Ultrathin Black Phosphorus-on-Nitrogen Doped Graphene for Efficient

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Ultrathin Black Phosphorus-on-Nitrogen Doped Graphene for Efficient Overall Water Splitting: Dual Modulation Roles of Directional Interfacial Charge Transfer Zhongke Yuan, Jing Li, Meijia Yang, Zhengsong Fang, Junhua Jian, Dingshan Yu, Xudong Chen, and Liming Dai J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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Journal of the American Chemical Society

Ultrathin Black Phosphorus-on-Nitrogen Doped Graphene for Efficient Overall Water Splitting: Dual Modulation Roles of Directional Interfacial Charge Transfer Zhongke Yuan,1‡ Jing Li,1‡ Meijia Yang,1 Zhengsong Fang,1 Junhua Jian,1 Dingshan Yu,1* Xudong Chen,1* and Liming Dai2* Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Key Laboratory of High Performance Polymer-based Composites of Guangdong Province, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China 2 Department of Macromolecule Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, USA 1

KEYWORDS: black phosphorus, N-doped graphene, heterostructures, water splitting, interfacial charge transfer

ABSTRACT: Few-layered exfoliated black phosphorus (EBP) has attracted surging interest for electronics, optoelectronics and catalysis. Compared to excellent progress in electronic and optoelectronic applications, very few reports are available for electrocatalysis by metal-free EBPs. Herein, we couple solution-processable ultrathin EBP nanosheets with higher Fermi level of N-doped graphene (NG) into a new metal-free 2D/2D heterostructure (EBP@NG) with well-designed interfaces and unique electronic configuration, as efficient and durable bifunctional catalysts towards hydrogen evolution and oxygen evolution reactions (HER/OER) for overall water splitting in alkaline media. By rational interface engineering, the synergy of EBP and NG is fully exploited, which not only improves the stability of EBP, but also effectively modulates electronic structures of each component to boost their intrinsic activities. Specifically, due to the lower Fermi level of EBP relative to NG, their electronic interaction induces directional interfacial electron transfer, which not only enriches the electron density over EBP and optimizes H adsorption/desorption to promote HER, but also introduces abundant positively charged carbon sites on NG and provides favorable formation of key OER intermediates (OOH*) to improve OER energetics. Thus, despite pure EBP or NG alone has poor or negligible activity, EBP@NG achieves remarkably enhanced bifunctional HER/OER activities, along with an excellent durability. This endows an optimized electrolyzer using EBP@NG as anode and cathode with a low cell voltage of 1.54 V at 10 mA cm-2, which is smaller than that of the costly integrated Pt/C@RuO2 couple (1.60 V).

1. INTRODUCTION Few-layered exfoliated black phosphorus (EBP) nanosheet, a two-dimensional (2D) building block that can be exfoliated from bulk BP, has received extensive attention for a variety of potential applications including electronics, optoelectronics and catalysis.1-8 Owing to its high carrier mobility, tunable electronic structure, large specific surface area with fully exposed surface atoms, and active lone-pairs, EBP has been regarded as a potential electrocatalyst for various electrochemical reactions.3,7 Particularly, its ultrathin sheetlike structure can render a good platform for fundamental understanding of the correlation between atomic, electronic structure and inherent properties.3,7 Compared to the great advances in electronic and optoelectronic applications,8-10 much less discussion was found in the literature for electrocatalysis by metal-free BPs, though few reports have recently appeared. For examples, Wang and Zhang et al discovered that bulk BP films or few-layered EBP sheets could be used as catalysts for oxygen evolution reaction (OER) in alkaline media.3,7 Bare EBP was also explored to catalyze hydrogen evolution reaction (HER) in acid.8 However, these plain BP materials demonstrated moderate or

even poor activities for OER or HER, possibly caused by either a low density of catalytic sites or a limited electrical conductivity. Further, an insightful mechanistic understanding of electrocatalysis by metal-free EBP is still lacking, while the instability of EBP under ambient conditions severely impedes its catalytic durability and application potential.3,8-10 In this context, designing and constructing desirable composites by assembling various building blocks is a good solution to tuning the properties of 2D materials and imparting new functions.11,12 EBP is usually used in many semiconductor heterojunctions for improving carrier transport or interfacial charge separation. More recently, several metal-based building blocks, including 0D nanoparticles (Ni2P, Co2P)13,14 and 2D nanosheet (MoS2),15 have been assembled onto EBP to promote HER or OER, which, however, demonstrate the main origin of catalytic sites from the active metal species rather than EBP itself. As such, the inherent electrocatalytic capability of metal-free EBP has not been fully exploited yet. Thus, it is important yet challenging to simultaneously enhance the ambient stability of EBP and fully explore its intrinsic catalytic activity for various reactions, and understand the related mechanism for expanding the application scope of EBP.

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One of the most significant applications of catalysts is electrolyzing water into H2 and O2, which potentially renders the abundant but intermittent renewable energy sources.16-18 Yet, the overall water splitting is an uphill process and the high overpotential for both cathodic HER and anodic OER hinders its realistic applications. The well-known catalysts for OER are Ru- or Ir-based materials19 and those for HER are Ptbased materials,20,21 yet their high cost and unsatisfying durability constrain the large-scale deployment. Although some advances were achieved in developing low-cost noblemetal-free alternatives, such as carbon and transition metalbased materials for HER and OER,17 it is still very challenging to pair two electrode reactions in an integrated electrolyzer, since most existing catalysts cannot offer effective HER/OER bifunctionality in the same pH and electrolyte. For example, some transition-metal-based layered double hydroxides were active for OER in base but inert for HER in the same medium.17 Also, the preparation of various single-function catalysts for HER or OER needs various processing and characterization facilities, resulting in high cost. On the other hand, metal-free catalysts are preferred for sustainable catalysis with reduced environmental effect. Among them, carbon materials such as heteroatom/defect doped carbon nanotubes,22 graphene23 and graphdiyne24 are particularly attractive for various catalytic reactions owing to their low cost, tunable electronic structure, excellent conductivity and stability.25,26 In spite of great advances, the bifunctional HER/OER catalysis of the existing carbon catalysts still requires further improvement. Therefore, it is urgently required to explore low-cost, yet efficient metal-free bifunctional catalysts that are capable of simultaneously catalyzing HER and OER in the same electrolyte to expedite the overall water splitting.27,28 Herein, we produced aqueous dispersion of few-layered EBP with a high concentration up to 0.15 mg mL-1 by exfoliating bulk sample in an eco-friendly water/ethanol (H2O/EtOH) mixture solvent and subsequently constructed a new metal-free 2D/2D heterostucture (EBP@NG) via electrostatic interaction of negatively-charged ultrathin EBP and positively-charged N-doped graphene (NG), as efficient, robust and bifunctional catalysts for overall water splitting in alkaline media. The resulting 2D/2D heterostructure has a well-designed heterointerface and unique electronic structure and is thus endowed with the following merits: 1) the face-toface direct contact with large interface area and facilitated charge separation and transfer; 2) the presence of NG can protect EBP from structural and chemical degradation in assembled hybrid films and improve the electrochemical stability; 3) owing to the lower Fermi level of EBP than that of NG, their electronic interaction can trigger effective electron injection from NG to EBP, which not only optimizes H adsorption and desorption to promote HER, but also tunes the adsorption energy of OER intermediates to improve OER energetics. Benefiting from the above advantages, though EBP or NG alone has poor or negligible activity, EBP@NG yields impressive bifunctional activities towards both HER and OER with an excellent durability. The optimized two-electrode electrolyzer with bifunctional EBP@NG as the anode and cathode achieves a low cell voltage of only 1.54 V at 10 mA cm-2, which outperforms the costly Pt/C@RuO2 (1.60 V) and is among the best results achieved so far.14,27,28 To our

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knowledge, this is the first successful attempt to convert inactive metal-free EBP-based materials into robust bifunctional catalysts for overall water splitting.

2. RESULTS AND DISCUSSION Figure 1a schematically illustrates the eco-friendly liquid exfoliation of bulk BP and the construction of the EBP@NG heterostructures. To begin with, bulk BP crystals were exfoliated into ultrathin nanosheets (EBP) via simple sonication in a deoxygenated water/ethanol (v/v=1:1) mixture. After centrifuging and rinsing with deionized (DI) water, EBP can be well re-dispersed in DI water to form a stable dispersion, showing a high concentration of 0.15 mg mL-1 without apparent aggregation over two weeks, as evidenced by the Tyndall effect (Figure 1b). Noteworthy, very recently, pure water or organic solvents such as N-methyl-2-pyrolidone (NMP) were also used to effectively exfoliate bulk BP into nanosheets but with a limited concentration (< 0.1 mg mL-1).7, 14 Generally, the oxidation rate of BP in solvents depends on the dissolved oxygen concentration and light intensity.29 To enhance the stability of EBP, we adopted N-doped graphene sheets (NG) (Figure S1a-b) to passivate EBP, forming stable heterostructures. Herein, NG was produced by chemical reduction of graphene oxide (GO) in the presence of ethylenediamine (EDA) according to the reported method30 (see more details in experimental section). X-ray photoelectron spectroscopy (XPS) confirms the efficient N doping into graphene with the doping level of 5 at% (Figure S1a). Thereafter, the EBP and NG dispersion in DI water was mixed to generate a precipitate (Figure S2), denoted as EBP@NG. The product formation is mainly driven by the electrostatic interaction between negatively charged EBP and positively charged NG based on the Zeta potential analysis (see Figure S1c-d). By varying the mass ratios of EBP to NG (1:10/1:8/1:4/1:2), a series of EBP@NG were achieved, respectively labelled as EBP@NG(1:10), EBP@NG(1:8), EBP@NG(1:4) and EBP@NG(1:2). Transmission electron microscopy (TEM) imaging of EBP in Figure 1c presents a free-standing nanosheet with a size of several hundred nanometers. The thickness of EBP is determined to be about 2.56-3.53 nm by atomic force microscopy (AFM) observation (Figure 1d-e), corresponding to few-layer (5-7 layers) sheets. High-resolution TEM (HRTEM) observation in Figure 1f reveals visible lattice fringes with the lattice spacing of 0.167 and 0.219 nm, ascribed to the (200) and (002) planes of BP, respectively.31 The selective-area electron diffraction pattern (SAED) discloses the crystalline nature of EBP with [010] preferential orientation. The TEM images of EBP@NG in Figure 1g reveal that some smaller EBP sheets are distributed over larger NG sheets, forming a 2D heterojunction, which is further verified by dark-filed TEM imaging with the corresponding element distribution mapping of C, N and P (Figure 1h). The variation in the Raman spectra from bulk BP to final EBP@NG is shown in Figure S3. After the liquid exfoliation, all three characteristic peaks of bulk BP centered at 361.0, 437.5 and 466.5 cm-1 corresponding to A1g, B2g and A2g vibrational modes exhibit an obvious shift, which is attributed to the reduced thickness and lateral dimensions of EBP sheets.29 After being coupled with NG, the peaks for B2g and A2g vibrational modes of all EBP@NG samples exhibit slight


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shift to higher wavenumber relative to those of EBP, implying the presence of the electronic interaction between the two components.15 As shown in Figure S4, we note that the combined TEM imaging and selected electron energy loss spectroscopy (EELS) unveil that the oxidation of EBP is more likely to occur at the sheet edge rather than the basal plane, since water tends to oxidize EBP at pre-existing defects (e.g., edges), as demonstrated in previous studies.29

Figure 1. (a) Schematic illustration for the liquid exfoliation of bulk BP and construction of EBP@NG. (b) Photograph of an EBP dispersion with the concentration up to 0.15 mg mL-1, showing a Tyndall effect. (c) TEM image, (d) AFM image, (e) corresponding height image, and (f) HRTEM image of EBP. Inset of f shows the corresponding SAED pattern. (g) Bright-field TEM image and (h) dark-field TEM image with corresponding element distribution image of EBP@NG(1:8). Scale bar: (c) 50 nm; (d, g, h) 500 nm; (f) 1 nm.

We further investigated the stability of EBP@NG in aqueous solution, a critical issue for broadening its practical applications.8-10 We utilized the inductively coupled plasma atomic emission spectroscopy analysis (ICP-AES) to monitor the concentration of degraded P species (phosphite ion, phosphate ion, or other PxOy species)31 of EBP@NG heterostructures and EBP (Table S1), which are dispersed in DI water under ambient condition. After two weeks in aqueous solution, the degradation concentration of EBP@NG(1:8) and EBP@NG(1:4) were 3.96 and 3.33 ppm respectively, far lower than that of bare EBP (43.20 ppm). XPS analysis also indicates that the high-resolution P 2p spectra of the EBP@NG heterostructures almost remain unchanged after two weeks, in sharp contrast to the case of the plain EBP that the component peak at 134 eV corresponding to degraded PxOy species becomes more pronounced (Figure S5). The above results imply that the degradation rate of EBP@NG is effectively inhibited with respect to EBP. Previous reports showed that surface or edge passivation by graphene32 or C6033 via covalent or noncovalent functionalization can improve the stability of BP in oxygenated water and some electrochemical environments. In our case, similar to previous studies,34 the electrostatic interaction enables intimate contact between EBP

and NG to effectively passivate P atoms on the puckered surface and hence stabilize EBP in the hybrid.33,34 NG can also act as an encapsulation reagent to protect EBP in our hybrid from the attacks of light, oxygen and water.33,34 The HER catalytic performance of EBP@NG with different mass ratios was firstly evaluated in 1 M KOH, along with EBP, NG and Pt/C for comparison (catalyst loading: 0.3 mg cm-2). All the potential was calibrated against the reversible hydrogen electrode (RHE) and all the data were collected without iR-compensation. The HER polarization curves as illustrated in Figure 2a and S6 reveal that EBP@NG heterostructures exhibit markedly enhanced HER activity with respect to each component. Among all investigated metal-free samples in this work, EBP@NG(1:4) yields the highest HER activity with a low overpotential (HER) of only 191 mV to afford the current density of 10 mA cm-2 (Figure 2b), much smaller than those of the plain EBP (370 mV) and NG (445 mV) under the same conditions. EBP@NG(1:4) also affords much larger cathodic current density relative to bare EBP and NG at the identical potentials (e.g. -0.20 and -0.25 V vs. RHE). These results manifest that the construction of 2D/2D heterostructures effectively boosts the performance for HER catalysis. Furthermore, EBP@NG(1:4) exhibits a much lower HER of 125 mV at 10 mA cm-2 with 95% iR-correction (Figure S7). Notably, the HER activity of our metal-free EBP@NG(1:4) is comparable, and even superior, to many other reported HER catalysts in alkaline media (Table S2). The excellent HER catalytic performance of EBP@NG(1:4) was also reflected by its smaller Tafel slope of 76 mV dec-1 than those of EBP (135 mV dec-1) and NG (102 mV dec-1), indicating more favorable reaction kinetics of EBP@NG(1:4) (Figure S8). The Nyquist plots in Figure S9 present much lower charge transfer resistances of 20  for EBP@NG(1:4) in comparison to EBP and NG, signifying faster reaction rate of EBP@NG(1:4). Furthermore, as shown in Figure 2c, the chronoamperometric curves reveal that EBP@NG(1:4) yields a stable catalytic current with < 10% current loss after continuous operation for 16 h, in sharp contrast to the bare EBP (45% loss after only 5 h) and the benchmark Pt/C (25% loss over 16 h). After the long-term stability test, the structure and composition of EBP@NG(1:4) exhibited negligible change as demonstrated by XPS and Raman analyses (see more details in Figure S10). The above results establish the superior catalytic durability of EBP@NG(1:4) for HER. In addition, EBP@NG(1:8) also displayed an excellent HER catalytic capability featuring a low overpotential of 210 mV, a small Tafel slope of 109 mV dec-1 and a superior durability. Given much lower overpotential and larger cathodic current density of EBP compared to NG, the high HER activity for EBP@NG(1:4) and EBP@NG(1:8) should mainly arise from the EBP species (Figure S6). To provide further insight into the excellent HER activity of the heterostructures, we calculated the electrochemical double capacitances (Cdl) in a nonfaradaic potential region to compare the electrochemical surface area (ECSA). As exhibited in Figure S11, both EBP@NG(1:8) and EBP@NG(1:4) possess large Cdl of 15.2 and 9.0 mF cm-2, respectively, which are far above those of the plain BP (0.56 mF cm-2) and NG (2.4 mF cm-2), implying significantly improved ECSA after the marriage of EBP and



NG.3 When normalizing the HER polarization curves by the Cdl difference, the EBP@NG heterostuctures still yield much better HER activity compared to two individual components (Figure S12). This result heralds that in addition to the high active surface area, the synergistic interaction between EBP and NG also contributes to the substantially improved HER catalytic performance.

Figure 2. (a) HER polarization curves, (b) overpotentials at 10 mA cm-2 and (c) chronoamperometric curves of EBP@NG and other reference catalysts in 1 M KOH without iR-compensation. (d) OER polarization curves, (e) overpotentials at 10 mA cm-2 and (f) chronoamperometric curves of EBP@NG and other reference catalysts in 1 M KOH without iR-compensation.

The OER performance of various catalyst samples was further examined in 1 M KOH with the same catalyst loading of 0.3 mg cm-2. All the data were collected without iRcompensation. Here, a potential region of 1-1.7 V was chosen to avoid over high oxidation potential to suppress possible degradation, similar to many previous studies.3,7,27,28 Figures 2d and S13 illustrate the OER polarization curves of EBP@NG with different mass ratios, NG, EBP and RuO2 without iR compensation. Clearly, among all metal-free samples, EBP@NG(1:8) yields the lowest OER potentials (OER) of 310 mV at 10 mA cm-2, much superior to other metal-free counterparts, such as plain EBP (> 500 mV) and NG (430 mV), even on a par with commercial RuO2 catalysts (300 mV) (Figure 2e). Noteworthy, EBP@NG(1:8) gives a much lower OER of 265 mV at 10 mA cm-2 with 95% iRcorrection (Figure S14). The OER activity of EBP@NG(1:8) is better than those of many state-of-the-art non-noble-metal OER catalysts in alkaline media (Table S3). EBP@NG(1:8) also gives a comparable Tafel slope (89 mV dec-1) to that of the RuO2 catalyst (78 mV dec-1), suggesting a facile OER kinetics (Figure S15). Similar good catalytic behavior was also found in EBP@NG(1:4), showing an overpotential of 350 mV and a Tafel slope of 82 mV dec-1. The rotating ring-disk electrode (RRDE) test indicates a negligible ring current as the

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ring electrode is set at 1.50 V vs RHE to monitor the generation of H2O2, signifying a favorable 4e- OER pathway over EBP@NG(1:8) (Figure S16). The OER Faradaic efficiency over the EBP@NG(1:8) electrode is determined to be 97% using the volumetric method (Figure S17), verifying that the detected catalytic current arises exclusively from the oxygen evolution rather than other side reactions. After a 12h continuous operation at 10 mA cm-2, EBP@NG(1:8) exhibits a high current retention with a minor current loss of < 4%, surpassing bare EBP (>50% activity loss after only 2h) and RuO2 (~50% loss after 5h). This result confirms the strong stability of EBP@NG(1:8) for OER in alkaline media. Further insights into the structure and composition for post-OER samples by Raman and XPS inspection verify the robustness of the EBP@NG(1:8) heterostructure catalyst after continuous oxygen evolution (Figure S18). As can be seen from Raman spectra for EBP@NG(1:8) (Figure S18a), the intensity ratio (A1g/A2g, 0.43) of the characteristic A1g and A2g peaks from EBP remains almost unchanged after the OER durability tests, while A1g/A2g (> 0.2) is an indicator of low oxidation degree of BP.29 XPS P 2p spectra of EBP@NG(1:8) particularly the component peak at 134 eV from the degraded PxOy exhibits subtle change. These results indicate that EBP in EBP@NG(1:8) remains almost unchanged in the composition and structure. Meanwhile, high-resolution C 1s and N 1s spectra of EBP@NG(1:8) before and after the OER durability test show similar profiles (Figure S18). Also, the oxygenrelated component peaks at 286-289 eV in C1s spectrum have no apparent change. No possible carbon corrosion product during OER (e.g., CO) was detected with gas chromatography within the detection limit (10 ppm). These results coupled with the observed high Faradaic efficiency of > 97% indicate a good stability for the NG in EBP@NG(1:8). Taken together, the above results imply that EBP@NG(1:8) is indeed a durable OER catalyst in our present case. Considering that EBP alone has negligible activity towards OER, the OER activity for EBP@NG heterostructures is mainly from NG. Similar to the above case for HER, the OER polarization curves normalized by the Cdl difference indicate that EBP@NG(1:8) affords a lower overpotential and a larger current density compared to individual components (Figure S19), highlighting the synergy between EBP and NG for boosting the OER activity of EBP@NG(1:8). It is noted that both HER and OER activities of EBP@NG can be readily modulated by varying the EBP/NG mass ratio, as depicted in Figures S6 and S13. This is because (1) HER active sites mainly come from EBP and OER active sites arise from NG in the heterostructures, yet the electronic interaction between the two components contributes to both reactions (vide infra); (2) NG has higher electron transport ability relative to EBP (Figures S9 and S20). The combined effect of the above factors will lead to the optimal mass ratio of two components for the best catalytic activity. Among all EBP@NG samples, both EBP@NG(1:4) and EBP@NG(1:8) yield excellent bifunctional activities and durability for HER and OER based on aforementioned results. Here, similar to many previous studies,35,36 3D highly conductive support (e.g., nickel foam, NF) was used to load the catalyst to optimize the half-cell and full-cell performance due to its excellent electron transfer capability. For half-cell tests, EBP@NG(1:4) on NF


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yields the lowest HER of 105 mV while EBP@NG(1:8) loaded on NF gives the lowest OER of 240 mV among all samples investigated in our work (Figure S21). In view of the observed impressive half-cell performance, we assembled two symmetric electrolytic cells (EBP@NG(1:8)||EBP@NG(1:8) and EBP@NG(1:4) ||EBP@NG(1:4), without iR-correction) using EBP@NG on NF as both the cathode and anode to assess the overall water splitting performance in alkaline media, similar to most literature-reported full-cell assemblies with NF as catalyst support (Table S4 and S5). Apparently, bare Ni foam has no activities for both OER and HER (Figure S22). EBP@NG (1:8) || EBP@NG (1:8) and EBP@NG (1:4) || EBP@NG (1:4) deliver excellent overall-water-splitting performance with small cell voltages of 1.59 and 1.67 V to afford the current density of 10 mA cm-2 (Figure 3a). In contrast, the pure EBP or NG alone gives rather poor performance. We also note that EBP@NG(1:4) exhibits the best HER activity while EBP@NG(1:8) gives the highest OER activity among all investigated heterostructure samples (see more details in Figures S6 and S13). Hence, an asymmetric cell (EBP@NG(1:4)||EBP@NG(1:8)) was further built to optimize the overall water splitting performance. As shown in Figure 3b, this asymmetric cell delivers a lower voltage of only 1.54 V to afford 10 mA cm-2 compared to other two symmetrical counterparts, outperforming the integrated Pt/C@RuO2 couple (1.60 V). Meanwhile, an H-type cell equipped with a separation membrane to avoid gas mixing is utilized to determine the amount of H2/O2, which indicates the faraday efficiencies (FE) of >97% and a molar ratio being close-to-2:1 between H2 and O2 (Figure 3c and S23). Such an excellent overall-water-splitting performance places our EBP@NG among the leading bifunctional catalysts for water splitting in alkaline media reported so far (Tables S4 and S5). We also constructed a solar energy-assisted electrolyzer. When using a plastic solar panel to power the device, the evolved hydrogen and oxygen bubbles can be clearly observed, which holds promise for distributed energy conversion and storage systems (Figure 3d). According to the above discussions, we can reason that the synergetic interaction between the EBP and NG plays a key role in remarkably enhanced HER and OER activities of EBP@NG. XPS analysis was used to probe the interaction of EBP and NG. As shown in Figure 4a, the P p1/2 and P p3/2 component peaks in the high-resolution P 2p spectra of EBP@NG(1:8) exhibit a shift of 0.2 eV towards lower binding energy direction relative to those of the plain EBP, indicating that EBP extracts electrons from NG after their contact, engendering the electron enrichment on the EBP side.15 Furthermore, the N-C peak located at 400.6 eV for EBP@NG(1:8) shifts by 0.2 eV compared to that of the bare NG, once again verifying the electronic coupling between NG and EBP (Figure S24).37 To further support the presence of strong electronic interaction between the EBP and NG, we performed kelvin probe force microscopy (KPFM) test to determine the work function (φ) of EBP and NG by using the Si substrate (φSi=4.6 eV) as the reference.38,39 As observed in Figure 4b, EBP gives a higher work function of ~4.7 eV compared to that of NG (~4.5 eV), indicating a lower Fermi level position of EBP. Thus, it is energetically favorable for the electron migration from NG to EBP after their direct contact, leading to the interfacial charge redistribution as

demonstrated in Figure 4c. Meanwhile, we also calculated the differential charge density of EBP@NG. The yellow and blue iso-surfaces in Figure 4d denote high density of holes and high density of electrons, respectively. This result further proves that EBP functions as acceptors and attracts electrons from neighboring NG, building an electron accumulated EBP surface and a hole-rich NG surface in EBP@NG heterostructures, which coincides with the above XPS results (Figure 4a).

Figure 3. (a) Polarization curves and (b) chronopotentiometric curves of EBP@NG and other reference samples used as HER and OER bifunctional catalyst for overall water splitting in 1 M KOH without iR-compensation. (c) Plots on the amount of evolved H2 and O2 versus time at a constant current density of 10 mA cm-2 for the EBP@NG(1:4)||EBP@NG(1:8) cell. (d) Photograph of a solar power driven water splitting device.

Density function theory (DFT) calculations were further performed to render some general insights into the marked improvement of both HER and OER activities over EBP@NG 40-42 (see more modeling details in supporting information). For HER, NG has a rather poor activity based on the above experimental results. Thus, we chose to focus on the BP species as the main HER active site. To assess the HER activity, we achieved the values of Gibbs free energy of hydrogen adsorption (GH*) on various sites of EBP@NG according to GH*= Ead + Gv + TS, where Ead refers to the adsorption energy of H, Gv refers to vibration free energy difference of the adsorbed H and H2 molecules, S refers to the translation and rotation entropy of H2 molecules and T refers to room temperature. All adsorption configurations are shown in Figure S25, with corresponding GH* values summarized in Table S6. Generally, the site with GH* closer to zero is suggested to have a better HER activity due to the balanced hydrogen adsorption and desorption. We note that the plain BP gives a larger positive GH* value (0.67 eV, Figure 4e) corresponding to very weak H adsorption and easy product desorption over its surface, unfavorable for HER. After coupling EBP with NG, GH* of EBP is drastically reduced to 0.1 eV - the smallest GH* value among all possible sites in EBP@NG (Figure 4e). This implies that the most active site for HER is indeed from the EBP and the coupling of EBP with NG engenders mediated adsorption and desorption (GH* →0)



and boost the inherent HER activity of EBP. Previous studies demonstrated that the electronic properties of a catalyst strongly affect the adsorption energy of reaction intermediates over the catalyst surface and consequently catalytic activity.40,41 Thus, in our case, it can be envisioned that the electron enrichment over EBP by the electron injection from NG favors the stabilization of adsorbed H over the surface and promote HER, similar to reported metal-based HER catalysts.14,15 Meanwhile, considering alkaline HER is generally described as follows: (2H2O + 2e- → H2 + 2OH-), such directional electron transfer from NG to electrochemical active EBP is conducive to rapid reduction of adsorbed H species on the surface into the final hydrogen molecules.15

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our recent studies.43 From the thermodynamics point of view, hydroperoxy species (OOH*) are usually regarded as a key intermediate for oxygen evolution. Thus, an efficient OER catalyst is anticipated to favor the OOH* formation, which is often regarded as the rate-determining step (RDS) for the oxygen evolution.35, 40 Figure 4f describes the free energy diagrams of OER over NG and EBP@NG at the potential of 1.23 V. As can be seen, the OOH* adsorption on NG is too weak and the conversion from O* to OOH* as RDS is inhibited due to high Gibbs free energy barrier (GO* → OOH* = 0.51 eV). In stark contrast, after forming the EBP@NG heterostructure, the adsorption energy for O* shows an obvious increase from 0.2 to 0.3 eV while the adsorption energies for both OH* and OOH* species slightly change due to the charge redistribution over NG. The above adsorption energy change of OER intermediates eventually results in an apparent reduction of the free energy of the OOH* formation (GO* → OOH* = 0.43 eV). Thus, EBP@NG presents a lower overpotential of EBP@NG (0.43 V) than that of NG (0.51 V). Overall, the above atomic-level HER and OER mechanistic insights on the EBP@NG surface clearly showcase the dual promotional roles of directional interfacial electron transfer for simultaneously boosting HER and OER activities of EBP@NG. Not limited to NG, our strategy could be applied to design other EBP-based heterostructures for bifunctional catalysis by pairing with other carbon materials with smaller work function relative to EBP (e.g., N-doped CNTs).

3. CONCLUSIONS

Figure 4. (a) XPS P 2p spectra of EBP@NG and reference samples. (b) Plots of work functions of NG and EBP samples on Si substrates derived from KPFM results (inset). (c) Illustration of interfacial charge redistribution between NG and EBP. (d) Illustration of the differential charge density of NG and EBP (blue for electron-rich area and yellow for hole-rich area). Calculated free energy diagrams of (e) the HER on BP and BP-NG and (f) OER on NG and BP-NG at the potential (U) of 1.23 V.

On the other hand, we focus on the NG species in EBP@NG as the main origin of OER activity, since EBP alone has almost no OER activity. In fact, our DFT calculations also confirm that the most active site is from NG (Figure S26, Table S7). In previous studies, it is widely accepted that doping N into carbon lattices can induce local charge redistribution by intramolecular charge transfer and generate positively charged carbon sites adjacent to N,22 facilitating the adsorption of OER intermediates.41 For our EBP@NG catalyst, as revealed by our theoretical analysis in Figure 4c, the interfacial charge transfer from NG to EBP also results in notable charge relocation and introduce rich activated carbon sites bearing positive charge across the whole interface, which is more favorable for OER, as pointed out in

In conclusion, we have achieved aqueous dispersion of fewlayered EBP with a high concentration of 0.15 mg mL-1 by exfoliating bulk BP in H2O/EtOH and built a new type of EBP@NG 2D/2D heterostructure via solution-based electrostatic assembly. Rational engineering of the EBP@NG heterointerface not only significantly enhances the ambient stability of EBP but also enables effective electronic modulation for each component to remarkably boost their inherent catalytic activities. As such, the optimized EBP@NG can serve as an efficient metal-free bifunctional catalyst for both HER and OER and afford ultralow HER and OER at 10 mA cm-2 together with an excellent catalytic durability. The HER and OER catalytic activities can be readily regulated merely by varying the mass ratios of components. Impressively, the two-electrode electrolyzer with bifunctional EBP@NG catalysts as the anode and cathode delivers a low cell voltage of only 1.54 V at 10 mA cm-2, which is among the best reported bifunctional activities to date. Combined theoretical and experimental studies reveal that the electronic interaction between EBP and NG not only optimizes H adsorption and desorption (GH* → 0) over EBP but also regulates the adsorption energies of OER intermediates on NG to favor the formation of OOH*, simultaneously improving HER and OER energetics. Our findings not only expand the application scope of EBP in energy-related fields, but also render fresh insight on the design of EBP-based catalysts for water splitting by electronic engineering. The atom-level mechanistic insights obtained in this study can be used as a general guideline for devising new bifunctional hybrid catalysts for diverse energy systems and beyond.


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4. EXPERIMENTAL SECTION 4.1. Synthesis of EBP. EBP nanosheets were synthesized by a simple liquid exfoliation method using an ethanol/water mixture solvent. Typically, 50 mg of black phosphorus (Smart elements, Germany) was ground in a N2-filled glove box and transferred into a N2-saturated ethanol/water (v/v=1:1) mixture. After 7 h of continuous water-bath sonication at 25 oC (S120H, ELMA), the obtained brown dispersion was centrifuged at 1500 rpm for 20 min, the supernatants were collected and vacuumed dry at 65 oC. EBP can be well re-dispersed in DI water to form a stable dispersion. The dispersion was sealed after purging with N2. EBP in water was negatively charged based on the Zeta potential analysis (Figure S1). 4.2. Synthesis of NG. NG was prepared by the literature reported method.24 In a typical process, 20 mg of GO was dispersed in 45 mL of DI water to generate a stable dispersion followed by the addition of 15 mL ethylenediamine. The mixture was stirred for 10 min and transferred into an autoclave (100 mL). After being subjected to hydrothermal treatment at 180 oC for 3 h, the resultant dark brown suspension was cooled down and centrifuged to obtain precipitates, followed by repeated rinsing with DI water and drying under vacuum. NG in water was positively charged based on the Zeta potential analysis (Figure S1). 4.3. Preparation of EBP@NG. Typically, 2 mg of EBP was dispersed in 10 mL of DI water. After sonication for ~10 min, a well-dispersed EBP suspension was obtained. Afterwards, the EBP dispersion was added into 20 mL of NG solution containing 16 mg of NG, where EBP was assembled onto the NG surface via electrostatic interaction between positively charged NG and negatively charged EBP. The EBP@NG heterostructures were then obtained. By varying the mass ratios of EBP to NG (1:10/1: 8/1: 4/1: 2), a series of EBP@NG were achieved, respectively labelled as EBP@NG (1:10), EBP@NG (1:8), EBP@NG (1:4) and EBP@NG (1:2). 4.4. Sample Characterization. The morphology and microstructure of samples were examined by scanning electron microscopy (SEM; SU-4800, Hitachi, Japan), transmission electron microscopy (TEM; JEM-2100, JEOL, operated at 200 kV) and atomic force microscopy (AFM; Bruker MM8). High-angle annular dark-field scanning transmission electron microscopic (HAADF-STEM) images and electron energy loss spectroscopy were taken on a FEI Tecnai G2F30 transmission electron microscope. Raman spectra were recorded with a 514 nm laser using a RENISHAW system. X-ray photoelectron spectroscopic (XPS) studies were performed using a Thermo ESCALAB 250 instrument. The XPS spectra were referenced to C 1s emission at 284.8 eV and fitted by Gauss type and the FWHM was constrained to < 3 eV while the Lorentzian-Gaussian constant was all set as 20 %. The atomic composition of samples was determined using a VISTA-MPX EL02115765 inductively coupled plasma atomic emission spectrometer (ICP AES). The KPFM measurements were performed using Dimension Icon (BrukerNano Surfaces). Amplitude modulation KPFM was used to obtain a high signal-to-noise ratio as opposed to that of frequency modulation. A contact potential difference (CPD) between the tip and the sample surface (φtip − φsample) is created, while the contrast in the CPD image is equivalent to

the local work function variation of the sample and the substrate to completely eliminate the topography effect. 4.5. Electrochemical Measurements. The electrochemical tests were performed in a three-electrode configuration connected to an electrochemical workstation (CHI 760E) at room temperature. An Ag/AgCl electrode filled with saturated KCl solution used as the reference electrode. A cylinder graphite rod was used as the counter electrode for HER tests while a platinum foil was used as the counter electrode for OER tests. The potential recorded against an Ag/AgCl electrode was converted to the reversible hydrogen electrode (RHE) according to the following equation ERHE = EAg AgCl +0.059 V × pH + 0.205 V (1) where ERHE is the potential calibrated against the RHE and EAg/AgCl is the potential performed against the Ag/AgCl reference electrode. For half-cell HER or OER test, the working electrode was prepared as follows: 6 mg of catalyst was dispersed in 2 mL aqueous solution consisting of 1.9 mL water and 100 μL Nafion solution (5 wt% in water) under sonication. The suspension was subjected to bath sonication for 10 min to produce a homogeneous catalyst ink. The obtained catalyst ink (20 μL) was deposited onto a mirror-polished glassy carbon electrode (disc area: 0.196 cm2). The catalyst loading was 0.3 mg cm-2, unless otherwise stated. LSV and RRDE measurements with a scan rate of 5 mV s-1 were performed at a rotation rate of 900 rpm in 1 M KOH. Prior to tests, the electrolyte was purged with N2 at room temperature. For overall water splitting tests, the catalyst was loaded on a piece of Ni foam (thickness: 2 mm, Sigma) as both anode and cathode. 6 mg of catalyst was dispersed in a mixture solution containing 1.9 mL of DI water and 100 μL of Nafion solution. After sonication for 10 min, a homogeneous catalyst ink was achieved. 100 μL of the catalyst ink was deposited onto a piece of Ni foam with a fixed area of 1 x 2 cm2 for electrochemical tests. LSV measurements with a scan rate of 5 mV s-1 were performed in 1 M KOH without iR compensation. To demonstrate a solar energy powered electrolyzer, a piece of Ni foam (1 x 1 cm2) loaded with 50 μL catalyst ink was used. 4.6. DFT calculations. DFT calculations were carried out on Dmol3 program on Materials studios. The Perdew-BurkeErnzerhof method with generalized gradient approximation (PBE-GGA) was used for exchange correlation. The supercell (9.89 Å *8.49 Å) of this system is composed by 3*2 unit cells of BP (9.94 Å *8.46 Å) and 4*2 unit cells of Gr (9.84 Å *8.52 Å). The vacuum layer was set around 20 Å to avoid the interaction along z-direction. During the structure optimization, the 2×2×1 k-points mesh was used to sample the entire Brillouin zone. The threshold was set to be 1.0×10-5 Hartree, 2.0×10-3 Hartree Å-1 and 5.0×10-3 Å for energy, force and displacement, respectively. After relaxation, the distance between NG and BP monolayer is ~3.35 Å, which means weak van der waals interaction between each layers. The differential charge density was also obtained by Materials studios. It was segmented into two parts, named as the BP area and NG area.

ASSOCIATED CONTENT Supporting Information



This material is available free of charge via the Internet at http://pubs.acs.org. DFT calculation details, analysis of the EBP degradation in water, electrocatalytic performance of controlled samples.

AUTHOR INFORMATION Corresponding Authors *[email protected] (D.Y.) * [email protected] (X.C.) * [email protected] (L.D.)

Author Contributions ‡Z.Y.

and J. L. contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (No. 51573214) and the Youth 1000 Talent Program of China.

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