Ruthenium Nanoparticles Decorated Tungsten Oxide as a Bifunctional

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Ruthenium Nanoparticles Decorated Tungsten Oxide as a Bifunctional Catalyst for Electrocatalytic and Catalytic Applications Chellakannu Rajkumar,†,⊥ Balamurugan Thirumalraj,†,‡,⊥ Shen-Ming Chen,*,† Pitchaimani Veerakumar,*,§,¶ and Shang-Bin Liu§ †

Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan § Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan ¶ Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan ‡

S Supporting Information *

ABSTRACT: The syntheses of highly stable ruthenium nanoparticles supported on tungsten oxides (Ru-WO3) bifunctional nanocomposites by means of a facial microwaveassisted route are reported. The physicochemical properties of these Ru-WO3 catalysts with varied Ru contents were characterized by a variety of analytical and spectroscopic methods such as XRD, SEM/TEM, EDX, XPS, N 2 physisorption, TGA, UV−vis, and FT-IR. The Ru-WO3 nanocomposite catalysts so prepared were utilized for electrocatalytic of hydrazine (N2H4) and catalytic oxidation of diphenyl sulfide (DPS). The Ru-WO3-modified electrodes were found to show extraordinary electrochemical performances for sensitive and selective detection of N2H4 with a desirable wide linear range of 0.7−709.2 μM and a detection limit and sensitivity of 0.3625 μM and 4.357 μA μM−1 cm−2, respectively, surpassing other modified electrodes. The modified GCEs were also found to have desirable selectivity, stability, and reproducibility as N2H4 sensors, even for analyses of real samples. This is ascribed to the well-dispersed metallic Ru NPs on the WO3 support, as revealed by UV−vis and photoluminescence studies. Moreover, these Ru-WO3 bifunctional catalysts were also found to exhibit excellent catalytic activities for oxidation of DPS in the presence of H2O2 oxidant with desirable sulfoxide yields. KEYWORDS: catalytic oxidation, diphenyl sulfide, electrochemical sensor, hydrazine, Ru nanoparticles, tungstate oxide

1. INTRODUCTION Tungstate-based nanostructured materials have received considerable research and development (R&D) attention recently due to their remarkable properties for perspective applications such as optical, photo, and electrochemical catalyses.1−4 For example, while metal nanoparticles (MNPs) supported tungstate nanocomposites have been used as electrode materials for high-performance supercapacitors,5,6 they have also been exploited as fast and highly sensitive electrochemical sensors for detection of volatile organic compounds (VOCs), biomolecules, and hazardous substances.7,8 Hydrazine (N2H4), a transparent oily liquid that has been widely employed as chemical corrosion inhibitor or reducing agent in chemical, pharmaceutical, agricultural industries as well as in bioimaging, and military and aerospace industries.9−11 Nevertheless, N2H4 is not only extremely unstable (flammable and highly explosive), unless handled in solution, but also highly toxic to humans and animals even at trace levels.12,13 Moreover, N2H4 may cause serious adverse effects to our digestion, kidney, liver, and neurological systems, when exposed by inhalation, oral, or dermal routes.14 United States Environ© 2017 American Chemical Society

mental Protection Agency (EPA) has classified N2H4 as a human carcinogen with a low threshold limit value (TLV) as low as 10 ppb in drinking water. As such, several analytical and spectroscopic techniques have been developed for the detection of N2H4, including high-performance liquid chromatography (HPLC),15 spectrophotometry,16 chemiluminescence,17 and electrochemical methods.18−21 Among them, electrochemical detection of N2H4 is recognized as a desirable technique not only because of its high sensitivity and selectivity but also because of its characteristics as eco-friendly, facile operation, and low cost. Over the past few years, tungsten trioxide (WO3) plays an important role, rendering a wide range of applications in materials sciences and chemistry.22 Because of these potential features and the unique property of the WO3, it has been studied as a promising material for electrodes in the electrooxidation reactions of N2H4.23 However, WO3 has suffered in the acidic as well as basic environments and exhibits poor Received: May 30, 2017 Accepted: August 29, 2017 Published: August 29, 2017 31794

DOI: 10.1021/acsami.7b07645 ACS Appl. Mater. Interfaces 2017, 9, 31794−31805

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ACS Applied Materials & Interfaces Scheme 1. Schematic Illustrations of the Procedures Used for the Syntheses of WO3 and Ru-WO3 Catalysts

electrocatalytic activity.24 Interestingly, the metal-supported WO3-based catalysts exhibit an excellent conductive substrate, when compared to bare WO3 working electrode (i.e., catalytic activity through metal−support interaction).25 Thus, the application of a WO3 matrix should increase the electrochemically active surface area and facilitate charge (electron, proton) distribution as well as diffusion of analysts. Herein, we report the microwave (MW)-assisted synthesis of a bifunctional Ru-WO3 catalysts and their application as electrochemical sensors for efficient detection of N2H4 and as oxidation catalysts. At the current time, to the best of our knowledge, there is no report in the literature of the use of RuWO3 composite in the electrochemical determination of N2H4. As will be shown subsequently, the Ru-WO3-modified glassy carbon electrodes (GCEs) exhibit excellent electrocatalytic activity, sensitivity, and selectivity for detection of N2H4, and hence, they are most suitable for applications as practical and cost-effective N2H4 sensors. Moreover, the Ru-WO3 catalyst also shows excellent activity for catalytic oxidation of diphenyl sulfide (DPS) to diphenyl sulfoxide (DPSO) in the presence of hydrogen peroxide (H2O2) under MW heating.

continuous stirring condition to form a dark red solution. Note that, here, the 1,2-propanediol was employed as a solvent as well as a reducing agent. After stirring for an additional 1 h, the mixture was subjected to microwave heating (power 300 W) at 180 °C for 2 h, during which the solution changed from dark brown color to black. As revealed by ultraviolet−visible (UV−vis) spectra shown in Figure S1 of the Supporting Information (hereafter denoted as SI), the Ru3+ ions were effectively reduced to metal Ru(0) state on the surfaces of the WO3 during the microwave irradiation. Finally, the precipitate was collected by centrifugation (at 8000 rpm), followed by washing consecutively with acetone and ethanol, then dried in vacuum at 60 °C for 6 h. The materials so obtained were denoted as Rux-WO3, were x = 0.5, 1.0, and 1.5 wt % represents the Ru loading. 2.3. Fabrication of the Ru-WO3-Modified Electrodes. To prepare the working electrode, first the bare GCE was polished by using alumina and were cleaned by ultrasonication in distilled water, ethanol, and subsequently dried in a hot air oven. Typically, 5.0 mg of Ru-WO3 composite was first dispersed in 1.0 mL of DI water and sonicated for 2 h; then 6.0 μL of Ru-WO3 catalyst was dropped onto the precleaned GCE and dried overnight for further measurements. 2.4. Catalyst Characterization and Electrochemical Measurements. The X-ray diffraction (XRD) patterns were recorded on a diffractometer (PANalytical X’Pert PRO) using Cu Kα radiation (λ = 0.1541 nm). Surface morphological studies of various samples were conducted using a scanning electron microscope (SEM; Hitachi S3000 H). Elemental compositions of the samples were carried out with an energy-dispersive X-ray (EDX) analyzer, which was an accessory of the SEM instrument. The structural morphology of various samples were studied by field-emission transmission electron microscopy (FETEM; JEOL JEM-2100F) at room temperature (25 °C) operating at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed using an ULVAC-PHI 5000 VersaProb apparatus. Nitrogen adsorption/desorption isotherm measurements were carried out on a Quantachrome Autosorb-1 volumetric adsorption analyzer at −196 °C. Prior to measurement, the sample was purged with flowing N2 at 150 °C for 12 h. Moreover, UV−vis and photoluminescence (PL) spectroscopies performed by using PerkinElmer LS-45 spectrophotometer instruments, respectively, were also employed to investigate the optical properties of various catalyst samples. Electrochemical measurements, including cyclic voltammetry (CV) and chronoamperometry (CA), were conducted on a CHI 1205b analyzer (CH Instruments). A conventional three-electrode cell system was utilized using the modified glassy carbon electrode (GCE) as the working electrode, Ag/AgCl (saturated KCl) as the reference electrode, and a platinum (Pt) wire as the counter electrode.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Research grade ruthenium(III) chloride (RuCl3, 98%), sodium tungstate dihydrate (Na2WO4·2H2O), oxalic acid (H 2 C 2 O 4 , 98%), cetyltrimethylammonium bromide (CTAB, C19H42NBr), polyvinylpyrrolidone (PVP, Mw ∼ 40 000), hydrazine (N2H4, 98%), 1,2-propanediol (C3H8O2), and hydrogen peroxide (H2O2, 30 wt % in water) were purchased from Sigma-Aldrich. The supporting electrolytes (pH = 3−11) were prepared by using 0.05 M Na2HPO4 and NaH2PO4 solutions. All other chemicals were of analytical grade and used without further purification. All solutions were prepared using Millipore DI water. 2.2. Preparation of WO3 and Ru-WO3 Catalysts. As illustrated in Scheme 1, the preparation of Ru-WO3 catalysts invoked a two-step synthesis procedure: Step I, first, 2.7 g of Na2WO4·2H2O and 0.895 g of CTAB were dissolved in 15 mL of distilled water while under magnetic stirring. The pH (from basic to acidic; 11 to 3) of the solution was adjusted by using hydrochloric acid (HCl; 2.0 M). Subsequently, ca. 1.0 g of H2C2O4 was added into the reaction system. The reaction mixture was then heated by means of microwave irradiation (power 300 W; Milestone’ START) at 150 °C for 1 h while under rigorous stirring condition (at 1200 rpm). The yellow precipitate was recovered by centrifugation, then subjected to washing (with DI water) and drying (at 110 °C overnight), followed by a calcination treatment in air at different temperatures (T = 200−500 °C) for 3 h. The resultant yellow powder was labeled as WO3-T, where T represents calcination temperature in °C. In Step II, the assynthesized WO3 (200 mg), RuCl3 (5−15 mg), and PVP (0.583 g) were dissolved in 1,2-propanediol (C3H8O2) (50 mL) under

3. RESULTS AND DISCUSSION 3.1. Structural and Physicochemical Properties of WO3 and Ru-WO3 Nanocomposites. The powder XRD patterns of the as-synthesized WO3 samples calcined at different temperatures are depicted in Figure S2 (SI). For WO3-T samples calcined at lower temperatures T ≤ 300 °C, 31795


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the temperature range of RT−220 °C was attributed to the loss of crystal water, whereas the peaks at ca. 286 °C was due to decompositions of CTAB and organic moiety.30 However, the weight-loss above 600 °C is due to desorption of oxygencontaining groups. Overall, a net weight-loss of ca. 18.8 wt % at 900 °C was obtained, indicating the formations of crystallized Ru−W−O inorganic phase. As revealed by the SEM image shown in Figure S6A (SI), the nanosized Ru1.0-WO3 composite showed random aggregates in surface morphology. Further analysis by EDX and element mapping clearly indicate a homogeneous distribution of O (56%), Ru (0.9%), and W (44%) elements throughout the Ru1.0-WO3 substrate, confirming the uniform dispersion of Ru NPs on the WO3 support (see Figures S6B−E; SI).31 Moreover, both the bare WO3 and the Ru1.0-WO3 samples may be homogeneously suspended in water after a brief sonication treatment (2 min) at room temperature (Figure S7; SI). The structural morphology of the WO3 and Ru-WO3 catalysts were also confirmed by FE-TEM study. As shown in Figure 2A−F, the corresponding FE-TEM images revealed that the bare WO3 exhibited crystalline platelet morphology with particle sizes in the range of 40−60 nm. Moreover, a welldispersed Ru NPs with sizes in the range of 3−6 nm on the surfaces of WO3 was also observed.32 By comparison, the uncalcined WO3 material showed micron-size crystalline aggregates with irregular shapes (Figure S8; SI). The XPS survey spectrum in Figure 3A clearly indicates the presences of Ru, W, and O elements on the surfaces of the Ru1.0-WO3 catalyst sample. The strong peak with binding energy (BE) centering at ca. 282 eV should be due to overlapping contributions from C 1s (ca. 289 eV), Ru 3d3/2 (284.3 eV), and Ru 3d5/2 (280.7 eV),31 as shown in Figure 3B. Moreover, as revealed earlier by UV−vis study (Figure S1; SI), a complete reduction of Ru3+ ions to Ru0 metal state on the surfaces of the WO3 support during the microwave irradiation may be inferred. The spectrum observed for the W 4f core-level (Figure 3C) revealed XPS peaks corresponding to W 4f7/2 (35.9 eV) and W 4f5/2 (38.2 eV), indicating the presence of W6+ oxidation state.27,28 Whereas, the peaks with BEs of ca. 530.5 and 531.8 eV in the O 1s core-level spectrum (Figure 3D) may be assigned to the oxygen atoms O2− in the lattice and the W−O bands in the WO3, respectively.33 Moreover, by comparing the FT-IR spectra obtained from the bare WO3 and the Ru1.0-WO3 materials with key synthesis ingredients such as CTAB and PVP, as shown in Figure 4A, various absorption bands may be assigned. The FT-IR bands at 812 and 1062 cm−1 observed for the structure-directing agent CTAB in Figure 4A(b) may be attributed to the C−N+ stretching modes, whereas the bands at 1378 and 1462 cm−1 were due to symmetric vibrational mode of the methylene (N+−CH3) moiety and CH2 scissoring mode, respectively.34 However, the bands in the range of 1600−3000 cm−1 are due to CH2 symmetric and asymmetric stretching vibrations. These characteristic bands observed for CTAB were also visible in the FT-IR spectrum of the uncalcined WO3 substrate, in which additional bands at 793 and 3418 cm−1 corresponding to stretching vibrations of O−W−O bonds and O−H stretching,35 respectively, were also observed. Nonetheless, the characteristic bands responsible for CTAB diminished in the FT-IR spectrum of WO3 after it was calcined at 500 °C in air, as can be seen in Figure 4A(c), indicating a complete removal of embedded CTAB moieties. Likewise, by comparing the IR spectra obtained from PVP in Figure 4A(d) with the Ru1.0-WO3

several broad diffraction peaks centering at 2θ angles of 23.6, 34.2, 47.5, and 55.7° were observed, revealing the characteristics of monoclinic WO3. This is confirmed by the sharp features at 2θ = 23.1, 23.6, 24.4, 33.3, and 34.2°, which corresponds to (002), (020), (200), (120), and (112) crystallographic planes of WO3, respectively (JCPDS card no. 00-043-1035),2,7 observed for the WO3-500 sample calcined at 500 °C, as shown in Figure 1A(a). On the other hand, the XRD

Figure 1. (A) XRD patterns and (B) N2 adsorption/desorption isotherms of (a) the as-prepared WO3, (b) Ru0.5-WO3, (c) Ru1.0-WO3, and (d) Ru1.5-WO3 catalysts.

patterns of the as-prepared Rux-WO3 (x = 0.5, 1.0, and 1.5 wt %) samples, especially the Ru1.5-WO3 with the highest Ru loading, exhibited weak characteristic diffraction peaks at 2θ = 38.3, 42.2, 44.0, 58.3, 69.4, and 78.4°, which may be assigned to the (100), (002), (101), (102), (110), and (103) planes of the hexagonal close-packed (hcp) Ru metal (ICDD JCPDS card no. 06-0663).26 The presences of weak diffraction peaks on top of the intense characteristic peaks accountable for the WO3 support reveal a well-dispersed Ru NPs on the surfaces of the WO3. By means of the FULLPROF software, the Rietveld refinement powder XRD spectrum of the Ru1.0-WO3 sample was obtained (see Figure S3; SI) with a reasonable goodness of fit of χ2 = 5.12. All diffraction peaks were well-fitted with the monoclinic structure with the P21/c space group and refined lattice parameters of a = 7.3099(4) Å, b = 7.5433(5) Å, c = 7.6989(6) Å, β = 90.7691(3), which are in good agreement with previous reports.27,28 Strong reflections accountable for RuO2 (peak at 28.1°) and hcp Ru metals (peaks at 42.2 and 69.4°) were found. However, no evidence accountable for RuO2 structure was found in XRD patterns observed for the Ru-WO3 samples. To gain information on textural properties of the WO3 and Ru-WO3 samples, N2 adsorption/desorption isotherm measurements were performed, as shown in Figure 1B. All samples showed typical type IV isotherm (IUPAC classification) with a type H 3 hysteresis loop, indicating the presence of mesoporosities.29 Further textural analyses revealed that the bare WO3 exhibited only low BET surface area (SBET), total pore volume (VTot), and BJH pore size (dBJH) of materials are about 12.8 m2 g−1, 0.062 cm3 g−1, and 6.4 nm, respectively (see Table S1; SI). Upon incorporating Ru NPs onto the WO3 support, progressive decreases in SBET, VTot, and dBJH with increasing Ru metal loading were observed, indicating the successful loading of Ru NPs in the pore walls of the WO3 support. The thermal stabilities of the bare WO3 and Ru-WO3 samples were studied by TGA, as shown in Figures S4 and S5 (SI). Typically, the calcined WO3 samples showed multiple weight-loss peaks. The initial weight-loss of ca. 12% occurred in 31796


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Figure 2. (A,B) FE-TEM images of the bare WO3 and (C−F) Ru1.0-WO3 catalysts at different magnifications; Insets in (A) and (F) are the corresponding SAED patterns.

composite in Figure 4A(e), successful capping of the binder

Furthermore, the optical properties of the bare WO3 sample were monitored by additional UV−vis and photoluminescence (PL) spectroscopic techniques. The UV−vis absorbance peak located at ca. 340 nm in Figure 4B(a) may be assigned to ligand-to-metal charge transition (O2p → W5d-O2p) of the WO3 for which the energy required for the transition depends strongly on concentration of W and oxidation temperature.37

onto the catalyst. In brief, the prominent IR bands at 1463 and 1424 cm−1 may be attributed to the characteristic absorptions of the pyrrolidinyl group, while the bands at 1661, 1018, and 3485 cm−1 may be ascribed due to CO, C−N, and −OH stretching vibrations in PVP, respectively.36 31797


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nearly null response for oxidation of N2H4 within the potential range of −0.8 to 0.7 V. By comparison, the Ru NPs-modified GCE showed a weak oxidation peak at −0.3 V. On the other hand, the Ru1.0-WO3modified GCE had a pronounced oxidation peak with an anodic peak potential (Epa) of 0.257 V and highest oxidation peak current (Ipa; ca. 100 μA) for oxidation of N2H4. Compared to the bare GCE, and Ru NPs-and WO3-modified electrodes, the excellent catalytic activity observed over the Ru1.0-WO3modified electrode during oxidation of N2H4 is due to their unique structural and physicochemical properties of the nanocomposite as well as the synergistic effect of the WO3 support and well-dispersed Ru NPs, which provoke formations of surface active sites favorable for enhancing the reversibility of the electron transfer process. It is notable that in the absence of the N2H4 analyte, the Ru1.0-WO3-modified GCE alone also exhibited a redox behavior, as shown in Figure 5B. In the presence of N2H4, the Ru1.0-WO3-modified GCE showed enhanced oxidation peak current at a lower potential (0.261 V), revealing an effective oxidation reaction, which is desirable as a binder-free electrochemical sensor for N2H4 detection. The electrooxidation of N2H4 over the Ru1.0-WO3-modified GCE could be established by the four-electron transfer process, which may be expressed as40,41

Figure 3. XPS (A) survey spectrum, and corresponding (B) Ru 3d, (C) W 4f, and (D) and O 1s core-level spectra of the Ru1.0-WO3 catalyst.

On the other hand, the as-prepared WO3 showed emission peaks in the visible light region at 440, 481, and 527 nm due to due to the nanosized particles and quantum confinement effect of the semiconductor material.38 As expected, the yellowish WO3 NPs suspension exhibited a strong blue luminescence emission under UV light (365 nm) excitation, as illustrated in Figure 4B (inset). The above results revel the presences of sizedependent and charge transition effects in the WO3 crystalline NPs. 3.2. Electrocatalytic Activity of Ru-WO3-Modified Electrodes for Oxidation of N2H4. The electrocatalytic activity of the Ru-WO3-modified electrodes for oxidation of N2H4 was investigated by cyclic voltammetry (CV) and chronoamperometry (CA) methods. To avoid impairing of catalytic activity by the binder,39 the Ru-WO3 catalyst was coated onto a polished glassy carbon electrode (GCE) substrate in the absence of a polymeric binder. Figure 5A shows the CV curves obtained from the bare GCE before and after modification by the WO3, Ru NPs, and Ru1.0-WO3 catalysts in 10 μM N2H4 containing N2-saturated phosphate buffer solution (PBS; pH = 7) at a scan rate of 50 mV s−1. It is obvious that the bare GCE and WO3-modified GCE showed

N2H4 + H 2O → N2H3 + H3O+ + e− (slow)

(1)

N2H3 + 3H 2O → N2 + 3H3O+ + 3e− (fast)

(2)

Here, eqn 1 represents the rate-determining step invoking a single-electron transfer, followed by a fast step involving threeelectron transfer processes to give N2 as a final product (eq 2). Thus, the overall mechanism for N2H4 oxidation can be expressed as N2H4 + 4H 2O → N2 + 4H3O+ + 4e−

(3)

The effects of Ru loading (x) on the electrocatalytic activities of Rux-WO3-modified GCEs in the presence of N2H4 were also investigated, the oxidation peak currents obtained for various sensors with x = 0.5, 1.0, and 1.5 wt % are summarized in Figure S9 (SI). Based on the oxidation peak currents obtained from CV measurements (scan rate 50 mV s−1), it is obvious that the Ru1.0-WO3-modified GCE showed slightly higher Ipa value than its counterparts with x = 0.5 and 1.5 wt %. Clearly, the growth and crystalline natures of Ru NPs and the WO3 support tend to change with the amount of Ru loading. Thus,

Figure 4. (A) FTIR of (a) CTAB, (b) uncalcined, and (c) calcined WO3, (d) PVP, and (e) Ru1.0-WO3 samples, and (B) UV−vis (a) absorption and (b) emission spectra of WO3. Inset: photoluminescence photographs of water suspended WO3 under sunlight and UV (365 nm) excitations. 31798


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Figure 5. (A) CV curves of bare GCE, WO3, Ru NPs, and Ru1.0-WO3-modified GCE with N2H4; (B) Ru1.0-WO3-modified GCE with and without the presence of N2H4; and (C) Ru1.0-WO3-modified GCE with N2H4 recorded at different scan rates (10−200 mV s−1). (D) The corresponding calibration plot of peak current (Ipa) vs square scan rate (10−200 mV s−1). All CV measurements were carried out using 10 μM N2H4 in N2saturated PBS solution (pH 7) at a scan rate of 50 mV s−1.

stable during oxidation of N2H4 at an electrolyte pH of 7. As such, the formation of hydrogen tungsten bronzes compound (HxWO3) may also be ruled out. Thus, it is conclusive that the oxidation of N2H4 over the RuWO3-modified GCE in the neutral PBS electrolyte solution readily invoked a four-electron process (eq 3) through the formations of reaction intermediates such as N2H3 and H3O+ (eqs 1 and 2) to result in N2 and 4H3O+ products. In this context, the enhanced catalytic activity may be due to the dispersed Ru NPs on the surfaces of the WO3 support, which tend to promote formations of active W5+ sites during the reaction. Such synergistic effect between the Ru NPs and the WO3 support was accountable for the superior performances during catalytic reactions.46,47 The effect of scan rate on electrochemical performances of the N2H4 sensor based on Ru1.0-WO3-modified GCE electrode was also investigated, as shown in Figure 5C. It is clear that, upon gradually increasing the scan rates from 10 to 200 mV s−1, a progressive increase in the oxidation peaks current (Ipa) and shifting of the corresponding peak potential toward positive direction were observed. Besides, the oxidation peak currents (Ipa) are linear over the square root of scan rates from 10−200 mV s−1 (Figure 5D), indicating the electro-oxidation of N2H4 was diffusion controlled kinetic process over the Ru1.0-WO3modified GCE.48 3.4. Reaction Kinetics and Proposed Mechanism for Electrooxidation of N2H4. The kinetics of the electrochemical sensor system were further studied by chronoamperometry (CA). Figure 6 displays the CA profiles observed for the Ru1.0-WO3-modified GCE without and with the presence of N2H4 in N2-saturated PBS solution (pH = 7). Compared to the bare modified GCE, which exhibited only very low response current (Ib, the limiting current without N2H4 analyte), a notable increase in CA response current (Ip) over the Ru1.0-

we have chosen the Ru1.0-WO3-modified GCE for the subsequent electrochemical studies. 3.3. Effect of Electrolyte pH and Scan Rate on Electrocatalytic Activity during Oxidation of N2H4. The pH of the electrolyte solution normally plays a major role during the electrooxidation process, and thus, its influence on electrocatalytic activity of N2H4 oxidation was also investigated. As shown in Figure S10A (SI), CV curves for the Ru1.0-WO3modified GCE with 10 μM N2H4 in controlled N2-saturated PBS solutions at different pH values (3−11) were recorded at a scan rate of 50 mV s−1. It can be seen that a maximum peak current was observed at a pH of 7, as shown in Figure S10B (SI). Further increasing the electrolyte pH beyond 7 resulted in a notable decrease in the observed peak current due to the protonation of N2H4. As such, an electrolyte pH of 7 was chosen for the subsequent experiments. Moreover, a linear correlation between the anodic peak potential (Epa) with pH of the electrolyte solution may be inferred with a correlation coefficient of 0.992. The slope observed for the Epa vs pH plot, − 54.8 mV, was in good agreement with that reported for other N2H4 sensors.42 The above results further verify that the electrooxidation of N2H4 over the Ru1.0-WO3-modified GCE indeed invoked a four-electron transfer process. Previously, it has been shown43,44 that metal−supported tungsten oxide (MWO3) catalysts showed inferior electrooxidation activity under either strong acid or strong base conditions. Although formations of several possible stable phases of tungsten oxides such as hydrogen tungsten bronzes (H0.18WO3 and H0.35WO3) and substoichiometric WO3−y (0 < y ≤ 1) have been proposed in acidic electrolyte systems.45 Experimental results reported herein reveal that the best electrooxidation activity of N2H4 over the Ru-WO3-modified GCE is under a neutral electrolyte solution with pH = 7. Because no catalyst aggregation was observed, it is indicative that the Ru1.0-WO3 catalyst remained 31799


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transfer number involved in the rate-determining step of N2H4 oxidation, K is a constant; R, T, and F have their usual significance (R = 8.314 JK−1 mol−1, T = 298 K, F = 96485 C mol−1). Assuming that one-electron transfer is the ratedetermining step (n = 1), the values of α and n involved during the electron transfer process were calculated as 0.34 and 3.72, respectively. On the basis of the above results, which are in accordance with previous literature reports,43−47,55 a plausible reaction pathway for electrooxidation of N2H4 is proposed, as illustrated in Scheme 2. In brief, during the oxidation reaction over the Scheme 2. Schematic Illustration of Electrooxidation of N2H4 over the Ru-WO3 Catalyst Figure 6. CA profiles of the Ru1.0-WO3-modified electrode with (blue curve) and without (red curve) the presence of N2H4 (10 μM) in PBS electrolyte solution (pH 7). Inset: variations of Ip/Ib with square root of time (t1/2).

WO3-modified GCE was observed when in the presence of N2H4. The rate equation of response current may be expressed as49 Ip = (πKCt )1/2 Ib (4) where Ip and Ib represents the response and limiting current with and without the presence of N2H4, respectively. C is the concentration (in mol cm−3) of the N2H4 analyte, t is the time (in second), and K denotes the reaction rate constant. Thus, on the basis of the Ip/Ib vs t1/2 plot shown in Figure 6 (inset), a K value of 2.81× 104 M−1 s−1 may be derived. In addition, the diffusion coefficient (D) of N2H4 may also be estimated by the Cottrell equation:50,51 Ip =

nFAC D πt

Ru-WO3 catalyst, the N2H4 molecules tend to adsorb on the surfaces of the catalyst at first, followed by interfacial electron transfers between the Ru NPs and the WO3 support, which result in formations of hydronium ions (H3O+) in aqueous solution and partial reduction of W6+ to W5+. This W5+ active site tends to accelerate the oxidation of N2H4, leading to an enhanced electrochemical activity. Thus, the rate-determining step for electrooxidation of H2N4 involved an one-electron transfer process, followed by a three-electron process to give N2 as the final product, as specified in (eqs 1−3). 3.5. Electrochemical Performances of the N 2 H 4 Sensor. To further assess the electrochemical performances of the Ru1.0-WO3-modified GCE during detection of N2H4, additional measurements by the amperometric (i−t) method were performed. Figure 7A shows the amperometric i−t response of different additions of N2H4 at Ru1.0-WO3-modified rotating disk electrode (RDE) in constantly stirred N2-saturated PBS (pH 7) at an applied potential of 0.261 V and rotation speed of 1200 rpm. The amperometric response of N2H4 shows a sharp oxidation peak current with the addition of 0.7 μM N2H4 into the constantly stirred N2-saturated PBS. The steadystate current of N2H4 oxidation was reached within 3 s, which indicated fast electro-oxidation of N2H4 on the electrode surface. As expected, it can be clearly seen that the oxidation peak current was gradually increased with the successive addition of N2H4 from the concentrations of 0.7−1129.9 μM, which indicated the rapid electro-oxidation of N2H4 at Ru1.0WO3-modified electrode. In addition, the anodic peak current of N2H4 oxidation had a linear relationship over the N2H4 concentrations from 0.7−709.2 μM with the correlation coefficient of 0.9903, as shown in Figure 7B. The calculated sensitivity of the sensor was 4.357 μA μM−1 cm−2. The limit of detection (LOD) was estimated to be 0.3625 μM based on the standard formula as mentioned below (5)20,48

(5)

where Ip denotes the peak current (in A), n is the number of electron, F = 96,485 C mol−1 is the Faraday constant, A is the surface area of the electrode (cm2), C represents bulk concentration of the analyte (mol cm−3), and t is time (s). Accordingly, by taking the eq 5, the value of D = 3.16 × 10−6 cm2 s−1 for N2H4. Moreover, the surface coverage of the electroactive species (Γ) on the Ru1.0-WO3-modified working electrode may be calculated from the equation:52 Ip =

n2F 2A Γυ 4RT

(6) −1

where υ is the scan rate (mV s ), R is the gas constant (8.314 J mol−1 K−1) and T is the temperature (in °C). By using eq 6, the Γ value was calculated to be 1.46 × 10−9 mol cm−2. The above values of K, D, and Γ so obtained are in close agreements with those reported earlier for the other N2H4 sensors.53 In addition, the plot of peak potential (Ep) showed a linear relationship over the log of scan rates. Based on the literature, the linear relationship can be expressed as the following eq 7:54 ⎡ 2.303RT ⎤ Ep = K + ⎢ ⎥log υ ⎣ 2αnαF ⎦

(7)

where, Ep is the peak potential of N2H4 oxidation, α is s the transfer coefficient for N2H4 oxidation, nα is s the electron 31800


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Figure 7. (A) Amperometric responses of the Ru1.0-WO3-modified GCE under consecutive injection of N2H4 within a total dosage range of 0.7− 1129.9 μM and (B) the corresponding calibration plot of response current vs N2H4 concentration. All measurements were conducted in N2-saturated PBS (pH = 7) at a rotation speed of 1200 rpm and an anodic potential (Epa) of +0.261 V.

Table 1. Comparison of Analytical Parameters at Ru-WO3-modified Electrode with Previously Reported N2H4 Sensors modified electrode

method

pH

linear range (μM)

detection limit (μM)

sensitivity (μA μM−1 cm−2)

ref

WO3 NPs WO3@DEDMABa WO3@TTABb CuNPs-PANIc-Nano-ZSM-5 ZnONRsd/SWCNTe Ni(OH)2-MnO2 Co3O4 NWsf MnO2/graphene NiHCF@TiO2 NPsg Ru1.0-WO3

amperometry amperometry amperometry amperometry amperometry LSVh amperometry amperometry DPVi Amperometry

7 7 7 --7 7 7 7 7 7

100−1000 100−1000 100−1000 0.004−800 0.5−50 5−18000 20−700 3−1120 0.2−1.0 0.7−709.2

144.73 28.8 29−59 0.001 0.17 0.12 0.5 0.16 0.11 0.3625

0.1847 9.39 3.38−10 1.6 0.10 25 μA mM−1 28.63 μA mM−1 1007 --4.357

22 23 24 56 57 58 59 60 61 this work

a Dodecylethyldimethylammonium bromide. bTetradecyltrimethylammonium bromide. cPolyaniline. dZinc oxide nanorods. eSingle-walled carbon nanotube. fCobalt oxide nanowires. gNickel hexacyanoferrate. hLinear sweep voltammetry. iDifferential pulse voltammetry.

LOD =

3S b q

(10)

where Sb is the standard deviation of the blank signal and q is the slope value (obtained from calibration plot). The analytical performance (sensitivity, LOD, and linear range) of the proposed sensor was compared with previously reported N2H4 sensor, and the results are summarized in Table 1.22−24,56−61 These findings demonstrate that the Ru1.0-WO3modified electrode showed an excellent electrocatalytic activity, good linear range, and lower LOD toward the oxidation of N2H4. 3.6. Selectivity, Stability, and Reproducibility of the N2H4 Sensor. Selectivity of the N2H4 sensor was normally affected by common metal ions and biological molecules such as dopamine (DA), uric acid (UA), ascorbic acid (AA) and glucose (Glu). Hence, we have investigated the selectivity of the sensor in the presence of common metal ions and biological molecule by amperometry, as shown in Figure 8A. It can be seen that a sharp peak was observed with the addition of 10 μM of N2H4 (a) in N2-saturated constantly stirred PBS. There is no change in the peak current even in the presence of 200-fold excess concentrations of Ni2+, Co2+, Zn2+, Ca2+, Br−, Cl−, I−, F−, SO32− and 50-fold higher concentration of DA, UA, AA, and Glu in N2-saturated PBS. These results further conclude that the proposed sensor exhibits an excellent anti-interference ability toward the detection of N2H4. In addition, the operational stability of the sensor was exhibited up to 93.6% of its initial response current in the presence of 10 μM of N2H4 containing PBS constantly run up to 2000 s as shown in Figure

Figure 8. (A) Amperometric response of Ru1.0-WO3-modified RDE containing 10 μM N2H4 (a), in the presence of a 200-fold excess concentration of metal ions (Ni2+, Co2+, Zn2+, Ca2+); anions (Br−, Cl−, I−, F−, SO32−) and 50-fold excess concentration of DA, UA, AA, and Glu; and (B) Stability of Ru1.0-WO3-modified electrode in the presence of 10 μM of N2H4 containing PBS constantly run up to 2000 s.

8B. This result suggested good operational stability of the RuWO3-modified electrode. The storage stability of the sensor was important for evaluating the material stability. Hence, we have also 31801


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catalysts have been receiving much more attention in the selective oxidation of sulfides to sulfoxides using H2O2 as oxidant. 69,70 Recently, our group reported the use of heterogeneous Ru/Al2O3 catalyst for the direct oxidation of sulfides by H2O2 in the acetonitrile (CH3CN) and water mixed solvent.70 In our report, for the first time, we used Ru-WO3 as a catalyst and H2O2 as an oxidant for the oxidation of diphenyl sulfide (DPS) to diphenyl sulfoxide (DPSO) using CH3CN as a solvent at 60 °C in 5 min under MW irradiation (Scheme 3) sulfoxides or sulfones

investigated the storage stability of the sensor by CV. The RuWO3-modified electrode was performed toward the oxidation of N2H4 in N2-saturated PBS (pH 7). The response current was carefully checked for every 2 days. The sensor retains 91.3% of its initial response current after 10 days. The electrode was stored at 4 °C, when not in use. This result authenticates the excellent storage stability of the sensor. In order to evaluate the reproducibility of the sensor, it was examined by CV toward the detection of 10 μM of N2H4 in N2-saturated PBS. Prior to analysis, five different modified electrodes were prepared and investigated by CV in the presence of 10 μM of N2H4containing PBS buffer. The relative standard deviation (RSD) was estimated to be 3.8%. The repeatability of the sensor was examined using a Ru1.0-WO3-modified electrode by CV. The 10 successive measurements were performed in PBS (pH 7) containing 10 μM of N2H4. The RSD value of the sensor retains 3.85%. Hence, the proposed Ru1.0-WO3-modified electrode shows excellent storage stability, good repeatability, and reproducibility toward the detection of N2H4. 3.7. Real Sample Test. The Environmental Protection Agency (EPA) declares that N2H4 is present in cigarette sample at a level of 50 ± 5 μg/gram.62,63 On this basis, we can use the cigarette sample for determination of N2H4 content for practical application. In order to determine the N2H4 level in a cigarette sample, first we need to prepare the sample. Briefly, the commercially available cigarette was purchased from the local market. Then, the cigarette sample was prepared in PBS. The unknown concentration of the N2H4-containing cigarette sample was studied by amperometry using the standard addition method. The cigarette sample was diluted 10 times before the experiment. After that, the known concentration of N2H4 was spiked into the PBS containing the cigarette sample. The recovery values of the sensor were ranging from 94.5% to 99.5%, suggesting accuracy of the proposed sensor. In addition, we have also performed the quantitative analysis N2H4 using the high-performance liquid chromatography (HPLC) method. The obtained recovery values are compared with our electrochemical method. The experimental results are summarized in Table 2.

Scheme 3. Oxidation of Diphenyl Sulfide Catalyzed by Ru1.0WO3 Catalyst

In the catalysis reaction, the mixture of catalyst (0.5 mol %), 1 mmol of DPS, and 30% H2O2 (1.5 mmol) in 3 mL of CH3CN were heated under MW irradiation at 60 °C for 5 min. After completion of the reaction, as indicated by thin-layer chromatography (TLC), the product was extracted with ethyl acetate (10 mL). The combined organic extracts were concentrated in vacuum and the resulting product was purified by column chromatography on silica gel with ethyl acetate and n-hexane as eluent to afford the product (yield 98%; colorless solid). However, when the reaction was conducted in the presence of the WO3 as a catalyst, the main product of DPSO was efficiently formed with yield (94%), compared to Ru-WO3 catalyst, indicating Ru1.0-WO3 was the active catalyst. Excess the amount of oxidant (H2O2), causes the formation of sulfones as a final product with highest yield (99%). The obtained products were confirmed with authentic sample. We have compiled the catalytic performance of our catalyst system with other catalysts in Table S2 (SI). Notably, our catalysts also show excellent catalytic performance for the oxidation of DPS in to DPSO was obtained in good to excellent yields. Even in the case of other tungstate−based catalysts afforded in moderate yields 52 and 55%, respectively (Table S2, SI). As can be clearly seen, all the catalysts were required long time, but our catalyst protocol needed only 5 min, which superior to those of the other catalysts. Since, the advantage of microwave-assisted oxidation reaction route is more energy efficient, cost-effective, and time-saving and so on. Moreover, considering these initial promising results and the selective oxidations of various challenging sulfides were explored under identical reaction conditions in future.

Table 2. Determination of N2H4 in Cigarette Sample at RuWO3-Modified Electrode by Amperometry sample

spiked (μM)

found (μM)

recovery (%)

RSD (%)

cigarette sample

unknown 2.0 2.0 2.0

10.68 12.57 14.66 16.43

− 94.5 99.5 95.8

− 3.8 3.5 4.1

Compared with HPLC method, our proposed sensor has almost reached the same recovery values for N2H4. This result confirms that the developed sensor is reliable and accurate determination of N2H4 in cigarette sample and can be employed for the determination of N2H4 for practical applications. 3.8. Catalytic Oxidation of Diphenyl Sulfide in H2O2. For catalysis applications, WO3 is a very promising material regarding its low cost and ease to synthesis, high thermal stability, good morphological, and structural properties.64 In addition, WO3 is a well-studied wide band gap semiconductor (∼2.75 eV) used for several applications including pH sensors,65 biosensors,66 catalysis,67,68 and so on. In the past few years, various types of tungsten-based heterogeneous

4. CONCLUSIONS In summary, Ru1.0-WO3 catalyst was synthesized via a facile microwave method have been developed and exploited as electrode supports for both electrocatalytic oxidation of N2H4 as well as catalytic oxidation of aromatic sulfides. The fabricated carbon-free nanostructured Ru-WO3 catalysts were characterized by a variety of analytical and spectroscopy techniques. The performance of bare WO3 was found to be poor compared with the Ru1.0/WO3 in terms of electro-oxidation of N2H4. A possible electrocatalytic reaction mechanism for the N2H4 over the Ru1.0/WO3 catalyst is proposed. However, it is a more stable phase during the reaction in the presence of H2O2 and is therefore a prospective material for catalytic applica31802


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ACS Applied Materials & Interfaces tions.64,67−69 Moreover, these results clearly demonstrate that the WO3-supported Ru catalyst possesses desirable electro catalytic and catalytic properties, which should facilitate prospective applications. Further investigations on other useful applications of this catalyst are in progress.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07645. Experimental results from UV−vis, XRD, TGA, SEM, TEM, EDX, and CV studies and textural property data of assorted WO3 and Ru-WO3 samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for S.-M.C.: [email protected]. *E-mail for P.V.: [email protected]. ORCID

Shen-Ming Chen: 0000-0002-8605-643X Pitchaimani Veerakumar: 0000-0002-6899-9856 Author Contributions ⊥

C.R. and B.T. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support (NSC 1012113-M-027-001-MY3 to S.M.C; NSC 104-2113-M-001-020MY3 to S.B.L.) from the Ministry of Science and Technology (MOST), Taiwan.

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