Efficient Charge Separation from F– Selective Etching and Doping of

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 19633−19638

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Efficient Charge Separation from F− Selective 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*,†

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†

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 ‡ 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 S Supporting Information *

ABSTRACT: TiO2 nanomaterials with coexposed {001} and {101} facets have aroused much interest owing to their outstanding photocatalytic performance. In this study, on the basis of its unique characteristics of photoinduced electron and hole transfer to different lattice planes, we synthesized F− selective etching and doping on {001} facets of anatase TiO2 nanosheets using TiO2 nanosheets with coexposed {001} and {101} facets as a precursor. Through a series of measurements, such as photoluminescence, transient photocurrent response, electrochemical impedance spectra, and Mott−Schottky measurements, it is proved that F− selective etching and doping on {001} facets of TiO2 can extremely accelerate the separation of photogenerated carriers by shortening the transfer pathway of holes and introducing Ti3+ and oxygen vacancies in {001} facets. Therefore, the asobtained sample shows excellent photocatalytic properties under the visiblelight irradiation; the highest rate of photocatalytic H2 evolution is up to 18270 μ mol h−1 g−1 and its quantum efficiency is up to 21.6% at λ = 420 nm. As an innovative exploration, this study provides a direct spatial charge separation strategy for developing highly efficient photocatalysts. KEYWORDS: TiO2, photogenerated carrier, {001} facets, photocatalytic hydrogen production, visible light

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 biggest 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 and decorating with narrow band gap quantum dots.15−17 However, all these approaches facilitate electron transfer in TiO2-based materials to a certain degree, and 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. Currently, TiO2 nanomaterials with exposed {001} and {101} facets have sparked considerable attention in developing highly efficient photocatalysts owing to their outstanding photocatalytic activity. Many previous studies have demon© 2018 American Chemical Society

strated 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 fivecoordinated Ti, whereas the {001} facets expose 100% fivecoordinated 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 m−2).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 an experimental technique and theory calculation; the photoinduced electrons transfer to {101} facets, while photoinduced holes transfer to {001} facets.22−25 Additionally, on the basis of 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 carrier recombination.26−28 Furthermore, Received: February 16, 2018 Accepted: May 28, 2018 Published: May 28, 2018 19633

DOI: 10.1021/acsami.8b02804 ACS Appl. Mater. Interfaces 2018, 10, 19633−19638

Research Article

ACS Applied Materials & Interfaces a previous study also indicates that {101} facets show an extreme resistibility when etching TiO2 nanosheets, while {001} facets easily corrode.29 Consequently, once F− selective etching and doping on {001} facets of anatase TiO2 nanosheets with coexposed {001} and {101} facets has been carried out, the photoinduced hole transfer distance is markedly shortened, and Ti3+ and oxygen vacancies can be introduced in {001} facets, which can dramatically accelerate the transportation of holes. Also, the holes that reached the surface of TiO2 nanosheets can be efficiently scavenged by a sacrificial agent;24,30,31 the separation efficiency of photoinduced carriers is enormously enhanced, thereby improving the rate of photocatalytic H2 generation. In terms of the abovementioned theory, a mechanism of selective etching and doping on {001} facets of anatase TiO2 nanosheets for H2 production is proposed in Scheme 1. Scheme 1. Mechanism of F− Selective Etching and Doping on TiO2{001}

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

to a lower energy position after F− selective 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. This obvious shift comes from a charge imbalance induced by F− doping in the TiO2 nanosheets. Also, the characteristic peaks of the Ti3+ are not found in F−TiO2 nanosheets, indicating that there is no Ti3+ present on the surface of F−TiO2 nanosheets. Figure 1c shows the O 1s spectra of TiO2 and F−TiO2 nanosheets, and the peak around 529.3 eV is assigned to the O−Ti−O bond. After F− selective etching and doping, the peak shifts to a 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 paramagnetic resonance (EPR) is performed. As shown in Figure 1d, a prominent peak at g = 1.97 confirms the existence of Ti3+ in the bulk,36 which indicates that the doped F− converts Ti4+ partially to Ti3+. Also, the weak signal at g = 2.00 signifies oxygen vacancies produced as a result of charge compensation in the etching and doping process.36 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. 71-1166). From the TiO2 diffraction peaks, there is no obvious shift due to the closer radius of F− (0.133 nm) and O2− (0.132 nm).37 Notably, compared with other characteristic peaks, the intensity of the

2. EXPERIMENTAL SECTION 2.1. Synthesis of TiO2 Nanosheets. TiO2 nanosheets were synthesized according to the literature,32 except that the reaction temperature was increased to 200 °C. 2.2. Preparation of F−TiO2 Nanosheets. F−TiO2 nanosheets (F content are 3.0, 5.3, 6.9, and 8.2 at. %) were prepared by a simple hydrothermal method, and 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 intensely stirred for 0.5 h. After that, the mixture was added into a 40 mL Teflon autoclave and subsequently kept at 180 °C for 20 h. Finally, the obtained product was gathered by centrifuging at 14 000 rpm for 10 min and then washed with distilled water and ethanol three times dried under vacuum at 70 °C overnight.

3. RESULTS AND DISCUSSION 3.1. Structure and Composition of TiO2 and F−TiO2 Nanosheets. First, X-ray photoelectron spectroscopy (XPS) measurement was performed to analyze the chemical states and composition of TiO2 and F−TiO2 nanosheets. Figure 1a shows the F 1s spectra of F−TiO2 nanosheets; it can be clearly seen that two peaks are 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 are 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

Figure 2. (a) XRD patterns of TiO2 and F−TiO2 nanosheets and (b) Raman spectra of TiO2 and F−TiO2 nanosheets. 19634

DOI: 10.1021/acsami.8b02804 ACS Appl. Mater. Interfaces 2018, 10, 19633−19638

Research Article

ACS Applied Materials & Interfaces

corresponding to the (101) and (001) planes of anatase TiO2, respectively. As shown in Figures 3a, S5a, and S6, TiO2 nanosheets became rougher after F− selective etching and doping on {001} facets. To determine the variety of effective surface areas in the etching and doping process, N 2 adsorption−desorption isotherm analysis is 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− selective etching and doping, varying from 52.69 to 151.48 m2 g−1. The increased surface area is favorable to the photocatalytic reaction. More importantly, the DE3 nanosheets (Figure 3e) show a smaller thickness of 4.24 nm compared to that of TiO2 (4.73 nm), which can be ascribed to the effect of etching. Moreover, EDS mapping (Figures 3h−k and S8) also verifies the distribution of the elements in DE3. Clearly, Ti and O elements are distributed uniformly on the {101} and {001} facets in a typical F−TiO2 nanosheet, while the F element is mainly distributed on the {001} facets (Figure S8). Only tiny amounts of the F element distributed on the {101} facets come from adsorption. This further demonstrates the F− selective etching and doping on the {001} facets of TiO2. 3.2. Chargers’ Separation in the Samples. It is wellknown 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−

(004) diffraction peak shows an obvious lower change after selective etching and doping on {001} facets of TiO 2 nanosheets, as shown in Figure S3. This fact is consistent with the 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 also be observed that the peak at 154 cm−1 shifts to high frequency and broadens 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 etching and doping. As shown in Figure S4, the Zeta potential of the 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 in agreement with the results of XRD and Raman measurements. The scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) images of TiO2 and F−TiO2 nanosheets with different doping amounts of F− are shown in Figures 3, S5 and S6. These TiO2 and F−TiO2 nanosheets possess uniform rectangle morphology, with a length of ∼40 nm, a width of ∼35 nm, and a thickness of ∼4.73 nm (Figures S5 and 3a). HRTEM (Figure 3d,f)shows the lattice spacing parallel to lateral facets and the top is 3.52 and 2.35 Å,

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

TiO2 nanosheets excited at 320 nm. Clearly, the 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 was effectively quenched after F− selective etching and doping on {001} facets, especially in DE3. This confirms that F− selective etching and doping on {001} facets in TiO2 nanosheets realizes a highly efficient spacial separation of photogenerated carriers. However, when the F− content in TiO2 nanosheets increased to 8.2 at. % (DE4), the emission intensity increased because excessive F− triggered 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

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; and (h− k) TEM image of an individual DE3 nanosheet and its EDS elemental mapping. 19635

DOI: 10.1021/acsami.8b02804 ACS Appl. Mater. Interfaces 2018, 10, 19633−19638

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ACS Applied Materials & Interfaces

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

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 exhibit a faint absorption in the visible-light region. Compared with pure TiO2 nanosheets, the absorption spectra of DE1, DE2, DE3, and DE4 after F− selective etching and doping on {001} facets show an enhanced absorption in the visible-light region. In addition, 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 F− selective 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 On the basis of 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 F− selective etching and doping on {001} facets, the band gap of TiO2 is not changed. The improved absorption is because F− etching and doping affects 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 the defect state. On the basis of the analysis of UV−vis DRS and charger separation, it can be found that the visible-light absorption of DE4 is 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 shows the valence band (VB) spectrum of TiO2 and F−TiO2 nanosheets. Obviously, the VB top values of TiO2 and DE3 are 2.13 and 2.05 eV below the Fermi level (EF), respectively. After calibration by the reference Fermi level, the valence band maximum values of TiO2 and DE3 are 2.45 and 2.26 eV (vs NHE), respectively. Compared with the TiO2 precursor, DE3 presents an 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 shown in Figure 5c. The conduction band minimum of the TiO2 and DE3 nanosheets is more negative compared to the hydrogen

done under the illumination of visible light. As shown in Figure 4b, after F− selective etching and doping on {001} facets, the transient photocurrent density of DE3 improved up to 0.71 mA cm−2, which is 5 times higher than that of TiO2 (0.11 mA cm−2). This outstanding improvement further evidences that F− selective etching and doping on {001} facets of anatase TiO2 nanosheets can remarkably accelerate 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,d, owing to improved electron conductivity, the arc radii of all samples under light irradiation are smaller than those without light irradiation. What is more, the arc radii of DE3 both under light irradiation and without light irradiation are much smaller than those of TiO2, DE1, DE2, and 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) are in the order of TiO2 (−0.22 V) < ED1 (−0.18 V) < ED2 (−0.15 V) < ED3 (−0.11 V) < ED4 (−0.09 V). More importantly, a positive shift in F−TiO2 nanosheets proves the decrease in bending of the band edge and acceleration of the separation and transportation of photogenerated carriers.14 Furthermore, the plots of TiO2 and F−TiO2 nanosheets depict a positive slope, indicating that 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 abovementioned experimental results, it is affirmatively confirmed that F− selective etching and doping on {001} facets of anatase TiO2 nanosheets exceedingly enhances the separation of photogenerated carriers. 3.3. Photocatalytic Hydrogen Production Performance. The conduction band minimum and the valence band maximum of TiO2 and F−TiO2 nanosheets were ascertained by 19636

DOI: 10.1021/acsami.8b02804 ACS Appl. Mater. Interfaces 2018, 10, 19633−19638

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ACS Applied Materials & Interfaces

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.

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−f. As shown in Figure 5d,e, DE3 nanosheets show a high quantum efficiency (21.6%) and outstanding photocatalytic hydrogen production performance (18270 μ mol h−1 g−1) under visible-light irradiation, which is much higher than those of pure TiO2 nanosheets (1320 μ mol h−1 g−1, 5.1%). All the abovementioned results convincingly prove that TiO2 nanosheets with F− selective etching and doping on {001} facets have excellent photocatalytic property because of 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 have high catalytic activity and stability.

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4. CONCLUSIONS In summary, F− selective etching and doping on {001} facets of TiO2 nanosheets has 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− selective etching and doping on {001} facets of TiO2 greatly facilitates the separation of photoinduced carriers. Accordingly, its photocatalytic H2 production under the visiblelight illumination is exceedingly enhanced. This effective strategy has a significant impact on improving the spatial separation of photogenerated carriers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02804. Surface atomic structure of {101} and {001} facets; the structure of TiO2 nanosheets with exposed {001} facets; enlarged (004) diffraction peaks in the XRD patterns; zeta potential of TiO2 and F−TiO2 nanosheets; SEM and TEM images of the obtained TiO2; a typical nanosheet and its corresponding enlarged HRTEM image; SEM images of the samples; nitrogen adsorption−desorption isotherms; TEM image of typical DE3 nanosheets and their corresponding EDS element mappings; Mott− Schottky plots; Mott−Schottky plots of FTO glass; carrier density; optical images; and optical band gaps (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.C.). *E-mail: [email protected] (P.G.). *E-mail: [email protected] (P.Y.). ORCID

Dianxue Cao: 0000-0001-9138-7295 Piaoping Yang: 0000-0002-9555-1803 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Program for National Natural Science Foundation of China (51672056), Natural Science Foundation of Heilongjiang Province of China (LC2015004), the Pandeng 19637

DOI: 10.1021/acsami.8b02804 ACS Appl. Mater. Interfaces 2018, 10, 19633−19638

Research Article

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DOI: 10.1021/acsami.8b02804 ACS Appl. Mater. Interfaces 2018, 10, 19633−19638