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10.1002/cssc.201802065
ChemSusChem
FULL PAPER
Surface nonpolarization on g-C3N4 for improved CO2
photoreduction by decoration of sensitized quantum dots
Huajun Feng[a] [b], Qiaoqi Guo[a] [b], Yingfeng Xu*[a] [b], Ting Chen[a] [b], Yuyang Zhou [a] [b], Yigang Wang[a]
[b], Meizhen Wang[a] [b], and Dongsheng Shen[a] [b]
Abstract: The concept of photocatalytic CO2 conversion provides
simultaneous solutions for global warming mitigation and fuels
generation, which, however, is limited by the unsatisfactory
photoconversion efficiency. Despite light harvesting and charge
separation, enhancing adsorption of CO2 via surface modification
could be an efficient way to improve CO2 photoconversion efficiency.
Herein, we doped nonpolar carbon quantum dots (denoted as CQDs)
on g-C3N4 to construct a metal-free heterojunction photocatalyst
(denoted as CQDs/g-C3N4). Besides the narrow bandgap and
electron-withdrawing of CQDs to improve the visible-light absorption
and photocarrier separation efficiency, we first discovered that
nonpolar CQDs could simultaneously improve adsorption of nonpolar
CO2, photoinduced H2, enhancing the reaction kinetics and altering
CO2 photoreduction pathways to generate CH4. Consequently, in
contrast to g-C3N4 that only generated CO and H2, CQDs/g-C3N4 could
generate 6-times CO and comparable CH4 without detectable H2
under the same condition. Therefore, this study first demonstrated the
promising nonpolar surface modification strategy for efficient CO2
adsorption, activation, and subsequent photoreduction.
visible-light absorption and charge separation efficiency,
appropriate surface modification ways to enhance CO2 adsorption,
activation, and lower the reaction barriers for the subsequent CO2
reduction could effectively improve the photocatalytic CO2
reduction performance.[6]
Different surface modification strategies, such as constructing
oxygen vacancies, the introduction of functional groups and
loading co-catalyst, have been developed to enhance CO2
adsorption and activation [17]. Theoretically, considering the
nonpolar property of CO2, altering the polarity of the
photocatalysts surface could potentially favour the adsorption of
CO2, enhancing the surface reaction kinetics of CO2
photoreduction. However, few studies reported thus far have
been focused on the surface polarity modification strategy to
enhance the CO2 photoreduction performance.
Carbon quantum dots, composed of sp2 hybridized carbon, exhibit
a unique electron reservoir, photo-induced electron transfer
properties, and tunable optical absorption.[8] Recent studies have
demonstrated the enhanced visible light absorption, photoexcited
charge carriers separation and photocatalytic activities for CQDs-
based composites.[9] Moreover, considering the nonpolar property
of CQDs and CO2, doping CQDs on photocatalyst could modify
the surface polarity of photocatalyst potentially enhancing CO2
adsorption to improve the surface reaction kinetics.[10] Herein, we
designed and demonstrated a series of CQDs modified graphitic
carbon nitride nanocomposite (denoted as CQDs/g-C3N4) for CO2
photoreduction. The photocatalytic properties of the CQDs/g-
C3N4 composites were systematically explored under visible light
irradiation. Due to the photo-sensitized effects, charge transfer
property of CQDs, CQDs/g-C3N4 exhibits efficiently enhanced
visible-light absorption and photocarrier separation efficiency.
Furthermore, we first discovered that nonpolar CQDs dopant
could simultaneously decrease the surface polarity of the
photocatalyst, which could efficiently enhance the adsorption of
nonpolar CO2 molecules and alter the CO2 photoreduction
pathways to generate a considerable amount of CH4 (20.78
μmol·h-1·g-1) and CO (23.38 μmol·h-1·g-1) with the presence of
H2O. In contrast, pristine g-C3N4 only generated one-sixth CO
(4.14 μmol·h-1·g-1) and H2 (4.57 μmol·h-1·g-1) without detectable
CH4 under the same condition. Therefore, this study first
demonstrated the promising nonpolar surface modification
strategy for efficient CO2 adsorption, activation, and subsequent
photoreduction into hydrocarbons.
Introduction
With the current rapid industrialization, the excessive burning of
fossil fuels results in a large amount of anthropogenic CO2
emissions, leading to adverse global warming and environmental
changes.[1] Meanwhile, CO2 as the main carbon source could be
transformed into a variety of C1 and C2 products, including CO,
CH4, and other hydrocarbon fuels.[2] Recently, photocatalytic
reduction of CO2 into renewable hydrocarbon fuels has attracted
worldwide attention, which could provide potential simultaneous
solutions for renewable fuels generation and global warming
mitigation.[3] However, due to unsatisfactory CO2 photoconversion
efficiency, photocatalytic reduction of CO2 for fuel production
remains a grand challenge.[4] It is attributed to the limited light
absorption and the rapid combination of photogenerated carriers.
In addition, it is kinetically difficult to photoreduce CO2 into CO
and CH4, because of the chemically inert property and the highest
oxidation state of carbon.[5] Therefore, apart from increasing
[a]
Prof. H. Feng, Q. Guo, Prof. Y. Xu, Prof. T. Chen, Dr. Y. Zhou, Y.
Wang, Prof. M. Wang, Prof. D. Shen
School of Environmental Science and Engineering
Zhejiang Gongshang University
Hangzhou 310013, P. R. China
Results and Discussion
[b]
Prof. H. Feng, Q. Guo, Prof. Y. Xu, Prof. T. Chen, Dr. Y. Zhou, Y.
Wang, Prof. M. Wang, Prof. D. Shen
Zhejiang Provincial Key Laboratory of Solid Waste Treatment and
Recycling
Carbon quantum dots (CQDs) were synthesized according to a
reported method, loaded on g-C3N4 with different amounts (1-4
wt%).[11] As shown in the transmission electron microscopy (TEM)
image, the synthesized CQDs are uniform with the average size
of 2.5 nm, which is well-crystallized (Fig. S1, Fig. 1c). TEM image
Hangzhou 310013, P. R. China
Supporting information for this article is given via a link at the end of
the document.
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