DOI: 10.1021/la900905h 13869Langmuir 2009, 25(24), 13869–13873 Published on Web 05/19/2009
pubs.acs.org/Langmuir
©2009 American Chemical Society
Graphene-Semiconductor Nanocomposites: Excited-State Interactions
between ZnO Nanoparticles and Graphene Oxide
†
Graeme Williams
‡
and Prashant V. Kamat*
Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame,
Indiana 46556.
‡
University of Waterloo cooperative education student
Received March 14, 2009. Revised Manuscript Received April 21, 2009
Graphene oxide sheets suspended in ethanol interact with excited ZnO nanoparticles and undergo photocatalytic
reduction. The luminescence quenching of the green emission of ZnO serves as a probe to monitor the electron transfer
from excited ZnO to graphene oxide. Anchoring of ZnO nanoparticles on 2-D carbon nanostructures provides a new
way to design semiconductor-carbon nanocomposites for catalytic applications.
Introduction
Graphene sheets are analogous to unrolled 2-D carbon nano-
tubes. They are individual sheets separated from the large,
stacked-sheet structure of graphite. The carbon sp
2
network of
single and bilayer graphene exhibits unique 2-D electronic trans-
port that hasbeen shown to produce strongconductivity.
1,2
Given
the economicalcost of graphene, thereis a significantdrive within
the scientific community to gain a greater understanding of its
properties and explore its possible applications. For example, the
potential for conductive graphene-based films and graphene-
sheet transparent conductive films has been explored.
3-6
How-
ever, photochemistry and photovoltaic aspects of 2-D carbon
nanostructures, graphene and graphene oxide (GO) sheets, are
relatively unstudied and provide an exciting new array of ideas
and applications.
Although graphite and graphite oxide have been known to be
in existence since the last century, it is onlyrecently that graphene
and graphene oxide sheets have been prepared and characterized
in a systematic way.
7
The most significant challenge in the
preparation of graphene is overcoming the strong exfoliation
energy of the π-stacked layers in graphite.
8
The recent focus on
graphene sheets involves the simple “micromechanical cleavage”
of graphite, where a piece of Scotch tape is used to remove
individual sheets of graphene. Micrographitic powder, however,
cannot be readily separatedinto individual sheetsor dispersed in a
solvent medium. Several methodsof preparation of graphene and
graphene oxide include the oxidation of graphite combined with
thermal exfoliation, the treatment of graphite fluorides with alkyl
lithium reagents, and theoxidation of graphitefollowed by strong
sonication.
8-10
The factors dictating stable dispersions of gra-
phene in various organic solvents and its interactions with
substituted organic compounds have also been discussed.
11,12
One obvious challenge is to utilize these 2-D carbon nanos-
tructures as conductive carbon mats so that one can anchor
semiconductor or metal nanoparticles and facilitate tailored
catalytic reactions (Scheme 1). Initial efforts to utilize graphene
as a 2-D carbon support to anchor metal nanoparticles have
already been reported.
13,14
ZnO and TiO
2
nanoparticles have
recently been examined in combination with carbon nanotubes,
showing an electron accepting and storing capacity of the
nanotubes.
15-18
Hence, it is reasonable to expect that GO sheets
may play a similar role of providing unique 2-D architecture to
support semiconductor catalyst nanoparticles. The ability of
semiconductor nanoparticles to partially reduce GO samples
when excited with UV light was demonstrated recently with
TiO
2
.
19
In our quest to further explore the interaction between
graphene oxide and semiconductor nanoparticles, we have now
probedthe ZnO emission to monitor theelectron transfer between
the semiconductor nanoparticles and GO sheets (Scheme 1). The
photochemical processes that illustrate the partial reduction of
GO in a ZnO nanoparticle suspension are presented here.
Experimental Section
Materials.
Graphene oxide was prepared by the Hummers’
method, where micrographitic powder was mixed with strong
oxidizing agents, filtered, and dried.
7
The oxidation process
functionalizes the graphene sheets with various hydroxyl and
epoxy groups, in addition to carbonyl and carboxyl groups along
†
Part of the “Langmuir 25th Year:Nanoparticles synthesis, properties, and
assemblies”special issue.
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