Synthesis and Structural Investigation of Erbium-Doped ZnWO4 Nanocrystals

INTRODUCTION

Zinc tungstate (ZnWO₄ or ZWO) is one of the most applicable divalent-transition metal tungstates with monoclinic wolframite structure and P2/c space group [1]. The ZWO structure consists of zigzag chains of distorted ZnO₆ and WO₆ octahedra composed of two groups of oxygen atoms (O₁ and O₂). Each octahedron (ZnO₆ and WO₆) shares its two corners with the adjacent octahedron (each of the ZnO₆ or WO₆ octahedra). The ZnO₆ octahedron comprises of four O₂ and two O₁, while the WO₆ octahedron consists of two O₂ and four O₁ atoms. Zn-O₁, W-O₁, Zn-O₂, and W-O₂ bond lengths are 0.20 nm, 0.19 nm, 0.22 nm, and 0.18 nm, respectively. O₂-Zn-O₁, O₂-W-O₁, O₁-Zn-O₁, O₁-W-O₁, O₂-Zn-O₂, and O₂-W-O₂ bond lengths are 94.82 nm, 89.29 nm, 106.35 nm, 79.41 nm, 76.32, and 102.94 nm, respectively [1, 2]. (The schematic diagram of the ZWO unit cell is shown in Fig. 1.)

ZWO, a self-activating phosphor, emits a broad blue-green light under deep UV excitation [1-4]. It possesses high refractive index, thermal and chemical stability, short decay time, non-toxicity, and high light yield. Due to these features, ZWO finds applications in optical devices, displays, scintillators, photoanodes, laser hosts, optical fibers, humidity sensors, microwave devices, photoluminescence, photovoltaics, photonics, and tomography [1-6]. Its monoclinic wolframite structure makes it a promising up-conversion host for lanthanides. Various synthesis methods, such as chemical precipitation, solid-state reaction, spray pyrolysis, co-precipitation, solvothermal, and sol-gel, enable the quick preparation of ZWO nanoparticles. Incorporating trivalent rare-earth ions, such as Er³⁺, enhance luminescence in the visible region. Er³⁺ doping in ZWO has been explored for temperature sensing, with potential applications in bioimaging [6-8]. Srivastava et al. synthesized ZnGa₂O₄:Yb³⁺, Er³⁺, and Cr³⁺ (ZGO-YEC) NPs that emit a bright single red emission under 980 nm excitation [9]. Biswas et al. incorporated Er³⁺-Yb³⁺-Na⁺ into ZWO, improving up-conversion and temperature sensing [10]. It was reported that synthesized Y2O3: Yb³⁺, Er³⁺ NPs demonstrated novel luminescence properties for bioimaging applications [10,11]. Although numerous studies have investigated the optical properties of pure/doped tungstate families and dual mixed oxide compounds [3-15], the structure of ZWO: xEr³⁺ has received less attention. As mentioned above, the unique structural, dielectric, and optical properties of zinc tungstate (ZnWO₄) have garnered significant attention in recent years [1-6]. This compound, with its intriguing characteristics, holds promise for diverse applications in various fields. Research on ZnWO₄ has been extended to explore its potential use in fields such as optoelectronics, photocatalysis, and photonic applications [2]. Understanding the structural intricacies, dielectric behavior, and optical features of ZnWO₄ is essential for harnessing its full potential. Recent studies have delved into the comprehensive analysis of ZnWO4’s properties. For instance, Dkhilalli et al. conducted a thorough investigation into the structural, dielectric, and optical properties of ZnWO₄, shedding light on its potential applications [2]. Additionally, the work of Xueyan Liu et al. (2023) introduced a novel p-n heterojunction photocatalyst involving CoWO₄/ZnWO₄, showcasing the compound’s versatility in enhancing photocatalytic activity under visible light irradiation [4]. Moreover, the research by Rafael W.R. Santana et al. (2023) explored the development of BiOBr/ZnWO₄ heterostructures for enhanced photocatalytic degradation, highlighting the compound’s potential in environmental remediation [3]. These recent investigations underscore the growing interest in ZnWO₄ and its multifaceted applications, paving the way for further exploration and utilization of this intriguing material. In this study, ZWO: Er³⁺ NPs were prepared using a simple method, and their structure was investigated for our future optical properties report.

EXPERIMENTAL Section

Synthesis of Nanoparticles

Pure ZWO and ZWO: Er³⁺ NPs were synthesized using a co-precipitation method followed by calcination. Initially, a solution was prepared by dissolving zinc acetate and sodium tungstate in 50 cc of deionized water. Subsequently, the tungstate solution was gradually introduced into the zinc solution. In the subsequent step, approximately 2 hours later, concentrations of 0.5 and 1at. %  Er (NO₃)₃.6H₂O  were mixed with 10 ccs of deionized water and added drop by drop to the aforementioned solution. The resulting mixture was subjected to centrifugation and subsequently dried at a temperature of 80 ℃ for a duration of 24 hours. Finally, the obtained powder was subjected to calcination at a temperature of 600 ℃ for a period of 2 hours. Erbium ions typically replace some of the zinc (Zn) ions in the crystal lattice. Erbium is commonly used as a dopant in various materials to introduce luminescent properties, and it can substitute for metal cations like Zn²⁺  or other cations in the host crystal lattice. Thus, in the case of ZnWO₄ with erbium doping, erbium ions are likely to replace zinc ions in the crystal structure. This substitution allows erbium to introduce its unique optical properties into the material, making it suitable for various applications in optics and photonics.

Characterization

The crystallinity, phase, and crystal structure of the powders were examined using a Bruker D8 X-ray powder diffractometer, operating with Cu Kα radiation (λ = 0.15406 nm) collected at 10°≤2θ≤70°. Fourier transform infrared spectroscopy (FTIR, Perkin Elmer RX) was employed to recognize the bending and stretching vibrations of Zn-O, Zn-O-W, and W-O bonds. Field-emission scanning electron microscopy (FE-SEM, MIRA III TESCAN) revealed the morphology and size distribution of NPs.

RESULTS AND DISCUSSION

Crystal Structure

The XRD patterns of the as-synthesized ZWO and ZWO: Er³⁺ phosphors and the JCPDS standard pattern (reference number 01-088-0251) presented in Fig. 2 indicated that the main phase of the powders was ZnWO₄ (JCPDS: 01-088-0251) with a monoclinic wolframite structure and space group P2/c. The XRD pattern of ZWO and ZWO: 1 at. % Er³⁺ nanopowders exhibited additional peaks marked with circles attributed to the low number of tungsten oxides [15]. A comparison between the two main diffraction peaks of ZWO (-111) and (111) in ZWO: x Er³⁺ (x=0, 0.5, and 1 at. %) was obtained (Fig. 1b), which demonstrates that the main peaks in doped samples, particularly in the state of x=1%at. The shift toward larger angles is attributed to the substitution of 3Zn²⁺ ions with 2Er³⁺ ions in the host lattice and the creation of a Zn²⁺ vacancy (V_Zn²⁺), which has caused the contraction of the crystal lattice, and then a reduction in the distance between the planes of the lattice. The intensity of the main diffraction peaks decreased in the doped samples, especially at x=0.5 at. %, indicating a decrease in crystallinity. The number of defects, such as V_Zn²⁺, interstitial O^2- vacancies, or other trapping defects in the crystal lattice of ZWO: 0.5 at. % Er was higher than that of the ZWO: 1 at. %  Er³⁺, leading to disorder in the local symmetries around the radiative centers and a reduction in crystallinity.

Using Williamson‒Hall’s method and XRD data, the crystal size and lattice strain values of NPs were calculated, as shown in Fig. 3. According to the Williamson-Hall results, the crystal sizes in ZWO, ZWO: Er (0.5 at. %), ZWO: Er (1 at. %), were 55 nm, 30 nm, and 45.2 nm, respectively. The crystal size in the doped samples and especially the ZWO: Er (0.5 at. %) diminished while the strain increased, indicating that Zn²⁺ ions were substituted well with Er³⁺ ions.

FTIR spectroscopy

Fig. 3 illustrates the FTIR spectra of the as-synthesized powders. The spectra confirmed the formation of zinc tungstate from the appearance of vibrational bands related to Zn-O, W-O, stretching, and symmetric vibrations of bridging oxygen atoms of Zn-O-W groups. As shown in Fig. 4, eight vibration modes in the fingerprint region of the infrared spectrum (1600-400 cm-1) are related to the W-O and Zn-O vibrations. The bands observed at 428.2 cm^-1 and 472 cm^-1 are attributed to the band and stretching vibrations of Zn-O, respectively. This attribution is consistent with previous articles (432 and 473 cm^-1) [16, 17].

The 3 bands at 532.32, 697, and 716 cm^-1 are due to the symmetric and asymmetric stretching vibrations of bridging atoms in WO₂ groups of distorted octahedra WO₆^-6, which are consistent with other studies [18, 19]. The bands at 532.32 and 716 cm^-1 correspond to the symmetric vibrations of the bridging oxygen atoms in the Zn-O-W group [20]. The symmetric vibrations of the bridging oxygen atoms of Zn-O-W groups produce broad absorption bands at 833.19 and 873 cm^-1 [18]. The vibrational modes at 1633.59 and 3460.06 cm^-1 correspond to the bending and stretching vibrations of the OH group reported to locate at 1644 and 3448 cm^-1, respectively [19, 21]. The results align with those of previous studies [22-26]. According to Fig. 3b, the transmission in the near-infrared region (NIR) increased in doped samples, especially in ZWO:1 at. % Er sample. It is expected that the pure ZWO will have better structural and optical characteristics due to having less transmission and more bonds in that frequency region. In the pure sample, the degree of crystallinity was higher, the bonds were complete, and a more complete structure was formed. ZWO:1 at. % Er transmittance percentage increased in 428-873 cm^-1, while it decreased in ZWO:0.5 at. % Er sample. The number of Zn-O-W and W-O bonds in ZWO:  1 at. % Er sample decreased, indicating the interstitial Er atoms in the ZWO structure, while the number of Zn-O-W and W-O bonds elevated in ZWO: 0.5 at. % Er. No significant displacements appeared in the location of the transmittance bands of samples.

Morphology

Fig. 5 displays the FE-SEM images, size distribution, and EDAX-map of the ZWO: Er³⁺ NPs prepared by the co-precipitation method. As shown in Fig. 5, the crystals in the ZWO: 1at. % Er³⁺ sample have a spherical morphology agglomerated in some parts with an average size of 140 nm. According to the EDS-map images, the distribution of Er, Zn, W, and O atoms was uniform.

CONCLUSION

In this article, pure and Er doped ZWO nanocrystals were prepared by using nanoscience and eco-friendly materials. The XRD results demonstrated that the samples had a monoclinic wolframite structure. The diffraction pattern of the ZWO: 0.5 at. % Er³⁺ was without an additional peak (related to tungsten oxide). The crystallinity of the pure sample was higher than that of the doped samples. Williamson-Hall’s results demonstrated that the size of the crystals in the ZWO: 0.5 at. % Er³⁺ sample decreased up to 30 nm and, the Er³⁺ ions replaced the Zn²⁺ ions in the ZWO lattice. SEM images revealed that nanoparticles agglomerated exhibited spherical morphology and an average size of 140 nm. The transmittance percentage in the doped sample changed concerning the pure one, indicating that interstitial Er³⁺ ions affected the number of W-O, Zn-O, and Zn-O-W bonds. The products have the potential for the production of flexible detectors and green lasers in future.

CONFLICT OF INTEREST

The authors declare no conflicts of interest.