Jinsheng Shi

Energy transfer from Bi3+ to Eu3+ triggers exceptional long-wavelength excitation band in ZnWO4:Bi3+, Eu3+ phosphors

Color-tunable phosphors of ZnWO4:x mol% Bi3+, 3 mol% Eu3+ (x = 0.5, 1, 2, 3, 4, 5) were prepared through a precipitation method and their luminescence properties were investigated as a function of Bi3+ concentration. The most intense 616 nm emission indicates that Eu3+ occupies the Zn2+ sites without inversion symmetry. The spectral overlap between the emission band of Bi3+ and the excitation band of Eu3+ supports the energy transfer from Bi3+ to Eu3+, which has been demonstrated to be of a resonant type via a dipole–dipole mechanism. Energy transfer from Bi3+ to Eu3+ triggers a long-wavelength excitation band at 340 nm originating from the Bi3+ 1S0 → 3P1 transition, which makes the phosphors fit for long-wavelength radiation. The decay curves of Bi3+ and Eu3+ emission were measured to understand the energy transfer processes. Interestingly, the critical concentration of Bi3+ for Eu3+ 616 nm emission in ZnWO4:Bi3+, Eu3+ was greatly increased by 400 times compared with that for Bi3+ 560 nm emission in ZnWO4:Bi3+. Color-tunable emission in ZnWO4:Bi3+, Eu3+ phosphors can be obtained by the modulation of the excitation wavelength and the ratio of Bi3+ and Eu3+. Our work provides a novel approach to develop phosphors which can be excited effectively under long-wavelength radiation.

Synthesis and photoluminescence of Bi3+,Eu3+ doped CdWO4 phosphors: application of energy level rules of Bi3+ ions

Bi3+,Eu3+ doped CdWO4 phosphors have been synthesized using a co-precipitation method and regular micro-rods were obtained in Bi3+ or Eu3+ single-doped samples. X-ray diffraction, scanning electron microscopy, UV-vis spectrophotometry, and photoluminescence and decay time measurements were used to characterize the as-prepared samples. CdWO4:Bi3+ and CdWO4:Bi3+,Eu3+ can be excited ranging from 250 to 400 nm. The excitation at 350 nm was assigned to be the Bi3+ 1S0 → 3P1 transition according to the energy level rules of Bi3+ ions. The origin of O–W charge transfer transition has been analyzed using the calculated band structure and density of states of CdWO4 based on density functional theory. The approach to charge compensation was two impurity ions substituting for three Cd2+ sites. Energy transfer properties from the WO6 group to Bi3+ as well as from Bi3+ to Eu3+ were discussed. The mechanism of energy transfer from Bi3+ to Eu3+ was determined to be the quadrupole–quadrupole interaction and the critical distance of energy transfer from Bi3+ to Eu3+ was calculated to be 15.31 A. The quantum efficiency, CIE chromaticity and thermal quenching properties have also been investigated.

Photoluminescence properties, crystal structure and electronic structure of a Sr2CaWO6:Sm3+ red phosphor

A novel Sm3+-doped Sr2CaWO6 (SCWO) red phosphor, synthesized though a solid-state reaction, was reported. Its crystal structure was analyzed and refined via the Rietveld full-pattern fitting method based on XRD patterns. The CASTEP module of Materials Studio was used to investigate the band structure and density of states of the SCWO. The optical band gap was calculated through the UV-vis diffuse reflectance spectrum and compared with the value predicted by the DFT method. Raman spectra were recorded to confirm the substitution of cations by Sm3+ ions. The broad W–O charge transfer band and narrow excitation band at 406 nm from Sm3+ ions have comparable intensities in SCWO:Sm3+. After the introduction of charge compensators Li+, Na+ or K+, the intensity of the near-UV excitation band at 406 nm was almost twice as much as before. The profiles of the emission spectra under excitation into the charge transfer absorption and f–f transition of Sm3+ are different, and low-symmetry Sm3+ centres are preferentially excited via f–f absorption transitions. The intensity ratios of electronic, magnetic dipole-allowed transitions under different radiations show that the charge compensators can influence the chemical environment around Sm3+.

Luminescence properties of nano and bulk ZnWO4 and their charge transfer transitions

We here investigated the photoluminescence properties of nano and bulk ZnWO4 phosphors with the aim to understand the observed altered properties of nanoparticles as compared to their bulk counterparts. Their excitation bands were decomposed into three components by Gaussian fitting. The influence of crystalline defects that are present due to the low synthesis temperature of ZnWO4 nanoparticles on the excitation spectra was studied. The structures of ZnWO4 samples were refined with the General Structure Analysis System. Making use of the estimated refractive indices and the refined lattice parameters, four chemical bond parameters of W–O bonds were calculated and integrated to give the environmental factor he, which can to some extent explain the broadened excitation band in bulk ZnWO4. A new factor H of every W–O bond was calculated, and based on the results, three components of the excitation band at about 250, 280 and 300 nm were assigned to be from W–O1, W–O2(2) and W–O2(1) charge transfer transitions, respectively.

Preparation of network-like ZnO–FeWO4 mesoporous heterojunctions with tunable band gaps and their enhanced visible light photocatalytic performance

Novel mesoporous ZnO–FeWO4 (Zn–FeWO4) heterojunctions with network-like structure were synthesized with a combination of thermal decomposition and hydrothermal methods. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), N2 sorption and Brunauer–Emmett–Teller (BET) surface area analysis, diffuse reflection spectroscopy (DRS) and photoluminescence spectroscopy (PL). The results showed that the size of FeWO4 was reduced by the modification of ZnO. With the increasing molar ratio of ZnO to FeWO4, the quantum size of FeWO4 decreased from 11 nm to 4 nm, as is followed by a surprisingly band gap broadening. Photocatalytic activity toward the degradation of RhB under visible light irradiation was investigated. The optimum decomposition rate of RhB using the prepared 1.5Zn–FeWO4 heterojunction was almost 39 and 9.7 times that of pristine ZnO and FeWO4, respectively. The active species trapping experiments showed that the holes exhibited an obvious influence on the photocatalytic degradation process. The study on the mechanism showed that the enhanced photocatalytic activity was mainly ascribed to heterojunction construction and band gap broadening, which enhance the efficient transfer and the oxidation potential of holes. This heterojunction shows a potential industrial application to remove undesirable organics from the environment.

Applications oriented design of Bi3+ doped phosphors

Based on the relationships between Bi3+ energy levels and environmental factor he in phosphors established before, positions of A band of Bi3+ in ZnWO4 and CdWO4 are predicted to lie in near ultraviolet region. ZnWO4:Bi3+ and CdWO4:Bi3+ were prepared via a precipitation method. By examining their photoluminescence, it was confirmed that Bi3+ 1S0-3P1 transition are located around 340 and 350 nm, matching well with predicted results, 357 nm and 377 nm, respectively. The emission spectra indicate that they can be effectively excited by 340 and 350 nm ultraviolet light, exhibiting a satisfactory green performance (560 and 537 nm), according with ultraviolet chip.

Remote Control Effect of Li+, Na+, K+ Ions on the Super Energy Transfer Process in ZnMoO4:Eu3+, Bi3+ Phosphors

Luminescent properties are affected by lattice environment of luminescence centers. The lattice environment of emission centers can be effectively changed due to the diversity of lattice environment in multiple site structure. But how precisely control the doped ions enter into different sites is still very difficult. Here we proposed an example to demonstrate how to control the doped ions into the target site for the first time. Alkali metal ions doped ZnMoO4:Bi3+, Eu3+ phosphors were prepared by the conventional high temperature solid state reaction method. The influence of alkali metal ions as charge compensators and remote control devices were respectively observed. Li+ and K+ ions occupy the Zn(2) sites, which impede Eu and Bi enter the adjacent Zn(2) sites. However, Na+ ions lie in Zn(1) sites, which greatly promoted the Bi and Eu into the adjacent Zn(2) sites. The Bi3+ and Eu3+ ions which lie in the immediate vicinity Zn(2) sites set off intense exchange interaction due to their short relative distance. This mechanism provides a mode how to use remote control device to enhance the energy transfer efficiency which expected to be used to design efficient luminescent materials.

Luminescence and energy transfer in a color tunable CaY4(SiO4)3O:Ce3+, Mn2+, Tb3+ phosphor for application in white LEDs

A Ce3+/Mn2+/Tb3+ co-activated CaY4(SiO4)3O phosphor was prepared through high temperature solid state reaction process. By means of dielectric theory of chemical bonding for complex crystals, covalence of chemical bonds in CaY4(SiO4)3O was calculated. Blue emission from Ce3+ in different crystallographic cation sites was discussed quantitatively based on the calculation results. Dual energy transfer of Ce3+ → Mn2+ and Ce3+ → Tb3+ occurs and color tunable emission can be realized by modulation of their relative PL intensity. Energy transfer efficiency has been investigated and the process has been demonstrated to be a resonant type via a dipole–quadrupole mechanism. Critical distance Rc calculated through quenching concentration method and spectral overlap route is 7.23 and 7.55 A, respectively. As-obtained samples show color tunable emission and the corresponding CIE chromaticity coordinates were given. White light with color coordinates (0.3339, 0.3055) and (0.3553, 0.3338), close to those of the ideal, was realized under lower energy UV light. All results show CaY4(SiO4)3O:Ce3+, Mn2+, Tb3+ phosphor could be a promising candidate for UV chip pumped light emitting diodes.

Rattle-type NiCo2O4–carbon composite microspheres as electrode materials for high-performance supercapacitors

In this work, NiCo2O4–carbon composite microspheres with rattle-type structure were successfully prepared by a template-engaged hydrothermal and subsequent calcination treatment. These rattle-type microspheres are composed of a solid carbon core and a porous shell with nanorods as building blocks. The calcination temperature of the NiCo2O4–carbon precursor has an obvious effect on the morphology as well as the resultant capacitive performances. Because of their unique structure and high specific surface area, these rattle-type NiCo2O4–carbon composite microspheres exhibited excellent electrochemical performances with high specific capacitance (790 F g−1 at 1 A g−1), and it even reached as high as 609 F g−1 at 10 A g−1. Additionally, excellent cycling stability with 99.4% specific capacitance retention after continuous 2000 cycles at a current density of 2 A g−1 was observed, suggesting their promising application in supercapacitors. The synergistic effect of different components and the rattle-type structure may contribute to the outstanding performance of the composite electrode.

A super energy transfer process based S-shaped cluster in ZnMoO4 phosphors: theoretical and experimental investigation

Efficient energy transfer from a sensitizer to an activator in phosphors is very important for white LEDs. Bi3+ and Eu3+ co-doped red phosphors are potential alternatives for white LEDs. However, energy transfer from Bi3+ to Eu3+ ions is still not efficient enough in most cases. Here, we have found that every six Zn sites form an S-shaped cluster in the ZnMoO4 crystal. Two Zn(2) sites will be occupied preferentially in ZnMoO4 according to the comparison between the calculated and experimental A band positions of Bi3+ in the ZnMoO4 host. Considering the S-shaped clusters and site occupation preference, a super energy transfer process from Bi3+ to Eu3+ ions is proposed. The distance between the Bi3+ and Eu3+ ions can be controlled by their total doping concentrations. When their total molar concentration is beyond 1/6, Bi3+ and Eu3+ begin to sit in two adjacent Zn(2) sites. Thus, a new super energy transfer from Bi3+ to Eu3+ emerges due to the adjacent Bi3+ and Eu3+ ions. When excited at 331 or 350 nm, which is assigned to the 1S0 → 3P1 transition of Bi3+, the phosphor emits intense red light. The relative intensity is about 6 times higher than that of an ordinary transfer process. It is a good example of how to utilize site occupation preference and provides a new way to design efficient phosphors.