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Efficient Charge Separation from F- Selectively Etching and Doping of Anatase-TiO2{001} for Enhanced Photocatalytic Hydrogen Production Yurong Yang, Ke Ye, Dianxue Cao, Peng Gao, Min Qiu, Li Liu, and Piaoping Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02804 • Publication Date (Web): 28 May 2018 Downloaded from http://pubs.acs.org on May 28, 2018

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ACS Applied Materials & Interfaces

Efficient Charge Separation from F- Selectively Etching and Doping of Anatase-TiO2{001} for Enhanced Photocatalytic Hydrogen Production Yurong Yang†,‡, Ke Ye†, Dianxue Cao*,†, Peng Gao*,†,§, Min Qiu‡, Li Liu‡, and Piaoping Yang*,† †

Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education,

College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin, Heilongjiang, 150001 (P. R. China), e-mail: [email protected]; [email protected]; [email protected]; ‡

College of Science, Heihe University, Heihe, Heilongjiang 164300, (P. R. China)

§

College of Materials Science and Chemical Engineering, Hangzhou Normal University,

Hangzhou, Zhejiang, 310026 (P. R. China), e-mail: [email protected]. KEYWORDS: TiO2, photogenerated carrier, {001} facets, photocatalytic hydrogen production, visible light

ABSTRACT: TiO2 nanomaterials with coexposed {001} and {101} facets have aroused much interest owing to its outstanding photocatalytic performance. In this study, based on its unique characteristics of photoinduced electrons and holes transfer to different lattice planes, we synthesized F- selectively etching and doping on {001} facets of anatase TiO2 nanosheets using

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TiO2 nanosheets with coexposed {001} and {101} facets as a precursor. Through a series of measurements, such as PL, transient photocurrent response, EIS and Mott-Schottky measurements, it is proved that F- selectively etching and doping on {001} facets of TiO2 can extremely accelerates the separation of photogenerated carriers by shorten the transfer pathway of holes, introduce Ti3+ and oxygen vacancies in {001} facets. Therefore, the as-obtained sample shows excellent photocatalytic properties under the visible-light irradiation, the highest rate of photocatalytic H2 evolution is up to 18270 µmol h-1 g-1 and it’s quantum efficiency is up to 21.6% at λ = 420 nm. As an innovative exploration, this study provides a directed spatial charge separation strategy for developing highly efficient photocatalysts.

1. INTRODUCTION TiO2, as a perspective material, plays an important role in photocatalysis, gas sensors, biomaterials and solar cells.1-3 However, one of the most impediments is that the recombination rate of photoinduced electrons and holes in TiO2 is extremely high (90% in 10 ns), which limits its feasible application of photoinduced carriers for chemical reactions.4-5 So far, a lot of strategies have been proposed to enhance the separation efficiency of photoinduced electrons and holes in TiO2, such as depositing with noble metals,6-8 constructing composites with other semiconductors,9-11 coupling with carbon materials,12-14 decorating with narrow band gap quantum dots.15-17 However, all the these approaches facilitates electron transfer in TiO2 based materials to a certain degree, numerous photoinduced electrons and holes have been recombined in the charge transportation process as a result of its random flow. Consequently, the highly efficient separation of photoinduced carriers in TiO2 based materials is still a challenge.


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Currently, TiO2 nanomaterials with exposed {001} and {101} facets have sparked considerable attention in developing highly efficient photocatalysts owing to its outstanding photocatalytic activity. Many previous studies have demonstrated that {001} facets can provide more active sites compared to {101} facets.18-21 This is mainly because {001} facets can provide additional active sites. As shown in Figure S1, the {101} facets expose both six-coordinated Ti and five-coordinated Ti, while the {001} facets expose 100% five-coordinated Ti, which leads to a more stronger activity in the photocatalytic reaction.22,23 In addition, the higher surface energies of {001} facets (0.90 J m-2) is also the reason for its high photocatalytic activity compared to that of {101} facets (0.44 J m2 18-21

).

Importantly, it is worth noting that many studies found that photoinduced electrons

and holes in TiO2 with coexposed {001} and {101} facets transfer to different crystal facets by experimental technique and theory calculation, the photoinduced electrons transfer to {101} facets, while photoinduced holes transfer to {001} facets.22-25 Additionally, based on the previous study, F- doped TiO2 nanomaterials can induce the formation of reduced Ti3+ and oxygen vacancies, extend optical absorption and reduce the rate of photoinduced carriers recombination.26-28 Furthermore, previous study also indicates that {101} facets shows an extreme resistibility when etching TiO2 nanosheets, while {001} facets are easy to corrosion.29 Consequently, once F- selectively etching and doping on {001} facets of anatase TiO2 nanosheets with coexposed {001} and {101} facets, the photoinduced holes transfer distance is markedly shortened, and Ti3+ and oxygen vacancies can be introduced in {001} facets, which can dramatically accelerate the transportation of holes. And the holes reached the surface of TiO2 nanosheets can be efficiently scavenged by sacrifice agent,24,30,31 the separation efficiency of photoinduced


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carriers are enormous enhanced, thereby the rate of photocatalytic H2 generation is improved. In terms of the above theory, a mechanism of selectively etching and doping on {001} facets of anatase TiO2 nanosheets for H2 production is proposed in Scheme 1.

Scheme 1. The mechanism of F- selectively etching and doping on TiO2{001}.

2. EXPERIMENTAL SECTION 2.1 Synthesis of TiO2 nanosheets. TiO2 nanosheets were synthesized according to the literature32 except that the reaction temperature was increased to 200 oC. 2.2 Preparation of F-TiO2 nanosheets. F-TiO2 nanosheets (F content are 3.0 at.%, 5.3 at.%, 6.9 at.%, 8.2 at.%) were prepared by a simple hydrothermal method, the obtained products were named as DE1, DE2, DE3 and DE4, respectively. In a typical procedure, TiO2 nanosheets were distributed in a HF aqueous solution (The molar ratios of TiO2 to HF were 1:0.5, 1:1, 1:1.5 and 1:2, respectively), and intense stired for 0.5 h. After that the mixture was added into a 40 mL Teflon autoclave and subsequently kept at 180 oC for 20 h. Finally, the obtained product was gathered by centrifuging at 14000 rpm for 10 min and then washed with distilled water and ethanol for three times, dried under vacuum at 70 °C overnight.

3. RESULTS AND DISCUSSION


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3.1 Structure and composition of TiO2 and F-TiO2 nanosheets. XPS measurement has been performed to analyze the chemical states and composition of TiO2 and F-TiO2 nanosheets firstly. Figure 1a shows the F 1s spectra of F-TiO2 nanosheets, it is clearly to see that two peaks located at 684.3 and 688.6 eV, which can be assigned to ≡Ti-F as a result of O substitution on the surface of TiO2 nanosheets and the substitution of the oxygen in TiO2 lattice,33,34 respectively. Figure 1b shows the Ti 2p spectra of TiO2 and F-TiO2 nanosheets, the two peaks of Ti 2p3/2 and Ti 2p1/2 in TiO2 nanosheets located at 458.6 and 464.4 eV respectively, indicating the Ti4+ in the crystal structure.14 Notably, it is obvious that the characteristic peaks of Ti4+ shift to a lower energy position after F- selectively etching and doping on TiO2{001}. Especially, two peaks in the Ti 2p spectra of DE4 nanosheets shifted to 457.9 and 463.7 eV, respectively. This obvious shift comes from a charge imbalance induced by F- doped in the TiO2 nanosheets. And it isn’t found the characteristic peaks of the Ti3+ in F-TiO2 nanosheets, indicating no Ti3+ present on the surface of F-TiO2 nanosheets. Figure 1c shows the O 1s spectra of TiO2 and F-TiO2 nanosheets, the peak around 529.3 eV is assigned to O-Ti-O bond. After F- selectively etched and doped, the peak shifts to lower energy position continuously, which indicates the presence of oxygen vacancies (Ov) in F-TiO2 nanosheets.33,35 To further determine the presence of Ti3+ and oxygen vacancies, electron para-magnetic resonance (EPR) are performed. As shown in Figure 1d, a prominent peak at g=1.97 confirms the existence of Ti3+ in the bulk,36 which indicates the doped F- convert partially Ti4+ to Ti3+. And the weak signal at g=2.00 signifies oxygen vacancies produced as a result of charge compensation in the etching and doping process.36


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Figure 1. XPS of TiO2 and F-TiO2 nanosheets and EPR spectrum of DE3 nanosheets. (a) F 1s spectra; (b) Ti 2p spectra; (c)O 1 s spectra; (d) EPR spectrum of DE3 nanosheets.

X-ray diffraction (XRD) measurements were performed to further examine the crystal structure of TiO2 and F-TiO2 nanosheets. As shown in Figure 2a, the main diffraction peaks displayed in TiO2 and F-TiO2 nanosheets are well indexed to the pure anatase phased TiO2 (JCPDS No. 711166). From the TiO2 diffraction peaks, it isn’t found an obvious shift due to the closer radius of F- (0.133 nm) and O2- (0.132 nm).37 Notably, compared with other characteristics peaks, the intensity of (004) diffraction peak shows a obvious lower change after selectively etching and doping on {001} facets of TiO2 nanosheets, as shown in Figure S3. This fact is consistent with prior study.26 Moreover, all the peaks in the Raman spectra (Figure 2b) of the samples are ascribed to anatase TiO2.38 In addition, it can be also observed the peak at 154 cm-1 shift to high frequency and broaden gradually, which implies the existence of Ti3+ and oxygen vacancies.39,40 It also indicates that the amount of Ti3+ and oxygen vacancies increases gradually with the increasing content of F- in TiO2 nanosheets. Zeta potentials were also applied to test the effects of


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etching and doping. As shown in Figure S4, the Zeta potential of TiO2 precursor is 0.03 mV. However, after F- etching and doping, the Zeta potential decreased greatly, varying from -1.19 to -4.64 mV. The significantly decreased potential is mainly due to the effect of etching and doping, which is agree with the results of XRD and Raman measurements.

Figure 2. (a) XRD patterns of TiO2 and F-TiO2 nanosheets; (b) Raman spectra of TiO2 and FTiO2 nanosheets.

The SEM, TEM and HRTEM images of TiO2 and F-TiO2 nanosheets with different doping amounts of F- are shown in Figure 3, S5 and S6. These TiO2 and F-TiO2 nanosheets possess uniform rectangle morphology, with length of ~40 nm, width of ~35 nm and thickness of ~ 4.73 nm (Figure S5, 3a). HRTEM (Figure 3d, f)shows the lattice spacing parallel to lateral facets and the top is 3.52 and 2.35 Å, corresponding to the (101) and (001) plane of anatase TiO2, respectively. As shown in Figure 3a, S5a and S6, TiO2 nanosheets became more rough after Fselectively etched and doped on {001} facets. In order to determine the variety of effective surface area in the etching and doping process, N2 adsorption-desorption isotherms analysis are carried out. As shown in Figure S7, clearly, the TiO2 precursor shows a small surface area of 31.66 m2 g-1, and the specific surface areas of F-TiO2 nanosheets increase significantly after F-


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selectively etched and doped, varying from 52.69 m2 g-1 to 151.48 m2 g-1. The increased surface area are favorable to the photocatalytic reaction. More importantly, the DE3 nanosheets (Figure 3e) shows a smaller thickness of 4.24 nm compared to that of TiO2 (4.73 nm), which can be ascribed to the effect of etch. Moreover, EDS mapping (Figure 3h-k, S8) also verify the distribution of the elements in DE3. Clearly, Ti, O elements are distributed uniformly on the {101} and {001} facets in a typical F-TiO2 nanosheet, while F element is mainly distributed on the {001} facets (Figure S8). Only tiny amounts F element distributed on the {101} facets comes from adsorption. This further demonstrates that F- selectively etching and doping on the {001} facets of TiO2.


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Figure 3. (a) SEM image of DE3 nanosheets; (b) TEM image of DE3 nanosheets; (c)-(f) HRTEM images of an individual nanosheet and its enlarged HRTEM image; (g) HRTEM images of nanosheets; (h)-(k) TEM image of a individual DE3 nanosheet and its EDS elemental mapping. 3.2 Chargers’ separation in the samples. It is well-known that photoluminescence (PL) techniques were applied to examine the behaviors of charge separation in photocatalysts.41,42 Figure 4a presents the PL spectra of TiO2 and F-TiO2 nanosheets excited at 320 nm. Clearly, the


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PL spectrum of TiO2 exhibits a greatly strong signal, indicating a higher recombination efficiency of photogenerated carriers.42 Moreover, the emission intensity of F-TiO2 nanosheets effectively quenched after F- selectively etching and doping on {001} facet, especially in DE3. This confirms that F- selectively etching and doping on {001} facet in TiO2 nanosheets realizes a highly efficient spacial separation of photogenerated carriers. But when the F- content in TiO2 nanosheets increased to 8.2 at.% (DE4), the emission intensity increased because excessive Ftriggered the introduction of more defects as the recombination centers of carriers. To further ascertain the separation efficiency of photoinduced carriers, the photocurrent response measurements of TiO2 and F-TiO2 nanosheets are employed under the illumination of visiblelight. As shown in Figure 4b, after F- selectively etching and doping on {001} facet, the transient photocurrent density of DE3 improved up to 0.71 mA cm-2, is 5 times higher than that of TiO2 (0.11 mA cm-2). This outstanding improvement further evidences that F- selectively etching and doping on {001} facets of anatase TiO2 nanosheets can remarkably accelerates the separation of photogenerated carriers. Then the photocurrent densities of DE4 (0.46 mA cm-2) decreased compared to that of DE3, which is in agreement with PL measurement results. To gain deep insights of the charge separation, electrochemical impedance spectra (EIS) measurement is performed. As depicted in Figure 4c and d, owning to improved electron conductivity, the arc radii of all samples under light irradiation are smaller than those of without light irradiation. What’s more, the arc radii of DE3 both under light irradiation and without light irradiation are much smaller than those of TiO2, DE1, DE2, DE4, indicating a more effective separation of photogenerated carriers in DE3. This finding was further confirmed by Mott-Schottky tests. The Mott-Schottky plots of TiO2 and F-TiO2 nanosheets are shown in Figure S9, their flat band potentials (vs NHE) turn out to be in the order of TiO2 (-0.22 V) < ED1 (-0.18 V) < ED2 (-0.15


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V) < ED3 (-0.11 V) < ED4 (-0.09 V). More importantly, a positive shift in F-TiO2 nanosheets proves decrease in bending of the band edge, and accelerating the separation and transportation of photogenerated carriers.14 Furthermore, the plots of TiO2 and F-TiO2 nanosheets depict a positive slope, indicating they are all n-type semiconductors. Moreover, in terms of the slope of Mott-Schottky plots, we calculated the carrier densities of the samples.43 The calculated carrier densities (Figure S11) of DE3 is 12.08×1018 cm-3, which is remarkable higher than that of pure TiO2 (3.42×1018 cm-3). The higher carrier density implying the carrier separation and transport is faster. In terms of the above experimental results, it is affirmatively confirmed that F- selectively etching and doping on {001} facets of anatase TiO2 nanosheets exceedingly enhances the separation of photogenerated carriers.

Figure 4. (a) PL spectra of TiO2 and F-TiO2 nanosheets; (b) Photocurrent response of TiO2 and F-TiO2 nanosheets; (c) and (d) EIS Nyquist plots of TiO2 and F-TiO2 nanosheets in the dark and under irradiation.


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3.3 Photocatalytic hydrogen production performance. The conduction band minimum and valence band maximum of TiO2 and F-TiO2 nanosheets were ascertained by combining UV-vis DRS with XPS measurements. From the UV-vis DRS of TiO2 and F-TiO2 nanosheets (Figure 5a), it is found that TiO2 nanosheets exhibits a faint absorption in the visible-light region. Compared with pure TiO2 nanosheets, the absorption spectra of DE1, DE2, DE3 and DE4 after F- selectively etching and doping on {001} facets show a enhanced absorption in the visible-light region. And with the content of F- increasing, the absorption intensities are obviously enhanced. The color change of the samples (Figure S12) also proves their enhanced absorption in the visible-light region. As a result of Fselectively etching and doping on {001} facets, Ti3+ and Ov defects are introduced in TiO2, which leads to significantly enhanced absorption with a red shift in the visible light region.44-48 Based on the Kubelka-Munk transformation of UV-vis DRS (Figure S13), the band gap of TiO2, DE1, DE2, DE3 and DE4 are all characterized as 3.08 eV. After Fselectively etching and doping on {001} facets, the band gap of TiO2 isn’t changed. The improved absorption is because F- etching and doping affect the surface of TiO2 nanosheets forming more defects or active sites, which may be responsible for the enhanced photocatalytic activity due to the enhanced charge separation by defect state. Based on the analysis of UV-vis DRS and charger separation, it can be found that the visible-light absorption of DE4 is the most strongest. However, the separation and transportation efficiency of photogenerated carriers in DE4 is lower than that of DE3. This fact indicates that excessive F- etching and doping will introduce more defects as the recombination of photogenerated carriers. Figure 5b is the valence band (VB) spectrum of TiO2 and F-TiO2 nanosheets. Obviously, the VB top of TiO2 and DE3 are 2.13 and 2.05


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eV below the Fermi level (EF), respectively. After calibrating by reference Fermi level, the valence band maximum of TiO2 and DE3 are 2.45 eV and 2.26 eV (vs NHE), respectively. Compared with TiO2 precursor, DE3 presents a upward shift of 0.19 eV in the valence band maximum and the conduction band minimum, which derived from the existence of Ti3+ and Ov. In terms of the aforementioned experimental results, the energy band structure of TiO2 and DE3 nanosheets are drawn in Figure 5c. The conduction band minimum of the TiO2 and DE3 nanosheets are more negative compared to the hydrogen production potential (Figure 3c), satisfying the thermodynamic requirements for hydrogen production under solar illumination. The quantum efficiency at λ=420 nm and photocatalytic hydrogen production performance of the samples are shown in Figure 5d, e and f. As shown in Figure 5d and e, DE3 nanosheets shows a high quantum efficiency (21.6%) and outstanding photocatalytic hydrogen production performance (18270 µmol h1

g-1) under visible-light irradiation, which is much higher than those of pure TiO2

nanosheets (1320 µmol h-1 g-1, 5.1%). All the above results convincingly prove that TiO2 nanosheets with F- selectively etching and doping on {001} facets have excellent photocatalytic property due to the rapid separation of photoinduced carriers. Furthermore, the high H2 evolution rate of DE3 nanosheets remains unchanged in a 40 h cyclic reaction, as shown in 5f. As a result, DE3 nanosheets has high catalytic activity and stability.


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Figure 5. (a) Optical absorption of TiO2 and F-TiO2 nanosheets; (b) Valence-band spectra of TiO2 and F-TiO2 nanosheets; (c) The energy band structure of TiO2 and DE3 nanosheets; (d) The quantum efficiency of photocatalytic H2 generation; (e) H2 generation rate of TiO2 and F-TiO2 nanosheets; (f) Photocatalytic H2 production of TiO2 and DE3 nanosheets in cycle reaction (10cycles, 4 h/cycle).

4. CONCLUSION In summary, F- selectively etching and doping on {001} facets of TiO2 nanosheets have been synthesized using TiO2 nanosheets with coexposed {001} and {101} facets as the template. Through a range of elaborate tests and analyses, it is affirmed that F- selectively etching and doping on {001} facets of TiO2 greatly facilitate the separation of photoinduced carriers. Accordingly, its photocatalytic H2 production under the visible-light illumination is exceedingly enhanced. This effective strategy have an significant impact on improving the spatial separation of photogenerated carriers. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:


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(1) The surface atomic structure of {101} and {001} facets (b); (2) The structure of TiO2 nanosheets with exposed {001} facets; (3) Enlarged (004) diffraction peaks in the XRD patterns; (4) Zeta potential of TiO2 and F-TiO2 nanosheets; (5) SEM and TEM images of the obtained TiO2 (a, b); A typical nanosheet and its corresponding enlarged HRTEM image (c, d, e, f); (6) SEM images of the samples; (7) Nitrogen adsorption-desorption isotherms; (8) TEM image of typical DE3 nanosheets and their corresponding EDS element mappings; (9) Mott-Schottky plots; (10) Mott-Schottky plots of FTO glass; (11) Carrier density; (12) Optical images; (13) Optical band gaps. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank the Program for National Natural Science Foundation of China (51672056), Natural Science Foundation of Heilongjiang Province of China (LC2015004), the Pandeng Plan Foundation of Hangzhou Normal University for youth scholars of materials, Natural Science Foundation of Heilongjiang Province (B201603 and E2017058), Nature Science Foundation of Zhejiang Provincial (LY18E020010), Heilongjiang Province Department of Education (18KYYWFCXY06) and Heihe University (KJY201702) for the financial support of this research.


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Selectively Etching and Doping of Anatase-TiO2 ... - ACS Publications

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