Photocatalytic and antibacterial properties of Au-TiO2 nanocomposite on monolayergraphene: From experiment to theoryWangxiao He, Hongen Huang, Jin Yan, and Jian Zhu Citation: Journal of Applied Physics 114, 204701 (2013); doi: 10.1063/1.4836875 View online: http://dx.doi.org/10.1063/1.4836875 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/114/20?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Synthesis of ZnO decorated graphene nanocomposite for enhanced photocatalytic properties J. Appl. Phys. 115, 173504 (2014); 10.1063/1.4874877 Cationic (V, Y)-codoped TiO2 with enhanced visible light induced photocatalytic activity: A combinedexperimental and theoretical study J. Appl. Phys. 114, 183514 (2013); 10.1063/1.4831658 Synthesis and characterization of zinc doped nano TiO 2 for efficient photocatalytic degradation of EriochromeBlack T AIP Conf. Proc. 1536, 103 (2013); 10.1063/1.4810121 Core/shell nano-structuring of metal oxide semiconductors and their photocatalytic studies AIP Conf. Proc. 1512, 34 (2013); 10.1063/1.4790898 Graphene oxide as a photocatalytic material Appl. Phys. Lett. 98, 244101 (2011); 10.1063/1.3599453

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Photocatalytic and antibacterial properties of Au-TiO2 nanocompositeon monolayer graphene: From experiment to theory

Wangxiao He,1 Hongen Huang,1 Jin Yan,1 and Jian Zhu1,2,a)

1School of Life Sciences and Technology, Xi’an Jiaotong University, Xi’an 710049, China2The Key Laboratory of Biomedical Information Engineering of Ministry of Education,Xi’an Jiaotong University, Xi’an 710049, China

(Received 19 August 2013; accepted 12 November 2013; published online 27 November 2013)

The formation of the Au-TiO2 nanocomposite on monolayer Graphene (GTA) by sequentially

depositing titanium dioxide particles and gold nanoparticles on graphene sheet was synthesized

and analyzed in our work. The structural, morphological, and physicochemical properties of

samples were thoroughly investigated by UV-Vis spectrophotometer, Raman spectroscopy, Fourier

transform infrared spectroscopy, atomic force microscopy, scanning electron microscope, and

transmission electron microscope. Photocatalytic performance of GTA, graphene (GR), TiO2, and

TiO2 -graphene nanocomposite (GT) were comparatively studied for degradation of methyl orange,

and it was found that GTA had highest performance among all samples. More importantly,

antibacterial performance of this novel composite against Gram-positive bacteria, Gram-negative

bacteria, and fungus was predominant compared to GR, TiO2, and GT. And the result of

biomolecules oxidation tests suggested that antimicrobial actions were contributed by oxidation

stress on both membrane and antioxidant systems. Besides, the rate of two decisive processes

during photocatalytic reaction, the rate of the charge transfer (kCT) and the rate of the electron-hole

recombination (kR) have been studied by Perturbation theory, Radiation theory, and Schottky

barrier theory. Calculation and derivation results show that GTA possesses superior charge

separation and transfer rate, which gives an explanation for the excellent oxidation properties of

GTA. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4836875]

I. INTRODUCTION

Graphene nanosheets have attracted a great deal of inter-

est due to their fundamental importance and promising appli-

cations in various fields, such as field effect transistors,

transparent conducting electrodes, supercapacitors, batteries,

photocatalysis, gas sensors, field emission devices, and

nanocomposite.1–5 Recently, with the advent of more evi-

dence that graphene nanosheets are not toxic towards mam-

malian cells,6 much more interest is being drawn to biological

applications of graphene nanosheets. The preliminary research

has showed that graphene can be an ideal candidate for bio-

medical applications, such as photothermal therapy of cancer,

gene transfection, and magnetic resonance imaging.5,7,8 As

one of the thinnest and strongest materials in the world,

graphene, the single atomic layer of graphite, with its high

mobility, high mechanical flexibility, high electric carrier con-

centration, optical transparency, intrinsic large surface, and

capability of chemical functionalization, can be an ideal me-

chanical support and electric charge carrier shuttle of photo

sensitizers and catalysts to construct nanocomposite photoca-

talysts with enhanced performance.9 Additionally, because of

the ballistic transport of graphene sheets at room temperature,

this functionalized graphene can dispersedly make itself to be

potentially ideal electron sinks or electron transfer bridges in

the composites.10 Therefore, it’s our aspiration to use gra-

phene carrying photosensitizers to construct a photocatalyst,

which has huge surface and excellent photocatalytic properties

to adsorb microorganisms and result in oxidative biological

systems damage of microbe.

Antibacterial agents broadly categorized as organic or

inorganic, are widely used in daily life for the prevention of

public health issues due to the ubiquitous micro-organisms

and their ability to establish themselves.11 Compared with

organic antibacterial agents, the inorganic agents have many

advantages such as higher heat resistance, lower decompos-

ability, and longer life expectancy.12 In this regard, the use

of nanostructured materials with antibacterial properties is

highly sought after owing to their ability to inhibit bacterial

growth attributed to their size, structure, and surface proper-

ties.13 Titanium dioxide (TiO2), one of those nanostructured

materials, has received much attention as a promising tech-

nology for pollution remediation and antibacterial properties

due to strong heterogeneous photocatalytic oxidation prop-

erty against organic compounds.

With its 3.2 eV electronic band gap, TiO2-based materi-

als are the most commonly used semiconductor photocata-

lysts because of their high oxidation capability, extreme

chemical stability, and excellent photostability.14 The effi-

ciency of the photocatalysis process is measured by quantum

yield (/), given by the simple relationship15

U / kCT

kCT þ kR¼ 1� kR

kCT þ kR; (1)

where it is proportional to the rate of the charge transfer (kCT)

and inversely proportional to the rate of the electron-hole

recombination (kR).15 The above equation reveals thata)Electronic mail: nanoptzj@163.com

0021-8979/2013/114(20)/204701/12/$30.00 VC 2013 AIP Publishing LLC114, 204701-1

JOURNAL OF APPLIED PHYSICS 114, 204701 (2013)

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expediting charge transfer processes and preventing the

electron/hole recombination under illumination would be criti-

cal for the improvement of the quantum yield, and a plausible

way is to immobilize the semiconductor on a suitable electron

acceptors.16 For this purpose, graphene, due to its superior

properties mentioned previously, used as mechanical support

and electric shuttle in composites for anchoring TiO2 nanopar-

ticles, has been reported,17–20 which will enhance the photoca-

talysis activity both by dramatically boosting reactant

adsorbability giving high reaction possibility and by acting a

photosensitizer to generate a greater density of electron/hole

pairs for high quantum efficiency.21 Besides, the heterojunc-

tion between graphene and TiO2 promotes separation of elec-

tron/hole pairs in TiO2 with electron injecting into graphene

that acts as electron sink hindering recombination,22 while the

hole remains in graphene are also capable of driving the oxi-

dation process.

However, unfortunately, the agglomeration of TiO2 NPs

on graphene prohibits the direct chemical contact between

the two components, which will dramatically diminish these

two photoelectron injection into graphene with the potential

for higher photocatalytic efficiency. What’s worse, although

the formation of Ti�O�C bonding using carbon as dopant

in TiO2 can extend TiO2 light absorption cutoff wavelength,

this photocatalytic also has very narrow range of light

absorption wavelength, which is due to the narrow absorp-

tion range of TiO2 (only sensitive to the light wavelengths

below 380 nm) and limited number of doping level. In order

to solve these problems, it is a good choice to use gold nano-

particles (Au NPs) modifying the photocatalytic. It has been

demonstrated that visible-light induced photocatalytic,

Au-modified TiO2 powder, can result in electron injection

from Au to TiO2 by the absorption of light around 550 nm,

caused by the localized surface plasmon resonance effect

and heterojunction barrier effect, and then the resultant

electron-deficient Au can oxidize organic compounds to be

recovered to its original metallic state.23,24 What’s more, the

excellent electrical conductivity of Au will help two kinds of

photoelectrons inject into graphene easily for higher photo-

catalytic efficiency, and the outstanding adsorption of

bio-macromolecular will enhance the oxidation efficiency to

kill microorganisms.

Even though graphene, TiO2 and Au NPs are promising

candidate in photocatalytic applications, the research about

the photocatalytic mechanism and theory is very limited and

remains wide-open. Hence, in this study, we first synthesized

a self-assembly nanophase materials modified by Au NPs

called GTA (TiO2-graphene-Au), and here, its synthetic

route, superior photocatalytic capability and outstanding

antimicrobial activity will be reported. Based on the basic

principles of semiconductor physics and quantum mechan-

ics, we established a model of electron excitation under regu-

lar perturbation and a model heterogeneous junction barrier

and electron trap, to explain the reason why GTA has the

most effective charge separation capacity, the most abundant

impurity levels, the highest oxidation ability, and the most

excellent antibacterial properties. Furthermore, an oxidation

test about some biological molecules will be presented, and

it will give explanation about the strong bactericidal ability

of GTA on the biochemical point of view. Moreover, it is

worth emphasizing the green synthetic strategy of GTA and

no toxic action to mammalian cells. Since the utilization

of nontoxic chemicals and environmental friendly solvents,

there exists a potential of GTA as antibacterial used in

human skin.

II. METHOD

A. Reagents

Nano-titanium dioxide particles (P25, Anatase), the nano

graphite powder with 40 nm average particle diameter, sodium

dodecyl benzene sulfonate, Chloroauric acid, and DNTB were

purchased from Aladdin Reagent. Thiobarbituric acid was pur-

chased from Sinopharm Chemical Reagent Co. Ltd., China.

Unless otherwise specified, Potassium permanganate (KMnO4),

sulfuric acid (H2SO4), hydrogen peroxide (H2O2), hydrochloric

acid (HCl), hydrazine hydrate, sodium hydroxide (NaOH), and

other reagents and materials were obtained commercially from

the Xi’an Chemical Reagent Factory (Xi’an, China) and used

as received without further purification. All the chemicals used

in this research were analytical grade and doubly distilled water

is used throughout the experiments. The experiments were car-

ried out at room temperature and humidity.

B. Synthesis of graphene oxide (GO)

The GO nanosheets were synthesized according to the

modified Hummer’s method25 and a reoxidation method

using nano-graphite powder as the starting material. To

begin with, 3 g of graphite was added into a mixture of 2.5 g

of K2S2O8, 2.5 g of P2O5, and 12 ml of concentrated H2SO4

(98%). After being heated to 80 �C and kept stirring for

4.5 h, 500 ml of deionized water was slowly added into the

mixture for dilution. Then, the preliminary oxides of the

graphite powder were recovered by filtration and vacuum

drying. After that, reoxidation was implemented by the solu-

tion of all preliminary oxides in 12 ml 98% H2SO4, while

keeping the temperature about 0 �C. Then, KMnO4 (15 g)

was gradually added to the above solution controlling the

temperature below 10 �C. The mixture was then stirred at

35 �C for 2 h. The reaction was terminated by the addition of

30% H2O2 solution (20 ml).The resulting solution was

diluted by adding 1000 ml of distilled water, and a dark

brown color suspension can be obtained. After being contin-

uously stirred for 2 h, the mixture was washed by repeated

centrifugation and filtration using a 5% HCl aqueous solu-

tion in order to remove any metal ions. Further, the centrifu-

gation process was repeated with distilled water until the pH

of the solution becomes neutral. The obtained brown colored

precipitate is graphitic oxide, and then dried it under vac-

uum. Finally, 160 ml of water was added to the resulting pre-

cipitate and sonicated well for nearly 1 h to obtain a uniform

suspension of GO.

C. Synthesis of composite photocatalysts

First, synthesize the TiO2 graphene nanocomposite

(GT). A simple hydrothermal method using methanol water

as solvent was used as the following.26 100 mg GO was

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ultrasonicated in a 100 ml of deionized water to disperse it

well; after that, 1 g TiO2 (P25), 100 mg sodium dodecyl ben-

zene sulfonate (SDBS) and 100 mg trisodium citrate were

added to the GO solution. Then, 30 ml of anhydrous ethanol

solution was added to the solution, because under such a sol-

vothermal condition, the solvent of ethanol water has a

strong help to reduce GO to GR.27 The mixing solution was

aged with vigorous stirring for 1 h to obtain a homogeneous

suspension. Then, this suspension was transferred to a

100 ml Teflon-sealed autoclave and maintained at 120 �C for

6.5 h. By this hydrothermal treatment, the reduction of GO to

GR and the deposition of TiO2 onto the GR sheet can be

simultaneously achieved.

Second, Synthesize the TiO2-gold graphene nanocompo-

site (GTA). The growth of Au on TiO2-graphene nanocom-

posite was also achieved by the hydrothermal method.26 Au

precursors were fabricated by preparing a solution of chlor-

oauric acid (0.067%), then 1 ml this solution was added into

100 ml boiling deionized water with 1 ml trisodium citrate

solution (0.1% mass fraction), and the resulting mixture was

stirred for 20 min at boiling temperature. Then, Au precur-

sors were expanded volume to 100 ml by adding deionized

water, and the products were mixed with GT by the volume

ratio of 1:1. After that, the mixing solution was stirred for

3 h at 90 �C to obtain a homogeneous suspension.

D. Characterization techniques

The optical properties of the samples were analyzed by

a Shimadzu UV-3600 UV-vis spectrometer. Samples were

dispersed in deionized water and placed in optically transpar-

ent 1 cm quartz cells. Atomic force microscopy (AFM)

images were obtained by using a Park Scientific model

CP-Research (VEECO) in continuous scan mode and tapping

mode. Scanning electron microscopy (SEM) images were

obtained on a FESEM instrument (SU-8010, HITACHI,

Japan). The dispersion was dropped onto a carbon coated

200 mesh copper grid and dried under room temperature

(25 �C) for transmission electron microscopy (TEM)

(TECNAI-F20 FEG TEM, Netherlands) observation with an

acceleration voltage of 200 kV. An etched silicon tip was

used as a probe for imaging the samples, which were

dropped and casted on freshly cleaved mica. Raman spectra

of the graphene nanosheets, TiO2, and GTA were produced

using LabRAM Aramis (HORIBA JOBIN YVON). The

Raman system was operated at 10 mV laser power and an ex-

citation wavelength of 514 nm with an Arþ ion laser. Fourier

transform infrared (FT�IR) spectra were obtained on

Thermo Fisher Nicolet 6700 FT-IR spectrophotometer

(USA).

E. Photocatalytic experiments

The photocatalytic liquid-phase degradation of methyl

orange (MO) was performed in a transparent beaker reactor

operating above a magnetic stirrer. 10 ml GTO and 90 ml

MO solution (20 mg/l) were loaded in the reactor shrouded

by a 40-watt incandescent bulb as the light resource. The

reaction temperature was controlled at 27 6 1 �C by an air-

cooling system. Before irradiation, the suspensions were

stirred in the dark for 2 h to ensure the establishment of

adsorption desorption equilibrium. A 3 ml sample solution

was taken at a 20 min interval during the experiment and

centrifuged to remove the catalyst completely. The solution

was analyzed on a UV-vis spectrophotometer (UV-3600,

Shimadzu). The percentage of degradation is reported as

C(x,t)/C(x,0). Here, C(x,t) is the absorption of dyes solution at

each irradiated time interval of the main peak of the adsorp-

tion spectrum, while C(x,0) is the absorption of the initial

concentration when the adsorption-desorption equilibrium

is reached, and x stands for the number of different

Photocatalysis catalysts.

F. Microorganism preparation

Three kinds of microorganism including a Gram-

negative bacterium (E. coli K12), a Gram-positive bacterium

(Rhodopseudanonas palustris), and a fungus (Candida) were

utilized in the study. All microorganisms were grown in

complete medium at 37 6 1 �C, and harvested in the midex-

ponential growth phase. Cultures were centrifuged at

6000 rpm for 10 min to pellet cells, and cells were washed

three times with isotonic saline solution to remove residual

macromolecules and other growth medium constituents. The

pellets were then resuspended in isotonic saline solution.

Bacterial cell suspensions were diluted to obtain cell samples

containing 106–107 CFU/ml.

G. Test to antibacterial properties

Microbe was incubated, respectively, with fresh GR, GT,

TiO2, GTA dispersions in isotonic saline solutions at 37 �Cunder 250 rpm shaking speed for 2 h and illuminated by a

40-watt fluorescent lamp. The loss of viability of Microbe

cells was evaluated by colony counting method. Briefly, series

of 10-fold cell dilutions (100 ll each) were spread onto LB

plates, and left to grow overnight at 37 �C. Colonies were

counted and compared with those on control plates to calcu-

late changes in the cell growth inhibition. Isotonic saline

solution without antibacterial materials was used as control.

All treatments were prepared in duplicate.

H. Biomolecules oxidation measurement

1. Lipid peroxidation measurement: The free radical

modulation activity of graphene-based materials was deter-

mined using a lipid peroxidation assay. Briefly, lipid peroxi-

dation was induced in yolk lecithin by adding 5 ll of

400 mM FeCl3 and 5 ll of 200 mM L-ascorbic acid.28 To

this, the graphene-based materials were added. A control

was prepared contained no compound. The samples were

incubated at 37 �C for 60 min with an illumination of 425 nm

wavelength. The reaction was inhibited by adding 1 ml of

stopping solution (0.25 N HCl, 1.5% Trichloroacetic acid,

and 0.375% Thiobarbituric acid). These reaction mixtures

were kept in a boiling water bath for 15 min, cooled and cen-

trifuged, then measured the absorbance of the resulting solu-

tion at 532 nm.

2. Thiol oxidation and quantification: The concentra-

tion of thiols in GSH was quantified by the Ellman’s

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assay.29,30 GTA, GT, or TiO2 dispersions (1 ml) in 50 mM bi-

carbonate buffer (pH 8.6) was added into 1000ll of GSH

(0.8 mM in the bicarbonate buffer) to initiate oxidation. All

samples were prepared in duplicate. The vessels filled with

GSH-GTA, GT, or TiO2 mixtures were covered with the illu-

mination by the 16 W UV-vis lamps with 425 nm wavelengths,

and placed in a shaker with a speed of 150 rpm at room tem-

perature for incubation of 2 h. After incubation, 4 ml of 0.05 M

Tris-HCl and 100ll of DNTB (5, 50-dithio-bis-(2-nitrobenzoic

acid)) were added into the mixtures to yield a yellow product.

GTA, GT, or TiO2 was removed from the mixtures by filtra-

tion through a 0.2 lm polyethersulfone filter. A 3 ml aliquot of

filtered solutions from each sample was then placed in an opti-

cally transparent 1 cm quartz cells. Their absorbance at 412 nm

was measured on a spectrophotometer. GSH solution without

graphene-based materials was used as a control. The loss of

GSH was calculated by the following formula:

Loss of GSH ¼ (absorbance of negative Control-

absorbance of sample)/absorbance of negative control*100%.

III. RESULT AND DISCUSSION

A. Synthesis of GTA

The synthesis of GTA involved three stages: the synthesis

of GO, adhesion of the titanium dioxide, and the deposition of

gold nanoparticles. Graphene nanosheets were successfully

synthesized by improved hydrothermal reduction of GO. At

first, the GO nanosheets were synthesized by the preoxidation

of graphite followed by harsh oxidation and exfoliation. The

obtained GO nanosheets were exfoliated to monolayers of GO

by ultrasonication prior to the reduction reaction. And as

showed in Figure 1(a), GO with several oxygenated functional

groups on its basal plane and at the edges can be deoxygen-

ated in alkaline solution and reduced by trisodium citrate. In

addition, SDBS helped titanium dioxide adhere to the gra-

phene. And the hydrothermal reaction enhanced the reaction

rate and kinetics due to the high pressure in hydrothermal con-

ditions.31 The change of color from brownish yellow (GO) to

black confirmed that the reduction of GO into graphene was

successfully achieved by the hydrothermal approach.28 After

that, prepared gold nanoparticles (radius of about 40 nm) will

be added to the mixture and stirred for 3 h at 90 �C until

obtained a homogeneous suspension. That the suspension’s

red color faded (the color bring by gold nanoparticles) and

the upper liquid become clear after stationary can confirm the

deposition of gold nanoparticles have been achieved. And the

photo of each production in the synthetic route was presented

in Figure 1(b).

B. Characterization of photocatalysts

Figure 2(a) shows an AFM image in continuous scan

mode of GTA nanosheets on a mica surface. In this mode,

continuous scanning probe will peel adhesion of nanopar-

ticles, so the figure is the morphology of the graphene. The

thickness of them is around 0.8 nm (see the AFM profile in

Figure 2(b)), indicating the single-layer GR sheets were

manufactured as the substrate of the GTA.6,32 Moreover,

Figure 2(c) shows the AFM image of GTA in tapping-mode,

which can indicate that nanoparticles have been attached to

the graphene substrate successfully and the radius of nano-

particles are about 40 nm. Figure 2(d) shows the surface

topography of GT in tapping-mode, according to the particle

size, the bright particles in the figure are the TiO2 NPs. And

it is easy to find that the density of particles on surface is

smaller than GTA, which is due to the no deposition of

Au NPs.

The morphology of the GT and GTA composites were

also characterized by TEM. Figure 3(a) presented the micro-

scale of these graphene sheets make that these TiO2 nanopar-

ticles can be indeed deposited on the graphene sheets. And

Figure 3(b) showed the structure of GTA that the TiO2 par-

ticles and Gold nanoparticles are densely anchored on gra-

phene sheets. Besides, SEM images of GTA (Figure 3(c))

and GT (Figure 3(d)) represent a typically wrinkled and dis-

ordered shape. And we can find that some small particles

have been attached to the surface of graphene sheets.

Raman spectroscopy was employed to study the crystal-

linity, disorder and defect levels of the TiO2, GR and GTA

(Figure 4). The Raman spectra of GR (black) show the typi-

cal features of graphene with the presence of D band locates

at 1350 cm�1 and G band at 1590 cm�1. G band provides in-

formation on the in-plane vibration of sp2 bonded carbon

atoms,33 while the D band is attributed to the presence of sp3

FIG. 1. (a) Schematic diagram of the preparation of GTA synthesized from

graphene oxide. (b) Photographs of GTA, GT, GO, GR, and TiO2 disper-

sions (at the concentration of 1 mg/ml).

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defects in graphene.34 Besides, the Raman lines for Eg, B1g,

A1g, or B1g modes of TiO2 anatase-phase (red) were also

observed. And the Raman spectra of GTA contained all char-

acteristic peaks of GR and TiO2. This demonstrates that the

graphene has been successfully exfoliated and synthesized

into the composite. In addition, compared with GR and

TiO2, GTA’s D band (1225 cm�1), G band (1480 cm�1), Eg,

B1g, A1g, and B1g modes have an apparent blue-shift. As a

general rule, Raman peak frequency shift is closely related

to the material particle size and internal stress changes.35,36

Hence, we concluded that GTA Raman peak blue shift and

broadening may be due to following three reasons: particle

size of the material increased after hydrated; TiO2 loaded to

GR, and changed the force between the atoms of the material

surface by physical adsorption and chemical action; gener-

ated new chemical bonds such as Ti-O-C, and changed the

structure of the material level.

In Figure 5, the bold solid lines are the FT-IR spectra of

TiO2, GT, and GTA, which have been baselined. And the

three dotted lines show the original property of each sample.

There are some characteristic peaks in FT-IR spectrum of

TiO2 and GT. In detail, the characteristic features of GT and

GTA spectra are the bands at 1153(C-O stretching vibra-

tions), 1706 (C-C stretching vibrations from graphene), 1304

(C-H bending vibration), and 3607 cm�1 (O-H stretching

vibrations). And the broader absorption below 1000 cm�1

can be regarded as Ti-O-C vibration (940 cm�1). The pres-

ence of Ti-O-C bonds indicates the firmly bonding between

the TiO2 nanoparticles and graphene. Besides, there exists a

characteristic peak in FT-IR spectrum of TiO2 at 656 cm�1

originates from Ti-O stretching vibration. But on the curve

of GT and GTA, the characteristic peak has a blue shift

FIG. 3. (a) TEM image of GT. (b) TEM image of GTA. (c) SEM image of

GTA. (d) SEM image of GT.

FIG. 4. Raman spectra of TiO2, GR, and GTA.

FIG. 2. (a) AFM image of GTA in tapping-mode and its corresponding

height profiles (b). (c) AFM image of GTA and (d) AFM image of GT in

continuous scan mode.

FIG. 5. Fourier transform infrared spectra of TiO2, GT, and GTA. Bold solid

lines are the baselined FT-IR spectra, and the dotted lines show the original

property.

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(at 810 cm�1). And the similar frequency shift was also

found in the Raman spectroscopy.

The optical properties of the samples are studied by

UV�vis absorption spectroscopy as shown in Figure 6. First,

the reduction of GO into graphene is characterized, as shown

in the figure: The UV-vis spectrum of GO shows a sharp

absorption peak at 231 nm (p–p* of the C–C aromatic

rings25), and GR shows a maximum absorption at 257 nm.

The occurrence of a red shift in the absorption spectra of gra-

phene is due to the increased electron concentration due to

removal of sp3 carbon atoms and is also consistent with the

restoration of sp2 carbon atoms.37 In addition, GT and GTA

also appear similar absorption peak to GR, but there exist

red-shifts of 8 nm to 10 nm38 and a certain degree of

enhanced absorption intensity in the visible and near infrared

region, which will effectively improve the optical and elec-

trical properties of the photocatalytic properties.17 At the

same time, though colloidal gold should have characteristic

absorption peak in the 400–700 nm, there is no obvious

absorption peak around 520 nm in the curve of GTA. The

occurrence confirmed the gold nanoparticles deposited well

and there existed almost no free gold nanoparticles in disper-

sion. Besides, in Figure 6, the purple line was the absorption

spectrum of P25 TiO2, which displayed an absorption edge

at 380 nm. As for the absorption spectrum of GT and GTA,

nearly all of TiO2 NPs have anchored on graphene sheets, so

the diffuse reflectance of the TiO2 powder is not necessary to

pay much attention.

C. Photocatalytic activity of GTA

Photocatalytic activity of each synthesized material was

comparatively investigated by evaluating the photodegradation

rate of MO under solar light. To measure the MO concentration

in the solution as time elapsed under solar radiation, a small vol-

ume of solution was withdrawn and the photocatalyst powder

was filtered out by 0.22lm membranes, and then the absorb-

ance was measured at the wavelength of 526 nm. The concentra-

tion of MO in the solution was plotted as a function of

irradiation time using Beer�Lamberts Law, and the results of

different photocatalysts were displayed in Figures 7(a) and 7(c).

It is obvious to conclude that the relative photodegradation

rate of GTA exhibited significant improvements compared to

GT and the bare TiO2 and GR. And the test was started after

the adsorption-desorption equilibrium is reached, so it can be

indicated that the decrease of MO concentration in solution

mainly results from the chemical oxidative decomposition,

not the physical adsorption on the photocatalyst surface.

After an hour irradiation, as showed in Figure 7(b), the pho-

tocatalytic degradation rate of GTA was 78%, which was the

2.1 times of GR, 1.5 times of TiO2, and 1.2 times of GT.

Comparing GTA with other materials, the enhanced photoca-

talytic activity can be attributed to the significantly enhanced

electron-hole pairs (EHPs) separation with the holes injec-

tion into graphene and gold nanoparticles to prohibit the

FIG. 6. UV�visible spectra of GO, GR, GT, GTA, and TiO2.

FIG. 7. (a) Photodegradation rate of MO by GR, TiO2, GT, and GTA sam-

ples under UV light irradiation (pH¼ 2). (b) Final photodegradation rate of

MO by four samples after 60 min. (c) GTA photodegradation rate.

204701-6 He et al. J. Appl. Phys. 114, 204701 (2013)

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photogenerated EHPs recombination, and the holes in A the

huge surface areas will be trapped at surface state to oxidize

MO. And the details of this process will be explained in the

last chapter (Model and calculation).

D. Antibacterial activity of GTA nanosheets

In this study, E. coli, Rhodopseudanonas palustris(PSB), and Candida were used as the model bacteria to eval-

uate antibacterial activities of the TiO2, GR, GT, and GTA.

The general mechanism of antibacterial activity of materials

relied on the size, structure, composition, and properties of

the individual materials. In addition, nanoscale size distribu-

tion, high surface to volume ratio, and other physicochemical

properties were also demonstrated to be viable factors for

governing anti-microbial effects.28

Herein, in order to test antibacterial ability of the differ-

ent materials, microbe cells (106–107 CFU/ml) were incu-

bated with the same concentration (150lg/ml) of TiO2, GR,

GT, and GTA dispersions in isotonic saline solution for 2 h

under solar light, respectively. The death rate of bacterial

cells was determined by the colony counting method. The

physiological saline containing the same amount of bacteria

without any antibacterial materials was used as a control.

And antibacterial rate is expressed as Ca�C0

C0� 100%, where

the Ca stands for antibacterial materials colony counts, and

C0 is on behalf of the control without antibacterial materials.

As shown in Figure 8(a), against E. coli, the TiO2 dis-

persion exhibits a moderate cytotoxicity with the cell inacti-

vation percentage at 57.06 6 2.85%. The GR dispersion

showed as light weaker antibacterial activity compared with

GT (68.93 6 3.45%), having the cell inactivation percentage

at 64.22 6 4.51%. GTA had the strongest bacterial activity

compared with others, and the loss of E. coli viability

increases to 90.21 6 6.1%. As for PSB (showed in Figure

8(b)), the TiO2 dispersion exhibits minimum cytotoxicity

with the cell inactivation percentage at 20.49 6 1.02%.

Antibacterial rates of GR and GT were 44.67 6 2.23% and

65.16 6 3.32%. GTA had the strongest bacterial activity

again with an excellent bacterial inactivation percentage of

100%, which were more than 4-fold compared with that of

pure TiO2. Besides, as showed in Figure 8(c), we also found

that GTA almost completely suppressed the growth of

Candida, leading to a viability loss up to 92.13 6 4.50%,

which was nearly three times more than the inactivation per-

centage of TiO2 at 38.51 6 1.92%. Additionally, GT and GR

had almost the same antibacterial rate, which were

71.64 6 3.58% and 77.64 6 3.90%. It also should be noted

that the shaking speed of 250 rpm was used in all antibacte-

rial assays, in order to make sure that the TiO2, GR, GT, and

GTA particles were well suspended in the saline solution

interacting with cells in all assays.

In short, in the fight against a microorganism, GTA

reflected the strongest antibacterial; GT and GR showed

almost the same cytotoxicity weaker than GTA. The occur-

rence is mainly due to the different photocatalytic activity of

each material (as showed in Figure 7). And the details of the

relationship between photocatalytic activity and antibacterial

will be analyzed in the last chapter, Model and calculation.

Besides, our results showed that GTA were more toxic to

Gram-positive bacteria than Gram-negative bacteria and

fungi. This is not in agreement with previous findings that

carbon nanomaterials are more toxic to Gram-negative bac-

teria than Gram-positive bacteria.6,39 It is well known that

the antibacterial activity of the materials resulted from the

loss of cellular integrity with the disruption of cell walls, and

Gram-negative bacteria possessing a thin peptidoglycan

layer (7–8 nm thickness) should be easier to be killed than

Gram-positive bacteria possessing a thick peptidoglycan

layer (about 20–80 nm thickness) and fungi with thicker cell

wall (100–200 nm). But it should be also noted that nanoma-

terials toxicity towards biological species not only relies on

the nature of the cell wall, but is also dependent on cell

membranes, cellular enzymes, and biochemical events. It has

FIG. 8. Antibacterial rate of E. coli (a), PSB (b), and fungi (c) after incuba-

tion with TiO2, GR, GT, and GTA dispersions.

204701-7 He et al. J. Appl. Phys. 114, 204701 (2013)

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been reported that carbon-based materials cause oxidative

stress in biological systems, which can lead to the inactiva-

tion of bacteria.29,40 Hence, we determined the free-radical

modulation activity of TiO2, GT, and GTA, by the oxidation

of the lecithin and glutathione.

E. Biomolecules oxidation measurement

As showed in Figure 9(a), compared with the control

group, lipid peroxidation has increased by 21.2%, 23.5%, and

29.1% after exposing to TiO2, GT, and GTA, respectively.

The oxidation of fatty acids leads to the generation of lipid

peroxides that stimulate a chain reaction to result in the disin-

tegration of a cell membrane followed by cell death.41 And

the mechanism of cellular toxicity such as elevated reactive

oxygen species (ROS) levels over the intra-cellular ROS level

is to cause cells entering a state of oxidative stress, which

would further result in damage to cellular components such as

DNA, lipids, and proteins.42 Oxidative stress is a condition in

which the delicate balance existing between prooxidant (free

radicals) production and their subsequent amelioration via the

antioxidant defense system becomes skewed. Hence, we simu-

late the state of oxidative stress, by oxidizing glutathione, and

further search the evidence of the GTA biological oxidation

toxicity.

Glutathione (GSH) is the main constituent of the bacte-

ria oxidation system, at a concentration ranging between

0.1 and 10 mM,43 and a large number of its consumption in a

short time will cause the collapse of the antioxidant system

resulting in oxidative stress. GSH is a tripeptide with thiol

groups. Its thiol groups can be oxidized to disulfide bond,

which converts GSH to glutathione disulfide, so GSH was

used as an oxidative stress indicator to incubate with TiO2,

GT, or GTA dispersions in this test. As showed in Figure 9(b),

a noteworthy fraction of GSH is oxidized after its exposure

to TiO2 (68.24 6 3.41%), GT (75.75 6 3.79%), and GTA

(91.56 6 4.10%). Among three types of graphene-based mate-

rials, GTA has the highest oxidation capacity toward GSH,

followed by GT and GR. And different oxidation capacities

toward GSH among them can be also attributed to their differ-

ent electronic properties and adsorption capacity. Integrating

the two experiments, it can be speculated that, GTA first oxi-

dizes the cell wall and membrane, and then continues to

undermine the intracellular antioxidant system, so that cells

are forced into oxidative stress.

F. Model and calculation

The previous research carried out has proposed a three-

step cytotoxicity mechanism for graphene-based materials.29,44

The first step is bacterial adhesion or deposition onto graphene

resulting in direct bacterium-antibacterial materials contact.

After that, the materials would make intimate, membrane stress

and disruptive interaction with bacteria. The last step involves

disrupting a specific microbial process by disturbing or oxidiz-

ing a vital cellular structure or component. In general, the nano-

materials toxicity against biological systems is due to oxidative

stress and physical properties. The former is due to the genera-

tion of elevated ROS levels, while the latter is due to direct con-

tact between the nanomaterial and cellular systems.45

Therefore, from the above two aspects, we will model specific

account for GTA to bring some light to the explanation fits

excellent antibacterial performance.

As for GTA, the generation of ROS levels is equivalent

to the efficiency of the photocatalytic process, which is

expressed by Eq. (1). Hence, generation of ROS level is

proportional charge transfer rate (kCT) and inversely propor-

tional to the electron-hole recombination rate (kR). In the

process of GTA absorbing photons to generate ROS, the kCT

is limited by the electronic transition rate (xt). So, it is nec-

essary to seek the expression of xt.

In an ideal physical system, the energy operator H0 (not

involving the time t) with the energy level En, has an

orthogonal-normalized eigenfunction wn (n is whole quan-

tum numbers). Assuming that, at the beginning, the system is

in a stationary state—wk, given by the simple relationship

wn x; tð Þ ¼ wk xð Þe�iEkt=�h; (2)

where v represents all independent variables referred by

wave functions and t is the time. When t> 0, system is sub-

jected to an external action of the interaction potential

H0 x; tð Þ 6¼ 0, so the wave function should satisfy the schro-

dinger equation46

i�h@

@tw x; tð Þ ¼ Hw ¼ H0 þ H0ð Þw x; tð Þ: (3)

FIG. 9. (a) The absorbance of lecithin oxidation (k¼ 525 nm) by TiO2, GT,

and GTA dispersions (10 lg/ml) after incubation 1 h. (b) GSH (0.4 mM) oxi-

dation by TiO2, GT, and GTA dispersions (10lg/ml) after incubation 1 h.

204701-8 He et al. J. Appl. Phys. 114, 204701 (2013)

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According to principle of superposition, w x; tð Þ can be repre-

sented as the equation

w x; tð Þ ¼X

Cnwn xð Þe�iEkt=�h; (4)

whose initial conditions are w x; 0ð Þ ¼ wk xð Þ and Cn 0ð Þ¼ dnk. Suppose when t¼T, according to the general possi-

bility theory of wave function, the possibility of H0 ¼ Ef is

C�f Tð ÞCf Tð Þ. And the C�f Cf time rate of change is called the

transition speed from initial state wk to terminate state wf ,

denoting as

xk!t tð Þ ¼ d

dtC�f tð ÞCf tð Þh i

: (5)

To obtain Cf ðtÞ, combined Eqs. (3) and (4), and then process

whole space integration in the orthogonal-normalized condi-

tionÐ

w�f wn xð Þdx ¼ df n, after that we can obtain an equation

i�hdCf

dte�

iEf t

�h ¼X

H0f nCne�iEnt

�h ; (6)

in which H0f n ¼Ð

w�f H0wndx ¼ hwf jH0jwni.Under some perturbations,

PC�n tð ÞCn tð Þ � 1 n 6¼ kð Þ,

so Eq. (6) approximates

i�hd

dtCf ¼ H0f k tð Þe�ixf k t: (7)

This approach neglects all Ck and takes CkðtÞ � 1. So after

the integration

Cf tð Þ ¼ 1

i�h

ðt

0

H0f k tð Þeixf k tdt: (8)

Due to the initial and terminal state of electronic, semicon-

ductor belongs to the separate energy spectrums. The one

stimulated electron absorbing one photon can be considered

as perturbation with single frequency H0. Therefore, when

t> 0, the perturbation H0 of matrix element is given by the

relationship47

H0f k tð Þ ¼ huf jH0juki ¼ Ff k eix0t þ e�ix0tð Þ: (9)

Among Eq. (9), Ff k is defined by the equation:

Ff k ¼ huf jFjuki ¼Ð

u�f xð ÞF xð Þuk xð Þdx, in which F is a her-

mite operator unrelated with time. In order to facilitate dis-

cussion, supposing xfk ¼ Ef�Ef

�h > 0, according to Eq. (8), it

is easy to obtain the relationship

Cf tð Þ ¼ Ff k

i�h

ðt

0

eix0t þ e�ix0tð Þeixf k tdt;

¼ Ff k

i�h

exp it xf k þ x0ð Þ� �� 1

i xf k þ x0ð Þ

(

þexp it xf k � x0ð Þ� �� 1

i xf k � x0ð Þ

): (10)

Owing to x0t� 2p (When t> 10�13s, due to particle-like

nature of light, x0 is approximate to 1015 1016 s�1), the

term whose denominator is xf k þ x0 can be neglected.

Therefore, Eq. (10) can be expressed as

Cf tð Þ ¼ Ff k

�h 1� exp½itðxf k � x0Þ�

xf k � x0

: (11)

So the speed of transition can be expressed as

xk!tðtÞ ¼d

dt½C�f ðtÞCf ðtÞ� ¼

2

�h2jFf kj2

sinðxf k � x0Þtxf k � x0

: (12)

Obviously, only when xf k ! x0, does the above equation

have salience value. Using the nature of dðxÞ, Eq. (12) can

be approximation to

xk!tðtÞ �2p

�h2jFf kj2dðxf k � x0Þ

¼ 2p

�h2jFf kj2dðEf � Ek � �hx0Þ: (13)

During the working of GTA, the perturbation slight

absorbed, so Ff k ¼ 12

ee0l r, and the rate of transition can be

given by the equation

xk!f ¼p2

e2e02

�hjðl rÞk;f j

2dðEf � Ek � �hxÞ: (14)

Introduced the density of material available optical energy

into the above equation, and defined u ¼Ð1x0

qðxÞdx. So, in

the conditions of conservation of energy, xt ¼ xk!f can be

given by the equation46

xt ¼ xk!f ¼4p2

�he2jðl rÞk;f j

2

ð1x0

qðxÞdx ¼ C

ð1E�h

qðxÞdx:

(15)

Here, C is a constant and E�h is equal to material available min-

imum light wave angular momentum x0.

Now the rate of electronic transition may be regarded as

the density integral of the available light, which is propor-

tional to the scope of light waves which can be absorbed.

Here, the scope will be subject to the influence of the two

factors. One is quantum size effects from the particle radius.

Another is barrier effect from the heterojunction interface.

Quantum size effects (QSE) occur for semiconductor par-

ticles possessing a conduction band and a valence band, in

which the pairs of electron and hole do not experience the

electronic delocalization.48 Hence, the confinement produces

increases the effective band gap and a quantization of dis-

crete electronic states. And the anomalies arise when the size

of the semiconductor particles become comparable to the de

Broglie wavelength of the charge carriers.49 Therefore, QSE

always occur for the semiconductor particles on the order of

1–10 nm in size. While photocatalyst particles have not

reached Q-particle sizes yet, there is no need to consider the

impact of the quantum size effect.

As for barrier effect from the heterojunction, the contact

between semiconductor and another phase (graphene or

metal) generally involves a redistribution of electric charges

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and the formation of a double layer. The transfer of mobile

charge carriers between the semiconductor and the contacted

phase, or the trapping of charge carriers at the interface, pro-

duces a space charge layer.

For TiO2-metal or TiO2-graphene phase interactions, the

heterojunction system represents a good example illustrating

space charge layers, band bending, and the formation of a

Schottky barrier as shown in Figure 10. Electrically neutral

and isolated from each other, the metal and the n-type semi-

conductor have different Fermi level positions. In the case dis-

cussed here, the TiO2 has a higher work function (Ws) than

Au or graphene (Wm). When those two kinds of materials are

connected electrically, electron migration from the Au or gra-

phene to the metal occurs until the two Fermi levels are

aligned,50 as shown in Figure 10. The electrical contact has

formed an accumulation layer. The band of the semiconductor

will bend down as one move toward the surface as a result of

the decrease of electron potential energy as one move toward

the positively charged outer layer. The barrier formed at the

metal-semiconductor interface is called the Schottky barrier,

and it is necessary to determine the height of the barrier.

Before contacting, the Fermi level of TiO2 is lower than

metal or graphene ððEFÞs < ðEFÞmÞ, and the condition satis-

fies the equation:50

ðEFÞm � ðEFÞs ¼ Ws �Wm (Ws > Wm). Connected by

conductor, metal, and semiconductor will be unified, so elec-

trons flow from metal to semiconductor until the different

Fermi levels become consistent. The potential difference at

the interface completely compensates for the difference of

Fermi level. At this time, with respect to the Fermi level of

the metal, the Fermi level of semiconductor has an incre-

mented of Ws �Wm, then

qðV0s � VmÞ ¼ Wm �Ws: (16)

Here, V0s is semiconductor potential, and Vm is metal poten-

tial. The result of contacting is engendering potential differ-

ence known as contact potential difference. As the decrease

of distance between metal and semiconductor, the positive

charge density of metal surface near semiconductor

increased. These positive electrons exist on semiconductor

surface engendering space charge region. A certain field

causes level bending in order to generate potential difference

Vs between internal and external surface of semiconductor.

Part of contact potential difference is assigned to space

charge region, and the rest is assigned to the surface between

metal and semiconductor (Vms).

Wm �Ws

q¼ Vms þ Vs: (17)

When the distance can be comparable with interatomic dis-

tance, electrons will be able to pass through gap. At that time

Vms is very small, and most of contact potential differences

are assigned to space charge region. Hence, the barrier height

of semiconductor (Schottky barrier) can be approximated as

qVDs ¼ Ws �Wm: (18)

And the barrier height of conductor is

q/Dc ¼ qVD þ En ¼ �qVS þ En ¼ WS �Wm þ En

¼ v�Wm; (19)

where v is the electron affinity of semiconductor, and En

¼ Ec � ðEFÞs.Therefore, the enhanced photocatalytic performance by

the barrier effect from the heterostructure can be explained by

assuming the formation of Schottky barrier, and a possible

proposed energy band structure of GTA is schematically elu-

cidated in Figure 10. Anatase TiO2 is an n-type wide band gap

(3.2 eV) semiconductor with work function of 5.50 eV51 and

electron affinity of 4.20 eV.52 While the work function of gra-

phene and Au are 4.42 eV53 and 5.10 eV.54 On the basis of

the above values, the barrier height of semiconductor at the

interface between graphene and TiO2, ðqVDsÞG�T ¼ 1.08 eV,

and the ðqVDsÞA�T between Au and TiO2 is 0.4 eV. Besides,

ðqVDcÞG�T ¼ 4.40–4.42¼�0.02 eV, and theðqVDcÞA�T ¼ 4.40

–5.50¼�0.90 eV.

When Au particles and TiO2 particles are attached onto

the graphene nanosheets surface, two kinds of nanohetero-

junctions are formed at the interface and electron transfer

from graphene and Au particles to TiO2 until their FermiFIG. 10. Schematic for the energy band structure of the two heterojunctions

(Au/TiO2 and TiO2/GR) in GTA.

204701-10 He et al. J. Appl. Phys. 114, 204701 (2013)

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levels align and the system reaches the thermal equilibrium

state.50 Therefore, due to alignment of Fermi level, band

bending would be expected, which is schematically shown in

Figure 10. And the bending will reduce distance between

conduction bands and valence bands, which can increase the

range of visible light absorption. Hence, according to

Eq. (15), the nanoheterojunctions at GTA will enhance the

charge transfer rate (kCT).

Because of carrier concentration gradients, electrons

will diffuse from metal to semiconductor engendering nega-

tive space charge region on the surface of semiconductor.

The electric field direction is from external to internal.

Meanwhile, VS > 0, energy band generates deflexibility.

That engendering two barriers in metal and semiconductor

impedes crossing between electrons and holes by serving as

an efficient electron trap. After excitation, the hole is free to

diffuse to the conductor surface and electron will stay in the

semiconductor surface. In addition, since the size of GTA

material, the width of the barrier at interface is almost impos-

sible to less than 10 A. So the tunnel effect of electron can be

negligible. Unless electrons or holes get enough energy to

pass through the barrier, they will not combine. So the

electron-hole recombination rate (kR) would slow down.

Therefore, during the photocatalytic reaction process at

GTA, since the role of the nanoheterojunctions, kCT is

increased and kR is slowed down, which leads to a significant

improvement of quantum yield (u) according to Eq. (1).

Additionally, it was observed that the photogenerated

electron and hole pairs were separated efficiently on the het-

erojunction interface, and then the electrons accumulated at

semiconductor surface participated in the chemical reaction

to produced powerful superoxide radical as well as oxidizing

agent (O2�, OH, and OOH), and then the holes were free

to diffuse around the broad surface of gold particles and gra-

phene, where oxidation of organic species can occur easily.

IV. CONCLUSION

A green synthetic strategy was employed to synthesize

GTA, and comparative studies were carried out to evaluate

the relative photocatalytic performance of GT, TiO2, GR, and

GTA. By photodegradation of MO under solar light, the GTA

had significantly higher performance over other comparisons.

After that, the antibacterial activity of GT, TiO2, GR, and

GTA aqueous dispersions towards three kinds of microorgan-

isms was compared. And our results on the antibacterial activ-

ity showed that GTA had the highest antibacterial activity and

had potential to act as a suitable antibacterial agent. And

based on the oxidation measurement of lecithin and glutathi-

one, we can infer that the bacterial cytotoxicity may be attrib-

uted to both the oxidative burst of membrane and the

destruction of the antioxidant system. Besides, the excellent

photocatalytic performance and antibacterial activity of GTA

benefit from enhanced improved photogenerated electron�hole pairs separation and transportation, and through our theo-

retical research, we attributed electron�hole pairs separation

to the height of barrier and attributed transportation to

the range of optical absorption. And through the derivation of

the relevant formulas, from the basic principle level, we

established models to explain the reason for GTA remarkable

photocatalytic properties. Currently, we are focusing on the

adsorption effect on the process of antibacterial and catalytic,

and regard it as theoretical foundation to further study how

their antibacterial and photocatalytic activities are concentra-

tion dependent. And we attempt to apply this material towards

biomedical applications such as antibacterial textiles and coat-

ing of medical instruments for future work.

ACKNOWLEDGMENTS

This work was partially supported by Program for New

Century Excellent Talents in University under Grant No.

NCET-10-0688, the Fundamental Research Funds for the

Central Universities under Grant No. 2011jdgz17, and by

Xi’an Jiaotong University National Innovation Fund for

Undergraduate Research Training and Practice (Grant No.

201310698044). The authors thank Dr. Shuiyun Yang for

helping with microbial culture, and thank Dr. Jianjun Li,

Yiling Zhu, and Chongbing Liao for helping with the charac-

terization of materials.

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