Preparation and characterization of PbO₂ electrodes from electro-deposition solutions with different copper concentration

Abstract

The present work focused on studying the effect of Cu²⁺ concentration on the electrochemical properties of lead dioxide electrodes prepared by electrochemical deposition method. The surface morphology and the structure of the electrodes were characterized by scanning electronic microscopy (SEM) and X-ray diffraction (XRD), respectively. The stability and electrochemical activity of the lead dioxide electrodes were investigated by accelerated life test, linear sweep voltammetry and bulk electrolysis. The results showed that Cu²⁺ significantly decreased the grain size of lead dioxide and formed a compact and fine surface coating. The service lifetime of the copper modified lead dioxide electrode was longer than that of the unmodified electrode. The electrode prepared from the solution containing 0.2 mol L⁻¹ copper nitrate (marked as PbO₂-0.2 M electrode) showed the longest service life (49 h). During the linear sweep test, the PbO₂-0.2 M electrode showed the highest electrochemical activity that can be attributed to its highest voltammetric charge quantity. Consequently, the PbO₂-0.2 M electrode showed the best performance on degradation of Acid Red G in simulated wastewater by bulk electrolysis. Its pseudo first-order kinetics coefficient was 0.02552 min⁻¹.

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1 Introduction

Electrochemical advanced oxidation processes have been developed for decades and reached a promising stage in degrading toxic or bio-refractory organic pollutants efficiently because of their versatility, high energy efficiency, environmental compatibility, and high cost effectiveness.^1–3 The electrode material is a crucial factor affecting the performance of an electrode.⁴ Thus, much attention has been paid on exploration of novel anode materials, especially “non-active” anodes,⁵ such as tin dioxide (SnO₂),^6–8 lead dioxide (PbO₂),^9–11 and boron-doped diamond (BDD),^12,13 which show high potential for oxygen evolution and high activity in producing hydroxyl radicals (˙OH).

Despite the high oxygen evolution potential (OEP) and high oxidation performance of the BDD electrode, its high surface resistance and difficulty to be prepared on a large-scale limit its application.^14–16 Ti/Sb–SnO₂, a typical dimensional stable anode (DSA), has been demonstrated to be efficient in the electro-oxidation of organic pollutants in wastewater treatment during the last 10 years.¹⁰ However, the main problem for the commercial application of SnO₂ electrode is its relative short service lifetime because of the weak combination between the titanium substrate and the SnO₂ layer.¹⁷ PbO₂ coating is more economical compared with those based on precious metals, and its high electrical conductivity (comparable to metals), high oxygen evolution potential and good stability lead to possible application in many process.¹⁸ For the last decade, it has been used to oxidate recalcitrant organic pollutants such as sulfamethoxazole,¹⁹ metalaxyl,²⁰ lignin,²¹ and real landfill leachate²² through the production of ˙OH during water electrolysis.

The typical solution used for the electro-deposition of β-PbO₂ layer is composed of lead nitrate, nitric acid and some additives. Most of the additives are used for the modification of PbO₂ electrode for further enhancement of the electrode oxidation performance and stability. For example, the element modification, such as Bi,^23,24 F,²⁵ Ce²⁶ and Fe,²⁷ could obviously accelerate the electrochemical oxidation process and enhance the electrode stability.

Among these additives, Cu²⁺ is different from others for its initial purpose in the electrochemical deposition solution is not to modify the PbO₂ electrode. Li¹⁸ et al. pointed out that the addition of Cu(NO₃)₂ into the electrochemical deposition solution was to avoid lead deposition and nitrate reduction on the cathode. However, different work used different Cu(NO₃)₂ concentration. For instants, V. Saez²⁸ et al. added 5.0 g L⁻¹ Cu(NO₃)₂·3H₂O into the deposition solution and controlled addition of CuCO₃ were carried out in order to maintain the level and concentration of the solution. Kong²⁹ et al. added 0.1 M Cu(NO₃)₂ during the electrochemical deposition process and Liu³⁰ et al. used 50 g L⁻¹ Cu(NO₃)₂. With the addition of Cu(NO₃)₂, there maybe some connection between the Cu²⁺ concentration and performance of the fabricated PbO₂ electrode. To the best of our knowledge, none of the present works focused on the modification effect of Cu²⁺ to the PbO₂ coating and the effect of Cu²⁺ concentration is still unknown.

Therefore, in the present work, for the sake of studying the effect of the copper modification, titanium-based β-PbO₂ electrodes under different Cu²⁺ concentration were fabricated by electrochemical deposition. The morphology, crystalline structure, stability, and electrochemical performance of the as-prepared electrode were characterized. In order to evaluate its electrocatalytic activity, Acid Red G (ARG) was used as a toxic biorefractory model organic pollutant for electrochemical degradation.

2 Materials and experiment

2.1 Materials

All chemicals used in the experiment were analytical reagent grade or higher and were used without further purification. Titanium (Ti) plate (purity: 99.6%, BaoTi Co. Ltd, China) with 0.5 mm thickness was used in this study. Deionized water prepared from an EPET-40TF system (EPET Co. Ltd, Nanjing, China) was used for aqueous solution preparation and Ti plate washing.

2.2 Electrode preparation

Ti plates with a dimension of 2 cm × 4.5 cm × 0.5 mm were used as the electrode substrate. Prior to the electrode preparation process, Ti plates were mechanically polished with 1000-grid abrasive papers, and then rinsed with deionized water. The plates subsequently underwent an ultrasonic cleaning (KQ2200DB, Kunshan Ultra Co. Ltd, Jiangsu, China) in the solution composed of acetone and 1 mol L⁻¹ NaOH (1 : 1 v/v) for 30 min to remove organic residues from the surface. Then the plates were etched in 10 wt% oxalic acid at 98 °C for 120 min. Finally, the plates were rinsed thoroughly with deionized water and dried for use.

The α-PbO₂ preliminary electro-deposition was carried out to prepare an interlayer between the rough Ti substrate and the surface β-PbO₂ layer. This can be helpful for the stability of the entire PbO₂ electrode. The deposition solution was composed of 0.11 mol L⁻¹ PbO and 3.5 mol L⁻¹ NaOH. The pre-treated Ti plate was used as the anode and the copper plate of the same area was used as the counter cathode. The deposition process was carried out in an undivided cylindrical vessel under galvanostatic conditions (10 mA cm⁻¹) for 30 min. The solution was maintained at 40 °C with water bath and stirred by a magnetic stirring bar in the deposition process. The electrochemical deposition system was powered by a DC power source (WYK-303B, China). After the deposition process, the as-prepared Ti/α-PbO₂ electrodes were rinsed thoroughly with deionized water. The average amount of α-PbO₂ oxide on the electrode surface is 23 mg cm⁻².

The surface β-PbO₂ layer was coated on the Ti/α-PbO₂ electrode through electrochemical deposition process. The deposition solution was composed of 0.5 mol L⁻¹ Pb(NO₃)₂, 0.01 mol L⁻¹ NaF, and Cu(NO₃)₂. The solution pH was adjusted to 2.0 using concentrated HNO₃. The deposition processes were carried out at 65 °C for 120 min and the current density was controlled at 10 mA cm⁻¹. The copper plate with the same size was used as the counter cathode. The amount of Cu was controlled by the concentration of Cu(NO₃)₂. The fabricated electrodes were marked as PbO₂-0 M, PbO₂-0.1 M, PbO₂-0.2 M, PbO₂-0.3 M and PbO₂-0.4 M, respectively, dependent on the electrode fabricated in different concentration of Cu(NO₃)₂ (0–0.4 M). The average amount of β-PbO₂ oxide on the electrodes surface is 35 mg cm⁻².

2.3 Analysis

The morphology of samples was characterized by a scanning electron microscopy (SEM, JEOL, JSM-6390A). Inductively coupled plasma atomic emission spectrometer (ICP-AES, ICPE-9000, Shimadzu, Japan) was used to determine the Cu²⁺ content in PbO₂ electrode. X-ray diffraction (XRD, D/MAX-2400X, Rigaku) analysis was performed using a diffractometer with Cu-K_α radiation, with an accelerating voltage of 40 kV and the electron probe current of 40 mA.

All electrochemical measurements were carried out on CHI 660D electrochemical workstation (Shanghai Chenhua Instrument Co. Ltd., China) with a conventional three-electrode cell at room temperature. Ti/α-PbO₂/β-PbO₂ electrode served as the working electrode, while Pt sheet served as the counter electrode and Ag/AgCl electrode as the reference electrode. Linear sweep voltammetry was performed to obtain their oxygen evolution potential in 0.5 mol L⁻¹ H₂SO₄ solution. Cycle voltammetry curves were recorded between 0 and 2 V in 0.5 mol L⁻¹ H₂SO₄ solution at a scan rate of 20 mV s⁻¹. The cycle voltammetry results were used to calculate the voltammetric charge quantity for different electrodes.

Anti-corrosion performance of the electrodes was investigated using accelerate lifetime test with a current density of 500 mA cm⁻² in 3 mol L⁻¹ H₂SO₄ solution at room temperature. The temperature of the sulfuric acid solution was kept at 45 °C ± 2 °C. During the accelerated lifetime test, the cell voltage was measured automatically by the electrochemical workstation and the test was considered to be end when the cell voltage was higher than 10 V.

2.4 Electrochemical oxidation

The bulk electrochemical oxidation tests were conducted in batch using an undivided electrolytic cell under galvanostatic condition, the current density of 10 mA cm⁻² was supplied by a WYK-303B potentiostat/galvanostat. The prepared electrodes served as the anodes and the cathode was stainless steel sheet (2 cm × 4.5 cm), with a distance of 2.0 cm between the two electrodes. The initial ARG concentration was 50 mg L⁻¹ and 0.1 mol L⁻¹ Na₂SO₄ was added to the aqueous solution as the supporting electrolyte. The experiments were carried out at room temperature for 120 min. During the experiments, liquid samples were withdrawn from the electrolytic cell every 20 min for the UV-vis analysis (Agilent 8453). The maximum adsorption wavelength of ARG molecular is 503 nm. The color removal efficiency of ARG in electrochemical oxidation can be calculated as follow:where A₀ is the absorbance value in 503 nm of initial wastewater sample and A_t is the absorbance value 503 nm of the wastewater samples at the given time t.

3 Results and discussion

3.1 Surface morphology of PbO₂ electrodes

Fig. 1 shows the SEM micrographs of the PbO₂ electrodes prepared with different Cu²⁺ concentration. Fig. 1a displays the morphology of the surface layer of the PbO₂-0 M electrode which is uniform and of typical pyramidal shape.^9–11,18,24 At low Cu²⁺ concentration (0.1 M and 0.2 M), the surface morphology of electrodes showed no significant change compared with that of PbO₂-0 M electrode except that the size of PbO₂ crystals was reduced. In particular, the surface of the PbO₂-0.2 M was more uniform than that of PbO₂-0 M and PbO₂-0.1 M. An increase in the concentration of copper nitrate to 0.3 M caused a pronounced change in the morphology of as-prepared PbO₂ electrode. As shown in Fig. 1d, it can be seen that a uniform structure consists of globular shape particles with a smaller size was formed. A further increase in copper nitrate to 0.4 M resulted that a well-defined structure consists of a pileup-pellets pattern and the smallest globular particles. Therefore, it is hypothesized that the increment of Cu²⁺ concentration in the electrochemical deposition solution can significantly reduce the particle size of PbO₂ crystal and obtain a compact and fine coating surface.

Fig. 1 SEM photographs and EDS result of the PbO₂ electrodes with different copper nitrate concentration ((a): 0 mol L⁻¹, (b): 0.1 mol L⁻¹, (c): 0.2 mol L⁻¹, (d): 0.3 mol L⁻¹, (e): 0.4 mol L⁻¹, (f): EDS result for (c)).

The EDS analysis results for the entire electrodes were shown in Fig. 1f and S1.† From the EDS results, it can be found that all the electrodes were composed of lead and oxygen. There was no copper element detected on the surface. However, the ICP-AES analysis results showed that the Cu²⁺ content was 0, 0.633, 0.829, 0.947, 0.981 mg Cu per g PbO₂ for PbO₂-0 M, PbO₂-0.1 M, PbO₂-0.2 M, PbO₂-0.3 M and PbO₂-0.4 M, respectively. This indicated that the copper embed into the PbO₂ matrix and formed a solid solution with lead dioxide via substitution.

3.2 Structure of PbO₂ electrodes

Fig. 2 shows XRD patterns of different PbO₂ electrode. The main diffraction peak of PbO₂-0 M at 36.2° is assigned to the (200) plane of β-PbO₂. When Cu²⁺ was introduced into the electrochemical deposition solution, the crystalline orientation of the prepared PbO₂ electrode changed and some new peaks appeared. The diffraction peaks observed at 2θ = 25.4°, 31.9°, 36.2°, and 49.0° are assigned to the (110), (101), (200), and (211) plane of β-PbO₂. The main crystal plane is β(211) plane for PbO₂-0.1 M, PbO₂-0.2 M and PbO₂-0.3 M and β(200) plane for PbO₂-0.4 M. A comparison of the XRD spectra in Fig. 2 shows that copper doping causes pronounced changes in the structure of the PbO₂ coating.

Fig. 2 XRD patterns of different PbO₂ electrodes.

The average grain sizes of PbO₂ crystals of electrodes were calculated using the Debye–Scherrer equation:²³

where D is the crystallite size, λ is the X-ray wavelength, β is the full width at half maximum of the peak and θ is the diffraction angle.

The result was shown in Table 1. It can be found that the trend of the crystal size was consistent with the SEM morphology in Fig. 1. These results suggest that the increment of copper nitrate decreases the grain size of the lead dioxide electrodes.

Table 1 Particle size of different PbO₂ electrodes from Debye–Scherrer's formula

Electrode	PbO2-0 M	PbO2-0.1 M	PbO2-0.2 M	PbO2-0.3 M	PbO2-0.4 M
Crystal size/nm	23.6	18.4	14.2	10.8	7.96

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The electrochemical deposition mechanism of PbO₂ electrode can be described as following:³¹

H₂O → ˙OH + H⁺ + e (3)

Pb²⁺ + ˙OH → Pb(OH)²⁺ (4)

Pb(OH)²⁺ + H₂O → Pb(OH)₂²⁺ + H⁺ + e (5)

Pb(OH)₂²⁺ → PbO₂ + 2H⁺ (6)

During the electrochemical deposition process, the ˙OH group generated from eqn (3) would adsorb on the PbO₂ crystal face and make crystal grow via eqn (4) to (6). The energies of different crystal faces are discriminatory. The crystals favor the growth along the crystal face with higher energy. However, Cu(II) occupies the crystal faces with higher energy more easily than ˙OH, due to the smaller radius. Therefore, the growth of PbO₂ crystals is blocked on the highest energy face and would grow on slightly lower energy faces, and thus, the growth of PbO₂ crystal grains is limited. The growth of the electrodeposited PbO₂ coating is a competition between the nucleation and crystal growth. Thus, the limitation of the growth of PbO₂ crystal grains increases the chance of crystal nucleus growth. This led to a uniform and smooth PbO₂ electrode surface.³²

3.3 Electrochemical stability test

Fig. 3 shows the time course of cell potential in the accelerated life test for different PbO₂ electrodes under 3 mol L⁻¹ H₂SO₄ solution with a current density of 500 mA cm⁻². According to Fig. 3, one can find that the addition of copper nitrate can obviously extend the lifetime of PbO₂ electrodes. Considering that the electrode is deactivated when the cell potential increases to 10 V, the PbO₂-0.2 M electrode displays the longest lifetime (49 h), then PbO₂-0.3 M electrode (45.25 h), PbO₂-0.4 M electrode (43.5 h) and last PbO₂-0.1 M electrode (28.25 h). The lifetimes of all the aforementioned modified electrodes are longer than that of unmodified PbO₂-0 M electrode (20.25 h). It indicated that the incorporation of Cu²⁺ into the deposition solution significantly improved the electrochemical stability of PbO₂, especially PbO₂-0.2 M.

Fig. 3 Variation of cell potential with the testing time in the accelerated life test for different PbO₂ electrodes.

There are two factors affecting the stability of PbO₂ electrodes. One is the reduction of inner stress in the PbO₂ matrix. The ionic radius of Cu(II) is 73 pm, which is very close to that of Pb(IV) (77.5 pm). Therefore, during the electrochemical deposition process, it is easy for copper to embed in the PbO₂ matrix and form a solid solution with lead dioxide via substitution, which could reduce its inner stress.^10,33 The other is the dense microstructure of the copper modified electrodes. As shown in Fig. 1, the decrease of the PbO₂ particle size can reduce the defect density of electrode surface and make a compact and fine surface layer. The compact surface of the modified PbO₂ electrodes can not only baffle the penetration of the electrolyte through the cracks and pores, but also prevent an increase of pressure inside the electrode caused by the internal O₂ evolution.³⁴ Thus, the probability of mechanical rupture of the electrode is diminished. These resulted in that the copper modified PbO₂ electrodes showed high electrochemical stability.

3.4 Electrochemical test

The electrochemical activity was related to real surface area and the number of active sites accessible to the electrolyte. The voltammetric charge quantity (q*), which is related to real surface area and the number of active sites, can reflect the electrochemical activity of an electrode.³⁵ For the same electrode material, larger q* indicates higher electrode activity. We employed the method reported in literature³⁴ to calculated q* for estimating the electrode activity. The equation can be expressed as follows:

q* = (q^*₀)⁻¹ + kν^−1/2 (7)

The outer charge quantity (q^*₀) stands for the quantity of theoretically electrochemical active sites of electrode surface, and ν stands for the scan rate of voltage, while k is a constant.

Fig. 4 shows the relationship of q* against the reciprocal of square root of scan rate for different PbO₂ electrodes. The q* was obtained by integration of the cycle voltammetric curves over the whole potential range from 0 to 2 V. The q* values increase in the order of PbO₂-0.4 M, PbO₂-0 M, PbO₂-0.1 M, PbO₂-0.3 M, and PbO₂-0.2 M. The results indicated that the PbO₂-0.2 M electrode showed the highest voltammetric charge quantity q*, which indicated that the PbO₂-0.2 M electrode had the highest active surface area.

Fig. 4 Relationship of voltammetric charge quantity (q*) versus the reciprocal of square root of scan rate in 0.5 mol L⁻¹ H₂SO₄ solution.

Fig. 5 shows the linear sweep voltammograms of PbO₂ electrodes in 0.5 mol L⁻¹ H₂SO₄ solution at a scan rate of 20 mV s⁻¹. The oxygen evolution potential increased as the following order of PbO₂-0.4 M, PbO₂-0 M, PbO₂-0.1 M, PbO₂-0.3 M, and PbO₂-0.2 M. Larger activity surface area led to higher electrochemical activity on oxygen evolution.³⁶

Fig. 5 Linear sweep voltammograms curves of different PbO₂ electrodes in 0.5 mol L⁻¹ H₂SO₄ solution, scan rate: 20 mV s⁻¹.

3.5 Electrochemical degradation test

To investigate the electrocatalytic degradation activity of the prepared electrodes, these PbO₂ electrodes were used as the anode in the treatment of aqueous ARG solution, and their ability on anodic decolorizaiton was also studied. The color removal efficiency data of those anodes are shown in Fig. 6. It is clear from Fig. 6 that the color removal rates by almost all the anodes are up to 85% within 120 min except for PbO₂-0.4 M electrode. PbO₂-0.2 M electrode showed the highest activity for color removal.

Fig. 6 Color removal efficiency as a function of degradation time for 50 mg L⁻¹ ARG in 0.1 mol L⁻¹ Na₂SO₄.

The curves of the normalized color removal efficiency with degradation time for different anodes are shown in the semilogarithmic plots in Fig. 7. According to the good linear correlation between the logarithm values of the normalized concentration and decolorization time, pseudo first-order kinetics can be considered in all cases, and the rate equation for the decolorization of ARG can be expressed as follows:

A_t = A₀e^−k_appt (8)

where k_app is the apparent kinetics coefficient.

Fig. 7 Kinetic analysis of the curves in Fig. 6.

The k_app values are listed in Table 2. It is apparent from Fig. 6 and Table 2 that PbO₂-0.2 M anode exhibited the best decolorization performance in the five electrodes for the degradation of ARG. The highest removal rate for PbO₂-0.2 M electrode can be ascribed to the highest active surface area. PbO₂-0.2 M electrode with larger surface area can provide more active sites centers in the gel layer of the coating to generated more ˙OH radicals. At the same time, a large surface area of PbO₂-0.2 M electrode increased the adsorption ability of reagent and ˙OH radicals, which resulted in an improvement of decolorization ability of PbO₂ anodes.³³

Table 2 Kinetics coefficients of different PbO₂ electrodes for degradation of ARG

Electrode	PbO2-0 M	PbO2-0.1 M	PbO2-0.2 M	PbO2-0.3 M	PbO2-0.4 M
kapp/min^−1	0.01604	0.01626	0.02552	0.0201	0.00727
R^2	0.9997	0.9987	0.9963	0.9991	0.9981

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4 Conclusions

The increment of Cu²⁺ concentration can significantly reduce the particle size of PbO₂ on the surface of electrode. The accelerated life test indicated that the introduction of Cu²⁺ can enhance the stability of PbO₂ electrode. The PbO₂ electrode prepared from the solution containing 0.2 mol L⁻¹ copper nitrate showed the longest service life (49 h). The highest voltammetric charge quantity of PbO₂-0.2 M indicated its highest electrochemical activity, and the PbO₂-0.2 M electrode showed the best degradation performance in the simulated wastewater treatment. Its pseudo first-order kinetics coefficient is 0.02552 min⁻¹. In summary, the optimization concentration of copper nitrate we obtained for electro-deposition of β-PbO₂ coating is 0.2 mol L⁻¹.

Acknowledgements

The authors gratefully acknowledge the financial support from China Postdoctoral Science Foundation (no. 2013M532053), the Postdoctoral Science Foundation of Shaanxi Province of P. R. China, and the Fundamental Research Funds for the Central University of P. R. China.

References

1. C. A. Martinez-Huitle and S. Ferro, Chem. Soc. Rev., 2006, 35, 1324–1340.
2. C. A. Martinez-Huitle and E. Brillas, Appl. Catal., B, 2009, 87, 105–145.
3. M. Panizza and G. Cerisola, Chem. Rev., 2009, 109, 6541–6569.
4. Y. Yao, C. Zhao, M. Zhao and X. Wang, J. Hazard. Mater., 2013, 263, 726–734.
5. B. Correa-Lozano, C. Comninellis and A. D. Battisti, J. Appl. Electrochem., 1996, 26, 683–688.
6. H. Xu, W. Yan and C. Tang, Chin. Chem. Lett., 2011, 22(03), 354–357.
7. Y. S. Young, C. Y. Sik and K. Soonhyun, et al., Appl. Catal., B, 2012, 111–112, 317–325.
8. T. Wu, G. Zhao and Y. Lei, et al., J. Phys. Chem. C, 2011, 115, 3888–3898.
9. H. Xu, J. Li, W. Yan and W. Chu, Rare Met. Mater. Eng., 2013, 42(5), 885–890.
10. Q. Li, Q. Zhang, H. Cui, L. Ding, Z. Wei and J. Zhai, Chem. Eng. J., 2013, 228, 806–814.
11. D. V. Girenko, A. B. Velichenko, E. Mahe and D. Devilliers, J. Electroanal. Chem., 2014, 712, 194–201.
12. G. H. Zhao, P. Q. Li and F. Q. Nong, et al., J. Phys. Chem. C, 2010, 114, 5906–5913.
13. X. P. Zhu, J. R. Ni and J. J. Wei, et al., J. Hazard. Mater., 2010, 184, 493–498.
14. X. Duan, F. Ma, Z. Yuan, L. Chang and X. Jin, J. Electroanal. Chem., 2012, 677–680, 90–100.
15. G. Zhao, Y. Zhang, Y. Lei, B. Lv, J. Gao, Y. Zhang and D. Li, Environ. Sci. Technol., 2010, 44, 1754–1759.
16. G. Zhao, P. Li, F. Nong, M. Li, J. Gao and D. Li, J. Phys. Chem. C, 2010, 114, 5906–5913.
17. G. Zhao, X. Cui, M. Liu, P. Li, Y. Zhang, T. Cao, H. Li, Y. Lei, L. Liu and D. Li, Environ. Sci. Technol., 2009, 43, 1480–1486.
18. X. Li, D. Pletcher and F. C. Walsh, Chem. Soc. Rev., 2011, 40, 3879–3894.
19. H. Lin, J. Niu, J. Xu, Y. Li and Y. Pan, Electrochim. Acta, 2013, 97, 167–174.
20. S. Chai, G. Zhao, Y. Wang, Y. Zhang, Y. Wang, Y. Jin and X. Huang, Appl. Catal., B, 2014, 147, 275–286.
21. K. Pan, M. Tian, Z. Jiang, K. Bruce and A. Chen, Electrochim. Acta, 2012, 60, 147–153.
22. M. Panizza and C. A. Martinez-Huitle, Chemosphere, 2013, 90, 1455–1460.
23. O. Shmychkova, T. Luk'yanenko, A. Velichenko, L. Meda and R. Amadelli, Electrochim. Acta, 2013, 111, 332–338.
24. W. Yang, W. Yang and X. Lin, Appl. Surf. Sci., 2012, 258, 5716–5722.
25. J. Cao, H. Zhao, F. Cao, J. Zhang and C. Cao, Electrochim. Acta, 2009, 54, 2596–2602.
26. Y. Liu, H. Liu, J. Ma and J. Li, Electrochim. Acta, 2011, 56, 1352–1360.
27. L. S. Andrade, L. A. M. Ruotolo, R. C. Rocha-Filho, N. Bocchi, S. R. Biaggio, J. Iniesta, V. Garcia-Garcia and V. Montiel, Chemosphere, 2007, 66, 2035–2043.
28. V. Saez, M. D. Esclapez, A. J. Frias-Ferrer, P. Bonete, I. Tudela, M. I. Diez-Garcia and J. Gonzalez-Garcia, Ultrason. Sonochem., 2011, 18, 873–880.
29. J. Kong, S. Shi, L. Kong, X. Zhu and J. Ni, Electrochim. Acta, 2007, 53, 2048–2054.
30. M. Liu, Y. Tang, L. Wang, Y. Hu, X. Jiao and W. Huang, Chem. Res. Chin. Univ., 2008, 24(3), 285–290.
31. A. B. Velichenko and D. Devilliers, J. Fluorine Chem., 2007, 128, 269.
32. H. Kong, W. Li, H. Lin, Z. Shi, H. Lu, Y. Dan and W. Huang, Surf. Interface Anal., 2013, 45(3), 715–721.
33. S. P. Tong, C. A. Ma and H. Feng, Electrochim. Acta, 2008, 53, 3002–3006.
34. Y. Yao, C. Zhao and Z. Jin, Electrochim. Acta, 2012, 69, 146–151.
35. H. Kong, W. Huang, H. Lin, H. Lu and W. Zhang, Chin. J. Chem., 2012, 30, 2059–2065.
36. H. Kong, H. Lu, W. Zhang, H. Lin and W. Huang, J. Mater. Sci., 2012, 47, 6709–6715.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra03235g

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