Hydrogenation of TiO2 nanosheets with exposed 001 facets for enhanced photocatalytic activity

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Hydrogenation of TiO2 nanosheets with exposed {001} facets for
enhanced photocatalytc activity{
Wang Wei, Ni Yaru, Lu Chunhua* and Xu Zhongzi
Received 26th May 2012, Accepted 20th July 2012
DOI: 10.1039/c2ra21049e
Hydrogenated {001}-facets-dominated anatase TiO2 nanosheets
have O–H bonds and O2 on their surface. The high reactive {001}
facets were maintained by the formation of Ti–H bonds. A large
number of Ti3+ ions and oxygen vacancies were produced by
hydrogenation, resulting in improved light absorption and
enhanced photocatalytic activity.
TiO2, the most popular wide-band-gap photocatalysts, which can use
the UV part of solar light to degrade organic pollutants and produce
hydrogen from water, have attracted world-wide attention in the last
two decades.1,2 The photoactivity performance of TiO2 is mainly
dependent on its morphology, crystal structure, light absorption, and
surface chemical environment. The {001} facets of anatase TiO2 have
been considered to have higher photocatalytic activity than the
traditional {101} facets because of their higher surface energy and
the 100% unsaturated Ti5c atoms located on the {001} facets.
3,4 To
obtain highly reactive TiO2, most works have been focused on the
controllable growth of {001}-facets-dominated TiO2. Liu et al.
prepared {001}-facets-dominated TiO2 microspheres, resulting in
high photocatalytic acitivity.5 Xie et al. successfully synthesized
hollow TiO2 boxes with a high percentage of exposed {001} facets.
6
However, systematic studies on the location position and effect of F,
the most commonly used agent to control the exposure of the {001}
facets, are scarce. On the other hand, it is also very important to
enhance the visible light, even infrared light, absorption of TiO2.
Recently, Chen et al. prepared hydrogenated TiO2 nanocrystals to
introduce a surface disorder structure, extending the light absorption
to the near infrared range (y1200 nm).7 Wang et al. reported that
the light absorption and water splitting ability of TiO2 nanowire
arrays were significantly enhanced by hydrogenation.8 To date, there
is no systematic experimental study on the hydrogenation of {001}-
facets-dominated anatase TiO2 nanosheets, especially those that are
F-modified (surface adsorption and inner substitution), to further
improve their photocatalytic activity. Systematic studies on the
nature of hydrogenation treatment and its correlation with the highly
reactive {001} facets are very interesting topics.
The hydrothermal method is very effective in controlling the
growth of crystal particles with certain morphologies. Herein, we
report the hydrogenation of F-modified anatase TiO2 nanosheets
with a high percentage of exposed {001} facets by annealing the fine
shaped pristine hydrothermal product under a high pressure
hydrogen atmosphere. The photocatalysts were characterized by
UV-visble diffuse reflectance spectroscopy, transmission electron
microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy,
electron paramagnetic resonance (EPR), and X-ray photoelectron
spectroscopy (XPS). We show that TiO2–H has Ti–H and O–H
chemical bonds and O2 species on its surface, and Ti3+ and an
oxygen vacancy located inside the crystal structure. The {001} facets
of TiO2–H are maintained by the formation of Ti–H chemical bonds,
resulting in the change from Ti5c to Ti6c. The light absorption of
TiO2–H is extended to the infrared range, and its photocatalytic
activity is highly enhanced.
The TEM images of TiO2 (Fig. 1a) and TiO2–H (Fig. 1b) reveal
that both of the photocatalysts are square sheets in shape. After
hydrogenation, the particles of TiO2–H are mutually bonded to each
other slightly, and the exposed {001} facets are maintained. The
HRTEM (Fig. 1c and d) images of TiO2 further depict that the
thickness of TiO2 is ca. 20 nm and prove the exposure of the {001}
facets.9 Besides the TEM analysis, the XRD patterns (Fig. 1e) also
prove the anatase phase of TiO2 and TiO2–H (JCPDS No. 21-1272).
More importantly, the (101) diffraction peak of TiO2–H gives a
slight shift toward a higher diffraction angle, indicating a smaller
interplanar crystal spacing (Fig. S1, ESI{). This result demonstrates
that structural changes have occured in TiO2 during the hydrogena-
tion process. The UV-visible diffuse reflectance spectra show that
{001}-facets-dominated TiO2–H exhibits a high and broad light
absorption band toward the infrared range (Fig. 1f). This result is in
good agreement with the color change of the photocatalysts from
white TiO2 to dark blue TiO2 (inset of Fig. 1f). The XPS valence
band spectra (Fig. S2, ESI{) depict that TiO2 and TiO2–H have a
nearly overlapped band edge below the Fermi energy, which
confirms that hydrogenation has a negligible effect on the valence
band position of TiO2–H.
8
Fig. 2a and b compares the Raman spectra differences of TiO2
and TiO2–H. Two features can be revealed by comparing the spectra:
the first feature is the weakening of all the diffraction peaks at
144 cm21, 396 cm21, 515 cm21 and 636 cm21 in TiO2–H. Second,
State Key Laboratory of Materials-Orient Chemical Engineering, College
of Materials Science and Engineering, Nanjing University of Technology,
Nanjing, 210009, PR China. E-mail: lchnjut@163.com;
Fax: +86-25-83587220; Tel: +86-25-83587252
{ Electronic Supplementary Information (ESI) available: details of the
synthesis and characterization of the photocatalysts, powder XRD data of
the (101) peak, valence band XPS spectra, O 1s and F 1s XPS spectra,
photocatalytic activity measurement of TiO2 and TiO2–H. See DOI: 10.1039/
c2ra21049e
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8286 | RSC Adv., 2012, 2, 8286–8288 This journal is � The Royal Society of Chemistry 2012
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the two Eg modes at 144 cm
21 and 636 cm21 of TiO2–H are shifted
to a higher frequency. These two features demonstrate the increase in
the number of oxygen vacancies in the lattice structure of TiO2–H.
9,10
According to the XRD analysis, the shift of the diffraction peak to
the higher angle indicates the reduction of the interplanar distance.
This can be ascribed to the escape of O atoms from the crystal lattice,
which often results in Ti3+.11 Fig. 2c shows the Ti 2p core level XPS
spectra of TiO2 and TiO2–H. The two peaks centered at y458.5 eV
and y464.4 eV correspond to the characteristic Ti 2p3/2 and Ti 2p1/2
peaks of Ti4+.7,12 The peak of TiO2–H shows a shift towards the
lower binding energy, indicating that Ti atoms in TiO2 and TiO2–H
have different chemical environments. More importantly, a detect-
able shoulder, which can be ascribed to Ti3+,9 appears in the low-
energy range in both of the samples. Regarding the origin of Ti3+
centers in both of the samples, low temperature EPR analysis
(Fig. 2d) is conducted to give a systematic study. It is well known
that HF will serve as the morphology control agent during the
preparation of pristine {001}-facets-dominated TiO2. Thus the
surface of TiO2 is adsorbed by the F atoms. However, TiO2 gives
a strong EPR signal and the calculated g values are g) = 1.992 and
gI = 1.962, which are the typical g values for a paramagnetic Ti
3+
center.13 This type of Ti3+ signal can be ascribed to originate from the
substitution of O22 ions by the F2 ions.14 The phenomenon shows
that the F atoms not only serve as morphology control agents but
also modify the inner structure of TiO2. No other signal, such as O
2
or O22, is detected, indicating the Ti3+ are located in the bulk of theTiO2 because surface Ti
3+ will adsorb O2.
13 Compared to TiO2,
TiO2–H gives a very different and stronger EPR signal which can be
ascribed to the Ti3+ and O2.15 The stronger Ti3+ signal is in good
agreement with the XPS analysis. The broadened Ti3+ signal of
TiO2–H may result from the different coordinations of Ti
3+. In TiO2,
the Ti3+ is produced by F substitution, which induces the chemical
bond Ti–F–Ti.14 In contrast, there will be Ti–%–Ti chemical bond in
TiO2–H as a result of the escape of the O and F atoms. The oxygen
atoms in TiO2 will interact with the H atoms, which will also produce
one kind of Ti3+. This result can also be obtained from the O 1s XPS
spectra (Fig. S3, ESI{). The signal centered at 529.8 eV and 531.3 eV
are the typical signals of Ti–O–Ti and surface OH species,
respectively.16 The O 1s spectra illustrate that TiO2–H has much
more surface OH species, which are produced by the O and H
interaction, than TiO2. Comparing to TiO2, the F atoms in TiO2–H
are almost removed (Fig. S4, ESI{). The F atoms may escape from
the surface and inner structure of TiO2–H because of the similar
chemical properties of O and F. Thus, more oxygen vacancies are
formed and the surface Ti–F bonds are broken.
Fig. 3a compares the photocatalytic activity of TiO2, TiO2–H, and
P25 in decomposing MB under UV-visible light irradiation. The
photocatalysis efficiency of TiO2–H is much higher than that of TiO2
and P25. The photocatalytic activity (Fig. S5, ESI{) of TiO2–H
Fig. 2 Raman (a, b), Ti 2p XPS (c), and EPR (d) spectra of TiO2 and
TiO2–H.
Fig. 3 Photocatalytic decomposition of MB (a) and ?OH generation
measurement (b) of TiO2 and TiO2–H under UV-visible light irradiation.
Schematic illustration (c) of the hydrogenation effect on the structural change
in TiO2 and TiO2–H.
Fig. 1 TEM images of TiO2 (a), TiO2–H (b) and high resolution TEM
images of TiO2 correlating to the (101) (c) and (001) (d) interplanar crystal
spacing. XRD patterns (e) and UV-visible diffuse reflectance spectra (f) of
TiO2 and TiO2–H.
This journal is � The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 8286–8288 | 8287
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decreased when the photocatalysts were irradiated only by visible light,
which may be ascribed to the decrease of the light intensity. More
importantly, the visible light photocatalytic activity of TiO2 is higher
than that of P25. As discussed before, this phenomenon may result
from the existence of the Ti3+ center created by F substitution. We also
tested the photocatalytic activity of the photocatalysts under UV (Fig.
S6, ESI{) and visible light (Fig. S7, ESI{) irradiation in decomposing
phenol. The results are very similar to those of MB degradation. The
formation of ?OH (Fig. 3b), which is considered to be very favorable
for photocatalysis reactions, in the photocatalytic reaction system was
also examined upon UV light irradiation. The PL intensity of
2-hydroxyterephthalic acid, which was formed by the interaction of
terephthalic acid with the generated ?OH, in the three reaction systems
are very different. TiO2–H gives a much stronger photo-oxidation
capability than P25, while TiO2 shows the weakest photo-oxidation
capability. This depicts that TiO2–H is a very effective photocatalyst
which can be used in a wide range of applications.
To get a deep insight on the hydrogenation effect on the structure
change of TiO2, the schematic illustration is shown in Fig. 3c. The
{001}-facets-dominated TiO2 is predominantly exposed with the
unsaturated Ti5c and O2c atoms. As calculated, the H atoms will
chemically bond with the O atoms, resulting in the formation of Ti3+
centers.17 On the other hand, the hydrogenation effect removes
almost all of the F atoms on the surface of the TiO2. The Ti5c atoms,
which were originally bonded with the F atoms, are now unsaturated
again. After the H atoms reached an interaction equilibrium with the
O atoms, the Ti5c atoms are very favorable for H absorption.
16 As a
result, the (001) surface of TiO2–H is now packed with Ti6c atoms,
which may be the reason for the retention of the {001} facets even
though the F atoms are removed from the surface of TiO2–H. On the
other hand, the removal of the O and bulk F atoms induces the
formation of oxygen vacancies and Ti3+, resulting in the band gap
narrowing and the surface O2 adsorption of TiO2–H.
In summary, we have hydrogenated the F-modified, both surface
adsorption and inner substitution, anatase TiO2 nanosheets with a
high percentage of the {001} facets. The hydrogenated {001} facets
dominated TiO2–H shows higher photocatalytic activity than P25
and pristine TiO2 under UV-visible and visible light irradiation.
Based on a systematic analysis, the surface of TiO2 is fully covered
with the Ti–H and O–H chemical bonds. There are also a lot of Ti3+
and oxygen vacancies that induce the adsorption of O2 species on
the surface of TiO2–H, resulting in a significant band-gap narrowing.
The Ti5c atoms on the {001} faces are fully coordinated by the
formation of Ti–H chemical bonds. As a result, a high percentage of
{001} facets is kept after the hydrogenation treatment. We believe
the high-performance TiO2–H with exposed {001} facets can be used
in other fields, such as self-cleaning and solar cells. This work gives
new insight into the development of TiO2 photocatalysts with a high
percentage of highly reactive facets and enhanced light absorption
ability.
Acknowledgements
This work was supported by the Innovation Foundation for
Graduate Students of Jiangsu Province China (CXLX11_0346)
and a project funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD).
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8288 | RSC Adv., 2012, 2, 8286–8288 This journal is � The Royal Society of Chemistry 2012
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