Direct Growth of Graphitic Carbon-Encapsulating Carbonate Apatite

Article Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Direct Growth of Graphitic Carbon-Encapsulating Carbonate Apatite Nanowires from Calcium Carbonate Namjo Jeong,*,† Soon-Chul Park,‡ Moon Seok Jang,§ and Sung-in Kim∥ Marine Energy Convergence & Integration Research Laboratory, Jeju Global Research Center, ‡Jeju Global Research Center, and § System Convergence & Integration Research Laboratory, Jeju Global Research Center, Korea Institute of Energy Research, 200, Haemajihaean-ro, Gujwa-eup, Jeju Special Self-Governing Province 695-971, Republic of Korea ∥ Measurement & Analysis Department, Korea Advanced Nano Fab Center, 109, Gwanggyo-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do, Republic of Korea Crystal Growth & Design Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 08/10/18. For personal use only.

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ABSTRACT: We report the direct chemical vapor deposition of graphitic carbonencapsulating carbonate apatite nanowires (GCECANs)/hexagonal portlandite-phased Ca(OH)2 hybrid structures from CaCO3 powder at 900 °C. The formation mechanism of the GCECANs is investigated in detail through a structural and morphological characterization of the final products. Spectroscopic analyses elucidate the effect of H2 concentration in the reactor on the transformation of the calcite-phased CaCO3 into vaterite-phased CaCO3 and subsequently portlandite-phased Ca(OH)2. The results show that GCECANs are formed on the surface of the derived Ca(OH)2. It is especially remarkable that a higher H2 concentration results in the precipitated growth of such hybrid structures. Microscopic images show that GCECANs 20 nm in diameter grew up to 3 μm in length along the direction corresponding to the [001] plane of the hexagonal apatite crystal. In addition, the electrical properties of the GCECANs are measured by using an electrode with 150 nm gap.

grafting substrate.10 Suetsugu et al. synthesized carbonate apatite crystal using a CaCO3 flux.11 Yoshimura et al. demonstrated hydrothermal conversion of calcite crystal to hydroxyapatite.12 The direct preparation of new and noble apatite nanostructures from CaCO3 thus provides a meaningful challenge for fundamental studies and the potential for practical uses in the biomaterial science. In our previous research, we synthesized carbon-encapsulating apatite nanowires on glass fibers containing CaO and highlighted their excellent microenvironment for osteogenic differentiation of human mesenchymal stem cells.13,14 In this work, we synthesize graphitic carbon encapsulating-carbonate apatite nanowires (GCECANs)-decorated Ca(OH)2 hybrid structure from CaCO3 powder by chemical vapor deposition (CVD)-based vapor−solid growth. Our particular focus is on the effect of H2 concentration in the reactor on such a transformation. The volume resistance and current density of the GCECAN deposited on electrode with 150 nm-gap are also investigated.

1. INTRODUCTION Bone is composed of an organic phase (collagen) and an inorganic phase (apatite) as well as some organic noncollagenous proteins and amorphous inorganic salts.1 Due to this, apatite structures are suitable for application in the medical field such as in bone repair, drug delivery, and implants.2−4 In particular, nanostructured apatite, including nanostructured apatite/ nanocarbon composites, have drawn keen attention to their unique properties and excellent capabilities in bone fusion and reinforcement, as these characteristics are not present in their bulk material counterparts.5−7 CaCO3 is a common and practical material found in many forms such as limestone, calcareous exoskeleton of marine animals, and various scales.8 It has been used in many industrial applications including as building materials in the construction industry, as a formation-bridging and filter cake-sealing agent in the oil industry, as a paint extender in the polymer industry, as a coating pigment for premium quality products in the paper industry, and as calcium-based antacid tablets in healthcare.9 Utilization of CaCO3 in the medical industry is of particular interest because it is one of the most abundant biological minerals in nature. CaCO3 is regarded as a good candidate for bone grafting substrate and/or starting inorganic precursor to induce the formation of bone minerals such as hydroxyapatite and carbonate apatite. For example, Walsh et al. reported the fabrication of CaCO3/hydroxyapatite composites for bone © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Material Preparation and Synthesis of GCECANs. CaCO3 powder was prepared as a starting material (Sigma-Aldrich). Received: January 19, 2018 Revised: June 27, 2018 Published: July 26, 2018 A

DOI: 10.1021/acs.cgd.8b00107 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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The prepared CaCO3 powder was put in a quartz boat with dimensions of 20 mm × 50 mm × 10 mm (L × W × h), which was then positioned in the center of a quartz tube with dimensions of 50 mm × 300 mm × 2 mm (Din × W × t). For synthesis of GCECANs, the reactor was heated up to 900 °C at 10 °C/min in Ar (99.999%) of 1000 mL/min and held at the elevated temperature for 60 min. During the synthesis, the concentrations of PH3 (99.999%) and C2H2 (99.9%) were maintained at 0.01 vol % and 3.0 vol % in Ar, respectively. To investigate the effect of H2 on the transformation of CaCO3 powder into a GCECAN-decorated Ca(OH)2 structure during the synthesis, the H2 concentration in the reactor was controlled between 10 vol % and 50 vol %. After the synthesis, the supply of reactant gases was stopped, and the reactor was cooled down to room temperature, while the flow of Ar was maintained at 1000 mL/min. Lastly, the final products were obtained from the reactor. 2.2. Characterization. The surface morphologies of the final products were characterized by using a field emission scanning electron microscope (FE-SEM: S-4700, HITACHI) operated at an accelerating voltage of 15 keV. X-ray diffraction (XRD) patterns were obtained using a Rigaku DMAX-2500 operated at 40 kV and 100 mA. The scanning range was controlled within 20−55 (2θ). A Cu−Kα source emitting at approximately 1.54 Å was used for the measurement. To observe the microstructure of the GCECANs, transmission electron microscopy (TEM) analysis was conducted using a field emission TEM (FE-TEM: FEI, Tecnai F30 Supertwin) operated at an accelerating voltage of 200 keV with a Gatan imaging filter (GIF) model 2002. For the TEM analysis, the sample was placed on a TEM grid (a carboncoated Cu grid with 1.2 μm holes). Energy dispersive X-ray spectroscopy (EDX: Genesis) was carried out to confirm the composition and configuration of the GCECANs. Electron energy loss spectroscopy (EELS: Gatan, Enia 1000) were carried out to confirm the composition and the core−shell configuration of the GCECANs. X-ray photoelectron spectroscopy (XPS) was performed using an AXIS-NOVA system (Kratos Inc.) with an X-ray source that emitted monochromatic Al−Kα radiation. The area analyzed was 1 mm × 2 mm, and the base pressure was 7.0 × 10−7 Pa. The Raman spectra were recorded using a backscattering geometry and were obtained under ambient conditions using the 514.5 nm line of an Ar-ion laser. The power level was maintained at 5 mW using a focus spot that was 1 μm in size and lasted for 30 s. To characterize the chemical structures of the GCECANs, infrared spectroscopy (IR, IFF66 V/S and HYPERION3000) was recorded using the attenuated total reflection method over the range of 600−4000 cm−1. To minimize the effect of H2O and CO2 on the samples, all spectroscopic measurements including XRD, XPS, IR, and Raman were immediately conducted through putting them in an Ar-filled bag after the growth process and then transferring them into the analysis equipment, and following TEM and SEM images were taken. To compare with the final products, we also prepared Ca(OH)2 as a reference material (SigmaAldrich). Gas chromatograph analysis was conducted using a 6890 series GC system from Agilent Technologies. The operation proceeded as follows. A 30 m × 320 μm × 0.25 μm 19091J-423 columns (Agilent) was used for a nitrogen phosphorus detector (NPD) with rubidium beads and a 30 m × 320 μm × 0 μm 113−4332 columns (J&W Scientific) for the frame ionization detector (FID). The gas conditions for the NPD were Ar at 5 mL/min, air at 60 mL/min, and H2 at 3 mL/min. For FID, He at 25 mL/min, air at 350 mL/min, and H2 at 30 mL/min were supplied. The injection volume was 600 μL, and auto injection was performed at a temperature of 250 °C. The temperature of the oven was initially set to 40 °C and was maintained for 4 min, and then was increased to 300 °C at the rate of 20 °C/min, and finally was maintained for 6 min. The detector temperature of NPD and FID was set to 300 °C. The run time was 16 min. To obtain electrical properties of the GCECAN, the electrode with 150 nm-gap was fabricated by using an e-beam lithography process and Cr/Au (5 nm/20 nm) deposition off.15 The deposition of the GCECAN on the electrode was processed as shown in our previous work.16 In brief, a 0.1 g of the GCECAN was dispersed in a 10 mL of ethanol for 5 min by using ultrasonicator. A 5 μL of the GCECANdispersed solution was dropped on the electrode. The sample was

dielectrophoretically (at 10 MHz and 5 V for 1 min) deposited between the electrode and then was washed with deionized water. Finally, the prepared sample was dried in an oven (∼50 °C) for 5 min.

3. RESULTS AND DISCUSSION 3.1. Synthesis of GCECANs. We analyzed the conversion of CaCO3 powder to GCECAN-decorated Ca(OH)2 hybrid structures by varying the concentration of H2 in the reactor from 10 vol % to 50 vol % using XRD graphs (Figure 1) and

Figure 1. (a) XRD graphs of sample obtained after synthesis with varying H2 concentration in the reactor from 10 to 50 vol %. (b) the detailed XRD graphs in the range of 2θ = 31−33°.

SEM images (Figure 2). The prepared CaCO3 had a calcite phase (space group R3̅c, compared with JCPDS 83-1762, a = 4.990 Å, and c = 17.063 Å) that is the most stable and common polymorph of CaCO3 at ambient temperature and atmosphere (Figure 1a).17 The main peaks at 23.1°, 29.4°, 36.1°, 39.4°, 43.1°, 47.5°, 48.5°, 56.5°, and 57.4° were assigned to (012), (104), (110), (113), (202), (018), (116), (122), and (214) reflections, respectively. In the previous study,18 we found that the calcite-phased CaCO3 (eggshell) was converted into CaO without the supply of H2 into the reactor. The formation of Ca(OH)2 following synthesis with 10 vol % H2 in Ar was remarkable. The reflections corresponding to (100), (101), (102), (110), and (111) were observed at 28.6°, 34.1°, 47.1°, 50.8°, and 54.3°, respectively. The main diffraction peaks were in good agreement with the JCPDS 87-0674 of a typical hexagonal portlandite-phased Ca(OH)2 (space group P3̅m1, a = 3.589 Å, and c = 4.911 Å).19 Also, the presence of vaterite-phased CaCO3 (space group P6522, compared with JCPDS 74-1867, B



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morphology of the calcite-phased CaCO3 crystal, which is supported by the XRD graph (Figure 1). An HRTEM image revealed that the nanostructures formed were nanowires of approximately 20 nm in diameter. The nanowires appeared uniform and grew up to 1 μm in length. The formation yield of the nanowires tended to rise with an increase in H2 concentration. At 30 vol % H2 concentration, it was observed that the surface of the micron-sized particle was cracked with nanowire formation (Figure 2b). At 50 vol % H2 concentration, the surface of micron-sized particles was mostly covered with nanowires (Figure 2c). The nanowires grew up to approximately 3 μm in length. From the XRD data in Figure 1, it can be confirmed that the nanowire-covered, micron-sized particles are Ca(OH)2 crystals and that the nanowires are hexagonal apatite crystals. Figure 3 shows the detailed XPS spectra of the samples prepared, including as-received CaCO3 and the products obtained after synthesis in 30 vol % and 50 vol % H2 in the reactor. Figure 3a shows the high-resolution XPS spectra of the Ca(2p) core levels of the three samples. Two peaks were observed for the CaCO3 sample, identified as Ca(2p3/2) and Ca(2p1/2) at 347.3 and 350.8 eV, respectively.23 The detailed Ca(2p3/2) peaks of the samples obtained after synthesis with 30 vol % H2 were classified into three components. The peak at 346.5 eV can be assigned to the Ca-(OH) bond.24 The peak at 347.6 eV denotes calcium atom related to the Ca-(PO4) species.25 The peak corresponding to CaCO3 was marked at 347.3 eV. As the H2 concentration rose to 50 vol %, the peaks corresponding to the Ca-(OH) and the Ca-(PO4) species increased significantly in intensity, indicating the transformation of CaCO3 to GCECAN-decorated Ca(OH)2 hybrid structures as shown in the SEM and XRD results. On the other hand, the peak corresponding to CaCO3 was very weakly detected, indicating the presence of carbonate ion in GCECAN. The P(2p) peak appeared at approximately 133.0 eV after the synthesis, which is typically observed in the XPS spectra of apatite crystals (Figure 3b).26 Peak intensity grew with an increase in H2 concentration. Figure 3c shows the O(1s) core levels of the samples. For the CaCO3 sample, the peak at 530.9 eV can be assigned to the carbonate bond,27 and the H2O effect of the surface was related to the peak at 532.2 eV. In the case of the synthesis with 30 vol % H2, the peak at 531.1 eV can be assigned to both carbonate and phosphate bonds. The peak measured at 531.1 eV indicated the presence of P−O−H bond and Ca(OH)2. The organic CO and − O−CO species were marked at high binding energies of 541.7 eV.28 After synthesis with 50 vol % H2 concentration, the peak related to carbonate was weakly recorded. The C(1s) core levels from the samples are shown in Figure 3d. For the CaCO3 sample, two clearly resolved peaks are at 285.0 eV, indicating hydrocarbon (CxHy), and at 289.5 eV, indicating carbonate.29 The C(1s) peaks of the samples obtained after synthesis with H2 supply was classified into six components (Figure 3b).30 The main peak was observed at 284.5 eV, which is related to the sp2hybridized graphite-like carbon atoms. The peak at 285.6 eV can be assigned to the sp3-hybridized carbon atom, implying disordered regions and structural defects in the graphite sheets. The peak at 286.6 eV denotes carbon atoms related to the C−O species. The −O−CO and carbonate species were marked at high binding energies of 287.9 and 289.3 eV, respectively. The peak at 290.6 eV corresponded to π−π* transitions. Figure 4 shows infrared (IR) spectra of the samples obtained with H2 concentration varying from 10 vol % to 50 vol %.

Figure 2. SEM images of the final products obtained after the synthesis with (a) 10, (b) 30, (c) 50 vol % H2 concentration in the reactor. The insets show the high-magnification SEM images of as-synthesized nanowires.

a = 0.729 Å, and c = 2.530 Å) was confirmed by the characteristic (004), (111), (112), (114), and (041) reflections at 24.8°, 27.1°, 32.7°, 49.9°, and 52.4°, respectively.20 The vaterite phase is a relatively unstable form compared to other anhydrous crystalline forms such as calcite and aragonite phases.17,21 As the H2 concentration in the reactor rose, the intensity of peaks corresponding to CaCO3 (both calcite and vaterite) decreased rapidly, whereas the formation of hexagonal apatite and hexagonal portlandite-phased Ca(OH)2 was more precipitated. From the XRD graphs detailed in the range of 2θ = 31−33° (Figure 1b), the (002), (211), (112), (300), (213), (303), and (322) reflections were detected at 25.9°, 31.8°, 32.2°, 32.8°, 49.5°, 52.1°, and 55.9°, respectively, which indicates the presence of hexagonal apatite.22 The sharpness and intensity of the peaks corresponding to the hexagonal apatite increased as the H2 concentration in the reactor increased, indicating the well-crystalline structure. The (006) peak of the calcite-phased CaCO3 was observed at around 31.4°, which gradually decreased as the H2 concentration in the reactor increased. The (300) peak of apatite detected at approximately 32.8° shifted to a lower 2θ value and became broad under conditions of lower H2 concentration. This is due to the presence of the (112) reflection at 32.7° assigned to the vaterite phase and the lower formation yield of the apatite crystal. It was found that CaCO3 was mostly converted to apatite Ca(OH)2 hybrid structure in the condition of 10 vol % H2 in Ar. Figure 2 shows the SEM images of the final products obtained after synthesis with 10 vol %, 30 vol %, and 50 vol % H2 concentration in the reactor. At 10 vol % H2 concentration, some nanostructures were partially observed on the surface of 20 μm particles (Figure 2a). Their formation yield appeared low. The unreacted particles exhibited a typical rhombohedral C



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Figure 3. XPS spectra of the samples obtained after synthesis with 30 and 50 vol % H2 concentration in the reactor: (a) Ca (2p), (b) P (2p), (c) O (1s), and (d) C (1s). CaCO3 was used as a reference data for comparison of the results.

The EELS analyses were performed to confirm the core− shell configuration of the GCECAN (Figure 6). The EELS mapping images shows calcium, phosphorus, and oxygen for the apatite core, and carbon for the graphitic shell (Figure 6a−e). At shell, only the C−K shell ionization edge was observed at 284.5 eV (Figure 6f). As the measurement points get closer to the center, on the other hand, P-L2,3, Ca-L2,3, and O−K shell ionization edges corresponding to the core were detected at 132.3, 347.2, and 530.8 eV. To prove the formation of graphitic carbon on the surface of the apatite nanowire core, Raman spectrum was taken from the sample obtained after synthesis with 50 vol % H2 concentration (Figure 7). There were two major Raman peaks in the firstorder Raman spectrum region: 1592 cm−1 (G-band) indicating graphitized structures, and 1347 cm−1 (D-band) implying defective structures. The G-band in multiwalled carbon nanotubes is generally found at approximately 1580 cm−1,39 but the G-band of the GCECAN shifted to higher frequencies, implying the presence of a strained microstructure in the crystalline graphitic structure.40 The ID/IG ratio of the filamentous nanocarbon was 1.07, indicating the presence of defects in their structures.41 In the second-order Raman spectrum region, two peaks were found at approximately 2710 cm−1 (G′-band) and 2940 cm−1 (D + G-band). For the core of the GCECAN, the peaks corresponding to the carbonate group were almost not observed in the sample. The peaks at 359 and 3616 cm−1 indicate the characteristic bands for the hydroxide group corresponding to the presence of Ca(OH)2. Also, the characteristic phosphate band corresponding to the apatite structure was weakly detected at 961 cm−1.

The CaCO3 and Ca(OH)2 powders were used as references for comparison of results. The peaks at 561 (ν4), 601 (ν4), 960 (ν1), 1025 (ν3), 1054 (ν3), and 1081 (ν3) cm−1 are characteristic of the phosphate group (Figure 4a).29,31 The characteristic OH bands corresponding to apatite at 3563 (ν) and 630 cm−1 (δ) are very weakly found (Figure 4a,b).32 A strong sharp band at 3640 cm−1 is due to the O−H stretching mode of the Ca(OH)2 powder (Figure 4b).33 For the CaCO3 powder, peaks were detected at 1390, 869, and 711 cm−1 (Figure 4a).34 The intensities of the bands at 1550, 1465, 1407, 871, and 707 cm−1, indicating the carbonate group,35 decreased as the H2 concentration rose. The peaks at 1550, 1465, and 871 cm−1, in particular, imply the presence of A and B type carbonate apatite.36 The phosphate and carbonate groups indicate the presence of a carbonate apatite structure. To better understand the morphologies and crystalline structures of the GCECAN formed, we performed TEM analysis as described in the Experimental Section. Figure 5a shows that the GCECANs have a core−shell configuration. The interplanar spacings of the core were calculated as 3.44, 4.71, and 2.78 Å, which can be assigned to the (002), (110), and (112) lattice planes of the hexagonal apatite, respectively.37 The growth of the apatite core was orientated to the [001] plane. The HRTEM image and fast Fourier transform (FFT) diffraction pattern, which was taken along the growth direction of the GCECAN, clearly revealed the hexagonal structure of the apatite crystal (Figure 5b,c). The interplanar spacing of the shell was 3.4 Å, indicating the [002] lattice fringe of bulk graphite.38 The EDX analyses supported the composition of the GCECAN formed (Figure 5d). D



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3.2. Growth Mechanism of GCECANs. In this work, the XRD results indicated that reaction 1 occurred more dominantly in the reactor in which various gaseous species existed (Figure 1).42 CaCO3 + 4H 2 = Ca(OH)2 + CH4 + H 2O

(1)

It is remarkable that the vaterite phase was formed as an intermediate product during the formation of apatite-Ca(OH)2 hybrid structures. Previous studies have reported that some additives and inhibitors can provide a favorable environment for transformation of the most thermodynamically stable calcite to the least stable vaterite.43,44 It is also noteworthy that calcite can be converted to calcium apatite crystals through vaterite in simulated body fluid.45,46 Therefore, it is speculated from the results that the supply of H2 plays a key role in the transformation of calcite to vaterite, and then to apatiteCa(OH)2 hybrid structures. Also, the thermal decomposition of the PH3 supplied during the synthesis can induce the formation of P4, resulting in the formation of P4O10 (g). Therefore, we suggest that apatite nanowires can be formed on the surface of Ca(OH)2 through vapor−solid growth by two possible reactions as given below. 10Ca(OH)2 (g) + 3/2P4 O10 (g) = Ca10(PO4 )6 (OH)2 (s) + 9H 2O(g)

(3)

The following set of reactions is also possible. P4 O10 (g) + 9H 2O(g) = 4H3PO4 (g)

(4)

10Ca(OH)2 (g) + 6H3PO4 (g) + = Ca10(PO4 )6 (OH)2 (s) + 18H 2O(g)

(5)

Figure 8 shows the high-resolution SEM image at the interface between GCECAN and Ca(OH)2. The nanoparticles with a diameter of 30 nm were formed on the surface of Ca(OH)2. It can be speculated that the nanoparticles act as a seed initiating the vapor−solid growth of GCECANs. To identify what led to the growth of nanowire structure from the nanoparticles, the GC analysis were performed as shown in Figure 9. As mentioned above, P4(g) was derived from the decomposition of PH3 at 900 °C. When C2H2 was spontaneously supplied with PH3, a strong peak was observed at about 8 min, which can be assigned to phosphorene (C5H5P). The C5H5P is a well-known phosphorus organic compound that can be a strong calcium binder,47 resulting in the formation of one-dimensional apatite nanostructure through continuous accumulation of gaseous calcium species on any specific lattice. Therefore, it is speculated that the C5H5P leads to the one-dimensional growth of apatite core along the [001] plane as shown in Figure 5a. Also, Gopi et al. reported that a nitrogen-based organic compound (HEDTA) led to the unusual transformation of calcite to vaterite above 170 °C,43 which motivates that the C5H5P may facilitate phase change of calcite into vaterite. Also, it is noteworthy that at 900 °C, C2H2 is thermally decomposed to various hydrocarbon, such as benzene, toluene, styrene, naphthalene, etc.48 Particularly, aromatic hydrocarbon provides a favorable environment for the formation of graphitic layers on the surface of apatite nanowire core. This can be supported from GC-FID graphs of Figure 9b. To prove the role of C5H5P in the initial formation of GCECANs, the GC-NPD analysis of exhaust gas as increase of synthesis time was investigated as shown in Figure 10.

Figure 4. (a) IR spectra of samples obtained after synthesis with 10, 30, 50 vol % H2 concentration in the reactor. (b) The detailed IR spectra in the range of 3000−4000 cm−1, indicating the hydroxide group.

Figure 5. HRTEM images of GCECAN (a) perpendicular and (b) parallel to growth axis. (c) FFT pattern corresponding to (b). (d) EDX spectrum corresponding to (b). E



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Figure 6. EELS results of GCECAN (a) TEM and (b−e) EELS mapping images. (f−h) EELS spectra at the points indicated by (1−3) arrows, respectively.

Figure 7. Raman spectra of the prepared samples. The synthesis was performed at 50 vol % H2 concentration in the reactor. To compare with the final products, we also prepared Ca(OH)2 as a reference material.

Figure 8. High-resolution SEM image at the interface between GCECAN and Ca(OH)2.

Figure 9. GC analysis after chemical reaction of gaseous reactants (Ar, H2, C2H2, and PH2) supplied for the synthesis of the GCECANs. (a) NPD and (b) FID results.

At a synthesis time of 0.5 min, the generation of P4 was observed. From the SEM image, no nanowires were found in the sample (Figure 10b). After 1.0 min, the formation of

C5H5P was remarkable, and very short nanowires was observed, which supports that C5H5P affects the formation of GCECANs. As the synthesis time increased, the length and F



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Figure 12. Electrical resistance test of GCECAN (a) schematic illustrating deposition of GCECAN on an electrode with 150 nm-gap and (b) room temperature current versus voltage curve measured on the GCECAN. The inset of panel b shows the high-magnification SEM images of the assembled GCECAN.

a typical I−V curve obtained by using the two probe configuration (see inset of Figure 12b) on the GCECAN. Conductivity of the GCECAN was calculated based on their dimension (25 nm in diameter) as follow equations.

Figure 10. (a) GC-NPD analysis of exhaust gas as increase of synthesis time. SEM images of the samples synthesized at (b) 0.5 min, (c) 1.0 min, (d) 2.0 min, and (e) 5.0 min. Synthesis temperature was 900 °C and the amount of H2 was 30 vol % in Ar.

yield of GCECANs increased. Also, the amount of C5H5P in exhaust gas increased up to 5.0 min, and it did not increase anymore after the time (Figures 9a and 10a). Therefore, we suggest the growth mechanism of the GCECANs from calcium carbonate crystal as shown in Figure 11. Figure 12a shows the schematic illustrating deposition of GCECAN on an electrode with 150 nm-gap. Figure 12b shows

volume resistance = R × A /L

(6)

current density = I /A

(7)

where R, I, A, and L represent resistance, current, cross area of GCECAN, and gap width of electrode, respectively. From the I−V curve, R is recorded to 115k Ω. Also, A is about 490 nm2. Therefore, a volume resistance of 3.76 × 10−2 Ω cm was

Figure 11. Growth mechanism of the GCECAN. G



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calculated and a current density at the 1 V of applied voltage was 1.77 × 106 A/cm2.

4. CONCLUSION We achieved the direct transformation of calcite-phased CaCO3 into GCECANs/hexagonal portlandite phased Ca(OH)2 hybrid structures using a CVD-based vapor−solid growth mechanism. The XRD and XPS graphs clearly revealed that an increase of H2 concentration in the reactor accelerated the formation of such hybrid structures. It was also determined that the vaterite phase was formed as an intermediate product during the transformation. The SEM and TEM images showed that the apatite nanowires of approximately 20 nm in diameter grew up to 3 μm in length along the [001] plane. Further, the IR results showed that the apatite formed was very consistent with the hexagonal carbonate apatite structures. The HRTEM images confirmed that the apatite nanowires were sheathed with graphitic carbon; this was supported by the Raman and EDX spectra. A volume resistance and current density at the 1 V of applied voltage were 3.76 × 10−2 Ω cm and 1.77 × 106 A/cm2.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82-64-800-2229, +82-42-860-3389. Fax: +82-42-860-3133. E-mail: [email protected]. ORCID

Namjo Jeong: 0000-0003-0939-2744 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was conducted under the framework of the Research and Development Program of the Korean Institute of Energy Research (KIER: B8-2441).

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REFERENCES

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Direct Growth of Graphitic Carbon-Encapsulating Carbonate Apatite

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