Three-dimensional core-shell nanorod arrays for efficient visible-light

Subscriber access provided by UNIV OF LOUISIANA

Energy, Environmental, and Catalysis Applications

Three-dimensional core-shell nanorod arrays for efficient visible-light photocatalytic H2 production Daotong You, Chunxiang Xu, Jing Wang, Wenyue Su, Wei Zhang, Jie Zhao, Feifei Qin, and Yanjun Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11988 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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 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 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.

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 23 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

ACS Applied Materials & Interfaces

Three-Dimensional Core-Shell Nanorod Arrays for Efficient Visible-Light Photocatalytic H2 Production Daotong Youa, Chunxiang Xua*, Jing Wangb, Wenyue Sub, Wei Zhanga, Jie Zhaoa, Feifei Qina and Yanjun Liua a. State Key Laboratory of Bioelectronics, School of Biological Sciences & Medical Engineering, Southeast University, Nanjing 210096, PR China b. State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, 350002, P. R. China

ABSTRACT

Constructing heterostructured nanomaterials with integrating different functional material into well-oriented nanoarchitecture is an efficacious tactics to obtain high performance photocatalyst. In this paper, we fabricated three-dimensional (3D) ZnO-WS2@CdS core-shell nanorod arrays as visible-light-driven photocatalysts for efficient photocatalytic H2 production. This unique core-shell heterostructure extends visible-light absorption and provides more active sites. More importantly, the ZnO-WS2@CdS nanorod arrays builds a beneficial energy level configuration and spatial structure to accelerate the generation, separation and transfer of the photogenerated electron hole. Based on the synergistic effects, the photocatalytic H2 rate of optimized ZnO-WS2@CdS nanorod arrays achieves 15.12 mmol h-1g-1 in visible light irradiation, which is 39, 9 and 8 times higher than pure CdS, ZnO-CdS and CdS-WS2 photocatalysts. The apparent quantum yield (AQY) is up to 14.92 % at 420 nm. Moreover, the core-shell heterostructure photocatalyst can recycle and maintain stability. Keywords: 3D heterostructure; core-shell; ZnO nanorod arrays; CdS; WS2; visible-light H2 generation

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

INTRODUCTION Harnessing solar energy to produce H2 through H2O splitting is a very prospective

strategy of renewable energy storage.1,2 In recent years, many semiconductor photocatalysts have been investigated for photocatalytic H2 production.3-6 It still have to face challenges such as fast recombination of photogenerated charge and inefficient of utilizing solar light although significant progress have been made in this area.7 Recently, much effort has been devoted to promote photocatalyst efficiency based on the structure-function relationship.8,9 For example, one-dimensional (1D) nanorods are beneficial for photocatalytic applications because their ideal geometrical structure offers a straight channel for photogenerated carriers transport as well as high surface areas. Moreover, the integration of 1D materials into 3D higher-order architectures is a highly efficient strategy to harvest light for photocatalysis.10-14 As typical semiconducting materials, ZnO has been widely studied as photocatalyst because of its suitable redox potential, high electron mobility and rich morphologies.15,16 However, optical absorption only in ultraviolet region (Eg = 3.37 eV) and rapid charge carrier recombination restrict the practical application of ZnO.17,18 Various strategies have been proposed to attack these issues. One of the prominent strategies is using another narrow band gap semiconductor to sensitize ZnO. CdS is frequently used as visible light sensitizing material for ZnO due to its narrow bandgap (2.4 eV) and the similar crystal structure as ZnO.19,20 The type-II band configuration of ZnO and CdS would promotes a spatial separation of the photogenerated electron and hole.21,22 Thus, controlling their interface between the ZnO and CdS is very essential and important to acquire more efficient photocatalyst. Loading cocatalyst is another effective approach to improve the photocatalytic performance.23,24 Noble metals such as Pt have shown great efficient cocatalyts for semiconductor, but they are high price.25-27 It is necessary to develop noble metal-free and highly active cocatalysts, WS2 is a promising cheap and highly efficient and abundant catalyst for large-scale production of hydrogen owing to high electron mobility, excellent structure, and abundant H2 producing active sites.28-30 Until now, the enhanced photocatalytic activity of WS2 has been reported in CdS and ZnO, 2


Page 2 of 23

Page 3 of 23 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


respectively.31-34 It is expected to obtain more excellent performance if introducing WS2 into CdS/ZnO composites. In this paper, WS2 and CdS nanoparticles were sputtered subsequently on vapor phase deposition-grown ZnO nanorod to construct 3D ZnO-WS2@CdS core-shell nanorod arrays in order to realize better photocatalytic performance by rational combination of multiple components. This 3D configuration not only widens visible-light absorption, supplies rich active sites, but also builds a beneficial energy level configuration and spatial structure to accelerate the separation and transfer of photogenerated electron hole. Owing to synergistic effect, the optimized ZnO-WS2@CdS exhibits high-performance photocatalytic hydrogen evolution rate of 15.12 mmol h-1g-1 in visible light irradiation, which was almost 39, 9 and 8 times greater than pristine CdS, CdS-ZnO and CdS-WS2 photocatalysts, respectively. The calculated AQY is as high as 14.92% at 420 nm wavelength. Moreover, compared with the powder photocatalyst, this photocatalyst in 3D nanorod arrays is easier to recycle and maintain stability.

EXPERRIMENTAL SECTION

Samples Preparation. The ZnO-WS2@CdS nanorod arrays were fabricated by three steps subsequently, as shown in Scheme. Firstly, the preparation of ZnO nanorod arrays mainly refer to our previous reports.35,36 Secondly, WS2 was grown on the ZnO nanorod arrays using radio frequency (RF) magnetron sputtering process by using 99.99 % pure WS2 as target. During process, the argon gas with a flow rate of 55 sccm was introduced into the chamber and maintained a work pressure of 1.5 pa, and the working power of 100 W. The reaction times are controlled by 3min, 5min and 8min, respectively, the obtained samples were marked as ZnO-WS2-1, ZnO-WS2-2 and ZnO-WS2-3, respectively. Finally, CdS nanoparticles were grown further on the surface of ZnO-WS2 nanorod arrays with similar RF magnetron sputtering process by using 99.99 % pure CdS as target. After the sputtering time about 10 min, the ZnO-WS2@CdS nanorod arrays were obtained and denoted as ZnO-WS2@CdS-1, ZnO-WS2@CdS-2 and ZnO-WS2@CdS-3, respectively, according to the different content of WS2. 3



Page 4 of 23

During the sputtering processes, pure WS2, pure CdS, ZnO-CdS and CdS-WS2 were also collected on sapphire substrate for comparison.

Scheme 1.Schematic diagram of preparation of ZnO-WS2@ CdS core-shell nanorod arrays.

Characterizations. The crystal structures of the samples were tested using X-ray diffractometer (XRD) on Bruker D8 Advance X-ray diffractometer system with Cu Kα radiation. Carl Zeiss Ultra Plus field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM, JOEL JEM-2100) were used to measure morphology and microstructures of samples, simultaneously the combining energy dispersive X-ray (EDS) spectra to analyze element composition and content. UV-Vis spectrophotometer (UV-2600) was used to measure the optical properties of samples. X-ray photoelectron spectroscopy (XPS) (ESCALAB 250) was used to test the surface chemical states and the density of states (DOS) of the valence band (VB) of samples, the binding energy of each element were calibrated by the C1s (284.6 eV). Fluorescence spectrophotometer (F4600) with 365 nm excitation wavelength was used to measure the the photoluminescence (PL) emission of samples. The Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA)

was

used

to

calculate

the

specific

surface

Brunauer-Emmett-Teller (BET) formula.

4


area

through

the

Page 5 of 23 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


Photoelectrochemistry Measurements. The photoelectrochemical properties of samples were tested by the ZAHNER IM6 (Germany) electrochemical workstation, this workstation is equipped with a standard three electrode system with a platinum plate, saturated Ag/AgCl electrode and obtained samples as the counter electrode, reference and working electrode, respectively. The prepared samples were directly spread on ITO glass electrode to prepare working electrodes and Na2SO4 (0.2 M, pH 7.0) aqueous solution was used for the electrolyte for the photocurrent measurement. The photosource was filter light (λ ≥ 420 nm) provided by CEL-S500/350 300 W Xe lamp. Photocatalytic Activity Measurements. The photocatalytic H2 reactions were evaluated by a gas-closed-circulation and evacuation system. Typically, the photocatalysts with the area of 1 cm × 1 cm were immersed into 10 % lactic acid solution (80 mL). The mass of the catalyst was ca. 20 mg, which was calculated by the difference in the mass of bare substrate and the sample coated-substrate. 300 W Xe lamp equipping a optical filter (HOYA; L42, λ ≥ 420 nm) as visible light source. On-line gas chromatography (GC-8A) was used to analyze the amount of H2 produced. The AQY was tested under the same reaction conditions and calculated by using the following equations 1: AQY ( %)=

=

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

（1）

The light intensity measurements were taken by a radiometer (Spectri Light ILT950) .

5



RESULTS AND DISCUSSION Crystal Structure and Morphology

Figure. 1. (a) Plan-view, (b) cross-section view SEM images and (c) XRD patterns of ZnO nanorods, inserted with an enlarged individual. (d) Plan-view, (e) cross-section view SEM images and (f) XRD patterns of ZnO-WS2@CdS-2 nanorods, inserted with an enlarged individual.

Figure. 1 present the SEM and XRD of ZnO and ZnO-WS2@CdS-2. As shown in Figure. 1a and b, the cross-section view SEM images display a well-oriented nanorod arrays grown perpendicular to the sapphire substrate, and the nanorods have smooth surface and hexagonal cross section average diameters in 300 ~ 400 nm and lengths of 4 µm. The XRD diffraction peaks located at 2θ = 34.46° and 72.59°, which can be assigned to the (002) and (004) planes of the wurtzite ZnO, respectively (JCPDS card No. 36-1451) as shown in Figure. 1c.36 The ZnO nanorod arrays were completely coated with WS2 and CdS nanoparticles from the top to the bottom after the two-step sputtering process (Figure. 1d and e). A typical XRD pattern of the obtained ZnO-WS2@CdS-2 in Figure. 1f clearly displays a diffraction peak at 26.5ºcorrespond to hexagonal CdS phase (002) planes (JCPDS No. 41-1049).37 It still maintains the diffraction signals from (002) and (004) planes of the wurtzite ZnO. No diffractions peaks assigned to the WS2 were observed, which probably due to the high dispersion of WS2. 32

6


Page 6 of 23

Page 7 of 23 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. 2. (a) SEM images of an individual ZnO-WS2@CdS-2 nanorod, and (b) the corresponding EDS spectrum and EDS mapping of element: Zn, O, S, Cd and W in (c), (d), (e), (f) and (j), respectively.

The detailed chemical composition of elements in ZnO-WS2@CdS-2 were analyzed using a SEM dispersive spectrometer (EDS). As shown in Figure 2a, the EDS analysis of the selected rectangular area of FESEM image of single ZnO-WS2@CdS-2 nanorod confirmed the consist of Zn, O, S, Cd and W in Figure. 2b. The EDS element mapping image of Figure. 2c-j further confirms the existence Zn, O, S, Cd and W in the sample. Obviously, the distribution of all the elements is well-distributed.

7



Figure. 3. (a) TEM images and (b) HRTEM images of ZnO, respectively. (c) TEM images and (d) HRTEM images of ZnO-WS2-2, respectively. (e) TEM images and (f) HRTEM images of ZnO-WS2@CdS-2, respectively.

Further observation of the morphologies and structures of the ZnO, ZnO-WS2-2 and ZnO-WS2@CdS-2 samples were investigated by TEM. The TEM image of the ZnO nanorod exhibits smooth surfaces and diameter with ~ 300 nm (Figure. 3a), which were in agreement with the SEM results. HRTEM in Figure. 3b indicates that ZnO nanorod has a single crystal structure.38 The TEM image of ZnO-WS2-2 (Figure. 3c) reveals WS2 nanofilms are covered on the surface of ZnO nanorod. Figure. 3d is the HRTEM image of ZnO-WS2-2, the two different lattice spacing were found to be 0.261 nm and 0.62 nm, which can be ascribed to the (002) planes of hexagonal ZnO and (002) planes of hexagonal WS2, respectively.39 As shown in Figure. 3e, the ZnO nanorod arrays were tightly coated with WS2 and CdS nanoparticles. Besides the (002) planes of wurtzite ZnO, the (002) plane of hexagonal CdS with a lattice spacing of 0.336 nm (JCPDS No. 41-1049)37 was found in the HRTEM image of ZnO-WS2@CdS-2 in Figure. 3f, while some interconnected WS2 nanofilm can be observed. It reveal that the three semiconductors coexist and connect intimately each other, which not only serve as pathways to facilitate the migration of charge carriers, but also induce synergistic effects for the improvement of photocatalytic efficiency.

8


Page 8 of 23

Page 9 of 23 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


XPS Analysis

Figure. 4. XPS spectra of ZnO-WS2@CdS-2: (a) full-range spectra, (b) W 4f states, (c) S 2p states, (d) Cd 3d states, (e) Zn 2p states and (f) O 1s states.

XPS characterization was performed to check the chemical composition of ZnO-WS2@CdS-2. The survey spectrum of Figure 4a can detect Cd, S, W, Zn, O and C element. The peaks at 35.7 eV and 37.8 eV can be corresponded to WS2 from Figure. 4b. The two peaks at 161.5 eV and 162.7 eV (Figure. 4c) can be corresponded to S 2p3/2 and S 2p1/2 of WS2, respectively.40 In the Cd 3d core level XPS spectrum (Figure. 4d), the peaks at 405.2 eV and 411.9 eV, which can be associated with Cd 3d5/2 and Cd 3d3/2 of CdS, respectively.41 In Zn 2p spectrum (Figure. 4e) the peaks at 1022.8 and 1045.7 eV are represent to Zn 2p3/2 and Zn 2p1/2, respectively. The O 1s XPS peak at 531.4 eV and 533.3 eV are ascribed to the O2- ions in the ZnO lattice and C/O or OH-1 adsorbed species on the surface as shown in Figure. 4f , respectively.42

9



Photocatalytic Performance

Figure. 5. (a), (b) Photocatalytic H2 evolution time courses and (c) average hydrogen production rates of photocatalysts under visible-light irradiation, respectively. (d) Photocatalytic H2 production recycling stability tests over ZnO-WS2@CdS-2 .

The photocatalytic activities of the ZnO-WS2@CdS-2 as well as the pure ZnO, pure ZnO-WS2-1, ZnO-WS2-2, ZnO-WS2-3, ZnO-WS2@CdS-1 and ZnO-WS2@CdS-3 were evaluated by examining H2 production by photocatalytic H2O splitting under visible-light irradiation (λ ≥ 420 nm). As delineated in Figure. 5a, no H2 production could be obtained in pure ZnO because the visible-light energy is not enough to excite it. Meanwhile, no H2 was detected in ZnO-WS2, which suggested that WS2 was very poor photocatalytic H2 evolution activity under visible-light. Furthermore, as introducing CdS, ZnO-WS2@CdS showed significant H2 production activity. Among them, ZnO-WS2@CdS-2 exhibited better performance than ZnO-WS2@CdS-1 and ZnO-WS2@CdS-3, indicating that the amount of WS2 with different sputtering time affects the photocatalytic performance, that low content of WS2 can not provide enough active sites while overmuch amount may effect the absorption of light. To investigate the role of each component, photocatalytic activities of CdS, 10


Page 10 of 23

Page 11 of 23 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


ZnO-CdS, CdS-WS2 and WS2 were compared, as shown in Figure. 5b. ZnO-CdS shows higher H2 production activity than pure CdS, this means that CdS and ZnO form a well heterojunction interface, which is conducive to separate electron hole. The photocatalytic activity results of ZnO-WS2@CdS-2 shows an even higher than ZnO-CdS and CdS-WS2 indicate that the synergistic effects ZnO, WS2 and CdS, which can provide more reactive active sites and promote the separation of photogenerated charge carrier. Figure. 5c estimates the average H2 evolution rates of sample according to Figure. 5a and b. ZnO-WS2@CdS-2 exhibits the most efficient H2 evolution with the largest H2 rate as high as 15.12 mmol h-1g-1, and the AQY reaches 14.92 % at 420 nm, this rate was in fact much higher than those of ZnO-WS2@CdS-1 (i.e., 13.15 mmol h-1g-1), ZnO-WS2@CdS-3 (i.e., 14.32 mmol h-1g-1), pure CdS (i.e., 0.39 mmol h-1g-1), ZnO-CdS (i.e., 1.62 mmol h-1g-1) and CdS-WS2 (i.e., 1.89 mmol h-1g-1) (Figure. 6c). That is to say, that the as-prepared ZnO-WS2@CdS-2 hybrid photocatalysts yielded a profound improvement in visible-light photocatalytic H2 production. The average H2 release rate of ZnO-WS2@CdS-2 is more than 39, 9 and 8 times greater than the pure CdS, ZnO-CdS and CdS-WS2 photocatalysts, respectively. The detailed comparison between ZnO-WS2@CdS and other recently reported H2 evolution photocatalysts has been listed in supporting information (Table S1). At the same time, we have directly chose outdoor sunlight as the light source for photocatalytic H2 production measurements of ZnO-WS2@CdS. This catalyst also can decompose water to produce continuous hydrogen bubbles as shown in Figure S1. The photocatalytic H2 production activity was 61.55 mmol/g after 4 hours of irradiation, which was higher than the results obtained using a 300 W Xe lamp (λ ≥ 420 nm) as the light source as shown in Figure S2. Figure. 5d shows the cyclic stability performance for the photocatalytic H2 production over ZnO-WS2@CdS-2 under the same conditions. The sample still remains nearly 95.7% H2 production in initial activity (1.06 mmol) after four run photocatalytic experiments without adding the sacrificial agents. The result indicates the ZnO-WS2@CdS was relatively stable photocatalysts. Besides, we have found the 11



morphology, crystal structure, optical properties and surface chemical compositions of ZnO-WS2@CdS remained no obvious change after photocatalytic reactions (16 h) by the comparison in Figure S3 and S4, further suggesting that the ZnO-WS2@CdS is relatively stable in visible-light photocatalysts.

Optical Property

Figure. 6. UV-vis diffuse reflectance spectra of the alone ZnO, ZnO-WS2-2 and ZnO-WS2@CdS-2.

The light absorption properties of the pure ZnO, ZnO-WS2-2 and ZnO-WS2@CdS-2 samples were measured using UV-vis diffuse reflectance spectra (DRS).43 As shown in Figure. 6 and Figure. S5, an absorption edge at ~ 390 nm of pure ZnO was observed, which corresponds to the intrinsic band gap at 3.18 eV. For the ZnO-WS2-2 composites, the absorption extend to the visible-light region, which can be attributed to the introduction of WS2 (Figure. S5).44 In contrast to the pure ZnO and ZnO-WS2-2, the light absorption of ZnO-WS2@CdS-2 sample extends further into the visible region (around 520 nm), which is originated in the intrinsic band gap absorption of CdS.45 The DRS results implies that ZnO-WS2@CdS sample has visible light response.

12


Page 12 of 23

Page 13 of 23 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


Surface Area

Figure. 7 Nitrogen adsorption-desorption isotherm of (a) ZnO, (b) ZnO-WS2-2, (c) ZnO-CdS, (d) ZnO-WS2@CdS-2.

The BET surface areas of ZnO, ZnO-WS2-2, ZnO-CdS, ZnO-WS2@CdS-2 were investigated via nitrogen absorption-desorption isotherms as shown in Figure. 7. The isotherms of all the samples are of type IV classification according to the IUPAC classification at high relative pressure (P/P0).46 BET surface areas are increased in the order ZnO-WS2@CdS-2 (85.54 m2g-1) > ZnO-CdS (58.73 m2g-1) > ZnO-WS-2 (54.68 m2g-1) > ZnO (53.73 m2g-1). These results indicate that the CdS and WS2 combined well with ZnO nanorod and provided more reactive sites, which thus are benefit to enhance photocatalytic performance.

13



Photoelectric Characteristics

Figure. 8. (a) PL spectra of ZnO, ZnO-CdS, ZnO-WS2@CdS-2 and ZnO-WS2-2; (b) Transient photocurrent response of ZnO, ZnO-CdS, ZnO-WS2@CdS-2, CdS-WS2 and ZnO-WS2-2.

In order to corroborate the above analysis, the PL spectra was used to investigate the process of photoinduced electron hole separation and migration. As shown in Figure. 8a, the pure ZnO presents an intense and broad emission peak at around 390 nm and 520 nm, corresponding to the intrinsic and defective luminescence of ZnO, respectively.47 Meanwhile, the PL intensity decreased significantly after the modification of WS2 and CdS, respectively. Especially, ZnO-WS2@CdS-2 shows an extremely low fluorescence intensity, indicating that ZnO-WS2@CdS possessed efficient migration and separation of interfacial charge carrier.48 Those analysis is further proved by transient photocurrent responses, the transient photocurrent responses of the pure ZnO, ZnO-WS2-2, ZnO-CdS, CdS-WS2 and ZnO-WS2@CdS-2 are displayed in Figure. 8b, the ZnO-WS2@CdS-2 composite shows the highest photocurrent density (17.97 mA cm-2), which was higher than those of ZnO (0 mA cm-2), ZnO-CdS-2 (0 mA cm-2), CdS-WS2 (2.34 mA cm-2), and ZnO-CdS (0.99 mA cm-2), and the photocurrent intensity maintains a stable value illumination, indicating the efficient separation of charge carrier by transportation the photogenerated electron-hole pairs from CdS to WS2 and ZnO, which is consistent with the excellent visible light photocatalytic H2.

14


Page 14 of 23

Page 15 of 23 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


Photocatalytic Mechanism

Figure. 9. VB XPS spectra of pure (a) ZnO, (b) CdS and (c) WS2; (d) Schematic illustration of the DOS of the pure ZnO, CdS and WS2.

In order to elucidate the photocatalytic mechanism and charge transfer process, the band structures of ZnO, CdS and WS2 were studied. Valence band XPS was tested with the spectra shown in Figure 9a-c, the VB energy levels of ZnO, CdS and WS2 were determined to be 2.51 eV, 1.36 eV and 0.94 eV, respectively. Therefore, in combination with the UV-vis absorption results above (Figure. S5), the conduction band (CB) edge of ZnO, CdS and WS2 were determined to -0.67 eV, -1.02 eV, and -0.58 eV by calculation, respectively.49,50 Therefore the band structure alignments can be well resolved as illustrated in Figure. 9d, the interface between CdS and ZnO can form type II heterojunction structure, and WS2 possesses a more negative CB as compared to ZnO. Based on the above results and analysis, the enhancement of photocatalytic performances can be ascribed to the following points: (1) The high surface areas of ZnO-WS2@CdS provide more adsorption sites and active sites. (2) CdS and WS2 nanoparticles were uniformly dispersed on the aligned ZnO nanorod arrays, which provide a favorable energy level alignment and spatial configuration for separation and transport of the photogenerated charge carriers. Thus the high H2 evolution mechanism in the ZnO-WS2@CdS-2 is proposed in Figure. 10. In the system of 15



ZnO-WS2@CdS-2, CdS are excited to generate electrons and holes under visible-light irradiation. Owing to the close interfacial contact between CdS, WS2 and ZnO, photogenerated electrons of CdS have two pathways of transfer, one way is directly transferred to WS2 surface, and the other can be indirectly migrated to WS2 via the ZnO due to the type II band structure between ZnO and CdS, thus the probability of photoelectron-hole recombination can be greatly reduced, and more electrons will participate in H+ reduction of H2, thereby enhancing hydrogen production activity of ZnO-WS2@CdS-2.

Figure 10. Proposed schematic for visible light (λ ≥ 420 nm) photocatalytic H2 production of ZnO-WS2@CdS-2 in lactic acid solution.

CONCLUSIONS In summary, this study successfully fabricates 3D ZnO-WS2@CdS core/shell

nanorod arrays structure combine ZnO nanorod arrays, WS2 and CdS nanoparticles through a VPD route and sputtering. It is clearly demonstrated that the ZnO-WS2@CdS nanorod arrays exhibits high-performance visible-light-driven photocatalytic H2 evolution with the rate of 15.12 mmol h-1g-1, corresponding to an AQY 14.92% at 420 nm, which was 39, 8 and 9 times greater than the pure CdS, CdS-WS2 and ZnO-CdS photocatalysts, respectively. The remarkably enhanced photocatalytic activity can be ascribed to not only beneficial energy level configuration and spatial structure to boost the separation and transfer of 16


Page 16 of 23

Page 17 of 23 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


photogenerated electron-hole, but also the positive synergistic effect between CdS, WS2 and ZnO, which provides large surface area (85.54 m2g-1) and more active sites. We believe this work offers a new strategy to construct efficient nanoarchitecture photocatalysts for H2 evolution from H2O splitting.

ASSOCIATED CONTENT Supporting Information Comparison the H2 production performance; H2 evolution under outdoor sunlight; SEM image, TEM image, XRD patterns, DRS and XPS before and after the photocatalytic reaction.

AUTHOR INFORMATION

Corresponding Author *Phone: 86-025-83790755. Fax: 86-025-83790362. E-mail address: [email protected]

ORCID Chunxiang Xu:0000-0001-8116-2869 Notes The authors declare no competing financial interest.

ACKONWLEDGMENTS We acknowledge support from NSFC (61475035, 11734005, 61704024), Science &

Technology Project of Jiangsu Province (BE2016177, BK20170696), and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1828), and Collaborative Innovation Center of Suzhou Nano Science and Technology.

REFERENCES

(1) Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Kang, Z. Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting via a Two-Electron Pathway. Science 2015, 347, 970-974.

17



(2) Zhang, Z.; Huang, Y.; Liu, K.; Guo, L.; Yuan, Q.; Dong, B. Multichannel-Improved Charge-Carrier Dynamics in Well-Designed Hetero-nanostructural Plasmonic Photocatalysts toward Highly Efficient Solar-to-Fuels Conversion. Adv. Mater. 2015, 27, 5906–5914. (3) You, D.; Pan, B.; Jiang, F.; Zhou, Y.; Su, W. CdS Nanoparticles/CeO2 Nanorods Composite with High-Efficiency Visible-Light-Driven Photocatalytic Activity. Appl. Sur. Sci. 2016, 363, 154-160. (4) Xu, H.; Song, P.; Yan, B.; Wang, J.; Guo, J.; Du, Y. Surface-Plasmon-Enhanced Photo-Electrocatalytic Ethylene Glycol Oxidation Based on Highly Open AuAg Nanobowls. ACS Sustain. Chem. Eng. 2018, 6, 4138-4146. (5) Li, K.; Zhang, C.; Liu, A.; Chu, D.; Zhang, C.; Yang, P., Du, Y.; Huang, J. Mesoporous Tungsten Oxide Modified by Nanolayered Manganese-Calcium Oxide as Robust Photoanode for Solar Water Splitting. J Colloid. Inter. Sci. 2018, 516, 145-152. (6) Xu, H.; Song, P.; Wang, J.; Shiraishi, Y.; Du, Y.; Liu, Q. Visible-Light-Driven 3D Dendritic PtAu@Pt Core-Shell Photocatalyst toward Liquid Fuel Electrooxidation. ACS Sustain. Chem. Eng. 2018, 6, 7159-7167. (7) Wang, H.; Zhang, L.; Chen, Z.; Hu, J.; Li, S.; Wang, Z.; Wang, X. Semiconductor Heterojunction Photocatalysts: Design, Construction, and Photocatalytic Performances. Chem. Soc. Rev. 2014, 43, 5234-5244. (8) Zhang, Z.; Huang, J.; Zhang, M.; Yuan, Q.; Dong, B. Ultrathin Hexagonal SnS2 Nanosheets Coupled with g-C3N4 Nanosheets as 2D/2D Heterojunction Photocatalysts toward High Photocatalytic Activity. Appl. Catal. B Environ. 2015, 163, 298-305. (9) Tong, Z.; Yang, D.; Shi, J.; Nan, Y.; Sun, Y.; Jiang, Z. Three-Dimensional Porous Aerogel Constructed by g-C3N4 and Graphene Oxide Nanosheets with Excellent Visible-Light Photocatalytic Performance. ACS Appl. Mater. Interfaces 2015, 7, 25693−25701 (10) Wang, M.; Zheng, D.; Ye, M.; Zhang, C.; Xu, B.; Lin, C.; Lin, Z. One-Dimensional Densely Aligned Perovskite-Decorated Semiconductor Heterojunctions with Enhanced Photocatalytic Activity. Small 2015, 11, 1436-1442. (11) Tian, J.; Zhao, Z.; Kumar, A.; Boughton, R. I.; Liu, H. Recent Progress in Design, Synthesis, and Applications of One-Dimensional TiO2 Nanostructured Surface Heterostructures: a Review. Chem. Soc. Rev. 2014, 43, 6920-6937. 18


Page 18 of 23

Page 19 of 23 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


(12) Wang, M.; Ioccozia, J.; Sun, L.; Lin, C.; Lin, Z. Inorganic-Modified Semiconductor TiO2 Nanotube Arrays for Photocatalysis. Energy Environ. Sci. 2014, 7, 2182-2202. (13) Tian, J.; Liu, Q.; Cheng, N.; Asiri, A. M.; Sun, X. Self-Supported Cu3P Nanowire Arrays as an Integrated High-Performance Three-Dimensional Cathode for Generating Hydrogen from Water. Angew. Chem. 2014, 126, 9731 –9735. (14) Liu, S.; Tang, Z. R.; Sun, Y.; Colmenares, J. C.; Xu, Y. J. One-Dimension-Based Spatially Ordered Architectures for Solar Energy Conversion. Chem. Soc. Rev. 2015, 44, 5053-5075. (15) Zhao, X.; Feng, J.; Liu, J.; Lu, J.; Shi, W.; Yang, G.; Cheng, P. Metal-Organic Framework-Derived ZnO/ZnS Heteronanostructures for Efficient Visible-Light-Driven Photocatalytic Hydrogen Production. Adv. Sci. 2018, 5, 1700590. (16) Wang, J.; Xia, Y.; Zhao, H.; Wang, G.; Xiang, L.; Xu, J.; Komarneni, S. Oxygen Defects-Mediated Z-Scheme Charge Separation in g-C3N4/ZnO Photocatalysts for Enhanced Visible-Light Degradation of 4-Chlorophenol and Hydrogen Evolution. Appl. Catal. B Environ. 2017, 206, 406-416 (17) Mahmood, K.; Swain, B. S.; Amassian, A. 16.1% Efficient Hysteresis-Free Mesostructured Perovskite Solar Cells Based on Synergistically Improved ZnO Nanorod Arrays. Adv. Energy Mater. 2015, 5, 1500568. (18) Bhirud, A.; Sathaye, S.; Waichal, R.; Park, C. J.; Kale, B. In Situ Preparation of N-ZnO/Graphene Nanocomposites: Excellent Candidate as a Photocatalyst for Enhanced Solar Hydrogen Generation and High Performance Supercapacitor Electrode. J. Mater. Chem. A 2015, 3, 17050-17063 (19) Zhao, H.; Dong, Y.; Jiang, P.; Wang, G.; Miao, H.; Wu, R.; Zhang, C. Light-Assisted Preparation of a ZnO/CdS Nanocomposite for Enhanced Photocatalytic H2 Evolution: an Insight into Importance of in Situ Generated ZnS. ACS Sustain. Chem. Eng. 2015, 3, 969-977 (20) Kuang, P. Y.; Su, Y. Z.; Xiao, K.; Liu, Z. Q.; Li, N.; Wang, H. J.; Zhang, J. Double-Shelled CdS-and CdSe-Cosensitized ZnO Porous Nanotube Arrays for Superior Photoelectrocatalytic Applications. ACS Appl. Mater. Interfaces 2015, 7, 16387−16394. (21) Ma, D.; Shi, J. W.; Zou, Y.; Fan, Z.; Ji, X.; Niu, C.; Wang, L. Rational Design of CdS@ZnO Core-Shell Structure via Atomic Layer Deposition for Drastically Enhanced Photocatalytic H2 Evolution with Excellent Photostability. Nano Energy 2017, 39, 183-191. 19



Page 20 of 23

(22) Zhao, K.; Yan, X.; Gu, Y.; Kang, Z.; Bai, Z.; Cao, S.; Zhang, Y. Self-Powered Photoelectrochemical

Biosensor

Based

on

CdS/RGO/ZnO

Nanowire

Array

Heterostructure. Small 2016, 12, 245-251. (23) Yu, X.; Shavel, A.; An, X.; Luo, Z.; Ibáñez, M.; Cabot, A. Cu2ZnSnS4-Pt and Cu2ZnSnS4-Au Heterostructured Nanoparticles for Photocatalytic Water Splitting and Pollutant Degradation. J. Am. Chem. Soc. 2014, 136, 9236-9239 (24) Gopannagari, M.; Kumar, D. P.; Reddy, D. A.; Hong, S.; Song, M. I.; Kim, T. K. In Situ Preparation of Few-Layered WS2 Nanosheets and Exfoliation into Bilayers on CdS Nanorods for Ultrafast Charge Carrier Migrations Toward Enhanced Photocatalytic Hydrogen Production. J Catal. 2017, 351, 153-160. (25) Pan, B.; Luo, S.; Su, W.; Wang, X. Photocatalytic CO2 Reduction With H2O over LaPO4 Nanorods Deposited with Pt Cocatalyst. Appl. Catal. B Environ. 2015, 168, 458-464. (26) Wang, M.; Ye, M.; Iocozzia, J.; Lin, C.; Lin, Z. Plasmon-Mediated Solar Energy Conversion via Photocatalysis in Noble Metal/Semiconductor Composites. Adv. Sci. 2016, 3, 1600024. (27) Wang, M.; Pang, X.; Zheng, D.; He, Y.; Sun, L.; Lin, C.; Lin, Z. Nonepitaxial Growth of Uniform and Precisely Size-Tunable Core/Shell Nanoparticles and Their Enhanced Plasmon-Driven Photocatalysis. J. Mater. Chem. A 2016, 4, 7190-7199. (28) Shi, J.; Tong, R.; Zhou, X.; Gong, Y.; Zhang, Z.; Ji, Q.; Liu, Z. Temperature-Mediated Selective Growth of MoS2/WS2 and WS2/MoS2 Vertical Stacks on Au Foils for Direct Photocatalytic Applications. Adv. Mater. 2016, 28, 10664-10672. (29) Reddy, D. A.; Park, H.; Ma, R.; Kumar, D. P.; Lim, M.; Kim, T. K. Heterostructured WS2-MoS2 Ultrathin Nanosheets Integrated on CdS Nanorods to Promote Charge Separation and Migration and Improve Solar-Driven Photocatalytic Hydrogen Evolution. ChemSusChem 2017, 10, 1563-1570. (30) Zhong, Y.; Shao, Y.; Ma, F.; Wu, Y.; Huang, B.; Hao, X. Band-Gap-Matched CdSe QD/WS2 Nanosheet

Composite:

Size-Controlled

Photocatalyst

for

High-Efficiency

Water

Splitting. Nano Energy 2017, 31, 84-89. (31) Chen, C.; Yu, W.; Liu, T.; Cao, S.; Tsang, Y. Graphene Oxide/WS2/Mg-Doped ZnO Nanocomposites for Solar-Light Catalytic and Anti-Bacterial Applications. Sol. Energ. Mat. Sol. C. 2017, 160, 43-53. 20


Page 21 of 23 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


(32) Zhong, Y.; Zhao, G.; Ma, F.; Wu, Y.; Hao, X. Utilizing Photocorrosion-Recrystallization to Prepare a Highly Stable and Efficient CdS/WS2 Nanocomposite Photocatalyst for Hydrogen Evolution. Appl. Catal. B Environ. 2016, 199, 466–472 (33) Xu, D.; Xu, P.; Zhu, Y.; Peng, W.; Li, Y.; Zhang, G.; Fan, X. High Yield Exfoliation of WS2 Crystals into 1-2 Layer Semiconducting Nanosheets and Efficient Photocatalytic Hydrogen Evolution from WS2/CdS Nanorod Composites. ACS Appl. Mater. Interfaces 2018, 10, 2810-2818. (34) Zhang, K.; Fujitsuka, M.; Du, Y.; Majima, T. 2D-2D Heterostructured CdS/WS2 with Efficient Charge Separation Improving H2 Evolution under Visible Light Irradiation. ACS Appl. Mater. Interfaces 2018, 10, 20458-20466 (35) You, D.; Xu, C.; Qin, F.; Zhu, Z.; Manohari, A. G.; Xu, W.; Liu, W. Interface Control for Pure Ultraviolet Electroluminescence from Nano-ZnO-Based Heterojunction Devices. Sci. Bull. 2018, 63, 38-45. (36) Lu, J.; Xu, C.; Dai, J.; Li, J.; Wang, Y.; Lin, Y.; Li, P. Improved UV Photoresponse of ZnO Nanorod

Arrays

by

Resonant

Coupling

with

Surface

Plasmons

of

Al

Nanoparticles. Nanoscale 2015, 7, 3396-3403. (37) Yu, Y.; Huang, Y.; Yu, Y.; Shi, Y.; Zhang, B. Design of Continuous Built-in Band Bending in Self-Supported

CdS

Nanorod-Based

Hierarchical

Architecture

for

Efficient

Photoelectrochemical Hydrogen Production. Nano Energy 2018, 43, 236-243. (38) Ren, X.; Zhang, X.; Liu, N.; Wen, L.; Ding, L.; Ma, Z.; Gao, Y. White Light-Emitting Diode From Sb-Doped p-ZnO Nanowire Arrays/n-GaN Film. Adv. Funct. Mater. 2015, 25, 2182-2188. (39) Gopannagari, M.; Kumar, D. P.; Reddy, D. A.; Hong, S.; Song, M. I.; Kim, T. K. In Situ Preparation of Few-Layered WS2 Nanosheets and Exfoliation into Bilayers on CdS Nanorods for Ultrafast Charge Carrier Migrations Toward Enhanced Photocatalytic Hydrogen Production. J. Catal. 2017, 351, 153-160. (40) Wu, X.; Gong, K.; Zhao, G.; Lou, W.; Wang, X.; Liu, W. MoS2/WS2 Quantum Dots as High-Performance Lubricant Additive in Polyalkylene Glycol for Steel/Steel Contact at Elevated Temperature. Adv. Mater. Interfaces 2018, 5, 1700859. (41) Qiu, B.; Zhu, Q.; Du, M.; Fan, L.; Xing, M.; Zhang, J. Efficient Solar Light Harvesting 21



Page 22 of 23

CdS/Co9S8 Hollow Cubes for Z-Scheme Photocatalytic Water Splitting. Angew. Chem. 2017, 129, 2728-2732. (42) Corro, G.; Vidal, E.; Cebada, S.; Pal, U.; Bañuelos, F.; Vargas, D.; Guilleminot; E. Electronic State of Silver in Ag/SiO2 and Ag/ZnO Catalysts and Its Effect on Diesel Particulate Matter Oxidation: An XPS Study. Appl. Catal. B Environ. 2017, 216, 1-10. (43) Zhao, Z.; Zhang, H.; Kim, D. Y.; Hu, W.; Bullock, E. S.; Strobel, T. A. Properties of The Exotic Metastable ST12 Germanium Allotrope. Nat. Commun. 2017, 8, 13909. (44) Kang, M.; Kim, B.; Ryu, S. H.; Jung, S. W.; Kim, J.; Moreschini, L.; Kim, K. S. Universal Mechanism of Band-gap Engineering in Transition-Metal Dichalcogenides. Nano Lett. 2017, 17, 1610-1615. (45) Yue, Z.; Liu, A.; Zhang, C.; Huang, J.; Zhu, M.; Du, Y.; Yang, P. Noble-Metal-Free Hetero-Structural

CdS/Nb2O5/N-Doped-Graphene

Ternary Photocatalytic

System

as

Visible-Light-Driven Photocatalyst for Hydrogen Evolution. Appl. Catal. B Environ. 2017, 201, 202-210. (46) Gu, X.; Wang, Y.; Lai, C.; Qiu, J.; Li, S.; Hou, Y.; Zhang, S. Microporous Bamboo Biochar for Lithium-Sulfur Batteries. Nano Res. 2015, 8, 129-139. (47) Zeng, H.; Duan, G.; Li, Y.; Yang, S.; Xu, X.; Cai, W. Blue Luminescence of ZnO Nanoparticles Based on Non-Equilibrium Processes: Defect Origins and Emission Controls. Adv. Funct. Mater. 2010, 20, 561-572. (48) Weng, B.; Yang, M. Q.; Zhang, N.; Xu, Y. J. Toward the Enhanced Photoactivity and Photostability of ZnO Nanospheres via Intimate Surface Coating with Reduced Graphene Oxide. J. Mater. Chem. A 2014, 2, 9380-9389. (49) Zhang, K.; Wang, L.; Kim, J. K.; Ma, M.; Veerappan, G.; Lee, C. L.; Park, J. H. An Order/Disorder/Water Junction System for Highly Efficient Co-Catalyst-Free Photocatalytic Hydrogen Generation. Energy Environ. Sci. 2016, 9, 499

(50) Khan, M. M.; Ansari, S. A.; Pradhan, D.; Ansari, M. O.; Lee, J.; Cho, M. H. Band Gap Engineered TiO2 Nanoparticles for Visible Light Induced Photoelectrochemical and Photocatalytic Studies. J. Mater. Chem. A. 2014, 2, 637

22


Page 23 of 23 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


Table of Contents graphic

23


Three-dimensional core-shell nanorod arrays for efficient visible-light

Recommend Documents