Materials Science and Engineering C 34 (2014) 304–310

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Electrochemical sensor for detection of hydrazine based on Au@Pd core–shell nanoparticles supported on amino-functionalized TiO2 nanotubes Xianlan Chen a,b,c, Wei Liu a, Lele Tang b, Jian Wang a, Haibo Pan b,c,⁎, Min Du c a b c

School of Science, Honghe University, Mengzi, Yunnan 661100, China College of Chemistry and Chemical Engineering, Qishan Campus, Fuzhou University, Fuzhou, Fujian 350108, China Fujian Key Lab of Medical Instrument & Pharmaceutical Technology, Yishan Campus, Fuzhou University, Fuzhou, Fujian 350002, China

a r t i c l e

i n f o

Article history: Received 4 April 2013 Received in revised form 29 August 2013 Accepted 18 September 2013 Available online 27 September 2013 Keywords: Hybrid nanostructures Amino-functionalized TiO2 nanotubes Flower-shaped Au@Pd nanostructures Electrocatalytic activity Hydrazine

a b s t r a c t In this paper, we reported a simple strategy for synthesizing well-defined TiO2NTs–Au@Pd hybrid nanostructures with prior TiO2 nanotube functionalization (F-TiO2NTs). TiO2NTs with larger surface area (BET surface area is 184.9 m2 g−1) were synthesized by hydrothermal method, and the NTs are anatase phase with a range of 2–3 μm in length and 30–50 nm in diameter after calcined at 400 °C for 3 h. 3-Aminopropyl-trimethoxysilane (APTMS) as a coupling agent was reacted with the surface hydroxyl groups as anchoring sites for flowershaped bimetallic Au@Pd nanostructures, self-assembling amine functionality on the surface of TiO2NTs. Note that two faces at the interface between F-TiO2NTs with (004) plane and Au@Pd nanostructures with (111) one of cubic Au and Pd nanoparticles are compatible, benefiting to the charge transfer between two components due to their crystalline coordination. The results showed that as-prepared F-TiO2NTs–Au@Pd hybrid nanostructures modified glassy carbon electrode (GCE) exhibits high electrocatalytic activity toward hydrazine (N2H4) at low potential and a linear response from 0.06 to 700 μM with the detection limit for N2H4 was found to be 1.2 × 10−8 M (S/N = 3). Based on scan rate effect during the hydrazine oxidation, it indicates that the number of electrons transferred in the rate-limiting step is 1, and a transfer coefficient (α) is estimated as 0.73. The selfassembled F-TiO2NTs–Au@Pd hybrid nanostructures as enhanced materials present excellent electrocatalytic activity, fast response, highly sensitive and have a promising application potential in nonenzymatic sensing. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Metal nanoparticles (MNPs) as catalysts have been vigorously investigated because of their specific properties and enormous potential applications as nanoelectronic devices [1], sensors [2], and surfaceenhanced Raman scattering [3]. Nevertheless, with the high surface area to volume ratios and surface energies, MNPs tend to lose reactivity as they precipitate or aggregate. More importantly, the performance of electrochemical sensor mainly depends on the morphology properties and surface area of MNPs on the modified electrodes. In order to retain its high electro-activity, the design of stable hybrid nanoparticles becomes one of the primary challenges for their applications [1]. Stabilization agents and/or substrates were used for MNP synthesis, including carboxylic acid [4], sodium citrate [5], carbon nanotubes (CNTs) [6], α-Fe2O3 [7], poly(vinyl-alcohol) polymer [8], ZrO2 [9], SiO2 [10] and TiO2 [11]. Among them, immobilization of the noble metal nanoparticles on an active substrate to form hybrid nanostructures may enhance the overall reactivity of the catalytic metal centers. As reported by Y.-C. Liu ⁎ Corresponding author at: College of Chemistry and Chemical Engineering, Qishan Campus, Fuzhou University, Fuzhou, Fujian 350108, China. Tel./fax: +86 591 22866127. E-mail address: [email protected] (H. Pan). 0928-4931/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2013.09.016

et al. [12], surface-enhanced Raman scattering (SERS) inactive SiO2 nanoparticles (NPs) are incorporated into the deposited SERS active Au NPs on substrates to synthesize effectively SERS active Au/SiO2 nanocomposites by using sonoelectrochemical pulse deposition, SERS Au/SiO2 nanocomposites with an enhancement factor of 5.4 × 108. Recently, hybrid nanostructures have been considered as another good catalyst candidate, Because of their excellent adsorption and charge transfer ability [13], the hybrid nanostructures provide the possibility of enhanced functionality and multifunctional properties compared with those of their single-component counterparts [14]. Relevant research has demonstrated that CdS or CdSe modified TiO2 nanocrystal hybrid nanostructures can improve photocatalytic activity of semiconductor [15,16]. CNTs have often been viewed as an excellent support for MNPs, forming hybrid nanostructures [17]. Among various substrates, TiO2 nanotubes (TiO2NTs), with high specific surface area, ion-changeable ability, biocompatible and photocatalytic ability, are more promising because of their regular structure, high active surface area, good chemical as well as thermal stability for immobilization of MNPs [18]. Moreover, it has been reported that the average apparent heterogeneous electron transfer rate constant (k) was 2.18 × 10− 3 cm s− 1 for TiO2NTs modified electrode. This result can be matched with the electron transfer rate constant of CNTs

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(7.53 × 10− 4 cm s− 1) [19,20]. Particularly, the immobilization of small MNPs on high-surface-area TiO2NTs improves the electrocatalytic activity [21]. Self-assembled monolayers (SAMs) are a class of molecular assemblies that are prepared by spontaneous adsorption of molecules from solution onto a solid substrate in order to further address the nanotechnology applications [22]. 3-Aminopropyltri-methoxysilane (APTMS) is one of the most useful molecules for the preparation of aminoterminated SAMs due to its methoxy groups, which play an important role in binding to the sample surface [23]. And also, APTMS is a kind of coupling agent that offers the advantage of chemical bonding of functional silane groups onto the surface of hydrophilic inorganic oxides. Most importantly, –NH2 groups in APTMS acted as a linker, which provides hydrogen bonding and metal–ligand bonding interaction with solid substrate or MNPs [24]. In the present work, APTMS react with the hydroxyl groups for self-assembling amine functionality on the surface of TiO2NTs. Au–Pd alloy nanoparticles have also been found to be an outstanding catalyst for ethanol oxidation, improving performance of direct alcohol fuel cells (DAFCs) [25]. And also, nonspherical bimetallic clusters not only increase the surface area, but easily form charge accumulation or valence change on prominent part of the surface, which can effectually improve the surface catalytic effect. In our previous work [26] has prepared the flower-shaped (FS) Au@Pd core–shell nanoparticles with a gold core diameter of ~20 nm and a palladium shell average thickness of ~3 nm by seed-mediated growth method. XPS analysis and Zeta potential indicated that the surface Pd atoms are positively charged, and HRTEM image demonstrates tens of small Pd nanoparticles aggregated on gold seeds, which increase the surface area of the formed three-dimensional FS nanoparticles, profitting the oxidation process of glucose. Further, a novel kind of MNPs, flower-shaped (FS) Au@Pd nanoparticles in this work, anchors onto the surface of TiO2NTs. In this work, major applications used as hydrazine electrochemical detectors can be derived from the F-TiO2NTs–Au@Pd nanocrystal hybrid nanostructures as enhanced materials. Hydrazine (N2H4), a liquid colorless compound, is now widely used in many industrial fields, such as antioxidant, corrosion inhibitor and catalyst in fuel for aircrafts and satellites [27]. However, it is still a neurotoxin and hence produces carcinogenic and mutagenic effects, causing damage to the lungs, liver and kidneys. The maximum recommended level of hydrazine in trade effluents is 1 × 10−6 M [28]. Therefore, it is highly desirable to fabricate a reliable, sensitive, economical and accurate online monitoring device for the effective detection of hydrazine. Recently, various noble nanocomposites, e.g. Pd/MWCNT [29], PEG-coated ZnS nanoparticles [30] and ZrHCF/Au-Pt inorganic–organic hybrid nanocomposite [31] have been used for the electrocatalytic oxidation of hydrazine with high sensitivity and good selectivity. To the best of our knowledge, there are no reports on the exploration of FS Au@Pd hybrid nanoparticles supported on TiO2NTs as enhanced materials for detecting hydrazine.

2. Experimental 2.1. Reagents and apparatus K2PdCl4 and HAuCl4 · 4H2O were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). APTMS and NaBH4 were obtained from J&K Chemical Co. Ltd. Hydrazine is the product of Guangzhou Chemical Reagent Company (Guangzhou, China), and its fresh solutions were prepared daily. TiO2 nanoparticles (Degussa P25, Germany) were 70% in anatase phase and 30% in rutile phase, and their primary particle diameters were in the range of 30–50 nm. A 0.1 M phosphate buffer solution (PBS) consisting of KH2PO4 and Na2HPO4 was employed as the supporting electrolyte. All other chemicals used were of analytical reagent grade. Ultra-pure water was obtained with a Milli-Q plus water purification system (Millipore Co. Ltd., USA) (18 MΩ).

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Cyclic voltammetric (CV) and chronoamperometric experiments were performed with a CHI 660B electrochemical workstation (CH Instrument Company, Shanghai, China). A conventional threeelectrode system was adopted. The working electrode was a modified glassy carbon electrode (GCE), a Pt wire and an Ag/AgCl electrode were used as the auxiliary and reference electrodes, respectively. All potentials were versus Ag/AgCl, and all experiments were carried out at room temperature. Zeta (ξ)-potential value measurement was performed on Zeta Sizer 3000 Laser Particle Size and Zeta Potential Tester (Malvern Corporation, UK) with the FS Au@Pd nanoparticles in aqueous solution. High-resolution transmission electron microscope (HRTEM) image was obtained using Tecnai G2 F20 S-TWIN, 200 kV (FEI Company, USA). Crystalline structures of TiO2NTs and F-TiO2NTs– Au@Pd nanocrystal hybrid nanostructures were analyzed by powder X-ray diffraction (XRD, X'pert, Philips, Holland). Nitrogen adsorption– desorption isotherms were obtained using Autosorb-1 (Quantachrome Instruments, USA), and specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method using adsorption isotherms. 2.2. Preparation of FS Au@Pd core–shell nanoparticles and TiO2 NTs FS Au@Pd core–shell nanoparticles were prepared according to seed-mediated growth method [26]. TiO2NTs were prepared by hydrothermal method [32]. Briefly, 0.4 g TiO2 nanoparticles (P25) were added into 40 mL NaOH solution with a concentration of 10 M. Then, the mixture was heated to 140 °C while stirring during 24 h in oil bath. After filtration, the white product was dispersed in 150 mL 0.1 M HCl by ultrasoundmachine, and then was treated with distilled water until pH value lower than 7. Subsequently, the products were dried at 80 °C and calcined at 400 °C for 3 h to remove any impurity on the TiO2NT surface. Finally, the resulted TiO2NTs with well crystallinity were obtained. 2.3. Preparation of F-TiO2NTs–Au@Pd hybrid nanostructures The assembly of oppositely charged interesting species has been widely used in fabricating multifunctional hybrid nanomaterials. Herein, we present an alternative strategy (SAMs) for surface functionalization of TiO2NTs by APTMS to prepare F-TiO2NTs, which is negatively charged in alkaline conditions as a result of the hydrolyzation of APTMS and offers versatile solid supports for FS Au@Pd nanostructures. The first step was to prepare the surface-functionalized TiO2 nanotubes. Briefly, 0.05 g TiO2NTs and 20 mL ethanol were placed in a beaker equipped with magnetic stirring bars, and 200 μL of APTMS was added to the above solution, followed by the addition of 2 mL of water and ammonia, respectively. The methoxy groups on the APTMS underwent hydrolysis and condensation reactions with the hydroxyl groups on the TiO2NT surface to yield the amine-functionalized TiO2NTs (F-TiO2NTs). After allowing the APTMS to react for 12 h, the suspensions were centrifuged and rinsed 4 times by ethanol and water to remove excess reactants. Finally, the white precipitation was dispersed in 10 mL of water and set aside. In the second step, FS Au@Pd particles were attached onto F-TiO2NTs. Briefly, F-TiO2NTs–Au@Pd hybrid structures were synthesized in a 5 mL vial by adding excess FS Au@Pd core–shell nanoparticles to 0.5 mL of F-TiO2NTs. The mixture was sonicated for 30 min, then centrifuged and washed 2–3 times with distilled water. Finally, the purified F-TiO2NTs–Au@Pd hybrid nanostructures were dispersed in 2 mL water, which formed a gray and homogeneous suspension that could stay stable for at least 1 month. 2.4. Preparation of the electrochemical sensor Prior to preparation of electrochemical sensor, a glassy carbon electrode (GCE) was polished to mirror smooth with 0.3 and 0.05 μm slurry, and then rinsed with water, ultrasonicated in water and ethanol

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bath. The bare electrode was dried under pure N2. Then 10 μL of the F-TiO2NTs–Au@Pd hybrid nanostructures was dropped on the GCE and dried under an infrared lamp. Thus, a uniform film modified electrode (F-TiO2NTs–Au@Pd/GCE) was obtained. The other modified electrodes, i.e., F-TiO2NTs/GCE and FS Au@Pd/GCE, as compared were fabricated through similar method. 3. Results and discussion 3.1. Physicochemical characterization The F-TiO2NTs–Au@Pd hybrid structures were synthesized by using F-TiO2NTs as catalyst support, and FS Au@Pd core–shell nanoparticles with the positive ξ-potential (+51.8 mV) were assembled on the negatively charged F-TiO2NTs (− 17.6 mV) mainly via the electrostatic interaction. The whole preparation strategy for constructing F-TiO2NTs–Au@Pd composite nanostructures is shown in Scheme 1. TiO2NTs with abundant hydroxyl groups on the surface were obtained via hydrothermal method. The methoxy groups of APTMS react with the native hydroxyl groups for modifying TiO2NTs surface to form F-TiO2NTs. At the end, methanol molecules are produced as byproducts [33]. The chemisorption of the APTMS molecule on the hydroxylated surface can be written as the following schematic reaction:

ð1Þ (1)

Obviously, APTMS played a crucial role in the formation of F-TiO2NTs, and –NH2 group acts as anchoring sites for flower-shaped bimetallic Au@Pd nanostructures. The structure and morphology of TiO2NTs and F-TiO2NTs–Au@Pd hybrid structures were characterized using HRTEM. Open-ended TiO2NTs with a range of 2–3 μm in length and 30–50 nm in diameter were obtained in Fig. 1. Fig. 2a and b represents typical HRTEM images of the as-prepared hybrid nanocatalysts at different magnifications. Compared with TiO2NTs (Fig. 1), the F-TiO2NTs–Au@Pd hybrid nanostructures have a rougher surface, indicating that FS Au@Pd

nanoparticles were readily adsorbed on the surface of TiO2NTs. To reveal the detailed structure of F-TiO2NTs–Au@Pd hybrid nanomaterials, the corresponding augmentation image is shown in Fig. 2b, where Au@Pd nanoparticles have an average diameter of 25 nm and high surface area, indicating the feasibility of our methods. The dependence of Au@ Pd nanoparticle particle size on the nature of the support is attributed to differences in metal/support interaction [34]. The higher Fermi level of anatase TiO2NTs can result in greater electronic interaction with Au@Pd particles, inhibiting Au@Pd agglomeration and leading to reduce the growth of Au@Pd particles. Furthermore, Au@Pd nanoparticles still keep their flower-shaped core–shell structure (inset of Fig. 2(b)) on the surface of TiO2NTs, which provide more selective docking points to electrocatalyze hydrazine in electrochemical measurements. According to the lattice planes of the hybrid nanostructures from HRTEM image (the inset of Fig. 2(c)), the lattice spacing of the inner core (Au) (0.235 nm) is similar as one of external flower layer (Pd) (0.225 nm), corresponding to the mean value of the (111) planes of face-centered cubic (fcc) Au and Pd, respectively, in FS Au@Pd core–shell nanoparticles. The lattice spacing of 0.351 nm is assigned to (101) plane of anatase TiO2NTs. On the other hand, the inset of Fig. 2(c) image shows that the (004) planes of TiO2NTs and (111) planes of Au@Pd MPs are parallel to each other, suggesting the coherent interfaces between two components. Moreover, the lattice spacing can be indexed to the (111) plane of the cubic Au@Pd MPs structure and matches well with the XRD pattern of the (004) plane of TiO2NTs (Fig. 3). It is possible to derive the heteroepitaxial deposition relationship between Au@Pd MPs and TiO2NTs. The lattices are indexed to be TiO2NTs (004) with a spacing of 0.237 nm and Au@Pd MPs (111) with a lattice spacing of 0.225 nm, and their mismatch is about 5.1%. In addition, it can even be identified that there are the stacking faults at the interface from careful observation in the inset of Fig. 2(c). From the electron diffraction patterns of F-TiO2NTs–Au@Pd hybrid nanomaterials (Fig. 2d), the nanodiffraction patterns of individual hybrid nanomaterial were obtained to identify the crystalline system, lattice parameters and 3D symmetry of atomic arrangement by concentrating the beam. The observed diffraction rings can be assigned to the (101), (004) and (200) diffractions of TiO2NTs, as well as (111), (200) and (220) diffractions with a face-centered cubic (fcc) structure of metal particles (MPs). Moreover, the distance of (004) face of TiO2NTs

Scheme 1. A proposed model for the formation of nanocomposites.

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Fig. 1. HRTEM image of TiO2 nanotubes. Insert: an augmentation of open-ended TiO2NTs.

is close to that of (111) face of cubic Au and Pd nanoparticles (the inset of Fig. 2(c)), indicating that the two faces at the interface are compatible and their diffraction rings appear partially overlapped as shown in Fig. 2d. Due to the bigger lattice spacing of TiO2NTs (004), the inside diffraction ring is assigned to the (004) face of TiO2NTs. To further confirm the existence of Au, Pd and TiO2NTs in the hybrid nanomaterials, an XRD experiment was employed for the structure analysis of hybrid nanostructures (Fig. 3). XRD results illustrate that after annealing at 400 °C for 3 h, all as-prepared TiO2 samples are converted to the anatase phases and the crystallinity is generally well. According to the standard JCPDs card (No. 21–1272), the diffraction peaks at 25.08°, 37.46° and 48.02° of 2θ are attributed to the (101), (004) and (200) planes of TiO2NTs, respectively. The corresponding XRD pattern of F-TiO2NTs–Au@Pd hybrid nanomaterials (line b) indicates that the hybrid nanostructures are composed of the elements Au, Pd and TiO2NTs, the resulting diffraction peaks located at 2θ values of 37.5°, 44.3°, 64.8° and 77.8° are attributed to the (111), (200), (220) and (311) planes of Au@Pd core–shell nanoparticles, respectively [35]. 3.2. Direct electrocatalysis and amperometric detection of hydrazine Fig. 4 shows CVs of bare GCE (line a), GCEs modified by F-TiO2NTs (line b), FS Au@Pd MPs (line c) and F-TiO2NTs-FS Au@Pd nanocomposites (lines d, e), in the absence (line e) and presence of 1 mM N2H4 (lines a, b, c, d) in 0.1 M PBS (pH 7.0). It can be observed that in contrast to the F-TiO2NTs-FS Au@Pd nanocomposites modified electrode without hydrazine (line e), a well-defined oxidation peak with 0.5 mM N2H4 in 0.1 M PBS(d) is observed with a peak potential of 0.025 V and a maximum oxidation current of ∼60 μA. As shown by the line c, the electrochemical oxidation of N2H4 at the Au@Pd MPs modified GCE shows a maximum current of ∼45 μA at around 0.02 V. It demonstrates that the oxidation of N2H4 on the FS Au@Pd MPs and F-TiO2NTs-FS Au@Pd nanocomposites shows low overpotential and high catalytic current, whereas there is no oxidation behavior observed on electrode modified with the F-TiO2NTs in the potential window from −0.4 to 0.4 V. The results indicate that the F-TiO2NTs are only used as a support and can’t catalyze N2H4 oxidation. While the present FS Au@Pd nanocomposites modified electrode enhances the conductivity of the F-TiO2NTs, the electron transfer within modified materials is more ready according to the

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comparability of both two faces at the interface, (004) plane of TiO2NTs and (111) one of cubic Au and Pd nanoparticles as shown above. Furthermore, it exhibits high electrocatalytic activity toward hydrazine oxidation. Based on these results, the following catalytic Scheme S1 describes the voltammetric response of electrochemical oxidation of hydrazine at the F-TiO2NTs-FS Au@Pd nanocomposites modified GCE. The electrochemical behavior of hydrazine is dependent on the pH value of an aqueous solution, so pH should be optimized for their catalytic determination. Certainly, the catalytic peak current (Ipa) of hydrazine oxidation at F-TiO2NTs-FS Au@Pd/GCE is up to a maximum at pH 7.0, and oxidation current reduces quickly with decreasing or increasing pH, as shown in Fig. S1(A). It is also found that the peak potential shifts positively with the decrease of pH, or shifts negatively with the increase of pH, indicating that higher oxidation overpotential is at pH 7.0. So pH 7.0 PBS is chosen for N2H4 determination. Fig. S1(B) shows the CVs of F-TiO2NTs-FS Au@Pd/GCE in the presence of N2H4 with different concentrations. The oxidation currents gradually increase and the peak potentials shift to more positive potentials with increasing N2H4 concentration. In the range 0.1 to 1.7 mM, the oxidation peak current (Ipa) represents a good linear relationship with hydrazine concentration (Fig. S1(C)). Scan rate (v) can influence the current responses of hydrazine, and corresponding electrochemical parameters could be deduced from the relationship between scan rate of potential sweep and current responses of hydrazine [28]. The dependence of oxidation peak current of 0.5 mM hydrazine on different scan rates at the F-TiO2NTs-FS Au@Pd/ GCE in 0.1 M phosphate buffer (pH 7.0) is illustrated in Fig. 5A. As the scan rate increases, the oxidation peak current (Ipa) increases linearly with the square root of the scan rate in the range of 20–180 mV s−1 (Fig. 5B), and the regression equation is Ipa = 6.030 + 0.022 × v1/2 with R2 = 0.996, suggesting that the oxidation process is controlled by diffusion. In addition, with increasing scan rate, the catalytic oxidation peak potential (Epa) shifts to more positive values with a linear correlation between the peak potential and the logarithm of scan rate, log(v), as illustrated in Fig. 5C. The Tafel slope b can be obtained from the linear relationship of Epa vs. log(v) by using the following equation [28]: Epa ¼ ðb=2Þ logðvÞ þ constant

ð2Þ

On the basis of Eq. (2), the slope of Epa vs. log(v) plot is b/2, where b indicates the Tafel slope. The plot of Epa vs. log(v) indicates a linear variation for scan rates ranging 20–180 mV s−1. The slope is Epa/log(v), which is found to be 0.111 V in this work, thus b = 2 × 0.111 = 0.222 V. The Tafel slope is b = 0.059/(1−α)na (α, the transfer coefficient; na, the number of electrons involved in the rate-determining), assuming the number of electrons is 1, and the transfer coefficient (α) is estimated as 0.73. If we assumed na = 2, α would then be equal to 0.87 beyond ranges for most electrode processes between 0.75 and 0.25 [36]. Therefore, it exhibits that one-electron transfer process is the rate-limiting step. Fig. S2 presents the Nyquist diagrams of the F-TiO2NTs-FS Au@Pd/ GCE at 0.0 V both in the absence and presence of N2H4 (0, 0.5, 2, 4, 8 and 16 mM) in 0.1 M 20 mL PBS (pH 7.0) solution. A depressed semicircle at high frequency in the absence of hydrazine and a steady decrease of the diameter of the semi-circle with the increase of hydrazine concentration in the presence of hydrazine were witnessed. This is due to the direct electro-oxidation of hydrazine taking place on the active sites of the F-TiO2NTs-FS Au@Pd hybrid nanostructures surface. According to the CV response with various N2H4 concentrations (Fig. S1(B)), the oxidation rate of hydrazine at F-TiO2NTs-FS Au@Pd/ GCE increases with the N2H4 concentration. It leads to the reduction of the charge transfer resistance, depending on the hydrazine concentration in the solution because the potential 0.0 V applied in the Nyquist diagrams is in the activation region of hydrazine oxidation. To interpret the electrochemical impedance results, an equivalent electric circuit is usually applied to fit the experimental impedance spectra. The inset

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a

b

shell

(Pd)

core (Au)

200 nm 20 nm

c

10 nm

d 0. 351 nm Mps (220) Mps(220)

0.225 nm 0.235 nm

NTs(200) NTs(200)

Mps (200) Mps(200)

Mps(111) Mps(111)

NTs(101) NTs(101)

NTs(004) NTs(004)

0. 351 nm

10 nm

5 1/nm

Fig. 2. (a) HRTEM images of the F-TiO2NTs-FS Au@Pd nanocomposites. (b) An augmentation of F-TiO2NTs-FS Au@Pd nanocomposites. Inset: an image of FS Au@Pd core–shell nanoparticles. (c) High-resolution TEM image displaying the lattice fringes of the F-TiO2NTs-FS Au@Pd nanocomposites. Insert: an augmentation of the interface of the F-TiO2NTs-FS Au@Pd nanocomposites. (d) Diffraction patterns of F-TiO2NTs-FS Au@Pd nanocomposites for different directions.

of the proposed modified electrode were compared with those previously reported in Table S1. As seen, the analytical parameters are comparable or better than the results reported for hydrazine determination at the surface of other modified electrodes [29,31,38–42].

Au@ Pd (111) (200)

Intensity/ a.u.

of Fig. S2 presents the equivalent circuit compatible with the corresponding Nyquist diagrams, where Rct, R and Q represent charge transfer resistance, solution resistance and constant phase element (CPE), respectively. The CPE is defined by Y0 and n. If n = 1, then the Y0 is considered a capacitor [37]. The parallel combination (Rct and Q) results in a depressed semi-circle at the Nyquist impedance plot. A descent of Rct develops with the increase of hydrazine concentration, corresponding to the analysis for the oxidation rate of hydrazine shown in CV profiles of Fig. S3. These results confirm that the F-TiO2NTs-FS Au@Pd/GCE exhibits significantly high catalytic activity towards the hydrazine oxidation. Amperometric methods are often used for the real-time and online monitor of many environmental pollutants. Fig. 6 illustrates the current-time plots for the present F-TiO2NTs-FS Au@Pd/GCE under optimized conditions (applied potential: 0.0 V) on successive addition of hydrazine. Once hydrazine was injected into the PBS blank, the modified electrode rapidly responds to change with the N2H4 concentration. The linear relationship between the catalytic current and N2H4 concentration is shown in the inset of Fig. 6. The F-TiO2NTs-FS Au@Pd/GCE based hydrazine chemical sensor shows high and reproducible sensitivity of 0.013 μA μM− 1 and a good linear response ranging from 0.06 to 700 μM, the linear progress equation is Ip = − 1.481 + 0.013 × c, with R2 = 0.997, a detection limit of 1.2 × 10− 8 M (S/N = 3) and a short response time (≤ 3 s). Detection limit, response time, sensitivity and linear calibration range

(220)

(311)

b TiO 2N Ts (004)

a

20

40

60

80

2 Theta/degree Fig. 3. XRD patterns of TiO2NTs (a) and F-TiO2NTs-FS Au@Pd nanocomposites (b).

X. Chen et al. / Materials Science and Engineering C 34 (2014) 304–310

70

a

60

100 80

50 40

d c b a e

30 20 10 0

Current / µA

Current / µA

309

60 40 20 0

-10 -0.4

-0.2

0.0

0.2

-20 -0.4

0.4

-0.2

0.0

Potential / V Fig. 4. CVs of hydrazine oxidation at the bare GCE (a) and GCEs modified by F-TiO2NTs (b), FS Au@Pd (c) and F-TiO2NTs-FS Au@Pd nanocomposites (d, e) in a 0.1 M PBS solution in the absence (e) and presence (a, b, c, d) of 0.5 mM hydrazine. The scan rate was 50 mV s−1.

100 90

I pa = 6.030 + 0.022×v1/2

80

R =0.996

2

Ipa / µA

70 60 50 40 20

40

60

80

100

v1/2

/

120

140

160

180

200

(Vs-1) 1/2

c 0.14 0.12

4. Conclusions

0.10

Epa / V

A novel hybrid nanostructure with enhanced electrocatalytic functions was prepared by self-assembled monolayers with sonication technique. It is found that as-prepared F-TiO2NTs is negatively charged in alkaline conditions as a result of the hydrolyzation of APTMS and offers versatile solid supports for new bifunctional nanocatalysts, FS Au@Pd core–shell nanoparticles with opposite charge. The resulting F-TiO2NTs–Au@Pd hybrid nanostructures were characterized by various methods and used as a promising catalyst in electrochemical sensors because of its enhanced electrocatalytic properties. It is interesting to note that the (004) face of TiO2NTs (0.237 nm) is similar to the (111) face of cubic Au@Pd nanoparticles at the interface between TiO2NT and metal nanoparticles, increasing the charge transfer between these two components. Electrochemical experiments indicate that the electrode modified with F-TiO2NTs–Au@Pd hybrid nanostructures shows prominent electrocatalytic activity toward hydrazine at low potential. The detection limit for hydrazine with a signal-to-noise ratio of 3 is found to be 1.2 × 10−8 M. The number of electrons transferred in the rate-limiting step toward the oxidation of hydrazine is 1, and a transfer coefficient (α) is estimated as 0.73. Based on these electrochemical behaviors of F-TiO2NTs–Au@Pd hybrid nanostructures, it is expected that the above hybrid materials will probably find applications in other important fields such as electronics, biomedicine and photocatalyst.

0.4

b

3.3. Interference tests by F-TiO2NTs-FS Au@Pd/GCE The influence of some possibly coexisting foreign substances including 2− 3+ − , Zn2+, HPO2− inorganic ions (K+, Ca2+, Na+, F−, CO2− 3 , SO4 , Fe 4 , Cl ) and an organic compound (2-mercaptobenzoxazole) presents in practical application. Moreover, nitrogen-containing compounds such as nitrate, nitrite and ammonia, often accompany with hydrazine in a variety of industrial processes. Possible interferences for the determination of hydrazine at the F-TiO2NTs-FS Au@Pd/GCE were investigated by adding various foreign species into the solution. The results showed that common ions had no interference on hydrazine determination, such as 300-fold quan2− 2+ − , Fe3+, Zn2+, HPO2− tities of K+, Ca2+, Na+, F−, CO2− 3 , SO4 , Cu 4 , Cl , 100-fold quantities of fructose, glucose, sucrose, 10-fold quantities of 2+ and 2-mercaptobenzoxazole. NH+ 4 , and 5-fold quantities of Cu

0.2

Potential / V

0.08

Epa = -0.122+0.111×log (v)

0.06

R2 =0.991

0.04 0.02 0.00 1.2

1.4

1.6

1.8

2.0

2.2

2.4

log (v) Fig. 5. (a) CVs of hydrazine oxidation at the F-TiO2NTs-FS Au@Pd nanocomposites modified GCE in 0.1 M 20 mL PBS (pH 7.0) containing 0.5 mM hydrazine at different scan rates (from bottom to top: 20, 40, 60, 80, 100, 120, 140, 160 and 180 mV s−1). (b) The relationship between the oxidation peak current (Ipa) and the scan rate (mV s−1). (c) The relationship between the oxidation peak potential (Epa) and log (v).

the National Science Foundation of China (NSFC) (21201035, 61201397), and Fujian Department of Science and Technology (2012J01204).

Acknowledgments

Appendix A. Supplementary data

The authors gratefully acknowledge the financial support from the Project for International S & T Cooperation of China (2012DFM30040),

Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.msec.2013.09.016.

310

X. Chen et al. / Materials Science and Engineering C 34 (2014) 304–310

8

8

Ip = -1.481+0.013c

Current / µA

6

Current / µA

6

4

R2 = 0.997

4 2 0 -2 0

2

100 200 300 400 500 600 700

c / uM 0

-2 0

150

300

450

600

750

900

Time / s Fig. 6. Typical amperometric current response of the F-TiO2NTs-FS Au@Pd nanocomposites modified GCE on successive injection of hydrazine into stirring N2-saturated PBS (pH 7.0) at an applied potential of 0.0 V. The inset is calibration curve for hydrazine concentration from 6 × 10−8 to 7 × 10−4 M.

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