First, the phase behavior of the system was studied to obtain the appropriate microemulsions. Figure shows the empirical phase diagram of the water/ethanol/sodium oleate (NaOA)/oleic acid (OA)/n-hexane mixtures at 298 K. Because of the complexity of the five-component system, the phase diagram was simplified to a ternary phase diagram, which is composed of total OA (including the part to generate NaOA with sodium hydroxide), water plus ethanol, and n-hexane. The composition is described using volume fractions. The water/ethanol ratio is always 1:1. The NaOA/OA molar ratio is always 2:3, and the total volume of OA is considered as the surfactant volume. The phase diagram is determined by gradual addition of n-hexane to a one-phase water/ethanol/NaOA/OA mixture with a constant volume fraction. For example, we begin from point A, and reach a critical point C where the solution starts showing a two-phase character.
Empirical phase diagram of the water/ethanol/NaOA/OA/n-hexane microemulsions.
The result shows that the one-phase/two-phase envelope extends from the point at 100% water plus ethanol to the point at 26.23% water plus ethanol, 20.45% OA, and 53.32% n-hexane, and the two-phase part is located in the lower OA region. Obviously, with an increase of the ratio of OA/(water plus ethanol), more n-hexane can be dissolved into their mixtures to form a stable system. The actual point (point B) we used is located in the right-bottom region, where the oil-in-water microemulsions are formed.
Figure shows the characterization data for the NaYF4:20% Yb3+, 2% Er3+ sample. The TEM image (Figure ) demonstrates that the synthesized particles are roughly spherical, monodisperse with the size uniformity of about 20 nm in diameter. The X-ray powder diffractometer (XRD) pattern (Figure ) shows well-defined peaks, indicating the high crystallinity of the synthesized material, and the peak positions and intensities from the experimental XRD pattern match closely with the calculated pattern for cubic phase of NaYF4 (JCPDS card, No. 77-2042). From the line broadening of the diffraction peaks, the crystallite size of the sample was determined to be approximately 18 nm using the Debye-Scherrer formula, which corresponds to the particle size determined from the TEM result.
Characterization data for NaYF4: 20% Yb3+, 2% Er3+ UCNPs. (A) TEM image (Inlet: HRTEM image of a single nanocrystal). (B) XRD pattern of the sample and the calculated line pattern for cubic phase of NaYF4 (JCPDS card, No. 77-2042).
UCNPs can easily be dispersed in nonpolar solvents (such as n
-hexane, cyclohexane) to form homogenous colloidal solutions. Figure shows images of a 1 wt.% solution of NaYF4
, 2% Er3+
UCNPs in n
-hexane, demonstrating its transparency. The visible upconversion luminescence can be observed when the solution is excited at 980 nm with a power density of 1.2 kW/cm2
(Figure ). The corresponding upconversion luminescence spectrum is also shown in Figure . There are three major emission bands at 520-530 nm (green light), 540-550 nm (green light), and 650-670 nm (red light), which are assigned to the 2
, and 4
transitions of Er3+
ion, respectively. Under 980 nm excitation, the absorption of the first photon can elevate Yb3+
ion to the 2
level from ground state, and then it can transfer the energy to the Er3+
ion. This energy transfer can promote Er3+
ion from 4
level to the 4
level and from the 4
level to the 4
by another energy transfer upconversion process (or a second 980 nm photon) if the 4
level is already populated. Then, the Er3+
ion can relax nonradiatively to the 2
levels, and the green emissions occur (2
). Alternatively, the ion can further relax and populate the 4
level leading to the red emission (4
]. The curve also shows that red emissions are much stronger than green emissions, so the products present light of orange color on the whole (Figure ).
Colloidal solutions of NaYF4:20% Yb3+, 2% Er3+ sample in n-hexane. (A) The solution showing its transparency. (B) Visible upconversion luminescence excited by 980 nm laser oxide. (C) Upconversion luminescence emission spectrum.
It is noted that the as-prepared nanoparticles are cubic phase, whose fluorescence efficiency is at least one-order of magnitude less than that of the hexagonal phase [8
]. A thermal treatment at ca. 400-600°C was reported to transform the cubic phase to the hexagonal phase, but which led to undesirable particle growth and agglomeration [2
]. We carried out the annealing of the as-prepared nanoparticles under N2
atmosphere by heating them to 600°C, and maintaining this temperature for 5 h. After annealing, the particles aggregated into larger clusters (Figure ), and the XRD pattern (Figure ) shows that hexagonal NaYF4
phase emerged in addition to the already existing cubic pattern (marked with asterisks), which implies that the particles transformed partially from cubic phase to hexagonal phase by annealing. In addition, upconversion luminescence emission spectrum (Figure ) was obtained after ultrasonic dispersion of a 1 wt.% solution of the products in n
-hexane, compared with the spectrum of nanoparticles before annealing, its green emission plays a dominant role, and the overall emissions are much stronger than those for cubic phase products.
Characterization data for NaYF4: 20% Yb3+, 2% Er3+ UCNPs after annealing. (A) TEM image. (B) XRD pattern (cubic phase is marked with asterisks) and the calculated line pattern for hexagonal phase of NaYF4 (JCPDS card, No. 28-1192).
Upconversion luminescence emission spectra of the nanoparticles before (dash line) and after (solid line) annealing.