Morphology-Controlled Self-Assembly and Nanostructured NiO: An Efficient and Robust Photocatalytic Water-Oxidation Catalyst

Article abstract of DOI:10.1002/cctc.201500308

Three α-NiO nanocompounds of different morphology, with nanorods, nanowires, and nanoplates, were synthesized by controlling the ratio of reactants and temperature. The shape and structure of the nanocompounds were confirmed by SEM, XRD, FTIR, Raman spectroscopy, energy-dispersive X-ray spectroscopy, BET, and X-ray photoelectron spectroscopy (XPS) analysis. These compounds were examined as catalysts in photocatalytic water oxidation with [Ru(2,2′-bipyridine)₃]²⁺ and S₂O₈^2- as a photosensitizer and a sacrificial oxidant, respectively. All of the samples exhibit high turnover frequencies and perfect stability in slightly alkaline conditions. A characteristic peak at around E=0.95 V versus Ag/AgCl assigned to a Ni³⁺ species was detected by cyclic voltammetry, which suggests that a high-valent nickel species may be responsible for water oxidation. The surface properties of the α-NiO nanorods also remain unchanged after examination by XPS before and after the photocatalytic reaction.

Full text of DOI:10.1002/cctc.201500308

DOI: 10.1002/cctc.201500308
Full Papers
Morphology-Controlled Self-Assembly and Nanostructured
NiO: An Efficient and Robust Photocatalytic Water-
Oxidation Catalyst
Xiaoqiang Du,[a] Yong Ding,*[a, b] and Chengqiang Li[a]
Three a-NiO nanocompounds of different morphology, with
nanorods, nanowires, and nanoplates, were synthesized by
controlling the ratio of reactants and temperature. The shape
and structure of the nanocompounds were confirmed by SEM,
XRD, FTIR, Raman spectroscopy, energy-dispersive X-ray spec-
troscopy, BET, and X-ray photoelectron spectroscopy (XPS)
analysis. These compounds were examined as catalysts in pho-
tocatalytic water oxidation with [Ru(2,2’-bipyridine)3]2+ and
ly. All of the samples exhibit high turnover frequencies and
perfect stability in slightly alkaline conditions. A characteristic
peak at around E=0.95 V versus Ag/AgCl assigned to a Ni3+
species was detected by cyclic voltammetry, which suggests
that a high-valent nickel species may be responsible for water
oxidation. The surface properties of the a-NiO nanorods also
remain unchanged after examination by XPS before and after
the photocatalytic reaction.
2À
S2O8 as a photosensitizer and a sacrificial oxidant, respective-
Introduction
The efficient harvesting and utilization of solar energy to pro-
duce hydrocarbon fuels is, so far, considered to be an appeal-
ing method to alleviate environmental pollution and the
energy crisis.[1] The overall photocatalytic water-splitting reac-
tion involves three major steps for molecular-based photocata-
lytic systems: 1) absorption of light by a photosensitizer to
generate the excited-state sensitizer for solar-energy harvest-
ing and utilization, 2) water oxidation that extracts electrons
and protons, and 3) reduction of protons or the conversion of
CO2 into hydrocarbon fuels by using the protons generated
during the second step.[2] Photocatalytic reactions could be re-
alized if these three sequential steps are completed. To make
this artificial system economically viable, the use of minor or
precious metals in each step should be avoided. To date, the
development of efficient water-oxidation catalysts (WOCs) is
a bottleneck for attaining artificial photosynthesis because
water oxidation is a challenging process that includes a multie-
lectron transfer coupled with a multiproton transfer.[3]
made from earth-abundant materials.[6] Various materials have
been studied for water oxidation, including polyoxometala-
tes,[5b,7] metal complexes with organic ligands,[8] and simple
salts.[9] Recently, first-row transition-metal hydroxide or oxide
materials have attracted much attention as potential materials
for water oxidation, because they are earth abundant, nontox-
ic, and inexpensive materials. In recent years, hierarchical nano-
structures with controllable morphology, orientation, and di-
mensionality have also received great attention for their
unique properties. For example, Boppana and Jiao[10] reported
a-MnO2 nanotubes, a-MnO2 nanowires, and b-MnO2 nanowires
as highly efficient and robust WOCs driven by visible light.
Nickel oxide (NiO) is an important transition-metal oxide and
has been extensively studied in many fields, including lithium-
ion batteries,[11] supercapacitors,[12] adsorbents in water treat-
ment,[13] and gas sensors and catalysis,[14] as a result of its out-
standing physical and chemical properties. In comparison with
certain metal oxides, the NiO crystals reported before have rel-
atively lower water-oxidation catalytic capacities.[5c] However,
the influence of the crystal morphology on the water-oxidation
activity has not been systematically explored for the nickel
oxide system.
Most WOCs reported previously contain precious metals
such as iridium[4] and ruthenium[5] as active species. However,
in order to be economically viable, the catalysts should be
In this paper, we report the investigation of a-NiO nanorods,
a-NiO nanowires, and a-NiO nanoplates as catalysts for water
oxidation driven by visible light. All of these materials exhibit-
ed excellent water-oxidation activity under visible light with
[a] X. Du, Prof. Y. Ding, C. Li
State Key Laboratory of Applied Organic Chemistry
Key Laboratory of Nonferrous Metal Chemistry and
Resources Utilization of Gansu Province
College of Chemistry and Chemical Engineering, Lanzhou University
Lanzhou 730000 (P.R. China)
2+
Ru(bpy)3 (bpy: 2,2’-bipyridine) as a sensitizer and Na2S2O8 as
E-mail: dingyong1@lzu.edu.cn
a sacrificial electron acceptor. We demonstrate that the crystal
morphology of a-NiO does not significantly influence its pho-
tocatalytic activity in water-oxidation reactions by comparing
nanostructured a-NiO nanorods, a-NiO nanowires, and a-NiO
nanoplates. The surface conditions of the a-NiO nanorods
remain unchanged after examination by X-ray photoelectron
[b] Prof. Y. Ding
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences
Lanzhou 730000 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500308.
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spectroscopy (XPS) before and after the photocatalytic reac-
tion. The Ni2+ atom is oxidized to a high-valent species, which
may be responsible for water oxidation.
The temperature also had a significant effect on the shape
1
of the product if the value of R was between 1 and = , besides
2
the [Ni2+]/[OHÀ] molar ratio. In the case of R=3= and a hydro-
4
thermal reaction for t=24 h, nanoplates were obtained at T=
1408C, a typical SEM image of which is shown in Figure 1e. It
can be seen that most of the nanoplates have a hexagonal
morphology with sizes in the range 100–150 nm. Many of
them are regular hexagons with the adjacent edges forming
angles of 1208 (Figure S1 in the Supporting Information). Fig-
ure 1b, d, and f show the X-ray diffraction (XRD) patterns of a-
NiO nanorods, nanowires, and nanoplates, respectively. The ob-
tained diffraction patterns reveal that all of the samples could
be perfectly indexed to the cubic structure of crystalline a-NiO
(JCPDS no. 78-0643).The spectra from electron-dispersive spec-
troscopy (Figure S8–S10 in the Supporting Information) further
confirmed the existence of Ni and O elements in the compo-
sites and that no impurities were present in the powder sam-
ples.
Results and Discussion
a-Ni(OH)2 nanorods were obtained after a hydrothermal reac-
tion for t=24 h at T=1208C with R=3 (R: molar ratio of NiSO4
to NaOH; that is, 9.8 mmol of NiSO4·6H2O and 3.3 mmol of
NaOH). Porous a-NiO nanorods could be obtained by thermal
decomposition of the as-synthesized precursor Ni(OH)2 nano-
rods at T=5008C for t=2 h. Figure 1a shows a representative
scanning electron microscopy (SEM) image of a-NiO nanorods.
From Figure 1a, it can be seen that the product was composed
of copious amounts of straight, thick nanorods with diameters
of 30–40 nm and lengths of several hundred nanometers.
If the amount of NaOH was increased to 4.9 mmol (R=2)
and the other conditions were kept the same as above, nano-
wires were generated, a typical SEM image of which is present-
ed in Figure 1c. The image indicates the large quantity and
good uniformity of the product; the nanowires can be seen to
have uniform width over their entire lengths. The smooth,
thin, wirelike structures have a width typically in the range of
20–30 nm and lengths of up to several micrometers; this di-
mension is greater than the length of the aforementioned
nanorods.
To investigate the specific surface areas and porous nature
of the a-NiO nanorods, nanowires, and nanoplates, Brunauer–
Emmett–Teller (BET) gas-sorption measurements were carried
out. The nitrogen adsorption–desorption isotherms of these
porous nanostructures are shown in Figure 2, and the insets
therein are the corresponding Barrett–Joyner–Halenda pore-
size distribution plots. The BET specific surface areas of the
samples were found to be 88.5, 65.5, and 83.0 m2 gÀ1 for the
a-NiO nanorods, a-NiO nanowires, and a-NiO nanoplates, re-
spectively, which indicates that there is no obvious difference
in the specific surface areas among the three kinds of mor-
phology.
The catalytic activity of the samples was examined in visible-
light-driven water oxidation. The reaction was performed by
photoirradiation (lꢀ420 nm) of
a borate buffer (80 mm,
pH 8.5, 15 mL) containing a NiO catalyst, Na2S2O8 as the sacrifi-
cial electron acceptor, and [Ru(bpy)3]Cl2 as the photosensitizer.
The catalytic cycle of the visible-light-driven water oxidation is
depicted in Scheme 1. In the [Ru(bpy)3]2+–S2O8 method, the
2À
sensitizer of [Ru(bpy)3]Cl2 absorbs visible light and gives elec-
trons to the sacrificial electron acceptor S2O82À, which is then
2À
reduced to the SO4 species. [Ru(bpy)3]2+ transforms into
[Ru(bpy)3]2+* (in which * denotes the excited state) because of
photoinduction, then the resulting [Ru(bpy)3]2+* loses elec-
2À
2À
trons to the S2O8 acceptor to generate [Ru(bpy)3]3+, SO4
,
and SO4 species. The radical species SO4 {[E (SO4 /SO42À)=
0
CÀ
CÀ
CÀ
+2.40 V vs. Ag/AgCl]},[15] for which the electrode potential is
higher than that of Ru(bpy)3]2+ {[E0([Ru(bpy)3]2+/[Ru(bpy)3]3+
)= +1.04 V vs. Ag/AgCl]},[16] can further oxidize another
Figure 1. a) SEM image and b) XRD pattern of a-NiO nanorods, c) SEM
image and d) XRD pattern of a-NiO nanowires, and e) SEM image and f) XRD
pattern of a-NiO nanoplates.
Scheme 1. Principal processes of O2 evolution in a light-driven water-oxida-
tion system. WOC=water-oxidation catalysts.
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two equivalents of water molecules, which are oxidized to pro-
duce one equivalent of oxygen molecules (Scheme 1).
Self-quenching and decomposition of the photosensitizer by
a nucleophilic attack of water or OHÀ on [Ru(bpy)3]3+ under
neutral or basic conditions are in competition with electron
transfer from the WOCs to [Ru(bpy)3]3+, which leads to the de-
composition of the photosensitizer and low O2 evolution
yields.[17] Thus, it is essential to find a better WOC that im-
proves the lifetime of the photosensitizer and, consequently,
the O2 yield.[18] Isotope-labeling water-oxidation experiments
with 18O-enriched water (10.8%) instead of H216O were carried
out with the a-NiO nanorod catalyst to confirm that the water
is the source of the evolved oxygen. The relative abundances
of the oxygen isotopes, which were determined from the in-
tensities of those three molecular ion peaks, is listed in Fig-
ure S16 in the Supporting Information. The ratio of
16O16O:16O18O:18O18O is determined to be 79:20:1, which is in
good agreement with the simulated ratio of 80:19:1 for
oxygen coming exclusively from water. The above data clearly
demonstrates that the evolved O2 originates exclusively from
the water. The oxygen-evolving activities of the samples were
measured under identical conditions (Figure 3). A turnover fre-
quency (TOF; the rate of O2 evolution in the initial 1 min/BET)
of 0.04 mmolsÀ1 mÀ2 for the a-NiO nanorods is calculated from
Figure 3. The a-NiO nanowires exhibit
a
TOF of
0.06 mmolsÀ1 mÀ2, whereas the TOF of the a-NiO nanoplates
obtained from Figure 3 is 0.05 mmolsÀ1 mÀ2. No O2 evolution
was confirmed from reaction solutions without either photo-
sensitizer or sacrificial electron acceptor (Table S2 in the Sup-
porting Information). A minimum amount of O2 evolution
(0.5 mmol) was detected in the absence of any catalysts. Simple
oxides based on cheap metals reported before have relatively
lower TOF numbers (Table S1 in the Supporting Information),
whereas the NiO catalysts in this system enhanced the water-
oxidation activity with higher TOF values.
Figure 2. Nitrogen adsorption isotherms of the porous NiO: a) nanorods,
b) nanowires, and c) nanoplates.
Figure 3. Kinetics of O2 formation in the photocatalytic system with different
catalysts (a-NiO nanowires, &; a-NiO nanorods, *; a-NiO nanoplates, ~.
Conditions: Light-emitting diode lamp (lꢀ420 nm); catalyst concentration:
0.5 glÀ1; 1.0 mm [Ru(bpy)3]Cl2; 5.0 mm Na2S2O8; 80 mm sodium borate buffer
(initial pH 8.5); total reaction volume: 15 mL; overall volume: approximately
28 mL; vigorous agitation with a magnetic stirrer.
[Ru(bpy)3]2+ to produce two equivalents of [Ru(bpy)3]3+ in the
overall photoinduced process. In the final step of this light-
driven electron-transfer process in the presence of NiO catalyst,
four equivalents of [Ru(bpy)3]3+ extract four electrons from
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To explore the differences among the photocatalytic activi-
ties, the TOFs were evaluated based on the BET results
(Table 1) for all of the samples. Notably, all of the nanostruc-
tured NiO photocatalysts exhibit similar TOFs, even with the
different morphologies (nanorods, nanowires, or nanoplates),
which have similar BET surface areas and composition in the
as-prepared samples. We conclude that the morphology of the
NiO catalyst does not significantly influence the water-oxida-
tion activity.
Table 1. Water oxidation catalyzed by different a-NiO catalysts in this
work.[a]
Catalyst
BET [m2 gÀ1
]
TOF[b]
[mmolsÀ1 mÀ2
TOF[c]
[sÀ1 per Ni]
]
a-NiO nanorods
a-NiO nanowires
a-NiO nanoplates
no catalyst
88.5
65.5
83.0
–
0.04
0.06
0.05
0
2.5”10À4
3.0”10À4
3.3”10À4
0
Figure 4. Kinetics of O2 formation in the photocatalytic system with fresh a-
NiO nanorods and recovered a-NiO nanorods. Conditions: Light-emitting
diode lamp (lꢀ420 nm); catalyst concentration: 0.5 glÀ1; 1.0 mm [Ru(b-
py)3]Cl2; 5.0 mm Na2S2O8; 80 mm sodium borate buffer (initial pH 8.5); total
reaction volume: 15 mL; overall volume: approximately 28 mL; vigorous agi-
tation with a magnetic stirrer.
[a] Conditions: Light-emitting diode lamp (lꢀ420 nm); catalyst concen-
tration: 0.5 gLÀ1; 1.0 mm [Ru(bpy)3]Cl2; 5.0 mm Na2S2O8; 80 mm sodium
borate buffer (initial pH 8.5); total reaction volume: 15 mL; overall
volume: approximately 28 mL; vigorous agitation with a magnetic stirrer.
[b] Turnover frequency normalized by a catalyst surface area for O2 evolu-
tion in one minute. [c] Turnover frequencies are calculated based on the
oxygen product rates (in one minute) divided by all of the Ni atoms in
the catalyst.
Despite the big difference in crystal morphology among the
NiO catalysts, large similarities in catalytic activity have been
found experimentally for the three oxides. However, we must
look deeper into how the reaction occurs and whether the cat-
alysts are stable or not. We concentrated on a detailed study
of the nanorods. a-NiO nanorods were collected without any
loss by centrifugation after a photocatalytic water oxidation. A
fresh buffer solution containing Na2S2O8 (5.0 mm) and [Ru(b-
py)3]Cl2 (1.0 mm) was added to the collected particles for repet-
itive examinations under photoirradiation. The high catalytic
activity of the a-NiO nanorods was maintained during the five
runs (Figure 4). The a-NiO nanorod catalysts were examined
before and after the reaction by powder XRD, SEM, FTIR,
Raman spectroscopy, HRTEM (see Figures S18–23 in the Sup-
porting Information), and TEM (Figure 5) measurements.
Almost no obvious variation in either the powder XRD pattern
or the morphology of the a-NiO nanorods catalysts was ob-
served. The high O2 yield with the a-NiO nanorods was main-
tained even after t=78 min (Figure S24 in the Supporting In-
formation).These results clearly indicate that a-NiO nanorods
are a highly active, stable, and robust catalyst in the photoca-
talytic system.
Figure 5. TEM images of fresh a) a-NiO nanorods and b) recovered a-NiO
nanorods.
water-oxidation reaction. Moreover, the half-wave potential of
2+/3+
Ru(bpy)3
in the same buffered system is much more posi-
tive (E=1.13 V vs. Ag/AgCl) than the onset potential of the cat-
alytic current (E=0.82 V vs. Ag/AgCl; Figure 6). Therefore,
[Ru(bpy)3]3+ is thermodynamically capable of promoting water
oxidation. At around E=0.95 V versus Ag/AgCl, a small oxida-
tion peak assigned to the redox of a Ni3+ species appears. The
similar oxidation peak at E=0.82 V versus Ag/AgCl has been
assigned to Ni2+/Ni3+ species for dilute Ni2+ solutions in
borate electrolyte at pH 9.2.[19] Moreover, the small irreversible
oxidation wave at E=0.68 V versus Ag/AgCl in the cyclic vol-
tammogram of [NiII(meso-L)](ClO4)2 (L: ligand) has also been as-
signed to the oxidation of Ni2+ to Ni3+ species.[8d] However,
the oxidation peak of Ni3+/Ni4+ species (E=1.34 V vs. Ag/AgCl)
is not detected in our system. The reason may be that the oxi-
We further analyzed the electrochemical properties of the a-
NiO nanorods with cyclic voltammetry. An anodic current was
observed at the onset potential (reference current value is
zero) at approximately E=0.82 V (vs. Ag/AgCl). In contrast,
only a minimum current was obtained in the same buffered
solution without the a-NiO nanorods (Figure 6). This is clearly
an indication of the ability of a-NiO nanorods to catalyze the
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(TOF=0.019 mmolsÀ1 mÀ2).[20] The O2 yield (the amount of O2
produced with respect to the persulfate) with the a-NiO nano-
rods (44.2%, pH 8.5, t=15 min) is also higher than those re-
ported with catalysts containing precious metals, like RuO2 par-
ticles (22.0%, pH 5.0),[5c] and abundant elements such as Fe2O3
(21.0%, pH 8.0)[21] and Fe3O4 (29.0%, pH 8.0).[21] In addition, the
O2 yield is higher than that of catalysts with abundant metals,
for example, NiO particles (24.0%, pH 8.0).[6a] Hence, a-NiO
nanorods composed of earth-abundant elements turn out to
be among the most active catalysts for photocatalytic water
oxidation.
The a-NiO nanorods are highly robust even during the pho-
tocatalytic water oxidation. A critical issue with some spinel
compounds is that their divalent metal species can be oxidized
under highly oxidizing conditions, which may result in micro-
phase separation and deactivation of the catalysts. Generally,
NiO is quite stable, but its transformation to nonstoichiometric
nickel oxide under highly oxidizing conditions has been report-
ed as a fact.[21] Therefore, it is of great necessity to confirm the
surface properties of each component after water oxidation. A
series of measurements was taken based on the knowledge
above, such as XRD, SEM, FTIR, Raman spectroscopy, HRTEM,
and TEM analyses. The change of the surface properties of the
a-NiO nanorods before and after the photocatalytic reaction
was observed by XPS. The XPS measurements were performed
for the full scan, Ni 2p and O 1s energy regions. The binding
energy of each element was corrected with the C 1s peak
Figure 6. 1) Cyclic voltammogram (CV) of 80 mm sodium borate buffer solu-
tion at pH 8.5. 2) CV of 80 mm sodium borate buffer solution at pH 8.5 with
0.5 gLÀ1 of a-NiO nanorods 3) CV of 80 mm sodium borate buffer solution at
pH 8.5 with 1.0 mm of [Ru(bpy)3]Cl2. Conditions: CHI660D electrochemical
analyzer with a glassy carbon pasting 0.01 mg a-NiO nanorods, Ag/AgCl,
and Pt wire electrodes as the working, reference, and auxiliary electrodes, re-
spectively, at RT with a scanning rate of 100 mVsÀ1
.
dation peak of Ni3+/Ni4+ species is overlapped with the in-
creasing oxygen-evolution-reaction current. The increase of the
catalytic current peak that starts just after the oxidation peak
suggests that a high-valent nickel species may be the active
species for the water oxidation. The catalytic cycle of the visi-
ble-light-driven water oxidation is depicted in Scheme 2.
3
(284.8 eV). Figure 7a exhibits the XPS spectra for Ni 2p = peaks;
2
1
The catalytic activities of heterogeneous catalysts are usually
investigated on the basis of normalization by the specific sur-
face area. So, to obtain the apparent TOFs, the rate of oxygen
evolution values calculated from the initial slope (t=1 min) of
the time courses were normalized with the Brunauer–Emmett–
Teller surface areas through N2 adsorption measurements at
T=77 K (88.5 m2 gÀ1 for a-NiO nanorods). An apparent TOF
value of 0.04 mmolsÀ1 mÀ2 was obtained with a-NiO nanorods.
To our knowledge, h-Co@200 possessed high catalytic activity
in the photocatalytic water oxidations. The apparent TOF value
with the a-NiO nanorods is larger than that with h-Co@200
Ni 2p = peaks appear at binding energies of 853.0, 854.6, and
2
871.9 eV with weak satellite peaks for the a-NiO nanorod sam-
ples before the reaction and at binding energies of 852.8,
854.5, and 872.1 eV with weak satellite peaks for the a-NiO
nanorod samples after the reaction. The binding energies of
these peaks indicate that the Ni species in the samples are
Ni2+ species, by comparison with reported peak positions of Ni
2p peaks of NiO.[22] The similar ratios of peak strength between
the Ni 2p main peak and satellite peak for both samples sup-
port the fact that the surface properties remain unchanged
even after the highly oxidizing conditions in photocatalytic
water oxidation. Figure 7b displays the XPS spectra
for the O 1s main peak, which appears at a binding
energy of 530.2 eV, with another peak at 528.3 eV for
the a-NiO nanorods before the reaction and at
a binding energy of 530.3 eV with another peak at
528.3 eV for the a-NiO nanorods after the reaction.
Although the main peaks of Ni 2p and O 1s from the
sample after the reaction are slightly shifted, the
identical separation between the main peak and sat-
ellite peak in both samples and the consistency of
the peak shapes clearly illustrate that there is no
change in the valence state of the Ni2+ and O2À spe-
cies. A similar phenomenon is reported in the related
research.[21] The surface properties of the a-NiO nano-
rods are the same before and after the photocatalytic
water-oxidation reaction.
Scheme 2. Proposed mechanism for light-driven water oxidation catalyzed by NiO.
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a typical preparation of a-Ni(OH)2 nanorods, an aqueous solution
(40 mL) containing NiSO4·6H2O (9.8 mmol) and NaOH (3.3 mmol)
was sealed into a 100 mL capacity Teflon-lined autoclave and
heated at T=1208C for 24 h (the reaction time was always 24 h
unless otherwise mentioned). After hydrothermal treatment, the re-
action mixture was separated into a green-paste-like precipitate
and a clear supernatant, which was decanted off. The solid product
was subsequently washed several times with deionized water and
then dried in air at T=608C for 12 h. Finally, a green powder was
obtained. By changing the molar ratio of NiSO4 to NaOH or the
temperature, a-Ni(OH)2 nanowires and a-Ni(OH)2 nanoplates could
be obtained. Porous NiO nanorods, nanowires, and nanoplates
were obtained if the as-synthesized Ni(OH)2 nanorods, nanowires,
and nanoplates were directly calcined at T=5008C for 2 h in
a muffle furnace.
Photocatalytic water oxidation
Photocatalytic water oxidation was performed as follows. The de-
sired concentration of catalyst (0.5 gLÀ1) was added to a buffer so-
lution (80 mm, pH 8.5 for borate buffer) containing [Ru(bpy)3] Cl2
(1.0 mm) and Na2S2O8 (5.0 mm). The above solution was deaerated
by purging with Ar gas for 10 min in a flask (28 mL) sealed with
a rubber septum (the volume of the reaction solution was 15 mL).
The reaction was then started by irradiating the solution with
a light-emitting diode light source (light intensity: 16 mW; beam
diameter: 2 cm) through a transmitting glass filter (lꢀ420 nm) at
RT. After each irradiation time, Ar (150 mL) was injected into the
flask and then the same volume of gas in the headspace of the
flask was withdrawn with an SGE gas-tight syringe and analyzed
by gas chromatography (GC). The O2 in the sampled gas was sepa-
rated by passing it through a 2 m”3 mm packed 5 Š molecular
sieve column with an Ar carrier gas and was quantified by a ther-
mal conductivity detector (Shimadzu GC-9A). The total amount of
evolved O2 was calculated from the concentration of O2 in the
headspace gas. Contamination of the headspace with air was cor-
rected by measuring the N2 present in the headspace (from the N2
peak in the GC traces). The solution pH value was measured after
the reaction by a Mettler Toledo FEP20 pH meter.
Figure 7. X-ray photoelectron spectra in the energy regions of a) Ni 2p and
b) O 1 s of a-NiO nanorods 1) before and 2) after the reaction. The binding
energy of each element was corrected by the C 1s peak (284.8 eV).
Conclusions
We report a-NiO nanorods, a-NiO nanowires, and a-NiO nano-
plates as highly efficient and robust water-oxidation catalysts
driven by visible light. All of the samples exhibit high turnover
frequencies and strong stability in slightly alkaline conditions.
We demonstrate that the morphology of the NiO catalyst does
not significantly influence its photocatalytic activity if the as-
prepared samples have similar Brunauer–Emmett–Teller surface
areas and composition. The a-NiO nanorods can be easily sep-
arated from the reaction solution by centrifugation separation
and still maintain excellent water-oxidation activity during five
cycles. The surface properties of the a-NiO nanorods also
remain unchanged after examination by X-ray photoelectron
spectroscopy before and after the photocatalytic reaction. The
Ni2+ species is oxidized to a high-valent species, which may be
responsible for water oxidation. Water-oxidation catalysis ach-
ieved by an oxide-based earth-abundant metal is a good ex-
ample of a design-efficient and cheap catalyst for artificial pho-
tosynthesis.
Materials: H218O (97% 18O) was purchased from Marshall Isotopes
Ltd. Purified water (18.2 MWcm) for the preparation of solutions
was obtained from a MilliQ system (Millipore, Direct-Q 3 UV). All
other chemicals and salts used were of the highest purity available
from commercial sources.
Characterization techniques
X-ray photoelectron spectra were measured by an ESCALAB250xi
instrument with X-ray monochromatization. GC–MS spectral analy-
ses of isotopically labeled O2 were performed on an Agilent Series
7890A model chromatograph interfaced with an Agilent Series
5975C model mass spectrometer. Powder X-ray diffraction data
were collected with a PANalytical X’Pert Pro diffractometer operat-
ed at 40 kV and 40 mA with CuKa radiation (step size: 0.0178; step
time: 10.34 s). Field-emission scanning electron microscopy (SEM)
observations were performed on a Hitachi S-4800 microscope op-
erated at an accelerating voltage of 5.0 kV. Transmission electron
microscopy (TEM) images were obtained with a TecnaiG2F30 instru-
ment. Infrared spectra (2–4 wt% sample in KBr pellets) were re-
corded with a Bruker Vertex 70v FTIR spectrometer. Raman scatter-
ing spectra were taken with a Renishaw Invia Raman microscope.
The laser light source was the 514.5 nm line radiation.
Experimental Section
Synthesis of a-NiO nanorods, a-NiO nanowires, and a-NiO
nanoplates
All of the chemicals were of analytical grade and were used with-
out further purification. Distilled water was used throughout. In
ChemCatChem 2015, 7, 2370 – 2376
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2375
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
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by light-emitting diode light (lꢀ420 nm) in a flask that was sealed
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Acknowledgements
This work was financially supported by the National Natural Sci-
ence Foundation of China (grant nos. 21173105 and 21172098)
and the ability to improve project of the National Fundamental
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Keywords: nanostructures
photochemistry · water chemistry
·
nickel
·
oxidation
·
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Received: March 24, 2015
Published online on June 26, 2015
ChemCatChem 2015, 7, 2370 – 2376
www.chemcatchem.org
2376
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim