Probing Surface Effects on α-NaYF₄ Nanoparticles by Nuclear Magnetic Resonance

Abstract

The structural properties of insulating α-NaYF 4 (cubic) nanoparticles with size ranging within 4 -25 nm were investigated by high-resolution 23 Na and 19 F solid-state Nuclear Magnetic Resonance (NMR) spectroscopy under magic angle spinning (MAS) with single pulse (SP-MAS), spin-echo (SE-MAS), inversion recovery, and 3Q-MAS

Multi-Step Crystallization and Chemical Evolution of Sodium Yttrium Fluoride

“…This is consistent with our observation that there are significant amorphous, poorly crystalline, and disordered regions in the gel and also emphasizes the propensity for this material to undergo solid-state diffusion, as we have observed. Single-pulse 23 Na NMR spectra of the gel and the α-NaYF samples, respectively (Figure S6), both contain major resonances centered at -18 ppm and -9.5 ppm, which were assigned respectively to Na + sites in the bulk nanoparticles, and to Na + sites at the surface or near defects, as originally reported by de Queiroz, et al 39 The fraction of the -9.5 ppm peak changes from 35% in α-NaYF to 75% in the gel sample, suggesting that the gel has over twice as many surface or defective Na + sites as compared to the final crystalline product.…”

“…Compared to orthorhombic YF 3 , which shows a relatively sharper peak at -58 ppm with a width of 12 ppm, and α-NaYF, which has a similarly broad resonance at -77 ppm with a width of 28 ppm, these data are again consistent with a gel that consists of some regions that are more similar to YF 3 and others that are closer to NaYF 4 . Because the broadness of the NaYF 19 F spectrum can be attributed to a large distribution of isotropic chemical shifts due to the random arrangement of Na + and Y 3+ around F -, 38,39 we show T 2 -filtered 19 F spectra in Figure 4D, which reduce signals from fasterrelaxing components and thereby allow for finer resolution of the remaining signal. The respective deconvolution of the resonances reveal that the gel does have some components similar to those in α-NaYF and YF 3 (-81 ppm and -69 ppm, respectively), however the main peaks in the region where we would expect to observe YF 3 are not consistent with the major peak of orthorhombic YF 3 at 58 ppm, due to the variation in the coordination geometries of bridging fluoride ions between the cubic and orthorhombic polymorphs of YF 3 as well as amorphous regions.…”

Relaxation Processes in Rare-Earth-Doped α-NaYF4 Nanoparticles by Nuclear Magnetic Resonance Spectroscopy

“…In addition, there is a small shoulder centered at −8.5 ppm due to 23 Na on the surface and nearby defects. 36 The peak attributed to the "bulk" NaYF 4 has T 1 of 45 ms and the one attributed to the surface/defects, 2 ms. 36 The 23 Na NMR signal of the non-paramagnetic RE-doped samples, Lu 3+ 2% and La 3+ 2%, also shows broad spectra and additional features, represented by three distinguishable peaks, Table 2. Best Fit Parameters from Numerical Deconvolution of the 23 Na SP-MAS Spectra of the 2% RE-Doped NPs (or Stated Otherwise) G, L) means the ratio between Gaussian (X) and Lorentzian (1 − X) in the curve, i.e., X = 0 stands for a full Lorentzian distribution and X = 1 for a full Gaussian distribution.…”

“…Furthermore, the second-order quadrupolar effect parameters (P Q ), given by 0.9−1.1 MHz, as calculated from 3Q-MAS experiments (see Supporting Information, Table S2 and Figure S4), 48 are similar to the undoped sample (given by 1.0−1.2 MHz). 36 As P Q is a measure of the magnitude of the electric field gradient (EFG), one can state that the addition of the REs does not add significant distortions to the α-NaYF 4 lattice.…”

“…21,36 Furthermore, NPs of ϕ ≈ 7 nm or those with high size polydispersity are prompt to present distortions due to surface effects and site defects. 36 context, we have prepared RE-doped α-NaYF 4 (from La 3+ to Lu 3+ ) NPs with a moderate doping level (≈2%), about 7 nm diameter and low polydispersity, and characterized their local structure by 23 Na and 19 F high-resolution NMR and magnetic relaxation. We also support our discussion by investigating 1% Lu 3+ -and 1% Gd 3+ -doped NPs.…”