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Interface control and catalytic performances of Au-NiSx heterostructures
Chemical Engineering Journal 382 (2020) 122794
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Interface control and catalytic performances of Au-NiSx heterostructures Yuepeng Lv
a,1
, Sibin Duan
a,1
a
b
, Yuchen Zhu , Haizhong Guo , Rongming Wang
a,⁎
T
a
Beijing Advanced Innovation Center for Materials Genome Engineering, Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, PR China b School of Physical Engineering, Zhengzhou University, Zhengzhou 450001, PR China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
Interface controlled Au-NiS hetero• structures were prepared by solx
vothermal method.
presence of Au improves the HER • The properties of NiS . transfer along the interface • Electron mainly determines the catalytic propx
erties.
A R T I C LE I N FO
A B S T R A C T
Keywords: Metal-semiconductor heterostructure Interface structure Nickel sulfide Structure evolution HER Structure-property relationship
Nickel sulfides (NiSx) as promising catalysts for hydrogen evolution reaction (HER) have attracted much interest. However, the HER catalytic activities of NiSx reported are relatively low for their poor electrical conductivity. The HER catalytic performance of NiSx is expected to be further enhanced by increasing their electronic conductivity. Constructing heterostructures consisting of noble metal and semiconductor has been proven to be an efficient method to promote their physicochemical performances benefitting from synergistic effects along the metal-semiconductor interface. Here, Au-NiSx heterostructures including core@shell, yolk-shell, and oligomerlike structures have been designed and synthesized by different solvothermal methods. A formation mechanism involving the Kirkendall effect has been proposed for the yolk-shell structure with an empty space around the Au core rather than the seeded grown core@shell structure. Catalytic performance measurements indicate that the Au@NiSx core@shell nanoparticles (NPs) exhibit superior HER catalytic property to Au-NiSx yolk-shell, oligomer-like structures, and “pure” NiSx NPs, resulting from the electron transfer between Au and NiSx interfaces. The overpotentials of Au-NiSx NPs with core@shell and yolk-shell nanostructures are 253 mV and 263 mV at 10 mA/cm2, respectively, which are lower than those of oligomer-like Au-NiSx (283 mV) and pure NiSx (321 mV) NPs. The Tafel slope of Au@NiSx core@shell structure (43.7 mV/dec) is also the lowest. These results demonstrate that the core@shell NPs possess the best HER performances, followed by their yolk-shell and oligomer-like counterparts. These findings confirm that the physicochemical performances of the metal-semiconductor heterostructures can be efficiently optimized by adjusting the electron transfer through the interface structure control.
⁎
Corresponding author. E-mail address: [email protected] (R. Wang). 1 These authors contribute equally to the paper. https://doi.org/10.1016/j.cej.2019.122794 Received 3 June 2019; Received in revised form 8 September 2019; Accepted 9 September 2019 Available online 10 September 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Chemical Engineering Journal 382 (2020) 122794
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1. Introduction
grade and used as received without further purification.
Constructing metal-semiconductor heterostructure composed of noble metals and semiconductor compounds with specific interface structure is one of the most effective way to optimize desired physicochemical properties and promote their applications in the fields of catalysis and energy [1]. The improved physicochemical properties of the heterostructure system mainly result from the coupling effect and synergistic effect of the interface between the noble metals and semiconductors. The geometrical structures and chemical properties of the interface, especially the electron transfer caused by the interface, determine the properties to a considerable extent [2–4]. Metal-semiconductor nanoparticles (NPs) with different heterostructures have been synthetized, including core@shell, yolk-shell, and oligomer-like structures [5–13]. The Au@Ni12P5 core@shell structure has been proved to possess excellent supercapacitor properties as a result of quick electron transfer due to its maximum interface between metal and semiconductor [14]. The Au@ZnS-AgAuS yolk-shell structure with a hollow space around the Au core exhibits good photocatalytic performance because of its better photoinduced electron/hole separation synergistically [15]. Due to their scientific significance, researchers have proposed various methods to prepare core@shell and yolk-shell structures, such as seeded growth [16–18], chemical deposition [19], in-situ conversion [14], chemical etching [20], ion exchange [21,22], and Kirkendall-based method [23]. However, the investigation of interface structure dependent physicochemical properties of metal-semiconductor NPs with different architectures, especially nickel sulfides based heterostructures, still has a large space. As main clean and renewable energy in the future, hydrogen is leading the research in the area. Water splitting is commonly used for hydrogen produce and it is urgent to find materials with low price, good stability, and high catalytic activity. As an important semiconductor, nickel sulfide (NiSx) has been used as batteries, supercapacitors, and catalysts [24–27]. Due to its good catalytic performance, low price, and the ability to be used under both acid and alkali environments, nickel sulfide has become a promising HER catalytic material [13,28]. However, the formation of sulfur-hydrogen bonds on the surface of nickel sulfide will severely suppress HER process [26,29]. This problem may be ameliorated with charge separation by constructing metal-semiconductor heterostructures. Previous work has shown that the Cu nanodots and Ni3S2 nanotubes in the Cu-Ni3S2 nanostructures are positively and negatively charged, separately, which weakens the sulfurhydrogen bonds formed on catalyst surfaces and improves their HER catalytic performances [13,30]. The separation of positive and negative charges between metal and NiSx can be modified with various interface structures. Then the HER performance of metal-NiSx heterostructures can be optimized by controlling the interfaces between the metal and NiSx. Herein, three kinds of Au-NiSx heterostructures including core@ shell, yolk-shell and oligomer-like structures and “pure” NiSx NPs were prepared. The growth mechanism and structural evolution of core@ shell and yolk-shell structures have been investigated with time-dependent experiments. Investigation on their HER performances indicates that the Au-NiSx heterostructures show optimized catalytic property. The relationship between the interface structures and properties are also studied.
2.2. Samples preparation 2.2.1. Synthesis of the Au@NiSx core@shell NPs Ni(acac)2 (0.3 g) was dissolved in OAm (16 ml) before being heated to 100 °C for 15 min to form a blue-green solution. Subsequently, a freshly prepared solution of HAuCl4‧4H2O (0.03 g in 3 ml of toluene) was added dropwise with magnetically stirring while maintained at 100 °C for 60 min. The solution was then heated to 180 °C, after which 1-dodecanethiol (0.78 ml) was added to the solution and kept stirred for 120 min before being cooled to room temperature. The product was collected by centrifugation, washed for several times with acetone and chloroform, and then preserved in toluene. 2.2.2. Synthesis of the Au-NiSx yolk-shell NPs Ni(acac)2 (0.3 g) was dissolved in OAm (16 ml) before being heated to 100 °C for 15 min to form a blue-green solution. Subsequently, a freshly prepared solution of HAuCl4‧4H2O (0.03 g in 3 ml of toluene) was added dropwise with magnetically stirring while maintained at 100 °C for 60 min. The solution was then heated to 230 °C for 120 min. Then 1-dodecanethiol (0.78 ml) was added to the dark purple solution and kept stirred for 180 min before being cooled to room temperature. Finally, the product was collected by centrifugation, washed several times with acetone and chloroform, and preserved in toluene. 2.2.3. Synthesis of “pure” NiSx NPs Ni(acac)2 (0.3 g) was dissolved in OAm (16 ml) before being heated to 230 °C. Subsequently, 1-dodecanethiol (0.78 ml) was added to the blue-green solution and magnetically stirred for 180 min before being cooled to room temperature. The product was collected by centrifugation, washed several times with acetone and chloroform, and then preserved in toluene. 2.2.4. Synthesis of the Au-NiSx oligomer-like NPs As-prepared “pure” NiSx NPs (0.11 g) were washed in 5 ml chloroform for several times and re-dispersed in OAm (16 ml). The mixture was then heated to 100 °C for 15 min. Afterwards, a freshly prepared solution of HAuCl4‧4H2O (0.03 g in 3 ml of toluene) was added dropwise with magnetically stirring while maintained at 100 °C for 60 min. After being cooled to room temperature, the product was collected by centrifugation, washed for several times with acetone and chloroform, and then preserved in toluene. 2.3. Characterization The crystal structures, morphologies, and chemical compositions and of the as-prepared products were investigated using X-ray diffraction (XRD, Rigaku, D/max 2200 PC diffractometer, Cu Kα radiation λ = 0.15406 nm), transmission electron microscopy (TEM, JEOL, JEM2200FS) and aberration-corrected transmission electron microscope (AC-TEM, FEI, Titan ETEM G2) operated at 300 kV. For the XRD measurements, the powder samples were dried in a vacuum drying oven. The specimen used for the TEM investigation was prepared by dispersing a drop of solution onto a porous carbon film supported on a copper grid, and then dried in vacuum drying oven. Elemental composition information was acquired by energy-dispersive X-ray spectroscopy (EDS, Oxford, X-Max80T). The electronic structures of the elements were measured by X-ray photoelectron spectroscopy (XPS, ULVAC-PHI, Phi5000 Versa Probe III, Al Kα). All XPS spectra were corrected using C 1s line at 284.6 eV, and background subtraction and curve fitting were also accomplished.
2. Material and methods 2.1. Chemicals Nickel (II) acetylacetonate (Ni(acac)2, 97.0%), chloroauric acid (HAuCl4‧4H2O), oleylamine (OAm, 98.0%), acetone (99.5%), chloroform (99.0%), 1-dodecanethiol (98%), toluene (99.5%), Nafion solution (5 wt%), and carbon black were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd (Beijing, China). All reagents were analytic
2.4. Electrochemical measurements All electrochemical measurements were tested in a three-electrode 2
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any pores, amorphous layers, or impurity phases. No additional diffraction contrasts coming from the strain can be found along the interface, indicating excellent bonding between the cores and the shells. The cores with darker contrast have a diameter of 4.5–7.5 nm, while the shells with lighter contrast have typical polyhedron structure with sizes of 15.0–25.0 nm. Detailed analysis indicates that the morphologies are dominantly truncated octahedrons. A schematic diagram for the core@ shell structure is depicted in the inset. The core@shell nature of the product was further confirmed by angle-dependent TEM characterization. As shown in Figs. S1a–d, the Au NPs always locate in the center, irrelevant to the viewing angle, demonstrating that the as-prepared sample is core@shell structured instead of a supported catalyst [14]. The bright field image in Fig. 2b shows another typical structure with distinct hollow space around the interface. The so-called yolk-shell NPs have typical truncated polyhedron structure with size of 16.0–32.0 nm with dark contrast cores of 4.5–7.5 nm in sizes. The interface between the core and the shell looks similar with the core@shell structrue as shown in Fig. 2a except for the hollow area. The cores in Fig. 2a and b have almost the same size for that the Au cores were synthesized at the same condition during the first step. Because the introduce of hollow space in the yolk-shell structure, the yolk-shell NPs are slightly bigger than the core@shell NPs. Truncated octahedrons also domninate the yolk-shell NPs and the schematic diagram is depicted in the inset. The nature that Au is located in the center of the product was further validated by angle-dependent TEM characterization from various viewing angles (Figs. S1e–h). Apart from the interface structures with a core in the center in Fig. 2a and b, another kind of interface structure with several small Au particles attached to the corners of the NiSx polyhedrons is synthesized, as shown in Fig. 3c. The so-called oligomer-like Au-NiSx NPs has typical sizes of 5.0–16.0 nm with attached small Au particles of 2.5–4.5 nm in sizes. The polyhedron NiSx NPs are also illustrated in Fig. 2d for comparison. The schematic diagrams for Fig. 2c and d are also depicted in the insets. The crystal structures of the samples were characterized by XRD, which proved that the as-synthesized products consisted of Au (cubic, JCPDS No. 99-0056) and NiSx. As shown in Fig. 3, two kinds of nickel sulfides, i.e. Ni3S2 (rhombohedral, JCPDS No. 85-1802) and Ni3S4 (cubic, JCPDS No. 76-1813) can be indexed in the products. No additional impurity phase was detected. By using the Scherrer's equation:
system with CHI 760E electrochemical workstation in 0.5 M H2SO4 solution. A glassy carbon electrode (GCE, 3 mm diameter) was used as the working electrode. Typically, the catalyst (5 mg) and carbon black (5 mg) were dispersed into a mixed solvent containing Nafion (100 μl) and ethanol (1 ml) to form a homogeneous ink by vigorously magnetically stirring. Then the homogeneous ink (2.5 μl) of the ink was dropped onto the surface of the GCE, and dried at room temperature. The mass loading of the catalyst is 0.161 mg/cm2. The graphite rod and Ag/AgCl were served as counter and reference electrode, respectively. Polarization curves for HER activities were characterized by linear sweep voltammetry (LSV) with a scan rate of 2 mV/s, 95% iR compensation. Electrochemical impedance spectroscopy was obtained by applying an AC voltage with a 5 mV amplitude in a frequency range from 0.1 to 100,000 Hz. The three-electrode method is also used for stability test. The catalyst was dropped onto carbon fiber papers and dried at room temperature as working electrodes. The catalyst mass loading was 0.2 mg/cm2 for the stability test. 3. Results and discussion In this work, the metal-semiconductor heterostructures composed of Au and NiSx with three specific morphologies, i.e. core@shell, yolkshell, and oligomer-like, were designed and controllably synthesized using seeded growth and in-situ conversion methods combined with Kirkendall effect. “Pure” NiSx NPs were also obtained for comparison. The specific synthesis methods and reaction processes are summarized in Fig. 1. 3.1. Morphology and structure The microstructures of the as-synthesized samples were characterized by AC-TEM. Four kinds of nanostructures, i.e. core@shell, yolkshell, oligomer-like, and “pure” polyhedron morphology were confirmed. For each sample, NPs have similar morphology with almost identical size. The sizes for NPs are mostly in the range of 10.0–20.0 nm. The diffraction contrast of the bright field images reveals well-crystallized nature of the samples. As shown in Fig. 2a, the NPs have typical core@shell structures. It should be noted that the interfaces between the cores and the shells are smooth and clean without
Fig. 1. Diagram for the synthesis processes of Au-NiSx heterostructures. The core@shell structure was formed by seeded growth of NiSx shells on Au NPs, while the yolk-shell structure was formed by seeded growth of Ni NPs on Au NPs followed by in-situ conversion from Au-Ni dumbbells to Au-NiSx yolk-shell NPs. The Au-NiSx oligomer-like structure was formed by seeded growth of Au NPs on NiSx NPs. In the figure, the colors of yellow, purple and green represent Au, Ni and NiSx NPs, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 3
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Fig. 2. Typical bright field TEM images of Au-NiSx heterostructures. The schematic diagram for each image is depicted in the inset. (a) Au@NiSx core@shell NPs; (b) Au-NiSx yolk-shell NPs; (c) Au-NiSx oligomer-like NPs; (d) NiSx NPs.
d = 0.89λ /(β ·cosθB )
(1)
where λ is the X-ray wavelength (0.15406 nm), θB is the Bragg diffraction angle and β is the full width at half maxima of the peak, the typical size for the nanostructures can be obtained. By fitting the peaks at about 31.2° corresponding to Ni3S2 ( −1 1 0) planes and Ni3S4 (3 1 1) planes, the typical sizes for NiSx in the Au-NiSx heterostructures can be calculated. The calculated size for NiSx in the Au@NiSx NPs is ~7.2 nm while those for the Au-NiSx yolk-shell and oligomer-like NPs are 10.8 nm and 6.7 nm, respectively. Compared with the TEM images, it can be found that the calculated typical sizes match with the sizes of the exposed polyhedrons or the thickness of the shells. By fitting the peaks at about 38.2° corresponding to Au (1 1 1) planes, the typical sizes for Au NPs in the Au-NiSx heterostructures can also be calculated. The calculated size for Au in Au@NiSx core@shell NPs is ~5.3 nm while those for Au-NiSx yolk-shell and oligomer-like NPs are 5.6 nm and 3.5 nm, respectively. The calculated sizes for Au particles are slightly smaller than the results from the TEM images, which may be contributed from the coupling broadening of the Au (1 1 1) peak with the Ni3S2 (1 1 1) peak and Ni3S4 (4 0 0) peak. The mole ratios of Ni3S2 to Ni3S4 in the heterostructures can also be estimated according to the equation:
Wx = IA Fig. 3. XRD patterns of NiSx based nanoparticles with different architectures including Au@NiSx core@shell, Au-NiSx yolk-shell, Au-NiSx oligomer-like and NiSx.
⎛ x ⎜KA ⎝
N
∑ (Ii i=A
⎞ K Ai) ⎟ ⎠
(2)
where Wx is the mass ratio of component X, IA is the XRD diffraction x A A KAl , KAl is the value intensity of phase A, KAx is the value of KAl 2 O3 2 O3 2 O3 of the integral strength of the strongest mixed peak of sample A and Al2O3 divided by the integral strength of the strongest mixed peak of 4
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Fig. 4. HRTEM images obtained by AC-TEM of: (a) a Au@NiSx core@shell NP, (b) a Au-NiSx yolk-shell NP, where a hollow space is distinguished near the Au core, (c) a Au-NiSx oligomer-like NP and (d) a NiSx NP.
exposed facets in the Au@NiSx NPs. Fig. 4b shows a typical HRTEM image of the Au-NiSx yolk-shell structure. The Au NP in the core is a single crystal. From the FFT pattern (inset), the shell is single crystallized and can be identified as cubic phase Ni3S4. The resolved lattice fringes with spacing of ~0.283 nm correspond to (3 1 1) planes of Ni3S4. Different from the Au@NiSx core@shell NP, Au and NiSx are not completely contacted with each other in the Au-NiSx yolk-shell NP. Hollow space near the Au core can be easily distinguished in the HRTEM image, as marked in Fig. 4b. By comparing the lattice fringes of Ni3S4 along the interface between Au and Ni3S4 and around the hollow area, no apparent difference can be observed, indicating no significant stress exists along the Au and Ni3S4 interface. Atomic steps can also be observed on the surface occasionally. Fig. 4c shows a typical HRTEM image of Au-NiSx oligomer-like NP. The inset FFT pattern reveals that the big particle is cubic phase Ni3S4. Two single crystal Au NPs are attached at the corners of Ni3S4 NP. The contact areas of the Au-NiSx oligomer-like NPs. interfaces are small compared with those in the AuNiSx core@shell and yolk-shell NPs. In Fig. 4c, the Ni3S2 part in a AuNiSx oligomer-like NP shows typical truncated octahedron morphology with almost perfect facets. Fig. 4d shows a typical NiSx polyhedron NP. FFT analysis indicates that it is cubic phase Ni3S4 nanocrystal with almost perfect facets, like that in Fig. 4c, because that the NiSx parts of Au-NiSx oligomer-like NPs are synthesized in the same conditions with that of “pure” NiSx NPs. The XPS spectra were used to characterize the chemical states of these four kinds of NiSx based NPs. The XPS spectra and corresponding peak deconvolution results are shown in Figs. 5, S3, and Table S2, respectively. Using a Gaussian fitting method, the Ni 2p spectrum was fitted by considering two spin orbit doublets and two shake-up
Al2O3 according to the mass ratio of 1:1. The mole ratio of Ni3S2 to Ni3S4 in Au@NiSx core@shell NPs is about 96:4 while those in Au-NiSx yolk-shell and oligomer-like NPs are about 50:50 and 63:37, respectively. Additionally, EDS measurements confirm the chemical composition of the Au-NiSx heterostructures. Fig. S2 shows the EDS spectra of the Au-NiSx core@shell, yolk-shell, and oligomer-like NPs as well as NiSx NPs, revealing that the products only consist of Au, Ni, and S. The signals of Cu and Si in the spectra come from the TEM specimen holder. The mass ratio for Au, Ni, and S for each sample is also illustrated in Table S2. The results obtained from the XRD analysis match well with that obtained from the EDS spectra. In order to further understand the atomic arrangements in the AuNiSx heterostructures, the as-prepared samples were characterized by high-resolution TEM (HRTEM), as shown in Fig. 4. The heterostructures are found to be well-crystallized. No impurity phases or amorphous layers are observed at atomic scale. The NiSx are single crystals and the Au NPs can be single crystals, icosahedron or decahedron. No matter how the structure of Au NP is, the interface between Au and NiSx is found to be smooth and clean. As shown in Fig. 4a, the diameter of Au core is about 4 nm with a five-fold symmetry, indicating icosahedron or decahedron structure of the particle. From the Fast Fourier Transformation (FFT) pattern of HRTEM as shown in the inset of Fig. 4a, the shell is single crystallized and can be indexed as rhombohedral phase Ni3S2. In the HRTEM image, the resolved lattice fringes with spacing of ~0.288 nm correspond to (−1 1 0) planes of Ni3S2. The lattice fringes penetrate throughout the shell structure, indicating single crystal nature of the shell. It also should be noted that the surface of Ni3S2 is clean but not smooth at atomic scale. Atomic steps can be observed on the surface, as marked by arrows, revealing the existing of high-index 5
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Fig. 5. High resolution XPS spectra for the (a) Ni 2p and (b) Au 4f of the Au@NiSx core@shell, Au-NiSx yolk-shell, Au-NiSx oligomer-like and NiSx NPs. (c) Schematic diagram of electron transfer in a Au@NiSx core@shell NP and a Au-NiSx yolk-shell NP.
satellites. The fitting peaks positioned at 853.05–852.70 and 870.54–869.90 eV are assigned to Ni0, while the peaks at 856.66–855.52 and 874.60–873.56 eV correspond to Ni2+ [31]. It is obvious that the atom ratios of Ni0 in the Au@NiSx core@shell and AuNiSx yolk-shell NPs are higher than those in the Au-NiSx oligomer-like and “pure” NiSx NPs, which means that Ni atoms accept more electrons from the Au cores in the Au@NiSx core@shell and Au-NiSx yolk-shell NPs. The S 2p XPS spectra are shown in Fig. S3. The S2−, S−, and S correspond to the peaks positioned at 161.46–161.04 eV, 162.60–162.19 eV, and 163.39–163.9 eV, respectively. The peaks positioned at 168.97–167.43 eV are assigned to SO42−/SO32−, which may
result from the surface oxidation [13]. As shown in Table S2, S atoms accept more atoms in the Au@NiSx core@shell and Au-NiSx yolk-shell NPs with the combination of Au cores, which is similar to the trend of Ni 2p peaks. For Au 4f XPS spectra of the four samples, it is obvious that there are small peaks beside Au0 4f peaks marked by blue lines, which can be assigned to the peaks of oxidative Au+ located in the direction of high binding energy of metallic Au peaks [32]. The peak fitting results demonstrate that the proportion of oxidative Au+ increases from 6% to about 20%, according to sequence of Au-NiSx oligomer-like, Au-NiSx yolk-shell, and Au@NiSx core@shell NPs. Taking the conclusions obtained from the Ni 2p and S 2p XPS spectra into account, obvious 6
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3.2.2. Au-NiSx yolk-shell NPs For Au-NiSx yolk-shell NPs, the sulfuration agent 1-dodecanethiol was added after the formation of Au-Ni dumbbell NPs. After that, samples were taken at 2, 5, 10, 15, 20, 30, 60, and 180 min. The TEM images to exhibit morphology evolution during the transformation from Au-Ni dumbbell to Au-NiSx yolk-shell NPs are shown in Fig. 7a–g, and the atomic ratios of Au to Ni and S to Ni obtained from the EDS analysis are listed in Fig. 7h and Table S4. Au NPs keep their morphologies and sizes during the transformation, and forming Au cores ultimately. As shown in Fig. 7h, the atomic ratios of Au to Ni are nearly constant during the whole process, meaning that the Ni sources have been used up in the formation of Au-Ni dumbbell NPs, and the atomic ratios of S to Ni increase quickly during sulfuration of Ni atom in the Au-Ni dumbbell NPs, and the morphology changes from dumbbell to yolk-shell at the same time. It is an in-situ transformation process like that exist in the growth of Au@Ni12P5 core@shell NPs [14,33]. Different from the reported in-situ transformation of Au@Ni12P5 core@shell NPs, a hollow space forms around a Au core, which may result from the Kirkendall effect [21,34]. During the sulfuration of Ni with 1-dodecanethiol, the rate at which S atoms diffuse into the interior of Ni NPs is lower than the rate at which Ni atoms diffuse outward, thus leaving a hollow space around the Au core and forming Au-NiSx yolk-shell NPs ultimately. Thus, the formation of Au-NiSx yolk-shell NPs is based on the in-situ transformation method combined with Kirkendall effect.
electron transfer from Au cores to NiSx shells happened in the Au-NiSx yolk-shell and Au@NiSx core@shell NPs. The contact area between the Au core and the NiSx shell in the core@shell NPs are higher than that in the yolk-shell NPs, resulting in the more remarkable electron transfer process, as illustrated in Fig. 5c. Therefore, the electronic structures of heterostructured metal-semiconductor NPs can be well regulated by elaborately design of interface structure, which will determine their electrocatalytic performances to some extent. 3.2. Growth mechanism Understanding the growth process not only is beneficial to the study of the growth mechanism, but also helps the design of interface structure to obtain core@shell or yolk-shell nanostructure with single crystal shells, while it is not easy to achieve it because of the large crystal structure difference between the noble metal cores and semiconductor shells. The time-dependent experiments were conducted combined with TEM and EDS analysis. 3.2.1. Au@NiSx core@shell NPs For the synthesis of Au@NiSx core@shell NPs, the sulfuration agent 1-dodecanethiol was added after the formation of Au NPs. After that, samples were collected at 5, 10, 20, 30, 60, 90, and 120 min. The morphology evolution of the obtained samples is shown in Fig. 6a–g, and the atomic ratios of Au to Ni and Ni to S obtained from the EDS analysis are listed in Fig. 6h and Table S3. During the evolution from Au NPs to Au@NiSx core@shell NPs, the size and morphology of Au NPs do not obviously change, while the shells are gradually thickening, and the truncated octahedral outer shape becomes more distinct as reaction time elapses. From Fig. 6h and Table S3, the atomic ratios of Au to Ni gradually decrease and it do not obviously change after 30 min, which means that the nickel source has been used up and the shell growth has completed at 30 min. It is also confirmed from that the sizes of the samples are similar from 30 to 120 min. It is noted that the atomic ratios of Ni to S are nearly constant from 5 to 120 min, meaning that the Ni and S atoms were attached to the surface of Au cores layer by layer at the same time. The growth process of Au@NiSx core@shell NPs is a typical epitaxial growth of NiSx on the Au seeds.
3.3. Electrochemical properties of Au-modified NiSx NPs To understand the synergistic effect and coupling effect between the Au and NiSx interface, the HER activities of the NiSx based NPs with different nanostructures were investigated in acidic solutions. All four NiSx based NPs behave as effective electrocatalysts towards HER, as shown in Fig. 8 and summarized in Table S5. From the polarization curves in Fig. 8a, the overpotentials at a current density of 10 mA/cm2 of Au@NiSx core@shell NPs (253 mV) and Au-NiSx yolk-shell NPs (263 mV) are obviously lower than that of Au-NiSx oligomer-like NPs (283 mV) and “ pure” NiSx NPs (321 mV), which are comparable to the value of metal-semiconductor heterostructure catalysts such as Pd/ MoSe2 (231 mV), and Au@CoP (160 mV, 1 mA/cm2) [35,36]. With the
Fig. 6. (a–g) TEM images of the intermediate products obtained at different times after adding 1-dodecanethiol during the preparation of Au@NiSx core@shell NPs, demonstrating that NiSx shells grow on the Au seeds, scale bar: 50 nm. (h) Curves of Au: Ni and S: Ni atomic ratios evolution during the formation of Au@NiSx core@ shell NPs. 7
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Fig. 7. (a–g) TEM images of the intermediate product obtained at different times after adding 1-dodecanethiol during the preparation of Au-NiSx yolk-shell NPs, demonstrating the transformation process from Au-Ni dumbbell NPs to Au-NiSx yolk-shell NPs, scale bar: 50 nm. (h) Curves of Au: Ni and S: Ni atomic ratios evolution during the transformation.
stability during the long-time HER catalytic process. All these results demonstrate that the NiSx based NPs with different nanostructures, i.e. Au@NiSx core@shell, Au-NiSx yolk-shell, Au-NiSx oligomer-like, and “pure” NiSx NPs, are all efficient HER catalyst, where the core@shell NPs outperform others. To further analyze the electrical factors affecting the HER activity of the samples and investigate the structure-property relationship, the electrochemical impedance spectra of the four samples were measured at room temperature with frequencies ranging from 100 kHz to 0.1 Hz. Fig. 8c shows that the diameter of semi-circular Nyquist plots of Au@NiSx core@shell, Au-NiSx yolk-shell, Au-NiSx oligomer-like and “pure” NiSx is gradually increasing. The equivalent circuit used to fit the impedance curves is given in the inset of Fig. 8c, consisting of an electrolyte resistance (Rs), a charge-transfer resistance (Rt), and a constant-phase element (CPE). The charge-transfer resistance Rt of the sample corresponds to the circular diameter in the Nyquist plot, representing the resistance of mass transfer during the electrocatalytic reaction, which is the decisive parameter for the HER catalytic efficiency [42]. Using ZSimpWin software (Princeton Applied Research, Oak Ridge, TN, USA), the chargetransfer resistance Rt of the Au@NiSx core-shell, Au-NiSx yolk-shell, AuNiSx oligomer-like, and “pure” NiSx electrodes are calculated to be 49.4 Ω, 62.3 Ω, 110.7 Ω, and 497.1 Ω, respectively. Obviously, the charge-transfer resistances of NPs with the inclusion of Au are lower than that of the “pure” NiSx, which can be attribute to the enhanced electrical conductivity and shortened charge diffusion length resulting
same loading, the Au@NiSx core@shell NPs exhibit an area-specific activity of 10 mA/cm2 at − 0.25 V vs RHE, which is 56%, 303% and 1513% higher than that of Au-NiSx yolk-shell NPs (6.43 mA/cm2), AuNiSx oligomer-like NPs (2.48 mA/cm2) and “pure” NiSx NPs (0.62 mA/ cm2) at −0.25 V vs RHE, respectively. It is obvious that the HER catalytic performance is greatly enhanced by the inclusion of Au cores, especially in the core@shell NPs. As shown in Fig. 8b, the Tafel slopes of Au@NiSx core@shell, Au-NiSx yolk-shell, Au-NiSx oligomer-like, and “pure” NiSx NPs are 43.7 mV/dec, 49.4 mV/dec, 50.6 mV/dec, and 56.1 mV/dec, respectively. The Tafel slopes of the Au-modified NiSx NPs are lower than that of the pure NiSx, and the Au@NiSx core@shell NPs are the lowest, followed by Au-NiSx yolk-shell, Au-NiSx oligomerlike and NiSx NPs. The lower Tafel slopes indicate the superior reaction kinetics and good HER activities of these Au-NiSx heterostructure NPs compared with “pure” NiSx NPs, which are comparable or lower than some heterostructure HER catalysts such as Au@CoP (52 mV/dec, 1 mA/cm2), Pd/NiS@Al2O3 (35 mV/dec), Au@Zn-Fe-C (130 mV/dec), Cu/Cu2O/Cu2S (107 mV/dec), Mo-CoP(65 mV/dec), and nanocomposite NbC (35 mV/dec) [36–41]. To test their stability, the products were dropped onto the carbon fiber paper. The chronoamperometric response for the Au@NiSx core@shell and Au-NiSx yolk-shell NPs were recorded, as shown in Fig. S4. The current densities of the HER activity of Au@NiSx core@shell and Au-NiSx yolk-shell NPs are relatively stable, maintaining 93.6% and 98.9% after 5 h, respectively. It confirmed that the as-prepared Au-NiSx heterostructure NPs have good
Fig. 8. Electrochemical performances of the HER catalysts: (a) polarization curves for the Au@NiSx core@shell, Au-NiSx yolk-shell, Au-NiSx oligomer-like and NiSx NPs on GC electrodes in 0.5 M H2SO4. (b) Tafel curves of the four sample in 0.5 M H2SO4. and (c) Nyquist plots of the four sample and a simulated circuit diagram. 8
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from synergistic effect between Au and NiSx. Among them, the core@ shell NPs have the lowest Rt and exhibit the highest catalytic activity, which may result from the fully contact between Au and NiSx. The synergistic effect between semiconductor and other compound with excellent electrical conductivity were also reported. Lin et al. have reported a dramatic enhancement of photocatalytic activity of the novel photocatalyst of Ag3PO4@MWCNTs@PANI due to the synergetic effect between MWCNTs and PANI on Ag3PO4 [43–45]. Besides, the single thin single crystalline shell also benefits much for good ionic diffusion and electron transport during the catalytic reaction, as demonstrated by the AC-TEM in Fig. 4. Atomic steps on the surfaces of Au@NiSx core@ shell NPs and Au-NiSx yolk-shell NPs generate high-index exposed facets, which also cause better catalytic activity. In contrast, the Au-NiSx oligomer-like NPs and “pure” NiSx NPs show almost perfect facets, without high-index exposed facets. In addition, it is reported that when NiSx are used as the HER catalyst, stable S-Hads bonds, which are difficult to desorb H2 are formed on the surface of the catalyst, which is a key factor to determine the catalytic activity [13]. Here in the Au-NiSx heterostructure NPs, due to the interaction between Au and NiSx NPs, surface NiSx are negatively charged resulting from the electron transfer between Au and NiSx, of which core@shell NPs are strongest, as shown from the XPS spectra in Fig. 5. The negatively charged NiSx weaken the S-Hads bonds formed on the surface, thus optimizing the desorption of H2, promoting the Heyrovsky process of HER and improving the HER activity accordingly [13,30]. Therefore, benefiting from the lower charge-transfer resistance, the single crystal NiSx shells and negatively charged NiSx surface, the Au modified NiSx NPs possess better HER activity than “pure” NiSx, of which the fully contacted core@shell NPs are the best.
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4. Conclusions In summary, Au@NiSx core@shell, Au-NiSx yolk-shell, Au-NiSx oligomer-like heterostructures, and “pure” NiSx NPs are controllably synthesized, and the evolution mechanisms of Au@NiSx core@shell and Au-NiSx yolk-shell NPs are revealed. Our HER catalysts results indicate that the catalytic activities of the Au-NiSx heterostructures are mainly dependent on their interface structures. The Au@NiSx core@shell NPs outperform the other three ones resulting from the synergistic effect and coupling effect between Au cores and NiSx shells. By controlling the interface between Au and NiSx, the electronic structures of the NiSxbased NPs are well controlled to optimize their catalytic performances. It is believed that the core@shell and yolk-shell structures will provide more opportunities for the application of metal-semiconductor heterostructure in energy storage, energy conversion and catalysis. Acknowledgements This research was supported by the National Key Research and Development Program of China (No. 2018YFA0703702), National Natural Science Foundation of China (Nos. 51971025, 51901012, 11674023, and 11280127), 111 project (No. B170003) and the Fundamental Research Funds for the Central Universities under FRFTP-19-022A2. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122794. References [1] X. Liu, J. Iocozzia, Y. Wang, X. Cui, Y. Chen, S. Zhao, Z. Li, Z. Lin, Noble metal-metal oxide nanohybrids with tailored nanostructures for efficient solar energy conversion, photocatalysis and environmental remediation, Energy Environ. Sci. 10 (2017) 402–434.
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hydrogen evolution reaction, J. Am. Chem. Soc. 137 (2015) 1587–1592. [31] R. Franke, X-ray absorption and photoelectron spectroscopy investigation of binary nickel phosphides, Spectrochim. Acta A 53A (1997) 933–941. [32] S.Y. Moon, H.C. Song, E.H. Gwag, C. Nedrygailov II, J.J. Lee, W.H. Kim, J.Y. Park Doh, Plasmonic hot carrier-driven oxygen evolution reaction on Au nanoparticles/ TiO2 nanotube arrays, Nanoscale 10 (2018) 22180–22188. [33] Y.Y. Xu, S.B. Duan, H.Y. Li, M. Yang, S.J. Wang, X. Wang, R.M. Wang, Au/Ni12P5 core/shell single-crystal nanoparticles as oxygen evolution reaction catalyst, Nano Res. 10 (2017) 3103–3112. [34] Y. Yin, R.M. Rioux, C.K. Erdonmez, S. Hughes, G.A. Somorjai, A.P. Alivisatos, Formation of hollow nanocrystals through the nanoscale Kirkendall effect, Science 304 (2004) 711–714. [35] M.D. Sharma, C. Mahala, M. Basu, Nanosheets of MoSe2@M (M = Pd and Rh) function as widespread pH tolerable hydrogen evolution catalyst, J. Colloid Interface Sci. 534 (2019) 131–141. [36] H.B. Liao, Y.M. Sun, C.C. Dai, Y.H. Du, S.B. Xi, F. Liu, L.H. Yu, Z.Y. Yang, Y.L. Hou, A.C. Fisher, S.Z. Li, Z.C.J. Xu, An electron deficiency strategy for enhancing hydrogen evolution on CoP nano-electrocatalysts, Nano Energy 50 (2018) 273–280. [37] Y.Y. Feng, Y.X. Guan, H.J. Zhang, Z.Y. Huang, J. Li, Z.Q. Jiang, X. Gu, Y. Wang, Selectively anchoring Pt single atoms at hetero-interfaces of gamma-Al2O3/NiS to promote the hydrogen evolution reaction, J. Mater. Chem. A 6 (2018) 11783–11789. [38] J. Lu, W.J. Zhou, L.K. Wang, J. Jia, Y.T. Ke, L.J. Yang, K. Zhou, X.J. Liu, Z.H. Tang, L.G. Li, S.W. Chen, Core-shell nanocomposites based on gold nanoparticle@Zinciron-embedded porous carbons derived from metal-organic frameworks as efficient dual catalysts for oxygen reduction and hydrogen evolution reactions, Acs Catal. 6
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Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Interface control and catalytic performances of Au-NiSx heterostructures Yuepeng Lv
a,1
, Sibin Duan
a,1
a
b
, Yuchen Zhu , Haizhong Guo , Rongming Wang
a,⁎
T
a
Beijing Advanced Innovation Center for Materials Genome Engineering, Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, PR China b School of Physical Engineering, Zhengzhou University, Zhengzhou 450001, PR China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
Interface controlled Au-NiS hetero• structures were prepared by solx
vothermal method.
presence of Au improves the HER • The properties of NiS . transfer along the interface • Electron mainly determines the catalytic propx
erties.
A R T I C LE I N FO
A B S T R A C T
Keywords: Metal-semiconductor heterostructure Interface structure Nickel sulfide Structure evolution HER Structure-property relationship
Nickel sulfides (NiSx) as promising catalysts for hydrogen evolution reaction (HER) have attracted much interest. However, the HER catalytic activities of NiSx reported are relatively low for their poor electrical conductivity. The HER catalytic performance of NiSx is expected to be further enhanced by increasing their electronic conductivity. Constructing heterostructures consisting of noble metal and semiconductor has been proven to be an efficient method to promote their physicochemical performances benefitting from synergistic effects along the metal-semiconductor interface. Here, Au-NiSx heterostructures including core@shell, yolk-shell, and oligomerlike structures have been designed and synthesized by different solvothermal methods. A formation mechanism involving the Kirkendall effect has been proposed for the yolk-shell structure with an empty space around the Au core rather than the seeded grown core@shell structure. Catalytic performance measurements indicate that the Au@NiSx core@shell nanoparticles (NPs) exhibit superior HER catalytic property to Au-NiSx yolk-shell, oligomer-like structures, and “pure” NiSx NPs, resulting from the electron transfer between Au and NiSx interfaces. The overpotentials of Au-NiSx NPs with core@shell and yolk-shell nanostructures are 253 mV and 263 mV at 10 mA/cm2, respectively, which are lower than those of oligomer-like Au-NiSx (283 mV) and pure NiSx (321 mV) NPs. The Tafel slope of Au@NiSx core@shell structure (43.7 mV/dec) is also the lowest. These results demonstrate that the core@shell NPs possess the best HER performances, followed by their yolk-shell and oligomer-like counterparts. These findings confirm that the physicochemical performances of the metal-semiconductor heterostructures can be efficiently optimized by adjusting the electron transfer through the interface structure control.
⁎
Corresponding author. E-mail address: [email protected] (R. Wang). 1 These authors contribute equally to the paper. https://doi.org/10.1016/j.cej.2019.122794 Received 3 June 2019; Received in revised form 8 September 2019; Accepted 9 September 2019 Available online 10 September 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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1. Introduction
grade and used as received without further purification.
Constructing metal-semiconductor heterostructure composed of noble metals and semiconductor compounds with specific interface structure is one of the most effective way to optimize desired physicochemical properties and promote their applications in the fields of catalysis and energy [1]. The improved physicochemical properties of the heterostructure system mainly result from the coupling effect and synergistic effect of the interface between the noble metals and semiconductors. The geometrical structures and chemical properties of the interface, especially the electron transfer caused by the interface, determine the properties to a considerable extent [2–4]. Metal-semiconductor nanoparticles (NPs) with different heterostructures have been synthetized, including core@shell, yolk-shell, and oligomer-like structures [5–13]. The Au@Ni12P5 core@shell structure has been proved to possess excellent supercapacitor properties as a result of quick electron transfer due to its maximum interface between metal and semiconductor [14]. The Au@ZnS-AgAuS yolk-shell structure with a hollow space around the Au core exhibits good photocatalytic performance because of its better photoinduced electron/hole separation synergistically [15]. Due to their scientific significance, researchers have proposed various methods to prepare core@shell and yolk-shell structures, such as seeded growth [16–18], chemical deposition [19], in-situ conversion [14], chemical etching [20], ion exchange [21,22], and Kirkendall-based method [23]. However, the investigation of interface structure dependent physicochemical properties of metal-semiconductor NPs with different architectures, especially nickel sulfides based heterostructures, still has a large space. As main clean and renewable energy in the future, hydrogen is leading the research in the area. Water splitting is commonly used for hydrogen produce and it is urgent to find materials with low price, good stability, and high catalytic activity. As an important semiconductor, nickel sulfide (NiSx) has been used as batteries, supercapacitors, and catalysts [24–27]. Due to its good catalytic performance, low price, and the ability to be used under both acid and alkali environments, nickel sulfide has become a promising HER catalytic material [13,28]. However, the formation of sulfur-hydrogen bonds on the surface of nickel sulfide will severely suppress HER process [26,29]. This problem may be ameliorated with charge separation by constructing metal-semiconductor heterostructures. Previous work has shown that the Cu nanodots and Ni3S2 nanotubes in the Cu-Ni3S2 nanostructures are positively and negatively charged, separately, which weakens the sulfurhydrogen bonds formed on catalyst surfaces and improves their HER catalytic performances [13,30]. The separation of positive and negative charges between metal and NiSx can be modified with various interface structures. Then the HER performance of metal-NiSx heterostructures can be optimized by controlling the interfaces between the metal and NiSx. Herein, three kinds of Au-NiSx heterostructures including core@ shell, yolk-shell and oligomer-like structures and “pure” NiSx NPs were prepared. The growth mechanism and structural evolution of core@ shell and yolk-shell structures have been investigated with time-dependent experiments. Investigation on their HER performances indicates that the Au-NiSx heterostructures show optimized catalytic property. The relationship between the interface structures and properties are also studied.
2.2. Samples preparation 2.2.1. Synthesis of the Au@NiSx core@shell NPs Ni(acac)2 (0.3 g) was dissolved in OAm (16 ml) before being heated to 100 °C for 15 min to form a blue-green solution. Subsequently, a freshly prepared solution of HAuCl4‧4H2O (0.03 g in 3 ml of toluene) was added dropwise with magnetically stirring while maintained at 100 °C for 60 min. The solution was then heated to 180 °C, after which 1-dodecanethiol (0.78 ml) was added to the solution and kept stirred for 120 min before being cooled to room temperature. The product was collected by centrifugation, washed for several times with acetone and chloroform, and then preserved in toluene. 2.2.2. Synthesis of the Au-NiSx yolk-shell NPs Ni(acac)2 (0.3 g) was dissolved in OAm (16 ml) before being heated to 100 °C for 15 min to form a blue-green solution. Subsequently, a freshly prepared solution of HAuCl4‧4H2O (0.03 g in 3 ml of toluene) was added dropwise with magnetically stirring while maintained at 100 °C for 60 min. The solution was then heated to 230 °C for 120 min. Then 1-dodecanethiol (0.78 ml) was added to the dark purple solution and kept stirred for 180 min before being cooled to room temperature. Finally, the product was collected by centrifugation, washed several times with acetone and chloroform, and preserved in toluene. 2.2.3. Synthesis of “pure” NiSx NPs Ni(acac)2 (0.3 g) was dissolved in OAm (16 ml) before being heated to 230 °C. Subsequently, 1-dodecanethiol (0.78 ml) was added to the blue-green solution and magnetically stirred for 180 min before being cooled to room temperature. The product was collected by centrifugation, washed several times with acetone and chloroform, and then preserved in toluene. 2.2.4. Synthesis of the Au-NiSx oligomer-like NPs As-prepared “pure” NiSx NPs (0.11 g) were washed in 5 ml chloroform for several times and re-dispersed in OAm (16 ml). The mixture was then heated to 100 °C for 15 min. Afterwards, a freshly prepared solution of HAuCl4‧4H2O (0.03 g in 3 ml of toluene) was added dropwise with magnetically stirring while maintained at 100 °C for 60 min. After being cooled to room temperature, the product was collected by centrifugation, washed for several times with acetone and chloroform, and then preserved in toluene. 2.3. Characterization The crystal structures, morphologies, and chemical compositions and of the as-prepared products were investigated using X-ray diffraction (XRD, Rigaku, D/max 2200 PC diffractometer, Cu Kα radiation λ = 0.15406 nm), transmission electron microscopy (TEM, JEOL, JEM2200FS) and aberration-corrected transmission electron microscope (AC-TEM, FEI, Titan ETEM G2) operated at 300 kV. For the XRD measurements, the powder samples were dried in a vacuum drying oven. The specimen used for the TEM investigation was prepared by dispersing a drop of solution onto a porous carbon film supported on a copper grid, and then dried in vacuum drying oven. Elemental composition information was acquired by energy-dispersive X-ray spectroscopy (EDS, Oxford, X-Max80T). The electronic structures of the elements were measured by X-ray photoelectron spectroscopy (XPS, ULVAC-PHI, Phi5000 Versa Probe III, Al Kα). All XPS spectra were corrected using C 1s line at 284.6 eV, and background subtraction and curve fitting were also accomplished.
2. Material and methods 2.1. Chemicals Nickel (II) acetylacetonate (Ni(acac)2, 97.0%), chloroauric acid (HAuCl4‧4H2O), oleylamine (OAm, 98.0%), acetone (99.5%), chloroform (99.0%), 1-dodecanethiol (98%), toluene (99.5%), Nafion solution (5 wt%), and carbon black were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd (Beijing, China). All reagents were analytic
2.4. Electrochemical measurements All electrochemical measurements were tested in a three-electrode 2
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any pores, amorphous layers, or impurity phases. No additional diffraction contrasts coming from the strain can be found along the interface, indicating excellent bonding between the cores and the shells. The cores with darker contrast have a diameter of 4.5–7.5 nm, while the shells with lighter contrast have typical polyhedron structure with sizes of 15.0–25.0 nm. Detailed analysis indicates that the morphologies are dominantly truncated octahedrons. A schematic diagram for the core@ shell structure is depicted in the inset. The core@shell nature of the product was further confirmed by angle-dependent TEM characterization. As shown in Figs. S1a–d, the Au NPs always locate in the center, irrelevant to the viewing angle, demonstrating that the as-prepared sample is core@shell structured instead of a supported catalyst [14]. The bright field image in Fig. 2b shows another typical structure with distinct hollow space around the interface. The so-called yolk-shell NPs have typical truncated polyhedron structure with size of 16.0–32.0 nm with dark contrast cores of 4.5–7.5 nm in sizes. The interface between the core and the shell looks similar with the core@shell structrue as shown in Fig. 2a except for the hollow area. The cores in Fig. 2a and b have almost the same size for that the Au cores were synthesized at the same condition during the first step. Because the introduce of hollow space in the yolk-shell structure, the yolk-shell NPs are slightly bigger than the core@shell NPs. Truncated octahedrons also domninate the yolk-shell NPs and the schematic diagram is depicted in the inset. The nature that Au is located in the center of the product was further validated by angle-dependent TEM characterization from various viewing angles (Figs. S1e–h). Apart from the interface structures with a core in the center in Fig. 2a and b, another kind of interface structure with several small Au particles attached to the corners of the NiSx polyhedrons is synthesized, as shown in Fig. 3c. The so-called oligomer-like Au-NiSx NPs has typical sizes of 5.0–16.0 nm with attached small Au particles of 2.5–4.5 nm in sizes. The polyhedron NiSx NPs are also illustrated in Fig. 2d for comparison. The schematic diagrams for Fig. 2c and d are also depicted in the insets. The crystal structures of the samples were characterized by XRD, which proved that the as-synthesized products consisted of Au (cubic, JCPDS No. 99-0056) and NiSx. As shown in Fig. 3, two kinds of nickel sulfides, i.e. Ni3S2 (rhombohedral, JCPDS No. 85-1802) and Ni3S4 (cubic, JCPDS No. 76-1813) can be indexed in the products. No additional impurity phase was detected. By using the Scherrer's equation:
system with CHI 760E electrochemical workstation in 0.5 M H2SO4 solution. A glassy carbon electrode (GCE, 3 mm diameter) was used as the working electrode. Typically, the catalyst (5 mg) and carbon black (5 mg) were dispersed into a mixed solvent containing Nafion (100 μl) and ethanol (1 ml) to form a homogeneous ink by vigorously magnetically stirring. Then the homogeneous ink (2.5 μl) of the ink was dropped onto the surface of the GCE, and dried at room temperature. The mass loading of the catalyst is 0.161 mg/cm2. The graphite rod and Ag/AgCl were served as counter and reference electrode, respectively. Polarization curves for HER activities were characterized by linear sweep voltammetry (LSV) with a scan rate of 2 mV/s, 95% iR compensation. Electrochemical impedance spectroscopy was obtained by applying an AC voltage with a 5 mV amplitude in a frequency range from 0.1 to 100,000 Hz. The three-electrode method is also used for stability test. The catalyst was dropped onto carbon fiber papers and dried at room temperature as working electrodes. The catalyst mass loading was 0.2 mg/cm2 for the stability test. 3. Results and discussion In this work, the metal-semiconductor heterostructures composed of Au and NiSx with three specific morphologies, i.e. core@shell, yolkshell, and oligomer-like, were designed and controllably synthesized using seeded growth and in-situ conversion methods combined with Kirkendall effect. “Pure” NiSx NPs were also obtained for comparison. The specific synthesis methods and reaction processes are summarized in Fig. 1. 3.1. Morphology and structure The microstructures of the as-synthesized samples were characterized by AC-TEM. Four kinds of nanostructures, i.e. core@shell, yolkshell, oligomer-like, and “pure” polyhedron morphology were confirmed. For each sample, NPs have similar morphology with almost identical size. The sizes for NPs are mostly in the range of 10.0–20.0 nm. The diffraction contrast of the bright field images reveals well-crystallized nature of the samples. As shown in Fig. 2a, the NPs have typical core@shell structures. It should be noted that the interfaces between the cores and the shells are smooth and clean without
Fig. 1. Diagram for the synthesis processes of Au-NiSx heterostructures. The core@shell structure was formed by seeded growth of NiSx shells on Au NPs, while the yolk-shell structure was formed by seeded growth of Ni NPs on Au NPs followed by in-situ conversion from Au-Ni dumbbells to Au-NiSx yolk-shell NPs. The Au-NiSx oligomer-like structure was formed by seeded growth of Au NPs on NiSx NPs. In the figure, the colors of yellow, purple and green represent Au, Ni and NiSx NPs, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 3
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Fig. 2. Typical bright field TEM images of Au-NiSx heterostructures. The schematic diagram for each image is depicted in the inset. (a) Au@NiSx core@shell NPs; (b) Au-NiSx yolk-shell NPs; (c) Au-NiSx oligomer-like NPs; (d) NiSx NPs.
d = 0.89λ /(β ·cosθB )
(1)
where λ is the X-ray wavelength (0.15406 nm), θB is the Bragg diffraction angle and β is the full width at half maxima of the peak, the typical size for the nanostructures can be obtained. By fitting the peaks at about 31.2° corresponding to Ni3S2 ( −1 1 0) planes and Ni3S4 (3 1 1) planes, the typical sizes for NiSx in the Au-NiSx heterostructures can be calculated. The calculated size for NiSx in the Au@NiSx NPs is ~7.2 nm while those for the Au-NiSx yolk-shell and oligomer-like NPs are 10.8 nm and 6.7 nm, respectively. Compared with the TEM images, it can be found that the calculated typical sizes match with the sizes of the exposed polyhedrons or the thickness of the shells. By fitting the peaks at about 38.2° corresponding to Au (1 1 1) planes, the typical sizes for Au NPs in the Au-NiSx heterostructures can also be calculated. The calculated size for Au in Au@NiSx core@shell NPs is ~5.3 nm while those for Au-NiSx yolk-shell and oligomer-like NPs are 5.6 nm and 3.5 nm, respectively. The calculated sizes for Au particles are slightly smaller than the results from the TEM images, which may be contributed from the coupling broadening of the Au (1 1 1) peak with the Ni3S2 (1 1 1) peak and Ni3S4 (4 0 0) peak. The mole ratios of Ni3S2 to Ni3S4 in the heterostructures can also be estimated according to the equation:
Wx = IA Fig. 3. XRD patterns of NiSx based nanoparticles with different architectures including Au@NiSx core@shell, Au-NiSx yolk-shell, Au-NiSx oligomer-like and NiSx.
⎛ x ⎜KA ⎝
N
∑ (Ii i=A
⎞ K Ai) ⎟ ⎠
(2)
where Wx is the mass ratio of component X, IA is the XRD diffraction x A A KAl , KAl is the value intensity of phase A, KAx is the value of KAl 2 O3 2 O3 2 O3 of the integral strength of the strongest mixed peak of sample A and Al2O3 divided by the integral strength of the strongest mixed peak of 4
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Fig. 4. HRTEM images obtained by AC-TEM of: (a) a Au@NiSx core@shell NP, (b) a Au-NiSx yolk-shell NP, where a hollow space is distinguished near the Au core, (c) a Au-NiSx oligomer-like NP and (d) a NiSx NP.
exposed facets in the Au@NiSx NPs. Fig. 4b shows a typical HRTEM image of the Au-NiSx yolk-shell structure. The Au NP in the core is a single crystal. From the FFT pattern (inset), the shell is single crystallized and can be identified as cubic phase Ni3S4. The resolved lattice fringes with spacing of ~0.283 nm correspond to (3 1 1) planes of Ni3S4. Different from the Au@NiSx core@shell NP, Au and NiSx are not completely contacted with each other in the Au-NiSx yolk-shell NP. Hollow space near the Au core can be easily distinguished in the HRTEM image, as marked in Fig. 4b. By comparing the lattice fringes of Ni3S4 along the interface between Au and Ni3S4 and around the hollow area, no apparent difference can be observed, indicating no significant stress exists along the Au and Ni3S4 interface. Atomic steps can also be observed on the surface occasionally. Fig. 4c shows a typical HRTEM image of Au-NiSx oligomer-like NP. The inset FFT pattern reveals that the big particle is cubic phase Ni3S4. Two single crystal Au NPs are attached at the corners of Ni3S4 NP. The contact areas of the Au-NiSx oligomer-like NPs. interfaces are small compared with those in the AuNiSx core@shell and yolk-shell NPs. In Fig. 4c, the Ni3S2 part in a AuNiSx oligomer-like NP shows typical truncated octahedron morphology with almost perfect facets. Fig. 4d shows a typical NiSx polyhedron NP. FFT analysis indicates that it is cubic phase Ni3S4 nanocrystal with almost perfect facets, like that in Fig. 4c, because that the NiSx parts of Au-NiSx oligomer-like NPs are synthesized in the same conditions with that of “pure” NiSx NPs. The XPS spectra were used to characterize the chemical states of these four kinds of NiSx based NPs. The XPS spectra and corresponding peak deconvolution results are shown in Figs. 5, S3, and Table S2, respectively. Using a Gaussian fitting method, the Ni 2p spectrum was fitted by considering two spin orbit doublets and two shake-up
Al2O3 according to the mass ratio of 1:1. The mole ratio of Ni3S2 to Ni3S4 in Au@NiSx core@shell NPs is about 96:4 while those in Au-NiSx yolk-shell and oligomer-like NPs are about 50:50 and 63:37, respectively. Additionally, EDS measurements confirm the chemical composition of the Au-NiSx heterostructures. Fig. S2 shows the EDS spectra of the Au-NiSx core@shell, yolk-shell, and oligomer-like NPs as well as NiSx NPs, revealing that the products only consist of Au, Ni, and S. The signals of Cu and Si in the spectra come from the TEM specimen holder. The mass ratio for Au, Ni, and S for each sample is also illustrated in Table S2. The results obtained from the XRD analysis match well with that obtained from the EDS spectra. In order to further understand the atomic arrangements in the AuNiSx heterostructures, the as-prepared samples were characterized by high-resolution TEM (HRTEM), as shown in Fig. 4. The heterostructures are found to be well-crystallized. No impurity phases or amorphous layers are observed at atomic scale. The NiSx are single crystals and the Au NPs can be single crystals, icosahedron or decahedron. No matter how the structure of Au NP is, the interface between Au and NiSx is found to be smooth and clean. As shown in Fig. 4a, the diameter of Au core is about 4 nm with a five-fold symmetry, indicating icosahedron or decahedron structure of the particle. From the Fast Fourier Transformation (FFT) pattern of HRTEM as shown in the inset of Fig. 4a, the shell is single crystallized and can be indexed as rhombohedral phase Ni3S2. In the HRTEM image, the resolved lattice fringes with spacing of ~0.288 nm correspond to (−1 1 0) planes of Ni3S2. The lattice fringes penetrate throughout the shell structure, indicating single crystal nature of the shell. It also should be noted that the surface of Ni3S2 is clean but not smooth at atomic scale. Atomic steps can be observed on the surface, as marked by arrows, revealing the existing of high-index 5
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Fig. 5. High resolution XPS spectra for the (a) Ni 2p and (b) Au 4f of the Au@NiSx core@shell, Au-NiSx yolk-shell, Au-NiSx oligomer-like and NiSx NPs. (c) Schematic diagram of electron transfer in a Au@NiSx core@shell NP and a Au-NiSx yolk-shell NP.
satellites. The fitting peaks positioned at 853.05–852.70 and 870.54–869.90 eV are assigned to Ni0, while the peaks at 856.66–855.52 and 874.60–873.56 eV correspond to Ni2+ [31]. It is obvious that the atom ratios of Ni0 in the Au@NiSx core@shell and AuNiSx yolk-shell NPs are higher than those in the Au-NiSx oligomer-like and “pure” NiSx NPs, which means that Ni atoms accept more electrons from the Au cores in the Au@NiSx core@shell and Au-NiSx yolk-shell NPs. The S 2p XPS spectra are shown in Fig. S3. The S2−, S−, and S correspond to the peaks positioned at 161.46–161.04 eV, 162.60–162.19 eV, and 163.39–163.9 eV, respectively. The peaks positioned at 168.97–167.43 eV are assigned to SO42−/SO32−, which may
result from the surface oxidation [13]. As shown in Table S2, S atoms accept more atoms in the Au@NiSx core@shell and Au-NiSx yolk-shell NPs with the combination of Au cores, which is similar to the trend of Ni 2p peaks. For Au 4f XPS spectra of the four samples, it is obvious that there are small peaks beside Au0 4f peaks marked by blue lines, which can be assigned to the peaks of oxidative Au+ located in the direction of high binding energy of metallic Au peaks [32]. The peak fitting results demonstrate that the proportion of oxidative Au+ increases from 6% to about 20%, according to sequence of Au-NiSx oligomer-like, Au-NiSx yolk-shell, and Au@NiSx core@shell NPs. Taking the conclusions obtained from the Ni 2p and S 2p XPS spectra into account, obvious 6
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3.2.2. Au-NiSx yolk-shell NPs For Au-NiSx yolk-shell NPs, the sulfuration agent 1-dodecanethiol was added after the formation of Au-Ni dumbbell NPs. After that, samples were taken at 2, 5, 10, 15, 20, 30, 60, and 180 min. The TEM images to exhibit morphology evolution during the transformation from Au-Ni dumbbell to Au-NiSx yolk-shell NPs are shown in Fig. 7a–g, and the atomic ratios of Au to Ni and S to Ni obtained from the EDS analysis are listed in Fig. 7h and Table S4. Au NPs keep their morphologies and sizes during the transformation, and forming Au cores ultimately. As shown in Fig. 7h, the atomic ratios of Au to Ni are nearly constant during the whole process, meaning that the Ni sources have been used up in the formation of Au-Ni dumbbell NPs, and the atomic ratios of S to Ni increase quickly during sulfuration of Ni atom in the Au-Ni dumbbell NPs, and the morphology changes from dumbbell to yolk-shell at the same time. It is an in-situ transformation process like that exist in the growth of Au@Ni12P5 core@shell NPs [14,33]. Different from the reported in-situ transformation of Au@Ni12P5 core@shell NPs, a hollow space forms around a Au core, which may result from the Kirkendall effect [21,34]. During the sulfuration of Ni with 1-dodecanethiol, the rate at which S atoms diffuse into the interior of Ni NPs is lower than the rate at which Ni atoms diffuse outward, thus leaving a hollow space around the Au core and forming Au-NiSx yolk-shell NPs ultimately. Thus, the formation of Au-NiSx yolk-shell NPs is based on the in-situ transformation method combined with Kirkendall effect.
electron transfer from Au cores to NiSx shells happened in the Au-NiSx yolk-shell and Au@NiSx core@shell NPs. The contact area between the Au core and the NiSx shell in the core@shell NPs are higher than that in the yolk-shell NPs, resulting in the more remarkable electron transfer process, as illustrated in Fig. 5c. Therefore, the electronic structures of heterostructured metal-semiconductor NPs can be well regulated by elaborately design of interface structure, which will determine their electrocatalytic performances to some extent. 3.2. Growth mechanism Understanding the growth process not only is beneficial to the study of the growth mechanism, but also helps the design of interface structure to obtain core@shell or yolk-shell nanostructure with single crystal shells, while it is not easy to achieve it because of the large crystal structure difference between the noble metal cores and semiconductor shells. The time-dependent experiments were conducted combined with TEM and EDS analysis. 3.2.1. Au@NiSx core@shell NPs For the synthesis of Au@NiSx core@shell NPs, the sulfuration agent 1-dodecanethiol was added after the formation of Au NPs. After that, samples were collected at 5, 10, 20, 30, 60, 90, and 120 min. The morphology evolution of the obtained samples is shown in Fig. 6a–g, and the atomic ratios of Au to Ni and Ni to S obtained from the EDS analysis are listed in Fig. 6h and Table S3. During the evolution from Au NPs to Au@NiSx core@shell NPs, the size and morphology of Au NPs do not obviously change, while the shells are gradually thickening, and the truncated octahedral outer shape becomes more distinct as reaction time elapses. From Fig. 6h and Table S3, the atomic ratios of Au to Ni gradually decrease and it do not obviously change after 30 min, which means that the nickel source has been used up and the shell growth has completed at 30 min. It is also confirmed from that the sizes of the samples are similar from 30 to 120 min. It is noted that the atomic ratios of Ni to S are nearly constant from 5 to 120 min, meaning that the Ni and S atoms were attached to the surface of Au cores layer by layer at the same time. The growth process of Au@NiSx core@shell NPs is a typical epitaxial growth of NiSx on the Au seeds.
3.3. Electrochemical properties of Au-modified NiSx NPs To understand the synergistic effect and coupling effect between the Au and NiSx interface, the HER activities of the NiSx based NPs with different nanostructures were investigated in acidic solutions. All four NiSx based NPs behave as effective electrocatalysts towards HER, as shown in Fig. 8 and summarized in Table S5. From the polarization curves in Fig. 8a, the overpotentials at a current density of 10 mA/cm2 of Au@NiSx core@shell NPs (253 mV) and Au-NiSx yolk-shell NPs (263 mV) are obviously lower than that of Au-NiSx oligomer-like NPs (283 mV) and “ pure” NiSx NPs (321 mV), which are comparable to the value of metal-semiconductor heterostructure catalysts such as Pd/ MoSe2 (231 mV), and Au@CoP (160 mV, 1 mA/cm2) [35,36]. With the
Fig. 6. (a–g) TEM images of the intermediate products obtained at different times after adding 1-dodecanethiol during the preparation of Au@NiSx core@shell NPs, demonstrating that NiSx shells grow on the Au seeds, scale bar: 50 nm. (h) Curves of Au: Ni and S: Ni atomic ratios evolution during the formation of Au@NiSx core@ shell NPs. 7
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Fig. 7. (a–g) TEM images of the intermediate product obtained at different times after adding 1-dodecanethiol during the preparation of Au-NiSx yolk-shell NPs, demonstrating the transformation process from Au-Ni dumbbell NPs to Au-NiSx yolk-shell NPs, scale bar: 50 nm. (h) Curves of Au: Ni and S: Ni atomic ratios evolution during the transformation.
stability during the long-time HER catalytic process. All these results demonstrate that the NiSx based NPs with different nanostructures, i.e. Au@NiSx core@shell, Au-NiSx yolk-shell, Au-NiSx oligomer-like, and “pure” NiSx NPs, are all efficient HER catalyst, where the core@shell NPs outperform others. To further analyze the electrical factors affecting the HER activity of the samples and investigate the structure-property relationship, the electrochemical impedance spectra of the four samples were measured at room temperature with frequencies ranging from 100 kHz to 0.1 Hz. Fig. 8c shows that the diameter of semi-circular Nyquist plots of Au@NiSx core@shell, Au-NiSx yolk-shell, Au-NiSx oligomer-like and “pure” NiSx is gradually increasing. The equivalent circuit used to fit the impedance curves is given in the inset of Fig. 8c, consisting of an electrolyte resistance (Rs), a charge-transfer resistance (Rt), and a constant-phase element (CPE). The charge-transfer resistance Rt of the sample corresponds to the circular diameter in the Nyquist plot, representing the resistance of mass transfer during the electrocatalytic reaction, which is the decisive parameter for the HER catalytic efficiency [42]. Using ZSimpWin software (Princeton Applied Research, Oak Ridge, TN, USA), the chargetransfer resistance Rt of the Au@NiSx core-shell, Au-NiSx yolk-shell, AuNiSx oligomer-like, and “pure” NiSx electrodes are calculated to be 49.4 Ω, 62.3 Ω, 110.7 Ω, and 497.1 Ω, respectively. Obviously, the charge-transfer resistances of NPs with the inclusion of Au are lower than that of the “pure” NiSx, which can be attribute to the enhanced electrical conductivity and shortened charge diffusion length resulting
same loading, the Au@NiSx core@shell NPs exhibit an area-specific activity of 10 mA/cm2 at − 0.25 V vs RHE, which is 56%, 303% and 1513% higher than that of Au-NiSx yolk-shell NPs (6.43 mA/cm2), AuNiSx oligomer-like NPs (2.48 mA/cm2) and “pure” NiSx NPs (0.62 mA/ cm2) at −0.25 V vs RHE, respectively. It is obvious that the HER catalytic performance is greatly enhanced by the inclusion of Au cores, especially in the core@shell NPs. As shown in Fig. 8b, the Tafel slopes of Au@NiSx core@shell, Au-NiSx yolk-shell, Au-NiSx oligomer-like, and “pure” NiSx NPs are 43.7 mV/dec, 49.4 mV/dec, 50.6 mV/dec, and 56.1 mV/dec, respectively. The Tafel slopes of the Au-modified NiSx NPs are lower than that of the pure NiSx, and the Au@NiSx core@shell NPs are the lowest, followed by Au-NiSx yolk-shell, Au-NiSx oligomerlike and NiSx NPs. The lower Tafel slopes indicate the superior reaction kinetics and good HER activities of these Au-NiSx heterostructure NPs compared with “pure” NiSx NPs, which are comparable or lower than some heterostructure HER catalysts such as Au@CoP (52 mV/dec, 1 mA/cm2), Pd/NiS@Al2O3 (35 mV/dec), Au@Zn-Fe-C (130 mV/dec), Cu/Cu2O/Cu2S (107 mV/dec), Mo-CoP(65 mV/dec), and nanocomposite NbC (35 mV/dec) [36–41]. To test their stability, the products were dropped onto the carbon fiber paper. The chronoamperometric response for the Au@NiSx core@shell and Au-NiSx yolk-shell NPs were recorded, as shown in Fig. S4. The current densities of the HER activity of Au@NiSx core@shell and Au-NiSx yolk-shell NPs are relatively stable, maintaining 93.6% and 98.9% after 5 h, respectively. It confirmed that the as-prepared Au-NiSx heterostructure NPs have good
Fig. 8. Electrochemical performances of the HER catalysts: (a) polarization curves for the Au@NiSx core@shell, Au-NiSx yolk-shell, Au-NiSx oligomer-like and NiSx NPs on GC electrodes in 0.5 M H2SO4. (b) Tafel curves of the four sample in 0.5 M H2SO4. and (c) Nyquist plots of the four sample and a simulated circuit diagram. 8
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from synergistic effect between Au and NiSx. Among them, the core@ shell NPs have the lowest Rt and exhibit the highest catalytic activity, which may result from the fully contact between Au and NiSx. The synergistic effect between semiconductor and other compound with excellent electrical conductivity were also reported. Lin et al. have reported a dramatic enhancement of photocatalytic activity of the novel photocatalyst of Ag3PO4@MWCNTs@PANI due to the synergetic effect between MWCNTs and PANI on Ag3PO4 [43–45]. Besides, the single thin single crystalline shell also benefits much for good ionic diffusion and electron transport during the catalytic reaction, as demonstrated by the AC-TEM in Fig. 4. Atomic steps on the surfaces of Au@NiSx core@ shell NPs and Au-NiSx yolk-shell NPs generate high-index exposed facets, which also cause better catalytic activity. In contrast, the Au-NiSx oligomer-like NPs and “pure” NiSx NPs show almost perfect facets, without high-index exposed facets. In addition, it is reported that when NiSx are used as the HER catalyst, stable S-Hads bonds, which are difficult to desorb H2 are formed on the surface of the catalyst, which is a key factor to determine the catalytic activity [13]. Here in the Au-NiSx heterostructure NPs, due to the interaction between Au and NiSx NPs, surface NiSx are negatively charged resulting from the electron transfer between Au and NiSx, of which core@shell NPs are strongest, as shown from the XPS spectra in Fig. 5. The negatively charged NiSx weaken the S-Hads bonds formed on the surface, thus optimizing the desorption of H2, promoting the Heyrovsky process of HER and improving the HER activity accordingly [13,30]. Therefore, benefiting from the lower charge-transfer resistance, the single crystal NiSx shells and negatively charged NiSx surface, the Au modified NiSx NPs possess better HER activity than “pure” NiSx, of which the fully contacted core@shell NPs are the best.
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