Interrogating solar photoelectrocatalysis on an exfoliated graphite-BiVO4/ZnO composite electrode towards water treatment

Article abstract of DOI:10.1039/c9ra02366f

A novel photoanode consisting of an exfoliated graphite-BiVO₄/ZnO heterostructured nanocomposite was fabricated. The material was characterised with scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). Photoelectrochemical studies were carried out with cyclic/linear sweep voltammetry and chronoamperometry. The solar photoelectrochemical properties of the heterojunction photoanode were investigated through the degradation of rhodamine B in water. The results revealed that the nanoparticles of BiVO₄ and ZnO were well entrapped within the interlayers of the exfoliated graphite (EG) sheets. Improved charge separation was achieved in the EG-BiVO₄/ZnO composite electrode which resulted in superior photoelectrochemical performance than individual BiVO₄ and ZnO electrodes. A higher degradation efficiency of 91% of rhodamine B was recorded using the composite electrode with the application of 10 mA cm^-2 current density and a solution pH of 7. The highest total organic carbon removal of 74% was also recorded with the EG-BiVO₄/ZnO. Data from scavenger studies were used to support the proposed mechanism of degradation. The electrode has high stability and reusability and hence lends itself to applications in photoelectrocatalysis, especially in water treatment.

Full text of DOI:10.1039/c9ra02366f

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Interrogating solar photoelectrocatalysis on an
exfoliated graphite–BiVO /ZnO composite
4
Cite this: RSC Adv., 2019, 9, 16586
electrode towards water treatment
a
a
a
Benjamin O. Orimolade, Babatunde A. Koiki, Busisiwe N. Zwane,
Gbenga M. Peleyeju, Nonhlangabezo Mabuba and Omotayo A. Arotiba *ab
a
ab
4
A novel photoanode consisting of an exfoliated graphite–BiVO /ZnO heterostructured nanocomposite was
fabricated. The material was characterised with scanning electron microscopy (SEM), energy dispersive
spectrometry (EDS) and X-ray diffraction (XRD). Photoelectrochemical studies were carried out with
cyclic/linear sweep voltammetry and chronoamperometry. The solar photoelectrochemical properties of
the heterojunction photoanode were investigated through the degradation of rhodamine B in water. The
results revealed that the nanoparticles of BiVO
the exfoliated graphite (EG) sheets. Improved charge separation was achieved in the EG–BiVO
composite electrode which resulted in superior photoelectrochemical performance than individual
BiVO and ZnO electrodes. A higher degradation efficiency of 91% of rhodamine B was recorded using
the composite electrode with the application of 10 mA cm current density and a solution pH of 7. The
highest total organic carbon removal of 74% was also recorded with the EG–BiVO /ZnO. Data from
4
and ZnO were well entrapped within the interlayers of
4
/ZnO
4
ꢀ2
Received 28th March 2019
Accepted 20th May 2019
4
scavenger studies were used to support the proposed mechanism of degradation. The electrode has
high stability and reusability and hence lends itself to applications in photoelectrocatalysis, especially in
water treatment.
DOI: 10.1039/c9ra02366f
rsc.li/rsc-advances
3
degradation process. PEC is a form of advanced oxidation
technique which deals with in situ production of active
1
Introduction
Catalysis, over the years, has been the cornerstone of the oxidizing species such as hydroxyl radicals, superoxide radicals
improvement and optimisation of many industrial processes and holes which oxidizes organics to water and carbon
1
4,8
including syntheses and production. The concept of catalysis is dioxide. In PEC, both light and electric energy are used to
also prominent in electrochemical processes where it is gener- generate these radicals and it combines both activities of pho-
ally referred to as electrocatalysis. In electrocatalysis, the rate of tocatalysis and electrolysis. In other to minimize the cost
an electrochemical reaction occurring on an electrode surface is associated with the use of light energy in generating electron–
improved. In fact, many electrochemical processes for energy hole pairs in PEC, solar light is used and the process is referred
2
9
and sensing are based on electrocatalysis and photo- to as solar photoelectrocatalysis (SPEC).
electrocatalysis (where light is involved). Recently, catalysis,
The electrode commonly used in PEC are oen prepared by
electrocatalysis and photoelectrocatalysis have found applica- the immobilization of a photoactive substance which are mostly
3,4
tion in the eld of water treatment. The challenge of water metal oxide semiconductors on a conducting substrate such as
pollution and incalcitrant pollutants, which are difficult to treat uorine doped tin oxide glass, anodized titanium sheet, pol-
with the conventional water treatment methods, are motiva- ished stainless steel and conducting quartz glass.
10–12
Evidently,
tions behind the quests for alternative or complementary the performance of PEC process depends largely on the semi-
approaches to water treatment. For example, the occurrence of conductor used as the anode. Zinc oxide (ZnO) has proven to be
dyes, pharmaceuticals and other organic pollutants are on the a suitable material for PEC applications. ZnO is a non-toxic
increase and they have been found to persist in water even aer semiconductor with excellent photocatalytic activities,
5–7
13
treatment.
outstanding chemical stability and high electron mobility. It
A promising environmentally friendly technique for a more has been extensively applied in the removal of organic dyes and
efficient removal of these dyes is the photoelectrocatalytic (PEC) pharmaceuticals through both photocatalysis and photo-
1
4–16
electrocatalysis.
3.4 eV), its performance is poor with the application of solar
energy in PEC process because it best be excited with UV-light
As a result of large band gap energy of ZnO
(
a
Department of Applied Chemistry, University of Johannesburg, South Africa. E-mail:
oarotiba@uj.ac.za
17
b
(which constitutes less than 5% of the total solar spectrum).
Centre for Nanomaterials Science Research, University of Johannesburg, South Africa
16586 | RSC Adv., 2019, 9, 16586–16595
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As a result of this drawback, much attention has been given to study. Results on the solar photoelectrochemical responses,
visible-light photoactive semiconductors such as tungsten degradation efficiency, scavenger study, reusability etc. are
1
8
19
20
21
trioxide, bismuth vanadate, copper(I) oxide, ferric oxide
presented. The positive effect of the inclusion of EG in the
22
and graphitic carbon nitride. Though these semiconductors heterojunction is conrmed through the enhanced the photo-
perform better than ZnO when visible light is applied in PEC, electrocatalytic degradation or rhodamine B dye at a relatively
ZnO is much readily available, easier to synthesize and has low photocurrent density.
higher electron mobility than some of them. Therefore, ZnO
has been combined with visible light active semiconductors to
form a heterojunction and such composite has shown higher
photocatalytic activity for both degradation of organic pollut-
23
2
Materials and method
2.1 Synthesis of ZnO
23–25
ants and water spilling for evolution of hydrogen.
Hydrothermal technique was employed for the synthesis of ZnO
Basically, an heterojunction is formed when two semi- nanoparticles. This was achieved by transferring a mixture
conductors of unequal energy band gaps are combined together containing zinc acetate (0.9041 g, 4 mmol) and 10 mL of PEG400
in a way that results in band alignment. Several methods such into a beaker containing 50 mL of absolute ethanol. The
as hydrothermal, chemical bath deposition, calcination, sol–gel mixture was stirred for 15 min and then transferred into
process and chemical deposition have been reportedly used for a 100 mL Teon-lined autoclave, which was lled with solid
26
formation of heterojunctions. The formation of hetero- NaOH (0.314 g, 8 mmol). The autoclave was placed in the oven
ꢁ
junction has been successfully applied to overcome the problem at 140 C for 24 h, and then allowed to cool to room tempera-
of rapid recombination of photogenerated electron–hole pairs ture. The precipitate was collected, washed with absolute
in visible light semiconductor photocatalysts and to improve ethanol and distilled water thoroughly, and then dried in
27
ꢁ
a vacuum at 60 C for 6 h.
36
their photoactivities.
Bismuth vanadate (BiVO
a high visible light photoactivity (absorbs up to 11% of visible 2.2 Synthesis of BiVO
4
) is a n-type semiconductor with
4
light) due to its relatively low band gap energy of 2.4 eV. With
the aforementioned properties and its appropriate band edge
Hydrothermal process was equally employed for the prepara-
tion BiVO . Briey, 1.940 g of bismuth salt and 2.208 g of
4
positions, BiVO can be a good material for the fabrication of
4
sodium hydrogen carbonate were dissolved in 70 mL of ethylene
glycol to form a solution (A). Another solution (B) containing
28
heterojunction with ZnO. Since the problem of recombination
of charge carriers are overcome in BiVO4 heterostructured
3 4
0.468 g of Na VO was dissolved in 10 mL of distilled water and
2
9
composites, such composites as BiVO
4
/Ag
/Cu
applied for degradation of organics and water spilling. Inter-
estingly, BiVO /ZnO heterostructured composites have also
3
VO
2
O
4
32
,
BiVO
4
/
was added to mixture A slowly under stirring. The mixture was
further stirred for 30 min and an orange emulsion was formed.
The pH of the emulsion was adjusted to 6 and subsequently
transferred into a 100 mL Teon-lined autoclave. The autoclave
30
gC
3
N
4
,
Bi
2
S
3
/BiVO
4
(ref. 31) and BiVO
4
has been
4
been reported to exhibit higher visible light photoactivity as well
as lower recombination rate of electron hole pairs suitable for
ꢁ
was sealed and placed in an oven at 180 C for 24 h. Aer the
reaction duration, the autoclave was allowed to cool to room
temperature and the resulting yellow product was collected,
washed with ethanol and distilled water several times and then
33
PEC applications. For instance, Feng et al. prepared BiVO4/
ZnO lms on FTO using liquid phase deposition technique in
combination with successive ionic layer adsorption and reac-
tion (SILAR) and achieved more than 90% removal efficiency of
tetracycline through PEC degradation. Similarly, in the report of
ꢁ
37
dried at 60 C for 6 h.
2
.3 Preparation of BiVO
A simple thermal treatment method as described by Singh et al.
was employed in preparing the BiVO /ZnO heterojunction. The
synthesized BiVO and ZnO was taken and mixed in the ratio
: 1 and mixed in mortar pestle until a homogeneous compo-
4
/ZnO composite
34
ꢀ2
Yang and Wu, photocurrent density of about 3.5 mA cm at
.23 V vs. RHE was achieved with the application of BiVO /ZnO
in water splitting.
1
4
4
4
The efficiency of BiVO /ZnO heterostructured composites in
4
1
PEC applications can further be improved by making use of
a conducting substrate that can also minimize the recombina-
tion of photogenerated electron–hole pairs by acting as sink for
the charge carriers. An example of such substrate is exfoliated
graphite (EG), a porous and easily compressible 2D carbon
material with high electrical and thermal conductivity. The
outstanding electron mobility of EG is essential in channelling
sition is obtained. The mixture was then annealed in
ꢁ
a programmable furnace at 450 C for one hour and allowed to
38
cool at room temperature. Other ratios were also prepared
following the same procedure.
4
2.4 Preparation of EG and EG–BiVO /ZnO composite
photogenerated charge carriers and thereby preventing recom- The exfoliated graphite was prepared through two steps: inter-
35
bination. In this study, a thermal treatment technique was calation and exfoliation. The intercalation was done by soaking
used to fabricate BiVO /ZnO heterostructured composite and natural graphite in concentrated nitric acid and sulphuric acid
the composite was combined with exfoliated graphite to form mixture (ratio of 1 : 3 by volume) for 72 h at room temperature
EG–BiVO /ZnO electrode. More importantly, we investigate the for. The intercalated graphite was washed thoroughly with
solar photoelectrocatalytic behaviours of EG–BiVO /ZnO pho- deionized water to a pH of 7, air-dried. Exfoliation of the
toanode using the degradation of rhodamine B dye as a case intercalated material was done by placing the material in
4
4
4
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ꢁ
39
a furnace at a temperature of 800 C for about a minute. The pattern of the analyte were observed using UV-Visible spectro-
EG–BiVO /ZnO composite was prepared by dispersion of the photometer (Cary 60, Agilent technologies, Australia) and the
4
synthesized BiVO /ZnO in absolute ethanol and sonicated for total organic carbon removal was recorded using TOC analyser
4
50 min to obtain uniformity of suspension of the nanoparticles (Teledyne Tekmar TOC fusion). The effects of current density
followed by the addition of EG and further sonicated for 30 min and pH on the removal efficiency were investigated.
(
equal masses of EG and BiVO
4
/ZnO were used). The mixture
ꢁ
was put in an oven to dry at 100 C overnight for complete
3
Results and discussion
evaporation of the ethanol. EG–BiVO
larly prepared using BiVO and ZnO respectively.
4
and EG–ZnO were simi-
3.1 X-ray diffractometry
4
The XRD pattern of the synthesized BiVO
4
, ZnO, exfoliated
2
.5 Fabrication of EG–ZnO, EG–BiVO and EG–BiVO /ZnO
4 4
graphite (EG) and the EG–BiVO /ZnO composite are shown in
4
composite electrodes
Fig. 1. The characteristic diffraction peak of EG was observed at
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
The prepared EG–BiVO /ZnO composite was compacted into 26.7 . The main peaks observe at 18.8 , 28.9 , 30.4 , 35.1 , 39.7
4
ꢁ
a pellet (13 mm in diameter) by a hydraulic press at a high and 42.1 in the XRD pattern of BiVO
4
has be indexed as (110,
0
11), (121), (040), (002), (211) and (150) respectively which
pressure of 10 000 psi. The pellet was employed for the
construction of an electrode with the use of a copper wire, non-
conductive epoxy resin and conductive silver paint. The pellet
was placed on the coiled part of the copper wire with the
assistance of the conductive silver paint and allowed to dry. The
edge of the composite pellet was subsequently sealed with the
correspond to the crystal lattice planes of monoclinic scheelite
BiVO
shows peaks at 31.8, 34.5, 36.4, 47.6, 56.7, 63.1, 67.8 and 69.1
which correspond to (100), (002), (101), (102), (110), (103), (112)
and (201) respectively to crystalline planes of hexagonal wurtzite
20,40
4
(JCPDS no. 14-0668).
The diffraction pattern of ZnO
resin to allow the ow of electricity from only the basal plane. ZnO (JPCDS no. 36-1451). All the characteristic peaks of BiVO
The surface area of the electrode was approximately 1.3 cm . ZnO and that of EG can be seen in the XRD pattern of the EG–
4
,
2
BiVO /ZnO composite which indicated that the composite was
successfully prepared.
The same procedure was also followed for the fabrication of EG–
BiVO and EG–ZnO electrodes.
4
4
2.6 Characterisation of prepared materials
3
.2 Morphological and optical studies of EG, BiVO , ZnO,
4
and EG–BiVO
The surface morphology of the prepared materials was exam-
ined using SEM combined with EDS. The SEM image of BiVO
Fig. 2a) revealed the irregular shapes of BiVO particles while
the ZnO nanoparticles (Fig. 2b) appear as ne agglomerated
particles. The BiVO particles are well integrated between the
4
/ZnO
The degree of crystallinity, particle size and purity of the
prepared photoanodes were determined with the aid of X-ray
diffractometer (Rigaku Ultima IV, Japan) using Cu Ka radia-
tion (k ¼ 0.15406) with K-beta lter at 30 mA and 40 kV. The
surface morphological studies and elemental composition of
the materials were carried out on a TESCAN Vega 3 (Czech
Republic) scanning electron microscope coupled with energy-
dispersive X-ray spectrometer (EDS).
4
(
4
4
particles of ZnO with increased agglomeration which revealed
the formation of BiVO /ZnO heterostructured composite
4
(
Fig. 2c). The graphitic sheets planes of exfoliated can be seen in
Fig. 2d with open gravity suitable for the entrapment of parti-
cles. The BiVO /ZnO particles were well distributed within the
planes of the graphitic sheets of the EG with lesser aggregation
2
.7 Electrochemical and photoelectrochemical experiments
All electrochemical and photoelectrochemical measurements
were conducted on an Autolab 302N (Netherlands) potentiostat/
galvanostat with three-electrode conguration. The fabricated
electrodes, platinum foil and Ag/AgCl (3.0 M KCl) were
employed as working electrodes, counter electrode and refer-
ence electrode respectively. A solar simulator (Oriel LCA-100)
equipped with a 100 W xenon lamp was used as the light
source for PEC experiments. The prepared electrodes were
positioned vertically facing the incident light of the simulator
and the distance between the photoelectrochemical cell and the
light source was kept constant at 10 cm. Photocurrent
measurements and linear sweep voltammetry were carried out
4
in a 0.1 M Na
in a 5 mM solution of [Fe(CN)
solution). For the degradation experiments, 50 mL of 10 mg L
of rhodamine B dye was used with 0.1 M Na SO solution as
2 4
SO solution. Cyclic voltammetry was carried out
3
ꢀ/4ꢀ
6
]
(prepared in a 0.1 M KCl
ꢀ1
2
4
supporting electrolyte. Electrochemical degradation experi-
ments were done in the dark. An aliquot of the solution was
collected from the reactor at certain time intervals using
4 4
Fig. 1 XRD patterns of EG (a), BiVO (b), ZnO (c) and EG–BiVO /ZnO
a disposable syringe. The concentration decay and degradation composite (d).
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Fig. 2 SEM images of BiVO
4
(a); ZnO (b); BiVO
4
/ZnO (c); EG (d); EG–BiVO
4
/ZnO (e) and EDS spectrum of EG–BiVO
4
/ZnO (f); UV-DRS spectra of
4 4
BiVO , ZnO and BiVO /ZnO (g).
4 4
within the BiVO /ZnO particles (Fig. 2e). The presence of C, O, preparation of EG–BiVO /ZnO composite electrode. The UV-
Bi, V and Zn elements can be seen the EDS spectrum (Fig. 2f) of Visible diffuse reectance spectra of the materials conrmed
the composite material which further conrmed the successful that the synthesized BiVO and BiVO /ZnO composite absorbs
4
4
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41
photons in the visible light region and their absorbance edges suggested that the electrode reaction rate is higher. Both the
can be traced to 535 nm and 520 nm respectively (Fig. 2g).
EG–ZnO and EG–BiVO4 electrode performed better than the
bare EG electrode and this could be attributed to that fact that
the addition of ZnO and BiVO to the EG resulted in increased
electroactive surface area and thereby increased the conduc-
4
3.3 Electrochemical characterisation of electrodes
The cyclic voltammograms of EG, EG–ZnO, EG–BiVO and EG– tivity. The electroactive surface area of all the electrodes were
4
ꢀ1
BiVO
KCl solution in 5 mM [Fe(CN)
in Fig. 3a. The EG–BiVO /ZnO electrode showed the highest
faradaic peak current than all the other electrodes which
/ZnO were recorded at a scan rate 20 mV s using 0.1 M calculated using the Randles–Sevcik equation:
4
3
/4
6
]
as the redox probe as shown
3
/2
1/2 1/2
v
ip ¼ kn AD
C
(1)
4
where A is the electroactive surface area, i
p
is the peak current, C
is the redox probe concentration, D is the diffusion coefficient
ꢀ
6
(
7.6 ꢂ 10
for ferrocyanide), v is the scan rate and k is
5
a constant (2.69 ꢂ 10 ). The electroactive surface areas obtained
2
2
2
2
are 0.196 mm , 0.239 mm , 0.248 mm and 0.309 mm for EG,
EG–ZnO, EG–BiVO and EG–BiVO /ZnO electrodes respectively.
4
4
The largest surface area of the EG–BiVO /ZnO justied the
4
highest faradaic current obtained with the electrode.
The linear sweep voltammetry of the EG, EG–ZnO, EG–BiVO
and EG–BiVO /ZnO were recorded using 0.1 Na SO as sup-
porting electrolyte and was carried out both in the dark and
light (Fig. 3b). The EG–BiVO /ZnO composite electrode gave the
4
4
2
4
4
highest current as compared to the other electrodes both in the
dark and in the light. The enhanced current observed is due to
the formation of heterojunction between the BiVO and ZnO
4
which resulted in better separation of the electron–hole pairs.
This was further conrmed by the photocurrent response with
the application of 1.5 V bias potential (Fig. 3c). The EG–BiVO
4
ꢀ
2
electrode gave a higher photocurrent (0.049 mA cm ) than that
ꢀ2
of EG–ZnO (0.031 mA cm ) and this is due to the fact that
visible light was used to irradiate the electrode and BiVO has
4
higher photoactivity than ZnO in the visible light region due its
lower band gap. Overall the EG–BiVO /ZnO electrode gave the
4
ꢀ2
best photocurrent response of 0.069 mA cm
and this
conrmed that the heterojunction formed between the BiVO
4
and ZnO resulted in increased interfacial charge transfer and
lowered the recombination rate of the charge carriers. More-
over, it is quite interesting to observe that the BiVO
4
, ZnO and
BiVO /ZnO electrodes with EG as the conducting substrate gave
4
higher photocurrent responses than their counterpart elec-
trodes with FTO used as the conducting substrate previously
33,34
reported.
This is mostly due to the fact that EG also
enhanced effective charge separation by acting as a reservoir for
the photogenerated charge carriers. In addition to this, carbon
materials can also serve as dopants for semiconductors gener-
ating internal electric eld contributing to formation of effec-
42–44
tive heterojunction.
3.4 Degradation of rhodamine B
The prepared electrodes were applied in the solar photo-
electrocatalytic (PEC) degradation of rhodamine B dye with
ꢀ1
initial concentration of 10 mg L . The degradation efficiency
was monitored using UV-Visible spectrophotometer at a wave-
length of 554 nm. As shown in Fig. 4a, there was decrease in the
intensity of the peak at 554 nm with increase in the reaction
time revealing the reduction in the concentration of the dye.
Degradation efficiency of 91% was achieved using the EG–
Fig. 3 (a) CVs of EG, EG–BiVO
4
, EG–ZnO, EG–BiVO
4
/ZnO composite
in 0.1 M KCl solution at a scan
rate of 20 mV s ; (b) LSV and (c) photocurrent responses of EG, EG–
BiVO , EG–ZnO and EG–BiVO /ZnO in 0.1 M Na SO
3
ꢀ/4ꢀ
electrodes using 5 mM [Fe(CN)
6
]
ꢀ1
4
4
2
4
.
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Fig. 4 UV-Vis spectra of PEC degradation of rhodamine B (a); normalised concentration decay versus time plot for photocatalytic, electro-
catalytic and photoelectrocatalytic degradation of rhodamine B dye on EG–BiVO /ZnO electrode (b) and corresponding kinetics plots (c);
normalised concentration decay versus time plot for PEC degradation of rhodamine B dye on EG–ZnO, EG–BiVO , EG–BiVO /ZnO electrodes
d); effects of current density (e) and pH (f) on degradation of rhodamine B dye; cycle experiments for the degradation of rhodamine B dye on
EG–BiVO /ZnO electrode (g).
4
4
4
(
4
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4
BiVO /ZnO composite electrode in the photoelectrocatalytic investigated by carrying out repeating experiments (cycles)
degradation process. When the composite electrode was using the same electrode. The electrode was cleaned with
applied in electrochemical (EC) degradation (without the deionized water aer each cycle. As shown in Fig. 4g, the
application of solar light), the degradation efficiency dropped to degradation efficiency dropped by less than 1% aer the third
56% while in photocatalysis (PC), 37% degradation efficiency cycle which implies that the electrode is relative stable and can
was recorded (Fig. 4b). The total organic carbon (TOC) removal be reused for PEC degradation.
obtained were 74%, 46% and 35% using PEC, EC and PC
respectively. This observation revealed that application of
3
.5 Proposed mechanism of degradation and scavenger
biased potential plays a vital role in the degradation process.
Additionally, when the electrode is irradiated with the solar
light, the electrode absorbs photons and electron–hole pairs are
studies
In PEC processes, photogenerated holes, superoxide and
hydroxyl radicals play active role in oxidizing the organic
compounds. The formation and reactions of these reactive
species with rhodamine B molecules are given in eqn (2)–(7).
generated within the BiVO and ZnO. The holes act as oxidants
4
for breaking down the dye molecules and also react with water
to generate hydroxyl radicals which are stronger oxidants
necessary for the rapid oxidation of the dye molecules. The
combined effect of electrical energy and solar energy accounted
for the highest degradation efficiency achieved in the PEC
process. The degradation rate constants for the three processes
+
CBꢀ
)
hv + BiVO
4
/ZnO / BiVO
4
/ZnO (hVB + e
(2)
(3)
(4)
(5)
(6)
(7)
CBꢀ
2
+ O / $O
2ꢀ
e
(PC, EC and PEC) were obtained by tting the data into Lang-
+
+
hVB + H O / $OH + H
2
muir–Hinshelwood pseudo rst order kinetic model equation.
As shown in Fig. 4c, the rate of the PEC process was the fastest
$
OH + rhodamine B / CO
2
+ H
+ H
+ H
2
O
ꢀ1
with apparent rate constant of 0.00921 min when compared
to both PC and EC processes with rate constants of
2ꢀ
$
O
+ rhodamine B / CO
2
2
O
ꢀ
1
ꢀ1
0
.00154 min and 0.00332 min respectively.
+
Among all the electrodes prepared, the bared EG electrode
h
VB + rhodamine B / CO
2
2
O
performed least with 27% degradation efficiency. The EG–BiVO4
electrode performed better (61% efficiency) than the EG–ZnO
To get a clearer picture of the roles of the active species in the
(
52%) electrode because the BiVO has higher photocatalytic PEC degradation of rhodamine B, scavenger study was carried
4
activity in the visible light region due to its lower band gap out. This was done by using ethylenediaminetetraacetate salt
Fig. 4d). The formation of BiVO /ZnO heterojunction was (EDTA), t-butanol (t-BuOH) and p-benzoquinone (p-BZQ) to
deduced through the improved photoelectrocatalytic activity of suppress the effect of holes, hydroxyl radicals and superoxide
(
4
47,48
all the composite electrodes with different ratios of BiVO
4
to radicals respectively.
The results are shown in Fig. 5a. Upon
ZnO. The formation of heterojunction enhanced better charge the addition of EDTA, the degradation efficiency dropped
separation and lowered the rapid recombination rate of elec- drastically to less than 10% which revealed that the photo-
tron–hole pairs resulting in better light harvesting. The EG– generated holes play a major role in the degradation of the
BiVO /ZnO consisting of BiVO and ZnO in ratio 1 : 1 gave the rhodamine B dye. With the addition of t-BuOH and p-BZQ the
4
4
highest degradation efficiency and was therefore used for all degradation efficiencies were 31% and 52% respectively sug-
other studies. gesting that both hydroxyl radicals and superoxide radicals play
The effects of parameters such as current densities and a lesser role in the degradation of the dye. The pronounced role
solution pH on the photoelectrocatalytic process were as of the photogenerated holes in degradation of the dye mole-
studied. The degradation efficiency increased with increased in cules could be attributed to a better separation of the electron–
ꢀ
2
ꢀ2
the applied current densities from 5 mA cm to 15 mA cm
(
4
hole pairs in the BiVO /ZnO heterojunction making more holes
ꢀ2
ꢀ2
Fig. 4e). The increase from 5 mA cm to 10 mA cm (62% to available to oxidize the dye molecules. This observation
9
1%) was more pronounced as compared to the increase provided a glimpse into the band alignment between ZnO and
ꢀ2
ꢀ2
observed from 10 mA cm to 15 mA cm (91% to 95%). This BiVO . Considering the band edge positions of intrinsic BiVO4
4
revealed the dependence of hydroxyl radicals production on the and ZnO, a type I heterojunction can be proposed in which the
applied current densities but when too high current densities charge carriers accumulate onto the BiVO4 and this will not
are applied, it could result in oxygen evolution which hinder the result in proper charge separation as the charge carriers can still
4
generation of hydroxyl radical and thereby limit the degradation recombine in the surface of the BiVO . This was unlikely to
45
efficiency. More so, the application of higher current densities happen since carbons (from EG) act as dopants for both ZnO
is not cost and energy effective. Increase in pH from 3 to 10 and BiVO4 leading to non-negligible internal electric eld.
(
Fig. 4f) resulted in decrease in the degradation efficiency which Therefore, Fermi energy levels (E ) of the two semiconductors
F
conrmed the suitability of acidic medium for degradation of aligned through the movement of electrons from the ZnO
46
rhodamine B dye.
(higher E ) to BiVO (lower E ) and the direction of internal
F 4 F
An ideal photoanode for degradation of organics is expected electric eld (E) was from ZnO to BiVO preventing the move-
4
to be stable and reusable. The stability and reusability of the ment of electrons in the same direction aer the formation of
prepared EG–BiVO /ZnO in PEC degradation process was also heterojunction. On the other hand the direction of the internal
4
16592 | RSC Adv., 2019, 9, 16586–16595
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4 4
Fig. 5 Scavenger studies of the PEC degradation of rhodamine B dye on EG–BiVO /ZnO (a); band alignment between ZnO and BiVO (b).
electric eld favored the movement of holes in the same electrode has a great potential for PEC applications in waste-
direction and hence holes were most efficient separated onto water treatment for the removal of organics.
4
the valence band of BiVO and available for oxidizing the
42
rhodamine B molecules. This deduction is illustrated in
Fig. 5b.
Conflicts of interest
There are no conicts to declare.
4
Conclusion
Acknowledgements
In this work, we have prepared a novel heterojunction photo-
anode of EG–BiVO /ZnO. The formation of BiVO /ZnO hetero-
junction with enhanced separation of charge carriers was
conrmed through the photocurrent response of the EG–BiVO
ZnO composite electrode which was higher than that of both the
EG–BiVO and EG–ZnO. The exfoliated graphite, used as the
conducting substrate, also improved charge separations by
acting as a sink for the charge carriers. When applied for the
photoelectrocatalytic degradation of rhodamine B, a degrada-
tion efficiency of 91% (from visible spectroscopy) and a total
organic carbon removal of 74% were recorded with the EG–
Financial supports from the following institutions in South
Africa are gratefully acknowledged: Faculty of Science, Univer-
sity of Johannesburg; Global Excellence and Stature (GES)
doctoral support, University of Johannesburg; Centre for
Nanomaterials Science Research, University of Johannesburg;
National Research Foundation (CPRR Grant numbers: 98887
and 118546) and Water Research Commission (Grant Number:
K5/2567). B. O. O. is grateful to University of Ilorin, Nigeria for
study leave.
4
4
4
/
4
4
BiVO /ZnO heterostructured photoanode. Scavenger studies
revealed that photogenerated holes played a major role in the
degradation of the dye since the band alignment between the
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