YVO4 Powders Research Articles

Defect control and optical performance of yttrium orthovanadate nanocrystals via a facile pH-sensitive synthesis

Nanocrystalline YVO4 powders have been synthesized via a facile co-precipitation method, which has been investigated over the wide pH range from 4 to 13. XRD patterns indicated that the phase structure purity of the precipitation is influenced by the pH value. The reaction mechanism of YVO4 crystal was suggested to illustrate the development stages beginning with precipitates to form impure amorphous phase structures such as amorphous yttrium polyvanadate and hydrous Y(OH)3. SEM and TEM techniques were performed to observe the changes of the morphology and size by adjusting the pH conditions after YVO4 nanocrystals were sintered under ambient conditions at 600 °C for 2 h. Luminescence properties showed that the self-activated emission of YVO4 matrix material shows an obvious peak at 486 nm in the 400–650 nm wavelength range, attributing to the charge transfer in the vanadate groups. With decreasing the pH value, the diffuse reflectance spectra of YVO4 nanocrystals demonstrated that the energy gap Eg showed a red-shift trend mainly ascribed to the oxygen vacancies. Moreover, the luminescence emission intensity of YVO4 nanocrystals increases obviously after sintering treatment, which can be explained by the sintering-induced grain growth and improved surface band bending due to the dehydration or the removal of carbonate species. The findings of this study could provide new insights into the understanding of the reaction mechanism of YVO4 crystal with variable optical band gap and offer an opportunity for realizing tunable optical performance by defect structure regulation for laser and optoelectronics applications.

Luminescence properties YVO4 powders doped with dysprosium and alkaline-earth metals

YVO4:Dy and YVO4:Dy,M (M = Mg, Ca, Sr, Ba) powders were prepared by sintering at 900°C. According to XRD patterns, all samples are well crystallized, single-phase YVO4. SEM showed that the morphology of the samples did not show apparent changes. Under 275 nm excitation, the strongest emission line is observed at 576.6 nm, corresponding to the transition from 4 F 9/2–6 H 13/2 level of Dy3+ ion. The yellow/blue (Y/B) intensity ratio 4 F 9/2–6 H 13/2 to 4 F 9/2–6 H 15/2 values of Dy3+ for the phosphors doped with different divalent alkaline-earth metal ions is changes slightly. For the YVO4:Dy3+ particles, this value is maximal, which indicates that the color of the with YVO4:Dy3+ samples is the most pure.

Hydrothermal Synthesis and Microstructural, Optical Properties Characterization of YVO4Phosphor Powder

The phonon energy of YVO4 crystal is lower than other usual compounds of salt. So it is suitable as host material for down-conversion materials. Hydrothermal method was adopted to synthesize YVO4 phosphor powder with the use of yttrium oxide and sodium vanadate as raw material. The change in the relative integral intensity of the (200) and (112) di raction peaks indicates that macroscopic stress in the lattice obviously changes with the elevated hydrothermal reaction temperature. The YVO4 phosphor powder synthesized involves a certain agglomeration of small particles. The phonon vibration in the YVO4 originates mainly from the internal vibrations in the vanadium oxygen tetrahedron, in addition to the Y O and O H vibrations. Due to a low phonon energy of only 2.8188 × 10−21 J, YVO4 helps to improve the down-conversion e ciency of rare-earth ions. A bandgap value of approximately 3.8 eV for the synthesized YVO4 powders leads to good absorption properties in the ultraviolet region. Upon excitation by the 320 nm ultraviolet photon, the intrinsic emission of YVO4 powders is annihilated, and a broadband emission of VO3− 4 near 450 nm is observed at room temperature. The YVO4 phosphor powder synthesized at 180 ◦C exhibits the maximum photoluminescence intensity because of its excellent crystallization.

Spectroscopic properties of cerium doped YVO4 crystals and analysis on valence state of cerium ion

High-quality cerium doped YVO4 crystals are grown by the Czochralski method in a furnace heated by medium frequency induction. Cerium carbonate and ceria of 2 at% are respectively doped into the different crystals. The X-ray diffraction spectra show that all samples present YVO4 crystallographic phase. The absorption spectra, excitation spectra and photoluminescence spectra of the samples are measured. It is found that the spectroscopic properties of the samples doped with the different cerium impurities are similar. The two kinds of the samples each radiate a wideband blue light centered at 440 nm when excited by 325 nm light. This blue light emission is due to the 5d → 4f intrinsic emission of Ce3+ ions. In addition, the two kinds of the samples each radiate a wideband red light centered at 620 nm when excited by 460 nm light. The X-ray photoelectron spectroscopy (XPS) show that the O(1s) peak splits into two peaks at 529.8 eV and 531.8 eV, which indicate that there are two types of coordinate oxygen ions in the crystal. One kind of coordinate oxygen ion is normal coordinated oxygen ion (O2-) and another kind of coordinate oxygen ion is a hole (O-). According to recorded 5 peaks of Ce(3d) XPS, it is deduced that the Ce3+ ions and Ce4+ ions coexist in each of these samples. It is considered that the wideband red light emission of Ce:YVO4 crystal is due to the charge transfer transition of the Ce4+-O2- ion pairs, and Ce:YVO4 powders may become a novel red phosphor used for white LED.

Synthesis of Nd:YVO4 Thin Films by a Sol‐Gel Method

Nd: YVO4 powders and thin films were successfuly synthesized by the sol–gel method using metal alkoxides. A homogeneous and stable solution was prepared by the reaction of Y(OEt)3, VO(OiPr)3, and Nd(OEt)3 in 2‐methoxyethanol. The precursor was a mixture of vanadium and yttrium double alkoxide. Precursor films were prepared by dip coating and crystallization to single‐phase YVO4 at 500°C. Nd:YVO4 films were crystallized with (200) preferred orientation on glass substrates, which showed the characteristic optical absorption of neodymium.

Preparation of YVO4 powder from the Y2O3 + V2O5 + H2O system by a hydrolysed colloid reaction (HCR) technique

Prior to the formation of YVO4 in the Y2O3 + V2O5 + H2O system, two intermediate, partially hydrophobic, complex colloidal mixtures with metastable characteristics can be produced at room temperature and atmospheric pressure. The ball-milled system, having both hydrophobic and hydrophilic species, transforms into the stable yttrium orthovanadate phase due to intensive hydrolysis. At room temperature an orange mixture (possessing dispersed Y2O3 and 4Y2O3−P(OH) + ·2VO 3 − , Y2O3−p(OH) + ·6VO 3 − ·xH2O-like heteroaggregations) formed by 20 h mixing at pH ca. 4.0 transforms slowly, another red-brown heavily flocculated colloidal mixture (with dispersed Y2O3 and Y2O3−p(OH) + ·V10O 28 6− ·yH2O-like aggregation) formed by 70 h mixing at pH ca. 4.5 transforms rapidly into YVO4 in water. During additional mixing of highly diluted red-brown mixtures this transformation can be completed at room temperature. At elevated temperatures (50–95 °C) the orange mixture precipitates into a red-brown decavanadate-type precipitatium which subsequently can also rapidly hydrolyse into an orthovanadate phase in the diluted aqueous systems. Both vanadium excess meta-and decavanadate-type aggregations exhibit amorphous character by X-ray diffraction. The semi-hydrophobic colloidal structure can modify the dissociation mechanism, which prevents the system from returning to the starting oxides, and gives a new HCR technique for YVO4 preparation with a simple hydrolysis process at low temperatures and atmospheric pressure.