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Cite this: DOI: 10.1039/c2jm35010f

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Controlled synthesis of Ag2O microcrystals with facet-dependentphotocatalytic activities†

Gang Wang,a Xiangchao Ma,b Baibiao Huang,*a Hefeng Cheng,a Zeyan Wang,a Jie Zhan,*a Xiaoyan Qin,a

Xiaoyang Zhanga and Ying Daib

Received 27th July 2012, Accepted 23rd August 2012

DOI: 10.1039/c2jm35010f

Ag2O microcrystals with different morphologies have been successfully synthesized by using various

complexing agents. To achieve kinetic control of the growth of the Ag2O microcrystals, [Ag(NH3)2]+

complexing ions are required to restrict the release rate of silver ions before adding NaOH solution.

The complexing anions play an important role in the growth process of the Ag2O microcrystals. This

kinetic control leads to five morphologies of Ag2O microcrystals (cubic, octahedral, rhombic

dodecahedra, polyhedra with 18 faces and rhombicuboctahedral), which exhibit facet-dependent

photocatalytic activity for the degradation of methyl orange (MO) under visible light irradiation. The

cubic Ag@Ag2O photocatalyst with exposed {100} facets showed the greatest activity of all the other

morphologies of the photocatalysts. The mechanism of dramatic enhancement of the photocatalytic

activity of Ag@Ag2O with exposed {100} facets was discussed in detail from three aspects, including

the highest surface energy of the {100} facet, the larger difference value between the weighted average

of the effective mass of holes and electrons along the [100] direction, and the suitable redox potentials of

the (100) surface.

1. Introduction

Due to the considerable attention paid to surface science, inor-

ganic crystals with highly reactive surfaces have long been

studied.1–12 Usually, the properties of materials are strongly

dependent on their surface morphologies.13–16 Different facets of

a single crystal exhibit distinctive chemical and physical prop-

erties. Therefore, methodological syntheses of exposed active

facets have always been important in materials chemistry. The

general concept of shape-dependent catalytic activity is that

crystals with high-energy planes generally exhibit much higher

catalytic activities than those with the most common, stable

facets. Nevertheless, the problem is that surfaces with high

reactivity diminish rapidly during the crystal growth process in

order to minimize surface energy. A typical example is anatase

titanium dioxide (TiO2) single crystals. According to the Wulff

construction, more than 94% of titanium dioxide crystals are

dominated by the thermodynamically stable {101} facets rather

than the high-energy {001} facets,16,17 and the highly active facets

({001}) of TiO2 crystals exhibit significantly greater photo-

catalytic properties than other facets do. Yang et al.18 have

successfully achieved high percentages of {001} reactive facets in

aState Key Laboratory of Crystal Materials, Shandong University, Jinan250100, P. R. China. E-mail: bbhuang@sdu.edu.cn; zhanjiesdu@yahoo.com.cn; Fax: +86 0531 88365969; Tel: +86 0531 88365969bSchool of Physics, Shandong University, Jinan 250100, P. R. China

† Electronic supplementary information (ESI) available: Fig. S1–S6. SeeDOI: 10.1039/c2jm35010f

This journal is ª The Royal Society of Chemistry 2012

TiO2 crystals, in which F� ions could efficiently depress the

surface energy of the TiO2 {001} facets during growth and

expose them. However, the controlled synthesis of other inor-

ganic materials with exposed reactive facets is still a challenge,

which needs further investigation.

Silver(I) oxide (Ag2O) is a brown powder with a narrow band

gap of 1.3 eV,19 and has been widely used in many industrial

products, for example, as a mild oxidizing agent, a buffing

compound, as colorants, in silver-oxide batteries, and as catalysts

for alkane activation and epoxidation.20,21 However, silver oxides

are seldom used as photocatalysts because of their unstable and

photo-sensitive properties under light irradiation. Recently,

Wang et al.22 reported a plasmonic photocatalyst Ag@AgCl,

which turned out to be stable and shows superior visible light

photocatalytic activity due to the plasmon resonance of Ag

particles on the surface of the AgCl particles. Ag2O nanoparticles

as active photocatalysts have been reported by Yu’s group,23

which could exhibit self-stability once the Ag2O–Ag structure

was formed during the photo-degradation process. However, to

the best of our knowledge, few literatures have reported Ag2O

crystals with controllable morphologies,24,25 or the morphology–

activity relationship of Ag@Ag2O. Therefore, it is of great

necessity to study the correlation between the active facets of

Ag2O microcrystals and their photocatalytic efficiencies.

Herein, we report a simple method to synthesize Ag2O

microcrystals with different morphologies. By introducing

various complexing agents, we could control the exposed facets

of Ag2O microcrystals. Meanwhile, ammonium ions in the

J. Mater. Chem.

Fig. 1 SEM images of the Ag2O microcrystals synthesized with various

morphologies: (A) cubes with exposed {100} facets, (B) octahedra with

exposed {111} facets, (C) rhombic dodecahedra with exposed {110}

facets, (D) polyhedra with 18 faces with exposed {100} and {110} facets,

(E) rhombicuboctahedra with exposed {100}, {110} and {111} facets.

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complexing agents are able to complex with Ag+ so as to release

silver ions slowly in the presence of hydroxyl ions. The anion in

the complexing agent solutions could adsorb onto the surface of

the Ag2O microcrystals, and thus adjust their growth conditions.

Different Ag2O microcrystals, such as cubic, octahedral,

rhombic dodecahedral, rhombicuboctahedral and polyhedra

with 18 faces, were facilely obtained by controlling the type or

concentration of complexing agents. The results of the photo-

catalytic process show that Ag@Ag2O exhibits facet-dependent

photocatalytic activities for the degradation of methyl orange

(MO) under visible light irradiation.

2. Experimental

2.1 Synthesis

Synthesis of Ag2O microcrystals. Ammonium acetate

(CH3COONH4), ammonium nitrate (NH4NO3), diammonium

phosphate ((NH4)2HPO4), ammonia, silver nitrate (AgNO3) and

sodium hydroxide (NaOH) were analytical grade and used

without further purification. CH3COONH4, NH4NO3,

(NH4)2HPO4, and NH3$H2O were dissolved in deionized water

to give different concentrations such as 0.01, 0.02, 0.03, 0.04, 0.05

and 0.5 M. The concentrations of AgNO3 solutions ranged from

0.01 to 0.5M. In a typical procedure, 2.5–5 mL complexing agent

solution was mixed with 2.5 mL AgNO3 solution. A certain

amount of 2 M NaOH was added to the mixed solution drop by

drop after stirring for 10 min. The suspension was placed in

darkness for 12 h, and the black precipitate was collected by

centrifuging at 5000 rpm, washed with water and ethanol several

times, and dried at 60 �C for 12 h in an oven.

Synthesis of Ag@Ag2O photocatalysts. Ag@Ag2O structured

photocatalysts with different morphologies were obtained

according to the procedure reported by Wang et al.22 0.2 g of

sample was dispersed into 100 mL methyl orange solution. The

suspension was constantly stirred under the irradiation of visible

light (l > 420 nm) for 10 min. The as-prepared Ag@Ag2O was

washed with deionized water and ethanol several times, and dried

in the oven at 60 �C overnight.

2.2 Characterization

X-ray diffraction (XRD) patterns were recorded on a Bruker

AXSD8 Advance powder diffractometer (CuKaX-ray tube, l¼0.154056 nm). The morphologies of the samples were obtained

using scanning electron microscopy (SEM, Hitachi S-4800).

2.3 Photocatalysis

The photocatalytic activities of the as-prepared samples were

evaluated by the photo-decomposition of MO solution under

visible light irradiation at room temperature. The visible light

source was a 300WXe arc lamp (PLS-SXE300, Beijing Trusttech

Co. Ltd) equipped with an ultraviolet cutoff filter. The vertical

distance between the suspension surface and the light source was

set at about 10 cm. In a typical photocatalytic experiment, 0.1 g

of sample was dispersed into 100 mL MO (20 mg L�1) solution

with constant stirring. The suspension was stirred in the dark

until reaching the adsorption–desorption equilibrium before

J. Mater. Chem.

irradiation. About 5 mL suspension was taken out for detection

after centrifugation. Concentrations of MO were analyzed at

464 nm as a function of irradiation time, using UV-vis spec-

troscopy (UV-7502 PC, Xinmao, Shanghai).

3. Results and discussion

3.1 Growth mechanism

Kinetic control is an effective approach to dominate the crystal

growth process of inorganic materials. Ammonium ions could

easily complex with the silver ions to form [Ag(NH3)2]+ com-

plexing ions, which could release silver ions slowly in the pres-

ence of OH� ions. The formed AgOH quickly decomposed to

form the Ag2O nucleus. This process is quite important to slow

down the growth rate of Ag2O microcrystals and create

favourable conditions for anions to adsorb onto the given facets

of Ag2O particles. Different morphologies of Ag2Omicrocrystals

could then be obtained when we introduced various anions into

the solutions.

Firstly, Ag2O microcrystals with a single exposed facet were

synthesized, including {100} cubes, {110} rhombic dodecahedra

and {111} octahedra. Cubic and octahedral crystals (Fig. 1A and

B) were obtained according to previous reports, which used

0.01 MNH4NO3 and 0.5 MNH3$H2O as the complexing agents,

respectively.19,21 Ag2O rhombic dodecahedra with exposed {110}

facets were synthesized by using a phosphate radical as the anion

of the complexing agent. In the experiment, a yellow precipitate

formed immediately after mixing the AgNO3 and (NH4)2HPO4

solutions, which could be Ag3PO4. The yellow Ag3PO4 slowly

This journal is ª The Royal Society of Chemistry 2012

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dissolved when NaOH solution was added dropwise to the

suspension. Excess NaOH solution led to the formation of brown

Ag2O nuclei deposits. Ag2O microcrystals with two exposed

facets were obtained according to the following equations.

AgNO3 + (NH4)2HPO4 / Ag3PO4Y + NH4NO3 (1)

NH4NO3 + NaOH $ NH3$H2O + NaNO3 (2)

Ag3PO4 + 6NH4OH $ 3[Ag(NH3)2]+ + PO4

3� + 6H2O (3)

[Ag(NH3)2]+ + OH� $ AgOH + 2NH3 (4)

2AgOH / Ag2OY + H2O (5)

Fig. 1C shows the SEM image of Ag2O microcrystals. We can

clearly see the Ag2O rhombic dodecahedron with {110} facets

completely exposed, and each facet is a normative diamond with

sides of 500 nm.

Secondly, Ag2O microcrystals with two exposed facets ({100}

and {110}) were successfully synthesized using CH3COONH4 as

the complexing agent. Fig. 1D shows the SEM image of the Ag2O

morphology with two exposed facets (a polyhedron with 18

faces); their surface morphologies are composed of six square

{100} facets and twelve hexagonal {110} facets with a size of

700–800 nm. During the synthetic process, the brown precipitate

immediately emerged, and then the solution became clear after

the addition of NaOH solution drop by drop. The brown

precipitate must be Ag2O nuclei, which quickly dissolved due to

the presence of the NH3$H2O that formed after adding the

NaOH solution. Finally, the brown precipitate formed again

with an excess of NaOH solution.

Fig. 1E shows the SEM image of Ag2O microcrystals with

three exposed facets, which were obtained by adding a certain

concentration of NH4NO3 solution as the complexing agent.24

The as-prepared samples possess the morphology of a rhombi-

cuboctahedron, with exposed {100}, {110} and {111} facets and

a crystal size of 500–700 nm.

3.2 The influence of complexing agent concentration

As a general rule, a high concentration of the reactants could

accelerate the diffusion rate of reactants to the crystal nucleus.

The concentration of CH3COONH4 and (NH4)2HPO4 plays a

crucial role in adjusting the area ratio of the Ag2O facets. In

addition, the molar ratio of Ag+ and NH4+ is set at 1 : 2 to form

[Ag(NH3)2]+ complex ions, which could slow down the release of

silver ions in the NaOH solution and restrain the growth of the

Ag2O microcrystals. The SEM images (Fig. S1, ESI†) show the

variation of the area of the facets {100} and {110} with changing

CH3COONH4 concentration. We can see that the areas of the

{100} and {110} facets were tunable as the concentration of

CH3COONH4 solution changed. The area of the {100} facets

grew smaller while the {110} facets became larger as the

CH3COONH4 concentration increased. The proportion of the

{100} facet ranged from 43.1% to 11.5% as the CH3COONH4

concentrations were about 0.01–0.05 M. A higher concentration

of NH4NO3 solution could also reduce the area of the {100}

facets of the Ag2Omicrocrystals (Fig. S2, ESI†). The {100} facets

This journal is ª The Royal Society of Chemistry 2012

of Ag2O microcrystals possess the highest surface energy among

the three facets, which leads to the preferential absorption of the

Ag2O nuclei and subsequently rapid growth. If the concentration

of the Ag2O nuclei in the solution is higher, {100} facets would

disappear rapidly during the crystal growth. In the same way,

Ag2O microcrystals with exposed {100} facets can be obtained

easily at a low concentration of Ag2O nuclei.

When we chose (NH4)2HPO4 as the complexing agent, the

yellow precipitate appeared and then quickly disappeared with

the addition of NaOH solution, and finally the brown precipitate

immediately emerged once the NaOH was in excess. The initial

yellow deposits were Ag3PO4 nuclei, and could be dissolved by

NH3$H2O. The formed [Ag(NH3)2]+ decreases the release rate of

silver ions. Finally, rhombic dodecahedral Ag2O with exposed

{110} facets was obtained after placing in darkness for 12 hours.

The octahedral Ag2O microcrystals quickly emerged when the

concentration of (NH4)2HPO4 was increased to 0.02M. Irregular

morphologies of Ag2Omicrocrystals were obtained in the 0.02M

(NH4)2HPO4 solution (Fig. S3, ESI†). The octahedral Ag2O

microcrystals seen in the SEM image were due to the rapid

diffusion of reactants at high concentration.

The different anions in complexing agent solutions could

adsorb onto different facets of the Ag2O microcrystals to alter

their corresponding surface energy, and thus adjusted their

growth conditions, resulting in Ag2Omicrocrystals with different

exposed facets. On the other hand, the high concentration of

reactants could increase the amount of Ag2O nuclei. In addition,

high concentrations lead to the accelerated growth rate of Ag2O

crystals. Therefore, the facets possessing high surface energy

would quickly disappear in the growth process.

3.3 The photocatalytic activities of different morphologies of

Ag@Ag2O

Ag@Ag2O photocatalysts were fabricated following the method

in our previous work.22 Fig. 2 shows the XRD patterns of the as-

synthesized cubic Ag@Ag2O, Ag@Ag2O with three facets

exposed, pure Ag2Omicrocrystals and cubic Ag@Ag2O after five

runs of photocatalytic experiments. The peaks of cubic

Ag@Ag2O and Ag@Ag2O with three facets exposed both match

well with those of standard Ag2O (JCPDS no. 41-1104), and the

rest of the peaks are assigned to silver (JCPDS no. 87-597). SEM

images of the as-synthesized Ag@Ag2O are also shown in Fig. 3.

The pictures clearly exhibit the existence of Ag nanoparticles

with a size of 40–80 nm on the surface of Ag2O. The morphol-

ogies of Ag@Ag2O were in accordance with the initial appear-

ance of the Ag2O microcrystals.

Methyl orange (MO) was chosen as a target to evaluate the

photocatalytic activities of the as-prepared samples. The pho-

tocatalytic efficiencies of Ag@Ag2O with different morphologies

are shown in Fig. 4. Ag@Ag2O with exposed {100} facets

exhibited the best photocatalytic activity among the five

morphologies of the photocatalysts. Table 1 displays the contrast

between the morphology-dependent photocatalytic activities.

Within 10 minutes, about 90% of the methyl orange was

decomposed under visible light irradiation. For Ag@Ag2O with

exposed {110} and {111} facets, the degradation efficiencies of

MO were approximately 40% and 21%, respectively. Ag@Ag2O

with exposed multi-facets (three or two facets) displayed

J. Mater. Chem.

Fig. 2 XRD patterns of Ag@Ag2O and pure Ag2O used for five

consecutive photodegradation cycles of MO dye. a: pure Ag2O, b: cubic

Ag@Ag2O, c: Ag@Ag2O with three facets exposed, d: cubic Ag@Ag2O.

Fig. 3 SEM images of Ag@Ag2O (A) cubic Ag@Ag2O (0.01 M), (B)

Ag@Ag2O with three facets exposed (0.02 M).

Fig. 4 Photocatalytic activities of Ag@Ag2O with different exposed

facets. A: cubic ({100}), B: rhombic dodecahedra ({110}), C: octahedral

({111}), D: polyhedra with 18 faces ({100} + {110}) (0.03 M), E:

rhombicuboctahedral ({100} + {110} + {111}) (0.03 M).

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relatively high activities, because the different surface energies

between the Ag2O crystal facets may drive electrons and holes to

different crystal facets, leading to the separation of electrons and

holes.26 We also found that the Ag@Ag2O photocatalyts with

three or two exposed facets likewise exhibited better activities

J. Mater. Chem.

when the concentration of the raw materials was low during the

synthetic process. According to the SEM images, Ag2O with

three and two exposed facets have relatively larger area {100}

facets when the concentrations of NH4NO3 and CH3COONH4

were lower. Therefore, further research has been carried out to

study the relationship between the size of the {100} facets and the

photocatalytic activity.

We controlled the area of the {100} facets by adjusting the

concentration of the complexing agents studied above. Samples

with different areas of the {100} facets were obtained by

controlling the concentrations of CH3COONH4 and NH4NO3

solutions (Fig. S1 and S2, ESI†). The concentrations of both

complexing agents ranged from 0.01 to 0.05 M. The results

indicated that photocatalytic activities of the as-synthesized

samples declined as area of the {100} facets decreased. The

photocatalytic activities of Ag@Ag2O with two exposed facets

decreased slowly with the reduction of the area of the {100}

facets (Fig. S4, ESI†). For the Ag@Ag2O photocatalyst with

three exposed facets, the activities similarly decreased with the

gradual disappearance of the {100} facets, which can be

controlled by the NH4NO3 concentration (Fig. S5, ESI†).

Therefore, the high surface energy of the {100} facets plays a key

role in the photocatalytic process. The photocatalytic activities

were tunable by controlling the area of the {100} facets, which

benefits the study of the practical process of the photocatalyst.

The stability of a photocatalyst is very important for its

application. Therefore, the stability of the Ag@Ag2O photo-

catalyst (cubic) has been further investigated by the recycling of

photocatalytic experiments. MO dye is quickly bleached after

injection of the MO solution 5 times, and the photocatalyst is

stable under repeated applications without exhibiting any

significant loss of activity (Fig. S6, ESI†). Furthermore, the XRD

patterns (Fig. 2) of the cubic Ag@Ag2O before and after the

recycling experiments are almost identical. The stability of

structure and properties ensures that cubic Ag@Ag2O could be

used as an efficient and stable visible-light-driven photocatalyst.

3.4 The possible mechanism of Ag@Ag2O facet-dependent

photocatalytic activity

To further investigate the facet-dependent photocatalytic activ-

ities of the Ag2O microcrystals, DFT calculations were employed

to study the surface energy of the {100}, {110} and {111} facets

of Ag2O. Actually, due to the special atomic arrangement char-

acteristics of Ag2O (111) surfaces, they can be divided into two

kinds of surfaces, (111) and (111)*, similar to that of Cu2O.27 As

the (111)* surface presents the highest surface energy (1.24 J

m�2), this kind of surface will not exist. Therefore, only three

kinds of surface are discussed in this paper, (100), (110) and (111)

surfaces. The calculation results show that the surface energy of

the (100) surface is estimated to be 0.94 J m�2, which is higher

than those of the (110) (0.76 J m�2) and (111) (0.65 J m�2)

surfaces, indicating that the {100} facet of Ag2O is more reactive

than the {110} and {111} facets, and thus it should facilitate dye

adsorption and provide more catalytically active sites.28,29

For the analysis of the recombination rate of photogenerated

e�–h+ pairs, we define the weighted average of the three degen-

erate holes at the valence band maximum (VBM) by the

following formula:

This journal is ª The Royal Society of Chemistry 2012

Table 1 Degradation rate of samples with different morphologies (RD ¼ Rhombic dodecahedra, RB ¼ rhombicuboctahedra, 18 faces ¼ polyhedrawith 18 faces)

Morphology

Name Cubic RD Octahedral 18 faces RBDegradation rate (10 min) 90% 40% 21% 72% 78%

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m* ¼P3i¼1

mðiÞ*h

Ð Et

E�jdt j NiðEÞdEP3i¼1

Ð Et

E�jdj NiðEÞdE

NiðEÞ ¼ VC

2p2

2m

ðiÞ*h

h-2

!3 =

2

ðEt � EÞ1

=

2

where Ni(E) is the density of states near the valence band

maximum (VBM), obtained using the nearly-free electron model.

m(i)*h is the effective mass of a hole, for which we have chosen the

value calculated in ref. 30. VC represents the crystal volume, Et is

energy of the VBM, and d is an infinitesimal value.

Based on the definition above, the weighted average of the

three degenerate holes along the [100], [110] and [111] directions

are calculated to be 2.038, 1.855 and 1.621, respectively. Since the

effective mass of electrons along the three directions are almost

the same (0.61),30 then a larger value of the weighted average of

the effective mass of a hole along [100] will distinguish the

mobility of the photogenerated electrons and holes very much,

thus separating them from each other in real space,30 and

lowering the recombination rate of the e�–h+ pairs, causing the

(100) surface to exhibit a higher photocatalytic activity.

To evaluate the photocatalytic energy conversion ability of

different surfaces, the relative redox potentials vs. the standard

hydrogen electrode (SHE) were calculated and are shown in

Fig. 5 (for the calculation method, see ESI†). We used the

position of the conduction band minimum (CBM) (+0.2 eV vs.

SHE) for bulk Ag2O determined by experiments as the reference

level.31 The position of the VBM for bulk Ag2O can be obtained

by subtracting the band gap from the value of CBM. The VBM

of each surface is determined by the difference of the theoretical

Fig. 5 Energetic diagrams of different facets of Ag2O and bulk Ag2O.

This journal is ª The Royal Society of Chemistry 2012

calculated bulk VBM and the surface VBM. For the determi-

nation of the CBM, we are confronted with the inability of DFT

to properly describe the unoccupied states. However, according

to the general theory of semiconductors, we are able to determine

the positions of CBM qualitatively; since the VBM of the (100)

and (110) surfaces are raised due to the dangling bonds on the

surfaces, we consider that the dangling bonds do not have much

influence on the CBM, so while for the (111) surface the VBM

hardly changes, we consider that the CBM are lowered due to the

dangling bonds, even though the value of the reduction cannot be

determined (indicated as X in Fig. 5). It can be seen from Fig. 5

that the (100) and (110) surface oxidation potentials have been

reduced, but they are still positive enough to guarantee that the

oxidation reaction for the photocatalytic decomposition of

methyl orange occurs. In terms of the reduction potentials, the

reduction potential of bulk Ag2O is relatively low and the

reduction potential of the (111) surface is further reduced,

probably resulting in the inactivation of the surface in the

reduction reaction for the photocatalytic decomposition of

methyl orange and thereby inhibiting the entire photocatalytic

reaction. This may be the reason that the (111) surface does not

exhibit any photocatalytic activity experimentally. It should be

noted at this point that, in the case of materials in contact with

electrolyte, the CBM and VBM values tend to vary with changes

in pH, concentration of solvent, and temperature of the medium.

But one can rely on the qualitative behavior of the system under

consideration.

4. Conclusions

In summary, we have adopted kinetic control of microcrystal

growth for preparing Ag2O microcrystals with different exposed

facets. During the preparation process, the formed [Ag(NH3)2]+

could restrict the growth of Ag2O microcrystals, and different

anions in solutions play an important role in the formation of

given facets. Five distinct morphologies have been obtained,

cubic, octahedral, rhombic dodecahedra, polyhedra with 18 faces

and rhombicuboctahedral. The different morphologies of

Ag@Ag2O photocatalysts exhibited distinct photocatalytic

activities due to the different surface structures of the exposed

facets. The Ag@Ag2O photocatalyst with exposed {100} facets

(cubic) exhibited the best photocatalytic activity, which was

evaluated by the degradation of MO under visible light irradia-

tion. The order of degradation rate was in accordance with that

of the Ag2O surface energies ({100} > {110} > {111}). It is also

found that a larger area of {100} facets resulted in a higher

photodegradation rate of MO. The probable mechanism for this

facet-dependent photocatalytic activity involves the highest

J. Mater. Chem.

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surface energy of the {100} facets, a larger difference value

between the weighted average of the effective mass of holes and

electrons along the [100] direction, and the suitable redox

potentials of the (100) surface in the cubic Ag@Ag2O photo-

catalyst. This work clarifies a feasible kinetic control method to

synthesize materials with facet-dependent photocatalytic activi-

ties, which may be of assistance to study the practical processes

of photocatalysts. At the same time, Ag@Ag2O extends the

scope of photocatalysts and will have more potential

applications.

Acknowledgements

This work was financially supported by the National Basic

Research Program of China (973 Program, No.2013CB632401),

the National Natural Science Foundation of China (no.

20973102, 21007031, 51021062, 51002091, 51072098) and the

Natural Science Foundation of Shandong Province

(ZR2010BQ005, ZR2010EM028).

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