Photocatalytic application of nanosized CdS immobilized onto functionalized MWCNTs

Article abstract of DOI:10.1039/c3dt53338g

Nanosized semiconductor CdS immobilized onto modified multi-walled carbon nanotubes (MWCNTs) carrying poly(amidoamine) dendron units were visualized by HR-TEM. Evidently, spherical CdS nanoparticles 3-5 nm in diameter were identified. Moreover, EDX spectr

Full text of DOI:10.1039/c3dt53338g

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Photocatalytic application of nanosized CdS
immobilized onto functionalized MWCNTs
Cite this: Dalton Trans., 2014, 43,
7429
D. D. Chronopoulos,a N. Karousis,a S. Zhao,b Q. Wang,b H. Shinoharab and
N. Tagmatarchis*a,b
Nanosized semiconductor CdS immobilized onto modified multi-walled carbon nanotubes (MWCNTs)
carrying poly(amidoamine) dendron units were visualized by HR-TEM. Evidently, spherical CdS nanoparti-
cles 3–5 nm in diameter were identified. Moreover, EDX spectroscopy gave additional spectroscopic
proof of the presence of CdS in the CdS-MWCNTs hybrid material. The photocatalytic activity of
CdS-MWCNTs toward the decomposition of rhodamine B (RhB) was examined by monitoring spectral
changes in the characteristic absorption band of RhB centred at 554 nm. The latter absorption band of
RhB was found to continuously depress during visible light irradiation in the presence of CdS-MWCNTs,
with faster kinetic rates as compared with the case when only reference CdS was present. The current
result was rationalized in terms of efficient photoinduced electron-transfer from CdS to MWCNTs within
the intrahybrid CdS-MWCNTs. In this frame, the suggested mechanism for the high and fast photo-
catalytic decomposition of RhB supports the accumulation of electrons in MWCNTs, which then react
with molecular oxygen, thus reducing it to superoxide radical anion O2•− responsible for the generation of
the highly reactive species of HO• and HOO•. The latter together with the holes generated in photo-
excited CdS were responsible for the decomposition of RhB. Finally, the photocatalyst CdS-MWCNTs was
recovered and efficiently reused for four consecutive catalytic cycles, thus highlighting its wider applica-
bility in removing organic pollutants from water.
Received 27th November 2013,
Accepted 23rd December 2013
DOI: 10.1039/c3dt53338g
www.rsc.org/dalton
cheap and chemically stable semiconductors, suffers from
poor absorption in an extended window of the solar spec-
Introduction
In recent years, there is great risk to human health and eco- trum,6 and thus it is photocatalytically active for the degra-
logical systems from the release of synthetic pigments, dyes dation of environmental pollutants only under UV irradiation.7
and pollutants in the environment.1 Therefore, it is absolutely Therefore, other semiconductor materials with increased solar
necessary to effectively remove such hazardous organic dyes energy conversion efficiency and a slow recombination rate of
from the atmosphere and in particular from wastewater. Along the photogenerated electrons and holes are sought.8 In this
these lines, photocatalysis is a promising technology for context, visible light can be more competently harvested by CdS
decomposition of noxious waste by oxidative reactive species.2 nanoparticles due to their narrow bandgap. In addition, inte-
The latter can be generated by molecular oxygen in the pres- gration of CdS to carbon nanotubes (CNTs) gives rise to novel
ence of accumulated electrons (i.e. by reduction of O2), those electron donor–acceptor hybrid nanostructures in which intra-
for example that are yielded in donor–acceptor ensembles hybrid electron-transfer events can lead to spatial separation of
upon electron-transfer phenomena at the acceptor site.
the photoinduced generated electrons and holes,9 thus deceler-
For such reasons, semiconductor nanoparticles attracted ating the recombination rate and resulting in an increase of the
increased interest during the last decade,3,4 especially due to corresponding lifetime and photocatalytic efficiency.
the advantageous function of inorganic quantum dots in cap-
Recently, a few ensembles in which the CdS were integrated
turing incident photons to induce charge separation.5 In par- into CNTs, either via covalent or supramolecular means,9,10
ticular, TiO2, although being among the most examined, mainly aiming to tackle energy conversion issues, have been
reported. However, to the best of our knowledge, the photo-
catalytic application of covalently functionalized CdS-CNT
aTheoretical and Physical Chemistry Institute, National Hellenic Research
hybrids in removing organic dyes from water yet awaits
Foundation, 48 Vassileos Constantinou Avenue, Athens 11635, Greece.
exploration. Even hybrid systems in which the CdS was inte-
E-mail: tagmatar@eie.gr; Fax: +30 210 7273794; Tel: +30 210 7273835
bDepartment of Chemistry and Institute for Advanced Research, Nagoya University,
Nagoya 464-8602, Japan
grated into CNTs via supramolecular means or in composite
forms, with the inherent handicap of weaker interactions
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between the two species due to inefficient stable anchoring of electronic interactions between CdS and MWCNTs and fur-
the semiconductor nanoparticles to CNTs, were rarely explored thermore suggested electron and/or energy transfer processes
as photocatalysts.11
from CdS to MWCNTs within the hybrid material,12 similar to
Having succeeded on higher loading of CdS onto covalently cases of other hybrid materials composed of CdS and CNTs.13
modified multi-walled carbon nanotubes (MWCNTs) carrying Focusing on the morphology, shape and size of
poly(amidoamine) (PAMAM) dendron units ending in an ethyl- CdS-MWCNTs, HR-TEM imaging revealed the immobilization
enediamine core,12 which served as a template for the stabiliz- of nanosized spherical CdS onto the PAMAM-modified
ation of monodispersed CdS nanoparticles, our research MWCNTs. Actually, HR-TEM imaging of CdS-MWCNTs under
efforts were directed on the examination of that CdS-MWCNTs low-magnification conditions showed high density coverage of
hybrid material toward the photocatalytic degradation of rho- the MWCNTs with CdS of around 15 nm in diameter (Fig. 1a).
damine B (RhB). Herein, it is shown that the presence of CdS However, detailed analysis under high-magnification conditions
nanoparticles uniformly immobilized onto PAMAM-modified illustrated that clustering of smaller-sized CdS nanoparticles
MWCNTs is able under visible illumination to induce (i.e. around 3–5 nm in diameter) occurs (Fig. 1b). Furthermore,
decomposition of aqueous RhB, with fast kinetic rates, due to direct proof that the aforementioned spherical nanoparticles
the electron-transfer processes that occur within the hybrid are due to immobilized CdS arose from energy dispersive X-ray
nanostructure.
(EDX) spectroscopy, since the latter is a widely applied surface
technique to study the elemental composition of nanostruc-
tured carbon-based hybrid materials. Thus, in the EDX spec-
trum of CdS-MWCNTs the presence of both cadmium and
sulfur in the hybrid material was verified, yet with a 1 : 1 average
atom ratio of Cd/S over the selected area (Fig. 1c).
Results and discussion
Preparation and characterization of CdS-MWCNTs
For comparison purposes and to be used as reference
material for the photocatalytic degradation of RhB (see below),
CdS nanoparticles were synthesized in the absence of
MWCNTs. In this frame, following similar experimental pro-
cedures like for the case of CdS-MWCNTs, reference CdS nano-
particles were stabilized upon treatment with 6-(BOC-amino)-
caproic acid. Likewise with the CdS-MWCNTs hybrid material,
HR-TEM imaging revealed the success of the reference CdS for-
mation, by observing aggregated spherical nanoparticles with
an average diameter of 50–70 nm (Fig. 1d), consisting of
smaller CdS nanoparticles of 5 nm in diameter (Fig. 1e). EDX
spectroscopy of reference CdS further proved their efficient for-
mation by observing the characteristic peaks for cadmium and
sulphur (Fig. 1f).
The preparation of CdS-MWCNTs material was accomplished
following our recently reported method.12 Briefly, amino-termi-
nated aryl-functionalized MWCNTs were used as a scaffold for
the step-by-step growth of a second-generation one-branched
dendron, carrying four terminal carboxyl groups per aryl unit
grafted onto the skeleton of MWCNTs. These carboxylic units
acted as anchor sites for the stabilization of Cd2+ ions, which
were subsequently converted to CdS nanoparticles after mild
thermal treatment with thiourea to yield CdS-MWCNTs
(Chart 1). The detailed spectroscopic characterization of
CdS-MWCNTs has already been reported, while fluorescence
emission studies showed that upon photoillumination of
CdS-MWCNTs the characteristic strong and broad emission of
CdS nanoparticles was quantitatively quenched by the pres-
ence of MWCNTs.12 The latter was indicative of strong
Photocatalytic degradation of RhB
The photocatalytic activity of nanosized CdS immobilized on
the PAMAM-functionalized MWCNTs was examined by moni-
toring the degradation of RhB under visible light irradiation.
In this frame, temporal changes in the concentration of RhB
were checked by inspecting variations occurring at the charac-
teristic absorption band of RhB centered at 554 nm. Initially, it
was confirmed that the absorption band of an aqueous solu-
tion of RhB (i.e. 554 nm) remained unchanged even after 3 h
of visible light irradiation. Then, for the current study, in an
aqueous solution of RhB (5 mL, 10−5 M),
5 mg of
CdS-MWCNTs (i.e. CdS loading: 0.14 μmol mg−1 as calculated
by thermogravimetry) as a photocatalyst was added and the
system was stirred in the dark for 30 min in order to establish
a suitable adsorption–desorption equilibrium between RhB
and the catalyst surface. UV-Vis spectroscopy (Fig. 2a) indi-
cated that the main band of RhB at 554 nm was practically flat-
tened after ca. 60 min of visible light irradiation in the
presence of air, thus suggesting the complete photocatalytic
degradation of RhB by the presence of CdS-MWCNTs. It is
Chart 1 Partial structure of nanosized CdS immobilized onto PAMAM-
modified MWCNTs.
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Fig. 1 Low (a, d) and high (b, e) magnification HR-TEM images and EDX spectra (c, f) of CdS-MWCNTs hybrid material and reference CdS respect-
ively. In the EDX spectra (c, f), stars denote the elements Cu and Si which are detected because of their presence in the microscope equipment,
sample holder, and crystal detector.
Fig. 2 UV-Vis spectral changes of aqueous RhB in the presence of (a) CdS-MWCNTs hybrid material, and (b) reference CdS, under visible light
irradiation in the presence of air. (c) Photocatalytic performance of CdS-MWCNTs (red), reference CdS (black) and reference PAMAM-modified
MWCNTs (grey) in the degradation of aqueous RhB under visible light irradiation in the presence of air.
noteworthy that continuous and significant quenching of the stepwise N-de-ethylation of RhB; namely, one and three ethyl
maximum absorbance of RhB took place during visible light groups were lost after 30 and 60 min of light irradiation, respect-
irradiation in the presence of CdS-MWCNTs, thus highlighting ively.16 The latter is justified by monitoring the formation of
the degradation of the xanthene conjugated system in RhB.14 At new absorption bands at 539 nm and 508 nm due to the for-
the same time, the maximum of the main absorption band of mation of N,N,N′-triethyl rhodamine and N-ethyl rhodamine,
RhB was gradually blue-shifted by 56 nm (i.e. from 554 to respectively, before accessing the fully N-de-ethylated rhodamine
498 nm) and completed within 90 min of photoirradiation, thus (Chart 2) with characteristic absorption centered at 498 nm.15 At
supporting the complete N-de-ethylation of RhB to rhoda- this stage it should also be mentioned that without visible light
mine.15 Actually, careful inspection on the temporal changes irradiation but in the presence of CdS-MWCNTs, the absorption
occurred in the absorption spectrum of photoirradiated RhB in profile of RhB remained unaltered, thus highlighting the neces-
the presence of CdS-MWCNTs as the photocatalyst revealed the sity of light for the decomposition process.
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these results the number of turnovers (TON), namely the
number of substrate molecules that a molecule of catalyst can
convert to products under a certain set of experimental con-
ditions, was calculated and found to be 14. At the same time,
the turnover frequency (TOF) for the same set of experiments
remained unchanged and found to be 9.5 h−1. To the best of
our knowledge, these TON and TOF values are the highest
obtained, although yet not optimized, for evaluating the
efficiency of recovery and reuse of CdS-MWCNTs for the photo-
catalytic degradation of RhB.
Mechanism for the photocatalytic degradation of RhB by
CdS-MWCNTs
The enhancement of the photocatalytic degradation of RhB by
CdS-MWCNTs as opposed to reference CdS is rationalized in
terms of the effective separation of holes–electrons in photo-
excited CdS. This is to say that electron-transfer from photo-
excited CdS to MWCNTs, within CdS-MWCNTs hybrid,
occurs.9 Then, the electrons accumulated in MWCNTs react
with molecular oxygen, thus reducing it to superoxide radical
anion O2•− responsible for the generation of the highly reactive
species of HO• and HOO•. Eventually, degradation of RhB
occurs by the holes generated in CdS and the powerful oxidiz-
ing species of HO• and HOO• capable of degrading most pollu-
tants.17 Based on the aforementioned mechanism, it is
understandable that charge separation in CdS-MWCNTs is the
process that governs the photodecomposition of RhB as
opposed to the case of reference CdS, since in the latter a
rapid recombination of the hole–electron pair is inevitable due
to the absence of suitable electron acceptor units such as
MWCNTs are. The photocatalytic mechanism for the action of
CdS-MWCNTs in the decomposition of RhB is schematically
illustrated in Fig. 3.
Chart 2 Molecular structures of RhB and the N-de-ethylated bypro-
ducts rhodamine, N,N,N’-triethyl rhodamine and N-ethyl rhodamine
formed during visible light photodegradation of RhB by CdS-MWCNTs
in the presence of air.
Control experiments performed in order to test if the photo-
catalytic degradation of RhB proceeds – and to what extent, if
it occurs – in the presence of PAMAM-modified MWCNTs (i.e.
without CdS) and in the presence of reference CdS nanoparti-
cles (i.e. without MWCNTs). Evidently, while upon visible light
irradiation, photodegradation of RhB in the presence of
PAMAM-modified MWCNTs was negligible (if not at all), refer-
ence CdS nanoparticles showed moderate photocatalytic
activity (i.e. the UV-Vis spectral changes of RhB in the presence
of reference CdS are displayed in Fig. 2b), however, with rather
slower kinetics as compared with the ones observed for the
CdS-MWCNTs hybrid material (cf. Fig. 2a). Thus, as the graph
for the temporal course of the photodegradation of RhB shows
(Fig. 2c), in 60 min only 50% of RhB was photodecomposed by
the reference CdS, while after the same elapsed period more
than 95% of RhB was degraded by CdS-MWCNTs. In addition,
control experiments in the absence of air were negative, thus
suggesting the necessity of the latter as discussed in the mech-
anism of photodegradation of RhB below.
Recovery and reuse of CdS-MWCNTs for the photocatalytic
degradation of RhB
In order to evaluate the photocatalytic stability of
CdS-MWCNTs hybrid material, the recyclability of the catalyst
in the photodegradation of RhB under visible light irradiation
was tested. In this context, CdS-MWCNTs were recovered after
completion of the first run by filtration, then washed with
water to remove any organic material entrapped and then re-
used by adding it to a freshly prepared aqueous solution of
RhB of the same concentration as the initial one (i.e. 5 mL,
10−5 M). Four successive catalytic cycles were performed and
all degradation efficiencies toward RhB obtained after 90 min
of visible irradiation were slightly compromised but always
Fig. 3 Schematic illustration for the electron-transfer process within
CdS-MWCNTs responsible for the photocatalytic degradation of
found above 90% as compared with the first run. Based on aqueous RhB under visible light irradiation in the presence of air.
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At given irradiation time intervals, 3 mL of the reaction
mixture was sampled and separated by centrifugation. The
Conclusions
Summarizing, the application of CdS-MWCNTs on the photo- concentration of RhB was determined by monitoring changes
degradation of RhB upon visible light irradiation in the pres- in the absorption band centered at 554 nm.
ence of air was successful. Initially, the immobilized CdS
nanoparticles on PAMAM-modified MWCNTs were monitored
by extensive HR-TEM imaging, highlighting the presence of
CdS as round objects with a diameter of 3–5 nm, which were
Acknowledgements
further spectroscopically identified by EDX analysis. Then, the Partial financial support by the Japan Society for the Pro-
high visible-light photocatalytic performance of CdS-MWCNTs motion of Science (JSPS) Program “FY2012 JSPS Invitation
toward decomposition of RhB was demonstrated by monitor- Fellowship for Research in Japan – Long Term” contract
ing temporal changes in the concentration of RhB as followed number L-12531 to NT is acknowledged.
by changes observed in the characteristic absorption band of
RhB centered at 554 nm. The CdS-MWCNTs hybrid material
was recovered simply by filtration, re-used and found to
efficiently catalyze the degradation of RhB for four successive
Notes and references
cycles. The high photocatalytic activity of CdS-MWCNTs was
facilitated by the separation of the photogenerated electron–
hole pair in CdS due to efficient photoinduced electron-trans-
fer from CdS to MWCNTs and the accumulation of electrons
in MWCNTs, which can then react with molecular oxygen,
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General
All solvents and reagents were purchased from Aldrich and
used without further purification unless otherwise stated. CdS-
decorated PAMAM-modified MWCNTs and reference CdS were
synthesized according to previously reported procedures.12
Steady-state UV-Vis electronic absorption spectra were
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JEM-2100F (JEOL) high-resolution field-emission gun TEM
operated at 80 keV at room temperature and under a pressure
of 10−6 Pa. HR-TEM images were recorded with a charge-
coupled device with an exposure time of typically 1 s. Energy
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formed using the same microscope equipped with a super
atmospheric thin-window X-ray detector.
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which was positioned 20 cm away from the reactor. The photo-
catalyst (5 mg, 0.14 μmol mg−1) was added into the aqueous
solution of RhB (5 mL, 10−5 M) with full stirring in the dark
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