Large disk electrodes of Ti/TiO₂-nanotubes/PbO₂ for environmental applications

Abstract

Large disk electrodes of Ti/TiO₂-nanotubes/PbO₂ (65 cm² of geometrical area) were successfully synthesized by anodization and electrodeposition procedures. Characterization of anodes was performed by SEM, EDS, AFM and electrochemical measurements, aiming towards environmental applications. PbO₂, an electrocatalytic material, promotes the production of strong oxidising species (hydroxyl radicals) that can be used for decontamination. Electrochemical treatment of synthetic dye effluent (2 L) containing 250 mg L⁻¹ of Acid Blue 113 dye (AB 113) was performed using a disk Ti/TiO₂-nanotubes/PbO₂ anode and an electrochemical flow cell. More than 85% of organic matter was removed by applying current densities of 20, 40 and 60 mA cm⁻². Moreover, colour decay achieved values of 60%, 90% and 100%, depending on the applied current density. Alternatively, this study allows us to understand how nanomaterials have prevented the corrosion phenomena on the anode surface (Pb²⁺ pollution) due to the homogeneous migration of PbO₂ within the TiO₂ nanotubes previously formed on the Ti support.

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Introduction

Lead dioxide coatings on inert substrates such as titanium and carbon now offer new opportunities as an anode material for environmental applications. It is now recognised that electrodeposition allows the preparation of stable coatings with different phase structures and a wide range of surface morphologies.¹ In addition, substantial modifications in the physical properties and catalytic activities of the coatings are possible through doping and fabrication of nanostructured deposits or composites.^1,2 Lead dioxide can be electrodeposited onto inert substrates such as titanium or carbon from a number of media in which Pb(II) is soluble.^2,3 These electrodes can only be used in applications that require a rather positive potential for electrochemical oxidation (EO). If a positive potential is applied, the coating will be protected from corrosion; but, at any significantly negative potential, cathodic reduction and dissolution of the lead dioxide coating is expected.^4,5 Ti is the usual substrate for PbO₂ coatings even when adhesion is a problem yet.^6–8 Ti is usually in the form of a plate or expanded metal mesh, which must be pre-treated before the anodic plating process in order to remove any existing TiO₂ scales from the surface to roughen the surface and to prevent passivation. This pre-treatment commonly involves sandblasting, alkaline degreasing followed by etching in heated oxalic acid or HCl for at least 30 min. However, pre-treatment is often insufficient, and hence various thin undercoats on Ti before deposition of the PbO₂ have been proposed—gold, platinum, tin dioxide, TiO₂/Ta₂O₅, PtO_x and TiO₂/RuO₂.^8–12 The main problem is that Ti self-generates an oxide layer that affects many of its properties such as charge transport and ability to adhere deposits on its surface. However, some research groups have recently developed nanostructured materials^13,14 of TiO₂ such as nanotubes, nanorods and nanowires, which act as a support to deposit PbO₂.^15–17 The unique properties of high aspect ratio TiO₂ nanotubes include large surface area, high cation exchangeability, high catalytic activity, easy separation and recyclability.¹⁸ Three popular synthesizing techniques for TiO₂ nanotubes have been investigated in recent years, including template-assisted, electrochemical anodization and hydrothermal treatment.^19–24 The method of electrochemical anodization is based on the anodization of titanium foil to obtain nanoporous TiO₂.²⁵ Essentially, TiO₂ nanotubes grow on the Ti surface with ordered arrangement and high aspect ratio, the dimensions of nanotubes formed are controlled by varying the electrolyte composition, applied voltage, pH and anodizing time.^26,27 TiO₂ nanotubes electrodes have superior photo-reactivity, non-toxicity, long term stability, high corrosion resistance and are available at low cost.^28–30 These features offer a useful method to fabricate nanoscale architectures of metal oxides using this method. In the case of PbO₂, it clearly emerges as an attractive material to be used anode for the direct oxidation of organic compounds due to its high oxygen evolution potential, low price, relative stability under the high positive potentials required and stability at high temperatures.³¹ The formation and growth of PbO₂ inside TiO₂ nanotubes have been already developed and used for environmental applications, but no significant dimensions of geometrical area have been produced.³² Therefore, the preparation of a more stable PbO₂ anode on a good support such as TiO₂ nanotubes could increase the economic value and accelerate the practical applications of these electrocatalytic materials. Herein, we report the first general preparation strategy for the synthesis of TiO₂ nanotube arrays to deposit PbO₂ in order to obtain large-disk electrodes. These synthesized large PbO₂ anodes show predominant electrochemical performances and are promising tools for the next generation of electrocatalytic materials for environmental applications.

Experimental

Preparation of Ti/TiO₂-nanotubes and Ti/TiO₂-nanotubes/PbO₂

All the electrochemical deposition experiments were performed with a MINIPA MPL-3305 (São Paulo, Brazil) power supply. TiO₂ nanotubes electrode was prepared according to the anodic oxidation method reported by Cerro-López and co-workers.³² The electrochemical procedure used a polished Ti disk with 10 cm diameter (1.8 mm thick and nominal surface area of 63.5 cm²). Before anodization of the Ti substrate (0.1 mm thick), the sample was successively sonicated in acetone, ethanol, and methanol for 15 minutes. Then, the anodizing process was performed using a two-electrode system (Ti disk serving as the anode and steel serving as the cathode) in a mixture of glycerol and ultrapure water in a 1.3 : 1 ratio (v/v) containing 0.5 wt% NaF and 0.2 M Na₂SO₄ as the support electrolyte by applying 30 V for 2 h (Fig. S1 of ESI†). After this procedure, the Ti/TiO₂-nanotubes disk was immediately picked up from the electrolyte, rinsed with ultrapure water and finally dried. The electrodeposition of PbO₂ onto a Ti/TiO₂-nanotubes disk array was achieved using a galvanostatic method using a solution of 0.25 M Pb(NO₃)₂ + 0.1 M HNO₃ by applying 30 mA cm⁻² for 10 min at 25 °C (see Fig. S2 (ESI†)). No gas bubbling was involved.

Surface characterization

Surface morphology of Ti/TiO₂-nanotubes and Ti/TiO₂-nanotubes/PbO₂ deposits was characterized using a scanning electron microscope (SEM, TESCAN VEGA model LSU at a 30 kV acceleration voltage), energy-dispersive X-ray spectroscopy (EDS, Shimadzu Electron Probe Microanalyzer EPMA-1720) and atomic force microscopy (AFM), while crystalline phases were determined using an X-ray diffractometer (DRX Bruker model D8Discover) using Cu Kα (λ = 1.54 Å) radiation. Electrochemical measurements were obtained using an AutoLab 302N (Metrohm workstation). For the study of electrochemical characteristics of Ti/TiO₂-nanotubes/PbO₂ and service life performances, cyclic voltammetry (CV) and polarization measurements of PbO₂ deposits were performed in 1.0 mol L⁻¹ H₂SO₄ between 0.5 and 3.5 V vs. Ag/AgCl (3 M) at 50 mV s⁻¹.

Electrochemical experiments

The electrocatalytic features of the Ti/TiO₂-nanotubes/PbO₂ electrode were tested during the EO of the dye solution by applying 20, 40 and 60 mA cm⁻² of current density. EO experiments were performed using an electrolytic flow cell with a single compartment with parallel plate electrodes for treating 2 L of 250 mg L⁻¹ of Acid Blue 113 dye (AB 113) at 25 °C (see Fig. S3†). The dye was obtained from a textile industry at Santiago de Chile (Chile) with no further purification prior to use. The characteristics of the dye Acid Blue 113 are shown in Table 1. Colour and chemical oxygen demand (COD) analyses were performed to evaluate the depollution performances. Moreover, analyte samples were subjected to GC-MS analysis using GC-FOCUS and MS-ISQ Thermo Scientific to identify the intermediates.

Table 1 The characteristics of Acid Blue 113 (AB 113)

Formula	C32H21N5Na2O6S2
Azo dye	C.I. Acid Blue 113
IUPAC name	Disodium 8-anilino-5-[[4-(3-sulfonatophenyl)azo-1-naphthyl]azo]naphthalene-1-sulfonate
λmax	570 nm
MW	681.66
CI	26 360
Chemical structure

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Results and discussion

Ti/TiO₂-nanotubes were successfully synthesized in a solution of glycerol and ultrapure water containing NaF and Na₂SO₄ by galvanostatic electrolysis with a potential of 30 V for 120 min. The surface morphology of deposits was observed by SEM, showing TiO₂ nanotubes with diameters of approximately 100–200 nm as observed in the images. The TiO₂ nanotubes grow vertically on the substrate and form a dense array, as shown in Fig. 1. Moreover, EDS measurements were obtained at different points along the TiO₂ nanotubes, and the typical EDS spectrum is shown in Fig. 1b. The composition analysis shows that the Ti/O stoichiometry corresponds to TiO₂, which demonstrates that the nanotubes were successfully synthesized. Furthermore, the AFM study reveals the profile of the lateral view of TiO₂ nanotubes (see Fig. S4†), confirming the nanometric diameter of the tubes formed (see Fig. 1). In addition, PbO₂ crystals were successfully deposited onto Ti/TiO₂-nanotubes arrays, according to Fig. 2. Deposits were composed of orderly arranged tetragonal PbO₂ crystals with sizes of about 15 μm (see inset and Fig. 2). These tetragonal crystals can assemble in tree form when the electrodeposition time is increased (see Fig. S5 (ESI†)). Fig. S6 (ESI†) clearly shows the growth of PbO₂ crystals on the TiO₂ nanotubes after 30 min of electrodeposition time. For this reason, the electrolysis time of 10 min is very important to obtain a homogenous PbO₂ deposit. The side lengths of PbO₂ are about 3.0–4.0 μm, and the thickness of the films was minimum, as shown in the inset in Fig. 2. EDS measurements were carried out at different points along the PbO₂ film, and the typical EDS spectrum is shown in Fig. 2. The additional composition analysis showed that the crystals formed in the TiO₂-nanotubes arrays consisted of pure PbO₂. Fig. S7 (ESI†) shows the XRD pattern of the synthesized PbO₂ crystals. All the peaks can be indexed to β-PbO₂ phases. No peaks of any other phases or impurities were detected, which also revealed that pure PbO₂ was obtained. The mechanism of formation of the PbO₂ morphology during electrodeposition is discussed in ESI.† Herein, the performances of large TiO₂-nanotubes/PbO₂ anodes for EO were studied (see Fig. S8†). Nevertheless, the preliminary examination was performed by CVs of TiO₂-nanotubes/PbO₂ (shown in Fig. 2) recorded at 50 mV s⁻¹ in 1.0 M H₂SO₄, as shown in Fig. 3. In Fig. 3, for each case, the 20^th cycle sweeping from 0.0 V to 2.0 V is shown. In the negative scanning, a slight reduction signal is observed at about 1.4 V for the PbO₂ deposit, and this can be attributed to the electroreduction of PbO₂ to PbSO₄ via the following electrochemical reaction: PbO_2(s) + HSO_4(aq) + 3H⁺ + 2e → PbSO_4(s) + 2H₂O. From the area under the cathodic signal, it can be clearly observed that the charge passed at the Ti/TiO₂-nanotubes/PbO₂ anode is several times less than that for the Pb/PbO₂ anode even when both the anodes were treated by similar Pb deposition procedures.

Fig. 1 (a) SEM image of anodized Ti/TiO₂-nanotubes. (b) and (c) are the magnified SEM images of the orderly arranged nanotubes. (d) EDS spectrum of the composition analysis, showing that it corresponds to TiO₂.

Fig. 2 (a) SEM images of PbO₂ deposit in Ti/TiO₂-nanotubes and the magnified orderly arranged PbO₂-tetragonal crystals. (c) EDS spectrum of the composition analysis, showing that the thin film corresponds to PbO₂.

Fig. 3 Electrochemical measurements obtained in H₂SO₄: (a) CV curves of PbO₂ (black curve) and Ti/TiO₂-nanotubes/PbO₂ (red curve) anodes, with 2 cm² of geometrical area. Inset: magnification of CV curve for Ti/TiO₂-nanotubes/PbO₂ electrode. (b) Polarization curves for both the electrodes.

On the reverse scan, only relatively small anodic signals are observed for both the samples, as shown in Fig. 3, suggesting the well-known passivating effect of PbSO₄. After several CV curves, the cathodic signal decreased in the successive cycles due to the reduced availability of PbO₂ for electroreduction. After 40 cycles, the charge passed at the Ti/TiO₂-nanotubes/PbO₂ anode is about 50 times less than that passed at the Pb/PbO₂ electrode, as shown in the inset of Fig. 3a. Therefore, the Ti/TiO₂-nanotubes/PbO₂ anode shows considerably better electrochemical properties than the Pb/PbO₂ anode, and this may be attributed to the considerably larger surface area of the TiO₂ nanostructures, providing considerably more open-edge morphologies and considerably larger network structures to the PbO₂ crystals formed. Fig. 3b also shows linear polarization curves of the Pb/PbO₂ and Ti/TiO₂-nanotubes/PbO₂ electrodes obtained in 1.0 M H₂SO₄ with a scan rate of 50 mV s⁻¹. The curves are very different and show that the oxygen evolution potential increases from 0.0 V to 2.8 V versus Ag/AgCl (3.0 M) for Pb/PbO₂ and Ti/TiO₂-nanotubes/PbO₂, respectively. This indicates that Pb/PbO₂ has a high oxygen evolution overpotential, and consequently is a poor electrocatalyst for the oxygen evolution reaction (o.e.r.); however, the potential of the o.e.r. was shifted to a more positive potential when Ti/TiO₂-nanotubes/PbO₂ was employed, and consequently noticeably disfavoring the production of oxygen.^33,34 This behavior indicates that the latter anode could exhibit good electrocatalytic properties for EO of organic pollutants in solution.^31,33,34

EO experiments were performed in order to evaluate the electrocatalytic features of disk Ti/TiO₂-nanotubes/PbO₂ anodes to remove the organic load and color of a dye synthetic effluent with an electrochemical flow cell by applying a current density of 20, 40 and 60 mA cm⁻². Fig. 4 shows color decay during the galvanostatic electrolysis of dye solutions containing 250 mg dm⁻³ AB 113. The removal changes were reasonably rapid, indicating that during the first treatment stages there are mechanisms involving dye oxidation to other simpler organics.³³ The oxidation of this complex molecule can result in the formation of many intermediates by the elimination of chromophore groups prior to the production of aliphatic carboxylic acids and carbon dioxide.^34,35 This can be explained because at the Ti/TiO₂-nanotubes/PbO₂ electrodes, ˙OH radicals formed by water oxidation (H₂O → ˙OH + H⁺ + e⁻) can be either electrochemically oxidized to dioxygen (˙OH → 1/2O₂ + H⁺ + e⁻) or contribute to the complete oxidation of the organic compounds, in this case, dyes (dyes + ˙OH → CO₂ + H₂O).^36,37 According to Comninellis,³⁸ this anode can be classified as a non-active anode, thus favoring the electrochemical combustion of organic pollutants. However, color removal decreased when the applied current density was increased. This behavior is due to the promotion of the oxygen evolution reaction^37,38 when an increase in the applied current density was attained. It can also be confirmed by polarization curves, which demonstrated the higher production of oxygen at higher current values (see Fig. 3b). Nevertheless, % of COD values (ESI†) clearly demonstrated that the oxidation power of this non-active electrode is independent of applied current density at different electrolysis times (see inset in Fig. 4). This outcome is in agreement with the data reported by other authors for the anodic oxidation of dyes.^33,39–42 During the electrolysis of synthetic dye effluent, the energy consumption (ESI†) seems to be proportional to the applied current density. For example, it increases from 39.2 to 70.3 kW h m⁻³ of effluent treated when the current density increases from 20 to 60 mA cm⁻². Conversely, a higher energy was required when Pb/PbO₂ was used as the anode (57 to 100 kW h m⁻³). After the EO of synthetic dye using the Ti/TiO₂-nanotubes/PbO₂ anode, by-products were identified by GC/MS. 1-Naphthalenol, naphthalene, 1,6-dimethyl-4-(1-methylethyl) and dibutyl phthalate were identified as intermediates, confirming the chromophore group cleavage as a first step followed by the formation of aromatic intermediates, giving lower coloration to the solution.^41,42 The electrochemical stability of the disk Ti/TiO₂-nanotubes/PbO₂ was examined by subjecting an electrode to fixed current density measurements for prolonged electrolysis times. Fig. S9 (ESI†) shows the variation of E_appl as a function of time. As revealed from these data, there is almost no increase in the values of E_appl up to 120 h. Therefore, the disk Ti/TiO₂-nanotubes/PbO₂ composed of orderly arranged tetragonal PbO₂ crystals showed high electrochemical stability for long-term applications. In addition, it is important to indicate that no pollution of Pb²⁺ was detected after longer times of electrolysis with the Ti/TiO₂-nanotubes/PbO₂ anode, in contrast with the results achieved when the Pb/PbO₂ electrode was used. The release of Pb²⁺ was monitored by electroanalytical measurements;⁴³ however, other techniques, such as ICP/MS and ICP/OES,^44–46 can be used to evaluate this parameter.

Fig. 4 Electrochemical treatment of synthetic dye effluent using a disk Ti/TiO₂-nanotubes/PbO₂ anode with 65 cm² of geometrical area using an electrochemical flow cell. Color decay, as a function of electrical charge passed, by applying different current densities. The inset is the percentage of COD removal, as a function of applied current density.

Conclusions

In summary, the large PbO₂ electrodes composed of orderly arranged TiO₂ nanotubes were synthesized via a simple, rapid, and efficient electrochemical approach. Compared with Pb/PbO₂ electrodes, the prepared Ti/TiO₂-nanotubes/PbO₂ can promote the remarkable enhancement in the performances of electrocatalytic materials for the treatment of wastewaters.^39,40 The higher electrochemical activities of Ti/TiO₂-nanotubes/PbO₂ than those of Pb/PbO₂ may be attributed to the larger surface area, more open-edge morphologies and larger network structures in the crystal body. This study will open a new approach in the search for new metal oxide structures from the preparation of TiO₂ nanotubes, giving more stability to non-active anodes for electrochemical devices with excellent performances.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra16723f

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