Published: October 04, 2011
r
2011 American Chemical Society
17652 dx.doi.org/10.1021/ja204838s
|
J. Am. Chem. Soc. 2011, 133, 17652–17661
ARTICLE
pubs.acs.org/JACS
Photogenerated Defects in Shape-Controlled TiO
2
Anatase
Nanocrystals: A Probe To Evaluate the Role of Crystal Facets in
Photocatalytic Processes
Massimiliano D’Arienzo,*
,†
Jaime Carbajo,
‡
Ana Bahamonde,
‡
Maurizio Crippa,
†
Stefano Polizzi,
§
Roberto Scotti,
†
Laura Wahba,
†
and Franca Morazzoni
†
†
INSTM, Department of Materials Science, University of Milano-Bicocca, Via R. Cozzi 53, I-20125 Milano, Italy
‡
Instituto de Catalisis y Petroleoquímica, ICP-CSIC, C/Marie Curie No. 2, 28049 Madrid, Spain
§
Dipartimento di Scienze Molecolari e Nanosistemi, Uni versity Ca’ Foscari of Venezia, Via Torino 155/b,
I-30172 Venezia-Mestre, Italy
b
S Supporting Information
’ INTRODUCTION
Nanocrystalline TiO
2
is an attractive material due to its
potential applications as photocatalyst in environmental pollu-
tion control, conversion and energy storage, sensors, Li batteries,
photovoltaics, and so on.
16
The activity of titania-based photo-
catalysts is attributed to photogenerated electrons (e
) and
holes (h
+
), located at the crystal surface, where they act as redox
sources leading to the mineralization of the chemisorbed
species.
25
Since catalyst irradiation frequently leads to pro-
ximal defects on the same particle, undesired electronhole
recombination occurs easily and suppresses the photocatalytic
activity.
79
Spherical TiO
2
nanoparticles are commonly employed as photo-
catalysts, due to their high surface-to-volume rati o, which guar-
antees a great number of surface active sites. Unfortunately, this
benefit is partially quenched by the electronhole recombination
process, favored by the small size of the crystals and by the
absence of geometric anisotropy.
8,9
By contrast, it has been demonstrated that anisotropic shaped
particles, such as nanorods,
10,11
nanobelts,
12
and nanotubes,
13
would guarantee high surface area as well as a lower charge re-
combination rate compared to the nanospheres. This assumption
arises from the hypothesis that e
and h
+
could in principle locate
in distal parts of anisotropic nanostructures, e.g., on different
crystal faces, ensuring a highly efficient charge separation.
12,13
The photocatalytic activity not only depends on the particle
shape but also is strictly related to the external surfaces exposed
and, particularly, to the arrangement and coordination of the
surface atoms on the differ ent crystal facets.
1424
Ohno et al.
Received: May 26, 2011
ABSTRACT: The promising properties of anatase TiO
2
nanocrystals
exposing specific surfaces have been investigated in depth both
theoretically and experimentally. H owever, a clear assessment of
the role of the crystal faces in photocatalytic processes is still under
debate. In order to clarify this issue, we have comprehensively
explored the properties of the photogenerated defects and in
particular their dependence on the exposed crystal faces in shape-
controlled anatase. Nanocrystals were synthesized by solvothermal
reaction of titanium butoxide in the presence of oleic acid and
oleylamine as morphology-directing agents, and their photocatalytic
performances were evaluated in the phenol mineralization in aqueous
media, using O
2
as the oxidizing agent. The charge-trapping centers,
Ti
3+
,O
, and O
2
, formed by UV irradiation of the catalyst were
detected by electron spin resonance, and their abundance and reactivity were related to the exposed crystal faces and to the
photoefficiency of the nanocrystals. In vacuum conditions, the concentration of trapped holes (O
centers) increases with
increasing {001} surface area and photoactivity, whi le the amount of Ti
3+
centers increases with the specific surface area of {101}
facets, and the highest value occurs for the sample with the worst photooxidative efficacy. These results suggest that {001} surfaces
can be considered essentially as oxidation sites with a key role in the photoxidation, while {101} surfaces provide reductive sites
which do not directly assist the oxidative processes. Photoexcitation experiments in O
2
atmosphere led to the formation of
Ti
4+
O
2
oxidant species mainly located on {101} faces, confirming the indirect contribution of these surfaces to the photooxida-
tive processes. Although this work focuses on the properties of TiO
2
, we expect that the presented quantitative investigation may
provide a new methodological tool for a more effective evaluation of the role of metal oxide crystal faces in photocatalytic processes.
17653 dx.doi.org/10.1021/ja204838s |J. Am. Chem. Soc. 2011, 133, 17652–17661
Journal of the American Chemical Society
ARTICLE
reported that rutile {011} and anatase {001} faces provide the
sites for oxidation, while the rutile {110} and ana tase {101} faces
offer the sites for reduction.
2530
This behavior was ascribed to
the presence of different energy levels of the conduction and
valence bands associated with the different faces because of the
atomic arrangements characteristic of their surfaces.
2527,29
The
difference in the energy levels drives the e
and h
+
to different
crystal faces, aiming to reach the most stable energy configura-
tion and leading to a charge separation.
Recently, both computati onal evidence
17,18,3032
and experi-
mental results
20,33,34
suggested that the enhanced reactivity of the
anatase {001} face for the dissociative adsorption of water is related
both to the high density of surface-undercoordinated Ti centers
and to the presence of enlarged TiOTi bond angles at the
surface, which makes titanium and oxygen centers very reactive.
18
These results have stimulated significant re search efforts
toward the synthesis of anatase nanocrystals with a relevant area
of exposed {001} faces.
17,20,3540
For instance, Lu et al. success-
fully obtained anatase crystals with 47% {001} faces, and these
showed high photoefficiency.
17
Li et al. synthesized anatase
nanoparticles with dominant {001} faces, which displayed
photocatalytic activity higher than that of nanoparticles with
predominant {101} faces, in the oxidation of toluene and
benzaldehyde.
37,38
In contrast with the previous results, other inves tigations
recently revealed that anatase nanocrystals with well-faceted
{101} surfaces exhibit enhanced photocatalytic activity.
12,29,41
For instance, Wu et al. synthesized anatase nanoparticles with
dominant {101} crystal faces, having high photoactivity and a
lower electronhole recombination rate than anatase nanospheres
with an identical crystal phase and a similar specific surface area.
12
Accordingly, Murakami et al. observed high photocatalytic
activity in the decomposition of acetaldehyde on anatase nano-
crystals with a large {101} surface area and a small {001} area.
29
In this scenario, it seems to us that some contradictory
interpretations concerning the catalytic activity mediated by
shape-controlled particles arise from insufficient characterization
work addressing how the crystalline facets exposed by TiO
2
nanocrystals affect the stability and the reactivity of the para-
magnetic species formed upon UV excitation. In fact, though the
mentioned studies agree that the photoactivity of TiO
2
nano-
crystals can be tailored by fine-tuning the particle morphology
and, specifically, the exposed specific crystal faces, and though
clever reports attribute to underc oordinated Ti centers and to
distorted OTiO moieties the responsibility for the enhanced
reactivity, there is hardly any investigation which directly relates
the reactivity of the different faces with the type, the amount, and
the location of the electronic defects, e
and h
+
, which are active
promoters in photocatalysis.
In an attempt to answer the above queries, the present study
associates the surface properties of anatase shape-controlled
nanocrystals, having specific exposed crystal faces, to the photo-
generated defects and to their photocatalytic activity. Spherical
particles, nanobars, and rhombic and rhombic elongated nano-
crystals were obtained by solvothermal reaction of titanium
butoxide precursor (TB) in the presence of oleic acid (OA)
and oleylamine (OM).
42
The OA and OM selective binding to
different crystal faces drives the growth of the nanocrystals along
specific crystallographic directions, depending on the TB/OA/
OM ratio and the reaction temperature.
After complete removal of the excess of capping agents bound
to the surfaces, the photocatalytic performances of the differently
shaped titani a samples were measured in the phenol mineraliza-
tion, using O
2
as the oxidizing agent, and then related with the
prominent faces of the nanocrystals.
The charge-trapping centers, Ti
3+
,O
, and O
2
, formed by
UV irradiation of the catalyst were detected by electron spin
resonance (ESR), and their abundance was related to the
exposed crystal faces and in turn to the photoefficiency of the
nanocrystals. To the best of our knowledge, this is the first time
that the properties of the photogenerated defects have been used
as a probe to evaluate the contribution of the morphology, and
specifically of the crystal faces, to the photocatalytic processes.
’ EXPERIMENTAL SECTION
Chemicals. Titanium(IV) butoxide (Ti(OBu)
4
or TB, 97%), oleic
acid (C
18
H
33
CO
2
H or OA, 90%), oleylamine (C
18
H
35
NH
2
or OM,
70%), and superhydride solution (1 M lithium triethylboronhydride,
LiEt
3
BH, in THF) were all purchased from Aldrich and used without
further purification.
Synthesis of Shape-Controlled TiO
2
Nanocrystals. Sol-
vothermal synthesis of shape-controlled anatase nanocrystals was per-
formed according to a previously reported procedure,
42
reacting TB in
the presence of OA and OM. The synthesis method is based on the
selective binding of OA to the anatase {001} face and of OM to the
anatase {101} face, which restricts the growth to the corresponding
direction. However, OA and OM act not only as capping agents but also
as an acidbase pair catalyst, increasing the condensation rate without
affecting the hydrolysis rate. Moreover, OA can react with TB or
hydroxyalkoxide to generate carboxyalkoxide species which slow down
the hydrolytic condensation process. OM, conversely, can promote the
non-hydrolytic condensation process by aminolysis reaction with tita-
nium carboxylalkoxide. Exploiting all the above properties, by simply
changing the TB/OA/OM molar ratio and the reaction temperature,
anatase nanocrystals with well-defined morphology were obtained.
42
In a typical experiment, TB (22 or 44 mmol) was added to a mixture
of X mmol of OA, Y mmol of OM (where X + Y = 217.5), in 25 mL of
absolute ethanol. X and Y were varied to gain different TB/OA/OM
molar ratios, which led to differently shaped nanocrystals: rhombic (R,
TB/OA/OM = 1:4:6), rhombic elongated (RE, TB/OA/OM = 2:4:6),
spherical (SP, TB/OA/OM = 1:6:4), and small nanobars (NB, TB/OA/
OM = 1:8:2). For example, to synthesize TiO
2
with rhombic shape, 22
mmol of TB was added to a mixture of 88 mmol of OA, 132 mmol of
OM, and 25 mL of absolute ethanol. The obtained mixture was stirred
for 15 min and then transferred into a 400 mL Teflon-lined stainless steel
autoclave containing 85 mL of absolute ethanol and 3.5 mL of Milli-Q
water. The system was then heated to 140 or 180 C for 18 h. After
decantation, TiO
2
powder was recovered from the autoclave, washed
several times with ethanol, filtered, and finally dried under vacuum (p <
10
2
mbar) at room temperature. Hereafter, TiO
2
nanoparticles with
rhombic, rhombic elongated, spherical, and nanobar shapes are labeled
as R, RE, S, and NB, respectively.
Removal of Capping Agent Residuals from the TiO
2
Nanocrystals. In order to avoid catalyst deactivation due to the pre-
sence of remnants OA and OM strongly bonded to the titania surface,
the following steps were performed.
First, dried TiO
2
nanocrystals were placed in a 100-mL round-
bottomed flask under an argon atmosphere. Next, 10 mL of super-
hydride solution was added dropwise under vigorous stirring at room
temperature; hydrogen gas release was observed due to reduction of the
OA on the surface of the TiO
2
nanocrystals.
43
After complete addition of
the superhydride solution, the resulting blue suspension underwent an
ultrasound treatment for 30 min and was kept overnight under stirring at
room temperature. In order to remove the unreacted superhydride and
the residual traces of THF, each sample was washed with Milli-Q water
17654 dx.doi.org/10.1021/ja204838s |J. Am. Chem. Soc. 2011, 133, 17652–17661
Journal of the American Chemical Society
ARTICLE
(two times with 10 mL), acetone (10 mL), and CH
2
Cl
2
(two times
with 10 mL).
To eliminate the excess of OM still coordinated to the surface, dried
nanocrystals were then resuspended in 250 mL of 0.4 M H
2
SO
4
solution
and treated under ultrasound for 30 min. The resulting naked TiO
2
nanocrystals were collected by centrifugation, washed two times with
10 mL of CH
2
Cl
2
, and dried under vacuum (p <10
2
mbar) at room
temperature.
Material Characterizations. Transmission electron microscopy
(TEM) and high-resolution TEM (HRTEM) images and selected area
electron diffraction (SAED) patterns were obtained by using a Jeol 3010
apparatus operating at 300 kV with a high-resolution pole piece (0.17 nm
point-to-point resolution) and equipped with a Gatan slow-scan 794
CCD camera. Samples were prepared by placing a 5 μL drop of a dilute
toluene dispersion of the nanocrystals on a holey carbon film supported
on a 3 mm copper grid.
According to the model developed by Jaroniec et al. for anatase TiO
2
nanoparticles,
44
the percentage of exposed {001} faces for R and RE
nanocrystals was calculated by the following equation:
%Sf001g
exp
¼ Sf001g=ðSf101gþSf001g
¼ 2b
2
=½2b
2
þða þ bÞða bÞ tanð68:3Þ100 ð1Þ
where 68.3 is the angle between the {101} and the {001} faces, and a
and b correspond to the minimum and maximums side of the square
{001} faces in anatase (Supporting Information, Figure S1A). The NB
shape was approximately that of a rectangular parallelepiped (Supporting
Information, Figure S1B), and the percentage of exposed {001} faces
was simply estimated by the following equation:
%Sf001g
exp
¼ Sf001g=ðSf101gþSf001g
¼ 2w
2
=½4lw þ 2w
2
100 ð2Þ
where w and l correspond to the sides of the {001} and of the poorly
pronounced {101} faces, respectively. The average values of a, b, l, and w
were evaluated by measuring the sizes of ∼100 particles in TEM images.
The crystal structure of the shape-controlled TiO
2
nanocrystals was
determined from the X-ray diffraction (XRD) patterns collected with a
Bruker D8 Advance diffractometer (Cu Kα radiation) in the range
2060 2θ (2θ step 0.025, count time of 2 s/step). The diffraction
peaks were indexed as pure anatase (JCPDS, no. 21-1272), and the
average crystallite size was estimated from the broadening of the [101]
XRD peak of anatase by using the Scherrer equation.
In order to verify the complete removal of the residual capping agents,
thermogravimetric analysis (TGA) and differential scanning calorimetry
(DSC) measurements before and after the cleaning procedure were
performed. TGA thermograms and DSC curves were collected by using
a Mettler Toledo TGA/DSC1 STARe system at constant gas flow
(50 cm
3
min
1
). The sample powders were heated in air from 30 to
1000 C at a heating rate of 10 C min
1
. TGA curves were used to
determine the temperature-dependent mass loss of the capping agents,
giving an indication of the strong interaction of these molecules with the
catalytic surface.
Nitrogen physisorption measurements were carried out by using a
Quantachrome Autosorb-1 apparatus. The specific surface area (SSA,
BET method),
45
pore volume (desorption cumulative pore volume,
DCPV), and pore size distribution (BJH method)
46
were measured after
evacuation at 200 C for 16 h.
Photocatalytic Experiments. Photodegradation experiments
were carried out in a 600 mL Pyrex discontinuous batch reactor
equipped with an external cooling jacket, enveloped by aluminum foil,
and a 125 W Hg high-pressure UV lamp placed in a coaxial quartz
cylinder. No optical filter was adopted. Titanium dioxide nanocrystals
(160 ( 5 mg) were suspended by ultrasound treatment in 600 mL of
water containing 121 ( 2 ppm of PhOH (93 ( 2 ppm as C). The
temperature was kept at 25 ( 2 C, and the suspension was recirculated
by a peristaltic pump (14 mL s
1
). Photodegradation was performed in
the presence of O
2
as oxidative agent. The slurry was saturated in an
online chamber by continuously bubbling oxygen (with a constant feed
of 100 mL min
1
) and circulated in the dark for 30 min before the UV
source was turned on. The excess gas was eliminated through a non return
check valve. Control experiments were carried out in the absence of TiO
2
(blank). To monitor the photoinduced degradation of phenol, aliquots
(6 mL) of the reaction solution were removed at regular intervals and
after centrifugation, and the clear solutions were analyzed for the total
organic carbon (TOC) using a Shimadzu TOC-V CSH analyzer.
Electron Spin Resonance Spectroscopy. The ESR investiga-
tion was performed by using a Bruker EMX spectrometer operating at
the X-band frequency and equipped with an Oxford cryostat working
in the temperature range of 4298 K. The nanocrystals were charged
in quartz glass tubes connected both to a high-vacuum pumping
system and to a controlled gas feed (O
2
). Spectra were recorded
under in vacuo conditions (10
5
mbar) at 130 K, before and after
30 min of UV irradiation inside the ESR cavity at the same temperature.
Figure 1. TEM images of th e shape-c ontrolled anatase samples: (a,b)
R,(c,d)SP,(e,f)NB, and (g, h) RE.Insetsinpanelsb,d,andfarehigh-
magnification images of the corresponding shapes of the smallest
nanocr ystals.
17655 dx.doi.org/10.1021/ja204838s |J. Am. Chem. Soc. 2011, 133, 17652–17661
Journal of the American Chemical Society
ARTICLE
The experimental procedure lowers the recombination of the photogen-
erated species. For each sample, the absence of a signal before irradiation
was checked. No significant differences resulted between the spectra
recorded just before and 20 min after switching off the UV irradiation,
except a small decrease of the signal intensity. Spectra were acquired with a
modulation frequency of 100 kHz, modulation amplitudes of 25G,
and microwave powers of 25 mW. Irradiation was performed by using a
150 W Xe UV lamp (Oriel) with the output radiation focused on the
samples in the cavity by an optical fiber (50 cm length, 0.3 cm diameter).
The g values were calculated by standardization with α,α
0
-diphenyl-β-
picryl hydrazyl (DPPH). The spin concentration was obtained by double
integration of the resonance lines, referring to the area of the standard
Bruker weak pitch (9.7 10
12
( 5% spins cm
1
). Accuracy on double
integration was (15%. Care was taken to always keep the most sensitive
part of the ESR cavity (1 cm length) filled. Spectra simulations and fits
were performed using the SIM 32 program.
47
’ RESULTS AND DISCUSSION
Structural and Morphological Characterization. Figure 1
summarizes the TEM investigation performed on the anatase
shape-controlled nanoparticles. According to the results reported
by Dihn et al.,
42
when the reaction is carried out at 180 C and
with low OA/OM ratio (TB/OA/OM = 1:4:6), R TiO
2
nano-
particles that are very homogeneous in shape and size (13.5 nm
length) are obtained (Figure 1a,b). At the same reaction tem-
perature, increasing the OA/OM ratio and leaving unchanged
the concentration of the titanium precursor (TB/OA/OM =
1:6:4), almost S nanocrystals with mean diameter of 9.5 nm are
produced (Figure 1c,d). Further increasing the OA concentra-
tion (TB/OA/OM = 1:8:2) at a lower reaction temperature
(140 C) leads to the formation of small NB TiO
2
, 13 nm long
(Figure 1e,f). Finally, under the same reaction conditions used to
obtain the R shaped nanoparticles, but with double the amount of
titanium precursor (TB/OA/OM = 2:4:6), the formation of
large RE particles is observed (Figure 1g,h), which are less
uniform in size (average length 4560 nm).
In order to study in detail the shape and the exposed crystal
faces, HRTEM images of R, RE,andNB TiO
2
nanocrystals were col-
lected (Figure 2). In particular, higher magnifications (Figure 2b,d)
of R and RE lattice fringes along the [010] direction clearly reveal
the presence of (101) and (002) crystallographic planes with
lattice space of 0.35 and 0.48 nm, respectively. The angles
indicated in the corresponding fast Fourier transform (FFT)
images (insets in Figure 2b,d) are ∼68.3 and ∼90, which are
identical to the theoretical values for the angles between the
{101} and {001} faces and between the {100} and {001} faces,
respectively. This information confirms that the R and RE anatase
nanocrystals mainly expose {001} and {101 } surfaces, with the
additional presence for RE nanocrystals of a small amount of
exposed {100} and {010} faces.
Table 1. Structural Parameters and Catalytic Performances of Shape-Controlled TiO
2
Nanocrystals
sample
L
XRD
(nm)
pore volume
(DCPV, cm
3
g
1
)
SSA
BET
(m
2
g
1
)
exposed {001}
crystal facets (%)
exposed {101}
crystal facets (%)
SSA
BET
of exposed
{001} crystal facets (m
2
g
1
)
SSA
BET
of exposed
{101} crystal facets (m
2
g
1
)
t
1/2
(min)
SP 7.6 0.47 178.8 130.0
NB 7.3 0.27 227.0 5.8 94.2 13.1 213.8 183.7
R 13.1 0.35 199.0 10.6 89.4 21.2 177.9 89.3
RE 16.5 0.21 170.5 9.2 90.8 15.7 154.8
a
124.1
a
Included the {010} and {100} minority facets.
Figure 2. Representative HRTEM images of the R (a,b), RE (c,d), and
NB (e,f) samples. The insets are the FFT images of the corresponding
nanoparticles.
17656 dx.doi.org/10.1021/ja204838s |J. Am. Chem. Soc. 2011, 133, 17652–17661
Journal of the American Chemical Society
ARTICLE
In the HRTEM images of NB nanoparticles, only the (101) and
(011) atomic planes, with space of ∼0.35 nm and interfacial angle
of 82.4, are detected (Figure 2e,f). This indicates that the crystal
growth occurs mainly on the {001} face, which becomes less
dominant. It is also very evident that the exposed {101} faces are
much less prominent than in the case of R and RE nanocrystals,
and the morphology resembles that of a rectangular parallelepiped.
Based on the TEM and HRTEM analysis, average a, b, l,andw
values were calculated, which correspond to the minimum and the
maximum sizes of the square {001} faces in R and RE nanocrystals,
and to the sides of the {001} and of the poorly pronounced {101}
faces of the NB (Supporting Information).
44
From these values
and according to eqs 1 and 2, the percentages of {001} exposed
crystal faces for R, RE,andNB nanocrystals were estimated as 10.6,
9.2, and 5.8%, respectively (see Table 1). For R and NB nano-
crystals, the remaining percentage can be easily assigned to the
{101} exposed crystal faces, while for RE nanoparticles it repre-
sents the sum of {101}, {010},and{100} faces. According to these
calculations, we can infer that NB particles present a higher
percentage of {101} faces than RE and R nanocrystals (Table 1).
XRD patterns (Figure 3) of all the synthesized nanocrystals
after the cleaning procedure indicate the exclusive presence of
anatase phase. A gradual increase in the relative intensity and
sharpening is observed for the (004) diffraction peak on going
from SP to R nanoparticles, suggesting a growth along the [001]
direction. The average crystallite sizes (L
XRD
), calculated by the
Scherrer equation, agreed in general with the crystal width
estimated from the TEM images (see Table 1).
TGA was performed on both as-prepared shape-controlled
TiO
2
nanocrystals and naked particles, obtained after removal of
the residual capping agents by washing with superhydride and
dilute sulfuric acid solutions.
43
In particular, Figure 4 shows the
weight loss curves and the corresponding derivatives of the RE
nanocrystals before (a and a
0
, black lines) and after the cleaning
procedure (b and b
0
, red lines). In the as-prepared sample (black
curve a), a first small weight loss (∼ 2%) beginning at nearly
Figure 3. XRD patterns of (a) spherical (SP), (b) nanobars (NB), (c)
rhombic elongated (RE), and (d) rhombic (R) TiO
2
nanocrystals
collected after the removal of capping agents.
Figure 4. TGA curves and corresponding derivative thermogravimetry
profiles of RE nanocrystals before (a,a
0
, black lines) and after the
cleaning procedure (b,b
0
, red lines) which removes the capping agents.
Figure 5. TEM images of (a) R, (b) RE, (c) SP, and (d) NB anatase
nanocrystals after the cleaning procedure with the superhydride solution
and the washing treatment with dilute sulfuric acid.
Figure 6. Adsorptiondesorption isotherm at liquid nitrogen tempera-
ture for R TiO
2
nanocrystals. The curve corresponds to a type IV iso-
therm with capillary condensation in the mesopores. Inset: pore-size
distribution.
