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Sulfur g-C3N4 Composites with Enhanced

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Printed in the United States of America

Science of Advanced Materials Vol.6,pp.2611–2617,2014

(https://www.wendangku.net/doc/3b82877.html,/sam )

Sulfur/g-C 3N 4Composites with Enhanced Visible Light Photocatalytic Activity

Yao Xu and Wei-De Zhang ?

School of Chemistry and Chemical Engineering,South China University of Technology,Guangzhou 510640,People’s Republic of China

ABSTRACT

Composite photocatalysts consisting of different ratios of sulfur and graphitic carbon nitride (S/g-C 3N 4)were prepared by chemical reduction.The resulted S/g-C 3N 4composite photocatalysts were characterized and their photocatalytic activity was evaluated using rhodamine B as a probe.The S/g-C 3N 4exhibits obviously enhanced photocatalytic activity under visible light irradiation,which is much higher than that of pure sulfur and g-C 3N 4.The S/g-C 3N 4composite was very stable during the reaction and can be used repeatedly.The synergistic effect between sulfur and g-C 3N 4was found to be responsible for the improvement of the separation of pho-togenerated electrons and holes.The holes (h +)and superoxide (?O ?2)were the main active species in the photocatalytic degradation of rhodamine B.The study provides new insight to develop high performance com-posite photocatalysts without any metal oxide or metal sul?de for environmental remediation.KEYWORDS:Sulfur,Graphitic Carbon Nitride,Photocatalysis,Rhodamine B.

1.INTRODUCTION

Photocatalysis has emerged as one of the most promising technologies because it represents an easy way to utilize the energy from either natural sunlight or arti?cial indoor illumination,and is thus ubiquitously abundantly available.In particular,photocatalysis offers a sustainable pathway to drive chemical reactions,such as degradation of organic pollutants,water splitting and carbon ?xation.1Ideal semi-conductor photocatalysts should at least have the charac-teristics of absorbing visible light,suitable band edges for targeted reactions,high stability in solution,environmental friendliness,and low cost.During the past decades,over-whelming attention has been focused on semiconductive metal oxides as photocatalysts,but the possible application of elemental semiconductors as photocatalysts has rarely been attempted.2Semiconductors based on non-metal con-taining compounds or elements could also be used as photocatalysts.

Recently,photocatalytic performance of graphite like carbon nitride (g-C 3N 4)was introduced as a prospec-tive photocatalyst for water reduction 3–5and oxidation 6due to its unique electronic band structure.g-C 3N 4only contains such elements as C,N and a small amount of H as the residual –NH 2group which is detectable by elemental analysis 7and FT-IR.8 9Moreover,g-C 3N 4

Author to whom correspondence should be addressed.Email:zhangwd@https://www.wendangku.net/doc/3b82877.html, Received:13April 2014Accepted:4June 2014

can be easily prepared by using common compounds,such as dicyandiamide,10–12melamine,13 14urea,15 16and thiourea 17 18as precursors.The optical bandgap of this organic semiconductor was determined to be 2.7eV .However,the photocatalytic activity of the as-prepared g-C 3N 4is low.In order to improve the photocatalytic activity of g-C 3N 4,Shalom and his colleagues used cya-nuric acid-melamine to prepare hollow carbon nitride.19In our previous studies,the photocatalytic performance of g-C 3N 4was greatly improved by acid-etching 20and ther-mal processing.21Many efforts have also been devoted to modifying the intrinsic properties of g-C 3N 4to enhance its photocatalytic activity with both inorganic and organic protocols,for example,doping with metal/non-metal elements,22–27protonation with HCl,28dye sensitizing,29and hybridization with other semiconductors.30–32On the other hand,although ionic sulfur is widely used in sul?de photocatalysts or as a dopant in oxide photocatalysts,1the possibility of using elemental sulfur as a photocatalyst or as a co-catalyst has little been explored,to the best of our knowledge.

Herein,we employ a wet chemical process to prepare S/g-C 3N 4hybrid material through direct growth of elemen-tal sulfur on the g-C 3N 4.The degradation of rhodamine B (RhB)solution under visible light irradiation over the obtained S/g-C 3N 4is explored.The effect of g-C 3N 4con-tent on photocatalytic activity and the electron-transfer mechanism of the S/g-C 3N 4hybrid photocatalyst are also discussed.

A R T I C L E

2.EXPERIMENTAL DETAILS

2.1.Preparation of Photocatalysts

All chemicals were of reagent grade and used without further puri?cation.The metal-free g-C 3N 4powders were synthesized by heating melamine in an alumina combus-tion boat under nitrogen gas ?ow (10mL/min)to 550 C at a heating rate of 10 C min ?1followed by annealing 4h at that temperature prior to cooling.The product was collected and ground into a powder.

S/g-C 3N 4composite photocatalysts were prepared as follows:0.32g g-C 3N 4and 2.50g Na 2S 2O 3·5H 2O were added into 80mL distilled water in a reaction vessel.The mixed solution was magnetically stirred for 30min to a uniform suspension containing g-C 3N 4particles.After that,3mol ·L ?1HCl solution was added to the suspension until the molar ratio of 1:2(Na 2S 2O 3:HCl)under magnetic stirring for 2h.In such a process,Na 2S 2O 3was discom-posed and the produced sulfur was deposited onto g-C 3N 4.The precipitated pale yellow S/g-C 3N 4composite powder was ?ltered and washed with distilled water until pH 7.Finally,the product was dried in vacuum,and the sample with S:g-C 3N 4of 1:1was obtained.The mass ratios of S:g-C 3N 4in the composite photocatalysts were 1:1,1:2,1:3,1:4and 1:5,and the resulting samples were denoted as CNS1,CNS2,CNS3,CNS4and CNS5,respectively.2.2.Characterization

The crystal structures and phase compositions of the pre-pared samples were determined by a powder X-ray diffrac-tometer (X’Pert PRO MPD).The particle size and surface morphology of the samples were observed using a scan-ning electron microscope (FESEM,LEO 1530VP).The UV-vis diffuse re?ectance spectra (DRS)were recorded on a UV-vis spectrometer (Hitachi U-3010)by using BaSO 4as a reference.The thermogravimetric analysis (TGA)was performed using a TA Instruments (SDT Q600)thermobal-ance.The Fourier transformation infrared spectroscopy (FT-IR)measurement was carried out using a Nicolet FT-IR760infrared spectrometer.The speci?c surface area was determined by nitrogen adsorption-desorption isotherms at 77K according to the Brunauer–Emmett–Teller analysis (ASAP 2020,Micromeritics,USA).

2.3.Photocatalytic Activity Reaction

The photocatalytic activity of the g-C 3N 4and S/g-C 3N 4composite samples was evaluated via photocatalytic degra-dation of RhB in an aqueous solution under visible light irradiation.A ?lament lamp (300W)with a 420nm cut-off ?lter provided visible light irradiation.In each experi-ment,0.10g photocatalyst was mixed with a 100ml RhB solution (5mg/L).Prior to irradiation,the suspension was magnetically stirred in dark for 30min to achieve saturated absorption of RhB onto the catalyst.At every 1h irradia-tion time interval,the sample suspension was collected and

centrifuged (12000rpm,5min)twice to remove the pho-tocatalyst particles.The RhB concentration was monitored at 554nm during the photodegradation process using a UV-vis spectrophotometer (Japan Shimadzu UV-vis 1700).

3.RESULTS AND DISCUSSION

3.1.XRD Analysis

The X-ray diffraction patterns of g-C 3N 4,CNSx (x =1,2,3,4,5)and pure sulfur are shown in Figure 1.As indicated in Figure 1(A),the pure g-C 3N 4has two distinct diffrac-tion peaks at 27.42 and 13.06 ,which can be indexed to graphitic materials as the (002)and (100)diffrac-tions (JCPDS 87-1526),respectively.These two diffraction peaks are in good agreement with the g-C 3N 4reported in the literature.33For the CNS composite catalysts,in addi-tion to the diffractions of (002)and (100)of g-C 3N 4,very sharp peaks corresponding to the orthorhombic sulfur are also observed.The strongest peak at 2 of 23.04 cor-responds to the re?ection of sulfur (222),while the other six characteristic diffraction peaks are also found in the

CNS5CNS4

CNS3CNS2

I n t e n s i t y /a . u .

2Theta/degree

CNS1(A)

g-C 3N 4

203040

I n t e n s i t y /a . u .

2Theta/degree

(B)

(444)

(400)

(040)

(026)(022)

(222)

(111)

Fig.1.XRD patterns of (A)g-C 3N 4and CNSx (x =1,2,3,4,5)photocatalysts,(B)primary peaks of pure sulfur.

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pro?les at 2 of 11.50 ,15.59 ,25.88 ,27.77 ,31.45 and 42.78 (JCPDS 08-0247),respectively.No other phases can be ?tted with these lines.Notably,the diffraction peaks of orthorhombic sulfur are stronger upon the increase of the sulfur content in the S/g-C 3N 4composites.The XRD analysis reveals the ?ne crystallinity of sulfur in the pre-pared samples.

3.2.FESEM Analysis

The FESEM of CNS4is shown in Figures 2(A)and (B).The overview of the sample at low magni?ca-tion (Fig.2(A))indicates the aggregate of the particles.Zoom-in observation (Fig.2(B))reveals that the sample contains nanoplatelets with thickness of ca.3to 4nm and some of the nanoplatelets are hexagonal with the length of ca.150–400nm,which presents the characteristic of orthorhombic S.

3.3.TGA Analysis

Thermogravimetric analysis was performed from room temperature to 800 C at a heating rate of 10 C/min.As can be seen in Figure 3(A),the pure g-C 3N 4was sta-ble in air and the starting temperature of weight loss was recorded at about 600 C and a sharp weight loss peak appeared at 700 C.For the CNS4composite,there are two steps,the S phase in the CNS4composite disappeared at the temperature above 240 C.The g-C 3N 4in the CNS4composite became unstable at temperature above 600 C,at which the g-C 3N 4was decomposed.At the end of the analysis total combustion of CNS4is achieved.Thus,

the

Fig.2.((A),(B))SEM images of CNS4.

100

200

300

400

500

600

700

800

0.0

0.20.40.60.81.0

W e i g h t c h a n g e

Temperature/oC

(A)4000350030002500200015001000500

T r a n s m i t t a n c e /%

Wavelength/nm

(B)

Fig.3.(A)TGA pro?les of the g-C 3N 4and CNS4.(B)FT-IR spectra of (a)the as-prepared g-C 3N 4,(b)CNS4and (c)CNS4after 4cycles.

weight loss after such temperature is directly related to the amounts of S and g-C 3N 4in the CNS4.The mass ratio of S is 19.0%,which is approximately the theoretical ratio in CNS4(S:g-C 3N 4is 1:4).

3.4.FT-IR Analysis

FT-IR spectra of the g-C 3N 4and CNS4are shown in Figure 3(B).For the pristine g-C 3N 4,the broad band at 3000–3300cm ?1is due to the stretching modes of termi-nal –NH 2or –NH–groups at the defect sites of the aro-matic ring.34The peaks at 1637,1574,1460,1405,1317,1238cm ?1are attributed to the typical stretching modes of CN heterocycles.35The peak at 809cm ?1?ts the charac-teristic breathing mode of triazine units.The FT-IR spec-trum of CNS4composite is almost the same as that of g-C 3N 4.The typical S S symmetric stretching vibration mode at 600–700cm ?1is too weak to be observed besides the absorption peaks of g-C 3N 4in the composite catalyst.3.5.DRS Analysis

Figure 4(A)shows the UV-visible absorption spectra of the pure g-C 3N 4,CNS4and pure S,respectively.The pure

A R T I C L E

200

300

400

500

600700800

0.00.20.40.60.81.0

1.21.4

A b s o r b a n c e

Wavenumber/nm

g-C 3N 4 Pure S CNS4

(A) 1.5

2.0

2.5

3.0

3.5

4.0

0.0

0.51.01.5

2.0

2.5

3.0

αh ν

Photo energy/eV

g-C 3N 4 Pure S

2.68 eV

2.65 eV

(B)Fig.4.(A)UV-vis diffuse re?ectance spectra of g-C 3N 4,pure S and CNS4.(B)Plot of (h 1/2versus energy (h )for the bandgap energy of the g-C 3N 4and pure S.

g-C 3N 4shows a strong absorption edge at about 453nm due to its bandgap of 2.70eV .36The nanocrystalline sul-fur has good absorption in ultraviolet region,showing its limited absorption in the visible region.After being loaded by S,the absorption of the CNS4increases a little bit.This will enhance the light harvesting capability of g-C 3N 4towards visible light.37The bandgaps were calculated to 2.68eV for the pure g-C 3N 4,and 2.65eV for the pure S in Figure 4(B).Both of them are visible light active.3.6.Photocatalytic Activity Reaction

The visible light activity of the S/g-C 3N 4catalysts towards the degradation of such model dye as RhB was investi-gated.As shown in Figure 5(A),the degradation of RhB is negligible without catalyst.For comparison,the reac-tion over pure g-C 3N 4and pure sulfur is also illustrated.Only 57.4%and 50.5%of RhB are decomposed after 5h over the pure g-C 3N 4and pure S,respectively.How-ever,the CNS1nanocomposite shows signi?cant improve-ment in the photodegradation of RhB compared with pure g-C 3N 4and pure S.The catalyst CNS4exhibits the degra-dation rate of 97%of RhB in 5h,proving that it is the best catalyst among all the synthesized catalysts.As

0.0

0.20.40.60.8

1.0

C /C 0

Time/h

(A)

0.0

0.10.20.30.40.5

0.6

0.7

A b s o r b a n c e /a .u .

Wavelength/nm

(B)Fig.5.(A)The kinetics of photodegradation of RhB (5.0mg/L)over different photocatalysts under visible light irradiation ( >420nm);(B)Time-dependent optical absorption spectra of RhB degradation over CNS4.

shown in Figure 5(B),the RhB shows a major absorp-tion band at 552nm before degradation.However,the spectrum shows a strong peak at 498nm instead of the peak at 554nm after 3h,indicating that RhB was de-ethylated in a stepwise manner.In such a process,ethyl groups were removed one by one and the gradual peak wavelength shifted toward the blue region,38that is,the RhB was completely decomposed to rhodamine.After 5h,more than 90%RhB was degraded.At the same time,the peak intensity in the UV region (280nm < <400nm)also decreases upon increasing irradiation time,which fur-ther indicates that both dye chromophore and aromatic ring have been destroyed.39The photocatalytic activity of the S/g-C 3N 4catalysts increases remarkably with increas-ing g-C 3N 4content,as shown in Figure 5(B),but at higher g-C 3N 4concentration,the photocatalytic activity decreases,suggesting that the optimal content is CNS4with the mass ratio of 1:4(S:g-C 3N 4).Because the con-tent of S increases in CNS5,the g-C 3N 4is covered by S,which prevents g-C 3N 4from reacting with the solution.Thus,electrons in g-C 3N 4cannot participate in the RhB degradation reaction and will accumulate on the interface

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of S and g-C 3N 4,which may lead to a serious recombina-tion of photogenerated electron–hole pairs.Consequently,the synergic effect between S and g-C 3N 4diminishes dras-tically and the photocatalytic activity decreases rapidly with further increase of the sulfur.

The surface area of a catalyst is an important factor for its activity.The BET surface area of the g-C 3N 4is 11.9m 2/g,while that of the pure sulfur is very small.After loading with S,however,the BET surface areas of the S/g-C 3N 4composites decline to 6.0–7.0m 2/g,which are much smaller than that of the pure g-C 3N 4.That is to say,the enhanced photocatalytic activity cannot be simply attributed to the surface area for the CNSx (x =1,2,3,4,5)photocatalysts.

3.7.Mechanism

It is essential to detect the main active species in the pho-tocatalytic process for revealing the photocatalytic mech-anism.The main active species include hydroxyl radicals (?OH),superoxide (?O ?2)and the holes,which could be detected by using isopropanol (IPA)as a radical scav-enger and EDTA-2Na as a hole scavenger.40As shown in Figure 6,the addition of IPA only causes a small change in the photodegradation of RhB.On the con-trary,the photocatalytic activity of CNS4is obviously sup-pressed by EDTA-2Na.Moreover,the sample shows lower photocatalytic activity when N 2is purged to reduce the absorbed O 2.This result suggests that the photogenerated holes and ?O ?2are the main oxidative species of the pho-tocatalytic degradation over CNS4.This process can be described as follows:

S/g-C 3N 4h

?→S h ++e ? /g-C 3N 4 h ++e ? S h ++e ? /g-C 3N 4 h ++e ? ?→S e ? /g-C 3N 4 h +

O 2+e ??→?O ?2

O ?2+h +

+RhB ?→product 0

0.0

0.2

0.4

0.6

0.8

1.0

C /C 0

Time/h

Fig.6.The plots of photogenerated carriers trapped in the system of

photodegradation of RhB under visible light.As discussed above,neither the surface area nor crystal

phase structure was obviously changed and the limited adsorption enhancement was not the major factor of the signi?cant enhancement of the photocatalytic activity of CNS4.The major enhancement of photocatalytic activity was mainly due to the high ef?ciency of charge separation induced by the synergistic effect of g-C 3N 4and sulfur.The enhancement of photocatalytic performance of the com-posite materials is supposed to be attributed to the more effective separation of the photogenerated electron–hole pairs.Based on the bandgap positions,the CB and VB edge potentials of polymeric g-C 3N 4were determined at ?1.12and +1.57eV at pH 7(vs.the normal hydrogen electrode),41respectively.The CB and VB of S were cal-culated according to the formula,E CB =X ?E e ?0 5E g .In this formula,E CB is the conduction band edge of a semiconductor at the point of zero charge.X is the abso-lute electronegativity of the semiconductor,expressed as the geometric mean of the absolute electronegativity of the constituent atoms,which is de?ned as the arithmetic mean of the atomic electron af?nity and the ?rst ionization energy.E e is the energy of free electrons on the hydrogen scale (ca.4.5eV).E g is the bandgap of the semiconductor,which can be obtained from Figure 4(B).Thus,the CB and VB edge potentials of pure S were calculated to be ?0.39and +2.26eV ,respectively.Both sulfur and g-C 3N 4can be excited by visible light and can generate electron–hole pairs,showing photocatalytic activity.Since the CB edge potential of g-C 3N 4(?1.12eV)is more negative than that of S (?0.39eV),the photo-induced electrons on g-C 3N 4particle surfaces transfer more easily to S.Similarly,the photo-induced holes on the S surface move to g-C 3N 4,due to the large difference in VB edge potentials.This reduces the probability of electron–hole recombination and leads to a larger amount of electrons on the S surface and holes on the g-C 3N 4surface,respectively.Thus,the photocat-alytic decomposition of RhB was promoted.A schematic for electron–hole separation and transportation at the CNS photocatalyst interface is illustrated in Figure 7.

E / V v s . N H E

0123

–1

–2

O 2

?O 2

–H 2O + CO 2

RhB Sulfur

h + h +h +e -e -e -

g-C 3N 4

RhB

CO 2 + H 2O

h + h +h +

e -e -e -

Fig.7.Schematic diagram of the band levels of the sulfur and g-C 3N 4composite and the possible reaction mechanism of the photocatalytic reaction.

A R T I C L E

3.8.Recycling

Recycling is a critical issue for the long-term use of a catalyst for practical applications.It is known that the pho-tocorrosion may occur on the photocatalyst surface in the photocatalytic reaction.To test the stability of RhB pho-todegradation on CNS4,we reused the catalyst four times.As shown in Figure 8(A),each experiment was carried out under identical conditions and,after four cycles,the photocatalytic activity of the CNS4photocatalyst remains almost unchanged.Further characterization of the CNS4catalyst after four cycles by FT-IR (Fig.3(B))and XRD (Fig.8(B))indicates that there is no obvious difference in the spectra of the as-prepared and the used catalyst.In addition,the CNS4sample is of nanosheet morphol-ogy and can be easily separated from an aqueous suspen-sion.This indicates that the CNS4photocatalyst displays an ef?cient photoactivity for the degradation of RhB under visible light irradiation and can be easily separated for reusing.In addition,by adding 0.10mol/L BaCl 2solution

into the solution after photoreaction,no SO 2?4/SO 2?

3was detected,suggesting that the S in CNS4is stable.

0.0

0.2

0.4

0.6

0.8

1.0

C /C 0

Time/h

(A)102030405060708090

CNS4 after 4 recycles

I n t e n s i t y /a .u .

2Theta/Degree

CNS4

(B)

Fig.8.(A)Stability of the CNS4photocatalyst after consecutive cycling photocatalytic degradation of RhB.(B)XRD patterns of CNS4and CNS4after 4cycles.

4.CONCLUSIONS

This study demonstrates the possibility of using the S/g-C 3N 4composite in photocatalytic degradation of organic pollutants.The prepared S/g-C 3N 4composites exhibit excellent performance in the degradation of RhB under visible-light ( >420nm).The optimized mass ratio is 1:4(S:g-C 3N 4)for the positive synergistic effect between the two components and the S phase in CNS4is 18.98%through the TGA pro?le.The photogenerated holes and ?O ?

2served as the major oxidative species in the photo-catalytic degradation of RhB.In addition,the S/g-C 3N 4composites can be easily recycled without decrease in the photocatalytic activity.We expect that the S/g-C 3N 4com-posites can provide new opportunities for the development of ef?cient visible light responsive photocatalysts.Acknowledgments:The authors thank the National Natural Science Foundation of China (21273080)for the ?nancial support.

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