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Bi SPR-promoted Z-scheme Bi2MoO6/CdS-diethylenetriamine composite with effectively enhanced visible light photocatalytic hydrogen evolution activity and stability Jiali Lv, Jinfeng Zhang, Jun Liu, Zhen Li, Kai Dai, and Changhao Liang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03032 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 3, 2017

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ACS Sustainable Chemistry & Engineering

Bi SPR-promoted Z-scheme Bi2MoO6/CdS-diethylenetriamine composite with effectively enhanced visible light photocatalytic hydrogen evolution activity and stability

Jiali Lv,‡a,b,c Jinfeng Zhang,‡a Jun Liu,b Zhen Li,a Kai Dai,*a and Changhao Liang*b,c

a.

College of Physics and Electronic Information, Anhui Key Laboratory of

Energetic Materials, Huaibei Normal University, 100 Dongshan Road, Huaibei, 235000, P.R. China.

b.

Email: [email protected]

Key Laboratory of Materials Physics and Anhui Key Laboratory of

Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, 350 Shushan Lake Road, Hefei, 230031, P.R. China.

Email:

[email protected]

c.

Department of Materials Science and Engineering, University of Science and

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

‡These authors contributed equally to this work.

1

ACS Paragon Plus Environment

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Abstract: Recently, CdS has been intensively investigated for its excellent photocatalytic hydrogen (H2) evolution property. However, the poor stability and serious photocorrosion severely influence its potential for practical application. In this work, we successfully synthesized Bi surface plasmon resonance (SPR)-promoted Z-scheme Bi2MoO6 nanosheet/CdS-diethylenetriamine (Bi-Bi2MoO6/CdS-DETA) composites via an in-situ solvothermal method. Crystalline structure, morphology, electrochemical properties along with the photocatalytic H2 evolution activity and stability

have

been

systematically

investigated.

The

as-prepared

Bi-Bi2MoO6/CdS-DETA composites with optimized structure exhibited advanced photocatalytic H2 evolution activity (reach to 7.37 mmol h-1 g-1) and high stability. Largely exposed active sites, effective Z-scheme charge separation and Bi metal SPR effect of the two-dimension (2D) heterostructure is contributed to improve H2 generation performance of Bi-Bi2MoO6/CdS-DETA composites. This work provides an effective way to develop solar energy utilization of CdS-based photocatalysts and guarantees a promising future for design and growth of visible light response and sustainable photocatalytic materials by taking both properties and architectural features into consideration. Keywords:

photocatalytic

hydrogen

evolution;

CdS;

Bi2MoO6;

Bi;

Diethylenetriamine

2


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INTRODUCTION With the aggravation of environmental pollution and energy shortage, reproducible clean energy technologies have attracted much attention more recently.1-4 Hydrogen (H2) has been regarded as the most promising candidates for sustainable energy development.5-9 Since Fujishima and Honda firstly reported the water splitting by TiO2 photoanode in 1972,10 photocatalytic water splitting method has been regarded as a clean, low-cost, and feasible strategy for H2 production. Many semiconductors, such as oxynitride-, oxide-, carbideand sulfide- based photocatalytic materials, have been applied for water splitting H2 evolution in the past few decades. Among them, transitional metal sulfides, such as MoS2,11,12 WS2,13,14 CdS,15,16 and Cu1.94S-ZnxCd1-xS17 possess unique optical performance and have been demonstrated as the effective visible-light-driven photocatalysts for H2 evolution. CdS, as one of the widely studied photocatalysts, has a suitable narrow band gap (about 2.40 eV), which matches well with the visible range of sun light irradiation.18,19 Moreover, its sufficient conduction band electron potential and enriched active sites also play important roles in H2 evolution activity.20,21 Unfortunately, CdS is unstable for the oxidization of its own sulfur ions by photogenerated holes, resulting in severely decreased activity of the catalyst.22,23 Varieties of approaches have been adopted to solve these defects and further improve its photocatalytic H2 evolution activity, such as controlling 3



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of crystal phase and morphology,24,25 formation of special nano-structured CdS,26,27 loading of co-catalysts and so on.28-30 Morphology regulation and CdS-based composites fabrication are two popular ways to enhance H2 production, because they could greatly improve the electrons and holes separation and increase the surface active sites.31,32 Furthermore, the economical, operation facile, and applicable features are still highly important for efficient solar H2 generation. To date, several different morphologies of CdS have been prepared, such as nano-ceramics, porous nanosheets (NSs) and hollow nanorods (NRs), polymer microarrays and microbores. Recently, it has been found that CdS with a 2D structure can help to enlarge surface area, enrich active sites and promote the separation of electron and hole pairs. Yu’s research group successfully synthesized CdS film nano-assembled flowers and illustrated that the as-prepared CdS photocatalysts exhibited three times superior H2 evolution activity than that of CdS nanoparticles (NPs).33 In Bera and his colleagues’ study, it had been clearly explained that the CdS nanosheet/RGO composite exhibits significantly higher photocatalytic activity compared to NRs and NPs composites.34 Despite tremendous achievement in the area of CdS, it is still a great challenge to develop cheap, highly efficient and long cycling solar-light-driven CdS-based catalysts. It is well known that semiconductor heterojunctions could effectively facilitate fast charge separation. Besides, recent research has found that two-dimensional (2D) layered semiconductor photocatalysts have sparked 4


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widespread interests because of abundant active sites and well-defined contact interface of composites.35,36 For example, Li et al. recently reported that Bi12O17Cl2/MoS2 2D photocatalyst was able to improve the H2 production activity and Bi-S at the heterojunction interface could greatly facilitate the separation of carriers and boost the photocatalytic performance.37 In this way, we intend to find a 2D structural, energy band matching photocatalyst to achieve the efficient advanced H2 production of CdS in visible light irradiation. Bi2MoO6 is a kind of Aurivillius oxide semiconductor, which is composed of Bi2O22+ layer sandwiched between MoO42-.38-40 And its narrow band gap energy (about 2.60 eV) suggests it can be easily captured by visible light.41-43 Usually, the layered Bi2MoO6 photocatalyst possesses good photocatalytic activity due to the large surface area and rich exposed active sites.44,45 However, the rapid recombination of photo-induced carriers results in the limitation of quantum yield and restricts the photocatalytic performance due to the inherent limitation of single photocatalytic materials. Thus, it is highly desirable for us to devise Bi2MoO6-based composites for improved photocatalytic performance. In this work, we prepared Bi surface plasmon resonance (SPR)-promoted Bi2MoO6/CdS-Diethylenetriamine (Bi-Bi2MoO6/CdS-DETA) composite for visible-light H2 generation. DETA, as one of the small organic amine molecules, was usually embedded into lattice to form the special morphology of organic-inorganic hybrids46-48. Subsequently, the Bi-Bi2MoO6/CdS-DETA composites were successfully synthesized via in-situ growth process for 5



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inhibited photocorrosion, enlarged surface area, raised stability and promoted visible-light absorption capacity. We also utilize density functional theory (DFT) calculations to explore the significance of Bi2MoO6 NSs. On the strength of theoretical studies, we reported a rational design of Bi-Bi2MoO6/CdS-DETA composites to realize the outstanding visible-light photocatalytic H2 production activity. As revealed by photocurrent measurements and photocatalytic performance tests, the as-prepared 5%Bi-Bi2MoO6/CdS-DETA composites exhibited the highest photocatalytic H2 evolution under 420 nm visible-light irradiation. Finally, the possible transfer mechanism of charge carriers is also discussed. This work not only provides a deep insight into the key role of a suitable matching semiconductor that impact on the formation of a composite, but also gives a direct way to convert solar energy into fuels in renewable energy field.

EXPERIMENTAL SECTION Materials Cadmium chloride (CdCl2·2.5H2O), ammonium molybdate ((NH4)2MoO4), ethylene glycol (C2H6O2, EG), sodium hydroxide (NaOH), ethylenediamine (EDA), diethylenetriamine (DETA), sodium sulfide (Na2S), bismuth nitrate pentahydrate (Bi(NO3)5·5H2O), sodium sulfite (Na2SO3) and sulfur powder (S) were bought from Sinopharm Chemical Reagent Corp. (P. R. China). 18 MΩ deionized water (DW) was prepared for the synthesis of the photocatalysts. Preparation of organic-inorganic CdS-DETA hybrid 6


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Firstly, 0.164g CdCl2·2.5H2O, 0.128g S, 24 mL DETA and 12 mL DW were mixed and stirred for 30 min. And then, the mixture was sealed into a Teflon-lined autoclave (50 mL) and maintained in oven at 80 oC for 48 h. Subsequently, the CdS-DETA product was collected by centrifugation with DW. Lastly, the final yellow product was collected by freeze vacuum drying. For comparison, pure CdS NRs were also synthesized using above method by changing DETA to EDA. Preparation of Bi-Bi2MoO6/CdS-DETA composites 0.20 g CdS-DETA photocatalysts, (NH4)6·Mo7O24 and Bi(NO3)5·5H2O with different stoichiometric ratio of 2.5%, 5%, 10% and 20%, were added into the mixture of 15 mL EG and 25 mL DW under magnetic stirring. Then, 10 M NaOH was used to adjust pH value to 9. Subsequently, the mixture was sealed into a 50 mL autoclave and maintained at 160 oC for 3 h. Finally, the Bi-Bi2MoO6/CdS-DETA composites were collected after washed several times. To investigate the effect of the Bi-Bi2MoO6 content on H2 production rates of the Bi-Bi2MoO6/CdS-DETA composites, the samples are presented as x%Bi-Bi2MoO6/CdS-DETA, where x is the weight content of Bi-Bi2MoO6 (x=2.5, 5, 10 and 20). Furthermore, pure Bi2MoO6 NSs were also made as previous reported.40 Bi-Bi2MoO6/CdS was prepared by changing DETA to EDA. Bi2MoO6/CdS-DETA was prepared by changing 15 mL EG and 25 mL DW to 40 mL DW.

Instruments and characterization 7



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The as-prepared Bi2MoO6 NSs, CdS-DETA, Bi-Bi2MoO6/CdS-DETA composites were characterized by various analytic techniques. Transmission electron

microscopy

(TEM)

and

high-resolution transmission electron

microscopy (HRTEM) images were collected by using JEOL JEM-2100 microscope. X-ray power diffraction (XRD) were recorded on a Panalytical Empyrean diffractometer, and Cu Kα radiation ( λ =1.5406 Å) was served as X-ray source. Surface composition and chemical states were performed on an ESCALAB

250

X-ray

photoelectron

spectrometer

(XPS).

The

Brunauer-Emmett-Teller specific surface area (SBET) values of samples were tested by a Micromeritics ASAP 2010 system at 77 K. The infrared (IR) absorption spectra were recorded by Nicolet 6700 Fourier transform IR (FT-IR). PerkinElmer Lambda 950 spectrophotometer was used to obtain the ultraviolet-visible diffuse reflectance spectra (UV-Vis DRS) and analyzed the light

absorption

properties

of

the

as-prepared

photocatalysts.

The

photoluminescence (PL) spectra were measured by a FLS920 combined fluorescence lifetime and steady state spectrometer. Electrochemical properties of as-prepared photocatalysts were investigated on a Shanghai Chenhua CHI-660D electrochemical workstation with a common 3 electrodes system. Platinum wire and calomel electrode were utilized as counter and reference electrode, respectively. H2 production activity tests

8


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Photocatalytic H2 production experiment was carried out in a 250 mL standard reaction vessel under room temperature and standard vacuum environment. The excitation light source is 300 W Xenon lamp equipped with UV-cutoff filter (>420 nm). 50 mg catalyst was added in 100 mL mixed solution containing 0.25 M Na2SO3 and 0.35 M Na2S. Then, 300 µl H2PtCl6·6H2O acid was added into the mixed solution, and about 0.6wt% Pt was reacted on the surface of photocatalyst by photoreduction method. Then, the solution was placed into the reaction system and then evacuated for 60 min. And the distance between Xenon lamp and the reaction vessel is 20 cm. The amount of produced H2 was determined with the gas chromatography (Ceaulight GC-7900). Computational details The density functional theory (DFT) calculations was used to investigate the electronic performance of CdS and Bi2MoO6 and carried out by CASTEP code.49

The

generalized

Perdew–Burke–Ernzerh

of

gradient (PBE)

approximation form

was

(GGA)

utilized

as

with the

exchange–correlation function. The interaction between ionic core and valence electrons was studied by the ultrasoft pseudopotential. The atomic coordinates and lattice parameters were relaxed with the cutoff of 450 eV and Monkhorst–Packgrids of 7 × 7 × 4 k-points for CdS and 5 × 2 × 5 k-points for Bi2MoO6. The convergence tolerance of a total energy, amaximum stress, maximum force and maximum atomic displacement for geometry optimization 9



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calculations were 5 ×10−6 eV/atom, 0.02 GPa, 0.01 eV / Å and 5 ×10−4 Å, respectively.

RESULTS AND DISCUSSION Bi-Bi2MoO6/CdS-DETA composites were prepared via a simple in-situ solvothermal treatment and the specific formation process is shown in Fig.1. Firstly, the bright yellow organic-inorganic CdS-DETA hybrid precursor was successfully synthesized by a typical solvothermal method. As indicated in Figure S1, DETA molecules in the autoclave were firstly protonated by reaction with water at 80 oC and then form positively charged ammonium ions. Then protonated DETA molecules were incorporated into neighboring CdS layers by coordination with S element. S as a nonmetal could control the speed of the reaction and the DETA organic molecules could be served as the matrix to modulate the morphology, crystal size, and performance of semiconductors.48,50 Secondly,

CdS-DETA

precursor

readily

reacted

with

Bi(NO3)3

and

(NH4)6Mo7O24 in EG solutions to form Bi2MoO6/CdS-DETA composites. Remarkably, the prefabricated organic-inorganic CdS-DETA hybrids possessed more active sites and higher surface area, which provided a platform and an easier route for the nucleation of Bi2MoO6. At the same time, some Bi3+ ions on the surface of the Bi2MoO6 can be reduced by EG, and thus a certain amount of Bi NPs were formed. Eventually, the novel Bi2MoO6/CdS-DETA with metal Bi-doped samples with large surface area were ultimately obtained. 10


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Figure 1. Schematic illustration of the preparation process for Bi-Bi2MoO6/CdS-DETA composites.

Figure 2. XRD patterns of CdS-DETA, x%Bi-Bi2MoO6/CdS-DETA (x=2.5, 5, 10, 20) and Bi2MoO6 NSs.

11



XRD patterns

of

CdS-DETA,

Bi2MoO6

and

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Bi-Bi2MoO6/CdS-DETA

composites are shown in Fig.2. For precursor CdS-DETA, the strong diffraction peaks at 2θ=24.93o, 26.66o, 28.33o, 36.82o, 43.90o, 48.12o, 52.11o and 53.08o are corresponding to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2) and (2 0 1) crystal planes of the hexagonal phase of CdS (JCPDS No. 80-0006), respectively. And the pure Bi2MoO6 NSs exhibits strong diffraction peaks at 2θ=10.89o, 28.25o, 32.59o, 33.07o, 46.72o, 47.07o, 75.40o and 56.16o, which corresponded to (0 2 0), (1 3 1), (0 0 2), (0 6 0), (2 0 2), (2 6 0), (3 3 3) and (1 9 1) crystal planes of Bi2MoO6 (JCPDS No. 76-2388) respectively. As for Bi-Bi2MoO6/CdS-DETA composites with different x, no obvious shift in peak position was investigated, which suggested that the deposited Bi2MoO6 NSs were sticked on the outer surface of CdS-DETA instead of incorporating into the lattice of CdS-DETA. Furthermore, although the peaks of Bi2MoO6 are very weak in Bi-Bi2MoO6/CdS-DETA composites, the existence of Bi2MoO6 in the composites can be described by the increased peak intensity of (0 0 2) crystal planes of Bi2MoO6 in the Bi-Bi2MoO6/CdS-DETA composites. However, the content of Bi is too little to be found in the patterns of Bi-Bi2MoO6/CdS-DETA composites.

12


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Figure 3 TEM images of (a) Bi2MoO6, (b) CdS-DETA and (c) 5% Bi-Bi2MoO6/CdS-DETA and (d) HRTEM images of 5% Bi-Bi2MoO6/CdS-DETA composites. TEM images can directly investigate the morphology and structure of the samples. As shown in Figure 3a, Bi2MoO6 has a clearly 2D nanosheet structure with arbitrary sizes. In Figure 3b, CdS-DETA shows a uniform ribbon size about 10~20 nm, and organic-inorganic CdS-DETA hybrids as a precursor also exhibit

layered

structure.

Figure

3c

shows

TEM

image

of

Bi-Bi2MoO6/CdS-DETA composites, the combination of two different materials can be clearly observed. Figure 3d shows HRTEM image, the lattice spacing values of 0.338, 0.396 and 0.286 nm, which corresponds to (1 1 1) crystal plane of CdS, (0 4 0) plane of Bi2MoO6 and (0 1 2) plane of Bi, respectively. What’s 13



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more, the HRTEM image also reveals that the Bi-Bi2MoO6 NSs were deposited tightly on the surface of CdS-DETA. To verify the organic-inorganic structure of CdS/DETA, the surface functional groups were characterized by FT-IR spectra. As indicated in Figure S2, the vibration bands of -CH2-, -NH2, C-N, and -NH in CdS-DETA suggest the incorporation of DETA molecule in the CdS-DETA hybrids.50 The weak intensity of vibration bands resulted from the low content of DETA in the hybrid structure. The unique shape and structural features of organic-inorganic CdS-DETA hybrids may bring extraordinary advantages in photocatalyst synthesizing and high photocatalytic activities. To further analyze and identify surface chemical composition and chemical status of the Bi-Bi2MoO6/CdS-DETA composites, XPS analysis was carried out as shown in Figure 4. Figure 4a shows a typical full scanning spectrum of 5%Bi-Bi2MoO6/CdS-DETA samples, and binding energies of N 1s, Cd 3d, S 2p, Mo 3d, Bi 4f and O 1s appear at corresponding photoelectron peaks, respectively. N 1s is from DETA of organic-inorganic CdS-DETA hybrids (Figure S3a). In Figure 4b, the two binding energy peaks at 404.7 and 411.5 eV are ascribed to Cd 3d5/2 and 3d3/2 for Cd2+ in CdS,51,52 respectively. XPS spectrum of S 2p in Figure 4c indicates the central peak at 161.3 eV, which is corresponding to the binding energy of S 2p for S2- in CdS.53 Figure 3d shows Mo 4d3/2 and 4d5/2 centered at 235.8 and 232.6 eV, 51 respectively. As shown in Figure 4e, there are three peaks for Bi element. The peaks of Bi 4f5/2 and 14


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4f7/2 are centered at 162.3 and 161.2 eV, in agreement with those of Bi2MoO6.54 but they show obvious shift to lower binding energies by ca. 0.2 eV compared with that of pure Bi2MoO6 (Figure S3b). The peak centered at 156.8 eV was attributed to the metallic Bi (Figure S3b).55 As shown in Figure 4f, O 1s spectrum can be divided into three different peaks of 529.2, 529.8 and 530.5 eV. Two peaks at 529.2 and 529.8 eV for Bi2MoO6 are related to Bi-O, Mo-O,56 respectively. The peak at 530.5 eV can be attributed to hydroxyl radicals, suggesting

that

hydroxyl

groups

(O-H)

formed

in

the

surface

of

Bi-Bi2MoO6/CdS-DETA composites, which will enhance the photocatalytic performance.57,58 Based on the results, it can be confirmed that CdS, Bi2MoO6 and metallic Bi exist in the as-prepared Bi-Bi2MoO6/CdS-DETA sample.

15



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Figure 4 (a) The overview and the corresponding high-resolution XPS spectra of (b) Cd 3d, (c) S 2p, (d)Mo 3d, (e)Bi 4f and (f) O 1s of 5%Bi-Bi2MoO6/CdS-DETA samples. Optical properties of Bi2MoO6, CdS-DETA, and Bi-Bi2MoO6/CdS-DETA composites have been shown in Figure 5a. CdS-DETA precursor displays a strong absorption of photons with wavelength less than 530 nm. As for pure Bi2MoO6, it can also absorb sun light with wavelength less than 473 nm. With the

increase

of

Bi-Bi2MoO6

content,

the

absorbance

edges

of

Bi-Bi2MoO6/CdS-DETA composites present a slightly blue shift compared to pure CdS-DETA. Figure S4 shows the color transformation of the different 16


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photocatalysts. The color of Bi2MoO6 is light yellow, and CdS-DETA is slightly dull yellow. The color transformation of Bi-Bi2MoO6/CdS-DETA composites from dull yellow to brown is induced by the appearance of metallic Bi. And the optical absorption is in according with the color transformation of the photocatalysts. As indicated in Figure 5a, an obvious SPR absorption in the range of 600~680 nm can be easily observed due to the SPR effects of Bi metal. The

results

demonstrate

that

the

as-prepared

Bi-Bi2MoO6/CdS-DETA

composites should possess potential excellent visible-light photoactivity. Moreover, the linear transformation absorption curve of CdS-DETA and Bi2MoO6 were shown in Figure 5b. The band-gap energy (Eg) for Bi2MoO6 and CdS-DETA can be calculated as follows:59

αhv ~ (hv − Eg )（n / 2）/ hv

（1）

Where: α is the absorption coefficient (at wave vector k=0) of the semiconductor. And the index number of the formula is n=1 or 4 for direct-gap (CdS) or indirect-gap semiconductor (Bi2MoO6). Therefore, on the basis of Eq. 1 and the tangent line of the curve, Eg of Bi2MoO6 and CdS are approximately 2.62 and 2.34 eV, respectively. Subsequently, the conduction band energy (ECB) and valence band energy (EVB) of a photocatalysis can be calculated according to the follows:60,61

EVB = X − E e + 0.5Eg ECB = EVB − E g

(2) (3)

17



Page 18 of 43

Where: X is electronegativity of photocatalysis, and Ee is definite value (about 4.5 eV). As for CdS-DETA and Bi2MoO6, the X values are about 5.15 and 5.50 eV, respectively. Thus, EVB and ECB of CdS-DETA are 1.82 and -0.52 eV, respectively. And EVB and ECB of of Bi2MoO6 are at 2.33 and -0.29eV, respectively.

Figure 5 (a) UV-Vis DRS spectra for Bi2MoO6, CdS-DETA, and X%Bi-Bi2MoO6/CdS-DETA composites and (b) Eg value for Bi2MoO6, CdS-DETA. Energy band structures and density of states (DOS) of CdS and Bi2MoO6 were calculated by DFT method and shown in Figure 6. It is clear that CdS is direct band gap semiconductor photocatalytic materials because its conduction band minimum (CBM) and valence band maximum (VBM) locate at same high symmetry point G. For Bi2MoO6, an indirect band gap was observed between VBM at G point and the CBM at P point along SX direction. Calculated band gap for CdS and Bi2MoO6 are 1.30 and 1.90 eV, respectively. It should be noted that calculated band gap values are much narrower than experimental results (2.34 eV for CdS and 2.62 eV for Bi2MoO6) due to the shortcoming of GGA function.62,63 According to the DOS shown in Figure 6c and 6d, valence band 18


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and conduction bands of CdS are composed of S3p, and a little Cd5s, 4p. As reported, S 3p orbitals facilitate suitable conduction band edges and narrow band gaps.64 Upper valence and conduction bands of Bi2MoO6 are mainly composed by 2p orbital of O and a little Mo 4d, Bi 6s, 6p orbital.

Figure 6. Band structure for (a) CdS and (b) Bi2MoO6 and DOS for (c) CdS and (d) Bi2MoO6 As shown in Figure 7, the 5%Bi-Bi2MoO6/CdS-DETA composites exhibits the lowest intensity compared with pure Bi2MoO6 and CdS-DETA, suggesting the formation of composition between Bi2MoO6 and CdS-DETA has been successfully established and can efficiently slow down the recombination of the photogenerated carries. According to above discussion, it can be concluded that the Bi-Bi2MoO6/CdS-DETA system is a typical Z-scheme photocatalytic 19



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composite rather than heterojunction composite. It is suggested that there are rich electrons in the CdS CB and holes in the Bi2MoO6 NSs VB in the water reduction and the S2- and SO32- oxidation reactions, respectively.

Figure 7. PL spectra for as-prepared photocatalysts. To further demonstrate the separation efficiency of photogenerated carriers of the catalysts, transient photocurrent response of pure Bi2MoO6, CdS-DETA and Bi-Bi2MoO6/CdS-DETA composites during two on-off cycles were measured as shown in Figure 8. In contrast to the CdS-DETA and pure Bi2MoO6 photocatalysts, all of the Bi-Bi2MoO6/CdS-DETA composites present the higher photocurrent intensity than that of single photocatalys. Note that the 5%Bi-Bi2MoO6/CdS-DETA composites provides the highest photocurrent intensity, which is about two times stronger than that of CdS-DETA under the same

experimental

condition,

indicating

that

Bi-Bi2MoO6/CdS-DETA 20


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composites could possibly achieve effective charge separation and enhanced photocatalytic activity. Moreover, the results are also in accordance with the consequence of the PL measurements.

Figure 8. Transient photocurrent response of the as-prepared photocatalysts. Figure 9a shows the H2 production rate on the pure Bi2MoO6, CdS-DETA, and Bi-Bi2MoO6/CdS-DETA composites under 420 nm visible-light irradiation. Pure Bi2MoO6 displays almost no photocatalytic activity for H2 evolution. The rate of H2 evolution on the CdS-DETA was about 4.88 mmol h-1 g-1. DETA organic molecules not only regulate the shape of CdS, but also change the physical and chemical performance of CdS. Figure S5a and S5b show the typical FESEM images of CdS NRs and CdS-DETA nanobelts (NBs). EDS analyses of CdS NRs and CdS-DETA NBs (Figure S5c and S5d) corroborate N element existence in CdS-DETA hybrids, which is in consistent with XPS result. Figure S6a shows the N2 gas adsorption-desorption isotherms for CdS NRs and 21



Page 22 of 43

CdS-DETA NBs. The SBET values are summarized in Figure S6b. It is found that CdS-DETA NBs (55.7 m2/g) have larger surface area than that of pure CdS NRs (13.4 m2/g). The results indicate that CdS-DETA NBs with larger surface area will provide more active site, which is also vital factor for excellent composite photocatalyst preparation. Figure S7a shows the UV-Vis DRS of CdS NRs and CdS-DETA hybrids. A very interesting behavior for the CdS-DETA NBs is a slight blue shift of band edge absorption with respect to CdS NRs. Thus, the inorganic-organic photocatalyst of CdS-DETA NBs have a wider band gap than CdS NPs (Figure S7b), and the stronger redox ability of CdS-DETA NBs could account for the higher H2 production. As shown in Figure S8, the inorganic-organic materials of CdS-DETA NBs exhibit much higher H2-production activity (4.88 mmol h-1 g-1) than the common CdS NRs (1.34 mmol h-1 g-1). After depositing different proportions of Bi-Bi2MoO6 NSs, the rate of H2 evolution increased in the different degree, indicating that Bi-Bi2MoO6 can promote the charge transfer on the interface and improve the performance of photocatalytic H2 generation. At the amount of Bi-Bi2MoO6 increased from 0 to 5, the photocatalytic H2 evolution rate sustained increased, and 5%Bi-Bi2MoO6/CdS-DETA exerted the optimal visible-light photocatalytic H2 production rate about 7.37 mmol h-1 g-1 with an apparent quantum efficiency of 20.7% at 420 nm, which is 1.5 and 5.5 times higher than that of CdS-DETA and common CdS NRs, respectively. Figure S8 also shows the vital function of Bi NPs and DETA molecules, 5%Bi-Bi2MoO6/CdS-DETA exhibits higher 22


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photocatalytic activity than 5%Bi-Bi2MoO6/CdS and 5%Bi2MoO6/CdS-DETA samples. Once the weight ratio of Bi-Bi2MoO6 increased to 10% and 20%, as shown in Figure 9a, the evidently declined H2 evolution rate could be attributed to the serious aggregation of Bi NPs and more recombination centers of photoinduced electrons and holes. Additionally, larger numbers of Bi-Bi2MoO6 covered on the surface of CdS-DETA NBs could also hinder the visible light absorption

of

the

photocatalyst.

Figure

S9

shows

the

N2

gas

adsorption–desorption isotherms for CdS-DETA, x%Bi-Bi2MoO6/CdS-DETA composites (x=2.5, 5, 10, 20) and Bi2MoO6 at the same condition of 77 K. Figure S9b summarizes SBET values of Bi-Bi2MoO6/CdS-DETA composites. Though SBET is important for photocatalysis, but it is not the main reason for high photocatalytic H2 production. Table 1 lists SBET and H2 production rate of some reported CdS-based samples. The four cycling runs for photocatalytic H2 production of 5%Bi-Bi2MoO6/CdS-DETA are shown in Figure 9b. In the former three cycle tests, the H2 production rate remains constant with time, and H2 production rate still remains 88% after four cycle tests, indicating that the as-prepared Bi-Bi2MoO6/CdS-DETA composites could effectively restrain the photocorrosion of CdS and possess a stable photocatalytic H2 production activity. Moreover, the crystal structure of reused 5%Bi-Bi2MoO6/CdS-DETA composite is further investigated by XRD (Figure S10). The structure of 5%Bi-Bi2MoO6/CdS-DETA four-cycling

photocatalytic

composite

can

measurements.

be

well

Thus,

maintained

this

result

after further 23



Page 24 of 43

demonstrated that Bi-Bi2MoO6/CdS-DETA presented the stability and high efficiency of photocatalytic H2 evolution performance.

Figure 9 (a) photocatalytic H2 production for pure Bi2MoO6, CdS-DETA, and x%Bi-Bi2MoO6/CdS-DETA composites, (b) cycling runs for photocatalytic H2 production of 5%Bi-Bi2MoO6/CdS-DETA samples and (c) Cyclic H2-evolution from Bi2MoO6, CdS-DETA, and 5%Bi-Bi2MoO6/CdS-DETA composites.

24


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Table 1. Summarized SBET and H2 production rate of CdS-based samples Sample

SBET 2

Co-catalyst

Light source Sacrificial agent

-1

7.12

Ref.

-1 -1

(solution)/Volume[mL] (mmol h g )

(m g ) CdS/BiVO4

H2 production

0.5wt % Pt

300 W

(25 v% methanol)/50

5.74

19

(10 v% lactic acid)/100

5.54

21

300 W

(0.35 M Na2S+ 0.25 M 4.54

65

Xe- lamp

Na2SO3)/100

Xe- lamp λ≥420nm CdS/Mo2C@C

276.10

0.5wt % Pt

300 W Xe- lamp λ≥420nm

CdS/MoS2/Mo

23.67

/

λ≥420nm g-C3N4/Au/CdS

/

/

300 W


5.3

66

Xe- lamp λ≥420nm CdS/g-C3N4

/

5wt% CoP

300 W

(0.35 M Na2S+0.25 M 23.536

Xe- lamp

Na2SO3) /50

67

λ≥400nm CdS

13.2

1wt%WS2

300 W


3.55

68


2.9

69

Xe- lamp λ≥420nm CdS/WO3

/

3wt% Pt

500 W Xe- lamp λ≥400nm

CdS

38.81

3wt% Co3O4 300 W Xe- lamp

(0.5 M Na2S + 0.5 M 4.72

70

Na2SO3)/100

λ≥420nm ZnO/CdS

/

/

300 W

(0.35 M Na2S + 0.25 M 1.725

Xe- lamp

Na2SO3)/10

20

λ≥400nm CdS NRs

13.4

0.6wt % Pt

300 W

(0.35 M Na2S+ 0.25 M 1.34

This

Xe- lamp

Na2SO3) /100

work

λ≥420nm CdS-DETA NBs

55.7

0.6wt % Pt

300 W

(0.35 M Na2S+ 0.25 M 4.88

This

Xe- lamp

Na2SO3) /100

work

λ≥420nm Bi-Bi2MoO6/CdS-DETA 40.7

0.6wt % Pt

300 W

(0.35 M Na2S+ 0.25 M 7.37

This

Xe- lamp

Na2SO3) /100

work

λ≥420nm

25



Page 26 of 43

From the above discussion, the possible photocatalytic H2-production mechanism of Bi-Bi2MoO6/CdS-DETA composite was proposed in Figure 10. As mentioned earlier, the Eg of Bi2MoO6 and CdS-DETA are 2.62 and 2.34 eV, respectively. When samples excited by visible light, the photogenerated electrons at VB potential of Bi2MoO6 and CdS-DETA can be easily transfer to their corresponding CB potential. The CB of Bi2MoO6 is more positive than that of CdS-DETA, whereas VB of CdS is more negative than that of Bi2MoO6. However, Bi2MoO6 has very poor photocatalytic H2 production activity due to the CB electron potential (-0.29 eV), which is lower than H+/H2 reduction potential (-0.41 eV).71 CdS-DETA possesses a superior H2 production performance for its appropriate CB potential position (-0.52 eV). Therefore, the photogenerated electrons can only transmit from the surface of Bi2MoO6 to CdS-DETA surface, and then the electrons were captured by Pt NPs on the surface of CdS. Furthermore, as we know, some metal NPs (such as Ag, Au, Pt) can strongly absorb visible light due to their SPR effect,72-75 which has created important applications in photocatalysts. As for Bi-Bi2MoO6/CdS-DETA composite, Bi NPs were anchored on the surface of Bi2MoO6 owing to the reduction of EG. With the help of Bi NPs, the Bi2MoO6 light absorption performance is apparently enhanced in the range of UV to near-infrared light region. Just as SPR effect of Pt NPs, the excellent visible light absorption intensity also benefited from the SPR influence of Bi NPs, and these results had been demonstrated by UV-Vis DRS measurement in Figure 5. The built-in 26


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electric field induced by the SPR effect of Bi NPs could facilitate the separation of photogenerated electrons and holes, thus, the SPR of Bi NPs is one crucial reason for the enhanced photocatalytic H2 production performance in visible region. Since the Fermi energy of Bi NP (about -0.17 eV) is lower than that of Bi2MoO6, the electrons would easily transfer from CB of Bi2MoO6 to the plasmonic Bi metal. Besides, the tightly anchored Bi NPs on the surface of Bi2MoO6 could act as electron traps to promote the separation of electron-hole pairs due to the fine electrical conductivity of Bi NPs. Then the captured electrons could easily transfer to CdS-DETA NBs because the VB position of CdS (+1.82 eV) is more positive than that of Fermi energy of Bi NPs. Then, the Z-scheme mechanism of Bi-Bi2MoO6/CdS-DETA composites was formed. And the Bi-Bi2MoO6/CdS-DETA heterojunction nanostructure itself can effectively hinder the recombination of photogenerated electron-hole pairs. Additionally, the 2D structure of Bi2MoO6 and CdS-DETA with much stronger bond strength and larger contact interface can also accelerate the efficient separation of photoexcited carriers. In consequence, as-prepared Bi-Bi2MoO6/CdS-DETA composites with SPR affected Bi NPs and superficial distributed Bi2MoO6 NSs can well control the corrosion of CdS and significantly improve the photocatalytic H2 production activity. These results are also consistent with the previous PL and transient photocurrent response study.

27



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Figure 10. Schematic diagram of charge transfer of Bi-Bi2MoO6/CdS-DETA composites under visible light irradiation.

CONCLUSIONS In summary, Bi-Bi2MoO6/CdS-DETA composites were synthesized via an in-situ solvothermal method. It was found that Bi-Bi2MoO6/CdS-DETA composites showed largely enhanced photocatalytic H2 evolution activity and cyclic stability. This kind of organic-inorganic CdS-DETA hybrid is very suitable for substrate material because of the activated surface group and large surface area. The DFT calculation results indicate that CdS and Bi2MoO6 28


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belong to direct and indirect band gap semiconductor materials, respectively. Benefited from novel organic-inorganic CdS-DETA hybrid substrate, Z-scheme heterojunction structure, Bi metal SPR effect, electrons and holes have been effectively separated during photocatalytic H2 evolution process, which lead to largely enhanced photocatalytic H2 evolution activity. Moreover, special separation of electron and hole in Z-scheme heterojunction composite has avoided enrichment of hole on valence band of CdS, which has effectively reduced photocorrosion of CdS. Thus, the stability of composite was found to be superior to that of pure CdS-DETA. This work has provided a new approach for design and synthesis of CdS-based photocatalysts with high activity and stable H2 generation by making good use of band structure of semiconducting materials.

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

SUPPORTING INFORMATION Figure S1 explains synthetic illustration of CdS-DETA hybrids. Figures (S2 to S10) explain FT-IR spectra, XPS spectra, the color, FESEM images, EDS spectra, SBET values, UV-Vis DRS spectra, H2 production and XRD measurements of photocatalysts.

29



Page 30 of 43

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (51572103 and 51502106), the Foundation for Young Talents in College of Anhui Province (gxyqZD2017051), the Key Foundation of Educational Commission of Anhui Province (KJ2016SD53) and Innovation Team of Design and Application of Advanced Energetic Materials.

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Graphic abstract

Bi SPR-promoted Z-scheme Bi2MoO6/CdS-DETA shows high visible light photocatalytic H2 evolution activity.


CdS ... - ACS Publications

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