Anisotropic N-Graphene-diffused Co3O4 nanocrystals with dense upper-zone top-on-plane exposure facets as effective ORR electrocatalysts

We provide strong evidence of the effectiveness of homogenously self-propelled particle-in-particle diffusion, interaction and growth protocol. This technique was used for one-pot synthesis of novel nitrogen-graphene oxide (N-GO)/Co3O4 nanocrystals with cuboid rectangular prism-shaped nanorods (NRs) along {110}-plane and truncated polyhedrons with densely-exposed, multi-facet sites along {311} and {111} planes. These hierarchal nanocrystals create electrode catalyst patterns with vast-range accessibility to active Co2+ sites, a vascular system for the transport and retention of captured O2 molecule interiorly, and low adsorption energy and dense electron configuration surfaces during the oxygen reduction reaction (ORR). The superior electrocatalytic ORR activity of the N-GO/Co3O4 polyhedron nanocrystals in terms of electrochemical selectivity, durability and stability compared with NRs or commercial Pt/C catalysts confirms the synergetic contribution of multi-functional, dense-exposed, and actively topographic facets of polyhedrons to significantly activate the catalytic nature of the catalyst. Our findings show real evidence, for the first time that not only the large number of catalytically active Co2+ cations at the top surface layer but also the dense location of active Co2+ sites on the upper-zone top-on-plane exposure, and the electron density configuration and distribution around the Co2+ sites were important for effective ORR.


Results and Discussion
A simple synthesis based on homogenously self-propelled diffusion of heterogeneous N-GO/Co 2+ -ions/NaOH/ urea (surfactant-free) composition domains was applied to control engineering of unique N-GO/Co 3 O 4 nanocrystal structures, axially hierarchal NRs, preferential nanocrystal polyhedron orientations, multi-exposed crystal {311}, {111}, and {110} facet surfaces using one-pot protocol. As Shown in Fig. 1 and S1, the building of N-GO/Co 3 O 4 hierarchal structures through self-propelled ion-to-ion diffusion pathways associated with particle-in-particle morphological shape growth of phase composition domains can be developed by time-dependent (i) stirring-, (ii) microwave/hydrothermal-, and (iii) annealing-assisted synthesis (see experimental section). The mechanistic growth of nanocrystals through homogenously self-propelled diffusion of distributed ions/particles/seeds of cobalt precursor into GO domains may occur consecutively via four steps. The first step is that the surface intercalation between cobalt and GO sheets was occurred through the Co 2+ -ions immigration to intercalate with the oxygen-functional groups of GO sheet through electrostatic interaction. The second step is the particle-in-particle diffusive surface interaction (Fig. 1B). The GO sheets were diffusively imbedded between parallel layers of cobalt and form complementary binding interactions with Co 2+ ions through balanced thermodynamic and kinetic processes. The third step is the formation of stable-centered Co(OH) x (CO 3 ) 0.5 •0.11H 2 O seed-growth units into GO mat cavities (Fig. 1C). After prolonged time of stirring at 60 °C, the intermolecular spacing enhances the movement velocity and diffusion between Co 2+ nuclei-site/GO. As a result of seed aggregation and folding, the cobalt seeds attach and cover the GO mats, forming sandwich like-structures as stable seed-growth step of crystal units. The fourth step is the homogenous cluster-building growth in single crystals with time-dependent hydrothermal (H.T.) and microwave (MW.T.) conditions. The seed growth unit coalesces and forms clusters during aggregation. Under H.T and MW.T conditions, the potentially anisotropic crystal growth might direct the aggregated unit cluster blocks to stabilize the high-energy surface of the preferentially arranged shape morphology and crystal geometry of nanorods and polyhedrons under time-dependent microwave (MW.T.) and hydrothermal (H.T.) conditions, respectively (Fig. 1D,E). Finally, the high-temperature annealing under N 2 gas flow led to the phase transition of GO/Co(OH) x (CO 3 ) 0.5 •0.11H 2 O to thermally-stable, mesoscopic N-doped-GO/Co 3 O 4 polyhedron and NR single crystals with relatively high surface area (≈50 m 2 /g) and mesopore space cavity of ≈20 nm (see Supporting Information, Figs S2 and S3) 26,27 . Fig. 1 show evidence that the homogenously anisotropic diffusion, interaction and growth of the crystal leads to produced polyhedron with newly multi-functional surface facets such as {311} and {111} dominants, and with highly truncated morphology, for the first time. Second, the randomly distributed and densely tilting-tangles along the vertical band networks of NR patterns led to create internal effects and voids (Fig. S3). This internal channels may then be produced continuous electron parallel to the c-axial orientation along {110} plane. The phase purity of Co 3 O 4 single nanocrystals with N-GO/Co 3 O 4 composite catalysts was supported by WA-XRD analyses as depicted in Fig. 2A. All diffraction patterns indicated that the face-centered cubic (fcc) phase of Co 3 O 4 (JCPDS 42-1467) can be formed in pure single crystal with the formation of N-GO/Co 3 O 4 NRs or polyhedrons 23 . The weak diffraction peak observed at 24.2° is mainly ascribed to the (002) reflection of carbon (graphene source). The Raman spectroscopies of N-GO/Co 3 O 4 composite NR and Polyhedron single crystals shows real evidence of the engineering of N-graphene-diffused into Co 3 O 4 single crystal structures through homogeneously particle-in-particle morphological shape growth of heterogeneously organometallic composition domains (Fig. 2B). Four evident peaks were located at approximately 480, 525, 615, and 682 cm −1 which correspond to E g , F 1 2m , F 2 2g , and A 1g modes of Co 3 O 4 , respectively 28 . In addition, the distinctive peaks observed at 1345 and 1600 cm −1 are mainly due to the characteristic D-and G-bands of the graphene component, respectively 29,30 . The D-band refers to the structural defects (A 1g symmetry), whereas the G−band represents the in-plane bond-vibration (sp 2 ) of carbon atoms (Fig. 2B).
XPS measurements were performed to illustrate the compositions and chemical status of N-GO/Co 3 O 4 polyhedrons as illustrated in Figs 2C,D and S4. The wide survey XPS spectrum exhibited four sharp peaks located at 284.6, 400, 530.1, and 780.1 eV corresponding to the characteristic features of C 1 s, N 1 s, O 1 s, and Co2p of carbon, nitrogen, oxygen, and cobalt elements of the investigated specimen, respectively (Fig. S4). The high -resolution Co2p spectrum results presented two major peaks at 782 and 797 eV that matched with the low energy band (Co 2p 3/2 ) and high energy band (2p 1/2 ) spin-orbit peaks of Co 2+ 31 . The energy difference between the peaks (Fig. 2C) is nearly 14 eV and is in good agreement with the literature and further demonstrates the presence of both Co 2+ /Co 3+ species in the NR or polyhedron structure [32][33][34][35] . In addition, the weak 2p satellite features observed at binding energies at 788.5 and 803.3 eV indicate the formation of spinel structure, in which Co 2+ cations conquer the tetrahedral sites and Co 3+ cations occupy the octahedral sites in the in the crystal lattice 33 . Notably, the surface-adsorbed species can effectively reduce oxygen. The XPS analysis results of N-GO/Co 3 O 4 polyhedrons and NR samples showed that the Co 3+ /Co 2+ ratio of polyhedron (0.74) is lower than that of the nanorod structure (0.82). Moreover, the N1s spectrum show its integration from three sub-peaks that appeared at 399.2, 401.0, and 403.3 eV binding energies due to pyridinic-N, pyrrolic-N, and quaternary-N, respectively 34,35 (Fig. 2D). The quantitative analyses show that the nitrogen content in the polyhedron nanostructure determined deconvoluted high-resolution N 1 s spectra that reached approximately 5.18%. The corresponding fractions of each component of nitrogen were found to be 41.3%, 34.23%, and 23.47% for pyridinic, quaternary, and pyrrolic type, respectively. Interestingly, the ORR activity of electrocatalysts could be significantly affected by pyridinic and quaternary components of N than pyrrolic N [36][37][38] .
Together, the XRD, Raman, and XPS analyses provide strong evidence of the effectiveness of homogenously self-propelled diffusion associated with particle-in-particle growth mechanism, on which the novel, N-GO/ Co 3 O 4 NRs and polyhedrons were formed in one-pot synthesis with hierarchal engineering of (i) arrangement of heterogeneous N-GO/Co 2+ /Co 3+ /O 2− atomic sites in NR or polyhedron nanocrystals, (ii) preferential polyhedron nanocrystal-sized orientations with truncated morphology, multi-functional, high-index exposure crystal {311}, {111}, {100} or {110} facets (Fig. 1), and (iii) building of one-dimensional, heterogeneous (N-GO/Co 3 O 4 ) composites in NR nanocrystals in band-like vases spreading out from the core of their orb and along preferential c-axial direction. These features in N-GO/Co 3 O 4 catalysts may attain low adsorption energy, and dense, well-distributed electron configuration on multi-exposed surface facets and around the catalytically active Co 2+ sites, as an avenue to design of electrode nano-pattern energy devices.
The top-view field emission scanning electron microscopy (FE-SEM) micrographs show evidence of well-grown axial NR bands in corolla of a flower with a predominance of exposed regular, sharp, and smoothly flatten surfaces ( Fig. 3A-C). The NRs have a length of sub-micrometers and a width of approximately 250-350 nm, which energetically favors axial branching-like bands ended with rectangular-shaped archery at the top-surfaces ( Fig. 3A-C). The NR are mainly consisting of cuboid rectangular prism or brick crystals grown along preferentially exposed-breadth {110} dominant planes, with an axial-lengthy direction of {1-10} and its opposite {100} planes. Those three plane sets are mutually perpendicular (Fig. 3G). This results elucidate that the fabrication strategy offers control over the anisotropic crystal growth of N-GO/Co 3 O 4 NRs in cuboid rectangular prism morphology along {110}-plane that spreads out vertically from the core/orb of NR band-like vases to the exposed rectangular-top-surfaces without defects or distortion, for the first time. Furthermore, the dense      fringes, indicating the structure of single crystal of N-GO/Co 3 O 4 truncated polyhedron. This genuine facet-dependent polyhedron may render the catalyst surfaces to have dense-exposed, multi-facet sites and then develop a mat-like tangle of stable surface patterns for vast-range accessibility to active sites, and create catalyst surfaces with low adsorption, and binding energies.
Significantly, Fig. 4E shows that nanoscale (2 nm-breadth) prickle-like ridges and hocked cavities were marginally covered the {311}-plane edges of the polyhedron, leading to decrease the s binding energy of O 2 molecules onto surfaces, and enhance dense location of active Co 2+ sites on the upper-zone top-on-plane (Fig. 4E). The HR-HAADF-STEM image shows well-distributed GO mat as grass-skin along all edge and bulk surface facets, indicating the effective of synthesis method to well-controlled, homogenous diffusion of GO into Co 3 O 4 nanocrsytals (Fig. 4E). The N-GO blade forming grassy surface mats at the edged scale ridges of {311}-N-GO/Co 3 O 4 (see Arrow, Fig. 4E) leads markedly to captured O 2 molecules and sustained relaxation of the active site Co 2+ atoms after 3000 times reuse/cycles of ORR. Figure 4H-L shows BF-STEM image of pronounced multi-exposed surfaces of N-GO/Co 3 O 4 polyhedrons and the corresponding elemental mapping results with the atomic configuration and distribution of Co, O, N, and C elements within (i) the truncated polyhedron architectures and (ii) crystal surface facets. The STEM-EDS images exhibited a marked atom-to-atom anisotropic growth geometry of truncated polyhedrons with well-distributed atomic surface active sites along all exposed low-and high-index planes. Increasing the number of topographic facets, well-distribution of actively atomic sites in polyhedron or NR particle-like islands are key to design catalytic electrodes for ORR.
The electrocatalytic performance of N-GO/Co 3 O 4 hybrid nanocrystals and commercially available Pt/C catalysts for ORR was initially evaluated by rotating disc electrode (RDE) measurements in O 2 -saturated KOH (0.1 M) at 1600 rpm under similar catalyst loadings. In O 2 -saturated solution, both nanorods and polyhedrons, such as N-GO/Co 3 O 4 hybrids (Fig. 5A, curves c and d), show higher activities than bare Co 3 O 4 polyhedrons, as demonstrated by their current densities. When GO was incorporated into Co 3 O 4 , the hybrids exhibit large cathodic currents. This finding indicates that the enhanced electrochemically electroactive surfaces of GO promoted Co 3 O 4 . The enhanced ORR electroactivity of the N-GO/Co 3 O 4 hybrids indicates that cobalt species synergistically coordinate with graphene matrix and nitrogen functionalities rather than stand alone as Co 3 O 4 . In addition, Co-N bonds play a vital role in increasing the electron density on the surface Co atoms of the N-GO/Co 3 O 4 hybrids, thereby enhancing their electrochemical properties.
The presence of numerous surface groups on the GO sheets enhances the valence state of the metal ions, leading to rapid reaction kinetics. The embedding of GO into the hierarchal structure introduces atomic charge density, which enables facile charge transfer from the carbon matrices to the adsorbed oxygen molecules and subsequent formation of superoxide ions. Consequently, the O-O bond weakens, and the ORR activity improves.
This finding demonstrate the synergetic role of N-graphene-promoted Co 3 O 4 nanocrystals to boost (i) the integrative GO mats with high in-plane conductivity that acts as an electronic trigger for shuttling electrons and (ii) N-heteroatoms in nanocrystals are more beneficial for enhancing the ORR catalytic performance of the electrocatalysts. As shown in Fig. 5A, the cathodic current density at 0.05 V (vs. RHE) of the investigated catalysts follow the order N-GO/Co 3 . Accordingly, the current density in the linear sweep voltammograms is dependent on the accessibility of the active sites for ORR. The onset potential confirms the intrinsic features of the catalytically active sites (Table S1).
Additionally, the ORR onset potential and half-wave potential of the N-GO/Co 3 O 4 polyhedrons (0.91 and 0.81 V, respectively) are lower than those of commercial Pt/C (0.94 and 0.84 V, respectively) (Fig. 4A, curve e) but relatively higher than those of the N-GO/Co 3 O 4 nanorods (0.88 and 0.78 V, respectively), Co 3 O 4 polyhedrons (0.85 and 0.76 V, respectively), and N-GO (0.81 and 0.72 V, respectively). The intrinsic ORR activity of the N-GO/ Co 3 O 4 polyhedron catalysts with close-matching features to that of commercially available Pt/C catalyst is mainly attributed to massive exposed surface, and high-index active site planes of truncated polyhedrons may lead to a vast-range accessibility of electron to pass along the top-charge Co 2+ -surfaces and also the in-depth center of the {111}, and {311} crystal planes. Therefore, the charge acceptance of the Co 2+ species remarkably differs between the polyhedrons and nanorods.
ORR is achieved at electroactive sites associated with the oxidation state cations (Co 2+ ) of the Co 3 O 4 surfaces 20 . These Co 2+ active sites serve as donor-acceptor reduction sites for Co 3 O 4 by capturing electrons and consolidate the electronic configuration of the O 2 molecules throughout ORR 39 . These reports suggested that Co 2+ cations are the main active sites for ORR; moreover, the amount of Co 2+ species formed on the N-GO/Co 3 O 4 polyhedron surface is higher than that on the other catalysts. The electrocatalytic performance of the proposed catalysts is comparable with that of similar electrocatalysts reported previously.
A series of linear sweep voltammograms (LSVs) were performed at different rotating speeds (400-2000 rpm) in O 2 -saturated KOH (0.1 M) solution to determine the number of electrons transferred (n) and illustrate the ORR kinetics of N-GO/Co 3 O 4 polyhedron nanocrystals and commercial Pt/C catalysts (Fig. S5A-D). Based on the collected LSV curves, the limiting current density of the catalysts increased with increasing rotation speed because of fast oxygen flux to the catalyst surface. The corresponding Koutecky−Levich (K−L) plots (J −1 vs. ω −0.5 ) within the potential range of 0.2-0.5 V versus RHE display good linearity of the fitted lines; hence, the reaction belongs to the first-order kinetics with respect to oxygen molecules (Fig. S5B,D). The number of electrons The ORR at the N-GO/Co 3 O 4 polyhedrons proceeds through direct four-electron pathway. These findings demonstrate the superior electrocatalytic ORR activity of the N-GO/Co 3 O 4 polyhedron nanocrystals and further confirm the synergistic contribution of (i) multifunctional and active topographic facets, (ii) high-index, dense-exposed, and homogenously well-distributed active sites (island-like), and (iii) prickle-like ridges, hocked cavities, and N-GO blade of grass surfaces covered the {311}-plane edges of polyhedrons to activate the catalytic nature of the catalyst significantly.
RRDE measurements were performed in O 2 -saturated KOH (0.1 M) solution to determine the amount of hydrogen peroxide species (HO 2− ) formed during ORR process at the disk electrode for the N-GO/Co 3 O 4 polyhedron catalyst (Fig. 5B-E). The RRDE analysis was further conducted to compare n values between N-GO/Co 3 O 4 polyhedrons and Pt/C catalysts (Fig. S5B,C)  proceeds through four-electron mechanisms similar to that catalyzed by Pt/C (Fig. S5D,E). As shown in Fig. S5C, the hybrid polyhedrons possess a small ring current due to HO 2− intermediates (hydrogen peroxide oxidation). According to the RRDE responses, the percentages of HO 2− species with respect to the oxygen reduction products are 9.27% and 7.12% for N-GO/Co 3 O 4 polyhedrons and Pt/C, respectively (Fig. S5D,E). Our finding indicated that the H 2 O 2 yield of the mesoporous N-GO/Co 3 O 4 polyhedrons is lower than that of the MnCo 2 O 4 /N-doped graphene hybrid 25 . Hence, the mesoporous N-GO/Co 3 O 4 polyhedrons hold considerable potential as energy technologies in the future.
The electrocatalytic durability is a significant factor that should be considered to develop the practical application of electrocatalysts. Continuous potential sweeps for 3000 cycles were performed in O 2 -saturated KOH (0.1 M) solution at room temperature under a rotation speed of 1600 rpm to compare the structural stability of the composite polyhedrons with that of the Pt/C catalyst (Fig. S6A,B). Under the same experimental conditions, the LSV measurements of the N-GO/Co 3 O 4 polyhedrons before and after 3000 cycles (Fig. S6A) exhibit remarkable long-term structure stability compared with that of Pt/C, indicating the remarkable cyclability and reproducibility of the polyhedrons. The N-GO/Co 3 O 4 polyhedrons display a small activity loss with negative shift in the E 1/2 of 15 mV for an extended period. For the commercially available Pt/C, the E 1/2 decreases by about 32 mV (Fig. S6B) after continuous potential scan, despite the more reactive property of Pt than that of the metal oxides. Pt nanoparticles undergo severe aggregation to form large particles because of their high surface energy 40,41 . Moreover, the weak contact of the Pt nanoparticles to the carbon support can substantially decrease the ORR activity of Pt 40 . Hence, we can conclude that the synthesized N-GO/Co 3 O 4 polyhedrons possess superior structural stability with efficient retention of electrocatalytic activity. The improved ORR performance of the N-GO/Co 3 O 4 polyhedrons can be ascribed to the (i) synergistic interaction between the Co species and GO surface groups, leading to distinct structural stability and fast charge mobility at the catalyst/electrolyte interfaces and facilitating the reaction pathway; and (ii) abundance of electroactive sites, which directly modify the relative positioning of the conduction bands and improves the electronic configuration of the active catalyst, thereby increasing the catalytic efficiency.
The durability of the as-synthesized catalysts and commercial Pt/C was assessed and compared with the chronoamperometric spectra at + 0.8 V versus RHF in an O 2 -saturated KOH (0.1 M) solution by using a rotating disk electrode to confirm the superior electrochemical performance of the N-GO/Co 3 O 4 hybrids. As shown in Fig. 5F, the ORR current densities of the as-grown N-GO/Co 3 O 4 polyhedrons and nanorods are 7.58% and 11.25% degradation, respectively, over 60,000 s of continuous operation. By contrast, the commercial Pt/C electrode shows remarkably decreased voltammetric current by about 22.32% under the same testing conditions relative to its initial activity; this finding indicates the poor stability of the Pt/C electrode. The electrochemical stability of the N-GO/Co 3 O 4 catalysts is significantly enhanced, and the as-grown nanostructures favor the stability, an indispensable feature for high-performance energy systems 42,43 .
Crossover influences due to methanol and stability are critical factors that affect the ORR electrocatalyst performance in fuel cells. To assess the tolerance of the N-GO/Co 3 O 4 polyhedrons and Pt/C catalyst to methanol crossover, we mixed methanol ( To investigate the crystal facet-dependent ORR catalytic activity, as schematically illustrated in Figs (6 and S9), we quantitatively determined the charge surface density, electron distribution and configuration around the active surface Co 2+ /Co 3+ sites, number of active-top-surface atoms, and adsorption surface energy along plane surfaces of {110}-NR, and {111}-, and {311}-polyhedron nanocrystal facets by DFT calculations. The distinct molecular design of nanocrystals for catalytic activity enhancement of the exposed surface sites is essential to improve the overall reaction kinetics. The densities of Co 3+ and Co 2+ active sites in the Co 3 O 4 catalyst can provide good electrocatalytic activity toward ORR because the concentration of the catalytically active Co 2+ sites on the surface of cobalt oxides positively influences the ORR reactivity. The spinel Co 3 O 4 revealed surface-dependent catalysis owing to the change in Co 3+ /Co 2+ percentages on the exposed surface facets [45][46][47] .
The ORR electroactivity of the as-synthesized cobalt oxides is sensitive to the number of Co 2+ cations by controlling the growth mechanism of their morphologies. Many Co 3 O 4 nanostructures are also enclosed by {111} and {110} planes. To the best of our knowledge, polyhedrons, such as Co 3 O 4 nanostructures with {111} and {311} exposed facets, have not been reported yet.
More importantly, DFT calculations (Figs 6 and S9) shows first evidence that those three key factors of (i) the large number of the catalytically active Co 2+ cations at the whole top surface layers of crystal, (ii) dense location of active Co 2+ sites on the upper-zone top-on-plane exposure, and (iii) the electron density configuration and distribution around the Co 2+ sites can favorably tune the electrochemical performances of the electrocatalyst for ORR. The configuration of surface-oxygen molecule adsorption on the first sublayer of Co 2+ sites is more complex than that on the topmost surface layer. Such configuration resulted in volcano-shaped dependence based on the effect of the activation and desorption features of O 2 molecules on the catalyst surface during ORR. Therefore, the location of Co 2+ sites on various exposed planes significantly influences the catalytic activity toward ORR. Additionally, the corresponding strength of oxygen binding influences the ORR kinetics. The improved ORR activities of the Co 3 O 4 nanocrystals might be associated with exposed facets and Co 2+ /Co 3+ valence states formed by oxygen vacancies. On the {111} and {311} exposed surfaces, O 2 is first selectively adopted on the subsurface oxygen vacancy due to the abundant negative charges. Consequently, the Co−O bond is formed. The adsorbed O 2 captures the free electrons at oxygen vacancies and produces radical groups, which subsequently enhance the reduction of O 2 through the four-electron pathway 48 .
In general, the O 2 molecules are attracted to the catalytic active sites (Co 2+ ) based on the Pauling mode. These active sites can efficiently transport electron to the absorbed O 2 molecules 47 , which clearly indicates that the active Co 2+ sites are catalytically active sites rather than Co 3+ sites.
Furthermore, the valence species of metal oxides are considered catalytically active sites for activating adsorbed O 2 and ion diffusion. Fast electron transport due to dense exposure of Co 2+ sites also favors O 2 − adsorption. The coexisting Co-O surface plays a synergistic role in breaking the O-O bond 49  − adsorption barrier on the active surfaces. The Co 2+ atom onto exposed {111} and {311} planes is preferentially oriented around the most-negative potential (electron clouds, red color) surfaces, leading to highly adsorbed oxygen molecule (O 2 ) reactants. Significantly, with {111}-and {113}-polyhedron crystal planes, the Co 2+ atom location was found in the dense electron-configuration and cloud-distribution on the upper-zone top-surfaces compared with {110} NR-top-surface exposures ( Fig. 6A-C). As shown from electrostatic potential energy map (ESP-EM) patterns, the 2-Co 2+ -{311}-top-surface showed a hyper electron-dense-cloud location compared with the exposed 4-Co 2+ -{111}-top-surface along the multi-functional facets of polyhedron crystals.
The as-grown cobalt oxide clusters become more conductive than that of the semiconducting cobalt oxide clusters, thereby promoting facile electron transport from the valence band to the conduction band because Co 2+ can more easily lose electrons than oxygen ions or other Co ions; hence, Co 2+ is the active site. These features accelerate the electron mobility onto Co 2+ centers, rate-determining processes, and ORR kinetics. In addition, the Co atoms are coordinated with the surrounding oxygen atoms at the top surface layers. The oxygen-rich Co 3 O 4 {111} and {311} surfaces with fully covered active sites present high catalytic activity for ORR due to facile electron transport from O 2 2− to the surfaces. Thus, the intermolecular charge transfer enables a net positive charge to enhance the ORR activity.
The catalytic mechanism allows the oxygen vacancies at the active top surfaces to significantly enhance the ORR activity [50][51][52] . The synergistic role of oxygen vacancies and exposed surfaces favors the improvement of the electrocatalytic performance for ORR. The DFT analysis indicates that the strong bonding between Co species and adsorbed O 2 molecules on the generated oxygen vacancies could stretch the O-O bonds, leading to dissociation of O-O bonds. This finding is in agreement with that reported by Tompsett et al. 50 , who confirmed that the presence of oxygen vacancy enhances the catalytic activity of MnO 2 toward ORR. Therefore, the generated oxygen vacancies are necessary for chemical reactions because they can strongly bind O 2 molecules and facilitate their dissociation.
The DFT calculation indicates that the adsorption energy of oxygen molecules onto exposed polyhedron and NR crystal surface facets was determined to be approximately −5.9, −5.86, and −5.67 kcal/mol for {311}, {111}, and {110} planes, respectively. This finding indicates that ORR activity onto exposed surfaces increases in this trend {110} < {111} < {311} planes. Thus, the oxygen molecules will easily adsorb onto the active {311} plane surfaces and strongly bond due to the unsaturated dangling bonds leading to better reaction kinetics than that of {111} and {110} planes. Figure S9 shows evidence that the upper-top-on-plane configuration of the single crystal NRs or polyhedrons is banged by catalytically active Co 2+ sites that significantly affected oxygen molecular diffusion and adsorption, and the electron/charge transport along the reactive facets (see also Fig. 6A-C). The Co 2+ density in the upper-zone top-surface planes decreases in the order of {311} > {110} ≥ {111}. The fully exposed Co 2+ top surfaces of the {111}, {311}, and {110} planes show four, two, and three catalytic active Co 2+ sites, respectively. Although the {111} facet affords a large number of active Co 2+ sites in both inner-/upper-zone plan of top-surface layers, but the high-index{311}-surface catalysts exhibited the highest ORR reactivity, as evidenced from the adsorption energy of oxygen molecules onto {311} active sites. Polyhedrons, such as Co 3 O 4 hybrid nanostructures with predominantly exposed {111}/{311} planes, possess significantly enhanced ORR electrocatalytic activity than that of the Co 3 O 4 hybrid nanorods due to abundant Co 2+ sites at the {111}/{311} reactive planes. Therefore, fast electron transfer from Co 2+ sites can considerably promote the catalytic activity for ORR 20,24 . With its facet-mediated behavior, cobalt spinel positive ions with dense charges (Co 2+ ) occupy holes generated by oxygen ions. Thus, the Co 2+ ions can effectively participate in the ORR due to enhanced dissociative adsorption and charge mobility. This result indicates that the process proceeds with the aid of Co 2+ ions due to its single e g occupancy 53 .
Our DFT calculations show real evidence, for the first time that not only the large number of catalytically active Co 2+ cations at the top surface layer but also the dense location of active Co 2+ sites on the upper-zone top-on-plane exposure. Moreover, the electron density configuration and distribution around the Co 2+ sites are important for effective ORR because they reduce the side reactions associated with carbon and electrolyte.
In general, the polyhedron nanocrystals with high index, multifunctional {111} and {311} exposure facets, reactive Co 2+ sites in both inner/upper zone top-on-plane surfaces, and a hyper electron-dense-cloud location are key components in improving the O 2 adsorption and diffusion and fast kinetics of electrons. The O 2 molecules are efficiently absorbed in the active sites, and the Co atoms are transformed into their oxidized state. Accordingly, the induced charge at the catalytically active centers can evolve the chemisorption mode of O 2 molecules. This reaction weakens the O-O bond and enhances the kinetics of ORR 54 . The ORR is a surface-structure-sensitive reaction on electrodes and is attributed to the accessible active sites corresponding to the cations on the catalyst surfaces. According to the DFT analysis, the exposed Co 2+ cations are more accessible than Co 3+ , indicating that the Co 2+ ions are determinant in catalyzing O 2 molecules and enabling high efficiency, reducing ORR overpotential, and enhancing cycle stability 55,56 . Thus, for polyhedrons, such as N-GO/Co 3 O 4 nanocrystals, the atomic binding energy of oxygen largely governs the electronic modification of Co 2+ sites engendered by the construction of an interfacial heterostructure between cobalt oxide and GO 57 . Notably, the Co 3+ /Co 2+ ratio measured by XPS observation is in agreement with the electrochemical results obtained for ORR. In addition to surface exposure, the Co 2+ active species play a pivotal role in the catalytic performance for ORR.
In summary, one-pot synthesis strategy offered a potentially effective control for (i) homogeneously atomic orientations of mesoscopic N-GO/Co 3 O 4 nanocrystals from heterogeneous composition (surfactant-free domain), (ii) preferential polyhedron nanocrystal-sized orientations with truncated morphology, multi-functional, high-index exposure crystal {311}, {111}, {100} or {110} facets, and (iii) building of one-dimensional, cuboid rectangular prism-shaped NR nanocrystals in band-like vases spreading out axially from the core of their orb to the exposed rectangular-top-surfaces in the same direction of axial-lengthy, tunnel-line channel {1-10} plane may act as a vascular system for the transport and retention of captured O 2 molecule interiorly. These features in N-GO/Co 3 O 4 architectures create effective catalysts with low adsorption energy, and dense, well-distributed electron configuration on multi-exposed surface facets and around the catalytically active Co 2+ sites, as an avenue to design electrode nano-pattern for ORR. Significantly, with {111}-and {113}-polyhedron crystal planes, the Co 2+ atom location was found in the dense electron-configuration and cloud-distribution on the upper-zone top-surfaces compared with {110} NR-top-surface exposures. As a result, the polyhedron catalysts show better ORR activity than that of NR nanocrystals.
In addition, the polyhedron nanocrystals revealed outstanding electrocatalytic ORR performance and superior stability comparable to commercial Pt/C. Results also demonstrate that polyhedrons can significantly catalyze oxygen molecules via a 4 − electron pathway with excellent methanol tolerance. In particular, the polyhedron nanostructures exhibit superior durability after 3000 continuous potential cycles comparable to commercially available Pt/C catalyst. The present study can serve as basis for future designing of cost-effective and efficient electrocatalysts as alternatives to Pt-based catalysts for practical application in fuel cells and other technological energy systems. Experimental section. Unique N-GO/Co 3 O 4 nanocrystals of cuboid rectangular prism-shaped NRs and truncated polyhedrons were fabricated by using homogenously self-propelled particle-in-particle diffusion, interaction and growth protocol under the peculiarity of heating treatment (Figs 1 and S1). The polyhedron and NR architectures were synthesized through one-pot, time-dependent hydrothermal (H.T.) and microwave (MW.T.) treatments, respectively. The N-enriched hierarchal surfaces were obtained by a simple high-temperature annealing process under N 2 gas flow, as evidenced from STEM-EDS profile (Fig. 4K).
First, N-GO/Co 3 O 4 nanocrystals of cuboid rectangular prism-shaped NRs were fabricated as follows: cobalt acetate tetrahydrate (Co(C 2 H 3 O 2 ) 2 .4H 2 O, 5 mM) and urea (CH 4 N 2 O, 12 mM) were dissolved in 37 mL of DI-H 2 O with stirring until a clear solution was obtained. Approximately 60 mg of solid GO, which was fabricated from graphite powder (see supporting information), was added to the Co 2+ ion/urea solution under stirring for 1 h. The mixture was vigorously stirred for 16 h at 60 °C to form a homogeneous mixture, and then transferred into a 75 mL Teflon-lined autoclave. The anisotropic crystal growth of N-GO/Co 3 O 4 NRs was occurred by treating the homogenous mixture thermally at 160 °C for 2 h under microwave irradiation (600 W). After cooling to 25 °C, the solid yield was rinsed several times using DI-H 2 O and absolute ethanol for removing the unreacted and soluble impurities and was subsequently dried at 60 °C overnight.
Second, N-GO/Co 3 O 4 nanocrystals of truncated polyhedrons were fabricated according to the typical synthesis as follows: 25 mM cobalt nitrate hexahydrate (Co(NO 3 ) 2 ·6H 2 O) and 6 mM NaOH were mixed under stirring in 60 mL of DI-H 2 O. Then, 60 mg of the as-prepared GO was immersed into the above solution under vigorous stirring for 16 h 60 °C to form a homogeneous mixture. The mixture was loaded into a Teflon-lined stainless steel autoclave and maintained at 180 °C for 8 h before cooling naturally to RT. The precursor was centrifuged, rinsed thoroughly with DI-H 2 Oand ethanol for removing the unreacted ions, and finally dried.
Third, the N-GO/Co 3 O 4 NR and polyhedron catalysts were obtained by annealing the precursors in a programmable furnace under nitrogen gas flow at 550 °C for 4 h with ramping rate 5 °C min −1 . The loading amount of GO in the N-GO/Co 3 O 4 NR and polyhedron composites was approximately of 8.44 wt%, and 8.15 wt%, respectively. The resulting N-GO/Co 3 O 4 NR and polyhedron catalysts were used for the electrochemical ORR (see supporting information).