Toward a Visible Light-Driven Photocatalyst: The Effect of Midgap-States-Induced Energy Gap of Undoped TiO2 Nanoparticles

Article abstract of DOI:10.1021/cs501539q

TiO₂ is one of the most promising candidate materials for clean-energy generation and environmental remediation. However, the larger-than 3.1 eV bandgap of perfectly crystalline TiO₂ confines its application to the ultraviolet (UV) r

Full text of DOI:10.1021/cs501539q

Research Article
pubs.acs.org/acscatalysis
Toward a Visible Light-Driven Photocatalyst: The Effect of Midgap-
States-Induced Energy Gap of Undoped TiO2 Nanoparticles
Houman Yaghoubi,*,†,‡,§ Zhi Li,∥ Yao Chen,‡ Huong T. Ngo,⊥ Venkat R. Bhethanabotla,⊥ Babu Joseph,⊥
Shengqian Ma,‡ Rudy Schlaf,∥ and Arash Takshi†
‡
§
†Bio/Organic Electronics Lab, Department of Electrical Engineering, Department of Chemistry, Nanotechnology Research and
∥
⊥
Education Center, Surface Science Lab, Department of Electrical Engineering, Department of Chemical and Biomedical
Engineering, University of South Florida, Tampa, Florida 33620, United States
S
* Supporting Information
ABSTRACT: TiO2 is one of the most promising candidate materials for clean-energy
generation and environmental remediation. However, the larger-than 3.1 eV bandgap
of perfectly crystalline TiO2 confines its application to the ultraviolet (UV) range. In
this study, the electronic and the optical properties of undoped mixed-phase TiO2
nanoparticles were investigated using UV and inverse photoemission, low intensity X-
ray photoelectron (XP), and diffused reflectance spectroscopy methods. The facile
solution-phase synthesized nanoparticles exhibited a midgap-states-induced energy gap
of only ∼2.2 eV. The diffused reflectance spectrum showed sub-bandgap absorption
due to the existence of a large Urbach tail at 2.2 eV. The UV photoemission spectrum
evidenced the presence of midgap states. The 2.2 eV energy gap enables the
nanoparticles to be photoactive in the visible energy range. The gas-phase CO2
photoreduction test with water vapor under visible light illumination was studied in
the presence of the synthesized TiO2 nanoparticles which resulted in the production of
∼1357 ppm gr−1
CO and ∼360 ppm gr−1
CH4, as compared to negligible amounts using a standard TiO2 (P25)
(catalyst)
(catalyst)
sample. The synthesized nanoparticles possessed a Brunauer−Emmett−Teller (BET) surface area of ∼131 m2 gr−1,
corresponding to a Langmuir surface area of ∼166 m2 gr−1. The determined interplanar distances of atomic planes by high-
resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD) methods were consistent. A detailed
elemental analysis using XPS and inductively coupled plasma mass spectrometry (ICP-MS) demonstrated that the synthesized
catalyst is indeed undoped. The catalytic activity of the undoped synthesized nanoparticles in the visible spectrum can be ascribed
to the unique electronic structure due to the presence of oxygen vacancy related defects and the high surface area.
KEYWORDS: visible light photocatalyst, midgap states, CO2 photoreduction, oxygen vacancy, TiO2
TiO2 photocatalysts for environmental applications.18−21 The
previous efforts to make use of TiO2 under solar radiation have
1. INTRODUCTION
In TiO2 photocatalysis, photogenerated electron−hole pairs
been based mainly on a doping approach to modify its
catalyze reactions on the surface of TiO2. This property has
electronic band structure.22−27 Recently, surface phases of pure
found numerous environmentally friendly applications in
TiO2 with lower bandgap have been suggested as an alternative
various fields, such as the production of carbonaceous solar
to the doping strategy.28 In bulk TiO2, the lower apparent
fuels, decontamination, water and air purification, and the self-
cleaning surfaces.1−9 Among TiO2 polymorphs, anatase has
bandgap of the surface is likely caused by surface-state-induced
band bending.29 These states are related to Ti interstitial
been acknowledged to have superior photocatalytic properties
in some previous works,10−12 partially due to the larger electron
species that diffuse to the surface in the presence of oxygen to
form a new TiO2 phase,30 or oxygen vacancies.31 However,
effective mass of anatase, which leads to a lower mobility of
charge carriers.13 However, some other groups demonstrated
these phases can only form under vacuum and at elevated
temperatures.28,30
that the catalytic activity is higher when TiO2 contains a
mixture of phases rather than just a single phase due to the
synergistic effect of a mixed-phase as well as the robust
separation of photoexcited charge carriers between several
The key aspects of surface and crystal facet engineering of
transition metal oxide photocatalysts have been summarized in
several recent reviews.28,32 The previous efforts mainly
phases.14−17 Nevertheless, the application of TiO2 has been
investigated the contribution of surface and subsurface defects
in the bulk TiO2 to the bandgap states.31,33,34 The focus of this
limited to the ultraviolet (UV) range, which makes it an
inefficient photocatalyst for the solar spectrum. Narrowing
down the TiO2 bandgap to turn it into an efficient material for
solar energy conversion has been an ongoing challenge. Several
reviews highlight the advances in the field of visible light active
Received: October 7, 2014
Revised: November 27, 2014
© XXXX American Chemical Society
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study is a quantitative evaluation of the electronic band
structure of surfactant-free undoped mixed-phase TiO2 nano-
particles with a reduced midgap-states-induced energy gap. The
TiO2 nanoparticles were obtained via a facile solution-phase
method at a low temperature. This method of synthesis enables
the tailoring of TiO2 physicochemical properties.35 In the
current study, a quantitative analysis of the bandgap states of
TiO2 nanoparticles in combination with visible light-induced
gas phase photoreduction of CO2 confirmed the significant role
of the midgap states in lowering the energy gap and the
catalytic response to the solar and the visible light.
cm pieces, they were mounted on sample holders with two
mounting screws, providing electrical contact between the Au
surface and the sample holder. Before the nanoparticle
deposition process, the substrates were sputter cleaned with
Ar+ ions (SPECS IQE 11/35 ion source) at 5 keV for about 40
min. The samples were then transferred to the deposition
chamber. The solutions containing the target materials were
injected into the deposition chamber through an orifice at rates
of 4 mL/h using electrospray.36 A high voltage (∼2000 V) was
applied between the syringe needle tip and the entrance orifice
assembly. A commercial multichamber system, SPECS GmbH
(Berlin, Germany), with 2 × 10−10 mbar base pressure was used
to perform inverse photoemission spectroscopy (IPES) and
PES measurements. The IPES setup consists of a Kimball ELG-
2/EGPS-1022 electron gun set to low energy mode, a Keithley
6485 Picoammeter to record the electron current at the
substrate, and a Geiger counter to quantify the emitted
photons. The Geiger-counter-based photon detector was
operated in proportional mode. A CaF2 window was used as
a low-pass filter with a 10.08 eV cutoff energy. Acetone vapor
mixed with argon was used as the ionization gas with a 9.69 eV
high-pass cutoff energy. This yielded a band-pass center-energy
of 9.9 eV at an energy resolution of about 0.4 eV. The PES
measurements were carried out with low-intensity X-ray
photoelectron spectroscopy (LIXPS) (Mg Kα, 1253.6 eV, 0.1
mA emission current), UPS (He I, 21.2182 eV) and XPS (Mg
Kα, 1253.6 eV, 20 mA emission current) in sequence.
2.4. Activity of CO2 Photoreduction. For each CO2
photoreduction test, 100 mg of the photocatalyst without any
cocatalyst was homogeneously spread on a ∼6.45 cm2 base area
of a stainless steel photoreactor with a quartz window. The
photocatalytic tests were performed both with a commercial
solar simulator (RST300S, Radiant Source Technology) at an
incident light intensity of 80 mW cm−2 (AM 1.0, which
represents the sun spectrum after traveling through the
atmosphere to the sea level with the sun directly overhead)
and a visible light source (using an Edmund Optics high
performance long-pass filter with the transmission wavelength
of 408 to 1650 nm which avoids any interference from UV
light). Both the solar and the visible light sources use a XL3000
PerkinElmer Fiber Optic Illumination (FOI) system that
employs a 300-W Cermax Xenon light source (see the
Supporting Information (SI) Figure S1 for the typical AM
1.0 solar spectral irradiance). The experiments were performed
in a closed chamber to avoid interference with ambient lights.
The reactor volume was ∼10 mL. The photoreactor was
vacuum-treated several times after which the high purity CO2
gas (99.99%, Airgas Inc.) was continuously passed through a
water bubbler to create a mixture of CO2 saturated with H2O
vapor. Then, the mixture was flowed through the reactor
system for a period of 30 min to reach equilibrium with the
catalyst prior to closing the inlet and outlet valves. The reactor
was operated in batch mode at a pressure of 20 psi and an
isothermal temperature of 40 °C. During the illumination,
samples of gas (each 80 μL) were taken from the reaction cell
by a gastight syringe (Hamilton, 100 μL) and were manually
injected to a gas chromatograph (GC, Agilent 6890N)
equipped with a thermal conductivity detector (TCD) for
subsequent concentration analysis. To evaluate the stability of
the catalyst (which also might imply the stability of the midgap
states), the same batch of catalyst used above, was tested again
under visible light illumination in a similar photocatalytic
experiment after 106 days.
2. EXPERIMENTAL SECTION
2.1. Synthesis. Titanium(IV) isopropoxide (TTIP,
99.999% trace metals basis) and hydrogen peroxide (H2O2,
30%) were purchased from Sigma-Aldrich and J.T. Baker,
respectively. The peroxotitanium complex (i.e., the TiO2
amorphous solution) was prepared as explained in our earlier
work.35 Initially, 3.75 mL of TTIP was added dropwise to a 30
mL stirring solution of H2O2 at room temperature. Then
deionized (DI) water was added to a final volume of 100 mL
after which the pH of the resultant solution was adjusted at 7.0
using NH4OH. To obtain the crystalline phase, the
peroxotitanium complex was refluxed for 10 h at 90 °C.12
TiO2 nanoparticles were electrophoretically separated from the
crystalline sol and dried at low temperature (90 °C) for 2 h in
an electric furnace to evaporate the solvent and its byproducts.
The remaining powders were collected for the analysis. The
electrophoresis apparatus was composed of stainless steel anode
and cathode electrodes. The electrodes were placed vertically in
a beaker containing the crystalline TiO2 nanoparticle solution.
The separation was carried out by applying 5 V across the cell,
until the solvent and the TiO2 particles were completely
segregated. The solvent was extracted using a syringe and the
gel was collected for subsequent drying.
2.2. Structural and Chemical Analysis. The diffused
reflectance spectrum (DRS) of the TiO2 sample was measured
using a JASCO V-670 UV−vis-NIR spectrophotometer in a
wavelength range of 200 to 800 nm with a scan speed of 100
nm min−1. The surface area, pore size, and pore volume of the
synthesized TiO2 samples were determined by collecting the N2
sorption isotherms at 77 K using the Micromeritics Surface
Area and Porosity analyzer ASAP-2020. In order to study the
crystallinity and the crystal orientation of the synthesized TiO2
nanoparticles, X-ray diffraction (XRD) measurements were
performed at room temperature (25 °C) using a D8 Advance
Bruker AXS X-ray diffractometer with Cu Kα radiation
(1.54060 Å), Kα1 (1.54060 Å), Kα2 (1.54443 Å), and Kβ
(1.39222 Å). The voltage and current of the X-ray generator
were adjusted at 40 kV and 40 mA, respectively. Further
multielemental analyses for detection of minor and trace
elements in the synthesized catalyst were performed using a
quadrupole inductively coupled plasma mass spectrometry
(ICP-MS, PerkinElmer ELAN DRC II). The argon gas utilized
was of spectral purity (99.9998%). An aqueous multielement
standard solution was used before any experiment, for
consistent sensitivity. The morphology of the samples was
observed using a high-resolution transmission electron micro-
scope (HR-TEM, Tecnai F30, FEI Company) operating at 300
kV.
2.3. Electronic Band Structure Analysis. Au thin-film
substrates (100 nm) on glass slides were purchased from EMF
Corp. (Ithaca, NY). After cutting the substrates into 1 cm × 1
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Figure 1. (a) X-ray diffraction (XRD) profiles of the synthesized TiO2 nanopowders (A: anatase, R: rutile, and B: brookite). (b) XPS survey
spectrum of TiO2 nanoparticle thin film. (c) Close-up view of the dashed rectangle in part a, showing absence of any nitrogen-containing groups in
the TiO2.
these observations demonstrate that the TiO2 nanostructure is
indeed undoped.
3.2. Size and Morphology of TiO2 Polymorphs. The
morphology of the synthesized TiO2 nanoparticles was
observed using a HR-TEM machine. As Figure 2a demonstrates
3. RESULTS AND DISCUSSION
3.1. Crystal Structure and Composition of TiO2
Polymorphs. Figure 1 shows the X-ray diffraction (XRD)
pattern of the synthesized TiO2 nanoparticles. The described
preparation method led to formation of a complex crystalline
mixture comprising anatase, rutile, and brookite. The Main
anatase TiO2 diffraction lines of (101), (004), (200), (105),
and (204) were observed at 2θ = 25.06°, 37.76°, 47.97°, 54.19°,
and 62.87°, respectively. The TiO2 diffraction lines of (110),
(101), (111), and (210) at 2θ = 27.25°, 35.97°, 41.14°, and
43.86°, respectively, can be indexed to the rutile polymorph.
The peak at 2θ = 30.60° can be assigned to (121) crystal plane
of brookite TiO2 (see SI Table S1 which shows a summary of
the XRD data). Annealing TiO2 nanopowders at higher
temperatures (200 °C) had only minor effects on the crystalline
properties of the three polymorphs (see SI Figure S2 for further
details). The average crystallite size of the anatase, rutile, and
brookite phases were calculated at 13.4, 14.7, and 17.8 nm,
respectively, using the Scherrer equation from the full-width at
half-maximum (fwhm) of the most intense diffraction peaks
(i.e., (101), (110), and (121)).6,35
X-ray photoelectron spectroscopy (XPS) has been employed
as a sensitive technique for nitrogen (N) detection by various
research groups.25−27,37−40 Irie et al. have demonstrated that for
a doped TiO2 structure with nitrogen (TiO2−xNx) with x as low
as 0.011, XPS can detect the N 1s core electrons.38 The XPS
survey spectrum and the N 1s core emission study of the TiO2
nanoparticle thin film in this work did not show any evidence of
the presence of any nitrogen (Figure 1b,c).
There is always a minor amount of C 1s emission (carbon
contamination) which may result from either the ex situ
preparation process or the transfer process of the sample into
the vacuum chamber, as it has been shown in other works.36,39
Liu et al. explained that the contamination carbon is not
incorporated into the TiO2 film and it only occurs on the
surface of the film as the intensity of C 1s signal can
significantly be decreased with Ar ion sputtering.39 As it is
shown later in “Activity of CO2 Photoreduction” (section 3.6),
background tests proved that the contamination carbon does
not contribute in creation of carbon-containing products.
Additionally, further multielemental analyses using ICP-MS
with a part-per-billion (ppb) detection resolution, showed only
a presence of 2.7 and 3.4 ppm of Ca and Cu, respectively,
which evidence the production of an ultrapure catalyst. Hence,
Figure 2. (a) TEM photograph of the synthesized TiO2 nanopowders
showing agglomerates of nanosize particles. (b) HR-TEM lattice image
of A(101). (c) HR-TEM lattice image of B(121), and A(004) or
R(101). (d) HR-TEM lattice image of R(110) and A(004). HR-TEM
images were labeled with examples of A: anatase, R: rutile, and B:
brookite.
the TiO2 sample mostly consists of irregularly shaped
nanoparticles, while some nanoflakes can be observed on the
edges of the aggregate structure. Figure 2b shows crystals with
interplanar spacing of 0.360 nm that matches the lattice spacing
of (101) atomic plane of anatase TiO2.41,42 In excellent
agreement with this observation, the XRD profile recorded in
Figure 1 demonstrates that the interplanar distance of the
(101) diffraction peak (i.e., the most intense reflection among
different polymorphs) is d101 = 0.355 nm (SI Table S1). It has
been shown previously that the lattice fringes of anatase (101)
atomic plane can vary from 0.349 to 0.360 nm.8,41,43,44
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Additionally, the interplanar spacing of 0.277 nm in Figure 2c is
likely associated with the distance between the (121) crystal
plane of the brookite phase.
The high-resolution TEM image (Figure 2c) also shows a
0.247 nm distance of the visible lattice fringes, which can be
related to the lattice spacing of anatase (004) or rutile (101)
atomic planes. The lattice fringes in Figure 2d suggest two
interplanar distances of ∼0.318 nm and ∼0.244 nm,
corresponding to rutile (110) and anatase (004), respec-
tively.8,43 These results are in agreement with the XRD pattern
analysis. Additionally, other families of facets were recognized
(SI, Figure S3), for example, rutile (101), rutile (111), and
anatase (010)-type facets.
Figure 4. Plot of transformed Kubelka−Munk function vs the energy
of the excitation source absorbed. The optical bandgap (narrow green
line) and the Urbach tail (bold red line) are shown.
3.3. Surface Area and Porosity of the Synthesized
TiO2 Nanoparticles. Figure 3a demonstrates the sorption
where R is the percentage of reflected light. The incident
photon energy (hν) and the optical bandgap energy (Eg) are
related to the transformed Kubelka−Munk function, [F(R) ·hν]
0.5 = A (hν − Eg), where Eg is the bandgap energy and A is the
constant depending on transition probability.
From the DRS spectrum, optical bandgapi.e., the thresh-
old for photons to be absorbedof the TiO2 sample is
determined to be ∼3.1 eV from the extrapolation of the liner
0.5
part of [F(R) ·hν]
plot, based on the indirect transition.
Interestingly, it is clear that a large Urbach tail is presented in
the absorption spectrum which gives a significantly lower
energy gap of ∼2.2 eV. In TiO2 photocatalysis, the photo-
induced electron hole pairs catalyze the reactions on the surface
of TiO2 and lead to its photocatalytic activity.48 These
photoinduced free electrons and holes are generated by
absorbing photons with energy matching or exceeding the
bandgap of the TiO2 nanoparticles. In our case, the tail
absorption (evidenced by DRS) can contribute significantly to
the absorption of photons with energies lower than the optical
gap, a phenomenon known as enhanced sub-bandgap
absorption.37,49 The presence of this tail in the DRS spectrum
may reflect the existence of disorders/defects which leads to the
formation of localized states extended in the bandgap (known
as midgap states).50,51 The subsequent UPS and XPS studies
later in this work provide further evidence on the existence of
the midgap states and their origin. Additionally, the effect of the
midgap states and the enhanced sub-bandgap absorption on the
TiO2 photocatalytic activity is studied later by the photo-
reduction of CO2 under both solar and visible light
illuminations.
3.5. Surface Physics and Electronic Band Structure
Analysis. In order to better comprehend the mechanism of the
photocatalytic activity as well as the band structure, the
electronic structure of the prepared TiO2 nanoparticles was
investigated using LIXPS, UPS, XPS, and IPES. In these
experiments, a thin-film of TiO2 nanoparticles was deposited on
an Ar+ sputter cleaned Au substrate. The secondary edge
spectral cutoffs acquired via LIXPS allowed for the determi-
nation of the work function (WF) of the TiO2 nanoparticles
(Figure 5a). Figure 5a demonstrates the corresponding LIXPS
measurements performed before and after UPS measurement.
Figure 3. (a) Adsorption (black circles) and desorption (open circles)
isotherms of N2 on the synthesized TiO2 nanoparticles. (b) Pore size
distributions of the synthesized TiO2 nanoparticles.
isotherms of N2 on the synthesized TiO2 sample. The plot in
Figure 3a shows the typical type-IV isotherm which occurs on
mesoporous adsorbents. The Langmuir and Brunauer−
Emmett−Teller (BET) specific surface areas were calculated
at 165.83 m2 gr−1 and 131.23 m2 gr−1, respectively. As surface
area plays a significant role in the photocatalytic reactions due
to the adsorption of the reactant and the more active sites for
the photoreaction,45,46 the relatively large surface area of the
synthesized nanoparticles can be favorable for an improved
photocatalytic activity.
The pore size distribution analysis in Figure 3b indicated that
the pore size of the TiO2 nanoparticles falls in the range of
12.69 Å to 233.93 Å with a total volume of 0.10842 cm3 gr−1.
Table 1 represents the measured surface areas, pore size, and
pore volume of the TiO2 sample.
3.4. Optical Bandgap and Sub-Bandgap Absorption
Analysis. The optical absorption of the TiO2 sample is shown
in Figure 4. The absorption is represented by the transformed
Kubelka−Munk spectrum calculated from the DRS, which
usually applies to highly scattering materials. High light
absorption by scattering is an important factor that determines
the photocatalytic activity of the TiO2. The optical absorption
coefficient (α) is calculated using reflectance data according to
the Kubelka−Munk equation,47 F(R) = α = (1 − R) /2R,
2
Table 1. BET and Langmuir Surface Areas (SA), Pore Size, and Pore Volume of the TiO2 Samples
sample
TiO2
BET SA (m2/gr)
131.23
Langmuir SA (m2/gr)
165.83
pore size a (Å)
>12.7
volume, pores a (cm3/gr)
0.01140
pore size b (Å)
volume, pores b (cm3/gr)
0.09702
≤234
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Figure 5. (a) Normalized secondary electron cutoff of low-intensity XP-spectra on a nanocrystalline TiO2 thin-film before and after UPS
measurements. Bottom spectra allow the determination of the WF before UV exposure. The shift of the secondary edge is due to the exposure to UV
light during the UPS measurements which resulted in a WF reduction. (b) The combined UPS and IPES spectra of the TiO2 nanoparticles. The left
spectrum refers to the valence region measured, using UPS after background subtraction. The right spectrum represents the conduction band above
the Fermi level, measured using IPES with 20 scans.
A difference between the LIXPS value before and after UPS is
related to the UV-induced photochemical hydroxylation of the
oxide surface.36,52 As it has been demonstrated earlier,
hydroxylation causes an oriented dipole on the surface with
the positively charged hydrogen atom directed outward,
resulting in a change in the secondary edges.52 The WF was
calculated by subtracting the cutoff binding energy value from
the He I excitation energy (21.2182 eV) and taking the analyzer
broadening of 0.1 eV into account. The WF of the TiO2
nanoparticle film was measured to be 4.5 eV (from the initial
LIXPS measurement before UPS, which is free from UV-
induced photochemical hydroxylation), which is in good
agreement with previously reported values.36 Figure 5b shows
the combined UPS and IPES spectra of the synthesized TiO2
nanoparticles.
TiO2.54 Hence, it is reasonable to assume that the midgap states
of the TiO2 nanoparticles can play a dominant role in its solar
and visible-light response and its photocatalytic properties.
Figure 6 illustrates the summarized electronic structure of the
The valence band maximum (VBM) was determined to be
3.3 eV below the Fermi level. The conduction band edge
(measured by IPES) was determined to be 0.5 eV above the
Fermi level. Hence, the corresponding transfer bandgap
(electronic bandgap) of the prepared TiO2 nanoparticles was
estimated to be about 3.8 eV. As TiO2 is a semiconductor
material with a large exciton binding energy, the transfer/
electronic bandgap (3.8 eV) of TiO2i.e. the threshold for
creating an unbound electron−hole pairis at a higher energy
than the optical bandgap (3.1 eV, Figure 4), which was
measured earlier in this work using DRS. Additionally, the
electronic bandgap of TiO2 nanoparticles are strongly depend-
ent on the synthesis method and the particle size.36
The ultraviolet photoemission (UP)-spectrum in Figure 5b
shows a clear feature with an edge of 1.7 eV. This feature is
related to the midgap states of the TiO2 nanoparticles,36 which
was earlier presented in this work with a large Urbach tail in the
DRS spectrum. Considering the UP-spectrum in Figure 5b, the
edge of the TiO2 midgap states, which was measured at 1.7 eV
below the Fermi level, establishes a 2.2 eV midgap-states-
induced energy gap, which also explains the observed Urbach
tail in the optical absorption spectrum. Therefore, these states
in the TiO2 bandgap may provide the major pathway for
electron transfer between the semiconductor nanoparticles and
the acceptor species.53 The excess electrons originating from
localized states can greatly affect the surface chemistry of
Figure 6. Schematic illustration of the electronic structure of the TiO2
nanoparticles combined with proposed diagram of CO2 photo-
reduction. Blue feature shows the presence of localized states extended
in the bandgap which give rise to a surface induced energy gap of 2.2
eV. The energy level at each layer is relative to the vacuum level. The
corresponding electrochemical potentials can be found from the
normal hydrogen electrode (NHE) axis at the right.
synthesized TiO2 nanoparticles combined with proposed
diagram of CO2 photoreduction. Under illumination with
energies equal or greater than 2.2 eV, the generated electrons at
the midgap states can be transferred to the conduction band of
the semiconductor, leaving holes behind. These photo-
generated electrons and holes can cause reduction and
oxidation reactions, respectively. The electrons can eventually
be transferred to the acceptor species such as CO2. In the
meantime, the possible hole reaction could be oxidation of
water vapor (H2O) to H+ by generated holes in the midgap
states. As it is shown in Figure 6, the top of the midgap states is
more positive than the oxidation potential of H2O to O2 (O2/
H2O energy level is located at +0.82 V vs NHE at pH 7.0).
Hence, it is reasonable to assume that photons from the visible
light with enough energy can provide the driving energy for this
reaction. As it has been explained in earlier works,55,56 the hole
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transport from the semiconductor (in this case from the
midgap states) to the water vapor might be sluggish, which
could slow down the kinetic and increase the overpotential.
The intent of the current study is to photoreduce CO2 for
production of hydrocarbon fuels using a novel transitional
metal oxide. Hence, although water vapor oxidation can likely
occur, as predicted by thermodynamics and based on the
presented band diagram, the sluggish kinetic is not an issue for
our anticipated application.
The localized electronic midgap states (responsible for the
Urbach tail) can originate from point defects (such as oxygen
vacancies), metal or nitrogen dopants, and/or lattice dis-
orders.39,57−60 The elemental analysis in the current study using
XPS (Figure 1b,c) showed no evidence of the presence of any
N. Further elemental analyses using both XPS and ICP-MS
demonstrated that the synthesized catalyst is indeed pure as the
amount of metal impurities in total was negligible (∼6 ppm).
Additionally, the high-resolution TEM image (Figure 2)
showed that the interplanar distances of the visible lattice
fringes are consistent with the lattice spacing of ordered anatase
and rutile in previous works.41−44 Hence, to further explore the
mechanism and the origin underlying the observed midgap
states, we carried out XPS Ti 2p and O 1s core emission study.
Figure 7 shows the corresponding XPS core level lines
measured after the deposition of the TiO2 nanoparticles on the
consistent with previous studies.22,63 Additionally, under
illumination, the photogenerated delocalized electrons can be
trapped at coordinatively unsaturated cation sites located in
anatase to form Ti3+ species.64,65 The fit of the O 1s XPS core
emission profile (Figure 7b was resolved using three different
components. The major signal at ∼530.5 eV is related to the
bulk Ti−O (crystal lattice oxygen).66,67 The higher-energy
peaks at 532.7 eV and ∼531.9 eV are assigned to the adsorbed
surface H2O (surface contamination layer) as well as oxygen
vacancies and surface hydroxyls, respectively, as was shown in
recent works.39,66,68,69 Because the depth of XPS analysis is
only limited to several atomic layers, these defects and adsorbed
oxygen species are likely located on the surface layer of the
oxide. Assigning the peak at 531.9 eV to oxygen vacancies
together with presence of Ti3+ signal, is in agreement with
earlier interpretations of core emission lines for identifying
oxygen deficient sites.68,70,71 The oxygen depletion generates
excess electrons that are stabilized into the reduced solid and
deeply influence its photocatalytic properties.53,54,64,72 Lin et al.
have shown that 3d Ti donor states that are present in the TiO2
with oxygen vacancies results in optical absorption across the
visible light region.73 Additionally, it has been shown in earlier
studies, using density functional theory (DFT) calculations,
that the presence of defects facilitates CO2 dissociation.74,75
A
detailed study on these defects and the behavior of the
structure in terms of charge dynamics in the dark and under
illumination is underway, and the results will be reported in
forthcoming papers.
3.6. Activity of CO2 Photoreduction. The photo-
reduction of CO2 to carbonaceous solar fuels has drawn
much attention because it helps to reduce global warming, to
overcome energy shortage, and positively affects the atmos-
pheric carbon balance.76 Several reviews highlight the photo-
induced activation of CO2 on Ti-based heterogeneous
catalysts.77,78 In this work, the heterogeneous photoelectro-
chemical reduction of CO2 under both the solar light and the
visible light illuminations in the presence of undoped TiO2
nanoparticles were studied (Figure 8). In order to rule out
possibilities of potential contamination, background tests were
first conducted with a mixture of N2 + H2O (without
introducing CO2 as the carbon source) passing through both
Figure 7. Core line emissions and defects characterization of the
undoped synthesized TiO2. (a) Ti 2p and (b) O 1s XPS core level
spectra for the TiO2 nanoparticles. The core levels in both parts are
shown by open circles and the corresponding fittings are shown either
by solid lines or closed symbols.
Au substrate. Figure 7a,b show the TiO2-related Ti 2p and O 1s
emission lines, respectively. A detailed deconvolution analysis
was performed on each spectrum to identify the contributing
components.
Figure 7a clearly shows the position of the Ti 2p spin doublet
with 2p3/2 and 2p1/2 binding energies of ∼459.0 and 464.7 eV,
respectively. These values are in agreement with the reported
figures for high-resolution Ti 2p XPS spectra of unmodified
TiO2 in an earlier study.37 The binding energy difference of 5.7
eV between the Ti 2p spin doublets indicates a valence state of
+4 for Ti.61 Additionally, noticeable Ti3+ signal was also
detected with a binding energy of 457.1 eV (Figure 7a, inset). It
has been demonstrated that the presence of small amount of
Ti3+ in the Ti 2p spectrum could derive from an oxygen
deficient surface.62 Succeeding the formation of an oxygen
vacancy, two Ti4+ cations should convert to two Ti3+ cations to
maintain the charge neutrality. These observations are
Figure 8. (a) CO and (b) CH4 evolutions over the prepared TiO2 and
P25 standard samples (in both parts (a) and (b), i and iv: synthesized
TiO2 and P25 TiO2 under solar light illumination, respectively. ii and
v: synthesized TiO2 and P25 TiO2 under visible light illumination,
respectively. iii: synthesized TiO2 under visible light illumination after
106 days (stability test). vi and vii: synthesized TiO2 and P25 TiO2
with a mixture of N2 + H2O (without any CO2) under solar light
illumination, respectively.
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the synthesized TiO2 and the standard P25 TiO2 (Figure 8a,b,
curves vi and vii) catalysts under photoillumination. The
analysis showed that no carbon-containing product was
observed. CO and CH4 as photoreduction products were
observed only in the presence of CO2, which supports the
interpretation that CO2 was indeed the carbon source.
Additionally, when the photocatalysts samples in the reactor
were kept in the dark (introducing CO2 but without
irradiation), no changes were observed in the CO2 photo-
reduction, confirming that the photoreduction of CO2 is a light-
driven reaction. By measuring the concentrations of CO and
CH4 at the reactor outlet, the production of CO and CH4, in
ppm·gr−1 during a 6 h photoillumination period was calculated,
and the data are shown in Figure 8. Figure 8 shows a negligible
production of CH4 over (P25) sample, which is in agreement
with a previous report.79 The overall production of CO and
CH4 on the synthesized TiO2 nanoparticles in 6 h were
measured to be ∼1818 and ∼477 ppm·gr−1 under solar light,
and ∼1357 and ∼360 ppm·gr−1 under visible light illumina-
tions, respectively; the photocatalytic activity of the synthesized
undoped TiO2 nanoparticles under visible light illumination is
6.75 and 7.66 times higher than that of TiO2 (P25) standard
sample in production of CO and CH4, respectively.
nanopowder, as an essential indicator for proton generation,80
was monitored which showed an increase from ∼0.26 for the
air ratio to above 0.65 and 0.55 after solar and visible light
irradiation, respectively. This data can be considered as a
further evidence for the photocatalytic reaction of CO2 with
H2O in the presence of the synthesized TiO2 sample, to form
CO/CH4 and O2 (Figure 9). Time dependence of the
Figure 9. Time dependence of the volumetric ratio of O2/N2 during
photoreduction of CO2 with H2O on the synthesized TiO2
nanopowder and P25 TiO2 standard sample. i and iii: synthesized
TiO2 and P25 TiO2 under solar light illumination, respectively. ii and
iv: synthesized TiO2 and P25 TiO2 under visible light illumination,
respectively.
The photochemical quantum yields (η %) of CO2 reduction
to CO and CH4 under solar irradiation (curve i, Figure 8a,b)
were calculated using the following equation as explained in an
earlier study.80
n × moles of reduction products (CO or CH4)
ηCO or CH (%) =
× 100%
4
moles of photon absorbed by catalyst
volumetric ratio of O2/N2 toward photoreduction of CO2 on
the synthesized nanopowder under visible illumination after
106 days, showed similar figures to the one in Figure 9, curve ii
(not shown here).
Overall, we reasoned that the photocatalytic activity of the
undoped TiO2 nanoparticles in the visible range can be ascribed
to the unique electronic structure of their surface (due to the
presence of midgap states which induce a surface energy gap of
2.2 eV) and their relatively large surface area. Under visible
light illumination, the electrons could be excited from the
midgap states to the conduction band. The CO2 molecules can
be adsorbed on a semiconductor surface and capture the
transferred electrons from the defect or the trapping sites to the
conduction band.78
(1)
where n is the number of electrons required to convert CO2 to
CO or CH4, that is two and eight, respectively. Because the
surface energy gap of the synthesized TiO2 was measured at
2.2. eV, photons with λ > 564 nm would not create an unbound
electron−hole. The incident light intensity of the source in the
effective UV and visible range (250−564 nm) was estimated to
be ∼28 mW cm−2 at the catalyst surface. Following the method
used by Wang et al.,80 the average photon energy in the range
of 250 to 564 nm was estimated to be ∼4.88 × 10−19 J. The
maximum CO and CH4 yields for the synthesized TiO2
catalysts were 0.157 and 0.041 μmol g−1 hr−1, respectively
(presented in ppm in Figure 8a,b, curve i). The moles of
photon absorbed by catalysts (or the photon flux)i.e., the
number of photons per second per unit areawas calculated at
57.38 × 1019 m−2·s−1. Considering the base surface area of the
photoreactor (6.45 cm2) and the amount of catalyst used (100
mg), the photochemical quantum yield for CO and CH4 was
calculated at 0.0141% and 0.0148%, respectively. The total
quantum yield (ηCO + ηCH4) was thus obtained as 0.0289%.
These results are comparable to the previously reported
figures.79
To study the stability of the synthesized catalyst, the same
batch of catalyst used above, was re-employed in a similar
photocatalytic experiment toward photoreduction of CO2
under visible light illumination, after 106 days. The obtained
results (Figure 8a,b, curve iii) demonstrate that the synthesized
catalyst is extremely stable because almost the same amount of
CO and CH4 could be obtained. This likely implies that the
midgap states associated with the synthesized structure are
pretty stable, as well.
In reality, the mechanism of CO2 photoreduction is very
complicated and requires determination of surface state energy
•−
levels for the CO2 (gas)/CO2 (surface) couples at the TiO2 surface
(solid−gas interface). A former review has applied an approach
comparable to that by Koppenol et al.,81 and has estimated this
energy level based on the adiabatic electron affinity of gaseous
•−
CO2, the heat of chemisorption of CO2 on the surface, and
the overall free energy change of the reaction.78 However,
because the former estimation is not accurate and ignores
different influential parameters such as the electrostatic
contributions, the standard electrochemical potentials of the
redox couples are commonly being used.77,80,82−84 Additionally,
it has been suggested that the mechanism of CO2 photo-
reduction in the presence of dissociated hydrogen atoms is
based on proton assisted multielectron transfer rather than a
single electron transfer process, because the −1.90 V (vs NHE)
•−
standard electrochemical potential of the CO2/CO2 redox
couple is highly unfavorable.79,80,85 Hence, other reactions such
Additionally, time dependence of the volumetric ratio of O2/
N2 during photoreduction of CO2 with H2O on the synthesized
as CO formation (Eo
= −0.52 V vs NHE) or CH4
redox
formation (Eo
= −0.24 V vs NHE) are energetically more
redox
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Research Article
favorable to occur. In the CO2 photoreduction process under
illumination, CO2 may interact with residing electrons at TiO2
surface sites (e.g., Ti3+) and reduces into CH4 via the reaction
of CO2 + 8H+ + 8e− → CH4 + 2H2O,82 while the
photogenerated holes in the valence band oxidize H2O into
oxygen and H+ via the half-reaction H2O → 1/2O2 + 2H+ +
2e− (Figure 6).82 Water can also act as a proton donor.79 To
elucidated the multiple roles of water in the overall photo-
catalytic reduction of CO2 to hydrocarbon fuels over TiO2, a
mechanistic study using electron paramagnetic resonance
technique is necessary which is outside the scope of the
current work.
Because our controlled experiments demonstrated no
evidence of CH4 formation in the dark or under illumination
in the absence of any catalyst, it is reasonable to conclude that
CO2 reduction is a light-driven catalytic reaction that occurs
over the photocatalyst. Several facts may explain the lower
production of CH4 comparing to CO in our study including the
following: multielectron transfer pathway of CH4 formation,
potential decomposition of CH4 to CO in a reaction with
photogenerated OH radicals,8 and higher number of electrons
and protons required for its formation. A thorough mechanistic
study to probe the reaction pathways is underway in our
laboratory and will be reported in forthcoming papers.
Houman Yaghoubi would like to acknowledge the University of
South Florida for partially supporting this work through an
Interdisciplinary Research Grant.
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ASSOCIATED CONTENT
* Supporting Information
The following file is available free of charge on the ACS
■
S
Publications website at DOI: 10.1021/cs501539q.
Additional information including the typical air mass
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Corresponding Author
*E-mail: hyaghoubi@mail.usf.edu. Tel.: +1-813-409-8192.
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ACKNOWLEDGMENTS
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The authors would like to thank Dr. Zachary Atlas and the
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