Chapter 7 O 2 Plasma-activated CuO-ZnO Inverse Opals as High-performance
7.3 Test of Methanol Reforming Reaction
Figure 7.8 compares the effects of material architecture and O2-plasma
treatment on the methanol conversion and rates of hydrogen production during
MRR, respectively. After hydrogen pre-reduction, all experiments were run for
20 hours with working catalysts (Cu-ZnO inverse opals). The chemical state of
metallic Cu (served as active species) could be determined via XAS
measurements (Figure 7.9). Minor deviations were observed in repeated runs,
which are indicated by the small error bars in the figures. Notably, the methanol
conversion and hydrogen production rates in a microreactor with O2-plasma
treated Cu-ZnO inverse opals for plasma-exposure time of 10 min are nearly
100% and 300 mmol gcat-1 h-1 at a temperature as low as 230 ℃, respectively,
which are significantly higher than those obtained with the conventional
dense-film microstructures (non-inverse opals). SEM image of the Cu-ZnO
non-inverse opals clearly shows presence of randomly oriented microstructures
(Figure 7.10). The 3-fold enhancement in catalytic activity and consequently
treatment. Owing to special pore structure, the occurrence of fast gas-solid
reactions benefits significantly from proceeding inside the porous transport
pathways of 3DOM nanoarchitectures. In addition, O2-plasma treatment is shown
to increase the surface concentration of Vo, which provides more active sites for
MRR, based on the equation:
Vo․․ + 2e' + O＊ → Oox + ＊,
where Vo․․, Oox, and ＊ are oxygen vacancy, lattice oxygen, and surface
active site, respectively.[129] Accordingly, it is well recognized that both
hierarchical pore-channel networks as well as surface density of Vo are crucial to
the observed excellent catalytic activity. On the other hand, only minor amounts
of CO were detected by CO detector in the outlet gas stream. The CO
concentration of only 130-170 ppm at 230 ℃ was detected for the O2-plasma
treated 3DOM nanoarchitectures with plasma exposure time of 10 min. This
result can be attributed to a higher surface density of Vo as a result of the
O2-plasma treatment. It is believed that the Vo can generate lattice oxygen that, in
turn, catalyzes the CO oxidation reaction.[122] Previous investigations have
pointed out that the presence of Vo plays an important role in CO oxidation
reaction due to the fact that they provide sites for oxygen activation by formation
nanoarchitecture developed in this study certainly provides a promising
alternative for catalytic generation of high purity hydrogen without requiring a
complex combination of multiple CO-clean systems to produce clean electrical
energy from small fuel cells for automotive applications. Last but not the least,
the advantage of O2-plasma treated inverse opals is further exemplified by the
outstanding stability (a minimal degradation rate of only 10% after 80 hours of
continuous operation) in MRR, which is significantly superior to the 31%
reduction for the commercial catalysts (Figure 7.8b). The XRD results on pre-
and post-reaction inverse opals show no apparent difference in the crystal
structure. SEM observations of the post-reaction inverse opals also confirm that
the microstructure is essentially unchanged, although some small fraction of
macropores appeared to have collapsed.
7.4 Summary
We have successfully demonstrated an easy route to fabricate a novel CuO-ZnO
catalyst with well-defined inverse opal nanostructure on the inner surface of
microchannels, via direct synthesis, for microreformer applications. The 3DOM
nanoarchitectures have distinct advantages in terms of enhanced catalytic activities
since they posse a high specific surface area as well as enable effective transport of
methanol, high hydrogen production rate (300 mmol gcat-1 h-1), low CO formation
(130-170 ppm), and outstanding stability (after continuous 80 hours of operation),
making it exceedingly promising toward MRR. The present results prove that
catalytic performance of Cu-ZnO catalysts for MRR can indeed be improved
through control not only of hierarchical pore-channel networks but also of Vo
induced by O2-plasma treatment. These efforts open up new opportunities in the
development of highly active and selective nanoarchitectures for a wide range of
different catalytic reaction systems.
Closed-packing
Figure 7.1 Schematic diagram of the novel catalyst CuO-ZnO inverse opals
fabricated on the inner surface of microchannel reactor.
Figure 7.2 Procedure for the preparation of CuO-ZnO inverse opals using
Figure 7.3 (a) SEM image and (b) HRTEM image of CuO-ZnO 3DOM
nanoarchitectures. (c) Micro- Raman spectra of CuO-ZnO inverse opals with
different O2-plasma exposure durations.
0.0 0.2 0.4 0.6 0.8 1.0 0
50 100 150 200 250 300
0 3 6 9 12
Pore-size Distribution
Pore Diameter (nm)
V o lu m e (c m
3/g ST P)
Relative Pressure (P/P
0
)
Adsorption Desorption
Figure 7.4 N2 adsorption-desorption isotherm and pore-size distribution (inset) of
CuO-ZnO inverse opals without O2-plasma treatment.
Figure 7.5 (a) TEM image of CuO-ZnO inverse opals. Corresponding
energy-filtered TEM (b) copper and (c) zinc mapping images of CuO-ZnO inverse
opals.
0 5 10 15 0.6
0.7 0.8 0.9
I
575/( I
575+I
438) ra tio
Plasma treated duration (min)
Figure 7.6 Relative ratio of the integrated 575 cm-1 peak intensity to the sum of the
438 and 575 cm-1 peaks as a function of O2-plasma exposure time for CuO-ZnO
inverse opals.
1010 1020 1030 1040
520 525 530 535 540
O 1s
520 525 530 535 540
O 1s
520 525 530 535 540
O 1s
520 525 530 535 540
O 1s
520 525 530 535 540
O 1s
520 525 530 535 540
O 1s
CuO-ZnO 3DOM nanoarchitectures after O2-plasma treatment for (a) 0, (b) 3, (c) 5,
(d) 10, and (e) 15 min, respectively.
(b)
8960 8980 9000 9020 9040 9060 9080
Cu K-edge
Normalized absorbance (a.u.)
Photon energy (eV)
O2-plasma treated CuO-ZnO inverse opals O2-plasma treated CuO-ZnO inverse opals after hydrogen pre-reduction
Figure 7.8 (a) Rates of Methanol conversion and H2 production for Cu-ZnO
catalysts with dense film, inverse opal, and O2-plasma treated inverse opal
nanostructure, respectively. (b) Stability tests of MRR with O2-plasma treated
Cu-ZnO inverse opal for plasma exposure time of 10 min (◇) and commercial
catalysts (○). Reaction conditions: H2O/O2/MeOH = 1/0.125/1, Reaction
temperature = 230 oC, W/F = 21 kgcat s mol-1methanol.
Figure 7.9 Cu K-edge X-ray absorption near-edge spectroscopy (XANES) of
O2-plasma treated CuO-ZnO inverse opals and O2-plasma treated CuO-ZnO inverse
1μm 1μm
Figure 7.10 SEM image of Cu-ZnO catalysts with non-inverse opal nanostructure.
Chapter 8
Enhanced Photocatalytic Activity with Carbon-Modified ZnO Inverse Opals for Solar Water-Splitting
8.1 Introduction
The success in the development of inverse opals in the past decade suggests their
indispensable role in a wide range of fields, from optical computing and
telecommunication to photocatalyst and photovoltaic cell.[111] Particularly,
increasing the effective optical path length of incident light to promote a more
efficient absorption of solar photons through highly periodic nanoarchitectures
presents a potential alternative and attractive approach to enhance the efficiency of
solar-related applications.[132,133] For example, by reducing the group velocity of
light at energies near the edge of the photonic stop-band, a higher probability of
absorption was achieved and the photodegradation efficiency of inverse opals
doubled.[133] In addition, well-ordered nanoarchitectures of photonic crystals have
also been successfully utilized in the modification of the absorption bands of dye
sensitizers due to the photonic bandgap effect in solar cells.[132] As a result of the
production of clean energy.[115,134] Hydrogen is an effective energy source that
can combine with oxygen in a fuel cell to produce electricity and water without
generating pollutants, and thus represents an environmentally benign fuel of the
future. However, today the main production of hydrogen comes from catalytic
reforming of hydrocarbon fuels, consuming natural resources and generating carbon
dioxide as an undesired byproduct.[35,100] Thus, central on direct solar energy
conversion to hydrogen via photoelectrochemical (PEC) water-splitting is a
fascinating route with advantages of green processing and energy savings.[23,25,42]
The light-harvesting efficiency of photoelectrodes is of great importance for
hydrogen generation using solar-powered water-splitting. In this regard, evolution of
hierarchical inverse-opal nanoarchitectures is exceedingly essential for their
immense potential of efficient solar driven hydrogen production due to the
extraordinary properties of photonic bandgap and slow photons, which can
significantly improve the confinement and localization effect of incident light
thereby enhancing the effective interaction of light with photoactive materials.
In the search for a semiconductor which can facilitate the efficient storage of solar
energy in the form of hydrogen via PEC cells, ZnO remains a favorable
material.[135-137] With the appropriate flat band potential, low electrical resistance,
demand is realistic.[138] The seminal achievements to study ZnO as an
oxygen-evolving photoanode for the light-driven decomposition of water detailed its
promising aspects and limitations. As major drawbacks, ZnO possesses quick
recombination of photoinduced electron-hole pairs, poor optical absorption ability
toward visible-light irradiation, and photoinstability in aqueous solution, which
intrinsically confine the integrated performance of solar energy conversion
devices.[139] Recently, extensive research efforts have been devoted to
straightforwardly overcome these disadvantages during the photocatalytic reactions
by surface modification of ZnO, such as doping by carbon and hybridization with
carbon-containing species.[140,141] Therefore, it is reasonable to expect that an
effective integration of ZnO inverse opal and carbon hybridization would lead to a
highly desirable photoanode with enhanced solar-hydrogen efficiency and long-term
durability in an aqueous environment. In this communication, we elucidate for the
first time the synergistic effect of optical amplification with surface hybridization on
the enhanced solar conversion efficiency of ZnO inverse opals modified by carbon
in a non-sacrificial electrolyte. By in-situ incorporating carbon into the matrix of
ZnO inverse opals through direct pyrolysis of the blends of ZnO and polymer opal,
higher probability of absorption is achieved and the lifetimes of photoinduced
8.2 Structural Characterization of Carbon-modified ZnO Inverse Opals
Figure 8.1 shows the morphology and microstructure of hierarchical inverse-opal
nanoarchitectures. The scanning electron microscopy (SEM) image illustrates a
continuous oxide framework consisting of an interconnected pore network
permeating throughout the structure (Figure 8.1a). The nanoarchitectures fabricated
exhibit a uniform pore size of ~ 360 nm, interconnected by ~220 nm windows
between the adjacent pores. This represents a shrinkage of about 30% during
calcinations since PS beads used as sacrificial templates were of ~ 500 nm size. One
of the most significant advantages of the inverse opals, which are clearly distinct
from the traditional photoelectrodes, is hierarchical pore-channel networks they
offer for effective surface contact between the incident light and photoelectrodes. A
high-resolution transmission electron microscopy (HRTEM) image of inverse opal is
presented in Figure 8.1b, yielding the spacing of the (101) lattice plane of the
hexagonal ZnO crystal to be 0.24 nm. Besides, Figure 8.1c and 8.1d provide
additional bright-field and high-angle annular-dark-field (HAADF) scanning TEM
images, showing that Zn atoms are homogeneously distributed in the whole
structures due to the large difference of atomic number between Zn and O. Further
8.2b). The formation of remained carbon could be attributed to the low calcination
temperature at 300 ℃, which is not enough to completely remove opal template.
In order to obtain better understanding on the electronic structures of the
carbon-modified ZnO inverse opals, we performed systematic XPS and XAS studies.
The C 1s core level spectra of carbon-modified ZnO inverse opals in Figure 8.2c
display a main peak at 285 eV and a shoulder peak around 288.3 eV. The main peak,
which is normally located at 284.6 eV, arises from adventitious elemental carbon or
graphite-like bonding, while the shoulder peak corresponds to C-O bonding
species.[93] Obviously, a slight up-shift of the main peak position with the
occurrence of a shoulder peak could be obtained as a result of the strong interaction
between carbon and ZnO by means of incorporation of carbon atoms into the ZnO
lattice. Furthermore, extended X-ray absorption fine structure (EXAFS) spectra
taken around the Zn K-edge of carbon-modified ZnO inverse opals are presented in
Figure 8.2d. A apparent decrease in relative intensity and interatomic distance of the
fist-shell and second-shell (Zn-O and Zn-Zn bonding) over carbon-modified ZnO
inverse opals would be observed compared to pure ZnO structure. The reason may
be mainly due to the strong distortion of the ZnO lattice resulted from positioning of
carbon in ZnO framework. Figure 8.3 shows the O K-edge XAS spectra of
between O 2p and Zn 4s states followed by the region between 539 and 550 eV
which is due to the hybridization between O 2p and Zn 4p states.[142] The increase
in relative intensity of spectral feature at ~531-535 eV observed in carbon-modified
ZnO inverse opals compared to pure ZnO structure indicates the increase of the
unoccupied density of states (DOS) of O 2p-Zn 4s hybridized states, which may be
caused by strong hybridization of C-O bonds.[142] In addition, the broadening of
the absorption peak at ~537 eV over carbon-modified ZnO inverse opals in
comparison with pure ZnO structure is assigned to the presence of oxygen vacancies,
suggesting that carbon incorporation would induce oxygen vacancies in ZnO
lattice.[142]
8.3 Test of Photoelectrochemical Reaction
In order to address the quantitative correlation between carbon incorporation and
light absorption of carbon-modified ZnO inverse opals, we performed
incident-photon-to-current-conversion efficiency (IPCE) measurements to study the
photoactive wavelength regime as shown in Figure 8.4a. In addition to achieve the
maximum IPCE value of 95%, it should be noted that the IPCE of carbon-modified
ZnO inverse opals at the incident wavelength of 400 nm is up to 26.6%, implying
that carbon modification substantially improves the light collection and conversion
Systematic electrochemical measurements were carried out to evaluate the PEC
properties of photoanodes fabricated from carbon-modified ZnO inverse opals.
Figure 8.4b shows a set of linear sweep voltammagrams without sacrificial reagents
in the dark and with illumination of 100 mW/cm2. Dark scan linear sweep
voltammagrams from -0.19 to +1.0 V show a small current in the range of 10-7
A/cm2. In comparison to ZnO noninverse-opal structures, carbon-modified ZnO
inverse opals exhibit a striking enhancement in photoresponse with a photocurrent
density of 1 mA/cm2 at +1.0 V, which is even superior to that of other ZnO-based or
TiO2-based photoanodes in recent reports.[135-137,143,144] Significantly, there is a
saturation of photocurrent at more positive potential observed in carbon-modified
ZnO inverse opals, indicating efficient charge separation induced by the surface
modification of ZnO with carbon upon illumination. The reason could be attributed
to the formation of oxygen vacancies resulted from carbon incorporation into ZnO
framework via XAS results. The proposed schematic diagram of charge-transfer
process in our system is illustrated in Figure 8.6. The oxygen vacancies may work as
electron acceptors and trap the photogenerated electrons temporarily to reduce the
surface recombination of electrons and holes. Here, the oxygen vacancies can be
considered to be the active sites of the ZnO photoanode. Furthermore, in order to
which the contribution due to applied potential is subtracted from the total
efficiency.[137] The plot of efficiency versus applied potential (Figure 8.4c) shows
the maximum value of efficiency is 0.75% at an applied potential of +0.3 V, which is
higher than the recent reported values for ZnO-based or TiO2-based
photoanodes.[136,137,143] Last but not least, the advantage of carbon-modified
ZnO inverse opals is further exemplified by the good stability (a minimal
degradation rate of only 7% after 5 h of continuous running) in the photo-oxidation
process at pH = 7 (Figure 8.4d). The photogenerated holes are the dominant roles for
ZnO materials during photocorrosion reaction. In our system, the photogenerated
holes rapidly transferring to the solution for water-oxidation reaction may
successfully facilitate the inhibition of the photocorrosion due to high efficiency of
charge separation induced by carbon modification of ZnO structures. Therefore, the
merit of the cooperative effect, i.e. optical amplification and surface hybridization,
can be unambiguously highlighted here via IPCE, linear sweep voltammagrams,
photon-to-hydrogen efficiency, and stability data.
8.4 Summary
We have successfully demonstrated an easy-to-fabricated route to directly prepare
carbon-modified ZnO inverse opals on the ITO substrate as photoanodes. While
modifications that can independently enhance the separation of photoinduced
electron-hole pairs, visible-light absorption, and photostability for ZnO photoanode.
It is anticipated that this model hybrid photoanode will enable us to design
high-activity, high-stability, visible-light-driven photoelectrodes in the future.
1 μm
Figure 8.1 (a) SEM image and (b) HRTEM image of carbon-modified ZnO
inverse opals. (c) Bright-field and (d) Z-contrast TEM images of
carbon-modified ZnO inverse opals.
100 nm
280 282 284 286 288 290 292 C 1s
280 282 284 286 288 290 292 C 1s
Normalized intensity (a.u.)
Binding energy (eV)
285 eV 284.6 eV
Figure 8.2 (a) Typical TEM image of carbon-modified ZnO inverse opals.
Corresponding energy-filtered TEM (b) carbon mapping image of carbon-modified
ZnO inverse opals. (c) XPS spectrum of C 1s peak of carbon-modified ZnO inverse
opals. (d) Zn K-edge EXAFS spectra of carbon-modified ZnO inverse opals and
pure ZnO structures.
525 530 535 540 545 550 O K-edge
Normalized absorbance (a.u.)
Photon energy (eV) Inverse Opal
ZnO standard
Figure 8.3 O K-edge XAS spectra of carbon-modified ZnO inverse opals and pure
ZnO structures.
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
300 400 500 600 700 800
0
300 400 500 600 700 800
0
300 400 500 600 700 800
0
Figure 8.4 (a) Measured IPCE spectra of carbon-modified ZnO inverse opals in the
region of 300 to 800 nm at a potential of +0.25 V (vs. Pt) in two-electrode system. (b)
Linear sweep voltammagrams, collected at a scan rate of 10 mV/s at applied
potentials from -0.19 to +1.0 V (vs. Ag/AgCl) for carbon-modified ZnO inverse
opals in the dark, ZnO noninverse-opal structures, and carbon-modified ZnO inverse
opals at 100 mW/cm2. (c) Photoconversion efficiency of the PEC cell with
carbon-modified ZnO inverse opal photoelectrode as a fuction of applied potential.
400 450 500 550 600
A b so rb a n c e ( a .u .)
Wavelength (nm)
Figure 8.5 UV-vis spectra of carbon-modified ZnO inverse opals showing a slight
red shift of absorption wavelength to the visible region.
V
o¨
Eg = 3.2 eV
hν
2.4 eV (G re en)
e
-e
-h
+H
2O
O
2V
o¨
Eg = 3.2 eV
hν hν
2.4 eV (G re en)
e
-e
-h
+H
2O
O
2Figure 8.6 Schematic diagram representing the charge-transfer process of the
carbon-modified ZnO inverse opals.
Chapter 9
Conclusions
9.1 Effects of Nitrogen-Doping on the Microstructure, Bonding and Electrochemical Activity of Carbon Nanotubes
Vertically aligned CNx NTs have been synthesized using microwave plasma
enhanced chemical vapor deposition with different nitrogen flow rate. The results
show that nitrogen incorporation provides a simple pathway to
engineering/modifying the microstructure and electronic bonding structure, which is
crucial to the ET kinetics of the CNx NT electrode. In-situ N doping of the CNTs at
an optimal 3.5 at. % N is effective in promoting substitutional graphite-like defect
structure, which is favorable for fast ET in electrochemistry.
9.2 Novel Copper-Zinc Oxide Nanoarchitectures as Microreformation Catalysts for Hydrogen Production
Firstly, we have successfully demonstrated an easy-to-fabricate route to prepare
arrayed ZnO NR@Cu NP heterostructures on the inner surface of microchannels.
The superb catalytic performance and stability of the Cu NP-decorated ZnO NR
Cu species, and the existence of SMSI effect.
Secondly, we have demonstrated an effective MW-treatment technique to enhance
the catalytic activity of CuO NT/ZnO NR nanostructures for MRR. It is believed
that formation of defects, microstrains and SMSI due to the selective heating by
MW-treatment may have contributed to such enhancement in catalyst performance.
Thirdly, we have successfully demonstrated an easy route to fabricate a novel
CuO-ZnO catalyst with well-defined inverse opal nanostructure on the inner surface
of microchannels. The Vo-rich Cu-ZnO inverse opal obtained in this study at a
low-reaction temperature of only 230 ℃ yields complete conversion methanol, high
hydrogen production rate (300 mmol gcat-1 h-1), low CO formation (130-170 ppm),
and outstanding stability (after continuous 80 hours of operation), making it
exceedingly promising toward MRR.
9.3 Enhanced Photocatalytic Activity with Carbon-Modified ZnO Inverse Opals for Solar Water-Splitting
We have successfully demonstrated an easy-to-fabricated route to directly prepare
carbon-modified ZnO inverse opals on the ITO substrate as photoanodes. While
hierarchical inverse-opal nanoarchitectures enthrall the idea of increasing the optical
electron-hole pairs, visible-light absorption, and photostability for ZnO photoanode.
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