Transcription of Nanofibrous Cerium Phosphate Using a pH-Sensitive Lipodipeptide Hydrogel Template

Abstract

A novel and simple transcription strategy has been designed for the template-synthesis of CePO4 xH2O nanofibers having an improved nanofibrous morphology using a pH-sensitive nanofibrous hydrogel (glycine-alanine lipodipeptide) as structure-directing scaffold. The phosphorylated hydrogel was employed as a template to direct the mineralization of high aspect ratio nanofibrous cerium phosphate, which in-situ formed by diffusion of aqueous CeCl3 and subsequent drying (60 C) and annealing treatments (250, 600 and 900 C). Dried xerogels and annealed CePO4 powders were characterized by conventional thermal and thermogravimetric analysis (DTA/TG), and Wide-Angle X-ray powder diffraction (WAXD) and X-ray powder diffraction (XRD) techniques. A molecular packing model for the formation of the fibrous xerogel template was proposed, in accordance with results from Fourier-Transformed Infrarred (FTIR) and WAXD measurements. The morphology, crystalline structure and composition of CePO4 nanofibers were characterized by electron microscopy techniques (Field-Emission Scanning Electron Microscopy (FE-SEM), Transmission Electron Microscopy/High-Resolution Transmission Electron Microscopy (TEM/HRTEM), and Scanning Transmission Electron Microscopy working in High Angle Annular Dark-Field (STEM-HAADF)) with associated X-ray energy-dispersive detector (EDS) and Scanning Transmission Electron Microscopy-Electron Energy Loss (STEM-EELS) spectroscopies. Noteworthy, this templating approach successfully led to the formation of CePO4 H2O nanofibrous bundles of rather co-aligned and elongated nanofibers (10–20 nm thick and up to ca. 1 m long). The formed nanofibers consisted of hexagonal (P6222) CePO4 nanocrystals (at 60 and 250 C), with a better-grown and more homogeneous fibrous morphology with respect to a reference CePO4 prepared under similar (non-templated) conditions, and transformed into nanofibrous monoclinic monazite (P21/n) around 600 C. The nanofibrous morphology was highly preserved after annealing at 900 C under N2, although collapsed under air conditions. The nanofibrous CePO4 (as-prepared hexagonal and 900 C-annealed monoclinic) exhibited an enhanced UV photo-luminescent emission with respect to non-fibrous homologues.