Visible Light Driven Photoelectrochemical Water Oxidation on
Nitrogen-Modified TiO
2
Nanowires
Son Hoang,
†
Siwei Guo,
†
Nathan T. Hahn,
†
Allen J. Bard,
‡
and C. Buddie Mullins*
,†,‡
†
Department of Chemical Engineering,
‡
Department of Chemistry and Biochemistry, Center for Electrochemistry, Texas Materials
Institute, Center for Nano and Molecular Science, University of Texas at Austin, 1 University Station C0400 Austin, Texas
78712-0231, United States
*
S
Supporting Information
ABSTRACT: We report hydrothermal synthesis of single crystalline
TiO
2
nanowire arrays with unprecedented small feature sizes of ∼5nm
and lengths up to 4.4 μm on fluorine-doped tin oxide substrates. A
substantial amount of nitrogen (up to 1.08 atomic %) can be
incorporated into the TiO
2
lattice via nitridation in NH
3
flow at a
relatively low temperature (500 °C) because of the small cross-section
of the nanowires. The low-energy threshold of the incident photon to
current efficiency (IPCE) spectra of N-modified TiO
2
samples is at
∼520 nm, corresponding to 2.4 eV. We also report a simple cobalt
treatment for improving the photoelectrochemical (PEC) performance
of our N-modified TiO
2
nanowire arrays. With the cobalt treatment, the IPCE of N-modified TiO
2
samples in the ultraviolet
region is restored to equal or higher values than those of the unmodified TiO
2
samples, and it remains as high as ∼18% at 450
nm. We propose that the cobalt treatment enhances PEC performance via two mechanisms: passivating surface states on the N-
modified TiO
2
surface and acting as a water oxidation cocatalyst.
KEYWORDS: Water photo-oxidation, N-modified TiO
2
, water oxidation catalyst, hydrothermal synthesis, single crystalline nanowire,
photocatalysis
T
itanium dioxide (TiO
2
) is well-known as a candidate for
water photo-oxidation as it is abundant, stable in aqueous
solution under irradiation, and has strong photocatalytic
activity.
1,2
However, due to its large band gap (∼ 3.0 eV for
rutile and 3.2 eV for anatase), TiO
2
is only active in the
ultraviolet (UV) region which contributes less than 5% of the
total energy of the solar spectrum.
3
Shifting the absorption of
TiO
2
to include visible light, which composes a greater portion
of the solar spectrum (45%), is one of the prerequisites to
enhancing the solar energy conversion efficiency of titania.
4−7
Another requirement of an effective photomaterial is good
electron− hole separation characteristics, which can be
improved by increasing the charge transfer (normally via
nanostructuring the morphology and doping with foreign ions)
and increasing the kinetics of water oxidation by holes and
water reduction by electrons (via loading of cocatalysts). TiO
2
has a short hole diffusion length (∼10 nm for the rutile single
crystal),
8
therefore it is necessary to reduce the TiO
2
characteristic size to decrease the diffusion pathway of
photoholes to the electrode/electrolyte interface. Moreover,
in a photoelectrochemical (PEC) cell, electrons generated in
the TiO
2
photoanode film have to travel within the TiO
2
film
to the back contact and then transfer to the cathode. Therefore
the optimum morphology is a one-dimensional, single
crystalline structure to enable electrons to travel to the back
contact and holes to diffuse to the electrode/ele ctrolyte
interface in the easiest manner without scattering at a grain
boundary. Co-catalysts, such as IrO
2
,
9
Co-based materials,
10
and Co-Pi
11
for water oxidation, are also needed to increase the
kinetics of the reactions, thus reducing the charge recombina-
tion rate.
Incorporating nitrogen has been said to narrow the band gap
of TiO
2
for water splitting applications since substitutional N
2p states hybridize with O 2p states, upshifting the valence
band edge while almost keeping the conduction band edge in
the same position.
4,12,13
However, there is an ongoing debate
regarding the red shift of the absorption edge of N-modified
TiO
2
. Some researchers believe substitutional N forms isolated
N 2p midgap states slightly above the top of the O 2p valence,
instead of mixing with O 2p to form a continuous valence band
as proposed above.
14−16
In this case, photogenerated holes may
be trapped in these localized states leading to a high
recombination rate, thus decreasing the quantum yields of N-
modified TiO
2
. Some other researchers suggest that high
doping of nitrogen in TiO
2
produces color centers with a
different local chemical composition and electronic struc-
ture.
17,18
In this picture, the color centers, including Ti
3+
, are
responsible for visible light absorption in the N-modified TiO
2
material.
Received: August 15, 2011
Revised: November 3, 2011
Published: November 23, 2011
Letter
pubs.acs.org/NanoLett
© 2011 American Chemical Society 26 dx.doi.org/10.1021/nl2028188 | Nano Lett. 2012, 12, 26−32
Nitrogen incorporation can be accomplished by calcining
TiO
2
under NH
3
. However, due to the low solubility of N in
the TiO
2
lattice, the reactions normally have to be conducted at
high temperatures (above 550 ° C) to yield sufficient N-dopant
incorporation for better visible light absorption. However,
annealing in NH
3
at such high temperatures leads to unwanted
side effects, such as defect formation within the TiO
2
lattice,
degradation of the transparent conductive substrate [fluorine-
doped tin oxide (FTO)], and sintering of the nanostructure.
In this Letter, we report a simple hydrothermal synthesis
route for growing densely packed, vertical, single crystalline
TiO
2
rutile nanowire arrays on FTO substrates of unprece-
dented small cross-sections with a characteristic dimension of
∼5 nm and lengths up to 4.4 μm. A significant amount of
nitrogen (up to 1.08 atomic %) can be incorporated into the
TiO
2
by annealing the films under NH
3
flow at a relatively low
temperature (500 °C) because of the exceptionally small
nanowire cross-section. Furthermore, we report a simple
surface treatment employing cobalt as a cocatalyst that we
believe has not been investigated previously with TiO
2
, in order
to improve the water oxidation performance of N-modified
TiO
2
. N-modified TiO
2
films without a cobalt cocatalyst
yielded a lower photocurrent under a full spectrum and lower
quantum yields in the UV region than similar unmodified TiO
2
samples, although the N-modified samples had higher visible
light photocurrents. A cobalt cocatalyst not only enhances the
quantum yield in the visible light region but also restores the
quantum yield in the UV region compared to the equivalent
values of the unmodified samples.
Synthesis of TiO
2
Nanowire Arrays. FTO-coated glass
substrates were first cleaned by sonication in a mixture of
ethanol and water for 30 min, subsequently rinsed with
deionized (DI) water, and finally dried in an air stream. In
order to enhance the sample integrity and shorten the growing
time, FTO substrates were also seeded with a thin layer of TiO
2
before growing the nanowire arrays. For seeding, clean FTO
substrates were first soaked in 0.025 M TiCl
4
in n-hexane for 30
min. They were then taken out, rinsed by ethanol, and finally
annealed in air at 500 °C for 30 min. In a typical hydrothermal
growth procedure, the seeded FTO substrates were placed on
the bottom of a Teflon-lined autoclave (125 mL, Parr
Instrument), containing 50 mL of n -hexane (extra dry, 96+%,
Acros Organics), 5 mL of HCl (ACS reagent grade 36.5−38%,
MP), and 2.5−5 mL of titanium(IV) isopropoxide (98+%,
Acros Organics). The hydrothermal synthesis was conducted at
150
o
C for certain amount of time in a box oven. After the
reaction was completed and the autoclave naturally cooled
down to room temperature, the TiO
2
nanowire films were
taken out and cleaned by rinsing with copious amount of
ethanol and water.
The hydrothermal growth of vertical TiO
2
nanowire arrays
on FTO with feature sizes of ∼20 nm via a nonpolar solvent/
hydrophilic solid substrate interfacial reaction was first reported
by Grimes and co-workers.
19
Using a similar strategy, we
developed the r ecipe (i.e., tita nium precursors, nonpolar
solvents) and the hydrothermal reaction conditi ons (i.e.,
reaction time and temperature) described above that enable
the synthesis of high-quality rutile TiO
2
single crystalline
nanowire arrays with smaller feature sizes (∼5 nm). As
proposed by Grimes et al.,
19
at room temperature, titanium
precursors [e.g., titanium tetraisopropoxide (TTIP)] and water
(from hydrochloric acid solution) are separated since the
precursors are soluble and water is immiscible in the nonpolar
solvents (e.g., n-hexane). Under hydrothermal conditions, to
minimize system energy, water diffuses to the hydrophilic FTO
surface where it hydrolyzes with TTIP to form TiO
2
nuclei on
the FTO surface. As the newly formed TiO
2
nuclei are also
hydrophilic, water continues to diffuse to the nuclei resulting in
further hydrolysis and crystal growth. The Cl
−
ions play an
important role in the hydrothermal growth as they promote
anisotropic growth of one-dimensional nanocrystals. The Cl
−
ions are inclined to absorb on the rutile (110) plane, thus
retarding further growth of this plane. We did not observe
nanowire array formations when HCl was replaced by HNO
3
or
H
2
SO
4
.
Characterization of TiO
2
Arrays. Shown in Figure 1a,b
are cross-sectional and top view scanning electron microscope
(SEM) images of a typical as-synthesized (with no further heat
treatment) nanowire film. The nanowire arrays consisting of
vertically aligned and tetragonal shaped nanowires are highly
uniform and densely packed with exceptionally small feature
sizes (average characteristic cross-sectional dimension is ∼5
nm). The grazing incidence X-ray diffraction (GIXRD) pattern
in Figure 1d shows that the as-synthesized nanowire arrays are
rutile TiO
2
with an enhancement in the (101) facet exposure
relative to the standard rutile powder pattern (JCPDS #88-
1175). The high-resolution transmission electron microscope
(HRTEM) image in Figure 1c further confirms that the
nanowires are single crystalline with an interplanar d-spacing of
0.327 nm, corresponding to (110) planes of rutile TiO
2
. The
atomic ratio of Ti to O was found to be ∼1:2 using energy
dispersive X-ray analysis (EDX) (the expected stoichiometric
values).
The length of the nanowire arrays is a function of the TTIP
to n-hexane volume ratio, the reaction conditions (i.e.,
temperature and time), and seeding layer. The thicknesses of
nanowire arrays versus reaction conditions, determined from
cross-sectional view SEM, are shown in Table 1. We are able to
grow nanowires with lengths varying from ∼500 nm up to 4.4
μm with no significant change in feature sizes. Moreover, if the
FTO substrates are coated with a thick TiO
2
layer (∼5 μm)
prior to hydrothermal reaction, we can grow nanowire arrays
with lengths up to 17 μm. Optimization of the thickness of a
Figure 1. Vertically aligned single crystalline TiO
2
rutile nanowire
arrays on FTO glass: (a) cross-sectional and (b) top view SEM images,
(c) HRTEM image, and (d) grazing incidence angle X-ray diffraction
(GIXRD) pattern.
Nano Letters Letter
dx.doi.org/10.1021/nl2028188 | Nano Lett. 2012, 12, 26−3227
photoelectrode for PEC applications involves balancing the
charge carrier mobility and the absorbance of photons. The
photoanode should be as thin as possible to allow electrons to
travel to the back contact in the shortest time while still being
thick enough to absorb the majority of the photons from
sunlight. In our study, the highest photocurrents were from
samples with thicknesses of ∼1.5 μm. Therefore, we focused on
the PEC characterization of TiO
2
nanowire films wit h
thicknesses of 1.59 ± 0.26 μm grown at 150 °C for 5 h with
TTIP/n-hexane ratios of 1:20.
The seeding layer enhances both the nanowire arrays’
adherence to the FTO substrate and the growth rate. For
example, the thickness of TiO
2
nanowire arrays grown with the
ratio TTIP/n-hexane of 1/10 at 150
o
C for 5 h with and
without the seeding layer is 2.60 ± 0.27 and 1.2 ± 0.15 μm,
respectively. As mentioned above, when FTO was coated with a
thick TiO
2
seeding layer of ∼5 μm, the same reaction
conditions resulted in a nanowire length of ∼17 μm.
We found that the combination of titanium precursor and
nonpolar solvent strongly affects the morphology of the
nanowire arrays. Using a combination of titanium(IV) tetra-n-
butoxide (TNBT) (Ti
4+
precursor) and n-hexane o r a
combination of TTIP and toluene (nonpolar solvent) resulted
in unoriented, wire bundle formation. We further investigated
nanowire array growth using a combination of TNBT and
toluene which resulted in oriented but shorter nanowire arrays
(∼1.3 μm) with bigger feature sizes (∼15 nm) (Figure S1,
Supporting Information).
PEC Properties of TiO
2
Nanowire Arrays. The PEC
characterization of TiO
2
nanowire samples was performed
using a three-electrode electrochemical cell with the FTO
supported nanowire arrays as the working electrode, a Ag/AgCl
(saturated KCl) reference electrode, a platinum wire counter
electrode, and 1 M KOH electrolyte (pH = 13.5). The working
electrode with exposed area of 0.16 cm
2
was illuminated from
the back side (through the FTO substrate−TiO
2
nanowire
interface) by a 100 W xenon lamp (Newport) through a UV/IR
filter (Schott, KG3) to remove infrared (>800 nm) and short
wavelength UV light (<300 nm). Using a Scientech calorimeter
(model 38-0101), the light intensity of the spectrum from 400
nm to 1.2 μm was measured as 37 mW/cm
2
. The fraction of
the total energy of the spectrum from 400 to 800 nm for our
lamp is estimated to be 85−90% of the total light energy,
therefore, we estimate the energy flux in our PEC measure-
ments to be ∼41−43 mW/cm
2
. Incident photon to current
conversion efficiencies (IPCEs) were calculated from amper-
ometry measurements using a monochromator (Newport) with
a bandwidth of 7.4 nm in conjunction with a power meter and
photodiode (Newport), given by
=
×
λ×
×
j
I
IPCE
1240
100%
ph
(1)
where j
ph
is the steady-state photocurrent density at a specific
wavelength, and λ is the wavelength of the incident light. I is
the light intensity for wavelength λ at the film surface, I ranges
from 80 to 300 μW/cm
2
over the spectrum of wavelengths
studied (320−550 nm).
The measured potentials versus the Ag/AgCl reference
electrode were converted to the reversible hydrogen electrode
(RHE) scale via the Nernst equation:
=+ +EE E0.059 pH
RHE Ag/AgCl
o
Ag/AgCl
(2)
where E
RHE
is the converted potential vs RHE, E
Ag/AgCl
is the
experimental potential measured against the Ag/AgCl reference
electrode, and E
o
Ag/AgCl
is the standard potential of Ag/AgCl at
25 °C (0.1976 V). We also used the same testing conditions for
other samples throughout this paper.
Before testing, the as-synthesized films were annealed in air
at 500 °C for 1 h to remove contaminants and increase the
adherence of the TiO
2
arrays to the SnO
2
layer. Figure 2a
shows the linear sweep voltammetry of the TiO
2
nanowire
sample. The onset potential of our TiO
2
nanowire arrays is
∼0.2 V
RHE
, around 0.2 V more negative compared to a TiO
2
nanotube sample.
5
In order to improve the PEC performance,
we also applied a cobalt treatment technique similar to that
reported by Gra
tzel et al.
10
in which the TiO
2
nanowire arrays
were soaked in 0.1 M Co(NO
3
)
2
for 1 min, followed by rinsing
Table 1. Thicknesses of Some TiO
2
Nanowire Arrays Grown
at 150 °C As a Function of TTIP/n-Hexane Ratio, Reaction
Time, and Seeding Layer
TTIP/n-hexane
volume ratio
reaction
time (hour) seeding
number of
sample(s)
length of
nanowires
1:20 5 no 1 500 nm
1:20 5 yes 20 1.59 ± 0.26 μm
1:10 5 no 4 1.2 ± 0.26 μm
1:10 5 yes 22 2.6 ± 0.27 μm
1:10 10 yes 4 4.4 ± 0.27 μm
Figure 2. (a) Linear sweep voltammetry measurements of TiO
2
nanowire arrays (1.6 μm) and the same film after cobalt treatment
and (b) chronoamperometry measurement (at 1.23 V
RHE
) of TiO
2
nanowire arrays and the same film after cobalt and silver treatments
(cobalt and silver treatments were performed on two different areas on
the same TiO
2
nanowire sample). All experiments were performed
with 1 M KOH electrolyte (pH = 13.5) and a 100 W xenon lamp
coupled with a UV/IR filter as the light source as described in the text.
Nano Letters Letter
dx.doi.org/10.1021/nl2028188 | Nano Lett. 2012, 12, 26−3228
with a copious amount of water. The photocurrent measured at
1.23 V
RHE
was improved by ∼20% due to the cobalt treatment,
from 0.38 mA/cm
2
(without treatment) to 0.46 mA/cm
2
.
Cobalt-based materials, such as Co-Pi, are well-known catalysts
for the water oxidation reaction.
11
However, to our knowledge,
there have not been any reports on PEC enhancement of TiO
2
due to loading cobalt as a cocatalyst, probably due to the high
intrinsic oxidative power of the holes photogenerated within
the valence band of TiO
2
. We speculate that in this case the
cobalt treatment improves the PEC performance mainly via the
saturation of dangling bonds on the TiO
2
surface, thus
passivating the surface states which act as charge recombination
centers. Employing a silver treatment (similar to cobalt
treatment), in which 0.05 M AgNO
3
replaced 0.1 M Co(NO
3
)
2
,
leads to a similar improvement (Figure 2b), supporting our
speculation.
We have also noticed that the orientation of the FTO placed
in the reactor, i.e., whether it ‘faces up’ or ‘faces down’ during
the nanowire growth affects the PEC performance, although it
does not affect the growth rate of the nanowire arrays. The
films grown with the FTO ‘facing up’ yielded a photocurrent
∼10−15% higher than films grown with the FTO ‘facing down’.
The samples grown with the FTO ‘facing up’ have some flower-
like microsize particles on top (Figure S2, Supporting
Information) which have been reported to enhance light
harvesting, thus improving the PEC performance.
20,21
Synthesis of N-Modified TiO
2
Nanowire Arrays.
Nitrogen-modified TiO
2
films were prepared by annealing
TiO
2
nanowire films in an NH
3
flow (100 mL/min) at
temperatures from 400 to 650 °C. The color of all films
changed from c loudy white to bright yellow, indicating
successful N incorporation. The average feature size of N-
modified TiO
2
nanowires is around 15 nm, larger than that of
the as-synthesized sample, probably due to sintering of the
nanowires at elevated temperatures (Figure S3, Supporting
Information). At calcination temperatures higher than 500 °C
(i.e., 550, 600, and 650 °C), FTO substrates were damaged and
not electrically conductive. Wang et al. reported that at
temperatures higher than 550 °C, NH
3
decomposes, releasing
H
2
and causing partial reduction of TiO
2
.
22
The appearance of
Sn signals in the XRD patterns of these films suggests that the
SnO
2
layer was also reduced (data not shown). Compared with
films annealed at lower temperatures (i.e., 400 and 450 °C),
films nitrided at 500 °C showed the highest photocurrent.
Therefore, we focused on characterizing films annealed in NH
3
at 500 °C.
Chemical Characterization of N-Modified TiO
2
Nano-
wire Arrays. The N 1s XPS spectra of TiO
2
nanowire films
annealed at 500 °CinNH
3
flow both for 1 and 2 h are shown
in Figure 3a,3 and 4, respectively. Two N 1 s binding energy
peaks around 400 and 394 eV in the films annealed in NH
3
clearly indicate that N has been successfully incorporated into
the TiO
2
lattice. The N 1s peak at ∼400 eV can be attributed to
either interstitial N
23,24
atoms or chemisorbed N-containing
gas, such as NH
3
or N
2
.
4,25,26
However, its origin and
contribution to visible light absorption are still under debate.
According to early XPS investiga tions on N-modified
TiO
2
,
12,22,24−26
the N 1s peak at 392−396 eV was assigned
to β−N in the Ti−N bond or N substituted at oxygen sites
(substitutional N). There is no TiN formation indicated in the
XRD and also no Ti
3+
in the XPS spectra (a typical one is
shown in Figure 3b,2) of these films, suggesting that the N 1s
peak at 394 eV in our N-modified TiO
2
samples may be
assigned as substitutional N, resulting in a composition that can
be described as TiO
2−x
N
x
. The substitutional N species is
commonly recognized as a contributor to visible light absortion
and changes in photocatalytic activity. For example, Irie et al.
25
reported a monotonic increase in visible light absorption, yet a
monotonic decrease in photocatalytic activity with an increase
in the substitutional N concentration. In addition, we did not
observe formation of Ti
3+
(Figure 3b), one of the most
important types of color centers. Therefore we believe that the
substitutional N species found at 394 eV is likely the main
contributor to visible light absorption and changes in the water
photo-oxidation performance in the TiO
2
nanowire films, as
shown in the next section.
Shown in Table 2 is our XPS analysis with atomic
percentages of substitutional N in films annealed for 1 and 2
h of 0.35 and 1.08%, respectively. Since the surface is rich in
oxygen, probably due to the adsorption of oxygen-containing
species on the surface, we calculate the values of x in TiO
2−x
N
x
as x = atomic % of N/atomic % of Ti, resulting in x value of
0.012 and 0.043 for samples annealed in NH
3
for 1 and 2 h,
respectively. Compared with other N-modified TiO
2
materials
Figure 3. (a) Core N 1s XPS spectra of (1) anatase powder annealed
in NH
3
for two hours, (2) TiO
2
nanowire film annealed in air for 30
min and then annealed in NH
3
for two hours, and (3) and (4) TiO
2
nanowire films annealed in NH
3
at 500 °C for 1 and 2 h, respectively.
(b) Core Ti 2p XPS spectra of (1) as-synthesized TiO
2
nanowire film
and (2) a TiO
2
nanowire film annealed in NH
3
at 500 °C for 2 h.
Table 2. N-Dopant Concentration in TiO
2
Nanowire Films
Annealed at 500 °CinNH
3
annealing
conditions
N content/peak position
(atomic %, eV)
x in
TiO
2−x
N
x
500 °C, NH
3
, 1 h 0.35%, 393.4 eV
0.012
3.41%, 401.3 eV
500 °C, NH
3
, 2 h 1.08%, 394.2 eV
0.043
3.04%, 399.3 eV
Nano Letters Letter
dx.doi.org/10.1021/nl2028188 | Nano Lett. 2012, 12, 26−3229
synthesized via nitridation of TiO
2
in NH
3
, such as a rutile
TiO
2
(110) single crystal,
27
anatase powder,
25
anatase
films,
28,29
and anatase nanobelts,
22
the substitutional N
concentrations in the present films are significantly higher
despite lower nitridation temperatures and/or shorter times.
We believe that the small feature size of the nanowire arrays
allows better nitrogen diffusion into the TiO
2
lattice and that
this is likely the key to the enhancement in the N-incorporation
level. We performed control tests in which a TiO
2
nanowire
sample with a larger average characteristic size (of ∼25 nm)
prepared by preannealing the as-synthesized TiO
2
nanowire
sample in air at 500 °C in 30 min (Figure 3a,2) and a
commercial TiO
2
anatase nanoparticle powder with average size
of 32 nm (Alfa Aesar; Figure 3a,1) were annealed in NH
3
under
the same conditions as our as-synthesized TiO
2
nanowire films
(500 °C for 2 h). No N was detected in the XPS spectra,
indicating the N uptake is very small, below the detection limit
of the XPS instrument (0.1%), thus supporting our hypothesis.
PEC Properties of N-Modified TiO
2
Nanowire Films.
Figure 4a shows current−voltage characteristics in dark (blue
dotted line) and white (solid blue line) lights for the
TiO
1.988
N
0.012
film. Compared with unmodified TiO
2
films
(Figure 2a), there is a positive shift in the onset potential, E
on
from 0.2 to 0.5 V
RHE
. Indeed, the transient photocurrent onset
potentials for the two samples are almost the same, at around
−0.15 V
RHE
, indicating that the flat-band potential does not
shift with the inclusion of N. In this case,
30
even if the band gap
is reduced, the apparent photocurrent onset potential relative
to the reference electrode (in a three-electrode cell) should
theoretically remain the same. We, therefore, believe that the
shift in E
on
may be due to either a larger banding requirement
for separating electrons and holes because of the material’s
likely possession of poorer charge-transport properties than
pure TiO
2
or a slower water oxidation kinetics at the surface of
N-modified TiO
2
sample. Mo reover, compared to an
unmodified sample of the same thickness (1.6 μm), the N-
modified sample shows a noticeably lower photocurrent,
reaching 0.23 mA/cm
2
at 1.23 V
RHE
, compared to 0.38
mA/cm
2
for the unmodified TiO
2
nanowire film (Figure 2a).
Although most authors report an enhancement in the visible
light response for N-modified TiO
2
films, they also observe a
significant decrease in quantum yields in the UV region that
leads to poor PEC performance under whole spectrum (i.e.,
white light) illumination.
13,15,25,31
Poor PEC performance has
often been explained as being due to the formation of isolated
N 2p states above the valence band edge, which act as
electron−hole recombination centers. Using time-resolved
absorption spectroscopy, Tang et al. reported two distinct
photohole populations that are trapped at the N-induced
states.
32
They also demonstrated that the lack of water
oxidation is due to either rapid electron−hole recombination
between charges trapped at the N-incorporation induced states
or the reduced oxidative power of the photoholes leading to a
lack of thermodynamic driving force. Additionally, Chambers et
al. reported that the hole trapping probability at the N-induced
states is crystallographically dependent.
33
The hole trapping
probability increases if the photogenerated holes diffuse along
⟨110⟩ and ⟨001 ⟩ directions, and the detrapping probability
increases if the holes diffuse along ⟨100⟩ direction. As other
authors have suggested, it could be that when incorporating
TiO
2
with N, substitutional N 2p states hybridize with O 2p.
4,34
Since the N 2p state has a higher orbital energy than the O 2p
state, the orbital hybridization shifts the valence band edge to
more negative potentials, thus decreasing the oxidative power
of photogenerated holes, which hinders hole transfer rates to
oxidizable species on the film surface (H
2
OorOH
−
). A water
Figure 4. (a) Linear sweep voltammetry of the TiO
1.988
N
0.012
sample and the same electrode after cobalt treatment in darkness (dotted lines) and
under illumination (solid lines). (b) IPCE spectra of N-modified TiO
2
films at 1.4 V
RHE
: blue and red curves are the corresponding IPCE spectra of
the TiO
1.988
N
0.012
photoelectrode in Figure 4a, black curve is the IPCE of unmodified TiO
2
sample after cobalt treatment, and green curve is the
IPCE of the TiO
1.957
N
0.043
pretreated with cobalt. (c) and (d) UV−vis absorbance and transmittance spectra of unmodified and N-modified TiO
2
nanowire samples, and an as-synthesized sample (black curve) was included as a reference.
Nano Letters Letter
dx.doi.org/10.1021/nl2028188 | Nano Lett. 2012, 12, 26−3230