TiO2 with Rapid Bulk to Surface

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Pt-Enhanced Mesoporous Ti3+/TiO2 with Rapid Bulk to Surface Electron Transfer for Photocatalytic Hydrogen Evolution Zichao Lian, Wenchao Wang, Guisheng Li, FengHui Tian, Kirk S. Schanze, and Hexing Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11494 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 22, 2016

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ACS Applied Materials & Interfaces

Pt-Enhanced Mesoporous Ti3+/TiO2 with Rapid Bulk to Surface Electron Transfer for Photocatalytic Hydrogen Evolution Zichao Lian,† Wenchao Wang,† Guisheng Li,*,†, ‡ Fenghui Tian,§ Kirk. S. Schanze,*,‡ and Hexing Li*,† †

Chinese Education Ministry Key Laboratory of Resource Chemistry, Shanghai Normal

University, Shanghai 200234, China. ‡

Department of Chemistry, University of Florida, Gainesville, UF

§

Institute of Computational Science and Engineering, Qingdao University, Qingdao, 266071,

China. KEYWORDS: Ti3+, TiO2, Pt doping, oxygen vacancies, H2 evolution

ABSTRACT: Pt-doped mesoporous Ti3+ self-doped TiO2 (Pt-Ti3+/TiO2) is in-situ synthesized via an ionothermal route, by treating metallic Ti in an ionic liquid containing LiOAc, HOAc and a H2PtCl6 aqueous solution under mild ionothermal conditions. Such Ti3+-enriched environment, as well as oxygen vacancies, is proven to be effective for allowing the in-situ reduction of Pt4+ ions uniformly located in the framework of the TiO2 bulk. The photocatalytic H2 evolution of PtTi3+/TiO2 is significantly higher than that of the photo-reduced Pt loaded on the original TiO2 1 ACS Paragon Plus Environment


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and commercial P25. Such greatly enhanced activity is due to the various valence states of Pt (Ptn+, n = 0, 2 or 3, forming Pt-O bonds embedded in the framework of TiO2 and ultrafine Pt metal nanoparticles on the surface of TiO2. Such Ptn+-O bonds could act as the bridges for facilitating the photo-generated electron transfer from the bulk to the surface of TiO2 with a higher electron carrier density (3.11 * 1020 cm-3), about 2.5 times of that (1.25 * 1020 cm-3) of photoreduced Pt-Ti3+/TiO2 sample. Thus, more photo-generated electrons could reach the Pt metal for reducing protons to H2.

1. INTRODUCTION Hydrogen has been widely proved as a potential energy carrier owing to its high calorific value and environmental friendly nature.1 In addition, hydrogen production is a potential candidate for solving the energy crises.2 Since the discovery of photoelectrochemical water splitting using TiO2 and a Pt electrode by Fujishima and Honda in 1972,3 various semiconductor materials and other elemental doped materials have been developed as photocatalysts for water splitting, including TiO2 doped with Fe,4 Ce,5 Cu,6 W7 and other elements8 using an in-situ doping method. For H2 evolution, co-catalysts, such as Pt,9 CoOx10 and NiOx11, are required to facilitate H2 production by enhancing charge separation or providing more active catalytic sites. Loading metallic Pt on the surface of TiO2 using a photoredox deposition method has often been employed as a co-catalyst.12,13 In this case, the Pt/TiO2 heterojunction may affect the charge transfer process forming a Schottky barrier between these components to separate the photoinduced electrons and holes, and the loaded Pt, which acts as an active site, could easily trap excited electrons and protons, which can catalyze H2 evolution.14 However, metallic Pt nanoparticles contribute little to H2 evolution due to an H2 desorption energy that is too high,15

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and hydrogen back-oxidation is catalyzed by metallic Pt co-catalysts.16,17 Therefore, the development of suitable co-catalysts is very important for water splitting Recently, different valence states of Pt modified TiO2 have attracted much attention because of their excellent ability to inhibit the hydrogen oxidation reaction in proton exchange membrane for fuel cells18 and H2 evolution process.15,17 Nevertheless, it is difficult to introduce the Pt species with various valence into the bulk framework of TiO2 owing to the difficulty in replacing Ti with Pt. Actually, such Pt species embedded in the framework of TiO2 could be very significant for accelerating the photogenerated electrons in the bulk to transfer to surface states, with a greatly enhanced quantum efficiency. Therefore, it is of great interest to explore a route for embedding Pt with various valence states in the framework of TiO2, towards the objective of achieving high quantum efficiency for photocatalytic reaction. In the present work, we report that platinum species, with various valence states, can be doped in the framework of TiO2 through an in-situ ionothermal method. Herein, the defect sites, including Ti3+-dopants and oxygen vacancies, in the TiO2 allow the Ptn+ to enter into the framework of mesoporous TiO2 via substituting Ti. The doped Pt ions embedded in the TiO2 lattice may facilitate the photogenerated electron transfer to metallic platinum on the surface of TiO2 for efficient H2 production via the Ptn+-O bonds. In addition, a mesoporous structure for the Pt-Ti3+/TiO2 microspheres was also constructed by such a metal-induced self-assembly assisted by an in-situ ionothermal method. 2. EXPERIMENTAL SECTION Preparation of Pt-doped mesoporous Ti3+ self-doped TiO2 (Pt-Ti3+/TiO2): : Analytical grade chemicals were received and used without any further purification. The typical synthesis process of Pt-Ti3+/TiO2 is same as our previously reported protocol

19

, except for introducing different 3

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amounts of an H2PtCl6 aqueous solution (1.0 g/100 mL) into a mixture containing 0.6 g of LiOAc.2H2O, 12 mL of DMF and 18 mL of glacial HOAc. Other reaction conditions were maintained unchanged. The as-obtained samples are represented by X wt% in-situ Pt-Ti3+/TiO2, where X represents the mass percentage ratio of Pt:Ti detected by ICP analysis. A black Ti3+/TiO2 sample was synthesized in the same manner without the H2PtCl6 aqueous solution according to our previously reported protocol.19 In addition, photoreduction of the platinum loading on commercial P25 and black Ti3+/TiO2 was performed in a 100 mL Pyrex flask containing 20 mL methanol and 60 mL water at room temperature, and the flask was sealed with a silicone rubber septum. The photoreduction time was 0.5 h. The samples are referred to as 0.20 wt% Pt-Ti3+/TiO2 and 0.20 wt% Pt-P25. Measurement and characterization: X-ray diffraction (XRD) patterns were recorded on a Rigaku Dmax-3C with Cu Kα irradiation. Transmission electronic microscopy (TEM) was employed to characterize the morphologies using a JEM-2010. Field emission scanning electron microscopy (FESEM) images were recorded on an S-4800. The Raman spectra were collected on a Dilor Super LabRam II instrument. The diffuse reflectance spectra of samples were performed on a Varian Cary 500 UV-Vis-NIR system-Labsphere diffuse reflectance accessory (200-800 nm) by using BaSO4 as a reference. X-ray photoelectron spectroscopy (XPS) analysis was recorded by a Perkin-Elmer PHI 5000C ESCA system. The contaminant carbon (C1s = 284.6 eV) was used as a reference for calibrating the binding energies. The electron paramagnetic resonance (EPR) spectra were recorded at 77 K using a Bruker EMX-8/2.7 EPR spectrometer. The metal Pt loadings were determined using an inductively coupled plasma optical emission spectrometer (ICP, Varian VISTAMPX). The photocurrents and electrochemical impedance spectroscopy (EIS) were measured using an electrochemical analyzer (CHI 660E Instruments, Chen Hua 4 ACS Paragon Plus Environment

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Instrument Co., Ltd.) in a standard three-electrode system. . The detailed information can be found in reference 19. A LED light (3.5 mW/cm2, single wavelength at 365 nm) was used as a light source. The EIS measurements were carried out over a frequency range of 0.1-105 Hz with an AC amplitude of 5 mV under irradiation with one LED light (λ = 365 nm) via a bias of 0.5 V vs SCE. The open circuit potential test was carried out in a three-electrode cell (0.50 M Na2SO4 as electrolyte) with a SCE electrode as a reference electrode, a platinum foil (99.99 %, 0.1 mm, 2.0 cm * 2.0 cm) as a counter electrode, and as-prepared electrodes as working electrodes in the potential range of -1~1 V. A LED lamp (λ = 365 nm) was used as light source. Both the flat-band potential and carrier density of the as-prepared samples were calculated based on the MottSchottky plots obtained at a fixed frequency of 1 KHz. Photocatalytic H2 generation: The photocatalytic H2 production experiments were performed in a 100 mL Pyrex flask at ambient temperature and atmospheric pressure, as reported by Yu et al.44 Four low-power 365 nm LEDs (3 W) (Shenzhen LAMPLIC Science Co. Ltd., China) were used as the light sources to induce the photocatalytic reaction, other reaction conditions were maintained unchanged. A significant factor in photochemical energy transformation is the quantum efficiency (QE), which was measured based on the following equation (1): number of reacted electrons × 100 number of incident photons number of evolved H 2 molecules × 2 = × 100 number of incident photons

QE[%] =

(1)

3. Results and discussion The morphologies of Ti3+/TiO2 and different amount of Pt loaded Ti3+/TiO2 samples were characterized by field emission scanning electron microscopy (FESEM). Figure 1a shows that the pure Ti3+/TiO2 was composed of stacked regular particles with an average size of about 25 5 ACS Paragon Plus Environment


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µm. It is interesting that the flower-like TiO2 consisting of nanorods could be produced by introducing the H2PtCl6 aqueous solution, as shown in Figure 1b-f. Herein, the introduced H2PtCl6 solution may have induced the self-assembly process, resulting in the formation of flower-like TiO2 spheres. As known, the acidity plays an important role in shaping the morphology of the metal oxide catalysts.20-22 In the present work, equal amounts of the reagents (i.e., H2O or HCl) were chosen to replace the H2PtCl6 solution for the fabrication of the catalysts. As shown in Figure S1, the morphology of the TiO2 prepared in the presence of added H2O or HCl was nearly same as that of the ionothermal Ti3+/TiO2 that was prepared without H2PtCl6. This is because the acidity of the reaction solution for Ti3+/TiO2 was effectively the same in the presence of a small amount of H2O or HCl, owing to the presence of excess glacial HOAc (18 mL). Thus, we conclude that the flower-like TiO2 morphology obtained in the presence of H2PtCl6 is caused by the Pt doping.

Figure 1. FESEM of Ti3+/TiO2 (a) and in-situ Pt-Ti3+/TiO2 with different Pt/Ti weight ratios (wt % Pt): (b) 0.18, (c) 0.19, (d) 0.20, (e) 0.21, and (f) 0.26.


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Figure 2. Scheme of formation process of in-situ Pt-Ti3+/TiO2 under ionothermal conditions. A possible Pt-Ti3+/TiO2 formation process is illustrated in Figure 2 according to a previously reported protocol.19 During the initial reaction stage, the fluoride enriched ionic liquid (IL) gradually dissolves the Ti foil. Then, the Ti foil reacts with the H+ ions resulting from the ionization of HOAc to form Ti4+/Ti3+ ions and hydrogen gas. The as-formed Ti4+ and Ti3+ ions may undergo ionothermal hydrolysis due to the presence of H2O. During the ionothermal hydrolysis process, Ti4+ transforms into anatase TiO2, and the Ti3+ ions may be incorporated into the TiO2 lattice. The defect sites in the Ti3+-doped lattice, and the oxygen vacancies may provide a growth environment that allows Pt0 and Ptn+ to enter the TiO2 lattice and produce a distribution of Pt valence states from the surface to the interior of TiO2. Moreover, the Pt4+ could not be reduced if Ti foil was replaced by other titanium(IV) precursors, such as titanium isopropoxide, tetrabutyl titanate, and titanium tetrachloride. This notion is supported by the XPS and ICP results in (Figure S2 and Table S1, respectively). In particular, nearly no Pt0 was observed when Ti4+ precursors were used as the titanium source. Moreover, it was also noted that nearly the same binding energies of Ti 2p and O 1s were observed in the TiO2 samples obtained based on the Ti4+ source as shown in Figure S3. However, positive shifts of Ti 2p and O 1s peaks were detected upon using Ti foil. The above XPS results revealed that a strong interaction exists between the Ptn+ and TiO2 in the ionothermal reaction system when using Ti foil as source. Thus, it can be concluded that the ionothermal-route induced a Ti3+-rich environment,19 which was suitable for in-situ reduction of Pt4+ to Pt0 or other valence states in the lattice of TiO2 during the 7 ACS Paragon Plus Environment


growth of TiO2 crystals under ionothermal conditions. It is reasonable that such in-situ Pt4+reduction process could also alter the crystal growth of TiO2 leading to formation of flower-like structures. (a)

(b) (F)

(F)

(E)

(E)

Intensity (a.u.)

Intensity (a.u.)

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(D) (C)

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(B)

(A) 20

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Figure 3. XRD plots (a) and the corresponding expanded XRD plots from 22 to 28 degrees (b) of Ti3+/TiO2 (A) and in-situ Pt-Ti3+/TiO2 with different Pt/Ti weight ratios (wt %): 0.18 (B), 0.19 (C), 0.20 (D), 0.21 (E), and 0.26 (F). To identify the crystal phase and structures of Ti3+/TiO2 and in-situ Pt-Ti3+/TiO2, X-ray diffraction (XRD) was carried out and the results are shown in Figure 3. The XRD spectra demonstrated that all of the samples consist of the TiO2 anatase phase (JCDPS 21-1272). However, no diffraction peaks corresponding to Pt metal or platinum oxide were observed, likely because of their very small size, high dispersity and low concentration. Compared with pure Ti3+/TiO2, the peaks of the (101) lattice plane in in-situ Pt-Ti3+/TiO2 are shifted substantially, as shown in Fig. 3b. The lattice parameters (a = b and c) of the pure Ti3+/TiO2 and Pt-Ti3+/ TiO2 crystals were obtained using the following equations (Bragg’s law): 2 2 −2 d ( hkl ) = λ / 2 sin θ , d (−hkl + k 2 b −2 + l 2 c −2 ) = h a

(2)


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All of the values are listed in Table S2. Compared to the pure Ti3+/TiO2, an increase in the a and b values was observed for the in-situ treated samples due to the substitution of Pt4+, which is a smaller ion (0.625 Å), for Ti3+, which is larger (0.670 Å), in the TiO2 lattice. This resulted in a shift of the peak shown in Figure 3b. The change in the c value was small. As expected, both the decrease (2.10 %) in a and b values and the decrease (0.52 %) in c value resulted in the cell volume shrinking by 1.61%. Therefore, it is reasonable to conclude that the platinum ions can replace Ti3+ ions in the crystal lattice. The XRD patterns suggest that platinum ions can be incorporated into the TiO2 lattice at low Pt levels. (a)

Pt

3+

Pt

2+

75.6 77.1

Pt

Photoreduced Surface 10 nm 20 nm

0

74.4

3+

Ti /TiO2

(b)

3+

0.20 wt% in-situ Pt-Ti /TiO2 458.7

71.2

Intensity (a.u.)

Intensity (a.u.)

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80

78

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74

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Binding energy (eV)

70

68

468

459.1

464

460

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Binding energy (eV)

Figure 4. XPS spectra of the Pt 4f (a) of 0.20 wt% in-situ Pt-Ti3+/TiO2 with various sputtering depths (surface, 10 nm, and 20 nm) and photoreduced 0.20 wt% Pt-Ti3+/TiO2, and Ti 2p (b) for Ti3+/TiO2 and 0.20 wt% in-situ Pt-Ti3+/TiO2 samples. X-ray photoelectron spectroscopy (XPS) was employed to analyze the elemental compositions of Ti3+/TiO2 and in-situ Pt-Ti3+/TiO2 to investigate the chemical states of Pt and acquire in-depth fundamental insight into the interaction between the in-situ deposited Pt and Ti3+/TiO2. The 0.20 wt% in-situ Pt-Ti3+/TiO2 sample exhibited a different lattice environment for the Pt ions. Figure 4a shows the Pt 4f spectra of the surface and the bulk with various sputtering depths for the 0.20



wt% in-situ Pt-Ti3+/TiO2 sample. For comparison, Pt loaded by photoreduction on Ti3+/TiO2 was also characterized by XPS analysis. The peaks at 70.4 eV and 74.0 eV were due to metallic Pt.17 In addition, the peaks at 71.2 eV and 74.4 eV originate from Pt0 (4f

7/2)

and Pt0 (4f

5/2),

respectively, on the surface of 0.20 wt% in-situ Pt-Ti3+/TiO2.23,24 Moreover, the peak at 75.6 eV for the surface and sputtering at a depth of 10 nm and 20 nm was attributed to Pt2+ (4f 5/2),17,25,26 and the peak at 77.1 eV for sputtering at a depth of 10 nm and 20 nm corresponded to Pt3+ (4f 5/2).

27-29

Therefore, the XPS data suggest that the Pt species in the in-situ Pt-Ti3+/TiO2 sample

were composed of different valence states of Ptn+ (i.e., Pt0, Pt2+, and Pt3+). In addition, a negative shift (-0.4 eV) for Ti 2p3/2 upon incorporation of Ptn+ (as shown in Figure 4b) and the slight shift to higher binding energy in O 1s (+ 0.4 eV) and Ti 2p (+ 0.3 eV) orbitals at the elevated sputtering depth of 20 nm (see Figure S4) revealed a strong electronic interaction between Ptn+ and TiO2 in the bulk of Ti3+/TiO2.30 These results demonstrate that both TiO2 doped with low concentrations of Pt ions and metallic Pt as well as the platinized TiO2 are unique with respect to their structure and the interaction between Pt and the TiO2 lattice. These materials feature a uniform distribution of Pt species from the surface to the interior bulk of TiO2. 3+

Ti /TiO2

1.938

0.20 wt% in-situ 3+ Pt-Ti /TiO2

Intensity (a.u.)

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2.4

2.111

2.072 1.930

2.157

2.2

1.997

2.0

1.8

1.6

1.4

g factor


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Figure 5. EPR spectra with the g values for Ti3+/TiO2 and 0.20 wt% in-situ Pt-Ti3+/TiO2 measured at 77 K. Electron paramagnetic resonance (EPR), has been widely utilized to probe charge carriers in oxide particles.31,32 In the present work, EPR spectroscopy was also utilized to investigate the electronic states of the metals (M) in the Ptn+ doped Ti3+/TiO2 samples owing to its high sensitivity for detection of the M3+ ions with unpaired electrons.33 As shown in Figure 5, pure Ti3+/TiO2 displayed strong g signals, ranging from 1.99 to 1.94, ascribed to the presence of Ti3+ species.34-36 Upon doping with the Ptn+ species, the intensity of the Ti3+ signals is decreased significantly, suggesting that Ti3+ ions could be partly oxidized by Pt4+ with the in-situ formation of reduced species, Ptn+. Nevertheless, the EPR spectrum of the in-situ Pt-Ti3+/TiO2 sample still exhibited clear signals at a g factor of 1.997 and 1.930 confirming the presence of Ti3+. Besides, the EPR results also demonstrated that both Ti3+ and oxygen vacancies coexist in the Ti3+/TiO2 bulk.37 The signals at g1 = 2.157, g2 = 2.111 and g3 = 2.072 indicate the presence of Pt3+ ions in the as-obtained samples.32, 33, 38-40 Such results suggest that Ptn+ may occupy some of the Ti3+ or oxygen vacancy positions by doping the TiO2 lattice. Therefore, these defect sites (Ti3+-dopants or oxygen vacancies) may provide an environment that allows Ptn+ to enter the TiO2 lattice and result in a distribution of Pt valence states from the surface (Pt0) to the bulk (Ptn+) of the TiO2 nanocrystal.



(a)

(b) 147 cm

-1

(D)

(D)

Intensity (a.u.)

Intensity (a.u.)

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(C)

(B)

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(B)

(A)

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400

600 -1

Wave numbers (cm )

800

100

120

140

160

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Figure 6. Raman spectra (a) and the corresponding expanded Raman spectra from 100 to 200 cm-1 (b) of in-situ Pt-Ti3+/TiO2 with different Pt/Ti weight percentage ratios (wt %): 0.18 (A), 0.19 (B), 0.20 (C), 0.21 (D). The oxygen vacancies of the as-obtained TiO2 samples via loading various amount of Pt were also probed by Raman spectroscopy, as shown in Figure 6. Generally, the pure anatase phase of TiO2 exhibits six typical peaks at approximately 144, 197, 639, 399, 519, and 513 cm-1, which correspond to the Eg(1), Eg(2), Eg(3), B1g(1), B1g(2) and A1g modes, respectively.41 Our previous results19 indicated that the pure Ti3+/TiO2 exhibits a primary peak at 157 cm-1, significantly shifted compared to 144 cm-1, suggesting a high concentration of oxygen vacancies existed in the Ti3+/TiO2 sample.19,42 However, doping Pt into the framework of Ti3+/TiO2 resulted in a shift to lower frequency to about 147 cm-1. Such a 10 cm-1 shift implied that substitution of Ti by Pt greatly decreased the number of oxygen vacancies. Such decrease of oxygen vacancies could be ascribed to the occupation of Ptn+ in the vacancy positions. These results suggest that Ptn+ could occupy the position occupied by Ti3+ or an oxygen vacancy, which resulted in a decrease of these sites compared with pure Ti3+/TiO2. The Raman results, together with XPS and ESR, suggest that Ptn+-O bonds were uniformly produced in the bulk of the as obtained TiO2.


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40

(a)

3 -1

-1

Desorption Pore Volume (cm g *nm )

3

Adsorption Volume (cm /g)

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Adsorption Desorption

10

0.0

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0.08

(b) 3.7 nm 0.06

0.04

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Pore Size (nm)

Figure 7. N2 adsorption-desorption isotherms (a) and pore size distributions (b) of 0.20 wt% insitu Pt-Ti3+/TiO2 recorded at 77 K. Figure 7a and b show the nitrogen adsorption-desorption isotherms and pore size distribution plots for mesoporous Pt-Ti3+/TiO2. The sample exhibited a type-IV isotherm that is representative of mesoporous solids. The specific surface area of the 0.20 wt% in-situ PtTi3+/TiO2 sample was 12 m2-g-1 using the Brunauer-Emmett-Teller (BET) method, as shown in Table S3. The pore diameter of the 0.20 wt% in-situ Pt-Ti3+/TiO2 sample was approximately 3.7 nm (estimated using the desorption branch of the isotherm) and had a very narrow pore size distribution. However, the specific surface area of Ti3+/TiO2 was only 0.30 m2-g-1. These results demonstrated that the doping with Pt significantly changed the morphology of TiO2 as well as its effective surface area. The structure changes indicated that the Pt species (Pt0 or Ptn+) may be embedded in the lattice of the mesoporous TiO2.



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Figure 8. TEM images (a) and the corresponding enlarged TEM spectrum (b) with the size distribution in the right inset of (b) and TEM-EDX elemental mapping of 0.20 wt% in-situ PtTi3+/TiO2. The fine structural morphology of the as-prepared sample was also imaged by TEM. As shown in Figure 8a and b, Pt-Ti3+/TiO2 exhibits flower-like microspheres, and the Pt species (dark dots in Figure 8b) are primarily distributed around the TiO2. For better comparison, the sample of 0.20 wt% photoreduced Pt-Ti3+/TiO2 was also investigated by TEM (Figure S5a and b). It was interesting that 0.20 wt% in-situ Pt-Ti3+/TiO2 exhibited a narrow size distribution of Pt species, centered at about 1.50 nm (Figure S6a), much lower than that (about 3.50 nm, Figure S6b) of the photoreduced sample. Such narrow size of Pt species may be attributed to aggregation of Pt clusters that are located in the mesoporous pore walls or contained in the mesoporous pores. The distribution of Pt species was also investigated by EDX elemental mapping. As shown in Figure 8b, the elemental maps for Pt, Ti, and O indicated that the elements are uniformly distributed in the as-prepared Pt-Ti3+/TiO2 microsphere. Note that a low concentration of Pt was observed in the elemental mapping results, consistent with the XRD analysis. 14 ACS Paragon Plus Environment

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30

photoreduced 0.20 wt% Pt-Ti /TiO2

photoreduced 0.20 wt%-Pt-Ti /TiO2 photoreduced 0.20 wt%-Pt/P25 3+ 0.20 wt% in-situ Pt-Ti /TiO2

25

photoreduced 0.20 wt% Pt/P25 3+ 0.20 wt% in-situ Pt-Ti /TiO2

20

-Z" (Ω)

0.3

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(b)

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Ti /TiO2 3+

Light on

3+

0.20 wt% in-situ Pt-Ti /TiO2

0.20 wt% in-situ Pt-Ti /TiO2

3+


8

3+


6

2

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-2

1/CSC *10 (F )

Open circuit potential (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

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0.2

Potential (V vs SCE)

Figure 9. The photocurrent responses (a), the Nyquist plot of electrochemical impedance spectra (b) of the photoreduced/in-situ 0.20 wt% Pt-Ti3+/TiO2 and photoreduced 0.20 wt% Pt/P25 samples in the on-off light process, time dependence of open circuit potential (c) of Ti3+/TiO2 and photoreduced/in-situ 0.20 wt% Pt-Ti3+/TiO2 and Mott-Schottky plots (d) of photoreduced/insitu 0.20 wt% Pt-Ti3+/TiO2. All of the experiments performed using a 365 nm LED light source (3.5 mW/cm2), with 3-electrode cell, immersed in 0.5 mol/L Na2SO4 aqueous solution electrolyte (pH = 6.0), Pt counter electrode and SCE reference electrode. To evaluate the photoelectrochemical responses of the as-prepared samples under UV light irradiation, the photocurrent densities were measured in a light on-off process with a pulse of 10 s using a potentiostatic technique in a standard three-electrode system, where the counter and reference electrodes were a platinum sheet and a saturated calomel electrode (SCE), respectively. As shown in Figure 9a, upon illumination with an LED source (λ = 365 nm) at an applied 15 ACS Paragon Plus Environment


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potential of 0.5 V vs. SCE, the 0.20 wt% in-situ Pt-Ti3+/TiO2 sample exhibited enhanced photocurrent compared with the photo-reduced 0.20 wt%-Pt-Ti3+/TiO2 and 0.20 wt%-Pt/P25 samples. Such photocurrent response presented a good reproducibility for numerous on-off cycles. The highest photocurrent density of the 0.20 wt% in-situ Pt-Ti3+/TiO2 sample indicated that the photo-induced electrons could be easily transferred from the conduction band of TiO2 to Ptn+, and further to the interface surface between Pt and TiO2, where the electrons are transported to the surface of Pt0. The interfacial properties between the electrode and the electrolyte were also investigated using EIS measurements. As known, the semicircle at a high frequency in the Nyquist plot may illustrate the charge-transfer process. The diameter of the semicircle is dependent on the charge-transfer resistance (Figure 9b). The arc for 0.20 wt% in-situ PtTi3+/TiO2 under LED light irradiation (λ = 365 nm) is considerably less than those of the photoreduced 0.20 wt% Pt-Ti3+/TiO2 and photoreduced 0.20 wt% Pt/P25 samples. This implied that in-situ doped Pt-Ti3+/TiO2 has significantly increased conductivity by reducing recombination of the electron-hole pairs, which in turn enhances the photocarrier density. The open circuit potential (Voc) of the as-obtained samples was measured both in the dark and under light irradiation to calculate the photovoltage (the difference between Voc in the dark and that under light). As shown in Figure 9c, the photovoltage of Ti3+-TiO2 was about 0.178 V, which could be further increased to 0.254 V after photoreduced deposition of Pt. It should be noted that the in-situ route for embedding Pt species (with the same amount of photoreduced deposition method) resulted in a significantly increased photovoltage of 0.521 V, more than twice that of photoreduced 0.20 wt% Ti3+-TiO2. This indicated that Pt species embedded in the bulk of Ti3+TiO2 enhances the accumulation of the photo-generated electrons. For better comparison, Mott-


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Schottky plots (Figure 9d) were also utilized to calculate the carrier density of the as-obtained samples, based on the following equation:43

ND =

(3)

2 dE ( ) eε 0ε d ( 1 ) C2

Where, the detailed value for various constants can be found in Reference 43. The ND values of 0.20 wt% in-situ Pt-Ti3+/TiO2 was determined as 3.11 x 1020 cm-3, about 2.5 times that of photoreduced 0.20 wt% Pt-Ti3+/TiO2 (1.25 x 1020 cm-3). Such enhanced electron carrier density could be attributed to the embedded Pt species in the bulk of TiO2. Thus, a lower resistance and larger charge transfer rate will be favorable for the photocatalytic reactions. 200 3+

3+

Photoreduced Pt-P25

-1 -2

160

Photoreduced Pt-Ti /TiO2

In-situ Pt-Ti /TiO2

H2 evolution (µmol*m *h )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


120

80

40

0.20 wt% 0.21 wt%

0.18 wt%

0.19 wt% 0.26 wt%

0.20 wt%

0.20 wt% 0

Figure 10. Hydrogen evolution rates of various Pt doped TiO2 samples obtained via both photoreduced and in-situ routes.



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Scheme 1. The mechanism of the accelerated photo-generated electron transfer from bulk to surface of TiO2 via Ptn+-O bond. As shown in Figure 10, the hydrogen evolution performances of Pt-Ti3+/TiO2 prepared by the traditional photoreduction of platinum loading on pure Ti3+/TiO2 and commercial P25 were used as the standard for better comparison. The H2 evolution rates of all the in-situ obtained Pt-doped samples were significantly higher compared to the standard samples. The 0.20 wt% in-situ PtTi3+/TiO2 sample exhibited an H2 evolution rate of 151 µmol-m-2-h-1, and a quantum efficiency of approximately 6.2 % under light irradiation (4 x 3 W LEDs, single wavelength at 365 nm). This is the highest rate observed among all of the Pt doped Ti3+/TiO2 samples. Such high activity was about 8 times greater than that of 0.20 wt% Pt-Ti3+/TiO2 (photoreduced) and 2.70 times greater than 0.20 wt% Pt/P25 (photoreduced). Such great enhancement of the photocatalytic activity for H2 evolution may result from the uniform bulk-to-surface Ptn+ (n = 0, 2 or 3) doped mesoporous hierarchical TiO2 spheres. The formed Ptn+-O bonds in the bulk of TiO2 may facilitate the photo-generated electrons to transfer from bulk to the catalyst outer surface, achieving a higher quantum efficiency, as illustrated in the Scheme 1.

4. Conclusions


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In summary, uniform and ultrafine Pt with various valence states was in-situ doped into the Ti3+/TiO2 framework providing materials that exhibit excellent performance for H2 evolution, much higher than those obtained by the traditional photoreduction method of metallic Pt loading within Ti3+/TiO2 and commercial P25. Such enhanced activity arises because the noble Pt metalinduced self-assembly resulted in the formation of flower-rod-like microspheres with mesoporous structure, leading to strong light absorption capability. More importantly, the high Ti3+ concentration as well as oxygen vacancies allow to dope various Pt valence states into the TiO2 lattice with the formation of well dispersed Ptn+-O species, possibly providing pathways for electron transfer, facilitating photogenerated charge transfer from the bulk to TiO2 to surface with a high electron carrier density of 3.11 x 1020 cm-3. Such defects site-enriched environments, including Ti-dopants and oxygen vacancies, seem to be effective for providing a growth environment for Pt dopants, resulting in the homogeneous distribution of Pt from surface (Pt0) to internal (Ptn+). The proposed route can be extended to the design of other hybrid semiconductorbased Ti3+/TiO2 materials, such as Au-Ti3+/TiO2, Ag-Ti3+/TiO2 and Pd-Ti3+/TiO2, for photocatalytic application.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected]. *Email: [email protected].



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Notes The authors declare no competing financial interest.

5. Acknowledgements This work was supported by the National Natural Science Foundation of China (21207090, 21477079, 21677099), Shanghai Government (14ZR1430900, 15QA1403300), and by a scheme administered by Shanghai Normal University (S30406). 6. Supporting Information Available FESEM images, XPS spectra, TEM images, ICP, XRD parameters and BET results of the asobtained samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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Table of Contents Graphic


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TiO2 with Rapid Bulk to Surface

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