Graphene oxide/titania hybrid films with dual-UV-responsive surfaces oftunable wettability{

Pengzhan Sun,a Miao Zhu,a Renzhi Ma,b Kunlin Wang,a Jinquan Wei,a Dehai Wu,a Takayoshi Sasakib and

Hongwei Zhu*ac

Received 6th August 2012, Accepted 31st August 2012

DOI: 10.1039/c2ra21699j

Ultrathin hybrid films of graphene oxide (GO) and monolayer

titania (TO) were assembled by layer-by-layer and drop-casting

methods. The photo-induced wettability modulation of the

hybrid films with different configurations was systematically

studied. Due to the photocatalytic reduction of GO by TO, GO

sheets in the hybrid films exhibited a tendency to undergo photo-

induced conversion from hydrophilic to hydrophobic upon UV

irradiation. On the contrary, TO nanosheets showed the reverse

trend. Both surfaces of the hybrid film showed opposite yet

tunable hydrophilicity under UV irradiation, demonstrating the

potential for future application in liquid transport engineering.

1. Introduction

It is of great importance to control the wettability of solid surfaces in

various fields such as coating and printing as well as adhesion.1,2

Typically, the wettability can be varied by chemical modification of

the surfaces3 and application of specific external factors such as

temperature gradient,4 electric fields,5–7 light irradiation8,9 and so on.

Among them, light irradiation is a potent method for controlling

the wettability of materials owing to its easy accessibility, high

conversion rate and destruction-free property.

Much attention has been attracted to this area in recent years since

the discovery of the photo-induced super hydrophilic properties of

TiO2 films, which have been applied in various kinds of field such

as for self-cleaning and anti-fogging glass, side mirrors, automobile

tiles, household glazing as well as buildings.10–14 TiO2 films show

amphiphilic properties under different conditions and a hydrophilic–

hydrophobic conversion can be achieved reversibly by alternating

UV irradiation and dark storage. At the same time, the mechanism

of photo-induced hydrophilic conversion has been investigated

extensively. The successful synthesis of monolayer titania (TO)

nanosheets by chemical exfoliation of layered titanates helps to better

understand the photo-induced hydrophilic conversion of the

surfaces, which lies in the fact that due to the structure of the TO

nanosheets it can be considered that the entire surface atoms are

arranged two-dimensionally.15,16 The TO nanosheets could be

assembled into multilayer films via the layer by layer (LBL)

electrostatic adsorption method and the thickness of the lamellar

multilayer films could be precisely controlled.17 The as-prepared

multilayer films exhibited efficient UV absorption properties18 and

excellent dielectric nature with dielectric constants of 90–140 in the

103–107 Hz range.19,20 Moreover, the LBL multilayer films exhibited

excellent photoinduced hydrophilic conversion properties and the

results from synchrotron radiation in-plane X-ray diffraction

demonstrated that slight but significant structural changes occurred

during the photo-induced hydrophilic conversion.21,22

Graphene, another two-dimensional monolayer material com-

posed of sp2 hybridized carbon bonds in a honeycomb-like network,

attracts much attention because of its unique and outstanding

properties.23 The fabrication of graphene oxide (GO) via oxidation

and exfoliation of graphite in aqueous solution is demonstrated to be

a promising method because it is cost-effective and easy to scale

up.24–26 There have been several methodologies for the reduction of

GO, such as chemical reduction,27–29 thermal reduction30,31 as well as

light reduction.32,33 The light reduction of GO with a photocatalyst

like TiO2 is a promising method due to its environment friendliness

and mild conditions. Moreover, graphene-based composites can be

directly fabricated during the reduction procedure.32–34

GO sheet can be regarded as graphene decorated with oxygen

functional groups on both sides of the sheet as well as around the

edges.35 These oxygen containing functional groups make GO super

hydrophilic. When GO sheets are reduced to rGO, the amount of

oxygen functional groups will decrease and a greater extent of the p

network will be restored within the graphene structure and thus it

will result in hydrophobic surfaces. As monolayer TO nanosheets

change from hydrophobic to hydrophilic upon UV irradiation, it is

of great importance to investigate the hybridization of GO and TO

nanosheets, which may result in the reduction of GO to rGO in the

presence of TO sheets as well as the dual tunable hydrophilicity of

the ultrathin hybrid films.

Herein, we wish to report the fabrication of ultrathin hybrid films

of GO and TO (Ti0.87O2) monolayers by using the LBL as well as

drop-casting (DC) methods. The ultraviolet light promoted the

aDepartment of Mechanical Engineering, Key Laboratory for AdvancedManufacturing by Materials Processing Technology, Tsinghua University,Beijing 100084, China. E-mail: hongweizhu@tsinghua.edu.cn;Fax: +86 10 62773637; Tel: +86 10 62781065bInternational Center for Materials Nanoarchitectonics, National Institutefor Materials Science Tsukuba, Ibaraki 305–0044, JapancCenter for Nano and Micro Mechanics (CNMM), Tsinghua University,Beijing 100084, China{ Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c2ra21699j

RSC Advances Dynamic Article Links

Cite this: RSC Advances, 2012, 2, 10829–10835

www.rsc.org/advances COMMUNICATION

This journal is � The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 10829–10835 | 10829

Publ

ishe

d on

03

Sept

embe

r 20

12. D

ownl

oade

d on

31/

10/2

014

08:2

8:21

. View Article Online / Journal Homepage / Table of Contents for this issue

decomposition of the interlayer PDDA, the in situ reduction of GO

and the surface modification of TO, resulting in the interesting dual

tunable wettability of the hybrid films (Fig. 1a). The mechanism and

potential applications of the hybrid films in photo-induced hydro-

philic/hydrophobic conversion are discussed.

2. Experimental

TO nanosheets were fabricated by exfoliating protonic titanate

crystals according to our previous work.15,16 GO sheets were

prepared by a modified Hummers’ method from worm-like

exfoliated graphite36 (see Experimental Section for details). The

fabrication process of the hybrid samples formed by the LBL

method is illustrated in Fig. S1a{: The surface-cleaned substrates

(quartz glass or Si wafer) were first immersed in a PDDA solution

(20 g L21, pH = 9) for 20 min to introduce positive charges onto the

substrates, and then thoroughly washed with deionized water.

PDDA-treated substrates were immersed in GO (or TO, the

corresponding photographs were shown in Fig. S2{) colloidal

suspension for another 20 min (pH = 9) and then the substrates

were rinsed with water. The LBL procedure was repeated to obtain

GOn–TOn or TOn–GOn multilayer lamellar films of different

configurations with the desired number of layers. The as-prepared

films were dried under nitrogen flow. Finally, the samples were

exposed to UV light for 48 h, during which time the PDDA layers

were completely removed (decomposed) and GO was reduced to

rGO (see XRD studies in the Experimental Section).

The fabrication of the hybrid samples by the DC method is

illustrated in Fig. S1b{: to obtain GOn–TOn or TOn–GOn lamellar

films, the surface-cleaned substrates were drop-cast with y1 mL of

0.2 mg mL21 GO colloidal suspension (or 0.16 mg mL21 TO

suspension) and dried in air at 80 uC. Then the GO (TO)-coated dry

substrates were drop-cast with y1 mL of 0.16 mg mL21 TO

colloidal suspension (or 0.2 mg mL21 GO suspension) and dried

under the above-mentioned conditions. To obtain GO/TO hybrid

films, the surface-cleaned substrates were drop-cast with y1 mL of a

mixture of GO (0.2 mg mL21) and TO (0.16 mg mL21) colloidal

suspensions (1 : 1 in volume) and then the samples were dried in air

at 80 uC.

3. Results and discussion

An atomic force microscopy (AFM) image of GO sheets deposited

on Si wafer by LBL method is displayed in Fig. 1b. GO sheets

adhered tightly on the Si wafer and the lateral size ranged from

several tens of nanometers to several micrometers. Most of the GO

sheets possessed a height of less than 2 nm and overlaps of several

layers were also found. A typical AFM image of TO deposited on Si

wafer by the LBL method is shown in Fig. 1c, revealing the self-

assembly of TO nanosheets with more uniform coverage compared

with previously reported result.34 Most of the monolayer nanosheets

were distributed uniformly on the substrate, but there were also some

patches showing overlapping as well as gaps between nanosheets.

Nearly all of them had a lateral size of several hundred nanometers

as well as a height of less than 2 nm for each sheet. An AFM image

of the GO–TO lamellar films fabricated by the LBL method is

displayed in Fig. 1d. It could be clearly observed that two layers of

nanosheets packed together on the substrate and the film possessed a

height of y4 nm, indicating that the GO and TO nanosheets

maintain a fine laminate structure and uniformly assemble together.

Fig. 1 Dual tunable wettability of the TO/GO hybrid film. (a) Schematic diagram of the dual tunable hydrophilicity of the hybrid film. AFM images of (b)

GO nanosheets, (c) TO nanosheets, (d) GO–TO film and (e) GO/TO hybrid film formed by LBL. The inset images show the height distributions of the

nanosheets marked by green lines. Scale bars: 0.5 mm. (f) Photograph of as prepared samples of GOn–TOn and (GO–TO)n. The samples in the first row are GO,

GO2–TO2, GO3–TO3, GO5–TO5, GO10–TO10. Samples of the second row are GO–TO, (GO–TO)2, (GO–TO)3, (GO–TO)5, (GO–TO)10. (g) Corresponding

photographs of samples shown in (f) after UV irradiation for 48 h.

10830 | RSC Adv., 2012, 2, 10829–10835 This journal is � The Royal Society of Chemistry 2012

Publ

ishe

d on

03

Sept

embe

r 20

12. D

ownl

oade

d on

31/

10/2

014

08:2

8:21

. View Article Online

An AFM image of the GO/TO hybrid films fabricated by the LBL

method from the GO (0.1 mg mL21) and TO (0.08 mg mL21) (1 : 1

in volume) colloidal suspension is shown in Fig. 1e. GO and TO

nanosheets were sporadically dispersed. Corresponding TEM images

are shown in Fig. S3{. The interlayer spacing changes of the hybrid

samples are revealed by X-ray diffraction (XRD) (see Fig. S4{ and

the Experimental Section for details).

During the LBL process, the brown color of the sample became

darker. For GOn–TOn and (GO–TO)n samples with the same n

value, there was little difference in color (Fig. 1f), which should be

attributed to the same number of TO and GO layers despite the

different stacking sequence. After UV irradiation for 48 h, all the

samples’ color changed from brown to black (Fig. 1g), which

revealed the effective reduction of GO to rGO in the presence of TO

nanosheets.

We also collected the UV-vis absorption spectra of the samples

fabricated by drop-casting and LBL methods as well as liquid phase

hybrid samples. The GO/TO hybrid suspension was prepared by

simply mixing two colloidal suspensions. Fig. S5,{ of the TO

colloidal suspension, shows a steep onset at around 324 nm,

corresponding to the band gap of TO (y3.84 eV). While no obvious

absorption edge for GO suspension was observed, the spectrum only

displayed an absorption peak located at y230 nm, which may

be dominated by the p–p* transition,37 and a shoulder located at

y300 nm, which may be attributed to the n–p transition of CLO.38,39

When mixing the two kinds of colloidal suspension, no obvious

aggregates were observed, even after UV irradiation for different

lengths of time (Fig. S2b–d{), which demonstrated the stability of the

hybrid suspension. UV-vis absorption spectra of the hybrid

suspension with different ratios are also shown in Fig. S5.{ These

results revealed that the absorption onset red shifted as the amount

of GO in the suspension was increased and the maximum absorption

intensity increased due to the presence of GO sheets, while the

maximum absorption edge remained the same as that of the TO

colloidal suspension. The zeta potential of GO (0.1 mg mL21),

TO (0.08 mg mL21) as well as the GO (0.1 mg mL21) and TO

(0.08 mg mL21) hybrid suspension (1 : 1 in volume) were 241 mV,

242.5 mV and 245.4 mV, respectively. These results demonstrated

the excellent stability of GO and TO as well as the GO and TO

hybrid suspensions.

We investigated the photo-induced hydrophilicity of GO/TO

hybrid films fabricated by the LBL as well as drop-casting methods

based on different configurations, as shown in Fig. 2 and Fig. 3. The

as-prepared GO/TO hybrid films fabricated via the LBL method

were irradiated with UV light for 48 h to remove the PDDA layer.

Then all the films were stored in the dark at 80 uC for 3 days. The

hybrid films fabricated via the drop-casting method were examined

directly after storage in the dark for 10 h due to their polymer-free

nature. In Fig. 2a, with UV irradiation the contact angles of the TO-

GO films fabricated by the drop-casting method increased at first

and then slowly decreased after UV irradiation for 12 h. The insets

show photographs of water drops deposited on the hybrid films after

UV irradiation for different lengths of time, which clearly show the

change in the contact angle. The mechanism of hybrophilicity

modulation can be explained as follows: each individual GO sheet

can be viewed as graphene decorated with oxygen functional groups

on both sides of the sheet as well as around the edges.35 These

functional groups make the GO sheets superhydrophilic. As UV

irradiation continued, electron–hole pairs were generated in the TO

nanosheets. The photo-generated electrons were directly captured by

the GO sheets which were then reduced to rGO sheets. With further

UV treatment, more electron–hole pairs were generated and the GO

sheets captured more photo-generated electrons, leading to a greater

extent of reduction, which represented the decrease in the amount of

oxygen functional groups and greater p network restoration within

the graphene structure. This modification resulted in an increase in

the contact angle. However, longer UV irradiation led to a decrease

in carbon content and an increase in carbon defects40 as well as the

easier absorption of dissociated water on defects, which resulted in a

decrease in contact angle. To further investigate the photo-induced

hydrophobic/hydrophilic conversion of GO/TO hybrid films fabri-

cated by the drop-casting method, we conducted the same

experiment with GO–TO and GO/TO samples. In Fig. 2b, the

contact angle decreased quickly with UV irradiation in an

exponential form, and agreed well with previous results.21,22 As the

TO layers were the topmost layer of the sample, upon UV

irradiation, the change of the contact angle mostly reflected the

properties of TO nanosheets. However, in Fig. S6,{ where the sample

was fabricated by drop-casting with GO/TO hybrid suspension as

the source, surprisingly it was found that the contact angle remained

Fig. 2 Photo-induced wettability evolution of (a) TO–GO and (b) GO–TO films formed by drop-casting.

This journal is � The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 10829–10835 | 10831

Publ

ishe

d on

03

Sept

embe

r 20

12. D

ownl

oade

d on

31/

10/2

014

08:2

8:21

. View Article Online

nearly constant. This could be attributed to the fact that the GO and

TO nanosheets self-assembled together in the suspension. When

drop-cast onto the substrate, these hybrid films self-assembled at the

interface of the substrates to form a nearly uniform arrangement

with a GO layer on the top and a TO layer on the bottom. As most

of the GO sheets were on the top, the hybrid films made by drop-

casting exhibited the properties of GO sheets, whose contact angles

increased slowly upon UV irradiation for a short period of time.

As shown in Fig. 3, for samples fabricated via the LBL method,

different structures exhibited different trends in the change in contact

angle under UV irradiation. For structures of (GO/TO)n, GOn–TOn

and (GO–TO)n, all the contact angles decreased with the increase of

UV irradiation time. While for structures of TOn–GOn and (TO–

GO)n, the contact angles fluctuated on a small scale. This could be

explained as follows: for structures of (GO/TO)n, GOn–TOn and

(GO–TO)n, the topmost layer is TO, whose contact angle decreased

Fig. 3 Photo-induced wettability evolution of different GO and TO hybrid structures formed by LBL. (a) n = 1. (b) n = 2. (c) n = 5. Right panels show the

corresponding photographs of water droplets.

10832 | RSC Adv., 2012, 2, 10829–10835 This journal is � The Royal Society of Chemistry 2012

Publ

ishe

d on

03

Sept

embe

r 20

12. D

ownl

oade

d on

31/

10/2

014

08:2

8:21

. View Article Online

upon UV irradiation. While for structures of TOn–GOn and (TO–

GO)n, GO sheets stayed on the top. After UV irradiation for 48 h,

they were reduced thoroughly and all converted into relatively

hydrophobic forms. A common phenomenon was demonstrated in

Fig. 3: after UV irradiation for 120 min, the change of contact angle

decreased in the order of rGOn–TOn, (rGO–TO)n and (rGO/TO)n

and the time for reaching saturation also decreased in this order. As

shown in Fig. 1e, the substrate coverage achieved via the LBL

method from the GO/TO hybrid colloidal suspension was smaller

than that of films made from only one component in suspension.

The nanosheets on the substrate were more isolated from others and

there were also fewer TO nanosheets, which led to the lowest change

of contact angle under UV irradiation. However, there remained a

question as to why the change of contact angle of GOn–TOn was

larger than that of (GO–TO)n. Although the mechanism of photo-

induced hydrophilic conversion of TO is still under discussion, two

key explanations are proposed here: (i) photocatalytic decomposition

of hydrophobic contaminates on the surface;41–43 (ii) structure

changes of TO.44–49 In our cases, if the first explanation was

responsible for the photo-induced hydrophilic conversion, according

to the previous work,34 the photo-generated electrons would be

quickly captured by the interlayer GO sheets between each two TO

layers, which would prohibit the recombination of electron–hole

pairs and lead to the enhancement of the photocatalytic capability of

TO. Then (GO–TO)n would more effectively decompose hydro-

phobic contaminates on the surface than GOn–TOn. However, the

results were opposite in our cases, which demonstrates that the

second explanation seems more reasonable for the photo-induced

hydrophilic conversion of TO. The change of contact angle of GOn–

TOn was larger than that of (GO–TO)n, which could be attributed to

the hypothesis that the GO sheets under the TO layers could

promote structure changes in TO nanosheets thanks to the removal

of the oxygen functional groups from the GO sheets during the

photo reduction process, and thus more effective photo-induced

hydrophilic conversion was obtained. The effect of GO on the

structure of the TO nanosheets needs to be further investigated.

Upon UV irradiation, there was a tendency for GO sheets to

exhibit an increase in contact angle. In other words, with UV

treatment, the surface of a GO sheet changed from hydrophilic to

relatively hydrophobic. As the TO nanosheets exhibited the reverse

tendency of surface hydrophilicity, we could imagine that, after

hybridization of GO and TO nanosheets, the two surfaces of the

hybrid films could achieve relatively different hydrophilic/hydro-

phobic properties upon UV irradiation, which will have potential

future applications in water transport engineering.

Based on the above idea, freestanding hybrid films of GO and TO

sheets were fabricated, as illustrated in Fig. 4a: first the GO

suspension was drop-cast on a piece of paper, followed by drying in

air, then the TO suspension was drop-cast on the formed GO film

followed by drying under the same conditions. The as-prepared

freestanding hybrid film shows excellent flexibility, mechanical

strength and a similar contact angle changing tendency to that in

Fig. 2: the GO surface changed from hydrophilic to relatively

hydrophobic upon UV irradiation, while the TO surface showed the

reverse tendency. The difference in wettability of both surfaces of the

hybrid film can be tuned by UV irradiation time (Fig. 4b and

Fig. 4c). To demonstrate the ability of water collection of both

surfaces of the freestanding hybrid film, after 8 h UV irradiation the

freestanding film was moisturized by an air humidifier. As shown in

Fig. 4d and Fig. 4e, lots of condensed water droplets were observed

on the TO surface, revealing its excellent hydrophilic nature. On the

contrary, many fewer water drops were formed on the reduced GO

surface. The above phenomena demonstrated the excellent dual

photo-induced tunable wettability of GO and TO hybrid films,

which will have potential future applications in water collection and

transportation.

4. Conclusions

In conclusion, we have fabricated GO and TO hybrid films via the

LBL and drop-casting methods and investigated the decomposition

of PDDA and GO reduction in lamellar films fabricated via the LBL

method. We investigated the photo-induced wettability evolution of

the hybrid films with different configurations. In the hybrid films of

GO and TO, the GO sheets exhibited photo-induced hydrophobic

conversion upon UV irradiation, while TO nanosheets showed the

reverse trend. After hybridization, both sides of the film show

opposite surface property change tendencies under UV irradiation.

The hybrid films of GO and TO nanosheets will have potential

future applications in liquid transport engineering.

Experimental section

GO preparation

Natural graphite flakes were mixed with concentrated sulfuric acid

and hydrogen peroxide, after that the mixture was stirred for 1 h and

then washed with deionized (DI) water until the pH reached 7. After

drying at 40 uC for 24 h, the obtained graphite intercalation

compounds were converted to expanded graphite through fast

heating at 900 uC for 10 s. Then the obtained worm-like graphite was

further treated with a modified Hummer’s method to obtain graphite

oxides. The GO colloidal suspension was further obtained by

sonication in water. The fabrication processes of the hybrid samples

by LBL and DC methods are illustrated in Fig. S1.{

Microscopic characterizations

The photographs of the as-prepared GO (0.1 mg mL21) and TO

(0.08 mg mL21) colloidal suspensions are shown in Fig. S2.{ Both of

these colloidal suspensions exhibit a clear, uniform nature and show

no obvious agglomerates after several weeks. Fig. S3a{ shows the

TEM image of GO sheets, which possess a lateral size in the

magnitude of micrometers. Fig. S3b{ shows the TEM image of TO

nanosheets, which possess a lateral size of several hundred

nanometers and distribute uniformly on the TEM grid. Fig. S3c{shows a TEM image of the GO–TO hybrid film assembled by the

LBL method, clearly showing two layers of nanosheets. Those with

larger lateral sizes are the GO sheets while the ones with smaller

lateral sizes are the TO nanosheets. Fig. S3d{ is a TEM image of the

GO/TO hybrid film fabricated by drop-casting using the GO and TO

(1 : 1 in volume) colloidal mixture suspension as the source. All the

nanosheets pack together and it is hard to distinguish between the

two different kinds of sheets.

XRD

The interlayer spacing changes of the hybrid samples were revealed

by XRD, as shown in Fig. S4a and S4b.{ The diffraction peaks of as-

prepared hybrid films of GOn–TOn and (GO–TO)n are located at

This journal is � The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 10829–10835 | 10833

Publ

ishe

d on

03

Sept

embe

r 20

12. D

ownl

oade

d on

31/

10/2

014

08:2

8:21

. View Article Online

2h = 5u and 4.98u, respectively. While after UV irradiation for 24 h,

the diffraction peaks shift to 2h = 6.92u and 5.98u, corresponding to a

decrease of the interlayer distance from 1.77 nm to 1.28 nm and

1.48 nm, respectively. After UV irradiation for 48 h, the diffraction

peaks further shift to 2h = 7.46u and 6.8u, corresponding to a further

decrease of the interlayer distance to 1.18 nm and 1.30 nm,

respectively. Both of as-prepared samples have been subjected to

annealing at 300 uC for 1 h instead of UV irradiation, and they show

a diffraction peak at 2h = 8.5u, corresponding to an interlayer

distance of 1.04 nm. Based on above results, we propose a

mechanism to explain the structural evolution of GOn–TOn and

(GO–TO)n samples under UV irradiation, as shown in Fig. S4c and

S4d.{ Upon UV irradiation, GOn–TOn and (GO–TO)n samples

showed different procedures of PDDA decomposition along with

reduction of GO to rGO. In (GO–TO)n, UV generated holes and

electrons in the titania layers were used for PDDA decomposition

and the reduction of GO to rGO and these two processes occurred

simultaneously, resulting in a relatively slow and uniform decrease in

the interlayer distance. As for the sample of GOn–TOn, the PDDA

layers beneath the TO layers decomposed faster than those beneath

Fig. 4 (a) Process schematics for fabricating the freestanding hybrid film of GO and TO sheets, and corresponding photographs (b) and (c) Dual tunable

hydrophilicity of the hybrid film under UV irradiation. (d) and (e) Photographs of two UV treated surfaces of the humidified film.

10834 | RSC Adv., 2012, 2, 10829–10835 This journal is � The Royal Society of Chemistry 2012

Publ

ishe

d on

03

Sept

embe

r 20

12. D

ownl

oade

d on

31/

10/2

014

08:2

8:21

. View Article Online

the GO layers and hence GO layers were reduced much more slowly,

resulting in a sharper decrease in the interlayer distance of the titania

layers on the top than that in (GO–TO)n. As the decomposition of

the PDDA layers beneath the TO layers was almost complete, the

decomposition of PDDA layers beneath GO sheets as well as the

reduction of GO to rGO became faster. The nearly identical

interlayer spacing of TO layers on the top as well as the sharp

decrease in the interlayer spacing of GO layers resulted in a relatively

slower decrease in the interlayer spacing than that in (GO–TO)n.

Finally, after further UV irradiation, GOn–TOn and (GO–TO)n

samples reached nearly the same interlayer spacing.

UV-vis absorption spectra

We have also collected UV-vis absorption spectra for the samples

fabricated by drop-casting and the LBL method as well as liquid

phase hybrid samples. In Fig. S5a,{ GO sheets drop-cast on quartz

glass exhibit an absorption peak at y225 nm, which is dominated by

p–p* transitions. While other GO (rGO)/TO hybrid films exhibit two

absorption peaks located at y225 nm and y265 nm, which are

characteristic of GO and TO nanosheets, respectively. After UV

irradiation for 48 h, due to the reduction of GO to rGO, both the

GO–TO and GO/TO hybrid films show stronger light absorption.

The UV-vis absorption spectra of the samples fabricated by the LBL

method are shown in Fig. S5b,{ in all the samples exists an intense

absorption peak at y265 nm and a weak shoulder at y225 nm. For

different stacking sequences of GO and TO nanosheets, as the

number of layers increases, the peak intensity located at y265 nm as

well as the shoulder located at y225 nm increase gradually,

suggesting a uniform configuration of GO and TO nanosheets. It is

interesting to note that after 48 h of UV irradiation, the peak

intensities of these two different structures change differently. For

GOn–TOn (n = 1, 2, 5), the peak intensity located at y265 nm and

the shoulder located at y225 nm increase after UV treatment, while

for (GO–TO)n (n = 1, 2, 5), the peak and the shoulder decrease after

UV irradiation for 48 h (Fig. S5d,e).{

Acknowledgements

This work was supported by the National Science Foundation of

China (50972067) and the Beijing Natural Science Foundation

(2122027).

References

1 X. Deng, L. Mammen, H. J. Butt and D. Vollmer, Science, 2012, 335, 67.2 J. Rafiee, X. Mi, H. Gullapalli, A. V. Thomas, F. Yavari, Y. F. Shi, P. M.

Ajayan and N. A. Koratkar, Nat. Mater., 2012, 11, 217.3 M. K. Chaudhury and G. M. Whitesides, Science, 1992, 256, 1539.4 A. M. Cazabat, F. Heslot, S. M. Troian and P. Caries, Nature, 1990, 346,

824.5 A. Quinn, R. Sedev and J. Ralston, J. Phys. Chem. B, 2003, 107, 1163.6 V. Peykov, A. Quinn and J. Ralston, Colloid Polym. Sci., 2000, 278, 789.7 M. Schneemilch, W. J. J. Welters, R. A. Hayes and J. Ralston, Langmuir,

2000, 16, 2924.8 S. Abbott, J. Ralston, G. Reynolds and R. Hayes, Langmuir, 1999, 15,

8923.9 N. Stevens, C. I. Priest, R. Sedev and J. Ralston, Langmuir, 2003, 19, 3272.

10 R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A.Kitamura, M. Shimohigoshi and T. Watanabe, Nature, 1997, 388, 431.

11 R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A.Kitamura, M. Shimohigoshi and T. Watanabe, Adv. Mater., 1998, 10, 135.

12 N. Sakai, R. Wang, A. Fujishima, T. Watanabe and K. Hashimoto,Langmuir, 1998, 14, 5918.

13 R. Wang, N. Sakai, A. Fujishima, T. Watanabe and K. Hashimoto,J. Phys. Chem. B, 1999, 103, 2188.

14 T. Watanabe, A. Nakajima, R. Wang, M. Minabe, S. Koizumi, A.Fujishima and K. Hashimoto, Thin Solid Films, 1999, 351, 260.

15 T. Tanaka, Y. Ebina, K. Takada, K. Kurashima and T. Sasaki, Chem.Mater., 2003, 15, 3564.

16 T. Sasaki, M. Watanabe, H. Hashizume, H. Yamada and H. Nakazawa,J. Am. Chem. Soc., 1996, 118, 8329.

17 T. Sasaki, Y. Ebina, T. Tanaka, M. Harada, M. Watanabe and G.Decher, Chem. Mater., 2001, 13, 4661.

18 T. Sasaki, Y. Ebina, M. Watanabe and G. Decher, Chem. Commun., 2000,2163.

19 M. Osada, Y. Ebina, H. Funakubo, S. Yokoyama, T. Kiguchi, K. Takadaand T. Sasaki, Adv. Mater., 2006, 18, 1023.

20 K. Akatsuka, M. Haga, Y. Ebina, M. Osada, K. Fukuda and T. Sasaki,ACS Nano, 2009, 3, 1097.

21 N. Sakai, K. Fukuda, T. Shibata, Y. Ebina, K. Takada and T. Sasaki,J. Phys. Chem. B, 2006, 110, 6198.

22 T. Shibata, N. Sakai, K. Fukuda, Y. Ebina and T. Sasaki, Mater. Sci.Eng., B, 2009, 161, 12.

23 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. S. Zhang, V.Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666.

24 D. Li, M. B. Muller, S. Gilje, R. B. Kaner and G. G. Wallace, Nat.Nanotechnol., 2008, 3, 101.

25 Y. Xu, H. Bai, G. Lu, C. Li and G. Shi, J. Am. Chem. Soc., 2008, 130,5856–5857.

26 Y. Si and E. T. Samulski, Nano Lett., 2008, 8, 1679.27 S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A.

Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff, Carbon,2007, 45, 1558.

28 C. Gomez-Navarro, R. T. Weitz, A. M. Bittner, M. Scolari, A. Mews, M.Burghard and K. Kern, Nano Lett., 2007, 7, 3499.

29 S. Gilje, S. Han, M. Wang, K. L. Wang and R. B. Kaner, Nano Lett.,2007, 7, 3394.

30 X. Wang, L. Zhi and K. Mullen, Nano Lett., 2008, 8, 323.31 H. A. Becerill, J. Mao, Z. Liu, R. M. Stoltenberg, Z. Bao and Y. Chen,

ACS Nano, 2008, 2, 463.32 G. Williams, B. Seger and P. V. Kamat, ACS Nano, 2008, 2, 1487.33 O. Akhavan and E. Ghaderi, J. Phys. Chem. C, 2009, 113, 20214.34 K. K. Manga, Y. Zhou, Y. Yan and K. P. Loh, Adv. Funct. Mater., 2009,

19, 3638.35 H. C. Schniepp, J. L. Li, M. J. McAllister, H. Sai, M. Herrera-Alonso,

D. H. Adamson, R. K. Prud’homme, R. Car, D. A. Saville and I. A.Aksay, J. Phys. Chem. B, 2006, 110, 8535.

36 W. Gu, W. Zhang, X. Li, H. Zhu, J. Wei, Z. Li, Q. Shu, C. Wang, K.Wang, W. Shen, F. Kang and D. Wu, J. Mater. Chem., 2009, 19, 3367.

37 J. Robertson, Mater. Sci. Eng., R, 2002, 37, 129.38 J. I. Paredes, S. Villar-Rodil, P. Sol| ?s-Fernandez, A. Mart| ?nez-Alonso

and J. M. D. Tascon, Langmuir, 2009, 25, 5957.39 M. F. Islam, E. Rojas, D. M. Bergey, A. T. Johnson and A. G. Yodh,

Nano Lett., 2003, 3, 269.40 O. Akhavan, M. Abdolahad, A. Esfandiar and M. Mohatashamifar,

J. Phys. Chem. C, 2010, 114, 12955.41 C.-Y. Wang, H. Groenzin and M. J. Shultz, Langmuir, 2003, 19, 7330.42 J. M. White, J. Szanyi and M. A. Henderson, J. Phys. Chem. B, 2003, 107,

9029.43 T. Zubkov, D. Stahl, T. L. Thompson, D. Panayotov, O. Diwald and J. T.

Jr. Yates, J. Phys. Chem. B, 2005, 109, 15454.44 N. Sakai, A. Fujishima, T. Watanabe and K. Hashimoto, J. Phys. Chem.

B, 2001, 105, 3023.45 N. Sakai, A. Fujishima, T. Watanabe and K. Hashimoto, J. Phys. Chem.

B, 2003, 107, 1028.46 R. Nakamura, K. Ueda and S. Sato, Langmuir, 2001, 17, 2298.47 A. Y. Nosaka, E. Kojima, T. Fujiwara, H. Yagi, H. Akutsu and Y.

Nosaka, J. Phys. Chem. B, 2003, 107, 12042.48 A. Y. Nosaka and Y. Nosaka, Bull. Chem. Soc. Jpn., 2005, 78, 1595.49 K. Uosaki, T. Yano and S. Nihonyanagi, J. Phys. Chem. B, 2004, 108,

19086.

This journal is � The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 10829–10835 | 10835

Publ

ishe

d on

03

Sept

embe

r 20

12. D

ownl

oade

d on

31/

10/2

014

08:2

8:21

. View Article Online