422
A. Mathur et al.
were studied and their oxidation states were also calculated by Xu et al. through
XPS. Where both Ni and Cu were found in their metallic states in addition to this
XPS spectra of O 1s reveals that by increasing the dopant Cu concentration the
peak intensity of O-Metal bond increases much faster than that of O–C. Implies
that introduction of Cu leads to metal oxidation and thus degree of oxidation
becomes stronger with Cu increment [10]. MnO 2 is a well-known material for
energy storage applications owing to its various phases—α, β, δ, γ etc. [5].
MnO 2 mainly exist in Mn
+4 oxidation state, whereas Mn
+3 also exist due to
Jahn-Teller Distortion. This Mn
+3 state is very active towards electrochemical
reactions. XPS aids in identifying this oxidation state of Mn and according
its composition. Activity of the material can also be explored on the basis of
percentage composition of Mn
+3 . Moreover, presence of multivalent states also
leads to more number of defect or vacancy sites which act as electroactive
centres for redox activity. Mathur et al. prepared a series of composite of αand δ-MnO 2 by varying their proportion [11]. The activity of the material was
correlated with amount of Mn
+3 and Mn
+2 defect sites and considered to play a
role in enhancing the conductivity of the material.
(e) Heterojunction determination: Heterojunction formation using in-situ XPS
technique has been observed by several researchers like heterojunction between
BiVO 4 /NiO, BiVO 4 /CoO x , BiVO 4 /Sn-doped In 2 O 3 (ITO) and MoO 3 /MoS 2
were observed [12, 13]. In brief, Lalita et al. monitored the heterojunction formation between MoO 3 and MoS 2 phase which was in-situ grown by
nonthermal O 2 plasma irradiation over MoS 2 nanoflakes [13]. Firstly, they
prepared MoS 2 nanoflakes and then those nanoflakes undergo O 2 plasma treatment in a controlled way. XPS was utilized to monitor the phase formation
of MoO 3 or to monitor the heterojunction formation when MoS 2 slowly transformed to MoO 3 matrix. Figure 6a represents the high-resolution spectra for Mo
3d obtained for samples undergo O 2 plasma treatment from 0 to 180 s. and corresponding S 2p spectra are shown in Fig. 6b. As the irradiation time increases, a
new peak starts appearing near the binding energy 170–172.0 eV, this could be
assigned to the oxy-sulfides bonds. The increment in the oxygen peak from the
barely present oxygen peak in untreated MoS 2 to O 2 treated sample indicates
the increased amount of O and slowly formed MoO 3 phase. The O 1s spectral
peak (Fig. 6c) situated near binding energy at 532 eV in molybdenum disulphide corresponds to the adsorbed oxygen species is in good agreement with
the literature [14].
In the O spectra (Fig. 6c) one new peak appeared with the O plasma irradiation,
near to binding energy of 530 eV which can be attributed to the binding energy
of Mo–O bond. The introduction of O in the lattice is proved by the binding
energy values corresponds to Mo–O bond. The presence of Mo–O bond directly
verify the introduction of O into the lattice rather than surface oxidation. One
big hump near to 531 eV corresponds to lattice oxygen which increase with the
increase in the oxygen plasma irradiation time. Thus, leads to increment in the
MoO 3 phase.
A. Mathur et al.
were studied and their oxidation states were also calculated by Xu et al. through
XPS. Where both Ni and Cu were found in their metallic states in addition to this
XPS spectra of O 1s reveals that by increasing the dopant Cu concentration the
peak intensity of O-Metal bond increases much faster than that of O–C. Implies
that introduction of Cu leads to metal oxidation and thus degree of oxidation
becomes stronger with Cu increment [10]. MnO 2 is a well-known material for
energy storage applications owing to its various phases—α, β, δ, γ etc. [5].
MnO 2 mainly exist in Mn
+4 oxidation state, whereas Mn
+3 also exist due to
Jahn-Teller Distortion. This Mn
+3 state is very active towards electrochemical
reactions. XPS aids in identifying this oxidation state of Mn and according
its composition. Activity of the material can also be explored on the basis of
percentage composition of Mn
+3 . Moreover, presence of multivalent states also
leads to more number of defect or vacancy sites which act as electroactive
centres for redox activity. Mathur et al. prepared a series of composite of αand δ-MnO 2 by varying their proportion [11]. The activity of the material was
correlated with amount of Mn
+3 and Mn
+2 defect sites and considered to play a
role in enhancing the conductivity of the material.
(e) Heterojunction determination: Heterojunction formation using in-situ XPS
technique has been observed by several researchers like heterojunction between
BiVO 4 /NiO, BiVO 4 /CoO x , BiVO 4 /Sn-doped In 2 O 3 (ITO) and MoO 3 /MoS 2
were observed [12, 13]. In brief, Lalita et al. monitored the heterojunction formation between MoO 3 and MoS 2 phase which was in-situ grown by
nonthermal O 2 plasma irradiation over MoS 2 nanoflakes [13]. Firstly, they
prepared MoS 2 nanoflakes and then those nanoflakes undergo O 2 plasma treatment in a controlled way. XPS was utilized to monitor the phase formation
of MoO 3 or to monitor the heterojunction formation when MoS 2 slowly transformed to MoO 3 matrix. Figure 6a represents the high-resolution spectra for Mo
3d obtained for samples undergo O 2 plasma treatment from 0 to 180 s. and corresponding S 2p spectra are shown in Fig. 6b. As the irradiation time increases, a
new peak starts appearing near the binding energy 170–172.0 eV, this could be
assigned to the oxy-sulfides bonds. The increment in the oxygen peak from the
barely present oxygen peak in untreated MoS 2 to O 2 treated sample indicates
the increased amount of O and slowly formed MoO 3 phase. The O 1s spectral
peak (Fig. 6c) situated near binding energy at 532 eV in molybdenum disulphide corresponds to the adsorbed oxygen species is in good agreement with
the literature [14].
In the O spectra (Fig. 6c) one new peak appeared with the O plasma irradiation,
near to binding energy of 530 eV which can be attributed to the binding energy
of Mo–O bond. The introduction of O in the lattice is proved by the binding
energy values corresponds to Mo–O bond. The presence of Mo–O bond directly
verify the introduction of O into the lattice rather than surface oxidation. One
big hump near to 531 eV corresponds to lattice oxygen which increase with the
increase in the oxygen plasma irradiation time. Thus, leads to increment in the
MoO 3 phase.