Anatase TiO2 mesocrystals enclosed by (001) and (101) facets: Synergistic effects between Ti3+ and facets for their photocatalytic performance

Article abstract of DOI:10.1002/chem.201201178

Let's facet it: TiO₂ mesocrystals (MCs; see figure) enclosed by (101) and (001) facets can be synthesized by a facile green approach and the ratio of (101)/(001) can be tuned simply by adjusting the solvothermal periods. The photocatalytic activity of TiO₂ MCs exposing high proportions of (101) facets possess higher photocatalytic activity than those with lower facets; this can be attributed to the synergistic effect of Ti³⁺ and the (101)/(001) facet ratio. Copyright

Full text of DOI:10.1002/chem.201201178

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DOI: 10.1002/chem.201201178
Anatase TiO2 Mesocrystals Enclosed by (001) and (101) Facets: Synergistic
Effects between Ti3+ and Facets for Their Photocatalytic Performance
Qifeng Chen,* Wanhong Ma, Chuncheng Chen, Hongwei Ji, and Jincai Zhao*[a]
Titanium dioxide (TiO2), is among the most widely inves-
tigated materials for its unique properties and many promis-
ing applications in environmental and energy areas.[1] Partic-
ularly, TiO2 is extensively utilized in photocatalysis.[1a,2] In
general, the properties and operational performances of
TiO2 are strongly dependent on its crystal phase, crystallini-
ty, morphology, surface area, and architecture.[3] The struc-
tural integrity of the photocatalyst also plays an important
effect on the photocatalytic activity of the photocatalysts.[3]
Defects in bulk and interface between subunit nanoparticles
can affect the photocatalytic activity; this can be attributed
to the capture of the photoinduced electrons and holes by
the defects and the transfer block of the electrons and holes
by the interface. Therefore, the construction of the architec-
ture of the particles can control the photocatalytic activity.
In addition, for the same crystal phase and composites,
such as anatase TiO2 single crystal (SC), it is found that the
photocatalytic activity of the TiO2 is highly influenced by
cently, there have been some efforts towards the direct mes-
oscale assembly of TiO2 MCs as well as their photocatalytic
reactivities.[9,10] In the mesoscale assembly processes, howev-
er, surfactants,[8] organic additives,[9b] solid surfactant[9f] or
hydrofluoric acid[10b] were used. MCs can be used as catalyst,
photocatalyst, sensor, and electrode materials.[9] High poros-
ity (or stacking defects) and high crystallinity are both re-
quired when aiming at the best performance.[7] There have
been few studies reported on the photocatalytic activity of
TiO2 MC. Furthermore, the photocatalytic activity of TiO2
MC with controllable proportion of (101) and (001) facets
has not been exploited yet. It is, therefore, highly desirable
to study the photocatalytic activities of TiO2 MC with differ-
ent (101)/
We have found that the photocatalytic activities enhance
with increasing (101)/(001) facet ratio when the TiO2 MCs
ACHTUNGTREN(NUGN 001) facet ratios.
AHCTUNGTRENNUNG
exhibit identical surface areas, crystallinity and structural in-
tegrity. It is shown that the photocatalytic activity of TiO2 is
determined by the synergistic effect between the Ti3+ and
the (101)/ACHTUNGTRENNUNG
(001) facet ratio.[4] This can be contributed to the
potential barriers of the reactions, which take place in the
valence band and conduction band, and the arrangement of
the atoms exposed on the various facets.[3,5] Recently, it has
been found that the average surface energy of anatase is
0.90 JmÀ12 for (001)>0.53 JmÀ12 for (100)>0.44 JmÀ12 for
(101), and it is expected that the higher surface energy (001)
has higher chemical activities, and the (001) facet therefore
is defined as the active facet.[2f,3,6] More recently, however,
Cheng and co-workers have proved that the photocatalytic
reactivity order of (001), (010), and (101) is (101)ꢀ(010)>
(001) in their experiments.[4]
TiO2 mesocrystal (MC), a highly ordered superstructure
of crystals with mesoscopic size (1–1000 nm), which exhibits
identical scattering pattern and behavior in polarized light
to TiO2 SC, can be regarded as an intermediate species be-
tween TiO2 nanopolycrystal (NPC) and TiO2 SC.[7] TiO2 MC
was firstly synthesized by topotactic conversion of
NH4TiOF3 MC in the presence of nonionic surfactants.[8] Re-
(101)/ACTHNGUTERN(UNG 001) facet ratio; the structural integrity of the com-
posed subunits as well plays an important role on the photo-
catalytic activity of the TiO2 MCs, compared with TiO2
nanoparticles.
We have synthesized regular shaped TiO2 MCs enclosed
with different proportion of (001) and (101) facets by a
facile green approach. The TiO2 MCs were prepared by
using formic acid (FA) and titanium isopropoxide (TTIP) as
original reactants without any other additives and surfac-
tants at 1608C, and the (101)/ACHTNUGTRENUNG(001) ratio of TiO2 MCs was
controlled by facilely varying the solvothermal treatment
periods. The obtained materials were denoted as MC-t,
where t represents the solvothermal periods (t=1, 2, 4, 6, 8,
10 h; see the Supporting Information). The TiO2 NPC parti-
cles, as a reference sample, were also synthesized by a simi-
lar procedure, with the original molar ratio of TTIP/FA 1:8
(Figure S1 in the Supporting Information); the commercial
photocatalyst, Degussa P25, was also used as a reference
sample.
Typical field emission scanning electron microscope
(FESEM) and transmission electron microscope (TEM)
images of a mesocrystal sample (MC-4) are shown in
Figure 1. The morphology of the sample is a highly truncat-
ed bipyramid with about 200 nm in thickness and 500 nm in
length (Figure 1a). As can be seen from Figure 1b, the as-
semblies are composed of subunits of about 30–40 nm in di-
ameter with similar shape to that of the assembly, which in-
[a] Dr. Q. Chen, Prof. W. Ma, Dr. C. Chen, Dr. H. Ji, Prof. J. Zhao
Key Laboratory of Photochemistry
Beijing National Laboratory for Molecular Science
Institute of Chemistry, Chinese Academy of Sciences
Beijing, 100190 (China)
E-mail: qfchen@126.com
jczhao@iccas.ac.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201178.
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netically metastable species or as intermediates in a crystal-
lization reaction leading to single crystals.[11]
To obtain the growth mechanism of the TiO2 MCs, sam-
ples were collected and investigated at various reaction peri-
ods. When titanium tetraisoproxide was introduced dropwise
into formic acid the transparent liquid immediately trans-
formed into a milk-like suspension. The obtained products
observed with TEM are composed of disordered amorphous
nanoparticles (Figure 2a); the XRD as well confirms the
amorphous phase of the obtained material (Figure S2b in
Figure 1. Representative images of TiO2 MCs (MC-4): a), b) FESEM
images; inset a) TiO2 MC particle enclosed by (101) and (001) facets;
c) TEM image; d) HRTEM image; insets: the SAED along the [001]
zone axis.
dicates the formation of TiO2 mesocrystals.[11] Furthermore,
an angle of 68.38 between the top and lateral facets (Fig-
ure 1a, inset), which is consistent with the interfacial angle
between the (001) and (101) facets, is observed on the as-
sembly, suggesting that the assemblies are enclosed by (001)
and (101) facets.[2f] There are, however, some defects on the
surface of the assembly particles, as shown in the FESEM
images (Figure 1b); this is in agreement with the character-
istics of mesocrystals.[7] Nitrogen absorption/desorption anal-
ysis further shows the existence of micropores among the
subunits in the assemblies (Figure S2a in the Supporting In-
formation), which is also a characteristic of the mesocrys-
tals.[11] X-ray diffraction (XRD) patterns (Figure S2b in the
Supporting Information) show that the obtained product
(MC-4) is of anatase phase with high crystallinity (JCPDS
No.: 21-1272). Further analysis of the peak broadening of
(101) reflection by the Scherrer equation indicates the aver-
age crystal size of 38 nm, suggesting that the TiO2 mesocrys-
tal particles are composed of nanocrystal subunits.[9e] The
high resolution TEM (HRTEM) images further prove the
formation of mesocrystals with high crystallinity, which is
consistent with the XRD result. As shown in Figure 1d, the
distance between the lattice fringe is 0.19 nm, corresponding
to the (001) crystal planes, and the patterns of the selected
area electron diffraction indexed as the [001] zone axis dif-
fraction indicates that the top and bottom square surfaces
are the (001) facets. The study on the morphology of MC-10
further proves the formation of TiO2 mesocrystals. The
HRTEM images of MC-10 reveal that the assemblies are
composed of subunits (Figure S3 in the Supporting Informa-
tion). The nanounits exhibit clear lattice fringes and the se-
lected area electron diffraction (SAED) is distorted to a cer-
tain extent, showing that the distortions came from small
mismatches between boundaries of the nanounits, which is a
typical characteristic for mesocrystals.[11] Mesocrystals with
typical defects and inclusions, such as boundaries and sur-
face defects, have been observed for various systems as ki-
Figure 2. SEM images of products synthesized by solvothermal reaction
of TTIP and FA for various periods: a) 1, b) 2, c) 4, d) 6, e) 8, and f) 10 h.
the Supporting Information). After solvothermal treatment
for 2 h (Figure 2b), the morphology of the products converts
into nanosheets, and anatase phase occurs as proven by
XRD (Figure S2b in the Supporting Information). Highly
truncated-bipyramid-shaped assemblies emerge when the
solvothermal treatment time reaches 4 h (Figure 2c). As the
solvothermal reaction period reaches 6 h (Figure 2d), the as-
sembly particles grow continuously along the [001] direction,
resulting in the decrease in the percentage of (001) facets
and the increase of (101) facets. As solvothermal time is
prolonged, the assemblies grow further along the [001] di-
rection and eventually they grow into the bipyramid shaped
particles at reaction time of 10 h (Figure 2e, f). The mor-
phologies of the products at different intervals indicate that
4 h of reaction time is sufficient for the formation of meso-
crystals and the TiO2 MCs grow along the [001] zone axis.
On the basis of the above-experimental results, the possi-
ble scheme for the formation of TiO2 MCs is shown in the
Figure 3. At the initial stage (step A) of the reaction, the ti-
tanium formate complexes, an intermediate, are formed im-
mediately via the coordination of FA to titanium centers by
À À
ligand exchange/substitution, and subsequently the Ti O Ti
bonds are formed by both hydrolysis-condensation and non-
hydrolytic condensation. The intermediates then convert
into TiO2 nanocrystals by a classical nucleation and nano-
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Anatase TiO2 Mesocrystals
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Scheme 1. Photocatalytic reduction and oxidation reaction of NSB under
different conditions.
Figure 3. Proposed formation scheme of the TiO2 MCs.
crystal growth; meanwhile they are temporarily stabilized
by the FA molecules, leading to the formation of leaf-like
nanosheets by orientation force in FA (step B).[9a,10a] The
nanosheets then undergo a mesoscale oriented self-assembly
process during which the FA molecules should be preferen-
tially attached onto the specific (101) surfaces of the nano-
sheets, and therefore lead to strongly anisotropic mutual in-
teractions between them, accompanied with further crystal-
linity enhancements (step C). Up to this period, nucleation
and crystallization are the dominant processes. As the reac-
tion time is prolonged, more subunits preferably adhere to
the (001) facets of the assemblies and the mesocrystals grow
along the [001] direction; this leads to the formation of trun-
cated bipyramid-like assemblies (step D), and eventually oc-
tahedral assemblies form, which can contribute to the high
surface energy of the (001) surface, often leading to assem-
bly into the oriented nanocrystals along the [001] direction
(step E). The process includes nucleation at the initial
period and non-classical growth, that is, particle-mediated
formation of mesocrystals, which is similar to the growth
process defined by Cçlfen and Antonietti.[11] In the growth
process, the proportion of (001) facets decreases and that of
(101) increases; FA plays a critical role in the formation of
TiO2 MC and acts as a binding molecule.
duced photocatalytically into aniline by electrons, and the
isopropanol is oxidized into acetone by holes. These photo-
catalytic reactions are presented in the Scheme 1. The pho-
tocatalytic reactivity was also characterized by hydroxyla-
tion of terephthalic acid (TA) to salicylic acid (TAOH) in
aqueous suspensions containing different photocatalysts.
The conversion of TA was used to evaluate the photocatalyt-
ic reactivity. The hydroxylation of TA to TAOH is a photo-
oxidation reaction, in which molecular oxygen acts as an ox-
idation reagent.
X-ray photoemission spectroscopy (XPS) was used to in-
vestigate the chemical composition of the particle surface.
The Ti 2p3/2 features were observed at 459.20 eV for samples
(Figure 4a). These features are all well-assigned to Ti4+, and
indicate that no Ti3+ exists on the surface (the XPS techni-
que characterizes the top 1–10 nm depth of the particle).[12]
The presence of Ti3+ was further investigated by low tem-
perature electron paramagnetic resonance (EPR; Fig-
ure 4b). No EPR signals appear for a g-value of about 2.02
À
corresponding to O2 produced from the reduction of adsor-
bed O2 by surface Ti3+, further confirming the absence of
surface Ti3+ as for the NPC and SC samples.[13] However,
À
for the MC samples, the signals for O2 and Ti3+ are ob-
served (Figure 4b), indicating the presence of Ti3+ in the
bulk of MC samples. The Ti3+ can be formed by the situ-re-
duction of Ti4+ by formic acid, which is as a reducing agent,
in an anaerobic system. The obtained product is blue in
color and displays a visible absorption (Figure S6 in the Sup-
The proportion of the (101) facet can be readily adjusted
by the variation of the solvothermal periods. At intervals of
4, 6, 8, and 10 h, it is shown that the obtained TiO2 MCs
have a (101)/ACHTUNGTRENNUNG(001) facet ratio of 60:40, 75:25, 88:12, 98:2,
corresponding to MC-4, -6, -8, and -10, respectively
(Table S1 in the Supporting Information).
porting Information), which is attributed to the existence of
[14]
Ti3+
.
The quantity of the Ti3+ can be increased as the sol-
Nitrosobenzene (NSB) is a relatively stable substrate at
room temperature, and it can be reduced photocatalytically
on TiO2 into aniline in the absence of molecular oxygen by
using isopropanol as sacrifice reagent, and it can also be oxi-
dized photocatalytically into nitrobenzene (NB) in the pres-
ence of molecular oxygen (Scheme 1). Furthermore, these
reactions facilitate the determination of the products. NSB,
therefore, was selected as a probe molecule to examine the
photocatalytic activity of the TiO2 MCs. The reaction be-
tween NSB and molecular oxygen in the photocatalytic oxi-
dation reaction of NSB to NB is performed by holes in the
valence band of TiO2. Simultaneously, the photocatalytic re-
duction of the molecular oxygen to hydrogen peroxide
(water molecule) by electrons in the conduction band hap-
pens. In the photoreduction of NSB, the reaction between
NSB and isopropanol was also carried out. The NSB is re-
vothermal treatment period is prolonged (Figure 4b). More-
over, the intensities of these signals enhance with an in-
crease in the proportion of the (101) facets; this suggests the
linear correlation between concentration of Ti3+ and the
proportion of (101) facet in this system.
The physical properties of the various TiO2 MCs facilitate
us to investigate the respective photocatalytic activities
(Table S1 in the Supporting Information). Little variation in
particle size and surface area exists among MC-4–MC-10;
the crystallinities of the TiO2 MCs hardly change (Fig-
ure S2b and Table S1 in the Supporting Information). The
variation in photoactivity can be attributed to the distinct
proportion of (001) and (101) facets in the TiO2 MCs. TiO2
MCs exhibiting a higher proportion of (101) facets possess
higher photo-oxidation and photoreduction activities
(Figure 5); this is similar to the TiO2 SCs with the different
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Q. Chen, J. Zhao et al.
Figure 5. The photocatalytic activity of different TiO2 MCs with various
proportion of (001) and (101) facets; The photo-oxidation reaction was
performed with nitrosobenzene (5.0 mm) and photocatalyst (1.0 gLÀ1
)
under ultraviolet visible light irradiation.
existence of oxygen vacancies can shift the valence band
maximum downwards as a result of band bending effects, as
well as the introduction of Ti3+ defects in band gap, which
facilitates the generation of strongly reductive electrons, as
proved by EPR (Figure 4b).[3,17] The Ti3+ in the bulk of
TiO2 MCs can be eliminated by calcination in the atmos-
phere and the color of the products alters from blue to
white (Figure S6 in the Supporting Information). The crys-
tallinity of the TiO2 MC after calcination does not alter evi-
dently compared with the one before calcination. The pho-
tocatalytic activities of the TiO2 MCs follow the same se-
quence (Figure S7 in the Supporting Information) and are
lower than those of the corresponding unsintered samples.
These results further confirm the existence of synergetic
Figure 4. Core Ti 2p: a) XPS spectra, and b) EPR spectra of the TiO2
NPC, SC, and MC samples.
effect between Ti3+ and the (101)/
ACTHNUTRGNE(UNG 001) facet ratio. Conse-
quently, the synergistic effects of the two factors are respon-
sible for the higher photocatalytic activity of the TiO2 MCs
with higher proportion of (101) facets.
(001)/ACHTUNGTRENNUNG
(101) facet ratios explored by Cheng et al.[4] The pho-
tocatalytic activity of hydroxylation of TA is also higher
using TiO2 MCs with a higher proportion of (101) facets
(Figure S4 in the Supporting Information). These results can
be attributed to both surface atom distribution and surface
electronic band structure.[3,6] The (001) facet is considered as
100% unsaturated Ti5c, which theoretically results in higher
photoreactivity of (001) than (101), whereas the (101) facet
possesses 50% Ti5c and 50% saturated Ti6c.[6,15] On the
other hand, the TiO2 MCs that have a higher percentage of
(101) facets exhibit a higher conduction band minimum than
that of the (001) facet, indicating that more strongly reduc-
tive electrons can be generated on (101) facets; the valence
band is identical, showing the similar mobility of charge car-
riers (Figure S5 in the Supporting Information).[4]
In addition, the normalized photocatalytic activity of TiO2
is higher than that of TiO2 NPC and P25 (Figure S8 in the
Supporting Information). The highly ordered junction be-
tween the subunits of TiO2 MCs plays a crucial role in the
photocatalytic activity. The highly ordered subunits of TiO2
nanoparticles assemble into TiO2 mesocrystals and the crys-
tal plane of (101) match well between the subunits (Fig-
ure S9 in the Supporting Information). The junctions should
be more regular than that of TiO2 NPC and P25 particles.
The photoinduced electrons and holes can more readily mi-
grate across the well-match junctions than in those irregular
junctions in TiO2 NPC and P25, and the holes migrate much
faster than the excited electrons in the anatase phase; this
reduces the recombination of holes and electrons, and there-
by improves the photocatalytic activity.[3,6] Additionally, the
high degree of crystallinity of TiO2 MCs is higher than that
of NPC and P25; this is indicative of a lower concentration
of bulk defects (oxygen and/or titanium vacancies) in the
subunits of MCs, and leads to lower hole-electron recombi-
In addition, the hole-trapped probability on the (101)
facet is higher than that of (001); this leads to a lower re-
combination of electron-hole on the (101) facet than (001),
thus the photocatalyst possessing the higher proportion of
(101) facet exhibits the higher photocatalytic activity.[16] The
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Anatase TiO2 Mesocrystals
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nation rate, and further results in the higher specific photo-
catalytic activity than that of NPCs.[3] Therefore, it is reason-
able that the photocatalytic activity of TiO2 MCs is higher
than that of NPCs with the identical normalized surface
areas.
Acknowledgements
We thank for the financial supporting of National Key Basic Research
Program of China (973 project, 2010CB933503) and National Science
Foundation (20920102034). We also thank Dr. J. M. Zhang (the Institute
of Physics, Perking University) for his ardent work on the HRTEM.
In summary, TiO2 MCs have been synthesized by a facile
green approach, with the use of TTIP and FA as the original
reactants. The obtained TiO2 MCs are enclosed by (001) and
(101) facets, and the proportion of (101) facet can be tuned
by adjusting the solvothermal periods. The TiO2 MCs expos-
ing a high proportion of (101) facets possess higher photoca-
talytic activity than those with a lower proportion; this can
be attributed to the synergistic effect of Ti3+ and the propor-
tion of (101) facets. In addition, the normalized photocata-
lytic activity of TiO2 MCs is higher than that of NPCs when
the proportion of (101) facets is equal; this indicates that
the structural integrity of crystal plays a key role in the pho-
tocatalytic activity.
Keywords:
mesocrystals · titanium
(101)/
E
·
anatase
·
catalysis
·
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Experimental Section
Synthesis and characterization of TiO2 mesocrystals: Formic acid (FA)
and titanium isopropoxide (TTIP) were purchased from Alfa Aesar
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ment of a mixture of formic acid and titanium isopropoxide at 1608C,
and the morphology of the TiO2 mesocrystals was controlled by variation
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propoxide (0.75 mL) was introduced dropwise into formic acid (50 mL)
under vigorous stirring. The obtained milky suspension was transferred
into a Teflon-lined autoclave (100 mL) and solvotreated at 1608C for a
desired period. The products were collected by centrifugation, washed
with ethanol four times, dried at 608C in a vacuum box for 12 h. The ob-
tained materials were denoted as MC-t, where t represents the solvother-
mal periods (t=1, 2, 4, 6, 8, 10 h). The TiO2 nanopolycrystal (NPC) was
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The morphology of obtained materials was characterized by field emis-
sion scanning electron spectroscopy (FESEM, Hitachi, 4300 and 4800)
and high resolution transmission electron spectroscopy (HRTEM, Tecnai,
F20). The crystal structure of the samples was recorded on an X-ray dif-
fractometer (Advance Bruker Axs diffractometer with CuKa irradiation,
40 kV and 100 mA). The BET surface areas of the samples were meas-
ured with a physical absorptometer (Tristar 3000, Micromeritics). The ob-
tained photocatalytic reaction products were analyzed by gas chromatog-
raphy/mass spectroscopy (Agilent 4975) and liquid chromatography/mass
spectroscopy (Agilent 6310).
Photoactivity measurements: Photocatalytic oxidation activity was evalu-
ated by photo-oxidation of nitrosobenzene in an aerobic TiO2 suspension
by using acetonitrile as a solvent. Photoreduction was examined by pho-
toreduction of nitrosobenzene in an anaerobic TiO2 suspension by using
isopropanol as both solvent and sacrifice reagent. The original concentra-
tions of nitrosobenzene and photocatalysts were 5.0 mmolLÀ1 (mm) and
1.0 gLÀ1, respectively. PLS-SXE-300/300UV (Trustech China) was used
as a light source in the photocatalytic reactions and the distance between
the reaction system and light lamp was 10 cm, and the light intensity was
about 0.3 mWcm2. Photocatalytic hydroxylation of terephthalic acid
(TA) to salicylic acid (TAOH) was performed in an aqueous suspension
containing different photocatalysts, the original concentration of TA was
0.5 mm and contained sodium hydroxide (2.5 mm), which favored the sol-
ubility of TA in water. The determination of conversion of TA was car-
ried out by LC/MS (Aligent 6300).
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www.chemeurj.org
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Q. Chen, J. Zhao et al.
P. Y. Feng, Angew. Chem. 2012, 124, 6327; Angew. Chem. Int. Ed.
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Received: April 6, 2012
Published online: && &&, 0000
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Anatase TiO2 Mesocrystals
COMMUNICATION
Let’s facet it: TiO2 mesocrystals (MCs;
see figure) enclosed by (101) and (001)
facets can be synthesized by a facile
Catalysis
Q. Chen,* W. Ma, C. Chen, H. Ji,
J. Zhao* ....................... &&&& — &&&&
green approach and the ratio of (101)/
ACHTUNG-
TRENNUNG
Anatase TiO2 Mesocrystals Enclosed
by (001) and (101) Facets: Synergistic
Effects between Ti3+ and Facets for
Their Photocatalytic Performance
the solvothermal periods. The photoca-
talytic activity of TiO2 MCs exposing
high proportions of (101) facets pos-
sess higher photocatalytic activity than
those with lower one; this can be
attributed to the synergistic effect of
Ti3+ and the (101)/
ACHTUNGTRENNUNG(001) facet ratio.
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!