Controlling the Core-Shell Structure of CuS@CdS Heterojunction via

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Controlling the Core-Shell Structure of CuS@CdS Heterojunction via Seeded Growth with Tunable Photocatalytic Activity Naixu Li, Wenlong Fu, Chen Chen, Maochang Liu, Fei Xue, Quanhao Shen, and Jiancheng Zhou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04606 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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ACS Sustainable Chemistry & Engineering

Revised MS# sc-2018-046063, 10/2018

Controlling the Core-Shell Structure of CuS@CdS Heterojunction via Seeded Growth with Tunable Photocatalytic Activity

Naixu Li†, Wenlong Fu‡, Chen Chen⊥, Maochang Liu‡,§,*, Fei Xue‡, Quanhao Shen†, Jiancheng Zhou†

†School

of Chemistry and Chemical Engineering, Southeast University, No. 2 Dongda Road, Nanjing, 211189,

P.R. China ‡International

Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power

Engineering, Xi’an Jiaotong University, No. 28 West Xianning Road, Xi’an, Shaanxi 710049, P. R. China ⊥College

of Water Conservancy and Hydropower Engineering, Hohai University, No. 1 Xikang Road, Nanjing,

210098, P. R. China §Suzhou

Academy of Xi’an Jiaotong University, No. 99 Renai Road, Suzhou, Jiangsu 215123, P. R. China

*To whom correspondence should be addressed. E-mail: [email protected]. Phone: +86-(29)-82668296-18; Fax: +86-(29)-82669033

0

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ABSTRACT We report the seed-mediated synthesis of CuS@CdS core-shell heterojunction with diverse morphology by introducing kinetic control. By using CuS nanoplates as seeds and a collective manipulation of the injection rate of Cd2+ precursor with a syringe pump and the reaction temperature, two distinctive growth modes, i.e., island and layer-by-layer modes, respectively, could be realized. It is found that the growth is, in principle, determined by the deposition rate relative to the diffusion rate of the CdS growth monomers. Specifically, at a high injection rate and a relatively low reaction temperature, the deposition of CdS monomers on the surface of the CuS nanoplate follows an island growth mode because of the lattice mismatch between CuS and CdS and distinct binding energies of them. We can facilitate surface diffusion of these deposited monomers by reducing the injection rate of Cd2+ and increasing the reaction temperature. In this case, growth can be switched to a layer-by-layer mode. The products are found with tunable and significantly improved photocatalytic performance toward dye degradation and H2 production from water under visible-light irradiation in comparison to the sole use of either CdS or CuS photocatalyst. This work provides an effective approach to the rational design of heterojunctions.

KEYWORDS: photocatalysis, heterojunction, kinetic control, surface diffusion, core-shell structure

1


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INTRODUCTION Wastewater containing organic dyes has been considered harmful to environment and human beings. Traditional methods based upon biological degradation have received attention, however, are still ineffective for decolorization and mineralization of these pollutants.1 Many advanced

oxidation

processes

have

therefore

been

developed.

Among

them,

semiconductor-based photocatalysis has been regarded as a simple yet powerful mean for degradation of organic dye pollutants involved in wastewater.2-4 For example, the last several decades have witnessed the successful use of TiO2 as photocatalyst for this application arising from its wide availability, non-toxicity, good stability, and excellent activity. However, TiO2 has a large band gap of 3.2 eV which impedes its large utilization of sun light. Developing visible-light-driven photocatalyst for rapid energy transfer is thus highly desired.5-7 On the other hand, the process has also relied on efficient mass transfer that involves charge carrier migration and reacting species transportation.8 To this end, semiconductor photocatalysts with heterostructures are attractive for use.9 This intense interest stems from the fact that heterostructures provide opportunities to control mass and energy flows, e.g., photons and charge carriers, in an engineered space, thus facilitate a wide variety of photoinduced chemical transformations. In principle, heterostructure usually represents as heterojunction in which a kind of interface forms between two layers or regions of dissimilar crystalline semiconductor.10, 11 As a result, the behavior of heterostructures for routing light and controlling the flow of energy, have depended crucially on the energy bands and spatial structure of semiconductor crystals. This notion explains why engineering the microstructure of a given heterojunction photocatalyst has fascinated us extensively with our increasing understanding and control of the atomic world.12 Generally, heterostructures made of metal sulfides are attractive for use because of their relatively narrow band positions for better solar spectrum response in comparison to oxides.13 As a result, shape-controlled sulfide heterojunctions are widely investigated photocatalysts for either organic pollutant degradation or water reduction. For example, Wang et al synthesized CuS/ZnS 2



hierarchical plate heterostructure via a combined solvothermal/cation-exchange method.14 It showed extended visible-light response and enhanced photocatalytic activity for H2O reduction. Deng et al reported a hydrothermal synthesis of CdS/CuS microflowers.15 The composite was found with excellent visible-light-driven catalytic activity toward methylene orange degradation. 93% methyl orange was removed after the photocatalytic reaction was proceeded for 150 min. Moreover, Hong et al. prepared ZnS/CuS/CdS ternary metal sulfide colloidal photocatalyst by simple cation-exchange and ionic reactions.16 The particulate composite also showed remarkably improved solar water reduction property due to the synergistic effects of the ternary nanoheterostructures. Despite these successful demonstrations with excellent photocatalytic performance, it is still noteworthy that most of the research have limitations on controlling each component of a given sulfide heterojunction. In this case, the mechanistic understanding and control toward the energy flow is significantly restricted. Over the last decade, seed-mediated growth has emerged as one of the most versatile approaches to synthesize nanocrystals with controlled microstructures.17 This strategy enables us not only to tune the morphology thus tailoring of the physicochemical properties of a given nanocrystal, but also to elucidate the underlying mechanism associate with the growth pattern. Herein, using CuS and CdS as model visible-light-driven photocatalysts, we demonstrated that the interface of core-shell CuS@CdS heterojunction could be readily manipulated by controlling the reaction kinetics during a seeded growth. The products showed improved and tunable photocatalytic activity toward dye degradation.

EXPERIMENTAL SECTION Synthesis of CuS hexagonal nanoplate. CuS nanoplate was synthesized by a one-step solvothermal method. Typically, 100 mg of polyvinyl pyrrolidone (PVP, MW ≈ 55,000) and 25 mg of sulfur (S) were dissolved into 6 mL diethylene glycol (DEG) and heated at 150 oC for 10 min in an oil bath under magnetic stirring. Subsequently, 2 mL DEG containing 28 mg of copper sulfate (CuSO4·5H2O) was added into the previous solution using a pipet. The reaction was then maintained for another 30 min. The product was collected through centrifugation, washed with 3


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acetone, ethanol and deionized water. Synthesis of CuS@CdS core-shell nanocrystals. This process was directed by a mini mechanical pump. In a typical process, 86.8 mg of the nanoplate seeds and 18 mL ethylene glycol (EG) were placed in a 100 mL three-necked flask that was heated to 190 oC in an oil bath under continuous magnetic stirring. 48.2 mg of cadmium acetate was then dissolved into a 10 mL EG, and was further introduced into the seed suspension by using a syringe pump. The reaction was allowed to proceed for an additional 5 mins after completing injection of the cadmium precursor. The final product was collected through centrifugation (at a rate of 10,000 rpm, 10 min each time), washed with acetone (one time to remove EG), ethanol (to remove soluble products), and deionized water (to remove soluble products) for further characterization. The reaction temperature as well as the injection rate could be changed in order to investigate their impact on the morphology variation. The water used in experiments and photocatalytic reactions are de-ionized water with a resistivity of 18.2 MΩ·cm. Photocatalytic reaction for the degradation of methylene blue (MB). Visible-light photocatalytic activities of CuS, CdS, and CuS@CdS were evaluated through the decoloration of MB with an initial dye concentration of 2×10-5 M. Photocatalytic reactor equipped with xenon lamp and a 430 nm cut-off filter was used in this study. In a typical run, 0.02 g of photocatalyst was dispersed in 40 mL of ultrapure water using ultrasonic probe for 30 min. Predetermined amount of organic dye (MB) was subsequently added into the catalyst suspension. After that, the suspension was stirred for 30 min in the dark to ensure adsorption/desorption equilibrium before light illumination. During visible light irradiation, aliquots of the reaction suspension were collected per 20 min and centrifuged to remove photocatalyst particles. The concentration of MB substrates was then determined by measuring the light absorbance using a UV-vis spectrophotometer (Hitachi U-4100). Photocatalytic reaction for H2 production from water. Photocatalytic tests of visible-light-driven H2 production were carried out by placing 0.02 g of CuS, or CdS, or CuS@CdS sample in a 100-mL aqueous solution containing 0.35 M Na2S and 0.25 M Na2SO3. 4



Similarly, photocatalytic reactor equipped with xenon lamp and a 430 nm cut-off filter was used in this study. The suspension was stirred for 10 min in the dark under continuous N2 flow before light illumination. The amount of H2 evolved from the reaction was determined by a gas chromatograph. Instrumentations. The crystallite morphologic micrographs of all CuS, CdS, and CuS@CdS samples were characterized with the aid of a JEOL JSM-7800F field emission scanning electron microscopy (FESEM), and an FEI Tecnai F30 transmission electron microscopy (TEM). Elemental mapping over the selected region of the photocatalyst was conducted by an energy-dispersive X-ray spectrometer (EDX) attached to the FEI Tecnai F30 TEM. X-ray diffraction (XRD) measurements were performed on an X'Pert PRO diffractometer with Cu Ka irradiation (λ = 1.5438Å). All the samples were scanned between 20 and 80 with a step size of 0.033o 2θ. The concentration of NH4+ was confirmed by the Thermo Scientific Integrion ion chromatography (IC).

RESULTS AND DISCUSSION A typical synthesis usually starts from generation of growth monomers, nucleation into seeds, and subsequently growth of seeds into nanocrystals.18 On one hand, with the introduction of well-controlled seeds into the synthesis, structure fluctuation at this stage is no longer an option. It thus becomes possible to separate growth from homo-nucleation and further investigate the explicit roles of reaction kinetics played at certain thermodynamic conditions during epitaxial crystallization. On the other hand, reaction kinetics that is determined by reaction velocity can be effectively manipulated by changing the growth potential. We therefore can begin the synthesis by injecting Cd2+ precursor using a syringe pump into a suspension containing CuS nanoplate seeds, the solvent, and an excess amount of S2-. Since CdS has a relatively small solubility product (Ksp = 8.0×10−27), Cd2+ will rapidly react with S2- to form CdS growth monomers. The reaction kinetics is expected to be controlled by simply adjusting the injection rate of Cd2+.19 Figure 1 schematically illustrates the growth process involving deposition of CdS monomers on the surface of CuS hexagonal nanoplate seeds that leads to the formation of CuS@CdS 5


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crystals with different core-shell patterns.20 In principle, CdS growth monomers will prefer to deposit on the surface of the seed via heterogeneous nucleation.21-23 This preference can be explained by the lower energy barrier of the process in comparison to homogeneous nucleation.24 On the other hand, the chalcogenide nature of the two materials would be beneficial for the close connection of CuS and CdS. In other words, Cu-S-Cd bonds could be the binding force of the core and shell. After deposition, these monomers may access other regions of the seed through surface diffusion in order to reduce the area of exposed facets thus the total surface free energy.25 As a result, the growth mode thus the final morphology of the nanocrystal is determined primarily by the relative magnitude of deposition rate (Vi, Figure 1) and diffusion rate (Vii, Figure 1). Significantly, the deposition rate that relies on the growth potential could be simply modulated by changing the injection rate of Cd2+ precursor solution and thereby the concentration of the CdS growth monomers involved in the reaction; while diffusion rate depends on temperature that could be described according to Arrhenius equation.26 To be more specific, at a fast injection rate and a relatively low reaction temperature, the concentration of CdS monomers around the seeds is rapidly increased, which results in relatively high supersaturation of CdS. Hetero-nucleation of CdS on the surface of CuS will thus frequently occur. This initial deposition event shall generate many CdS nuclei serving as active sites for further growth. Notably, although some portions of the CuS seed are still uncovered, later formed CdS monomers shall still prefer to deposit on these activated sites to form many islands governed by the correlation of lattice mismatch and interfacial energy barrier between CuS and CdS. In this case, Vi > Vii. The final products will be CuS@CdS core-shell nanocrystal consists of multiple islands anchored on the nanoplate (denoted as CuS@CdS-I). This growth pattern can be altered by enabling accelerated surface diffusion of the deposited CdS growth monomers. It could be achieved through simply extending the deposition time and increasing the reaction temperature. In this case, fewer CdS monomers will be produced at the beginning of the synthesis, leading to a reduced supersaturation of CdS. As a result, only limited CdS nuclei could be generated on the surface of the CuS nanoplate for further growth. Since surface diffusion of 6



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deposited monomers is significantly improved and Vi < Vii, newly deposited monomers could be more active and have more time to migrate to other sites on the surface of the seed. Eventually, the initial formed island fragments are eliminated, leading to the switch of overgrowth from island mode to a layer-by-layer mode. The resultant product could be denoted as CuS@CdS-L. It is worth pointing out that although Vi and Vii can be simply modulated by this method, determination of their exact values is still a challenge to date. We then carried out a series of syntheses according to the above rationales. Figure S1 shows the representative scanning electron microscopy (SEM) of the as-prepared CuS nanoplate seeds. They are characterized by hexagonal morphology with an average diameter of about 100 nm and a mean thickness of about 10 nm. The growth was investigated by increasing the injection rate of cadmium acetate solution at a relatively high temperature of 190 oC. Figure 2 represents the typical scanning electron microscopy (SEM) images of the as-prepared CuS@CdS core-shell nanocrystals. Clearly, if the Cd2+ precursor solution was rapidly introduced, e.g., with one-shot injection, the growth proceeded by island mode. Basically, owing to a strong interaction between CdS, those deposited CdS monomers were essentially bound to the original deposited site. The surface of the CuS seed was finally covered by many salient CdS islands (Figure 2a, denoted as CuS@CdS-I). With the decrement of the injection rate of the precursor solution, the number of CdS islands was gradually reduced (Figure 2, b and c, 10 mL/h and 2 mL/h, respectively), indicating promoted surface diffusion. The growth could be dominated by a layer-by-layer mode, when an injection rate of 1 mL/h was applied (Figure 2d, denoted as CuS@CdS-L). To confirm above results, the structure and composition of the CuS@CdS nanocrystals were further investigated by transmission electron microscopy (TEM), high-resolution TEM (HRTEM),

high-angle

annular

dark-field

scanning

TEM

(HAADF-STEM),

and

energy-dispersive X-ray (EDX) analysis (Figure 3). We restricted the characterizations to the two extreme situations, i.e., the CuS@CdS-I and CuS@CdS-L nanocrystals. Because of the relatively large difference of the atomic number between Cd and Cu, the CdS composite can be simply resolved from CuS by the image contrast.27 Clearly, for the CuS@CdS-I nanocrystals, 7


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diverse CdS nanoparticles with a mean size of about 5 nm were spatially confined on the CuS plate seed, leading to a rough appearance of the products (Figure 3a). The CuS@CdS-L nanocrystal, on the contrary, was found without showing any CdS islands (Figure 3b). It was found that the surface of the seed was partially covered by a slice of single-crystal CdS layer. This asymmetric growth behavior can be attributed to a process involving initial growth activation and surface diffusion, (like self-catalytic growth and lattice-mismatch induced diffusion).28-30 Specifically, if the steady concentration of the growth monomers in the reaction suspension was kept relatively low, only limited nuclei can be heterogeneously formed and further severed as active sites for subsequent growth. The heterostructure with atomic-level contact between CdS and CuS as well as the single-crystal feature of CdS nanocrystals was further demonstrated by HRTEM images shown in Figure 3, c and d. In addition, the island and layered features of the two kind crystals were confirmed by our EDX mapping observation. As shown in Figure 3, e-l, while both nanocrystals were composed of CuS core and CdS shell, the homogeneity of Cd in the two samples was clearly different. For example, for the CuS@CdS- I sample, localized Cd islands can be simply resolved as indicated by the white arrows shown in Figure 3h. It should also be noted that the Cd map has a distinct uncoated area of the CuS surface as indicated by the white dashed line shown in Figure 3l, in accordance with the TEM characterization. It is worth mentioning that this selective exposure is important for reactions taking place on CuS. In fact, as aforementioned, surface diffusion can be thermally promoted. In this regard, surface diffusion could be significantly suppressed in a lower reaction temperature. For example, when the reaction temperature dropped to 140 oC, the CdS islands could not be eliminated even the injection rate of the Cd2+ precursor reduced to 1 mL/h (Figure S2, S3). In another aspect, a relatively low injection rate limits supply of the growth monomers, which, in turn, allows sufficient diffusion of the deposited monomers to a more stable site. If the precursor solution was one-shot injected, we only obtained island CdS nanoparticles anchored on the surface of the seed, even the reaction was maintained up to 15 h (Figure S4). Consequently, both high temperature of 8



the reaction and low introduction rate of the precursor are required for the growth to follow a layer-by-layer mode instead of island mode, and give the product CuS@CdS-L. Pure CuS, CdS (obtained by a synthetic procedure similar to CuS@CdS-I, however, without adding CuS seeds), and CuS@CdS nanocrystals shown in Figure 2 were further investigated by X-ray diffraction (XRD) and ultraviolet-visible (UV-vis) spectrophotometer. As shown in Figure S5a, both CuS and CdS were characterized by identical XRD peaks that adopted standard hexagonal phases (reference JCPDS cards: #01-078-2391 and #00-041-1049, respectively). Since CuS nanoplate served as seeds, the intensity of its diffraction peaks has changed little during the growth, indicating the well-maintained crystallinity. On the contrary, the diffraction intensity of CdS was continuously increased from CuS@CdS-I (obtained by one-shot injection of Cd2+) to CuS@CdS-L (obtained by injection of Cd2+ at a rate of 1 mL/h), as also confirmed by increasing peak ratio between CdS-(0002) and CuS-(0006) diffractions. Generally, when the growth changes from an island to a layer-by-layer mode, better crystallinity will be gained, which, in turn, will lead to sharper XRD peaks. These results were in well consistent with our TEM observations. Since CuS and CdS possess similar absorption edges (~ 540 nm), no obvious difference of the UV-vis absorption property was detected for the CuS@CdS nanocrystals except drops of absorption in the spectra range less than 525 nm compared to either CuS or CdS (see Figure S5b), as well as successive increased absorption from CuS@CdS-I to CuS@CdS-L in the same region. The phenomenon may be a result of the strong interaction at the interface of CuS and CdS nanocrystals associated with their microstructures. It also excludes the possibility of loose contact between the two materials. We next sought to investigate the morphology and composite dependent photocatalytic activity of these semiconductor nanocrystals. Prior to the photocatalytic reaction, 0.02 g of the photocatalyst was dispersed into 40 mL aqueous solution contained 2 × 10-5 M methylene blue (MB). The suspension was then stirred for 30 min in the dark to reach adsorption/desorption equilibrium. During visible light irradiation (λ ≥ 430 nm), aliquots of the reaction suspension were collected and centrifuged to remove photocatalyst particles. The concentration of MB 9


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substrates was then determined by measuring the light absorbance using a UV-vis spectrophotometer. For comparison, the activities of pure CuS and CdS were also tested. Figure 4, a and b present, by taking CuS@CdS-I and CuS@CdS-L catalyzed reactions as examples, the temporal visible-light absorption spectra of the dye solution during the reaction.31-35 The spectra of MB solution showed two characteristic peaks at around 613 and 663 nm, respectively. The kinetic degradation rates of these nanocrystals were therefore measured by monitoring the intensity of wavelength of maximum absorption (λmax = 663 nm). As shown in Figure 4c, either CuS or CdS could not completely have MB degraded within the test time (6 h), although CdS exhibited a better performance. The reaction, however, could be largely activated when the two materials were combined together. It is worth pointing out that the dye can be slowly decomposed under visible light irradiation. As shown in Figure S6, after six hours, the concentration of MB was dropped to 80%. This degradation rate is comparable to that by using a pure CuS photocatalyst. All the reaction solutions were decolored and finally became transparent when catalyzed by CuS@CdS core-shell nanocrystals. This notable improvement suggested a synergic effect created by the two materials. Specifically, as reported by Zhang et al., CuS could trap photogenerated electrons through a so-called interfacial charge transfer mechanism.23,36 In this case, photogenerated electrons shall be localized on CuS to reduce dissolved O2 into O2−; while the holes will be repelled to CdS, reacting with surface hydroxyl groups to form OH· radicals and subsequently have MB oxidized. This separation behavior is of crucial importance to the effective use of photogenerated charges and hence the enhanced photocatalytic activity. More importantly, the degradation rate dramatically increases from CuS@CdS-I to CuS@CdS-L. As analyzed above, dropwise injection of Cd precursor leads to better crystallinity of CdS nanocrystals on the surface of CuS seed. This preferential growth in principle will promote the charge mobility in CdS. As a combination of aforementioned effects, CuS@CdS-L undoubtedly possessed superiority toward MB degradation. It is noticed that MB could be completely removed within 3 h by using CuS@CdS-L nanocrystals. The rapid degradation by using CuS@CdS indicates the reaction can be photocatalytically accelerate. We also investigated 10



the pH dependent photodegradation property of CuS@CdS-L. The tests were carried out at similar reaction conditions except that the pH value was adjusted to 4 or 10, by adding HCl and NaOH. As shown in Figure S7, the photocatalytic activity of the photocatalyst is highly dependent on pH values. By adding small amount of NaOH, the degradation was further improved. MB could be fully degraded within 2 h. In alkaline condition, more hydroxyl ions are provided. It leads to the large-scale formation of hydroxyl radicals by reaction with photogenerated holes.37 However, only about 40% MB can be degraded after 2 h photoirradiation in acidified solution. In this case, the degradation has been significantly deactivated after 20 min. The reason shall be attributed to the instability of CuS@CdS in HCl solution, which passivates the reaction. Similar phenomenon was also observed by Maleki et al.38 The decomposition products were also checked by ion chromatography. NH4+ was found after the photocatalytic reaction (Table S1). The result indicating that MB degradation was a mineralization phenomenon. Moreover, the superiority of the heterojunction was also certified by the photocatalytic tests of H2 evolution from water reduction. As shown in Figure 5, while both CuS and CdS showed negligible activity for H2 production, CuS@CdS heterojunctions were found with significantly improved H2-generation property. Similarly, the highest activity was achieved over the CuS@CdS-L photocatalyst, with an average H2 evolution rate of 222.8 μmol h-1 (8.2, 27.2, and 146 μmol h-1 for CuS, CdS, and CdS@CuS-I, respectively). We also measured the H2-evolution rate by using CuS@CdS prepared with two middle injection rates (2 mL/h and 10 mL/h) of the cadmium precursor (Figure S8). The H2 evolution rates over the two photocatalyst were 203.6 and 222.8 μmol h-1, respectively. Although the activity is not changed too much, the activity trend is clear. This result indicates, in addition to the effect of heterojunction, the crystallinity is also important to the enhanced photocatalytic activity. A 100-h photocatalytic test toward H2 evolution over the CdS@CuS-L composite without notable activity decrease indicated that the heterojunction is generally stable for photocatalytic reaction (Figure S9). Compared with other similar composition and structure,36, 39-42 the CdS@CuS in this work displayed a relatively good photocatalytic performance toward hydrogen evolution and MB 11


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degradation (see Table S2 for comparison). Basically, the photocatalytic activity has a strong dependence on the crystallinity of a photocatalyst. Although the heterojunction can effectively separate photogenerated charge carries at the interface, electrons and holes still need to migrate in CuS and CdS, respectively, to surface reaction sites. Since CuS is the same, the fate of the photoreaction relies on the transportation of excited holes thus the crystallinity of CdS. As demonstrated by HRTEM images (Figure 3) and XRD patterns (Figure S5a), CdS in CdS@CuS-L with a better crystallinity thus could provide higher photocatalytic activity. It is also worth pointing out that both CdS@CuS-I and CdS@CuS-L showed increased BET surface area in comparison to CuS (Table S3). While CdS@CuS-I possessed the biggest surface area, the difference between these photocatalysts is not too large. In this case, the impact taken by surface area would be limited. Moreover, it is worth pointing out that lattice mismatch between CuS and CdS would induce interfacial strain, contributing to the electronic band level engineering.43,44 This effect is usually influenced by the growth pattern thus the interfacial behavior, and can be estimated by the Williamson-Hall method according to the XRD data (see Supporting information for detailed description).43-38 While the strain value is 0.53% for pure CuS, they are -0.27%, -0.28%, -0.31%, and -0.32% for the core-shell crystals obtained by successively reducing the injection rate of cadmium precursor (see Table S4). The negative values imply net compressive strain involved in the core-shell structure,43 yet, only with slight difference. This notion can be explained by the similar contact between CuS and CdS. We thus hypothesize that the effect taken by lattice strain is generally at the same level for all CuS@CdS samples. However, as such interfacial lattice strain can cause changes in both ground and excited hole states,43 it is indeed deserved to be quantitatively studied in the future. Taken together, CdS@CuS-L presented the highest photocatalytic activity toward both dye degradation and H2 evolution. The corresponding reaction mechanism responsible for the two photocatalytic procedures was schematically shown in Figure 6, a and b. To validate the charge flow proposed above, we next sought to clarify the band structure of the heterojunction. The band-edge potential of conduction band (CB) and valence band (VB), 12



designated as ECB and EVB, could be calculated from the following equation: ECB = χ - E0 - 1/2 Eg

(1)

EVB = ECB + Eg

(2)

in which χ is the absolute electronegativity of the semiconductor, determined by the geometric mean of the absolute electronegativity of constituent atoms, which is defined as the arithmetic mean of the atomic electron affinity and the first ionization energy; E0 is the energy of free electrons on the hydrogen scale; and Eg is the band gap of the semiconductor. The band gaps of both CuS and CdS were about 2.4 eV according to the UV-vis spectra shown in Figure S5. Hence, the conduction and valence band values could be calculated to be -0.4 and 2.0 eV, respectively, for CuS with respect to NHE; while the values are -0.5 and 1.9 eV for CdS. The obtained band level diagram was shown in Figure 6c. The band offsets for both conduction and valence bands were about 0.1 eV, which indicates a type II band structure between CdS and CuS. The band position offsets were also measured by XPS valence band spectra, as shown in Figure S10. The valence band maximum values of CuS and CdS were measured to be 1.0 eV and 0.9 eV relative to the Fermi energy level of the instrument, respectively. The band level difference (0.1 eV) is in good agreement with the theoretical calculation. The interfacial band structure between the two materials have thus been well-demonstrated. In this case, the photoinduced electrons transfer from the CB of CdS to that of CuS while the holes are repelled to the side of CdS. We also employed an in-situ photoreduction method to revel the reduction sites. In principle, Pt can be photoreduced and nucleated in-situ on the surface of a photocatalyst where photoelectron evolves. Figure 7 shows the STEM mapping results of the CdS@CuS-L nanocrystal obtained after photodeposition of 1 wt% Pt. Clearly, a small portion of the seed was covered by CdS, which is consistent with above observation. Significantly, most Pt nucleated on CuS nanoplates, however, with a notable disappearance at the region covered by CdS (as marked by the dashed cycle). The results provide a direct evidence of our proposed band structure diagram shown in Figure 6c. These results clearly demonstrate the effectiveness of forming a heterojunction, as well as the importance of rationally designing the interfacial structure. 13


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CONCLUSION In summary, we have successfully prepared a series of well-shaped CuS@CdS core-shell heterojunction photocatalyst. The success relies on the use of conformal CuS nanoplate seeds as well as a syringe pump for manipulating the reaction kinetics. Two distinct growth patterns, i.e., island growth and layer-by-layer growth were revealed. The obtained nanocrystals were further served as photocatalysts for visible-light-driven MB degradation and water splitting. Among them, CuS@CdS-L photocatalyst showed the highest activity which was supposed to be a result of both the intimate heterojunction structure and the best crystallinity of CdS for excellent charge separation and transfer abilities. We believe the research provides a versatile method to control the heterostructures of semiconductors and further investigate the shape-dependent physicochemical properties. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (No. 51502240 and 51602052) and the Natural Science Foundation of Jiangsu Province (No. BK20150378 and BK20150604), and the China Fundamental Research Funds for the Central Universities.

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Figure 1. (a) A schematic illustration showing the overgrowth of CdS on CuS nanoplate seeds, which could be divided into two major steps: (i) deposition of CdS monomers at an activated growth sites and (ii) surface diffusion of the deposited CdS monomers from the initial sites to other sites. (b) Two possible growth modes may be involved by controlling the rates of deposition and diffusion. When Vi > Vii, most of the deposited CdS monomers will stay at the nucleation sites in an island growth mode, producing CuS@CuS-I nanocrystals. When Vi < Vii, the CdS can evenly spread on the CuS {0001} faces through a layer-by-layer growth mode, generating CuS@CdS-L nanocrystal.

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Figure 2. SEM images of CuS@CdS nanocrystals obtained with the reaction temperature set to be 190 C and the injection of Cd2+ precursor by (a) one-shot injection, (b) 10 mL/h, (c) 2 mL/h, and (d) 1 mL/h, respectively. Scale bars in the insets are 100 nm. The total injection amount of the precursor solution for each synthesis was kept to be 10 mL.

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Figure 3. (a), (b) TEM images of CuS@CdS-I and CuS@CdS-L photocatalysts and (c), (d) corresponding HRTEM images. Inset in (c) presents an atomic structure of hexagonal CdS viewed from [0001] direction. (e) STEM image and (f-h) corresponding EDX mapping images of a selected CuS@CdS-I nanocrystal. (i) STEM image and (k-l) corresponding EDX mapping images of a selected CuS@CdS-L nanocrystal. Scale bars in (e- l) are 100 nm, respectively.

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Figure 4. Temporal visible-light absorption spectra during MB degradation by using (a) CuS@CdS-L and (b) CuS@CdS-I photocatalysts. (c) The time-coursed degradation performance by plotting C/Co vs. reaction time over different photocatalysts.

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Figure 5. (a) Time-course visible-light-driven photocatalytic H2 evolution from water and (b) the corresponding average H2 evolution rate by using CuS, or CdS, or CuS@CdS-I, or CuS@CdS-L as photocatalyst.

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Figure 6. Proposed photocatalytic reaction mechanism involved in (a) MB degradation and (b) H2 production from water; (c) the band structure of CuS and CdS.

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Figure 7. STEM mapping images of (a) Cu, (b) Pt, (c) S, and (d) Cd for the CdS@CuS-L sample with photodeposit Pt. Clearly, Pt was rarely found on CdS, as marked by the dashed cycle in (b) and (d).

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For Table of Contents Use Only

This paper reports a versatile strategy to synthesizing CuS@CdS core-shell heterojunctions with controllable interfaces for efficient solar H2 production and organic pollutant degradation.

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Controlling the Core-Shell Structure of CuS@CdS Heterojunction via

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