AIP Advances 6, 115010 (2016); https://doi.org/10.1063/1.4967387 6, 115010
© 2016 Author(s).
Probing the charge recombination in rGO
decorated mixed phase (anatase-rutile) TiO
2
multi-leg nanotubes
Cite as: AIP Advances 6, 115010 (2016); https://doi.org/10.1063/1.4967387
Submitted: 03 October 2016 • Accepted: 24 October 2016 • Published Online: 23 November 2016
Y. Rambabu, Manu Jaiswal and Somnath C. Roy
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AIP ADVANCES 6, 115010 (2016)
Probing the charge recombination in rGO decorated mixed
phase (anatase-rutile) TiO
2
multi-leg nanotubes
Y. Rambabu, Manu Jaiswal, and Somnath C. Roy
a
Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India
(Received 3 October 2016; accepted 24 October 2016; published online 3 November 2016)
Recombination of photo-generated charges is one of the most significant challenges
in designing efficient photo-anode for photo electrochemical water oxidation. In
the case of TiO
2
, mixed phase (anatase-rutile) junctions often shown to be more
effective in suppressing electron-hole recombination compared to a single (anatase
or rutile) phase. Here, we report the study of bulk and surface recombination pro-
cess in TiO
2
multi-leg nanotube (MLNTs) anatase-rutile (A-R) junctions decorated
with reduced graphene oxide (rGO) layers, through an analysis of the photo-current
and impedance characteristics. To quantify the charge transport/transfer process
involved in these junctions, holes arriving at the interface of semiconductor/electrolyte
were collected by adding H
2
O
2
to the electrolyte. This enabled us to interpret the
bulk and surface recombination process involved in anatase/rutile/rGO junctions
for photo-electrochemical water oxidation. We correlated this quantification to the
electrochemical impedance spectroscopy (EIS) measurements, and showed that in
anatase/rutile junction the increase in PEC performance was due to suppression
in electron-hole recombination rate at the surface states that effectively enhances
the hole transfer rate to the electrolyte. On the other hand, in rGO wrapped A-R
MLNTs junction it was due to both phenomenon i.e decrease in bulk recombination
rate as well as increase in hole transfer rate to the electrolyte at the semiconduc-
tor/electrolyte interface. © 2016 Author(s). All article content, except where oth-
erwise noted, is licensed under a Creative Commons Attribution (CC BY) license
(
http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4967387]
I. INTRODUCTION
An efficient production of hydrogen through the photo electrochemical water splitting requires
that the semiconducting material should efficiently absorb light to generate electron hole pairs and
these charges can be separated quickly to minimize recombination.
1
A single material that can satisfy
all the criteria has not been found so far and the practical solar-to-hydrogen conversion efficiencies
are limited only to a few percent.
TiO
2
has so far been the most extensively studied photo-catalyst because of several qualities
such as ease of processing, lack of toxicity, efficient electron-hole pair generation under the UV
radiation, chemical stability and photo-corrosion resistance.
2,3
Various TiO
2
nanostructures such
as, nanotubes, nanorods, nanofibers, nanoparticles etc. have been investigated as photo-electrodes
for water splitting. However, the major bottle necks such as limited absorption of the visible light
and higher recombination of the photo-generated charges are addressed through several strategies
including doping,
4
decoration with metal and semiconductor nano-particles
5
and composite forma-
tion using other semiconductor materials.
6,7
In recent times, graphene and its derivatives (reduced
graphene oxide-rGO) are also being investigated for improving the photo-catalytic/photovoltaic
properties by forming composite with TiO
2
.
8–10
It is reported that excellent electronic conduc-
tivity of this 2D material helps in the separation of photo-generated charges which, in turn,
a
Corresponding Author. Email:
somnath@iitm.ac.in
2158-3226/2016/6(11)/115010/9 6, 115010-1 © Author(s) 2016
115010-2 Rambabu, Jaiswal, and Roy AIP Advances 6, 115010 (2016)
improves the catalytic activities. Furthermore, the layer morphology also helps in easy trans-
port of the charge in the composite material.
11–13
Although, graphene composites with various
nanostructures of TiO
2
had been investigated over the past couple of years, effective composite
formation with the electrochemically anodized TiO
2
nanotubes remained a challenge due to com-
pact morphology that prevented access to the outer surfaces of the nanotubes. We have recently
shown that by using well-separated multi-leg single-phase TiO
2
nanotubes, a partial wrapping
of rGO layers over the outer surfaces could be achieved, in which, rGO layers were also found
to make interconnections between the adjacent nanotubes. An enhanced photocurrent has been
observed in the rGO wrapped TiO
2
nanotubes that was attributed to the reduction of electron-hole
recombination.
11
On the other hand, it has also been shown that mixed phase (anatase-rutile) is more effective
in improving PEC performance compared to single phase of TiO
2
having either anatase or rutile
component.
14–16
This has been attributed to the ability of mixed phase junction (anatase/rutile) in
suppressing charge carrier recombination rates by prolonging life time of electron-hole pair due to
charge transfer from one phase to the other, thus increases the charge separation rate.
16,17
However,
there is no general conclusion obtained in the direction of charge transfer from one phase to the
other.
18
In a recent work, we have demonstrated that the MLNTs annealed at 800
◦
C have mixed
phase with an anatase/rutile ratio ∼20:80 and stable multi-leg morphology.
19
Here, we report on the study of charge recombination in TiO
2
multi-leg nanotube (MLNTs)
anatase-rutile (A-R) junctions decorated with reduced graphene oxide (rGO) layers with the help of
hole scavenger (H
2
O
2
).
20
A comparative systematic investigations shown for charge recombination
phenomenon associated with rGO functionalized mixed phase TiO
2
nanotubes (A/R-rGO MLNTs),
rGO functionalized anatase phase TiO
2
nanotubes (A-rGO MLNTs), bare anatase phase TiO
2
nanotubes (A-MLNTs) and bare mixed phase TiO
2
nanotubes (A/R-MLNTs) for PEC water split-
ting. Such an analysis so far has not been reported for rGO decorated mixed phase junctions of
TiO
2
. To get a deeper understanding about the synergistic effect of anatase/rutile/rGO junction, we
quantitatively analyzed the obtained photocurrent j
photo
of A/R-rGO MLNTs junctions by splitting
into the product j
abs
× η
ct
× η
tr
,
20
where j
abs
is the maximum photocurrent density in ideal case
under no losses in the system, η
tr
is the fraction of holes that reach the surface of semiconductor
(without recombination in the bulk of the semiconductor) and η
ct
is the fraction of holes injected
into the electrolyte (without recombination on the surface). The data obtained from the analysis
of the photocurrent values have been correlated with Electrochemical Impedance Spectra (EIS) to
achieve a detailed understanding of the charge recombination and transport occurring inside the
semiconductor as well as at the semiconductor-electrolyte interface. This comprehensive study of
the charge transfer process occurring at rGO modified TiO
2
mixed phase junctions show that the
enhancement in photo-current corresponding to A/R-MLNTs junction is due the decrease in electron-
hole recombination rate at the surface states of semiconductor (η
ct
). However, in A/R-rGO MLNTs,
the improvement in photocurrent is attributed to the enhancement of charge separation and trans-
port (reduction of recombination) both in the bulk semiconductor (η
tr
), as well as across at the
semiconductor/electrolyte interface (η
ct
).
II. SYNTHESIS OF MULTI-LEG NANOTUBES
TiO
2
multi-leg nanotubes (MLNTs) were synthesized by electrochemical anodization
method.
11,19
Prior to anodization Ti foil was cleaned with soap solution as well as ultrasonicated
in a solution mixture consists of acetone, isopropyl alcohol and de-ionized water in equal volumes
for 30 min. The Ti foil was dried under nitrogen stream before mounting onto anodization set-up.
The anodization was carried out for 2 hr. in an electrolyte mixture comprising of 96 ml Di-Ethylene
Glycol (DEG, Fisher Scientific) and 0.6 wt% ammonium bi-fluoride salt (NH
4
F.HF) and 4ml de-
ionized water (Millipore) at a constant voltage of 60V. After anodization, the samples were rinsed
thoroughly with isopropyl alcohol and de-ionized water and kept at room temperature for 1 hr. to dry.
Subsequently as-prepared samples were annealed at 500
◦
C for 3hrs with heating and cooling rates
of 1
◦
C per minute for anatase phase formation and 800
◦
C for 15 minutes for desired mixed phase
formation.
115010-3 Rambabu, Jaiswal, and Roy AIP Advances 6, 115010 (2016)
A. Electrophoretic deposition of reduced graphene oxide layers on TiO
2
nanotubes
For synthesis of reduced graphene oxide modified anatase and mixed phase nanotubes elec-
trophoretic deposition method was employed. A two electrode setup, with TiO
2
nanotubes (on Ti
metal foil) as anode and a Pt foil cathode were used to carry out deposition of rGO layers. Reduced
graphene oxide aqueous solution (0.05 mg/mL) was used as electrolyte.
11
A constant voltage of 50 V
was applied between two electrodes separated at distance of 2 cm for a time interval of 20-90 sec.
to obtain MLNTs wrapped or inter connected with rGO layers. The amount of rGO deposition can
be controlled via deposition time. Figure S1 shows the deposition time vs. photocurrent density, the
optimum time found to be ∼ 40 sec, to get maximum photocurrent density with optimum wrapping
or interconnections.
B. Characterization and PEC measurements:
For surface morphology characterization of as prepared samples, a field-emission scanning elec-
tron microscope (FEI Quanta 400) was used. High resolution transmission electron microscopy
(HR-TEM) measurements were performed using JEOL JEM 3010 microscope. X-ray diffrac-
tion studies were performed using Panalytical X’pert-pro instrument. Photo-electro-chemical mea-
surements were carried out using CHI 6005E (CH Instruments, USA) electrochemical analyzer
in 1M NaOH aqueous solution as well as in 1M NaOH+0.5 M H
2
O
2
aqueous solution at
room temperature using three-electrode configuration. All electrolytes were prepared using Mil-
lipore de-ionized water (18.2M Ω). As prepared samples of exposed area 0.25 cm
2
was used
as the anode, a Pt wire and Ag/AgCl (KCl saturated) were used as counter and reference elec-
trodes. Measured potential vs. Ag/AgCl was converted into potential vs. RHE using the formula
E
RHE
= E
Ag/AgCl
+ E
0
Ag/AgCl
+ 0.059 × pH, where E
0
Ag/AgCl
=0.1976V at 25
◦
C was the standard poten-
tial of reference electrode (Ag/AgCl).The measurements were performed under 1 Sun illumination
(100mW/cm
2
) using 300 watt Xenon lamp (Oriel) equipped with AM 1.5G filter. Diffuse Reflectance
measurements were performed using JASCO V-660 UV-VIS-NIR spectrophotometer equipped with
an integrating sphere and standard BaSO
4
sample used as a reference. The maximum achievable
photocurrent density under no losses in system, j
abs
, was obtained from optical absorbance (A) using
calculation shown in the
supplementary material (Figure S2). The electrochemical impedance mea-
surements (EIS) were performed in a frequency range of 0.1 Hz to 100 KHz; impedance spectra were
fitted using Z-view software.
III. RESULTS AND DISCUSSION
Figure
1 shows the FESEM image of rGO modified TiO
2
mixed phase A/R-MLNTs. The verti-
cally oriented, well-separated nanotubes with a stable multi-leg morphology present a robust platform
for the wrapping of rGO onto the TiO
2
nanotubes annealed at 800
◦
C. As evident, the rGO layers
FIG. 1. FESEM shows the rGO layers wrapped/inter connected MLNTs with mixed phase A/R-rGO MLNTs.
115010-4 Rambabu, Jaiswal, and Roy AIP Advances 6, 115010 (2016)
FIG. 2. Raman spectra of rGO wrapped/decorated anatase multi-leg nanotubes (A-rGO MLNTs) and mixed phase multi-leg
nanotubes. The observation of D and G peaks confirm the presence of reduced graphene oxide layers on TiO
2
multi-leg
nanotubes.
are not only wrapped around the nanotubes but also form inter connections between the adjacent
nanotubes.
The weight ratio of anatase to rutile phases for rGO decorated TiO
2
MLNTs was estimated using
the intensities of characteristic XRD peaks (Figure S3) corresponding to anatase and rutile phases and
it was found to be ∼20:80, consistent with our previous work.
19
Figure 2 shows the Raman spectra
of rGO decorated anatase/rutile TiO
2
MLNTs. After deposition of rGO layers on TiO
2
MLNTs, the
additional two peaks D-band and G-band were observed for A/R-rGO MLNTs, besides the TiO
2
peaks.
19,21,22
The D-band at 1328 cm
-1
was attributed to disordered sp
2
bonded carbon atoms or
defects and the G-band at 1596 cm
-1
was attributed to ordered sp
2
bonded carbon network (graphitic
structure, E
2g
mode),
23,24
confirms deposition of rGO layers on TiO
2
MLNTs.
Figure
3 Shows the TEM and HR-TEM images of A-MLNTs obtained after annealing at 500
◦
C
and A/R-MLNTs obtained after annealing at 800
◦
C. Figure
3(a) shows the HR-TEM image of lattice
fringe spacing 0.35nm corresponding to (101) plane of anatase phase; inset shows the TEM image
FIG. 3. HR-TEM images of as prepared photo anodes (a) lattice fringe spacing of anatase multi leg nano tubes annealed at
500
◦
C (A-MLNTs), inset shows nanotubes with two legs (b) lattice fringe spacing and phase boundary corresponding to
anatase/rutile multi-leg nanotubes (A/R-MLNTs). (MLNT annealed at 800
◦
C), inset shows nanotubes with two legs.