Pompon Dahlia-like Cu2O/rGO Nanostructures for Visible Light Photocatalytic H2 Production and 4-Chlorophenol Degradation

Article abstract of DOI:10.1002/cctc.201902048

Hierarchical Cu₂O nanospheres with a Pompon Dahlia-like morphology were prepared by a one-pot synthesis employing electrostatic self-assembly. Nanocomposite analogues were also prepared in the presence of reduced graphene oxide (rGO). Photophysical properties of the hierarchical Cu₂O nanospheres and Cu₂O/rGO nanocomposite were determined, and their photocatalytic applications evaluated for photocatalytic 4-chlorophenol (4-CP) degradation and H₂ production. Introduction of trace (<1 wt %) rGO improves the apparent quantum efficiency (AQE) at 475 nm of hierarchical Cu₂O for H₂ production from 2.23 % to 3.35 %, giving an increase of evolution rate from 234 μmol.g^?1.h^?1 to 352 μmol.g^?1.h^?1 respectively. The AQE for 4-CP degradation also increases from 52 % to 59 %, with the removal efficiency reaching 95 % of 10 ppm 4-CP within 1 h. Superior performance of the hierarchical Cu₂O/rGO nanocomposite is attributable to increased visible light absorption, reflected in a greater photocurrent density. Excellent catalyst photostability for >6 h continuous reaction is observed.

Full text of DOI:10.1002/cctc.201902048

Accepted Article
Title: Pompon Dahlia-like Cu2O/rGO nanostructures for visible light
photocatalytic H2 production and 4-chlorophenol degradation
Authors: Sekar Karthikeyan, Kassam Ahmed, Amin Osatiashtiani,
Adam Fraser Lee, Karen Wilson, Keiko Sasaki, Ben Coulson,
Will Swansborough-Aston, Richard Douthwaite, and Wei Li
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Link to VoR: http://dx.doi.org/10.1002/cctc.201902048
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10.1002/cctc.201902048
ChemCatChem
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Pompon Dahlia-like Cu2O/rGO nanostructures for visible light
photocatalytic H2 production and 4-chlorophenol degradation
Sekar Karthikeyan,*[a],[b] Kassam Ahmed,[a] Amin Osatiashtiani,[a] Adam F. Lee,*[c] Karen Wilson,[c]
Keiko Sasaki,[b] Ben Coulson,[d] Will Swansborough-Aston,[d] Richard E. Douthwaite,[d] and Wei Li*[a]
Abstract: Hierarchical Cu2O nanospheres with a Pompon Dahlia-like
morphology were prepared by a one-pot synthesis employing
electrostatic self-assembly. Nanocomposite analogues were also
prepared in the presence of reduced graphene oxide (rGO).
Photophysical properties of the hierarchical Cu2O nanospheres and
Cu2O/rGO nanocomposite were determined, and their photocatalytic
applications evaluated for photocatalytic 4-chlorophenol (4-CP)
degradation and H2 production. Introduction of trace (<1 wt%) rGO
improves the apparent quantum efficiency (AQE) at 475 nm of
hierarchical Cu2O for H2 production from 2.23 % to 3.35 %, giving an
increase of evolution rate from 234 μmol.g-1.h-1 to 352 μmol.g-1.h-1
respectively. The AQE for 4-CP degradation also increases from 52 %
to 59 %, with the removal efficiency reaching 95 % of 10 ppm 4-CP
within 1 h. Superior performance of the hierarchical Cu2O/rGO
nanocomposite is attributable to increased visible light absorption,
to efficiently separate photoexcited electron(e-)-hole(h+) pairs.[4]
Significant research effort has focused on strategies to tune the
photophysical properties of oxide semiconductors by modifying
their surface/interface properties through crystal facet
engineering, the formation of phase junctions or heterojunctions,
and the incorporation of co-catalysts, with the goal of efficient
solar light harvesting and improved charge carrier
separation/energy matching (and hence high activity and
selectivity) to the selected reactant and desired product.[5] Copper
(I) oxide is an abundant and low-cost p-type semiconductor with
a direct (forbidden) band gap of 2.17 eV and optical band gap of
2.62 eV,[6] which is favourable for overall photocatalytic water
splitting to produce H2 under visible light (600 nm) irradation.[7]
Cu2O has a high optical absorption coefficient, with a high
theoretical H2 conversion efficiency of 18 % for water splitting,[8]
and power conversion efficiency of 20 %, and hence finds
widespread application in photocatalysis environmental pollutant
remediation[9] and solar cells.[10] However, the reduction and
oxidation potentials of Cu2O lie within its bandgap resulting in poor
photostability,[8] and rapid recombination of photogenerated
charge carriers occurs.[11]
reflected in
a greater photocurrent density. Excellent catalyst
photostability for >6 h continuous reaction is observed.
Introduction
Various structural modifications of Cu2O have been
investigated to overcome these limitations, with different
morphologies such as nano-wires[12], cubes[13], flowers,[14] and
spheres,[15] offering significant improvements in photophysical
properties for photocatalytic applications. Size and morphology of
Cu2O nanostructures determine their resulting chemical and
physical properties.[16] Hierarchical semiconductors have gained
recent interest as they can offer additional control of electronic
and optical properties.[17] We reported a hierarchical Cu2O
photocatalyst comprised of individual nanoparticles assembled
into porous nanocubes that exhibited a promising hydrogen
productivity (water splitting) 15 mol g-1.h-1 corresponding to an
apparent quantum efficiency (A.Q.E) of 1.2 % in the presence of
a Pt co-catalyst.[18] Hierarchical structures also offer improved
mass transport to confer higher photocatalytic activity,[19] although
existing fabrication routes often employ disposable templates,
high temperatures (170 C),[20] and/or coatings, which increase
catalyst production cost and time.[21]
Graphene is a two dimensional monolayer of sp2 hybridised
carbon atoms, which due to its unique physical and electronic
properties, has attracted global scientific interest and investment
since its formal discovery/isolation in 2004.[22] The reduced form
of the oxide of graphene, reduced graphene oxide (rGO), exhibits
a high surface area, tunable band gap, and excellent electron
mobility.[23] rGO surfaces also possess a variety of chemically
reactive oxygen functionalities that render it a versatile catalyst
support and amenable to mixing with other semiconductors to
form hybrid semiconductor composites with potentially superior
photocatalytic properties.[24] Indeed, graphite and rGO
nanosheets have been explored as supports for dispersing Cu2O
Global energy and health challenges arising from anthropogenic
fossil fuel usage (and resulting climate change) and
contamination of aquatic environments are driving the
development of environmentally benign technologies for energy
production/storage and wastewater treatment.[1] Solar energy has
emerged as a key resource to address such challenges,[2] both
through direct electric power generation, and harnessing by
semiconductor photocatalysts for aqueous phase H2 evolution,
CO2 reduction and pollutant degradation, and antimicrobial
coatings.[3]
Many transition metal oxide semiconductors possess tunable
bandgaps and favourable conduction and valence band energies
[a]
Dr S. Karthikeyan, K. Ahmed, Dr A. Osatiashtiani, Dr W. Li
European Bioenergy Research Institute
Aston University
Birmingham B4 7ET (UK)
E-mail: karthik.keyan02@gmail.com; w.li8@aston.ac.uk
Dr S. Karthikeyan, Prof K. Sasaki
Department of Earth Resources Engineering
Kyushu University
Fukuoka 819-0395 (Japan)
Prof A.F. Lee, Prof K. Wilson
[b]
[c]
[d]
RMIT University
Melbourne VIC3000 (Australia)
E-mail: adam.lee2@rmit.edu.au
Dr B. Coulson, W. Swansborough-Aston, Dr R.E. Douthwaite
Department of Chemistry
University of York
York YO10 5DD (UK)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201902048
ChemCatChem
FULL PAPER
and TiO2 respectively. Carbonaceous supports are reported to
facilitate photoexcited charge separation, and hence improve the
photocatalytic activity and photostability of Cu2O,[9a, 25] while thin
protective carbon layers arising from glucose carbonisation can
significantly increase the photocurrent density and photostability
of Cu2O for photoelectrochemical water splitting.[26]
Herein we report the one-pot hydrothermal synthesis and
photocatalytic application of hierarchical Cu2O nanospheres and
corresponding Cu2O/rGO nanocomposites for 4-chlorophenol (4-
CP) degradation and H2 production under visible light. 4-CP was
selected as a hazardous, recalcitrant organic compound present
in waste water effluent from diverse industrial processes (e.g. pulp
and paper, textile and petroleum sectors), and unlike organic dyes
does not act as a catalyst photosensitiser. Intermixing rGO and
hierarchical Cu2O nanospheres improved photocatalytic activities
(and apparent quantum efficiencies) for both reactions relative to
the nanospheres alone, without requiring a precious metal co-
catalyst or external bias, associated with improved visible light
absorption by the nanocomposite. In addition, photodegradation
of 4-CP favoured less toxic organic oxidation products
ions in parallel with reduction of the graphene framework[27] to
form a hierarchical Cu2O/rGO nanocomposite. PEG likely acts as
a structure-directing agent promoting the formation of discrete
Cu2O nanoparticles which coalesce at the rGO surface.
Transformation of Cu2O agglomerates into hierarchical structure
may proceed by Ostwald ripening and/or self-aggregation and the
simultaneous reduction of GO yielding an integrated
nanocomposite. A related (albeit template-free) aggregation of
hollow Cu2O microstructures via hydrazine reduction is reported
in the literature, however neither the photophysical properties nor
catalytic performance were described.[28]
The morphologies of hierarchical Cu2O and the Cu2O/rGO
nanocomposite were examined by TEM and SEM (Figure 1 and
2). TEM of hierarchical Cu2O shows ~400-500 nm aggregates of
spherical Cu2O nanoparticles (mean size ~50 nm) in good
agreement with SEM images (Figure S1).
Corresponding TEM images of the hierarchical Cu2O/rGO
nanocomposite also present semi-transparent graphene oxide
sheets that exhibit folds and wrinkles (Figure 2a-d) which are in
intimate contact with the Cu2O aggregates possibly driven by
electrostatic interactions arising from reduction of the parent
graphene oxide.[29] Cu2O aggregates and individual particles in
the nanocomposite were slightly smaller than those of the free
hierarchical Cu2O, being 250-400 nm (Figure 2a inset) and 15-30
nm (Figure 2c inset) respectively. Aggregates still exhibited a
Pompon Dahlia-like morphology (Figure 2b inset and Figure S1).
Phase analysis by XRD confirmed the exclusive formation of
crystalline Cu2O (Figure 3a) in the hierarchical Cu2O and
Cu2O/rGO nanocomposite, indicated by peaks at 29.56°, 36.41°,
42.31°, 61.36°, and 73.50° associated with characteristic (110),
(111), (200), (220), (311), and (222) reflections of pure Cu2O
phase (JCPDS 03-0898)[30] and lattice constants a=5.19; b=5.08,
c=11.69 and =90.38. The absence of reduced graphene oxide
Results and Discussion
Structure, photophysical and electronic properties
The synthetic route to Pompon Dahlia-like Cu2O/rGO is illustrated
in Scheme 1. The Cu(II)-PEG complex was added to graphene
oxide to form a hybrid inorganic-organic nanostructure wherein
polar groups on the graphene oxide nanosheets promote
multivalent bridging (via hydrogen bonding) of GO with ligand
coordinated Cu2+ ions. Subsequent hydrazine reduction in the
presence of NaOH likely promotes a transition from ion-by-ion
growth to particle-mediated crystallization of complexed Cu(I)
Scheme 1. Synthesis of a Pompon Dahlia-like Cu2O/rGO nanocomposite photocatalyst.
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10.1002/cctc.201902048
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exerts little impact on the precipitation and reduction of the copper
precursor. Textural properties of the hierarchical Cu2O and
Cu2O/rGO nanocomposite revealed identical, low BET surface
areas of 13 m2.g-1 (Table 1) comparable to those previously
reported for single crystal Cu2O-rGO composites,[32] and identical
BJH pore size distributions (Figure S2) indicative of ~2 nm
mesopores presumably associated with voids between individual
Cu2O nanoparticles in the aggregates. The mesopore volume of
the nanocomposite was similar to that of the hierarchical Cu2O.
Optical absorption properties of the two materials were studied by
DRUVS (Figure 3b); although both exhibited broad absorbance
between 200-600 nm consistent with literature reports,[32-33] the
band edge of the Cu2O/rGO nanocomposite was red-shifted.
Optical band gaps Eg were calculated from the corresponding
Tauc plots (Figure 3c) using Eq. 4:

ꢀꢁꢂ ꢃ ꢄꢅꢁꢂ ꢆ ꢇꢈꢉ
4
where A is the absorption coefficient and  the linear absorption
coefficient determined from the Kubelka-Munk formalism in Eq. 5:
ꢎ
ꢊ ꢃ ꢅꢋꢌꢍꢉ
5
ꢏꢍ
Figure 1. (a-d) TEM images of Pompon Dahlia-like hierarchical Cu2O. Insets
show particle size distributions (a) for the individual Cu2O nanoparticles and (b)
for the aggregates, and (c) a corresponding SEM image of the aggregates.
The resulting direct band gaps were 2.42 eV and 2.13 eV for
hierarchical Cu2O and the Cu2O/rGO nanocomposite respectively.
Since rGO shows weak adsorption >400 nm,[34] the red shift in the
nanocomposite must arise from interfacial contact between rGO
sheets and Cu2O and a change in the oxide valence band (VB)
and/or conduction band (CB) energies, as previously reported.[35]
Such band gap narrowing increase light absorption which could
enhance visible light photocatalysis.[32] Note that the Cu2O band
gap is reported to vary between 2.1-2.6 eV, being sensitive to
quantum confinement effects and heterojunction formation.[10, 36]
Table 1. Photophysical properties of Pompon Dahlia-like hierarchical Cu2O
and Cu2O/rGO nanocomposite.
Sample
Crystal
size[a]
/ nm
Particle
size[b]
/ nm
BET
area[c]
/ m2.g-1
Band
gap[d]
/ eV
CB
edge[e]
/ eV
VB
edge[e]
/ eV
Hierarchical
Cu2O
15.3
14.7
400-
500
13
13
2.42
2.13
-1.12
-1.03
+1.3
+1.1
Hierarchical
Cu2O/rGO
250-
400
[a] XRD. [b] TEM. [c] N2 porosimetry. [d] DRUVS. [e] Calculated from valence
band XPS and DRUVS.
Figure 2. (a-d) TEM images of Pompon Dahlia-like hierarchical Cu2O/rGO
nanocomposite. Insets show particle size distributions (a) for the aggregates
and (c) for individual Cu2O nanoparticles, and (b) a corresponding SEM image
and (d) high resolution TEM image of the aggregates.
The surface copper oxidation state was quantified from high
resolution Cu 2p XP spectra (Figure 4a), with the hierarchical
Cu2O and Cu2O/rGO nanocomposite dominated by spin-orbit split
doublets with 2p3/2 binding energies of 932.2 eV and 934.4 eV
indicative of Cu2O and Cu(II) carbonate dihydroxide respectively,
and a weak shake-up satellite at 943.4 eV associated with Cu(II)
species. The absence of Cu(II) XRD features suggests that
Cu2(OH)2CO3 arises from the post-synthetic reaction of Cu2O
nanoparticle surfaces with the surrounding atmosphere.[37]
reflections ~25  peaks is ascribed to its very low concentration
(<0.4 wt%) in the nanocomposite.[31] Volume-averaged crystallite
sizes calculated from the Scherrer equation reveal similar (15 nm)
Cu2O nanoparticles for both materials, suggesting the rGO matrix
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10.1002/cctc.201902048
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0.7
0.6
0.5
0.4
0.3
0.2
0.1
(a)
(b)
Cu2O
C O
Cuu22O
Cu2O/rGO
Cu2O/rGO
Cu2O/rGO
200
400
600
800
5
15 25 35 45 55 65 75
2 / °
Wavelength / nm
2O
Cu2O
(c)
(d)
Cu O/rGO
Cu2O/rGO
2
Cu22OO
Cu22OO//rrGGOO
1.9
2.2
2.5
2.8
3.1
3.4
0.0
0.2
0.4
0.6
0.8
1.0
Bandgap / eV
Relative Pressure /P/P0
Figure 3. (a) XRD patterns, (b) DRUV spectra and (c) corresponding Tauc plots, and (d) N2 adsorption-desorption isotherms of Pompon Dahlia-like hierarchical
Cu2O and Cu2O/rGO nanocomposite.
(a)
(b)
C-OH/C-O
O=C-O
Cu(I)
C-C/C-H
Cu2O/rGO
Cu2O/rGO
Cu(II)
Cu(II) satellite
Cu2O
958
Cu2O
948
938
928
292
89
289
6
286
3
283
0
280
77
Binding energy / eV
Binding energy / eV
Figure 4. (a) Cu 2p and (b) corresponding C 1s XP spectra of Pompon Dahlia-like hierarchical Cu2O and Cu2O/rGO nanocomposite.
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10.1002/cctc.201902048
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Spectral fitting reveals that the surfaces of both hierarchical
materials predominantly comprise Cu2O (Table S1) with that of
the nanocomposite somewhat enriched (88 % versus 79 %).
Corresponding C 1s XP spectra revealed almost identical
distributions of three distinct chemical environments for both
hierarchical materials at 284.6, 286.2, and 288.3 eV (Figure 4b),
respectively assigned to the alcohol and ether functions of PEG
and surface carbonate.[9a, 38] A small increase in the sp2 carbon
environment was observed for the Cu2O/rGO nanocomposite
consistent with graphene incorporation (Table S2).[37] O 1s
spectra were consistent with these assignments, exhibiting three
distinct chemical environments at 531.4, 533.4, and 535.8 eV
arising from Cu2O, carbonate, and PEG ether species (Figure S3),
with an enhanced Cu2O contribution for the nanocomposite
consistent with a higher Cu(I):Cu(II) atomic ratio and less surface
carbonate.
Charge carrier separation and hence photocatalytic
performance depend on the electronic band structure, band
alignment and interfacial contact of photocatalysts.[39] Band
energies were investigated by valence band XPS (Figure S4a-c);
the VB potential maxima of hierarchical Cu2O and the Cu2O/rGO
nanocomposite were +1.30 and +1.10 eV respectively relative to
the Fermi level, and corresponding CB minima edges (derived
from the optical band gap and valence band XP spectra) were -
1.12 eV and -1.03 eV for Cu2O/rGO. The CB minimum is therefore
unaffected by formation of the Cu2O/rGO heterojunction, albeit
more negative than previous reports (e.g. -0.42 for oxygen-
deficient Cu2O nanoparticles[40]), and in both cases much greater
than required for photocatalytic hydrogen production from water
(−0.65 eV at pH 7).[41]
nanostructures on a silicon wafer (AQE in water = 0.3 %) under
visible light,[45] demonstrating advantageous photophysical
properties of our Pompom Dahlia-like aggregates. Hydrogen
production over various Cu2O photocatalysts is summarised in
Table S3.
44
(a)
Cu2O
Cu2O/RGO
36
28
20
12
0
2
4
6
8
10 12 14
Irradiation time / h
400
350
300
250
200
150
(b)
Cu2O
Cu2O/ rGO
Photocatalytic H2 production
The photocatalytic activity of the hierarchical Cu2O and
Cu2O/rGO-0.4 wt% nanocomposite for H2 evolution for sacrificial
water splitting was assessed under visible light irradiation in the
presence of methanol as a sacrificial hole scavenger (Figure 6a-
b). No evolved oxygen was observed for either catalyst. Hydrogen
productivities of 18 μmol.g-1.h-1 and 31 μmol.g-1.h-1 were
measured for the hierarchical Cu2O and Cu2O/rGO
nanocomposite respectively, with negligible deactivation during
14 h operation. Increasing the methanol concentration to 10 vol%
conferred an almost quantitative increase in H2 productivity, which
reached 234 μmol.g-1.h-1 for the hierarchical Cu2O and 352
μmol.g-1.h-1 for the hierarchical Cu2O/rGO nanocomposite,
suggesting that charge carrier recombination is rate-determining
for hydrogen evolution over both materials. The greater activity of
the nanocomposite equates to an apparent quantum efficiency
(AQE) of 3.35 % versus 2.23 % for the hierarchical Cu2O (Figure
S5). Interfacing the Cu2O semiconductor with rGO nanosheets to
form a heterojunction almost doubles the specific activity,
consistent with greater visible light absorption.[42] Hydrogen
production over the hierarchical Cu2O was superior to that of non-
porous (13 μmol.g-1.h-1)[43] and Cu2O nanoparticles (10 μmol.g-1.h-
0
2
4
6
8
10 12 14
Irradiation time / h
Figure 5. Visible light photocatalytic H2 production over Pompon Dahlia-like
hierarchical Cu2O and Cu2O/rGO nanocomposite with (a) 1 vol% and (b) 10
vol% methanol in water as a sacrificial hole scavenger. Reaction conditions:
0.02 g catalyst, 200 W Hg-Xe (420 nm).
Photocatalytic 4-CP removal
)
1 [7] of comparable size, and the AQE. higher than reported for Pt-
Visible light photocatalytic degradation of 4-CP was subsequently
studied over the Pompon Dahlia-like hierarchical Cu2O and
Cu2O/rGO nanocomposite (Figure S6). 4-CP was selected as a
model recalcitrant organic compound that does not exhibit visible
decorated 500 nm Cu2O nanocubes (AQE = 1.2 %),[41b] 375 nm
hierarchical Cu2O nanocubes (AQE = 1.2 %),[44] and Pt-free 300-
500 nm Cu2O powder (AQE = 0.3 %)[7] and 150 nm Cu2O
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60
50
40
30
20
10
0
0.18
0.16
0.14
0.12
0.1
(a)
(b)
0.08
0.06
0.04
0.02
0
Commercial
Cu2O
Cu2O
Cu2O/rGO
Commercial
Cu2O
Cu2O
Cu2O/rGO
4CP removal
(c)
120
Cl-HQ selectivity
4 Cl-CC selectivity
FA selectivity
100
80
60
40
20
0
Cu2O/rGO
Cu2O
Figure 6. Visible light photocatalytic 4-CP degradation over Pompon Dahlia-like hierarchical Cu2O and Cu2O/rGO nanocomposite and commercial Cu2O
nanoparticles: (a) initial rates of 4-CP removal and (b) corresponding apparent quantum efficiencies after 15 min reaction; and (c) 4-CP removal efficiency and
product selectivity after 60 min reaction. Experimental conditions: 0.02 g catalyst, 50 mL of 7.78 ×10-5 M aqueous 4-CP, 200 W Hg-Xe arc lamp (420 nm).
light absorption and hence cannot act as a photosensitiser which
is problematic in mechanistic investigations of photocatalytic dye
degradation.[46] Initial rates and AQE for 4-CP removal (Figure
6a-b) by the nanocomposite were slightly higher than for the
hierarchical Cu2O aggregates (0.18 versus 0.16 mmol.g-1.min-1,
and 59 versus 52 % respectively). However, the conversion of 4-
CP reached 95 % for hierarchical Cu2O/rGO after 60 min reaction,
compared with 73 % for the Cu2O aggregates alone (and only
negligible photolysis in the absence of any catalyst), likely
associated with increased light absorption. In comparison to both
hierarchical catalysts, commercial Cu2O nanoparticles exhibited
poor activity for 4-CP photocatalytic degradation, achieving only
32 % removal after 60 min, attributed to its low surface area (5
m2.g-1 versus 13 m2.g-1 for hierarchical Cu2O/rGO). There are no
reports of 4-CP photodegradation over Cu2O/rGO photocatalysts,
however the present activity far exceeds other semiconductor
photocatalysts (Table 2), even those employing UV light and/or
high power light sources. The initial rate of 4-CP degradation and
1 h removal efficiency showed a modest dependence on rGO
loading, with 0.4 wt% rGO exhibiting the optimal performance,
however pure rGO was almost inactive (Figure S7).
The major products of 4-CP decomposition were
chlorohydroquinone (CL-HQ), 4-chlorocatechol (4Cl-CC), and
fumaric acid (FA) (Figure 6c). Formation of polyoxygenated
intermediates is consistent with a radical mechanism involving
photogenerated holes transferred to adsorbed water or surface
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10.1002/cctc.201902048
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hydroxyls to form hydroxyl radicals (OH), or the reaction of
photoexcited electrons with oxygen to produce OH via H2O2,
although direct oxidation of 4-CP cannot be excluded. The
hierarchical Cu2O/rGO favours deeper oxidation and the
formation of FA (Figure 6c), consistent with more oxidizing
equivalents potentially resulting from longer charge carrier
lifetimes, and faster charge transfer kinetics (vide infra).[47]
Futhermore, Cu2O/rGO exhibited excellent photostability over five
consecutive reactions (Figure S8).
the CB of Cu2O to rGO, leaving photogenerated holes in the Cu2O
VB.[49] TRPL decay curves (Figure S10b) were best fit to a bi-
exponential function ( Eq. 6).[50]
ꢅꢗꢘꢉ
ꢅꢗꢘꢉ
ꢅꢎꢉ
ꢐꢑꢒ ꢃ ꢄ ꢓ ꢄꢋꢔꢕꢖꢅꢙꢉ ꢓ ꢄꢏꢔꢕꢖ
6
where, A is the baseline correction constant, t is time, A1 and A2
are the contributions of the exponential factors, which include the
lifetimes 1 and 2 of the two excited states. Both lifetimes are
short lived consistent with direct radiative emission. The average
charge carrier lifetime  was determined from Eq. 7:
 ꢃ ꢄꢋ1ꢏ ꢓ ꢄꢏ2ꢏ/ꢄꢋ1 ꢓ ꢄꢏ2
7
Table 2. Comparison of 4-CP removal efficiency over different photocatalysts.
and shows that photoinduced charge carrier lifetimes are
essentially identical for the Cu2O/rGO nanocomposite and
hierarchical Cu2O (Table 3). We therefore find no evidence for
Photocatalyst
Experimental details
Rate
constant
10-2.min-1
Ref.
59
60
61
62
63
64
significant
interfacial
charge
separation
across
the
heterojunction.[51]
N-Doped
TiO2
0.1 g catalyst, 500 W Xe lamp
(λ>420 nm), 100 mL of 10 mg
L-1 4-CP, 180 min.
4.6
Table 3. TRPL fitting of Pompon Dahlia-like hierarchical Cu2O and Cu2O/rGO
nanocomposite.
TiO /WO3
1.2 g/L catalyst, 50 W lamp
(λ>435 nm), 2×10-4 M 4-CP,
180 min.
0.84
0.042
0.049
5.26
0.41
7.9
Photocatalyst
1
/ ns
2
/ ns
A1/(A1+A2)
/ %
A2/(A1+A2)
/ %

/ ns
2
TiO2
–
CoPc
0.1 g catalyst, 128 W Lightex
LT50 lamp, 100 mL of 0.013 M
4-CP, 30 min.
nanocomposite
Hierarchical
Cu2O
1.601
1.662
2.056
2.251
23.6
23.0
76.4
77.0
1.97
2.14
1.441
1.314
Combustion
synthesized TiO2
1 g/L catalyst, 250 W Xe lamp
(λmax 470 nm), 0.15 mmol/L 4-
CP, 240 min.
Hierarchical
Cu2O/rGO
Mesoporous
C3N4
g-
40 mg catalyst, 300 W Xe lamp
(λ>420 nm), 1.2 ×10-4 M 4-CP,
60 min.
Photoelectrochemical measurements showed transient
photocurrents (Figure 7a) of the Cu2O/rGO composite was
approximately double that of the Cu2O aggregates indicating
more redox equivalents are available for photocatalytic reactions.
Electrochemical impedance spectroscopy can also provide
insight into photoelectrode phenomena. Ideally, an equivalent
circuit can be found to model specific photophysical and
photoelectrochemical phenomena. More generally, the radius of
the impedance curve on a resulting Nyquist plot reflects the
resistance in the system. Comparison of data acquired in the dark
and under illumination show the reduced radius of the lower
frequency feature of the Cu2O/rGO nanocomposite under
illumination (Figure 7b) indicating that the introduction of rGO
facilitates electron migration across the electrode or at the
electrode/electrolyte interface.[52] Corresponding Mott Schottky
plots under illumination (Figure 7c) provide insight into the flat
band potential and doping density. Negative slopes were obtained
for the hierarchical Cu2O and Cu2O/rGO nanocomposite,
consistent with p-type semiconductors. The x-axis intercept
shows the flat band potentials of Cu2O and Cu2O/rGO
nanocomposite are similar at around 1.1 V and 0.98 V,
respectively, which compares to 0.55 V reported for a continuous
film of cubic Cu2O nanocrystals.[52] A more positive flat band
potential will increase the rate of oxidation of MeOH and water,
supporting H2 production and 4-CP degradation, respectively
(Figure 5 and 6). The slope of the linear portion of the curve in a
Pt/TiO2
0.5 g/L catalyst, 11 W Hg lamp
(λ 200-280 nm), 0.5 mM 4-CP.
120 min.
Heirarchical
Cu2O/rGO
20 mg catalyst, 200 W Hg-Xe
arc lamp (λ ≥ 420 nm), 50 mL
of 4.2 ×10-2 mM 4-CP, 60 min.
This
work
The chemical state of copper post-reaction was subsequently
investigated. XRD and DRUVS revealed negligible change in the
phase or electronic properties of the as-prepared hierarchical
Cu2O/rGO nanocomposite (Figure S9), however XPS evidenced
an increase in the proportion of surface Cu(II) species (to 60 %)
associated with photo-oxidation of the exterior of Cu2O
nanoparticles.
Mechanistic studies
Photocatalytic activity for CO2 reduction is also reported to
increase following the addition of rGO to Cu2O,[9b] being attributed
to increased charge separation across the Cu2O/rGO interface.
Charge separation and recombination effects were investigated
in water suspensions using steady state photoluminescence and
time-resolved photoluminescence (TRPL) which showed very
little difference between Cu2O and Cu2O/rGO (Figure S10a-b). It
is reported that rGO acts as an electron trap in heterojunction
nanocomposites,[48] and photoexcited electrons can transfer from
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10.1002/cctc.201902048
ChemCatChem
FULL PAPER
Figure 7. (a) Transient photocurrent, (b) EIS (Nyquist) plot at 0 V vs. RHE, (c) Mott-Schottky plot of Cu2O, Cu2O/rGO, and Cu2O/rGO (200 W Hg-Xe arc lamp and
0.5 M NaSO4 electrolyte), and (d) proposed charge transfer mechanism for Cu2O/rGO.
Mott-Schottky plot is used to calculate the majority carrier density
from Eq. 8:
charge transfer rates or the driving force for charge separation
between our two catalysts (in contrast to Cu2O/CuO bilayered
composites[52]). Hence the higher photoactivity of the hierarchical
Cu2O/rGO nanocomposite for hydrogen production and 4-CP
degradation compared to hierarchical Cu2O appears solely
associated with its broader absorption of visible light, and not
reduced recombination.
ꢠꢡꢢ
ꢔ
1
2
ꢃ
ꢅꢞꢆꢇꢐꢟꢆ
)
8
ꢚ2
ꢛꢛ0ꢜꢝꢄ
where ꢛ is the dielectric constant (7.60 for Cu2O[53]), ꢛꢣ is the
permittivity of free space, ꢜ is the electron charge, ꢝꢤ is the
majority carrier density, ꢞ is the applied potential, ꢇꢥꢦis the flat
band potential, ꢠꢧ is the Boltzmann’s constant and T is the
temperature. The majority carrier densities were similar at 1.5 x
1019 cm-3 and 1.7 x 1019 cm-3 for hierarchical Cu2O and Cu2O/rGO
respectively, both higher that for Cu2O (3.07ꢀxꢀ1017ꢀcm−3), CuO
(2.41ꢀx1018ꢀcm−3), and Cu2O/CuO bilayered composite
A tentative mechanism for photocatalytic 4-CP oxidative
degradation over the hierarchical Cu2O/rGO nanocomposite is
illustrated in Scheme 2. Under illumination, electrons (e-) within
the Cu2O valence band are photoexcited into the Cu2O
conduction band, and subsequently migrate to rGO sheets. Resu-
lting Cu2O valence band holes (h+) may then react with hydroxyl
ions from the aqueous solution to form OH radicals, while
photoexcited electrons trapped by rGO react with dissolved
(2.58ꢀx1018ꢀcm−3) photoanodes prepared by thermal oxidation[52]
,
though less than for electrodeposited/annealed p-type Cu2O-CuO
thin films[54] (at 1.3 xꢀ1020ꢀcm−3). Together with the similar flat band
potentials, these values suggest there is little difference in either
-
oxygen to form superoxide O2 radicals. The latter may further
react with water to produce additional OH through redox
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10.1002/cctc.201902048
ChemCatChem
FULL PAPER
acetonitrile HPLC grade (Sigma, 99.93 %) were used without purification.
Deionised water was used for all solutions.
reactions. Reactive intermediates identified by HPLC included
chlorohydroquinone (Cl-HQ), chlorocatechol (Cl-CC) and fumaric
acid (FA), indicating that the 4-CP photooxidation pathway
processes according to Scheme 2, with OH• radicals the key
oxidant (Eqs. 9-15).
Synthesis of hierarchical Cu2O/rGO: A Pompon Dahlia (flower)-like
Cu2O/rGO catalyst was synthesised by solution phase chemistry under
ambient conditions. GO (2 mg) prepared following a literature method[55]
was ultrasonicated in 10 mL water for 30 min, to which a mixture of 50 mL
of 0.2 M copper chloride and 5 mL of 0.06 M PEG-600 was added and
followed by a further 10 min ultrasonication. The resulting mixture was then
heated to 60 C under stirring for 30 min, resulting in a deep blue solution.
Subsequently, 8 mL of 2 M NaOH was added into the preceding deep blue
solution, and followed by the dropwise addition of 1 M aqueous hydrazine
monohydrate (H4N2.H2O) (1 mL in 5 mL of water) under stirring for an
additional 5 min. The reaction mixture was then transferred to 20 mL cold
water in a multi-neck round-bottomed flask and purged under N2 for 30 min
to promote the formation of Cu2O (brownish-yellow colour change). The
resulting solid was separated from the reaction mixture by centrifugation
at 6000 rpm for 10 min, washed with H2O and then ethanol to remove
residual PEG, and finally vacuum dried for 24 h and stored in a vacuum
desiccator. Cu2O is formed through the initial complexation of Cu(II) ions
with PEG and rGO, and subsequent precipitation as the Cu(I) oxide by
hydrazine reduction in the presence of NaOH. Hydroxyl groups from PEG
and rGO likely play an important role in controlling the Cu(II) ion density
and directing the formation of hierarchical Cu2O structures. The preceding
synthesis corresponds to a nominal rGO mass loading of 0.4 % in the
hierarchical Cu2O/rGO nanocomposite, and was extended to prepare
nanocomposites with nominal rGO mass loadings spanning 2-100 %, and
in the absence of GO to prepare pure Dahlia (flower)-like Cu2O with
morphologies akin to those reported by Kow et al.[56] In all cases the final
catalysts were a reddish-orange colour, produced in ~540 mg yield, and
exhibited essentially identical electronic and crystalline properties (Figure
S11), although high rGO loadings result in encapsulation of Cu2O
nanoparticles (Figure S12) which may hinder photoexcitation and 4-CP
adsorption by the metal oxide.
ꢚꢨꢏꢩ/ꢪꢫꢩ ꢓ ꢁꢂ → ꢚꢨꢏꢩ/ꢪꢫꢩ∗ꢅꢁꢬꢭꢧ ꢓ ꢔꢮꢌꢧ
ꢉ
9
ꢚꢨꢏꢩ/ꢪꢫꢩ∗ ꢅꢁꢬꢭꢧꢉ ꢓ ꢯꢏꢩ → ꢚꢨꢏꢩ/ꢪꢫꢩ ꢅꢯꢩ∙ꢉ ꢓ ꢯꢭ
ꢚꢨꢏꢩ/ꢪꢫꢩ∗ꢅꢔꢚꢌꢡꢉ ꢓ ꢩꢏ → ꢚꢨꢏꢩ/ꢪꢫꢩꢅꢩꢏꢌ∙ꢉ
ꢚꢨꢏꢩ/ꢪꢫꢩꢅꢩꢏꢌ∙ꢉ ꢓ ꢯꢭ → ꢚꢨꢏꢩ/ꢪꢫꢩ∗ꢅꢯꢩꢏ∙ ꢉ
10
11
12
13
ꢚꢨꢏꢩ/ꢪꢫꢩ∗ꢅꢯꢩꢏ∙ ꢉ ꢓ ꢚꢨꢏꢩ/ꢪꢫꢩ∗ꢅꢯꢩꢏ∙ ꢉ
→ ꢚꢨꢏꢩ/ꢪꢫꢩꢅꢯꢏꢩꢏꢉ ꢓ ꢩꢏ
ꢚꢨꢏꢩ/ꢪꢫꢩꢅꢯꢏꢩꢏꢉ ꢓ ꢁꢂ → ꢚꢨꢏꢩ/ꢪꢫꢩ∗ꢅ2ꢯꢩ∙ꢉ
14
15
ꢚꢨꢏꢩ/ꢪꢫꢩ∗ꢅꢯꢩ∙ꢉ ꢓ 4ꢚꢰ ꢓ ꢁꢂ
→ ꢱꢔꢊꢲꢒꢑꢂꢔ ꢑꢳꢒꢔꢪꢴꢔꢵꢑꢊꢒꢔꢶ ꢓ ꢯꢏꢩ
Scheme 2. Proposed 4-CP photodegradation pathway.
Conclusions
A hierarchical Cu2O/rGO nanocomposite was fabricated by
electrostatic self-assembly and subsequent low temperature
hydrothermal processing. The resulting nanocomposite
comprised 300-500 nm aggregates of 15-30 nm Cu2O
nanocrystals arranged in a Pompom Dahlia (flower)-like structure,
in contact with 1 wt% of rGO nanosheets. This architecture offers
broad visible light absorption and excellent stability, resulting in
high activity for photocatalytic H2 production from water-alcohol,
and 4-CP degradation predominantly to (low toxicity) fumaric acid,
without recourse to precious metal co-catalysts. Such hierarchical
Cu2O/rGO nanocomposites may provide a low cost approach to
solar fuels and chemical (via CO2 reduction) production, and the
environmental remediation of recalcitrant wastewater pollutants.
Physicochemical characterization: Crystallinity and phase indexing was
performed by powder X-ray diffraction (XRD) using a Bruker-AXS D8
ADVANCE diffractometer operated at 40 kV and 40 mA and Cu K
radiation (=0.15418 nm) between 2 10-80 in 0.02 steps. X-ray
photoelectron spectroscopy (XPS) was undertaken on a Kratos Axis HSi
spectrometer with monochromated Al K X-ray source operated at 90 W
and normal emission, with magnetic focusing and a charge neutraliser.
Spectra were fitted using CasaXPS version 2.3.16, with energy
referencing to adventitious carbon at 284.6 eV, and surface compositions
derived through applying appropriate instrumental response factors. TEM
microscopy was performed on a JEM-2100Plus microscope operated at
200 kV (Warwick University, UK); samples were dispersed in ethanol and
ultrasonicated for 5 min and then drop coated on Cu grid coated with
carbon film. Brunauer–Emmett-Teller (BET) surface areas were obtained
by N2 physisorption at 77 K using a Quantachrome NOVA 4000e
porosimeter on samples degassed at 120 C for 4 h. Surface areas were
calculated over the relative pressure range 0.01-0.2, and BJH pore size
distributions calculated from the desorption branch of the isotherm for
relative pressures >0.35. Diffuse reflectance UV-vis absorption spectra
(DRUVS) were recorded on a Thermo Scientific Evo220 spectrometer
using an integrating sphere, and KBr as a standard, with band gaps
determined between 200-800 nm. Steady state photoluminescence (PL)
spectra were measured on a F-4500FL spectrophotometer using 560 nm
excitation. Time-resolved photoluminescence (TRPL) spectra were
measured on an Edinburgh Photonics FLS 980 spectrometer using pulsed
picosecond LED light and 560 nm excitation.
Experimental Section
Chemicals: Copper (II) chloride (CuCl2, 97 %, Aldrich), polyethylene glycol
(Alfa Aesar, MW600), sodium hydroxide (NaOH, Sigma), hydrazine
monohydrate (H4N2.H2O, Alfa Aesar, 98 %), ethanol (Fisher chemicals,
9.8 %), rGO (Sigma), Cu2O nanoparticles (Sigma, 97 %, 5000 nm particle
size, 47 nm crystallite size, 5 m2.g-1), 4-chlorophenol (C6H5ClO, Acros
organics, 99 %), chlorohydroquinone (Sigma, 85 %) Chlorocatechol
(Sigma, 97 %), fumaric acid (Sigma, 99 %), sodium sulfate (Na2SO4,
Sigma, 99 %), sodium sulfite (Na2SO3, Sigma, 98 %), potassium bromide
(KBr, Sigma, 99 %), Nafion (Sigma), H2O HPLC grade (Sigma), and
Photoelectrochemical
characterization:
A
three
electrode
photoelectrochemical cell was used, comprising a Pt wire counter
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10.1002/cctc.201902048
ChemCatChem
FULL PAPER
꣉꣊꣋꣌ ꣂ꣍꣊꣎꣏꣐꣑
꣒ꣀꣁꣂ ꢌꣀꣁꣂ
electrode and Hg/Hg2SO4 reference electrode. The photoelectrochemical
measurements were converted to the reversible hydrogen electrode (RHE)
using Eq. 1:
% Selectivity ꢃ
꣈ 100
꣓
꣇ꣃ꣄ꣅ꣆
3
ꣃ꣄ꣃꢘꣃꣅ꣆
Acknowledgements
Εꢬ ꢷꢸ.
ꢃ Ε ꢬ ꢷꢸ.
ꢓ Ε
ꢓ 0.059 ꢖ
1
ꢻꢼ
ꢻꢼ ꢽꢾ
ꢻꢼ
ꢻꢼ ꢽꢾ
ꢿ
ꢍꢹꢺ
ꢎ
ꢿ
ꢎ
We thank the Royal Society and Science and Engineering
Research Board for the award of a Royal Society-SERB “Newton
International Fellowship (NF NF151399)” to S.K. We thank the
Biotechnology and Biological Sciences Research Council
(BBSRC; via grant BB/P022685/1). S. K. and K. S. would like to
thank the Japan Society for the Promotion of Science (JSPS) for
providing a postdoctoral fellowship for foreign researchers
(P18387) and the research grant (KAKENHI JP18F18387).
The working electrode was prepared by dropcasting
5
μL of a
homogeneous colloidal suspension on a 3 mm diameter glassy carbon
electrode. The colloid was prepared by 30 min sonication of a catalyst and
Nafion dispersed in a water/ethanol mixture (0.5 mL, 1:1 v/v). Nitrogen
degassed 0.5 M Na2SO4 was used as the electrolyte with a pH of 6.8.
Irradiation was performed using a 200 W Hg-Xe arc lamp ((Oriel
Instruments 66002, 420 nm). Nyquist plots were recorded under
illumination and in the dark on an Autolab potentiostat with Nova software
using a 10 mV AC signal applied between 100 kHz to 0.1 Hz, and Mott-
Schottky plots were recorded at 1000 Hz (under illumination and in the
dark) using a DC signal spanning -1 to 0 V in 10 mV potential steps.
Keywords: Cu2O • reduced graphene oxide • 4-chlorophenol •
hydrogen • photocatalysis
[1]
[2]
R. D. Cortright, R. R. Davda, J. A. Dumesic, Nature 2002, 418, 964.
Y. Hou, B. L. Abrams, P. C. K. Vesborg, M. E. Björketun, K. Herbst, L.
Bech, A. M. Setti, C. D. Damsgaard, T. Pedersen, O. Hansen, J.
Rossmeisl, S. Dahl, J. K. Nørskov, I. Chorkendorff, Nat. Mater. 2011, 10,
434.
Photocatalytic H2 evolution: Photocatalytic H2 production was performed
using hierarchical Cu2O and Cu2O/rGO photocatalysts in a sealed quartz
photoreactor (384 mL volume) with a 200 W Hg-Xe arc lamp (Oriel
Instruments 66002) and 420 nm cut-off filter to remove UV light; the light
intensity inside the reactor was 43.7 mW.cm-2. Catalysts (20 mg) were
dispersed in 45 mL water with 5mL methanol as a sacrificial hole
scavenger and sonicated for 5 min to obtain a uniform distribution. The
photoreactor was then purged with He for 1 h in the dark to remove
dissolved oxygen, prior to visible light irradiation. Aliquots of gas from the
reactor headspace were periodically withdrawn during irradiation using a
1 mL gas syringe and injected into a Shimadzu Tracera GC-2010 Plus gas
chromatography fitted with a Carboxen1010 capillary column (30 m×0.53
mm×0.1 µm) and barrier ionization detector (using a He carrier) for gas
analysis. Mass-normalised photocatalytic activities are reported to enable
quantitative benchmarking of different catalysts, as described in the
literature.[57]
[3]
a) F. Fresno, R. Portela, S. Suárez, J. M. Coronado, J. Mat. Chem. A
2014, 2, 2863-2884; b) M. A. Fox, M. T. Dulay, Chem. Rev. 1993, 93,
341-357.
[4]
[5]
P. D. Tran, L. H. Wong, J. Barber, J. S. Loo, Energy Environ. Sci. 2012,
5, 5902-5918.
a) J. Ran, J. Zhang, J. Yu, M. Jaroniec, S. Z. Qiao, Chem. Soc. Rev.
2014, 43, 7787-7812; b) H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z.
Wang, J. Liu, X. Wang, Chem. Soc. Rev. 2014, 43, 5234-5244.
M. Heinemann, B. Eifert, C. Heiliger, Phys. Rev. B 2013, 87, 115111.
M. Hara, T. Kondo, M. Komoda, S. Ikeda, J. N. Kondo, K. Domen, M.
Hara, K. Shinohara, A. Tanaka, Chem. Commun. 1998, 357-358.
[6]
[7]
[8]
[9]
J. Luo, L. Steier, M.-K. Son, M. Schreier, M. T. Mayer, M. Gratzel, Nano
̈
Lett. 2016, 16, 1848-1857.
4-CP photocatalytic degradation: Photocatalytic 4-CP degradation was
performed in a sealed quartz photoreactor (260 mL) using a 200 W Hg-Xe
arc lamp with 420 nm cut-off filter, and the temperature maintained at 25
C by a Huber Minichiller. Catalysts (20 mg) were dispersed in 50 mL of
7.78 ×10-5 M aqueous 4-CP solution by 7 min ultrasonication in the dark
and stirred for a subsequent 120 min in the dark to equilibrate molecular
adsorption. Aliquots (1 mL) were periodically withdraw from the reaction
mixture for HPLC analysis. Post-reaction catalysts were separated by
centrifugation at 8000 rpm for 10 min, and then vacuum dried and stored
in a vacuum desiccator for characterisation. The catalyst mass was
selected to enable measurement of intrinsic reaction kinetics (Figure S13).
Concentrations of 4-CP and chlorohydroquinone (Cl-HQ), 4-
chlorocatechol (4Cl-CC) and fumaric acid (FA) products were determined
from multi-point calibration curves of reference compounds using an
Agilent 1260 Infinity Quaternary HPLC equipped with UV diode array and
refractive index detectors; an Agilent Zorbax Eclipse plus C18 column was
employed at 35 °C using 1 mL/min of a 30 vol% acetonitrile/70 vol% water
(HPLC grade) mobile phase, and 280 nm detection. The extent of 4-CP
removal, and product selectivity’s were calculated from Eqs. 2 and 3
respectively:
a) A. Chakravarty, K. Bhowmik, A. Mukherjee, G. De, Langmuir 2015, 31,
5210-5219; b) X. An, K. Li, J. Tang, ChemSusChem 2014, 7, 1086-1093;
c) R. Chen, S. Pang, H. An, J. Zhu, S. Ye, Y. Gao, F. Fan, C. Li, Nat.
Energy 2018, 3, 655.
[10] A. S. Zoolfakar, R. A. Rani, A. J. Morfa, A. P. O'Mullane, K. Kalantar-
Zadeh, J. Mat. Chem. C 2014, 2, 5247-5270.
[11] S. Sahoo, S. Husale, B. Colwill, T.-M. Lu, S. Nayak, P. M. Ajayan, ACS
Nano 2009, 3, 3935-3944.
[12] W. Z. Wang, G. Wang, X. S. Wang, Y. Zhan, Y. Liu, C. L. Zheng, Adv.
Mater. 2002, 14, 67-69.
[13] W.-C. Huang, L.-M. Lyu, Y.-C. Yang, M. H. Huang, J. Am. Chem. Soc.
2011, 134, 1261-1267.
[14] S. Jiao, L. Xu, K. Jiang, D. Xu, Adv. Mater. 2006, 18, 1174-1177.
[15] H. Xu, W. Wang, Angew. Chem. Int. Ed. 2007, 46, 1489-1492.
[16] S. Karthikeyan, S. Kumar, L. J. Durndell, M. A. Isaacs, C. M. Parlett, B.
Coulson, R. E. Douthwaite, Z. Jiang, K. Wilson, A. F. Lee,
ChemCatChem 2018, 10, 3554-3563.
[17] C. M. A. Parlett, K. Wilson, A. F. Lee, Chem. Soc. Rev. 2013, 42, 3876-
3893.
[18] S. Kumar, C. M. Parlett, M. A. Isaacs, D. V. Jowett, R. E. Douthwaite, M.
C. Cockett, A. F. Lee, Appl. Catal. B-Environ. 2016, 189, 226-232.
[19] H. Zhang, Q. Zhu, Y. Zhang, Y. Wang, L. Zhao, B. Yu, Adv. Funct. Mater.
2007, 17, 2766-2771.
ꣀꣁꣂ
ꢌꣀꣁꣂ
ꣃ꣄ꣃꢘꣃꣅ꣆
꣇ꣃ꣄ꣅ꣆
% 4CP removal ꢃ
꣈ 100
2
ꣀꣁꣂ
ꣃ꣄ꣃꢘꣃꣅ꣆
where, the 4CPinitial and 4CPfinal are the mols of 4-CP at the start and end
of the irradiation period.
[20] a) L. Chen, Y. Zhang, P. Zhu, F. Zhou, W. Zeng, D. D. Lu, R. Sun, C.
Wong, Sci. Rep. 2015, 5, 9672; b) L. Zhang, P. Ying, B. Yu, L. Wu, J.
Wang, X. Gu, S. Chen, R. Zhou, Z. Ni, RSC. Adv. 2014, 4, 42892-42898.
This article is protected by copyright. All rights reserved.

10.1002/cctc.201902048
ChemCatChem
FULL PAPER
[21] M. Hartmann, Angew. Chem. Int. Ed. 2004, 43, 5880-5882.
[22] a) K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V.
Dubonos, I. V. Grigorieva, A. A. Firsov, Science 2004, 306, 666-669; b)
A. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183.
[23] F. Li, L. Zhang, J. Tong, Y. Liu, S. Xu, Y. Cao, S. Cao, Nano Energy 2016,
27, 320-329.
[40] M. Singh, D. Jampaiah, A. E. Kandjani, Y. M. Sabri, E. Della Gaspera, P.
Reineck, M. Judd, J. Langley, N. Cox, J. van Embden, E. L. H. Mayes,
B. C. Gibson, S. K. Bhargava, R. Ramanathan, V. Bansal, Nanoscale
2018, 10, 6039-6050.
[41] a) G. Nagabhushana, G. Nagaraju, G. Chandrappa, J. Mat. Chem. A
2013, 1, 388-394; b) S. Karthikeyan, S. Kumar, L. J. Durndell, M. A.
Isaacs, C. M. A. Parlett, B. Coulson, R. E. Douthwaite, Z. Jiang, K. Wilson,
A. F. Lee, ChemCatChem 2018, 10, 3554-3563.
[24] a) Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Nat.
Mater. 2011, 10, 780; b) Q. Xiang, J. Yu, J. Phys. Chem. Lett. 2013, 4,
753-759; c) Q. Xiang, B. Cheng, J. Yu, Angew. Chem. Int. Ed. 2015, 54,
11350-11366.
[42] D. Mateo, I. Esteve-Adell, J. Albero, A. Primo, H. García, Appl. Catal. B-
Environ. 2017, 201, 582-590.
[25] Z. Gan, X. Wu, M. Meng, X. Zhu, L. Yang, P. K. Chu, ACS Nano 2014,
8, 9304-9310.
[43] S. Kakuta, T. Abe, Electrochem. Solid State Lett. 2009, 12, P1-P3.
[44] S. Kumar, C. M. A. Parlett, M. A. Isaacs, D. V. Jowett, R. E. Douthwaite,
M. C. R. Cockett, A. F. Lee, Appl. Catal. B-Environ. 2016, 189, 226-232.
[45] D. Barreca, P. Fornasiero, A. Gasparotto, V. Gombac, C. Maccato, T.
Montini, E. Tondello, ChemSusChem 2009, 2, 230-233.
[26] Z. Zhang, R. Dua, L. Zhang, H. Zhu, H. Zhang, P. Wang, ACS Nano 2013,
7, 1709-1717.
[27] C. K. Chua, M. Pumera, Chem. Commun. 2016, 52, 72-75.
[28] W. Wang, P. Zhang, L. Peng, W. Xie, G. Zhang, Y. Tu, W. Mai,
CrystEngComm 2010, 12, 700-701.
[46] N. Barbero, D. Vione, Environ. Sci. Technol. 2016, 50, 2130-2131.
[47] Y. Li, Z. Sun, S. Zhu, Y. Liao, Z. Chen, D. Zhang, Carbon 2015, 94, 599-
606.
[29] S. Liu, Z. Chen, N. Zhang, Z.-R. Tang, Y.-J. Xu, J. Phys. Chem. C 2013,
117, 8251-8261.
[48] a) G. Katsukis, J. Malig, C. Schulz-Drost, S. Leubner, N. Jux, D. M. Guldi,
ACS Nano 2012, 6, 1915-1924; b) Y. Hou, A. B. Laursen, J. Zhang, G.
Zhang, Y. Zhu, X. Wang, S. Dahl, I. Chorkendorff, Angew. Chem. Int. Ed.
2013, 52, 3621-3625; c) P. N. O. Gillespie, N. Martsinovich, ACS Appl.
Mater. Interfaces 2019, 11, 31909-31922.
[30] S. Sun, X. Zhang, X. Song, S. Liang, L. Wang, Z. Yang, CrystEngComm
2012, 14, 3545-3553.
[31] a) Q. Li, B. Guo, J. Yu, J. Ran, B. Zhang, H. Yan, J. R. Gong, J. Am.
Chem. Soc. 2011, 133, 10878-10884; b) J. T.-W. Wang, J. M. Ball, E. M.
Barea, A. Abate, J. A. Alexander-Webber, J. Huang, M. Saliba, I. Mora-
Sero, J. Bisquert, H. J. Snaith, R. J. Nicholas, Nano Lett. 2014, 14, 724-
730.
[49] Y.-C. Pu, H.-Y. Chou, W.-S. Kuo, K.-H. Wei, Y.-J. Hsu, Appl. Catal. B-
Environ. 2017, 204, 21-32.
[50] a) S. Cao, Q. Huang, B. Zhu, J. Yu, J. Power Sources 2017, 351, 151-
159; b) M. Athanasiou, R. Smith, B. Liu, T. Wang, Sci. Rep. 2014, 4, 7250.
[51] D. K. Padhi, K. Parida, J. Mat. Chem. A 2014, 2, 10300-10312.
[52] Y. Yang, D. Xu, Q. Wu, P. Diao, Sci. Rep. 2016, 6, 35158.
[53] Z. Zhang, P. Wang, J. Mat. Chem. 2012, 22, 2456-2464.
[54] P. Wang, H. Wu, Y. Tang, R. Amal, Y. H. Ng, J. Phys. Chem. C 2015,
119, 26275-26282.
[32] W. Zou, L. Zhang, L. Liu, X. Wang, J. Sun, S. Wu, Y. Deng, C. Tang, F.
Gao, L. Dong, Appl. Catal. B-Environ. 2016, 181, 495-503.
[33] C. H. Kuo, C. H. Chen, M. H. Huang, Adv. Funct. Mater. 2007, 17, 3773-
3780.
[34] A. Al Nafiey, A. Addad, B. Sieber, G. Chastanet, A. Barras, S. Szunerits,
R. Boukherroub, Chem. Eng. J. 2017, 322, 375-384.
[35] J. Liu, J. Ke, D. Li, H. Sun, P. Liang, X. Duan, W. Tian, M. O. Tadé, S.
Liu, S. Wang, ACS Appl. Mater. Interfaces 2017, 9, 11678-11688.
[36] Y. Chang, J. J. Teo, H. C. Zeng, Langmuir 2005, 21, 1074-1079.
[37] S. Deng, V. Tjoa, H. M. Fan, H. R. Tan, D. C. Sayle, M. Olivo, S.
Mhaisalkar, J. Wei, C. H. Sow, J. Am. Chem. Soc. 2012, 134, 4905-4917.
[38] L. Xu, F. Zhang, X. Song, Z. Yin, Y. Bu, J. Mat. Chem. A 2015, 3, 5923-
5933.
[55] W.-K. Jo, S. Kumar, M. A. Isaacs, A. F. Lee, S. Karthikeyan, Appl. Catal.
B-Environ. 2017, 201, 159-168.
[56] J. Kou, A. Saha, C. Bennett-Stamper, R. S. Varma, Chem. Commun.
2012, 48, 5862-5864.
[57] a) T. Maschmeyer, M. Che, Angewandte Chemie International Edition
2010, 49, 1536-1539; b) T. Maschmeyer, M. Che, Angewandte Chemie
International Edition 2010, 49, 9590-9591; c) H. Kisch, Angewandte
Chemie International Edition 2010, 49, 9588-9589.
[39] Y. Xu, A. Li, T. Yao, C. Ma, X. Zhang, J. H. Shah, H. Han,
ChemSusChem 2017, 10, 4277-4305.
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