Disordered Co1.28Mn1.71O4 as a visible-light-responsive photocatalyst for hydrogen evolution

Article abstract of DOI:10.1002/chem.201203683

Catching the sunlight: The disordered Co_1.28Mn _1.71O₄ was synthesized by a facile reduction method in the presence of amorphous MnO₂ precursors and Co²⁺. It exhibits relatively high hydrogen-production activity under visible-light irradiation (see figure). The larger number of active sites in the bulk catalyst endow nanoparticles with improved water-reduction activity. This is the first demonstration of disordered ternary oxides for visible-light-driven hydrogen evolution in the absence of a cocatalyst. Copyright

Full text of DOI:10.1002/chem.201203683

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DOI: 10.1002/chem.201203683
Disordered Co1.28Mn1.71O4 as a Visible-Light-Responsive Photocatalyst for
Hydrogen Evolution
Zu Peng Chen, Jun Xing, Hai Bo Jiang, and Hua Gui Yang*[a]
Utilizing semiconductor photocatalysts and solar energy
for hydrogen evolution has attracted intense worldwide in-
terest because it is an ideal and renewable method to supply
recyclable hydrogen, which provides a potential solution to
counteract the energy problem in the near future. Over the
past 40 years, plenty of photocatalysts exhibiting high photo-
catalytic activity for hydrogen production from water have
been developed.[1] However, most of these photocatalysts
have a wide band gap (>3.0 eV) and only work under UV-
light irradiation. Therefore, it is essential to search for more
suitable semiconductors as visible-light-responsive photo-
Herein,
we
successfully
developed
disordered
Co1.28Mn1.71O4, which absorbs the full visible-light spectrum
and demonstrates a hydrogen evolution capability even in
the absence of any cocatalyst. The structural disorder con-
tributes a lot to the radical change in the improved hydro-
gen evolution properties, which results in mid gap states and
yields a band tail in the band gap states, merging with the
valence band. To the best of our knowledge, “disorder-engi-
neering” of ternary oxides (MMO) as well as their photoca-
talytic activity has not been explored so far. The synthetic
strategy provided in this work may potentially be extendable
to other materials with a fast electron–hole recombination
rate to boost their catalytic activity. The CoxMn3ÀxO4 pow-
ders were prepared through a wet-chemical method under
ambient conditions, modifying a procedure reported by
Chen et al. for preparing nanocrystalline spinel-type com-
ACHTUNGTRENNUNG
catalysts.[2] Recently, Mao and co-workers synthesized black
TiO2 through hydrogenation, thereby turning the surface
into a highly defective amorphous layer, which leaded to
greatly enhanced photocatalytic properties.[3] It is the highly
defective amorphous layer causing an up shift of the valence
band (VB) edge of TiO2, which narrows the band gap and
eventually makes the visible-light-responsive TiO2. This in-
teresting discovery will certainly open a door to develop
other photocatalysts for visible-light-driven hydrogen evolu-
tion.
[9]
pounds MxMn3ÀxO4 (see the Supporting Information for
the detailed synthetic procedure). Sequential X-ray fluores-
cence spectrometry (XRF) and indicatively coupled plasma
atomic emission spectroscopy (ICP-AES) (Table S1 in the
Supporting Information), confirm that the product has the
chemical composition Co1.28Mn1.71O4. From the elemental
maps of Co, Mn, and O shown in Figure S1 in the Support-
ing Information, it can be seen that all elements are evenly
distributed in the disordered Co1.28Mn1.71O4 photocatalyst.
Figure 1a shows a digital camera image of a monolith of
disordered dark black Co1.28Mn1.71O4, which indicates its visi-
ble-light responsivity. According to the experimental data
derived from an X-ray diffraction (XRD) pattern (Fig-
ure 1b), it can be deduced that the synthesized sample ex-
hibits a typical pattern of amorphous phase. A broadened
peak belonging to amorphous materials is showed around
238 and there is no typical diffraction peak of Co1.28Mn1.71O4,
this structural characteristic can also be confirmed by high-
resolution transmission electron microscopy (HRTEM) and
corresponding selected area electron diffraction (SAED)
(Figure 1c). A representative HRTEM image presents that
the prepared Co1.28Mn1.71O4 features a disordered morpholo-
gy and structure, and no detectable lattice fringes or crystal-
like regions can be observed. Moreover, the SAED pattern
reveals a full halo ring without any obvious diffraction spots.
As a comparison, the XRD pattern of the prepared crystal-
line CoMn2O4 shown in Figure 1b matches well with the tet-
ragonal structure of the spinel-phase CoMn2O4 (JCPDS
Card No. 77-0471). Meanwhile, Raman spectra were also ap-
plied to examine structural differences (Figure 1d) of these
In general, most of the existing efficient photocatalysts
consist of transition-metal cations with the electronic config-
uration d0 (e.g., Ti4+ and Ta5+) or post-transition-metal cat-
ACHTUNGTRENNUNG
ions with d10 configuration (e.g., In3+ and Sn4+),[4] whereas
other transition-metal ions have been scarcely studied.
Cobalt manganese ternary oxides (e.g., CoMn2O4) have at-
tracted a great deal of research interest because of their ver-
satile applications in various fields, including catalysts,[5] Li-
ion batteries,[6] and magnetic materials.[7] Meanwhile, the
special material has the advantages of high abundance, low
cost, and low toxicity.[8] Despite of the above-mentioned ad-
vantages and the unique structural characteristics, no re-
search has been reported on photocatalytic hydrogen evolu-
tion properties of CoMn2O4.
[a] Z. P. Chen, Dr. J. Xing, Dr. H. B. Jiang, Prof. Dr. H. G. Yang
Key Laboratory for Ultrafine Materials of Ministry of Education
School of Materials Science and Engineering
East China University of Science & Technology
130 Meilong Road, Shanghai, 200237 (P. R. China)
Fax : (+86)21-64252127
E-mail: hgyang@ecust.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203683.
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ferred into Co1.28Mn1.71O4 (Fig-
ure S2b in the Supporting In-
formation).
The
average
particle size of these two sam-
ples is around 50 nm, whereas
the crystalline CoMn2O4 dem-
onstrates a cube-like morpholo-
gy with a mean edge length
around 70 nm (Figure S2c in
the Supporting Information).
Meanwhile, the average size of
N-TiO2 particles is comparable
(Figure S2d in the Supporting
Information).
The surface chemical bonds
of the disordered Co1.28Mn1.71O4
were investigated by X-ray pho-
toelectron spectroscopy (XPS)
(Figure 2). Figure 2a shows the
survey spectra of the as-synthe-
sized disordered Co1.28Mn1.71O4.
The Elements Co, Mn, O, and
adventitious C coexist in the
obtained Co1.28Mn1.71O4 sample.
As indicated in Figure 2b, the
Co 2p spectra shows two well-
separated branches of 2p1/2 at
Figure 1. a) Digital camera image of the disordered Co1.28Mn1.71O4. XRD patterns (b) and Raman spectra (d)
of the crystalline and disordered cobalt manganese ternary oxides (lex =514 nm). c) HRTEM image and the
corresponding SAED of the disordered Co1.28Mn1.71O4.
disordered and crystalline samples. In terms of the crystal-
line CoMn2O4, there were six Raman peaks with frequencies
at n˜ =178.9, 317.1, 369.0, 488.9,
780.9 eV with a satellite around 785.9 eV, and 2p3/2 at
797.3 eV with a satellite around 803.2 eV. The two intensive
577.1, and 660.9 cmÀ1, respec-
tively, which agree well with the
pure CoMn2O4 spinel oxide re-
ported by other researchers.[10]
A
somewhat broader and
weaker Raman-active mode
was observed in the disordered
sample, which could be attribut-
ed to the local lattice imperfec-
tions and lower crystallinity.[11]
These indicate that both of the
disordered and crystalline sam-
ples are in similar local geomet-
ric construction.
The typical scanning electron
microscope (SEM) images of
the disordered and crystalline
cobalt
manganese
ternary
oxides, the amorphous MnO2
precursor, as well as nitrogen-
doped TiO2 (N-TiO2) are shown
in Figure S2 in the Supporting
Information. The morphology
of the amorphous MnO2 precur-
sor (Figure S2a in the Support-
ing Information) was well pre-
Figure 2. XPS spectra of the as-prepared disordered Co1.28Mn1.71O4: a) survey spectra of the sample, b) Co 2p,
served even after it was trans- c) Mn 2p, and d) O 1s; the red curves are the fitted spectra.
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A Visible-Light Responsive Photocatalyst
COMMUNICATION
peaks (780.9 and 797.3 eV) and the corresponding satellites
are typical for divalent cobalt.[12] The Mn 2p XPS spectra
(shown in Figure 2c), which were also investigated, exhibit-
ing Mn 2p3/2 =642.5 eV and Mn 2p1/2 =654.0 eV. Specifically,
the Mn 2p3/2 XPS reveals the Mn3+ peak at 641.9 eV and the
[8,13]
broad peak at 643.9 eV can be assigned to Mn4+
.
Mean-
while, the weak signals at about 640.4 and 647.5 eV contrib-
[8,14]
ute to Mn2+
.
Therefore, the oxidation state of Mn in the
disordered Co1.28Mn1.71O4 definitely coexisted as mixed
phase. The O 1s XPS spectra of the prepared sample shown
in Figure 2d, can be resolved into two distinguishable peaks
at 530.0 and 531.1 eV, which can be assigned to the lattice
oxide oxygen of the metal oxides and a surface-absorbed
À
oxygen containing species (possibly metal OH or water
molecules).[3,9,15] Furthermore, the density of states (DOS)
of the valence band of these two samples were measured by
valence-band XPS spectra (Figure S3 in the Supporting In-
formation). The crystalline CoMn2O4 reveals the edge of the
maximum energy of the valence band at about 0.4 eV,
whereas that of the disordered one is at about À0.2 eV. This
could be contributed to the extension of VB maximum to
the band tail states, resulting from the disordered es-
sence.[3,16]
Figure 3a shows the UV/Vis absorption spectra of disor-
dered Co1.28Mn1.71O4 and crystalline CoMn2O4. The absorp-
tion spectra indicate that the crystalline CoMn2O4 exhibits a
high photon absorbance band up to l=1100 nm though it
declines from l=700 nm. The band gap was estimated to be
approximately 1.46 eV, according to the Eg =1240/l (the ab-
sorption edge is l=850 nm). Interestingly, the disordered
Co1.28Mn1.71O4 absorbs not only the full visible-light spec-
trum, but also a part of the near-infrared region with barely
any decline in the absorbance, which is consistent with the
dark black color of the sample. This means that the disor-
dered structure of Co1.28Mn1.71O4 could capture low-energy
photons and then enhance its light utilization.
The presence of defect levels is revealed by photolumines-
cence spectra of the disordered and crystalline samples (Fig-
ure 3b). It has been suggested that the PL signals are
mainly originating from the surface states resulting from the
defects on the surface of the nanoparticles. It can be ob-
served that the PL signal intensity around l=430 nm for
disordered Co1.28Mn1.71O4 is much greater than that of crys-
talline CoMn2O4, which indicates that there exist more de-
fects in the disordered Co1.28Mn1.71O4. This is consistent with
the structural features of these two samples.
Figure 3. a) UV/Vis absorption spectra and b) room temperature photolu-
minescence (PL) spectra (lex =290 nm) of disordered Co1.28Mn1.17O4 and
crystalline CoMn2O4 .
largely inactive for the reaction, even when deposited with
the Pt cocatalyst. This interesting finding is not consistent
with the conventional principle that the highly crystalline
structure of inorganic photocatalysts is advantageous for
photocatalytic activity. However, other factors, including the
material microstructures, the specific surface area, and the
composition also significantly determine the catalytic per-
formance of photocatalysts.[17] The disordered Co1.28Mn1.71O4
system produced a total amount of 40.7 mmol H2 with
100 mg photocatalyst during the course of 8 h visible-light ir-
radiation without noticeable catalyst deactivation. This high
activity is considered to be a result of the numerous disor-
dered structures in the amorphous phase, which provide
trapping sites for excited photocharges and reduce the op-
portunity of rapid recombination, thus promoting electron
transfer and photocatalytic reactions. Because of the vast
defects and unsaturated bonds in the disordered
Co1.28Mn1.71O4, there exist an increasing number of active
sites, so that electrons could produce H2 in these sites quick-
ly. At the same time, the disordered structure possesses a
much higher specific surface area than that of a crystalline
sample (as shown in Figure S4 in the Supporting Informa-
The disordered Co1.28Mn1.71O4 exhibits substantial photo-
catalytic activity for hydrogen evolution under visible-light
irradiation (l>420 nm). Figure 4a shows the time course of
hydrogen evolution by using differently modified disordered
Co1.28Mn1.71O4 and crystalline CoMn2O4, coexisting with
methanol as a sacrificial reagent. Co1.28Mn1.71O4 shows a
steady H2 evolution even in the absence of any cocatalyst.
We deposited 1% wt. Pt nanoparticles onto the surface of
disordered Co1.28Mn1.71O4 and crystalline CoMn2O4 by a
photo
ACHTUNGTRENNUNGreduction method. An improved photocatalytic activi-
ty was achieved by Co1.28Mn1.71O4, whereas CoMn2O4 was
Chem. Eur. J. 2013, 19, 4123 – 4127
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H. G. Yang et al.
To further investigate the electron behavior in the disor-
dered Co1.28Mn1.71O4, the time-resolved fluorescence decay
spectra of both disordered Co1.28Mn1.71O4 and crystalline
CoMn2O4 were performed, and the fitting of the decay spec-
trum are given in Table S2 in the Supporting Information,
showing three radiative lifetimes with different percentages.
In detail, the shortest lifetime of t=0.117 ns in the crystal-
line CoMn2O4 increases up to t=0.158 ns in the disordered
Co1.28Mn1.71O4, and its percentage increases from 80.04 to
83.75%. Both the shortest lifetime and the percentage of
charge carriers of these two samples increase from t=
0.117 ns and 80.04% in the crystalline CoMn2O4 to t=
0.158 ns and 83.75% in the disordered Co1.28Mn1.71O4. The
medium lifetime of t=1.22 ns in the crystalline CoMn2O4
increases up to t=1.282 ns in the disordered sample, though
its percentage decreases from 15.31 to 13.17%. These results
mean that the radiative lifetime of around 97% of the
charge carriers has been increased by simply disordered en-
gineering, which plays an important role in improving the
probability of their involvement in photocatalytic reactions
before recombination and thus enhances its photocatalytic
activity.
The action spectrum for H2 evolution from water contain-
ing 20 vol% methanol as an electron donor by 1 wt% Pt-de-
posited disordered Co1.28Mn1.71O4 as a function of the wave-
length of the incident light is shown in Figure S5 in the Sup-
porting Information. This result indicated that visible light
up to l=550 nm can be used. Furthermore, some blank
tests showed that indeed no reaction took place in the dark
or without methanol as electron donor.
In conclusion, this is the first demonstration of disordered
ternary oxides for hydrogen evolution under visible-light ir-
radiation even in the absence of a cocatalyst. Although this
new photocatalyst requires further improvement in its effi-
ciency, the present study provides new insights into the syn-
thesis of more suitable photocatalysts responding to visible
light and new ideas for boosting the catalytic performance.
Furthermore, the facile and inexpensive preparation strategy
of the disordered Co1.28Mn1.71O4 described here enables the
possibility of large-scale application of hydrogen evolution
from water under solar light.
Figure 4. a) Time courses of H2 production from water containing
20 vol% methanol as an electron donor under visible-light irradiation
(l>420 nm) by 100 mg 1 wt% Pt-deposited disordered Co1.28Mn1.71O4
(red dot) crystalline CoMn2O4 (green star), and N-TiO2 (purple star), as
well as unmodified Co1.28Mn1.71O4 (blue triangle). b) Cycling measure-
ments of H2 generation from water with 1 wt% Pt-loaded disordered
Co1.28Mn1.71O4 in four consecutive runs with intermittent evacuation
every 5 h under visible-light irradiation.
tion), which contributes to better photocatalytic proper-
ties.[18] In addition, the use of amorphous MnO2 as precursor
results in shorter diffusion distances and higher surface
areas, which are useful for the solid state diffusion and the
interfacial contact. For comparison purpose, the well-known
N-TiO2 that is visible-light sensitive for H2 evolution was
prepared[19] and the hydrogen evolution rate was tested
under the same experimental conditions. As illustrated in
Figure 4a, it could only produce 2.7 mmol H2 in 8 h. To eval-
uate the stability of the disordered sample, we also carried
out four consecutive runs of the photocatalytic reaction with
intermittent evacuation every 5 h under visible-light irradia-
tion. A little deterioration of the photocatalytic activity
occurs after the first run and it eventually tends towards
steady (Figure 4b). It was found that the disordered
Co1.28Mn1.71O4 exceeds N-TiO2 overwhelmingly, as soon as
light irradiation commenced.
Acknowledgements
This work was financially supported by a Major Basic Research Program
of Science and Technology Commission of Shanghai Municipality
(10JC1403200), a Shuguang Talents Program of Education Commission
of Shanghai Municipality (09SG27), National Natural Science Foundation
of China (20973059, 91022023, 21076076), Fundamental Research Funds
for the Central Universities (WJ0913001), a Program for Professor of
Special Appointment (Eastern Scholar) at Shanghai Institutions of
Higher Learning and Program for New Century Excellent Talents in Uni-
versity (NCET-09-0347). H.G.Y. conceived the project and contributed to
the design of the experiments, analysis of the data, and revising the
paper. Z.P.C. designed and carried out the experiments, analyzed the
data, and wrote the paper. J.X. assisted with some of the experiments
and was involved in the interpretation of the data. H.B.J. discussed the
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A Visible-Light Responsive Photocatalyst
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Received: October 16, 2012
Published online: February 20, 2013
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