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Upconversion-P25-graphene composite as an advanced sunlight drivenphotocatalytic hybrid material†

Long Ren, Xiang Qi,* Yundan Liu, Zongyu Huang, Xiaolin Wei, Jun Li, Liwen Yang and Jianxin Zhong*

Received 23rd January 2012, Accepted 4th April 2012

DOI: 10.1039/c2jm30457k

Herein, a new nanocomposite consisting of up-conversion (UC) material (YF3:Yb3+,Tm3+), TiO2 (P25)

and graphene (GR) has been prepared and shown to be an advanced sunlight activated photocatalyst.

During the facile hydrothermal method, the reduction of graphene oxide and loading of

YF3:Yb3+,Tm3+ and P25 were achieved simultaneously, and the functionalities of each part were

integrated together. The as-prepared ternary UC–P25–GR nanocomposite photocatalyst exhibited

great adsorptivity of dyes, a significantly extended light absorption range, efficient charge separation

properties and superior durability. Indeed, the photocatalytic activity of this novel ternary

nanocomposite under sunlight was improved compared with those of P25–GR nanocomposites and

bare P25. Overall, this work could provide new insights into the fabrication of ternary composites as

high performance photocatalysts and facilitate their application in environmental protection issues.

1. Introduction

As the most investigated functional material in semiconductor

photocatalysis, titanium dioxide (TiO2) has been widely used in

the fields of energy conversion and environmental pollutant

degradation owing to its non-toxicity, effectiveness, low cost,

and chemical stability.1–4 For pure TiO2, under ultraviolet (UV)

illumination electrons are excited from the valence band to the

conduction band, forming electron–hole pairs, which are

responsible for the photocatalytic activity of TiO2.5 The main

drawbacks of TiO2 that greatly limit its practical applications are

the fast recombination of photogenerated electron–hole pairs

and the narrow optical response, being limited to UV light.6

Therefore, many attempts have been made to improve the pho-

tocatalytic activity of TiO2 by inhibiting the recombination of

photogenerated electron–hole pairs and extending the optical

absorption to the visible light region. For instance, to extend the

optical absorption of TiO2, surface modification,7,8 structure

optimization9 and doping of metal10 or nonmetal elements11,12 are

the common routes. Meanwhile, deposition of noble metals on

the TiO2 surface,13,14 formation of a composite with a semi-

conductor,15,16 utilization of electron donors–acceptors and hole

scavengers17,18 are typical approaches to retard the bulk and

surface recombination of photogenerated electron–hole pairs in

TiO2 during a photocatalytic process.

Recently, particular attention has been paid to the coupling of

graphene with TiO2 which has shown a significant improvement

Laboratory for Quantum Engineering andMicro-Nano Energy Technologyand Faculty of Materials and Optoelectronic Physics, Xiangtan University,Hunan 411105, P. R. China. E-mail: xqi@xtu.edu.cn; jxzhong@xtu.edu.cn

† Electronic supplementary information (ESI) available. See DOI:10.1039/c2jm30457k

This journal is ª The Royal Society of Chemistry 2012

of the photoelectrochemical catalytic ability owing to the supe-

rior charge transport properties, the intense light absorption and

the unique flexible sheet-like structure of the graphene compo-

nent.19–25 Due to the excellent electrical conductivity for storing

and shuttling electrons, graphene is considered as an ideal plat-

form for scavenging photogenerated electrons when in contact

with TiO2.26,27 Furthermore, these graphene-based composites

also show an improving adsorption for organic dyes and an

extended light absorption range.19,28 Despite all the advantages,

these TiO2 (or Degussa P25)/graphene nanocomposites prepared

using different recipes still suffer from a key factor that limits the

photocatalytic activity under solar irradiation, i.e. the low-usage

of natural sunlight. Compared with the pure TiO2 and Degussa

P25 nanoparticles, the efficient photocatalysis of these TiO2 (or

Degussa P25)/graphene nanocomposites also responds only to

UV light and a rather limited part of the visible light region.

However, the percentage of UV light in the solar spectrum is only

5%, which is extremely low compared to that of visible light

(�48%) and near-infrared (NIR) light (�44%).29 Therefore, the

modification of TiO2 to reduce its band gap and make it sensitive

to the longer-wavelength light is one of the most important

objectives in related realms. Unfortunately, a large fraction of

the NIR of sunlight remains untapped for photocatalysis.

Recently, several incorporations of up-conversion (UC) agents

with TiO2 were reported to extend the absorption edge up to the

visible region or even the NIR region by UC luminescence of rare

earths. For example, Qin et al. had reported a near-infrared

photocatalysis based on YF3:Yb3+,Tm3+/TiO2 core–shell nano-

particles, which showed an effective photocatalysis under NIR

light.29 However, the efficient utilization of light was limited

because the light could not penetrate the UC nanocrystal cores.

Moreover, the problem of a fast recombination of

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photogenerated electron–hole pairs in these simple hybrid UC–

TiO2 core–shell structures could not be ignored as they affected

the photocatalytic efficiency.

In this work, we utilize both the up-conversion luminescence

of UC materials and the excellent electrical properties of gra-

phene to design a new advanced-sunlight-activated photo-

catalyst. Benefiting from the high specific surface area and the

flexible sheet-like structure, graphene emerged as an excellent

platform on which to load both UC nanocrystals and TiO2

nanoparticles to form UC material–P25–graphene (UC–P25–

GR) ternary nanocomposites which can achieve a uniform

distribution of these loaded nanomaterials without aggregation.

As illustrated in Scheme 1, in this photocatalyst, the loading of

UC nanocrystals is expected to emit UV light after absorbing

NIR light of the solar spectrum and the optical response of the

UC–P25–GR nanocomposites is enhanced from UV to NIR. As

for the P25 nanoparticles, TiO2 is activated to form photo-

generated electron–hole pairs after absorbing the UV light, along

with parts of the violet-blue light (caused by the chemical bonds

of Ti–O–C existing in the composites and the good transparency

of graphene19), from the sunlight directly and from the secondary

energy transferred from the UC nanocrystals. In addition, gra-

phene could act as an acceptor of the photogenerated electrons

by P25 and ensure fast charge transportation in view of its high

conductivity. The introduction of graphene will not only accel-

erate the separation of photogenerated electron–hole pairs, but

also enhance the adsorption capacity of the photocatalyst.

Consequently, the photocatalytic activity of TiO2 could realize

a considerable improvement under solar irradiation.

2. Experimental section

2.1 Catalyst preparation

First, graphene oxide (GO) was synthesized from graphite

powder by the modified Hummer’s method30 using a mixture of

H2SO4, NaNO3, and KMnO4. A previous graphite oxidation

procedure with H2SO4, K2S2O8, and P2O5 was carried out before

the synthesis of GO. Second, YF3:Yb3+,Tm3+ nanocrystals were

prepared by a simple hydrothermal method.31 YCl3$6H2O

Scheme 1 Schematic structure of UC–P25–GR nanocomposites and

tentative processes of the photocatalysis.

11766 | J. Mater. Chem., 2012, 22, 11765–11771

(1.59 mmol), YbCl3$6H2O (0.4 mmol), TmCl3$6H2O

(0.01 mmol) were dissolved in distilled water. Then hydrofluoric

acid (40%) was added dropwise to form a colloidal solution

under stirring. After reacting at 130 �C for 20 h, the product was

isolated by centrifugation and dried at 60 �C, then annealed at

500 �C for 1 h under an argon atmosphere. The UC–P25–GR

composite was obtained via a hydrothermal method based on

Rajamathi’s work, with modifications.32 In detail, 4 mg of GO

was dissolved in a solution of distilled H2O (20 mL) and ethanol

(10 mL) by ultrasonic treatment for 1 h, then 0.2 g of P25

(Degussa) and 0.1 g YF3:Yb3+,Tm3+ nanocrystals were added

into the obtained GO solution and stirred for another 2 h to get

a homogeneous suspension. The suspension was then placed in

a 50 mL Teflon-sealed autoclave and maintained at 200 �C for 6 h

to simultaneously achieve the reduction of GO and the deposi-

tion of P25 and UC on the graphene support. Finally, the

resulting composite was recovered by filtration, rinsed by

deionized water several times, and dried at room temperature.

For comparison, the P25–GR nanocomposite (prepared with the

same content, 4 mg GO and 0.2 g P25) was obtained by reducing

GO via a hydrothermal route under the same conditions, but

without the addition of UC materials. Another reference sample,

the UC–P25 sample, was obtained by mixing the UC materials

(0.1 g YF3:Yb3+,Tm3+ nanocrystals) with P25 (0.2 g).

2.2 Characterization

The crystal structures of the as-prepared samples were deter-

mined by X-ray diffraction (XRD) using the Cu Ka radiation.

The morphologies and microstructures of the samples were

characterized using scanning electron microscopy (SEM, JEOL,

JSM-6360) and transmission electron microscopy (TEM,

JEM2100) with an energy dispersive spectroscope (EDS). The

photoluminescence (PL) spectra were acquired at room temper-

ature using a fluorescence spectrometer (Hitachi F-4500) under

the excitation of a 980 nm diode laser. The absorption spectra

were measured under the diffuse reflection mode using a Shi-

madzu UV-3600 UV-VIS-NIR spectrophotometer.

Photoelectrochemical test systems were composed of an CHI

660D Electrochemistry workstation, a 980 nm diode laser, and

a homemade three-electrode cell using platinum as the counter

electrode, Ag/AgCl as the reference electrode, and Na2SO4

(0.5 M) as the electrolyte. The working electrode was prepared

on indium-tin oxide (ITO) conductor glass. The sample powder

(10 mg) was ultrasonicated in 1 mL of anhydrous ethanol to

disperse it evenly to get a slurry. The slurry was spread onto ITO

glass whose side part was previously protected using Scotch tape.

The working electrode was dried overnight under ambient

conditions. A copper wire was connected to the side part of the

working electrode using a conductive tape. Uncoated parts of the

electrode were isolated with epoxy resin.

2.3 Photocatalytic experiments

The photodegradation of methyl orange (MO) dyes was

observed based on the absorption spectroscopic technique. In

a typical process, 30 mg of the photocatalyst (UC–P25–GR,

P25–GR, UC–P25 or P25) was suspended in 100 mL of MO dyes

in aqueous solution (0.02 g L�1) contained in a 100 mL

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cylindrical quartz vessel. Before irradiation, the suspensions were

stirred in the dark for 1 h to ensure the establishment of an

adsorption–desorption equilibrium. A 150 W high-pressure

Xenon lamp (CHF-XM150W33,34) was placed 10 cm away from

the reaction vessel, which was used to provide a full-spectrum

emission with an AM 1.5 filter to simulate the sunlight source

(the simulated sunlight system is from Beijing Trusttech Co. Ltd.,

China). The illumination intensity was 100 mW cm�2. Besides

testing the performance of photocatalysts under simulated

sunlight, the decomposition of MO was also carried out under

a 980 nm laser to verify the photocatalytic activity of the samples

under NIR irradiation. At given irradiation time intervals, 4 mL

aliquots were sampled and centrifuged to remove remnant pho-

tocatalyst. Supernatant aliquots were subsequently analyzed by

UV-visible spectroscopy using the Lambda 25 spectrophotom-

eter. In the durability test of the UC–P25–GR catalyst in the

photodegradation of MO under simulated sunlight, four

consecutive cycles were tested. At the beginning, 30 mg of UC–

P25–GR was dispersed in 100 mL of MO solution (0.02 g L�1).

Then the mixture underwent four consecutive cycles, each lasting

for 60 min. After each cycle, the catalyst was filtrated and washed

thoroughly with water, and then added into the fresh MO solu-

tion (0.02 g L�1). The percentage of degradation is reported as

C/C0. Here, C is the absorption of dye solution at each irradiated

time interval of the main peak of the adsorption spectrum, while

C0 is the absorption of the initial concentration when the

adsorption–desorption equilibrium is reached.

3. Results and discussion

3.1 Structure and morphology characterizations

The powder X-ray diffraction pattern of the prepared UC–P25–

GR nanocomposites was compared with those of UC nano-

crystals (YF3:Yb3+,Tm3+) and P25 nanoparticles, as depicted in

Fig. 1. The XRD pattern of the UC–P25–GR nanocomposites

indicates that the sample is well crystallized. Meanwhile, it is easy

to recognize that all diffraction peaks of the composite can be

easily assigned to Y0.795Yb0.2Tm0.005F3 (which is indexed to

Fig. 1 The XRD patterns of P25, up-conversion (UC) nanomaterials

(YF3:Yb3+,Tm3+) and UC–P25–GR nanocomposites.

This journal is ª The Royal Society of Chemistry 2012

orthorhombic YF3, JCPDS file no. 32-1431) and the pure P25

(which is indexed to anatase TiO2, JCPDS file no. 21-1272, and

rutile TiO2, JCPDS file no. 21-1276). However, no diffraction

peaks for carbon species were observed in the composite, which

might be due to the low amount and relatively low diffraction

intensity of graphene.

The morphologies and microstructures of the UC–P25–GR

nanocomposites were characterized by SEM and TEM. As

shown in Fig. 2, the SEM images were conducted to reveal the

morphological characteristics of different samples. Being regar-

ded as the two construction units of the ternary composites, fine

nanoparticles of P25 and cambiform-like morphologies of UC

nanocrystals with uniform size were observed as in Fig. 2(a) and

(b), respectively. For comparison, the binary composite of P25

loading on the graphene was synthesized and it can be observed

from the SEM image (Fig. 2(c)) that P25 nanoparticles are

loaded on the high quality ultra-thin graphene layers derived

from the solvothermal synthesis. As for the UC–P25–GR

nanocomposites, it is clearly discerned that the graphene

platform is covered by both the fine nanoparticles and the

cambiform-like nanomaterials. The results propose that

a ternary UC–P25–GR composite had been successfully

prepared by our simple solvothermal process. Moreover, the

uniform distribution of UC nanocrystals and P25 nanoparticles

on graphene help both of them to absorb the light illumination

directly in the photocatalytic process. For further detailed

structure analysis, the characterization of the ternary nano-

composites was carried out by TEM equipped with SAED and

EDS. Fig. 3 shows a typical TEM image of the ternary UC–P25–

GR composites with a polycrystalline diffraction pattern and

component analysis. The profile of a single TiO2 particle (which

is assigned to B in this work) and part of cambiform-like UC

nanocrystals aggregate (assigned to A) can be clearly distin-

guished, since the aggregation of these component units was well

prevented with the benefit of the graphene carrier. Meanwhile,

the fringe of graphene (assigned to C) can be explicitly identified,

convincing us of the presence of graphene under the TiO2 and

UC materials layer. The SAED pattern taken from a large area

Fig. 2 SEM images of (a) P25, (b) UC nanomaterials, (c) P25–GR, and

(d) UC–P25–GR.

J. Mater. Chem., 2012, 22, 11765–11771 | 11767

Fig. 3 Typical TEM image of UC–P25–GR, with up-conversion materials (A) and P25 (B) loading on the surface of graphene (C). The images on the

right are the SAED and EDS patterns of UC–P25–GR nanocomposites.

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of the composite confirms the high quality of the graphene sheets

derived from the solvothermal synthesis. Moreover, distinct

electron diffraction patterns from the (101), (200) planes of

anatase TiO2 and (121), (321) planes of YF3:Yb3+,Tm3+ were

observed. The elemental mappings of the composites obtained by

EDS also indicate that the atomic ratios of the elements are

similar to the original molar ratio of the feed. The above

results suggest that, as a robust and flexible supporter, graphene

effectively combines the UC materials together with P25

nanoparticles with uniform distribution.

3.2 Optical characterization

Under the excitation of a 980 nm laser, the photoluminescence

spectra of YF3:Yb3+,Tm3+ nanocrystals were recorded to eval-

uate energy transformation of the UC material after absorbing

Fig. 4 PL spectra of the up-conversion (UC) material (YF3:Yb3+,Tm3+

nanocrystals) under 980 nm NIR excitation.

11768 | J. Mater. Chem., 2012, 22, 11765–11771

IR light. As shown in Fig. 4, the emission peaks at 347 nm and

362 nm were assigned to 1I6 /3F4 and

1D2 /3H6 transitions,

respectively, of Tm3+ ions doped in YF3 nanocrystals. Except the

above mentioned UV emissions, two blue emission peaks at 452

nm and 476 nm come from the 1D2 / 3F4 and 1G4 / 3H6

transitions of Tm3+ ions. The UC emission of as-prepared

YF3:Yb3+,Tm3+ nanocrystals is roughly coincident with those in

the literature.29,31 It is noteworthy that the intensity of UV

emission is higher than those of the others which would be

propitious to the efficient photocatalysis of the UC–P25–GR

nanocomposites.

As mentioned above, the absorption range of light is a key

factor in the photocatalysis. Therefore, UV-vis-NIR spectro-

scopic measurements were carried out to examine the optical

response of UC–P25–GR nanocomposites and of the control

samples. The absorption spectra (see ESI, Fig. S1†) indicate that

the narrowing of the band gap of P25 occurred with the graphene

introduction19 and does not disappear with the presence of UC

nanocrystals. What’s more, the as-prepared ternary nano-

composites exhibit a powerful absorption in the NIR region. By

the way, the P25–GR composites also show NIR absorption,

which is caused by the light absorption of graphene. As a result

of the wondrous extended photo-responding range, an extremely

efficient utilization of the solar spectrum could be achieved by the

photocatalyst.

3.3 Photoelectrochemistry measurements

To ascertain the generation of electron–hole pairs in the UC–

P25–GR nanocomposites under NIR light, photo-electro-

chemical experiments were performed. Fig. 5 displays the

photocurrent transient response under 980 nm irradiation for

UC–P25–GR, UC–P25, P25–GR and bare P25 electrodes,

respectively. It is noteworthy that there is a fast and uniform

photocurrent responding to each switch-on and switch-off event

in the UC–P25–GR electrode. Moreover, the photocurrent is so

This journal is ª The Royal Society of Chemistry 2012

Fig. 5 Photocurrents of UC–P25–GR nanocomposites, P25–GR

nanocomposites, UC–P25 composites and P25 nanoparticles under

intermittent irradiation by 980 nm laser at a bias potential of 0.2 V.

Fig. 6 Bar plot showing the remaining methyl orange (MO) in solution

after reaching the adsorption equilibrium in the dark over P25, UC–P25,

P25–GR and UC–P25–GR.

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stable that no obvious photocurrent decay is observed. In

contrast, the photocurrent density for P25 is not existent at all.

The P25–GR sample shows NIR absorption, but there scarcely

exists any photo-response under the NIR irradiation. As for the

UC–P25 sample, it exhibits a faint but legible photocurrent

density which is much lower than the photocurrent density of the

UC–P25–GR sample. The above photocurrent transient

response suggested that, in the four kinds of electrodes, only the

UC–P25 and UC–P25–GR samples, with the presence of UC,

can respond to the NIR light. What’s more, the photocurrent

intensity of the UC–P25–GR is nearly 0.2 mA cm�3, a thousand

times more than the density of UC–P25, and the sharp separation

of electron–hole pairs with the introduction of GR21,27 is

considered to contribute greatly to the generation of this strong

photocurrent after absorbing the UV light emitted by UC

material. In detail, as represented in Scheme 1, the photo-

generated electron–hole pairs are generated after the P25 nano-

particles receive the secondary energy emitted by the UC

nanocrystals under NIR irradiation, and the photocurrent grows

fast due to the transport of photo-generated electrons. After the

equilibration of competitive separation and recombination of

photogenerated electron–hole pairs, the photocurrent reaches

a relatively high constant value. Thereby, a highly efficient

photocatalytic activity under solar irradiation is to be expected.

Fig. 7 Photodegradation of MO by P25, UC–P25, P25–GR composites

and UC–P25–GR with a reaction time of 60 min under irradiation of

simulated sunlight.

3.4 Photocatalytic performance

The photocatalytic activities of P25, P25–GR, UC–P25 and UC–

P25–GR were measured by the photodegradation of methyl

orange (MO) as a model reaction under simulated sunlight. MO

is considered as a suitable probe chemical for photocatalytic

activity tests because MO would not sensitize TiO2.35 Before

irradiation, a dark adsorption test was carried out to estimate the

adsorptivity of UC–P25–GR nanocomposites. Fig. 6 displays

bar charts that show the remaining solution ofMO after reaching

the adsorption equilibrium in the dark over the UC–P25–GR,

P25–GR, UC–P25 and bare P25 photocatalysts. It was obvious

that, after equilibrium in the dark for 1 h, most dye molecules

This journal is ª The Royal Society of Chemistry 2012

remained in the solution with bare P25 or UC–P25 as the cata-

lyst, whereas a large amount of dye molecules was adsorbed on

the surface of P25–GR and UC–P25–GR. The enhanced

adsorptivity was attributed to the introduction of graphene.28 As

a prerequisite for good photocatalytic activity, the enhanced

adsorptivity of the ternary nanocomposites would prefigure the

advanced performance in the photodegradation of MO.

Under the simulated solar irradiation, it is evident from Fig. 7

that the UC–P25–GR composite showed significant improve-

ments in the photodegradation of MO compared to P25, UC–

P25 and P25–GR. As a reference substance, the photo-

degradation of MO without any photocatalyst was tested and no

degradation of MO under the irradiation was verified. About

30% of MO molecules were degraded in the first 10 min with the

aid of as-prepared UC–P25–GR nanocomposite; in contrast,

only 12%, 7% and 4% MO molecules were degraded in

the control reactions with P25–GR, UC–P25 and bare P25,

respectively. Furthermore, after 60 min of simulated sunlight

irradiation, more than 78% of the initial dyes were decomposed

by UC–P25–GR. Contrastingly, only 53% and 46% of the initial

J. Mater. Chem., 2012, 22, 11765–11771 | 11769

Fig. 10 Photodegradation ofMO by P25, UC–P25, P25–GR composites

and UC–P25–GR with a reaction time of 3 h under irradiation of the

980 nm laser with 4 W excitation power in 0.1256 cm2.

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contaminants diminished after 60 min for P25–GR and UC–P25,

and nearly 73% of the initial dye still remained in the solution

after the same time period for bare P25. Manifestly, this

UC–P25–GR ternary composite photocatalyst presented here

shows remarkable improvements in the photodegradation rate

under solar irradiation. To test the stability of the UC–P25–GR

catalyst for the degradation of MO under sunlight, the photo-

degradation of MO was monitored for four consecutive cycles,

each for 60 min. From the cycling runs in the photodegradation

of MO (see ESI, Fig. S2†), it was noted that no significant

decrease was observed in the photodegradation rate during the

four consecutive cycles, which indicates the good stability of the

prepared UC–P25–GR photocatalyst. The UC–P25–GR

samples also display the best catalytic activities compared to pure

P25, UC–P25 and P25–GR on decomposing methylene blue

under simulated solar irradiation (see ESI, Fig. S3†). To test the

abilities of this new photocatalyst in real life, real wastewater

from the dye industry was collected to evaluate photo-

degradation activity of the as-prepared photocatalyst. As shown

in Fig. 8 and 9, it is clear that the UC–P25–GR possessed the best

photocatalytic activities among these four photocatalysts under

Fig. 8 The schematic illustration of photodegradation of wastewaters by

the as-prepared UC–P25–GR composites under simulated sunlight.

Fig. 9 Photodegradation of wastewaters by P25, UC–P25, P25–GR and

UC–P25–GR composites with reaction time of 120 min under irradiation

of simulated sunlight.

11770 | J. Mater. Chem., 2012, 22, 11765–11771

the same model photodegradation reaction. Furthermore,

similar photocatalysis experiments were carried out under

a 980 nm laser to check the photocatalytic activities of the P25,

P25–GR, UC–P25 and UC–P25–GR under NIR light. Fig. 10

indicates that the UC–P25–GR photocatalyst have the best

photocatalytic activity among these four samples under NIR

irradiation.

4. Conclusion

In this work, we have demonstrated a new strategy by integrating

the NIR-to-UV UC property of YF3:Yb3+,Tm3+ with the excel-

lent electrical properties of graphene to enhance the photo-

catalytic efficiency of TiO2. Following such ideas, a UC–P25–GR

photocatalyst with high performance has been successfully and

directly produced via a one-step hydrothermal method. The

enhanced photocatalytic activity is associated with the large

extended photoresponsive range, great adsorptivity of dyes and

high electron–hole separation efficiency due to the synergetic

interactions among TiO2, graphene and UC material. This work

is anticipated to promote practical applications of photocatalysts

under solar irradiation in addressing various environmental

issues.

Acknowledgements

This work was supported by the Grants from National Natural

Science Foundation of China (nos 51002129, 51172191,

11074211, and 10802071), the Cultivation Fund of the Key

Scientific and Technical Innovation Project (708068), Ministry of

Education of China, the Doctoral Program of Higher Education

(no. 200805300003) and the China Postdoctoral Science Foun-

dation funded project (no. 20100480068).

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