TiO2 anatase nanoparticles are among the relevant players
in the field of light-responsive semiconductor nanomaterials used
to face environmental and energy issues. In particular, shape-engineered
TiO2 anatase nanosheets with dominant {001} basal facets
gained momentum because of the possibility to exploit different and/or
improved functional behaviors with respect to usual bipyramidal TiO2 anatase nanoparticles, mainly exposing {101} facets. Nevertheless,
such behavior depends in a significant extent on the physicochemical
features of surfaces exposed by nanosheets. They can vary in dependence
on the presence or removal degree of capping agents, namely, fluorides,
used for shape-engineering, and experimental investigations in this
respect are still a few. Here we report on the evolution of interfacial/surface
features of TiO2 anatase nanosheets with dominant {001}
facets from pristine nanoparticles fluorinated both in the bulk and
at their surface to nanoparticles with F– free surfaces
by treatment in a basic solution and to totally F– free nanoparticles by calcination at 873 K. The nanoparticles fluorine
content and its subsequent evolution is determined by complementary
techniques (ion chromatography, TOF-SIMS, XPS, AES, SEM-EDX), probing
different depths. In parallel, the evolution of the electronic properties
and the Ti valence state is monitored by UV–vis spectroscopy
and XPS. The calcination treatment results in {001} facets poorly
hydroxylated, hydrated, and hydrophilic, which appear as surface features
consequent to the expected (1 × 4) reconstruction. Moreover,
IR spectroscopy of CO adsorbed as probe molecule indicates that the
Lewis acidity of Ti4+ sites exposed on (1 × 4) reconstructed
{001} facets of calcined TiO2 nanosheets is weaker than
that of cationic centers on {101} facets of bipyramidal TiO2 anatase nanoparticles. The samples have also been tested in phenol
photodegradation highlighting that differences in surface hydration,
hydroxylation, and Lewis acidity between TiO2 nanoparticles
with nanosheet (freed by F– by calcination at 873
K) and bipyramidal shape have a strong impact on the photocatalytic
activity that is found to be quite limited for the nanoparticles mainly
exposing (1 × 4) reconstructed {001} facets.
Currently established and projected regulatory frameworks require the classification of materials (whether nano or non-nano) as specified by respective definitions, most of which are based on the size of the constituent particles. This brings up the question if currently available techniques for particle size determination are capable of reliably classifying materials that potentially fall under these definitions. In this study, a wide variety of characterisation techniques, including counting, fractionating, and spectroscopic techniques, has been applied to the same set of materials under harmonised conditions. The selected materials comprised well-defined quality control materials (spherical, monodisperse) as well as industrial materials of complex shapes and considerable polydispersity. As a result, each technique could be evaluated with respect to the determination of the number-weighted median size. Recommendations on the most appropriate and efficient use of techniques for different types of material are given.Graphical AbstractElectronic supplementary materialThe online version of this article (doi:10.1007/s11051-016-3461-7) contains supplementary material, which is available to authorized users.
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