Highly graphitic carbon nanosheets synthesized over tailored mesoporous molecular sieves using acetylene by chemical vapor deposition method

Abstract

Graphitic carbon nanosheets (GCNS) were synthesized using mesoporous Ti-MCM-41 molecular sieves as catalytic template and acetylene as carbon precursor following chemical vapor deposition method, under atmospheric pressure. The mesoporous Ti-MCM-41 molecular sieves with various Si/Ti ratios were synthesized by direct hydrothermal method. The materials so obtained were characterized by various physico-chemical techniques such as X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The analytical results indicated that the obtained GCNS possess high thermal stability and good graphitization with an inter layer distance of around 3.45 Å. The influence of reaction parameters such as temperature and catalytic templates was studied to improve the quality and quantity of GCNS. The excellent graphitized GCNS material on a large scale was achieved from Ti-MCM-41 at moderate reaction temperature. This work proposes a way for the large scale synthesis of GCNS with applications suitable for nanoelectronics, nanocomposites and so on.

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1. Introduction

Generally, carbon nanostructured materials are of great interest in catalysis, electronics, thermomechanical and biomedicine. Among the carbon nanostructured materials such as fullerenes, carbon nanotubes, and mesoporous carbon, study of graphitic carbon nanosheets is comparatively limited. These types of carbon nanostructures and their composites possess unique thermomechanical and electronic properties, leading to their potential application in different important areas; such as, as adsorbents, supports for use in catalysis, hydrogen storage, drug delivery, supercapacitors and lithium ion batteries, etc.^1–9 Graphitic carbon nanostructures such as fullerenes, CNTs, graphene, and mesoporous carbon are synthesized by various methods such as chemical vapor deposition (CVD), arc discharge, laser ablation, wet chemical etc. Among these methods, CVD has attracted great interest because of its unique properties such as large scale synthesis of highly pure graphitized carbon with tunable size. The size in CVD can be easily be tailored by adjusting the reaction time, reaction temperature and flow rate of the carbon precursor. The focus remains on carrying out reactions at moderate temperatures to give successful commercialized nanostructures at a reasonable cost.^10,11 The catalyst is an important factor for efficient transformation of carbon clusters into graphitic carbon (graphene sheets). The catalytically produced carbon nanomaterials are adequate for many applications, especially in electronic devices because they can be directly synthesized without any major contamination by carbonaceous impurities. Various catalytic chemical vapor deposition (CCVD) methods are already known for the production of graphene like carbon nanomaterial using different carbon precursors.^12–16 Acetylene is considered as a good carbon source for production of graphitic carbon nanostructures because it contains fewer number of hydrogen atoms with rich carbon atom per molecule. Its greater activity in comparison to other hydrocarbons such as benzene, ethylene and so on, also favours its use as the carbon source.^17,18 In general, mesoporous MCM-41 molecular sieves are of interest because of their remarkable properties such as high surface area (>1000 m² g⁻¹), large pore volume (>0.8 cm³ g⁻¹), narrow pore size distribution and easy surface functionalization. Pure siliceous MCM-41 (Si-MCM-41) molecular sieve has limited catalytic activity but active catalytic sites can be generated in it by isomorphously substituting silicon with transition metal.¹⁹ Several studies have been dedicated to the investigation of transition metal substituted MCM-41 because of their wide range of applications in catalysis for the fine chemical synthesis. Apart from catalysis, the metal incorporated MCM-41 is also used as a catalytic template for the synthesis of CNTs and related nanostructure materials.^20,21 The GCNS with graphitization were synthesized from various source and different methods, however, obtaining the GCNS with elevated graphitized nanosheets still remains a challenge in the field of nanotechnology for industrial upgradation.

In the present investigation, the novel GCNS with high degree of graphitization on large scale were synthesized by CVD method. For first time, titanium incorporated mesoporous MCM-41 molecular sieves (Ti-MCM-41) have been used as a catalytic templates and acetylene used as carbon precursor for the production of GCNS. The reaction temperature and metal concentration over the catalytic template was optimized for the quantitative and qualitative studies of GCNS. The synthesized carbon material was investigated by various physico-chemical techniques. Moreover, the results strongly suggest that the obtained GCNS are pure with excellent graphitization.

2. Experimental

2.1. Materials

The chemicals used for the synthesis of mesoporous MCM-41 molecular sieves were sodium metasilicate nonahydrate (Na₂SiO₃·9H₂O) and titanium tetraisopropoxide (Ti{OCH(CH₃)₂}₄) as sources of silica and titanium, respectively. Cetyltrimethylammonium bromide (CTAB) was used as the structure-directing agent. Sulphuric acid (H₂SO₄) was used to adjust the pH of the medium. Acetylene gas (99.5%), nitrogen gas (99.9%) and hydrogen gas (99.9%) as carbon source, carrier gas and reducing agent, respectively. Hydrofluoric acid (HF) and hydrochloric acid (HCl) were used for purification of synthesized GCNS. All the chemicals were purchased from Qualigens Fine Chemicals and used without any further purification. The double distilled (DD) water was used throughout this study.

2.2. Synthesis of mesoporous Si-MCM-41 and Ti-MCM-41 molecular sieves

Mesoporous Ti-MCM-41 molecular sieves with various Si/Ti ratios were synthesized by the direct hydrothermal method with the gel composition of SiO₂ : xTi : 0.2CTAB : 0.89H₂SO₄ : 120H₂O.^22,23 In a typical synthesis procedure, sodium metasilicate in water was combined with an appropriate amount of titanium source and the pH of the solution was adjusted to 10.5 by adding 4 N H₂SO₄ with constant stirring to form a gel. After 30 min, an aqueous solution of CTAB was added to it and the mixture was stirred for 1 h at room temperature. The suspension was then transferred into a Teflon-lined stainless steel autoclave, sealed carefully subsequently heated at 145 °C for 48 h in a hot air oven. After hydrothermal treatment (crystallization), Ti-MCM-41 molecular sieves were recovered by filtration, washed with DD water several times. The obtained precipitate was dried at 100 °C for 5 h in a hot air oven under static condition. The final active catalyst was obtained by removing the occluded surfactant by calcining the sample at 550 °C for 5 h in a muffle furnace under static condition.²⁴ Similar procedure was adapted for the synthesis of mesoporous Si-MCM-41 molecular sieves, except the titanium source.

2.3. Synthesis of GCNS

The reaction for the synthesis of GCNS was carried out using the mesoporous Ti-MCM-41 molecular sieves with various Si/Ti ratios as a catalytic template by CVD method. A simple CVD setup containing horizontal tubular furnace, and gas flow control units were used to produce GCNS for conducting the experiments which is shown in Fig. 1. In a typical synthesis, about 100 mg of catalytic template was placed in a quartz boat inside a middle of the quartz tube. The catalytic template was purged with nitrogen gas at a flow rate of 100 SCCM (SCCM denotes standard cubic centimeter per minute) for 30 min in order to remove physically absorbed/adsorbed water molecules from the host mesoporous Ti-MCM-41 molecular sieves. Subsequently hydrogen gas was purged at a flow rate of 100 SCCM for 30 min in order to prevent the oxidation of metal nanoparticles. The reaction was carried out using acetylene as carbon source at particular reaction temperature (800–950 °C) with a flow rate of 100 SCCM for 30 min. The furnace was then cooled to room temperature under nitrogen atmosphere, and the final product was collected after the completion of the reaction. The reaction was carried out over various Si/Ti ratios of mesoporous Ti-MCM-41 molecular sieves (Ti-MCM-41 (25), Ti-MCM-41 (50), Ti-MCM-41 (75) and Ti-MCM-41 (100)) at different reaction temperatures (800, 850, 900 and 950 °C) for the qualitative and quantitative production of carbon nanosheets. The obtained material was weighed, purified and then characterized by various physico-chemical techniques. The percentage of the carbon deposition (CD) depends on the weight percentage (wt%) of catalytic template was calculated from the following eqn (1):where, M_tot and M_cat is the total mass of carbon product with catalytic template and mass of catalytic template, respectively.

Fig. 1 Schematic of the CVD setup employing for the production of GCNS.

2.4. Purification of GCNS

The silica phase and metals nanoparticles were removed by treating with hydrofluoric acid at ambient temperature as per earlier reports.^25,26 In a typical procedure, the as-synthesized carbon sample was mixed with an appropriate amount of hydrofluoric acid, subsequently sonicated for 20 min and followed by stirring for 30 min. The mixture was filtered, washed with DD water few times. The obtained sample was further mixed with hydrochloric acid and stirred for 2 h at 60 °C under static condition to dissolve the metal nanoparticles and diminish the damaged carbon nanosheets. The final mixture was filtered, washed with DD water until the aqueous layer becomes neutral for removal of metal nanoparticle from product. The obtained residue (black solid) was dried at 100 °C for 5 h in a hot air oven under static condition.^27–29 The resulting dark final product was collected and stored for further use.

2.5. Characterization methods

The synthesized mesoporous Si-MCM-41/Ti-MCM-41 molecular sieves and GCNS were characterized by various physico-chemical techniques such as inductive coupled plasma-atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD), N₂ physisorption isotherm, Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric and derivative thermogravimetric analysis (TG/DTA), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM with EDS), high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The amount of metal loaded over the mesoporous MCM-41 molecular sieves was determined by ICP-AES (Perkin-Elmer OPTIMA 3000). The samples were dissolved in a mixture of hydrofluoric acid and nitric acid before the measurement. The XRD patterns were obtained on a PANalytical X'Pert diffractometer using Cu Kα radiation (K = 1.54 Å) equipped with a liquid nitrogen cooled germanium solid-state detector. The XRD patterns of Ti-MCM-41 and GCNS were recorded in the 2θ range of 1–10° and 5–80° respectively, and at the step interval of 0.02° with the counting time of 5 s at each point. Nitrogen physisorption isotherms were measured at −197 °C using a Micromeritics ASAP 2000. Prior to the experiments, the samples were dried at 130 °C and evacuated for 8 h in flowing argon at the flow rate of 60 SCCM at 200 °C. Surface area, pore size, and pore volumes were obtained from isotherms using the conventional Brunauer–Emmet–Teller (BET) equation and Barrett–Joyner–Halenda (BJH) method. The total pore volumes were estimated at P/P₀ = 0.98. Thermogravimetric measurements were performed using a Mettler TA 3001 analyser equipped with a gas flow system. Samples of approximately 10 mg were treated under air atmosphere, increasing the temperature by 10 °C min⁻¹, from ambient temperature to 1000 °C.

The FT-IR spectra were recorded in transmittance mode on a Shimadzu IRPrestige-21 FTIR spectrometer in the wavenumber range of 400–4000 cm⁻¹ by the co-addition of 50 scans at a resolution of 16 cm⁻¹. SEM was performed on a JEOL with acceleration voltage of 4 kV. The solid sample was dispersed in acetone and placing on silicon wafer. The HRTEM images were recorded on a JEOL 3010 electron microscope operated at an accelerating voltage of 300 kV. Samples for HRTEM were prepared by placing droplets of a suspension of the sample in acetone on a carbon-coated polymer micro grid supported on a Cu grid. Raman spectra were recorded with a Raman spectrometer Almega X/Thermo using a laser excitation line at 532 nm.

3. Results and discussion

3.1. Characterization of mesoporous Si-MCM-41 and Ti-MCM-41 molecular sieves

3.1.1. Elemental analysis. The amount of metal nanoparticles present in the MCM-41 decides the activity of the catalysts towards synthesis of fine carbon nanostructure material. Elemental analysis of hydrothermally synthesized mesoporous Ti-MCM-41 molecular sieves with various Si/Ti ratios was analyzed by ICP-AES and Si/Ti ratios, which is summarized in Table S1.† Under the synthesis conditions used, the Si/Ti ratio of mesoporous Ti-MCM-41 molecular sieves derived by the elemental analysis was nearly close to the gel composition as observed in the mesoporous Ti-MCM-41 molecular sieves. This result indicates that almost all the metal (Ti) nanoparticles were incorporated in the mesoporous MCM-41 molecular sieves during the hydrothermal treatment.

3.1.2. Surface area analysis. The nitrogen physisorption isotherm technique was used for the identification of surface area, pore size and pore volume of the synthesized material. The nitrogen adsorption and desorption isotherms of calcined mesoporous Si-MCM-41 and Ti-MCM-41 molecular sieves with various Si/Ti ratios are shown in Fig. 2A. All the synthesized mesoporous MCM-41 molecular sieves exhibit a type IV isotherm with H1 hysteresis loops at intermediate relative pressures indicating that the order of hexagonal arrays of well-defined pore structures. Three well-defined stages are identified: (i) a slow increase in nitrogen uptake at low relative pressures, corresponding to monolayer–multilayer adsorption on the pore walls, (ii) a sharp step at intermediate relative pressures indicative of capillary condensation within mesopores and (iii) a plateau with a slight inclination at high relative pressures associated with multilayer adsorption on the external surface of the mesoporous structure.^30,31 The BET surface area of synthesized mesoporous Si-MCM-41 molecular sieve is around 1150 m² g⁻¹. The steepest hysteresis loops were observed at P/P₀ between 0.2 and 0.4 for the synthesized mesoporous Si-MCM-41 molecular sieve, implying that the pore size distribution for this sample is relatively narrow. The BET surface area of synthesized mesoporous Ti-MCM-41 molecular sieve with Si/Ti ratios from 25 to 100 is from 870 to 1100 m² g⁻¹ respectively. The hysteresis loops varied slightly with the increased amount of Ti in the mesoporous frame work. The steepest hysteresis loops were observed at P/P₀ between 0.2 and 0.4 for the mesoporous Ti-MCM-41 molecular sieve with Si/Ti ratios. But the P/P₀ value was little shifted to the lower region while increasing the Ti concentration over the mesoporous MCM-41 molecular sieves, implying that the pore size distribution for these samples are relatively narrow but pore shrinkage occurred during the Ti incorporation which is supported by pore size distribution graph. Fig. 2B shows the pore size distribution graph of calcined mesoporous Si-MCM-41 and Ti-MCM-41 molecular sieves with various Si/Ti ratios. While incorporating the Ti over the mesoporous MCM-41 molecular sieves and increasing the Ti concentration over mesoporous MCM-41 molecular sieves, the pore diameter and pore volume are gradually decreased which is clear from Fig. 2B. BET surface area, pore size, and pore volume of calcined mesoporous Si-MCM-41 and Ti-MCM-41 molecular sieves with various Si/Ti ratios are summarized in the Table S1.†

Fig. 2 (A) Nitrogen physisorption isotherms and (B) pore size distribution of calcined Si-MCM-41 and Ti-MCM-41.

3.1.3. Morphological analysis. The surface morphology of calcined mesoporous Si-MCM-41 and Ti-MCM-41 molecular sieves with various Si/Ti ratios was revealed using the SEM analysis. These samples did not show a tendency to form particles/materials with a particular morphology, the surface morphology is smooth and spongy-like porous in nature which is shown in Fig. S4.† The surface smoothness faintly decreases with increasing the metal content in the mesoporous structure as seen in SEM micrographs. This might be due to the shrinkage of pore or damage of pore occurring within mesostructured material. The pore structure and long range order of mesoporous Si-MCM-41 and Ti-MCM-41 molecular sieves is revealed by HRTEM analysis. The structural morphology of the calcined mesoporous Si-MCM-41 and Ti-MCM-41 molecular sieves with various Si/Ti ratios and different view are shown in Fig. 3. HRTEM images show a honeycomb-like structure of the mesoporous MCM-41 molecular sieves. There are virtually regular hexagonal pores and arrays of fine pore arrangement existing in these samples.³² HRTEM images of mesoporous Si-MCM-41, Ti-MCM-41 (100), Ti-MCM-41 (75), Ti-MCM-41 (50) and Ti-MCM-41 (25) molecular sieves with electron beams parallel to the pore channels are shown in Fig. 3a, c, e, g and i and perpendicular to the pore channels are shown in Fig. 3b, d, f, h and j, respectively. These images clearly show that the formed mesoporous MCM-41 molecular sieves are highly ordered (long range order) with fine hexagonal pore structure. The interlayer distance and pore size faintly decreases while increasing the Ti content in the mesoporous structure. The bond shrinkage occurs with the Ti incorporation over the mesoporous MCM-41 molecular sieves which were strongly supported by BET surface area analysis. The mesoporosity was not much affected and the absence of Ti nanoparticles on the surface of mesoporous Ti-MCM-41 molecular sieve even at high amount of Ti. These HRTEM images strongly imply that all the Ti nanoparticles were incorporated into the framework of mesoporous MCM-41 molecular sieves during the hydrothermal process.

Fig. 3 TEM images of calcined Si-MCM-41, Ti-MCM-41 (100), Ti-MCM-41 (75), Ti-MCM-41 (50) and Ti-MCM-41 (25) with electron beams parallel to the pore channels (a), (c), (e), (g) and (i); perpendicular to the pore channels (b), (d), (f), (h) and (j), respectively.

3.2. Characterization and discussion of synthesized GCNS

3.2.1. XRD analysis. The phase structure and degree of crystallinity of the synthesized product was determined by XRD analysis. Fig. 4A shows a typical wide-angle XRD pattern of the GCNS obtained over the mesoporous Ti-MCM-41 molecular sieves with various Si/Ti ratios at the reaction temperature 850 °C. The pattern clearly shows that there is intense diffraction peak around 2θ = 26° that can be indexed as the C(002) plane and the weak diffraction peaks at about 2θ = 43, 53 and 78° are corresponding to C(100) C(004) and C(110) planes, respectively. The diffraction reveals the high-quality graphitic nature of nanosheets. The C(002) peaks position of 2θ value of GCNS faintly increases with increase in Ti content of the mesoporous MCM-41 molecular sieves, this might be due to shrinkage of interlayer between the GCNS. While increasing the metal content over the mesoporous MCM-41 molecular sieves, the d-spacing value of GCNS calculated using Bragg eqn (S1)† decreases on the position of C(002) reflection peak. The amount of carbon nanosheet increases with increase in the metal nanoparticles because metal nanoparticles are used as seed for the growth of graphitic carbon nanostructures. The d-spacing value decreased due to high concentration of GCNS in specific area. Increasing the incorporation of Ti in the mesoporous structure doesn't affect the degree of crystallinity (graphitization) of obtained GCNS much. Among the synthesized catalytic templates, the mesoporous Ti-MCM-41 (75) molecular sieves yield GCNS with high degree of crystallinity compared to other catalytic templates that we have used (Ti-MCM-41 (100), Ti-MCM-41 (50) and Ti-MCM-41 (25)). Thus, mesoporous Ti-MCM-41 (75) molecular sieves was chosen as a suitable catalytic template for the optimization for different reaction temperatures for the better formation of GCNS. The wide-angle XRD pattern of GCNS obtained over the mesoporous Ti-MCM-41 (75) molecular sieves at different temperatures from 800 to 950 °C is shown in Fig. 4B. As shown in Fig. 4B, the intensity of the diffraction peak at 26° increases with increase in the reaction temperature, indicating higher graphitization degree of GCNS at higher temperature. Beyond the optimum reaction temperature, the intensity of the diffraction peak decreased indicating low degree of crystallinity (reason for the graphitization of GCNS will be discussed later). The well graphitized GCNS were obtained at 850 and 900 °C compared to other temperatures. The interlayer distance (d-spacing value) was calculated using Bragg eqn (S1)† based on the position of C(002) reflection peak. The calculated interlayer distance (∼3.4–3.5 Å) are closer to those observed for bulk hexagonal graphite (∼3.4 Å).^33,34 The interlayer distance of GCNS decreases with increase in reaction temperatures beyond the optimum temperature it increased. The interlayer distance of GCNS obtained from various Si/Ti ratios of Ti-MCM-41 and different reaction temperatures are summarized in Tables 1 and 2, respectively. Later on, HRTEM analysis was performed to confirm the interlayer distance and high order of GCNS.

Fig. 4 XRD patterns of GCNS (A) obtained over the (a) Ti-MCM-41 (100), (b) Ti-MCM-41 (75), (c) Ti-MCM-41 (50), (d) Ti-MCM-41 (25) at 850 °C and (B) obtained over the Ti-MCM-41 (75) at (a) 800, (b) 850, (c) 900, (d) 950 °C; TG/DTA curves of as-synthesised GCNS (C) over the Ti-MCM-41 with various Si/Ti ratios at 850 °C and (D) over the Ti-MCM-41 (75) at different temperatures.

Table 1 Influence of catalytic template for the yield of CD/GCNS synthesized at 850 °C

S. No	Catalytic template	d-spacing value of GCNS^a (Å)	Yield of CD using gravimetric calculation^b (%)	Yield calculated from thermogram
				CD^c (%)	GCNS^d (%)	GCNS^e (%)
a The values are calculated from the XRD pattern.b ^,c,d&e The values are calculated using the equation (1), (2), (3) & (4) respectively. Carbon source: C2H2; flow rate of C2H2/N2: 100/500 SCCM; reaction temperature: 850 °C; reaction time: 30 min.
1	Ti-MCM-41 (100)	3.48	160	163	158	97
2	Ti-MCM-41 (75)	3.46	207	213	210	99
3	Ti-MCM-41 (50)	3.45	200	203	200	99
4	Ti-MCM-41 (25)	3.42	190	194	182	94

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Table 2 Influence of reaction temperature for the growth yield of CD/GCNS synthesized over the mesoporous Ti-MCM-41 molecular sieves

S. No	Growth temperature (°C)	d-spacing value of GCNS^a (Å)	Yield of CD using gravimetric calculation^b (%)	Yield calculated from thermogram
				CD^c (%)	GCNS^d (%)	GCNS^e (%)
a The values are calculated from the XRD pattern.b ^,c,d&e The values are calculated using the equation (1), (2), (3) & (4) respectively. Carbon source: C2H2; flow rate of C2H2/N2: 100/500 SCCM; catalytic template: Ti-MCM-41 (75); reaction time: 30 min.
1	800	3.49	160	170	164	96
2	850	3.46	207	213	210	99
3	900	3.45	204	212	209	99
4	950	3.47	175	186	177	94

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3.2.2. Thermogravimetric analysis. The percentage of carbon deposition, purity and thermal stability of the carbon nanostructure material is revealed by thermograms. TG/DTA curves of as-synthesised carbon nanomaterial over the mesoporous Ti-MCM-41 molecular sieves with various Si/Ti ratios at the reaction temperature of 850 °C are presented in Fig. 4C. The TGA curve for GCNS shows initially negligible amount of weight loss observed below 150 °C due to desorption/decomposition of physisorbed water molecules over the as-synthesized GCNS. The combustion of amorphous carbon occupied over the surface of GCNS take place between 300 and 450 °C, which is lower than the decomposition temperature of GCNS.³⁵ The major weight loss observed between 550 and 650 °C is due to the decomposition of well-structured GCNS. The DTA shows the maximum endothermic peak around 590 °C for GCNS obtained from mesoporous Ti-MCM-41 (75) molecular sieves. The maximum endothermic peak is less than 590 °C for the GCNS obtained from other Si/Ti ratios of mesoporous Ti-MCM-41 molecular sieves because of graphitization of GCNS. The product shows high thermal stability indicating the high degree of graphitization and the major weight loss indicates the presence of high yield of GCNS over the mesoporous Ti-MCM-41 molecular sieves. Apart from this weight losses an additional weight were not observed in the thermograms. It shows that the synthesized carbon materials are highly pure with well graphitization. About 30% of the sample remained behind after performing TGA up to 1000 °C as residue and this residue remains mainly due to the presence of catalytic template found in the as-synthesized GCNS. The yield of residue depends on the Si/Ti ratios overs the mesoporous MCM-41 molecular sieves that mean yield of CD varied depends on catalytic templates. The percentage of CD based on wt% of catalytic template was calculated using the following eqn (2):where, P_twl and P_res is the percentage of total weight loss of carbon product and percentage of residue, respectively.

The percentage of GCNS based on the wt% of catalytic template was calculated using the following eqn (3):

where, P_GCNS and P_res is the percentage of total weight loss of GCNS and percentage of residue, respectively.

The percentage of GCNS based on the wt% of CD was calculated using the following eqn (4):

where, P_GCNS and P_twl is the percentage of total weight loss of GCNS and percentage of total weight loss of carbon product, respectively.

In general, the metal particles are the seed for the growth of carbon nanostructures such as CNT, mesoporous carbon and graphitic carbon nanosheets, etc. The yield of GCNS increases with increase in metal content over the support (mesoporous MCM-41 molecular sieves). However, beyond the optimum metal content Ti-MCM-41 (75), the yield of obtained GCNS decreased. The deposition of GCNS depends on Ti concentration and on the surface area of the catalytic templates. The yield of GCNS obtained over the Ti-MCM-41 (75) is higher than GCNS obtained over the Ti-MCM-41 (100), Ti-MCM-41 (50) and Ti-MCM-41 (25). The mesoporous Ti-MCM-41 (100) molecular sieve have high surface area but inadequate amount of metal nanoparticle present over the specific surface area of support and thus obtained yield was low. Ti-MCM-41 (50) and Ti-MCM-41 (25) have adequate amount of metal particles present over the support but the surface area of catalytic template is low so that yield of GCNS is low. The small amount of amorphous carbon increased for GCNS obtained from mesoporous Ti-MCM-41 (25) molecular sieves. This might be due to inadequate amount of specific surface area for growth of longer GCNS with graphitization. The above result reveals that the deposition of GCNS depends on surface area of the catalytic template and concentration of metal nanoparticles over the support. Apart from this, the reaction temperature plays important role in the yield of obtained GCNS. Fig. 4D shows the TG/DTA curves of the as-synthesised GCNS with mesoporous Ti-MCM-41 (75) molecular sieves at different reaction temperatures. The yield of GCNS increases with reaction temperature, beyond 850 °C (optimum temperature) yields of GCNS decreased. The yield at reaction temperatures 850 and 900 °C are almost similar however, the yields are high when compared to that obtained at other reaction temperatures. The low decomposition of carbon precursor and self-pyrolysis of product occurs at low and high reaction temperature, respectively. The amount of amorphous carbon increased slightly for GCNS obtained at 950 °C compared to GCNS obtained at other reaction temperatures. The calculated yield of CD/GCNS depends on the mesoporous Ti-MCM-41 molecular sieves with various Si/Ti ratios and different reaction temperatures are summarized in the Tables 1 and 2, respectively.

Fig. S5† shows the TG/DTA curves of the GCNS resulting from mesoporous Ti-MCM-41 (75) molecular sieves at the reaction temperatures 850 and 900 °C. The measurement was carried out over a temperature ranging from ambient temperature to 1000 °C under air atmosphere. After purification, the TG/DTA curves of GCNS show one major weight loss like the curves of as-synthesised GCNS. The weight loss observed between 550 and 650 °C is due to the finely constructed GCNS. The remaining negligible amount of residue is credited to the yield of charred carbon. The residue is nearing to the zero which confirms the absence of catalytic temple over the deposited carbon material after purification (after acid treatment).³⁶ The DTA shows the maximum endothermic peak around 585 °C for GCNS is obtained at 850 °C. This endothermic peak value is slightly shifted to the lower region of temperature compared to value of as-synthesized GCNS, this might be due to elimination of some carbon nanosheets from the GCNS bunch or relaxation of carbon nanosheets within GCNS during the purification. The resulted thermogram shows that the formed GCNS are well graphitized with high yield and purity, and the graphitization is not affected much even after acid treatment.

3.2.3. Raman spectroscopy analysis. Raman spectroscopy is a one of the most powerful noninvasive tools for the characterization of carbon nanomaterials such as graphene, CNTs and related carbon nanostructured materials. Fig. 5A shows the Raman spectra of the GCNS resulting from mesoporous Ti-MCM-41 molecular sieves with various Si/Ti ratios at the reaction temperature 850 °C. Graphitic carbon usually has two common bands below 2000 cm⁻¹ in their Raman spectrum i.e., D-band and G-band.³⁷ The D-band is related to the defects in the graphitic sheets and the G-band to the vibration of the sp²-hybridized carbon atoms in the graphitic sheets. The peak for the disorder-induced D-band was seen around 1340 cm⁻¹ and those for the G-bands around 1580 cm⁻¹ for all the samples.³⁸ The peak intensities of the D-band and G-band are denoted as I_D and I_G, respectively. The graphitization of carbon nanomaterial was determined by the value of I_D/I_G.^39,40 The I_D/I_G values are around 0.68, 0.64, 0.62 and 0.63 for obtained GCNS from Ti-MCM-41 (25), Ti-MCM-41 (50), Ti-MCM-41 (75) and Ti-MCM-41 (100), respectively. The low intensity ratio of D to G band (I_D < I_G) indicates that the obtained GCNS are well graphitized. The I_D/I_G values decreases with an increase in the Si/Ti ratio (decreasing the Ti concentration) in the mesoporous MCM-41 molecular sieves from 25 to 75 as is obvious from Raman spectra (Fig. 5A). The I_D/I_G values of GCNS obtained from mesoporous Ti-MCM-41 (100) molecular sieves slightly increased, this might be due to trace amount of carbonaceous impurities present over the GCNS. The carbonaceous impurities may be present because of inadequate amount of metal nanoparticles present in specific surface area of the mesoporous MCM-41 molecular sieves. The yield of GCNS is high over the Ti-MCM-41 (75) when compared to other ratios which was confirmed by TGA calculation. The degree of graphitization slightly increases with increase in metal content and beyond the optimum metal content it decreased. Graphitic sheets were affected mildly during the purification depending on high concentration of metal nanoparticles over the mesoporous MCM-41 molecular sieves. Further the mesoporous Ti-MCM-41 (75) molecular sieve was used as an optimized catalytic template and was treated at different reaction temperature for the quality with quantity production of GCNS. Raman spectra of the GCNS obtained over the mesoporous Ti-MCM-41 (75) molecular sieves at different reaction temperatures are shown in Fig. 5B. The intensity ratios of D to G band are 0.69, 0.62, 0.63 and 0.80 for the reaction temperatures 800, 850, 900 and 950 °C, respectively. When the reaction temperature increases from 900 to 950 °C there is an increase in I_D/I_G value revealing the low degree of graphitization of the samples. In general, the value of I_D/I_G is less than 1 which indicates the high graphitic degree of carbons.^41,42 Among all Si/Ti ratio of the catalytic templates and different reaction temperature used in this study (Si/Ti: 25–100; temperatures: 800–950 °C), Ti-MCM-41 (75) produced GCNS are well graphitized at 850 °C which is confirmed by Raman spectrum (the I_D/I_G values is around 0.62). Yield obtained is also high in this case which is revealed by TGA studies also (the yield of GCNS is above 210% depending on catalytic template). Low degree of graphitization is observed at low and high temperature. The low degree of graphitization is due to poor formation of graphene sheets. Poor formation is due to interference of carbonaceous impurities such as amorphous carbons and microcrystalline carbons during the growth of GCNS. The carbonaceous impurity result at high temperature due to high rate of decomposition of carbon precursor. Self-pyrolysis of GCNS also occurs at high temperature that might reduce the degree of graphitization. The data illustrate that the formation of GCNS over the mesoporous Ti-MCM-41 (75) molecular sieves at 850 °C without major carbonaceous impurities such as amorphous carbon and damaged carbon layers.

Fig. 5 Raman spectra of GCNS (A) obtained over the Ti-MCM-41 with various Si/Ti ratios (a) 100, (b) 75, (c) 50, (d) 25 at 850 °C, inset: the plot of I_D/I_G vs. Si/Ti ratio and (B) obtained over the Ti-MCM-41 (75) at (a) 800 (b) 850, (c) 900, (d) 950 °C, inset: the plot of I_D/I_G vs. reaction temperature.

3.2.4. Morphology analysis. The surface morphology of the GCNS was investigated by the SEM technique. SEM images of GCNS obtained from the various Si/Ti ratio of Ti-MCM-41 at the reaction temperature of 850 °C are shown in Fig. S6.† The SEM images indicate that the formed GCNS are well packed flakes like smooth morphology. Among the catalytic templates, mesoporous Ti-MCM-41 (75) molecular sieves produced large amount of GCNS with excellent graphitization. This might be due to adequate amount of active nanoparticles and available active sites for growing the GCNS without any distraction. Beyond increasing the ratio of Si/Ti (75) in mesoporous Ti-MCM-41 molecular sieves, the active nanoparticles are in inadequate amount in specific surface area. The calculated yield of the GCNS was less for both low and high Si/Ti ratios. The large amount of GCNS produced over the mesoporous Ti-MCM-41 (75) molecular sieves was selected as an ideal catalytic template for the further optimization of reaction temperature. Fig. S7† shows the SEM images of GCNS obtained over the Ti-MCM-41 (75) with different reaction temperatures such as 800, 850, 900 and 950 °C. The surface roughness of the GCNS is significantly increased with increasing the growth temperature as shown in Fig. S7(a)–(d).† This might be due to agglomeration of carbon nanosheets at high temperature. The yield of GCNS and related discussions are already mentioned (refer to Sections 3.2.2. and 3.2.3.). The yield of CD/GCNS was estimated using gravimetric calculation (eqn (1)) and investigated by TGA calculation (eqn (2)–(4)) and the values are summarized in the Tables 1 and 2. SEM with EDS images of GCNS resulting from mesoporous Ti-MCM-41 (75) molecular sieves at the reaction temperature 850 and 900 °C are shown in Fig. 6. EDS measurement is used as a quantitative analysis for the presence of the oxygen and metal components over the GCNS. Carbon grosses the predominant part in the GCNS with absence of impurities like catalytic templates, which is clear from EDS spectrum. Absence of catalytic templates and other impurities confirms the obtained GCNS is highly pure. This also ensures that the surface morphology of the GCNS has not been damaged by the acid treatment. The high purity of GCNS obtained at 850 °C is clear from SEM with EDS images. The Raman spectrum also confirmed the high purity of GCNS obtained at 850 °C.

Fig. 6 SEM with corresponding EDS images of GCNS obtained over the Ti-MCM-41 (75) at (a) 850 and (b) 900 °C.

Insight structure morphology of GCNS was revealed by HRTEM analysis. Fig. 7 shows the HRTEM images of GCNS with different magnifications obtained over the mesoporous Ti-MCM-41 (75) molecular sieves with different reaction temperatures from 800 to 950 °C. The degree of graphitization increases with increase in reaction temperatures; beyond the optimum level the degree of graphitization decrease, which is clear from HRTEM images. The HRTEM image of GCNS shows compressed/charred carbon at high temperature (Fig. 7g). The interlayer distance decreases with increase in reaction temperature but beyond the optimum reaction temperature it increased. Lattice fringes of GCNS are much clear for the reaction temperatures 850 and 900 °C compared to other reaction temperatures. If the rate of decomposition of carbon source is equal to the formation of carbon nanosheets, the GCNS obtained are without major defect. As the low decomposition of carbon precursor and self-pyrolysis of product occurs at low and high reaction temperature, respectively, the reaction temperature 850–900 °C is optimum for the formation of GCNS with excellent graphitization. The thickness of the obtained GCNS is ∼30 nm with high yield at the reaction temperature of 850 °C. The graphitization also enhanced which was confirmed by Raman and XRD results. The interlayer distance of GCNS calculated from HRTEM images is around 3.45 Å, close to the result obtained from XRD.^43,44

Fig. 7 TEM images of GCNS obtained over the Ti-MCM-41 (75) at (a and b) 800, (c and d) 850, (e and f) 900 and (g and h) 950 °C.

HRTEM images and selected area of electron diffraction pattern (SAED) of GCNS obtained over the mesoporous Ti-MCM-41 (75) molecular sieves with different reaction temperatures 850 and 900 °C are shown in Fig. 8. HRTEM images and SAED patterns show the formed GCNS have a high degree of graphitization. The synthesized GCNS at 850 °C is better graphitization than obtained at 900 °C, which is clear from HRTEM images with SAED pattern. The SAED pattern of GCNS matches well with XRD pattern of GCNS. Moreover, the metal concentration over the mesoporous MCM-41 molecular sieves and reaction temperature significantly affects the morphology and graphitization of GCNS.

Fig. 8 HRTEM images and corresponding SAED pattern of GCNS obtained over the Ti-MCM-41 (75) at (a) 850 and (b) 900 °C.

3.3. Catalytic activity and mechanism of formation of GCNS

The mesoporous MCM-41 molecular sieves have high thermo-chemical stability and the ease with which their surface can be functionalized.²³ Transition metal can be incorporated into the pore walls of the mesoporous MCM-41 molecular sieves that stabilize dispersed catalytic sites and also exhibit good structural stability. Their uniform and tuneable pore diameters make them well adapted for good catalytic supports. The quantity and quality of the GCNS depends on the reaction parameters such as reaction temperature, flow rate of carbon precursor, concentration of metal nanoparticles over the catalytic support, nature of the catalytic template and so on. Titanium nanoparticles were embedded in the hexagonal structure of mesoporous MCM-41 molecular sieves. The metal nanoparticles being the seed for the growth of GCNS, the much more active titanium nanoparticles were used as the seed. Acetylene was used as the carbon precursor for the formation of GCNS over the Ti-MCM-41 by a simple CVD method. The formation mechanism of GCNS is as follows: while acetylene gas enters at the appropriate temperature, acetylene produces carbon radicals. The active carbon radicals react easily with other carbon radicals and make carbon–carbon linkage because of their high catenation properties. Subsequently, the catenation and Ti-carbide formation might occur at the same time.⁴⁵ Therefore, the formation of graphene layers over the Ti-MCM-41 act as an intermediate and argon gas controls the oxidation of GCNS during the reaction. The graphitic GCNSs were obtained from intermediate treated with the inorganic acids such as HF and HCl. The experiment for the synthesis of GCNS was carried out with mesoporous siliceous MCM-41 molecular sieves (without Ti incorporation), but absence of graphitic carbon nanosheets was observed. Small amount of amorphous carbon only observed over the Si-MCM-41 which confirms the formation of GCNS over the surface of the Ti-MCM-41. The deposition yield of GCNS depends on amount of Ti nanoparticle incorporated over the mesoporous MCM-41 molecular sieves and depends on the reaction temperatures. The quality of GCNS is related to the thickness and fine construction of graphene layers, the reasons are as follows. The thickness and yield of the GCNS depend on the nature of catalytic templates is one of the factors. The concentration of metal nanoparticles over the MCM-41 and reaction temperatures will control the growth of GCNS over the catalytic templates. The interlayer distance of resulted GCNS was decreased with increasing the concentration of Ti and the yield of GCNS were increased with increasing the concentration of Ti. The yield increased means, the number of graphene layer increased within GCNS. So that thickness of the GCNS were increased with increasing the yield of GCNS; thus the inter layer distance also decreased with increasing the yield. The result suggested that the mesoporous Ti-MCM-41 molecular sieves can control the yield and thickness of the GCNS. The graphitization was increased with increasing the metal concentration and the reaction temperatures until the optimum level. The thickness of the GCNS might be depends on nature of the catalytic templates. While increasing the yield, the d-spacing value (interlayer distance) will decrease due to fine construction of graphene sheets within GCNS and because of that graphitization will increase. While increasing the amount of Ti over the MCM-41, the yield (thickness) and graphitization (quality) were also improved until the optimum level. The yield and graphitization of GCNS, and related explanation were discussed elaborately in previous sessions.

4. Conclusions

To the best of our knowledge, this is the first report on the synthesis of GCNS with excellent graphitization over the mesoporous Ti-MCM-41 molecular sieves as the catalytic template and acetylene used as carbon precursor by simple CVD method. Our studies showed that the catalytic activity for the production of GCNS with high yield was mainly attributed to the concentration of metal nanoparticle over mesoporous Ti-MCM-41 molecular sieves and surface area of catalytic templates and also the reaction temperature. The GCNS with quantity and quality was resulted at 850 °C where interlayer distance was around 3.45 Å. The tailored mesoporous MCM-41 molecular sieves assisted CVD method will be an ideal choice for the production of GCNS with high degree of graphitization at reasonable cost.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2014R1A2A1A11052391), the Nano Material Technology Development Program (2012M3A7B4049675), and Priority Research Centers Program of the Ministry of Education (2014R1A6A1031189).

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Footnote

† Electronic supplementary information (ESI) available: Textural properties, XRD pattern, FT-IR spectra, TG/DTA curves and SEM images of the Si-MCM-41 and Ti-MCM-41; TG/DTA curves, SEM and HRTEM images of GCNS. See DOI: 10.1039/c5ra15288g

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