, 2 %Er³⁺ nanofibers become rougher than those of the composite nanofibers. FESEM image of Y₂O₂S:10 %Yb³⁺, 2 %Er³⁺ nanofibers is manifested in Fig. 2c. It reveals that the morphology and diameter of Y₂O₂S:10 %Yb³⁺, 2 %Er³⁺ nanofibers are nearly similar to those of Y₂O₃:10 %Yb³⁺, 2 %Er³⁺ nanofibers. Preliminarily, we can safely conclude that the sulfur atmosphere plays an important role in retaining the morphology of the precursor nanofibers. In order to study the diameters distribution of these samples, Image-Pro Plus 6.0 software is used to measure diameters of 100 nanofibers from SEM images, and the results are analyzed with statistics. The histograms of diameters distribution of these fibers are indicated in Fig. 3. It is found from Fig. 3 that under the 95 % confidence level, the diameters of these nanofibers analyzed by Shapiro–Wilk method are normal distribution. The diameters of PVP/[Y(NO₃)₃ + Yb(NO₃)₃ + Er(NO₃)₃] composite nanofibers, Y₂O₃:10 %Yb³⁺, 2 %Er³⁺ nanofibers and Y₂O₂S:10 %Yb³⁺, 2 %Er³⁺ nanofibers are 176 ± 26, 134 ± 15 and 105 ± 13 nm, respectively.

Fig. 2

FESEM images of PVP/[Y(NO₃)₃ + Yb(NO₃)₃ + Er(NO₃)₃] composite fibers (a), Y₂O₃:10 %Yb³⁺, 2 %Er³⁺ nanofibers (b) and Y₂O₂S:10 %Yb³⁺, 2 %Er³⁺ nanofibers (c)

Fig. 3

Histograms of diameters distribution of PVP/[Y(NO₃)₃ + Yb(NO₃)₃ + Er(NO₃)₃] composite fibers (a), Y₂O₃:10 %Yb³⁺, 2 %Er³⁺ nanofibers (b) and Y₂O₂S:10 %Yb³⁺, 2 %Er³⁺ nanofibers (c)

TEM image of the Y₂O₂S:10 %Yb³⁺, 2 %Er³⁺ nanofibers is illustrated in Fig. 4. It is found that the Y₂O₂S:10 %Yb³⁺, 2 %Er³⁺ nanofibers are composed of nanoparticles with size of 40–70 nm, and the diameter of the Y₂O₂S:10 %Yb³⁺, 2 %Er³⁺ nanofibers is ca.100 nm. The result is well consistent with that of SEM analysis and further confirms that Y₂O₂S:Yb³⁺, Er³⁺ nanofibers are successfully prepared.

Fig. 4

TEM image of Y₂O₂S:10 %Yb³⁺, 2 %Er³⁺ nanofibers

EDX spectrum of Y₂O₂S:10 %Yb³⁺, 2 %Er³⁺ nanofibers is manifested in Fig. 5. The presence of Y, O, S, Yb and Er elements corresponds to Y₂O₂S:10 %Yb³⁺, 2 %Er³⁺ nanofibers and no other impurity elements are found, indicating that the pure Y₂O₂S:Yb³⁺, Er³⁺ nanofibers are obtained. Au peak is from the conductive film of Au plated on the sample for SEM observation.

Fig. 5

EDX spectrum of Y₂O₂S:10 %Yb³⁺, 2 %Er³⁺ nanofibers

3.3 Up-conversion luminescent characteristics

Upon the excitation of a 980-nm diode laser, up-conversion emission spectra of Y₂O₂S:10 %Yb³⁺ nanofibers doped with different concentration of Er³⁺ ions are obtained, as shown in Fig. 6. The nanofibers emit strong green and weak red up-conversion emissions centering at 526, 548 and 668 nm, respectively. The strong green emissions are respectively assigned to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 energy levels transitions of Er³⁺, and the red emission is attributed to the transitions of ⁴F_9/2 → ⁴I_l5/2 energy levels of Er³⁺ ions. The comparison among the up-conversion luminescence spectra of these nanofibers shows that the intensity of the green up-conversion luminescence is increased more than that of the red one, and the relative intensity ratio I_green/I_red (ca. 2, 2.11, 2.67 and 3.46 for Er³⁺-doping concentration of 0.5, 1, 2 and 3 %, respectively) increases with the increase of Er³⁺ concentration. When the Er³⁺ concentration is higher than 2 %, the concentration quenching effect occurs and thus the luminescence intensity is decreased. Therefore, the optimum molar ration of Yb³⁺ to Er³⁺ in the Y₂O₂S:Yb³⁺, Er³⁺ nanofibers is 5:1.

Fig. 6

Up-conversion emission spectra of Y₂O₂S:10 %Yb³⁺, x %Er³⁺ nanofibers excited by a 980-nm diode laser

In the upconversion (UC) emission process, the UC emission intensity (I_up) is proportional to the n power of the pump power (P) according to the following formula [37, 38]: I_up∝Pⁿ, Where n is the number of infrared photons required to absorb for emitting one visible photon. The pump power dependence of the visible emission bands was measured in these samples. A plot of natural logarithm I_up versus natural logarithm P yields a straight line with slope n, as shown in Fig. 7. It is seen that the luminescence intensities of both green and red up-conversion increase with the increase of the pump powers. For the green (548 nm) and red (668 nm) emissions, the values of n (the slope) for ⁴S_3/2 → ⁴I_15/2 (548 nm) and ⁴F_9/2 → ⁴I_15/2 (668 nm) energy levels transitions of Er³⁺ ions in Y₂O₂S:Yb³⁺, Er³⁺ nanofibers are determined to be 1.82 and 1.99, respectively. Generally, a straight line with the slope approximately equal to 2 for the up-conversion luminescence indicates two photons involve in this up-conversion luminescence process. From the above results, it is proved that both the green and red emissions of Y₂O₂S:Yb³⁺, Er³⁺ nanofibers are two-photon process.

Fig. 7

Plots of natural logarithm intensity of the up-conversion emission versus natural logarithm pumped power

In principle, four basic population mechanisms may be involved in the UC process, namely energy transfer (ET), ground state absorption (GSA), excited state absorption (ESA) and photo avalanche (PA) [39]. We can immediately rule out PA as a mechanism of up-conversion in Y₂O₂S:Yb³⁺, Er³⁺ nanofibers because no inflection point is observed in the power study. The two-photon process in Y₂O₂S:Yb³⁺, Er³⁺ nanofibers may happen via the following GSA, ESA and ET processes and their possible schematic diagram is shown in Fig. 8 [32]. When the Yb³⁺ and Er³⁺ ions are co-doped into the host lattices, the ET process become dominant because Yb³⁺ ion has a much larger absorption cross section as compared with that of Er³⁺ around 980 nm [39]. Therefore, the Er³⁺ ions are excited from the ground state to ⁴I_11/2 level by general multi-photon processed through the ET from the excited Yb³⁺ ions [⁴I_15/2 (Er) + ²F_5/2 (Yb) → ⁴I_11/2 (Er) + ²F_7/2 (Yb)]. Then the ions in ⁴I_11/2 level are raised to ⁴F_7/2 level via the ESA or the ET from Yb³⁺ ions to the excited Er³⁺ ions. The electrons in ⁴F_7/2 level undergo multi-photon relaxation to the luminescent level ²H_11/2 and ⁴S_3/2 resulting from the small energy gap between them. The radiative transition from ²H_11/2 and ⁴S_3/2 to ground state can result in the green emissions at 526 and 548 nm. For the red emissions at 668 nm, the electrons at the excited state ⁴I_11/2 relax nonradiatively to the ⁴I_13/2 level, then the electrons at ⁴I_13/2 level can be excited to the ⁴F_9/2 level by the ESA or ET processes from Yb³⁺. The red emission at 668 nm originates from the transition from the ⁴F_9/2 to ⁴I_15/2.

Fig. 8

Up-conversion mechanism and population processes in Yb³⁺ and Er³⁺ co-doped Y₂O₂S nanofibers

3.4 Possible formation mechanism for the Y₂O₂S:Yb³⁺, Er³⁺ nanofibers

Possible formation mechanism of Y₂O₂S:Yb³⁺, Er³⁺ nanofibers is shown in Fig. 9. PVP, Y(NO₃)₃,Yb(NO₃)₃ and Er(NO₃)₃ are mixed with DMF to form spinning solution. Y³⁺, Yb³⁺, Er³⁺, NO₃ ⁻ are mixed or absorbed onto PVP to form sol with certain viscosity, then PVP/[Y(NO₃)₃ + Er(NO₃)₃ + Yb(NO₃)₃] composite nanofiber are fabricated under electrospinning. PVP acts as template during the formation of Y₂O₃:Yb³⁺, Er³⁺ nanofibers. In the process of calcination, PVP is oxidized to break the chain, and then to volatilize. Nitrates are decomposed and oxidized to produce NO₂, and Y³⁺/Yb³⁺/Er³⁺ are oxidized to form Y₂O₃:Yb³⁺, Er³⁺ crystallites, many crystallites are combined into nanofiber. Then, Y₂O₃:Yb³⁺, Er³⁺ nanofiber is sulfurized using S powers as sulfurization agent. In the sulfurization process, S is gasificated at about 350 °C. With the increase of calcination temperature, gasificated sulfur reacts with Y₂O₃:Yb³⁺, Er³⁺ nanofibers to produce Y₂O₂S:Yb³⁺, Er³⁺ nanofibers. During the process, sulfur powders and Y₂O₃:Yb³⁺, Er³⁺ nanofibers are separated by carbon rods which prevent Y₂O₃:Yb³⁺, Er³⁺ nanofibers from morphology damage and also play a key role in reduction through reacting with oxygen species of Y₂O₃:Yb³⁺, Er³⁺ 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 Y₂O₃:Yb³⁺, Er³⁺ nanofibers, but also can fabricate Y₂O₂S:Yb³⁺, Er³⁺ nanofibers with pure phase at relatively low temperature. Reaction schemes for formation of Y₂O₂S:Yb³⁺, Er³⁺ nanofibers proceed as follows:

Fig. 9

Possible formation mechanism of Y₂O₂S:Yb³⁺, Er³⁺ nanofibers

$$ {\text{PVP}}/\left[ {{\text{Y}}\left( {{\text{NO}}_{ 3} } \right)_{ 3} {\text{ + Yb}}\left( {{\text{NO}}_{ 3} } \right)_{ 3} {\text{ + Er}}\left( {{\text{NO}}_{ 3} } \right)_{ 3} } \right] \, + {\text{ O}}_{ 2} \mathop\rightarrow\limits^{700^\circ{\mathrm{C}}}{\text{ CO}}_{ 2} + {\text{H}}_{ 2} {\text{O }} + {\text{NO}}_{ 2} + {\text{Y}}_{ 2} {\text{O}}_{ 3} {:}{\text{Yb}}^{{ 3 { + }}} ,{\text{Er}}^{{ 3 { + }}} $$

$$ {\text{Y}}_{ 2} {\text{O}}_{ 3}{:}{\text{Yb}}^{{ 3 { + }}} ,{\text{ Er}}^{{ 3 { + }}} {\text{ + S + C }}\mathop\rightarrow\limits^{800^\circ{\mathrm{C}}}{\text{ Y}}_{ 2} {\text{O}}_{ 2} {\text{S:Yb}}^{{ 3 { + }}} ,{\text{ Er}}^{{ 3 { + }}} {\text{ + CO}} $$

$$ {\text{Y}}_{ 2} {\text{O}}_{ 3}{:}{\text{Yb}}^{{ 3 { + }}} ,{\text{ Er}}^{{ 3 { + }}} {\text{ + S + CO }}\mathop\rightarrow\limits^{800^\circ{\mathrm{C}}}{\text{ Y}}_{ 2} {\text{O}}_{ 2} {\text{S: Yb}}^{{ 3 { + }}} ,{\text{ Er}}^{{ 3 { + }}} {\text{ + CO}}_{ 2} $$

4 Conclusions

In summary, pure hexagonal phase Y₂O₂S:Yb³⁺, Er³⁺ nanofibers with space group ${\text{P}}\overline{ 3} m 1$were fabricated via sulfurization of the electrospinning-derived cubic-phase Y₂O₃:Yb³⁺, Er³⁺ nanofibers. The morphology of Y₂O₃:Yb³⁺, Er³⁺ nanofibers can be inherited to Y₂O₂S:Yb³⁺, Er³⁺ using sulfur powders as sulfurization reagent via the double-crucible method. The diameters of Y₂O₂S:Yb³⁺, Er³⁺ nanofibers analyzed by Shapiro–Wilk method are normal distribution and are 105 ± 13 nm. Upon the excitation of a 980-nm diode laser, Y₂O₂S:Yb³⁺, Er³⁺ nanofibers emit green and red up-conversion emissions centering at 526, 548 and 668 nm, respectively attributed to the ²H_11/2/⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_l5/2 energy levels transitions of Er³⁺ ions. The double-crucible sulfurization technique we propose here is of universal importance to fabricate other rare earth oxysulfide nanomaterials with various morphologies.