Metal-organic framework-derived highly active cobalt-nitrogen-carbon hybrid arrays for efficient hydrogen evolution reaction

The design of efficient catalysts with abundant active sites is fascinating but still is a great challenge for hydrogen evolution reaction (HER). Herein, a highly active cobalt-embedded nitrogen-doped carbon nanotube hybrid array on nickel foam (Co-NCNT/NF) was constructed to enhance the HER activity. The 3D hydroxide nanosheets were utilized as a template and reactant to form the ordered ZIF-8 array by a solid–liquid interfacial release of Zn ions and coordination reaction, and the growth of ZIF-67 on the surface of ZIF-8 arrays assures the introduction of cobalt species into the precursor, catalyzing the formation of active cobalt-nitrogen-carbon (Co-N-C) sites. Based on the structure analysis and density functional theory calculations, an optimal configuration of the cobalt (111) face coated by carbon layer with one Co atom coordination to one N and C reveals a multi-site nature of the designed Co-NCNT/NF catalyst, which provided the improved conductivity, 3D hierarchical structure, the uniform distribution, and efficient exposure of active Co–N-C sites. A small overpotential of 157 mV at a current density of − 10 mA cm−2 for HER in 1 M KOH was obtained, proving that the optimized proton adsorption on the multi-sites structure promotes the HER activity of Co-NCNT/NF.


Introduction
As an attractively renewable sources, the hydrogen has been paid more and more attention in the recent research due to the theory development of the hydrogen storage and production [1][2][3][4][5][6][7].By using the green electricity from the solar and wind energy, the hydrogen evolution reaction (HER) by water electrolysis is one of much more popular ways to produce hydrogen [8].The state-of-the-art catalysts for HER are noble metals like Pt, but their high price restricts the large-scale implementation of water electrolysis [9,10].The transition metal cobalt, with low price, high abundance and excellent electrical conductivity, has shown a great promise in electrocatalysis, especially various cobalt-based derivatives [11,12].For example, the cobalt-nitrogen-carbon catalysts (Co-N-C) [13,14], using N-doped carbon to disperse metal cobalt atoms or cobalt clusters, have been considered as a promising class of non-precious metal catalysts for HER with their unique electronic properties and structural features [15][16][17][18][19].It has been proved that the coordinated Co-N x structures act as the high active site, which contributes the intermediate H atoms (H*) to combine on the catalyst surface and proceeds the first Volmer process [20,21].In addition, the tiny Co nanoparticles or clusters coated by the N-doped carbon shell also optimize the conductivity by combining the advantage of metal and carbon [22,23].However, the preparation of Co-N-C materials must undergo the high-temperature process, during which the formation of active sites is usually limited by the agglomeration effect, inevitably disturbing the improvement of the catalyst activity.
The distribution of the active species on the substrate, such as graphene, carbon nanosheets, and various doped carbon [24][25][26][27][28][29][30], has been confirmed to be an important way to overcome the agglomeration effect, because the substrate guarantees the stronger anchoring of the active sites.Besides, the support also introduces more fluent mass transfer and 157 Page 2 of 13 faster electron conduction [31].The controllable composition and ordered structure of metal-organic framework (MOF) provides an exercisable way to obtain the Co-N-C catalyst with the expected structure, in which the Co-N-C-based catalysts could disperse on the self-supporting multi-level substrate, possessing abundant catalytic sites and high conductivity [32][33][34].For example, benefiting from the controllable annealing of a ZIF-67 thin film, Jia et al. gained a catalyst of aligned N-doped carbon nanotubes (NCNT) on a 3D macroporous polymeric substrate, which provided the abundant sites to catalyze the water decomposition, showing an enhanced electrocatalytic HER activity of 233 mV at 10 mA cm −2 [35].Later, a dense NCNT was also synthesized from ZIF-67 on the polyacrylonitrile fiber films, which exhibited a high performance in alkaline-metal-ion batteries [36].Such NCNT on self-supporting substrate with improved performance proves that the design of the Co-N-C by ZIF-67 has helped to achieve an initially spatial Co-N-C structure with the uniform sites; however, these active sites with random and single-dimension distribution are needed to be adjusted to achieve the better exposure.The aggregation and instability of MOF structures during the whole operation is a great challenge for achieving this high-quality active structure.How to construct the modified three-dimension (3D) NCNT array on the conducting materials by MOF routes, how to change the surface state and control the microenvironment around the active centers is the further focuses to improve the Co-N-C activity.
Herein, a self-supporting Co-based NCNT array anchored on nickel foam (Co-NCNT/NF) is obtained with the 3D spatial structure and abundant Co-N-C sites.For the formation and exposure of active sites, the subtly transition from zinc nitrate hydroxide nanosheets to the ordered ZIF-8 and the further epitaxy growth of ZIF-67 were performed to construct the initial 3D frame; the subsequent annealing process guarantees the successful 3D Co-NCNT nano arrays.Based on the DFT calculation and catalyst structure analysis, this multi-level Co-NCNT/NF assembled by the interlacing carbon nanotubes contains the anticipative spital environment and the local coordination structure of Co metal sites.Specially, one Co atom coordinated to one N and three C atoms above the cobalt (111) plane (Co/CoN 1 C) is found to show the multi-site feature for H* adsorption.The structure design of Co-NCNT derived from the MOF in present work is aim to overcome the aggregation of Co-based sites, retain much more active sites for HER reaction, and provide a fluent mass transfer process by utilizing the hierarchical ZIF growth.As a result, the prepared Co-NCNT/NF with the optimized structure shows the superior HER activity with a small overpotential of 157 mV at a current density of − 10 mA cm −2 for HER in 1 M KOH.
Before synthesis of zinc nitrate hydroxide nanosheets array (Zn-NSA), the Ni foam was first treated by ultrasonic in 1 M HCl and then washed with ethanol and water.In a typical synthesis, 7.5 mmol of Zn(NO 3 ) 2 •6H 2 O and 25 mmol of hexamethylenetetramine were dissolved in 100 mL of methanol, and then, 10 mL of 2 M HNO 3 aqueous solution was added.The Ni foam was sunk in this mixed solution at 60 °C for 48 h [37].After cooling down to room temperature, the surface of Ni foam was covered by white-colored Zn-NSA (Zn-NSA/NF).By rinsing the final product with methanol several times and then drying it at the oven overnight, the Zn-NSA/NF was obtained.
In order to achieve the transformation from Zn-NSA to ZIF-8, the obtained Zn-NFA/NF was simply immersed into 20 mL of 2-methylimidazole (Hmim) aqueous solution (3 M) for 6 h in indoor temperature.After that, whole sample was treated by water and then dried at 60 °C overnight, and ZIF-8/NF was obtained.The further growth of ZIF-67 on ZIF-8 layers was carried out through the similar procedures, only that the mother liquid was replaced by the Co(NO 3 ) 2 (0.05 M) and Hmim (0.4 M) methanol solution for 1 h.After taking the sample out of the above vessel, the ZIF-67/ZIF-8/ NF was washed by methanol several times and dried at the oven overnight.The single ZIF-67/NF was prepared by directly immersing the Zn-NSA/NF into the mixed solution of Co(NO 3 ) 2 and Hmim.
To prepare the Co-NCNT/NF, the ZIF-67/ZIF-8/NF precursor placed in a ceramic boat was heated to 350 °C and maintained for 1.5 h in a tube furnace and then the temperature is further increased to 700 °C at a ramp rate of 2 °C min −1 and kept for 3.5 h under an Ar/H 2 flow (95%/5% in volume ratio).The controlled samples without ZIF-67 or ZIF-8 were prepared by annealing the ZIF-8/NF or ZIF-67/ NF at 700 °C, defined as Zn-NC/NF and Co-NC/NF, respectively.For further comparison, samples annealed at different temperatures (600 °C, 700 °C, 800 °C, and 900 °C) termed as Co-NCNT-600, Co-NCNT-700, Co-NCNT-800, and Co-NCNT-900, respectively, were also prepared.

Structural characterization
The morphology of different samples was recognized by scanning electron microscopy (SEM, Hitachi S-4800, Japan) and transmission electron microscopy (TEM, JEOL, JEM-2100, Japan).X-ray diffractometry (XRD, Bruker D8 Advance diffractometer, Cu Kα radiation source) and Raman microscopy (Renishaw, 532 nm excitation laser, the light power at the exit of the laser was 10 mW) were used to test the crystalline information, and the Raman data were fitted by the Peakfit software with four typical peaks of (I peak, D peak, D" peak and G peak) [38,39].X-ray photoelectron spectroscopy (XPS, Thermal Fisher, Al Kα) was used to test the structure and composition of the prepared materials, and the Casa XPS software was used to fit the XPS data by a Shirley-type background and 30% Gaussian-Lorentzian ratio [40].

Electrochemical measurement
A three-electrode system was used to conduct the electrochemical measurement in a CHI 660D electrochemical station in 1 M KOH solution.The obtained material with 0.5 cm × 0.5 cm area was directly used as a working electrode for the HER characterization.The counter electrode was a graphite rod (99.9995%,Alfa Aesar) and reference electrode was Ag/AgCl electrode (saturated KCl).The electrode potential was given versus the reversible hydrogen electrode (RHE) in whole article.Linear sweep voltammetry (LSV) was utilized to evaluate the activity of materials, and cyclic voltammetry (CV) was used to determine the stability.Electrochemical impedance spectra (EIS) were tested from a frequency of 10 5 to 0.1 Hz at an overpotential of 175 mV.As a contrast, the HER performance of 10wt% Pt/C (Aladdin Reagent, Shanghai) was also tested.The working electrode of Pt/C catalyst was obtained by ultrasonicating its powder in a Nafion ionomer (Dupont) ethanol solution to form the slurry and then dipping it on Ni foam.

Density functional theory calculations
The density functional theory (DFT) calculations were carried out using the Vienna ab initio software package (VASP) [41].The projector augmented wave method was used, and the generalized gradient approximation (GGA) was parameterized by the Perdew-Burke-Ernzerhof (PBE) as the electron exchange-correlation function [42].The cutoff energy of 400 eV and a 2 × 2 × 1 k-point was used.The model of Co encapsulated by the N coordinated Co graphene (N x CoC) was constructed by employing a slab model to mimic the structure of Co-NCNT, in which the (111) face of fcc Co metal was covered by a monolayer of N x CoC graphene.During the structural optimization, the two bottom layers of Co atoms were fixed to mimic a semi-infinite metal solid.
The force on each atom is set to be below 0.02 eV/Å and the energy was converged within 5 × 10 −5 eV.Moreover, all calculations were spin polarized, and the dispersion correction was performed by Grimme's semiempirical DFT-D3 scheme to dispose the van der Waals (vdW) interactions in this layered system [43].
The adsorption free energy (ΔG) of the intermediates H* atom was calculated by in which ΔE H* represents the binding energy of H species and the ΔZPE and ΔS are the zero point energy correction and entropy change of adsorption H, respectively.ΔZPE and ΔS were obtained by the NIST-JANAF thermodynamics table for gaseous molecules combining with calculating the vibrational frequencies for the H* intermediates at the temperature of 298 K.The calculation of ΔZPE and ΔS for the different structure is provided in the supporting information.

Morphology structures of precursors and prepared nano arrays
To achieve the successful construction of the 3D Co-NCNT nanoarrays, three steps are referred in the precursor preparation.First, the Zn-NSA nanosheets were uniformly grown on Ni foam by a simple solvothermal reaction and the morphological detail of the Zn-NSA is shown in Fig. 1a.The thickness of every nanosheet is about 100 nm, which intertwines with each other to form an initial 3D space frame, proving the primary success of the structure design.Then, the Zn-NSA sheets were used as the sacrificial template to react with the Hmim in water to form the ZIF-8 array.As shown in Fig. 1b, the 3D configuration is maintained after that the Zn-NSA transfers to ZIF-8 array, and the thickness of lamellas is up to about 0.42 μm, which is about 4 times thicker than the Zn-NSA.As the last step of the template construction, the epitaxy growth of ZIF-67 on ZIF-8 is performed by immersing the ZIF-8 array into the mixed methanol solution of Co(NO 3 ) 2 and Hmim.The structure of the ZIF-67/ ZIF-8 is shown in Fig. 1c, and the lamellas thickness of 3D structure is further getting larger as the 2.4 μm, showing that the thickness of ZIF-67 is about 2 μm, which would provide abundant carbon, nitrogen, and cobalt source to derive the prospective Co-NCNT arrays in the subsequent process.
Besides the regular evolution of the morphology, the X-ray diffraction (XRD) patterns in Fig. 1d show the specific crystalline structure of each precursor.The obvious diffraction peak for the Zn-NSA at 9.4° is attributed to the (002) plane of layered zinc nitrate hydroxide (Zn 5 (NO 3 ) 2 (OH) 8 ), and the corresponding interplanar spacing value is 9.41 Å [37].These broad interspaces of the layered Zn 5 (NO 3 ) 2 (OH) 8 could facilitate the movement of protons and deprotonation of Hmim (Mim − ) in the inside layers, achieving a perfectly formation of ZIF-8 as displayed in the typical XRD pattern.To be detail, the diffusion of H + into the interlayer space breaks the Zn-OH bond of zinc nitrate hydroxide, and then, the free Zn 2+ ion dissociates to the surface and coordinates with the Mim − to form the ZIF-8 nuclei.The crystallization of ZIF-8 promotes more Hmim to deprotonate, and the produced H + further diffuses to react with Zn-OH of the substrate, thus resulting in the growth of ZIF-8 [37,44].The resembled XRD peaks of ZIF-67/ZIF-8/NF with the ZIF-8/NF show the successful formation of ZIF-67, and the increased intensity of diffraction peaks is from a lot massive growth of ZIF-67, which is in common with the SEM results in Fig. 1c.
The annealing treatment of the metal-organic frame (MOF) is an effective way to obtain the M-N-C materials in recent preparation of catalyst, which is benefited from the abundant metal centers and various ligand structure of MOF [45].According to our previous experiences, the catalysts with a hierarchical Co-NCNT array structure could be obtained by an annealing process for the ZIF precursor arrays on the Ar/H 2 flow at a high temperature [46].Figure 2 shows the morphology of different M-N-C catalysts derived from our designed precursors.It could be found that there is the formation of carbon layers on the Zn-NC/NF as shown in Fig. 2a.The sparse and discrete NCNT on the surface of Co-NC/NF in Fig. 2b shows that the Co sources is imperative for the formation of NCNT, but the indigent sources of C, N, and Co impede the efficient growth of NCNT due to the limited growth of ZIF-67 on the Zn-NSA as shown in Fig. S1a. Figure 2c is corresponding to the Co-NCNT/NF structure, which is obviously different from the Zn-NC/NF and Co-NC/NF by the feature of more bushy carbon nanotubes on the 3D walls.The further enlarged image certifies that the sheet-like structure is assuredly assembled by carbon nanotubes and the whole thickness is about 300 nm.From the above images of the M-N-C structure, it certifies that the ZIF structure derived from the metal hydroxide template effectively achieved 3D design idea, because all the Zn-NC/NF, Co-NC/NF, and Co-NCNT/NF maintained the 3D spatial structure after high-temperature annealing.Apart from the structure information, the elementary composition of Co-NCNT was performed by the SEM-EDS technique, and the result is shown in Fig. S1b the Co-NCNT from Co-NC, making the uniform distribution of active sites for the subsequent catalytic reaction.

Identify the carbon frame and the cobalt species
As the catalyst for the electrochemical reaction, two important parts are mainly concerned, which is the fluent electron transfer and highly active sites [13].Raman spectra are very useful technique to analyze the carbon structure.It has accurately identified the vibration of different carbon structure, such as carbon defects, graphene layers, and multi-wall carbon nanotubes [48].The typical Raman peaks of carbon-based materials could reflect the integrity of the structure and further predict their property, such as the conductivity [32].Based on this, the Raman spectra of the Zn-NC/NF, Co-NC/NF, and Co-NCNT/NF were provided in Fig. 3a.Three main peaks in Fig. 3a at about 670 cm −1 , 1355 cm −1 , and 1585 cm −1 are corresponding to the Co-O vibration, D and G peaks, respectively [18], which exhibits the successful carbonization of the different precursor and incorporation of cobalt species into the carbon frame for the Co-NC/NF and Co-NCNT/NF.The G peak is the E 2g vibration mode, reflecting in-plane C-C bond stretching and the change of carbon atoms with the bond angle.D peak is corresponding to an A 1g vibrational mode for the C-C breathing mode without change of bond angle at carbon layer edges [49][50][51].The deconvolution of Raman spectra ranging from 1000 to 1800 cm −1 was performed to reveal the structure of the Co-NCNT array in Fig. 3b.Apart from the traditional G peak and D peak, two additional peaks of I peak at 1180-1200 cm −1 and D" peak at 1450 cm −1 were introduced [39], which are ascribed to the sum and difference combinations of C = C chain stretching and C-H wagging modes, respectively [52].The change of Raman shift of D peak and I D /I G peak intensity ratio in the Zn-NC/NF, Co-NC/NF, and Co-NCNT/NF reflect the degree of graphitization and the N-doped degree of catalysts.It could find that from Zn-NC/NF to Co-NC/NF to Co-NCNT/NF, the D peak has a red shift (from 1346 to 1335 cm −1 ) in Fig. S2, suggesting a relatively decrease of N content [53].At the same time, the introduction of the Co leads to the decrease of the I D /I G (from 0.97 to 0.88), suggesting that the enhanced graphitization of the Co-NC and Co-NCNT compared with Zn-NC [54].The higher graphitization degree and lower N-doped content make the Co-NCNT have an excellent conductivity.

Highly active sites
An important aspect for HER catalysts is the highly active site, and the X-ray photoelectron survey spectrum (XPS) of the Co-NCNT/NF is provided to analyze the detailed sites structure as shown in Figs.3c, d and S3.The full spectrum in Fig. S3a shows the presence of C, N, O, and Co elements.The high-resolution spectra of C 1 s, O 1 s, N 1 s, and Co 2p are shown in Figs.3c, d and S3b, c, respectively.The binding energies of 284.8 eV and 285.7 eV in the high-resolution C 1 s spectrum (Fig. S3b) are consistent with the C-C/C = C and C-N in the carbon matrix, respectively [55], showing that N atoms have doped into the graphitic structure.To elucidate the modes of existing N species, the N 1 s spectrum of Co-NCNT is deconvoluted, which includes the pyridinic-N (398.3 eV), Co-N x (399.0 eV), pyrrolic-N (399.8 eV), and graphitic-N (401.1 eV) (Fig. 3c), respectively [16].The pyridinic-N and pyrrolic-N bonds usually introduce the obvious defects in carbon skeleton.The graphitic-N always refers to the improvement of the conductivity.Last, the highresolution Co 2p spectrum in Fig. 3d shows the existence of metallic Co (778.5 eV and 793.2 eV), Co-O (779.9 eV and 795.2 eV), and Co-N x (781.1 eV and 797.6 eV) species [32,56].Figure 3e is the proportion summary of different N and Co structure from Fig. 3c, d, in which the Co proportion is obtained from the Co 2p 3/2 .The high proportion of 36% for Co-N x in N 1 s and 22% for Co-N in Co 2p show that the successful formation of catalytic active sites [31].
To gain atomistic insight into the nature of electrochemical HER activity, the first-principles calculation based on density function theory (DFT) is performed to resolve the structure and active sites of Co-NCNT.According to the experimental results, the Co (111) face encapsulated by a CoN x coordinated graphene layer (Co/N x CCo) is designed as shown in Figs.4a and S4 to filtrate optimal structure.After geometry optimization, the carbon layer shows a strong interaction with the Co (111) slab by forming C-N, C-Co, Co-N, and Co-Co bonds, corresponding to above experiment bond structure and implying a potential change of the chemical environment of carbon surface.The intermediate H* adsorption free energies (ΔG H* ) on different models were systematically performed, especially for the Co/N x CCo with different N atoms coordinated with Co atoms on carbon layer.From the Table S2, it could find that the increasing number of N atom in carbon layer makes the ΔG H* of the Co site (Co-site) and the C site next to Co atom (Co-C-site) increasing, and the ΔG H* of the C site next to N atoms (Co-N-C-site) also shows a slightly increased tendency, suggesting that the Co/N 1 CCo with one N atom coordination is more suitable for the hydrogen adsorption.It is worth noting that the ΔG H* on Co/N 1 CCo is 0.21 eV, 0.21 eV, and 0.25 eV for the Co-site, Co-C-site, and Co-N-C-site, respectively.For comparison, the model of Co (111) face encapsulated by N-doped graphene (Co/NC) was also designed to calculate the ΔG H* , and it is regrettable that the only one active C site on Co/NC shows a relatively larger ΔG H* value of 0.34 eV.In addition, metallic Co shows a negative ΔG H* value of − 0.49 eV and the pure graphene shows a positive ΔG H* value of 1.79 eV, implying their low activity toward HER.After encapsulating Co within graphene, the ΔG H* of graphene surface was effectively tuned (0.82 eV), but it is still too high to adsorb hydrogen intermediate.As shown in a classical three-state diagram in Fig. 4b, all ΔG H* of other catalysts model is larger than the Co/N 1 CCo; therefore, it shows the optimal potential activity for the hydrogen production.The unambiguous sites of the Co-NCNT on the HER with the optimized H adsorption energy will be coupling with the practical electrochemical reaction in next.

Enhanced HER reactions
The HER performance of Co-NCNT/NF and reference samples, including Zn-NC/NF, Co-NC/NF, NF, and commercial Pt/C, was performed next using a standard three-electrode system in 1 M KOH.In Fig. 5a, the commercial Pt/C shows the best catalytic activity, and the non-noble Co-NCNT/ NF catalyst only requires an overpotential of 157 mV at a current density of 10 mA cm −2 .It could be found that the Co-NCNT/NF exhibits higher HER catalytic activity than Zn-NC/NF (290 mV) and Co-NC/NF (220 mV) in the same catalytic current density.The Tafel slopes originated from the polarization curves in Fig. 5b were used to analyze the HER catalytic mechanism.The commercial Pt/C shows the value of 46 mV dec −1 and resembles to the previous value [57].The Co-NCNT/NF gives a lower value of 88 mV dec −1 compared with that of Co-NC/NF (150 mV dec −1 ) and Zn-NC/NF (207 mV dec −1 ), indicating its much faster kinetics [58].The activity of Zn-NC/NF, Co-NC/NF, and Co-NCNT/NF is further summarized in Fig. 5c, and the introduction of cobalt effectively decreases the kinetic barrier, showing the reasonable analysis of sites in Fig. 4a.The obvious difference of Co-NCNT/NF with Co-NC/NF shows that the active sites dispersed in the 3D carbon nanotube arrays is very important for the high HER activity.The Co-NCNT/NF with the unique structure of 3D nanotubes arrays provides the abundant cobalt-based N-doped carbon sites and sufficient space to expose these sites, resulting in the more improved performance.The Nyquist plot for samples in Fig. 5d at an overpotential of 175 mV further certified this superiority, and the semi-circle of Co-NCNT/NF shows a smallest diameter compared with those of Zn-NC/NF and Co-NC/NF, suggesting its faster charge transfer process.At the same time, the inset shows that the Co-NCNT/NF has a smaller series resistance of 3.6 Ω compared with the Co-NC of 3.8 Ω, showing the improved electron conduction.The electrochemical active surface area (ECAS) is an effective way to reflect the exposure degree of active sites, which is usually obtained by measuring the double-layer capacitance (C dl ) [59].The C dl in Fig. 5e is from cyclic voltammogram curves in a non-faradic potential range under different scan rates (Fig. S5a-d), showing that the Co-NCNT/NF has a higher value of 158 mF cm −2 than that of Co-NC/NF (73 mF cm −2 ) and Zn-NC/NF (32 mF cm −2 ).Due to the proportion relationship between C dl and ECAS, the larger C dl means that there are more active sites in the Co-NCNT/NF, which results from the rich and uniform distribution of cobaltbased active sites on the dense NCNT array as certified by the above structure analysis.The stability of Co-NCNT/NF catalyst is assessed by cyclic voltammogram and currenttime curves testing, which was operated at potential from 0 V to 0.3 V vs. RHE with 1000 cycles and at 10 mA cm −2 current density, respectively.The almost no decay is observed based on the LSV curves before and after cycles in Fig. 5f, revealing the outstanding durability of Co-NCNT/ NF.The current-time curve in Fig. 5f inset also proves that Combining the relationship between structure and activity, it could be found that the N 1 CoC/Co sites have a natural advantage to maximize the hydrogen production on the basic condition.The 3D spatial structure and the NCNT distribution seem to be playing an important role for anchoring N 1 CoC/Co sites.Proving this point, the ZIF-8/ZIF-7/ NF precursor annealed at different pyrolysis temperatures was performed.As shown in Fig. 6a, the tiny NCNT on the dodecahedra surface appears when the ZIF-67/ZIF-8/NF is heated at 600 °C.The much longer CNT on dodecahedra arrays surfaces emerges with the temperature increasing to 700 °C (Fig. 6b) since Co nanoparticles on the tip of CNT could effectively drive the nanotubes to grow at higher temperature.However, further rising the temperature to 800 and 900 °C, the dimeter of NCNT becomes larger and Co nanoparticles appear to seriously aggregate (Fig. 6c-d).At the same time, the 3D array of Co-NCNT-900 also starts to collapse, implying the severely destruction of 3D structure.This regular change of the NCNT structure corresponds to their HER activity as shown in Fig. 6e.The overpotential of Co-NCNT/NF catalysts changes with the annealing temperature increasing as evidenced by the LSV curves.To be specific, the overpotential of Co-NCNT-700 is 157 mV at the catalytic current density of 10 mA cm −2 , which is lower than that of 168 mV of Co-NCNT-600, 213 mV of Co-NCNT-800, and 236 mV of Co-NCNT-900.There is an interesting detail that the activity of Co-NCNT-600 is superior than the Co-NCNT-800, which is probable to much more N 1 CCo sites on the Co-NCNT-600 due to the maintaining of strong coordination bond between cobalt ions and organic ligands at the lower annealing temperature.When the calcination temperature is higher than 800 °C, a lot of agglomeration of Co nanoparticles cause a quick loss of catalytic N x CCo sites, leading to the decrease of the HER activity.Therefore, the optimal activity of Co-NCNT-700 is attributed to the balance of the N 1 CCo sites and carbon conductivity.The maintain of the 3D porous structure and suitable nanotube size of Co-NCNT arrays derived from the multi-level MOF precursor is important to provide highly active sites for the practical catalysis, because the elaborately designed 3D structure assures the effective infiltration of electrolytes and sufficient exposure of dense active sites.The Tafel plots in Fig. 6e further certify this trend as reflected from LSV curves, where the Co-NCNT-700 gives a small slope value of 88 mV dec −1 compared with other samples, suggesting that the Co-NCNT-700 has a much more efficiently catalytic dynamic process.The lower value of x-intercept in Nyquist (Fig. S7a) means the small contact resistance of all samples, and a smaller diameter of Co-NCNT-700 shows its faster charge transfer in the HER.The improved contact resistance of Co-NCNT-700 is benefited from the optimal structure.To be detail, the higher annealing temperature of the Co-NCNT-700 provides much higher carbonization degree compared with the Co-NCNT-600, which gives the excellent intrinsic resistance of the active material.But for the Co-NCNT-800 and Co-NCNT-900, the higher annealing temperature leads to the structure collapse of Co-NCNT arrays, impeding the electron transfer between Ni foam and the catalyst, resulting in the increase resistance of the Co-NCNT-800.The C dl of the samples from Fig. S7b further proved the importance of structure, in which Co-NCNT-700 has a largest value of 158 mF cm −2 compared with other control samples.The Raman spectra in Fig. 6f of Co-NCNT catalysts show the I peak, D peak, D" peak, and G peak, and the value of I D /I G (0.79, 0.88, 0.89, 0.91) was gradually increasing as the annealing temperature increasing in Fig. S7c, which shows that the cobalt aggregation catalyzes the carbon decomposition and introduces much more carbon defects [60], leading to a loss of active sites.The Co-NCNT-700 with a balanced structure of conductivity and abundant active sites shows an improved HER activity.
Based on above results, the abundant active sites, improved conductivity and hierarchical structure of Co-NCNT/NF, achieve the improvement of HER electrocatalytic activity of Co-N-C catalyst.First, the uniformly dispersed active species on the nanotube array assures abundant active sites to adsorb the hydrogen atoms.Second, the hierarchical 3D structure facilitates to protect this Co-N-C sites and provides a fluent mass transfer process.Finally, the in situ growth of Co-NCNT nanosheets on substrate without using the Nafion or other polymeric assures the excellent electron conductivity.

Conclusion
In summary, the 3D Co-based NCNT arrays on nickel foam were obtained through designing a multi-level MOF precursor.The formation of Co-NCNT array guarantees the effective exposure of abundant Co-based sites, which exhibited superior electrocatalytic performance toward HER.The in situ nucleation of ZIF-8 on Zn-based substrate and subsequent formation of ZIF-8/ZIF-67/NF laid the foundation of the construction of superior 3D Co-NCNT arrays.The concurrence of ZIF-8 and ZIF-67 is an important factor for the high conductivity, rich active sites of Co-NCNT arrays.Moreover, the reasonable annealing temperature provides the external condition for the formation of the active sites and the maintenance of the 3D structure.The excellent HER activity is from both the abundant surface sites and the 3D space, as well as the cooperative effect from the structure of cobalt coated by carbon layer.Finally, the epitaxial growth from metal hydroxide to metal-organic framework is a potential method to design the catalyst structure, especially the multi-level structure.The synthetic method in this study is universal and could be widely extended to other application by controlling the precursor structure and reaction conditions.

Fig. 1
Fig. 1 The structure of the step-by-step precursors.a SEM images of Zn-NSA, b ZIF-8/NF, c ZIF-67/ZIF-8/NF, the bottom figure is the largescale part of the corresponding top one for every group.d XRD patterns of the NF, Zn-NSA, ZIF-8/NF, ZIF-67/ZIF-8/NF

Fig. 2
Fig. 2 The morphology structure of the Zn-NC/NF, Co-NC/NF and Co-NCNT/NF.SEM images of a Zn-NC/NF, b Co-NC/NF, c Co-NCNT/NF, the bottom figure is the large-scale part of the corresponding top one for every group.d TEM and HRTEM images of Co-NCNT/NF

Fig. 3
Fig. 3 Binding environment of the Co-NCNT probed by Raman spectroscopy and XPS. a and b the Raman spectra of Zn-NC/NF, Co-NC/ NF and Co-NCNT/NF, respectively; the XPS spectra for the c N 1 s,

Fig. 4
Fig. 4 Atomic structure of the Co-NCNT interfacial perimeter for DFT calculations.a Schematic illustration of the N 1 CoC/Co model; the Co, C, and N atoms are shown in pink, gray, and dark blue color, respectively; b the calculated free-energy diagram of the HER on C, Co, NC, C/Co, NC/Co, and N 1 CoC/Co

Fig. 5
Fig. 5 Hydrogen evolution performance.a LSV curves at a scan rate of 5 mV s −1 , b Tafel plots from the Tafel equation (η = a + blogj), c the summary of the overpotential at the current density of 10 mA cm −2