Size-Dependent Visible Light Photocatalytic Performance of Cu2O Nanocubes

Article abstract of DOI:10.1002/cctc.201800439

Well-defined Cu₂O nanocubes with tunable dimensions and physicochemical properties have been prepared using a simple one-pot reaction. Reduction of Cu(II) salts by ascorbic acid in the presence of PEG as a structure-directing agent affords crystalline Cu₂O nanocubes of between 50 to 500 nm. Optical band gap, band energies, charge-carrier lifetimes and surface oxidation state systematically evolve with nanocube size, and correlate well with visible light photocatalytic activity for aqueous phase phenol degradation and H₂ production which are both directly proportional to size (doubling between 50 and 500 nm). HPLC reveals fumaric acid as the primary organic product of phenol degradation, and selectivity increases with nanocube size at the expense of toxic catechol. Apparent quantum efficiencies reach 26 % for phenol photodegradation and 1.2 % for H₂ production using 500 nm Cu₂O cubes.

Full text of DOI:10.1002/cctc.201800439

Accepted Article
Title: Size-dependent visible light photocatalytic performance of Cu2O
nanocubes
Authors: Sekar Karthikeyan, Santosh Kumar, Lee J Durndell, Mark
A Isaacs, Christopher M.A. Parlett, Ben Coulson, Richard
Douthwaite, Zhi Jiang, Karen Wilson, and Adam Fraser Lee
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of the final Version of Record (VoR). This work is currently citable by
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the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
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Size-dependent visible light photocatalytic performance of Cu2O
nanocubes
Sekar Karthikeyan,[a] Santosh Kumar,[b] Lee J. Durndell, [a] Mark. A. Isaacs, [a] Christopher M.A.
Parlett,[a] Ben Coulson,[c] Richard E. Douthwaite,[c] Zhi Jiang,[d] Karen Wilson[e], and Adam F. Lee*[e]
Abstract: Well-defined Cu2O nanocubes with tunable dimensions
and physicochemical properties have been prepared using a simple
one-pot reaction. Reduction of Cu(II) salts by ascorbic acid in the
presence of PEG as a structure-directing agent affords crystalline
Cu2O nanocubes of between 50 to 500 nm. Optical band gap, band
energies, charge-carrier lifetimes and surface oxidation state
systematically evolve with nanocube size, and correlate well with
visible light photocatalytic activity for aqueous phase phenol
degradation and H2 production which are both directly proportional to
size (doubling between 50 and 500 nm). HPLC reveals fumaric acid
as the primary organic product of phenol degradation, and selectivity
increases with nanocube size at the expense of toxic catechol.
overall water splitting.[6]. A wide range of Cu2O morphologies are
amenable to synthesis, including cubes,[7] rhombicuboctahedra,[8]
polyhedra,[9] nanowires,[4] nanocages,[10] and hollow structures,[11]
and consequently their facet-dependent photocatalytic properties
have been investigated for the destruction of hazardous organic
compounds.[12] However, few reports have explored particle size
effects of copper oxides on photocatalytic performance, despite
the well-known evolution of photophysical properties,[13] because
the synthesis of well-defined Cu2O nanostructures of tunable
dimensions remains challenging.[14]
Wastewater depollution due to the unregulated discharge of
recalcitrant organic compounds (ROCs), notably from textile,
paper, drug, and food manufacturing, cannot be removed by
conventional biological and/or physicochemical processing (e.g.
microorganisms, flocculation or chlorination)[15] and represent a
significant hazard for many developed and developing
countries.[16] Such ROCs include toxic phenolics,[17] and their
removal has been investigated using so-called advanced
oxidation processes (AOPs) which utilise a range of highly
oxidising species, either alone or in combination (e.g. O3, O3/H2O2,
UV/O3, UV/H2O2, O3/UV/H2O2, and Fe2+/H2O2),[18] to decompose
phenol in aqueous solution. Fenton-type AOPs which utilise the
redox properties of certain transition metals, exhibit high removal
efficiencies for diverse ROCs including phenols, but share a
common requirement for H2O2 addition, and heterogeneous
variants are susceptible to metal leaching.[19] Hence
photocatalytic wastewater treatment, ideally employing visible
light and O2 (air), could offer a more cost-effective and sustainable
alternative to current AOPs.
Apparent quantum efficiencies reach 26
%
for phenol
photodegradation and 1.2 % for H2 production using 500 nm Cu2O
cubes.
Introduction
Metal oxide based semiconductor photocatalysts are widely
employed for environmental remediation[1] and solar fuels
production via water splitting[2] and CO2 reduction[3], due to their
earth-abundance, low cost, and environmental sustainability.[4]
Copper (I) oxide (Cu2O) is an attractive semiconductor for large-
scale applications such as wastewater treatment,[5] and
possesses a narrow bandgap (2.0-2.2 eV) enabling visible light
utilisation. In addition the position of the conduction band
minimum (CBM) and valence band maximum (VBM) span the
potentials for proton reduction and water oxidation required for
To date, various metal oxide nanostructures including
[22]
TiO2,[20] TiO2/carbon,[21] metal/non-metal doped-TiO2
and
ZnO,[23] graphitic carbon nitride,[24] and Cu2O composites[17] have
been studied for phenol degradation, although quantitative
comparisons between different photocatalysts is hampered by the
lack of a standard testing protocol. Indeed, the majority of
photocatalytic studies do not report either mass or surface area
normalised rates. However, we recently reported that hierarchical
Cu2O nanocubes[5a] show high activity for methylene blue
degradation (0.6 µmolg-1min-1 versus 0.008 µmolg-1min-1 for Cu2O
nanowires),[4] and a Cu2O/TiO2 p-n heterojunction photocatalyst
has shown promise in aqueous phase p-nitrophenol degradation
under artificial sunlight. Cu2O is also an effective photocatalyst for
H2 evolution from water splitting,[2],[5a] with 300-500 nm Cu2O
nanocrystals exhibiting H2 productivity of 0.16 µmolg−1h−1.[25]
These compare to the composites 1 wt% MoS2 /200-400 nm Cu2O
nanospheres[26] and 3 wt% Pt/Cu2O-g-C3N4 nanocomposites
giving 250 µmolg−1h−1,[27] although both studies used high power
light sources (350 W and 300 W respectively) and sacrificial
alcohol donors, and did not report quantum efficiencies. In
contrast, we recently achieved 15 µmolg−1h−1 H2, and an apparent
[a]
Dr. S. Karthikeyan, Dr. L. J. Durndell, Dr. C. M. A. Parlett, Dr. M. A.
Isaacs
European Bioenergy Research Institute
Aston University
Aston Triangle, Birmingham B4 7ET, UK
Dr. S. Kumar
Department of Chemical Engineering
University of Bath
[b]
[c]
[d]
[e]
Bath BA2 7AY, UK
B. Coulson, Dr. R. E. Douthwaite
Department of Chemistry
University of York
York YO10 5DD, UK
Dr Z. Jiang
Research Center for Combustion and Environment Technology
Shanghai Jiao Tong University
Shanghai, P.R. China
Prof. A.F. Lee, Prof. K. Wilson
School of Science
RMIT University
Melbourne VIC 3001, Australia
E-mail: adam.lee2@rmit.edu.au
Supporting information for this article is given via a link at the end of
the document.
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100 nm
50 nm
200 nm
100 nm
300 nm
500 nm
500 nm
500 nm
Figure 1. TEM images of Cu2O nanocubes and (inset) corresponding particle size distributions.
quantum efficiency of 1.2 %, over 375 nm hierarchical Cu2O
nanocubes in the absence of any sacrificial donor.[28]
resolved photoluminescence, and N2 porosimetry. Optical band
gaps, electronic band energies, rates of charge recombination,
apparent quantum efficiency, and the surface copper oxidation
state, all evolve monotonically with nanocube size, and correlate
with photocatalytic activity towards aqueous phase phenol
degradation, and hydrogen production, under visible light
irradiation.
Herein we report a systematic investigation of the impact of
Cu2O size on both the photocatalytic degradation of aqueous
organics, and production of H2. Previous efforts to prepare
monodispersed Cu2O nanocubes of tunable size have utilised
nitrate, sulphate or chloride precursors, and capping agents such
as SDS, CTAB or PEG.[7, 14b, 14c, 29] However, access to a wide size
range of Cu2O nanostructures has to date required a seed-
mediated approach, and there are no systematic studies of their
corresponding size-structure-activity relationships.
Results and Discussion
A new synthetic route to monodispersed Cu2O nanocubes
with sizes spanning 50-500 nm employing an acetate precursor is
described, enabling facile tuning of their photophysical properties
and corresponding photocatalytic performance. Physicochemical
properties were characterised by bulk and surface analytical
methods including XRD, XPS, HRTEM, SEM, DRUVS, time-
Photophysical characterisation
Successful synthesis of uniform Cu2O nanocubes with tunable
dimensions was first demonstrated using a simple solution phase
approach with ascorbic acid as a reductant and PEG as a
structure-directing agent. The morphology and size distribution of
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61.38 °, 73.52 °, and 77.38 ° indexed as the (111), (200), (211),
(220), (311), and (222) reflections of cubic Cu2O (JCPDS #73-
0687); no reflections attributable with either CuO or fcc Cu metal
were observed. Volume-averaged crystallite sizes estimated from
the Scherrer equation (Table 1) suggest that in all cases the
dense Cu2O nanocubes comprise compact agglomerations of 20-
30 nm crystallites, whose size increased slightly with the overall
nanocube dimensions. Textural properties were also examined by
nitrogen porosimetry, with the resulting type II adsorption-
desorption isotherms (Figure 2b) characteristic of macroporous
materials (or non-porous materials possessing large interparticle
voids) with very small H3-type hysteresis loops; corresponding
BJH pore size distributions (Figure S3) confirmed that the
nanocubes are essentially non-porous. BET surface areas
decreased with increasing nanocube size, albeit not by the order
of magnitude predicted by simple geometric considerations.
Although small nanoparticles may be advantageous in
photocatalysis due to their higher specific surface area,[32] other
size-dependent factors such as the surface termination,[33] and
mobility and recombination of photoexcited charge carriers may
favour large particles.[34]
50 nm
a
100 nm
300 nm
500 nm
10
20
30
40
50
60
70
80
2 / 
100
90
80
70
60
50
40
30
50 nm
300 nm
100 nm
500 nm
Optical properties of the Cu2O nanocubes were determined
by DRUVS (Figure 3a).
All nanocubes exhibited strong
absorbances between 200-500 nm, in close agreement with
previous reports on Cu2O nanocubes.[7, 14c] Corresponding Tauc
plots (Figure 3b) were obtained using Equation 1:
훼ℎ푣 = 퐴(ℎ푣 − 퐸푔)
(1)
where , h, , Eg, and A are the absorption coefficient, Planck’s
constant, light frequency, band gap energy, and a proportionality
constant, respectively. As a direct band gap material  = 0.5 for
Cu2O, this enables the band gap to be calculated using absorption
coefficients determined from the Kubelka-Munk formalism
(Equation 2):
2
푎 = (1−푅)
(4)
2푅
The resulting band gaps decreased with nanocube size from 2.25
eV to 1.96 eV (Table 1) consistent with literature values for Cu2O
nanostructures.[5a, 35] The electronic band structure of the Cu2O
nanocubes was determined by valence band XP measurements.
Valence band maximum (VBM) energies, derived from the
intercept of the tangent to the density of states at the Fermi edge
(Figure S4a), decreased with increasing particle size from 0.81
(50 nm) to 0.35 eV (500 nm). This is in good agreement with XPS
and optical absorption studies of Cu2O nanoparticles prepared by
reactive evaporation, attributed to an increase in the dopant
ionization energy and associated shift in the Fermi level.[36]
Corresponding conduction band minimum (CBM) energies,
derived from these VBM and the optical band gap energies
spanned -1.44 eV to -1.61 eV, significantly greater than that
required for hydrogen reduction (-0.65 eV at pH 7).[37]
The oxidation state of copper in the nanocubes was
explored by Cu 2p XP spectra (Figure S4b), which evidence a
strong size-dependence. Small cubes exhibit sharp
photoemission features at 932.3 and 951.8 eV binding energy,
consistent with the spin-orbit split 2p3/2 and 2p1/2 components of
Cu2O and/or copper metal;[5a] the latter can be discounted since
there is no evidence of metallic copper from XRD. In addition, the
50-300 nm nanocubes exhibit a second weak 2p spin-orbit split
b
0.0
0.2
0.4
0.6
0.8
1.0
P/P0
Figure 2. (a) XRD patterns, and (b) N2 adsorption-desorption isotherm of Cu2O
nanocubes.
Cu2O nanocubes was determined by TEM (Figure 1) and SEM
(Figure S1) and revealed dense nanocubes with smooth surfaces
and dimensions of between 50-500 nm. Cubes could be prepared
with excellent monodispersity simply by increasing the PEG
concentration, consistent with previous reports,[7] which shows
that ligand capping concentration has an important role in
controlling particle size and structure[30]. High resolution TEM
images of the 300 nm Cu2O nanocubes (Figure S2) reveal
interplanar lattice spacings of 0.24 nm, indicative of (111) facets
of cubic Cu2O.[31]
The phase purity and crystallinity of the Cu2O nanocubes
was subsequently investigated by powder XRD (Figure 2a). All
nanocubes exhibited peaks at 2= 29.6 °, 36.42°, 42.31°, 52.46°,
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Table 1. Physicochemical properties of Cu2O nanocubes.
Nanocube size / nm[a]
Crystallite size[b] / nm
BET surface area / m2.g-1[c]
Band gap / eV[d]
CBM[e] / eV
Cu(I)[f] / atom%
Cu(I):Cu(II)[f]
50
18.5
25.3
26.7
29.3
20
19
15
13
2.25
2.10
2.05
1.96
-1.44
-1.56
-1.60
-1.61
77.1
80.8
81.3
89.6
3.4
4.2
4.5
8.6
100
300
500
[a] TEM. [b] XRD. [c] N2 porosimetry. [d] DRUVS. [e] calculatyed from valence band XPS and DRUVS. [f] Cu 2p XPS.
0.4
0.35
0.3
1.4
1.2
1
50 nm
100 nm
300 nm
500 nm
0.25
0.2
0.8
0.6
0.4
0.2
0
0.15
0.1
50 nm
100 nm
300 nm
500 nm
0.05
0
200
400
600
800
1.6
2.1
2.6
3.1
3.6
Bandgap / eV
Wavelength / nm
Figure. 3. (a) DRUVS absorption spectra, and (b) corresponding Tauc plot of Cu2O nanocubes.
doublet at 935.6 eV and 955.4 eV, accompanied by a weak 943
eV satellite feature, both characteristic of Cu(OH)2 (these features
Table 2. Fitted parameters from time-resolved photoluminescence for Cu2O
are almost absent for the 500 nm cubes).[38] Spectral fitting
reveals that the proportion of Cu2O relative to Cu(OH)2 increased
monotonically from 77.1 % in the 50 nm sample to 89.6 % for the
500 nm nanocubes. Recombination of photoexcited charge
carriers within the Cu2O nanocubes was subsequently
investigated by steady state (Figure 4a) and time-resolved (Figure
4b) photoluminescence (PL). For 560 nm excitation, the principal
emission at 620 nm (Figure 4a) was inversely proportional to
particle size, indicative of slower radiative recombination of
photoexcited electron-hole pairs for the larger cubes.[39] This
conclusion is supported by corresponding time-resolved PL decay
spectra (Figure 4b) which exhibited longer radiative lifetimes for
the 500 nm (1=0.628 ns) versus 50 nm (1=0.585 ns) nanocubes
(Table 2).[4, 40]
nanocubes.
[b]
Nanocube size
/ nm[a]
1
/ ns
B1
2[c]
50
0.585
0.619
0.628
0.628
361.2
364.1
192
1.44
1.38
1.19
1.18
100
300
500
143.1
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0.035
0.03
0.025
0.02
0.015
0.01
0.005
0
a
a
5300
50 nm
100 nm
300 nm
500 nm
4300
3300
2300
1300
300
600
630
660
690
0
200
400
600
Wavelength / nm
Cu2O size / nm
1000
900
800
700
600
500
400
300
200
100
0
0.0025
0.002
0.0015
0.001
0.0005
0
b
b
50 nm
100 nm
300 nm
500 nm
R² = 0.9931
0.0
2.0
4.0
6.0
8.0 10.0
0
200
400
600
Time / ns
Cu2O size / nm
Figure 4. (a) Steady state, and (b) time-resolved PL spectra of Cu2O nanocubes
under 560 nm excitation.
Figure 5. (a) Mass normalised, and (b) surface area normalised initial rates of
phenol degradation over Cu2O nanocubes under visible light. Experimental
conditions: 0.127 mmol phenol in 50 mL water, 20 mg catalyst, 200 W Hg-Xe
arc lamp with 420 nm visible cut-off filter, 15 min reaction.
Photocatalytic phenol degradation
The photocatalytic performance of the family of Cu2O nanocubes
was first studied for the aqueous phase degradation of phenol
under visible light irradiation. In addition to being a prototypical
organic pollutant,[41] phenol has negligible absorbance in the
visible region, and hence is not prone to the artefacts that arise in
dye degradation studies due to catalyst sensitisation (which in
500 nm results in a three-fold enhancement in photocatalytic
activity. The linear relationship between the surface area
normalised degradation rate (essentially the turnover frequency)
and nanocube size demonstrates that phenol decomposition is
heavily influenced by either bulk electronic properties (e.g. band
gap, CBM, and/or rate of charge carrier recombination) or the
surface density of Cu(I) species, rather than a unique structure-
sensitivity to (100) facets. Similar observations are reported for
WO3 nanoparticles towards photocatalytic water oxidation,
wherein a four-fold rate-enhancement in the specific activity was
observed upon increasing particle size from 100 to 800 nm
turn
hinder
mechanistic
insight
and
performance
benchmarking).[42] The resulting reaction profiles (Figure S5)
reveal only a small (<10 %) contribution from photolysis in the
absence of photocatalyst. Corresponding initial rates of phenol
degradation reveal a direct relationship between nanocube size
and both specific (mass-) and surface area normalised
degradation rates (Figure 5); increasing cube size from 50 to
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100
employ higher power light sources and catalyst loadings than the
present work.
Phenol removal
Catechol selectivity
FA selectivity
a
90
80
70
60
50
40
30
20
10
0
A
critical and oft-neglected aspect of photocatalytic
depollution studies is the fate of organic reactants, i.e. the nature
of the resulting degradation products, with most literature reports
simply assuming complete mineralisation.[43] The phenol
degradation pathway, and possible role of in-situ generated
radicals, was therefore subsequently investigated by HPLC as a
function of Cu2O nanocube size, with catechol and fumaric acid
identified as the principal organic by-products after 240 min
irradiation. As anticipated, for all nanocube sizes catechol
appears as the primary product of phenol oxidation, being itself
consumed at longer reaction times, whereas fumaric acid is
formed throughout the reaction as a secondary decomposition
product (Figure 6a). Tryba et al report that the formation of
catechol as a primary product of phenol decomposition (rather
than hydroquinone or benzoquinone) favours subsequent
carboxylic acid formation and mineralisation.[44] The greater rate
observed for increasing nanocube size from 50 to 500 nm leads
to enhanced phenol removal from 36 to 50 % after 6 h, and
concomitant absolute selectivity to fumaric acid from 7.8 to 25 %.
The latter observation is significant since the toxicity of fumaric
acid is far lower than that of catechol, with LD50 Oral of ~10,500
mg.kg-1 and 250 mg.kg-1 respectively.[45] Apparent quantum
efficiencies (see Supporting Information for calculation) tracked
the rates of phenol degradation, increasing from 14 to 26 % with
Cu2O nanocube size (Figure 6b). The latter value is 1500 times
that reported for Pt/TiO2 under UV irradiation.[46]
The photocatalytic oxidation of phenol to catechol and
fumaric acid likely occurs through hydroxyl radicals which initially
form hydroxyl substituted aromatic derivatives,[47] with negligible
hydroquinone detected (which exists in equilibrium with
benzoquinone). Hydroxyl radical attack is proposed to follow
previous reports[48] as summarised in Scheme 1, and initiated by
hydroxyl radical formation at the photocatalyst surface via
reaction of photoexcited holes with hydroxide ions.[49]
Concomitant superoxide radical anion (O2•−) formation can occur
through the reduction of O2 via photoexcited electrons from the
conduction band,[50] which in turn reacts with water to form H2O2,
another strong oxidant.
50 nm 100 nm 300 nm 500 nm
50 100 300 500
Cu2O size / nm
30
25
20
15
10
5
b
0
50
100
300
500
Catalyst stability towards visible light driven phenol
degradation was assessed for the 300 nm Cu2O nanocubes over
multiple recycles. Figure 7a evidenced negligible activity loss over
four consecutive reactions, consistent with retention of the initial
crystallinity and particle size observed by XRD (Figure 7b), and
hence excellent long-term stability. Photocorrosion is often
problematic for Cu2O photocatalysts, and hence the size and
morphology of 300 nm nanocubes was examined by SEM and
TEM post-reaction following aqueous phase phenol degradation.
The resulting micrographs (Figure S6) evidence no significant
changes in either particle size or shape following photocatalytic
phenol degradation, confirming the stability of our Cu2O
nanocubes, consistent with Figure 7a. Corresponding Cu 2p XP
spectra also show little increase in the 943 eV Cu(II) satellite
(Figure S7) post-reaction.
Cu2O size /nm
Figure 6. (a) Phenol removal and relative selectivity to organic by-products over
Cu2O nanocubes under visible light, and (b) corresponding apparent quantum
efficiency. Experimental conditions: 0.127 mmol phenol in 50 mL water, 20 mg
catalyst, 200 W Hg-Xe arc lamp with 420 nm visible cut-off filter, 240 min
reaction.
particles, attributed to slower surface recombination of
photogenerated electrons and holes.[34] Smaller semiconductor
nanoparticles often exhibit high defect densities (notably surface
defects), which may promote undesired e--h+ charge carrier
surface recombination, or the creation of new states localised
within the band gap that trap photogenerated charge carriers,
thereby lowering photocatalytic activity.[33] The first order rate
constant for phenol degradation over the 500 nm Cu2O cubes of
0.009 min-1 significantly exceeds literature reports for alternative
metal oxide photocatalysts (Table S1),[43] which also typically
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6.5
6
a
5.5
5
4.5
4
3.5
3
2.5
2
0
200
400
600
800
Reaction time / min
Scheme 1. Proposed pathways for photocatalytic phenol degradation over
Cu2O nanocubes under visible light.
b
Photocatalytic hydrogen production
Cuprous oxide has been proposed as a promising candidate for
photo-assisted water splitting, due to its favourable conduction
band position for proton reduction, although its shallow valence
band only offers (at best) a low overpotential for water oxidation.[37,
51] Since the measured CBM energies for our Cu2O nanocubes far
exceed that required for proton reduction (>1.5 eV versus -0.4 eV),
their performance was therefore assessed for photocatalytic H2
production in the presence of a 1 wt% platinum co-catalyst and
Na2SO3 as a hole scavenger. HRTEM of the platinised 300 nm
Cu2O nanocube revealed highly dispersed and uniform 0.6-1.2
nm Pt nanoparticles (Figure S8 a-b). The resulting visible light
photocatalytic activity (Figure 8a-b) reveals a similar size-
dependency as observed for phenol degradation; specific (mass-
normalised) and surface area normalised H2 productivity are
proportional to particle size. These trends mirror the
corresponding rise in CBM energy (Table 1), a key parameter in
controlling activity,[52] and apparent quantum efficiency (Figure S9,
which reached 1.2 % for 500 nm Cu2O), across the nanocube
family. This performance is comparable to that of Hara and co-
workers who observed 0.16 µmolg−1h−1 over unstructured 300-
500 nm Cu2O particles, but with an apparent quantum efficiency
of only 0.3 %.[53]
Charge recombination is believed to play an important role
in regulating photocatalytic reactivity. Hence the question arises
whether the variation in charge carrier lifetimes between different
nanocubes accounts for their size-dependent photocatalytic
activity. If so, then normalising hydrogen productivity to the charge
recombination rates from time-resolved photoluminescence
spectroscopy (Table 2) should result in a common value for all
photocatalysts. Such normalisation does not yield a common
value (Figure S10), and hence charge carrier lifetimes do not
dominate our photocatalyst performance.
Fresh
Used
10
20
30
40
2 / 
50
60
70
80
Figure 7. (a) Photocatalyst stability during phenol degradation over 300 nm
Cu2O nanocubes under visible light, and (b) corresponding powder XRD of fresh
and used nanocubes following four consecutive reactions. Experimental
conditions: 0.127 mmol phenol in 50 mL water, 20 mg catalyst, 200 W Hg-Xe
arc lamp with 420 nm visible cut-off filter.
Conclusions
A new one-pot, solution phase synthesis offers uniform Cu2O
nanocubes of tunable size between 50 to 500 nm. Nanocube
photophysical properties, notably the optical band gap,
conduction band minimum, and charge-carrier recombination rate,
evolve continuously with particle size and are closely correlated
with visible light photocatalytic activity for phenol degradation and
H2 production. Despite their lower surface area,
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(Sigma, 99 %) were used without further purification. Deionised water was
used for all solution preparation.
0.016
a
0.014
Cu2O nanocube synthesis
A range of Cu2O nanocubes were prepared by a solution phase approach.
Typically, 5 mL of 0.02 M copper acetate was mixed with 6 mL of 0.04 M
polyethylene glycol during continuous stirring at 550 rpm and 65 C for 10
min, resulting in a deep blue solution. In a separate vessel, 2 mL of 1 M
NaOH and 2 mL of 0.05 M ascorbic acid were mixed with 35 mL of
deionised water. The NaOH/ascorbic acid mixture was then added
dropwise to the copper acetate/PEG solution, and stirring continued at 65
C for another 5 min. The mixture was then transferred to a sealed round
bottom flask and purged with N2 for 30 min to allow formation of a
brownish-yellow Cu2O nanocube precipitate. The nanocubes were
isolated by 7 min centrifugation at 8000 rpm, washed twice with H2O, and
three times with ethanol to remove any residual PEG, and finally dried
overnight at 65 C and stored in air. Different size nanocubes were
prepared with side lengths between 50 and 500 nm by changing the
volume of PEG between 2-9 mL respectively. The synthesis is summarised
in Scheme 2: PEG first complexes to the Cu(II) ions which are
subsequently reduced on ascorbic acid addition, and precipitated as Cu(I)
oxide in the presence of NaOH; hydroxyl groups in the PEG matrix are
likely responsible for controlling the density of Cu ions and resulting Cu2O
nanocube dimensions.[5a, 7]
0.012
0.01
0.008
0.006
0.004
0.002
0
0
200
400
600
Cu2O size / nm
0.0012
b
NaOH+ascorbic acid
0.001
0.0008
0.0006
0.0004
0.0002
300 nm
10 min
5 min
H2O+ethanol
N2
65 C
65 C
8000 rpm
centrifugation
1 m
Cu(CH3COO)2 + PEG
Scheme 2. Synthesis of Cu2O nanocubes.
Pt functionalisation of Cu2O nanocubes
Cu2O nanocubes were also functionalised with a Pt co-catalyst by in-situ
photodeposition to aid water splitting.[54] 100 mg Cu2O nanocubes were
dispersed in 20 mL deionised water, to which 30 mL methanol and an
appropriate amount of aqueous H2PtCl6 was added (2.10 mg of H2PtCl6 in
20 mL deionised water, equating to a nominal 1 wt% Pt loading). The
resulting mixture was ultrasonicated for 5 min, and then irradiated by a 200
W Hg-Xe light source for 1 h under stirring within a stainless steel reactor.
The Pt/Cu2O nanocubes were then separated by centrifugation and dried
in vacuo at 65 C for 6 h.
0
0
200
400
600
Cu2O size / nm
Figure 8. a) Photocatalytic hydrogen evolution under visible light, b) surface
area normalised rates of hydrogen production over 1 wt% Pt promoted Cu2O
nanocubes (experimental conditions: 50 mL water, 50 mg catalyst, 200 W Hg-
Xe arc lamp with 420 nm visible cut-off filter, 5 h reaction.
Catalyst characterisation
Crystallinity was examined 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 performed on a Kratos Axis
HSi spectrometer with monochromated Al K X-ray source operated at 90
W and normal emission, employing 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. Cu2O
and Pt/Cu2O nanocubes were visualised using a JEOL JEM-2100 TEM
microscope operated at 200 kV. 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,
larger Cu2O nanocubes offer superior phenol mineralisation, and
selectivity to more benign by-products (fumaric acid versus
catechol), and also exhibit excellent stability over four consecutive
re-uses. Future work will explore the impact of Cu2O shape and
facet on photoactivity.
Experimental Section
Chemicals
Copper (II) acetate (Aldrich, 98.0 %), NaOH (Aldrich, ≥98 %), L-ascorbic
acid (Aldrich, 99 %), polyethylene glycol (Alfa Aesar, MW 600),
hexachloroplatinic (IV) acid hydrate (Aldrich, 99.9 %), phenol (Aldrich,
99 %) acetonitrile HPLC grade (Sigma, 99.93 %) and sodium sulfate
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10.1002/cctc.201800439
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FULL PAPER
with band gaps determined between 200-800 nm. Steady state
photoluminescence (PL) spectra of Cu2O were measured on a F-4500FL
SERB “Newton International Fellowship” for financial support. We
thank Dr Amin Osatiashtiani for assistance with XPS analysis.
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. Phenol concentrations were determined by HPLC using
an Agilent 1260 Infinity Quaternary HPLC equipped with both a UV diode
array and refractive index detectors; an Agilent Zorbax Eclipse plus C18
column was employed at 35 C, with 25 μL injection volume, and 1 mL/min
of a 10 vol% acetonitrile/90 vol% water (HPLC grade) mobile phase, and
270 nm detection wavelength.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: Cu2O • photocatalysis • nanocubes • phenol •
hydrogen
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Engineering Research Board for the award of a Royal Society-
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Entry for the Table of Contents
FULL PAPER
Tuning Cu2O nanocube size enables
facile control over photocatalytic
performance for environmental and
energy applications.
Sekar Karthikeyan, Santosh Kumar, Lee
J. Durndell, Mark. A. Isaacs, Christopher
M.A. Parlett, Ben Coulson, Richard
Douthwaite, Zhi Jiang, Karen Wilson,
and Adam F. Lee*
Page No. – Page No.
Size-dependent visible light
photocatalytic performance of Cu2O
nanocubes
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