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Topics in Current Chemistry (2019) 377:24
key properties controlling activity. In spite of this, such characterization has been
shown fundamental to establishing the scientific ground of the field.
There are a significant number of techniques to extract information about heterogeneous photo-catalysts, but a simple classification procedure can be based on
the type of information. Mostly morphological, structural, and electronic types of
information are relevant to any photo-catalyst. Morphology is frequently analyzed
by physisorption of gases (nitrogen, carbon dioxide) and mercury, although in recent
times this is combined with information from other techniques such as X-ray diffraction, nuclear magnetic resonance, or microscopy [7, 8]. Most frequent morphological properties analyzed consider primary and secondary (or aggregate) size
and porosity. A challenge is to have a complete view of the full distribution of the
observable(s) in the highly heterogeneous samples or at least rigorous information
about a few moments (aside the order zero one) of the corresponding observable
distribution.
Such basic morphological information is combined with structural and electronic
information. To illustrate the most relevant techniques used to achieve these two
types of physico-chemical characterizations, we selected a number of techniques
applied to an anatase-TiO 2 material in the presence of a heterocation (tungsten). A
first challenge when the two cations (Ti and W) coexist in the solid matrix is to
distinguish between a true doped, single-phase material, and a composite material,
even in the case of diffraction (and Raman) silent surface species, which may be present over a dominant anatase phase [9]. The general system consisting of a dominant
anatase material having a heteroatom dopand at bulk or surface position(s) and thus
rendering single or multiple (composite) phase materials can therefore provide a
general view of the complex task pursued when characterizing a solid photo-catalyst.
Figure 1 contains a summary of the most common techniques applied to obtain
structural information. Customary, structural information is divided into that intrinsic to long-range order, requiring periodicity in atomic arrangement above a few
nanometers, and characteristic of crystalline materials, and that taking place at a
local level, below 1 nm, describing the atom organization around a specific position and existing in all (amorphous, nano of fully crystalline) materials. As a primary source of information, X-ray or neutron diffraction and Raman spectroscopy
renders information about existent crystalline phases. As is well known, diffraction
comes from the in-phase sum of the elastically scattered photons taking place by
interaction with the electron cloud of the solid while Raman is an inelastic scattering process of photons controlled by the polarizability of the electronic cloud. Such
techniques can detect a contribution from a solid phase if superior to roughly 0.5
weight percentage. Both techniques can inform about the properties of the crystalline domains present in the materials. Concerning nanomaterials, the signal of both
techniques is sensitive to particle size (crystalline domain), a fact typically detected
in the peak(s) positions and/or line widths [10]. The complete analysis of such techniques by specific mathematical procedures (such as the well-known Rietveld refinement for diffraction techniques) can render insights into the unit cell parameters and
volume, tetragonallity in the case of anatase, defect nature and distribution, strain,
and cation/anion fractional occupation. Information relative to the crystalline size
(somehow related to the primary particle size) and shape is also accessed from both
165
Reprinted from the journal
Topics in Current Chemistry (2019) 377:24
key properties controlling activity. In spite of this, such characterization has been
shown fundamental to establishing the scientific ground of the field.
There are a significant number of techniques to extract information about heterogeneous photo-catalysts, but a simple classification procedure can be based on
the type of information. Mostly morphological, structural, and electronic types of
information are relevant to any photo-catalyst. Morphology is frequently analyzed
by physisorption of gases (nitrogen, carbon dioxide) and mercury, although in recent
times this is combined with information from other techniques such as X-ray diffraction, nuclear magnetic resonance, or microscopy [7, 8]. Most frequent morphological properties analyzed consider primary and secondary (or aggregate) size
and porosity. A challenge is to have a complete view of the full distribution of the
observable(s) in the highly heterogeneous samples or at least rigorous information
about a few moments (aside the order zero one) of the corresponding observable
distribution.
Such basic morphological information is combined with structural and electronic
information. To illustrate the most relevant techniques used to achieve these two
types of physico-chemical characterizations, we selected a number of techniques
applied to an anatase-TiO 2 material in the presence of a heterocation (tungsten). A
first challenge when the two cations (Ti and W) coexist in the solid matrix is to
distinguish between a true doped, single-phase material, and a composite material,
even in the case of diffraction (and Raman) silent surface species, which may be present over a dominant anatase phase [9]. The general system consisting of a dominant
anatase material having a heteroatom dopand at bulk or surface position(s) and thus
rendering single or multiple (composite) phase materials can therefore provide a
general view of the complex task pursued when characterizing a solid photo-catalyst.
Figure 1 contains a summary of the most common techniques applied to obtain
structural information. Customary, structural information is divided into that intrinsic to long-range order, requiring periodicity in atomic arrangement above a few
nanometers, and characteristic of crystalline materials, and that taking place at a
local level, below 1 nm, describing the atom organization around a specific position and existing in all (amorphous, nano of fully crystalline) materials. As a primary source of information, X-ray or neutron diffraction and Raman spectroscopy
renders information about existent crystalline phases. As is well known, diffraction
comes from the in-phase sum of the elastically scattered photons taking place by
interaction with the electron cloud of the solid while Raman is an inelastic scattering process of photons controlled by the polarizability of the electronic cloud. Such
techniques can detect a contribution from a solid phase if superior to roughly 0.5
weight percentage. Both techniques can inform about the properties of the crystalline domains present in the materials. Concerning nanomaterials, the signal of both
techniques is sensitive to particle size (crystalline domain), a fact typically detected
in the peak(s) positions and/or line widths [10]. The complete analysis of such techniques by specific mathematical procedures (such as the well-known Rietveld refinement for diffraction techniques) can render insights into the unit cell parameters and
volume, tetragonallity in the case of anatase, defect nature and distribution, strain,
and cation/anion fractional occupation. Information relative to the crystalline size
(somehow related to the primary particle size) and shape is also accessed from both
165
Reprinted from the journal