1. Probing the role of thermally reduced graphene oxide in enhancing
performance of TiO2 in photocatalytic phenol removal from aqueous
environments
Haruna Adamu a
, Prashant Dubey b
, James A. Anderson a,c,⇑
a
Surface Chemistry and Catalysis Group, Department of Chemistry, University of Aberdeen, AB24 3UE, UK
b
Centre of Material Sciences, Institute of Interdisciplinary Studies, University of Allahabad, Allahabad 211002, India
c
Materials and Chemical Engineering Group, School of Engineering, University of Aberdeen, AB24 3UE, UK
h i g h l i g h t s
One-pot syntheses of titania graphene
oxide (GO) and titania thermally
reduced graphene oxide (TGO)
composites.
Enhanced photocatalytic behaviour of
composites with respect to titania.
Improved adsorption and reduced
hole/electron recombination rates for
composites.
Reaction under nitrogen shows TGO
to acts in electron transport and
storage.
g r a p h i c a l a b s t r a c t
a r t i c l e i n f o
Article history:
Received 18 June 2015
Received in revised form 6 August 2015
Accepted 11 August 2015
Available online 9 September 2015
Keywords:
Phenol
Photodegradation
Graphene oxide
Thermally reduced graphene oxide
Adsorption
a b s t r a c t
A simple one-pot syntheses of TiO2-graphene oxide (GO)/thermally reduced graphene oxide (TGO) com-
posites were performed with different concentrations of GO/TGO (0.25, 0.5 and 1.0 wt%). The materials
were characterised by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spec-
troscopy, Raman spectroscopy, and nitrogen adsorption–desorption isotherms in order to evaluate struc-
tural and physiochemical properties of synthesised materials. The TiO2-0.25% TGO exhibited the highest
photocatalytic activity for phenol degradation in aqueous solution and this was attributed to optimal
adsorption efficiency of phenol along with prolonged lifetime of electron–hole pairs. Photocatalytic activ-
ity in the absence of dissolved oxygen (under nitrogen) was also performed and, in the case of TGO, con-
firmed the role of graphene as an electron sink and transporter for suppression of electron–hole pair
recombination.
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
Following the work of Novoselov and Geim on graphene [1], this
material has received great attention in the arena of nanotechnol-
ogy and photocatalysis due to its outstanding properties, especially
those which exploit its high electron mobility and chemical stabil-
ity [2,3], and its high adsorption capacity for organic pollutant
[4,5]. Therefore, in recent years, graphene has been embraced in
the design of photocatalysts as a tool for enhancing their photocat-
alytic performance. For example, many graphene-based semicon-
ductor photocatalysts have been developed, evaluated and
reported, the most common being graphene-coupled with CdS,
TiO2 and ZnO [6–8]. Results from these studies indicate that gra-
phene is an excellent co-catalyst in enhancing photocatalytic
http://dx.doi.org/10.1016/j.cej.2015.08.147
1385-8947/Ó 2015 Elsevier B.V. All rights reserved.
⇑ Corresponding author at: Surface Chemistry and Catalysis Group, Department
of Chemistry, University of Aberdeen, AB24 3UE, UK.
E-mail address: j.anderson@abdn.ac.uk (J.A. Anderson).
Chemical Engineering Journal 284 (2016) 380–388
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2. oxidation and reduction activity. For instance, many reports have
shown that integration of graphene into the matrix of semiconduc-
tor can improve photocatalytic performance in the degradation of
organic pollutants [6–8]. Based on its excellent electron mobility,
it is generally believed that graphene can quickly separate and
transfer photogenerated electrons and thus impede recombination
[7,9]. Its high adsorption capacity is also deemed to provide advan-
tages [4,5]. Phenols and their derivatives are important aqueous
pollutants which result from their use as a key industrial feedstock
for manufacturing household cleaning commodities (such as disin-
fectants, antiseptics and detergents), agricultural inputs (e.g., her-
bicides) and numerous pharmaceutics into the waste streams,
particularly surface-water bodies [10]. Consequently, phenol and
its derivatives are often chosen as model pollutants in the evalua-
tion of photocatalytic activity. Similarly, in this study, phenol was
chosen to be the model aqueous organic pollutant for the evalua-
tion of photocatalytic activity of the prepared composites.
Of all the most widely used graphene-based semiconductor
photocatalysts, innovative composite materials of TiO2-graphene
have emerged and are considered to be a viable and effective
method for complete mineralisation of organic pollutants in water
[11,12]. Many studies have reported high photocatalytic perfor-
mance of TiO2-graphene composites and attributed this to signifi-
cant improvement of interfacial charge transfer, which is a vital
key issue for photocatalytic activity [7,13]. It is suggested that
the participation of graphene results in the reduction of photogen-
erated electron–hole pairs recombination rate via its effective
interfacial charge transfer, which has been confirmed by a number
of experimental approaches including use of photovoltaic
response, electrochemical impedance spectra, photoluminescence,
laser pulse excitation and gaseous phase photocurrent [7,12–14].
However, evidence of effective interfacial charge transfer by gra-
phene through photocatalytic oxidative reaction in the absence
of molecular oxygen as an electron scavenger is limited. In this
study, the ability of graphene to accept and shuttle photogenerated
electrons in the absence of electron scavenger is demonstrated and
is implicated in reducing the rate of recombination of photogener-
ated charge carriers that improved photodegradation of phenol.
A simple one-pot synthesis of TiO2-graphene oxide (GO)/ther-
mally reduced graphene oxide (TGO) composite materials with
varying concentrations of GO/TGO is reported along with its photo-
catalytic performance for the removal of phenol from aqueous
solutions. The ratio of GO/TGO within TiO2 matrix has been opti-
mised for photocatalytic degradation of phenol. Photocatalytic
oxidative reactions in the presence and absence (under nitrogen)
of molecular oxygen were conducted in order to explore the rela-
tionship between photocatalytic activity and the integration of
GO/TGO within TiO2 matrix.
2. Experimental
2.1. Materials
All chemicals were used as received without further purifica-
tion. Phenol (C6H5OH) was used as model pollutant. Distilled and
ultra-pure deionised water (MilliQ P 18.2 Mῼ-cm) were used
throughout the study.
2.2. Synthesis of GO and TGO
GO was prepared from natural graphite (crystalline, 325 mesh,
Alfa Aesar) by a modification to Hummer’s method [15]. The as-
synthesised GO was used for the synthesis of TGO. In a typical syn-
thesis of TGO, 100 mg of dried GO was measured into in a 250 ml
beaker. It was heated in a preheated muffle furnace at around
250 °C for 10 min. During the thermal reduction, the brown-
tinted GO turned into fine-fluffy black powder and this obtained
powder is denoted TGO.
2.3. Synthesis of TiO2-GO/TGO composite materials
In a typical synthetic method for TiO2-GO/TGO composite mate-
rials, 7.4 ml of titanium tetraisopropoxide was added drop-wise to
30 ml of aqueous 1 M HNO3. The resulting white precipitate was
agitated for 2 h with continuous stirring to obtain a clear solution.
The desired amount of GO/TGO (0.25/0.5/1.0 wt%) was sonicated in
20 ml of DI water for 30 min and added to the clear TiO2 solution.
The resulting mixture was further sonicated for 30 min before add-
ing 50 ml of DI water and agitated for another 1 h. The mixture pH
was adjusted to 3 by adding 1 M NaOH aqueous solution resulted
in a turbid GO/TGO-TiO2 hybrid material in the form of a colloid
which was then agitated for a further 1 h at room temperature.
The colloidal composite material was collected by centrifugation
followed by washing with DI water until the pH of the supernatant
became neutral and the resulting solid was then dried overnight at
80 °C. The as-synthesised composite materials were subjected to
heat treatment at 300 °C under nitrogen for 1 h to encourage
crystal growth of TiO2 nanoparticles. Pure TiO2 nanoparticles was
also synthesised under the same procedure but without addition
of GO/TGO.
2.4. Characterisation
Field emission scanning electron microscopic (FESEM) images
were obtained by using a SUPRA 40VP FESEM (Carl Zeiss NTS GmbH,
Oberkochen) microscope under high-vacuum mode operated at
10 kV. Samples were prepared by drop casting of ethanol suspen-
sion of GO and TGO on to the conducting carbon substrate and
evaporated to dryness. In addition to scanning electron microscopy
(SEM), the morphology of the sample powder of the composites
were observed by high resolution transmission electron micro-
scopy (HRTEM, JEOL JEM-2000 EX).
Fourier transform infrared (FTIR) spectra were measured using
a Spectrum 2 PerkinElmer FTIR spectrometer using KBr discs.
Raman spectra were acquired on a PerkinElmer Raman spectrom-
eter (Raman Micro-200, 514.5 nm). To identify the crystalline phase
composition of the samples, a powder X-ray diffraction (XRD) pat-
terns were obtained on a Phillips X’Pert Pro X-ray diffractometer
(PANalytical) with Cu Ka radiation (k = 0.15418 nm) and the oper-
ational voltage and current were maintained at 40 kV and 40 mA,
respectively. A scan rate of 5°/min was used to record the diffrac-
tion patterns in the 2h range of 5–80°. The optical properties of the
materials were characterised in diffuse reflectance mode a using
UV–VIS spectrophotometer (Cary 60 UV–VIS), in which BaSO4
was used as the internal reference standard. To establish the band
gap of the materials, [F(a)hm]1/2
versus Ephoton for indirect transition
was employed.
A Micromeritics Tristar-3000 was used for nitrogen adsorption–
desorption isotherms on samples at À196 °C and in the pressure
range (P/P0
) of 0.05–1.0. All samples were degassed at 200 °C for
4 h prior to analysis. The specific surface area (SBET) of these mate-
rials was determined using the multiple point Brunauer–Emmett–
Teller (BET) procedure, whereas the pore volume and size distribu-
tion profiles were obtained from Barrett–Joyner–Halender (BJH)
method.
2.5. Adsorption tests
The adsorption tests were conducted to extract equilibrium iso-
therms. Adsorption experiments were carried out by shaking
0.25 g of TiO2-0.25 wt% GO/TGO composites and pure TiO2 in
H. Adamu et al. / Chemical Engineering Journal 284 (2016) 380–388 381

3. 100 ml Pyrex bottles filled with 25 ml of phenol solution with con-
centrations of 5, 10, 25, 50, and100 mg LÀ1
at a constant tempera-
ture of 25 ± 0.1 °C in a thermostat water bath-cum-shaker for 24 h.
The suspensions were then centrifuged, filtered with 0.45 lm syr-
inge filter (Millipore) and then analysed to obtain the equilibrium
concentration of phenol using a UV–visible spectrophotometer
(Lambda 25, PerkinElmer).
To determine the maximum adsorption uptake, the amount of
phenol adsorbed per unit mass of the materials used (mg gÀ1
)
was calculated from the following expression:
qe ¼ ðCi À CeÞ V=m ð1Þ
where qe is the amount adsorbed at equilibrium (mg gÀ1
), Ci is the
initial concentration (mg LÀ1
), Ce is the equilibrium concentration
(mg LÀ1
), V is the volume of the aqueous phase (L), and m is the
mass of photocatalyst used.
2.6. Photocatalytic tests
Photocatalytic reactions were carried out in a stirred, batch
reactor fitted with a primary cooler (Fischer Scientific 3016S) to
maintain a constant reaction temperature of 25 ± 0.1 °C. A sec-
ondary cooling system was also required to maintain constant
temperature due to the heat given out by the UV lamp (Heraeus
400W 135V). Water was used as the coolant in this secondary cool-
ing system by running a constant flow through a Pyrex cooler
which encased the UV lamp. The temperature of the reactions
was monitored by digital thermometer inserted in the reaction pot.
To a total volume of 1.7 L phenol solution with concentration of
50 mg LÀ1
in the reactor, 0.25 g of tested photocatalyst material
was suspended. The suspension was magnetically stirred for 1 h
in the dark in order to attain adsorption–desorption equilibrium
between the phenol and solid. Immediately thereafter, sample
was withdrawn and considered as the initial concentration. The
lamp was then turned on and allowed to stabilise for 5 min before
the first sample under illumination was taken. Subsequently, sam-
pling of the solution was then carried out at 20 min intervals over a
3 h period. Each 3 ml sample was filtered with a 0.45 lm syringe
filter and the residual concentration of phenol in the filtered solu-
tion was analysed by measuring the maximum absorbance of phe-
nol at 210 nm using UV–visible spectrophotometer (Lambda 25,
PerkinElmer). The photocatalytic degradation of phenol in the
absence of molecular oxygen as electron scavenger was also con-
ducted following the same procedure as above, but by replacing
the bubbled gas with nitrogen to remove dissolved oxygen.
3. Results and discussion
3.1. Characterisation of materials
Following an initial screening of samples of varying wt% of GO
and TGO in the TiO2 matrix, 0.25 wt% in both cases was found to
be the most active in the photodegradation of phenol and there-
fore, characterisation details are limited to those for this particular
loading.
The FESEM micrographs of synthesised GO, TGO and their com-
posites with 0.25% concentrations in TiO2 are presented in Fig. 1.
From the micrographs, the layered structure for GO and TGO was
observed, but TGO showed better layer separation than GO. The
thickness of the graphene layers was significantly reduced in the
case of TGO. Due to the typical wrinkled structure of TGO, it has
the tendency to fold at the edge of the graphene sheets [7,15].
The FESEM images of both composites (Fig. 1c and 1d) clearly show
the formation of TiO2 nanoparticles and the surface of GO and TGO
are mostly covered by TiO2, confirming intimate contact between
the two components. Result from the HRTEM (Figs. S1 and S2)
shows significant differences between the two composites. The
titania aggregates were smaller in the case of TiO2-0.25% GO and
the graphene appeared to be relatively uniformly dispersed. On
the other hand, TiO2-0.25% TGO showed larger aggregates of titania
and the graphene appeared to be less well distributed over the tita-
nia phase.
From results of the UV–Vis diffuse reflectance measurements,
band gaps of 3.29, 3.08 and 3.03 eV were calculated for TiO2,
TiO2-0.25% GO and TiO2-0.25% TGO, respectively. The reduced band
gaps for the composite materials with respect to pure titania sup-
port the idea of significant interaction between components and
also should enhance light absorption properties of the sample with
respect to the lower frequency component of radiation applied.
Fig. 2a shows FTIR spectra of GO and TGO samples. Several char-
acteristic peaks such as those at 1732 (C@O carboxyl or carbonyl
stretching vibration), 1404 (O–H deformations in the C–OH
Fig. 1. FESEM images of (a) GO, (b) TGO, (c) TiO2-0.25% GO and (d) TiO2-0.25% TGO.
382 H. Adamu et al. / Chemical Engineering Journal 284 (2016) 380–388

4. groups), 1224 (C–OH stretching vibration) and 1063 cmÀ1
(C–O
stretching vibrations in C–O–C of epoxy/ether) were observed for
GO [16]. The broad peak centred at around 3412 cmÀ1
is assigned
to adsorbed water or –OH groups linked to the graphene sheets.
The peak at around 1623 cmÀ1
may be attributed to the deforma-
tion mode of adsorbed water and non-oxidised graphitic domains.
After thermal treatment of GO, the FTIR spectrum of TGO reveals
almost removal of the C@O band at around 1732 cmÀ1
, which con-
firms the successful removal of –COOH groups linked to GO [17].
The presence of an intense peak at around 1076 cmÀ1
is assigned
to epoxy groups still linked with TGO. It would appear that thermal
treatment of GO samples at 250 °C was effective in the reduction of
GO, as indicated by significant removal of –COOH groups and also
by the exfoliation of GO.
Synthesised graphene materials were further characterised by
Raman spectroscopy (Fig. 2b). Two typical peaks were observed,
namely the D-band (disordered sp3
carbon atoms) and the
G-band (graphitic sp2
carbon atoms) for GO and TGO, which are
peaks attributed to graphitic materials [18]. In the case of GO,
the D- and G-band was found at $1332 and $1590 cmÀ1
, respec-
tively, while TGO showed corresponding features at $1356 and
$1594 cmÀ1
, respectively. The relative intensity of the disordered
D-band and crystalline G-band (ID/IG) that reflects order of defects
was lower for TGO ($0.7), than for GO ($1.1). This may be attrib-
uted to removal of defect level in TGO during removal of –COOH
groups via thermal treatment as confirmed by FTIR [19]. In other
words, TGO showed a decrease in ID/IG ratio compared to GO, sug-
gesting an increase of the average size of the in-plane sp2
domains
of carbon atoms in TGO, resulting from removal of oxygen
containing groups. Therefore, it can be inferred that TiO2-0.25 wt
% TGO contained more sp2
domains of carbon atoms than
TiO2-0.25 wt% GO.
The XRD patterns of synthesised GO, TGO, TiO2, and the com-
posites TiO2-GO/TGO (0.25 wt% compositions) are presented in
Fig. 3. The XRD pattern of GO shows one major intense peak at
2h = 11.8°, which corresponds to a d-spacing of 0.75 nm. This value
is much larger than the d-spacing of natural graphite (0.335 nm),
and is attributed to effective oxidation of graphite to form GO
[20]. After thermal treatment of GO to form TGO, this characteristic
peak of GO completely disappeared and a new peak at 2h = 26.0° is
present which corresponds to a d-spacing of 0.34 nm, which char-
acterises the thermal reduction of GO to TGO [21]. The diffrac-
togram of pure TiO2 synthesised by sol–gel method, which shows
good crystallinity, and peaks at 2h = 25.3°, 37.9°, 48.0°, 54.4°,
62.8°, 69.4° and 75.1° which are ascribed to reflections from
(101), (004), (200), (211), (204), (220), and (215) planes of ana-
tase phase of TiO2, respectively [22,23]. The diffraction patterns of
the as-prepared composites are similar to that of pure TiO2. The
FWHM of the most intense peak for all of the materials
(2t % 25.4°) allowed calculation of an approximate crystallite size
of 7.3, 9.6 and 13.8 nm for pure TiO2, TiO2-0.25% GO and
TiO2-0.25% TGO, respectively. The larger crystallite dimensions
for the latter are consistent with data form the TEM micrographs
(Figs. S1 and S2). Separate diffraction features due to GO and
TGO were not observed in the synthesised composites, which
may be due to their low content and consequently low diffraction
intensities [7,22].
In order to determine the specific surface area, pore volume and
size distribution of synthesised composites along with pure TiO2,
GO and TGO, nitrogen adsorption–desorption measurements were
carried out (Fig. 4a). The isotherms are of type IV according to the
IUPAC classifications, as they show open, large hysteresis loops
[24] which are indication of mesoporous materials [25]. The iso-
therms show comparatively more open hysteresis loop for both
composites than pure TiO2, which may be attributed to the open
nature of the hysteresis loops for GO and TGO (inset of Fig. 4a)
and resulted in the presence of large pores in the composites
(Table 1). However, integration of GO and TGO into the TiO2
resulted in a decrease in surface area and porosity enhancement
compared to pure TiO2 (Table 1). The decrease in surface area is
significantly greater in the TGO composite compared to GO com-
posite. This may have occurred due to higher penetration of TiO2
particles into exfoliated 2D graphene sheets of TGO than GO (being
thinner than GO as confirmed by FESEM) and thus, more of an
agglomeration effect was found in the case of TiO2-0.25 wt% TGO
composite [26]. The relatively greater loss in surface area for TGO
after incorporation into the titania is consistent with the larger
estimated crystallite size of the titania measured by XRD for this
sample and the large aggregate size for this sample in TEM
(Figs. S1 and S2). Due to the incorporation of small amounts of
Fig. 2. FTIR (a) and Raman spectra (b) of GO and TGO.
Fig. 3. XRD patterns of GO, TGO, TiO2, TiO2-0.25% GO and TiO2-0.25% TGO.
H. Adamu et al. / Chemical Engineering Journal 284 (2016) 380–388 383

5. GO/TGO into TiO2, the synthesised composites show characteristics
which are more akin to the surface properties of TiO2 [27]. The
specific surface area and pore size distributions of all samples are
summarised in Table 1.
The pore size distributions lie within the range of 2–20 nm
(Fig. 4b) reflecting the mesoporous nature of all materials (Table 1).
The pore size of the prepared composites was found to be even
greater than that of pure TiO2, which could be an advantage in
respect to the photocatalytic reaction processes involving the com-
posites where larger molecules are involved [26]. After the combi-
nation of TiO2 with GO and TGO, the pore size distribution
broadened significantly, even though the GO and TGO show a
narrow range of pore size distribution (inset of Fig. 4b) and thus,
confirm the hybrid surface structure of the composites. Conse-
quently, the pore size distribution of both composites comprises
a small amount of micropores and the majority as mesopores
(Fig. 4b). The result of the pore size distribution and pore volume
pattern obtained, particularly the TiO2-0.25 wt% TGO, are similar
to those reported by other researchers [22,28].
3.2. Adsorption isotherms of phenol
Adsorption isotherms were measured to understand the inter-
action of phenol with the surface of pure TiO2 and prepared com-
posites. The rates of photodegradation of pollutants may depend
on the affinity and surface coverage of the reactants on the photo-
catalyst surface and therefore, combining GO and TGO with TiO2
could result in improved interaction and proximity of phenol with
the active centres of TiO2, potentially enhancing degradation rates
[29].
Phenol uptake as a function of equilibrium concentration on
pure TiO2, TiO2-0.25 wt% GO and TiO2-0.25 wt% TGO are presented
in Fig. 5a. As expected, the amount adsorbed increased with
increased phenol solution concentration and then reached a max-
imum consistent with attainment of a monolayer. On a per mass
basis, adsorption capacity increased in the sequence pure TiO2 fol-
lowed by TiO2-0.25 wt% GO and with TiO2-0.25 wt% TGO adsorbing
the highest amount. This trend in adsorption capacity is not a func-
tion of surface area and thus is likely to be related to the surface
functionality (Table 1). The high adsorptivity shown towards phe-
nol by TiO2-0.25 wt% TGO may be attributed to selective adsorp-
tion of the phenol on the composite photocatalyst, resulting from
p–p interaction between the aromatic rings of phenol and the gra-
phene planes. Additionally, as confirmed by FTIR (Fig. 2a) the pres-
ence of remnant epoxides in TGO, could lead to the formation of
hydrogen bonds with the hydroxyl groups of phenol. Conse-
quently, these two types of TiO2-0.25 wt% TGO-phenol interactions
could account for the higher adsorption of phenol shown by the
composite compared to the other materials. This has been
observed by other researchers [5]. The high level of carboxylate
functional groups on TiO2-0.25 wt% GO compared with
TiO2-0.25 wt% TGO may have explain the higher capacity of the lat-
ter if it is assumed that these groups inhibit the otherwise favour-
able p–p interaction between the aromatic rings of phenol and the
graphene planes.
Although the surface areas of TiO2 and TiO2-0.25 wt% GO were
similar (224 and 217 m2
gÀ1
, respectively), TiO2-0.25 wt% GO
adsorbed twice as much phenol as pure TiO2 (Table 1). This may
be attributed to the phenol interactions with the composite via
hydrogen bonds formation between the hydroxyl groups of phenol
and oxygen-containing groups of GO. Furthermore, it was also
reported that GO is capable of interacting with aromatic com-
pounds through p–p conjugation [30] and this may also have con-
tributed to the enhanced adsorption of phenol by TiO2-0.25 wt%
GO compared with pure TiO2. However, based on the wide varia-
tion in the respective adsorption capacity of the two composites,
Table 1
Samples characteristics based on the adsorption of nitrogen (À196 °C) and phenol (25 °C).
Samples SBET (m2
gÀ1
) Pore size (nm) Pore volume (cm3
gÀ1
) Phenol Uptake
(qe) (mg/g)
a
KL (L mgÀ1
)
TiO2 224 3.1 0.29 4.08 Â 10À1
7.64 Â 10À2
GO 28 4.9 0.13
TGO 371 17.6 1.56
TiO2-0.25 wt% GO 217 3.9 0.33 9.67 Â 10À1
1.54 Â 10À1
TiO2-0.25 wt% TGO 180 4.2 0.27 20.9 Â 10À1
2.27 Â 10À1
a
Langmuir constant from phenol adsorption.
Fig. 4. (a) Nitrogen adsorption–desorption isotherms and (b) pore size distributions
of pure TiO2, TiO2-0.25% GO and TiO2-0.25% TGO composites. Inset of (a) represents
the isotherms and inset of (b) shows corresponding plots of the pore size
distributions of GO and TGO.
384 H. Adamu et al. / Chemical Engineering Journal 284 (2016) 380–388

6. one could infer that the extent of phenol adsorption on the
TiO2-0.25 wt% GO was governed by hydrogen bonding. This
assumption is based on the reasoning that, even though remnants
of graphitic sp2
domain exists in the body of GO (Fig. 2b), the abun-
dance of oxygen-containing functional groups in its basal plane
and at the plane edges can restrain delocalisation of p-electrons
or likely to restrict p–p conjugation of the sp2
domain and in effect,
the possible p–p stacking between the GO sheets and phenol aro-
matic rings could be relatively restrained and therefore has little
effect on the adsorption of phenol. It has been reported that the
carbon atoms in GO are of sp3
hybridisation and connected to oxy-
gen [31]. In a similar assertion, GO can only interact via p–p stack-
ing on sp2
networks (if present) that are not oxidised or engaged in
hydrogen bonding between –OH and –CO2H functionalities of the
GO [32]. Overall, the loss of the peak at 2h = 26.4° that charac-
terises the (002) plane of graphite in the diffractogram of GO indi-
cates that graphite was completely oxidised to GO. This further
implies dominance of sp3
carbon atoms in GO after oxidation.
Hence, adsorption of phenol by TiO2-0.25 wt% GO may be mainly
through hydrogen bonding and augmented due to availability of
other surface sites.
Langmuir and Frendlich isotherm models were used in order to
describe the adsorption behaviour of the materials. The adsorption
data of phenol was best fitted to the Langmuir model with a corre-
lation coefficient (r2
0.99) compared to r2
0.85 with Frendlich.
Equilibrium data of adsorption of phenol using the Langmuir equa-
tion, are presented in Fig. 5b for pure TiO2, TiO2-0.25 wt% GO and
TiO2-0.25 wt% TGO. A linearised Langmuir model was employed
(Eq. (2)):
Ce=qe ¼ ð1=KLqmÞ þ ð1=qmÞCe ð2Þ
where qe is the amount of phenol adsorbed at equilibrium (mg/g);
Ce is the equilibrium concentration of phenol (mg/L); KL (L/mg)
and qm (mg/g) are the Langmuir constant and maximum adsorption
capacity. The constant KL was determined from the intercept and
slope of the linear plot fitted to the experimental data of Ce/qe vs Ce.
The experimental adsorption capacity (qe) for each of the mate-
rials and derived Langmuir constants are summarised in Table 1.
The phenol uptake by TiO2-0.25 wt% TGO is 5 and 2 times higher
than that of the TiO2 and TiO2-0.25 wt% GO, respectively. The Lang-
muir adsorption coefficient, KL, for TiO2-0.25 wt% TGO was the
highest, suggesting greater affinity for phenol than the other mate-
rials despite the larger surface areas. These values are consistent
with the proposal that two kinds of adsorbate–adsorbent interac-
tions could be responsible for the higher adsorption of phenol on
TiO2-0.25 wt% TGO. However, the amounts of phenol adsorbed
was significantly improved after the integration of GO and TGO
into the TiO2 matrix regardless of the decrease in surface area
(Table 1), which suggests that the adsorption of phenol by the
composites were not only a function of surface area. This is in con-
sistent with other reports that GO and/or reduced graphene oxide
used as co-adsorbents in composites resulted in enhancement in
the adsorption of aqueous organic pollutants [30].
3.3. Photocatalytic performance evaluation
Photocatalytic tests using phenol as a model under UV light illu-
mination were measured, and the results are summarised in
Fig. 6a and b for TiO2-GO and TiO2-TGO composites, respectively.
The photocatalytic behaviour of pure TiO2 under the same condi-
tions was also compared. Photolysis reaction was also investigated
Fig. 6. Photocatalytic degradation of phenol under UV light irradiation of (a) for various compositions of TiO2-GO and (b) TiO2-TGO composites along with pure TiO2 and
photolysis.
Fig. 5. (a) Adsorption isotherms of phenol on pure TiO2, TiO2-0.25% GO and TiO2-0.25% TGO based on equilibrium concentrations, (b) data fits to the Langmuir expression.
H. Adamu et al. / Chemical Engineering Journal 284 (2016) 380–388 385

7. and the results indicates that the concentration of phenol changed
only slightly (ca. 11%) after 3 h exposure. In contrast, 62% pho-
todegradation of phenol was achieved in the presence of pure
TiO2. Photodegradation of phenol was considerably improved by
addition of GO and TGO and photocatalytic efficiency was max-
imised in both cases using 0.25 wt% loadings (Fig. 6a and b). 81%
and 96% of phenol degradation after 3 h exposure was achieved
with TiO2-0.25 wt% GO and TiO2-0.25 wt% TGO, respectively. How-
ever, photocatalytic performance was progressively worsened for
higher concentrations of both GO and TGO in TiO2-matrix. This
may be due to blockage of the light harvesting centres of TiO2 that
were disadvantageously occupied by GO and TGO with their
agglomerative nature at higher concentrations in their respective
composites and thus, prevented UV light due to shielding or scat-
tering effects to reach the surface of TiO2 where the photocatalytic
reaction takes place [28,33].
Under the conditions employed, TiO2-0.25 wt% TGO was by far
the best photocatalyst, and almost completely removed phenol
(Fig. 6b). No initial induction period was observed for this sample
unlike the case of pure TiO2 and TiO2-0.25 wt% GO (Fig. 7a and b).
This effect which gave the impression of increased solution con-
centration remained significant up to 40 and 20 min after UV illu-
mination was initiated during photodegradation of phenol with
pure TiO2 and TiO2-0.25 wt% GO, respectively. As this phenomenon
was not observed in the case of TiO2-0.25 wt% TGO, it suggests
properties specific to the other two samples were responsible.
The apparent concentration increase was previously observed
and attributed to desorption of phenoxyl radicals from the surface
of photocatalyst and partially stabilised in solution due to free
electron delocalisation within the aromatic ring [34]. In addition,
it was also observed that shortly after the UV light initiation, a pink
colour appeared in the reaction solution, which remained much
longer before fading during the photodegradation process with
pure TiO2 than TiO2-0.25 wt% GO but was very short for
TiO2-0.25 wt% TGO. The pink colour was due to a mixture of
catechol and hydroquinone in aqueous solution [35]. Even though
peaks attributed to aromatic intermediates and ring cleavage as a
result of phenol degradation have been reported [36,37], in the
present case, no obvious maxima which could be assigned to such
phenol degradation products were detected in the UV–visible
absorption spectra (Fig. 7a–c). As such, it can be inferred that the
mixture of the said intermediates produced were not easily
degraded by pure TiO2 and TiO2-0.25 wt% GO, while with TiO2-
0.25 wt% TGO the intermediates were degraded rapidly, again
highlighting advantages of the TiO2-0.25 wt% TGO material.
It has been established that the introduction of GO and TGO into
the TiO2 matrix is responsible for improved photocatalytic activity.
The observed enhancement can be attributed to the increased
adsorptive properties. The phenol was adsorbed on the GO and
TGO surfaces with different orientation until adsorption–desorp-
tion equilibrium was attained. Under UV light, the adsorption equi-
librium was disrupted because of the decomposition of the
adsorbed phenol by the photogenerated carriers, which facilitated
rapid transfer of more phenol from solution to the interface and
successively decomposed via redox reactions. A similar case may
be made for the reaction intermediates. Therefore, enhanced
adsorption and photocatalytic reaction was achieved in a single
process and led to improvement in photodegradation of phenol.
This process, perhaps, accelerated by the transfer of photogener-
ated carriers to the nearby adsorbed phenol, consequently retarded
charge recombination and leaving hole/electron pairs to partici-
pate in redox reactions that led to higher photocatalytic activity
compared to pure TiO2. However, the observed improvement in
the photodegradation of phenol by the composites, particularly
the TiO2-0.25 wt% TGO, may be influenced by other characteristics
beyond adsorption. This is due to the ability of graphene to accept
electrons due to its 2D structure composed of p-conjugation [7,9].
Therefore, in the case of TiO2-0.25 wt% TGO, there could be a trans-
fer of photogenerated electrons from the TiO2 conduction band to
TGO and can be stored in the abundant p–p domain within the
composite, which could result in an effective interface charge sep-
aration and suppress h+
/eÀ
recombination. The process of mitigat-
ing electron–hole recombination could follow the same
mechanism in TiO2-0.25 wt% GO, however, due to greater defect
abundance, it has less possibility of delocalisation of electrons,
and consequently exhibits lower photocatalytic activity.
Photocatalytic performance of a photocatalyst is enhanced by
prolonging the lifetime and trapping of eÀ
/h+
pairs, the use of pho-
toluminescence and photocurrent emission techniques are often
employed for probing the efficiency of charge carrier separation
and to understand the fate of eÀ
/h+
pairs on the surface of photo-
catalyst [9]. In this study, in order to probe the role of GO and
TGO in the composites with respect to electrons transfer and stor-
age efficiency, photocatalytic degradation of phenol was conducted
under nitrogen condition to remove molecular oxygen as a poten-
tial electron scavenger. The change of phenol concentration was
monitored by UV–visible absorption spectra over a 3 h period
(Fig. 8). Under these conditions using pure TiO2 and TiO2-0.25 wt
% GO, no conversion was observed. This demonstrates more evi-
dence of poor delocalization of electron carrier in TiO2-GO compos-
ite. In contrast, a decrease in phenol concentration of more than
50% was found for TiO2-0.25 wt% TGO (Fig. 8). This demonstrates
that the presence of TGO in this composite acted to suppress the
recombination of eÀ
/h+
pairs by mediating photogenerated elec-
trons away from the TiO2 surface, exploiting its 2D p-conjugated
structure and thus, could be responsible for the increased photo-
catalytic oxidation of TiO2. This observation is consistent with
the previous report that the calculated Fermi level of graphene
was found to be À0.08 V vs NHE (normal hydrogen electrode)
[38], which is below the conduction band of TiO2. As a result, gra-
phene can serve as a sink for the photogenerated electrons and can
also be stored in the giant p–p network of graphene sheets in the
Fig. 7. UV–visible absorbance spectra of phenol during photocatalytic degradation as a function of time over (a) pure TiO2, (b) TiO2-0.25% GO, and (c) TiO2-0.25% TGO.
386 H. Adamu et al. / Chemical Engineering Journal 284 (2016) 380–388

8. composite. Therefore, it could be inferred that the observed
decrease in phenol concentration was mainly due to efficient inter-
facial electron transfer by TGO within the composite.
Overall, TiO2-0.25 wt% TGO possess greater affinity for phenol
as well as effective charge separation and transportation proper-
ties, which are the basis for significant improvement in conversion
efficiency over pure TiO2 and TiO2-0.25 wt% GO for the pho-
todegradation of phenol. Hence, the mechanistic pathway for the
enhanced photodegradation of phenol influenced by GO and TGO
in their respective composites is different and the proposed reac-
tion pathway is depicted in Fig. 9. Besides the reaction model
depicted, as the result reflects the role of TGO in interfacial electron
transfer, the following reaction mechanism, as demonstrated in the
photodegradation of methylene blue (MB) [7], can be proposed.
The electron is transferred to TGO following the excitation of
valence electrons of TiO2 by the use of UV light to the conduction
band. These electrons on TGO can react with the dissolved oxygen,
when present, to yield oxygen superoxide species. On the other
hand, the photogenerated electrons on the surface of TiO2 could
also react directly with dissolved oxygen and form oxygen super-
oxide species, which can subsequently react with water to give
hydroxyl radicals. The hydroxyl radicals then degrade adsorbed
phenol. In the same way, the highly oxidising holes of TiO2 can
react with H2O or hydroxyl group to yield surface hydroxyl radi-
cals, which can also degrade adsorbed phenol. Similarly, the holes
can oxidise adsorbed phenol directly. However, the major steps in
the reaction mechanism are illustrated below.
TiO2 À TGO !
hv
TiO2ðh
þ
Þ þ TGOðeÀ
Þ
TGOðeÀ
Þ þ O2 ! TGO þ OÀ
2
OÀ
2 þ H2O ! Å
OH
TiO2ðh
þ
Þ þ H2O ! TiO2 þ Å
OH
PhenolðadsÞ þ Å
OH ! CO2 þ H2O
PhenolðadsÞ þ TiO2ðh
þ
ÞCO2 þ H2O
3.4. Kinetics studies
The Langmuir–Hinshelwood kinetics model has been used to
describe the rates of photocatalytic degradation of organic sub-
stances, where the photocatalytic reaction rate depends on the
concentration of organic substance [34,37]. (Eq. (3)):
r ¼ dC=dt ¼ ðkr  Kads  CÞ=ð1 þ Kads  CÞ ð3Þ
where kr and Kads are the reaction rate constant and adsorption
coefficient, respectively. The photocatalytic reaction showed appar-
ent first order behaviour and the rate equation can be rearranged
into linear form as:
lnC0=C ¼ kr  Kads  t ¼ kapp  t ð4Þ
where C0 is the initial phenol concentration and C represents con-
centration at time t, kapp is the apparent first order rate constant.
As such, the initial photodegradation rate of phenol can be written
in the form-
r0 ¼ kapp  C0 ð5Þ
The plots of lnC0/C against time (t) are depicted in Fig. 10 for the
photodegradation of phenol by pure TiO2, TiO2-0.25 wt% GO and
TiO2-0.25 wt% TGO. The results of the kinetics data for phenol pho-
todegradation generate straight line graphs, which indicate that
the photodegradation of phenol followed pseudo-first order reac-
tion kinetics. This suggest that the reactions were operating in
the regime where KC ( 1 despite the fact that the initial concen-
trations used (50 mg/L) lies above a concentration needed to attain
a monolayer according to the measured isotherms (Fig. 5a). This
may indicate that phenol adsorption was diminished under UV
radiation compared to the uptake measurements in the dark. It
may also imply that the overall kinetics of the phenol photodegra-
dation was not only influenced by the concentration but also by
other processes, as well as the intrinsic and extrinsic parameters.
The photodegradation rate constant, kapp, and the initial pho-
todegradation rate, r0 were calculated from the plots and the val-
ues listed in Table 2. With the exception of TiO2-0.25 wt% TGO,
the kinetics parameters of pure TiO2 and TiO2-0.25 wt% GO were
calculated after the period considered to be the photoinduction
time [34]. The rate constants in decreasing order were
TiO2-0.25 wt% TGO, TiO2-0.25 wt% GO and TiO2. TiO2-0.25 wt%
TGO was the most active in the photodegradation of phenol with
a rate constant 2.5 times higher than pure TiO2. This is attributed
Fig. 9. Proposed mechanisms of phenol degradation in oxygen by (a) TiO2-0.25% GO and (b) TiO2-0.25% TGO composites.
Fig. 8. Photocatalytic degradation of phenol under nitrogen.
H. Adamu et al. / Chemical Engineering Journal 284 (2016) 380–388 387

9. to its distinctive surface properties such as electronic conduction
and high adsorptivity. The enhancement of photocatalytic perfor-
mance was not as significant following addition of GO to TiO2 as
it was by adding TGO.
4. Conclusion
TiO2-GO and TiO2-TGO composites were synthesised by a sim-
ple one-pot integration method. TiO2 particles are present in the
anatase phase in both composites. TiO2-TGO exhibited good
photocatalytic activity towards the removal and mineralisation of
phenol. However, photocatalytic activity declined with increasing
loadings of both GO and TGO in the composites, which may be
attributed to agglomerative or light scattering at high concentra-
tions. The best performance was observed with 0.25 wt% loadings
which of GO and TGO, resulted in 81 and 96% phenol degradation,
respectively, after 3 h under UV illumination and which were both
enhanced performances with respect to pure TiO2. The photo-
oxidative degradations of phenol under the conditions employed
followed pseudo first order kinetics, with TiO2-0.25 wt% TGO
showing a rate constant 2.5 times higher than that of pure TiO2.
The enhanced photocatalytic activity observed in TiO2-0.25 wt%
TGO can be explained by hybridised adsorption-carrier charge sep-
aration cooperation, These findings may be of significance not only
in environmental catalysis, but also in other applications that
requires electrons storage and shuttling in graphene-based
materials.
Acknowledgements
We thank the INSA-RSE bilateral exchange programme for
financial assistance (PD) and the Petroleum Technology Develop-
ment Fund (PTDF, Nigeria) for the award of PhD scholarship, as
well as Abubakar Tafawa Balewa University, Bauchi-Nigeria for
the granted fellowship (H.A.).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cej.2015.08.147.
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Table 2
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388 H. Adamu et al. / Chemical Engineering Journal 284 (2016) 380–388