Efficient Visible-Light-Driven Splitting of Water into Hydrogen over

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Efficient visible-light-driven splitting of water into hydrogen over surface-fluorinated anatase TiO2 nanosheets with exposed {001} facets/layered CdS-diethylenetriamine nanobelts Kai Dai, Jiali Lv, Jinfeng Zhang, Guangping Zhu, Lei Geng, and Changhao Liang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02064 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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ACS Sustainable Chemistry & Engineering

Efficient visible-light-driven splitting of water into hydrogen over surface-fluorinated anatase TiO2 nanosheets with exposed {001} facets/layered CdS-diethylenetriamine nanobelts Kai Dai*a, Jiali Lvb, c, Jinfeng Zhanga, Guangping Zhua, Lei Genga, Changhao Liang*b a. College of Physics and Electronic Information, Anhui Key Laboratory of Energetic Materials, Huaibei Normal University, 100 Dongshan Road, Huaibei, 235000, P. R. China. Email: [email protected] b. Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and

Nanotechnology, Institute of Solid State Physics, Hefei Institutes of Physical

Science, Chinese Academy of Sciences, 350 Shushan Lake Road, Hefei, 230031, China. E-mail: [email protected] c.

Department of Materials Science and Engineering, University of Science and

Technology of China, 96 Jinzhai Road, Hefei, 230026, China.

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

Abstract: Cadmium sulfide

(CdS),

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as one of superior visible-light-driven

photocatalysts, has a prosperous and practical future in hydrogen (H2) production from water splitting for addressing the deteriorating environmental problems, such as environmental contamination and energy shortage. But the inherent drawback of serious photocorrision always limits its photocatalytic performance. Here, we fabricated a layered nanojunction to enhance H2 generation of surface-fluorinated TiO2/CdS-diethylenetriamine (F-TiO2/CdS-DETA) system. Loading of F-TiO2 nanosheets (NSs) with exposed {001} facets on inorganic-organic CdS-DETA nanobelts (NBs) greatly improve the interfacial contact. The layered nanojunction structure efficiently inhibits the charge carriers’ recombination and enhances its H2-production stability of CdS. At an optimal ratio of 30%F-TiO2, F-TiO2/CdS-DETA composite exhibits the highest H2 production rate of 8342.86 µmol h-1 g-1, which is 6.6 times and 1.7 times as high as that of CdS nanoparticles and CdS-DETA NBs, respectively. The apparent quantum yield of H2 evolution system reaches 24.9% at 420 nm with Pt co-catalyst. More importantly, the surface of F-TiO2 NSs enriches a large amount of trapping centers of photogenerated holes, which can thus efficiently promote the charge carriers’ separation and enhance the photocatalytic H2 evolution of CdS-DETA. And effective charges transfer route of F-TiO2/CdS-DETA is also demonstrated by photoluminescence test and photocurrent response. This work provides an ideal model to the design of stable and efficient H2-production photocatalysts. Keywords: Hydrogen evolution; Surface-fluorinated TiO2; CdS; Diethylenetriamine;

2


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Photocatalysis INTRODUCTION Conversion of solar light into hydrogen (H2) fuel by photocatalysts is considered to be one of the most effective strategies to mitigate increasingly aggravating environmental contamination and energy crisis.1-6 Cadmium sulfide (CdS), as an outstanding photocatalytic candidate, has got much attention because of narrow band gap energy (about 2.4 eV), suitable band edge position and low material costs.7-9 Nevertheless, the inherent photo-corrosion and poor separation efficiency greatly limited

its

photocatalytic

ability.10

Additionally,

CdS

nanoparticles

(NPs)

conglomerating also severely influences the reacted contact interface and results in fast recombination rate of photoinduced carriers during the photocatalytic reaction.11-12 To address these problems, innovation in terms of morphology control and development of material design are required. Inorganic-organic hybrids perfectly combine advantages of both inorganic and organic materials which can better serve in optoelectronic, magnetic and catalytic applications.13-15 Huang et al. had reported that the covalent organic-inorganic network could design the thickness of inorganic materials and control the optical performance.16 Yu’s research group also synthesized CoSe2-amine mesostructured nanomaterials with unique shapes and structural features.17 Zhang et al. had reported hollow inorganic-organic CdxZn1-xSe nanoframes for outstanding photocatalytic H2 evolution.18 Moreover, in our previous work, we also reported that CdS-DETA (DETA=diethylenetriamine) nanobelt (NB) hybrids with extensive surface area can

3



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improve photocatalytic H2 evolution performance.19 Additionally, as we all know, constructing CdS with other suitable semiconductors is an effective way to weaken some intrinsic drawbacks of single semiconductor materials,20-21 such as the rapid charge recombination and serious photo-corrosion. The CdS-based heterojunction structures not only offer complementary light absorption in different spectral regions, but also accelerate spatial separation of electrons from holes.22-23 Thus, fabricating CdS-based heterojunction is a feasible way to further improve the photocatalytic H2 evolution of the as-prepared inorganic-organic CdS-DETA hybrids.24 In 1972, Fujishima and Honda first studied photocatalytic water-splitting on titanium dioxide (TiO2) electrode,25 this conventional photocatalyst with high chemical stability, fine photocatalytic activity, environment-friendly society and easily synthesized property has been highlighted over the past few years.26-27 And the anatase phase surface-fluorinated TiO2 (F-TiO2) with well exposed active {001} facets possesses higher surface energy and efficient chemical adsorbability,28-29 which leads to an excellent documented photocatalytic capacity. To simultaneously enhance the photocatalytic H2 production of CdS and expand the visible-light response range of TiO2, some pursuers have made attempts in building TiO2/CdS nanojunction structures.30-31 Tada et al. had successfully constructed visible-light-driven CdS-Au-TiO2 three component system to enhance the electrons transfer and photocatalytic activity.32 Wei et al. also designed Pt@CdS core-shell photocatalyst for conversion of CO2 to methane.33 However, the novel structural engineering of planar nanojunction with high photocatalytic efficiency and superior stability is still

4


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confronted with many difficulties and dire challenges.34-35 Our group had reported facet coupled structure between F-TiO2-{0 0 1} nanosheet and g-C3N4-{0 0 2} nanosheet, this novel heterojunction area within the stacked nanosheet/nanosheet (NS/NS) is much larger than that between quasi spherical NPs and thus much higher electron-hole separation effect are expected than that of spherical/spherical heterojunction. Thus, it presented a significantly enhanced photocatalytic activity.36 From prophase investigation, in the presence of hydrogen fluoride (HF), the as-prepared F-TiO2 presented a unique two-dimension NS structure,37 which has more active sites and stronger chemical synergistic effect for building TiO2-based composites.38 And the exposed (0 0 1) facet of TiO2 equipped with many trapping centers of holes,39 can perfectly separate the photoinduced holes and electrons and thus improve its catalytic activities.40-41 Furthermore, the ideal energy band matching between CdS and TiO2 is also favorable for enhancing the photocatalytic H2 evolution. Therefore, it is desirable to obtain an effective method to increase the H2 production through incorporating F-TiO2 into CdS-DETA. Herein, we reported an in-situ growth rout to design F-TiO2/CdS-DETA hybrids with improved visible-light-response H2 production. The inorganic-organic CdS-DETA nanocrystals as the precursors were firstly prepared, owing to their special nanobelt structure and multiple surface radicals, which facilitate materials composition. The F-TiO2 NSs with highly active exposed {0 0 1} plane provide another superiority for the enhanced separation efficiency of photoexcited carriers.42 Furthermore, the in-situ growth offers a simple and effective approach for forming high-quality interfacial

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contacts between these two semiconductors with atomic precision.43 Thirdly, the large heterojunction area will supply more carrier transfer channel. Finally, the addition of F-TiO2 NSs will prevent photocorrosion of CdS. Thus, based on a combination of hydrothermal treatment, the projected F-TiO2/CdS-DETA system was successfully constructed, in which the F-TiO2 NSs are spatially distributed on CdS-DETA NBs. As a consequence of efficient holes and electrons separation and transmission driven by interaction of CdS-DETA and F-TiO2, this composite system exhibits a superior candidate for visible-light-driven photocatalytic H2 evolution.

EXPERIMENTAL SECTION

Materials P25

TiO2

was

provided

from

Degussa,

Germany.

Cadmium

chloride

(CdCl2•2.5H2O), sublimed sulfur (S), tetrabutyl titanate (Ti(OC4H9)4) and DETA were purchased by Shanghai Chemical Reagent Co. Ltd., China. HF (40 wt.%) was obtained by Sinopharm Chemical Reagent Co. Ltd., China. Fabrication of F-TiO2/CdS-DETA composite Firstly, F-TiO2 NSs were prepared by the methods in the previous report.36 In typical preparation, HF (3.0 mL) and Ti(OC4H9)4 (25.0 mL) were stirred and then transferred into autoclave (50 mL) and subsequently heated at 453 K for 24 h. F-TiO2 NSs were collected by repeated washing with 18.2 MΩ distilled water and dried at 333 K. A certain amount of as-prepared F-TiO2 NSs, CdCl2•2.5H2O (0.1642 g), DETA (24 mL) and 18.2 MΩ distilled water (12 mL) were initially magnetic stirred for 1 h. Then, S (0.1281 g) was added and further stirred for the whole day. After that, the

6


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mixture was heated at 353 K for 48 h in autoclave. The precipitates were collected by repeated centrifugation and dried in a freeze-dryer at 213 K. Besides, the CdS-DETA NBs were also prepared by the same method without F-TiO2. Characterization X-ray diffraction (XRD Rigaku D/MAX 24000) diffractometer was applied to investigate phase structure of F-TiO2, CdS-DETA and F-TiO2/CdS-DETA composite with Cu Kα radiation ( λ = 1.5406 Å). The structure and morphology of the as-prepared samples were observed on field emission scanning electron microscopy (FESEM, Hitachi S5500). High resolution transmission electron microscopy (HRTEM JEOL JEM-2010) with scanning TEM energy dispersive X-ray spectroscopy (STEM-EDS) mapping was used to further record the shape and element composites of samples. The infrared absorption spectra were recorded on a Fourier transform infrared (FT-IR Nicolet 6700). UV-Vis diffuse reflectance spectroscopy (DRS Perkin Elmer UV/VIS/NIR Spectrometer labda 950) measurements and photoluminescence (PL FLS920) spectra were used the study the optical performance of F-TiO2/CdS-DETA composite. Chemical status of F-TiO2/CdS-DETA composite was investigated by X-ray photoelectron spectra (XPS Thermo ESCALAB 250). The photoelectrochemical

measurement

spectrum

was

received

on

CHI-660D

electrochemical system with a conventional three-electrode cell in 1.0 M Na2SO4 electrolyte solution. And Pt wire and saturated calomel electrode (SCE) worked as the counter and reference electrodes.

Photocatalytic H2 evolution test

7



Page 8 of 36

As-synthesized photocatalyst (50 mg) was stirred in a reactor containing 100 mL of mixed aqueous solution containing 0.35 mg L-1 Na2S and 0.25 mg L-1 Na2SO3 and then bubbled with N2 gas for 45 min to remove the air. Pt cocatalyst (0.6 wt%) was photodeposited on the surface of photocatalysts by dissolving H2PtCl6 into the reactor. 300 W Xe lamp (CEL-HXF300, Ceaulight, China) equipped with a 420 nm-cut-off filter was used as light source. And the distance between the reactor and Xe lamp is 20 cm. The produced H2 was determined with the online gas chromatography (Ceaulight GC-7900, TCD). Furthermore, the apparent quantum efficiency (QE) for H2 production was calculated according to Eq. (1):

QE (%) = =

number of reacted electrons ×100 number of incident photons number of evolved H 2 molecules × 2 number of incident photons

× 100

(1)

RESULTS AND DISCUSSION F-TiO2/CdS-DETA NBs were designed by the facile solvothermal process as

indicated in Scheme 1. F-TiO2 NSs were firstly synthesized through a typical solvothermal method. Then, S, Cd2+ ions and F-TiO2 NSs were blended and interacted in the mixed solution of DETA and H2O at low temperature. During this progress, DETA molecule in the solution is firstly protonated by reaction with H2O. Afterwards, the protonated DETA molecules cooperated with S on the surface of F-TiO2 NSs in the heating. Finally, the inorganic-organic F-TiO2/CdS-DETA composite was successfully prepared.

8


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Scheme 1 Schematic representation of the preparation of F-TiO2/CdS-DETA hybrids

Figure 1 XRD patterns of different rate of F-TiO2, F-TiO2/CdS-DETA and CdS-DETA As-prepared samples of F-TiO2/CdS-DETA nanostructures are characterized by using XRD technique to investigate the crystalline phase. Figure 1 exhibits the XRD patterns of F-TiO2, F-TiO2/CdS-DETA composites and CdS-DETA. The characteristic peaks of CdS-DETA at 25.02o, 26.59o, 28.36o, 36.77o, 43.90o and 48.07o, which can be indexed to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) diffraction plane with hexagonal phase and the line matches well with the reported value (JCPDS NO. 080-0006, space group: P63mc/186, a=b=4.121 nm and c=6.682 nm). For F-TiO2, XRD peaks are observed at 2θ = 25.45o, 38.08o, 48.16o, 54.13o, 55.15o, 62.85o and 9



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70.43o, corresponding to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4) and (2 2 0) diffraction planes respectively, which are well matched with TiO2 (JCPDS NO. 21-1272). The F-TiO2/CdS-DETA composite samples exhibit diffraction peaks corresponding to both F-TiO2 and CdS. But, the peaks at (1 0 0) of CdS have slightly shifted and strengthened with increasing concentration of F-TiO2, which reflect their contents in the F-TiO2/CdS-DETA hybrids. The morphologies and structures of F-TiO2/CdS-DETA hybrids can be directly recorded by FESEM characterization, and we can specifically observe the influence of F-TiO2 NSs on CdS-DETA NBs. The FESEM micrograph in Figure 2a shows aggregation of F-TiO2 NSs with the length of 50-60 nm. Figure 2b depicts that F-TiO2/CdS-DETA composites express multilevel structure. As comparison, CdS NPs with the size of 80~200 nm are also fabricated (Figure S1). The TEM image of F-TiO2 and CdS-DETA NBs are shown in Figure 2c and 2d, respectively. Most of CdS NBs are very thin with the width of 15~30 nm and the length of hundred nanometers. Figure 2e shows the TEM image of F-TiO2/CdS-DETA system, where F-TiO2 NSs cover the surface of CdS-DETA NBs. HRTEM image of F-TiO2/CdS-DETA hybrids in Figure 2f shows the spacing of lattice fringes are perpendicular to the growth direction about 0.35 nm, which is corresponding to the lattice parameter in (0 0 1) facet of TiO2. Furthermore, the lattice fringes of CdS NBs can also be observed clearly, the fringes with a lattice spacing of 0.32 nm consistent with the (1 0 0) facet of CdS. STEM-EDS mapping image in Figure 2g is employed to investigate elemental distribution of F, O, Ti, Cd, S and N in F-TiO2/CdS-DETA composites. The

10


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green, red and orange regions in images are the Ti-containing, O-containing and F-containing portions of the as-prepared photocatalysts, whereas the cyan, yellow and blue regions are Cd-containing, S-containing and N-containing elements, respectively. As it can be seen that, the elements of Ti, O, F, S, Cd and N are well dispersed in F-TiO2/CdS-DETA composites. Hence, F-TiO2 NSs are uniformly dispersed on CdS-DETA NBs.

Figure 2 SEM images of (a) F-TiO2, (b) CdS-DETA, and (c) F-TiO2/CdS-DETA photocatalysts; TEM images of (d) CdS-DETA NBs and (e) the hybirds of F-TiO2/CdS-DETA; (f) HRTEM images of the F-TiO2/CdS-DETA NBs composites; (g) STEM-EDS elemental maps of F-TiO2/CdS-DETA NBs composites overlap, Ti (green), O (red), F (orange), Cd (blue), S (yellow) and N (cyan), respectively. Scale bar: 500 nm. 11



Page 12 of 36

In order to make sure the existing elements and understand surface chemical state of F-TiO2/CdS-DETA composites, XPS spectra were used in this work. Figure 3a exhibits the XPS survey spectrum of the as-synthesized photocatalysts. The binding energies of F 1s, Ti 2p, Cd 3d, O 1s, S 2p, C 1s and N 1s are observed and respectively corresponding to photoelectron peaks at 683.6 (F 1s), 530.7 (O 1s), 458.2 (Ti 2p), 405.5 (Cd 3d), 163.2 (S 2p), 285 eV (C 1s), and 398.6 eV (N 1s). The two strong peaks in Figure 3b at 404.2 and 410.9 eV are assigned to Cd 3d5/2 and Cd 3d3/2 for Cd2+ ion for CdS crystal,44 respectively. Furthermore, the XPS spectrum for S 2p in Figure 3c demonstrates the main peak at 161.1 eV, which is assigned to the binding energy of S 2p for S2-.45-46 As shown in Figure 3d, the measured binding energies of Ti 2p3/2 and Ti 2p1/2 are respectively centered at 458.37 and 463.94 eV, which are very similar with those of pure TiO2.47-48 The XPS spectra for O1s in Figure 3e can be separated into two peaks centered at 531.08 and 529.22 eV, which can be attributed to hydroxyl radicals and lattice oxygen of TiO2.49-50 In addition, the peak located at 683.88 eV can be indentified from F 1s in Figure 3f, and the typical feature peak can be ascribed to Ti-F species from the defects on F-TiO2 crystal surface.43 N 1s centered at 398.62 eV in Figure 3g and C 1s centered at 284.9 eV in Figure 3h are from DETA molecule of CdS-DETA.51-52 Thus, the XPS results confirm that both CdS-DETA and F-TiO2 are well composited in F-TiO2/CdS-DETA system.

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b

1000

800

600

400

410.9

Intensity (a. u.)

C1s 1200

200

414

0

412

410 408 406 Binding energy (eV)

Binding energy (eV)

c S2p

d Ti2p Intensity (a. u.)

161.1

Intensity (a. u.) 172

170

168

166

164

162

160

468

158

O1s

463.94

466

464

532

530

460

458

456

454

528

Intensity (a. u.)

683.88

526

690

688

Binding energy (eV)

686

684

682

680

678

Binding energy (eV)

g N1s 398.62

400

462

f F1s

529.22

534

402

Binding energy (eV)

531.08

Intensity (a. u.) 536

404

Binding energy 458.37(eV)

Binding energy (eV)

e

404.2

Cd3d

S2p

O1s Ti2p

F1s

N1s

Intensity (a. u.)

Cd3d

a

h C1s

284.9

Intensity (a. u.)

Intensity (a. u.)

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


399

398

397

396

Binding energy (eV)

395

394

298

296

294

292

290

288

286

284

282

280

Binding energy (eV)

Figure 3 The overview (a) and the corresponding high-resolution XPS spectra (b) Cd 3d, (c) S 2p, (d) Ti 2p, (e) O 1s, (f) F 1s, (g) N1s, and (h) C1s of F-TiO2/CdS-DETA system.

The existence of TiO2 and CdS in the F-TiO2/CdS-DETA system can be further confirmed by FT-IR analysis. The comparison of the FT-IR spectra of F-TiO2,

13



CdS-DETA NBs and F-TiO2/CdS-DETA hybrids is shown in Figure 4. As for CdS-DETA, some strong vibration bands are observed in 3416, 1626, 1398 and 864 cm-1, which are corresponding to -NH, -NH2, -CH2 and -CH, respectively,

19, 53

suggesting the existence of DETA in the CdS-DETA NBs. Additionally, the result is also equal to that of the XRD experiment. DETA not only supplies the functional group, but also facilitates the special uniform shape with large surface area (Figure S2). Transmittance (a. u.)

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

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F-TiO2 40% F-TiO2/CdS-DETA

30%F-TiO2/CdS-DETA 20% F-TiO2/CdS-DETA 10% F-TiO2/CdS-DETA

CdS-DETA

-CH -NH2 -CH

-NH 4000

3500

3000

2500

2000

1500

2

1000

500

Wavenumber (cm-1)

Figure 4 FT-IR spectra of CdS-DETA, F-TiO2/CdS-DETA and pure F-TiO2.

UV-Vis absorption spectra of F-TiO2 nanosheets, F-TiO2/CdS-DETA hybrids and CdS-DETA NBs are shown in Figure 5. The colour transformation of /CdS-DETA, F-TiO2/CdS-DETA and F-TiO2 is shown in Figure S3. The colour of CdS-DETA is slightly yellow and F-TiO2 is white. As indicated in Figure 5a, the absorption edge of CdS-DETA is 552 nm, while the absorption wavelength of F-TiO2 is shorter than 420 nm. In addition, the absorption bands in the visible region at about 450-550 nm for

14


Page 15 of 36

those F-TiO2/CdS-DETA samples gradually declined with the increase of F-TiO2, which can be attributed to the intrinsic band gap absorption of CdS and TiO2. This result indicates that F-TiO2/CdS-DETA is coupled well and can be regarded as visible-light-driven photocatalysis. The following equation can be used to calculate the band gap energy for F-TiO2 NSs and CdS-DETA NBs:54

α hν = A(hν − Eg ) n /2

(2)

where n is determined by the type of optical transition of a photocatalysis. The values of n are 1 and 4 for F-TiO2 NSs and CdS-DETA NBs, respectively. α , hν , A and

Eg are the absorption coefficient, light energy, constant value and energy band gap, respectively. Thus, according to Eq. (2), the Eg of CdS-DETA NBs is calculated as 2.33 eV, while the Eg of F-TiO2 is calculated as 3.07 eV (Figure 5b). The test results are in agreement with previous report.20, 47 Moreover, Eg of F-TiO2/CdS-DETA is broadens increasingly with the growth of F-TiO2 in visible light region.

a

CdS-DETA

b40

2.0

30

1.5

30 % F-TiO2/CdS-DETA 40 % F-TiO2/CdS-DETA F-TiO2

(α υ )2 (eV)2 α hυ

20 % F-TiO2/CdS-DETA

1.0

20

CdS-DETA

F-TiO2 0.5

10 Eg = 2.33 eV

300

400

500

600

Wavelength (nm)

700

800

0 1.5

1/2 1/2 (α α hυ υ ) (eV)

10 % F-TiO2/CdS-DETA

Absorbance (a. u.)

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


2.0

Eg = 3.07 eV

2.5

3.0

3.5

0.0 4.0

hυ υ (eV)

Figure 5 UV-Vis DRS spectra of F-TiO2, CdS-DETA and F-TiO2/CdS-DETA composite photocatalysts and (b) plots of (αhν)2 versus energy (hν) for F-TiO2 and CdS-DETA NBs.

15



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The optimized crystal structures and the calculated electronic band structures of hexagonal CdS and anatase TiO2 are shown in Figure 6. As indicated in Figure 6a and 6b, it can be found that CdS belongs to the direct band gap photocatalyst because valence band maximum (VBM) and conduction band minimum (CBM) locate at same high symmetry point. However, TiO2 is an indirect band gap semiconductor because CBM and CBM locate at different high symmetry point. The calculated Eg values of CdS and TiO2 are 1.40 and 2.13 eV, respectively, which are much smaller than their experimental results (2.33 eV for CdS and 3.07 eV for TiO2) because of the drawback of GGA function.55-56

The

GGA

function

most

extensively

applied

usually

underestimate the semiconductor band gaps by about 30–100% compared with experimental results. This underestimation of band gap is closely related to the inherent lack of derivative discontinuity and delocalization error in the exchange–correlation functional derivative. Further observation Figure 6c and 6d, the upper VB and CB of CdS are composed of S 3p, and Cd 5s states. In addition, the upper VB of TiO2 mainly consist of O 2p, which is mixed with a few Ti 3d states. Moreover, the CB of TiO2 is mainly composed of Ti 3d states and a few Ti 3p and O 2p states.

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3

3

Energy / eV

b4

Energy (eV)

a4 2 1

1.40 eV

0 -1 -2

2 2.13 eV

1 0 -1

G

A

H

G

K

M

L

-2

H

6

A M

Z

R X

G Z

G

d

4

DOS (electrons / eV )

c DOS (electrons / eV )

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


Cd 4d Total S 3s

2

Cd 5s

0 -2

0

2

4

Energy / eV

6

16 Total 12

8 O 2p

Ti 3d

4 Ti 3p 0 -5

-4

-3

-2

-1

0

1

2

3

4

5

Energy / eV

Figure 6. Band structure for (a) CdS and (b) TiO2. DOS for (c) CdS and (d) TiO2.

Figure 7 shows the PL spectra of pure CdS-DETA and F-TiO2/CdS-DETA samples. The highest emission peak was centered at 590 nm for pure CdS-DETA, and it is worth noting that the position of the peak in PL spectrum was slightly red shifted with the increase of F-TiO2, but the emission intensity significantly reduced, which suggested that F-TiO2/CdS-DETA composites could efficiently decrease the recombination rate of photoinduced charge carriers. A new peak at 460.8 nm appeared, which may be attribute to the defects of F-TiO2/CdS-DETA composites. Furthermore, 30%F-TiO2/CdS-DETA hybrids exhibit the lowest emission intensity, suggesting that 30%F-TiO2/CdS-DETA samples have superior separation efficiency than those in the others. Thus, the as-prepared F-TiO2/CdS-DETA hybrids could effectively reduce the recombination rate of photo-generated electrons and holes and improve the

17



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photocatalytic activity of CdS-DETA by adding of F-TiO2 NSs.

Figure 7 PL spectra of pure CdS-DETA and F-TiO2/CdS-DETA samples.

Photocurrent measurement can intuitively detect the total amount of electron and hole pairs generation within the photocatalysis. Usually, the higher the intensity of photocurrent appears, the more effectively photoinduced charge carriers are separated. Thus, to further demonstrate the photocatlytic mechanism of as-prepared samples, photocurrent-time measurement was utilized in an on-and-off cycle mode as shown in Figure

8.

The

photocurrent-time

figures

of

CdS-DETA,

F-TiO2

and

F-TiO2/CdS-DETA composites were shown with two on-off intermittent illumination cycles. It can be seen that the electrodes of the photocatalysts present a quick photocurrent response once the switch with visible light irradiation is turned on. Moreover, 30%F-TiO2/CdS-DETA exhibits the highest photocurrent intensity comparing with F-TiO2, CdS-DETA and other F-TiO2/CdS-DETA composites. The conclusions of photocurrent measurement provide another powerful evidence to 18


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illuminate the high separation rate of F-TiO2/CdS-DETA hybrids, which is profitable for photocatalytic activities under visible light excitation.

Figure 8. Photocurrent-time curves of CdS-DETA, F-TiO2/CdS-DETA composites and F-TiO2.

Photocatalytic H2 evolution performance of CdS-DETA, F-TiO2/CdS-DETA composites and F-TiO2 is measured and compared in Figure 9a and S4. For pure F-TiO2 NSs, the photocatalytic H2 production can almost ignorable within the visible light irradiation. But CdS-DETA NBs and F-TiO2/CdS-DETA composites exhibit obviously H2 evolution activity under identical condition. In addition, the optimal photocatalytic performance of F-TiO2/CdS-DETA was obtained when the loading of 30%F-TiO2 with the H2 production rate about 8342.86 µmol h-1 g-1, which is 6.6and 1.7 times as high as that of CdS NPs and CdS-DETA NBs, respectively. The apparent quantum yield of H2 evolution of F-TiO2/CdS-DETA system reaches 24.9%. The limited loading of F-TiO2 can not bring about abundant visible light absorption, while overweight loading of F-TiO2 may easily produce defects and then affect the H2 19



Page 20 of 36

evolution activity. Moreover, the photocatalytic performance of physical mixture of F-TiO2 NSs and CdS-DETA NBs are tested as well, with a weight ratio of 3:7, it exhibits an obviously lower H2 production rate than the as-synthesized F-TiO2/CdS-DETA composites as shown in Figure 9a. This consequence can be attributed to the poor contact between the physical mixture of F-TiO2 and CdS-DETA NBs, which block the interface photoinduced charge transfer. Hence, the as-prepared F-TiO2/CdS-DETA photocatalysts can efficiently suppress the recombination of photoexcited holes and electrons, which greatly enhances the photocatalytic activity in H2 evolution. Figure S5a exhibits the N2 gas adsorption−desorption curves for F-TiO2, F-TiO2/CdS-DETA composites and CdS-DETA at 77 K. Figure S5b exhibits the SBET values of different catalysts. We can find that SBET value of 30%F-TiO2/CdS-DETA is not the highest. Though surface area is important for catalysis,57 it is not key reason for high H2 evolution. In addition, the 30%F-TiO2/CdS-DETA also exhibits stable photocatalytic activity in H2 production as investigated in Figure 9b. There is almost no change in the rate of H2 production even after 9 hours under visible light irradiation. But the pure CdS-DETA only maintains about 78 % H2 production under the same condition. Moreover, the crystal structure of reused 30%F-TiO2/CdS-DETA sample is further tested by XRD instrument (Figure S6). The crystal structure of 30%F-TiO2/CdS-DETA can be well maintained after three-cycling testing. Thus, it suggested that F-TiO2/CdS-DETA nanocomposite exhibited the high photostability and efficient H2 production.

20


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Figure 9. (a) Comparison of photocatalytic H2-production rates of CdS-DETA, F-TiO2/CdS-DETA and F-TiO2 samples from 0.35 M Na2S and 0.25 M Na2SO3 mixed aqueous solutions under visible light (λ>400 nm); (b) Time cycles of photocatalytic H2 evolution for 30%F-TiO2/CdS-DETA samples.

The band structure of F-TiO2 and CdS can be valued by the concepts of electronegativity and calculated as follows:58

ECB = X − E e − 0.5E g

(3)

EVB = ECB + E g

(4)

Where Ee is 4.5 eV, X is the electronegativity for semiconductor, EVB and ECB are the VB and CB potential, respectively. X values for the F-TiO2 and CdS-DETA are 5.25 and 5.81 eV, respectively. Therefore, the ECB and EVB of F-TiO2 are calculated as -0.225 and 2.845 eV, while the ECB and EVB of CdS-DETA are -0.415 and 1.915 eV. On the basis of the above discussion, it is clear that the rate of H2 is lower over pure CdS-DETA NBs than that of F-TiO2/CdS-DETA due to its rapid recombination of photoinduced carriers. And the large overpotential of F-TiO2 NSs also impacts the H2 production efficiency. Thus, F-TiO2/CdS-DETA photocatalysis could increase the 21



Page 22 of 36

photocatalytic H2 production by loading a balance amount of F-TiO2 NSs with exposed (1 0 0). A possible mechanism of the H2 evolution reaction on the as-prepared F-TiO2/CdS-DETA hybrids is put forward as shown in Figure 10. Firstly, as we have known it, the pure F-TiO2 cannot work under visible light excitation because of its large energy band gap (3.07 eV), but organic-inorganic CdS-DETA hybrid exhibits an excellent H2 production under visible light. When the CdS-DETA is excited by visible light, the photo-induced electrons can easily transfers from CB of CdS-DETA NBs to the CB of F-TiO2. Thus, loading proper portion of F-TiO2 NSs on the CdS-DETA surface to develop a close interaction between F-TiO2 and CdS-DETA,59 which is a easy way to hinder the photoinduced charge carriers’ recombination

and

significantly

enhance

H2

production.

And

the

lower

redox-potential position of F-TiO2/CdS-DETA makes the photoexcited electrons from CB of CdS-DETA NBs to the CB of F-TiO2.60 Furthermore, Pt NPs which were deposited on the CdS-DETA by photoreduction may serve as electron captures to promote the photoinduced electrons’ separation and migration.61 Secondly, the as-prepared F-TiO2 NSs with exposed (0 0 1) faces have more electrons which could participate in the relevant reaction. As shown in Figure 10, the photoinduced electrons and holes are located separately on CdS-DETA and F-TiO2, which induce the charges separation and improve the photostability of the as-prepared F-TiO2/CdS-DETA composites. As the charges gradually accumulate, the interpolar electric field produced by polarization can further promote the separation of the photoinduced electrons and holes and then transmit them to the surface fast.62-63 Therefore, the

22


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interpolar electric field can accelerate the transport of the photoinduced charges and enhance photocatalytic H2 evolution. Furthermore, the large combination area structure will supply enough light absorption and charge transition path, which significantly improve the H2 evolution. Thus, it is reasonable that the photocatalytic H2 production is greatly enhanced with the appropriate content of F-TiO2. But we also could find that the H2 production activity declined rapidly when the proportion of F-TiO2 is more than 30%. This is because the overweight of F-TiO2 can prevent visible light absorption, and the recombination center of photogenerated carriers could be another important reason for the depressed H2 evolution.

Figure 10. The possible charge transfer process under visible light irradiation. CONCLUSIONS In conclusion, F-TiO2/CdS-DETA nanostructure photocatalyst was successfully prepared by a solvothermal treatment. The photocatalytic H2 evolution of the as-prepared samples are remarkably improved with the combination of F-TiO2 exposed (0 0 1). The optimal F-TiO2 loading concentration is 30% and H2 evolution

23



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rate is 8342.86 µmol h-1 g-1 with Pt as co-catalyst. F-TiO2/CdS-DETA photocatalysts can be well excited under visible light, which can lead to effective photoinduced holes and electrons separation and thus enhance the photocatalytic H2 evolution. This study offers a kind of novel and high-efficiency F-TiO2/CdS-DETA photocatalysts with economical applications in photocatalytic H2 evolution.

ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxxxxxxxxxx

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of

China (51572103 and 51502106), the Distinguished Young Scholar of Anhui Province (1808085J14), the Key Foundation for Young Talents in College of Anhui Province (gxyqZD201751), the Key Foundation of Educational Commission of Anhui Province (KJ2016SD53) and Application of Advanced Energetic Materials (22700200).

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Graphic abstract Surface-fluorinated anatase TiO2 nanosheets with exposed {001} facet/layered CdS-diethylenetriamine nanobelt composite shows high visible light photocatalytic H2 evolution activity.

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Efficient Visible-Light-Driven Splitting of Water into Hydrogen over

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