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Graphene quantum dots prepared from chemical exfoliation of multiwall carbon nanotubes: An efficient photocatalyst promoter
Catalysis Communications 74 (2016) 104–109
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Graphene quantum dots prepared from chemical exfoliation of multiwall carbon nanotubes: An efficient photocatalyst promoter Shu Wei a,b, Ran Zhang a, Yan Liu c, Hong Ding d, Yong-Lai Zhang a,⁎ a
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, China College of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun, Jilin 130022, China Key Laboratory of Bionic Engineering, Ministry of Education, Jilin University, Changchun 130012, China d State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China b c
a r t i c l e
i n f o
Article history: Received 22 September 2015 Received in revised form 8 November 2015 Accepted 13 November 2015 Available online 15 November 2015 Keywords: Graphene quantum dots Multiwall carbon nanotubes Exfoliation Visible-light photocatalysts
a b s t r a c t We report here a facile preparation of graphene quantum dots (GQDs) by chemical exfoliation of multiwall carbon nanotubes (MWCNTs) using a modified hummers' method. The resultant GQD samples possess strong electronic property, revealing great potential for photocatalyst design. As an efficient promoter, GQDs/P25 nanocomposites have been successfully prepared by simple wet impregnation and subsequent thermal annealing at 200 °C. In the tests of photocatalytic degradation of organic dyes under visible-light irradiation, the GQDs promoted P25 samples which shows much higher photocatalytic activity compared to the pure P25, indicating the crucial roles of GQDs. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Recently, zero-dimensional carbon materials, represented by fullerene and carbon dots, have attracted enormous research interests due to their unique physical/chemical properties [1–10]. Especially, carbon quantum dots (CQDs) including graphitic nanoparticles, amorphous carbon dots, and graphene quantum dots (GQDs) have reveal great potential for a wide range of applications, such as fluorescent probes [11], photovoltaic devices [12], photocatalysis [13–14], light-emitting diodes [15] and bioimaging [16–18]. As compared with conventional semiconductor quantum dots, CQDs exhibit a series of unique advantages. For instance, CQDs are generally nontoxic and thus biocompatible, since they consist of only carbon and some oxygen functional groups. Additionally, they could be excited over a broad spectral range from the visible to near IR region, and usually show clear up-conversion PL properties, thus CQDs have been considered as a promising candidate for rational design of novel visible-light driven photocatalysts [19–20]. Moreover, CQDs are excellent electron donors and acceptors in photoexcited states; which makes them promising for various optoelectronic devices [21]. The exceptional superiorities continuously stimulate the rapid progress of methodologies for CQDs preparation. Generally, the preparation strategies could be classified into two broad categories: top-down and bottom-up approaches. The former route usually generate ultra-small fragment of carbon materials (e.g., graphite, graphene, carbon soot ⁎ Corresponding author. E-mail address: [email protected] (Y.-L. Zhang).
http://dx.doi.org/10.1016/j.catcom.2015.11.010 1566-7367/© 2016 Elsevier B.V. All rights reserved.
and fullerene) using laser ablation, ultra-sonic treatment, electrochemical etching and hydrothermal cutting; whereas, the later method resorts to carbonization or organic molecules, or graphitization of polycyclic aromatic hydrocarbon [22–27]. However, despite the rapid development of synthetic methodologies, current preparation methods still suffer from serious problems with respect to complex procedures, the requirement of special equipment, incontrollable morphology and size, as well as low yields, which significantly limits their broad applications in various scientific fields. In this regard, novel methods for allowing facile preparation of high quality CQDs/GQDs with high-yield are highly desired. Herein, we present a facile chemical exfoliation of multiwall carbon nanotubes (MWCNTs) for preparation of GQDs via a modified hummers' method [28]. The resultant GQDs were very uniform in size, and possess strong electronics property. Using our GQDs as photocatalyst promoters, we demonstrate the design and preparation of GQDs/P25 (TiO2) nanocomposites, which show significantly promoted photocatalytic performance for the degradation of organic dyes under visible-light irradiation. 2. Experimental section 2.1. Preparation of GQDs from CNTs GQDs were prepared from chemical exfoliation of purified MWCNTs by a modified hummers' method. In a typical synthesis, 200 mg of MWCNTs were immersed in 46 mL conc. H2SO4 and then 6 g of KMnO4 was added slowly in small quantities under mild stirring while
S. Wei et al. / Catalysis Communications 74 (2016) 104–109
the temperature was maintained between 0 and 5 °C by using an ice bath. After the complete addition of KMnO4, the mixture was heated to 37 °C and kept at this temperature for about 1 h. Then, distilled water (92 mL) was added slowly. The temperature of the mixture was raised to 95 °C and maintained for about 12 h. The mixture was further diluted with 280 mL of water and later 20 mL of 30% H2O2 was added. The resultant mixture was purified by following procedure: firstly, low-speed centrifugation was done at 4000 rpm for 15 min about 3–5 times to the removal of the unexfoliated MWCNTs and some watersoluble by-product. Secondly, high-speed centrifugation was implemented at 12,000 rpm for 15 min to collect GQDs samples. Then, the precipitates were redispersed in water with mild sonication and then the remainder mixture was dialyzed in a dialysis bag for 48 h to ensure the complete removal of residual metallic impurities and acid. 2.2. Preparation of GQDs/P25 nanocomposites In a typical preparation of GQDs/P25 nanocomposites, 200 mg of P25 fine powder was added into 50 mL as-synthesized GQDs aqueous solution with a concentration of 2 mg·mL−1 with the aid of ultrasound. Then, water in the mixture was evaporated under stirring in an uncovered beaker at room temperature. The resultant composite was designated as GQDs/P25. To further reduce the GQDs, the composite was annealed at 200 °C for 2 h; and the resultant was designated as GQDs/P25-R. 2.3. Photocatalytic tests Photocatalytic degradation of rhodamine B (RhB) was carried out in 80 mL quartz cuvette containing 20 mL (10 mg·mL−1) RhB solution and
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a suitable amount (20 mg) P25 nanoparticles, GQDs/P25, or GQDs/P25R nanocomposites as catalyst. A 350 W xenon lamp was used for illumination. Photocatalytic degradation of methyl orange (MO) was carried out under the same condition. 2.4. Characterization Transmission electron micrographs (TEM) and high-resolution TEM (HR-TEM) images were obtained with a JEOL JEM-1011 transmission electron microscope and a Hitachi H-7650 transmission electron microscope, respectively. Raman spectra were obtained with a Renishaw Raman system model 1000 spectrometer. UV–vis spectra were measured with a PE Lambda 20 spectrometer. Fluorescence spectra were measured with a Shimadzu RF-5301PC spectrofluorimeter. X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250 spectrometer. Spectra were baseline corrected using the instrument software. Thermogravimetric (TG) curves were carried out on a NETZSCH STA 449C with a heating rate of 20 °C·min−1 from room temperature to 800 °C. 3. Results and discussion 3.1. Characterization of GQDs Fig. 1a shows the TEM image of both pristine MWCNTs. The diameter of our MWCNTs was about 15 nm, and the wall thickness is ~5 nm. The diameter of CNTs is very important for the size control of the resultant GQDs, thick CNTs would significantly broaden the size distributions of the resultant GQDs. Fig. 1b shows the TEM image of typical GQDs; it reveals that the monodisperse GQDs are very uniform in size, about
Fig. 1. (a) TEM images of MWCNTs with diameters under 20 nm, (b) TEM images of GQDs with diameters under 5 nm (inset: HR-TEM image of a typical GQDs), (c) raman spectrum of GQDs, and (d) UV–vis absorption and photoluminescent (PL) emission spectra of an aqueous dispersion of GQDs.
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Fig. 2. Survey X-ray photoelectron spectrum of (a) GQDs/P25, and (b) GQDs/P25-R; C 1s XPS spectrum of (c) GQDs/P25, and (d) GQDs/P25-R.
4 nm. The HR-TEM image (the insert of Fig. 1b) indicates that the GQD is highly crystalline, with a lattice parameter of 0.24 nm, in good agreement with the (1120) lattice fringes of graphene. Raman spectrum of GQDs display two broad picks at 1332 and 1603 cm−1, corresponding to D and G band, respectively (Fig. 1c). The G band peak is attributed to an E2g mode of graphite associated with the vibration of sp2 bonded carbon atoms; whereas the D band peak is related to the vibrations of carbon atoms with dangling bonds in plane terminations of disordered graphite. The presence of a G band indicates the graphitic character of the GQDs, which is in good agreement with the HR-TEM image. The D band peak indicates the presence of defects (sp3 carbon). As compared with graphene oxides prepared by chemical exfoliation of graphite, the D band peak intensity of GQDs sample is much larger than that of its G band peak, the intensity ratio (ID/IG) is as high as 1.33, indicating the presence of more defects than GO whose ID/IG ratio is around 1. The obviously increased defects could be mainly attributed to the small pieces of GQDs, whose edge carbon atoms become significant when their size decreases to only a few nanometers. Fig. 1d shows the UV–vis absorption and photoluminescent (PL) emission spectra of the GQDs aqueous solution. It can be observed from the UV–vis spectrum that the GQDs show absorption in a broad
spectrum range. Additionally, a shoulder peak exits at 320 nm in the UV–vis spectrum, which possibly attributable to n–π* transition of C_O bonds [29]. The PL spectrum shows that the sample exhibits a PL emission peak at 498 nm, when the excitation wavelength was fixed at 424 nm. 3.2. Photocatalysts design based on GQDs and P25 Photocatalysis provides a promising approach to completely eliminate toxic contamination in the surroundings [30–33]. Due to the efficiency and broad applicability, TiO2 photocatalysts have attracted much interest. However, a major obstacle to its effective utilization lies in the inefficient use of sunlight, especially in visible-light region, because less than 5% of solar light and less than 0.1% of indoor lighting are usable or captured by undoped TiO2 due to its large intrinsic band gap. Considering the electronic property of GQDs, as a proof of concept, we design and prepared a composites photocatalyst based on our GQDs and conventional TiO2 (Degussa P25) as efficient visible-light driven photocatalysts. To get further insight into the surface chemical composition of the GQDs/P25 nanocomposites, X-ray photoelectron spectroscopy (XPS) was implemented. According to the XPS survey
Fig. 3. (a) TG curves and (b) UV–vis adsorption spectra of P25, GQDs/P25, and GQDs/P25-R.
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GQDs/P25 (Fig. 2c) and GQDs/P25-R (Fig. 2d) have been deconvoluted into three peaks that corresponding to C\\C, C\\O, and C_O, respectively. It is well known that the reduction of GO would render certain conductivity to the reduced GO (RGO). The thermal annealing treatment of GQDs would undoubtedly recover its conductivity and the reduced GQDs would facilitate the charge separation, and therefore, promote the photocatalytic activity of the GQDs/P25-R sample significantly. To evaluate the reduction degree and investigate the mass ratio of GQDs in the nanocomposite samples, thermogravimetric (TG) analysis has been carried out in air. Fig. 3a shows the TG curves of P25, GQDs/ P25, and GQDs/P25-R samples. Obviously, GQDs is not thermally stable due to the pyrolysis of labile oxygen-containing-groups such as \\OH and\\COOH. As shown in Fig. 3a, the total burning of GQDs was accomplished at ~ 550 °C; and the total weight loss of P25, GQDs/P25, and GQDs/P25-R samples are 1.1%, 7.4% and 5.1%, respectively. The weight loss of P25 (~1.1%) is due to the desorption of adsorbates and the decomposition of impurities; whereas the weight loss for GQDs/P25, and GQDs/P25-R samples are mainly attributed to the burning of GQDs. In this case, the contents of GQDs in GQDs/P25, and GQDs/P25-R samples are calculated to be 7.4% and 5.1%, respectively. The UV–vis adsorption spectra of P25, GQDs/P25, and GQDs/P25-R samples were also collected to evaluate their light harvesting properties. As shown in Fig. 3b, the three samples exhibit a representative adsorption with intense transition in the range of 240–420 nm in UV region due to the band gap transition of TiO2 semiconductor. Importantly, as compare with P25, GQDs/P25 and GQDs/P25-R nanocomposites have continuous wide adsorption in the visible region, which suggests the presence of GQDs improved the light absorption obviously. The extension of adsorption edge to visible region indicates that, under visiblelight irradiation, the nanocomposites photocatalysts may exhibit much higher photocatalytic activities than that of pure P25 nanoparticles.
Fig. 4. Photocatalytic degradation of (a) RhB, and (b) MO with P25, GQDs/P25 and GQDs/ P25-R nanocomposites as catalysts.
spectrum (Fig. 2a, b), both GQDs/P25 and GQDs/P25-R samples consist of Ti, O, and C; the chemical binding energies of Ti 2p, O 1s, C 1s are 460.0, 531.9 and 284.6 eV, respectively. The presence of carbon is ascribed to GQDs. After thermal annealing at 200 °C, O 1s peak intensity of GQDs/P25-R sample slightly decreased as compared with that of GQDs/P25, indicating the reduction of GQDs. The C 1s spectra of
3.3. Photocatalytic activity and proposed mechanism To evaluate their photocatalytic activities, the photodegradation of rhodamine B (RhB, 10 ppm) solutions and methyl orange (MO, 10 ppm) solutions under visible-light irradiation have been carried out at room temperature. Fig. 4a shows the degradation rates of RhB over P25, GQDs/P25 and GQDs/P25-R samples, respectively. As can be seen, after visible light irradiation for 120 min, the degradation efficiency of RhB was found to be about 29% when solely used pristine P25 nanoparticles as photocatalyst, while using GQDs/P25 and GQDs/P25-
Fig. 5. Schematic model for photocatalytic process of GQDs promoted P25 photocatalysts under visible-light irradiation.
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R samples both led to much higher degradation efficiency of RhB aqueous solution. Specifically, when the mixtures were exposed to visible-light irradiation for 120 min, about 62% of RhB was degraded by GQDs/P25 sample as photocatalysts. More excellently, for GQDs/ P25-R sample, all of RhB molecules have been degraded. On the other hand, Controlled experiments based on RhB aqueous solution without catalysts and photocatalysts (GQDs/P25-R sample) adsorption experiments without light have also been carried out, whereas both showed neglectable degradation of RhB. To further confirm the superiority of our nanocomposites photocatalysts, photocatalytic degradation of MO solution was also studied. As shown in Fig. 4b, P25 exhibits poor photocatalytic performance under visible-light irradiation due to its low absorption in visible-light region; whereas the GQDs promoted samples show much higher activities. After reaction for 2 h, about 57% of MO aqueous solution was degraded by GQDs/P25 sample, and nearly all of MO solution was degraded by GQDs/P25-R sample. On the contrary, P25 gives a very low degradation efficiency of ~ 21%, and no obvious degradation of MO could be observed without the assistance of photocatalyst under visible-light irradiation. In addition, there is nearly no adsorption capacity of the photocatalyst (GQDs/P25-R) for MO, which result is the same as the adsorption experiments carried out above. All these results indicate that the GQDs significantly promote the photocatalytic performance of P25 under visible-light irradiation. The above-mentioned experimental results suggested that GQDs act as an efficient promoter for general UV-active TiO2 photocatalysts, such as P25 (a well-known photocatalysts). UV–vis adsorption spectra of the nanocomposites confirmed that the presence of GQDs enhanced the light harvest, which extends their absorption edge to visible-light region. It has been demonstrated that carbon nanostructures have a large electron-storage capacity [34], thus, for our case, the GQDs would act as electron acceptor and facilitate charge transfer, accordingly. As shown in Fig. 5, the photogenerated electrons from P25 particles upon light irradiation can be effectively trapped and shuttled freely in the conducting network of GQDs, which effectively hindered the recombination of electron–hole pairs formed by P25. The electron–hole pairs react with the adsorbed oxidants/reducers (usually O2 and OH−) to produce a large number of − active oxygen radicals, such as •O− 2 and •OH , with very strong oxidation capability, which subsequently cause degradation of organic dyes [35]. This effect would guarantee the high photocatalytic activities of the GQDs promoted P25 samples.
4. Conclusions In summary, we have demonstrated a facile preparation of GQDs by chemically exfoliation of MWCNTs, and the design of high-performance visible-light driven photocatalysts based on this kind of GQDs and general Degussa P25. The as-prepared GQDs were very uniform in size, and possess strong electronic property. Using this kind of GQDs as an efficient promoter, the nanocomposite photocatalysts have been successfully prepared by a simple wet impregnation method, and the resultant GQDs promoted P25 photocatalysts exhibit much higher photocatalytic activities in the degradation of organic dyes (e.g., RhB and MO) under visible-light irradiation. The development of functional GQDs would not only contribute rational design and preparation of high-performance photocatalysts, but also hold great promise for the other innovations, for instance, novel optoelectronic devices.
Acknowledgments The authors would like to acknowledge the National Basic Research Program of China under grant nos. 2011CB013000 and 2014CB921302, and the NSFC under grant nos. 61376123, 91323301 and 61435005.
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Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Graphene quantum dots prepared from chemical exfoliation of multiwall carbon nanotubes: An efficient photocatalyst promoter Shu Wei a,b, Ran Zhang a, Yan Liu c, Hong Ding d, Yong-Lai Zhang a,⁎ a
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, China College of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun, Jilin 130022, China Key Laboratory of Bionic Engineering, Ministry of Education, Jilin University, Changchun 130012, China d State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China b c
a r t i c l e
i n f o
Article history: Received 22 September 2015 Received in revised form 8 November 2015 Accepted 13 November 2015 Available online 15 November 2015 Keywords: Graphene quantum dots Multiwall carbon nanotubes Exfoliation Visible-light photocatalysts
a b s t r a c t We report here a facile preparation of graphene quantum dots (GQDs) by chemical exfoliation of multiwall carbon nanotubes (MWCNTs) using a modified hummers' method. The resultant GQD samples possess strong electronic property, revealing great potential for photocatalyst design. As an efficient promoter, GQDs/P25 nanocomposites have been successfully prepared by simple wet impregnation and subsequent thermal annealing at 200 °C. In the tests of photocatalytic degradation of organic dyes under visible-light irradiation, the GQDs promoted P25 samples which shows much higher photocatalytic activity compared to the pure P25, indicating the crucial roles of GQDs. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Recently, zero-dimensional carbon materials, represented by fullerene and carbon dots, have attracted enormous research interests due to their unique physical/chemical properties [1–10]. Especially, carbon quantum dots (CQDs) including graphitic nanoparticles, amorphous carbon dots, and graphene quantum dots (GQDs) have reveal great potential for a wide range of applications, such as fluorescent probes [11], photovoltaic devices [12], photocatalysis [13–14], light-emitting diodes [15] and bioimaging [16–18]. As compared with conventional semiconductor quantum dots, CQDs exhibit a series of unique advantages. For instance, CQDs are generally nontoxic and thus biocompatible, since they consist of only carbon and some oxygen functional groups. Additionally, they could be excited over a broad spectral range from the visible to near IR region, and usually show clear up-conversion PL properties, thus CQDs have been considered as a promising candidate for rational design of novel visible-light driven photocatalysts [19–20]. Moreover, CQDs are excellent electron donors and acceptors in photoexcited states; which makes them promising for various optoelectronic devices [21]. The exceptional superiorities continuously stimulate the rapid progress of methodologies for CQDs preparation. Generally, the preparation strategies could be classified into two broad categories: top-down and bottom-up approaches. The former route usually generate ultra-small fragment of carbon materials (e.g., graphite, graphene, carbon soot ⁎ Corresponding author. E-mail address: [email protected] (Y.-L. Zhang).
http://dx.doi.org/10.1016/j.catcom.2015.11.010 1566-7367/© 2016 Elsevier B.V. All rights reserved.
and fullerene) using laser ablation, ultra-sonic treatment, electrochemical etching and hydrothermal cutting; whereas, the later method resorts to carbonization or organic molecules, or graphitization of polycyclic aromatic hydrocarbon [22–27]. However, despite the rapid development of synthetic methodologies, current preparation methods still suffer from serious problems with respect to complex procedures, the requirement of special equipment, incontrollable morphology and size, as well as low yields, which significantly limits their broad applications in various scientific fields. In this regard, novel methods for allowing facile preparation of high quality CQDs/GQDs with high-yield are highly desired. Herein, we present a facile chemical exfoliation of multiwall carbon nanotubes (MWCNTs) for preparation of GQDs via a modified hummers' method [28]. The resultant GQDs were very uniform in size, and possess strong electronics property. Using our GQDs as photocatalyst promoters, we demonstrate the design and preparation of GQDs/P25 (TiO2) nanocomposites, which show significantly promoted photocatalytic performance for the degradation of organic dyes under visible-light irradiation. 2. Experimental section 2.1. Preparation of GQDs from CNTs GQDs were prepared from chemical exfoliation of purified MWCNTs by a modified hummers' method. In a typical synthesis, 200 mg of MWCNTs were immersed in 46 mL conc. H2SO4 and then 6 g of KMnO4 was added slowly in small quantities under mild stirring while
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the temperature was maintained between 0 and 5 °C by using an ice bath. After the complete addition of KMnO4, the mixture was heated to 37 °C and kept at this temperature for about 1 h. Then, distilled water (92 mL) was added slowly. The temperature of the mixture was raised to 95 °C and maintained for about 12 h. The mixture was further diluted with 280 mL of water and later 20 mL of 30% H2O2 was added. The resultant mixture was purified by following procedure: firstly, low-speed centrifugation was done at 4000 rpm for 15 min about 3–5 times to the removal of the unexfoliated MWCNTs and some watersoluble by-product. Secondly, high-speed centrifugation was implemented at 12,000 rpm for 15 min to collect GQDs samples. Then, the precipitates were redispersed in water with mild sonication and then the remainder mixture was dialyzed in a dialysis bag for 48 h to ensure the complete removal of residual metallic impurities and acid. 2.2. Preparation of GQDs/P25 nanocomposites In a typical preparation of GQDs/P25 nanocomposites, 200 mg of P25 fine powder was added into 50 mL as-synthesized GQDs aqueous solution with a concentration of 2 mg·mL−1 with the aid of ultrasound. Then, water in the mixture was evaporated under stirring in an uncovered beaker at room temperature. The resultant composite was designated as GQDs/P25. To further reduce the GQDs, the composite was annealed at 200 °C for 2 h; and the resultant was designated as GQDs/P25-R. 2.3. Photocatalytic tests Photocatalytic degradation of rhodamine B (RhB) was carried out in 80 mL quartz cuvette containing 20 mL (10 mg·mL−1) RhB solution and
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a suitable amount (20 mg) P25 nanoparticles, GQDs/P25, or GQDs/P25R nanocomposites as catalyst. A 350 W xenon lamp was used for illumination. Photocatalytic degradation of methyl orange (MO) was carried out under the same condition. 2.4. Characterization Transmission electron micrographs (TEM) and high-resolution TEM (HR-TEM) images were obtained with a JEOL JEM-1011 transmission electron microscope and a Hitachi H-7650 transmission electron microscope, respectively. Raman spectra were obtained with a Renishaw Raman system model 1000 spectrometer. UV–vis spectra were measured with a PE Lambda 20 spectrometer. Fluorescence spectra were measured with a Shimadzu RF-5301PC spectrofluorimeter. X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250 spectrometer. Spectra were baseline corrected using the instrument software. Thermogravimetric (TG) curves were carried out on a NETZSCH STA 449C with a heating rate of 20 °C·min−1 from room temperature to 800 °C. 3. Results and discussion 3.1. Characterization of GQDs Fig. 1a shows the TEM image of both pristine MWCNTs. The diameter of our MWCNTs was about 15 nm, and the wall thickness is ~5 nm. The diameter of CNTs is very important for the size control of the resultant GQDs, thick CNTs would significantly broaden the size distributions of the resultant GQDs. Fig. 1b shows the TEM image of typical GQDs; it reveals that the monodisperse GQDs are very uniform in size, about
Fig. 1. (a) TEM images of MWCNTs with diameters under 20 nm, (b) TEM images of GQDs with diameters under 5 nm (inset: HR-TEM image of a typical GQDs), (c) raman spectrum of GQDs, and (d) UV–vis absorption and photoluminescent (PL) emission spectra of an aqueous dispersion of GQDs.
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Fig. 2. Survey X-ray photoelectron spectrum of (a) GQDs/P25, and (b) GQDs/P25-R; C 1s XPS spectrum of (c) GQDs/P25, and (d) GQDs/P25-R.
4 nm. The HR-TEM image (the insert of Fig. 1b) indicates that the GQD is highly crystalline, with a lattice parameter of 0.24 nm, in good agreement with the (1120) lattice fringes of graphene. Raman spectrum of GQDs display two broad picks at 1332 and 1603 cm−1, corresponding to D and G band, respectively (Fig. 1c). The G band peak is attributed to an E2g mode of graphite associated with the vibration of sp2 bonded carbon atoms; whereas the D band peak is related to the vibrations of carbon atoms with dangling bonds in plane terminations of disordered graphite. The presence of a G band indicates the graphitic character of the GQDs, which is in good agreement with the HR-TEM image. The D band peak indicates the presence of defects (sp3 carbon). As compared with graphene oxides prepared by chemical exfoliation of graphite, the D band peak intensity of GQDs sample is much larger than that of its G band peak, the intensity ratio (ID/IG) is as high as 1.33, indicating the presence of more defects than GO whose ID/IG ratio is around 1. The obviously increased defects could be mainly attributed to the small pieces of GQDs, whose edge carbon atoms become significant when their size decreases to only a few nanometers. Fig. 1d shows the UV–vis absorption and photoluminescent (PL) emission spectra of the GQDs aqueous solution. It can be observed from the UV–vis spectrum that the GQDs show absorption in a broad
spectrum range. Additionally, a shoulder peak exits at 320 nm in the UV–vis spectrum, which possibly attributable to n–π* transition of C_O bonds [29]. The PL spectrum shows that the sample exhibits a PL emission peak at 498 nm, when the excitation wavelength was fixed at 424 nm. 3.2. Photocatalysts design based on GQDs and P25 Photocatalysis provides a promising approach to completely eliminate toxic contamination in the surroundings [30–33]. Due to the efficiency and broad applicability, TiO2 photocatalysts have attracted much interest. However, a major obstacle to its effective utilization lies in the inefficient use of sunlight, especially in visible-light region, because less than 5% of solar light and less than 0.1% of indoor lighting are usable or captured by undoped TiO2 due to its large intrinsic band gap. Considering the electronic property of GQDs, as a proof of concept, we design and prepared a composites photocatalyst based on our GQDs and conventional TiO2 (Degussa P25) as efficient visible-light driven photocatalysts. To get further insight into the surface chemical composition of the GQDs/P25 nanocomposites, X-ray photoelectron spectroscopy (XPS) was implemented. According to the XPS survey
Fig. 3. (a) TG curves and (b) UV–vis adsorption spectra of P25, GQDs/P25, and GQDs/P25-R.
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GQDs/P25 (Fig. 2c) and GQDs/P25-R (Fig. 2d) have been deconvoluted into three peaks that corresponding to C\\C, C\\O, and C_O, respectively. It is well known that the reduction of GO would render certain conductivity to the reduced GO (RGO). The thermal annealing treatment of GQDs would undoubtedly recover its conductivity and the reduced GQDs would facilitate the charge separation, and therefore, promote the photocatalytic activity of the GQDs/P25-R sample significantly. To evaluate the reduction degree and investigate the mass ratio of GQDs in the nanocomposite samples, thermogravimetric (TG) analysis has been carried out in air. Fig. 3a shows the TG curves of P25, GQDs/ P25, and GQDs/P25-R samples. Obviously, GQDs is not thermally stable due to the pyrolysis of labile oxygen-containing-groups such as \\OH and\\COOH. As shown in Fig. 3a, the total burning of GQDs was accomplished at ~ 550 °C; and the total weight loss of P25, GQDs/P25, and GQDs/P25-R samples are 1.1%, 7.4% and 5.1%, respectively. The weight loss of P25 (~1.1%) is due to the desorption of adsorbates and the decomposition of impurities; whereas the weight loss for GQDs/P25, and GQDs/P25-R samples are mainly attributed to the burning of GQDs. In this case, the contents of GQDs in GQDs/P25, and GQDs/P25-R samples are calculated to be 7.4% and 5.1%, respectively. The UV–vis adsorption spectra of P25, GQDs/P25, and GQDs/P25-R samples were also collected to evaluate their light harvesting properties. As shown in Fig. 3b, the three samples exhibit a representative adsorption with intense transition in the range of 240–420 nm in UV region due to the band gap transition of TiO2 semiconductor. Importantly, as compare with P25, GQDs/P25 and GQDs/P25-R nanocomposites have continuous wide adsorption in the visible region, which suggests the presence of GQDs improved the light absorption obviously. The extension of adsorption edge to visible region indicates that, under visiblelight irradiation, the nanocomposites photocatalysts may exhibit much higher photocatalytic activities than that of pure P25 nanoparticles.
Fig. 4. Photocatalytic degradation of (a) RhB, and (b) MO with P25, GQDs/P25 and GQDs/ P25-R nanocomposites as catalysts.
spectrum (Fig. 2a, b), both GQDs/P25 and GQDs/P25-R samples consist of Ti, O, and C; the chemical binding energies of Ti 2p, O 1s, C 1s are 460.0, 531.9 and 284.6 eV, respectively. The presence of carbon is ascribed to GQDs. After thermal annealing at 200 °C, O 1s peak intensity of GQDs/P25-R sample slightly decreased as compared with that of GQDs/P25, indicating the reduction of GQDs. The C 1s spectra of
3.3. Photocatalytic activity and proposed mechanism To evaluate their photocatalytic activities, the photodegradation of rhodamine B (RhB, 10 ppm) solutions and methyl orange (MO, 10 ppm) solutions under visible-light irradiation have been carried out at room temperature. Fig. 4a shows the degradation rates of RhB over P25, GQDs/P25 and GQDs/P25-R samples, respectively. As can be seen, after visible light irradiation for 120 min, the degradation efficiency of RhB was found to be about 29% when solely used pristine P25 nanoparticles as photocatalyst, while using GQDs/P25 and GQDs/P25-
Fig. 5. Schematic model for photocatalytic process of GQDs promoted P25 photocatalysts under visible-light irradiation.
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R samples both led to much higher degradation efficiency of RhB aqueous solution. Specifically, when the mixtures were exposed to visible-light irradiation for 120 min, about 62% of RhB was degraded by GQDs/P25 sample as photocatalysts. More excellently, for GQDs/ P25-R sample, all of RhB molecules have been degraded. On the other hand, Controlled experiments based on RhB aqueous solution without catalysts and photocatalysts (GQDs/P25-R sample) adsorption experiments without light have also been carried out, whereas both showed neglectable degradation of RhB. To further confirm the superiority of our nanocomposites photocatalysts, photocatalytic degradation of MO solution was also studied. As shown in Fig. 4b, P25 exhibits poor photocatalytic performance under visible-light irradiation due to its low absorption in visible-light region; whereas the GQDs promoted samples show much higher activities. After reaction for 2 h, about 57% of MO aqueous solution was degraded by GQDs/P25 sample, and nearly all of MO solution was degraded by GQDs/P25-R sample. On the contrary, P25 gives a very low degradation efficiency of ~ 21%, and no obvious degradation of MO could be observed without the assistance of photocatalyst under visible-light irradiation. In addition, there is nearly no adsorption capacity of the photocatalyst (GQDs/P25-R) for MO, which result is the same as the adsorption experiments carried out above. All these results indicate that the GQDs significantly promote the photocatalytic performance of P25 under visible-light irradiation. The above-mentioned experimental results suggested that GQDs act as an efficient promoter for general UV-active TiO2 photocatalysts, such as P25 (a well-known photocatalysts). UV–vis adsorption spectra of the nanocomposites confirmed that the presence of GQDs enhanced the light harvest, which extends their absorption edge to visible-light region. It has been demonstrated that carbon nanostructures have a large electron-storage capacity [34], thus, for our case, the GQDs would act as electron acceptor and facilitate charge transfer, accordingly. As shown in Fig. 5, the photogenerated electrons from P25 particles upon light irradiation can be effectively trapped and shuttled freely in the conducting network of GQDs, which effectively hindered the recombination of electron–hole pairs formed by P25. The electron–hole pairs react with the adsorbed oxidants/reducers (usually O2 and OH−) to produce a large number of − active oxygen radicals, such as •O− 2 and •OH , with very strong oxidation capability, which subsequently cause degradation of organic dyes [35]. This effect would guarantee the high photocatalytic activities of the GQDs promoted P25 samples.
4. Conclusions In summary, we have demonstrated a facile preparation of GQDs by chemically exfoliation of MWCNTs, and the design of high-performance visible-light driven photocatalysts based on this kind of GQDs and general Degussa P25. The as-prepared GQDs were very uniform in size, and possess strong electronic property. Using this kind of GQDs as an efficient promoter, the nanocomposite photocatalysts have been successfully prepared by a simple wet impregnation method, and the resultant GQDs promoted P25 photocatalysts exhibit much higher photocatalytic activities in the degradation of organic dyes (e.g., RhB and MO) under visible-light irradiation. The development of functional GQDs would not only contribute rational design and preparation of high-performance photocatalysts, but also hold great promise for the other innovations, for instance, novel optoelectronic devices.
Acknowledgments The authors would like to acknowledge the National Basic Research Program of China under grant nos. 2011CB013000 and 2014CB921302, and the NSFC under grant nos. 61376123, 91323301 and 61435005.
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