Semiconductor-based luminescent materials have recently attracted considerable attention due to their advantages in absorbing light for many applications in solving environmental and energy problems [1,2]. For example, long-life, energy-efficient, and environmentally friendly solid-state white lighting equipment based on semiconductor chips and inorganic phosphors has become a popular alternative for ambient lighting. LEDs have already displaced incandescent and fluorescent lights as a new lighting source. A white LED can be made in several ways. Using red, green, and blue phosphors with UV chips solves the problem of phosphor deterioration and color-changing [3,4]. In addition, using a UV chip with a single-phase phosphor with several emission centers produces a warm-white light. Further, bismuth tungstate (Bi2WO6 ) is considered as a superior matrix material of the tungstate family for photoluminescence materials due to the charge transfer from oxygen to the tungsten under UV light [5-7]. In a recent study, porous metal-organic frameworks were modified by Bi2WO6 nanosheets using a facile hydrothermal method for fabricating a photocatalyst with direct Z-scheme heterojunction . In another study, the high mineralization efficiency and excellent durability of the Bi2WO6 catalyst were demonstrated to remove organic dyes . Generally, few reports have investigated photosensitivity and luminescent properties of Bi2WO6 NPs. This paper reportes a facile synthesis of Bi2WO6 nanopowders via a simple method. The structural, morphological, and optical properties of the synthetic samples were investigated, and the luminescence mechanism of NPs was discussed.
EXPERIMENTAL WORKS AND CHARACTERIZATION
Raw materials and utilized reagents were analytical grades without additional purification. Proper amounts of Na2WO4.2H2O and Bi (NO3)3·5H2O were dissolved in 25 mL distilled water to make solutions A and B, respectively. After 15 min, solution A was progressively added to solution B dropwise under continual magnetic stirring, and instantly a milk-white suspension solution was generated. The precipitate was collected and washed several times with distilled water and absolute ethanol, and then dried for 24 h. Finally, powders were calcined at 600°C for 2 h to obtain Bi2WO6 nanoparticles. A schematic representation of the experimental work is shown in Fig. 1. The phase purity of the as-prepared Bi2WO6 particles was examined using X-ray powder diffraction (XRD) with Cu Kα radiation. The particle morphology was investigated by field-emission scanning electron microscope (FESEM Mira3 TESCAN). FT-IR was performed in the range of 400–4000 cm-1 for studying the functional groups, and photoluminescence (PL) spectroscopy was used to examine the optical characteristics of the produced nanopowders (Cary fluorescence spectrophotometer with which featured a xenon lamp, producing 80 powerful flashes per second).
RESULTS AND DISCUSSION
Fig. 2a. shows the XRD patterns of synthesized Bi2WO6 NPs in which the diffraction peaks of Bi2WO6 particles can be indexed in terms of the tetragonal Bi2WO6 phase (PDF card number 96-200-3106). It seems the sample is not sufficiently pure since there are some unknown peaks without Miller indices. This can be solved by increasing the temperature of the calcination and more rinsing (Fig. 2b). The calculated crystallite size of the nanopowders according to the Debye-Sherrer equation was 32 nm. Using equation (1), the crystallite size was calculated from the full width at half maximum (FWHM) of the preferred crystalline plane of the peak :
Where D is the crystallite size; k refers to the shape factor; is the wavelength of the X-ray beams ( =1.5406 nm for CuK); is the FWHM, and is the peak point in radians. It should be noted that this calculation is only a relative estimate of the size along one determined crystallographic direction, and that the values obtained using the Scherrer formula are not absolute at all. The Scherrer formula was used to estimate the preferred crystalline plane. In addition, the XRD parameters and lattice constants of the synthesized nanopowder were calculated and shown in Table 1.
The morphology and structure were investigated by FESEM (Fig. 3). Cubic-like and spherical-like particles were observed in the average diameter size of 0.92 m. Some aggregates were detected in the prepared Bi2WO6 nanopowder, attributed to the larger particles.
The FT-IR spectrum of the Bi2WO6 NPs is shown in Fig. 4. The bands at 1700 and 3500 relate to OH stretching, and the peaks at 550 and 750 correspond to the Bi–O–Bi W–O asymmetric stretches. The in-plane deformation of the W group is responsible for the bands at 660 cm-1.
PL analysis is usually used for examining photonic carriers’ migration, transfer, and separation. The PL spectrum of Bi2WO6 NPs is shown in Fig. 5. As is observed with 290 nm UV excitation, three weak violets, blue, and red emission peaks as well as strong yellow emission peaks are obtained in the visible region of wavelengths. The 1A1-3T1 transitions inside the Wcomplex is responsible for blue-green emission in the WO4 structure [11,12]. The yellow emission in the spectrum suggests the energy transfer from the WO6 group to lattice ions . The photoluminescence measurement was conducted at room temperature using different excitation wavelengths. The obtained results are shown in Fig. 6 according to which with an increase in the excitation wavelength (410 nm), the position of the PL peaks is blue-shifted, and strong blue-green emission was obtained. Further, based on Fig. 7, under an excitation wavelength of 250 nm, strong green emission appeared in the PL spectrum of synthesized nanopowders. Excitation wavelength dependence can arise when the distribution of the molecules in the ground state differ in their solvation sites and energies [13-15]. As a result, tunable emission colors under different excitation wavelengths were obtained at room temperature.
Synthesized nanoparticles (0.5 wt.%) were imbedded in polyvinyl alcohol (PVA) matrix by direct mixing method for preparing flexible nanocomposite film, and their PL properties were investigated at room temperature. Luminescence spectrum of the pure PVA and prepared nanocomposite together with an inset schematic image of the prepared sample are shown in Fig. 8. The prepared nanocomposite exhibits strong luminescence peaks while the spectrum of PVA does not show any specific characteristic emission peaks. Based on Fig. 8, flexible Bi2WO6/PVA composite film exhibited blue, yellow, and orange emission colors compared with PVA. Therefore, prepared nanocomposite can be a proper candidate for optoelectronic devices applications. In addition, PVA is excited by absorbing light and produces an oscillating dipole, which de-excites the ground state by transferring energy to Bi2WO6 non-radiatively. This nonradiative energy stimulates many electrons in the nanocomposite’s valence band to the conduction band . Furthermore, the emission state of Bi2WO6 releases the majority of the electrons’ energy. The augmentation of emission results from nonradiative energy transfer from matrix to Bi2WO6. Shallow levels as an activator ion introduce extra ground-state and excited-state levels of energies between the valence and conduction bands of the Bi2WO6 nanocomposite. An electron with sufficiently high excitation energy is elevated to the conduction band, travels away from the center, and may be trapped by a lattice defect or return to an ionized center. There it initially occupies an exciting level and then emits a photon to return to the ground state of the activator center [15,17]. Generally, few reports have investigated photosensitivity and luminescent properties of Bi2WO6 NPs. This research reports a facile synthesis of highly luminescent Bi2WO6 nanopowders via a simple method.
Bi2WO6 nanoparticles with an average size of 0.9 m were successfully prepared via a simple route. The facile synthesized nanopowders exhibited visible emission colors under UV excitation at room temperature. In fact, tunable emission colors under different excitation wavelengths were obtained at room temperature which can be considered as a good candidate for future white emitting diode applications.
The authors would like to acknowledge the Iranian National Elites Foundation for the financial support of this research.
COMPLIANCE WITH ETHICAL STANDARDS
All the authors declare that they have no conflict of interest.