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biocompatible, and have been used in many applications in
our daily lives, including as drug carriers and in cosmetics.[10]
The role of metal ions in the central cavity of the porphyrin in
the proximity of the host semiconductor for efficient decon-
tamination of drinking water has been a subject of several
recent reports.[11] Tuning the photo-response,[12] the efficiency
of photo-injected electrons,[13] and the stability of dyes upon
metalation[14] have been addressed in a series of reports.[15] In
our recent studies, we explored the critical role of the central
metal ions (Fe3+/Fe2+) incorporated into hematoporphyrin-TiO2
nanohybrids and their implications in photocatalysis.[16]
From the practical application point of view, the use of por-
phyrin-based photocatalytic devices for water decontamination
is very important, given the fact that water from natural re-
sources contains metal ions (especially Fe3+ and Cu2+). In the
present study, we have synthesized and characterized a PP-
ZnO nanohybrid for a flow-type photocatalytic solar device for
a prototype water decontamination plant using visible light.
We have explored the role of metal ions, specifically iron(III)
and copper(II), in the test water, and have deployed a model
contaminant, methylene blue (MB), a hazardous waste product
from the textile industry,[17] in the photocatalytic device under
visible light. Femtosecond time-resolved transient absorption
studies have clearly unraveled the key time component associ-
ated with ground-state recovery of the sensitized PP upon
metalation for the change in overall photocatalytic efficiency.
In addition, picosecond-resolved fluorescence studies of the
nanohybrids in the absence and presence of metal ions have
clearly shown that excited-state electron-transfer dynamics is
responsible for the photocatalytic action. Moreover, the role of
UV light excitation of the nanohybrid, in which the host semi-
conductor is expected to be excited, is also discussed. Our
studies are expected to be of relevance to the large-scale use
of porphyrin-based nanomaterials for the decontamination of
drinking water by solar light catalysis.
Figure 1. (a) HRTEM images of ZnO NPs. (b) X-ray diffraction patterns of
ZnO, PP-ZnO, (Fe)PP-ZnO, (Cu)PP-ZnO. (c) FTIR spectra of PP, PP-ZnO, (Cu)PP-
ZnO, (Fe)PP-ZnO. The spectra of PP-ZnO, (Fe)PP-ZnO, and (Cu)PP-ZnO were
taken on a ZnO background. (d) FTIR spectra of PP, PP-ZnO, (Cu)PP-ZnO, and
(Fe)PP-ZnO.
FTIR study
Fourier-transform infrared (FTIR) spectroscopy was used to
confirm the binding mode of PP on the ZnO surface. For free
PP, stretching frequencies of the carboxylic group are located
at 1696 and 1402 cmꢀ1 for the antisymmetric and symmetric
stretching vibrations, respectively, as shown in Figure 1c. In
PP-ZnO, the stretching frequencies of the carboxylic groups
are located at 1618 and 1405 cmꢀ1 for the antisymmetric and
symmetric stretching vibrations, respectively, providing clear
evidence for deprotonation of the carboxylic group upon addi-
tion of ZnO NPs. The difference between the carboxylate
stretching frequencies, D=nasꢀnsym, is useful in identifying the
binding mode of the carboxylate ligand.[20] The observed D
value for the PP-ZnO nanohybrid was 213 cmꢀ1, smaller than
that of free PP (294 cmꢀ1). This suggests that the binding
mode of PP on ZnO is predominantly bidentate. However,
nanohybrids incorporating FeIII and CuII also show bidentate
covalent binding of PP to ZnO NPs through the carboxylic
groups. The NꢀH stretching frequency has been used to inves-
tigate the attachment of the metal ions to the PP associated
with the ZnO host. In free PP, the NꢀH stretching frequency is
at 3441 cmꢀ1 (Figure 1d). In the case of the PP-ZnO nanohy-
brid, the NꢀH stretching frequency of the PP cavity remains
unperturbed as PP anchors onto the ZnO surface through its
carboxylic group. In the presence of iron or copper, the NꢀH
bond is perturbed, indicating that iron(III) and copper(II) bind
to the PP through the pyrrole nitrogen atoms of the porphy-
rin.[16] The binding between PP and ZnO was also confirmed by
Raman spectroscopy. Raman spectra were collected from PP,
ZnO NPs, and PP-ZnO nanohybrids in the wavenumber region
300–600 cmꢀ1. PP molecules do not show an obvious peak in
the experimental range. However, four vibration peaks at 328,
Results and Discussion
Structural characterization of the nanohybrids
A typical high-resolution transmission electron microscope
(HR-TEM) image of ZnO NPs is shown in Figure 1a. From the
TEM study, an average size of the ZnO NPs of 25 nm was esti-
mated. TEM study of a single NP revealed crystal fringes with
an interplanar distance of 0.26 nm (inset in Figure 1a), corre-
sponding to the spacing between two (002) planes of ZnO.[18]
XRD study (Figure 1b) on the bare ZnO NPs (2q range from
208 to 708) and upon sensitization with PP, in the absence and
presence of the metal ions (FeIII, CuII), showed the characteristic
planes of ZnO (100), (002), (101), (102), (110), and (103). Intact-
ness of the crystal planes of ZnO upon sensitization of the PP
dye and metal ions was also clear from this study. Wurtzite
ZnO exhibits well-defined crystallographic faces, that is, polar
(002) and nonpolar (100), (101) surfaces. McLaren et al.[19]
showed the terminal polar faces to be more active surfaces for
photocatalysis than the nonpolar surfaces perpendicular to
them.
Chem. Eur. J. 2014, 20, 10475 – 10483
10476
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim