Shape-control And Luminescence Properties Of Two Types Of Phosphate Phosphors

Energy conservation and consumption reduction is the mainstream of the world for a sustainable development today. In lighting field, the merits of energy savings, eco-friendly and reliability are particularly prominent. Owing to these properties, phosphor-converted white-LEDs have aroused a fast-growing interest. Currently, there are two main methods to obtain a white light. One of the approaches is solely based on the combination of a blue LED and a yellow-emitting phosphor (YAG:Ce). However, it shows a poor color rendering index (CRI) and unstable correlated color temperature due to the deficiency of red emission. One solution to this problem is to make LEDs by coating a near-ultraviolet (n-UV) or UV emitting LED with a mixture of trichromatic phosphors (blue, green, and red emitting phosphors) or just a single-phased multicolor phosphor, which exhibits a spectral distribution covering the whole visible range and therefore can realize a high quality white light. Accordingly, the development of trichromatic or single-phased multicolor phosphors, with good luminescence properties such as high luminous efficiency and low thermal quenching and so on, has become one of the hotspots in the field of white LED. What’s more, the microenvironment of lattice matrix and crystal morphology features also play key roles in phosphor usability, thus they become other research hot topics.In our work, two types of phosphate phosphors excited by UV light were synthesized. They were synthesized by the high temperature solid-state or hydrothermal processes, and discussed in detail from the angles of crystal structure, energy transfer, preparation methods, micro-morphologies and cation replacing in matrix. The main results are listed as follows:1. A series of blue-green tunable emitting phosphors Ba3(PO4)2:Ce3+, Tb3+were successfully synthesized via a high temperature solid-state route. Their excitation spectra exhibit a broad band in the range of 230-380 nm originating from the fâ†'d electron transition of Ce3+. And the emission spectra show not only one broad band of 370-480 nm coming from Ce3+, but also a series of characteristic emission lines of Tb3+peaked at 487,541,582 and 621 nm. The energy transfer mechanism from Ce3+ to Tb3+is determined to be dipole-dipole interaction and the energy transfer efficiency can reach 44.69% at the current doping level. Meanwhile, as the temperature reaches 150℃, the integrated emission intensity of Ba2.66(PO4)2:0.18Ce3+,0.16Tb3+phosphor is 76.73% of that at room temperature.2. An intrinsic emitting phosphor Ba3(PO4)2 was successfully prepared by hydrothermal and high temperature solid-state routes, respectively. In hydrothermal synthesis, when the pH value of precursor solution is 6.0,7.0,8.0 and 9.0, BaHPO4 can be obtained; Otherwise, Ba3(PO4)2 or Ba5(PO4)3OH is obtained with the pH value adjusted to be 6.0-9.0 and 10.0, respectively. The as-obtained BaHPO4 powders are highly dispersed flower-like spheres with diameter at about 10μm. Ba3(PO4)2 and Ba5(PO4)3OH are around 100 nm nano-particles. Under the excitation of 280-400 nm UV light, the three materials all show their intrinsic emitting with a broad emission band ranging from 380 to 625 nm, which is assigned to the transition of PO43-. The decay lifetimes of the hydrothermal samples BaHPO4, Ba3(PO4)2 and Ba5(PO4)3OH are determined to be 13.05,12.25 and 169.64 ns, respectively. Secondly, in the high temperature solid-state synthesis, when 5 mass% H3BO3 is added into the raw materials as a flux, the as-formed Ba3(PO4)2 samples are smooth, highly dispersed and uniform-sized regular particles, however, some irregular and shaggy ones are obtained with non-flux. In both cases, the particle sizes are at 2～5μm. The emission spectra of Ba3(PO4)2 samples obtained in the high temperature solid-state process also exhibit a broad blue band in the range of 380-475 nm, of which the emission intensity is weaker than that of the hydrothermal samples. The decay lifetime of Ba3(PO4)2 (5% H3BO3) is 31.54 ns. Lastly, with the temperature reaching 150℃, the emission intensity of hydrothermal sample Ba3(PO4)2 (pH= 9.7) is 54.59% of that at room temperature. Accordingly, for Ba3(PO4)2 (5% H3BO3) obtained in the high temperature solid-state synthesis, that is 89.62%. What’s more, its CIE coordinates nearly keep the same at different temperatures.3. A series of white emitting Ca3Gd7(PO4)(SiO4)5O2:Dy3+and blue emitting Ca3(Gd,Y/La)6.65(PO4)(SiO4)5O2:0.35Ce3+phosphors were successfully formed by a high temperature solid-state method. The excitation spectrum of Ca3Gd7(PO4)(SiO4)5O2:Dy3+contains two parts. One is coming from Gd3+in the host peaked at 275 and 313 nm, the other is assigned to the characteristic absorption of Dy3+, of which the main peaks are at 323,337,349,364 and 388 nm. The emission spectrum also shows the Dy3+ characteristic emission lines at 480,572 and 665 nm. The CIEs of all the Ca3Gd7(PO4)(SiO4)5O2:Dy3+samples are located at white light area, meanwhile, that of Ca3Gd6.3(PO4)(SiO4)5O2:0.7Dy3+ is (0.328,0.343), which is close to the ideal one (0.33,0.33). In Ca3Gd7(PO4)(SiO4)5O2 host, the concentration quenching mechanism between Dy3+ions is electric dipole-dipole interaction.4. The excitation spectrum of Ca3Gd6.65(PO4)(SiO4)5O2:0.35Ce3+is a broad band of 250-400 nm containing two peaks at 305 and 335 nm. Under excitation of 303 or 305 nm, the emission spectra both demonstrate a broad band ranging in 380-600 nm originating from the 5dâ†'4f transition of Ce3+, but the peak positions are different, the former is at 416 nm and the latter is at 422 nm. This phenomenon certifies that there are two Ce3+luminescence centers in the host, corresponding to 4f(S1) and 6h(S2) sites.5. Y3+and La3+are used to replace Gd3+in the host of Ca3Gd6.65(PO4)(SiO4)5O2, respectively. The ionic radius of Y3+is smaller than that of Gd3+, and accordingly, La3+ is larger. In the case of Y3+substituting for Gd3+, the matrix lattice shrinks, thus the emission peak of Ce3+from the 4f(S1) luminescence center shows a red shift. And the substituting of La3+for Gd3+has the opposite effect. When Gd3+is replaced by 100 mol% Y3+ and 80 mol% La3+ respectively, the corresponding emission intensity is 6 and 5.5 times of that of Ca3Gd6.65(PO4)(SiO4)5O2:0.35Ce3+.6. With the temperature reaching 150℃, the emission intensity of Ca3Gd6.65(PO4)(SiO4)5O2:0.35Ce3+keeps at 35.87% of that at room temperature. In the series of Ca3(Gd0.95-xYx)7(PO4)(SiO4)502:0.35Ce3+(x= 0.1,0.2,0.4,0.06,0.8, 0.95) phosphors, Ca3(Gd0.85Y0.1)7(PO4)(SiO4)5O2:0.35Ce3+shows the best thermal quenching property, of which the emission intensity at 150℃ is 41.54% compared to that at room temperature. And for the Ca3(Gd0.95-yLay)(PO4)(SiO4)5O2:0.35Ce3+(y= 0.1,0.2,0.4,0.06,0.8,0.95) phosphors, Ca3(Gd0La0.95)7(PO4)(SiO4)5O2:0.35Ce3+ exhibits the best, it is 74.22%, moreover, its CIEs nearly remain at (0.166,0.071) at different temperatures.