English

• Oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are of importance to determine the overall performance of various electrochemical energy conversion technologies such as water splittings, fuel cells and rechargeable metal-air cells [1-3]. However, the kinetics that governing these mechanisms are impeded by their low energy conversion efficiency [4]. For example, we note that noble metals-based electrocatalysts in the case of Pt/C, RuO₂ and IrO₂, hold as competitive OER or ORR catalysts. However, these materials are quite scarce which restrict their widespread usage [5]. To this end, the employment of earth-abundant materials, including transition metal oxides, phosphides, hydroxides, sulfides and nitrides, are economically-feasible as an attractive strategy to tackle this hindrance [6-9]. Unfortunately, simple single of them normally exhibt dissatisfied oxygen bifunctional electrocatalytic activity, since the poor electrical conductivity, inactivated catalytic active sites and limited electron transfer rate. Therefore, the construction of heterostructure electrocatalysts comprised of different active components is urgently needed for acquiring the efficient oxygen bifunctional electrocatalytic performance.

  Black phosphorus (BP) as an up-rising member of two-dimensional (2D) graphene-like materials has aroused soaring interest in optical and electronic applications since the first report in 2014, owing to its appealing advantages, such as the unique anisotropic properties, high carrier mobility and puckered-honeycomb configuration [10-12]. Particularly, the abundance of active lone pair electrons in BP supply favorable chemisorption sites for the oxygen-related molecules or ions, endowing BP as a potential oxygen electrocatalyst [13, 14]. Ideally, these lone pair electrons exposed to the surface of ultrathin BP all can be served as oxygen electrocatalytic active sites [15]. However, the presence of rich lone pair electrons makes BP extremely vulnerable to oxidative degradation, especially in strong oxidation and high anodic potential environment towards OER catalytic processes [16]. To date, the common strategies to stabilize BP is either to passivate the exposed lone pairs electrons by coating the BP NSs with various protective materials, which inevitably weakens the intrinsic electrocatalytic activity of BP [17, 18]. Therefore, developing an effective strategy to take advantage of those active electrons to achieve electronic structural innovation is highly desirable for enhancing the OER electrocatalytic activity and stability. Additionally, exploiting an excellent BP-based oxygen bifunctional electrocatalyst toward OER and ORR remains as an emergent challenge that needs to be tackled.

  Encouraged by these findings, herein, we construct a unique electronic acceptor-donor heterointerfacial interaction between BP and Co₃O₄ (Co₃O₄/BP heterojunction) through wet ball milling process. The intensive Co₃O₄/BP heterointerfacial interaction is disclosed explicitly by the formation of Co–O–P and Co–P bonds over the Co₃O₄/BP between BP and adjacent Co₃O₄. Consequently, the active electrons exposed on the surface of BP will migrate to Co₃O₄ since the higher Fermi level of BP compared with that of Co₃O₄. Such charge transfer not only impedes the oxidative deterioration of BP effectively, but also convert partially Co³⁺ to the oxygen electrocatalytic active species of Co²⁺, thus donating the improvement of oxygen bifunctional electrocatalysis.

  Typically, the synthesis process of Co₃O₄/BP heterojunction can be designed via wet ball-milling (Fig. 1, experimental details are shown in Supporting information). Firstly, the Raw-Co₃O₄ is prepared by a universal oxidation treatment (Fig. S2a in Supporting information) of commercial Co powders (particle size: around 400 nm, Fig. S1a in Supporting information). Subsequently, the formed Raw-Co₃O₄ (particle size: around 850 nm, Fig. S1b in Supporting information) and Bulk-BP are ultrasonic dispersed in N₂-saturated NMP (N-methyl-2-pyrrolidone). Finally, the Co₃O₄/BP heterojunction is developed by wet ball-milling under N₂ protection.

  Figure 1

  

  Figure 1.  Schematic demonstration of the formation of Co₃O₄/BP.

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  X-ray diffraction (XRD) patterns displayed in Fig. 2a clarify the crystalline nature as-grown samples. The distinct diffraction peaks located at the approximately of 16.8°, 26.7°, 34.1°, 34.9°, 52.4°and 56.1° correspond to the (020), (021), (040), (111), (060) and (151) diffraction planes of BP. While the additional peaks centered in 31.3°, 36.9°, 59.4° and 65.2° are attributed to the (220), (311), (511) and (440) reflections of crystalline Co₃O₄, which demonstrates the successful formation of Co₃O₄/BP heterojunction. Noticeably, the almost disappeared (021) diffraction peaks of Co₃O₄/BP can be ascribed to the reduction of BP crystallinity and the facilitation of BP lamellar exfoliation during the Co₃O₄-coexisting ball milling, which is consistent with the results previously reported [19, 20]. In addition, the identical diffraction peak positions observed in Figs. S2b and c (Supporting information) reveal the unchanged crystalline nature of Co₃O₄ and BP after ball-milling with N₂-saturated NMP. Therein, the obviously weakened peak intensity after wet ball milling can be attributed to the reduction in the size of Co₃O₄ and BP.

  Figure 2

  

  Figure 2.  (a) XRD patterns of BP, Co₃O₄ and Co₃O₄/BP. (b) TEM images and SAED patterns of BP and (c) Co₃O₄/BP (inset is the histogram of Co₃O₄ particle size distributions). (d) AFM image and (e) EDX element mappings of Co₃O₄/BP. (f) N₂ sorption isotherms and the corresponding pore size distribution of Co₃O4/BP. (g, h) HRTEM images of Co₃O₄/BP. (i) Raman spectra of BP, Co₃O₄ and Co₃O₄/BP.

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  Fig. 2b presents a typical transmission electron microscopy (TEM) image of BP, where the lattice spacing of 0.26 nm is indexed to (040) facets of BP, which is consistent with the discrete spots from the selected-area electron diffraction (SAED) presented in the inset. Moreover, the well-defined two-dimensional (2D) lamellar morphology demonstrates a definitive conversion of BP from the blocks (Fig. S3 in Supporting information) to nanosheets (Fig. S4 in Supporting information). Fig. 2c and Fig. S5 (Supporting information) show the TEM and scanning electron microscopy (SEM) images of Co₃O₄/BP heterojunction, which not only display flake-like nanostructure, but also reveal a fair amount of Co₃O₄ nanoparticles with a diameter of about 40 nm embedded in the lamellate BP skeleton. Moreover, the main discrete spots observed in corresponding SAED pattern match well with the structures of BP and Co₃O₄. The topographic detail of Co₃O₄/BP heterojunction is further probed by atomic force microscope (AFM, Fig. 2d), which gives a thickness of ~17 nm for BP nanosheets and a diameter of ~38 nm for Co₃O₄ nanoparticles. The elemental composition and distribution of Co₃O₄/BP heterojunction is investigated by energy dispersive X-ray spectroscopy (EDX) measurements. The corresponding element mappings in Fig. 2e demonstrate the coexistence of P, Co and O elements over the Co₃O₄/BP sample. Moreover, the mapping patterns of Co incorporated with the same domain of O signal reveals the successful construction of Co₃O₄/BP heterojunction. The specific surface area and pore properties of Co₃O₄/BP are investigated by recording the corresponding N₂ sorption isotherms (Fig. 2f). Noteworthy, the surface area and average pore size of the Co₃O₄/BP are calculated to be 15.39 m²/g and 6.37 nm, which are larger than those of Co₃O₄ and BP (Fig. S6 and Table S1 in Supporting information). Such considerable improvement on the surface area and porosity are highly anticipated since these features provide rich active sites and high electron transfer rates, thereby heightening the oxygen bifunctional catalysis [21, 22].

  Fig. 2g exemplifies a typical high-resolution TEM (HRTEM) image of heterointerface between Co₃O₄ and BP, in which the interplanar lattice spacing of 0.26 nm is indexed to the (440) facet of orthorhombic BP (Fig. 2h), while the lattice distance of 0.29 and 0.23 nm are attributed to the (220) and (311) planes of cubic Co₃O₄ (Fig. 2h and Fig. S7 in Supporting information). Moreover, we found that Co₃O₄ are well connected with several BP layers. Interestingly, the unique microscopic configuration of heterointerface not only induces the interfacial electron transfer between Co₃O₄ and adjacent BP, but also inhibits the agglomeration and dissolution of Co₃O₄ nanoparticles, thus resulting in the improvement of oxygen catalytic activity and stability.

  Raman spectroscopy is employed to examine the heterointerfacial interaction of Co₃O₄/BP. Fig. 2i compares the Raman spectra of BP, Co₃O₄ and Co₃O₄/BP samples, where the three peaks located at around 462, 438 and 467 cm⁻¹ belong to the A_1g out-of-plane, B_2g armchair and A_2g zig-zag vibrational modes of BP, respectively [23, 24]. Besides, the remainder signals centered at 469 and 673 cm⁻¹ associated to the E_g tetrahedral site (Co²⁺) and A_1g octahedral site (Co³⁺) vibrational modes of Co₃O₄ [25]. Notably, the positions of such characteristic vibration peaks observed in Co₃O₄/BP are shifted to lower and higher wavenumbers with respect to the pristine BP and Co₃O₄, indicating the existence of intimate heterointerface interactions between different constituents in the Co₃O₄/BP heterojunction [26]. In order to clarify the essence of such interaction, the zeta potentials of BP, Co₃O₄ and Co₃O₄/BP are initial recorded as presented in Fig. S8 (Supporting information). The opposite charge distributions of BP and Co₃O₄ resulted intensive electronic interactions at the heterointerface, which is conducive to the electronic transmission between BP and Co₃O₄.

  Accordingly, the electronic states and interactions of the surface elemental configuration for the Co₃O₄/BP heterojunction are further explored by X-ray photoelectron spectroscopy (XPS) technique (Tables S2 and S3 in Supporting information). Fig. 3a and Fig. S9 (Supporting information) reveal the high-resolution XPS spectra of O 1s for the Co₃O₄, BP and Co₃O₄/BP samples, in which the obvious peak at ~528.5 eV corresponds to the Co–O bond of Co₃O₄ [27], while other peak which was observed at ~530.4 eV originates from low-coordinated defects of oxygen, and the located at 531.6 eV attributes to the P–O bond of BP [28]. Remarkably, an additional signal is detected significantly at ~533.0 eV towards Co₃O₄/BP, which can be assigned to the P–O–Co bond coupling by the oxygenic functional groups of Co₃O₄ and BP [29], resembling that of C–O-Ni and C–O–Co reported earlier [30, 31]. The formed P-O-Co bond strongly suggests the forceful coupling between 2D BP nanosheets and Co₃O₄ nanoparticles.

  Figure 3

  

  Figure 3.  High-resolution XPS spectra of (a) O 1s, (b) Co 2p, and (c) P 2p for the Co₃O₄, BP or Co₃O₄/BP. (d) UPS spectra and (e) the corresponding energy band diagram of Co₃O₄ and BP. (f) Schematic diagram of the electron transfer in Co₃O₄/BP heterointerfaces, where the violet and yellow isosurfaces depict the heterointerfacial interaction.

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  Fig. 3b displays the high-resolution Co 2p spectra of Co₃O₄ and Co₃O₄/BP, where the peaks centered at 779.4 and 794.5 eV confirm the existence of Co³⁺ species, while the signals detected at 781.7 and 796.6 eV reveal the presence of Co²⁺ in the crystalline Co₃O₄ [27]. Meanwhile, the high-resolution P 2p spectra of BP and Co₃O₄/BP are also deconvoluted in Fig. 3c. Therein, the prominent peaks located at 129.2, 130.0 and 133.7 eV are assigned to the 2p_3/2, 2p_1/2 and P–O species, respectively [32]. In particular, the peak that appeared at the binding energy of 132.7 eV indicated the chemical bonding between P and Co, confirming the intense electron affinity of BP to Co [33]. Additionally, such an obvious electronic interaction is also proved by comparing the Co 2p and P 2p binding energies of Co₃O₄/BP with those of BP or Co₃O₄. Generally, the positive shift of binding energy indicates the attenuated electronic screening effect due to the decrease of the electron density, whereas the corresponding negative shift suggests an enhanced electron density [19]. Notably, the negative and positive binding energies shifts of Co 2p and P 2p observed in Figs. 3b and c demonstrate an increase and decrease in the electron density of Co and BP over the Co₃O₄/BP heterojunction. In other words, electrons tend to transfer from BP to Co₃O₄ through the Co/BP heterointerface (Fig. 2h) after the formation of Co₃O₄/BP heterojunction. It is worth mentioning that the P–O signal was reduced explicitly in the high-resolution P 2p spectrum of Co₃O₄/BP (Fig. 3c), which suggests the electron transfer from BP to Co effectively inhibits the oxidative degradation of BP, thereby improving the oxygen electrocatalytic stability.

  In addition, the ultraviolet photoelectron spectra (UPS) are recorded at the excitation energy of 21.22 eV (He I. vs. the vacuum level) to evaluate the direction of electron transfer between BP and Co₃O₄ catalytic active sites (Fig. 3d). The work functions of BP and Co₃O₄ are calculated by subtracting the electron cutoff energy, which stands for the difference between the vacuum level (0 eV) and Fermi level (E_f) [34]. Consequently, the E_f of BP and Co₃O₄ are found to be −3.89 and −4.57 eV (Fig. 3e). Therefore, to reach E_f equilibrium, the electron will migrate from BP to Co₃O₄ (Fig. 3f), which is in accordance well with the XPS results. We propose such effective electron migrations not only create massive oxygen electrocatalytic active sites of Co²⁺ in Co₃O₄, but also this effort protect BP from oxidative degradation, hence improving the oxygen electrocatalytic properties.

  The OER electrocatalytic properties are assessed firstly by linear sweep voltammetries (LSVs). Before further analyses, the mass ratio between Bulk-BP and Raw-Co₃O₄ (P/Co) is optimized (Fig. S10 in Supporting information). Noted that the superordinate onset potential (E₀) and prominent overpotential at the current density of 10 mA/cm² (η₁₀) reveal the best electrochemical performance of Co₃O₄/BP obtained at the P/Co of 2/5 (Experimental section in Supporting information). Fig. 4a compares the LSVs of BP, Co₃O₄, Co₃O₄/BP in O₂-saturated 1.0 mol/L KOH, while employs RuO₂ as a benchmark. Remarkably, the Co₃O₄/BP heterojunction possesses the most positive E₀ (1.448 V) and lowest η₁₀ (253 mV) in comparison to those of BP and Co₃O₄ (Table S4 in Supporting information), and even exceeded than RuO₂, confirming the excellent OER catalytic activity of Co₃O₄/BP. Moreover, to the best of our knowledge, the OER catalytic activity of Co₃O₄/BP is one of the highest photocatalytic activities for other BP-based catalysts recently reported (Table S5 in Supporting information). Such an outstanding catalytic performance can be further verified by contrasting the corresponding Tafel slopes (Fig. 4b). Co₃O₄/BP heterojunction exhibits a Tafel slope of 68 mV/dec (Fig. 4c), which is slightly smaller than RuO₂, highlighting a more favorable OER kinetics of Co₃O₄/BP. The electrochemical impedance spectroscopy (EIS) and double-layer capacitance (C_dl) tests are carried out to evaluate the electron transfer efficiency and electrochemical active surface area (ECSA). Figs. 4d and e and Fig. S11 (Supporting information) show the results of C_dl and EIS measurements, in which the smallest nyquist semicircle and largest C_dl imply the lowest electron transfer resistance and highest intrinsic oxygen electrocatalytic activity of the Co₃O₄/BP heterojunction compared with those of single-catalysts (BP and Co₃O₄). In addition to catalytic activity, the durability is also necessary for an excellent oxygen catalyst. Fig. 4f exhibit the LSVs and chronoamperometric response of Co₃O₄/BP, in which the attenuations of E₀ and η₁₀ for the Co₃O₄/BP are almost negligible after 1000 continuous OER cycles. Moreover, the loss of current density for the Co₃O₄/BP is only around 24% after 10 h continuous OER operation (Fig. S12a in Supporting information), confirming the robust durability of developed Co₃O₄/BP heterojunction. The HRTEM image after the repetitive cycles reveals that Co₃O₄ nanoparticles still well dispersed on the lamellar BP substance, again demonstrating its good structure stability (Fig. S13 in Supporting information). The high-resolution XPS spectra of Co 2p and P 2p of Co₃O₄/BP before and after OER cycles so as to clarify the OER process (Fig. S14 and Table S6 in Supporting information). The shift towards lower binding energies of Co 2P_1/2 and Co 2P_3/2 indicates the oxidation process of Co²⁺ to Co³⁺, which also can be confirmed by the increased atomic ratio of Co³⁺/Co²⁺ after OER test (Tables S3 and S6 in Supporting information). Such results suggest that the low valent Co ions in Co₃O₄ provides the actual active sites for OER electrocatalysis. Additionally, the negligible increase in the P-O ratio from 11.8% to 19.4% results from the slight oxidation of BP during the OER process.

  Figure 4

  

  Figure 4.  (a) LSVs, (b) Tafel plots and (c) the corresponding Tafel slopes, E₀ and η₁₀ of BP, Co₃O₄, Co₃O₄/BP and RuO₂ towards OER. (d) CVs and the calculated C_dl of Co₃O₄/BP. (e) EIS plots of BP, Co₃O₄ and Co₃O₄/BP. (f) LSVs of Co₃O₄/BP before and after 1000 repetitive cycles, as well as the chronoamperometric response towards OER. (g) LSVs, (h) Tafel plots and (i) the corresponding calculated Tafel slopes, E₀ and E_1/2 of BP, Co₃O₄, Co₃O₄/BP and Pt/C towards ORR.

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  Considering of the structural and electronic characterizations, the Co₃O₄/BP heterojunction possesses outstanding OER catalytic activity and stability, which can be attributed to the following several critical aspects. The first consideration is the high carrier mobility of BP endows high electrical conductivity of Co₃O₄/BP, thereby enhancing the electronic transmission rate. Secondly, the emblematic laminar nanoskeleton accompanied by rich porosity enhances the transport efficiency of OER-related reactants, intermediates and products [35]. Thirdly, the preferential transfer of active electron from BP to Co₃O₄ effective inhibits the oxidative degradation of BP active substance, thus ensuring the excellent OER durability of Co₃O₄/BP (Figs. S12a and b in Supporting information). Most importantly, the intensive heterointerfacial coupling between BP and Co₃O₄ via the formation of Co–P bonds endows a unique electronic acceptor-donor interaction, which compels partial Co³⁺ to be reduced to the highly-active sites of divalent Co²⁺, further resulting in the vastly promotion of OER electrocatalysis [29]. The significant role of heterointerface in boosting the OER catalytic performance can also be highlighted by comparing the LSVs of Co₃O₄+BP (direct mixing of Co₃O₄ and BP) and Co₃O₄/BP heterojunction. As presented in Figs. S12c and S15 (Supporting information), in the absence of interfacial electron interaction, a poorer OER activity and stability is observed over the Co₃O₄+BP sample, which unambiguously indicate the indispensability of heterointerface in the OER electrocatalysis.

  Motivated by the beneficial heterointerfacial coupling between BP and Co₃O₄, the ORR catalytic activities of BP, Co₃O₄ and Co₃O₄/BP are evaluated additionally in O₂-saturated 0.1 mol/L KOH, while Pt/C catalyst is benchmarked. Notably, the tendency of the ORR activity is in agreement well with that of OER (Figs. 4a and g). The Co₃O₄/BP heterojunction possesses the optimum ORR electrocatalytic activity in terms of the most positive E₀ and half-wave potential (E_1/2) in comparison with those of single BP and Co₃O₄ (Table S7 in Supporting information), even the catalytic performance is comparable to the Pt/C reference catalyst. The corresponding Tafel plots are further analyzed to investigate the ORR catalytic kinetics (Fig. 4h). We confirm Co₃O₄/BP heterojunction display a Tafel slope of 98 mV/dec (Fig. 4i), which is close to that of Pt/C, suggesting an equally efficient ORR kinetic process of Co₃O₄/BP as Pt/C. The Co₃O₄/BP displays enhanced ORR catalytic activity than that of the substrate can be attributed to the high electrical conductivity of Co₃O₄/BP and the intensive heterointerfacial coupling between BP and Co₃O₄, leading to the accelerated electron transport. Moreover, the afforded Co₃O₄/BP catalyst also exhibits a satisfactory ORR durability with a slight loss of activity after 6000 continuous ORR cycles (Fig. S16 in Supporting information). The above results persuasively reveal that the afforded Co₃O₄/BP heterojunction is a promising oxygen bifunctional catalyst.

  In conclusion, a compact heterointerface between Co₃O₄ and BP has been elaborately constructed by wet ball-milling to boost the oxygen bifunctional electrocatalytic performance. The developed Co₃O₄/BP heterojunction possesses a discernible two-dimensional lamellar morphology with abundant porosity. Moreover, the formation of Co–O–P and Co–P bonds over the Co₃O₄/BP heterojunction reflect an intensive junction between BP and Co₃O₄. In particular, such heterointerface provides a unique electronic acceptor-donor interaction between Co₃O₄ and BP. Therein, due to higher Fermi level position of BP than that of Co₃O₄, facilitating the effective electrons transfer from BP to Co₃O₄. This energetically suppress the oxidative degradation of BP, and partially convert the Co³⁺ to its reduced active sites of divalent Co²⁺. Therefore, the explored Co₃O₄/BP heterojunction exhibits superior oxygen bifunctional electrocatalytic properties compared to the BP and Co₃O₄ catalysts. Importantly, by constructing the Co₃O₄/BP heterointerface provides a referential strategy to design highly active BP-based oxygen bifunctional catalysts.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (No. 62004143), the Natural Science Foundation of Hubei Province (No. 2021CFB133), the Central Government Guided Local Science and Technology Development Special Fund Project (No. 2020ZYYD033), the Opening Fund of Key Laboratory of Rare Mineral, Ministry of Natural Resources (No. KLRM-KF 202005), the Innovation Project of Engineering Research Center of Phosphorus Resources Development and Utilization of Ministry of Education (No. LCX2021003), the Open Research Fund of Key Laboratory of Material Chemistry for Energy Conversion and Storage (HUST), Ministry of Education (No. 2021JYBKF05), the Opening Fund of Key Laboratory for Green Chemical Process of Ministry of Education of Wuhan Institute of Technology (No. GCP202101), and the 13^th Graduate Education Innovation Fund of Wuhan Institute of Technology. This work is dedicated to celebrating the 50^th anniversary of Wuhan Institute of Technology.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.03.101.

  
  
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  Figure 1  Schematic demonstration of the formation of Co₃O₄/BP.

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  Figure 2  (a) XRD patterns of BP, Co₃O₄ and Co₃O₄/BP. (b) TEM images and SAED patterns of BP and (c) Co₃O₄/BP (inset is the histogram of Co₃O₄ particle size distributions). (d) AFM image and (e) EDX element mappings of Co₃O₄/BP. (f) N₂ sorption isotherms and the corresponding pore size distribution of Co₃O4/BP. (g, h) HRTEM images of Co₃O₄/BP. (i) Raman spectra of BP, Co₃O₄ and Co₃O₄/BP.

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  Figure 3  High-resolution XPS spectra of (a) O 1s, (b) Co 2p, and (c) P 2p for the Co₃O₄, BP or Co₃O₄/BP. (d) UPS spectra and (e) the corresponding energy band diagram of Co₃O₄ and BP. (f) Schematic diagram of the electron transfer in Co₃O₄/BP heterointerfaces, where the violet and yellow isosurfaces depict the heterointerfacial interaction.

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  Figure 4  (a) LSVs, (b) Tafel plots and (c) the corresponding Tafel slopes, E₀ and η₁₀ of BP, Co₃O₄, Co₃O₄/BP and RuO₂ towards OER. (d) CVs and the calculated C_dl of Co₃O₄/BP. (e) EIS plots of BP, Co₃O₄ and Co₃O₄/BP. (f) LSVs of Co₃O₄/BP before and after 1000 repetitive cycles, as well as the chronoamperometric response towards OER. (g) LSVs, (h) Tafel plots and (i) the corresponding calculated Tafel slopes, E₀ and E_1/2 of BP, Co₃O₄, Co₃O₄/BP and Pt/C towards ORR.

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