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 μm 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.
Highlights
A novel and relatively simple transcription strategy has been designed for the template-synthesis of cerium(III) phosphate nanofibers having an improved nanofibrous morphology using a pH-sensitive nanofibrous hydrogel
A phosphorylated hydrogel nanofibrous template was first prepared by soft acidification of a C12GA basic solution with H3 PO4, which exhibited a morphology based on long nanoribbons or nanotapes (60–100 nm width, 100–140 nm thick and hundreds of microns length) merged laterally forming broader tapes and a relatively entangled
According to differential thermal and thermogravimetric analysis (DTA/TG) analysis), as it was confirmed by X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) characterization, with a much better-grown and more homogeneous fibrous morphology with respect to a reference CePO4 sample prepared under similar conditions
Summary
Rare-earth phosphates ( on, RPO4 ) are being prolifically investigated during the last decades given the wide variety of interesting properties they may exhibit, such as optical, electronical (or optoelectronical), ion-exchange, catalytic, heat-resistance, and biocompatibility, among others.These advanced properties have enabled the application of RPO4 , especially CePO4 and related solid solutions (with Tb, Gd, or other lantanides) and composite systems (Au/CePO4 ,...) in a plethora of technological applications, such as optoelectronic devices (luminescent materials, phosphors, displays, green light-emitting diodes or switches, solid-state lasers, redox sensors, non-linear optical devices,...) [1,2,3,4,5,6,7,8,9,10,11,12,13], fluorescent probes for chemical sensing [14,15,16] and detection or removal of heavy metals (e.g., Pb2+ , Co2+ ,...) [17,18], bio-sensing, bio-imaging and cell-labeling applications [19,20,21,22,23], ceramic materials (with high thermal and mechanical properties) [24], dielectrics [25], catalysts [26,27,28,29,30], ion-exchange or ion-conducting materials (proton-conducting membranes) for solid-oxide fuel cells (SOFCs) [25,31] and solar-cells, ceramic pigments [32,33,34], UV filters for sunscreens [35], and so on.It is well-known that one-dimensional (1D) nanostructured materials may exhibit improved or even novel properties with respect to their bulk counterparts, due to quantum-sized nano-confinement effects, becoming especially suited to design better performant electronic, optoelectronic, electrochemical or electromechanical devices [36,37]. Rare-earth phosphates ( on, RPO4 ) are being prolifically investigated during the last decades given the wide variety of interesting properties they may exhibit, such as optical, electronical (or optoelectronical), ion-exchange, catalytic, heat-resistance, and biocompatibility, among others These advanced properties have enabled the application of RPO4 , especially CePO4 and related solid solutions (with Tb, Gd, or other lantanides) and composite systems (Au/CePO4 ,...) in a plethora of technological applications, such as optoelectronic devices (luminescent materials, phosphors, displays, green light-emitting diodes or switches, solid-state lasers, redox sensors, non-linear optical devices,...) [1,2,3,4,5,6,7,8,9,10,11,12,13], fluorescent probes for chemical sensing [14,15,16] and detection or removal of heavy metals (e.g., Pb2+ , Co2+ ,...) [17,18], bio-sensing, bio-imaging and cell-labeling applications [19,20,21,22,23], ceramic materials (with high thermal and mechanical properties) [24], dielectrics [25], catalysts [26,27,28,29,30], ion-exchange or ion-conducting materials (proton-conducting membranes) for solid-oxide fuel cells (SOFCs) [25,31] and solar-cells, ceramic pigments [32,33,34], UV filters for sunscreens [35], and so on. Many studies have revealed the need of using high P:Ce ratios (much higher than 1:1) or very acidic conditions (pH values < 1.5) to obtain more uniform and highly-elongated (high aspect ratio) morphologies with a better crystallinity, and these characteristics become essential to improve the photoluminescent properties of CePO4 [4,39,41,48,52,54,57]
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