Photocatalytic degradation of acetic acid in the presence of visible light-active TiO2-reduced graphene oxide photocatalysts

Article abstract of DOI:10.1016/j.cattod.2016.05.055

Visible light-active TiO₂–reduced graphene oxide photocatalysts were prepared using simple mechanical mixing of titanium dioxide with different amounts of rGO (0.1, 0.5, 1.0 and 2.0?wt.%) in the presence of 1-butyl alcohol. Structures and morphologies of the samples were examined by means of FTIR/DRS, UV–vis/DR, XRD, SEM, TEM and Raman spectroscopy. The photocatalytic properties were checked on the basis of acetic acid photooxidation (the steady rate of linear increase of the CO₂ yield was used for the estimation of photocatalytic activity). The maximum photodegradation rate was observed for TiO₂ decorated with 0.5 wt.% of rGO. The enhancement of photodegradation efficiency should be related to π-conjugation system, two-dimensional planar structure and efficient charge separation of reduced graphene oxide nanosheets.

Full text of DOI:10.1016/j.cattod.2016.05.055

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Photocatalytic degradation of acetic acid in the presence of visible
light-active TiO2-reduced graphene oxide photocatalysts
A.W. Morawskia,∗, E. Kusiak-Nejmana, A. Wanaga, J. Kapica-Kozara, R.J. Wróbela,
B. Ohtanib, M. Aksienionekc, L. Lipin´ skac
a West Pomeranian University of Technology, Institute of Inorganic Technology and Environment Engineering, Pułaskiego 10, 70-322 Szczecin, Poland
b Hokkaido University, Research Institute for Catalysis, Sapporo 001-0021, Japan
c Institute of Electronic Materials Technology, Wólczyn´ska 133, 01-919 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Visible light-active TiO2–reduced graphene oxide photocatalysts were prepared using simple mechanical
mixing of titanium dioxide with different amounts of rGO (0.1, 0.5, 1.0 and 2.0 wt.%) in the presence of
1-butyl alcohol. Structures and morphologies of the samples were examined by means of FTIR/DRS,
UV–vis/DR, XRD, SEM, TEM and Raman spectroscopy. The photocatalytic properties were checked on
the basis of acetic acid photooxidation (the steady rate of linear increase of the CO2 yield was used
for the estimation of photocatalytic activity). The maximum photodegradation rate was observed for
TiO2 decorated with 0.5 wt.% of rGO. The enhancement of photodegradation efficiency should be related
to ␲-conjugation system, two-dimensional planar structure and efficient charge separation of reduced
graphene oxide nanosheets.
Received 30 January 2016
Received in revised form 15 May 2016
Accepted 23 May 2016
Available online xxx
Keywords:
Photocatalysis
Titanium dioxide
Graphene
© 2016 Elsevier B.V. All rights reserved.
Reduced graphene oxide
Acetic acid photooxidation
Visible light
1. Introduction
generated electrons across the graphene 2D–sheets, which mini-
mizes the electron- hole recombination and enhances the oxidative
In recent years, graphene, a two-dimensional novel carbon
nanomaterial with zero band gap, large specific surface area, excel-
lent mechanical, electrical, optical and thermal properties and its
applications in sensors and biosensors, electronic devices, liquid
crystalline displays, capacitors solar cells, H2 production, energy
storage and nanocomposites including TiO2/graphene materials,
has been intensively studied [1–10]. Graphene is composed of sin-
gle or less than 10 planar sheets of sp2–bonded carbon atoms
forming six–membered rings [11]. Singh et al. [5] described in detail
the most common methods of graphene preparation: exfoliation
and cleavage, chemical vapour deposition CVD, graphene oxide
reduction, total organic synthesis or un-zipping carbon nanotubes.
Recently, the combination of graphene and semiconductors,
especially titanium dioxide, as a promising route to obtain new
graphene–TiO2 nanocomposites with enhanced charge separation
in electron–transfer, has been intensively studied. Incorporation of
graphene or graphene oxide into TiO2 provides large specific sur-
face area and high charge carrier mobility due to the moving of TiO2
reactivity [11–13]. Huang et al. [13] explained the enhancement of
TiO2–graphene nanocomposites photocatalytic activity by inter-
facial charge transfer through a C Ti bond, which increased the
number of holes participating in the photocatalytic process due
to markedly decreasing recombination of electron- hole pairs. The
formation of Ti
O C bonds results expanding the light absorption
to longer wavelengths and the possibility of TiO2 excitation with
visible or solar light.
Wang et al. [11] proposed the graphitization of melamine
used as carbon source dispersed in methanol and mixed with
P25 TiO2 and then homogenized in ultrasonic bath. The sig-
nificant enhancement of photocatalytic activity of new hybrid
TiO2–graphene materials, calculated on the basis of Methylene Blue
(MB) decomposition under UV light source, was caused by a rapid
photoinduced charge separation and the inhibition of recombina-
tion for electron–hole pairs. This results in the increase of number
of holes participating in the photooxidation process of used thi-
azine dye. Thus the synergetic effect between graphene–like carbon
and titanium dioxide was also proved. Ni et al. [14] discussed new
photocatalytic mechanisms of MB degradation under visible light
in the presence of graphene supported TiO2 and graphene strongly
wrapped–TiO2 nanocomposites. Firstly, the higher photoactivity of
∗
Corresponding author.
E-mail address: amor@zut.edu.pl (A.W. Morawski).
http://dx.doi.org/10.1016/j.cattod.2016.05.055
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: A.W. Morawski, et al., Photocatalytic degradation of acetic acid in the presence of visible light-active
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graphene–modified TiO2 samples was found in comparison with
bare TiO2 and commercial P25 due to enhanced light absorption,
separation of photogenerated carriers, and improved dye adsorp-
tion. Secondly, it was found that in the case of graphene supported
TiO2 nanocomposites the photogenerated electrons from excited
MB adsorbed on graphene can flow on the graphene surface freely,
but these electrons cannot inject into the titania conduction band
due to the slightly lower graphene energy level in comparison with
TiO2. Hence, the electrons rapidly recombined to the MB ground
state. The excitation of graphene strongly wrapped–TiO2 nanocom-
posites and MB molecules allowed the electrons from the excited
MB be transferred to the TiO2 conduction band via graphene. Most
of the electrons will be trapped to form reactive oxygen species
(ROS), which can easily oxidize MB, causing degradation of organic
contamination.
has been expanded at a temperature of about 1000 ◦C. Next the
appropriate amounts of NaNO3 (pure p.a., CAS: 7631-99-4) and
concentrated H2SO4 (pure p.a., CAS: 7664-93-9) were added to
sample of expanded graphite. The KMnO4 (pure p.a., CAS: 7722-
64-7) used as the oxidant, was gradually added into the mixture
maintaining the reaction temperature below 5 ◦C. The process was
conducted with continuous stirring the reactants. In the next step
the mixture was heated to 35 ◦C and left at this temperature for
3 h. Then, the reaction mixture was heated to 95 ◦C. To complete
the reaction, deionized water was added and a small amount of
hydrogen peroxide. The product was washed with 3% HCl (pure p.a.,
CAS: 7647-01-0) solution to remove of sulfate ions and finally with
deionized water. The purified product was subjected to exfoliation
using ultrasonic processor. Such obtained GO sample was dried
under vacuum and then thermally reduced. Used reagents of ana-
lytical grade were purchased from Avantor Performance Materials
Poland S.A., Poland.
Furthermore, Meng et al. [15] prepared graphene–based TiO2
nanocomposites by atomic layer deposition and Peining et al. [7]
proposed a very simple method of TiO2–graphene nanomateri-
als fabrication by electrospinning. However, modified Hummer’s
method is the preparation method most frequently used for
TiO2–graphene nanocomposites [16–18].
2.3. Preparation of TiO2/rGO nanocomposites
A 5 g of titanium dioxide (named as starting TiO2) and appro-
priate mass ratio (0.1, 0.5, 1.0 and 2.0 wt.%) of reduced graphene
oxide-to-titania were mechanically mixed in the presence of
100 mL 1–butyl alcohol and then ultrasonicated for 30 min. After
sonication step, a slurry of TiO2, 1–butanol and rGO was partially
evaporated in order to remove the excess of primary aliphatic alco-
hol. Then, each sample was dried at 100 ◦C for 24 h in a muffle
furnace to remove water and residues of 1–butyl alcohol from the
obtained photocatalysts surface. The final products are denoted
as TiO2–rGO(x), where x is appropriate mas ratio of added rGO.
The TiO2–rGO nanocomposites were prepared without calcination
process.
In this paper, the study on the photocatalytic performance
of TiO2-graphene nanocomposites activated by visible light is
reported. A large number of already published articles describe
the activity of TiO2-graphene photocatalysts usually under UV
light. According to the facts that graphene is a carbonaceous
material and carbon doping of titania generally contributes to
bathochromic shift of absorption band, the investigation of TiO2-
rGO photoactivity under visible light seems to be crucial. In our
case the photocatalytic activity of prepared nanomaterials was
calculated on the basis of acetic acid decomposition. Addition-
ally, TiO2–graphene nanocomposites were obtained in different
TiO2–to–graphene mass ratio by mechanical mixing of starting TiO2
supplied by sulfate technology and reduced graphene oxide (rGO)
in the form of flakes synthesized according to modified Hummersꢀ
method.
2.4. Photocatalysts characterization
The surface properties of tested materials were examined using
a FTIR/DRS spectrophotometer FTIR 4200 (Jasco, Japan) equipped
with a diffuse reflectance accessory (Harrick, USA). The FTIR band
under examination covered wave numbers ranging from 400 to
4000 cm−1, and each spectrum was collected with 4 cm−1 resolu-
tion.
2. Experimental
2.1. Materials and reagents
The light reflectance abilities of the samples were obtained by
UV–vis/DR spectrometry using a V-650 UV–vis spectrophotome-
ter (Jasco, Japan) equipped with an integrating sphere accessory
for studying DR spectra. BaSO4 (purity 98%, CAS: 7727-43-7,
Avantor Performance Materials, Poland) was used as a reference
material. Crystal structures and relative phase compositions of
obtained photocatalysts were analyzed using X-ray powder diffrac-
tion analysis (XꢀPRO Philips diffractometer) with Cu-K␣ radiation
(␭ = 1.54056 Å). Titania anatase-over-rutile ratio was calculated in a
way described previously [20,21]. The standard uncertainties were
estimated from fivefold XRD measurements of the same sample.
Subsequently, phase content and crystallite sizes were evaluated.
For obtained values the average and standard deviation was calcu-
lated. The surface morphologies of obtained materials was checked
via transmission electron microscopy (TEM) on an electron micro-
scope (FEI Tecnai F30, Frequency Electronics Inc., Mitchel Field, NY,
USA) operated at an acceleration voltage of 200 kV. Additionally,
scanning electron microscopy (SEM) Hitachi SU8020 Ultra-High
Resolution Field Emission Scanning Electron Microscope (Japan)
was used. The 5 kV electrons energy was applied for the exci-
tation of studied samples. Raman spectrum was recorded on a
Renishaw Micro Raman spectrometer (␭ = 785 nm). The N2-BET
specific surface area measurements of the photocatalysts were
carried out based on N2 adsorption isotherms at 77 K using the
Quadrasorb Si analyzer (Quantachrome, USA). Prior to analyses,
A crude titanium dioxide slurry supplied by sulfate technology
in Grupa Azoty Zakłady Chemiczne “POLICE” S.A. (Poland) contain-
ing post-production residues of sulfuric acid (2.1 wt.%) was rinsed
with aqueous solution of ammonia water (solution 25%, CAS: 1336-
21-6, Avantor Performance Materials Poland S.A.) and further with
distilled water to pH = 6.8. The solution of ammonia water was used
in order to remove the residues of sulphuric acid through forma-
tion of ammonium sulfate, which is easily dissolved in water. The
prepared material was dried at 105 ◦C for 24 h in a muffle furnace to
remove water molecules adsorbed on the surface of starting TiO2.
1-butyl alcohol (purity 99.5%, CAS: 71-36-3) was purchased from
Avantor Performance Materials (Poland). Natural graphite (Asbury
Carbons, USA) was a starting material in a process of graphene
preparation. All the chemicals were used as received without fur-
ther purification. The ultrapure water from Millipore ultrafiltration
system was used in all the experiments. Commercial titanium diox-
ide KRONOClean 7000 (Kronos International, Inc., Germany) was
used as the reference carbon-modified TiO2 photocatalyst.
2.2. Preparation of reduced graphene oxide (rGO)
Graphene oxide (GO) was obtained by modified Hummers
method [19] as follows. The starting material was the flake graphite
(Asbury Carbons, USA) intercalated with mineral acids, which
Please cite this article in press as: A.W. Morawski, et al., Photocatalytic degradation of acetic acid in the presence of visible light-active
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each sample was degassed at 105 ◦C for 12 h under high vacuum
to remove residual water adsorbed on the surface of titania pho-
tocatalysts. The values of the specific surface areas (SBET) were
determined using multi-point analysis of adsorption isotherms
applying Brunauer–Emmet–Teller equation.
2.5. Photocatalytic measurements
The photocatalytic properties of tested materials was deter-
mined on the basis of CO2 evolution rate during decomposition
of acetic acid in aqueous solutions in air atmosphere under visible
light irradiation. Acetic acid (CAS: 64-19-7, Wako Pure Chemical
Industry, Japan) was used a model organic pollutant, which does
not absorb the visible light. As a source of light a xenon lamp with a
cut-off filter providing the wavelengths of light >450 nm was used.
The photocatalytic tests were performed in sealed glass tubes. A
50 mg portion of each photocatalyst powder was suspended in
5.0 mL of an aqueous solution containing 5 vol.% of acetic acid. Dur-
ing experiments, reaction mixtures were continuously stirred. The
molar amount of products evolved in the gas phase of reaction mix-
tures was measured using a FID-Gas chromatograph (Shimadzu
GC-14B) for carbon dioxide detection. The steady rate of linear
increase of the product (CO2) yield was used for the estimation
of photocatalytic activity. Previous study showed with applied to
same conditions CO2 was released by complete oxidation of acetic
acid [22]. The stoichiometry of acetic acid oxidation is assumed to
be follows:
CH3COOH + 2O2 → 2CO2 + 2H2O
(1)
For the best active sample the action spectra measurement was
performed. Action spectra analysis was performed employing the
photocatalytic decomposition of acetic acid in aerated liquid sus-
pensions as a test reaction, by measuring the reaction rate as a
function of irradiation wavelength, in rectangular quartz cells with
a ca. 10.5 mL volume. 30 mg of each photocatalysts was suspended
in 3 mL of aqueous acetic acid (5 vol.%). The tube was purged of air
with argon for at 10 min to remove the CO2 adsorbed on the surface
of TiO2-rGO photocatalysts and then stirred in the dark for 15 min.
After this time, the sample was irradiated at monochromatic wave-
lengths for 60 min using a diffraction grating-type illuminator
(Jasco, CRM-FD), equipped with a 300 W Xenon lamp (Hamamatsu,
C2578-02). During the experiments, the reaction mixtures were
continuously stirred. The irradiation wavelength was selected in
the 450–480 nm range. The light intensity was measured by an opti-
cal power meter (HIOKI 3664). During the irradiation, every 20 min
a 0.2 mL of the gas phase of the reaction mixture was withdrawn
with a syringe and injected into a gas chromatograph for carbon
dioxide analysis (Shimadzu GC-14B). The standard uncertainties
were estimated from twofold measurements of CO2 concentration.
For obtained values the average and standard deviation was cal-
culated. The wavelength-dependent apparent quantum efficiency
Fig. 1. FTIR/DRS spectra of: (1) KRONOClean 7000, (2) starting TiO2, (3) TiO2-
rGO(0.1), (4) TiO2-rGO(0.5), (5) TiO2-rGO(1.0), (6) TiO2-rGO(2.0).
bands typical for titania are possible to observe in the spec-
tra of all studied photocatalysts: the broad absorption band in
3800–2600 cm−1 range attributed to the O H stretching mode
of interacting hydroxyl groups, symmetric and asymmetric OH
stretching modes molecular water coordinating with Ti4+ ions [21]
and the narrow band at 1630 cm−1 assigned to the molecular water
bending mode [22]. It was noted that the increase of rGO addi-
tive in TiO2-rGO nanomaterials causes the partial reduction of the
intensity of OH groups at 3800–2600 cm−1 due to the changes
in concentration of surface hydroxyl groups. The strong band at
950 cm−1 found for starting TiO2 and reference KRONOClean 7000
is attributed to the self-absorption of titanium [23,24]. The first
thing that could be concluded from rGO-modified TiO2 FTIR/DRS
spectrum is that the decrease of the intensity of band attributed
to the self-absorption of titanium for all modified photocatalysts
is possible to observe. This is related with increasing rGO mass
ratio in prepared samples. Secondly, the band at 950 cm−1 found
for starting TiO2 shifted to lower wavenumbers for all new pho-
tocatalysts. New bands for the TiO2-rGO nanocomposites could be
also observed. Bands found at 3000 and 2850 cm−1 are attributed
to the aliphatic groups characteristic for 1-butanol added to TiO2-
rGO mixture in the preparation process [25,26]. The presence of
peak at 1383 cm−1 found for all samples mixed with the rGO may
be indicative of incomplete reduction of graphene oxide [20]. Fur-
thermore, the characteristic band of rGO found at 1540 cm−1 was
assigned to the skeletal vibration of the graphene sheets [27,28].
In the case of commercial titania KRONOClean 7000 a broad
band at 1580 cm−1 assigned to the asymmetric and symmetric
stretching vibrations of an arylcarboxylate group and band at
1330 cm−1 attributed to the CO stretching vibrations are possible
˚
app
was calculated as the ratio of the electron consumption rate
from the rate of CO2 generation to the flux of incident photons.
According to the stoichiometry of the reaction (see Eq. (1)), it is
8-electron process (reduction of one O2 molecule requires 4 elec-
trons) with carbon dioxide and water as the main products. Due
to the fact that carbon dioxide concentration was measured it was
assumed that no water molecule was produced. Thus, for that kind
of simplified reaction, only 4 electrons are required.
3. Results and discussion
The surface properties of the photocatalysts were investigated
by means of FTIR/DRS spectroscopy. Fig. 1 shows the FTIR/DRS
spectra of reference KRONOClean 7000, unmodified (starting) TiO2
and reduced graphene oxide-modified TiO2 photocatalysts. Several
Please cite this article in press as: A.W. Morawski, et al., Photocatalytic degradation of acetic acid in the presence of visible light-active
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(1)
(2)
90
60
30
0
(3)
(4)
(5)
(6)
(1) KRONOClean 7000
(2) starting TiO2
(3) TiO2-rGO(0.1)
(4) TiO2-rGO(0.5)
(5) TiO2-rGO(1.0)
(6) TiO2-rGO(2.0)
250
400
550
700
Wavelength[nm]
Fig. 4. Raman spectra of: (1) starting TiO2, (2) TiO2-rGO(0.5), (3) reduced graphene
oxide.
Fig. 2. UV–vis/DR spectra of: (1) reference KRONOClean 7000, (2) unmodified TiO2
and (3–6) TiO2-decorated graphene hybrid materials.
as though there is a 5% of rutile phase. It is clear that the rutile phase
is present in starting TiO2 sample, what can be observed in XRD
pattern of starting material (a weak diffraction line at 27.5◦). The
presence of rutile in the starting sample is directly involved in the
production process. It can be also noted from Table 1 that no phase
transition occurs after graphene modification what is rather logical
because no thermal treatment was used to obtain new nanocom-
posites. The addition of appropriate amount of reduced graphene
oxide to TiO2 samples leads to the decrease of specific surface area
from 312 m2/g to c.a. 220–230 m2/g for rGO-TiO2 photocatalysts.
Fig. 4 shows the Raman spectra of reduced graphene oxide,
unmodified TiO2, and TiO2–rGO(0.5). The four specific vibration
modes are located at 146 cm−1 (Eg), 395 cm−1 (B1g), 514 cm−1 (A1g
)
and 639 cm−1 (Eg) indicating the presence of the anatase phase
in all of these samples. Furthermore, the band related to reduced
graphene oxide could be observed in TiO2-rGO(0.5) sample and it
is similar to rGO. In the spectrum of both reduced graphene oxide
and modified sample two typical modes are observed: a D peak
at 1321 cm−1 and a G peak at 1613 cm−1. The G peak is common
to all sp2 carbon forms and provides information on the in-plane
vibration of sp2 bonded carbon atoms and the D peak suggests
the presence of sp3 defects [33]. In TiO2-rGO(0.5) nanocomposite
the position of G peak and D peak remains unaltered. In addition,
the increase in the D/G ratio in each Raman spectrum of rGO and
TiO2-rGO was observed. Similar observations have been reported
in the literature [34,35]. The large ratio of D/G was explained by
the presence of smaller but more numerous sp2 domains in the
carbon which shows the presence of defects in rGO and TiO2-rGO.
Moreover, the D/G intensity ratio in spectrum of TiO2-rGO is more in
comparison with rGO what could be caused by the presence of more
defects in the TiO2-rGO composite, probably due to the immobi-
lization of TiO2 particles [36]. The 2D band at around 2477 cm−1
originates from two phonon double resonance.
The presence of graphene flakes in the obtained nanocomposites
are also proved by means of SEM and TEM microscopy (presented
in Fig. 5). As it can be seen from Fig. 5a and c the TiO2 particles
form agglomerates with the average size of 200–250 nm. The crys-
talline form of starting TiO2 seems to contain poor anatase phase.
The Fig. 5b presents the reduced graphene oxide thin flake. It is
possible to observe that the rGO material obtained according to
modified Hummersꢀ method forms the sheet of few layers (up to
5 layers maximally). The rGO flake is rather thin with the surface
free from the contaminants. This was also checked using thermo-
gravimetric analysis (the lack of mass had a value of 98%). Both SEM
(Fig. 5c) and TEM (Fig. 5d) analysis confirm that the TiO2 nanopar-
ticles covers the graphene flakes to form TiO2-decorated graphene
hybrid materials.
Fig. 3. XRD patterns of: (1) KRONOClean 7000, (2) starting TiO2, (3) TiO2-rGO(0.1),
(4) TiO2-rGO(0.5), (5) TiO2-rGO(1.0), (6) TiO2-rGO(2.0).
to observe [29]. It was found that the material contains 0.96 wt.%
of carbon what was reported in our previous paper [30].
Fig. 2 presents the UV–vis/DR spectra of the commercial TiO2
KRONOClean 7000, starting TiO2 and rGO-modified TiO2 samples.
All obtained samples showed a typical high absorption in the UV
region. This can be related to the intrinsic band gap absorption of
TiO2 resulting from the electron transitions from the valence band
to the conduction band O 2p → Ti 3d [31]. It can be also seen that
the additive of rGO increases light absorption intensity in the visi-
ble region. Moreover, the light absorption intensity increases with
increase of rGO mass ratio added to the photocatalysts sample. This
phenomenon corresponds to the surface plasmon resonance effect
of the graphene unit [32]. It is worth mentioning that for commer-
cial titania KRONOClean 7000 the absorption of visible light was
also noted. It is strongly related with the slightly beige colour of
studied photocatalyst due to the presence of 0.96 wt.% of carbon.
Fig. 3 shows the XRD patterns of starting TiO2, TiO2 photo-
catalysts modified with reduced graphene oxide and reference
KRONOClean 7000. The physicochemical properties, carbon con-
tent and energy band gap values of studied samples were listed
in Table 1. It can be seen that the starting TiO2 and all modified
samples consist of anatase phase with a small amount of rutile (up
to 5%). KRONOClean 7000 photocatalyst contains 100% of anatase
crystalline phase of TiO2 with an average crystal size of about
11 nm. This data as well as the measured SBET value (242 m2/g) cor-
responds to those given by a producer. Titanium dioxide used as a
starting material contains 95% of anatase phase and, it might seem
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Table 1
Phase composition, average crystallites size and specific surface area of studied TiO2-based photocatalysts.
Sample code
Anatase in crystallite phase [%]a
Anatase crystallite size [nm]a
Rutile crystallite size [nm]a
SBET [m2/g]
KRONOClean7000
starting TiO2
TiO2-rGO(0.1)
TiO2-rGO(0.5)
TiO2-rGO(1.0)
TiO2-rGO(2.0)
100
11 0.4
10 0.4
12 0.4
12 0.4
11 0.4
11 0.4
–
242
312
227
232
232
226
95
97
97
96
97
1
1
1
1
1
22 5.2
20 5.2
22 5.2
19 5.2
23 5.2
a
The standard uncertainties with 0.95 level of confidence (k≈2) are given.
Fig. 5. SEM images of: (a) starting TiO2, (b) graphene oxide, (c) TiO2-rGO(0.5). TEM image of: (d) TiO2-rGO(0.5) nanocomposite.
In Fig. 6 the CO2 photocatalytic evolution during acetic acid
decomposition under visible light irradiation using xenon lamp
(>450 nm) is presented. The activity under visible light of all new
obtained materials is higher in comparison with starting TiO2. As
it was mentioned above, no characteristic changes in the crys-
talline structure, phase composition as well as light absorption
abilities were observed after mechanical mixing of titanium diox-
ide with different amounts of reduced graphene oxide and in this
case these properties do not seem to influence the photocatalytic
activity under visible light irradiation. This is due to the fact that in
order to increase the visible light activity of TiO2 the rGO mod-
ification is essential. It can be observed that with the increase
of reduced graphene oxide amount the photocatalytic activity
increases. The best result is observed for sample containing 0.5 wt.%
of rGO. Considering that, it can be concluded that the optimal rGO
concentration for TiO2-rGO samples is 0.5 wt.%. This increase in
photoactivity for some samples is probably due to the presence
of Ti3+ and Ti4+ and oxidation of free ␲-bonds during preparation
[37].
0.03
0.02
0.01
0.0
0
starting
TiO2
TiO -rGO(0.5)
TiO -rGO(2.0)
KRONOClean
7000
TiO2-rGO(0.1)
TiO2-rGO(1.0)
2
2
Fig. 6. CO2 photocatalytic evolution during acetic acid decomposition under mer-
cury lamp visible light (>400 nm) irradiation in the presence of commercial
KRONOClean 7000, starting TiO2 and TiO2 modified with different mass ratio of
reduced graphene oxide.
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A.W. Morawski et al. / Catalysis Today xxx (2016) xxx–xxx
graphene oxide works as electron acceptor and photosensitizer to
efficiently increase the acetic acid photooxidation.
(1) starting TiO2
(2) TiO2-rGO(0.5)
0.4
0.4
Acknowledgement
(2)
(1)
This work was supported by grant DEC-2012/06/A/ST5/00226
from the National Science Centre, Poland.
0.2
0.2
References
[1] D.C. Wei, Y.Q. Liu, Adv. Mater. 22 (2010) 3225.
[2] A. Dideykin, A.E. Aleksenskiy, D. Kirilenko, P. Brunkov, V. Goncharov, M.
Baidokova, D. Sakseev, Diamond Relat. Mater. 20 (2011) 105.
[3] J. Shen, M. Shi, B. Yan, H. Ma, N. Li, M. Ye, Nano Res. 4 (2011) 795.
[4] Z. Qianqian, B. Tang, H. Guoxin, J. Hazard. Mater. 198 (2011) 78.
[5] V. Singh, D. Joung, L. Zhai, S. Das, S.I. Khondaker, S. Seal, Prog. Mater. Sci. 56
(2011) 1178.
[6] J. Liang, W. Wei, D. Zhong, Q. Yang, L. Li, L. Guo, ACS Appl. Mater. Interfaces 4
(2012) 454.
[7] Z. Peining, A. Sreekumaran Nair, P. Shengije, Y. Shengyuan, S. Ramakrishna,
ACS Appl. Mater. Interfaces 4 (2012) 581.
0
0
450
460
470
480
Wavelength[nm]
Fig. 7. Action spectra of acetic acid decomposition obtained in the presence of: (1)
starting TiO2 and (2) TiO2-rGO(0.5).
[8] C. Xu, A. Cui, Y. Xu, X. Fu, Carbon 62 (2013) 465.
[9] D. Zhang, X. Pu, G. Ding, X. Shao, Y. Gao, J. Liu, M. Gao, Y. Li, J. Alloy. Compd.
572 (2013) 199.
Additionally, in this case this sample shows higher activity than
commercial carbon TiO2 KRONOClean 7000. With the increase of
rGO content in the new obtained photocatalysts more rGO particles
are able to get anchored onto the TiO2 sheet, which has the ability
to effectively transfer charge carrier mobility due to the moving of
TiO2 generated electrons across the graphene 2D-sheets [10,38]. In
TiO2-rGO photocatalysts the excited electrons of TiO2 can rapidly
transfer from the conduction band of TiO2 to rGO, which suppresses
the electron- hole recombination and enhances the oxidative reac-
tivity. This allows more charge carriers (e− and h+) to form highly
reactive species and promote the degradation [39]. The larger con-
tent of reduced graphene oxide caused a decrease in photoactivity.
This may be caused by the blocking of the active places by too much
content of reduced graphene.
Additionally, in order to better elucidate the enhanced photoac-
tivity of rGO materials and verify their activation in the visible
region the photocatalytic oxidation of the transparent acetic acid
substrate was systematically investigated as a function of irradia-
tion wavelength. Fig. 7 shows the action spectra in the 450–480 nm
range with a 15 nm wavelength step, obtained with the rGO pho-
tocatalysts compared with starting TiO2 in the photooxidation of
the transparent acetic acid substrate. The action spectra analysis
confirms that rGO modification of TiO2 ensured a better apparent
quantum efficiency in all range. In visible light the rGO samples
reveals little visible activity but it is higher than the starting mate-
rial. The action spectra measurement confirms higher photoactivity
of TiO2- rGO sample.
[10] W.-K. Jo, Vacuum 99 (2014) 22.
[11] Y. Wang, R. Shi, J. Lin, Y. Zhu, Appl. Catal. B: Environ. 100 (2010) 179.
[12] I.V. Lightcap, T.H. Kosel, P.V. Kamat, Nano Lett. 10 (2010) 577.
[13] Q. Huang, S. Tian, D. Zeng, X. Wang, W. Song, Y. Li, W. Xiao, C. Xie, ACS Catal. 3
(2013) 1477.
[14] Y. Ni, W. Wang, W. Huang, C. Lu, Z. Xu, J. Colloid Interface Sci. 428 (2014) 162.
[15] X. Meng, D. Geng, J. Riu, R. Li, X. Sun, Nanotechnology 22 (2011) 1.
[16] G. Jiang, Z. Lin, C. Chen, L. Zhu, Q. Chang, N. Wang, W. Wei, H. Tang, Carbon 49
(2011) 2693.
[17] M. Wojtoniszak, B. Zielin´ ska, X. Chen, R.J. Kalen´ czuk, E. Borowiak-Palen´ , J.
Mater. Sci. 47 (2012) 3185.
[18] T. Liu, R. Zhang, C. Hu, F. Chen, S. Duo, Q. Hu, Phys. Chem. Chem. Phys. 15
(2013) 12963.
[19] W.S. Hummers, R.E.J. Offeman, Am. Chem. Soc. 80 (1958) 1339.
[20] X. Rong, F. Qiu, C. Zhang, L. Fu, Y. Wang, D. Yang, Ceram. Int. 10 (2014) 072.
[21] M. Winter, D. Hamal, X. Yang, H. Kwen, D. Jones, S. Rajagopalan, K.J. Klabunde,
Chem. Mater. 21 (2009) 2367.
[22] D. Dolat, B. Othani, S. Mozia, D. Moszyn´ ski, N. Guskos, Z. Lendzion-Bielun´ ,
A.W. Morawski, Chem. Eng. J. 239 (2014) 149.
[23] A.J. Maira, J.M. Coronado, V. Augugliaro, K.L. Yeung, J.C. Conesa, J. Soria, J.
Catal. 202 (2001) 413.
[24] K. Hadjiivanov, Appl. Surf. Sci. 135 (1998) 331.
[25] D. Dolat, N. Quici, E. Kusiak-Nejman, A.W. Morawski, G. Li Puma, Appl. Catal. B
115–116 (2012) 81.
[26] E. Kusiak-Nejman, M. Janus, B. Grzmil, A.W. Morawski, J. Photochem.
Photobiol. A:Chem. 226 (2011) 68.
[27] H. Zhang, X. Lv, Y. Li, Y. Wang, J. Li, ACS Nano 4 (2010) 380.
[28] C. Nethravathi, M. Rajamathi, Carbon 46 (2008) 1994.
[29] P. Za˛bek, J. Eberl, H.E. Kisch, Photochem. Photobiol. Sci. 8 (2009) 264.
[30] E. Kusiak- Nejman, A. Wanag, Ł. Kowalczyk, B. Tryba, J. Kapica-Kozar, A.W.
Morawski, J. Adv. Oxid. Technol. 18 (2015) 204.
[31] L.L. Tan, W.J. Ong, S.P. Chai, A.B. Mohamed, Nanoscale Res. Lett. 8 (2013) 465.
[32] K. Li, J. Xiong, T. Chen, L. Yan, Y. Dai, D. Yong, Y. Lv, Z. Zeng, J. Hazard. Mater.
250 (2013) 19.
[33] C. Chen, W. Cai, M. Long, B. Zhou, Y. Wu, D. Wu, Y. Feng, ACS Nano 4 (2010)
6425.
4. Conclusion
[34] T.N. Lambert, C.A. Chavez, B. Hernandez-Sanchez, P. Lu, N.S. Bell, A. Ambrosini,
T. Friedman, T.J. Boyle, D.R. Wheeler, D.L. Huber, J. Phys. Chem. C 113 (2009)
19812.
[35] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y.
Wu, S.T. Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558.
Mixing of reduced graphene oxide with TiO2 photocatalyst in
the presence of aliphatic alcohol contributes to the enhancement
of photocatalytic activity mainly by efficient charge carriers separa-
tion. It is also attributed to the presence of oxygen sites of rGO that
are able to accept photoinduced electrons. The decreased activity
at higher rGO contents is related to the increase of absorbance and
scattering of photons through excess carbon in the photosystem,
which means that rGO content should be optimized (0.5 wt.% of rGO
in our case) to maximize the photocatalysis performance. Reduced
[36] Y.F. Zhao, Y. Xie, Z.Y. Sun, H.Y. Zhang, R.T. Tao, C.L. Huang, Z.M. Liu, Sci. China
Chem. 55 (2012) 1294.
ˇ
[37] M. Stengl, S. Bakkarolijeva, T.M. Grygar, J. Bludská, M. Kormunda, Cental J. 7
(2013) 1.
[38] I.V. Lightcap, T.H. Kosel, P.V. Kamat, Nano Lett. 10 (2010) 577.
[39] Y. Liu, L. Chen, J. Hu, J. Li, R. Richards, J. Phys. Chem. C 114 (2010) 1641.
Please cite this article in press as: A.W. Morawski, et al., Photocatalytic degradation of acetic acid in the presence of visible light-active
TiO2-reduced graphene oxide photocatalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.05.055