Growth of TiO2 nano-wall on activated carbon fibers for enhancing the photocatalytic oxidation of benzene in aqueous phase

Article abstract of DOI:10.1016/j.cattod.2016.11.019

In the present study, TiO₂ nano-wall networks (TNWs) were successfully grown on the surface of activated carbon fibers (ACFs) pre-coated with TiO₂ nanoparticles (TNPs) via a hydrothermal process. The TNWs with an average length of 0.7–0.9 μm grew on the ACFs (ACF-TNW) surface in a three-dimensional direction to a complex structure with good uniformity and high crystallinity, as confirmed by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. The maximum removal capacities of benzene in a combined process of photo-oxidation and sorption affinity were 111.6 and 144.8 mg/g for ACF-TiO₂ and ACF-TNW, respectively, at pH 5, under UV light irradiation within 80 min. The highest benzene removal efficiency of 98.7% in the ACF-TNW system (in 100 mL of 50 mg/L), also confirmed by the CO₂ emission results, was accompanied by total organic carbon (TOC) and chemical oxygen demand (COD) reductions of 79.5 and 82.3%, respectively. The ACF-TNW nanocomposite also showed a higher photocatalytic oxidation of benzene than that of TNPs due to the minimization of electron-hole recombination resulting from the transfer of photo-induced electrons through the ACFs surface. The ACF-TNW nanocomposite was also easily separated from aqueous solution for regeneration and showed good stability after multiple uses.

Full text of DOI:10.1016/j.cattod.2016.11.019

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Contents lists available at ScienceDirect
Catalysis Today
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Growth of TiO2 nano-wall on activated carbon fibers for enhancing
the photocatalytic oxidation of benzene in aqueous phase
Ajit Sharma, Byeong-Kyu Lee∗
Department of Civil and Environmental Engineering, University of Ulsan, Nam-gu, Daehak ro 93, Ulsan 680-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 June 2016
Received in revised form 7 October 2016
Accepted 6 November 2016
Available online xxx
In the present study, TiO2 nano-wall networks (TNWs) were successfully grown on the surface of activated
carbon fibers (ACFs) pre-coated with TiO2 nanoparticles (TNPs) via a hydrothermal process. The TNWs
with an average length of 0.7–0.9 ␮m grew on the ACFs (ACF-TNW) surface in a three-dimensional direc-
tion to a complex structure with good uniformity and high crystallinity, as confirmed by scanning electron
microscopy (SEM) and X-ray diffraction (XRD) analysis. The maximum removal capacities of benzene in
a combined process of photo-oxidation and sorption affinity were 111.6 and 144.8 mg/g for ACF-TiO2 and
ACF-TNW, respectively, at pH 5, under UV light irradiation within 80 min. The highest benzene removal
efficiency of 98.7% in the ACF-TNW system (in 100 mL of 50 mg/L), also confirmed by the CO2 emission
results, was accompanied by total organic carbon (TOC) and chemical oxygen demand (COD) reductions
of 79.5 and 82.3%, respectively. The ACF-TNW nanocomposite also showed a higher photocatalytic oxida-
tion of benzene than that of TNPs due to the minimization of electron-hole recombination resulting from
the transfer of photo-induced electrons through the ACFs surface. The ACF-TNW nanocomposite was also
easily separated from aqueous solution for regeneration and showed good stability after multiple uses.
© 2016 Elsevier B.V. All rights reserved.
Kewords:
ACFs
TiO2
Nano-wall
UV light
Photo-oxidation
Benzene
1. Introduction
become a very attractive and promising alternative because of its
high retention capacity, easy handling, and high benzene removal
Benzene is a natural constitute of crude oil and an important
component of gasoline because of its high octane number. The
major sources of benzene in water are atmospheric deposition,
spills of petrol and other petroleum products, and chemical plant
effluents [1]. Levels of up to 179 ␮g/L have been reported in chem-
ical plant effluents [2]. In seawater, levels were reported to be
in the range 5–20 ng/L (coastal area) [3]. The U.S. National Pri-
mary Drinking Water Regulations has set a maximum contaminant
level for benzene in drinking water at 0.005 mg/L (5 ppb) [4]. The
maximum contaminant level goal, a non-enforceable health goal
that would allow an adequate margin of safety for the preven-
tion of adverse effects, is zero benzene concentration in drinking
water. Unfortunately, many conventional treatment processes,
such as membrane separation, and physiochemical methods, such
as catalytic oxidation-reduction process and biological methods,
are unable to minimize or neutralize the environmental harm of
aquatic benzene [1,5–7]. Recently, benzene removal by a combi-
nation of sorption and catalytic oxidation-reduction processes has
efficiency [8,9]. The separation of benzene molecules over water is
often treated by a sorption method using carbon-based composites
[10,11]. However, the relatively high initial and regeneration costs
of such carbon-based composites have encouraged researchers to
study the applicability of other more economic and environmen-
tally friendly removal techniques.
Titanium dioxide (TiO2) exhibits high photocatalytic activ-
ity under UV light irradiation with a bandgap of 3.2 eV, but its
low quantum efficiency creates negligible utilization under vis-
ible phase [12,13]. The photocatalytic degradation of aqueous
phase volatile organic compounds by titanium dioxide (TiO2) is
a relatively slow process with many limitations for industrial or
large-scale applications. These limitations include safety issues, use
of powder materials and high-energy consumption due to UV irra-
diation requirement [14–16]. Catalytic or sorbent-based powder
materials cause channeling and flow problems in real industrial
applications. Powder photocatalysts cannot be recycled easily and
may cause secondary pollution [17]. Thus, it is desirable to prepare
TiO2 photocatalysts deposited on the support materials with high
surface areas. Activated carbon fibers (ACFs) are a newly developed
type of support material with a large surface area and its prepara-
tion has attracted research attention [18,19]. Shi et al. [20] reported
∗
Corresponding author.
E-mail addresses: bklee@ulsan.ac.kr, bklee.ulsan.kr@gmail.com (B.-K. Lee).
http://dx.doi.org/10.1016/j.cattod.2016.11.019
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Sharma, B.-K. Lee, Growth of TiO2 nano-wall on activated carbon fibers for enhancing the photo-
catalytic oxidation of benzene in aqueous phase, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.019

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that ACFs-supported TiO2 exhibited high photocatalytic activity.
Ao et al. [21] reported that Ag nanoparticles loaded TiO2 array can
be fabricated on ACFs and that the resulting composite exhibited
excellent photocatalytic activity. However, no study has focused on
the preparation and photocatalytic activity of metallic nanoparticle
(NP)-coated ACFs on which TiO2 nano-wall networks (TNWs) are
grown.
at 80 ◦C for 12 h. In order to enhance the TiO2 crystallization, TiO2-
doped ACFs was calcined at 600 ◦C in nitrogen atmosphere for 2 h
to afford the ACF-TiO2 sample.
The TNPs were synthesized using the above procedure except
that no ACFs pieces were added [12].
2.3. Growth of TiO2 nano-wall networks (TNWs) on ACF
The first aim of the present study is to prepare TiO2-doped ACFs
as a photocatalytic filter for the removal of benzene from aqueous
environments. ACFs have been recognized for its multifunctional
properties based on its hierarchical porous structure, high thermal
and tensile stability [18]. ACFs are attractive due to its versatile
roles as an adsorbent and catalyst, as well as a support material for
metallic catalysts in environmental pollution remediation [22–24].
In the current study, the ACFs surface was modified by TiO2 doping
to improve the sorption affinity and photocatalytic property in the
sorption-oxidation of aqueous benzene. To date, most of the TiO2
nanostructures prepared on ACFs have been limited to amorphous
TiO2 NPs (TNPs) obtained by sol-gel process [25]. These amorphous
TNPs are not crystalline, but a good crystalline structure needs to
be formed for secure treatment at high temperature. Moreover, the
quality of the resulting composite material is not good enough for
photocatalysis/sorption because of its uneven surface, salt forma-
tion and blockage of porous structure [20,24,25].
Thus, the growth of TNWs on the ACFs surface can enhance
the photocatalytic activity under UV light irradiation. Moreover,
we hypothesized that the growth would increase the sorption
affinity of the photocatalyst, thereby significantly enhancing the
benzene removal rate by the combination of sorption and photo-
degradation. In addition, the ACFs surface modified with TNWs
(ACF-TNW) may increase the photocatalytic activity by improving
the electron transfer efficiency. In this study, TNWs were grown
on the ACF surface and the phase structure, vibrational mode,
doped metal oxide crystallinity, optical behavior and surface mor-
phology of the resulting nanocomposite were investigated. The
sorption affinity and photocatalysis of ACF-TNW were also ana-
lyzed by evaluating the sorption-oxidation of benzene under UV
light irradiation.
Sample pieces of ACF-TiO2 were put into 80 mL of 10 M NaOH
aqueous solution, then transferred into a 100 mL Teflon-lined stain-
less steel autoclave, and sealed. The autoclave was put into an oven,
heated at 140 ◦C for 20 h, cooled naturally in air and washed with
deionized water several times until the sample pH reached 7. The
morphological structure of TNWs on the ACFs surface (ACF-TNW)
was determined by surface morphology analysis.
2.4. Instruments
The surface morphology for monitoring of the TNWs growth was
analyzed by Hitachi S-4700 scanning electron microscopy (SEM).
Semi-quantitative analyses for elemental composition of the ACFs
surface were performed on an energy-dispersive X-ray (EDX) spec-
trometer connected to a Hitachi S-4700. Differences in the light
absorption spectra of TiO2, ACF-TiO2 and ACF-TNW were identified
by comparing UV–vis absorption spectra at 300–800 nm wave-
lengths using a UV–vis spectrophotometer (UV-1700 Shimadzu).
The crystallinity of the TiO2 and ACF-TNW surfaces was charac-
terized by X-ray diffraction (XRD, Bruker AXN) in the 2ꢀ range of
5–80◦. The chemical oxidation states of ACF-TNW were character-
ized by X-ray photoelectron spectroscopy (XPS, Thermo Scientific
K-Alpha XPS spectrometer).
Stock solutions of benzene were prepared in the concentration
range of 2 to 50 mg/L with double distilled H2O. The concentra-
tions of benzene were determined by UV–vis spectrophotometry
(Genesys 10S UV–vis spectrophotometer, Thermo Scientific) at the
wavelength (␭max) range of 190–212 nm [1].
Analysis of chemical oxygen demand (COD) was used to express
the COD of benzene solution before and after the photo-oxidation.
The total organic carbon (TOC) of the benzene solution was deter-
mined by TOC 500 A (Shimazu, Japan). The mineralization efficiency
of complete benzene removal was measured by a carbon dioxide
(CO2) analyzer (Alpha Omega Instruments 9510 analyzer).
2. Materials and methods
2.1. Chemicals used
2.5. Oxidation-sorption of benzene
Titanium (IV) isopropoxide (TIP) as a source of TNWs and ben-
zene as a pollutant were purchased from Daejung Chemical and
Metals Co. Ltd. All regents were prepared with double distilled H2O
using analytical grade chemicals. ACFs substrate was purchased
from Osaka Gas Chemical Co., Ltd. (Japan). Before being used as the
substrate for TNWs growth, the ACFs substrate was pretreated as
follows. Firstly, the ACFs substrate was cut into small square pieces
uniformly sized 2 cm × 2 cm, and then soaked in hydrochloric acid
(1%) for 1.5 h to remove all impurities [18]. After washing several
times with distilled water, the ACFs pieces were dried in a hot-
air oven at 60 ◦C. The ACFs pieces were then placed in a nitrogen
furnace at 120 ◦C for 2 h, and used in subsequent experiments.
The complete removal of benzene using TiO2, ACF, ACF-TiO2
and ACF-TNW as photocatalytic sorbent materials was carried out
under optimized pH conditions in a temperature-controlled water
bath shaken at 110 rpm and 30 ◦C for 2 h under UV light irradia-
tion and dark phase. Photocatalysis was executed in a thermostatic
water bath equipped with UV lamps (Superstar Dulux EL, 20 W). The
distance between the light source and the photocatalyst was 20 cm.
The power density in the reaction chamber was 0.05 W/cm2. For the
pH optimization test, the pH levels were adjusted in the range of 2 to
10 using 0.1 N HCl or NaOH. The ACF-TNW pieces sized 2 cm × 2 cm
were added into 100 mL of the benzene solution. The reaction solu-
tion was exposed in the presence of UV light at room temperature
before starting the reaction. After the reaction, the flask was shaken
for the desired time in UV light irradiation and dark phase. The
reaction solution was filtered through a Whatman 0.45 ␮m filter
membrane. The aqueous benzene concentration was evaluated by
measuring the intensity of absorption peak at 190–212 nm using a
UV–vis spectrophotometer [1]. The time equilibration was deter-
mined using 100 mL of benzene solution at concentrations of 20
and 50 mg/L, containing 2 cm × 2 cm sized ACF-TNW pieces under
UV light irradiation, and the reaction times with shaking varied
2.2. Preparation of TiO2 nanoparticle-deposited ACF
Titanium (IV) isopropoxide (TIP) (30 mL) was dissolved in
100 mL of ethanol with stirring for 20 min to obtain a homoge-
neous solution. The ACFs pieces sized 2 cm × 2 cm were used as the
substrate for the deposition of TNPs. Then, the ACFs substrate was
dipped in the homogeneous solution. After sonicating for 20 min,
the ACFs substrate was taken out and washed with ethanol to
remove any excess TIP precursor between the fibers and then dried
Please cite this article in press as: A. Sharma, B.-K. Lee, Growth of TiO2 nano-wall on activated carbon fibers for enhancing the photo-
catalytic oxidation of benzene in aqueous phase, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.019

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from 0.1 to 3 h. The effects of various experimental design parame-
ters, such as the effect of H2O2, on the benzene oxidation were also
examined under UV light irradiation. The used ACF-TNW compos-
ite was treated with H2O2, washed by deionized water with ethanol
and dried at 80 ◦C for regeneration experiment under identical con-
ditions. The molecules of benzene removed per gram of TiO2, ACF,
ACF-TiO2 and ACF-TNW were calculated by the difference between
the initial and the final benzene concentration using Eq. (1) [12].
of benzene (mg/L), Ce the equilibrium concentration of benzene
(mg/L), V the volume of the benzene sample solution used (L),
and W (g) the weight of TiO2, ACF, ACF-TiO2 and ACF-TNW used.
A blank solution (no addition of sorbent-catalyst) was used in all
experiments to evaluate the amount of evaporation loss of benzene
during the study. A cap-closed reaction flask was used to control the
evaporation loss of benzene during the oxidation-sorption process.
qe = (Co − Ce) × V/W
(1)
3. Results and discussion
The benzene removal efficiency after the photo-oxidation and
the mineralization efficiency of benzene were calculated using the
following Eqs. (2) and (3), respectively [1,9]:
3.1. Materials characteristics
3.1.1. Morphology and elemental compositions
Removalefficiency(%) = [Co − Ce/Co] × 100
(2)
The morphology and elemental compositions of ACF, ACF-TiO2
and ACF-TNW were analyzed by SEM/EDX analysis (Fig. 1). As
shown in Fig. 1(a), the ACFs surface was almost clean after pre-
treatment with many micropores that can function as active sites
for adsorption. Fig. 1(b) shows SEM images of the ACF-TiO2 compos-
Mineralization(%) = [CO2produced/benzeneconversion] × 100(3)
where qe is the amount of the eliminated benzene molecules (mg/g)
on TiO2, ACF, ACF-TiO2 and ACF-TNW, Co the initial concentration
Fig. 1. SEM and EDX analysis of ACF (a, d), ACF-TiO2 (b, e) and ACF-TNW (c, f), respectively.
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Fig. 2. UV–vis spectra of all samples.
Fig. 3. XRD patterns of all samples tested.
ite prepared by the sol gel process. TNPs were uniformly deposited
on the ACFs surface. As shown in Fig. 1(c), the growth of TNWs
on the ACFs surface is in three-dimensional direction to a com-
plex structure was produced. The TNW structures with an average
length of 70–90 nm were confirmed in the SEM image (Fig. 1c).
The highly uniform TNWs structures of TiO2 were deposited on the
ACFs surface with high crystallinity. The TNWs structures of TiO2
on the ACFs surface exhibited great adsorption capacity and photo-
catalytic activity for complete oxidation of benzene from aqueous
media. In the EDX spectra of ACF, ACF-TiO2 and ACF-TNW (Fig. 1d–f,
respectively), the elemental peaks of Ti, O, and C were attributed
to the TiO2 doping on the carbon-based ACF surface. In addition, Ti
peaks were not observed on the surface of the parent ACF. How-
ever, other elements such as Pt and Cl originated from SEM-EDX
sample characterization processes.
According to the SEM morphology of ACF-TNW (Fig. 1c), the
growth of TiO2 nano-wall network on to ACF was obtained by
the “dissolve and grow” method [27]. In the reaction process, the
NaOH played as a reactant to dissolve Ti and also act to control
the hydrolysis rate of Ti(III) by providing an alkaline environment.
In the presence of NaOH, Ti reacts with H+ at high temperature
and pressure and gradually dissolves, continually releasing the
Ti(III) precursors into the reaction solution. Thus, TiOH2+ is pro-
duced by hydrolysis of Ti(III) because Ti(III) is unstable in aqueous
solution and TiOH2+ is oxidized to Ti(IV) by reaction with dis-
solved oxygen. The Ti(IV) complex ions are thus used as the growth
units with chainlike cluster structures, and thus, a large number
of the polycrystalline cluster structures are formed in the solution
by homogeneous nucleation. Subsequently, these cluster struc-
tures are deposited on the ACF surface and act as the nucleating
centers for further growth of TiO2 nano-wall network. This may
be attributed to the good bonding force and low surface tension
between the ACF surface and TiO2 nano-wall network.
completely absorb UV and visible irradiation [20]. Therefore, ACF,
ACF-TiO2 and ACF-TNW exhibited strong absorption throughout
the UV–Vis wavelength range employed due to the carbon-based
black color of ACFs. In the spectra of the ACF-TiO2 and ACF-TNW
materials, absorbance in the UV region was higher than in the vis-
ible region due to the TiO2 metal oxide deposition on the ACFs
surface. However, the absorption of ACF-TNW was greater than
that of ACF-TiO2. The slight increase in the absorbance of ACF-TNW
following TNWs growth may have been due to obstruction of the
light by the nano-wall structures [21]. In addition, ACF-TNW had
increased absorption at 400–500 nm, indicating enhancement of
the photocatalytic properties in the UV phase compared to TiO2.
The light absorption was increased by the interaction of the car-
bon basal surface of ACFs with TiO2. This indicates that ACFs,
having a conjugated structure, enhanced the charge separation in
the electron-transfer processes [18,26]. Furthermore, this can be
attributed to charge transition transfer and excited photon energy
reduction, which combined to give UV–vis lower absorption than
that of TiO2. These UV–vis spectra results suggest that ACF-TNW has
photocatalytic activity under UV light irradiation. Thus, the photo-
catalytic activity of TiO2 and adsorption by ACFs facilitate complete
removal of benzene from aqueous media.
3.1.3. Crystallography
XRD was used to analyze the crystallinity and phase compo-
sition of the TNP, ACF, ACF-TiO2 and ACF-TNW materials (Fig. 3).
Major XRD peaks of the synthesized TNPs were observed at 2ꢀ of
53.8◦, representing the rutile phase, while the peaks at 2␪ values
of 25.3o, 37.7o, 38.5, 47.9o, 55.1o, 62.8o 68.6o 70.3o and 75.1o indi-
cated the anatase phase of TNPs [12,28]. In the XRD patterns of ACF,
however, the major peaks at 2ꢀ of 23.4◦ and 44.2◦ are attributed to
the crystalline plane of amorphous carbon [18]. Furthermore, all
the XRD patterns of the ACF-TiO2 and ACF-TNW composite mate-
rials showed the typical peaks of a TiO2 crystalline structure and
crystalline plane of amorphous carbon, indicating the presence of
TiO2 doping in the carbon texture of ACFs. This crystalline charac-
teristic indicates that the texture of ACFs was not damaged by TiO2
fabrication. However, all peaks in the XRD patterns of ACF-TNW
have higher intensity with better crystalline structure than those
3.1.2. Optical absorption
Fig. 2 shows the optical absorption in the range of 200–800 nm
of the synthesized TNPs, ACF, ACF-TiO2 and ACF-TNW materials. As
shown in Fig. 2, the spectra of TiO2 indicated absorption in the UV
region at 360–405 nm, which can be attributed to photo-excitation
from a valence band to a conduction band of TiO2 [28]. However,
UV–vis absorption spectra of ACF show that carbon-based ACF can
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Fig. 4. XPS core-level spectra of all samples tested.
of ACF-TiO2. Therefore, ACF-TNW provides better photocatalytic
properties that can generate more electron-hole pairs for the oxi-
dation of benzene during irradiation with UV light and the porous
carbon surface in ACF-TNW provides more sorption ability.
(in TiO2) to 464.5 eV. In addition, the Ti2p peaks of TiO2 in ACF-
TNW were less intense than those in TNPs, implying that the ionic
state of TiO2 was not simply Ti4+ in both composites. Subsequently,
the multi-peak shapes of the Ti 2p peaks of titanium in ACF-TNW
revealed that they were composed of a combination of Ti3+ and Ti4+
peaks. The formation of Ti3+ in ACF-TNW was due to the reduc-
tion of Ti4+, which could be attributed to the crystalline plane of
amorphous carbon of the ACF surface [20,32]. In the C1s core level,
the leading peaks were observed at 284.4 and 284.5 eV for ACF
and ACF-TNW, respectively. The peak for ACF-TNW showed less
intensity than that of ACF. Additionally, the XPS spectrum of C
1s core level was resolved into four individual peaks represent-
ing carbon associations: C C (BE = 284.3 eV), C O (BE = 284.8 eV),
3.1.4. Chemical oxidation states
The chemical oxidation states of TNPs, ACF and ACF-TNW were
elucidated based on the core level XPS spectra. The full range of
XPS spectra (0–1300 eV) of TiO2, ACF and ACF-TNW are shown in
Fig. 4. The TiO2 and ACF-TNW hierarchical hetero-structures are
composed of C, O and Ti. No significant impurities were observed
in the XPS spectra of ACF-TNW. However, the oxygen peak in the
XPS spectra of ACF was less intense than that of TiO2 and no Ti ele-
ments were observed. In the Ti2p core, the binding energies (BE)
at 464.9 eV and 459.3 eV were accredited to the 2p1/2 and 2p3/2
peaks of TiO2, respectively, indicating that the ionic state of TiO2
was Ti+4 [12,28]. The XPS spectra of TiO2 also presented the nar-
row and sharp shape of the Ti 2p peaks, indicating that all the
titanium in TNPs existed in the Ti+4 state [9]. The binding energy
of the Ti2p1/2 peaks in ACF-TNW slightly diverged from 464.9 eV
C
O H (BE = 285.8 eV) and C C (BE = 288.3 eV) [33]. In the O1s core
level, the leading peaks were observed at 530.6 and 532.1 eV for
ACF-TNW and ACF, respectively. The peak intensity for ACF-TNW
was broader and stronger compared to that for ACF. In addi-
tion, the asymmetric spectrum of oxygen in ACF-TNW evidently
indicated the presence of multicomponent oxygen species on the
surface. Three chemical states of oxygen corresponding to Ti
O Ti
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Fig. 5. UV–vis absorption spectra of benzene solutions according to duration of con-
tact with ACF-TNW.Concentration of benzene = 50 mg/L, amount of ACF-TNW = 2 cm
x 2 cm, reaction volume = 100 mL, reaction time = 80 min, temperature = 30 ◦C..
Fig. 6. Effect of solution pH on the photo-oxidation and sorption of benzene under
dark phase and UV light irradiation.
Concentration of benzene = 50 mg/L, amount of samples = 0.01 g of TiO2 powder and
size of ACF, ACF-TiO2 and ACF-TNW = 2 cm × 2 cm (equal to 0.01 g of TiO2 powder),
reaction volume = 100 mL, H2O2 = 0.1%, reaction time = 80 min, temperature = 30 ◦C.
(BE) = 530.3 eV), O C (BE = 531.5 eV) and O H (BE = 532.8 eV) were
detected [34]. The hydroxyl groups of water molecules absorbed
on the composite surface could function as electron donors for
photo-generation of H+ and be oxidized into hydroxyl radicals
(•OH), which play the key role in the photo-oxidation of benzene
molecules [9,20,32].
ACF-TNW (pHzpc = 8.2). In the strongly acidic solutions (pH ≤ 4),
the whole surface of the sorbent-catalyst becomes mostly positive
charged and thus produces an electrostatic repulsion towards the
acidic moiety of benzene [36]. The ACF-TNW has a greater anionic
charge at pH 5 or higher pH. Thus, the improved sorption of benzene
at pH 5 is because the acidic form of benzene and ACF-TNW with
anionic charge has a strong electrostatic attraction that improves
the sorption ability. However, at pH 6, benzene has a relatively
lower acidic form and relatively less electrostatic attraction than
those at pH 5, resulting in relatively decreased sorption ability.
At alkaline pH, the whole surface of the sorbent-catalyst becomes
negatively charged and an electrostatic repulsion toward the nega-
tively charged benzene dominates [35]. Additionally, at alkaline pH,
the removal capacity is decreased due to the competitive sorption
onto the sorbent between adsorbate molecules and hydroxide ions.
In the presence of UV light irradiation, the ACF-TNW surface was
3.2. Oxidation-sorption of benzene
3.2.1. Temporal photo-oxidation
In order to explore the reaction mechanism, a kinetic study was
carried out for ACF-TNW. Fig. 5 shows the UV–vis absorbance val-
ues of benzene during photocatalytic oxidation. Without adding
any photocatalyst samples, the benzene degradation was negligible
even under UV light irradiation. This indicates that photoly-
sis is not effective for benzene degradation [1]. The reaction
mixture was stirred under dark condition for 80 min to attain
adsorption–desorption equilibrium between benzene and ACF-
TNW. When the reaction solution was irritated under UV light for
different times (10, 20, 40, 80 min), the benzene peak significantly
decreased or almost disappeared with increasing UV light irradia-
tion. This observation indicates that the time must be optimized to
generate the electrons and holes (e−/h+) that are required to initiate
the complete photo-oxidation of benzene.
hydrated with −Ti
O Ti and Ti O C [28]. It is also possible that
OH−/H2O reacted with localized holes through electron transfer
process to generate numerous hydroxyl radicals that can enhance
the benzene oxidation under UV light irradiation [37].
3.2.3. H2O2 effect
3.2.2. pH effect
Fig. 7 shows the photocatalytic activity of ACF-TNW in the
photo-oxidation of benzene under UV light irradiation in the pres-
ence and absence of H2O2. H2O2 can provide more •OH radicals,
thereby enhancing the photo-oxidation with formation of col-
ored intermediates during photocatalysis. The test run of benzene
photo-oxidation was conducted over a range of added H2O2 (30%)
from 0.01 and 0.2% in 100 mL of benzene solution (50 mg/L), under
UV light irradiation. The photo-oxidation efficiency was 47.9%
without H2O2 addition. As the concentration of H2O2 was increased
from 0.01 to 0.1%, the photo-oxidation of benzene increased from
86.5 to 95.3%. Further increase in the H2O2 concentration up to 0.2%
resulted in a decrease to 72.1% degradation. H2O2 can trap photo-
induced e− to stabilize the paired e−/h+, which is attributed to the
combination of photo-Fenton and photocatalytic reactions occur-
ring on the ACF-TNW surface to generate more active radicals in
aqueous medium. The •OH radicals could be generated via reac-
Fig. 6 depicts the influence of pH on the oxidation-sorption
removal of benzene by TNPs, ACF, ACF-TiO2 and ACF-TNW under
UV light irradiation and dark phase. For all the sorbent sam-
ples, the benzene elimination efficiency was very low in strongly
acidic media (pH ≤ 2) and relatively low in basic media (pH > 7)
as compared to that in weakly acidic conditions around pH 5.
The benzene elimination was maximized at pH 5, with maximum
removal capacities of 56.6, 87.8, and 106.2 mg/g for ACF, ACF-TiO2
and ACF-TNW, respectively, under dark condition. Under UV light
irradiation, the maximum amount of removed benzene was 9.6,
111.6, and 144.8 mg/g for TiO2, ACF-TiO2 and ACF-TNW, respec-
tively, at pH 5. These values were greatly improved as compared
those in dark phase. The surface charge of the sorbent-catalyst is
mainly responsible for benzene removal [35]. The zero point of
charge (pHzpc) of benzene was 4.45, which is lower than that of
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Fig. 9. Evolution of CO2 emission with variable UV light irradiation time.
Concentration of benzene = 50 mg/L, amount of ACF-TNW = 2 cm × 2 cm, reaction
volume = 100 mL, H2O2 = 0.1%, reaction time = 80 min, temperature = 30 ◦C.
Fig. 7. Effect of H2O2 addition to ACF-TNW on the photo-oxidation of benzene under
UV light irradiation.
firmed by CO2 emissions during the photo-oxidation of benzene
(Fig. 9). The emission concentration of CO2 gradually increased
with increasing irradiation time up to 50 min, but decreased slowly
after that. This decrease in CO2 concentration emitted indicates a
decrease in the benzene mineralization rate after 50 min because
of the decrease in benzene molecules available for the photo-
oxidation, resulting in low conversion of benzene to CO2 [1,39].
Concentration of benzene = 50 mg/L, amount of ACF-TNW = 2 cm × 2 cm, reaction
volume = 100 mL, reaction time = 80 min, temperature = 30 ◦C.
3.2.5. Reusability and stability
The ACF-TNW nanocomposite was reused in ‘n’ number of cycles
to evaluate its physical stability and economical use for the photo-
oxidation of benzene. ACF-TiO2 has more active sites where the
photo-generated charge carriers react with absorbed molecules to
form hydroxyl and superoxide radicals. However, powder-based
TiO2 material reduces the number of active sites during regen-
eration experiment handling, which decreases the photocatalytic
efficiency. These drawbacks can be overcome by forming nano-
wall structures on the ACFs surface via a facile process. After being
used for benzene photo-oxidation, ACF-TNW in fiber form can eas-
ily be separated from the reaction solution and re-activated for
further process. The re-activation or regeneration test was con-
ducted by treating ACF-TNW (2 cm × 2 cm/10 mL) with 30% H2O2
solution for 30 min. The removal efficiency of benzene was 36%
in the 4th cycle and removal efficiency of benzene was decreased
with increasing number of regeneration cycles up to the fourth
cycle (Fig. 10). The H2O2 treatment can remove the surface-trapped
compounds by oxidizing them to H2O and CO2, thereby provid-
ing free active sites and a negatively charged surface [24,28]. This
result indicated that ACF-TNW was stable and exhibited a high
photocatalytic performance over four utilization and regeneration
cycles. A comparison study of benzene removal efficiency by ACF-
TNW using combined process of oxidation and sorption with other
sorbent/photo-catalysts was added in Table 1.
Fig. 8. Variations in TOC, COD and benzene removal efficiency according to duration
of UV light irradiation.
Concentration of benzene = 50 mg/L, amount of ACF-TNW = 2 cm × 2 cm, reaction
volume = 100 mL, H2O2 = 0.1%, reaction time = 80 min, temperature = 30 ◦C.
tion between H2O2 and e− or O2 [1,38]. Therefore, the addition
•−
of H2O2 to ACF-TNW enhanced the benzene oxidation by minimiz-
ing the recombination of electrons and holes with the generation
of more •OH radicals. As the H2O2 concentration further increased,
the additional H2O2 trapped the •OH radicals generated to form
•
weaker oxidant HO2 radicals. Thus, the addition of excess H2O2
decreases the amount of •OH radicals and acts as an inhibitor [28].
3.2.6. Photo-oxidation-sorption affinity mechanism
Scheme 1 shows the photo-oxidation and sorption affinity
mechanism for improved benzene removal due to the porous car-
bon texture of ACFs and the generation of more electron-hole pairs
in the ACF-TNW photocatalytic system under UV light irradiation
and dark phase. The porous ACF substrate can improve the sorption
affinity of benzene depending upon two main interactions: electro-
static and dispersive [20,22]. The dispersive attraction through ␲-␲
interactions between the ␲ orbital of the ACFs surface on the car-
3.2.4. Degree of mineralization
Fig. 8 shows the degree of benzene mineralization by photo-
oxidation using ACF-TNW as a function of irradiation of UV light
with variations of TOC and COD. After only 80 min irradiation under
UV light, benzene photo-oxidation efficiency by ACF-TNW of 98.8%
was achieved with a maximum reduction in COD and TOC of 82.3
and 79.5%, respectively. The benzene mineralization was also con-
Please cite this article in press as: A. Sharma, B.-K. Lee, Growth of TiO2 nano-wall on activated carbon fibers for enhancing the photo-
catalytic oxidation of benzene in aqueous phase, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.019

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A. Sharma, B.-K. Lee / Catalysis Today xxx (2016) xxx–xxx
Table 1
Comparison of benzene removal efficiency by sorption/oxidation.
Catalyst/sorbent
Benzene (mg/L)
Removal efficiency (mg/g)
Removal efficiency (%)
Ref.
Granular activated carbon (GAC)
Carbon-nanoparticle polysulfone
ACF
100
100
10
98.3
67
66
–
–
–
[29]
[35]
[30]
TiO2/zeolites
Cu–TiO2/PU
ACF-TiO2
ACF-TNW
25
100
50
–
–
∼100
[31]
[9]
This study
This study
91
111.6
144.8
88.4
98.7
50
Scheme 1. Proposed schematic mechanism for the photo-oxidation of benzene by ACF-TNW.
oxygen on Ti+4 leads to the formation of a (Ti+4 — O2•−) complex,
as shown in Eq. (4);
bon basal planes and the electronic density in the benzene aromatic
rings can improve the sorption affinity of benzene [35]. Further-
more, the electrostatic interaction between benzene molecules and
the protonated structure of the ACF-TNW surface may also provide
additional benzene sorption on ACFs. Yakout et Al. [40] suggested
that benzene can also be adsorbed on the weakly acidic or nonacidic
oxygen groups by the interaction of benzene ring ␲ electrons with
the positive charge of those groups. In the photo-oxidation of ben-
zene by ACF-TNW under UV light irradiation, the ACF-TNW surface
acts as a photocatalyst for benzene decomposition or mineral-
ization. The positive holes of TiO2 nano-wall break apart water
molecules to form hydrogen and hydroxyl radicals, while the neg-
ative electrons react with oxygen molecules to form superoxide
radicals. These hydroxyl radicals and superoxide radicals are effi-
−
Ti+4 + O2 → (Ti+4 − O2
)
(4)
•
The second possibility is the cleavage of Ti+4 — O2− peroxo-
complexes in the presence of oxygen, as shown in Eq. (5):
−
+
Ti+4 + O22− ) + O2 →2(Ti+4 − O2
)
(5)
•
In these reactions, Ti+4 complexes can also be present in ACFs
through high O C and Ti
O
Ti bonding. Then the formed superox-
ide complex (Ti+4 — O2
)
can react with a benzene radical cation
•−
+
from the benzene sorption to form a phenyl peroxy radical (see
equation 6);
•
−
cient oxidants of photo-oxidation. The superoxide anion O2
,
−
•
)
C6H6 + + (Ti+4 − O2
+ → C6H5OO•
(6)
(7)
•
which can be formed via two different mechanisms on ACF-TNW,
is a possible reactive species [39]. In the first case, the adsorption of
2C6H5OO• → (C6H5OO)2 → 2C6H5O• + O2
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(2013R1A1A2065796) and (Grants No. C0398793) was supported
by Business for Cooperative R&D between Industry, Academy,
and Research Institute funded Korea Small and Medium Business
Administration in 2016.
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This work was supported by
a grant from the basic
science research program through the National Research Foun-
dation of Korea (NRF) funded by the Ministry of Education
Please cite this article in press as: A. Sharma, B.-K. Lee, Growth of TiO2 nano-wall on activated carbon fibers for enhancing the photo-
catalytic oxidation of benzene in aqueous phase, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.019