LaTiO2N as a visible-light (a??600 nm)-driven photocatalyst (2)

Article abstract of DOI:10.1021/jp026767q

LaTiO₂N, a Ti-based oxynitride, was studied as a visible-light (420-600 nm)-driven photocatalyst. DFT calculation for LaTiO₂N indicated that the top of the valence band consists predominantly of N 2p orbitals with a small contribution by O 2p orbitals, while the bottom of the conduction band is made up entirely of empty Ti 3d orbitals. LaTiO₂N was synthesized from a La₂Ti₂O₇ precursor at 1123 K under NH₃ flow. The relationship between the preparation condition and the photocatalytic activities of H⁺ reduction to H₂ and H₂O oxidation to O₂ was examined. It was found that the photocatalytic activity for O₂ evolution increased with nitridation time, reaching a maximum at 72 h. Further treatment beyond 72 h lowered the rate of O₂ evolution. The rate of H₂ evolution was almost independent of nitridation time. The relationship between nitridation time and the rate of O₂ evolution is discussed on the basis of X-ray photoelectron spectroscopy (XPS) analyses.

Full text of DOI:10.1021/jp026767q

J. Phys. Chem. B 2003, 107, 791-797
791
LaTiO2N as a Visible-Light (e600 nm)-Driven Photocatalyst (2)
Asako Kasahara,† Kohta Nukumizu,† Tsuyoshi Takata,† Junko N. Kondo,† Michikazu Hara,†
Hisayoshi Kobayashi,‡ and Kazunari Domen*,†,#
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama
226-8503, Japan, Department of Chemistry and Bioscience, Kurashiki UniVersity of Science and the Arts,
Nishinoura 2640, Tsurajima, Kurashiki 712-8505, Japan, and Core Research for EVolutional Science and
Technology, Japan Science and Technology Co. (CREST, JST), 2-1-13 Higashi-Ueno, Taito-ku 110-0015, Japan
ReceiVed: August 15, 2002; In Final Form: NoVember 19, 2002
LaTiO2N, a Ti-based oxynitride, was studied as a visible-light (420-600 nm)-driven photocatalyst. DFT
calculation for LaTiO2N indicated that the top of the valence band consists predominantly of N 2p orbitals
with a small contribution by O 2p orbitals, while the bottom of the conduction band is made up entirely of
empty Ti 3d orbitals. LaTiO2N was synthesized from a La2Ti2O7 precursor at 1123 K under NH3 flow. The
relationship between the preparation condition and the photocatalytic activities of H+ reduction to H2 and
H2O oxidation to O2 was examined. It was found that the photocatalytic activity for O2 evolution increased
with nitridation time, reaching a maximum at 72 h. Further treatment beyond 72 h lowered the rate of O2
evolution. The rate of H2 evolution was almost independent of nitridation time. The relationship between
nitridation time and the rate of O2 evolution is discussed on the basis of X-ray photoelectron spectroscopy
(XPS) analyses.
1. Introduction
Photocatalytic reactions are currently applied in various fields
to solve energy problems and to meet the increasingly stringent
standards set by environmental regulations. Heterogeneous
photocatalysts offer great potential for converting photon energy
into chemical energy and for decomposing pollutants in air or
in solution. In particular, photocatalysts that catalyze overall
water splitting under visible light irradiation would have great
potential in solar energy applications. However, to date, no
effective visible-light-driven photocatalyst for the reaction has
been devised. Although some Ti4+, Nb5+, and Ta5+-based oxides
such as SrTiO3, K2La2Ti3O10, K4Nb6O17, and NaTaO3 function
as stable and efficient photocatalysts for overall water splitting
to form stoichiometric H2 and O2,1-4 these materials are not
activated by visible light because the tops of the valence bands
are composed of O 2p orbitals with low potential energies,
resulting in large band gap energies of over 3 eV.5
Figure 1. Schematic structures of (a) La2Ti2O7 and (b) LaTiO2N.
Recently, (oxy)nitrides containing Ti4+ or Ta5+, such as
LaTiO2N,6 TaON,7 and Ta3N5,8 have been reported as potential
visible-light-driven photocatalysts. These stable inorganic ma-
terials are synthesized by nitriding the corresponding metal
oxides under NH39,10 and absorb visible light up to wavelengths
of 500-600 nm. Density functional theory (DFT) calculations
for TaON and Ta3N5 have revealed these materials to have the
following common features. The formal electronic configura-
tions of transition metal cations are d0, the bottoms of the
conduction bands consist of empty d orbitals, and the tops of
the valence bands consist mainly of N 2p orbitals.
(λ e 600 nm) functions as a photocatalyst for the reduction of
H+ to H2 and oxidation of H2O to O2 in the presence of a
sacrificial electron donor (methanol) and acceptor (Ag+). These
photoreactions were confirmed to proceed via a band gap
transition of 2.1 eV on the basis of the wavelength dependence
of the activity. In this paper, more detailed results of the
preparation conditions and the photocatalytic activity of LaTiO2N
are presented.
The schematic structures of La2Ti2O7, a precursor, and
LaTiO2N are shown in Figure 1. La2Ti2O7 is a monoclinic
compound with space group P21 formed of alternating perovs-
kite-like blocks of nTiO6 octahedron slabs.11 On the other hand,
the oxynitride is a triclinic compound with space group P(-1),
the same structure as a perovskite oxide represented by ABO3
(A, B ) metal cations), and is composed of a TiOxNy octahedral
structure (x + y ) 6).9,12
The present authors have already reported briefly on LaTiO2N
as a photocatalyst.6 LaTiO2N, when irradiated with visible light
† Tokyo Institute of Technology.
‡ Kurashiki University of Science and the Arts.
# CREST.
10.1021/jp026767q CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/28/2002

792 J. Phys. Chem. B, Vol. 107, No. 3, 2003
Kasahara et al.
2. Experimental Section
2.1. Synthesis of LaTiO2N. LaTiO2N was prepared by
nitriding La2Ti2O7 under NH3 flow.6,9,12 La2Ti2O7 was obtained
from a corresponding oxide precursor containing stoichiometric
amounts of La3+ and Ti4+ cations (1:1) by oxidation at 1423 K
for 10 h in air.13,14 The resulting La2Ti2O7 powder was nitrided
at 1123 K under a flow of NH3 at 20 cm3 min-1 in a quartz
tube reactor. Effluent gas with entrained water produced during
nitridation was passed into a silicon oil bubbler at 420 K. The
bubbler was directly connected to the nitridation reactor to
prevent contamination of water and O2 in air. The powder
obtained after nitridation varied in color from orange to dark
brown with increasing nitridation time.
2.2. Characterization of Catalysts. The prepared samples
were studied by X-ray powder diffraction (XRD, Rigaku
Geigerflex RAD-B, Cu KR), scanning electron microscopy
(SEM, S-4700, Hitachi), energy-dispersive X-ray spectroscopy
(EDX, EMAX 7000, Horiba), and UV-visible diffuse reflec-
tance spectroscopy (UV-vis DR, V-560, Jasco). The surface
of the prepared sample was examined by X-ray photoelectron
spectroscopy (XPS, ESCA 3200, Shimadzu). The binding energy
was corrected against Au 4f3/2 (83.8 eV). The amount of N in
a sample was measured by elemental analysis (CHNS-932,
LECO).
2.3. Photocatalytic Reactions. The reactions were carried
out in a Pyrex reaction vessel connected to a closed gas
circulation and evacuation system. Photoreduction of H+ to H2
and photooxidation of H2O to O2 in the presence of a sacrificial
electron donor (methanol) and acceptor (Ag+) were examined
as test photoreactions. H2 evolution was examined in an aqueous
solution (200 mL) containing 0.20 g of the sample loaded with
3 wt % of Pt and 20 mL of methanol. The sample was reduced
in H2 at 573 K after loading with Pt from [Pt(NH3)4]Cl2 by
impregnation. For a typical photooxidation of water into O2,
the reaction was performed in an aqueous AgNO3 solution (0.01
mol dm-3, 200 mL) containing 0.20 g of the catalyst and 0.20
g of La2O3 powder, a basic oxide added to maintain the solution
at a pH of approximately 8 during the reaction (see section 3.3).
The reaction solution was evacuated several times to remove
air and then irradiated with a 300-W Xe lamp equipped with a
cutoff filter. The evolved gas was analyzed by gas chromatog-
raphy. Quantum efficiencies (Φ) were calculated using the
following equation:
Figure 2. Four models of atomic arrangement for (LaTiO2N)4. In
model A, Ti atom has “axial” Ti-N bonds. In models B and C, Ti
atom has “equatorial” Ti-N bonds with trans and cis configurations,
respectively. In model D, Ti atom has “axial” and “equatorial” mixed
Ti-N bonds. Models A-D preserve P(-1) symmetry.
coordinates were referenced from Clarke et al.12 The unit cell
consisted of (LaTiO2N)4 with three distinct sites for N atoms.
Four N atoms were placed in a unit cell. The P(-1) crystal
symmetry was preserved to give models A-D in Figure 2. Each
unit cell contained 160 electrons. Model C gave the lowest total
energy of -14 519.98 eV, and relative energies measured from
that of model C were 1.21, 0.22, and 0.44 eV for models A, B,
and D. The calculated band gap energies for models A-D were
0.63, 0.66, 0.67, and 0.66 eV, respectively. Although current
DFT methods predict smaller band gaps than are seen experi-
mentally, this discrepancy is very large when applied to
oxynitrides. Major reason is ascribed to the fact that the N atoms
are placed randomly to the O atom sites and only the averaged
atomic coordinates are available from the literature.12 The unit
cell structure for model C was optimized with a constraint of
lattice parameters. The total energy only decreased by 0.82 eV,
but the band gap greatly increased to 1.57 eV, which is 75% of
the experimental value (2.1 eV).
The band dispersion relation and density of states are shown
in Figure 3 for the optimized structure of model C. The orbitals
are numbered and the 80 occupied orbitals are classified into
eight bands. The lowest band consists of the Ti 3s orbitals. The
Ti 3p and La 5s bands appear in almost the same energy range.
The O 2s band then appears, followed by the La 5p band. The
N 2s band lies in an energy region 5-6 eV higher than the O
2s band. The valence bands are comprised of O 2p orbitals for
the lower-energy side and N 2p orbitals for the higher-energy
side. The splitting of these orbitals is less remarkable compared
to that between the O 2s and N 2s orbitals. Figure 4 shows the
Φ (%) ) (AR/I) × 100
where A is a coefficient for a particular photoreaction (for H2
evolution, A ) 1;15 for O2 evolution, A ) 4), R is the H2 and
O2 evolution rate (molecules h-1), and I is the rate of
introduction of incident photons into a reaction vessel. The
absorption rate of incident photons was measured using a Si
photodiode, typically 9.6 × 1021 photons h-1 at 420 < λ <
600 nm. It was assumed that the quantum efficiencies were
constant in the wavelength range of 420-600 nm for both
reactions. Therefore, the quantum efficiencies obtained in this
work are regarded as averaged apparent quantum efficiencies.
3. Results and Discussion
3.1. DFT Electronic Structure Calculations. The electronic
structure of LaTiO2N was calculated using a planewave-based
DFT program, CASTEP.16 The core orbitals were replaced by
ultrasoft core potentials,17 and La 5s25p65d16s2, Ti 3s23p64s2-
3d2, N 2s22p3, and O 2s22p4 electrons were treated explicitly.
The kinetic energy cutoff was set at 260 eV. The atomic

LaTiO2N as a Photocatalyst
J. Phys. Chem. B, Vol. 107, No. 3, 2003 793
Figure 3. Band dispersion and density of states for optimized structure of model C of LaTiO2N. A total of 80 occupied orbitals are classified into
eight bands.
four density contour maps from the lower to the upper parts of
the valence band.
morphology among the samples prepared by nitridation for more
than 48 h. The TEM image and electron diffraction pattern of
LaTiO2N reveal that well-crystallized LaTiO2N was formed by
nitridation for 72 h.
The bottom of the conduction band is made up of Ti 3d
orbitals. Then the La 4f and also 5d orbitals appear in a little
higher energy range. Figure 5 shows the four density contour
maps for orbitals 81, 90, 91, and 100. The Ti 3d orbitals are
dominant for the former two, and the La 4f, 5d orbitals are major
contribution for the latter two.
Figure 10 shows the UV-vis spectra for the samples. The
absorption edge of La2Ti2O7 is found to be ca. 320 nm, and the
band gap energy is estimated to be 3.9 eV. The UV-vis spectra
of the sample after nitridation consists of two broad bands
located above and below 600 nm, and both bands increase with
nitridation time. There is no significant difference in the UV-
vis spectra among the samples obtained by nitridation for more
than 72 h. Heating a sample, prepared by nitridation for 72 h,
at 573 K for 24 h in air produced no change in the XRD pattern
but reduced the band intensity above 600 nm, as shown in
spectrum e in Figure 10. Absorption in the near-infrared region
above 600 nm is attributable to a small amount of Ti3+ species.
Therefore, the band gap transition is estimated to extend to as
long as ca. 600 nm, indicating that the band gap energy is 2.1
eV.
3.3. Photocatalytic Reactions. Figure 11 shows the time
courses of O2 evolution on LaTiO2N under visible light
irradiation (λ > 420 nm) in an aqueous AgNO3 solution under
several pH conditions. LaTiO2N prepared by nitridation for 72
h was used in the reaction because this LaTiO2N sample
exhibited the highest activity for O2 evolution (see section 3.4).
O2 evolution did not take place in the dark. When the pH was
maintained at about 8 by the addition of La2O3 (Figure 11a),
O2 evolved efficiently with a small amount of N2 formation, as
demonstrated in our previous paper.6 In the presence of a basic
oxide, La2O3, the pH of the solution during the photoreaction
is buffered at pH ) 8-9 by the dissolution (La2O3 is a basic
oxide), and the efficient oxidation of water by (oxy)nitrides
including LaTiO2N proceed under such relatively high concen-
tration of OH-.6-8 The initial quantum efficiency (QE) was
estimated to be 1.5%. Although the N2 evolution may suggest
some degradation of the catalyst, it is attributed to surface
With the optimized structure of model C, the bandgap energy
of 1.57 eV was estimated. The effect of optimization is not a
simple decrease in Ti-N bond lengths. The sum of four Ti-O
bond lengths and two Ti-N bond lengths increased by ca. 0.1
Å. By the optimization, Ti atoms move asymmetrically, that is,
one Ti-N(O) bond length reduces and the other one is stretched.
3.2. XRD, SEM, TEM, and UV-vis Spectra. Figure 6
shows the XRD patterns for samples obtained by nitriding La2-
Ti2O7 at 1123 K for several time periods. After 48 h of
nitridation, the structure of La2Ti2O7 disappeared and the XRD
pattern became characteristic of a perovskite-type LaTiO2N.9
Nitridation for more than 48 h increased the diffraction peak
intensities yet did not promote impurity phases. The bulk atomic
ratios of Ti/La, O/La, and N/La for each sample are shown in
Figure 7. The Ti/La ratios were measured by EDX, while the
O/La and N/La ratios were estimated on the basis of the quantity
of N in the samples by elemental analysis and the charge balance
among La3+, Ti4+, O2-, and N3-: assuming that the charge
valance is set up among La3+, Ti4+, O2-, and N3- in a sample
without defects, the O/La ratio was estimated. The O/La ratio
values could contain some error. The N/La ratio increased with
nitridation time to a constant (N/La ) 0.96) after 94 h. This
result indicates that nitridation for more than 94 h yields near-
stoichiometric LaTiO2N.
SEM and TEM images of the samples before and after
nitridation for 72 h are shown in Figures 8 and 9. The
introduction of N3- into La2Ti2O7 from NH3 results in a rough
LaTiO2N morphology. There is no noticeable difference in

794 J. Phys. Chem. B, Vol. 107, No. 3, 2003
Kasahara et al.
Figure 5. Density contour maps of the conduction band: (A) density
for the lowest unoccupied orbital 81 and exclusive contribution from
Ti 3d orbitals; (B) density for orbital 90 and almost all density on Ti
3d orbitals; (C) density for orbital 91 and primary contribution from
La 4f and 5d orbitals; (D) density for orbital 100 and primary
contribution from La 4f and 5d orbitals.
Figure 4. Density contour maps of the valence band: (A) density for
the lowest orbital in the valence band 45 and dominant contribution
from O 2p orbitals; (B) density for orbital 55 and mutual contribution
from O 2p and N 2p orbitals; (C) density for orbital 75 and mutual
contribution from O 2p and N 2p orbitals; (D) density for the highest
occupied orbital 80 and larger contribution from N 2p orbitals.
nitrogen species because the evolution rate became almost
negligible after 2 h. The estimated amount of surface nitrogen
is about 11 µmol, which is very similar to the amount of N2
that evolves during photocatalytic reactions.18 Ca2+-doped
LaTiO2N modified by an IrO2 colloid exhibited much higher
activity (QE ) 5%)6 with excellent stability, demonstrating the
functionality of this material.6 A similar effect was also observed
for LaTiO2N loaded with the IrO2 colloid.
The strong pH dependence of O2 evolution from LaTiO2N is
noteworthy. Figure 11b,c shows gas evolution on the same
LaTiO2N (72 h of nitridation) sample in an aqueous AgNO3
solution without La2O3. The pH of the solutions was adjusted
before the reactions to 6.0 and 9.0 with 0.01 M HNO3 and
NaOH aqueous solutions. The pH of the solutions after the
reaction is indicated in the figure. In the case of pH 6.0, the
rate of O2 evolution was suppressed remarkably and O2
evolution was accompanied by the evolution of about 50 µmol
of N2. In fact, only 2.1 µmol of O2 evolved over 5 h, and the
pH of the solution dropped to 4.3. This decrease in pH of the
200-mL solution indicates that about 10.0 µmol of H+ was
produced for the evolution of about 2.5 µmol of O2, as given
by the reaction
Figure 6. XRD patterns for (a) La2Ti2O7 and samples obtained by
nitriding La2Ti2O7 at 1123 K after nitridation for (b) 22, (c) 48, (d) 72,
(e) 94, and (f) 120 h.
µmol). The decrease in pH is therefore attributed to the
production of H+ through the oxidation of water. However, it
is noted that degradation of LaTiO2N proceeds to some extent
in an acidic solution to form N2, following the reaction
2N3- + 6h+ f N2
4Ag+ + 2H2O f 4H+ + O2 + 4Ag0
This is in good agreement with the amount of evolved O2 (2.1
In an alkaline solution (Figure 11c, initial pH ) 9.0) without

LaTiO2N as a Photocatalyst
J. Phys. Chem. B, Vol. 107, No. 3, 2003 795
Figure 7. Variation in Ti/La, O/La, and N/La atomic ratios of samples
with nitridation time.
Figure 8. SEM images of (a) La2Ti2O7 and (b) LaTiO2N (72 h
nitridation).
Figure 9. TEM images and electron diffraction patterns of La2Ti2O7
and LaTiO2N (72 h nitridation).
Figure 10. UV-vis DR spectra of (a) La2Ti2O7, samples obtained by
nitriding La2Ti2O7 at 1123 K after nitridation for (b) 22, (c) 48, and
(d) 72 h, and sample obtained (e) by heating LaTiO2N (72 h of
nitridation) in air at 573 K for 24 h.
Figure 11. O2 evolution from LaTiO2N under visible light irradiation
(>420 nm) for LaTiO2N (72 h of nitridation) ) 0.2 g, AgNO3 solution
) 0.01 mol dm-3, 200 mL: (a) La2O3 ) 0.2 g, initial pH ) 8.4; (b)
without La2O3, initial pH ) 6.0; (c) without La2O3, initial pH ) 9.0.
La2O3, the initial rate of O2 evolution was higher than that in
the acidic solution but O2 evolution ceased after 2 h. The pH
was reduced to 3.8 by the end of reaction. These results are
interpreted as follows. Under basic conditions, oxidation of water
on LaTiO2N produces O2 and H+, thereby lowering the pH of
the solution. When the solution becomes acidic, O2 evolution
slows and finally stops. The amount of evolved N2 was almost
the same (ca. 50 µmol) in both experiments (Figure 11b,c),
suggesting similar degradation of the catalyst.
Figure 12 shows the time courses of H2 evolution on LaTiO2N
(72 h) under visible light irradiation (λ > 420 nm) in aqueous

796 J. Phys. Chem. B, Vol. 107, No. 3, 2003
Kasahara et al.
Figure 14. Dependence of surface Ti/La, O/La, and N/La atomic ratios
on nitridation time for LaTiO2N.
Figure 12. H2 evolution from LaTiO2N under visible light irradiation
(>420 nm) for Pt-coated LaTiO2N (72 h of nitridation) ) 0.2 g,
methanol solution ) 200 mL (distilled water 180 mL, methanol 20
mL): (a) without La2O3; (b) La2O3 ) 0.2 g.
Figure 15. AES spectra of Ta foil behind La2Ti2O7 powder in the
quartz reactor (a) before nitridation and (b) after nitridation under NH3
flow (20 cm3 min-1) at 1123 K for 120 h.
the growth of LaTiO2N crystal. However, the formation of
stoichiometric LaTiO2N reduces the rate of O2 evolution.
Each sample was investigated by XPS to clarify the depen-
dence of the rate of O2 evolution on nitridation time. The surface
atomic ratios of Ti/La, O/La, and N/La of each sample are
shown in Figure 14. These atomic ratios were obtained from
the peak intensities of La 3d, Ti 2p, O 1s, and N 1s orbitals in
the XPS spectrum for each sample. During the initial 22 h of
nitridation, the surface N/La ratio increases rapidly to 0.6, and
after 22 h, the amount of surface N gradually reaches saturation
(N/La ) 0.85) at somewhat less than the bulk concentration,
as shown in Figure 7 (N/La ) 0.96). The bulk Ti/La ratio
remains almost unchanged (Ti/La ) 1) with nitridation, as
shown in Figure 7, whereas the surface Ti/La ratio is reduced
to about 0.7 by nitridation for 120 h. After nitridation for 120
h, the quartz tube holding the sample in the nitridation reactor
was colored dark blue, suggesting that some material was
leached from the sample into the flow of NH3 during nitridation.
To detect the desorbed species, a Ta foil was placed behind the
sample powder in the reactor and nitridation was carried out at
1123 K for 120 h under 20 cm3 min-1 flow of NH3. Figure 15
shows the Auger electron spectroscopy (AES) differential
spectra of the Ta foil before and after nitridation. A peak
assigned to the Ti LMM Auger electron transition emerges
clearly after nitridation, indicating that surface Ti is desorbed
during nitridation. Although the desorption mechanism remains
Figure 13. Evolution rates of H2 and O2 from LaTiO2N prepared with
various nitridation times (>420 nm): (H2) Pt-coated LaTiO2N (0.2 g),
methanol solution (200 mL; distilled water 180 mL, methanol 20 mL);
(O2) LaTiO2N (0.2 g), La2O3 (0.2 g), AgNO3 solution (200 mL, 0.01
mol dm-3).
methanol solutions with and without La2O3 (initial pH ) 6.5
and 7.6). In both cases H2 evolution was observed, but N2 was
not detected by gas chromatography, indicating that LaTiO2N
reduces H+ to H2 without appreciable degradation.
3.4. Variation in Photocatalytic Activity with Nitridation
Time. Figure 13 shows the dependence of the rates of H2 and
O2 evolution under visible light irradiation on nitridation time.
The rates plotted in this figure were obtained during the initial
stages of the reactions (1-2 h). The rate of O2 evolution
increases with nitridation time up to a maximum rate at 72 h.
Further nitridation results in a decrease in activity for water
oxidation. In contrast, after 48 h, the rate of H2 evolution
becomes independent of nitridation time. H2 and O2 evolution
occurs and a LaTiO2N phase emerges at 22 h (according to
XRD), as shown in Figure 6. The evolution rates increase with

LaTiO2N as a Photocatalyst
J. Phys. Chem. B, Vol. 107, No. 3, 2003 797
unclear, the deep blue color of the reactor implies that reduced
Ti species are liberated from the surface. One possible explana-
tion for the sharp activity drop with nitridation longer than 120
h is the desorption of surface Ti. The rate of O2 evolution
increases with the formation of LaTiO2N, but a considerable
amount of surface Ti desorbs during the long nitridation period,
and thereby photocatalytic activity for the oxidation of water
decreases. Another possibility is the increase of defects. As
shown in Figure 14, the Ti/La ratio decreased from an initial
stage of nitridation, and the O2 evolution rate drop with
nitridation longer than 120 h cannot be ascribed to only Ti
removal. LaTiO2N is formed by reduction of La2Ti2O7, followed
by nitriding, and at the nitridation temperature (1143 K), a
considerable amount of NH3 is decomposed to N2 and H2. Many
defects would occur by nitriding for a long period under such
a reductive atmosphere, resulting in a decrease in photocatalytic
activity. Nitridation in the presence of a small amount of oxidant,
such as H2O or O2, may prevent surface Ti from desorbing and
thereby improve the photocatalytic activity of LaTiO2N.
No significant difference in the rate of H2 evolution was
observed among the present samples for all nitridation periods,
although the observed formation of perovskite LaTiO2N is
known to be essential for H2 evolution. This difference between
O2 evolution and H2 evolution is thought to be due to the
differing behavior of active sites for water oxidation and
reduction.
Acknowledgment. This work was supported by the Core
Research for Evolutional Science and Technology (CREST)
program of the Japan Science and Technology Corporation
(JST).
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(13) The metal oxide precursor was prepared by a polymerized complex
method, a new sol-gel method, as follows:14 Titanium tetraisopropoxide
(5.7 g) and 8.7 g of La(NO3)‚6H2O were dissolved in 98.8 g of ethylene
glycol at room temperature, followed by addition of 76.4 g of anhydrous
citric acid and 102.0 g of methanol. The mixture was then stirred at 403 K
until a transparent gel was formed. The polymer was then carbonized at
623 K and calcined in air at 923 K for 2 h to remove carbon.
(14) Kakihana, M. J. Sol-Gel Sci. Technol. 1996, 6, 7.
(15) In the case of methanol, one photon was assumed to form a H2
molecule because of the current-doubling effect.
(16) Payne, M. C.; Teter, M. P.; Allan, D. C.; Arias, T. A.; Joannopoulos,
J. D. ReV. Mod. Phys. 1992, 64, 1045.
(17) Vanderbilt, D. Phys. ReV. 1990, B41, 7892.
4. Conclusion
According to DFT calculations, LaTiO2N is revealed to have
a valence band with a top composed mainly of N 2p orbitals
with a small contribution by O 2p orbitals, while the bottom of
its conduction band consists of Ti 3d orbitals. In an alkaline
solution, LaTiO2N efficiently oxidizes water into O2 under
visible light (e600 nm) irradiation, while in an acid solution,
it does not function as a viable photocatalyst. H2 evolution on
LaTiO2N proceeds without a strong dependence on the pH of
the solution. A perovskite phase of LaTiO2N was observed and
is essential for maintaining the photocatalytic activity of this
catalyst. Nitridation of La2Ti2O7 at 1123 K for more than 72 h
formed near-stoichiometric LaTiO2.06N0.96 in the bulk, but the
surface concentration of Ti was found to decrease with nitri-
dation time, resulting in lower activity for water oxidation.
(18) The amount of surface nitrogen species was estimated as follows:
N2 (mol) ) {s(cat)(2.67/2)(3.13 × 10-19) m2}/AN, where s(cat) is the surface
area of LaTiO2N used in the photocatalytic reaction (BET surface area 11
m2 g-1). The catalyst surface was assumed to consist entirely of (100)
oriented crystals and to contain 2.67 N atoms per 3.13 × 10-19 m2.12 AN is
Avogadro’s number. The amount of surface nitrogen species was determined
by XPS to be 8.0 µmol. Using the surface atomic ratio (O/N ) 2.26:0.71)
obtained by the peak intensities of La 3d, Ti 2p, O 1s, and N 1s in the XPS
spectrum gives a value of 8.0 µmol of nitrogen species.