Facile synthesis and photoelectric properties of carbon dots with upconversion fluorescence using arc-synthesized carbon by-products

Abstract

We have demonstrated an easy chemical oxidation approach toward fluorescent carbon dots (CDs) using arc-synthesized carbon by-products. The as-synthesized CDs possess a strong excitation-independent photoluminescence emission located at ca. 502 nm and strong upconversion fluorescence under long wavelength radiation (from 900 to 550 nm). Importantly, the CD-based device has been investigated and exhibits excellent photoresponse under UV irradiation, highlighting the potential for optoelectronic applications.

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Since their discovery from the products of arc-synthesized carbon nanotubes (CNTs),¹ carbon dots (CDs) have attracted growing attention in recent years due to their multiphoton fluorescent, low toxicity, tailored surface modification, and bio-compatibility,^2–4 which has been considered as promising candidates to replace the heavy-metal semiconductor quantum dots in the applications such as solar cells, light emitting devices and bio-imaging.^5–8 Numerous efforts have been made to synthesise CDs, and the synthesis strategies can be classified into two main groups: top-down and bottom-up methods. In the top-down methods including arc discharge,⁹ laser ablation,¹⁰ and electrochemical oxidation,¹¹ the CDs are usually obtained by cutting a micron-sized graphitic structure into small pieces. While the bottom-up methods synthesize the CDs through thermal oxidation,¹² microwave irriadiation,¹³ or hydrothermal treatment¹⁴ of suitable organic precursors. Although the CDs synthesized by the bottom-up methods exhibit easily strong and size-dependent photoluminescence, the structural stability is lower than that of graphitic CDs synthesized by top-down methods due to the poor crystallinity.

As a typical synthesis approach to carbon nanotubes (CNTs), direct current arc discharge has also been used to synthesize nano-structural graphitic carbon,¹⁵ Xu et al.⁹ demonstrated that fluorescent CDs can be obtained by oxidizing arc-synthesized carbon nanoparticles and serve the function of typical chromophores to harvest visible photons for energy conversions. However, the carbon nanoparticles with different sizes are formed simultaneously in the arc discharge process, resulting in a wide size distribution of fluorescent CDs. Herein, we present a chemical oxidation approach toward fluorescent graphitic CDs using arc-synthesized carbon by-products, which are often removed as impurities during the purification of single-walled CNT (SWNT). Combinating the control of particle sizes using centrifugal separation and chemical oxidation, fluorescent graphitic CDs with narrow size-distribution have been successfully synthesized and possess strong upconversion fluorescence under long wavelength radiation (near infrared light (NIR) to visible light). More importantly, fluorescent CD-based prototype device exhibits an excellent photoelectric response under UV irradiations.

The graphitic CDs with narrow size distribution are synthesized by chemical oxidization of the carbon by-products with controlled particle sizes. The synthesis procedure has been shown in Scheme 1 (Experimental details in ESI†). Briefly, the SWNT products¹⁶ were adequately dispersed in aqueous solution of 1 wt% sodium dodecyl sulfate (SDS) under ultrasonic vibrations, followed by centrifugation at 6000 rpm to remove large carbon particles and catalysts. The required carbon particles with controlled size distribution (Fig. S1, ESI†) would be obtained after further centrifugation at 9000 rpm of the stable dispersions. Fluorescent graphitic CDs with narrow size distribution can be synthesized by controlling the temperature and reaction time of strong acid reflux.

Scheme 1 Synthetic procedure of fluorescent graphitic CDs using arc-synthesized carbon by-products.

The morphologies of the as-synthesized CDs were observed by transmission electron microscopy (TEM), as shown in Fig. 1a. One can see clearly that the CDs are well-dispersed spherical nanoparticles with good mono-dispersity. The corresponding size distribution (inset of Fig. 1a) was calculated by measuring several hundred particles, indicating that the CDs have a narrow particle-size distribution in the range of 3.2–8.0 nm with average diameter of 5.6 nm. Fig. 1b shows a typical Raman spectrum of the as-synthesized CDs with excitation wavelength of 532 nm, which has been usually used to estimate the quality of CDs. Two Raman peaks at 1350 cm⁻¹ (D band) and 1570 cm⁻¹ (G band) in Fig. 1b can be seen clearly, corresponding to the vibrations of carbon atoms with dangling bonds in the termination plane of disordered graphite and the vibration of sp²-bonded carbon atoms in a two-dimensional hexagonal lattice.¹⁷ The intensity ratio of the D and G bands (I_D/I_G) is ca. 0.80, suggesting a relatively high degree of graphitization of the as-synthesized CDs.

Fig. 1 (a) A typical TEM image and (b) Raman spectrum of the as-synthesized CDs. Inset is corresponding particle size distribution.

To further characterize the optical properties of the as-synthesized CDs in details, the UV-vis absorption spectrum and PL spectrum were recorded accordingly, as shown in Fig. 2a. The CDs give an apparent absorption at ca. 375 nm, which is assigned to the presence of aromatic π orbitals of larger CDs.¹⁸ A strong PL emission peak centred at ca. 502 nm can be observed when excited at 360 nm. At an excitation wavelength of 350 nm, the quantum yield of the as-synthesized CDs in aqueous solution was measured to be 3.31% using quinine sulphate as a standard, which is comparable to previous reports.^19,20 One can see clearly from Fig. 2a inset that the as-synthesized CDs give excellent aqueous dispersibility and exhibit distinct green luminescence under UV irradiation (365 nm). The detailed PL measurements were also performed with different excitation wavelengths increasing from 300 nm to 380 nm in 20 nm increments. Unlike most other fluorescent CDs,² the as-synthesized CDs in this work exhibit an excitation-independent PL behaviour. When the excitation wavelength changes from 300 to 380 nm, the corresponding PL spectra remain almost invariable and show a strong peak at ca. 502 nm (Fig. 2b). Since the excitation-dependent PL behaviour is attributed to different energy levels associated with different “surface states”, which are formed by different functional groups,²¹ this special feature may result from a relatively uniform CD surface with few kinds of functional groups.

Fig. 2 (a) UV-vis absorption (dark) and PL spectrum (green). Inset: optical photograph of the CD aqueous solution under visible (left) and UV light (right, 365 nm). (b) PL spectra with different excitation wavelengths. (c) UV-vis absorption before and after NaBH4 reduction. (d) PL spectra of the reduced CDs with different excitation wavelengths.

To investigate the effect of functional groups on the optical properties of the CDs, NaBH₄ was used to reduce the as-synthesized CDs at room temperature. The chemical structures of CDs before and after reduction were characterized using Fourier transform infrared (FTIR) spectroscopy (Fig. S2, ESI†). After NaBH₄ reduction, the absence of a small peak at 1429 cm⁻¹ (C–OH deformation vibration)²² and the formation of a new peak at 1093 cm⁻¹ (C–O–C stretching vibration)²³ are observed, suggesting that hydroxyl (C–OH) groups were probably partly reduced to ether (C–O–C) groups during reduction.

Fig. 2c shows UV-vis absorption spectra of the CDs before and after NaBH₄ reduction. One can see clearly that the absorption peak of reduced CDs shifts from ca. 375 to ca. 270 nm, which is attributed to the change of functional groups on the surfaces. The PL spectra of the reduced CDs with different excitation wavelengths are shown in Fig. 2d. Compared with those of pristine CDs, a significant blue-shift of PL peaks can be seen clearly after NaBH₄ reduction, corresponding to the blue-shift in UV-vis absorption spectra. More importantly, the PL emission peak shifts from 428 to 468 nm when the excitation wavelength varies from 300 to 380 nm, exhibiting an excitation-dependent PL behaviour, which is different from those of pristine CDs. Since the CD sizes have no obvious change during the reduction,²² the functional groups on CD surface are obviously changed, resulting in a significant change of the surface states, which has been confirmed by FTIR spectra.

More importantly, the upconverted PL properties of the as-synthesized CDs have also been investigated with excitation at long-wavelength, as shown in Fig. 3a. Remarkably, the as-synthesized CDs exhibit good upconversion fluorescent properties, and give the upconverted PL emissions located at ca. 502 nm when excited by long-wavelength light (from 550 to 900 nm), which are the same as the downconversion PL peaks. Most interestingly, similar to the downconversion PL spectra, an excitation-independent PL feature has also been observed in the upconversion PL spectra. After NaBH₄ reduction, the reduced CDs also exhibit an excitation-dependent upconverted PL behaviour (Fig. 3b), and the PL emission peak shifts from 431 to 460 nm when the excitation wavelength varies from 550 to 750 nm. The upconverted PL properties of CDs should be attributed to the multiphoton active process similar to that of previously reported CDs.^3,17 The above results indicated that CDs may be employed as photovoltaic materials with broadband absorption for solar cell and light emitting devices.

Fig. 3 Upconversion fluorescent properties of the as-synthesized CDs before (a) and after (b) NaBH₄ reduction.

To explore the photoelectric properties of the CDs, we fabricated a prototype photodetector device by drop casting the CDs dispersed in deionized water onto between two Au electrodes on SiO₂ substrate with a 10 μm spacing. The typical current–voltage (I–V) curves of the device without and with UV (365 nm) irradiation are shown in Fig. 4. One can see that both I–V curves exhibit a nonlinear behaviour. Compared with the dark current, the current is dramatically increased by over 60% when UV light is turned on, revealing the excellent photoresponse of the as-synthesized CDs. This photoelectric property further demonstrates that the CDs can be considered as promising candidates for new optoelectronic devices.

Fig. 4 Typical I–V curves of the CD-based devices without (black) and with (red) UV irradiation. Inset is a schematic of the prototype photodetector device.

In conclusion, fluorescent graphitic CDs have been synthesized by chemical oxidation approach using arc-synthesized carbon by-products. The as-synthesized CDs possess a strong excitation-independent PL emission located at ca. 502 nm and strong upconversion fluorescence under long wavelength radiation (from 900 to 550 nm). Meanwhile, the reduced CDs exhibit significant blue-shifts of UV absorption and PL emissions due to the changes in the functional groups on CD surface. More importantly, fluorescent CD-based prototype device has also been investigated and exhibits an excellent photoelectric response under UV irradiation. We believe that our findings would highlight the potential application of the CDs for new optoelectronic devices.

Acknowledgements

This work was supported by National High-Tech R&D Program of China (no. 2011AA050504), National Natural Science Foundation of China (no. 61376003), Shanghai Natural Science Foundation (no. 13ZR1456600), Shanghai Science and Technology Grant (no. 12NM0503800). We also thank Instrumental Analysis Center of SJTU for the analysis supports.

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Footnote

† Electronic supplementary information (ESI) available: Detailed experiment procedure, SEM image of the acquired carbon particles and FTIR spectra of the CDs before and after NaBH₄ reduction. See DOI: 10.1039/c3ra45453c

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