carbon dots: Boosting the hydrolytic dehydrogenation of ammonia borane

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Interface electron collaborative migration of Co–Co3 O4 /carbon dots: Boosting the hydrolytic dehydrogenation of ammonia borane Han Wu , Min Wu , Boyang Wang , Xue Yong , Yushan Liu , Baojun Li , Baozhong Liu , Siyu Lu PII: DOI: Reference:

S2095-4956(19)30944-1 https://doi.org/10.1016/j.jechem.2019.12.023 JECHEM 1049

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

27 November 2019 19 December 2019 19 December 2019

Please cite this article as: Han Wu , Min Wu , Boyang Wang , Xue Yong , Yushan Liu , Baojun Li , Baozhong Liu , Siyu Lu , Interface electron collaborative migration of Co–Co3 O4 /carbon dots: Boosting the hydrolytic dehydrogenation of ammonia borane, Journal of Energy Chemistry (2019), doi: https://doi.org/10.1016/j.jechem.2019.12.023

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Interface electron collaborative migration of Co–Co3O4/carbon dots: Boosting the hydrolytic dehydrogenation of ammonia borane Han Wua,b, Min Wuc, Boyang Wanga, Xue Yongd, Yushan Liua, Baojun Lia, Baozhong Liub, Siyu Lua,* a

College of Chemistry, Zhengzhou University, Zhengzhou 450001, Henan, China

b

College of Chemistry and Chemical Engineering, Henan Polytechnic University,

Jiaozuo 454000, Henan, China c

College of Materials Science and Engineering, Zhejiang University of Technology,

Hangzhou 310014, Zhejiang, China d

Department of Physics and Engineering Physics, University of Saskatchewan,

Saskatoon, S7N5E2, Canada *Corresponding author. E-mail address: [email protected] (S. Lu). Abstract Ammonia borane (AB) is an excellent candidate for the chemical storage of hydrogen. However, its practical utilization for hydrogen production is hindered by the need for expensive noble-metal-based catalysts. Herein, we report Co–Co3O4 nanoparticles (NPs) facilely deposited on carbon dots (CDs) as a highly efficient, robust, and noblemetal-free catalyst for the hydrolysis of AB. The incorporation of the multi-interfaces between Co, Co3O4 NPs, and CDs endows this hybrid material with excellent catalytic activity (rB = 6816

min–1 gCo–1) exceeding that of previous non-noble-metal NP

systems and even that of some noble-metal NP systems. A further mechanistic study suggests that these interfacial interactions can affect the electronic structures of interfacial atoms and provide abundant adsorption sites for AB and water molecules,

resulting in a low energy barrier for the activation of reactive molecules and thus substantial improvement of the catalytic rate. Keywords: Ammonia borane; Hydrogen evolution; Co–Co3O4 interface; Carbon dots; Nanoparticles 1. Introduction Nowadays, the excessive consumption of fossil fuels and growing environmental concerns have made the need to develop a hydrogen economy more urgent [1,2]. However, many issues must be overcome for the transition to hydrogen energy; in particular, the efficient generation of hydrogen at room temperature presents enormous challenges [3]. The main current hydrogen production method via the electrolysis of water involves sacrificing one energy source for another, which is not economically desirable [4]. Therefore, a safe and efficient hydrogen production method is necessary for sustainable hydrogen utilization. Hydrogen production from chemical hydrogen storage materials has thus attracted research attention [5]. Its high hydrogen gravimetric density (19.6 wt%) and low molecular weight (30.86 g mol–1) have led ammonia borane (NH3BH3, AB) to be regarded as competitive for chemical hydrogen storage [6,7]. Since the first AB synthesis reported by Shore and Parry in 1955, many methods with considerable yields have been developed [8]. Recently, the hydrolysis of AB has been widely investigated because of the high amount of released hydrogen. When appropriate catalysts are used, hydrogen can be released

under

NH3 BH3 2H2

ambient NH

B

2

conditions -

via

the

following

reaction:

3H2 . To date, fast hydrogen evolution from AB has

been achieved using homogenous molecular systems and metallic nanoparticles (NPs),

especially Pt- and Ru-based noble metals or their alloys, as the catalysts [9,10]. However, the high cost and limited reserves of noble metals have greatly limited their usage. Compared with noble-metal catalysts, non-noble-metal materials exhibit lower catalytic activity. Because of their relatively inert catalytic nature, non-noble metal catalysts require extensive morphological and interfacial engineering to be effective [11]. To date, many excellent non-noble-metal-based materials, using such as Mo [12], Ni [13], and Co [14], have been shown to be efficient catalysts for AB hydrolysis. Many efforts have been made to develop new structures (e.g., amorphous [15], NP [16,17], alloyed [18], core–shell bimetallic [19], and metal–metal oxide [14]) to enhance the catalytic activity in AB hydrolysis compared with that of equivalent crystalline or monometallic counterparts. For example, Xu et al. [20] reported that a novel Au@Co core–shell NP material synthesized via a one-step seeding–growth process exhibited higher catalytic activity for AB dehydrogenation than its pure metal counterpart. However, pure metals and their derivatives still face the problems of poor dispersity, low conductivity, and ease of agglomeration. To overcome these limitations, some functional carbon supports, such as metal–organic frameworks (MOFs) [21], carbon nanotubes [22], graphene [23], and organic polymers [24], have been applied. In particular, because of their microporous cages, MOFs can confine ultrafine metal particles, resulting in high turnover frequency (TOF) for the AB hydrolysis. However, these catalysts are usually prepared via complicated syntheses, typically involving toxic organic reagents, and only a small volume of products can be obtained. Therefore, the design and synthesis of efficient carbon-based non-noble-

metal composites for AB hydrolysis in a simple and green way is critical to the popularization and application of chemical hydrogen storage materials. Carbon dots (CDs), a new photoluminescent material, have attracted considerable attention because of their tunable photoluminescence (PL) and unique electron-transfer abilities as well as their low cost, nontoxicity, and good hydrophilicity [25–28]. Recently, many excellent CDs have been prepared from inexpensive organic small molecules and biomass without the use of toxic organic reagents [29,30]. In addition, the strong complexing action between surface groups and many metal ions (such as Fe3+, Ru3+, Co2+, and Cu2+), combined with their large specific surface areas, have made CDs an excellent class of materials to support catalysts for photocatalysis and electrocatalysis [31,32]. To the best of our knowledge, there have been no reports on the use of CDs for AB hydrolysis. We anticipated that the excellent physicochemical properties of CDs would also endow the resultant catalysts with high catalytic performance for AB hydrolysis. In addition, the structures of active sites in these materials must also be clearly understood for the rational design of new efficient catalyst systems. Herein, we first synthesized excellent PL CDs with a uniform size of approximately 2.54 nm via a simple hydrothermal process using inexpensive citric acid. Then, Co–Co3O4 hybrid NPs facilely deposited on CDs (simplified as Co– Co3O4/CDs) were constructed via direct calcination of the Co2+–CDs complexes and subsequent activation treatment in air. When used as a catalyst for AB hydrolysis, the material exhibited excellent catalytic activity (rB = 6816

min–1 gCo–1). In

addition, even after five cycles, it retained very high catalytic activity. Further mechanistic study suggested that the high activity could be attributed to the interfacial interaction between the Co, Co3O4 NPs, and CDs, which provided abundant active sites for dehydrogenation of AB. 2. Experimental 2.1. Chemicals Cobalt nitrate hexahydrate (Co(N citric acid monohydrate (C6H8

7·H2

3)2·6H2

, Aladdin Industrial Co., Ltd., AR),

, Shanghai Macklin Biochemical Co., Ltd.,

GR), cobalt nanoparticles (Co-50 nm, Shanghai Macklin Biochemical Co., Ltd., 99.9%), sodium hydroxide (Na H, Sino Pharm Chemical Reagent Co. Ltd., China, AR), ammonia borane (AB, Tianjin Chemical Reagent Co., Ltd., China, 97%) and deionized (DI) water are purchased from commercial suppliers. All of the chemicals are used as received without any further purification. 2.2. Synthesis of materials Synthesis of CDs. The CDs were synthesized by the typical hydrothermal method. Briefly, a solution composed of

mmol C6H8

7·H2

and 30 mL DI water

was added into a 60 mL Teflon-lined autoclave followed by being heated at 200 °C for 8 h. After natural cooling to room temperature, the as-prepared production was filtered through a 0.22 μm filter membrane and finally dialyzed with a membrane (500 Da) for 2 h to remove the large particles and small molecules. As a result, the yellowish filtrate was obtained. After freeze-drying further, the CDs powders were obtained and stored at characterization.

°C, which can be redispersed in DI water for further

Synthesis of Co-Co3

/CDs NPs. Typically, 296 mg Co(NO3)2·6H2O was

dissolved in the above CDs solution followed by ultrasound for 30 min. The obtained light red solution was kept to be further stirred for 10 h to get an absolutely homogeneous solution. Then, the excess liquid was removed over a rotary evaporator. Finally, the dark red powders (referred to as Co 2+-CDs complex) were obtained after being dried at 60 °C for 48 h under vacuum. The collected Co2+-CDs complex powders were annealed at 660 °C for 4 h under argon atmosphere. The as-obtained sample is simplified as Co/CDs. Then, the Co/CDs powders were further activated at 200 °C for 22 h in air and simplified as Co-Co3O4/CDs. Following similar procedures, the Co-Co3O4/CDs-I, Co-Co3O4/CDs-II, and Co-Co3O4/CDs-III were also produced under the same preparation condition except changing the annealing time to 1,2 and 6 h, respectively. Moreover, a series of Co-Co3O4/CDs powders with different oxidation degrees were obtained via activating Co/CDs powders under air atmosphere at 200 °C for different durations to obtain Co-Co3O4/CDs. The resulting materials are named Co-Co3O4/CDs-t, t represents activation time. Synthesis of Co3O4/CDs NPs. A certain amount of Co NPs (the average particle size is 50 nm) was annealed at 600 °C for 2 h in air to obtain Co3O4 NPs. Then, 81 mg Co3O4 NPs were dissolved in the above CDs solution followed by ultrasound for 30 min to get a uniform system. The system was stirred in a water bath at 70 °C until the liquid evaporated. The obtained solid was dried under vacuum to get Co3O4/CDs powders. Synthesis of Co-Co3O4 NPs. A certain amount of Co NPs (the average particle

size is 50 nm) was placed in porcelain boat. Then, the porcelain boat was heated under air atmosphere for different temperatures and durations to get a series of CoCo3O4 powders with different oxidation degrees. The resulting materials are named Co-Co3O4-T-t, T represents activation temperature, t represents activation time. Synthesis of the mixture of Co and Co3

NPs. The mixture was prepared by

direct physical mixing according to the mass ratio of Co to Co3O4 (1:1). 2.3. Materials characterization Powder X-ray diffraction (XRD) analyses were performed on a Rigaku Dmax3C X-ray diffraction with Cu Kα radiation and the 2θ angles were at 5°–90°. A FEI Talos, F200S instrument were used to obtained transmission electron microscope (TEM) images and Energy-dispersive X-ray spectroscopy mapping. The HAADFSTEM images were obtained via a FEI TITAN Chemi STEM equipped with a CEOS probe corrector (Heidelberg, Germany). X-ray photoelectron spectroscopy (XPS) was performed on a PHI quantera SXM spectrometer. Thermal gravimetric analysis (TGA) was studied on a STA 409PC/PG (NETZSCH Germany). Raman spectra were studied via a LabRAM HR Evolution (HORIBA). Infrared spectroscopy was performed on an IR Spectrometer NEXUS 470. UV-vis was acquired on a TU-1810PC. PL emission and PL excitation spectra were acquired on an RF-6000 (Shimadzu, Japan). 2.4. Catalytic hydrolysis test of AB The catalytic activities of the as-prepared materials were evaluated with the following method. The catalyst material (10 mg) was firstly placed in a roundbottomed glass flask containing 5 mL DI water. After ultrasonic dispersion for 30 min,

the flask was immersed in a temperature-controlled water bath with a magnetic stirrer at 600 rpm. A 10 mL aqueous solution composed of AB (1.5 mM) and NaOH (0.5 g) was added to the flask through a dropping funnel. Hydrogen generation was studied via the traditional water displacement method. All samples are finely ground before being tested. Moreover, the data from the stabilizing stage of the whole reaction was used to calculate the specific hydrogen generation rate (rB) and TOF according the following two formulas (Eqs. 1 and 2): [ [

Where and

(Eq. 1)

]

(Eq. 2)

]

is the time (min) which it takes to generate 75 mL of hydrogen

(min) for 25 mL,

(g) represents the mass of Co in the catalyst.

mole of generated H2 during the 25–75 mL, while

is the

is the total mole of Co in the

catalyst. The reusability of catalyst was tested as follows: after the previous cycle was completed, the catalyst was kept at the bottom of the flask using a magnet. After removing supernatant liquid, the catalyst was washed with water and ethyl alcohol. The obtained catalyst was re-dispersed into 5 mL ID water followed ultrasonic dispersion for 10 min. A fresh AB aqueous solution (5 mL, 1.5 mM) was added to trigger the subsequent recycle run. This process is repeated four times to evaluate the durability of the catalyst. 3. Results and discussion 3.1. Physicochemical properties of the as-synthesized CDs

Fig. 1. Characterizations of the as-prepared CDs. (a) TEM image (inset: size distribution) and (b) TEM image (inset: HRTEM image) of CDs. (c) XRD pattern of CDs. (d) FT-IR spectrum of CDs. (e) excitation-emission PL spectrum. (f) UV-vis, PL emission, and PL excitation spectra of CDs. (g)-(i) XPS full spectrum and highresolution C 1s and

1s XPS spectrum of CDs.

A simple hydrothermal method was used to synthesize the CDs from dehydration and carbonization of citric acid molecules. The morphology of the as-prepared CDs was characterized using TEM. The TEM images (Fig. 1a, b) revealed a uniform dispersion of the as-prepared CDs with an average diameter of approximately 2.74 nm.

The high-resolution TEM (HRTEM) image shown in the inset of Fig. 1(b) shows the high crystallinity of the CDs with an interlayer spacing of 2.05 Å, corresponding to the d-spacing of the graphene (002) planes [33]. Furthermore, the XRD pattern (Fig. 1c) shows a broad peak at approximately 26.3°, which is typical for CDs. In general, the oxygen-containing groups in low organic molecules are a key to the formation of CDs during the hydrothermal process [34,35]. To validate this point, we further investigated the chemical composition and functional groups on the surface of the CDs. The Fourier-transform infrared (FTIR) spectrum in Fig. 1(d) shows a broad peak at 3084 cm–1, which is attributed to the stretching vibration of –OH. Here, the redshift of the hydroxyl group indicates that the as-prepared CDs are more likely to form hydrogen bonds. The peaks at 1713 and 1408 cm–1 are assigned to C=O stretching vibrations and C–H deformation vibrations, respectively [36,37]. The synthesized CDs clearly contained abundant carboxyl and hydroxyl groups on the surface derived from the citric acid molecules. The presence of these oxygen-containing groups not only increased the water solubility of the as-prepared CDs but also caused their surfaces to exhibit a certain negative potential, which greatly promoted the complexation between them and metal cations [38]. In addition, the C–O–C/C–O stretching vibration peak observed at 1212 cm–1 indicates that before carbonization, dehydration and polymerization of several citric acid molecules occurred to form macromolecules during the hydrothermal process [39]. The optical properties of the as-synthesized CDs were investigated using UV–vis and PL spectroscopy (Fig. 1e, f). When excited by 340 nm, the CDs exhibited strong

blue PL centered at 417 nm with significant excitation-wavelength-dependent PL (Fig. 1e). Moreover, the UV–vis spectra (Fig. 1f) of the CDs show an obvious absorption peak in the range of 200–300 nm, which is assigned to the π π* transition of C= . The surface elemental composition and chemical states of CDs were determined using XPS (Fig. 1g–i). The XPS survey spectrum reveals that the surfaces of the CDs were mainly composed of C and O (Fig. 1g). The proportion of oxygen in the assynthesized CDs was much higher than that of carbon because of the higher number of oxygen-containing groups on their surfaces. The high-resolution C 1s spectrum (Fig. 1h) can be deconvoluted into three peaks at 285.2 eV (C–C), 286.8 eV (C–O/C– O–C), and 289.2 eV (C=O) [40]. The high-resolution O 1s spectrum (Fig. 1i) contains two peaks at 532.1 and 533.2 eV for C=O and C–OH/C–O–C, respectively [41]. These XPS results further demonstrate that the surfaces of the CDs are capped with abundant oxygen-containing functional groups, which indicates not only the formation of CDs but also their surface modification and functionalization. 3.2. Structure and interaction of catalysts (Co-Co3O4/CDs)

Fig. 2. (a) Schematic illustration of the preparation of the Co/CDs and Co-Co3O4/CDs NPs. (b, c) TEM images of Co-Co3O4/CDs NPs. (d) HR-TEM image and (e) the selected area electron diffraction of the core. (f, g) Element mapping images of Co-Co3O4/CDs NPs. Benefiting from their small size, strong ion complexation, and unique electrontransfer abilities, the CDs were expected to improve the catalytic performance of the catalysts through modification of their morphology and interfaces during synthesis. Here, the Co–Co3O4/CDs material was successfully synthesized and used as an efficient catalyst for dehydrogenation of AB (the detailed synthesis process is shown

in Fig. 2a). Briefly, the CDs and metal ions (Co2+) were first bonded through ion exchange to form a stable and uniform system. Then, annealing facilitated the nucleation and growth of metal particles, accompanied by the sintering of the surrounding CDs into shells. Finally, the required samples were obtained via secondary activation under air atmosphere. More details of the material were elucidated by TEM. The TEM images of the Co–Co3O4/CDs (Figs. 2b and S1a) show that NPs with sizes between 60 and 80 nm were firmly anchored in the carbon matrix and connected to each other via outer carbon layers. The high-magnification TEM image (Figs. 2c and S1b) reveals some novel three-dimensional core–shell structures with shell thicknesses between 3 and 5 nm in these NPs. Surprisingly, some cavities existed between the shells and cores, which could originate from the expansion of the gases from oxygen-containing groups on the surfaces of CDs during annealing. The novel configuration not only effectively inhibited the agglomeration of metal–metal oxide particles (the core) during the reaction process but also facilitated the electron transfer. Moreover, the presence of the cavities greatly alleviates the volume changes of the cores and increases the stability of the material structure. In addition, the cavities can also act as ion buffer reservoirs to accelerate the mass transfer, thus increasing the reaction rate. The HRTEM image (Fig. 2d) shows that the core consists of two phases (the yellow dotted line represents the crystal interfaces), where the lattice fringes with a spacing of 2.05 Å are matched to the (111) plane of Co (JCPDS Card No. 15-0806), and those with spacings of 4.64 and 2.81 Å are assigned to the (111) and (220) planes of Co3O4 (JCPDS Card No. 42-1467), respectively. The

selected area electron diffraction pattern (Fig. 2e) of the core also confirms the polycrystalline structure composed of Co0 and Co3O4. Moreover, the energydispersive X-ray spectroscopy elemental mapping images (Fig. 2f–g) clearly indicate the presence of elemental C, O, and Co; in addition, the NPs also clearly exhibit a core–shell structure with a carbon shell on the outside. For comparison, Co/CDs was also prepared without further activation treatment in air. The TEM image (Fig. S2a) shows that Co/CDs possess a similar configuration to the Co–Co3O4/CDs except that the core was composed of a pure metallic cobalt phase. Moreover, the highmagnification TEM image (Fig. S2b) further confirms that the outer carbon shell was composed of CDs (as indicated by the yellow circle in Fig. S2b).

Fig. 3. Characterizations of the as-prepared Co/CDs and Co-Co3O4/CDs NPs. (a) XRD patterns of Co/CDs and Co-Co3O4/CDs NPs. (b) Raman spectra of Co/CDs and

Co-Co3O4/CDs NPs. The (c) XPS full spectra and high-resolution (d) C 1s, (e) Co 2p, and (f)

1s XPS spectra of Co/CDs and Co-Co3O4/CDs NPs.

XRD patterns of the Co/CDs and Co–Co3O4/CDs are presented in Fig. 3(a). As expected, the XRD pattern of Co/CDs includes peaks at 44.2°, 51.5°, and 75.8°, corresponding to the (111), (200), and (220) lattice facets of cubic Co (JCPDS Card No. 15-0806), respectively. After activation treatment in air, a series of new peaks emerged at 31.2° (220), 36.8° (311), 59.3° (511), and 65.2° (440), which are assigned to cubic Co3O4 (JCPDS Card No. 42-1467). This result indicates that the metallic cobalt phase in Co/CDs was successfully oxidized into a hybrid phase of Co and Co3O4 (i.e., Co–Co3O4/CDs), which is consistent with the above TEM results. Here, the weak diffraction peak intensity may be due to the fact that Co and Co3O4 are covered by outer carbon layers. Furthermore, Raman spectra (Fig. 3b) confirmed the compositions of the two catalysts: the typical D bands (1346 cm–1) and G bands (1587 cm–1) further confirm the formation of graphitic carbon in both. The value of ID/IG is an important parameter to characterize the state of carbon atoms [42]. Here, the greater ID/IG value calculated for Co–Co3O4/CDs (1.10) than that for Co/CDs (1.01) indicated that the carbon structure became more disordered upon activation treatment in air. In addition, the high ratio also implies an increase in structural defects. The defects can effectively adjust the electronic structure of carbon materials and provide more active sites [43]. In addition, several bands at 472, 512, and 673 cm–1 can only be detected in the spectrum of the Co–Co3O4/CDs, which is attributable to the Ramanactive modes of Co3O4 [44]. To characterize the material structure differences before

and after activation treatment, the nitrogen adsorption–desorption isotherms (Fig. S3a) of Co–Co3O4/CDs and Co/CDs were measured at 77 K: both displayed type-IV isotherms with a distinct hysteresis loop in the range of 0.4–1.0 P/P0, affirming the mesoporous nature of these catalysts [45]. Moreover, the sharp peaks at relative pressure P/P0 < 0.1 in the adsorption isotherms can be observed, indicating that they also possess substantial micropores [9]. From Table S1, SBET values of 220.2 and 165.6 m2·g–1 were obtained for Co/CDs and Co–Co3O4/CDs with average pore diameters of 5.11 and 6.51 nm, respectively. The slight decrease in the specific surface areas of Co–Co3O4/CDs is due to the partial oxidation of the carbon shells during the activation process, which causes an increased volume in the mesopores and decreased volume in the micropores (see the red circle in Fig. S3b). Here, a high specific surface area can provide more adhesion sites for reactive molecules. In addition, the presence of a porous structure not only facilitates the full contact between the reaction substrate and catalyst but also promotes the desorption of generated hydrogen. More details for the Co–Co3O4/CDs and Co/CDs were elucidated from XPS spectra (Fig. 3c–f). The XPS survey spectra of Co–Co3O4/CDs and Co/CDs confirm the presence of C, Co, and O surface species. The relative atomic ratios of elemental C, O, and Co in the Co–Co3O4/CDs and Co/CDs were also calculated from the XPS spectra (Table S2). Here, the high oxygen content in the Co/CDs is due to the high oxygen content in the as-prepared CDs (see Fig. 1g). After activation treatment in air, the decrease of the carbon content is due to the partial oxidation of the external carbon

shells during the activation process, resulting in increased porosity, consistent with the above results. In addition, the oxygen content of the Co–Co3

/CDs increased

slightly. The high-resolution C 1s spectra (Fig. 3d) of the Co/CDs and Co–Co3O4/CDs can both be deconvoluted into three peaks at 284.8 eV (C–C), 285.9 eV (C–O/C–O– C), and 289.0 eV (C=O), indicating that abundant oxygen-containing groups remained on the surface of the materials. The high-resolution Co 2p spectrum (Fig. 3e) of the Co/CDs can be divided into some peaks. The peaks at 778.5 eV (Co 2p3/2) and 793.8 eV (Co 2p1/2) are attributed to Co0. The peaks at 781.7 eV (Co 2p3/2) and 797.0 eV (Co 2p1/2) are assigned to Co2+, which could be caused by the inevitable surface oxidation of metallic cobalt at room temperature in air. Moreover, the peaks at 786.7 and 802.9 eV are shake-up satellite peaks [46]. After activation treatment in air, two additional peaks at 782.7 eV (Co 2p3/2) and 795.0 eV (Co 2p1/2) attributable to Co3+ appeared in the Co 2p spectrum of Co–Co3O4/CDs [44]. In addition, the peak at 793.8 eV (Co 2p1/2) in the Co 2p spectrum of Co/CDs disappears, which suggests that the metallic Co was partially converted into Co3O4 during activation treatment. Moreover, the high-resolution O 1s spectra (Fig. 3f) of the Co/CDs and Co–Co3O4/CDs also confirm the presence of the Co–O, C=O, and C–O/C–O–C bonds. Peng et al. [47] confirmed the interfacial charge transfer (from metal to oxygen) between metal and metal oxide through theoretical calculation and experiment. Here, the special electron transfer can also be confirmed by the obvious increase in the binding energy of Co0 in the Co 2p spectra of Co–Co3O4/CDs compared with that in the Co 2p spectra of Co/CDs. Moreover, because of the phase transition from CoO to Co3O4 on the surface,

the Co–O peak in the Co 2p spectrum of Co–Co3O4/CDs was slightly shifted toward high binding energy, which made the charge transfer (from Co0 to O) less visible [48]. In summary, in view of its large specific surface area, 3D porous structure, high crystallinity, and hydrophilicity, the novel Co–Co3O4/CDs hybrid material is expected to be an efficient catalyst for the hydrolysis of AB. 3.3. Enhanced catalytic AB hydrolysis of Co–Co3O4/CDs

Fig. 4. (a) Hydrogen production from AB catalyzed by various catalysts at 298 K and (b) corresponding hydrogen production rates. (c) Catalytic hydrogen production of

Co-Co3O4/CDs at different AB concentrations, the corresponding plot of AB concentration vs reaction rate both in the logarithmic scale (inset). (d) Catalytic hydrogen production of Co-Co3

/CDs at various amount of catalyst, the

corresponding plot of the amount of catalyst vs. reaction rate both in the logarithmic scale (inset). (e) Catalytic hydrogen production of Co-Co3O4/CDs at different temperatures, corresponding Arrhenius plot of lnk vs. 1000/T (inset). (f) The stability test of Co-Co3

/CDs at 298 K.

Herein, a typical drainage method was used to test the catalytic activities of the as-synthesized materials for AB hydrolysis. The reaction was performed at 600 rpm in self-stirring mode. The materials, Co–Co3O4/CDs-I, Co–Co3O4/CDs-II, Co– Co3O4/CDs, and Co–Co3O4/CDs-III, were prepared under the same conditions except for adjustment of the annealing time to 1, 2, 4, and 6 h, respectively. Moreover, Co– Co3O4 and Co3O4/CDs were also produced. The corresponding XRD patterns were obtained (Fig. 5a, b). Hydrogen production reactions catalyzed by these materials were performed at 298 K in alkaline solution (1 M Na H) (Fig. 4a), and Fig. 4(b) shows the specific rates obtained from Fig. 4(a). Among these catalysts, Co– Co3O4/CDs enabled completion of the entire hydrogen production process within 3 min, whereas the pure CDs material exhibited no catalytic activity even after 60 min. It is apparent that all of the Co–Co3O4/CDs-x (x = ∅, I, II, III) materials exhibited better catalytic activities than Co/CDs and Co3O4/CDs, which implies that the Co– Co3O4 hybrid structure is more favorable for catalytic hydrolysis of AB. In addition, because of the good hydrophilicity, large surface area, and fast charge transferability

of the CDs, the Co–Co3O4/CDs-x materials also exhibited better catalytic properties than Co–Co3O4 NPs, indicating that the carbon shells (from the CDs) greatly enhanced the catalytic activity. Among the Co–Co3O4/CDs-x materials, Co– Co3O4/CDs exhibited the best catalytic activity. Considering results from TGA (Fig. S4), the rB of Co–Co3O4/CDs was calculated to be 6816 17.93

min–1 gCo–1 (TOF =

min–1 molCo–1) based on the cobalt content in the material. The catalytic

activity of Co–Co3O4/CDs exceeded that of previous non-noble-metal NP systems, and was even better than that of some noble-metal NP systems (Table S3). Furthermore, the Raman spectra of the Co–Co3O4/CDs-x materials (Fig. S5a) further suggest that the outstanding performance of the Co–Co3O4/CDs may be attributed to the more structural defects resulting from the higher intensity ratio (ID/IG). The abundant defects will greatly change the overall electronic structure of the carbon materials, and will further enhance their catalytic activity [49]. Fig. S5(b) shows that a number of hydrophilic functional groups remain on the surfaces of the Co– Co3O4/CDs-x materials, which can vastly increase the dispersion of these catalysts in aqueous solution. Moreover, the duration of activation treatment can also significantly affect the catalytic activities of materials (Fig. 5c, d). It is noticeable that with increasing oxidation degree of the Co–Co3O4/CDs-x materials, their activities show a volcano-type trend. We think that this trend may be related to the activity source of these materials (see details in the mechanism section below).

Fig. 5. (a) XRD patterns of Co-Co3O4 and Co3O4/CDs, (b) Co-Co3O4/CDs-x (x=∅, Ⅰ, Ⅱ, Ⅲ). (c) Catalytic hydrogen generation of Co-Co3O4/CDs materials prepared for different activation time, and (d) corresponding XRD patterns. (e) Catalytic hydrogen production of Co-Co3O4/CDs at different mixing modes, corresponding hydrogen production rates. (f) Catalytic hydrogen production of Co-Co3O4/CDs with/without NaOH (1 M) at 298 K, and the corresponding hydrogen production rates. In addition, Co–Co3O4/CDs catalyst was selected for further analysis of the kinetic characteristics of AB hydrolysis. First, the hydrogen production reactions were

investigated at various concentrations of AB under identical conditions. As observed in Fig. 4(c), the kinetic curves were close to parallel. Further calculations (inset of Fig. 4c) revealed that the reaction rate somewhat depends on the AB concentration but does not follow zero-order reaction kinetics. This may be because the high concentration of the reaction substrate can increase the probability of contact between reactant molecules and the catalyst, thereby facilitating the reaction kinetics. Moreover, hydrogen production at various amounts of catalyst was also studied (Fig. 4d). Conversely, the reaction rate is strongly affected by the catalyst amount, following first-order kinetics. To obtain the apparent activation energy (Ea) of the catalytic reaction, hydrogen production was also investigated from 298 K to 318 K (Fig. 4e). Increasing the reaction temperature significantly increased the reaction rate because of the rapid movement of ions and water molecules at high temperatures, which will facilitate mass transfer during the reaction. An Arrhenius plot for the tested temperature range was constructed (inset of Fig. 4e), and Ea was calculated from the Arrhenius equation (Eq. 3): lnk0 = where

-Ea T

ln

represents the rate constant,

(Eq. 3) is the ideal gas constant,

is the reaction

temperature, and A denotes the pre-exponential factor. The slope of the Arrhenius plot gave an activation energy, Ea, of 40 kJ mol–1, which approximately equal to or lower than that of other previous non-noble metal NP systems (Table S3). The cyclic stability of the Co–Co3O4/CDs was further tested at 298 K, as shown in Fig. 4(f). During five cycles, the catalyst maintained high catalytic activity without

a sharp decay, indicating its excellent cycling stability. The TEM images (Fig. S6) of the used Co–Co3O4/CDs after five cycles reveal no significant agglomeration of the NPs and that their core–shell structure remained intact with no obvious collapse. Furthermore, the slight attenuation of catalytic performance might be due to the permeation of the boron phase (the amorphous phase covering the cores), which may have caused the absence of obvious diffraction peaks in the XRD patterns of the used Co–Co3O4/CDs after five cycles (Fig. S7). Moreover, considering practical application requirements, the operating conditions of the catalyst were also studied. Fig. 5(e) shows that the entire hydrogen production process is quickest under self-stirring, followed by the static state, and the longest reaction time (6 min) is required for magneton stirring mode. As known from previous reports [50], self-stirring is efficient for magnetic momentum transfer between the external magnetic field and the reaction system to ensure sufficient mixing of the catalysts and reactants. Under magneton stirring, the catalyst was firmly fixed on the surface of the magneton because of its magnetic nature (inset of Fig. 5e), which prevented it dispersing sufficiently, thus reducing its efficiency. Furthermore, we studied the role of the solution environment during hydrogen production. As observed in Fig. 5(f), after adding sodium hydroxide, the rB of the reaction system significantly increased, which is related to the hydrolysis mechanism of AB (Fig. 6d). As Hou et al. reported [51], several SN2 reaction processes exist in the entire AB hydrolysis process, where the OH* attacks the –BH3 group in NH3BH3*, and then, the B–N bond of NH3BH3 is broken to form the BH3OH* group, a pivotal medium during

all the reactions. After three cycles, BH2(OH)2* and BH(OH)3* are also generated. Therefore, when there is a large amount of free hydroxide in the solution, abundant BH3OH* (BH2(OH)2* and BH(OH)3*) groups can be rapidly generated, which will facilitate the entire hydrogen production process. However, too much hydroxide will induce competitive adsorption with water molecules near the active sites of the catalyst and thus inhibit hydrolysis (Fig. S8). Therefore, the appropriate concentration of hydroxide can effectively promote reaction and increase the reaction rate. 3.4. Synergistic effect and activity enhancement mechanism

Fig. 6. (a) The comparison of Co-Co3O4 and mixtures of Co and Co3O4 NPs for AB hydrolysis. (b) Catalytic hydrogen generation of Co-Co3O4 hybrids prepared for different activation time, and (c) corresponding XRD patterns. (d) A schematic diagram of the AB hydrolytic dehydrogenation process under appropriate catalyst.

To further understand the active centers of the as-synthesized Co–Co3O4/CDs, a series of Co–Co3O4 hybrids with different oxidation degrees and mixture of Co and Co3O4 NPs were prepared as catalysts for AB hydrolysis. By comparing the catalytic activities of the Co–Co3O4 and the mixture of Co and Co3O4 NPs (Fig. 6a), we observed that the two-phase hybrid structure exhibited stronger catalytic activity than the two-phase mixtures, suggesting that there is some interaction between Co and the Co3O4 phase and that the interaction can provide more active sites for AB hydrolysis. Furthermore, with increasing oxidation degree of the Co–Co3O4 hybrids, a volcanotype activity trend was observed (Fig. 6b, c). From the above results, we speculate that the interface-induced interaction between Co and Co3O4 is responsible for improving the catalytic activity of the Co– Co3O4/CDs for AB hydrolysis. Combined with the dehydrogenation process of AB (Fig. 6d), the following detailed collaborative catalytic process is proposed: Because of the strong interfacial charge transfer, the introduction of interfaces greatly changes the electronic structure of cobalt metal atoms at the interface. The cobalt metal atoms can efficiently adsorb AB molecules and activate B–H bonds to reduce the activation energy at this step [52]. The resulting H* atoms can be rapidly desorbed from the cobalt metal surface because of the weak binding energy of cobalt metal atoms to H* at the interfaces [47]. In addition, the O atoms nearby also work as active sites via hydrogen bonds with H2O molecules to activate the O–H bonds to facilely form H* atoms and OH* radicals on the surface of Co3O4 surrounding the interfaces. Moreover, OH– in the solution can preferentially attach to a Co3O4 site at the interface because of

strong electrostatic affinity to the locally positively charged Co2+/Co3+ species and more unfilled d orbitals in Co2+/Co3+ than in Co metal. Briefly, the adsorbed water molecules on Co3O4 dissociate to form OH* radicals bonded to the surface Co atom and H atoms bonded to the neighboring O atom. As a result, the interfaces will accumulate abundant H* atoms and OH* radicals, which not only contribute to the production of H2 molecules by increasing the probability of collisions between H* atoms but also promote the progress of SN2 reactions and increase the reaction rate. Based on these findings, a simple simulation process for the synergistic mechanism of this composite material is presented in Fig. 7. Accordingly, more hydrogen evolution sites will be generated with increasing number of metal–metal oxide interfaces. However, excessive interfaces also cause a large amount of metal to be converted into metal oxide, which will reduce the conductivity of the material and be detrimental to its catalytic activity. Therefore, we speculate that there is a balance between the number of interfaces and the conductivity of the material, which may be the main reason for the volcano-type trend in catalytic activity observed for Co–Co3O4/CDs-x materials and Co–Co3O4 hybrids (Figs. 5c and 6b). Overall, the interfacial interaction of metal–metal oxides can effectively enhance their catalytic activity; in addition, the high activity should result from a good balance between the number of interfaces and the conductivity of the material.

Fig. 7. A simple synergistic mechanism for the Co–Co3

/CDs.

The presence of CDs is also important for the high catalytic activity of the Co– Co3O4/CDs. The Co–Co3O4 hybrid NPs protected by CDs exhibit very strong structural stability. In addition, rapid charge transfer at the interfaces between the metal hybrid NPs and CDs was observed because of the high conductivity of the CDs (Fig. S9). The hydrophilic and porous surface also enabled reactive molecules to rapidly reach the surface of the Co–Co3O4 hybrid NPs to participate in dehydrogenation. The generated hydrogen could also be released rapidly along the interfaces. In general, this unique synergy ensures that the entire hydrolysis process is performed efficiently, rapidly, and orderly, thereby increasing the utilization efficiency of the internal active components and greatly stimulating the potential catalytic activity of the material. 4. Conclusions In summary, we successfully synthesized Co–Co3O4 NPs facilely deposited on CDs as an efficient noble-metal-free catalyst for the hydrolysis of AB. Benefiting from the improved surface area, high dispersibility, good conductivity, and interfacial interactions between the Co, Co3O4 NPs, and CDs, the Co–Co3O4/CDs exhibited

excellent catalytic activity (rB = 6816

min–1 gCo–1) and good cyclic stability.

Further mechanistic study suggested that the high interfacial synergy can provide abundant adsorption sites for reactive molecules, which is beneficial to further activation of AB and water molecules. In addition, the need to balance the number of interfaces and the conductivity of the material was also proposed. These findings provide significant and promising insight for the design and optimization of metal and metal-oxide hybrid materials for hydrogen storage and related applications. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21774041, 51433003) and the China Postdoctoral Science Foundation (2018M640681, 2019T120632).

Declaration of Interest Statement This work is original, our own, and has not been published previously and they have no conflict of interest. We believe this work will be of wide general interest to readership, and await your views.

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TOC

The Co-Co3O4/CDs was proved to be a highly efficient noble-metal-free catalyst for AB hydrolysis. The incorporation of the multi-interfaces between Co, Co3O4 NPs, and CDs endows the hybrid material with excellent catalytic activity.