1 3
Topics in Current Chemistry (2019) 377:24
frequently utilized in catalysis. They are frequently combined with electron diffraction and X-ray energy dispersive spectroscopy in order to complete the structural information already described and extracted from XRD or Raman [14, 15]. In
particular, in the system under investigation (Fig. 1), the use of STEM (scanning
transmission electron microscopy here with a high angle annular dark field detector) was able to identify tungsten atoms at the materials. They are located at the
more brilliant spots in the image(s) included in Fig. 1. The use of mathematical tools
(classification techniques based in a K-means clustering algorithm) allow to obtain
the W–W distance(s) taking place in a material having ca. 20 atomic (cation basis)
tungsten content. The plot of such an observable (see bottom of Fig. 1) shows welldefined distance(s) between tungsten atoms, which are exclusively located at specific (bulk and surface positions) of anatase planes, showing the true doped nature
of the sample [9]. Of course, microscopy can capture additional details concerning
the presence of amorphous phases (frequently occurring at surface positions) and/
or details of the defect nature and distribution. Apart from point defects, dislocations, twinning and stacking faults, and antiphase domains are subjects of microscopy studies [14, 15].
In a brief comment, here we can mention some techniques specifically dedicated
to analyzing surface groups present at photo-catalytic materials. Among them, infrared spectroscopy is the most utilized, although it can also provide structural (and
electronic) information coming from the phonon modes of the solids [16, 17]. Surface species, particularly surface hydroxyl groups, are analyzed in photo-catalysts
with the help of infrared (but also with EPR under light excitation) using or not
probe molecules [11, 16, 17]. In addition, surface chemical composition can be analyzed using low-energy surface scattering (LEIS) [18]. In the case of Ti–W systems,
the Raman band at ca. 970 cm
−1
(Fig. 1) is indicative of the presence of surface
tungsten atoms detected by the formation of W=O bonds [9, 13]. Raman is sensitive
to the formation of different M–O and (as previously mentioned) M–O–M bonds
originated by the presence of cations at surface (and bulk) positions in a significant
number of mixed or composite oxides. Similarly, Raman can detect bonds of other
(sulfides, nitrides, etc.) chemical compounds [10].
The structural characterization of the solid photo-catalysts is completed with
the study of the local order of the materials. Typical techniques (which, as mentioned, can allow the study of amorphous, nanocrystalline, and crystalline samples) to handle this issue are extended X-ray absorption fine structure (EXAFS) and
X-ray photoelectron spectroscopy (XPS). EXAFS is a technique mostly carried out
in synchrotrons which analyze the signal coming from the interference between the
incident and scattered photo-electron(s), and gives a picture of the local order for
atoms (shells) at distances below ca. 6–7 Angstroms from the absorber atom [19,
20]. In recent times, X-ray total scattering techniques are also utilized to study the
local order as they offer information at essentially all (local) distances present in
the nanostructured material [21]. Both techniques differ in the maximum distance
achievable but also in the fact that EXAFS describes the local structure around the
absorbing atom (allowing to study all atoms of the sample) while total scatting signal contains the sum of all pairs of atoms present at the structure. On the other hand,
XPS scans the unoccupied density of state (far from the edge) as a function of the
167
Reprinted from the journal
Topics in Current Chemistry (2019) 377:24
frequently utilized in catalysis. They are frequently combined with electron diffraction and X-ray energy dispersive spectroscopy in order to complete the structural information already described and extracted from XRD or Raman [14, 15]. In
particular, in the system under investigation (Fig. 1), the use of STEM (scanning
transmission electron microscopy here with a high angle annular dark field detector) was able to identify tungsten atoms at the materials. They are located at the
more brilliant spots in the image(s) included in Fig. 1. The use of mathematical tools
(classification techniques based in a K-means clustering algorithm) allow to obtain
the W–W distance(s) taking place in a material having ca. 20 atomic (cation basis)
tungsten content. The plot of such an observable (see bottom of Fig. 1) shows welldefined distance(s) between tungsten atoms, which are exclusively located at specific (bulk and surface positions) of anatase planes, showing the true doped nature
of the sample [9]. Of course, microscopy can capture additional details concerning
the presence of amorphous phases (frequently occurring at surface positions) and/
or details of the defect nature and distribution. Apart from point defects, dislocations, twinning and stacking faults, and antiphase domains are subjects of microscopy studies [14, 15].
In a brief comment, here we can mention some techniques specifically dedicated
to analyzing surface groups present at photo-catalytic materials. Among them, infrared spectroscopy is the most utilized, although it can also provide structural (and
electronic) information coming from the phonon modes of the solids [16, 17]. Surface species, particularly surface hydroxyl groups, are analyzed in photo-catalysts
with the help of infrared (but also with EPR under light excitation) using or not
probe molecules [11, 16, 17]. In addition, surface chemical composition can be analyzed using low-energy surface scattering (LEIS) [18]. In the case of Ti–W systems,
the Raman band at ca. 970 cm
−1
(Fig. 1) is indicative of the presence of surface
tungsten atoms detected by the formation of W=O bonds [9, 13]. Raman is sensitive
to the formation of different M–O and (as previously mentioned) M–O–M bonds
originated by the presence of cations at surface (and bulk) positions in a significant
number of mixed or composite oxides. Similarly, Raman can detect bonds of other
(sulfides, nitrides, etc.) chemical compounds [10].
The structural characterization of the solid photo-catalysts is completed with
the study of the local order of the materials. Typical techniques (which, as mentioned, can allow the study of amorphous, nanocrystalline, and crystalline samples) to handle this issue are extended X-ray absorption fine structure (EXAFS) and
X-ray photoelectron spectroscopy (XPS). EXAFS is a technique mostly carried out
in synchrotrons which analyze the signal coming from the interference between the
incident and scattered photo-electron(s), and gives a picture of the local order for
atoms (shells) at distances below ca. 6–7 Angstroms from the absorber atom [19,
20]. In recent times, X-ray total scattering techniques are also utilized to study the
local order as they offer information at essentially all (local) distances present in
the nanostructured material [21]. Both techniques differ in the maximum distance
achievable but also in the fact that EXAFS describes the local structure around the
absorbing atom (allowing to study all atoms of the sample) while total scatting signal contains the sum of all pairs of atoms present at the structure. On the other hand,
XPS scans the unoccupied density of state (far from the edge) as a function of the
167
Reprinted from the journal