Ti4+ Redox Shuttle Enhancing Photocatalytic H2

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Surface Ti / Ti Redox Shuttle Enhancing Photocatalytic H Production in Ultrathin TiO Nanosheets/ CdSe Quantum Dots 2

Yunfang Ji, Wei Guo, Huihui Chen, Longshuai Zhang, Song Chen, Mutian Hua, Yuhan Long, and Zhuo Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09055 • Publication Date (Web): 13 Nov 2015 Downloaded from http://pubs.acs.org on November 14, 2015

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The Journal of Physical Chemistry

Surface Ti3+/ Ti4+ Redox Shuttle Enhancing Photocatalytic H2 Production in Ultrathin TiO2 Nanosheets/ CdSe Quantum Dots Yunfang Ji1, Wei Guo2, Huihui Chen1, Longshuai Zhang1, Song Chen1, Mutian Hua1, Yuhan Long1 and Zhuo Chen*1 1

Department of Materials Physics and Chemistry, School of Materials Science and Engineering,

Beijing Institute of Technology, Beijing 100081, People’s Republic of China 2

Department of Physics, Beijing Institute of Technology, Beijing 100081, People’s Republic of

China *Email: [email protected] Tel:+861068913469 KEYWORDS Photocatalytic, Surface defect, Titanium oxide nanosheet, Hydrogen generation, Synergistic effect

ABSTRACT Constructing unique nanocomposites to facilitate charge transfer and promote catalytic activity provides a promising approach for enhancing solar-to-hydrogen efficiency. However, revealing the catalytic surface state and understanding their roles in the photocatalytic process remain elusive. Here, we develop a facile and mild solution method for ultrathin TiO2

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nanosheets with a large specific surface area and enhanced surface Ti3+ accumulation. Moreover, we demonstrate a hybrid nanosystem composed of ultrathin 2D TiO2 nanosheets directly coupled with 0D CdSe QDs for hydrogen generation. The 0D CdSe QDs increases visible light absorption ability after coupled with the TiO2 nanosheets. Owing to the synergistic effect between the quantum dots and nanosheets, the apparent quantum efficiency increases to 45% at 380 and 450 nm and the H2 evolution exhibits a ten-fold enhancement. EPR results indicate that the surface Ti3+ greatly promotes H free radical production due to the redox couple Ti3+/Ti4+ providing an efficient route for the photoexcited electron-hole separation. Our findings provide new insights towards understanding and exploiting high efficient photocatalyst.

1 Introduction Solar-to-hydrogen conversion through a semiconductor photocatalytic process became one of the hottest research topics due to the increasing concern on clean and renewable energy and environmental issues.1-10 Usually, a photocatalytic process based on semiconductor involves the following steps:1 1) the photoinduced charge carrier generation; 2) the photogenerated charge carriers migrate from bulk to surface; 3) the photogenerated charge carriers react with chemical species at the photocatalyst/electrolyte interface. After generated, the light-induced charge carriers can be trapped or transferred, but they can also recombine. Trapping and transferring of charge carriers will compete with recombination, which strongly influence on the overall quantum efficiency. For the photocatalytic process, improvement of charge separation and suppression of charge carrier recombination are essential for enhancing the photocatalytic efficiency.


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The charge transfer process not only depends on the size and shape of the photocatalyst, but also is strictly related to the surface charge states, which strongly influence the equilibrium between the Fermi level potential of photocatalyst and the chemical potential of the absorbed species.1 Constructing unique nanostructures has been successfully used to suppress charge recombination and increase photo-generated carriers.11-13 Among various nanostructures, twodimensional (2D) nanomaterials have recently attracted considerable attention due to their short diffusion length of charge carrier from the interior to the surface, large absorption cross-section and large specific surface area.11 To date, for H2 evolution the current research mostly focuses on graphene, graphene oxide and MoS2, 9,11,14,15 but relatively few research have been reported on 2D TiO2 nanosheets.6 Usually the charge generation and transfer occur fast enough and efficiently at picosecond to nanosecond timescale, while the proton reduction by photoelectrons are much slower at microsecond to millisecond timescales.1,13 This kinetic discrepancy leads to energy losses occurring at the different interfacial charge transfer reactions, which strongly depend on the interface local structures. Therefore, revealing the interface and understanding its multifunctionality are essential to promote efficient interfacial charge transfer and facilitate photocatalytic reaction at the photocatalyst/electrolyte interfaces. To the best our knowledge, there is no report on the surface states of 2D ultrathin TiO2 nanosheets and their effect on hydrogen evolution. Although TiO2 is a promising photoactive semiconductor for hydrogen production because of its long minority diffusion length, high chemical stability and low cost, it only absorb UV light due to its wide band gap, leading to extremely low quantum efficiency.1,10 To date, numerous researches have been reported to enhance the visible light response of TiO2 by metal or nonmetal doping because it can induce the formation of new states closed to the valence band and


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conduction band, resulting in significant energy gap narrowing together. 1,10 But these dopants also act as charge carrier recombination centers, which result in the limited improvement of photocatalytic activity or even lower than pure TiO2.16-18 An alternative method for extending the light absorption is to couple TiO2 with a narrow bandgap semiconductor.1,16 Herein, zerodimensional (0D) semiconductor quantum dots (QDs) have been popularly used as photosensitizers due to their adjustable conduction and valance band-edge energies and absorption spectrum.19 These tunable properties of semiconductor QDs provide a convenient way to realize energetically favorable band alignment between QDs and TiO2. Ab initio simulations show that a set of electron bridging state is key to fast charge separation at the interface20. Besides small QD size, the coupling between the QDs and TiO2 is another key factor. Consequently, construction of hybrid photocatalysts with well-coupled 0D /2D interface stands for a promising approach for hydrogen production. 9,11,14,15 Here, we demonstrate a nanocomposites consisted of 2D TiO2 nanosheets prepared by a mild solution method and 0D CdSe QDs for hydrogen generation. The photoexcited electrons in ultrathin TiO2 nanosheets are easily trapped at surface and can reduce Ti4+ to Ti3+, benefiting from the ultrathin 2D nanostructure with large specific surface area. The redox couple Ti3+/Ti4+ acts as a shuttle, which provides an efficient transport road for photoexcited electrons from the CdSe QDs to the TiO2 nanosheets and then to the protons. The surface Ti3+ significantly promote H free radical production. These are responsible for the apparent quantum efficiencies to 45% at 450 nm and 380 nm and the 10-fold enhancement of H2 evolution compared with unloaded TiO2 nanosheets. Furthermore, the mechanism of H2 production were investigated by electron paramagnetic resonance (EPR). The findings of our work have an impact on the understanding of the multifuncationality of the CdSe/TiO2/Pt system and their effects on electron transfer in


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nanostructured photocatalysts and provide new opportunities towards designing and exploiting high efficient photocatalyst.

2 Experimental Section 2.1 Materials: Titanium(Ⅳ)(triethanolaminato)isopropoxide (TTEAIP) was ordered from Sigma-Aldrich Inc. Oleic acid (OA), oleylamine (OLA), trioctylphosphine

(TOP), 1-octadecene (ODE),

mercaptopropionic acid (MPA) and H2PtCl6·6H2O were purchased from Aladdin Reagent Inc. in Shanghai. Cadmium oxide, cadmium chloride, selenium, sodium borohydride, sodium sulfide, sodium sulfite and ethanol were purchased from Beijing chemical company without any further purification. Deionized water was purchased from Weisi chemical company. 2.2 Synthesis TiO2 nanosheets: In a typical synthesis process, 0.2 ml of TTEAIP, 1.5ml of OA and 1ml of OLA were added in a mixture solvent composed of 1 ml ethanol and 0.5 ml distilled water. The resulting solution was transferred to a 10mL Teflon-lined stainless steel autoclave with stirrer and then kept it at 200 oC for 12 hours. When the autoclave was cooled down to room temperature, the white products were washed several times by ethanol and distilled water. After washed, the samples were used for measurements or further decoration with CdSe quantum dots. 2D TiO2 nanosheets /0D CdSe nanocrystals without MPA: In a typical decoration process, 20ml of CdCl2 aqueous solution and TiO2 nanosheets were mixed and loaded in a 50ml three-neched flask under a nitrogen atmosphere. 20ml of NaHSe aqueous solution was prepared by dissolved Se powder into degassed NaBH4 solution under a


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nitrogen atmosphere. The NaHSe solution was swiftly injected into the CdCl2 and TiO2 precursor solution. The mole ratios of TiO2 to CdSe and Cd to Se were fixed 1:1 and 5:1, respectively. 2.3 Charaterizations Transmission Electron Microscopy (TEM) samples were prepared by immerging a 300 mesh copper grid with a ultrathin carbon film into a solution containing various samples. TEM images were collected on Tecnai F20 microscopes. Energy Dispersive Spectroscopy (EDS) measurements were performed using a Tecnai F20 TEM operated at 200kV. Data analysis was performed using the INCA software. The Powder X-ray Diffraction (XRD) measurements were performed using PANalytical X’ Pert PRO MPD with a 240 mm radius goniometer using an accelerating voltage of 40 kV and a current of 40 mA. Copper Kα radiation was used (1.5406 Å). Absorption spectra were collected on a Hitachi U4100 spectrometer at room temperature. The Brunauer-Emmett-Teller (BET) specific surface area of the products was analyzed by nitrogen adsorption in a Beishide 3H-2000PS2 nitrogen adsorption apparatus. All of the prepared samples were degassed at 150℃ prior to nitrogen adsorption measurements. The BET surface area was determined by a multipoint BET method. To evaluate the H2 evolution, the samples were illuminated through a quartz window used a Xe solar simulator PLS-SXE300 with/or without a band pass filter as the light source. The light intensity was double checked by a standard Si solar cell (SRC-1000-TC-QZ, SN 10510-0309) and a power meter (LPE-1A). For full solar spectrum testing, the measured light intensity was 200 mW/cm2. The following band pass filters, with center wavelengths at 380, 450, 500, and 550 nm were employed, and the measured light intensity was 22.3, 26.4, 25.7 and 27.5 mW/cm2, respectively. The photocatalytic cell was evacuated after pouring 100ml of aqueous solution


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(containing 50mg catalyst and 50 mM Na2SO3 and 50 mM Na2S as sacrificial reagents and 1 wt % Pt as co-catalysts) into the reactor (Perfectlight Labsolar ℃-AG). H2 production was analyzed by a gas chromatographer (GC-7900). The Electron Paramagnetic Resonance (EPR) spectra were collected by a JES-FA200 spectrometer. For low temperature (123K) measurements, after degassing the samples were cooled by use of a temperature controller under a liquid nitrogen flow. In EPR signal of TEMPO testing, the concentration of TEMPO and samples in solution was 10-2 mmol/L and 0.25 mg/ml, respectively, where the solution contains 50mg catalyst and 50 mM Na2SO3 and 50 mM Na2S as sacrificial reagents and 1 wt % Pt as co-catalysts.

3 Results and Discussion To date the synthesis methods for TiO2 nanosheets can be classified into two main kinds: exfoliation of layered titanates4,21.22 and HF-assistant hydrothermal method.23-25 The complicated former usually involves solid-state calcination, acid exchange and exfoliation of layered titanates.22 The relative simple latter needs HF acid, which is very corrosive.23-25 In this work, we develop a facile and mild solution method for ultrathin TiO2 nanosheets.


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Figure 1. XRD patterns of TiO2 nanosheets before and after annealing.

The phase structure and crystallinity of TiO2 nanosheets were measured by X-ray diffraction (XRD) technique. Figure 1. shows the XRD patterns of the as-synthesized TiO2 nanosheets. After annealing 2 hours at 450 ℃ in air, the crystallinity of the samples was significantly enhanced. All the peaks can be indexed to anatase TiO2 (JCPDS No. 21-1272). Structural characterizations of the samples were performed by transmission electron microscopy (TEM) and High-resolution transmission electron microscopy (HRTEM). Figure S1 shows the TEM and HRTEM images of the as-synthesized samples, revealing that the morphology of the TiO2 samples was nanosheet. After decorated with CdSe QDs, the TiO2 nanosheets exhibit a uniform distribution of nanodots, as shown in Figure 2a. The decoration of CdSe QDs on the nanosheets was further analyzed by Energy-dispersive X-ray spectroscopy (EDS) mapping. Variations in the O, Ti, Cd and Se signals along the 2D nanosheet are given in Figure 2c. Clearly, the CdSe QDs were successfully attached to TiO2 nanosheets. The corresponding HRTEM shown in Figure 2b gives a lattice fringe of about 0.35 nm, being consistent with the d101 space of the bulk anatase


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TiO2. The thickness of the TiO2 nanosheet was measured to be ~5nm by atomic force microscope (AFM) (insets in Figure 2). The band gap of the TiO2 nanosheet estimated by its absorption spectrum (as shown in Figure S2) are 3.72 eV. Compared with the bulk anatase TiO2 (3.2eV), the band gap of the TiO2 nanosheet becomes wider due to quantum confinement effect.

Figure 2. (a) TEM, (b) HRTEM and (c) EDS mapping images of ultrathin TiO2 nanosheets/CdSe QDs nanocomposite. Insets are AFM images of the sample.

Currently, most researches on TiO2 nanosheets are focused on organic pollutant degradation,2226

rather few research reports their hydrogen generation performance.6,27 Usually crystallinity of

photocatalysts has key influence on their performance. We firstly evaluate the effect of annealing temperature on the photocatalytic H2 production of TiO2 nanosheets. Before annealing, the H2 evolution is 0.78 mmolg-1h-1, but after annealing 2 hours at 450 ℃, the H2 evolution decreases to 0.11 mmolg-1h-1 as shown in Figure 3. Although the crystallinity of TiO2 nanosheets is improved as the annealing temperature increases, their H2 evolution is decreased due to a decrease in the


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surface area of the nanosheet, which is confirmed by the nitrogen adsorption/desorption isotherm shown in Figure S4. The Brunauer–Emmett–Teller (BET) surface area of the as-synthesized TiO2 nanosheet is measured to be 396 m2 g-1. After annealing 2 hours at 450 ℃ in air, the surface area rapidly decreases to 108 m2 g-1, which is due to an aggregation induced by annealing. Thus, maintaining large surface area of the sample is important to an efficient photocatalytic process. Furthermore, EPR spectroscopy was used to examine their surface characteristics (both with or without annealing samples). The EPR spectra of TiO2 nanosheets with or without annealing were recorded at 123K under argon atmosphere, as shown in Figure S5. After solar irradiation, the spectrum of the sample without annealing shows a broad signal with

, which can be

assigned to photogenerated surface Ti3+ species.28 Clearly, after annealing, the Ti3+ signal disappears. The surface Ti3+ is assumed to be critical for photocatalytic reaction, as they serve as active site to promote charge transfer.1 Therefore, annealing reduces not only the surface area but also the surface activity, leading to the decrease of H2 evolution.


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Figure 3. H2 evolution of 1 wt% Pt loaded TiO2-450,TiO2, CdSe, TiO2/ MPA/CdSe and TiO2/CdSe in a aqueous solution containing 50 mM Na2SO3 and 50 mM Na2S as sacrificial reagents under solar irradiation.

To extend its visible light response, the TiO2 nanosheets were decorated with CdSe QDs. The absorption spectra show a narrow size distribution of the QD size, which is about 2 nm. H2 production rate is enhanced due to quantum confinement effect for such small QDs29. In our system, the H2 production rate is further boosted due to a synergistic effect between the CdSe QDs and TiO2 nanosheets30. The hydrogen evolution tests were carried out in an aqueous solution containing 50 mM Na2SO3 and 50 mM Na2S as sacrificial reagents and 1 wt % Pt as cocatalysts. Here, we compare two kinds of CdSe QDs. One is oil-soluble CdSe QDs transferred to water by replacing the initial hydrophobic surfactants with mercaptopropionic acid (MPA). The other is prepared in aqueous solution without any surfactants. The H2 production in Figure 3 significantly increased after loading CdSe QDs onto TiO2 nanosheets, reaching 4.35 mmolg-1h-1 for the former and 7.96 mmolg-1h-1 for the latter. Compared with pure TiO2 nanosheets, the enhancement factors are around 5-fold and 10-fold, respectively. The increases can be attributed to the improvement of visible light absorption ability, which is supported by the absorption spectra in Figure S6. For the sample without MPA, the 0D CdSe QDs and 2D ultrathin TiO2 nanosheets are coupled directly, resulting in a remarkable synergistic effect due to faster charge separation and better photon adsorption efficiency. For understanding the interplay between the H2 evolution and the light absorption of these samples, we further investigated their wavelength-dependent apparent quantum efficiencies


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(AQE) by band pass filters, as shown in Figure 4. AQE are calculated based on the equation: . The AQE values of the TiO2/CdSe sample increase slowly from 1.2% at 550 nm to 9.9% at 500 nm, and then rapidly increase to 45.6% at 450 nm, and maintain relatively high efficiencies of 45.8% at 380 nm, which is almost coincident with its absorption spectrum. Clearly, the TiO2/CdSe sample significantly improves the photocatalytic activity not only in UV region, but also in visible light region in contrast to the unloaded TiO2 nanosheets with 1.2% at 380 nm. Moreover, the TiO2/CdSe sample exhibits relatively high efficiencies because the surfactant is absent here and the photogenerated charge carrier transfer is not hindered by MPA.

Figure 4. Wavelength-dependent apparent quantum efficiencies of TiO2, CdSe, TiO2/CdSe / MPA and TiO2/CdSe and absorption spectrum of TiO2/CdSe.

Next, EPR technique was conducted to investigate the origin of surface Ti3+ in the TiO2/CdSe nanocomposites. The EPR signal at 123K under argon atmosphere was recorded as a function of


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time during solar irradiation as shown in Figure 5. The spectrum of the TiO2/CdSe sample in the , which can be assigned to bulk anatase Ti3+.5,28 The weak

dark exhibits a weak signal at

signal is hardly influenced by illumination. Considering the EPR test condition without any reactant, we infer that the signal is associated with O vacancy in TiO2 nanosheets. Additionally, a broad signal with

5,28

appears after irradiation, which has been assigned to

surface Ti3+ as mentioned above. The signal intensity of the photoinduced surface Ti3+ is found to gradually increase with the illumination time, indicating an accumulation of surface Ti3+. We deduce that the surface Ti3+ originates from reduced Ti4+ on the surface of TiO2 sample by the photoexcited electron . The formation of surface Ti3+ could be related to the following reasons: (1) The upward band bending of the TiO2 nanosheet can be neglected due to its only ~5 nm thickness, and thus the photoexcited electrons are feasible to migrate to the surface. (2) The lattice relaxation induced by Ti3+ is easier at surface than in the bulk due to fewer bulk lattice confinements. (3) The surface Ti3+ will induce deep energy levels in the bandgap, driving the electrons to transfer to the surface.1

Figure 5. The EPR spectra of the TiO2/CdSe sample at 123K under solar irradiation and argon atmosphere as a function of time.


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To further reveal the photoexcited electron transfer in TiO2/CdSe, the EPR signal of surface Ti3+ at 123K was collected as a function of wavelength, as shown in Figure 6. Obviously, no detectable EPR signal was recorded during irradiation at wavelength of 550nm. When the light switched from 550 to 500, 450 and 380 nm, the signal intensity of the surface Ti3+ increases gradually. The shorter the wavelength is, the stronger the signal becomes. It is reasonable that the strongest signals appears at 380 nm because TiO2 absorbs UV light mostly. More interestingly, the surface Ti3+ can be also produced by yellow light at 500 nm. Given that TiO2 is not able to absorb visible light, we infer that under visible light illumination the photoexcited electrons are injected into TiO2 from CdSe, and then reduce Ti4+ to Ti3+ at the surface, as evident from that the wavelength-dependent alteration of EPR signal intensity is in agreement with the absorption spectrum of TiO2/CdSe sample.

Figure 6. The EPR spectra of the TiO2/CdSe sample at 123K after irradiation 20 min as a function of irradiation wavelength.


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To further explore the photoinduced catalytic reactions on the surface of the TiO2/CdSe nanocomposites under practical photocatalytic conditions, the EPR measurements were performed in the same conditions for H2 evolution (an aqueous solution containing 50 mM Na2SO3 and 50 mM Na2S as sacrificial reagents and 1 wt % Pt as co-catalysts) except that 2,2',6,6'-tetramethylpiperidine N-oxyl (TEMPO) was added to the solution as a good hydrogen abstractor31. The H free radical production can be monitored by double integral the EPR signal of TEMPO, as shown in Figure 7. The intensity of the EPR signal slowly decreases as illumination time under 550 nm. The signal intensity drops faster as irradiation wavelength becomes shorter, indicating that the H free radical production increases with shortening wavelength. The trend of the wavelength-dependent H free radical production is consistent with the wavelength dependence of H2 evolution and surface Ti3+ formation. Thus, we infer that the photoexcited electron trapped at surface Ti3+ reduces H+ to form H, according to equation Ti3++H+=Ti4++H·, where the redox couple Ti3+/Ti4+ as a shuttle provides an efficient transport route for the photoexcited electrons to migrate from the CdSe QDs to the TiO2 nanosheets and then to the protons. In addition, there exists another feasible electron transfer path, i.e., the photoinduced electron trapped at surface Ti4+ transfers to Pt island and then to reduce proton. It is well known that Pt as a co-catalyst can also facilitate the charge transfer and require a low overpotential for proton reduction.1,7 In order to further clarify the impact of Pt on the electron transfer processes in TiO2/CdSe, the H2 and H free radical evolution for TiO2/CdSe in the absence of Pt were measured. The H2 evolution is 1.76 mmolg-1h-1, which is 4.5-fold decreased. However, there is little change for the H free radical production after removal of Pt. This results imply that the photoinduced surface Ti3+ plays a dominate role for proton reduction, while Pt is the active site for H-H association. The adsorption energy of H atom on anatase (101) surface is reported to be


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2.35 eV by DFT calculations32, which is 0.25 eV weaker than on Pt(111) surface33. Considering that H atom diffusion barrier on anatase (101) can be as low as 0.70 eV, we infer that the H free radicals diffuse to Pt surface and are catalyzed by Pt to produce hydrogen.

Figure 7. Normalized double integrals of the EPR signal of TEMPO and kinetic fits as a function of irradiation time at room temperature.

Based on the above results, a possible photocatalytic mechanism is described as follows. Type Ⅱ band structure between TiO2 and CdSe is beneficial for the photogenerated charge carrier separation. The high surface area of the unique 2D ultrathin TiO2 nanosheets makes it easier to trap the electron, greatly facilitating the charge separation. The photogenerated holes in the valence band of CdSe oxidize SO32- to form SO42-, according to equation29 H2O+SO32-+ 2h+ = SO42-+2H+. The photoexcited electron trapped at surface Ti3+ reduce the H+ to form H free radical, then H free radicals diffuse onto Pt and associate as H2 gas, as shown in Figure 8. By analyzing the reduction potentials at −0.83 V for H2 reduction from water and at −0.67 V for Ti4+


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reduction in TiO2,34 we conclude that the photoexcited electrons are trapped at surface to reduce Ti4+ rather than reducing water, as evident from the above EPR results. Therefore, the path of water reduction to hydrogen immediately could be ignored.

Figure 8. A possible photocatalytic mechanism for the TiO2 nanosheets/CdSe quantum dots/ Pt system.

It is well known that electrons generated by UV irradiation of TiO2 tend to transfer quickly to Pt particles loaded on TiO2 surface35,36, our ultrathin TiO2 nanosheet shows very different feature, where the photoinduced electrons seem to be trapped in anatase TiO2 surface to form Ti3+. Excess electrons injected into the conduction band of TiO2 may couple to the lattice distortion induced by the electrons themselves, which is referred to polaronic effect37. There have been debates about the nature of the polarons in TiO238. Some calculations indicate a non-localized state in anatase TiO237, alternative studies also demonstrate that the lattice relaxation associated with the electron trapping is more favorable at the (101) surface of anatase1,39. This is due to the fact that the self-trapped electron state lies deeper in the band gap with respect to the bulk levels. If the trapped state in the gap is lower than the Fermi level of Pt, electrons will not migrate to Pt particles. This may explain our experiment that H free radical production is not effected by Pt


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loading. We infer that the unusual electron trapping may result from stronger lattice relaxations in our ultrathin TiO2 nanosheet . In conclusion, we develope a facile and mild solution method for ultrathin TiO2 nanosheets, which exhibit large surface area and high photocatalytic activity induced by an enhancement in surface Ti3+ accumulation. For TiO2 nanosheets, annealing reduces not only the surface area but also the surface activity, leading to the decrease of the H2 evolution. Through loaded CdSe QDs onto TiO2 nanosheets, the light absorption are extended to visible light region. The apparent quantum efficiency increases to 45% at 380 and 450 nm. H2 evolution also exhibits the 10-fold enhancement compared with unloaded TiO2 nanosheets, owing to the synergistic effect between the quantum dots and the nanosheets. EPR results indicate that the redox couple Ti3+/Ti4+ as a shuttle provides an efficient transport route for photoexcited electrons from CdSe QDs to TiO2 nanosheets and then to reduce protons, which are responsible for the enhancement. Furthermore, the EPR results reveal that the photoinduced surface Ti3+ plays a dominant role for proton production, and Pt is the active site for H2 association. Thus, our CdSe/TiO2/Pt system serves as a multifunctional catalyst. The present findings are important not only for the understanding of the enhancement in photocatalysis efficiency in nanostructured composites, but also for how to design high efficient photocatalyst by 0D QDs directly coupled with 2D ultrathin nanosheets. Supporting Information Prepared process of 2D TiO2 nanosheets /0D CdSe nanocrystals with MPA; TEM images of assynthesized TiO2 nanosheets; Absorption spectrum of TiO2 nanosheets; XPS Ti 2p spectrum of TiO2 nanosheets; Nitrogen adsorption–desorption isotherm of TiO2 nanosheets before and after annealing; The EPR spectra of TiO2 nanosheets before and after annealing under solar


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irradiation. Absorption spectra of CdSe and TiO2/CdSe with/or without MPA; The EPR signal of TEMPO at room temperature under solar irradiation and argon atmosphere as a function of time.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant No. 51472031 and 51102017). REFERENCES (1) Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Yu, H.; Anpo, M.; Bahnemann, D.W. Understanding TiO2 Photocatalysis: Mechanisms and Materials Chem Rev., 2014,114, 9919-9986. (2) Fujishima, A. ; Honda, K.; Electrochemical Photolysis of Water at a Semiconductor Electrode Nature, 1972, 238, 37–38. (3) Kronawitter, C. X.; Vayssieres, L.; Shen, S.; Guo, L.; Wheeler, D. A.; Zhang, J. Z.; Antoun, B. R.; Mao, S. S. A Perspective on Solar-driven Water Splitting with All-oxide HeteroNanostructures Energy Environ. Sci., 2011,4, 3889-3899. (4) Ida, S.; Kim, N.; Ertekin, E.; Takenaka, S.; Ishihara, T. Photocatalytic Reaction Centers in Two-Dimensional Titanium Oxide Crystals J. Am. Chem. Soc., 2015,137, 239-244. (5) Priebe, J. B.; Karnahl M.; Junge, H.; Beller, M.; Hollmann, D.; Brückner, A. Water Reduction with Visible Light: Synergy between Optical Transitions and Electron Transfer in AuTiO2 Catalysts Visualized by In situ EPR Spectroscopy Angew. Chem. Int. Ed., 2013, 52, 1142011424.


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Ti4+ Redox Shuttle Enhancing Photocatalytic H2

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