applied sciences Article
Direct Synthesis of Graphene Quantum Dots with Different Fluorescence Properties by Oxidation of Graphene Oxide Using Nitric Acid Meilian Zhao College of Medical Technology, Chengdu University of Traditional Chinese Medicine, Chengdu 611137, China;
[email protected]; Tel.: +86-286-180-1313 Received: 3 July 2018; Accepted: 2 August 2018; Published: 5 August 2018
Abstract: Graphene quantum dots (GQDs) play a critical role in many applications in the electrical and optical fields. We develop a simple three-step hydrothermal etching method to prepare GQDs by adopting graphene oxide (GO) as a precursor and nitric acid as an oxidant. We discuss the formation mechanism of GQDs by the characterization of products and intermediates with Scanning electronic microscopy (SEM), Transmission electron microscopic (TEM), Raman, Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Two kinds of GQDs have been obtained after the treatment of GO with different concentrations of nitric acid. The sizes of GQDs are small, with diameters of 3.38 nm and 2.03 nm on average, respectively. When excited with 365 nm UV light, the two kinds of GQDs exhibit green and yellow luminescence; the different optical properties can be attributed to the differences in degree of oxidation and nitrogen doping. The result is important for GQDs in synthesizing and optical field. Keywords: graphene quantum dots (GQDs); fluorescence; hydrothermal method; nitrogen doping
1. Introduction Graphene, a unique type of two-dimensional semiconductor with superior electronic, thermal, and mechanical properties and chemical stability, has been applied in diverse fields such as electronics, magnetics, and optics [1–4]. Graphene quantum dots (GQDs), a new kind of luminescent zero-dimensional carbon materials with finite size, have attracted tremendous interest from scientists. Because of the quantum confinement in GQDs, it is expected that they will yield many interesting phenomena which do not exist in other semiconductor materials [5,6]. Compared with traditional QDs, GQDs show many advantages, e.g., low toxicity, low-cost synthesizing strategies, good solubility in water, and organic solvents [7]. Therefore, GQDs are considered as promising candidates to replace traditional QDs, and have been integrated into many fields such as bioimaging [8,9], sensing [10], catalysts [11–13], and photovoltaic devices [14]. Nowadays, many methods have been developed to prepare GQDs using large graphene sheets as raw materials through physical and chemical methods, such as hydrothermal and solventhermal treatments, ultrasonic shearing, electrochemical oxidation, and oxygen plasma treating [15–21]. Pan et al. [15,16] synthesized strong blue and green luminescent GQDs by cutting graphene sheets through the hydrothermal route. Concentrated H2 SO4 and HNO3 were used to oxidize the graphene sheets. Zhou et al. [17] obtained GQDs by photo-Fenton reaction of graphene oxide (GO) under the UV irradiation, the GQDs have uniform crystallinity, and present a strong photoluminescence property. Li et al. [19] reported an electrochemical approach with a filtration-formed graphene film as a working electrode in PBS for direct preparation of functional GQDs, which exhibited green luminescence. Roy et al. [22] prepared GQDs from plant leaves through a green one-pot hydrothermal
Appl. Sci. 2018, 8, 1303; doi:10.3390/app8081303
www.mdpi.com/journal/applsci
Appl. Sci. 2018, 8, 1303
2 of 10
method; GO formed after 2 h and GQDs formed after 8 h of the hydrothermal reaction. The obtained GQDs displayed intense green photoluminescence (PL). Our previous work successfully synthesized different colored GQDs by cutting oxidized GO through a hydrothermal method; the oxidized GO was obtained by oxidation of GO with O3 [23]. Oxidation is an important part in synthesizing GQDs using graphene, GO, and CNT as starting materials, because it can introduce oxygen-containing groups or defects onto the surface or edge of the carbon materials, and increase active sites. Generally, oxidants used in oxidation include H2 SO4 -KMnO4 , concentrated H2 SO4 and HNO3 , Fenton reagent, O3 , and so on [15,17,24–26]. Some of the techniques mentioned above are complex and expensive. Dong et.al. [27] prepared single-layered GQDs using SWCNT as a precursor through a simple and green, three-step hydrothermal etching method. They chose different concentrations of HNO3 as oxidant; the as-prepared GQDs present green and yellow luminescence. Concentrated HNO3 is excellent in oxidation, introducing nirtrogen (N) and oxygen-containing groups into GQDs. GO, with oxygen-containing groups on the surface and edge has similar sp2 carbon networks to SWCNT, while it has a layered structure; therefore, it is predicted that GO should be an ideal raw material for preparing GQDs, adopting concentrated HNO3 as oxidant. However, to the best of our knowledge, no attention has been paid to cutting GO into GQDs by using HNO3 as oxidant. In this work, we present a simple, three-step hydrothermal method to synthesize fluorescent GQDs by using GO as raw material and HNO3 as the oxidant. Two kinds of GQDs with different degrees of oxidation and N doping have been obtained. The mechanism for the formation of GQDs and their fluorescence properties have been discussed in detail. 2. Materials and Methods 2.1. Materials All chemicals were analytically pure and used as received. Graphite powder (spectral pure, >99.85%) was purchased from Huayi Company in Shanghai, China, concentrated nitric acid (AR, 65–68%) Guanghua Sci.-Tech Co., Ltd. (Guangzhou, China). Graphene oxide sheets were synthesized with a modified Hummers’ method. The water used throughout the experiments was freshly deionized by an ultrapure water system. 2.2. Equipments Scanning electronic microscopy (SEM) images were obtained with a Field-emission scanning electron microscope (S-4800, Hitachi, Tokyo, Japan). High resolution transmission electron microscopic (HR-TEM) images were taken by a TECNAL G2 F20 (FEI Company, Hillsboro, OR, USA) electron microscope operating at 200 kV. Fourier transform infrared (FT-IR) spectra were obtained on a FT-IR spectrometer (NICOLET-6700, Thermo Scientific, Waltham, USA). Raman spectra were recorded on a Lab RAM HR with a 785 nm wavelength laser beam (HORIBA Jobin Yvon S.A.S., Les Ulis, France). X-ray photoelectron spectroscopy (XPS) data were measured by XSAM800 (Kratos, Manchester, UK) with an Al source for determining the composition and chemical bonding configurations. Fluorescence spectra were measured with a F-7000 spectrometer (Hitachi, Tokyo, Japan), which was operated with photomultiplier tube (PMT) voltage at 700 V, slit widths for excitation and emission 2.5 nm and 5.0 nm, respectively. 2.3. Synthesis of graphene quantum dots (GQDs) We prepared GO from graphite adopting a modified Hummers’ method [28,29]. The obtained GO was used as the precursor synthesizing GQDs through a three-step, top-down method. Firstly, the proper amount of GO was refluxed with 8 M HNO3 at 100 ◦ C for 24 h. During the reaction, the color of solvent darkened. When the reaction was complete, the suspension was centrifuged at 2770 g for 10 min after being cooled to room temperature in order to remove the supernatant. The
Appl. Sci. 2018, 8, 1303
3 of 10
Appl. Sci. 2018, 8, x FOR PEER REVIEW Appl. Sci. 2018, 8, x FOR PEER REVIEW
3 of 10 3 of 10
deposit (labelled as oxidized graphene oxide, O-GO) was collected after being thoroughly washed After that, the obtained suspension was centrifuged with the force of 2770 g for 10 min; a light brown with (DI) water and centrifuged. Secondly,with the the obtained O-GO 20 mL DI Afterdeionized that, the obtained suspension was centrifuged force of 2770was g fordispersed 10 min; a in light brown supernatant and black deposit (labelled as reduced O-GO, R-O-GO) was collected. The supernatant ◦ water, and then hydrothermally in reduced a Teflon-lined 200supernatant C for 8 h. supernatant andheated black deposit (labelled as O-GO,stainless R-O-GO)steel wasautoclave collected.at The showed green fluorescence under 365 nm UV light irradiation, indicating that GQDs were obtained After that, the obtained suspension wasnm centrifuged with the force of 2770 gthat for 10 min;were a light brown showed green fluorescence under 365 UV light irradiation, indicating GQDs obtained in this procedure [27]. The supernatant was dialyzed over DI water in a dialysis bag with a retained supernatant and black deposit (labelled as reduced O-GO, R-O-GO) was collected. The supernatant in this procedure [27]. The supernatant was dialyzed over DI water in a dialysis bag with a retained molecular weight of 3500 Da for two weeks, in order to remove acid and impurity ions; the obtained showed green fluorescence 365weeks, nm UV that GQDs were molecular weight of 3500 Daunder for two inlight orderirradiation, to remove indicating acid and impurity ions; the obtained obtained solution was labelled as GQD1. The black sediment was collected, as numerous graphene in this procedure [27]. The dialyzed over DI water in a dialysis bag with agraphene retained solution was labelled as supernatant GQD1. Thewas black sediment was collected, as numerous nanosheets still existed in it. Therefore, the sediment was further refluxed with 15 M HNO3 at 100 °C molecular of 3500 for two weeks, in order was to remove and impurity ions; the3 obtained nanosheetsweight still existed in Da it. Therefore, the sediment furtheracid refluxed with 15 M HNO at 100 °C for 12 h; after the reaction, the color of solution changed to transparent brown. The strong yellow solution as GQD1. sediment was collected, as numerous graphene nanosheets for 12 h;was afterlabelled the reaction, the The colorblack of solution changed to transparent brown. The strong yellow fluorescence under 365 nm UV light irradiation suggested that a lot of GQDs were released into the ◦ C for still existed in it. Therefore, the sediment was further refluxed with 15 M HNO at 100 12the h; fluorescence under 365 nm UV light irradiation suggested that a lot of GQDs were 3 released into solution. After cooling to room temperature, the solution was dialyzed over DI water to remove acid after the reaction, the color of solution changed transparent strong yellow solution. After cooling to room temperature, thetosolution was brown. dialyzedThe over DI water to fluorescence remove acid and impurity ions; as a result, GQD2 were collected. under 365 nm ions; UV light suggested that a lot of GQDs were released into the solution. After and impurity as a irradiation result, GQD2 were collected. cooling to and roomDiscussion temperature, the solution was dialyzed over DI water to remove acid and impurity 3. Results 3. Results and Discussion ions; as a result, GQD2 were collected. 3.1. Characterization and Formation Mechanism of GQDs 3. Results and Discussion 3.1. Characterization and Formation Mechanism of GQDs Oxidation and hydrothermal treatment modified the structure of GO. Different Oxidation and hydrothermal treatment modified the structure of GO. Different 3.1. Characterization andperformed Formation Mechanism of GQDs characterizations were to explain the formation mechanism of GQDs. Figure 1 shows the characterizations were performed to explain the formation mechanism of GQDs. Figure 1 shows the surface morphologies of GO, O-GO, and R-O-GO. Dried O-GO,ofand exist in sheets, and Oxidation and hydrothermal treatment modified theGO, structure GO.R-O-GO Different characterizations surface morphologies of GO, O-GO, and R-O-GO. Dried GO, O-GO, and R-O-GO exist in sheets, and O-GO and R-O-GO both present metallic appearance. The SEM images show that there were performed to explain the formation mechanism of GQDs. Figure 1 shows are thewrinkles surface O-GO and R-O-GO both present metallic appearance. The SEM images show that there are wrinkles at the surface of GO, O-GO and R-O-GO (Figure 2). HR-TEM images (Figure 3) show that and morphologies of GO, O-GO, and R-O-GO. Dried GO, O-GO, and R-O-GO exist in sheets, GQD1 and O-GO at the surface of GO, O-GO and R-O-GO (Figure 2). HR-TEM images (Figure 3) show that GQD1 and GQD2 are nanosheets with small appearance. size and circular shape. GQD1 arethat distributed diameters and R-O-GO both present metallic The SEM images show there are with wrinkles at the GQD2 are nanosheets with small size and circular shape. GQD1 are distributed with diameters ranging from 2.25 to 5.03 nm (3.38 nm in average) and GQD2 1.20 nm to 2.98 nm (2.03 nm on surface of GO, O-GO and R-O-GO (Figure 2). HR-TEM images (Figure 3) show that GQD1 and GQD2 ranging from 2.25 to 5.03 nm (3.38 nm in average) and GQD2 1.20 nm to 2.98 nm (2.03 nm on average). are nanosheets with small size and circular shape. GQD1 are distributed with diameters ranging from average). 2.25 to 5.03 nm (3.38 nm in average) and GQD2 1.20 nm to 2.98 nm (2.03 nm on average).
Figure1.1.Photographs Photographs of (a) (a) GO, (b) (b) O-GO and and (c) R-O-GO R-O-GO taken under under visible light. Figure light. GO: GO: graphene graphene Figure 1. Photographs of of (a) GO, GO, (b) O-GO O-GO and (c) (c) R-O-GO taken taken under visible visible light. GO: graphene oxide; O-GO: oxidized graphene oxide; R-O-GO: reduced O-GO. oxide; O-GO: oxidized graphene oxide; R-O-GO: reduced O-GO. oxide; O-GO: oxidized graphene oxide; R-O-GO: reduced O-GO.
Figure 2. Scanning electronic microscopy (SEM) images of (a) GO, (b) O-GO, (c) R-O-GO dispersed Figure2.2.Scanning Scanningelectronic electronic microscopy microscopy (SEM) (SEM) images images of (a) GO, (b) O-GO, (c) R-O-GO dispersed Figure in water. of (a) GO, (b) O-GO, (c) R-O-GO dispersed in water. in water.
FT-IR analysis of GO, O-GO, and R-O-GO (Figure 4) shows that there are four characteristic peaks at ∼3430, ∼2360, ∼1630, and ∼1070 cm−1 , corresponding to -OH, C=O, C=C, and C-O-C
Appl. Sci. 2018, 8, x FOR PEER REVIEW
4 of 10
Appl. Sci. 2018, 8, 1303
4 of 10
groups, respectively [30]. The absorption intensity of -OH (3437 cm−1 ) and C-O-C (1070 cm−1 ) groups weakens in R-O-GO, suggesting that -OH and C-O-C groups disappear gradually during the Appl. Sci. 2018, 8, xtreatment. FOR PEER REVIEW 4 of 10 hydrothermal
Figure 3. High resolution transmission electron microscopy (HR-TEM) images of (a) GQD1 and (b) GQD2. The insets in (a,b) are the corresponding representative images of individual GQD1 and GQD2. The right column shows the diameter distribution of GQD1 and GQD2.
FT-IR analysis of GO, O-GO, and R-O-GO (Figure 4) shows that there are four characteristic peaks at ∼3430, ∼2360, ∼1630, and ∼1070 cm−1, corresponding to -OH, C=O, C=C, and C-O-C groups, Figure 3.[30]. High resolution transmission images (a)−1(a) GQD1 andweakens (b) respectively The absorption intensityelectron of -OHmicroscopy (3437 cm−1)(HR-TEM) and C-O-C (1070ofcm ) groups Figure High resolution transmission electron microscopy (HR-TEM) images of GQD1 and GQD2. The insets in (a,b) are the corresponding representative images of individual and (b) GQD2. The insets in (a,b) are the corresponding representative images of individual GQD1 and in R-O-GO, suggesting that -OH and C-O-C groups disappear gradually during the hydrothermal GQD2. GQD2. The right column shows the diameter distribution distribution of of GQD1 GQD1 and and GQD2. GQD2. treatment. FT-IR analysis of GO, O-GO, and R-O-GO (Figure 4) shows that there are four characteristic peaks at ∼3430, ∼2360, ∼1630, and ∼1070 cm−1, corresponding to -OH, C=O, C=C, and C-O-C groups, respectively [30]. The absorption intensity of -OH (3437 cm−1) and C-O-C (1070 cm−1) groups weakens in R-O-GO, suggesting that -OH and C-O-C groups disappear gradually during the hydrothermal treatment.
Figure 4. Fourier transform infrared (FT-IR) spectra of GO, O-GO and R-O-GO. Figure 4. Fourier transform infrared (FT-IR) spectra of GO, O-GO and R-O-GO.
Furthermore, chemical oxidation can create a lot of disordered structures in GO, which is Furthermore, chemical oxidation lot ofisdisordered in GO, which is revealed revealed by the Raman spectra (Figurecan 5).create The Gaband associatedstructures with E2g vibrational modes of the by2 the Raman spectra (Figure 5).represents The G band is associated with E2g vibrational modes of the sp2 sp carbon atom, and the D band the edge defect and amorphous graphitic system [31,32]. carbon atom, ratio and the band represents thetoedge defect and amorphous system [31,32]. The intensity ID/IGD , “disorder structure” the “crystalline structure” isgraphitic adopted to evaluate the The intensity ratio I /I , “disorder structure” to the “crystalline structure” is adopted to evaluate D G Figure 4. Fourier transform infrared (FT-IR) spectra of GO, O-GO and R-O-GO. structure of different oxidized GO. An increase in the ID/IG means an increase in the amount of the structuredisorder of different oxidized Anand increase in thein IDthe /IGsize means an increase in the amount of topological in the graphiteGO. layer a decrease of nanocrystalline graphite [30]. topological disorder in the graphite layer and a decrease in the size of nanocrystalline graphite [30]. Furthermore, oxidation can create a lot of disordered structures in GO, whichare is The Raman Spectrachemical of GO, O-GO and R-O-GO given in Figure 5 show that the D and G bands The Raman of GO, O-GO and R-O-GO inassociated Figure 5 show the D and modes G bands revealed by Spectra the Raman spectra (Figure 5). The Ggiven band is with that E2g vibrational of are the sp2 carbon atom, and the D band represents the edge defect and amorphous graphitic system [31,32]. The intensity ratio ID/IG, “disorder structure” to the “crystalline structure” is adopted to evaluate the structure of different oxidized GO. An increase in the ID/IG means an increase in the amount of topological disorder in the graphite layer and a decrease in the size of nanocrystalline graphite [30]. The Raman Spectra of GO, O-GO and R-O-GO given in Figure 5 show that the D and G bands are
Appl. Sci. 2018, 8, 1303 Appl. Sci. 2018, 8, x FOR PEER REVIEW
5 of 10 5 of 10
respectively. The The intensity ratio ID /IGGdecreases decreasesfrom from1.02 1.02(GO) (GO)to to0.96 0.96 (O-GO), (O-GO), 1345 and 1588 1588 cm cm−−11, ,respectively. D/I general rule. We deduce that after treatment of GO by , the obtained which isiscontrary contrarytotothethe general rule. We deduce that the after the treatment of HNO GO by HNO 3, the 3 O-GO is of relatively quality. However, O-GO exhibits a broader D band, which that obtained O-GO is of high relatively high quality.the However, the O-GO exhibits a broader D indicates band, which the intercalation of N atoms and oxygen-containing groups leads to somewhat structures. indicates that the intercalation of N atoms and oxygen-containing groupsdisordered leads to somewhat This result isstructures. similar to aThis previously-reported conclusion [20]. It was predicted that the intensity disordered result is similar to a previously-reported conclusion [20]. It ratio was 2 ID /IG of GO significantly after reduction as the spdecrease structure restored. However, the2 predicted thatwould the intensity ratio decrease ID/IG of GO would significantly after reduction as the sp ID /IG wasrestored. found toHowever, be 1.15 in R-O-GO afterfound hydrothermal treatment, which larger than that in O-GO. structure the ID/IG was to be 1.15 in R-O-GO afterishydrothermal treatment, Similarisresults reported elsewhere the also literature [33,34]. We deduce that theliterature increase which largerhave than also that been in O-GO. Similar results in have been reported elsewhere in the of the IDWe /IGdeduce ratio after reduction attributed to the decrease incan thebe average size to of the [33,34]. thatchemical the increase of the Ican D/IGbe ratio after chemical reduction attributed sp2 domains upon reduction which newly-created graphitic domains are more numerous decrease in the average size of of O-GO, the sp2indomains upon reduction of O-GO, in which newly-created in number, but smaller in size, than those present but in O-GO. Besides increased of graphitic domains are more numerous in number, smaller in size,that, thanthe those presentfraction in O-GO. graphene edges also contribute the increase in could the ID /I ratio, which is in accordance with Besides that, thecould increased fraction ofto graphene edges also contribute to the increase in the G results [33,35]. Ithe D/IGreported ratio, which is in accordance with the reported results [33,35].
Figure Figure 5. 5. Raman Raman spectra spectra of of GO, GO, O-GO O-GO and and R-O-GO. R-O-GO.
The chemical structure and the type of oxygen-containing groups of GO changed after The chemical structure and the type of oxygen-containing groups of GO changed after oxidation oxidation and hydrothermal treatment. XPS analysis indicates that N atoms are introduced into GO and hydrothermal treatment. XPS analysis indicates that N atoms are introduced into GO after after oxidation by HNO3, as shown in Figure 6. The high-resolution XPS spectra of C 1s further oxidation by HNO3 , as shown in Figure 6. The high-resolution XPS spectra of C 1s further confirm the confirm the chemical structure change during the oxidation and hydrothermal treatment. As shown chemical structure change during the oxidation and hydrothermal treatment. As shown in Figure 7, in Figure 7, different types of C atoms exist, including C-C/C=C (∼284.5 eV), C-N (∼285.6 eV), different types of C atoms exist, including C-C/C=C (∼284.5 eV), C-N (∼285.6 eV), C-O-C/C-OH C-O-C/C-OH (∼286.5 eV) and C=O/COOH (∼287.8 eV) [30,36]. There are mainly carboxyl and epoxy (∼286.5 eV) and C=O/COOH (∼287.8 eV) [30,36]. There are mainly carboxyl and epoxy groups in groups in GO prepared by Hummers’ method, the amount of C-O increases while C-C and C=C GO prepared by Hummers’ method, the amount of C-O increases while C-C and C=C decreases after decreases after oxidation by HNO3, which indicates that oxidation makes the C-C and C=C bonds oxidation by HNO3 , which indicates that oxidation makes the C-C and C=C bonds break and attach break and attach to oxygen-containing groups. The amount of C increases and O decreases in to oxygen-containing groups. The amount of C increases and O decreases in R-O-GO, which can be R-O-GO, which can be explained from the fact that a lot of oxygen-containing groups are released in explained from the fact that a lot of oxygen-containing groups are released in the form of CO2 , CO, and the form of CO2, CO, and H2O during the hydrothermal treatment [23]. The decreased amount of H2 O during the hydrothermal treatment [23]. The decreased amount of C-O-C and C-O in R-O-GO C-O-C and C-O in R-O-GO is in good agreement with the results of FT-IR. Moreover, the is in good agreement with the results of FT-IR. Moreover, the high-resolution XPS spectra of N 1s high-resolution XPS spectra of N 1s (Figure 8a,b) also show that C-N exists in O-GO and R-O-GO, (Figure 8a,b) also show that C-N exists in O-GO and R-O-GO, suggesting that N atoms are introduced suggesting that N atoms are introduced into GO in different forms including C-N-C (∼399.5 eV) and into GO in different forms including C-N-C (∼399.5 eV) and N-(C)3 (∼401.5eV) [37–39]. The XPS N-(C)3 (∼401.5eV) [37–39]. The XPS results indicate that both GQD1 and GQD2 are composed of results indicate that both GQD1 and GQD2 are composed of carbon, oxygen, hydrogen, and nitrogen carbon, oxygen, hydrogen, and nitrogen (Figures 7 and 8). N 1s analysis shows that N exists in (Figures 7 and 8). N 1s analysis shows that N exists in GQD1 in the forms of C-N-C and N-(C)3 , GQD1 in the forms of C-N-C and N-(C)3, while in GQD2, besides the two forms of N, -NO2 (406.4 eV) while in GQD2, besides the two forms of N, -NO2 (406.4 eV) also exists [37,39–42]; see in Figure 8c,d. also exists [37,39–42]; see in Figure 8c,d. XPS analysis gives more information about the chemical XPS analysis gives more information about the chemical structure of the studied systems. Element structure of the studied systems. Element analysis of GO, O-GO and R-O-GO by FT-IR and XPS analysis of GO, O-GO and R-O-GO by FT-IR and XPS presents similar results; these techniques are presents similar results; these techniques are complementary and exhibit more accurate conclusions complementary and exhibit more accurate conclusions when used together. when used together.
Appl. Sci. 2018, 8, 1303 Appl. Sci. 2018, 8, x FOR PEER REVIEW Appl. Sci. 2018, 8, x FOR PEER REVIEW
6 of 10 6 of 10 6 of 10
Figure 6. 6. XPS survey of GO, GO, O-GO, R-O-GO, R-O-GO, GQD1 and and GQD2. Figure Figure 6. XPS XPS survey survey of of GO, O-GO, O-GO, R-O-GO, GQD1 GQD1 and GQD2. GQD2.
Figure 7. High resolution XPS spectra of C 1s for (a) GO, (b) O-GO, (c) R-O-GO, (d) GQD1 and (e) Figure 7. High resolution XPS spectra of C 1s for (a) GO, (b) O-GO, (c) R-O-GO, (d) GQD1 and (e) Figure GQD2. 7. High resolution XPS spectra of C 1s for (a) GO, (b) O-GO, (c) R-O-GO, (d) GQD1 and GQD2. (e) GQD2.
Appl. Sci. 2018, 8, 1303 Appl. Sci. 2018, 8, x FOR PEER REVIEW
7 of 10 7 of 10
Figure 8. High resolution XPS spectra of N 1s for (a) O-GO, (b) R-O-GO, (c) GQD1 and (d) GQD2. Figure 8. High resolution XPS spectra of N 1s for (a) O-GO, (b) R-O-GO, (c) GQD1 and (d) GQD2.
Based on the results mentioned above, a possible mechanism for the formation of the GQDs can Based on GO the results mentioned above, possible mechanism for theoxidation formationwith of the8 GQDs can be proposed. was initially oxidized to aO-GO during the chemical M HNO 3, be proposed. GO was initially oxidized to O-GO during the chemical oxidation with 8 M HNO 3, producing a lot of oxygen-containing groups and introducing N atoms. After that, the structure of producing lot of oxygen-containing groups and introducing N atoms. After that, theforming structurea O-GO was adecomposed, because the oxygen-containing groups were partially removed, of was decomposed, the oxygen-containing were partially removed, forming lotO-GO of nanosheets (R-O-GO)because by the hydrothermal method groups under the continuous high-temperature aand lot high-pressure of nanosheets (R-O-GO) by the hydrothermal method under the continuous high-temperature and conditions. Meanwhile, plenty of graphene quantum dots (GQD1) were formed high-pressure conditions. Meanwhile, plenty of graphene quantum dots (GQD1) were formed and released and released into the solution. Finally, the collected R-O-GO was further oxidized with concentrated into the solution. Finally, the collected R-O-GO was further oxidized with concentrated HNO3 . Chemical HNO 3. Chemical oxidation is an effective method to release GQDs from their aggregates into oxidation is anAs effective method from theirinto aggregates into solution [43]. Aswas a result, a lot solution [43]. a result, a lot to ofrelease GQD2GQDs were released the solution after R-O-GO refluxed of GQD2 released into the solution after R-O-GO was refluxed with 15 M HNO3 for 12 h. with 15 Mwere HNO 3 for 12 h.
3.2. Fluorescence Properties of GQDs 3.2. Fluorescence Properties of GQDs The aqueous solutions of GQD1 and GQD2 are light brown under visible light, but they have The aqueous solutions of GQD1 and GQD2 are light brown under visible light, but they have different fluorescence under an excitation of a beam of 365 nm. GQD1 and GQD2 exhibit green and different fluorescence under an excitation of a beam of 365 nm. GQD1 and GQD2 exhibit green and yellow fluorescence, respectively. Figure 9 demonstrates the PL spectra of the two GQDs. The emission yellow fluorescence, respectively. Figure 9 demonstrates the PL spectra of the two GQDs. The spectra of GQD1 show obvious excitation-dependence. With the gradual increase of the excitation emission spectra of GQD1 show obvious excitation-dependence. With the gradual increase of the wavelength from 280 nm to 480 nm, the maximum emission wavelength of GQD1 is red-shifted from excitation wavelength from 280 nm to 480 nm, the maximum emission wavelength of GQD1 is 408 nm to 500 nm. However, the emission wavelength of GQD2 keeps stable at about 535 nm during red-shifted from 408 nm to 500 nm. However, the emission wavelength of GQD2 keeps stable at the excitation wavelengths we employed. The different PL properties of GQD1 and GQD2 may be about 535 nm during the excitation wavelengths we employed. The different PL properties of GQD1 attributed to their size and surface state, which are affected by functional groups [7,36,44,45]. Surface and GQD2 may be attributed to their size and surface state, which are affected by functional groups decoration degree and size can change the energy gap of GQD, thus affecting the PL properties. Our [7,36,44,45]. Surface decoration degree and size can change the energy gap of GQD, thus affecting analysis of HR-TEM shows that GQD1 are larger than GQD2 in size; this can lead to a red shift of the PL properties. Our analysis of HR-TEM shows that GQD1 are larger than GQD2 in size; this can GQD1 compared to GQD2, which is contrary to our result. Therefore, surface decoration is the major lead to a red shift of GQD1 compared to GQD2, which is contrary to our result. Therefore, surface reason for the different PL properties. XPS analysis shows that N and O atoms exist in both GQD1 and decoration is the major reason for the different PL properties. XPS analysis shows that N and O GQD2; C 1s XPS indicates that the amounts of N and O are greater in GQD2 than GQD1 (Figure 7d,e). atoms exist in both GQD1 and GQD2; C 1s XPS indicates that the amounts of N and O are greater in In addition, N atoms doped in GQDs with different forms: besides C-N-C and N-(C)3 , -NO2 can be GQD2 than GQD1 (Figure 7d,e). In addition, N atoms doped in GQDs with different forms: besides found in GQD2 (Figure 8c,d). From the above analysis, it is reasonable to conclude that the difference C-N-C and N-(C)3, -NO2 can be found in GQD2 (Figure 8c,d). From the above analysis, it is reasonable to conclude that the difference in optical properties between GQD1 and GQD2 should be attributed to their differences in degree of oxidation and N doping.
Appl. Sci. 2018, 8, 1303
8 of 10
in optical properties between GQD1 and GQD2 should be attributed to their differences in degree of oxidation and doping. Appl. Sci. 2018, 8, xN FOR PEER REVIEW 8 of 10
Figure 9. Fluorescence spectra of (a) GQD1 and (b) GQD2 under different excitation wavelengths. Figure 9. Fluorescence spectra of (a) GQD1 and (b) GQD2 under different excitation wavelengths.
4. Conclusions 4. Conclusions A simple three-step method to prepare small-sized GQDs, adopting GO as precursor, has been A simple three-step method to prepare small-sized GQDs, adopting GO as precursor, has been developed. In the first step, GO was oxidized into O-GO by 8 M HNO3. Secondly, the obtained developed. In the first step, GO was oxidized into O-GO by 8 M HNO3 . Secondly, the obtained O-GO O-GO was dispersed in DI water and heated hydrothermally ◦to 200 °C, resulting in the formation of was dispersed in DI water and heated hydrothermally to 200 C, resulting in the formation of GQD1 GQD1 and R-O-GO. Finally, R-O-GO was refluxed with 15 M HNO3 under◦ 100 °C, forming GQD2. and R-O-GO. Finally, R-O-GO was refluxed with 15 M HNO3 under 100 C, forming GQD2. The The diameters of GQD1 and GQD2 are 3.38 nm and 2.03 nm on average, respectively. Oxidation by diameters of GQD1 and GQD2 are 3.38 nm and 2.03 nm on average, respectively. Oxidation by HNO3 HNO3 results in the introduction of N atoms and oxygen-containing groups. N atoms doped in results in the introduction of N atoms and oxygen-containing groups. N atoms doped in GQDs in the GQDs in the form of C-N-C and N-(C)3 and -NO2. The GQDs show different optical properties, form of C-N-C and N-(C)3 and -NO2 . The GQDs show different optical properties, which is caused by which is caused by the different degree of oxidation and N doping. the different degree of oxidation and N doping. Author Contributions: Meilian Zhao conceived, designed, performed the experiments and wrote the paper. Author Contributions: Meilian Zhao conceived, designed, performed the experiments and wrote the paper. Funding: was funded funded by by the the National NationalNatural NaturalScience ScienceFoundation FoundationofofChina China(81503366) (81503366)and and Funding: This This research research was Natural Science Foundation of the Education Department of Sichuan ProvinceProvince (16ZA0109). Natural Science Foundation of the Education Department of Sichuan (16ZA0109). Acknowledgments: We sincerely thank Prof.Yong Guo and Dan Xiao and Dr. Feng Yang in College of Chemistry, Acknowledgments: We sincerelyimportant thank Prof.Yong Guo Xiao and Dr. Feng Yang in College of Sichuan University for providing suggestions andand helpDan in this work. Chemistry, Sichuan University for providing important suggestions and help in this work. Conflicts of Interest: The author declares no conflict of interest. Conflicts of Interest: The author declares no conflict of interest.
References References 1. 1. 2. 2. 3. 3.
4. 4. 5. 5. 6. 6. 7. 7. 8.
9.
Novoselov, K.S.; Geim, A.K.; Morozov, S.V.; Jiang, D.; Zhang, Y.; Dubonos, S.V.; Grigorieva, I.V.; Firsov, A.A. Novoselov, Geim, A.K.; Morozov, S.V.;films. Jiang,Science D.; Zhang, Dubonos, Grigorieva, I.V.; Firsov, Electric fieldK.S.; effect in atomically thin carbon 2004,Y.; 306, 666–669. S.V.; [CrossRef] [PubMed] A.A. Electric field effect in atomically thin carbon films. Sci. 2004, 306, 666–669. Kuila, T.; Mishra, A.K.; Khanra, P.; Kim, N.H.; Lee, J.H. Recent advances in the efficient reduction of graphene Kuila, T.; its Mishra, A.K.; as Khanra, Kim,electrode N.H.; Lee, J.H. Recent advances the efficient reduction of oxide and application energy P.; storage materials. Nanoscale 2013, 5,in52–71. [CrossRef] [PubMed] graphene oxide and its application energyT.;storage electrode materials. Nanoscale 5, 52–71. Shen, F.; Wang, D.; Liu, R.; Pei, X.; as Zhang, Jin, J. Edge-tailored graphene oxide 2013, nanosheet-based field Shen, F.; Wang, D.; Liu, R.; Pei, X.; Zhang, T.; Jin, J. Edge-tailored graphene oxide nanosheet-based field effect transistors for fast and reversible electronic detection of sulfur dioxide. Nanoscale 2013, 5, 537–540. effect transistors for fast and reversible electronic detection of sulfur dioxide. Nanoscale 2013, 5, 537–540. [CrossRef] [PubMed] Fowler, Kaner, R.B.; R.B.; Weiller, B.H. Practical Practical chemical chemical sensors Fowler, J.D.; J.D.; Allen, Allen, M.J.; M.J.; Tung, Tung, V.C.; V.C.; Yang, Yang, Y.; Y.; Kaner, Weiller, B.H. sensors from from chemically chemically derived derived graphene. graphene. ACS ACS Nano Nano 2009, 2009, 3, 3, 301–306. 301–306. [CrossRef] [PubMed] Son, Cohen, M.L.; M.L.; Louie, Louie, S.G. S.G. Energy Energy gaps gaps in in graphene graphene nanoribbons. nanoribbons. Phys. Son, Y.W.; Y.W.; Cohen, Phys. Rev. Rev. Lett. Lett. 2006, 2006, 97, 97,216803. 216803. Nakada, K.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M.S. Edge state in graphene ribbons: Nanometer size [CrossRef] [PubMed] effect andK.; edge shape Rev. B 1996, M.S. 54, 17954–17961. Nakada, Fujita, M.;dependence. Dresselhaus,Phys. G.; Dresselhaus, Edge state in graphene ribbons: Nanometer size Shen, J.; Zhu, Yang, X.; Li, C. Graphene dots: emergent nanolights effect and edgeY.;shape dependence. Phys. Rev.quantum B 1996, 54, 17954–17961. [CrossRef] for bioimaging, sensors, catalysis and photovoltaic devices. Chem. Commun. 2012, 48, 3686–3699. Shen, J.; Zhu, Y.; Yang, X.; Li, C. Graphene quantum dots: emergent nanolights for bioimaging, sensors, Liu, Q.; Guo, Rao, Z.; Zhang, B.; Chem. Gong, Commun. J.R. Strong two-photon-induced fluorescence from photostable, catalysis and B.; photovoltaic devices. 2012, 48, 3686–3699. [CrossRef] [PubMed] biocompatible nitrogen-doped graphene quantum dots for cellular and deep-tissue imaging. Nano Lett. 2013, 13, 2436–2441. Lv, O.Y.; Tao, Y.X.; Qin, Y.; Chen, C.X.; Pan, Y.; Deng, L.H.; Liu, L.; Kong, Y. Highly fluorescent and morphology-controllable graphene quantum dots-chitosan hybrid xerogels for in vivo imaging and pH-sensitive drug carrier. Mater. Sci. Eng. C 2016, 67, 478–485.
Appl. Sci. 2018, 8, 1303
8.
9.
10.
11. 12. 13.
14.
15. 16.
17.
18. 19.
20. 21.
22. 23.
24. 25. 26. 27. 28.
9 of 10
Liu, Q.; Guo, B.; Rao, Z.; Zhang, B.; Gong, J.R. Strong two-photon-induced fluorescence from photostable, biocompatible nitrogen-doped graphene quantum dots for cellular and deep-tissue imaging. Nano Lett. 2013, 13, 2436–2441. [CrossRef] [PubMed] Lv, O.Y.; Tao, Y.X.; Qin, Y.; Chen, C.X.; Pan, Y.; Deng, L.H.; Liu, L.; Kong, Y. Highly fluorescent and morphology-controllable graphene quantum dots-chitosan hybrid xerogels for in vivo imaging and pH-sensitive drug carrier. Mater. Sci. Eng. C 2016, 67, 478–485. [CrossRef] [PubMed] Tan, F.; Cong, L.; Li, X.; Zhao, Q.; Zhao, H.; Quan, X.; Chen, J. An electrochemical sensor based on molecularly imprinted polypyrrole/graphene quantum dots composite for detection of bisphenol A in water samples. Sens. Actuat. B-Chem 2016, 233, 599–606. [CrossRef] Liu, K.; Song, Y.; Chen, S. Oxygen reduction catalyzed by nanocomposites based on graphene quantum dots-supported copper nanoparticles. Int. J. Hydrogen Energy 2016, 41, 1559–1567. [CrossRef] Tian, H.W.; Shen, K.; Hu, X.Y.; Qiao, L.; Zheng, W.T. N, S co-doped graphene quantum dots-graphene-TiO2 nanotubes composite with enhanced photocatalytic activity. J. Alloy. Compd. 2017, 691, 369–377. [CrossRef] Shen, K.; Xue, X.; Wang, X.Y.; Hu, X.Y.; Tian, H.W.; Zheng, W.T. One-step synthesis band-tunable N, S co-doped commercial TiO2 /graphene quantum dots composite with enhanced photocatalytic activity. RSC Adv. 2017, 7, 23319–23327. [CrossRef] Moon, B.J.; Lee, K.S.; Shim, J.; Park, S.; Kim, S.H.; Bae, S.; Park, M.; Lee, C.L.; Choi, W.K.; Yi, Y.; et al. Enhanced photovoltaic performance of inverted polymer solar cells utilizing versatile chemically functionalized ZnO@graphene quantum dot monolayer. Nano Energy 2016, 20, 221–232. [CrossRef] Pan, D.Y.; Zhang, J.C.; Li, Z.; Wu, M.H. Hydrothermal route for cutting graphene sheets into blue-luminescent graphene quantum dots. Adv. Mater. 2010, 22, 734–738. [CrossRef] [PubMed] Pan, D.Y.; Guo, L.; Zhang, J.C.; Xi, C.; Xue, Q.; Huang, H.; Li, J.H.; Zhang, Z.W.; Yu, W.J.; Chen, Z.W.; et al. Cutting sp(2) clusters in graphene sheets into colloidal graphene quantum dots with strong green fluorescence. J. Mater. Chem. 2012, 22, 3314–3318. [CrossRef] Zhou, X.; Zhang, Y.; Wang, C.; Wu, X.; Yang, Y.; Zheng, B.; Wu, H.; Guo, S.; Zhang, J. Photo-fenton reaction of graphene oxide: a new strategy to prepare graphene quantum dots for DNA cleavage. ACS Nano 2012, 6, 6592–6599. [CrossRef] [PubMed] Zhuo, S.; Shao, M.; Lee, S.T. Upconversion and downconversion fluorescent graphene quantum dots: ultrasonic preparation and photocatalysis. ACS Nano 2012, 6, 1059–1064. [CrossRef] [PubMed] Li, Y.; Hu, Y.; Zhao, Y.; Shi, G.Q.; Deng, L.E.; Hou, Y.B.; Qu, L.T. An electrochemical avenue to green-luminescent graphene quantum dots as potential electron-acceptors for photovoltaics. Adv. Mater. 2011, 23, 776–780. [CrossRef] [PubMed] Li, Y.; Zhao, Y.; Cheng, H.; Hu, Y.; Shi, G.; Dai, L.; Qu, L. Nitrogen-doped graphene quantum dots with oxygen-rich functional groups. J. Am. Chem. Soc. 2012, 134, 15–18. [CrossRef] [PubMed] Gokus, T.; Nair, R.R.; Bonetti, A.; Boehmler, M.; Lombardo, A.; Novoselov, K.S.; Geim, A.K.; Ferrari, A.C.; Hartschuh, A. Making graphene luminescent by oxygen plasma treatment. ACS Nano 2009, 3, 3963–3968. [CrossRef] [PubMed] Roy, P.; Periasamy, A.P.; Chuang, C.; Liou, Y.R.; Chen, Y.F.; Joly, J.; Liang, C.T.; Chang, H.T. Plant leaf-derived graphene quantum dots and applications for white LEDs. New J. Chem. 2014, 38, 4946–4951. [CrossRef] Yang, F.; Zhao, M.L.; Zheng, B.Z.; Xiao, D.; Wu, L.; Guo, Y. Influence of pH on the fluorescence properties of graphene quantum dots using ozonation pre-oxide hydrothermal synthesis. J. Mater. Chem. 2012, 22, 25471–25479. [CrossRef] Galande, C.; Mohite, A.D.; Naumov, A.V.; Gao, W.; Ci, L.J.; Ajayan, A.; Gao, H.; Srivastava, A.; Weisman, R.B.; Ajayan, P.M. Quasi-molecular fluorescence from graphene oxide. Sci. Rep. 2011, 1, 1–5. [CrossRef] [PubMed] Chen, J.L.; Yan, X.P.; Meng, K.; Wang, S.F. Graphene oxide based photoinduced charge transfer label-free near-infrared fluorescent biosensor for dopamine. Anal. Chem. 2011, 83, 8787–8793. [CrossRef] [PubMed] Liu, L.V.; Tian, W.Q.; Wang, Y.A. Ozonization at the vacancy defect site of the single-walled carbon nanotube. J. Phys. Chem. B 2006, 110, 13037–13044. [CrossRef] [PubMed] Dong, Y.; Pang, H.; Ren, S.; Chen, C.; Chi, Y.; Yu, T. Etching single-wall carbon nanotubes into green and yellow single-layer graphene quantum dots. Carbon 2013, 64, 245–251. [CrossRef] Xu, Y.X.; Bai, H.; Lu, G.W.; Li, C.; Shi, G.Q. Flexible graphene films via the filtration of water-soluble noncovalent functionalized graphene sheets. J. Am. Chem. Soc. 2008, 130, 5856–5857. [CrossRef] [PubMed]
Appl. Sci. 2018, 8, 1303
29.
30. 31. 32. 33. 34. 35. 36.
37.
38. 39. 40. 41. 42.
43. 44.
45.
10 of 10
Kovtyukhova, N.I.; Ollivier, P.J.; Martin, B.R.; Mallouk, T.E.; Chizhik, S.A.; Buzaneva, E.V.; Gorchinskiy, A.D. Layer-by-layer assembly of ultrathin composite films from micron-sized graphite oxide sheets and polycations. Chem. Mater. 1999, 11, 771–778. [CrossRef] Lin, Z.; Yao, Y.; Li, Z.; Liu, Y.; Li, Z.; Wong, C.P. Solvent-assisted thermal reduction of graphite oxide. J. Phys. Chem. C 2010, 114, 14819–14825. [CrossRef] Casiraghi, C.; Hartschuh, A.; Qian, H.; Piscanec, S.; Georgi, C.; Fasoli, A.; Novoselov, K.S.; Basko, D.M.; Ferrari, A.C. Raman spectroscopy of graphene edges. Nano Lett. 2009, 9, 1433–1441. [CrossRef] [PubMed] Malard, L.M.; Pimenta, M.A.; Dresselhaus, G.; Dresselhaus, M.S. Raman spectroscopy in graphene. Phys. Rep. 2009, 473, 51–87. [CrossRef] Ren, P.G.; Yan, D.X.; Ji, X.; Chen, T.; Li, Z.M. Temperature dependence of graphene oxide reduced by hydrazine hydrate. Nanotechnology 2011, 22, 055705. [CrossRef] [PubMed] Qian, M.; Feng, T.; Ding, H.; Lin, L.; Li, H.; Chen, Y.; Sun, Z. Electron field emission from screen-printed graphene films. Nanotechnology 2009, 20, 425702. [CrossRef] [PubMed] Bo, Z.; Shuai, X.; Mao, S.; Yang, H.; Qian, J.; Chen, J.; Yan, J.; Cen, K. Green preparation of reduced graphene oxide for sensing and energy storage applications. Sci. Rep. 2014, 4, 4684. [CrossRef] [PubMed] Li, L.L.; Ji, J.; Fei, R.; Wang, C.Z.; Lu, Q.; Zhang, J.R.; Jiang, L.P.; Zhu, J.J. A facile microwave avenue to electrochemiluminescent two-color graphene quantum dots. Adv. Funct. Mater. 2012, 22, 2971–2979. [CrossRef] Lu, W.; Qin, X.; Liu, S.; Chang, G.; Zhang, Y.; Luo, Y.; Asiri, A.M.; Al-Youbi, A.O.; Sun, X. Economical, green synthesis of fluorescent carbon nanoparticles and their use as probes for sensitive and selective detection of mercury (II) ions. Anal. Chem. 2012, 84, 5351–5357. [CrossRef] [PubMed] Wang, Y.; Shao, Y.; Matson, D.W.; Li, J.; Lin, Y. Nitrogen-doped graphene and its application in electrochemical biosensing. ACS Nano 2010, 4, 1790–1798. [CrossRef] [PubMed] Shao, Y.; Zhang, S.; Engelhard, M.H.; Li, G.; Shao, G.; Wang, Y.; Liu, J.; Aksay, I.A.; Lin, Y. Nitrogen-doped graphene and its electrochemical applications. J. Mater. Chem. 2010, 20, 7491–7496. [CrossRef] Matter, P.H.; Zhang, L.; Ozkan, U.S. The role of nanostructure in nitrogen-containing carbon catalysts for the oxygen reduction reaction. J. Catal. 2006, 239, 83–96. [CrossRef] Arrigo, R.; Haevecker, M.; Schloegl, R.; Su, D.S. Dynamic surface rearrangement and thermal stability of nitrogen functional groups on carbon nanotubes. Chem. Commun. 2008, 4891–4893. [CrossRef] [PubMed] Yang, F.; Zhao, M.L.; Ji, H.Y.; He, D.H.; Wu, L.; Zheng, B.Z.; Xiao, D.; Guo, Y. Solvothermal synthesis of oxygen/nitrogen functionalized graphene-like materials with diversified morphology from different carbon sources and their fluorescence properties. J. Mater. Sci. 2015, 50, 1300–1308. [CrossRef] Liu, R.; Wu, D.; Feng, X.; Müllen, K. Bottom-up fabrication of photoluminescent graphene quantum dots with uniform morphology. J. Am. Chem. Soc. 2011, 133, 15221–15223. [CrossRef] [PubMed] Zhu, S.; Zhang, J.; Tang, S.; Qiao, C.; Wang, L.; Wang, H.; Liu, X.; Li, B.; Li, Y.; Yu, W. Surface chemistry routes to modulate the photoluminescence of graphene quantum dots: From fluorescence mechanism to up-conversion bioimaging applications. Adv. Funct. Mater. 2012, 22, 4732–4740. [CrossRef] Bao, L.; Zhang, Z.L.; Tian, Z.Q.; Zhang, L.; Liu, C.; Lin, Y.; Qi, B.; Pang, D.W. Electrochemical tuning of luminescent carbon nanodots: From preparation to luminescence mechanism. Adv. Mater. 2011, 23, 5801–5806. [CrossRef] [PubMed] © 2018 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
