CdS Nanorods Coupled with WS2 Nanosheets for ... - ACS Publications

Subscriber access provided by EDINBURGH UNIVERSITY LIBRARY | @ http://www.lib.ed.ac.uk

Article

CdS Nanorods Coupled with WS2 Nanosheets for Enhanced Photocatalytic Hydrogen Evolution Activity Jie He, Lang Chen, Zi-Qi Yi, Chak-Tong Au, and Shuang-Feng Yin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01511 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 18, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

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

Industrial & Engineering Chemistry Research

CdS Nanorods Coupled with WS2 Nanosheets for Enhanced Photocatalytic Hydrogen Evolution Activity Jie He a+, Lang Chen a+, Zi-Qi Yia, Chak-Tong Aub, Shuang-Feng Yina* a

State Key Laboratory of Chemo/Biosensing and Chemometrics, College of

Chemistry and Chemical Engineering, Provincial Hunan Key Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing Carbon-dioxide Emissions，Hunan University, Changsha 410082, Hunan, People’s Republic of China b

College of Chemistry and Chemical Engineering, Hunan Institute of Engineering, Xiangtan 411104, Hunan, China.

Keywords：： Water splitting, Hydrogen production, Tungsten sulfide, Cadmium sulfide, Photocatalyst

ACS Paragon Plus Environment


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 2 of 26

ABSTRACT WS2

nanosheets–CdS

nanorods

with

heterojunctions

were

prepared

by

ultrasonic/exfoliation method using dimethylformamide as dispersing agent. The CdS nanorods were coupled with small WS2 nanosheets as a result of exfoliation. Excellent hydrogen production rate of 1222 µmol h−1 (20 mg catalyst) was achieved over the WS2-CdS composite with WS2-to-CdS mass ratio = 1.6:1 under visible light irradiation (λ ≥ 400 nm). The efficient evolution of hydrogen is attributed to the promotion of the separation of photo-generated charge carriers due to the presence of the heterojunctions created on the surface. 1. INTRODUCTION The use of nanoscaled photocatalysts for hydrogen evolution is a choice for the generation of clean energy.1 Many researchers are interested in the use of graphene-like

two-dimensional

layered

materials

such

as

transition-metal

dichalcogenides (TMDs).2 TMDs have a general formula of MX2 (M represents a transition metal element and X represents a chalcogen), and there exist the planar layers of X–M–X which are stacked together only by Van der Waals interaction.3 Thus they are easily exfoliated into thin-layer entities and exhibit excellent performance towards the electrocatalytic hydrogen evolution reaction (HER).4-9

Among the known photocatalysts, CdS is attractive for its excellent absorption of visible light. The CdS-based photocatalysts are commonly adopted for fundamental studies because CdS is not expensive and simple to prepare.10-12 Furthermore, with positive intrinsic properties such as variable band gaps, large surface-to-volume ratios,


Page 3 of 26

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


and tunable conduction/valence band energies, the quantum dots (QDs), nanorods (NRs) and nanowires (NWs) of CdS were investigated extensively for photocatalytic hydrogen evolution from water splitting.13−24 Nevertheless, pure CdS is poor in photocatalytic hydrogen production under visible light. It is only when it is combined with another semiconductor that satisfactory photocatalytic activity can be achieved.25-27

Recently, we found that one-dimensional CdS decorated with ultrathin MoS2 nanosheets is photocatalytically efficient in hydrogen evolution reaction.25 The exfoliated ultrathin MoS2 nanosheets that expose rich active edge sites for HER on the surface of CdS not only promote the separation of photogenerated electrons and holes but also facilitate the reaction processes at the interface. It was pointed out that the enhanced photocatalytic performance is attributed to the unique structure of the MoS2-CdS composite. From the literature, we found that WS2, CuS, Cu2S and CoS were selected to enhance the hydrogen evolution activity of TiO2, CdS, and g-C3N4.28-31 Nonetheless, whether a structure similar to that of MoS2-CdS can be constructed by using other dichalcogenide compounds remained unclear. As a member of the transition metal dichalcogenide compounds, WS2 possesses crystal structure and chemical property similar to those of MoS2, and by itself is photocatalytic active for hydrogen evolution from water. We anticipate that enhanced photocatalytic performance can also be achieved through the combination of exfoliated WS2 nanosheets with CdS NRs. Herein we report the fabrication of CdS nanorods that were coupled with WS2 nanosheets through ultrasonic exfoliation using



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 4 of 26

dimethylformamide (DMF) as dispersing agent. The as-obtained WS2-CdS composites have unique structure and show enhanced hydrogen evolution activities under visible light (λ ≥ 400 nm). The relationship between surface properties of composites and performance in the generation of hydrogen from water was studied. 2. EXPERIMENTAL SECTION All of the reagents used in the experiment were commercially available and were used without further purification.

2.1. Synthesis of CdS NRs We

prepared

CdS

NRs

through

a

solvothermal

method.

Briefly,

Cd(NO3)2·4H2O (1.92 g) and thiourea (1.42 g) were dissolved in ethanediamine (50 mL). The solution was then transferred into a polyphenylene-lined autoclave of 100 mL, sealed and heated at 180 °C for 24 h. The product yellow in color was collected by filtration, washed by de-ionized water and ethanol for three times and then dried at 80 °C in an oven for 12 h.

2.2. Synthesis of WS2-CdS The WS2-CdS composites with different WS2-to-CdS mass ratios (viz. 0.4:1, 0.8:1, 1.2:1, 1.6:1, 2.0:1 and 2.4:1) using DMF as dispersing agent were prepared by ultrasonic exfoliation method. They are denoted as WC-DMF but individually as WC-0.4, WC-0.8, WC-1.2, WC-1.6, WC-2.0, and WC-2.4, respectively. Using the WC-1.6 as an example, WS2 (0.4 g) was dispersed in DMF for 3 h by ultrasonic


Page 5 of 26

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


treatment. After that, CdS NRs (0.25 g) was added and the as-resulted mixture was subject to ultrasonic treatment for another 1 h and stirring for another 12 h. The WS2-CdS composite was collected by vacuum distillation (to remove the dispersing agent). For comparison studies, WS2-CdS composites with WS2-to-CdS mass ratio = 1.6:1 were synthesized likewise except that H2O or ethanol was used instead as dispersing agent (denoted herein as WC-H2O and WC-ethanol, respectively).

2.3. Characterization The crystal structures of the as-prepared samples were characterized by XRD (D/MAX-2000/PC, Rigaku Corporation) with monochromatized CuKα radiation (λ = 0.15406 nm). The scanning range was 5° to 80°. The surface composition of the WC composite was determined by X-ray photoelectron spectroscopy (XPS). The scanning electron microscope (SEM) images were collected by using a field emission scanning electron microscope (FE-SEM) (Hitachi S-4800). The morphologies and elemental mapping images were also obtained from transmission electron microscopy (TEM, Tecnai G2 F20) at an accelerating voltage of 200 kV. The BET surface area was measured using Gemini VII 2390 instrument (Micromeritics Instrument Corp.). The UV-vis

diffuse

reflectance

spectra

of

samples

were

investigated

on

a

spectrophotometer (Cary 100, Agilent). Photoluminescence (PL) spectra of samples were recorded over a fluorescence spectrophotometer under the excitation light of 350 nm.

Photoelectrochemical

activity

measurements

were

recorded

over

an

electrochemical analyzer (CHI660D, Shanghai Chenhua electrochemical workstation)



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

in a standard three-electrode system using the obtained sample as working electrode, a Pt wire as counter electrode, and a saturated calomel electrode (SCE) as reference electrode.

2.4. Photocatalytic hydrogen generation

Hydrogen production experiments were performed in a sealed quartz reactor of 250 mL at 5 °C and under negative pressure. An arc lamp (HSX-F300, Beijing NBET Corp.) with a cut-off filter of λ ≥ 400 nm was positioned 5 cm away from the reactor. Firstly, 20 mg of photocatalyst was added in an aqueous solution containing lactic acid (50 mL, concentration = 20 vol%) with constant stirring for 30 min. Then argon was bubbled through the solution before irradiation. The hydrogen was quantified using a TCD gas chromatograph (Agilent, 7820A) with a 5A molecular sieve column.

The apparent quantum yield (AQY) upon 420 nm irradiation was measured according to the method reported in our previous work,25 except for the intensity test of monochromatic light. The AQY was calculated using the following equation:

AQY(%) = =

The number of reacted electrons

× 100% The number of incident photons 2 × The number of evolved H2 molecules

The number of incident photons 2 × NH 2 = × 100% Ni 2 × NH 2 = × 100% I×A× t ×λ h×c

3. RESULTS AND DISCUSSION


× 100%

Page 6 of 26

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


3.1. Phase structure

The XRD results of the WS2-CdS samples are shown in Figure 1. All peaks can be indexed to wurtzite CdS (JCPDS card NO. 89-2944) and WS2 (JCPDS card NO. 84-1398). There are no other diffraction peaks except those of CdS and WS2.

(g) Intensity (a.u.)

Page 7 of 26

(f) (e) (d) (c) (b) (a) 10

20

30

40

50

2 Theta (degree)

60

70

80

Figure 1. XRD patterns of (a) CdS NRs, (b) WS2, (c) WC-0.8, (d) WC-1.6, (e) WC-2.4, (f) WC-H2O, and (g) WC-ethanol.

All the WC-DMF, WC-H2O and WC-ethanol samples are similar in diffraction pattern, indicating that DMF is not the only dispersing agent capable of synthesizing WS2-CdS composites.

3.2. Composition of catalyst

The WC-1.6 sample was subject to XPS analysis with all the peak positions calibrated against the C1s signal (binding energy = 284.6 eV) of contaminant carbon. As shown in Figure 2(a), only Cd, W, S and trace amount of O, C were detected from the survey spectrum. The high-resolution spectra of Cd3d, W3d and S2p signals were carried out to figure out the oxidation states of the elements. Figure 2(b) shows the



Cd3d5/2 and Cd3d3/2 signals at 405.1 and 411.9 eV that are typical of CdS.28 There are four W4f peaks at 32.7, 34.8, 35.8, and 37.9 eV. The signals at 32.7 and 34.8 eV can be assigned to WS2 while those at 35.8 and 37.9 eV to WO328. There are small WO3 peaks in the XPS results mainly because of the oxidation of surface WS2 in the synthetic process. It was reported that the presence of WO3 does not contribute to the enhancement of H2 evolution.28

Intensity (a.u.)

411.9

(c)

42

800 600 400 200 Binding energy (eV)

0

32.7

(d)

34.8

37.9 35.8

40

420

415 410 405 Binding energy (eV) 161.7

Intensity (a.u.)

1000

405.1

(b)

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

Page 8 of 26

38 36 34 32 Binding energy (eV)

30

28

162.7

174 172 170 168 166 164 162 160 158 Binding energy (eV)

Figure 2. XPS spectra of WC-1.6: (a) survey (b) Cd3d, (c) W4f and (d) S2p spectrum.

The W4f and S2p binding energies are consistent with those reported by Zong et al. for WS2;28 thus we confirmed the presence of WS2 on account of the combined results of XRD and XPS investigations. What is more, no N1s signal was detected (not shown here) which suggests that DMF was completely removed. XPS results further confirm the presence of WS2 and CdS in the composites. The WC-0.8,


Page 9 of 26

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


WC-1.6 and WC-2.4 composites were analyzed by ICP-AES to figure out the real element composition (Table S1, Supporting Information). The theoretical and real mass fractions of CdS are close which confirms the successful preparation of the WC composites.

3.3. Morphological structure The SEM images of the CdS NRs and those of the purchased WS2 are displayed in Figure 3. As shown in Figure 3(a), the as-prepared CdS NRs are about 30 nm to 50 nm in diameter. From the high-resolution SEM image, one can see that the CdS NRs have smooth surface showing no presence of any nanosheets or nanoparticles (Figure 3(b)). The low- and high-resolution SEM images are shown in Figure 3(c) and (d) respectively. And it is revealed that the purchased WS2 is composed of nanosheets that are closely packed together. The SEM images of WC-0.8, WC-1.6 and WC-2.4 samples are also displayed in Figure 3.



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

Figure 3. SEM images of (a) (b) CdS NRs, (c) (d) purchased bulk WS2, (e) (f) WC-0.8, (g) (h) WC-1.6, and (i) (j) WC-2.4.

The WC-DMF samples are made up of nanorods (Figure 3(e)), and with the increase of WS2 content in the exfoliation process, there is increase of surface roughness. It is noted that in excess amount of WS2, there is presence of unexfoliated WS2 particles on the surface of CdS NRs (Figure 3(i) and (j)), plausibly due to the inadequate dispersion of WS2 particles in the adopted amount of DMF.


Page 10 of 26

Page 11 of 26

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


The selection of dispersing agent in the preparation process has a significant influence on the physical and chemical properties of WS2. To determine the effect of dispersing agents, we investigated the surface structures of CdS NRs, purchased WS2, WC-H2O and WC-1.6 by TEM. In consistent with the SEM results, the low magnification TEM image of the CdS nanorods reveals smooth surfaces (Figure 4(a)) while that of the WS2 shows nanosheets that are closely packed (Figure 4(b)). From Figure 4(c) and (d), one can see that WC-1.6 is more enriched with WS2 edge sites than WC-H2O. The TEM image of WC-H2O reveals multilayer of WS2 nanosheets (black box in Figure 4(c)) that are unevenly distributed on the surface. The exfoliated WS2 nanosheets (red box in Figure 4(d)) of WC-1.6 which are smaller than those of WC-H2O are more uniformly distributed. Compared to the protrusive MoS2 nanosheets on the CdS surface of the MoS2-CdS system, 25 the WS2 nanosheets of the WS2-CdS system are more capable of covering the CdS surface (Figure 4(d)). Figure 4(e) is the HRTEM image of WC-1.6, and the lattice spacing of 0.62 nm can be ascribed to the (002) plane of WS2. Moreover, CdS with a lattice spacing of 0.33 nm ascribable to the (002) plane is in close contact with WS2.



(b)

(a)

100 nm

100 nm

(d)

(c)

100 nm

100 nm

(e)

(f)

Cu S S

300

Cd

WS2 (002) 0.62 nm

W

200

Count

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

Cu Cd

100 O

CdS (002) 0.33 nm 5 nm

W

Cd

WW

Cd

0 0

10

20

Energy (keV)

30

40

Figure 4. TEM images of (a) CdS NRs, (b) purchased WS2, (c) WC-H2O, (d) WC-1.6; HRTEM image of (e) WC-1.6; and (f) EDX image corresponding to (e).

Figure 4(f) is the energy-dispersive X-ray spectrum (EDX; selected area) of WC-1.6 which suggests the sole existence of Cd, W, and S elements. In addition, the image of TEM element mapping confirms the microstructure and the chemical composition of

WC-1.6 (Figure S1, Supporting Information). We propose that

unlike bulk WS2, the exfoliated WS2 nanosheets are more flexible and more readily adsorbed on the surface of CdS NRs in a more closely manner. Consequently, the coupling of exfoliated WS2 nanosheets on the surface of CdS NRs is facile, and there is stronger binding force between WS2 and CdS.

3.4. Nitrogen adsorption-desorption analysis


Page 12 of 26

Page 13 of 26

Shown in Figure 5 are the N2 adsorption-desoprtion isotherms and the inset table illustrates the specific surface areas of the CdS NRs and WC-1.6 samples. Both of the samples show type-IV isotherms. The hysteresis loops start at P/P0 = 0.8 and show high adsorption at P/P0 = 1.0, indicating the coexistence of mesopores and macropores.

60 SBET (m2/g)

50

Total Volume (cc/g)

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


40

CdS NRs

26.1

WC-1.6

79.5

30 20

WC-1.6 CdS NRs

10 0 0.0

0.2

0.4

0.6

0.8

1.0

P/P0

Figure 5. Nitrogen adsorption-desorption isotherms of WC-1.6 and CdS NRs. (Inset: table of specific surface areas.)

As depicted in the inset table, the specific surface areas of the CdS NRs and WC-1.6 composites are 26.1 and 79.5 m2/g respectively. The higher value of the latter is ascribed to the presence of exfoliated WS2 nanosheets on the surface.

3.5. Optical properties

It is important to figure out the light absorption abilities of the samples. According to UV-visible diffuse reflectance spectroscopy (UV-vis DRS), all the samples have absorption with wavelength ranging from 350 nm to 600 nm (Figure 6).



With the increasing of WS2 content, the visible light absorption intensity also increases, and this could be related to the relatively strong ability of WS2 for visible light absorption. The results suggest that the as-prepared WC composites have visible light driven ability for catalytic actions.

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

Bulk WS2 WC-2.4 WC-2.0 WC-1.6 WC-1.2 WC-0.8 WC-0.4 CdS NRs

350

400

450 500 550 Wavelength (nm)

600

Figure 6. UV-vis absorption property of WS2-CdS samples prepared with different WS2-to-CdS mass ratios. Also shown are those of CdS NRs and bulk WS2.

Photoluminescence (PL) spectra are commonly collected to assess the separation efficiency of photogenerated electron-hole pairs. Generally, it is recognized that a lower PL intensity indicates higher efficiency of electron-hole separation. Figure 7(a) and (b) are the PL spectra of the samples recorded with a 350 nm laser source. Apparently, WC-1.6 shows the lowest PL intensity among all the samples. In other words, the WC-1.6 composite is the most efficient based on the separation efficiency of photogenerated charge carriers. There is a peak shift at around 550 nm of the samples. Generally, peak shift is always caused by impurities. According to the report of Ye et al.,34 the peak of CdS in the PL spectra is blue shifted. It is worth pointing out that with the decrease of layer number of MoS2, the shift becomes more obvious. It


Page 14 of 26

Page 15 of 26

can be deduced that the thinner MoS2 nanosheets have stronger influence on CdS. As to the WC-1.6 composite, blue shift is also found in PL spectra. We deduce that the blue shift mainly results from the influence of WS2 on CdS. When WS2 is exfoliated to single or a few layers, the shift becomes obvious. CdS NRs WC-1.6 WC-ethanol WC-H2O

400

450

(a)

CdS NRs WC-1.6 WC-0.8 WC-2.4

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



650

400

450


(b)

650

Figure 7. Room temperature PL spectra of (a) CdS NRs, WC-1.6, WC-ethanol and WC-H2O; and those of (b) CdS NRs, WC-0.8, WC-1.6 and WC-2.4

With more photo-generated electrons transferred to the WS2 nanosheets, the interaction of the electrons with H+ in water occurs directly, resulting in efficient production of H2.

3.6. Photocatalytic properties

Experiments of photocatalytic hydrogen evolution upon visible light irradiation (λ > 400 nm) were carried out to evaluate the activities of the WC composites. Displayed in Figure 8(a) are the photocatalytic performance of WC-1.6, WC-H2O and WC-ethanol. There is steady generation of H2, and the 3-h average hydrogen evolution rates over WC-1.6, WC-H2O and WC-ethanol are 61.1, 15.4 and 20.8 mmol/g/h, respectively. Given in Table 1 are the performances of representative



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 16 of 26

noble-metal-free catalysts ever reported for the photocatalytic evolution of hydrogen from water splitting; it is obvious that the WC-1.6 catalyst is the most superior among them.[28, 35-42] Due to the different experimental conditions, there is different in factors such as light intensity, sacrifice agent and its concentration, catalyst concentration and loading which are all closely related to hydrogen evolution amount. The AQY of WC-1.6 is calculated to be 28.9% under monochromatic irradiation of 420 nm. It is relatively higher than those reported in the literatures. We ascribe the higher photocatalytic activity to the exfoliated structure of the WS2 nanosheets as well as to the rich heterojunctions between CdS and WS2. Table 1. Comparison of representative noble-metal-free photocatalysts ever reported for photocatalytic HER from water splitting.

Photocatalyst

WS2/CdS WS2-CdS nanohybrids MoS2/CdS NiOx/CdS CuS/CdS PdS/CdS

Light

Reactant

Activity

source

solution

[mmol h-1 g-1]

Lactic acid

λ≥400 nm, Xe λ≥400 nm, Xe λ≥420 nm λ≥400 nm, Xe 500W Xe arc 300W Xe, >430nm

AQY

Reference

4.2

－

28

Lactic acid

2.0

－

35

Lactic acid

5.4

－

36

Na2S-Na2SO3

5.9

Na2S-Na2SO3

3.3

－

38

Na2S-Na2SO3

4.45

－

39

NiS/CdS

300W Xe, >420nm

Na2S-Na2SO3

1.15

g-C3N4/WS2

300W Xe, >420nm

Methanol

0.1


8.6% at 400 nm

6.1% at 420 nm －

37

40

41

Page 17 of 26

WS2 sensitized

350W Xe,

mesoporous TiO2

>430nm

WC-1.6

λ≥400 nm, Xe

Na2S

0.89

Lactic acid

61.1

－

42

28.9% at

this work

420 nm

40

20

(b)

WC-0.4 WC-0.8 WC-1.2 WC-1.6 WC-2.0 WC-2.4 CdS NRs

3500

Amount of H2 (µmol)

－1

mg

－1

4000

(a)

60

Rate of H2 evolution / µmol 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


3000 2500 2000 1500 1000 500 0

0 WC-1.6

WC -H2O

WC-ethanol

0

1

2

3

Irradiation time (h)

Figure 8. Photocatalytic activity of WS2-CdS for hydrogen evolution reaction: (a) Average H2 evolution rate of WS2-CdS synthesized using different dispersing agents and (b) Time-dependent photocatalytic evolution of H2 for WC-0.4, WC-0.8, WC-1.2, WC-1.6, WC-2.0, WC-2.4 and CdS NRs.

We also studied the effect of WS2 content in WS2-CdS on the photocatalytic performance (Figure 8(b)). All the WS2-CdS composites show activities higher than that of WS2 or CdS alone. And WC-1.6 is the one that performs the best, showing the highest HER rate of 61.1 mmol/g/h. The variation of WS2 content has a great influence on the hydrogen evolution activity. With the increasing of WS2 content, the WC nanocomposite showed increasing hydrogen evolution activity. However, it is noted that excess WS2 on the surface hinders the light absorption of CdS, causing a reduction of photo-generated electrons. Furthermore, the ability of a certain volume of DMF to disperse bulk WS2 is limited, and bulk WS2 in excess could not be fully exfoliated. Both factors would result in the decrease of photocatalytic activity.



3.7. Possible mechanism for enhanced photocatalytic activities

With the construction of heterojunctions between the CdS and the WS2, there is diffusion of photo-generated electrons to the WS2 nanosheets (Figure 9(a) and (b)).

(a)

(b)-2 e-

-1 Potential

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 18 of 26

eH2O

Visible light

0 CdS

WS2

H2

1

h+

h+

2

Figure 9. (a) Schematic of the photocatalytic process of WS2-CdS in lactic acid solution, (b) graphical representation of visible light induced charge transfer between CdS and WS2 for photocatalytic hydrogen evolution reaction

To confirm the enhanced efficiency in the separation of photogenerated charge carriers, we measured the photocurrent of the samples (Figure 10). All the samples show good reproducibility of photocurrent and the intensity of WC-1.6 is stronger than that of CdS NRs and purchased WS2, implying WC-1.6 has higher separation efficiency of photogenerated electrons and holes.


2

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


Current density (µA/cm )

Page 19 of 26

c

2

1 b a 0 0

40

80 120 Time (second)

160

200

Figure 10. Photocurrent responses of (a) purchased WS2, (b) CdS NRs and (c) WC-1.6 electrodes in on/off cycles under visible light irradiation

4. CONCLUSIONS

Composites of CdS nanorods coupled with WS2 nanosheets were prepared by a solvothermal/exfoliation method. The one with optimized WS2-to-CdS mass ratio shows excellent photocatalytic activities for hydrogen production. The out-standing performance of the WS2-CdS photocatalysts arises from abundant active sites of the exfoliated WS2 nanosheets as well as the surface heterojunctions created between WS2 and CdS.

ASSOCIATED CONTENT

Supporting Information Figures S1 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION



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 20 of 26

Corresponding author *Phone:

86-731-88821711,

Fax:

86-731-88821171,

E-mail

address:

[email protected] Author Contributions +

These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This project was financially supported by NSFC (Grant No. 21401054, 21476065 and J1210040), the China Postdoctoral Science Foundation (2014M562098), the Fundamental Research Funds for the Central Universities, Hunan Provincial Natural Science Foundation (2015JJ3033) and Hunan Provincial Science and Technology Project (2015JC3051). C. T. Au thanks the HNU for an adjunct professorship.

REFERENCES [1] Kudo, A.; Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 2009, 38, 253–278. [2] Yang, J.; Shin, H. S. Recent advances in layered transition metal dichalcogenides for hydrogen evolution reaction. J. Mater. Chem. A, 2014, 2, 5979–5985. [3] Rao, C. N. R.; Matte, H. S. S. R.; Maitra, U. Graphene analogues of inorganic layered materials. Angew. Chem. Int. Ed., 2013, 52, 13162–13185. [4] Zhang, H. Ultrathin two-dimensional nanomaterials. ACS Nano, 2015, 9, 9451–9469.


Page 21 of 26

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


[5] Tana, C. L.; Zhang, H. Two-dimensional transition metal dichalcogenide nanosheet-based composites. Chem. Soc. Rev. 2015, 44, 2713–2731. [6] Li, H.; Wu, J.; Yin, Z. Y.; Zhang, H. Preparation and Applications of Mechanically Exfoliated Single-Layer and Multilayer MoS2 and WSe2 Nanosheets. Acc. Chem. Res. 2014, 47, 1067–1075. [7] Coleman, J. N.; Lotya, M.; O’Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; Shvets, I. V.; Arora, S. K.; Stanton, G.; Kim, H.Y.; Lee, K.; Kim, G. T.; Duesberg, G. S.; Hallam, T.; Bolan, J. J.; Jing, J.; Donegan, J. F.; Grunlan, J. C.; Moriarty, G.; Shmeliov, A.; Nicholls, R. J.; Perkins, J. M.; Grieveson, E. M.; Theuwissen, K.; McComb, D. W.; Nellist, P. D.; Nicolosi, V. Two-dimensional nanosheets produced by liquid exfoliation of layered materials. Science 2011, 331, 568–571. [8] Zhang, Y. S.; Shi, J. P.; Han, G. F.; Li, M. J.; Ji, Q. Q.; Ma, D. L.; Zhang, Y.; Li, C.; Lang, X Y.; Zhang , Y. F.; Liu, Z. F. Chemical vapor deposition of monolayer WS2 nanosheets on Au foils toward direct application in hydrogen evolution. Nano Res., 2015, 8, 2881–2890. [9] Zirak, M.; Zhao, M.; Moradlou, O.; Samadi, M.; Sarikhani, N.; Wang, Q.; Zhang, H. L.; Moshfegh, A. Z. Controlled engineering of WS2 nanosheets–CdS nanoparticle heterojunction with enhanced photoelectrochemical activity. Sol. Energ. Mat. Sol. C., 2015, 141, 260–269. [10] Yan, H. J.; Yang, J. H.; Ma, G. J.; Wu, G. P.; Zong, X.; Lei, Z. B.; Shi, J. Y.; Li, C. Visible-light-driven hydrogen production with extremely high quantum efficiency on Pt–PdS/CdS photocatalyst. J. Catal. 2009, 266, 165–168. [11] Li, Y. X.; Tang, L. F.; Peng, S. Q.; Li, Z. C.; Lu, G. X. Phosphate-assisted hydrothermal synthesis of hexagonal CdS for efficient photocatalytic hydrogen evolution. CrystEngComm 2012, 14, 6974–6982. [12] Peng, T. Y.; Li, K.; Zeng, P.; Zhang, Q. G.; Zhang, X. G. Enhanced photocatalytic hydrogen production over graphene oxide–cadmium sulfide nanocomposite under visible light irradiation. J. Phys. Chem. C 2012, 116, 22720–22726.



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

[13] Sakamoto, M.; Xiong, A.; Kanakubo, R.; Ikeda, T.; Yoshinaga, T.; Maeda, K.; Domen, K. Teranishi, T. Chem. Lett. 2012, 41, 1325−1327. [14] Zhao, J.; Holmes, M. A.; Osterloh, F. E. Quantum confinement controls photocatalysis: a free energy analysis for photocatalytic proton reduction at CdSe nanocrystals. ACS Nano 2013, 7, 4316−4325. [15] Ben-Shahar, Y.; Scotognella, F.; Waiskopf, N.; Kriegel, I.; Dal Conte, S.; Cerullo, G.; Banin, U. Effect of surface coating on the photocatalytic function of hybrid CdS–Au nanorods. Small 2015, 11, 462−471. [16] Berr, M.; Vaneski, A.; Susha, A. S.; Rodríguez-Fernández, J.; Döblinger, M.; Jäckel, F.; Rogach, A. L.; Feldmann, J. Colloidal CdS nanorods decorated with subnanometer sized Pt clusters for photocatalytic hydrogen generation. Appl. Phys. Lett. 2010, 97, 093108. [17] Tongying, P.; Plashnitsa, V. V.; Petchsang, N.; Vietmeyer, F.; Ferraudi, G. J.; Krylova, G.; Kuno, M. Photocatalytic hydrogen generation efficiencies in one-dimensional CdSe heterostructures. J. Phys. Chem. Lett. 2012, 3, 3234−3240. [18] Tongying, P.; Vietmeyer, F.; Aleksiuk, D.; Ferraudi, G. J.; Krylova, G.; Kuno, M. Double. Heterojunction nanowire photocatalysts for hydrogen generation. Nanoscale 2014, 6, 4117−4124. [19] Han, Z. J.; Qiu, F.; Eisenberg, R.; Holland, P. L.; Krauss, T. D. Robust photogeneration of H2 in water using semiconductor nanocrystals and a nickel catalyst. Science 2012, 338, 1321−1324. [20] Wilker, M. B.; Shinopoulos, K. E.; Brown, K. A.; Mulder, D. W.; King, P. W.; Dukovic, G. Electron transfer kinetics in CdS nanorod–[FeFe]-hydrogenase complexes and implications for photochemical H2 generation. J. Am. Chem. Soc. 2014, 136, 4316−4324. [21] Amirav, L.; Alivisatos, A. P. Photocatalytic hydrogen production with tunable nanorod heterostructures. J. Phys. Chem. Lett. 2010, 1, 1051−1054. [22] Wu, K. F.; Chen, Z. Y.; Lv, H. J.; Zhu, H. M.; Hill, C. L.; Lian, T. Q. Hole removal rate limits photodriven h2 generation efficiency in CdS-Pt and


Page 22 of 26

Page 23 of 26

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


CdSe/CdS-Pt semiconductor nanorod–metal tip heterostructures. J. Am. Chem. Soc. 2014, 136, 7708−7716. [23] Simon, T.; Bouchonville, N.; Berr, M. J.; Vaneski, A.; Adrović, A.; Volbers, D.; Wyrwich, R.; Döblinger, M.; Susha, A. S.; Rogach, A. L.; Jäckel, F.; Stolarczyk, J. K.; Feldmann, J. Redox shuttle mechanism enhances photocatalytic H2 generation on Ni-decorated CdS nanorods. Nat. Mater. 2014, 13, 1013−1018. [24] Liu, S. Q.; Tang, Z. R.; Sun, Y. G.; Colmenares, J. C.; Xu, Y. J. One-dimension-based spatially ordered architectures for solar energy conversion. Chem. Soc. Rev. 2015, 44, 5053−5075. [25] He, J.; Chen, L.; Wang F.; Liu, Y.; Chen, P.; Au, C. T.; Yin, S. F. CdS Nanowires decorated with ultrathin MoS2 nanosheets as an efficient photocatalyst for hydrogen evolution. ChemSusChem 2016, 9, 624–630. [26] Wang, Y. B.; Wang, Y. S.; Xu, R. Photochemical deposition of Pt on CdS for H2 evolution from water: markedly enhanced activity by controlling Pt reduction environment. J. Phys. Chem. C 2013, 117, 783–790. [27] Zhang, J.; Yu, J. G.; Jaroniec, M.; Gong, J. R. Noble metal-Free reduced graphene oxide-ZnxCd1–xS nanocomposite with enhanced solar photocatalytic H2-production performance. Nano Lett. 2012, 12, 4584–4589. [28] Zong, X.; Han, J. F.; Ma, G. J.; Yan, H. J.; Wu, G. P.; Li, C. Photocatalytic H2 evolution on CdS loaded with WS2 as cocatalyst under visible light irradiation. J. Phys. Chem. C 2011, 115, 12202–12208. [29] Zhang, L. J.; Jiang, T. F.; Li, S.; Lu, Y. C.; Wang, L. L.; Zhang, X. Q.; Wang, D. J.; Xie, T. F. Enhancement of photocatalytic H2 evolution on Zn0.8Cd0.2S loaded with CuS as cocatalyst and its photogenerated charge transfer properties. Dalton Trans. 2013, 42, 12998–13003. [30] Chen, Y. B.; Qin, Z. X.; Wang, X. X.; Guo, X.; Guo, L. J. Noble-metal-free Cu2S-modified photocatalysts for enhanced photocatalytic hydrogen production by forming nanoscale p–n junction structure. RSC Adv. 2015, 5, 18159–18166.



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

[31] Zhu, Y. S.; Xu, Y.; Hou, Y. D.; Ding, Z. X.; Wang, X. C. Cobalt sulfide modified graphitic carbon nitride semiconductor for solar hydrogen production. Int. J Hydrogen Energ. 2014, 39, 11873–11879. [32] Yuan, J. L.; Wen, J. Q.; Gao Q. Z.; Chen, S. C.; Li, J. M.; Li X.; Fang, Y. P. Amorphous Co3O4 modified CdS nanorods with enhanced visible-light photocatalytic H2-production activity. Dalton Trans., 2015, 44, 1680–1689. [33] Zhou, X.; Jin J.; Zhu X. J.; Huang J.; Yu J. G.; Wong W. Y.; Wong, W. K. New Co(OH)2/CdS nanowires for efficient visible light photocatalytic hydrogen production. J. Mater. Chem. A, 2016, 4, 5282–5287. [34] Chang, K.; Li, M.; Wang, T.; Ouyang, S. X.; Li, P.; Liu, L. Q.; Ye, J. H. Photocatalytic H2 Evolution over nMoS2/CdS ( n≥1) Under Visible Light. Adv. Energy Mater. 2015, 5, 1402279. [35] Chen, J. Z.; Wu, X. J.; Yin, L. S.; Li, B.; Hong, X.; Fan, Z. X.; Chen, B.; Xue, C.; Zhang, H. One-pot synthesis of CdS nanocrystals hybridized with single-layer transition-metal dichalcogenide nanosheets for efficient photocatalytic hydrogen evolution. Angew. Chem. Int. Ed. 2015, 54, 1210–1214. [36] Zong, X.; Yan, H. J.; Wu, G. P.; Ma, G. J.; Wen, F. Y.; Wang, L.; Li, C. Enhancement of photocatalytic H2 evolution on CdS by loading MoS2 as cocatalyst under visible light irradiation. J. Am. Chem. Soc., 2008, 130, 7176–7177. [37] Chen, X. P.; Chen, W.; Gao, H. Y.; Yang, Y.; Shangguan, W. F. In situ photodeposition of NiOX on CdS for hydrogen production under visible light: Enhanced activity by controlling solution environment. Appl. Catal. B: Environ. 2014, 152, 68–72. [38] Zhang, L. J.; Xie, T. F.; Wang, D. J.; Li, S.; Wang, L. L.; Chen, L. P.; Lu, Y. C. Noble-metal-free CuS/CdS composites for photocatalytic H2 evolution and its photogenerated charge transfer properties. Int. J. Hydrogen Energ. 2013, 38, 11811–11817.


Page 24 of 26

Page 25 of 26

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


[39] Chen, Q. Y.; Suo, C.; Zhang, S.; Wang, Y. H. Effect of PdS on photocatalytic hydrogen evolution of nanostructured CdS under visible light irradiation. Int. J. Photoenergy 2013, 149586. [40] Zhang, J.; Qiao, S. Z.; Qi, L. F.; Yu, J. G. Fabrication of NiS modified CdS nanorod p–n junction photocatalysts with enhanced visible-light photocatalytic H2-production activity. Phys. Chem. Chem. Phys. 2013, 15, 12088–12094. [41] Akple, M. S.; Low, J.; Wageh, S.; Al-Ghamdi, Ahmed. A.; Yu, J. G.; Zhang, J. Enhanced visible light photocatalytic H2-production of g-C3N4/WS2 composite heterostructures. Appl. Surf. Sci. 2015, 358, 196–203. [42] Jing, D. W.; Guo. L. J. WS2 sensitized mesoporous TiO2 for efficient photocatalytic hydrogen production from water under visible light irradiation. Catal. Commun. 2007, 8, 795–799.



CdS Nanorods Coupled with WS2 Nanosheets for Enhanced Photocatalytic Hydrogen Evolution Activity

WS2 Nanosheets 2.4 eV

60

－1

mg

－1

e-

Rate of H2 evolution / µmol 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

40

20

h+ CdS NRs

0 WC-1.6 WC -H2O WC-ethanol CdS NRs

Composites of CdS nanorods coupled with WS2 nanosheets were synthesized by ultrasonic/exfoliation method using dimethylformamide as dispersing agent. The optimized composite shows high photocatalytic efficiency for hydrogen evolution.


Page 26 of 26

CdS Nanorods Coupled with WS2 Nanosheets for ... - ACS Publications

Recommend Documents