Analytica Chimica Acta 819 (2014) 26–33
Contents lists available at ScienceDirect
Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
In situ synthesis of ceria nanoparticles in the ordered mesoporous carbon as a novel electrochemical sensor for the determination of hydrazine Yue Liu b , Yijun Li a,b,∗ , Xiwen He b a b
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, PR China College of Chemistry, Nankai University, 94 WeiJin Road, Tianjin 300071, PR China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• CeO2 –OMC composites were prepared via a hydrothermal method.
• CeO2 –OMC had electrocatalytic ability to oxidation of hydrazine.
• The sensor had high sensitivity, excellent stability and reproducibility. • The sensor was successfully employed to detect hydrazine in real water samples.
a r t i c l e
i n f o
Article history: Received 11 December 2013 Received in revised form 15 February 2014 Accepted 18 February 2014 Available online 21 February 2014 Keywords: Ceria nanoparticles Ordered mesoporous carbon Electrocatalysis Hydrazine
a b s t r a c t A novel ceria (CeO2 )–ordered mesoporous carbon (OMC) modified electrode for the sensitive amperometric determination of hydrazine was reported. CeO2 –OMC composites were synthesized via a hydrothermal method at a relatively low temperature (180 ◦ C) and characterized by scanning electron microscopy (SEM), transmission electron microcopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The CeO2 –OMC modified glassy carbon electrode was characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) and indicated good electrocatalytic effect to the oxidation of hydrazine. Under the optimized conditions, the present sensor could be used to measure hydrazine in wide linear range from 40 nM to 192 M (R2 = 0.999) with a low detection limit of 12 nM (S/N = 3). Additionally, the sensor has been successfully applied to detect hydrazine in real water samples and the recoveries were between 98.2% and 105.6%. Eventually, the sensor exhibited an excellent stability and reproducibility as a promising method for determination of hydrazine. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Ordered mesoporous carbon (OMC) is a kind of 3-D nanostructured porous materials which has attracted enormous interest since it was first synthesized by Ryoo et al. [1]. Compared to multiwall carbon nanotubes [2] and graphene [3], OMC has better
∗ Corresponding author. Tel.: +86 22 23494885; fax: +86 22 23502458. E-mail address: [email protected] (Y. Li). http://dx.doi.org/10.1016/j.aca.2014.02.025 0003-2670/© 2014 Elsevier B.V. All rights reserved.
electrocatalytic and electrochemical properties because of its unique structure such as ordered pore structure, well-defined pore size, high specific surface area and excellent conductivity. Due to these advantages, it could be widely used in energy storage, catalysis, batteries, supercapacitor and sensors [4–8]. Moreover, many metal and metallic oxides like Au [9], Ag [10], Pt [11], CuO [12], MnO2 [13], NiO [14] was embedded into the surface and pores of OMC, which improved the specificity catalysis of OMC and extended the applications. Ceria (CeO2 ), as a low-cost and efficiently catalytic Lanthanide rare earth oxide, was applied widely
Y. Liu et al. / Analytica Chimica Acta 819 (2014) 26–33
in fuel cell, catalyst and batteries [15–18]. A few sensors based on CeO2 nanoparticles have been reported in recent years [19–22]. However, the CeO2 nanoparticles often aggregate and easily break off from the electrode surface [22], resulting in weakening the electrocatalytic ability and stability of the electrodes. What’s more, CeO2 has poor conductivity, which also limits its application in electrochemistry. To solve these problems, OMC can be used as a substrate to disperse and immobilize CeO2 due to its high surface area and excellent electrical conductivity. The CeO2 –OMC composites are expected to be a potential candidate as electrode material owing to the combination of the catalytic properties of CeO2 and the fast electron transfer ability of OMC. Hydrazine (N2 H4 ) is an important chemical compound which has been widely used in chemical industry, fuel cell, rocket fuel, agriculture [23–25] attributed to its strong reducibility, strong alkalinity and hygroscopicity. However, hydrazine is harmful to the environment and human health as a kind of highly toxic material as well. It was reported that hydrazine could cause some diseases such as headache, liver and kidney damage, even DNA damage [26,27]. Hence, it is essential to provide a reliable and sensitive method for rapid detection of hydrazine. Up to date, flow injection analysis (FIA) [28], ion chromatography [29], spectrophotometry [30,31] and chemiluminescence (CL) [32,33] have been reported for the determination of hydrazine. However, these methods are complicated, laborious and unable to fulfill the real-time determination. By comparison, chemical modified electrodes (CMEs) can play as an important role to detect hydrazine rapidly based on their easy, economical, and labor-free operation. Many CMEs have been reported such as Au–SH–SiO2 @Cu–MOF [34], CoHCF–MWCNT/GE [35], CeHCF–OMC/GCE [36] and ZnO–MWCNT/GCE [37] and some of them were used for the determination of hydrazine. The previous
27
work had some limitations in cost, linear range, stability, sensitivity and other characteristics. Thus, it is necessary to develop a novel and low-cost material for hydrazine determination. In this work, CeO2 –OMC composites were prepared at a relatively low temperature (180 ◦ C) via a hydrothermal method, which could avoid damaging the structure of OMC. Simultaneously, the CeO2 –OMC modified electrode was also prepared to serve as an amperometric sensor of hydrazine for the first time. The CeO2 nanoparticles were well dispersed in OMC, and the hybrid composites increased the electron transfer rate and showed excellent electrocatalytic effect to the oxidation of hydrazine and showed excellent stability and reproducibility.
2. Experimental 2.1. Materials Ordered mesoporous carbon (OMC, 3–5 nm of pore size) was purchased from XF NANO (Nanjing, China). Nafion solution (5 wt% in 15–20% water/lower aliphatic alcohols) was obtained from Alfa Asear. Ceric ammonium nitrate ((NH4 )2 Ce(NO3 )6 ), hydrous hydrazine (N2 H4 ·H2 O, 80%) and sodium hydroxide (NaOH) were purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). The phosphate buffer solution (PBS, 0.1 M) was prepared by Na2 HPO4 and NaH2 PO4 and the pH value was adjusted by H3 PO4 or NaOH. All the chemicals were at least analytical grade and used without further treatment. Pure water was prepared from a UP water purification system (>18 M cm). All the solutions were prepared daily and purged with nitrogen.
Fig. 1. SEM images of (A) OMC, (B) CeO2 –OMC; TEM images of (C) OMC and (D) CeO2 –OMC.
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Y. Liu et al. / Analytica Chimica Acta 819 (2014) 26–33
2.2. Apparatus Electrochemical measurements were performed on Epsilon electrochemical workstation (BAS, USA) with a modified GCE as the working electrode, a platinum wire as the auxiliary electrode and a saturated calomel electrode (SCE) as the reference electrode, respectively. The morphology of composites was characterized by Tescan Vega 3 SBH scanning electron microscope (SEM) and Tecnai G2 T2 S-TWIN transmission electron microscope (TEM). Xray diffraction (XRD) measurement was performed on a Rigaku D/max/2500v/pc (Japan) with Cu K␣ source. The 2 angles probed were from 10 ◦ to 80 ◦ at a rate 4◦ min−1 . The X-ray photoelectron spectra were obtained on a Shimadzu (Japan) Kratos AXIS Ultra DLD X-ray photoelectron spectrometer (XPS) with Mg K␣ anode (15 kV, 400 W) at a takeoff angle of 45◦ . Electrochemical impedance spectroscopy (EIS) experiments were performed on LK2010 (Lanlike Chemical Electronic High Technology Company, China) at a potential of 0.17 V in 5 mM K3 [Fe(CN)6 ] and 0.1 M KCl with the frequency range from 10 kHz to 0.1 Hz. 2.3. Procedures 2.3.1. Synthesis of CeO2 –OMC composites CeO2 –OMC composites were prepared according to literature [38] . Thirty milligrams OMC was dispersed in 20 mL pure water by 30 min ultra sonication to obtain black suspension. Then 15 mL 0.02 M (NH4 )2 Ce(NO3 )6 solution was added to OMC suspension under magnetic stirring. After several minutes, 1 wt% NaOH (aq) was added dropwise into the mixture to adjust pH value to 10.0. The mixture was separated by centrifugation and then mixed with 10 mL 5 M NaOH solution and transferred into a 50 mL stainless steel autoclave. After heated at 180 ◦ C for 45 h, the composites were separated by centrifugation, washed by pure water and ethanol for several times, then dried at 60 ◦ C for 12 h. The reactions involved are shown below:
obvious that the CeO2 nanoparticles were synthesized successfully on the surface of OMC. The X-ray photoelectron spectroscopy (XPS) was used to characterize CeO2 –OMC composites. In Fig. 2A, the peaks of Ce, C and O elements were observed in XPS spectra. As shown in Fig. 2B, the Ce 3d5/2 and Ce 3d3/2 peaks appeared at binding energies 882.7 eV and 901.3 eV, respectively [20]. It also demonstrated that the CeO2 nanoparticles were successfully synthesized on the surface of OMC. Fig. 3 shows the XRD patterns of the OMC (curve a) and CeO2 –OMC (curve b) composites. Compared to OMC, it could be clearly observed that eight peaks appeared in CeO2 –OMC composites. According to standard card (JPCDS 34-0394) eight peaks could be assigned to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1) and (4 2 0) crystal facets in fluorite structured CeO2 . It could confirm that the CeO2 particles were grown on the OMC. 3.2. Characterization of electrochemical behavior of CeO2 –OMC/GCE The electrochemical impedance spectroscopy (EIS) was employed to investigate the electron transfer capability of different modified electrodes using 5 mM K3 [Fe(CN)6 ] at the potential of 0.17 V. In the Nyquist diagrams, the diameter of the semicircle presents the interfacial electron transfer resistance and the linear portion corresponds to the diffusion process. As shown in Fig. 4,
Ce4+ + 4OH− −→ Ce(OH)4 Ce(OH)4 −→ CeO2 + 2H2 O 2.3.2. Fabrication of the modified electrode Prior to the modification, the glass carbon electrode (4 mm of diameter) was polished with alumina slurry and successively sonicated in 1:1 nitric acid, acetone and pure water for 20 min. Then the electrode was dried in air. Typically, 10 mg CeO2 –OMC composites were dispersed in 5 mL DMF under sonication for 30 min and 5 L suspension was dropped on the cleaned electrode surface. After dried under the infrared lamp, 5 L 0.1 wt% Nafion was coated on it and then dried to obtain CeO2 –OMC modified electrode. Single-component OMC or CeO2 modified electrodes were also prepared by the above method for comparison. 3. Results and discussions 3.1. Characterization of CeO2 –OMC composites The morphology of OMC and CeO2 –OMC were characterized by SEM (Fig 1A and B) and TEM (Fig 1C and D). From Fig. 1A, OMC exhibited well-defined morphology with small bundles structure. For comparison in Fig. 1B, the CeO2 –OMC exhibited the form of small rods with rough surface and some particles embedded on the wall of OMC but the structure of OMC was not changed. The TEM images indicated clearly that a number of nanoparticles (about 30 nm) were uniformly growing on the surface of OMC. It was
Fig. 2. XPS spectra of the obtained (A) CeO2 –OMC and (B) Ce 3d.
Y. Liu et al. / Analytica Chimica Acta 819 (2014) 26–33
29
Fig. 3. XRD pattern of OMC (curve a) and CeO2 –OMC (curve b).
The OMC/GCE showed the smallest interfacial electron transfer resistance, indicating the fast electron transfer capability of OMC. And the CeO2 /GCE showed the largest interfacial electron transfer resistance, suggesting the poor conductivity of CeO2 . When CeO2 nanoparticles were well dispersed on the OMC, the interfacial electron transfer resistance of CeO2 –OMC/GCE obviously reduced compared to CeO2 /GCE and bare GCE, which illustrated that OMC not only provided large surface area to immobilize and disperse the CeO2 nanoparticles but also increased the electron transfer rate. 3.3. Electrochemical behavior of hydrazine on the CeO2 –OMC modified electrode Cyclic voltammetry was used to study the electrochemical behavior of hydrazine in 0.1 M phosphate buffer solutions (PBS, pH 8.0). As shown in Fig. 5, the voltammograms of different electrodes in the absence (curves A1, B1, C1 and D1) and presence
(curves A2, B2, C2 and D2) of 0.5 mM hydrazine were presented. From Fig. 5A, it is observed that hydrazine was oxidized on the bare GCE with only a low oxidation current and high overpotential, which indicated the kinetically sluggish electron transfer between hydrazine and GCE. While in the Fig. 5B, the oxidation current of hydrazine on the CeO2 /GCE was increased comparing to that on the GCE, which cloud be owing to the facile Ce3+ /Ce4+ redox cycle of CeO2 as catalyst [39]. However, the oxidation current of hydrazine was still not high enough due to the poor conductivity of CeO2 . In Fig. 5C, the oxidation current of hydrazine was enhanced on the OMC/GCE with the negative shift of peak potential (at about 0.4 V), which could be due to high surface area and large quantities of edge-plane-like defective sites of OMC [36]. On CeO2 –OMC/GCE (Fig. 5D), hydrazine was oxidized at lower potential (0.36 V) as well as a 2.3-fold increase in the peak current comparing with that on OMC/GCE, which might be attributed to the synergistic effect of the CeO2 –OMC composites. Comparing with previous reports, the presented electrode gave the more negative peak potential for
Fig. 4. Nyqusit plots of 5 mM K3 [Fe(CN)6 ] in 0.1 M KCl from 10 kHz to 0.1 Hz for (a) OMC/GCE; (b) CeO2 –OMC/GCE; (c) GCE; (d) CeO2 /GCE.
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Y. Liu et al. / Analytica Chimica Acta 819 (2014) 26–33
Fig. 5. CVs of (A) GCE, (B) CeO2 /GCE, (C) OMC/GCE and (D) CeO2 –OMC/GCE in the absence (curves A1, B1, C1, D1) and the presence (curves A2, B2, C2, D2) of 0.5 mM hydrazine in 0.1 M pH = 8.0 PBS. Scan rate: 100 mV s−1 .
the oxidation of hydrazine (0.517 V on ZnO/MWCNTs/GCE [37] and 0.5 V on CeO2 /Au/GCE [39]). All the electrochemical responses of electrodes were irreversible, as no reduction current was observed during the reverse scan. According to the previous reports [34,46], the electrochemical oxidation of hydrazine was proposed to be as follows: N2 H4 + 4OH− −→ N2 + 4H2 O + 4e
3.4. Optimization of the concentration of the (NH4 )2 Ce(NO3 )6 During the hydrothermal process, different concentration of (NH4 )2 Ce(NO3 )6 influenced the amount of CeO2 nanoparticles grown on the OMC, which could affect the electrocatalytic activity of CeO2 –OMC composites to oxidation of hydrazine. Excess CeO2 embedded in OMC could reduce the conductivity of the composites and insufficient CeO2 embedded in OMC caused an insufficient catalytic activity of OMC–CeO2 composites. Thus it was important to choose an appropriate amount of CeO2 in OMC. From Fig. 6, the oxidation peak current of 0.1 mM hydrazine increased with the (NH4 )2 Ce(NO3 )6 concentration from 0 to 0.3 mM. The oxidation peak current decreased when the concentration of (NH4 )2 Ce(NO3 )6 was higher than 0.3 mM. As a result, 0.3 mM (NH4 )2 Ce(NO3 )6 was chosen as the optimum concentration for synthesis of CeO2 –OMC composites. 3.5. Effect of pH value According to previous work [34,39], the electrochemical reaction of hydrazine was dependent on pH value of the buffer solution.
Therefore, different buffers with pH values ranging from 6.0 to 10.0 were tested to study the electrochemical behavior of hydrazine by cyclic voltammetry. The oxidation peak potential of hydrazine on CeO2 –OMC/GCE was shifted negatively with the increased pH values of PBS. As shown in Fig. 7, the peak current increased from pH 6.0 to pH 8.0 and decreased at pH values above 8.0. The peak current changed with different pH values could be due to the form of hydrazine (the pKa of hydrazine was 8.1). When the pH value of PBS was below the pKa , the hydrazine exited as the protonated form and it was repulsed from the electrode surface, which led to a decrease of oxidation peak current. When the pH value was above 8.0, the change of current might result from the deprotonation form of hydrazine [34]. In conclusion, pH = 8.0 PBS was chosen as the supporting electrolyte in the following experiments. 3.6. Amperometric determination of hydrazine The current–time curve was utilized for quantitative analysis of hydrazine. The different operating potentials were tested for the determination of 30 M hydrazine. As a result, the current response of hydrazine was increased slowly from 0.2 V to 0.6 V. At last, 0.4 V was chosen as the working potential to achieve a good sensitivity. Simultaneously, the low working potential could avoid the interference of some phenol compounds (often co-exist in waste water). As shown in Fig. 8A, current–time curves illustrated the responses of CeO2 –OMC/GCE by successive additions of different concentrations of hydrazine under stirring in 0.1 M PBS (pH 8.0). The oxidation currents increased with each addition of hydrazine and reached a steady-state soon (
Contents lists available at ScienceDirect
Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
In situ synthesis of ceria nanoparticles in the ordered mesoporous carbon as a novel electrochemical sensor for the determination of hydrazine Yue Liu b , Yijun Li a,b,∗ , Xiwen He b a b
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, PR China College of Chemistry, Nankai University, 94 WeiJin Road, Tianjin 300071, PR China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• CeO2 –OMC composites were prepared via a hydrothermal method.
• CeO2 –OMC had electrocatalytic ability to oxidation of hydrazine.
• The sensor had high sensitivity, excellent stability and reproducibility. • The sensor was successfully employed to detect hydrazine in real water samples.
a r t i c l e
i n f o
Article history: Received 11 December 2013 Received in revised form 15 February 2014 Accepted 18 February 2014 Available online 21 February 2014 Keywords: Ceria nanoparticles Ordered mesoporous carbon Electrocatalysis Hydrazine
a b s t r a c t A novel ceria (CeO2 )–ordered mesoporous carbon (OMC) modified electrode for the sensitive amperometric determination of hydrazine was reported. CeO2 –OMC composites were synthesized via a hydrothermal method at a relatively low temperature (180 ◦ C) and characterized by scanning electron microscopy (SEM), transmission electron microcopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The CeO2 –OMC modified glassy carbon electrode was characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) and indicated good electrocatalytic effect to the oxidation of hydrazine. Under the optimized conditions, the present sensor could be used to measure hydrazine in wide linear range from 40 nM to 192 M (R2 = 0.999) with a low detection limit of 12 nM (S/N = 3). Additionally, the sensor has been successfully applied to detect hydrazine in real water samples and the recoveries were between 98.2% and 105.6%. Eventually, the sensor exhibited an excellent stability and reproducibility as a promising method for determination of hydrazine. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Ordered mesoporous carbon (OMC) is a kind of 3-D nanostructured porous materials which has attracted enormous interest since it was first synthesized by Ryoo et al. [1]. Compared to multiwall carbon nanotubes [2] and graphene [3], OMC has better
∗ Corresponding author. Tel.: +86 22 23494885; fax: +86 22 23502458. E-mail address: [email protected] (Y. Li). http://dx.doi.org/10.1016/j.aca.2014.02.025 0003-2670/© 2014 Elsevier B.V. All rights reserved.
electrocatalytic and electrochemical properties because of its unique structure such as ordered pore structure, well-defined pore size, high specific surface area and excellent conductivity. Due to these advantages, it could be widely used in energy storage, catalysis, batteries, supercapacitor and sensors [4–8]. Moreover, many metal and metallic oxides like Au [9], Ag [10], Pt [11], CuO [12], MnO2 [13], NiO [14] was embedded into the surface and pores of OMC, which improved the specificity catalysis of OMC and extended the applications. Ceria (CeO2 ), as a low-cost and efficiently catalytic Lanthanide rare earth oxide, was applied widely
Y. Liu et al. / Analytica Chimica Acta 819 (2014) 26–33
in fuel cell, catalyst and batteries [15–18]. A few sensors based on CeO2 nanoparticles have been reported in recent years [19–22]. However, the CeO2 nanoparticles often aggregate and easily break off from the electrode surface [22], resulting in weakening the electrocatalytic ability and stability of the electrodes. What’s more, CeO2 has poor conductivity, which also limits its application in electrochemistry. To solve these problems, OMC can be used as a substrate to disperse and immobilize CeO2 due to its high surface area and excellent electrical conductivity. The CeO2 –OMC composites are expected to be a potential candidate as electrode material owing to the combination of the catalytic properties of CeO2 and the fast electron transfer ability of OMC. Hydrazine (N2 H4 ) is an important chemical compound which has been widely used in chemical industry, fuel cell, rocket fuel, agriculture [23–25] attributed to its strong reducibility, strong alkalinity and hygroscopicity. However, hydrazine is harmful to the environment and human health as a kind of highly toxic material as well. It was reported that hydrazine could cause some diseases such as headache, liver and kidney damage, even DNA damage [26,27]. Hence, it is essential to provide a reliable and sensitive method for rapid detection of hydrazine. Up to date, flow injection analysis (FIA) [28], ion chromatography [29], spectrophotometry [30,31] and chemiluminescence (CL) [32,33] have been reported for the determination of hydrazine. However, these methods are complicated, laborious and unable to fulfill the real-time determination. By comparison, chemical modified electrodes (CMEs) can play as an important role to detect hydrazine rapidly based on their easy, economical, and labor-free operation. Many CMEs have been reported such as Au–SH–SiO2 @Cu–MOF [34], CoHCF–MWCNT/GE [35], CeHCF–OMC/GCE [36] and ZnO–MWCNT/GCE [37] and some of them were used for the determination of hydrazine. The previous
27
work had some limitations in cost, linear range, stability, sensitivity and other characteristics. Thus, it is necessary to develop a novel and low-cost material for hydrazine determination. In this work, CeO2 –OMC composites were prepared at a relatively low temperature (180 ◦ C) via a hydrothermal method, which could avoid damaging the structure of OMC. Simultaneously, the CeO2 –OMC modified electrode was also prepared to serve as an amperometric sensor of hydrazine for the first time. The CeO2 nanoparticles were well dispersed in OMC, and the hybrid composites increased the electron transfer rate and showed excellent electrocatalytic effect to the oxidation of hydrazine and showed excellent stability and reproducibility.
2. Experimental 2.1. Materials Ordered mesoporous carbon (OMC, 3–5 nm of pore size) was purchased from XF NANO (Nanjing, China). Nafion solution (5 wt% in 15–20% water/lower aliphatic alcohols) was obtained from Alfa Asear. Ceric ammonium nitrate ((NH4 )2 Ce(NO3 )6 ), hydrous hydrazine (N2 H4 ·H2 O, 80%) and sodium hydroxide (NaOH) were purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). The phosphate buffer solution (PBS, 0.1 M) was prepared by Na2 HPO4 and NaH2 PO4 and the pH value was adjusted by H3 PO4 or NaOH. All the chemicals were at least analytical grade and used without further treatment. Pure water was prepared from a UP water purification system (>18 M cm). All the solutions were prepared daily and purged with nitrogen.
Fig. 1. SEM images of (A) OMC, (B) CeO2 –OMC; TEM images of (C) OMC and (D) CeO2 –OMC.
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Y. Liu et al. / Analytica Chimica Acta 819 (2014) 26–33
2.2. Apparatus Electrochemical measurements were performed on Epsilon electrochemical workstation (BAS, USA) with a modified GCE as the working electrode, a platinum wire as the auxiliary electrode and a saturated calomel electrode (SCE) as the reference electrode, respectively. The morphology of composites was characterized by Tescan Vega 3 SBH scanning electron microscope (SEM) and Tecnai G2 T2 S-TWIN transmission electron microscope (TEM). Xray diffraction (XRD) measurement was performed on a Rigaku D/max/2500v/pc (Japan) with Cu K␣ source. The 2 angles probed were from 10 ◦ to 80 ◦ at a rate 4◦ min−1 . The X-ray photoelectron spectra were obtained on a Shimadzu (Japan) Kratos AXIS Ultra DLD X-ray photoelectron spectrometer (XPS) with Mg K␣ anode (15 kV, 400 W) at a takeoff angle of 45◦ . Electrochemical impedance spectroscopy (EIS) experiments were performed on LK2010 (Lanlike Chemical Electronic High Technology Company, China) at a potential of 0.17 V in 5 mM K3 [Fe(CN)6 ] and 0.1 M KCl with the frequency range from 10 kHz to 0.1 Hz. 2.3. Procedures 2.3.1. Synthesis of CeO2 –OMC composites CeO2 –OMC composites were prepared according to literature [38] . Thirty milligrams OMC was dispersed in 20 mL pure water by 30 min ultra sonication to obtain black suspension. Then 15 mL 0.02 M (NH4 )2 Ce(NO3 )6 solution was added to OMC suspension under magnetic stirring. After several minutes, 1 wt% NaOH (aq) was added dropwise into the mixture to adjust pH value to 10.0. The mixture was separated by centrifugation and then mixed with 10 mL 5 M NaOH solution and transferred into a 50 mL stainless steel autoclave. After heated at 180 ◦ C for 45 h, the composites were separated by centrifugation, washed by pure water and ethanol for several times, then dried at 60 ◦ C for 12 h. The reactions involved are shown below:
obvious that the CeO2 nanoparticles were synthesized successfully on the surface of OMC. The X-ray photoelectron spectroscopy (XPS) was used to characterize CeO2 –OMC composites. In Fig. 2A, the peaks of Ce, C and O elements were observed in XPS spectra. As shown in Fig. 2B, the Ce 3d5/2 and Ce 3d3/2 peaks appeared at binding energies 882.7 eV and 901.3 eV, respectively [20]. It also demonstrated that the CeO2 nanoparticles were successfully synthesized on the surface of OMC. Fig. 3 shows the XRD patterns of the OMC (curve a) and CeO2 –OMC (curve b) composites. Compared to OMC, it could be clearly observed that eight peaks appeared in CeO2 –OMC composites. According to standard card (JPCDS 34-0394) eight peaks could be assigned to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1) and (4 2 0) crystal facets in fluorite structured CeO2 . It could confirm that the CeO2 particles were grown on the OMC. 3.2. Characterization of electrochemical behavior of CeO2 –OMC/GCE The electrochemical impedance spectroscopy (EIS) was employed to investigate the electron transfer capability of different modified electrodes using 5 mM K3 [Fe(CN)6 ] at the potential of 0.17 V. In the Nyquist diagrams, the diameter of the semicircle presents the interfacial electron transfer resistance and the linear portion corresponds to the diffusion process. As shown in Fig. 4,
Ce4+ + 4OH− −→ Ce(OH)4 Ce(OH)4 −→ CeO2 + 2H2 O 2.3.2. Fabrication of the modified electrode Prior to the modification, the glass carbon electrode (4 mm of diameter) was polished with alumina slurry and successively sonicated in 1:1 nitric acid, acetone and pure water for 20 min. Then the electrode was dried in air. Typically, 10 mg CeO2 –OMC composites were dispersed in 5 mL DMF under sonication for 30 min and 5 L suspension was dropped on the cleaned electrode surface. After dried under the infrared lamp, 5 L 0.1 wt% Nafion was coated on it and then dried to obtain CeO2 –OMC modified electrode. Single-component OMC or CeO2 modified electrodes were also prepared by the above method for comparison. 3. Results and discussions 3.1. Characterization of CeO2 –OMC composites The morphology of OMC and CeO2 –OMC were characterized by SEM (Fig 1A and B) and TEM (Fig 1C and D). From Fig. 1A, OMC exhibited well-defined morphology with small bundles structure. For comparison in Fig. 1B, the CeO2 –OMC exhibited the form of small rods with rough surface and some particles embedded on the wall of OMC but the structure of OMC was not changed. The TEM images indicated clearly that a number of nanoparticles (about 30 nm) were uniformly growing on the surface of OMC. It was
Fig. 2. XPS spectra of the obtained (A) CeO2 –OMC and (B) Ce 3d.
Y. Liu et al. / Analytica Chimica Acta 819 (2014) 26–33
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Fig. 3. XRD pattern of OMC (curve a) and CeO2 –OMC (curve b).
The OMC/GCE showed the smallest interfacial electron transfer resistance, indicating the fast electron transfer capability of OMC. And the CeO2 /GCE showed the largest interfacial electron transfer resistance, suggesting the poor conductivity of CeO2 . When CeO2 nanoparticles were well dispersed on the OMC, the interfacial electron transfer resistance of CeO2 –OMC/GCE obviously reduced compared to CeO2 /GCE and bare GCE, which illustrated that OMC not only provided large surface area to immobilize and disperse the CeO2 nanoparticles but also increased the electron transfer rate. 3.3. Electrochemical behavior of hydrazine on the CeO2 –OMC modified electrode Cyclic voltammetry was used to study the electrochemical behavior of hydrazine in 0.1 M phosphate buffer solutions (PBS, pH 8.0). As shown in Fig. 5, the voltammograms of different electrodes in the absence (curves A1, B1, C1 and D1) and presence
(curves A2, B2, C2 and D2) of 0.5 mM hydrazine were presented. From Fig. 5A, it is observed that hydrazine was oxidized on the bare GCE with only a low oxidation current and high overpotential, which indicated the kinetically sluggish electron transfer between hydrazine and GCE. While in the Fig. 5B, the oxidation current of hydrazine on the CeO2 /GCE was increased comparing to that on the GCE, which cloud be owing to the facile Ce3+ /Ce4+ redox cycle of CeO2 as catalyst [39]. However, the oxidation current of hydrazine was still not high enough due to the poor conductivity of CeO2 . In Fig. 5C, the oxidation current of hydrazine was enhanced on the OMC/GCE with the negative shift of peak potential (at about 0.4 V), which could be due to high surface area and large quantities of edge-plane-like defective sites of OMC [36]. On CeO2 –OMC/GCE (Fig. 5D), hydrazine was oxidized at lower potential (0.36 V) as well as a 2.3-fold increase in the peak current comparing with that on OMC/GCE, which might be attributed to the synergistic effect of the CeO2 –OMC composites. Comparing with previous reports, the presented electrode gave the more negative peak potential for
Fig. 4. Nyqusit plots of 5 mM K3 [Fe(CN)6 ] in 0.1 M KCl from 10 kHz to 0.1 Hz for (a) OMC/GCE; (b) CeO2 –OMC/GCE; (c) GCE; (d) CeO2 /GCE.
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Fig. 5. CVs of (A) GCE, (B) CeO2 /GCE, (C) OMC/GCE and (D) CeO2 –OMC/GCE in the absence (curves A1, B1, C1, D1) and the presence (curves A2, B2, C2, D2) of 0.5 mM hydrazine in 0.1 M pH = 8.0 PBS. Scan rate: 100 mV s−1 .
the oxidation of hydrazine (0.517 V on ZnO/MWCNTs/GCE [37] and 0.5 V on CeO2 /Au/GCE [39]). All the electrochemical responses of electrodes were irreversible, as no reduction current was observed during the reverse scan. According to the previous reports [34,46], the electrochemical oxidation of hydrazine was proposed to be as follows: N2 H4 + 4OH− −→ N2 + 4H2 O + 4e
3.4. Optimization of the concentration of the (NH4 )2 Ce(NO3 )6 During the hydrothermal process, different concentration of (NH4 )2 Ce(NO3 )6 influenced the amount of CeO2 nanoparticles grown on the OMC, which could affect the electrocatalytic activity of CeO2 –OMC composites to oxidation of hydrazine. Excess CeO2 embedded in OMC could reduce the conductivity of the composites and insufficient CeO2 embedded in OMC caused an insufficient catalytic activity of OMC–CeO2 composites. Thus it was important to choose an appropriate amount of CeO2 in OMC. From Fig. 6, the oxidation peak current of 0.1 mM hydrazine increased with the (NH4 )2 Ce(NO3 )6 concentration from 0 to 0.3 mM. The oxidation peak current decreased when the concentration of (NH4 )2 Ce(NO3 )6 was higher than 0.3 mM. As a result, 0.3 mM (NH4 )2 Ce(NO3 )6 was chosen as the optimum concentration for synthesis of CeO2 –OMC composites. 3.5. Effect of pH value According to previous work [34,39], the electrochemical reaction of hydrazine was dependent on pH value of the buffer solution.
Therefore, different buffers with pH values ranging from 6.0 to 10.0 were tested to study the electrochemical behavior of hydrazine by cyclic voltammetry. The oxidation peak potential of hydrazine on CeO2 –OMC/GCE was shifted negatively with the increased pH values of PBS. As shown in Fig. 7, the peak current increased from pH 6.0 to pH 8.0 and decreased at pH values above 8.0. The peak current changed with different pH values could be due to the form of hydrazine (the pKa of hydrazine was 8.1). When the pH value of PBS was below the pKa , the hydrazine exited as the protonated form and it was repulsed from the electrode surface, which led to a decrease of oxidation peak current. When the pH value was above 8.0, the change of current might result from the deprotonation form of hydrazine [34]. In conclusion, pH = 8.0 PBS was chosen as the supporting electrolyte in the following experiments. 3.6. Amperometric determination of hydrazine The current–time curve was utilized for quantitative analysis of hydrazine. The different operating potentials were tested for the determination of 30 M hydrazine. As a result, the current response of hydrazine was increased slowly from 0.2 V to 0.6 V. At last, 0.4 V was chosen as the working potential to achieve a good sensitivity. Simultaneously, the low working potential could avoid the interference of some phenol compounds (often co-exist in waste water). As shown in Fig. 8A, current–time curves illustrated the responses of CeO2 –OMC/GCE by successive additions of different concentrations of hydrazine under stirring in 0.1 M PBS (pH 8.0). The oxidation currents increased with each addition of hydrazine and reached a steady-state soon (
