Graphene quantum dots (GQDs) exhibit great potential for various optoelectronic applications due to their size-dependent and edge-sensitive photoluminescence properties. Previously, GQDs were mainly synthesized from graphene oxides by chemical exfoliation under strongly acidic environment or by multi-step lithographic methods including masking, patterning and lift-off, which hindered the efficient preparation of high-quality GQDs for practical applications. On the other hand, nitrogen-functionalization or doping is known to be very helpful to tailor the intrinsic properties of GQDs, but it needs further complicated wet-chemical reactions. Atom-thick GQDs can be called graphene quantum sheets (GQSs), which is expected to show stronger quantum effects derived from monolayer graphene, compared to GQDs obtained from graphene oxides (GOs) or carbon fibers. Here we report a very simple solvent-free method to prepare nitrogen-doped graphene quantum sheets (N-GQSs) by directly applying nitrogen plasma to as-grown graphene on Cu. The resulting N-GQSs can be transferred as a film-like layer or easily dispersed in an organic solvent to be transferred on arbitrary substrates. We also demonstrate that a porous Si-cathode decorated with N-GQSs exhibits enhanced photochemical and electrochemical activities advantageous for solar-driven hydrogen evolution reaction. Thus, it is expected that the N-GQSs converted from monolayer graphene would be useful for a wide range of optoelectronic, electrochemical, and energy storage applications in the future.
Recently, GQD has been attracting much attention in bio-imaging, light-emitting, and photovoltaic applications[1-3] due to its unique optical properties depending on size and functional edge as well as the accessibility to solution chemistry.[4-6] This has stimulated tremendous efforts to develop various synthesizing methods such as hydrothermal cutting, patterning by nanolithography, and electrochemical scissoring of graphene sheets, as well as bottom up synthesis by wet chemistry to produce GQDs with different sizes and functionalities. Usually, these methods require strongly acidic environment or time-consuming multi-step processes, which is a drawback for more efficient synthesis of high-quality GQDs. Moreover, in order to be used for various optoelectronic and energy applications, GQDs often need to be fabricated as a thin-film structure on a solid interface. However, the GQDs in aqueous solvent are hardly processible because they are unstable in aqueous environment. In addition, preparing GQD films from aqueous dispersion is challenging because spin-coating or drop-casting method doesn't provide enough control over thickness and uniformity as well as it requires rigid and flat surface rather than flexible or conformal substrates.
Plasma treatment is one of the facile ways to tune the intrinsic properties of graphene, and previously, oxygen plasma was applied to prepare chemically functionalized graphene showing uniform photoluminescence and Raman spectral changes originated from its defective structures.[10, 11] On the other hand, chemical doping is an effective way to tune the optical, chemical, and electronic properties of graphene. Likewise, the band-gap of GQDs can be engineered by changing size, shape, edge-, and surface functionalities,[6, 13, 14] leading to tunable photoluminescence with higher intensity. In particular, the functional modification with nitrogen could offer more active sites needed for higher catalytic activities, which is important for various energy applications.
Herein, we introduce a simple one-step method to prepare large-scale N-doped GQSs by directly applying nitrogen plasma to as-grown graphene on Cu, which can be transferred as a film-like layer or easily dispersed in an organic solvent. Moreover, we confirm that the N-GQSs transferred on flat Si and porous Si can be a good photoelectrochemical catalyst for hydrogen evolution reaction (HER). Atomic force microscopy (AFM) and transmission electron microscopy (TEM) images show that the average size of N-GQSs is 4.84 nm, and the substitution of carbon with nitrogen is evidenced by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The unique optical properties of N-GQSs were confirmed by absorption and photoluminescence spectra, showing strong emission with the maximum wavelength of 430 nm when excited by 365 nm radiation source (Xe lamp).
First, the monolayer graphene was synthesized by using 10 × 10 cm2 Cu foils as catalytic substrates in 1000 °C quartz reactor with flowing 50 sccm CH4 and 5 sccm H2 for 30 min under 8 Torr. Next, the N-GQSs were prepared by irradiating nitrogen plasma (10 W RF power with varying exposure time under 120 mTorr) to as-grown CVD graphene on Cu as shown in Figure 1. Finally, the N-GQSs were transferred using conventional polymer-assisted dry-transfer methods on a target substrate after removing Cu by 0.1 M aqueous ammonium persulphate etchant. Alternatively, the floating N-GQSs after removing Cu without PMMA can be dispersed into common organic solvent such as dichloromethane using solvent extraction techniques.
The AFM images in Figure 2a-d show the gradual increase of surface roughness with increasing nitrogen plasma treatment time, indicating that the as-grown graphene on Cu is directly converted N-GQSs. The AFM height profile of N2-plasma treated graphene for 16 sec shows the average height of 1.64 ± 0.06 nm (Figures S1 and S2).
Figure 3a-b show the N-GQSs film directly transferred from Cu to a SiO2 substrate after coating with poly(methyl methacrylate) (PMMA) layer. The PMMA can be easily removable by acetone. Figure 3c-d show the N-GQDs drop-cased onto a SiO2 substrate from the suspension in dichloromethane. The atomic structures of N-GQSs were investigated by high-resolution transmission electron microscopy (HRTEM) as shown in Figure. 3e-g. The sample was prepared by drop-drying the N-GQSs solution on a graphene-supported TEM grid. Most of N-GQSs show a size distribution from 3 to 7 nm with an average value of 4.84 nm (Figure 3h). The clear atomic lattice structure shown in Figure 3g indicates that the N-GQSs are highly crystalline.
After exposure to nitrogen plasma, the D peaks related structural defects at the edges of graphene were significantly increased in Raman spectra (Figure 4a).[17, 18] On the other hand, the shift of D and 2D peaks indicates that the graphene is doped with nitrogen atoms. X-ray photoelectron spectroscopy (XPS) measurements were performed to determine the chemical composition of N-GQSs (Figure 4b-d). No N 1s peak was observed in monolayer graphene. The strong C 1s peak at 284.8 eV corresponding to sp2 carbon indicates that the conjugated honeycomb lattices are mostly maintained after N2-plasma treatment. We suppose that the oxygen-related sub peaks such as CO (286.6 eV), CO (288.3 eV), and OCO (289 eV) are originated from the reaction of unstable N-GQS edges or defects with oxygen when exposed to air. The C-N bond peak at 285.2 eV in C 1s spectrum (Figure 4c) as well as the pridinic (398.5 eV) and pyrrolic (399.9 eV) peaks in N 1s spectrum (Figure 4d) indicate that nitrogen-to-carbon ratio is ∼2.7%. The UV-vis absorption spectrum of the N-GQSs shows an absorption band with a peak maximum (λmax) at 270 nm (Figure 4e). The PL spectrum exited at 370 nm shows a strong peak at λmax = 430 nm. Thus, the N-GQDs emitted intense blue luminescence under 365 nm wavelength irradiation by UV lamp (Figure 4e, inset). We found that the λmax in PL spectra is almost invariable with varying excitation wavelength from 360 nm to 420 nm.
Recently, graphene based catalyst becomes an attractive candidate for photoelectrochemical reaction.[20, 21] Figure 5 demonstrates the catalyst application of N-GQSs on a flat bare Si and a porous Si substrates for hydrogen production. The scanning electron microscopy (SEM) images in Figure 5a-b show the cross section of a bare Si and a porous Si and each inset shows the top-view of the sample. To evaluate the photocathodic behavior, N-GQSs were loaded on a bare Si with the dry transfer and on a porous Si with the solution drop-casting. The photocurrent density was measured as potential sweep from 0.4 V to –0.8 V vs. Reversible Hydrogen Electrode (RHE) in a three electrode cell. A light source of a 300 W Xe lamp (100 mW cm−2) with an Air Mass 1.5 Global condition filter was illuminated on the samples in an aqueous 1 M perchloric acid solution (pH 0) (Figure S3). Interestingly, the N-GQSs exhibit the superior catalytic activity for HER. As shown in Figure 5c, the photocurrent density-potential (J–E) curve of N-GQSs/bare Si dramatically is shifted approximately ∼0.35 V toward positive potential compared to that of the bare Si as well as the onset potential of N-GQSs also positively shifted by 0.29 V (Table S1). Compared to bare Si, porous Si exhibits enhanced limiting current density and positive shift of onset potential ascribed to light trapping effect. In N-GQSs on a porous Si substrate, the positive shift in 0.09 V of the onset potential also shows higher activity for HER compared to that of a porous Si. Figure 5d shows the electrocatalytic activity of N-GQSs, CV curves of the samples were obtained without illumination with rotating disk electrode (RDE) system. For the working electrode, N-GQSs were transferred to a glassy carbon tip which is inert in aqueous solution. The J–E curves were swept from 0.1 V to –0.35 V and the onset potential were obtained –5 mA cm−2 of HER current density of as-grown graphene and N-GQSs (Table S1). The onset potential of N-GQSs is –0.22 V with respect to RHE, which has positive shift by 0.07 V compared to that of as-grown graphene. This result is similar to the photoelectrochemical behavior of the J–E curves, a positive shift in the overall J–E curve induced by N-GQSs. From the above results, we conclude that the N-GQSs show the superior electrocatalytic effect for hydrogen production when combined with Si photocathodes with arbitrary morphologies.
In summary, we have demonstrated the formation of N-doped GQSs from as-grown monolayer graphene on Cu using nitrogen plasma. Various spectroscopic analyses including AFM, TEM, XPS, Raman, and PL indicate the direct formation of high-quality N-GQSs from CVD graphene. The N-GQSs can be transferred onto an arbitrarily shaped photocathode surface to enhance the catalytic activity for photoelectrochemical hydrogen evolution, which would be also useful for various display, energy, and biological applications.