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4.5. Cyclic voltammetry studies
the 300 nm peak, revealed 75% degradation while the 500 nm peak
revealed 85% degradation. Likewise when the nanocomposite was used
as a photocatalyst, under UV irradiation, Fig. 6(b), the 300 nm peak,
revealed 60% degradation while 50% degradation was noticed for the
500 nm peak. The plots of ln C/Co vs. time, Fig. 7 (a), (b) revealed the
degradation kinetics of AR dye degradation using PNA and 1-ZnO/PNA
as catalysts under microwave and UV irradiation respectively. The kinet-
ics was found to be of first order in all the cases. The rate constant
(k) values using PNA as catalyst under microwave irradiation were ob-
served to be 0.009 and 0.007 for 300 nm and 550 nm peaks respectively.
The k values for UV exposed solutions using the same catalyst, were cal-
culated to be 0.005 and 0.004 for the 300 nm and 500 nm peaks respec-
tively. Hence the kinetics of degradation under microwave was twice as
fast as compared to UV light irradiation. Using ZnO/PNA as catalyst
under microwave irradiation, Fig. 7 (a), the rate constant values were
observed to be 0.052 and 0.036 for the 300 nm and 500 nm peaks respec-
tively while under UV irradiation using the same catalyst, the rate
constant values were observed to be 0.009 and 0.007 for the 300 nm
and 500 nm peaks respectively. The kinetics of degradation was noticed
to be higher under microwave than under UV irradiation and remarkably
high when ZnO/PNA was used as catalyst.
The cyclic voltammogram of pure PNA, Fig. 5(a), exhibited an oxida-
tion peaks at 0.7 and 0.1 V while the reduction peak was observed at
0.35 V and 0.1 V respectively. The peaks observed were not sharp and
also confirmed that the oxidation and reduction were irreversible in
PNA. The nanocomposite 1-PNA/ZnO, Fig. 5(b), showed only one oxida-
tion peak at 0.7 V while 2 reduction peaks were observed at 0.45 V and
0.1 V respectively. Likewise for 3-PNA/ZnO, Fig. 5(c), the oxidation peak
was observed at 0.7 V while reduction peak was found at 0.1 V and 0.4 V
respectively. The 5-PNA/ZnO nanocomposite, Fig. 5(d), showed similar
oxidation peak as observed incase of the other two nanocomposites at
0.7 V while the reduction peak appeared at 0.4 V. The results revealed
that loading of ZnO caused a slight shift in the reduction peaks of the
nanocomposites as compared to pure PNA. The HOMO energy was
calculated to be −5.5 eV for all the samples while the LUMO energy
was observed to vary from −4.5-5 eV.
4.6. Dye degradation studies
The degradation of AR dye solution was monitored
spectrohphotomerically upon exposure to microwave irradiation for
40 min and under UV irradiation for 120 min respectively in presence
of PNA and ZnO/PNA as catalyst. The UV–visible spectrum of PNA-250
solution upon exposure to microwave irradiation for 40 min showed a
decrease in the characteristics absorbance peaks at 300 nm and
⁎
500 nm. The peak at 300 nm were assigned to π-π transitions of the
benzene ring in AR dye while the peak at 500 nm was associated with
that of substituted benzene. As the exposure time increased from 5 to
40 min, the absorbance maxima decreased in both the peaks. The
peak at 300 nm showed a decrease in the absorbance value from 1.4
in 5 min to 0.6 in 40 min. Likewise, the peak at 500 nm showed a de-
crease from 1.7 to 1.0 in 40 min. The spectra of PNA-250, upon exposure
to UV irradiation also showed significant reduction in the absorbance
values i.e. from 1.4 in 30 min to 0.8 in 120 min incase of the peak ob-
served at 300 nm while the peak at 500 nm shows a decrease from
1.6 to 1.0 in 120 min. The UV–visible spectrum of 1-ZnO/PNA-AR-250
also revealed a decrease in the absorbance value from 1.5 to 0.3 in
40 min for the peak at 300 nm while for the 500 nm peak, the absor-
bance values showed a decrease from 1.2 to 0.4 in 40 min. Similarly,
for UV irradiated 1-ZnO/PNA-AR-250 solution, the decrease in the
absorbance of 300 nm peak was noticed from 1.5 to 0.6 in 120 min
and for the 550 nm peak, the absorbance values decreased from 1.0 to
0.6 in 120 min (supplementary information).The decrease in the absor-
bance intensity of the peaks in the UV and visible region thus confirmed
the degradation of the AR dye. It appeared that under microwave irradi-
ation, upon addition of ZnO/PNA, the decrease in the absorbance inten-
sity of the peak in the visible region was enhanced which confirmed the
catalytic behaviour of the nanohybrid under microwave-irradiation.
Hence it can be concluded that dye degradation was enhanced when
ZnO/PNA nanocomposite was used as a catalyst under microwave irra-
diation. Under UV irradiation, the degradation time taken was three
times slower than the time taken under microwave irradiation. As
compared to pristine PNA, the rate of degradation under microwave
irradiation was enhanced using ZnO/PNA as catalyst. The mechanism
is explained in the later section.
4.7. Comparison of degradation kinetics
The plot of C/Co vs. time using PNA as catalyst under microwave ir-
radiation, Fig. 6(a), showed 35% degradation for the 300 nm peak while
for 500 nm peak, degradation was observed to be 25%. Similarly, for PNA
as photocatalyst under UV irradiation, Fig. 6(b), the % degradation was
observed to be 40% and 36% respectively. Under microwave irradiation,
when the nanocomposite 1-ZnO/PNA was used as catalyst, higher
degradation was observed Fig. 6(a). In presence of the nanocomposite,
Fig. 6. C/Co vs. time plot for AR dye solution containing PNA and 1-ZnO/PNA as catalyst
exposed to (a) microwave irradiation (b) UV irradiation.