Photocatalytic when the polarization vector is positive at C-

Photocatalytic
activity of ?-S8, BaTiO3 and BaTiO3/?-S8
composite was evaluated for the degradation of FR under UV/solar light
irradiation. The photocatalytic activity of the
catalysts under UV light shows the following decreasing order: BaTiO3/?-S8
> BaTiO3 > ?-S8 (Fig. 7) while under solar light the activity of the catalysts changes to: BaTiO3/?-S8
> ?-S8 > BaTiO3 (Fig. 8).  The high activity of
the composite under both UV/solar light compared
to individual counterparts BaTiO3 and ?-S8 is due to the efficient vectorial charge carrier
migration and separation. The degradation experiments showed two to
three fold increase in the rate constant values for BaTiO3/?-S8
composite (Table 3 & 4). Specific redox reactions can take place within ferroelectric
domains of BaTiO3 which is also reported in the literature 8. The
unit cell of BaTiO3 is non-centrosymmetric and hence possess a
permanent spontaneous polarization (P0). This non-centrosymmetric crystal structure leads to the
generation of discrete stable polarization states which generate specific electric
fields and drives electrons and holes in the opposite directions 8. The
resultant surface polarization causes upward band bending when the polarization
vector is positive at C- domain
preventing the movement of electrons and downward band bending takes place when
the polarization is negative at C+
domain facilitating the electron transfer process (Fig. 9). Under light
illumination, charge carriers are photogenerated and the electrons are driven
towards the C+ domain and
holes to the C- domain. This
band bending results in distinct redox chemistry at the surface 9. Reduction occurs
at the C+ domain due to
electron accumulation and oxidation at the C-
domain due to the accumulation of holes 33.Therefore, the extent of band
bending and the magnitude of the spontaneous polarization has a potential
impact on increasing the lifetime of the charge carriers. Under the UV light
both BaTiO3 and ?-S8 undergoes electron excitation from
their respective valence band (VB) to
conduction band (CB) generating electron-hole
pairs 33. The electrons in the CB of BaTiO3
and also from ?-S8 can be
immediately trapped by surface adsorbed molecular oxygen to yield superoxide
radicals due to the perfect matching of CB energy levels with redox potential
of molecular oxygen. Simultaneously the holes in the VB of BaTiO3
have the possibility of moving into the VB of ?-S8 which further
participates in the formation of hydroxyl radicals 34. Both these processes
reduces the rate of recombination in the composite (Fig. 10 a).

The lower activity of BaTiO3 under visible
light is due to its poor ability to absorb the visible light (due to its higher
band gap). ?-S8 exhibited better activity under visible light  due to its narrow ?-?* band gap which is usually
referred to as non-vertical excitation process 31. The BaTiO3/?-S8
composite exhibited enhanced activity under visible light when compared to individual
catalysts. The higher activity of the composite is ascribed to the formation of
a junction between the two catalysts with favorable band edge positions which
leads to the vectorial charge transfer process which
in turn enhances the efficient charge separation (Fig. 10 b). Under solar light
illumination only ?-S8 undergoes excitation and it almost
acts as a sensitizer by transferring the photogenerated electrons to the CB of BaTiO3
which could efficiently activate molecular oxygen to generate superoxide
radicals. Alternatively, the photogenerated electrons from the CB of ?-S8 can also be trapped by the
molecular oxygen. The redox potentials for O2/O2?•
is -0.33 V and CB minimum of ?-S8 was found at -0.45 V (vs
NHE) which is negative enough to reduce oxygen to yield superoxide radical (O2?•)
15, 35.  In the first process where
photosensitization takes place the positive domain of the static dipolar fields in the BaTiO3 is
oriented towards its surface which attracts the excited CB electrons of ?-S8.
The positions of CB minimum of ?-S8 was
found at -0.45 V (vs NHE) while CB minimum of BaTiO3 was found to be
– 0.15 V 15. The CB minimum of BaTiO­3 is 0.30 V lower than the CB
minimum of ?-S8 which thermodynamically favors the electron transfer
process efficiently. The VB maximum of ?-S8 was found at +2.39 V and
the redox potential for •OH/OH- is at +1.99 V. Therefore
the oxidation potential of VB holes in ?-S8 was positive enough to
oxidize hydroxyl anion to hydroxide free radical 36. The hydroxyl free radicals formed from the above processes
further participate in the degradation reaction and helps in the mineralization
of the pollutant dye molecule. The coupling of a wide band gap semiconductor
with a metal free photocatalyst could efficiently promote the vectorial charge
carrier transfer dynamics under both UV/solar light irradiation.

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Further the oxidation of surface sulfur leads to the
formation of

on the surface of the catalyst and this anchored sulphate group acts as reactive electron trapping
site which in turn reduces the electron-hole recombination. The sulfate
species introduces strong acidic nature on the surface and can be referred as
Bronsted acidic sites whose acidity increases by the presence of neighboring Ti4+
ions which acts as strong Lewis acidic sites 37. In addition, the inductive
effect of S=O of the sulfate ions makes the Lewis acid strength of Ti stronger,
since it is easier for the bonded sulfate ion to pull the covalent electrons
away from the Ti4+ ions. The Lewis sites are
converted to Bronsted acid site through the covalent bonding of water to the
same titanium ion. This conversion promotes the formation of hydroxyl radicals
on the photocatalyst surface 38. The strong interaction between the sulfate
anion and titanium cation increases the positive polarity on the titanium
cation. The high electronegativity of sulfur can also induce polarization of
neighboring hydroxyl groups 37. The acidity of the photocatalyst favors the
adsorption of the organic contaminant and any dissolved oxygen in solution. The
highly polarized state and super acidity would favor the trapping of electrons
results in an improved quantum efficiency by producing highly reactive hydroxyl
radicals. The polarization effect within the BaTiO3 lattice and the
polarization effect caused by the presence of sulfate groups on the surface can
be compared to a situation where two parallel plate capacitors are in series
which enhances the efficiency of photocatalysis.