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

Autor(es)

de Queiroz, Thiago B. ^[1] ; Cabrera-Baez, Michael ^[2] ; Menegasso, Paulo ^[3] ; Martinez, Eduardo D. ^[4] ; Garcia Flores, Ali F. ^[3] ; Rettori, Carlos ^[1] ; Urbano, Ricardo R. ^[3]

Afiliação do(s) autor(es)

^[1] Univ Fed ABC, Ctr Ciencias Nat & Humanas, Santo Andre 09510580, SP - Brazil

^[2] Univ Fed Pernambuco, Dept Fis, Recife 50670901, PE - Brazil

^[3] Univ Estadual Campinas, Inst Fis Gleb Wataghin, Campinas 13083859, SP - Brazil

^[4] Consejo Nacl Invest Cient Tecn CONICET, Comis Nacl Energia Atom CNEA, Ctr Atom Bariloche, San Carlos De Bariloche, Rio Negro - Argentina

• Tipo de documento: Artigo Científico
• Fonte: Journal of Physical Chemistry C; v. 124, n. 17, p. 9523-9535, APR 30 2020.
• Citações Web of Science: 0

Resumo

The structural properties of insulating alpha-NaYF4 (cubic) nanopartides with size ranging within 4-25 nm were investigated by high-resolution Na-23 and F-19 solid-state nuclear magnetic resonance (NMR) spectroscopy under magic an spinning (MAS) with single-pulse (SP-MAS), spin-echo (SEMAS), inversion recovery, and 3QMAS experiments. The Na-23 SP-MAS spectra show a broad peak around -18 ppm with a shoulder around -9 ppm, which becomes more prominent for the smallest nanoparticles. The Na-23 nuclei resonating around -9 ppm demonstrate a longitudinal relaxation time of a few milliseconds, while the ones resonating around -18 ppm are on the order of 50-125 ms. This feature is noticed for all studied nanopartides, but it is more evident for the smallest ones (phi less than or similar to 7 nm), especially among the batches with higher polydispersity. On the basis of these relaxation times, field-dependent measurements, and Na-23 3QMAS, we attributed the signal around -18 ppm to Na-23 in the bulk of the nanopartides and the signal around -9 ppm to surface or/and sites near defects, featuring higher fluctuations in the electric field gradient (EFG). The Na-23 3QMAS spectra provide evidence for two (and sometimes three) distinct Na sites in alpha-NaYF4 with similar quadrupole coupling but slightly different chemical shifts. The F-19 SE-MAS spectra show a broad peak around -75 ppm with a small shoulder around -120 ppm corresponding to only 1% of the signal. The peak around -75 ppm is attributed to the stoichiometric NaYF4 composition, and its broadening is attributed to a distribution of Na- and Y-rich environments. The minor shoulder around -120 ppm is associated with the F-deficient NaYF4 structure. The F-19 spin-spin relaxation time indicates some degree of mobility of the fluorine atoms, possibly due to the presence of F vacancies triggering hopping-like ion motion. The signal related to the F-deficient structure is greatly enhanced for the smallest nanoparticles (phi = 4 nm), i.e., along with the increase of 23 Na surface effects and defects. Therefore, we correlate several NMR techniques to provide a fundamental structural view for nanopartides used as upconversion host systems with prominent technological applications. Particularly for alpha-NaYF4, significant surface effects and defects must be expected for nanoparticles with dimensions in the order of few nanometers (phi less than or similar to 7 nm). (AU)

• Processo FAPESP: 16/14436-3 - Estudos das correlações eletrônicas e magnéticas nos supercondutores a base de ferro e condutores moleculares através de Ressonância Magnética Nuclear (NMR) e Quadrupolar (NQR)
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• Processo FAPESP: 16/15780-0 - Estudo das propriedades físicas de materiais nanoestruturados e interações fundamentais em sistemas bulk
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