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| Is Graphene Brand New in Carbon-Based Semiconductor Photocatalysts
for Organic Pollutants Degradation? |
| Xincun Dou |
| Laboratory of Eco-Materials and Sustainable Technology (LEMST), Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Urumqi, Xinjiang
830011, China |
| *Corresponding author: |
Dr. Prof. Xincun Dou, Laboratory of Eco-Materials and
Sustainable Technology (LEMST), Xinjiang Technical Institute of Physics &
Chemistry, Chinese Academy of Sciences, Urumqi, Xinjiang 830011, China, Tel:
+86-991-367-7875; Fax: +86-991-383-8957; E-mail: xcdou@ms.xjb.ac.cn |
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| Received April 25, 2012; Accepted April 25, 2012; Published April 30, 2012 |
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| Citation: Dou X (2012) Is Graphene Brand New in Carbon-Based Semiconductor
Photocatalysts for Organic Pollutants Degradation? J Thermodyn Catal 3:e104.
doi:10.4172/2157-7544.1000e104 |
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| Copyright: © 2012 Dou X. This is an open-access article distributed under the
terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and
source are credited. |
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| Graphene-based semiconductor photocatalysts have been
extensively studied for the photocatalytic degradation of organic
compounds and have proven more efficient than the bare semiconductor
photocatalysts [1-5]. However, graphene, as a “rising star” material
and another allotrope of carbon (activated carbon, fullerenes, and
carbon nanotubes), is it really superior to other carbon materials
when combined with semiconductor photo catalysts, or just because
graphene is a newly discovered “weirdie” and people are blindly keen
on? To get a comprehensive but still open answer to this question, the
unique properties of graphene and the physical and chemical factors
that matters the photodegradation activity of carbon-semiconductor
photocatalysts should be addressed. In addition, the characteristics of
different carbon-semiconductor system should be emphasized. |
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| Graphene has many exceptional properties, such as high electron
mobility, theoretically high surface area of 2600 m2g-1, and high
transparency [6]. For the synthesis of graphene-based semiconductor
photocatalysts, the most widely used technique to obtain graphene is
Hummers’ method [7]. The exfoliated graphite oxide (GO) sheets which
are the intermediate products in the synthesis process usually possess
a rich assortment of oxygen-containing groups, such as carboxylic,
hydroxyl, and epoxide functional groups [4]. The presence of oxygen
functionalities in GO allows interactions with the cations and provides
reactive sites for the nucleation and growth of nanoparticles, which
results in the rapid growth of various graphene-based composites.
Furthermore, high quality graphene sheets permit ballistic transport,
making them potentially ideal electron sinks or electron transfer bridges
[8]. Based on these intrinsic physical and chemical characteristics,
the enhanced photocatalytic degradation of organic compound for
Graphene-TiO2 system could be attributed to the following reasons:
(1) the interactions between organic molecules and the aromatic
regions of graphene enhance the adsorption on photocatalysts, (2)
the formation of the Ti–O–C chemical bonds narrow the band gap of
TiO2 and extends the photoresponse range, (3) the transfer of excited
electrons from TiO2 to graphene suppresses the charge recombination
[3]. It is also found that the electrons in the upper valence band could
be directly excited from graphene to the conduction band of TiO2
under visible light irradiation [9], thus graphene acts as the sensitizer
to TiO2. It is unassailable that the photocatalytic activity of TiO2-
Graphene Composite is higher than that of TiO2 alone. However, to
compare graphene’s superiority over other carbon, on one side, it was
advocated that graphene is superior than carbon nanotubes (CNTs) in
carbon-TiO2 composites [5]. On the other side, it is strongly argued
that TiO2-Graphene composite is in essence the same as other TiO2-
carbon (activated carbon, fullerenes, and carbon nanotubes) composite
materials on enhancement of photocatalytic activity of TiO2 [10]. |
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| To clarify whether graphene provides brand new insights into
carbon-semiconductor photocatalysts, the role of graphene’s forebear
should be addressed. Activated carbon (AC) has been extensively
studied as a support for heterogeneous catalysis due to its high surface
area (typically 900~1200 m2g-1) structure over which TiO2 particles may
be distributed and immobilized [11]. AC–TiO2 mixtures/composites
can improve the photocatalytic activity due to the high adsorption
capacity and ready passage of reacting species to the TiO2 particles
[12]. Besides, AC support itself is capable of a significant level of selfphotocatalytic
activity, out-performing the AC–TiO2 composite under
UV light irradiation [13]. C60 has unique electronic properties and it
absorbs moderately the visible light and strongly the UV light [14,15].
In C60-TiO2 photocatalysts, the fullerene can act as an electron acceptor
to minimize the electron–hole recombination or as an electron donor
to sensitize TiO2 [16]. CNTs can provide large amounts of surface
reaction sites due to the large surface area (200~900 m2g-1). In addition,
the surface chemistry of CNTs may be tailored to promote specificity
towards adsorbents. CNTs may act as extremely effective electron sinks
since they exhibit high electrical conductivity and high electron storage
capacity (one electron for every 32 carbon atoms). As a result, they
can synergistically enhance the photocatalytic activity of TiO2 through
the retardation of electron–hole recombination. The presence of the
C-O-Ti bond in the system has been proven to extend light absorption
to longer wavelengths. CNTs may also enhance TiO2 photocatalytic
activity by acting as a photosensitizer, transferring electrons to TiO2
[17]. The mid band-gap states introduced by defects or CNT oxidation
also result in higher photocatalytic activity [9]. |
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| Based on the above discussion, it is clear that although graphene is
much more popular than its forebears at present, the graphene-based
semiconductor photocatalysts basically exhibit four positive factors
that can potentially enhance the photocatalytic activity (provision of
high quality highly adsorptive active sites, minimization of electronhole
recombination, band-gap tuning, and photosensitization). All
the four factors well established in other carbon semiconductor
photocatalysts. Nevertheless, it is still too early to get a decisive answer
for graphene’s superiority to other carbon materials when combined
with other semiconductor photocatalysts. Graphene is a new carbon
material applied for photocatalysis, it is still too young since it was
only born in 2004. Although the number of the reports on graphene semiconductor photocatalysts is increasing unprecedentedly, the fully
understanding of the role of graphene to the photocatalytic properties
and mechanisms of graphene based photocatalysts still need more
evidence. Furthermore, as pointed by Leary and Westwood [8] the
development and endorsement of standardized testing methods is
needed for the investigation of photocatalysis activity. |
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