2D materials due to their unique physical and chemical properties assume significance in a wide variety of applications. Among the various strategies employed for the synthesis of 2D materials, liquid phase exfoliation from layered crystalline precursors (bottom-up route) is considered to be beneficial. Nevertheless, use of aggressive chemicals and formation of fragmented or chemically modified nanosheets limit the applicability of this methodology. Researchers at University College London, Imperial College London, University of Bristol, United Kingdom and École Polytechnique Fédérale de Lausanne, Switzerland have demonstrated a liquid phase dissolution route for the synthesis of 2D carbon nitride (CN) nanosheets using poly(triazine imide)-lithium bromide (PTI-LiBr) as the crystalline precursor and aprotic polar solvents as the liquid phase. The spontaneous dissolution of PTI-LiBr in organic solvents yield solutions containing defect-free, crystalline, 2D CN nanosheets.
Dicyandiamide (DCDA), lithium bromide (LiBr) and potassium bromide (KBr) were used as the starting materials. 2 g of DCDA was mixed with 10 g of the LiBr/KBr (52%:48%) and thoroughly ground. 7 g of the ground homogeneous powder was heated to 400 °C under flowing N2 and soaked at 400 °C for 6 h. 4 g of this pretreated mixture was placed inside a quartz tube sealed at one end. The quartz tube was evacuated to < 10-6 mbar and sealed. The quartz ampoule was heated to 600 °C for 12 h. The resultant brown coloured material was removed from the ampoule, repeatedly washed with hot deionized water, centrifuged at 4000 rpm and the retrieved PTI-LiBr was washed with methanol. The structural and morphological properties of PTI-LiBr are shown in Fig. 1
Fig. 1 (a) XRD pattern of crystalline PTI·LiBr (Inset: one unit cell of a PTI·LiBr); (b) SEM image of an aggregate of hexagonal prismatic PTI·LiBr crystallites (Inset: TEM image of hexagonal PTI·LiBr crystallites).
Dissolution of as-synthesized PTI-LiBr crystals in N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) indicates a change in the color of the liquid over time (Fig. 2(a)). The extent of dissolution of PTI-LiBr crystals is enhanced under UV-light illumination (Fig. 2(b)).
Fig. 2 Time-lapse photographs depicting spontaneous dissolution of PTI-LiBr in DMSO up to 48 h under visible and UV illuminations.
The high-resolution TEM images of CN nanosheets deposited from solutions containing PTI-LiBr dissolved in NMP (Figs. 3(a)-3(c)) indicate that the CN nanosheets are atomically intact with well-defined edges and maintained the hexagonal shape with its lateral dimensions close to that of the precursor bulk crystals. No evidence of any dislocations or point defects could be observed.
Fig. 3 (a-c) HR-TEM images of CN nanosheets deposited from solutions containing PTI-LiBr dissolved in NMP
Both bulk and exfoliated CN exhibit luminescence in the UV/visible range. The normalized photoluminescence (PL) emission spectra of CN nanosheets dissolved in DMF exhibit a broad peak ∼380 nm, which slightly shift toward blue-green range with an increase in wavelength excitation from 260 to 330 nm (Fig. 4(a)). The PL spectra of stacked or aggregated films of CN nanosheets deposited from dissolved solution also exhibit a broad peak centered ∼480 nm (red-shift when compared to PL spectra of dissolved CN nanosheet) (Fig. 4(b)). The broadening of the PL spectra of CN nanosheets dissolved in DMF as well as the stacked or aggregated CN film deposited from dissolved solution indicates that they could be composed of 9 to 40 layers in thickness. These inferences indicate that depending on the thickness of CN nanosheets, it would be possible to tune the PL wavelength from narrow UV to broad-band white.
Fig. 4 PL spectra of CN nanosheets at varying excitation wavelength: (a) CN nanosheets dissolved in DMF; (b) stacked or aggregated CN film deposited from dissolved nanosheets
The methodology employed for the synthesis of 2D CN nanosheets is simple and easily scalable. The spontaneous dissolution of PTI-LiBr crystals in NMP, DMF, and DMSO results in the formation of stable solutions of pristine, defect-free CN nanosheets with well-defined functional properties. The luminescence property of dissolved as well as stacked film of CN nanosheets indicate that they can be explored as potential next-generation materials for photocatalysis. The tunability of PL spectra depending on the stack thickness of CN nanosheets makes them as suitable candidate materials for UV-blue and white LED emitters. The CN nanosheets prepared by this method can be used for a wide range of optoelectronic devices.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Thomas S. Miller et al., Single Crystal, Luminescent Carbon Nitride Nanosheets Formed by Spontaneous Dissolution, Nano Lett. 2017, 17, 5891−5896.
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