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Facile-green synthesis of nitrogen-doped carbon-supported ultrafine silver catalyst with enhanced electrocatalytic property
Electrochimica Acta 108 (2013) 66–73
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Facile-green synthesis of nitrogen-doped carbon-supported ultrafine silver catalyst with enhanced electrocatalytic property Yichen Wang a,b , Haibin Wu a , Xiue Jiang a,∗ a b
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 21 February 2013 Received in revised form 13 June 2013 Accepted 18 June 2013 Available online 2 July 2013 Keywords: Folic acid Heterogeneous catalysis N-doped carbon nanostructure Oxygen reduction reaction Ultrafine Ag nanoparticles
a b s t r a c t We have demonstrated a facile and green strategy to synthesize ultrafine silver nanoparticles monodispersed on N-doped three-dimensional carbon nanocloud surfaces without any toxic reagent. Folic acid was employed as the carbon precursor for forming N-doped carbon nanoflakes by a hydrothermal method. The as-prepared products can serve as both reducing agent and substrate, on which a high density of ultrafine Ag nanocrystals is stably grown in a homogeneously dispersive state spontaneously at room temperature. A feasible synthesis mechanism has been proposed by characterization of carbon precursor, nanomaterials composited without and with silver nanoparticles. It was found that the ethylenic and oxygenated groups led to the reduction process. The nanohybrids showed an enhanced electrocatalytic activity toward oxygen reduction reaction (ORR) in alkaline solution via a four-electron pathway. The catalyst also exhibited strong duration of methanol and good stability compared to commercial Pt/C catalysts. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Over the past several decades, considerable attention has been paid to prepare high performance catalyst since it directly restrict the development of the chemical industry, such as liquid fuel cells [1,2]. In all of these areas, the performance of the cell is limited by the oxygen reduction reaction (ORR) rate at the cathode [3–5]. As the particle size decrease, the surface area and the number of active metal atoms are increased, which greatly improve catalytic activities of nanoparticles. Therefore, ultrafine nanoparticles attracted particular interest in preparation of catalyst due to highly enhanced catalytic activities [6–8]. However, the enhanced surface energy often leads to aggregation or sintering of the catalysts in the applications [9]. One of the effective strategies for avoiding this problem is to use suitable support to keep the activity and size of ultrafine nanoparticles [10]. The synergetic effect of the nanomaterials with support might further improve the catalytic performance [11–13]. Carbon nanomaterials, including carbon nanotube [14], graphene [15] and mesoporous carbon [13,16] are the most frequently used supports to enhance the catalytic performance of metal catalyst due to their high specific surface areas, good mechanical and thermal stability. Recently, nitrogen-doped carbons have attracted much attention in the wide world either as metal-free catalyst [3,17,18]
∗ Corresponding author. Tel.: +86 0431 85262426; fax: +86 0431 85685653. E-mail address: [email protected] (X. Jiang). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.06.100
or as fascinating supports to construct high-performance catalyst [13,19,20] because the incorporation of nitrogen atom in the carbon architecture can enhance the functional properties of the carbon [21]. Although their enhanced catalytic properties have extensively been proved, most of the fabrication methods, such as chemical vapor deposition nitrogen containing compounds [17], treatment of carbons with ammonia at high temperature [20] and high temperature carbonization of nitrogen-containing precursors [13,19], are complicated and even toxic to people and environment. To this point, new strategy for rapid production of nitrogen-containing support is highly desired. Currently, carbon-supported silver nanoparticle catalysts have attracted particular interest because of their lower cost and high performance [22–24], especially promising activities toward ORR under alkaline conditions [12,25,26], and silver also has the highest electrical and thermal conductivity among all metals. Before use as host materials to anchor silver nanoparticles, the most used carbon materials have to be purified or acid-treated by refluxing in concentrated sulfuric and nitric acids to introduce charged groups and strengthen the interaction of silver nanoparticles and the support [22,23]. Due to the application of various chemical reagents or some complicated equipments, the protocols of material synthesis are not simple and economical. In addition, the adding of reducing agents or capping molecules may cause the environment pollution and increase the cost. Therefore, it should be of particular importance to develop energy-efficient, low-cost and environment friendly methods for the preparation of high-quality silver catalyst.
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Fig. 1. (a, b) TEM images and (c) HRTEM image of the N-CNFs. (d) XPS survey spectrum of the N-CNFs. High resolution XPS spectra of (e) N1s and (f) C1s of the N-CNFs.
Here we report a synthetic strategy that enables facile, economical and green synthesis of well dispersed silver nanoparticles with ultrafine sizes anchored on new nitrogen-doped carbon nanoflakes (N-CNFs), which are prepared by a hydrothermal treatment using folic acid (FA) as source of nitrogen-containing carbon precursor for the first time. The growth of ultrafine Ag nanocrystals on the surface of novel N-CNFs forms AgNPs-decorated N-doped threedimensional carbon nanoclouds (AgNPs/N-TDCNCs), which occurs via a spontaneous process under room temperature without adding any more agents. Most importantly, the as-prepared nanocomposites exhibited enhanced electro-catalytic activity and stability toward ORR in alkaline solution via a four-electron pathway. 2. Experimental 2.1. Reagents and materials FA was obtained from Aladin Ltd. (Shanghai, China). AgNO3 was purchased from Beijing Chemical Corp. (Beijing, China). All
chemicals were used without further purification. The water used throughout all experiments was purified through a Millipore system.
2.2. Synthesis of N-CNFs and AgNPs/N-TDCNCs N-CNFs were prepared by a hydrothermal process. In a typical preparation experiment, 100 mg of FA was added to 30 mL of H2 O with stirring, and then the cloudy mixture was transferred into a 50 mL Teflon-lined autoclave and heated at 200 ◦ C for a period of 10 h. The as-prepared N-CNFs were separated by removing the insoluble products though centrifugation at 12,000 rpm for 10 min and the supernatant was collected for further characterization and use. 1 mL of 0.1 M AgNO3 was added drop by drop to 5 mL of N-CNFs aqueous solution kept in a vial under vigorous stirring and the reaction was sustained for 30 min at room temperature. After that, the suspension was centrifuged and washed with water for 3 times
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Fig. 2. (a, b) TEM images and (c) HRTEM image of the AgNPs/N-TDCNCs. (d) XPS survey spectrum of the AgNPs/N-TDCNCs. High resolution XPS spectra of (e) N1s and (f) C1s of the AgNPs/N-TDCNCs.
to remove the remaining reagent. The resultant AgNPs/N-TDCNCs were dispersed in water for characterization and use. 2.3. Measurements The transmission electron microscopy (TEM) measurements of the as-prepared sample were made on a HITACHI H-8100EM (Hitachi, Tokyo, Japan). High-resolution TEM (HRTEM) images and the corresponding EDX were all carried out on a JEM-2010 (HR) microscope. The X-ray diffraction (XRD) measurements were performed on a PW1700 powder diffractometer using Cu-K␣ radiation with a Ni filter ( = 0.154059 nm at 30 kV and 15 mA). The X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 X-ray photoelectron spectrometer with a monochromated X-ray source (AlK␣ h=1486.6 eV). UV–vis spectra were recorded on a VARIAN CARY 50 UV/vis spectrophotometer.
The Fourier-transformed infrared spectroscopy (FTIR) study was conducted with a VERTEX 70 FTIR. Cyclic voltammetry and rotating disk electrode (RDE) experiments were carried out by using a computer-controlled CHI832C electrochemical analyzer (CH Instruments, Inc., Shanghai) in a home-made three-electrode electrochemical cell consisting of a glassy carbon electrode (GCE) as a working electrode, a platinum coil as a counter electrode and an Ag/AgCl (in saturated KCl) as a reference electrode. The modified AgNPs/N-TDCNCs GCE (AgNPs/N-TDCNCs/GCE) was prepared by a simple casting method. Prior to the surface coating, the GCE was polished with 1.0 and 0.3 m alumina powder, respectively, and rinsed with double distilled water, followed by sonication in ethanol solution and double distilled water successively. Then, the electrode was dried in a stream of nitrogen. After that, 10 L of AgNPs/N-TDCNCs (∼2 mg/ml) was dropped onto the surface of pretreated GCE and left to dry at room temperature. Prior
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to experiment, 2 L of Nafion (0.5%) was additionally dropped onto the surface of the resultant-modified electrode and dried at room temperature for 2 h. N2 -saturated and O2 -saturated electrolyte were prepared by blowing the electrolyte solution with ultrahigh purity nitrogen and oxygen for 15 min then sealing the solution with nitrogen and oxygen atmospheres, respectively, during the experimental procedure. All electrochemical experiments were performed at the room temperature.
3. Results and discussion 3.1. Formation of the materials In our experiment, a yellow dispersion (Supporting Information, Fig. S1a) was formed after hydrothermal treatment of FA at 200 ◦ C for 10 h. Fig. 1a shows the low magnification TEM of the NCNFs. It can be seen that the as-prepared products are composed of some unordered nanoflakes, which is further evidenced by the high magnification TEM (Fig. 1b). The HRTEM picture of the as-prepared N-CNFs (Fig. 1c) exhibits crystal lattices with the measured lattice spacing of 0.35 nm (Fig. S2). XPS measurement was utilized to identify the surface composition of the N-CNFs. As shown in Fig. 1d, three peaks are observed in the XPS spectrum of the N-CNFs at 285.0, 400.0 and 531.0 eV, which have been attributed to C1s, N1s and O1s, respectively [27–29]. The elemental analysis indicates that the N/C atom ratio is 0.202 The N1s spectrum (Fig. 1e) can be decomposed to three peaks at 399.0, 399.9, and 401.5 eV, which are attributed to pyridinic, pyrrolic and quaternary N, respectively [30–32]. The C1s spectrum (Fig. 1f) decomposed at 285.7, 286.5 and 288.2 eV are assigned to C O, Epoxy and C O [27], respectively. The fraction of the oxygencontaining groups in the C1s spectrum is 0.43. These results indicate that the N-CNFs are N-doped carbon materials with oxygen-rich functional groups. When AgNO3 was mixed with the N-CNFs aqueous solution at room temperature for 30 min with vigorous stirring, the yellow N-CNFs dispersion turned to dark green suspension gradually (Fig. S1b), implying the interaction of AgNO3 and the N-CNFs. The resulting products were collected by centrifugation and washed several time with water. Fig. 2a shows the low magnification TEM of the AgNPs/N-TDCNCs. It seems like a three-dimensional nanocloud inserted with pores, which is obviously different with that of the NCNFs. High magnification TEM (Fig. 2b) shows the decoration of the nanomaterials with lots of ultrafine particles. The corresponding EDX spectroscopy (Fig. S3) of the nanomaterials shows the peaks corresponding to Ag, C, and N elements, indicating the ultrafine particles are Ag nanoparticles (AgNPs). The size of AgNPs is uniform, with an average diameter of 3.96 nm and a narrow standard deviation of 0.58 nm (Fig. S4). The HRTEM image (Fig. 2c) of the products shows a crystalline structure of AgNPs with a lattice spacing of 0.23 nm (Fig. S5), which corresponds to the (1 1 1) lattice plane of Ag. The XRD pattern (Fig. 3) of the products indicates a face-centered cubic (fcc) structure of Ag with a strong peak standing for the (1 1 1) lattice plane. Similarly, the XPS spectrum reveals that the main compositions of the products are C1s, N1s, O1s and Ag. However, the percentage of pyridinic N in total N atoms increases as reflected by the decomposition of the N1s spectrum (Fig. 2e). Meanwhile, the C1s spectrum shows the decrease of the oxygencontaining functional groups to 0.35 (Fig. 2f). All these results indicate that the novel N-CNFs can be used as support to load and reduce AgNO3 to ultrafine AgNPs accompanied by structural changes of the N-CNFs to form AgNPs/N-TDCNCs via a spontaneous process under room temperature. The measurement of inductively coupled plasma optical emission spectroscopy (ICP-AES) reveals
Fig. 3. XRD patterns of the N-CNFs and AgNPs/N-TDCNCs along with the bulk XRD pattern of Ag (black bars, from JCPDS no. 04-0783).
that the loading content of Ag in the prepared nanohybrids is 33.39%. In order to reveal the reactive mechanism of the AgNPs/NTDCNCs, we monitored the reaction processes with molecular spectroscopy. Fig. 4a shows the UV–vis absorption spectra of the FA (solid line), N-CNFs (dashed line) and AgNPs/N-TDCNCs (dotted line). The spectrum of the FA solution shows an absorption band at 280 nm attributed to the –* transition of pterin (PT) ring [33]. After the hydrothermal treatment for 10 h, the PT peak decreases as well as the appearance of the peak at 230 nm assigned
Fig. 4. (a) UV–vis absorption spectra of FA, N-CNFs, AgNPS /N-TDCNCs, N-CNFs 48 h, and AgNPS /N-TDCNCs 48 h. (b) TEM image of AgNPS /N-TDCNCs 48 h.
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to the ethylenic linkages [34]. The peak dramatically decreases after formation of the AgNPs/N-TDCNCs. This might suggest the involvement of the ethylenic linkages in the reduction of silver ions. The adsorbed silver ions were reduced to AgNPs by newly formed ethylenic linkages on the surface of the N-CNFs, which results in the decrease of the peak at 230 nm. Increase of hydrothermal treatment time of the FA to 48 h, the resulted solution shows the enhanced optical density at 230 nm (N-CNFs 48 h, dash dotted line). With the addition of AgNO3 to this solution, lots of bigger AgNPs were formed (Fig. 4b) accompanied by the decrease of the peak at 230 nm in UV–vis spectrum (AgNPS /N-TDCNCs 48 h, dash dot dotted line), which further suggests the action of the ethylenic linkages. Stronger reduction led by more ethylenic linkages resulted in the formation of bigger AgNPs. The exact information about the functional groups involved in the processes was further acquired by FTIR spectroscopy. The FTIR spectrum of the FA shows typical peaks at 1696, 1606, 1486 and 1410 cm−1 (Fig. 5), which are assigned to asymmetric stretching vibration of C O [35], as(COOH) , bending mode of NH-vibration [35], absorption band of the PT in FA [35] and the ring C C symmetric stretching, s(C C) [36]. After the hydrothermal treatment, the peaks at 1486 and 1606 cm−1 disappear with appearance of the peaks at 1578, 1298, and 1240 cm−1 assigned to asymmetric vibration of the carboxylate groups, as(COO–) [37], symmetric stretching vibration of C O [38], and C O stretching of carboxylic acid or C N stretching vibration of amides [39]. The peaks of as(COOH) and s(C C) become broad, suggesting their couple with s(C C) of alkenes[40] and s(COO–) [37], respectively. These indicate that the PT ring of the FA was decomposed upon the hydrothermal treatment accompanied by the formation of compounds containing C C or oxygenated components. After formation of AgNPs/N-TDCNCs, the peaks corresponding to C C vibration and oxygenated components disappear with the appearance of two peaks at 1515 and 1365 cm−1 attributed to the vibration of phenyl [35] and C–N–C of imide [41]. These results indicate that the oxygenated and C C
Fig. 5. FTIR spectra of the FA, N-CNFs and AgNPs/N-TDCNCs.
groups on the surface of N-CNFs may be involved in the reduction of silver ions [15]. Therefore, a possible mechanism can be deduced as shown in Scheme 1. The FA precursors were transformed to NCNFs through the hydrothermal treatment with the formation of the ethylenic and oxygenated groups. When AgNO3 was mixed with the N-CNFs, the silver ions were reduced by the ethylenic and oxygenated groups, which resulted in the formation of ultrafine AgNPs anchored on the carbon supporter to form AgNPs/N-TDCNCs. 3.2. Electrocatalysis of the AgNPs/N-TDCNCs toward oxygen reduction To illustrate the usefulness of these attractive nanohybrids, the activity of the catalyst for ORR was tested. Fig. 6a shows cyclic voltammograms (CVs) of O2 reduction at the AgNPs/NTDCNCs/GCE in 0.1 M KOH solution at a scan rate of 50 mV s−1 .
Scheme 1. Scheme of the formation mechanism of AgNPs/N-TDCNCs.
Fig. 6. CVs of (a) AgNPs/N-TDCNCs/GCE and (b) commercial Pt/C/GCE in N2 -saturated 0.1 M KOH, O2 -saturated 0.1 M KOH, and O2 -saturated 0.1 M KOH containing 1 M CH3 OH solutions at a scan rate of 50 mV s−1 .
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In the N2 -saturated solution (solid line), no obvious current peak is obtained in the potential window range from 0.1 to −0.8 V. In comparison, when the electrolyte solution was saturated with O2 (dotted line), distinct reduction peak appears at the potential of −0.32 V, indicating that the synthesized nanohybrids have high electrocatalytic activity toward ORR. The onset potential was at −0.06 V. Addition of 1 M methanol to the solution nearly did not affect the reduction of the oxygen at the AgNPs/N-TDCNCs/GCE (dotted line). To evaluate the catalytic activity of the AgNPs/NTDCNCs, we studied the ORR catalyzed by commercial Pt/C catalyst (20 wt% Pt on carbon black). Fig. 6b demonstrates CVs of O2 reduction at the commercial Pt/C catalyst-modified GCE (Pt/C/GCE) in the same experiment condition. There is a reductive peak at around −0.18 V with the onset potential at around 0 V in the O2 -saturated (dotted line) 0.1 M KOH solution in comparison to N2 -saturated (solid line) KOH solution. However, this peak completely disappeared in the presence of 1 M methanol with the appearance of methanol oxidation peak (dashed line). Although the O2 reduction potential at the AgNPs/N-TDCNCs/GCE exhibits a 0.14 V negative shift than that of at the Pt/C/GCE, the onset potential is nearly the same. Most importantly, the O2 reduction reaction at the AgNPs/N-TDCNCs/GCE was not affected by methanol, suggesting the AgNPs/N-TDCNCs possesses remarkable tolerance to methanol. To further investigate the reaction kinetic of oxygen reduction, the linear sweep voltammetric (LSV) experiment on RDE was employed. Fig. 7a shows the rotating disk voltammograms (RDVs) of the ORR recorded at the AgNPs/N-TDCNCs/GCE in an oxygensaturated 0.1 M KOH solution at different rotation rates ranging from 225 to 3600 rpm. The transferred electron number per oxygen molecule involved in the ORR process was determined using the Koutecky–Levich (K–L) equation and K–L plots at different potentials by importing the corresponding parameters into the equation as given below [3,17]: 1 1 1 1 1 = + = + J JK JL JK Bω1/2
(1)
B = 0.2nF(DO2 )2/3 −1/6 CO2
(2)
Here n is the number of electron transfer in the reduction of one O2 molecule, F is the Faraday constant (F = 96,485 C mol−1 ), DO2 is the diffusion coefficient of O2 in 0.1 M KOH (DO2 = 1.9 × 10−5 cm2 s−1 ), is the kinematics viscosity for KOH (v = 0.01 cm2 s−1 ) and CO2 is concentration of O2 in the solution (CO2 = 1.2 × 10−6 mol cm−3 ). The constant 0.2 is adopted when the rotation speed is expressed in rpm. As seen from Fig. 7b, the corresponding K–L plots, which depict the inverse current density (J−1 ) as a function of the inverse of the square root of the rotation rate (ω−1/2 ), exhibit good linearity at different electrode potentials. The number of electron transfer can be obtained from the slope of the K–L plots by calculating the K–L equation. The slops obtained from the plots at the potentials range from −0.3 to −0.7 V are approximately constant, which indicates the electron transfer numbers for ORR at different potentials are similar. The n value calculated from the slopes of K–L plots within the potential range from −0.3 to −0.7 V was 3.7–4.4 (Fig. 7c). Such results indicate that the ORR goes through the most efficient four-electron process. Furthermore, this conclusion was testified by the rotating ring disk electrode (RRDE) in a 0.1 M KOH solution saturated with O2 at a rotation rate of 900 rpm with the Pt ring electrode polarized at 0.5 V. Compared to the disk current, a negligible ring current is recorded, which implies the negligible existence of hydrogen peroxide (Fig. S6). Since the stability is one of the most important concerns in current alkaline fuel cell technology, the stability of the prepared nanohybrids was investigated by the chronoamperometric method at a constant potential of −0.6 V for 10,000 s in a 0.1 M
Fig. 7. (a) RDE curves of AgNPs/N-TDCNCs/GCE in O2 -saturated 0.1 M KOH at a scan rate of 20 mV s−1 with different rotation rates. (b) The K–L plots at different potentials. (c) The numbers of electrons transferred for ORR at different potentials.
KOH solution saturated with O2 at a rotation rate of 900 rpm. The current at AgNPs/N-TDCNCs/GCE (black line) shows a much slower attenuation than that at the Pt/C/GCE (gray line) in Fig. 8. Continuous ORR current loses 40% on the Pt/C/GCE, while only loses 20% on the AgNPs/N-TDCNCs/GCE after 10,000 s. Impressively, after about 1000 s, the current of the nanohybrids exhibits subtle decay while the Pt/C/GCE has obvious decrease with the lapse of time. This phenomenon indicates that the AgNPs/N-TDCNCs possess better stability during the ORR. Overall, the ORR performance of the easy-prepared AgNPs/N-TDCNCs is comparable to that of
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Fig. 8. The chronoamperometric curves of AgNPS/N-TDCNCs/GCE and Pt/C/GCE at −0.6 V in an O2 -saturated 0.1 M KOH solution at a rotation rate of 900 rpm.
AgNPs-modified carbon nanotubes based materials with complex steps [12,26]. 4. Conclusion In summary, we have developed a facile method to synthesize a novel N-doped carbon nanoflake. Furthermore, we have successfully synthesized ultrafine Ag nanoparticles anchored on the novel N-doped carbon materials by a simple and green method at room temperature without using additional reduction reagent. The nanohybrid exhibited high electrocatalytic activity toward oxygen reduction reaction via a four-electron pathway. This study provides an economical and simple strategy toward green preparing N-doped carbon materials and hybrids of noble metal nanomaterials on N-doped carbon materials for catalysis, photothermic therapy, sensing and other applications. Acknowledgments This work was supported by the Youth Foundation of China (21105097), President Funds of the Chinese Academy of Sciences and the Natural Science Foundation of Jilin Province (201215092) Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2013.06.100. References [1] B.C.H. Steele, A. Heinzel, Materials for fuel-cell technologies, Nature 414 (2001) 345–352. [2] M.K. Debe, Electrocatalyst approaches and challenges for automotive fuel cells, Nature 486 (2012) 43–51. [3] K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction, Science 323 (2009) 760–764. [4] V.R. Stamenkovic, B. Fowler, B.S. Mun, G. Wang, P.N. Ross, C.A. Lucas, N.M. ´ Improved oxygen reduction activity on Pt3Ni(111) via increased Markovic, surface site availability, Science 315 (2007) 493–497. [5] L. Xiao, L. Zhuang, Y. Liu, J. Lu, H.D. Abrun˜a, Activating Pd by morphology tailoring for oxygen reduction, J. Am. Chem. Soc. 131 (2008) 602–608. [6] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chemistry and properties of nanocrystals of different shapes, Chem. Rev. 105 (2005) 1025–1102. [7] M. Pang, J. Hu, H.C. Zeng, Synthesis, morphological control, and antibacterial properties of hollow/solid Ag2 S/Ag heterodimers, J. Am. Chem. Soc. 132 (2010) 10771–10785. [8] J.A. Farmer, C.T. Campbell, Ceria maintains smaller metal catalyst particles by strong metal–support bonding, Science 329 (2010) 933–936.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Facile-green synthesis of nitrogen-doped carbon-supported ultrafine silver catalyst with enhanced electrocatalytic property Yichen Wang a,b , Haibin Wu a , Xiue Jiang a,∗ a b
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 21 February 2013 Received in revised form 13 June 2013 Accepted 18 June 2013 Available online 2 July 2013 Keywords: Folic acid Heterogeneous catalysis N-doped carbon nanostructure Oxygen reduction reaction Ultrafine Ag nanoparticles
a b s t r a c t We have demonstrated a facile and green strategy to synthesize ultrafine silver nanoparticles monodispersed on N-doped three-dimensional carbon nanocloud surfaces without any toxic reagent. Folic acid was employed as the carbon precursor for forming N-doped carbon nanoflakes by a hydrothermal method. The as-prepared products can serve as both reducing agent and substrate, on which a high density of ultrafine Ag nanocrystals is stably grown in a homogeneously dispersive state spontaneously at room temperature. A feasible synthesis mechanism has been proposed by characterization of carbon precursor, nanomaterials composited without and with silver nanoparticles. It was found that the ethylenic and oxygenated groups led to the reduction process. The nanohybrids showed an enhanced electrocatalytic activity toward oxygen reduction reaction (ORR) in alkaline solution via a four-electron pathway. The catalyst also exhibited strong duration of methanol and good stability compared to commercial Pt/C catalysts. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Over the past several decades, considerable attention has been paid to prepare high performance catalyst since it directly restrict the development of the chemical industry, such as liquid fuel cells [1,2]. In all of these areas, the performance of the cell is limited by the oxygen reduction reaction (ORR) rate at the cathode [3–5]. As the particle size decrease, the surface area and the number of active metal atoms are increased, which greatly improve catalytic activities of nanoparticles. Therefore, ultrafine nanoparticles attracted particular interest in preparation of catalyst due to highly enhanced catalytic activities [6–8]. However, the enhanced surface energy often leads to aggregation or sintering of the catalysts in the applications [9]. One of the effective strategies for avoiding this problem is to use suitable support to keep the activity and size of ultrafine nanoparticles [10]. The synergetic effect of the nanomaterials with support might further improve the catalytic performance [11–13]. Carbon nanomaterials, including carbon nanotube [14], graphene [15] and mesoporous carbon [13,16] are the most frequently used supports to enhance the catalytic performance of metal catalyst due to their high specific surface areas, good mechanical and thermal stability. Recently, nitrogen-doped carbons have attracted much attention in the wide world either as metal-free catalyst [3,17,18]
∗ Corresponding author. Tel.: +86 0431 85262426; fax: +86 0431 85685653. E-mail address: [email protected] (X. Jiang). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.06.100
or as fascinating supports to construct high-performance catalyst [13,19,20] because the incorporation of nitrogen atom in the carbon architecture can enhance the functional properties of the carbon [21]. Although their enhanced catalytic properties have extensively been proved, most of the fabrication methods, such as chemical vapor deposition nitrogen containing compounds [17], treatment of carbons with ammonia at high temperature [20] and high temperature carbonization of nitrogen-containing precursors [13,19], are complicated and even toxic to people and environment. To this point, new strategy for rapid production of nitrogen-containing support is highly desired. Currently, carbon-supported silver nanoparticle catalysts have attracted particular interest because of their lower cost and high performance [22–24], especially promising activities toward ORR under alkaline conditions [12,25,26], and silver also has the highest electrical and thermal conductivity among all metals. Before use as host materials to anchor silver nanoparticles, the most used carbon materials have to be purified or acid-treated by refluxing in concentrated sulfuric and nitric acids to introduce charged groups and strengthen the interaction of silver nanoparticles and the support [22,23]. Due to the application of various chemical reagents or some complicated equipments, the protocols of material synthesis are not simple and economical. In addition, the adding of reducing agents or capping molecules may cause the environment pollution and increase the cost. Therefore, it should be of particular importance to develop energy-efficient, low-cost and environment friendly methods for the preparation of high-quality silver catalyst.
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Fig. 1. (a, b) TEM images and (c) HRTEM image of the N-CNFs. (d) XPS survey spectrum of the N-CNFs. High resolution XPS spectra of (e) N1s and (f) C1s of the N-CNFs.
Here we report a synthetic strategy that enables facile, economical and green synthesis of well dispersed silver nanoparticles with ultrafine sizes anchored on new nitrogen-doped carbon nanoflakes (N-CNFs), which are prepared by a hydrothermal treatment using folic acid (FA) as source of nitrogen-containing carbon precursor for the first time. The growth of ultrafine Ag nanocrystals on the surface of novel N-CNFs forms AgNPs-decorated N-doped threedimensional carbon nanoclouds (AgNPs/N-TDCNCs), which occurs via a spontaneous process under room temperature without adding any more agents. Most importantly, the as-prepared nanocomposites exhibited enhanced electro-catalytic activity and stability toward ORR in alkaline solution via a four-electron pathway. 2. Experimental 2.1. Reagents and materials FA was obtained from Aladin Ltd. (Shanghai, China). AgNO3 was purchased from Beijing Chemical Corp. (Beijing, China). All
chemicals were used without further purification. The water used throughout all experiments was purified through a Millipore system.
2.2. Synthesis of N-CNFs and AgNPs/N-TDCNCs N-CNFs were prepared by a hydrothermal process. In a typical preparation experiment, 100 mg of FA was added to 30 mL of H2 O with stirring, and then the cloudy mixture was transferred into a 50 mL Teflon-lined autoclave and heated at 200 ◦ C for a period of 10 h. The as-prepared N-CNFs were separated by removing the insoluble products though centrifugation at 12,000 rpm for 10 min and the supernatant was collected for further characterization and use. 1 mL of 0.1 M AgNO3 was added drop by drop to 5 mL of N-CNFs aqueous solution kept in a vial under vigorous stirring and the reaction was sustained for 30 min at room temperature. After that, the suspension was centrifuged and washed with water for 3 times
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Fig. 2. (a, b) TEM images and (c) HRTEM image of the AgNPs/N-TDCNCs. (d) XPS survey spectrum of the AgNPs/N-TDCNCs. High resolution XPS spectra of (e) N1s and (f) C1s of the AgNPs/N-TDCNCs.
to remove the remaining reagent. The resultant AgNPs/N-TDCNCs were dispersed in water for characterization and use. 2.3. Measurements The transmission electron microscopy (TEM) measurements of the as-prepared sample were made on a HITACHI H-8100EM (Hitachi, Tokyo, Japan). High-resolution TEM (HRTEM) images and the corresponding EDX were all carried out on a JEM-2010 (HR) microscope. The X-ray diffraction (XRD) measurements were performed on a PW1700 powder diffractometer using Cu-K␣ radiation with a Ni filter ( = 0.154059 nm at 30 kV and 15 mA). The X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 X-ray photoelectron spectrometer with a monochromated X-ray source (AlK␣ h=1486.6 eV). UV–vis spectra were recorded on a VARIAN CARY 50 UV/vis spectrophotometer.
The Fourier-transformed infrared spectroscopy (FTIR) study was conducted with a VERTEX 70 FTIR. Cyclic voltammetry and rotating disk electrode (RDE) experiments were carried out by using a computer-controlled CHI832C electrochemical analyzer (CH Instruments, Inc., Shanghai) in a home-made three-electrode electrochemical cell consisting of a glassy carbon electrode (GCE) as a working electrode, a platinum coil as a counter electrode and an Ag/AgCl (in saturated KCl) as a reference electrode. The modified AgNPs/N-TDCNCs GCE (AgNPs/N-TDCNCs/GCE) was prepared by a simple casting method. Prior to the surface coating, the GCE was polished with 1.0 and 0.3 m alumina powder, respectively, and rinsed with double distilled water, followed by sonication in ethanol solution and double distilled water successively. Then, the electrode was dried in a stream of nitrogen. After that, 10 L of AgNPs/N-TDCNCs (∼2 mg/ml) was dropped onto the surface of pretreated GCE and left to dry at room temperature. Prior
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to experiment, 2 L of Nafion (0.5%) was additionally dropped onto the surface of the resultant-modified electrode and dried at room temperature for 2 h. N2 -saturated and O2 -saturated electrolyte were prepared by blowing the electrolyte solution with ultrahigh purity nitrogen and oxygen for 15 min then sealing the solution with nitrogen and oxygen atmospheres, respectively, during the experimental procedure. All electrochemical experiments were performed at the room temperature.
3. Results and discussion 3.1. Formation of the materials In our experiment, a yellow dispersion (Supporting Information, Fig. S1a) was formed after hydrothermal treatment of FA at 200 ◦ C for 10 h. Fig. 1a shows the low magnification TEM of the NCNFs. It can be seen that the as-prepared products are composed of some unordered nanoflakes, which is further evidenced by the high magnification TEM (Fig. 1b). The HRTEM picture of the as-prepared N-CNFs (Fig. 1c) exhibits crystal lattices with the measured lattice spacing of 0.35 nm (Fig. S2). XPS measurement was utilized to identify the surface composition of the N-CNFs. As shown in Fig. 1d, three peaks are observed in the XPS spectrum of the N-CNFs at 285.0, 400.0 and 531.0 eV, which have been attributed to C1s, N1s and O1s, respectively [27–29]. The elemental analysis indicates that the N/C atom ratio is 0.202 The N1s spectrum (Fig. 1e) can be decomposed to three peaks at 399.0, 399.9, and 401.5 eV, which are attributed to pyridinic, pyrrolic and quaternary N, respectively [30–32]. The C1s spectrum (Fig. 1f) decomposed at 285.7, 286.5 and 288.2 eV are assigned to C O, Epoxy and C O [27], respectively. The fraction of the oxygencontaining groups in the C1s spectrum is 0.43. These results indicate that the N-CNFs are N-doped carbon materials with oxygen-rich functional groups. When AgNO3 was mixed with the N-CNFs aqueous solution at room temperature for 30 min with vigorous stirring, the yellow N-CNFs dispersion turned to dark green suspension gradually (Fig. S1b), implying the interaction of AgNO3 and the N-CNFs. The resulting products were collected by centrifugation and washed several time with water. Fig. 2a shows the low magnification TEM of the AgNPs/N-TDCNCs. It seems like a three-dimensional nanocloud inserted with pores, which is obviously different with that of the NCNFs. High magnification TEM (Fig. 2b) shows the decoration of the nanomaterials with lots of ultrafine particles. The corresponding EDX spectroscopy (Fig. S3) of the nanomaterials shows the peaks corresponding to Ag, C, and N elements, indicating the ultrafine particles are Ag nanoparticles (AgNPs). The size of AgNPs is uniform, with an average diameter of 3.96 nm and a narrow standard deviation of 0.58 nm (Fig. S4). The HRTEM image (Fig. 2c) of the products shows a crystalline structure of AgNPs with a lattice spacing of 0.23 nm (Fig. S5), which corresponds to the (1 1 1) lattice plane of Ag. The XRD pattern (Fig. 3) of the products indicates a face-centered cubic (fcc) structure of Ag with a strong peak standing for the (1 1 1) lattice plane. Similarly, the XPS spectrum reveals that the main compositions of the products are C1s, N1s, O1s and Ag. However, the percentage of pyridinic N in total N atoms increases as reflected by the decomposition of the N1s spectrum (Fig. 2e). Meanwhile, the C1s spectrum shows the decrease of the oxygencontaining functional groups to 0.35 (Fig. 2f). All these results indicate that the novel N-CNFs can be used as support to load and reduce AgNO3 to ultrafine AgNPs accompanied by structural changes of the N-CNFs to form AgNPs/N-TDCNCs via a spontaneous process under room temperature. The measurement of inductively coupled plasma optical emission spectroscopy (ICP-AES) reveals
Fig. 3. XRD patterns of the N-CNFs and AgNPs/N-TDCNCs along with the bulk XRD pattern of Ag (black bars, from JCPDS no. 04-0783).
that the loading content of Ag in the prepared nanohybrids is 33.39%. In order to reveal the reactive mechanism of the AgNPs/NTDCNCs, we monitored the reaction processes with molecular spectroscopy. Fig. 4a shows the UV–vis absorption spectra of the FA (solid line), N-CNFs (dashed line) and AgNPs/N-TDCNCs (dotted line). The spectrum of the FA solution shows an absorption band at 280 nm attributed to the –* transition of pterin (PT) ring [33]. After the hydrothermal treatment for 10 h, the PT peak decreases as well as the appearance of the peak at 230 nm assigned
Fig. 4. (a) UV–vis absorption spectra of FA, N-CNFs, AgNPS /N-TDCNCs, N-CNFs 48 h, and AgNPS /N-TDCNCs 48 h. (b) TEM image of AgNPS /N-TDCNCs 48 h.
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to the ethylenic linkages [34]. The peak dramatically decreases after formation of the AgNPs/N-TDCNCs. This might suggest the involvement of the ethylenic linkages in the reduction of silver ions. The adsorbed silver ions were reduced to AgNPs by newly formed ethylenic linkages on the surface of the N-CNFs, which results in the decrease of the peak at 230 nm. Increase of hydrothermal treatment time of the FA to 48 h, the resulted solution shows the enhanced optical density at 230 nm (N-CNFs 48 h, dash dotted line). With the addition of AgNO3 to this solution, lots of bigger AgNPs were formed (Fig. 4b) accompanied by the decrease of the peak at 230 nm in UV–vis spectrum (AgNPS /N-TDCNCs 48 h, dash dot dotted line), which further suggests the action of the ethylenic linkages. Stronger reduction led by more ethylenic linkages resulted in the formation of bigger AgNPs. The exact information about the functional groups involved in the processes was further acquired by FTIR spectroscopy. The FTIR spectrum of the FA shows typical peaks at 1696, 1606, 1486 and 1410 cm−1 (Fig. 5), which are assigned to asymmetric stretching vibration of C O [35], as(COOH) , bending mode of NH-vibration [35], absorption band of the PT in FA [35] and the ring C C symmetric stretching, s(C C) [36]. After the hydrothermal treatment, the peaks at 1486 and 1606 cm−1 disappear with appearance of the peaks at 1578, 1298, and 1240 cm−1 assigned to asymmetric vibration of the carboxylate groups, as(COO–) [37], symmetric stretching vibration of C O [38], and C O stretching of carboxylic acid or C N stretching vibration of amides [39]. The peaks of as(COOH) and s(C C) become broad, suggesting their couple with s(C C) of alkenes[40] and s(COO–) [37], respectively. These indicate that the PT ring of the FA was decomposed upon the hydrothermal treatment accompanied by the formation of compounds containing C C or oxygenated components. After formation of AgNPs/N-TDCNCs, the peaks corresponding to C C vibration and oxygenated components disappear with the appearance of two peaks at 1515 and 1365 cm−1 attributed to the vibration of phenyl [35] and C–N–C of imide [41]. These results indicate that the oxygenated and C C
Fig. 5. FTIR spectra of the FA, N-CNFs and AgNPs/N-TDCNCs.
groups on the surface of N-CNFs may be involved in the reduction of silver ions [15]. Therefore, a possible mechanism can be deduced as shown in Scheme 1. The FA precursors were transformed to NCNFs through the hydrothermal treatment with the formation of the ethylenic and oxygenated groups. When AgNO3 was mixed with the N-CNFs, the silver ions were reduced by the ethylenic and oxygenated groups, which resulted in the formation of ultrafine AgNPs anchored on the carbon supporter to form AgNPs/N-TDCNCs. 3.2. Electrocatalysis of the AgNPs/N-TDCNCs toward oxygen reduction To illustrate the usefulness of these attractive nanohybrids, the activity of the catalyst for ORR was tested. Fig. 6a shows cyclic voltammograms (CVs) of O2 reduction at the AgNPs/NTDCNCs/GCE in 0.1 M KOH solution at a scan rate of 50 mV s−1 .
Scheme 1. Scheme of the formation mechanism of AgNPs/N-TDCNCs.
Fig. 6. CVs of (a) AgNPs/N-TDCNCs/GCE and (b) commercial Pt/C/GCE in N2 -saturated 0.1 M KOH, O2 -saturated 0.1 M KOH, and O2 -saturated 0.1 M KOH containing 1 M CH3 OH solutions at a scan rate of 50 mV s−1 .
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In the N2 -saturated solution (solid line), no obvious current peak is obtained in the potential window range from 0.1 to −0.8 V. In comparison, when the electrolyte solution was saturated with O2 (dotted line), distinct reduction peak appears at the potential of −0.32 V, indicating that the synthesized nanohybrids have high electrocatalytic activity toward ORR. The onset potential was at −0.06 V. Addition of 1 M methanol to the solution nearly did not affect the reduction of the oxygen at the AgNPs/N-TDCNCs/GCE (dotted line). To evaluate the catalytic activity of the AgNPs/NTDCNCs, we studied the ORR catalyzed by commercial Pt/C catalyst (20 wt% Pt on carbon black). Fig. 6b demonstrates CVs of O2 reduction at the commercial Pt/C catalyst-modified GCE (Pt/C/GCE) in the same experiment condition. There is a reductive peak at around −0.18 V with the onset potential at around 0 V in the O2 -saturated (dotted line) 0.1 M KOH solution in comparison to N2 -saturated (solid line) KOH solution. However, this peak completely disappeared in the presence of 1 M methanol with the appearance of methanol oxidation peak (dashed line). Although the O2 reduction potential at the AgNPs/N-TDCNCs/GCE exhibits a 0.14 V negative shift than that of at the Pt/C/GCE, the onset potential is nearly the same. Most importantly, the O2 reduction reaction at the AgNPs/N-TDCNCs/GCE was not affected by methanol, suggesting the AgNPs/N-TDCNCs possesses remarkable tolerance to methanol. To further investigate the reaction kinetic of oxygen reduction, the linear sweep voltammetric (LSV) experiment on RDE was employed. Fig. 7a shows the rotating disk voltammograms (RDVs) of the ORR recorded at the AgNPs/N-TDCNCs/GCE in an oxygensaturated 0.1 M KOH solution at different rotation rates ranging from 225 to 3600 rpm. The transferred electron number per oxygen molecule involved in the ORR process was determined using the Koutecky–Levich (K–L) equation and K–L plots at different potentials by importing the corresponding parameters into the equation as given below [3,17]: 1 1 1 1 1 = + = + J JK JL JK Bω1/2
(1)
B = 0.2nF(DO2 )2/3 −1/6 CO2
(2)
Here n is the number of electron transfer in the reduction of one O2 molecule, F is the Faraday constant (F = 96,485 C mol−1 ), DO2 is the diffusion coefficient of O2 in 0.1 M KOH (DO2 = 1.9 × 10−5 cm2 s−1 ), is the kinematics viscosity for KOH (v = 0.01 cm2 s−1 ) and CO2 is concentration of O2 in the solution (CO2 = 1.2 × 10−6 mol cm−3 ). The constant 0.2 is adopted when the rotation speed is expressed in rpm. As seen from Fig. 7b, the corresponding K–L plots, which depict the inverse current density (J−1 ) as a function of the inverse of the square root of the rotation rate (ω−1/2 ), exhibit good linearity at different electrode potentials. The number of electron transfer can be obtained from the slope of the K–L plots by calculating the K–L equation. The slops obtained from the plots at the potentials range from −0.3 to −0.7 V are approximately constant, which indicates the electron transfer numbers for ORR at different potentials are similar. The n value calculated from the slopes of K–L plots within the potential range from −0.3 to −0.7 V was 3.7–4.4 (Fig. 7c). Such results indicate that the ORR goes through the most efficient four-electron process. Furthermore, this conclusion was testified by the rotating ring disk electrode (RRDE) in a 0.1 M KOH solution saturated with O2 at a rotation rate of 900 rpm with the Pt ring electrode polarized at 0.5 V. Compared to the disk current, a negligible ring current is recorded, which implies the negligible existence of hydrogen peroxide (Fig. S6). Since the stability is one of the most important concerns in current alkaline fuel cell technology, the stability of the prepared nanohybrids was investigated by the chronoamperometric method at a constant potential of −0.6 V for 10,000 s in a 0.1 M
Fig. 7. (a) RDE curves of AgNPs/N-TDCNCs/GCE in O2 -saturated 0.1 M KOH at a scan rate of 20 mV s−1 with different rotation rates. (b) The K–L plots at different potentials. (c) The numbers of electrons transferred for ORR at different potentials.
KOH solution saturated with O2 at a rotation rate of 900 rpm. The current at AgNPs/N-TDCNCs/GCE (black line) shows a much slower attenuation than that at the Pt/C/GCE (gray line) in Fig. 8. Continuous ORR current loses 40% on the Pt/C/GCE, while only loses 20% on the AgNPs/N-TDCNCs/GCE after 10,000 s. Impressively, after about 1000 s, the current of the nanohybrids exhibits subtle decay while the Pt/C/GCE has obvious decrease with the lapse of time. This phenomenon indicates that the AgNPs/N-TDCNCs possess better stability during the ORR. Overall, the ORR performance of the easy-prepared AgNPs/N-TDCNCs is comparable to that of
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Fig. 8. The chronoamperometric curves of AgNPS/N-TDCNCs/GCE and Pt/C/GCE at −0.6 V in an O2 -saturated 0.1 M KOH solution at a rotation rate of 900 rpm.
AgNPs-modified carbon nanotubes based materials with complex steps [12,26]. 4. Conclusion In summary, we have developed a facile method to synthesize a novel N-doped carbon nanoflake. Furthermore, we have successfully synthesized ultrafine Ag nanoparticles anchored on the novel N-doped carbon materials by a simple and green method at room temperature without using additional reduction reagent. The nanohybrid exhibited high electrocatalytic activity toward oxygen reduction reaction via a four-electron pathway. This study provides an economical and simple strategy toward green preparing N-doped carbon materials and hybrids of noble metal nanomaterials on N-doped carbon materials for catalysis, photothermic therapy, sensing and other applications. Acknowledgments This work was supported by the Youth Foundation of China (21105097), President Funds of the Chinese Academy of Sciences and the Natural Science Foundation of Jilin Province (201215092) Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2013.06.100. References [1] B.C.H. Steele, A. Heinzel, Materials for fuel-cell technologies, Nature 414 (2001) 345–352. [2] M.K. Debe, Electrocatalyst approaches and challenges for automotive fuel cells, Nature 486 (2012) 43–51. [3] K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction, Science 323 (2009) 760–764. [4] V.R. Stamenkovic, B. Fowler, B.S. Mun, G. Wang, P.N. Ross, C.A. Lucas, N.M. ´ Improved oxygen reduction activity on Pt3Ni(111) via increased Markovic, surface site availability, Science 315 (2007) 493–497. [5] L. Xiao, L. Zhuang, Y. Liu, J. Lu, H.D. Abrun˜a, Activating Pd by morphology tailoring for oxygen reduction, J. Am. Chem. Soc. 131 (2008) 602–608. [6] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chemistry and properties of nanocrystals of different shapes, Chem. Rev. 105 (2005) 1025–1102. [7] M. Pang, J. Hu, H.C. Zeng, Synthesis, morphological control, and antibacterial properties of hollow/solid Ag2 S/Ag heterodimers, J. Am. Chem. Soc. 132 (2010) 10771–10785. [8] J.A. Farmer, C.T. Campbell, Ceria maintains smaller metal catalyst particles by strong metal–support bonding, Science 329 (2010) 933–936.
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