Influence of post-treatment operations on structural properties and photocatalytic activity of octahedral anatase titania particles prepared by an ultrasonication-hydrothermal reaction

Article abstract of DOI:10.3390/molecules191219573

The influence of changes in structural and physical properties on the photocatalytic activity of octahedral anatase particles (OAPs), exposing eight equivalent {101} facets, caused by calcination (2 h) in air or grinding (1 h) in an agate mortar was studied with samples prepared by ultrasonication (US; 1 h)-hydrothermal reaction (HT; 24 h, 433 K). Calcination in air at temperatures up to 1173 K induced particle shape changes, evaluated by aspect ratio (AR; d001/d 101 = depth vertical to anatase {001} and {101} facets estimated by the Scherrer equation with data obtained from X-ray diffraction (XRD) patterns) and content of OAP and semi-OAP particles, without transformation into rutile. AR and OAP content, as well as specific surface area (SSA), were almost unchanged by calcination at temperatures up to 673 K and were then decreased by elevating the calcination temperature, suggesting that calcination at a higher temperature caused dull-edging and particle sintering, the latter also being supported by the analysis of particle size using XRD patterns and scanning electron microscopic (SEM) images. Time-resolved microwave conductivity (TRMC) showed that the maximum signal intensity (I max), corresponding to a product of charge-carrier density and mobility, and signal-decay rate, presumably corresponding to reactivity of charge carriers, were increased with increase in AR, suggesting higher photocatalytic activity of OAPs than that of dull-edged particles. Grinding also decreased the AR, indicating the formation of dull-edged particles. The original non-treated samples showed activities in the oxidative decomposition of acetic acid (CO₂ system) and dehydrogenation of methanol (H₂ system) comparable to and lower than those of a commercial anatase titania (Showa Denko Ceramics FP-6), respectively. The activities of calcined and ground samples for the CO₂ system and H₂ system showed almost linear relations with AR and Imax, respectively, suggesting that those activities may depend on different properties.

Full text of DOI:10.3390/molecules191219573

Molecules 2014, 19, 19573-19587; doi:10.3390/molecules191219573
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Influence of Post-Treatment Operations on Structural Properties
and Photocatalytic Activity of Octahedral Anatase Titania
Particles Prepared by an Ultrasonication-Hydrothermal Reaction
Zhishun Wei, Ewa Kowalska * and Bunsho Ohtani
Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan;
E-Mails: wei@cat.hokudai.ac.jp (Z.W.); ohtani@cat.hokudai.ac.jp (B.O.)
* Author to whom correspondence should be addressed; E-Mail: kowalska@cat.hokudai.ac.jp;
Tel.: +81-11-706-9130; Fax: +81-11-706-9133.
External Editor: Pierre Pichat
Received: 25 September 2014; in revised form: 13 November 2014 / Accepted: 17 November 2014 /
Published: 26 November 2014
Abstract: The influence of changes in structural and physical properties on the
photocatalytic activity of octahedral anatase particles (OAPs), exposing eight equivalent
{101} facets, caused by calcination (2 h) in air or grinding (1 h) in an agate mortar was
studied with samples prepared by ultrasonication (US; 1 h)–hydrothermal reaction (HT; 24 h,
433 K). Calcination in air at temperatures up to 1173 K induced particle shape changes,
evaluated by aspect ratio (AR; d001/d101 = depth vertical to anatase {001} and {101} facets
estimated by the Scherrer equation with data obtained from X-ray diffraction (XRD)
patterns) and content of OAP and semi-OAP particles, without transformation into rutile.
AR and OAP content, as well as specific surface area (SSA), were almost unchanged by
calcination at temperatures up to 673 K and were then decreased by elevating the
calcination temperature, suggesting that calcination at a higher temperature caused
dull-edging and particle sintering, the latter also being supported by the analysis of particle
size using XRD patterns and scanning electron microscopic (SEM) images. Time-resolved
microwave conductivity (TRMC) showed that the maximum signal intensity (Imax),
corresponding to a product of charge-carrier density and mobility, and signal-decay rate,
presumably corresponding to reactivity of charge carriers, were increased with increase in
AR, suggesting higher photocatalytic activity of OAPs than that of dull-edged particles.
Grinding also decreased the AR, indicating the formation of dull-edged particles. The
original non-treated samples showed activities in the oxidative decomposition of acetic

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acid (CO2 system) and dehydrogenation of methanol (H2 system) comparable to and lower
than those of a commercial anatase titania (Showa Denko Ceramics FP-6), respectively.
The activities of calcined and ground samples for the CO2 system and H2 system showed
almost linear relations with AR and Imax, respectively, suggesting that those activities may
depend on different properties.
Keywords: titania; photocatalytic activity; octahedral anatase particles; post-treatments;
time-resolved microwave conductivity
1. Introduction
Titanium(IV) oxide (titania) has been the most frequently used photocatalyst in various areas [1,2].
In almost all practical applications of photocatalysis, titania has been used as a photocatalyst because
of its many advantages including low price, high photostability, nontoxicity and superior redox ability [3].
Although studies on titania photocatalysis [3] have suggested the desired structural and/or physical
properties for high-level photocatalytic activity, e.g., anatase crystallites rather than rutile ones and
smaller particles, i.e., higher specific surface area, intrinsic interpretation for possible structure-activity
correlations has not yet been obtained, at least to the authors’ knowledge [4–6].
Particle morphology has recently attracted much attention from scientists as a possible key
parameter for controlling the activity of photocatalyst particles [7–14]. Various methodologies have
been developed to control particle morphology by using diverse treatments such as ultrasonication,
grinding, washing, microwave irradiation, and thermal and pressure treatments [15–17], and by
selective preparation of single-crystalline facetted photocatalyst particles [18–20]. Along this line,
octahedral anatase particles (OAPs), exposing eight equivalent {101} facets, with sizes of several tens
of nanometers, have been prepared by ultrasonication (US) of partially proton-exchanged potassium
titanate nanowires followed by hydrothermal reaction (HT) [21,22]. As is suggested by the fact that
natural anatase titania minerals have been found in an octahedral shape, the {101} facets are
thermodynamically most stable and tend to be exposed when titania is prepared under equilibrium
conditions such as HT. In the previous study of our group, it was shown that OAPs prepared by HT
after US showed relatively high activity for oxidative decomposition of acetic acid under an aerobic
atmosphere (CO2 system) but low activity for methanol dehydrogenation under an argon atmosphere
(H2 system) [21–23].
As has been often and commonly observed for particulate photocatalysts, photocatalytic activities
of OAP-containing samples prepared under different US and HT conditions have different
physical/structural properties, morphologies and photocatalytic activities. However, it was accidentally
observed that particles with almost the same structural activities, such as specific surface area,
crystalline composition and particle size, except only for OAP content, could be prepared by changing
only US time, and the photocatalytic activity, especially for a CO2 system, was almost proportional to
the OAP content; i.e., the morphology of particles directly determines the photocatalytic activity [23].
In the present study, an OAP-containing sample was calcined or ground in an agate mortar in air to
change mainly the particle morphology, and the influence of change in morphology on photocatalytic

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activity was examined in order to clarify the intrinsic reason for the morphology-dependent
photocatalytic activity of anatase titania photocatalyst particles.
2. Results and Discussion
2.1. Preparation of Original OAP-Containing Sample
The previously reported US-HT process [21] led to conversion of partially protonated potassium
titanate nanowires (TNWs) into anatase titania particles of predominantly octahedral shape. SEM
images of the original sample (without post-treatment) are shown in Figure 1. The obtained particles
were smaller than 50 nm and exhibited various morphologies, i.e., OAP, semi-OAP and others (see the
Experimental Section). In the XRD pattern, only peaks assigned to anatase crystallites appeared
(Figure 2a). Images of high-resolution transmission electron microscopy (HRTEM) supported the
presence of single crystals of anatase as shown in Figure 2b; i.e., lattice fringes with a spacing
of 0.35 nm and an angle between two kinds of fringes of 68.3° agreed well with the previously
reported (101) lattice spacing and angle between (101) and (001) [21].
Figure 1. SEM images with (a) low magnification and (b) high magnification of the
original OAP-containing sample. Scale bar: 100 nm.
(a)
(b)
Figure 2. (a) XRD pattern and (b) HRTEM image of the original OAP-containing sample.
(a)
(b)

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2.2. Influence of Calcination on the Structure of Particles
The influence of calcination on the structural properties of samples was studied by changing
calcination temperatures (573–1173 K; 2 h) and keeping the other process conditions, i.e., HT time (24 h),
HT temperature (433 K), US time (1 h), TNW amount of titanate nanowires (267 mg) and Milli-Q
water volume (80 mL) the same. Figure 3a shows the influence of calcination temperature on
crystallinity (anatase; content of crystalline phases) and specific surface area (SSA). It should be noted
that no peaks assigned to rutile crystalline were observed even with calcination at 1173 K. This feature
will be discussed later. Figure 3b shows changes in crystallite size estimated from a 101 XRD peak
(“XRD size”; same as in Figure 3a), particle size estimated by SEM observation (“SEM size”), and
particle size expected from SSA (“SSA size”) calculated with the assumption of spherical uniform-sized
anatase particles [4]. It is expected that SEM size, the longest part of each particle, may be larger than
XRD size, depth of the particle measured in the direction vertical to the {101} lattice plane.
Furthermore, SEM size may be the size of aggregated particles if they are seen as a single crystal,
while XRD size corresponds to the average size of a single crystalline part in aggregated particles.
Considering those problems, use of XRD size seems better to discuss the change in particle size. As
seen in Figure 3a, with elevation of calcination temperature, XRD size slightly increased at ≤973 K but
was much larger in the temperature range higher than 973 K, while SSA decreased even at a lower
temperature, i.e., 773 K. Therefore, at a temperature higher than 773 K, a difference between XRD size
and SSA size can be seen in Figure 3b, indicating partial sintering, with lattice mismatch, i.e.,
formation of grain boundaries, of anatase crystallites by calcination. The decrease in crystallinity in the
temperature range of 673–873 K is attributable to the formation of grain boundaries. (The increase in
crystallinity at the lower temperature might be due to dehydration and/or crystallization of amorphous
phase.) Thus, calcination at temperatures of 773–1073 K in air induced sintering of some of the
particles without lattice matching as the difference between XRD size and SSA size shows; XRD size
became comparable to or larger than that of SSA size by calcination at 1173 K, suggesting fusion of
crystallites to a larger single crystal.
These findings indicate that the original OAP-containing particles (content of OAP and semi-OAP:
62%) are stable toward heat-induced crystal transformation at the temperature at lowest below 1073 K,
presumably due to their exposure of ordered {101} facets with less defects, which may trigger crystal
transformation into larger single crystals (fusion) or rutile. As was stated above, all of the samples,
even those heated at 1173 K, reported here included no rutile phase, though it has been reported that
conversion of anatase to rutile usually occurs at much lower temperatures, e.g., 823–973 K [24–27].
The high level of heat tolerance of the present OAP-containing particles is attributable to the
above-mentioned particle morphology exposing ordered facets. It has been reported that rice-like
titania nanorods were only stable enough to convert 2% of anatase into rutile at 1173 K [28]. Another
possible reason for the heat tolerance is the presence of stabilizers, i.e., potassium cations possibly
contaminated from TNWs. It was reported that the presence of impurities enhanced the heat tolerance
of anatase; e.g., lanthanum(III) oxide-modified titania underwent phase transformation by calcination
at >1073 K [29]. Study on this heat tolerance is now under way and the details will be published
elsewhere [30].

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Figure 3. (a) Correlations between crystalline size (XRD size), crystallinity, specific
surface area and calcination temperature; (b) Comparison of XRD size, SEM size and SSA
size of samples as functions of calcination temperature; (c) Changes in OAP/semi-OAP
content and aspect ratio (AR) as functions of calcination temperature; (d) OAP/semi-OAP
content as a function of AR.
(b)
(a)
100
80
60
40
20
0
120
100
80
60
40
20
0
120
90
60
30
0
350
300
250
200
150
100
50
crystallinity
SSA size
XRD size
SEM size
specific surface area
XRD size
0
1273
273
473
673
873
1073
1273
273
473
673
873
1073
calcination temperature/K
calcination temperauture/K
(c)
(d)
80
60
40
20
0
1.8
1.5
1.2
0.9
0.6
0.3
0.0
80
60
40
20
0
ground
OAP/semi-OAP content
aspect ratio
273
473
673
873
1073
1273
0.6
0.9
1.2
1.5
1.8
aspect ratio
calcination temperature/K
Calcination also changed the morphology of samples. As Figure 4 shows, sharply angulate particles
became round-shaped particles by calcination accompanied by the above-mentioned sintering, and the
higher the calcination temperature was, the higher was the extent of round edging. These observations
were quantified using OAP (and semi-OAP) content and aspect ratio (AR) as shown in Figure 3c. The
former was evaluated by counting numbers of OAPs, semi-OAPs and others in SEM images, and the
latter was evaluated as the ratio of crystallite sizes calculated from 004 and 101 XRD-peak widths and
might be closely related to the OAP content. With calcination at < 675 K, there was little change in
either OAP content or AR, i.e., morphology was not changed in this low temperature region. This is
consistent with results of the above-mentioned particle-size analysis. In a higher temperature region,
≥773 K, both OAP content and AR were decreased to 3% and 0.72 at 1173 K, respectively, suggesting
that sintering (or fusion) of crystallites induced a change in morphology to a round-edged shape. As
Figure 3d shows, OAP and semi-OAP contents decreased linearly with decrease in AR > 1, indicating
that both are measures of particle shape; the content of semi-OAPs, which include particles with
round-edged summits, increased with decrease in AR. The x-intercept of the extrapolated linear part of
plots was ca. 0.9. Based on the assumption that an octahedral particle is changed to a decahedral
particle that exposes eight {101} and two {001} facets, the AR value giving the lowest surface

Molecules 2014, 19
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area/volume (weight) ratio is estimated to be ca. 0.95 [31], similar to the observed intercept. It seems
that the calcination-induced AR decrease is accounted for by modification of particle shape to give
smallest surface area/volume ratio. Considering the arbitrary property of OAP content, AR will be
used when discussing shape dependence in the following sections.
Figure 4. SEM images of OAPs with calcination temperatures of (a) 673 K; (b) 873 K;
(c) 1073 K and (d) 1173 K. Scale bar: 100 nm.
(a)
(b)
(c)
(d)
2.3. Influence of Calcination on Photocatalytic Activities
Photocatalytic activity for two reaction systems, i.e., decomposition of acetic acid (CO2 system) and
methanol dehydrogenation (H2 system), was examined. The dependence of photocatalytic activities on
calcination temperature is shown in Figure 5a. The highest photocatalytic activity in the CO2 system
was obtained for the sample calcined at 673 K. The influence of calcination was less evident in the H2
system than in the CO2 system, and the sample calcined at 873 K showed 1.7 times higher
photocatalytic activity than that of the original uncalcined sample. Although this activity trend can be
explained, in a conventional manner, by a good balance of higher specific surface area and higher
crystallinity [32], such an explanation does not give any intrinsic insights into the correlation between
the physical/structural property and photocatalytic activity. In a previous study on photocatalytic
activity of OAP-containing particles, it was found that photocatalytic activities for CO2 and H2 systems
are governed only by the OAP content of samples; the activities were linearly increased with increase
in OAP content for samples with almost the same other structural properties such as SSA, crystallinity,
XRD size and total density of electron traps [23]. This tendency was also observed for the present
samples, especially for activity for the CO2 system as shown in Figure 5b as a function of AR, since
AR seems to be a slightly better measure for content of OAPs as shown in Figure 3d. The plots (not
shown) of photocatalytic activities in both CO2 and H2 systems as a function of SSA showed trends,

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increase with SSA, but had poor linearity. Thus, the activity of OAP-containing samples seems to be
regulated by the shape of particles. The meaning of this shape-dependent activity and another
dependence of activity in the H2 system will be discussed later.
Figure 5. (a) Correlation between photocatalytic activity and calcination temperature (data
of the original uncalcined sample shown at “273 K”); (b) Photocatalytic activities as
functions of aspect ratio (AR). Plots for the original uncalcined sample are on a dotted line.
Open (CO2) and closed (H2) triangles are of the ground sample.
(a)
(b)
120
100
80
60
40
20
0
60
40
20
0
120
100
80
60
40
20
0
80
60
40
20
0
,
ground
CO2 system
H2 system
CO2 system
H2 system
0.6
0.8
1.0
1.2
1.4
1.6
1.8
273
473
673
873
1073
1273
calcination temperature/K
aspect ratio
2.4. Time-Resolved Microwave Conductivity
As one of the physical properties closely related to photocatalytic activity, time-resolved microwave
conductivity (TRMC) was measured for the present samples. Such carrier dynamics may be also
studied by photocurrent measurements as was reported previously [33], but due to the possible
complexity in interpretation of the measurements owing to the change in sample structural properties
during the electrode preparation process, we used TRMC measurement, which can be performed in the
form of powder. Intensity of the TRMC signal generally shows microwave absorption by migration of
charge carriers and is considered to be a product of charge-carrier density and their mobility [34]. It is
also thought that positive holes, one of the charge carriers, generated in titania particles, are quickly
trapped in certain sites within the time of a nanosecond laser pulse to result in negligible migration.
Therefore, the TRMC signal may reflect only migration of photoexcited electrons [34]. Figure 6 shows
parts of time-course curves of TRMC signals; all samples exhibited a rise of the signal within a
355-nm laser-pulse duration (ca. 10 ns), and decay was observed in the µs time region. First, we
assume that electrons photoexcited in a conduction band (CB) are trapped in traps, the energy level of
which is lower than the bottom of the CB, within a ps time scale [35], i.e., faster than a process
detected in TRMC measurement. Then, charge migration giving maximum signal intensity (Imax) can
be assigned to electron migration through the trap-detrap mechanism, i.e., trapping in shallow traps
and detrapping to the CB thermally, since electron hopping between deep traps is very slow [36] and
thereby electrons in deep traps may have little involvement in the TRMC response. In such a
circumstance, the higher the density of shallow traps is, the higher the mobility is. On the basis of these
considerations, density of electrons trapped in shallow traps was slightly increased by calcination at
<873 K and then decreased by calcination at >873 K. On the other hand, decay of the signal 10–20 ns

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after a laser pulse is attributable to trapping of electrons (once trapped in shallow traps) in deep traps,
prohibiting further migration of electrons. The rate of this decay was evaluated, for convenience, by
calculating the ratio of intensities at 4000 ns and the maximum (I4000/Imax). The decay became slightly
faster with calcination at ≤873 K, suggesting an increase in density of deep traps presumably caused by
particle sintering, but, overall, the trends of Imax and I4000/Imax were of mirror images, i.e., the higher
Imax was, the lower was I4000/Imax, indicating that the rate of trapping by deep traps depends also on the
density of shallow traps.
Figure 6. (a) Influence of calcination temperature on TRMC signal; (b) Imax and I4000/Imax
as functions of AR; (c) Correlation between photocatalytic activities in CO2 and H2
systems and maximum intensity of the TRMC signal (Imax). Data for the original samples
are plotted on dotted lines. Open (CO2) and closed (H2) triangles are of the ground sample.
(a)
(b)
0.5
0.4
0.3
0.2
0.1
0.0
Imax
400
400
873 K
773 K
673 K
573 K
uncalcined
973 K
1073 K
300
200
100
0
300
200
100
0
I4000/Imax
0
10
20
30
40
50
60
1.0
1.2
1.4
1.6
1.8
time/ns
aspect ratio
(c)
120
100
80
60
40
20
0
60
50
40
30
20
10
0
,
ground
CO2 system
H2 system
150
200
250
300
350
400
Imax (a.u.)
Figure 6c shows the dependence of photocatalytic activities for CO2 and H2 systems on Imax. A
fairly linear relation was observed for activity in the H2 system. In the H2 system, platinum was in-situ
deposited as a catalyst for hydrogen liberation by photoexcited electrons, and it has been observed that
deposition of platinum reduces, in a short time region, the density of trapped electrons by capturing
them [37]. Therefore, it seems reasonable that there is such a linear relation between activity in the H2
system and Imax with the assumption that electrons that are able to be trapped in shallow traps migrate
efficiently to platinum deposits to liberate hydrogen without being trapped by deep traps. On the other
hand, oxygen as a possible electron acceptor in the CO2 system is known to have a negligible influence

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on the nanosecond transient behavior of photoexcited electrons [38] and this is one of the reasons for
the absence of a clear correlation with the TRMC data. Although the intrinsic reason is still
ambiguous, the activity for the CO2 system was governed by the particle morphology as is represented
by AR (Figure 5b).
2.5. Influence of Grinding on Structure and Photocatalytic Activity of OAP-Containing Particles
Grinding is one of the most commonly used post-treatment operations to make samples
homogeneous by separating loosely bound agglomerates. However, grinding may also change the
structure of samples and their photocatalytic activity. In this regard, the influence of grinding was
studied for OAP-containing samples. After 1 h grinding in an agate mortar, the morphology of
OAP-containing samples was obviously changed to round-edged as shown in Figure 7; OAPs were
changed to semi-OAPs and others, as schematically shown in Figure 7.
Figure 7. SEM images with (a) low magnification and (b) high magnification of the
ground OAP-containing sample (scale bar: 100 nm) and schematic representation of
change in morphology.
(a)
(b)
This SEM observation is consistent with the decrease in AR (1.33) from that of the original sample
(1.64) as shown in Figure 3d. (A deviation of a plot for the ground sample from a linear relation in
Figure 3d might be caused by the arbitrary property of counting the numbers of semi-OAPs and others
in SEM images; 30% semi-OPA content is expected from the plots.) Photocatalytic activities in CO2
and H2 systems are plotted in Figure 5b,c, respectively. Again, the activity in the CO2 system of the

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ground sample seems to be a linear relation of activity with AR (Figure 5b) but not being explained by
max dependence (Figure 6c). On the other hand, the activity in the H2 system was not plotted in a linear
I
relation with Imax (Figure 6c). It has been observed that grinding titania particles in an agate mortar for
a long time (>24 h) caused the formation of deep traps, leading to negligible photocatalytic activity
presumably due to enhanced recombination at the deep traps [39]. Assuming that similar deep traps
were also produced in the present case with grinding for a relatively short time, decreases in Imax and
I
4000/Imax evaluated in TRMC measurements were accounted for by fast trapping in such deep traps in
the ground sample. However, loaded platinum in the H2 system may capture photoexcited electrons
before being trapped in the deep traps, resulting in unexpectedly higher photocatalytic activity of the
ground sample in the H2 system. Of course, the possibility that preferable platinum loading on the
ground sample led to higher photocatalytic activity in the H2 system cannot be excluded. Study to find
the true activity-controlling structural property is now in progress.
3. Experimental Section
3.1. Preparation of OAPs Samples
Partially proton-exchanged potassium titanate nanowires (TNWs, 267 mg) were ultrasonically
dispersed in Milli-Q water (40 mL) for 1 h at 298 K. The suspension was placed in a sealed Teflon
bottle (100 mL), into which was poured an additional 40 mL of Milli-Q water. The bottle was heated
for 24 h at 433 K in an oven without agitation. After cooling, titania as white precipitate was
centrifuged, washed with RO (reverse osmosis) water, and dried under vacuum (353 K, 12 h). The
thus-obtained OAP-containing samples were used as the original starting samples for further studies.
Two post-treatment operations, calcination and grinding in air, were performed. For calcination, a
1.2-g sample in an air-open ceramic crucible was placed in an oven, and the temperature was raised to
a given temperature (573–1173 K) at the rate of 10 K·min−1, kept at that temperature for 2 h, and
cooled down to ambient temperature. For grinding, a 1.2 g sample was ground in an agate mortar
for 1 h in air.
3.2. Characterization
Specific surface area of samples was evaluated by nitrogen adsorption at 77 K using the
Brunauer–Emmett–Teller (BET) equation. The morphology was studied by scanning electron
microscopy (SEM, JEOL JSM-7400F, Akishima, Japan), scanning transmission electron microscopy
(STEM, HITACHI HD-2000, Tokyo, Japan) and transmission electron microscopy (TEM, JEOL
JEM-2100F). Particles in a sample were classified into three groups based on the results of SEM
analysis: (a) OAP, an octahedral particle without observable defects; (b) semi-OAP, an octahedral
particle with a defect (defects) and (c) others, an irregular shaped non-octahedral particle. Composition
of these particles in each sample was measured by counting at least 200 particles in several SEM
images of a sample [23].
XRD analysis was performed using the SmartLab intelligent X-ray diffraction system (Rigaku,
Akishima, Japan) equipped with a sealed tube X-ray generator (a copper target; operated at 40 kV and
30 mA), a D/teX high-speed position-sensitive detector system and an ASC-10 automatic sample

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changer. Data acquisition conditions were as follows: 2θ range, 10–90°; scan speed, 1.00°·min−1; and
scan step, 0.008°. The obtained XRD patterns were analyzed by Rigaku PDXL, a crystal structure
analysis package including Rietveld analysis [40], installed in a computer controlling the
diffractometer. Crystallite size (XRD size) was evaluated from corrected width of an anatase 101
diffraction peak using the Scherrer equation. Crystallinity of a sample was evaluated using an internal
standard, highly crystalline nickel oxide (NiO). The standard (20.0 wt %) was mixed thoroughly with a
sample (80.0 wt %) by braying in an agate mortar. Since Rietveld analyses give composition of each
crystal among total crystal content, the composition of the standard (formally 20.0 wt %) is measured
to be larger if the sample contains a non-crystalline component. Therefore, crystalline and non-crystalline
compositions are estimated by re-calculation to make the standard composition to be 20.0 wt %. At
present, the authors regard the non-crystalline part to be water, which can be estimated by
thermogravimetry, and amorphous titania and/or titanates.
3.3. Photocatalytic Activity Test
Photocatalytic activities of samples were examined by measuring the amount of evolved carbon
dioxide (CO2) and hydrogen (H2) from continuously stirred (1000 rpm) suspensions of a sample
(50 mg) in an aerated aqueous acetic acid solution (5.0 mL, 5.0 vol %) (CO2 system) and a deaerated
aqueous methanol solution (5.0 mL, 50 vol %) containing chloroplatinic acid (corresponding to
2.0 wt % (as platinum) of a photocatalyst) for in-situ platinum photodeposition (H2 system),
respectively. Photoirradiation (>290 nm) was performed with a 400-W high-pressure mercury lamp
(Eiko-sha, Osaka, Japan) at 298 K. Amounts of liberated CO2 and H2 in gas phase were measured by
gas chromatography (TCD-GC). The photocatalytic activities are presented with reference to those of a
commercial titania photocatalyst, Showa Denko Ceramics FP-6 (anatase, SSA: ca. 100 m2·g−1, XRD
size: 15 nm). FP-6 is known to exhibit a high level of photocatalytic activity among commercial titania
powders, similar to well-known Evonik (Degussa, Essen, Germany) P25 [41,42]. The average rates of
FP-6 and P25 were, respectively, ca. 0.54 and 0.91 µmol·h−1 in the H2 system and 0.039 and
0.046 µmol·h−1 in the CO2 system [23]. Due to negligible solubility of H2 in water and CO2 in an
aqueous solution of acetic acid, no correction was made for gas dissolution in reaction suspensions.
3.4. Time-Resolved Microwave Conductivity Measurements
Charge-carrier dynamics was studied by measuring time-resolved microwave conductivity (TRMC)
induced by a ns time-scale laser pulse in Laboratory of Physical Chemistry, University of Paris-Sud.
Incident 36.8-GHz microwaves and UV laser pulses were generated by a Gunn diode of the Ka band
and third harmonic of a 1064-nm Nd:YAG laser (10 Hz) with full width at half maximum of ca. 10 ns,
respectively [43].
4. Conclusions
Post-treatments, calcination and grinding, of OAP-containing particles changed structural
properties, including OAP/semi-OAP content, aspect ratio (AR) and particle size, and TRMC
responses, e.g., Imax and I4000/Imax reflecting change in charge-carrier dynamics. The changes caused by

Molecules 2014, 19
19584
calcination were interpreted by annealing of single-crystal OAPs and sintering to be aggregates of
single crystals of OAPs, leading to, respectively, higher crystallinity/higher density of shallow traps
and slightly lower crystallinity/lower aspect ratio, i.e., lower OAP content/higher density of deep traps
due to grain boundaries, while the changes caused by grinding were interpreted by round-edging,
i.e., decrease in OAP (and semi-OAP) content/higher density of deep traps. As a result, photocatalytic
activities in CO2 and H2 systems were linearly increased or decreased depending on AR reflecting
OAP content and maximum TRMC signal intensity (Imax), respectively. Although few supporting
experimental results have been obtained, a working hypothesis is that photocatalytic activity is
governed by electron traps, the density of which is influenced by the method of preparation and
post-treatment [44], and that most electron traps are located on the surface of particles and their energy
depends on the structure of exposed surfaces such as {101} facets on OAPs. Analyses of
energy-resolved density of electron traps [45] in the present OAP-containing particles are now in
progress and results will be published in the near future.
Acknowledgments
The authors are grateful to Christophe Colbeau-Justin, Jonathan Verrett and Hynd Remita for
technical assistance and discussion on the results of TRMC analyses. This study was partly supported
by CONCERT-Japan Program (Japan Science and Technology Agency) and a Grant-in-Aid
(KAKENHI) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of
Japan (Grant No. 2510750303). One of the authors (Z.W.) appreciates China Scholarship Council
(CSC) for support.
Author Contributions
ZW, EK and BO designed research, analyzed the data and wrote the paper; ZW performed
experiments. All authors read and approved the final manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are not available from the authors.
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