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Photochemical &Photobiological Sciences

PAPER

Cite this: Photochem. Photobiol. Sci., 2013,

12, 1091

Received 24th November 2012,

Accepted 25th March 2013

DOI: 10.1039/c3pp25398h

www.rsc.org/pps

Photoelectrochemical oxidation of p-nitrophenol on anexpanded graphiteTiO2 electrode

B. Ntsendwana,a S. Sampath,a,b B. B. Mambaa and O. A. Arotiba*a

In the quest for more efficient photoanodes in the photoelectrochemical oxidation processes for organic

pollutant degradation and mineralisation in water treatment, we present the synthesis, characterisation

and photoelectrochemical application of expanded graphiteTiO2 composite (EGTiO2) prepared using

the solgel method with organically modified silicate. The BrunauerEmmettTeller surface area analyser,

ultravioletvisible diffuse reflectance, scanning electron microscopy, energy dispersive spectroscopy, X-ray

diffractometry, Raman spectrometry and X-ray photoelectron spectroscopy were employed for the

characterisation of the composites. The applicability of the EGTiO2 as photoanode material was investi-

gated by the photoelectrochemical degradation of p-nitrophenol as a target pollutant in a 0.1 M Na2SO4

(pH 7) solution at a current density of 5 mA cm2. After optimising the TiO2 loading, initial p-nitrophenol

concentration, pH and current density, a removal efficiency of 62% with an apparent kinetic rate constant

of 10.4 103 min1 was obtained for the photoelectrochemical process as compared to electrochemical

oxidation and photolysis, where removal efficiencies of 6% and 24% were obtained respectively after

90 min. Furthermore, the EGTiO2 electrode was able to withstand high current density due to its high

stability. The EGTiO2 electrode was also used to degrade 0.3 104 M methylene blue and 0.1 104 M

Eosin Yellowish, leading to 94% and 47% removal efficiency within 120 reaction time. This confirms the

suitability of the EGTiO2 electrode to degrade other organic pollutants.

1. Introduction

Advanced oxidation processes (AOPs) have been identified as

promising alternative techniques for water treatment due to

their ability to effectively degrade a wide range of organic pol-

lutants by means of in situ generated powerful oxidants such

as hydroxyl radicals.19 Among the AOPs, electrochemical oxi-

dation of aqueous wastes containing biorefractory organics

such as phenolic compounds has been shown to possess some

advantages such as ease of automation, high efficiency and

environmental compatibility.1012 However, this process is

limited by mass transfer of organic matter in the bulk solution

to the surface of the electrode and is generally accompanied by

side reactions such as oxygen or chlorine evolution due to the

high voltage required to destroy the organic molecules inaqueous solutions. These shortcomings can be improved by

the combination of electrochemical oxidation systems with

other processes.

The application of heterogeneous photocatalysis involving

the use of a semiconducting photocatalyst (e.g. TiO2) in the

degradation of organic pollutants has been extensively studied.

TiO2 is capable of destroying more than 3000 types of refractory

organic compounds under ultraviolet radiation.13 Upon light

irradiation with energies greater than its band gap, a TiO2semiconductor will be excited to generate electronhole pairs.

The electrons and holes will migrate from the conduction and

valence bands to the solid surface to respectively initiate reduc-

tive and oxidative reactions. The holes will oxidize the surface

adsorbed water or hydroxyl ions to form hydroxyl radicals

(OH), while the electron can reduce the dissolved O2 mole-

cules to form various species, such as O2, H2O, H2O2 and

OH.14 Such oxygen-containing species can be photocatalyti-

cally active in the mineralization of organic contaminants andthe inactivation of microorganisms such as bacteria and

viruses. However, photocatalytic oxidation technology suffers

from low photo efficiency owing to the slow rate of electron

transport and high rates of recombination of the photogene-

rated electronhole pairs.11,12 It has been reported that the

application of electrical energy can alleviate the problems

associated with electronhole recombination by driving away

the photoelectrons from the semiconductorelectrolyte inter-

face, leading to more efficient degradation of organic contami-

nants due to the increased concentration of OH radicals.10Electronic supplementary information (ESI) available. See DOI:

10.1039/c3pp25398h

aDepartment of Applied Chemi stry, University of Johannesburg, P.O. Box 17011,

Doornfontein, 2028, South Africa. E-mail: [email protected] of Inorganic and Physical Chemistry, Indian Institute of Science,

Bangalore, 560012, India

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The combination of heterogeneous photocatalysis and elec-

trochemical oxidation techniques generally termed photoelec-

trochemical processes is fast becoming a plausible route to

organic waste degradation in water treatment.1517 A typical

combination of heterogeneous photocatalysis and electroche-

mical oxidation involves the immobilization of TiO2 powder

(a photoactive material) on a conductive material upon which

a bias anodic potential is applied.17 Photoelectrochemical oxi-

dation is more effective than photocatalytic oxidation since thephotoactive and the conducting materials can simultaneously

generate hydroxyl radicals which are responsible for degra-

dation. The main advantages of the immobilization of the

catalyst are the avoidance of losses in the recovery process and

the enhanced photocatalytic efficiency through the use of an

external electric field.16 The anodic applied potential should

be kept low to prevent the photocatalyst from falling off and

thus prolong the service life of the photoelectrode. Conversely,

the use of very low anodic potentials can reduce the efficiency

of the electrochemical oxidation process. Therefore, the photo-

electrochemical oxidation technique for the degradation and

mineralization of organic pollutants should be optimised forenhanced photoefficiency and high stability.

The use of TiO2 photo anodes for degradation of pollutants

has been explored.1822 The preparation of these TiO2 photo-

anodes involves the use of expensive conducting materials such

as Indium doped Tin Oxide (ITO), Fluorine doped Tin Oxide

(FTO) glass and Ti sheet. ITO and FTO glass substrates suffer

from low conductivity. The scarcity and increasing price of

indium and low production volume of high quality FTO limit

the wide application of ITO and FTO glass as suitable sub-

strates.23,24 The highly conductive Ti is also limited due to high

cost.25 Recently, carbon materials such as activated carbon,

carbon nanotubes and graphite have been used as substrates

for immobilization of TiO2 due to their low cost and high con-

ductivity. These carbon nanoTiO2 composites, such as carbon

coating of photoactive anatase-type TiO2, carbonTiO2 photo-

catalyst, TiO2-mounted activated carbon, and other modified

nanoTiO2 particles, have shown good photocatalytic activity

for the decomposition of organic compounds.2630 Palmisano

et al.25 also reported the preparation of a photoanode by sup-

porting TiO2 onto graphite rods for photoelectrochemical oxi-

dation of p-nitrophenol. However, these forms of carbon suffer

some challenges such as difficulty in preparation on a large

scale. In addition, the processing of these composites into elec-

trodes requires substrates such as Ti or ITO glass or the use of

an adhesive. The use of substrates affects the loading of thecatalyst and the performance of these composites tends to

deteriorate due to the use of adhesive during fabrication.31,32

Contrary to its counterparts, expanded graphite (EG) can be

easily processed to electrodes without using an adhesive or a

binder that can be degraded during treatment. Expanded graph-

ite is a low density carbon material which exhibits a series of

unique properties such as stability to aggressive media, develo-

ped specific surface, electrical conductivity, high temperature

resistance, compatibility and flexibility.3336 It is produced when

graphite intercalation compounds (GICs) are given a thermal

shock, the intercalates vaporise and tear the layers apart. This

leads to an expansion in the c direction, resulting in a puffed-

up material. EG exhibits a porous structure which can trap or

accommodate foreign compounds leading to the formation of

composite materials. The graphite particles in EG can be recom-

pressed or re-stacked without any binder and the restacking

mechanism results in the interlocking of the layers during com-

pression.37 EG has a preferential orientation of the basal plane

when compressed. It is a very good adsorption substrate due toits near perfect crystallographic face and has better homogen-

eity of the surface than any other form of graphite. EG is used

for seals, high temperature gaskets, catalyst supports and elec-

tromagnetic shielding materials.38,39

In addition, EG has been used as a substrate to trap titania

powder for the removal of heavy oils and photocatalytic degra-

dation of chlorinated phenoxyacetic acids.40,41 The composite

material demonstrated high stability and enhanced efficiency.

However, to the best of our knowledge no attempt has been

made at using EG material as a support anode (substrate) for

photoelectrochemical oxidation of organic pollutants in water.

This paper focuses on the preparation and characterisationof expanded graphiteTiO2 composite (EGTiO2) photoanode

using the solgel derived organically modified silicate binder

method and its application in the photoelectrochemical degra-

dation of p-nitrophenol in aqueous medium. The EGTiO2composite preparation method affords a high stability of the

catalyst.42 The enhanced stability can enable the photoelec-

trode to withstand high current density and anodic potential,

which will then increase the production of hydroxyl radicals

and reduce the recombination rate of photogenerated elec-

trons and holes respectively.

2. Experimental procedure

2.1 Materials and apparatus

Methyltrimethoxysilane (MTMOS), p-nitrophenol, methyl blue,

eosin yellowish, methanol and HCl were purchased from

Sigma-Aldrich. Titania, P-25 was supplied by Degussa Corp.

The characterisation of expanded graphite and its composite

was carried out using SEM/EDS (ESEM quanta), specific surface

areas were measured by BrunauerEmmettTeller (BET) nitro-

gen adsorptiondesorption (Shimadzu, Micromeritics ASAP

2010 Instrument), UV-Vis diffuse reflectance spectroscopy (Shi-

madzu 2450) of dry powders using BaSO4 was used as a reflec-

tance standard, and X-ray photoelectron spectra were recordedwith an ESCA-3 Mark II spectrometer (VG Scientific, UK) using

Al K radiation (1486.6 eV), an X-ray diffractometer (Bruker D8

with Cu-K) and Raman spectroscopy (Lab RAM HIR, Horida

Jobin Xvon, France; using 514.5 nm air cooled with Ar+ laser)

with 50 objective and a laser intensity of 1.3 mW.

The experiments were carried out in a photoreactor with an

approximate volume of 100 mL. EG and EGTiO2 with 1 cm2

area were used as working electrodes, Ag/AgCl (3.0 M KCl) and

platinum foil as a reference and a counter electrode respect-

ively. A potentiostat/galvanostatic with a voltage range of 13 V

Paper Photochemical & Photobiological Sciences

1092 | Photochem. Photobiol. Sci., 2013, 12, 10911102 This journal is The Royal Society of Chemistry and Owner Societies 2013

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and a current range of 2.515 mA was employed as the power

supply for electrochemical degradation of p-nitrophenol

(60 mL) in 0.1 M sodium sulphate supporting electrolyte. A

250 W quartz tungsten-halogen (QTH) lamp equipped with a

liquid filter was employed as a light source and the light inten-

sity at the outside surface of the lamp was 0.27 W. The dis-

tance between the reactor and the light source was 2 cm.

The effects of the TiO2 loading, the pH of the solution, the

current density (2.5 mA15 mA), initial concentration(00.8 mM) and applied potential 1 V3 V on the degradation

efficiency ofp-nitrophenol were investigated.

Aliquots were withdrawn from the electrochemical cell at

30 min intervals. The decay of nitrophenol concentration was

studied using a UV-Vis spectrophotometer (Perkin Elmer

model Lambda 35). Absorption bands at 310 nm were chosen

to convert the absorbance to concentration using linear

regression: y = 8.5x + 0.13667 obtained from a plot of absor-

bance vs. concentration (00.8 mM).

2.2 Preparation of supported titania films

Methanol (about 10 mL), titania P-25 (80 mg), and methyltri-methoxysilane (MTMOS) (100 mg) were mixed in a vial (20 mL)

and sonicated for 15 min to ensure uniform dispersion of

titania. EG (100 mg) was subsequently added and the mixture

was shaken for 5 min. After the addition of 1 M HCl (0.1 mL),

the mixture was allowed to hydrolyze for 3 h in a closed vessel

and poured into a Petri dish to dry. The contents of the Petri

dish were constantly mixed for uniform distribution of titania

on the EG. After drying for 24 h at room temperature, the float-

ing catalyst was incubated at 90 C for 24 h to remove residual

methanol. The percentages of EG and TiO2 in the EGTiO2composite material were 56% and 44% respectively.

2.3 Fabrication of the EG

TiO2 electrode

The prepared composite material was processed into pellets,

which were then used for the fabrication of the EGTiO2 elec-

trode using a glass rod, copper wire and conduction silver

paint (ESI). The clean (oxygen-free) Cu wire was coiled on one

end to form a flat surface where the EGTiO2 pellets were

placed. The conduction between the Cu wire and EGTiO2 was

achieved by using a silver paste. The EGTiO2 was then left to

air-dry for several minutes. The edges of the EGTiO2 were

then covered using epoxy resin so that the current contribution

is only from the basal plane. The copper wire is to transmit

the electrons to the external circuit since the electrode was

immersed in the solution. Owing to the compressibility of EGand its ability to interlock, it is possible to fabricate the elec-

trode into various shapes, thicknesses and sizes.

3. Results and discussion

3.1 Characterisation of expanded graphiteTiO2 composite

material

3.1.1 BET measurements. The commercially available P-25

titania was modified using a methyl silicate binder (MTMOS)

to improve its hydrophobicity and thus avoid detachment of

the film when wetted. The MTMOS serves as a binder for TiO2,

both inside and outside the graphite particle. It also interpene-

trates the catalyst, resulting in high loading of TiO2, and

increases its rigidity. Table 1 presents the BET surface areas of

the EG, P-25 titania, unmodified and methylsilane modified

EGTiO2. The unmodified EGTiO2 showed a high surface area

and pore volume in comparison with the methyl silane modi-

fied EGTiO2. The decrease in pore volume is due to interpene-tration of TiO2 into the porous matrix of EG facilitated by the

MTMOS.

3.1.2 UV-Vis diffuse reflectance. The effect of methyl sili-

cate binder on the photocatalytic properties of TiO2 was

studied and is given in Fig. 1. The characteristic band of TiO2was observed below 390 nm. The methyl silane modified EG

TiO2 showed high absorbance than the unmodified EGTiO2composites. The high absorbance of methyl silane modified

EGTiO2 is attributed to the presence of a higher amount of

TiO2 loaded on graphite particles. The high amount of TiO2results in enhanced photocatalytic efficiency. This result also

shows that the binder has no negative or quenching eff

ect onthe absorbance of TiO2. As supported by the BET data, the

primary role of the binder is to enhance TiO2 loading. Thus

methyl silane modified EGTiO2 was selected as suitable com-

posites for degradation ofp-nitrophenol.

3.1.3 SEM/EDS. The expanded graphite (EG) consists of

both closed and open pores which provide surface roughness

and porous cavities for titania incorporation (Fig. 2a). As seen

Table 1 BET data of EG, TiO2, unmodified and methyl silane modified EGTiO2

Sample BET surface area/m2 g1 Pore volume/cm3 g1

EG 14.66 0.06584TiO2 13.89 0.08641EGTiO2 10.36 0.07265EGTiO2 (MTMOS) 4.745 0.02139

Fig. 1 UV-Vis diffuse reflectance spectra of (a) TiO2, (b) methyl silane modified

EGTiO2, (c) unmodified EGTiO2 and (d) EG.

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in Fig. 2b, the TiO2 was well dispersed on the surface of EG

without clogging the pores. This dispersion allows the possibi-

lities of both photocatalysis and adsorption on the EGTiO2.

The presence of Ti K, O K and Si K in the EDX spectrum of the

composite materials confirms a successful insertion of TiO2on the matrix of EG.

3.1.4 X-ray diffractometry. Prominent characteristic peaks

of TiO2 occurring at 2= 25.4 and 27.4 are due to the reflec-

tions for anatase and rutile phases of the commercial P25respectively as shown in Fig. 3a. Since commercial TiO2-P25 is

a mixture of rutile (20%) and anatase (80%) crystallite, the

peaks corresponding to both phases are expected.

An estimate of the particle size from the broadening of the

main (101) anatase peak observed at 2= 25.4 can be done by

using the following Scherrer formula:

dK

cos

where K is the shape factor which is dimensionless and has a

typical value of about 0.9, is the Cu K radiation wavelength, is the line broadening at half the maximum intensity

(FWHM) in radians and is the Bragg angle. The particle size

of the TiO2-P25 was found to be 25 nm.

Fig. 2 SEM/EDS of (a) EG and (b) EGTiO2 materials.

Fig. 3 XRD pattern of (a) TiO2 and (b) EGTiO2 materials. R: rutile; A: anatase.



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The XRD pattern of the EGTiO2 composite material con-

sists of a characteristic peak of EG occurring at 2 = 26.7

(Fig. 3b). The anatase and rutile peaks, which show the incor-

poration of TiO2 into the matrix of EG, were also observed on

the spectrum of the composite. Since the region at which the

peaks of the composites appear is narrow, the EG peak over-

laps the peak corresponding to the rutile phase in Fig. 3b.

However, the intensity of the XRD peaks of the P25 in the com-

posite materials decreased remarkably. This is inferred to be adilution effect caused by the presence of EG.

3.1.5 Raman spectroscopy. Raman spectra of the anatase

phase of TiO2 has six Raman active modes (A1g + 2B1g + 3Eg) at

147, 198, 398, 515, 640 and 796 cm1 while rutile has four

active modes (A1g + B1g + B2g + Eg) situated at 144, 448, 612

and 827 cm1, respectively.43 The TiO2-P25 used in the study is

a mixture of anatase (80%) and rutile (20%). Thus the Raman

peaks of titania (Fig. 4a) at 148, 522 and 644 cm1 are assigned

to the anatase phase while one peak at 400 cm1 corresponds

to the rutile phase. These signature peaks were also observed

in the Raman spectrum of EGTiO2 (Fig. 4b) with the G band

at 1587 cm1

corresponding to the sp2

hybridized carbon ofthe EG.

3.1.6 X-Ray photoelectron spectroscopy. The high resolu-

tion spectra of the prepared EGTiO2 composite materials and

the location of binding energies are given in Fig. 5(ad) and

Table 2.

The spectrum of EGTiO2 exhibits Ti 2p, O 1s, Si 2p and

C 1s. The Ti 2p1/2 and Ti 2p3/2 spin orbital splitting photo-

electrons is located at binding energies of 465.360 and 459.613

eV respectively as shown in Fig. 5(a). The prominent peak at

459.61 eV indicates that the Ti element mainly existed as

Ti(IV).44,45

The deconvolution of the C 1s spectrum (Fig. 5b) revealed

the presence of graphitic (sp2) a n d CH from the methyl

portion of the MTMOS structure at 284.57 and 285.750 eV

respectively. The curve resolution of the O 1s signal (Fig. 5c)

indicated the presence of peaks located at 530.37 eV and

531.13 eV which are assigned to bulk oxide and hydroxyl

species respectively. The O1s binding energy of the SiOTi

species observed at 532.39 eV confirms the bonding of the

MTMOS to the TiO2 surface. The SiOSi, SiOCH3 and Si 2p

peaks observed at 533.19, 533.39 and 103.75 eV are character-

istic peaks of the silane binder.

3.2 Electrochemical characterisation of the EGTiO2

electrodeThe electrochemical properties of EG and EGTiO2 electrodes

were studied using 2 mM [Fe(CN)6]3/4 in 0.1 M KNO3 solu-

tion as a redox probe at a scan rate of 20 mV s1 as shown in

Fig. 6. The EG electrode exhibited enhanced current in com-

parison with the EGTiO2 electrode. The electron transfer peak

to peak separations (Ep) of 117 mV and 170 mV for EG and

EGTiO2 electrodes respectively were obtained. It is known

that the closer the Ep to 59 mV for a reversible system, the

faster the electron transfer rate. Hence the EG electrode exhi-

bits faster electron transfer kinetics than the EGTiO2 elec-

trode. The slow electron transfer in the EGTiO2solution

interface is due to electrochemical inactiveness of the TiO2film which reduces the conductivity of EG.

The charge transfer resistance of the redox couple at EG

and EGTiO2 electrodes was studied using electrochemical

impedance spectroscopy (Fig. 7). The charge transfer resist-

ances (Rct) of EG and EGTiO2 electrodes in 5 mM [Fe(CN)6]3/4

were found to be 0.326 k cm2 and 1.47 k cm2 respectively

using the simple Randles equivalence circuit fitting. The

lower charge transfer resistance at the EG electrode indicates

faster electron transfer kinetics than at the EGTiO2. The

charge transfer rate constant (Kapp) was obtained from the

Nyquist plot using the following equation:

Kapp RT=F2

RctC

where Kapp is the apparent rate constant at the EG and EG

TiO2 electrodes, R is the gas constant, T is the temperature in

Kelvin, F is the Faraday constant, Rct is the charge transfer

resistance and C is the concentration of the redox couple.46

Fig. 4 Raman spectra of (a) TiO2 and (b) expanded graphiteTiO2 materials.



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Fig. 5 High resolution curves of (a) Ti 2p, (b) C 1s, (c) O 1s and (d) Si 2p found in EG TiO2 composite material.

Table 2 Location of binding energies for the EGTiO2 photoemissions

Ti B.E./eV O 1s B.E./eV C 1s B.E./eV Si B.E./eV

2p3/2 465.360 Bulk O2 530.37 Graphitic (sp2) 284.57 Si 2p 103.75

2p1/2 459.613 OH 531.13 CH 285.75SiOSi 533.19SiOTi 532.39SiOCH3 533.39

Fig. 6 Cyclic voltammograms of (a) EG and (b) EGTiO2 electrodes in 5 mM [Fe (CN)6]3/4 at 20 mV s1.



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The Kapp of 16.3 105 cm s1 and 3.61 105 cm s1 were

obtained for EG and EGTiO2 respectively. These kinetic data

show that the EGTiO2 has a slower kinetics owing to the elec-

trochemical inactiveness of TiO2 as explained using the cyclic

voltammetric data.

3.3 Photoelectrochemical oxidation ofp-nitrophenol

3.3.1 Degradation kinetics. The Langmuir Hinshelwood

model is usually employed to describe the kinetics of electro-

chemical degradation of aquatic organics.43,44 It basically

relates the degradation rate (r) and reactant concentration in

water at time t (C), which is expressed as follows:

r dC

dt

krKadC

1KadC

where kr is the rate constant and Kad is the adsorption equili-

brium constant. When the solution is highly diluted and the

adsorption is relatively weak, KadCbecomes1; the reaction is

essentially an apparent first-order reaction. The equation can

then be simplified to the pseudo-first-order kinetics with an

apparent first-order rate constantKapp:

lnC0

Ct krKadt Kappt

where C0 is the initial concentration (mM) and Ct is the con-

centration after a period of time. A plot of ln(C0/Ct) versus t

gives a linear relationship, confirming that the oxidationprocess displays pseudo-first-order kinetic behaviour. The

apparent kinetic rate constant is obtained from the magnitude

of the slopes of the straight lines and their corresponding half-

life values (Table 3).

The degradation of p-nitrophenol under the three different

processes (Fig. 8) showed pseudo-first-order kinetic behaviour

and the kinetic parameters are given in Table 3. Electrochemi-

cal oxidation exhibited low removal efficiency of the electroche-

mical process (6%) in comparison with the photocatalytic

process. This is attributed to the fact that electrochemical

oxidation involves adsorption of hydroxyl group on the active

surface of the electrode before the formation of hydroxyl radi-

cals. These adsorbed hydroxyl groups have a much stronger

tendency to combine with each other and generate oxygen

molecules. The hydroxyl radicals responsible for oxidation of

organic pollutant face the competition from the oxygen evol-

ution processes and this result in low degradation efficiency.

These parasitic oxygen molecules can also prevent contact

between hydroxyl radicals and organic pollutants and this

result in poor mass transfer of the organic pollutant to the

electrode surface.

In photocatalysis, the photogenerated electron is used to

reduce the oxygen molecules to form superoxide radicalswhich can later form hydroxyl radicals. Therefore, in photoca-

talysis there are no parasitic oxygen side reactions that can

affect the degradation efficiency.

Under the same conditions, the photoelectrochemical oxi-

dation process resulted in an enhanced degradation efficiency

ofca. 62% with a fast kinetics rate of 10.4 103 min1 signify-

ing a synergic combination of electrochemical oxidation and

photolysis processes. It has been reported that a combination

of the oxygen released by the electrochemical method can be

reutilized as the electron acceptor for the oxidation in the pho-

tolysis process, promoting the degradation rate by indirect oxi-

dation.47

In addition to this possibility, the TiO2 used in ourcomposite is capable of generating hydroxyl radicals upon

Fig. 7 Nyquist plot of (a) EG and (b) EGTiO2 electrodes in 5 mM [Fe(CN)6]3/4.

Table 3 Degradation kinetic parameters of p-nitrophenol

ProcessKapp/ 10

3

min1t1/2/min

Removalefficiency/%

Photolysis 1.56 444 24Electrochemical oxidation 0.72 962 6Photoelectrochemical oxidation 10.4 66.7 62

Fig. 8 Normalized concentration decay with time of 0.4 mM p-nitrophenol in

a 0.1 M Na2SO4 solution (pH 7) at a current density of 5 mA cm2 under (a)

direct photolysis, (b) electrochemical oxidation and (c) photoelectrochemical

oxidation.



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irradiation with light of energy higher than its band gap and

thus enhance the overall efficiency.

Therefore, the advantage of combining the electrochemical

oxidation system with photolysis is that the by-product of elec-

trolysis (newly generated oxygen species) could be utilized to

elevate the photolysis efficiency and in this way the enhanced

removal efficiency ofp-nitrophenol was achieved.

In order to optimize the photoelectrochemical degradation

of the p-nitrophenol and to further probe the photoelectrocata-lytic process, a series of studies discussed below were

conducted.

3.3.2 Optimisation of experimental conditions. The effects

of TiO2 loading, pH of the solution, current density, initial

concentration of the p-nitrophenol and applied potential were

studied and given in Fig. 9.

Upon increasing the amount of TiO2 content from 80 mg to

180 mg (Fig. 9a), the degradation rate increased from 7.93

103 min1 to 10.4 103 min1 respectively.

The enhanced reaction rate is attributed to the generation

of more electrons and hole pairs and thus higher concen-

tration of hydroxyl radicals for degradation of p-nitrophenol.However, the enhancement of degradation efficiency was not

higher upon increasing the TiO2 content as would be expected.

This is attributed to the fact that at high TiO2 concentration,

some TiO2 particles are shielded from the applied UV light

rendering them inactive.

The pH values of the reaction solution can affect the

surface charge of the active species and thus alteration of the

pH of the solution leads to different interactions between the

organic pollutant and the electrode depending on the surface

charge of the electrode material. According to Wang et al.48

the functional groups from hydrated titanium dioxide may be

TiO, TiOH and/or TiOH2+. The point of zero charge ( pHpzc) ofthe Degussa P25 TiO2 normally ranges from 6.25 to 6.6.

49,50 At

pH values higher than pHpzc, TiO is the predominant group

on the TiO2 surface.

Most phenolic compounds act like weak acids and thus dis-

sociation of hydrogen ions from the phenolic compounds

depends on the pH of the solution. It is known that p-nitro-

phenol exists as a nitrophenolate anion when the pH of the

solution is greater than its pKa value (7.2) and as a molecule

when the pH of the solution is lower than its pKa value. There-

fore, the neutral molecular form of p-nitrophenol could be

adsorbed easily on the surface of TiO2 by a weak electrostatic

attraction with TiO

via hydrogen bonding.47

Thus, at neutralpH, the degradation was more efficient than at low pH as

shown in Fig. 9b. The low degradation efficiency at low pH is

attributed to the fact that in a highly acidic medium, there is

Fig. 9 Dependence of photoelectrochemical degradation of 0.4 mM p-nitrophenol in Na2SO4 at pH 7, 5 mA cm2 and 0.27 W output power on (a) TiO2 loading,

(b) pH of the solution, (c) initial concentration of p-nitrophenol and (d) current density.



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high production of hydrogen ions (H+) which can compete

with molecules of p-nitrophenol for adsorption sites on the

TiO surface, leading to low photoelectrocatalytic degradation

ofp-nitrophenol.

The influence of the initial p-nitrophenol concentration on

the photodegradation of p-nitrophenol is given in Fig. 9c. An

increase of the initial concentration from 0.2 mM to 0.4 mM

p-nitrophenol resulted in enhanced degradation efficiency.

However further increase of the concentration to 8 mM led to

diminished degradation efficiency because the generated elec-

tronhole pairs are insufficient to decompose every p-nitro-

phenol molecule as the number of molecules increases with

an increase ofp-nitrophenol concentration.Fig. 9d reveals the effect of current density during photo-

electrochemical oxidation of p-nitrophenol. The degradation

efficiency increased with an increase of current density from

2.5 mA cm2 to 5 mA cm2. The increase of degradation

efficiency is attributed to higher production of hydroxyl radical

upon an increase of the current density. An increase of current

density from 5 mA cm2 to 15 mA cm2 did not really increase

the degradation efficiency as would have been expected. This

is because at higher current density, the rate of formation of

intermediates also increases. These rapidly formed intermedi-

ate products scavenge the in situ generated hydroxyl radicals

that are also needed by the parent organic pollutant, thusslowing down the degradation kinetics of the entire process

relatively. Based on the point of energy cost, it is favourable to

Fig. 10 Dependence of photoelectrochemical degradation of 0.4 mM p-nitro-phenol in Na2SO4 at pH 7 and 0.27 W output power on applied potential for

the EGTiO2 photoelectrode.

Fig. 11 UV-Vis spectra during degradation of 0.3 104 M MB at (a) photolysis, (b) electrochemical oxidation (1.5 V), (c) photoelectrochemical oxidation (1.5 V)

and (d) their normalized concentration decay versus time.



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be informed that an increase of the current density may not

necessarily increase the degradation efficiency, and for this

work, 5 mA cm2 can be said to be the optimum. Furthermore,

the ability of the EGTiO2 photoanode to withstand a high

current density suggests that this composite electrode exhibits

high stability and thus can be considered for real practical

applications.

3.3.3 Effect of the electrode potential. The importance of

selecting a suitable potential for the degradation of 0.4 mMp-nitrophenol solutions was studied by the application of

different electrode potentials ranging from +1 V to +3 V as

shown in Fig. 10.

The increase in biased potential helps in the separation of

photogenerated electrons and holes, leading to higher concen-

tration of hydroxyl radicals and thus increased degradation.

The degradation efficiency increased as a function of biased

potential. At an applied potential of 3 V, about 62% removal

efficiency of 0.4 mM p-nitrophenol (60 mL) was achieved

within 90 min reaction time. In comparison with a report by

Palmisano et al.,25 where 70% removal efficiency of 0.072 mM

p-nitrophenol (130 mL) was obtained at 3 V applied potentialfor 6.5 h degradation time using a graphite rodTiO2 anode,

the EGTiO2 composite electrode used in this study demon-

strated improved efficiency. The enhanced efficiency of this

composite electrode can be attributed to the porous matrix of

EG materials which allows high loading of TiO2. The high

loading of TiO2 is beneficial for the production of hydroxyl

radical and thus enhanced efficiency at high concentration of

p-nitrophenol at a shorter reaction time was achieved. In

addition, no detachment of TiO2 from expanded graphite was

observed upon increasing the potential. Therefore, the EG

TiO2 composite electrode exhibited high stability.

3.4 Suitability of the EGTiO2 electrode towards degradation

of other organic pollutants

To demonstrate the photoelectrochemical degradation versati-

lity of the EGTiO2 electrode, two model organic dyes methyl-

ene blue (a cationic dye) and eosin yellowish (an anionic dye)

were employed as target pollutants as shown in Fig. 11 and 12

respectively. Since the photoanode is positively charged in

acidic solutions and negatively charged in alkaline solutions,

the efficiency of methylene blue (MB) photodegradation is

expected to increase with pH owing to the electrostatic inter-

actions between the negative surface of the photoanode and

the cationic nature of MB. Thus the degradation of MB wascarried out in a highly alkaline (pH 12) medium. The photo-

electrochemical degradation of MB was compared to photo-

lysis and electrochemical degradation as shown in Fig. 11(a)(c).

Fig. 12 UV-Vis spectra during degradation of 0.1 104 M Eosin Y at (a) photolysis, (b) electrochemical oxidation (1.5 V), (c) photoelectrochemical oxidation (1.5

V) and (d) their normalized concentration decay versus time.



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The absorbance of the maximum absorption band at 680 nm

decreased with time for the photolysis process reaching 29%

removal efficiency at a rate constant of 3.08 103 min1. The

electrochemical oxidation process resulted in an improved

removal efficiency of 80% and a faster kinetics 11.9 103

min1 over photolysis. The combination of these two processes

resulted in the highest degradation efficiency of 94% and the

most facile kinetic rate of 20.9 103 min1 when compared to

the individual processes (Fig. 11c). These results demonstratethe possible applicability of the EGTiO2 photoanode in the

photoelectrochemical degradation of organic dyes such as MB.

For the degradation of eosin yellowish (Eosin Y), UV-Vis

reflectance spectra obtained before and after light irradiation

are observed at 400 nm600 nm absorption band intervals. As

an anionic dye, Eosin Y interacts more efficiently with the EG

TiO2 photoanode at low pH, where the surface of the photoa-

node is positively charged. Thus the degradation of Eosin Y

was conducted at pH 4. The absorbance of Eosin Y decreased

with irradiation time using the photolysis process resulting in

37% removal efficiency with a rate constant of 10.4 102

min3

(Fig. 12a) while a resistance to electrochemical oxi-dation was observed owing to a poor removal efficiency of 6%

with a rate constant of 1.55 102 min3 (Fig. 12b). However,

the combination of the two processes improved the removal

efficiency to 47% with a 1.55 102 min3 rate constant under

the same conditions (Fig. 12c). The improvement of the

removal efficiency is attributed to the synergistic character of

the photoelectrochemical process and also supports the wider

applicability of the EGTiO2 photoanode. The low removal

efficiency recorded for Eosin Y suggests that the photoanode

may be more suitable for cationic pollutants than for anionic

pollutants.

4. Conclusion

This work successfully reports the synergic effect of photoelec-

trochemistry on the degradation of p-nitrophenol in compari-

son with individual electrochemical oxidation and photolysis

processes. The removal efficiency of 62% after just 90 minutes

under the optimum conditions of neutral pH, a relatively mild

3 V potential and the low current density suggest the suit-

ability of the prepared EGTiO2 as a photoanode in the photo-

electrochemical degradation of phenolic compounds, which

are toxic and relatively resistant to biological degradation. The

potential of the EGTiO2 photoanode for wider photodegrada-tion applications is evidenced by its ability to degrade two

other classes of organic pollutants, namely: methyl blue and

Eosin Y. The low cost of graphite and the ease of preparation

of the EGTiO2 also lead to possible large-scale applications.

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