Morphology-controllable templated synthesis of three-dimensionally structured graphenic materials

Carbon 111 (2017) 476e485

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Morphology-controllable templated synthesis of three-dimensionally structured graphenic materials Je-Ruei Wen, Chung-Yuan Mou* Department of Chemistry, National Taiwan University, Taipei 106, Taiwan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 June 2016 Received in revised form 4 October 2016 Accepted 10 October 2016 Available online 11 October 2016

The physical and chemical properties of graphene are highly related to its nanostructure. Yet, existing preparation methods cannot easily obtain free-standing 3-D graphenic materials with controllable morphology. Here we report a growth strategy for graphenic materials with various micron- and nanoarchitectures by templating g-alumina, which was transformed from morphology-controllable boehmite. The resulting graphenic materials show perfectly replicated fine structures of the corresponding templates, and they give high specific surface areas in the range of 1149e1867 m2 g
1 which are desirable for energy applications. Hydrogen adsorption properties of the templated graphenic materials were evaluated at 77 K, and the samples presented 1.34e1.66 wt% storage capacities near 1.1 bar. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Controlling morphology is one of the important tasks for nanomaterials since it determines the size, shape, and exposed crystal facet which are intimately related to the physical and chemical properties of the materials, such as surface texture, chemical activity, electrical conductivity, etc. For graphene, a twodimensional nanomaterial with atomic thickness, the morphology effects on physicochemical properties become very significant. For instance, it is known that the electrical property of graphene can be readily altered by the number of layers [1], edge/boundary conditions [2e4], stress and strain [5e7], and size of nanostructures [8]. Furthermore, owing to the fact that it easily suffers from severe interlayer stacking and agglomeration [9e11], free-standing graphene-based materials usually demonstrate low accessible surface areas (<500 m2 g
1) not suitable for many applications. Accordingly, it is desirable to produce graphene and its derivatives with 3D extended single or few layered constructions which may be adjustable to fulfill various application requirements. Methods have been developed to make 3-D graphenic materials with specific shapes. Among them, chemical vapor deposition (CVD) is a facile and scalable route for synthesis of graphene on metal or metal oxide substrates. By Ni foam or assembled Fe [12e14], Ni wires [15,16], Pd nanowires [17], and Ni spheres [18] as

* Corresponding author. E-mail address: [email protected] (C.-Y. Mou). http://dx.doi.org/10.1016/j.carbon.2016.10.023 0008-6223/© 2016 Elsevier Ltd. All rights reserved.

templates, graphene with 3D interconnected network, tube, edgeclosed ribbon, and ball architectures are produced, respectively. On the other hand, MgO [19e21], MgAl-layered double oxide [22], ZrO2 [23], anodic aluminum oxides (AAO) [24e26], and SiO2 frustules [27] have also been used as structure-directing substrates for carbon materials exhibiting either irregular sphere, hexagonal meshed flake, tube, or replicated diatom architectures. However, good morphological control of 3-D nearly single layer graphenic carbon materials still remains a challenge. It has been reported that high-quality CVD graphene can be obtained by epitaxial growth of graphene on sapphire without a metal catalyst is presented [28e30]. It would be reasonable to expect some CVD process be developed for graphene growth using high surface area g-alumina which can be obtained by calcination of boehmite (g-AlOOH) [31]. At temperatures above 400  C, boehmite will undergo isomorphous phase transformation to gamma-alumina which the morphology can be preserved. In our previous report [32], alumina nanosheets with curtain-like crumpled form were obtained through calcination of crumpled boehmite nanosheets, which were synthesized by a two-stage hydrothermal method. Preparation of boehmite materials having a variety of 1-D to 3-D morphologies by facile hydrothermal or solvothermal approaches have been reported as well [33e35]. Given the characteristic of topotactic transition of boehmite to alumina, a general route of obtaining varied micron- and nano-structured graphenic materials via a fine morphology-controllable template system is demonstrated in this work. We are able to control the morphology of CVD graphenic materials by replication of the

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boehmite-derived alumina architectures. The resulting graphenic structures exhibit perfectly duplicated morphologies and large specific surface areas (SSAs) which would be very useful in energy applications such as catalysis, electric and gas storages. Moreover, hydrogen adsorptions on the templated graphenic carbons were measured at 77 K. The materials demonstrate high uptake values as predicted by an empirical rule. 2. Experimental section 2.1. Materials All reagents were used as received without further purification. Aluminum sulfate octadecahydrate (Al2(SO4)3$18H2O), aluminum nitrate nonahydrate (Al(NO3)3$9H2O), hexamethylenetetramine (C6H12N4, HMTA), acetone (þ99%), ethanol (99.5%), and hydroflu€s. Aluminum oric acid (HF, 48e51%) were purchased from Acro chloride hexahydrate, (AlCl3$9H2O) was purchased from J.T. Baker. Ammonium hydroxide (NH4OH, 35%) and sulfuric acid (H2SO4, 98%) were purchased from Fisher Scientific. Trisodium citrate dihydrate (C6H5Na3$2H2O) was purchased from SHOWA. 2.2. Sample preparation 2.2.1. Synthesis of boehmite curtain-like crumpled nanosheets Boehmite curtain-like crumpled nanosheets (CCNS) were synthesized by a two-stage hydrothermal process as in our previous report [32]. Typically, 0.8247 g of Al2(SO4)3$18H2O was dissolved into 100 mL distilled water in a round flask, and distilled sulfuric acid was added for adjusting the pH to 2.50. While being heated to 70  C in oil bath under vigorous stirring, a desired amount of 2 M HMTA aqueous solution was added into the solution. It was gradually heated from 70 to 95  C over 5 h, and was transferred into Teflon-lined stainless steel autoclaves with ~70% fill of total volume for another 20-h heating at 150  C. Subsequently, the autoclaves were allowed to cool to room temperature. The resulting precipitates were collected by filtration, washed with distilled water and absolute ethanol, and dried in air at 60  C overnight. 2.2.2. Synthesis of boehmite nanoflake-assembled hollow spheres A method adjusted from that reported by Cai et al. [34] was utilized to prepare boehmite nanoflake-assembled hollow spheres (HS-F). 1.666 g of Al2(SO4)3$18H2O was dissolved in 100 mL distilled water with addition of 2 mL of 5 M urea aqueous solution. Being stirred at room temperature for 10 min, the mixed solution was then transferred into Teflon-lined stainless steel autoclaves with ~70% fill of total volume and heated at 180  C for 3 h. After naturally cooling down, the white precipitates were filtered, washed with distilled water and absolute ethanol, and then dried in air environment at 60  C overnight. 2.2.3. Synthesis of boehmite nanowire-assembled hollow spheres Boehmite nanowire-assembled hollow spheres (HS-W) were produced following a method reported by Zhang et al. [35]. 4.6891 g Al(NO3)3$9H2O was dissolved in 100 mL distilled water, followed by addition of 125 mL of acetone and 25 mL of 0.125 M trisodium citrate. The mixed solution was stirred at room temperature for 30 min, and then was transferred into Teflon-lined stainless steel autoclaves with ~70% fill of total volume, being heated at 200  C for 48 h. After cooling to room temperature, the precipitates were recovered by filtration, washed with distilled water and absolute ethanol, and finally dried in air at 60  C overnight.

477

2.2.4. Synthesis of boehmite nanowires Two kinds of boehmite nanowires with different diameters and lengths were prepared. The first kind (L ¼ ~1.5 mm, D ¼ 20 nm) was derived by modifying a method reported in a previous literature [31]. Firstly, 1.45 g AlCl3$6H2O was added in 40 mL of absolute ethanol, stirring at 40  C vigorously until totally being dissolved. 80 mL of distilled water was then added into the transparent solution, which was later transferred into Teflon-lined stainless steel autoclaves and maintained at 200  C for 72 h. The autoclaves were allowed to cool to room temperature, and the precipitates were filtered, washed with distilled water and absolute ethanol, and then dried in air at 60  C overnight. The other kind (L ¼ ~0.5 mm, D ¼ 6 nm) is prepared by adjusting a method reported by He et al. [33]. 2.4145 g of AlCl3$6H2O was dissolved in 20 mL of distilled water, and 20 mL of 1.5 M NH4OH was added into the solution under vigorous stirring for 10 min. The precipitates were collected via centrifugation, and washed by distilled water and absolute ethanol for several times to remove the adsorbed chlorides. The precipitates were then redispersed with 100 mL of distilled water, adjusting the pH value to ca. 4.0 by dilute H2SO4. The solution was put into Teflon-lined stainless steel autoclaves with ~70% filled, heated to 200  C and kept for 24 h. After cooling to r.t., the precipitates were recovered by centrifugation, washed with distilled water and absolute ethanol, and finally dried in air at 60  C overnight. 2.2.5. Templated synthesis of graphenic carbon materials The aforementioned boehmite samples were transformed into alumina templates by calcination at 600  C for 4 h at a heating rate of about 1.5  C/min. In a typical synthesis of templated graphenic materials, the alumina powders were loaded on a ceramic boat and put in the isothermal zone of a hot walled horizontal tube furnace. The furnace was heated at a rate of ~1.67  C/min under nitrogen flow with a rate of 100 sccm and atmospheric pressure. After reaching to 1000  C, alumina templates were annealed for 5 min with an additional H2 flow at a rate of 10 sccm. The nitrogen flow was then terminated while 1% C2H2/Ar feedstock was introduced simultaneously with a rate of 250 sccm. The growth time of graphenic carbons varied from 3 to 8 min, depending on the type of alumina materials. At the end of the growth, the feedstock and hydrogen flows were shut off, and the system was allowed to cool down naturally under a nitrogen flow. To remove the alumina templates, the as-prepared graphenic materials were treated with a mixed solution of HF, ethanol, and distilled water by a volume ration of 1:5:4 for one day. The purified sample was collected via vacuum filtration through a polycarbonate membrane, washed by distilled water and absolute ethanol, and dried at 60  C overnight. 2.3. Characterization Transmission electron microscopy (TEM) images were taken on a Hitachi H-7100 Transmission Electron Microscope operating at 100 kV. Scanning electron microscopy (SEM) images were taken on a Hitachi S-4800 Field Emission Scanning Electron Microscope. High-resolution transmission electron microscopy (HRTEM) images were taken on a Philips/FEI Tecnai 20 G2 S-Twin Transmission Electron Microscope operating at 200 kV. Powder X-ray diffraction patterns were collected on a PANalytical's X'PertPRO X-ray diffractometer using filtered Cu Ka radiation (l ¼ 0.154 nm). N2 adsorption-desorption isotherms were measured at liquid N2 temperature (77 K) on a Micromeritics ASAP 2020. Samples were degassed at 120  C under 5  10
3 torr overnight before analyses. SSAs were derived from the isotherms by Brunauer-Emmett-Teller (BET) method, and pore size distributions were calculated by

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Barrett-Joyner-Halenda (BJH) method. Thermogravimetric analysis (TGA) was performed on a METLER TGA/DSC 1 STAR System under air flow at a heating rate of 10  C/min. The degrading temperature was determined by the infection point of the weight loss curve. Xray photoemission spectroscopy (XPS) measurements were performed on a Thermo Scientific Theta Probe with Al Ka irradiation (1486.80 eV). Raman spectra was recorded by a Jobin-Yvon LabRAM HR800 using a 633 nm excitation He-Ne laser. The purified samples were drop-coated on 300 nm SiO2/Si wafers by absolute ethanol, drying at 60  C for more than 1 h before experiments. Hydrogen adsorption isotherms were derived by a Micromeritics ASAP 2020 Plus at 77 K. Samples were outgassed at 120  C under 1  10
6 torr for more than 12 h before measurements. 3. Results and discussion 3.1. Synthesis and characterizations of templated graphenic carbon materials TEM and SEM images of the boehmite nanomaterials are shown in Fig. 1. In Fig. 1aed, boehmite CCNS exhibits intensely crumpled thin nanosheet form, while the folds and wrinkles are approximately oriented in certain directions over the nanosheet. In Fig. 1eeh, boehmite HS-F presents hollow sphere structures with outer and inner diameters of about 3e5 mm and 2e4 mm, respectively, constructed by 10e20 nm thick nanoflakes. On the other hand, boehmite HS-W demonstrates another type of hollow spheres built up by nanowires (Fig. 1iel). The diameter and shell thickness of the hollow spheres are around 1.5 mm and 100 nm, respectively, while the diameter of the nanowire unit is c.a. 2e4 nm. Crystal structures of the samples were characterized by XRD, as shown in Fig. S1a. Most the diffraction peaks can be well indexed to orthorhombic g-AlOOH (JCPDS No. 21-1307), except a tiny peak of CCNS appears at 29.7, which belongs to

ammonioalunite ((NH4)Al3(SO4)2(OH)6) (JCPDS No. 42-1334) [32]. After treatments at 600  C, the products manifest patterns of facecentered cubic g-Al2O3 phase (JCPDS No. 29-0063) (Fig. S1b). Meanwhile, the alumina samples reveal almost identical morphologies (Fig. S2) as before calcination, suggesting a successful phase transformation of the materials accompanied with excellent preservation of geometries. These alumina materials served as platforms for graphene growth during CVD process and were subsequently removed by HF solutions, leaving the deposited carbon materials. Fig. 2 illustrates TEM and SEM images of the final carbon products after purification. As can be seen in Fig. 2aed, CCNS-templated curtain-like crumpled graphene materials (CCG) exhibit a highly corrugated sheet morphology. Most importantly, the wrinkles and folds within a single sheet of CCG are roughly unidirectional rather than randomly scattered, evidently different from those extrinsically crumpled graphene materials [9,11,36,37]. In Fig. 2eeh, nanoflake-assembled graphenic hollow spheres (GHS-F) also display defined shapes as the corresponding HS-F template. No evident collapse of the micron sized hollow architectures is observed. As for nanotube/ nanoribbon-assembled graphenic hollow spheres (GHS-TR) (Fig. 2iel) templating from HS-W, the urchin-like hollow structure is duplicated as well. However, the building blocks are nanotubes and nanoribbons instead of nanowires, as indicated by arrows in Fig. 2j. When taking a closer examination by HRTEM (Fig. 3), there are plenty of tiny corrugations, ripples, and creases found throughout the surfaces, which present quite rough texture as comparing with general CVD graphene grown on a Cu surface. Such minute crumpling may be originated from the surface roughness of alumina substrates (Figs. S2b, f, and j), the lattice mismatch, or the difference of thermal expansion coefficients between alumina and graphene. Moreover, single and couple lines are mostly observed, as identified by arrows in Fig. 3, elucidating that the templated materials are constituted by one to two layers of graphene.

Fig. 1. TEM and SEM images of the boehmite nanomaterials: (aed) CCNS, (eeh) HS-F, and (iel) HS-W.

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Fig. 2. TEM and SEM images of the templated graphenic materials: (aed) CCG, (eeh) GHS-F, and (iel) GHS-TR. The blue and red arrows in (j) indicate nanotubes and nanoribbons, respectively. (A colour version of this figure can be viewed online.)

Fig. 3. HRTEM images of the purified templated graphenic materials: (a,b) CCG, (c, d) GHS-F, and (e, f) GHS-TR.

Fig. 4 gives the nitrogen sorption isotherms of the boehmite nanomaterials, alumina templates and resulting graphenic materials, while the calculated pore size distributions of templated graphenic carbons are provided in Fig. S3. All samples gave broad size distributions between 2 and 8 nm, reflections of the broad pore size distribution of the templates. The boehmite CCNS and its

alumina and graphenic derivatives display type II isotherms with H3 type hysteresis loops. The loops between P/P0 0.45e0.85 are originated from slit-like pores between the nanosheets, while the increased adsorption above P/P0 0.85 can be attributed to interparticle voids. As for HS-F series, type II adsorption curves and H3 loops are observed as well. Nevertheless, the isotherms reveal

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Fig. 4. Nitrogen adsorption-desorption isotherms of the (a) boehmite nanomaterials, (b) alumina templates, and (c) templated graphenic materials. (A colour version of this figure can be viewed online.)

larger loops than the CCNS products, indicating larger voids between the nanoflakes. Furthermore, significant cavitations appeared at P/P0 0.5e0.8 suggest existence of plugged pore structures due to the inter-connected nanoflakes throughout the spheres. In contrast, HS-W and its graphenic replicate present type IV isotherms and type H1 loops, which are evidence of mesoporous structures ascribed to the radially aligned nanowires and nanotubes/nanoribbons. It is noticed that GHS-TR shows an additional ramp of adsorption at P/P0 0.85e0.95 that represents capillary condensation occurred from the confined hollow spheres. Obviously, samples classified to the same geometry give similar isotherms, corroborating the successful morphology preservation of alumina as well as the structural replication of graphenic carbons. In addition, CCG, GHS-F, and GHS-TR manifest large SSAs of 1609, 1149, and 1867 m2 g
1, respectively (see Table 1). Given that the theoretical SSA of monolayer graphene is 2630 m2 g
1 [38], the high values of surface area of our materials provide another evidence for the single-to double-layer composition of the carbon products. It is worthy to mention that reduced graphene oxide materials generally present SSAs lower than 700 m2 g
1, regardless of being processed by thermal expansion or extrinsically crumpling [9,37]. The much higher accessible surface areas of our templated graphenic materials are attributed to the tiny corrugations as well as the excellent preservation of 3-D structures, which greatly prevent graphene sheets from face-to-face stacking and aggregation even after removal of the hard templates.

Table 1 BET surface areasa of the boehmite, alumina, and graphenic carbon samples calculated from Fig. 4. Morphology

Boehmite

Alumina

Graphenic carbon

CCNS HS-F HS-W

360.2 141.9 289.8

341.4 189.1 222.0

1609 1149 1867

a

Unit: m2g
1.

The yield and thermal stability of graphenic materials were characterized by TGA (Fig. 5a, b), and the results are summarized in Table 2. In Fig. 5a, the as-received CCG, GHS-F, and GHS-TR show weight losses of 12.47%, 7.89%, and 12.92% around 500e600  C, respectively. In light of that the growth of graphene is limited at surface of the template, we can estimate carbon content in each sample with an assumption of only single graphene layer synthesized on each surface (see Supplementary data for more details). Our estimation shows that the carbon content of CCG, GHS-F, and GHS-TR should be around 11.67%, 7.33%, and 12.51%, respectively. These values are close to the weight losses derived from experiments, substantiating the monolayer formation mechanism. After purification process, all the samples exhibited weight losses around 550e750  C (Fig. 5b), suggesting better thermal stability and thus better crystallinity of our graphenic carbons than those of reduced graphene oxide materials which, in general, are completely degraded at temperature lower than 500  C in air environment [39]. Besides, the TG curves of the purified samples can be distinguished into two weight loss sections: 550e585  C and 585e750  C. The first section is ascribed to degradation of a trace amount of surface functional groups (as evidenced by XPS later), whereas the second part shows a major weight loss that corresponds to oxidative thermal degradation of sp2 honeycomb carbon structure. At temperature higher than 750  C, materials are totally degraded and no residue is left after the thermal analysis, confirming the complete removal of alumina templates. In order to have a better understanding about chemical nature of the purified graphenic materials, the samples were characterized by X-ray photoelectron and Raman spectroscopies. Fig. 5cee and Fig. S4 show the XPS spectra of purified samples, and the results are summarized in Table 3. In Fig. 5cee, the C1s spectra have primary peaks located at 284.6 eV, indicating the sp2 carbons in graphenic structures [40]. There are another two small shoulders at 286.2 and 289.1 eV, which are assigned to hydroxyl/epoxy and carboxylate functional groups on the surface, respectively, whereas no carbonyl

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Fig. 5. Thermogravimetric analysis results of the (a) as-received and (b) purified templated graphenic materials. (cee) C1s XPS spectra and (f) Raman spectra of the purified graphenic carbons. (A colour version of this figure can be viewed online.)

Table 2 Results of TGA of the graphenic materials before and after purification. Sample

Estimated carbon content (%)

Experimental carbon content (%)

Degrading temp. (as-received) ( C)

Degrading temp. (purified) ( C)

CCG GHS-F GHS-TR

11.67 7.34 12.51

12.47 7.89 12.92

573 564 573

710 718 701

Table 3 XPS and Raman spectra results of the purified graphenic materials. Sample

CCG GHS-F GHS-TR a

XPS

Raman

CeC (%)

CeO (%)

eCOOe (%)

D band (cm
1)

G band (cm
1)

2D band (cm
1)

I(D)/I(G)a

82.5 79.3 74.1

14.7 15.2 20.8

2.8 5.5 5.1

1317 1320 1313

1588 1586 1589

2632 2617 2614

1.60 1.54 1.94

D band-to-G band intensity ratio.

group was identified near 287.8 eV that would have been found in oxygen-functionalized graphene. Besides, p to p* satellite peaks centered around 291 eV are also observed, implying the well graphenic sp2 structure of the templated products [41]. Meanwhile, Al2p spectra in Fig. S4b exhibit no discriminate signal, confirming the complete removal of alumina without contamination. Raman spectra of the products in Fig. 5f display typical D band, G band, 2D band, and D þ G band. As comparing to Cu-grown CVD graphene, the alumina-templated graphenic samples demonstrate higher D band over G band intensity ratios (ID/IG) that represent relatively smaller crystallite domain sizes [42], similar to those oxide-grown graphenic materials [20,22]. While GHS-TR shows a little bit higher ID/IG value than the other two (Table 3), the sample has a very small crystalline domain size, probably attributed to the higher oxygen content in the graphenic structure as evidenced by XPS, as well as the highly crumpled surface and small dimension of the nanowire templates that graphene single-crystalline domains could hardly

grow into larger ones. It is known the relative intensity of 2D band drastically weakens for graphene of lateral dimension of 5 to 2 nm [42]. The positions of 2D bands in our samples also downshifted due to small domain sizes [20] as compared with those of typical mono- and bilayer graphene centered around 2650 cm
1. Based on the results above, a proposed mechanism of templated synthesis of graphenic materials is illustrated in Fig. 6. For growth on alumina CCNS (Fig. 6a), carbon depositions are adapted to the undulation of alumina surface and replicate the corrugations throughout the nanosheets. After removing the underlying template, the graphene layers formed at both sides of the alumina nanosheets would either stay as separate monolayer by means of steric barrier arisen from the wrinkles, or adhere to each other as double-layer due to van der Waals attraction. In contrast, graphene layers deposited on a nanoflake unit of HS-F are relatively smooth (Fig. 6b), forming case-like shell that covers the nanoflake. The subsequent extraction of template would leave the graphene shell

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Fig. 6. CVD templated growth mechanism of the graphenic materials. (A colour version of this figure can be viewed online.)

as the huge nanoflake unit that builds up the hollow-spherical construction. The formation of nanotube and nanoribbon structures of GHS-TR is depicted in Fig. 6c. Since graphenic carbon was grown around the nanowire unit, alumina@graphene core-shell nanowire was produced after CVD. When the alumina core is removed, the graphenic shell, i.e. carbon nanotube would be left. On the contrary, if the nanowire template is not fully covered during carbon deposition, the graphenic layer would eventually become the graphene nanoribbon. According to the proposed growth mechanism, if only monolayer graphene materials were produced, their SSAs would be a constant value of 2630 m2 g-1, independent of the SSA of template. Yet, in reality, the graphene sheets would partially adhere to each other, forming stacked layers as shown in Fig. 6. The phonomonum is highly related to the varied degrees of stacking after removing the template and thus to different accessible surface areas. Besides, as a result of the varied morphologies, it is expected that the materials will be favorable for various applications. For instance, the aligned crumples in CCG would benefit charge transport within the structure, making the material promising for electrochemical applications. Furthermore, GHS-F and GHS-TR show different graphenic nanotextures and mesoporosities, which would accommodate them to different adsorption and filtration purposes, such as microbeads and long-chain polymers filtration. In addition to the nanotubes that constructed GHS-TR, templated synthesis of bundles of carbon nanotubes with varied diameters and lengths were also carried out (Figs. S5 and S6). The perfect replication of different morphologies, ranging from micron size to nanoscale, corroborates the feasibility of templated growth of graphenic carbons on alumina materials with controllable shapes and sizes, which are not attainable by the former reports [20,22,25] due to the limitation in preparing the templates into different morphologies. In contrast, fabrication of boehmite with various forms and scales is much easier, and thus it is expected to extend this method for obtaining graphenic materials having more structural possibilities. Additionally, alumina is a high dielectric constant material widely utilized as back gate in field-effect transistors; a direct growth of graphene on alumina can eliminate the

conventional transfer from catalysts to destination substrates, where the transfer process usually results in fractures or contamination of graphene [43]. Accordingly, our boehmite-to-aluminato-graphene preparation approach suggests great potential for facile fabrication of CVD graphene composites with designed 1-D to 3-D architectures, far beyond the limit at a smooth plane. Finally, we would like to make some comments on the chemical mechanism of graphene formation on alumina. There have been previous reports on the graphitization on alumina for the formation of carbon nanostructures [44,45]. However, the mechanism was not discussed. It is well-known that there are rich Lewis acid sites on galumina. The aromatization reaction of C2H2 over alumina nanoparticles has been reported [46]. Recently, it is further found cooperative Al sites on alumina promotes hydrogen transfer and carbon
carbon bond formation on alumina [47]. The combination of aromatization and de-hydrogenation could explain well the formation of graphene on the alumina surface. Since the catalytic active sites are richly located over the surface of alumina, conformal monolayer formation of graphene on the surface of alumina can be thus expected.

3.2. H2 sorption measurements Hydrogen is one of the clean energy resources for substitution of fossil fuels in the future; yet, the lack of appropriate storage methods for hydrogen prevents it from wide use. Due to the large SSA, light weight, and good stability, graphene-based materials are believed to be promising candidates for hydrogen storage that gravimetric capacity of 6e12.8 wt% is predicted [48e52]. Herein, hydrogen sorption properties of the templated graphenic materials were assessed by volumetric adsorption study at 77 K. As shown in Fig. 7a, all the samples presented Langmuir adsorption behaviors, together with uptakes of 1.57, 1.25, and 1.49 wt% at 1 bar and higher values of 1.66, 1.34, and 1.57 wt% near 1.1 bar for CCG, GHS-F, and GHS-TR, respectively. Besides, hystereses are observed for CCG and GHS-F, inferring some chemisorption interactions between H2 and graphene other than that of typical physisorption. Yet, the sorption behaviors can be fully regenerated after degas treatments at a mild

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Fig. 7. (a) Hydrogen adsorption isotherms of the templated graphenic materials at 77 K. (b) Comparison of hydrogen storage capacity at 77 K & 1 bar for different carbon materials with respect to BET SSA. The dashed line denotes the linear relationship (1.0  10
3 wt% m
2 g) obtained at 77 K & 1 bar [58]. (A colour version of this figure can be viewed online.)

temperature of 150  C. It has been reported that adsorption of molecular hydrogen could be slightly more favorable on a concave graphene surface than on a flat one, that is, chemisorption of hydrogen atoms could be an energetically more favorable process on the convexity due to the distorted sp2 hybridization [51,53]. These highly curved sites exist richly in our crumpled graphenic 3D structures. Since CCG and GHS-F exhibited graphene sheets with abundant corrugations, it is considered that the hystereses are associated with the chemisorption process on convex side of the wrinkles, or with confinement effect originated from the bottleneck-like cavities. Both the storing pathways suggest higher uptake probabilities at room temperature than physisorption does, and the stored gas could be completely released via treatments like mild heating for introducing local deformations. Typically in recent reports, reduced graphene oxide materials and pillared graphene architectures exhibit storage capacities of 0.6e1.2 wt% at 77 K and 1 bar [54e57], (Fig. 7b), and an empirical linear relationship for hydrogen sorption on carbon materials at 77 K and 1 bar gave a slope 0.5 wt% of hydrogen molecules stored per 500 m2 g
1 (i.e. 1.0  10
3 wt% m
2 g) [58]. It is seen that the templated graphenic carbons demonstrate better storage capacity than those previously reported graphene-based materials. These behaviors are associated with the resistance to interlayer stacking and interparticle aggregation of our 3-D graphenic structures for maintaining large SSAs. Also, the uptake capacities of the samples follow the same reported empirical trend, meaning that the uptakes are mainly regulated by physisorption. However, the mild hysteresis in sorption also indicates some minor contributions from chemisorption. To better assess the feasibility of the products at room temperature condition, a linear relationship between capacities measured at 77 K (1 bar) and 298 K (100 bar) (H(298K,
3  H(77K, 1 bar)) extrapolated by Yang et al. [59] is 100 bar) ¼ 283  10 applied for estimation. It turns out that CCG, GHS-F, and GHS-TR are expected to exhibit 0.44, 0.35, and 0.42 wt% storage capacities at 298 K and 100 bar, respectively. Such values are far from satisfactory yet for energy applications. Yet, based on DFT calculations, it has been predicted that Al-decorated graphene could achieve higher storage capacity of 10.5e13.79 wt% [60,61]. A further modification of the templated graphenic materials with doping by Al, B or N may greatly enhance the hydrogen uptake towards fulfilling future application requirements. 4. Conclusions In summary, we have established a method for morphology-

controllable synthesis of graphenic materials. Simply by manipulating hydrothermal or solvothermal conditions, one can obtain boehmite with different morphologies, which are readily transformed to alumina as templates for CVD graphenic carbons. The whole transformation from boehmite to graphenic material is morphology-preserving. This new approach may significantly expand the applications of graphene by means of the fine micronto-nanoscale morphology controllability and 3-D structure realization. Moreover, taking advantage of the large SSAs, the surface carboxylate groups, and the extensive crumpling, one may expect future applications of the 3-D graphenic materials in energy fields such as catalysis [62], battery electrodes [11,63], stretchable electronics [64], and supercapacitors [37,39,65]. As for storing hydrogen, the materials show reasonably high uptake capacities compared to empirical prediction. It could serve as a starting point for better hydrogen adsorption with other modifications, such as Al-modification and novel design of architectures of graphene [44,66]. Acknowledgements This work was supported by Ministry of Science and Technology of Taiwan. The TEM, SEM, and HRTEM characterizations were assisted by Ms. Chin-Yan Lin and Ms. Ya-Yun Yang of Instrumentation Center at National Taiwan University (NTU). The XPS experiment was assisted by Ms. Xiao-Ping Xu (Department of Chemical Engineering, NTU). The hydrogen adsorption analysis was assisted by Prof. Yi-Hsin Liu of Department of Chemistry at National Taiwan Normal University as well as Prof. Soofin Cheng and Dr. Yu-Wei Huang of Department of Chemistry at NTU. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2016.10.023. References [1] K. Nagashio, T. Nishimura, K. Kita, A. Toriumi, Mobility variations in monoand multi-layer graphene films, Appl. Phys. Express 2 (2) (2009) 025003. [2] J. Tian, H. Cao, W. Wu, Q. Yu, Y.P. Chen, Direct imaging of graphene edges: atomic structure and electronic scattering, Nano Lett. 11 (9) (2011) 3663e3668. [3] Q. Yu, L.A. Jauregui, W. Wu, R. Colby, J. Tian, Z. Su, et al., Control and characterization of individual grains and grain boundaries in graphene grown by chemical vapour deposition, Nat. Mater. 10 (6) (2011) 443e449. [4] A.W. Tsen, L. Brown, M.P. Levendorf, F. Ghahari, P.Y. Huang, R.W. Havener, et

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