Electronic structure of titanium oxide nanotubulesDepartment of Polymer Science and Engineering, State Key Laboratory of Silicon Materials, Institute of Polymer Composites, Zhejiang Univ
Trang 1Electronic structure of titanium oxide nanotubules
Department of Polymer Science and Engineering, State Key Laboratory of Silicon Materials, Institute of Polymer Composites, Zhejiang University, Hangzhou 310027, PR China
Received 19 May 2003; in final form 18 August 2003
Published online: 2 October 2003
Abstract
We investigated the electronic states of titanium oxide nanotubules (TiOx-NTs) by using field induced surface photovoltage spectroscopy (FISPS) Compared with common TiO2 P25 nanocrystals, new surface photovoltage re-sponse bands extending to 550 nm were found under the effect of external electric field Based on the principle of FISPS, these responses were ascribed to the surface state transitions X-ray diffraction (XRD) indicated that the crystalline structure changed remarkable during the hydrothermal synthesis process The existence of oxygen vacancies contrib-uting to the surface states was further confirmed by the sub-band gap photoluminescence
Ó 2003 Elsevier B.V All rights reserved
1 Introduction
Recently, one-dimensional (1D) nanostructured
TiO2 materials such as nanotubules, nanowires
have gained considerable attention for their
bril-liant prospects in photocatalyst, environment
pu-rification, solar cell and gas and humidity sensor
[1–4] Various methods have been introduced to
acquire the 1D nanostructured TiO2 Using sol–gel
strategy, Hoyer [5] prepared TiO2 nanotubes with
diameters of 70–100 nm Applying porous alumina
template, Imai et al [6] successfully fabricated
TiO2 nanotubes in a tunable way The simple
method of hydrothermal synthesis, first developed
by Kasuga et al [7], was also extensively employed
to synthesize TiO2 nanotubes, nanoribbons and nanowires Among these researches, however, the efforts focus on the characterization of crystallo-graphic structures and microscopic morphologies, only little attentions have been paid to the elec-tronic properties of these 1D nanostructured materials In fact, the electronic properties play a key role in determining their photo/electronic performance
Considering that the surface photovoltage spectrum (SPS) and electric field-induced surface photovoltage spectrum (FISPS) are highly sensi-tive tools to study the photophysics of the photo-generated species or excited states without any sample-contamination and destruction [8,9], we have investigated the surface electronic properties
of the titanium oxide nanotubules using the SPS, FISPS techniques and photoluminescence
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Corresponding authors Fax: +86-571-8795-1635.
E-mail addresses: sunjz@zju.edu.cn (J Sun), mwang@zju.
edu.cn (M Wang).
0009-2614/$ - see front matter Ó 2003 Elsevier B.V All rights reserved.
doi:10.1016/j.cplett.2003.09.037
Trang 2(PL) spectroscopy In this Letter, we report
our experimental results, besides photovoltaic
re-sponse corresponding to band-to-band transition;
new surface photovoltage responses extended to
550 nm have been observed under the effect of the
external electric field Based on the principle of
FISPS and the feature of PL, the new responses
can be assigned to the surface states, which are
resulted from oxygen vacancies formed in the
structural evolution of the raw materials TiO2P25
to the nanotubules
2 Experimental
The method of hydrothermal synthesis, as
de-scribed by Kasuga et al [7] was employed to
prepare titanium oxide nanotubules In a typical
experiment, 500 microgramme titanium dioxide
power (Degussa P25) and 50 ml 5–10 M NaOH
aqueous solutions was put together into a
Teflon-lined autoclave The autoclave was sealed into a
stainless tank, and maintained the temperature at
180°C for 24 h, without shaking or stirring And
after it was cooled to room temperature, a white
tousy power was obtained by centrifugation The
precipitate was washed with hydrochloric acid and
distilled water several times until the pH was 7,
and dried at 60 °C for 12 h, then annealed at
350 °C for 2 h, finally a soft white powder was
obtained
SPS is a measurement of the relation between
the surface photovoltage vs the light wavelength
That is to say, the signal detected by SPS is
equivalent to the change in the surface potential
barrier on illumination: dV ¼ Vs V0
s, where Vs
and V0
s are the surface potential height with and
without illumination, respectively The surface
photovoltaic spectra of the as-synthesized
prod-ucts were measured with a Metal–Insulator–
Semiconductor (MIS) approach, using steady state
chopped light source monochromator-lock-in
de-tection technique [8] Periodic excess carriers
gen-eration and subsequent redistribution changes the
surface potential; this change is picked up by a
transparent ITO glass electrode in a capacitor
ge-ometry (Fig 3b inset) Monochromatic light was
obtained by passing light from a 500 W xenon
lamp through a grating monochromator A lock-in amplifier (Stanford SR 830), synchronized with a light chopper (Stanford SR 540) was employed to amplify the photovoltage signal FISPS is a tech-nique that combines the field-effect principle with SPS With an external voltage applied to the two sides of the sample, the mobile direction and the diffusion length of the photogenerated charge carriers can be altered Moreover, the space charge density and the electronic state of the molecular can be changed So the two factors will have direct effects on the SPV intensity and the photovoltaic characteristics The principle and the illustration
of the FISPS were discussed in detail by Zhang
et al [9]
The morphology of the nanotubules was ob-served on a JEM-200CX transmission electron microscopy (TEM) Samples for observation were prepared by ultrasonic dispersion of a small amount of sample in absolute ethanol; then a drop
of the solution was dipped onto a copper micro-grid with carbon film X-ray diffraction (XRD) patterns were obtained using 98-XD UV-visible absorption spectra of the samples were recorded
on a Varian Cary bio100 spectrometer PL spectra were measured on a Hitachi 800 spectrometer with
a Xe lamp as the excitation light source Colloid solutions in absolute ethanol were prepared ul-trasonically for the UV–Visible and the PL measurements
3 Results and discussion
A typical TEM image of the as-synthesized TiO2 nanostructured materials is shown in Fig 1
It is apparent that the products are consisted of uniform tubules of about twenty nanometers in diameter and several hundred in length Further-more, the titanium oxide nanotubules are quite pure without raw materials on the surface of the tubules
XRD measurements (Fig 2) show that the as-prepared titanium oxide nanotubules have a new crystalline structure The diffraction patterns are not only different from well-defined anatase and rutile phase, but also different from the reported phases of known titanate [10,11] Based on the
Trang 3data of literature [11], we mark the obtained
na-notubules as a mixture of anatase TiO2 and
tita-nate that has not been completely transformed
into anatase phase in the thermal process
There-fore, ignoring the possible existence of light
ele-ments such as H, the obtained nanotubules can in
general be referred as TiOx [12]
UV–Visible spectra of the raw materials TiO2
P25 and the obtained TiOx nanotubules (TiOx
-NTs) are depicted in Fig 3a The spectral lines for
both samples exhibit only one characteristic
ab-sorption band, which is assigned to the intrinsic
transition from the valence band (VB) to the
conduction band (CB); the correspondent
thresh-old values are 375 nm for TiO P25 powders while
295 nm for the TiOx-NTs The 80 nm blue shift of absorption maximum can be attributed to the nano-size effect, because the average diameter of the nanotubules (as shown by the TEM image in Fig 1) is 12 nm smaller than that of TiO2 P25 particles
The SPS of TiO2 P25 and TiOx-NTs without external field are illustrated in Fig 3b The profiles
of the SPS and the UV–Visible absorption spec-trum resemble each other The symbolic feature between the absorption and surface photovoltaic action spectrum suggests that the band-to-band transition is the major contribution to the surface photovoltage [8] In the framework of band the-ory, electron–hole pairs are generated in the TiO2
under the illumination Driven by the built-in field,
0
1000
2000
3000
(2) TiOx-NTs
(2)
(1)
R R
R A A
A
Fig 2 XRD of TiO 2 P25 and TiO x -NTs (peaks resulting from
the anatase and rutile phase are denoted by A and R,
respec-tively).
0 2 4
To lock-in Amplifier
Bias
R
ITO Sample Optical glass
(2)
(1)
Wavelength (nm)
(1) TiOx-NTs (2) TiO2P25 powders
0.0 0.4 0.8 1.2 1.6
Wavelength (nm)
(1) TiO
x -NTs (2) TiO2P25 powders
(2) (1)
(a)
(b)
Fig 3 (a) UV–Visible spectra of TiO 2 P25 and TiO x -NTs (b) SPS of TiO 2 P25 and TiO x -NTs without an external electric field (inset shows the structure of the photovoltaic cell for measuring the SPS and FISPS).
Fig 1 A typical TEM image of TiO x -NTs.
Trang 4the photogenerated holes move in the valance
and the electrons in the CB The displacement of
the photogenerated electrons and holes leads to
the change of the surface net charges thereby the
surface photovoltage is produced [13]
Interestingly, the FISPS of the TiO2 P25 and
TiOx-NTs are distinctly different Figs 4a,b show
the discrepancy of the TiO2 P25 and TiOx-NTs
under the effect of a series external electric field
For TiO2 P25 (Fig 4a), the intensity of surface
photovoltage response systematically enhanced
whereas no new photovoltage responses appeared
when the applied bias varied from 0 to 1 V For
TiOx-NTs, in contrast, distinct scenery emerged
when bias were applied to the samples As shown
in Fig 4b, the intensity of intrinsic photovoltage
response greatly increases with the increase of bi-ases from 0 to 1 V At the same time, the low energy part of the photovoltage response extends
to 550 nm, and a new feature is found at about
388 nm According to the principle of FISPS [9], the SPV response of the surface states transition is sensitive to external field, while the intrinsic band-to-band transition is insensitive Therefore the new photovoltage response could be rationally attrib-uted to the extrinsic sub-band gap or surface state transitions In generally, the sub-band gap energy levels localize between the CB and VB, the charge carriers populating in these levels are bounded and the electronic transitions of local states are for-bidden Consequently, the SPV response is weak even undetectable without the induction of the external field But if an external field is applied, the surface states energy band tilts and its optical constant changes [14] These two effects enlarge the transition momentum of local states and increase the probability of electronic transitions As a re-sult, the enhanced surface photovoltage responses can be observed under the effect of the external field It is reasonable to associate the new feature
of photovoltage response with the surface state transitions of the TiOx-NTs It is noted that we have not observed the SPV response associated with the subgap surface states in the TiO2 P25 Although many investigations have demonstrated that the anatase, rutile, their single nanocrystal and doped TiO2 have subgap [15], it is also found that the subgap states are close related to the di-mension of the nanocrystals and the preparation conditions In fact, TiO2P25 has surface state, too (it can be seen from the PL spectroscope); but because of the synthesis condition and the
sensi-tivity of our SPS instrument, we havenÕt observed
the response of subgap in TiO2 P25
To further testify the existence of the surface states, the PL spectra of the TiOx-NTs and TiO2
P25 powders were compared at room temperature, the excitation wavelength was 310 nm From the spectral line (1) and (2) in Fig 5a, it is noted that the PL intensity of the TiOx-NTs is much stronger than that of TiO2 P25 powders; a quantitative comparison demonstrates that the luminescence quantum efficiency for TiOx-NTs is up to 40 times
of the TiO P25 counterpart The evident
Fig 4 FISPS of TiO P25 and TiO -NTs.
Trang 5enhancement in PL efficiency implies the more
multitudinous existence of surface states in TiOx
-NTs On the other hand, the investigations done
by other groups [16–19] suggested that the traps
were the main contribution to the PL of TiO2
nanomaterials Furthermore the oxygen vacancies
had been considered to be the origin of the
ob-served PL of TiO2 nanowires [16] Therefore, it is
reasonable to assign the new band of photovoltage
response under the external electric field to the
local surface states caused by oxygen vacancies in
the TiOx-NTs The schematic energy level diagram
showing the position of the surface state inside the
band gap in relation to the VB is illustrated in
Fig 5b The energy space between the VB and the surface state was calculated to be0.08 eV, which was equal to the energy difference of band-to-band and local state transitions
4 Conclusions
In summary, TiOx-NTs have electronic prop-erties different from common TiO2 nanocrystals, the photovoltage response band expands to 550
nm and new feature exhibits under the effect of the external electric field Based on the experimental data of FISPS and PL, in the framework of band theory, we ascribe the new photovoltage responses
to the electronic transitions of surface states, which are mainly caused by the oxygen vacancies
at the surface These results together with previous studies on TiO2 nanomaterials indicate the simul-taneous evolutions of crystallographic and elec-tronic structures in the formation of TiOx-NTs using a hydrothermal synthesis method It can be expected the repopulation of surface electronic states may provide new opportunities for the ap-plication of TiOx-NTs in the fields of photo/elec-tronic devices
Acknowledgements
The authors would like to thank the financial support of the National Natural Science Founda-tion of China with granted number of 90101008
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