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A. Mathur et al.
energy loss due to inelastic scattering form the background of the spectrum. Apart
from these, shake-up satellites and Auger electron peaks are also present in the
spectra.
Auger Electron: Due to high energy primary incident electron, an electron is
ejected from the core shell of an atom (K shell), leaving behind a hole. The emitted
electron from the K-shell along with the incident scattered electron, both are ejected
from the atom. According to the conservation of energy principle, another electron
from higher orbital (L shell), loses energy to fill the hole. Due to this transition from L
to K shell, X-ray photons are emitted. This X-ray may cause another electron to eject
from the shell (L 1 shell). Hence, as a result three electrons are ejected from the atom
and hence Auger electrons are represented as for example KLL 1 , representing the
shells from where the electrons have been emitted. With increasing atomic number,
the yield of Auger electrons decreases.
To identify the chemical environment of an atom, its core energy level spectra is
analysed, because due to bonding, the chemical environment of the core spectra is
affected. The peaks show a shift in the binding energy if their chemical environment
is changed. For example, carbon bonded with other carbon (C–C), carbon bonded
with a single oxygen (C–O) and carbon bonded with two oxygen atoms (O–C=O)
all are located at different binding energy. Moreover, a single element with variable
oxidation state can also be distinguished because they correspond to different binding
energy values.
Applications of XPS: A significant work exists using XPS as a tool to characterize
catalyst’s surface used in the field of environmental remediation, catalysis, energy
storage, energy conversion devices, etc. Most of the catalytic reactions in this field
rely on the catalyst’s surface and XPS plays critical role to find out the surface
functional groups of the catalyst. Owing to its high sensitivity it is equally good
to detect the change if any happened before and after the reaction occurred on the
catalyst’s surface. In addition, the presence of any chelating group which can bind
the catalyst’s surface and thus could reduce the effective concentration of surface
catalytic site, can be easily find out with the help of XPS technique. XPS is being
used in several ways by researchers for detailed knowledge of the catalysts surface,
from literature following are some examples.
(a) To determine the active site blockage: XPS being a surface sensitive technique is vastly used to find out any blocking element (or chelating agent) on
the catalyst’s surface. For example, Ravinder et al. reported the synthesis of
TiO 2 nanoparticles with the help of DETA (diethylenetriamine), a N containing
chelating agent [2]. This chelating agent has a major role to maintain the flakes
like structure of TiO 2 having high surface area. However, it blocks the active
surface site of TiO 2 nanoflakes (as governed by its photocatalytic activity), thus
removal of this chelating agent was necessary to unblock the catalyst surface
sites. In such case, XPS played crucial role to monitor the presence of chelating
agent, if any, over the TiO 2 nanoflakes. In brief, Fig. 2a represents the XPS
survey spectra of catalysts calcined at 200 and 400 °C for 3 h. The presence of
A. Mathur et al.
energy loss due to inelastic scattering form the background of the spectrum. Apart
from these, shake-up satellites and Auger electron peaks are also present in the
spectra.
Auger Electron: Due to high energy primary incident electron, an electron is
ejected from the core shell of an atom (K shell), leaving behind a hole. The emitted
electron from the K-shell along with the incident scattered electron, both are ejected
from the atom. According to the conservation of energy principle, another electron
from higher orbital (L shell), loses energy to fill the hole. Due to this transition from L
to K shell, X-ray photons are emitted. This X-ray may cause another electron to eject
from the shell (L 1 shell). Hence, as a result three electrons are ejected from the atom
and hence Auger electrons are represented as for example KLL 1 , representing the
shells from where the electrons have been emitted. With increasing atomic number,
the yield of Auger electrons decreases.
To identify the chemical environment of an atom, its core energy level spectra is
analysed, because due to bonding, the chemical environment of the core spectra is
affected. The peaks show a shift in the binding energy if their chemical environment
is changed. For example, carbon bonded with other carbon (C–C), carbon bonded
with a single oxygen (C–O) and carbon bonded with two oxygen atoms (O–C=O)
all are located at different binding energy. Moreover, a single element with variable
oxidation state can also be distinguished because they correspond to different binding
energy values.
Applications of XPS: A significant work exists using XPS as a tool to characterize
catalyst’s surface used in the field of environmental remediation, catalysis, energy
storage, energy conversion devices, etc. Most of the catalytic reactions in this field
rely on the catalyst’s surface and XPS plays critical role to find out the surface
functional groups of the catalyst. Owing to its high sensitivity it is equally good
to detect the change if any happened before and after the reaction occurred on the
catalyst’s surface. In addition, the presence of any chelating group which can bind
the catalyst’s surface and thus could reduce the effective concentration of surface
catalytic site, can be easily find out with the help of XPS technique. XPS is being
used in several ways by researchers for detailed knowledge of the catalysts surface,
from literature following are some examples.
(a) To determine the active site blockage: XPS being a surface sensitive technique is vastly used to find out any blocking element (or chelating agent) on
the catalyst’s surface. For example, Ravinder et al. reported the synthesis of
TiO 2 nanoparticles with the help of DETA (diethylenetriamine), a N containing
chelating agent [2]. This chelating agent has a major role to maintain the flakes
like structure of TiO 2 having high surface area. However, it blocks the active
surface site of TiO 2 nanoflakes (as governed by its photocatalytic activity), thus
removal of this chelating agent was necessary to unblock the catalyst surface
sites. In such case, XPS played crucial role to monitor the presence of chelating
agent, if any, over the TiO 2 nanoflakes. In brief, Fig. 2a represents the XPS
survey spectra of catalysts calcined at 200 and 400 °C for 3 h. The presence of