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Co3O4@(Fe-doped)Co(OH)2 Microfibers: Facile Synthesis, Orientedassembly, Formation Mechanism and High Electrocatalytic Activity Yao Zhou, Yijin Wu, Pengfang Zhang, Jian De Chen, Baihua Qu, and Jun-Tao Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10402 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 24, 2017

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ACS Applied Materials & Interfaces

Co3O4@(Fe-doped)Co(OH)2 Microfibers: Facile Synthesis, Orientedassembly, Formation Mechanism and High Electrocatalytic Activity Yao Zhou,a* Yijin Wu,a Pengfang Zhang,a Jiande Chen,a Baihua Qub and Jun-Tao Lia* a

College of Energy, Xiamen University, Xiamen, 361005, China;

b

Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, China.

E-mail: [email protected]; [email protected] KEYWORDS: Synthesis, Self-assembly, Co3O4@(Fe-doped)Co(OH)2 microfibers, Magnetic-responsive behavior, Electrocatalysis

ABSTRACT: Cobalt oxide or hydroxide nanoarchitectures, often synthesized via solvothermal or electrodeposition or templated approaches, have wide technological applications owing to their inherent electrochemical activity and unique magnetic responsive properties. Herein, by revisiting the well-studied aqueous system of Co/NaBH4 at room temperature, the chain-like assembly of Co3O4 nanoparticles is attained with the assistance of an external magnetic field; more importantly, an one-dimensional hierarchical array consisting of perpendicularly-oriented and interconnected Co(OH)2 thin nanosheets could be constructed upon such wellaligned Co3O4 assembly, generating biphasic core-shell-structured Co3O4@Co(OH)2 microfibers with permanent structural integrity even upon the removal of the external magnetic field; isomorphous doping was also introduced to produce Co3O4@Fe-Co(OH)2 microfibers with similar structural merits. The cobalt-chemistry in such Co/NaBH4 aqueous system was illustrated to reveal the compositional and morphological evolutions of the cobalt species and the formation mechanism of the microfibers. Owing to the presence of Co3O4 as the core, such anisotropic Co3O4@(Fe-doped)Co(OH)2 microfibers demonstrated interesting magneticresponsive behaviors, which could undergo macro-scale oriented-assembly in response to a magnetic stimulus; and with the presence of a hierarchical array of weakly crystallized thin (Fe-doped) Co(OH)2 nanosheets with polycrystallinity as the shell, such microfibers demonstrated remarkable electrocatalytic activity towards oxygen evolution reactions in alkaline conditions.

1. Introduction Transition metal oxide or hydroxide or their mixture, especially those of cobalt, have attracted huge interests owing to their intrinsic outstanding electrochemical performances as electrode materials for electrochemical energy conversion and storage technologies such as Li-ion battery, supercapacitors,1,2 or efficient and cost-effective non-noble electrocatalysts for oxygen evolution reaction (OER) in water splitting.3,4 The hybrids of cobalt oxide and cobalt hydroxides with different structural features have also been reported occasionally and employed for supercapacitors and OER process, which are shown to demonstrate synergestic effect in their electrochemical performance.1,2,5 Most of such cobalt oxide or hydroxides with different phases in the existing literature were prepared by solvothermal methods or electrodeposition approaches, which often assume two-dimensional nanosheet or nanosheet-like morphologies.6 In addition to their catalytic activity, another interesting property of Co-relevant nanocrystals, such as metallic cobalt or cobalt oxides, is their magnetic-responsive nature which enables multiple important applications. Driven by external magnetic fields, self-assembly of the room-temperature

ferrimagnetic nanoparticles (NPs) such as cobalt or cobalt oxide NPs could be achieved conveniently in a controllable manner,7,8 compared with that of the non-responsive ones which either require availability of uniform primary NPs as building blocks or extensive usage of amphiphiles.9-11 Onedimensional (1D) chain-like discrete assemblages of cobalt oxide NPs with lengths over centimeter long could be realized easily in different solutions.11 However, such linear assemblages generally have simple morphology and lose their structural integrity easily upon the removal of the applied external magnetic field, which hence have limited applications.11 Nevertheless, they could serve as a 1D backbone, upon which a second chemical composition, usually Au12 or Pt13, so far as reported, could be introduced as a core or shell to form a hybrid.9 Such a hybrid is stable even upon the removal of the external magnetic field as the integral shape of the assemblages was registered by the second solid chemical composition, and synergistic effects were often resulted in owing to the interactions of two different chemical components. Furthermore, surface functionalizations of such linear assemblages with polymeric molecules are also possible via colloidal polymerization strategies under the presence of

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external magnetic fields, forming an interconnected, stable inorganic-organic chain-like nanocomposite.14,15 Regardless of the above researches, such magnetic-driven self-assembly phenomena were limited to the metal oxides and is not applicable to the relevant metal hydroxides (e.g.,Co(OH)2) which are generally non-magnetically responsive at room temperature. In another relevant issue, namely the generation of H2 from hydrolysis of NaBH4 with cobalt species as a catalyst, cobaltrelated chemistry is also of great importance to understand the catalytic mechanism as well as the process kinetics.16-20 Hydrolysis of NaBH4 is a continuously evolving process which leads to dynamic changes in the chemicophysical environment of the system (e.g., the pH value, the population and concentration of ionic species); as a consequence, the cobalt species might also evolve constantly, which could potentially exist as Co3O4, CoO, Co(OH)2, CoBx, Co3(BO3)2 and metallic Co etc. Though being intensively investigated, the actual activation and deactivation Co species remains quite controversial; and most of the relevant studies in this regard focus on understanding the chemical states of the cobalt species via X-ray-based techniques, whereas the structural evolution of those cobalt-based solids at micro- or meso-scale, which is also an important feature for such a heterogeneous catalytic process, are rarely addressed. Herein, the cobalt-chemistry was explored within the classical Co/NaBH4 aqueous system with distinctively low concentrations of NaBH4 in the presence of an external magnetic field, which was found to demonstrate interesting morphological and compositional evolutions; and biphasic core-shelled hybrids of Co3O4@(Fe-doped)Co(OH)2 microfibers which have chain-like assembly of Co3O4 NPs as the core and well-organized (Fe-doped) Co(OH)2 thin nanosheets as the shell were fabricated facilely with permanent compositional and structural merits via a selfassembly process; such hybrids with delicate nanoarchitectures depict interesting magnetic-responsive behaviors as well as remarkable electrocatalytic activity towards for OER process. 2. Experimental Section 2.1 Chemicals and materials The following chemicals were used as received without further purification: sodium dodecylbenzenesulfonate (SDBS), NaBH4 (99.99%), polyvinyl pyrrolidone (PVP, K30), Co(NO3)2⋅6H2O (≥98.0%), FeCl3. 6H2O (≥98.0%), were from Sigma Aldrich; KOH (85%) was from Merck; carbon black was from Nacalai Tesque Inc; perfluorosulfonic acid-PTFE copolymer (nafion solution, 5% w/w) was from Alfa Aesar; deionized water was collected through the Elga MicroMeg purified water system. 2.2 Synthesis of chain-like assembly of Co3O4 NPs Towards 10.0 ml of water, aqueous solutions of SDBS (2.0 ml, 0.1 M) and Co(NO3)2 (1.0 ml, 0.1 M) was successively added and the mixture was stirred for two hours. Then 3.0 ml of the above precursor solution was well mixed with 10.0 ml of 8.0% wt PVP aqueous solution. Subsequently, upon the addition of 4 mg NaBH4 which was dissolved in 0.4 ml deionized water, the mixture was stirred vigorously with a cylindric magnet for 30 seconds, which then was kept static

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together with the magnet at ambient condition for at least 3 h. Afterward, the samples adsorbed on the magnet were harvested and washed with ethanol to obtain the chain-like assembly of Co3O4 NPs. 2.3 Synthesis of Co3O4@Co(OH)2 microfibers Towards 10.0 ml of 8.0% wt PVP aqueous solution, an aliquot of Co(NO3)2 solution (0.5 ml, 0.1 M) was added. Then 1.0 ml of freshly prepared NaBH4 aqueous solution (10 mg/ml) was added, accompanied by vigorous magnetic stirring with a cylindric magnet for 30 seconds. The solution together with the magnet was kept static at ambient condition for at least 3 h. Afterward, the samples adsorbed on the magnet were harvested and washed with ethanol to give the hierarchical Co3O4@Co(OH)2 microfibers. 2.4 Synthesis of Co3O4@Fe-Co(OH)2 microfibers Towards 10.0 ml of 8% wt PVP aqueous solution, the aqueous mixture of Co(NO3)2 solution (0.5 ml, 0.1 M) and FeCl3 solution (0.2 ml, 0.1 M) was added. Then 1.0 ml of freshly prepared NaBH4 solution (10 mg/ml) was added, accompanied by vigorous magnetic stirring with a cylindric magnet for 30 seconds. The mixture together with the magnet was kept static at ambient condition for at least 3 h. Afterwards, the samples adsorbed on the magnet were harvested and washed with ethanol. 2.5 Oxygen evolution reaction (OER) catalyzed by Co3O4@(Fe-doped)Co(OH)2 microfibers The as-prepared microfibers were evaluated as electrocatalysts for OER in alkaline condition. The glassy carbon electrode (GCE, 3 mm in diameter) was polished successively with 1.0 µm, 0.3 µm and 0.05 µm alumina slurries prior to modification, and between the slurries the electrode was sonicated in deionized water for 10 s. 3.5 mg of the dry powder of the as-prepared microfibers and 3.5 mg of carbon black was dispersed into 0.25 ml of nafion solution (which was prepared by well mixing 112.5 µl of deionized water, 112.5 µl of ethanol and 25 µl of 5%w/w nafion solution) and the mixture was sonicated to obtain a homogeneous thick ink. 2.0 µl of the ink was drop-casted on the GCE, which was dried at room temperature naturally. Electrochemical measurements were recorded using a computer-controlled potentiostat (Autolab, PGSTAT 302N) with a standard-threeelectrode configuration. The counter and reference electrodes were Pt gauze and Ag/AgCl with 3.0 M KCl, respectively. The working electrode was firstly scanned 10 cycles at 50 mV/s to obtain a stable response before data recording. Cyclic voltammograms (CVs) and linear sweep voltammograms (LSV) of the working electrodes were obtained with a scan rate of 10 mV/s in 0.1 M freshly prepared KOH solution. The potentials for LSV curves were iR-compensated and referenced to the reversible hydrogen electrode according to E(V vs RHE) = E(V vs Ag/AgCl/3 M KCl) + 0.978 V at pH = 13. The overpotentials and tafel slopes were calculated from the relevant polarization curves. 2.6 Material characterizations The above formed products were characterized by transmission electron microscopy (TEM, JEM-2010, 200 kV) and high-resolution TEM (HRTEM, JEM-2100F, 200 kV). Xray photoelectron spectroscopy (XPS, AXIS-HSi, Kratos Analytical) analysis was conducted using a monochromatized Al Kα exciting radiation (hν=1286.71 eV) with a constant


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analyzer-pass-energy of 40.0 eV. All binding energies were referenced to C 1s peak (its binding energy set at 284.5 eV) arising from C=C bonds. Crystallographic information was obtained by Panalytical X'Pert-pro MPD X-ray power diffractometer (a Cu Kα irradiation source, λ = 1.54056 Å). The magnetic properties was measured at room temperature by a VSM system at room temperature (Lakeshore, 7404). 3. Results and discussion 3.1 Formation of chain-like assembly of Co3O4 NPs The synthetic strategy, as illustrated in Scheme 1, in brief involves the static aging of the PVP aqueous solution of Co(NO3)2 and NaBH4 with a magnet at room temperature. Interactions of NaBH4 with Co2+ with or without a polymer or surfactant have been long explored for synthesis of cobaltrelevant nanostructures10 or H2 generation technologies.18,19 The reaction conditions, e.g., the presence of O2, the concentration of NaBH4, or the addition of NaOH, could affect

each nanochain, individual Co3O4 NPs with average size of 26 ± 6 nm could be observed which are generally aligned into almost a line, and a narrow spacing could be identified between each two neighboring NPs. The Co3O4 NPs are quite polycrystalline. At high resolution, they exhibit heterogeneous TEM image contrast across each individual NPs; there are also multiple internal cracks within the particle and small nanocrystals on the surface (Figure 1d); occasionally, particles with significant defects on the surface could be spotted (Figure S1c). Note that the mixture becomes brown and then black immediately upon the addition of the fresh NaBH4 aqueous solution into the Co2+ precursor solution. Metallic Co nanocrystals were highly likely to be formed at the early stage, which were then oxidized into tiny Co3O4 nanocrystals during the course of oriented-diffusion towards the magnet which sank at the bottom of the reactor. When approaching the magnet, such tiny Co3O4 nanocrystals would coalesce with

Scheme 1. Flow chart for preparation of the chain-like assembly of Co3O4 NPs and Co3O4@(Fe-doped)Co(OH)2 microfibers. Note that the formation of the Co3O4 NP assembly also involves addition of SDBS in the Co2+ precursor solution.

the eventual chemical composition and the structural features of the solid cobalt-based endproducts profoundly. In most of the relevant researches, the NaBH4 solutions were often purged with inertial gas to remove the dissolved oxygen, and the dosages of NaBH4 applied were far in excess of the Co2+ cations.10,16 Nevertheless, herein, all of the experiments were conducted under ambient conditions with the presence of air (that is, oxygen); the NaBH4 concentrations were distinctively lower than those reported; and the presence of a magnetic field is also expected to play a role considering the magnetic responsive nature of cobalt-relevant species. Prior to any further discussion on the Co3O4@Co(OH)2 microfibers, simultaneous formation and assembly of Co3O4 NPs into 1D chain-like structures driven by an external magnetic field were briefly reported first. Shown in Figure 1a, TEM characterization reveals the formation of microrods with lengths longer than ten micrometers under the low magnification mode (Figure S1a,b). At higher magnifications, within each microrods a number of 1D discrete nanochains which are almost straight and paralleling to each other in a quite organized manner could be found (Figure 1b,c). The nanochains, though quite close to each other, are barely entangled with its neighbors. Such a meso-scale ordered arrangement occurs because of the presence of the magnetic forces; otherwise, these nanochains with lengths more than ten micrometers would have existed in an unorganized and random manner. Further increasing the magnification, within

Figure 1. (a) low-magnification and (b,c,d) high-magnification TEM images of the chain-like assemblages of the Co3O4 NPs.

each other spontaneously, ultimately leading to the polycrystalline nature of the Co3O4 NPs shown in Figure1. Ostwald ripening where NPs with relatively large sizes grow spontaneously at the sacrifice of smaller NPs was expected to occur.21,22 Accordingly, the surface of some of polycrystalline NPs looks relatively uneven (Figure 1d and Figure S1c,d); their XRD pattern indicates very poor crystalliniity and the coexistence of some Co(OH)2 species within the sample (Figure S2). Note that a highly viscous PVP aqueous solution (8% wt) was employed, therefore the tiny Co and Co3O4 nanocrystals was expected to diffuse relatively slow, which provides the necessary kinetic control for the eventual relocation and re-orientation of the Co3O4 NPs on the magnet. Actually, it takes more than 3 hours for the bulk solution to become colorless and transparent. Depicted in Scheme 1, at the end of the reaction, the cylindric magnet was covered by black substances shown in Figure 1a.



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3.2 Formation of Co3O4@(Fe-doped)Co(OH)2 microfibers With slight modification in the reaction conditions used for the synthesis of chain-like assembly of Co3O4 NPs, core-shell structured nanocomposite of Co3O4@Co(OH)2 or Co3O4@FeCo(OH)2 with hierarchical microfiber-like morphology could be obtained easily. As shown in the FESEM images in Figure 2a-d, numerous 1D hierarchical microfibers with lengths over several tens of micrometers were formed. Quite resembling a verticillate branch with affluent leaves, each microfiber consists of numerous thin 2D nanosheets which are standing perpendicularly to the long axis of the microfiber, forming an array of 2D nanosheets (Figure 2b,i). These randomlyarranged nanosheets have irregular shapes and are hundreds of nanometers to even a few micrometers in size. Such nanosheets within the microfibers are inter-linked with each other permanently. Therefore, even upon the removal of the external magnetic field, they could still well mantain their overall structural integrity regardless of the thermal motion or mechanical stirring. With the edges of neighboring nanosheets well inter-connected, numerous meso-scale open-end voids are formed within each microfiber (Figure 2c,d), which could support fast mass transfer of external reactants when used as electrocatalysts. The core-shell structure of such hierarchical microfibers could be verified under TEM characterization. The internal core which consists of numerous linearly aligned Co3O4 NPs have much higher TEM image contrast than the external shell which are formed by interconnected thin Co(OH)2 nanosheets (Figure 2i). Shown in Figure 2e,f, inside each microfiber there ar several paralleling nanochains which were embedded as the backbone, each of which is an assembly of multiple spherical NPs, quite like those depicted in Figure 1. The nanosheets could be clearly observed, which are curved and have light TEM image contrast close to that of the background, indicating their thin nature. And as the neighboring nanosheets are connected with each other, TEM image contrast of those nanosheets are heterogeneous across each individual microfiber (Figure 2g,h). Note that all of the Co(OH)2 nanosheets are immobilized perpendicularly on the backbone of the Co3O4 NP assembly, with few freestanding nanosheets observed outside the microfibers.

Figure 2. (a,b,c,d) FESEM; (e,f,g,h) TEM images (with different magnifications) and (i) schematic depictions with different views of the Co3O4@Co(OH)2 microfibers.

The chemical composition of the as-prepared microfibers was further revealed. The coexistences of the Co(OH)2 and Co3O4 are well reaffirmed by the relevant XRD pattern shown in Figure 3a (curve 1). The two peaks with 2θ angles at 20.0o and 33.4o and a weak one at 45.4o are assigned to the (002), (101) and (018) crystal planes in α-Co(OH)2 (PDF #511731);23 and one more well-defined peak at 59.4o is attributed to the (511) crystal plane of Co3O4 nanocrystals (PDF #431003). Compared with those prepared via hydrothermal meth-


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ods, the as-formed Co3O4@Co(OH)2 microfibers which were produced at room temperature have relatively poor crystallinity and hence exhibit generally weak and wide XRD peaks. XPS

Figure 3. (a) XRD patterns of (curve 1) Co3O4@Co(OH)2 and (curve 2) Co3O4@Fe-Co(OH)2 microfibers, note that the red and the blue lines are the standard XRD patterns of α-Co(OH)2 and Co3O4, respectively; (b) XPS spectra of Co 2p and O 1s for the Co3O4@Co(OH)2 microfibers; (c) XPS spectra of Fe 2p, Co 2p and O 1s for the Co3O4@Fe-Co(OH)2 microfibers. The black dots are the experimental data and the colored curves are calculated.

analysis of the elements Co 2p and O 1s in the as-prepared sample was shown in Figure 3b. The deconvolution of Co 2p3/2 reveals only one peak with binding energy at 781.2 eV and a relevant satellite peak, which is assigned to Co 2p in Co(OH)2 species. Accordingly, the analysis of the O 1s spectrum reveals a strong peak at 530.9 eV, which originates from the typical hydroxyl groups in Co(OH)2. One more peak centred at 533.3 eV is also found, which should be assigned to the O species in the glass substrate used for XPS analysis. In both cases of Co 2p and O 1s no signal of Co3O4 was found. Note that XPS analysis could only detect elements existing at the locations deep to 12 nm down from a surface. Hence the XPS analysis indicates that all of the Co3O4 spherical NPs are well wrapped beneath the Co(OH)2 nanosheets, as depicted in Figure 2i. This is also consistent with the FESEM observation where no Co3O4 NP was exposed (Figure 2a-d).

Figure 4. (a,b,c,d) FESEM and (e,f,g,h) HRTEM images (with different magnifications) of the Co3O4@Fe-Co(OH)2 microfibers and (i) their relevant elemental mapping patterns.

The synthetic strategy of such 1D core-shell hierarchical structure is also applicable in the presence of Fe3+, which generates the Co3O4@Fe-Co(OH)2 nanocomposite where the as-formed Co(OH)2 nanosheets were doped with Fe3+ successfully. As shown in Figure 4a-c, numerous microfibers with length as long as hundreds of micrometers were fabricated under similar conditions. The microfibers demonstrate high structural integrity, each of which has a number of perpendicularly oriented thin nanosheets, similar to the case of Co3O4@Co(OH)2 microfibers. TEM observations clearly support the formation of the core-shell structure where linearly arranged spherical Co3O4 NPs are embedded within the hierarchical shells formed by interconnected Fe-doped Co(OH)2 nanosheets (Figure 4e,f,g). Their chemical compositions were also verified by the elemental mapping and the EDX spectrum (Figure S3). As demonstrated in Figure 4i, the mapping patterns of Co, Fe and O overlap very well with the corresponding TEM image. In the high resolution TEM image of the thin nanosheets, according to the orientations of



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the lattice fringes, numerous small crystal domains with average sizes less than 5 nm could be well identified, revealing their polycrystalline nature (Figure 4h). The as-formed Co3O4@Fe-Co(OH)2 nanocomposite was also characterized by XRD and XPS analyses. The peaks with 2θ angles at 37.4o and 59.4o are assigned to the Co3O4 nanocrystals (Figure 3a, line 2), and those at 23.4o and 33.8o are from the Fe-doped Co(OH)2. The peaks for (001) shift to much higher 2θ angles compared with that in the case of Co3O4@Co(OH)2 microfibers, indicating the evident change in the lattice constant c. As shown in Figure 3c, the XPS spectrum of Fe 2p3/2 reveals a single peak with binding energy at 711.4 eV and a corresponding satellite peak, which well verifies the successful doping of Fe3+ into the sample; and the XPS spectra of Co 2p and O 1s confirms the formation of Co(OH)2, similar to the previous case without Fe-doping. The percentage of iron doped within the microfibers could be controlled by adjusting the mole ratio of the relevant Fe(III) and Co(II) precursors. The EDX analysis revealed that the atomic ratio of Fe to Co is 1:3.6 when the mole ratio of Fe(III) to Co(II) was 1:2.5 in the precursor solution. 3.3 Oriented assembly of Co3O4@(Fe-doped)Co(OH)2 microfibers Within the microfibers, the (Fe-doped) Co(OH)2 nanosheets are well connected with each other which forms a continuous hierarchical shell (Figure 2i), which maintains the overall structural stability. Therefore, the as-prepared Co3O4@(Fedoped)Co(OH)2 microfibers are stable even upon the removal of the external magnetic field, and can be well dispersed in solvents through mechanical stirring or sonication, without compromising their structural integrity. Actually, the microfibers in Figure 2 and 3 were harvested via vigorous sonication or stirring to detach them from the magnet. This is distinct from those linear assemblies of Co3O4 NPs which are temporarily organized by the external magnetic field.11 To evaluate the magnetic-respnsive properties of the microfibers, their moment (M) as a function of the strength of the applied magnetic field (H) has been acquired. As depicted in Figure 5, for both the cases, a well-defined magnetic hysterisis loop was observed, clearly proving the ferromagnetic nature of the our microfibers. The remanence and the coercivity for the Co3O4@Fe-Co(OH)2 microfibers are 0.01 emu/mg and 225 Oe, much larger than those of the Co3O4@Co(OH)2 case. Such a difference indicates that the weight percentage of Co3O4 in the doped case is larger than that in the undoped case. This seems to be consistent with their relevant FESEM images, as the Co(OH)2 nanosheets in the undoped microfibers (Figure 2) seem to be larger in sizes than the Fe-Co(OH)2 nanosheets in the doped case (Figure 4). Due to the presence of the Co3O4 NPs as the internal core, the Co3O4@(Fe-doped)Co(OH)2 nanocomposites possess interesting magnetic responsive behaviors, which enables easy separation from the solvent as well as repetitive mesoscale

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Figure 5. H-M plots of the Co3O4@(Fe-doped)Co(OH)2 microfibers at room temperature.

or even macroscale self-assembly and disassembly of such hierarchical microfibers in solution. Particularly, as the 1D microfibers are anisotropic in dimensions, they demonstrate oriented self-assembly behaviors in response to a magnetic field.

Figure 6. (a,b,c) photos to show the oriented assembly of the Co3O4@Co(OH)2 microfibers into a 2D array on the surface of a flat magnetic bar; (d,e) the photo of the oriented assembly of Co3O4@Fe-Co(OH)2 microfibers on a cylindric magnet; and (f) a schematic depiction of the oriented assembly of the microfibers.


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An example of this regard is exhibited in Figure 6a-c. In this scenario, a glass container with aqueous solution of freely dispersed Co3O4@Co(OH)2 microfibers was kept static on the surface of a flat magnetic bar (Figure 6f). Those 1D microfibers were found to flow towards the magnetic bar at the bottom gradually (Figure 6a), and after several minutes all the microfibers precipitate at the bottom and the up solution becomes clear (Figure 6c). More importantly, as depicted in Figure 6f, those long microfibers became well oriented when approaching the bottom of the container, with their long axis standing normal to the surface of the flat magnetic bar, i.e., the bottom of the container (Figure 6b). Eventually, a 2D array of such freestanding microfibers was formed in the solution, which could even be observed directly with naked eyes (Figure 6c). The Co3O4@Fe-Co(OH)2 microfibers demonstrate similar magnetic-responsive oriented behavior. Shown in Figure 6d and the enlarged photo in Figure 6e, the microfibers were spontaneously assembled on the cylindric magnet and are positioned perpendicularly to the surface of the magnet, which form a macroscale assembly. Note that, without efficient external interference, freestanding 1D structures such as nanowires or microfibers obtained via wet-chemistry strategies entangle with each other readily in a random manner and form a network.24-26 Furthermore, displayed in Figure 7, when the

micro-magnet which is not only responsive to the external magnetic field but also could relocate their positions and adjust their orientations so that their long axis parallels to the direction of the external magnetic field, which eventually become aligned in a ordered 2D array in a fluid (Figure 6f). Note that such macroscale 2D arrays of microfibers in Figure 6c depends on the presence of the external magnetic field, which hence could be disassembled via mechanical stirring and reassembled again repetitively. 3.4 Formation mechanism: revisit of the Co2+/NaBH4 system Both the chain-like assembly of the Co3O4 NPs and the Co3O4@(Fe-doped)Co(OH)2 microfibers were produced at ambient conditions from the interaction between the Co2+ and NaBH4 (Scheme 1), a classical system that has been widely investigated and employed for various technological purposes.10,17-19 As illustrated in equation 1, the aqueous solution of NaBH4 will undergo hydrolysis spontaneously, releasing H2 bubbles; and the hydrolysis product sodium metaborate NaBO2 could further undergo hydrolysis, leading to an alkaline pH condition of the solution (equation 2); meanwhile, Co2+ cations could also be reduced by NaBH4 and converted into metallic Co (equation 3, note that the formation of CoBx is not considered in this case due to the low concentration of NaBH4 in our work). With the presence of O2, the metallic Co is susceptible to oxidation and becomes cobalt oxides easily such as CoO or Co3O4 (equation 4); direct precipitation of Co(OH)2 could also occur readily with sufficient OH- anions (equation 5). NaBH 2H O NaBO 4H ↑ (Equation 1) (Equation 2) BO 2H O ↔ BOH OH NaBH Co 8OH → NaBO Co 6H O (Equation 3)

Co O → CoO

Figure 7. (a,b,c,d) FESEM images (with different magnifications) of 2D array of the microfibers after the solvent evaporated away with the presence of the external magnetic field.

solvent (e.g., ethanol in this case) in the container in Figure 6c is evaporated away, particularly, in the presence of the external magnetic field, the 2D array of the microfibers, which are originally freestanding in the solvent, collapse on the substrate due to the forces such as gravity. However, one can see that, instead of falling down on the substrate randomly in all the directions, all of the microfibers lying on the substrate assume almost the same orientation, with their long axis paralleling to each other (Figure 7a,b). Such a phenomenon reaffirms that the oriented assembly of microfibers in Figure 6c is normal to the bottom of container. For the sake of observation, the sample for FESEM and TEM characterization in Figure 2 and 4 were actually prepared similarly to avoid random scattering or extensive entanglement of the microfibers on the substrate. In view of such oriented assembly behaviors, owing to the presence of the internal well-aligned Co3O4 NPs, each of the microfibers is actually a

(Equation 4)

Co 2OH ↔ CoOH ↓ (Equation 5) Note that the instantaneous kinetics of the above reactions could vary widely throughout the whole process, due to continuous changes in the concentrations of the relevant reactants. As such, the chemical state and the morphology of the cobalt species could be quite versatile.16,19 While the chemical states have been intensively investigated to understand the essential catalytic active species responsible for activation and deactivation of H2 generation from NaBH4 hydrolysis, the morphological evolutions at micro- and/or meso- scale of the cobalt species have been rarely attempted so far as we know. The technical conditions to produce the linear chain of Co3O4 NPs (Figure 1) and those to form the hierarchical Co3O4 NP@(Fe-doped)Co(OH)2 microfibers (Figures 2&3) are quite similar, both of which involves the usage of NaBH4 (with limited dosage) to convert or precipitate the Co2+ cations in the PVP aqueous solution. For both cases, upon the addition of the aqueous NaBH4 solution into the Co2+ precursor solution, the mixture became brown within 30 seconds. This early stage is likely to be dominated by the reduction of Co2+ cations to tiny metallic Co species, i.e., equation 3.16 However, the as-formed Co0 species are extremely unstable in the aqueous solution of NaBH4, even in cases where the amount of NaBH4 is far in excess of Co2+.17 In view of this, the as-formed tiny Co0



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nanocrystals, when migrating towards the magnet at the bottom of the container driven by the magnetic forces, would be oxidized and converted to cobalt oxides, typically Co3O4 species, i.e., equation 4. Thus-formed Co3O4 nanocrystals would also migrate towards the magnet and coalesce with each other via Ostwald ripening, eventually forming the chain-like assemblages of Co3O4 NPs on the magnet (Figure 1). Similarly, the internal core in the case of Co3O4@(Fe-doped)Co(OH)2 microfibers, i.e., the chain-like assembly of Co3O4 NPs , are also speculated to be formed at the early stage prior to the formation of the Co(OH)2 nanosheets. Meanwhile, accompanied with the oxidation of the Co0 and the directed diffusion of Co3O4 nanoclusters, hydrolysis of NaBH4 occurs continuously, which could last for hours, as supported by the evolution of bubbles observed on the magnetic bar at the end of the reaction (Figure 6d). The NaBO2 would further undergo hydrolysis to release OHgradually (equation 2). Therefore, the solution becomes increasingly alkaline along with prolonging storage time, which consequently favors the occurrence of equation 5, i.e., the formation of Co(OH)2 nanocrystals. Moreover, due to the catalytic effect of the cobalt species towards NaBH4 hydrolysis,20 the local concentration of OH- on the immediate surface of Co or Co3O4 nanocrsytals might be slightly higher than those in the bulk solution due to accelerated hydrolysis of NaBH4; this is also applicable to the area around the magnet where the cobalt species were concentrated. Therefore, the pH value around the magnet might be higher than the other areas. As such, when the concentration of OH- is sufficiently high, Co(OH)2 nanocrystals starts evolving gradually on the backbone of the Co3O4 NPs. Actually, the pH value was found to play a determining role in regulating the ultimate chemical compositions of the cobalt species in such a Co/NaBH4 system. A major difference in the experimental conditions between the case of the Co3O4 chain-like assembly and the Co3O4 NP@(Fe-doped)Co(OH)2 case is that the absolute concentration of Co2+ cations and NaBH4 is higher in the latter case, yet the molar ratio of NaBH4 to Co2+ is similar between these two cases. Specifically, the absolute concentration of NaBH4 in the former case is less than 0.3 mg/ml, whereas in the microfiber case it is around 0.9 mg/ml. Therefore, the solution in the microfiber case, especially at the later stage, would experience much higher concentration of OH-, which thus favors the formation of Co(OH)2. Actually, as a control experiment, further increasing the absolute concentration of NaBH4 and Co2+, blue Co(OH)2 floccules would be obtained as the only endproduct which are non-magnetic responsive at all at room temperature. Therefore, similar to the NaBH4/Co(II) system for H2 generation, for cases involving such low NaBH4 concentrations, the chemical compositions of the endproducts vary widely depending on both the concentrations of NaBH4 and Co(II) as well as their mole ratio. Our preliminery results revealed that, to generate Co3O4@(Fe-doped)Co(OH)2 microfibers, the final NaBH4 concentration could vary within the range of 0.5~0.9 mg/ml and the Co(II) concentration could be adjusted from 2.5 mM to 6.0 mM. A systematic investigation is yet needed for a thorough understanding in this regard. It shall also be pointed out that in the formation of chain-like assemblage of Co3O4 NPs, SDBS, an anionic surfactant, was introduced. Its presence is found to support the formation of Co3O4 species,

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possibly via forming the Co(DBS)2 complex on the interface of the Co3O4 NPs which hence inhibits the generation of Co(OH)x species, as supported by the control experiment in Figure S4. Furthermore, the presence of the external magnetic field was proved necessary to induce the self-assembly of the Co3O4 NPs which then serve as the backbone to support and organize the hierarchical growth of the Co(OH)2 nanosheets. When the magnetic bar was removed from the solution after the initial stirring of 30 seconds, disordered mixtures of irregular aggregates, thin nanosheets and spherical particles are formed under the same condition (Figure S5a,b). Note that we have tried the synthesis with different experimental scales using magnetic bars with various sizes and shapes. It seemed that the shape and the size (i.e., the magnetic strength) of the magnet has no noticeable influence on the general structural features of the as-obtained core-shell microfibers. This is understandable, since the magnetic field works directly on the assembly behaviors of the internal spherical Co3O4 NPs which in general will be linearly aligned whereas the formation and the structure of the external (Fe-doped) Co(OH)2 shell is fundamentally regulated by the chemistry principles and their inherent crystallographic properties. Besides, it has been repetitively demonstrated that the generation of organic or inorganic nanostructures with hierarchical complicacy often requires good control over the process kinetics.27,28 In view of this, the usage of PVP aqueous solution is important for the morphological evolution of our hierarchical microfibers. Its high viscosity leads to relatively slow diffusion of the primary nanocrystals and thus provides the necessary kinetic control for the subsequent coalescence and reorientation of the primary nanocrystals on the magnet. And compared with deionized water, the usage of a highly concentrated PVP solution could also slow down the hydrolysis of the NaBH4, which thus prevents the pH value from rapid increasing. As a comparison, when the PVP solution was replaced with deionized water as the solvent, though Co3O4@Co(OH)2 core-shell nanocomposite could still be observed, they are mostly irregular which lost the 1D structural integrity of those hierarchical microfibers (Figure S5c,d). This is because that, with relatively fast hydrolysis of NaBH4 and rapid diffusion of cobalt species, the formation of Co(OH)2 would occur early and dominate over that of the Co3O4 NPs, as a result of which the nanocomposite is much less ordered, due to less presence of the magnetic-responsive Co3O4 NPs within the Co(OH)2 shell (Figure S5c,d). Note that a PVP concentration of 8% wt was employed throughout the work. Our control experiment revealed that such a relatively high PVP concentration is necessary. Using a PVP concentration of 5% wt, microfibers could be formed but with less structural integrity, and there are also formation of irregular aggregates (Figure S5e,f). 3.5 OER electrocatalytic activity of Co3O4@(Fedoped)Co(OH)2 microfibers Nanocomposites of transition metal oxide and metal hydroxides have been found with very important applications


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Figure 8. (a,b) double layer capacitance measurements obtained within the non-Faradaic region with various scan rates and (c) asobtained linear plots of cathodic charging currents versus the scan rate; (d) the CVs (e) the LSVs; and (f) the relevant Tafel plots to evaluate the OER catalytic activity of the as-formed chain-like assemblage of Co3O4 NPs ( the wine curve), Co3O4@Co(OH)2 (the purple curves or dots) and Co3O4@Fe-Co(OH)2 (the green curves or dots) microfibers.

as electrode materials for chemical energy storage and conversion technologies.1 Particularly, cobalt oxide and hydroxide and their relevant nanocomposites with various morphologies have been widely explored as efficient and lowcost electrocatalysts to enhance the oxygen evolution reaction (OER) process in water electrolysis.29,30 Herein, the electrocatalytic property of such hierarchical Co3O4@(Fedoped)Co(OH)2 microfibers were further explored via a preliminary test using the OER as a demo in 0.1 M KOH. Prior to the OER test, the electrochemically active surface area (ECSA) was evaluated from the electrochemical double layer capacitance of the catalytic surface.31 As shown in Figure 8a for the Co3O4@Co(OH)2 case and Figure 8b for the Co3O4@Fe-Co(OH)2 case, cyclic voltammogram(CV) curves were obtained with a series of scan rates (υ) within the potential range where no Faradaic current occurs. And thus obtained cathodic charging currents (i) were plotted linearly against the scan rates at the open-circuit potential to give the relevant electrochemical double layer capacitance Cdl using the equation i =υCdl. Shown in Figure 8c, the Cdl was obtained as 0.148 mF and 0.124 mF and for the case with and without Fedoping, respectively. The ECSA was then estimated from the equation ECSA= Cdl/Cs where Cs is the specific capacitance. Herein the general specific capacitance in alkaline solution － was employed, which was around 0.04 mF cm 2 according to

a recent benchmarking study on metal oxide-based OER catalysts.31 On this basis, the ECSA were obtained accordingly. And when the ECSA was further divided by the geormetric area of the glasscarbon electrode (3.0 mm in diameter), the roughness factor was acquired, which is an important index which depicts the electrocatalytic interface texture and is associated with the reaction kinetics and activity. Accordingly, the working electrode prepared with Co3O4@Co(OH)2 microfibers gave a roughness factor of 44, and the one with Fe-doping gave a value of 52. Compared with a wide variety of typical cobalt or nickel oxide-based OER catalysts prepared by electrodeposition,31 the roughness factors of the working electrode prepared with our hierarchical microfibers are larger by at least one order of magnitude. Such large roughness factor indicates the explosure of numerous catalytic active sites as well as favorable diffusion kinetics for the reactants. This is highly desirable for the solid-liquid interfacial catalytic process. Such comparative results clearly confirmed the structural merits of our microfibers. As supported in Figure 2a-d and Figure 4a-d, with perpendicularly-oriented thin (Fedoped) Co(OH)2 nanosheets organized along the longitudinal axis, such 3D-hierarchially architectured microfibers entangled with each other, forming an interface with a much less compact and rough topology, compared with those conventional discrete spherical particles or unorganized individual nanoflakes. To evaluate their OER catalytic activity, the CVs of the two catalysts, the microfibers with and without Fe-doping, was compared in Figure 8d. The case of Co3O4@Co(OH)2 demonstrates a well-defined anodic oxidation peak at 1.156 V (vs RHE, similarly thereafter) and a relevant cathodic reduction peak at 1.075 V, which are the typical oxidation and reduction peaks of Co(OH)2 species (Figure 8d, the purple curve). Cobalt species with higher oxidative valence such as CoIII or CoIV might be formed and are the factual catalytic active sites.32 And the case with Fe-doping demonstrates a pair of anodic and cathodic peak at relatively higher voltages due to the coexistence of Fe and Co ionic species (Figure 8d, the green curve). The relevant polarization curves of the microfibers, and as well, the chain-like assembalge of Co3O4 NPs, were obtained with a scan rate of 10 mV/s, as shown in Figure 8e. Oxygen starts evolving at around 1.560 V for both cases, which give the onset overpotential of 0.34 V; subsequently, the current density increases dramatically versus increasing voltage. At the current density of 10 mA/cm2, as indicated by the horizontal dashed grey line in Figure 8e, the overpotential was calculated to be 0.386 V for the case with Fe-doping and 0.408 V for the case without Fe-doping, respectively. Both are evidently smaller than that of the pure Co3O4 NPs which gives an overpotential of 0.53 V (Figure 8e, the wine curve). The tafel plot was also calculated from the relevant polarization curve of the microfibers (Figure 8f), from which a tafel slope of 56 and 85 mV/dec was obtained for the cases with and without Fe-doping, respectively. Compared with most of the cobalt or nickle hydroxide-based electrocatalysts,4,33 such relatively small tafel slopes further confirms their high catalytic activity. Note that the overpotential at 10 mA/cm2 is a widely used Figure of merit to compare the catalytic performances of different OER electrocatalysts. The electrocatalytic activities of our nanocomposites were also



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compared with those reported in the literature with similar chemical composition, as listed in Table 1. One can see that, compared with most of the Co(OH)2 and cobalt oxides with various morphologies reported in the literature, our Co3O4@Co(OH)2 microfiber exhibits evidently lower overpotential. Such high catalytic activity mainly originates from its hierarchical 1D structure; with well-organized array of thin nanosheets of Co(OH)2, relatively large specific area and thus large population of active sites could be exposed and are easily accessible to the external reactants, as supported by their large roughness factor. Moreover, produced at room temperature, one may expect that our Co3O4@(Fedoped)Co(OH)2 microfibers are poorly crystallized (Figure 3a) compared with those produced via hydrothermal treatment or electrodeposition. Such poor crystallinity supports the existence of defects which could act as active catalytic sites.33 It shall also be mentioned that compared with other synthetic approaches, the facile preparation and easy separation with high repeatability is another obvious advantage of our Co3O4@(Fe-doped)Co(OH)2 microfibers. The case with Fedoping displays improved OER catalytic activity, clearly showing a synergetic effect from the coexistence of the two transition metal cations. And those prepared with a ratio of Fe(III)/Co(II) close to 1/3 in the precursor solution demonstrated the optimal OER catalytic activity (Figure S6). The catalytic durability of the microbibers was also evaluated through the chrono potentiometry test at the current density of 10 mA/cm2 for several hours. Both cases demosntrate remarkable catlytic stability, with only slight increase in the overpotential after more than 7 hours (Figure S7). Table 1. Comparisons of OER catalytic activity (overpotential η at 10 mA/cm2) and ease of prepration for different cobalt oxide/hydroxide based electrocatalysts.

Electroatalysts Co3O4@Co(OH)2

η/V 0.408

pH 13

Synthetic strategy this work

Co3O4@Fe-Co(OH)2

0.386

13

this work

0.390

13

hydrothermal method

0.450 0.405 0.430

13 13 14

hydrothermal method ZIF 67 as template electrodeposition

0.499

14

electrodeposition

14

polystyrene sphere as template

Fe3O4/Co(OH)2 nanosheet34 Co(OH)2 nanoplates6 Co3O4 hollow sphere35 CoOx on FTO31 Co3O4/Co(OH)2 on steel mesh5 C-dopedCo/Co3O4 hollow sphere36

0.352

However, it shall be pointed out that, compared with the classical Fe-Co layered double hydroxides, our Co3O4@FeCo(OH)2 demonstrate a much lower catalytic activity. Actually, Fe-Co layered double hydroxides have been observed as the best OER catalysts so far as we know. Therefore, such comparative results indicate that, for the FeCo based OER catalysts, how the Fe3+ interacts with the Co2+ at the molecular level have profound effects on their catalytic activity, which are yet to be studied. In addition, it has been reported that the coexistence of Co3O4 and Co(OH)2 could lead to synergetic effects towards their electrochemical propeties.1,2 However, herein, lacking of a hollow shell of hierarchically organized (Fe-doped) Co(OH)2 nanosheets for

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comparison, at this stage it remains unknown whether there exists a synergetic effect between the coexistence of the two components towards the OER catalytic activity or not. 4. Conclusions In summary, by revisiting the well-studied Co/NaBH4 aqueous system, the chain-like assembly of Co3O4 NPs or nanocomposite of Co3O4@Co(OH)2 microfibers which has chain-like assembly of Co3O4 NPs as the core and perpendicularly-oriened inter-connected Co(OH)2 thin nanosheets as the shell were realized facily at ambient conditions with the assistance of an external magnetic field; the Co(OH)2 could also be doped, generating the Co3O4@FeCo(OH)2 microfibers at room temperature. The microfibers could well maintain their structural integrity even upon the removal of the external magnetic field. The pH value, which is closely affected by the concentration of NaBH4, was found to play a determining role in regulating the eventual composition and morphology of the cobalt endproducts. With the internal Co3O4 core, such 1D Co3O4@(Fe-doped)Co(OH)2 microfibers exhibit macro-scale oriented self-assembly behaviors in response to a magnetic stimulus. And due to their poor crystallinity and hierarchical structure of the shell, the Co3O4@(Fe-doped)Co(OH)2 microfibers show remarkable electrocatalytic activity towards the oxygen evolution reaction in 0.1 M KOH, with an overpotential of 0.408 V obtained for the case without Fe-doping and 0.386 V for the case with Fedoping. The ease of preparation and the remarkable magneticelectrochemical dual activity of the microfibers makes our synthetic strategy highly competitive for fabrication of cobalt oxide or hydroxide nanoarchitectures with favorable structual merits.

ASSOCIATED CONTENT Supporting Information. TEM, XRD pattern, EDX spectrum, photos, OER stability test. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author [email protected]; [email protected].

Conflict of interest There are no conflicts to declare. Acknowledgements The authors would like to acknowledge National University of Singapore for part of the TEM observation. This work was supported by the National Natural Science Foundation of China (NO. 21703182 and 21373008). References (1) Qorbani, M.; Naseri, N.; Moshfegh, A. Z. Hierarchical Co3O4/Co(OH)2 Nanoflakes as a Supercapacitor Electrode: Experimental and Semi-Empirical Model. ACS Appl. Mater. Inter. 2015, 7, 1117211179. (2) Pan, X.; Ji, F.; Kuang, L.; Liu, F.; Zhang, Y.; Chen, X.; Alameh, K.; Ding, B. Synergetic Effect of Three-Dimensional Co3O4@Co(OH)2


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