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Surface energy-driven solution epitaxial growth of anatase TiO2 homostructures for overall water splitting
Journal Pre-proof Surface energy-driven solution epitaxial growth of anatase TiO2 homostructures for overall water splitting Shi Li, Luoyuan Ruan, Shanpeng Wang, Zhiyu Wang, Zhaohui Ren, Gaorong Han
PII:
S1005-0302(20)30123-7
DOI:
https://doi.org/10.1016/j.jmst.2020.01.038
Reference:
JMST 1972
To appear in:
Journal of Materials Science & Technology
Received Date:
29 November 2019
Revised Date:
1 January 2020
Accepted Date:
3 January 2020
Please cite this article as: { doi: https://doi.org/ This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier.
Letter Surface Energy-Driven Solution Epitaxial Growth of Anatase TiO2 Homostructures for Overall Water Splitting Shi Li
a, #,
Luoyuan Ruan
a, #,
Shanpeng Wang b, Zhiyu Wang a, Zhaohui Ren
a, b, *,
a
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Gaorong Han a State Key Laboratory of Silicon Materials, School of Materials Science and
Engineering, Cyrus Tang Center for Sensor Materials and Application, Zhejiang
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100,
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b
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University, Hangzhou 310027, China
Corresponding author.
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China
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E-mail address: [email protected].
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# Shi Li and Luoyuan Ruan contributed equally to this work.
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[Received 29 November 2019; Received in revised form 1 January 2020; Accepted 3 January 2020]
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Graphical Abstract
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Abstract
Titanium dioxide (TiO2) has been extensively investigated as a photocatalyst for
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water splitting to produce H2. However, an overall water splitting by using anatase
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TiO2 is extremely difficult due to the short lifetime of holes. In this work, we propose that a surface energy decrease from {001} to {101} of anatase TiO2 is able to drive an
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epitaxial growth. A novel anatase TiO2 homostructure has been successfully synthesized via a facile hydrothermal route, where {101} semi-pyramid nanoparticles
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epitaxially grew on the both sides of the {001} nanosheets. The epitaxial relationship between the nanoparticles and the nanosheets has been characterized to be
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{001}//{001} of anatase TiO2. For the first time, it is interesting to find that the homostructure with 12 wt% of {101} semi-pyramid can significantly improve the H2 evolution rate by nearly 5 times compared to the pure nanosheets under the ultraviolet irradiation. More importantly, such homostructure enables 10.78 mol•g-1•h-1 of O2 production whereas the pure nanosheets cannot evolve detectable O2 gas. Meanwhile,
the time-resolved photoluminescence analysis indicates that the mean lifetime of the holes is increased from 2.20 ns of the nanosheets to 3.59 ns of the homostructure, accounting for the observed overall water splitting. The findings suggest that constructing a homostructure by a surface energy strategy could be promising towards overall water splitting, which may be applicable to other photocatalytic materials.
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Keywords: {001}, {101}, homostructure, anatase TiO2, overall water splitting
1. Introduction
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Photocatalytic reactions on TiO2 powders have attracted much attention because of its
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nontoxicity, low-cost, environmental compatibility and suitable semiconductor characteristics, such as the photocatalysis of water and photocatalytic CO2 reduction.
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Nevertheless, anatase TiO2 performs poorly for the overall pure water splitting because only evolve H2 without simultaneous production of O2 [1]. It has been widely
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accepted that the lifetime of the photogenerated charge carriers is a decisive factor
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during the overall water splitting, especially the short-lived holes. The lifetime of holes within anatase TiO2 has been determined to be 2.3 ns while the electron lifetime
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is longer than 1 s under the conditions measured
[2].
In particular, producing one
molecule of gaseous oxygen require four holes during the artificial water oxidation [3], making photocatalytic O2 evolution a big challenging. In addition, the photogenerated oxygen may remain being adsorbed on the TiO2 surface and further reduction by the photoelectrons to form O2-, HO2•, H2O2 and etc
[4-5] .
It has been discussed that the
holes can be trapped at the active sites on the surface of anatase TiO2 to form •OH radicals which seems benefit for alcohol oxidation rather than water oxidation
[1,6-7].
Thus, the overall water splitting for anatase TiO2 is subjected to the O2 production from the water oxidation. As reported previously, anatase TiO2 exposes the most stable {101} planes to minimize the surface energy [8-9]. In contrast, {001} crystal plane has been determined
plane
[9-10].
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to have a larger specific surface energy of 0.90 J/m2 than 0.44 J/m2 for {101} crystal
It was discovered that an introduction of HF can stabilize {001} crystal [11].
Anatase TiO2 with
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planes of anatase TiO2 to achieve a exposure ratio of 47%
{001} exposed has demonstrated superior photocatalytic activity towards water [12-19].
Meanwhile, the exposed ratio of
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splitting and organic pollutant degradation
[16].
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{001} crystal facet at anatase TiO2 nanosheets can be further raised up to 98.7%
However, there remains some arguments on the relationship between the {001}
groups
[5,21-22].
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crystal plane exposed ratio and the whole photocatalytic activity among various An appropriate addition of {101} appears to be beneficial for the
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photocatalytic performance instead of simply pursuing high exposed ratio of {001} crystal facet
[15,17,23].
Theoretically, {001} and {101} crystal planes of anatase TiO2
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nanocrystals have a slight difference on band level which can selectively drive photogenerated electrons and holes to migrate into different crystal planes
[23-25].
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fascinating trapping behavior of electrons and holes is dependent on the crystal planes of anatase TiO2. For example, an excess electron is favorably trapped by the aqueous {101} interface while the {001} surface is attractive for holes
[22].
Within the same
single crystal, the {001}/{101} ratio is generally adjusted through controlling the exposed ratio of {001} planes
[23-24].
However, little is known about a practical
epitaxial growth and catalytic performance of a {001}-{101} homostructure. Herein, we developed an approach employing a difference in the specific surface energy to induce an epitaxial growth of the anatase TiO2 homostructures consisting of {001} and {101}, which is achieved by consuming the ratio of {101} to reduce the system
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energy. By using a facile hydrothermal method, {101} semi-pyramid nanoparticles have been characterized to epitaxially grow on the both sides of the {001} nanosheets
to form a novel anatase TiO2 homostructure. And the epitaxial relationship between
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the nanosheets and the nanoparticles has been characterized to be {001}//{001} of
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anatase TiO2. To explore the potential of photocatalytic overall water splitting without Pt loading, the homostructures were used under the ultraviolet irradiation compared to
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the pure nanosheets. The results indicate that the homostructure with 12 wt% of
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{101} semi-pyramid can evolve H2 with simultaneously producing O2 while the pure nanosheets only evolve H2 during the same overall water splitting process. By a
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time-resolved photoluminescence spectroscopy, it is inferred that an increase of mean lifetime of holes may contribute to the enhancement of the photocatalytic
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performance of the overall water splitting. 2. Material and Methods 2.1 Preparation of {001}-exposed TiO2 Nanosheets Anatase TiO2 nanosheets (NS) was prepared by a facile solvothermal method. Tetrabutyl titanate (Ti(OBu)4), hydrofluoric acid (HF), absolute ethanol, methanol,
aqueous ammonia, hydrochloroplatinic acid (H2PtCl6) and sodium borohydride (NaBH4) were used as received without further purification. Caution! HF is extremely corrosive and toxic and it should be handled with extreme care. Firstly, 10 g of Ti(OBu)4 and 5 mL of hydrofluoric acid (with a concentration of 40 wt %) were mixed in a dried Teflon-lined stainless steel autoclave with a capacity of 50 mL at room temperature. Subsequently, 10 mL of absolute ethanol was added into the
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above solution under magnetic stirring for 15 minutes, and then kept at 180℃for 16 hours. The obtained white products were washed thoroughly with 0.1 M NaOH
aqueous solution, deionized water and absolute ethanol for several times to remove
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the residual contamination and fluoride ions absorbed, respectively. After dried at 80℃ ,
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{001}-exposed anatase TiO2 NSs with F- ions removed were harvested.
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2.2 Preparation of Anatase TiO2 Homostructures
Anatase TiO2 homostructures were prepared by a facile secondary solvothermal
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method. At first, 25 mL of absolute ethanol was poured into a dried Teflon-lined stainless autoclave with a capacity of 50 mL. Secondly, 300 L, 400 L and 500 L
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(denoted as SH-300, SH-400 and SH-500, respectively) of Ti(OBu)4 were injected
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into the solvent then magnetic stirring for 15 minutes. Subsequently, 5 mL of aqueous ammonia was dropped into the mixture then stirring for 20 minutes. And 5 mL of deionized water was added in and stirred for another 20 minutes. At last, 1 g of F--removed TiO2 NSs were poured into the aqueous solution under stirring 2 hours. The reaction was carried out in a Teflon-lined autoclave at 200℃for 12 hours. The final products were washed completely with deionized water and absolute ethanol
respectively then dried at 80℃for 12 hours. Considering the statistical difficulties of the {101}-{001} ratio of the homostructure, here we assume that the used Ti(OBu)4 are basically converted to the anatase TiO2 nanoparticles based on the mass conservation. And we use the mass fraction between the anatase TiO2 nanoparticles converted by the used Ti(OBu)4 and the F--removed anatase TiO2 nanosheets to define the {101}-{001} ratio, which are 7 wt% (SH-300), 9 wt% (SH-400) , 12 wt%
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(SH-500), respectively. 2.3 Characterization
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X-ray diffraction analysis (XRD) were collected on a Thermo ARL X’TRA X-ray diffractometer with Cu K-ray radiation ( = 1.54056 Å). The microstructure and
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morphology of the materials were investigated by scanning electron microscopy
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(SEM, Hitachi SU-70), transmission electron microscopy and high-resolution transmission electron microscopy (TEM&HRTEM, FEI Tecnai G2 F20 S-TWIN).
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UV-vis diffuse reflectance spectra of the samples were recorded on SHIMADZU UV-2550. Fourier transform infrared spectrum (FTIR) was detected by NICOLET
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830. The photoluminescence spectra (PL) and time-resolved photoluminescence
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decay spectra were conducted on Edinburg FLS 920 using a 405 nm laser with a pulse width of 58.6 ps. The excitation intensity is ~ 4 nJ cm-2. The operated frequency was 200 kHz.
2.4 Photocatalytic performance of overall water splitting
Photocatalytic tests of overall water splitting using the synthesized samples was carried out in Labsolar-Ⅲ AG testing system. In a typical photocatalytic experiment, 30 mg of TiO2 powder was suspended in 100 mL of deionized water in a quartz reactor under vigorous magnetic stirring. Prior to the photocatalytic reaction, the quartz cell was deaerated by a vacuum pump for 20 minutes. The photocatalysts were irradiated with ultraviolet light (320 nmnm) using a 300 W Xe lamp. The temperature
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of the reaction was held at 5 °C by the cooling water circulating around the entire quartz cell. The hydrogen and oxygen gas were quantitatively determined by a gas
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chromatography (FULI 9790) online using Ar as carrying gas. 3. Results and Discussion
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To address our proposition, a scheme to illustrate the growth model of the anatase
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TiO2 homostructures is displayed as Figure 1a. As shown in the left side of Figure 1a, the anatase TiO2 would shape a sheet-like with a large {001} exposed while shape a [8].
An anatase TiO2 homostructure may be
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bipyramid with a large {101} exposed
formed by the {101} semi-pyramid epitaxially growing on the both sides of {001}
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sheet to minimize the surface energy. The lateral and top views of the homostructure
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are displayed in the right side of Figure 1a. Figure 1b shows the XRD patterns of the nanosheets (denoted as NS) and the homostructures. These XRD patterns can be indexed to the typical anatase phase of TiO2 (JCPDS No. 21-1272), belonging to the I41/amd space group [13,15]. The main diffraction peaks at 25.3°, 37.8°, 48.0° and 55.0° are ascribed to the (101), (004), (200), (211) planes of anatase TiO2, respectively. It can be concluded that the homostructures synthesized by the second hydrothermal
method can still retain pure anatase phase with a high crystallization. The morphology of the resultant samples was detected by SEM images (Figure 1c-f). As shown in Figure 1c, the NS shapes typical regular rectangle with a length range of 40~140 nm. After the hydrothermal process, no obvious morphological change of the NS can be observed according to Figure 1d-1f. Moreover, it is clear that some nanoparticles exist on the both sides of the NS with a diameter range of 7 – 20 nm.
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A futher investigation by using TEM was conducted to clarify the crystal facet of the
NS and the SH-500. From Figure 2a, these NSs are ~ 100 nm in length and ~6.5 nm in
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thickness. HRTEM (Figure 2c) image shows the lattice spacing of 0.238 nm,
confirming that the exposed facet is (004) crystal plane of anatase phase (JCPDS No.
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21-1272). Figure 2j and Figure 2k represent the typical vertical view and horizontal
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view of the SH-500, respectively. However, some {101}-exposed anatase TiO2 nanoparticles still had a minor fraction of {001} facets according to the Wulff
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construction shown in the Figure 2e, f, h, and i, which shaped inconsistently with the semi-pyramids described in the Figure 1a
[9].
HRTEM image (Figure 2l) of the
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SH-500 distinctly shows the lattice spacing of 0.352 nm, which matches well with that of the {101} crystal plane. An epitaxial relationship between the NS and the
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SH-500 has been characterized to be {001}//{001} of anatase TiO2. These results strongly support the epitaxy relationship between the NS and the {101}-exposed anatase TiO2 nanoparticles. As shown in Figure 2k, the {101}-exposed anatase TiO2 nanoparticles with a diameter of 20 nm grew up on the both sides of the NSs. And the geometric projections are visualized by the insets of Figure 2j and Figure 2k,
respectively. The {101}-exposed TiO2 nanoparticles projects a rectangle onto a horizontal plane while projects a triangle onto a perspective plane. Thus, the {101}-exposed TiO2 nanoparticles are semi-pyramid configuration spatially. Besides, the SH-300 and SH-400 display the similar microstructural characteristics as SH-500, shown at Figure. 2d-f and 2g-i, respectively. Furthermore, the optical absorption and bandgap of the samples were characterized by
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UV-vis spectroscopy. By Kubelka-Munk function, the bandgap of pure NS was fitted to be 3.3 eV, as shown in Figure 3a. The SH-300, SH-400 and SH-500 have a same [26].
The
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bandgap value of 3.4 eV, which are larger than that of bulk anatase (3.2 eV)
increase of the bandgap of the anatase TiO2 samples could be attributed to the [27-29].
An epitaxial growth of anatase TiO2 nanoparticles on the
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quantum size effect
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nanosheets cannot give rise to an obvious change in the shift of the bandgap. To clarify the surface of the samples synthesized, the FTIR spectra are carried out as
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presented in Figure 3b. All FTIR spectra display the similar pattern. The absorption band appear at above 3200 cm-1 and at 1627 cm-1 can be attributed to the stretching
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vibration mode and the bending mode of hydroxyl groups, respectively the absorption band at 450 cm-1 reveals the framework of Ti-O-Ti
[13,31].
[13,29-31].
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The NS and
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the homostructures exhibit a small peak at 1400 cm-1, which is ascribed to the organic group from impurities [32]. To investigate the photocatalytic ability of the homostructures, performance tests for overall water splitting were carried out under the ultraviolet radiation (320 nmnm). As indicated in Figure 4a and b, the pure NS display a H2 evolved
rate of 4.01 mol•g-1•h-1 without the O2 production. The H2 evolution rate of the NS weakly decrease over time. As for the homostructures, the H2 evolution rates are 6.10 mol • g-1 • h-1 (SH-300), 3.72 mol • g-1 • h-1 (SH-400) and 19.31 mol • g-1 • h-1 (SH-500), respectively. It is proved that the anatase TiO2 homostructures are more superior than the NSs on the photocatalysis. The H2 evolution rate of the SH-500 is enhanced by roughly 5 times more than the NS. Moreover, no O2 can be detected
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during the photocatalytic processes of the pure NSs, the SH-300 and the SH-400 as
shown in Figure 4b. For the SH-500, after 1 hour of induction period, O2 evolved
during the next 2 hours. The O2 evolution rate after three hours is 10.78 mol•g-1•h-1.
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To make the photoactivity in overall water splitting more convincing, chromatograph
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with the co-existence of hydrogen and oxygen are provided, shown in Figure S1 and Table S1 in the supporting information (denoted as SI) file. The O2 production is a
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four-hole consuming process and takes ~ 1 s by anatase TiO2, probably causing the O2 [3].
The sluggish
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evolution rate much slower than the H2 evolution rate as well
desorption of the produced O2 molecules from the photocatalyst may also lead to the
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appearance of induction period. And reducing O2 by electrons to form O2- in tens of microseconds again may cause the disappearance of O2 [3-4]. The color change from
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white to bluish is observed on the homostructures during the photocatalytic reaction. As a consequence, only a tiny fraction of holes can be effectively used to produce O2. Moreover, a complementary test using methanol as sacrificial agent also shows that the SH-500 has a higher H2 evolution rate than the NS (see Figure S2 at SI), demonstrating the homostructure has more superior photocatalytic performance over
the pure NS. The specific surface areas of NS and SH-500 were determined to be 121.6 m2/g and 105.8 m2/g, respectively (see Table S2 at SI). Thus, we suggest that an overall water splitting of the SH-500 is mainly due to the formation of the homostructures. To understand the observed photocatalytic performances, the time-resolved photoluminescence on the samples were collected and analyzed. Figure 4c shows PL
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spectra of the NS, SH-300, SH-400 and SH-500 in the wavelength range of 430-790
nm. The broad PL band peaking at 540 nm (2.29 eV) shows a large Stokes shift about
the self-trapping excitons
[26, 33-34].
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0.91 eV from the band gap energy (3.2 eV) due to the Frank-Condon effect, caused by
Although the SH-300 increases the emission
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intensity compared to the NS, a decrease in emission intensity on the SH-400 and
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SH-500 compared to the NS in Figure 4c. This indicates that an appropriate amount of introducing {101} crystal facets can significantly reduce the radiative recombination
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rate of photo-generated charge carriers in TiO2. Time-resolved PL was employed to investigate the dynamical behavior of the photogenerated charge carriers (Figure 4d).
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One should note that the PL decay curves can be used to discuss the relaxation dynamics of holes in anatase TiO2 single crystals under an extremely low excitation [2].
The experimental scatter dots can be fitted by a double exponential
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density
function (equation (1)) indicating by the colorful solid lines in Figure 4d. The mean lifetime of the holes can be obtained by the equation (2) as following, where I(t) is the PL intensity at time t. The measured parameters are listed in the Table 1. Although the lifetime of the shorter part is almost the same for all the samples, the longer part is
evidently different, especially the longer lifetime of the SH-500 is twice more than the NS. The calculated mean lifetime of the holes within these samples are 2.20 ns, 1.85 ns, 2.33 ns, 3.59 ns, respectively, which convincingly prove the elongated lifetime of the charge carriers can be achieved by the homostructure. (1)
𝜏 = (𝐴1 𝜏1 + 𝐴2 𝜏2 )/(𝐴1 + 𝐴2 )
(2)
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𝐼(𝑡) = 𝐴1 exp(− 𝑡⁄𝜏1 ) + 𝐴2 exp(− 𝑡⁄𝜏2 )
4. Conclusion
In conclusion, anatase TiO2 homostructures have been successfully prepared through
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a facile hydrothermal method, consisting of the {001} crystal facets and the {101}
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crystal facets through an interfacial epitaxy that could be driven by the surface energy difference. It was found for the first time that the SH-500 exhibits overall water
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splitting under ultraviolet radiation, where H2 and O2 production rate are 19.31 mol•
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g-1•h-1 and 10.78 mol•g-1•h-1, respectively. The average lifetime of holes for the SH-500 has been determined to be 3.59 ns, longer than that 2.20 ns of the NS,
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possibly accounting for the enhancement in the efficiency of photogenerated electron and hole separation. It is suggested that a photocatalytic homostructure via a surface
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energy-driven crystal facet epitaxy could be particularly attractive towards overall water splitting.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China [Nos. U1809217 and 51472218], State Key Laboratory of Crystal Materials (KF1807) and Fundamental Research Funds for the Central Universities
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[2019XZZX005-4-01].
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Figure List
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Figure 1. (a) Scheme of the growth model of anatase TiO2 homostructures. (b) XRD patterns of NS, SH-300, SH-400 and SH-500. (c-f) SEM images of NS (c), SH-300
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(d), SH-400 (e), SH-500 (f), respectively.
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Figure 2. (a-c) TEM and HRTEM images of the pure NS; (d-f) TEM and HRTEM images of the SH-300; (g-i) TEM and HRTEM images of the SH-400; (j-l) TEM and HRTEM images of the SH-500.
Figure 3. (a) Bandgap spectra transformed by Kubelka-Munk (KM) function. (b)
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FTIR spectra of the NS, SH-300, SH-400, SH-500, respectively.
Figure 4. (a) Comparison of the H2 evolution between the NS, SH-300, SH-400 and
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SH-500 from overall water splitting under ultraviolet light (320 nmnm). (b) Comparison of the O2 evolution between the NS, SH-300, SH-400 and SH-500 from overall water splitting under ultraviolet light (320 nmnm).(c) PL emission spectra of the NS, SH-300, SH-400, SH-500, respectively. (d) Time-resolved photoluminescence decay spectra of the emission peak at 540 nm of all samples.
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Table List Table 1. The lifetime results of the samples studied by time-resolved PL techniques. em (nm)
ex (nm)
1 (ns)
A1
2 (ns)
A2
avg (ns)
NS
540
405
1.37
0.72
5.04
0.21
2.20
SH-300
540
405
1.03
0.78
4.90
0.21
1.85
SH-400
540
405
1.55
0.70
7.80
0.10
2.33
SH-500
540
405
1.83
0.48
13.00
0.09
3.59
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Samples
PII:
S1005-0302(20)30123-7
DOI:
https://doi.org/10.1016/j.jmst.2020.01.038
Reference:
JMST 1972
To appear in:
Journal of Materials Science & Technology
Received Date:
29 November 2019
Revised Date:
1 January 2020
Accepted Date:
3 January 2020
Please cite this article as: { doi: https://doi.org/ This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier.
Letter Surface Energy-Driven Solution Epitaxial Growth of Anatase TiO2 Homostructures for Overall Water Splitting Shi Li
a, #,
Luoyuan Ruan
a, #,
Shanpeng Wang b, Zhiyu Wang a, Zhaohui Ren
a, b, *,
a
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Gaorong Han a State Key Laboratory of Silicon Materials, School of Materials Science and
Engineering, Cyrus Tang Center for Sensor Materials and Application, Zhejiang
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100,
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b
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University, Hangzhou 310027, China
Corresponding author.
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China
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E-mail address: [email protected].
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# Shi Li and Luoyuan Ruan contributed equally to this work.
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[Received 29 November 2019; Received in revised form 1 January 2020; Accepted 3 January 2020]
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Graphical Abstract
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Abstract
Titanium dioxide (TiO2) has been extensively investigated as a photocatalyst for
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water splitting to produce H2. However, an overall water splitting by using anatase
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TiO2 is extremely difficult due to the short lifetime of holes. In this work, we propose that a surface energy decrease from {001} to {101} of anatase TiO2 is able to drive an
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epitaxial growth. A novel anatase TiO2 homostructure has been successfully synthesized via a facile hydrothermal route, where {101} semi-pyramid nanoparticles
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epitaxially grew on the both sides of the {001} nanosheets. The epitaxial relationship between the nanoparticles and the nanosheets has been characterized to be
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{001}//{001} of anatase TiO2. For the first time, it is interesting to find that the homostructure with 12 wt% of {101} semi-pyramid can significantly improve the H2 evolution rate by nearly 5 times compared to the pure nanosheets under the ultraviolet irradiation. More importantly, such homostructure enables 10.78 mol•g-1•h-1 of O2 production whereas the pure nanosheets cannot evolve detectable O2 gas. Meanwhile,
the time-resolved photoluminescence analysis indicates that the mean lifetime of the holes is increased from 2.20 ns of the nanosheets to 3.59 ns of the homostructure, accounting for the observed overall water splitting. The findings suggest that constructing a homostructure by a surface energy strategy could be promising towards overall water splitting, which may be applicable to other photocatalytic materials.
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Keywords: {001}, {101}, homostructure, anatase TiO2, overall water splitting
1. Introduction
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Photocatalytic reactions on TiO2 powders have attracted much attention because of its
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nontoxicity, low-cost, environmental compatibility and suitable semiconductor characteristics, such as the photocatalysis of water and photocatalytic CO2 reduction.
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Nevertheless, anatase TiO2 performs poorly for the overall pure water splitting because only evolve H2 without simultaneous production of O2 [1]. It has been widely
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accepted that the lifetime of the photogenerated charge carriers is a decisive factor
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during the overall water splitting, especially the short-lived holes. The lifetime of holes within anatase TiO2 has been determined to be 2.3 ns while the electron lifetime
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is longer than 1 s under the conditions measured
[2].
In particular, producing one
molecule of gaseous oxygen require four holes during the artificial water oxidation [3], making photocatalytic O2 evolution a big challenging. In addition, the photogenerated oxygen may remain being adsorbed on the TiO2 surface and further reduction by the photoelectrons to form O2-, HO2•, H2O2 and etc
[4-5] .
It has been discussed that the
holes can be trapped at the active sites on the surface of anatase TiO2 to form •OH radicals which seems benefit for alcohol oxidation rather than water oxidation
[1,6-7].
Thus, the overall water splitting for anatase TiO2 is subjected to the O2 production from the water oxidation. As reported previously, anatase TiO2 exposes the most stable {101} planes to minimize the surface energy [8-9]. In contrast, {001} crystal plane has been determined
plane
[9-10].
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to have a larger specific surface energy of 0.90 J/m2 than 0.44 J/m2 for {101} crystal
It was discovered that an introduction of HF can stabilize {001} crystal [11].
Anatase TiO2 with
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planes of anatase TiO2 to achieve a exposure ratio of 47%
{001} exposed has demonstrated superior photocatalytic activity towards water [12-19].
Meanwhile, the exposed ratio of
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splitting and organic pollutant degradation
[16].
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{001} crystal facet at anatase TiO2 nanosheets can be further raised up to 98.7%
However, there remains some arguments on the relationship between the {001}
groups
[5,21-22].
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crystal plane exposed ratio and the whole photocatalytic activity among various An appropriate addition of {101} appears to be beneficial for the
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photocatalytic performance instead of simply pursuing high exposed ratio of {001} crystal facet
[15,17,23].
Theoretically, {001} and {101} crystal planes of anatase TiO2
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nanocrystals have a slight difference on band level which can selectively drive photogenerated electrons and holes to migrate into different crystal planes
[23-25].
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fascinating trapping behavior of electrons and holes is dependent on the crystal planes of anatase TiO2. For example, an excess electron is favorably trapped by the aqueous {101} interface while the {001} surface is attractive for holes
[22].
Within the same
single crystal, the {001}/{101} ratio is generally adjusted through controlling the exposed ratio of {001} planes
[23-24].
However, little is known about a practical
epitaxial growth and catalytic performance of a {001}-{101} homostructure. Herein, we developed an approach employing a difference in the specific surface energy to induce an epitaxial growth of the anatase TiO2 homostructures consisting of {001} and {101}, which is achieved by consuming the ratio of {101} to reduce the system
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energy. By using a facile hydrothermal method, {101} semi-pyramid nanoparticles have been characterized to epitaxially grow on the both sides of the {001} nanosheets
to form a novel anatase TiO2 homostructure. And the epitaxial relationship between
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the nanosheets and the nanoparticles has been characterized to be {001}//{001} of
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anatase TiO2. To explore the potential of photocatalytic overall water splitting without Pt loading, the homostructures were used under the ultraviolet irradiation compared to
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the pure nanosheets. The results indicate that the homostructure with 12 wt% of
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{101} semi-pyramid can evolve H2 with simultaneously producing O2 while the pure nanosheets only evolve H2 during the same overall water splitting process. By a
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time-resolved photoluminescence spectroscopy, it is inferred that an increase of mean lifetime of holes may contribute to the enhancement of the photocatalytic
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performance of the overall water splitting. 2. Material and Methods 2.1 Preparation of {001}-exposed TiO2 Nanosheets Anatase TiO2 nanosheets (NS) was prepared by a facile solvothermal method. Tetrabutyl titanate (Ti(OBu)4), hydrofluoric acid (HF), absolute ethanol, methanol,
aqueous ammonia, hydrochloroplatinic acid (H2PtCl6) and sodium borohydride (NaBH4) were used as received without further purification. Caution! HF is extremely corrosive and toxic and it should be handled with extreme care. Firstly, 10 g of Ti(OBu)4 and 5 mL of hydrofluoric acid (with a concentration of 40 wt %) were mixed in a dried Teflon-lined stainless steel autoclave with a capacity of 50 mL at room temperature. Subsequently, 10 mL of absolute ethanol was added into the
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above solution under magnetic stirring for 15 minutes, and then kept at 180℃for 16 hours. The obtained white products were washed thoroughly with 0.1 M NaOH
aqueous solution, deionized water and absolute ethanol for several times to remove
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the residual contamination and fluoride ions absorbed, respectively. After dried at 80℃ ,
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{001}-exposed anatase TiO2 NSs with F- ions removed were harvested.
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2.2 Preparation of Anatase TiO2 Homostructures
Anatase TiO2 homostructures were prepared by a facile secondary solvothermal
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method. At first, 25 mL of absolute ethanol was poured into a dried Teflon-lined stainless autoclave with a capacity of 50 mL. Secondly, 300 L, 400 L and 500 L
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(denoted as SH-300, SH-400 and SH-500, respectively) of Ti(OBu)4 were injected
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into the solvent then magnetic stirring for 15 minutes. Subsequently, 5 mL of aqueous ammonia was dropped into the mixture then stirring for 20 minutes. And 5 mL of deionized water was added in and stirred for another 20 minutes. At last, 1 g of F--removed TiO2 NSs were poured into the aqueous solution under stirring 2 hours. The reaction was carried out in a Teflon-lined autoclave at 200℃for 12 hours. The final products were washed completely with deionized water and absolute ethanol
respectively then dried at 80℃for 12 hours. Considering the statistical difficulties of the {101}-{001} ratio of the homostructure, here we assume that the used Ti(OBu)4 are basically converted to the anatase TiO2 nanoparticles based on the mass conservation. And we use the mass fraction between the anatase TiO2 nanoparticles converted by the used Ti(OBu)4 and the F--removed anatase TiO2 nanosheets to define the {101}-{001} ratio, which are 7 wt% (SH-300), 9 wt% (SH-400) , 12 wt%
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(SH-500), respectively. 2.3 Characterization
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X-ray diffraction analysis (XRD) were collected on a Thermo ARL X’TRA X-ray diffractometer with Cu K-ray radiation ( = 1.54056 Å). The microstructure and
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morphology of the materials were investigated by scanning electron microscopy
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(SEM, Hitachi SU-70), transmission electron microscopy and high-resolution transmission electron microscopy (TEM&HRTEM, FEI Tecnai G2 F20 S-TWIN).
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UV-vis diffuse reflectance spectra of the samples were recorded on SHIMADZU UV-2550. Fourier transform infrared spectrum (FTIR) was detected by NICOLET
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830. The photoluminescence spectra (PL) and time-resolved photoluminescence
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decay spectra were conducted on Edinburg FLS 920 using a 405 nm laser with a pulse width of 58.6 ps. The excitation intensity is ~ 4 nJ cm-2. The operated frequency was 200 kHz.
2.4 Photocatalytic performance of overall water splitting
Photocatalytic tests of overall water splitting using the synthesized samples was carried out in Labsolar-Ⅲ AG testing system. In a typical photocatalytic experiment, 30 mg of TiO2 powder was suspended in 100 mL of deionized water in a quartz reactor under vigorous magnetic stirring. Prior to the photocatalytic reaction, the quartz cell was deaerated by a vacuum pump for 20 minutes. The photocatalysts were irradiated with ultraviolet light (320 nmnm) using a 300 W Xe lamp. The temperature
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of the reaction was held at 5 °C by the cooling water circulating around the entire quartz cell. The hydrogen and oxygen gas were quantitatively determined by a gas
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chromatography (FULI 9790) online using Ar as carrying gas. 3. Results and Discussion
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To address our proposition, a scheme to illustrate the growth model of the anatase
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TiO2 homostructures is displayed as Figure 1a. As shown in the left side of Figure 1a, the anatase TiO2 would shape a sheet-like with a large {001} exposed while shape a [8].
An anatase TiO2 homostructure may be
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bipyramid with a large {101} exposed
formed by the {101} semi-pyramid epitaxially growing on the both sides of {001}
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sheet to minimize the surface energy. The lateral and top views of the homostructure
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are displayed in the right side of Figure 1a. Figure 1b shows the XRD patterns of the nanosheets (denoted as NS) and the homostructures. These XRD patterns can be indexed to the typical anatase phase of TiO2 (JCPDS No. 21-1272), belonging to the I41/amd space group [13,15]. The main diffraction peaks at 25.3°, 37.8°, 48.0° and 55.0° are ascribed to the (101), (004), (200), (211) planes of anatase TiO2, respectively. It can be concluded that the homostructures synthesized by the second hydrothermal
method can still retain pure anatase phase with a high crystallization. The morphology of the resultant samples was detected by SEM images (Figure 1c-f). As shown in Figure 1c, the NS shapes typical regular rectangle with a length range of 40~140 nm. After the hydrothermal process, no obvious morphological change of the NS can be observed according to Figure 1d-1f. Moreover, it is clear that some nanoparticles exist on the both sides of the NS with a diameter range of 7 – 20 nm.
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A futher investigation by using TEM was conducted to clarify the crystal facet of the
NS and the SH-500. From Figure 2a, these NSs are ~ 100 nm in length and ~6.5 nm in
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thickness. HRTEM (Figure 2c) image shows the lattice spacing of 0.238 nm,
confirming that the exposed facet is (004) crystal plane of anatase phase (JCPDS No.
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21-1272). Figure 2j and Figure 2k represent the typical vertical view and horizontal
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view of the SH-500, respectively. However, some {101}-exposed anatase TiO2 nanoparticles still had a minor fraction of {001} facets according to the Wulff
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construction shown in the Figure 2e, f, h, and i, which shaped inconsistently with the semi-pyramids described in the Figure 1a
[9].
HRTEM image (Figure 2l) of the
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SH-500 distinctly shows the lattice spacing of 0.352 nm, which matches well with that of the {101} crystal plane. An epitaxial relationship between the NS and the
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SH-500 has been characterized to be {001}//{001} of anatase TiO2. These results strongly support the epitaxy relationship between the NS and the {101}-exposed anatase TiO2 nanoparticles. As shown in Figure 2k, the {101}-exposed anatase TiO2 nanoparticles with a diameter of 20 nm grew up on the both sides of the NSs. And the geometric projections are visualized by the insets of Figure 2j and Figure 2k,
respectively. The {101}-exposed TiO2 nanoparticles projects a rectangle onto a horizontal plane while projects a triangle onto a perspective plane. Thus, the {101}-exposed TiO2 nanoparticles are semi-pyramid configuration spatially. Besides, the SH-300 and SH-400 display the similar microstructural characteristics as SH-500, shown at Figure. 2d-f and 2g-i, respectively. Furthermore, the optical absorption and bandgap of the samples were characterized by
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UV-vis spectroscopy. By Kubelka-Munk function, the bandgap of pure NS was fitted to be 3.3 eV, as shown in Figure 3a. The SH-300, SH-400 and SH-500 have a same [26].
The
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bandgap value of 3.4 eV, which are larger than that of bulk anatase (3.2 eV)
increase of the bandgap of the anatase TiO2 samples could be attributed to the [27-29].
An epitaxial growth of anatase TiO2 nanoparticles on the
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quantum size effect
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nanosheets cannot give rise to an obvious change in the shift of the bandgap. To clarify the surface of the samples synthesized, the FTIR spectra are carried out as
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presented in Figure 3b. All FTIR spectra display the similar pattern. The absorption band appear at above 3200 cm-1 and at 1627 cm-1 can be attributed to the stretching
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vibration mode and the bending mode of hydroxyl groups, respectively the absorption band at 450 cm-1 reveals the framework of Ti-O-Ti
[13,31].
[13,29-31].
And
The NS and
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the homostructures exhibit a small peak at 1400 cm-1, which is ascribed to the organic group from impurities [32]. To investigate the photocatalytic ability of the homostructures, performance tests for overall water splitting were carried out under the ultraviolet radiation (320 nmnm). As indicated in Figure 4a and b, the pure NS display a H2 evolved
rate of 4.01 mol•g-1•h-1 without the O2 production. The H2 evolution rate of the NS weakly decrease over time. As for the homostructures, the H2 evolution rates are 6.10 mol • g-1 • h-1 (SH-300), 3.72 mol • g-1 • h-1 (SH-400) and 19.31 mol • g-1 • h-1 (SH-500), respectively. It is proved that the anatase TiO2 homostructures are more superior than the NSs on the photocatalysis. The H2 evolution rate of the SH-500 is enhanced by roughly 5 times more than the NS. Moreover, no O2 can be detected
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during the photocatalytic processes of the pure NSs, the SH-300 and the SH-400 as
shown in Figure 4b. For the SH-500, after 1 hour of induction period, O2 evolved
during the next 2 hours. The O2 evolution rate after three hours is 10.78 mol•g-1•h-1.
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To make the photoactivity in overall water splitting more convincing, chromatograph
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with the co-existence of hydrogen and oxygen are provided, shown in Figure S1 and Table S1 in the supporting information (denoted as SI) file. The O2 production is a
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four-hole consuming process and takes ~ 1 s by anatase TiO2, probably causing the O2 [3].
The sluggish
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evolution rate much slower than the H2 evolution rate as well
desorption of the produced O2 molecules from the photocatalyst may also lead to the
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appearance of induction period. And reducing O2 by electrons to form O2- in tens of microseconds again may cause the disappearance of O2 [3-4]. The color change from
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white to bluish is observed on the homostructures during the photocatalytic reaction. As a consequence, only a tiny fraction of holes can be effectively used to produce O2. Moreover, a complementary test using methanol as sacrificial agent also shows that the SH-500 has a higher H2 evolution rate than the NS (see Figure S2 at SI), demonstrating the homostructure has more superior photocatalytic performance over
the pure NS. The specific surface areas of NS and SH-500 were determined to be 121.6 m2/g and 105.8 m2/g, respectively (see Table S2 at SI). Thus, we suggest that an overall water splitting of the SH-500 is mainly due to the formation of the homostructures. To understand the observed photocatalytic performances, the time-resolved photoluminescence on the samples were collected and analyzed. Figure 4c shows PL
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spectra of the NS, SH-300, SH-400 and SH-500 in the wavelength range of 430-790
nm. The broad PL band peaking at 540 nm (2.29 eV) shows a large Stokes shift about
the self-trapping excitons
[26, 33-34].
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0.91 eV from the band gap energy (3.2 eV) due to the Frank-Condon effect, caused by
Although the SH-300 increases the emission
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intensity compared to the NS, a decrease in emission intensity on the SH-400 and
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SH-500 compared to the NS in Figure 4c. This indicates that an appropriate amount of introducing {101} crystal facets can significantly reduce the radiative recombination
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rate of photo-generated charge carriers in TiO2. Time-resolved PL was employed to investigate the dynamical behavior of the photogenerated charge carriers (Figure 4d).
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One should note that the PL decay curves can be used to discuss the relaxation dynamics of holes in anatase TiO2 single crystals under an extremely low excitation [2].
The experimental scatter dots can be fitted by a double exponential
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density
function (equation (1)) indicating by the colorful solid lines in Figure 4d. The mean lifetime of the holes can be obtained by the equation (2) as following, where I(t) is the PL intensity at time t. The measured parameters are listed in the Table 1. Although the lifetime of the shorter part is almost the same for all the samples, the longer part is
evidently different, especially the longer lifetime of the SH-500 is twice more than the NS. The calculated mean lifetime of the holes within these samples are 2.20 ns, 1.85 ns, 2.33 ns, 3.59 ns, respectively, which convincingly prove the elongated lifetime of the charge carriers can be achieved by the homostructure. (1)
𝜏 = (𝐴1 𝜏1 + 𝐴2 𝜏2 )/(𝐴1 + 𝐴2 )
(2)
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𝐼(𝑡) = 𝐴1 exp(− 𝑡⁄𝜏1 ) + 𝐴2 exp(− 𝑡⁄𝜏2 )
4. Conclusion
In conclusion, anatase TiO2 homostructures have been successfully prepared through
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a facile hydrothermal method, consisting of the {001} crystal facets and the {101}
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crystal facets through an interfacial epitaxy that could be driven by the surface energy difference. It was found for the first time that the SH-500 exhibits overall water
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splitting under ultraviolet radiation, where H2 and O2 production rate are 19.31 mol•
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g-1•h-1 and 10.78 mol•g-1•h-1, respectively. The average lifetime of holes for the SH-500 has been determined to be 3.59 ns, longer than that 2.20 ns of the NS,
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possibly accounting for the enhancement in the efficiency of photogenerated electron and hole separation. It is suggested that a photocatalytic homostructure via a surface
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energy-driven crystal facet epitaxy could be particularly attractive towards overall water splitting.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China [Nos. U1809217 and 51472218], State Key Laboratory of Crystal Materials (KF1807) and Fundamental Research Funds for the Central Universities
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[2019XZZX005-4-01].
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Figure List
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Figure 1. (a) Scheme of the growth model of anatase TiO2 homostructures. (b) XRD patterns of NS, SH-300, SH-400 and SH-500. (c-f) SEM images of NS (c), SH-300
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(d), SH-400 (e), SH-500 (f), respectively.
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Figure 2. (a-c) TEM and HRTEM images of the pure NS; (d-f) TEM and HRTEM images of the SH-300; (g-i) TEM and HRTEM images of the SH-400; (j-l) TEM and HRTEM images of the SH-500.
Figure 3. (a) Bandgap spectra transformed by Kubelka-Munk (KM) function. (b)
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FTIR spectra of the NS, SH-300, SH-400, SH-500, respectively.
Figure 4. (a) Comparison of the H2 evolution between the NS, SH-300, SH-400 and
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SH-500 from overall water splitting under ultraviolet light (320 nmnm). (b) Comparison of the O2 evolution between the NS, SH-300, SH-400 and SH-500 from overall water splitting under ultraviolet light (320 nmnm).(c) PL emission spectra of the NS, SH-300, SH-400, SH-500, respectively. (d) Time-resolved photoluminescence decay spectra of the emission peak at 540 nm of all samples.
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Table List Table 1. The lifetime results of the samples studied by time-resolved PL techniques. em (nm)
ex (nm)
1 (ns)
A1
2 (ns)
A2
avg (ns)
NS
540
405
1.37
0.72
5.04
0.21
2.20
SH-300
540
405
1.03
0.78
4.90
0.21
1.85
SH-400
540
405
1.55
0.70
7.80
0.10
2.33
SH-500
540
405
1.83
0.48
13.00
0.09
3.59
Jo
ur
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lP
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Samples