Design and fabrication of new electrodes for photo-electrolysis using a material that is photo-active, stable, corrosion resistant, and cost effective.

1. Photoelectrochemical characterization of titania photoanodes fabricated using
varying anodization parameters
Rajesh Sharma*, Senior Member, IEEE, Keith Arnoult, Student Member, IEEE, Kevin Hart,
Maqsood Mughal, Robert Engelken
Arkansas State University
State University, AR 72467, USA
*rsharma@astate.edu
Abstract -- Titanium dioxide (TiO2) has long been considered
a model photoanode material for electrolysis of water using solar
energy. A number of studies have looked into the synthesis
methods to optimize physical as well as chemical properties of
titania photoanodes. Electric field assisted anodic oxidation of
titanium (Ti) for fabrication of titania photoanodes is a relatively
new synthesis technique. This paper presents a systematic study
of this technique by varying anodization parameters. The
current-time behavior of Ti anodization was also studied. The
current–voltage (I-V) characteristic of these samples was
measured under dark and illumination conditions. The electrode
fabricated using 20 Volts for 20 minutes demonstrated the best
performance among all the samples tested. The photocurrent
density obtained under visible radiation was 0.528 mA/cm2
. This
study will assist in design and fabrication of new electrodes for
photoelectrolysis using a material that is photoactive, stable,
corrosion resistant, and cost effective.
Index Terms—anodes, energy conversion, hydrogen,
photoelectricity, titanium
I. INTRODUCTION
It is believed that a transition from fossil fuel based energy
resources would be necessitated in a not too distant future.
Hydrogen is a promising energy resource that could replace
hydrocarbon-based fuels. This would require a sustainable
hydrogen production using renewable resources. Photo-
electrochemical water splitting is one such method for
hydrogen production where the only inputs are water and
solar energy. Fujishima et al. [1] successfully demonstrated
the photoelectrochemical decomposition of water into oxygen
and hydrogen without application of any external voltage
using a n-type TiO2 (rutile) semiconductor electrode in 1972.
Since then, numerous studies have reported various aspects of
photoelectrolysis [2, 3]. Although significant advances have
been made in this area, a number of challenges still remain.
One of the major challenges is finding a photoactive, stable,
corrosion resistant, and cost effective material that can utilize
a large spectrum of the solar radiation.
Titanium dioxide has long been considered a model
photoanode material for electrolysis of water using solar
energy. However, it has not been able to meet the potential
that has been expected of it. It has several advantages, which
are desirable for application in photoelectrochemical systems.
It is photostable, chemically inert, and cost effective. One of
the major disadvantages of TiO2 is its large bandgap (3.2 eV
for anatase and 3.0 eV for rutile). As a result, it can be
photoactive only under UV radiation (wavelengths shorter
than 400 nm) utilizing only about 4% of incident solar
radiation. A number of studies have looked into the synthesis
methods to optimize physical as well as chemical properties
of titania photoanodes [4]. Solgel [5], hydrothermal [6],
solvothermal [7], chemical vapor deposition [8], electrostatics
spray hydrolysis [9] and many other methods have been used
in synthesis of a variety of titania nanostructures for
photoanode fabrication. However, the search for a titania
electrode that can efficiently harness solar energy for photo-
electrolysis continues.
Electric field-assisted anodic oxidation of titanium for
fabrication of titania photoanodes is a relatively new synthesis
technique. Zwilling et al. [10] first reported the formation of
thin oxide compact films using both voltammetric and
chrono-amperometric methods. Followed by that, a large
number of studies have looked into synthesis of various kinds
of nanostructures [11-16]. These nanostructures have been
used in a variety of applications ranging from dye-sensitized
solar cells, sensors, photocatalysis and photoelectrolysis. This
paper presents a systematic study of electric field-assisted
anodic oxidation of titanium by varying anodization
parameters. The objective of this study is to elucidate the
impact of anodization voltage and duration on
phtotoelectrochemcial properties of TiO2 photoanodes.
II. EXPERIMENTAL
A. Electrochemical synthesis of titania nanostructures
Titanium dioxide samples were synthesized using
anodization of Ti foil. Electrochemical synthesis of TiO2
nanotubes has been previously reported in several studies
following Zwilling et al [10]. Ti foil (99.9% pure, 0.5 mm
thick) was anodized in 0.5 M phosphoric acid (H3PO4, Sigma-
Aldrich, 85% in water) and 0.14 M ammonium fluoride
(NH4F, Fisher, 100%) environment. A two-electrode
configuration was used for anodization. A square shaped
platinum (Pt) electrode (thickness: 1 mm; area: 6.25 cm2
)
served as a cathode. The distance between the two electrodes
was kept at 25 mm in all of the experiments. Fig. 1 shows the
experimental setup. Anodization was carried out at a constant
potential using a dc power supply (Matsusada Model R4K-
80M-LUs1). A software application for use with the
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2. Microsoft Windows platform was written using the Delphi
programming language. Communications between the
computer and Matsusada power source was via a universal
serial bus (USB). The application recorded the voltage and
current at a specified interval. This interval was adjustable
via a user defined field. Once the recording process was
complete, the data was saved in a comma delimited text file.
The data was then imported into Microsoft Excel using the
standard importing process for comma delimited text files.
During anodization, ultrasonic waves were applied to the
solution to enhance the mobility of the ions in the solution.
The anodized samples were washed with distilled water to
remove the occluded ions from the anodized solutions, dried
in an air-oven, and sent for plasma treatment. Samples were
anodized at 20 V and 40 V. The anodization was performed
for 5, 10, 20, and 60 min. The anodized titania nanotubular
arrays were annealed in an oxygen atmosphere at 500 °C for 2
h in a furnace (ThermoScientific- Lindberg/Blue M-Model
number BF51848A-1) at a heating rate of 1 °C/min.
Fig. 1. Experimental setup for anodization of Ti samples
B. Photoelectrochemical analysis
The photocurrent response of the samples was measured
using a standard three-electrode photoelectrochemical cell.
Titania samples were used as a photoanode (working
electrode), and a platinum mesh (Alpha-Aesar) (2.5 cm2
) was
used as a counter electrode (Fig. 2). A capillary tube filled
with Ag/AgCl electrolyte and an insert of a salt-bridge
(saturated KCl)- Luggin probe was used as a reference
electrode. The distance between the photoanode and the Pt
counter electrode was fixed at 10 cm. The solar spectrum was
simulated using a halogen lamp. The power density of the
Fig. 2. Photoelectrochemical characterization setup
lamp as measured was 1000 W/m2
. Light was illuminated
onto the surface of the test photoanode through a 3 inch
diameter optical quartz window. The photocurrent density
was measured using a potentiostat/galvanostat (Wavenow
USB Potentiostat Model AFTP1, Pine Research
Instrumentation).
The electrolyte used was 1M KOH (pH ~ 14) aqueous
solution. The electrolyte was prepared using reagent grade
chemicals and doubly distilled water. No aeration was carried
out to purge out the dissolved gases in the electrolyte. The
samples were anodically polarized at a scan-rate of 5 mV/s
under darkness and illuminated conditions, while the
photocurrent was recorded. No electron mediators were added
to the electrolyte to increase the charge separation efficiency.
All of the tests were conducted without any co-catalysts or
sacrificial agents.
III. RESULTS AND DISCUSSIONS
Electric field-assisted anodic oxidation of titanium for
fabrication of titania photoanodes is a multistep process. It
starts with the growth of oxide layer on the Ti metal surface
as it is immersed in the electrolyte and it reacts with oxygen
(O2-
) and hydroxyl (OH-
) ions [17]. These ions continue
reacting with the metal surface, even though they will have to
traverse through the oxide layer, further increasing the
thickness of the oxide layer. This oxidation process is further
assisted by the electric field between anode and cathode. Next
step is the field-assisted migration of Ti metal ions from
metal-oxide layer [18]. Electric field weakens Ti-O bond
promoting dissolution of metal cations in the electrolyte. At
the same time, the oxide layer also keeps on getting eroded
due to electric field. All of these processes are accompanied
by chemical dissolution of Ti4+
and TiO2 due to etching by
fluoride ions. The surface structure of the TiO2 layer is the
result of interplay between all the above-mentioned processes.
Figs. 3 and 4 show the anodization current–time behavior
for Ti samples anodized at 20 V and 40 V respectively. The
anodization current dropped almost immediately after the
application of electric field. After an initial increase-decrease
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3. (a) (b)
(c) (d)
Fig. 3. Anodization current –time behavior for Ti sheet at 20 V. Samples were anodized for (a) 5 min (b)10 min (c) 20 min, and (d) 60 min
transient, the current reached a steady-state value. This
indicates formation of the oxide layer on the surface. The
oxide layer is highly resistive, hence, the drop in current.
Even though there were some minor differences between
samples, this overall behavior was exhibited by all samples.
The anodization current of samples anodized for 60 min at 20
V and 40 V (Fig. 3d and 4d), respectively, was almost zero,
indicating the formation of a continuous and thick oxide
layer, whereas in other samples, the anodization current
density varied from approximately 0.01 to 0.04 mA/cm2
,
indicating the presence of pits and cracks in the oxide layer.
Figs. 5 and 6 show the photoelectrochemcial response of
samples anodized at 20 V and 40 V, respectively under dark
and illuminated conditions. The sample anodized at 20 V for
20 min exhibited the highest photocurrent density (0.528
mA/cm2
) among all the samples tested (Table 1). The
anodization voltage or treatment duration did not have
TABLE I
SUMMARY OF THE PHOTOCURRENT DENSITY OF TITANIA
SAMPLES
Anodization parameters Photocurrent density
(mA/cm2
)
Applied voltage
(V)
Treatment duration
(min)
20 5 0.166
20 10 0.132
20 20 0.528
20 60 0.209
40 5 0.106
40 10 0.175
40 20 0.393
40 60 0.209
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4. (a) (b)
(c) (d)
Fig. 4. Anodization current –time behavior for Ti sheet at 40 V. Samples were anodized for (a) 5 min (b)10 min (c) 20 min, and (d) 60 min
a strong correlation with the photoelectrochemical
performance of the titania photoanodes. It seems that 5 and 10
minutes of anodization at 20 V did not provide sufficient film
thickness and surface area for efficient light absorption. 20
minutes of anodization at 20 V provided a continuous oxide
film without pits and cracks, and surface microstructure that
enhanced light absorption. Anodization at 40 V did not
provide any advantage over anodization at 20 V. The
performance of the photoanodes decreased beyond 20
minutes for both 20 and 40 minutes anodization. It could
possibly be due to an increased thickness of the oxide layer
that hindered the charge flow.
Table 2 shows the open circuit potential (OCP)
measurements for samples anodized at 20 V and 40 V,
respectively, under dark and illuminated conditions. The OCP
for all of the samples was more negative under illuminated
TABLE II
SUMMARY OF THE OPEN-CIRCUIT POTENTIAL (OCP) AND
PHOTOVOLTAGE OF TITANIA SAMPLES
Anodization parameters Open-Circuit Potential
(OCP)
Photovoltage
(Vph)
Applied
voltage (V)
Treatment
duration
(min)
Dark Illuminated
20 5 -174 -221 47
20 10 -174 -339 165
20 20 -163 -336 173
20 60 -180 -355 175
40 5 -293 -449 156
40 10 -368 -478 110
40 20 -262 -397 135
40 60 -275 -443 168
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5. (a) (b)
(c) (d)
Fig. 5. The photocurrent density versus potential (V vs Ag/AgCl) for samples anodized at 20 V for (a) 5 min (b) 10 min (c) 20 min, and (d) 60 min
conditions as compared to under dark conditions. The
negative shift in OCP upon illumination indicates that the
material possess n-type conductivity. All the samples tested
exhibited n-type conductivity. The difference between the
OCP under illuminated and dark conditions is the
photovoltage (Vph), [18], which is a good indicator of the
performance of the material. The sample with the highest
photocurrent density also had the second highest
photovoltage.
IV. CONCLUSIONS
Titania photoelectrodes were fabricated using electric-field
assisted anodic oxidation of titanium. Samples were
synthesized by varying anodization voltage and duration. It
was found that the sample fabricated using 20 V anodization
voltage for 20 min demonstrated the best
photoelectrochemical performance. The increased
photoactivity of this sample could be due to a combination of
several factors (1) high photovoltage, (2) efficient charge
separation, (3) optimal thickness of for efficient charge
transfer, and (4) increased surface area and hence enhanced
electrode-electrolyte area to provide maximum optical
absorption and efficient charge transfer.
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6. (a) (b)
(c) (d)
Fig. 6. The photocurrent density versus potential (V vs Ag/AgCl) for samples anodized at 40 V for (a) 5 min (b) 10 min (c) 20 min, and (d) 60 min
ACKNOWLEDGMENT
This work was partially funded by a NASA EPSCoR
Research Infrastructure Development (RID) grant and a
Arkansas State University Faculty Research Award.
REFERENCES
[1] A. Fujishima, and K. Honda, “Electrochemical Photolysis of Water at
a Semiconductor Electrode,” Nature, vol. 238, pp. 37, July 7, 1972.
[2] A. J. Bard, “Design of Semiconductor Photoelectrochemical Systems
for Solar Energy Conversion,” J. Phys. Chem., vol. 86, pp.172-177,
1982.
[3] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, “Visible-
Light Photocatalysis in Nitrogen-Doped Titanium Oxides,” Science,
vol. 293. pp. 269-271, 13 July 2001.
[4] X. Chen and S. S. Mao, “Synthesis of Titanium Dioxide
Nanomaterials,” Journal of Nanoscience and Nanotechnology, Vol 6,
906 – 925, 2006.
[5] N. Suzuki, X. Jiang, V. Malgras, Y. Yamauchi, A. Islam, L. Han,
“Synthesis of Thin Titania Photoanodes with Large Mesopores for
Electricity- generating Windows,” Chemistry Letters, vol. 44, no. 5,
pp. 656-658, 2015.
[6] W. Wu, T. Shih and J. Ting, “Hydrothermally synthesized TiO2
nanopowders and their use as photoanodes in dye-sensitized solar
Page 6 of 7
978-1-4799-8374-0/15/$31.00 © 2015 IEEE
2015-EPC-0533

7. cells,” International Journal of Energy Research, vol. 37, no.8, pp.
964–972, 25 June 2013.
[7] J. Liu , Y. Zhao, L. Shi, S. Yuan, J. Fang , Z. Wang , and M. Zhang,
“Synthesis of Crystalline Phase and Shape Controlled Sn4+
-Doped
TiO2 Nanocrystals: Effects of Reaction Solvent,” ACS Appl. Mater.
Interfaces, vol.3, no.4, pp 1261–1268, 2011.
[8] S. H. Nam, J. Hyun, and J. Boo, “Synthesis of TiO2 thin films using
single molecular precursors by MOCVD method for dye-sensitized
solar cells application and study on film growth mechanism,”
Materials Research Bulletin, vol. 47, pp. 2717-2721, 2012.
[9] D.G. Park and J.M. Burlitch, “Nanoparticles of anatase by electrostatic
spraying of an alkoxide solution,” Chem. Mater. Vol. 4, pp. 500–502,
1992.
[10] V. Zwilling, V, M. Aucouturier, and E. Darque-Ceretti, “Anodic
oxidation of titanium and TA6V alloy in chromic media. An
electrochemical approach,”Electrochimica Acta, vol. 45, no. 6, pp.
921-929, December 1999.
[11] C. A. Grimes, “Synthesis and application of highly ordered arrays of
TiO2 nanotubes,” J. Mater. Chem., vol. 17, pp.1451-1457, 2007.
[12] A. Ghicov, S. Aldabergenova, H. Tsuchyia, and P. Schmuki, “ TiO2-
Nb2O5 Nanotubes with Electrochemically Tunable Morphologies,”
Angew. Chem. Int. Ed. vol. 45, pp. 6993-6996, 2006.
[13] A. Ghicov, H. Tsuchiya, J.M. Macak, P. Schmuki, “Titanium oxide
Nanotubes prepared in phosphate electrolytes,” Electrochemistry
Communications, vol. 7, pp. 505-509, 2005.
[14] G. K. Mor, O. K. Varghese, M. Paulose, K. Shankar, and C. A. Grimes,
“A review on highly ordered, vertically oriented TiO2 nanotube arrays:
Fabrication, material properties, and solar energy applications,” Solar
Energy Materials and Solar Cells, vol. 90, no.14, pp.2011-2075, 2006.
[15] K. S. Raja, M. Misra and K. Paramguru “Formation of Self-Ordered
Nano-Tubular Structure of Anodic Oxide Layer on Titanium,”
Electrochmica Acta, vol. 51, pp. 154-164, 2005,
[16] R. Sharma, P. P. Das, V. Mahajan, J. Bock, S. Trigwell, A. S. Biris, M.
K. Mazumder, M. Misra, “Enhancement of Photoelectrochemical
Conversion Efficiency of Nanotubular TiO2 Photoanodes using
Nitrogen Plasma Assisted Surface Modification,” Nanotechnology,
Vol. 20, no.7, pp.075704 2009.
[17] V. P. Parkhutik and V.I. Shershulsky, “Theoretical modeling of porous
oxide-growth on aluminum,” J Physics D, vol. 25, pp, 1258- 1263,
1992.
[18] Z. Chen, H.N. Dinh, and E. Miller, “Flat-Band Potential Techniques,”
in Photoelectrochemical Water Splitting, Standards, Experimental
Methods and Protocols, Springer Briefs in Energy, Springer, 2013, pp.
63-85.
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