High chlorine evolution performance of electrochemically reduced TiO₂ nanotube array coated with a thin RuO₂ layer by the self-synthetic method†

Teayoung Lee^a, Woonghee Lee^b, Seongsoo Kim^a, Changha Lee^a, Kangwoo Cho^b, Choonsoo Kim*^c and Jeyong Yoon*^ad
aSchool of Chemical and Biological Engineering, Institute of Chemical Processes (ICP), Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea. E-mail: jeyong@snu.ac.kr
^bDivision of Environmental Science & Engineering, POSTECH, 77 Chungam-ro, Nam-gu, Pohang 37673, Republic of Korea
^cDepartment of Environmental Engineering, Institute of Energy/Environment Convergence Technologies, Kongju National University, 1223-24, Cheonan-daero, Cheonan-si 31080, Republic of Korea. E-mail: choonsoo@kongju.ac.kr
^dKorea Environment Institute, 370 Sicheong-daero, Sejong-si 30147, Republic of Korea

First published on 25th March 2021

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1. Introduction

The electrochemical oxidation process (EOP) has emerged as an alternative to the conventional oxidation process because of its relatively simple facilities, maintenance, and accessibility.^1–8 In addition, as the need for a decentralized water treatment system has increased, the spectrum of EOP's application is broadening from urban to rural areas and developing countries.^2,4,9–11 The process controls contaminants by generating oxidants on-site. In particular, chlorine (Cl₂) has been widely used to remove microorganisms, organic matters, and ammonia effectively along with other oxidants such as ozone, hydroxyl, and sulphate radicals.^{1,4,5,10,12–16} The generated Cl₂ diffuses to the bulk solution (below eqn (1)–(3)), and they exist three major species including Cl₂ (pH < 3), HOCl (pH 3–8), and OCl⁻ (pH > 8).¹

2Cl− → Cl2(aq) + 2e− (E0 = 1.36 V vs. NHE)	(1)

Cl2(aq) + H2O → HOCl + Cl− + H+	(2)

HOCl ↔ OCl− + H+	(3)

Moreover, Cl₂ is considered as a disinfectant to protect from the infectious COVID-19 in the water, sanitation, and hygiene (WASH) field by World Health Organization (WHO),¹⁷ and several researchers suggested using it for treating the wastewater from hospital or household against potentially dangerous coronavirus.^18,19 Thus, practically, the Cl₂ generation system by EOP has a lot of attention with the advantages and is thought to be a suitable technology for small communities to overcome the disease.

For the high efficiency of EOP, the anode material is a pivotal factor governing oxidant species, energy consumption, and cost-effectiveness. A dimensionally stable anode (DSA; RuO₂, IrO₂, etc.) has an excellent electrochemical property for chlorine evolution reaction (ClER),^5,14,20–27 but, one obstacle in the effective use of DSA is the high manufacturing cost based on the inclusion of expensive noble metals.

In this regard, recently, a reduced TiO₂ nanotube array (r-TiO₂), which can be simply fabricated by electrochemical self-doping of an anatase TiO₂ nanotube array (a-TiO₂), has attracted much attentions as a promising electrode in electrochemical ClER.^11,28–30 The self-doping simply converts Ti⁴⁺ in a-TiO₂ to Ti³⁺ via the intercalation of protons as self-dopants, potentially leading to high electrocatalytic activity in the generation of oxidants with high surface area and low-cost.^28,31 In spite of its advantages, r-TiO₂ has unfortunately suffered from a poor long-term service time, low current efficiency and high energy consumption in chlorine production.

Therefore, this study aimed to enhance the chlorine generation performance of r-TiO₂ with a simple thin layer coating of RuO₂ as an excellent anode material (RuO₂@r-TiO₂). This RuO₂ thin layer was coated on the surface of r-TiO₂ via the spontaneous reduction of ruthenium precursor resulting from a partial conversion reaction of Ti³⁺ to Ti⁴⁺ (ref. 32 and 33) in r-TiO₂ and the followed thermal treatment, successfully leading to the improvement of electrocatalyst for chlorine evolution. To fully understand its surface properties, we used field emission scanning electron microscopy (FE-SEM), field emission transmission electron microscopy (FE-TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The electrocatalytic activity for ClER was investigated with cyclic voltammetry (CV) measurement and the N,N-diethyl-p-phenylenediamine (DPD) method. Furthermore, scanning electrochemical microscopy (SECM) was employed as an in situ analysis to investigate the enhanced ClER from the uniformly coated RuO₂ on the r-TiO₂.

2. Materials and methods

2.1 Preparation of r-TiO₂ and RuO₂@TiO₂

r-TiO₂ was prepared by a typical two-step anodization method and electrochemical self-doping. First, the Ti foil was anodized at 60 V for 2 h in an ethylene-glycol-based electrolyte containing DI water (2.5 wt%) and NH₄F (0.2 wt%). The formed nanotube film was peeled off by a compressed air stream, and then a second-anodization was conducted at 40 V for 7 h under the same electrolyte condition. By annealing the as-prepared TiO₂ NTA at 450 °C for 1 h in air, the crystal structure was converted into an anatase-dominant phase (a-TiO₂).³⁴ Then, electrochemical doping was performed on the a-TiO₂ under cathodic polarization with constant current (16.7 mA cm⁻²) for 90 s in a phosphate buffer solution ([KH₂PO₄]₀ = 0.1 M with KOH, pH = 7.02).^28–30 The prepared r-TiO₂ had a thickness of approximately 13.8 μm and the width of the nanotubes was 130 ± 30 nm (Fig. S1†).

Prior to the RuO₂ coating process, the r-TiO₂ was fully dried at room temperature, then immersed in an aqueous ruthenium precursor solution (5 mM RuCl₃·H₂O in deionized (DI) water) under the dark condition for 24 h to produce the self-synthesized coating (Ru@r-TiO₂). After that, the treated r-TiO₂ was fully washed with DI water to remove the remained ruthenium precursor in the nanotubes, then annealed in 450 °C (air) for 1 h to be oxidized (RuO₂@r-TiO₂). The process is presented briefly in Fig. 1.

2.2 Characterization of RuO₂@r-TiO₂

The morphologies of r-TiO₂ and RuO₂@r-TiO₂ were observed with field emission scanning electron microscopy (FE-SEM, JSM-6701F, JEOL, Japan) at 20 kV, and field emission transmission electron microscopy (FE-TEM, JEM-F200, JEOL, Japan) was employed to confirm the deposited RuO₂ layer on the wall of r-TiO₂ including energy-dispersive X-ray spectroscopy (EDS). An X-ray diffractometer (XRD, Bruker D8 DISCOVER, Germany) and X-ray photoelectron spectroscope (XPS, Sigma Probe, ThermoVG, UK) were used to examine the material species of the TiO₂ NTA-based electrodes.

2.3 Evaluation of electrochemical properties of RuO₂@TiO₂

The electrochemical properties of RuO₂@r-TiO₂ were investigated by cyclic voltammetry (CV) measurement with a three-electrodes system (reference electrode: Ag/AgCl in sat. KCl, counter electrode: Pt mesh) at a scan rate of 5 mV s⁻¹. To understand the electrocatalytic activity of RuO₂@r-TiO₂, chlorine was electrochemically produced in a two-electrodes system that consisted of RuO₂@r-TiO₂ as an anode and Pt mesh as a cathode with a constant current density of 16.7 mA cm⁻² in 0.1 M NaCl. The produced chlorine concentration was monitored by the N,N-diethyl-p-phenylenediamine (DPD) method with a spectrophotometer (DR 900, Hach Co., USA, 530 nm). The current efficiency (%) and the energy consumption (W h g⁻¹) of chlorine generation were calculated by eqn (4) and (5).

	(4)

	(5)

where c is the concentration of generated chlorine (g L⁻¹), V is the electrolyte volume (L), n is the electrons' number (1 eq. mol⁻¹), F is the faradaic constant (96485 C eq.⁻¹), M(Cl₂) is the molecular weight of chlorine (71 g mol⁻¹), I is the applied constant current (C s⁻¹), t is the operation time (s), and e is the cell voltage (V).

To confirm the chlorine evolution mechanism of RuO₂@r-TiO₂, the hydroxyl radical production was investigated with a degradation of terephthalic acid (TA) as a probe compound. In addition, the effect of hydroxyl radical on chlorine evolution was examined by adding 1 M t-BuOH. The TA degradation was measured by high-performance liquid chromatography (HPLC; Ultimate 3000, Dionex, Sunnyvale, CA, USA) in the methanol and formic acid (0.1%) mixture eluent (v/v, 60:40).³⁵ Furthermore, scanning electrochemical microscopy [SECM; SP-300 (bipotentiostat), M470 (SECM Workstation, Bio-Logics SAS), France] was performed to scrutinize the activities for ClER on samples as an in situ measurement. This visualized the scanned area (500 μm × 500 μm of the electrodes) with a colour gradation from blue to red.

3. Results and discussion

3.1 Morphology of RuO₂@r-TiO₂

Fig. 2 shows FE-SEM images (top and cross sectional view) of r-TiO₂ and RuO₂@r-TiO₂. As can be seen in Fig. 2, significant differences on the nanotube edge and sidewall of RuO₂@r-TiO₂ were found. The RuO₂@r-TiO₂ revealed that a large number of nano-grains were formed on the verges of the nanopore entrances compared to the r-TiO₂ in Fig. 2a and c. In addition, the sidewall of RuO₂@r-TiO₂ was thicker after deposition on r-TiO₂ (Fig. 2b and d). It seems that the assembles of nano-grains on the sidewall of the inner pore were formed to a thin layer (see inset image of Fig. 2d).

The sidewall condition of RuO₂@r-TiO₂ is inspected meticulously by FE-TEM in Fig. 3. Considering the thickness of the r-TiO₂ sidewall (ca. 14–22 nm), the sidewall thickness of RuO₂@r-TiO₂ (ca. ∼45.3 nm) was approximately three times thicker. With the distinguished interface layer at RuO₂@r-TiO₂ (red dashed square in inset image of Fig. 3b) and the results of EDS in Fig. S2,† we see that RuO₂ was successfully deposited on the sidewall of RuO₂@r-TiO₂. It is plausible that the RuO₂@r-TiO₂ revealed a fine coating on the overall surface of r-TiO₂ with open-top and hollow nanotube structure via the self-synthesized coating method.

3.2 Characteristics of the deposits on RuO₂@r-TiO₂

To better understand the deposits on RuO₂@r-TiO₂, the XRD patterns and XPS spectra of as-prepared r-TiO₂ and RuO₂@r-TiO₂ are further analysed in Fig. 4. From the results of the XRD patterns in Fig. 4a, an elusive peak appears at 28° with regard to RuO₂ at RuO₂@r-TiO₂.^36–38 Hence, the coated RuO₂ is examined in detail through the XPS results of Ru 3d, O 1s, and Ti 2p in Fig. 4b–d. As shown in Fig. 4b, the RuO₂@r-TiO₂ exhibited a clear RuO₂ peak at 280.4 eV in the XPS spectra of Ru 3d_5/2.^38–43 From the shoulder peak (529.4 eV) in XPS spectra of O 1s (red line in Fig. 4c), the deposition of RuO₂ was further confirmed.^{38,39,44–46} Moreover, this is clearly supported by the peak shift (1.2 eV) from 458.9 to 457.7 eV in Fig. 4d of Ti 2p_3/2 indicating a heterojunction of RuO₂ and TiO₂.^40,42,47

These results suggest that the RuO₂ thin layer was well-formed on the overall surface of RuO₂@r-TiO₂ via the self-synthesized coating method. Note that the self-synthesis method has been reported in the nanoparticles of noble metals on a TiO₂ sphere with the spontaneous redox reaction between metal ions and the reduced TiO₂.^32,33 To the best of our knowledge, this is the first report to prepare a thin RuO₂ layer coating on electrochemically reduced TiO₂ NTA via the previous phenomenon including the following thermal treatment, without severe pore blockage and toxic chemicals when compared to other methods for treating active materials on the TiO₂ NTAs.^37,48–53

3.3 Electrocatalytic activities of RuO₂@r-TiO₂

Fig. 5 shows the improved electrochemical properties of RuO₂@r-TiO₂. From the result in Fig. 5a, where the RuO₂@r-TIO₂ initiated an oxygen evolution reaction (OER) at a potential of approximately 1.1 V vs. Ag/AgCl, the over-potential of which decreased significantly by up to 1.0 V compared to the r-TiO₂. This means that the surface of RuO₂@r-TiO₂ had a higher electrocatalytic activity for OER when assisted by the thin RuO₂ layer. In the cathodic biased potential regime ranging from 0 to −1.5 V vs. Ag/AgCl on the RuO₂@r-TiO₂, there were no reactions regarding proton inter/deintercalation (−0.6/−0.9 V vs. Ag/AgCl) which was obviously found on r-TiO₂ (blue dash line in the inset image of Fig. 5a) as a unique electrochemical feature of r-TiO₂.^28,54–57 Additionally, as shown in Fig. S3,† in contrast to RuO₂@r-TiO₂, the r-TiO₂ lost its electrochemical property after the thermal treatment. This implies that the self-synthesized coating covered the entire surface of the r-TiO₂ and that the r-TiO₂ under the deposit of RuO₂@r-TiO₂ was prevented from oxidizing during the thermal treatment. Accordingly, its electrochemical property did not vanish even when it was annealed in the air condition; rather, this property was improved by the formed RuO₂.

For oxygen evolution under a constant current condition (Fig. 5b), RuO₂@r-TiO₂ led to a lower initial operational cell voltage (∼3.0 V) than r-TiO₂ (∼4.2 V), and it showed significantly enhanced stability with a value that was approximately 20 times higher (∼22 h) than that of r-TiO₂ (∼1.2 h). This can be evaluated based on the drastic increase in cell voltage. In addition, under a polarity reversal operation (switching constant current for +16.7 mA cm⁻² vs. −16.7 mA cm⁻²), the RuO₂@r-TiO₂ was highly stable on the stress from the harsh reversal condition compared to r-TiO₂. This indicates that RuO₂@r-TiO₂ is a more reliable material than r-TiO₂ in various environmental and industrial applications. Nevertheless, for the further success of RuO₂@r-TiO₂, the long-term performance stability must be improved via controlling the doping level of r-TiO₂ (Fig. S4 in ESI†). It is required to further study the effect of type of catalyst (i.e., IrO₂, Pt and carbon, etc.) for thin layer coating, coating thickness, and temperature of thermal treatment on CIER.

Fig. 6 shows the enhanced chlorine evolution performance and the pathway for ClER of RuO₂@r-TiO₂ compared to r-TiO₂. As shown in Fig. 6a, the chlorine production rate of RuO₂@r-TiO₂ was estimated to be approximately 17.85 mg L⁻¹ min⁻¹. This is approximately 20% faster than that of r-TiO₂ (14.35 mg L⁻¹ min⁻¹). Correspondingly, RuO₂@r-TiO₂ exhibited a current efficiency of 81.0% with an energy consumption of 3.0 W h g⁻¹, indicating higher electrocatalytic activity for chlorine production compared to r-TiO₂ (current efficiency of 64.7% and energy consumption of 5.2 W h g⁻¹ in Fig. 6b). The high chlorine evolution performances can be explained by the uniformly organized nanotube structure with the thin RuO₂ layer.

Furthermore, with the thin RuO₂ layer, the surface of RuO₂@r-TiO₂ was converted to be more attractive for chlorine than hydroxyl radical (Fig. 6c). This resulted in excellent chlorine production performances, namely the RuO₂@r-TiO₂ can be defined as an active electrode (high efficiency for chlorine production; RuO₂, IrO₂, etc.) rather than an inactive electrode (high efficiency for hydroxyl radical; r-TiO₂, boron doped diamond electrode, SnO₂, PbO₂, etc.).^{5,14,20,28,37} Note that, in common, the active electrode produces CIER via direct electron transfer with chloride ions whereas the inactive electrode leads to CIER by the indirect pathway mediated by hydroxyl radicals. This is well supported by the effect of the hydroxyl radical scavenger on the chlorine evolution (Fig. S5†). With the addition of t-BuOH as hydroxyl radical scavenger, the chlorine evolution efficiency of RuO₂@r-TiO₂ did not meaningfully decrease, whereas that of r-TiO₂ was significantly reduced. This means the small effect of hydroxyl radical on the chlorine evolution of RuO₂@r-TiO₂ instead of r-TiO₂ produced chlorine mediated by hydroxyl radical.³⁹ Considering the effect of hydroxyl radical on CIER of RuO₂@r-TiO₂ and r-TiO₂, the surface of RuO₂@r-TiO₂ behaves active electrodes. Eventually, we see the surface of RuO₂@r-TiO₂ was converted to an active electrode from an inactive electrode. It is attributed to that the RuO₂ thin layer was uniformly coated on the surface of nanopores of r-TiO₂, and thus, the surface of r-TiO₂ only worked as substrate, not catalytic material.

3.4 Effect of the immersion time on chlorine production efficiency of RuO₂@r-TiO₂

To optimize the deposition of RuO₂ on RuO₂@r-TiO₂, we further investigate the effect of the RuO₂ loading amount controlled via Ru precursor dipping time (ranging from 1 to 24 h) on the deposition and chlorine evolution efficiency of RuO₂@r-TiO₂. Fig. 7 shows the morphologies of the RuO₂@r-TiO₂ samples prepared with the time of 1, 3, 6, 12, and 24 h. As shown in Fig. 7, as the immersion time increased, the size of the nano-grains on the nano-pore edge and the thickness of the sidewall (inset images) gradually increased. In particular, regardless of the time, surfaces of all samples were more attractive for chlorine evolution than hydroxyl radical (Fig. 8a). However, compared to the chlorine evolution performances of RuO₂@r-TiO₂ prepared with the immersion times of 6, 12 and 24 h (current efficiency of 79.2, 79.4, and 81.0%; and energy consumption of 3.0, 3.0, 3.0 W h g⁻¹, respectively), the RuO₂@r-TiO₂ prepared with immersion times of 1 and 3 h resulted in relatively low chlorine evolution performance with current efficiencies of 72.2, 77.6% and energy consumption of 3.6, 3.2 W h g⁻¹, respectively (see Fig. 8b, and refer to all data of chlorine generation in Fig. S6a†). Correspondingly, a similar trend in the long-term stability was found (Fig. S6b†).

Moreover, this is well supported by the results obtained with the sample generation/tip collection (SG/TC) mode of SECM (Fig. 9) which visualized the electrocatalytic activity for chlorine production via a chlorine reduction reaction at −0.2 V vs. Ag/AgCl (the detailed experimental condition are shown in Fig. S7 and S8†).^58,59 As shown in Fig. 9, the electrocatalytic activity was evenly enhanced across the entire surface after 12 h according to the condition of deposited RuO₂ confirmed previously in Fig. 7 with FE-SEM. Particularly, compared to the pristine r-TiO₂ (Fig. 9a) which revealed nano-patterns of emerald-colored rings, reddish cores were observed after commencing RuO₂ deposition on r-TiO₂ (Fig. 9b–f). The RuO₂ coated inner pores improved the chlorine generation inside of the nanotubes, so that the higher activity (red) was measured at the center of the patterns than the circular edge. In Fig. 9f, circular nano-patterns on RuO₂@r-TiO₂ (24 h) were uniformly distributed in red. This color change verifies that the RuO₂ was effectively coated at the top edge of the tubes and even at its inner-pores as previously shown in images of SEM and TEM (Fig. 2 and 3), resulting in the enhanced the electrocatalytic performance for the CIER. As such, the morphological and electrochemical properties of r-TiO₂ are feasible to be facilely controlled with the self-synthesis coating method that can be extended to various fields of studies utilizing TiO₂ NTA-based electrocatalysts.

4. Conclusions

In this study, we successfully fabricated the RuO₂@r-TiO₂ with a self-synthesized coating method leading to a RuO₂ thin layer coating on r-TiO₂ and demonstrated considerably enhanced electro-catalytic activity for chlorine production. The fine RuO₂ coating was achieved via the self-synthesized coating originating from the conversion of Ti³⁺ to Ti⁴⁺ on r-TiO₂ and thermal treatment under the atmospheric condition. Using various surface analysis including FE-SEM, FE-TEM, EDS and XPS, the formation of a RuO₂ thin layer (thickness of ∼27.5 nm) on the inner pore sidewall of RuO₂@r-TiO₂ was clearly proven. The RuO₂@r-TiO₂ exhibited the highly enhanced electrocatalytic activity for chlorine production with the production rate of 17.85 mg L⁻¹ min⁻¹, the high current efficiency of 81.0%, energy consumption of 3.0 W h g⁻¹, and long-term stability of ∼22 h compared to r-TiO₂ (production rate of 14.35 mg L⁻¹ min⁻¹, the current efficiency of 64.7%, energy consumption of 5.2 W h g⁻¹, and long term service time of 1.2 h). In addition, the performances of RuO₂@r-TiO₂ was optimized by controlling the immersion time in the precursor. These results provide a new approach to the thin metal oxide coating on r-TiO₂ and provide opportunities for various applications such as electrolysis, photo-catalyst, and energy storage devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the Technology Innovation Program (10082572, Development of Low Energy Desalination Water Treatment Engineering Package System for Industrial Recycle Water Production) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea) and the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT of the Korea Government (MSIT) (NRF-2019R1G1A1003336).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra09623g

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