Structural and Optical Properties of ZnWO₄:Er³⁺ Crystals

Abstract

Erbium ion- (Er³⁺-) doped ZnWO₄ crystals were synthesized via a hydrothermal method with different erbium concentrations. The prepared materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman, UV-Vis, and photoluminescence (PL) spectroscopy. For ZnWO₄:Er³⁺ crystals, a pure phase of ZnWO₄ was obtained without any evidence of impurity present. The SEM images show that the grain size and morphology of ZnWO₄:Er³⁺ material depends on the Er³⁺-dopant concentration. The UV-Vis spectra of ZnWO₄:Er³⁺ compounds exhibited an absorption band at about 323 nm (3.83 eV) stemming from the [WO₄]²⁻. Other absorption bands centered at 367, 379, 408, 490, and 522 nm are related to Er³⁺-ion transitions. Room temperature PL spectra of the ZnWO₄:Er³⁺ compounds exhibited visible emission at 515–540 and 545–565 nm corresponding to the ²H_11/2−⁴I_15/2 and ⁴S_3/2−⁴I_15/2 transitions of Er³⁺ ions, respectively.

1. Introduction

Rare-earth compounds have been widely used as phosphors in high-performance luminescent devices [1–3]. For such applications, rare-earth compounds have been introduced into a variety of host materials, and a lot of research effort has been focused on the development of novel and more facile synthesis procedures and on the tuning of the photoluminescence properties of rare-earth centers. ZnWO₄ is a very promising host material since it is nonhygroscopic and nontoxic, and it exhibits intrinsic photoluminescence properties. In the literature, various studies report on the doping of ZnWO₄ with a variety of rare-earth ions such as Eu [4, 5], Y [6], and Ho [7]. Doping other rare-earth element ions such as the Er³⁺ ion has barely been investigated from a synthesis and photoluminescence point of view. On the other hand, the spectroscopy of the Er³⁺ incorporated in a variety of other host materials has received much attention in the recent years, focusing especially on the development of green and infrared eye-safe laser [8].

Previously, Er³⁺-doped ZnWO₄ has been prepared via the Czochralski method [8]; however, this method requires high calcination temperatures. Reports on the hydrothermal synthesis of this material were not found. Furthermore, control of crystal morphology and size as well as the effects of surface area, crystallinity, and dispersion on the luminescent properties of Er³⁺-doped ZnWO₄ was not discussed in detail. In this work, we report on a facile hydrothermal synthesis procedure at low temperature (180°C) of Er³⁺-doped ZnWO₄, and we discuss the effect of the composition on the structural and optical properties.

2. Experiments

Erbium-doped zinc tungstate (ZnWO₄) nanoparticles were prepared by the hydrothermal reaction of Zn(NO₃)₂·6H₂O, Na₂WO₄·2H₂O, and Er(NO₃)₃·5H₂O at 180°C for 6 hrs. The typical procedures for the preparation of these samples consist of adding 10 mL of an aqueous solution of Zn(NO₃)₂·6H₂O (1 mmol) to 20 mL of an aqueous solution containing Na₂WO₄·2H₂O (1 mmol) and Er (NO₃)₃·5H₂O under vigorous stirring. The volume of this solution was increased to 40 mL by adding distilled water, and the pH of the solution was adjusted to 7 by dropwise addition of a 30% aqueous ammonia solution. After that, the solution was transferred into a Teflon-lined stainless steel autoclave of 100 mL capacity. The autoclave was heated to 180°C for 6 hrs without shaking or stirring, and it was then allowed to slowly cool to room temperature. From this autoclave, a white precipitate was collected and washed for four times with distilled water, after which the solid powder was retrieved. The solid powder was heated at 80°C and dried at this temperature under vacuum for 2.5 hrs before calcination at 700°C for 2 hrs.

Structural characterization was performed by means of X-ray diffraction using a D5005 diffractometer with Cu Kα radiation. The FE-SEM observations were carried out by using a S4800 (Hitachi) microscope. Raman measurements were taken in a backscattered geometry using the Jobin Yvon T 64000 triple spectrometer equipped with a cryogenic charge-coupled device (CCD) array detector, using the 514.5 nm line of an Ar-Ion laser. The absorption spectra were recorded by using the Jasco 670 UV-Vis spectrometer, and the room temperature luminescence spectra were recorded on a spectrofluorometer (PL, Fluorolog-3,) using 325 nm laser excitation.

3. Results and Discussion

Figure 1 shows the diffraction patterns of the pure ZnWO₄ and ZnWO₄:Er³⁺ with different erbium concentrations after the calcination. The pure zinc tungstate diffractogram is in good accordance with the standard JCPDS card (no. 73 0554; no. 89-0447) [9, 10]. The different Er³⁺-doped ZnWO₄ exhibits almost the same XRD patterns as those of the undoped ZnWO₄. An expanded view of the (111) peak of these different materials on the right shows that the diffraction peaks are slightly shifted to lower angles. This observation suggests that Er³⁺ is successfully substituting Zn²⁺ in the lattice and that Er³⁺ is homogeneously incorporated into the lattice. The minor lattice expansion resulting from the Er³⁺ incorporation is reasonable considering the relative ionic radius of Zn²⁺ (0.74 Å) and Er³⁺, (0.89 Å) [11] as shown in Figure 2. In pure ZnWO₄ crystals, zinc ions occupy octahedral position (𝑍=8) of the wolframite structure [12], and, when doped in ZnWO₄, Er³⁺ should replace Zn²⁺ to be in O_h position [8].

Figure 1 

XRD patterns of ZnWO4 and Er3+-doped ZnWO4 samples.

Figure 2 

Cell parameter versus Er content.

Figure 3 shows the SEM images of pure ZnWO₄ and Er³⁺-doped ZnWO₄ materials prepared by the hydrothermal route at 180°C from the solution of pH 7 with various doping concentrations (2, 4, 6, 8, 10, and 15 mol%). It is clear that the morphologies and dimensions of the samples strongly depend on the doping concentration. SEM micrographs for the pure ZnWO₄ crystals show an average length of about 50 nm (Figure 3(a)). However, when Er³⁺ ions were doped with concentration of 2 mol%, the resulting powder consists of nanorods with a diameter of about 20 nm and a length ranging from 100 nm to 250 nm (Figure 3(b)). With increasing Er-doping concentration, the size of the rods becomes more inhomogeneous, and the length of crystals decreased (Figures 3(c)–3(e)). When the erbium concentration was raised to 15 mol%, nanoparticles tend to aggregate into large pigment particles (Figure 3(f)).

(a)

(b)

(c)

(d)

(e)

(f)

Figure 3 

SEM images of different Er3+-doped ZnWO4 samples: (a) 0%, (b) 2%, (c) 4%, (d) 6%, (e) 8%, and (f) 15%.

ZnWO₄ materials have a monoclinic wolframite structure with C_2h point group symmetry and P2/c space group. It has two formula units per unit cell. The W–O interatomic distance is substantially smaller than that of Zn–O; therefore, to a first-order approximation, the lattices can be separated into internal vibrations of the octahedra and the external vibrations in which an octahedron vibrates as a unit. Group theory analysis for wolframite-type ZnWO₄ predicts 36 lattice modes, of which 18 even vibrations (8𝐴_𝑔+10𝐵_𝑔) are Raman active [13]. The recorded Raman spectra of ZnWO₄:Er³⁺ crystals with various erbium doping (0–15 mol%) are shown in Figure 4.

Figure 4 

Raman spectra of ZnWO4:Er3+ crystals. The inset shows a zoom on the peak at 907 cm−1.

In fact, all expected lines of 907, 787, 709, 679, 547, 408, 343, 276, 193, and 162 cm⁻¹ are observed in the Raman spectra shown in Figure 4. In particular, the presence of six internal vibration modes (stretching modes) of 𝐴_𝑔 and 𝐵_𝑔 (907, 787, 709, 679, 547, and 408 cm⁻¹) should be noted as an important property of monoclinic wolframite ZnWO₄; the vibration modes arise from the six internal stretching modes caused by each of the six W–O bonds in the WO₆ octahedrons [14]. Note that the highest-frequency line at 907 cm⁻¹ corresponding to the 𝐴_𝑔 mode has a linewidth of about 9 cm⁻¹ for all samples.

When the particle size decreases to the nanometer scale, the effect on the vibrational properties of these materials might occur. A volume contraction occurs within the nanoparticles that are due to the size-induced radial pressure, which leads to increases in the force constants as a result of the decreases in the interatomic distances. In vibrational transitions, the wavenumber varies approximately in proportion to 𝑘^1/2, where 𝑘 is the force constant. Consequently, the Raman bands shift towards a higher wavenumber due to the increasing force constants [15].

Figure 5 shows the diffuse reflectance spectra of the ZnWO₄:Er³⁺ sample with Er concentrations of 4 mol%. Five distinct absorption peaks at wavelength 367, 379, 408, 490, and 522 nm in the absorption spectrum are attributed to transitions from ground state to excited states of Er³⁺ ions: ⁴I_15/2 → ²K_15/2, ⁴I_15/2 → ⁴G_11/2, ⁴I_15/2 → ²H_9/2, ⁴I_15/2 → ⁴F_7/2, and ⁴I_15/2 → ²H_11/2, respectively [16]. In addition, the absorption spectrum consists of a broad intense band at around 264 nm which is contributed by the [WO₄]²⁻ group.

Figure 5 

Absorption spectra of ZnWO4:Er3+ (4 mol%).

ZnWO₄ material is an indirect-gap semiconductor [17, 18]. For a crystalline semiconductor, the optical absorption near the band edge follows the following equation: 𝑎ℎ𝜈=𝐴(ℎ𝜈−𝐸_𝑔)^1/𝑛, where 𝑎, 𝜈, 𝐸_𝑔, and 𝐴 are the absorption coefficient, the light frequency, band gap, and constant, respectively [19]. For the ZnWO₄, 𝑛 was determined to be 2. Using this equation to calculate, the band gap of the crystal Er³⁺-doped ZnWO₄ (4 mol%) was roughly estimated to be 3.83 eV, while those of other ZnWO₄:Er³⁺ compounds (2%–15%) were determined to be between 3.82 and 3.85 eV.

PL spectra of ZnWO₄:Er³⁺ with Er³⁺ concentration of 2, 4, 6, 8, and 15 mol% under 325 nm excitation were shown in Figure 6. Three emission lines centered around wavelengths 537, 550, and 562 nm were observed in the PL spectra of the ZnWO₄:Er³⁺ nanoparticles. The emissions at 515–540 and 543–565 nm correspond to the ²H_11/2--⁴I_15/2 and ⁴S_3/2--⁴I_15/2 transitions of Er³⁺ ions, respectively [20]. The transition ⁴S_3/2 → ⁴I_15/2 at 550 nm is suitable for green laser [8]. The absorption spectra in Figure 4 confirmed that this erbium emission can result only from the energy transfer from the UV-excited tungstate groups to the erbium ions.

Figure 6 

PL spectra of ZnWO4:Er3+ with different molar concentrations of Er3+.

4. Conclusions

A hydrothermal synthesis procedure for the generation of erbium-doped ZnWO₄ materials has been developed. For ZnWO₄:Er³⁺ crystals, a pure ZnWO₄ phase was obtained without any evidence of impurities present; however, small distortions in the lattice parameters because of the difference in size between erbium and zinc could be observed. The 𝑐-parameter changes from 0.5734 nm to 0.575 nm, while the 𝑎- and 𝑏-parameters do not almost change. The Raman peak of these materials shifts to lower frequency with increasing Er-dopant concentration which again supports the successful substitution of Zn²⁺ by Er³⁺ in ZnWO₄ lattice. The SEM images clearly show that the grain size and morphology of ZnWO₄:Er³⁺ material depends on concentration of the doped Er³⁺ ion. The absorption spectra of ZnWO₄:Er³⁺ show a combination of the tungstate absorption in combination with the Er³⁺-ion transitions at 367, 379, 408, 490, and 522 nm. From these measurements, the band gap of Er³⁺-doped ZnWO₄ was estimated to be about 3.83 eV. Room temperature PL spectra of the ZnWO₄:Er³⁺ compounds exhibited emission in the visible ranges 515–540 and 545–565 nm stemming, respectively, from the ²H_11/2--⁴I_15/2 and ⁴S_3/2--⁴I_15/2 transitions of Er³⁺ ions.

Our work suggests that the idea of using ZnWO₄ doped with Er³⁺ could be a plausible strategy to develop a green laser.

Conflict of Interests

We declare that there is no conflict of interests regarding the publication of this article.

Acknowledgments

This work was supported by the National Foundation for Science and Technology Development (NAFOSTED) of  Vietnam and the Research Foundation Flanders (FWO) of Belgium (Code no. FWO.2011.23).