Enhanced photocatalytic activity of chemically bonded TiO 2/graphene composites based on the effective interfacial charge transfer through the C-Ti bond

Article abstract of DOI:10.1021/cs400080w

Recently, graphene-based semiconductor photocatalysts have attracted more attention because of their enhanced photocatalytic activity caused by interfacial charge transfer (IFCT). However, the effect of a chemical bond is rarely involved for the IFCT. In this work, TiO₂/graphene composites with a chemically bonded interface were prepared by a facile solvothermal method using tetrabutyl orthotitanate (TBOT) as the Ti source. The chemically bonded TiO₂/graphene composites effectively enhanced their photocatalytic activity in photodegradation of formaldehyde in air, and the graphene content exhibited an obvious influence on the photocatalytic activity. The prepared composite with 2.5 wt % graphene (G2.5-TiO₂) showed the highest photocatalytic activity, exceeding that of Degussa P25, as-prepared pure TiO₂ nanoparticles, and the mechanically mixed TiO ₂/graphene (2.5 wt %) composite by a factor of 1.5, 2.6, and 2.3, respectively. The enhancement in the photocatalytic activity was attributed to the synergetic effect between graphene and TiO₂ nanoparticles. Other than the graphene as an excellent electron acceptor and transporter, the enhanced photocatalytic activity was caused by IFCT through a C-Ti bond, which markedly decreased the recombination of electron-hole pairs and increased the number of holes participating in the photooxidation process, confirmed by XPS analysis, the gaseous phase transient photocurrent response, electrochemical impedance spectroscopy, and photoluminescence spectra. This work about effective IFCT through a chemically bonded interface can provide new insights for directing the design of new heterogeneous photocatalysts, which can be applied in environmental protection, water splitting, and photoelectrochemical conversion.

Full text of DOI:10.1021/cs400080w

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Article
Enhanced photocatalytic activity of chemically
bonded TiO2/graphene composites based on the
effective interfacial charge transfer through C-Ti bond
Qingwu Huang, Shouqin Tian, Dawen Zeng, Xiaoxia Wang,
Wulin Song, Yingying Li, Wei Xiao, and Changsheng Xie
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs400080w • Publication Date (Web): 22 May 2013
Downloaded from http://pubs.acs.org on May 30, 2013
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Enhanced photocatalytic activity of chemically bonded TiO /graphene composites based on the
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effective interfacial charge transfer through C-Ti bond
†
, §
┴
†, ‡*
‡
§
Qingwu Huang , Shouqin Tian , Dawen Zeng
, Xiaoxia Wang , Wulin Song ,
║
║
‡*
Yingying Li , Wei Xiao , Changsheng Xie
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†
State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and
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Technology, No. 1037, Luoyu Road, Wuhan 430074, PR China
‡
Nanomaterials and Smart Sensors Research Lab (NSSRL), Department of Materials Science and Engineering, Huazhong
University of Science and Technology, No. 1037, Luoyu Road, Wuhan 430074, PR China
§
Analytical and Testing Center of Huazhong University of Science and Technology, Huazhong University of Science and
Technology, No. 1037, Luoyu Road, Wuhan 430074, PR China
┴
State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, No. 122, Luoꢀshi Road,
Wuhan, 430070, PR China
║
State Nuclear Power Research Institute, Beijing 100029, P. R. China
*
Corresponding author. Tel.: +86ꢀ027ꢀ87556544; Fax: +86ꢀ027ꢀ87543778.
Eꢀmail address: dwzeng@mail.hust.edu.cn (D. Zeng), csxie@mail.hust.edu.cn (C. Xie).
Abstract
Recently, grapheneꢀbased semiconductor photocatalysts have attracted more attention due to their enhanced
photocatalytic activity caused by the interfacial charge transfer (IFCT). However, it is rarely involved for the effect of
chemical bond on IFCT. In this work, TiO /graphene composites with chemical bonding interface were prepared by a facile
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solvothermal method using tetraꢀbutyl orthoꢀtitanate (TBOT) as Ti source. The chemically bonded TiO /graphene
2
composites effectively enhanced their photocatalytic activity in photodegradation of formaldehyde in air, and the graphene
content exhibited an obvious influence on photocatalytic activity. The prepared composite with 2.5 wt% graphene
(
G2.5ꢀTiO ) showed the highest photocatalytic activity, exceeding that of Degussa P25, asꢀprepared pure TiO nanoparticles
2 2
and the mechanically mixing TiO /graphene (2.5 wt%) composite by a factor of 1.5, 2.6 and 2.3, respectively. The
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enhancement in the photocatalytic activity was attributed to the synergetic effect between graphene and TiO nanoparticles.
2
Other than the graphene as an excellent electron acceptor and transporter, the enhanced photocatalytic activity was caused
by IFCT through CꢀTi bond, which markedly decreased the recombination of electronꢀhole pairs, and increased the number
of holes participating in the photooxidation process, confirmed by XPS analysis, the gaseous phase transient photocurrent
response, electrochemical impedance spectroscopy (EIS) and Photoluminescence (PL) spectra. This work about effective
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IFCT through chemical bonding interface can provide new insights for directing the design of new heterogeneous
photocatalysts, which can be applied in environmental protection, water splitting and photoꢀelectrochemical conversion.
Keywords: Chemical bonding; CꢀTi bond; photocatalytic activity; interfacial charge transfer; gaseous phase photocurrent
1. Introduction
In recent years, semiconductor (SC) photocatalysis has emerged as an advanced green technology for environmental
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pollution purification. In photocatalysis, photogenerated electron and (or) hole involve mainly two process. One is driving
the photocatalytic reactions, and another is recombination and generation heat, which is harmful to photocatalysis. Further
studies indicate that only a small fraction of photogenerated carries can successfully transfers to the interface to initiate
redox reactions. So, effective electron transfer at SC surface has been widely acclaimed to be of great importance, which is
a fundamental process relevant to photocatalytic applications.
To date, titania has proven to be the most suitable photocatalyst, largely due to its strong oxidizing power, biological and
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chemical inertness, and low cost. Unfortunately, the rapid recombination rate of photogenerated electronꢀhole pairs within
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TiO results in its low quantum efficiency, thus limiting its practical applications. In the past decades, various strategies
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2b, 4
have been developed in an attempt to modify the photocatalytic process and improve the photocatalytic performance.
In
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particular, carbonꢀtitania hybrid materials have been receiving much attention as a new class of photocatalysts, which could
potentially offer desirable efficiency for separating electronꢀhole pairs.
Recently, owing to the high specific surface area and superior electron mobility of graphene, numerous efforts have been
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paid to combine graphene with semiconductor photocatalysts to enhance the catalytic performance. It is believed that
photoexcited electrons from TiO transfer to nanocarbons, such as carbon nanotubes or graphene, and hinder the
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recombination process, thereby enhancing the oxidative reactivity. Further studies indicate that the interaction between
graphene and TiO can significantly determine the interfacial electron transfer properties, which is a key issue for
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photocatalytic activity. Dong et al. find that RGO/TiO composite will significantly increase the photovoltaic response and
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significantly prolong the mean life time of electronꢀhole pairs compared with pure TiO , which is experimentally supported
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by transient photovoltage (TPV) result. Li et al reported that chemical bonded P25ꢀgraphene composite as a high
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performance photocatalyst for degradation methylene blue. In Zhang and Wang’s reports, P25/RGO composite with the
most intensive interaction fabricated by hydrothermal method shows the highest H evolution activity. However, there has
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been rarely reported that chemical bonding plays a critical role on the IFCT and photocatalytic performance, especially
compared with mechanically mixing semiconductor/grapheme without chemical bonding interface.
In this paper, we demonstrate that TiO /graphene composites with CꢀTi chemical bonding interface were prepared by a
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facile solvothermal method using TBOT as Ti source. XPS analysis confirmed the existence of chemical CꢀTi bond between
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the graphene and TiO nanoparticles. What’s more, chemical bonding is of great importance for the efficiency of
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photoꢀinduced interfacial electron transfer. Due to the photoinduced chemicalꢀbonded interfacial charge transfer (CBꢀIFCT),
TiO /graphene nanocomposites with CꢀTi bond exhibit an exceptional photocatalytic reactivity towards removing HCHO in
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air compared to the pure TiO and the corresponding mechanical mixing sample (without chemical bonding). It is proposed
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that the photoinduced CBꢀIFCT can effectively enhance photocatalytic reactivity by decreasing the possibility of
recombination of electronꢀhole pairs, and increasing the number of holes participating in the photooxidation process.
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. Experimental
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.1. Sample Preparation
All chemicals used in this study were analytical grade and were used without further purification. Distilled water was
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used in all experiments. The graphene used in all experiments were prepared based on Stride’s reports. In a typical
preparation procedure for graphene/TiO nanocomposites, 0.02 mol of tetraꢀbutyl orthoꢀtitanate (TBOT) was dissolved into
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0 mL of ethanol and then this TBOT solution was dropwisely added under magnetically stirring to 10 mL of graphene
water suspension, which contains a specific amount of the graphene. The designed mass ratio of graphene to titania is 0, 0.5,
.5, 2.5 and 3.0 wt%, and the corresponding final products are denoted as GxꢀTiO , where x is 0, 0.5, 1.5, 2.5 and 3.0,
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respectively. After stirring for another 120 min, the mixed suspension was transferred to a 70 ml Teflonꢀlined autoclave, and
maintained at 200°C for 10 h. The obtained white or black–white precipitates were collected and washed thoroughly with
distilled water and absolute ethanol for several cycles, and then dried in vacuum at 80°C for 12 h to get the GxꢀTiO2
nanocomposites. For comparison, a sample with graphene loading about 2.5 wt% was also prepared by a simple mechanical
mixing of G0ꢀTiO and graphene, which was noted as Mixing.
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.2 Characterization
Powder Xꢀray diffraction (XRD) patterns were obtained on a Philips X'Pert Pro Xꢀray diffractometer (PANalytical,
ꢀ1
Holland) with Cu Kα radiation (λ = 0.15418 nm) at a scan rate (2θ) of 0.05° s . The accelerating voltage and the applied
current were 40 kV and 80 mA, respectively. Raman spectra were recorded at room temperature using a microꢀRaman
spectrometer (Horiba Jobin Yvon, LabRAM HR800) in the backscattering geometry with a 488 nm laser as an excitation
source. Transmission electron microscope (TEM) measurements were performed with a JEMꢀ2100F STEM microscope
working at 200 kV. Xꢀray photoelectron spectroscopy (XPS) measurements were carried out with a VG Multilab 2000
spectrometer employing Mg Kα radiation. UVꢀvisible absorbance spectra were obtained for the dryꢀpressed disk samples
with a UVꢀvisible spectrophotometer (Lambda 35, PerkinElmer, America). BaSO was used as a reflectance standard in a
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UVꢀvisible diffuse reflectance experiment. The thermogravimetric analysis differential thermal analysis (TGꢀDTA) was
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carried out using a PerkinElmer Pyris Diamond analyzer in air at a heating rate of 10°Cꢁmin . Photoluminescence (PL)
emission spectra were acquired under excitation at 325 nm using an Edinburgh Instruments PLSP920 spectrometer.
For the EIS measurement, the asꢀprepared photocatalyst powders were fabricated to the film electrodes by the method as
below. First, the powders and ethanol were mixed homogeneously (200 mg/mL), and the obtained paste was then spread on
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the conducting fluorineꢀdoped SnO glass substrate (FTO, with a sheet resistance of 15 ꢂ) with a glass rod. Finally, the
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resultant films with a ca. 2 ꢃm thickness and 2 cm active areas were calcinated at 450°C for 2 h in inert atmosphere to
achieve good electronic contact between the photocatalyst and FTO glass. The EIS measurements were carried out on an
IM6eX electrochemical workstation (Zahner, INC. Germany) by using threeꢀelectrode cells. EIS measurements were carried
out in H SO solution (25%) by using a threeꢀelectrode system. The resultant electrode served as the working electrode, with
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a platinum wire as the counter electrode and a Ag/AgCl (saturated KCl) electrode as the reference electrodes, which was
performed in the presence of a 2.5 mM K [Fe(CN) ]/K [Fe(CN) ] (1:1) mixture as a redox probe in 0.1 M KCl solution. The
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impedance spectra were recorded under an AC perturbation signal of 5 mV over the frequency range of 1 MHz to 10 mHz
at 0.5 V.
For the gaseous phase photocurrent measurement under HCHO gas atmosphere, the asꢀprepared photocatalyst powders
were fabricated to the film electrodes by the method as below. First, the powders and ethanol were mixed homogeneously
(
200 mg/mL), and the obtained paste was then spread on alumina thin flats (ca. 6×8 mm), which was preprinted with the Au
interdigital electrode. The condition of photocurrent measurement was kept as the same as gaseous photocatalytic
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degradation experiments (200 ppm HCHO gas and 36 W/m UV irradiation) and the bias voltage is 5 V.
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.3. Photocatalytic activity
Photocatalytic activity of the samples was evaluated by degrading ca. 200 ppm HCHO under irradiation of a UV (365 nm)
LED light at ambient temperature with a 1 L reactor. The powder photocatalysts were coated onto a square groove (5 × 5
cm), and dried in an oven at 80°C for 2 h. The weight of the photocatalysts used for each experiment was kept at about 0.05
g. After the square groove coated with photocatalyst powders were placed into the reactor, the HCHO gas with
concentration of ca. 200 ppm was passed through the reactor and reached adsorptionꢀdesorption equilibrium with the
catalyst before light irradiation. The initial temperature was 25 ± 1°C. Finally, a 2.8 W UV LED array lamp (1 cm above the
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groove) was switched on to trigger the photocatalytic reaction. The measured UV intensity was ca. 36 W/m . The analysis
for the HCHO concentration and carbon dioxide in the reactor was performed onꢀline with a Photoacoustic Field
GasꢀMonitor (INNOVA Air Tech Instruments, Model 1412). Each set of experiment was followed for 60 min under UV
irradiation.
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3. Results and discussion
3.1. Enhanced photocatalytic properties
To analyze the application potential of graphene/TiO nanocomposites in volatile organic compounds (VOCs) gas
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purification, photocatalytic degradation was determined to select HCHO gas as target gas. The photocatalytic
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activities of the asꢀprepared graphene/TiO nanocomposites were evaluated by the oxidation of 200 ppm HCHO gas in
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air. Note that, under dark conditions without light illumination, or illumination in the absence of catalyst, did not
result in the photocatalytic decomposition of HCHO. Therefore, the presence of both illumination and catalyst was
necessary for the efficient degradation. These results clearly indicated that the decomposition of HCHO in air was
caused by photocatalytic reactions on graphene/TiO composite powders under UV illumination.
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The kinetics of the degradation reaction was fitted to a pseudoꢀfirstꢀorder reaction and a much larger apparent rate
constant (κ) for the hybrid was quantitatively evaluated, as Fig. 1 shows. Without graphene, the sample G0ꢀTiO shows
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ꢀ3
ꢀ1
good photocatalytic activity, and its κ reaches 11.03×10 min . Even the loading amount of graphene (0ꢀ3.0 wt%) is low,
the adding graphene exhibits a significant influence on the photocatalytic activity of the graphene/TiO nanocomposites.
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Especially, the photocatalytic activity increases significantly with increasing loaded graphene content, and the κ reaches the
ꢀ3
ꢀ1
highest value of 28.52×10 min at 2.5 wt% graphene (G2.5ꢀTiO ). In this regard, the photocatalytic activity exceeds that
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of pure TiO by 2.6 times. However, further increasing the graphene content to 3.0 wt%, its κ value decreases to 23.48×10
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ꢀ1
min . For comparison, the photocatalytic activity of P25 (Degussa, TiO ) and Mixing was also carried out, with a respective
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ꢀ1
ꢀ3
ꢀ1
κ value of 19.52×10 min and 12.2×10 min , under the same test conditions. The highest photocatalytic activity
(
G2.5ꢀTiO ) exceeds that of P25 by 1.5 times, which means the excellent photocatalytic activity of GxꢀTiO composite. The
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Mixing sample only shows κ value slightly higher than that of G0ꢀTiO sample, which indicates that the simple
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mechanically adding graphene without chemical bonding interface has small effect on photocatalytic activity. Also, even the
Mixing sample and G2.5ꢀTiO sample have the same graphene loading, their photocatalytic activity shows great difference,
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by a factor of ca. 2.3. This difference in the photocatalytic activity means that the interaction state between graphene and
TiO nanoparticles has vital effect on photocatalytic activity.
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For the practical application of photocatalysts, mineralization ratio in the catalysis process and the stability of
photocatalyst are two key issues. The photocatalytic process is complex, and many intermediate products are produced,
especially when the initial pollutant is complicated. Some intermediate products may be more harmful to human health than
the initial pollutant. So a thorough decomposition of pollutant is of much necessarity. Fortunately, the concentration of
HCHO and CO decreases and increases linearly, respectively, for all the prepared photocatalysts. Moreover, the
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concentration of the produced carbon dioxide is about the same as that of the decomposed HCHO (shown in Fig. 2a),
suggesting that HCHO is completely degraded to CO by the prepared TiO /graphene photocatalysts.
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The stability of photocatalyst is another important practical issue. The intermediate products are usually adsorbed on the
surface active position of the photocatalyst. The photocatalytic activity will decrease dramatically after several cycles of
usage. Fig. 2b shows the photocatalytic stability of G2.5ꢀTiO photocatalysts for five cycles of usage. The κ value kept
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rarely the same in the first four cycles and only decreased slightly in the fifth cycle.
The excellent mineralization efficiency and high photocatalytic stability demonstrate the good practical application
potential of the prepared TiO /graphene nanocomposite. The excellent photocatalytic activity of asꢀprepared TiO /graphene
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nanocomposites is interesting, and detailed characterizations are carried out to reveal the photocatalysis mechanism.
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.2. Structure and morphology of TiO /graphene nanocomposites
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Fig. 3a shows the XRD patterns for the TiO /graphene nanocomposites synthesized with different contents of graphene
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compared to pure TiO . The present peaks clearly represent the formation of anatase crystallites (JCPDS no. 01ꢀ089ꢀ4921).
2
Otherwise, no apparent peaks for graphene were observed. The trace amount of loaded graphene with low atomic number
11
(
Z=6) can not be resolved by XRD. However, the existence of graphene can be clearly elucidated by Raman analysis as
discussed next.
The local structure of TiO /graphene composite is investigated by comparing its Raman spectra with those of the pure
2
ꢀ1
TiO and graphene (Fig. 3b). The four bands located at around 141 (Eg(1)), 391 (B1g(1)), 514 (A + B ) and 634 cm (Eg(2))
2
1g
1g(2)
12
are characteristic for pure anatase TiO (G0–TiO , Fig. 3b). The two typical bands located at around 1355 (Dꢀband) and
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2
ꢀ1
13
1
595 cm (Gꢀband) correspond to graphene (Fig. 3b). As for the Mixing and G2.5ꢀTiO (Fig. 3b), all the Raman bands for
2
anatase TiO and graphene are basically retained. In addition, a smaller intensity ratio of the D band to G band was found in
2
Mixing (I /I = 0.75) and G2.5ꢀTiO (I /I = 0.79) compared with pure graphene (I /I = 0.94), which can be assigned to
D
G
2
D
G
D G
11a
lower defects and disorder of the graphene structures. This demonstrated that TiO and graphene existed in Mixing and
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G2.5ꢀTiO2.
The SEM images of asꢀprepared GxꢀTiO nanocomposite are shown in Fig. 4. The pure TiO nanoparticles (G0ꢀTiO )
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2
were evenly distributed (in Fig. 4a). After adding graphene, the TiO nanoparticles were located or wrapped by graphene
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nanosheets (in Fig. 4bꢀd). The external morphology and microstructures of the TiO /graphene nanocomposites with 2.5 wt%
2
6c
graphene were further studied by TEM (Fig. 5). Due to interfacial interactions and preferential heterogeneous nucleation,
numerous TiO nanocrystals were densely deposited onto the graphene sheets (in Fig. 5a and b). The corresponding
2
HRTEM image (in Fig. 5d) showed clear lattice fringes, which allowed for the identification of crystallographic spacing.
The fringe spacing of ca. 3.51 Å matched that of the (101) crystallographic plane of anatase TiO . From the inset of Fig. 5d,
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the (ꢀ101) facet was also observed. According to the crystallographic knowledge and imaging theory of TEM, the exposed
facet (010) of TiO was perpendicular to these two crystal facets. In addition, the edge of the graphene can also be observed
2
and indicated in Fig. 5d. This suggested that the exposed facet (010) of TiO was attached to the surface of graphene, and
2
thus probably forming an interface between TiO and graphene. In addition, the (010) facet with more Ti atoms exhibited a
2
14
higher surface energy than (101) plane, indicating that there probably was an interaction between Ti atoms and graphene.
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To study the interaction between graphene and TiO in GxꢀTiO , XPS analysis was utilized. Fig. 6a shows the high
2
2
resolution XPS spectra of C 1s region for Mixing. The binding energy of 284.8 eV was a typical peak position for graphite
2
11a
carbon, which demonstrated the sp ꢀhybridized carbon in the graphene state. Furthermore, the deconvoluted peaks
certered at the binding energy of 285.9 eV and 289.0 eV were attributed to the CꢀO and C=O oxygenꢀcontaining
5j
carbonaceous bands. C 1s spectra for the asꢀprepared G2.5ꢀTiO composite are shown in Fig. 6b. Two distinct peaks at
2
2
84.7 eV and 286.0 eV corresponded to graphitic carbon in grapheme and oxygenated carbons in CꢀO bond. Besides, an
additional shoulderꢀpeak located at 281.2 eV was found, which was usually assigned to the formation of chemical bond
15
between carbon atom and titanium atom in the lattice of TiO , which resulted in formation of CꢀTi bonds. Furthermore,
2
lower amounts of the oxygenꢀcontaining carbonaceous bands were detected in the carbon peak of G2.5ꢀTiO as the peak
2
area ratio of CꢀO bonds were decreased obviously and the peak of C=O were not observed in Fig. 6b, indicating less oxygen
deficiencies. Formation of the CꢀTi bond also can be examined and confirmed by analysis of the Ti (2p) core level of the
XPS spectra, as shown in Fig. 6c and d. Fig. 6c shows XPS spectra for Mixing at Ti (2p) binding energy regions. The bands
16
located at binding energies of 458.8 eV and 464.6 eV were assigned to OꢀTi bond in TiO2. In Fig. 6d, in addition to the
two characteristic peaks of TiO at 458.8 eV (Ti 2p ) and 464.6 eV (Ti 2p ), two other weak peaks centered at 455.1 eV
2
3/2
1/2
and 461.1 eV (relating to the Ti 2p3/2 and 2p ) were found and probably coming out of a CꢀTi bond between TiO and
1/2
2
graphene in G2.5ꢀTiO composite. This demonstrated that the CꢀTi chemical bond was present in G2.5ꢀTiO and absent in
2
2
Mixing. In addition, the formation of CꢀTi chemical bond was also confirmed by the Calculation results in Fig. S1 and 2. As
can be seen that the (010) facet of TiO with a higher energy surface and more Ti atoms can adsorb graphene easily and thus
2
CꢀTi chemical bond can be formed easily between the (010) facet of TiO and graphene. Furthermore, the DOS of anatase
2
(
010)/graphene in Fig. S3 showed that the DOS profiles of Ti and C were coincide with each other, further suggesting the
formation of the chemical bond between Ti and C atoms.
The TGꢀDTA analysis was carried out to further confirm the chemical bonding by carbon atom in G2.5ꢀTiO composite.
2
Fig. 7 shows the TGꢀDTA curve of G2.5ꢀTiO and mechanical mixing TiO /graphene (with 2.5% graphene loading)
2
2
ꢀ1
composite, which were performed in air atmosphere with a heating rate of 10°Cꢁmin . With increasing temperature, the
mechanical mixing TiO /graphene showed a gradual mass loss until ca. 400°C and a very sharp exothermic peak at 427°C
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in the DTA curve (Fig. 7b), which was attributed to the combustion of graphene layers. In the contrast, G2.5ꢀTiO2
composite exhibited a later onset of weight loss than the mechanical mixing TiO /graphene composite, and showed an
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obvious exothermic peak centered at 533°C (Fig. 7a). This suggested that the thermal stability of the nanocomposite
samples was enhanced, with the graphene nanosheets being stabilized by the deposited TiO nanoparticles due to the strong
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chemical coupling between them. Likewise, this result proved that there was a CꢀTi bond in G2.5ꢀTiO composite, in line
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with the above results.
The optical properties of the asꢀprepared TiO /graphene nanocomposites were measured by UVꢀvis diffuse reflectance
2
spectra (in Fig. S4a). All of these samples displayed the typical absorption with an intense transition in the UV region of the
spectra, which was assigned to the intrinsic band gap absorption of TiO due to the electron transitions from the valence
2
18
band to conduction band (O2p → Ti3d). The TiO /graphene nanocomposites exhibited increasing absorption in the visible
2
7c
region after adding graphene. In line with the previous reports, a red shift to higher wavelength in the absorption edge of
GxꢀTiO composites has been observed, indicating a narrowing of the band gap of GxꢀTiO . However, it was difficult to
2
2
determine the value for such a red shift. A plot of the transformed KubelkaꢀMunk function dependent on the energy of light
is shown in Fig. S4b. The roughly estimated band gaps were 2.91, 2.94, 3.00, 2.94 and 3.02 eV for G3.0ꢀTiO , G2.5ꢀTiO ,
2
2
G1.5ꢀTiO , G0.5ꢀTiO and pure TiO , respectively. This red shift could be attributed to the chemical bonding between TiO
2
2
2
2
7c
and the specific sites of GO, which was consistent with the DOS result in Fig. S3. In addition, there was an obvious
decease in UVꢀlight absorption for GxꢀTiO compared with G0ꢀTiO , which was in a good agreement with the previous
2
2
9
reports.
.3. Mechanism of enhanced photocatalytic activity
Based on the above characterization results, the mechanism of enhanced photocatalytic activity could be mainly attributed
3
to chemically bond interfacial contact between TiO and graphene. Charge carrier transfer plays a pivotal role in
2
photocatalytic processes, for once electronꢀhole pairs are generated by light excitation, most of them recombine generating
heat and only a small fraction can successfully transfer to the interface to initiate redox reactions. It is reported that the
19
20
factors affecting the efficiency of the electron transfer property includes the nanoparticles surface, size
and
21
morphology. As for grapheneꢀbased photocatalysts, the enhanced carrier transfer property is mainly contributed by the
7c, 22
enhanced special charge transportation properties of graphene.
Also, the interaction extent between graphene and
9
semiconductor nanoparticles determines the electron transportation, which further determines the photocatalytic activity.
Our previous work about graphene/BiOCl photocatalyst reveals that the formation of CꢀBi bond contributes much for the
11a
photocatalytic performance. Herein, we propose an effective chemicalꢀbonded interfacial charge transfer (CBꢀIFCT) to
11a, 23
explain the enhanced photocatalytic activity of TiO /graphene composite.
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This is ascribed that mechanical mixing process is not able to create effective interfacial contact between TiO and
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grapheme, while TiO /graphene composite with chemical bonding causes an intimate interaction between TiO
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2a, 24
nanoparticles and graphene nanosheets (illustrated in Fig. 8a).
In this regard, the graphene loading with intimate
chemical bonding can efficiently facilitate the interfacial electron transfer and electronꢀhole separation. TiO has a high
2
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5
potential of conduction band bottom (2.81 eV vs. NHE, normal hydrogen electrode), so the free electrons have powerful
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reducibility. Oxygen molecules adsorbed on the TiO surface could react with free electrons. Thereby, a depletion layer is
2
26
created with low conductivity near the surface. Within TiO material, the CB electrons have to travel through the grain
2
boundaries (GB). Thus, when potential barrier is formed at the GB regions, the mobility of electron carriers could be
27
limited. In other words, the electrons transfer from one TiO grain to the neighbored one should get through the potential
2
barrier. So, if TiO and graphene are chemically coupled, they can decrease the potential barrier at the GB regions. The
2
chemical binding could provide a good spatial condition for charge transport from TiO to graphene via the interfaces.
2
However, the mechanical mixing could not provide intimate spatial condition for charge transport.
25
For the TiO materials, the reported energy level of valence band (VB) and conduction band (CB) is 2.81 V and ꢀ0.39 V
2
28
vs. NHE (normal hydrogen electrode), respectively. The calculated Fermi energy level of graphene is ꢀ0.08 V vs. NHE. So,
the photoꢀgenerated electron on the CB of TiO is energetically feasible to transfer to graphene (as showed in Fig. 8b). As
2
the Fermi energy of graphene is much lower than the CB of TiO , the graphene can act as a sink for the photogenerated
2
electrons. The excited electrons can be stored in the huge πꢀπ network of graphene nanosheets in the composites, which can
retard the electronꢀhole recombination on TiO . This process facilitates effective interface charge separation and hinders
2
carrier recombination.
The electron transfer between TiO and graphene nanosheets can be expressed as:
2
ꢀ
+
TiO + hγ → e (CB, TiO ) + h (VB, TiO )
(1)
(2)
2
2
2
ꢀ
ꢀ
graphene + e (CB, TiO ) →e (graphene) + TiO
2
2
On the other hand, the adsorbed HCHO molecule could directly transfer electron with chemical bonded TiO /graphene
2
29
composite. DFT calculation results show that, for HCHO molecule, the highest occupied molecular orbital (HOMO) is 2.5
eV below the Fermi level and the lowest unoccupied molecular orbital (LUMO) is 0.6 eV above the Fermi level. So, as Fig.
8
b illustrated, the electron located at the LUMO of HCHO can transfer to graphene, and the holes stayed in the VB of TiO2
are reacted with the absorbed HCHO, as expressed as follows:
ꢀ
ꢀ
graphene + e (LUMO, HCHO) →e (graphene) + HCHO
(3)
(4)
(5)
+
+
H O + h (VB, TiO ) → ꢁOH + H
2
2
HCHO (ad) + ꢁOH → CO + H O
2
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The key of improving the oxidation efficiency for the GxꢀTiO composite is attributed to IFCT effect, which decreases
2
the possibility of recombination of electronꢀhole pairs, increases the number of holes participating in the photooxidation
process and thus enhances the photocatalytic activity. The IFCT effect is experimentally supported by EIS, gaseous phase
photocurrent and PL test results.
22a, 30
In the previous studies, EIS analysis is commonly used to confirm the above proposition.
Fig. 9a shows the EIS
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Nynquist plots of the asꢀprepared and the mechanical mixing TiO /graphene nanocomposites UV irradiation. The arc radius
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on EIS Nynquist plot of G2.5ꢀTiO is the smallest of the three samples; and that of the Mixing sample is only smaller than
2
G0ꢀTiO sample. In EIS Nynquist plot, the smaller the semicircle size indicates an effective separation of photogenerated
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electronꢀhole pairs and fast interfacial charge transfer to the electron donor or acceptor. Since the radius of the arc on the
EIS spectra reflects the reaction rate occurring at the surface, suggesting that a more effective separation of photogenerated
electronꢀhole pairs and a faster interfacial charge transfer occur on G2.5ꢀTiO photocatalyst under this condition. This result
2
clearly indicates that the chemical combination of TiO and graphene could effectively enhance the separation of
2
photogenerated electronꢀhole pairs.
The EIS test is carried out under liquid conditions, which is not clearly the same as gaseous degradation experiments. To
further support the above proposition, the transient gaseous phase photocurrent responses are recorded for photoelectrodes
consisting of pure TiO , mechanical mixing and chemical bonding TiO /graphene nanocomposites under the same
2
2
conditions as the photocatalytic reaction, i.e., 200 ppm HCHO gas in air and UV LED irradiation. Fig. 9b shows the Iꢀt
curves for the three samples with UVꢀirradiation. It is suggested that the photocurrent is mainly determined by the
separation efficiency of photoꢀgenerated electronꢀhole pairs within the photocatalyst. The photocurrent of the chemical
bonding composite (G2.5ꢀTiO ) is enhanced ca. 15.3 times than that of pure TiO (G0ꢀTiO ), which indicates that the
2
2
2
separation efficiency of photoinduced electrons and holes is improved through the electronic interaction between graphene
nanosheets and TiO nanoparticles. Also, the photocurrent of the chemical bonding composite (G2.5ꢀTiO ) is enhanced ca.
2
2
4
.1 times than that of mechanical mixing composite (the Mixing), which is mainly due to the chemical interactions and the
synergetic effect graphene nanosheets and TiO nanoparticles.
2
PL emission spectra resulting from the recombination of photoinduced charge transportation are powerful demonstrations
11a
of enhanced charge transportation and separation properties. PL signals for G0ꢀTiO , Mixing and G2.5ꢀTiO under
2
2
excitation at 325 nm are given in Fig. 10. The lowest PL intensity for G2.5ꢀTiO indicates the lowest recombination rate of
2
photoinduced electronꢀhole pairs, consistant with the photocatalytic activity of the sample. In addition, Mixing also exhibits
a slightly lower PL intensity than G0ꢀTiO , but its intensity is much higher than that of G2.5ꢀTiO . This is mainly attributed
2
2
to the less defects in G2.5ꢀTiO , confirmed by the above Raman and XPS results. More importantly, this PL result
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demonstrates that only the TiO /graphene composite with chemical bonding interface can effectively facilitate the charge
2
transportation and separation, in a good agreement with the results of EIS and gaseous phase photocurrent.
4. Conclusion
In conclusion, TiO /graphene nanocomposites with high photocatalytic activity were synthesized by a facile solvothermal
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approach at 200°C for 10 h. The anatase TiO nanocrystals were densely supported on graphene nanosheets with close
2
interfacial contacts. Further characterization results indicate that the TiO nanocrystals were chemical bonding with the
2
graphene nanosheets, which is confirmed by the formation of CꢀTi bond in XPS. The chemical bonding nanocomposite
(G2.5ꢀTiO ) exhibits 2.6 times enhancement of photocatalytic activity than pure TiO . Based on the EIS, gaseous phase
2
2
photocurrent and PL test results, this enhancement can be explained by the IFCT effect, which could provide a good spatial
condition for charge transport from TiO to graphene via the interfaces, decrease the possibility of recombination of
2
electronꢀhole pairs, and thus lead to a higher photocatalytic activity.
Acknowledgements
This work was supported by the National Basic Research Program of China (Grant Nos. 2009CB939702 and
2
009CB939705), Nature Science Foundation of China (No. 50772040 and 50927201). The authors are also grateful to
Analytical and Testing Center of Huazhong University of Science and Technology.
References
(
(
1) Linsebigler, A.; Lu, G.; Yates, J. Chem. Rev. 1995, 95, 735.
2) (a) Inagaki, M.; Kojin, F.; Tryba, B.; Toyoda, M. Carbon 2005, 43, 1652; (b) Wang, Z.; Cai, W.; Hong, X.; Zhao, X.;
Xu, F.; Cai, C. Appl. Catal. B 2005, 57, 223; (c) Divalentin, C.; Finazzi, E.; Pacchioni, G.; Selloni, A.; Livraghi, S.;
Paganini, M.; Giamello, E. Chem. Phys. 2007, 339, 44.
(
3) (a) Mozia, S.; Toyoda, M.; Inagaki, M.; Tryba, B.; Morawski, A. J. Hazard. Mater. 2007, 140, 369; (b) Byrappa, K.;
Dayananda, A. S.; Sajan, C. P.; Basavalingu, B.; Shayan, M. B.; Soga, K.; Yoshimura, M. J. Mater. Sci. 2008, 43, 2348.
(
(
4) Kobayakawa, K.; Murakami, Y.; Sato, Y. J. Photochem. Photobiol. A: Chem. 2005, 170, 177.
5) (a) Sun, L.; Zhao, Z.; Zhou, Y.; Liu, L. Nanoscale 2012, 4, 613; (b) Lambert, T. N.; Chavez, C. A.; Bell, N. S.;
Washburn, C. M.; Wheeler, D. R.; Brumbach, M. T. Nanoscale 2011, 3, 188; (c) Li, M.; Tang, P. S.; Hong, Z. L.; Wang, M.
Q. Colloid Surf. A 2008, 318, 285; (d) Li, M.; Hong, Z. L.; Fang, Y. N.; Huang, F. Q. Mater. Res. Bull. 2008, 43, 2179; (e)
Li, M.; Zhou, S. F.; Zhang, Y. W.; Chen, G. Q.; Hong, Z. L. Appl. Surf. Sci. 2008, 254, 3762; (f) Khalid, N. R.; Ahmed, E.;
Hong, Z. L.; Zhang, Y. W.; Ahmad, M. Curr.Appl. Phys. 2012, 12, 1485; (g) Khalid, N. R.; Ahmed, E.; Hong, Z. L.;
Ahmad, M. Appl. Surf. Sci. 2012, 263, 254. (h) Khalid, N. R.; Hong, Z. L.; Ahmed, E.; Zhang, Y. W.; Chan, H.; Ahmad, M.
ACS Paragon Plus Environment

ACS Catalysis
Page 12 of 24
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5
5
5
5
5
6
Appl. Surf. Sci. 2012, 258, 5827; (j) Perera, S. D.; Mariano, R. G.; Vu, K.; Nour, N.; Seitz, O.; Chabal, Y.; Jr, K. J. B. ACS
catal. 2012, 2, 949.
(6) Chang, K.; Chen, W. Chem. Commun. 2011, 47, 4252.
(7) (a) Wang, G.; Wang, B.; Park, J.; Yang, J.; Shen, X.; Yao, J. Carbon 2009, 47, 68; (b) Williams, G.; Seger, B.; Kamat,
P. V. ACS Nano 2008, 2, 1487; (c) Zhang, H.; Lv, X.; Li, Y.; Wang, Y.; Li, J. ACS Nano 2010, 4, 380.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
(
(
(
8) Wang, P.; Zhai, Y.; Wang, D.; Dong, S. Nanoscale 2011, 3, 1640.
9) Fan, W. Q.; Lai, Q. H.; Zhang, Q. H.; Wang, Y. J. Phys. Chem. C, 2011, 115, 10694.
10) Choucair, M.; Thordarson, P.; Stride, J. A. Nat. Nanotechnol. 2008, 4, 30.
11) (a) Gao, F.; Zeng, D.; Huang, Q.; Tian, S.; Xie, C. Phys. Chem. Chem. Phys. 2012, 4, 613; (b) Huang, Q.; Zeng, D.; Li,
H.; Xie, C. Nanoscale 2012, 4, 5651.
12) (a) Li, N.; Liu, G.; Zhen, C.; Li, F.; Zhang, L.; Cheng, H. Adv. Funct. Mater. 2011, 21, 1717; (b) Liu, G.; Yang, H. G.;
Wang, X.; Cheng, L.; Lu, H.; Wang, L.; Lu, G. Q.; Cheng, H. J. Phys. Chem. C 2009, 113, 21784.
(
(
(
(
(
(
(
(
13) Malard, L. M.; Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S. Phys. Rep. 2009, 473, 51.
14) Lei, Y. K.; Liu, H. J.; Xiao, W. Modelling Simul. Mater. Sci. Eng. 2010, 18, 025004.
15) Wu, Z.; Dong, F.; Zhao, W.; Wang, H.; Liu, Y.; Guan, B. Nanotechnology 2009, 20, 235701.
16) Zhang, L.; Koka, R. V. Mater. Chem. Phys. 1998, 57, 23.
17) Jiang, G.; Lin, Z.; Chen, C.; Zhu, L.; Chang, Q.; Wang, N.; Wei, W.; Tang, H. Carbon 2011, 49, 2693.
18) Liu, G.; Wang, L.; Yang, H. G.; Cheng, H. M.; Lu, G. Q. M. J. Mater. Chem. 2010, 20, 831.
19) Makarova, O. V.; Rajh, T.; Thurnauer, M. C.; Martin, A.; Kemme, P. A.; Cropek, D. Environ. Sci. Technol. 2000, 34,
4
797.
(
(
(
20) Wang, W.; Germanenko, I.; ElꢀShall, M. S. Chem. Mater. 2002, 14, 3028.
21) Murray, C.; Kagan, C.; Bawendi, M. Annu. Rev. Mater. Sci. 2000, 30, 545.
22) (a) Cao, A.; Liu, Z.; Chu, S.; Wu, M.; Ye, Z.; Cai, Z.; Chang, Y.; Wang, S.; Gong, Q.; Liu, Y. Adv. Mater. 2010, 22,
1
03; (b) Zhang, Y.; Tang, Z.; Fu, X.; Xu, Y. ACS Nano 2011, 5, 7426; (c) Wang, Y.; Shi, R.; Lin, J.; Zhu, Y. Appl. Catal., B
010, 100, 179.
2
(
(
(
(
23) Liang, Y.; Wang, H.; Sanchez Casalongue, H.; Chen, Z.; Dai, H. Nano Res. 2010, 3, 701.
24) Zong, X.; Yan, H.; Wu, G.; Ma, G.; Wen, F.; Wang, L.; Li, C. J. Am. Chem. Soc. 2008, 130, 7176.
25) Xu, Y.; Schoonen, M. Am. Mineral. 2000, 85, 543.
26) Zhou, J.; Gu, Y.; Hu, Y.; Mai, W.; Yeh, P. H.; Bao, G.; Sood, A. K.; Polla, D. L.; Wang, Z. L. Appl. Phys. Lett. 2009,
9
4, 191103.
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(27) Muraoka, Y.; Takubo, N.; Hiroi, Z. J. Appl. Phys. 2009, 105, 103702.
(28) (a) Tang, Y.; Lee, C.; Xu, J.; Liu, Z.; Chen, Z.; He, Z.; Cao, Y.; Yuan, G.; Song, H.; Chen, L.; Luo, L.; Cheng, H.;
Zhang, W.; Bello, I.; Lee, S. ACS Nano 2010, 4, 3482; (b) Wang, X.; Zhi, L.; Mullen, K.. Nano Lett. 2007, 8, 323.
(29) Chi, M.; Zhao, Y. Comp. Mater. Sci. 2009, 46, 1085.
(
(
30) Gao, E.; Wang, W.; Shang, M.; Xu, J. Phys. Chem. Chem. Phys. 2011, 13, 2887.
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Captions for figures:
Figure 1. The photocatalytic activity (κ) of GxꢀTiO composites (x=0, 0.5, 1.5, 2.5 and 3.0), P25 and Mixing samples.
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Figure 2. (a) The degradation curves of HCHO and increasement of CO by GxꢀTiO composite (x=0, 0.5, 1.5, 2.5 and 3.0);
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b) the stability test of G2.5ꢀTiO photocatalyst.
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Figure 3. (a) XRD patterns of TiO /graphene nanocomposites; (b) Raman spectra of G2.5ꢀTiO , Mixing, G0ꢀTiO and pure
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grapheme.
Figure 4. SEM images of GxꢀTiO nanocomposite, where x=0 (a), 0.5 (b), 1.5 (c) and 2.5(d).
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Figure 5. (a, b) TEM and (c, d) HRTEM images of G2.5ꢀTiO nanocomposite.
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Figure 6. The C 1s (a, b) and Ti 2p (c, d) XPS spectra of mechanical mixing (a, c) and chemical bonded (b, d)
TiO /graphene nanocomposites, respectively.
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Figure 7. The TGꢀDTA curves of chemical bonded (a) and mechanical mixing (b) TiO /graphene nanocomposites,
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respectively.
Figure 8. Schematic illustrations for effective IFCT effect for TiO /graphene composites.
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Figure 9. (a) EIS spectra and (b) gaseous phase photocurrent curves of G0ꢀTiO , G2.5ꢀTiO and the mechanical mixing
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composite samples.
Figure 10. PL emission spectra of G0ꢀTiO , Mixing and G2.5ꢀTiO .
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Figure 1. The photocatalytic activity (κ) of GxꢀTiO composites (x=0, 0.5, 1.5, 2.5 and 3.0), P25 and Mixing samples.
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Figure 2. (a) The degradation curves of HCHO and increasement of CO by GxꢀTiO composite(x=0, 0.5, 1.5, 2.5 and 3.0);
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Figure 4. SEM images of GxꢀTiO nanocomposite, where x=0 (a), 0.5 (b), 1.5 (c) and 2.5(d).
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Figure 6. The C 1s (a, b) and Ti 2p (c, d) XPS spectra of mechanical mixing (a, c) and chemical bonded (b, d)
TiO /graphene nanocomposites, respectively.
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Figure 7. The TGꢀDTA curves of chemical bonded (a) and mechanical mixing (b) TiO /graphene nanocomposites,
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Figure 8. Schematic illustrations for effective IFCT effect for TiO /graphene composites.
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Figure 9. (a) EIS spectra and (b) gaseous phase photocurrent curves of G0ꢀTiO , G2.5ꢀTiO and the mechanical mixing
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Fig. 10 PL emission spectra of G0ꢀTiO , Mixing and G2.5ꢀTiO .
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Herein, we have employed a facile solvothermal method to prepare TiO /graphene composites with
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chemical bonding interface, which was confirmed by XPS analysis and TG-DTA analysis. The
chemically bonded TiO /graphene composites effectively enhanced their photocatalytic activity in
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photodegradation of formaldehyde. The prepared composite with 2.5 wt% graphene showed the highest
photocatalytic activity, exceeding that of P25, pure TiO and the mechanically mixing TiO /graphene
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through the chemical bond, which markedly decreased the recombination of electron-hole pairs, and
increased the number of holes participating in the photooxidation process. The explanation was further
confirmed by gaseous phase transient photocurrent response, EIS and PL.
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