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Co3O4 composites
Materials Research Bulletin 48 (2013) 2648–2653
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Synthesis and electrocatalytic performance for p-nitrophenol reduction of rod-like Co3O4 and Ag/Co3O4 composites Lu Pan a,b,*, Jing Tang a, 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
A B S T R A C T
Article history: Received 26 October 2012 Received in revised form 28 January 2013 Accepted 19 March 2013 Available online 1 April 2013
Rod-like precursors of Co3O4 and Ag/Co3O4 composites with different Ag contents were synthesized via a co-precipitation method. Co3O4 and Ag/Co3O4 composite samples were fabricated by calcining each precursor at 400 8C for 3 h. The as-prepared samples were characterized by thermogravimetric analysis and differential thermal gravimetric analysis (TGA/DTA), X-ray diffraction (XRD), transmission electron microscopy (TEM), and field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), respectively. Co3O4 and Ag/Co3O4 composites were used as electrocatalyst modified on a glassy carbon electrode for p-nitrophenol reduction in basic solution. The results showed that pnitrophenol could be reduced effectively on the modified electrode. By comparison with a bare glassy carbon electrode, peak current increased markedly with Co3O4 and Ag/Co3O4 samples, and peak potential decreased obviously with Ag/Co3O4 samples. Ag/Co3O4 composites with 4% Ag exhibited the highest electrocatalytic activity for p-nitrophenol reduction. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures D. Electrochemical properties D. Catalytic properties
1. Introduction Over recent decades, one-dimensional (1D) nanoscaled materials have been studied widely due to the extraordinary performances and applications in many spheres, such as electronic device [1], photoswitch [2], catalyst [3–5], magnetism [6], lithium ion battery, gas sensors [7], and so on. So far, many methods, including hydrothermal [8], solvothermal [9], electrochemical deposition [10], molten salt [11], and precipitation methods [12], have been used successfully to synthesize one-dimensional (1D) nanomaterials. To realize the preparation of 1D nanomaterials in a large scale, new method with inexpensive and practical advantages should be developed positively. Co3O4, an important transition metal oxide, not only has a normal spinel crystal structure of AB2O4 (A ! Co2+, B ! Co3+) in which Co2+ ions occupy 1/8 tetrahedral A sites and Co3+ ones occupy 1/2 octahedral B sites but is an intrinsic p-type semiconductor with (direct optical bandgaps at 1.48 and 2.19 eV) [13]. Nanosized Co3O4 is an extremely significant and functional material, which is used widely as catalyst [14], lithiumion battery electrode [15], magnetic material [16], sensor [17], and so on. However, Co3O4 material with single composition cannot
* Corresponding author at: Department of Chemistry and Chemical Engineering, Huainan Normal University, Huainan 232001, China. Fax: +86 554 6672650. E-mail address: [email protected] (L. Pan). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.03.037
meet with the increasing requests in many spheres. To effectively expand performances of Co3O4, it is an effective route to fabricate Co3O4 composites by mingling with or doping other elements in Co3O4. Frequently, Co3O4 composites may exhibit novel chemical and physical properties that the single composition cannot possess. By mingling and doping, the Co3O4 composites can exhibit novel performances [18–21]. Of the composites, the Co3O4 nanomaterials doped with noble metals have been investigated predominantly [22–25]. Among the noble metals, silver with lowest price and excellent properties has been studied and applied widely. Frequently, the composite silver/semiconductor oxides with nanostructure have been studied widely for their optical, electrical, magnetic, and chemical properties that are not found in the single individual component [26–28]. However, the synthesis and application of Ag/Co3O4 composites are reported rarely. From the limited literatures, multi-step reactions often are adopted to synthesize nanosized Ag/Co3O4 composite, namely Co3O4 is prepared first then Ag particles is doped in Co3O4 [29,30]. Clearly, multi-step preparation of Ag/Co3O4 composite has the drawback of procedure complexity, which leads to the operations to be difficult in controlling. To avoid the complicated procedures, we designed a new synthesis method in which Ag/Co3O4 composites with different Ag contents were fabricated by calcining precursor prepared via co-precipitation reaction of Ag+ and Co2+ ions using oxalic acid as assistant reagent. With the method, Ag content in the composite was easily controlled. Furthermore, the final samples displayed rod-like morphology.
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Although Ag/Co3O4 composite often is used in electrochemical sphere [30,31], the report on its electrocatalysis is obtained seldom. In the work, rod-like Co3O4 and Ag/Co3O4 composites with different Ag contents were used as electrocatalyst modified on a glassy carbon electrode (GCE) for p-nitrophenol in a basic solution. The enhanced electrocatalytic performances suggest their potential application in electrochemistry.
2. Experiments
Fig. 1. SEM images of precursor of Co3O4 (a) and Ag/Co3O4 with 5%Ag (b).
2.1. Materials AgNO3, Co(NO3)26H2O, H2C2O42H2O, polyethyleneglycol PEG (6000), and p-nitrophenol were purchased from Shanghai Chemical Reagent Co., China. All reagents were of analytical grade and used as received without further purification. 2.2. Preparation Co3O4 and Ag/Co3O4 samples AgNO3 and Co(NO3)26H2O with total 10.0 mmol as well as 1 g of PEG-6000 were dissolved completely under vigorous agitation in 100 mL of distilled water in a 250 mL of beaker, then 100 mL of H2C2O4 solution containing 12 mmol of H2C2O4 was added by dropwise. Subsequently, the beaker was bathed in water at 60 8C for 6 h. It could be seen a large quantity of colloid appeared. As the precipitate subsided completely, the precursor was filtered, washed with distilled water first then absolute alcohol for several times, finally dried in vacuum at 80 8C for 6 h. By calcining the precursor at 400 8C for 3 h according to TG experiment, the pure Co3O4 and Ag/Co3O4 samples with 1–5% Ag contents, respectively, were prepared. 2.3. Characterization Thermogravimetric and differential thermal analysis were performed on a Shimadzu TA-50WS analyzer in N2 gas in the temperature range from room temperature to 600 8C. The phases analysis of the as-synthesized samples was identified by a DX2000 X-ray diffraction (XRD) with Cu Ka radiation (l = 0.154178 nm), using an operation voltage and current of 40 kV and 50 mA. XPS was performed using an ESCALAB 250 VG Lited XPS operated at 15 kV (hn = 1486.6 eV). 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.
GCE was prepared and used directly for electrochemical measurement. 3. Results and discussion Fig. 1 shows the SEM images of the precursors of Co3O4 and Ag/ Co3O4 with 5% Ag, respectively. From Fig. 1a and b, both precursors are composed of microrods with the mean diameter of 600 nm or so, but their length is not uniform. The two precursors have no obvious difference in morphology except the length. The Co3O4 precursor has a big span in length and the longest rod can reach 20 mm but the shortest merely has several hundreds of nanometers. However, the Ag/Co3O4 one with 5% Ag content has comparatively uniform length. The SEM images of the precursors of two samples suggest that the resultant Co3O4 and Ag/Co3O4 composite samples obtained by calcining each precursor might be rod-like in morphology. To determine the calcination temperature to fabricate the final sample, thermogravimetric and differential thermal analysis of the precursor of Co3O4 were performed. The corresponding TG-DTA curves are shown in Fig. 2. From Fig. 2, two clear loss weights occur at 170–230 8C and 310–370 8C, respectively, which indicate that the precursor has experienced two reactions with the peaks around at 201 8C and 318 8C, respectively. The weight change (wt.%) for the two reactions is 18.5% and 36.9%, respectively, and the values agree well with that CoC2O42H2O ! CoC2O4 + 2H2O (theory wt. change = 19.6%) and CoC2O4 + 2/3O2 ! 1/ 3Co3O4 + 2CO2 (theory wt. change = 36.6%) except a minor difference between the actual and theoretic weight changes in the first weight loss. By increasing the temperature in the range from 370 to 600 8C, the sample weight keeps unchangeable, which indicates that the precursor has decomposed completely and the
2.4. Electrochemical determination Electrocatalytic measurements of a bare GCE and a GCE modified with the as-synthesized sample were carried out 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. A GCE (4.0 mm in diameter) was used as working electrode, a platinum plate (Pt) as counter electrode and a Ag/AgCl electrode as reference electrode. Prior to each determination, the surface of a GCE was polished carefully on an abrasive paper first, further polished with 0.3 and 0.05 mm a-Al2O3 paste in turn, and then rinsed completely with doubly distilled water and absolute alcohol. A 20 mg of sample was dispersed in 4 mL of doubly distilled water under ultrasonication conditions to obtain a suspension solution. Of the suspension solution, 50 mL was taken out and covered on the surface of carbon of the GCE in good reversible condition. After dried automatically in air, a modified
Fig. 2. TG and DTG curves of precursor of Co3O4.
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resultant Co3O4 product has been prepared. The further experiments confirmed that the Ag/Co3O4 samples with different Ag contents all could be fabricated by calcining each precursor at 400 8C. From Fig. 2, we can deduce the formation mechanism of Co3O4 and Ag/Co3O4 samples. Co2+ and Ag+ ions react with H2C2O4 under 60 8C water bath, and the reactions can be represented as the following Eqs. (1) and (2): Co2þ þ H2 C2 O4 þ 2H2 O ! CoC2 O4 2H2 O þ 2Hþ
(1)
2Agþ þ H2 C2 O4 ! Ag2 C2 O4 þ 2Hþ
(2)
2
Because free C2O4 concentration from H2C2O4 is very low, so CoC2O42H2O and Ag2C2O4 precipitates form slowly, which can make them grow fully along a certain direction and onedimensional precursors of Co3O4 and Ag/Co3O4 samples yield. As the CoC2O42H2O and Ag2C2O4 precipitates are filtered and dried, they decompose into one-dimensional Co3O4 and Ag/Co3O4 samples by calcination as the following reactions: CoC2 O4 2H2 O ! CoC2 O4 þ 2H2 O
(3)
CoC2 O4 þ2=3O2 ! 1=3Co3 O4 þ 2CO2
(4)
Ag2 C2 O4 þ ! 2Ag þ 2CO2
(5)
Fig. 3 shows the XRD patterns of Co3O4 (a) and Ag/Co3O4 with 3% Ag (b). From curve a, all diffraction peaks are in good agreement with cubic spinel Co3O4 phase, no other peaks belonging to CoOOH, CoO or Co2O3 phases is detected, verifying formation of pure Co3O4. From curve b, not only all the diffraction peaks of cubic phase of Co3O4 that are similar to the ones in curve a appear, but another three diffraction peaks around at 388, 448 and 648 in 2u which correspond to the (1 1 1), (2 0 0), (2 2 0) planes of metallic Ag with face-centered cubic (fcc) structure are detected distinctly. Comparing to the intensity of diffraction peaks of Co3O4, the one of Ag peaks is weaker, which may be due to the lower content of Ag in the composite. To further verify the compositions of the sample, the XPS experiment was carried out using Ag/Co3O4 composite with 3% Ag.
Fig. 3. XRD patterns of Co3O4 (a) and Ag/Co3O4 with 3% Ag (b).
The results are shown in Fig. 4. From the survey XPS spectrum (Fig. 4a), C1s peak locates at 284.6 eV, and except C, Ag, Co, and O, no peaks of other elements can be observed, which indicates that the sample ought to be consisted of silver and cobalt composite oxide. The high-resolution XPS spectra of Co2p, Ag3p and O1s of the sample are shown in Fig. 4b–d, respectively. From Fig. 4b, two peaks locate at binding energies of about 779.6 eV for Co2p3/2 and 795.5 eV for Co2p1/2, and Co2p3/2–Co2p1/2 energy separation is approximately 15.9 eV, which are well consistent with the values reported in the literature [32]. Lack of prominent shake-up satellite peaks in the Co2p spectra further suggests the presence of mainly Co3O4 phase. Fig. 4c shows that Ag3d binding energies are 367.95 and 373.9 eV, confirming the existence of Ag. From Fig. 4d, the broaden peak at binding energy of 529.7 eV is attributed to O1s. In addition, there is a secondary peak locating at around 532 eV, which may be attributed to the presence of superficial lattice oxygen in Co3O4 [33]. Based on the XPS data, the examined sample is identified as Ag/Co3O4 composite. The calculated molar ratio of Ag:Co is very close to the theoretical value, further confirming the composition of the Ag/Co3O4 composite with 3% Ag. Fig. 5 shows the TEM images of Co3O4 and Ag/Co3O4 composite with 5% Ag. From Fig. 5a and b, Co3O4 exhibits rod-like morphology, the length of the rods reaches over 10 mm. From the low (Fig. 5a) and high magnification (Fig. 5b) images, Co3O4 is actually composed of nanoparticles with mean size of 35 nm or so. In addition, there are numerous caves in these rods, which further verifies that the rods are assembled by a great deal of nanoparticles. By comparing Fig. 5c and d to Fig. 5a and b, Ag/ Co3O4 shows the similar morphology of Co3O4. No bigger particles can be observed on these rods, which reveal that Ag nanoparticle has a smaller size. Generally, Ag particles obtained by calcining a precursor such as silver carbonate have a large size, which is the result of heavy agglomeration of silver particles prepared by sintering at 400 8C [34]. In the work, Ag+ ions reacted with H2C2O4 and yielded Ag2C2O4 precipitation. Ag2C2O4 decompose at 140 8C or so as the following reaction: Ag2C2O4 ! 2Ag + 2CO2 [35,36]. Because of the rushing of a large mount of CO2 gas, Ag nanoparticles are preserved well, which preventing Ag particles from aggregating. Hence, no bigger Ag particles can be observed in the Ag/Co3O4 composite. Fig. 6 shows the FE-SEM images of Ag/Co3O4 composite with 5% Ag. From the low magnification images (Fig. 6a and b), the composite is mainly composed of microrods in a large scale. In addition, a few nanoparticles scatter beside the rods. From the high magnification images shown in Fig. 6c and d, it can be seen clearly that the Ag/Co3O4 composite microrods actually are assembled by a few stratiform structures which are composed of nanoparticles. In spite of the inconsistent size of these rods, Ag/Co3O4 composite displays a novel structure. The electrocatalytic performances of Co3O4, and Ag/Co3O4 composites with different Ag contents modified on a GCE for pnitrophenol reduction in a basic solution were investigated, respectively. The results are shown in Fig. 7. From Fig. 7, a bare GCE was used and the result is displayed in curve 1. From curve 1, the bare GCE shows rather poor electrocatalytic performance, for the peak current is low and the corresponding peak potential is rather high (its peak current vs. peak potential is 46 mA vs. 1.083 V). As a GCE modified with Co3O4, the peak current increases to 115 mA, which is 2.6 times bigger than that with a bare GCE, and the corresponding peak potential is 1.039 V, which has a slight decrease by comparing to that with a bare GCE. Clearly, a GCE modified with Co3O4 shows enhanced electrocatalytic activity comparing to a bare GCE. When a GCE modified with Ag/Co3O4 composite with 1–5% Ag contents, respectively, not only the peak currents all increase markedly but the corresponding peak potentials decrease in turn by comparison with a bare GCE. Their
L. Pan et al. / Materials Research Bulletin 48 (2013) 2648–2653
Fig. 4. XPS spectrum of Ag/Co3O4 with 3% Ag.
Fig. 5. TEM images of Co3O4 (a and b) and Ag/Co3O4 with 5% Ag (c and d).
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Fig. 6. FE-SEM images of Ag/Co3O4 with 5% Ag.
corresponding reduction peak current vs. peak potential are 123 mA vs. 0.877 V, 128 mA vs. 0.874 V, 144 mA vs. 0.805 V, 173 mA vs.0.712 V, and 145 mA vs. 0.687 V, respectively. Obviously, by comparing to a bare GCE, the GCE modified with Ag/Co3O4 composites all exhibit enhanced electrocatalytic performances for p-nitrophenol reduction, and their electrocatalytic activities are 2.7, 2.8, 3.1, 3.8 and 3.1 times bigger than that with a bare GCE. In addition, the Ag/Co3O4 composites also show higher activity than Co3O4 for p-nitrophenol reduction, because not only all peak currents are bigger but all peak potentials decrease. Although the peak current increases first (Ag content from 1% to 4%) then decreases (Ag content from 4% to 5%), the corresponding peak potential has an obvious decreasing tendency. The Ag/Co3O4 composite with 4% Ag content shows the highest electrocatalytic
activity. From above results, a GCE modified with Co3O4 can improve electrocatalytic activity only by increasing peak current, however, the one modified with Ag/Co3O4 composite shows enhanced electrocatalytic activity by not only increasing peak current but reducing peak potential. The results show that addition of Ag nanoparticles into Co3O4 can effectively reduce the peak potential. It is well known that Ag particles have strong ability of electron transportation than most of other metals. With addition of Ag nanoparticles into Co3O4, the elcetrocatalyst has much more strong electron transportation, which leads to p-nitrophenol can be reduced at lower potential. However, as Ag content increases from 4% to 5%, the electrocatalytic activity decreases. We speculated on that the main reason might be that Ag particles with a higher content in the composite aggregate severely, as a
Fig. 7. Cyclic voltammograms of a bare GCE and a GCE modified with Co3O4 and Ag/ Co3O4 with different Ag contents in 1 mol L1 NaOH + 1.0 mmol L1 p-nitrophenol (scanning rate 0.02 V s1).
Fig. 8. The plot of Ipc vs. square root of scan rate with Co3O4 and Ag/Co3O4 with different Ag contentS in 1 mol L1 NaOH +1.0 mmol L1 p-nitrophenol.
L. Pan et al. / Materials Research Bulletin 48 (2013) 2648–2653
result the Ag atoms in the surface of the electrocatalyst decrease or the catalytic activity centers reduce, which causes the catalytic activity to decline [37]. Additionally, to further investigate the electrocatalytic performances of the samples, the effect of scanning rate on the catalytic activity were examined using a GCE modified with Co3O4 and Ag/ Co3O4 composites with Ag content of 1–5%, respectively. The results are shown in Fig. 8. From Fig. 8, all the reduction peak currents increase by increasing scanning rate in the range from 0.02 to 0.10 V s1. Furthermore, it can be seen clearly that the peak currents all increase linearly with the square root of scan rate from 0.02 to 0.10 V s1 (all linear coefficients are over 0.99), indicating p-nitrophenol reduction on the Co3O4/GCEs and Ag/Co3O4/GCEs is attributed to diffusion controlled reactions [38]. 4. Conclusions Co3O4 and Ag/Co3O4 composites with different Ag contents were synthesized successfully via a co-precipitation process. The resultant samples exhibited rod-like morphology and there are numerous caves in them. The Ag/Co3O4 composite rods are actually assembled in levels by nanoparticles. The samples were used as electrocatalysts modified on a GCE for p-nitrophenol reduction in a basic solution. The samples all showed an enhanced electrocatalytic activity by comparing to a bare GCE. With Co3O4, the peak current increase but the peak potential almost kept unchangeable, while with Ag/Co3O4 composites, not only the peak current increased markedly but the peak potential decreased clearly, by comparing with a bare GCE. The Ag/Co3O4 composite with 4% Ag showed the highest electrocatalytic activity. References [1] [2] [3] [4] [5]
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Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Synthesis and electrocatalytic performance for p-nitrophenol reduction of rod-like Co3O4 and Ag/Co3O4 composites Lu Pan a,b,*, Jing Tang a, 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
A B S T R A C T
Article history: Received 26 October 2012 Received in revised form 28 January 2013 Accepted 19 March 2013 Available online 1 April 2013
Rod-like precursors of Co3O4 and Ag/Co3O4 composites with different Ag contents were synthesized via a co-precipitation method. Co3O4 and Ag/Co3O4 composite samples were fabricated by calcining each precursor at 400 8C for 3 h. The as-prepared samples were characterized by thermogravimetric analysis and differential thermal gravimetric analysis (TGA/DTA), X-ray diffraction (XRD), transmission electron microscopy (TEM), and field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), respectively. Co3O4 and Ag/Co3O4 composites were used as electrocatalyst modified on a glassy carbon electrode for p-nitrophenol reduction in basic solution. The results showed that pnitrophenol could be reduced effectively on the modified electrode. By comparison with a bare glassy carbon electrode, peak current increased markedly with Co3O4 and Ag/Co3O4 samples, and peak potential decreased obviously with Ag/Co3O4 samples. Ag/Co3O4 composites with 4% Ag exhibited the highest electrocatalytic activity for p-nitrophenol reduction. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures D. Electrochemical properties D. Catalytic properties
1. Introduction Over recent decades, one-dimensional (1D) nanoscaled materials have been studied widely due to the extraordinary performances and applications in many spheres, such as electronic device [1], photoswitch [2], catalyst [3–5], magnetism [6], lithium ion battery, gas sensors [7], and so on. So far, many methods, including hydrothermal [8], solvothermal [9], electrochemical deposition [10], molten salt [11], and precipitation methods [12], have been used successfully to synthesize one-dimensional (1D) nanomaterials. To realize the preparation of 1D nanomaterials in a large scale, new method with inexpensive and practical advantages should be developed positively. Co3O4, an important transition metal oxide, not only has a normal spinel crystal structure of AB2O4 (A ! Co2+, B ! Co3+) in which Co2+ ions occupy 1/8 tetrahedral A sites and Co3+ ones occupy 1/2 octahedral B sites but is an intrinsic p-type semiconductor with (direct optical bandgaps at 1.48 and 2.19 eV) [13]. Nanosized Co3O4 is an extremely significant and functional material, which is used widely as catalyst [14], lithiumion battery electrode [15], magnetic material [16], sensor [17], and so on. However, Co3O4 material with single composition cannot
* Corresponding author at: Department of Chemistry and Chemical Engineering, Huainan Normal University, Huainan 232001, China. Fax: +86 554 6672650. E-mail address: [email protected] (L. Pan). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.03.037
meet with the increasing requests in many spheres. To effectively expand performances of Co3O4, it is an effective route to fabricate Co3O4 composites by mingling with or doping other elements in Co3O4. Frequently, Co3O4 composites may exhibit novel chemical and physical properties that the single composition cannot possess. By mingling and doping, the Co3O4 composites can exhibit novel performances [18–21]. Of the composites, the Co3O4 nanomaterials doped with noble metals have been investigated predominantly [22–25]. Among the noble metals, silver with lowest price and excellent properties has been studied and applied widely. Frequently, the composite silver/semiconductor oxides with nanostructure have been studied widely for their optical, electrical, magnetic, and chemical properties that are not found in the single individual component [26–28]. However, the synthesis and application of Ag/Co3O4 composites are reported rarely. From the limited literatures, multi-step reactions often are adopted to synthesize nanosized Ag/Co3O4 composite, namely Co3O4 is prepared first then Ag particles is doped in Co3O4 [29,30]. Clearly, multi-step preparation of Ag/Co3O4 composite has the drawback of procedure complexity, which leads to the operations to be difficult in controlling. To avoid the complicated procedures, we designed a new synthesis method in which Ag/Co3O4 composites with different Ag contents were fabricated by calcining precursor prepared via co-precipitation reaction of Ag+ and Co2+ ions using oxalic acid as assistant reagent. With the method, Ag content in the composite was easily controlled. Furthermore, the final samples displayed rod-like morphology.
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Although Ag/Co3O4 composite often is used in electrochemical sphere [30,31], the report on its electrocatalysis is obtained seldom. In the work, rod-like Co3O4 and Ag/Co3O4 composites with different Ag contents were used as electrocatalyst modified on a glassy carbon electrode (GCE) for p-nitrophenol in a basic solution. The enhanced electrocatalytic performances suggest their potential application in electrochemistry.
2. Experiments
Fig. 1. SEM images of precursor of Co3O4 (a) and Ag/Co3O4 with 5%Ag (b).
2.1. Materials AgNO3, Co(NO3)26H2O, H2C2O42H2O, polyethyleneglycol PEG (6000), and p-nitrophenol were purchased from Shanghai Chemical Reagent Co., China. All reagents were of analytical grade and used as received without further purification. 2.2. Preparation Co3O4 and Ag/Co3O4 samples AgNO3 and Co(NO3)26H2O with total 10.0 mmol as well as 1 g of PEG-6000 were dissolved completely under vigorous agitation in 100 mL of distilled water in a 250 mL of beaker, then 100 mL of H2C2O4 solution containing 12 mmol of H2C2O4 was added by dropwise. Subsequently, the beaker was bathed in water at 60 8C for 6 h. It could be seen a large quantity of colloid appeared. As the precipitate subsided completely, the precursor was filtered, washed with distilled water first then absolute alcohol for several times, finally dried in vacuum at 80 8C for 6 h. By calcining the precursor at 400 8C for 3 h according to TG experiment, the pure Co3O4 and Ag/Co3O4 samples with 1–5% Ag contents, respectively, were prepared. 2.3. Characterization Thermogravimetric and differential thermal analysis were performed on a Shimadzu TA-50WS analyzer in N2 gas in the temperature range from room temperature to 600 8C. The phases analysis of the as-synthesized samples was identified by a DX2000 X-ray diffraction (XRD) with Cu Ka radiation (l = 0.154178 nm), using an operation voltage and current of 40 kV and 50 mA. XPS was performed using an ESCALAB 250 VG Lited XPS operated at 15 kV (hn = 1486.6 eV). 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.
GCE was prepared and used directly for electrochemical measurement. 3. Results and discussion Fig. 1 shows the SEM images of the precursors of Co3O4 and Ag/ Co3O4 with 5% Ag, respectively. From Fig. 1a and b, both precursors are composed of microrods with the mean diameter of 600 nm or so, but their length is not uniform. The two precursors have no obvious difference in morphology except the length. The Co3O4 precursor has a big span in length and the longest rod can reach 20 mm but the shortest merely has several hundreds of nanometers. However, the Ag/Co3O4 one with 5% Ag content has comparatively uniform length. The SEM images of the precursors of two samples suggest that the resultant Co3O4 and Ag/Co3O4 composite samples obtained by calcining each precursor might be rod-like in morphology. To determine the calcination temperature to fabricate the final sample, thermogravimetric and differential thermal analysis of the precursor of Co3O4 were performed. The corresponding TG-DTA curves are shown in Fig. 2. From Fig. 2, two clear loss weights occur at 170–230 8C and 310–370 8C, respectively, which indicate that the precursor has experienced two reactions with the peaks around at 201 8C and 318 8C, respectively. The weight change (wt.%) for the two reactions is 18.5% and 36.9%, respectively, and the values agree well with that CoC2O42H2O ! CoC2O4 + 2H2O (theory wt. change = 19.6%) and CoC2O4 + 2/3O2 ! 1/ 3Co3O4 + 2CO2 (theory wt. change = 36.6%) except a minor difference between the actual and theoretic weight changes in the first weight loss. By increasing the temperature in the range from 370 to 600 8C, the sample weight keeps unchangeable, which indicates that the precursor has decomposed completely and the
2.4. Electrochemical determination Electrocatalytic measurements of a bare GCE and a GCE modified with the as-synthesized sample were carried out 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. A GCE (4.0 mm in diameter) was used as working electrode, a platinum plate (Pt) as counter electrode and a Ag/AgCl electrode as reference electrode. Prior to each determination, the surface of a GCE was polished carefully on an abrasive paper first, further polished with 0.3 and 0.05 mm a-Al2O3 paste in turn, and then rinsed completely with doubly distilled water and absolute alcohol. A 20 mg of sample was dispersed in 4 mL of doubly distilled water under ultrasonication conditions to obtain a suspension solution. Of the suspension solution, 50 mL was taken out and covered on the surface of carbon of the GCE in good reversible condition. After dried automatically in air, a modified
Fig. 2. TG and DTG curves of precursor of Co3O4.
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resultant Co3O4 product has been prepared. The further experiments confirmed that the Ag/Co3O4 samples with different Ag contents all could be fabricated by calcining each precursor at 400 8C. From Fig. 2, we can deduce the formation mechanism of Co3O4 and Ag/Co3O4 samples. Co2+ and Ag+ ions react with H2C2O4 under 60 8C water bath, and the reactions can be represented as the following Eqs. (1) and (2): Co2þ þ H2 C2 O4 þ 2H2 O ! CoC2 O4 2H2 O þ 2Hþ
(1)
2Agþ þ H2 C2 O4 ! Ag2 C2 O4 þ 2Hþ
(2)
2
Because free C2O4 concentration from H2C2O4 is very low, so CoC2O42H2O and Ag2C2O4 precipitates form slowly, which can make them grow fully along a certain direction and onedimensional precursors of Co3O4 and Ag/Co3O4 samples yield. As the CoC2O42H2O and Ag2C2O4 precipitates are filtered and dried, they decompose into one-dimensional Co3O4 and Ag/Co3O4 samples by calcination as the following reactions: CoC2 O4 2H2 O ! CoC2 O4 þ 2H2 O
(3)
CoC2 O4 þ2=3O2 ! 1=3Co3 O4 þ 2CO2
(4)
Ag2 C2 O4 þ ! 2Ag þ 2CO2
(5)
Fig. 3 shows the XRD patterns of Co3O4 (a) and Ag/Co3O4 with 3% Ag (b). From curve a, all diffraction peaks are in good agreement with cubic spinel Co3O4 phase, no other peaks belonging to CoOOH, CoO or Co2O3 phases is detected, verifying formation of pure Co3O4. From curve b, not only all the diffraction peaks of cubic phase of Co3O4 that are similar to the ones in curve a appear, but another three diffraction peaks around at 388, 448 and 648 in 2u which correspond to the (1 1 1), (2 0 0), (2 2 0) planes of metallic Ag with face-centered cubic (fcc) structure are detected distinctly. Comparing to the intensity of diffraction peaks of Co3O4, the one of Ag peaks is weaker, which may be due to the lower content of Ag in the composite. To further verify the compositions of the sample, the XPS experiment was carried out using Ag/Co3O4 composite with 3% Ag.
Fig. 3. XRD patterns of Co3O4 (a) and Ag/Co3O4 with 3% Ag (b).
The results are shown in Fig. 4. From the survey XPS spectrum (Fig. 4a), C1s peak locates at 284.6 eV, and except C, Ag, Co, and O, no peaks of other elements can be observed, which indicates that the sample ought to be consisted of silver and cobalt composite oxide. The high-resolution XPS spectra of Co2p, Ag3p and O1s of the sample are shown in Fig. 4b–d, respectively. From Fig. 4b, two peaks locate at binding energies of about 779.6 eV for Co2p3/2 and 795.5 eV for Co2p1/2, and Co2p3/2–Co2p1/2 energy separation is approximately 15.9 eV, which are well consistent with the values reported in the literature [32]. Lack of prominent shake-up satellite peaks in the Co2p spectra further suggests the presence of mainly Co3O4 phase. Fig. 4c shows that Ag3d binding energies are 367.95 and 373.9 eV, confirming the existence of Ag. From Fig. 4d, the broaden peak at binding energy of 529.7 eV is attributed to O1s. In addition, there is a secondary peak locating at around 532 eV, which may be attributed to the presence of superficial lattice oxygen in Co3O4 [33]. Based on the XPS data, the examined sample is identified as Ag/Co3O4 composite. The calculated molar ratio of Ag:Co is very close to the theoretical value, further confirming the composition of the Ag/Co3O4 composite with 3% Ag. Fig. 5 shows the TEM images of Co3O4 and Ag/Co3O4 composite with 5% Ag. From Fig. 5a and b, Co3O4 exhibits rod-like morphology, the length of the rods reaches over 10 mm. From the low (Fig. 5a) and high magnification (Fig. 5b) images, Co3O4 is actually composed of nanoparticles with mean size of 35 nm or so. In addition, there are numerous caves in these rods, which further verifies that the rods are assembled by a great deal of nanoparticles. By comparing Fig. 5c and d to Fig. 5a and b, Ag/ Co3O4 shows the similar morphology of Co3O4. No bigger particles can be observed on these rods, which reveal that Ag nanoparticle has a smaller size. Generally, Ag particles obtained by calcining a precursor such as silver carbonate have a large size, which is the result of heavy agglomeration of silver particles prepared by sintering at 400 8C [34]. In the work, Ag+ ions reacted with H2C2O4 and yielded Ag2C2O4 precipitation. Ag2C2O4 decompose at 140 8C or so as the following reaction: Ag2C2O4 ! 2Ag + 2CO2 [35,36]. Because of the rushing of a large mount of CO2 gas, Ag nanoparticles are preserved well, which preventing Ag particles from aggregating. Hence, no bigger Ag particles can be observed in the Ag/Co3O4 composite. Fig. 6 shows the FE-SEM images of Ag/Co3O4 composite with 5% Ag. From the low magnification images (Fig. 6a and b), the composite is mainly composed of microrods in a large scale. In addition, a few nanoparticles scatter beside the rods. From the high magnification images shown in Fig. 6c and d, it can be seen clearly that the Ag/Co3O4 composite microrods actually are assembled by a few stratiform structures which are composed of nanoparticles. In spite of the inconsistent size of these rods, Ag/Co3O4 composite displays a novel structure. The electrocatalytic performances of Co3O4, and Ag/Co3O4 composites with different Ag contents modified on a GCE for pnitrophenol reduction in a basic solution were investigated, respectively. The results are shown in Fig. 7. From Fig. 7, a bare GCE was used and the result is displayed in curve 1. From curve 1, the bare GCE shows rather poor electrocatalytic performance, for the peak current is low and the corresponding peak potential is rather high (its peak current vs. peak potential is 46 mA vs. 1.083 V). As a GCE modified with Co3O4, the peak current increases to 115 mA, which is 2.6 times bigger than that with a bare GCE, and the corresponding peak potential is 1.039 V, which has a slight decrease by comparing to that with a bare GCE. Clearly, a GCE modified with Co3O4 shows enhanced electrocatalytic activity comparing to a bare GCE. When a GCE modified with Ag/Co3O4 composite with 1–5% Ag contents, respectively, not only the peak currents all increase markedly but the corresponding peak potentials decrease in turn by comparison with a bare GCE. Their
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Fig. 4. XPS spectrum of Ag/Co3O4 with 3% Ag.
Fig. 5. TEM images of Co3O4 (a and b) and Ag/Co3O4 with 5% Ag (c and d).
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Fig. 6. FE-SEM images of Ag/Co3O4 with 5% Ag.
corresponding reduction peak current vs. peak potential are 123 mA vs. 0.877 V, 128 mA vs. 0.874 V, 144 mA vs. 0.805 V, 173 mA vs.0.712 V, and 145 mA vs. 0.687 V, respectively. Obviously, by comparing to a bare GCE, the GCE modified with Ag/Co3O4 composites all exhibit enhanced electrocatalytic performances for p-nitrophenol reduction, and their electrocatalytic activities are 2.7, 2.8, 3.1, 3.8 and 3.1 times bigger than that with a bare GCE. In addition, the Ag/Co3O4 composites also show higher activity than Co3O4 for p-nitrophenol reduction, because not only all peak currents are bigger but all peak potentials decrease. Although the peak current increases first (Ag content from 1% to 4%) then decreases (Ag content from 4% to 5%), the corresponding peak potential has an obvious decreasing tendency. The Ag/Co3O4 composite with 4% Ag content shows the highest electrocatalytic
activity. From above results, a GCE modified with Co3O4 can improve electrocatalytic activity only by increasing peak current, however, the one modified with Ag/Co3O4 composite shows enhanced electrocatalytic activity by not only increasing peak current but reducing peak potential. The results show that addition of Ag nanoparticles into Co3O4 can effectively reduce the peak potential. It is well known that Ag particles have strong ability of electron transportation than most of other metals. With addition of Ag nanoparticles into Co3O4, the elcetrocatalyst has much more strong electron transportation, which leads to p-nitrophenol can be reduced at lower potential. However, as Ag content increases from 4% to 5%, the electrocatalytic activity decreases. We speculated on that the main reason might be that Ag particles with a higher content in the composite aggregate severely, as a
Fig. 7. Cyclic voltammograms of a bare GCE and a GCE modified with Co3O4 and Ag/ Co3O4 with different Ag contents in 1 mol L1 NaOH + 1.0 mmol L1 p-nitrophenol (scanning rate 0.02 V s1).
Fig. 8. The plot of Ipc vs. square root of scan rate with Co3O4 and Ag/Co3O4 with different Ag contentS in 1 mol L1 NaOH +1.0 mmol L1 p-nitrophenol.
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result the Ag atoms in the surface of the electrocatalyst decrease or the catalytic activity centers reduce, which causes the catalytic activity to decline [37]. Additionally, to further investigate the electrocatalytic performances of the samples, the effect of scanning rate on the catalytic activity were examined using a GCE modified with Co3O4 and Ag/ Co3O4 composites with Ag content of 1–5%, respectively. The results are shown in Fig. 8. From Fig. 8, all the reduction peak currents increase by increasing scanning rate in the range from 0.02 to 0.10 V s1. Furthermore, it can be seen clearly that the peak currents all increase linearly with the square root of scan rate from 0.02 to 0.10 V s1 (all linear coefficients are over 0.99), indicating p-nitrophenol reduction on the Co3O4/GCEs and Ag/Co3O4/GCEs is attributed to diffusion controlled reactions [38]. 4. Conclusions Co3O4 and Ag/Co3O4 composites with different Ag contents were synthesized successfully via a co-precipitation process. The resultant samples exhibited rod-like morphology and there are numerous caves in them. The Ag/Co3O4 composite rods are actually assembled in levels by nanoparticles. The samples were used as electrocatalysts modified on a GCE for p-nitrophenol reduction in a basic solution. The samples all showed an enhanced electrocatalytic activity by comparing to a bare GCE. With Co3O4, the peak current increase but the peak potential almost kept unchangeable, while with Ag/Co3O4 composites, not only the peak current increased markedly but the peak potential decreased clearly, by comparing with a bare GCE. The Ag/Co3O4 composite with 4% Ag showed the highest electrocatalytic activity. References [1] [2] [3] [4] [5]
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