Template free mild hydrothermal synthesis of core-shell Cu2O(Cu)?CuO visible light photocatalysts for N-acetyl-para-aminophenol degradation

Article abstract of DOI:10.1039/c9ta07009e

Solar photocatalytic processes are a promising approach to environmental remediation, however their implementation requires improvements in visible light harvesting and conversion and a focus on low cost, Earth abundant materials. Semiconducting copper oxides are promising visible light photocatalysts for solar fuels and wastewater depollution. Here we report the mild, hydrothermal (template-free) synthesis of core-shell Cu₂O(Cu)?CuO photocatalytic architectures for the visible light photocatalytic degradation of N-acetyl-para-aminophenol (APAP). Hollow and rattle-like core-shell nanosphere aggregates with diameters between 200 nm and 2.5 μm formed under different synthesis conditions; all comprised an inner Cu₂O shell, formed of 10-50 nm nanoparticles, surrounded by a protective corona of CuO nanoparticles. High reductant and structure-directing agent concentrations promoted the formation of a yolk-like Cu₂O/Cu core, associated with improved photophysical properties, notably a high oxidation potential and suppressed charge carrier recombination, that correlated with the highest apparent quantum efficiency (8%) and rate of APAP removal (7 μmol g^-1 min^-1). Trapping experiments demonstrated hydroxyl radicals were the primary active species responsible for APAP oxidation to quinones and short chain carboxylic acids. Rattle-like core-shell Cu₂O/Cu?CuO nanospheres exhibited excellent physiochemical stability and recyclability for APAP photocatalytic degradation.

Full text of DOI:10.1039/c9ta07009e

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Template free mild hydrothermal synthesis of core-shell Cu2O(Cu)@CuO visible liVgiehwtArticle Online
DOI: 10.1039/C9TA07009E
photocatalysts for N-acetyl-para-aminophenol degradation
Sekar Karthikeyan, † Chitiphon Chuaicham, † Radheshyam R Pawar, † Keiko Sasaki, †*Wei Li, ‡ Adam F.
Lee,∥* Karen Wilson∥
†Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University,
744 Motooka, Nishiku, Fukuoka 819-0395, Japan
‡European Bioenergy Research Institute, Aston Institute of Materials Research, Aston University,
Birmingham B4 7ET, UK
∥Applied Chemistry & Environmental Science, RMIT University, Melbourne VIC 3000, Australia.
*Corresponding Author: E-mail: keikos@mine.kyushu-u.ac.jp, adam.lee2@rmit.edu.au ; Tel: +81-92-802-
3338, +61 399252623
Abstract
Solar photocatalytic processes are a promising approach to environmental remediation, however their
implementation requires improvements in visible light harvesting and conversion and a focus on low cost, Earth
abundant materials. Semiconducting copper oxides are promising visible light photocatalysts for solar fuels and
wastewater depollution. Here we report the mild, hydrothermal (template-free) synthesis of core-shell
Cu2O(Cu)@CuO photocatalytic architectures for the visible light photocatalytic degradation of N-acetyl-para-
aminophenol (APAP). Hollow and rattle-like core-shell nanosphere aggregates with diameters between 200 nm
and 2.5 m formed under different synthesis conditions; all comprised an inner Cu2O shell, formed of 10-50
nm nanoparticles, surrounded by a protective corona of CuO nanoparticles. High reductant and structure-
directing agent concentrations promoted the formation of a yolk-like Cu2O/Cu core, associated with improved
photophysical properties, notably a high oxidation potential and suppressed charge carrier recombination, that
correlated with the highest apparent quantum efficiency (8 %) and rate of APAP removal (7 mol.g-1.min-1).
Trapping experiments demonstrated hydroxyl radicals were the primary active species responsible for APAP
oxidation to quinones and short chain carboxylic acids. Rattle-like core-shell Cu2O/Cu@CuO nanospheres
exhibited excellent physiochemical stability and recyclability for APAP photocatalytic degradation.
Keywords: Photocatalysis; copper; core-shell nanoparticles; N-acetyl-para-aminophenol; depollution
1. Introduction
Solar energy utilisation by semiconductor nanomaterials is the subject of intensive fundamental research and
technological developments for applications in environmental remediation,1 and fuels2 and fine chemicals3, 4
production. Titania has been the inorganic semiconductor photocatalyst of choice for hydrogen production,
1

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wastewater treatment, pathogen inactivation, and in personal care products,5, 6 however as a wide band gap (~3.2
DOI: 10.1039/C9TA07009E
eV) semiconductor it can only absorb up to 5 % of sunlight reaching the Earth’s surface.7 Cuprous oxide (Cu2O)
is a low cost and Earth abundant p-type narrow band gap (2.0 to 2.5 eV) semiconductor, with conduction and
valence band energies suited to visible light driven for solar fuels production, water splitting and environmental
depollution.8 Various strategies have been adopted to engineer the photophysical properties of Cu2O
nanomaterials through controlling their size,9 morphology10 and surface termination,11, 12 and the introduction
of co-catalysts13 and heterojunctions,14 with the goal of improved light absorption and photoexcited charge
carrier lifetimes (and consequently quantum efficiency) in addition to enhanced photocatalytic stability.15 Cu2O
nanostructures can exhibit unexpected photophysics, for example charge separation across the surface of cubic
Cu2O single crystal photocatalysts induced by asymmetric illumination; visible light excited holes (h+) and
electrons (e-) migrate to illuminated and shadowed regions respectively as a result of different charge carrier
mobilities.16 Cupric oxide (CuO, band gap 1.2 to 1.7 eV) has also been used as a co-catalyst to stabilise Cu2O
and affect charge separation across heterojunction interfaces.17-19 Diverse Cu2O composite nanostructures are
reported, enabling tuning of textural, electronic, and optical properties for thermal, photo- and electrocatalysis.20-
22
Among these, are core-shell/hollow Cu/Cu2O and CuO/Cu2O architectures, synthesised by low (230 nm to
260 nm single crystal Cu2O hollow spheres)23 or high temperature (4-10 nm single crystal Cu2O@CuO
nanoparticles)24 surfactant templating, or templateless high temperature(<450 C),25, 26 electrodeposition,17, 27
28
solvothermal,20,
or hydrothermal29 processing. Yang et al report that Cu2O/CuO bilayered
composite photocathodes are advantageous for photoelectrochemical hydrogen evolution due to injection of
photoexcited electrons from the Cu2O conduction band (CB) to that of CuO and concomitant reverse injection
of photoexcited holes from the CuO valence band (VB) to that of Cu2O, increasing carrier transport rates through
the composite and across the photocathode/solution interface.17 However, core-shell/hollow CuO/Cu2O
architectures have rarely been applied to wastewater depollution, and only then for dye degradation29 25, 26
wherein photosensitisation is problematic.30
We recently reported the synthesis of hierarchical31 and size controlled (50-500 nm) non-porous9 Cu2O
nanocubes for photocatalytic H2 production and phenol degradation respectively, and hence were curious to
explore the potential of core-shell/hollow CuOx systems for removing organic micropollutants from water.32 N-
acetyl-para-aminophenol (APAP) is an important nonsteroidal anti-inflammatory, analgesic and antipyretic
drug that is extensively used in global healthcare, and a xenobiotic found in wastewater at ng.L−1 to μg.L−1 levels
in ground and drinking water,33 and whose long-term adverse effects on biological and pharmacokinetic activity
are of significant concern. In Japan, levels as high as 92 μg.L-1 are reported in river water downstream of a
hospital effluent sewage treatment plant,33 while levels reached 65 µg.L-1 in the Tyne river (UK),34 6 μg.L−1 and
10 μg L−1 for sewage effluent in Europe and natural water in the USA respectively,35 0.033 and 6.8 μg.L−1 for
surface and river water in South Korea.36
2

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Here we describe the mild (60 C) hydrothermal synthesis, characterisation and application of core-shell
DOI: 10.1039/C9TA07009E
Cu2O(Cu)@CuO nanospheres with diameters between from 200 nm and 2 m. Photophysical properties were
investigated by bulk and surface analytical techniques including XRD, SEM, HR(S)TEM, XANES, XPS, N2
porosimetry, and UV-visible spectroscopies. Cu2O(Cu)@CuO hollow nanospheres comprised shells of Cu2O
nanoparticle aggregates decorated by a CuO nanoparticulate corona, attributed to Ostwald ripening of Cu2O
nuclei during the reduction/ageing; high reductant and PEG concentrations also resulted in a rattle-like core of
Cu2O and Cu nanoparticles. The rattle-like core-shell Cu2O/Cu@CuO nanospheres exhibited the highest
quantum efficiency and activity for visible light photocatalytic degradation of APAP by hydroxyl radicals,
demonstrating potential use in wastewater treatment.
2. Experimental
2.1.Chemicals
Copper (II) chloride dihydrate (CuCl2.2H2O, Wako 99.9 %), polyethylene glycol (Wako 560-640 M.W.),
sodium hydroxide (NaOH, Wako 98 %), hydrazine monohydrate (H4N2.H2O, Wako 98 %), ethanol (Wako 99.5
%), acetonitrile (Wako 99.8 %), HPLC grade water (Wako), N-acetyl-para-aminophenol (i.e. Paracetamol or
acetaminophen, TCI 98 %).
2.2.Core-shell Cu2O@CuO photocatalyst synthesis
A core-shell Cu2O@CuO photocatalyst was fabricated by a template-free hydrothermal method (Scheme 1). 50
mL of 0. 2 M copper chloride dihydrate and 3 mL of 0.01 M PEG-600 were added to a three neck round-
bottomed flask along with 6 mL of water and stirred at 600 rpm at 60 C for 5 min until a deep blue solution
was obtained. Subsequently, 10 mL of 2 M NaOH and then 1 mL of 2 M hydrazine monohydrate with 10 mL
of water were added dropwise to the preceding solution, and stirred for a further 15 min. The resulting mixture
was transferred to a sealed round-bottomed flask under N2 for 30 min to allow the formation of a yellowish-red
Cu2O@CuO precipitate, which was then filtered and washed three times with water and three times with ethanol
to ensure removal of residual PEG, centrifuged in a TOMY Suprema 21 high speed refrigerated centrifuge for
6 min at 10000 rpm, and then freeze dried in an EYELA Freeze Dryer FDU-2100 for 24 h. The resulting solid
was stored in a desiccator and is termed Cu2O@CuO-B. We believe that PEG complexes the initial Cu (II) ions
and controls the morphology of mixed Cu (I/II) oxide nanostructures formed during subsequent
NaOH/hydrazine induced (partial) reduction and precipitation according to Eqn. 1.
4퐶푢퐶푙2 + 푁2퐻4.퐻2푂 + 8푁푎푂퐻→2퐶푢2푂 + 푁2 + 8푁푎퐶푙 + 7퐻2푂
(1)
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DOI: 10.1039/C9TA07009E
0.2 M CuCl2 (50 mL) + 0.01 M
PEG-600 (3 mL)
2 M NaOH (10 mL) +2 M
N2H4.H2O (1 mL)
N2, 30 min
H2O+ethanol
6000 rpm/5 min
centrifugation
5 min
60 °C
15 min
60 °C
Cu2O@CuO
Scheme 1. Synthesis of core-shell Cu2O@CuO photocatalysts.
Three related materials were also synthesised: Cu2O@CuO-A in which 0.5 mL of 2 M hydrazine monohydrate
volume was used; Cu2O@CuO-C in which 10 mL of 2 M NaOH was added prior to the addition of 0.5 mL of
1 M hydrazine monohydrate; and Cu2O@CuO-D adopting the same protocol as for Cu2O-C but in which only
0.5 mL of PEG was used in the first step of the synthesis.
2.3.Catalyst characterisation
Photophysical properties of the four catalysts were characterized by bulk and surface analytical methods.
Powder X-ray diffraction (PXRD) was performed using a Rigaku Ultima IV diffractometer with Cu Kα radiation
at 40 kV/40 mA with a step size of 0.02°. Band gaps (Eg) were calculated by diffuse reflectance UV-Vis
spectroscopy (DRUVS) on a Shimadzu UV-2450 spectrophotometer equipped with an ISR-2200 integrating
sphere. Solid state photoluminescence spectroscopy (PL) was performed using a JASCO F-6600
spectrofluorometer. Transmission electron microscopy (TEM) imaging was conducted using a JEOL JEM-
2100HCKM instrument, and field emission scanning electron microscopy (FE-SEM) imaging on a ZEISS
ULTRATM 55 (344999-9055-100) instrument. (S)TEM samples were prepared by dispersion in ethanol,
sonication, drop casting on carbon coated copper grids (molybdenum grids were used for EDX elemental
mapping using a JED-2300T detector) and drying. X-ray photoelectron spectra (XPS) were collected on an
ULVAC-PHI ESCA 5800 spectrometer using a monochromated Al Kα X-ray source operated at 200 W. Energy
referencing was performed using the C 1s peak for adventitious carbon set to 284.6 eV, and peak fitting was
undertaken using Casa XPS Version 2.3.16. Specific surface area and pore size analysis was performed using a
BEL-Max N2 porosimeter on degassed samples. Cu K-edge XANES spectra of the as-prepared catalysts and Cu
foil, Cu2O and CuO reference materials were measured on BL15 of the Kyushu Synchrotron Light Research
Center (SAGA-LS; Tosu, Japan) in transmission mode. Samples were prepared by dilution with BN and
mounted with Kapton tape. The photon energy was scanned using a Si double crystal monochromator from 8.9
to 9.2 keV. The storage ring operating current was 171.9 mA at 1.40 GeV. Linear combination fitting (LCF)
analysis was performed using the Athena-Demeter 0.9.26 program in the IFEFFIT software package.
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DOI: 10.1039/C9TA07009E
2.4.N-acetyl-para-aminophenol (APAP) photodegradation
20 mg of photocatalyst was dispersed in 50 mL of 0.066 mM APAP (10 mg.L-1) aqueous solution by sonication
for 5 min in the dark, and then transferred to a 250 mL glass photoreactor covered with a quartz plate and
illuminated from above The reaction mixture was stirred for a further 30 min in dark to equilibrate any molecular
adsorption. An aliquot of the reaction mixture was then collected (t0) prior to visible light irradiation using a
500 W Xe lamp with a 420 nm cut-off filter: solution temperature was maintained at 25 C and stirring at 500
rpm throughout. Light intensity at the catalyst was 1.82 mW/cm2, over an effective irradiation area of 4.5 cm
(distance from light source to the photoreactor centre was 14. 9 cm). Quantum efficiency calculations are
detailed in the Supporting Information. Aliquots were periodically sampled and filtered using a CPO20AN,
prior to HPLC analysis. APAP degradation was monitored using a JASCO LC-netII/ADC HPLC with UV-2075
(243 nm) detector and a Shodex C18M4E analytical column (4.6 mm I.D × 250 mm) held at 25 C, with pressure
minimum and maximum of 0.2 MPa and 20 MPa respectively, and a mobile phase of 30:70 (v/v)
acetonitrile:water with a flow rate of 0.6 mL/min. APAP conversion was calculated according to Eqn. 2.
APAP intial ― APAP final
=
× 100
APAP removal efficiency (%)
(2)
퐴푃퐴푃 푖푛푖푡푖푎푙
where initial and final are the molar concentrations of N-acetyl-para-aminophenol in the t0 sample and at the
end of reaction respectively. Reaction products were identified by HPLC calibration with reference compounds
(benzoquinone, hydroquinone, and maleic, fumaric and oxalic acids) and LC-MS analysis on an Agilent 6545
LC/Q-TOF using a ZORBAX SB-C1” Rapid Resolution HD column (2.1 I.D. x 150 mm x 1.8 m) at 25 C
with 95:5 (v/v) water:methanol (acidified with 0.1% HCOOH) and acetonitrile mobile phases using a gradient
program ramping the acetonitrile co-feed from 5 to 100 % over 12 min.
2.5.Trapping experiments
Scavengers: 1 mmol of p-benzoquinone, isopropyl alcohol, or disodium ethylenediaminetetraacetic acid
(EDTA) were added to a reaction mixture of 20 mg Cu2O@CuO-B in 50 mL of 0.06 mM APAP aqueous
•−
solution, as trapping agents for superoxide radicals (O2 ), hydroxyl radicals (•OH), and holes (hvb+) respectively.
Visible light photocatalytic degradation of APAP was then performed as described above.
Hydroxyl radicals: Hydroxyl-mediated oxidation of terephthalic acid (TA) to 2-hydroxyterephthalic acid (HTA)
was conducted by adding 0.9 mmol TA to an ultrasonicated mixture of 20 mg Cu2O@CuO-B in 50 mL water,
and then irradiated with visible light in the above quartz reactor. Aliquots (1 mL) of the reaction mixture were
periodically sampled and HTA emission spectra measured using a JASCO F-6600 spectrofluorometer under
315 nm excitation. DMSO trapping of reactively-formed formaldehyde was performed by ultrasonicating 20
mg of Cu2O with 50 mL of 250 M DMSO followed by visible light irradiation in the above quartz reactor.
Aliquots (1 mL) of the reaction mixture were periodically sampled and derivatised using 20L H3PO4-NaH2PO4
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buffer and 200L of 240 mol/L DNPH. The resulting DNPH-HCHO complexe was analysed using a JASCO
DOI: 10.1039/C9TA07009E
LC-netII/ADC HPLC with UV-2075 (355 nm) detector and a Shodex C18M4E analytical column (4.6 mm I.D
× 250 mm) held at 32.2 C and 17.4 MPa, with a mobile phase of 60:40 (v/v) methanol:water and flow rate of
0.8 mL/min.
3. Results and discussion
The phase composition and crystallinity of Cu2O@CuO materials was characterized by PXRD (Fig. 1). All
samples were dominated by reflections characteristic of Cu2O (JCPDS 01-078-2076). Cu2O@CuO-B and
Cu2O@CuO-D also exhibited weak reflections for CuO (JCPDS 01-073-6372), while Cu2O@CuO-A and
Cu2O@CuO-B exhibited weak reflections for Cu2(OH)Cl (JCPDS 01-087-0679), and fcc Cu metal (JCPDS 01-
07-3038).21, 37 Cu2O@CuO-C only contained crystalline Cu2O. Volume-averaged crystallite sizes for the Cu2O
phase calculated from the Scherrer equation varied between 12 and 52 nm (Table 1), being largest for
Cu2O@CuO-B (greatest hydrazine concentration) and Cu2O@CuO-D (least PEG), possibly reflecting faster
reduction kinetics (fewer Cu2O nucleation centres) and less disruption of the nucleation network respectively.
Metallic Cu nanoparticles were respectively ~12 and 15 nm diameter for Cu2O@CuO-A and Cu2O@CuO-D.
Cu22OO@-ACuO-A
2O-B
Cu2O@CuO-B
2O-C
Cu2O@CuO-C
2O-D
Cu2O@CuO-D
 Cu2O
 CuO
 Cu2(OH)Cl
Cu
20
30
40
50
60
70
80
2 / 
Fig. 1. Powder XRD patterns of Cu2O@CuO.
The morphology of Cu2O@CuO materials was subsequently explored by FE-SEM (Fig. 2a-d and Fig. S1).
Addition of hydrazine before NaOH or use of 3 mL PEG resulted in 250-500 nm cauliflower aggregates,
Cu2O@CuO-A, Cu2O@CuO-B and Cu2O@CuO-C, whereas delayed addition of hydrazine and the use of a
lower amount of PEG (0.5 mL) favoured larger and more uniform spherical structures with a textured surface,
Cu2O@CuO-D, indicating that the structure-directing agent strongly influenced overall nanostructure
dimensions. The external appearance of these is similar to larger CuO/Cu2O composite hollow microspheres
prepared by high temperature (200 C) hydrothermal ageing of an aqueous Cu(CH3COO)2 solution.29 Nitrogen
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porosimetry resulted in type II adsorption-desorption isotherms with small H3-type hysteresis loops (Fig. S2);
DOI: 10.1039/C9TA07009E
there was no systematic variation in surface areas with synthesis protocol, which spanned 12-25 m2.g-1 (Table
1) comparable to hierarchical Cu2O nanocubes38 and CuO/Cu2O hollow microspheres.29
a
b
c
d
Fig. 2. FE-SEM images of (a) Cu2O@CuO-A, (b) Cu2O@CuO-B, (c) Cu2O@CuO-C, (d) Cu2O@CuO-D.
(S)TEM imaging provided more insight into the nature and internal architecture of the preceding nanostructures
(Fig. 3 and Fig. S3) formed by Ostwald ripening (and surface oxidation) of Cu2O crystallites within the parent
spherical aggregates.39 Cu2O@Cu2O-A, Cu2O@Cu2O-B and Cu2O@Cu2O-C comprised spherical nanoparticles
with differential contrast between the lighter core and denser shell regions (indicative of either a hollow or
separated core, Fig. 3a-d). The cauliflower-like appearance of these three materials arises from small crystallites
(likely Cu2O or CuO, Fig. 1) that irregularly decorate the external shell. Cu2O@Cu2O-D also exhibits a core-
shell structure, albeit with a denser core and lighter shell which we attribute to a dense polycrystalline interior
resulting from the lower concentration of PEG in the synthesis and hence minimal disruption to the initial Cu2O
nuclei aggregation process (i.e. no intraparticle voids). Cu2O@Cu2O-A and Cu2O@Cu2O-C comprised hollow
shells almost identical to those obtained by symmetric hollowing of Cu2O spherical particles in the presence of
polyvinylpyrrolidone (PVP).39 In contrast, Cu2O@Cu2O-B exhibited a rattle-like core-shell structure (Fig. 3b
and Fig. S4a-c) akin to that previously reported during the asymmetric hollowing of Cu2O spherical particles
in the presence of PEG,39 attributed to inhomogeneous packing of primary Cu2O crystallites in the spherical
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aggregate formed on hydrazine reduction; subsequent Ostwald ripening was not initiated inDtOhIe: 1c0.e10n3t9re/C9oTfA0th70e09E
spherical aggregate resulting in a residual Cu2O/Cu core. EDX mapping of Cu2O@Cu2O-B confirms a uniform
distribution of Cu and O between different core-shell particles (Fig. 3e-f). Higher resolution images of
Cu2O@Cu2O-B (Fig. 3g inset) revealed lattice fringes characteristic of CuO [(111) at 2.52 Å] in the corona of
irregular crystallites, Cu2O [(111) at 2.46 Å] in the main shell, and metallic copper [(200) at 1.81 Å) within the
core,9, 26, 40, 41 consistent with the phases observed by XRD. Quantitative EDX microanalysis across an individual
rattle-like core-shell nanoparticles reveals an increase in the Cu:O stoichiometry from 1:1 in the corona to 1.43:1
in the centre of the core, consistent with a transition from a pure CuO exterior to a Cu2O-like interior. Note that
core analysis is complicated by contributions from the encapsulating shell, and hence the measured Cu:O
stoichiometry does not reach 2:1 as anticipated for a ‘naked’ Cu2O nanoparticle. The composition of core and
shell regions was verified by spatially resolved electron energy loss spectroscopy (EELS), with the signatures
for CuO and Cu2O42 identified in the respective corona and shell of Cu2O@Cu2O-B (Fig. 3h-i); note that copper
metal does exhibit a detectable energy loss spectrum.43, 44 Formation of a core-shell Cu2O@CuO structure
indicate that Cu2O nanoparticles formed by hydrazine reduction of Cu(II) ions complexed within PEG micelles
are kinetically-stabilised towards subsequent oxidation by a passivating CuO outer shell45 (itself comprised on
nanoparticle aggregates, possibly formed by an Ostwald ripening process mediated by the PEG23, 39, 46). Similar
size (~400 nm) core-shell structures are reported by electrodeposition of CuSO4 over a H-terminated p-type Si
(100) substrate.27
a
b
g
h
3200
3000
2800
2600
2400
2200
2000
1800
1600
1400
1200
L3
L2
500 nm
200 nm
c
d
900
950
1000
Electron energy loss / eV
7500
7000
6500
6000
5500
5000
i
j
2.52Å
L3
L2
Cu(200)
1.81Å
e
f
2.46Å
900
950
1000
Electron energy loss / eV
Fig. 3. Bright-field low resolution TEM images of (a) Cu2O@CuO-A, (b) Cu2O@CuO-B (inset STEM), (c)
Cu2O@CuO-C, (d) Cu2O@CuO-D, and corresponding (e-f) EDX elemental maps, (g-j) high resolution TEM
images, and (h-i) EELS spectra of Cu2O@CuO-B.
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The electronic and optical absorption properties of Cu2O@CuO samples were determined by DRUVS (Fig. 4).
DOI: 10.1039/C9TA07009E
All samples exhibited broad absorbances around 200-600 nm consistent with previous reports for copper
oxides.9, 24 CuO and Cu2O are indirect and direct band gap semiconductors respectively.47 Since CuO was
present in all Cu2O@CuO samples (see XAS fitting below) and exhibits a narrower energy gap than that of
(direct band gap) Cu2O, indirect band gaps were calculated from Tauc plots (Fig. S5) according to Eqn. (4):
훼ℎ푣 = 퐴(ℎ푣 ― 퐸푔)
(4)
where A is absorption coefficient,  is linear absorption coefficient, h is Planck’s constant, h is the photon
energy, and Eg the band gap determined for  determined using the Kubelka-Munk formalism in Eqn. (5):
(1 ― 푅)2
푎 =
(5)
2푅
Resulting indirect band gaps spanned 2.48 eV (Cu2O@CuO-B) to 2.01eV (Cu2O@CuO-D), similar values to
those for hollow Cu2O nanoshells (formed by PVP-mediated aggregation of nuclei into solid spheres in the
initial reduction step of the synthesis). It is unlikely that this variation is due to quantum confinement effects,
since these typically occur for sub-10 nm for Cu2O nanoparticles,24 i.e. smaller than any crystallites in our
Cu2O@CuO materials (Table 1), and is more likely due to morphology-dependent light absorption and
scattering by Cu2O (and CuO) nanoparticles.
0.7
2O-A
Cu2O@CuO-A
Cu22OO@-BCuO-B
Cu22OO@-CCuO-C
2O-D
Cu2O@CuO-D
0.6
0.5
0.4
0.3
0.2
0.1
0
200 300 400 500 600 700 800
Wavelength / nm
Fig. 4. DRUV spectra of Cu2O@CuO.
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DOI: 10.1039/C9TA07009E
Table 1. Physicochemical properties of Cu2O@CuO.
Catalyst
Cu2O
Surface areab Band gapc VB edge CB edge Mean particle
crystallite sizea / m2.g-1
/ nm
/ eV
potentiald potentiald sizee
/ eV / eV / nm
Cu2O@CuO-A
Cu2O@CuO-B
Cu2O@CuO-C
Cu2O@CuO-D
11.8
37.1
13.1
52.0
25
2.43
1.55 -0.88 240-300
21
18
12
2.48
2.25
2.00
1.57
1.33
1.24
-0.91
-0.92
-0.76
350-500
350-400
2700
aXRD; bN2 porosimetry; cDRUVS; dValence band XPS and DRUVS; eCu 2p XPS. [f] TEM
The surface oxidation state of core-shell Cu2O@CuO materials was also examined by XPS (Fig. 5a). Cu 2p XP
spectra of all samples evidenced multiple chemical environments, with spin-orbit split core-level binding
energies of 931.7±0.5 eV and 951.6±0.2 characteristic of Cu(I)/Cu(0) species, and shakeup satellites around of
940 eV and 960 eV indicative of Cu(II),24 consistent with the observation of Cu2O and CuO features by XRD
(Fig. 1), TEM and EELS (Fig. 3). The Cu(II) satellite was weakest for Cu2O@CuO-C, in accordance with XRD
for which no crystalline CuO phase was detected (although this sample also exhibited the most clearly defined
Cu(II) core-level photoemission ~933.5 eV), however for all materials the intensity of CuO features was greater
than apparent from XRD (a bulk analytical method). This is in accordance with the core-shell structure
visualised by TEM (Fig. 3) wherein a thin corona of CuO nanoparticles protects an underlying Cu2O shell
against oxidation, resulted in the CuO surface enrichment observed by XPS. Surface passivation of Cu2O
nanoparticles is also reported using organic ligands (3-mercaptopropionic acid).40 Oxidation potentials for the
Cu2O@CuO materials were subsequently determined from valence band XPS measurements (Fig. S6). The
valence band maximum (VBM) values were larger than reported for hierarchical nanocubes (~0.6 eV)38 but
smaller than those for single crystal Cu2O films (2.09 eV),48 and in all cases are significantly higher than that
required to generate hydroxyl radicals (0.82 V versus NHE at pH 7). Corresponding conduction band maximum
(CBM) values were approximately -0.9 eV, and hence sufficient to generate reactive oxygen species (-0.16 V
at pH 7).49 Cu K-edge XANES provided additional insight into the local chemical environment of copper species
(Fig. 5b), with all materials exhibiting a rising-edge feature around 8982 eV and an intense white line around
8995 attributed to 1s→4pz (or 4px,ypz for Cu(I) species) and 1s→4p (continuum) transitions respectively. 50, 51
A very inflection was also observed around 8977 eV for Cu2O@CuO-A and Cu2O@CuO-B, attributed to the
dipole-forbidden 1s→3d transition, pre-edge feature characteristic of Cu(II), and indeed the overall spectra for
both resembled that of pure CuO. In contrast, the spectra of Cu2O@CuO-C and Cu2O@CuO-D closely
resembled pure Cu2O, exhibiting a pronounced 8981 eV rising-edge feature. Linear combination fitting to
reference materials confirmed that Cu2O@CuO-A and Cu2O@CuO-B were predominantly comprised of CuO,
while Cu2O@CuO-C and Cu2O@CuO-D were predominantly Cu2O (Table 2). Note that the absence of
detectable CuO for Cu2O@CuO-A by XRD (Fig. 1) suggests that this phase must either be present as very small
(<2 nm) crystallites or as amorphous particles, consistent with their location within the corona as observed by
10

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XPS. Trace copper metal could also be fitted for Cu2O@CuO-A and Cu2O@CuO-B, in agreDemOI:e1n0t.10w39it/hC9XTAR07D009E
and TEM for the latter material (Fig. 3 g and j).
CuO
Cu2OO@@CCuu2OO-A-A
O@Cu2O-B
Cu2O@CuO-B
O@Cu2O-C
Cu2O@CuO-C
O@Cu2O-D
Cu2O@CuO-D
2O
Cu2O
Cu2O@CuO-A
Cu2O@CuO-B
a
b
Cu+/Cu0
Cu2+ satellite
Cu metal
Cu2O@CuO-C
Cu2O@CuO-D
964
954
944
934
924
8900
8950
9000
9050
9100
Binding energy / eV
Energy / eV
Fig. 5. (a) Cu 2p XP and (b) Cu K-edge XANES spectra of Cu2O@CuO and reference materials.
Table 2. Linear combination fitting of Cu2O@CuO XANES.
Catalyst
Cu2O / % CuO /% Cu / % Cu(I):Cu(II) R-factor
Cu2O@CuO-A 13.8
Cu2O@CuO-B 22.1
84.7
75.8
1.5
2.1
0.16
0.29
0.0030
0.0042
Cu2O@CuO-C 84. 6
Cu2O@CuO-D 81.3
15.4
18.7
-
5.49
4.34
0.0002
0.0008
Photocatalytic APAP oxidation
Photocatalytic degradation of APAP under visible light irradiation was subsequently conducted using the
preceding Cu2O@CuO materials. APAP does not absorb visible light, and hence cannot act as a photosensitiser
which is problematic in mechanistic investigations of e.g. photocatalytic dye degradation30. Negligible
adsorption or reaction occurred over any Cu2O@CuO material in the dark (<3.6 % APAP loss). Cu2O@CuO-B
was the most active towards APAP removal, achieving 48 % removal in 90 min, equating to an apparent
quantum efficiency (AQE) of 8 % (Fig. 6a-b). Initial rates for APAP decomposition under visible light were
2.5, 7.0, 5.0 and 3.5×10-3 mmol.g-1.min-1 for Cu2O@CuO-A, Cu2O@CuO-B, Cu2O@CuO-C and Cu2O@CuO-
D respectively, (Fig. 6c) outperforming literature systems (Table S1), and quantitatively mirroring the trend in
AQE values. The latter correlation suggests that the higher activity of Cu2O@CuO-B must be associated with
11

Journal of Materials Chemistry A
Page 12 of 19
either increased visible light absorption or improved charge carrier transport and/or lifetimes. RegaVridewinAgrticltehOenline
DOI: 10.1039/C9TA07009E
former, DRUVS measurements reveal Cu2O@CuO-B possesses the widest band gap and hence will only be
photoexcited by the smallest proportion of the incident spectrum, however light absorption and scattering is
strongly dependent on particle morphology and shell thickness,39, 52 with rattle-like core-shell mesoporous TiO2
spheres outperforming hollow analogues for phenol degradation, attributed to increased absorption through
internal light scattering.
9
8
7
6
5
4
3
2
1
0
60
50
40
30
20
10
0
a
b
0.008
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0
3.5
3
c
d
2.5
2
1.5
1
0.5
0
Fig. 6. (a) Removal efficiency, (b) apparent quantum efficiency, (c) specific activity, and (d) surface area
normalised rate of APAP photocatalytic degradation over Cu2O@CuO under visible light. Reaction
conditions: 0.06 mmol APAP; 50 mL water; 20 mg catalyst; 300 min for (a-b) and 90 min for (c-d).
12

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To investigate whether charge carrier lifetimes also differ between the Cu2O@CuO photocatalysts, their steady
DOI: 10.1039/C9TA07009E
state photoluminescence was compared under 380 nm and 560 nm excitation (Fig. 7a-b). Under both excitation
sources Cu2O@CuO-B exhibited the weakest emission at 420, 572 nm and 616 nm, consistent with slower
radiative recombination of photoexcited electron-hole pairs, and hence longer charge carrier lifetimes.38, 53, 54
The higher activity of Cu2O@CuO-B for photocatalytic APAP degradation is thus likely associated with both
increased light absorption due to strong scattering between the shell and rattle-like core, and suppressed charge
recombination (due to transfer between Cu2O and CuO/Cu).
a
b
2O-A
Cu2O@CuO-A
2O-A
Cu2O@CuO-A
2O -B
Cu2O@CuO-B
2O-B
Cu2O@CuO-B
2O-C
Cu2O@CuO-C
2O-C
Cu2O@CuO-C
U2O-D
Cu2O@CuO-D
Cu22OO@-DCuO-D
585
615
645
675
705
735
400
450
500
550
600
650
Wavelength / nm
Wavelength / nm
Fig. 7. Photoluminescence spectra of Cu2O@CuO under (a) 380 nm excitation and (b) 560 nm excitation and
emission. Excitation bandwidth 3 nm; emission bandwidth 10 nm, response 0.5 s, 900 V, 200 nm.min-1 scan.
By-products of APAP degradation over Cu2O@CuO-B were determined by HPLC (Fig. S7), revealing the
formation of aromatic hydroquinone and benzoquinone, and ring-opened fumaric, maleic, and oxalic acids
(Scheme 2): preliminary LC-MS analysis confirmed hydroquinone as a reactive intermediate. Hydroxyl radicals
(•OH) are proposed the active species responsible for photocatalytic APAP degradation over P25 TiO2, with
initial hydroxylation of the aromatic at the para (and ortho and meta) position with respect to the -OH to form
hydroxyquinone, and subsequent •OH oxidation forming p-benzoquinone (BQ) which undergoes ring opening
and further oxidation to produce maleic and other carboxylic acids.55, 56
O
3
4
HO
1
OH
OH•
2
O
3
6
O
fumaric acid
OH•
4
OH
OH•
O
2
1
OH
5
2
O
OH•
HO
1
1
OH
4
5
2
1
O
N
O
HO
2
OH•
6
3
H
oxalic acid
HO
O
benzoquinone
hydroquinone
N-acetyl-para-aminophenol
O
1
2
4
OH
3
maleic acid
Scheme 2. Proposed reaction pathways in APAP photocatalytic degradation over Cu2O@CuO-B.
13

Journal of Materials Chemistry A
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The nature of the active species responsible for APAP photocatalytic degradation over Cu2O@CuVOiew-BArtiwcleaOsnline
DOI: 10.1039/C9TA07009E
explored through additional experiments in the presence of either p-benzoquinone (BQ), isopropyl alcohol
•−
(IPA), or disodium ethylenediaminetetraacetic acid (EDTA), as trapping agents for superoxide radicals (O2 ),
hydroxyl radicals (•OH), and holes (hvb+) respectively (Fig. 8a). IPA addition had the greatest impact on the rate
of APAP removal, causing it to decrease from 7×10-3 mmol.g-1.min-1 to only 1.5×10-3 mmol.g-1.min-1 mmol.g-
1.min-1, a significantly lower rate than observed on the addition of either BQ or EDTA. This strongly suggests
•
that OH species are responsible for APAP oxidation, as reported for titania photocatalysts.34 Additional
evidence for hydroxyl radical formation by Cu2O@CuO-B was obtained undertaken through fluorescence
spectroscopy. Briefly, an aqueous solution of terephthalic acid (TA) was irradiated in the presence of
Cu2O@CuO-B and the subsequent emission spectra monitored as a function of irradiation time: in the presence
of •OH radicals TA undergoes oxidation to 2-hydroxyterephthalic acid (HTA), which is characterised by a 425
nm emission (Fig. 8b). Continuous HTA formation was observed over the course of 300 min (the duration of
•
APAP photocatalytic degradation experiments) confirming that Cu2O@CuO-B is very active for OH radical
production under visible irradiation. As final verification of the efficacy of Cu2O@CuO-B for •OH formation
under visible light, formaldehyde (HCHO) production by DMSO trapping of hydroxyl radicals was
demonstrated; a standard analytical method in advanced oxidation processes.57 The resulting formaldehyde was
derivatised with dinitrophenylhydrazine (DNPH), and evolution of the DNPH-HCHO complex followed by
HPLC (Fig. S8).
0.008
O
O
HO
HO
a
b
1
4
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0
3
2
Cu2O@ CuOHO
2
6
5
5
6
3
4
1
OH
O
OH
Terephthalic acid
O
2-Hydroxy terephthalic acid
0 min
30 min
60 min
90 min
120 min
180 min
240 min
300 min
337
387
437
487
537
587
None
IPA
BQ
EDTA
Wavelength / nm
Fig. 8. (a) Influence of trapping agent addition on the rate of APAP removal over Cu2O@CuO-B under visible
light: 0.06 mmol APAP; 1 mmol scavenger; 50 mL water; 20 mg catalyst. (b) HTA emission spectra as a
function of reaction time over Cu2O@CuO-B under visible light: 0.9 mmol TA; 50 mL water; 20 mg catalyst;
315 nm excitation; excitation bandwidth 3 nm; emission bandwidth 10 nm, response 0.5 s, 900 V, 200
nm.min-1 scan.
14

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Photocatalyst stability is critical for practical application, and hence was also investigated for the most active
DOI: 10.1039/C9TA07009E
Cu2O@CuO-B material. Negligible deactivation was observed over four consecutive cycles of APAP
photodegradation under visible light (Fig. S9), consistent with the excellent physicochemical stability observed
by SEM, XRD and XPS (Fig. 9); particle morphology, phase/crystallinity and oxidation state were unchanged
even after 20 h continuous APAP photocatalytic degradation.
Fresh
Post-reaction
Fresh
Spent
e
Post-reaction
a
b
20
30
40
50
60
70
80
2 / 
c
d
Fresh
Post-reaction
f
962
952
942
932
922
Binding energy /eV
Fig. 9. (a-d) High and low magnification SEM, (e) XRD and (f) XPS of Cu2O@CuO-B before and after 20 h
APAP photocatalytic degradation.
3.1.Photocatalytic APAP removal Mechanism
Visible light photocatalytic APAP degradation over core-shell Cu2O@CuO nanospheres is proposed to occur
by e- photoexcitation into the conduction band of the Cu2O shell, and subsequent transfer across the
25, 47
heterojunction interface into the conduction band of the CuO corona.24,
These photoexcited e- are
sufficiently energetic (-0.8 to -0.9 eV) to reduce dissolved oxygen (O2) and produce superoxide radical anions
•
(O2•−) which in turn can react with water to form OH radicals able to oxidise APAP (Scheme S1).
Simultaneously, photoexcited h- formed in the valence band of the CuO corona may inject across the
heterojunction into the valence band of the Cu2O shell, and subsequently reacted with hydroxyl ions to form
•
additional OH radicals. Charge separation across the Cu2O/CuO interface suppresses bulk recombination of
photogenerated e--h+ pairs, increasing charge carrier lifetimes and hence the probability of migration to the
surfaces of both semiconductor nanoparticles of the nanosphere aggregates. The high activity and quantum
efficiency of the Cu2O@CuO-B photocatalyst is likely associated with a combination of factors. This material
•
possesses the greatest oxidation and reduction potentials for OH radical formation and exhibits the weakest
photoluminescence. The latter is consistent with a high degree of photoexcited e--h+ separation and may reflect
15

Journal of Materials Chemistry A
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electron trapping by Cu metal nanoparticles in contact with the Cu2O shell; Chen et al reported that Cu played
DOI: 10.1039/C9TA07009E
an important role in trapping electrons photoexcited into the Cu2O conduction band, thereby promoting
photocatalytic degradation of methylene blue.58 Ongoing investigations aim to elucidate the relative importance
of core-shell nanosphere morphology and composition to further enhance visible light absorption, charge
separation and mobility, and ultimately photocatalytic efficiency for organic micropollutant degradation and
solar fuels production.
4. Conclusions
A family of core-shell Cu2O(Cu)@CuO visible light photocatalysts were synthesised by a template-free, mild
hydrothermal route. The resulting nanosphere aggregates, formed by Ostwald ripening of Cu2O nuclei, exhibited
tunable dimensions spanning 200 nm to 2 m, and well-defined shells of 10-50 nm Cu2O nanoparticles
decorated by a (protective) corona of CuO nanoparticles. Photophysical properties, notably band gaps,
conduction and valence band edges, and photoluminescence, varied with nanosphere size and composition.
High concentrations of hydrazine reductant and PEG structure-directing agent promoted asymmetric Ostwald
ripening resulting in rattle-like Cu2O/Cu metal cores, whereas lower concentrations resulted in hollow cores.
All photocatalysts were active for the aqueous phase degradation of N-acetyl-para-aminophenol (APAP), even
at concentrations of 10 mg.L-1 far exceeding those encountered in wastewater streams. Trapping studies
•
employing a variety of molecular scavengers provide strong evidence that OH radicals are responsible for
APAP oxidation, consistent with the quinone and carboxylic acid by-product formation. Rattle-like core-shell
Cu2O/Cu@CuO exhibited the highest apparent quantum efficiency and activity for visible light photocatalytic
degradation of APAP, and showed excellent structural and chemical stability over 20 h testing. These results
highlight the potential of composite semiconductor photocatalysts using Earth abundant elements for
environmental remediation and solar (photo)electrocatalytic energy conversion.
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgments
S.K. and K.S. would like to thank the Japan Society for the Promotion of Science (JSPS) for providing
postdoctoral fellowship for foreign researchers (P18387) and the research grand (KAKENHI JP18F18387). We
thank Prof. Kuniyoshi Shimizu and Mr. Dedi Satria at Graduate School of Agriculture, Kyushu University for
supporting LC-MS analysis. XANES measurements were performed at Kyushu University Beamline (SAGA-
LS /BL06 with proposal No. 2019IK004). FE-SEM and HR(S)TEM performed at Ultra Microscopic Analysis
Center at Kyushu University and X-ray photoelectron spectra were collected at Advanced Analytical Centre
(Nanotechnology Platform), Kyushu University under the proposal No. S-18-KU-0043.
16

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DOI: 10.1039/C9TA07009E
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Journal of Materials Chemistry A
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DOI: 10.1039/C9TA07009E