Morphology-controllable synthesis of carbon nanomaterials directly on Al₂O₃ substrates, and their photoluminescence

Abstract

By controlling the growth temperature, different categories of carbon nanomaterials (CNMs) such as carbon nanocoils (CNCs) and chain-like carbon nanospheres (CNSs) can be synthesized directly over Al₂O₃ substrate without using any transition-metal catalysts. It is proposed that the Al₂O₃ particles play a key role in the CNM growth, and the reaction temperature has a great impact on the morphology of the obtained CNMs. Furthermore, the photoluminescence studies indicate that the obtained CNCs and chain-like CNSs show different optical properties, which suggest that the optical properties of the obtained CNMs may be tuned by controlling their structures.

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1. Introduction

Owing to their peculiar properties and potential applications such as field emission displays, hydrogen storage, micro-sensors and electromagnetic wave absorbers, different categories of carbon nanomaterials (CNMs) such as carbon nanotubes (CNTs), carbon nanocoils (CNCs), and carbon nanodots (CNDs) have gained much attention in the past decade.^1–6 In order to study their properties and realize these applications, it is necessary to produce CNMs in high selectivity.^7–9 Besides the solution reaction process,^10,11 as we all know that the techniques for the production of CNMs can be classified roughly into three main categories: (i) laser ablation, (ii) arc discharge, and (iii) chemical vapor deposition.^12,13 Common to these methods is the use of transition-metal catalysts, typically iron-group elements (Fe, Co and Ni) and their alloys. Because of the limitation of synthesis route and the understanding about catalyst, the growth of CNMs directly over substrate is thought to be impossible all the time, which is the major obstacle for the development of CNM-based nanoelectronics.¹² Therefore, the role of catalyst and the growth of CNMs have attracted much attention in the past years, and many other metal and semiconductors nanoparticles such as Au, Ge and SiC, which were regarded as inactive catalysts for the growth of CNMs in the past, have been proved to be active for CNM growth.^14–16 These findings challenge the traditional thinking about the growth of CNMs, and the increasing attention is being paid toward exploring new catalysts for the controllable and efficient growth of CNMs.

Recent experimental studies show that single-walled carbon nanotubes (SWCNTs) could growth directly over Si/SiO₂ substrate without using any transition-metal catalysts.^17–19 Enlightened by these ideas and based on our previous work,^20,21 in this study, we present a simple and efficient method for realizing the selective growth of CNMs in high selectivity directly over Al₂O₃ substrate.

2. Experimental

2.1 Synthesis of carbon nanomaterials

In a typical procedure, alumina (99.6% Al₂O₃, purchased from Heifei Ke Jing Materials Technology Co., Ltd) substrate was commercially available and analytically pure. After washed with ethanol and distilled water, Al₂O₃ substrate (1 cm² × 1 cm²) was dispersed on a ceramic plate which was placed inside a quartz tube. With argon (99.999% Ar, purchased from Shen Jiang Gas Company) flowing through (flow rate = 0.03 L min⁻¹) the reaction tube, the temperature of furnace was raised from room temperature (RT) to a designed temperature. Then Ar was cut off and acetylene (dissolved C₂H₂, 99.999%, purchased from Shen Jiang Gas Company) was introduced into (flow rate = 0.03 L min⁻¹) the tube at 450, 600 and 650 °C for 1 h, respectively. After cooling to RT in Ar naturally, black samples could be observed clearly over the Al₂O₃ substrate. For easy description, the products generated at the decomposition temperature of 450, 600 and 650 °C are denoted hereinafter as C-450, C-600 and C-650, respectively.

2.2 Characterization of products

The samples were examined on an X-ray powder diffractometer (XRD) at RT for phase identification using CuKα radiation (model D/Max-RA, Rigaku). The morphologies of the samples were examined using a transmission electron microscope (model JEM-2000EX, operated at an accelerating voltage of 200 kV), and a field emission scanning electron microscope (FE-SEM) (model FEI Sirion 200, operated at accelerating voltages of 5 kV). Raman spectroscopic investigations were performed using a Jobin-Yvon Labram HR800 instrument with 532 nm laser excitation. The photoluminescence (PL) spectra of the obtained sample were obtained at RT by a spectrofluorophotometer using Xe lamp as the light source.

3. Results and discussion

3.1 Phase and microstructures of the substrate and samples

In order to understand the growth of CNMs over Al₂O₃ substrate, Fig. 1 gives the phase and microstructure of the raw substrate. As shown in Fig. 1a, one can find that all the diffraction peaks can be indexed as the (012), (104), (110), (006), (113), (024), (116), (018), (214), (300) and (1010) crystal faces of rhomb centred hexagonal phase Al₂O₃, which is consistent with the reported data (JCPDS file no. 82-1467). No other peaks can be observed, indicating that the raw substrate is Al₂O₃ substrate. Fig. 1b shows the FE-SEM image of the raw Al₂O₃ substrate. Different shapes and sizes of cavities can be observed frequently and clearly on the surface. The enlarged FE-SEM observation indicates some Al₂O₃ particles with different sizes (80–450 nm) appeared around these cavities.

Fig. 1 (a) XRD pattern, and (b) FE-SEM images of the raw Al₂O₃ substrate.

For comparison, the microstructures of the annealed Al₂O₃ substrate and the obtained C-450 are shown in Fig. 2. As shown in Fig. 2a, many cavities can be seen obviously and the whole morphology of the substrate is almost unchanged. However, compared to the raw Al₂O₃ substrate, the high resolution FE-SEM investigation (as shown in Fig. 2b) indicates that much more Al₂O₃ particles with different sizes can be observed around these cavities. In other words, the anneal process can make the formation of much more Al₂O₃ particles around the cavity. Fig. 2c and d give the FE-SEM investigation of the obtained C-450. As shown in Fig. 2c, large-scale of CNCs can be seen obviously around the cavities. The result indicates that the growth of CNMs should come from the area of Al₂O₃ particles, which is consistent with the growth of SWCNTs reported by Cheng and Huang et al., respectively.^17,19 Fig. 2d shows the high resolution FE-SEM image of the obtained C-450. The closer investigation reveals that the diameters of the obtained CNCs are very uniform. The average diameter of CNCs is ca. 25 nm, which is much smaller than those of CNCs reported before.^22–24 In order to obtain high quality morphology images of C-450, the obtained sample was dispersed in ethanol and dropped on the copper screen for FE-SEM and TEM observations. As shown in Fig. 3a and b, large scale and size uniform CNCs can be observed clearly. Moreover, as shown in Fig. 3c and d, if acetylene was decomposed at 500 °C, the TEM and FESEM observations indicated that high selectivity of CNCs and carbon microcoils (CNMs) could be found in the obtained sample (C-500).²⁵

Fig. 2 FE-SEM images of (a and b) the annealed Al₂O₃ substrate, and (c and d) the as-synthesized C-450.

Fig. 3 (a) FE-SEM and (b) TEM images of C-450; (c) FE-SEM and (d) TEM images C-500.

3.2 Effect of temperature and possible formation mechanism of CNMs

In order to investigate the effect of decomposition temperature, sample was synthesized at 600 °C (C-600). As shown in Fig. 4a, the obtained C-600 is mainly made up of CNCs with different sizes and chain-like spheroidal carbon particles. The closer investigation (as shown in Fig. 4b) shows that the morphology of some CNMs is very difficult to define as CNCs or chain-like CNSs. Based on the FE-SEM investigation of C-450 and C-600, we think that the morphology of CNMs involves the conversion from CNCs to chain-like CNSs with the increasing of decomposition temperature, and C-600 should be the interim of the transformation process. To prove the idea further, C-650 was obtained through the decomposition of acetylene at 650 °C. Fig. 4c shows the FE-SEM image of the obtained sample at the case, which reveals the obtained C-650 is chain-like spheroidal carbon particle, and the diameter ranges from 400 nm to 1 μm. According to the classification given by Serp et al.,²⁶ the obtained carbon material in the case is called chain-like CNSs. From the closer FE-SEM image (as shown in Fig. 4d), it can be seen that no CNCs are observed at the case and the obtained chain-like CNSs show a large diameter distribution. In general, all the results show that CNMs can grow directly over the Al₂O₃ substrate and the morphology of CNMs can be tunable by controlling the decomposition temperature. Moreover, because no metal precursors were used over the Al₂O₃ substrate, the experiment here is much more simple and different from that reported by Liu et al.²⁷

Fig. 4 FE-SEM images of the obtained (a and b) C-600, and (c and d) C-650.

In comparison with the obtained results, we find that much more Al₂O₃ particles (as shown in Fig. 2b and 4b and d) have formed on the substrate with the temperature increasing from 450 to 650 °C. Therefore, much larger scale of CNMs should be produced over the Al₂O₃ substrate, which is consistent with the obtained results (as shown in Fig. 2c and 4a and c). Generally, all the obtained results show that different morphologies of CNMs can be synthesized directly over Al₂O₃ substrate. The growth of CNMs was proved by the fact that Al₂O₃ particles can serve catalytically as nucleation points, which was consistent with those of oxides such as SiO₂, TiO₂ for the SWCNT growth.^18,19 In order to confirm the point further, Al₂O₃ nanoparticles on Si substrate (Al₂O₃/Si) were prepared by the method reported elsewhere,¹⁹ and the decomposition of acetylene was conducted over Al₂O₃/Si at 500 °C. As shown in Fig. 5a and b, besides the observation of carbon nanofibers (CNFs) occasional, different sizes of CNCs/CNMs were seen in large-scale. Moreover, if the flat Al₂O₃ substrate was used to grow CNMs at 650 °C, as shown in Fig. 5c and d, few of chain-like CNSs could be observed on the substrate, which may be caused by the thermal decomposition of C₂H₂ at the high temperature or the existence of few Al₂O₃ nanoparticles. Generally, combined with the aforementioned results, one can find that Al₂O₃ nanoparticles are very crucial for the growth of CNMs.

Fig. 5 FE-SEM images of the samples obtained over: (a and b) the Al₂O₃/Si substrate, and (c and d) the flat Al₂O₃ substrate.

As reported before, some materials such as SiC, Al₂O₃, TiO₂ and SiO₂ were proved experimentally to be active for CNM growth.^28,29 Although, unlike metal catalyst, Al₂O₃ dose not have a catalytic function to decompose hydrocarbon molecules. The previous density functional theory calculations indicated that oxygen atoms in some oxides can improve the capture of –CH_x.³⁰ Based on these previous results, it is expected that the Al₂O₃ nanoparticles can capture the molecules of –CH_x and give the formation of carbon atoms. The carbon atoms generated on the catalyst surface differ in mobility and assemble into hexagonal, pentagonal, and heptagonal rings.³¹ As shown in Fig. 6, a flat graphite layer is made up of hexagonal carbon rings. The introduction of pentagon and heptagon into the hexagonal carbon rings can make the curvature of the flat graphite flake. Moreover, it has been proved that the temperature has a great impact on the formation and creation rates of pentagonal or/and heptagonal carbon rings, and the formation of CNCs or CNSs can be attributed to the existence and creation rate of pentagonal–heptagonal carbon rings in the CNTs (as shown in Fig. 6).^32–34 Therefore, in our study, it is thought that the adsorption of –CH_x is a first step toward the growth of CNMs, and the pyrolysis temperature is crucial for the structure of CNMs. Although the exact formation mechanism for CNMs cannot be given, the selective growth of CNCs and chain-like CNSs directly over Al₂O₃ substrate is not difficult to be understood in our study. More importantly, the method for the morphology-controllable synthesis of CNMs directly on Al₂O₃ substrates was not reported before, which may expand the utilization of CNMs in nanoelectronic effectively.

Fig. 6 (a–c) The hexagonal, pentagonal and heptagonal carbon rings used to assemble the graphitic flakes; (d) formation of CNCs;³⁴ and (e) schematic structural model of carbon spheres proposed by Kroto and Mckay.³⁵

3.3 Optical and Raman spectrum analysis

Fig. 7 shows the Raman spectra of the obtained samples. All the Raman spectra exhibit two peaks of the D and G band. The D band in the Raman spectrum is associated with structural defect and partially disordered structures of sp² domains, while G band is an intrinsic property of the two-dimensional graphene lattice.^36,37 Generally, the intensity ratio of D band to G band (I_D/I_G) is used to estimate the disorder of graphene.³⁸ One can find that C-650 show higher I_D/I_G value than C-600 and C-450, which indicates higher crystallinity of C-450 and C-600.

Fig. 7 Raman spectra of the obtained C-450, C-600 and C-650.

In order to explore the optical properties of the obtained samples, PL study was performed. Shown in Fig. 8a are the PL spectra of C-450, C-600 and C-650 with an excitation wavelength of 280 nm. It can be seen that there are two weak emission peaks for the obtained C-450, one is in the UV region at ca. 362 nm and another is in blue region at ca. 465 nm, which is similar to the results of CNCs reported previously.^39,40 As reported previously by Shukla et al.,⁴¹ the strong emissions in the UV region may be related to the effects of quantum confinement, and the blue emission may result from the recombination of electron–hole pairs, which originates from various structure-related defects.^42,43 Moreover, compared with C-450, two interesting phenomena can be observed clearly over the obtained C-600 and C-650: (i) the PL enhancement in both of the two peaks, and (ii) the appearance of new peak in the violet region at ca. 416 nm, which was not observed over other CNMs such as CNCs, CNTs or CNFs previously.^38–44 The observation of PL may imply the presence of a band gap in the electronic structure of some CNCs and CNSs in the obtained samples.⁴¹ In comparison with the aforementioned results of the obtained samples, one can suggest that the peak enhancement and the appearance of new peak in the violet region should be related to the change of CNM morphology or structure. However, because of the complexity of CNM structure, further theoretical simulation is needed to explain the obtained experimental results clearly. Fig. 8b–d gives the PL spectra of the obtained C-450, C-600 and C-650 with the different excitation wavelength. Common to these samples is the intensity of UV PL peak decreases rapidly with the excitation wavelength increasing from 220 to 300 nm, and the obtained samples exhibit an excitation-dependent PL behavior, which is consistent with carbon materials reported previously.⁴⁵ Interestingly, a clearly red shift of the UV PL peak can be observed over the obtained C-450 and C-600, while the UV PL peak of C-650 exhibits a obvious blue shift. In general, the obtained results suggest that the optical properties of the obtained CNMs may be tuned by controlling their structure, which may favor CNMs can be used in nanophotonics effectively.

Fig. 8 PL spectra of C-450, C-600 and C-650 with excitation wavelength (a) 280 nm, and (b–d) from 220 to 300 nm.

4. Conclusions

In summary, without using any transition-metal catalysts, we have demonstrated that CNCs could be synthesized directly over Al₂O₃ substrate through the decomposition of acetylene at 450 °C. By controlling the reaction temperature, the morphology transformation of the obtained CNMs can be observed clearly. It is proposed that the Al₂O₃ particles play a key role in the CNM growth, and the Al₂O₃ particles show different catalytic activity at different growth temperature. Furthermore, the PL studies indicate that the obtained CNCs and chain-like CNSs show different optical properties, which suggest that the optical properties of the obtained CNMs may be tuned by controlling their structure.

Acknowledgements

This work was supported by the International Cooperation Project of Guizhou Province (2012-7002), the National Science Foundation of Guizhou province (2014-2059), the Postdoctoral Science Foundation of China (2015M570427), the National Science Foundation of China (Grant No. 11364005 and 11174132), and the Foundation of the National Key Project for Basic Research (2012CB932304 and 2011CB922102) for financial support.

Notes and references

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