Reaction of Co3O4 Nanocrystals on Graphene Sheets to Fabricate

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Reaction of Co3O4 Nanocrystals on Graphene Sheets to Fabricate Excellent Catalysts for Hydrogen Generation Xianli Wu, Xiaoyu Zhang, Guosheng Han, Yanyan Liu, Baozhong Liu, Jie Gao, Yanping Fan, and Baojun Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00572 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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ACS Sustainable Chemistry & Engineering

Reaction of Co3O4 Nanocrystals on Graphene Sheets to Fabricate Excellent Catalysts for Hydrogen Generation Xianli Wu,† Xiaoyu Zhang,† Guosheng Han,† Yanyan Liu,† Baozhong Liu,‡ Jie Gao,§ Yanping Fan,*, ‡ and Baojun Li*,† †

College of Chemistry and Molecular Engineering, Zhengzhou University, 100 Science Road,

Zhengzhou 450001, P R China ‡

College of Chemistry and Chemical Engineering, Henan Polytechnic University, 2001

Century Avenue, Jiaozuo 454000, P R China §

Integrated Analytical Laboratories, 273 Franklin Rd, Randolph, NJ 07869, USA

* Corresponding Author. E-mail: [email protected] (Y.P. Fan) and [email protected] (B.J. Li). Abstract: The development of functionalized graphene is of significant importance for the application of graphene in solving current energy and environmental problem. In this article, a series of graphene-based hybrid materials including porous graphene (PG) and carbon nanofibers anchored graphene oxide (GCNFs) were synthesized adopting a controllable solid-phase reaction strategy. Co3O4 nanocrystals (NCs) were used as oxidants to modify and adjust graphene sheets. The PG and GCNFs were formed crucially through the red-ox reaction between graphene and Co3O4 NCs during calcinations. As catalyst for NaBH4 hydrolysis at room temperature, CoOx NCs anchored onto PG (CoOx-PG) and CoOx NCs anchored onto GCNFs provided hydrogen generation specific rates of 1472 mL·min−1·gCo−1 and 2696 mL·min−1·gCo−1, respectively. These outstanding catalytic activities are attributed to the synergistic effect of CoOx NCs and PG/GCNFs. A general strategy was proposed for the oxidative cutting and reconstruction of graphene sheets, and the preparation of high active catalysts for hydrogen generation from NaBH4 hydrolysis. Keywords: hydrogen generation; NaBH4 hydrolysis; porous graphene (PG); punch holes

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Introduction Since its successful isolation in 2004, graphene as an important two-dimensional (2D) material has been enthusiastically investigated in various fields, because of its fantastic properties, such as extraordinary electrical conductivity, excellent optical performance, high mechanical strength combined with outstanding surface area.1−4 The syntheses of large area and high quality graphene have been developed in the past years.5,6 Due to the zero band gap of single layer graphene, it is difficult to realize the cutting and processing of graphene to flexible size and shape. This problem seriously limits the commercial applications of graphene in electronic industry.7−9 Effective control of cutting graphene into diversiform designed patterns,12,13 constructing defects, tuning its band gap and edge state (zigzag or armchair edged), and further manufacturing integrated microelectronic circuit of the nanoelectronic equipments,7 is the key step for widely large-scale applications of graphene in the future. Although various functionalized graphene has been synthesized to explore its full potential, there is no substantial improvement for cutting and machining the large area and high quality graphene with high precision, owing to its distinctive stable physical properties.10,11 To date, a variety of physical and chemical methods have been tentatively demonstrated for cutting graphene. These methods includes conventional electron beam or plasma lithography,14 electric field cutting technique dependent on STM/AFM and energy beam machining technology,15,16 thermally driven Fe,17 Co,18,19 and Ni20−22 metal nanoparticles (NPs) etching at high temperatures under Ar/H2 atmosphere, the oxidative unzipping and cutting of graphene.23 Compared with these complicated methods, the oxidative cutting based on carbon combustion mechanism, is a facile route to decorate graphene with oxygen containing functional groups and defects (holes or cracks).24 These metal oxide NPs with various geometric shapes can be used as etchants to transform graphene into CO2/CO gas. Recently, there are several reports about metal oxide NPs such as ZnO,25 SnO2,26 Fe3O4,27 RuO2,28 and SiO2,29 used as etchants to obtain graphene nanoribbons, nanomeshes and porous graphene. These advances have clearly showed possibility to change the band gap and edge state of graphene. The excellent properties of functionalized graphene 2


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may be further discovered in these etching graphene. However, these processes have not been well understood and there are still many unanswered questions. On one hand, the species of metal oxides are very limited, and many metal oxides anchored on graphene cannot induce carbothermal reaction.25 On the other hand, through in situ synthesis of metal oxides anchored on graphene sheets from metal salts, the morphology and size of metal oxides cannot be accurately manipulated.30 It is still a serious challenge to uniformly load these precisely controllable NPs onto graphene or reduced graphene oxide (rGO) so as to perform controllable cutting of graphene. As a clean, renewable and alternative energy carrier, hydrogen is expected to be widely used in various mobile energy devices in the future. Due to high hydrogen content up to 10.8 wt%, NaBH4 is recognized as an ideal hydrogen-storage material. From NaBH4, high pure hydrogen without CO, which is notorious toxic for the catalysts of fuel cells, will be obtained readily.31−33 Therefore, various catalysts have been explored to catalyze NaBH4 hydrolysis for facile hydrogen production.34,35 From the viewpoint of awkwardness of high cost and limited resource for noble metals, the development of non noble metal catalysts, such as Co-based catalysts, is of special significance and expected to replace their noble metal counterparts (Pt, Ru) in future practical application.36,37 The interfaces and Mott-Schottky junctions between two nanostructures generally will show superior catalytic performances because of the unique electronic and geometric interaction. In this work, a simple effective strategy was proposed to achieve oxidative cutting graphene by controlling the solid phase reaction between graphene oxide (GO) and transition metal oxide nanocrystals (NCs). The pure porous graphene (PG) and carbon nanofibers anchored rGO (GCNFs) were synthesized using Co3O4 NCs as etchants through a three-step approach. The resultant CoOx NCs anchored PG or GCNFs exhibited high and stable catalytic activity toward hydrogen generation from NaBH4 hydrolysis. These catalysts can be easily separated from reaction system and still remain their high activity after reused for five times. This promising strategy will provide inexpensive alternatives for those noble metal catalysts in hydrogen generation. 3



Experimental section Synthesis of catalysts Co3O4-GO, and Co3O4-rGO. Co3O4-GO composite was prepared through a two-step process: Co3O4 NCs were prepared by a modified method reported by Li and coworkers.38 GO sheets were prepared through a modified Hummers’ method. Co3O4-GO hybrids were fabricated through a chemical bonding reaction. In a typical synthesis, GO (50 mg) dispersed in ethanol (50 mL), and Co3O4 NCs (50 mg) dispersed in ethanol (25 mL) were prepared by ultrasonic mixing. The mixed suspension was stirred vigorously for 12 h at room temperature. After being stopped stirring and set aside for a while subsequently, the supernatant was poured out after the solid was completely deposited at the beaker bottom. Co3O4-GO was obtained after the solid was dried in air at 50 °C for 8 h. Co3O4-rGO was obtained by a similar procedure to the above process, except GO was firstly reduced to rGO. The reduction was performed by dropping hydrazine hydrate (N2H4·H2O, 0.1 g, 80 wt%) into the mixed suspension after being stirred for 2 h, followed by further 10 stirring. CoOx-PG, and CoOx-GCNFs. CoOx-PG and CoOx-GCNFs were obtained after Co3O4-GO and Co3O4-rGO were calcined in a tubular furnace at 600 °C for 2 h with a heating rate of 3 °C·min−1 under N2 gas flow (200 mL·min−1) (Caution! The materials have to be poured into ethanol under N2 gas protection to prevent spontaneous combustion after calcinations). PG, and GCNFs. The resulting CoOx-PG and CoOx-GCNFs were immersed in HCl (6 M) for three days to remove large size CoOx NCs, then the PG and GCNFs were obtained. Characterization The crystallographic structures of samples were determined by X-ray Diffractometer (XRD, X’Pert PRO, Holland) produced by PANalytical company with a Cu Kα radiation (λ = 1.5418 Å). The measurements were performed at an acceleration voltage of 40 kV and a current of 45 mA in a scanning 2 theta range of 10 °-80 °. The morphology and inner structures of samples were characterized by FEI transmission electron microscope (TEM, TECNAI G2F20-S-TWIN, America) with an acceleration voltage of 200 kV. The thermal gravimetric 4


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analyses (TGA) were conducted on STA 449 F3 (NETZSCH, Germany) with a heating rate of 10 °C·min−1. The nitrogen adsorption/desorption isotherms were carried out on an Automated Surface Area and Pore Size Analyzer (ASAP2420-4MP, Micromeritics, America). Before the test, the samples were degassed at 200 °C for 2 h under vacuum. X-ray photoelectron spectroscopy (XPS) were conducted on a PHI Quantera SXM spectrometer (Al excitation source, Kα=1486.60 eV, America). The binding energies were adjusted according to C1s peak (284.8 eV). Raman spectra were performed with the 514 nm line of an argon-ion laser as excitation source on a Renishaw RM-1000 (England). The catalytic properties in hydrogen generation from NaBH4 hydrolysis were studied assisted by a typical water displacement method (Figure S1). Generally, catalyst (20 mg) was added into a round-bottom flask (50 mL) placed in a water bath on a magnetic stirrer. NaBH4 aqueous solution (0.1 M, 20 mL) was added to the flask using a constant pressure dropping funnel (25 mL). The volume of obtained hydrogen was measured by a water-filled cylinder (250 mL). Considering the instability in initial and ending stage of catalytic reaction, the catalytic performances in stable stage corresponding to hydrogen generation amount of 60 mL-140 mL were used to assess the performances of catalysts. Hydrogen generation specific rates were evaluated by the following calculation mode (formula (1)): r=

80(mL) [t140 - t60 ](min) ∗ mc (g)

(1)

Here, r represents hydrogen generation specific rate, t140 is the time of obtaining 140 mL hydrogen, t60 is the same time for 60 mL hydrogen, mc denotes the content of cobalt element in Co-based catalyst.

Results and discussion

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Figure 1. Illustration of the synthesis processes of CoOx-PG, CoOx-GCNFs, PG, and GCNFs. The synthetic routes for 2D hybrid CoOx-PG and CoOx-GCNFs from GO, rGO and Co3O4 NCs are shown in Figure 1. The GO sheets prepared by the modified Hummers’ method contain abundant oxygen containing groups, such as carboxylic (-COOH) and hydroxyl groups (-OH) at their edges, which provide active sites for chemical modification of graphene (Figure S2).39,40 Co3O4 NCs were uniformly deposited on the surface of GO sheets depending on noncovalent bonds, such as hydrogen bonding, Van der Waals force and electrostatic interactions (Figure 2a, and Figure S3).41 Carbothermal reduction reaction occurred during high temperature calcination stage. Co3O4 NCs freely glided on GO sheets to punch ordered holes. Some O atoms react with C atoms in rGO and released CO or CO2 (Figure S4), meanwhile Co3O4 NCs were partially reduced to CoOx NPs. The resultant CoOx NPs were firmly embedded into the middle of holes in rGO sheets by the connection of noncovalent bonds between rGO and CoOx NPs (Figure S5). The reaction process is expressed as follows (formula (2)): Calcination Co3O 4 +Cgraphite  → CoO x + CO/CO 2

(2) 6


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Figure 2. TEM images of (a) 2D Co3O4-GO nanosheets, and (b-h) 2D hybrid CoOx-PG. Inset in (d) is the illustration of CoOx-PG. The density of holes will be regulated by tuning the loading amount of metal oxides. Influenced by the oxidation degree of GO, when GO was reduced to rGO with N2H4·H2O, the surface oxygen-containing functional groups significantly decreased (Figure S2). The reduction results in a more stable inert surface of rGO in high temperature than that of GO. Carbothermal reaction was not enough to occur to generate holes on graphene. It is surprising that Co3O4 NCs catalyzed the transformation of graphene to GCNFs under high temperature condition. The formation of GCNFs may be ascribed to the catalytic reconstruction effect of small amount of low valence Co species generated in the reduction process. This catalytic effect is similar to those observed in growth processes of carbon nanotubes and graphene. The detailed morphology and structure of Co3O4-GO, CoOx-PG, Co3O4-rGO, and CoOx-GCNFs were observed by TEM analysis (Figure 2-4). The well dispersed Co3O4 NCs of about 10 nm in size were uniformly spread on the surface of GO to yield 2D Co3O4-GO nanosheets. The Co3O4 NCs were successfully anchored on GO sheet and retained good crystallinity (Figure 2a, and Figure S3a). A large number of holes and defects generated on GO sheet, evidencing that the carbothermal reaction occurred and some regions on GO sheets were oxidized to CO or CO2 during the calcination process (Figure 2b-2d, and Figure S4). 7



The size of holes (>10 nm) was larger than that of Co3O4 NCs to some extent. This difference is because of the agglomeration, stack and migration of Co3O4 NCs and CoOx NPs at high temperature (Figure 2b). The structure stability was maintained, because the Co3O4 NCs and CoOx NPs were uniformly embedded in the middle of holes through the connection of noncovalent bonds with GO sheets (Figure 2e-2g, and Figure S6a-S6c). These holes and defects further confirmed the disorder motion of Co3O4 NCs on the surface of GO sheets (Figure 2e). The motion was driven by the negative Gibbs free energy (∆G) of carbothermal reaction. The clear cutting lines along with the motion of Co3O4 NCs were observed (Figure S6d). The width of cutting groove is similar to the diameter of Co3O4 NCs. The distinct lattice fringe with d-spacing of 0.28 nm corresponds to the (220) planes of Co3O4 (Figure 2h).42,43 Besides, the lattice fringe (d=0.246 nm) corresponds to the (111) plane of CoO (JCPDS card no. 48-1719). These lattice fringes confirm that the solid-phase transform from Co3O4 to CoO via red-ox reaction took place in the calcinations process.44−46 It is interesting to note that there are some core-shell structures on graphene sheets (Figure 2f). The reaction between Co3O4 and carbon lead to the formation of carbon shells surrounding CoOx cores. The existence of carbon shells may play a role of linker between graphene sheets and CoOx NPs to strengthen their linkage, as a result, to improve the robust stability in catalysis reaction. After being etched by HCl solution to remove CoOx NCs, the PG sheets were obtained (Figure 3). Compared to pristine GO sheets, a large number of ordered holes are obviously presented into GO or rGO sheets (Figure 3a, 3b). The diameter of holes was as large as 100 nm. Without the support of CoOx NCs, the rough surface of PG composed of single or several layers of folded graphene sheets was no longer smooth (Figure 3c, 3d).

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Figure 3. TEM images (a) pure GO sheets, and (b-d) porous 2D PG sheets. Inset in (b) is the illustration of PG.

Figure 4. TEM images of (a, b) Co3O4-rGO, (c, d) 2D hybrid CoOx-GCNFs, and (e-h) GCNFs. Inset in (d, e) are the illustration of CoOx-GCNFs and GCNFs. It was also found that the utility of hydrazine hydrate is critical to the growth of CNFs on rGO sheet. Before calcination, the Co3O4 NCs were uniformly loaded on rGO sheets (Figure 4a, 4b). In the process of calcination, the disorder migration of Co3O4 NCs and the following catalytic carbon-reconstruction led to the formation of CNFs. An obvious difference from 9



CoOx-PG is found that there is no core-shell structure on graphene sheets, demonstrating that carbon cannot migrate onto the surface of CoOx NPs to react. This different reaction mode may be the reason for the generation of various products in two reaction processes. As a result, the final CoOx NCs were gathered on rGO sheets (Fig 4c, 4d). A large amount of coiled CNFs were entrapped among rGO nanosheet (Figure 4e-4h). The length of CNFs was up to several microns. The diameter of CNFs was approximately in the range of 5 to 20 nm. In addition, it is worthy to note that the CNFs were densely grown on the edge of rGO sheets for the abundant defects on the edge of rGO sheets (Figure S7).

Figure 5. XRD patterns of (a) Co3O4-GO and CoOx-PG, (b) Co3O4-rGO and CoOx-GCNFs, and Raman spectra of (c) Co3O4-GO and CoOx-PG, and (d) Co3O4-rGO and CoOx-GCNFs. The XRD patterns display the crystallographic structures of precursors and as-prepared samples (Figure 5a, 5b). The diffraction peaks of pure Co3O4 at 2θ values of 19.0, 31.4, 36.9, 38.7, 44.9, 55.8, 59.6, 65.5, and 77.6 could be well indexed to the (111), (220), (311), (222), (400), (442), (511), (440), and (533) crystal planes of Co3O4 (JCPDS card no. 65-3103) 10


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(Figure S3d).47−49 Compared with that in Co3O4-GO, the typically peak of GO sheets is absence at 10.6 º (001) and the peaks of rGO appeared at 26.2 º (002) in CoOx-PG, suggesting that GO has been reduced to rGO with decreasing layer spacing and generating new defects in PG sheets (Figure 5a).50,51 The peaks of CoO appeared at 42.4 º (200), 61.5 º (220) and 73.6 º (311) suggests that Co3O4 NCs were reduced to CoOx-GCNFs (Figure 5b). In the Raman spectra, the existence of D bands reflects the properties of sp3 carbon atoms in graphene edges, defects, and vacancies, etc (Figure 5c, 5d). The G bands reveal the in-plane vibrations of sp2 carbon atoms. The intensity ratio of ID/IG is often seen as an important parameter to characterize the density of defects in graphene or rGO sheets.52,53 The characteristic D and G bands were observed at 1341 and 1580 cm−1 for Co3O4-GO and CoOx-PG (Figure 5c). The ratios of ID/IG for Co3O4-GO and CoOx-PG were 1.10 and 1.13, respectively, indicating that the formation of holes introduced more defects and vacancies into the GO sheets during the carbothermal reaction. Figure 5d represents the D (1347 cm−1) and G (1575 cm−1) bands for Co3O4-rGO and CoOx-GCNFs, the ID/IG intensity ratios were 1.37 and 1.39, respectively. The CoOx-GCNFs sample gave a higher ID/IG value than Co3O4-rGO, suggesting that the surface states of rGO sheets have substantially changed to disorder and defect-rich. The elemental compositions and chemical states in four materials were tested by XPS spectra (Figure 6, and Figure S8a-S8d). Four elements of C, O, N, and Co were observed in all four materials (the obtained contents of each element are summarized in Table S1). The interactions between CoOx and PG or GCNFs could be further confirmed by the existence of C−O−Co in O1s fine spectrum (Figure S8e-S8h). The deconvoluted C1s peaks for Co3O4-GO and CoOx-PG corresponded to C=C (284.0 eV), C−O/C−N (285.7 eV), and C=O (287.8 eV) (Figure 6a-6d).54 After calcinations, the C−O and C=O bonds diminished while the C=C bond dominated (in CoOx-PG), demonstrating that the reaction of oxygen-containing functional groups is preferential in the carbothermal process. After reduction, the C−O and C=O bonds also diminished (in Co3O4-rGO). The N1s peaks for CoOx-PG and CoOx-GCNFs display a low intensity (Figure S8a-S8d). Two main peaks of Co2p3/2, Co2p1/2 of Co2p spectra 11



around 778-782 and 793-800 eV were assigned to Co0 atoms, Co2+ and Co3+ and ions (Figure 6e-6h). Three peaks of Co(II) and one peak of Co(III) are fitted well for Co3O4-GO. For Co3O4-rGO, three peaks of Co(II) and one peak of Co0 can be fitted well, due to the surface reduction of Co3O4 NPs by hydrazine hydrate (Figure 6g). After thermal treatment, the peaks of Co0 and Co2+ also are presence for CoOx-PG and only peaks of Co2+ were fitted well for CoOx-GCNFs (Figure S8b-S8d). The changes of carbon contents before and after calcination for two groups of samples were studied by TGA analysis in air (Figure S9). The weight losses below 100 °C were assigned to the loss of water. After calcinations, the carbon contents of Co3O4-GO and Co3O4-rGO obviously dropped. The calculated loss ratios were 33.0% and 23.4%, respectively, indicating that the thermal decomposition of GO (rGO) sheet was through a CO/CO2 elimination pathway during calcinations process. Nitrogen adsorption-desorption isotherms and pore sizes distribution of Co3O4-GO and CoOx-PG were conducted to confirm the production of holes into GO sheets (Figure S10). CoOx-PG exhibited higher specific surface area and large pore volume (278 m2·g−1, 0.55 cm3·g−1) compared with Co3O4-GO (141 m2·g−1, 0.41 cm3·g−1). Concurring with expectations, the increased specific surface area and pore volume were attributed to the generation of holes and defects in carbothermal reaction. Due to the reduction of GO to rGO sheets, some agglomeration of rGO layers taken placed and caused a significant decreasing surface areas of Co3O4-rGO (20 m2·g−1). The reconstruction of carbon increases the surface area of CoOx-GCNFs (28 m2·g−1).

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Figure 6. XPS fine spectra of (a-d) C 1s and (e-h) Co 2p of Co3O4-GO, CoOx-PG, Co3O4-rGO, and CoOx-GCNFs. The catalytic activities of CoOx-PG, PG, CoOx-GCNFs, GCNFs, Co3O4-GO, Co3O4-rGO, 13



Co3O4 NCs, GO, rGO and Raney Ni toward the hydrogen generation from NaBH4 hydrolysis were evaluated in an alkaline solution (Figure 7a, 7b and Table S2). Unexpectedly, the CoOx-PG and CoOx-GCNFs needed shorter time for obtaining identical H2 under the same experimental conditions as those for Co3O4-GO and Co3O4-rGO. The GO and rGO sheets exhibited only very weak and ignorable catalytic activities. The composites of Co3O4-GO and Co3O4-rGO showed higher activities than that of their single counterpart, Co3O4 NCs. The CoOx-PG and CoOx-GCNFs provided significantly higher hydrogen generation rates (1472, 2696 mL·min−1·gCo−1) than those of Co3O4 NCs (970 mL·min−1·gCo−1) and Raney Ni (a widely used industrial catalyst, 385 mL·min−1·gCo−1), as well as some reported noble metal-containing catalysts prepared through complex steps (Table S3), demonstrating the excellent catalytic activities of CoOx-PG and CoOx-GCNFs. These improvements could be due to their unique structures, which provide effective synergistic effect between CoOx NCs and PG/GCNFs.56-58 These high catalytic activities are derived from the size effect of CoOx NCs (with diameter about 10 nm). Meanwhile, as a stable substrate, the 2D PG sheets effectively prevented CoOx NCs form severe accumulation by the strong interaction based on noncovalent bonds. For ideal catalytic activity, the chemical state of active metal in catalyst is of high importance. In the hydrolysis of NaBH4, some previous studies have demonstrated that the valence states of Co and phase compositions of active oxides are key factors. Compared to the pristine phase of Co3O4, a mixed Co-Co3O4 exhibits higher catalytic activity.55,58 This viewpoint also is effective for current research objects. The different chemical states of Co in CoOx-PG and Co3O4-GO are partially responsible for their catalytic activity (Figure 6e, 6f). After high temperature oxidation treatment, the edges and near areas of holes on PG and GCNFs sheets were rich in oxygen-containing functional groups with significant hydrophilic property (Table S1), so it is well dispersed in alkaline aqueous solution of NaBH4. Due to the existence of oxygen-containing species, the PG sheets displayed uneven charge distribution with somewhat basic or acidic properties. These basic/acidic sites will quickly recover and release H+ from adsorbed water, and then transferred to the negative active hydrogen (H-) of NaBH4 to produce H2 on CoOx species. 14


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This intermediate procedure reduced the apparent activation energy (Ea) of overall reaction system, and then accelerated hydrogen generation rate. In addition, the effect of NaBH4 concentration was investigated from 0.1 M to 1.0 M. The hydrogen generation rate had little change, proving that this reaction mainly belonged to zero-order reaction (Figure S11). The loading amount of catalyst determines the reaction rate of hydrogen generation.55−58

Figure 7. Hydrogen generation from NaBH4 hydrolysis with (a) CoOx-PG, PG, CoOx-GCNFs, GCNFs, Co3O4-GO, Co3O4-rGO, Co3O4 NCs, GO, rGO and Raney Ni as catalyst at 303 K, and (b) corresponding specific rates of the above catalysts, (c-f) hydrogen generation with (c, d) CoOx-PG and (e, f) CoOx-GCNFs as catalyst at different temperature 15



and the corresponding Arrhenius plots (insets in (c), (e)), and (d, f) cycling stability tests of CoOx-PG and CoOx-GCNFs catalysts at 303 K. A series of experiments at different temperatures ranging from 303 K to 328 K were performed to investigate the apparent Ea of NaBH4 hydrolysis reaction catalyzed by CoOx-PG and CoOx-GCNFs, respectively (Figure 7c, 7e). The hydrogen generation rate obviously increased from 1472 mL·min−1·gCo−1 at 303 K to 6922 mL·min−1·gCo−1 at 328 K. The increase of temperature accelerated the transmitting of BH4− ions. The apparent Ea for catalytic reaction was calculated according to the Arrhenius formula (formula (3)):57,58 r = k0 exp( −

Ea ) RT

(3)

The resultant apparent Ea is 51.3 kJ·mol−1 for CoOx-PG and 28.3 kJ·mol-1 CoOx-GCNFs (Figure 7c, 7e). A variety of catalytic properties of noble metals catalysts for NaBH4 hydrolysis were revealed in Table S2. The CoOx-PG and CoOx-GCNFs significantly reduced the activation energy of this system. It is noteworthy that the order of Ea values is not exactly consistent with that of reaction rates. These differences are derived from the various structures of catalysts and the inconsistency of the preceding factors of Arrhenius formula. The catalytic reaction rate is a result of overall reaction system. The apparent Ea generally is considered as a descriptor for the intrinsic activity of catalytic active sites. Besides, the cycling stability tests of CoOx-PG and CoOx-GCNFs were performed for five runs (Figure 7d, 7f). The CoOx-PG and CoOx-GCNFs exhibited stable hydrogen generation properties. The hydrogen generation rates still maintained 1023 and 1325 mL·min−1·gCo−1 with little decay after five runs. These excellent catalytic stabilities of CoOx-PG and CoOx-GCNFs are their distinct advantages compared to those reported catalysts.33,34 More Co3O4 NCs were reduced to CoOx NCs and other low valance Co species in the catalytic reaction. So the used catalysts exhibited stronger magnetism after the first run (Figure S12a, S12c), which were further confirmed by XRD patterns (Figure S12b, S12d). The intensity of peaks became lower because of the smaller sizes and amorphous nature of NCs after reduction. As a result, 16


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the material could be easily separated from reactor, then washed and dried naturally for next cycling run. To confirm the superior stability of these structures, TEM images of used CoOx-PG and CoOx-GCNFs were conducted after first runs (Figure S13). The structures were perfectly maintained after first runs, demonstrating the robust cycling stability in catalysis.

Figure 8. STEM-HAADF images and EDX elemental images of the (a) PG, and (b) GCNFs. It is noteworthy that PG and GCNFs also exhibit somewhat substantial catalytic activity 17



(Figure 7a, 7b). Though the activities of PG and GCNFs are lower than those of CoOx-PG and CoOx-GCNFs, they are significantly higher than those of GO and rGO. The origination of these catalytic activities is very interesting because most of Co element has been removed from carbon substrates in the etching process. In order to understand the catalytic mechanism over PG and GCNFs, and the inductively coupled plasma-mass spectrometer measurement and STEM-HAADF images with EDX elemental mapping were conducted (Figure 8). The surprising results are the presence of trace Co element (﹤0.25‰) in these two carbon structures (Table S4, Figure S14). These very low amounts of Co (0.015wt‰ in PG and 0.242wt‰ in GCNFs) are responsible for the obvious improvement of activities (ca. ten to twenty times) compared with GO and rGO. The specific activity of per gram of Co of PG and GCNFs are much higher than those of CoOx-PG and CoOx-GCNFs. These very surprising results are worthy of further study. At present, there is a lack of in-depth study on the mechanism of hydrolysis of NaBH4. Due to the lack of high quality research instruments, we have to try to only speculate on the mechanism of this reaction by imitating the hydrolysis mechanism of ammonia borane (NH3·BH3) at present. Generally, the activation of BH4- anion is considered as the most difficult step in overall reaction.59 In the presence of acidic ions, it will be very easy to react with BH4- anion. Using transition metal-based catalysts (Co), the activation of water molecule to form H and OH will become the key step because of the lack of acid sites. The synergy effect between transition metal atom and adjacent non-metal atom is key factor for the activation of BH4- anion and water molecule.60 As pointed out in literature, the existence of graphene will modulate the electronic state of active phase to improve their catalytic activity. The acid and basic property of oxidized graphene often becomes the source of catalytic activity for some reaction. In these systems, the coexistence of CoOx and PGs/GCNFs is indispensable for the high catalytic activity. In the next study, some spectroscopy methods will be tried to illustrate these problems.

Conclusion In conclusion, a controllable solid-phase reaction strategy was demonstrated to cut graphene 18


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sheets to PG and convert rGO sheet to GCNFs with Co3O4 NCs as etchants. These different transformations were mainly attributed to the different oxidation degrees of graphene. The CoOx-PG and CoOx-GCNFs possessed a perfect synergistic structure between CoOx NCs and PG/GCNFs. The tiny CoOx NCs were firmly embedded into 2D PG or GCNFs to effectively avoid further accumulation. The creative holes and defects on graphene sheets generated high specific surface areas and large pore volumes, and promoted the effective electron transfer in catalysis. Therefore, the CoOx-PG and CoOx-GCNFs showed excellent catalytic properties for high purity hydrogen generation from NaBH4 hydrolysis superior to their single-component counterparts. This study established a mild way to get porous graphene and carbon fibres on graphene sheets by oxidative cutting or reconstruction, and provided high performance catalysts for facile hydrogen generation. These perfect structures may be applied in other aspects to resolve the current severe energy and environmental problems. ASSOCIATED CONTENT

Supporting Information UV-vis spectra, XRD patterns, TEM images, XPS spectra, TGA curves, N2-sorption curves, materials information, experimental devices, and some hydrogen generation properties STEM-HAADF and EDX results. The Supporting Information is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION

Notes The authors declare no competing financial interest.

Acknowledgment

Financial supports from the National Natural Science Foundation of China (no. 21401168, 19



21004055, 51671080, and 51471065), and the Key Scientific Research Project of Colleges and Universities in Henan Province (no. 15A150081) are acknowledged. All the authors thank the great Communist Party of China.

REFERENCES

1.

Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306 (5696), DOI 10.1126/science.1102896.

2.

Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6 (3), DOI 10.1038/nmat1849.

3.

Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. The Electronic Properties of Graphene. Rev. Mod. Phys. 2009, 81 (1), DOI 10.1103/RevModPhys.81.109.

4.

Geim, A. K. Graphene: Status and Prospects. Science 2009, 324 (5934), DOI 10.1126/science.1158877.

5.

Li, X. S.; Cai, W. W.; An, J. H.; Kim, S.; Nah, J.; Yang, D. X.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils., Science 2009, 324 (5932), DOI 10.1126/science.1171245.

6.

Tu, Z. Q.; Liu, Z. C.; Li, Y. F.; Yang, F.; Zhang, L. Q.; Zhao, Z.; Xu, C. M.; Wu, S. F.; Liu, H. W.; Yang, H. T.; Richard, P. Controllable Growth of 1-7 Layers of Graphene by Chemical Vapour Deposition. Carbon 2014, 73, DOI 10.1016/j.carbon.2014.02.061.

7.

Balog, R.; Jorgensen, B.; Nilsson, L.; Andersen, M.; Rienks, E.; Bianchi, M.; Fanetti, M.; Laegsgaard, E.; Baraldi, A.; Lizzit, S.; Sljivancanin, Z.; Besenbacher, F.; Hammer, B.; Pedersen, T. G.; Hofmann, P.; Hornekaer, L. Bandgap Opening in Graphene Induced by Patterned Hydrogen Adsorption. Nat. Mater. 2010, 9 (4), DOI 10.1038/nmat2710.

8.

Zhang, Z. X.; Huang, H. L.; Yang, X. M.; Zang, L. Tailoring Electronic Properties of Graphene by pi-pi Stacking with Aromatic Molecules. J. Phys. Chem. Lett. 2011, 2 (22), DOI 10.1021/jz201273r.

9.

Weiss, N. O.; Zhou, H. L.; Liao, L.; Liu, Y.; Jiang, S.; Huang, Y.; Duan, X. F. Graphene: An Emerging Electronic Material. Adv. Mater. 2012, 24 (43), DOI 10.1002/adma.201201482.

10. Lei, Z. B.; Zhang, J. T.; Zhang, L. L.; Kumar, N. A.; Zhao, X. S. Functionalization of Chemically Derived Graphene for Improving its Electrocapacitive Energy Storage Properties. Energ. Environ. Sci. 2016, 9 (6), DOI 10.1039/C6EE00158K. 11. Georgakilas, V.; Tiwari, J. N.; Kemp, K. C.; Perrnan, J. A.; Bourlinos, A. B.; Kim, K. S.; Zboril, R. Noncovalent Functionalization of Graphene and Graphene Oxide for Energy Materials, Biosensing, Catalytic, and Biomedical Applications. Chem. Rev. 2016, 116 (9), DOI 10.1021/acs.chemrev.5b00620. 20


Page 20 of 26

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12. Zhou, Y.; Loh, K. P. Making Patterns on Graphene. Adv. Mater. 2010, 22 (32), DOI 10.1002/adma.201000436. 13. Zhang, L. M.; Diao, S. O.; Nie, Y. F.; Yan, K.; Liu, N.; Dai, B. Y.; Xie, Q.; Reina, A.; Kong, J.; Liu, Z. F. Photocatalytic Patterning and Modification of Graphene. J. Am. Chem. Soc. 2011, 133 (8), DOI 10.1021/ja109934b. 14. Baringhaus, J.; Settnes, M.; Aprojanz, J.; Power, S. R.; Jauho, A. P.; Tegenkamp, C. Electron Interference in Ballistic Graphene Nanoconstrictions. Phys. Rev. Lett. 2016, 116 (18), DOI 10.1103/PhysRevLett.116.186602. 15. Fuierer, R. R.; Carroll, R. L.; Feldheim, D. L.; Gorman, C. B. Patterning Mesoscale Gradient Structures with Self-assembled Monolayers and Scanning Tunneling Microscopy based Replacement Lithography. Adv.

Mater.

2002,

14

(2),

154−157.

DOI

10.1002/1521-4095(20020116)14:23.0.CO;2-B. 16. Tang, X.; Lai, K. W. C. Substrate Effect on Atomic Force Microscopy-Based Nanolithography of Graphene. Ieee. T. Nanotechnol. 2016, 15 (4), DOI 10.1109/TNANO.2016.2558620. 17. Datta, S. S.; Strachan, D. R.; Khamis, S. M.; Johnson, A. T. C. Crystallographic Etching of Few-layer Graphene. Nano Lett. 2008, 8 (7), DOI 10.1021/nl080583r. 18. Schaffel, F.; Warner, J. H.; Bachmatiuk, A.; Rellinghaus, B.; Buchner, B.; Schultz, L.; Rummeli, M. H. Shedding Light on the Crystallographic Etching of Multi-Layer Graphene at the Atomic Scale. Nano Res. 2009, 2 (9), DOI 10.1007/s12274-009-9073-0. 19. Schaffel, F.; Wilson, M.; Bachmatiuk, A.; Rummeli, M. H.; Queitsch, U.; Rellinghaus, B.; Briggs, G. A. D.; Warner, J. H. Atomic Resolution Imaging of the Edges of Catalytically Etched Suspended Few-Layer Graphene. ACS Nano 2011, 5 (3), DOI 10.1021/nn103035y. 20. Ci, L. J.; Song, L.; Jariwala, D.; Elias, A. L.; Gao, W.; Terrones, M.; Ajayan, P. M. Graphene Shape Control by Multistage Cutting and Transfer. Adv. Mater. 2009, 21 (44), DOI 10.1002/adma.200900942. 21. Lukas, M.; Meded, V.; Vijayaraghavan, A.; Song, L.; Ajayan, P. M.; Fink, K.; Wenzel, W.; Krupke, R. Catalytic Subsurface Etching of Nanoscale Channels in Graphite. Nat. Commun. 2013, 4, DOI 10.1038/ncomms2399. 22. Campos, L. C.; Manfrinato, V. R.; Sanchez-Yamagishi, J. D.; Kong, J.; Jarillo-Herrero, P. Anisotropic Etching and Nanoribbon Formation in Single-Layer Graphene. Nano Lett. 2009, 9 (7), DOI 10.1021/nl900811r. 23. Sun, T.; Fabris, S. Mechanisms for Oxidative Unzipping and Cutting of Graphene. Nano Lett. 2012, 12 (1), DOI 10.1021/nl202656c. 24. Fujii, S.; Enoki, T. Cutting of Oxidized Graphene into Nanosized Pieces. J. Am. Chem. Soc. 2010, 132 21



(29), DOI 10.1021/ja101265r. 25. Ta, H. Q.; Bachmatiuk, A.; Warner, J. H.; Zhao, L.; Sun, Y. H.; Zhao, J.; Gemming, T.; Trzebicka, B.; Liu, Z. F.; Pribat, D.; Rummeli, M. H. Electron-Driven Metal Oxide Effusion and Graphene Gasification at Room Temperature. ACS Nano 2016, 10 (6), DOI 10.1021/acsnano.6b02625. 26. Kim, H. K.; Bak, S. M.; Lee, S. W.; Kim, M. S.; Park, B.; Lee, S. C.; Choi, Y. J.; Jun, S. C.; Han, J. T.; Nam, K. W.; Chung, K. Y.; Wang, J.; Zhou, J. G.; Yang, X. Q.; Roh, K. C.; Kim, K. B. Scalable Fabrication of Micron-scale Graphene Nanomeshes for High-performance Supercapacitor Applications. Energ. Environ. Sci. 2016, 9 (4), DOI 10.1039/c5ee03580e. 27. Bhuvaneswari, S.; Pratheeksha, P. M.; Anandan, S.; Rangappa, D.; Gopalan, R.; Rao, T. N. Efficient Reduced Graphene Oxide Grafted Porous Fe3O4 Composite as a High Performance Anode Material for Li-ion Batteries. Phys. Chem. Chem. Phys. 2014, 16 (11), DOI 10.1039/c3cp54778g. 28. Kim, J. Y.; Kim, K. H.; Yoon, S. B.; Kim, H. K.; Park, S. H.; Kim, K. B. In Situ Chemical Synthesis of Ruthenium Oxide/Reduced Graphene Oxide Nanocomposites for Electrochemical Capacitor Applications. Nanoscale 2013, 5 (15), DOI 10.1039/c3nr01233f. 29. Krauss, B.; Nemes-Incze, P.; Skakalova, V.; Biro, L. P.; von Klitzing, K.; Smet, J. H. Raman Scattering at Pure Graphene Zigzag Edges. Nano Lett. 2010, 10 (11), DOI 10.1021/nl102526s. 30. Jun, Y. W.; Choi, J. S.; Cheon, J. Shape Control of Semiconductor and Metal Oxide Nanocrystals through Nonhydrolytic Colloidal Routes. Angew. Chem. Int. Ed. 2006, 45 (21), DOI 10.1002/anie.200503821. 31. Demirci, U. B.; Miele, P. Cobalt-based Catalysts for the Hydrolysis of NaBH4 and NH3BH3. Phys. Chem. Chem. Phys. 2014, 16 (15), DOI 10.1039/c4cp00250d. 32. Wang, D.; Su, C.; Wang, S.; Fan, Y.; Chen, M.; Xie, Y. Porous Co-P-Pd Nanotube Arrays for Hydrogen Generation by Catalyzing the Hydrolysis of Alkaline NaBH4 Solution. J. Mater. Chem. A 2015, 3 (16), DOI 10.1039/c5ta00904a. 33. Mahmood, J.; Jung, S. M.; Kim, S. J.; Park, J.; Yoo, J. W.; Baek, J. B. Cobalt Oxide Encapsulated in C2N‑h2D Network Polymer as a Catalyst for Hydrogen Evolution. Chem. Mater. 2015, 27 (13), DOI 10.1021/acs.chemmater.5b01734. 34. Zhang, H.; Ling, T.; Du, X. W. Gas-Phase Cation Exchange toward Porous Single-Crystal CoO Nanorods for Catalytic Hydrogen Production. Chem. Mater. 2015, 27 (1), DOI 10.1021/cm504076n. 35. Hung, T. F.; Kuo, H. C.; Tsai, C. W.; Chen, H. M.; Liu, R. S.; Weng, B. J.; Lee, J. F. An Alternative Cobalt Oxide-supported Platinum Catalyst for Efficient Hydrolysis of Sodium Borohydride. J. Mater. Chem. 2011, 21 (32), DOI 10.1039/c1jm11720c. 36. Gao, J.; Zhao, H. B.; Yang, X. F.; Koel, B. E.; Podkolzin, S. G. Geometric Requirements for 22


Page 22 of 26

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Hydrocarbon Catalytic Sites on Platinum Surfaces. Angew. Chem. Int. Ed. 2014, 53 (14), DOI 10.1002/anie.201309043. 37. Gao, J.; Zhao, H. B.; Yang, X. F.; Koel, B. E.; Podkolzin, S. G. Controlling Acetylene Adsorption and Reactions on Pt–Sn Catalytic Surfaces. ACS Catal. 2013, 3 (6), DOI 10.1021/cs400198f. 38. Hu, L. H.; Peng, Q.; Li, Y. D. Selective Synthesis of Co3O4 Nanocrystal with Different Shape and Crystal Plane Effect on Catalytic Property for Methane Combustion. J. Am. Chem. Soc. 2008, 130 (48), DOI 10.1021/ja806400e. 39. Bellunato, A.; Tash, H. A.; Cesa, Y.; Schneider, G. F. Chemistry at the Edge of Graphene. ChemPhysChem 2016, 17 (6), DOI 10.1002/cphc.201500926. 40. Khan, M.; Tahir, M. N.; Adil, S. F.; Khan, H. U.; Siddiqui, M. R. H.; Al-warthan, A. A.; Tremel, W. Graphene based Metal and Metal Oxide Nanocomposites: Synthesis, Properties and Their Applications. J. Mater. Chem. A 2015, 3 (37), DOI 10.1039/c5ta02240a. 41. Zhou, Y.; Bao, Q. L.; Tang, L. A. L.; Zhong, Y. L.; Loh, K. P. Hydrothermal Dehydration for the "Green" Reduction of Exfoliated Graphene Oxide to Graphene and Demonstration of Tunable Optical Limiting Properties. Chem. Mater. 2009, 21 (13), DOI 10.1021/cm9006603. 42. Lu, Y. Y.; Zhan, W. W.; He, Y.; Wang, Y. T.; Kong, X. J.; Kuang, Q.; Xie, Z. X.; Zheng, L. S. MOF-Templated Synthesis of Porous Co3O4 Concave Nanocubes with High Specific Surface Area and Their Gas Sensing Properties. ACS Appl. Mater. Interfaces 2014, 6 (6), DOI 10.1021/am405858v. 43. Reddy, M. V.; Prithvi, G.; Loh, K. P.; Chowdari, B. V. R. Li Storage and Impedance Spectroscopy Studies on Co3O4, CoO, and CoN for Li-Ion Batteries. ACS Appl. Mater. Interfaces 2014, 6 (1), DOI 10.1021/am4047552. 44. Liu, T.; Guo, Y. F.; Yan, Y. M.; Wang, F.; Deng, C.; Rooney, D.; Sun, K. N. CoO Nanoparticles Embedded in Three-dimensional Nitrogen/Sulfur co-doped Carbon Nanofiber Networks as a Bifunctional Catalyst for Oxygen Reduction/Evolution Reactions. Carbon 2016, 106, DOI 10.1016/j.carbon.2016.05.007. 45. Gao, R.; Liu, L.; Hu, Z. B.; Zhang, P.; Cao, X. Z.; Wang, B. Y.; Liu, X. F. The Role of Oxygen Vacancies in Improving the Performance of CoO as a Bifunctional Cathode Catalyst for Rechargeable Li-O2 Batteries. J. Mater. Chem. A 2015, 3 (34), DOI 10.1039/c5ta03885e. 46. Ni, W. P.; Liu, S. M.; Fei, Y. Q.; He, Y. D.; Ma, X. Y.; Lu, L. J.; Deng, Y. Q. CoO@Co and N-doped Mesoporous Carbon Composites Derived from Ionic Liquids as Cathode Catalysts for Rechargeable Lithium-oxygen Batteries. J. Mater. Chem. A 2016, 4 (20), DOI 10.1039/c6ta01222a. 47. Xu, D.; Xie, Y.; Song, Y. J.; Deng, W. Q. A Green and Facile Method toward Synthesis of Waste Paper-derived 3D Functional Porous Graphene via in Situ Activation of Cobalt (II). J. Mater. Chem. A 23



2015, 3 (31), DOI 10.1039/c5ta03220b. 48. Gu, D.; Li, W.; Wang, F.; Bongard, H.; Spliethoff, B.; Schmidt, W.; Weidenthaler, C.; Xia, Y. Y.; Zhao, D. Y.; Schuth, F. Controllable Synthesis of Mesoporous Peapod-like Co3O4@Carbon Nanotube Arrays for High-Performance Lithium-Ion Batteries. Angew. Chem. Int. Ed. 2015, 54 (24), DOI 10.1002/anie.201501475. 49. Sun, H. T.; Sun, X.; Hu, T.; Yu, M. P.; Lu, F. Y.; Lian, J. Graphene-Wrapped Mesoporous Cobalt Oxide Hollow Spheres Anode for High-Rate and Long-Life Lithium Ion Batteries. J. Phys. Chem. C 2014, 118 (5), DOI 10.1021/jp408021m. 50. Ryu, W. H.; Yoon, T. H.; Song, S. H.; Jeon, S.; Park, Y. J.; Kim, I. D. Bifunctional Composite Catalysts Using Co3O4 Nanofibers Immobilized on Nonoxidized Graphene Nanoflakes for High-Capacity and Long-Cycle Li-O2 Batteries. Nano Lett. 2013, 13 (9), DOI 10.1021/nl401868q. 51. Raccichini, R.; Varzi, A.; Passerini, S.; Scrosati, B. The Role of Graphene for Electrochemical Energy Storage. Nat. Mater. 2015, 14 (3), DOI 10.1038/nmat4170. 52. Krauss, B.; Nemes-Incze, P.; Skakalova, V.; Biro, L. P.; von Klitzing, K.; Smet, J. H. Raman Scattering at Pure Graphene Zigzag Edges. Nano Lett. 2010, 10 (11), DOI 10.1021/nl102526s. 53. Zhang, L. L.; Xiong, Z. G.; Zhao, X. S. Pillaring Chemically Exfoliated Graphene Oxide with Carbon Nanotubes for Photocatalytic Degradation of Dyes under Visible Light Irradiation. ACS Nano 2010, 4 (11), DOI 10.1021/nn102308r. 54. Liu, Y. Y.; Zhang, J.; Zhang, X. J.; Li, B. J.; Wang, X. Y.; Cao, H. Q.; Wei, D.; Zhou, Z. F.; Cheetham, A. K. Magnetic Catalysts as Nanoactuators to Achieve Simultaneous Momentum-transfer and Continuous-flow Hydrogen Production. J. Mater. Chem. A 2016, 4 (11), DOI 10.1039/c5ta10697d. 55. Xing, C. C.; Liu, Y. Y.; Su, Y. H.; Chen, Y. H.; Hao, S.; Wu, X. L.; Wang, X. Y.; Cao, H. Q.; Li, B. J. Structural Evolution of Co-Based Metal Organic Frameworks in Pyrolysis for Synthesis of Core-Shells on Nanosheets: Co@CoOx@Carbon-rGO Composites for Enhanced Hydrogen Generation Activity. ACS Appl. Mater. Interfaces 2016, 8 (24), DOI 10.1021/acsami.6b04058. 56. Duan, S. S.; Han, G. S.; Su, Y. H.; Zhang, X. Y.; Liu, Y. Y.; Wu, X. L.; Li, B.J. Magnetic Co@g-C3N4 Core-Shells on rGO Sheets for Momentum Transfer with Catalytic Activity toward Continuous-Flow Hydrogen Generation. Langmuir 2016, 32 (25), DOI 10.1021/acs.langmuir.6b01248. 57. Zhang, J.; Hao, J. H.; Ma, Q. L.; Li, C. Q.; Liu, Y. S.; Li, B. J.; Liu, Z. Y. Polyvinylpyrrolidone Stabilized-Ru Nanoclusters Loaded onto Reduced Graphene Oxide as High Active Catalyst for Hydrogen Evolution. J. Nanopart. Res. 2017, 19 (6), DOI 10.1007/s11051-017-3924-5. 58. Liu, Y. Y.; Han, G. S.; Zhang, X. Y.; Xing, C. C.; Du, C. X.; Cao, H. Q.; Li, B. J. Co-Co3O4@Carbon Core–Shells Derived from Metal−Organic Framework Nanocrystals as Efficient Hydrogen Evolution 24


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Catalysts. Nano Res. 2017, 10 (9), DOI 10.1007/s12274-017-1519-1. 59. Peng, C. Y.; Kang, L.; Cao, S.; Chen, Y.; Lin, Z. S.; Fu, W. F. Nanostructured Ni2P as a Robust Catalyst for the Hydrolytic Dehydrogenation of Ammonia–Borane. Angew. Chem. Int. Ed. 2015, 54 (52), DOI 10.1002/anie.201508113. 60. Feng, K.; Zhong, J.; Zhao, B. H.; Zhang, H.; Xu, L.; Sun, X. H.; Lee, S. T. CuxCo1-xO Nanoparticles on Graphene Oxide as A Synergistic Catalyst for High-Efficiency Hydrolysis of Ammonia–Borane. Angew. Chem. Int. Ed. 2016, 55 (39), DOI 10.1002/anie.201604021.

25



Table of Content Graphic

A controllable strategy via solid-phase reaction between Co3O4 nanocrystals and graphene sheets is suggested to provide high active catalysts for hydrogen generation from hydrolysis of NaBH4.

T-1


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Reaction of Co3O4 Nanocrystals on Graphene Sheets to Fabricate

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