Materials Letters 198 (2017) 188–191
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Materials Letters
journal homepage: www.elsevier.com/locate/mlblue
Peculiar behavior of TiO2 tubular structure for water reduction
in dark and under natural light exposure
U.H. Shah a, K.M. Deen b, Z. Rahman c, H. Asgar a, W. Haider a,c,⇑
a
School of Engineering and Technology, Central Michigan University, Mt. Pleasant, MI 48859, USA
Department of Materials Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada
c
Science of Advanced Materials, Central Michigan University, Mt. Pleasant, MI 48859, USA
b
a r t i c l e
i n f o
Article history:
Received 20 March 2017
Received in revised form 3 April 2017
Accepted 5 April 2017
Available online 6 April 2017
Keywords:
Photocatalytic
Electrochemistry
a b s t r a c t
TiO2 tubes were formed by electrochemical anodization process. The electrochemical reduction of water
on TiO2 tubular structure (TiO2–TS) at various pH in dark and natural light was investigated by potentiodynamic cathodic polarization, cyclic voltammetry (CV) and impedance spectroscopy analyses. The negative shift in open circuit potential, relatively low Tafel slope and higher exchange current density
presented by TiO2–TS under natural light depicted increased electrochemical activity for water reduction.
Under light exposure, CV scan showed high cathodic current density with lower overpotential (0.3 V)
when compared to dark (>0.8 V). Furthermore, under natural light, the decrease in charge transfer resistance and oxide film resistance also validated the increase in the electrochemical activity of TiO2–TS
towards water dissociation.
Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction
The demand for cleaner and cheaper fuel is one of the biggest
challenge because of energy crisis and pollution caused by conventional energy sources [1]. Titanium dioxide is a well-known as
semiconductor material owing to its potential to be used for
energy applications such as hydrogen gas production. It has been
shown that titanium has an photocatalytic ability to produce
hydrogen gas under solar radiation [2]. The properties of material
can be enhanced by moving towards nanoscale due to quantum
confinement effect [3]. Anodization is the simple method to obtain
firmly attached titanium dioxide nanotubes perpendicular to the
surface. During anodization, the overlapping of Ti and O2 results
in the formation of band gap between 3.2 and 3.0. The oxygen deficiency during anodization form band gaps with lower energy levels
which could be the reason of its increased photocatalytic behavior
[4]. A review by Roy et al. [5] and Ge et al. [6] and many references
cited therein explains the synthesis and use of TiO2 nanotubes in
many advanced applications e.g. as photocatalytic materials for
water splitting and organic pollutants degradation, in photocells,
electrochormic devices, energy storage, drug delivery systems
and biomedical implants.
⇑ Corresponding author at: School of Engineering and Technology, Central
Michigan University, Mt. Pleasant, MI 48859, USA.
E-mail address: [email protected] (W. Haider).
http://dx.doi.org/10.1016/j.matlet.2017.04.031
0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
In this study, photocatalytic potency of TiO2 tubular structure
(TiO2–TS) was evaluated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and cathodic potentiodynamic polarization scans at various pH in dark and natural light.
2. Experimental
The 5 mm thick circular samples (/ = 16 mm) were cut from the
rod and sequentially ground on SiC papers from 180 to 1200 grit
size. Polished samples were degreased ultrasonically in acetone,
ethanol and deionized water for five minutes successively.
Anodization was carried out in a two electrode cell for 90 min at
60 V containing electrolyte (92 ml C2H6O2, 0.1 M NH4H2PO4,
0.3 M NH4F, 0.5 ml C3H8O3 and 8 ml DI-H2O) using DC source
(Sorensen-SGA100X50C-0AAA). The surface morphology was
examined under scanning electron microscope (FEI Nova200nanolab) coupled with EDS. During electrochemical testing,
anodized samples were used as working electrodes, platinum and
saturated Ag/AgCl were used as counter and reference electrodes,
respectively. The electrochemical behavior of TiO2–TS was determined in deionized-water at pH 6, 7 & 8 (using KOH) in dark and
natural light. Open circuit potential(OCP) was measured before
potentiodynamic cathodic polarization measurements (from 0 V
to 1.0 V vs. OCP) at scan rate of 1 mV/s. Cyclic voltammograms
were obtained at various sweep rates (100, 50, 25, 15, 10, 5 and
1 mV/s) within 1.0 V potential range. EIS were obtained by
U.H. Shah et al. / Materials Letters 198 (2017) 188–191
perturbing 5 mVrms AC amplitude w.r.t OCP from 10 mHz to
100KHzfrequency range.
3. Results & discussions
Fig. 1 presents the SEM micrograph of ordered TiO2–TS after
anodization of CpTi2. The chemical composition and cross sectional
view of anodized surface is shown as inset of Fig. 1. The average
189
length, wall thickness and average inner diameter of the tubes
was measured to be 2.5 mm, 17 nm and 134 ± 8 nm (as shown
on the distribution curve in Fig. 1b), respectively. The OCP was measured in water at pH 6, 7 and 8 in dark and under natural light. The
OCP was appreciably shifted to negative values under natural light
(317 ± 65 mV) when compared to dark (488 ± 102 mV). It was
observed that with the increase in pH, the OCP was slightly shifted
to more active region (negative) in both dark and natural light. The
Fig. 1. a) SEM of tubular-structure obtained after anodization. b) Normal distrubution of inner diameter(nm) c) EDS data.
Fig. 2. (a) Potentiodynamic polarization curves (b) exchange current density calculated from polarization curves.
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U.H. Shah et al. / Materials Letters 198 (2017) 188–191
significant decrease in potential towards active region under natural light could be associated to the charge separation (electrons/holes)within the TiO2 structure due to photoexcitation [4]. The
cathodic potentiodynamic polarization curves were obtained to
evaluate the kinetic behavior of TiO2–TS are shown in Fig. 2a. The
larger cathodic Tafel slope (bc) presented by TiO2–TS in dark could
be associated with the limited kinetic response due to polarization
effects. However, relatively small bc values and more negative OCP
observed under natural light could be associated with the increase
electrochemical activity of TiO2–TS. The increase electrochemical
activity was further confirmed from high exchange current density
(io) values under natural light than in dark as shown in Fig. 2b.
Among various pH, the io was highest at pH 6 and an order of magnitude larger under natural light when compared to dark. Fig. 3a
present the CV scans of TiO2–TS at various pH in both dark and
under natural light. In dark, the TiO2–TS showed the skewed rectan-
gular curves with relatively small current response at all pH without any obvious peak for redox reaction. However, under natural
light, the rapid increase in current during reverse scan (cathodic)
and a broad anodic peak observed at 1.10 V (vs. OCP) could be
associated with the hydrogen evolution (H2)due to water reduction
and desorption of any intermediate species (Hads) (reactions (2) and
(3)), respectively during reduction of water. At pH 6, the relatively
higher io and larger current density observed in natural light at
higher sweep rate could be related with the fact that diffusion layer
could grow fast on TiO2–TS at lower sweep rates due to fast kinetic
reactions. The sweep rate dependent current response could also be
attributed to the double layer charging coupled with the redox
reactions at the surface of porous TiO2–TS. However, the relatively
smaller current response in dark (Fig. 3b) could be associated with
non–faradaic double layer charging and the limited diffusion of
electrolyte within the porous structure.
Fig. 3. Cyclicvoltammograms (a) at scan rate 5 mV/s and effect of sweep rate at pH 6 in (b) dark and (c) natural light.
Fig. 4. EIS spectra obtained in natural light and dark. Inset shows electrical equivalent circuit (EEC).
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U.H. Shah et al. / Materials Letters 198 (2017) 188–191
Table 1
The parameters obtained by simulating the experimental spectra with EEC model.
X
X
pH
Ecorr (mV vs. Ag/AgCl)
RS
Dark
6
7
8
461.2
432.0
407.8
3885
4608
2480
1271
3666
4102
Light
6
7
8
271.8
305.0
335.0
95
27
59
210
634
367
cm2
RP
cm2
S:sm
UDL lcm
2
m
S:sn
UOx lcm
2
n
S:sn
W lcm
2
Goodness of Fit
5655
1186
6616
666
555
329
0.898
0.964
0.595
89
146
608
0.929
0.544
0.617
5
4
482
5.1E6
3.7E6
4.82E6
1500
226
371
4
168
118
0.884
0.757
0.961
32
927
3
0.723
0.804
0.521
9000
2248
7196
9E3
2E3
7E6
ROL
X
cm2
2H2 O þ 2e ! H2 þ 2OH
ð1Þ
TiO2 þ Hþ þ e ! TiO2 :Hads
ð2Þ
2Hads ! H2 "
ð3Þ
Fig. 4, represents the Nyquist plots of TiO2–TS in both dark and
under natural light at pH 6, 7 and 8. The relatively higher real
impedance of TiO2–TS at high frequency was evident in the dark
than under natural light. The high frequency impedance behavior
could be related with the double layer charging and the faradaic
reactions whereas, low frequency regime corresponded to the diffusion controlled reactions at the TiO2–TS/electrolyte interface. The
impedance spectra were simulated with equivalent electrical circuit (EEC) as shown in the inset of Fig. 4 and the quantitative information is provided in Table 1. The appreciable decrease in solution
resistance (Rs) under exposure of natural light could also be associated with the photo excitation of charge (electrons/holes) carries
within the crystal lattice of TiO2. The relatively lower charge transfer resistance (Rct) and oxide film resistance (Rox) of TiO2–TS under
natural light also validated the effective catalytic activity and
improvement in the charge mobility within its lattice structure.
In dark, the charge was greatly relaxed within the double layer
(large /dl) than was transported through the TiO2–TS/electrolyte
interface, which was in agreement with the lower current response
under applied potential as shown in CV curves. The higher admittance values for semi–infinite Warburg (W) diffusion under natural
light could be related with the depletion of reacting species at the
TiO2–TS/electrolyte interface due to relatively fast kinetic reac-
tions. Furthermore, upon exposing TiO2–TS to natural light, the
corrosion potential (Ecorr) was significantly shifted from
+451 mV to 300 mV (vs. Ag/AgCl).
4. Conclusions
The TiO2–TS was produced by electrochemical anodization of
cpTi2 in the optimized conditions. The relatively large negative
shift of OCP, high exchange current density (io) and lower Tafel
slopes (bc) under natural light was evident from the cathodic polarization scans and this behavior could be affiliated with the
improved electrochemical activity of TiO2–TS towards reduction
of water. CV scans further confirmed that relatively large overpotential (>+0.8 V) would be required for water reduction over
TiO2–TS in the dark conditions when compared to (+0.3 V) in the
natural light. Impedance spectra also confirmed that the low
charge transfer resistance and oxide film resistance with relatively
large Warburg diffusion coefficient could be affiliated with the
effective catalytic activity of TiO2–TS due to enhanced charge
mobility within its structure and relatively fast kinetic response
under natural light.
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