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Large-scale solvothermal synthesis of fluorescent carbon nanoparticles
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 395601 (http://iopscience.iop.org/0957-4484/25/39/395601) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 74.217.196.6 This content was downloaded on 30/09/2014 at 02:28
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 395601 (9pp)
doi:10.1088/0957-4484/25/39/395601
Large-scale solvothermal synthesis of fluorescent carbon nanoparticles Kahoe Ku1, Seung-Wook Lee2, Jinwoo Park1, Nayon Kim1, Haegeun Chung3, Chi-Hwan Han4 and Woong Kim1,2 1
Department of Materials Science and Engineering, Korea University, Seoul 136-713, Korea Department of Nano-Semiconductor Engineering, Korea University, Seoul 136-713, Korea 3 Department of Environmental Engineering, Konkuk University, Seoul 143-701, Korea 4 Photovoltaic Research Center, Korea Institute of Energy Research, Daejeon 305-343, Korea 2
E-mail: [email protected] Received 7 May 2014, revised 16 July 2014 Accepted for publication 4 August 2014 Published 11 September 2014 Abstract
The large-scale production of high-quality carbon nanomaterials is highly desirable for a variety of applications. We demonstrate a novel synthetic route to the production of fluorescent carbon nanoparticles (CNPs) in large quantities via a single-step reaction. The simple heating of a mixture of benzaldehyde, ethanol and graphite oxide (GO) with residual sulfuric acid in an autoclave produced 7 g of CNPs with a quantum yield of 20%. The CNPs can be dispersed in various organic solvents; hence, they are easily incorporated into polymer composites in forms such as nanofibers and thin films. Additionally, we observed that the GO present during the CNP synthesis was reduced. The reduced GO (RGO) was sufficiently conductive (σ ≈ 282 S m−1) such that it could be used as an electrode material in a supercapacitor; in addition, it can provide excellent capacitive behavior and high-rate capability. This work will contribute greatly to the development of efficient synthetic routes to diverse carbon nanomaterials, including CNPs and RGO, that are suitable for a wide range of applications. S Online supplementary data available from stacks.iop.org/NANO/25/395601/mmedia Keywords: carbon nanoparticles, fluorescence, composites, nanofibers 1. Introduction
graphite [5], chemical oxidation of arc-discharge carbon nanotubes or candle soot [6, 7], electrochemical preparation from graphite or carbon nanotubes [8, 9], thermal decomposition or pyrolysis of various molecules [10–12], chemical synthesis based on halohydrocarbon precursors [14–16] and the chemical breakdown of larger carbon materials such as graphite [13]. Recently, several synthetic routes to the production of CNPs with high QY involving hydrothermal reactions, pyrolysis and microwave synthesis have been reported [3, 17–19]. However, reports on the synthesis of CNPs with high QY at gram-scale production levels are rare. More recently, a simple heating method and a microwaveassisted technique were developed that produce CNPs in neargram-scale quantities [1, 20]. Therefore, there is still a need for synthetic strategies that produce high-QY CNPs in large amounts. In this work, we report a novel synthetic route for the production of fluorescent CNPs in large quantities. A simple
Photoluminescent carbon nanoparticles (CNPs) are promising novel carbon-based nanomaterials suitable for biomedical, photocatalytic and light-emitting-diode applications [1–3]. Since CNPs are mostly composed of carbon, they are fundamentally different from semiconductor nanoparticles that contain toxic heavy metals such as Cd, Hg and Pb. Given their reduced environmental impact, CNPs have great potential in practical applications for which toxic semiconductor nanoparticles are not suitable [4]. However, both the production yield and the quantum yield (QY) of the CNPs need to be significantly improved before they can attain viability in various applications. Most of the synthetic methods reported to date produce CNPs with relatively low QY (1–10%) or in very small quantities (98.5%) in the benzaldehyde was incorporated into the CNPs, which corresponds to the highest carbon conversion efficiency reported so far [6, 7, 12, 23, 24]. Additionally, the GO was reduced at the same time. The RGO product was around 0.26 g and showed decent conductivity (∼282 S m−1) and a specific surface area (∼38.5 m2 g−1). XPS analysis indicated that the CNPs were composed mostly of carbon, and the GO can be reduced to RGO. The atomic C:O ratio of the CNPs estimated from the XPS is approximately 97:3 (figure 2(a)); this is consistent with the energy-dispersive x-ray spectroscopy results, which determined that C:O = 93:7. More specifically, the carbon that was 1 s peak of the CNP material was located at 285 eV, which indicated that the C atoms present were mostly bonded to other C atoms (figure 2(b)). The peak fitting result indicated that the ratio of the sp2 to sp3 carbon was 9:1. On the other hand, the XPS data also showed that the GO was reduced during the solvothermal CNP synthesis. The GO materials showed the strongest C–C carbon peak at 285 eV, as well as
2.6. Preparation of the RGO supercapacitors
The RGO (30 mg) was dispersed in propylene carbonate (PC) (30 ml) and ultrasonicated for ∼90 min. Part of the solution was vacuum-filtered on carbon paper (Toray TGP-H-090). The RGO layer (mass = 0.41 mg) that formed on the paper (size = 1 cm × 1 cm) was dried in an oven at 80 °C for 2 h. A PTFE membrane was used as a separator between the two RGO/carbon-paper electrodes. The electrode/separator/electrode assembly was wrapped with Parafilm and placed in a Teflon frame containing an electrolyte solution. 1 M tetraethylammonium tetrafluoroborate (TEABF4, 99.0%, SigmaAldrich) in a PC solution was used as the electrolyte. The cell was placed in a glass container and tested under a vacuum. 3
Nanotechnology 25 (2014) 395601
K Ku et al
Figure 2. (a) XPS spectrum of the CNP, and the C1s spectra of the (b) CNPs, (c) GO, and (d) RGO.
strong peaks in the range of 286–289 eV, indicating that a significant amount of carbon was incorporated in alcohol, epoxide, aldehyde, ketone and/or carboxylic acid moieties (figure 2(c)) [21, 25]. When the GO was reduced to RGO, the carbonyl peak (287.4 eV) and the carboxyl peak (289.1 eV) nearly disappeared, and the intensity of the hydroxyl peak (286.3 eV) was significantly reduced (figure 2(d)). Consistently, the atomic ratio of carbon to oxygen that was calculated from XPS increased from 1.4 to 5.7 upon reduction. In addition, the red-shift of the G-peak in the Raman spectra upon reduction consistently indicated the conversion of GO to RGO (figure S3 in the SI) [26]. The presence of the 2D peak of the RGO implies that the RGO has a graphitic structure. The disappearance or reduction of the functional-group peaks in the FT-IR spectrum implies a successful reduction as well (figure S4 in the SI). We can attribute the reduction of the GO to ethanol; it has been demonstrated that GO can be converted to RGO in the presence of ethanol [27–30]. Generally, the CNPs were
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My IOPscience
Large-scale solvothermal synthesis of fluorescent carbon nanoparticles
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 395601 (http://iopscience.iop.org/0957-4484/25/39/395601) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 74.217.196.6 This content was downloaded on 30/09/2014 at 02:28
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 395601 (9pp)
doi:10.1088/0957-4484/25/39/395601
Large-scale solvothermal synthesis of fluorescent carbon nanoparticles Kahoe Ku1, Seung-Wook Lee2, Jinwoo Park1, Nayon Kim1, Haegeun Chung3, Chi-Hwan Han4 and Woong Kim1,2 1
Department of Materials Science and Engineering, Korea University, Seoul 136-713, Korea Department of Nano-Semiconductor Engineering, Korea University, Seoul 136-713, Korea 3 Department of Environmental Engineering, Konkuk University, Seoul 143-701, Korea 4 Photovoltaic Research Center, Korea Institute of Energy Research, Daejeon 305-343, Korea 2
E-mail: [email protected] Received 7 May 2014, revised 16 July 2014 Accepted for publication 4 August 2014 Published 11 September 2014 Abstract
The large-scale production of high-quality carbon nanomaterials is highly desirable for a variety of applications. We demonstrate a novel synthetic route to the production of fluorescent carbon nanoparticles (CNPs) in large quantities via a single-step reaction. The simple heating of a mixture of benzaldehyde, ethanol and graphite oxide (GO) with residual sulfuric acid in an autoclave produced 7 g of CNPs with a quantum yield of 20%. The CNPs can be dispersed in various organic solvents; hence, they are easily incorporated into polymer composites in forms such as nanofibers and thin films. Additionally, we observed that the GO present during the CNP synthesis was reduced. The reduced GO (RGO) was sufficiently conductive (σ ≈ 282 S m−1) such that it could be used as an electrode material in a supercapacitor; in addition, it can provide excellent capacitive behavior and high-rate capability. This work will contribute greatly to the development of efficient synthetic routes to diverse carbon nanomaterials, including CNPs and RGO, that are suitable for a wide range of applications. S Online supplementary data available from stacks.iop.org/NANO/25/395601/mmedia Keywords: carbon nanoparticles, fluorescence, composites, nanofibers 1. Introduction
graphite [5], chemical oxidation of arc-discharge carbon nanotubes or candle soot [6, 7], electrochemical preparation from graphite or carbon nanotubes [8, 9], thermal decomposition or pyrolysis of various molecules [10–12], chemical synthesis based on halohydrocarbon precursors [14–16] and the chemical breakdown of larger carbon materials such as graphite [13]. Recently, several synthetic routes to the production of CNPs with high QY involving hydrothermal reactions, pyrolysis and microwave synthesis have been reported [3, 17–19]. However, reports on the synthesis of CNPs with high QY at gram-scale production levels are rare. More recently, a simple heating method and a microwaveassisted technique were developed that produce CNPs in neargram-scale quantities [1, 20]. Therefore, there is still a need for synthetic strategies that produce high-QY CNPs in large amounts. In this work, we report a novel synthetic route for the production of fluorescent CNPs in large quantities. A simple
Photoluminescent carbon nanoparticles (CNPs) are promising novel carbon-based nanomaterials suitable for biomedical, photocatalytic and light-emitting-diode applications [1–3]. Since CNPs are mostly composed of carbon, they are fundamentally different from semiconductor nanoparticles that contain toxic heavy metals such as Cd, Hg and Pb. Given their reduced environmental impact, CNPs have great potential in practical applications for which toxic semiconductor nanoparticles are not suitable [4]. However, both the production yield and the quantum yield (QY) of the CNPs need to be significantly improved before they can attain viability in various applications. Most of the synthetic methods reported to date produce CNPs with relatively low QY (1–10%) or in very small quantities (98.5%) in the benzaldehyde was incorporated into the CNPs, which corresponds to the highest carbon conversion efficiency reported so far [6, 7, 12, 23, 24]. Additionally, the GO was reduced at the same time. The RGO product was around 0.26 g and showed decent conductivity (∼282 S m−1) and a specific surface area (∼38.5 m2 g−1). XPS analysis indicated that the CNPs were composed mostly of carbon, and the GO can be reduced to RGO. The atomic C:O ratio of the CNPs estimated from the XPS is approximately 97:3 (figure 2(a)); this is consistent with the energy-dispersive x-ray spectroscopy results, which determined that C:O = 93:7. More specifically, the carbon that was 1 s peak of the CNP material was located at 285 eV, which indicated that the C atoms present were mostly bonded to other C atoms (figure 2(b)). The peak fitting result indicated that the ratio of the sp2 to sp3 carbon was 9:1. On the other hand, the XPS data also showed that the GO was reduced during the solvothermal CNP synthesis. The GO materials showed the strongest C–C carbon peak at 285 eV, as well as
2.6. Preparation of the RGO supercapacitors
The RGO (30 mg) was dispersed in propylene carbonate (PC) (30 ml) and ultrasonicated for ∼90 min. Part of the solution was vacuum-filtered on carbon paper (Toray TGP-H-090). The RGO layer (mass = 0.41 mg) that formed on the paper (size = 1 cm × 1 cm) was dried in an oven at 80 °C for 2 h. A PTFE membrane was used as a separator between the two RGO/carbon-paper electrodes. The electrode/separator/electrode assembly was wrapped with Parafilm and placed in a Teflon frame containing an electrolyte solution. 1 M tetraethylammonium tetrafluoroborate (TEABF4, 99.0%, SigmaAldrich) in a PC solution was used as the electrolyte. The cell was placed in a glass container and tested under a vacuum. 3
Nanotechnology 25 (2014) 395601
K Ku et al
Figure 2. (a) XPS spectrum of the CNP, and the C1s spectra of the (b) CNPs, (c) GO, and (d) RGO.
strong peaks in the range of 286–289 eV, indicating that a significant amount of carbon was incorporated in alcohol, epoxide, aldehyde, ketone and/or carboxylic acid moieties (figure 2(c)) [21, 25]. When the GO was reduced to RGO, the carbonyl peak (287.4 eV) and the carboxyl peak (289.1 eV) nearly disappeared, and the intensity of the hydroxyl peak (286.3 eV) was significantly reduced (figure 2(d)). Consistently, the atomic ratio of carbon to oxygen that was calculated from XPS increased from 1.4 to 5.7 upon reduction. In addition, the red-shift of the G-peak in the Raman spectra upon reduction consistently indicated the conversion of GO to RGO (figure S3 in the SI) [26]. The presence of the 2D peak of the RGO implies that the RGO has a graphitic structure. The disappearance or reduction of the functional-group peaks in the FT-IR spectrum implies a successful reduction as well (figure S4 in the SI). We can attribute the reduction of the GO to ethanol; it has been demonstrated that GO can be converted to RGO in the presence of ethanol [27–30]. Generally, the CNPs were
