This document describes research on using carbon dots as both reducing agents and stabilizing agents in the synthesis of palladium nanoparticle-carbon dot composites. The carbon dots were able to reduce palladium chloride to form palladium nanoparticles coated with an ultrathin carbon dot shell. These palladium@carbon dot composites showed high catalytic activity for carbon-carbon bond formation reactions like Heck and Suzuki couplings. The document provides details on the synthesis and characterization of the carbon dots and palladium@carbon dot composites, as well as evaluation of their catalytic activity.

1. Dalton
Transactions
COMMUNICATION
Cite this: Dalton Trans., 2013, 42, 13821
Received 10th May 2013,
Accepted 30th July 2013
DOI: 10.1039/c3dt51234g
www.rsc.org/dalton
Carbon dot reduced palladium nanoparticles as active
catalysts for carbon–carbon bond formation†
Deepa Dey,* Tamalika Bhattacharya, Biju Majumdar, Sonam Mandani,
Bhagwati Sharma and Tridib K. Sarma*
Carbon dots were used as a reducing agent for the synthesis of Pd
nanoparticles coated with ultrathin carbon dot shells of ca. 4 nm.
The resulting composite nanoparticles showed high catalytic
activity for the Heck and Suzuki coupling reactions.
Carbon nanodots (C-dots) constitute a fascinating new class of
carbon structures showing size and excitation wavelength
dependent photoluminescence (PL) behaviour.1
With their
high photostability and lack of known cytotoxicity, C-dots are
considered to be a green alternative to fluorescent semicon-
ductor nanoparticles and have shown potential use in optical
detection, bioimaging, light emitting diodes, fluorescent ink
and photocatalysts.2
The presence of carboxylic and hydroxyl
moieties at their surface endows excellent water solubility as
well as biocompatibility. This optimism has led to increased
interest recently in developing methods for their synthesis,
involving approaches such as laser ablation, pyrolysis, wet oxi-
dation, ultra-sound and microwave assisted synthesis, hydro-
thermal synthesis and electrochemical etching.1,3
Several
natural sources such as carbohydrates, proteins, amino acids,
biopolymers etc. have been used for the synthesis of C-dots.4
Recently there have been several reports on C-dots derived
from food products such as orange juice, banana juice, soy
milk, egg, sugar, bread, jaggery etc.5
However the potential of
C-dots remains relatively unexplored as compared to the other
carbon based counterparts such as carbon nanotubes and
graphene oxide. Composites of metallic nanoparticles (NPs)
with carbonaceous materials have shown tremendous techno-
logical importance, ranging from catalysis, sensing, fuel cells
and optoelectronics.6
Specifically in catalysis, metal nano-
particles embedded in carbon nanotubes and graphene as
supports act as excellent heterogeneous catalysts for organic
transformations.7
C-dots might also function as excellent sup-
ports for nucleation and growth of nanoparticles leading to
the formation of new functional materials where C-dots play a
critical role in prevention of agglomeration and effective cataly-
sis by the metallic component.
Herein we report a new method for the milligram to gram
scale synthesis of C-dots via thermal carbonization of clotted
cream, a traditional method for the production of butter oil.
This method enables large scale synthesis of highly fluorescent
C-dots having high water solubility without any post-treatment
with acids or surface passivating agents. The hydroxy, carboxy
and epoxy functionalized groups on the C-dot surface act as
nucleation centres for the growth of metallic nanoparticles.
However, C-dots have an inherent reducing property that
enables using them for reduction of metal salts to the corres-
ponding nanoparticles. In our quest for finding new appli-
cations of C-dots, we used the C-dots as reducing as well as
stabilizing agents for the synthesis of Pd NPs, where the
C-dots formed a thin layer around the nanoparticle surface. Pd
nanoparticle composites have been used as an effective hetero-
geneous catalyst in important organic transformations such as
C–C bond formation through Suzuki and Heck coupling.8
We
have studied the efficacy of these novel Pd@C-dot composites
as a catalyst for C–C coupling reactions further expanding the
current paradigm of applicability of C-dots.
A simple, low cost preparative strategy involves the thermal
simmering of clotted cream forming butter oil along with a
brown residue as illustrated in Fig. 1A. The dried brown
residue thus obtained after separating from the butter oil
could be dispersed in water and other organic solvents
(Fig. S2†). The dried solids recovered by lyophilizing water dis-
persed C-dots showed a yellow emission when excited with
365 nm UV light (Fig. 1B). While these brown residues were
dispersed in water, the solution turned pale yellow in the
absence of any surface passivating agent and exhibited blue
fluorescence under UV light (365 nm) (Fig. 1C). When these
C-dots were excited at the excitation edge of 340 nm, a maximum
emission peak at 432 nm was observed. In addition, with an
increase in the excitation wavelength from 290 to 530 nm, the
†Electronic supplementary information (ESI) available: Experimental section
and supporting figures. See DOI: 10.1039/c3dt51234g
Discipline of Chemistry, School of Basic Sciences, Indian Institute of Technology
Indore, IET Campus, DAVV, Khandwa Road, Indore 452017, Madhya Pradesh, India.
E-mail: deepa@iiti.ac.in, tridib@iiti.ac.in; Fax: +91 731 2364182;
Tel: +91 731 2438706
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2. emission from C-dots gradually shifted to higher wavelength
with decreased fluorescence intensity (Fig. 1D). These exci-
tation dependent emissions from C-dots have also been
reported previously1,3–5
and presumably occur due to nanodots
of different sizes along with a considerable contribution from
emission trap sites on each C-dot. The PL quantum yield
measured using quinine sulphate as a reference was 1.4%
comparable with those of the reported luminescent C-dots.9
Transmission electron microscopic (TEM) images of C-dots
demonstrated well-dispersed and spherical NPs of average size
6.6 nm (Fig. 2A). The high resolution TEM (HRTEM) image of
C-dots showed high crystallinity with the appearances of
lattice fringes signifying the (102) lattice of graphitic (sp2
)
carbon (Fig. 2C). The high crystallinity of the C-dots was
further supported by the corresponding selected area electron
diffraction (SAED) pattern (Fig. 2A). Atomic force microscopy
measurements also validated the formation of C-dots with par-
ticle sizes in the range of 4–10 nm (Fig. 2D). Their topographic
heights are mostly between 1 and 2 nm. Fig. 2E presents the
fluorescence decay profile of C-dots, which shows double expo-
nential decay kinetics. The mean lifetime τ¯ was calculated to
be 4.9 nS (χ2
= 1.08) which was comparable with previously
reported values.10
The short lifetime of the fluorescence of the
C-dots indicates the radiative recombination of excitons giving
rise to fluorescence.
The inherent reducing capability of C-dots was realized by
reduction of metal salts leading to the growth of metallic
nanoparticles.11
When H2PdCl4 was reacted with C-dots in
water under refluxed conditions (100 °C), Pd@C-dots were
formed with an ultrathin C-dot layer of ca. 3.8 nm around the
Pd NP surface. As observed in the TEM image (Fig. 3A), each
composite NP had a low contrast shell of continuous C-dot
layers wrapping a high contrast Pd core, signifying the core–
shell structure. The Pd NPs appeared to form chain like struc-
tures embedded within a carbon matrix, suggesting that the
growth of the NPs was initiated from the C-dot surface. Prob-
ably the peripheral carboxyl groups facilitated the binding and
subsequent reduction of the metal salts. The SAED pattern of
Pd@C-dots displayed high crystallinity and the ring patterns
corresponding to Pd metal with fcc structure were observed
(Fig. S5†). The absorption spectrum of C-dots shows a narrow
peak at 280 nm assigned to the π–π* transition of nanocarbon.
However, in the Pd@C-dots, this characteristic peak dis-
appeared as shown in Fig. 3B. Further evidence for the reduction
of Pd2+
salts by C-dots was obtained by observing the dramatic
Fig. 1 (A) Digital image of gram scale crude C-dot samples synthesized by
thermal caramelization of clotted cream. (B) Water soluble C-dots extracted
from the crude product; image of the dried sample in daylight and excited by
365 UV lamp. (C) Photograph of the water dispersed C-dots excited by daylight
and a 365 UV lamp. (D) Photoluminescence spectra of C-dots at different exci-
tation wavelengths as indicated; normalized spectra (inset).
Fig. 2 (A) TEM image (scale bar 100 nm), corresponding SAED pattern (inset)
and (B) particle size distribution of the C-dots dispersed in water; (C) HRTEM
image showing the lattice fringes; (D) AFM image of C-dots deposited on mica
(scale bar 100 nm) and corresponding height profile along the line; (E) fluor-
escence decay profile (λex = 375 nm and λem = 430 nm) of C-dots.
Fig. 3 (A) TEM image (scale bar 20 nm) and HRTEM image of Pd@C-dot nano-
particles (inset). (B) UV-visible and fluorescence spectra (inset) of (a) C-dots and
(b) Pd@C-dots dispersed in water. (C) Powder XRD spectra of (a) C-dots and (b)
Pd@C-dots deposited on a glass slide. (D) FTIR spectra of (a) C-dots and (b)
Pd@C-dots dried from aqueous solution using KBr pellets.
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3. quenching of the signatory fluorescence emissions of C-dots
(Fig. 3B, inset) in aqueous solution. This strengthens the
mechanistic aspect that radiative recombination of electrons
and holes trapped at the surface sites of the C-dots is respon-
sible for the emission properties.12
The electrons reduce the
Pd2+
salts to corresponding nanoparticles and the nucleation
is initiated at the surface sites that host the electrons, thus
being effective in disrupting the radiative recombinations
leading to quenching of the fluorescence emissions. The
powder XRD spectra of C-dots showed a broad peak centred at
2θ = 23° corresponding to 3.8 Å, whereas Pd@C-dot showed a
characteristic (111) reflection peak of Pd in addition to C-dot
characteristic diffraction (Fig. 3C). From the FTIR spectra
(Fig. 3D), it was observed that the peaks due to CvO stretching
frequency in the 1700–1780 cm−1
region in the case of C-dots
disappeared when they were involved in Pd NP formation.
Further the intensity of the C–O (alkoxy) stretching vibration at
1060 cm−1
was significantly diminished (ESI 2†). In order to
get further structural evidence, we performed X-ray photo-
electron spectroscopy (XPS) measurements on C-dots and
Pd@C-dot composites. The high resolution spectrum of the
C1s region of C-dots (Fig. 4A) revealed the presence of C–C
(284.8 eV), C–O (286.8 eV), CvO (287.8 eV) and COOH (289.0
eV) bonds,13
indicating the enrichment of hydroxyl, carbonyl
and carboxylic acid groups on the C-dot surface. In the case of
Pd@C-dot composites (Fig. 4B), there was a dramatic
reduction in the intensity of the oxygenated peaks present on
the surface of C-dots. By analyzing the XPS spectra (Fig. S7†), it
was observed that pristine C-dots were 71.65% carbon and
28.35% oxygen, whereas Pd@C-dots were 75.38% carbon and
24.62% oxygen. An increase of carbon content and a decrease
of oxygen indicated deoxygenation of C-dots during the Pd
nanoparticle formation. From FTIR and XPS studies, it was
evident that part of the oxygenated groups on the C-dots were
involved in the redox reaction between C-dots and PdCl4
2−
salts.
Pd catalyzed C–C coupling reactions are recognized as
powerful and convenient synthetic methods in organic chem-
istry.14
On the other hand, carbon in different forms has been
used as efficient supports for nanoparticle dispersion to
prevent agglomeration and provide a large surface area. There-
fore we were encouraged to carry out Suzuki–Miyuara and
Heck coupling as model reactions to investigate the catalytic
efficacy of Pd@C-dot nanoparticles in water medium. When
Pd@C-dot nanoparticles were used as a catalyst for the coup-
ling of phenylboronic acid and bromobenzene, the catalytic
activity of Pd decreased during the reaction, yielding a
maximum of 45% biphenyl conversion even at elevated temp-
erature and high catalytic loading (Table S5†). The Pd@C-dots
precipitated during the reaction suggesting that the ultrathin
C-dot layer was incapable of preventing Pd agglomeration
during the catalytic reaction. When a small amount of poly-
(N-vinyl-2-pyrrolidone) (PVP) was used as a co-stabilizer,14c
the
reaction was complete within 12 h with a biphenyl yield of
95%. In order to find out the optimized conditions, we varied
the amount of catalyst, temperature and base in water
(Table S5†). In the presence of PVP, the Pd@C-dot catalyst was
stable without any noticeable precipitation that led to the com-
pletion of the reaction with encouraging yield. We observed
that there was a substantial enhancement in the yield when
the Pd@C-dot-PVP catalyst amount was increased from 0.3 to
0.5 mol%. For Suzuki–Miyaura coupling, the catalytic activity
was evaluated for the coupling of aryl bromides, as the acti-
vation of C–Br bonds is very difficult compared to C–I bonds.
We were interested in studying the general trend of this reac-
tion using Pd@C-dot-PVP catalyst with diverse substrates
under optimized conditions and the results are shown in
Table 1. The presence of electron withdrawing groups in the
aryl ring had a considerable effect on the reaction rate with
lower yield of the desired products (Table 1, entry 5). The
Suzuki coupling could be successfully performed with hetero-
cyclic substrates also (Scheme S1†). The synthetic efficacy of
this catalyst was also evaluated for aqueous Heck coupling
reactions of styrene and substituted styrene with iodobenzene
(Table 2). The reactions gave good yields of the corresponding
products at 40 °C in the presence of K2CO3. Whereas the elec-
tron-donating groups had little effect on the kinetics of the
reaction, the presence of the electron-withdrawing group
resulted in lower yield of the product (Table 2, entry 4).
Fig. 4 C1s XPS of (A) C-dots and (B) Pd@C-dots.
Table 1 Suzuki–Miyaura coupling of arylbromides using Pd@C-dot-PVP
catalysta
Entry R1 R2 Time (h) Yieldb
(%)
1 H H 6 95
2 p-CH3 H 6 93
3 p-CH3–CH2 H 6 80
4 p-CH2vCH H 8 86c
5 o-CHO H 10 74
6 p-OCH3 p-CH3 6 89
7 p-OCH3 H 6 78
8 p-OCH3 p-Br 10 67
a
Reaction conditions: aryl bromide (0.7 mmol), aryl boronic acid
(1.0 mmol), Pd catalyst (0.5 mol% with respect to aryl boronic acid),
KOH (3.0 equiv.), water (5 ml). b
Isolated product. c
Homocoupling
product.
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4. Isolation of the catalyst from the reaction mixture and
using them for successive reactions makes the process
effective for industrial applications. For studying this, we
reused the Pd@C-dot-PVP catalyst in subsequent reactions and
the results showed efficient catalytic activity even after the
third cycle (Table S6†). In order to study any structural changes
of the catalyst during their catalytic activity, we performed
TEM and XPS studies of the catalyst after the 1st cycle. As
shown in the TEM image (Fig. 5A), there was no noticeable
agglomeration of the Pd nanoparticles, and the C-dot layers
were intact in the Pd nanoparticles. In order to get further
information about the chemical state of Pd in the Pd@C-dot-
PVP catalyst before and after they were involved in the Suzuki
coupling reaction, we performed high resolution XPS analysis
in the range of 350–330 eV. As shown in Fig. 5B, there was no
significant shift in the Pd3d3/2 and Pd3d5/2 peaks with binding
energies of 340.2 eV and 335.3 eV suggesting the stability and
minimal structural changes of Pd nanoparticles during the
catalytic reaction. We studied the preliminary reaction mechan-
ism through Pd leaching experiments coupled with ICP-AES
analysis (ESI 4†). The results suggested that Suzuki reactions
might be catalyzed largely by trace amounts of active Pd
species in the reaction solution under the conditions
employed in the study. The results are consistent with the
earlier observations of “quasi-heterogeneity” of Pd
nanoparticles in C–C coupling reactions.15
It is worth mention-
ing that we required a higher catalytic loading of Pd@C-dot-
PVP catalyst compared to other commonly used catalysts such
as Pd-PVP and Pd/C for the Suzuki coupling reaction (ESI 5†).
We believe that the presence of C-dot layers on the Pd nano-
particle surface might inhibit the kinetics of the catalyzed reac-
tion. The evolution of Pd2+
ions, mechanistically perceived to
be the driving force behind the catalyzed reaction, might be
hindered due to in situ reduction of the metal ions to Pd0
by
C-dots simultaneously.
In conclusion, we have shown the fabrication of C-dots
from a bio-precursor and used them for the reduction of
PdCl4
2−
salts leading to the formation of Pd@C-dot core–shell
nanostructures. Although bare C-dots were not capable
enough to prevent agglomeration of the Pd NPs during the
Suzuki and Heck reactions, addition of a co-stabilizer in the
form of PVP led to an efficient composite that showed high cat-
alytic activity towards the formation of C–C bonds. Further
improvement of the stability of the catalyst as well as detailed
mechanistic investigation will promote C-dot composite
materials as a green and efficient catalyst for practical appli-
cations. Realizing the potential of C-dots as reducing and
stabilizing agents towards the development of functional
materials will definitely lead to eco-friendly routes for chemi-
cal transformations.
This work was supported by research grants from DST
research funding (SR/WOS-A/CS-50/2010 and SR/S1/PC-32/
2010). We acknowledge help from SAIF, NEHU, Shillong and
the UGC-DAE Consortium for Scientific Research, Indore
for TEM, powder-XRD and XPS facilities. B. S. and
S. M. acknowledge research fellowships from UGC and CSIR
respectively. We thank Dr Anjan Chakraborty and Ms Raina
Thakur for helpful scientific discussions.
Notes and references
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