Photoactivity and charge trapping sites in copper and vanadium doped anatase TiO2 nano-materials

Article abstract of DOI:10.1039/c5cy00929d

Two series of Cu²⁺ and V⁴⁺ doped anatase TiO₂ samples were prepared using the microemulsion synthetic route by varying the metal/Ti ratio. The samples were characterized by UV-vis absorption, nitrogen physisorption, XRD, XPS, Raman and EPR spectroscopy. Their photocatalytic activity against toluene photo-oxidation was evaluated under both sunlight-type and pure UV light irradiation. The photoactivity was drastically affected by the dopant content. Low or moderate metal doping levels were beneficial for toluene photo-degradation, while high doping levels suppressed the photoactivity. Electron paramagnetic resonance (EPR) spectroscopy was employed to study the structure and electronic characteristics of the prepared catalysts and elucidate the physicochemical aspects governing the photoactivity. The presence of different Cu and V species was detected as a function of the metal content in the catalyst. EPR spectroscopy revealed that photoinitiated charge formation takes place in both the Cu and V doped TiO₂ series and that charge separation may be enhanced depending on the dopant content. A direct correlation between the dopant species formed and hole formation was observed in both series. Photoactivity is directly correlated with the formation of holes, which in turn is tuned by the dopant content regulating the isolated metal centers versus metal cluster formation. Low or moderate doping levels enhanced the separation of photo-produced electron-hole pairs via accepting e^-, eliminating trapping sites related to localized Ti³⁺ states and increasing the abundance of hole species. At higher levels where metal clusters prevailed, dopants acted as recombination centers, deteriorating the photoactivity in both TiO₂-doped series. The results indicate a general mechanism potentially applicable to similar photocatalytic systems.

Full text of DOI:10.1039/c5cy00929d

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ARTICLE
Photoactivity and charge trapping sites in Copper and Vanadium
doped TiO2 anatase nano-materials
Konstantinos C. Christoforidisa,b,* and Marcos Fernández-Garcíaa
Received 00th January 20xx,
Accepted 00th January 20xx
Two series of Cu2+ and V4+ doped TiO2 anatase samples were prepared using the microemulsion synthetic route by varying
the metal/Ti ratio. The samples were characterized by UV-Vis absorption, nitrogen physisorption, XRD, Raman, XPS and
EPR spectroscopy. Their photocatalytic activity against toluene photo-oxidation was evaluated under both sunlight-type
and pure UV light irradiation. The photoactivity was drastically affected by the dopant content. Low or moderate metal
doping was beneficial for toluene photo-degradation, while high doping levels suppressed photoactivity. Electron
paramagnetic resonance (EPR) spectroscopy was employed to study the structure and electronic characteristics of the
prepared catalysts and to elucidate the physicochemical aspects governing photoactivity. The presence of different Cu-
and V-species was detected as a function of the metal content in the catalyst. EPR spectroscopy revealed that
photoinitiated charge formation takes place in both Cu and V doped TiO2 series and that charge separation may be
enhanced depending on the dopant content. A direct correlation between the dopant species formed and the holes-
formation was observed in both series. Photoactivity is directly correlated with the formation of holes, which in turn is
tuned by the dopant content regulating the isolated metal centers versus metal clusters formation. Low or moderate
doping levels enhanced the separation of photo-produced electron-hole pairs via accepting e-, eliminating trapping sites
related to localized Ti3+ states and increasing the abundance of hole species. At higher levels where metal clusters
prevailed, dopants acted as recombination centers deteriorating photoactivity in both TiO2-doped series. The results
indicate a general mechanism potentially applicable in similar photocatalytic systems.
DOI: 10.1039/x0xx00000x
www.rsc.org/
degradation reactions is usually defined by the available
photogenerated charges,2,5,6 efforts have been devoted on
extending the life time of such centers either through the
1. Introduction
Photocatalysis using semiconductor nanomaterials is an
attractive method for environmental protection and energy
conversion since it can be applied at ambient conditions (room
temperature and atmospheric pressure) using solar energy as
the energy source.1,2 Among the different photocatalysts
presented in the literature, nanostructured titanium dioxide
(TiO2) is definitely the most frequently employed due to its
inherent properties (non-toxic, low cost, high stability).1,2
Many attempts have been made to improve and optimize the
properties of TiO2 towards realistic application of
heterogeneous photocatalysis. Much efforts have been
devoted on narrowing the relatively wide band gap of TiO2 (~
3.2 eV for anatase) targeting on its activation by visible light.
Towards this objective, doping with transition metals has been
largely employed to decrease the band gap energy or to
introduce energy levels inside the band gap.1,3,4
synthesis of multi-phase materials, composites or metal-doped
TiO2.7-17 In the case of doping, in addition to the desired red-
shift of the adsorption spectrum and in some cases the
reported efficient separation of the photogenerated charges,
the presence of transition metals has also been related with
potential negative impacts such as acting as recombination
centers or introducing defective sites in the crystal lattice and
increasing the probability of electron/hole (e-/h+)
recombination.1,3,18-20 Studies have been performed on charge
trapping and recombination21,22 as well as on understanding
the factors affecting such processes.23,24 Nevertheless,
conflicting results have been presented in the literature
regarding TiO2 doped with transition metals. This is related to
the complex physicochemical effects originating from the
doping process which show dependence of a significant
number of structural/electronic variables, which in turn
originate already from the preparation method and would
ultimately control the performance of the photocatalyst.
Since the photocatalytic efficiency of titania in organic
Copper and vanadium are among the most used transition
metals as TiO2 dopants25-30 since they can lead to significant
absorption in the visible region of light. TiO2 nanomaterials
loaded with Cu or V have been developed and studied in a
variety of photo-initiated reactions30-36 presenting either
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Catal. Sci. Technol.
increased or decreased photoactivity. Mechanistic schemes vigorously stirred for 24 h, centrifuged, decanted, thoroughly
including the introduction of new energy levels in the band gap rinsed under stirring with methanol in order to eliminate any
of TiO2 and the participation in the charge transfer processes portion from the organic and surfactant media, and dried at
o
such as trapping of photogenerated electrons and holes and O2 110 C for 24 h. The dried precipitates were calcined in air at
o
o
reduction have been proposed.31,36,37 Gray and co-workers 500 C for 2h using a ramp of 1 C min-1. A reference material
studied the copper loading effect on the electron was also synthesized using the same procedure without the
paramagnetic resonance (EPR) characteristics of surface presence of the doped metal. Sample labels are: T for pure
modified rutile TiO2.38 Mixed phase anatase/rutile TiO2, TCux and TVx for Cu- and V-doped TiO2 respectively
nanomaterials doped with vanadium presented high activity (where x corresponding to the atomic percentage of the
under visible light irradiation.39 Co-doped materials have been metal).
also studied where copper and vanadium proposed to act as 2.2 Characterization techniques
electron scavenger facilitating charge separation.36,40 However,
Brunauer, Emmett and Teller (BET) surface area and porosity
despite the great interest and the numerous studies in the
measurements were carried out by N2 adsorption at 77K using
literature in transition metal doped TiO2, very few works
a Micromeritics 2010 instrument. The nitrogen adsorption
report a systematic study and detailed combined analysis of
volume at the relative pressure (P/P0) of 0.98 was used to
the dopants’ species present, the photogenerated charge
determine the pore volume and average pore diameter. XRD
patterns were recorded in the range 10o< 2θ< 120o using 0.02o
carriers, the direct observation of their trapping sites in the
final nanomaterial and their effect in photoactivity. Particularly
steps, using a Siemens D-501 diffractometer with Ni filter and
for the case of Cu2+ and V4+ in anatase-based nanomaterials,
graphite monochromator with a Cu Ka X-ray source. UV-visible
such study has never been reported to the best of our
diffuse reflectance spectroscopy experiments were performed
knowledge.
with a Shimadzu UV2100 apparatus with a nominal resolution
The present work outlines the preparation of two series of
doped anatase titania using Cu2+ and V4+ prepared by
of ca. 1 nm using BaSO4 as reference. Band gap analysis was
done following standard procedures by plotting (hνα)1/2 (hν =
excitation energy, α = absorption coefficient) vs. energy.18
chemically interaction with species coming from the titanium
isopropoxide precursor using the microemulsion procedure in
Raman spectra were recorded using
a Horiba iH320
an effort to minimize the differences between the samples
prepared. Materials with primary particle size and textural
properties in a comparable range were developed. The
catalysts were evaluated against sunlight and UV-light gas-
phase photo-oxidation of toluene and EPR spectroscopy was
applied to study both the structure as well as the
photogenerated charges in the nanomaterials. Electron and
hole pairs generated by light irradiation and trapped in TiO2
were investigated in order to relate their abundance and
stability with the catalytic activity and the catalyst’s
composition linked with the dopants species. The relationship
between the structure, the dopants species, the
photogenerated charges and the catalytic activity is discussed
on the basis of the combined catalytic and spectroscopic study.
The two series prepared using different and widely applied
transition metals as TiO2 dopants were studied in an effort to
validate the consistency of the mechanism proposed herein.
spectrometer and He:Ne laser excitation (632.8 nm).
XPS data were recorded on 4 × 4 mm2 pellets, 0.5 mm
thick, prepared by slightly pressing the powder materials
which were outgassed in the prechamber of the instrument at
room temperature up to a pressure < 2 × 10−8 Torr to remove
chemisorbed water from their surfaces. The SPECS
spectrometer main chamber, working at a pressure <10−9 Torr,
was equipped with a PHOIBOS 150 multichannel hemispherical
electron analyser equipped with a dual X-ray source working
with Al Kα (hꢀ = 1486.2 eV). The C 1s peak at 284.6 eV was
used to correct charging effects.
EPR measurements were carried out with a Bruker ER200D
instrument operating in the X-band. All spectra were recorded
at 77 K in a double T type cavity. The frequency of the
microwave was calibrated using diphenyl-picryl-hydrazyl
(DPPH) as standard (with g = 2.0036) located in the second
cavity. For outguessing treatments, 60 mg of the Ti-sample
were placed in a quartz cell and vacuum treated at pressures
down to 2×
10-5 Torr at room temperature for 1 h. Irradiation
2. Materials and Methods
treatments were carried out at liquid nitrogen temperature
(77 K), by placing the EPR cell in a quartz Dewar flask, unless
otherwise stated. The samples were irradiated with UV-light
for specific periods using four lamps (Philips TL 6W/08,
identical to those used in the catalytic experiments)
symmetrically positioned outside of the sample. EPR spectra
were simulated using EasySpin software package.41
2.1 Catalyst preparation
Titania precursor materials were prepared using
a
microemulsion synthetic route by addition of titanium(IV)
isopropoxide (Aldrich) to an inverse emulsion containing an
aqueous phase (50 ml) dispersed in n-heptane (85/10 v/v vs.
H2O; Panreac), and using Triton X-100 (variable quantity;
Aldrich) as surfactant, in 1-hexanol (105/100 v/v vs. surfactant;
Aldrich). The metal (Cu2+ or V4+) was introduced from copper
nitrate and vanadyl acetylacetonate (Aldrich). The final atomic
content of the metal (x %) varied from 0.05 to 1 %
(stoichiometry: Ti1-0.0xM0.0xOy). The resulting mixture was
2.3 Photo-catalytic experimental details
Activity for the gas-phase photo-oxidation of toluene was
tested in a continuous flow annular photoreactor (see Figure
S1 at Electronic Supplementary Information)42 containing ca.
2 | Catal. Sci. Technol., 2015, 00, 1-12
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101
Eg
200
211
204
105
004
215
200
116
a
b
c
A1g
B1g
Eg
B1g
Eg
TiO2
TCu005
TCu01
TCu05
d
e
TCu1
TV005
f
TV01
TV1
g
100
200
300
400
500
600
700
Wavenumber (cmꢀ1)
20
30
40
50
60
70
80
2
θ
Figure 2. Raman spectra of the indicated pure (T) and metal doped (TCux and TVx)
Figure 1. XRD patterns of the pure TiO2 (a), the TCux (TCu005 (b), TCu01 (c), TCu1 (d))
samples.
and the TVx nanomaterial (TV005 (e), TV01 (f) and TV1 (g)).
Table 1 Characteristics of the pure TiO2,TCux and TVx samples.
40 mg of photocatalyst as a thin layer coating on a pyrex tube,
corresponding to TiO2 surface density of 0.5 mg/cm2. The
corresponding amount of catalyst was suspended in 1 ml of
water, painted on a pyrex tube (cut-off at ca. 290 nm) and
dried at room temperature. The reacting mixture (100 ml/min)
was prepared by injecting toluene (Panreac, spectroscopic
grade) into a wet (ca. 75% relative humidity) 20 vol.% O2/N2
flow before entering at room temperature to the
photoreactor, yielding an organic inlet concentration of ca. 800
ppmv (velocity 1.33 cm/s). After flowing the mixture for 3-4 h
in the dark (control test), the catalyst was irradiated by four
lamps symmetrically positioned outside the photoreactor. The
Sample Crystal size
(nm)
BET Pore volume Porosity
Pore size
(nm)
5.7
Eg
(m2 g-1)
77.6
68.1
70.0
76.5
81.9
73.0
75.9
74.8
76.0
(ml g−1
0.13
0.11
0.11
0.11
0.12
0.11
0.11
0.11
0.13
)
(%)
TiO2
11.6
12.5
12.7
12.6
12.0
11.8
11.9
11.5
11.6
33.2
30.5
29.1
30.3
31.1
29.3
30.9
31.5
33.5
3.18
3.16
3.15
3.13
3.09
3.16
3.14
3.09
3.02
TCu0.05
TCu0.1
TCu0.5
TCu1
5.8
5.2
5.5
5.0
TV0.05
TV0.1
TV0.5
TV1
5.1
5.3
5.6
5.9
Crystal size of anatase determined by XRD using Scherrer equation and the
most intense (1 0 1) diffraction peak. Total pore volume taken at relative
pressure P/P0 = 0.98. Porosity: estimated based on the density of anatase
(3.9 g cm-3) and the pore volume determined using the adsorption branch
of the N2 isotherm at 0.98 relative pressure. Pore size: calculated from the
adsorption branch of the N2 isotherm using the BJH method. Eg was
calculated considering the oxides indirect band gap semiconductors.
photocatalytic tests were performed under
a radiation
spectrum simulating sunlight (Philips TL 6W/54-765; Color
rendering index 75; main peaks at 410, 440, 540, and 580 nm;
6 W) as well as under pure UV-light irradiation (Philips TL
6W/08; 350 nm; 6 W). The external surface of the catalyst’s
thin film was exposed to an irradiance of 12.5 and 8 mW/cm2
under sunlight and pure UV-light respectively. Reaction rates
were evaluated under steady state conditions, typically
achieved after 4-5 h from the beginning of irradiation. No
change in activity was detected for all samples within 24 h
after reaching steady state conditions. The concentration of
reactants and products was analyzed using an on-line gas
chromatograph (Agilent GC 6890) equipped with HP-PLOT-
Q/HP-Innowax columns (0.5/0.32 mm I.D. × 30 m) and TCD/FID
detectors for the quantification of CO2 and organic substances
(toluene, photo-catalytic products) respectively.
consisted with 100 % anatase crystal structure since. No peaks
assigned to other oxide phases were found, suggesting that
either the dopant metal ions are incorporated in the TiO2
lattice or Cu and V oxides are very small and dispersed, non-
detectable by XRD.43 The cell parameters of anatase are not
affected in the case of Cu due to the low metal doping and the
compensation effect related to defect creation, particularly Ti
vacancies, vs. the larger Cu ionic radius.4,47 In the case of V4+
the isoelectronic character of V and Ti as well as the similar
ionic radius (0.58 Å; compared to Ti4+ 0.605 Å) justify the
absence of changes in anatase cell parameters. Anatase
crystallinity is not affected significantly by the introduction of
the metal. The primary crystal size of anatase as estimated by
the Scherrer formula using the full width at half maximum
(FWHM) of the most intense (1 0 1) diffraction peak is
approximately 12 nm. Minimal variations in the anatase crystal
size were thus observed within the samples of both series
(Table 1).
3. Results and discussion
3.1 Catalysts characterization
Figure 1 shows the XRD patterns of the pure and doped TiO2
samples. Typical spectra of anatase TiO2 are observed. The
main Bragg reflections are indexed in the figure according to
the JCPDS card 21-1272 corresponding to the anatase
structure. In both TCux and TVx series the XRD patterns are
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Ti 2p
2p3/2
O 1s
A
2p1/2
B
TiO2
A
B
1.8x10ꢀ2
1.2x10ꢀ2
TiO2
TCu005
TCu1
TV005
TV1
TiO2
TiO2
TCu1
TV1
TCu005
TCu1
TV1
TCu1
6.0x10ꢀ3
TV005
456 458 460 462 464 466
528 529 530 531 532 533 534
Cu 2p
C
D
TV1
2p3/2
V 2p
2p1/2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
10
Relarive pressure (P/P0)
Pore Diameter (nm)
TCu1
Satelite
peak
TV1
Figure 3. Representative nitrogen adsorption-desorption isotherms (A) and pore size
distribution calculated from the adsorption branch of the N2 isotherm using the BJH
method (B) of the pure and metal doped TiO2 nanomaterials.
TCu0.5
932 936 940 944 948 952 956
Binding Energy (eV)
512 514 516 518 520 522 524 526
Binding Energy (eV)
The phase composition of the prepared nanomaterials was
further studied by Raman spectroscopy. Representative
Raman spectra of the undoped and the Cu- and V-doped TiO2
materials are shown in Figure 2. The exclusive presence of
anatase phase is evidenced with peaks at ca. 144, 195, 399,
517 and 639 cm-1, corresponding to the Eg(1), Eg(2), B1g(1),
B1g(2), A1g(1) and Eg(3) modes respectively.18 No crystalline
phases of either Cu or V oxide are detected. This is expected
for materials with low metal doping levels as those prepared
here. The detection of Cu or V oxides would be expected for
dopant’s concentrations > 1.5 %.44 Nevertheless, the Raman
results are consistent with the XRD.
Figure 4. Core level XPS spectra in the Ti 2p (A), O 1s (B), Cu 2p (C) and V 2p (D) region.
were fitted using two symmetric peaks centred at 529.80 and
531.45 eV, attributed to the oxygen lattice ions of the anatase
network and surface hydroxyl groups.47,48
Figure 4C presents the Cu 2p region of the XPS spectra.
Typical Cu 2p XPS spectra are observed. The peaks centred
approximately at 933.7 and 953.6 eV, ascribed to the Cu 2p3/2
and Cu 2p1/2, accompanied by a satellite peak (clearly observed
only for the main Cu 2p3/2 peak due to the low signal to noise
ratio) located at approximately at 941-944.5 eV, indicate the
presence of Cu at the (+2) oxidation state.49,50 The presence of
Cu1+ is expected at lower binding energies.49,51 Cu 2p peaks
were only detected in the materials with Cu content higher
than 0.5 % (Figure 4C). In the TVx series, a weak peak at
approximately 516.7 eV originating from a single contribution
was detected only for the high load sample (TV1) attributed to
the V 2p3/2 at the (+4) oxidation state (Figure 4D).36
The BET surface area, total pore volume, porosity and
average pore size of the investigated catalysts are listed in
Table 1. First to mention is that most of the samples present
textural properties in a comparable range. N2 adsorption-
desorption isotherms of the materials are presented in Figure
3A. All samples exhibited typical features of type-IV isotherms
with H2 hysteresis loop, attributed to the development of a
mesoporous material.12,45 The surface area is not affected
significantly by the presence of the dopant as extracted using
the standard BET method.
Light absorption properties were studied by UV–Vis
spectroscopy. Starting with the analysis of the band gap
(Figure S2A and B), we note firstly that anatase is known to be
an indirect band gap semiconductor, although relaxation of
energy and momentum rules on the nanoscale may induce
direct band gap character.2,4,52 In any case, the band gap
behaviour throughout the series is similar for both cases (e.g.
direct and indirect). Table 1 gives the result of the indirect
band gap calculation.
The adsorption isotherms were used to determine the pore
size distribution using the Barret-Joyner-Halender (BJH)
method (Figure 3B). The presence of mesoporosity with
narrow pore size distributions was observed, implying good
homogeneity of the pores. In addition, the average pore
diameter is not significantly altered within the samples of both
series ranging from 5 to 6 nm.
Figure 5 displays plots used for band gap estimation as well as
the corresponding UV-Vis absorption spectra of the prepared
materials (insets). The increase of the absorption at
wavelength lower than 390 nm can be assigned to the intrinsic
band gap absorption of pure anatase TiO2. Upon the
introduction of the metal, a noticeable shift of the absorption
spectra towards the visible-light region can be observed, which
increased with increasing dopant’s concentration. By
extrapolating the linear portion of (hνα)1/2 versus energy, the
band gap (Eg) values were estimated (Table 1). The UV-visible
information provides evidence that the dopants induce band
gap modification (by altering band structure) even from the
lower doping quantity tested here. The trend indicates a
The surface composition and chemical state were further
investigated by means of X-ray photoelectron spectroscopy
(XPS). In the XPS survey spectra, peaks due to Ti, O, Cu and V
were detected. Figure 4 presents representative XPS spectra of
Ti 2p, O 1s, Cu 2p and V 2p core level profiles of the prepared
materials. The peaks at approximately 458.6 and 464.3 eV are
ascribed to the Ti 2p3/2 and 2p1/2 spin-orbit components
characteristic of anatase TiO2 (Figure 4A).46,47 No obvious
differences in the binding energies of the Ti 2p peaks were
observed within the samples of both series (i.e. TCux and TVx),
indicating that the increase of the dopant’s concentration does
not affect significantly the TiO2. The O 1s peaks (Figure 4B)
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A⊥
A
TCu1
TCu05
TCu01
TCu005
TCu0.05
TCu0.1
400 500 600 700
α
ν
Wavelength (nm)
h
TCu1
A
CuꢀII
||
3.8 3.6 3.4 3.2 3.0 2.8 2.6
CuꢀI
Energy (eV)
TCu0.05
TCu0.1
TV1
TV05
TV01
TV005
TiO2
B
g = 1.93
3500 3600 3700
2700 2800 2900 3000
2800
3000
3200
3400
3600
400
500
600
α
ν
Wavelength (nm)
Magnetic Field (G)
h
Figure 6. Normalized EPR spectra of the TCux samples taken in dark under vacuum
(dashed green spectrum: computer simulation of the broad unstructured Cu2+ signal).
Left inset: hyperfine splitting in the parallel Cu2+ spectral region specifying the two
distinct Cu2+ species (Cu-I and Cu-II). Right inset: Ti3+ centers formed in the TCu0.05
sample. Experimental conditions: 4 G modulation amplitude, 12.4 mW microwave
power. For the TCu1 sample a 10-fold smaller receiver gain was used due to signal
saturation.
3.8 3.6 3.4 3.2 3.0 2.8 2.6
Energy (eV)
Figure 5. Plot used to estimate band gap considering the oxides indirect
semiconductors and diffuse reflectance UV-Vis absorption spectra (insets) of the TCux
(A) and the TVx nanomaterials (B).
Table 2. EPR characteristics of the paramagnetic species in the TCux and the TVx
series.
A∥
gꢁ
Aꢁ
~ 18
~ 26
48
giso
Aiso
Species
Cu-I
g∥
decrease of the band gap energy with increasing dopant
concentration in the nanomaterial. In addition, absorption in
the visible region is also detected in all but the pure TiO2
reference sample. In the case of the TCux series, the intensity
of the shoulder at ~440 nm and the absorption at ~700-800
nm increases continuously with increasing dopant
concentration. The same is observed for the spectra in the TVx
series in the range 450-700 nm. The light absorption in the
visible region can be attributed to the d–d transitions of Cu25,53
and V,14,28 similar to that of Fe.18
~ 2.09
~ 2.09
1.964
1.962
1.971
2.434
2.370
1.914
1.919
1.928
75
Cu-II
V-I
109
182
170
162
1.947
1.948
1.957
92.7
91.3
95.3
V-II
52
V-III
62
V-IV
1.93
magnetic field region. Two Cu2+ EPR signals can be clearly
observed in the parallel region with g1∥=2.435, A∥1=75 G and
g2∥=2.370 and A2∥=109 G (named Cu-I and Cu-II respectively,
Table 2). They can be attributed to isolated or nearly isolated
copper ions and they present similarities in terms of g and A
values with Cu2+ species previously reported for copper ions
incorporated in anatase matrix,54 as well as for copper-
alumina55,56 and Cu-TiO2-ZrO2 ternary samples.57 The hyperfine
spitting of the spectral high field component reveal the
presence of at least two Cu2+ species with A⊥ = 26 and 18 G
having approximately the same g-values (g⊥~2.09). Comparing
the relative intensity of signal Cu-I and Cu-II, the Cu-II signal is
approximately two times smaller in the TCu0.1 compared to
TCu0.05.
3.2 EPR characterization
3.2.1 Cu2+ - doped TiO2
The EPR spectra of the Cu-doped samples recorded in dark are
shown in Figure 6. Two sets of Cu2+ EPR signals can be
observed. The first dominates the low Cu2+ concentration
samples presenting well resolved hyperfine splitting and the
second, a broad unstructured signal, dominates the high Cu2+
loaded samples.
In the low Cu2+ doped nanomaterials (samples TCu0.05 and
TCu0.1), the EPR signals are characteristic of paramagnetic
Cu2+ (3d9, S= 1/2) ions in oxygen environment of distorted
octahedral geometry. They are characterized by a set of axial
or near axial g-values. Hyperfine structure due to I = 3/2
(multiplicity 4) of Cu2+ ion can be observed, both in the parallel
and perpendicular component of the spectra (low and high
magnetic field region of the spectra respectively). Due to the
complexity of the experimental spectra, no reasonable
computer simulated spectra could be performed.
Nevertheless, distinct Cu2+ species were detected at the low
The spectral resolution decreases with increasing Cu
content. The sample containing 1% of Cu2+ is dominated by a
broad unstructured signal, extended from 2600 to 3500 G and
centered at g~2.1, while a hyperfine structure similar to the
low Cu-loaded samples is partially resolved in the
perpendicular region. The increase of the line width and the
loss of the hyperfine structure can be assigned to the dipole-
dipole interaction occurring between neighboring Cu2+ sites
within CuO clusters formed upon increasing Cu-content. These
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are probably nanoclusters of copper ions not detectable with
XRD and Raman. This signal accounts for approximately 70 %
of the TCu0.5, 20% of the TCu0.1 and less than 3 % of the
TCu0.05 spectra.
TV0.05
TV0.1
An additional signal at g = 1.93 is also observed in the TCux
series. This signal is not present in the pure anatase TiO2
reference sample and is attributed to Ti3+ centers23,31 (Figure 6,
right inset) previously assigned to surface trapping sites,23
most likely formed as charge neutrality species due to the
incorporation of Cu2+ in the TiO2 lattice. Comparing the
normalized EPR spectra of sample TCu0.05 and TCu0.1, the g =
1.93 signal intensity is almost two times smaller in the TCu0.1
and it follows the loss of the Cu-II signal. This may suggest that
by increasing the Cu content from 0.05 to 0.1 %, the portion of
Cu2+ incorporated into the anatase lattice is decreased with
respect to the total Cu2+ added, resulting to the decrease of
the Ti3+ EPR signal with respect to the Cu2+. It may also suggest
that low Cu loadings favor incorporation of Cu2+ into the
anatase lattice while for Cu2+ > 0.1 % surface Cu-species are
also present. Alternatively, it may suggest that charge
compensation occurs by different physical phenomena at
loadings below/above ca. 0.1 %. Nevertheless, the presence of
such signal verified that part of the initial added Cu2+ occupied
substitutional cation sites in TiO2 In the normalized spectra in
Figure 6, the Ti3+ signal is not clearly observed in the high load
samples because the spectrum is dominated by the Cu2+ signal.
The inability to detect Ti3+ species with XPS spectroscopy is
most likely due to the low Cu-loading (i.e. < 0.1 %) which is
overcome with the high sensitivity of EPR. The high sensitivity
of EPR is also evidenced on the Cu2+ (Figure 6) and V4+ (Figure
7, see in the following) EPR signal when compared to the XPS
data (Figure 3).
TV1
VꢀI
VꢀII
VꢀIII
VꢀIV
2800 3000 3200 3400 3600 3800 4000 4200
Magnetic Field (G)
Figure 7. Normalized EPR spectra of the TVx samples in dark under vacuum.
Spectra specification: Blue
- experimental spectra; Green - total simulated
spectra; Orange - individual simulated spectra (V-I to V-IV). Experimental
conditions: 10 G modulation amplitude, 4.9 mW microwave power.
the TV0.05 sample, the hyperfine structure in the parallel
region cannot be clearly resolved due to the low signal
intensity.
At higher vanadium concentrations, the interaction
between different V4+ ions increases giving rise to a dipolar
broadening (see spectrum TV1). This results to an additional
broad, unstructured, singlet EPR spectrum centered at g = 1.93
with linewidth of approximately 400 G (specie V-IV). Similar
EPR signals have been previously reported for V-doped TiO2
anatase samples.59-61 Nevertheless, hyperfine splitting due to
isolated V4+ species can still be detected. In addition to the two
V4+ spectra detected in the low V-loaded samples (TV0.05 and
TV0.1), a new V4+ spectrum with g and A principle components
g⊥ = 1.971, g|| = 1.928, A⊥ = 62 G and A|| = 162 G was detected
(specie V-III). The contribution of the broad singlet EPR signal is
approximately 85 % for the TV1 sample and 39 % for the TV0.5
(based on double integration analysis of the simulated
spectra), attributed to magnetically interacting vanadium
species, indicating the formation of oxide. As in the case of the
TCux nanomaterials, these are nanoclusters not detectable
with XRD and Raman.
Overall, the present EPR data verify the isolated nature of
the cupper species formed in the samples with low metal
loading (TCu0.05 and TCu0.1) due to the hyperfine splitting
observed in both the parallel and perpendicular component
and the formation of Cu clusters upon increasing Cu
concentration. In addition, Ti3+ centers are formed due to the
charge imbalance generated as a consequence of the dopant
introduction in the TiO2 lattice.
3.2.2 V4+ - doped TiO2
Figure 7 presents EPR spectra of the V-TiO2 samples under
vacuum and in dark. In all samples, an eight-component
hyperfine structure is observed due to the dipole-dipole
interaction of the electronic (S = 1/2) and magnetic moment (I
= 7/2, multiplicity 8) of V4+ ions (51V, natural abundance of 99.8
%).
Based on the analysis of the giso and Aiso values62 of the
three isolated V4+ spectra, the two species (V-I and V-II)
detected at low V4+ doping do not present pronounced vanadyl
character compared to the third specie (V-III) detected only for
V4+> 0.5 %. This indicates that the third specie might be
located on the surface of the material as surface vanadyl sites.
Of importance is to mention that, in contrast to the Cu-doped
samples, no EPR signal attributed to Ti3+ was detected for the
V-doped materials which can be attributed to the absence of
charge imbalance due to the isolectronic properties of Ti4+ and
V4+.
Magnetically isolated species, characterized by axial g and
A tensors, have been detected. The TV0.05 and TV0.1 spectra
present basically the same profile and can be analyzed as a
superposition of two different V4+ species, with slightly
different principle values (Table 2), suggesting minor
differences in the coordination environment (specie V-I and V-
II). V4+ presenting similar g and A tensors have been previously
attributed to V4+ centers located within the anatase bulk.58 In
At this point we highlight the fact that while no difference
was observed within the samples containing different amount
of dopant with XRD and Raman and only the nanomaterials
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detected on the selectivity towards CO2 or benzaldehyde
within the samples of both series. The time evolution of
toluene for representative examples is presented in Figure S2
of the supporting information.
3.0x10ꢀ9
2.0x10ꢀ9
1.0x10ꢀ9
8.0x10ꢀ10
6.0x10ꢀ10
4.0x10ꢀ10
2.0x10ꢀ10
A
Pꢀ25
A first important observation is that most of the prepared
nanomaterials improve the performance of P-25. In particular,
the most active doped TiO2 nanomaterial (TV0.1) presents a
six-fold higher photoactivity compared to pristine P-25. In
addition, a number of the doped-TiO2 nanostructures are more
active than the pure TiO2 nanomaterial having a similar
particle size and obtained using the same preparation
procedure.
0.00
0.05
0.10
Catalyst dopant content (%)
Pꢀ25
0.2
0.0
0.4
0.6
0.8
1.0
Catalyst dopant content (%)
8.0x10ꢀ10
6.0x10ꢀ10
4.0x10ꢀ10
2.0x10ꢀ10
B
In the TCux series (Figure 8A), the photoactivity under solar
light irradiation is continuously decreased with increasing Cu-
content. Compared to the pure TiO2 reference sample,
photoactivity is higher only for the low-doped sample
(TCu0.05). In the TVx series (Figure 8B) photoactivity is
increased for the two low V-doped nanomaterials presenting
higher efficiency than the pure TiO2 reference sample and it is
decreased at higher dopant concentration. Therefore, toluene
photo-excitation appears to be favored at low metal doping
and seems clearly detrimental for high levels. This suggests the
presence of a turning point where a beneficial/detrimental
effect on photoactivity is detected with respect to the
reference pure TiO2 sample in both TCux and TVx series where
the dopant content varies. This turning point is observed at
relatively higher dopant amount in the TVx series (i.e. TV0.5)
compared to the TCux nanomaterials (i.e. TCu0.1).
Nevertheless, in both cases it is observed at low dopant
concentration, it is approximately at similar levels as previously
reported7,14 and it falls within the optimal area defined by Bloh
et al..63
Pꢀ25
0.0
0.2
0.4
0.6
0.8
1.0
Catalyst dopant content (%)
Figure 8. Surface-area-normalized steady state reaction rates for toluene photo-
oxidation under sunlight irradiation by the Cu-doped (A) and V-doped (B)
nanomaterials. Inset: photocatalytic results under pure UV-light irradiation for selected
samples (TCux: squares, TVx: cycles, triangle: P-25).
with relatively high dopant content could be studied with XPS,
EPR provided structural information for both the low and high
doped catalysts in the two series.
The structural and electronic effects of Cu and V doping on
the anatase can be thus summarized as being related to the
introduction of the dopant in anatase matrix, the formation of
isolated species and clusters at low and high dopant
concentration respectively, altering the band gap energy and
in some cases introducing Ti3+ centers in the crystal structure.
The dopants introduced do not appear to substantially modify
primary particle size, the crystal structure as well the texture
properties of the nanomaterials.
The inset in Figure 8 shows results for selected samples in
similar conditions to those reported in Figure 8 but using pure
UV light excitation. A similar trend of the photoactivity with
respect to the dopant level is observed irrespective of the light
source nature. However, smaller differences were detected
within the samples tested under pure UV light irradiation,
since the improvement of photocatalytic activity is more
pronounced under solar light irradiation. This indicates that
visible light activity is induced in the doped nanomaterials.
However, although electronic properties linked with the light
absorption properties of the nanomaterials might have an
impact, is not the dominant parameter determining
photoactivity.
3.3 Photocatalytic activity
Figure 8 shows steady state reaction rates for the gas-phase
photo-oxidation of toluene by the two TiO2 series under
artificial sunlight and pure UV irradiation (inset), taking into
account the different BET surface area of the samples.
Expressing the rate per weight unit (mol s-1 g-1) gives
essentially a similar general behavior as the one presented in
Figure 8, since the specific surface area is not significantly
altered especially for low dopant’s content (Table 1). For
comparison, the photocatalytic results obtained for the
commercial reference TiO2 Degussa P-25 is also included in the
figure. Catalytic tests carried out without photocatalysts or
irradiation confirmed that the measured activities were fully
attributed to photo-induced processes on TiO2. CO2 and
benzaldehyde were the only gaseous products detected during
toluene photo-oxidation. No significant differences were
Regarding photocatalytic activity, one should mention that
it is a result of different factors, including crystalline phases
present in the nanomaterial, particle size, specific surface area,
porosity, band gap, composition, charge separation and many
others. Herein, precise control of the synthesis process and the
mild conditions applied for the nanomaterials modification,
resulted to practically unchanged nanostructures regarding
crystal structure, crystallinity and texture properties,
minimizing the differences within samples of the two series. As
was stated above, pure electronic properties are not of
significance in interpreting fully photoactivity. Therefore,
additional EPR experiments were contacted to study the
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Cu2+
h+
Ti3+ localized
(a)
Oꢀ
g⊥ = 1.991
g|| = 1.961
TCu0.05
(b)
(b')
g = 1.90
×
10
Ti3+ surface
3500
3600
3700
(c)
h+ꢀI
TV0.1
h+ꢀII
No light
Irradiated
eꢀ
3200
3300
3400
3400
3500
3600
3700
3800
Magnetic Field (G)
Magnetic Field (Gauss)
Figure 10. EPR spectra of the TCu0.05 (upper) and TV0.1 nanomaterial (lower) taken in
dark (blue) and under 20 min UV irradiation (orange). Experimental conditions: same as
in Figure 9.
Figure 9. EPR spectra of (a) the pure TiO2 sample in dark, (b) after 20 min UV irradiation
with the corresponding simulated spectrum (b΄, sum of the three individual signals), (c)
after 20 min UV irradiation in the presence of iso-propanol. The individual simulated
spectra (h+-I, h+-II and e-) are given at the bottom of the figure. Experimental
conditions: 2 G modulation amplitude, 4.9 mW microwave power.
Table 3. g-values of the photogenerated species observed in the nanomaterials
prepared. Error: ± 0.0002.
Sample
TiO2
H-I
O
H-II
gꢁ
2.0182 2.0145 2.0040 2.0102 2.0020 1.991 1.961
Ti-I (Ti3+
gꢁ
g||
)
ꢃ
gꢆ
ꢂꢄ
ꢃ
ꢂꢄ
O
charge carriers formation and abundance under light irradiation.
gꢅ
gꢇ
g||
3.4 Charge trapping sites on TiO2
Figure 9 presents EPR spectra of the pure TiO2 anatase
reference sample in dark (a) and under light irradiation (b and
c). No EPR signal is detected in the spectra recorded in dark
(Figure 9a). After 20 min UV irradiation at liquid N2
temperature, two sets of EPR signals are detected (Figure 9b).
The low field feature is assigned to oxygen species, holes (h+)
trapped at oxygen anions (Oꢂ). As summarized in Table 3,
computer simulation of the experimental data allowed the
determination of two overlapping signals (h+-I and h+-II)
attributed to h+ trapped at two different Oꢂ sites.24,62 The g-
values of both signals are similar to h+ species previously
observed in TiO2 anatase prepared and treated in a similar way
as those here.21,22,64-72
TCu005 2.0185 2.0142 2.0040 2.0105 2.0022
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
TCu01
TCu05
TCu1
TV005
TV01
TV05
TV1
2.0187 2.0145 2.0040 2.0105 2.0022
–
–
–
–
–
–
–
–
–
–
2.0185 2.0142 2.0040 2.0105 2.0022
2.0185 2.0142 2.0040 2.0105 2.0022
2.0184 2.0142 2.0040 2.0105 2.0022
2.0180 2.0140 2.0032 2.0105 2.0022
h+-II) and e- (Ti-I) trapped at localized states reveals that e- are
less than 10 % of the trapped h+. The signal intensity of the
trapped charges remains essentially constant at T = 77 K and in
the dark during the course of the experiment. After
subsequent warming of the sample at room temperature, both
h+ and e- EPR signals are lost (data not shown) as a result of
the h+/e- recombination process.
The high field component of the spectrum contains a sharp
EPR feature of axial symmetry, gꢁ
= 1.991 and g|| = 1.961 (Ti-
I). Based on the EPR characteristics in terms of g-values and
spectral shape, this signal can be assigned to electrons trapped
at localized states in the anatase structure as Ti3+
centers.68,69,71-73 These species have been previously attributed
to centers located in the inner part of the particle.22,68,72-74 The
intensity of the Ti3+ signal is not affected by the presence of
atmospheric O2 which further confirmed their location in the
lattice of anatase (data not shown). It should be noted that Ti3+
centers formed in the bulk of the material act as
recombination centers, reducing the activity of the surface
reactions.71,72 These centers are different from Ti3+ centers
that give rise to a broad signal at higher field (the negative
contribution of which can be observed in the inset in Figure 9
at g ~ 1.90) and have been previously attributed to centers at
disordered environment at the crystals surface.73,74 Double
integration of the single spectra corresponding to h+ (h+-I and
In the presence of isopropanol,
a well known hole
scavenger, the EPR signal assigned to trapped holes disappears
completely (Figure 9c). The sharp Ti-I signal with g⊥ = 1.991, g||
= 1.961 remains essentially unchanged and a new signal at g ~
1.95 assigned to different Ti3+ centers appears.31,65 This second
species is located at the surface of the solid and is extended
over 320 G due to the relatively disordered environment.74,75
These observations are in accordance with previous
reports31,71 and clearly demonstrate that the holes are trapped
at the surface since they easily react with hole scavengers such
as isopropanol. Therefore, they are consumed during photo-
oxidation of toluene.
Overall, based on the above observations and in
accordance with the well established charge transfer processes
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feature identical to the one of the pure TiO2 reference sample
assigned to trapped holes is observed in the majority of the
doped TiO2 materials. No hole signal was detected for the
TCu0.5 and TCu1 samples. The experimental spectra were
simulated (red spectra in Figure 11) by assuming two
overlapping signals similar to those used for the pure TiO2
reference; ii) the Ti3+ EPR signal of the e- trapped at localized
centers (Ti-I) detected in the undoped TiO2 reference sample
disappears completely in the presence of the dopant in both
TCux and the TVx series (Figure 11). These results clearly
indicate that in the metal doped anatase nanomaterials
photoinitiated charge formation takes place, forming identical
h+ species as those in the pure reference TiO2 catalyst.
TCu0.05
TCu0.1
TV0.05
TV0.1
TV0.5
TV1
Regarding the Oꢂ signal, clear differences in the intensity
of the hole signal were observed by varying the amount of the
dopant. The relative signal intensity corresponding to trapped
holes as Oꢂ centers versus the dopant’s concentration is
presented in Figure 12. The amount of trapped holes which
ultimately drives the surface photooxidation processes follows
a belt-like shape with optimum conditions achieved at low or
moderate dopant concentration. In the case of the Cu2+-doped
materials, the intensity of the hole signal when compared to
the pure TiO2 reference is higher only for the TCu0.05 sample
and it is not detected in the high-loaded samples (Figure 12,
samples TCu0.5 and TCu1). In the case of the TVx series, the
Oꢂ signal intensity is higher in the TV0.05 and TV0.1 compared
to the pure TiO2 sample and deteriorates at higher V4+
concentrations. This loss in both series for dopant content
higher than 0.5 % cannot be attributed to shading/shielding
effect taking into account the low dopant to Ti surface ratio
based on XPS. The relative contribution of the two individual
hole signals (h+-I and h+-II), calculated as the % of the total
intensity of the hole signal (I + II), is essentially stable in all
samples and is not affected by the dopant concentration. At
this point, we highlight the clear resemblance of the EPR h+
signal intensity variation with the catalytic activity presented in
Figure 8 for both series. It has been shown previously in V-
doped TiO2 that the photocatalytic degradation of dyes in
solution follows the hydroxyl radical formation,43 which in turn
is related to h+ production.
3350
3400
3450
3500
Magnetic Field (Gauss)
Figure 11. Normalized EPR spectra of metal doped TiO2 taken after 20 min UV
irradiation in vacuum after subtracting a proper amount of the corresponding spectra
recorded in dark. Dashed red lines: computer simulated spectra using two overlapping
h+ signals. Experimental conditions: same as in Figure 9.
TCu
TV
x
x
0.0
0.2
0.4
0.6
0.8
1.0
Catalyst dopant content (%)
Figure 12. Relative EPR intensity of the hole signal in the pure (solid symbol) and the
TCux (blue color) and TVx (green color) series.
in similar systems,1,68,69 we indicate the following charge
transfer steps under light irradiation:
The absence of the Ti-I EPR signal assigned to localized sites
of trapped electrons (i.e. Ti3+) in all doped nanomaterials under
irradiation can be exclusively assigned to the presence of the
dopant. This is based on the general mechanism of charge
photo-generation and transfer process described in reactions
(1)-(3)1,68,69 and is further supported by the holes formation in
the doped materials (detected as Oꢂ centers) as well as the
detection of Ti3+ centers as localized electron trapping sites in
the pure TiO2 material. In such a scheme, the straightforward
explanation would be the presence of an effective electron
transfer process where the dopant plays the role of the
acceptor for the photogenerated electrons,31,38,73 minimizing
TiO2 + ℎꢈ → eꢂ
+
hꢉ
(1)
(2)
(3)
ꢉ
ꢉ
eꢂ
hꢉ
+
+
Ti4
O2
→
→
Ti3
ꢂ
Oꢂ
indicating the charge carriers formation under irradiation (1)
and the electron (2) and hole (3) trapping at localized states.
Changes in the EPR spectra were also detected for the
doped materials under irradiation. Representative spectra of
the catalysts taken in the dark and under irradiation are
presented in Figure 10. Figure 11 presents EPR spectra of the
doped materials in the region of the trapped h+ and e-
normalized to their second integral. The spectra were taken
under irradiation and in vacuum and are presented after
removing a proper amount of the corresponding spectra
recorded in dark. Two important information can be extracted
when compare the pure with the doped materials: i) an EPR
the Ti3+ signal at gꢁ
= 1.991 (i.e. Ti-I specie). In fact, a decrease
of the Cu2+ EPR signal intensity was detected in the TCu0.05
nanomaterial upon light irradiation (Figure 10, marked area).
This suggests reduction of Cu2+ to the EPR silent Cu+ via
accepting photogenerated e-.31,38 The signal intensity was
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and
V
series (Figure
6
and 7 respectively) is inversely
7.0x10ꢀ10
6.0x10ꢀ10
5.0x10ꢀ10
4.0x10ꢀ10
3.0x10ꢀ10
2.0x10ꢀ10
1.0x10ꢀ10
A
100
80
60
40
20
0
proportional to the EPR hole signal intensity.
The interpretation of such correlation plots would imply
that photocatalytic degradation of toluene is directly
correlated with the hole formation, which in turn is tuned by
the dopant’s species in the nanomaterial. Isolated and highly
dispersed Cu2+ and V4+ species are beneficial for toluene
photodegradation while the presence of metal oxide clusters
reduces photoactivity via deteriorating hole formation. This
mechanism is controlled by the dopant’s content in the
nanostructures and in our materials the turning points are 0.1
% and 0.5 % for Cu-doped and V-doped anatase TiO2
respectively. The isolated dopant’s species restrain the
recombination rate of electron and hole pairs by accepting
photogenerated electrons demonstrated by the absence of Ti3+
centers as localized electron traps in the doped materials
EPR Oꢀ Signal Intensity (ar. un.)
8.0x10ꢀ10
7.0x10ꢀ10
6.0x10ꢀ10
5.0x10ꢀ10
4.0x10ꢀ10
3.0x10ꢀ10
2.0x10ꢀ10
1.0x10ꢀ10
B
100
80
60
40
20
0
(Figure
8
and 9). Once efficiently separated, the
photogenerated charges can be more effectively involved in
oxidation and reduction reactions enhancing toluene’s
photodegradation. On the other hand, by increasing the
dopant’s content above
a certain point in the final
nanomaterial, metal clusters are formed acting as
recombination centers38,63 of photogenerated e- and h+
suppressing photoactivity. Despite the difference in the actual
metal content linked with the turning point of the catalytic
activity (0.1% for Cu2+ and 0.5% for V4+), the data presented
herein verify that photoactivity is governed by the same
mechanism in both TiO2 series, suggesting that this
interpretation may find application in similar systems.
EPR Oꢀ Signal Intensity (ar. un.)
Figure 13. Correlation plots between the EPR hole signal intensity and the steady state
reaction rates of toluene photodegradation (open symbols, left Y-axis) and the EPR
hole signal intensity with the cluster formation (solid symbols, right Y-axis) for the TCux
(A) and the TVx (B) series. The pure TiO2 reference sample is indicated with the red
symbol. Lines are guide for the aids.
restored after exposing the sample to ambient atmosphere.
No such difference was detected for the V-doped materials.
However, the vanadium case is more complicated due to the
possible participation of three different oxidation states (+5,
+4, +3). This electron transfer process is expected to have an
impact on the photoactivity, via eliminating the Ti-I specie
formed in the bulk of the material22,68,72,73 and facilitating
charge separation.
Conclusions
Two series of Cu2+ and V4+ doped TiO2 anatase samples were
prepared via the microemulsion synthetic route. The electronic
and structural properties of the metal containing titania
nanomaterials were fully characterized. Nanomaterials with
primary particle size and texture properties in a comparable
range were developed while band gap narrowing was
observed for the doped nanostructures. EPR spectroscopy
revealed the presence of different dopant species via varying
their nominal concentration and their effect on the abundance
and stability of photogenerated charge carriers, e- and h+ pairs.
Complete structural and electronic characterization combined
with catalytic results of toluene photo-oxidation under both
pure UV and artificial solar light irradiation allowed the
determination of the factors affecting photoactivity. EPR
provided direct experimental evidence that photoinitiated
charge formation took place in the nanomaterials. Different
dopant species were identified. Isolated and highly dispersed
Cu2+ and V4+ species favored hole formation while at the same
time decrease the amount of Ti3+ centers (i.e. trapped e- at
localized states) under irradiation. It is proposed that the
presence of such dopant species is responsible for the
enhanced photocatalytic activity via increasing the charge
separation efficiency acting as efficient electron scavengers
and increasing the surface availability of the key charge carrier,
Figure 13 shows correlation plots between the EPR hole
signal intensity versus the steady state reaction rates for the
gas-phase toluene photodegradation (left Y-axis) and the
cluster formation (right Y-axis) for the TCux (Figure 13A) and
TVx series (Figure 13B). A linear relation is observed in both
cases. A linear trend between catalytic activity as well as the
metal cluster formation is observed as a function of hole
formation in both cases. In the second case, note that some
samples do not have a cluster contribution to the doping metal
distribution and generates two linear response regions (one
trivial, with null slope). Given that holes are the primary
photogenerated species responsible for toluene degradation
by TiO2,5,6,73 the difference in photoactivity detected between
samples in both series (Figure 8) is due to the relative
abundance of such species in the nanomaterial, as expressed
in Figure 12 and indicated by the correlation plots in Figure 13.
On the other hand, the trend of cluster formation as expressed
by the broad unstructured signal in the EPR spectra of both Cu
10 | Catal. Sci. Technol., 2015, 00, 1-12
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Article
23 D.C. Hurum, K.A. Gray, T. Rajh and M.C. Thurnauer, J. Phys.
Chem. B, 2005, 109, 977.
24 R. Scotti, I.R. Bellobono, C. Canevali, C. Cannas, M. Catti, M.
D’Arienzo, A. Musinu, S. Polizzi, M. Sommariva, A. Testino
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the holes. At higher dopant levels, metal clusters were formed
acting as recombination centers deteriorating photoactivity.
Acknowledgements
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6898.
,
Financial support by project MINECIO_Spain ENE2013-46624-
C4-1-R (partially supported by FEDER) is fully acknowledged.
K.C.C. acknowledges the TALENTS FVG Programme for a Post-
doctoral fellowship funded from the European Social Fund
(Operational Programme 2007-2013, Objective 2 Regional
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Graphical abstract
Isolated dopant’s species and metal-cluster formation regulate photoactivity and charge
carriers formation via accepting e- and eliminating Ti3+ states.