Application of X-Ray Photoelectron Spectroscopy in Materials …
427
and ∼167.3 eV respectively. Thiopenic S, pyridinic N and graphitic N has been
reported to participate and enhance electrocatalytic activity [28–30].
(j) Identification of active sites: From XPS, it was revealed that metal-N 2 and
metal-N 4 are the active centers for ORR [27, 31–33]. However, Metal-N 4 are
energetically more favourable than metal-N 2 sites [34], because oxygen or
reducing atmosphere stabilizes metal-N 4 sites, as supported by DFT calculations also [35]. Moreover, annealing temperature parameters can also be adjusted
according to XPS analysis for obtaining enhanced ORR performance. Kateryna
et al. varied annealing temperature of their materials from 200 to 1000 °C and
observed the evolution of species with temperature [32]. They observed that with
increasing temperature from 0 to 1000 °C, changes in the elemental composition of the materials was observed. Oxygen, nitrogen and Co atomic percentage
decreased with increase in C concentration. From this it was inferred that structural reorganization occurs with change in pyrolysis temperature and a potential
mechanism is happening wherein cobalt is incorporated into carbon matrix in
bulk, thereby decreasing Co concentration on the surface. The potential mechanism might involve breaking of Co–N 4 sites, leading to formation of metal
nanoparticles and some cobalt metal particles getting involved in formation of
graphitic carbon, with small amount of oxide layer around them. Thus, change
in the molecular structure impacts its catalytic activity also by modifying the
electronic structure. After evaluating the ORR activity of the material pyrolyzed
at 700 °C, the role of different forms of Co was demonstrated. Both the two types
of Co–CoN x and Co particles covered by cobalt oxides (Co x O y ) are required
for complete reduction of O 2 [36]. It was hypothesized that Co follow a dualsite mechanism for ORR. Co–N x sites might be the first sites where O 2 gets
adsorbed and reduces to peroxide. Later on Co x O y sites further break peroxide
to water to complete the reduction of O 2 . Further, the material with higher
amount of Co x O y showed the highest activity. The material with lesser amount
of Co x O y and higher amount of aromatic C and pyridinic N showed intermediate activity. Higher amount of pyrrolic N and lower amount of Co x O y showed
the least activity. This is how structure to property relationship was established
using XPS analysis. By optimizing the percentage of various components in a
structure, its property can be tuned.
(k) Biomaterials: XPS is used in a limited manner to characterize bio based materials may be because of their relatively shorter life-time, damage caused due to
X-ray exposure, limited resources for reference, etc. [37]. Biomaterials, such
as peptides, can be used to passivate monolayer semiconductors like InAs, so
as to avoid growth of oxide layer (Fig. 11) [38]. XPS is used to characterize
these peptides to determine their structural properties and interaction among
the constituents [39]. Moreover, biological lysozymes are class of enzymes
that can be used to synthesize silica particles from silicic acid precursor [40].
This biologically obtained silica was used to oxidize glucose through composite
silica/single-walled-carbon nanotube/lysozyme enzyme. XPS was used to quantify the enzyme immobilized/adsorbed on the composite, by determining N:C
427
and ∼167.3 eV respectively. Thiopenic S, pyridinic N and graphitic N has been
reported to participate and enhance electrocatalytic activity [28–30].
(j) Identification of active sites: From XPS, it was revealed that metal-N 2 and
metal-N 4 are the active centers for ORR [27, 31–33]. However, Metal-N 4 are
energetically more favourable than metal-N 2 sites [34], because oxygen or
reducing atmosphere stabilizes metal-N 4 sites, as supported by DFT calculations also [35]. Moreover, annealing temperature parameters can also be adjusted
according to XPS analysis for obtaining enhanced ORR performance. Kateryna
et al. varied annealing temperature of their materials from 200 to 1000 °C and
observed the evolution of species with temperature [32]. They observed that with
increasing temperature from 0 to 1000 °C, changes in the elemental composition of the materials was observed. Oxygen, nitrogen and Co atomic percentage
decreased with increase in C concentration. From this it was inferred that structural reorganization occurs with change in pyrolysis temperature and a potential
mechanism is happening wherein cobalt is incorporated into carbon matrix in
bulk, thereby decreasing Co concentration on the surface. The potential mechanism might involve breaking of Co–N 4 sites, leading to formation of metal
nanoparticles and some cobalt metal particles getting involved in formation of
graphitic carbon, with small amount of oxide layer around them. Thus, change
in the molecular structure impacts its catalytic activity also by modifying the
electronic structure. After evaluating the ORR activity of the material pyrolyzed
at 700 °C, the role of different forms of Co was demonstrated. Both the two types
of Co–CoN x and Co particles covered by cobalt oxides (Co x O y ) are required
for complete reduction of O 2 [36]. It was hypothesized that Co follow a dualsite mechanism for ORR. Co–N x sites might be the first sites where O 2 gets
adsorbed and reduces to peroxide. Later on Co x O y sites further break peroxide
to water to complete the reduction of O 2 . Further, the material with higher
amount of Co x O y showed the highest activity. The material with lesser amount
of Co x O y and higher amount of aromatic C and pyridinic N showed intermediate activity. Higher amount of pyrrolic N and lower amount of Co x O y showed
the least activity. This is how structure to property relationship was established
using XPS analysis. By optimizing the percentage of various components in a
structure, its property can be tuned.
(k) Biomaterials: XPS is used in a limited manner to characterize bio based materials may be because of their relatively shorter life-time, damage caused due to
X-ray exposure, limited resources for reference, etc. [37]. Biomaterials, such
as peptides, can be used to passivate monolayer semiconductors like InAs, so
as to avoid growth of oxide layer (Fig. 11) [38]. XPS is used to characterize
these peptides to determine their structural properties and interaction among
the constituents [39]. Moreover, biological lysozymes are class of enzymes
that can be used to synthesize silica particles from silicic acid precursor [40].
This biologically obtained silica was used to oxidize glucose through composite
silica/single-walled-carbon nanotube/lysozyme enzyme. XPS was used to quantify the enzyme immobilized/adsorbed on the composite, by determining N:C