Introduction

The anodic oxidation of organic compounds in wastewater has attracted great interest due to its ability to purify and clean effluents in a rapid and controlled manner. The methodology has demonstrated high efficiency, especially in research works for pollutant decolourisation and chemical oxygen demand (COD) removal that utilize boron-doped diamond (BDD) electrodes in filter press flow cells [1, 2]. In many cases, however, the application of BDD is limited to laboratory scale, since BDD electrodes are expensive and considerable effort is required to set up a treatment plant. Therefore, there is a need to develop cost-effective coatings, which operate in a similar fashion to BDD and remain stable in a filter press flow cell during the mineralisation, decolourisation, and removal of organic compounds. Over the last decade, TiO2-based materials and coatings have been adopted as photocatalytic materials with the aid of ultraviolet (UV) light to degrade organic compounds. The increasing interest in electrochemical methods [2], such as electro-Fenton [3], anodic oxidation [4], electrocoagulation [5], solar photoelectro-Fenton [6], and photoassisted electrochemical methods [2], have attracted the attention of researchers in the field of water treatment as efficient, cost-effective, and environmental friendly technologies.

Anodic oxidation has been reported to be effective for partial and complete mineralisation with colour removal of organic compounds by using mixed metal oxides of Ru, Ti, Sb, Sn or Ir, and PbO2 [7]. Anodic oxidation is widely employed and recognized as much easier in comparison to other electrochemical technologies, such as electrochemical reduction (e.g. direct electrochemical reduction of amaranth azo dye) [2]. Anodic oxidation produces physisorbed hydroxyl radicals •OH that discharge over the anode surface (M) during the electrolysis of water:

$$ \mathrm{M}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{M}\left({}^{\bullet}\mathrm{OH}\right)+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} $$
(1)

The species M(˙OH) react with organic material until mineralisation and produce carbon dioxide and water [4]. Metal oxide electrode material plays an important role during the anodic oxidation reaction, since both chemical reactivity and ability to electrogenerate M(•OH) are related to the nature of the electrode. Different anode materials have been investigated to observe the stability and efficiency in batch as well as flow studies [8,9,10,11]. The anode materials should be selected on their high over potential for the oxygen evolution reaction (OER) as well as for their large surface area. On BDD electrodes for example, OER occurs at ca. + 2.3 V vs. SCE and it is more suitable for the production of M(˙OH) species than other materials [12,13,14].

Titanate nanotube (TiNT) coating has recently emerged as an attractive alternative for wastewater remediation due to their stability at high temperature, ease of preparation, low price, and relatively high oxygen evolution potential of 1.8 V vs. SCE in 0.1-mol L−1 Na2SO4 solution. For example, TiNT/Sb-doped SnO2 electrodes showed complete mineralisation and decolourisation for anodic oxidation of benzoic acid at 20 mA cm−2 [15]. Other nanomaterials, such as nano PbO2/TiO2 and TiO2 nanosheets, have been used for decolourisation and demineralisation of methyl orange and chloroethene [16, 17]. Recent research has shown that the preparation of PbO2 anodes decorated with TiO2 nanotubes was found effective for the complete mineralisation and decolourisation of reactive blue-194 at a current density of 150 mA cm−2 [18]. A modified Ti/SnO2-Sb/PbO2electrode has also being utilized for the removal of the azo dye acid black-194 at a current density of 30 mA cm−2 [19].

Other approaches, seeing to replace expensive metals, such as platinum [20], involve the formation of PbO2 inside the TiO2 nanotubes formed on a titanium substrate [21], which show high catalytic activity and large surface areas for electrodegradation. PbO2 acts as a conductive bridge during the anodic oxidation [22] demonstrating their suitability as electrodes for wastewater treatment [23, 24].

PbO2 coatings on RVC or a carbon-polymer substrate obtained by electrodeposition in methanesulfonic acid electrolytes have been found effective for electrochemical water treatment [25]. The inclusion of TiNT films has been claimed to be 100% efficient for the mineralisation and removal of benzoic acid from wastewater [15]. The anatase phase of TiNT provides active surface area of 20 m2 g−1, larger than the traditional Degussa P25 TiO2 particles that have a 7-m2 g−1 surface area and are 20% more efficient for the photo decolourisation of rhodamine-B dye [26]. The potential advantage of using a substrate with defined regular structure, such as RVC, provides efficient mass transport of the effluent over the surface, which ultimately provides efficient degradation [27]. RVC possesses a honeycomb structure [28] and has been employed for the electro Fenton degradation of azo dyes [1].

Polypyrrole/anthroquinone disulphonate composite film-modified graphite cathodes were used for the oxidation of azo dyes and achieved up to 80% mineralisation by electro-Fenton [29]. RVC was also employed to produce hydrogen peroxide to form the Fenton reagent and oxidase formic acid [30] and for removing metal ions [31, 32]. RVC substrate coated with TiNT produced inexpensive novel electrodes for advanced processes, such as anodic oxidation and photocatalytic degradation. In another example, TiO2 nanotubular arrays and titanium-based electrodes have showed considerable electrocatalytic behavior of organic compounds like ascorbic acid, glucose, dopamine, and alcohols [33, 34]. The use of lower cost carbonaceous alternatives, such as carbon foam PbO2 composite coatings with TiNT, is worth study due to their morphology and porosity, which increase the stability of TiNT during anodic oxidation [35].

The inclusion of photocatalytically active titanium dioxide produces electron-hole pairs, as depicted in reaction (2), under UV light.

$$ {\mathrm{TiO}}_{2\kern0.5em }+\mathrm{UV}\to \kern0.75em {e}_{\mathrm{CB}}^{-}+{h}_{\mathrm{VB}}^{+} $$
(2)

The adsorbed photons over the titanium dioxide nanoparticulate photocatalyst are initiated at an energy greater than 3.2 eV, which corresponds to the energy necessary to excite an electron from the valance to the conductive band with the formation of positive hole at the valance band [36]. This creates electron-hole pairs, and the active electrons can form superoxide ions \( \left({\mathrm{O}}_2^{-\bullet}\right) \)and peroxide radicals by reacting with O2 as shown in reactions (3) and (4) [37, 38].

$$ {\mathrm{e}}_{\mathrm{CB}}^{-}+{\mathrm{O}}_{2\kern0.5em }\to {\mathrm{O}}_2^{-\bullet }+{\mathrm{H}}^{+}\kern0.5em $$
(3)
$$ {\mathrm{O}}_2^{-\bullet }+{\mathrm{H}}^{+}\to \mathrm{H}{\mathrm{O}}_2^{-} $$
(4)

Such photocatalysis is reported in the literature for TiO2 under UV light irradiation [39]:

$$ 2\mathrm{H}{\mathrm{O}}_2^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2^{-}\kern0.5em $$
(5)
$$ {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2^{-\bullet}\to {\;}^{\bullet}\mathrm{OH}+{\mathrm{O}\mathrm{H}}^{-}+{\mathrm{O}}_2 $$
(6)
$$ {\mathrm{H}}_2\;\mathrm{O}+{\mathrm{h}}_{\mathrm{VB}}^{+}\to {\;}^{\bullet}\mathrm{O}\mathrm{H}+{\mathrm{H}}^{+} $$
(7)

Reaction (5) involves production of hydrogen peroxide, while reactions (6) and (7) represent the formation of •OH radicals. The oxidation of a water molecule forms the short-lived and powerful oxidant •OH radical (redox potential = 2.7 V vs. SCE) [40]. This oxidant is capable of decolourisation and mineralising organic matter residue to lower molecular weight compounds and ultimately to carbon dioxide and water [41].

The aim of this study is to investigate the electrochemical and photochemical performance of TiNS/PbO2 coating on RVC for the decolourisation of reactive black-5 (RB-5) dye and characterize the structural and morphological properties of the coatings. The decolourisation of this dye is important due to its widely commercial usage in the leather, wool, polymer, rubber, and silk industries.

Experimental details

Reagent grade 50% Pb (II) methanesulfonate (Pb(CH3SO3)2, 70% methanesulfonic acid (MSA), tetrabutylammonium hydroxide (TBAOH), cesium carbonate (Cs2CO3), Degussa TiO2 (P25), and reactive black-5 (RB-5) dye were purchased from Sigma Aldrich, while hydrochloric acid, sulfuric acid, sodium hydroxide, sodium sulphate, and acetone obtained from Fischer scientific and used as received.

Synthesis of titanate nanosheets

A single-layer of titanate nanosheets (TiNS) was developed using an adapted methodology from Sasaki et al. [42]. TiNS were prepared by the solid reaction between the Cs2CO3 and P25 at molar ratio 1:5.3 and 800 °C for 90 min. Once cooled, the resultant mixture was ground into fine powder. The powder was reheated at 800 °C for two cycles of 20 h each; after each interval of high-temperature treatment, the resulting mixture was cooled overnight and ground to fine powder. The resulting white powder was lepidocrocite-like cesium titanate, Cs0.7Ti1.82□0.175O4, where □ represents a vacancy [43].

Ion exchange method was used to produce smectite-like acid titanate from a mixture that contained 2 g of lepidocrocite-like cesium titanate and 80 mL of 1-mol L−1 HCl. The solution was stirred with a magnetic follower over 4 days at 700 rpm. The solution was replaced with a fresh acid solution every day to maintain the amount of H+ ions at 1 mol L−1 for acid leaching of Cs+ ions. The remaining solution was vacuum filtered by employing a nylon membrane and ultimately cleaned with distilled water until the conductivity reached around 10 μS cm−1. The resulting smectite-like acid titanate was recovered and used to produce exfoliated single-layer TiNS. This was achieved by stirring 0.4 g of smectite-like acid titanate with 100 mL of aqueous solution of 0.0165-mol L−1 tetrabutylammonium hydroxide (TBAOH) at 200 rpm and room temperature (25 °C) for 2 weeks. This process replaced the protons (H+) present in the TiNS with bulky (TBA+) ions. The exfoliated nanosheets were obtained as an aqueous suspension.

Electrodeposition of PbO2 on RVC

A piece of RVC substrate from ERG materials of 100 pores per linear inch (ppi, porosity grade of RVC) of 2-cm × 2-cm × 0.15-cm dimensions was treated with 70% nitric acid (Fisher Scientific) at 110 °C for 1 h and thoroughly rinsed and left in deionized water overnight, then dried at 90 °C overnight. A Cu wire was glued (Leit-C, Agar scientific) to the RVC to provide electrical connection. The RVC electrode was placed in an undivided electrochemical glass cell fitted with a water jacket connected to a water thermostat, Grant LT D6G model (Fig. 1), and was surrounded by a cylindrical mesh of stainless steel to provide uniform potential distribution. The cell was filled with an electrolyte solution containing 1-mol L−1 Pb(CH3SO3)2 and 0.2-mol L−1 MSA 100 mL of electrolyte. The electrodeposition of PbO2 on the RVC anode was carried out at 2.5 A at 60 °C for 30 min with a constant stirring of electrolyte at 700 rpm with a 0.25-cm long PTFE-coated magnetic follower [44, 45].

Fig. 1
figure 1

Electrodeposition of PbO2 from a solution containing 1-mol L−1 Pb (CH3SO3)2 and 0.2-mol L−1 MSA at 2.5 A and 60 °C for 30 min in a small undivided glass cell containing 100 mL of electrolyte with constant stirring at 700 rpm

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Anodic electrophoretic deposition

The RVC substrate coated with PbO2 was used as an anode for the electrophoretic deposition of TiNS from a solution containing exfoliated TiNS with TBAOH. The cathode was graphite plate of 1.5-cm × 6-cm × 1.2-cm dimensions in a parallel plate configuration with an interelectrode gap, d, of 1 cm. A cell potential difference of 15 V was applied between the electrodes, which created a potential gradient of 15 V cm−1. The electrolyte was vigorously stirred at 700 rpm with a magnetic stirrer, as shown in Fig. 2.

Fig. 2
figure 2

a Schematic diagram of cell for deposition of TiNS suspension containing 100 mL of TiNS exfoliated with tetrabutylammonium hydroxide (TBAOH) at 25 °C. b Schematic diagram of the arrangement for photocatalytic decolourisation of the dye

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Heat treatment

The TiNS/PbO2/RVC samples obtained from electrophoretic deposition were calcined at 450 °C for 1 h. The sections of RVC that were not coated with TiNS/PbO2 film due to the fact that material outside the electrolyte, disintegrated at such a high temperature, whereas those coated with the film resisted calcination. The TiNS film calcined after the heat treatment exhibited uniform coating over the RVC substrate.

Characterisation of the coated substrate

The surface morphology of the TiNS/PbO2/RVC samples was characterized by field emission scanning electron microscope (FESEM), using a JEOL 6500F at an accelerating voltage of 20 kV. Raman spectra were obtained using a confocal microscope (Renishaw, RM 2000) fitted with a light source of 632.8-nm wavelength to measure the peaks for the presence of deposited species. The exposure time was 30 s with a 10% intensity of laser radiation.

Electrochemical experiments

The electrochemical experiments were performed by means of a computer aided PGSTAT302 N potentiostat/galvanostat from Autolab (EcoChemie, Netherlands) by using Nova 1.11 software. The obtained coatings including calcined TiNS/PbO2/RVC, PbO2/RVC, and acid treated RVC were washed with ultra-pure water and dried. Each electrode was used to electrolyze a solution of 100 mL containing 10 ppm of RB-5 dye in 0.5 mol L−1 of sodium sulphate at pH = 3 and 25 °C at 2 mA cm−2. The electrolyte was stirred and maintained a stable pH throughout the electrochemical studies. Platinum mesh and Hg/HgO (0.6-mol L−1 NaOH) were used as a counter and reference electrodes, respectively.

Photocatalytic experiments

The calcined TiNS/PbO2/RVC sample was cut into a disc of 4-mm diameter and 2-mm thickness (0.919 g) and was fitted in a glass tube holder which was immersed in a 10-mL solution containing 10 ppm of RB-5 dye in 0.5 mol L−1 of sodium sulphate at pH 3 in Fig. 2b). The photocatalytic experiments were performed under a UV lamp with 20-mW cm−2 intensity. Prior to the photocatalytic experiments, the solution with tube holder was kept in the dark for 30 min in order to achieve maximum adsorption of RB-5 dye on the calcined TiNS/PbO2/RVC. This ensured that the decolourisation measurements in solution were due to the UV irradiation and not to the adsorption of the dye on the composite. Following this, the UV irradiation was provided every 1 min to the solution. The absorbance of the RB-5 in solution was measured in a Hitachi U3010 UV-Vis spectrophotometer at a wavelength of 597 nm. A calibration curve was used to evaluate the concentration of RB-5 dye in the solution.

Results and discussion

Surface characterisation of TiNS/PbO2 coated RVC

Figure 3a shows PbO2 layer coated over the strut of the 100-ppi RVC after electrodeposition at 2.5 A, whereas Fig. 3b shows the micrograph image of the PbO2 agglomerates covering the surface of RVC at larger magnification. TiNS films were anodically deposited on the PbO2/RVC surface by electrophoretic deposition as seen in Fig. 3c, which provides more uniform morphology than the PbO2/RVC film. The more magnified SEM image as seen in Fig. 3d revealed the presence of smooth and elongated strands of TiNS on the top of the PbO2/RVC. The TiNS/PbO2/RVC coating covers the typical tetrahedron structure of the RVC (100 ppi) substrate. The high resolution image (30,000 magnification) of TiNS/PbO2/RVC coating exhibit that TiNS are nanosize particles distributed over the substrate.

Fig. 3
figure 3

FESEM images of TiNS/PbO2 coatings over the RVC substrate obtained by electrodeposition and anodic electrophoretic deposition. a RVC strut coated with PbO2 after anodic electrodeposition. b PbO2 film at higher magnification. c TiNS/PbO2/RVC layer showing fully covered layer of TiNS film over PbO2/RVC. d TiNS/PbO2/RVC layer at higher magnification. e TiNS/PbO2/RVC at 30,000 magnification at a scale of 100 nm

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After calcination at 450 °C, the TiNS/PbO2/RVC coatings remain stable as shown in Fig. 4a. The high temperature of the calcination creates surface defects over the TiNS/PbO2/RVC coatings. The image also shows that the nanosheets are still attached to the PbO2/RVC coating even after the calcination at 450 °C.

Fig. 4
figure 4

a TiNS/PbO2/RVC after calcination at 450 °C. b Calcined TiNS/PbO2/RVC layer having undulated surfaces at higher magnification

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Figure 4b is an image obtained for similar coating, which denotes an undulated surface with ridges and valleys of TiNS/PbO2/RVC coatings after the transformation of the TiNS from the titanate to the anatase phase which imparts the photocatalytic active nature to the film and can be used to degrade reactive black-5 dye. The uncoated RVC (used to connect to the power supply during electrodeposition) oxidised at high temperature, while the carbon on the coated areas remained intact, which suggests that TiNS/PbO2 coating preserves the mechanical integrity of the RVC structure.

The elemental analysis of the coating was determined by EDX from the micrograph shown in Fig. 5a. It confirmed that the TiNS/PbO2/RVC coating contained titanium, lead, and carbon elements. Figure 5b–d shows the elemental maps revealing particles uniformly distributed over the substrate.

Fig. 5
figure 5

EDX images for elemental analysis of calcined TiNS/PbO2/RVC obtained by anodic electrophoretic deposition. a Calcined TiNS/PbO2/RVC. b Titanium elemental analysis. c Lead elemental analysis. d Carbon (RVC) elemental analysis

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The structural characteristics related to the transformation of the TiNS after the heat treatment were seen by Raman spectroscopy as shown in Fig. 6. The peaks seen at 177, 200, 397, 517, 639, and 747 cm−1 in the curve a indicate that heat treatment converted the film from titanate to anatase phase, which have been previously reported in the literature [46]. The peaks observed at 279 and 464 cm−1 in curve b for non-calcined coatings disappeared after calcination.

Fig. 6
figure 6

Raman spectra of the TiNS/PbO2/RVC obtained by anodic electrophoretic deposition a calcined at 450 °C and b non-calcined

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Electrochemical studies of the coatings

The OER is an unwanted reaction during the direct electrochemical anodic oxidation of organic compounds. The first step is the formation of hydroxyl radical (•OH) from the water oxidation at the anode substrate (Eq. (1). The following steps depend upon the nature of electrode substrate. Two types of substrates can be classified for direct anodic oxidation: “active” and “non-active” electrodes [4]. In active anodes, strong interaction of the hydroxyl radicals over electrodes takes place due to oxides generation [2], which leads to the OER (4). Platinum is as an active anode, as it is capable of attracting hydroxyl radicals over its surface, due to its greater adsorption enthalpy. In the case of non-active electrodes like PbO2, there is weak interaction of the electrodes with hydroxyl (•OH) radicals which are more prone to react in the electrolyte and oxidise organic compounds dissolved in solution [8].

Cyclic voltammetry studies using calcined TiNS/PbO2/RVC and PbO2/RVC electrodes in 0.5-mol L−1 Na2SO4 at a scan rate of 10 mV s−1 are shown in Fig. 7a. The electrode potential at which the OER starts to be significant (> 1 mA cm−2) on the calcined TiNS/PbO2/RVC, (curve 1) was 2.2 V vs. Hg/HgO, whereas in the case of PbO2/RVC, curve 2, the oxygen evolution started at > 1.8 V vs. Hg/HgO. The higher overpotential for the OER observed in calcined TiNS/PbO2/RVC suggests that this coating can potentially be a better catalyst for the production of hydroxyl radicals.

Fig. 7
figure 7

a Polarisation curve by using coating 1 (TiNS/PbO2/RVC) as anode after calcination at 450 °C in 0.5-mol L−1 Na2SO4, pH = 3.0. 2) (PbO2/RVC) as anode in 0.5-mol L−1 Na2SO4. Experimental conditions: potential sweep rate 10 mV s−1, temperature 298 K. b Polarisation curve by using novel coating 1 (TiNS/PbO2/RVC) as anode in an electrolyte 10-ppm R.B-5 dye in 0.5-mol L−1 Na2SO4, pH = 3.0. 2) (PbO2/RVC) as anode after calcination at 450 °C in an electrolyte 10-ppm R.B-5 dye in 0.5-mol L−1 Na2SO4, pH = 3.0, Experimental conditions: potential sweep rate 10 mV s−1, temperature 298 K

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When 10 ppm of RB-5 dye was added to the solution of 0.5-mol L−1 Na2SO4, the oxidation process on the calcined TiNS/PbO2/RVC started earlier at around 0.5 V vs. Hg/HgO but the current was around 1 mA cm−2 at 1.4 V vs. Hg/HgO. This suggests that the oxidation of the dye on this catalyst started before the OER as can be seen in Fig. 7b (curve 1). In the case of the PbO2/RVC electrode (curve 2), no significant oxidation of the dye was observed in comparison to curve 1 and the current density observed is significantly higher than the calcined TiNS/PbO2/RVC after 2.5 V vs. Hg/HgO. This might be due to the fact that by increasing the potential, the formation of the hydroxyl radical (•OH) competes with the direct oxidation of the dye and this oxidation is undetectable due to lower current density recorded in the voltammograms [10, 44].

Electrochemical decolourisation and colour removal of RB-5 dye

Electrolysis at constant current of 2 mA cm−2 was carried out in order to remove the colour from the RB-5 dye solutions using different coatings. The electrolyte consisted of 100 mL of solution containing 10 ppm of RB-5 dye in 0.5-mol L−1 Na2SO4 at pH 3. UV-Vis spectra were used to follow the colour removal of RB-5 dye, which shows a maximum absorption band in the visible light region at λmax = 597 nm. The oxidation of the azo dye complex molecule leads to low molecular weight intermediates, such as aliphatic and aromatic organic molecules [4]. The formation of these compounds is due to the displacement of the chromophore functional group and followed by the oxidation of the organics to carbon dioxide and organic acids (carboxylic acids) [19,20,21].

The time-dependent profiles of the normalized concentration c/co, for the electrochemical decolourisation of the dye using RVC, PbO2/RVC, and calcined TiNS/PbO2/RVC anode electrodes are shown in Fig. 8. The data of the concentration decay for the decolourisation of RB-5 dye on different electrodes can be fitted into the following logarithmic relationship:

$$ \ln \left(\frac{c}{c_0}\right)=- kt $$
(7)

where k is the rate constant, c the concentration, and t the reaction time. The fitting suggests a pseudo-first-order reaction kinetics for the decolourisation. The comparison of rate constant (k) is presented in Table 1.

Fig. 8
figure 8

Electrochemical remediation of RB-5 dye by (■) RVC, (□) photoassisted Fenton using iron oxide on activated alumina support [47], (▲) RVC/PbO2, and (○) calcined TiNS/PbO2/RVC

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Table 1 Comparison of apparent rate constants for decolourisation of RB-5 dye by RVC, calcined TiNS (TiO2)/PbO2/RVC, and PbO2/RVC and other related values from selected literature
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It can be seen that the pseudo-first-order rate constant decolourisation kinetics obtained using different anodes and from the data from the literature depends upon the nature of the electrocatalytic activity of the coating [40]. The anode electrodes utilized are able to decolourate the solution < 99% except for RVC electrode. This may be due to the tendency of PbO2/RVC and calcined TiNS/PbO2/RVC anodes to utilize •OH radicals produced from water discharge reaction and employ the radicals for the decolourisation of organic dye as shown in Eq. (8) below:

$$ \mathrm{RB}-5 +^{\bullet}\mathrm{O}\mathrm{H}\to \mathrm{Intermediates}\to {\mathrm{H}}_2\;\mathrm{O}+{\mathrm{CO}}_2 $$
(8)

The generation of hydroxyl radicals for the decolourisation of dyes over active electrodes plays an important part; however, they are also part of the mechanism to generate oxygen. If the OER is favored, the decolourisation of organics will be compromised [5]. In order to avoid this competition, the electrodes with large overpotential for oxygen evolution like TiNS/PbO2/RVC and PbO2/RVC are preferred. The logarithm of the normalized concentration of RB-5 decay vs. time when a current density of 2 mA cm−2 was applied on the anodes is shown in Fig. 9. The comparison of these electrodes shows that using TiNS/PbO2/RVC removes 98% of the colour with linear pseudo-first-order decolourisation and rate constant k = − 0.060 min−1.

Fig. 9
figure 9

Electrochemical remediation kinetics of RB-5 dye by (■) RVC, (▲) RVC/PbO2, and (●) calcined TiNS/PbO2/RVC

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Colour removal is also substantial when using PbO2/RVC electrode as shown by the normalized concentration data in Fig. 9, which is around 98%. This behavior is due to the production of Pb (•OH) as explained in Eq. (1) [41]. Negligible decolourisation results were obtained in case of RVC alone, which were about 36% colour removal after 60 min which is lower compared to the previously mentioned electrodes. Some reported works showed only 70% conversion of RB-5 dye by using photoassisted Fenton, which is lower than the values reported here [47]. This revealed that the •OH radical is weakly adsorbed over the surface of non-active electrode (PbO2) [8] and ultimately favors the production of •OH radical as indicated in Eq. (1) and also subsequent adsorption of the dye molecules over the active sites provided by positively charged TiNS under acidic conditions over the PbO2/RVC substrate.

Photocatalytic decolourisation of RB-5 dye

In order to evaluate the photocatalytic activity of the anatase phase of the calcined TiNS (TiO2)/PbO2/RVC coating characterized by Raman studies (Fig. 6), photocatalytic studies of the decolourisation of RB-5 dye in Na2SO4 and pH = 3 were carried out. The coating showed noticeable photocatalytic activity towards decolourisation of RB-5 dye. The activity of the coating depends on factors like protonation of TiO2 nanosheets in acidic media [53] and the generation of holes and electrons over the nanosheet surface in the presence of UV irradiation [54, 55]. In acidic solutions, the TiO2 nanosheets become positively charged as indicated by Eq. (10) and attract the negatively charged RB-5 dye [49].

$$ \mathrm{TiOH}+{\mathrm{H}}^{+}\to \kern0.5em \mathrm{TiO}{\mathrm{H}}_2^{+}\left(\mathrm{acidic}\ \mathrm{solution}\right) $$
(10)

Under UV light, there is the formation of electrons (e−) in the conduction band and holes (h+) in the valance band of the TiO2 nanosheet anatase as seen in Eq. (11). The photoinduced holes possess oxidative properties, which can attack the dye molecule directly leading to its decolourisation or indirectly forming hydroxyl radicals by promoting the reaction between the hydroxyl anions and water as shown by Eq. (12). The hydroxyl radicals react with the organic matter. However, the electrons can also react with an electron acceptor like O2 to produce oxidizing radicals \( {\mathrm{O}}_2^{-\bullet}\kern0.50em \)as shown by reaction (13) [49].

$$ {\mathrm{TiO}}_{2\kern0.5em }+ hv\to \kern0.75em {e}_{\mathrm{CB}}^{-}+{h}_{\mathrm{VB}}^{+} $$
(11)
$$ {\mathrm{OH}}^{-}+{h}_{\mathrm{VB}}^{+}\to {\;}^{\bullet}\mathrm{OH}+{\mathrm{H}}^{+} $$
(12)
$$ {e}_{\mathrm{CB}}^{-}+{\mathrm{O}}_{2\kern0.5em }\to {\mathrm{O}}_2^{-\bullet}\kern0.75em $$
(13)
$$ \mathrm{RB}-5 +^{\bullet}\mathrm{OH}\to \mathrm{Decompose} $$
(14)

Overall, the holes and radicals are responsible for the photocatalytic activity and decolourisation of RB-5 dye as indicated by Eqs. (4), (5), and (6) producing •OH radical, which ultimately decompose the dye as shown in Eq. (14). In this case, PbO2 dominates the base of the conduction band of TiNS (anatase TiO2), while the valance band belongs to the TiNS (anatase TiO2) nanoparticle states [56]. The formation of the new band gap will intensify the electron-hole pair transfer upon photoexcitation. The photocatalytic decolourisation is shown in Fig. 10 and indicates that the RB-5 dye follows a pseudo-first-order reaction kinetics on the calcined TiNS/PbO2/RVC coating, as revealed in Fig. 11.

Fig. 10
figure 10

Photocatalytic remediation of RB-5 dye by (▼) RVC, (●) calcined TiNS/PbO2/RVC, (■) using calcined TiNS/PbO2/RVC

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Fig. 11
figure 11

Photocatalytic remediation kinetics by (▼) RVC, (●) calcined TiNS/PbO2/RVC

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The comparison of rate constant (k) calculated by using Eq. (7) for the photocatalytic decolourisation (− 0.383 min−1) with electrochemical decolourisation (− 0.060 min−1) reveals that photocatalytic activity is higher by 84%. The annealing converts TiNS to the single-crystal anatase phase, which improved the photocatalytic activity when irradiated with UV light. The calcined TiNS/PbO2/RVC decolourized the reactive black-5 solution in 15 min. The improved photocatalytic activity of a coating, i.e. TiNS/PbO2/RVC ascribed due to the formation of holes and radicals by the incident UV light and ultimately participates in the photocatalytic decolourisation of the dye.

Conclusions

Lead dioxide was electrodeposited on RVC, and the resulting coating decorated with TiNS was firstly prepared via anodic electrophoretic deposition followed by calcination.

The following are the main findings:

  • Compared with RVC alone and PbO2/RVC electrodes, the calcined TiNS/PbO2/RVC coatings were able to decolourise RB-5 dye from wastewater more effectively.

  • The calcined TiNS/PbO2/RVC was also found effective for the photocatalytic decolourisation of the dye.

  • The nature of the electrode plays an important part in decolourisation and colour removal of the organic chromophore in the dye. The hydroxyl free radical (•OH) generates on the electrode surface provides an efficient and clean method for the decolourisation of RB-5 dye. Using calcined TiNS/PbO2/RVC electrodes; a first-order decolourisation kinetics has been determined with higher rate of reaction in comparison with some reported values in the literature.

  • Raman results indicated the transformation of the titanate phase of TiNS/PbO2/RVC to the anatase structure after calcination at 450 °C.

  • The anatase phase in the coating also found effective in photocatalytic studies and removal results revealed a first-order kinetics decolourisation of RB-5 dye in acidic conditions.

  • The photocatalytic effect resulted from three mechanisms, which involve photoinduced holes in valance band, the protonation of TiNS in acidic solutions, and the formation of oxidative radicals evolved from the reaction between the electrons from the conduction band and the electron acceptor (•O2).

This study opens up a promising approach in developing metal oxide structures decorated with TiO2 nanosheets over inexpensive carbon based substrate, for applications in wastewater treatment. TiNS coating on RVC also improved thermal resistance of RVC. Therefore, these coatings could potentially serve the purpose of decontamination of wastewater by using cheaper and flow-through electrodes such as RVC with the combination of PbO2 and TiNS.