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Novel Composition Graded Ti/Ru−Sb−SnO2 Electrode Synthesized by Selective Electrodeposition and Its Application for Electrocatalytic Decolorization of Dyes Tigang Duan, Ye Chen,* Qing Wen,* and Ying Duan Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin, 15001 Heilongjiang, China S Supporting Information *

ABSTRACT: Compositionally continuous gradient electrode of Ru−Sb−SnO2 was fabricated successfully by the selective potentialpulse electrodeposition method in a single electrolytic solution. Cyclic voltametry was used to determine the deposition potentials, obtaining a three-step potential mode. Applying the selected threestep potential pulse, a uniform layer with controllable composition gradient was obtained and had morphology of vertically aligned sheets. The Ru content in the deposition layer displays a parabolic variation and has a maximum value of 2.4 at. % with 200 deposition cycles. Furthermore, the growth mechanism of perpendicular sheet morphology and composition gradient was investigated and is related to the diffusion layer and applied potentials. Finally, the electrocatalytic dye decolorization experiments show that first-order kinetics constants on Ti/Ru−Sb−SnO2 electrode reach 23.0 × 10−3 min−1 (for methylene blue), 25.1 × 10−3 min−1 (for orange II), and 25.2 × 10−3 min−1 (for methyl orange), respectively. These results demonstrate that the compositionally continuous gradient electrode possesses a good catalytic activity and wide use for dye decolorization.

1. INTRODUCTION As one of the unique semiconductor materials, tin oxide (SnO2) has possessed widespread applications in the fields of gas sensors, solar cells, transparent conducting electrodes, catalyst support materials, and photo/electro-catalysis.1−5 In the wastewater treatment application, SnO2 electrode, which has characteristics of low cost, high oxygen evolution potential, and efficient production of hydroxyl radicals, has been thought to be superior for the electrocatalytic removal of aqueous organic contaminants.6−9 Pure SnO2 is an n-type semiconductor and can not be used directly as the electrocatalytic electrode material. Some dopants such as Ir, Sb, and so on have been commonly considered to improve the conductivity, activity, and stability of SnO2 electrodes: IrO2−SnO2 electrodes have possessed low oxygen evolution potential and have been applied for oxygen evolution; Sb has been considered as one of the best dopants, the introduction of Sb has made very slight effects on the oxygen evolution potential, and the optimal Sb doping level can improve the conductivity and provide the best electrocatalytic characteristics of organic contaminant degradation for SnO2 electrodes.6,7,10 To improve further the properties of Sb-doped SnO2 electrodes, numerous modification efforts have been taken considerably, and can be summarized into two main research directions, that is, doping modification and structure modification. Doping modification efforts have included the © XXXX American Chemical Society

metal ion doping modification (Ce, Eu, Gd, Dy, Pt, Pd, Ru, Fe, Ni, etc.),11−13 and nanomaterial doping modification (Cr3C2, TiN, carbon nanotube, graphene, etc.).14−17 Doping metal ion efforts for electrode modification have been mostly taken into practice through the thermal decomposition method and the sol−gel method. However, due to the dilatability difference between active oxide layer and substrate, it is unavoidable to produce a crack so that the precursor of metallic oxide may not be closely attached to the substrate.18 Recently, nanomaterial modification efforts have been practiced through the electrodeposition method, which shows many advantages, such as improvement of the distribution and adhesion of deposit layer, convenient and manageable operation, low fabrication cost, and expedient control on layer thickness and morphology.14,19,20 Hu et al.21 employed a single electrolytic bath instead of previous multiple baths to prepare composite SnO2−Sb2O4 electrode through electrocodeposition. Furthermore, Zhang et al.22 used a new-typed electrolytic bath to fabricate a nanocrystalline composite SnO2−Sb film electrode by pulse electrodeposition. These nanomaterial-modified electrodes showed superior electrocatalytic performance and stability. However, the content of components in the catalytic layer was Received: January 12, 2015 Revised: March 15, 2015

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DOI: 10.1021/acs.jpcc.5b00323 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. SEM images of (a) Ti/Ru−Sb−SnO2(200) and (b) Ti/Ru−Sb−SnO2(DC) electrodes. (c) TEM and (d) HRTEM images of Ti/Ru−Sb− SnO2 electrode. The insets in (a) and (b) are the corresponding magnified images, and the inset in (c) is the corresponding SAED pattern.

photovoltaic materials,37 and catalysts.38 Jiang et al.33 employed the wet powder spray technique to construct the compositionally continuously graded layers with high electrochemical performance for the application of solid oxide fuel cells. It is considered that the gradient property is beneficial to diminish the boundary hierarchy, considerably improving the performance of electrode. In this Article, a straightforward approach to obtain the compositional gradient Ru−Sb−SnO2 electrode via selective electrodeposition was presented, and the electrochemical dynamics control of compositional gradient was realized. Electrodes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive Xray spectroscopy (EDS). The formation of composition gradient was discussed. Meanwhile, the growth mechanism of the vertically aligned sheet layer was investigated, revealing a dependence on the diffusion layer and applied potentials. Finally, the electrocatalytic methylene blue decolorization was performed to investigate the catalytic activity of Ru−Sb−SnO2 electrode, and a good catalytic performance was obtained, revealing the positive effects of composition gradient and vertically aligned sheet structure.

hard to control precisely. For the structure modification efforts, there have been some ideas such as the interlayer introduction,23 the special structure idea,24−26 and so on. Thereinto, the special structure idea has been considered as one of the most innovative thoughts. Recently, there have been several different kinds of newly special structures created, which include ordered mesoporous Sb-doped SnO2 thin films with adjustable doping levels and high electrical conductivity,27 assembled 2D macroporous SnO2/1D TiO2 nanotubes,26 nanocrystalline antimony-doped tin oxide disordered macroporous 3D electrodes, 28 and ordered porous Sb:SnO 2 electrode.29 Yet the conditions for preparing specially structured electrodes are complicated. On the basis of the above analysis, to simplify the work and boost the performance, we combined the advantage of single-bath electrodeposition and the idea of special structure to fabricate compositional gradient electrodes. Gradient materials are the advanced materials with continuously and smoothly varying composition in the preferred orientation, possess unique properties of mechanics, electrics, chemistry, and so on, and are superior to uniform material composed of similar constituents.30−33 The gradual variation of properties, which is a key characteristic of gradients, is of eminent importance in technology; physicochemical properties of gradients, such as the chemical composition, the topography of a surface, and many others, can be tuned in length and shape, which is a key attribute of gradients.34 On the basis of the above consequence, the application performance of gradients can be enhanced, and has been employed in various technological applications including electroplating,31 energy storage,32 new dielectric thin film,35 sensing materials,36

2. EXPERIMENTAL SECTION 2.1. Preparation of Ti/Ru−Sb−SnO2 Electrode. Prior to the electrodeposition, Ti sheets (0.5 mm thickness, TA0 Type) went through mechanical polishing, alkaline degreasing in a 10% NaOH solution at 85 °C for 1 h, and acid etching in a 10% oxalic acid solution at 85 °C for 2 h. Pretreated Ti sheets then were preserved in a 3% oxalic acid solution. B


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Figure 2. (a) XRD pattern of Ti/Ru−Sb−SnO2(200) electrode; (b−d) XPS spectra of Ti/Ru−Sb−SnO2(200) electrode.

2.3. Electrochemical Decolorization Experiment. The electrochemical decolorization experiments for dyes (methylene blue, orange II, and methyl orange) were performed in 50 mL of 50 mg L−1 dye solution with the supporting electrolyte of 0.25 M Na2SO4 solution. The electrolysis was performed in the galvanostatic condition of 20 mA cm−2 with a working electrode area of 1 × 2 cm2. The electrochemical decolorization processes of dyes were monitored by the UV−vis absorbance spectroscopy, and dye solution concentrations were measured by the absorbance intensity at the characteristic wavelengths of 664 nm for methylene blue, 484 nm for orange II, and 463 nm for methyl orange.

Electrodeposition process was performed using a CHI760C electrochemical workstation (CH Instruments, China) in a three-electrode system. The Ti sheet served as the working electrode with a deposition area of 1 × 4 cm2. A platinum sheet (2 × 2 cm2) and saturated calomel electrode (SCE) were used as a counter and reference electrode, respectively. The electrodeposition solution consisted of 0.1 M SnCl4·5H2O, 0.011 M SbCl3, 2 × 10−4 M RuCl3·3H2O, and 0.01 M EDTA2Na. For the deposition, a potential pulse deposition method with a three-stage pulse (−0.55 V, 10 s; −0.8 V, 3 s; −0.5 V, 10 s) was applied at 30 °C for 200 cycles. Ti substrates with deposited layers were then dried at 100 °C and annealed in a muffle furnace at 600 °C for 2 h. Finally, alloy layers were transferred to metal oxide layers, and hierarchical Ti/Ru−Sb− SnO2 electrodes were obtained. To prepare the control sample, Ru- and Sb-codoped SnO2 eletrode, the constant current electrodeposition was conducted for 1 h with a current density of 5 mA cm−2, followed by an annealing of 600 °C for 2 h. 2.2. Characterization of Electrode. As-prepared electrodes were characterized by a scanning electron microscope (SEM, INSPECT S50, MAKE FEI, America) equipped with an energy dispersive X-ray spectrometer (EDS), transmission electron microscope (TEM, JEM 2100, Japan), X-ray diffractometer (XRD, Cu Kα radiation, 40 kV and 150 mA; Rigaku D/Max2500, Japan), and X-ray photoelectron spectroscope (XPS, Al Kα radiation, hν = 1486.6 eV; Thermo ESCALAB 250, UK), respectively. The deposit thicknesses of electrodes were measured via a microprocessor coating thickness gauge (MiniTest 600, ElectroPhysik, Germany).

3. RESULTS AND DISCUSSION 3.1. Characterization. Figure 1 displays typical SEM and TEM images of the compositionally gradient Ru−Sb−SnO2 electrode prepared under the pulse-potential deposition condition. The morphology of Ti/Ru−Sb−SnO2 electrode with 200 deposition cycles shown in Figure 1a displays a homogeneous hierarchical lamellar structure decorated with well-distributed clusters. The high-magnification SEM image (inset of Figure 1a) displays morphology of flower buds or flower pistils decorating on the vertically aligned sheets. Figure 1c and d displays the TEM and HRTEM images of Ti/Ru− Sb−SnO2 electrode. Samples for the TEM were ultrasonically obtained from the titanium substrate, prepared by ultrasonically dispersing the product in ethanol, and placed on Cu grids. The TEM image shows a flake-like structure with irregular particles. The HRTEM image can provide more information about structure such as crystal lattice. A uniform interplanar spacing C


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Figure 3. Cyclic voltammograms of Ti sheet in the solutions containing (a) 0.1 M HCl, 0.01 M EDTA-2Na, and 2 × 10−4 M RuCl3·3H2O; (b) 0.1 M HCl, 0.01 M EDTA-2Na, and 0.011 M SbCl3; (c) 0.01 M EDTA-2Na and 0.1 M SnCl4·5H2O; and (d) 0.1 M SnCl4·5H2O, 0.011 M SbCl3, 2 × 10−4 M RuCl3·3H2O, and 0.01 M EDTA-2Na. The sweep rate is 10 mV s−1.

fitted using XPS Peak Processing Program, and the Sb 3d5/2 peak is separated from the mixed spectrum (Sb 3d5/2 plus O 1s). Furthermore, the O 1s peak is split into two peaks: the peak at 530.4 eV is attributed to lattice oxygen species (OL), which are incorporated into SnO2 crystal lattice, and the peak at 531.4 eV is ascribed to adsorbed hydroxyl oxygen containing species (Oad), which are adsorbed hydroxyl/oxygen group.40 The calculated atom ratio of Oad and OL is 69.5%. Our previous reports15−17 indicated that the electrocatalytic activity is closely dependent on the content of adsorbed hydroxyl oxygen containing species. Thus, Ti/Ru−Sb−SnO2 electrode can be expected to have a good electrocatalytic activity for dye decolorization. 3.2. Electrodeposition of Hierarchical Ru−Sb−SnO2. To determine the deposition potentials for the elements, cyclic voltammetry was performed on the titanium substrates, and Figure 3 displays the cyclic voltammograms of several deposition processes. EDTA-2Na has often been utilized as surfactant/complexing agent in the morphology-controlling synthesis, and thus was introduced to control the growth of deposit layer in this work. The electrochemical behavior of ruthenium in a solution of 2 × 10−4 M RuCl3·3H2O, 0.01 M EDTA-2Na, and 0.1 M HCl is shown in Figure 3a. Two reductive peaks can be observed: one reductive peak at 0 V corresponds to the electroreduction reaction of Ru(III) → Ru(II), and other peak at 0.2 V corresponds to the electroreduction reaction of Ru(II) → Ru(0). The electrochemical behavior of Sb(III) is shown in Figure 3b, and the electroreduction of Sb(III) appears at −0.24 V. For the Sn(IV)

with the value of 0.34 nm is in agreement with the (110) plane of tetragonal rutile SnO2. The corresponding SAED pattern reflects the polycrystallinity of the active oxide layer. To study the crystal structure of Ti/Ru−Sb−SnO2 electrode, XRD characterization was performed. Figure 2a shows the XRD pattern of Ti/Ru−Sb−SnO2 electrode with 200 deposition cycles. The strong diffraction peaks appear at 26.6°, 33.9°, 37.9°, 51.8°, 54.8°, 57.8°, 61.9°, 64.7°, 65.9°, and 71.3°, which are well matched with the (110), (101), (200), (211), (220), (002), (310), (112), (301), and (202) of tetragonal rutile SnO2 (PDF no. 41-1445), respectively. Moreover, the strongest peak is at (110) plane, revealing the preferred orientation along the (110) crystallographic direction. XPS is one of the effective characterization methods to give a good comprehension about the electrode surface state, and thus is used to analyze the chemical states of elements on the electrode surface. Figure 2b shows the Sn 3d spectrum of Ti/Ru−Sb−SnO2. The Sn 3d5/2 state appears at the binding energy of 486.9 eV, while the Sn 3d3/2 state is observed at the binding energy of 495.3 eV. The gap between the Sn 3d5/2 and Sn 3d3/2 levels is 8.4 eV, which is ascribed to Sn4+ in SnO2. To study the antimony oxidation states, the peak of Sb 3d3/2 is split into two peaks, Sb5+ state and Sb3+ state. Observed from Figure 2c, the peak of Sb3+ appears at 539.9 eV, and the peak of Sb5+ at 540.4 eV. The calculated atom ratio of Sb3+ and Sb5+ is 2.05. The high ratio can be ascribed to the antimony surface segregation at high temperature, and the redundant antimony existing as Sb2O3 on the surface can act as a catalyst to assist the catalytic process.39 As shown in Figure 2d, the XPS spectrum of Sb 3d5/2 and O 1s is overlapped, thus D


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Figure 4. SEM images and EDS results of deposits at different potentials: (a, c) −0.55 V with 1000 s, and (b, d) −0.8 V with 1000 s. SEM images of Ti/Ru−Sb−SnO2 electrodes with 200 deposition cycles: (e) without −0.55 V step, and (f) with −0.55 V step.

voltammogram of titanium substrate in the solution of 0.1 M SnCl4·5H2O, 0.011 M SbCl3, 2 × 10−4 M RuCl3·3H2O, and 0.01 M EDTA-2Na is shown in Figure 3d. The reductive peak C1 appears around −0.37 V, and can be attributed to the codeposition of Ru and Sb. The reductive peak C2 appearing around 0.58 V should be the codeposition of Ru, Sb, and Sn. According to the results in Figure 3b and c, the oxidative peaks A1 and A2 should correspond to the oxidative dissolution of Sb and the oxidative dissolution of Sn, respectively. The CV results

behavior in Figure 3c, no significant reductive peak is observed possibly due to the effect of fierce hydrogen evolution, but the oxidative peak of Sn appears at −0.39 V. These results indicate that the deposition behaviors of Ru, Sb, and Sn are the overpotential deposition processes, which take place at more negative potentials than the Nernst equilibrium potential. The overpotential deposition is determined by electrode overpotential, electroactive species concentration, and deposit− substrate and deposit−deposit interactions.41 The cyclic E


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Figure 5. SEM images of Ru−Sb−SnO2 electrodes with different cycles: (a) 50, (b) 150, (c) 200, (d) 300, (e) 350, and (f) 400. Insets are the corresponding magnified images.

attaching to the titanium substrate (deducting the substrate element content, the Sb−Ru alloy sphere is approximately 75% Sb and 25% Ru as determined by EDS). The deposit at −0.8 V for 1000 s is shown in Figure 4b, and displays a compact film with acanthosphere-like grains (these grains consist of 83.46% Sn, 16.06% Sb, and 0.48% Ru). Figure S1 in the Supporting Information shows the deposit at −0.7 and −0.9 V. These results reveal the effect of deposition potential on the composition. Differing from the single potential pulse case, observed from Figure 4e, double-pulse mode (−0.8; −0.5 V) was employed to obtain a hierarchical porous structure with plenty of uniform dendritic grains. Figure 4f shows the threepulse mode (−0.55; −0.8; −0.5 V), and displays a distinct hierarchical structure: a compact granular layer attaches to the substrate, vertically aligned sheets grow on it, and some grains decorate on sheets. On the basis of the above analysis, a threestep pulse electrodeposition is used to fabricate electrode. The first step is the predeposition step, in which many small nuclei

provide opportunities for shape-controlled deposition through modulating the applied potentials to control the deposit morphology and composition. According to the results shown in Figure 3, a three-stage potential pulse (−0.55; −0.8; −0.5 V) deposition was selected preliminarily to fabricate the electrode: the pulse stage of −0.55 V as the predeposition step is applied to the codeposition of Ru and Sb; the pulse stage of −0.8 V acting as the fast growth step is applied to the codeposition of Ru, Sb, and Sn and simultaneously avoids severe hydrogen evolution; the stage of −0.5 V is applied to the anodic dissolution of deposition layer, modulating the growth and playing a role of morphology modification. The effect of potential pulse will be investigated in the following. To particularly investigate the selection of potential pulse, the deposition processes were performed under different potentials, and the resulting SEM images are shown in Figure 4a and b. Figure 4a shows the deposit morphology at −0.55 V for 1000 s, and displays uniform Sb−Ru alloy microspheres F


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Figure 6. Variations of element molar content with the deposition cycles: (a) Ru, (b) Sn, and (c) Sb. (d) Variation of deposition thickness with the deposition cycle.

are electro-generated on the substrate, to provide heterogeneous nuclei. These nuclei serve as active sites in the following step. Figure 4e and f illustrates important effects of the predeposition step on the morphology-controlling electrodeposition, and clearly demonstrate that skipping the predeposition step leads to the formation of porous structure mainly composed of microscale grains. This result indicates that the predeposition step plays an important role in controlling the morphology and growth. 3.3. Dependence of Morphology and Composition Gradient on Electrodeposition Cycle. The morphology and chemical composition of deposit layer are also investigated with the variation of electrodeposition cycle. Figure 5 shows the SEM images for the morphology evolution of deposit layer with electrodeposition cycle. Within the first 50 cycles of electrodeposition, the deposit layer presents a growth tendency to form preliminary compact grains on the substrate and to produce many standing fragments clustering together on gains, as observed in Figure 5a. In the 150 cycles of electrodeposition, plenty of standing fragments protrude from the grains and grow up to form uniform and vertically aligned sheets (Figure 5b). With the deposition going on, the vertical sheet layer is gradually consolidated. When the deposition cycle is increased to 300, lots of dense grain clusters form on the top of vertically aligned sheets (Figure 5d), indicating the morphology transformation from vertical sheets to vertical dendritic grains. Therefore, when the cycle reaches 350, a uniform dendriticgrain layer comes out and presents an approximately rod-array structure. However, when the growth cycle is longer than 400, the deposition layer displays a compact layer decorated by

some dendrites, showing distinctly different morphology. These results for the morphology evolution of deposition layer may be related to the diffusion of bulk concentration, and will be discussed in the following. As mentioned above, a three-potential pulse electrodeposition was used to fabricate electrode. The heterogeneous nuclei of Ru−Sb alloys are first produced on the substrate at −0.55 V. In the following growth step, perpendicularly oriented fragments are produced at the more negative potential of −0.8 V, followed by mild morphology modification through anodically etching at −0.5 V. With the deposition going on, vertically aligned sheets are obtained with the applied potentials. However, with the deposition further proceeding, the overdeposition appears and results in the destruction of uniform and vertically aligned sheets and the formation of a compact granular layer decorated with some dendrites. This distinct growth process may be attributed to the effects of diffusion layer and bulk concentration. With electrodeposition going on, a diffusion layer on the substrate forms and results in the increasing concentration gradient of composition ions.42 That is, the nearer is the substrate, the lower is the concentration. Because of the concentration difference along the direction perpendicular to the substrate, more metal ions diffused to the top of standing flakes; thus the standing flakes grow faster than those on the substrate. Therefore, the diffusion layer is beneficial to the growth of standing flakes, and the vertically aligned sheets are the main products within 300 deposition cycles. Moreover, under the further action of diffusion layer, the overdeposition, which is the electrodeposition with too long time, can produce standing dendritic G


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Figure 7. Schematic illustration for the variations of Ru, Sn, and Sb contents with the deposition cycle.

Figure 8. (a) Variations of the decolorization efficiency with electrolysis time on different Ru−Sb−SnO2 electrodes; (b) the corresponding firstorder kinetics curves of methylene blue decolorization.

Ru bulk concentration caused by the further increase of deposition cycle. The composition variations of Sb and Sn can be interacted with the Ru composition. 3.4. Electrocatalytic Decolorization Performance. Supporting Information Table S1 shows the EDS results of electrodes. To investigate the effect of morphology and composition on the electrocatalytic activity of electrode, different Ru−Sb−SnO2 electrodes are used to degrade dyes. During the electrolysis process, SnO2 electrode can electrocatalytically generate hydroxyls as the strong oxidizing species so as to efficiently oxidize target pollutants to CO2 and H2O. Figure 8a shows the decolorization efficiency (η) with the variation of electrolysis time. With the electrolysis proceeding, methylene blue decolorization efficiency increases. During 60 min electrolysis, the methylene blue decolorization efficiencies for all electrodes reach 60.8%, 66.7%, 74.6%, 70.2%, 68.3%, and 63.1%, respectively. After 120 min, the methylene blue decolorization efficiencies are 85.2%, 89.0%, 94.2%, 89.0%, 90.4%, and 87.0%, respectively, revealing the most efficient decolorization activity of Ru−Sb−SnO2 electrode with 200 cycles. Figure 8b shows the semilog relationship of methylene blue concentration with electrolysis time. The dye decolorization process follows the first-order kinetics model:

grains and even compact layer with dendrites. It should also be pointed that the selected potential-pulse mode plays a significant part in the growth of vertically sheets because different potentials are applied to the selective deposition (−0.55 V for Ru−Sb codeposition and −0.8 V for Ru−Sb−Sn codeposition). In addition, these results may be closely related to the surface composition. The surface composition variations will be discussed next. To investigate the effect of deposition cycle on the composition of Ru−Sb−SnO2 electrode, the molar contents of Ru, Sb, and Sn were detected by EDS characterization. Figure 6 displays the variations of Ru, Sb, and Sn contents of deposition layer with the deposition cycle. Observed from Figure 6a, with increasing deposition cycle, the content of Ru continuously increases and then decreases, revealing a continuous composition gradient. The content of Sb in the deposition layer continuously decreases because of the decrease of Sb ion bulk concentration. However, for the variation of Sn content, it appears slightly reduced and then rapidly ascendant, which can be the influence of Ru and Sb deposition. Figure 6d displays the increase of deposition thickness with deposition cycles. The composition gradient variation is schematically illustrated in Figure 7. The case for the variation of Ru composition gradient can be explained as the increase of Ru content is probably on account of the case that the increased electroactive sites on the deposit surface is preferential for Ru deposition, while the decrease can be ascribed to the decreased

ln(c0/c) = kt

(1)

where c0 and c are the concentrations of methylene blue at initial and given time, respectively, and k is the kinetic rate H


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To investigate sufficiently the electrocatalytic ability of Ti/ Ru−Sb−SnO2 electrodes, the electrocatalytic decolorization processes of orange II and methyl orange were also performed, and the results are shown in Figure 10. Figure 10a shows the decolorization performance of different Ru−Sb−SnO2 electrodes against orange II. After 120 min, the decolrization efficiencies are 90.4%, 91.7%, 95.2%, 83.3%, 86.8%, and 82.0%, respectively. The first-order kinetics rate constants are 19.2 × 10−3, 20.7 × 10−3, 25.1 × 10−3, 15.4 × 10−3, 16.6 × 10−3, and 14.3 × 10−3 min−1, respectively. This result shows that Ru−Sb−SnO2(200) electrode exerts the best performance for the electrocatalytic orange II decolorization. For the methyl orange decolorization, the situation is similar: Ru−Sb− SnO2(200) has the best decolorization efficiency for methyl orange after 120 min (95.2%) and the largest decolorization rate constant (25.2 × 10−3 min−1). These results show that Ti/ Ru−Sb−SnO2 (200) electrode has a wide use for the decolorization of different dyes. 3.5. Electrode Stability. To compare the stability, the accelerated service life experiments were performed using chronopotentiommetry with a constant anodic current density of 100 mA cm−2. Figure 11 shows the accelerated service life

constant. Obtained from the slope of the inset curve in Figure 8b, the kinetic rate constants of methylene blue decolorization are 16.0 × 10−3, 19.0 × 10−3, 23.4 × 10−3, 20.1 × 10−3, 19.4 × 10−3, and 17.2 × 10−3 min−1, respectively. The rate constant of Ti/Ru−Sb−SnO2(200) is 1.44 times as much as that of Ti/ Ru−Sb−SnO2 (DC). These show that the Ti/Ru−Sb− SnO2(200) electrode has the best electrocatalytic performance for methylene blue decolorization. Figure 9 shows the UV−vis spectrum of methylene blue on Ti/Ru−Sb−SnO2(200) electrode. The UV−vis absorbance

Figure 9. Changes in the UV−vis absorbance spectrum of methylene blue after different time intervals on Ti/Ru−Sb−SnO2 electrode with 200 cycles.

peaks of initial methylene blue solution appear at wavelengths of 664, 293, and 246 nm: the peak at 664 nm is attributed to dimenthylamino group as the chromophore, and the other two peaks are attributed to the benzene rings derived from the decomposition of methylene blue molecule.43 All characteristic peaks of methylene blue decrease with the electrocatalytic decolorization process proceeding. The absorbance peak at 664 nm quickly decreases because of the N-demethylation and deamination of methylene blue, and the peaks at 293 and 246 nm decrease as a consequence of benzene ring cracking. During the electrocatalytic decolorization process, no new peaks appear, implying a complete oxidative decomposition of methylene blue.

Figure 11. Accelerated service life tests on Ti/Ru−Sb−SnO2(DC) and Ti/Ru−Sb−SnO2(200) electrodes in a 0.25 M Na2SO4 solution with a constant anodic current density of 100 mA cm−2.

curves of electrodes. The anode potential is recorded as a function of time, and it indicates that the electrode is

Figure 10. Decolorization efficiencies of (a) orange II and (b) methyl orange on Ru−Sb−SnO2 electrodes. Insets are the corresponding decolorization kinetics curves. I


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developments in thin films, nanowires, and nanorods. Chem. Rev. 2014, 114, 7442−7486. (4) Wang, H.; Rogach, A. L. Hierarchical SnO2 nanostructures: recent advances in design, synthesis, and applications. Chem. Mater. 2014, 26, 123−133. (5) Manjula, P.; Boppella, R.; Manorama, S. V. A facile and green approach for the controlled synthesis of porous SnO2 nanospheres: application as an efficient photocatalyst and an excellent gas sensing material. ACS Appl. Mater. Interfaces 2012, 4, 6252−6260. (6) Comninellis, Ch.; Vercesi, G. P. Problems in DSA coating deposition by thermal decomposition. J. Appl. Electrochem. 1991, 21, 136−142. (7) Kotz, R.; Stucki, S.; Carcer, B. Electrochemical waste water treatment using high overvoltage anodes. part I: physical and electrochemical properties of SnO2 anodes. J. Appl. Electrochem. 1991, 21, 14−20. (8) Comninellis, Ch.; Pulgarin, C. Electrochemical oxidation of phenol for wastewater treatment using SnO2 anodes. J. Appl. Electrochem. 1993, 23, 108−112. (9) Wu, T.; Zhao, G.; Lei, Y.; Li, P. Distinctive tin dioxide anode fabricated by pulse electrodeposition: high oxygen evolution potential and efficient electrochemical degradation of fluorobenzene. J. Phys. Chem. C 2011, 115, 3888−3898. (10) Chen, Y.; Hong, L.; Xue, H.; Han, W.; Wang, L.; Sun, X.; Li, J. Preparation and characterization of TiO2-NTs/SnO2-Sb electrodes by electrodeposition. J. Electroanal. Chem. 2010, 648, 119−127. (11) Cui, Y.-H.; Feng, Y.-J.; Liu, Z.-Q. Influence of rare earths doping on the structure and electro-catalytic performance of Ti/Sb-SnO2 electrodes. Electrochim. Acta 2009, 54, 4903−4909. (12) Adams, B.; Tian, M.; Chen, A. Design and electrochemical study of SnO2-based mixed oxide electrodes. Electrochim. Acta 2009, 54, 1491−1498. (13) Yang, S. Y.; Choo, Y. S.; Kim, S.; Lim, S. K.; Lee, J.; Park, H. Boosting the electrocatalytic activities of SnO2 electrodes for remediation of aqueous pollutants by doping with various metals. Appl. Catal., B 2012, 111−112, 317−325. (14) Hu, F.; Dong, Z.; Cui, X.; Chen, W. Improved SnO2-Sb2O4 based anode modified with Cr3C2 and CNT for phenol oxidation. Electrochim. Acta 2011, 56, 1576−1580. (15) Duan, Y.; Wen, Q.; Chen, Y.; Duan, T.; Zhou, Y. Preparation and characterization of TiN-doped Ti/SnO2-Sb electrodeby dip coating for Orange II decolorization. Appl. Surf. Sci. 2014, 320, 746−755. (16) Duan, T.; Chen, Y.; Wen, Q.; Duan, Y. Enhanced electrocatalytic activity of nano-TiN composited Ti/Sb-SnO2 electrode fabricated by pulse electrodeposition for methylene blue decolorization. RSC Adv. 2014, 4, 57463−57475. (17) Duan, T.; Wen, Q.; Chen, Y.; Zhou, Y.; Duan, Y. Enhancing electrocatalytic performance of Sb-doped SnO2 electrode by compositing nitrogen-doped graphene nanosheets. J. Hazard. Mater. 2014, 280, 304−314. (18) Fan, C. M.; Hua, B.; Wang, Y.; Liang, Z. H.; Hao, X. G.; Liu, S. B.; Sun, Y. P. Preparation of Ti/SnO2-Sb2O4 photoanode by electrodeposition and dip coating for PEC oxidations. Desalination 2009, 249, 736−741. (19) Chen, Z.; Li, S.; Zhang, W. Pulse electrodeposition of ZnO/ SnO2 composite films: growth mechanism, structural and optical studies. J. Alloys Compd. 2013, 557, 274−278. (20) Lai, M.; Lim, J.-H.; Mubeen, S.; Rheem, Y.; Mulchandani, A.; Deshusses, M. A.; Myung, N. V. Size-controlled electrochemical synthesis and properties of SnO2 nanotubes. Nanotechnology 2009, 20, 185602−185607. (21) Hu, F.; Cui, X.; Chen, W. Pulse Electro-codeposition of Ti/ SnO2-Sb2O4-CNT electrode for phenol oxidation. Electrochem. SolidState Lett. 2010, 13, F20−F23. (22) Zhang, L.; Xua, L.; He, J.; Zhang, J. Preparation of Ti/SnO2-Sb electrodes modified by carbon nanotube for anodic oxidation of dye wastewater and combination with nanofiltration. Electrochim. Acta 2014, 117, 192−201.

deactivated when the potential increased 5 V from its initial value.6 Ti/Ru−Sb−SnO2(DC) electrode displayed a continuous increase by 5 V in potential and got deactivated within 11.9 h. Ti/Ru−Sb−SnO2(200) demonstrated a similar tendency in the potential variation with electrolysis time. As observed from Figure 11, the accelerated service lives of Ti/Ru−Sb− SnO2(DC) and Ti/Ru−Sb−SnO2(200) electrodes are 11.9 and 18.3 h, respectively. The accelerated life of modified electrode is 1.54 times as much as that of control electrode. The improved accelerated lifetime can be ascribed to the morphology and composition caused by the application of selective electrodeposition.

4. CONCLUSIONS In summary, we report herein that compositionally continuous gradient Ti/Ru−Sb−SnO2 electrode with vertically aligned sheets was successfully fabricated by the selective potential pulse electrodeposition method. The experimental results and analysis suggest that the distinct morphology and surface composition for deposition layer strongly depend on the electrodeposition cycle and can be attributed to the effect of diffusion layer and the effect of applied potentials. The electrocatalytic dye decolorization experiments confirm that Ti/Ru−Sb−SnO2 electrode possesses a good catalytic performance. We expect that this method can be extended to prepare electrode and to improve the electrocatalytic activity. In the next experiment, we will further investigate and optimize the potential pulse to improve the electrode performance.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images for deposition at −0.7 and −0.9 V; the EDS results of electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare the following competing financial interest(s): The Doctoral Program of the Ministry of Education (no. 20132304110027), the Fundamental Research Funds for the Central Universities, and the Fundamental Research Operating Expense for the Central Universities (HEUCFD1417).

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ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (nos. 51179033 and 21476053), the Doctoral Program of the Ministry of Education (no. 20132304110027), the Fundamental Research Funds for the Central Universities, and the Fundamental Research Operating Expense for the Central Universities (HEUCFD1417).

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REFERENCES

(1) Batzill, M.; Diebold, U. The surface and materials science of tin oxide. Prog. Surf. Sci. 2005, 79, 47−154. (2) Das, S.; Jayaraman, V. SnO2: a comprehensive review on structures and gas sensors. Prog. Mater. Sci. 2014, 66, 112−255. (3) Chen, Z.; Pan, D.; Li, Z.; Jiao, Z.; Wu, M.; Shek, C.-H.; Wu, C. M. L.; Lai, J. K. L. Recent advances in tin Dioxide materials: some J


Article

The Journal of Physical Chemistry C (23) Li, G.; Wang, Y.-H.; Chen, Q.-Y. Influence of fluoride-doped tin oxide interlayer on Ni-Sb-SnO2/Ti electrodes. J. Solid State Electrochem. 2013, 17, 1303−1309. (24) Li, P.; Zhao, G.; Cui, X.; Zhang, Y.; Tang, Y. Constructing stake structured TiO2-NTs/Sb-doped SnO2 electrode simultaneously with high electrocatalytic and photocatalytic performance for complete mineralization of refractory aromatic acid. J. Phys. Chem. C 2009, 113, 2375−2383. (25) Chai, S.; Zhao, G.; Li, P.; Lei, Y.; Zhang, Y.; Li, D. Novel sievelike SnO2/TiO2 nanotubes with integrated photoelectrocatalysis: fabrication and application for efficient toxicity elimination of nitrophenol wastewater. J. Phys. Chem. C 2011, 115, 18261−18269. (26) Li, P.; Zhao, G.; Li, M.; Cao, T.; Cui, X.; Li, D. Design and high efficient photoelectric-synergistic catalytic oxidation activity of 2D macroporous SnO2/1D TiO2 nanotubes. Appl. Catal., B 2012, 111− 112, 578−585. (27) Wang, Y.; Brezesinski, T.; Antonietti, M.; Smarsly, B. Ordered mesoporous Sb-, Nb-, and Ta-doped SnO2 thin films with adjustable doping levels and high electrical conductivity. ACS Nano 2009, 3, 1373−1378. (28) Moir, J.; Soheilnia, N.; O’Brien, P.; Jelle, A.; Grozea, C. M.; Faulkner, D.; Helander, M. G.; Ozin, G. A. Enhanced hematite water electrolysis using a 3D antimony-doped tin oxide electrode. ACS Nano 2013, 7, 4261−4274. (29) Wang, L.; Palacios-Padrós, A.; Kirchgeorg, R.; Tighineanu, A.; Schmuki, P. Enhanced photoelectrochemical water splitting efficiency of a hematite-ordered Sb:SnO2 host-guest system. ChemSusChem 2014, 7, 421−424. (30) Shukla, A.; Jain, N.; Chona, R. A review of dynamic fracture studies in functionally graded materials. Strain 2007, 43, 76−95. (31) Jha, D. K.; Kant, T.; Singh, R. K. A critical review of recent research on functionally graded plates. Compos. Struct. 2013, 96, 833− 849. (32) Paravannoor, A.; Nair, A. S.; Ranjusha, R.; Praveen, P.; Subramanian, K. R. V.; Sivakumar, N.; Nair, S. V.; Balakrishnan, A. Camphoric carbon-grafted Ni/NiO nanowire electrodes for highperformance energy-storage systems. ChemPlusChem 2013, 78, 1258− 1265. (33) Jiang, T.; Wang, Z.; Ren, B.; Qiao, J.; Sun, W.; Sun, K. Compositionally continuously graded cathode layers of (Ba0.5Sr0.5)(Fe0.91Al0.09)O3‑δ-Gd0.1Ce0.9O2 by wet powder spraying technique for solid oxide fuel cells. J. Power Sources 2014, 247, 858−864. (34) Krabbenborg, S. O.; Huskens, J. Electrochemically generated gradients. Angew. Chem., Int. Ed. 2014, 53, 9152−9167. (35) Dover, R. B. V.; Schneemeyer, L. F.; Fleming, R. M. Discovery of a useful thin-film dielectric using a composition-spread approach. Nature 1998, 392, 162−164. (36) Potyrailo, R. A.; Mirsky, V. M. Combinatorial and highthroughput development of sensing materials: the first 10 years. Chem. Rev. 2008, 108, 770−813. (37) Caselli, D.; Liu, Z.; Shelhammer, D.; Ning, C.-Z. Compositiongraded nanowire solar cells fabricated in a single process for spectrumsplitting photovoltaic systems. Nano Lett. 2014, 14, 5772−5779. (38) Jayaraman, S.; Hillier, A. C. Electrochemical synthesis and reactivity screening of a ternary composition gradient for combinatorial discovery of fuel cell catalysts. Meas. Sci. Technol. 2005, 16, 5−13. (39) Dusastre, V.; Williams, D. E. Sb(III) as a surface site for water adsorption on Sn(Sb)O2 and its effect on catalytic activity and sensor behavior. J. Phys. Chem. B 1998, 102, 6732−6737. (40) He, H.; Dai, H. X.; Au, C. T. Defective structure, oxygen mobility, oxygen storage capacity, and redox properties of RE-based (RE = Ce, Pr) solid solutions. Catal. Today 2004, 90, 245−254. (41) Biçer, M.; Köse, H.; Şişman, I.̇ Selective electrodeposition and growth mechanism of thermoelectric bismuth-based binary and ternary thin films. J. Phys. Chem. C 2010, 114, 8256−8263. (42) Xu, D.; Yan, X.; Diao, P.; Yin, P. Electrodeposition of vertically alighed palladium nanoneedles and their application as active substrates for surface-enhanced Raman scattering. J. Phys. Chem. C 2014, 118, 9758−9768.

(43) Liang, X.; Zhong, Y.; Zhu, S.; Ma, L.; Yuan, P.; Zhu, J.; He, H.; Jiang, Z. The contribution of vanadium and titanium on improving methylene blue decolorization through heterogeneous UV-Fenton reaction catalyzed by their co-doped magnetite. J. Hazard. Mater. 2012, 199−200, 247−254.

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