COMMUNICATION
pubs.acs.org/JACS
Carbon Nanoparticles as Visible-Light Photocatalysts for Efficient CO2
Conversion and Beyond
Li Cao,† Sushant Sahu,† Parambath Anilkumar,† Christopher E. Bunker,*,‡ Juan Xu,† K. A. Shiral Fernando,§
Ping Wang,† Elena A. Guliants,§ Kenneth N. Tackett, II,† and Ya-Ping Sun*,†
†Department of Chemistry and Laboratory for Emerging Materials and Technology, Clemson University, Clemson, South Carolina
29634, United States
‡Propulsion Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433, United States
§Sensors Technology Office, University of Dayton Research Institute, Dayton, Ohio 45469, United States
ABSTRACT: Increasing atmospheric CO2 levels have
generated much concern, driving the ongoing carbon se-
questration effort. A compelling CO2 sequestration option
is its photocatalytic conversion to hydrocarbons, for which
the use of solar irradiation represents an ultimate solution.
Here we report a new strategy of using surface-functiona-
lized small carbon nanoparticles to harvest visible photons
for subsequent charge separation on the particle surface in
order to drive the efficient photocatalytic process. The
aqueous solubility of the catalysts enables photoreduction
under more desirable homogeneous reaction conditions.
Beyond CO2 conversion, the nanoscale carbon-based
photocatalysts are also useful for the photogeneration of
H2 from water under similar conditions.
Figure 1. Aqueous soluble PEG-functionalized carbon nanoparticles
before (left, fluorescent) and after (right, photocatalytic for CO2
reduction in aqueous solution) coating with metal.
emission in the visible spectral region that may also extend into
the near-IR.7,8 The fluorescence emissions are attributed to
radiative combinations of electrons and holes confined on the
carbon nanoparticle surface (Figure 1),7,9,10 phenomenologically
resembling those found in nanoscale semiconductors. In this
study, the carbon particle surface was also doped with gold or
platinum to concentrate the photogenerated electrons to enable
more efficient CO2 reduction.
Carbon powders containing sub-10 nm nanoparticles were
refluxed in aqueous nitric acid (2.6 M) for 12 h, dialyzed in
membrane tubing (pore size equivalent to MW ∼ 1000) against
fresh water, and then centrifuged to retain the supernatant as an
aqueous suspension of small carbon nanoparticles bearing sur-
face carboxylic acid moieties.7 The particles recovered from the
suspension were functionalized with oligomeric poly(ethylene
glycol) diamine (PEG1500N; Figure 1) in a classical amidation
reaction that involved the acylation of the particle-surface
carboxylic acids in neat thionyl chloride as an intermediate
step.7,10 The functionalized carbon nanoparticles were readily
dissolved in water to form stable aqueous solutions of various
concentrations, with solution colors ranging from yellow at low
concentrations to black at very high concentrations. A UVꢀvis
absorption spectrum of the solution used in subsequent photo-
lysis for gold coating is shown in Figure 2.
he significant rise in atmospheric CO2 levels due to the
T
combustion of hydrocarbon fuels has generated much con-
cern. Among various CO2 sequestration options, a compelling
approach is photocatalytic conversion to recycle CO2 back to
hydrocarbon fuels, for which the use of solar irradiation may
represent an ultimate solution. However, there are major chal-
lenges in finding potent photocatalysts for such a task.1ꢀ3
Nanoscale wide-band-gap semiconductors such as titanium
dioxide (TiO2) and cadmium sulfide (CdS) were originally used
and have since been quite popular in CO2 photoreduction and
related photocatalytic reactions,1ꢀ4 but their limitations in terms
of the requirement for UV excitation and generally low conver-
sion efficiencies have also become evident.2,3 Various improve-
ment strategies, including the use of TiO2 nanotubes and dye
sensitization to extend the absorption of the photocatalysts into
the visible region, have been developed.5,6 Here we report a new
approach involving the use of surface-functionalized small carbon
nanoparticles to harvest visible photons for subsequent charge
separation on the particle surface in order to drive the photo-
catalytic process. The aqueous solubility of the catalysts enables
photoreduction under essentially homogeneous reaction condi-
tions. Beyond CO2 conversion, the nanoscale carbon-based
photocatalysts are also useful to the photogeneration of H2 from
water under similar experimental conditions.
The functionalized particles were coated with gold or plati-
num by simple solution-phase photolysis, as these metals are
known in the literature for their photocatalytically enhanced
reduction reactions.11ꢀ13 The photoexcited carbon particles
acted as electron donors to reduce the gold or platinum salt
It is now known that small carbon nanoparticles can be
covalently functionalized to give them strong absorption and
Received: January 26, 2011
Published: March 14, 2011
r
2011 American Chemical Society
4754
dx.doi.org/10.1021/ja200804h J. Am. Chem. Soc. 2011, 133, 4754–4757
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