Suli Wu

Strong red and NIR emission in NaYF4:Yb3+,Tm3+/QDs nanoheterostructures

Due to the strong tissue absorption of short-wave-length light below 600 nm, in vivo imaging based on UCNPs as luminescent probes is still limited. The NIR spectral range (700–1100 nm) and the red region (600–700 nm) are referred to as the “optical window” of the biological tissues because the light scattering, absorbance and autofluorescence of tissue are minimum in this range. Here we synthesized NaYF4:Yb3+,Tm3+/CdS nanoheterostructures. In the nanoheterostructures, the red (700 nm) and NIR (802 nm) emissions of NaYF4:Yb,Tm were enhanced by 7.3 times, while the blue emission peak at 476 nm nearly disappeared and no peak of CdS appeared. Transmission electron microscopy (TEM) images show that the morphology of the binary nanoparticles is predominantly peanut-like and the X-ray diffraction (XRD) patterns of the NaYF4:Yb3+,Tm3+/CdS nanoheterostructure were assignable to both wurtzite phase CdS and hexagonal phase NaYF4:Yb3+,Tm3+. In comparison, introducing Cd2+ ions or S2− ions or preparation of physical mixture of NaYF4:Yb3+,Tm3+ and CdS did not enhance the red (700 nm) and NIR (802 nm) emissions of NaYF4:Yb, Tm. These results indicated that the interactions in the NaYF4:Yb3+,Tm3+/CdS nanoheterostructure played a key role for enhancing the NIR emission of NaYF4:Yb3+,Tm3+ nanocrystals. QDs act as a sub energy level to mediate energy transfer of Tm3+. To evaluate the effect of other QDs on the upconverted fluorescence spectrum of NaYF4:Yb3+,Tm3+, NaYF4:Yb3+,Tm3+/CdSe nanoheterostructures were also prepared by a similar method. The upconverted fluorescence spectra indicated that by conjugation with CdSe, the NIR emission at 802 nm of NaYF4:Yb3+,Tm3+ was also enhanced greatly.

Facile Synthesis of Monodispersed Polysulfide Spheres for Building Structural Colors with High Color Visibility and Broad Viewing Angle

The synthesis and assembly of monodispersed colloidal spheres are currently the subject of extensive investigation to fabricate artificial structural color materials. However, artificial structural colors from general colloidal crystals still suffer from the low color visibility and strong viewing angle dependence which seriously hinder their practical application in paints, colorimetric sensors, and color displays. Herein, monodispersed polysulfide (PSF) spheres with intrinsic high refractive index (as high as 1.858) and light-absorbing characteristics are designed, synthesized through a facile polycondensation and crosslinking process between sodium disulfide and 1,2,3-trichloropropane. Owing to their high monodispersity, sufficient surface charge, and good dispersion stability, the PSF spheres can be assembled into large-scale and high-quality 3D photonic crystals. More importantly, high structural color visibility and broad viewing angle are easily achieved because the unique features of PSF can remarkably enhance the relative reflectivity and eliminate the disturbance of scattering and background light. The results of this study provide a simple and efficient strategy to create structural colors with high color visibility, which is very important for their practical application.

Modulation of the emission intensity and color output of NaYF4:Yb3+,Er3+ nanocrystals by OH−

Oleic acid together with sodium hydroxide (NaOH) was widely used to control the size, phase and morphology of NaYF4 UCNCs. However, reports are scarce about the modulation of the luminescence intensity and color output of NaYF4 UCNCs by using different amounts of NaOH. Here, we synthesized NaYF4:Yb3+, Er3+ nanocrystals by a hydro/solvent thermal method using oleic acid as ligand with different amounts of NaOH and systematically investigated their upconverted fluorescence spectra. The results indicated that the emission intensity of NaYF4 : Yb3+,Er3+, especially the green emission at 540 nm was dramatically increased as NaOH decreased. The enhancement factors of 540 nm emissions were 48, 42 and 32 for samples prepared with 0 g, 0.2 g and 0.4 g NaOH (the corresponding pH of the reaction solution were 5.9, 6.9 and 7.4), respectively, compared with that of the product prepared with the usually used 0.7 g NaOH (the corresponding pH was 8.3). Importantly, the ratio of green to red (RGR) increased from 0.4 to 5.9 when the pH decreased from 8.3 to 5.9. That is to say, the color output was tuned from orange yellow to nearly pure green. This resulted from the high energy vibration of OH−, which led to nonradiative relaxation from green emission levels (2H11/2 and 4S3/2) to the red emission level (4F9/2) and nonradiative relaxation from the red emission level (4F9/2) to the no emission level (4I11/2) of Er3+.

Multiple Colors Output on Voile through 3D Colloidal Crystals with Robust Mechanical Properties

Distinguished from the chromatic mechanism of dyes and pigments, structural color is derived from physical interactions of visible light with structures that are periodic at the scale of the wavelength of light. Using colloidal crystals with coloring functions for fabrics has resulted in significant improvements compared with chemical colors because the structural color from colloidal crystals bears many unique and fascinating optical properties, such as vivid iridescence and nonphotobleaching. However, the poor mechanical performance of the structural color films cannot meet actual requirements because of the weak point contact of colloidal crystal particles. Herein, we demonstrate in this study the patterning on voile fabrics with high mechanical strength on account of the periodic array lock effect of polymers, and multiple structural color output was simultaneously achieved by a simple two-phase self-assembly method for printing voile fabrics with 3D colloidal crystals. The colored voile fabrics exhibit high color saturation, good mechanical stability, and multiple-color patterns printable. In addition, colloidal crystals are promising potential substitutes for organic dyes and pigments because colloidal crystals are environmentally friendly.

Ligand dynamic effect on phase and morphology control of hexagonal NaYF4

In this work, the ligand dynamic effect was utilized to synthesize hexagonal NaYF4 (β-NaYF4) crystals with controllable morphology by a hydrothermal method. A series of fatty acids was chosen as organic ligands to investigate the ligand dynamic effect on the crystal morphology. Confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray power diffraction (XRD), the dynamic nature of ligands has strong influences on the crystal morphology, crystal growth and phase transition processes. Moreover, possible crystal growth processes with different chain length ligands have been proposed.

Structural Color Patterns on Paper Fabricated by Inkjet Printer and Their Application in Anticounterfeiting

Inkjet-printed structural color patterns have attracted great attention in recent years because of their broadly promising applications. However, the patterns are usually fabricated on pretreated plastic substrates. Herein a convenient inkjet printing method was developed to fabricate large-scale computer-designed structural color patterns on photo paper without any treatment using inks containing monodisperse CdS spheres. By this strategy, not only were the single-color and multicolor structural color patterns on paper successfully obtained, but also invisible photonic anticounterfeiting was achieved without any external stimuli. The key point of this anticounterfeiting technique is printing patterns and the background with inks containing uniformed CdS spheres with different diameters but similar intrinsic colors, so that the invisible patterns can be observed clearly by simply changing the viewing angle. The invisible and visible can be realized without the change of intrinsic structure, and the patterns are all solids. The patterns will have long lifetime and good durability, which is beneficial for their practical usage.

Prolonging the lifetime of excited electrons of QDs by capping them with π-conjugated thiol ligands

The lifetimes of the excited electrons (photoluminescence lifetime) of quantum dots (QDs) have an important effect on the electron transfer efficiency between QDs and other substances and thus determine their application. Here, a new strategy to prolong the photoluminescence (PL) lifetime of quantum dots (QDs) by using π-conjugated ligands together with thioglycolic acid (TGA) as the QDs ligand shell is reported. 4-Mercaptobenzoic acid (4-MBA), 4-methylbenzeneththiol (4-MBT) or 2-mercaptobenzothiazole (2-MBTH) was selected as π-conjugated ligand together with thioglycolic acid (TGA) to synthesize aqueous CdTe quantum dots (QDs). The decay curves of TGA–CdTe, 4-MBA–TGA–CdTe, 4-MBT–TGA–CdTe and 2-MBTH–TGA–CdTe (λem = 550 nm) were recorded and their average PL lifetimes were calculated. The HOMO and LUMO energy levels of 4-MBA, 4-MBT and 2-MBTH were calculated with Gaussian 09W. By comparing lifetimes of QDs capping by different ligands relating to the HOMO and LUMO energy levels of ligands, we presume that in the QDs/π-conjugated ligand hybrid, an electron delocalized field is formed by mixing the LUMO energy levels of π-conjugated ligand with conductive band-edge energy (Ecb) of CdTe QDs, which will supply the excited electron with a more stable environment. As a result, the PL lifetime of CdTe QDs is prolonged greatly (from 49 ns to 80 ns) when a π-conjugated ligand with appropriate LUMO energy level was used as ligand shell.

Monodisperse TiO2 Spheres with High Charge Density and Their Self-Assembly.

Titanium dioxide (TiO2 ) spheres are potential candidates to fabricate three-dimensional (3D) photonic crystals owing to their high refractive index and low absorption in the visible and near-infrared regions. Here, TiO2 spheres with both high surface charge density and uniform size, which are necessary for the self-assembly of TiO2 spheres, have been prepared by means of sol-gel methods in ethanol in the presence of thioglycolic acid as ligand. Thioglycolic acid, which contains two functional groups, not only acts as coordinating ligand for stabilizing and controlling the growth of TiO2 spheres but also endows the resulting TiO2 spheres with high charge density as based on ζ-potential analysis when the pH of the TiO2 aqueous dispersion was 6.5 or higher. The SEM images illustrate that the diameter of the prepared TiO2 spheres can be tuned from 100 to 300 nm by simply controlling the concentration of H2 O. FTIR spectra confirm that thioglycolic acid bonded to the surface of TiO2 spheres through carboxylic groups. As anticipated, the obtained TiO2 spheres could self-assemble to form a 3D opal photonic crystal structure by means of a simple gravity sedimentation method. Then the TiO2 spheres in the 3D opal photonic crystal structure were able to transform into a pure anatase phase by annealing at different temperatures.

Decoration of Si-nanowires-grafted Si micropillar array with Ag nanoparticles for photoelectrocatalytic dechlorination of 4-chlorophenol

Use of Si materials as photoelectrodes in aqueous solutions is fundamentally required to inhibit the formation of an insulating SiO2 layer as well as promote the transfer of photogenerated holes and electrons from the interior of Si to the solid–liquid interface. In order to meet these requirements, an Si material, with hierarchical structure and Si nanowires standing on the surface of Si micropillar array (SiNW/SiMP), was prepared for facilitating the transfer of photogenerated carriers to the solid–liquid interface. Decoration of SiNW/SiMP by Ag nanoparticles (Ag/SiNW/SiMP) via depositing Ag nanoparticles on the surface of SiNW/SiMP successfully restrained the generation of SiO2 through preventing the contact of SiNW/SiMP with water. Pristine SiNW/SiMP photocathode exhibited enhanced photoelectrochemical activity with a photocurrent of approximately −31 mA cm−2 at −1 V (vs. SCE), which was about one order of magnitude larger than that of SiMP; however, the photocurrent decayed with prolonged illumination. By comparison, Ag/SiNW/SiMP photocathode exhibited stable photocurrent of approximately −37.5 mA cm−2 at −1 V (vs. SCE) during 10 cycles of CV testing, which was 21% higher than that of the pristine SiNW/SiMP. Ag/SiNW/SiMP photocathode exhibited excellent photoelectrocatalytic activity towards dechlorination of 4-chlorophenol (4-CP). Over 95% of 4-CP (initial concentration of 10 mg L−1) was rapidly degradaded after 20 min, which were 3 and 1.1 times higher than those of the SiMP and SiNW/SiMP. The good reproducibility was verified by the results of six consecutive experiments.

Increasing electrical conductivity of upconversion materials by in situ binding with graphene

Upconversion nanoparticles (UCNPs) hold promise as near-infrared light converters to enhance the efficiency of solar cells. However, the prevalent use of UCNPs in solar cells is restricted by their poor electrical conductivity and low emission efficiency. Here reduced graphene oxide (rGO)-NaYF4:Yb(3+)/Er(3+) composites are proposed to achieve good electrical conductivity due to the high charge carrier mobility of rGO. Composites of rGO and UCNPs combined by a chemical bond are in situ synthesized by the hydrothermal method, followed by a reduction process. The contact of UCNPs with rGO is proved by SEM, and the binding between the rGO-UCNP composites is confirmed by Fourier transform infrared spectroscopy. The composites are doped into the photoanode of a solar cell. As anticipated, electrochemical impedance spectroscopy confirms the good electrical conductivity of the in situ synthesized rGO-UCNPs. Furthermore, the use of rGO-UCNPs in solar cells enables an enhancement in short-circuit current density and overall efficiency by about 10%. These findings reveal that the combination of UCNPs with rGO opens up new opportunities of extending the use of UCNPs in the area of solar energy harvesting.