Electrodeposition preparation of a cauliflower-like Sb–SnO₂ electrode from DMSO solution for electrochemical dye decolorization

Abstract

We have developed a simple and effective way to fabricate Ti/Sb–SnO₂ electrodes by electrodeposition from a dimethyl sulfoxide solution. The electrode surface exhibits a distinct cauliflower-like structure with a small grain size. Its electrochemical properties have been evaluated by electrochemical impedance spectroscopy and accelerated service life tests; furthermore, the results were compared with those obtained for an electrode prepared from aqueous solution. The DMSO-prepared electrode has low charge transfer resistance (22.71 vs. 107.60 Ω) and high stability (59.3 vs. 2.8 h). Moreover, its ability to electrochemically decolorize methyl orange and rhodamine B is greatly improved.

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1. Introduction

In recent years, electrochemical oxidation processes (EOP) have been the focus of extensive research for use in the treatment of organic pollutant wastewater. The high efficiency, easy operation and environmental compatibility of EOP make them ideal for water treatment.^1–3 In particular, antimony-doped SnO₂ is an important material for EOP because of its low cost, excellent electrocatalytic performance, and high ability to generate hydroxyl radicals compared with Pt anodes, boron-doped diamond, and other dimensionally stable anodes (DSA).^4–6 At present, there are many different techniques used to prepare Sb–SnO₂ electrodes, such as sol–gel processing,⁷ dip-coating,⁸ ultrasonic spray pyrolysis,⁹ hydrothermal synthesis,¹⁰ and electrodeposition.¹¹ Among different synthetic strategies, electrodeposition has proven to be a simple, low cost and high yield method for the large-scale production of electrodes. Most processes for preparing Ti/Sb–SnO₂ electrodes by electrodeposition are performed in aqueous solution. Nevertheless, metal coatings plated from aqueous solutions may contain hydrogen, which is known to cause high internal stress and cracks in the plated films, which adversely affects the mechanical properties of the coatings.¹² Another drawback associated with Sb–SnO₂ electrodes prepared by electrodeposition in aqueous solution is the poor solubility and easy hydrolysis of antimony salts in water; consequently, the number of elements that can be deposited from aqueous media is limited. Additionally, the quality of the coating surfaces may be affected by several side effects, such as water electrolysis.¹³ Finally, the Ti/Sb–SnO₂ electrodes have a short service life due to the weak adhesion of the Ti substrate and SnO₂ layer, which leads to substrate passivation and consumption of the active substance.¹⁴

Many attempts have been made to overcome these disadvantages. The addition of rare earths and noble metals, such as Ru, Au, and Pt into the Sb–SnO₂ coating can increase the service life of the electrodes, but the electrocatalytic ability is decreased.^15–17 Another method is the modification of the Ti substrate; that is, the formation of TiO₂ nanotubes.⁵ TiO₂ nanotube interlayers can improve the stability of the electrode without decreasing its electrochemical ability, but because it requires more stringent laboratory-conditions, this has been only realized at the laboratory scale.

As an alternative to aqueous solutions, electrodeposition in organic media can address the aforementioned issues. Therefore, we prepared a Ti/Sb–SnO₂ electrode by electrodeposition using a dimethylsulfoxide (DMSO) solution containing metal chlorides. DMSO is a common polar aprotic organic solvent that has been successfully employed for the electrochemical growth of Ag₂Se, ZnO and CdO films.^18–21 The results showed that these films revealed a good adhesion to the substrate and also high quality microstructure. And due to the non-aqueous solvent in the synthesis, there was only one phase on the films without the presence of other phases. However, there have been no reports of the preparation of Sb–SnO₂ films in DMSO solution for use as DSAs to treat wastewater.

In this work, a Ti/Sb–SnO₂ electrode was successfully prepared by electrodeposition in a DMSO solution (referred to as Ti/Sb–SnO₂(D)). The Sb–SnO₂ films with dense and cauliflower-like structures were used as electrodes and are thought to not only enhance the adhesion of coatings on the Ti substrate but also provide a large active surface area for electrocatalytic oxidation processes.

2. Experimental

2.1 Synthesis of electrodes

All reagents were analytical grade and used without further purification. DMSO, SnCl₂·2H₂O, SnCl₄·5H₂O, SbCl₃ and tartaric acid were purchased from Aladdin Chemicals (Shanghai, China). The preparation of the Ti/Sb–SnO₂(D) was carried out in a 100 mL DMSO solution containing 0.1 M SnCl₂·2H₂O and 0.009 M SbCl₃, using a two-electrode system with an graphite rod as the anode and a preprocessed Ti plate as the cathode. The direct current electrodeposition was then performed using an E3612A DC power (Agilent, America) at a constant current of 10 mA cm⁻² for 20 min under slow stirring at 25 °C. Following the electrodeposition, the Ti plate was dried at 100 °C, before being annealed in a muffle furnace at 600 °C for 1 h.

Meanwhile, an electrode was also prepared in an aqueous solution as a control; this is referred to as Ti/Sb–SnO₂(A). The required reagents for this method have been reported previously,²² and the same electrodeposition and heat treatment processes were used as those described for the Ti/Sb–SnO₂(D) electrode.

2.2 Characterization

Scanning electron microscopy (SEM) was studied by an INSPECT S50 scanning electron microscope with an energy dispersive X-ray spectrometer (EDS). The X-ray diffraction (XRD) measurement was performed on a Rigaku D/MAX2500 X-ray diffractometer using Cu-Kα radiation.

2.3 Electrochemical experiments

Electrochemical impedance spectroscopy (EIS) and accelerated service life tests were made using an SP-240 electrochemical workstation (BioLogic Science Instruments, France) in a conventional three-electrode cell. The as-prepared electrode (1.0 × 1.0 cm) served as the working electrode; a platinum sheet (2.0 × 2.0 cm) and saturated calomel electrode (SCE) were used as a counter and reference electrode, respectively. A 0.25 M Na₂SO₄ solution was used as the electrolyte.

The EIS measurements were performed in the frequency range from 10⁴ to 10⁻¹ Hz at a potential of 1.9 V (vs. SCE) with signal amplitude of 5 mV. Accelerated service life tests were performed using chronopotentiometry with an anode density of 100 mA cm⁻². The potential of the anode was recorded as a function of time, and it was defined that the electrode was deactivated when the potential increased 5 V from its initial value.

2.4 Electrochemical decolorization tests

The electrochemical decolorization experiments for methyl orange (MO) and rhodamine B (RhB) were performed in a 0.25 M Na₂SO₄ dye solution (50 mL, 50 mg L⁻¹). The electrolysis was performed at a current density of 20 mA cm⁻², with the prepared electrode as the anode and a Ti plate with the same dimensions (1.0 cm × 2.0 cm) as the cathode. Samples were monitored by the UV-Vis absorbance spectroscopy every 15 min at the characteristic wavelength of 465 nm (MO) and 554 nm (RhB).

3. Results and discussion

Fig. 1 shows the surface morphologies of the two electrodes. The surfaces are significantly different. Fig. 1(a) and (b) represent SEM images of Ti/Sb–SnO₂(A), showing that the surface is sparse and rough with some depressions. The metal oxide has an irregular spherical form on the surface of electrode, a morphology that is similar to that of previous electrodes prepared by direct current electrodeposition in aqueous solution.^14,23 In contrast, the electrode prepared in DMSO solution has a dense and uniform coating, and the particles are compact without holes. Fig. 1(d) shows the magnified SEM image of the electrode, and the electrode surface particles have cauliflower-like shapes. These cauliflower-like particles have a larger specific surface area than that of the spherical metal particles, and this shape is expected to provide more active sites for electrocatalytic oxidation.

Fig. 1 SEM images of the Ti/Sb–SnO₂ electrodes prepared in (a and b) aqueous and (c and d) DMSO solutions; XRD patterns of the Ti/Sb–SnO₂ electrodes prepared in different solutions.

XRD analysis was applied to study the crystalline properties of SnO₂ films. As shown in Fig. 1(e), the two electrodes have similar XRD spectra. In addition to the sharp peaks related to the Ti support, all the diffraction peaks of the electrodes can be indexed as standard cassiterite SnO₂. Compared with Ti/Sb–SnO₂(A), the SnO₂ surface synthesized in DMSO solution has more intense reflections with wider half-peak width, suggesting that a more crystalline surface composed of smaller SnO₂ crystallites was obtained from deposition in organic solvent. The grain size of the SnO₂ crystallites were estimated using the Scherrer-equation and were found to be approximately 48 nm and 28 nm for Ti/Sb–SnO₂(A) and Ti/Sb–SnO₂(D), respectively. This result indicates that the DMSO system had a grain refinement effect on SnO₂. Smaller crystallites are considered favorable to enhance the electrocatalytic ability; therefore, the Ti/Sb–SnO₂(D) electrode is expected to have a good electrochemical degradation ability.

Fig. 2(a) illustrates the preparation of the Ti/Sb–SnO₂ electrodes. For the Ti/Sb–SnO₂(A), the prepared coating contained hydrogen, leading to many holes on the electrode surface; these greatly influenced the binding force between the metal coating and the Ti substrate. The structure of the coatings is determined by their composition. In the case of the organic system, DMSO molecules partially decomposed during electrodeposition and the decomposition products (probably dimethyl sulfide (CH₃–S–CH₃) and sulfur molecules¹²) were subsequently incorporated into the deposit, promoting SnO₂ grain refinement. Fig. 2(b) and (c) are the EDS results of Ti/Sb–SnO₂(D) electrode before and after heat treatment, and it can be seen that there was a small amount of sulfur (7.18 at%) on the electrode surface before heat treatment. After calcination, the sulfur was oxidized into gas oxide and it can't detect the peak of S element in the EDS image.

Fig. 2 (a) Schematic illustration for the preparation of Ti/Sb–SnO₂ electrodes; EDS results of Ti/Sb–SnO₂(D) electrode (b) before heat treatment and (c) after heat treatment.

During the electrodeposition process, the reactions on the Ti plate surface (cathode) were expressed as follows:

Sn²⁺ + 2e⁻ → Sn (1)

Sb³⁺ + 3e⁻ → Sb (2)

The metal ions were reduced to the metals deposited on the electrode surface. After heat treatment of 600 °C, the Sn and Sb metals are oxidized to metal oxides, leading to the increase of O element content in the surface of the electrode. Besides, the atom ratio of O/Sn was 1.60, not equal to 2, it was because the doping of Sb made a change of the oxygen vacancy distribution in the SnO₂ crystal, and caused the lattice distortion.²⁴ The n_Sb/n_Sn was 0.18, which was larger than the ratio of n_Sb/n_Sn in the electrodeposition solution (0.09), indicating that Sb was enriched.²²

EIS and accelerated service life tests were performed to investigate the electrochemical performance and stability of the electrodes. As shown in Fig. 3(a), similar profiles were observed for the two kinds of electrodes. The Nyquist plots indicate resistance at high frequency and a depressed capacitive-resistive semicircular shape in the middle-low frequency. To simulate the EIS data, the equivalent circuit model was used to fit the EIS behavior. In this model, R₁ represents the solution resistance, R₂ is the charge transfer resistance and CPE₂ is the constant phase element, which is parallel to the R₂. The charge transfer resistance represents the electrochemical discharge process, and we were able to measure the electrochemical performance of the electrode. The EIS analysis result is shown in Table 1, and it was found that the Ti/Sb–SnO₂(D) electrode had a smaller R₂, only 22.71 Ω, less than a quarter of that of the Ti/Sb–SnO₂(A). This result indicates good electron-transfer capability and high conductivity of the Ti/Sb–SnO₂(D). Fig. 3(b) shows the result of the accelerated service life tests. For the Tb/Sb–SnO₂(A), at high current density, the electrode was deactivated after 2.8 h following extreme oxygen evolution and an increasing potential. In contrast, the Ti/Sb–SnO₂(D) electrode prolonged the accelerated service lifetime to 59.3 h, 21.2 times longer than that of the Ti/Sb–SnO₂(A) electrode. Meanwhile, the electrode prepared in DMSO solution also had a lower initial electrode potential, indicating that the electrical conductivity increased.

Fig. 3 (a) Nyquist plots of the Ti/Sb–SnO₂ electrodes, the inset shows the equivalent circuit model; (b) accelerated service life test curves of the electrodes at a current density of 100 mA cm⁻²; SEM image (c) and EDS results (d) of Ti/Sb–SnO₂(D) electrode after accelerated service life test.

Table 1 Accelerated service lifetime comparison for different Ti/Sb–SnO₂ electrodes

Electrode	Electrolyte	Current density	Accelerated life	Ref.
TiO2-NTs/SnO2–Sb2O5	0.1 M Na2SO4	100 mA cm^−2	42 h	26
Ti/SnO2–Sb2O4–CNT	1 M H2SO4	100 mA cm^−2	4.31 h	23
Ti/Sb–SnO2–TiN	0.25 M Na2SO4	100 mA cm^−2	5.22 h	14
Ti/Sb–SnO2-(c)PED	0.25 M Na2SO4	100 mA cm^−2	22.12 h	22
Ti/Sb–SnO2	0.5 M H2SO4	100 mA cm^−2	38.32 h	4
Ti/Sb–SnO2(A)	0.25 M Na2SO4	100 mA cm^−2	2.80 h	This work
Ti/Sb–SnO2(D)	0.25 M Na2SO4	100 mA cm^−2	59.30 h	This work

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Besides, Table 1 presents the accelerated life comparison of different Ti-based Sb–SnO₂ electrodes which have been reported. It can be seen that the Ti/Sb–SnO₂(D) electrode has a relative long accelerated service lifetime under similar experimental condition. The real service life of the electrodes will be much longer than the accelerated service life because of the much lower current density used in practical applications.

There are many reasons for the deactivation of the electrodes, such as metal base passivation, coating detachment, coating consumption, and mechanical damage.²⁵ Fig. 3(c) and (d) show the SEM and EDS images of the Ti/Sb–SnO₂(D) electrode after stability testing. We found that most of the electrode surface was still covered with metal oxide, and no exposure to the titanium substrate was detected. EDS was used to measure the elemental concentrations for the electrode before and after stability testing. The EDS results indicate that the deactivation of Ti/Sb–SnO₂(D) was due to consumption of the coating, rather than coating detachment leading to exposure of the Ti substrate.

Two types of dyes, an azo dye (MO) and azo-free dye (RhB) were chosen as the targets for decolorization tests to investigate the electrocatalytic activity of the Ti/Sb–SnO₂ electrodes. As Fig. 4 shows, a sharp decrease in both MO and RhB concentration occurred with the use of both types of electrodes; however, dye removal was faster on the Ti/Sb–SnO₂(D). For Ti/Sb–SnO₂(D), within 1 h of electrolysis, MO was almost completely electrochemically decolorize. In contrast, only 76% was decolorized using the aqueous-prepared electrode. After 2 h electrolysis, the MO removal rates were 100% and 86% for Ti/Sb–SnO₂(D) and Ti/Sb–SnO₂(A), respectively, and the values for RhB removal were 97% and 83%, respectively. The insets of Fig. 4 show the semi-log relationship of MO and RhB concentrations with electrolysis time. The electrochemical decolorization of both dyes was found to follow pseudo-first order reaction kinetics, and the first order kinetic constants are shown in Table 2. The results showed that the target pollutants were removed most rapidly when using the Ti/Sb–SnO₂(D) electrode.

Fig. 4 Electrochemical decolorization of MO (a) and RhB (b) on Ti/Sb–SnO₂ electrodes. The insets show the corresponding kinetic analysis associated with the pseudo-first order reaction kinetics.

Table 2 Parameters of the Ti/Sb–SnO₂ electrodes

		Ti/Sb–SnO2(A)	Ti/Sb–SnO2(D)
R2/Ω		107.60	22.71
Kinetic constants/min^−1	MO	0.01651	0.06257
	RhB	0.01508	0.02589

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In order to investigate the durability of electrode, we made reusability tests on electrocatalytic decolorization of methyl orange and rhodamine B, that is, reuse the Ti/Sb–SnO₂(D) electrodes after first electrochemical decolorization tests and redo the decolorization tests in the new dye solutions. Fig. 5 shows the electrochemical docolorization efficiency changes with the electrode use time. The durability results show that, after 5 times decolorization, the electrochemical decolorization efficiencies for MO and RhB are still higher than 99% and 96%, respectively. This indicates that the electrodes prepared in the DMSO solution have a high stability and good electrochemical decolorization ability.

Fig. 5 Decolorization efficiency changes with the electrode use time on the Ti/Sb–SnO₂(D) electrode.

4. Conclusion

A high efficiency catalytic Ti/Sb–SnO₂ electrode was prepared by electrodeposition from DMSO solution. This electrode exhibited a dense cauliflower-like coating with small crystallite size. Furthermore, it had low charge transfer resistance, long accelerated service lifetime and high kinetic constants for electrochemical decolorization of the dyes MO and RhB. In summary, electrodeposition from organic DMSO solution provides a novel method to prepare Ti/Sb–SnO₂ electrocatalytic electrodes in organic solvents for the treatment of contaminated water.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21476053 and 51179033), the Doctoral Program of the Ministry of Education (No. 20132304110027), the Fundamental Research Funds for the Central Universities (No. HEUCF201403019), and the Fundamental Research Operating Expense for the Central Universities (No. HEUCFD1417).

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