Ni or FeO nanocrystal-integrated hollow (solid) N-doped carbon nanospheres: preparation, characterization and electrochemical properties†‡

Yucang Liang *^a, Jonathan David Oettinger ^a, Peng Zhang ^b and Bin Xu *^b
aInstitut für Anorganische Chemie, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany. E-mail: yucang.liang@uni-tuebingen.de
^bState Key Laboratory of Organic-Inorganic Composites, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, P. R. China. E-mail: xubin@mail.buct.edu.cn

First published on 19th June 2020

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Introduction

Hollow micro-/nano-structured (multi)metal oxide and carbon spheres have a crucial role in cutting edge innovations due to their unique structural (pore structure, morphology, hollow cavity capacity, etc.) and functionalization (surface properties, etc.) characteristics, giving them promising potential for energy conversion/storage,^1–3 catalysis,^4–7 adsorption,⁸ and biomedicine^9,10 applications. In terms of the practical application of catalyst and electrode materials, the fabrication of controllable hollow cavities can not only offer a void space that can serve as an efficient electrochemical reaction chamber and reservoir for mass filling, diffusion and transportation,¹¹ but also can effectively buffer the volume expansion of an electrode material during the charge and discharge process; hence, this can be greatly beneficial for the assuagement of capacity fading and the development of high-energy electrodes.^3,12 Much effort has been focused on the preparation of hollow materials, especially hollow carbon nanospheres (HCNSs), via using a metal or metal oxide/silica sphere-based hard templating approach; however, this is a complicated and tedious multistep process with low yield and significant time and reagent costs.^2,6,8,9,12 Soft templating or template-free strategies are becoming a facile and efficient route for preparing hollow spherical materials with high yields; for example, a microemulsion pathway and carbonization were adopted to fabricate (metal-integrated) HCNSs for catalysis.⁴ The kinetically controlled growth of aminophenol–formaldehyde resin spheres and post-treatment can also be used to prepare single-/multi-shell HCNSs with different cavities in the absence of a templating reagent.¹³

In these above-mentioned HCNSs, the integration of a metal, metal oxide or metal complex often can alter or promote the chemical and physical properties of the final composite, such as the catalytic activity^4–7 and battery-suitable properties.¹¹ Hence, metal/metal oxide or sulfide-based (nitrogen-doped) (N)HCNSs have attracted wide attention for the synthesis and application of battery materials and catalysts. Fe₂O₃@C hollow nanospheres derived from metal–organic frameworks were used as an anode material in lithium-ion batteries (LIBs) to investigate and understand the lithium storage mechanism.¹⁴ Fe₃O₄@HCNSs as an anode material for LIBs showed a high reversible capacity, excellent cycling stability and a high-rate capability.¹⁵ N-Doped carbon-coated hollow Fe₃O₄ also showed appealing electrochemical performance in LIBs.¹⁶ Co₃O₄/NHCNSs, a hierarchically structured material prepared via a templating approach and post-calcination treatment, and Co_xS@NHCNSs are promising electrode materials for high-performance supercapacitors¹⁷ and superb sodium-/lithium-ion batteries.¹⁸ Recently, Co₃O₄/C hybrid hollow spheres were suggested as a promising material for advanced sodium-ion batteries (SIBs) thanks to their enhanced structural stability and fast electrode kinetics.¹⁹ Manganese oxide-based hollow carbon nanospheres MnO_x@HCNSs,²⁰ rGO@MnO_x@HCNSs,²¹ and mesoporous hollow S@C@MnO₂ nanospheres²² have been explored for use in high-performance supercapacitors and lithium–sulfur batteries. In terms of Ni(II)-based hollow carbon nanospheres as battery materials, NiO nanosheet-anchored hollow carbon bowls (NiO@HCNBs),²³ NiS@HCNSs,²⁴ and NiCo₂S₄/HCNSs²⁵ were studied as anode materials and exhibited enhanced lithium storage properties, high adsorption abilities for polysulfides in high performance lithium–sulfur batteries, and remarkable specific capacitance and cycle stability. Moreover, Mo-based materials have been widely investigated in terms of rational design and precise preparation for applications in lithium/sodium storage, L/SIBs, and supercapacitors, including materials such as MoO₂@HCNSs,²⁶ MoS₂@hollow mesoporous carbon spheres (MoS₂@HMCSs),²⁷ MoS₂/MoO₂-@NHCNSs,²⁸ MoS₂-rGO/HCNSs (rGO: reduced graphene oxide),²⁹ and MoSe₂@HCNSs.³⁰ In fact, before these investigations, nanostructured SnO₂@HCNSs,³¹ prepared via a hard-templating approach, subtly integrated the advantages of both SnO₂ and hollow carbon spheres together and showed exceptional cycling performance, charge rate capability, and high capacity as an anode material. In 2016, SnO₂/N-doped carbon submicro-boxes were further fabricated and showed enhanced lithium storage properties.³² Furthermore, V-/Ti-/Zn-/Ce-based battery materials, including V₂O₃@HCNSs,³³ Na₂Ti₃O₇@NHCNSs,³⁴ TiO₂@HCNSs,³⁵ ZnSe@HCNSs,³⁶ and NCNSs@HCeO₂³⁷ have been successively reported. It is interesting to find that investigations into metals with low oxidation states or metal nanoparticles on hollow carbon nanospheres have been almost neglected. TiO@HCNSs with low-valent Ti prepared through a templating method and carbonization showed improved capacitance and prolonged cycle life in advanced Li–S batteries.³⁸ A previous study revealed that the reaction Sn + xLi⁺ + xe⁻ ↔ Li_xSn (0 ≤ x ≤ 4.4) in LIBs is reversible, which is beneficial for the recyclability of the alloying and dealloying processes and the repeated release and storage of lithium,³⁹ and Sn(0)@C nanoboxes or nanofibers were hence rationally designed and prepared for use as battery materials, showing enhanced electrochemical performance.^40–43 Metallic cobalt nanoparticles were encapsulated with nitrogen-enriched graphene (NG) to generate a Co@NG composite showing bifunctional electrocatalysis properties, and it could be applied in zinc-air batteries.⁴⁴ In addition, NiCo/N-doped carbon acted as a catalyst, showing high catalytic activity and durability for robust oxygen reduction reaction performance.⁴⁵

Herein, series of metallic Ni(0) and FeO-integrated hollow N-doped carbon nanospheres, Ni(0)-HNCNSs and FeO-HNCNSs, were designed and successfully prepared via the monodisperse NiO or FeO_x nanocrystal (NC)-integrated microemulsion-directed formation of hollow phenolic resin nanospheres (PRNSs) and a subsequent carbonization process. The metal content in the final Ni(0) or FeO-HNCNSs depended upon the original amount of metal oxide nanocrystal used. For the as-made NiO or FeO_x-PRNSs, in terms of a single hollow nanosphere, a typical core–shell hollow structure was observed, and the visible nanocrystals depended upon the particle size of the original nanocrystals used. After carbonization, the enrichment of metal Ni and FeO nanoparticles in Ni(0)-HNCNSs and FeO-HNCNSs led to the heterogeneous distribution of the metal or metal oxide. To increase and enhance the uniform distribution of metal in Ni(0)-HNCNSs, NiCl₂·6H₂O was used to replace NiO nanocrystals; Ni(0)-integrated hollow (solid) N-doped carbon microspheres (Ni(0)-H(S)NCMSs) were obtained, and the size and distribution of the Ni(0) nanoparticles were well controlled. Moreover, the electrochemical properties of Ni(0)-HNCNSs and FeO-HNCNSs as anode materials for lithium-ion batteries were studied.

Results and discussion

Monodisperse NiO nanocrystals

As a (photo)catalyst and precursor material for the preparation of nanostructured thin film that is applied to photocathodes in p-type dye-sensitized solar cells, monodisperse NiO NCs were previously prepared by thermal decomposition via the air oxidation of monodisperse metallic Ni nanoparticles.^46,47 However, the synthesis of monodisperse phase-pure NiO nanocrystals is still an enormous challenge,⁴⁸ due to the easy formation of a Ni and NiO mixture without the assistance of other reagents.^49–52 Herein, a temperature-controlled thermal decomposition reaction was applied to prepare monodisperse snowflake-like arrayed NiO nanocrystals using nickel oleate as the Ni precursor in the presence of oleic acid and 1-octadecene with a high boiling point at 290–295 °C for a time of between 20 min and 2 h. TEM image (Fig. 1a) clearly uncovered that the NiO nanocrystals were encapsulated by the oleic acid stabilizer to form a monodisperse snowflake- or rice-like structure with an average size of around 3 nm. Infrared resonance spectrum (not shown) indicated that the characteristic vibrations of C–H, CO, C–O, and Ni–O bonds appeared at 2954–2850, 1556, 1462, 1410, and 500 cm⁻¹ from oleate groups and NiO. According to elemental analysis, the NiO and stabilizer content values in 100 g of NiO nanocrystals are 76.71 and 23.29 wt%, respectively. To understand the formation of monodisperse and snowflake-like NiO nanocrystals, the reaction-time-dependent structural evolution was investigated by TEM. As can be seen in Fig. 1b–e, after being reacted at 290 °C for 20 min, loose or flocculent NiO aggregation with an average size of 17.9 ± 1.8 nm was formed. Upon increasing the reaction time from 20 to 60 min, more and more NiO nanocrystals aggregated together to form dense and bigger snowflake-like colloidal NiO clusters combined with oleate stabilizer; the size of the snowflake-like aggregation changed from 21.2 ± 2.1 nm to 25.7 ± 2.3 nm, while the size of a single NiO nanoparticle did not show any significant alteration. Afterwards, no obvious changes in size were observed in the reaction-time range of 60 to 105 min. However, a drastic change occurred after the reaction lasted for 2 h; the aggregation of NiO nanocrystals disintegrated into smaller fragments and individual nanograins, but single NiO nanocrystal still retained the same particle size as before. As a representative example, an SEM image of NiO aggregation after a reaction time of 45 min indicates the sphere-like aggregation (Fig. 1f), clearly showing the presence of the oleate stabilizer. Furthermore, NiO crystals showed face-centred cubic symmetry (Fmm), as confirmed by the XRD pattern in Fig. 2a.

However, when the reaction temperature and time were changed to 295 °C for 1.5 h and 300 °C for 30 min and 1.5 h, pure NiO and phase-mixed Ni and NiO nanocrystals were obtained in different cases, as verified by XRD patterns (Fig. 2a) and TEM images (Fig. 3). It is clear that the solid obtained at 295 °C for 1.5 h shows sphere-like aggregation with an average size of 54.9 ± 5.6 nm (Fig. 3a) and snowflake-like NiO NC arrays (Fig. 3b). The average size of an individual NiO NC is 3.4 ± 0.4 nm. For such large NiO nanocrystal aggregation, the actual content values of NiO and stabilizer are 80.4 and 19.6 wt%, respectively, and the NiO content is higher than in monodisperse snowflake- and rice-like structures with small size. This result is in good agreement with the idea that highly monodisperse nanocrystals need more stabilizer molecules. When the reaction was run at 300 °C for 30 min, mixed phases of NiO (Fmm) and Ni NCs (P6₃/mmc) were simultaneously formed (Fig. 2a), and the NiO NCs showed loose spherical aggregation with an average size of 2.6 ± 0.2 nm from TEM images (4.0 ± 0.3 nm from SEM images), while most of the Ni NCs had spherical morphology with an average size of 17.7 ± 3.2 nm from SEM images (17.3 ± 4.1 nm from TEM images). With the extension of the reaction time, dominant Ni NCs with hexagonal symmetry (P6₃/mmc) were generated, and this was accompanied by the formation of a trace amount of face-centred cubic Ni NCs (Fmm) and the preservation of a trace amount of NiO NCs. Under identical conditions but with a reaction temperature less than 290 °C, it is difficult to obtain high-quality NiO nanocrystals with good yield. These investigations clearly revealed that phase-pure NiO NCs could only be prepared in a narrow temperature range in the present system, revealing sensitive temperature dependence.

Ni-HNCNSs-n (n = 1, 2, 3)

According to the schematically illustrated strategy in Fig. 4a, the preparation of Ni-HNCNSs included two steps: NiO nanocrystals were first encapsulated via an in situ phenolic resin reaction involving 2,4-dihydroxybenzoic acid and formaldehyde (derived from the reaction of hexamethylenetetramine (HMT)) with water ((CH₂)₆N₄ + 6H₂O = 4 NH₃ + 6 HCHO) in an aqueous microemulsion system to form NiO-PRNSs; then, the product was carbonized under Ar protection to obtain Ni(0)-HNCNSs. In this study, all nitrogen contents derived from the doping of HMT in NiO-PRNSs. Note that the reaction of HMT with water produced formaldehyde and ammonia during the initial reflux process; the former then reacted with 2,4-dihydroxybenzoic acid in the presence of the latter as a basic mediator to form PR, while the latter as a reaction reagent also reacted with carboxylic acid groups from 2,4-dihydroxylbenzoic acid to form ammonium carboxylate species. During carbonization, the decomposition of partially integrated HMT in PR with ammonium carboxylate species produced ammonia, which was doped into the carbon framework to form pyridinic N and graphitic N species. These species in the carbon spheres can be further confirmed via X-ray photoelectron spectroscopy (XPS) analysis.

Herein, when monodisperse NiO nanocrystals prepared at 290 °C for 2 h were used with increasing NiO NC content from 0.08 to 0.15 g, as-made NiO-PRNSs-n (n = 1, 2, 3) were obtained. As can be seen in Fig. 5, for NiO-PRNSs-1, a representative SEM image clearly shows the formation of NiO-integrated phenolic resin nanospheres, while bowl-like morphology was also observed (Fig. 5a). The TEM image reveals semi-moon-shaped hollow carbon nanospheres accompanied by the existence of NiO-filled solid nanospheres, but it is difficult to observe small NiO nanocrystals in the hollow nanospheres (Fig. 5b). The average diameter of the hollow nanospheres measured from TEM images is 184.7 ± 4.6 nm. After carbonization, the NiO nanocrystals were converted into metallic Ni(0) nanoparticles and Ni(0)-HNCNSs-1 was obtained. To the best of our knowledge, these Ni(0)-integrated hollow N-doped graphite carbon nanospheres are here prepared for the first time, although graphene-encapsulated metallic nickel nanoparticles derived from the pyrolysis of Ni-MOF spheres were recently reported.⁵³

For Ni(0)-HNCNSs-1, SEM image confirmed the preservation of spherical morphology and TEM image showed that metallic Ni particles were inlaid into the carbon shell; the average size was 4.3 ± 0.2 nm and, meanwhile, the average particle size of Ni(0)-HNCNSs-1 decreased to 136.6 ± 2.3 nm due to structural contraction during carbonization. In fact, in all hollow N-doped carbon nanospheres, the distribution of Ni particles is not homogeneous, and some HNCNSs did not contain Ni particles. This can be attributed to the inhomogeneous integration of hydrophobic NiO nanocrystals into the aqueous micellar solution of sodium oleate (SO) and the triblock copolymer P123. Only hydrophobic NiO nanocrystals were integrated into the partially hydrophilic micelles and encapsulated by phenolic resin to form NiO-PRNSs, while the phenolic resin reaction also simultaneously occurred in the micellar system. As a result, HPRNSs with and without NiO nanocrystals were obtained at the same time, and this is readily understandable.

To further investigate the chemical composition and valence states of the elements of Ni(0)-HNCNSs-1, X-ray photoelectron spectroscopy (XPS) analysis was performed. As seen in Fig. 6, the high-resolution C 1s XPS spectrum features three peaks at 284.9, 285.7, and 288.9 eV, which derive from the C 1s binding energies of CC, C–N, and CO, respectively. The O 1s XPS spectrum shows two characteristic peaks at 532.8 and 533.8 eV that can be correspondingly designated to the binding energies of C–OH and CO groups. Due to the doping of N originating from the decomposition of partially integrated HMT and PR with ammonium carboxylate species during the synthetic process, two N 1s peaks are detected at 398.2 and 401.2 eV, which can be attributed to the electronic states of pyridinic N and graphitic N, respectively. For the doping of N into carbon framework, a previous investigation uncovered that the main N source was NH₃ or N₂.⁵⁴ Moreover, the Ni 2p XPS spectrum indicates two sets of relatively weak peaks at 853.5 and 870.9 eV, and 857.1 and 874.4 eV; the first two peaks correspond to the binding energies of Ni 2p_3/2 and Ni 2p_1/2 from metallic Ni, and the last two peaks are very weak, relating to the binding energies of Ni 2p_3/2 and Ni 2p_1/2 from a very trace amount of Ni²⁺ being oxidized by air. The appearance of weak Ni 2p peaks in the XPS spectrum also implies a low Ni content in Ni(0)-HNCNSs-1. These results are in good agreement with the experimental set-up. For comparison, elemental analysis confirmed that the C, N, and Ni content values are 82.42, 0.44 and 11.47 wt%, respectively, in Ni(0)-HNCNSs-1. The mass ratio of C and Ni reaches 7.19.

When 0.102 g of NiO nanocrystals was used, the obtained NiO-PRNSs-2 showed three different morphologies: spheres, ring buoy, and semi-bowl (Fig. 7a). The semibowl- and ring-buoy-like shapes were markedly increased in comparison with NiO-PRNSs-1, implying that an increase in the NiO content strongly influences the micellar structure composed of SO and P123 in the reaction system. TEM image indicated that every ring-buoy-like shape was solid and the spheres were hollow, except the spheres with large NiO aggregation (Fig. 7b). Although three shapes were observed, their particle sizes were close to each other, with an average size of 103.1 ± 1.5 nm. After carbonization, Ni(0)-HNCNSs-2 showed similar morphology to its parent (Fig. 7c and d), but the overall particle size contracted to an average size of 72.0 ± 1.5 nm. Note that the size of Ni particles inlaid into the carbon shell was 13.4 ± 0.6 nm, which is much bigger in comparison with that in Ni(0)-HNCNSs-1. By comparing NiO-PRNSs-1 and NiO-PRNSs-2, and Ni(0)-HNCNSs-1 and Ni(0)-HNCNSs-2 in terms of particle size, upon increasing the NiO content, the resultant average size of whole particles markedly decreased and the average size of Ni nanoparticles greatly increased, revealing that the amount of NiO nanocrystals in the reaction system alters the size of the micelles. A high NiO content tends to result in small-sized micelles with a small hydrophilic interface in aqueous solution. In this case, the protective layer of original NiO nanocrystals was broken and new NiO was stabilized by oleate and P123 located at the hydrophilic interface of the micelles, resulting in the conversion of the NiO nanocrystals from hydrophobicity to hydrophilicity. As a result, in situ formed PR was deposited onto the hydrophilic interface of micelles containing NiO nanocrystals via a heterogeneous nucleation mechanism, hence leading to the formation of NiO-PRNSs with small size, and the correspondingly formed Ni nanoparticles became bigger.

On the basis of the above-mentioned hypothesis, if a large amount of large aggregated NiO nanocrystals is used, the obtained Ni(0)-HNCNSs should show small particle size with an inhomogeneous distribution of large-sized Ni particles in comparison with the highly monodisperse NiO nanocrystals, thanks to the difficult integration of large NiO aggregations in the micelles. To confirm this speculation, snowflake-like arrayed NiO nanocrystals with an average size of 54.9 ± 5.6 nm prepared at 295 °C for 1.5 h were used, the resultant Ni(0)-HNCNSs-3 obtained were relatively uniform nanospheres with an average size of 110.4 ± 2.1 nm, while the average Ni particle size was 41.6 ± 5.6 nm, less than the size of the initial NiO nanocrystals. Note that most of the NiO nanocrystals could not be integrated into the micelles of SO and P123, but the unintegrated NiO still influenced the size of the formed PRNSs. The resultant Ni particles in Ni(0)-HNCNSs-3 showed a severely inhomogeneous distribution. Some HNCNSs did not contain any Ni particles (ESI, Fig. S1‡). This result clearly verifies the reorganization of NiO nanocrystals and the effect of this on the sizes of the micelle and PRNSs. Moreover, such snowflake-like arrayed NiO nanocrystal aggregation is difficult to use as the core to form core–shell structured NiO-PR with NiO as the core and PR as the shell. However, when monodisperse single FeO nanocrystals were used, core–shell structured FeO-PRNSs were formed. This will be addressed below.

To confirm the phase structures of Ni(0)-HNCNSs-n (n = 1, 2, 3), X-ray diffraction (XRD) analysis was carried out, and the patterns are shown in Fig. 2b. A series of diffraction peaks appeared at 2θ = 26.6, 43.5, 44.6, 51.9 and 76.2°; the first two peaks are indexed to the (002) and (100) planes of graphite carbon, and the last three peaks belong to the (111), (200) and (220) reflections of face-centred cubic Ni particles with Fmm symmetry, corroborating the formation of graphite carbon spheres and the conversion of hexagonal NiO to metallic Ni during carbonization, as well as the successful fabrication of Ni-HNCNs. On the basis of the results from XPS and XRD studies, the reduction of NiO nanocrystals to metallic nickel Ni⁰ during carbonization can be said to be caused by the instability of NiO nanocrystals at high temperatures (>300 °C), which has been observed in the preparation of NiO nanocrystals. In addition, for Ni(0)-HNCNSs-n (n = 2, 3), upon increasing the NiO nanocrystal content used in the initial reaction system, the corresponding Ni content values increased to 15.04 wt% and 20.64 wt% with C/Ni mass ratios of 5.31 and 3.63, respectively, revealing that the Ni content increased with an increase in the initial amount of NiO used.

Ni-H/SNCMSs-m (m = 4, 5)

In fact, for the Ni(0)-HNCNSs-n series, the inhomogeneous distribution of a low content of Ni particles in the carbon nanospheres was observed. To improve the uniform distribution of Ni particles and enhance the Ni content in the composite materials, a one-pot strategy was attempted to prepare Ni(II)-integrated N-doped carbon nanospheres based on the reaction between the functional groups of 2,4-dihydroxybenzoic acid and Ni(II) ions in an aqueous micellar system of SO and P123 (Fig. 4b). Ni(II) ions first reacted with the carboxylic acid group of 2,4-dihydroxybenzoic acid to form a Ni coordination complex in aqueous solution. After adding an aqueous micellar solution of SO, P123 and HMT, the Ni coordination complex probably covalently bonded to HMT and further reacted with formaldehyde derived from the reaction of HMT and water under heating conditions to form a Ni(II)-coordinated solid or hollow N-doped phenolic resin composite (Ni(II)-PR).

When a small amount of Ni(II) ions was used (the molar ratio of a Ni²⁺ to 2,4-dihydroxybenzoic acid was 0.122), Ni(II)-coordinated solid N-doped phenolic resin microspheres (Ni(II)-PRMSs-4) were obtained. If a large amount of Ni(II) ions was employed (the molar ratio of a Ni²⁺ to 2,4-dihydroxybenzoic acid was 0.977), Ni(II)-coordinated N-doped solid phenolic resin microspheres (Ni(II)-PRMSs-5) were obtained. After carbonization, Ni(II)-PRMSs-4 and Ni(II)-PRMSs-5 were converted into corresponding Ni(0)-HNCMSs-4 and Ni(0)-SNCMSs-5. As shown in Fig. S2,‡ both Ni(II)-PRMSs-4 and Ni(II)-PRMSs-5 were composed of solid microspheres and irregular flake-like materials, but the latter contained a lot of flake-like material in comparison to the former. For Ni(II)-PRMSs-4, particles had a very broad size distribution, from hundreds of nanometers to thousands of nanometers. For Ni(II)-PRMSs-5, the average size is 739.3 ± 37.4 nm. After carbonization, as viewed in Fig. 8a and b, solid-structured Ni(II)-PRMSs-4 converted into hollow-structured Ni(0)-HNCMSs-4 microspheres, and irregular flake-like material still existed around the microspheres. Note that metallic Ni particles are inlaid into the carbon shell and irregular flakes with a relatively uniform distribution, and crystalline lattice fringes can be readily observed (Fig. 8c). The average diameters of the hollow microspheres and crystalline metallic Ni nanoparticles are about 1.43 μm and 63.7 ± 3.9 nm, respectively. Moreover, energy-dispersive X-ray (EDX) spectroscopic elemental mapping of C, N, O, and Ni in Ni(0)-HNCMSs-4 clearly uncovered the chemical composition of the composite and the related distributions (Fig. 8d–h).

However, Ni(0)-SNCMSs-5 still retained a solid microsphere structure and the average size contracted to 642.8 ± 36.5 nm, while the PR flakes converted into a carbon thin layer after carbonization and crystalline metallic Ni particles were formed, as seen by observing the crystal lattice fringes (Fig. 9). Note that Ni nanoparticles with an average size of 8.5 ± 1.1 nm are uniformly distributed in the microspheres and carbon flakes, indirectly confirming the presence of a highly regular dispersion of Ni(II) ions in the initially formed Ni(II)-coordinated PR frameworks via chemical bond interactions and not simply physical adsorption. Based on SEM, TEM and EDX information, the Ni particles were seen to be almost uniformly distributed in the N-doped carbon spheres in comparison with Ni(0)-HNCNSs-n (n = 1, 2, 3), directly confirming an effective improvement in homogeneously distributing metallic Ni(0) nanoparticles in Ni(0)-HNCMSs-4 and Ni(0)-SNCMSs-5. Moreover, the Ni content values in Ni(0)-HNCMSs-4 and Ni(0)-SNCMSs-5 are 11.40 and 17.28 wt%, respectively, and the corresponding C/Ni mass ratios are 7.26 and 4.52. In addition, an interesting phenomenon is found. Both Ni(II)-PRMSs-4 and Ni(II)-PRMSs-5 are solid microspheres before carbonization. However, after carbonization, solid Ni(II)-PRMSs-4 is converted into hollow Ni(0)-HNCMSs-4 microspheres; Ni(II)-PRMSs-5 is converted into Ni(0)-SNCMSs-5 but still preserves its solid structure. We think that the formation of hollow or solid spheres during carbonization is related to the initially formed loose or compact spherical structures. In fact, Ni(II)-PRMSs-4 contains low amounts of the Ni(II)-2,4-dihydroxybenezoate complex framework compared to Ni(II)-PRMSs-5; hence it easily contracts and forms cavity voids at the centers of the spheres. Oppositely, Ni(II)-PRMSs-5 contains more of the Ni(II)-2,4-dihydroxybenezoate skeleton composite; therefore it is more difficult for it to contract compared to the organic phenolic resin framework and, hence, during carbonization the initial spheres only uniformly contract to form solid spheres with a reduced particle size and they cannot convert into more-or-less hollow spheres with cavity voids.

In order to directly corroborate the chemical composition, valence states, and distribution of elements in the Ni(0)-SNCMSs-5 composite, XPS and EDX spectroscopic elemental mapping of C, N, O, and Ni were performed. As viewed in Fig. S3,‡ the high-resolution C 1s, O 1s, and N 1s XPS spectra are quite similar to those from Ni(0)-HNCNSs, revealing the existence of the same species, and the Ni 2p XPS spectrum also shows two strong peaks corresponding to metallic Ni, and two very weak peaks, implying traces of Ni²⁺ derived from the air oxidation of metallic Ni. Moreover, the EDX spectroscopic elemental mapping of C, N, O, and Ni in Fig. S4‡ show the formation of a chemical composite, especially confirming the doping of N and the uniform distribution of Ni. For Ni(0)-HNCMSs-4 and Ni(0)-SNCMSs-5, their phases, including graphite carbon and face-centered cubic metallic Ni, were verified via XRD patterns, as shown in Fig. 2b.

FeO-HNCNSs-6

Herein, monodisperse cube-like FeO nanocrystals with an average size of 22.5 ± 0.3 nm were prepared according to literature with a slight modification.⁵⁵ Such nanocrystals were encapsulated by in situ formed phenolic resin to form core–shell structured FeO-PRNSs with single/multiple FeO nanocrystals as the core (Fig. 10a–c), and they had various morphologies such as ring-buoy-like, semibowl-like and spherical. Note that the size of the FeO nanocrystals that acted as the cores is same as the original ones, and the multiple FeO nanocrystal-fabricated core–shell particles have a bigger size while a few hollow PR particles did not contain a FeO nanocrystal as the core. These FeO nanocrystal-(incompletely or fully)filled core–shell structures involved a kind of unique pomegranate-like structure with or without an empty cavity. The average size of FeO-PRNSs is 137.7 ± 3.2 nm. After carbonization, the FeO phase in FeO-HNCNSs-6 did not change, as determined by comparing the XRD pattern (Fig. 2b). Most FeO-HNCNSs-6 retained the same morphology as the parent (Fig. 10d and e); the average size decreased to 92.5 ± 2.4 nm, but the FeO nanocrystals still kept an average size of 22.7 ± 1.6 nm. Note that most FeO nanocrystals are still located in the hollow cavity or inlaid into the carbon layer, but some FeO nanocrystals migrated to the carbon surface layer to form carbon-coated FeO (Fig. 10f), revealing FeO-attached and carbon-coated hollow structures or empty-inner-cavity, partially-filled-cavity, and full-filled pomegranate-like structures. In FeO-HNCNSs-6, the Fe content is 15.29 wt% and the corresponding C/Fe mass ratio is 4.84. Due to the filling of FeO, such hollow structures probably have a small available void volume for LIB use.

Moreover, via changing the ratio of the micellar composite in the reaction system, FeO nanocrystals can be uniformly arrayed on the in situ formed PR thin surface to fabricate a FeO-PR composite (Fig. S5a and b‡) with high FeO loading (Fe content of 14.97 wt%); but after carbonization, all FeO nanocrystals were enriched to form an FeO-N-doped carbon composite (FeO-NCC) with naked and large FeO particles (Fig. S5c and d‡). This attempt does not allow for the effective embedding of FeO nanocrystals in the microporous or mesoporous carbon layer and, hence, no further characterization was carried out.

N₂ physisorption

For all Ni- or FeO-integrated hollow/solid N-doped carbon materials, N₂ physisorption studies were performed to determine the specific surface area and pore volume. As shown in Fig. 11, all metal-integrated carbon materials showed a typical type-IV isotherm with different hysteresis loops at P/P₀ = 0.46 and 0.90, corroborating the existence of a mesoporous metal-integrated (S)/HCN(M)S structure and a textural structure involving particle aggregation. The appearance of various hysteresis loops can probably be attributed to the formation of Ni-incomplete filling-induced mesoporous cavities, the micelle-induced non-uniform mesoporous structure, and the particle package-induced textural structure, as well as the microporous structure during carbonization. For Ni- and FeO-HNCNSs, the specific surface areas varied between 404 and 505 m² g⁻¹, and the micropore volumes are quite similar: around 0.16 cm³ g⁻¹. The single point total pore volumes at a relative pressure of 0.99 vary from 0.68 to 0.93 cm³ g⁻¹. Moreover, the specific surface areas of Ni(0)-HNCMSs-4 and Ni(0)-SNCMSs-5 are 387 and 422 m² g⁻¹, respectively, but the total pore volumes are very small: 0.25 and 0.29 cm³ g⁻¹, respectively. In addition, the BJH pore size distributions reveal that all samples are mesoporous materials without uniform pore size from the adsorption branches (Fig. S6a‡), and the mesopore sizes range from 3.5 to 3.9 nm from the desorption branches of the isotherms (Fig. S6b‡). Moreover, a microporous structure still exists in all samples, which is confirmed from the appearance of micropore volumes from N₂ physisorption. The detailed pore parameters are listed in Table S1.‡

Electrochemical properties of Ni(0)-HNCNSs-1 and FeO-HNCNSs-6

To further investigate the electrochemical properties of metallic Ni(0) and FeO-integrated N-doped hollow carbon nanospheres, two materials, Ni(0)-HNCNSs-1 and FeO-HNCNSs-6, were chosen as representatives. Although the integration of metal probably occupied the empty cavities of the hollow carbon spheres, the doping of nitrogen on carbon is beneficial for lithium storage due to the lithiophilicity of N.^56,57 The unique hollow structures of the samples with high surface area can actually facilitate the permeation of electrolyte and shorten the ion diffusion and electron transfer pathways, thus enabling Ni(0)-HNCNSs-1 and FeO-HNCNSs-6 to achieve good lithium storage properties in terms of specific capacities and rate capabilities. Ni(0)-HNCNSs-1 delivers initial charge and discharge capacities of 522.3 and 1030.0 mA h g⁻¹, respectively, corresponding to an initial coulombic efficiency of 50.7% (Fig. 12b ). Fig. 12a shows the cycling performance of Ni(0)-HNCNSs-1 at a current density of 100 mA g⁻¹. Ni(0)-HNCNSs-1 shows a specific capacity of 380 mA h g⁻¹ after 130 cycles, indicating its super-stable lithium storage performance. The rate capability of Ni(0)-HNCNSs-1 was further evaluated and, as shown in Fig. 12c, the capacities of Ni(0)-HNCNSs are 445, 367, 318, 275, 248, 226 mA h g⁻¹ at 50, 100, 200, 300, 1000, and 2000 mA g⁻¹, respectively. When the current density is returned to 50 mA g⁻¹, the capacity recovers to 375 mA h g⁻¹, corroborating the superior reversibility during repeated lithiation/delithiation processes.

The lithium storage properties of FeO-HNCNSs-6 were also studied, as shown in Fig. 12d–f. The initial charge and discharge capacities of FeO-HNCNSs-6 are measured to be 443.4 and 955.9 mA h g⁻¹, corresponding to an initial coulombic efficiency (ICE) of 46.4%. The capacity loss of FeO-HNCNSs-6 might be attributed to the formation of a solid electrolyte interphase (SEI) layer and the high surface area. Notably, a platform was observed at around 0.9 V in the discharge process, which might result from the reduction of FeO to metallic Fe, while the distortions at around 1.57 V and 1.90 V during the charge process could be ascribed to the oxidation of Fe to FeO.⁵⁸ After 200 charge/discharge cycles at 100 mA h g⁻¹, FeO-HNCNSs-6 delivers a specific capacity of 398.7 mA h g⁻¹, indicating its excellent cycling stability. FeO-HNCNSs-6 also shows excellent rate capability, showing capacities of 424.2, 363, 320.1, 277.9, and 246.3 mA h g⁻¹ at 50, 100, 200, 500, and 1000 mA g⁻¹, respectively. Moreover, a capacity of 380.5 mA h g⁻¹ could be achieved when the current density was returned to 50 mA g⁻¹, reflecting the superior reversibility.

Conclusions

In this study, temperature-dependent phase-pure monodisperse NiO nanocrystals were prepared via a thermal decomposition approach, showing sphere-like shapes and snowflake-like NiO arrays. Such hydrophobic NiO nanocrystals were integrated into hydrophilic micelles of SO and P123 in an aqueous system, undergoing conversion from hydrophobicity to hydrophilicity. In situ formed PR was successfully deposited onto the hydrophilic area of the NiO-SO-P123 micelles via a heterogeneous nucleation mechanism to form uniform NiO-PRNSs. Via adjusting the size and amount of NiO nanocrystals used, the diameter of the obtained NiO-PRNSs can be effectively controlled from 185 to 103 nm, and every sample showed a narrow size distribution, revealing the influence of the NiO nanocrystals on the reconstructed NiO-integrated micellar size. Moreover, the appearance of different morphologies, such as ring buoy-like and semi-bowl-like shapes, relies on the initial amount of NiO used. After carbonization, NiO was converted to Ni(0) and inlaid in HNCNSs to form Ni(0)-HNCNSs; meanwhile, the size and distribution of metallic Ni particles were strongly affected by the amount and size of the initial NiO nanocrystals used. The sizes of Ni(0)-HNCNSs and metallic Ni(0) particles can be tuned from 136 nm to 72 nm, and from 4 nm to 42 nm, respectively. However, when monodisperse and polyhedral FeO nanocrystals were used, the obtained FeO-free/-incompletely-filled/-fully-filled core–shell structured Fe-PRNSs showed relatively uniform particle size, while multiple FeO cores tended to form large FeO-PRNSs based on uniformly sized original FeO. The carbonized FeO-HNCNSs still preserved pomegranate-like core–shell structures with a uniform size of 92.5 nm, but no change in the size of FeO was observed. Moreover, a one-pot method can also be used to prepare high-loaded Ni(0)-integrated hollow or solid N-doped carbon microspheres or flakes with a broad size range, but with a highly uniform distribution of Ni with a size as small as 8.5 nm. Note that Ni(0)- and FeO-HNCNSs are prepared here for the first time to the best of our knowledge. Finally, low-loaded Ni/FeO-HNCNSs with uniform morphology and size were chosen as representative samples to investigate their electrochemical properties for LIB use; the samples showed excellent lithium storage properties and superior reversibility. This study provides a potential strategy to control the sizes and morphologies of metal-integrated carbon materials for further adjusting their electrochemical properties.

Experimental

General

All chemicals were used as received and purchased from the following companies: Pluronic P123 (average M_n = 5800), concentrated hydrochloric acid (≥37%), nickel chloride hexahydrate (NiCl₂·6H₂O, ≥98%), 2,4-dihydroxybenzoic acid (97%), hexamethylenetetramine (HMT, ACS reagent, ≥99%), oleic acid (90%), 1-octadecene (90%), and oleylamine (70%) were obtained from Sigma-Aldrich; ethanol (≥99.8%) was obtained from Honeywell; dichloromethane (99.99%) was obtained from Fisher Chemical; sodium hydroxide (99%) was obtained from AnalaR NORMAPUR; and n-hexane (≥97%) was obtained from EMD Millipore. Sodium oleate (SO) was synthesized via the reaction of oleic acid and NaOH in ethanol. Nickel oleate was prepared via the reaction of nickel chloride hexahydrate and sodium oleate in a mixed solvent of water, ethanol, and n-hexane. FeO nanocrystals were synthesized according to the literature with slight modifications.⁵⁵ Deionized water was used for all reactions.

NiO nanocrystals

Nickel oleate (5.08 g) in 1-octadecene (70 mL) and oleic acid (1.60 g) was stirred at 100 °C to form a clear solution, then heated to 290 °C at a heating rate of 1.2 °C min⁻¹ and kept at this temperature for 2 h. During this period, the reaction solution changed from green to yellowish green, and finally to dark red. After being cooled down to room temperature, 160 mL of ethanol was added and the mixture was stirred for 5 min. NiO nanocrystals were separated via centrifugation at 2 × 10⁴ rotations per minute (RPM) for 15 min, then re-dispersed in hexane and centrifuged again. This process was repeated twice to get the slightly yellowish green product. Elemental analysis (wt%): C, 17.89; H, 2.61; N, 0.01.

Ni(0)-Integrated hollow nitrogen-doped carbon nanospheres (Ni(0)-HNCNSs-n (n = 1, 2, 3))

NiO nanocrystals (0.080 g) in CH₂Cl₂ (10 mL) were first sonicated at room temperature for 2 h to get a highly dispersed suspension. Then, this suspension was added to a pre-prepared micellar solution of P123 (0.087 g) and SO (0.1472 g) in H₂O (30 g) at 40 °C under stirring. Afterwards, the temperature of the suspension was slowly increased to 50 °C to completely evaporate the solvent CH₂Cl₂. The resulting suspension was added to a pre-prepared aqueous solution composed of H₂O (120 g), HMT (0.2832 g) and 2,4-dihydroxybenzoic acid (0.3813 g) and stirred for 5 min. Afterwards, the resultant suspension was heated to 120 °C and this temperature was maintained for 2 h under reflux. After being cooled down to room temperature, the solid was collected via centrifugation at 2 × 10⁴ RPM for 15 min, washed with ethanol once, and dried under air overnight; this was denoted as NiO@phenolic resin-1 (NiO-PR-1). This solid was then carbonized at 800 °C for 5 h (at a heating rate of 1.5 °C per minute from 25 °C to 800 °C) to obtain Ni-HNCNS-1. Elemental analysis (wt%): C, 82.42; H, 0.88; N, 0.44; Ni, 11.47.

When the NiO content was increased to 0.102 g, NiO-PR-2 was obtained. 0.150 g of NiO nanocrystals with large size was used to prepare NiO-PR-3. The corresponding carbonized samples were marked as Ni-HNCNS-2 and Ni-HNCNS-3. Elemental analysis (wt%): Ni-HNCNS-2: C, 79.88; H, 0.72; N, 0.63; Ni, 15.04; Ni-HNCNS-3: C, 74.90; H, 0.81; N, 0.37; Ni, 20.64.

Ni(0)-Integrated solid nitrogen-doped carbon microspheres (Ni(0)-H/SNCMSs-m (m = 4, 5))

NiCl₂·6H₂O (0.1422 mg) and 2,4-dihydroxybenzoic acid (0.7626 g) in H₂O (240 g) were stirred at 40 °C for 2 h to form a clear greenish solution. A pre-prepared micellar solution of P123 (0.087 g) and SO (0.1476 g) dissolved in H₂O (40 g) at 40 °C was then added. After being stirred at 40 °C for 10 min, HMT (0.140 g) was added and the mixture was stirred for 10 min. Afterwards, the solution was heated to 120 °C and refluxed for 2 h. After being cooled down to room temperature, the suspension was centrifuged at 2 × 10⁴ RPM for 15 min, washed with ethanol once, and dried under air overnight; this was denoted as Ni(II)-PRMSs-4. This solid was then carbonized at 800 °C for 5 h (heating rate: 5 °C per minute from 25 °C to 800 °C) to obtain Ni-HNCNSs-4. Elemental analysis (wt%): C, 82.73; H, 1.11; N, 0.59; Ni, 11.40.

Similarly, when 1.1375 g of NiCl₂·6H₂O was used, the obtained as-made sample was denoted as Ni(II)-PRMSs-5, and the carbonized sample was Ni(0)-SNCNSs-5. Elemental analysis (wt%): C, 78.03; H, 1.01; N, 0.48; Ni, 17.28.

Iron(II) oxide-integrated hollow nitrogen-doped carbon nanospheres (FeO-HNCNSs-6)

The procedure was similar to that of Ni-HNCNSs-1, however, FeO nanocrystals (0.110 g) were used instead of NiO (80 mg); the other conditions were identical. The as-made sample was denoted as FeO-PRNSs, and the carbonized sample as FeO@HNCSs-6. Elemental analysis (wt%): C, 74.07; H, 0.68; N, 0.25; Fe, 15.29.

Iron(II) oxide-integrated nitrogen-doped carbon composite (FeO-NCC)

FeO nanocrystals (0.220 g) in CH₂Cl₂ (12 mL) were first sonicated at room temperature for 2 h to get a highly dispersed suspension. Then, this suspension was added to a micellar solution containing P123 (0.261 g) and SO (0.442 g) in H₂O (35 g) prepared at 40 °C under stirring. Afterwards, the temperature of the suspension was slowly increased to 50 °C to completely evaporate the solvent CH₂Cl₂. The resulting suspension was added to a pre-prepared aqueous solution composed of H₂O (115 g), HMT (0.2832 g) and 2,4-dihydroxybenzoic acid (0.3813 g), and stirred for 5 min. Afterwards, the resultant suspension was heated to 120 °C and this temperature was maintained for 2 h under reflux. After being cooled down to room temperature, the solid was collected via centrifugation at 2 × 10⁴ RPM for 20 min, washed with ethanol once, and dried under air overnight; this was denoted as FeO@phenolic resin composite (FeO-PR). Elemental analysis for FeO-PR (wt%): C, 47.25; H, 6.35; N, 1.83; Fe, 14.97. FeO-PR was then carbonized at 800 °C for 5 h (a heating ramp of 1.5 °C min⁻¹ from 25 °C to 800 °C) to obtain the FeO–N–C composite FeO-NCC. No further characterization except for TEM measurements was performed, due to the irregular shape and large aggregation of FeO.

Characterization

Low and wide-angle powder X-ray diffraction (PXRD) patterns were collected with a Bruker Advance D8 instrument using monochromatic CuKα radiation (λ = 1.5406 Å) in the 2θ range of 0.50–10° and 10–100° with scan speeds of 2 and 5 second per step, respectively. SEM images were obtained using Hitachi SU8030 apparatus operating at 30 kV. Some TEM images were obtained using Hitachi SU8030 apparatus operating at 30 kV and high-resolution (HR) TEM images were obtained using JEOL JEM2100 apparatus accompanied with energy dispersive X-ray (EDX) microanalytic apparatus operating at 200 kV. Nitrogen physisorption was performed using an ASAP 2020 volumetric adsorption instrument from Micromeritics to obtain adsorption–desorption isotherms, specific Brunauer–Emmett–Teller (BET) surface areas and pore volumes. Prior to analysis the samples were degassed at 250 °C for 4 h under vacuum pressure less than 10⁻³ mbar. A Thermo Scientific Nicolet 6700 FTIR spectrometer was used to measure diffuse reflectance infrared Fourier-transform (DRIFT) spectra with a KBr reference spectrum. Elemental analyses were performed using an Elementar Vario MICRO cube. X-ray photoelectron spectroscopy (XPS) data were recorded using Thermo Scientific ESCALAB 250Xi apparatus. C, N, and H elemental analyses were performed using an Elementar Vario MICRO cube, while the Ni and Fe content values were determined via an inductively coupled plasma-optical emission spectrometer (ICP-OES) from the Thermo Scientific iCAP 7000 Series.

Electrochemical measurements

All electrochemical measurements were conducted at room temperature using CR2025 coin cells. The working electrodes were prepared via homogenizing active material, a conductive agent (Super-P), and carboxymethylcellulose sodium (CMC) in deionized water at a mass ratio of 80:10:10 before finally coating on Cu foil. To standardize measurements, the mass loading of active material on the Cu foil was fixed at 1 mg cm⁻². Lithium foil, Celgard 3500 membrane, and 1 M LiPF₆ in a mixture of ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 by volume) with the addition of 5 wt% fluoroethylene carbonate (FEC) were used as the counter electrode, separator, and electrolyte for the LIBs, respectively. All cells were assembled in a glove box with water and oxygen content values below 0.1 ppm. All electrochemical measurements (i.e., galvanostatic charge/discharge, cycling performance, and rate performance) were performed in the voltage range of 0.01–3 V using a Land BT2000 battery tester (Wuhan, China).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Dr Yucang Liang is grateful to Prof. Dr Reiner Anwander for his financial support for this research and Elke Nadler for SEM/TEM measurements. Prof. Dr Bin Xu acknowledges financial support from the National Key R&D Program of China (Grant No. 2017YFB0102204).

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Footnotes

† This article is dedicated to the 60th anniversary of the establishment of the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, P. R. China.

‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nr03019h

This journal is © The Royal Society of Chemistry 2020