Acid-Exfoliated Metallic Co0.85Se Nanosheets as Cocatalyst on Cd0

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Acid-Exfoliated Metallic Co0.85Se Nanosheets as Cocatalyst on Cd0.5Zn0.5S for Photocatalytic Hydrogen Evolution under Visible Light Irradiation Xiaohui Sun, and Hong Du ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b03361 • Publication Date (Web): 04 Sep 2019 Downloaded from pubs.acs.org on September 4, 2019

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ACS Sustainable Chemistry & Engineering

Acid-Exfoliated Metallic Co0.85Se Nanosheets as Cocatalyst on Cd0.5Zn0.5S for Photocatalytic Hydrogen Evolution under Visible Light Irradiation

Xiaohui Sun, Hong Du* Center of Electrochemical Technology and Application Engineering Research, College of Chemistry and Chemical Engineering, Xinjiang Normal University, 102 Xinyi Road, Urumqi 830054, China *Email：175790509@ qq.com

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ABSTRACT: Co0.85Se, Nonstoichiometric phase, as an excellent electrocatalytic activity, was used as a cocatalyst to modify Cd0.5Zn0.5S nano-photo catalyst. The Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S Schottky junction shows the greatest activity for H2 generation (759.3 μmol h-1), which is 2.1 folds than that of bare Cd0.5Zn0.5S (350.6 μmol h-1). It turns out that Co0.85Se nanoplates can be exfoliated into nanosheets by diluted hydrochloric acid. It is noted that exfoliated metallic Co0.85Se nanosheets show a little better HER property and a smaller the charge transfer resistance than those of Co0.85Se nanoplates, implying that exfoliated Co0.85Se nanosheets have more active sites and higher electrical conductivity. The distinct properties of exfoliated metallic Co0.85Se nanosheets enhance the efficient spatial separation of photo-induced charged particle carriers, and results in an excellent enhancement in the photoelectric current and photocatalytic hydrogen evolution performance of Cd0.5Zn0.5S. The amount of Co0.85Se in the heterojunction has an obvious effect on the hydrogen evolution property. The consequences show that 1.5 wt% exfoliated Co0.85Se nanosheets added into Cd0.5Zn0.5S sample turns out the maximum water splitting rate of 759.3 μmol h-1. For other semiconductor photocatalyst systems, our work demonstrates that introducing exfoliated metallic Co0.85Se nanosheets electrocatalyst as an efficient co-catalyst to improve photocatalytic capacity without noble metals is feasible. KEYWORDS: Acid-exfoliated, Co0.85Se, Schottky junction, Photocatalytic H2 evolution

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INTRODUCTION Transition metal chalcogenides (TMCs) was found to be a high-active materials, including cobalt selenide (CoSe,1 CoSe2,2,3 Co3Se4,4 Co7Se8,5 Co9Se8,6 Co0.85Se7,8) has attracted much attention ascribed to its excellent electrocatalytic properties. And compared to noble metals (Pt, Ag, Au, etc.), cobalt selenide as the free-noble metals has high conductivity, low cost, high catalytic activity and chemical stability. Co0.85Se shows similar performance of zero-valent metals, which is different from stoichiometric phase cobalt selenide (CoSe2, CoSe), owing to the strong hybridization between the Co 3d and Se 4p spin-up (alpha) electrons, making it presenting the half-metallic characterization (high conductivity). The half-metallic behavior of Co0.85Se can significantly promote electron transportation in the photocatalyst.9,10 Such as Yu et al.7,11 synthesized hexagonal-type nanocrystalline Co0.85Se. Co0.85Se embedded on reduced graphene nanosheets exhibits excellent electrocatalytic hydrogen production performance. The excellent electrocatalytic property, stability, earth abundance, except for low-cost preparation, make Co0.85Se an outstanding candidate alternative to noble metal electrocatalysts. Electrochemical water splitting affords a feasible way to obtain H2 with high purity while it consumes a lot of electric energy. So photocatalytic water splitting has become a challenging and remarkable study in solving the energy and environmental crisis.12,13 CdS nanostructure, with a narrow direct band gap (2.4 eV), is a most common excellent photocatalyst ascribing to its high visible light harvesting capacity and suitable band edge potentials.14-16 However, it has low photocatalytic efficiency and serious photocorrosion12,17 attributed to fast recombination of electron-hole pairs. Thus, in order to obtain stable and efficient photocatalyst, ternary chalcogenide Cd1-xZnxS solid solution

18-20

was synthesised has attracted increasing attention in the field of photocatalytic hydrogen

generation. The appropriate band gap of Cd1-xZnxS can be adjusted from a small band gap 2.4 eV (CdS) to a large bandgap 3.7 eV (ZnS) by changing its composition.21-23 Photo-stability and photocatalytic activity of Cd1-xZnxS can be improved in some extent. However, similar to CdS, pure Cd1-xZnxS tends to form photocorrosion layer and the aggregation of large particle, issuing in unstable structures and a high electron-hole (e−/h+) recombination rate. It inhibited the further enhancement of photocatalytic performance.24,25 Meanwhile, in order to promote the photocatalytic activity of Cd1-xZnxS, a large

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number of researches has been done in hunting suitable cocatalysts to improve the performance of photocatalytic materials. In 2015, Du26 designed a novel MoO2-Zn0.5Cd0.5S schottky junction. The results show that Zn0.5Cd0.5S sample with 2 wt% metallic MoO2 exhibits the best H2 evolution (252.4 μmol h-1), whereas MoO2/Zn0.5Cd0.5S was prepared by high-temperature calcined method, which has some drawbacks (energy consumption and high risk). Moreover, the Schottky junction formed between g-C3N4 and Cd1-xZnxS showed high photocatalytic activity.25,27,28 Recently, Zhan et al.29 synthesized metallic 1D CoP nanowires as a cocatalyst to form a well-designed integrated with Zn0.5Cd0.5S photocatalyst. The experimental results show that the hydrogen evolution efficiency of CoP/Zn0.5Cd0.5S heterojunction has attained to 12175.8 μmol·h-1∙g-1 under solar light irradiation. Herein, we synthesized a highly efficient photocatalytic H2 generation system with Cd0.5Zn0.5S nanorods working as the light absorber and metallic Co0.85Se nanoparticles as the cocatalyst. First, Co0.85Se nanoplates were prepared by facile solvothermal synthesis at 455 K. Then Co0.85Se nanoplates were exfoliated into nanosheets by diluted hydrochloric acid. The Co0.85Se/Cd0.5Zn0.5S Schottky junction was prepared under hydrothermal conditions. Interestedly, the rate of H2 evolution of the optimized Co0.85Se/Cd0.5Zn0.5S (759.3 μmol h-1) higher than that of pure Cd0.5Zn0.5S (350.6 μmol h-1), which revealed that Co0.85Se/Cd0.5Zn0.5S heterojunction have super photocatalytic activity attributed to loading metallic Co0.85Se cocatalyst. Our results indicated that owning earth-rich, high efficiency, and durability, metallic Co0.85Se is a capable candidate for in other photocatalytic hydrogen production systems.

RESULTS AND DISCUSSION X-ray diffraction (XRD) was employed to measure the crystal structure of obtained samples. As shown in Figure 1a, the XRD patterns of Co0.85Se nanoplates and exfoliated Co0.85Se nanosheets. The XRD peaks at 33.3°, 44.7°, 50.6°, 60.4°, 61.9° and 69.9° respectively corresponding to (101), (102), (110), (103), (112) and (202) of Co0.85Se hexagonal phase (JCPD 52-1008). After 0.01 M HCl treatment, the main peak of Co0.85Se is unchanged. Furthermore, the exfoliated Co0.85Se nanosheets only show a much weaker peak at 33.3°, 44.7°,50.6°, suggesting the successful exfoliation of Co0.85Se nanoplates

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into few-layer-thick nanosheets after acid-assisted treatment.30 The morphology of Co0.85Se nanoplates and exfoliated Co0.85Se nanosheets are shown in Figure 1b and c. It is clearly seen from Figure 1b that the morphology represents irregular nanoplate, and the size is approximately 100 nm and the surface of Co0.85Se nanoplates is smooth. Figure 1c shows the typical SEM image of exfoliated Co0.85Se nanosheets, which indicates that they consist of a smaller thickness of individual nanosheets and a larger overall surface size after acid-assisted liquid treatment. In particular, the surface of Co0.85Se becomes rough and incomplete after treated with hydrochloric acid, and the structure of which is similar with “flower” can expose abundant active sites for photocatalysis. The XRD patterns of Cd0.5Zn0.5S and Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S nanocomposites are shown in Figure 1d. The diffraction peak at 26.2°, 27.7°, 29.5°, 38.3°, 45.6°, 50.2° and 53.7° can be well indexed to (100), (002), (101), (102), (110), (103) and (112) of ternary compound Cd0.5Zn0.5S (JCPD 49-1302). No characteristic diffraction peaks of Co0.85Se are detected probably because of the low loading content (1.5 wt%) in nanocomposites. exfoliated

Co0.85Se nanosheets

Intensity (a.u.)

(a)

(b)

Co0.85Se nanoplates

(101)

(102)

(110)

100 nm

(103)(112) (202) (004)

10

20

30

40

50

60

70

80

2-theta (degree)

(c)

(d)

Cd0.5Zn0.5S

(112)

(110) (103)

(102)

(100)

(002)

Co0.85Se (1.5wt%)/Cd0.5Zn0.5S

(101)

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


100 nm 10

20

30

40

50

2-theta (degree)

60

70

Figure 1. XRD patterns of Co0.85Se nanoplates and exfoliated Co0.85Se nanosheets (a), SEM of Co0.85Se nanoplates (b) and exfoliated Co0.85Se nanosheets (c), XRD pattern of Cd0.5Zn0.5S and Co0.85Se (1.5 5



wt%)/Cd0.5Zn0.5S (d). The structure change of Co0.85Se sample before and after acid-assisted treatment are investigated by the Raman spectra. As can be seen from Figure 2a, The distinct Raman peaks of Co0.85Se nanoplates at 185 cm−1 and 664 cm−1 belong to the Ag and A1g modes of CoSe2, respectively, and the characteristic peaks at 460 cm−1, 506 cm−1, and 603 cm−1 are indexed to the Eg, F12g and F22g modes of trivalent cobalt compounds, respectively.31-33 The characteristic peaks of Ag, A1g, Eg, F12g and F22g of exfoliated Co0.85Se nanosheets shift to a higher wavenumber, which suggests the successful exfoliation of Co0.85Se nanoplates.34 Besides, the peak at 476 cm-1 of Co0.85Se nanoplates, ascribed to hexagonal cobalt element,35 disappeared when Co0.85Se nanoplates were treated with HCl, which may be attributed to the reaction between HCl and Co0.85Se. Meanwhile, the tiny peak at 145 cm-1 of exfoliated Co0.85Se nanosheets is assiged to trigonal selenium that maybe the reason for lattice selenium atom,35 The peak at 168 cm-1 of exfoliated Co0.85Se nanosheets confirms to the Se-Se tensile model of cubic Co0.85Se.36 In addition, Figure 2b revealed Raman spectrums of Cd0.5Zn0.5S and Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S. The Ramtan peaks at 317 cm−1 and 625 cm−1 are allocated to the 1 LO and 2 LO longitudinal optical phonon peaks of Cd0.5Zn0.5S, respectively.37 After the Co0.85Se nanosheets doped in Cd0.5Zn0.5S, the Raman intensity of LO phonon modes of Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S are still strong and narrow, indicating the good crystallinity and ordered structure of sample.38

Se-Se 168

Se 145

Ag 191

Eg 471

1

F2g 514

2

F2g

A1g

676

611

Co0.85Se nanoplates Ag 185

200

(b)

exfoliated Co0.85Se nanosheets

Co 476 Eg 460

400

1

F2g F 2g 2

A1g 664

506 603

600

800

Intensity (a.u.)

(a) 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

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Co0.85Se(1.5wt%)/Cd0.5Zn0.5S

315

1LO 625

1000

-1

Raman shift (cm )

200

400

2LO

600

Cd0.5Zn0.5S

800

1000

-1

Raman shift (cm )

Figure 2. Raman spectra of (a) Co0.85Se nanoplates and exfoliated Co0.85Se nanosheets, (b)Cd0.5Zn0.5S and Co0.85Se/Cd0.5Zn0.5S. Transmission electron microscopy and high−resolution TEM images are showed in Figure 3. The 6


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morphology of Co0.85Se before and after treated with hydrochloric acid represents a smaller thickness, which is in accordance with the results of SEM investigations and Raman spectroscopy(Figure 3a and 3c). The lattice space value is measured to be 0.27 nm, which matches well with the d-spacing values for the (101) crystal planes of Co0.85Se (Figure 3b). Meanwhile, the “flower” structure of Co0.85Se nanosheets superimposed on each other leads that the size of nanosheets becomes larger revealed in Figure 3c. Moreover, the fringes with a lattice spacing of 0.27 nm still remain unchanged after acid-assisted treatment and the (110) crystal planes are exposed with a lattice distance of 0.18 nm (Figure 3d). The interplanar space value of d = 0.31 nm is assigned to the (110) plane of wurtzite Cd0.5Zn0.5S and the lattice sapce value of d = 0.18 nm corresponds to Co0.85Se hexagonal phase (Figure 3f). In addition, Figure 3f also displays a HRTEM pattern of the Co0.85Se/Cd0.5Zn0.5S image which clearly verifies the formation of heterojunction between exfoliated Co0.85Se nanosheets and Cd0.5Zn0.5S NPs as indicated by a red line . In order to research the elemental distribution maps of Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S nanocomposites, EDS spectrum and corresponding element mapping (STEM) analyses were performed as shown in Figure S1 and Figure 3g. The results of EDS spectrum indicate Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S contains Cd, Zn, S, Co, and Se elements. Moreover, STEM elemental mappings disclose that Co, Se, Zn, Cd and S elements are uniformly distributed in Co0.85Se (nanosheets)/Cd0.5Zn0.5S composites. All above observations further confirmed the successful synthesis of the composites in keeping well with TEM result (Figure 3e).

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Figure 3. TEM images of Co0.85Se nanoplates (a), exfoliated Co0.85Se nanosheets (c) and Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S (e); HRTEM images of Co0.85Se nanoplates (b), exfoliated Co0.85Se nanosheets (d) and Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S (f); STEM and the corresponding element mapping images of Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S (g). Brunauer–Emmett–Teller (BET) gas sorptometry measurement are conducted to research porous structure of nanocomposites. It can be seen that Cd0.5Zn0.5S and Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S show the H3 hysteresis loops and exhibit type IV isotherms (Figure 4 and Figure S2). Hence, it is demonstrated that the presence of mesopores in the samples (Figure 4a).37,39 The corresponding DFT pore size distribution is shown in Figure 4b, with three main peaks at ~ 2.4 nm, ~ 4.5 nm and ~ 15.3 nm for Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S. However, Cd0.5Zn0.5S pore size distribution is ~15.3 nm (Figure S2), indicating that the introduction of exfoliated Co0.85Se nanosheets leads to the decrease of pore volume of Cd0.5Zn0.5S.40 Noteworthy, the BET specific surface areas of Cd0.5Zn0.5S and Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S are ca. 94.18 m2 g-1 and 34.51 m2 g-1, respectively(Figure S2), suggesting that there

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were more active sites on the surface of Co0.85Se/Cd0.5Zn0.5S composite, which was beneficial to the improvement of catalytic performance. 250

(a)

0.3

Adsorption Desorption

200

dV/dD(a.u.)

3

Adsorbed Volume (cm /g STP)

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


150 100

(b)

0.2

2 -1

0.1

BET= 94.18 m g

50 0

0.0

0.0

0.2

0.4

0.6

0.8

1.0

4

Relative Presssure (P/Po)

8

12

16

Pore Diameter (nm)

20

24

Figure 4. N2 adsorption-desorption isotherm (a) and pore size distribution curve of Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S (b). X-ray photoelectron spectroscopy (XPS) are measured to analyze the chemical bonding states and composition of Co0.85Se nanosheets shown in Figure 5(a-c). The XPS survey spectrum reveals the presence of C 1s, Se 3d and Co 2p (Figure 5a). The signal of C 1s might correspond to adventitious carbon. As is shown in Figure 5b, high-resolution Co 2p spectrum of Co0.85Se nanosheets is composed of Co 2p3/2 and Co 2p1/2. In addition, Co 2p3/2 and Co 2p1/2 are deconvoluted into three consecutive peaks, indicating the coexistence of Co2+, Co3+ and shakeup satellite peaks.41,42 The peaks observed at 778.8 and 793.8 eV indicated the existence of Co0.43 The peaks observed at 781.2 and 797.2 eV are assigned to Co2+ cations, while those at 783.4 eV and 798.8 eV belong to Co3+, 785.3 eV and 802.7 eV to the shakeup satellite peaks, respectively.3 The presence of two satellite peaks indicates that the electronic state of Co2+ ions is in high-spin arrangement.44 It can be obtained from the XPS deconvoluted peaks of Co 2p that the peaks area ratio of Co2+/Co3+ is 4.47, indicating many Co2+ ions existed on the surface of Co0.85Se.9 For Se 3d XPS spectrum (Figure 5c), it is remarkable that Co 3p3/2 and Co 3p1/2 of Co2+ correspond to the peak at 58.9 and 60.1 eV peak. The peak at 55.1 eV arises from Se 3d3/2 of metal selenides.8,45,46

9



(b)

Co 2p

Co 2p

Intensity (a.u.)

Se 3d

Intensity (a.u.)

C 1s

sat.

800

600

400

200

0

3+ 793.8 eV Co

Co3+

778.8 eV

60.1 eV

Se 3d3/2

Co 3p3/2

55.1 eV

58.9 eV

sat. 786.1 eV

62

60

Co 2p

(e)

58

56

54

52

Binding Energy (eV)

Binding Energy (eV)

(d)

(f)

Se 3d

S 2p Se 3d

800

600

400

200

Binding Energy (eV)

(h)

Zn 2p1/2

1050 1045 1040 1035 1030 1025 1020

Binding Energy (eV)

800

795

790

785

Binding Energy (eV)

(g)

Zn 2p Zn 2p3/2

Co 2p 1/2

805

0

Intensity (a.u.)

C 1s

Co 2p 3/2

780

Cd 3d5/2 405.3 eV

404.8 eV

411.9 eV 410.7 eV

414

56

Cd 3d Cd 3d3/2

416

775

412

410

408

406

404

Binding Energy (eV)

54

52

402

166

50

Binding Energy (eV) S 2p

(l)

Intensity (a.u.)

Co 2p

Intensity (a.u.)

Cd 3d

Zn 2p

Intensity (a.u.)

Intensity (a.u.)

Co0+

783.4 eV

Se 3d

Co 3p1/2

810 805 800 795 790 785 780 775

Binding Energy (eV)

1000

781.2 eV

0+ 797.2 eV Co

803.1 eV

(c)

2+

Co

Co2+

798.8 eV 1000

Co 2p3/2

Co 2p1/2

Intensity (a.u.)

(a)

Intensity (a.u.)

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S 2p1/2

S 2p3/2

160.3 eV

161.7 eV

164

162

160

158

Binding Energy (eV)

Figure 5. The survey XPS spectrum (a) and XPS spectrum of Co 2p (b), Se 3d (c) of Co0.85Se nanosheets and XPS survey spectrum (d), high-resolution XPS spectrum of Cd 3d (e), Zn 2p (f), S 2p (g), Co 2p (h), Se 3d (i) of Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S. Moreover, as shown in Figure 5(d-i), the XPS spectra of Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S are characterized. And the survey XPS spectrum shows that Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S including Co, Se, Cd, Zn, S, and C elements (Figure 5d). The result is consistent with Figure 3g and EDS results (Figure S1). However, due to the low doping amount of Co0.85Se (1.5 wt%), the peak intensity is not obvious in the survey spectrum. The peaks at 793.78 eV and 778.33 eV could be assigned to Co 2p1/2 and Co 2p3/2 (Figure 5e). In Figure 5f, the high resolution of Se 3d5/2 spectrum is 54.2 eV.47 The two 10


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typical peaks situated at 411.9 eV and 405.3 eV are assigned to Cd 3d spectra, and the intensity ratio of two peaks area was 2:3 (Figure 5g). The above result demonstrated that the chemical state of Cd was Cd2+.48 However, the Cd 3d peaks are not sharp, but obtuse owing to the formation of Schottky junction between Co0.85Se and Cd0.5Zn0.5S. The Cd 3d peaks at 404.8 eV and 410.7 eV could be ascribed to Cd-Se bond, indicating the strong combination of Co0.85Se and Cd0.5Zn0.5S.49 Furthermore, the stronger electron paramagnetic resonance (EPR) signal g=2.002 of Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S Schottky junction should be the overlapped signals of Cd or Zn-vacancies in Cd0.5Zn0.5S and small Co3+ on the Co0.85Se surface(Figure S3), which also proved that metallic Co0.85Se nanosheets decorated Co0.85Se/Cd0.5Zn0.5S heterojunction was successfully fabricated. In addition, Figure 5h exhibits the peaks for Zn 2p3/2 at 1021.88 eV and Zn 2p1/2 at 1044.73 eV, respectively, which verifies the existence of chemical state Zn2+.40 Besides, the S 2p spectrum is divided into two peaks located at 161.7 eV and 160.3 eV, which are typical of S 2p3/2 and 2p1/2 binding energies of sulfur ions (Figure 5i).50 Cd0.5Zn0.5S

(a)

1.2

Co0.85Se (1.0wt%)/Cd0.5Zn0.5S

0.5

Co0.85Se (1.2wt%)/Cd0.5Zn0.5S

1.0

Co0.85Se (1.5wt%)/Cd0.5Zn0.5S

0.4

Co0.85Se (2.0wt%)/Cd0.5Zn0.5S

0.3

0.8 0.6

0.2

0.4

0.1

0.2

0.0 300

(b)

(hv)1/2

0.6

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


400

500

600

Wavelength (nm)

700

0.0 2.0

2.2

2.4

2.6

Photo Energy (eV)

2.8

3.0

Figure 6. The UV–vis diffuse reflectance spectra of Cd0.5Zn0.5S and Co0.85Se/Cd0.5Zn0.5S photocatalysts (a), (αhv)1/2 vs. photon energy plots of Cd0.5Zn0.5S (b). The UV-vis diffuse reflectance spectra of Cd0.5Zn0.5S, Co0.85Se/Cd0.5Zn0.5S photocatalysts with different amounts of Co0.85Se loadings are shown in Figure 6a. The absorption edge of Cd0.5Zn0.5S and Co0.85Se/Cd0.5Zn0.5S nanocomposites are totally situated at the visible region (≥ 400 nm), and also Co0.85Se (x wt%)/Cd0.5Zn0.5S (x = 0, 1, 1.2, 1.5, 2) nanocomposites show a red shifted to longer wavelengths, which can be attributed to the presence of black Co0.85Se in the composites. Moreover, the absorption edge of Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S is situated at about 575 nm. Figure 6b displays that 11



Cd0.5Zn0.5S with a band gap around 2.32 eV can absorb visible light (about 530 nm). Meanwhile, for black Co0.85Se, absorption edge exists in whole range of visible light (Figure S4). After coupling with Co0.85Se onto the Cd0.5Zn0.5S surface, the enhancement of visible light absorption for Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S heterojunction attributed to the contribution from black Co0.85Se, which is conducive to the excitation of electron hole pairs and improves the hydrogen production efficiency of Cd0.5Zn0.5S.

Cd Zn S 0.5

0.5

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

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Co

Se (1.5 wt%)//Cd Zn S 0.5

0.85

600

610

0.5

620

630

Wavelength (nm) Figure 7. Photoluminescence spectrum of Cd0.5Zn0.5S and Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S. Photoluminescence spectrum of Cd0.5Zn0.5S and Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S with an excitation wavelength 400 nm were exhibited in Figure 7. The Cd0.5Zn0.5S samples exhibits intensive band edge luminescence at about 610 nm. However, for Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S, the intensity of the emission peak significantly decreases, which indicates that adding Co0.85Se will promote carrier transfer and restrain their recombination at the Co0.85Se/Cd0.5Zn0.5S interface.20 The photocatalytic H2 evolution under visible light irradiation (λ ≥ 420 nm) on Cd1−xZnxS was presented in Figure S5. With the x values increasing, the H2 evolution rate increases, which is possibly ascribed the formation of solid solution resulting in lower conduction band (CB) position. It is obvious that the optimal sample of Cd0.5Zn0.5S displayed the best efficiency of photocatalytic H2 production (759.3 μmol h−1). However, with increasing Zn2+ content, the photocatalytic activity of Cd1−xZnxS solid solutions (x >0.5) decline due to the decrease in visible light absorption.

12


(a)

Cd0.5Zn0.5S

3000

Co0.85Se (1.2 wt%)/Cd0.5Zn0.5S

2500

Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S Co0.85Se (2.0wt%)/Cd0.5Zn0.5S

2000 1500 1000 500 0

700

400

2

Time (h)

3

4

(b) 544.3

476.4

500

350.6

300 200

0 1

620.9

600

100 0

759.3

800

Co0.85Se (1.0 wt%)/Cd0.5Zn0.5S

Rate of H2(mol/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


Amount of H2 production ( mol)

Page 13 of 25

a

b

0 wt%

1 wt%

c

d

e

1.2 wt% 1.5 wt% 2 wt%

Sample

Figure 8. the kinetic curves of H2 production (a) and Photocatalytic H2 evolution rates (b) for Co0.85Se/Cd0.5Zn0.5S nanocomposites with various mass ratio of exfoliated Co0.85Se nanosheets under visible-light irradiation (λ≥ 420 nm). The kinetic curves for H2 evolution of Cd0.5Zn0.5S and Co0.85Se (x wt%)/Cd0.5Zn0.5S (x = 1, 1.2, 1.5, 2) are revealed in Figure 8a. From Figure 8a, Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S revealed the highest visible light photocatalytic H2 evolution rate from water, and the averageH2 production rate reaches to 30370 μmol·h-1·g−1 under 4 h irradiation. In addition, the apparent quantum yield (AQE) of Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S is about 15.9% measured at 420 nm. Thus, it could be inferred that Co0.85Se/Cd0.5Zn0.5S S composite is an efficient visible-light-induced photocatalyst. Photocatalytic H2 production rates of Co0.85Se/Cd0.5Zn0.5S nanocomposites with variable proportions of Co0.85Se loadings are presented in Figure 8b. The photocatalytic activity of Cd0.5Zn0.5S has a remarkable improvement due to the introduction of metallic Co0.85Se cocatalyst, which demonstrates that the strong interface coupling interaction between metallic Co0.85Se and Cd0.5Zn0.5S would facilitate the electrons vectorial transfer from Cd0.5Zn0.5S to metallic Co0.85Se.51 Moreover, with the increasing mass ratio of Co0.85Se, the photocatalytic H2 production activity of Cd0.5Zn0.5S shows a trend of increasing and then decreasing. The hydrogen production rate of Cd0.5Zn0.5S alone is 350.6 μmol h-1 ascribed to e−/h+ pair rapid recombination. However, metallic Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S Schottky junction displays the most excellent photocatalytic activity at a maximum rate of 759.3 μmol·h-1. Nevertheless, much more Co0.85Se (2.0 wt%) loading leads to the decrease in H2 production, which could be caused by insufficient light absorption. The reason may be that the presentation in a 13



large percentage of black Co0.85Se could result in the deeper color of Co0.85Se/Cd0.5Zn0.5S nanocomposites (the color changes from orange to bottle green). It means that the light absorption of Cd0.5Zn0.5S is declined which is ascribed to the remarkable decrease of transparency and preventing the light from irradiating on the photocatalyst surface.27 Therefore, by loading appropriate amounts of Co0.85Se on Cd0.5Zn0.5S is significant to imporve the photocatalytic efficiency of the nanocomposites.

Cd0.5Zn0.5S

40

Co Se(1.5wt%)/Cd0.5Zn0.5S 0.85

30 20 10 0 0

10

20

30

40

Z' ()

8

Current Density (mA/cm2)

(a)

50

-Z'' ()

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

Page 14 of 25

Co Se 0.85

(b)

Cd0.5Zn0.5S Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S

6

on

on

on

on

4

off

off

off

off

2

0 0

40

80 120 160 200 240 280 320

Time (s)

Figure 9. (a) Electrochemical impedance spectra of Cd0.5Zn0.5S and Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S Schottky junction in 0.5 M H2SO4. (b) Transient photocurrent responses of Co0.85Se, Cd0.5Zn0.5S and Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S sample. The role of metallic Co0.85Se in effective charge transfer was characterized by the EIS spectra (Figure 9a). It can be clearly seen that a semicircle could be observed for Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S with a narrow semicircle compared to that of bare Cd0.5Zn0.5S, indicating a faster electron transfer rate of Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S composites.17 This result means that the introduction of metallic Co0.85Se boosts the electrical conductivity of the composite and the fast charge transfer of photoexcited charge carriers of Cd0.5Zn0.5S. In other words, metallic Co0.85Se can effectively enhance the efficient separation of photoexcited e-/h+ pairs.52 Moreover, the polarization curves and EIS curves of Co0.85Se nanoplates and exfoliated Co0.85Se nanosheets are performed to further research the Co0.85Se electrocatalytic property shown in Figure S6(a, b). As shown in Figure S6a, the Co0.85Se nanoplates display an onset potential of about 200 mV, but the exfoliated Co0.85Se nanosheets have an onset potential of 180 mV. These results indicate that exfoliated Co0.85Se nanosheets exhibit higher efficient 14


Page 15 of 25

hydrogen evolution activity (HER) due to its more activity sites. Moreover, the consequence presents that the semicircle is much shorter for exfoliated Co0.85Se nanosheets than Co0.85Se nanoplates, revealing its lower resistance, faster charge transfer along with favorable HER reaction kinetics (Figure S6b).53 Apparently, the cause of this results might be more active sites and higher electrical conductivity of exfoliated Co0.85Se nanosheets. Transient photocurrent experiments was carried out to further verify the function of exfoliated Co0.85Se nanosheets of charge transport (Figure 9b). It can be observed that Co0.85Se exhibited lowest photocurrent under visible-light illumination. Bare Cd0.5Zn0.5S exhibits a low photocurrent response due to the fast recombination of the photo-induced charge carriers. After adding exfoliated Co0.85Se nanosheets cocatalyst, the Co0.85Se/Cd0.5Zn0.5S nanocomposite exhibits the highest photocurrent intensity, which indicates that the charge transfer rate from Cd0.5Zn0.5S to exfoliated Co0.85Se nanosheets is increased. Meanwhile, the recombination of e−/h+ pairs is inhibited and the photo-induced e−/h+ pairs have a longer lifetime. Furthermore, it is noted that the photogenerated electrons of Cd0.5Zn0.5S prefer to vectorial transfer to metallic Co0.85Se surface, which is resulted from the strong coupling at the interface of metallic Co0.85Se and Cd0.5Zn0.5S.25 Thus, the radiative recombination of e−/h+ pairs is delayed,25,54 in accordance with the higher photocatalytic activity of Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S. 3,000

2nd run

1st run

Rate of H2(mol )

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


3rd run

4th run

10

14

2,500 2,000 1,500 1,000 500 0 0

2

4

6

8

Time (h)

12

16

Figure 10. Long-time stability test of Co0.85Se (1.5 wt %)/Cd0.5Zn0.5S under the wavelength larger than 420 nm visible light irradiation.

15



Page 16 of 25

The photocatalytic activities are investigated by 16 h visible light illumination to verify the durability and recycling performance of Co0.85Se (1.5 wt %)/Cd0.5Zn0.5S. Obviously, after four cycles, the photocatalytic activity of Co0.85Se/Cd0.5Zn0.5S Schottky junction was no noticeable decrease, implying the remarkable stability of Co0.85Se (1.5 wt %)/Cd0.5Zn0.5S (Figure 10). However, Cd0.5Zn0.5S has an unstable photocatalytic activity due to its strong aggregation and photocorrosion. Therefore, Co0.85Se as a co-catalyst to prolong the lifetime of photogenerated electron-hole pair of Cd0.5Zn0.5S is worth to study. Figure S7 presents the XRD patterns of Co0.85Se (1.5 wt %)/Cd0.5Zn0.5S before and after four runs under visible light irradiation. It could be clearly seen that the heterojunction is stable during the photocatalytic process. The potential schematic mechanism is clearly shown in Scheme 1. When Co0.85Se/Cd0.5Zn0.5S Schottky junction is exposed to visible light, the photo-induced electrons are photo-excited from the valence band of Cd0.5Zn0.5S NPs into its conduction band, thereby forming the photo-induced e−/h+ pairs. Particularly, the strong interface coupling interaction between metallic Co0.85Se and Cd0.5Zn0.5S is beneficial for the vectorial transport of the photo-induced electrons from conduction band of Cd0.5Zn0.5S NPs to Co0.85Se nanosheets with high conductivity, thus suppressing charge recombination efficiently and prolonging the lifetime of charge carriers.55 Owing to the half-metallic characterization of Co0.85Se, electrons trapped by hydrogen ions on the surface of metallic Co0.85Se perform the reduction process, leading in the effective evolution of H2. As a remarkable electron acceptor and transporter, exfoliated metallic Co0.85Se nanosheets can be used to transfer the photo-induced electrons and prolong the lifetime of the photo-induced charge just like graphene.56 The fast electron transfer to metallic Co0.85Se nanosheets with high conductivity results in efficient spatial charge separation of photo-induced charge carriers. Thus the recombination of e-/h+ is inhibited effectively and the lifetime of photo-generated charge carriers is prolonged. As a result, the H2 evolution activity is significantly improved. The results further suggest that a novel way for enhancing photocatalytic H2 rate of Cd0.5Zn0.5S photocatalyst by doping metallic Co0.85Se nanosheets as cocatalyst.

16


Page 17 of 25 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 0.5Zn 0.5S Eg=2.32 eV

Se 85 . o 0 C e e ts h s o nan

Visible light CB

●

H2

e-

H+

VB 2-

h+

4 SO

-

S2 2SO 3 Scheme 1. Schematic diagram of the possible electron transfer process occurring in Co0.85Se/Cd0.5Zn0.5S system under visible illumination. CONCLUSIONS In conclusion, hexagonal Co0.85Se nanoplates had been prepared via a simply one-step in situ hydrothermal experiments under ambient conditions. As a photocatalytic cocatalyst, Co0.85Se is used to improve photocatalytic activity and stability of Cd0.5Zn0.5S. Co0.85Se (1.5 wt%)/Cd0.5Zn0.5S Schottky junction exhibits the highest hydrogen efficiency (759.3 µmol h-1), up to 2.1 folds of that bare Cd0.5Zn0.5S. The recycling experiments demonstrated that Co0.85Se/Cd0.5Zn0.5S has better stability. Further study on the transfer performance of the photo-induced e−/h+ pairs with the help of EIS and photocurrent could afford a clear understanding that the crucial role that Co0.85Se has in cocatalyzed photocatalytic hydrogen generation. The formation of Schottky junction of Co0.85Se /Cd0.5Zn0.5S nanocomposites can not only supports the photo-induced charge separation and transfer, but also inhibits the photo-induced e−/h+ recombination via the cocatalytic role of Co0.85Se. Therefore, it is expected that metallic Co0.85Se nanosheets were examined as a potential cocatalyst to ehance the photocatalytic efficiency due to its its abundance, low cost.

ASSOCIATED CONTENT Supporting Information 17



Page 18 of 25

Experimental Section; EDX spectra of Co0.85Se (1.5wt%)/Cd0.5Zn0.5S; Nitrogen adsorption-desorption isotherms of Cd0.5Zn0.5S; Polarization curves and EIS spectra of Co0.85Se nanoplates and exfoliated Co0.85Se nanosheets; XRD patterns of sample before and after test; UV–vis spectra of Co0.85Se; EPR spectrum of Cd0.5Zn0.5S and Co0.85Se(1.5wt%)/Cd0.5Zn0.5S; Photocatalytic hydrogen evolution of Cd1-xZnxS solid solutions. AUTHOR INFORMATION Corresponding Authors: *E-mail: [email protected]. Notes The authors declare no competing finanicial interest. ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of Xinjiang (2017D01A56).

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Table of Content Cd 0.5Zn 0.5S Eg=2.32 eV

140 120

-Z'' (ohm)

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


exfoliated Co 0.85Se nanosheets

100

Se C o 0 .8 5 h e e t s s nano

Visible light

Co 0.85Se nanoplates

CB

●

H2

e-

H+

80 60

VB 2-

h+

40

4 SO

20 -

0

0

50

100

150

200

S2 2SO 3

250

Z' (ohm)

In this work, metallic Co0.85Se nanosheets were introduced as an effective cocatalyst to enhance photocatalytic H2 evolution over Co0.85Se/Cd0.5Zn0.5S Schottky junction.

25


Acid-Exfoliated Metallic Co0.85Se Nanosheets as Cocatalyst on Cd0

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