No abstract

A simple, mild, and effective self-templated approach has been developed to directly convert solid SiO2 microspheres into hollow structures. The reaction involves initial partial dissolution of silica cores in a NaBH4 solution and subsequent shell formation due to the redeposition of the silicate species back onto the colloid surfaces. The increasing concentration of NaBO2 as the result of the slow decomposition of NaBH4 in water is found to be responsible for the regrowth of the silica shell. This method allows the production of hollow silica spheres with sizes ranging from ∼70 nanometers to several micrometers, largely determined by the size of the starting silica colloids. The solid-to-hollow transformation mechanism is investigated in detail by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier Transform Infrared (FTIR) spectrometry, X-ray absorption spectroscopy (XAS), N2 adsorption−desorption, and X-ray diffraction (XRD). We also study the reaction conditions that allow control over the wall thickness, surface morphology, and shell porosity.

The electronic structure of the phospho-olivine Li(x)FePO4 was studied using soft-x-ray-absorption (XAS) and emission spectroscopies. Characteristic changes in the valence and conduction bands are observed upon delithation of LiFePO4 into FePO4. In LiFePO4, the Fe-3d states are localized with little overlap with the O-2p states. Delithiation of LiFePO4 gives stronger hybridization between Fe-3d states and O-2p states leading to delocalization of the O-2p states. The Fe L-edge absorption spectra yield "fingerprints" of the different valence states of Fe in LiFePO4 and FePO4. Resonant soft-x-ray-emission spectroscopy at the Fe L edge shows strong contributions from resonant inelastic soft x-ray scattering (RIXS), which is described using an ionic picture of the Fe-3d states. Together the Fe L-edge XAS and RIXS study reveals a bonding character of the Fe 3d-O2p orbitals in FePO4 in contrast to a nonbonding character in LiFePO4.