Hierarchical TiO2 microsphere assembled from nanosheets with high photocatalytic activity and stability

Article abstract of DOI:10.1016/j.cplett.2019.136989

TiO₂ hierarchical microsphere assembled from anatase TiO₂ nanosheets has been successfully prepared and exhibited excellent catalytic activities of H₂ production and benzyl alcohol selective oxidized to benzaldehyde. These

Full text of DOI:10.1016/j.cplett.2019.136989

Journal Pre-proofs
Research paper
Hierarchical TiO Microsphere Assembled from Nanosheets with High Photo-
2
catalytic Activity and Stability
Jie Hu, Yi Lu, Xiao-Fang Zhao, Yu-Qian Tang, Yuan-Zhou Li, Yu-Xuan
Xiao, Zhi-Yi Hu, Bao-Lian Su, Xiao-Yu Yang
PII:
S0009-2614(19)30970-4
DOI:
https://doi.org/10.1016/j.cplett.2019.136989
CPLETT 136989
Reference:
To appear in:
Chemical Physics Letters
Received Date:
Revised Date:
Accepted Date:
3 September 2019
31 October 2019
25 November 2019
Please cite this article as: J. Hu, Y. Lu, X-F. Zhao, Y-Q. Tang, Y-Z. Li, Y-X. Xiao, Z-Y. Hu, B-L. Su, X-Y.
Yang, Hierarchical TiO Microsphere Assembled from Nanosheets with High Photocatalytic Activity and
2
Stability, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett.2019.136989
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Hierarchical TiO Microsphere Assembled from Nanosheets with High
2
Photocatalytic Activity and Stability
Jie Hu, § Yi Lu, § , a Xiao-Fang Zhao, Yu-Qian Tang, Yuan-Zhou Li, Yu-Xuan
, a
a
b
a
a
*a, c
Bao-Lian Su*a, d and Xiao-Yu Yang,*a, e
Xiao, Zhi-Yi Hu,
§
These authors contributed equally to this work.
a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing & School
of materials science and engineering, Wuhan University of Technology, 122, Luoshi Road,
Wuhan, 430070, China.
E-mail: xyyang@whut.edu.cn, baoliansu@whut.edu.cn.
b
School of Chemical Engineering, Northeast Electric Power University, 169, Changchun Road,
Jilin, 132012, China.
c
Nanostructure Research Centre (NRC), Wuhan University of Technology, 122, Luoshi Road,
Wuhan, 430070, China.
Email: zhiyi.hu@whut.edu.cn.
d
Laboratory of Inorganic Materials Chemistry (CMI), University of Namur, 61, rue de Bruxelles,
B-5000 Namur, Belgium. E-mail: bao-lian.su@unamur.be.
e
School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts
0
2138, USA. E-mail: xyyang@seas.harvard.edu.
Graphical abstract
Highlights



Ti-based hierarchical microsphere assembled from nanosheets was prepared.
TM-S exhibited excellent catalytic activities of H production.
2
TM-S achieved photo-assisted benzyl alcohol selective oxidized to
benzaldehyde.
1


Surface F and nanosheets structure were suggested to explain enhanced
performance.
Abstract
TiO hierarchical microsphere assembled from anatase TiO nanosheets has been
2
2
successfully prepared and exhibited excellent catalytic activities of H production and
2
benzyl alcohol selective oxidized to benzaldehyde. These enhanced photoactivity can
be mainly ascribed to the charge separation promoted by the surface F and nanosheet
units, enhancement of substrate molecular transport profited from the hierarchical
structure.
Keywords: TiO , hierarchical structure, nanosheet, photocatalysis
2
1
. Introduction
Titania(TiO ) is one of the most important semiconductors for advanced energy
2
conversion and storage applications.[1-3] Previous reports have demonstrated that
catalytic activity and selectivity of TiO materialsare strongly dependent on their
2
nano-architecture.[4, 5] To date, nanosheets are emerging as a class of key materials in
energy and environmental fields due to their atomic/electronic structures and intrinsic
[6]
properties. These nanosheet materials in particular have large specific surface area
and nano-scaled thickness, which not only offer the more reaction sites for catalytic
reaction, but also prolong the separation of photoexcited carriers by the minimized
transfer pathway.[7, 8] Hierarchical functional structures have attracted great interest in
recent years to enhance the structure stability and photoactivity through the oriented
assembly of nanosized units or their aggregates.[9, 10] Pre-formed nanosheets can be
integrated with a three-dimensional (3D) material as a new building block to fabricate
2

interfacial heterostructures.[11] However, it is still a big challenge to design and
synthesize a perfect hierarchical functional structure with enhanced photocatalytic
performances.
Herein, we report a template-free approach to synthesize TiO hierarchical
2
microspheres assembled from anatase TiO nanosheets with activated {001} facets.
2
As shown in Scheme 1, nanosheets will grow within the microemulsion droplets and
develop into a microsphere with a mixed phase of anatase TiO and TiOF (TM)
2
2
under the presence of toluene. However, it has been reported by many researchers
[12, 13]
that TiOF shows poor photocatalytic activity.
After heat treatment in air or
2
solvothermal treatment in isopropyl alcohol,[14, 15] TiOF crystals can be converted
2
into anatase TiO with surface absorbed fluorine (F). The sample prepared with
2
further solvothermal treatment exhibits highest photocatalytic activity on both
hydrogen generation and selective oxidation of benzyl alcohol, which can be mainly
ascribed as: (1) the charge separation promoted by the existence of surface fluorine
(F) and nanosheets structure with exposed active facets, (2) enhancement of substrate
molecular transport profited from the hierarchical structure.
Scheme 1. Schematic formation of Ti-based hierarchical microsphere (TM) assembled
from nanosheets.
2
Experimental section
3

2
.1. Materials
Hydrofluoric acid (SCRC, AR grade), toluene (SCRC, AR grade), titanium
butoxide (TBT, Aladdin, 98.0%), benzyl alcohol (SCRC, 98%), commercial TiO2
anatase, 40 nm, SCRC) were used as purchased. All other chemicals were of AR
(
grade and used as purchased.
Caution! Hydrofluoric acid is extremely corrosive and should be handled very
carefully.
2
.2. Synthesis of TM, TM-S, TM-C and TM-S-A
Ti-based microspheres (TM) constructed of nanosheets were synthesized
through a solvothermal approach in a toluene-HF mixed solution using TBT as
titanium source. The sample consists of anatase TiO and TiOF . Typically, HF (40
2
2
wt %, 1.0 mL) was drop wised into a mixture solution of TBT (12.5 g) and toluene
2.0 mL) under vigorous stirring. And then, the reaction mixture was heated to 200
(
oC for 24 h. The light blue products were collected through membrane filtering (pore
size, 0.45 μm) and washed with ethanol.
Sample named TM-S was prepared by solvothermal treatment of sample TM in
suitable isopropyl alcohol (TBA) at 200℃ for 24 h. Followed by washing TM-S
powders with NaOH (0.1 M) and water several times, the control sample without
surface F was prepared (donated as TM-S-A). For the purpose of comparison, TiOF2
crystals in TM also have been transformed into anatase TiO by heat-treating of TM
2
under 500℃ in the air (donated as TM-C).
2
.3. Synthesis of TiO (101) and TiOF
2 2
The {101}-exposed anatase TiO and TiOF were prepared by the route in the
2
2
literature and denoted as TiO (101) and TiOF2[15], respectively.
[
16]
2
2
.4. Characterization
The samples were characterized by X-ray diffraction (XRD, D8 ADVANCE, 40
kV, 40 mA) patterns with Cu Kα radiation (λ=1.54056 Å). The morphology, element
distribution and structure were performed on a field-emission gun scanning electron
4

microscope (SEM, Hitachi S-4800, operated at 5 kV) and scanning/transmission
electron microscope (S/TEM, Thermo Fisher Scientific Talos-F200S, fitted with
Super-X EDS system, operated at 200 kV). The nitrogen adsorption-desorption
isotherms were performed on a Micrometrics (TriStar II 3020 system, 77 K) after the
o
samples were degassed at 120 C for 12 h. Raman spectra were characterized by an
Invia Raman Microscope (Invia Microscope, Renishaw, UK, 632.8 nm) at room
temperature. The surface electronic states of F were obtained byX-ray photoelectron
spectroscope (XPS, VG Multilab 2000). The UV/vis spectroscopy diffuse reflectance
spectra (UV/vis DRS) were performed on a UV/vis spectrophotometer (UV-2550).
Time-resolved PL fluorescence spectra of samples suspended in aqueous solutions
(
pH 2.4) were characterized with a fluoromax-4 spectrofluorometer by excitation at
86 nm (HORIBA Scientific).
.5. Evaluation of photocatalytic activity
2
2
2
.5.1. Photocatalytic aerobic oxidation
Photocatalytic oxidation of benzyl alcohol was performed as follow: Firstly, the
catalyst (16 mg) was added to the mixture solution of benzotrifluoride (1.5 mL),
benzyl alcohol (0.1 mM) and dodecane (0.1 mM). The above reaction solution was
transferred to a Pyrex glass bottle. And then, the solution was irradiated by a lamp
(PLS-SXE 300C) under stirring. After being irradiated for 4h, the catalysts were
removed through centrifugation and the remaining solution was analyzed by a gas
chromatograph (GC-Agilent 7890A, FID). Conversion of benzyl alcohol and
selectivity of benzaldehyde were calculated.
2
.5.2 Photocatalytic hydrogen generation
Photocatalytic hydrogen generation was tested in a closed circulation system as
follow: The catalyst (20 mg) was suspended in a mixture of deionized water (80 mL)
and methanol (20 mL). Followed by adding a chloroplatinic acid aqueous solution (10
mM, 52 µL), the reaction mixture was sealed in a quartz vessel and vacuumed. And
then, the reaction mixture was irradiated by a lamp (PLS-SXE 300C) under stirring
5

and cooling. The generted H was analyzed by gas chromatograph (GC-Agilent
2
7
890A, TCD).
3
. Results and discussion
The microstructure of as-prepared Ti-based microsphere (TM) was studied by
scanning electron microscopy (SEM). It is apparent that the as-prepared microspheres
with diameters about 5 µm are constructed of nanosheets with a length of 30-50 nm
and a thickness of about 20 nm (Figure 1a). SEM images point out that the
morphology of the sample was remained after solvothermal treatment (Figure S1a).
However, the microsphere morphology of the material prepared was destroyed after
alkali treatment, as exhibited in Figure S1b. And, calcination promotes the growth of
crystallite and leads to the increasing of nanosheets’ thickness and the gap between
assembled nanosheets (Figure S1c, Table S1). As shown in Figure S1d, commercial
TiO is constructed by nanoparticles. The crystal compositions of the samples were
2
confirmed by X-ray diffraction (XRD, Figure 1b). The diffraction peaks (at 23.4° and
2
0
0
5.3°) correspond to the (100) plane diffraction of cubic TiOF (ICDD no.
2
0-08-0060) and the (101) plane diffraction of anatase TiO2 (ICDD no.
1-0086-1157), respectively, which indicates that the TM is composed of mixed
phases. The peak at 23.4° disappears after solvothermal treatment in TBA at 200 °C
for 24 h (Figure 1b red line), which suggests the beginning of TiOF hydrolysis. It
2
could be ascribed to the generation of a small amount of water during dehydration of
isopropyl alcohol. Additionally, the XRD peak intensities of anatase phase in the
TM-S sample become stronger and slightly narrower than the TM sample, indicating
the enhanced crystallinity. The calcined sample (TM-C, Figure 1b brown line) also
shows similar stronger intensities and narrower width of XRD diffraction peaks
contributed to anatase phase. These experimental results indicate that cubic TiOF2
phase can be transformed into anatase TiO phase through calcination or solvothermal
2
treatment. In addition, the crystal structure has no significant change after alkali
6

treatment (TM-S-A). The nitrogen adsorption-desorption isotherms were performed to
confirm the porous structure and specific surface area of the samples (Figure 1c).
These isotherm curves are type IV with type H3 hysteresis loop, indicating the
existence of mesopores in aggregated nanosheets. This is consistent with the SEM
image (Figure 1a). The specific surface areas of TM, TM-S, TM-S-A, TM-C,
2
-1
commercial TiO were estimated to be 84, 82, 92, 57 and 7 m ·g , respectively,
2
which enables the different rate of diffusion in the solution and leads to the
distinguished photocatalytic activity. From the pore size distribution data, the TM,
TM-S, TM-S-A, TM-C show broad distribution of pore diameters centred at 22.7,
2
5.6, 15.7 and 36.9 nm, respectively (Figure S2 and Table S1). Corresponding to the
isotherm curves, commercial TiO does not show porous structure. Raman spectra
2
(Figure 1d) were displayed to identify the well-crystallized structure of TM-S,
−
1
−1
TM-S-A, TM-C and commercial TiO . The peaks ranging from 150 cm to 700 cm
2
were attributed to the anatase crystallites of TiO . Meanwhile, crystallites of TiOF
2
2
show no significant Raman peaks.
7

Figure 1. (a) SEM image of TM sample, (b) XRD patterns of TM, TM-S, TM-S-A,
TM-C and commercial TiO , (c) Nitrogen adsorption-desorption isotherm curves of
2
TM, TM-S, TM-S-A, TM-C, and commercial TiO , (d) Raman spectra of the TM,
2
TM-S, TM-S-A, TM-C, commercial TiO and TiOF .
2
2
The HAADF-STEM image (Figure 2a) and corresponding EDSelemental maps
Figure 2b-d) of TM-S sample show the nature of two-dimensional F-doping TiO2
(
nanosheets. Corresponding to the XPS spectra (Figure S3), the element mapping
image of F exhibits that surface F are uniformly distributed on the nanosheets. Surface
F ions exhibit strong holes capture ability, which can reduce the recombination of
electron-hole pairs and improve the photoactivity.[17] The high resolution HAADF-
STEM image (Figure 2e) and corresponding FFT (Figure 2e inset) reveal that the side
surface of nanosheet is perpendicular to TiO [100] zone axis and the {001} facet is
2
the maximum exposed surface. Further, the atomic resolution HAADF-STEM image
(
Figure 2f) demonstrates the clean surface with different TiO crystal facets, as the
2
model is shown (Figure 2f inset). The TM-C sample has a similar structure composed
of smaller and thicker TiO2
8

Figure 2. (a) HAADF-STEM image of TM-S sample, (b-d) corresponding EDS
elemental maps: Ti (red), F (green), and O (blue), (e) HR-HAADF-STEM image of
the area indicated by red box in (a) and corresponding FFT pattern (inset), (f)
enlarged atomic resolution HAADF-STEM image of the area indicated by red box in
(
e) and corresponding model of TiO nanoparticle (inset).
2
nanosheets but without surface F, making it possible to distinguish the effect of
surface F ions without the disturbing of structure (Figure S4).
The photocatalytic activities of the samples were investigated through selective
photo-assisted oxidation of benzyl alcohol to benzaldehyde, which is a common
chemical reaction and very important for laboratory and commercial processes.[18-20]
Performances of the TM, TM-S, TM-S-A, TM-C, commercial TiO under light
2
irradiation and dark are presented in Figure 3a and Table S2. All of these samples
show the selectivity of more than 99% for the photocatalytic oxidation of benzyl
alcohol under irradiation and trace of photocatalytic activity under dark. TM and
TM-S display obviously higher photoactivity (42.5% and 44.9%, respectively)
9

compared to TM-C (25.7%). The enhanced activity could be due to the existence of
surface Ti-F groups, which exhibits strong electron capture ability and could reduce
the recombination of electron-hole pairs and improve the photoactivity. It is further
proved by the fact that TM-S-A and TM-S-2nd being free of F show similar
conversion rate (23.0% and 23.5% respectively) to TM-C. H generation from a
2
water/methanol mixture solution (Figure 3b-c) and the change of the fluorescence
intensity of terephthalic acid with irradiation time have been performed (Figure S5).
TM-S (Figure 3b-c) not only shows the highest hydrogen production rate of 45
-1
-1
mmol·h ·g , but also exhibits high stability after 5 cycles. STEM images exhibit that
Pt nanoparticles are uniformly loaded onto the surface of TM-S and TM-C (Figure
S6), indicating the performance difference is independent of Pt deposited.
Corresponding to the results on the selective oxidation of benzyl alcohol to
benzaldehyde, the surface F and hierarchal structure of TM-S benefit the enhanced
photoactivity. To further confirm the beneficial effects of the surface F and
hierarchical structure on the photocatalytic activity and chemistry of the samples,
photoactivity (Figure S7), PL analysis (Figure S8a) and photocurrent density of
samples were studied (Figure S8b). According to the XPS valence band and band gap
calculated (Figure S7), sample TM-S, TM-S-A, TM-C and commercial TiO show
2
similar band structure, indicating the different photoactivity is irrelevant with band
structure in this work. As shown in Figure S8a, the TM-S exhibits the lowest
fluorescence intensity with excitation wavelength of 320 nm. The photocurrent
density of TM, TM-S, TM-S-A and TiOF under UV/vis light irradiation were carried
2
out. As shown in Figure S8b, TM-S shows the highest photocurrent intensity during
these samples, indicating the most efficient charge seperation and transfer. After
treating by NaOH, the photocurrent of TM-S-A decreased, which mainly caused by
the remove of surface F ions. Time-resolved PL decays of the TM, TM-S, TM-S-A,
TM-C and commercial TiO were performed to further compare the efficiency of
2
charge separation (Figure S9). Due to the prolonged lifetime of photogenerated
10

carriers, the samples with efficient charge separation will show a slow PL signal
decay.[2, 22] As we can see in Figure S9 and the corresponding fitting and calculating
results in Table S3, TM-S shows the longest photoluminescence lifetime (3.66 ns)
because of the high efficiency in electron-hole separation benefited from surface F
ions.[23]
Figure 3. (a) Photocatalytic selective oxidation conversion of benzyl alcohol to
nd
benzaldehyde over TM, TM-S, TM-S-2 , TM-S-A, TM-C, commercial TiO , TiO
2
2
(
101) and blank,a, [21] (b) comparison of the photogeneration of H from methanol
2
aqueous solution after loading 0.5 wt% Pt on TM, TM-S, TM-S-A, TM-C and
commercial TiO under UV/vis light irradiation, and (c) cycle performance of the
2
photogeneration of H from methanol aqueous solution after loading 0.5 wt% Pt on
2
a
TM-S, TM-S-A and commercial TiO under UV/vis light irradiation. Reaction
2
conditions: 1.5 mL of BTF; 0.1 mmol of benzyl alcohol; 0.1 mmol dodecane; 16 mg
of catalysts; reaction for 4 h at room temperature.
11

The proposed reaction mechanism is briefly shown in Figure 4. According to the
DFT calculations, Yu et al. pointed out that the {101} and {001} facets of anatase
TiO within a single crystal can form a surface heterojunction for they show different
2
band edge positions.[24] Under light irradiation, the photogenerated holes (h ) tend to
move to {001} facets as oxidative sites, while photogenerated electrons are inclined to
move to {101} facets as reductive sites.[25] Moreover, the abundant surface
undercoordinated Ti atoms and active oxygen atoms lead to the high reactivity of
anatase {001}, which is provided by first-principles calculations.[26-27] Thus the {001}
facets become more active than {101} facets in photo-assisted oxidation reactions. In
+
+
our case, the surface F anions with negative electric will attract the h to migrate to
[28]
the surface of TiO and inhibit the migration of photogenerated electrons, which
2
further prevents electron-hole recombination. During the photocatalytic oxidation of
benzyl alcohol, the O adsorbed at defect sites will bond with photoelectrons and
2
-
-
produce O . These O can cleave the O-H bond of the alcohol and then evolve into an
2
2
alkoxide intermediate which will combine with a proton from the reaction between h+
[29]
and H O to produce H O and yield benzaldehyde.
The nanosheet structure and
2
2
2
surface F ions mainly contribute to the enhanced photoactivity of TM-S.
Figure 4. Proposed mechanism for the photocatalytic properties of TM-S.
4
. Conclusions
12

In summary, we presented a synthetic strategy toward TiO hierarchical
2
microspheres assembled from anatase TiO nanosheets. Through the evaluation of
2
photocatalytic performance, these TiO nanosheet spheres manifest a nearly twice of
2
conversion efficiency (44.9%) in benzyl alcohol selective oxidized to benzaldehyde
than counterpart (TM-S-A), relatively high ability of photocatalytic hydrogen
-1
-1
generation (45 mmol·h ·g ) and high stability. The improved photocatalytic ability
can be mainly attibuted to the charge separation promoted by the existence of surface
F and exposed {001} high-energy facets, enhancement of substrate molecular
transport profited from the hierarchical structure.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or
personal relationships that could have appeared to influence the work reported in this
paper.
Acknowledgments
This work supported by National Key R&D Program of China
(2017YFC1103800), PCSIRT (IRT_15R52), NSFC (U1663225, U1662134,
2
1711530705, 51503166, 51861135313), ISTCP (2015DFE52870), HPNSF
(2016CFA033), the Fundamental Research Funds for Central Universities
(WUT:2019III012GX), Jilin Province Science and Technology Development Plan
(20180101208JC). The authors thank the National Center for Magnetic Resonance in
Wuhan for Time-resolved PL.
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