A sol-gel derived, copper-doped, titanium dioxide-reduced graphene oxide nanocomposite electrode for the photoelectrocatalytic reduction of CO2 to methanol and formic acid

Article abstract of DOI:10.1039/c5ra12525a

A classic Cu-RGO-TiO₂ photoelectrocatalyst was fabricated by a facile sol-gel method, deposited on ITO film via electrophoretic deposition and characterized by XRD, FESEM, UV-Vis and FT-IR spectroscopy. A uniformly distributed porous composite film was observed on the ITO substrate with an average particle size of 18 nm. A lower photoluminescence response of the Cu-RGO-TiO₂ sample indicates better electron/hole separation upon irradiation. A maximum 1.31 mA cm^-2 photocurrent density was observed at -0.61 V bias potential under solar simulator irradiation during CO₂ photoelectrocatalysis. Formic acid and methanol were the main products, but longer reaction times led to increased methanol formation. The estimated current efficiency of the production of formic acid and methanol was 32.47%, and the estimated rates of formation of formic acid and methanol were 255 μmol h^-1 cm^-2 and 189.06 μmol h^-1 cm^-2, respectively.

Full text of DOI:10.1039/c5ra12525a

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PAPER
A sol–gel derived, copper-doped, titanium
dioxide–reduced graphene oxide nanocomposite
electrode for the photoelectrocatalytic reduction
of CO2 to methanol and formic acid
a
Cite this: RSC Adv., 2015, 5, 77803
Md. Rakibul Hasan, Sharifah Bee Abd Hamid,a Wan Jeffrey Basirun,b
*
Syazwan Hanani Meriam Suhaimya and Ahmad Nazeer Che Matb
A classic Cu-RGO–TiO2 photoelectrocatalyst was fabricated by a facile sol–gel method, deposited on ITO
film via electrophoretic deposition and characterized by XRD, FESEM, UV-Vis and FT-IR spectroscopy. A
uniformly distributed porous composite film was observed on the ITO substrate with an average particle
size of 18 nm. A lower photoluminescence response of the Cu-RGO–TiO2 sample indicates better
electron/hole separation upon irradiation. A maximum 1.31 mA cmꢀ2 photocurrent density was observed
at ꢀ0.61 V bias potential under solar simulator irradiation during CO2 photoelectrocatalysis. Formic acid
and methanol were the main products, but longer reaction times led to increased methanol formation.
The estimated current efficiency of the production of formic acid and methanol was 32.47%, and the
estimated rates of formation of formic acid and methanol were 255 mmol hꢀ1 cmꢀ2 and 189.06 mmol hꢀ1
cmꢀ2, respectively.
Received 29th June 2015
Accepted 7th September 2015
DOI: 10.1039/c5ra12525a
www.rsc.org/advances
active photocatalyst or photoelectrocatalysis that can facilitate
high rates of conversion while drastically decreasing the
necessary energy input.6
1. Introduction
The recycling of greenhouse gases (i.e., CO2 and CH4) offers
long term solutions for energy and environmental related
problems.1 The presence of excess carbon dioxide (CO2) in the
earth's atmosphere has a great negative impact which is the
warming of the planet. Various methods have been developed to
efficiently convert and recycle CO2. Renewable energy can be
utilized in the CO2 recycling process because it is a valuable raw
material for the production of combustible chemicals such as
formic acid, methanol and methane.2,3 CO2 photoconversion is
challenging for several reasons: inefficient absorption of visible
light by present day photocatalysts, low quantum efficiency due
to rapid eꢀ/h+ recombination upon irradiation, facile backward
redox reactions that lead to product dissociation and lack of
product selectivity. These disadvantages are the main obstacles
for the commercial success of CO2 recycling.4
The earliest report of CO2 photoconversion was published by
Halmann in 1978.7 Honda and Fujishima rst reported the use
of titanium dioxide in the photoelectrolysis of water, but did not
report the photoreduction of CO2 with titanium until 1979.8
Since that time, TiO2 has remained the most preferred photo-
catalyst and photoelectrocatalyst for CO2 reduction. TiO2
nanoparticles doped with N, Pt, Ag and Cu, among others,
exhibited signicant CO2 photoconversion.9–11 The wide band
gap (i.e., 3.2 eV for the anatase phase) of TiO2 is favorable due to
its strong redox ability and high resistance to photocorrosion.
Metal doping of TiO2 could decrease the recombination process
because the metal centers function as electron traps. Moreover,
due to the work function difference, a Schottky barrier appears
in the bimetallic TiO2 structure. This phenomenon also helps to
decrease the recombination of charge carriers.9 TiO2 structures
incorporated with crystalline carbon structures, such as gra-
phene, reduced graphene oxide (RGO) and carbon nanotubes,
exhibit high catalytic efficiencies.11–13 Graphene–TiO2
composite materials have been described as future generation
photocatalysts due to the low toxicity and extended photo-
activity.14,15 Carbon structures can also promote simple reaction
mechanisms during catalysis, which result in higher selectivity
and desired yields.16 Thus, further modication of the TiO2
nanostructures can enhance the overall photoconversion of
CO2.
However, current markets for CO2-based products are
developing, rather than considering it a waste chemical. Pho-
toelectrocatalytic reduction of CO2 is able meet this high energy
conversion demand by utilizing free and abundant solar
energy.5 Successful conversion of CO2 depends on a highly
aInstitute of Nanotechnology
& Catalysis Research (NanoCat), Institute of
Postgraduate Studies (IPS), University of Malaya, 3rd Floor, Block A, 50603 Kuala
Lumpur, Malaysia. E-mail: rakibacctdu@gmail.com; sharifahbee@um.edu.my;
hanani_ms@yahoo.com.my
bDepartment of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala
Lumpur, Malaysia. E-mail: jeff@um.edu.my; hmd_naz@yahoo.com
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Reduced graphene oxide (RGO) as a catalyst promoter with
TiO2 was reported in our previous work.17 Composite photo-
catalysts consisting of several elements are promising mate-
rials for visible light activity which enhance the kinetic
properties of the desired reactions.18 Mohamed et al. reported
RGO–TiO2 composites for the photocatalytic reduction of CO2
to methane gas.19 They reported that the incorporation of the
RGO into the composite increased the photocatalytic perfor-
mance of TiO2 compared to pure TiO2 by decreasing the TiO2
band-gap into the visible region and prolonging the recom-
bination time of the charge carriers.19 On the other hand, Cu is
a representative d-block element with attractive catalytic
properties for CO2 electroreduction process. Copper elec-
trodes have shown great success in hydrocarbon gas produc-
tion such as hydrogen (H2), methane (CH4) etc. Moreover, the
Cu atoms can act as a channel for the dispersion of photo-
electrons, which suppress the eꢀ/h+ recombination in the
photoelectrocatalysis process. Hence, a Cu-RGO–TiO2 nano-
composite photoelectrode was conceived and designed to
convert CO2 in aqueous methyl diethanolamine (MDEA)
solution as the CO2 absorption electrolyte.
2.3 Preparation of the Cu-RGO–TiO2 nanocomposite
To prepare the TiO2 nanocomposite, 20 mL of Ti(OBu)4 (TBOT)
was diluted in absolute ethanol and stirred continuously. Then,
6 mL of ultrapure water, 6 mL of ethanol, 1 mol% Cu(NO3)2-
$5H2O and 1 wt% RGO powder were mixed and sonicated for 1.5
hours. This mixture was added to the TBOT solution drop-wise
for 30 minutes. Acetic acid was added to lower the pH of the
solution to less than 3. The total mixture was vigorously stirred
for 30 minutes to obtain a homogeneous solution. The dark
colored solution was then stored at room temperature for 48
hours to complete the gel formation and was dried at 80 ꢁC
overnight. The dry porous gel was milled using a steel mortar
and pestle. The ne powder catalyst was then calcined at 550 ꢁC
for 4 hours.
2.4 Fabrication of Cu-RGO–TiO2/ITO photoanodes
The Cu-RGO–TiO2 was deposited on the ITO substrate by elec-
trophoretic deposition in a two-electrode cell. The prepared
composite powder was dispersed in DI water with a concen-
tration of 0.01 mg mLꢀ1. The pH of the solution was maintained
by determining the zeta potential value. The composite powder
mixture was sonicated in DI water to ensure a homogeneous
dispersion before the electrophoretic deposition process.
2. Experimental methods
2.1 Materials
2.5 Photoelectrochemical measurements
Graphite akes with 7–10 mm nominal size were procured from
Alfa Aesar. Tetra-n-butyl orthotitanate [Ti(OBu)4] was procured
from Sigma-Aldrich, and all other chemicals used in this work
were of analytical grade. The indium tin oxide (ITO)-coated
conducting glass plates (0.7 mm thickness) were procured
from Osaka, Japan. Deionized water (18.2 MU cm) used in these
experiments was from a Milli-Q system (Millipore, Bedford, MA;
Mega-Pure System, Model MP-290). The carbon dioxide gas had
99.9% purity.
All photoelectrochemical measurements were performed in a
cubic quartz photoelectrochemical cell using a potentiostat/
galvanostat (Autolab PGSTAT30, Ecochemie, Netherlands). A
standard three-electrode system was used, where the photo-
anode with an area of 1 cm2 was the working electrode (WE). A
platinum (Pt) wire and a saturated calomel electrode (SCE) were
the counter (CE) and reference (RE) electrodes, respectively. The
electrolyte in these experiments was 0.1 M Na2SO4. The poten-
tial in the voltammetric measurements was swept from 0.0 V to
ꢀ1.5 V at 50 mV sꢀ1. Electrochemical impedance spectroscopy
(EIS) was measured at the open circuit potential (OCP) both in
dark and under solar irradiation. The EIS experimental data
were simulated using analog circuits with NOVA 1.10 soware
on a computer interfaced with the potentiostat. The frequency
range was from 10ꢀ1 Hz to 105 Hz with an acquisition of 10
points per decade and an amplitude of 5 mV around the OCP.
2.2 Preparation of reduced graphene oxide (RGO)
Graphene oxide (GO) was prepared by the modied Hummer's
method.20 Initially, 1 g of graphite akes were mixed with
sulfuric acid and phosphoric acid (120 mL : 13 mL). Then, 6 g
of KMnO4 was added gradually, and the mixture was stirred
continuously for 3 days. This was followed by the addition of 7
mL of H2O2 solution (30%) and 135 mL of ice water until the
mixture turned brown in color. The mixture was then centri-
fuged at 4000 rpm for 10 minutes. Aer centrifugation, the GO
suspension was washed with 1 M HCl with sonication. The GO
suspension was then dried in an oven and later subjected to
hydrazine treatment to obtain the reduced graphene oxide
(RGO).21 The GO powder (300 mg) was then added into 600 mL
of ultrapure Milli-Q water and sonicated for 15 minutes. This
was followed by the addition of 3 mL of hydrazine hydrate and
the solution was stirred vigorously at 90 ꢁC for 6 hours,
precipitating the RGO as a black solid.21 Finally the solution
was ltered and washed with ethanol and water several times
to obtain the RGO powder. The powder was dried in an oven at
ꢁ
80 C.
Fig. 1 Quartz cell photoelectrocatalytic (PEC) reactor.
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The irradiation intensity was ꢂ10 mW cmꢀ2. Prior to the
reaction, CO2 was bubbled for 1 hour to saturate the solution,
and the nal pH of the CO2-saturated solution was 7.6. The
liquid phase product was analyzed by HPLC, and only the peaks
for formic acid and methanol were considered.
2.7 Instrumentation
A scanning electron microscope (SEM, Quanta FEI 200) was used
to study the morphology of the as-prepared composite samples.
X-ray diffraction (XRD) was performed on a powder X-ray
diffractometer (Bruker D8 Advance equipped with EVA diffract
soware, Germany) over a range of 10ꢁ # 2q # 80ꢁ, at 40 kV and
˚
30 mA with Cu Ka radiation (k ¼ 1.5418 A). UV-Vis diffuse
reectance spectra were obtained on a Lambda 35 series equip-
ped spectrophotometer (Perkin Elmer) with a thin lm slot.
Fourier transformed infrared spectroscopy (FTIR) was recorded
by a Bruker IFS 66 V/S. The light source, a 150 W (Xe arc lamp)
solar simulator, was from OSRAM photo optic (Germany). It had
a consistent spectrum prole of AM 1.5G (i.e., similar charac-
teristic features to a 1.5 global radiation solar light under stan-
dard air conditions). X-ray photoelectron spectroscopy (XPS,
Thermo Scientic K-alpha instrument) with an unmonochrom-
atized Mg Ka radiation (photon energy 1253.6 eV) source,
vacuum less than 10ꢀ9 Torr and spectral resolution of 0.1 eV,
were carried out on at gold (Si/10 nm Ti/200 nm Au) as the
Fig. 2 XRD of TiO2, RGO–TiO2 and Cu-RGO–TiO2.
2.6 Constant potential photoelectrolysis
The photoelectrocatalytic conversion of CO2 was performed in
the quartz cell containing aqueous 10% methyl diethanolamine
(MDEA). Fig. 1 shows a schematic diagram of the photo-
electrocatalytic reduction process with a three-electrode system,
where the CE and WE had the same surface area.
Fig. 3 XPS spectra of Cu-doped TiO2 nanoparticles: (a) Ti 2p, (b) O 1s and (c) Cu 2p.
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substrate and reference. The XPS core levels were aligned to the
Au 4f 7/2 binding energy (BE) of 84 eV.
3. Results and discussions
3.1 Fabrication and characterization of the Cu-RGO–TiO2/
ITO composite electrode
The catalyst powder was characterized by crystallographic,
imaging and optical techniques. Fig. 2 shows the XRD pattern of
the prepared catalyst. From the JCPDS ref. code 01-086-1157, the
characteristic peaks at 25.3 (101), 37.9 (103), 48.1 (200), 53.9
(105) and 75.1 (215) are attributed to the titanium tetragonal
system of the anatase phase.
ꢁ
The calcination temperature of 550 C played an important
role in the production of the crystalline nanocomposites in the
TiO2 anatase phase. The phase change from anatase to rutile was
absent during the calcinations process. The Cu diffraction peaks
at 2q ¼ 43ꢁ and 45ꢁ and Cu2O peaks at 36ꢁ were not observed due
to the low concentration and incorporation in the TiO2 lattice
structure. Fig. 2 shows the overlap of the RGO characteristic peak
at 25.8ꢁ with the TiO2 (101) plane. Cu could be incorporated into
the TiO2 lattice due to the similarity of the Cu and Ti atomic radii
(132 pm and 160 pm, respectively). Furthermore, the peaks
attributed to Cu or Cu2O were not observed. The average crys-
tallite size is given by the Scherrer's formula (eqn (1)):
Kl
D ¼
;
(1)
b cos q
where, D ¼ crystallite size, l ¼ wavelength of the X-ray, b ¼ full
width at half maximum of the peak (in radians) and q ¼ angle of
diffraction (in degrees). The average crystallite size of the Cu-
RGO–TiO2 composites (with respect to the anatase (101) peak) is
between 15–28 nm. Thus, the doping of RGO did not introduce
any crystalline phase changes, and we can conclude that RGO
had no effect on crystallite size.
Fig. 3 shows the XPS spectra of the Cu-doped TiO2 nano-
particles. The electron-binding energy (BE) of the Ti 2p photo-
electron peak (Fig. 3a) illustrates the existence of Ti4+ in the
TiO2 nanostructures at 458.2 eV. The oxygen peak at 529.5 eV
(Fig. 3b) conrms the formation of the metal–oxygen bond,
while the Cu 2p spectrum indicates the co-formation of Cu2+
and Cu+ in the structure (Fig. 3c). The binding energy of 932.2
eV is a characteristic of Cu2O. The shake-up satellite peaks at
940.7 and 943.3 eV (green dashed lines) are an indication of
Cu(II) formation.22,23 The existence of the shoulder (934.2 eV) in
the copper spectrum also indicates the presence of Cu+ in the
structure. The deconvoluted peaks in the copper spectrum
indicate that 56% of the copper exists as Cu(I).
Fig. 4 shows the FESEM images of the thin lm electrodes.
The TiO2 nanoparticles are quite homogeneous with sizes
between 30–45 nm. The RGO–TiO2 composites are also homo-
geneous, but more porous in the thin lm form. The particle
size varies within the same range as the TiO2 nanoparticles, but
the RGO–TiO2 nanocomposites could be clearly distinguished
from the single TiO2 nanoparticles.
Fig. 4 FESEM images of (a) TiO2, (b) RGO–TiO2 and (c) Cu-RGO–TiO2.
size is larger than the TiO2 and RGO–TiO2 nanoparticles. This
morphological change could be due to the formation of grain
boundaries at high calcination temperatures. Moreover, RGO
has a tendency to aggregate into larger particles because of the
weak van der Waals interactions.24 The RGO layers could not be
clearly seen in the FESEM micrographs, which could be due to
the charging effect of the electron-rich RGO.17,25
Fig. 5 shows the HRTEM image of Cu-RGO–TiO2. The esti-
mated particle sizes (Fig. 5) are in good agreement with the XRD
and SEM results. The relative d-spacing values resulted in
successful Cu doping into the TiO2 lattice. The Cu-RGO–TiO2
lm showed a slight red-shi in the absorbance edge and a
A different morphology was observed for the Cu-RGO–TiO2
thin lm. The lm bed is dense and packed, and the particle
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Fig. 5 TEM image of Cu-RGO–TiO2.
signicant enhancement of light absorption between 400–800
nm. Fig. 6 shows the corresponding UV-Vis diffuse reectance
spectra of the fabricated thin lms. The doping of Cu into the
TiO2 lattice creates intermediate sites for the transfer of pho-
togenerated electrons, hence facilitates the electron–hole pair
separation.26
Fig. 6 UV-Vis diffuse reflectance spectra of the TiO2, RGO–TiO2 and
Cu-RGO–TiO2 photocatalysts (a) and the plot of ahn1/2 vs. hn (b).
The incorporation of RGO, on the other hand, facilitates
electron transfer and also increases the visible absorption of
TiO2. Thus, the doping of TiO2 with Cu and incorporation of
RGO are effective for enhanced photocatalytic activity. The
optical band-gap was determined from the Kubelka–Munk
equation as follows:
ahn ¼ A(hn ꢀ Eg)n
(2)
where, a ¼ absorption coefficient, A ¼ constant (ꢂ1), v ¼
frequency of light (3 ꢃ 108 msꢀ1), h ¼ Plank's constant (4.136 ꢃ
10ꢀ15 eV), Eg ¼ band gap energy, n ¼ 2 for indirect allowed
transitions.
From eqn (2) and the plot in 6(b), the Eg values of TiO2, RGO–
TiO2 and Cu-RGO–TiO2 are 3.23, 3.10 and 2.98, respectively. The
lower band-gap of Cu-RGO–TiO2 can be attributed to the
formation of Ti–Cu–O, which lowers the conduction band level
of TiO2. The rapid recombination of the photogenerated elec-
trons and holes emits photoluminescence (PL). A lower PL
emission intensity indicates a decrease in the radiative recom-
bination and better separation of the excitons.27 Fig. 7 compares
the PL spectra of the prepared samples. Broad peaks were
Fig. 7 Photoluminescence spectra of TiO2, RGO–TiO2 and Cu-RGO–
TiO2.
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Table 1 The charge-transfer resistance (RCT) of the TiO2, RGO–TiO2
and Cu-RGO–TiO2 photoelectrodes from simulation of the EIS results
Samples
Irradiation condition
Rct/U cm2
TiO2
Dark
Solar simulator
Dark
Solar simulator
Dark
Solar simulator
891.6
832.7
488.3
465.1
536.9
216.8
RGO–TiO2
Cu-RGO–TiO2
Ti stretching vibrational peak is observed below 1000 cmꢀ1, and
the broader peak in the 2400–3400 cmꢀ1 range is attributed to
the H-bonding between the –OH groups present on the TiO2
surface.28 A smaller peak at 2358 cmꢀ1 could be due to the
atmospheric CO2 molecules adsorbed on the TiO2 surface.
The most critical drawback in photoelectrocatalysis is the
recombination process.29 EIS is a useful tool to investigate
the charge transfer and recombination processes at the
semiconductor/electrolyte interface.30 The EIS responses of
TiO2/ITO, RGO–TiO2/ITO and Cu-RGO–TiO2/ITO in the dark
and under illumination are shown in the Nyquist plots (Fig. 9a).
The impedance values of the photocatalysts were measured at
the open circuit potential in dark and illuminated conditions.
The Cu-RGO–TiO2 thin lm showed lower resistance than TiO2
and RGO–TiO2 under both conditions. Upon irradiation, the
semicircle diameter decreases, and the charge transfer resistance
(RCT), which is the resistance of electron transfer process across
the electrode/electrolyte interface, also decreases.31 The inset of
Fig. 9a shows the equivalent circuit model across the electrode/
electrolyte interface, where a Warburg diffusion model is
proposed. Here, Re refers to the bulk resistance between the WE
and RE at the high frequency intercept of the semicircle with
respect to the real axis. The interfacial resistance is represented
by RCT in the equivalent circuit model.32 Fig. 9b shows the
equivalent circuit model which represents the electrode–electro-
lyte interface. Table 1 gives the RCT values for all three photo-
electrodes in both dark and irradiated conditions.
Fig. 8 FTIR spectra of TiO2, RGO–TiO2 and Cu-RGO–TiO2.
observed in the TiO2 and RGO–TiO2 samples between 500–700
nm. This indicates direct emission aer excitation and poor
separation of the photogenerated electrons and holes. The
lower emission intensity of the Cu-RGO–TiO2 sample could be
due to the lower radiative combination of excitons, which
indicates a slower recombination process.
The inclusion of Cu nanoparticles generates considerable
uorescence quenching indicative of efficient electron transfer
from TiO2 to Cu, due to the favorable electrical conduction
between them. Under light irradiation, the excited electrons will
follow the Cu-assisted path towards the RGO structure, which
eventually decreases the direct recombination of the charge
carriers.
The FTIR spectra of the prepared catalyst samples are shown
in Fig. 8. The prominent absorption bands are observed at 666,
1551 and 1626 cmꢀ1. In the RGO spectrum, most of the C]O
bonds were eliminated, and the peak at 1720 cmꢀ1 was absent,
which conrms a well-reduced RGO structure. The peak at 1551
cmꢀ1 is attributed to the graphene skeletal vibration. The Ti–O–
Fig. 9 Nyquist plots of the TiO2, RGO–TiO2 and Cu-RGO–TiO2 film electrodes at open circuit potentials both in the dark and under visible
irradiation (9a) and conceptualization of the equivalent circuit model at the electrode–electrolyte interface (9b).
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octahedral titania structure. But the charge transfer resistance
decreases signicantly (ꢂ217 U cm2) upon irradiation. This can
be attributed to the electron channeling in the Ti–Cu–C structure
which decreases the recombination of charge carriers and
increases the photocurrent density. In fact, this proposed struc-
ture may have the rectifying characteristics due to the formation
of Schottky barriers in the metal–semiconductor junction.33
To investigate the effect of an applied bias on the modied
TiO2 photoelectrode, a Mott–Schottky plot was obtained at
room temperature. The at-band potential can be determined
from the equation:
ꢀ
ꢁ
1
2
kT
q
¼
VB ꢀ Vfb
ꢀ
(3)
CS2C 330qND
where, CSC ¼ space charge capacitance, V ¼ at band
potential, ND ¼ charge carrier conc. VB ¼ applied potential, 3 is
the dielectric constant of the semiconductor and 30 is the
vacuum permittivity. Thus, the impedance measurement
results can be plotted according to eqn (3) and are shown in
Fig. 10. The estimated at-band potential and the
Fig. 10 Mott–Schottky plot for Cu-RGO–TiO2/ITO.
It can be observed that the Cu-RGO–TiO2 electrode shows
higher resistance (536.9 U cm2) in the dark. This is probably due
to the incorporation of Cu and corresponding defects in regular
Fig. 12 Cu-RGO–TiO2 photoelectrode: (a) dependence of the
Fig. 11 Voltammetry in 10% MDEA aqueous solution: (a) TiO2, RGO– photocurrent on reaction time in the photoelectrocatalytic reduction
TiO2 and Cu-RGO–TiO2 electrodes under illumination and (b) the Cu- of CO2 and (b) the photocurrent density profile for a six-hour reaction
RGO–TiO2 electrode in the dark and under illumination.
time.
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Fig. 13 HPLC detection of (a) methanol and (b) formic acid, and (c) the yield of formic acid and methanol with time.
corresponding donor density are ꢀ0.76V and 5.76 ꢃ 1015
Cu-RGO–TiO2 gives a maximum photocurrent of 1.31 mA
cmꢀ2 at ꢀ0.61 V vs. SCE. Fig. 11b shows the effect of light and
cmꢀ3, respectively.
It should be noted that the onset potential for the anodic dark conditions of the Cu-RGO–TiO2 photoelectrode in the CO2-
current is ꢀ0.82 V, and is not necessarily equal to the at band saturated solution. The photocurrent density was unsteady, and
potential. In fact, the at band potential is a region where the uctuated during the reaction (Fig. 12a). In fact, CO2 reduction
recombination process tends to suppress the threshold of the process is a multi-electron transfer process and it is difficult to
photocurrent. Some factors i.e. crystallographic structure of TiO2, maintain the selectivity of a single product. Moreover, the
doping, thin lm properties etc. play a vital role in determining competition between different electroactive species present in
the at band region. Hence the applied bias was chosen a little the electrolyte such as CO2 and H+ decreases the selectivity of the
higher (ꢀ0.61 V) than the estimated at band potential to overall reaction. The interaction and catalytic effect of Cu atoms
maintain the continuous ow of anodic photocurrent.
towards CO2 in the electro-reduction process has been reported
in earlier reports.34 But pristine TiO2 are not reactive with CO2
molecules. This is probably due to the difficulties of the angular
COc2 radicals to come in contact with the regular TiO2 structure.35
However, the initial photocurrent density is 4.56 mA cmꢀ2 but
decreases to 0.63 mA cmꢀ2 at the end of the reaction. The slow
diffusion process of CO2 in the electrolyte may be responsible for
the sudden decrease of photocurrent density and this can be
understood from the impedance measurement Nyquist plots
(Fig. 9a). This is evident by the appearance of the onset of a
Warburg element at lower frequencies which denotes a diffusion
3.2 Photoelectrocatalytic reduction of carbon dioxide into
formic acid and methanol
Fig. 11 shows the voltammetric response of the catalyst lms
under illumination by the solar simulator. Prior to this, CO2 gas
was bubbled in the 10% MDEA solution for 1 hour to saturate
the solution. No signicant photocurrent was observed at the
TiO2/ITO and RGO–TiO2/ITO electrodes. Moreover, blank
experiments (without CO2) were also carried out on all three
electrodes and no photocurrent was observed in the process.
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Table 2 Comparison of photocatalytic and photoelectrocatalytic reduction of CO2 to methanol and formic acid on TiO2 composites in the
literature
No Electrode material/light source/solution
Catalytic method
Photocatalytic
Products
Methanol
Highest yield
Ref.
1
Cu doped TiO2, 8 W Hg lamp UVC (254 nm), CO2
saturated 0.2 M NaOH
19.7 mmol gꢀ1 hꢀ1
450 mmol gꢀ1 hꢀ1
37 and 38
2
3
Cu doped TiO2, 10 W UV lamp, CO2 saturated 1 M KHCO3 Photocatalytic
Methanol
Methanol
39
Cu(I)/TiO2/365 nm, 16 W cmꢀ2 UV, 1.2 wt%-Cu/TiO2
catalyst at 1.29 bar of CO2 saturated pure water
CdSe quantum dot (QD)-sensitized TiO2/visible light >
420 nm, CO2 saturated pure water. 300 W Xe lamp
Cu–TiO2 on molecular sieve 5A/UV light, CO2 saturated
\0.2 M NaOH. 250 W Hg lamp
Photocatalytic
Photocatalytic
Photocatalytic
Photocatalytic
40 and 41
4
5
6
7
8
Methanol
Methanol
3.3 ppm gꢀ1 hꢀ1
18
42
43
10
0.78 mg hꢀ1 gꢀ1
Pd nanoparticles bismuth titanate, 0.1 CO2 saturated
H2SO4, solar simulator 300 W, 1.5 AM
Formic acid 110–160 mMol hꢀ1 gꢀ1
Nitrogen doped TiO2. Cu counter electrode, CO2 bubbled
KHCO3 electrolyte, 2V vs. SCE. 100 W Xe lamp
(CdS) and (Bi2S3) on TiO2 nanotube. 500 W Xe lamp,
wavelength less than 400 nm, CO2 saturated 0.1 M
NaOH, 0.1 M NaSO3
Photo-electrocatalytic Methanol
formic acid lower than 8%
Methanol
Faradaic efficiency
Photocatalytic
44.9 mmol hꢀ1 cmꢀ2 (Bi2S3) 44
31.9 mmol hꢀ1 cmꢀ2 (CdS)
9
Cu–TiO2–RGO, 0.1 M Na2SO4, CO2 saturated 10% Methyl
diethanolamine (MDEA), ꢀ0.61 V vs. SCE. 150 W
Xe arc lamp, 1.5 AM
Photo-electrocatalytic Methanol
255 mMol hꢀ1 cmꢀ2
This work
formic acid 189.06 mmol hꢀ1 cmꢀ2
ꢂ
ꢃ
limiting process for the Cu-RGO–TiO2 electrode upon irradiation
(Fig. 9a). A full photocurrent prole for a six-hour reaction period
is shown in Fig. 12b.
ðICÞ ꢀ ðICÞ
0
t
Current efficiency ¼
ꢃ FV 100 %AM1:5G
(4)
8 I Dt
where (IC)0 and (IC)t are the total inorganic carbon (g Lꢀ1) at
times 0 and t, respectively, I is the current (A), F is the Faraday
constant (26.8 Ah), V is the volume (L) and t is the time of
treatment (h). Thus, the estimated current efficiency for the
process is 32.47%. Table 2 compares the literature results for
the reduction of CO2 to methanol and formic acid using TiO2
3.3 Product analysis and efficiency
The products from the photoelectrolysis experiments were
collected at one hour intervals. The saturated CO2 solution in
10% MDEA contained an estimated concentration of 6950 ppm
(ꢂ0.16 M) dissolved CO2. Aer a six-hour period, the solution photoelectrocatalysis and photocatalysis.
was analyzed, and the nal concentrations of formic acid
The photo-oxidation of Cu-RGO–TiO2 releases electrons and
(Fig. 13a) and methanol (Fig. 13b) were 98 ppm and 242 ppm, produces reactive holes, which react with water to release
oxygen:45
Initial oxidation:
respectively. Aer 2 hours of reaction, the maximum formic acid
concentration was 157 ppm, which decreased further (Fig. 13).
This could be due to the reduction of formic acid to meth-
anol during the course of the reaction. The current efficiency
was determined using eqn (4):36
4 h+ + 2H2O / O2 + 4H+
Initial reduction:
eꢀ + H+ / Hc
and
eꢀ + CO2/ cCO2
Subsequent reactions:
cCO2 + 2Hc + h+ / HCOOH
cCO2 + 6Hc + h+ / CH3OH
A simple mechanism is shown in Fig. 14.
Fig. 14 CO2 photoelectrocatalysis mechanism in aqueous media.
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15 K. Zhou, Y. Zhu, X. Yang, X. Jiang and C. Li, New J. Chem.,
2011, 35, 353–359.
4. Conclusions
16 L. Yuan, Q. Yu, Y. Zhang and Y. J. Xu, RSC Adv., 2014, 4,
15264–15270.
17 M. R. Hasan, S. B. A. Hamid, W. J. Basirun, Z. Z. Chowdhury,
A. E. Kandjani and S. K. Bhargava, New J. Chem., 2014, 39,
69–76.
18 C. Wang, R. L. Thompson, J. Baltrus, C. Matranga and
J. Phys, Chem. Lett., 2010, 1, 48–53.
19 L. L. Tan, W. J. Ong, S. P. Chai and A. R. Mohamed, Nanoscale
Res. Lett., 2013, 8, 465.
20 D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii,
Z. Sun, A. Slesarev, L. B. Alemany, W. Lu and J. M. Tour,
ACS Nano, 2010, 4, 4806–4814.
21 S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas,
A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and
R. S. Ruoff, Carbon, 2007, 45, 1558–1565.
22 M. A. Mahmoud, W. Qian and M. A. El-Sayed, Nano Lett.,
2011, 11, 3285–3289.
23 N. Pauly, S. Tougaard and F. Yubero, Surf. Sci., 2014, 620, 17–
22.
Crystalline Cu-RGO–TiO2 nanoparticles were prepared by sol–
gel synthesis and deposited onto an ITO glass substrate via
electrophoretic deposition. XRD and XPS analysis conrmed
the incorporation of Cu dopant atoms into the TiO2 lattice.
Photo-absorption was observed in the visible region, and the
band gap was 2.98 eV. The slow recombination of the photo-
generated electrons and holes was conrmed by PL spectra.
The composite WE was stable for more than 6 hours of reaction
time. Formic acid (HCOOH) and methanol (CH3OH) were the
main products from the reduction of CO2 with H2O (or proton
solvents) in MDEA solution, with rates of 255 mmol hꢀ1 cmꢀ2
and 189.06 mmol hꢀ1 cmꢀ2, respectively. HCOOH was initially
produced (maximum concentration of 157 ppm), but it later
became an intermediate for the formation of methanol and
higher hydrocarbons. Cu doping directs the product selectivity
towards HCOOH and CH3OH and promotes the RGO–TiO2
catalyst to produce larger amounts of CH3OH (maximum 242
ppm) when the reaction time is more than 3 hours.
24 M. Shi, J. Shen, H. Ma, Z. Li, X. Lu, N. Li and M. Ye, Colloids
Surf., A, 2012, 405, 30–37.
25 M. R. Hasan, S. B. A. Hamid and W. J. Basirun, Appl. Surf.
Sci., 2015, 339, 22–27.
26 N. R. Khalid, E. Ahmed, Z. Hong, M. Ahmad, Y. Zhang and
S. Khalid, Ceram. Int., 2013, 39, 7107–7113.
27 J. Shi, J. Chen, Z. Feng, T. Chen, Y. Lian, X. Wang and C. Li, J.
Phys. Chem. C, 2007, 111, 693–699.
Acknowledgements
The authors would like to thank the University of Malaya for
funding this work with High Impact Research (HIR-F-000032),
RP005B 13AET and FP033 2013A research grants for their
cordial support.
28 J. Shen, B. Yan, M. Shi, H. Ma, N. Li and M. Ye, J. Mater.
Chem., 2011, 21, 3415–3421.
References
1 A. Goeppert, M. Czaun, J. P. Jones, G. K. S. Prakash and 29 X. Chen and S. S. Mao, Chem. Rev., 2007, 107, 2891–2959.
G. A. Olah, Chem. Soc. Rev., 2014, 43, 7957–8194.
30 F. F. Santiago, E. M. Barea, J. Bisquert, G. K. Mor, K. Shankar
2 W. N. Wang, Aerosol Air Qual. Res., 2014, 14, 533–549.
and C. A. Grimes, J. Am. Chem. Soc., 2008, 130, 11312–11316.
3 T. Yui, A. Kan, C. Saitoh, K. Koike, T. Ibusuki and O. Ishitani, 31 W. H. Leng, Z. Zhang, J. Q. Zhang and C. N. Cao, J. Phys.
ACS Appl. Mater. Interfaces, 2011, 3, 2594–2600.
Chem. B, 2005, 109, 15008–15023.
4 A. Dhakshinamoorthy, S. Navalon, A. Corma and H. Garcia, 32 X. I. He, Y. Y. Cai, H. M. Zhang and C. H. Liang, J. Mater.
Energy Environ. Sci., 2012, 5, 9217–9233. Chem., 2011, 21, 475–480.
5 Q. Zhai, S. Xie, W. Fan, Q. Zhang, Y. Wang, W. Deng and 33 Z. Zhang and J. T. Yates Jr., Chem. Rev., 2012, 112(10), 5520–
Y. Wang, Angew. Chem., Int. Ed., 2013, 52, 5776–5779. 5551.
6 Z. Jiang, T. Xiao, V. L. Kuznetsov and P. P. Edwards, Philos. 34 Y. Hori, Modern Aspects of Electrochemistry, ed. C. Vayenas et
Trans. R. Soc., A, 2010, 368, 3343–3364.
7 M. Halmann, Nature, 1978, 275, 115–116.
8 T. Inoue, A. Fujishima, S. Konishi and K. Honda, Nature,
1979, 277, 637–638.
9 Y. Wang, Q. Lai, F. Zhang, X. Shen, M. Fan, Y. He and S. Ren,
RSC Adv., 2014, 4, 44442–44451.
10 Y. P. Peng, Y. T. Yeh, S. I. Shah and C. P. Huang, Appl. Catal.,
B, 2012, 123–124, 414–423.
11 Y. T. Liang, B. K. Vijayan, K. A. Gray and M. C. Hersam, Nano
Lett., 2011, 11, 2865–2870.
12 Z. Li, B. Gao, G. Z. Chen, R. Mokaya, S. Sotiropoulos and
G. L. Puma, Appl. Catal., B, 2011, 110, 50–57.
13 J. H. Yun, R. J. Wong, Y. H. Ng, A. Du and R. Amal, RSC Adv.,
2012, 2, 8164–8171.
14 C. Chen, W. Cai, M. Long, B. Zhou, Y. Wu, D. Wu and
Y. Feng, ACS Nano, 2010, 4(11), 6425–6432.
al., Springer, New York, 2008, vol. 42, pp. 89–189.
35 V. P. Indrakanti, J. D. Kubicki and H. H. Schobert, Fuel
Process. Technol., 2011, 92, 805–811.
36 T. T. Guaraldo, S. H. Pulcinelli and M. V. B. Zanoni, J.
Photochem. Photobiol., A, 2011, 217, 259–266.
37 I. H. Tseng, W. C. Chang and J. C. S. Wu, Appl. Catal., B, 2002,
37, 37–48.
38 I. H. Tseng, J. C. S. Wu and H. Y. Chou, J. Catal., 2004, 221,
432–440.
39 H. W. N. Slamet, E. Purnama, S. Kosela and J. Gunlazuardi,
Catal. Commun., 2005, 6, 313–319.
40 J. C. S. Wu, H. M. Lin and C. L. Lai, Appl. Catal., A, 2005, 296,
194–200.
41 J. C. S. Wu and H. M. Lin, Int. J. Photoenergy, 2005, 7, 115–
119.
77812 | RSC Adv., 2015, 5, 77803–77813
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
42 B. Srinivas, B. Shubhamangala, K. Lalitha, P. A. K. Reddy, 44 X. Li, H. Liu, D. Luo, J. Li, Y. Huang, H. Li, Y. Fang, Y. Xu and
V. D. Kumari, M. Subrahmanyam and B. R. De, Photochem.
Photobiol., 2011, 87, 995–1001.
L. Zhu, Chem. Eng. J., 2012, 180, 151–158.
45 S. Ahmed, M. A. Mansoor, W. J. Basirun, M. Sookhakian,
¨
43 K. S. Raja, Y. R. Smith, N. Kondamudi, A. Manivannan,
M. Misra and V. Subramanian, Electrochem. Solid-State
Lett., 2011, 14, F5–F8.
N. M. Huang, K. M. Lo, T. Sohnel, Z. Arin and
M. Mazhar, New J. Chem., 2015, 39, 1031–1037.
This journal is © The Royal Society of Chemistry 2015
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