- Email: [email protected]
Fe3O4 (M = Ag, Cu) composites using hexamethylentetramine and their electrocatalytic properties
Materials Chemistry and Physics 134 (2012) 177–182
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis of nanostructured M/Fe3 O4 (M = Ag, Cu) composites using hexamethylentetramine and their electrocatalytic properties Lu Pan a,b,∗ , Yonghong Chen a,b , Fengwu Wang a a b
Department of Chemistry and Chemical Engineering, Huainan Normal University, Huainan 232001, China Anhui Key Laboratory of Low temperature Co-fired Material, Huainan Normal University, Huainan 232001, China
a r t i c l e
i n f o
Article history: Received 3 November 2011 Received in revised form 14 February 2012 Accepted 18 February 2012 Keywords: Ag/Fe3 O4 Cu/Fe3 O4 Nanostructres Electrochemical properties
a b s t r a c t Nanoscaled Ag/Fe3 O4 hybrids with different Ag contents and Cu/Fe3 O4 nanoshpere and microsphere were successfully synthesized with assistance of sodium citrate and (CH2 )6 N4 via a hydrothermal process. The as-prepared samples were identified and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), and X-ray photoelectron spectroscopy (XPS), respectively. All samples were used as electrocatalysts modified on a glassy carbon electrode for p-nitrophenol reduction in a basic solution. The catalytic activity of Ag/Fe3 O4 samples increased first and then decreased by increasing Ag content from 0% to 8%, and the one with 6% Ag displayed the highest catalytic activity. All the Cu/Fe3 O4 samples exhibited enhanced catalytic activity by comparison with a glassy carbon electrode, and the one prepared with the molar ratio of Cu2+ , Fe3+ , citrate anion, and (CH2 )6 N4 with 1:1:3:5 exhibited the highest catalytic activity. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Magnetite (Fe3 O4 ), an important member of spinel-type ferrite, has been extensively utilized as a gas sensor, pigment, recording material, electrophotographic developer, catalyst, biomedical treatment and spintronic devices, etc. [1–4]. Recently, interesting study on Fe3 O4 composites or hybrids doped with noble or other transition metals have been attracted widely, because of their doping or synergetic effect, which have significantly broadened the property and potential application of Fe3 O4 materials [5–10]. Although some noble metals such gold, palladium and platinum in Fe3 O4 always exhibit higher catalytic activity in many spheres, in consideration of the higher price, their practical application usually are restricted greatly. Thus, the research on other cheaper metals substituting these noble ones that doped in Fe3 O4 or combining with Fe3 O4 shows not only exceeding interest but also significantly potential application value [11,12]. In our previous work, almost mono-dispersed Ag/Fe3 O4 microspheres with identical size of 175 nm were synthesized using one-step procedure via a hydrothermal method [13]. Generally, two synthetic strategies usually are used to prepare Ag/Fe3 O4 composites, namely Fe3 O4 is prepared first, and then Ag particles
reduced by suitable agents grow on Fe3 O4 particles or other shapes [14,15]. Because Ag nanoparticles are easy to aggregate during its reduction process, so some surfactants often are utilized to prevent them from aggregating. Inevitably, such synthesis processes are often full of much complication, and furthermore, silver ion concentration usually is rather low, which lead to it is difficultly for such synthesis technique to be applied widely in a large scale production. In this work, a facile and easily controlled procedure was designed to synthesize Ag/Fe3 O4 and Cu/Fe3 O4 samples using only one-step strategy. Sodium citrate was used to serve as ligand to Ag+ , Fe3+ and Cu2+ ions and (CH2 )6 N4 as reaction agent, Ag/Fe3 O4 with a smaller size and Cu/Fe3 O4 nano- and microspheres were successfully prepared, which could overcome above disadvantages. 2. Experimental 2.1. Chemicals AgNO3 , CuSO4 ·5H2 O, NH4 Fe2 (SO4 )2 ·12H2 O, sodium citrate dihydrate (C6 H5 Na3 O7 ·2H2 O), and (CH2 )6 N4 , all were purchased from Shanghai Chemical Reagent Co., China. All reagents were of analytical grade and used as received without further purification. 2.2. Samples synthesis
∗ Corresponding author at: Department of Chemistry and Chemical Engineering, Huainan Normal University, Huainan 232001, China. Tel.: +86 554 6672651; fax: +86 554 6672650. E-mail address: [email protected] (L. Pan). 0254-0584/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.02.048
Synthesis of Ag/Fe3 O4 samples: AgNO3 and NH4 Fe2 (SO4 )2 ·12H2 O with total 2.0 mmol were dissolved in 45 mL of distilled water, then 3.0 mmol of sodium citrate dihydrate
178
L. Pan et al. / Materials Chemistry and Physics 134 (2012) 177–182
and 6.0 mmol of (CH2 )6 N4 were added in turn, and a tea-green homogeneous solution was obtained. The resultant mixture was transferred to 60 mL of Teflon-lined stainless steel autoclave, which was sealed and maintained at 160 ◦ C for 12 h. The autoclave was cooled to room temperature naturally, and the black precipitate was filtered and washed with distilled water then ethanol for several times, and dried in vacuum at 50 ◦ C for 6 h. By changing molar ratio of Ag+ and Fe3+ , several Ag/Fe3 O4 samples with different Ag contents were prepared. Synthesis of Cu/Fe3 O4 samples: CuSO4 ·5H2 O and NH4 Fe2 (SO4 )2 ·12H2 O with equal 1.25 mmol were dissolved in 45 mL of distilled water, then 2.5 mmol of mmol of sodium citrate dehydrate and 6.25 mmol of (CH2 )6 N4 were added in turn under rigorous stirring, and a transparent and homogeneous solution was obtained. The solution was transferred to 60 mL of Teflon-lined stainless steel autoclave, which was sealed and maintained at 160 ◦ C for 24 h. The autoclave was cooled to room temperature naturally, and the precipitate was filtered and washed with distilled water then ethanol for several times, and dried in vacuum at 60 ◦ C for 6 h. A few Cu/Fe3 O4 samples were synthesized by varying molar ratios of the total metal salts, sodium citrate and (CH2 )6 N4 . 2.3. Characterization The phases analysis of the as-synthesized samples was performed on a Philips X’Pert PROSUPER X-ray diffraction (XRD) with Cu K␣ radiation ( = 0.154178 nm), using an operation voltage and current of 40 kV and 50 mA. The SEM images were carried on a JEOL-6300F field-emission scanning electron microscopy (FE-SEM) with accelerating voltage of 15 kV. The TEM images were collected on a Hitachi Model H-800 transmission electron microscopy, using an accelerating voltage of 200 kV. XPS was performed using an ESCALAB 250 VG Lited XPS operated at 15 kV (h = 1486.6 eV). 2.4. Electrochemical determination The electrocatalytic measurements of the as-synthesized samples modified on a glassy carbon electrode (GCE) for reduction of p-nitrophenol in a basic solution were performed on LK 98 microcomputer-based electrochemical system (Tianjin Lanlike Chemical and Electron High Technology Co. Ltd., Tianjin in China). A three-electrode single compartment cell was used for cyclic voltammetry determination. A GCE (3.7 mm diameter) and a platinum plate were used as working and counter electrode, respectively, and a Ag/AgC1 electrode was used as reference electrode. Before each measurement, the GCE surface was carefully polished on an abrasive paper first, then further polished with 0.3 and 0.05 m ␣-Al2 O3 paste in turn, finally rinsed thoroughly with doubly distilled water and absolute alcohol, ensuring to keep the GCE in a good reversible condition. A 20 mg of sample was dispersed in 4 mL of doubly distilled water under ultrasonication to obtain a suspension solution. Of the suspension mixture, 50 L was taken out and covered on the carbon surface of a GCE in a good reversible state. After dried spontaneously in air, the modified GCE was prepared and used directly for electrochemical determination. 3. Results and discussion Fig. 1 shows the XRD patterns of Fe3 O4 (curve a) and Ag/Fe3 O4 with 6% Ag content (curve b). Without addition of Ag+ ion, cubic crystalline Fe3 O4 was synthesized. According to curve a, the characteristic diffraction peaks of the Fe3 O4 sample are in good agreement with the standard spectrum of cubic Fe3 O4 (JCPDS card. No. 190629). When a certain quantity of Ag+ was added, Ag/Fe3 O4 hybrid was synthesized. From curve b, the characteristic diffraction peaks
Fig. 1. XRD patterns of Fe3 O4 (a) and Ag/Fe3 O4 with 6% Ag (b).
of cubic metallic Ag and Fe3 O4 were detected clearly. Additionally, it is found that the characteristic diffraction peaks broadened, which indicated the sample could be composed of small-sized nanoparticles. It is well known that two synthetic strategies have been used frequently to prepare mental nanoparticles with heterostructures. For synthesis of Ag/Fe3 O4 hybrid, the key factor is to control the nucleation speed and dispersion of Ag particles. Wiley et al. synthesized Ag/Fe3 O4 samples via a solvothermal method in which Ag0 and Fe2+ were produced from reduction of Ag+ and Fe3+ with solvent, which could not only prevent the sample from aggregating but also Ag particles from being oxidized by Fe3+ ion [16]. In this work, the added citrate anion acted as coordinating agent to both Fe3+ and Ag+ ions. As a result the concentration of free Fe3+ or Ag+ in the solution was very low, and the nucleus concentration was so low that Ag and Fe3 O4 particles, both of which were reduced by HCHO released from the hydrolysis of (CH2 )6 N4 [17], could not form rapidly and aggregation of the samples could be avoided. According to curve b in Fig. 1, the diffraction peaks of Ag and the ones of Fe3 O4 all clearly broadened, furthermore, the diffraction peaks of Ag were not much stronger than those of Fe3 O4 , suggesting that both Ag and Fe3 O4 possessed a smaller size. Zhang et al. had synthesized Ag/Fe3 O4 composite, and the characteristic diffraction peaks of Ag were much stronger than those of Fe3 O4 , which revealed that the Ag nanoparticles had obvious agglomeration [12]. XPS technology was used to further identify the Ag/Fe3 O4 sample with 6% Ag content. The results show in Fig. 2. The survey scan shows clear peaks corresponding to the binding energies of Ag3d (368.3 eV), Fe2p and O1s (Fig. 2a), respectively. Further fine scanning around the Fe2p peaks revealed two characteristic binding energy peaks at 710.9 and 724.6 eV for Fe2p3/2 and Fe2p1/2 , respectively, which were in good agreement with the reported values of Fe3 O4 in the literature, and no characteristic peaks of Fe3+ in ␥Fe2 O3 locating at 719.0 eV was observed [18]. Other peaks in the spectrum (Fig. 2a) were ascribed to the carbon contamination on the substrate. Based on the XRD and XPS analysis of the sample, it was confirmed that the final sample was Ag/Fe3 O4 hybrid. Fig. 3 shows the XRD patterns of the samples prepared without (curve a) and with addition of Cu2+ ion (curve b) to Fe3+ solution. According to curve a, the characteristic diffractions of the sample was in good consistence with cubic Fe3 O4 . As equal amount of Cu2+ and Fe3+ were mixed, the resultant sample demonstrated a composite Cu/Fe3 O4 structure. The characteristic diffractions of Cu and Fe3 O4 were detected, respectively. The XRD pattern of Fe3 O4
L. Pan et al. / Materials Chemistry and Physics 134 (2012) 177–182
179
Fig. 2. XPS spectrum of Ag/Fe3 O4 with 4% Ag.
sample was extremely similar to that of the one shown in curve a in Fig. 1. The three diffraction peaks (2) locating at 3.25◦ , 40.42◦ and 71.15◦ , respectively, were in good agreement with face-center cubic Cu (JCPDF Card No.: 85-1326). From Fig. 3, the diffraction peaks of Cu were much stronger than that of Fe3 O4 , which verified the crystallinity of Cu particles was higher than the one of Fe3 O4 . For synthesis of Cu/Fe3 O4 , sodium citrate was added and citrate anion served as ligand to Cu2+ and Fe3+ . Because of the higher stability constants of FeL and CuL (1 g Kf (FeL) = 25.0, 1 g Kf (CuL) = 18.0, L representing citrate anion), the free Fe3+ and Cu2+ ions were extraordinarily low, which leaded to the nucleus concentration of Cu and Fe3 O4 particles reduced from Cu2+ and Fe3+ by HCHO released from hydrolysis of (CH2 )6 N4 were so low that Cu and Fe3 O4 particles could not easily aggregate. Likewise, the Cu/Fe3 O4 sample was further verified using XPS technology. The results show in Fig. 4. From Fig. 4a, the survey spectrum revealed the existence of the Cu, Fe and O elements. From Fig. 4b, two strong peaks locating at 710.9 and 724.6 eV corresponded to Fe2p3/2 and Fe2p1/2 , respectively, which were in good agreement with the reported values of Fe3 O4 . From Fig. 4c, two strong peaks at 932.5 and 952.5 eV were finely due to the binding
Fig. 3. XRD patterns of Fe3 O4 (a) and Cu/Fe3 O4 (b).
energy of Cu2p3/2 and Cu2p5/2 of Cu atomic rather than Cu2+ in CuO [19]. The XPS analysis further verified the Cu/Fe3 O4 hybrid. Fig. 5 shows the TEM images of Ag/Fe3 O4 samples with 2% and 6% contents, respectively. From Fig. 5, it can be seen clearly that there were two phases of Ag particles (the black with mean size of 25 nm or so) and Fe3 O4 particles (the gray with mean size of 10 nm or so). With a lower Ag content, Ag nanoparticles dispersed in Fe3 O4 ones (Fig. 5a and b). With a higher Ag content, Ag nucleus generating quickly, leading to a large size of the sample. Because the stability constant of Ag2 HL (1 g Kf (Ag2 HL) = 7.1) was much lower than that of FeL (1 g Kf (FeL) = 25.0), the free concentration of Ag+ was much higher than that of Fe3+ , the formation speed of Ag+ was much faster than that of Fe3+ , leading to the size of Ag was larger than that of Fe3 O4 . Fig. 6 reveals the TEM and FE-SEM images of Cu/Fe3 O4 sample prepared with different molar ratios of total metal ions, citrate anion and (CH2 )6 N4 . From Fig. 6, it can be observed that the samples displayed sphere-like shapes. From Fig. 6a, c, e and f, the sizes of the samples were uniform in a large scale and all were sphere-like in shape. With a fixed molar ratio of total metal ions and citrate anion (1:1), the addition amount of (CH2 )6 N4 had a significant effect on the size and morphology. With a higher molar ratio of (CH2 )6 N4 and total metal ions (5:2), the corresponding sample was empty sphere with mean size of around 60 nm, which was composed of numerous nanoparticles (Fig. 6a and b). As the molar ratio of (CH2 )6 N4 and total metal ions decreased to 1:1, not only size of the sphere increased clearly (the mean value with 110 nm) but also the main spheres were not empty (from Fig. 6c–f). With a concentrated (CH2 )6 N4 , the amount of HCHO generated by hydrolysis of (CH2 )6 N4 was larger, Cu and Fe3 O4 particles were produced fast, and more importantly, a large quantity of volume of NH3 gas rushed and a certain of cavity volume in the sphere generated, resulting in a empty sphere structure. With addition of lower concentration of (CH2 )6 N4 , the HCHO reductant generated slowly, leading to a slow reduction of the particles which had sufficient time to arrange. Moreover, a little volume of NH3 gas released, and the cavity volume of sphere was less. As a result, the sphere with a full structure formed. Additionally, the effect of addition amount of sodium citrate on the morphology and structure of sample was investigated. The result shows in Fig. 7. As the molar ratio of citrate anion and total metals was controlled at 1.5:1, the particles of the sample agglomerated heavily, and a great deal of nanoparticles piled into microspheres. Because both Cu2+ and Fe3+ formed ML (1:1) complexes with citrate anion, and the exceeded half amount of citrate
180
L. Pan et al. / Materials Chemistry and Physics 134 (2012) 177–182
Fig. 4. XPS spectrum of Cu/Fe3 O4 .
anion caused much lower free concentrations of Cu2+ and Fe3+ , in consequence, the as-synthesized Cu and Fe3 O4 particles were so smaller that they agglomerated easily without addition of any surfactant. The electrocatalytic properties of Fe3 O4 and Ag/Fe3 O4 samples modified on a GCE for p-nitrophenol reduction in a basic solution were investigated. The results show in Fig. 8. From curve 1 in Fig. 8, a bare GCE exhibited weak catalytic activity, for the corresponding reduction peak current was low and its value was only 40 A. When a GCE modified with Fe3 O4 and Ag/Fe3 O4 hybrids with different Ag
contents in the range from 2 to 8% were used, respectively, the corresponding reduction peak current increased to 83, 102, 112, 134, 140, and 119 A, respectively, which was 2.1, 2.6, 2.8, 3.4, and 3.0 time bigger than the one with a bare GCE. Obviously, by comparison to Fe3 O4 , the electrocatalytic activity for p-nitrophenol reduction had a clear increase using Ag/Fe3 O4 hybrids. However, the catalytic activity of the samples did not increase all the time by increasing Ag content in the sample. The catalytic performance of Ag/Fe3 O4 hybrids had an increasing tendency accompanying Ag content increasing from 2 to 6%, but decreased by further
Fig. 5. TEM images of Ag/Fe3 O4 , Ag content with 2% in (a) and (b), 6% in (c).
L. Pan et al. / Materials Chemistry and Physics 134 (2012) 177–182
181
Fig. 6. TEM and FE-SEM images of Cu/Fe3 O4 : TEM images in (a)–(d), FE-SEM images in (e) and (f). Molar ratios of Cu2+ :Fe3+ :sodium citrate:(CH2 )6 N4 = 1:1:2:5 in (a) and (b), 1:1:2:2 in (c) to (f).
increasing Ag content to 8%. The sample with 6% Ag exhibited the highest electrocatalytic activity for p-nitrophenol reduction. We speculated that Ag particles in the sample with a higher Ag content would aggregate severely, leading to decrease of Ag atom number or catalytic activity centers in the surface of the sample, which caused the catalytic activity to decline. The electrocatalytic properties of three Cu/Fe3 O4 samples modified on a GCE for p-nitrophenol reduction in a basic solution also were investigated. The results display in Fig. 9. By comparison to a bare GCE, the three samples all exhibited significantly enhanced catalytic performance. Among the samples, the one prepared with molar ratio of Cu2+ , Fe3+ , citrate anion, and (CH2 )6 N4 with 1:1:3:5 Fig. 7. TEM image of Cu/Fe3 O4 citrate:(CH2 )6 N4 = 1:1:3:5.
with molar ratios of Cu2+ :Fe3+ :sodium
Fig. 8. Cyclic voltammorgrams of a bare GCE and a GCE modified with Ag/Fe3 O4 samples with different Ag contents in 1.0 mol L−1 sodium hydroxide + 1.0 mmol L−1 p-nitrophenol (scanning velocity: 0.02 V s−1 ).
Fig. 9. Cyclic voltammorgrams of a bare GCE and a GCE modified with Cu/Fe3 O4 with molar ratios of Cu2+ :Fe3+ :sodium citrate:(CH2 )6 N4 = 1:1:2:5 (curve 2), 1:1:3:5 (curve 3), and 1:1:2:2 (curve 4) in 1.0 mol L−1 sodium hydroxide + 1.0 mmol L−1 pnitrophenol (scanning velocity: 0.02 V s−1 ).
182
L. Pan et al. / Materials Chemistry and Physics 134 (2012) 177–182
exhibited the highest catalytic activity, because not only its peak current was highest and reached 232 A but also the corresponding peak potential was lowest at −0.948 V (shown in curve 3). Additionally, p-nitrophenol could be reduced effectively at lower potential with the sample, for example, the peak current reached 120 A at −0.75 V. However, the peak currents were below 35 A using the other two samples at the same potential. Based on the TEM images shown in Figs. 6 and 7, the samples prepared with molar ratios of Cu2+ , Fe3+ , citrate anion, and (CH2 )6 N4 with 1:1:2:5 and 1:1:2:2 were sphere-like shape, but the one prepared with molar ratio of Cu2+ , Fe3+ , citrate anion, and (CH2 )6 N4 with 1:1:3:5 was microsphere piled with numerous nanoparticles. Under ultrasonication, the microsphere was completely dispersed into numerous nanoparticles, so the GCE modified with the sample with a smaller size had a great deal of activity centers, resulting in a higher catalytic activity. 4. Conclusions Ag/Fe3 O4 hybrids with different Ag contents and Cu/Fe3 O4 samples were synthesized with assistance of sodium citrate and (CH2 )6 N4 without addition of any surfactant via a hydrothermal procedure. Ag/Fe3 O4 samples were nanoparticles in which Ag particles were larger than Fe3 O4 ones. Cu/Fe3 O4 exhibited sphere-like shape with addition of equal molar amount of citrate anion and total metals. The size of the Cu/Fe3 O4 sphere could be controlled by addition amount of (CH2 )6 N4 . The electrocatalytic performances of Ag/Fe3 O4 and Cu/Fe3 O4 samples modified on a GCE for reduction p-nitrophenol in a basic solution were investigated. The Ag/Fe3 O4
and Fe3 O4 all displayed enhanced elctrocatalytic property, and the sample with 6% Ag content revealed the highest catalytic activity. For the Cu/Fe3 O4 samples, the one prepared with excessive citrate anion showed the highest catalytic activity, and p-nitrophenol could be reduced effectively under lower potential with the sample. References [1] J. Wang, Y.J. Wu, Y.J. Zhu, Mater. Chem. Phys. 106 (2007) 1. [2] G.F. Zou, K. Xiong, C.G. Jiang, H. Li, T.W. Li, J. Du, Y.T. Qian, J. Phys. Chem. B 109 (2005) 18356. [3] H. Hayakawa, H. Tanaka, K. Fujimoto, Catal. Commun. 8 (2007) 1820. [4] Z. Durmus, H. Kavas, M.S. Toprak, A. Baykal, T.G. Altıncekic, A. Aslan, A. Bozkurt, S. Cosgun, J. Alloys Compd. 484 (2009) 371. [5] J. Jiang, H. Gu, H. Shao, E. Devlin, G.C. Papaefthymiou, J.Y. Ying, Adv. Mater. 20 (2008) 4403. [6] F.H. Lin, W. Chen, Y.H. Liao, R.A. Doong, Y.D. Li, Nano Res. 4 (2011) 1223. [7] T. Schalowa, B. Brandta, D.E. Starra, M. Laurina, S. Schauermanna, S.K. Shaikhutdinova, J. Libuda, H.J. Freund, Catal. Lett. 107 (2006) 189. [8] J.H. Hsu, S.Y. Chen, W.M. Chang, T.S. Jian, C.R. Chang, J. Appl. Phys. 93 (2003) 7702. [9] C.M. Yu, J.W. Guo, H.Y. Gu, Microchim. Acta 166 (2009) 215. [10] Z.B. Li, Y.D. Deng, B. Shen, W.B. Hu, Mater. Sci. Eng. B 164 (2009) 112. [11] M. Estrella, L. Barrio, G. Zhou, X. Wang, Q. Wang, W. Wen, J.C. Hanson, A.I. Frenkel, J.A. Rodriguez, J. Phys. Chem. C 113 (2009) 14411. [12] D.H. Zhang, G.D. Li, J.X. Li, J.S. Chen, Chem. Commun. (2008) 3414. [13] L. Pan, L. Li, M. Xu, Z. Zhang, Mater. Sci. Eng. B 176 (2011) 1123. [14] J.G. Liu, B.L. Huang, X.Y. Li, P. Li, X.Y. Zeng, J. Electron. Mater. 39 (2010) 2702. [15] C.H. Lai, T.F. Wu, M.D. Lan, IEEE Trans. Magn. 41 (2005) 3397. [16] B. Wiley, Y. Sun, B. Mayers, Y. Xia, Chem. Eur. J. 11 (2005) 454. [17] Y.G. Zhang, S.T. Wang, Y.T. Qian, Z.D. Zhang, Solid State Sci. 8 (2006) 462. [18] D.H. Zhang, Z.Q. Liu, S. Han, C. Li, B. Lei, M.P. Stewart, J.M. Tour, C.W. Zhou, Nano Lett. 4 (2004) 2151. [19] U.S. Chen, Y.L. Chueh, S.H. Lai, L.J. Chou, H.C. Shih, J. Vac. Sci. Technol. B 24 (2006) 139.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis of nanostructured M/Fe3 O4 (M = Ag, Cu) composites using hexamethylentetramine and their electrocatalytic properties Lu Pan a,b,∗ , Yonghong Chen a,b , Fengwu Wang a a b
Department of Chemistry and Chemical Engineering, Huainan Normal University, Huainan 232001, China Anhui Key Laboratory of Low temperature Co-fired Material, Huainan Normal University, Huainan 232001, China
a r t i c l e
i n f o
Article history: Received 3 November 2011 Received in revised form 14 February 2012 Accepted 18 February 2012 Keywords: Ag/Fe3 O4 Cu/Fe3 O4 Nanostructres Electrochemical properties
a b s t r a c t Nanoscaled Ag/Fe3 O4 hybrids with different Ag contents and Cu/Fe3 O4 nanoshpere and microsphere were successfully synthesized with assistance of sodium citrate and (CH2 )6 N4 via a hydrothermal process. The as-prepared samples were identified and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), and X-ray photoelectron spectroscopy (XPS), respectively. All samples were used as electrocatalysts modified on a glassy carbon electrode for p-nitrophenol reduction in a basic solution. The catalytic activity of Ag/Fe3 O4 samples increased first and then decreased by increasing Ag content from 0% to 8%, and the one with 6% Ag displayed the highest catalytic activity. All the Cu/Fe3 O4 samples exhibited enhanced catalytic activity by comparison with a glassy carbon electrode, and the one prepared with the molar ratio of Cu2+ , Fe3+ , citrate anion, and (CH2 )6 N4 with 1:1:3:5 exhibited the highest catalytic activity. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Magnetite (Fe3 O4 ), an important member of spinel-type ferrite, has been extensively utilized as a gas sensor, pigment, recording material, electrophotographic developer, catalyst, biomedical treatment and spintronic devices, etc. [1–4]. Recently, interesting study on Fe3 O4 composites or hybrids doped with noble or other transition metals have been attracted widely, because of their doping or synergetic effect, which have significantly broadened the property and potential application of Fe3 O4 materials [5–10]. Although some noble metals such gold, palladium and platinum in Fe3 O4 always exhibit higher catalytic activity in many spheres, in consideration of the higher price, their practical application usually are restricted greatly. Thus, the research on other cheaper metals substituting these noble ones that doped in Fe3 O4 or combining with Fe3 O4 shows not only exceeding interest but also significantly potential application value [11,12]. In our previous work, almost mono-dispersed Ag/Fe3 O4 microspheres with identical size of 175 nm were synthesized using one-step procedure via a hydrothermal method [13]. Generally, two synthetic strategies usually are used to prepare Ag/Fe3 O4 composites, namely Fe3 O4 is prepared first, and then Ag particles
reduced by suitable agents grow on Fe3 O4 particles or other shapes [14,15]. Because Ag nanoparticles are easy to aggregate during its reduction process, so some surfactants often are utilized to prevent them from aggregating. Inevitably, such synthesis processes are often full of much complication, and furthermore, silver ion concentration usually is rather low, which lead to it is difficultly for such synthesis technique to be applied widely in a large scale production. In this work, a facile and easily controlled procedure was designed to synthesize Ag/Fe3 O4 and Cu/Fe3 O4 samples using only one-step strategy. Sodium citrate was used to serve as ligand to Ag+ , Fe3+ and Cu2+ ions and (CH2 )6 N4 as reaction agent, Ag/Fe3 O4 with a smaller size and Cu/Fe3 O4 nano- and microspheres were successfully prepared, which could overcome above disadvantages. 2. Experimental 2.1. Chemicals AgNO3 , CuSO4 ·5H2 O, NH4 Fe2 (SO4 )2 ·12H2 O, sodium citrate dihydrate (C6 H5 Na3 O7 ·2H2 O), and (CH2 )6 N4 , all were purchased from Shanghai Chemical Reagent Co., China. All reagents were of analytical grade and used as received without further purification. 2.2. Samples synthesis
∗ Corresponding author at: Department of Chemistry and Chemical Engineering, Huainan Normal University, Huainan 232001, China. Tel.: +86 554 6672651; fax: +86 554 6672650. E-mail address: [email protected] (L. Pan). 0254-0584/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.02.048
Synthesis of Ag/Fe3 O4 samples: AgNO3 and NH4 Fe2 (SO4 )2 ·12H2 O with total 2.0 mmol were dissolved in 45 mL of distilled water, then 3.0 mmol of sodium citrate dihydrate
178
L. Pan et al. / Materials Chemistry and Physics 134 (2012) 177–182
and 6.0 mmol of (CH2 )6 N4 were added in turn, and a tea-green homogeneous solution was obtained. The resultant mixture was transferred to 60 mL of Teflon-lined stainless steel autoclave, which was sealed and maintained at 160 ◦ C for 12 h. The autoclave was cooled to room temperature naturally, and the black precipitate was filtered and washed with distilled water then ethanol for several times, and dried in vacuum at 50 ◦ C for 6 h. By changing molar ratio of Ag+ and Fe3+ , several Ag/Fe3 O4 samples with different Ag contents were prepared. Synthesis of Cu/Fe3 O4 samples: CuSO4 ·5H2 O and NH4 Fe2 (SO4 )2 ·12H2 O with equal 1.25 mmol were dissolved in 45 mL of distilled water, then 2.5 mmol of mmol of sodium citrate dehydrate and 6.25 mmol of (CH2 )6 N4 were added in turn under rigorous stirring, and a transparent and homogeneous solution was obtained. The solution was transferred to 60 mL of Teflon-lined stainless steel autoclave, which was sealed and maintained at 160 ◦ C for 24 h. The autoclave was cooled to room temperature naturally, and the precipitate was filtered and washed with distilled water then ethanol for several times, and dried in vacuum at 60 ◦ C for 6 h. A few Cu/Fe3 O4 samples were synthesized by varying molar ratios of the total metal salts, sodium citrate and (CH2 )6 N4 . 2.3. Characterization The phases analysis of the as-synthesized samples was performed on a Philips X’Pert PROSUPER X-ray diffraction (XRD) with Cu K␣ radiation ( = 0.154178 nm), using an operation voltage and current of 40 kV and 50 mA. The SEM images were carried on a JEOL-6300F field-emission scanning electron microscopy (FE-SEM) with accelerating voltage of 15 kV. The TEM images were collected on a Hitachi Model H-800 transmission electron microscopy, using an accelerating voltage of 200 kV. XPS was performed using an ESCALAB 250 VG Lited XPS operated at 15 kV (h = 1486.6 eV). 2.4. Electrochemical determination The electrocatalytic measurements of the as-synthesized samples modified on a glassy carbon electrode (GCE) for reduction of p-nitrophenol in a basic solution were performed on LK 98 microcomputer-based electrochemical system (Tianjin Lanlike Chemical and Electron High Technology Co. Ltd., Tianjin in China). A three-electrode single compartment cell was used for cyclic voltammetry determination. A GCE (3.7 mm diameter) and a platinum plate were used as working and counter electrode, respectively, and a Ag/AgC1 electrode was used as reference electrode. Before each measurement, the GCE surface was carefully polished on an abrasive paper first, then further polished with 0.3 and 0.05 m ␣-Al2 O3 paste in turn, finally rinsed thoroughly with doubly distilled water and absolute alcohol, ensuring to keep the GCE in a good reversible condition. A 20 mg of sample was dispersed in 4 mL of doubly distilled water under ultrasonication to obtain a suspension solution. Of the suspension mixture, 50 L was taken out and covered on the carbon surface of a GCE in a good reversible state. After dried spontaneously in air, the modified GCE was prepared and used directly for electrochemical determination. 3. Results and discussion Fig. 1 shows the XRD patterns of Fe3 O4 (curve a) and Ag/Fe3 O4 with 6% Ag content (curve b). Without addition of Ag+ ion, cubic crystalline Fe3 O4 was synthesized. According to curve a, the characteristic diffraction peaks of the Fe3 O4 sample are in good agreement with the standard spectrum of cubic Fe3 O4 (JCPDS card. No. 190629). When a certain quantity of Ag+ was added, Ag/Fe3 O4 hybrid was synthesized. From curve b, the characteristic diffraction peaks
Fig. 1. XRD patterns of Fe3 O4 (a) and Ag/Fe3 O4 with 6% Ag (b).
of cubic metallic Ag and Fe3 O4 were detected clearly. Additionally, it is found that the characteristic diffraction peaks broadened, which indicated the sample could be composed of small-sized nanoparticles. It is well known that two synthetic strategies have been used frequently to prepare mental nanoparticles with heterostructures. For synthesis of Ag/Fe3 O4 hybrid, the key factor is to control the nucleation speed and dispersion of Ag particles. Wiley et al. synthesized Ag/Fe3 O4 samples via a solvothermal method in which Ag0 and Fe2+ were produced from reduction of Ag+ and Fe3+ with solvent, which could not only prevent the sample from aggregating but also Ag particles from being oxidized by Fe3+ ion [16]. In this work, the added citrate anion acted as coordinating agent to both Fe3+ and Ag+ ions. As a result the concentration of free Fe3+ or Ag+ in the solution was very low, and the nucleus concentration was so low that Ag and Fe3 O4 particles, both of which were reduced by HCHO released from the hydrolysis of (CH2 )6 N4 [17], could not form rapidly and aggregation of the samples could be avoided. According to curve b in Fig. 1, the diffraction peaks of Ag and the ones of Fe3 O4 all clearly broadened, furthermore, the diffraction peaks of Ag were not much stronger than those of Fe3 O4 , suggesting that both Ag and Fe3 O4 possessed a smaller size. Zhang et al. had synthesized Ag/Fe3 O4 composite, and the characteristic diffraction peaks of Ag were much stronger than those of Fe3 O4 , which revealed that the Ag nanoparticles had obvious agglomeration [12]. XPS technology was used to further identify the Ag/Fe3 O4 sample with 6% Ag content. The results show in Fig. 2. The survey scan shows clear peaks corresponding to the binding energies of Ag3d (368.3 eV), Fe2p and O1s (Fig. 2a), respectively. Further fine scanning around the Fe2p peaks revealed two characteristic binding energy peaks at 710.9 and 724.6 eV for Fe2p3/2 and Fe2p1/2 , respectively, which were in good agreement with the reported values of Fe3 O4 in the literature, and no characteristic peaks of Fe3+ in ␥Fe2 O3 locating at 719.0 eV was observed [18]. Other peaks in the spectrum (Fig. 2a) were ascribed to the carbon contamination on the substrate. Based on the XRD and XPS analysis of the sample, it was confirmed that the final sample was Ag/Fe3 O4 hybrid. Fig. 3 shows the XRD patterns of the samples prepared without (curve a) and with addition of Cu2+ ion (curve b) to Fe3+ solution. According to curve a, the characteristic diffractions of the sample was in good consistence with cubic Fe3 O4 . As equal amount of Cu2+ and Fe3+ were mixed, the resultant sample demonstrated a composite Cu/Fe3 O4 structure. The characteristic diffractions of Cu and Fe3 O4 were detected, respectively. The XRD pattern of Fe3 O4
L. Pan et al. / Materials Chemistry and Physics 134 (2012) 177–182
179
Fig. 2. XPS spectrum of Ag/Fe3 O4 with 4% Ag.
sample was extremely similar to that of the one shown in curve a in Fig. 1. The three diffraction peaks (2) locating at 3.25◦ , 40.42◦ and 71.15◦ , respectively, were in good agreement with face-center cubic Cu (JCPDF Card No.: 85-1326). From Fig. 3, the diffraction peaks of Cu were much stronger than that of Fe3 O4 , which verified the crystallinity of Cu particles was higher than the one of Fe3 O4 . For synthesis of Cu/Fe3 O4 , sodium citrate was added and citrate anion served as ligand to Cu2+ and Fe3+ . Because of the higher stability constants of FeL and CuL (1 g Kf (FeL) = 25.0, 1 g Kf (CuL) = 18.0, L representing citrate anion), the free Fe3+ and Cu2+ ions were extraordinarily low, which leaded to the nucleus concentration of Cu and Fe3 O4 particles reduced from Cu2+ and Fe3+ by HCHO released from hydrolysis of (CH2 )6 N4 were so low that Cu and Fe3 O4 particles could not easily aggregate. Likewise, the Cu/Fe3 O4 sample was further verified using XPS technology. The results show in Fig. 4. From Fig. 4a, the survey spectrum revealed the existence of the Cu, Fe and O elements. From Fig. 4b, two strong peaks locating at 710.9 and 724.6 eV corresponded to Fe2p3/2 and Fe2p1/2 , respectively, which were in good agreement with the reported values of Fe3 O4 . From Fig. 4c, two strong peaks at 932.5 and 952.5 eV were finely due to the binding
Fig. 3. XRD patterns of Fe3 O4 (a) and Cu/Fe3 O4 (b).
energy of Cu2p3/2 and Cu2p5/2 of Cu atomic rather than Cu2+ in CuO [19]. The XPS analysis further verified the Cu/Fe3 O4 hybrid. Fig. 5 shows the TEM images of Ag/Fe3 O4 samples with 2% and 6% contents, respectively. From Fig. 5, it can be seen clearly that there were two phases of Ag particles (the black with mean size of 25 nm or so) and Fe3 O4 particles (the gray with mean size of 10 nm or so). With a lower Ag content, Ag nanoparticles dispersed in Fe3 O4 ones (Fig. 5a and b). With a higher Ag content, Ag nucleus generating quickly, leading to a large size of the sample. Because the stability constant of Ag2 HL (1 g Kf (Ag2 HL) = 7.1) was much lower than that of FeL (1 g Kf (FeL) = 25.0), the free concentration of Ag+ was much higher than that of Fe3+ , the formation speed of Ag+ was much faster than that of Fe3+ , leading to the size of Ag was larger than that of Fe3 O4 . Fig. 6 reveals the TEM and FE-SEM images of Cu/Fe3 O4 sample prepared with different molar ratios of total metal ions, citrate anion and (CH2 )6 N4 . From Fig. 6, it can be observed that the samples displayed sphere-like shapes. From Fig. 6a, c, e and f, the sizes of the samples were uniform in a large scale and all were sphere-like in shape. With a fixed molar ratio of total metal ions and citrate anion (1:1), the addition amount of (CH2 )6 N4 had a significant effect on the size and morphology. With a higher molar ratio of (CH2 )6 N4 and total metal ions (5:2), the corresponding sample was empty sphere with mean size of around 60 nm, which was composed of numerous nanoparticles (Fig. 6a and b). As the molar ratio of (CH2 )6 N4 and total metal ions decreased to 1:1, not only size of the sphere increased clearly (the mean value with 110 nm) but also the main spheres were not empty (from Fig. 6c–f). With a concentrated (CH2 )6 N4 , the amount of HCHO generated by hydrolysis of (CH2 )6 N4 was larger, Cu and Fe3 O4 particles were produced fast, and more importantly, a large quantity of volume of NH3 gas rushed and a certain of cavity volume in the sphere generated, resulting in a empty sphere structure. With addition of lower concentration of (CH2 )6 N4 , the HCHO reductant generated slowly, leading to a slow reduction of the particles which had sufficient time to arrange. Moreover, a little volume of NH3 gas released, and the cavity volume of sphere was less. As a result, the sphere with a full structure formed. Additionally, the effect of addition amount of sodium citrate on the morphology and structure of sample was investigated. The result shows in Fig. 7. As the molar ratio of citrate anion and total metals was controlled at 1.5:1, the particles of the sample agglomerated heavily, and a great deal of nanoparticles piled into microspheres. Because both Cu2+ and Fe3+ formed ML (1:1) complexes with citrate anion, and the exceeded half amount of citrate
180
L. Pan et al. / Materials Chemistry and Physics 134 (2012) 177–182
Fig. 4. XPS spectrum of Cu/Fe3 O4 .
anion caused much lower free concentrations of Cu2+ and Fe3+ , in consequence, the as-synthesized Cu and Fe3 O4 particles were so smaller that they agglomerated easily without addition of any surfactant. The electrocatalytic properties of Fe3 O4 and Ag/Fe3 O4 samples modified on a GCE for p-nitrophenol reduction in a basic solution were investigated. The results show in Fig. 8. From curve 1 in Fig. 8, a bare GCE exhibited weak catalytic activity, for the corresponding reduction peak current was low and its value was only 40 A. When a GCE modified with Fe3 O4 and Ag/Fe3 O4 hybrids with different Ag
contents in the range from 2 to 8% were used, respectively, the corresponding reduction peak current increased to 83, 102, 112, 134, 140, and 119 A, respectively, which was 2.1, 2.6, 2.8, 3.4, and 3.0 time bigger than the one with a bare GCE. Obviously, by comparison to Fe3 O4 , the electrocatalytic activity for p-nitrophenol reduction had a clear increase using Ag/Fe3 O4 hybrids. However, the catalytic activity of the samples did not increase all the time by increasing Ag content in the sample. The catalytic performance of Ag/Fe3 O4 hybrids had an increasing tendency accompanying Ag content increasing from 2 to 6%, but decreased by further
Fig. 5. TEM images of Ag/Fe3 O4 , Ag content with 2% in (a) and (b), 6% in (c).
L. Pan et al. / Materials Chemistry and Physics 134 (2012) 177–182
181
Fig. 6. TEM and FE-SEM images of Cu/Fe3 O4 : TEM images in (a)–(d), FE-SEM images in (e) and (f). Molar ratios of Cu2+ :Fe3+ :sodium citrate:(CH2 )6 N4 = 1:1:2:5 in (a) and (b), 1:1:2:2 in (c) to (f).
increasing Ag content to 8%. The sample with 6% Ag exhibited the highest electrocatalytic activity for p-nitrophenol reduction. We speculated that Ag particles in the sample with a higher Ag content would aggregate severely, leading to decrease of Ag atom number or catalytic activity centers in the surface of the sample, which caused the catalytic activity to decline. The electrocatalytic properties of three Cu/Fe3 O4 samples modified on a GCE for p-nitrophenol reduction in a basic solution also were investigated. The results display in Fig. 9. By comparison to a bare GCE, the three samples all exhibited significantly enhanced catalytic performance. Among the samples, the one prepared with molar ratio of Cu2+ , Fe3+ , citrate anion, and (CH2 )6 N4 with 1:1:3:5 Fig. 7. TEM image of Cu/Fe3 O4 citrate:(CH2 )6 N4 = 1:1:3:5.
with molar ratios of Cu2+ :Fe3+ :sodium
Fig. 8. Cyclic voltammorgrams of a bare GCE and a GCE modified with Ag/Fe3 O4 samples with different Ag contents in 1.0 mol L−1 sodium hydroxide + 1.0 mmol L−1 p-nitrophenol (scanning velocity: 0.02 V s−1 ).
Fig. 9. Cyclic voltammorgrams of a bare GCE and a GCE modified with Cu/Fe3 O4 with molar ratios of Cu2+ :Fe3+ :sodium citrate:(CH2 )6 N4 = 1:1:2:5 (curve 2), 1:1:3:5 (curve 3), and 1:1:2:2 (curve 4) in 1.0 mol L−1 sodium hydroxide + 1.0 mmol L−1 pnitrophenol (scanning velocity: 0.02 V s−1 ).
182
L. Pan et al. / Materials Chemistry and Physics 134 (2012) 177–182
exhibited the highest catalytic activity, because not only its peak current was highest and reached 232 A but also the corresponding peak potential was lowest at −0.948 V (shown in curve 3). Additionally, p-nitrophenol could be reduced effectively at lower potential with the sample, for example, the peak current reached 120 A at −0.75 V. However, the peak currents were below 35 A using the other two samples at the same potential. Based on the TEM images shown in Figs. 6 and 7, the samples prepared with molar ratios of Cu2+ , Fe3+ , citrate anion, and (CH2 )6 N4 with 1:1:2:5 and 1:1:2:2 were sphere-like shape, but the one prepared with molar ratio of Cu2+ , Fe3+ , citrate anion, and (CH2 )6 N4 with 1:1:3:5 was microsphere piled with numerous nanoparticles. Under ultrasonication, the microsphere was completely dispersed into numerous nanoparticles, so the GCE modified with the sample with a smaller size had a great deal of activity centers, resulting in a higher catalytic activity. 4. Conclusions Ag/Fe3 O4 hybrids with different Ag contents and Cu/Fe3 O4 samples were synthesized with assistance of sodium citrate and (CH2 )6 N4 without addition of any surfactant via a hydrothermal procedure. Ag/Fe3 O4 samples were nanoparticles in which Ag particles were larger than Fe3 O4 ones. Cu/Fe3 O4 exhibited sphere-like shape with addition of equal molar amount of citrate anion and total metals. The size of the Cu/Fe3 O4 sphere could be controlled by addition amount of (CH2 )6 N4 . The electrocatalytic performances of Ag/Fe3 O4 and Cu/Fe3 O4 samples modified on a GCE for reduction p-nitrophenol in a basic solution were investigated. The Ag/Fe3 O4
and Fe3 O4 all displayed enhanced elctrocatalytic property, and the sample with 6% Ag content revealed the highest catalytic activity. For the Cu/Fe3 O4 samples, the one prepared with excessive citrate anion showed the highest catalytic activity, and p-nitrophenol could be reduced effectively under lower potential with the sample. References [1] J. Wang, Y.J. Wu, Y.J. Zhu, Mater. Chem. Phys. 106 (2007) 1. [2] G.F. Zou, K. Xiong, C.G. Jiang, H. Li, T.W. Li, J. Du, Y.T. Qian, J. Phys. Chem. B 109 (2005) 18356. [3] H. Hayakawa, H. Tanaka, K. Fujimoto, Catal. Commun. 8 (2007) 1820. [4] Z. Durmus, H. Kavas, M.S. Toprak, A. Baykal, T.G. Altıncekic, A. Aslan, A. Bozkurt, S. Cosgun, J. Alloys Compd. 484 (2009) 371. [5] J. Jiang, H. Gu, H. Shao, E. Devlin, G.C. Papaefthymiou, J.Y. Ying, Adv. Mater. 20 (2008) 4403. [6] F.H. Lin, W. Chen, Y.H. Liao, R.A. Doong, Y.D. Li, Nano Res. 4 (2011) 1223. [7] T. Schalowa, B. Brandta, D.E. Starra, M. Laurina, S. Schauermanna, S.K. Shaikhutdinova, J. Libuda, H.J. Freund, Catal. Lett. 107 (2006) 189. [8] J.H. Hsu, S.Y. Chen, W.M. Chang, T.S. Jian, C.R. Chang, J. Appl. Phys. 93 (2003) 7702. [9] C.M. Yu, J.W. Guo, H.Y. Gu, Microchim. Acta 166 (2009) 215. [10] Z.B. Li, Y.D. Deng, B. Shen, W.B. Hu, Mater. Sci. Eng. B 164 (2009) 112. [11] M. Estrella, L. Barrio, G. Zhou, X. Wang, Q. Wang, W. Wen, J.C. Hanson, A.I. Frenkel, J.A. Rodriguez, J. Phys. Chem. C 113 (2009) 14411. [12] D.H. Zhang, G.D. Li, J.X. Li, J.S. Chen, Chem. Commun. (2008) 3414. [13] L. Pan, L. Li, M. Xu, Z. Zhang, Mater. Sci. Eng. B 176 (2011) 1123. [14] J.G. Liu, B.L. Huang, X.Y. Li, P. Li, X.Y. Zeng, J. Electron. Mater. 39 (2010) 2702. [15] C.H. Lai, T.F. Wu, M.D. Lan, IEEE Trans. Magn. 41 (2005) 3397. [16] B. Wiley, Y. Sun, B. Mayers, Y. Xia, Chem. Eur. J. 11 (2005) 454. [17] Y.G. Zhang, S.T. Wang, Y.T. Qian, Z.D. Zhang, Solid State Sci. 8 (2006) 462. [18] D.H. Zhang, Z.Q. Liu, S. Han, C. Li, B. Lei, M.P. Stewart, J.M. Tour, C.W. Zhou, Nano Lett. 4 (2004) 2151. [19] U.S. Chen, Y.L. Chueh, S.H. Lai, L.J. Chou, H.C. Shih, J. Vac. Sci. Technol. B 24 (2006) 139.