Bi2MoO6 heterostructure for enhanced photocatalytic activities and gas-sensing properties

Author’s Accepted Manuscript Anion-exchange Engineering of Cookie-like Bi2S3/Bi2MoO6 Heterostructure for Enhanced Photocatalytic Activities and Gas-Sensing Properties Yu Pei, Xiaoguang Li, Hang Chu, Yuancai Ge, Pei Dong, Robert Baines, Liyuan Pei, Mingxin Ye, Jianfeng Shen

PII: DOI: Reference:

www.elsevier.com/locate/talanta

S0039-9140(16)30956-0 http://dx.doi.org/10.1016/j.talanta.2016.12.015 TAL17105

To appear in: Talanta Received date: 23 September 2016 Revised date: 25 November 2016 Accepted date: 6 December 2016 Cite this article as: Yu Pei, Xiaoguang Li, Hang Chu, Yuancai Ge, Pei Dong, Robert Baines, Liyuan Pei, Mingxin Ye and Jianfeng Shen, Anion-exchange Engineering of Cookie-like Bi2S3/Bi2MoO6 Heterostructure for Enhanced Photocatalytic Activities and Gas-Sensing Properties, Talanta, http://dx.doi.org/10.1016/j.talanta.2016.12.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Anion-exchange Engineering of Cookie-like Bi2S3/Bi2MoO6 Heterostructure for Enhanced Photocatalytic Activities and Gas-Sensing Properties

Yu Pei1, Xiaoguang Li1, Hang Chu1, Yuancai Ge1, Pei Dong2, Robert Baines2, Liyuan Pei1, Mingxin Ye1*, Jianfeng Shen1* 1

Institute of Special Materials and Technology, Fudan University, Shanghai, 200433,

P. R. China 2

Department of Materials Science and NanoEngineering, Rice University, 6100 Main

Street, Houston, TX 77005, USA

Corresponding Authors: E-mail: [email protected], [email protected]

Abstract Developing efficient visible-light-driven photocatalysts will advance alternative energy technologies, ultimately curbing the enviromental pollution associated with fossil fuels. In this work, Bi2S3/Bi2MoO6 photocatalysts with a heterogeneous cookie-like structure were prepared for the first time by in-situ anion exchange at relatively low temperatures. The catalysts exhibited enhanced photocatalytic activity, which we attributed to the photocurrent response, a diminished recombination rate of photogenerated electron-hole pairs, and the existence of a large heterojunction interface. These governing factors were discerned by photoelectrochemical measurements, calculated energy band positions and photoluminescence spectra. Bi2S3/Bi2MoO6 nanocomposites also exhibit better performance in response to gas than bare Bi2MoO6 according to gas sensing tests. Our work, in relaying a feasible method to synthesize Bi2S3/Bi2MoO6-based heterojunction superstructures, and documents a universal preparation method of synthetic heterogeneous complexes, and provides

necessary

groundwork

for

the

development

of

next

generation

semiconductor photocatalytic technology and gas sensor.

Keywords: anion exchange; Bi2S3/Bi2MoO6 nanocomposite; heterostructures; photocatalytic degradation; Gas-Sensing

1. Introduction Photocatalysts are types of catalysts that, when exposed to light radiation, facilitate chemical reactions but do not undergo chemical change themselves. Photocatalysts have applications across a number of fields, including water splitting, photodegrading of organic pollution, and photo-reduction of carbon dioxide, etc. [1-3]. Metal oxide semiconductors, in particular, have attracted captured critical attention, because they are environmentally friendly and efficient photocatalysts [4,5]. Unfortunately, traditional metal oxide photocatalysts like TiO2, ZnO, ZnS, and other similar semiconductor materials usually have a wide band gap. This characteristic renders them responsive only to UV light, which accounts for a mere fraction of sunlight. From the perspective of photocatalytic efficiency, a fast recombination rate of photogenerated electron-hole pairs also undermines photocatalyst efficiency [6-8]. Broadening the light-response range, especially into the visible-spectrum, and ameliorating the microstructure are keys to improving photocatalytic efficiency. Recently,

researchers

have

developed

several

novel

semiconductor

photocatalysts with relatively high efficiencies and wide visible-light response ranges, such as Bi-based photocatalysts [9], Ag-based photocatalysts [10], and W-containing photocatalysts [11]. Among Bi-based photocatalysts, bismuth molybdate (Bi2MoO6) demonstrates outstanding photocatalytic performance and has frequently been applied to degrade organic pollutants under visible light radiation [12]. Unfortunately, the light conversion efficiency of Bi2MoO6 photocatalyst is low due to fast recombination between electrons and holes. The incorporation of Bi2MoO6-based heterojunctions, such

as

Ag3PO4/Bi2MoO6

[13],

TiO2/Bi2MoO6/Bi3.64Mo0.36O6.55

[14],

and

Bi2MoO6/Zn-Al [15], proved to be the best way to enrich photocatalytic properties. Since bismuth sulfide (Bi2S3), a type of Bi-based semiconductor, has a narrow band gap and excellent capacity to absorb visible-light [16], it is well disposed to the modification of other semiconductor photocatalysts [17,18]. Specifically, the renovated Bi2S3/Bi2MoO6 composite exhibited better separation efficiency of visible-light-induced pairs as well as excellent photocatalytic performance. On the other hand, as a typical multicomponent metal oxide with a band gap of

approximately 2.66 eV Bi2MoO6 has also got wide attention because of its potential applications as gas sensor material in recent years [49]. Compared to simple binary oxides, multicomponent oxides have great advantages as gas-sensing materials. For this reason, the design and synthesis of modified Bi2MoO6 nanocomposites with high surface area via a simple and low cost method still have important scientific and practical significance. Many approaches, such as hydrothermal treatment, have been employed to synthesize modified nanocomposites. Hydrothermal treatment yields a product with satisfactory morphology and dispersion [19], but the process itself takes a long time to react, requiring high reaction pressure, and temperature [20,21]. In comparison, anion exchange poses huge advantages in terms of reaction conditions. Anion exchange synthesis processes takes less time [22]. Additionally, anion exchange can synthesize a product with robust microstructure, even under normal pressure and fairly low temperature [23]. To the best of our knowledge, almost no literature has reported the study of anion exchange synthesis of Bi2S3/Bi2MoO6 nanocomposites and the effect of Bi2S3 content on photocatalytic and gas-sensing systems. Herein, for the first time, we prepared Bi2S3/Bi2MoO6 nanocomposites through a fast, simple ion exchange method using Bi2MoO6 nanosheets as precursors and CH3CSNH2 (TAA) as a sulfur source. Different Bi2S3/Bi2MoO6 nanocomposites were prepared by varying the contents of Bi2S3 in the reaction process. In their synthesized form, the Bi2S3/Bi2MoO6 nanocomposites form a heterogeneous cookie-like structure. Photocatalytic properties of the composites were assessed by decomposing a typical model pollutant, Rhodamine B (RhB) dye, under visible light radiation. Results show that the Bi2S3/Bi2MoO6 nanocomposite exhibits significantly greater photocatalytic activity than bare Bi2MoO6, we investigated the mechanisms governing the photocatalytic activity of Bi2S3/Bi2MoO6 nanocomposite and the contribution of bismuth sulfide to its photocatalytic effects. Finally, we also investigated gas-sensing properties of Bi2S3/Bi2MoO6 nanocomposite by response to alcohol and n-hexane. Bi2S3/Bi2MoO6 nanocomposite demonstrated better gas-sensing properties than bare Bi2MoO6, further promoting its potential application as gas sensor.

2. Experimental 2.1 Reagents and Chemicals Bismuth nitrate (Bi(NO3)3·5H2O), sodium molybdate (Na2MoO4·2H2O), nitric acid (HNO3), sodium hydroxide (NaOH), Sodium sulfate (Na2SO4), ethanol (CH3CH2OH), hexane (C6H14), thioacetamide (CH3CSNH2), and RhB (C28H31ClN2O3) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were analytically pure and used without further purification. Deionized water was employed in this study. 2.2 Preparation of the Bi2S3/Bi2MoO6 nanocomposites The Bi2MoO6 precursor was synthesized through a hydrothermal method according to a previous report [24]. The Bi2S3/Bi2MoO6 heterojunction was prepared via anion exchange with Bi2MoO6 precursor and TAA. In brief, 1 mmol yellow Bi2MoO6 powder was dispersed in 100 mL deionized water and continuously stirred for 1h. According to the stoichiometric ratios, varying amounts of TAA (0.3 mmol, 0.6 mmol, 0.9 mmol respectively) were dissolved in the yellow suspensions. Then the suspensions were set at 60 °C for 2 h. The products were extracted by filtration, washed by deionized water and ethanol, and dried at 60 °C for 12 h. In accordance with the dissolved amount of Bi2S3, we denoted this series of composites Bi2S3/Bi2MoO6-1, Bi2S3/Bi2MoO6-2, and Bi2S3/Bi2MoO6-3. Pure Bi2S3 was synthesized by adopting the same hydrothermal method but without Bi(NO3)3·5H2O and TAA. 2.3 Characterization The composition and phase of the composites were characterized by Powder X-ray diffraction (XRD, Bruker, D8 advance) at 40 kV and 40 mA Cu Kα radiation. X-ray photoelectron spectroscopy (XPS, Oxford, IncaX-max50) was used to record the binding energy and status of the elements in the as-prepared samples. The morphologies and microstructures of the heterostructures were investigated with scanning electron microscopy (SEM, Tescan, Maia3 XMH) and transmission electron microscopy (TEM, JEOL, JEM-2010). The contents and distribution of characteristic

elements were examined by an energy dispersive spectrometer (EDS, Bruker, XFlash660). Photocurrent density-potential curves (I-V curve) and electrochemical impedance spectroscopy (EIS) under UV-visible light irradiation were measured on an electrochemical workstation (Autolab PG 302N) in a standard three-electrode system, while the visible-light absorption abilities of the samples were collected through a UV-vis spectrophotometer (DRS, Shimadzu, UV-3600) using BaSO4 as reference. Lastly, the transition ability of the light-induced pairs was assessed with a fluorescence spectrophotometer (PL, Shanghai Lengguang, F97pro) at room temperature. 2.4 Measurement of photocatalytic activity and gas sensing We investigated photoelectrochemical with a standard three-electrode cell. It contained a working electrode, a Pt wire as the counter electrode, and a mercurous sulfate electrode as the reference electrode all within an electrochemical workstation under a Xe lamp. A 0.1 M Na2SO4 solution was used as the supporting electrolyte. The working electrode was consisted of an indium-tin oxide (ITO) glass deposited with production samples and was prepared as follows: 0.1 g of the sample was ground with 0.02 g polyvinylidene fluoride (PVDF) and 1 mL ethanol. The mixture was then stirred for 3 hours to make slurry, which was coated on an indium tin oxide (ITO) glass electrode by the doctor blade technique, and the active area was controlled to be 1.0 cm2. The electrode was dried in air and then was ready for further characterization. Gas sensing tests were performed on an electrochemical workstation (CHI600E, CH Instruments Ins). The test gas was ethanol and hexane. Sensor unit were prepared as follows: 0.1 g of the sample was grounded with 0.02 g polyvinylidene fluoride (PVDF) and 1 mL ethanol, then the mixture was stirred for 3 hours to make slurry. The slurry was coated onto a titanium plate, and the active area was controlled to be 1.0 cm2. The electrode was dried in air and then was ready for further characterization. Photocatalytic activities of the samples were evaluated by degradation of RhB in aqueous solution, under visible light irradiation, at room temperature, using a 200 W

Xe lamp with a cut-off filter (>420 nm) as the light source. In each experiment, 100 mg photocatalyst was added to 100 mL RhB solution (10-5 mol L-1, pH=7) and magnetically stirred for an hour in the dark to reach adsorption-desorption equilibrium between the photocatalyst and dye. Afterwards, solution was exposed to visible light while undergoing magnetic stirring at the same speed. About 3 mL of solution was extracted after 30 min, and centrifuged (15000 rpm, 5 min, twice) to produce a clear dye solution. The concentration of RhB was then determined by analyzing the photoabsorption intensity using a UV-vis spectrophotometer with a 554 nm wavelength beam. 3. Results and discussion 3.1 Synthesis and Characterization of Bi2S3/Bi2MoO6 nanocomposites The Bi2S3/Bi2MoO6 nanocomposites were prepared via a simple anion exchange method by using Bi2MoO6 as the precursor and TAA as the sulfur source. First, Bi2MoO6 nanosheets templates were obtained via the aforementioned hydrothermal method. We then synthesized the Bi2S3/Bi2MoO6 nanocomposites (Scheme 1) by introducing S2- through Bi2S3. More details on the synthesis procedure can be found in the Experimental Section. Powder X-ray diffraction (XRD) gave us insight into the crystal structures of pure Bi2MoO6, pure Bi2S3, and the Bi2S3/Bi2MoO6 nanocomposites, as shown in Fig. 1a. Importantly, diffraction peaks located at 28.3°, 32.6°, 46.8°, and 55.5° are responses to the (131), (002), (062), and (133) lattice planes of pure Bi2MoO6 [25]. Moreover, the diffraction peaks at 25.0°, 28.6°, 31.7°, 35.7°, and 39.8° are correspond to the (130), (211), (221), (240), and (141) lattice planes of the pure Bi2S3 [26]. Results affirm that all diffraction peaks from pure Bi2MoO6 and pure Bi2S3 are attributable to orthorhombic phase Bi2MoO6 (JCPDS No. 21-0102) and orthorhombic phase Bi2S3 (JCPDS No. 17-0320). No Bi2S3 characteristic peaks are evident in the Bi2S3/Bi2MoO6-1 or Bi2S3/Bi2MoO6-2 composites’ spectra due to a well-dispersed, relatively low concentration of Bi2S3 in each [27]. However, some new diffraction peaks are notable in the enlarged image (Fig. 1b) of Bi2S3/Bi2MoO6-3. These peaks

indicate the five main lattice planes of pure Bi2S3. In other words, the composites’ Bi2S3 diffraction peaks accentuated relative to the content of Bi2S3 suggesting stepwise formation of Bi2S3 on the Bi2MoO6 surface [28,29]. No other diffraction peaks can be observed, validating the high purity of Bi2S3 and Bi2MoO6 in the composites. We next investigated the structural characteristics of the synthesized composites with different microscopic techniques. Scanning electron microscopy (SEM), energy dispersive spectrometetry (EDS) and transmission electron microscopy (TEM) images of the samples are presented in Fig. 2. Microscopic imaging reveals that most of the pure Bi2MoO6 grew into nanoplates, self-propagating with smooth surfaces (Fig. S1a and b, Supporting Information). The lateral size of the nanoplates is approximately 400 nm and their thickness is about 50 nm. The morphology of the Bi2S3/Bi2MoO6-2 composite also resembles nanoplates, as illustrated in Fig. 2a. Unlike Bi2MoO6, the surface of Bi2S3/Bi2MoO6-2 is coarse and includes small particles on its edges. The red rectangle in Fig. 2b highlights the particulates. It is likely that the edges serve as anion exchange reaction sites. Observing the Bi2MoO6 nanosheets with lateral size of 200~500 nm in Fig. S2a, we note a 2D structure, smooth surface, square linkage architecture. Fig. S2b illustrates how the nanosheets in Bi2S3/Bi2MoO6-2 composite are coarse and irregular, which suggests the existence of heterojunction nanostructure. High-resolution TEM (HRTEM) imaging (Fig. 2c) indicates some small nanoparticles located on the surface of the nanosheets. According to the heterostructure defined in Fig. 2d, the d-spacing (0.397 nm) corresponds to specific crystal planes of the (001) faces of Bi2S3 [30], while the (0.315 nm) and (0.327 nm) spacings are respectively associated with the (131) and (140) faces of Bi2MoO6 [31,32]. Bi2MoO6 could be easily converted into Bi2S3 in the presence of S2- ions due to the low solubility (Ksp = 1×10-97) of Bi2S3 relative to Bi2MoO6 and the decomposition of TAA in S2- at 333 K in aqueous solution [33]. A crystal lattice defect in Bi2S3 is delimited by the white line in Fig. 2d. Results convey that Bi2S3 nanoparticles interlaced in the Bi2MoO6 nanoplatelet matrices, and

then formed large interface heterostructures in the Bi2S3/Bi2MoO6 nanocomposites [34]. The existence of these heterostructures separates photoinduced electron-hole pairs and consequently is pivotal to the composites’ photocatalytic activities. Moreover, TEM analysis also reaffirmed the co-existence of Bi2S3 and Bi2MoO6 in the samples. Elements in the Bi2S3/Bi2MoO6-2 composites were further studied with EDS to confirm the formation of interface heterostructures. Bi, Mo, O, and S elements were all detected, as highlighted by the white rectangle in Fig. 2e. This elemental swath is consistent with the XPS results. Observing Fig. 2e it’s noted that indeed Bi, Mo, and O elements are evenly dispersed on the surface of the sample, while the red rectangle frame in Fig. 2b indicates S elements distributed on the edges of the nanosheets, EDS thus confirms we successfully obtained a composite consisting of Bi2S3 particles and Bi2MoO6 nanoplates through an anion exchange reaction. XPS analysis was performed to elucidate the surface chemical states and composition of heterostructures. Fig. 3a displays the full spectra of Bi2MoO6 and Bi2S3/Bi2MoO6-2. Here, all the diffraction peaks can be indexed to C, Bi, Mo, S, and O elements. The C 1s (284.6 eV) was used as a criterion to calculate the binding energy of other elements (Fig. S3a). The binding energies of Bi 4f5/2 and Bi 4f7/2 are 163.6 eV and 158.4 eV (Fig. 3b), respectively, indicating a trivalent oxidation state for bismuth in the samples [35]. Interestingly, the binding energy of the Bi-O bond in Bi2MoO6 is higher than the binding energy of the Bi-S bond in Bi2S3. The resulting Bi 4f peaks in the Bi2S3/Bi2MoO6-2 composite appears in a lower energy region than the pure Bi2MoO6. To some extent, electron transfer between Bi2S3 and Bi2MoO6 occurred in the compounds. Fig. 3c shows peaks located at 232.0 and 235.0 eV which are characteristic of Mo 3d5/2 and Mo 3d3/2, indicate that Mo is in a sexavalent element state in the composite [36]. The weak signal located at 227.0 eV, close to the Mo 3d peaks, can be assigned to S 2s owing to trace amounts of Bi2S3. On the other hand, Fig. S3b presents the high-resolution O 1s spectrum with a 531.4 eV binding energy. The binding energy peaks of both O 1s and S 2s are consistent with previous reports [37], illustrating the co-existence of Bi2S3 and Bi2MoO6 components in the

Bi2S3/Bi2MoO6-2 sample, and aligning with XRD results. 3.2 Photocatalytic Activity After the Bi2MoO6 and Bi2S3/Bi2MoO6 nanocomposites were synthesized, their photoelectrochemical (PEC) activity was discerned via UV-light illumination. Namely, photocurrent density-potential curves (I-V curve) in Fig. 4a were obtained by linear sweep voltammetry. The photocurrent of Bi2S3/Bi2MoO6-2 nanocomposites turned out to be about 3 times higher than that of pure Bi2MoO6. It also had a pronounced lower onset potential under UV-light irradiation, indicating that the polarization current had been enhanced after the addition of bismuth sulfide. The outstanding photocurrent of the Bi2S3/Bi2MoO6 nanocomposites was further investigated by electrochemical impedance spectroscopy (EIS) measurements. Fig. 4b portrays the radius of every arc and its relation to the charge-transfer process. Smaller radii correspond to lower charge-transfer resistance and faster electron transfer kinetics in the redox reaction. The Bi2S3/Bi2MoO6 nanocomposites’ arc radius in particular is quite smaller than the arc of pure Bi2MoO6, reflecting its lower charge-transfer resistance and therefore enhanced photocurrent. Our series of PEC investigations demonstrates that the Bi2S3/Bi2MoO6 nanocomposites possess favorable photoresponses for PEC activity directly attributable to specific morphological and structural characteristic. The photocatalytic activities of Bi2S3, Bi2MoO6, and Bi2S3/Bi2MoO6 nanocomposites were tested by degradation of RhB under visible-light irradiation. For a standard of comparison, RhB degradation of pure Bi2S3 and pure Bi2MoO6 were performed. This setup also allowed us to explore any possible synergistic effects between Bi2S3 and Bi2MoO6. Fig. 5a presents the degradation spectrum of RhB by the Bi2S3/Bi2MoO6-2 nanocomposites under visible light irradiation as a function of time. Apexes of the absorption peaks attenuate as visible light irradiation time increases. Optically, during the photocatalytic degradation process of RhB, the solution color gradually fades from an initial deep pink. We also observe blue shift when the highest absorption peak changes from 554 nm to 544 nm. Conversely, apices of the absorption peaks (Fig. S4) for bare Bi2MoO6 under the

same conditions attenuate to a lesser degree than those in the Bi2S3/Bi2MoO6-2 nanocomposites. The highest absorption peak changes from 554 nm to 547 nm and undergoes blue shift, but is notably weaker than the shift that occurred with the nanocomposites. The blue shift phenomenon denotes molecular remove ethylated with the reaction of incomplete mineralization in the degradation process of RhB [38]. The absorbance intensity did not markedly increase in the UV-light region, conveying that most of the aromatic structure of RhB is destroyed in the degradation process [35]. Fig. 5b portrays the degradation rates of Bi2S3, Bi2MoO6, and Bi2S3/Bi2MoO6 nanocomposites with varying Bi2S3 contents. It is clear that Bi2S3/Bi2MoO6 nanocomposites have better photocatalytic properties than pure Bi2S3 or pure Bi2MoO6 as they exhibit 29.5% degradation. In the case of Bi2S3/Bi2MoO6-2, photodegradation rates reach 5.10×10-3 min-1, or 2.63 times the photodegradation rate of pure Bi2MoO6 (Fig. 5c). Comparing Bi2S3/Bi2MoO6 nanocomposites with differing Bi2S3 contents, we are able to see how photocatalytic degradation depends on Bi2S3 content. Namely, the concentration of Bi2S3 is proportional to the photodegradation properties of the composite. But the photodegradation property of Bi2S3/Bi2MoO6-3 is less than that of Bi2S3/Bi2MoO6-2. The Langmuir–Hinshelwood (L–H) kinetics model (Fig. 5d) demonstrates this difference, and the photocatalytic degradation process of RhB can be expressed as the following pseudo-first-order kinetics equation [39-41], In

C0 =k t C app

where C0 is the initial concentration of RhB solution, C is the RhB concentration at time t, and kapp is the apparent pseudo-first-order rate constant (min-1). On basis of photocatalytic degradation performances, we assert that the content of Bi2S3

greatly

influences

the

photocatalytic

efficiency

of

Bi2S3/Bi2MoO6

nanocomposite. In particular, Bi2S3 nanoparticles promote the recombination of photo-generated carriers of electron-hole pairs rather than an electron conduction path [42]. This might help explain why the photodegradation property of Bi2S3/Bi2MoO6-3

diminishes as the content of Bi2S3 increases. 3.3 The mechanism of photocatalytic activity Heterojunction photo-absorption properties were examined with UV-vis diffuse reflectance spectra (DRS); results are depicted in Fig 6a. Bi2S3 demonstrates constant, strong absorption intensity over the entire range of visible light (Fig S5). Of particular interest is how the absorption edge of Bi2MoO6 is situated at ~469 nm, remaining consistent with the reported value [43]. Compared to pure Bi2MoO6, the absorption edge of the Bi2S3/Bi2MoO6 composites underwent significant red shift after the introduction

of

Bi2S3.

This

potentially

enhanced

its

visible-light-induced

photocatalytic activity and proves that the Bi2S3 existed in Bi2S3/Bi2MoO6 heterojunctions prepared through the anion exchange method. We estimated the composites’ optical band gaps based on the intercept of the tangents on a plot of (αhv)2 versus photo energy (hv), as shown in Fig 4a. The band gap energies (Eg) of Bi2S3, Bi2S3/Bi2MoO6-3, Bi2S3/Bi2MoO6-2, Bi2S3/Bi2MoO6-1, and Bi2MoO6 turned out to be 1.40 eV, 1.60 eV, 1.84 eV, 2.01 eV, and 2.80 eV, respectively. Among them, the Eg values of Bi2S3 and Bi2MoO6 closely approach those reported in literature [44,45]. As photoluminescence (PL) emission is primarily a factor of recombination of free carriers, PL spectra is an apposite technique to survey the separation efficiency of the photogenerated charge carriers in a semiconductor [37]. Fig 6b shows a comparison of the PL spectra (excited at 300 nm) of bare Bi2MoO6 and the Bi2S3/Bi2MoO6 nanocomposites at room temperature. While Bi2MoO6 has a broad blue-green emission peak at 420~520nm, the PL emission intensity of the Bi2S3/Bi2MoO6 nanocomposites are dramatically diminished, indicating that the recombination of photogenerated charge carriers was inhibited by the heterojunction nanostructure. In other words, the coupling of Bi2S3 and Bi2MoO6 is conducive to separating the photogenerated charge carriers. To further investigate the mechanism behind the Bi2S3/Bi2MoO6 nanocomposites’ enhanced photocatalytic performance, the relative band positions of the two semiconductors were studied since the band edge position dictates the separation of

the photoexcited electrons and holes. The conduction band (CB) and valance band (VB) potentials can be calculated with the formula [46,47] 1 ECB =X-E0 - Eg 2 where X is the electronegativity of the semiconductor, E0 is the energy of free electrons on the hydrogen scale (~4.50eV), and Eg is the band gap of the semiconductor. The valence band tops (EVB) can be calculated by EVB =ECB +Eg According to these formulas, the X values for Bi2S3 and Bi2MoO6 are 5.30 eV and 6.13 eV. The conduction band bottom (ECB) of Bi2S3 and Bi2MoO6 are calculated as 0.10 eV and 0.23 eV, respectively. Finally, the valence band tops (EVB) of Bi2S3 and Bi2MoO6 are 1.50 e and 3.03 eV, respectively. Evidently, the conduction band bottom (ECB) and the valence band tops (EVB) of Bi2S3 are both higher than those of Bi2MoO6. This means photoelectrons can automatically flow into the conduction band of Bi2MoO6 from the conduction band of Bi2S3. Additionally, photoholes can easily flow into the valence band of Bi2MoO6 from the Bi2S3 valence band. As scheme 2 shows, the conduction band and valence band of Bi2S3 and Bi2MoO6 align well; in fact, they form the heterogeneous structure [48]. Therefore, the photoinduced electrons and holes can be efficiently separated, the recombination of electron−hole pairs can be reduced, and photocatalytic performance is consequently improved. To investigate whether the Bi2S3/Bi2MoO6 has response to particular gas as sensor, we studied the potential change when there existed ethanol or n-hexane gas. As shown in Fig. 7a, When the sample surface comes into contact with alcohol gas, both Bi2MoO6 and Bi2S3/Bi2MoO6 sensors have change in potential. The process of the response and recovery indicates that they can respond to ethanol gas as a sensor. The Bi2S3/Bi2MoO6 sensor shows higher potential changes when contact with ethanol gas, it demonstrates that Bi2S3/Bi2MoO6 sensor has better gas sensing property. In order to explore the sensing of Bi2S3/Bi2MoO6 nanocomposites to different gases, we also employed n-hexane as test gas. In Fig. 7b, it shows that the Bi2S3/Bi2MoO6 sensor

can also produce a potential change with the contact of n-hexane gas, it indicates the Bi2S3/Bi2MoO6 sensor can response to different gas. The results suggest that Bi2S3/Bi2MoO6 nanocomposite has better gas sensing property than bare Bi2MoO6 and the potential applications on gas sensors.

4. Conclusion In summary, Bi2S3/Bi2MoO6 nanocomposites have been successfully synthesized by employing Bi2MoO6 as a precursor and TAA as a sulfur source via a simple anion exchange method. These Bi2S3/Bi2MoO6 nanocomposites demonstrate idiosyncratic morphologies and enhanced photocatalytic activities—some even around 3 times higher than that of bare Bi2MoO6. Photoelectrochemical measurements, PL spectrum testing, and energy band theory calculations were conducted to expound on the mechanisms behind the photocatalytic activity of the Bi2S3/Bi2MoO6 nanocomposite. The prepared Bi2S3/Bi2MoO6 sensor shows higher potential changes than bare Bi2MoO6 sensor and has response to different gases, such as ethanol and n-hexane, etc. It was demonstrated that a heterojunction forms between Bi2S3 and Bi2MoO6, which enhances charge transfer and separation, separates electron-holes pairs, and inhibits their recombination. Ultimately, our research has established a simple method to fabricate heterojunctions that ameliorate photocatalytic performances. The approach could be extended to prepare other photocatalysts with heterojunctions and gas sensor. References [1] D. P. Serrano, J. M. Coronado, V. A. de la Peña O'Shea, P. Pizarro, J. Á. Botas, Journal of Materials Chemistry A, 1 (2013) 12016. [2] F. Fresno, R. Portela, S. Suárez, J. M. Coronado, Journal of Materials Chemistry A, 2 (2014) 2863-2884. [3] M. Miyauchi, H. Irie, M. Liu, X. Qiu, H. Yu, K. Sunada, K. Hashimoto, The Journal of Physical Chemistry Letters, 7 (2016) 75-84. [4] J. Li, W. Fang, C. Yu, W. Zhou, L. Zhu, Y. Xie, Applied Surface Science, 358 (2015) 46-56.

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Scheme 1 Schematic illustrating the synthesis process of Bi2S3/Bi2MoO6 nanocomposites with anion exchange method.

Fig. 1 XRD patterns of the samples, (a) Bi2S3, Bi2S3/Bi2MoO6 composites (1, 2, 3), Bi2MoO6; (b) The enlarged image of Bi2S3 and Bi2S3/Bi2MoO6-3 composites, rhombus (♦) present the peaks of Bi2S3.

Fig. 2 SEM images of (a, b) Bi2S3/Bi2MoO6-2 nanocomposite; (c, d) HRTEM image of the interface of Bi2S3/Bi2MoO6-2 composite; (e) EDS pattern of Bi2S3/Bi2MoO6-2 composite.

Fig. 3 XPS spectra of the samples, (a) Full spectrum of Bi2MoO6 and Bi2S3/Bi2MoO6-2; (b) Bi 4f spectrum of Bi2MoO6 and Bi2S3/Bi2MoO6-2; (c) Mo 3d spectrum.

Fig. 4 The photoelectrochemical performance of pure Bi2MoO6 and Bi2S3/Bi2MoO6 nanocomposites in Na2SO4 solution under Xe light, (a) linear sweep voltammetry curves of pure Bi2MoO6 and Bi2S3/Bi2MoO6 -2 nanocomposites with UV-visible light illumination; (b) EIS Nyquist plots of pure Bi2MoO6 and Bi2S3/Bi2MoO6 nanocomposites with UV-visible light illumination.

Fig. 5 (a) The temporal evolution of the spectrum mediated by Bi2S3/Bi2MoO6 -2 nanocomposites; (b) Degradation efficiency of phenol as a function of time by the as-prepared samples under visible light irradiation; (c) The degradation rate in bar graph form; (d) The pseudo-first-order kinetic plots of degradation by the samples.

Fig. 6 (a) Plot of (αhv)2 vs. energy (hv) for the band gaps of samples; (b) The photoluminescence (PL) spectrum of the samples at room temperature (λ nm).

excitation=300

Scheme 2 Diagram for energy band levels of Bi2S3/Bi2MoO6 composites and the possible charge separation process.

Fig. 7 (a) Voltage change of Bi2MoO6 and Bi2S3/Bi2MoO6 with and without ethanol gas; (b) Potential difference of Bi2S3/Bi2MoO6 with ethanol and n-hexane gas.

Highlights 1. For the first time, novel cookie-like Bi2S3/Bi2MoO6 heterostructure was obtained via ion exchange engineering. 2. The formation process and mechanism of synergetic effect of Bi2S3/Bi2MoO6 heterostructure were deeply investigated, which makes the design of similar heterostructures possible. 3. The Bi2S3/Bi2MoO6 nanocomposites exhibit enhanced photocatalytic activities and gas sensing properties, which was mainly attributed to the heterojunction between Bi2S3 and Bi2MoO6.