- Email: [email protected]
Au hybrid nanoparticles as electrocatalyst for hydrogen evolution reaction
Accepted Manuscript Title: Graphene quantum dots/Au hybrid nanoparticles as electrocatalyst for hydrogen evolution reaction Author: Peihui Luo Linqin Jiang Weilong Zhang Xiangfeng Guan PII: DOI: Reference:
S0009-2614(15)00798-8 http://dx.doi.org/doi:10.1016/j.cplett.2015.10.042 CPLETT 33367
To appear in: Received date: Revised date: Accepted date:
17-5-2015 18-10-2015 19-10-2015
Please cite this article as: P. Luo, L. Jiang, W. Zhang, X. Guan, Graphene quantum dots/Au hybrid nanoparticles as electrocatalyst for hydrogen evolution reaction, Chem. Phys. Lett. (2015), http://dx.doi.org/10.1016/j.cplett.2015.10.042 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Graphene quantum dots/Au hybrid nanoparticles as electrocatalyst for hydrogen evolution reaction
ip t
Peihui Luoa,*[email protected], Linqin Jianga,b, Weilong Zhanga, Xiangfeng Guana,b
a
cr
College of Electronics and Information Science, Fujian Jiangxia University, Fuzhou 350108, People’s Republic of China b
an
Tel: 86-591-23531652; Fax: 86-591-23531375
us
Institute of Advanced Photovoltaics, Fujian Jiangxia University, Fuzhou 350108, People’s Republic of China
M
Abstract
te
d
Graphene quantum dots/Au hybrid nanoparticles (denoted as GQDs-Au) were prepared by heating HAuCl4 with GQDs, and they showed higher electrocatalytic activity for hydrogen evolution reaction than that of pure Au nanoparticles.
Ac ce p
Keywords: Graphene quantum dots; Gold; Electrocatalyst; Hydrogen evolution reaction
1. Introduction Graphene quantum dots (GQDs) recently discovered are a new class of quantum dots
Page 1 of 9
emitters with near graphene structure and sizes usually below 10 nm [1−3]. Due to quantum confinement and edge effects, GQDs possess a discrete bandgap and unique photoluminescence (PL) properties. In addition, they have low-toxicity, chemical inertness, biocompatibility and no photo-bleaching
properties,
facilitating their
applications
in
many fields,
including
ip t
photoelectronics and catalysis, etc [1−11]. To achieve the full potential of GQDs, GQDs-based nanocomposites have been fabricated, which could integrate properties of GQDs and functional
cr
components for many specific applications. For example, the hybrids of GQDs with optical active oxides [7,12−14], polymers [15] and graphene [16] showed applications in supercapacitor
and
sensing
fields,
respectively.
Recently,
several
us
photoelectronics,
nanocomposites based on GQDs with noble metals such as Au, Ag, Pt and Pd were also prepared [17−21]. Especially for GQDs/Pt nanocomposites [21], it exhibited markedly enhanced
an
electrocatalytic activity in oxygen reduction reactions, in comparison with comerical Pt/C catalysts. GQDs contain many carboxylic and hydroxyl groups at their surface, imparting them
M
with excellent water solubility and inducing many structural defects, which improving the electrocatalytic performance of the composites. Pt is widely used as the electrocatalyst, but its
d
high cost largely limits the commercialization. In comparison, the cost of Au is lower than Pt, and Au nanoparticles supported on metal oxides exhibit high catalytic activity, inspiring
te
numerous scientists for exploring nanoscale Au catalysis [22,23]. In order to further explore electrocatalytic properties of GQDs, we synthesized GQDs/Au hybrid nanoparticles (denoted as
Ac ce p
GQDs-Au) according to our method reported previously [24]. The obtained GQDs-Au formed a Au@GQDs core-shell nanostructure with an average diameter of 15.6 nm, and showed much higher electrocatalytic activity than pure Au nanoparticles for hydrogen evolution reaction (HER).
2. Experimental section
2.1. Synthesis of GQDs-Au
The GQDs-Au were prepared according to our method reported previously [24]. Typically, 0.5 mL aqueous solution of HAuCl4 (0.83 mg/mL) was added to 30 mL aqueous suspension of GQDs (0.03 mg/mL). The mixed solution was kept at 60 oC for 80 min to yield a stable pink suspension of GQDs-Au. The diameter of GQDs-Au composite nanoparticles was measured to be 15.6 nm.
Page 2 of 9
2.2. Electrocatalytic HER All the electrochemical measurements were performed on a CHI 440 potentiostat under computer control. A glassy carbon electrode (diameter =3 mm) with a catalyst loading of 0.17
ip t
mg/cm2 was used as the working electrode and a graphite rod was used as the counter electrode. The electrolyte was a 0.5 M aqueous solution of H2SO4. All the potentials were referred to a saturated calomel electrode (SCE). They were further calibrated with respect to reversible
cr
hydrogen electrode (RHE). The calibrations were simply performed according to the formula:
us
ERHE = ESCE + 0.273 V [25]. 3. Results and discussion
an
Fig. 1 shows the synthesis procedure of GQDs-Au. GQDs-Au were synthesized by simply heating GQDs with HAuCl4 (see Experimental section). The mechanism for reducing Au nanoparticles was proposed to be Galvanic displacement and redox reaction by the relative
M
potential difference, similar to the reduction of metal ions under heating in the presence of reduced graphene oxide [26−28]. GQDs, which contain various oxygenated groups, were
d
equalent to small graphene oxide sheets. Oxygenated groups functioned as nucleation sites, and
te
aromatic conjugated domains on GQDs acted as an electron-donating source to reduce Au3+ ions. Meanwhile, the GQDs also functioned as stabilizing agents, and
encapsulated naked Au
Ac ce p
nanoparticles and prevented their aggregation. Eventually, the GQDs-Au with core-shell structure were formed. The residual oxygenated groups of the GQDs coatings made the composite nanoparticles could be stably dispersed in water [24].
Fig. 1 Schematic illustration of the strategy for the GQDs-Au preparation.
In this paper, GQDs were obtained via electrooxidation of graphite rods [24]. They reveal the average size of 3.1 0.6 nm (Fig. 2a), and fine crystalline patterns with a lattice spacing of 0.238 nm, corresponding to the (100) facet of graphite (Fig. 2b). The heights of typical GQDs measured by AFM were around 0.7 nm and 1.4 nm, equivalent to single-layer and double-layer
Page 3 of 9
graphene sheets, respectively (see Fig. S1 in the Supporting material) [29]. TEM image shown in Fig. 2c indicates the synthetic GQDs-Au were nearly spherical in shape with an average diameter of 15.6 1.7 nm. HRTEM image of single composite nanoparticle clearly shows the high-contrast lattice fringes assigned to Au (200) and (111) planes inside, and the low-contrast
ip t
lattice fringes of graphite (100) facets on the periphery (Fig. 2d). And the size of graphite crystalline region in Fig. 2d was similar to that of GQDs in Fig. 2a and 2b. The SAED pattern
cr
(see Fig. S2 in the Supporting material) of GQDs-Au also displays the diffraction rings assigned to Au and graphite lattice planes, respectively. It suggests the formation of GQDs coated Au
us
nanostructure. However, the intact GQDs shell layer of single nanoparticle in Fig. 2d was not observed obviously, probably due to that an ultra-thin carbon film was used as the substrate for
an
taking out this image [21]. The HRTEM image of single nanoparticle on a carbon nanotube grid clearly shows a low-contrast halo ring associated with aggregated GQDs wrapping around a high-contrast Au core (see Fig. S3 in the Supporting material). This observation further reflects
M
our prepared GQDs-Au with a core-shell structure. The GQDs-Au can be stably dispersed in water for several weeks. Although GQDs-Au with average diameters smaller that 15.6 nm can be
d
obtained by decreasing the concentration of HAuCl4 in the reaction system, they were tested to
Ac ce p
sizes is in progress.
te
be unstable and easy to form large aggregates. Improvement for stability of GQDs-Au of smaller
Fig. 2 TEM images of GQDs (a) and GQDs-Au (c); HRTEM images of double GQDs (b) and single GQDsAu (d).
The UV-visible spectra of GQDs and GQDs-Au dispersions are shown in Fig. 3a. The spectrum of GQDs-Au has a plasmon absorption peak associated with Au nanoparticles at 528 nm, while that of GQDs does not show any peaks in the range of 300 to 650 nm. As a result, the aqueous dispersions of GQDs and GQDs-Au are yellowish and pink under daylight, respectively. The fluorescence intensity of GQDs-Au dispersion is much weaker than that of GQDs dispersion (Fig. 3b). This phenomenon can be explained by the photoinduced electron transfer from GQDs shells to Au cores and the aggregation of GQDs [24]. And the emission peak of GQDs-Au
Page 4 of 9
shows a little blue-shift compared to that for pure GQDs. This phenomenon was relevant to oxygenated groups at GQDs surface. Because GQDs acted as a reducing agent in synthesis of GQDs-Au, and part of their oxygenated groups were further oxidized by Au3+ ions upon heating
us
cr
Au produces less surface defects, resulting in blue-shift of emission [29] .
ip t
and removed from the reaction system. A lower degree of surface oxidation of GQDs in GQDs-
Fig. 3 UV-visible absorption spectra (a) and fluorescence spectra excited at 330 nm. (b) of GQDs and GQDsAu dispersed in water. Insets of (a) are photographs of GQDs and GQDs-Au aqueous dispersions under
an
daylight.
Hydrogen has been recognized as a clean energy carrier [30,31], and it can be produced by
M
electrolysis of water [32]. Platinum (Pt) has been tested to be the most effective catalyst for the electrochemical HER because of its low overpotential () [33]. But the high cost and scarcity of
d
Pt have limited its widespread commercialization. Therefore, it is important to explore highly active HER catalysts based on more abundant materials at lower costs. We investigated the
te
electrocatalytic activity of our GQDs-Au for HER by coating them on a glassy carbon (GC)
Ac ce p
electrode. The experiments were carried out in a 0.5 M H2SO4 solution using a typical threeelectrode cell (Experimental section). The polarization curve of the GC electrode modified with GQDs-Au (sizes = 15.6 1.7 nm) shows a small overpotential of 0.14 V for HER, while that of a Pt electrode is close to zero (Fig. 4a). In comparison, the GC electrode coated by pure Au nanoparticles with the same sizes (see Experimental section in the Supporting material) exhibits a higher overpotential of 0.27 V, and GQDs have a negligible HER activity (Fig. 4a). The linear portions of the polarization curves were fit to the Tafel equation ( = b log j + a, where j is current density and b is Tafel slope) [33], yielding Tafel slopes of about 30, 78, 75, 140 mV/decade for Pt, GQDs-Au, Au and GQDs, respectively (Fig. 4b). Although the Tafel slopes of GQDs-Au and Au nanoparticles are almost the same, the overpotential of the former catalyst is about 0.13 V lower than that of the latter, indicating a much higher electrocatalytic activity. The improved electrocatalysis performance of the GQDs-Au nanoparticles is mainly attributed to the strong chemical coupling/interactions between their both components, which afforded a
Page 5 of 9
strong GQDs coating layer at Au surface and prevented Au nanoparticles aggregation together. Furthermore, single Au nanoparticle was encapsulated by many GQDs with abundant structural defects, benefiting for producing more catalytically active Au sites at near supports. High durability is important for a good electrocatalyst. To assess this, we cycled our GQDs-Au
ip t
catalyst continuously for 1000 cycles. The GQDs-Au exhibited similar j–V curve as initial cycle, with negligible loss of the cathodic current (see Fig. S4 in the Supporting material). It is
cr
probably that if graphene is introduced into nanocomposites or the size of Au nanoparticle in GQDs-Au is decreased, the overpotential and Tafel slope will be further decreased. In addition,
us
GQDs have merits including low-toxicity, chemical inertness and biocompatibility, these will
M
an
facilitate the commercialization of nanocomposites based on GQDs with Au in catalysis.
Fig. 4 Polarization curves (a) obtained with several catalysts as indicated and corresponding Tafel plots (b)
d
recorded on glassy carbon electrodes with a catalyst loading 0.17 mg/cm2 for each electrode.
te
4. Conclusion
In summary, the GQDs-Au with a core-shell nanostructure have been successfully prepared
Ac ce p
by directly heating HAuCl4 with GQDs. The GQDs coating layers on Au nanoparticles strongly enhanced their electrocatalytic activity for HER. The result was ascribed to the ultrasmall sizes and abundant structural defects of GQDs. Considering the low cost of GQDs-Au, they are a much cheaper catalyst than Pt with high electrocatalysis performance on HER. Acknowledgments
This work was supported by National Natural Science Foundation of China (51503036, 51402050), Natural Science Youth Foundation of Fujian Province (2015J05091) and Yong Scientific Research Personnel Cultivation Fund of Fujian Jiangxia University (JXZ2014001). Appendix A. Supplementary data Supplementary data associated with this article can be found online at.
Page 6 of 9
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Figure captions Fig. 1 Schematic illustration of the strategy for the GQDs-Au preparation.
Page 8 of 9
Fig. 2 TEM images of GQDs (a) and GQDs-Au (c); HRTEM images of double GQDs (b) and single GQDs-Au (d). Fig. 3 UV-visible absorption spectra (a) and fluorescence spectra excited at 330 nm. (b) of
ip t
GQDs and GQDs-Au dispersed in water. Insets of (a) are photographs of GQDs and GQDs-Au aqueous dispersions under daylight.
cr
Fig. 4 Polarization curves (a) obtained with several catalysts as indicated and corresponding Tafel plots (b) recorded on glassy carbon electrodes with a catalyst loading 0.17 mg/cm2 for
M
an
us
each electrode.
Ac ce p
te
d
Graphene quantum dots/Au hybrid nanoparticles with a core-shell structure, show excellent electrocatalytic activity towards hydrogen evolution reactions.
Page 9 of 9
S0009-2614(15)00798-8 http://dx.doi.org/doi:10.1016/j.cplett.2015.10.042 CPLETT 33367
To appear in: Received date: Revised date: Accepted date:
17-5-2015 18-10-2015 19-10-2015
Please cite this article as: P. Luo, L. Jiang, W. Zhang, X. Guan, Graphene quantum dots/Au hybrid nanoparticles as electrocatalyst for hydrogen evolution reaction, Chem. Phys. Lett. (2015), http://dx.doi.org/10.1016/j.cplett.2015.10.042 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Graphene quantum dots/Au hybrid nanoparticles as electrocatalyst for hydrogen evolution reaction
ip t
Peihui Luoa,*[email protected], Linqin Jianga,b, Weilong Zhanga, Xiangfeng Guana,b
a
cr
College of Electronics and Information Science, Fujian Jiangxia University, Fuzhou 350108, People’s Republic of China b
an
us
Institute of Advanced Photovoltaics, Fujian Jiangxia University, Fuzhou 350108, People’s Republic of China
M
Abstract
te
d
Graphene quantum dots/Au hybrid nanoparticles (denoted as GQDs-Au) were prepared by heating HAuCl4 with GQDs, and they showed higher electrocatalytic activity for hydrogen evolution reaction than that of pure Au nanoparticles.
Ac ce p
Keywords: Graphene quantum dots; Gold; Electrocatalyst; Hydrogen evolution reaction
1. Introduction Graphene quantum dots (GQDs) recently discovered are a new class of quantum dots
Page 1 of 9
emitters with near graphene structure and sizes usually below 10 nm [1−3]. Due to quantum confinement and edge effects, GQDs possess a discrete bandgap and unique photoluminescence (PL) properties. In addition, they have low-toxicity, chemical inertness, biocompatibility and no photo-bleaching
properties,
facilitating their
applications
in
many fields,
including
ip t
photoelectronics and catalysis, etc [1−11]. To achieve the full potential of GQDs, GQDs-based nanocomposites have been fabricated, which could integrate properties of GQDs and functional
cr
components for many specific applications. For example, the hybrids of GQDs with optical active oxides [7,12−14], polymers [15] and graphene [16] showed applications in supercapacitor
and
sensing
fields,
respectively.
Recently,
several
us
photoelectronics,
nanocomposites based on GQDs with noble metals such as Au, Ag, Pt and Pd were also prepared [17−21]. Especially for GQDs/Pt nanocomposites [21], it exhibited markedly enhanced
an
electrocatalytic activity in oxygen reduction reactions, in comparison with comerical Pt/C catalysts. GQDs contain many carboxylic and hydroxyl groups at their surface, imparting them
M
with excellent water solubility and inducing many structural defects, which improving the electrocatalytic performance of the composites. Pt is widely used as the electrocatalyst, but its
d
high cost largely limits the commercialization. In comparison, the cost of Au is lower than Pt, and Au nanoparticles supported on metal oxides exhibit high catalytic activity, inspiring
te
numerous scientists for exploring nanoscale Au catalysis [22,23]. In order to further explore electrocatalytic properties of GQDs, we synthesized GQDs/Au hybrid nanoparticles (denoted as
Ac ce p
GQDs-Au) according to our method reported previously [24]. The obtained GQDs-Au formed a Au@GQDs core-shell nanostructure with an average diameter of 15.6 nm, and showed much higher electrocatalytic activity than pure Au nanoparticles for hydrogen evolution reaction (HER).
2. Experimental section
2.1. Synthesis of GQDs-Au
The GQDs-Au were prepared according to our method reported previously [24]. Typically, 0.5 mL aqueous solution of HAuCl4 (0.83 mg/mL) was added to 30 mL aqueous suspension of GQDs (0.03 mg/mL). The mixed solution was kept at 60 oC for 80 min to yield a stable pink suspension of GQDs-Au. The diameter of GQDs-Au composite nanoparticles was measured to be 15.6 nm.
Page 2 of 9
2.2. Electrocatalytic HER All the electrochemical measurements were performed on a CHI 440 potentiostat under computer control. A glassy carbon electrode (diameter =3 mm) with a catalyst loading of 0.17
ip t
mg/cm2 was used as the working electrode and a graphite rod was used as the counter electrode. The electrolyte was a 0.5 M aqueous solution of H2SO4. All the potentials were referred to a saturated calomel electrode (SCE). They were further calibrated with respect to reversible
cr
hydrogen electrode (RHE). The calibrations were simply performed according to the formula:
us
ERHE = ESCE + 0.273 V [25]. 3. Results and discussion
an
Fig. 1 shows the synthesis procedure of GQDs-Au. GQDs-Au were synthesized by simply heating GQDs with HAuCl4 (see Experimental section). The mechanism for reducing Au nanoparticles was proposed to be Galvanic displacement and redox reaction by the relative
M
potential difference, similar to the reduction of metal ions under heating in the presence of reduced graphene oxide [26−28]. GQDs, which contain various oxygenated groups, were
d
equalent to small graphene oxide sheets. Oxygenated groups functioned as nucleation sites, and
te
aromatic conjugated domains on GQDs acted as an electron-donating source to reduce Au3+ ions. Meanwhile, the GQDs also functioned as stabilizing agents, and
encapsulated naked Au
Ac ce p
nanoparticles and prevented their aggregation. Eventually, the GQDs-Au with core-shell structure were formed. The residual oxygenated groups of the GQDs coatings made the composite nanoparticles could be stably dispersed in water [24].
Fig. 1 Schematic illustration of the strategy for the GQDs-Au preparation.
In this paper, GQDs were obtained via electrooxidation of graphite rods [24]. They reveal the average size of 3.1 0.6 nm (Fig. 2a), and fine crystalline patterns with a lattice spacing of 0.238 nm, corresponding to the (100) facet of graphite (Fig. 2b). The heights of typical GQDs measured by AFM were around 0.7 nm and 1.4 nm, equivalent to single-layer and double-layer
Page 3 of 9
graphene sheets, respectively (see Fig. S1 in the Supporting material) [29]. TEM image shown in Fig. 2c indicates the synthetic GQDs-Au were nearly spherical in shape with an average diameter of 15.6 1.7 nm. HRTEM image of single composite nanoparticle clearly shows the high-contrast lattice fringes assigned to Au (200) and (111) planes inside, and the low-contrast
ip t
lattice fringes of graphite (100) facets on the periphery (Fig. 2d). And the size of graphite crystalline region in Fig. 2d was similar to that of GQDs in Fig. 2a and 2b. The SAED pattern
cr
(see Fig. S2 in the Supporting material) of GQDs-Au also displays the diffraction rings assigned to Au and graphite lattice planes, respectively. It suggests the formation of GQDs coated Au
us
nanostructure. However, the intact GQDs shell layer of single nanoparticle in Fig. 2d was not observed obviously, probably due to that an ultra-thin carbon film was used as the substrate for
an
taking out this image [21]. The HRTEM image of single nanoparticle on a carbon nanotube grid clearly shows a low-contrast halo ring associated with aggregated GQDs wrapping around a high-contrast Au core (see Fig. S3 in the Supporting material). This observation further reflects
M
our prepared GQDs-Au with a core-shell structure. The GQDs-Au can be stably dispersed in water for several weeks. Although GQDs-Au with average diameters smaller that 15.6 nm can be
d
obtained by decreasing the concentration of HAuCl4 in the reaction system, they were tested to
Ac ce p
sizes is in progress.
te
be unstable and easy to form large aggregates. Improvement for stability of GQDs-Au of smaller
Fig. 2 TEM images of GQDs (a) and GQDs-Au (c); HRTEM images of double GQDs (b) and single GQDsAu (d).
The UV-visible spectra of GQDs and GQDs-Au dispersions are shown in Fig. 3a. The spectrum of GQDs-Au has a plasmon absorption peak associated with Au nanoparticles at 528 nm, while that of GQDs does not show any peaks in the range of 300 to 650 nm. As a result, the aqueous dispersions of GQDs and GQDs-Au are yellowish and pink under daylight, respectively. The fluorescence intensity of GQDs-Au dispersion is much weaker than that of GQDs dispersion (Fig. 3b). This phenomenon can be explained by the photoinduced electron transfer from GQDs shells to Au cores and the aggregation of GQDs [24]. And the emission peak of GQDs-Au
Page 4 of 9
shows a little blue-shift compared to that for pure GQDs. This phenomenon was relevant to oxygenated groups at GQDs surface. Because GQDs acted as a reducing agent in synthesis of GQDs-Au, and part of their oxygenated groups were further oxidized by Au3+ ions upon heating
us
cr
Au produces less surface defects, resulting in blue-shift of emission [29] .
ip t
and removed from the reaction system. A lower degree of surface oxidation of GQDs in GQDs-
Fig. 3 UV-visible absorption spectra (a) and fluorescence spectra excited at 330 nm. (b) of GQDs and GQDsAu dispersed in water. Insets of (a) are photographs of GQDs and GQDs-Au aqueous dispersions under
an
daylight.
Hydrogen has been recognized as a clean energy carrier [30,31], and it can be produced by
M
electrolysis of water [32]. Platinum (Pt) has been tested to be the most effective catalyst for the electrochemical HER because of its low overpotential () [33]. But the high cost and scarcity of
d
Pt have limited its widespread commercialization. Therefore, it is important to explore highly active HER catalysts based on more abundant materials at lower costs. We investigated the
te
electrocatalytic activity of our GQDs-Au for HER by coating them on a glassy carbon (GC)
Ac ce p
electrode. The experiments were carried out in a 0.5 M H2SO4 solution using a typical threeelectrode cell (Experimental section). The polarization curve of the GC electrode modified with GQDs-Au (sizes = 15.6 1.7 nm) shows a small overpotential of 0.14 V for HER, while that of a Pt electrode is close to zero (Fig. 4a). In comparison, the GC electrode coated by pure Au nanoparticles with the same sizes (see Experimental section in the Supporting material) exhibits a higher overpotential of 0.27 V, and GQDs have a negligible HER activity (Fig. 4a). The linear portions of the polarization curves were fit to the Tafel equation ( = b log j + a, where j is current density and b is Tafel slope) [33], yielding Tafel slopes of about 30, 78, 75, 140 mV/decade for Pt, GQDs-Au, Au and GQDs, respectively (Fig. 4b). Although the Tafel slopes of GQDs-Au and Au nanoparticles are almost the same, the overpotential of the former catalyst is about 0.13 V lower than that of the latter, indicating a much higher electrocatalytic activity. The improved electrocatalysis performance of the GQDs-Au nanoparticles is mainly attributed to the strong chemical coupling/interactions between their both components, which afforded a
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strong GQDs coating layer at Au surface and prevented Au nanoparticles aggregation together. Furthermore, single Au nanoparticle was encapsulated by many GQDs with abundant structural defects, benefiting for producing more catalytically active Au sites at near supports. High durability is important for a good electrocatalyst. To assess this, we cycled our GQDs-Au
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catalyst continuously for 1000 cycles. The GQDs-Au exhibited similar j–V curve as initial cycle, with negligible loss of the cathodic current (see Fig. S4 in the Supporting material). It is
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probably that if graphene is introduced into nanocomposites or the size of Au nanoparticle in GQDs-Au is decreased, the overpotential and Tafel slope will be further decreased. In addition,
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GQDs have merits including low-toxicity, chemical inertness and biocompatibility, these will
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facilitate the commercialization of nanocomposites based on GQDs with Au in catalysis.
Fig. 4 Polarization curves (a) obtained with several catalysts as indicated and corresponding Tafel plots (b)
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recorded on glassy carbon electrodes with a catalyst loading 0.17 mg/cm2 for each electrode.
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4. Conclusion
In summary, the GQDs-Au with a core-shell nanostructure have been successfully prepared
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by directly heating HAuCl4 with GQDs. The GQDs coating layers on Au nanoparticles strongly enhanced their electrocatalytic activity for HER. The result was ascribed to the ultrasmall sizes and abundant structural defects of GQDs. Considering the low cost of GQDs-Au, they are a much cheaper catalyst than Pt with high electrocatalysis performance on HER. Acknowledgments
This work was supported by National Natural Science Foundation of China (51503036, 51402050), Natural Science Youth Foundation of Fujian Province (2015J05091) and Yong Scientific Research Personnel Cultivation Fund of Fujian Jiangxia University (JXZ2014001). Appendix A. Supplementary data Supplementary data associated with this article can be found online at.
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Figure captions Fig. 1 Schematic illustration of the strategy for the GQDs-Au preparation.
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Fig. 2 TEM images of GQDs (a) and GQDs-Au (c); HRTEM images of double GQDs (b) and single GQDs-Au (d). Fig. 3 UV-visible absorption spectra (a) and fluorescence spectra excited at 330 nm. (b) of
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GQDs and GQDs-Au dispersed in water. Insets of (a) are photographs of GQDs and GQDs-Au aqueous dispersions under daylight.
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Fig. 4 Polarization curves (a) obtained with several catalysts as indicated and corresponding Tafel plots (b) recorded on glassy carbon electrodes with a catalyst loading 0.17 mg/cm2 for
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each electrode.
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Graphene quantum dots/Au hybrid nanoparticles with a core-shell structure, show excellent electrocatalytic activity towards hydrogen evolution reactions.
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