On the mechanism of photocatalytic reactions on CuxO@TiO2core-shell photocatalysts

Article abstract of DOI:10.1039/d0ta12472a

Titania (titanium(iv) oxide) is a highly active, stable, cheap and abundant photocatalyst, and is thus commonly applied in various environmental applications. However, two main shortcomings of titania,i.e., charge carrier recombination and inactivity under visible-light (vis) irradiation, should be overcome for widespread commercialization. Accordingly, titania has been doped, surface modified and coupled with various ions/compounds, including narrower bandgap semiconductors, such as oxides of copper and silver. Unfortunately, these oxides are not as stable as titania, and thus loss of activity under long-term irradiation (photo-corrosion) has been observed. Therefore, this study has focused on the preparation of stable coupled photocatalysts,i.e., Cu_xO@TiO₂core-shell nanostructures, by the microemulsion method from commercial Cu₂O as a core and TiO₂(ST01-fine anatase) as a shell. The photocatalysts have been characterized by DRS, SEM, TEM, XRD, XPS and reversed double-beam photo-acoustic spectroscopy (RDB-PAS) methods, and activity tests under UV (anaerobic dehydrogenation of methanol and oxidative decomposition of acetic acid) and vis (phenol oxidation) irradiation. The higher activities of coupled photocatalysts than their counterparts have been found in all studied systems under UV/vis irradiation. Moreover, long-term experiments (10 h) have shown high stability of Cu_xO@TiO₂. However, the change of oxidation state of copper has also been observed,i.e., to negative and positive values, confirming the charge transfer according to the Z-scheme under UV irradiation and type-II heterojunction under vis irradiation, respectively. The property-governed activity and the mechanism have been discussed in detail.

Full text of DOI:10.1039/d0ta12472a

Showcasing research from Institute for Catalysis (ICAT),
Hokkaido University in collaboration with groups from
Poland: Poznan University of Technology, Gdansk
University of Technology, University of Warsaw and West
Pomeranian University of Technology, Szczecin.
As featured in:
On the mechanism of photocatalytic reactions on
CuxO@TiO2 core–shell photocatalysts
CuxO@TiO2(anatase) photocatalysts, prepared by a
microemulsion method, exhibit high activity and stability
under long-term UV irradiation. Based on XPS analysis, it
is proposed that improved photocatalytic performance
under UV irradiation originates from the Z-scheme
mechanism, whereas inactivity under vis irradiation
(type-II heterojunction) results from the charge-carrier
recombination.
See Marcin Janczarek,
Ewa Kowalska et al.,
J. Mater. Chem. A, 2021, 9, 10135.
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On the mechanism of photocatalytic reactions on
CuxO@TiO2 core–shell photocatalysts†
Cite this: J. Mater. Chem. A, 2021, 9,
10135
ac
a
d
Kunlei Wang,ab Zuzanna Bielan,
Maya Endo-Kimura, Marcin Janczarek,
*
Dong Zhang,ae Damian Kowalski,f Anna Zielinska-Jurek,
Agata Markowska-
c
´
g
ae
ae
*
Szczupak,
Bunsho Ohtani
and Ewa Kowalska
Titania (titanium(IV) oxide) is a highly active, stable, cheap and abundant photocatalyst, and is thus
commonly applied in various environmental applications. However, two main shortcomings of titania,
i.e., charge carrier recombination and inactivity under visible-light (vis) irradiation, should be overcome
for widespread commercialization. Accordingly, titania has been doped, surface modified and coupled
with various ions/compounds, including narrower bandgap semiconductors, such as oxides of copper
and silver. Unfortunately, these oxides are not as stable as titania, and thus loss of activity under long-
term irradiation (photo-corrosion) has been observed. Therefore, this study has focused on the
preparation of stable coupled photocatalysts, i.e., CuxO@TiO2 core–shell nanostructures, by the
microemulsion method from commercial Cu2O as a core and TiO2 (ST01-fine anatase) as a shell. The
photocatalysts have been characterized by DRS, SEM, TEM, XRD, XPS and reversed double-beam photo-
acoustic spectroscopy (RDB-PAS) methods, and activity tests under UV (anaerobic dehydrogenation of
methanol and oxidative decomposition of acetic acid) and vis (phenol oxidation) irradiation. The higher
activities of coupled photocatalysts than their counterparts have been found in all studied systems under
UV/vis irradiation. Moreover, long-term experiments (10 h) have shown high stability of CuxO@TiO2.
However, the change of oxidation state of copper has also been observed, i.e., to negative and positive
values, confirming the charge transfer according to the Z-scheme under UV irradiation and type-II
heterojunction under vis irradiation, respectively. The property-governed activity and the mechanism
have been discussed in detail.
Received 25th December 2020
Accepted 28th February 2021
DOI: 10.1039/d0ta12472a
rsc.li/materials-a
in water treatment, air purication, synthesis of chemical
compounds, self-cleaning surfaces and renewable energy
1. Introduction
processes, such as photocurrent generation and water splitting
for hydrogen production.1–6 Although titania has been recog-
nized as one of the best photocatalysts, two main shortcomings
must be overcome for broader and commercial application, i.e.,
charge carrier recombination (typical for all semiconductors),
and inactivity under vis irradiation (due to the wide bandgap of
titania, i.e., ca. 3.0–3.2 eV).
The development of photocatalytic processes is still mainly
connected with materials based on titanium(IV) oxide (TiO2,
titania), which has been widely known as an efficient, stable and
green semiconductor with photocatalytic performance and used
aInstitute for Catalysis (ICAT), Hokkaido University, N21 W10, 001-0021 Sapporo,
Japan. E-mail: kowalska@cat.hokudai.ac.jp
To overcome these limitations, different strategies for the
preparation of titania-based materials have been considered,
e.g., (i) doping with cations and anions, (ii) surface modication
with various molecules and particles, (iii) formation of
advanced-morphology materials (faceted, mesocrystals, and
inverse opals), and (iv) coupling with other semiconductors.7
Accordingly, designing coupled photocatalysts, composed of
TiO2 and other metal oxides, to establish the heterojunction is
an important way to enhance the charge carrier (electron–hole)
separation (increasing the lifetime of charge carriers) and to
obtain vis-responsive materials (in the case of oxides with
a narrower bandgap than that of titania).8 Copper oxides (p-type
semiconductors) have been considered as good candidates to
bNorthwest Research Institute, Co. Ltd. of C.R.E.C., 730000, Lanzhou, P. R. China
cDepartment of Process Engineering and Chemical Technology, Faculty of Chemistry,
Gdansk University of Technology (GUT), G. Narutowicza 11/12, 80-233 Gdansk,
Poland
dInstitute of Chemical Technology and Engineering, Faculty of Chemical Technology,
Poznan University of Technology, 60-965 Poznan, Poland. E-mail: marcin.
janczarek@put.poznan.pl
eGraduate School of Environmental Science, Hokkaido University, 060-0810 Sapporo,
Japan
fFaculty of Chemistry, Biological and Chemical Research Centre, University of Warsaw,
Zwirki i Wigury 101, 02-089 Warsaw, Poland
gDepartment of Chemical and Process Engineering, West Pomeranian University of
Technology in Szczecin, Al. Piastow 42, 71-065 Szczecin, Poland
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ta12472a
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form an efficient heterojunction with TiO2, i.e., Cu2O and CuO ¼ 52 m2 gꢀ1 and crystal sizes equal to ca. 25 nm and 40 nm for
with bandgap energies of ca. 2.1 eV and 1.7 eV, respectively. In anatase and rutile, respectively,27 Nippon Aerosil, Tokyo, Japan)
particular, among copper oxides, the selection of Cu2O provides and ST01 (ST-01, ne anatase, SSA ¼ 298 m2 gꢀ1 and crystal size
a promising strategy to prepare hybrid photocatalysts with high of 8 nm, Ishihara Sangyo, Osaka, Japan), have been used in this
activity and good antipathogenic properties.9,10 Moreover, the study, i.e., P25 as a reference titania sample (known as one of
Cu2O/TiO2 p–n heterojunction has been described as an effi- the most active titania photocatalysts) and ST01 as a shell layer.
cient photocatalyst for degradation of organic compounds and Cuprous oxide, used as a core, was supplied by Wako Pure
hydrogen generation.8,11–14 It has been reported that even simple Chemicals, Tokyo, Japan. Other chemicals, including cyclo-
physical mixing of copper(I) oxide and titania (grinding) results hexane, isopropanol, acetone, hydrochloric acid, ammonia
in the formation of an efficient photocatalyst.13 The high effi- solution, methanol, acetic acid, cetyltrimethylammonium
ciency of such material has been observed for UV/Vis-induced bromide (CTAB, 98%) and dioctyl sulfosuccinate sodium salt
acetic acid oxidation, and two mechanistic variants of this (AOT), were provided by Wako Pure Chemicals. All reagents
heterojunction depending on the form of TiO2 have been sug- were of analytical grade, and were thus used without further
gested: Z-scheme and heterojunction-type II for anatase and purication.
rutile-based samples, respectively. Furthermore, Cu2O/TiO2
photocatalysts have exhibited high bactericidal and fungicidal
Preparation of the Cu2O@TiO2 core–shell heterojunction
properties.13 However, loss in the activity during long-term
The water-in-oil (w/o) microemulsion method was applied for
irradiation has been observed because of the low stability of
the preparation of CuxO@TiO2 core–shell photocatalysts. In this
Cu2O (photocorrosion), i.e., dissolution of copper from the
photocatalyst surface (e.g., Cu+ / Cu2+).15,16 Therefore, pre-
system, the nanodroplets of the aqueous phase have been
dispersed in a continuous oil phase, stabilized by the surfactant
venting copper loss during photocatalytic reactions is the most
and co-surfactant at the w/o interface. The Cu2O core was
successfully covered with the titania shell layer using the
changes in zeta potential of Cu2O and TiO2 as a function of pH
important goal for efficient and stable Cu2O/TiO2 photo-
catalysts. For example, Wang et al. reported that a titania
protective layer on Cu2O signicantly increased the stability and
activity of the Cu2O/CuO photocatalyst.17
´
value (Fig. 1), rstly proposed by Zielinska-Jurek et al. for Fe3-
O4@SiO2@TiO2 (core/interlayer/shell) particles.26 The Cux-
O@TiO2 photocatalysts were obtained via two variants of the
microemulsion method, i.e., under acidic (pH < 5) and basic
(pH > 11) conditions. In the acidic variant, cuprous oxide was
dispersed in an anionic AOT water solution and the pH was set
to a value of 5 by adding of 0.1 M HCl solution. The molar ratio
between water and surfactant was set at 30. The prepared
solution was then dropwise added to a cyclohexane/isopropanol
(100 : 6 volume ratio) oil phase and stirred for 2 h to stabilize
the microemulsion. Aer stabilization, the corresponding
amount of TiO2 was added to the system and stirred for 18 h.
Aerwards, the microemulsion was destabilized with acetone,
washed with deionized water, dried at 80 ꢁC for 24 h and ground
in an agate mortar. A similar procedure was performed in the
basic variant, but with cationic CTAB as the surfactant and at
a pH value of 11, set by adding ammonia solution. The junction
The design of the coupled photocatalysts is also connected with
the character of interactions between the two main components
(metal oxides) forming the heterojunction. It has been proposed
that the formation of core–shell structures is a promising strategy
since such materials show the integration of individual compo-
nents into a functional system that brings about an improvement
in the physical and chemical properties (e.g., stability, dis-
persibility, and multi-functionality), which could not be achieved
for the components without good and/or stable contact.18,19
Particularly, core@shell TiO2-structured materials have been rec-
ommended not only due to their enhanced activity and stability,
but also improved separability, e.g., when magnetic oxides are
used as a core.20,21 Considering Cu2O, the main problem arises
from its poor stability (photocorrosion) in an aqueous environ-
ment. Therefore, the preparation of Cu2O(core)@TiO2(shell)
systems might allow this phenomenon to be avoided, and conse-
quently improve the overall photocatalytic performance. Su et al.
obtained stable core–shell Cu2O/TiO2 nanocomposites, in which
the TiO2 layer uniformly covers each Cu2O octahedral particle.22 In
other studies, Cu2O photocathodes, protected by a thin coating of
Al-doped ZnO/TiO2 or TiO2 layer, exhibit high activity for photo-
electrochemical water reduction, photoelectrocatalysis and CO2
reduction.23–25 In the present study, the microemulsion method,
successfully applied to synthesize core–shell structured titania-
based materials (e.g., magnetic photocatalysts),26 has been
applied for the rst time to prepare core–shell CuxO@TiO2
photocatalysts.
2. Materials and methods
Two commercially available TiO2 photocatalysts, P25 (AERO-
Fig. 1 Zeta potential of Cu2O and TiO2 (ST01) suspensions as a func-
XIDE® TiO2 P25, mixed-phase titania, specic surface area (SSA) tion of pH value.
10136 | J. Mater. Chem. A, 2021, 9, 10135–10145
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between the Cu2O core and TiO2 shell was promoted by their analyzed by reversed double-beam photoacoustic spectroscopy
opposite surface charges, provided by the presence of anionic (RDB-PAS) and photoacoustic spectroscopy (PAS), respectively,
and cationic surfactants under acidic and basic conditions, as described elsewhere.30 In brief, for RDB-PAS measurement,
respectively. The Cu2O content was set to 1, 2, 5, 10 and 50 wt% the sample was lled in a stainless-steel sample holder in
of the composite.
a home-made PAS cell equipped with an electret condenser
The radius of the water in oil droplets (Rw) has been calcu- microphone and a Pyrex window on the upper side. Methanol-
lated, based on eqn (1),28 and found to reach ca. 2.3 mm. saturated nitrogen was owed through the cell for 30 min, the
Therefore, it is assumed that efficient coverage of the Cu2O core cell was irradiated by a 625 nm light-emitting diode beam
by the TiO2 shell inside the micelle is possible considering the (Luxeon LXHL-ND98, LUMILEDS, San Jose, CA, USA), modu-
sizes of their crystallites, i.e., ca. 88 nm for Cu2O and 8 nm for lated at 35 Hz by a function generator (DF1906, NF Corporation,
TiO2 (and even aggregated particles, i.e., 100–1000 nm and 20– Yokohama, Japan) as modulated light, and a monochromatic
60 nm, respectively, as shown in Fig. S1–S4 in the ESI†).
light beam from a Xe lamp (ASB-XE-175, Spectral Products,
Putnam, CT, USA) equipped with a grating monochromator
(CM110 1/8 m, Spectral Products, CT, USA) as continuous light.
The continuous light was scanned from 650 to 300 nm with
a 5 nm step. The RDB-PA signal was detected using a digital
lock-in amplier (LI5630, NF Corporation). The obtained spec-
trum was differentiated from the lower-energy side and cali-
brated with the reported total electron-trap density in units of
mmol gꢀ1, measured using a photochemical method31 to obtain
an ERDT pattern. For PAS measurements, the cell window was
irradiated from 650 to 300 nm by a light beam from a Xe lamp
(ASB-XE-175, Spectral Products) with a grating monochromator
(CM110 1/8m, Spectral Products, CT, USA) modulated at 80 Hz
3Vaq ½H2Oꢂ
Rw
¼
½nmꢂ
(1)
s
½Sꢂ
where Vaq is the volume of water molecules (water volume)
[dm3], s is the area per polar head [nm2], and [H2O]/[S] is the
water/surfactant ratio. For CTAB the s value is 4.23 ꢃ 10ꢀ4 nm2,
according to Haq et al.,29 and Vaq and [H2O]/[S] have been set to
0.0108 dm3 and 30, respectively, resulting in Rw of 2.29 ꢃ
103 nm.
Characterization of photocatalysts
The morphology of samples and distribution of elements were by a light chopper (5584A, NF Corporation, Japan) to detect the
investigated using a scanning transmission electron micro- PAS signal using a digital lock-in amplier, and then photo-
scope equipped with an energy-dispersive X-ray spectrometer acoustic (PA) spectra were calibrated using the PA spectrum of
(STEM-EDS, HD-2000, HITACHI, Tokyo, Japan). The specic graphite as a reference. The CBB, as energy from the top of the
surface area of the samples was estimated by nitrogen adsorp- valence band (VBT), of the samples was calculated from the
tion at 77 K using the Brunauer–Emmett–Teller (BET) equation. onset wavelength corresponding to the bandgap of the samples.
The photoabsorption properties of the prepared photocatalysts
were analyzed by diffuse reectance spectroscopy (DRS; JASCO
V-670 equipped with a PIN-757 integrating sphere, JASCO, LTD.,
Photocatalytic activity tests
Pfungstadt, Germany). Barium sulfate and bare titania (ST01) The photocatalytic activity of the samples was evaluated under
were used as references for DRS analysis. Crystalline properties UV/vis irradiation for (1) oxidative decomposition of acetic acid
were analyzed by X-ray powder diffraction (XRD; Rigaku Intel- (CO2 system; CO2 evolution), and (2) anaerobic dehydrogenation
ligent XRD SmartLab with a Cu target, Rigaku, LTD., Tokyo, of methanol (H2 system; H2 evolution), and under vis irradiation
Japan). Crystallite sizes of anatase, rutile and copper were esti- for (3) oxidation of phenol. For activity tests, (1 and 2) 50 mg and
mated using the Scherrer equation. The chemical composition (3) 10 mg of photocatalyst was suspended in 5 mL of aqueous
of the surface (content and chemical state of elements, i.e., solution of (1) acetic acid (5 vol%), (2) methanol (50 vol%), and (3)
titanium, oxygen and copper) was determined by X-ray photo- 0.21 mmol Lꢀ1 phenol in 35 mL Pyrex test tubes. The tubes were
electron spectroscopy (XPS; JEOL JPC-9010MC with MgKa X- sealed with rubber septa, and the suspensions were continuously
rays, JEOL, LTD., Tokyo, Japan). The dependence of the stirred in a thermostated water bath and irradiated by (1 and 2)
surface charge of bare semiconductors on the pH value was Hg lamp (l > 290 nm) and (3) Xe lamp (l > 420 nm; water IR lter,
measured as zeta potential. 0.1 g of semiconductor (Cu2O or cold mirror and cut-off lter Y-45, in a reactor shown in ref. 32).
TiO2) was dispersed in 100 mL of distilled water and continu- For methanol dehydrogenation, suspensions containing titania
ously stirred using a magnetic stirrer. The pH value was set by and methanol were pre-bubbled with Ar for 15 min to remove
adding 0.1 M HCl and 0.1 M NaOH solutions, and the changes oxygen from the system. The amounts of (1) generated hydrogen,
were monitored using an Elmetron multifunction meter, CX- (2) liberated carbon dioxide, and (3) phenol and benzoquinone
505, equipped with an EPS-1 14600/14 pH electrode (Elme- (main degradation product) were determined by chromatog-
tron, Zabrze, Poland). Aer the pH value was set, the sample was raphy: (1 and 2) GC-TCD (detection of the gas phase amount every
taken with a syringe and placed in a disposable folded capillary (1) 20 min and (2) 15 min) and (3) HPLC.
cell (DTS1070, Malvern Instruments Ltd., Malvern, UK). The
Commercial titania P25 (Degussa/Evonik) was used as
measurement of zeta potential was performed using a Malvern a reference sample since P25 has one of the highest photo-
Nano Zetasizer (Malvern Instruments Ltd., Malvern, UK).
catalytic activities among various titania samples in different
The energy-resolved distribution of electron traps (ERDT) reaction systems (oxidation and reduction27,33,34), and is thus
patterns and conduction band bottom (CBB) position were commonly used as a “standard” titania sample.
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Cu2O (Fig. 2c), as conrmed by the absorption spectra (Fig. 2a
and b). However, the spectra in the vis range do not resemble
the shape of Cu2O absorption (intense absorption in the range
of 400–650 nm), except for that of the 50% CuxO@TiO2 sample
(due to the insufficient content of TiO2 to fully cover the Cu2O
core, as shown in Fig. 3a–c and S3†). Interestingly, samples
prepared by grinding (the same Cu2O and TiO2; previous study9)
have shown a different color (pink) and DRS spectra resembling
Cu2O absorption in the vis range (a clear peak with a maximum
at ca. 550 nm). Similarly, other reports on Cu2O-modied pho-
tocatalysts have shown photoabsorption spectra correlating
with Cu2O absorption in the vis range, e.g., for Cu2O/TiO2
(ref. 16 and 35–38) and Cu2O/Si39 samples. Therefore, it is ex-
pected that mainly different forms of copper are responsible for
vis absorption, i.e., (i) in the range of 400–500 nm: mixed oxides
of copper, i.e., Cu+/Cu40 and CuxO/TiO2 (x: 1–2, due to interfacial
charge transfer (IFCT) from the valence band of TiO2 to the
CuxO),41 and (ii) at wavelengths longer than 500 nm: CuO,42,43
Cu(OH)2 (blue samples),44 and d–d transitions of Cu2+ species.45
The most similar spectra (to data shown in Fig. 2a) have been
obtained by Park et al. for CuxO/TiO2, prepared by annealing
a Cu@Cu2O nanocomposite mixed with a titania precursor
(TiCl4).46 It has been suggested that the formation of a hetero-
junction between CuxO and TiO2 is responsible for vis absorp-
tion in the range of 400–600 nm, while absorption at 600–
3. Results and discussion
Preparation of Cu2O@TiO2 core–shell particles
The zeta potential values of the suspensions of bare Cu2O and
TiO2 samples as a function of pH value are shown in Fig. 1. The
analysis is crucial for the proper selection of the microemulsion
environment, particularly pH value and surfactant kind, in
order to ensure efficient coverage of Cu2O by TiO2. It is known
that titania nanoparticles (NPs) are positively charged at pH
values below 5. Indeed, in the case of the ST01 titania suspen-
sion, the isoelectric point (IEP; zeta potential equal to zero)
appears at a pH value of ca. 6.3. The IEP of cuprous oxide occurs
almost in the whole range of pH values between 8 and 9. Above
and below these values, NPs are negatively and positively
charged, respectively.
In the present study, CuxO@TiO2 photocatalysts have been
obtained via two variants of the microemulsion method, i.e.,
under acidic (pH < 5) and basic (pH > 11) conditions. For the
successful covering of Cu2O by the TiO2 shell, it is necessary to
introduce charge-changing surfactants since the substrates
possess the same surface charge (both positively and negatively
charged at pH values of 5 and 11, respectively).26 Therefore,
under acidic conditions, the anionic AOT surfactant changes
the surface charge of Cu2O in order to self-assemble with
positively charged TiO2. An analogous procedure, with the use
of cationic CTAB surfactant, has been applied under basic
conditions. The samples prepared under basic conditions show
higher activity, and thus this manuscript presents these
samples.
2
2
1000 nm is caused by the Eg / T2g interband transitions in
the Cu2+ clusters. Accordingly, it is proposed that the conditions
during microemulsion might inuence the properties of less
stable oxides than titania, such as Cu2O, changing their oxida-
tion state, and thus resulting in the formation of mixed-
oxidation-state materials (as conrmed also by XPS data dis-
cussed in the following part).
Characterization of Cu2O@TiO2 samples
The DRS spectra of all modied and bare samples are shown in
Fig. 2a and b. The absorption in the UV range is due to the
intrinsic interband absorption of titania (ca. 3.2 eV), while the
absorption in the vis range is caused by the presence of copper.
All CuxO@TiO2 samples are colored, and the color intensity
(from light blue to dark blue-grey) increases with the content of
The XRD patterns for all photocatalysts are shown in Fig. 2d,
while the crystalline composition and crystallite sizes are listed
Fig. 2 Properties of the samples: (a and b) DRS spectra recorded with
BaSO4 (a) and bare TiO2 (b) as references; (c) photographs of the Fig. 3 STEM images of CuxO@TiO2 samples with different Cu2O
samples; (d) XRD patterns.
contents: 50% (a–c) and 2% (d–f).
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Table 1 The properties of the bare and composite samplesa
TiO2 (anatase)
(%)*
Cu2O
Specic surface
Sample
Color
Size (nm)
(%)*
Size (nm)
area (m2 gꢀ1
)
TiO2
Cu2O
White
Red
100
—
8.6
—
—
100
0.9
1.2
4.7
—
310.8
2.6
87.9
72.4
54.4
68.0
60.4
63.6
1% CuxO@TiO2
2% CuxO@TiO2
5% CuxO@TiO2
10% CuxO@TiO2
50% CuxO@TiO2
Light blue
Light blue
Blue
Blue/grey
Dark blue/grey
99.1
98.8
95.5
88.8
57.9
8.6
8.6
8.6
8.3
8.2
202.4
251.2
238.3
231.3
170.4
11.2
42.1
a
(%)* – crystalline content in wt%; all crystallite sizes were analyzed using the main peak in the XRD patterns.
in Table 1. Titania (ST01) is composed of anatase, as proven by Cu2O by a thick and porous shell of titania (in the form of
clear peaks at ca. 25ꢁ, 37–39ꢁ, 48ꢁ, 52–55ꢁ, 62–63ꢁ, 74–76ꢁ and aggregated core/shell structures), and with ne Cu2O NPs in the
82–83ꢁ, with ne crystallites of ca. 8.6 nm. In the case of bare range of 3–20 nm. On the other hand, the 5% Cu2O sample
Cu2O and 50% Cu2O samples, clear peaks for cubic cuprous exhibits a thinner layer of titania (Fig. 4 and S2†), which is
oxide could be detected at 29.63ꢁ, 36.37ꢁ, 42.45ꢁ, 61.64ꢁ and reasonable considering the lower content of TiO2, as clearly
73.75ꢁ. For the 10% Cu2O sample, only the main {111} peak of shown by the distribution of shell thickness in Fig. S4.†
Cu2O could be observed in the XRD pattern. However, for the
The oxidation states of elements and surface composition of
samples with lower than 10 wt% content of Cu2O only highly samples have been investigated by XPS, and deconvoluted data
enlarged patterns and Rietveld analysis could conrm the are shown in Fig. 5 and Table 2 (summarized in the next section
presence of copper species. It should be pointed out that the including results aer photocatalytic tests). Titanium exists
formation of core/shell structures with Cu2O by the micro- mainly in the form of TiO2 (Ti4+), but a low content of reduced
emulsion method does not inuence the crystalline properties titanium (Ti3+) has also been detected, reaching ca. 5%. The
of titania, as expected since titania is well known for its high formation of the core–shell structure results in a decrease in the
chemical and thermal stability. However, a decrease in the content of Ti3+ to 0.8%. Accordingly, it is proposed that Ti3+
crystalline size of Cu2O has been observed (from 88 nm to 54 could be an active site for bonding with the CuxO core, similar
nm), probably as a result of alkaline treatment (pH value change to surface titania modication with other elements/
during microemulsion). The crystalline content of titania in the compounds, e.g., noble metals,47 rare-earth elements48 and
coupled samples correlates well with that used for sample ruthenium complexes.49 Moreover, the content of hydroxyl
preparation, i.e., reaching 99.1%, 98.8%, 95.5%, 88.8% and
57.9% of TiO2 for 1%, 2%, 5%, 10% and 50% CuxO@TiO2
samples, respectively.
The morphology of the samples has been investigated by
STEM microscopy with EDS mapping, and the obtained results
are shown in Fig. 3, 4 and S1–S3 (ESI†). It should be remem-
bered that commercial samples of TiO2 and Cu2O have been
used for core/shell structure preparation without controlled
morphology. Although, similar sizes of titania NPs could be
clearly seen (9–20 nm, and aggregated NPs up to 60 nm
(Fig. S4†)), cuprous oxide is characterized by high polydispersity
in size (10–1000 nm (Fig. S4†)) and shape (including spherical
and cubic particles, building-block structures and rods
(Fig. S3†)). One can distinguish two types of morphological
conguration of the analysed core/shell structures. The rst one
is composed of ne titania NPs localized on large CuxO particles
(Fig. 3a, e, and f and S1–S3†), and the covering of Cu2O depends
on the content of titania, i.e., efficient coverage for 98 wt%
(Fig. 3d–f and S1†) and only partial coverage for 50 wt% (Fig. 3a–
c and S1†). In the case of smaller CuxO particles of ca. 10 nm
(possibly the amorphous form undetected by XRD), they are also
covered with ne titania, forming aggregated CuxO@TiO2
particles (Fig. 3d). The transmission electron (TE) image of the
Fig. 4 STEM images with EDS mapping for the 5% CuxO@TiO2 sample:
1% CuxO@TiO2 sample has also conrmed the coverage of
oxygen (red), titanium (green), and copper (yellow).
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Fig. 6 ERDT patterns (bars) and CBB positions (bottom of grey box) for
bare TiO2, and core–shell (a) and physically mixed (ground) (b) samples
containing 1% and 5% CuxO; values in h i denote the total density of ETs
in units of mmol gꢀ1
.
used. Bare and modied titania (ST01) samples, i.e., with 1%
and 5% Cu2O, prepared by two methods, i.e., microemulsion
(this study: “@” samples) and physical mixing (grinding: “/”
samples; previous study),13 have been investigated, and the
obtained results are shown in Fig. 6. It has been proposed that
the conduction band bottom (CBB), energy-resolved distribu-
tion of electron traps (ERDT) patterns and total electron-trap
(ET) density might reect the bulk structure, surface structure
and bulk/surface size, respectively.51 Interestingly, only samples
prepared as core@shell structures exhibit a slight narrowing of
the bandgap (energy shi of the conduction band bottom
(CBB)), probably due to IFCT (as discussed earlier – DRS data).
The ERDT patterns of bare TiO2, and 1% and 5% CuxO@ST01
clearly show that titania modication with Cu2O has induced
the shi of ETs (electron traps) into the conduction band
depending on Cu2O content (Fig. 6a). On the basis of the
principle of RDB-PAS measurement, in which electrons in the
valence band of a sample are excited directly to ETs, and the
density of electron-lled traps is measured, such high-energy
shi of ERDT to the higher energy side is quite unusual and
might be attributable to, at least, two phenomena. (1) Some
treatment of the sample (during core/shell structure prepara-
tion) causes a change in the sample structure to induce the
disappearance of lower energy traps and appearance of higher
energy traps. (2) The treatment causes coverage of the sample
surface to inhibit contact with methanol, used as a hole-
trapping agent in RDB-PAS measurement. Since, during RDB-
PAS analysis, a light beam for excitation is scanned from the
Fig. 5 XPS spectra for Ti 2p3/2, O 1s and Cu 2p3/2 of bare TiO2 (1st line),
Cu2O (2nd line), and 1% CuxO@TiO2 (3rd line) samples.
groups on the titania surface (red part in the oxygen spectrum)
has decreased, which indicates that some hydroxyl groups
participate in the core–shell formation. In the case of copper,
the main peak at ca. 532 eV matches with that of Cu–OH, while
Cu–O appears at a lower energy of ca. 529 eV.50 The oxygen peak
for the 1 wt% CuxO@TiO2 sample could well resemble the
respective counterparts, i.e., Cu2O and TiO2, considering the
low content of Cu2O. In the case of the copper peak, four peaks
might be estimated, i.e., CuO, Cu(OH)2, Cu2O and zero-valent
Cu, reaching ca. 26.7%, 59.0%, 12.5% and 1.8%, respectively.
The co-existence of various forms of copper on the surface of
a commercial sample is not surprising since it is well known
that Cu2O is reactive and easily oxidized. Interestingly, in the
core@shell particles, the content of Cu2O has signicantly
increased, reaching ca. 45%. Accordingly, it is proposed that
alkaline treatment during sample preparation resulted in
dissolution of the surface layer (containing Cu2+), leaving
mainly Cu2O as a core. Nevertheless, the mixed composition of
copper forms has been conrmed (as suggested by the DRS
spectra), and thus the photocatalysts have been re-named as
CuxO@TiO2 (instead of Cu2O@TiO2).
For detailed characterization of photocatalysts, reversed
double-beam photo-acoustic spectroscopy (RDB-PAS) has been
Table 2 Surface composition of samples after deconvolution of XPS peaks
Ti
O
Cu
Samples
Ti3+
Ti4+
O–H
M–OH/C]O
Metal oxide
Cu(OH)2
CuO
Cu2O
Cu
TiO2
Cu2O
5.1
—
0.8
3.3
3.4
49.7
94.9
—
99.2
96.7
96.6
50.3
5.3
7.5
1.7
4.4
4.5
34.2
61.9
54.1
25.1
21.5
23.4
60.5
30.6
44.2
70.5
74.0
61.4
—
—
—
—
26.7
10.2
—
—
16.4
59.0
40.5
—
2.9
68.3
12.5
45.4
71.2
89.7
1.5
1.8
3.9
28.8
7.4
13.8
1% CuxO@TiO2 initial
1% CuxO@TiO2 8 h H2 (UV)
1% Cu2O@TiO2 8 h CO2 (UV)
1% CuxO@TiO2 8 h (vis)
15.2
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higher energy side to the lower energy side, when methanol CuxO@TiO2 core/shell photocatalysts exhibit high photo-
supply is limited, the signal accumulation might be delayed, catalytic activity for H2 evolution, and the sample with the
resulting in a higher-energy shi of the ERDT peak.
lowest content of CuxO has been the most active. Interestingly,
Nevertheless, the total ET density of the 1% CuxO@TiO2 the samples prepared by grinding (previous study)13 have also
sample is much higher (almost double) than that of other shown high activity enhancement aer Cu2O addition, but the
samples. It should be noted that it has been proposed that best performance has been obtained for larger content of Cu2O,
shallow ETs with high energy might be responsible for high i.e., 5%. It should be pointed out that in ground samples, Cu2O
photocatalytic activity.52,53 In addition, it must be remembered was the predominant form of copper (pink sample) and an
that the total ET density might reect the specic surface area irregular mixed-oxide structure has been formed. Here, the
(SSA),51 and thus an increase in specic surface area should formation of a core/shell structure with a porous titania layer is
result in enhanced photocatalytic activity.30 Interestingly, the highly benecial for photocatalytic performance, reaching ca.
physically mixed (ground) samples (1% and 5% Cu2O/TiO2) 1.5 times higher activity that that by the most active ground
show similar tendencies (Fig. 6b), i.e., the higher the content of sample.
Cu2O, the deeper the ET distribution inside the CB. Moreover,
Two possible mechanisms for the enhanced activity of the
the 5% Cu2O/TiO2 sample with the highest total ET density (83 CuxO@TiO2 photocatalysts have been proposed, i.e., electron
mmol gꢀ1) and the deepest ET distribution inside the CB has transfer via type-II heterojunction and charge separation in the
shown the highest activity in both reaction systems under UV/ Z-scheme mechanism. In the case of heterojunction type II
vis irradiation, i.e., H2 and CO2 evolution.13 In the present (Fig. 8a), the electron transfer from the CB of CuxO into the CB
study, the most active samples for H2 and CO2 evolution (dis- of TiO2, and reverse hole transfer (i.e., from the VB of TiO2 into
cussed in the next section) are 1% CuxO@TiO2 and 5% Cux- the VB of Cu2O) might result in efficient charge carrier separa-
O@TiO2 samples, respectively. Accordingly, it might be tion, but also in lower redox ability (electrons in a less negative
proposed that the total ET density (in the case of high energy CB and holes in a less positive VB). Therefore, the Z-scheme
traps) and the shi of the ERDT peak to higher energy might mechanism with recombination of electrons from TiO2 and
result in efficient reduction and oxidation reactions, holes from Cu2O (Fig. 8b) is preferable for efficient redox reac-
respectively.
tions. It should be remembered that CuxO has been used as
a core, and thus the change of oxidation state of copper might
conrm the mechanism of action, i.e., oxidation and reduction
of copper should indicate heterojunction type II and Z-scheme,
Activity of Cu2O@TiO2 samples
The photocatalytic activities of the samples have been tested respectively. Indeed, XPS analysis (Fig. 9 and Table 2) performed
under UV/vis irradiation in two systems, i.e., oxidative decom- aer prolonged irradiation (8 h; stability study discussed in the
position of acetic acid with CO2 evolution (CO2 system) and following part; Fig. 11) suggests that copper has been reduced,
dehydrogenation of methanol with H2 evolution (H2 system), resulting in the formation of a sample with a high content of
and the obtained data are shown in Fig. 7.
zero-valent copper (an increase from ca. 4 to 28%), as shown by
the color change of samples (Fig. 7b) and schematic drawings in
The low activity of bare titania (ST01 and P25) in the H2
system (black bars) is not surprising since titania needs a co- Fig. 8c and d. Wang et al. have found similar reduction of
catalyst for efficient H2 evolution. Although usually zero-valent copper (XAFS analysis) even for copper NPs deposited on the
noble metals (NPs or nanoclusters) have been used as co-cata- titania surface during H2 generation under UV irradiation, i.e.,
lysts,47,51,54 other compounds have also been successfully
applied, e.g., graphene,53 Cu2O13 and CuxO.14,55 Similarly, the
Fig. 8 The schematic drawings of (a) heterojunction type II between
CuxO and TiO2, (b) basic Z-scheme mechanism of charge transfer
between CuxO and TiO2, (c) modified Z-scheme mechanism of charge
Fig. 7 Photocatalytic activity of samples under UV-vis irradiation in transfer between CuxO and TiO2 with participation of zero-valent Cu,
a H2 system and CO2 system (a), and photographs of the samples' (d) oxidative decomposition of acetic acid on a porous CuxO@TiO2
color changes (b) during irradiation (from the top: 0, 15, 30 and 45 min photocatalyst, and (e) methanol dehydrogenation on a porous Cux-
of irradiation) in the H2 system.
O@TiO2 photocatalyst with simultaneous formation of zero-valent Cu.
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reaching almost three times higher activity (for 1–5% CuxO
samples) than that of bare titania. It should be pointed out that
the color of the samples has not changed signicantly during
short (60 min) irradiation under aerobic conditions, and thus
the photocatalytic activity might mainly correlate with the
specic surface area (SSA). It is well known that a larger SSA
results in more efficient adsorption of reagents, and thus faster
reactions, especially in the CO2 system, which has already been
conrmed by systematic studies performed on more than 40
titania samples in different reaction systems.30 Indeed,
a signicant activity drop has been obtained for the sample with
reduced SSA, i.e., containing 50% CuxO. Like in the H2 system,
both mechanisms (Z-scheme and type-II heterojunction; Fig. 8a
and b) should be considered in the CO2 system. XPS data (Fig. 9)
have conrmed that also in the case of oxidation reaction,
copper has been reduced. Accordingly, the Z-scheme has been
proven in both reaction systems under UV irradiation, con-
rming efficient formation of the core@shell structure, which
prevents the photo-corrosion of CuxO and its leakage.
For the CO2 system, oxidation of acetic acid might proceed
by two pathways, i.e., via (i) photo-generated holes in TiO2 and/
or (ii) the superoxide ion (O2cꢀ) formed from O2 reduction by
electrons from CuxO. Since the photocatalysts show high
stability (Fig. 11b and 12b; discussed later), it is expected that
oxidation should also proceed via O2cꢀ (if not the activity should
stop since for an efficient Z-scheme both an electron acceptor
on the CuxO core and an electron donor on the TiO2 shell are
necessary). In the case of O2cꢀ, two possibilities should be
considered, i.e., (i) the penetration of O2 inside the TiO2 shell or
(ii) an electron transfer via the TiO2 network to the adsorbed
oxygen on the shell surface. It is proposed that the latter is more
possible since oxygen is probably too large to penetrate via the
titania shell and due to negligible activity of the samples under
vis irradiation (lack of electron donor inside the titania shell, as
discussed in the next section). Accordingly, the electron transfer
via the titania shell has been proposed for efficient activity of
core@shell photocatalysts in the CO2 system, as shown in
Fig. 8e. This mechanism might also explain the increase in
activity with an increase in CuxO content from 1% to 5%
(99 wt% and 95 wt% titania, respectively), resulting from the
formation of core@shell particles with a thinner titania shell,
thus allowing fast electron transfer via the TiO2 network.
Moreover, the experiments in the presence of different scaven-
gers (Fig. S5 in the ESI†) have conrmed that O2cꢀ radicals are
mainly responsible for oxidative decomposition of acetic acid,
i.e., activity has dropped by ca. 50% in the presence of 4-
Fig. 9 XPS spectra for Ti 2p3/2, O 1s and Cu 2p3/2 of the 1% CuxO@TiO2
sample before (1st line; same data as that in Fig. 3) and after 8 h irra-
diation during methanol dehydrogenation under UV irradiation (2nd
line), acetic acid oxidation under UV irradiation (3rd line) and phenol
oxidation under vis irradiation (4th line).
from Cu2+ via Cu+ to Cu0.56 Additionally, it should be pointed
out that zero-valent copper is not stable (due to fast oxidation),
and the obtained XPS data have conrmed that the core@shell
structure has been formed, keeping metallic copper safely
inside the titania porous layer. However, it is also expected that
proton reduction might be impossible on the surface of copper
when it is inside the titania shell. Therefore, it has been
proposed that the titania layer is not compact, but rather
composed of ne NPs, resulting in the formation of a highly
porous layer that is permeable to small atoms/molecules (e.g.,
H+/H2), as shown in Fig. 8e. Additionally, considering the dark
coloration of the samples during irradiation (the formation of
zero-valent copper), the highest activity of the sample with the
lowest content of copper should be caused by the less dark
color. It is clear that light transmittance through such dark
suspensions is hindered (due to the shielding effect; similarly
for Pt/TiO2),51 and thus only a part of the sample is photo-
activated by the incident light.
hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl
(TEMPOL).
It
should be mentioned that isopropanol as a HOc scavenger also
decreases activity signicantly (by ca. 30%), which might also
indirectly conrm the Z-scheme mechanism.
Since all coupled samples are coloured, the photocatalytic
activity under vis irradiation has also been tested for oxidative
decomposition of phenol as a model compound. It must be
pointed out that the disappearance of phenol (concentration
decrease in an aqueous phase) does not necessarily imply its
degradation, but could also be caused by its adsorption on the
photocatalyst surface (as conrmed by 30 min pre-stirring in the
In the case of the CO2 system, the photocatalytic activity of
almost all coupled samples (except the 50% sample) has been
signicantly increased aer titania modication with Cu2O,
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Fig. 10 Vis-activity of the bare samples and coupled photocatalysts in
terms of the rate of phenol disappearance (a), and the rate of ben-
zoquinone generation (b), and the possible mechanism under vis
irradiation (c).
dark), and thus the determination of oxidation products is
highly recommended. Consequently, activity data are presented
for both (i) a decrease in the phenol concentration (Fig. 10a) and
(ii) formation of the main intermediate, i.e., benzoquinone
(Fig. 10b). In the case of vis activity, only CuxO could be excited,
and thus the mechanism of the type-II heterojunction with only
CuxO excitation should be considered, i.e., an electron transfer
from the CB of CuxO into the CB of TiO2. Accordingly, oxidation
of organic compounds (here phenol) could proceed on the
titania surface. However, for efficient and stable photocatalytic
performance of CuxO, both redox reactions should take place,
i.e., reduction and oxidation. In the case of the CuxO core, there
is no hole acceptor inside the titania shell, and thus the pho-
togenerated holes rst self-oxidize the CuxO core, as conrmed
by the change of oxidation state of copper (Cu+ / Cu2+; Fig. 9
and Table 2), and then no further reaction might occur. Inter-
estingly, an electron transfer from CuxO into TiO2 has also been
conrmed by XPS data for titanium since a signicant reduction
of Ti4+ has been noticed (Ti3+ content increases from ca. 1 to
50%). Indeed, the obtained data have conrmed the negligible
vis-response of the CuxO@TiO2 core/shell photocatalysts.
Interestingly, bare commercial TiO2 (P25) exhibits an even
higher vis response than core@shell photocatalysts, which
might be cause by the presence of ETs inside the bandgap. A
slight vis-response for commercial and faceted titania samples
with shallow ETs has already been reported,57 and the defect-
rich titania (“self-doped”) has been proposed as an efficient
vis-responsive material.58
Fig. 11 Long-term photocatalytic activity of samples in (a) the CO2
system for 1% and 5% CuxO@TiO2 samples, and (b) the H2 system for
1% CuxO@TiO2, and (c and d) photographs of the samples during
irradiation in the H2 system: (c) first irradiation (after 0, 1, 2, 3, 4, 5, 6, 7
and 8 h of irradiation (from the top)), (d) second irradiation (after 0,
15 min, 30 min, 45 min, 1 h and 2 h of irradiation (from the top)).
Stable activity has been observed in the CO2 system during
4 h of irradiation for fresh and pre-irradiated samples (Fig. 11a).
Extended irradiation (>4 h) caused a slight decrease in reaction
rate, because of insufficient content of dissolved oxygen, which
has been conrmed by the increase in reaction rate during the
subsequent irradiation of air-bubbled samples. Accordingly, it
is proposed that even when the surface state of copper changes
during irradiation from Cu2+ into mainly Cu+, stable activity
without sample discoloration is achieved. In the case of the H2
system, although stable activity is observed during 8 h irradia-
tion (Fig. 11b), opening of the test tube and Ar-bubbling cause
a decrease in reaction rate by about half. Therefore, it is
proposed that small amounts of copper (e.g., as Cu+ or Cu0
clusters) could also penetrate through the titania layer, acting as
a co-catalyst on its surface for H2 evolution. However, post-
bubbling could cause their detachment from the photo-
catalyst surface, and thus the re-irradiated sample shows lower
activity, resulting from a smaller number of active sites for H2
evolution. This hypothesis could be conrmed by the slight
colour change of samples during irradiation, i.e., (i) slight
discoloration during the rst irradiation (loss of copper species;
Fig. 11c), and (ii) stable colour during re-irradiation (no more
copper leakage from the photocatalyst surface; Fig. 11d).
Accordingly, it is proposed that even if a slight loss of activity in
the initial stage of the photocatalytic process is observed (due to
imperfect CuxO coverage by the TiO2 layer), the coupled pho-
tocatalysts are quite stable, and could be successfully applied
for photocatalytic reactions under UV irradiation. It should be
pointed out that samples prepared by grinding are not stable at
all with complete discoloration during longer irradiation.13
In order to check the stability of samples, long-term photo-
catalytic activity has been investigated, and the results are
shown in Fig. 11. Since high accumulation of gas products (H2
and CO2) inside the test tubes during 8 h irradiation and
insufficiency of dissolved oxygen in the CO2 system were
observed, the test tubes were re-opened, purged with Ar or air
(in the case of H2 and CO2 systems, respectively) and once more
irradiated for 120 min and 180 min, respectively.
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efficiency of the CuxO@TiO2 photocatalyst. The core@shell
particles have only one, but quite serious, shortcoming, i.e.,
negligible activity under vis irradiation. However, efficient
reduction of copper oxides inside the titania layer under UV
irradiation might enable the formation of a stable and efficient
vis-responsive material, i.e., plasmonic photocatalyst
(Cu0@TiO2). The above results unequivocally show that coupled
CuxO@TiO2 with a core@shell nanostructure has signicant
potential to be an efficient and stable photocatalyst dedicated to
various reaction systems.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors thank Professor Zhishun Wei (Hubei University of
Technology, China) for fruitful discussion on the samples'
characterization. This research has been nancially supported
by the Polish National Science Centre (2016/21/B/ST5/03387)
and “Yugo-Sohatsu Kenkyu” for an Integrated Research
Consortium on Chemical Sciences (IRCCS) project from the
Ministry of Education and Culture, Sports, Science and Tech-
nology, Japan (MEXT).
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Fig. 12 Long-term photocatalytic activity of a thermally treated (2 h,
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