Surface activation of cobalt oxide nanoparticles for photocatalytic carbon dioxide reduction to methane

Article abstract of DOI:10.1039/c9ta04323c

Cobalt oxide nanoparticles, by surface activation, successfully promoted photocatalytic CO₂ reduction with a high efficiency and selectivity in aqueous media. Treatment with N-bromosuccinimide (NBS) resulted in the formation of an active form of Co₃O₄, removed surfactants, and coordinated Br on the surface, thereby enhancing the catalytic efficiency of CO₂ reduction to methane.

Full text of DOI:10.1039/c9ta04323c

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Surface Activation of Cobalt Oxide Nanoparticles for
Photocatalytic Carbon Dioxide Reduction to Methane
Ji Yong Choi,a Chan Kyu Lim,a Bumjin Park,a Minjun Kim,a Aqil Jamal,b and Hyunjoon Song*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Cobalt oxide nanoparticles by surface activation successfully although a few studies recently demonstrated their high
promoted photocatalytic CO2 reduction with high efficiency and potential for efficient CO2RR.19,20
selectivity in aqueous media. The treatment with N-
To overcome the low activity in desired reactions, we focused
bromosuccinimide (NBS) formed an active form of Co3O4, removed on the surface activation of nanoparticles. There has been much
surfactants, and coordinated Br on the surface, thereby enhancing work to improve catalytic properties by proper surface
the catalytic efficiency of CO2 reduction to methane.
treatments. Within the scope of CO2 reduction, Yin and
coworkers developed surface fluorination on TiO2-x, modulating
the conduction band edge to facilitate methane generation.21
Goodenough and coworkers synthesized carbon-coated In2O3
nanobelts, on which the surface carbon enhanced visible
absorption as well as the chemisorption of CO2 to increase the
activity.13
In this study, we treated CoO nanoparticles with N-
bromosuccinimide (NBS), which plays multiple roles in surface
oxidation to Co3O4, removal of surfactants, and coordination of
Br on the surface. Using these surface-activated CoO/Co3O4
nanoparticles as photocatalysts, CO2 was successfully converted
to methane with remarkable activity and selectivity in aqueous
medium. The surface treatment with sulfur and NBS together,
additionally enhanced the activity with quantum efficiency up
to 2.3% at 405 nm. This approach outperformed the other
catalysts reported thus far, used in aqueous media.
CoO nanoparticles were synthesized via thermal
decomposition of Co(acac)3 and oleylamine.22 The transmission
electron microscopy (TEM) image in Fig. 1a shows a morphology
having multiple facets with an average length of 9.1 ± 0.9 nm
(Fig. S1, ESI†). The high resolution TEM (HRTEM) image in the
inset shows lattice spacing of 0.248 nm, corresponding to the
CoO(111) planes. The X-ray diffraction (XRD) pattern in Fig. 1c
is assigned to cubic CoO (JCPDS No. 48-1719), and the peak at
780 eV and a broad satellite at 785.3 eV in the Co 2p3/2 region
of X-ray photoelectron spectroscopy (XPS) also match the
signals of Co2+ species from CoO (Fig. S2, ESI†).23 The CoO
nanoparticles were then treated with NBS in toluene at high
temperature. After the treatment, the morphology was
unchanged and the average diameter was estimated to be 8.8 ±
0.8 nm (Figs. 1b and S1b, ESI†). However, the lattice spacing of
peripheral region in the HRTEM image is 0.188 nm,
Production of desired carbon chemicals using the carbon
dioxide reduction reaction (CO2RR) has been considered one of
the best potential methods to resolve both energy and
environmental disruption problems.1-3 There exist numerous
approaches used to convert CO2 to other chemicals,4 but the
photochemical approach is regarded as the ultimate goal in
terms of utilizing renewable and pollutant-free solar energy.5-7
Metal oxide semiconductors have been employed as
photocatalytic materials due to their diverse structure and
bandgap, high stability, resistance against photooxidation, and
natural abundance.8 TiO2 is a well-known representative
material, but others such as Ga2O, Cu2O, ZnO, In2O3, and C3N4
have also been widely investigated.9-14
Cobalt oxides have recently attracted much attention due to
their superior catalytic activity for water oxidation.15 Cobalt
oxides have bandgaps in a visible region (2.6 eV for CoO and 2.1
eV for Co3O4),16 and the band edges can be modulated by
structural manipulation in a nanometer scale. Co metal has
loosely bonded d-electrons that allow it to participate in
catalytic reactions with high electrical conductivity.17 In
addition, partial oxidation of Co enhances CO2 adsorption
capacity.18 Based on these properties, cobalt oxides are
expected to be well suited for photocatalytic CO2 reduction.
However, the attempts have mostly been unsuccessful thus far,
a. Department of Chemistry, Korea Advanced Institute of Science and Technology
(KAIST), Daejeon 34141, Republic of Korea. E-mail: hsong@kaist.ac.kr
b. Research and Development Center, Saudi Aramco, Dhahran 31311, Kingdom of
Saudi Arabia
†Electronic Supplementary Information (ESI) available: Methods, additional TEM
images,
XPS
data,
and
photocatalytic
reaction
results.
See
DOI: 10.1039/x0xx00000x.
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Fig. 2 (a) Photocatalytic evolution amounts along the reaction progress, and (b) evolution
rates of the gaseous products using CoO/Co3O4 nanoparticles. The same amount (2.7 mg)
of catalyst was used for each reaction.
as a reactant produced 13CH4 and 13CO, which were confirmed
by strong m/z peaks at 17 and 29, respectively (Fig. S5b and c,
ESI†). Three runs of the repetitive reaction for 2 h with CO2
purging demonstrated the stability of the photocatalyst (Fig.
S6a, ESI†), and after the reaction, the particle morphology and
composition as a mixture of CoO and Co3O4 were unchanged
(Fig. S6b and c, ESI†).
CoO and Co3O4 are known as indirect and direct band gap
semiconductors. The band gaps were estimated to be 2.6 eV for
CoO and 2.1 eV for Co3O4 nanoparticles, by Tauc plots from the
UV-visible absorption spectra (Fig. S7, ESI†). The absorbance of
CoO/Co3O4 around 800 nm was due to O2--Co3+ excitation, in
which the Co(III) centers created sub-bands (Eg1 = 1.4 eV) inside
the band gap.25,26 The flat-band potentials were measured using
Mott-Schottky plots from the impedance measurement (Fig. S8,
ESI†). The band-edge potentials were known to be largely
affected by surface states or morphology of nanostructures,
and thereby they diverged from the values of bulk materials.27
Based on these data, the absolute band edge positions were
successfully estimated (Fig. 3a). In this diagram, both CoO and
Co3O4 can provide sufficient overpotentials for the
electrochemical CO2RR into CH4.
To clarify the active site between two domains, the CoO
nanoparticles were treated with NBS for distinct periods (Fig. S9,
ESI†). Up to 8 min of treatment when the CoO : Co3O4 ratio was
22 : 78 as measured by XPS and XANES, the CH4 evolution rate
increased to the maximum rate of 10.4 μmol h-1. Complete
oxidation to Co3O4 occurred in 30 min, and the catalyst still
exhibited a high evolution rate of 8.8 μmol h-1, indicating that
the Co3O4 domain provided active sites. On this basis, as shown
in the band diagram of Fig. 3a, photoexcited electrons on the
conduction band edge of CoO could be readily transferred to
that of Co3O4 at a lower energy level, which facilitated CO2
reduction. Photogenerated holes also moved from the valence
band edge of CoO to that of Co3O4 and promoted water
oxidation.
Fig. 1 TEM and HRTEM images of (a) CoO and (b) CoO/Co3O4 nanoparticles. (c) XRD and
(d) XPS (Co 2p) spectra of CoO/Co3O4 nanoparticles. The bars represent (a,b) 20 nm and
(insets) 5 nm.
corresponding to the Co3O4(331) planes (Fig. 1b inset). The XRD
pattern assignable to cubic Co3O4 (JCPDS No. 43-1003) is hardly
discernible (Fig. 1c), but the deconvolution of the XPS peaks in
the Co 2p3/2 region shows the evolution of new peaks at 779.5
and 781.5 eV, which correspond to the Co3+ and Co2+ species
from Co3O4 (Fig. 1d).24 X-ray absorption near edge structure
(XANES) analysis also indicates the formation of Co3O4 from CoO
by the treatment with NBS (Fig. S3, ESI†). The linear combination
fitting of the Co K-edge XANES spectra indicates that the ratio
of Co3O4 in the nanoparticles changed from 58% for 3 min to
82% for 8 min and 100% for 30 min of treatment.
The CoO/Co3O4 nanoparticles were well dispersed in aqueous
solution. Using the nanoparticles as photocatalyst, CO2RR was
driven by irradiation using a Xe lamp (300 W) in CO2-saturated
water without any hole scavengers. In Fig. 2a, two major
products, CH4 and CO, were measured along the reaction
progress. The other liquid carbon products were not detected
by nuclear magnetic resonance (NMR) spectroscopy (Fig. S4,
ESI†). CH4 was the main product with an evolution rate of 10
μmol h-1, which is equivalent to catalytic activity of 3.8 mmol
-1
gcat h-1. The quantum efficiency of CH4 production was
estimated to be 0.94% with irradiation using a light emitting
diode at 405 nm. CO and H2 were also generated, but the rates
were only 0.73 and 0.13 μmol h-1; thus, the selectivity of CH4
production was calculated to be 98%. The oxidation product, O2,
was detected with the rate of 2.8 μmol h-1, which was smaller
than expected from the ideal stoichiometric reaction. It seems
that photogenerated holes were being consumed through other
pathways, such as a backward reaction from CO to CO2.
The mechanism of photochemical CO2RR to CH4 is speculated
to occur through a formaldehyde or carbene pathway.28 In
either process, CO is the key intermediate in the initial stage of
the reaction. It is well known that Co3O4 is an active oxide in the
catalytic CO oxidation to CO2,29 and in this regard, we attempted
to check the photocatalytic activity of CO using our catalyst.
To verify the origin of the carbon products, a control
experiment under N2 atmosphere was performed that yielded
negligible products (Fig. S5a, ESI†). The experiment using 13CO2
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Fig. 4 (a) TEM image of S-doped CoO/Co3O4 nanoparticles. (b) Elemental mapping of Co
(red), O (skyblue), and S (green) on a single particle. (c) XPS spectrum in the S 2p region
and (d) evolution rates of photocatalytic CO2RR using S-doped CoO/Co3O4 nanoparticles.
The bars represent (a) 10 nm and (b) 4 nm.
Fig. 3 (a) Band alignment of CoO/Co3O4 with electrochemical reduction potentials of
related reactions at pH 7. (b) Photocatalytic CO reaction using CoO/Co3O4 nanoparticles.
(c) XPS spectra in the Br 3p region before and after the NBS treatment. (d) Photocatalytic
CO2 reduction to CH4 using CoO/Co3O4 nanoparticles with NBS (red line) or DIB (black
line) treatment.
the treatment, all the bands had disappeared, and the particles
were readily dispersed in water, but not in toluene (Fig. S10,
ESI†).
Interestingly, after the NBS treatment, Br was coordinated on
the particle surface. NBS is an oxidant, but also behaves as a Br
radical source. In the XPS spectrum, the peaks at 182 and 188
eV appeared in the Br 3p region (Fig. 3c). It was reported that
the surface Br can effectively trap photogenerated holes, and
enhance the lifetime of charge separation.32 To prove the
importance of surface Br, CoO nanoparticles were treated with
another oxidant, (diacetoxyiodo)benzene (DIB), which does not
offer halogen radicals during the reaction. After the surface
oxidation, the XPS spectrum in the Co 2p region and the XRD
spectrum revealed that the Co oxidation state was similar to
that of the NBS-treated nanoparticles (Fig. S11, ESI†). However,
there were no XPS signals in the I 3d region, which confirmed
no coordination of halogen atoms on the surface. The resulting
CoO/Co3O4 nanoparticles were partly dispersible in aqueous
medium. In photocatalytic CO2RR, the DIB-treated
nanoparticles exhibited only half the evolution rate of CH4 with
NBS-treated catalyst (Fig. 3d), indicating that the surface Br
atoms played a critical role during the reaction.
To extend the strategy of surface activation with
heteroatoms, sulfur was the next target for modifying CoO
nanoparticles. Sulfur is used as a dopant that increases CO2
binding on the surface, and CoS is a well-known cocatalyst for
photocatalytic hydrogen evolution reactions.33 A small amount
of dodecanethiol was added during the growth of CoO
nanoparticles, followed by NBS treatment under the given
conditions. The TEM image in Fig. 4a shows that the
morphology with multiple facets resembles CoO/Co3O4
nanoparticles. The energy dispersive X-ray spectroscopy (EDS)
analysis indicates that the nanoparticles contain 2.6 atomic% of
sulfur, and elemental mapping shows that sulfur is evenly
distributed on the surface (Fig. 4b). In the XPS spectrum, the
Under the identical conditions used for CO2RR, CO was bubbled
into the reaction medium. Although CO solubility in water is
very low, the evolution rates of CH4 and CO2 were 0.92 and 2.3
μmol h-1, respectively (Fig. 3b). Based on this observation, we
could imagine the complete picture of the reaction. On the
catalyst surface, photoexcited electrons reduced adsorbed CO2
to CO intermediate, and then further reduced it to CH4 in the
presence of surface-adsorbed hydrogen atoms. On the other
hand, photogenerated holes were transferred to either water
or CO intermediate, and oxidized them to O2 or CO2. If the
conversion of CO to CH4 or CO2 is rapid enough, CH4 is only the
final reduced product without the detection of CO,21 which
explains the extreme selectivity for CH4 in our reaction. This
result is opposite to the reaction study using Co3O4 as a
photocatalyst by Wang and coworkers, in which CO was a main
product.16 However in this case, the reaction was carried out in
gas phase, which could not supply enough protons from the
reaction environment. In aqueous medium, it became easier to
capture protons on the catalyst surface, which promoted the
CH4 production.30
NBS readily oxidized CoO to CoO/Co3O4 nanoparticles
without changing morphology. Moreover, the NBS treatment
on the surface has another advantage. The pristine CoO
nanoparticles were hard to disperse in aqueous medium due to
the strong coordination of oleylamine on the surface. However,
the simple NBS treatment built up surface charges and
completely removed the surfactants. Before the treatment, the
particles were only dispersible in toluene, and the infrared
spectrum showed typical vibration bands of alkylamine, such as
C-H stretching (2920 and 2852 cm-1), N-H bending (1741 cm-1),
CH2 bending (1458 cm-1) and C-N stretching (1376 cm-1).27 After
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peak at 779 eV in the Co 2p3/2 was assigned to Co2+ of CoS,34 but
it is hard to identify in the mixture of other species (Fig. S12,
ESI†). However, the S peaks at 161.8 and 163.0 eV in the S2p
region are clearly assigned from CoS before the NBS
treatment.35 After the treatment, the peaks shift to higher
binding energies, indicating that sulfur is also oxidized to
sulphate (Fig. 4c).36 The photocatalytic CO2RR was performed
using the S-doped CoO/Co3O4 nanoparticles. The evolution rate
was 28 μmol h-1 with irradiation using the Xe lamp, which
corresponded to catalytic activity of 10 mmol gcat-1 h-1 (Fig. 4d)
with sufficient photochemical stability (Fig. S13, ESI†). The
quantum efficiency of CH4 production was estimated to be 2.3%
using a light emitting diode at 405 nm. The activity was
improved 2.7 times compared to that of the undoped
CoO/Co3O4 catalyst. It is difficult to directly compare these
values to other catalysts, because there have been no studies
reporting the quantum efficiency of photochemical CO2RR,
presumably due to very low activity under a monochromatic
wavelength. However, it is worth noting that our photocatalytic
system marks a recordable photochemical activity in aqueous
medium. Comparison with other reported photocatalysts is
provided in Table S1. We also performed photochemical CO2RR
under the irradiation with a AM 1.5G filter (Fig. S14, ESI†). As
summarized in Table S2, the S-doped CoO/Co3O4 nanoparticles
exhibited a comparable conversion rate to the other catalysts
even though no hole scavengers were used during the reaction.
Notes and references
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DOI: 10.1039/C9TA04323C
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Conclusions
In summary, surface activation with NBS was applied to CoO
nanoparticles. The NBS treatment promoted surface oxidation
to Co3O4, complete removal of the surfactant, and Br
coordination on the catalyst surface. In photochemical CO2RR,
the resulting CoO/Co3O4 nanoparticles exhibited a remarkable
activity of 10 μmol h-1 (3.8 mmol gcat-1 h-1), quantum efficiency
of 0.94% at 405 nm, and CH4 selectivity of 98%. The activity was
further enhanced by surface modification with sulfur, reaching
28 μmol h-1 (10 mmol gcat-1 h-1) with the quantum efficiency of
2.3%. We expect that this surface activation approach can be
extended to other metal oxide semiconductors, thereby
enhancing photocatalytic performance in various reactions.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work was supported by the Saudi-Aramco and KAIST CO2
management center. This work was also supported by the
National Research Foundation (NRF) funded by the Ministry of
Science and ICT, Republic of Korea (NRF-2018R1A2B3004096
and 2018R1A5A1025208). The authors acknowledge the
Pohang Accelerator Laboratory (PAL) for beamline use.
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Table of contents entry
Surface treatment on cobalt oxide nanoparticles by N-bromosuccinimide and sulfur
exhibited high photocatalytic CO2 reduction efficiency into methane.