Charge carrier dynamics and photocatalytic behavior of TiO2 nanopowders submitted to hydrothermal or conventional heat treatment

Article abstract of DOI:10.1039/c5ra13291f

The sol-gel technique followed by conventional (TiO₂-1) and hydrothermal (TiO₂-2) thermal treatment was employed to prepare TiO₂-based photocatalysts with distinct particle sizes and crystalline structures. The as prepared metal oxides were evaluated as photocatalysts for gaseous HCHO degradation, methanol, and dye oxidation reactions. Additionally, metallic platinum was deposited on the TiO₂ surfaces and H₂ evolution measurements were performed. The photocatalytic activities were rationalized in terms of morphologic parameters along with the electron/hole dynamics obtained from transient absorption spectroscopy (TAS). TiO₂-2 exhibits smaller particle size, poorer crystallinity, and higher surface area than TiO₂-1. Moreover the hydrothermal treatment leads to formation of the metastable brookite phase, while TiO₂-1 exhibits only the anatase phase. TAS measurements show that the electron/hole recombination of TiO₂-2 is faster than that of the latter. Despite that, TiO₂-2 exhibits higher photonic efficiencies for photocatalytic oxidation reactions, which is attributed to its larger surface area that compensates for the decrease of the surface charge carrier concentration. For H₂ evolution, it was found that the surface area has only a minor effect and the photocatalyst performance is controlled by the efficiency of the electron transfer to the platinum islands. This process is facilitated by the higher crystallinity of TiO₂-1, which exhibits higher photonic efficiency for H₂ evolution than that observed for TiO₂-2. The results found here provide new insights into the correlations between thermal treatment conditions and photocatalytic activity and will be useful for the design of high performance photocatalysts.

Full text of DOI:10.1039/c5ra13291f

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Charge carrier dynamics and photocatalytic behavior of TiO2
nanopowders submitted to hydrothermal or conventional heat
treatment
A. O. T. Patrocinio*§, J. Schneider‡, M. D. França§, L. M. Santos§, B. P.
Caixeta§, A. E. H. Machado§ and D. W. Bahnemann‡, †
§
Laboratory of Photochemistry and Materials Science; Institute of Chemistry,
Universidade Federal de Uberlandia, Uberlandia, 38400-902, Brazil
‡Institut für Technische Chemie, Leibniz Universität Hannover, Callinstrasse 3,
D-30167, Hannover, Germany
†
Laboratory for Nanocomposite Materials, Department of Photonics, Faculty of
Physics, Saint-Petersburg State University, Ulianovskaia str. 3, Peterhof, Saint-
Petersburg, 198504, Russia
* to whom the correspondence should be addressed:
otaviopatrocinio@iqufu.ufu.br;

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Abstract
The solꢀgel technique followed by conventional (TiO2ꢀ1) and hydrothermal
(TiO2ꢀ2) thermal treatment was employed to prepare TiO2ꢀbased photocatalysts
with distinct particle sizes and crystalline structures. The as prepared metal
oxides were evaluated as photocatalysts for gaseous HCHO degradation,
methanol, and dye oxidation reactions. Additionally, metallic platinum was
deposited on the TiO2 surfaces and H2 evolution measurements were
performed. The photocatalytic activities were rationalized in terms of
morphologic parameters along with the electron/hole dynamics obtained from
transient absorption spectroscopy (TAS).TiO2ꢀ2 exhibits smaller particle size,
poorer crystallinity, and higher surface area than TiO2ꢀ1. Moreover the
hydrothermal treatment leads to formation of the metastable brookite phase,
while TiO2ꢀ1 exhibits only the anatase phase. TAS measurements show that the
electron/hole recombination of TiO2ꢀ2 is faster than that of the later. Despite of
that, TiO2ꢀ2 exhibits higher photonic efficiencies for photocatalytic oxidation
reactions, which is attributed to its larger surface area that compensates the
decrease of the surface charge carrier concentration. For H2 evolution, it was
found that the surface area has only a minor effect and the photocatalyst
performance is controlled by the efficiency of the electron transfer to the
platinum islands. This process is facilitated by the higher crystallinity of TiO2ꢀ1,
which exhibits higher photonic efficiency for H2 evolution than that observed for
TiO2ꢀ2. The results found here provide new insights into the correlations
between thermal treatment conditions and photocatalytic activity and will be
useful for the design of high performance photocatalysts.
1

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Introduction
The research in renewable energy sources is intense due to the concerns about
the global warming and other environmental issues related to the huge
dependence of the global energy matrix on fossil fuels 1ꢀ3. Photocatalysis calls
particular attention, due the possibility to harvest the abundant solar energy and
to convert it into fuels, such as hydrogen 4ꢀ6
.
TiO2ꢀbased materials are the most intensely investigated photocatalytic system
7
following the breakthrough by Fujishima and Honda . They exhibit chemical
stability, nontoxicity, high reactivity, and can be prepared in different forms and
sizes with highly ordered porosity and crystallinity 8ꢀ10. Typically, the oxide is
prepared from a molecular precursor, followed by a thermal treatment to ensure
crystallinity. Conventional heating at atmospheric pressure as well as the use of
pressurized hydrothermal reactors are the most popular thermal treatments
employed 11ꢀ15
.
Through the use of molecular engineering, the main limitations of TiO2 have
been superposed. For example, visible light activity has been obtained by the
16ꢀ20
21ꢀ24
introduction of dopants
or by the formation of nanocomposites
.
Moreover, H2 evolution without the necessity of an external bias has been
observed in the presence of coꢀcatalysts on the oxide surface 25ꢀ28
.
In parallel to the advances of material preparation, large research efforts have
been devoted to investigations of wellꢀdefined photocatalytic reaction systems
and their detailed reaction mechanisms and kinetics using a variety of
spectroscopic techniques 29ꢀ37. Particularly, the use of timeꢀresolved methods,
38ꢀ41
such as transient absorption spectroscopy (TAS)
and microwave
42, 43
conductivity measurements (TRMC)
has allowed to enhance the
2

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fundamental understanding of electron/hole trapping and recombination, with
the latter being recognized as the limiting step of the photocatalytic process 44.
Despite the remarkable progress, it is not an easy task to establish clear
relationships between the photocatalytic properties and the metal oxide
morphological and electronic properties. Serpone45 and Zhang 46, among others
47, 48
have investigated the role of particle size on charge carrier dynamics and
photocatalytic activity with the results of these studies being not always similar.
More recently, Carneiro and coꢀworkers described the dependence of the
methylene blue degradation rate on the concentration of hydroxyl groups and
holes at TiO2 surface among with their correlations with the particle size 43.
In this work, two different TiO2ꢀbased photocatalysts were prepared by the solꢀ
gel technique, which allows an easy control of the particle size. The as prepared
TiO2 particles were submitted to a conventional thermal treatment at 400 ºC in
an oven and also to a hydrothermal reactor (200 ºC, 12.4 bar), yielding different
crystalline phases, surface defects, etc. The samples were fully characterized
and evaluated by several photocatalytic oxidation tests and also for H2
evolution, with the aim to provide new insights into the underlying structureꢀ
reactivity relationship. The observed photonic efficiencies were rationalized in
terms of the morphological properties of TiO2ꢀbased photocatalysts and the
photogenerated charge carrier dynamics studied by nanosecond transient
absorption spectroscopy.
Experimental
Materials. All chemicals were of analytical or HPLC grade and were used as
received. TiO2 photocatalysts were prepared through the hydrolysis of Ti(IV)
3

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isopropoxide (Aldrich, 97%). 5 cm3 of the alkoxide were dissolved in 30 mL of
isopropanol under argon atmosphere. The precursor solution was cooled in an
ice bath and, under vigorous stirring, 50 cm3 of deionized water were slowly
added dropwise to yield a white precipitate. The photocatalyst 1 (TiO2ꢀ1) was
obtained by stirring the mixture for thirty minutes at low temperature. The
resulting powder was separated by centrifugation (6000 rpm, 20 minutes),
washed with deionized water and thermally treated at 400ºC for 6 hours in a
conventional oven. The photocatalyst 2 (TiO2ꢀ2) was obtained by stirring the
hydrolysed precursor solution for 12 hours at room temperature. The white
powder was separated by centrifugation (6000 rpm, 20 minutes) and washed
with deionized water to eliminate any residual alcohol. The powder was
subsequently suspended in 100 mL of water and transferred to a stainless steel
reactor, which was heated at 200 ºC for 8 hours at 12.4 bar. Afterwards, the
photocatalyst was separated by centrifugation and dried at 70°C by 24 h.
Deposition of metallic platinum on the TiO2 surfaces (0.5 %wt) was carried out
49,
as described elsewhere
50. Briefly, 50 mg of the photocatalyst were
suspended in 50 cm3 of 10% (v/v) methanol aqueous solution in a borosilicate
reactor. After purging the suspension with N2, 130 ꢀL of a 0.1 mol Lꢀ1 H2PtCl6
aqueous solution was added and the system was exposed to UVꢀA light (Philips
90 W Hg lamps, 1 mW cmꢀ2) by 8 hours. The resulting pale gray powders were
separated by centrifugation and dried at 70 ºC under reduced pressure.
Characterization. The powders were characterized by Xꢀray diffraction analysis
(XRD) using an XRD600 powder diffractometer (Shimadzu) operating at 40 kV
and 30 mA employing Cu Kα radiation. The diffractograms were collected
between 20 to 90º at 0.5º/min. Scanning transmission electron microscopy
4

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(STEM) images were obtained in a FEI Inspect F50 located at the Brazilian
Nanotechnology National Laboratory (LNNano) and operated at 30 kV. N2
adsorptionꢀdesorption isotherms were obtained in an ASAP 2020 analyzer
(Micrometrics). The sorption data were analyzed using the BarrettꢀJoynerꢀ
Halenda (BJH) model. Diffuse reflectance measurements were carried out in an
UVꢀ1650PC spectrometer (Shimadzu) with the band gap energy being
estimated by the Tauc method 51.
Transient absorption spectroscopy (TAS) was carried out employing a LKS80
nanosecond laser flash photolysis spectrometer (Applied Photophysics)
equipped with the proper diffuse reflectance accessory. The samples were
excited in a quartz cuvette by a LPX 200 excimer laser (XeF; λexc = 351 nm; 14
mJ per pulse) from Lambda Physics. The transient signals were collected by a
PMT connected to a computer interfaced to a DSO9064A oscilloscope (Agilent).
The data points were converted to absorbance values according to equation (1),
where A = absorbance; I0 = analyzing light level; Ir = light reflected by the
absorbing sample; Ia = light absorbed by the absorbing sample. In these
experiments, the samples were employed as dry powders.










I
I0


0 
(1)
ꢁA = log
= log

Ir
I0 − Ia


Photocatalytic Studies. The photocatalytic activity of the as prepared TiO2
powders was evaluated by different methods and compared to the activity of the
wellꢀknown photocatalyst Evonik Degussa Aeroxide TiO2 P25 (TiO2ꢀP25).
Acetaldehyde photoꢀoxidation was carried out accordingly to the ISO 22197ꢀ2
52
standard method
in an experimental setup described previously 21, 53. The
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active area of the samples was 15 cm2. All samples were cleaned under UVꢀ
irradiation (10 W/m², 365 nm) prior the experiments. The gaseous reaction
mixture was prepared by mixing streams of dry air (500 mL/min), wet air (500
mL/min, relative humidity of 50%), and 10% of a CH3CHO/N2 mixture
(approximately 50 mL/min) to obtain a final CH3CHO concentration of 1 ppm at
297 K. Prior to the photocatalytic tests, the photoreactor was purged with the
CH3CHO/water vapor/air mixture without illumination until a steady CH3CHO
concentration was achieved at the reactor outlet. Afterwards, the sample was
irradiated for approximately 120 min.
For methanol photoꢀoxidation, 50 mg of TiO2 were suspended in 3.0x10ꢀ2 mol Lꢀ
1 aqueous methanol solution placed in a round borosilicate reactor. The system
was irradiated using UVꢀA light (3.04x10ꢀ7 Einstein sꢀ1). The amount of
formaldehyde produced at different time intervals was determined by the Nash
method 49, 54, 55. The photonic efficiency values (ξ), which is defined as the ratio
of CH3CHO degradation or HCHO formation and the incident photon flux, were
calculated as previously described 53, 56
.
The activity of the TiO2 samples was also evaluated through UVꢀA degradation
of the azoꢀdye Ponceauꢀ4R (P4R) or trisodium (8Z)ꢀ7ꢀoxoꢀ8ꢀ[(4ꢀ
sulphonatonaphthalenꢀ1ꢀyl)hydrazinylidene]naphthaleneꢀ1,3ꢀdisulphonate.
Detailed description of the experimental setup can be found elsewhere 57. The
photon flux in the range between 300 and 815nm was (3±1)x10ꢀ6 Einstein sꢀ1 as
determined by a radiometric/ photometric setup described previously 58. The
degradation rate was evaluated by Total Organic Carbon measurements (TOC)
carried out in a Shimadzu TOCꢀVCPH Analyzer. Control experiments were
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taken in the absence of any photocatalyst to evidence the role of the TiO2 on
the photochemical reaction.
Platinized samples were employed for hydrogen evolution measurements using
25
the experimental setup described previously . Briefly, the TiO2 photocatalysts
were suspended in 50 mL of 10% (v/v) methanol aqueous solution placed in a
double jacket quartz reactor. The system was connected to a quadrupole mass
spectrometer (QMS) for gas analysis (Hiden HPRꢀ20) and purged with argon
until no traces of O2 and N2 could be observed. UVꢀA irradiation was performed
using a 150 W Xe lamp. The photon flux was 1.90x10ꢀ6 Einstein sꢀ1.
Results
Electronic and morphological characterization. The two TiO2ꢀbased
photocatalysts are constituted of agglomerates of spherical nanoparticles, as
visualized in the respective STEM images (Figure 1). The particle sizes
estimated from these images are (25 ± 2) nm and (12 ± 3) nm, respectively for
TiO2ꢀ1 and TiO2ꢀ2.
Figure 1. STEM images of TiO2ꢀ1 (a) and TiO2ꢀ2 (b) photocatalysts.
7

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XRD analyses of the powders, Figure 2, show that TiO2ꢀ1 is constituted only by
the anatase polymorph, while TiO2ꢀ2 exhibits peaks characteristic of anatase
and brookite. Rietveld refinement of the XRD data (see Table S1 and Figures
S1, S2, and S3 in the Supplementary Information) reveals that the brookite
content is approximately 45%. The appearance of the brookite phase in the
TiO2 samples prepared by the hydrothermal method agrees with previous
59
studies carried out by Isley and Penn , as well as by other authors 60, 61. This
photocatalyst also exhibits broader diffraction peaks as compared with TiO2ꢀ1
and the commercial sample (P25), which is in agreement to the smaller particle
size observed in the STEM images. Crystallite sizes calculated by the Scherrer
equation were 18 nm and 10 nm, respectively for TiO2ꢀ1 and TiO2ꢀ2, similar to
the values found by electron microscopy. Moreover, the lattice parameters
obtained for TiO2ꢀ2 (Table S1) indicate a more distorted structure in relation to
TiO2ꢀ1 or TiO2ꢀP25, especially in the
c axis.
As expected, both photocatalysts exhibit absorption only in the UV region with
an optical bandgap of 3.2 eV, similar to the values reported for TiO2 anatase 62.
Diffuse reflectance spectra of the as prepared powders can be found in the
Supplementary Information (Figure S4).
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A
R
P25
B
TiO -2
2
TiO -1
2
20
30
40
50
60
70
80
90
2θ
Figure 2. XRD patterns of TiO2 photocatalysts. The peaks marked as A, R, and
B, refer to the main diffraction peaks of anatase, rutile and brookite,
respectively.
The samples were also characterized by N2ꢀadsorption/desorption isotherms
using the B.E.T. method (Figure S5). Type IV sorption curves were observed,
according to the IUPAC classification 63, which are characteristic of mesoporous
materials. The surface areas calculated for the samples were 66 and 164
m2 gꢀ1, respectively for TiO2ꢀ1 and TiO2ꢀ2. In addition to the higher surface area,
the photocatalyst prepared via hydrothermal treatment also exhibits greater
porosity, 27%, as compared with 15%, for TiO2ꢀ1.
The morphological characterization of the as synthesized photocatalysts
confirmed the expected differences between samples thermally treated in a
hydrothermal reactor at relatively low temperature and those treated in a
conventional oven. TiO2ꢀ2 exhibits smaller particle size and higher surface area,
since it was treated at low temperatures thus avoiding grain boundaries.
Another consequence of the hydrothermal treatment at 200 ºC is the smaller
9

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crystallinity of TiO2ꢀ2 in relation to TiO2ꢀ1, which was sintered at 400 ºC and
exhibited only anatase XRD patterns, with no signal from the metastable
brookite phase.
Photocatalytic activity. The effectiveness of the TiO2 samples as
photocatalysts was evaluated comparing the efficiency of the oxidation of a dye
(Ponceau 4R), gaseous acetaldehyde, and methanol, and also the
photocatalytic hydrogen evolution, using platinum as coꢀcatalyst. A summary of
the observed rates and the respective photonic efficiencies is presented in
Table 1 along with the main morphological data.
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Table 1. Main morphological parameters and photocatalytic performances of
bare and platinized TiO2ꢀbased photocatalysts.
Gaseous
CH3CHO
Dye
Degradation
Particle
diameter
(nm)
MeOH oxidation
H2 evolution
SBET
Sample
(m2 gꢀ1)
degradation
rate& x10ꢀ4
C/C0 (%)
rate*
5.0
4.8
6.4
5.2
5.2
3.5
rate*
ξ (%)
ξ (%)
8.3
ξ (%)
42
66
30 ± 2
25 ± 2
12 ± 3
P25
82
33
75
3.3
2.0
2.4
5.5
3.4
4.0
TiO2ꢀ1
7.9
164
TiO2ꢀ2
10.6
8.5
P25ꢀPt
TiO2ꢀ1ꢀPt
TiO2ꢀ2ꢀPt
0.15
0.11
0.10
7.7
5.6
5.0
8.5
5.8
& in ꢀmol sꢀ1; * in ꢀmol Lꢀ1 sꢀ1 gꢀ1
In Figure 3, the time profile of the gaseous CH3CHO photodegradation is shown
using the two photocatalysts prepared here in comparison to the commercial
titanium dioxide P25. Apparently, the commercial photocatalyst exhibits a higher
efficiency as compared with both samples prepared by the solꢀgel method.
Among the synthesized powders, TiO2ꢀ2 exhibits the best performance. The
same trend is observed for the degradation of the Pounceau 4R dye (time
profile TOC curves for the different photocatalysts can be found in the
Supplementary Information, Figure S6).
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1.2
1.0
0.8
0.6
0.4
P25
TiO2ꢀ1
TiO2ꢀ2
Light on
Light on
Light on
0
100 200 300
0
100 200 300 400
0
100 200 300 400
time (min)
Figure 3. Time profiles of the CH3CHO photodegradation upon UV(A)
irradiation (1 mW cmꢀ2) in the presence of different TiO2ꢀbased photocatalysts.
For the photocatalytic methanol oxidation, the performance of bare TiO2ꢀ2 is
higher than that observed for the commercial oxide and approx. 35% better than
that observed when using TiO2ꢀ1, Figure 4. Interestingly, after platinization, the
photocatalytic activity of TiO2ꢀ2 decays by 50%, while for TiO2ꢀ1 and P25, the
photonic efficiencies increase by 7.5% and 2.4%, respectively.
The time profiles of the photocatalytic hydrogen evolution obtained by mass
spectrometry are shown in Figure 5. For both samples, one can observe a fast
increase of the H2 production after the light is turned on. Then, the H2 evolution
rate reaches a plateau and remains almost constant until the irradiation is
turned off. The slight decrease on H2 evolution rate over time is likely related to
the decrease of the sacrificial agent concentration. The photocatalytic
production of hydrogen follows the same trend as that observed for the
methanol oxidation, with TiO2ꢀ1ꢀPt exhibiting a higher performance than TiO2ꢀ2ꢀ
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Pt. It is worthwhile to note that the efficiency of TiO2ꢀ2 decreases after Pt
deposition.
1.2
P25
TiO2ꢀ1
1.0
TiO2ꢀ2
P25ꢀPt
0.8
0.6
0.4
0.2
0.0
TiO2ꢀ1ꢀPt
TiO2ꢀ2ꢀPt
0
10
20
30
40
50
60
time (min)
Figure 4. Time evolution of the HCHO concentration as a result of the
photocatalytic methanol oxidation under UV(A) irradiation (1 mW cmꢀ2) in the
presence of bare and platinized TiO2 photocatalysts.
Light off
TiO2ꢀ1ꢀPt
TiO2ꢀ2ꢀPt
400
300
200
100
0
0
50
100
150
200
250
300
350
time (min)
Figure 5. Time profile for the H2 evolution from 10% (v/v) aqueous methanol
solution upon UV irradiation in the presence of platinized TiO2ꢀ1 (____) and
TiO2ꢀ2 (____) photocatalysts.
Time-resolved absorption spectroscopy (TAS). In order to better understand
the role of morphological parameters and thermal treatment conditions on the
photocatalytic performance of TiO2 photocatalysts, the reaction dynamics of the
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photogenerated charge carriers were investigated by means of nanosecond
TAS, employing bare and platinized samples. In TAS measurements carried out
at the nanosecond time domain, the absorption signatures originate from
trapped electrons and holes, since free charge carriers tend to recombine (or be
4
trapped) at the picosecond time scale . Typical transient absorption spectra
obtained under N2 atmosphere for the TiO2 photocatalysts prepared here are
shown in Figure 6.
(b)
time slices
200 ns
time slices
(a)
200 ns
500 ns
500 ns
1
2
5
8
ꢀs
ꢀs
ꢀs
ꢀs
1
2
5
8
ꢀs
ꢀ
s
ꢀ
s
ꢀs
400
500
600
700
800
400
500
600
700
800
λ
/nm
λ
/nm
Figure 6. Transient absorption spectra measured for TiO2ꢀ1 (a) and TiO2ꢀ2 (b)
under N2 atmosphere at different time scales following the laser pulse.
One can observe a peak at 380ꢀ500 nm, a broad band between 500ꢀ700 nm
and a further peak at 760ꢀ890 nm. For TiO2ꢀ1, the high energy absorption
maximum exhibits a hypsochromic shift at longer times after the laser excitation,
which is not observed for TiO2ꢀ2. The introduction of metallic platinum at the
photocatalyst surface induces some changes on the transient absorption
spectra, as exemplified for TiO2ꢀ1 in Figure 7.
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(a)
TiO2ꢀ1
1.0
0.8
0.6
0.4
0.2
0.0
TiO2ꢀ1ꢀPt
(b)
0.2
0.0
ꢀ0.2
ꢀ0.4
400
500
600
700
800
λ
/nm
Figure 7. (a) Transient absorption spectra of TiO2ꢀ1 and TiO2ꢀ1ꢀPt measured
200 ns after the laser excitation under N2 atmosphere. (b) Difference spectrum
between the bare and the platinized photocatalyst.
It is well known that metallic platinum acts as electron acceptor on the TiO2
4
surface, with electrons being transferred within 10 ps . Thus, in nsꢀ transient
absorption spectra of the TiO2ꢀPt photocatalysts, the signals from trapped
electrons should be greatly reduced in comparison to the bare photocatalysts.
The difference spectrum shown in Figure 7(b) thus evidences a strong
contribution of the trapped electron absorption between 400 and 650 nm. Below
400 nm and above 700 nm, the transient absorption spectra seem to be
dominated by trapped holes. Such interpretations are corroborated by previous
studies carried out by Serpone et. al. 45 attributing the transient absorption in the
range of 490ꢀ650 nm to electrons trapped as Ti3+ centers and also in different
energy midgap levels. However, absorption features of trapped holes have also
been reported in the 400ꢀ700 nm region 11, overlapping with the contribution of
photogenerated electrons and hence making it difficult to identify the individual
absorption signatures of the trapped carriers in the investigated spectral range.
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64
Zawadzki
performed calculations based on the Density Functional Theory
and assigned the transient hole absorption in the 400ꢀ700 nm range to
interpolaron transitions from Oꢀ centers located in the bulk to neighboring
oxygen lattice sites. This author showed that holes trapped in the bulk are
responsible for the lower energy transient absorption signals and that this
feature is blueꢀshifted as the holes migrate to the surface. Hence, these
calculations can explain the hypsochromic shift observed for the high energy
band in the TiO2ꢀ1 transient absorption.
The TAS decay traces observed at three selected wavelengths (400, 500, and
780 nm) are presented in Figure 8 for the bare as well as for the platinized
photocatalysts. As shown in Figure 8c, in the presence of platinum (as electron
scavenger), the transient absorption signal at 780 nm does not even decay in
the microsecond timeꢀrange. This behavior is different to that observed at 400
nm and evidences that the broad band observed in the range between 750 and
890 nm contains a strong contribution of holes located in a different trap state
as compared with those responsible for the peak at higher photon energies
(Figure 8a). The lifetime of the photogenerated holes increases considerably
once the electrons are transferred to the Pt islands on the oxide surface. This
33
result is qualitatively similar to that reported by Yoshirara et al.
for TiO2
nanoparticles with diameters of 10ꢀ15 nm. These authors observed two
transient absorption peaks at 520 nm and 1200 nm, both of which were
attributed to holes. The high energy feature was ascribed to holes trapped more
deeply than the ones responsible for the transient absorption at 1200 nm. This
conclusion is corroborated by the observations made here. The decay at 400
nm is less affected by the presence of metallic platinum than the signal at 780
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nm, which is an indicative that the holes responsible for the lower energy
absorption signature are also the main responsible for the photocatalytic
activity.
(b)
(c)
(a)
TiO2ꢀ1
TiO2ꢀ2
TiO2ꢀ1 Pt
TiO2ꢀ2 Pt
0
2
4
6
8
0
2
4
6
8
0
2
4
6
8
time (
ꢀs)
time (
ꢀ
s)
time (ꢀs)
Figure 8. Absorption time profile of bare and platinized photocatalysts at (a)
400 nm, (b) 500 nm, and (c) 780 nm.
Other interesting features can be taken from the decay profiles at 400 nm and
500 nm (Figure 8a and b, respectively). Both decay curves do not exhibit single
exponential behavior, with some residual absorption still being observed after
10 ꢁs. Results for the biexponential fits of the transient absorption data are
shown in Table 2.
Table 2. Biexpontential fits for the transient absorption decay of bare and
platinized TiO2ꢀbased photocatalysts under N2 atmosphere.
Sample
TiO2-1
λprobe
τ (ꢀs)
τ (ꢀs)
1
2
1.0 (38%)
9.0 (65%)
TiO2-1-Pt
TiO2-2
0.78 (50%) 7.1 (50%)
0.73 (67%) 5.0 (33%)
0.80 (69%) 6.0 (31%)
400
TiO -2-Pt
2
TiO -1
0.51 (65%)
0.46 (77%)
0.29 (80%)
0.30 (95%)
2.2 (46%)
15 (35%)
13 (23%)
12 (20%)
>10 (5%)
29 (54%)
2
TiO2-1-Pt
TiO2-2
500
780
TiO -2-Pt
2
TiO -1
2
TiO2-2
0.88 (58%) 6.6 (42%)
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At all investigated wavelengths the decay kinetics of the TiO2ꢀ2 photocatalyst
are approx. 30% faster than those observed for TiO2ꢀ1. For TiO2ꢀ2, the decay at
400 nm does not change after platinum deposition, thus the signal can possibly
be related to strongly trapped holes that do not contribute to any photocatalytic
reaction. For TiO2ꢀ1, the presence of platinum increases the rate of these decay
processes in relation to the bare powder. This can be explained by the
migration of holes between different trapping sites, once the electrons have
been transferred to the platinum islands.
At 500 nm, where the contribution of trapped electrons is higher, one can
observe that the initial decay is faster than that at 400 nm, particularly for the
TiO2ꢀ2 photocatalyst. Such faster decay rate can be clearly seen in the
normalized timeꢀresolved absorption spectra (see Supplementary Information,
Figures S7 and S8) and can be related to a faster charge carrier recombination
rate in this photocatalyst. As can be seen in Figure 8b and also in the transient
spectra (Figure 6), the relative intensity of the TiO2ꢀ1 transient absorption
between 500 and 600 nm is higher than that in TiO2ꢀ2. Apparently, more
electron/hole pairs are recombining in TiO2ꢀ2 than in TiO2ꢀ1.
The charge carrier dynamics observed for TiO2ꢀ2 can be correlated to the
hydrothermal treatment applied during its preparation. The low temperature
employed (200 ºC) should lead to the formation of more defects in the
crystalline structure. These defects may act as recombination centers,
decreasing the electron/hole lifetime. Another key aspect influencing the
efficiency of the charge separation in TiO2 is the particle size. Serpone et al.
studied the influence of the particle size on the charge carrier dynamics and
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found that the fastest recombination rate is observed for the smallest TiO2
nanoparticles 45. Herein TiO2ꢀ2 exhibits the smallest particle size.
Discussion
One of the objectives of this work is to establish relationships between the
photocatalyst preparation, its morphological and electronic properties, and,
finally, its photocatalytic activity. Given the large number of variables and
factors that have to be taken into account, such task can be puzzling and it is
usually not easy to establish a direct correlation valid for all photocatalysts.
However, the data collected herein allow us to determine some interesting
features that can be helpful for the development of highly efficient materials for
photocatalysis.
The photocatalytic tests performed showed that in comparison to TiO2ꢀ1 the
TiO2ꢀ2 photocatalyst exhibits better efficiencies for dye degradation, gaseous
acetaldehyde oxidation, and methanol oxidation. The performance of the
photocatalyst prepared via hydrothermal treatment was even better than that
observed for the commercial photocatalyst P25 for methanol oxidation. In these
experiments the higher surface area of TiO2ꢀ2 seems to play the major role
determining the photocatalytic effectiveness. All these processes involve the
transfer of photogenerated holes to adsorbed species or the indirect transfer to
•
an adsorbed water molecule yielding OH radicals at the surface, which will
further oxidize the substrate molecules 65. In both mechanisms, higher surface
areas should lead to higher photonic efficiencies and, additionally, faster hole
transfer processes should result in increased degradation rates. In the case of
the photocatalytic methanol oxidation, the interfacial charge transfer occurs
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4
within 300 ps
and partially compensates the faster recombination rate
observed for TiO2ꢀ2 in relation to TiO2ꢀ1.
If we normalize the degradation rates by the surface areas of the respective
photocatalysts, the role of the electron/hole dynamics turns more evident.
Taking the methanol oxidation, the normalized rates are 12x10ꢀ2, 7x10ꢀ2 and
4x10ꢀ2
ꢀ
mol Lꢀ1 sꢀ1 mꢀ2, respectively for TiO2ꢀP25, TiO2ꢀ1, and TiO2ꢀ2. These
values follow the same trend observed for the electron/hole lifetimes obtained
by TAS. Thus, in a given time after excitation, more charge carries are available
on the TiO2ꢀ1 than in TiO2ꢀ2, but as the surface are of the former is lower, fewer
molecules are oxidized.
Another interesting relationship that can be established between the TAS data
and the photocatalytic activity arise from the comparison of the degradation
rates (corrected by surface area) and the relative intensity of the peaks at low
and high energy. The ꢁA (800 nm)/ꢁA(400 nm) ratio immediately after the laser
pulse is around 0.7 in TiO2ꢀ1 and 0.45 in TiO2ꢀ2. This decrease is in the same
order than that observed for the corrected degradation rates for methanol
oxidation and confirms that the holes responsible for the absorption signature at
lower energy are the photoactive ones.
The crystal structure as well as the crystallinity also plays an important hole on
the observed photocatalytic behaviour. Initially, TiO2ꢀ2 was expected to exhibit a
more efficient charge separation as compared with TiO2ꢀ1, as the former
exhibits both, brookite and anatase crystallites in its structure. As reported
earlier 66, 67, the conduction band edge of brookite TiO2 is approx. 0.14 eV more
negative than that of anatase, which should impose an energy barrier for the
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back electron transfer. However, in the case of TiO2ꢀ2 the larger amount of
defects within its crystalline structure along with its smaller particle size seem to
facilitate the charge recombination, reducing the charge carriers available for
photocatalytic processes.
Charge carrier recombination can obviously be expected to be dependent on
the light intensity employed in the laser experiments and further studies are
necessary to understand more deeply the reason of the faster recombination
rates observed for TiO2ꢀ2. As stated previously 68, the electron/hole
recombination kinetics depend on the number of charge carriers generated per
particle, being much faster at very high concentrations of electron/hole pairs in
each TiO2 particle. Therefore, it can be expected that light intensity effects will
be more evident in TiO2ꢀ2 due to its smaller primary particle size. The results
found here for the bare photocatalysts are in good agreement with those
reported by Carneiro et al.43 These authors concluded that, for powders
consisting of primary particles with 7ꢀ15 nm diameter, the surface area and the
available adsorption sites play a major hole for the photocatalytic performance,
while for 15ꢀ35 nm particles, the higher hole concentration and the greater
crystallinity decreases the dependency on the surface area.
After platinum deposition on the TiO2 surface, the photocatalytic activities for
methanol oxidation and H2 evolution increase for TiO2ꢀ1 and TiO2ꢀP25, as a
result of the improved charge separation efficiency. However, the photonic
efficiency of TiO2ꢀ2 for methanol oxidation decreases by 50% in relation to the
bare photocatalysts with its performance for H2 evolution being the worst among
the investigated powders. Metal deposition leads to a decrease of the available
adsorption sites on the TiO2 surface, as previously observed for other TiO2ꢀ
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based photocatalysts 69. In fact, the surface areas of the as synthesized TiO2
powders decrease c.a. 10% after Pt deposition. Although, as the surface area
reduction is observed for all catalysts, the decrease in the TiO2ꢀ2 photocatalytic
activity should be related to the concentration of charge carriers on each
catalyst surface.
For all samples, the presence of platinum leads to a decrease in the charge
carrier recombination rates in relation to those of the bare photocatalysts, since
the metal islands act as efficient electron acceptors. Therefore, the results
indicate that the lower crystallinity and the reduced particle size of TiO2ꢀ2 lead
to a smaller concentration of electrons effectively transferred to the Pt islands. It
has previously been shown that the soꢀcalled antenna mechanism provides an
important pathway for H2 evolution on PtꢀTiO2 photocatalysts 69. This
mechanism is based on the interparticle charge carrier transport within TiO2
agglomerates until the electrons reach the Pt islands (or the holes reach
oxidizable adsorbed substrate molecules, respectively). Hence, the difference in
the conduction band energies of anatase and brookite present in TiO2ꢀ2
impedes the electron mobility, further decreasing the amount of H2 evolved,
scheme 1. For the bare TiO2ꢀ2 photocatalyst, the larger surface area
compensates the decrease of the charge carrier concentration resulting in an
overall efficiency higher than that observed for the other oxides. However, this
parameter only has a minor effect for the H2 evolution in the presence of a coꢀ
catalyst.
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e-
e-
e-
e-
TiO2ꢀ2, prepared at 200 °C (12.4 bar)
efficient eletron transfer
TiO2ꢀ1, prepared at 400 °C (1 bar)
Scheme 1. Diagram illustrating the effect of the as prepared TiO2 crystal
structures on the electron transfer process to Pt islands.
Conclusions
In this work two different TiO2 powders were prepared by the solꢀgel method
followed by thermal treatment in a conventional oven at 400 ºC (TiO2ꢀ1) or in a
hydrothermal reactor at 200 ºC and 12.4 bar (TiO2ꢀ2). The later material
exhibited higher surface area than the former, and its XRD pattern evidences
the presence of approx. 45% of brookite along with the anatase polymorph.
Rietveld refinement indicates a higher distortion on the TiO2ꢀ2 crystalline
structure. The photocatalytic activities of the as prepared powders were
evaluated using different reactions and were compared to those of the
commercial photocatalyst TiO2ꢀP25. The bare TiO2ꢀ2 exhibited better
performance in photocatalytic processes involving an adsorption step, which
can be directly related to its higher surface area. When metallic platinum is
deposited on the TiO2 surfaces and the samples are tested for H2 evolution
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using methanol as the hole scavenger, the photonic efficiency of TiO2ꢀ1 is
approx. 10% higher than that of TiO2ꢀ2. These results were rationalized using
nanosecond transient absorption spectroscopy (TAS). It was found that:
1. The electron/hole recombination rates in TiO2ꢀ2 are faster than in TiO2ꢀ1
due to the more distorted crystalline structure and smaller particle size of
the former. This negative aspect is overcompensated by the higher
surface area of TiO2ꢀ2 in the absence of any coꢀphotocatalyst.
2. In the presence of platinum as coꢀcatalyst, the photocatalytic efficiency is
dominated by the availability of charge carriers at the metaloxide surface.
Thus, the thermal treatment conditions determine two main factors, which
influence very strongly the photocatalytic activity of the samples studied here,
namely, the crystallinity and the surface area. The high crystallinity, obtained
under higher temperatures, is particularly important for the transport of the
charge carriers and this seems to be crucial for the production of molecular
hydrogen, in which the electrons are transported to the Pt islands, through the
oxide particles.
On the other side, the defects (lower crystallinity) are important for the
harvesting of the charge carriers after they reach the particle surface, where a
higher surface area yields an increase of the reaction rate constants. This
behaviour is clearly observed for the TiO2 particles prepared by the
hydrothermal treatment. An important conclusion of this work is that the best
photocatalyst for a given photoreaction will not necessarily be the best choice
for all photocatalytic processes! This will become particularly critical in the case
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of the photocatalytic H2 evolution where the presence of coꢀcatalysts is
necessary.
Acknowledgments
This work was supported by Fundacão de Amparo à Pesquisa do Estado de
Minas Gerais (FAPEMIG), Conselho Nacional de Desenvolvimento Cientifíco e
Tecnologíco (CNPq) and Coordenacão de Aperfeiçoamento de Pessoal de
Nível Superior (CAPES). AOTP is thankful to the DLR Green Talents program
for the research stay in Germany. Financial support by the Deutsche
Forschungsgemeinschaft (DFG) is gratefully acknowledged (Project Number BA
1137/8ꢀ2). The authors are thankful to I. Ivanova and P. Esteban respectively
for the assistance on H2 evolution measurements and TAS experiments.
Electronic Supplementary Information: Rietveld analysis of XRD diffraction
patterns, UVꢀVis reflectance spectra and N2 adsorption/desorption isotherms of
the TiO2 samples, timeꢀprofile curves for dye degradation in the presence of
different photocatalysts and additional timeꢀresolved absorption data.
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Table of Contents (TOC) Image
The role of thermal treatment on the electron/hole dynamics and on the
photocatalytic activity of TiO2 is investigated by transient absorption
spectroscopy.
28