Accepted Manuscript Title: Enhancing Electrocatalytic Performance of Sb-doped SnO2 Electrode by Compositing Nitrogen-doped Graphene Nanosheets Author: Tigang Duan Qing Wen Ye Chen Yiding Zhou Ying Duan PII: DOI: Reference:

S0304-3894(14)00674-8 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.08.018 HAZMAT 16192

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

27-4-2014 14-8-2014 17-8-2014

Please cite this article as: T. Duan, Q. Wen, Y. Chen, Y. Zhou, Y. Duan, Enhancing Electrocatalytic Performance of Sb-doped SnO2 Electrode by Compositing Nitrogen-doped Graphene Nanosheets, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.08.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhancing Electrocatalytic Performance of Sb-doped SnO2 Electrode

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by Compositing Nitrogen-doped Graphene Nanosheets Tigang Duan, Qing Wen*, Ye Chen*, Yiding Zhou, Ying Duan

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Key Laboratory of Superlight Materials and Surface Technology of Ministry of

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Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin, 15001, Heilongjiang, China

Tel.: +86-13039978811, +86-13059004260.

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*Corresponding author.

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Highlights

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Email-address: [email protected] (Q Wen), [email protected] (Y Chen).

Sb-doped SnO2 electrode is modified with nitrogen-doped graphene nanosheets.

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Accelerated life of Ti/Sb-SnO2-NGNS is 4.45 times as long as that of Ti/Sb-SnO2. Electroactive sites of Ti/Sb-SnO2-NGNS are 1.43 times more than that of Ti/Sb-SnO2.

The decolorization rate constant of MB on Ti/Sb-SnO2-NGNS is 36.6 min-1. The decolorization rate constant of OII on Ti/Sb-SnO2-NGNS is 44.0 min-1.

Abstract: An efficient Ti/Sb-SnO2 electrode modified with nitrogen-doped graphene nanosheets (NGNS) was successfully fabricated by the sol-gel and dip coating method. 1

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Compared with Ti/Sb-SnO2 electrode, the NGNS-modified electrode possesses smaller unite crystalline volume (71.11 Å3 versus 71.32 Å3), smaller electrical resistivity (13 Ω·m versus 34 Ω·m), and lower charge transfer resistance (10.91 Ω

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versus 21.01 Ω). The accelerated lifetime of Ti/Sb-SnO2-NGNS electrode is

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prolonged significantly, which is 4.45 times as long as that of Ti/Sb-SnO2 electrode.

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The results of X-ray photoelectron spectroscopy measurement and voltammetric charge analysis indicate that introducing NGNS into the active coating can increase

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more reaction active sites to enhance the electrocatalytic efficiency. The electrochemical dye decolorization analysis demonstrates that Ti/Sb-SnO2-NGNS

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presents efficient electrocatalytic performance for methylene blue and orange II decolorization. And its pseudo-first order kinetic rate constants for methylene blue

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and orange II decolorization are 36.6 and 44.0 min-1, respectively, which are 6.0 and 7.1 times as efficient as those of Ti/Sb-SnO2, respectively. Considering the significant

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electrocatalytic activity and low resistivity of Ti/Sb-SnO2-NGNS electrode, the cost of

wastewater treatment can be expected to be reduced obviously and the application prospect is broad.

Key words: Nitrogen-doped graphene; Sb-doped SnO2; Electrochemical dye

decolorization.

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1. Introduction Electrochemical oxidation technology (EOT), with hydroxyl radical (·OH) as

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oxidant, has been widely known as a green and effective technology for the treatment of wastewater containing toxic or non-biodegradable organic contaminant, due to its

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high oxidation efficiency, fast reaction rate and easy operation [1 - 3]. EOT is closely

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associated with the heterogeneous electron transfer reaction on electrode/electrolyte interface, so the effectiveness of electrocatalytic oxidation process depends largely on

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the properties of electrode materials, such as composition, crystal structure,

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morphology, etc. [4]. Desirable electrodes must fulfil three requirements: (i) high electrocatalytic activity in pollutant degradation; (ii) high stability under anodic

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polarization conditions; (iii) low production costs and long lifetime [5 - 7].

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Up to date, numerous types of electrodes including graphite [8], boron-doped

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diamond (BDD) [9], Pt [10], IrO2 [11], RuO2 [12], MnO2 [13], PbO2 [14], and Sb-doped SnO2 [15] electrodes have been investigated. Among all these electrodes, some researchers have found that Ti/Sb-SnO2 electrode is one of the most promising

electrodes due to the high oxygen evolution over-potential and favorable electrocatalytic characteristic [16]. In order to further improve the performance of Ti/Sb-SnO2 electrode, many efforts have been done. One of effective and easy methods is introducing dopants into the oxide layers of electrodes. Some foreign materials, such as rare earths, noble metals and other materials, have been chosen to modify the Ti/Sb-SnO2 electrodes [17]. It has been proved that the doping of these foreign materials can improve of electrocatalytic oxidation activity or stability of 3

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electrodes. For example, the introduction of rare earth Ce, Eu, Gd and Dy can increase the electrocatalytic performance of Ti/Sb–SnO2 electrodes to some degree [18], but doping some rare earth, such as Y, can slightly reduce the accelerated service life [19].

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The incorporation of Pt, Ir in the Ti/Sb-SnO2 electrodes can greatly increase the

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service lifetime [7, 20], but the addition of Ir reduces the efficiency for wastewater

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treatment as the existence of Ir results in a decrease of oxygen evolution potential [21, 22].

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In recent years, doping carbon nanomaterials into oxide layers of Ti/Sb-SnO2 electrodes during their formation has become a new highlight for the modification of

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electrocatalytic SnO2 electrodes. Hu et al. [23] have reported that carbon nanotube modified Ti/SnO2-Sb2O4 electrode possesses improved stability and activity. Zhang et

te

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al. [24] have had an investigation on the service life and electrocatalytic performance of carbon nanotube modified Ti/SnO2-Sb electrode. The results of their works show

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that the introduction of carbon nanotubes gives rise to the formation of compact active layer, increase of oxygen evolution potential, and increase of active sites for electrocatalytic oxidation; this may be ascribed to the aspects of growth guiding and energy buffering effects caused by carbon nanotubes [25]. To further investigate the effects of carbon nanomaterials on the electrocatalytic

electrode, herein we introduce nitrogen-doped graphene nanosheets to modify the Sb-doped SnO2 electrodes. As one of new two-dimensional carbon nanomaterials, graphene has possessed higher mechanical strength, faster electronic transport rate, greater specific area and more excellent electrochemical performance than carbon

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nanotube, due to its ideal two-dimensional structure and peculiar electronic characteristics [26]. Some properties of graphene such as electronic, thermal vibration, mechanical and chemical properties are changed qualitatively when nitrogen atoms

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are doped into graphene [27]. Consequently, nitrogen-doped graphene shows some

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peculiar features and its application performance is enhanced. Based on the above

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consequence, nitrogen-doped graphene has greatly attracted attentions of many researchers and has been comprehensively investigated in various applications

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including fuel cells, supercapacitors, lithium-ion batteries, electronic devices and biosensors [28]. Zhang et al. [29] have studied the performance of nitrogen-doped

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graphene as oxygen reduction electrocatalyst. It is considered that doping nitrogen into graphene can modulate the electronic property and broaden the energy gap to

te

d

introduce asymmetry spin density and atomic charge density, making it possible for nitrogen-doped graphene to show high electroncatalytic activities. However, few

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works have been reported about the nitrogen-doped graphene modification of electrocatalytic electrodes for water treatment. On account of the superior properties of nitrogen-doped graphene, it is expected

that some performance of electrocatalytic electrodes applied for water treatment, such as the stability, electric conductivity and catalytic activity, will be significantly enhanced through doping nitrogen-doped graphene nanosheets (NGNS). In this paper, Ti/Sb-SnO2-NGNS composite electrodes were fabricated by the sol-gel and dip

coating method. The electrode surface characterization was investigated through X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray

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photoelectron spectroscopy (XPS). To estimate the electrocatalytic performance of electrodes, we selected methylene blue (MB, as the azo-free dye) and orange II (OII, as the azo dye) as the targets, investigated the electrochemical decolorization

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efficiencies of MB and OII, and discussed the effects of current densities, initial pH

cr

values and dye concentrations. Some electrochemical measurements were conducted

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to analyze oxygen evolution potential, stability, and electrochemical activity. The results proved that the performance of Sb-doped SnO2 electrode was advanced greatly

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due to the introduction of nitrogen-doped graphene nanosheets.

2. Experimental

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2.1. Preparation of Ti/ Sb-SnO2-NGNS composite electrodes The graphite oxide was synthesized from natural graphite powder by the modified

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Hummers method [30]. 500 mg graphite oxide was added into 500 mL distilled water and ultrasonically dispersed for 2 h to obtain a graphene oxide homogeneous aqueous dispersion (1 mg mL-1). And 8 g glucose was added into the aqueous dispersion and

magnetically stirred for 30 min. Next 4 mL ammonia was dropped into the resulting dispersion. Then the mixture was kept in a 95 °C water bath for 90 min. Finally, the resulting black dispersion was filtrated, washed with distilled water and dried to obtain graphene nanosheets (GNS). Nitrogen-doped graphene nanosheets (NGNS) were synthesized by the nitric acid (as the nitrogen source) treatment of graphene nanosheets. 300 mg as-prepared 6

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graphene nanosheets were ultrasonically dispersed into 100 mL 5 % nitric acid solution. And the resulting solution was heated and reflowed at 105 °C for 6 h. Finally, nitrogen-doped graphene nanosheets were obtained through filtrating, washing with

cr

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distilled water, and dried at 80 °C for 8 h.

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The tin precursor solution was obtained through dissolving 11.3 g SnCl2·2H2O into 100 mL ethyl alcohol and reflowing at 83 °C for 2 h. The preparation of the

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antimony precursor solution was done in the same way, with an addition of 1.27 g SbCl3 instead of stannous chloride in 50 mL ethyl alcohol. Then, both precursor

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solutions were mixed uniformly and reflowed at 83 °C for 2 h. Finally, the resulting

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solution was aged in a 40 °C water bath to obtain a flavescent sol precursor.

Ti sheets (0.5 mm thickness, 99.6 % purity) went through mechanically polishing,

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degreasing in a 10 % NaOH solution at 85 °C for 1 h, and etching in a 10 % oxalic acid solution at 85 °C for 2h. Then pretreated Ti sheets were preserved in a 3 % oxalic acid solution.

Ti/Sb-SnO2-NGNS electrodes were prepared by the sol-gel and dip coating

method. Before dip coating, 20 mL ethylene glycol homogeneous suspension containing 17.3 mg as-prepared NGNS was added into 50 mL sol precursor solution, and the mixed solution was evaporated to 50 mL at 80 °C and ultrasonically dispersed for 10 min. The pretreated Ti sheet was dipped and coated four times in the sol precursor solution using a dip coater. And the coated Ti sheet was dried at 100 °C for

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10 min, and calcinated at 350 °C for 15 min in a muffle furnace. This procedure was repeated 15 times. Finally, the coated Ti sheet was calcinated at 600 °C for 2 h to obtain Ti/Sb-SnO2-NGNS electrode. The procedure to the preparation of

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Ti/Sb-SnO2-GNS electrode was the same as mentioned above, except adding GNS

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instead of NGNS, while Ti/Sb-SnO2 electrode was prepared through the above

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process except mixing NGNS.

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2.2. Characterization of electrodes

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The morphologies of Ti/Sb-SnO2 and Ti/Sb-SnO2-NGNS electrodes were scanned by scanning electron microscope (SEM ， INSPECT S50, MAKE FEI). X-ray

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diffraction (XRD, Rigaku D/Max2500) was performed with a Cu Kα radiation at 40

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kV and 150 mA to obtain the crystalline patterns of tin oxide. X-ray photoelectron

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spectroscopy (XPS) measurements were carried out in a high vacuum system using a Thermo ESCALAB 250 Electron Spectrometer (Al Kα radiation; hν = 1486.6 eV) to

analyze the composition and chemical state of the surface elements. The binding energies were calibrated with respect to the signal of contamination carbon (284.62 eV). The XPS core level spectra were analyzed using XPS Peak Processing software with a Gaussian-Lorentzian sum function after a background subtraction and the Gaussian-Lorentzian

mixing

ratio

was

kept

in

the

range

of

0.2.

The

Brunauer-Emmett-Teller (BET) surface area was obtained by measuring N2 adsorption isotherms using a TriStar 3000 at 77 K.

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2.3. Electrochemcial dye decolorization experiments

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The electrochemical decolorization experiments were performed in 50 mL 50 mg

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L-1 dye (MB and OII, respectively) with 0.25 M Na2SO4 solution. The electrodes,

prepared as described above, served as the working electrodes, a platinum sheet was

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used as the counter electrode, and saturated calomel electrode (SCE) as the reference

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electrode. 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

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process of dyes was monitored by the UV-Vis spectrophotometer, and the concentrations of methylene blue solution and orange II solution were measured by

te

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respectively.

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the absorbance intensity at the characteristic wavelength of 664 nm and 498 nm,

2.4. Electrochemical experiments

Electrochemical experiments were performed in a conventional three-electrode

cell recorded by an electrochemical workstation (SP-240, BioLogic Science Instruments, France). The prepared Ti/Sb-SnO2 and Ti/Sb-SnO2-NGNS electrodes

served as the working electrode with a test area of 1 × 1 cm2. A platinum sheet (2 × 2 cm2) and saturated calomel electrode (SCE) were used as a counter and reference electrode, respectively. The electrolyte was 0.25 M Na2SO4 solution. Linear sweep

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voltammetry (LSV) measurements were performed with a sweep speed of 10 mV s-1. Cyclic voltammetry (CV) was performed to test electrochemical properties of electrodes in 0.1 M KCl solution using 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] redox couple,

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and the sweep rate was 20 mV s-1 with the potential region of - 0.2 V to 0.6 V.

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Electrochemical impedance spectroscopy (EIS) was conducted in a range of 104

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Hz-10-1 Hz and at a potential of 1.9 V (vs. SCE) with an amplitude signal of 5 mV. Before each measurement, the system was stabilized at open circuit voltage for 5-10

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min.

Electrode stability tests were characterized using chronopotentiommetry with an

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anodic current density of 100 mA cm-2 in the three-electrode system as above. The electrolyte was 0.25 M Na2SO4 solution. The potential was recorded as a function of

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initial value [31].

te

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time, and it indicated electrode deactivation that the potential increased 5 V from its

3. Results and discussion

3.1. Electrode characterization

Crystal patterns and composition of electrodes make a great difference on the

electrode performance. XRD can be used to study the crystalline structure and lattice parameters of the electrode active coating. The XRD patterns of Ti/Sb-SnO2, Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS electrodes are shown in Fig. 1. The diffraction peak positions for all samples coincide with those of tetragonal rutile SnO2 10

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(PDF#41-1445) with the diffraction peaks appearing at (110), (101), (200), (211) and (301). The peaks corresponding to antimony oxide are not found, which may be connected with either the low content of antimony element or the doping of antimony

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ions into the stannic oxide phase.

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Observed from Fig. 1, the diffraction intensities for Ti/Sb-SnO2-GNS and

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Ti/Sb-SnO2-NGNS are much stronger than those for Ti/Sb-SnO2, demonstrating an increase of crystallinity. And the diffraction intensities of (211) and (301) planes for

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Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS increase significantly, suggesting the preferred orientation along the (211) and (301) directions. These results may be

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ascribed to growth guiding of graphene nanosheets and nitrogen-doped graphene nanosheets. Furthermore, the peak intensity of (211) plane for Ti/Sb-SnO2-NGNS

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are larger than those for Ti/Sb-SnO2-GNS, suggesting the effect of nitrogen-doped graphene nanosheets on the preferred growth of SnO2 crystal along the (211)

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direction.

The lattice parameters (a=b and c) and unit cell volume (V) for Ti/Sb-SnO2,

Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS electrodes were obtained from the main diffraction peaks of SnO2 phase. Table 1 shows the average values of lattice parameters calculated by Bragg’s formula. The lattice parameters of SnO2 for

electrodes are smaller than those of the standard SnO2, which can be ascribed to the SnO2 lattice distortion caused by the lattice doping of antimony element. Furthermore, with doping nitrogen-doped graphene nanosheet, the unit cell volume of SnO2 decreases from 71.32 Å3 to 71.11 Å3, showing a decrease of crystallite size in the

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lattice shape; this is in accordance with the result of carbon nanotube modified SnO2 electrode [24]. It can be expected that Ti/Sb-SnO2-NGNS electrode has an improved

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electrocatalytic activity. Fig. 1

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cr

Table 1

Morphologies of electrodes were characterized by SEM shown in Fig. 2. The

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Sb-doped SnO2 coatings for all electrodes exhibit a cracked mud-like structure which is typical for oxide electrodes prepared using the sol-gel and thermal decomposition

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method. Due to the possibility that there exist different thermal expansion coefficients for the titanium substrate and oxide coating [32], the coating of Ti/Sb-SnO2 electrode

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has plenty of cracks, which is not easy to prevent the formation of titanium oxide in use and affects the stability of electrodes. In contrast with that of Ti/Sb-SnO2

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electrode, the coatings of Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS electrodes present

an improved crack-mud structure with fewer cracks, indicating a good coverage of the Ti substrate. This result shows that introducing graphene nanosheets or nitrogen-doped graphene nanosheets into coating is favorable to significantly decrease cracks on the coating surface as well as to enhance the stability. Furthermore, Ti/Sb-SnO2-NGNS

electrode

presents

smoother

and

more

compact

than

Ti/Sb-SnO2-GNS. As a consequence, Ti/Sb-SnO2-NGNS electrode can be expected to have better stability. Additional, BET tests show that the BET surface areas of Ti/Sb-SnO2, Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS electrodes are 50.4, 63.2 and

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73.2 m2 g-1, respectively. This denotes that Ti/Sb-SnO2-NGNS electrode was prepared with high specific surface area. The high specific surface area is closely related to electrochemical activity of electrodes and thus is one advantage for NGNS-modified

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electrode.

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cr

Fig. 2

XPS measurements of electrodes were conducted to analyze the chemical states

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of elements (shown in Fig. 3). Fig. 3(a) shows the XPS survey scan of Ti/Sb-SnO2-NGNS electrode. Sample has the Sn, Sb, O and C. The presence of C can

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be ascribed to the introduction of nitrogen-doped graphene. Fig. 3(b) shows the core level N1s spectrum for Ti/Sb-SnO2-NGNS. A low fitted peak is observed in the BE

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region of 397 - 402 eV, indicating the successful addition of nitrogen-doped graphene into the active coating. Figs. 3(c), (d) and (e) show the core level O1s spectra of

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Ti/Sb-SnO2, Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS electrodes. They were fitted

with the XPS Peak Processing software using Lorentzian-Gaussian peak shapes, and the fitted values were given in Table 2. The O1s peak is observed in the BE region of 529 - 535 eV. Two O1s peaks, which are a lower binding energy peak at about 530.51 - 530.75 eV and a higher binding energy peak at about 531.38 - 531.70 eV, can be observed. They are likely to be assigned to the lattice oxygen species (OL) and

adsorbed hydroxyl oxygen species (Oad), respectively [33, 34]. Seen from Table 2, the content of Oad for Ti/Sb-SnO2-GNS electrode is 26.19 %, which is higher than that for Ti/Sb-SnO2 electrode. Furthermore, with introducing

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nitrogen-doped graphene nanosheets into the oxide active coating, the content of Oad further is increased to 27.14 %, and the atomic ratio of Oad: OL is accordingly boosted to 37.24 %. These results are probably attributed to the higher degree of crystallinity

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and the preferred growth of SnO2 crystal for Ti/Sb-SnO2-NGNS electrode (as shown

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in Fig. 1). For the case that adsorbed hydroxyl oxygen species is the most active

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oxygen species and influences the catalytic activity of electrodes [35], therefore, the increased Oad content implies that the electrochemical activity of Ti/Sb-SnO2-NGNS

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electrode will be reasonably influenced by nitrogen-doped graphene. Fig. 3

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Table 2

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3.2. Electrochemical dye decolorization results analysis

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3.2.1.Comparison of decolorization performance between Ti/Sb-SnO2 electrode ， Ti/Sb-SnO2-GNS electrode and Ti/Sb-SnO2-NGNS electrode Ti/Sb-SnO2, Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS electrodes were applied to

the treatment of dye contaminants to investigate the electrocatalytic activity. Electrochemical methylene blue and orange II decolorization processes were performed in a galvanostatic condition of 20 mA cm-2 with the initial dye concentration of 50 mg L-1. Figs. 4(a) and (b) show the decolorization efficiency (η),

calculated according to Equation (1), with the variation of time:

 (%)  [( A0  At ) / A0 ] 100

(1)

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Where A0 and At are the absorbance at the initial time and t (min), respectively. Observed from Fig. 4(a), the methylene blue decolorization efficiency increases with

electrolysis

time,

and

the

decolorization

on

Ti/Sb-SnO2-GNS

and

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Ti/Sb-SnO2-NGNS are much more efficient than that on Ti/Sb-SnO2. After 60 min

cr

electrolysis, the methylene blue decolorization efficiencies of Ti/Sb-SnO2,

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Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS electrodes rise and reach 35.3 %, 77.7 % and 87.0 %, respectively. And after 100 min, the methylene blue decolorization

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efficiencies are 43.8 %, 93.6 % and 97.7 %, respectively. The curve variation of electrochemical azo dye orange II decolorization process is similar to that of azo-free

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dye methylene blue decolorization. These results reveal that Ti/Sb-SnO2-NGNS electrode has the best dye decolorization efficiency. The insets of Figs. 4(a) and (b)

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show the semi-log relationship of dye concentration with electrolysis time, and dye decolorization process is considered to follow the pseudo-first order kinetic model [36,

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37]:

ln(c0 / ct )  kt

(2)

Where c0 and ct are the concentrations of dye at the initial time and given time, respectively, and k is the kinetic rate constant. The kinetic rate constants for dye

decolorization were obtained from the slope of the inset curves in Figs. 4(a) and (b). The kinetic rate constants for methylene blue and orange II decolorization on Ti/Sb-SnO2-NGNS are 36.6×10-3 min-1 and 44.0×10-3 min-1, respectively, which are

higher than those on Ti/Sb-SnO2 and Ti/SbSnO2-GNS. Table 3 shows the pseudo-first order kinetic rate constant comparison of different electrodes. The rate constant of

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Ti/SnO2-Sb electrode for the C.I. Acid Red 73 decolorization is increased to 1.62 times through the yttrium modification, and it is increased to 1.93 times through carbon nanotube modification. In this work, the rate constants of Ti/Sb-SnO2-NGNS

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electrode for methylene blue and orange II decolorization are increased to 6.0 and 7.1

cr

times, respectively. These results reveal that doping nitrogen-doped graphene

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nanosheets into the catalyst coating can boost the electrocatalytic activity of electrode for the wastewater treatment.

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Figs. 4(c) and (d) show the variations of potential with electrolysis time for Ti/Sb-SnO2, Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS electrodes during the

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electrochemical dye decolorization process. Similarly, the potentials for all electrodes increase with electrochemical dye decolorization. However, the potential of

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Ti/Sb-SnO2 electrode is obviously higher than those of Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS, and it increases faster. And the potentials of Ti/Sb-SnO2-GNS and

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Ti/Sb-SnO2-NGNS electrodes are similar. This result means that Ti/Sb-SnO2-GNS

and Ti/Sb-SnO2-NGNS electrodes are much more stable and have a lower resistance. Meanwhile, with the introduction of highly conductive nitrogen-doped graphene nanosheets, the electrical resistivity of Ti/Sb-SnO2-NGNS electrode is reduced significantly from 34 Ω·m to13 Ω·m. The low resistivity of Ti/Sb-SnO2-NGNS

electrode can be beneficial to reduce the corresponding energy consumption, consequently reducing the cost of wastewater treatment. On the basis of the above results, introducing nitrogen-doped graphene nanosheets can largely enhance the reaction rate and lower the potential for the

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electrochemical dye decolorization, which is very significant to reduce the disposal costs in practice.

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Fig. 4

cr

Table 3

of Ti/Sb-SnO2-NGNS electrodes 5

shows

the

electrochemical

dye

decolorization

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Fig.

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3.2.2. Influence of numbers of coating preparation on the decolorization performance

processes

on

Ti/Sb-SnO2-NGNS electrodes with different coating preparation numbers. After 60 electrolysis,

the

decolorization

efficiencies

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min

of

methylene

blue

on

Ti/Sb-SnO2-NGNS electrodes with coating number of 10, 15 and 20 are 77.8 %, 87.0

te

d

% and 75.4 %, respectively. After 100 min, the decolrization efficiencies of methylene blue are 95.3 %, 97.7 % and 88.3 %, respectively. The pseudo-first order kinetics rate

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constants of methylene blue are 28.4, 36.6 and 20.7 min-1, respectively. This denotes

that Ti/Sb-SnO2-NGNS electrode with coating number of 15 possesses the best decolorization performance for methylene blue. The situation of orange II decolorization is similar to that of methylene blue decolorization. These results show that the optimal coating number of electrode is 15. Fig. 5

3.2.3. Influence of electrolysis conditions on dye decolorization Fig. 6(a) shows the changes of dye decolorization efficiency and kinetics rate

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constant with the initial dye concentration after 100 min electrolysis. With increasing the initial dye concentration, the decolorization efficiencies and kinetics rate constants decrease. When the concentrations of methylene blue and orange II are 50 mg L-1, the

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decolorization efficiencies are 97.7 % and 99 %, respectively. As the dye

cr

concentration is increased from 50 to 200 mg L-1, although the decolorization efficiencies are 70.1 % and 72.3 %, respectively, the dye decolorization amounts

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increase from 48.85 and 49.5 mg L-1 to 140.2 and 144.6 mg L-1, respectively.

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Although the decolorization efficiency of dye decreases, actual dye removal amount increases with increasing the initial dye concentration. Considering that the dye

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decolorization is almost 100 % when the concentration reaches 50 mg L-1, the level of 50 mg L-1 of dye is selected in subsequent experiments.

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Sb-doped SnO2 electrodes have been known as efficient electrocatalytic electrodes for electrochemically generating ·OH radicals [31], so the electrochemical

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decolorization of dye on Ti/Sb-SnO2-NGNS electrode greatly depends on the

oxidation from hydroxyl radicals formed on the anode surface. Besides, the direct electron transfer from dye to the anode is not ignorable. Both of the above processes, especially the oxidation from hydroxyl radicals, depend on the current density. Fig. 6(b) shows the effects of current density on dye decolorization efficiency and kinetics rate constant. When the current density is 10 mA cm-2, the decolorization rate

constants for methylene blue and orange II are only 13.5 and 24.8 min-1, respectively, after 100 min electrolysis. When the current density is increased from 20 mA cm-2 to 30 mA cm-2, only a slight increase occurs for orange II decolorization, but a decrease

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occurs for methlene blue decolorization. The situation of decolorization efficiency changes is similar to that of kinetics rate constant changes. With increasing current density, the rate of electrochemical process, including the hydroxyl radical oxidation

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and direct electron transfer, increases, which results in the increase of dye

cr

decolorization efficiency. However, when the current density increases further, the

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competitive electrochemical reaction such as the oxygen evolution gets obvious. As a consequence, taking account of the degradation rate and disposal cost, the current

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density of 20 mA cm-2 is applicable.

The effects of initial pH value on dye decolorization efficiency are presented in

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Fig. 6(c). The initial pH of the solution was adjusted using H2SO4 and NaOH solutions. For the methylene blue decolorization, the decolorization efficiencies in

te

d

acidic, neutral and alkaline conditions are 70.6 %, 97 % and 82.6 %, respectively, revealing that the neutral condition is beneficial. And for the orange II decolorization,

Ac ce p

the decolorization efficiencies in the acidic and neutral conditions are almost same but it decreases significantly in the alkaline situation, suggesting the alkaline condition is disadvantageous. The decolorization kinetics rate constants in the neutral condition are also the largest. Thus, it can be concluded that the neutral condition is beneficial to the electrochemical dye decolorization. Fig. 6

19

Page 19 of 49

3.3. Electrode stability

The electrode stability is one of important factors related with the electrode

ip t

quality. The stabilities of Ti/Sb-SnO2, Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS electrodes were investigated through accelerated service life tests. At a high current

cr

density, the severe oxygen evolution reaction occurring on the electrode surface

us

accelerates the electrode deactivation. Fig. 7 shows the accelerated service life curves of electrodes. Ti/Sb-SnO2 electrode demonstrates a sigmoid type increase in potential,

an

and its accelerated life curve towards stabilization in the high potential indicates the

M

deactivation. The situation for Ti/Sb-SnO2-NGNS electrode is different. The accelerated life curve of Ti/Sb-SnO2-NGNS is stable in the low potential and exhibits

d

a relatively fast increase in the high potential which suggests the electrode

te

deactivation. The accelerated lifetime of Ti/Sb-SnO2 is 6.0 h, and doping graphene

Ac ce p

nanosheets and doping nitrogen-doped graphene nanosheets prolong the lifetime to 18.1 h and 26.7 h, respectively, which are 3.02 and 4.45 times, respectively, as long as that of Ti/Sb-SnO2. By the accelerated service life test, the stability of Ti/Sb-SnO2-NGNS electrode is enhanced largely, which confirms our assumption. Fig. 7

3.4. Effects of doping NGNS on the electrochemical performance of electrode

Fig. 8 shows the LSV curves of Ti/Sb-SnO2, Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS electrodes. All electrodes have a wide electrochemical potential 20

Page 20 of 49

window from 0 V to about 1.6 V (vs. SCE), and present high oxygen evolution potentials. Observed from Fig. 8, the oxygen evolution potential of electrodes are about 1.75, 1.8 and 1.9 V (vs. SCE), respectively. Evidently, doping nitrogen-doped

ip t

graphene nanosheets into the coating advances the oxygen evolution potential. This

cr

may be ascribed to the preferred growth and the high degree of crystallization of

us

Ti/Sb-SnO2-NGNS electrode. Higher oxygen evolution potential can restrain the oxygen evolution reaction and reduce the energy consumption, which is favorable to

an

the electrocatalytic activity of electrodes. Thus Ti/Sb-SnO2-NGNS has the best electrocatalytic performance, which is confirmed by the electrochemical dye

M

decolorization.

d

Fig. 8

te

The behavior of the [Fe(CN)6]3-/[Fe(CN)6]4- redox couple was investigated for

Ac ce p

studying electrochemical performance of electrodes. All the cyclic voltammograms of Ti/Sb-SnO2, Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS electrodes show well-defined anodic and cathodic current peaks in Fig. 9. Compared with that of Ti/Sb-SnO2, the

anodic peak current density of Ti/Sb-SnO2-GNS increases from 0.96 to 1.19 mA cm-2,

and that of Ti/Sb-SnO2-NGNS further increases to 1.47 mA cm-2. It implies that

Ti/Sb-SnO2-NGNS electrode has the best electrochemical activity. Fig. 9

Fig. 10 shows the experimental and fitted EIS spectra for Ti/Sb-SnO2,

21

Page 21 of 49

Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS electrodes. EIS experiments were performed with a potential of 1.9 V (vs. SCE). In order to interpret the EIS spectra, the equivalent circuit was used to fit and to model the EIS behavior shown in the inset

ip t

of Fig. 10, and the simulation parameters of EIS data were listed in Table 4. The

cr

Nyquist plots for all electrodes are similar in form, and are characterized by one

us

resistance in higher frequency region and one depressed capacitive-resistive semicircle in middle-low frequency region. The resistance in higher frequency region

an

corresponding to resistance (R1) in the model represents the solution resistance. The depressed semicircle in middle-low frequency, which behaves as the constant phase

M

element (CPE2) and charge transfer resistance (R2) in the equivalent circuit, represents the electrochemical discharge process, and its behavior demonstrates the

te

d

electrochemical performance of electrode. A small depressed semicircular arc is displayed for Ti/Sb-SnO2-NGNS electrode, revealing a good electron transfer. As

Ac ce p

shown from Table 4, the R2 value of Ti/Sb-SnO2-NGNS are 10.91 ohm, only about 3/4

of Ti/Sb-SnO2-GNS (15.31 ohm) and 1/2 of Ti/Sb-SnO2 (21.01 ohm), respectively.

This result indicates that doping nitrogen-doped graphene can improve the electrochemical action of electrode. Fig. 10 Table 4

The roughness factor (Rf) and voltammetric charge of all electrodes were determined from the cyclic voltammetry (CV) to explore effects of doping graphene

22

Page 22 of 49

nanosheets and effects of doping nitrogen-doped graphene nanosheets. Roughness factor is connected with the real surface area of electrode, voltammetric charge is related to the amounts of electroactive sites, and both of them reflect the

ip t

electrocatalytic activity of electrodes [40].

cr

The roughness factor is defined as real surface area per apparent geometric area

of electrode, and is calculated by comparing determined capacitance with the

us

capacitance of a smooth oxide surface (60 μF cm-2) [41]. Fig. 11 shows the plots of

an

current densities at 0.7 V against sweep rates for all electrodes. The current densities of Fig. 11 were measured in the points of 0.7 V from the CV with 0.6 - 0.8 V potential

M

range for different sweep rates. The capacitances were obtained from the plot slopes in Fig. 11, and they were compared with the value of 60 μF cm-2 to estimate the

te

d

roughness factor. Compared with that of Ti/Sb-SnO2, the roughness factor of Ti/Sb-SnO2-GNS is boosted (91.67 versus 35.17, Table 5), and that of further

increases

Ac ce p

Ti/Sb-SnO2-NGNS

to

146.0. Thus,

the

introduction of

nitrogen-doped graphene nanosheets can improve remarkably the roughness of electrode surface and is favorable to the electrocatalytic activity. Fig. 11

The voltammetric charge (q*) is closely related to the real specific surface area and the amounts of electroactive sites, which greatly determines the electrocatalytic performance of electrode [40]. The total voltammetric charge qtotal*, which is related to the total electrochemically active surface area of the oxide coating, can be obtained

23

Page 23 of 49

through plotting the reciprocal of q* against the square root of the potential scan rate by using the following equation [41]: ( q* ) 1  (qtotal * ) 1  kv1/2

(3)

ip t

In addition, the outer voltammetric charge qouter*, which is the charge related to the

cr

most accessible electroactive surface area, is obtained through the extrapolation to v → ∞ in the plot of q* versus v-1/2 according to the following equation [42]:

q*  qouter *  k 'v 1/2 shows

these

plots

for

us

12

Ti/Sb-SnO2,

Ti/Sb-SnO2-GNS

and

an

Fig.

(4)

Ti/Sb-SnO2-NGNS electrodes and good linear fittings can be observed. As seen from

M

Fig. 12(a), Ti/Sb-SnO2-NGNS electrode appears the largest qtotal* value, showing that introducing nitrogen-doped graphene into the coating significantly improves the entire

te

d

electrochemically active surface area. The BET test result also indicates that the NGNS-modified electrode has the largest specific surface area, and thus it is not

Ac ce p

surprising that Ti/Sb-SnO2-NGNS possesses the largest total electroactive surface area. Fig. 12(b) shows the relationship between q* and v-1/2, and there are some

deviation at the high scan rate region, which may be owed to the uncompensated ohm drop that decreases the q* value [31]. It is observed that the value of outer voltammetric charge is obviously increased, which can be resulted from the increase of total voltammetric charge. This result also shows that the NGNS-modified electrode can provide with the most active sites for the electrocatalytic process. The values of voltammetric charges for all electrodes were listed in Table 5. Seen from Table 5, the values of qtotal* and qouter* for Ti/Sb-SnO2-NGNS are 2.03 and 2.43 times

24

Page 24 of 49

as large as those for Ti/Sb-SnO2, respectively, and they are 1.36 and 1.78 times as large as those for Ti/Sb-SnO2-GNS, respectively. These results indicate that the electrode modified with nitrogen-doped graphene nanosheets has higher effective

ip t

electrochemical surface area. Furthermore, these results are also confirmed through

cr

the above electrochemical dye decolorization experiments.

an

Table 5

us

Fig. 12

M

Conclusions

An efficient Ti/Sb-SnO2-NGNS electrode was successfully fabricated by the and

dip

coating

method.

Compared

d

sol-gel

with

Ti/Sb-SnO2

electrode,

te

Ti/Sb-SnO2-NGNS electrode possesses lower charge transfer resistance, higher

Ac ce p

oxygen evolution potential and larger active specific area. As a consequence of introducing nitrogen-doped graphene nanosheets, the accelerated service lifetime of Ti/Sb-SnO2-NGNS electrode is significantly improved. The electrochemical dye decolorization analysis shows that the decolorization process coincides with the pseudo-first order kinetics and that the rate constants on Ti/Sb-SnO2-NGNS electrode

are much more efficient than those on Ti/Sb-SnO2. Finally, considering the improved electrocatalytic activity and long service lifetime, the cost of the organic contaminant wastewater treatment using Ti/Sb-SnO2-NGNS electrode can be expected to be reduced significantly, revealing a good practical application prospect.

25

Page 25 of 49

Acknowledgment

ip t

The authors gratefully acknowledge the financial support provided by National

cr

Natural Science Foundation of China (No. 51179033), the Doctoral Program of

the Ministry of Education (No. 20132304110027), the Fundamental Research Funds the

Central

Universities

(HEUCF201403019),

us

for

Heilongjiang

an

Postdoctoral Science-Research Foundation (LBH-Q12118), and Special Fund Research Program for Talents of Science Technology Innovation in Harbin (No.

te

References

d

M

2009RFXXG204).

Ac ce p

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ip t

[5] S. Kim, S.K. Choi, B.Y. Yoon, S.K. Lim, Effects of electrolyte on the

cr

electrocatalytic activities of RuO2/Ti and Sb-SnO2/Ti anodes for water treatment,

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Appl. Catal. B: Environ. 97 (2010) 135-141.

[6] G. Zhao, X. Cui, Electrochemical degradation of refractory pollutant using a

an

novel microstructured TiO2 nanotubes/Sb-doped SnO2 electrode, Environ. Sci. Technol. 43 (2009) 1480-1486.

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[7] F. Montilla, E. Morallón, J.L. Vázquez, Evaluation of the electrocatalytic activity of antimony-doped tin dioxide anodes toward the oxidation of phenol in aqueous

te

d

solutions, J. Electrochem. Soc. 152 (2005) B421-B427. [8] J.L. Chen, G.C. Chiou, C.C. Wu, Electrochemical oxidation of 4-chlorophenol

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with granular graphite electrodes, Desalination 264 (2010) 92-96.

[9] S.C. Elaoud, M. Panizza, G. Cerisola, T. Mhiri, Electrochemical degradation of sinapinic acid on a BDD anode, Desalination 272 (2011) 148-153.

[10]A.G. Vlyssides, D. Papaioannou, M. Loizidoy, P.K. Karlis, A.A. Zorpas, Testing an electrochemical method for treatment of textile dye wastewater, Waste Manage. 20 (2000) 569-574. [11]E. Chatzisymeon, S. Fierro, I. Karafyllis, D. Mantzavinos, N. Kalogerakis, A. Katsaounis, Anodic oxidation of phenol on Ti/IrO2 electrode: experimental studies, Catal. Today 151 (2010) 185-189.

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[13]P.K. Sen, K. Das, Electrodeposited MnO2 as electrocatalyst for carbohydrate

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[14]S. Song, L. Zhan, Z. He, L. Lin, J. Tu, Z. Zhang, J. Chen, L. Xu, Mechanism of the anodic oxidation of 4-chloro-3-methyl phenol in aqueous solution using

an

Ti/SnO2-Sb/PbO2 electrodes, J. Hazard. Mater. 175 (2010) 614-621. [15]H. Lin, J. Niu, S. Ding, L. Zhang, Electrochemical degradation of acid

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M

perfluorooctanoic

Ti/SnO2-Sb/PbO2

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te

d

[16]M. Panizza, G. Cerisola, Direct and mediated anodic oxidation of organic pollutants, Chem. Rev. 109 (2009) 6541-6569.

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[17]S.Y. Yang, Y.S. Choo, S. Kim, S.K. Lim, J. Lee, H. Park, Boosting the electrocatalytic activities of SnO2 electrodes for remediation of aqueous

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[18]Y.-H. Cui, Y.-J. Feng, Z.-Q. Liu, Influence of rare earths doping on the structure and electro-catalytic performance of Ti/Sb-SnO2 electrodes, Electrochim. Acta 54 (2009) 4903-4909. [19]L. Xu, Z. Guo, L. Du, J. He, Decolourization and degradation of C.I. Acid Red 73 by anodic oxidation and the synergy technology of anodic oxidation coupling

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ip t

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an

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[26]V. Georgakilas, M. Otyepka, A.B. Bourlinos, V. Chandra, N. Kim, K.C. Kemp, P. Hobza, R. Zboril, K.S. Kim, Functionalization of graphene: covalent and non-covalent approaches, derivatives and applications, Chem. Rev. 112 (2012) 6156-6214. [27]H. Wang, T. Maiyalagan, X. Wang, Review on recent progress in nitrogen-doped graphene: synthesis, characterization, and its potential applications, ACS Catal. 2 (2012) 781-794.

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[30]W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339-1341.

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[31]Y. Chen, L. Hong, H. Xue, W. Han , L. Wang, X. Sun, J. Li, Preparation and characterization of TiO2-NTs/SnO2-Sb electrodes by electrodeposition, J.

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Electroanal. Chem. 648 (2010) 119-127.

[32]C.M. Fan, B. Hua, Y. Wang, Z.H. Liang, X.G. Hao, S.B. Liu, Y.P. Sun,

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[37]G. Zhao, J. Li, X. Ren, J. Hu, W. Hu, X. Wang, Highly active MnO2 nanosheet

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synthesis from graphene oxide templates and their application in efficient

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oxidative degradation of methylene blue, RSC Adv. 3 (2013) 12909-12914.

[38]Y. Yao, C. Zhao, M. Zhao, X. Wang, Electrocatalytic degradation of methylene

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blue on PbO2-ZrO2 nanocomposite electrodes prepared by pulse electrodeposition,

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[39]A.I. del Río, J. Fernández, J. Molina, J. Bonastre, F. Cases, On the behaviour of doped SnO2 anodes stabilized with platinum in the electrochemical degradation

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of reactive dyes, Electrochim. Acta 55 (2010) 7282-7289. [40]R. Berenguer, C. Quijada, E. Morallón, Electrochemical characterization of SnO2

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electrodes doped with Ru and Pt, Electrochim. Acta 54 (2009) 5230-5238.

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[42]F. Montilla, E. Morallón, A. De Battisti, J.L. Vázquez, Preparation and characterization of antimony-doped tin dioxide electrodes, part 1, electrochemical characterization, J. Phys. Chem. B 108 (2004) 5036-5043. [43]L.K. Xu, J.D. Scantlebury, Electrochemical surface characterization of IrO2-Ta2O5 coated titanium electrodes in Na2SO4 solution, J. Electrochem. Soc.

31

Page 31 of 49

an

us

cr

ip t

150 (2003) B288-B293.

M

Figure captions

te

Ti/Sb-SnO2-NGNS electrode.

d

Fig. 1 XRD patterns of Ti/Sb-SnO2 electrode, Ti/Sb-SnO2-GNS electrode and

Fig. 2 SEM images of (a) Ti/Sb-SnO2 electrode, (b) Ti/Sb-SnO2-GNS electrode and (c)

Ac ce p

Ti/Sb-SnO2-NGNS electrode; (d) Nitrogen sorption-desorption isotherms of electrodes.

Fig. 3 XPS spectra: (a) survey scanning spectrum and (b) N1s spectra of

Ti/Sb-SnO2-NGNS electrode; O1s spectra on the surfaces of (c) Ti/Sb-SnO2, (d)

Ti/Sb-SnO2-GNS and (e) Ti/Sb-SnO2-NGNS electrodes. Fig. 4 Variations of (a) MB and (b) OII decolorization efficiency with electrolysis time on Ti/Sb-SnO2, Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS electrodes; variations of potential (E vs. SCE) for (c) MB and (d) OII decolorization with electrolysis time. The insets are corresponding pseudo-first order of MB and OII decolorization.

32

Page 32 of 49

Fig. 5 Variations of (a) MB and (b) OII decolorization efficiency with electrolysis time on Ti/Sb-SnO2-NGNS electrodes with different coating numbers. The insets are corresponding pseudo-first order of MB and OII decolorization.

ip t

Fig. 6 Effects of (a) initial dye concentration, (b) current density and (c) initial pH

cr

value on dye decolorization efficiency and rate constant of Ti/Sb-SnO2-NGNS

us

electrode. The supporting electrolyte is 0.25 M Na2SO4.

Fig. 7 Accelerated service life curves for Ti/Sb-SnO2, Ti/Sb-SnO2-GNS and

an

Ti/Sb-SnO2-NGNS electrodes in a 0.25 M Na2SO4 solution with an anodic current density of 100 mA cm-2.

M

Fig. 8 Linear sweep voltammogram performed in 0.25 M Na2SO4 solution with a sweep speed of 10 mV s-1.

te

d

Fig. 9 Cyclic voltammograms curves of Ti/Sb-SnO2, Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS electrodes in 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] solution with 0.1 M

Ac ce p

KCl supporting electrolyte. The sweep rate is 20 mV s-1. Fig. 10 Electrochemical impedance spectra (Nyquist plots) of Ti/Sb-SnO2,

Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS electrodes. The inset is corresponding equivalent circuit model.

Fig. 11 Evolution of the current density at 0.7 V versus the sweep rate for Ti/Sb-SnO2,

Ti/Sb-SnO2-GNS

and

Ti/Sb-SnO2-NGNS

electrodes.

The

inset

is

cyclic

voltammgrams between 0.6 and 0.8 V at different sweep rates for Ti/Sb-SnO2-NGNS in 0.25 M Na2SO4 solution. Fig. 12 (a) Reciprocal voltammetric charge (q*)

-1

versus the square root of the

33

Page 33 of 49

voltammetric

scan

(v1/2)

rate

for

Ti/Sb-SnO2,

Ti/Sb-SnO2-GNS

and

Ti/Sb-SnO2-NGNS electrodes; (b) voltammetric charge (q*) versus the reciprocal square root of the voltammetric scan rate (v-1/2) for Ti/Sb-SnO2, Ti/Sb-SnO2-GNS and

ip t

Ti/Sb-SnO2-NGNS electrodes. Data obtained from the cyclic voltammograms

cr

obtained between 0 and 1.5 V (vs. SCE) at various scan rates in a 0.25 M Na2SO4

Ac ce p

te

d

M

an

us

solution.

34

Page 34 of 49

Table 1 Lattice parameters (a = b and c) and unit cell volume (V) for Ti/Sb-SnO2,

Unit cell parameter

Cassiterite (Standard SnO2)

4.738

3.187

Ti/Sb-SnO2

4.732

Ti/Sb-SnO2-GNS

4.733

Ti/Sb-SnO2-NGNS

4.727

V (Å3)

cr

c (Å)

71.54

us

a = b (Å)

ip t

Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS electrodes.

71.32

3.185

71.35

an

3.185

71.11

M

3.183

d

Table 2

te

XPS data of chemical states of the oxygen element on the electrode surface. Binding energy / eV Electrode

Oad content / %

Atomic ratio of Oad: OL /

O1s (Oad)

( Content  O OadO  100% ) ad L

%

Ti/Sb-SnO2

530.51

531.38

23.98

31.54

Ti/Sb-SnO2-GNS

530.64

531.75

26.19

35.49

Ti/Sb-SnO2-NGNS

530.75

531.70

27.14

37.24

Ac ce p O1s (OL)

35

Page 35 of 49

Table 3

References

[38]

Electrode

Electrolysis Condition

k / 10-3 min-1

30 mg L-1 MB, 0.25 M Na2SO4,

ip t

Pseudo-first order kinetic rate constant comparison of different electrodes.

Ti/PbO2-ZrO2

24.3

-2

cr

50 mA cm Ti/SnO2-Sb;

1 g L-1 C.I. Acid Red 73, 0.1 M Na2SO4,

Ti/SnO2-Sb-Y

50 mA cm-2

us

[19]

13.1; 21.2

0.8 g L-1 C.I. Reactive Orange 4, 0.1M Na2SO4, Ti/SnO2-Sb-Pt

an

[39]

1.05

125 mA cm-2

1 g L-1 C.I. Acid Red 73, 0.1 M Na2SO4,

8.3;

Ti/SnO2-Sb-CNT

50 mA cm-2

16.0

M

Ti/SnO2-Sb;

This work

te

Ti/Sb-SnO2;

d

[24]

6.11; -1

50 mg L MB, 0.25 M Na2SO4,

Ti/Sb-SnO2-GNS;

28.3; 20 mA cm-2 36.6

Ac ce p

Ti/Sb-SnO2-NGNS Ti/Sb-SnO2;

This work

6.13; -1

50 mg L OII, 0.25 M Na2SO4,

Ti/Sb-SnO2-GNS;

27.9; 20 mA cm-2 44.0

Ti/Sb-SnO2-NGNS

36

Page 36 of 49

Table 4 EIS fitting results of Ti/Sb-SnO2, Ti/Sb-SnO2-GNS and Ti/Sb-SnO2-NGNS

ip t

electrodes. R1/ohm

R2/ohm

CPE2/F

n

Ti/Sb-SnO2

5.27

21.01

5.36×10-3

0.88

Ti/Sb-SnO2-GNS

4.99

15.31

3.45×10-3

Ti/Sb-SnO2-NGNS

3.60

10.91

17.88×10-3

cr

Electrode

us

0.85

an

0.87

Table 5

M

The roughness factors and voltammetric charges of Ti/Sb-SnO2, Ti/Sb-SnO2-GNS and

d

Ti/Sb-SnO2-NGNS electrodes.

Ti/Sb-SnO2

Ac ce p

Ti/Sb-SnO2-GNS

qtotal* / mC cm-2

qouter* / mC cm-2

35.17

30.14

14.80

91.67

44.99

20.21

146.0

61.09

35.95

Rf

te

Electrode

Ti/Sb-SnO2-NGNS

37

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Ac ce p

te

d

M

an

us

cr

ip t

Fig. 1

38

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us

cr

ip t

Fig. 2

(b)

Ac ce p

te

d

M

an

(a)

(c)

(d)

39

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us

cr

ip t

Fig. 3

(b)

(d)

Ac ce p

(c)

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d

M

an

(a)

(e)

40

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us

cr

ip t

Fig. 4

(b)

(d)

Ac ce p

(c)

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d

M

an

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41

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M

an

us

cr

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Fig. 5

Ac ce p

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d

(a)

(b)

42

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us

cr

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Fig. 6

Ac ce p

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d

M

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(a)

(b)

(c)

43

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Ac ce p

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d

M

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cr

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Fig. 7

44

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Ac ce p

te

d

M

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us

cr

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Fig. 8

45

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Ac ce p

te

d

M

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us

cr

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Fig. 9

46

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Ac ce p

te

d

M

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us

cr

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Fig. 10

47

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Ac ce p

te

d

M

an

us

cr

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Fig. 11

48

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M

an

us

cr

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Fig. 12

Ac ce p

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d

(a)

(b)

49

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