Topics in Current Chemistry (2019) 377:24
1 3
1 Introduction
Heterogeneous photo-catalysis is a field of research intensively investigated
which includes both energy and environmental applications. This field is essentially defined by the use of photons as the energy source to drive the chemical
reaction(s) of interest [1, 2]. An inherent part of the research in this and, in general, in any field related to catalysis, concerns the physico-chemical characterization of the materials. Heterogeneous photocatalysts are frequently nanostructured
materials in the form of powders or films. Moreover, such materials are dominantly constituted by semiconductors that can handle light and transform photons
into chemical energy but also by other nanosized materials such as metals, and,
more frequently, by composite materials having several components of different
chemical nature [1, 3]. In all active photo-catalytic materials, light is transformed
in hole and electron-related charge carrier species after absorption. Such species
would interact with chemical reactants in subsequent steps of the process but such
chemical steps always compete with the recombination of charge [1, 2]. Both the
nanostructure as well as the composite nature of the materials posse important
challenges to the characterization of photo-catalysts and here we will attempt to
provide a general view of the problem and the current status of the research.
Traditionally, the characterization of such materials was performed in pre and
post-reaction specimens and was focused on obtaining pertinent morphological,
structural, and electronic information of the photo-catalysts. Nowadays, such a
characterization is a pillar of any investigation in photo-catalysis but a push for
operando studies can be noticed [4, 5]. Moreover, further to such works, there
are some interesting contributions that try to combine operando spectroscopic
tools and kinetic studies under the so-called spectro-kinetic approach [6]. Considering these points, here we will provide a first look into the traditional studies
but will emphasize results corresponding to more detailed analyses or examples
concerning advanced methods closely connected with operando or spectrokinetic approaches. Operando studies mainly (although not exclusively) focus
on the analysis of the (surface and/or bulk) of the catalyst while spectro-kinetic
approaches complete the information attempting to obtain mechanistic and relevant kinetic information of the process under investigation.
The combination of the information from characterization studies attempts to
give an answer to the most challenging question in catalysis; the settlement of
activity–structure links that can rationalize and, more importantly, predict activity, setting up the bases for the future developments in the field.
2 Static View of the Photo‑Catalysts: Traditional Characterization
As mentioned, the use of light to trigger a photo-catalytic process makes that a
significant number of scientific contributions carried out characterization studies
of the samples before and after reaction. This may render a restrictive view of the
164
Reprinted from the journal
1 3
1 Introduction
Heterogeneous photo-catalysis is a field of research intensively investigated
which includes both energy and environmental applications. This field is essentially defined by the use of photons as the energy source to drive the chemical
reaction(s) of interest [1, 2]. An inherent part of the research in this and, in general, in any field related to catalysis, concerns the physico-chemical characterization of the materials. Heterogeneous photocatalysts are frequently nanostructured
materials in the form of powders or films. Moreover, such materials are dominantly constituted by semiconductors that can handle light and transform photons
into chemical energy but also by other nanosized materials such as metals, and,
more frequently, by composite materials having several components of different
chemical nature [1, 3]. In all active photo-catalytic materials, light is transformed
in hole and electron-related charge carrier species after absorption. Such species
would interact with chemical reactants in subsequent steps of the process but such
chemical steps always compete with the recombination of charge [1, 2]. Both the
nanostructure as well as the composite nature of the materials posse important
challenges to the characterization of photo-catalysts and here we will attempt to
provide a general view of the problem and the current status of the research.
Traditionally, the characterization of such materials was performed in pre and
post-reaction specimens and was focused on obtaining pertinent morphological,
structural, and electronic information of the photo-catalysts. Nowadays, such a
characterization is a pillar of any investigation in photo-catalysis but a push for
operando studies can be noticed [4, 5]. Moreover, further to such works, there
are some interesting contributions that try to combine operando spectroscopic
tools and kinetic studies under the so-called spectro-kinetic approach [6]. Considering these points, here we will provide a first look into the traditional studies
but will emphasize results corresponding to more detailed analyses or examples
concerning advanced methods closely connected with operando or spectrokinetic approaches. Operando studies mainly (although not exclusively) focus
on the analysis of the (surface and/or bulk) of the catalyst while spectro-kinetic
approaches complete the information attempting to obtain mechanistic and relevant kinetic information of the process under investigation.
The combination of the information from characterization studies attempts to
give an answer to the most challenging question in catalysis; the settlement of
activity–structure links that can rationalize and, more importantly, predict activity, setting up the bases for the future developments in the field.
2 Static View of the Photo‑Catalysts: Traditional Characterization
As mentioned, the use of light to trigger a photo-catalytic process makes that a
significant number of scientific contributions carried out characterization studies
of the samples before and after reaction. This may render a restrictive view of the
164
Reprinted from the journal