Structure, Morphology, and Faceting of TiO2 Photocatalysts by the Debye Scattering Equation Method. The P25 and P90 Cases of Study.

Federica Bertolotti,Antonio Cervellino,Anna Vivani,Norberto Masciocchi,Daniele Moscheni,Antonietta Guagliardi,Fabio Ferri

doi:10.3390/nano10040743

Abstract

Characterization of functional nanocrystalline materials in terms of quantitative determination of size, size dispersion, type, and extension of exposed facets still remains a challenging task. This is particularly the case of anisotropically shaped nanocrystals (NCs) like the TiO2 photocatalysts. Here, commercially available P25 and P90 titania nanopowders have been characterized by wide-angle X-ray total scattering techniques. Synchrotron data were modelled by the reciprocal space-based Debye scattering equation (DSE) method using atomistic models of NC populations (simultaneously carrying atomic and nanoscale structural features) for both anatase and rutile phases. Statistically robust descriptors are provided of size, morphology, and {101} vs. {001} facet area of truncated tetragonal bipyramids for anatase, jointly to polymorph quantification. The effects of using the proper NC shape on the X-ray diffraction pattern are analyzed in depth through DSE simulations by considering variable bipyramid aspect ratios (resulting in different {101} vs. {001} surface) and relative dispersion in a bivariate manner. We demonstrate that using prismatic NCs having equal volume and aspect ratio as bipyramids provides reasonably accurate sizes and {101} and {001} surface areas of the parent morphology.

Highlights

The functionality of crystalline nanomaterials, and of those possessing anisotropic shapes (as a consequence of low(er) symmetry structures or synthesis conditions), is heavily dependent on the surface extension and on the type of exposed crystal faces [1]
We demonstrate that using prismatic NCs having equal volume and aspect ratio as bipyramids provides reasonably accurate sizes and {101} and {001} surface areas of the parent morphology
Influence of Nanocrystals (NCs) Morphology and Facets Surface Area on the WAXTS Pattern of Anatase. Among those NCs which are well known to adopt a clear morphology with non–orthogonal faceting, titania, in the form of anatase, is a paradigmatic case

Summary

Introduction

The functionality of crystalline nanomaterials, and of those possessing anisotropic shapes (as a consequence of low(er) symmetry structures or synthesis conditions), is heavily dependent on the surface extension and on the type of exposed crystal faces [1]. This widely encompasses, to mention a few, chemical reactivity, catalysis, rheology, luminescence, and photocatalytic activity [2,3]. The most photocatalytically active titania polymorph, it exhibits a tetragonal crystal structure [1], and surface energy calculations predict that a slightly truncated square bipyramid, exposing {101} and {001}. Conflicting results have been reported in this regard, likely related to the difficulty of separately quantifying the activity of the different facets and the concurrence of surface hydroxylation (ubiquitous in low temperature syntheses), making the correlation between facets extent and photocatalytic properties still a debated issue [10,11,12]

Methods

Results

Conclusion