Synergistic Effect of Charge Generation and Separation in Epitaxially

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Surfaces, Interfaces, and Applications

Synergistic Effect of Charge Generation and Separation in Epitaxially Grown BiOCl/Bi2S3 Nano-Heterostructure Yanjie Wang, Jiarui Jin, Weiguo Chu, David Cahen, and Tao He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03390 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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ACS Applied Materials & Interfaces

Synergistic Effect of Charge Generation and Separation in Epitaxially Grown BiOCl/Bi2S3 Nano-Heterostructure Yanjie Wang,a,b Jiarui Jin,a Weiguo Chu,a David Cahen,c Tao He a,b * a CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China. b University of Chinese Academy of Sciences, Beijing 100049, China c Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel * Corresponding author: E-mail: [email protected]; Fax: +86 10 6265 6765; Tel: +86 10 8254 5655

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ABSTRACT Nano-heterostructures are widely used in the field of optoelectronic devices and an optimal proportion usually exists between the constituents that make up the structures. Investigation on the mechanism underlying the optimal ratio is instructive for fabricating nano-heterostructures with high efficiency. In this work, BiOCl/Bi2S3 type-II nano-heterostructures with different Bi2S3 / BiOCl ratios have been prepared via epitaxial growth of Bi2S3 nanorods on BiOCl nanosheets with solvothermal treatment at different sulfuration temperatures (110 ~ 180 ºC) and their photoelectrochemical (PEC) performances as photoanodes have been studied. Results indicate that the Bi2S3 content increases with the sulfuration temperature. BiOCl/Bi2S3-170 (i.e., sulfurized @ 170 ºC) exhibits the highest PEC performance under visible-light illumination, while BiOCl/Bi2S3-180 with the maximum Bi2S3 content shows the highest visible-light absorption, i.e., possessing the best potential for charge generation. Further analysis indicates that the BiOCl/Bi2S3 heterojunction interface is also crucial in determining the PEC performance of the obtained heterostructures by influencing the charge separation process. With increasing Bi2S3 content, the interface area in the BiOCl/Bi2S3 nano-heterostructures increase firstly, and then decrease due to the mechanical fragility of the nanosheet-nanorod structure and the structural instability in the (010) direction of Bi2S3 with higher Bi2S3 content. Therefore, the increasing content of the Bi2S3 does not necessarily correspond to higher heterojunction area. The optimal performance of BiOCl/Bi2S3-170 results from the maximum of the synthetic coordination of the charge generation and separation. This is the first time ever to figure out the detailed explanation of the optimal property in the nano-heterostructures. The result is inspiring in designing high performance nano-heterostructures from the point of synthesizing


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morphological mechanical robust heterostructure and structural stable constituents to reach a high interfacial area, as well as high light-absorption ability.

KEYWORDS: BiOCl/Bi2S3, photoelectrochemical performance, heterojunction interface, charge generation, charge separation INTRODUCTION Constructing heterojunction between two semiconductors makes it feasible to combine two suitable materials into a single monolithic structure and expand the photoresponse spectrum so as to increase the efficiency of generation and accelerate the separation of photogenerated electronhole pairs and, thus, improve light utilization efficiency for, e.g., photoelectrochemical application. Various heterostructures have been designed over the past decades, such as type I (straddling gap), type II (staggered gap) and Van der Waals heterostructures.1-3 However, it still remains a challenge to tackle the limit of charge extraction. One major reason is heterogeneity and defects at the interface between the component materials due to lattice mismatch and resulting structural defects and strain fields. In-situ growth of semiconductor A on semiconductor B on the nanoscale can help ensure a high quality interface with little mismatch. Indeed, many nano-heterostructures have been fabricated for gas sensing, photocatalytic H2 evolution and CO2 reduction, and the like, such as ZnO/NiO nanorods,4 ZnS/CuS porous nanosheets,5 Bi2S3/CeO2 nanoparticles,6 CuO/TiO2-xNx hollow nanocubes,7 and SrTiO3/TiO2 nanotubes.8 Usually there is an optimal ratio between the two semiconductors, at which the constructed nano-heterostructures have optimal performance in the respective application field as mentioned above. Under many circumstances, the optimal nano-heterostructure contains very little of one constituent, but shows much higher performance


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than the pure major constituent. For instance, 2% CuS/ZnS porous nanosheets display the highest H2-production rate with an apparent quantum efficiency of 20% at 420 nm compared with a negligible activity for pure ZnS.5 It is speculated that > 2% CuS would shield the incident light. Song et al. reported that TiO2/CdS with a molar ratio of 8:1 exhibits the highest photocatalytic activity for CO2 reduction in cyclohexanol under UV-Vis irradiation. They suggested that further increase of CdS would lead to aggregation of CdS nanoparticles on the TiO2 nanosheets, which has an adverse effect on the light harvesting with no evidence provided.9 Even for the same nano-heterostructure system the optimized ratio might not be consistent with each other. Gao et al. claimed that 28% wt. FeTiO3/TiO2 is the most efficient in decomposing organic compounds,10 while Truong et al. reported that 20% wt. FeTiO3/TiO2 has the optimal performance in CO2 reduction, possibly due to formation of recombination centers at high iron concentration.11 Moreover, as above mentioned, diverse explanations have been proposed for optimal performance of different heterostructures with a given component ratio. Thus, it is worth to investigate the mechanism underlying the optimal performance, where the component ratio is the key point, and it will be instructive for us to design nano-heterostructures with high efficiency. BiOCl is a promising candidate in the field of photocatalysis because of its layered structure and the self-built internal electric field that favors the separation of photogenerated electron-hole pairs.12 It has been widely used in the photocatalysis like degradation of the pollutants in water, CO2 reduction and water splitting.13-15 However, BiOCl with a bandgap of 3.2 eV can respond only to the UV region of solar spectrum, which limits its efficient use of solar energy. Owing to its strong visible-light absorption, Bi2S3 with a bandgap of 1.2~1.7 eV has attracted great attention in fields like photovoltaics, photocatalysis, photodetection, and Schottky diodes.16-19


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Combining BiOCl and Bi2S3 can extend the photoresponse range to the visible-light region. Moreover, the presence of common cation Bi may be in favor of the epitaxial growth of one material onto another. Previous work has indicated that Bi2S3-sensitized BiOCl microflowers and nanoparticles show improved photocatalytic activity in the degradation of Rhodamine B under visible-light irradiation, with an optimized quantity of Bi2S3 showing the highest efficiency;20, 21 it is still not clear, though, why there is an optimal BiOCl / Bi2S3 ratio. To figure out what are the causes for this behavior, we prepared BiOCl nanosheet/Bi2S3 nanorod heterostructures with different ratios of Bi2S3 to BiOCl by in-situ epitaxial growth of Bi2S3 nanorods on BiOCl nanosheets. As a result, nano-heterojunctions were formed between the two components. The resulting BiOCl/Bi2S3 heterostructures were evaluated as photoanodes so as to seek for the one(s) with optimal photoelectrochemical (PEC) activity and BiOCl/Bi2S3-170 (i.e., sulfurized @ 170 ºC) was found to possess the highest PEC activity. Based on detailed analysis of the morphology and structure for different heterostructures, we find that the Bi2S3 content and the area of the nano-heterojuction interface are the decisive factors for PEC performance, specifically for charge generation and separation, respectively. Furthermore, analysis on the optimized heterojunction reveals that the shapes of the two constituents and their respective structure can influence greatly on the area of the heterojunction interface. Thus, we can provide a reasonable explanation how to optimize a heterostructure for PEC activity and provide a general strategy for nano-heterojunction design towards functionality. RESULTS AND DISCUSSION Composition and Crystal Structure


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X-ray photoelectron spectrum (XPS) was performed to investigate the chemical nature of the asprepared samples. Bi, O, Cl and S are observed in all the samples, as shown in Figure 1a using BiOCl/Bi2S3-170 sample as the example. Two characteristic S peaks appear at 162.0 and 160.9 eV after deconvolution for Bi2S3, which can be assigned to S 2p1/2 and S 2p3/2 due to spin-orbit separation (Figure S1a). Such peaks can also be observed in the BiOCl/Bi2S3-170 sample, but with a slight shift in the peak values (Figure S1b), suggesting the existence of S2- in the heterostructure. The slight difference in binding energy of the S 2p XPS signal among different samples can be attributed to the difference in chemical environment.22 The binding energy at around 197.6 and 199.0 eV observed in BiOCl and BiOCl/Bi2S3-170 samples is ascribed to Cl 2p3/2 and Cl 2p1/2 (Figure S1c), respectively. Two peaks are observed at 164.3 and 159.0 eV for BiOCl/Bi2S3-170, and at 164.6 and 159.3 eV for BiOCl and Bi2S3 (Figure 1b), corresponding to Bi 4f5/2 and Bi 4f7/2, respectively.23 The 5.3-eV difference in the above doublet peaks is typical for the spin-orbit splitting of the Bi 4f core level. It is noted that another two Bi 4f peaks shift to lower binding energy (163.3 and 158.0 eV for BiOCl/Bi2S3-170; 163.8 and 158.5 eV for BiOCl; 163.4 and 158.2 eV for Bi2S3). Previous study has indicated that such shift originates from the existence of Bi in (+3–x) valence state,24,25 which may be formed due to the deficiency in oxygen and/or sulfur. This deficiency is generated possibly because of the corrosion effect from the carboxyl group in oleic acid during the synthesis that can create oxygen and sulfur vacancy in the vicinity of Bi cation.26 The above conclusion is confirmed by the results from low temperature electron spin resonance (ESR) (Figure S2). The typical g value of 2.001 can be ascribed to oxygen vacancies in BiOCl,27 while those of 1.974 and 1.994 are assigned to sulfur vacancies in Bi2S3 and BiOCl/Bi2S3-170.28 Such anion deficiencies can lead to the n-type characteristics in BiOCl nanosheets, Bi2S3 nanorods and BiOCl/Bi2S3.29


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Figure 1c shows X-ray diffraction (XRD) patterns of the BiOCl nanosheets, Bi2S3 nanorods and the corresponding heterostructures. The as-prepared BiOCl nanosheets and Bi2S3 nanorods can be indexed to tetragonal BiOCl (PDF#06-0249) and orthorhombic Bi2S3 (PDF#17-0320). In the XRD patterns for the BiOCl/Bi2S3 heterostructure, three weak peaks appear as shown in BiOCl/Bi2S3-150, which can be assigned to the (211), (221) and (002) reflections of orthorhombic Bi2S3. The intensity of these three peaks becomes stronger with increasing temperature, in accordance with an increase of Bi2S3 content, which fits with the increasingly dark color of the samples with increasing annealing temperature (Figure S3). Other characteristic peaks from Bi2S3 can also be observed in BiOCl/Bi2S3-170 and BiOCl/Bi2S3-180 samples, while diffraction peaks of Bi2S3 in BiOCl/Bi2S3 prepared at a temperature lower than 150 ºC are hardly seen, due to the very low content of Bi2S3.

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Bi2S3 (PDF#17-0320)

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Figure 1. (a) XPS survey spectrum of BiOCl/Bi2S3-170, (b) high resolution XPS spectra of Bi 4f for Bi2S3, BiOCl and BiOCl/Bi2S3-170, and (c) XRD patterns of Bi2S3, BiOCl and heterostructures of BiOCl/Bi2S3, prepared at 6 different temperatures. The inset in (c) is the zoom-in pattern of Bi2S3 (211) for different samples, which displays the intensity increase for the series samples of BiOCl/Bi2S3-150, BiOCl/Bi2S3-160, BiOCl/Bi2S3-170 and BiOCl/Bi2S3-180.

Optical Properties and Energy Level Alignment


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Optical absorption from near-IR to the UV was measured by diffuse reflectance spectroscopy and results for all the samples are shown in Figure 2a, where the Kubelka–Munk transformation of the diffuse reflection is plotted against light wavelength. Absorbance of the BiOCl/Bi2S3 heterostructures tends to increase in the visible-light range with increasing sulfuration temperature, implying that more charge carriers are generated upon irradiation. Accordingly, the color of the heterostructure changes gradually from white to black (Figure S3), consistent with increasing Bi2S3 content. In addition, the bandgap (Eg) can be determined to be 3.10 eV for BiOCl (Figure 2b) and 1.46 eV for Bi2S3 (Figure 2c) from the above data by using the equation of 𝛼ℎ𝜈 = 𝐴(ℎ𝜈 − 𝐸𝑔 )

𝑛⁄ 2,

where α, h, ν and A represent the absorption coefficient,

Plank constant, light frequency and a constant, respectively. The value of n is dependent on the optical transition characteristics of a semiconductor, which is 4 for an indirect transition in

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2.00E+012

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Figure 2. (a) UV-Vis diffuse reflection spectra of as-prepared BiOCl/Bi2S3 heterostructures, as well as of the pure BiOCl nanosheets and Bi2S3 nanorods; (b) and (c): Tauc plots of (b) BiOCl nanosheets and (c) Bi2S3 nanorods and the corresponding diffuse reflection spectra (insets), Mott-Schottky plots of (d) BiOCl


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nanosheets and (e) Bi2S3 nanorods, and (f) energy level alignment scheme, derived from the results obtained on the individual components, for a BiOCl/Bi2S3 heterojunction.

To get the energy level alignment of the as-prepared BiOCl nanosheets and Bi2S3 nanorods, capacity measurements are carried out to determine their flat band potential. Figures 2d and 2e are the resulting Mott-Schottky plots (1/C2 vs. applied potential) for BiOCl nanosheets and Bi2S3 nanorods, respectively. The flat band potential can be obtained by extrapolating to infinity capacity, C, the linear fit of the 1/C2 to zero, i.e., eliminating the depletion layer by bias. The bias values that are found to be 0.37 and 0.28 V (vs RHE) for BiOCl nanosheets and Bi2S3 nanorods, respectively. If we assume vacuum level alignment (no interface reactions, no interface dipoles, no charge transfer between the two materials) then the band alignment between the as-prepared BiOCl and Bi2S3 can be schematically shown as in Figure 2f, indicating the formation of a typeII staggered heterostructure. Clearly, the valence band of Bi2S3 is more negative than BiOCl. So the photogenerated holes in Bi2S3 upon visible-light illumination will directly participate in the water oxidation reaction at the exposed Bi2S3 surface/electrolyte interface. Meanwhile, electrons generated in the Bi2S3 nanorods will migrate to the conduction band of BiOCl nanosheets, then transport to the FTO substrate under a bias and flow to the counter electrode to take part in the reduction reaction like hydrogen evolution and CO2 reduction. The electrons and holes can thus be separated spatially, resulting in suppressed recombination. So, the narrow bandgap semiconductor in the BiOCl/Bi2S3 heterostructure (here Bi2S3) mainly contributes to the charge generation under visible-light illumination, while the heterojunction at the interface is responsible for efficient separation of the photogenerated charge carriers. Moreover, the slope of the Mott-Schottky curve for both samples is positive, implying that both BiOCl and Bi2S3 in this work are n-type semiconductors, which makes them promising as photoanodes.


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PEC Characteristics To be able to characterize PEC performance of the BiOCl/Bi2S3 heterostructures as well as BiOCl nanosheets and Bi2S3 nanorods, the as-prepared samples were casted onto fluorine-doped tin oxide (FTO) electrodes to probe photoinduced charge generation, separation and transfer. Figure S4 provides the images of scanning electron microscope (SEM) of the as-prepared electrodes, demonstrating that the surface of the films are uniform and their thicknesses are similar, which ensures the reliability of the PEC performance among different samples. Figure 3a compares the photogenerated open circuit potential (OCP) for all the samples. The potentials for all the electrodes shift negatively within a few seconds after illumination starts, indicating that the photogenerated electrons are injected from Bi2S3 to BiOCl and then to the FTO substrate. The circuit is completed by the holes at each Bi2S3 surface exposed to the electrolyte, the electrochemical reaction and the reduction reaction on the cathode. We note that OCP values of all the heterostructures are higher than those obtained with pure BiOCl nanosheets and pure Bi2S3 nanorods. BiOCl/Bi2S3-170 has the highest OCP value (0.15 V) among all the samples, while BiOCl/Bi2S3-180 shows almost the same OCP value as BiOCl/Bi2S3-150 (~0.1 V). In Fig. 3 we show the anodic photocurrent as observed in chopped light voltammetry (CLV) (Figure 3b) and I-t (Figure 3c) measurements. The photocurrent in the CLV curve with BiOCl/Bi2S3-170 is also higher than that with the other heterostructures. The onset potential of BiOCl/Bi2S3-170 is the least positive ~0.37 V (vs RHE) among all the samples, which favors the anodic reaction (Figure 3b). A similar trend is observed for the transient photocurrents measured at 1.23 V (vs RHE) bias (Figure 3c). This behavior of BiOCl/Bi2S3-170 suggests that with it we achieve the fastest charge transfer rate; while all samples indeed show good switching (light on/off) behavior in terms of the photocurrents. Thus, the BiOCl/Bi2S3 heterojunction can


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facilitate efficient charge separation and decrease electron-hole recombination, compared to pure

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Figure 3. (a) open circuit potential curves, (b) chopped light voltammetry plots, (c) transient photocurrent, and (d) incident photon to current conversion efficiency curves of the as-prepared BiOCl/Bi2S3 heterostructures as well as pure BiOCl nanosheets and Bi2S3 nanorods electrodes under visible-light illumination (＞ 420 nm, 100 mW/cm2).

To further investigate the PEC performance, incident photon to current conversion efficiency (IPCE) measured under 1.23 V bias (vs. RHE) was calculated from 𝐼𝑃𝐶𝐸 = (1240 × 𝐽) ∕ (𝜆𝑃𝜆 ), where J is the photocurrent density (mA/cm2), λ is the wavelength of the incident light (nm), and Pλ is the corresponding light power density (mW/cm2). The IPCE value of BiOCl/Bi2S3-170 is much higher over the whole spectrum than that of all the other samples


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(Figure 3d), though the obtained junction shows lower optical absorbance than BiOCl/Bi2S3-180 (Figure 2a). Considering that the IPCE value is determined by the light harvesting and charge carrier behavior (including the charge generation, recombination, separation, and subsequent participation in the electrochemical reaction), the high IPCE value of BiOCl/Bi2S3-170 implies its superior charge generation, separation and utilization ability, especially compared to BiOCl/Bi2S3-180. Electrochemical impedance spectroscopy (EIS) obtained under a bias of 1.23 V (vs RHE) with visible-light illumination can also shed light on the charge separation and charge transfer at the semiconductor/electrolyte interface. Figure S5 shows the Nyquist diagram, in which the radius of the partial semicircle in the lower left represents charge transfer resistance (Rct) at the corresponding electrode/electrolyte interface. The Nyquist plots are fitted using the equivalent circuit that is presented in the inset of Figure S5. The resultant value of Rct for all the samples is listed in Table S1. The BiOCl/Bi2S3-170 system has the lowest Rct value (186 kΩ), followed by BiOCl/Bi2S3-160 (390 kΩ) and BiOCl/Bi2S3-180 (447 kΩ), and BiOCl/Bi2S3-110 exhibits the highest value (2.28 × 1011 kΩ) among all the heterostructured samples due to the presence of a large amount of BiOCl that exhibits almost no response to the visible light. This trend agrees well with the PEC results as discussed above. Hence, the BiOCl/Bi2S3-170 is the most efficient in facilitating hole transfer from the Bi2S3 nanorods to the electrolyte. Since the type-II heterojunction exists in all the heterostructured samples, in-depth study on the heterostructure is used to probe the reason for the optimal PEC performance of BiOCl/Bi2S3-170. Morphology, Microstructure and Growth Orientation


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Figure 4. TEM images of (a) BiOCl nanosheets and (d) Bi2S3 nanorods, HRTEM of (b) a single BiOCl nanosheet and (e) Bi2S3 nanorod as well as the corresponding FFT shown in the insets, and schematic illustration of the crystal orientation of (c) BiOCl nanosheet and (f) Bi2S3 nanorod.

The morphology and microstructure of the obtained samples are studied by transmission electron microscopy (TEM), and the crystal structure is investigated by high-resolution TEM (HRTEM). Figure 4a shows the TEM image of the as-prepared BiOCl square nanosheets, with ~80 nm width on each side. A typical HRTEM image of BiOCl nanosheets and the corresponding fast Fourier transform (FFT) are shown in Figure 4b and its inset, respectively. The interlayer spacing -

of the orthogonal lattice that is parallel to the lateral facets is 0.275 nm, indexed as (110) and (11 0) planes of the tetragonal BiOCl. It indicates that the top and bottom facets of the nanosheets are the (001) faces, indexed as the [001] zone axis diffraction, as shown in the schematic illustration (Figure 4c). Figure 4d is a TEM image of the Bi2S3 nanorods. Although they tend to agglomerate


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to form bundles, the shape of Bi2S3 nanorods is clear in the zoom-in image (Figure 4e). The distance of the perpendicular adjacent interlayer spacing is 0.2 nm, corresponding to the distance of (002) planes in Bi2S3, which demonstrates the [001] growth direction of the Bi2S3 nanorods (Figure 4f). The ratio of BiOCl to Bi2S3 in the BiOCl/Bi2S3 heterostructures prepared under different sulfuration temperature is determined by energy dispersive X-ray spectroscopy (EDX), and shows that the amount of Bi2S3 in the samples increases with increasing temperature (Table S2). Furthermore, it is found that the Bi2S3 nanorods become longer and thicker when the sulfuration temperature increases; meanwhile the BiOCl nanosheets shrink, with their periphery broken first (Figures 5a-f), i.e., some heterojunctions may become broken as growth continues. Figure S6 shows the typical TEM images of an individual BiOCl/Bi2S3 nanosheet-nanorod heterostructure, demonstrating two representative growth sites. Specifically, the Bi2S3 nanorods mainly attach to the sides and corners of the BiOCl nanosheets. We used HRTEM imaging to look at the BiOCl/Bi2S3 interfaces to probe the interfacial orientation relationship.

Figure 5. TEM images of the BiOCl/Bi2S3 heterostructures, prepared with different sulfuration temperatures, (a) 110, (b) 130, (c) 150, (d) 160, (e) 170, and (f) 180 ºC, and three typical heterojunction


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interfaces on the (g) corner, (h) side and (i) side near the corner of a BiOCl nanosheet, as well as the FFT of the corresponding areas of (g-1, h-1 and i-1) BiOCl and (g-2, h-2 and i-2) Bi2S3.

Figures 5g-i clearly displays the lattice fringes of the heterostructures, which illustrate that the Bi2S3 nanorods grow epitaxially on both the sides and corners of the BiOCl nanosheets. The red lines indicate the separation between the two materials. FFT images of each region show the same pattern as that for the respective BiOCl nanosheets and Bi2S3 nanorods (Figures 4b, 4e), indicating that in the heterostructure the BiOCl nanosheets grow with their (001) planes as the top surface and the Bi2S3 nanorods grow preferentially along the [001] direction, consistent with the above discussion. The interlayer spacing of 0.195 nm can be indexed to (200) or (020) planes in tetragonal BiOCl, while the lattice distance of 0.199 nm in orthorhombic Bi2S3 corresponds to the (002) plane, respectively. Calculations demonstrate that the lattice mismatch between the (200) plane (equivalent (020) plane) of tetragonal BiOCl and (002) plane of orthorhombic Bi2S3 is 0.02, implying that BiOCl and Bi2S3 can form a nearly coherent interface. Thus, the epitaxial growth direction is BiOCl (200) or (020) ∥Bi2S3 (002), of which Figure 6a shows the ball and _

stick model structure. Since the angle between (200) (or (020)) planes and (110) (or (11 0)) planes (parallel to the lateral facets) in BiOCl is 45º, therein the Bi2S3 nanorods grow with an angle of 45ºto the sides of BiOCl nanosheet (Figure 6b).


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Figure 6. (a) Simulated crystal structure for BiOCl/Bi2S3 interface and (b) growth orientation model of Bi2S3 nanorods on BiOCl nanosheets.

Synergistic Effect of Charge Generation and Separation Based on the above discussion and analysis, an ion exchange-erosion mechanism is proposed to illustrate the formation and break-up of the BiOCl/Bi2S3 heterojunction. During the solvothermal reaction process, BiOCl nanosheets first form at 170 ºC after adding oleic acid. BiOCl can be easily converted into Bi2S3 in the presence of S2- due to the much lower solubility of Bi2S3 (Ksp = 1  10‒97) than BiOCl (Ksp = 1.8  10‒31). Then the sulfur powder is dissolved in the solution during the sulfuration process and changes into S2- ions. The amount of S2- in the solution increases with increasing temperature. Considering that the nanorod grows at an angle of 45ºto the sides of the nanosheets, regardless of the exact growth sites, the corner of a BiOCl nanosheet is used as an example for the growth site of the Bi2S3 nanorods. Thus, three scenarios can be considered in terms of the sulfuration temperature. -1-

Low sulfuration temperature like 110 and 130 ºC. The S2- ions exchange with OCl3‒ to

form small Bi2S3 particles and/or clusters, which grow preferentially along (200) or (020) facets of the BiOCl nanosheets (Figures 7a, a-1, and a-2). Bi2S3 particles attach firmly to the corner of the BiOCl nanosheet, i.e., most of the nascent Bi2S3 particles form a strong nano-heterojunction with the BiOCl nanosheets. As demonstrated previously by calculation, Bi2S3 prefer to grow along the [001] direction, thermodynamically and kinetically.32 Thus, Bi2S3 nanoparticles prefer to form into nanorods parallel to the [001] direction at higher [S2-] in the solution (Figures 7b, b1, and b-2), with larger heterojunction interfaces than that shown in Figure 7a. Since Bi2S3 is the light-harvesting material in the obtained heterostructures (Figure 2f), charge generation in the


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heterostructures will be enhanced with increasing amount of the Bi2S3. In this case, the heterojunction interface area becomes larger and larger with the increase of the Bi2S3 content, leading to improved charge generation and separation. Accordingly, the PEC performance increases with increasing sulfuration temperature, i.e., BiOCl/Bi2S3-130 is higher than BiOCl/Bi2S3-110.

Figure 7. (a - d) HRTEM images illustrating different BiOCl/Bi2S3 heterostructures, and corresponding schematic diagrams of the (a-1 - d-1) side view and (a-2 - d-2) top view.

-2-

Medium sulfuration temperature like 150 and 160 ºC. With further increase of

temperature (thereby, increasing [S2-] in the solution), Bi2S3 nanorods become longer and thicker with a larger interface area (Figures 7c, c-1, and c-2). Bi2S3 has a laminated structure with each pseudo-layer containing [Bi2S3]∞, connected by weak Bi-S interactions along the [010] direction


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(Figure 6a). The Bi2S3 nanorods can be easily cleaved between the layers along the [010] direction.32 Moreover, since the BiOCl nanosheets are very thin in the [001] direction, the region around the BiOCl/Bi2S3 interface will become thinner when more BiOCl is consumed during the reaction, which may also make it easy for the Bi2S3 nanorods to be detached in view of the mechanical stability (Figures 7d, d-1, and d-2). In this case, the Bi2S3 content and heterojunction interface continue to grow with increasing sulfuration temperature, while the growth rate of the interface area becomes slower. So the charge generation and separation in the heterostructure is further enhanced with increasing sulfuration temperature, while the increase in the separation rate of electron-hole pairs starts to slow down. Nevertheless, BiOCl/Bi2S3-160 still exhibits higher PEC performance than BiOCl/Bi2S3-150. -3-

High sulfuration temperatures like 170 and 180 ºC. As discussed above, more Bi2S3

nanorods can be generated at a higher sulfuration temperature, while a number of isolated Bi2S3 nanorods and BiOCl nanosheets can be observed in this case (Figure 5f). Accordingly, the area of the heterojunction interface can diminish in a single heterojunction. Specifically, considering many Bi2S3 nanorods can be disconnected from the BiOCl nanosheets when they become longer and thicker, the entire interface area in the system will be decreased greatly. Even if more Bi2S3 means generation of more charge carriers, the decreased interface area is detrimental to charge separation. So the interface area will eventually become the rate controlling factor in the PEC process, as it will limit optimal use of the photogenerated charge carriers, if the heterostructure is prepared at a high temperature. This explains why BiOCl/Bi2S3-170 shows better PEC performance than BiOCl/Bi2S3-180. Therefore, PEC performance is dependent on the synergistic effect between the charge generation and separation in the BiOCl/Bi2S3 nano-heterojunction. The production of more Bi2S3


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nanorods with increase of sulfuration temperature will facilitate generation of more charge carriers; meanwhile, a larger area of the heterojunction interface between BiOCl and Bi2S3 favors separation of the photogenerated electrons and holes. However, further increase of the Bi2S3 amount at higher sulfuration temperature will make it easier for the Bi2S3 nanorods to be detached from the BiOCl nanosheets due to the mechanical fragility of the BiOCl nanosheetsBi2S3 nanorods structure and the interlayer instability of the Bi2S3 structure, leading to the disconnection of Bi2S3 from BiOCl and, consequently, decrease in the interface area. Eventually, even if more Bi2S3 nanorods produced means higher charge generation ability, the decreased heterojunction interface area will result in an adverse effect in the separation of electron/hole pairs. Therefore, there is an optimal relation between the amount of Bi2S3 nanorods and interface area of BiOCl/Bi2S3 heterostructures to achieve the highest PEC performance. This optimum is achieved by sulfurization at 170 °C (i.e., the BiOCl/Bi2S3-170 sample). That is to say, to achieve a heterostructure with efficiency as high as possible, it is equally important to have both the high light-absorption ability via increasing the amount of narrow bandgap semiconductor and high heterojunction interface area through modulating the structure and morphology. CONCLUSIONS BiOCl/Bi2S3 nano-heterostructures with different Bi2S3 amount have been prepared successfully via epitaxial growth of Bi2S3 nanorods on BiOCl nanosheets. The obtained BiOCl/Bi2S3 heterostructures exhibit much higher PEC performance than pure BiOCl and Bi2S3 due to the formation of a type-II heterojunction at the interface. Increasing the amount of Bi2S3 nanorods is not necessarily always accompanied by the formation of a larger heterojunction interface area. With increasing Bi2S3 content in the heterostructures, the BiOCl/Bi2S3 interface area increases first and then decreases. Owing to the synergistic effect between the charge generation


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(dependent on the amount of Bi2S3) and charge separation (determined by both the interface energy level alignment and the interface area) in the heterostructures, BiOCl/Bi2S3-170 shows the highest PEC performance under visible light illumination. Therefore, to achieve the PEC performance as high as possible for a heterostructure, it is critical not only to prepare the materials with high light-harvesting performance, but also to generate a sufficiently large heterojunction interface area so as not to be limited (in terms of current density /current crowding). Our analyses of the growth mechanism show that the shape and structure of the two constituents in a heterostructure can play an important role in forming large heterojunction interface areas. The general strategy for rational design of an efficient heterostructure is that it should be mechanical stable in shape and crystalline robust in structure so as to achieve a large, efficient interfacial heterojunction area. EXPERIMENTAL METHODS Reagents and Materials. All reagents were used without any further purification. BiCl3 (98%), oleyamine and 1-octadecene (90%) were bought from J&K Scientific Ltd., Beijing, China. Tri-noctylphosphine (98%) (TOPO) was bought from Alfa Aesar. KHCO3 (AR) was purchased from Xilong Scientific, China. Sulphur powder was obtained from Sinopharm Chemical Reagent Co., Ltd. Oleic acid (＞ 85.0%) was bought from Tokyo Chemical Industry Co. Ltd. n-Hexane (AR) was purchased from Beijing Chemical Works. FTO glass (14Ω/sq) was got from Wuhan JingeSolar Energy Technology Co., Ltd, China. FTO was ultrasonic cleaned successively by Milli-Q water, ethanol, acetone and isopropanol before use. Synthesis of BiOCl Nanosheets. BiOCl nanosheets were synthesized by a solvothermal method. 1 mmol of BiCl3, 2 mmol of TOPO, 16 mmol of oleic acid and 20 mL of 1-octadecene were


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mixed in a three-necked bottle. Then, the mixture was held at 90 ºC for 30 min to remove the water, followed by quick injection of 10 mmol of oleylamine into the mixture once the temperature was heated to 170 ºC and then held for another 10 min. After the resultant mixture was cooled down to room temperature, the precipitate was centrifuged and then washed with nhexane for 8 times. The obtained product was dried at 70 ºC in a vacuum oven for 8 h. Fabrication of BiOCl/Bi2S3 Heterostructures. BiOCl/Bi2S3 heterostructure was synthesized by in-situ sulfuration of the BiOCl nanosheets synthesized above. The amount of the Bi2S3 in the heterostructures was adjusted by controlling the sulfuration temperature. The initial synthesis procedure is the same as that used for preparation of BiOCl nanosheets. After 10 mmol of oleylamine was injected into the mixture at 170 ºC and heated for 10 min, the temperature of the system was changed to the target one respectively as follows, 180, 170, 160, 150, 130 and 110 ºC. As soon as it reached the objective temperature, a mixture with 0.75 mmol of S powder, 8 mmol of oleic acid and 3 mmol of oleylamine was injected into the three-necked bottle and then the system was heated for 5 min. The resultant mixture was cooled down to room temperature and the precipitate was centrifuged and then washed with n-hexane for 8 times. The obtained product was dried at 70 ºC in a vacuum oven for 8 h. The heterostructures prepared under different sulfuration temperatures are denoted as BiOCl/Bi2S3-110, BiOCl/Bi2S3-130, BiOCl/Bi2S3-150, BiOCl/Bi2S3-160, BiOCl/Bi2S3-170 and BiOCl/Bi2S3-180. Synthesis of Bi2S3 Nanorods. Bi2S3 nanorods were also prepared by a solvothermal method. First, 0.78 mmol of BiCl3, 16 mmol of oleic acid and 20 mL of 1-octadecene were mixed in a three-necked bottle. After the mixture was held at 150 ºC for 30 min under nitrogen atmosphere, the temperature was elevated to 170 ºC. Then a mixture with 1.4 mmol of S powder, 8 mmol of oleic acid and 3 mmol of oleylamine was injected into the system. The mixture was cooled down


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to room temperature after it was kept at 170 ºC for 5 min. The resultant precipitate was centrifuged and washed with n-hexane for 8 times. The obtained product was dried at 70 ºC in a vacuum oven for 8 h. Characterizations. The crystalline phase of the obtained samples was determined by powder XRD (Smartlab, 45 kV, 200 mA) with Cu kα radiation. The data was collected by a step mode with a speed of 0.02 degree/step and 1 second/degree. XPS spectra were recorded with Thermo Scientific ESCALAB 250 instrument. Low-temperature ESR spectra were collected on an Electron Paramagnetic Resonance Spectrometer (Bruker, E500) at 90 K. SEM images were obtained by field emission scanning electron microscope (SU8220, Hitachi). TEM and HRTEM images were acquired by using field emission transmission electron microscopy (Tecnai G2 F20 U-TWIN, FEI) at 200 kV and the annexed energy disperse X-ray (EDX) spectrometer was used to analyze the Bi2S3 amount in the as-prepared heterostructures. UV–vis diffuse reflectance spectra were recorded on a UV/Vis/NIR spectrophotometer (Lambda 750, Perkin-Elmer) with BaSO4 as the background reference. Photoelectrochemical Measurements. All the samples were casted onto FTO substrates to form a film electrode with a controlled size of 1 cm  1 cm. Photoelectrochemical measurements were operated on an electrochemical workstation (IM6, Zahner) equipped with a white light source (100 mW/cm2, WLC02, Zahner) and a UV cut-off filter (λ > 420 nm). Typically, a threeelectrode configuration was used, with sample electrode as the working electrode, platinum as the counter electrode and saturated calomel electrode (SCE) as the reference electrode. 0.1 mol/L of KHCO3 aqueous solution saturated by CO2 (pH = 6.8) was employed as the electrolyte in all the measurements, considering that HCO3‒ ions involved in hole transfer will facilitate the oxidation reaction.33 Mott-shottcky measurements were carried out from 0.15 to 1.15 V (vs RHE)


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under a frequency of 1 kHz. OCP was operated both in the dark and under visible-light illumination (100 mW/cm2). CLV was performed from 0.25 to 1.30 V (vs RHE) with chopped light (100 mW/cm2) with a potential scan rate of 2 mV/s. Transient photocurrent was carried out under 1.23 V (vs RHE) with chopped light (100 mW/cm2). Photocurrent action spectroscopy was collected by the electrochemical workstation (IM6, Zahner) equipped with a tunable light source (TLS 03, Zahner) under 1.23 V (vs RHE). For the EIS measurements, 1.23 V (vs RHE) was applied to the working electrode with a frequency range of 0.4 Hz ~ 100 kHz and an amplitude of 5 mV. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org, including XPS spectra of S2p and Cl2p, ESR spectra, optical pictures, SEM images of the asprepared electrodes, EIS spectra, TEM images of an individual BiOCl/Bi2S3 nanosheet-nanorod heterostructure, charge transfer resistance, and EDX results of the as-prepared samples. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID David Cahen: 0000-0001-8118-5446. Tao He: 0000-0001-6336-2402.


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Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of China (2015DFG62610).

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Synergistic Effect of Charge Generation and Separation in Epitaxially

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