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Enhanced photoelectrocatalytic oxidation of small organic molecules by gold nanoparticles supported on carbon nitride
Journal of Electroanalytical Chemistry 719 (2014) 86–91
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Enhanced photoelectrocatalytic oxidation of small organic molecules by gold nanoparticles supported on carbon nitride Shouqin Chang a, Aiyun Xie a, Shu Chen a,⇑,1, Juan Xiang a,b,c,⇑ a
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, PR China Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, Central South University, Changsha 410083, PR China c State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, PR China b
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Article history: Received 18 September 2013 Received in revised form 23 January 2014 Accepted 26 January 2014 Available online 12 February 2014 Keywords: Au/g-C3N4 nanocomposite Photoelectrocatalytic oxidation Small organic molecules Gold nanoparticles
a b s t r a c t Graphitic carbon nitride (g-C3N4) loaded with gold nanoparticles (Au NPs) was first explored as a catalyst for the photoelectrooxidation of small organic molecules (SOMs) in this paper. Au/g-C3N4 nanocomposite possesses significantly enhanced photoelectrocatalytic activity under UV light illumination, compared with pure g-C3N4 and Au NPs. The prepared catalysts with different Au loading were characterized and compared by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflection spectra (DRS) and electrochemical impedance spectroscopy (EIS). Close combination between g-C3N4 sheets and well-dispersed Au NPs is beneficial to the efficient separation of electron–hole pairs and charge transportation. The synergic effect of metallic Au NPs and polymeric g-C3N4 is responsible for the superior photoelectrocatalytic performance and is effective to mitigate the surface-poisoning species. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Photoelectrocatalysis (PEC) over metal–semiconductor nanocomposites has attracted significant attention due to their potential applications especially in hydrogen production [1–6], environmental decontamination [7–11] and catalytic reactions for solar energy conversion [12–16]. Different metallic nanoparticles and oxides can be either added into the crystallattice by doping, implanting and coprecipitating [17–19], or anchored onto the semiconductor surface by coating and depositing [20–24], which are effective approaches to promote the electron–hole pairs separation and improve the photoelectrocatalytic performance [25,26]. Graphitic carbon nitride (g-C3N4), a novel semiconductor photocatalyst, has recently attracted great interest because of its high stability, nontoxicity, and ease of preparation in large scale [27–30]. Most of research has been emphasized in the field of photosplitting water [27,31,32], photodecomposition of organic pollutants [33–35] and chemosensors [36]. However, its application in PEC is still relatively lacking. ⇑ Corresponding authors. Address: College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, PR China (J. Xiang). Tel.: +86 731 88836964; fax: +86 731 88879616. E-mail addresses: [email protected] (S. Chen), [email protected] (J. Xiang). 1 School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan, 411201, PR China. http://dx.doi.org/10.1016/j.jelechem.2014.01.026 1572-6657/Ó 2014 Elsevier B.V. All rights reserved.
Supported gold nanoparticles (Au NPs) incorporated with types of semiconductors (e.g., TiO2, ZnO, Al2O3, CeO2, etc.) demonstrated unexpectedly high catalytic activity towards various oxidative transformations [37] for the so-called support effect or synergistic effect [24,38]. For the oxidation of small organic molecules (SOMs), Au NPs were found to be more effective than Pt for mitigating the CO poison. Therefore, Au/g-C3N4 nanocomposite is expected to be a promising candidate for the photoelectrochemical application, which is employed as catalyst for water spliting recently [39]. However, less attention has been paid to the application of g-C3N4-based materials in PEC of SOMs. In this work, Au/g-C3N4 nanocomposite was prepared by a facile citrate-reduction method and used as a photoelectrocatalyt for SOMs oxidation firstly. Highly dispersed Au NPs were successfully loaded on the surface of g-C3N4 sheet and then characterized in detail by different techniques. The photoelectrocatalytic results demonstrated that Au/g-C3N4 significantly enhanced the oxidation of formic acid and some other SOMs by lowering oxidation potential, increasing the oxidation current and being free from CO poisoning. 2. Experimental 2.1. Chemicals and materials HAuCl44H2O (Sinopharm. Chemical Reagent Co., Ltd.), sodium citrate, perchloric acid, sodium hydroxide, formic acid,
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formaldehyde and ethanol were all of analytical grade. Deionised water with a resistivity higher than 18.2 MX cm was used to prepare all solutions. The g-C3N4 was kindly provided by Prof. Xinchen Wang from Fuzhou University (Fuzhou, China), which was prepared and well characterized as previously reported [27]. Before the Au loading, g-C3N4 was ground into very fine powder with a mortar. 2.2. Apparatus and measurements Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were carried out on a Tecnai G2 20 S-TWIN, a JSM-2100F microscope, respectively. X-ray diffraction (XRD) measurements were conducted on a Bruker D8 Advance diffractometer with Cu Ka radiation (k = 0.15406 nm). X-ray photoelectron spectroscopy (XPS) data were recorded using a Thermo K-Alpha 1063 instrument with a monochromatized Al Ka line source (72 W). UV–vis diffuse reflection spectra (DRS) was obtained for the dry-pressed disk samples at a Shimadzu UV-2450 spectrophotometer equipped with an integrating sphere assembly, using BaSO4 as reference. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and chronoamperometry experiments were performed on a CHI650D electrochemical workstation in a conventional three-electrode system, which consisted of a modified glassy carbon electrode (GCE, d = 3 mm, working electrode), Pt wire (counter electrode) and a saturated Ag/AgCl (reference electrode). An integrated Xe lamp (PLS-SXE300CUV, 300 W) with the wavelength range of 250–380 nm was used as a UV light source to measure the photoelectrochemical properties of the nanocomposites. All potentials in this study are reported with respect to Ag/AgCl. 2.3. Synthesis of Au/g-C3N4 nanocomposites The Au/g-C3N4 nanocomposites with different Au loading, namely, from 20 wt.% (weight percentage) to 90 wt.% were prepared by a facile citrate-reduction method. Typically, for the 20 wt.% sample, 100 mg g-C3N4 powder was suspended in HAuCl4 solution (100 mL, 2.36 mM) under ultrasonic for 30 min. The suspension was heated at 65 °C under stirring, and then sodium citrate solution (1.35 mL, 0.296 M) was added. After stirring for 4 h at 65 °C, the product was collected with centrifuge, rinsed and dried for 2 h at 150 °C. For different Au loading, the required amount of g-C3N4 was added. 2.4. Preparation of the modified electrodes The bare GCE was polished with 0.05 lm alumina slurry, followed by ultrasonically cleaned in ethanol and deionized water, respectively. In a typical procedure, 7.5 mg 20 wt.% Au/g-C3N4 catalyst was dispersed in 90 lL 0.5 wt.% nafion–ethanol solution under ultrasonic for 30 min. After sonication, 3 lL of the welldispersed catalyst ink was dropped onto the GCE and dried naturally in air. For different catalyst, the amounts of g-C3N4 in all inks were the same.
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almost similar with different Au loading. As shown in the HRTEM images (Fig. 1E and F), well attachment between g-C3N4 surface and Au NPs was formed, and the Au NPs were highly crystallized as evidenced from the Au (1 1 1) (0.24 nm) crystalline lattices. XRD data in Fig. 2 further elucidated the structures and crystallinities of Au/g-C3N4 samples. The weak peak at 27.4° is a characteristic interlayer stacking structure of the conjugated aromatic system, matched well with the (0 0 2) crystal planes of g-C3N4 [27]. The intense diffraction peaks at 38.0°, 44.3°, 64.5° and 77.4° are typical for the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of facecentered cubic (fcc) Au (JCPDS No. 04-0784), respectively. The peak intensity increases along with the increasing Au loading, indicating the rise of long-range order and crystallinity. Among the peaks observed, the intensity of the (1 1 1) peak is the highest, indicating that (1 1 1) plane is the predominant crystal facet. The Au crystallite size in the 80 wt.% Au/g-C3N4 is estimated as 24 nm from the half peak width of the (1 1 1) peak according to Scherrer equation, which is consistent with the TEM observation. In order to acquire more accurate surface composition and chemical information, XPS was utilized to analyse the chemical status of C, N and Au in the Au/g-C3N4 samples. From the data of C1S shown in Fig. 3A, two peaks can be distinguished to be centered at 284.6 or 284.8 eV and 287.9 or 288.0 eV, respectively. The peak at 284.6 or 284.8 eV is typically assigned to graphitic carbon or sp2 CAN in the literature, while the other one at 287.9 or 288.0 eV is identified as the sp2-hybridized carbon in the aromatic rings (NAC@N) [40,41]. Fig. 3B shows the N1S XPS spectra of the samples. The Gaussian multipeak fit for each plot shows that the N1S spectra consist of two strong peaks at 398.5 eV and 399.8 or 400.0 eV, which could be attributed to CAN@C and NA(C)3 functionalities, respectively [42,43]. Those results suggest that there is no change in the C or N electronic structures with the addition of Au. Fig. 3C shows binding energy of Au4f7/2 and Au4f5/2, which is in accordance with the formation of Au0, according to the reference [44]. The extinction spectrum of Au/g-C3N4 samples displayed two regions around 440 and 525 nm, corresponding to the edge absorption of g-C3N4 and typical surface plasmon resonance (SPR) absorption of Au NPs, as demonstrated in Fig. 4. The significant increase in SPR absorption intensity indicates increasing Au content on g-C3N4. Au/g-C3N4 samples all show a slight shift of the g-C3N4 band gap and the absorption edges shift with Au loading. These observations may be attributed to the interactions between g-C3N4 and Au NPs in the composite samples [45]. Furthermore, the dielectric properties of Au/g-C3N4 samples were investigated by EIS method. Fig. 5 depicts the corresponding Nyquist impedance plots for a series of Au/g-C3N4 samples at a potential of 0.2 V in 0.1 M HClO4 solution. Smaller diameters of arc radius than bare g-C3N4 are presented on each composites, which implies more efficient charge immigration across the electrode/ electrolyte interface and better conductivity of g-C3N4 material after Au loading [42,46]. However, lower conductivity is observed for the sample with 90 wt.% loading. It can be ascribed to weaker adhesive ability for excess loading and the aggregation of Au NPs, as featured by TEM in Fig. 1D. 3.2. PEC of formic acid on the Au/g-C3N4 modified GCE electrodes
3. Results and discussion 3.1. Characterization of the Au/g-C3N4 nanocomposites Fig. 1 shows some representative TEM images of the prepared Au/g-C3N4 samples and the corresponding histograms of the particle size distribution. From the TEM images (Fig. 1A–D), most of the Au NPs were dispersed in the range of 15–35 nm on g-C3N4 support with different density. The average particle sizes of Au NPs were
Here we focus on the Au/g-C3N4 nanocomposite samples for photoelectrocatalytic activities towards SOMs under UV light irradiation. Among familiar SOMs, formic acid has attracted particularly interest. The photoelectrocatalytic performance of bare g-C3N4, Au NPs and Au/g-C3N4 nanocomposite towards formic acid oxidation was readily tested by CV in a mixed solution of 0.1 M HClO4 and 0.1 M HCOOH. As shown in Fig. 6, formic acid oxidation on the Au/g-C3N4 modified GCE (curve a) is well evidenced with a
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Fig. 1. TEM images and corresponding size distribution histograms of the Au/g-C3N4 samples with different Au loading: (A–D) 20, 60, 80 and 90 wt.% and HRTEM of 80 wt.% Au/g-C3N4 sample (E and F).
Fig. 2. XRD patterns of the Au/g-C3N4 samples with different Au loading.
lower onset potential at 0.15 V and a peak at 0.8 V, while on the bare g-C3N4 (curve c) and pure Au NPs (curve b) modified GCE electrodes, no obvious redox peak could be found under the same condition, almost as same as the behavior in 0.1 M HClO4 background solution (not shown here). Different from traditional Pt and Pd based catalysts [47], the formation of CO poisonous species on Au/g-C3N4 in the positive and negative going sweeps is nearly
complete suppressed, and formic acid is likely oxidized directly to carbon dioxide via a dehydrogenation (HCOOH ? H2 + CO2) route [48,49]. Even though pure Au is almost inert towards HCOOH electrooxidation, it is observed that Au/g-C3N4 can catalyze formic acid oxidation even in the dark (curve d). One possible reason for the unexpected activity is that new active sites are formed at the Au/g-C3N4 interface [12]. These sites also facilitate the oxidation of adsorbed formate, which spillover from Au NPs [50]. When the Au/g-C3N4 modified GCE electrode was exposed to the UV irradiation, a great anodic current was observed, which was ca. 3 times that of Au/g-C3N4 in the dark. Undoubtedly, the remarkable current is attributed to the synergic effect of Au NPs and g-C3N4, which is closely related with the efficient electron–hole pairs separation and charge transportation. It is already known that, a Schottky barrier could generate at the metal–semiconductor interface, which would effectively capture the photogenerated electrons, reduce the rate of electron–hole recombination [8]. Under UV illumination, the excited electron could readily migrate from the conduction band of g-C3N4 to Au NPs, and then, the hole is free to diffuse to g-C3N4 semiconductor surface. It is obvious that, there are rich oxidation sites on the composite upon illuminating, which can oxidize formic acid, resulting in the remarkable photocurrent. To evaluate the effect of the Au loading on the catalytic activity, different working electrodes were prepared using Au/g-C3N4 samples with different Au loading. The current densities (Fig. 7) were
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Fig. 3. XPS spectra of the Au/g-C3N4 samples with different Au loading: (A) C1S, (B) N1S, (C) Au4f.
Fig. 4. The UV–vis DRS of g-C3N4 and Au/g-C3N4 nanocomposites.
Fig. 6. CVs of 80 wt.% Au/g-C3N4 (curve a), Au NPs (curve b), g-C3N4 (curve c) under UV irradiation and 80 wt.% Au/g-C3N4 in the dark (curve d) in 0.1 M HClO4 + 0.1 M HCOOH solution at the scan rate of 20 mV/s.
excess loading may result in the aggregation of Au nanoparticles in Au/g-C3N4 nanocomposite and reduction in the efficiency of charge separation [53]. Therefore, the Au/g-C3N4 sample with the loading of 80 wt.% was chosen for the following investigation. 3.3. Stability and reproducibility of the Au/g-C3N4 modified GCE electrode
Fig. 5. Nyquist impedance plots of g-C3N4 and Au/g-C3N4 nanocomposites recorded in the frequency range 10 3–105 Hz with an ac amplitude of 5 mV.
normalized by the electrochemical surface area (ECSA), which were calculated by the reduction of surface oxides on Au using the charge factor of 400 lC cm 2 [51,52]. As shown in Fig. 7A, the oxidation peak potential of formic acid was not obviously influenced by the Au loading, while the peak current density varied with the increase of Au loading. It can be clearly seen in Fig. 7B that the catalytic activity of Au/g-C3N4 samples changed in the order: 80 wt.% > 70 wt.% > 90 wt.% > 60 wt.% > 50 wt.% > 20 wt.%, which was in accordance with the resistance sequence in Fig. 5. This is because an increase in Au loading amount contributes to increasing the density of active sites for formic acid oxidation, however,
The stability and reproducibility of the Au/g-C3N4 modified GCE were evaluated by polarizing the electrode at 0.8 V, as illustrated in Fig. 8. It can be clearly seen from Fig. 8A that the current for formic acid oxidation on the catalyst dropped rapidly in the first 400 s and then became relatively stable beyond this period. Moreover, after stored at room temperature for 3 weeks, the active oxidation of formic acid retained more than 95% of its original response for the Au/g-C3N4 catalyst. As demonstrated in Fig. 8B, the rise and fall of the current corresponded well to the illumination being switched on and off. Moreover, this pattern of photocurrent was highly reproducible for numerous on–off cycles of illumination. These results show that the GCE modified with Au/g-C3N4 nanocomposite has competitive stability and repeatability. 3.4. PEC of some other SOMs on the Au/g-C3N4 modified GCE electrodes Fig. 9 shows the CVs of formaldehyde and ethanol on the Au/gC3N4 modified GCE. As can be seen from the figure, the photoelectrochemical behaviors of formaldehyde and ethanol are similar to formic acid. Be analogous to the case of formic acid, the bare g-C3N4 shows no catalytic activity for the oxidation of formaldehyde
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Fig. 7. Influence of Au loading on the peak current density of HCOOH in 0.1 M HClO4 solution. (A) CVs of Au/g-C3N4 nanocomposites. Scan rate: 20 mV/s. (B) Peak current density of HCOOH at different Au loading value. Error bars represent standard errors of the means from three replicates.
Fig. 8. (A) Chronoamperometry curve recorded at 0.8 V under UV irradiation. (B) Intermittently illuminated chronoamperometry curve recorded at 0.8 V.
Fig. 9. CVs of 80 wt.% Au/g-C3N4 (curve a), Au NPs (curve b) and g-C3N4 (curve c) under UV irradiation in different solutions: (A) 0.1 M NaOH + 0.1 M HCHO and (B) 0.1 M NaOH + 0.1 M C2H5OH solution. Scan rate: 20 mV/s.
and ethanol (curve c in Fig. 9A and B). The dramatically current peak appeared at about 0.2 V on Au/g-C3N4 (curve a in Fig. 9A) is attributed to the formaldehyde oxidation, which is much higher than that on pure Au NPs (curve b in Fig. 9A). The current peak at 0.4 V in the forward scan (curve a in Fig. 9B) is attributed to ethanol oxidation on Au/g-C3N4 nanocomposite with higher activity than pure Au NPs (curve b in Fig. 9B).
4. Conclusion In summary, Au/g-C3N4 nanocomposite with highly dispersed Au NPs on the surface of g-C3N4 has been synthesized and proved to be an efficient catalyst for the photoelectrooxidation of SOMs. The structural, optical and electronic properties were carefully characterized by XRD, XPS, DRS, TEM, HRTEM and photoeletrochemical
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techniques. Excellent photoelectrocatalytic activity can be attributed to the Au NPs-facilitated separation of electron–hole pairs, and interfacial charge transfer. We believe that this metal nanoparticles supported on g-C3N4 may provide a promising functional material for light driven electrocatalysis. Acknowledgments This work was supported by the National Natural Science Foundation of China (21273288, 20773165), the Specialized Research Fund for the Doctor Program of Higher Education (20120162110018), the Hunan Provincial Natural Science Foundation of China (09JJ1002), the Program for New Century Excellent Talents in University (NCET-07-0865), and the exploration innovation fund for Graduate student (2013zzts178). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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Enhanced photoelectrocatalytic oxidation of small organic molecules by gold nanoparticles supported on carbon nitride Shouqin Chang a, Aiyun Xie a, Shu Chen a,⇑,1, Juan Xiang a,b,c,⇑ a
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, PR China Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, Central South University, Changsha 410083, PR China c State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, PR China b
a r t i c l e
i n f o
Article history: Received 18 September 2013 Received in revised form 23 January 2014 Accepted 26 January 2014 Available online 12 February 2014 Keywords: Au/g-C3N4 nanocomposite Photoelectrocatalytic oxidation Small organic molecules Gold nanoparticles
a b s t r a c t Graphitic carbon nitride (g-C3N4) loaded with gold nanoparticles (Au NPs) was first explored as a catalyst for the photoelectrooxidation of small organic molecules (SOMs) in this paper. Au/g-C3N4 nanocomposite possesses significantly enhanced photoelectrocatalytic activity under UV light illumination, compared with pure g-C3N4 and Au NPs. The prepared catalysts with different Au loading were characterized and compared by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflection spectra (DRS) and electrochemical impedance spectroscopy (EIS). Close combination between g-C3N4 sheets and well-dispersed Au NPs is beneficial to the efficient separation of electron–hole pairs and charge transportation. The synergic effect of metallic Au NPs and polymeric g-C3N4 is responsible for the superior photoelectrocatalytic performance and is effective to mitigate the surface-poisoning species. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Photoelectrocatalysis (PEC) over metal–semiconductor nanocomposites has attracted significant attention due to their potential applications especially in hydrogen production [1–6], environmental decontamination [7–11] and catalytic reactions for solar energy conversion [12–16]. Different metallic nanoparticles and oxides can be either added into the crystallattice by doping, implanting and coprecipitating [17–19], or anchored onto the semiconductor surface by coating and depositing [20–24], which are effective approaches to promote the electron–hole pairs separation and improve the photoelectrocatalytic performance [25,26]. Graphitic carbon nitride (g-C3N4), a novel semiconductor photocatalyst, has recently attracted great interest because of its high stability, nontoxicity, and ease of preparation in large scale [27–30]. Most of research has been emphasized in the field of photosplitting water [27,31,32], photodecomposition of organic pollutants [33–35] and chemosensors [36]. However, its application in PEC is still relatively lacking. ⇑ Corresponding authors. Address: College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, PR China (J. Xiang). Tel.: +86 731 88836964; fax: +86 731 88879616. E-mail addresses: [email protected] (S. Chen), [email protected] (J. Xiang). 1 School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan, 411201, PR China. http://dx.doi.org/10.1016/j.jelechem.2014.01.026 1572-6657/Ó 2014 Elsevier B.V. All rights reserved.
Supported gold nanoparticles (Au NPs) incorporated with types of semiconductors (e.g., TiO2, ZnO, Al2O3, CeO2, etc.) demonstrated unexpectedly high catalytic activity towards various oxidative transformations [37] for the so-called support effect or synergistic effect [24,38]. For the oxidation of small organic molecules (SOMs), Au NPs were found to be more effective than Pt for mitigating the CO poison. Therefore, Au/g-C3N4 nanocomposite is expected to be a promising candidate for the photoelectrochemical application, which is employed as catalyst for water spliting recently [39]. However, less attention has been paid to the application of g-C3N4-based materials in PEC of SOMs. In this work, Au/g-C3N4 nanocomposite was prepared by a facile citrate-reduction method and used as a photoelectrocatalyt for SOMs oxidation firstly. Highly dispersed Au NPs were successfully loaded on the surface of g-C3N4 sheet and then characterized in detail by different techniques. The photoelectrocatalytic results demonstrated that Au/g-C3N4 significantly enhanced the oxidation of formic acid and some other SOMs by lowering oxidation potential, increasing the oxidation current and being free from CO poisoning. 2. Experimental 2.1. Chemicals and materials HAuCl44H2O (Sinopharm. Chemical Reagent Co., Ltd.), sodium citrate, perchloric acid, sodium hydroxide, formic acid,
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formaldehyde and ethanol were all of analytical grade. Deionised water with a resistivity higher than 18.2 MX cm was used to prepare all solutions. The g-C3N4 was kindly provided by Prof. Xinchen Wang from Fuzhou University (Fuzhou, China), which was prepared and well characterized as previously reported [27]. Before the Au loading, g-C3N4 was ground into very fine powder with a mortar. 2.2. Apparatus and measurements Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were carried out on a Tecnai G2 20 S-TWIN, a JSM-2100F microscope, respectively. X-ray diffraction (XRD) measurements were conducted on a Bruker D8 Advance diffractometer with Cu Ka radiation (k = 0.15406 nm). X-ray photoelectron spectroscopy (XPS) data were recorded using a Thermo K-Alpha 1063 instrument with a monochromatized Al Ka line source (72 W). UV–vis diffuse reflection spectra (DRS) was obtained for the dry-pressed disk samples at a Shimadzu UV-2450 spectrophotometer equipped with an integrating sphere assembly, using BaSO4 as reference. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and chronoamperometry experiments were performed on a CHI650D electrochemical workstation in a conventional three-electrode system, which consisted of a modified glassy carbon electrode (GCE, d = 3 mm, working electrode), Pt wire (counter electrode) and a saturated Ag/AgCl (reference electrode). An integrated Xe lamp (PLS-SXE300CUV, 300 W) with the wavelength range of 250–380 nm was used as a UV light source to measure the photoelectrochemical properties of the nanocomposites. All potentials in this study are reported with respect to Ag/AgCl. 2.3. Synthesis of Au/g-C3N4 nanocomposites The Au/g-C3N4 nanocomposites with different Au loading, namely, from 20 wt.% (weight percentage) to 90 wt.% were prepared by a facile citrate-reduction method. Typically, for the 20 wt.% sample, 100 mg g-C3N4 powder was suspended in HAuCl4 solution (100 mL, 2.36 mM) under ultrasonic for 30 min. The suspension was heated at 65 °C under stirring, and then sodium citrate solution (1.35 mL, 0.296 M) was added. After stirring for 4 h at 65 °C, the product was collected with centrifuge, rinsed and dried for 2 h at 150 °C. For different Au loading, the required amount of g-C3N4 was added. 2.4. Preparation of the modified electrodes The bare GCE was polished with 0.05 lm alumina slurry, followed by ultrasonically cleaned in ethanol and deionized water, respectively. In a typical procedure, 7.5 mg 20 wt.% Au/g-C3N4 catalyst was dispersed in 90 lL 0.5 wt.% nafion–ethanol solution under ultrasonic for 30 min. After sonication, 3 lL of the welldispersed catalyst ink was dropped onto the GCE and dried naturally in air. For different catalyst, the amounts of g-C3N4 in all inks were the same.
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almost similar with different Au loading. As shown in the HRTEM images (Fig. 1E and F), well attachment between g-C3N4 surface and Au NPs was formed, and the Au NPs were highly crystallized as evidenced from the Au (1 1 1) (0.24 nm) crystalline lattices. XRD data in Fig. 2 further elucidated the structures and crystallinities of Au/g-C3N4 samples. The weak peak at 27.4° is a characteristic interlayer stacking structure of the conjugated aromatic system, matched well with the (0 0 2) crystal planes of g-C3N4 [27]. The intense diffraction peaks at 38.0°, 44.3°, 64.5° and 77.4° are typical for the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of facecentered cubic (fcc) Au (JCPDS No. 04-0784), respectively. The peak intensity increases along with the increasing Au loading, indicating the rise of long-range order and crystallinity. Among the peaks observed, the intensity of the (1 1 1) peak is the highest, indicating that (1 1 1) plane is the predominant crystal facet. The Au crystallite size in the 80 wt.% Au/g-C3N4 is estimated as 24 nm from the half peak width of the (1 1 1) peak according to Scherrer equation, which is consistent with the TEM observation. In order to acquire more accurate surface composition and chemical information, XPS was utilized to analyse the chemical status of C, N and Au in the Au/g-C3N4 samples. From the data of C1S shown in Fig. 3A, two peaks can be distinguished to be centered at 284.6 or 284.8 eV and 287.9 or 288.0 eV, respectively. The peak at 284.6 or 284.8 eV is typically assigned to graphitic carbon or sp2 CAN in the literature, while the other one at 287.9 or 288.0 eV is identified as the sp2-hybridized carbon in the aromatic rings (NAC@N) [40,41]. Fig. 3B shows the N1S XPS spectra of the samples. The Gaussian multipeak fit for each plot shows that the N1S spectra consist of two strong peaks at 398.5 eV and 399.8 or 400.0 eV, which could be attributed to CAN@C and NA(C)3 functionalities, respectively [42,43]. Those results suggest that there is no change in the C or N electronic structures with the addition of Au. Fig. 3C shows binding energy of Au4f7/2 and Au4f5/2, which is in accordance with the formation of Au0, according to the reference [44]. The extinction spectrum of Au/g-C3N4 samples displayed two regions around 440 and 525 nm, corresponding to the edge absorption of g-C3N4 and typical surface plasmon resonance (SPR) absorption of Au NPs, as demonstrated in Fig. 4. The significant increase in SPR absorption intensity indicates increasing Au content on g-C3N4. Au/g-C3N4 samples all show a slight shift of the g-C3N4 band gap and the absorption edges shift with Au loading. These observations may be attributed to the interactions between g-C3N4 and Au NPs in the composite samples [45]. Furthermore, the dielectric properties of Au/g-C3N4 samples were investigated by EIS method. Fig. 5 depicts the corresponding Nyquist impedance plots for a series of Au/g-C3N4 samples at a potential of 0.2 V in 0.1 M HClO4 solution. Smaller diameters of arc radius than bare g-C3N4 are presented on each composites, which implies more efficient charge immigration across the electrode/ electrolyte interface and better conductivity of g-C3N4 material after Au loading [42,46]. However, lower conductivity is observed for the sample with 90 wt.% loading. It can be ascribed to weaker adhesive ability for excess loading and the aggregation of Au NPs, as featured by TEM in Fig. 1D. 3.2. PEC of formic acid on the Au/g-C3N4 modified GCE electrodes
3. Results and discussion 3.1. Characterization of the Au/g-C3N4 nanocomposites Fig. 1 shows some representative TEM images of the prepared Au/g-C3N4 samples and the corresponding histograms of the particle size distribution. From the TEM images (Fig. 1A–D), most of the Au NPs were dispersed in the range of 15–35 nm on g-C3N4 support with different density. The average particle sizes of Au NPs were
Here we focus on the Au/g-C3N4 nanocomposite samples for photoelectrocatalytic activities towards SOMs under UV light irradiation. Among familiar SOMs, formic acid has attracted particularly interest. The photoelectrocatalytic performance of bare g-C3N4, Au NPs and Au/g-C3N4 nanocomposite towards formic acid oxidation was readily tested by CV in a mixed solution of 0.1 M HClO4 and 0.1 M HCOOH. As shown in Fig. 6, formic acid oxidation on the Au/g-C3N4 modified GCE (curve a) is well evidenced with a
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Fig. 1. TEM images and corresponding size distribution histograms of the Au/g-C3N4 samples with different Au loading: (A–D) 20, 60, 80 and 90 wt.% and HRTEM of 80 wt.% Au/g-C3N4 sample (E and F).
Fig. 2. XRD patterns of the Au/g-C3N4 samples with different Au loading.
lower onset potential at 0.15 V and a peak at 0.8 V, while on the bare g-C3N4 (curve c) and pure Au NPs (curve b) modified GCE electrodes, no obvious redox peak could be found under the same condition, almost as same as the behavior in 0.1 M HClO4 background solution (not shown here). Different from traditional Pt and Pd based catalysts [47], the formation of CO poisonous species on Au/g-C3N4 in the positive and negative going sweeps is nearly
complete suppressed, and formic acid is likely oxidized directly to carbon dioxide via a dehydrogenation (HCOOH ? H2 + CO2) route [48,49]. Even though pure Au is almost inert towards HCOOH electrooxidation, it is observed that Au/g-C3N4 can catalyze formic acid oxidation even in the dark (curve d). One possible reason for the unexpected activity is that new active sites are formed at the Au/g-C3N4 interface [12]. These sites also facilitate the oxidation of adsorbed formate, which spillover from Au NPs [50]. When the Au/g-C3N4 modified GCE electrode was exposed to the UV irradiation, a great anodic current was observed, which was ca. 3 times that of Au/g-C3N4 in the dark. Undoubtedly, the remarkable current is attributed to the synergic effect of Au NPs and g-C3N4, which is closely related with the efficient electron–hole pairs separation and charge transportation. It is already known that, a Schottky barrier could generate at the metal–semiconductor interface, which would effectively capture the photogenerated electrons, reduce the rate of electron–hole recombination [8]. Under UV illumination, the excited electron could readily migrate from the conduction band of g-C3N4 to Au NPs, and then, the hole is free to diffuse to g-C3N4 semiconductor surface. It is obvious that, there are rich oxidation sites on the composite upon illuminating, which can oxidize formic acid, resulting in the remarkable photocurrent. To evaluate the effect of the Au loading on the catalytic activity, different working electrodes were prepared using Au/g-C3N4 samples with different Au loading. The current densities (Fig. 7) were
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Fig. 3. XPS spectra of the Au/g-C3N4 samples with different Au loading: (A) C1S, (B) N1S, (C) Au4f.
Fig. 4. The UV–vis DRS of g-C3N4 and Au/g-C3N4 nanocomposites.
Fig. 6. CVs of 80 wt.% Au/g-C3N4 (curve a), Au NPs (curve b), g-C3N4 (curve c) under UV irradiation and 80 wt.% Au/g-C3N4 in the dark (curve d) in 0.1 M HClO4 + 0.1 M HCOOH solution at the scan rate of 20 mV/s.
excess loading may result in the aggregation of Au nanoparticles in Au/g-C3N4 nanocomposite and reduction in the efficiency of charge separation [53]. Therefore, the Au/g-C3N4 sample with the loading of 80 wt.% was chosen for the following investigation. 3.3. Stability and reproducibility of the Au/g-C3N4 modified GCE electrode
Fig. 5. Nyquist impedance plots of g-C3N4 and Au/g-C3N4 nanocomposites recorded in the frequency range 10 3–105 Hz with an ac amplitude of 5 mV.
normalized by the electrochemical surface area (ECSA), which were calculated by the reduction of surface oxides on Au using the charge factor of 400 lC cm 2 [51,52]. As shown in Fig. 7A, the oxidation peak potential of formic acid was not obviously influenced by the Au loading, while the peak current density varied with the increase of Au loading. It can be clearly seen in Fig. 7B that the catalytic activity of Au/g-C3N4 samples changed in the order: 80 wt.% > 70 wt.% > 90 wt.% > 60 wt.% > 50 wt.% > 20 wt.%, which was in accordance with the resistance sequence in Fig. 5. This is because an increase in Au loading amount contributes to increasing the density of active sites for formic acid oxidation, however,
The stability and reproducibility of the Au/g-C3N4 modified GCE were evaluated by polarizing the electrode at 0.8 V, as illustrated in Fig. 8. It can be clearly seen from Fig. 8A that the current for formic acid oxidation on the catalyst dropped rapidly in the first 400 s and then became relatively stable beyond this period. Moreover, after stored at room temperature for 3 weeks, the active oxidation of formic acid retained more than 95% of its original response for the Au/g-C3N4 catalyst. As demonstrated in Fig. 8B, the rise and fall of the current corresponded well to the illumination being switched on and off. Moreover, this pattern of photocurrent was highly reproducible for numerous on–off cycles of illumination. These results show that the GCE modified with Au/g-C3N4 nanocomposite has competitive stability and repeatability. 3.4. PEC of some other SOMs on the Au/g-C3N4 modified GCE electrodes Fig. 9 shows the CVs of formaldehyde and ethanol on the Au/gC3N4 modified GCE. As can be seen from the figure, the photoelectrochemical behaviors of formaldehyde and ethanol are similar to formic acid. Be analogous to the case of formic acid, the bare g-C3N4 shows no catalytic activity for the oxidation of formaldehyde
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Fig. 7. Influence of Au loading on the peak current density of HCOOH in 0.1 M HClO4 solution. (A) CVs of Au/g-C3N4 nanocomposites. Scan rate: 20 mV/s. (B) Peak current density of HCOOH at different Au loading value. Error bars represent standard errors of the means from three replicates.
Fig. 8. (A) Chronoamperometry curve recorded at 0.8 V under UV irradiation. (B) Intermittently illuminated chronoamperometry curve recorded at 0.8 V.
Fig. 9. CVs of 80 wt.% Au/g-C3N4 (curve a), Au NPs (curve b) and g-C3N4 (curve c) under UV irradiation in different solutions: (A) 0.1 M NaOH + 0.1 M HCHO and (B) 0.1 M NaOH + 0.1 M C2H5OH solution. Scan rate: 20 mV/s.
and ethanol (curve c in Fig. 9A and B). The dramatically current peak appeared at about 0.2 V on Au/g-C3N4 (curve a in Fig. 9A) is attributed to the formaldehyde oxidation, which is much higher than that on pure Au NPs (curve b in Fig. 9A). The current peak at 0.4 V in the forward scan (curve a in Fig. 9B) is attributed to ethanol oxidation on Au/g-C3N4 nanocomposite with higher activity than pure Au NPs (curve b in Fig. 9B).
4. Conclusion In summary, Au/g-C3N4 nanocomposite with highly dispersed Au NPs on the surface of g-C3N4 has been synthesized and proved to be an efficient catalyst for the photoelectrooxidation of SOMs. The structural, optical and electronic properties were carefully characterized by XRD, XPS, DRS, TEM, HRTEM and photoeletrochemical
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techniques. Excellent photoelectrocatalytic activity can be attributed to the Au NPs-facilitated separation of electron–hole pairs, and interfacial charge transfer. We believe that this metal nanoparticles supported on g-C3N4 may provide a promising functional material for light driven electrocatalysis. Acknowledgments This work was supported by the National Natural Science Foundation of China (21273288, 20773165), the Specialized Research Fund for the Doctor Program of Higher Education (20120162110018), the Hunan Provincial Natural Science Foundation of China (09JJ1002), the Program for New Century Excellent Talents in University (NCET-07-0865), and the exploration innovation fund for Graduate student (2013zzts178). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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