Article
pubs.acs.org/JPCC
Probing the Nature of Bandgap States in Hydrogen-Treated TiO2
Nanowires
Damon A. Wheeler,† Yichuan Ling,† Robert J. Dillon,‡ Robert C. Fitzmorris,† Christopher G. Dudzik,†
Liat Zavodivker,† Tijana Rajh,§ Nada M. Dimitrijevic,§ Glenn Millhauser,† Christopher Bardeen,‡
Yat Li,*,† and Jin Z. Zhang*,†
†
Department of Chemistry and Biochemistry, University of California, Santa Cruz, 1156 High Street, Santa Cruz, California 95064,
United States
‡
Department of Chemistry, University of California, Riverside, 501 Big Springs Road, Riverside, California 92521 United States
§
NanoBio Interface Group, Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States
ABSTRACT: Hydrogen treatment of TiO2 has been demonstrated to significantly alter its optical properties, including
substantially enhanced visible light absorption that has important
implications for various applications. The chemical nature of the
bandgap states responsible for the increased visible absorption is
not yet well understood. This work reports a detailed study of the
structural, optical, electronic, and ultrafast properties of hydrogentreated TiO2 (H:TiO2) nanowires (NWs) using a combination of
experimental techniques including high-resolution transmission
electron microscopy (HRTEM), electron spin resonance spectroscopy (ESR), time-resolved fluorescence (TRF), and femtosecond transient absorption (TA) spectroscopy in order to
explain the origin of the strong visible absorption. The combined
TEM, ESR, TRF, and TA data suggest that the presence of a localized mid-bandgap oxygen vacancy (VO) occupied by a lone electron
in an antibonding orbital situated at a surface site is likely responsible for the visible absorption of the material. The data further indicate
that while untreated TiO2 NWs are fluorescent, the hydrogen treatment leads to quenching of the fluorescence and highly efficient
charge carrier recombination from the VO state following excitation with visible light. With UV excitation, however, the charge carrier
recombination of the H:TiO2 NWs exhibits a larger component of a slow decay compared to that of untreated TiO2, which is
correlated with enhanced photoelectrochemical performance. Both the treated and untreated samples exhibit a fast decay that
dominates the TA signals, which is likely caused by a high density of surface trap states. A simple model is proposed to explain all the
key optical and dynamic features observed. The results have provided deeper insight into the chemical nature and photophysical
properties of bandgap states in chemically modified TiO2 nanomaterials.
characteristics of TiO2 since ∼45% of the solar spectrum is
composed of visible-wavelength photons. To that end, an extensive
amount of research has been performed on improving and gauging
the efficiency of TiO2 in various architectures. Varying platforms
have been explored, including TiO2-quantum dot (QD) heterostructures8−10 and elemental doping by elements such as
nitrogen,11,12 chromium,13 and other transition metals.14−16
One recently developed strategy for improving the photoelectrochemical properties of TiO2 involves the treatment of the
TiO2 with molecular hydrogen at elevated temperatures which
renders the TiO2 a black color.17−20 It has been proposed that the
dark coloration of H:TiO2 is due to the formation of either H or
H2 impurities in the bandgap from the hydrogen treatment.21,22
It is also suggested that the black color is caused by the existence
1. INTRODUCTION
In 1972, Fujishima and Honda discovered that titanium dioxide
(TiO2) was a promising material as a photoanode for
photoelectrochemical (PEC) water splitting.1 In the intervening
four decades, research on the material has exploded. Much effort
has been devoted to providing a fundamental understanding of
its photocatalytic properties as well as enhancing its overall
efficiency and performance as a PEC material. In particular,
research on TiO2 has gained significant traction due to the
advantageous characteristics of the material, including its strong
optical absorption, favorable band alignment, resistance to
chemical degradation and photobleaching, abundance, low
toxicity, and low cost.2−6
One of the limitations to TiO2 has been its wide bandgap that
prevents it from absorbing the majority of the solar spectrum.
This is shown by its low photoconversion efficiency (2.2%)
under 1 W, 1.5 AM global solar irradiation.7 Because of this, an
emphasis is being placed on improving the visible absorption
© 2013 American Chemical Society
Received: October 3, 2013
Revised: November 27, 2013
Published: December 2, 2013
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vapor deposition (CVD) system in which the CVD tube was
filled with ultrahigh purity hydrogen gas (Praxair).
2.3. UV−vis Measurements, Electron Microscopy, and
Electron Spin Resonance. Absorption measurements were
performed on an HP 8452A diode array spectrophotometer with
spectral resolution set to 2 nm. High-resolution transmission
electron microscopy (HRTEM) was carried out on a Philips
CM300-FEG at the National Center for Electron Microscopy at
Lawrence Berkeley National Laboratory with the accelerating
voltage set to 300 kV. X-band continuous wave electron spin
resonance (ESR) spectroscopy experiments were conducted at
the Center for Nanoscale Materials at Argonne National
Laboratory on a Bruker Elexsys E580 spectrometer equipped
with an Oxford CF935 helium flow cryostat with an ITC-5025
temperature controller. Samples were purged with nitrogen prior
to measurements to remove air/oxygen. Excitation of the sample
in the ESR spectrometer was carried out with a broadband xenon
lamp equipped with water as IR cutoff filter. The g-factors were
calibrated for homogeneity and accuracy by comparison to a coal
standard, g = 2.00285 ± 0.00005.
2.4. Time-Resolved Fluorescence. Time-resolved fluorescence experiments were performed on a regeneratively
amplified Spectra-Physics Spitfire laser system with a repetition
rate of 40 kHz. The 400 nm pump beam was generated by
frequency doubling the 800 nm fundamental with a barium
borate (BBO) crystal. Residual fundamental was removed with a
hot mirror and a Schott glass BG39 filter. Pump power
dependence study was conducted by setting the per-pulse
fluence to 0.4 and 1.3 μJ/cm2. The samples were kept under
vacuum in a Janus ST100 cryostat and excited in the front-face
geometry. Pump scatter was filtered from the signal using a
400 nm long-wave-pass filter and two Schott glass OG420 filters.
The signal was detected with a Hamamatsu C4334 Streakscope.
2.5. Femtosecond Transient Absorption System. The
transient absorption (TA) system utilized in this study has been
described previously.26 2.5 mW and 110 fs seed pulses with a
repetition rate of 33 MHz were obtained from a frequencydoubled Er-doped fiber oscillator and amplified in a Ti-sapphire
regenerative amplifier using chirped-pulse (multipass) amplification. The seed pulse from the oscillator was stretched
temporally to ∼200 ps using a grating stretcher, regeneratively
amplified at a repetition rate of 752 Hz by an intracavity-doubled,
Q-switched Nd:YLF laser, and recompressed by a grating
compressor. The final output pulses that were obtained were
typically 150 fs with a pulse energy of 1 mJ and centered at
795 ± 10 nm. The amplified output was beam split such that 90%
of the light was directed into an optical parametric amplifier
(OPA) while the remaining 10% was passed through a sapphire
crystal to generate a white light continuum (WLC) probe pulse
with a spectral range from 450 to 750 nm. The pump pulse
repetition rate was halved by an optical chopper which was
monitored by a CCD and used to produce a differential
absorption spectrum. The delay between the pump and probe
pulses was managed by a motor-controlled optical delay stage
with a temporal resolution of 10 fs. The pump and probe pulses
were focused on a 5 cm focal length curved mirror to overlap
spatially and temporally on the surface of the sample. For UVpumped experiments, a 380 nm pump pulse was selected from
two phase-matched bismuth borate (BiBO) crystals in the OPA,
while for visible-pumped experiments a 450 nm excitation was
chosen. The TA spectra were analyzed via a singular value
decomposition (SVD) global fitting procedure written at UCSC
for use in Matlab, in which the data at all wavelengths and all
of a lone electron occupying a Ti 3d state within the bandgap of
the TiO2.18 Because the hydrogen gas used to treat the sample has
a strong reductive ability and generates a high density of electron
donors, this Ti 3d state has been termed an “oxygen vacancy”
(VO).18,23 Electronically, this VO serves as a potential gateway
for optical transitions, which contribute to the strong optical
absorption, thereby contributing to the black color of the H:TiO2.
Because of the hydrogen treatment, the solar-to-hydrogen
efficiency of TiO2 was significantly enhanced.18 The improvement was attributed to elevated charge transport due to increased
donor density from the VO. It resulted in a 5-fold enhancement of
the incident-photon-to-current-conversion efficiency (IPCE) in
the UV region.
Despite the promising results that have been obtained by using
H:TiO2 as a photoanode, there is still some dissension with
regard to the chemical nature of the black color, specifically if the
photons absorbed can be utilized for PEC water splitting.
Because H:TiO2 samples display consistently improved PEC
performance relative to pristine TiO2, an improved understanding
of the fundamental reason behind this improvement is of great
interest.17,18,23−25
In this work, we carried out structural, optical, electronic, and
ultrafast transient absorption characterizations of the midbandgap states by systematically investigating hydrogen-treated
and pristine TiO2 NWs using transmission electron microscopy
(TEM), electron spin resonance (ESR), time-resolved fluorescence (TRF), and transient absorption pump−probe spectroscopy (TA). The TEM indicated that the smooth, dense, and
untreated NWs gave yield to the presence of a roughened surface
structure upon hydrogen treatment. ESR analyses indicated the
presence of a lone electron localized at a Ti 3d state,
characterized by an axial g-tensor at g⊥ = 1.975 and g∥ = 1.943,
typical of radicals in which an unpaired electron acquires orbital
angular momentum. Meanwhile, TRF and TA studies were used
to probe the dynamics of the charge carrier in the VO and related
states which provide insight into the constitution of the midbandgap state and how it gives rise to improved PEC
performance. From all of these results in conjunction, we
obtained a more complete picture about the electronic states that
give rise to the color and the overall improved PEC response of
the H:TiO2 relative to the untreated TiO2.
2. EXPERIMENTAL SECTION
2.1. TiO2 Nanowire Synthesis. The TiO2 NW arrays were
synthesized based on a previously reported hydrothermal
method.5 Briefly, 15 mL of deionized water was mixed with
15 mL of concentrated (37%) HCl in a 100 mL beaker with
continuous stirring. Subsequently, 0.5 mL of titanium n-butoxide
was added to the dilute HCl solution. The prepared solution and
a piece of fluorine-doped tin oxide (FTO) glass substrate were
transferred to a 40 mL Teflon-lined stainless steel autoclave with
the FTO being fully submerged in the solution. The sealed
autoclave was heated for 5 h in an oven set at 150 °C and allowed
to cool naturally to room temperature. A uniform white film
composed of TiO2 was found coated on the FTO glass. The
sample was then washed sequentially with ethanol and water.
Finally, the sample was annealed in air at 550 °C for 3 h in order
to increase the crystallinity of the NWs and increase their contact
with the FTO substrate.
2.2. Hydrogen Treatment of TiO2 Nanowires. The asprepared TiO2 NWs were annealed in a hydrogen atmosphere at
a temperature range from 350 to 550 °C for 30 min. The
hydrogen treatment was performed in a home-built chemical
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times are fit simultaneously to a multiexponential process,
ensuring that the minimal number of exponentially decaying
intermediates needed to fit the spectra can be determined.30,31
3. RESULTS
3.1. UV−Vis Electronic Absorption Spectra and
Electron Microscopy Images. Figure 1 shows representative
Figure 1. UV−vis spectra of TiO2 and H:TiO2 NWs annealed at various
temperatures.
UV−vis spectra of the TiO2 and H:TiO2 samples. The overall
extinction coefficient increases from the red to the UV region.
The “apparent” peak at 330 nm is not real but a result of
inaccurate measurement at wavelength shorter than 300 nm. The
signal at wavelengths redder than the expected bandgap of TiO2
is mostly from scattering due to the fact that the nanowires have
length scales comparable to the wavelength of light. There may
also be some minor contributions from absorption of bandgap
states.
Figure 2a presents HRTEM images which show the crystal
lattices of the untreated TiO2 sample. A lattice spacing of 3.5 Å,
indicative of the d-spacing of the (001) plane of rutile-phase
TiO2, has been highlighted. Also shown in Figure 2a (inset) is the
electron diffraction pattern of the samples. After hydrogen
treatment, the H:TiO2 samples show an increased roughness
when compared to the untreated TiO2 (Figure 2b). The image in
Figure 2b was a sample annealed at 450 °C. As the annealing
temperature of the H:TiO2 samples increased, the observed
surface roughness likewise increased. The rough surfaces are
notably absent in the untreated TiO2 samples. Finally, Figure 2c
shows a 3D representation of what we believe the morphological
changes manifest as on the unit cell scale. Figure 2c (left) is a
schematic of untreated TiO2, while Figure 2c (right) is a
schematic of H:TiO2 NWs, in which the VO are present. Because
VOs are generated when an oxygen atom at a bridging oxygen site
renders two subsurface Ti3+ ions exposed, the schematic
representation of the right side of Figure 2c takes on a twisted
shape.
3.2. Electron Spin Resonance (ESR) Spectra. ESR was
performed in order to validate the notion that the VO was
composed of a lone electron in the localized21,27 Ti 3d state of the
H:TiO2. In Figure 3, we present the ESR spectra of hydrogentreated TiO2 in different crystalline modifications: rutile, P25,
and nanowires (a), rutile nanoparticles (b), hydrogen-treated
P25 (c), and hydrogen-treated rutile TiO2 NWs (d). All
hydrogen-treated samples display two sets of signals: signals
for electrons localized on Ti centers (g < 2.00) and those for
oxygen centered radicals (g ≥ 2.00).28,29 The electrons on the
hydrogen-treated rutile sample were seen to localize mainly on Ti
rutile lattice sites (g = 1.972, g∥ = 1.948) and a small fraction on
Figure 2. Representative HRTEM images of (a) TiO2 NWs, showing a
dense and smooth architecture, (b) TiO2 NWs hydrogen treated at
450 °C, showing a rough surface, and (c) a proposed image of (left)
pristine TiO2 and (right) hydrogen-treated TiO2 in which bridging
oxygen atoms have been removed, thereby exposing subsurface Ti3+
ions. White spheres correspond to Ti and red spheres to O.
rutile surface Ti sites (g = 1.953, g∥ = 1.975) and was the most
intense of the spectra obtained. The hydrogen-treated P25
displayed a small rutile-like signal which was somewhat shifted
from the lattice signal (g⊥ = 1.978, g∥ = 1.943 compared to rutile
g⊥ = 1.975, g∥ = 1.940), indicative that the signal is localized at the
interface between anatase and rutile crystalline structures, and a
large anatase surface signal (g = 1.926), which indicates some
surface-trapped electrons on anatase sites, consistent with the
hydrogen reduction process. Additionally, a small oxygencentered radical with the same characteristics as holes on anatase
TiO2 is observed. Finally, hydrogen-treated TiO2 NWs displayed a
rutile lattice signal (g⊥ = 1.975, g∥ = 1.943). Moreover, an oxygencentered radical is observed with the g-tensor indicative of an “extra”
electron in the antibonding orbital (g⊥ = 1.999, g∥ = 2.044).
3.3. Time-Resolved Fluorescence and Ultrafast Transient Absorption. Time-resolved fluorescence (TRF) was
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Figure 3. (a) ESR plot of hydrogen-treated TiO2 in the forms of rutile (black curve), P25 (blue curve), and nanowires (red curve). (b) ESR plot of
hydrogen-treated rutile (black) and hydrogen-treated rutile under differing illumination times: intermediate (red) and long (orange). (c) ESR
comparison of untreated P25 (blue curve), dark H:P25 (black curve), and illuminated P25 (orange). (d) ESR comparison of illuminated TiO2
nanowires (orange), H:TiO2 nanowires (light blue curve), and 15 MA, 2 MW H:TiO2 (black curve).
collected on the H:TiO2 NW samples following a 400 nm
excitation from a frequency-doubled 800 nm fundamental.
Figure 4 represents the time-resolved fluorescence (TRF) of
H:TiO2 NWs annealed at various temperatures: 350, 400, 450,
and 500 °C. A global fitting algorithm was employed to
determine a single set of time constants that would fit the kinetic
traces across all probe wavelengths for a given sample with a
minimal number of exponentials.30,31 For both treated and
untreated samples, all decays were fit well with a triple
exponential. The TiO2 sample had a decay that was fit to time
constants of 35 ps, 120 ps, and >1 ns regardless of pump fluence.
For the 0.4 μJ/cm2 fluence runs, the H:TiO2 samples displayed a
faster decay for the fast component with increasing annealing
temperature with lifetimes of 30 ps for the samples. The middle
component fit well to a 200 ps lifetime, and the long component
had a lifetime of greater than 1 ns for all samples. For the 1.3 μJ/cm2
fluence runs, the fast component lifetime fit well to a triple
exponential of 25 ps for the samples. Again, the middle component
fit to a 200 ps lifetime and a slow component of greater than 1 ns for
all samples. Additionally, as seen in Figure 4b, the global fitting
revealed that the relative intensity of the fast component of the
decays increased from 68 to 77% as the annealing temperature
increased from 350 to 550 °C. Despite that the integrated intensity
of the fast component remained at 30% for all treatment
temperatures.
Ultrafast TA data were collected of the TiO2 and H:TiO2 NWs
at UV pump (380 nm, 475 nm probe) and visible pump (450 nm,
505 nm probe) wavelengths and are shown in Figure 5. For the
untreated TiO2 sample, visible wavelength excitation produced
no noticeable signal as the bandgap of pristine TiO2 is greater
than the wavelength of excitation light used; thus data could not
be collected on that sample at that pump wavelength. However,
for the UV pump, the TiO2 exhibited a fast initial decay followed
by a medium decay and a long-lived slow decay manifesting as a
persistent y-offset. The data (summarized in Table 1) were fit to a
triple exponential with time constants of 30 ps, 100 ps, and >1 ns.
The UV-pumped H:TiO2 displayed a slower medium decay than
the UV-pumped TiO2; yet, as with the untreated TiO2 NW
sample, the H:TiO2 also retained the persistent offset. This is
indicative of long-lived electron−hole pairs. All of the data of the
UV-pumped H:TiO2 samples were fit well to a triple exponential
with time constants of 60 ps, 200 ps, and >1 ns. For the 450 nmpumped H:TiO2, a fast initial decay was noticed, followed by a
nearly complete relaxation to baseline, indicative of very efficient
electron−hole recombination. Those data, essentially independent of annealing temperature, were fit to a double exponential
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Figure 4. (a) Intensity vs time (ps) of H:TiO2 NWs along with fitting curves and (b) global fitting results of the PL lifetime of H:TiO2, annealed at 350,
400, 450, and 500 °C, showing the contribution from each lifetime component to the overall integrated spectrum at each wavelength. The H:TiO2
samples required triple-exponential functions to fit their decays.
with time constants of 30 and ∼125 ps. For all of the samples
studied, no power dependence was observed. Figures 5b and 5c
are three-dimensional false-colored plots of the H:TiO2 and
TiO2 TA data with UV excitation which indicates the spectral
location of the maximum transient bleach feature. Noticeably,
the transient bleach is seen to extend for a longer time period for
H:TiO2 (Figure 5b) than for pristine TiO2 (Figure 5c).
Additionally, it is apparent from Figures 5b and 5c that there is
an overt spectral evolution of TA signals over time, and the
maximum bleach feature for both is consistently situated at probe
wavelengths of 475 nm. Figure 5d illustrates a three-dimensional
false-colored plot of H:TiO2 following visible pumping. From
Figure 5d, one can see that the bleach feature is centered near
460 nm and has components red-shifted to 600 nm. The bleach
feature is also seen to recover quickly, regardless of wavelength,
and no overt spectral evolution is noticeable.
As with the time-resolved fluorescence data, SVD fitting
indicated that the relative intensity of the fast component
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Figure 5. (a) Transient bleach relaxation traces of TiO2 and H:TiO2 NWs following 450 nm excitation (red curve) and 380 nm excitation (blue and
green curves). No noticeable differences in the bleach relaxation traces were evident based upon annealing temperature. (b, c) 3D plot of H:TiO2 and
TiO2 NWs, respectively, with optical density plotted as a function of wavelength (nm) and time (ps) following UV excitation. (d) 3D plot of H:TiO2
NWs following visible pumping. (e−g) Example of SVD fit results for H:TiO2 under visible illumination, TiO2 under UV illumination, and H:TiO2
under UV illumination, respectively. Shown for (e), (f), and (g) are the B spectra showing the wavelength dependence of the initial amplitude of the
various time constants.
the fitting revealed a component centered at 455 nm with a 30 ps
lifetime. This feature is also noticeable in Figure 5a in which the
fast component is mixed with the substrate response and does
not appear to have an overt time-dependent red-shift. The fast
increased with increasing annealing temperatures. Finally,
Figures 5e−g show the global fitting results representative of
H:TiO2 under visible illumination, TiO2 under UV illumination,
and H:TiO2 under UV illumination, respectively. In Figure 5e,
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position of the signal is in accordance with previously reported
ESR signatures from Ti3+.37,38 Illuminated rutile samples
displayed a broad weak signal for photogenerated holes and a
sharp signal for electrons in rutile environment. As the rutile was
illuminated at liquid helium temperatures, only lattice electrons
are formed as they do not have enough energy, at that
temperature, to be transferred via a hopping mechanism to
surface sites. The hydrogen-treated rutile displayed three
interesting characteristics: (i) a rutile lattice signal that grew
with illumination (g = 1.972), (ii) a fraction of tetrahedral sites
that decrease upon illumination, suggestive that they are located
near the surface (g = 1.977) and disappear in the reaction with
surface generated holes, and (iii) surface rutile sites that
disappear upon illumination also due to recombination with
surface holes. When the H:P25 was investigated, interfacial rutile
and surface anatase Ti3+ signals were observed. More
importantly, surface trapped electrons on anatase sites were
observed, consistent with chemical reduction of H2 that should
affect surface sites primarily. Interestingly, oxygen radicals were
observed and associated with holes on anatase TiO2. When this
sample was illuminated, new electrons on lattice rutile centers
were formed (g = 1.975) in addition to anatase photogenerated
holes while surface-level anatase electrons disappeared in the
reaction with photogenerated holes. The H:TiO2 NW signal was
very small and was composed primarily of rutile signal. The
sample contained no hole-like oxygen-centered radicals that were
observed in illuminated samples but rather oxygen-centered
radicals, indicative of an “extra” unpaired electron in an
antibonding orbital. When this sample was illuminated, more
rutile centers were formed and a signal typical of photogenerated
holes was observed. This signal resembles the signal obtained by
illumination of untreated NWs. Effectively, the H:TiO2 NWs are
composed mainly of rutile with a small fraction of anatase
(smaller than that of P25, at least) which is still photoactive. All
ESR experiments indicate that hydrogen treatment of TiO2
nanoparticles, independent of their crystalline structure, results
in VO associated with the presence of extra electrons primarily
localized on Ti sites both within the nanoparticle lattice and on
the surface. The charge migration between electron trapping
sites under illumination shows behavior typical of the stabilized
Ti3+ centers.
4.3. Charge Carrier Dynamics. Traditionally, the PL from
rutile is considered to be weak and is likely mediated by a high
density of defect states.39,40 Interestingly, the weak PL was
reduced further upon hydrogen treatment: 35 ps for the TiO2
NWs and ∼30 ps for the H:TiO2. This suggests that hydrogen
treatment fundamentally alters the quantity and energetic
position or depth of trap states within the bandgap since a
higher density of states within the bandgap is expected to quench
PL and reduce the lifetime observed. This is in line with the
HRTEM results, indicating an increase in surface defect density
in the crystalline lattice of the samples upon hydrogen treatment.
However, the fact that the medium component becomes slower
from TiO2 (120 ps) to H:TiO2 (200 ps) indicates that the PL
decay process is slowed on this time scale due to some longerlived trap state with hydrogen treatment, likely deeper trap states.
The overall slower decay of the TA of the H:TiO2 sample with
UV excitation as compared to the untreated sample indicates that
hydrogen treatment lengthened the lifetime of the charge
carriers, most likely in trap states or the VO state. Based on the PL
peak position (∼460 nm, or 2.7 eV), there is one fluorescent trap
state estimated to be ∼0.3 eV below the CB since the bandgap of
rutile is known to be 3.0 eV. While there are many trap states,
Table 1. Summary of Fitting Time Constants to the TA Data
sample
τ1 (ps)
τ2 (ps)
τ3 (ns)
TiO2 (UV illumination)
H:TiO2 (UV illumination)
H:TiO2 (visible illumination)
30
60
30
100
200
125
>1.0
>1.0
30 ps decay gives way to the 125 ps slow component which is
centered at 462 nm. Importantly, the fast component is seen to
account for ∼85% of the overall signal, indicating that the
H:TiO2 NW dynamics are driven primarily by surface disorderoriented states. For Figure 5f, the fitting revealed a fast
component centered at 470 nm, a middle component of
100 ps centered also at 470 nm, and a long-lived component in
excess of 1 ns. Here, the fast component comprised ∼65% of the
overall signal. Finally, in Figure 5g for UV-pumped H:TiO2, a fast
component of 60 ps centered at 450 nm, a middle component of
200 ps centered at 455 nm, and a slow component of >1 ns were
present. In this sample, the fast component accounts for ∼75% of
the overall signal.
4. DISCUSSION
4.1. Structural Properties. Pristine rutile TiO2 is composed
of O anions and Ti cations in the native 4+ state with a 6-fold
coordination to oxygen atoms.23 It is known that heating the
sample at elevated temperatures in excess of 350 °C results in
desorption of surface-adsorbed oxygen atoms, which results in
oxygen vacancies, or VO.18,32,33 In terms of structure, an absent
oxygen atom at a bridging oxygen site renders two subsurface
Ti3+ ions exposed. As has been established previously,17,18 the
inclusion of Ti3+-based disorder into the lattice of pristine, rutilephase TiO2 during the high-temperature treatment with
molecular hydrogen has the effect of reducing Ti4+ to Ti3+
through the occupation of vacant Ti 3d orbitals of TiO2 that are
otherwise empty. The generation of Ti3+ states is therefore
believed to follow a stoichiometric equation of
x H+ + TiO2 + x e− → HxTiO2 − x
These isolated and localized states are known to exist ∼0.75 eV
below the CB of TiO2 that is composed of 3d Ti3+.21,27,34
Structurally, it is the TiO2 (110) plane that has been thought
to have defect stoichiometry leading to a photocatalytically
active Ti3+ state.23,35,36 There also exists a number of defects in the
lattice. One other interesting feature of the H:TiO2 NWs, relative
to the TiO2 NWs, is that the high-temperature annealing yields
substantial roughness in the nanowires. Prior to annealing, the
nanowires were largely smooth and dense. The high-temperature
annealing afforded an abrupt change to a much more polycrystalline form.
4.2. Electronic States Probed by ESR. All fully oxidized
samples were ESR silent in the dark while upon illumination they
displayed signals for photogenerated electrons and holes in
different crystalline environments. In previous studies, it was
found that photogenerated electrons in P25 are localized on both
rutile and anatase sites, and the position of the signals in
illuminated P25 can be used as a standard for decoding the
crystalline environment in which photogenerated electrons and
holes reside. Illuminated P25, which should contain no unpaired
electrons indicative of the VO, displayed an ESR signal indicative
of surface holes localized on anatase sites and electrons in both
rutile and anatase environment. The signal in water is however
was found to be shifted to higher g-values, which is consistent
with the existence of tetrahedral interfacial sites. The spectral
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component as with UV excitation. The lack of a long-lived
component is consistent with lack of photocurrent in the visible
due to the very fast charge carrier recombination.
4.4. Nature of the Bandgap State in H:TiO2. Figure 6
presents a model to illustrate the relevant energy levels including
the bandgap states of TiO2 and H:TiO2 and associated charge
carrier relaxation process following UV or visible excitation. For
non-hydrogen-treated TiO2, above-bandgap excitation leads to
an electron−hole pair. The initial relaxation that the electron will
experience is into a state approximately 0.3 eV below the CB
which is determined to be a fluorescent state. Subsequent to that
relaxation occurs through the manifold of trap states endemic to
metal oxides before a final recombination occurs with the hole.
Upon hydrogen treatment of TiO2, however, an increasing
number of defects are introduced into the system, thereby
serving to quench the PLboth in intensity and in PL lifetime.
Additionally, the treatment of the TiO2 with molecular hydrogen
has the ability to donate electrons, thereby reducing some Ti4+
ions into Ti3+ ions and creating a VO, within the bandgap,
approximately 0.75 eV below the bottom of the CB.45 Upon
probing the state with UV-pumped transient absorption, an
electron−hole pair is photogenerated, the electron of which
relaxes down 0.75 eV into the VO. Compared to untreated TiO2,
the relaxation is relatively slow since the TiO2 has a fluorescent
state located ∼0.3 eV below the CB, making the initial electronic
relaxation into that state relatively fast. Visible light excitation
directly populates the VO state. Because of the high-temperature
treatment and thereby the generation of a dense manifold of PLquenching states below the VO, the recombination of the visibly
pumped electron and hole is fast and efficient. This is shown
schematically in Figure 6. However, further work may be done to
provide a more insightful look at the model.
It has been shown before that when Ti3+ centers are
introduced into a TiO2 system, the Fermi level is raised and
electron accumulation occurs on the surface.21 This accumulation of electrons on the surface of the TiO2 will not only reduce
the rate of surface recombination but also act to decrease the
quantum yield, which is what is seen with the decrease in PL
upon hydrogen treatment. Meanwhile, hydrogen treatment of
TiO2 was found to be capable of prolonging hole lifetimes by
reducing the number of bulk recombination centers.46
For visible pumping of the H:TiO2, the electronic transition
involved in the excitation of the H:TiO2 should therefore result
in the transfer of an electron from the valence band to the VO
state, which is a singly occupied orbital of Ti3+. Surface Ti3+
states, which act as electron traps, aid in charge separation by
slowing charge recombination, as discussed earlier, resulting in
not only slower electron−hole recombination relative to pristine
samples but also by resulting in a persistent, long-lived offset of the
data, both of which are seen in Figure 5a. For UV pumping, in which
the conduction band states are the electron acceptor, the rate of
relaxation depends on the density of conduction band states. It has
been argued before that because TiO2 has a large density of such
states, the overall relaxation event for pristine TiO2 should be quick.
For hydrogen-treated samples, however, since the initial relaxation
dynamics of UV-pumped samples is slower, it can be argued that the
density of these states has diminished or at least that the energetic
distance separating them has increased.
If one considers that TiO2 has, overall, a large number of
electron traps and a relatively low number of free electrons, then
for any recombination to occur, an electron must be excited to
the conduction band before migrating to the surface of the
nanostructure. From this, the lifetime will increase as the ratio of
we tentatively assign the 30 ps decay observed in the TA data to
electron relaxation from the bottom of the CB to the fluorescent
trap state. With hydrogen treatment, the fast decay is slowed
down to 60 ps, and this could be due to contribution of the VO
state created or change in the location or density of the
fluorescent trap states. It is challenging to pinpoint this at this
point. As can be seen in Figures 5b and 5c also, the TA intensity
of H:TiO2 at red wavelengths decreases more slowly than that of
pristine TiO2, indicative of longer-lived charge carriers,
consistent with the single wavelength analysis.
The VO state is estimated to lie ∼0.75 eV below the CB.18
Electrons in this deep trap state could be longer lived than in the
0.3 eV shallow fluorescent trap state. This would be consistent
with the longer lifetime observed for the H-TiO2 sample. This
would be consistent with the localized nature of the VO state.21,27
Of course, other states in the bandgap could contribute to the
charge carrier dynamics. For example, interstitial hydrogen,
introduced during the hydrogen treatment, is shown to locate
in positions close to surface oxide ions.41−43 These hydrogen−
oxygen species, taking the form of −OH functionalities, are
known to have to primary energetic locations at 0.6 eV below the
CB and 0.82 eV below the CB.44 As with the VO, these bands are
also greatly localized42 which will aid in the slower recombination
time subsequent to UV pumping.
For visible-pumped H:TiO2, we suggest that the visible
excitation takes the electron from the VB to the VO. We cannot
completely rule out the possibility of the transition being from
the VO to the CB. The overall fast decay of the charge carriers
with visible excitation is likely due to a dense manifold of states
under the VO, as illustrated in Figure 6. There is no long-lived
Figure 6. Proposed model for energy levels related to the optical
properties and dynamics studies. CB and VB represent the conduction
band and valence band, respectively. For H:TiO2 NWs, visible pumping
excites an electron to the VO state which is followed by nonradiative
exciton decay mediated by a manifold of trap states. UV pumping for the
same sample excites an electron across the bandgap which is followed by
relaxation to the VO state and subsequent relaxation through the
manifold of trap states. For TiO2 NWs, UV pumping (visible pumping
not possible) likewise excites an electron across the bandgap, which is
followed by relaxation into a fluorescent trap state situated ∼0.3 eV
below the CB. Subsequent recombination was fast due to trap states in
the bandgap.
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trapped to free electrons increases, which changes exponentially
as the Fermi level moves through the bandgap.47,48 This is most
true when recombination occurs primarily from the conduction
band and becomes different when considering surface-statemediated recombination.49,50 Assuming that the surface defect
states have a similar distribution as that of bulk trap states, the
distributed density of trap states near the TiO2 surface can be
described by the volume fraction of bulk traps near enough to the
surface of the nanostructure to participate in electron relaxation.
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5. CONCLUSION
We have carried out a systematic study in order to gain
fundamental insight into the chemical nature of the bandgap
states in hydrogen-treated TiO2 NWs that results in visible light
absorption. Pristine, rutile-phase TiO2 strongly absorbs UV light
due to its bandgap of 3.0 eV. Hydrogen treatment, however,
introduces mid-bandgap states, attributed to oxygen vacancies or
VO, as well as nonfluorescent trap states situated below the
localized VO state. This VO state is partly responsible for the
visible absorption of the hydrogen-treated TiO2. Moreover, the
manifold of states below the VO leads to fast recombination of
charge carriers following visible excitation into the VO state. ESR
data, collected on hydrogen-treated rutile, P25, and NWs, indicate
that the VO state is a singly occupied state of largely Ti3+ character. In
addition, an oxygen-centered radical, possibly obtained by thermalization of some electrons on surface oxygen sites, is observed which
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Upon excitation with visible light, however, the charge carrier
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trap states below the VO. The new bandgap states introduced from
hydrogen treatment led to PL quenching due to increased
nonradiative decay. The overall short lifetime of charge carriers
generated with visible light excitation is unfavorable for device
applications such as PEC since they are too short-lived for efficient
collection. It would be highly desirable to design and create bandgap
states with long lifetime for PEC or photovoltaic applications. It is
unclear at this point if and how the bandgap states probed in TA are
responsible for PEC. Correlation between the lifetime of bandgap
states and the PEC performance is likely more complicated than
anticipated due to the multiple steps or time scales in charge
transport and the multiple time scales of charge carrier relaxation.
■
Article
AUTHOR INFORMATION
Corresponding Authors
*E-mail [email protected], Ph (831)-459-1952 (Y.L.).
*E-mail [email protected], Ph (831) 459-3776 (Z.Z.).
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
J.Z.Z. acknowledges the BES Division of the U.S. DOE (DEFG02-ER46232) for financial support. Y.L. acknowledges the
support of this work partially by U.S. NSF (DMR-0847786) and
UCSC faculty startup funds. Use of the Center for Nanoscale
Materials was supported by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under
Contract DE-AC02-06CH11357.
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