Ultrathin Metal–Organic Framework Nanosheets-Derived Yolk–Shell Ni 0.85 Se@NC with Rich Se-Vacancies for Enhanced Water Electrolysis

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Nov 2021Ultrathin Metal–Organic Framework Nanosheets-Derived Yolk–Shell Ni0.85[email protected] with Rich Se-Vacancies for Enhanced Water Electrolysis Zhao-Di Huang, Chao Feng, Jian-Peng Sun, Ben Xu, Tian-Xiang Huang, Xiao-Kang Wang, Fang-Na Dai and Dao-Feng Sun Zhao-Di Huang College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 Google Scholar More articles by this author , Chao Feng State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao, Shandong 266580 Google Scholar More articles by this author , Jian-Peng Sun College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 Google Scholar More articles by this author , Ben Xu College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 Google Scholar More articles by this author , Tian-Xiang Huang State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao, Shandong 266580 Google Scholar More articles by this author , Xiao-Kang Wang College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 Google Scholar More articles by this author , Fang-Na Dai *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 Google Scholar More articles by this author and Dao-Feng Sun *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao, Shandong 266580 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000537 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail We present a controlled fabrication of selective ultrathin metal–organic framework (MOF) nanosheets as preassembling platforms, yolk–shell structured with a few-layered N-doped carbon (NC) shell-encapsulated Ni0.85Se core (denoted as Ni0.85[email protected]) via an oriented phase modulation (OPM) strategy. The ultrathin nature of the MOF nanosheets gave rise to the modification of structure at the electronic level with abundant Se-vacancies and effective electronic coupling via an Ni–Nx coordination at the interface between the Ni0.85Se core and NC shell. The Ni0.85[email protected] obtained exhibited low overpotentials for both oxygen evolution reaction (OER; 300 mV) and hydrogen evolution reaction (HER; 157 mV) at 10 mA·cm−2 under an alkaline condition, outperforming their corresponding bulk MOF-derived counterparts. By exploiting Ni0.85[email protected] as anode and cathode catalysts, a low cell voltage of 1.61 V was achieved by performing alkaline water electrolysis. Remarkably, it also reached a high activity in natural seawater (pH = 7.98) and simulated seawater (pH = 7.86) electrolytes, even surpassing integrated Pt/C-RuO2/CC electrodes. Density functional theory (DFT) studies illustrated that abundant Se-vacancies effectively regulated the electronic structure of Ni0.85[email protected] by accelerating electron transfer from Ni to N atoms at the interface, and thus, enabling the Ni0.85[email protected] to attain a near-optimal electronic configuration that stimulated ideal adsorption-free energy toward key reaction intermediates. Download figure Download PowerPoint Introduction Electrochemical water splitting, including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), holds great potential as a promising technology for sustainable storage, conversion, and transportation of hydrogen energy, and it strongly depends on the thermodynamics, along with kinetic parameters of its two half-reactions.1–3 Considering the sustainability of H2 production, seawater electrolysis is an eminently desirable path owing to its earth-abundant reserves, accounting for 96.5% of the world’s total water resources. Although Pt- and Ru/Ir-based electrocatalysts are recognized as reducing reaction energy barriers and boosting seawater splitting efficiency, their limited earth-abundance and expensive market-price hamper their extensive applications enormously.4,5 Accordingly, tremendous endeavors have been undertaken to search for advanced alternatives to precious metal electrocatalysts. Transition metal–based nanomaterials with the feature of earth-abundant, low cost, and high efficiency have been widely explored as promising overall seawater splitting electrocatalysts.6,7 Among them, Ni–Se nanomaterials (e.g., Ni0.85Se, NiSe, Ni3Se2, and NiSe2) stand out as they bear the advantages of large anionic size, accelerated electron mobility, and suitable bandgaps, which could accelerate electron transfer and enlarge their inherent catalytic sites.8–11 However, most Ni–Se particles easily agglomerate in seawater electrolysis and have difficulty in holding durative catalytic active sites.12,13 Therefore, the development of effective bifunctional electrocatalysts for water splitting, and especially show good catalytic activity, as well as durability in seawater, are demonstrated to be exceedingly desirable. By targeting the improvement in stimulating intrinsic activity and bearing better tolerance, heteroatom-doped carbon matrix (HCM)-encapsulated transition metal (TM)-based core–shell nanomaterials ([email protected]) would provide abundant electrocatalytic active sites, high electrical conductivity, and suitable carbon protective layers.14,15 Using metal–organic frameworks (MOFs) as superior precursors to controllably fabricate [email protected] nanomaterials could partially achieve the above-mentioned goals.16–19 Compared with bulk MOF, a thinner MOF tends to acquire more accessible active sites with enhanced catalytic activity.20–23 For example, Li et al.24 selected a thick-layered MOF (∼30 nm) as a template to construct Co3O4/CBDC nanosheet arrays and used it as an OER electrocatalyst. However, ultrathin MOF nanosheets with higher aspect ratio and ultrathin characteristics than bulk MOFs and thick-layered MOFs are supposed to be ideal preassembling platforms: First, the ultrathin nature of MOF nanosheets enables it to curl naturally during a calcination process, such that the carbon skeleton from the MOF ligands form a stable, protective layer, thereby avoiding a corrosion. Meanwhile, it builds an inimitable functional interface, rendering it convenient for systematic investigation of its different components’ interaction mechanisms. Second, the ultrathin characteristics of MOF nanosheets enable them to form a few-layers, not a multilayer, and an HCM shell after pyrolysis, leading to a well-defined core–shell/yolk–shell structure. Such a particular architecture could provide a high percentage of exposed active centers to enhance electron-transfer ability, aiming to optimize the intrinsic adsorption free energy. Third, under pyrolysis conditions, the ultrathin MOF nanosheets could effectively trigger the creation of anion-vacancies or defects, which are conductive based on the modification of intrinsic catalytic activity at the electronic level, thereby improving the seawater-splitting performance. Herein, inspired by the above-mentioned expected merits, we chose a well-defined ultrathin MOF nanosheets (∼5 nm, [Ni(HBTC)(DABCO)·3DMF], HBTC = trimesic acid, DABCO = 1,4-diazabicyclo [2.2.2] octane, DMF = N,N-dimethylmethanamide) as preassembling platforms. Favorably, it was possible to synthesize the core–shell NiSe2@NC nanomaterials through pyrolysis, followed by selenization of ultrathin MOF nanosheets. The oriented structural and compositional transformations from orthorhombic NiSe2@NC to hexagonal Ni0.85[email protected] were revealed, resulting in the formation of yolk–shell Ni0.85[email protected] nanomaterials. The ultrathin nature of MOF nanosheets contributed to the formation of a few-layer N-doped carbon (NC) shell and abundant Se-vacancies for effective electronic coupling, thereby effectively tuning the electronic structure of the Ni0.85[email protected] nanomaterial. As expected, the Ni0.85[email protected] obtained exhibited low overpotentials in the overall-water-splitting to produce O2 and H2 with a low cell voltage of 1.61 V at 10 mA·cm−2 in an alkaline medium, outperforming the corresponding bulk MOF-derived counterparts. Remarkably, the synthesized Ni0.85[email protected] also demonstrated excellent performance in natural seawater (pH = 7.98) and simulated seawater (pH = 7.86), even outperforming integrated Pt/C-RuO2/CC electrodes. Density functional theory (DFT) studies illustrated that the C atoms next to the pyrrolic N could serve as the most active OER sites. In contrast, the C atoms in the para-position of pyrrolic N facilitated HER. Meanwhile, the abundant Se-vacancies enhanced the binding strength of O-containing intermediates, resulting in a change of the rate-determining step (RDS) from O* → OOH* to O* → OH*, which was conducive to the optimized OER kinetics. Also, it further elevated the charge-transfer efficiency significantly in the coupling interface via the chemical bond Ni−Nx, which synergistically favored the H- and O-containing intermediates’ adsorption and activation. Experimental Section Preparation of ultrathin MOF nanosheets The ultrathin MOF nanosheets were synthesized according to a method described previously,25 with a slight modification. First, 0.145 g (0.5 mmol) of Ni(NO3)2·6H2O, 0.056 g (0.5 mmol) of DABCO, 0.053 g (0.25 mmol) of 1,3,5-Benzenetricarboxylic acid (H3BTC), and 1 g of polyvinylpyrrolidone (PVP) were dissolved in DMF (10 mL) solution. The mixture was stirred at room temperature for 30 min at 600 rpm. Then it was transferred to a 25 mL reaction vessel, packaged, and transferred to an oven at 120 °C for 24 h. After cooling naturally to room temperature, the product was separated by centrifugation (8000 rpm for 30 min) three times with DMF and methanol. Then a final pale green powder was obtained as ultrathin MOF nanosheets, which were activated by drying in vacuum at 80 °C for 12 h. Synthesis of ultrathin MOF nanosheets-derived [email protected], NiSe2@NC, and Ni0.85[email protected] For the synthesis of [email protected] nanomaterials, activated MOF nanosheets were transferred to a square porcelain boat, followed by placing in a furnace, and heating to 600 °C for 2 h with a heating rate of 5 °C·min−1 under an argon atmosphere. The NiSe2@NC nanomaterials were synthesized by mixing the as-obtained [email protected] powder with selenium (Se) powder evenly in the mortar at a mass ratio of 1∶2, and then the mixture transferred to a furnace. The furnace was heated at 350 °C for 2 h to obtain NiSe2@NC. Subsequently, the Ni0.85[email protected] was synthesized by further annealing the as-prepared NiSe2@NC at 850 °C for 2 h. Preparation of bulk MOF The preparation of bulk MOF was similar to that of ultrathin MOF nanosheets, except that no PVP was used. Synthesis of bulk MOF-derived [email protected], B-NiSe2@NC, and B-Ni0.85[email protected] nanomaterials The preparations of [email protected], B-NiSe2@NC, and B-Ni0.85[email protected] nanomaterials were similar to those of ultrathin MOF nanosheets-derived nanomaterials, except that the precursor for the reaction was the bulk MOF. Characterization of the synthesized nanomaterials Powder X-ray diffraction (XRD) was performed on a Bruker AXS D8 (Cu Kα radiation, λ = 1.5406 Å) under an operating voltage of 40 kV and a current of 30 mA. The morphology was characterized by scanning electron microscopy (SEM; JEOL JSM-6330, Beijing, China) and high-resolution transmission electron microscopy (HRTEM; Hitachi JEM-2100F, Beijing, China). Thermogravimetric analysis (TGA) curves were recorded on a Mettler Toledo instrument (Shanghai, China) at a heating rate of 10 °C·min−1 in the range of 40–800 °C under an O2 atmosphere. Nitrogen physisorption isotherms were recorded at 77 K using an Autosorb-iQ nitrogen volumetric adsorption instrument (Quantachrome Instruments, Shanghai, China). Before measurement, the samples were degassed at 100 °C for 12 h. X-ray photoelectron spectra (XPS) analysis was performed on an ESCALAB 250 (Thermo Electron Corporation, Shanghai, China) with Al Kα radiation (1486.6 eV). The in situ electron paramagnetic resonance (EPR) measurement was performed using an ENDOR spectrometer (JEOL ES-ED3X, Beijing, China) at a liquid nitrogen temperature of 77 K. The g factor was obtained using manganese (Mn) signal as an internal standard. Preparation for electrochemical measurements Working electrodes The prepared nanomaterial samples (5 mg) and Nafion solution (50 μL, 5 wt %) were dispersed in a mixture of deionized (DI) water (450 μL) and ethanol (500 μL). The mixture was sonicated for 30 min to form a homogeneous ink. Then, 5 μL of the ink was drop-dried on a polished glassy carbon electrode (GCE) with the requirement of a diameter of 3 mm (loading: 0.35 mg·cm−2). Before taking measurements, the GCE loaded with active materials was allowed to dry at room temperature for 1 h. As a comparison, 5.0 mg of commercial Pt/C and RuO2 powders were also dispersed on a polished GCE using the same approach. Electrochemical measurements The electrocatalytic activity was measured on a Gamry Potentiostat Reference 3000 electrochemical workstation (Ningbo Gamry Optical Instrument Co. Ltd., Shanghai, China) with a standard three-electrode system. An Ag/AgCl (KCl saturated) electrode and a carbon rod (3 mm in diameter) were used as reference and counter electrode, respectively. All current densities were normalized to the geometrical surface area, and all potentials were converted to the reversible hydrogen electrode (RHE), according to the equation ( E RHE = E Ag / AgCl + 0.22 + 0.059 pH ) . OER measurements For the OER test, 1.0 M KOH solution was used as an aqueous electrolyte and bubbled with O2 for at least 30 min before taking the electrochemical measurement. The linear sweep voltammetry (LSV) curves were obtained by sweeping the potential from −1 to −1.6 V (vs RHE) at a scan rate of 5 mV·s−1. The electrochemical surface areas (ECSAs) were investigated by measuring the double-layer capacitance (Cdl) via cyclic voltammograms (CVs), according to the equation (ECSA = Cdl/Cs, where Cs is the specific capacitance and taken as 0.85 mF·cm−2).26 The CV curves were measured at various scan rates (40–200 mV·s−1) in the potential range from 0.25 to 0.35 V (vs RHE). The electrochemical impedance spectroscopy (EIS) was performed on the Gamry potentiostat, evaluating a frequency range from 105–10−2 Hz at −1.5 V (vs RHE) using a 10 mV amplitude. The stability measurements were carried out using CV between −1.05 and −1.35 V (vs RHE) for 2000 cycles and long-term chronoamperometry under a fixed voltage of 1.57 V. HER measurements The process of HER measurements was similar to that of OER, but a flow of N2 was employed instead of a flow of O2 to ensure an H2/H2O equilibrium. Overall water splitting Typically, 5 mg active samples were dispersed in a water/ethanol solution (500 μL, 3:1 v/v) with 25 μL of Nafion solution by sonicating for 2 h to form a homogeneous ink. Then, 200 μL of the uniform ink was drop-casted on carbon cloth (CC; 1.0 × 1.0 cm2) and dried at room temperature (mass loading: 1.0 mg·cm−2). A piece of CC was carefully pretreated by sonication in 6.0 M HCl, DI water, and ethanol for 15 min, respectively, to remove surface oxide. In this study, CC was chosen as the conductive substrate because it is porous and has a negligible catalytic or noncatalytic activity in the investigated potential region. Overall seawater splitting For overall seawater splitting measurements, the typical process was similar to the overall water splitting, except for electrolyte differences. Natural seawater (pH = 7.98) electrolyte was collected from Golden Beach (Qingdao, Shandong Province) and filtered to remove visible impurities before use. Simulated seawater (pH = 7.86) electrolyte was prepared by mixing 6.683 g of NaCl, 0.565 g MgCl2, 0.813 g MgSO4, 0.280 g CaCl2, 0.048 g NaHCO3, 0.87 g Na2SO4, and 0.180 g KCl in 250 mL of DI water (18 MΩ), according to a previous report.27 Computational details Spin-polarized DFT calculations were performed using the Vienna Ab initio Simulation Packages (VASP; the Chinese Academy of Sciences, Beijing) and employed using the generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) functional to describe the exchange and correlation energy in all calculations.28 The projector augmented wave (PAW) method was used to represent the interactions between valence electrons and ionic cores. The plane wave cutoff energy was fixed at 400 eV. To model Ni0.85[email protected] nanohybrids, an N-doped graphene layer adsorbed on Ni-terminated Ni0.85Se slab with exposed (1 1 1) surface was used, resulting in a model with the lowest lattice mismatch for the following calculation. The supercell consists of 2 × 2 unit cells for Ni0.85Se slab and 4 × 4 unit cells for the N-doped graphene layer with a 15 Å vacuum region to simulate the adsorption. For the Ni0.85[email protected] model, the top two layers together with the adsorbates were fully relaxed in all dimensions until the maximum force on a single atom was smaller than 0.02 eV·Å−1, and the energy and force convergence was set to 1 × 10−4 Ha. The Brillouin zone was sampled by the Monkhorst–Pack method with a 2 × 2 × 1 k-point mesh. For the HER, the Gibbs free energy (ΔGH*) is calculated as follows29: Δ G H * = Δ E H + Δ E ZPH − T Δ S H ΔEH, ΔEZPE, and ΔSH are the adsorption energy of hydrogen, the zero-point energy difference, and the entropy difference, respectively. In standard conditions, ΔEZPE − TΔSH is ∼0.24 eV. Hence, ΔGH* is calculated using the equation ΔEH + 0.24. The OER estimation follows four elementary steps. The free energies of the intermediates at 298.15 K were obtained by: Δ G = Δ E + Δ E ZPE − T Δ S − e U where ΔS and U are the zero-point energy changes, entropy changes, and applied potentials. ΔE is the binding energy of adsorption species HO*, O*, and HOO*, with defined as follows: Δ E = E substrate + adsorbate − E substrate − E adsorbate Finally, the theoretical overpotential η is determined by the potential limiting step: η = max [ Δ G HO * , Δ G O * , Δ G HOO * , Δ G O 2 ] / e − 1.23 [ V ] Results and Discussion Ultrathin MOF nanosheets [Ni(HBTC)(DABCO)·3DMF] were synthesized and characterized initially, as described in the Experimental Section. The XRD pattern, Brunauer–Emmett–Teller (BET) measurements, and Flourier transformation infrared (FT-IR) spectroscopy spectra of the MOF framework agreed well with previous reports, suggesting their high-phase purity ( Supporting Information Figures S1 and S2).25,30 Further, combined characterizations of TEM, coupled with atomic force microscopy (AFM) ( Supporting Information Figure S3a), unveiled the MOF possessed loosely packed nanosheet morphology with a 5 nm thickness ( Supporting Information Figure S3b). Since the theoretical interlayer distance of the ultrathin MOF nanosheets was calculated to be 0.96 nm ( Supporting Information Figures S4a and S4b), it could be inferred rationally that the layer number of the as-obtained ultrathin MOF nanosheets is ∼5 ± 1. Using the expected ultrathin MOF nanosheets as a chemically tunable platform, the yolk–shell Ni0.85[email protected] with rich Se-vacancies was fabricated by an exquisite oriented phase modulation (OPM) strategy, as illustrated schematically in Figure 1, and described in the following three steps as “ligand cleavage,” “selenization reaction,” and “Se-vacancies creation.” Initially, the ultrathin MOF nanosheets were calcinated at 600 °C under Ar atmosphere, through which the ligands and Ni2+ were carbonized and reduced, constructing into [email protected] core–shell nanomaterials. The nanosheet surfaces became rough, as shown in the TEM images with Ni nanoparticles uniformly distributed ( Supporting Information Figure S5a). The XRD diffraction peaked at ∼44.5°, 51.85°, and 76.37° indicate metallic Ni species (PDF no. 04-0850; Supporting Information Figure S5b). Subsequently, the prepared [email protected] nanomaterials were further converted into NiSe2@NC nanomaterials through a controllable selenization reaction at 350 °C. Finally, the Ni0.85[email protected] nanomaterials with rich Se-vacancies were synthesized successfully by calcining the NiSe2@NC nanomaterials at 850 °C. Figure 1 | The conversion process of core–shell NiSe2@NC into yolk–shell Ni0.85[email protected] derived from ultrathin MOF nanosheets. Download figure Download PowerPoint EPR spectra of Ni0.85[email protected] and NiSe2@NC structures provided valuable fingerprinting information about the unpaired electrons with the observed g value of 2.003, indicating the existence of Se-vacancies (Figure 2a). Also, it demonstrated that strongly increased EPR signals were generated in Ni0.85[email protected], compared with NiSe2@NC, verifying the effectiveness of an OPM strategy in stimulating abundant Se-vacancies.31,32 We also performed TGA differential scanning calorimetry (TGA–DSC) measurements of NiSe2@NC in the N2 atmosphere to dynamically monitor the compositional transformation progress. As depicted in Figure 2b, three temperature ranges of mass loss are apparent, corresponding to the carbonization of organic ligands (area A, 350–450 °C), the generation of Se vacancies (area B, 450–650 °C), and the oriented structure transformation from orthorhombic NiSe2@NC to hexagonal Ni0.85[email protected] (area C, 650–850 °C), respectively. The corresponding DSC curve proved that the three processes were exothermic, evidenced by concave valley peaks (marked with a, b, and c). In contrast, the Ni0.85[email protected] nanomaterials obtained exhibited excellent thermal stability. No apparent mass loss was observed in the tested TGA temperature range from 40 to 850 °C ( Supporting Information Figure S6). In previous studies, similar phase transformations from CoSe2@NC to [email protected] nanomaterials were reported, and the fabrication of CoSe2@NC was achieved through direct selenization of ZIF-67.33,34 However, the direct-selenization strategy could not be applied to our MOF precursors for the fabrication of NiSe2@NC nanomaterials. Instead, the initial ligand cleavage treatment was used for further selenization and phase transformation. It is worth mentioning that the phase transformation from NiSe2@NC to Ni0.85[email protected] nanomaterials was accompanied by evolution from the core–shell to the yolk–shell. The formation of the yolk–shell structure was due to the Kirkendall effect caused by the different diffusion rates of metal atoms during the Se-vacancies creation process.35,36 As characterized by SEM and TEM images, it was clear that NiSe2 and Ni0.85Se nanoparticles with an average diameter of ∼15–20 nm were anchored on the surface of an onion-like carbon shell (Figures 2c and 2e, Supporting Information Figure S7). The HRTEM images in Figures 2d and 2f show that the interspacing lattice fringes observed in the nanoparticle were 0.244 and 0.35 nm, corresponding to NiSe2 (211) and Ni0.85Se (110), respectively. The lattice fringes of the carbon shell with the 0.34 nm belong to C (002) plane. Besides, the emergence of internal voids effectively proves the evolution from the core–shell to yolk–shell. Such a unique yolk–shell structure could protect the active species from migration and pulverization, shorting the charge-transfer paths and facilitate the electrolyte’s permeation.37,38 The elemental distribution mapping ( Supporting Information Figure S8) and energy-dispersive spectra (EDS) ( Supporting Information Figure S9) of Ni0.85[email protected] nanomaterials showed the coexistence and homogeneous distribution of Se, Ni, C, and N. Based on EDS, the atomic ratio of Ni/Se was calculated to be 0.89, implying the formation of Ni0.85[email protected] nanomaterials. The selected area electron diffraction (SAED) pattern of Ni0.85[email protected] nanomaterials, shown in Supporting Information Figure S10, displays a series of well-defined rings. The rings labeled with white were assigned to the (101), (102), and (201) planes of hexagonal-type Ni0.85Se, while a green ring was assigned to the (002) plane of carbon, consistent with the HRTEM results. Figure 2 | (a) EPR spectra of NiSe2@NC and Ni0.85[email protected] nanomaterials. (b) TGA–DSC curves of NiSe2@NC nanomaterials performed in N2 atmosphere. (c and e) TEM images, and (d and f) HRTEM images of NiSe2@NC and Ni0.85[email protected] nanomaterials. Download figure Download PowerPoint The powder XRD studies further illustrated the phase transformation from NiSe2@NC to Ni0.85[email protected] As shown in Figure 3a, the XRD pattern of NiSe2@NC nanomaterials could be indexed easily to orthorhombic-type NiSe2 in space group Pnnm (58) with lattice parameters of a = 4.89, b = 5.96, c = 3.67, α = β = γ = 90 (PDF no.18-0886). In contrast, the XRD pattern of Ni0.85[email protected] could be indexed to hexagonal-type Ni0.85Se in space group P63/mmc (194) with lattice parameters of a = b = 3.624, c = 5.288, α = β = 90, γ = 120 (PDF no.18-0888). Moreover, inductively coupled plasma optical emission spectrometry (ICP-OES) results further verified the measured atomic ratio of Ni to Se changes from 0.5 (NiSe2@NC) to 0.86 (Ni0.85[email protected]). The aforementioned results strongly indicated the phase transformation from NiSe2@NC to Ni0.85[email protected] Note that because of the much higher crystallization of Ni–Se phases than that of NC shells, no diffraction peaks could be identified for carbon in the thermally-treated products. Yet, based on the Raman spectra in Figure 3b, the presence of graphite D- and G-bands indicated the existence of carbon, and the ratio of ID/IG increased from NiSe2@NC (0.84) to Ni0.85[email protected] (1.01), suggesting that more disordered and defective carbons were present in the Ni0.85[email protected] nanomaterials. Nitrogen adsorption–desorption isotherms and pore-size distribution curves of Ni0.85[email protected] nanomaterials are shown in the Supporting Information Figure S11. Accordingly, the BET surface area is 148.5 m2·g−1, and the pore-size distribution were in the range of 2–10 nm. The unique mesoporous channels and large surface areas could accelerate the mass/charge transfer, further guaranteeing a highly exposed electrochemically active area for H2 and O2 production.39 To analyze the surface electronic properties and bonding states of elements during the oriented structural transformation process, XPS was conducted toward the Ni0.85[email protected] and NiSe2@NC nanomaterials. The XPS survey spectra undoubtedly revealed the existence of main reactive elements, including Se, Ni, C and N, and O elements possibly arise from inevitable air oxidation ( Supporting Information Figure S12). For the NiSe2@NC nanomaterials, the Se 3d spectrum could be fitted into two peaks at the binding energies of 54.3 and 55.2 eV (Figure 3c), corresponding to the Se2− 3d5/2 and Se2− 3d3/2. After conversion to Ni0.85[email protected], the Se 3d spectrum emerged at another valence state centered at 56.7 and 55.8 eV and was ascribed to Se− 3d5/2 and Se− 3d3/2.40,41 Meanwhile, the peak intensity of Se− was higher than that of Se2−. Collectively, the above-mentioned results showed that Ni0.85[email protected] had a mixed-valence state of Se2− and Se−, and Se− was the main valence state, which implied the creation of Se-vacancies. The Ni 2p spectra for NiSe2@NC and Ni0.85[email protected] nanomaterials displayed the same shape, as illustrated in Figure 3d. For Ni 2p, two peaks at 852.9 eV (±0.2 eV) and 855.7 eV (±0.2 eV) was attributed to Ni–Se bonds, and binding energy peaks at ∼870.2 eV (±0.2 eV) and 873.6 eV (±0.1 eV) depicted the appearance of Ni–O bonds. The corresponding satellite peaks appear at 878.85 eV (±0.05 eV) and 860.5 eV (±0.5 eV). Besides, the C 1s spectra (Figure 3e) could be fitted in