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2018
Preparation and photoluminescence properties of graphene quantum dots by decomposition of
graphene-encapsulated metal
nanoparticles derived from Kraft lignin and transition metal salts
Temerov, Filipp
Elsevier BV
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© Elsevier B.V.
CC BY-NC-ND https://creativecommons.org/licenses/by-nc-nd/4.0/
http://dx.doi.org/10.1016/j.jlumin.2018.10.093
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Preparation and Photoluminescence Properties of Graphene Quantum Dots by Decomposition of Graphene-encapsulated Metal Nanoparticles Derived from Kraft Lignin and Transition Metal Salts
Filipp Temerov, Andrei Beliaev, Bright Ankudze, Tuula T. Pakkanen
PII: S0022-2313(18)31011-1
DOI: https://doi.org/10.1016/j.jlumin.2018.10.093 Reference: LUMIN16041
To appear in: Journal of Luminescence Received date: 7 June 2018
Revised date: 3 October 2018 Accepted date: 21 October 2018
Cite this article as: Filipp Temerov, Andrei Beliaev, Bright Ankudze and Tuula T. Pakkanen, Preparation and Photoluminescence Properties of Graphene Quantum Dots by Decomposition of Graphene-encapsulated Metal Nanoparticles Derived from Kraft Lignin and Transition Metal Salts, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.10.093
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1
Preparation and Photoluminescence Properties of Graphene Quantum Dots by Decomposition of Graphene-encapsulated Metal Nanoparticles Derived from Kraft Lignin and Transition Metal Salts
Filipp Temerov, Andrei Beliaev, Bright Ankudze, Tuula T. Pakkanen
Department of chemistry, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland
Abstract
Graphene quantum dots (GQDs) are nanometer-sized pieces of a graphene sheet possessing a number of fascinating electrical and optical properties. In this work, GQDs were synthesized by a top-down method using lignin as a carbon source due to its high carbon content and high abundance in the nature. GQDs were prepared via an intermediate step, by the synthesis of graphene-encapsulated metal nanoparticles (GEMNs). First the GEMNs were synthesized from lignin and three different transition metal chlorides (Fe, Co, Ni), and then metal nanoparticles were removed, and graphene capsules thus obtained were then decomposed to GQDs by using an alkaline hydrothermal treatment. GQDs have a diameter around 20-25 nm and exhibit a yellowish emission under UV light. Characterization results obtained by using the IR, Raman and UV-vis spectroscopies showed that the GQDs obtained from the three types of GEMNs have a graphene structure with various oxygen-containing functional groups. A comprehensive study on the luminescence properties of the GQDs revealed that the three kinds of GQDs show an excitation- wavelength-dependent photoluminescence. The life times are in the range of 5.2-5.5 ns and the quantum yield values vary within the limits of 11.7-12.4 %.
2
Graphical abstract fx1
Keywords
: Graphene quantum dots; Graphene encapsulated metal nanoparticles; Lignin;Graphene capsules; Transition metal salts.
1 Introduction
Graphene quantum dots (GQDs) are small pieces of a graphene sheet having the size below 30 nm and possessing fascinating electrical [1], optical [2] and low toxicity properties [3].
Due to their excellent stable photoluminescence and chemical stability, GQDs have been considered as novel materials for biomedical, optical, electronic, energy, and environmental applications and have already been used in many different utilizations such as organic light emitting diodes (OLED) [4],solar cells [5], bioimaging [6], and tissue engineering [7].
Over the past few years, a large number of novel synthesis methods has been introduced for GQDs [8,9] and they are usually divided in two main groups: top-down and bottom- up methods based on the carbon source and product relationship. The top-down strategies involve decomposition of cheap carbon–rich bulk materials to smaller by physical, chemical, or electrochemical methods and techniques [9–12], while the bottom-up methods involve a growth strategy based on a synthesis of GQDs from small organic molecule precursors acting as a carbon source [9].There is a whole range of materials with a high carbon content such as coal [10], carbon fibers [13], graphene oxide (GO) [11], carbon nanotubes (CNTs) [12], and fullerenes [14], which have been utilized as precursors for a synthesis of GQDs by the top-down approach. Usually for the preparation of GQDs, these starting compounds are exposed to a chemical or electrochemical oxidation, a hydrothermal or solvothermal treatment and a microwave- or ultrasound-assisted treatment[10,11,15] . By the top-down methods, GQDs with surface functional groups can be produced by relatively simple operations [16]. However, the number of bottom-up methods has been growing exponentially, because these methods allow to produce GQDs with well-defined
3 sizes, shapes and properties by a controlled synthesis from small molecule precursors. The primary bottom-up methods are a stepwise organic synthesis [17], a chemical vapor deposition (CVD) [18], and carbonization of organic precursors [4]
Here we introduce a new approach of preparation of GQDs by a decomposition of graphene capsules obtained from graphene-encapsulated metal nanoparticles (GEMNs), synthesized from lignin as the carbon source. GEMNs consist of small metal nanoparticles, which have been coated with several layers of graphene [19,20]. There are several pathways for preparation of GEMNs such as a thermal decomposition [19,21] and an arc discharge method [22–24]. Carbon sources used for GEMNs preparation are metal organic frameworks [21], graphite [22–24] and lignin[19]. Graphene-encapsulated metal nanoparticles have been prepared from metals such as Fe [22], Co [23], Ni [24] and Cu [19,16]. Preparation of graphene-encapsulated copper nanoparticles from lignin has been reported by J. Zhang’s research group [19,25]. The idea of utilizing lignin in the preparation of GEMNs is very promising due to the high abundance of lignin in the nature and its high aromatic carbon content [26]. On treatment with an acid, metal nanoparticles are released from GEMNs and empty graphene capsules can be obtained and examined [19,25]. The structure and properties of graphene capsules are important aspects in their potential applications, since they can be modified by varying the temperature, pressure and inert atmosphere [25]. We will in this study show that graphene capsules can be further decomposed to GQDs, the size and luminescence properties of which are extremely important features in potential utilizations of GQDs.
In this work, we have prepared GEMNs from lignin and three different transition metal chlorides (Fe (III), Co (III) and Ni (II)) in order to study a relationship between the chosen metal and the structure of graphene capsules, a removal of metal nanoparticles and a decomposition of graphene capsules to GQDs by using alkaline hydrothermal conditions. The structure and properties of the graphene capsules and GQDs were examined using different characterization methods such as FTIR, Raman, and UV-Vis spectroscopies. The photoluminescence properties of
4 the graphene quantum dots were determined by measuring emission and excitation spectra as well as lifetimes and quantum yields of the emission.
2 Materials and methods 2.1 Materials
Alkaline Kraft lignin (MW = 60000 g/mol), potassium hydroxide (85 %), cobalt (III) chloride (99.99 % trace metals basis), nickel (II) chloride (99.9 % trace metals basis), iron (III) chloride (99.95 % trace metals basis) and nitric acid (60 %) were purchased from Sigma-Aldrich and were used without purification. The deionized water was obtained from the Millipore water purification system.
2.2 Preparation of graphene-encapsulated metal nanoparticles (GEMNs) and graphene capsules from lignin
Syntheses of GEMNs from Kraft lignin using three metallic salts (CoCl3·6H2O, NiCl2·6H2O, FeCl3·6H2O) were performed. So, 4 parts of salt (1g), 4 parts of lignin (1g) and 10 parts of deionized water (2,5g) were placed in a round bottom flask. The mixtures were heated to 80
°C and kept in an oil bath for 12 h. After the heating treatment, the three mixtures were dried in an oven at 103 °C overnight [19,25].
The procedure of a high temperature treatment was carried out inside a high temperature furnace. A quartz tube was used as a reactor. The ceramic boat holding about 1.5 g of metal-salt-lignin mixture was placed inside the reactor tube and was heated to 500 °C. Before the heat treatment, air from the reactor was removed by flowing argon gas for 15 min. Then the temperature was raised to 500 °C and held at 500 °C for 30 min. The sample was cooled down to room temperature under the argon atmosphere and then transferred in a desiccator[19,25].
5 After the heat treatment, the metal particles were removed in a dissolution procedure.
GEMNs (0.05 g) were dispersed in 30 ml of 20 % HNO3. Then the mixture was heated to the boiling point and was kept boiling for 30 min. The obtained mixtures were filtered through a membrane (VWR, Randor, PA, with pore size 0.45 µm) and rinsed with 600 ml of deionized water.
The powders from the three GEMNs were dried in the oven at 60 °C for 6 h and then at 103 °C overnight [19,25].
2.3 Preparation of GQDs from graphene capsules.
After the purification step, empty graphene capsules were obtained. Graphene capsules (0.05 g) were mixed with 25 ml of 1M KOH in a conical flask and then ultrasonicated for 10 min. The reaction mixture was poured into a Teflon liner and placed into an autoclave with a temperature of 250 °C for 12 h. After the procedure yellowish liquids were obtained. At the purification stage, the liquids obtained were transferred into a dialysis tube (with a retained molecular weight: 500-1000 Da) for 2 days in order to remove all potassium hydroxide and to reach a pH value around 7.
2.4 Characterization
A FTIR analysis of graphene capsules and GQDs was made with a Bruker VERTEX 70 FT-IR spectrometer. The GQDs (1 mg) and KBr (250 mg) were grounded in a mortar and then obtained powder was introduced to a hydro press for 5 min at 8 kPa. After that the FT-IR spectrum of the obtained tablet was measured in the scanning range from 4000 to 400 cm-1 (20 scans).
Raman spectra of graphene capsules and GQDs were collected on a Renishaw® inVia Raman microscope with a wire 3.4TM Softwear and a Class 3B laser system with a 514 nm green laser. Elemental analysis of GQDs was done by a varioMICRO, CHNS mode.
6 UV-Vis spectra of GQDs were obtained with a Perkin Elmer Lambda 900 UV/VIS/NIR and Hitachi (U-3310) spectrophotometers. The measurements of GQDs were carried out in the range of 250 to 800 nm.The dried GQDs (1 mg) were dissolved in 10 ml of water and then the solution was poured in a cuvette and introduced to the spectrometer. The steady-state excitation and emission spectra of size distributed GQDs were recorded on Edinburgh (FS920) and HORIBA FluroMax-4P spectrofluorometers, using a spectral width of 1 nm. Both the wavelength dependent excitation and emission response of the fluorimeter were calibrated. Lifetime studies were performed with an Edinburgh FL 900 photon-counting system using a hydrogen-filled lamp as the excitation source. The absolute emission quantum yield was determined by Vavilov's method using a hydrogen-filled lamp pumping and Rhodamine 6G (Φem = 0.95) and Coumarin 480 (Φem = 0.87) as standards.Measurements of emission and excitation spectra along with lifetime of excited state were performed in water solution using concentration of ca. 1×10-5 M.
3 Results and discussion
3.1 Characterization of graphene capsules
It is well known that it is possible to wrap metal nanoparticles inside graphene capsules by a high temperature treatment. Many studies [19,20,23] have shown successful preparation of graphene-encapsulated metal nanoparticles with different transition metals such as Fe, Co, Cu and Ni. The authors usually use materials with a high carbon content, for example graphite as the carbon source. J. Zhang’s research group [19] was the first one to prepare GEMNs using lignin as the carbon source. In our research work we used the established procedure by Zhang et. al. for preparation of our empty graphene capsules.
For the synthesis of the graphene encapsulated metal nanoparticles (GEMNs), we used alkaline Kraft lignin as a carbon source and three different metallic salts: iron (III) chloride, cobalt (III) chloride and nickel (II) chloride. Each GEMNs mixture was dried and purified with an acid in
7 order to remove metal nanoparticles and to get empty graphene capsules. The graphene capsules were studied with FT-IR and Raman spectroscopies to find out influence of the chosen metal on the chemical structure of the graphene capsules. Specific functional groups in graphene capsules samples were analyzed with infrared spectroscopy.
The IR spectra of the three graphene capsules obtained from GEMNs of Fe, Co and Ni are shown in Figure 1. All three samples contained a broad band around 3430-3432 cm-1, which corresponds to an O-H vibration. Peaks at 2918 cm-1 and 2849 cm-1,thatcorrespond to the C-H stretching vibrations, are clearly observed in the Co sample. The capsules samples of Ni, Co and Fe have an absorbance at 1713 cm-1,which matches to a carboxylic C=O group stretching vibration.
The peak around 1619 cm-1 observedin all three samples correspond to a C=C vibrational mode in the conjugated carbon backbone. The sharp signal at 1384 cm-1 in all samples is very distinctive and often observed in graphene samples [12], and this peak corresponds to a C-H vibration. At the same time, the capsule samples of Co and Fe have stretching vibrations of epoxy groups at 1116 cm−1and 1123 cm−1, and a stretching vibration of secondary alcoholic groups C-OH at 1033 cm−1[27]. Based on the IR study, the Ni sample does not seem to contain these groups, which could be explained by the high carbon dissolution property of nickel [20,28].
8 Figure 1. IR spectra of graphene capsules.
To confirm the presence of a graphene structure in the empty capsules, Raman spectroscopy was used. Graphene capsules have two bands around 1355-1360 cm−1 and 1580-1585 cm−1 (Figure 2). The band at 1355cm−1 is called D band and corresponds to destruction of the sp2 network by the sp3-bound carbon atoms. The band at 1580cm−1 is G band and it corresponds to the vibration of the sp2- bound carbon atoms in the graphene materials [5,29]. In the three Raman spectra, two bands were observed and their intensity ratios, ID/IG, were used to evaluate the perfectness of the graphene structures. Graphene capsules from Co GEMNs showed the lowest ID/IG ratio (Table 1), indicating that the structure had a quite low level of defects while graphene capsules from Ni GEMNs with the ID/IG ratio of 0.82 had the highest number of defects and disorders. The Raman results are in a good correlation with the IR results. The IR spectrum of the Co graphene capsules contains less IR bands from functional groups in the low wavenumber region and hence a lower number of sp3-hybridized carbon atoms and less defects (Table 1).
The elemental analysis results of the graphene capsules (Table 1) proved that carbonization has been done in a proper way, because the carbon content of each sample is higher
9 than 50 %. The presence of nitrogen can be explained with nitrogen residues originating from the HNO3 treatment used for dissolution of metal nanoparticles.
Table 1. Elemental analysis and ID/IG ratios from Raman spectra analysis of graphene capsules.
Sample ID N, (wt %) C, (wt %) H, (wt %) ID/IG
Fe graphene capsules 3.57 53.16 3.21 0.72
Co graphene capsules 2.89 60.35 2.50 0.66
Ni graphene capsules 3.36 54.18 3.04 0.82
Figure 2. Raman spectra of graphene capsules.
Different mechanisms are possible for formation of graphene-encapsulated metal nanoparticles [19,20]. However, the mechanisms presented so far do not fully describe formation of the graphene layers. It is well known that formation of graphene on the surface of metal particles can take place either from an amorphous carbon source or from a gas [19]. As the temperature increases above 300 °C, metal salt particles undergo transformation to metal atoms, which start forming metal nanoparticles. At the same time an amorphous carbon shell forms around
10 nanoparticles[30]. Depending on the isolation effect [31], a different number of graphene layers could be synthesized. On the other hand, the graphene-layer formation on metal nanoparticles might be explained as a methane deposition on the metal surface, because lignin starts to release methane at 400 °C and CH4is extensively used as the gaseous carbon source to synthesize graphene (Figure 3) [31]. In the case of transition metals such as Fe, Co, and Ni, formation of graphene shells could also occur via a dissolution-precipitation pathway [32].
Figure 3. A schematic presentation of the graphene quantum dots formation via a fabrication of GEMNs and a hydrothermal cutting of empty graphene capsules.
3.2 Fabrication and characterization of graphene quantum dots
GQDs were obtained by a hydrothermal cutting of empty graphene capsules.
Graphene capsules were mixed with an aqueous solution of potassium hydroxide and introduced to an autoclave at 250°C for 12 h. Figure 3 presents a schematic illustration of graphene quantum dots formation.
The water solutions of each capsule sample, showing a brownish color under a visible light, were examined under a UV irradiation. All three samples possessed a yellowish luminescence (Figure 4) and the solutions of GQDs stayed stable and steady for a month. On the basis of the emission color, it is possible to estimate the average size of GQDs, because the emission wavelength has been shown to depend on the size of GQDs. GQDs of the different sizes have been
11 found to possess different shapes and edges. The smallest GQDs (around 5 nm) usually have a circular shape with mixed edges of zig-zag and armchair while the biggest (around 35 nm) have square or rectangular shapes mainly with armchair edges. GQDs with the size between these two ranges can have elliptical or hexagonal shapes. So based on the size-dependent shape and corresponding edge variations of GQDs, it is possible to estimate the size of GQDs on the basis of the emission wavelength. On the basis of the yellowish luminescence color, we suggest that all GQD samples have an average size around 20 - 25 nm [8].
Figure 4.The luminescent colors of the water solutions of GQDs derived (A) from Co graphene capsules, (B) from Ni graphene capsules, and (C) from Fe graphene capsules, under a 385 nm UV irradiation.
The field emission scanning transmission electron microscopy (FE-STEM) was used to study the shape and size of GQDs. Figure 5 presents STEM images of GQDs derived from the graphene capsules of Fe GEMNs. According to the images, the size of single GQDs is varying in the range, where the smallest diameter is 15 nm and the largest around 25 nm. We can also observe agglomeration of two or more GQDs in one image (Figure 5C), with the average diameter of 90 nm.
The shape of the non-agglomerated dots is almost spherical. Based on the color of the emitted light and the STEM images we can deduce that the sample consist of GQDs with the diameter mainly in
12 the range of 20-25 nm [33]. During the formation of graphene capsules around metal nanoparticles, graphene layers can probably possess different kinds of surface defects and based on these defects graphene capsules can break up in the hydrothermal treatment along different grain boundaries [19,25].
Figure 5. Scanning transmission electron microscopy images of GQDs obtained from Fe graphene capsules, (A) and (B) GQDs with the average diameter of 25 nm, (C) agglomerated GQDs with the diameter of 90 nm, and (D) GQDs with the average diameter of 15 nm.
The IR spectra of the three graphene quantum dot samples are presented in Figure 6.
All three samples contain the same broad OH band around 3430 - 3438 cm-1 as the graphene capsules. All GQDs have a weak shoulder at 1724 cm-1, which fits to the carboxylic C=O group stretching vibration. The two very distinctive peaks at around 1619 cm-1 and 1384 cm-1 in all three samples correspond to the C=C stretching vibration of the graphene backbone and to the C-H stretching vibration, respectively. The intensities of these two signals, characteristic for the
13 graphene domain, have increased relatively to that of the hydroxyl band, indicating partial deoxidization of the quantum dot structures [34]. Peaks at 2918 cm-1 and 2849 cm-1, which are observed in all three samples, belong to the C-H stretching vibrations. However, there are several distinctive narrow bands in the low wavenumber region in all samples. There are four characteristic signals in the Co and Ni samples and only two are observed in the case of the Fe sample. In all samples, the peak at 1123 cm−1 corresponds to the vibrations of epoxy groups and the peak at 1033 cm−1 fits to the stretching vibrations of C-O groups. The Co and Ni samples also contain peaks at 810 cm−1 and 694 cm−1, which are due to the C-H bending vibrations [27,35].
Figure 6. IR spectra of graphene quantum dots.
Raman spectroscopy was used to examine the level of defects and disorders in graphene quantum dots. In the three Raman spectra (Figure 7), two bands at 1355-1360 cm-1 and 1580-1585 cm-1 were observed and the ratio of their intensities was used to evaluate the perfection degree of GQDs [29]. The structure with the lowest ID/IG peak intensity ratio is having the least number of defect sites and in this case the most perfect structure belongs to the GQDs derived from
14 graphene capsules of Co GEMNs and the least perfect one is GQDs obtained from graphene capsules of Fe GEMNs (Table 2) [29,34].
Based on the Raman spectral analysis results of the graphene capsules (Table 1) and the GQDs (Table 2) we can compare the state of perfection in these two types of structures. The graphene capsules with the lowest and highest ID/IG ratios gave in the hydrothermal cutting treatment GQDs with the similar degrees of perfection.
Figure 7. Raman spectra of graphene quantum dots.
Table 2. Elemental analysis results and ID/IG ratios from the Raman spectra analysis of GQDs.
Sample ID N, (wt %) C, (wt %) H, (wt %) ID/IG
Fe GQDs 0.61 44.47 4.47 0.86
Co GQDs 0.58 43.89 4.90 0.75
Ni GQDs 0.60 46.27 5.58 0.81
The elemental analysis results (Table 2) of GQDs derived from graphene capsules showed that all samples have a relatively high amount of carbon, close to 50 %, indicating high carbonization yields. The remaining amount probably belongs to oxygen, mainly in OH and epoxy
15 groups. The yields of GQDs derived from graphene capsules were in the range 14-17 % for all three samples.
3.3 Photoluminescence properties of GQDs
In the absorption spectra of GQDs in Figure 8, there is a broad absorption in the UV region and an absorption tail extending to the visible part of the electromagnetic spectrum, till 800 nm, showing no well-defined absorption bands [36].The peak around 280 nm appears due to a n- π*
transition in the C=O functional groups [4] (see also Figure 10 B).
Figure 8. UV-vis spectra of the three types of GQDs measured in a water solution.
The photoluminescence (PL) mechanism of GQDs is not very well understood and has been explained with different factors such as a quantum-confinement phenomenon[37], emissive traps [38], electronic conjugate structures [39], and free zig-zag sites [34]. Graphene quantum dots contain sp2-hybridized carbon atoms in the graphene core, and sp3-hybridized carbons in the functional groups on the edges or on the sides of the graphene sheet. The PL properties are determined by the π energy states of the sp2-hybridized carbons and are due to radiative recombination of localized electron-hole pairs in the sp2 structure [39]. The energy band gap depends on the size, shape and fraction of the sp2 domains [39]. GQDs are considered having different fluorophore centers, which are subsystems of the aromatic π-conjugated graphene core
16 surrounded by the sp3 hybridized carbon atoms [40]. Usually, the graphene core governs the intrinsic emission, while the attached oxygen-containing functional groups control the defect state emission [41]. The defect state emission arises from a defect site or energy traps, while the intrinsic state emission comes from the quantum size effect, recombination of localized electron-hole pairs [42]. Different other factors can influence PL properties, for example GQDs prepared under alkaline and acidic conditions have different PL intensities [34] and the PL intensity can correlate with the thickness of graphene layers where a single layer of graphene shows a bright PL while multilayers revealed a rather poor PL [43].
The photoluminescence spectra of GQDs were measured at three different excitation wavelengths (400, 450 and 500 nm) and an excitation-dependence was observed (Figure 9). When the excitation wavelength was increased, the emission signals shifted to the high wavelength region with no major change in the emission intensities. The FWHMs (full width of half maximum) of the emission signals were, however, found to decrease with the increasing excitation wavelength. The emission signals can be considered as a transition from the lowest unoccupied molecular orbital (LUMO) to the highest occupied molecular orbital (HOMO). Molecules at the electronic excited state can lose some of the excitation energy in a radiationless decay before a radiative transition to the electronic ground state (Figure 10 B).
The excitation-wavelength-dependence is a common feature observed in the context of photoluminescence of GQDs [44]. This phenomenon can be possibly explained in terms of electronic conjugate structures, free zigzag sites, and emissive traps [45]. According to the IR and Raman results, the three GQDs structures contain different oxygen-containing functional groups (hydroxyl, carboxyl, carbonyl and epoxy), which can have various energy levels, resulting in a series of emissive traps and having influence on the PL emission [38]. [4,39]. The shift of PL to the high wavelength can also be attributed to a size distribution of GQDs having different energy band gaps. The STEM images obtained indicated a broad size distribution due to an agglomeration of
17 quantum dots. According to the quantum size effect the band gap decreases as the size of GQDs increases.
Figure 9. Emission spectra of GQDs derived from (A) Fe graphene capsules, (B) Co graphene capsules and (C) Ni graphene capsules, at three excitation wavelengths.
Figure 10
.
(A) Photoluminescence excitation spectra of the three types of GQDs (the detection wavelength of 600 nm) and (B) a proposed presentation of electronic transitions of GQDs derived from Fe and Co graphene capsules.18 The photoluminescence excitation (PLE) spectra are almost similar to each other (Figure 10 A) and contain three maximum bands around 280, 365, and 490 nm, except in the case of the Ni GQDs sample. The absence of the 280 nm band in the PLE spectrum of the Ni GQDs indicates that absorption to this high-energy electronic excited state does not produce fluorescence when the state is deactivated to the ground state. The first maximum at 280 nm corresponds to a - π* transition in the C=O bond at aromatic rings [13], the second maximum at 365 nm corresponds to a π - π* transition in C=O bonds [34], and the intense maximum at 490 nm can be assigned to a π - π* transition in the aromatic system [40].
The two electronic transitions observed at the low wavelength region in the PLE spectra of the Fe and Co samples are regarded as transitions from the σ (280 nm, 4.43 eV) and π (365 nm, 3.3 eV) orbitals (HOMO) to the π* orbital (LUMO) (Figure 10B). The localized π and π*
energy levels of sp2 domains have been theoretically shown to be between the σ and σ* states of sp3 domains [40,46,47]. The energy difference E between the σ and π orbitals of 1.13 eV is below 1.5 eV required for the emissive triplet carbenes at zig-zag sites, because carbene-like structures are most abundant at zig-zag edges [18, 37].
Graphene quantum dots prepared under alkaline hydrothermal conditions from graphene sheets have been found to emit a strong blue PL, because zig-zag sites of GQDs are free and emissive triplet carbene ground state is active in PL [29]. The graphene quantum dots obtained from the graphene capsules of Fe, Co and Ni under the similar alkaline hydrothermal conditions show, however, a strong PL in the longer wavelength region most likely due to emission from oxygen-containing defect states [35].
The lifetimes of all three GQDs were monitored at 550 nm and appeared to be in the range of 5.2-5.5 ns (Table 2, Figure 11). These short lifetimes can be attributed to the fluorescence process [34]. The average lifetime decay curve of GQDs was fitted with a two-exponential function.
The curve contains two slow components (∼ 1.5 and 6.3 ns), which are consistent with the reported
19 lifetimes of GQDs prepared by an exfoliation method [13]. The slow components involve a rapid bandgap transition and a long decay of oxygen-related emission [48].
Figure 11. The PL decay curves of GQDs derived from (A) Fe graphene capsules, (B) Co graphene capsules and (C) Ni graphene capsules.
The quantum yields of all three samples have similar values around 12 %. Compared to the results of different types of GQDs recently synthesized [36], the obtained quantum yields are in the usual range observed for oxygen containing graphene quantum dots. However, it is known that quantum yields can be enhanced up to 24 % by a simple chemical reduction of oxygen functional groups [35].
Table 3. The photophysical data of three types of GQDs.
Emission (excitation at 450 nm)
Excitation (monitoring
at 600 nm) Lifetime, ns Quantum yield,
%
Fe GQDs 545 280, 365, 490 5.5 11.7
Co GQDs 545 280, 365, 490 5.2 12.4
Ni GQDs 550 370, 495 5.4 12.2
20
4 Conclusions
Graphene quantum dots (GQDs) with the yellowish luminescence and the diameter in the range of 20-25 nm were produced from the graphene capsules of the three graphene- encapsulated metal nanoparticles (GEMNs). GEMNs were obtained from lignin and three different transition metal salts. The structure and properties of graphene capsules and quantum dots were compared. It was found that on formation of graphene layers around metal particles the structure of graphene capsules depends slightly on the chosen metal. The photoluminescence results showed that three GQDs have similar PL properties; the emission wavelength, lifetime (in the range of 5.5- 5.2 ns) and quantum yield (within the range of 12.4-11.7 %). However, there is a slight dissimilarity in the structure of GQDs, such as the discrepancy in the ID/IG ratios of the Raman spectra and in the presence of oxygen-containing functional edge groups, according to the IR results. Based on the structural and photoluminescence results, the GQDs obtained from lignin resemble those graphene quantum dots synthesized by using a bottom-up method from glucose or obtained from resin-based carbon fibers or graphene oxide sheets.
Acknowledgements
A.B. gratefully acknowledges a financial support from the Saastamoinen Foundation. The authors thank professor Pi-Tai Chou and the Center for Optical and Laser Materials Research in National Taiwan University for their help with the photophysical measurements.
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