Electronic Modulation of Electrocatalytically Active Center of Cu7S4

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Electronic Modulation of Electrocatalytically Active Center of Cu7S4 Nanodisks by CobaltDoping for Highly Efficient Oxygen Evolution Reaction Downloaded via UNIV OF TOLEDO on June 29, 2018 at 14:30:39 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Qun Li, Xianfu Wang,* Kai Tang, Mengfan Wang, Chao Wang, and Chenglin Yan* Soochow Institute for Energy and Materials InnovationS, College of Physics, Optoelectronics and Energy, Soochow University, Suzhou 215006, China Jiangsu Provincial Key Laboratory for Advanced Carbon Materials and Wearable Energy Technologies, Soochow University, Suzhou 215006, China Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China S Supporting Information *

ABSTRACT: Cu-based electrocatalysts have seldom been studied for water oxidation because of their inferior activity and poor stability regardless of their low cost and environmentally benign nature. Therefore, exploring an efficient way to improve the activity of Cu-based electrocatalysts is very important for their practical application. Modifying electronic structure of the electrocatalytically active center of electrocatalysts by metal doping to favor the electron transfer between catalyst active sites and electrode is an important approach to optimize hydrogen and oxygen species adsorption energy, thus leading to the enhanced intrinsic electrocatalytic activity. Herein, Co-doped Cu7S4 nanodisks were synthesized and investigated as highly efficient electrocatalyst for oxygen evolution reaction (OER) due to the optimized electronic structure of the active center. Density-functional theory (DFT) calculations reveal that Coengineered Cu7S4 could accelerate electron transfer between Co and Cu sites, thus decrease the energy barriers of intermediates and products during OER, which are crucial for enhanced catalytic properties. As expected, Co-engineered Cu7S4 nanodisks exhibit a low overpotential of 270 mV to achieve current density of 10 mA cm−2 as well as decreased Tafel slope and enhanced turnover frequencies as compared to bare Cu7S4. This discovery not only provides low-cost and efficient Cu-based electrocatalyst by Co doping, but also exhibits an in-depth insight into the mechanism of the enhanced OER properties. KEYWORDS: electronic structure, charge-transfer, metal-doping, energy barriers, oxygen evolution reaction metal-based oxides,10−14 hydroxides,15−18 chalcogenides,19−21 nitrides,22−24 and phosphides.4,25,26 However, their electrocatalytic performances remain below those of the state-of-art water oxidation catalysts. The design of simple and effective strategies to obtain highly active transition-metal-based catalysts will be crucial for the development of electrocatalytic water oxidation. Recent studies have revealed that modifying electronic structure to favor the electron transfer between catalyst active sites and electrode thus avoiding Ohmic potential drop and

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xygen evolution reaction (OER), which generates oxygen through electrochemical oxidation of water, holds a key to water splitting technology, and plays an important role in various renewable energy systems.1−4 However, owing to its complex four-electron redox process, OER with sluggish kinetics often requires a high overpotential (η) and thus water oxidation electrocatalyst is usually needed to promote the reaction rate.5−7 Currently, the state-of-art OER catalysts are noble metal-based materials, such as IrO2 and RuO2,8,9 but their high cost could impose a big barrier on their widespread application in energy-related areas. As a result, the development of highly efficient, low-cost, stable, and environmental friendly electrocatalysts toward OER has been an imperative yet challenging issue. Therefore, tremendous efforts have been devoted to non-noble materials based on transition© 2017 American Chemical Society

Received: August 7, 2017 Accepted: November 27, 2017 Published: November 27, 2017 12230

DOI: 10.1021/acsnano.7b05606 ACS Nano 2017, 11, 12230−12239

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Cite This: ACS Nano 2017, 11, 12230−12239

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Figure 1. (a) XRD patterns of the as-synthesized catalysts. SEM image of the catalysts: (b) Cu7S4 and (c) Co−Cu7S4-0.07, inserts show a single nanodisk. (c) HRTEM image of the as-prepared Co−Cu7S4-0.07. (d) HAADF-STEM image and (e−g) EDX elemental mapping images of Co, S, and Cu for the Co−Cu7S4-0.07.

Cu2‑xS@Ru nanoplates, have been investigated as efficient electocatalysts for hydrogen evolution reaction, and their good electrocatalytic properties were attributed to the abundant MoS2 active edge sites and the heterointerfaces between metallic Ru/Ru oxide and copper sulfide.37,38 Nevertheless, the electrocatalytic activities of copper sulfide still gained little attention. Herein, for optimizing the electronic structure, Codoped Cu7S4 nanodisks (denoted as Co−Cu7S4) were synthesized and applied as the electrocatalyst for water oxidation. The Co-engineered Cu7S4 catalyst with modified electronic structure of the active sites shows enhanced electron transfer between Co and Cu sites, and it can improve the electrocatalytic kinetics. As expected, Co-engineered Cu7S4 nanodisks exhibit a low overpotential of 270 mV to achieve current density of 10 mA cm−2 as well as decreased Tafel slope and enhanced turnover frequencies as compared to bare Cu7S4. This discovery not only provides low-cost and efficient Cubased electrocatalyst, but also expands the scope of nonprecious metal elements for water oxidation.

consequently energy loss is a key point that influences the activity of OER catalysts, which are strongly correlated with the electronic configuration of the electrocatalyst.27−30 Highervalence ion substituting was shown to be a promising method to optimize the electronic structure of the electrocatalysts to improve the kinetics of electrocatalysis. For instance, CoO showed negative shift of conduction band edge by ∼0.8 eV when 50% of Co positions are replaced by Mn.31 The bandgap narrowing leads to the enhanced excitation of charge carriers to the conduction band, which is beneficial for the electrical conductivity increase and electrochemical properties. N-doped Ni3S2 exhibited stronger density of states (DOS) near the Fermi level than that of pristine Ni3S2, indicating its more metallic behavior and higher electronic conductivity.28 The higher electrical conductivity will be favor of the improvement of the electron transfer capacity, which could lower the energy barriers of the intermediates and products, thus enhancing the activity of electrocatalysts. For example, Co-doped iron pyrite (FeS2) electrocatalysts could lower the kinetic energy barrier by promoting H−H bond formation,32 and V-doped FeOOH also decreased the binding energy of the oxygen species on both Fe and V sites during the OER.33 As the second cheapest metal among the first-row transition metals, such as manganese, cobalt, nickel, and iron, copper provides wide prospects for practical catalysis but it has seldom been studied for water oxidation because of the severe anodic corrosion.34 To solve this problem, copper oxide (CuO) has been reported to activate OER due to its rich redox properties that enable rich redox reactions.35 However, the reported CuO catalysts still suffered from inferior electrocatalytic activity. For example, the obtained CuO nanowires required large overpotential of 430 mV to achieve very low current density of 0.1 mA cm−2.36 Therefore, seeking novel Cu-based electrocatalysts and exploring efficient ways to improve the activity are very urgent for their practical application. Recently, Copper sulfide based composites, including Cu7S4@MoS2 nanoframes and

RESULTS AND DISCUSSION Cu7S4 nanodisks were prepared by the fast thermolysis of copper thiocyanate according to a method reported by Yoon et al.,39 and Co-doped Cu7S4 (namely Co−Cu7S4-x, x is the atomic ratio of Co/(Co+Cu), shown in Table S1) was obtained by adding cobalt acetylacetonate in the mather solution. X-ray diffraction (XRD) patterns of all the samples in Figure 1a can be assigned to roxbyite Cu7S4 (PDF no.23−958, system: monoclinic, space group: C2/m), which rules out the possibility of forming cobalt sulfide compounds on the surface. Scanning electron microscopy (SEM) image of the obtained Cu7S4 is shown in Figure 1b, where uniform hexagonal nanoplates are clearly seen. The insert in Figure 1b shows a side length of about 60 nm with transmission electron microscopy (TEM). The plate-like structure keeps well after little Co-incorporation, for example, Co−Cu7S4-0.035 and Co−Cu7S4-0.07 show the 12231


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Figure 2. Crystal structure of (a) Cu7S4 and (b) Co-doped Cu7S4 (Cu atoms were randomly replaced with Co atoms). The partial density of state (pDOS) for (c) Cu7S4 and (d) Cu7S4 nanosheets, the Fermi level is defined as zero. (e) Comparison of temperature-dependent resistance of (Co-doped) Cu7S4.

Figure 3. (a) XPS survey spectrum of as-synthesized Cu7S4 and Co−Cu7S4-0.07. High-resolution XPS spectra of (b) Co 2p, (c) Cu 2p, and (d) S 2p region. The binding energy shifts of (e) Cu 2p, and (f) S 2p (x is the atomic ratio of Co/(Co+Cu)).

similar hexagonal structure (Figures S1, 1c, and insert). Highresolution TEM (HRTEM) image of Co−Cu7S4-0.07 in Figure 1d displays the lattice fringe with an interplanar distance of around 3.3 Å, which are consistent well with the spacing of the

(16 0 0) planes of the Cu7S4, indicating the crystal structure remained well after Co-doping. The elements distribution investiaged by the high-scanning TEM energy dispersive X-ray spectroscopic (STEM-EDX) elemental mappings was shown in 12232


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Figure 4. OER performance of Cu7S4 and Co−Cu7S4 with different Co-doping amounts in 1.0 M KOH. (a) Polarization curves, (b) overpotential required for J = 10 mA cm−2, (c) current densities at η = 350 mV, (d) Tafel plots, and (e) long-term durable operation with a constant current density of 20 mA cm−2 for Co−Cu7S4 and 10 mA cm−2 for Cu7S4.

Besides, the DOS of Co−Cu7S4 near the Fermi level is stronger than that of pristine Cu7S4 because the Co atoms clearly contribute many states though DOS of Cu is reduced after Codoping, which indicates that the electrical behavior of Cu7S4, especially the concentration of charge carrier and the electronic conductivity, can be effectively enhanced by the introduction of Co atoms. To confirm this theoretical prediction, temperaturedependent resistance tests were performed. As shown in Figure 2e, the resistance gradually increased with increasing temperature, implying the metallic behavior of (Co-doped) Cu7S4. Moreover, Co-engineered Cu7S4 exhibits lower resistance than that of bare Cu7S4, in agreement with the theoretical studies. In addition, visible-NIR optical absorbance of the sample was measured (Figure S7). Compared with pure Cu7S4, Co−Cu7S40.07 exhibited increased absorbance intensity in the NIR region that was associated with free carrier intraband absorption.41−43 The enhanced NIR localized surface plasmon resonance (LSPR) reveals the improved free carrier concentration after moderate Co-doping. In this case, Co−Cu7S4 nanodisks allow much faster electron transfer and prove more conducting paths to endow them with faster kinetics and better OER catalytic activities.2,44 X-ray photoelectron spectroscopy (XPS) is further performed to analyze the surface chemical states and electrontransfer of the as-synthesized Co-doped Cu7S4 electrocatalyst along with that of bare Cu7S4. The overall survey spectrum of Co−Cu7S4-0.07 (Figure 3a) shows obvious additional peaks at around 61.2, 781, and 798 eV corresponding the Co signal

Figure 1d−g. The homogeneously distributed Cu, S, and Co suggests that Co has been uniformly incorporated into Cu7S4 nanodisks. However, the structure collapse and other new particles (cubic Co3O4 phase identified using TEM, HRTEM, and EDX mapping in Figures S3−S5) could be found for highly Co-doped sample (Co−Cu7S4-0.14). The occurred structure transition and generated side products may cause decreased electrical conductivity and consequently affecting OER property.29,40 The Co doping effect on the electronic structure of Cu7S4 was first illustrated by density-functional theory (DFT) calculations (where, as a model example, 7.7% of Cu positions are replaced by Co). As known, the high density of states (DOS) contributes to fast electron transport and admirable catalytic performance. Therefore, DOS was calculated to explore the electronic structure changes of Cu7S4 after Coincorporation. Based on this regard, Cu7S4 and Co−Cu7S4 were first subjected to geometry optimization. Figure S6 displays the XRD patterns of Cu7S4 crystal, in which XRD pattern of the optimized Cu7S4 keeps well with that of the sample from experiment, demonstrating the reasonable optimization of Cu7S4 crystal structure model. As shown in Figure 2a and b, the optimized Co−Cu7S4 shows total energy of −741.81 eV, which is much lower than that of the optimized Cu7S4, revealing the more stable structure of Co−Cu7S4. As depicted by the results of DFT calculations (Figure 2c and d), both the calculated DOS of Cu7S4 and Co−Cu7S4 are continuous around the Fermi level, suggesting their intrinsic metallic characters. 12233


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Figure 5. (a) CV curves at scan rates from 10 to 50 mV s−1 of the Co−Cu7S4-0.07 catalyst. (b) Capacitive J versus scan rate for the as-prepared electrocatalysts. The linear slope, equivalent to twice of the double-layer capacitance Cdl, was used to represent the ECSA. (c) Nyquist plots and (d) TOF calculations at an overpotential of 350 mV of the as-prepared electrocatalysts.

when compared with that of pure Cu7S4 phase, which indicates the successful incorporation of Co. Figure 3b shows the high resolution Co 2p spectra of Co−Cu7S4-0.07, and Co 2p spectrum presents two main structures arising from Co 2p3/2 (776−791 eV) and Co 2p1/2 (791−811 eV). Co 2p3/2 peaks at 780.8 and 782.6 eV can be attributed to Co3+ and Co2+ species,45,46 and the peak at 778.1 eV is associated with the binding energy of Co−S bond,47 further confirming part of Cu has been substituted with Co atoms. The deconvoluted Cu 2p profile clearly shows the strong Cu 2p3/2 and 2p1/2 peaks at 931.76 and 951.62 eV, respectively, as well as their satellite peaks on bare Cu7S4 (Figure 3c). After Co-doping, Cu 2p peaks are obviously shifted to higher binding energies, suggesting that Cu 2p presents lower electron density along with Coincorporation, which agree with the DFT calculation results in Figure 2c and d. Meanwhile, the positive shifts of Cu 2p in Co−Cu7S4-0.07 indicate improved electron transfer ability in Co-doped Cu7S4 nanodisks.48,49 With respect to S 2p core level spectra, Co−Cu7S4-0.07 exhibits binding energies of 161.1, 162.2, and 163.26 eV for bridging S2− (Figure 3d), which are negatively shifted ∼0.24, 0.25, and 0.7 eV when compared with pristine Cu7S4. The negative shift of S 2p3/2 binding energy reveals the increased electron occupation, resulting in enhanced electron-donating ability. The peaks at 168.25 and 169.5 eV are associated with residual SO42− species,50 and its high intensity in Co−Cu7S4-0.07 suggests the oxidation of surface sulfur. Figures S8 and S9 also show the XPS spectra of Cu 2p and S 2p in Co−Cu7S4-0.035 and Co−Cu7S4-0.14 samples, and the energy shifts are concluded in Figure 3e and f. Clearly, Co− Cu7S4-0.07 has the maximum energy shifts, with the highest partial positive (Cu) and negative (S) charge. In brief, the positive shifts of Cu 2p and negative shifts of S 2p3/2 will lead to increased electric dipole and more electron transfer from Cu to S, thus reduce the energy barrier of the electrocatalytic process because electrons diffuse from metallic centers Cu to S, which is beneficial to the adsorption and desorption process between reactant and resultant molecules.51

To evaluate the electrocatalytic performence toward OER, the Co-doped Cu7S4 nanodisks were tested in 1.0 M KOH solution using a standard three-electrode configuration. For comparison, the pristine Cu7S4 and commercial IrO2 with the same mass loading were also tested. Figure 4a displays their polarization curves without iR correction. From Figure 4b, it is obvious that Co−Cu7S4-0.07 exhibits the best OER activity with a low overpotential of 270 mV to achieve a current density of 10 mA cm−2 among the tested samples. Whereas, Co− Cu7S4-0.035, Co−Cu7S4-0.14, bare Cu7S4, and IrO2 exhibit relatively poorer OER properties with overpotenrials of 320, 336, 440, and 510 eV to reach a current density of 10 mA cm−2, respectively. The higher OER activity of Co−Cu7S4-0.07 is also evident from the larger current density at fixed overpotential. As displayed in Figure 4c, Co−Cu7S4-0.07 electrode exhibits a large electrocatalytic current of 30.8 mA cm −2 at an overpotential of 0.35 V, which is about 1.75, 2.22, 11.3, and 20.2 times higher than those of Co−Cu7S4-0.035, Co−Cu7S40.14, bare Cu7S4, and IrO2 electrodes, respectively, indicating the efficient OER behavior of the Cu7S4 nanodisks with a moderate Co-doping, which are also better than some Cu and Co based catalysts (Table S3). Meanwhile, Co−Cu7S4-0.14 shows much lower OER activity with respect to Co−Cu7S40.07, which can be attributed to the reduced charge transfer along with Co-doping amount as confirmed by the decreased binding energy shifts from the XPS results, as well as the crystal structure transformation and impurity at high Co-doping content. Tafel slopes of the as-prepared samples were shown in Figure 4d. As known, a smaller Tafel slope is more favorable for practical applications, as a greatly increased OER rate will be achieved with a low increase in overpotential. The bare Cu7S4 nanodisks show a high Tafel slope of 179 mV dec−1, which was mainly due to the poor electron transfer capability. In addition, the value for Co-doped Cu7S4 is about 130 mV dec−1, indicating the relatively rapid OER kinetics of Co-doped Cu7S4 nanodisks, which is consistent with their increased electron 12234


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Figure 6. (a) Adsorption geometries of the intermediates. (b) Gibbs free energy landscape. (c) The schematic illustrating the feasible OER mechanism for the Co-engineered Cu7S4.

much faster electron transfer process during OER. To further study the inherent activity on per-catalytic site toward OER, the turnover frequencies (TOFs) at an overpotential of 0.35 V were estimated assuming that all the metal ions in the materials were catalytically active.53 As displayed in Figure 5d, Co−Cu7S4-0.07 nanodisk exhibits the highest TOF. Specifically, the TOF of Co−Cu7S4-0.07 electrode is 0.08 s−1 at η = 0.35 V, which is 1.78, 2.22, and 11.4 time higher than those of Co−Cu7S4-0.035, Co−Cu7S4-0.14, and pristine Cu7S4 electrodes, respectively. Overall, the fast OER kinetics and excellent electrocatalytic performances have been achieved at our well-designed Coengineered Cu7S4 electrodes. DFT calculation was further performed to understand the underlying mechansim. In alkaline media, the OER is supposed to involve five reaction steps (Figure S15) and each step involves electron transfer accompanied by proton expulsion.54 Figure 6a shows the crystal structure model after being optimized and the corresponding OER interpretation on Cu7S4 and Co-doped Cu7S4. The Cu7S4 (0 16 0) surface with five layers was selected for the OER steps. Figure 6b displays profiles of the free energy changes for the steps of OER under the catalytic roles of Cu7S4 and Co-doped Cu7S4. Obviously, Co-doped Cu7S4 could achieve lower energy levels on both Cu and Co sites than bare Cu7S4. Therefore, as depicted in Figure 6c, the Co-engineered Cu7S4 with modified electron structure will accelerate electron transfer between the active sites, which, therefore, could lower the energy barriers, improve the intrinsic activity of the active sites, and improve the electrocatalytic performance. The catalysts after OER tests were fully characterized, including the morphology, crystal structure, and chemical composition. TEM images of Cu7S4 and Co−Cu7S4-0.07 samples after OER reveal the well-kept plate-like structure (Figures S16 and 7a). However, it is interesting that the phase

transfer between the active sites. As shown in Figure 4e, Co− Cu7S4-0.07 catalyst exhibits the highest stability among the samples with negligible change in potential after 24 h of OER reaction at current density of 20 mA cm−2, while pure Cu7S4 shows the lowest stability even at current density of 10 mA cm−2, further demonstrating the higher electrocatalytic activity of Co-engineered Cu7S4. It is supposed that the electrochemical surface area (ECSA) of the catalyst, generally evaluated by the double layer capacitance (Cdl) as it is presumed to be linearly proportional to the Cdl of the catalyst, is directly proportional to the amount of active sites for OER.52 Therefore, cyclic voltammetry (CV) curves under different scan rates in 1.0 M KOH (Figures 5a and S10−S12) were performed to evaluate the enhanced OER activity of Co-engineered Cu7S4 electrodes. Figure 5b clearly shows that the Cdl of Co-doped Cu7S4 is much higher than that of the bare Cu7S4 electrode (except Co−Cu7S4-0.14), and Co− Cu7S4-0.07 catalyst exhibits the highest value (176.2 mF cm−2) among the samples, demonstrating that the active sites of Codoped Cu7S4 electrodes for OER can be well modified by Codoping and Co−Cu7S4-0.07 possesses the highest active sites, which is consistent well with the LSV results. The increased ECSA is attributed to the incorporation of Co, which may cause surface roughening and consequently lead to the significant improvement of active surface. N2 adsorption−desorption isotherms shown in Figure S13 displays the highest BET surface of 19.65 m2 g−1 of the Co−Cu7S4-0.07 sample, revealing its highest electrochemical active surface that agrees well with the ECSA. It is more evident for the fast OER kinetics of the Co-engineered Cu7S4 electrodes by electrochemical impedance spectroscopy (EIS) measured at a potential of 1.58 V vs. RHE. The EIS dates (Figures 5c and S14) display much smaller charge transfer resistance (Rct) of Co-doped Cu7S4 electrodes and Co−Cu7S4-0.07 catalyst exhibits the smallest Rct, indicating 12235


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Figure 7. (a) TEM, (b) HRTEM, and (c) SAED images of the Co−Cu7S4-0.07 sample after OER test. (d) Cu 2p XPS spectrum in Cu7S4 after OER. (e) Cu 2p and (f) Co 2p XPS spectrum in Co−Cu7S4-0.07 sample after OER. (g) Co3+/Co2+ ratios in Co-doped Cu7S4 before and after OER. (h) Energy level diagram for Cu7S4 and Co−Cu7S4-0.07. (i) Schematic illustration of formation of uncoordinated Cu (δ+) and Co (δ+) centers during the OER and the plausible reaction mechanism.

The electronic modulation of the electrocatalytically active center of the catalysts also contributed greatly to the high electrocatalytic activity. Concentration of SO42− species on the surface of the samples (calculated from the XPS results, Figure S20) increased slightly after OER (Table S2), indicating the weak oxidation occurred on the surface during the OER. Ultraviolet photoelectron spectroscopy (UPS) was also used to determine the values of energy level of the pure Cu7S4 and Co−Cu7S4-0.07, which are calculated to be 4.48 and 4.26 eV (Figures 7h and S21−22), by subtracting the width of the He I UPS spectra at excitation energy of 21.22 eV. It is interesting that the energy levels of the samples after OER were decreased to 3.59 and 0.99 eV for pure Cu7S4 and Co−Cu7S4-0.07 (Figure 7h and S23−24), respectively, indicating Co-induced phase transition of Cu7S4 could greatly reduce the energy required for the electrons to get away from the surface, thus lowering the barrier of OER processes. As depicted in Figure 7i, to produce one O2 molecule, four electrons will be generated. A fast OER kinetics could be realized only if the electrons could be easily transferred away. While the Co-induced low energy level exactly supplied the possibility of fast electron transfer, which would significantly improve the OER activity.

of Co−Cu7S4-0.07 sample after OER was changed from monoclinic to hexagonal system, as confirmed from the HRTEM image (Figure 7b) and SAED pattern (Figure 7c). The lattice fringe displays an interplanar spacing of 0.19 nm, which matches well with the (110) lattice plane of covellite CuS. Nevertheless, the HRTEM image of Cu7S4 (Figure S17a) shows two different domains of monoclinic Cu7S4 and hexagonal CuS, which can be further confirmed form the SAED pattern with two distinguished diffraction spots (Figure S17b). XPS measurement was further carried out to identify the phase transition during the OER process. Figure 7d displays the Cu 2p XPS spectra in Cu7S4 after OER, in which the Cu+/Cu2+ ratio decreased compared with that before OER (Figure 3c), revealing Cu+ in Cu7S4 has been partly oxidized during OER. As for the Co−Cu7S4-0.07, Cu 2p peaks showed obviously positive shift, and all the peaks at 933.25, 934.89, 953.14, and 954.85 can be ascribed to Cu2+ signal. The results indicate that Co-engineered Cu7S4 rendered more severe phase transition during OER, which is consistent well with the HRTEM and SAED results. To uncover the reason behind the Co-induced phase transition, Co 2p XPS spectra of the samples after OER were analyzed (Figures 7f, S18, and S19). As revealed, the Co3+/Co2+ ratios of the Co-doped Cu7S4 after OER were decreased from 1.56 to 0.83, 0.66, and 0.63, respectively, along with the increase of Co-doping (Figure 7g). Thus, it can be concluded that high valence Co3+ existing in Cu7S4 would accelerate the phase transition rate from monoclinic Cu7S4 to hexagonal CuS during the OER process. Compared with the Cu and Co 2p XPS spectra of Co−Cu7S4-0.07 before and after OER test, one can clearly find that the Cu 2p peaks shift positively while Co 2p peaks shift negatively after OER, indicating electron transferred from Cu sites to Co sites during OER, that cause the oxidation of Cu+ and reduction of Co3+.

CONCLUSIONS In summary, Co-engineered Cu7S4 nanodisks were developed as highly efficient electrocatalysts for OER in alkaline solutions with a low overpotential of 270 mV to achieve current density of 10 mA cm−2 as well as decreased Tafel slope and enhanced turnover frequencies compared to bare Cu7S4. The excellent electrocatalytic performance of Co-doped Cu7S4 electrodes can be mainly attributed to the following aspects: (1) The incorporation of Co in Cu7S4 can enhance the intrinsic activity of active sites by electronic modulation of the electrocatalytically active center that causes enhanced electron transfer 12236


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For standardization, all the tested potentials were converted to the hydrogen electrode (RHE) based on the equation of E(RHE) = E(Ag/ AgCl) + 0.059 × pH + E0(Ag/AgCl), where E(Ag/AgCl) is the potential against Ag/AgCl measured in the experiment, and E0(Ag/ AgCl) is the standard potential (0.196 V) of Ag/AgCl at room temperature. The catalysts were first activated by a CVs scan with a scan rate of 5 mV s−1 until reaching a stable state. LSV curves were obtained at a scan rate of 5 mV s−1. Tafel slopes were obtained from the LSV curves by plotting the potential against log (current density). Chronopotentiometry measurements were carried out at a current density of 10 and 20 mA cm−2 to evaluate the long-term stability of the samples. The ECSA was represented by the double-layer capacitance (Cdl) obtained from CVs at the potential of 0.2−0.3 V versus Ag/AgCl with the scan rates of 10, 20, 30, 40, and 50 mV s−1. The Cdl values could be estimated by plotting the ΔJ = (Ja − Jc) versus Ag/AgCl against the scan rate. The linear slope is twice of the Cdl. The EIS was measured using AC impedance spectroscopy over a frequence range of 0.01 Hz−10000 Hz. The TOF was calculated according to the equation of TOF = (j × A)/(4 × F × m). Where the j is the current density at the overpotential of 350 mV, A is the surface area of the electrode (1 cm−2), F is the Faraday constant, and m is the number of moles of the active materials that deposited onto the carbon paper. DFT Calculation. The simulations were performed by spinunrestricted density functional therory as implemented by Vienna Ab initio Simulation Package.55 The electron exchange-correlation interaction functional was generalized gradient approximation in Perdew−Burke−Ernzerhof (PBE) functional.56 The projector augmented wave was applied to describe the electrion−ion interaction and the planewave energy cutoff was 350 eV. For geometric optimization, the atomic positions were allowed to relax untial the total energy and force are less than 10−4 eV and −0.02 eV Å−1. Electron smearing of σ = 0.2 eV was used following the Methfessel−Paxton scheme.57 Brillouin zone sampling was employed using a Methfessel−Paxton grid with 7 × 7 × 7. And 11 × 11 × 11 K-point grid was used to calculate the DOS. The free energies of the intermediates was calculated at 298.15 K, and (0 16 0) surface with a vacuum layer of 20 Å was selected. Adsorption energy of intermediates (A = O, OH, OOH, or O2 group) on substrate followed the approach of Nøeskov et al.:58

between Co and Cu sites, thus lowering the energy barriers of intermediates and products. (2) The optimized electronic structure of Co-engineered Cu7S4 nanodisks exhibits higher density of states around the Fermi level, which allows much faster electron transfer thus endow them with faster OER kinetics. (3) Co-induced phase transition could also reduce the energy required for the electrons to get away, thus improving the kinetics during OER. Therefore, this work not only develops a facile route to better the electrocatalytic activity of Cu-based catalysts, but also provides an in-depth insight into the mechanism of enhanced OER properties.

EXPERIMENTAL SECTION Materials. Copper(I) thiocyanate (CuSCN, 99%), cobalt(III) acetylacetonate [95%, Ni(acac)2·xH2O], oleylamine [OLA, tech. 70%, C18H37N], and nafion (5 wt %) were purchased from AlfaAesar. Potassium hydroxide (KOH) was purchased from Aladdin Ltd. (Shanghai, China). All reagents were used as received without further purification. Synthesis of Cu7S4 Nanodisks. Cuprous thiocyanate (CuSCN, 0.6 g) and 100 mL oleylamine (OLA) were added into a three-necked flask. The flask was kept at 140 °C for 30 min with vigorous stirring after being aerated argon for 30 min. Then the system was heated at 240 °C and reacted for 30 min. Afterward, the reacted solution was rapidly cooled down to room temperature in a water bath. The obtained product was centrifuged with ethanol, and collected after freeze-drying. Synthesis of Co−Cu7S4-x (x is the Atomic Ratio of Co/(Co +Cu)). CuSCN (0.193 g) and 0.0407 g cobalt(III) acetylacetonate (molar ratio = 1:0.1) were dissolved into 100 mL OLA. The mixture was then added into a three-necked flask. The flask was heated at 140 °C and sustained for 30 min with vigorous stirring after being aerated argon for 30 min. Then the system was heated at 240 °C and reacted for 30 min. Then, the reacted solution was rapidly cooled down to room temperature in a water bath. The obtained product was centrifuged with ethanol, collected after freeze-drying. The as-obtained product was denoted as Co−Cu7S4-0.07. Other Co−Cu7S4 with different Co-doping amounts were also obtained by adjudting the amounts of CuSCN and cobalt(III) acetylacetonate with molar ratio of 1:0.05 and 1:0.2, which were named as Co−Cu7S4-0.035 and Co− Cu7S4-0.14, respectively. Materials Characterization. The as-obtained catalysts were characterized by X-ray diffraction pattern (XRD, D8 Advance, Bruker) with a Cu target (Kα, λ= 0.15406 nm). The morphology and structures of the samples were explored using TEM and HRTEM (Titan Themis Cubed G2 300). The chemical compositions of the prepared catalysts were examined using X-ray photoelectron spectrometer (Escalab 250Xi, Thermo Fisher). The binding energy was calibrated to the C 1s peak of 284.5 eV. Temperature-dependent resistance of (Co-doped) Cu7S4 samples was measured on a probe station with a heater. The distance between two probes contacted with (Co-doped) Cu7S4 film was about 1 mm. (Co-doped) Cu7S4 film was first prepared by pressing the powder into a sheet. The current− voltage curves were collected using an electrochemical working station to obtain the resistance of the film at different temperatures. The metal content was determined by ICP mass spectroscopy using the PerkinElmer Optima 8300 ICP-OES instrument. Electrochemical Measurements. To prepare the working electrode, 8 mg active material and 2 mg acetylene black were dispersed into 350 μL ethanol and 95 μL 5 wt % Nafion solution. Then 35 μL ink was dropped on the carbon paper of 1 cm2 after ultrasonic for 30 min and dried for 15 min in air. Electrochemical performances, including cyclic voltammetry (CV), LSV, EIS, and chronopotentiometry, were performed using a three-electrode electrochemical system. In the system, catalysts coated in carbon papers were directly used as the working electrode, Ag/AgCl (saturated with 4 M KCl) and platinum wire were used as the reference and counter electrodes, respectively. 1 M KOH solution was used as electrolyte.

ΔE O = E(sub/O) − E(sub) − [E(H 2O) − E(H 2)] * ΔE OH = E(sub/OH) − E(sub) − [E(H 2O) − E(H 2)/2] * ΔE OOH = E(sub/OOH) − E(sub) * − [2 × E(H 2O) − 3 × E(H 2)/2]

ΔE O2 = E(sub/O2 ) − E(sub) − [2 × E(H 2O) − 2 × E(H 2)] * where E(sub/O), E(sub/OH), and E(sub/OOH) denote the total energies of O, OH, and OOH groups on substrate, respectively; E(sub), E(H2O), and E(H2) are the total energies of bare substrate, water, and hydrogen gas, respectively. Gibbs free energy change (ΔG) of chemical reaction was calculated by ΔG = ΔE + ΔZPE − T ΔS where E, ZPE, T, and S denote the calculated total energy, zero point energy, temperature, and entropy, ΔZPE is usually very small, hence neglected here.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b05606. SEM images of Co−Cu7S4-0.035 and Co−Cu7S4-0.14 samples; TEM, HRTEM, and EDX mapping of Co− Cu7S4-0.14; XRD patterns of the as-synthesized Cu7S4 nanodisks and optimized Cu7S4 crystal structure model; 12237


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absorption spectra of Cu7S4 and Co−Cu7S4-0.07 nanodisks; XPS spectra of Cu 2p and S 2p in Co−Cu7S4-0.035 and Co−Cu7S4-0.14; CV curves at different scan rates of the Cu7S4 samples; N2 adsorption−desorption isotherms of the as-prepared samples; Nyquist plots of the Co− Cu7S4-0.07 catalyst; TEM, HRTEM, and SAED pattern TEM image of the Cu7S4 after OER; Co 2p and S 2p XPS spectra of the samples after OER test; and UPS spectrum of the samples before and after OER (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Qun Li: 0000-0001-9121-0297 Xianfu Wang: 0000-0002-2066-7473 Kai Tang: 0000-0002-3462-2057 Mengfan Wang: 0000-0003-3370-6395 Chao Wang: 0000-0002-1486-0525 Chenglin Yan: 0000-0003-4467-9441 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21603157, 51402202), Natural Science Foundation of Jiangsu Province (no. BK20150311), College Natural Science Foundation of Jiangsu Province (no. 16KJB430025), and Postdoctoral Science Foundation of China (2016T90488, 2015M580459). We also thank the support of Suzhou Key Laboratory for Advanced Carbon Materials and Wearable Energy Technologies. REFERENCES (1) Bashyam, R.; Zelenay, P. A Class of Non-Precious Metal Composite Catalysts for Fuel Cells. Nature 2006, 443, 63−66. (2) Xu, K.; Chen, P.; Li, X.; Tong, Y.; Ding, H.; Wu, X.; Chu, W.; Peng, Z.; Wu, C.; Xie, Y. Metallic Nickel Nitride Nanosheets Realizing Enhanced Electrochemical Water Oxidation. J. Am. Chem. Soc. 2015, 137, 4119−4125. (3) Guo, C.; Zheng, Y.; Ran, J.; Xie, F.; Jaroniec, M.; Qiao, S.-Z. Engineering High-Energy Interfacial Structures for High-Performance Oxygen-Involving Electrocatalysis. Angew. Chem., Int. Ed. 2017, 56, 8539−8543. (4) Zhang, Y.; Ouyang, B.; Xu, J.; Jia, G.; Chen, S.; Rawat, R. S.; Fan, H. J. Rapid Synthesis of Cobalt Nitride Nanowires: Highly Efficient and Low-Cost Catalysts for Oxygen Evolution. Angew. Chem., Int. Ed. 2016, 55, 8670−8674. (5) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383− 1385. (6) Mirzakulova, E.; Khatmullin, R.; Walpita, J.; Corrigan, T.; VargasBarbosa, N. M.; Vyas, S.; Oottikkal, S.; Manzer, S. F.; Hadad, C. M.; Glusac, K. D. Electrode-Assisted Catalytic Water Oxidation by a Flavin Derivative. Nat. Chem. 2012, 4, 794−801. (7) Meng, F.; Zhong, H.; Bao, D.; Yan, J.; Zhang, X. In Situ Coupling of Strung Co4N and Intertwined N-C Fibers toward Free-Standing Bifunctional Cathode for Robust, Efficient, and Flexible Zn AirBatteries. J. Am. Chem. Soc. 2016, 138, 10226−10231. (8) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for 12238


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