Efficient CoO nanowire array photocatalysts for H2 generationXueying Zhan, Zhenxing Wang, Fengmei Wang, Zhongzhou Cheng, Kai Xu, Qisheng Wang, Muhammad Safdar, and Jun He Citation: Applied Physics Letters 105, 153903 (2014); doi: 10.1063/1.4898681 View online: http://dx.doi.org/10.1063/1.4898681 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/15?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Synthesis, characterization and magnetic properties of ultrafine Co3O4 octahedra AIP Advances 5, 087122 (2015); 10.1063/1.4928494 Surface plasmon resonance enhanced light absorption of Au decorated composition-tunedZnO/ZnxCd1−xSeyTe1−y core/shell nanowires for efficient H2 production Appl. Phys. Lett. 106, 123904 (2015); 10.1063/1.4916397 Mesoporous coupled ZnO/TiO2 photocatalyst nanocomposites for hydrogen generation J. Renewable Sustainable Energy 5, 033118 (2013); 10.1063/1.4808263 Structure and magnetic properties of Co-doped ZnO dilute magnetic semiconductors synthesized viahydrothermal method AIP Conf. Proc. 1461, 87 (2012); 10.1063/1.4736875 Effect of catalyst nanoparticle size on growth direction and morphology of InN nanowires AIP Advances 2, 022150 (2012); 10.1063/1.4729916

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Efficient CoO nanowire array photocatalysts for H2 generation

Xueying Zhan,a) Zhenxing Wang,a) Fengmei Wang, Zhongzhou Cheng, Kai Xu,Qisheng Wang, Muhammad Safdar, and Jun Heb)

National Center for Nanoscience and Technology, Beijing 100190, China

(Received 23 August 2014; accepted 7 October 2014; published online 16 October 2014)

CoO nanowire arrays for efficient water-splitting were fabricated via a facile hydrothermal and sub-

sequent annealing method. The CoO nanowire is composed of assembled CoO nanoparticles and

the particle size can be controlled by annealing temperatures. CoO nanowire array exhibits advan-

tages of easy fabrication, recyclability, and high stability. The origin of the difference of photocata-

lytic activity among CoO bulk, CoO nanowires annealed under different temperatures, can be

contributed to remarkable shift in the position of the band edge due to different CoO particle sizes.

Our finding may provide an avenue in design and fabrication of Co-based nanosturctures for practi-

cal applications. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4898681]

To address the environmental crises and energy shortage

issues, the generation of hydrogen from water utilizing solar

energy is considered as an attractive strategy.1–3 Currently,

the main challenge is how to design and prepare the high-

performance photocatalysts. As we know, practical hydrogen

production requires the photocatalysts have high photocata-

lytic activity, high quantum efficiency, low cost, as well as

significant stability. In addition, because of its simplicity and

low energy cost, bias-free photocatalysts are of particular in-

terest and desired.4–7 In the past few decades, the photocata-

lysts based on semiconductor nanomaterials have been

received a widespread attention due to their large surface-to-

volume ratio and high charge separation efficiency.8–12

Various semiconductors, such as TiO2,13–15 ZnO,16–19

CdS,20,21 and WO3,22,23 have been extensively studied for

water splitting. Cobalt monoxide (CoO) has a relatively nar-

row direct band gap around 2.6 eV.24 Its corresponding

absorption edge locates in the visible light region, which

shows great potential applications in photocatalytic hydrogen

generation. However, only very few previous studies have

been focused on CoO. On one hand, because of the special

requirement necessary to force cobalt in a low valance state,

CoO is hard to be synthesized. On the other hand, its

conduction-band edge locates very closely to hydrogen-

evolution potential; generally, bulk CoO is difficult to be

used in photocatalytic hydrogen production.

Strikingly different from its bulk form, CoO nanostruc-

tures demonstrate strong photocatalytic ability to decompose

water into H2 and O2 using solar energy. Recently, Liao

et al., reported an efficient CoO nanocrystal photocatalyst to

carry out water-splitting with a solar-to-hydrogen efficiency

of around 5%.25 The high photocatalytic activity was

ascribed to the remarkable band edge shift in CoO nanocrys-

tals. However, the CoO nanoparticles suffer from a short

lifetime and become deactivated after about one hour of

reaction, which arises from the aggregation of the CoO nano-

particles. In addition, the nanoparticles are too small to be

recycled, which may increase the cost of the photocatalyst.

For practical applications of CoO nanocrystal photocatalyst

in the future, it is necessary to improve the chemical stability

and further lower the cost.

In this study, we fabricate a CoO nanowire (NW) array

bias-free photocatalyst for water-splitting. CoO NW array is

grown on carbon fiber papers and composed of a large num-

ber of CoO nanoparticles inside an individual NW. In con-

trast to the conventional powder catalysts,25 the NWs grown

on a flexible substrate make it possible to be recycled. More

importantly, due to the CoO NW arrays are fixed on carbon

fiber papers, the aggregation phenomenon are effectively

eliminated during the reaction process thus the stability is

significantly enhanced. After 12 h H2 evolution reaction, the

photocatalytic activity has no obvious degradation. The CoO

NW arrays are synthesized through a facile two-step

approach: first, to form Co(OH)2 NW arrays via a hydrother-

mal method and second to be transformed to CoO NW arrays

by an annealing process. Different annealing temperatures

significantly affect the morphologies and size of the CoO

NWs. The photocatalytic H2 evolution results indicate the

CoO NW arrays annealed at 500 �C have higher rate of H2

evolution than 700 �C, which is attributed to the smaller par-

ticle size. The electrochemical impedance spectroscopy

(EIS) further reveals the CoO NWs annealed at 500 �C have

higher conduction band position which implies the better

hydrogen production capacity. These results provide very

useful guidelines in designing the next-generation Co-based

photocatalyst and are expected to accelerate the pace of the

practical applications of Co-based photocatalyst.

The three-dimensional CoO NW arrays were synthe-

sized on a carbon cloth template by a hydrothermal pro-

cess.26,27 First, the carbon fiber paper was fully soaked in an

ethanol solution of 0.1 M CoCl2 and 0.5 M CO(NH2)2 for

10 min and then calcined under a flow of argon at 450 �C for

4 h. The seed formed during the calcination. After that a seed

layer was formed on the carbon fiber paper. Co(OH)2 NW

arrays were then grown on the seed carbon fiber paper in a

aqueous solution of 0.1 M CoCl2 and 0.5 M CO(NH2)2 at

450 �C for 4 h. The as-prepared NWs were annealed under a

100 sccm argon flow at different temperatures 500 �C and

700 �C. For convenience, we refer CoO NWs annealed at

500 �C and 700 �C to S500 and S700, respectively.

a)X. Zhan and Z. Wang contributed equally to this work.b)Author to whom correspondence should be addressed. Electronic mail:

hej@nanoctr.cn

0003-6951/2014/105(15)/153903/5/$30.00 VC 2014 AIP Publishing LLC105, 153903-1

APPLIED PHYSICS LETTERS 105, 153903 (2014)

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Morphologies and microstructures of the as-prepared sam-

ples were measured by Hitachi S-4800 field-emission scan-

ning electron microscopy (FE-SEM) and FEI Tecnai F20

high-resolution transmission electron microscopy (HRTEM).

The chemical composition of the as-prepared samples was

examined with X-ray photoelectron spectroscopy (XPS,

ESCALAB250Xi). The X-ray diffraction (XRD) patterns of

the samples were measured on an (Philips X’Pert Pro Super)

X-ray powder diffractometer with Cu Ka radiation

(k¼ 1.5418 A). UV-vis diffuse reflectance spectra (DRS)

were measured by a UV-vis spectrometer (Hitachi U-3010).

The Mott-Schottky plots of samples were measured in a

three-electrode electrochemical cell with a Pt counter elec-

trode and an Ag/AgCl reference electrode in 0.1 M Na2SO4

solution. The photocatalytic H2 evolution by water-splitting

experiments was conducted in a 300 ml pyrex reactor under

photoirradiation conditions that was coupled with a gas chro-

matograph (TianmeiGC-7900) equipped with a thermal

conductivity detector. A 300 W xenon lamp with a cutoff fil-

ter (250< k< 780 nm, 751.6 mW/cm2) was used to irradiate

the suspension. In a typical H2-production experiment, the

as-prepared sample was placed at the bottom of a quartz re-

actor containing 100 ml of a mixed aqueous solution with

0.25 M Na2SO3 and 0.35 M Na2S as sacrificial agents.

Before irradiation, the system was bubbled with Argon for

1 h to remove the air and to ensure that the system was under

anaerobic conditions. The generated H2 was analyzed by gas

chromatography.

Fig. 1(a) shows the carbon fiber is around 10 lm in di-

ameter and has a clean surface. It is noted that a dense

Co(OH)2 seed film forms on the surface after a hydrothermal

pretreatment to the carbon fibers (see experimental section),

as shown in the inset of Fig. 1(b). This process of seed deco-

ration ensures the Co(OH)2 NWs can grow along the carbon

fiber with high density, as shown in Fig. 1(c). From the mag-

nified SEM image shown in Fig. 1(d), the length of Co(OH)2

NWs is estimated to be almost above 10 lm and the diameter

is mainly in the range from 100 to 200 nm. The NWs grow

straightly along the radial direction of the carbon fibers and

vertically stands on the surface. After annealing at 500 �C for

4 h, the morphologies of NWs slightly change: the NWs are

mildly bended, as shown in Fig. 1(e). In contrast, while

annealing at 700 �C the collapse of the NWs happens. This is

most likely due to the more severe dehydration reaction at

the higher temperature.

The chemical composition of the NWs was investigated

by x-ray photoelectron spectra XPS. Fig. 2 presents the Co

2p spectra of the as-prepared Co(OH)2 NWs, S500 and

S700. Two strong peaks of Co 2p3/2 at 781.2 eV and Co 2p1/2

at 797.2 eV confirm its Co(OH)2 chemical nature. Compared

with Co 2p spectra of Co(OH)2, corresponding Co 2p peaks

of CoO have a redshift of �0.6 eV, which are well consistent

with the previous reports.28 The difference arises from dif-

ferent chemical conditions around Co atoms. It is worth to

note that S500 and S700 have similar binding energies,

which implies that S500 and S700 have the same chemical

composition. The two peaks at �6 eV above the main peaks

Co 2p3/2 and Co 2p1/2 are the satellites peaks, which is due to

the paramagnetic and high-spin Co(II) electronic configura-

tion in CoO and Co(OH)2.29 The XPS results indicate the

production after annealing of Co(OH)2 are CoO NWs.

The microstructures of as-prepared NWs were examined

by HRTEM. Fig. 3(a) depicts one typical single Co(OH)2

NW has a smooth surface and uniform diameter of �150 nm.

FIG. 1. (a) and (b) SEM images of car-

bon fiber before and after Co(OH)2

seed decoration by hydrothermal

method. Inset: zoomed in a single car-

bon fiber surface. (c) SEM image of as

prepared Co(OH)2 NWs on carbon

fibers. (d) A magnified SEM image of

Co(OH)2 NWs on a single carbon fiber.

(e) and (f) SEM images of obtained

CoO NWs after annealing of Co(OH)2

NWs at 500 �C and 700 �C.

FIG. 2. X-ray photoelectron spectra of Co(OH)2 NWs, S500, and S700.

153903-2 Zhan et al. Appl. Phys. Lett. 105, 153903 (2014)

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From the inset of Fig. 3(a), the clear lattice fringe reveals

that the Co(OH)2 NW grows along [100] direction (JCPDF

74-1057). Once Co(OH)2 NWs were annealed at high tem-

perature, the original single crystalline structure of Co(OH)2

was destroyed and the smooth single crystalline Co(OH)2

NW was converted into rough polycrystalline CoO NW, as

shown in Figs. 3(b) and 3(c). Every individual CoO NW is

composed of a large number of CoO nanoparticles. Previous

study reported micropowder and bulk CoO could not photo-

catalyze water-splitting, however, CoO nanocrystals had the

ability to do this.25 So we believe this kind of structure char-

acteristic of CoO NW will be very beneficial for hydrogen

evolution reaction. From the HRTEM images, left bottom of

Fig. 3(b) and the inset of Fig. 3(c), the nanoparticles have

obvious CoO lattice fringe: the plane distances of �2.16 A,

corresponding to (200) plane (JCPDF 78-0431). XRD pat-

terns (Fig. 3(d)) were further used to confirm the CoO chemi-

cal nature. In addition, there is no other crystal phase but

CoO, which indicates our CoO NW arrays have high purity.

Fig. 3(e) shows the histograms of the size distribution of

CoO nanoparticles inside the NWs. For S500, the most com-

mon particle size is �34 nm, much smaller than �65 nm

obtained at 700 �C. This predicts that S500 have better

behavior in hydrogen generation than S700.

Hydrogen generation experiments were carried out in an

aqueous solution containing Na2S and Na2SO3 under the

photoirradiation conditions without external bias. Fig. 4

shows the photocatalytic H2 production activity of S500 and

S700. As shown in Fig. 4(a), the amount of H2 generated

based on S500 reaches 0.325 mmol/g in 4 h, which is over

2 times than that of S700 (0.141 mmol/g). Moreover, as

shown in Fig. 4(b), the rate of H2 evolution based on S500 is

around 17.76 lmol h�1 g�1. This is almost 2 times than that

of S700 (9.36 lmol h�1 g�1). Worth noting is that CoO NWs

annealed at different temperatures have the same lattice

plane signals from XRD results but different particle sizes in

HRTEM images. Therefore, we believe that the size of nano-

particles which form individual NW is the key factor to influ-

ence the production of hydrogen. Moreover, to evaluate the

stability of S500, testing cycles were displayed in Fig. 4(c).

Through three continuous testing cycles during 12 h, S500

did not exhibit obvious degradation, indicating its high H2

production capability. The stability is much better than the

CoO nanocrystals,25 which benefits from the carbon fiber

template. It should be noted that when the electron scav-

engers were employed in reaction system, the O2 also can be

produced.

FIG. 3. (a) and (c) TEM images of as-

prepared Co(OH)2 NWs and S700.

Insets: relevant HRTEM images. (b)

Left top: TEM image of a single CoO

NW annealed at 500 �C; left bottom:

HRTEM image of CoO nanoparticles

inside the CoO NW. Right: magnified

TEM image. The partial nanoparticles

are indicated by the red circles. (d)

XRD patterns of CoO NWs. (e)

Histograms of CoO nanoparticle sizes,

obtained from a lot of TEM images.

FIG. 4. (a) Photocatalytic H2-production activity of a carbon fiber paper,

S500, and S700 during 4 h light illumination. (b) The rates of H2 evolution

of S500 and S700. (c) Time course of photocatalytic H2 production activity

of S500. For every cycle H2 inside was removed after every 4 h.

153903-3 Zhan et al. Appl. Phys. Lett. 105, 153903 (2014)

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In order to investigate the overall water-splitting per-

formance of our samples, we have carried out overall water-

splitting experiments without any sacrificial reagents. For

our CoO nanowires, we do not observe the production of H2

or O2 without sacrificial reagents. We believe this is due to

the following two reasons: (1) The annealing process may

introduce defects, vacancies, and trap states in CoO nano-

crystals. These recombination centers would deteriorate the

separation efficiency of electron-hole, thus reduce the

capacity to generate H2 and O2 simultaneously. (2) Our CoO

nanowires actually are composed of large number of CoO

nanoparticles in one-dimensional direction. The transport of

photogenerated electrons through the nanoparticle network

by particle-to-particle hopping method will generate undesir-

able loss of electrons, which limits the overall water-splitting

performances. In order to realize high-performance overall

water-splitting, the crystal quality should be improved and

the grain boundaries should be depressed.

The photocatalytic H2 production activity of catalysts is

affected by two crucial factors: the light absorption capabil-

ity of materials and the position of conduction band.30 To

investigate the light absorption capability of CoO NWs

annealed at different temperatures, we measured UV-vis dif-

fuse reflectance spectra of each sample. As shown in the

inset of Fig. 5(a), S500 shows a similar absorption range

with that of S700. The band gaps of the samples were

obtained by fitting the optical transition at the absorption

edges using the Kubelka-Munk function described by the

equation F¼ (1�R)2/2R, where R is reflectance. R can be

calculated from the inset of Fig. 5(a) by the equation

Abs¼�ln(1�R). From Fig. 5(a), it can be seen that the

band gaps of both S500 and S700 are very similar and close

to 2.6 eV. Furthermore, we measured the electrochemical

performance of CoO nanowires grown on carbon fiber using

a three electrode set-up with 0.1 M Na2SO4 as electrolyte so-

lution at 0.0 V (vs SCE), as shown in Fig. 5(b). It is clear that

both samples have good visible light response. Based on

above results, the light absorption capability should not be

the reason resulting in the difference of photocatalytic ability

between the two samples.

To further uncover the difference in photocatalytic ac-

tivity CoO NWs annealed under different temperatures, the

position of conduction band was studied in detail. Fig. 5(c)

shows the Mott-Schottky plots of both S500 and S700, C�2

as a function of electrochemical potentials. The negative

slopes of two curves in Fig. 5(c) show CoO has a typical

p-type semiconductor nature, which is consistent with the

reported literature.31 The flat-band potential, which is the

difference between the Fermi level and water reduction

potential, can be deduced from the intercepts of the Mott-

Schottky plots. In Fig. 5(c), the flat-band potentials, 1.13 V

for S500 and 1.31 V for S700, were obtained. Generally, the

Fermi level was assumed to be 0.3 eV above the valence

band of CoO.25,32 The band-edge position of CoO bulk can

be found in a literature.24 Based on band position of CoO

bulk, the band gaps obtained from Fig. 5(a), the flat-band

potential values gotten from Fig. 5(c) and the assumption of

about Fermi level, the energy diagram can be drawn in Fig.

5(d). The conduction band edge of CoO bulk is very close to

the hydrogen evolution potential which means CoO bulk

have almost no photocatalytic H2 production activity. This is

the reason why micropowder and bulk CoO cannot generate

H2 by photocatalytic reaction.25 But the conduction band

edges of both S500 and S700 rise above the hydrogen evolu-

tion potential. So S500 and S700 exhibit good hydrogen pro-

duction performance in Fig. 4(a). In particular, S500 has

higher conduction band position implying better hydrogen

production capacity in agreement with the results in Fig. 4.

According to previous calculations in a literature,33 the con-

duction band edges of nanomaterials rise with the nanopar-

ticles size becoming smaller. It can be concluded that the

nanoparticles size of NWs further affect the photocatalytic

H2 production activity.

In conclusion, efficient CoO NW array photocatalysts

for water-splitting grown on carbon fiber papers were fabri-

cated via a facile hydrothermal method and a consequent

annealing process. Every individual CoO NW is composed

of assembled CoO nanoparticles. The annealing tempera-

tures strongly affect the nanoparticle size. TEM analysis

indicates the particle size in S500 mainly distribute around

�34 nm, much smaller than �65 nm, obtained from S700.

As a result, the amount of H2 generated based on S500

reaches 0.325 mmol/g in 4 h, over 2 times than S700.

Meanwhile, the rate of H2 evolution based on S500 is also

2 times than S700. Compared with CoO nanocrystals, our

CoO NW arrays on carbon fiber papers show higher chemi-

cal stability. Further, through EIS, the origin of the differ-

ence of photocatalytic activity among CoO bulk, S500 and

S700, is contributed to remarkable shift in the position of the

band edge. These results provide useful insights in the design

and fabrication of unique, recyclable, highly stable, and low

cost Co-based photocatalysts for practical applications in the

future.

This work at National Center for Nanoscience and

Technology was supported by 973 Program of the Ministry

of Science and Technology of China (No. 2012CB934103),

FIG. 5. (a) (Fh�)2 as a function of photon energy (h�) of S500 and S700.

Inset: UV-vis diffuse reflectance spectra of carbon fiber paper, S500, and

S700. (b) Amperometric j-t curves of S500 and S700, at a zero versus SCE

voltage with the visible light on/off. Light intensity: 377 mW/cm2. (c) Mott-

Schottky plots of S500 and S700. (d) The band-edge positions of CoO bulk,

S500, and S700.

153903-4 Zhan et al. Appl. Phys. Lett. 105, 153903 (2014)

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the 100-Talents Program of the Chinese Academy of

Sciences (No. Y1172911ZX), the National Natural Science

Foundation of China (No. 21373065), and Beijing Natural

Science Foundation (No. 2144059).

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