Decolourisation of reactive black-5 at an RVC substrate decorated with PbO2/TiO2 nanosheets prepared by anodic electrodeposition

Decolourisation of reactive black-5 at an RVC substrate decorated with PbO2/TiO2 nanosheets... Reticulated vitreous carbon (RVC) substrates were coated with a composite of PbO and titanate nanosheets (TiNS) by anod- ic electrophoretic depostion. The structure and morphological characteristics of the coating were evaluated by field emission scanning electron microscopy (FESEM) and Raman spectroscopy. The TiNS/PbO /RVC coating contained the anatase phase and showed a well-defined, microporous morphology with hydrophilic character along the length and thickness of the RVC struts. Electrochemical and photocatalytic activity of the coatings facilitated RB-5 dye degradation as a model organic pollutant in wastewater. The electrochemical decolourisation involves the generation of hydroxyl free radicals over the TiNS/PbO /RVC anode composite surface, whereas photocatalytic decolourisation was driven by the synergetic photocatalytic effect imparted by the photoinduced holes and free electron acceptors. The photocatalytic properties of the TiNS/PbO coating were achieved by calcination at 450 °C for 60 min in air which converted the titanate phase to anatase and modified its surface area. This enabled 98% electrochemical decolourisation of the RB-5 dye solution (measured by visible absorption at 597 nm) in a time of 60 min. . . . Keywords Anodic electrophoretic deposition PbO RVC TiNS Introduction need to develop cost-effective coatings, which operate in a similar fashion to BDD and remain stable in a filter press flow The anodic oxidation of organic compounds in wastewater has cell during the mineralisation, decolourisation, and removal of attracted great interest due to its ability to purify and clean organic compounds. Over the last decade, TiO -based mate- effluents in a rapid and controlled manner. The methodology rials and coatings have been adopted as photocatalytic mate- has demonstrated high efficiency, especially in research works rials with the aid of ultraviolet (UV) light to degrade organic for pollutant decolourisation and chemical oxygen demand compounds. The increasing interest in electrochemical (COD) removal that utilize boron-doped diamond (BDD) methods [2], such as electro-Fenton [3], anodic oxidation electrodes in filter press flow cells [1, 2]. In many cases, how- [4], electrocoagulation [5], solar photoelectro-Fenton [6], and ever, the application of BDD is limited to laboratory scale, photoassisted electrochemical methods [2], have attracted the since BDD electrodes are expensive and considerable effort attention of researchers in the field of water treatment as effi- is required to set up a treatment plant. Therefore, there is a cient, cost-effective, and environmental friendly technologies. Anodic oxidation has been reported to be effective for par- tial and complete mineralisation with colour removal of or- Highlights � Stable TiNS/PbO films on 100-ppi RVC were achieved by 2 ganic compounds by using mixed metal oxides of Ru, Ti, Sb, electrophoretic deposition. Sn or Ir, and PbO [7]. Anodic oxidation is widely employed � SEM and Raman spectroscopy showed uniform layers of PbO /TiNS. and recognized as much easier in comparison to other electro- � PbO /TiNS/RVC allowed 98% decolourisation of 100-mL RB-5 dye in 60 min. chemical technologies, such as electrochemical reduction (e.g. direct electrochemical reduction of amaranth azo dye) [2]. * C. Ponce de León Anodic oxidation produces physisorbed hydroxyl radicals capla@soton.ac.uk OH that discharge over the anode surface (M) during the electrolysis of water: Electrochemical Engineering Laboratory, Faculty of Engineering and � þ − Environment, Engineering Sciences, University of Southampton, M þ H O→ MðÞ OH þ H þ e ð1Þ Highfield, Southampton SO17 1BJ, UK J Solid State Electrochem The species M(˙OH) react with organic material until Polypyrrole/anthroquinone disulphonate composite film- mineralisation and produce carbon dioxide and water [4]. modified graphite cathodes were used for the oxidation of Metal oxide electrode material plays an important role during azo dyes and achieved up to 80% mineralisation by electro- the anodic oxidation reaction, since both chemical reactivity Fenton [29]. RVC was also employed to produce hydrogen and ability to electrogenerate M( OH) are related to the nature peroxide to form the Fenton reagent and oxidase formic acid of the electrode. Different anode materials have been investi- [30] and for removing metal ions [31, 32]. RVC substrate gated to observe the stability and efficiency in batch as well as coated with TiNT produced inexpensive novel electrodes for flow studies [8–11]. The anode materials should be selected advanced processes, such as anodic oxidation and photocata- on their high over potential for the oxygen evolution reaction lytic degradation. In another example, TiO nanotubular ar- (OER) as well as for their large surface area. On BDD elec- rays and titanium-based electrodes have showed considerable trodes for example, OER occurs at ca. + 2.3 V vs. SCE and it electrocatalytic behavior of organic compounds like ascorbic is more suitable for the production of M(˙OH) species than acid, glucose, dopamine, and alcohols [33, 34]. The use of other materials [12–14]. lower cost carbonaceous alternatives, such as carbon foam Titanate nanotube (TiNT) coating has recently emerged as PbO composite coatings with TiNT, is worth study due to an attractive alternative for wastewater remediation due to their morphology and porosity, which increase the stability their stability at high temperature, ease of preparation, low of TiNT during anodic oxidation [35]. price, and relatively high oxygen evolution potential of The inclusion of photocatalytically active titanium dioxide −1 1.8 V vs. SCE in 0.1-mol L Na SO solution. For example, produces electron-hole pairs, as depicted in reaction (2), under 2 4 TiNT/Sb-doped SnO electrodes showed complete UV light. mineralisation and decolourisation for anodic oxidation of − þ −2 TiO þ UV→ e þ h ð2Þ CB VB benzoic acid at 20 mA cm [15]. Other nanomaterials, such as nano PbO TiO and TiO nanosheets, have been used for 2/ 2 2 The adsorbed photons over the titanium dioxide decolourisation and demineralisation of methyl orange and nanoparticulate photocatalyst are initiated at an energy greater chloroethene [16, 17]. Recent research has shown that the than 3.2 eV, which corresponds to the energy necessary to preparation of PbO anodes decorated with TiO nanotubes 2 2 excite an electron from the valance to the conductive band was found effective for the complete mineralisation and with the formation of positive hole at the valance band [36]. decolourisation of reactive blue-194 at a current density of −2 This creates electron-hole pairs, and the active electrons can 150 mA cm [18]. A modified Ti/SnO -Sb/PbO electrode 2 2 −� form superoxide ions O and peroxide radicals by reacting has also being utilized for the removal of the azo dye acid 2 −2 with O as shown in reactions (3)and (4)[37, 38]. black-194 at a current density of 30 mA cm [19]. 2 Other approaches, seeing to replace expensive metals, such − − � þ e þ O →O þ H ð3Þ CB 2 as platinum [20], involve the formation of PbO inside the − � þ − TiO nanotubes formed on a titanium substrate [21], which O þ H →HO ð4Þ 2 2 show high catalytic activity and large surface areas for Such photocatalysis is reported in the literature for TiO electrodegradation. PbO acts as a conductive bridge during under UV light irradiation [39]: the anodic oxidation [22] demonstrating their suitability as electrodes for wastewater treatment [23, 24]. − − 2HO → H O þ O ð5Þ 2 2 2 2 PbO coatings on RVC or a carbon-polymer substrate ob- − � � − H O þ O → OH þ OH þ O ð6Þ 2 2 2 tained by electrodeposition in methanesulfonic acid electro- 2 þ � þ lytes have been found effective for electrochemical water H O þ h → OH þ H ð7Þ VB treatment [25]. The inclusion of TiNT films has been claimed Reaction (5) involves production of hydrogen peroxide, to be 100% efficient for the mineralisation and removal of benzoic acid from wastewater [15]. The anatase phase of while reactions (6)and (7) represent the formation of OH 2 −1 TiNT provides active surface area of 20 m g , larger than radicals. The oxidation of a water molecule forms the the traditional Degussa P25 TiO particles that have a 7- short-lived and powerful oxidant OH radical (redox poten- 2 −1 m g surface area and are 20% more efficient for the photo tial = 2.7 V vs. SCE) [40]. This oxidant is capable of decolourisation of rhodamine-B dye [26]. The potential ad- decolourisation and mineralising organic matter residue vantage of using a substrate with defined regular structure, to lower molecular weight compounds and ultimately to such as RVC, provides efficient mass transport of the effluent carbon dioxide and water [41]. over the surface, which ultimately provides efficient degra- The aim of this study is to investigate the electrochemical dation [27]. RVC possesses a honeycomb structure [28]and and photochemical performance of TiNS/PbO coating on has been employed for the electro Fenton degradation of azo RVC for the decolourisation of reactive black-5 (RB-5) dye and characterize the structural and morphological properties dyes [1]. J Solid State Electrochem of the coatings. The decolourisation of this dye is important electrical connection. The RVC electrode was placed in an due to its widely commercial usage in the leather, wool, poly- undivided electrochemical glass cell fitted with a water jacket mer, rubber, and silk industries. 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 −1 Experimental details an electrolyte solution containing 1-mol L Pb(CH SO ) and 3 3 2 −1 0.2-mol L MSA 100 mL of electrolyte. The electrodeposition Reagent grade 50% Pb (II) methanesulfonate (Pb(CH SO ) , of PbO on the RVC anode was carried out at 2.5 A at 60 °C for 3 3 2 2 70% methanesulfonic acid (MSA), tetrabutylammonium hy- 30 min with a constant stirring of electrolyte at 700 rpm with a droxide (TBAOH), cesium carbonate (Cs CO ), Degussa 0.25-cm long PTFE-coated magnetic follower [44, 45]. 2 3 TiO (P25), and reactive black-5 (RB-5) dye were purchased from Sigma Aldrich, while hydrochloric acid, sulfuric acid, Anodic electrophoretic deposition sodium hydroxide, sodium sulphate, and acetone obtained from Fischer scientific and used as received. The RVC substrate coated with PbO was used as an anode for the electrophoretic deposition of TiNS from a solution con- Synthesis of titanate nanosheets taining exfoliated TiNS with TBAOH. The cathode was graphite plate of 1.5-cm × 6-cm × 1.2-cm dimensions in a par- A single-layer of titanate nanosheets (TiNS) was developed allel plate configuration with an interelectrode gap, d, of 1 cm. using an adapted methodology from Sasaki et al. [42]. TiNS A cell potential difference of 15 V was applied between the −1 were prepared by the solid reaction between the Cs CO and electrodes, which created a potential gradient of 15 V cm . 2 3 P25 at molar ratio 1:5.3 and 800 °C for 90 min. Once cooled, The electrolyte was vigorously stirred at 700 rpm with a mag- the resultant mixture was ground into fine powder. The pow- netic stirrer, as shown in Fig. 2. der was reheated at 800 °C for two cycles of 20 h each; after each interval of high-temperature treatment, the resulting mix- Heat treatment ture was cooled overnight and ground to fine powder. The resulting white powder was lepidocrocite-like cesium titanate, The TiNS/PbO /RVC samples obtained from electrophoretic Cs Ti □ O ,where □ represents a vacancy [43]. deposition were calcined at 450 °C for 1 h. The sections of 0.7 1.82 0.175 4 Ion exchange method was used to produce smectite-like acid RVC that were not coated with TiNS/PbO film due to the fact titanate from a mixture that contained 2 g of lepidocrocite-like that material outside the electrolyte, disintegrated at such a −1 cesium titanate and 80 mL of 1-mol L HCl. The solution was high temperature, whereas those coated with the film resisted stirred with a magnetic follower over 4 days at 700 rpm. The calcination. The TiNS film calcined after the heat treatment solution wasreplacedwithafreshacid solutionevery dayto exhibited uniform coating over the RVC substrate. + −1 maintain the amount of H ions at 1 mol L for acid leaching of Cs ions. The remaining solution was vacuum filtered by Characterisation of the coated substrate employing a nylon membrane and ultimately cleaned with dis- −1 tilled water until the conductivity reached around 10 μScm . The surface morphology of the TiNS/PbO /RVC samples was The resulting smectite-like acid titanate was recovered and used characterized by field emission scanning electron microscope to produce exfoliated single-layer TiNS. This was achieved by (FESEM), using a JEOL 6500F at an accelerating voltage of stirring 0.4 g of smectite-like acid titanate with 100 mL of 20 kV. Raman spectra were obtained using a confocal micro- −1 aqueous solution of 0.0165-mol L tetrabutylammonium hy- scope (Renishaw, RM 2000) fitted with a light source of droxide (TBAOH) at 200 rpm and room temperature (25 °C) 632.8-nm wavelength to measure the peaks for the presence for 2 weeks. This process replaced the protons (H )present in of deposited species. The exposure time was 30 s with a 10% the TiNS with bulky (TBA ) ions. The exfoliated nanosheets intensity of laser radiation. were obtained as an aqueous suspension. Electrochemical experiments Electrodeposition of PbO on RVC The electrochemical experiments were performed by means of A piece of RVC substrate from ERG materials of 100 pores per a computer aided PGSTAT302 N potentiostat/galvanostat linear inch (ppi, porosity grade of RVC) of 2-cm × 2-cm × from Autolab (EcoChemie, Netherlands) by using Nova 0.15-cm dimensions was treated with 70% nitric acid (Fisher 1.11 software. The obtained coatings including calcined Scientific) at 110 °C for 1 h and thoroughly rinsed and left in TiNS/PbO /RVC, PbO /RVC, and acid treated RVC were 2 2 deionized water overnight, then dried at 90 °C overnight. A Cu washed with ultra-pure water and dried. Each electrode was wire was glued (Leit-C, Agar scientific) to the RVC to provide used to electrolyze a solution of 100 mL containing 10 ppm of J Solid State Electrochem Fig. 1 Electrodeposition of PbO from a solution containing 1- −1 mol L Pb (CH SO ) and 0.2- 3 3 2 −1 mol L 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 −1 RB-5 dye in 0.5 mol L of sodium sulphate at pH = 3 and the micrograph image of the PbO agglomerates covering the −2 25 °C at 2 mA cm . The electrolyte was stirred and main- surface of RVC at larger magnification. TiNS films were anod- tained a stable pH throughout the electrochemical studies. ically deposited on the PbO /RVC surface by electrophoretic −1 Platinum mesh and Hg/HgO (0.6-mol L NaOH) were used deposition as seen in Fig. 3c, which provides more uniform as a counter and reference electrodes, respectively. morphology than the PbO /RVC film. The more magnified SEM image as seen in Fig. 3d revealed the presence of smooth Photocatalytic experiments and elongated strands of TiNS on the top of the PbO /RVC. The TiNS/PbO /RVC coating covers the typical tetrahedron struc- The calcined TiNS/PbO /RVC sample was cut into a disc of 4- ture of the RVC (100 ppi) substrate. The high resolution image mm diameter and 2-mm thickness (0.919 g) and was fitted in a (30,000 magnification) of TiNS/PbO /RVC coating exhibit that glass tube holder which was immersed in a 10-mL solution TiNS are nanosize particles distributed over the substrate. −1 containing 10 ppm of RB-5 dye in 0.5 mol L of sodium sul- After calcination at 450 °C, the TiNS/PbO /RVC coatings phate at pH 3 in Fig. 2b). The photocatalytic experiments were remain stable as shown in Fig. 4a. The high temperature of the −2 performed under a UV lamp with 20-mW cm intensity. Prior calcination creates surface defects over the TiNS/PbO /RVC to the photocatalytic experiments, the solution with tube holder coatings. The image also shows that the nanosheets are still was kept in the dark for 30 min in order to achieve maximum attached to the PbO /RVC coating even after the calcination at adsorption of RB-5 dye on the calcined TiNS/PbO /RVC. This 450 °C. ensured that the decolourisation measurements in solution were Figure 4b is an image obtained for similar coating, which due to the UV irradiation and not to the adsorption of the dye on denotes an undulated surface with ridges and valleys of TiNS/ the composite. Following this, the UV irradiation was provided PbO /RVC coatings after the transformation of the TiNS from every 1 min to the solution. The absorbance of the RB-5 in the titanate to the anatase phase which imparts the photocata- solution was measured in a Hitachi U3010 UV-Vis spectropho- lytic active nature to the film and can be used to degrade tometer at a wavelength of 597 nm. A calibration curve was used reactive black-5 dye. The uncoated RVC (used to connect to to evaluate the concentration of RB-5 dye in the solution. the power supply during electrodeposition) oxidised at high temperature, while the carbon on the coated areas remained intact, which suggests that TiNS/PbO coating preserves the Results and discussion mechanical integrity of the RVC structure. The elemental analysis of the coating was determined by Surface characterisation of TiNS/PbO coated RVC EDX from the micrograph shown in Fig. 5a. It confirmed that the TiNS/PbO /RVC coating contained titanium, lead, and Figure 3ashows PbO layer coated over the strut of the 100-ppi carbon elements. Figure 5b–d shows the elemental maps re- RVC after electrodeposition at 2.5 A, whereas Fig. 3bshows vealing particles uniformly distributed over the substrate. J Solid State Electrochem Fig. 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 The structural characteristics related to the transformation strong interaction of the hydroxyl radicals over electrodes of the TiNS after the heat treatment were seen by Raman takes place due to oxides generation [2], which leads to the spectroscopyasshowninFig. 6. The peaks seen at 177, OER (4). Platinum is as an active anode, as it is capable of −1 200, 397, 517, 639, and 747 cm in the curve a indicate that attracting hydroxyl radicals over its surface, due to its greater heat treatment converted the film from titanate to anatase adsorption enthalpy. In the case of non-active electrodes like phase, which have been previously reported in the literature PbO , there is weak interaction of the electrodes with hydrox- −1 � [46]. The peaks observed at 279 and 464 cm in curve b for yl ( OH) radicals which are more prone to react in the electro- non-calcined coatings disappeared after calcination. lyte and oxidise organic compounds dissolved in solution [8]. Cyclic voltammetry studies using calcined TiNS/PbO / −1 Electrochemical studies of the coatings RVC and PbO /RVC electrodes in 0.5-mol L Na SO at a 2 2 4 −1 scan rate of 10 mV s are shown in Fig. 7a. The electrode The OER is an unwanted reaction during the direct electro- potential at which the OER starts to be significant (> −2 chemical anodic oxidation of organic compounds. The first 1mA cm ) on the calcined TiNS/PbO /RVC, (curve 1) was step is the formation of hydroxyl radical ( OH) from the water 2.2 V vs. Hg/HgO, whereas in the case of PbO /RVC, curve 2, oxidation at the anode substrate (Eq. (1). The following steps the oxygen evolution started at > 1.8 V vs. Hg/HgO. The depend upon the nature of electrode substrate. Two types of higher overpotential for the OER observed in calcined TiNS/ substrates can be classified for direct anodic oxidation: PbO /RVC suggests that this coating can potentially be a bet- Bactive^ and Bnon-active^ electrodes [4]. In active anodes, ter catalyst for the production of hydroxyl radicals. J Solid State Electrochem Fig. 3 FESEM images of TiNS/ PbO coatings over the RVC PbO film substrate obtained by electrodeposition and anodic electrophoretic deposition. a RVC strut coated with PbO after anodic electrodeposition. b PbO film at higher magnification. c TiNS/PbO /RVC layer showing fully covered layer of TiNS film over PbO /RVC. d TiNS/PbO / 2 2 RVC layer at higher magnification. e TiNS/PbO /RVC at 30,000 magnification at a scale of 100 nm b) a) 1m 10 m TiNS film c) d) 1m 10 m TiNS film 30,000 magnification e) 100 nm When 10 ppm of RB-5 dye was added to the solution of 0.5- of the dye was observed in comparison to curve 1 and the −1 mol L Na SO , the oxidation process on the calcined TiNS/ current density observed is significantly higher than the cal- 2 4 PbO /RVC started earlier at around 0.5 V vs. Hg/HgO but the cined TiNS/PbO /RVC after 2.5 V vs. Hg/HgO. This might be 2 2 −2 current was around 1 mA cm at 1.4 V vs. Hg/HgO. This due to the fact that by increasing the potential, the formation of suggests that the oxidation of the dye on this catalyst started the hydroxyl radical ( OH) competes with the direct oxidation before the OER as can be seen in Fig. 7b (curve 1). In the case of the dye and this oxidation is undetectable due to lower of the PbO /RVC electrode (curve 2), no significant oxidation current density recorded in the voltammograms [10, 44]. 2 J Solid State Electrochem Fig. 4 a TiNS/PbO /RVC after calcination at 450 °C. b Calcined TiNS/PbO /RVC layer having TiNS film undulated surfaces at higher magnification after calcination b) a) 10 m 100 m Electrochemical decolourisation and colour removal electrodes are shown in Fig. 8. The data of the concentration of RB-5 dye decay for the decolourisation of RB-5 dye on different elec- trodes can be fitted into the following logarithmic relationship: −2 Electrolysis at constant current of 2 mA cm was carried out in order to remove the colour from the RB-5 dye solutions c ln ¼ −kt ð7Þ using different coatings. The electrolyte consisted of 100 mL −1 of solution containing 10 ppm of RB-5 dye in 0.5-mol L Na SO at pH 3. UV-Vis spectra were used to follow the where k is the rate constant, c the concentration, and t the 2 4 colour removal of RB-5 dye, which shows a maximum ab- reaction time. The fitting suggests a pseudo-first-order reac- sorption band in the visible light region at λ =597 nm. The tion kinetics for the decolourisation. The comparison of rate max oxidation of the azo dye complex molecule leads to low mo- constant (k) is presented in Table 1. lecular weight intermediates, such as aliphatic and aromatic It can be seen that the pseudo-first-order rate constant decolourisation kinetics obtained using different anodes and organic molecules [4]. The formation of these compounds is due to the displacement of the chromophore functional group from the data from the literature depends upon the nature of the electrocatalytic activity of the coating [40]. The anode and followed by the oxidation of the organics to carbon diox- ide and organic acids (carboxylic acids) [19–21]. electrodes utilized are able to decolourate the solution < 99% The time-dependent profiles of the normalized concentra- except for RVC electrode. This may be due to the tendency of tion c/c , for the electrochemical decolourisation of the dye PbO /RVC and calcined TiNS/PbO /RVC anodes to utilize 2 2 using RVC, PbO /RVC, and calcined TiNS/PbO /RVC anode OH radicals produced from water discharge reaction and 2 2 Fig. 5 EDX images for elemental analysis of calcined TiNS/PbO / RVC obtained by anodic electrophoretic deposition. a Calcined TiNS/PbO /RVC. b Titanium elemental analysis. c Lead elemental analysis. d 1m Carbon (RVC) elemental analysis a) b) Ti Ka1 c) Pb Ma1 d) C Ka1-2 J Solid State Electrochem a) 2) 1) Fig. 6 Raman spectra of the TiNS/PbO /RVC obtained by anodic b) electrophoretic deposition a calcined at 450 °C and b non-calcined employ the radicals for the decolourisation of organic dye as shown in Eq. (8)below: RB−5 þ OH→ Intermediates→ H O þ CO ð8Þ 2 2 1) The generation of hydroxyl radicals for the decolourisation 2) of dyes over active electrodes plays an important part; how- ever, 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 elec- trodes with large overpotential for oxygen evolution like TiNS/PbO /RVC and PbO /RVC are preferred. The logarithm 2 2 of the normalized concentration of RB-5 decay vs. time when −2 a current density of 2 mA cm was applied on the anodes is Fig. 7 a Polarisation curve by using coating 1 (TiNS/PbO /RVC) as shown in Fig. 9. The comparison of these electrodes shows 2 −1 anode after calcination at 450 °C in 0.5-mol L Na SO , pH = 3.0. 2) 2 4 that using TiNS/PbO /RVC removes 98% of the colour with 2 −1 (PbO /RVC) as anode in 0.5-mol L Na SO . Experimental conditions: 2 2 4 −1 linear pseudo-first-order decolourisation and rate constant k = potential sweep rate 10 mV s , temperature 298 K. b Polarisation curve −1 − 0.060 min . by using novel coating 1 (TiNS/PbO /RVC) as anode in an electrolyte 10- −1 ppm R.B-5 dye in 0.5-mol L Na SO , pH = 3.0. 2) (PbO /RVC) as 2 4 2 Colour removal is also substantial when using PbO /RVC anode after calcination at 450 °C in an electrolyte 10-ppm R.B-5 dye in electrode as shown by the normalized concentration data in −1 0.5-mol L Na SO , pH = 3.0, Experimental conditions: potential sweep 2 4 −1 Fig. 9, which is around 98%. This behavior is due to the rate 10 mV s , temperature 298 K 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 Photocatalytic decolourisation of RB-5 dye 60 min which is lower compared to the previously mentioned electrodes. Some reported works showed only 70% conver- In order to evaluate the photocatalytic activity of the anatase sion of RB-5 dye by using photoassisted Fenton, which is phase of the calcined TiNS (TiO )/PbO /RVC coating charac- 2 2 lower than the values reported here [47]. This revealed that terized by Raman studies (Fig. 6), photocatalytic studies of the the OH radical is weakly adsorbed over the surface of non- decolourisation of RB-5 dye in Na SO andpH= 3were car- 2 4 active electrode (PbO )[8] and ultimately favors the produc- ried out. The coating showed noticeable photocatalytic activ- tion of OH radical as indicated in Eq. (1) and also subsequent ity towards decolourisation of RB-5 dye. The activity of the adsorption of the dye molecules over the active sites provided coating depends on factors like protonation of TiO nano- by positively charged TiNS under acidic conditions over the sheets in acidic media [53] and the generation of holes and PbO /RVC substrate. electrons over the nanosheet surface in the presence of UV 2 J Solid State Electrochem Fig. 9 Electrochemical remediation kinetics of RB-5 dye by (■) RVC, Fig. 8 Electrochemical remediation of RB-5 dye by (■)RVC, (□) (▲)RVC/PbO ,and (●) calcined TiNS/PbO /RVC 2 2 photoassisted Fenton using iron oxide on activated alumina support [47], (▲)RVC/PbO , and (○) calcined TiNS/PbO /RVC 2 2 −� electron acceptor like O to produce oxidizing radicals O as shownbyreaction(13)[49]. irradiation [54, 55]. In acidic solutions, the TiO nanosheets − þ become positively charged as indicated by Eq. (10) and attract TiO þ hv→ e þ h ð11Þ CB VB the negatively charged RB-5 dye [49]. − þ � þ OH þ h → OH þ H ð12Þ VB − − � þ þ e þ O → O ð13Þ TiOH þ H → TiOHðÞ acidic solution ð10Þ 2 CB 2 RB−5 þ OH→ Decompose ð14Þ Under UV light, there is the formation of electrons (e ) in the conduction band and holes (h )inthevalance band of Overall, the holes and radicals are responsible for the pho- the TiO nanosheet anatase as seen in Eq. (11). The photo- tocatalytic activity and decolourisation of RB-5 dye as indi- induced holes possess oxidative properties, which can at- cated by Eqs. (4), (5), and (6)producing OH radical, which tack the dye molecule directly leading to its decolourisation ultimately decompose the dye as shown in Eq. (14). In this or indirectly forming hydroxyl radicals by promoting the case, PbO dominates the base of the conduction band of reaction between the hydroxyl anions and water as shown TiNS (anatase TiO ), while the valance band belongs to the by Eq. (12). The hydroxyl radicals react with the organic TiNS (anatase TiO ) nanoparticle states [56]. The formation of matter. However, the electrons can also react with an the new band gap will intensify the electron-hole pair transfer Table 1 Comparison of apparent rate constants for decolourisation of RB-5 dye by RVC, calcined TiNS (TiO )/PbO /RVC, and PbO /RVC and other 2 2 2 related values from selected literature −1 Substrate − k/min Decolourisation % Time/min Reference RVC by using anodic oxidation 0.0064 36 60 This study PbO /RVC by using anodic oxidation 0.050 97 60 This study Calcined TiNS(TiO )/PbO /RVC by using anodic oxidation 0.060 98 60 This study 2 2 Calcined TiNS(TiO )/PbO /RVC by using photocatalytic decolourisation 0.383 85 5 Photocatalytic rate 2 2 in this study TiO (P25) by using photocatalytic degradation 0.038 ≈ 90 60 [48] TiO nanofiber-nanoparticle composite by using photocatalytic degradation 0.039 94.4 60 [49] TiO nanofiber composite by using photocatalytic degradation 0.026 75.5 60 [50] Photoassisted Fenton by using iron oxide on activated alumina support N.G 70 480 [47] TiO -P25 by using photocatalytic degradation N.G 98 150 [51] UV/H O by using Iron salt N.G 99.3 180 [52] 2 2 J Solid State Electrochem 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 PbO /RVC electrodes, the calcined TiNS/PbO /RVC coatings were able to decolourise RB-5 dye from wastewater more effectively. & The calcined TiNS/PbO /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 chro- mophore in the dye. The hydroxyl free radical ( OH) gen- Fig. 10 Photocatalytic remediation of RB-5 dye by (▼)RVC,(●) calcined TiNS/PbO /RVC, (■) using calcined TiNS/PbO /RVC erates on the electrode surface provides an efficient and 2 2 clean method for the decolourisation of RB-5 dye. Using upon photoexcitation. The photocatalytic decolourisation is calcined TiNS/PbO /RVC electrodes; a first-order decolourisation kinetics has been determined with higher shown in Fig. 10 and indicates that the RB-5 dye follows a pseudo-first-order reaction kinetics on the calcined TiNS/ rate of reaction in comparison with some reported values PbO /RVC coating, as revealed in Fig. 11. in the literature. & Raman results indicated the transformation of the titanate The comparison of rate constant (k) calculated by using Eq. −1 (7) for the photocatalytic decolourisation (− 0.383 min )with phase of TiNS/PbO /RVC to the anatase structure after −1 calcination at 450 °C. electrochemical decolourisation (− 0.060 min ) reveals that photocatalytic activity is higher by 84%. The annealing con- & The anatase phase in the coating also found effective in pho- tocatalytic studies and removal results revealed a first-order verts TiNS to the single-crystal anatase phase, which im- proved the photocatalytic activity when irradiated with UV kinetics decolourisation of RB-5 dye in acidic conditions. & The photocatalytic effect resulted from three mechanisms, light. The calcined TiNS/PbO /RVC decolourized the reactive black-5 solution in 15 min. The improved photocatalytic ac- which involve photoinduced holes in valance band, the tivity of a coating, i.e. TiNS/PbO /RVC ascribed due to the protonation of TiNS in acidic solutions, and the formation formation of holes and radicals by the incident UV light and of oxidative radicals evolved from the reaction between ultimately participates in the photocatalytic decolourisation of the electrons from the conduction band and the electron acceptor ( O ). the dye. This study opens up a promising approach in developing metal oxide structures decorated with TiO nanosheets over inexpensive carbon based substrate, for applications in waste- water 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 PbO and TiNS. Acknowledgements S.Z.J. Zaidi is grateful to the Faculty of Engineering and the Environment at the University of Southampton for the Rayleigh studenship, to the Bestway Foundation, Charity No. 297178, UK and to the European Commission Project H2020 CO2EXIDE, for the economic support.The authors acknowledge D.V. Bavykin for useful discussions. The raw data presented in this paper can be found at the repository of the University of Southampton: https://doi.org/10.5258/SOTON/D0530 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// Fig. 11 Photocatalytic remediation kinetics by (▼)RVC,(●) calcined creativecommons.org/licenses/by/4.0/), which permits unrestricted use, TiNS/PbO /RVC 2 J Solid State Electrochem distribution, and reproduction in any medium, provided you give appro- and its application in organic degradation. Electrochim Acta 201: priate credit to the original author(s) and the source, provide a link to the 240–250 Creative Commons license, and indicate if changes were made. 17. Biyoghe BNL, Ibondou MP, Gu X, Xu M, Lu S, Qiu Z, Mbadinga SM (2014) Efficiently synthetic TiO Nano-sheets for PCE, TCE, and TCA degradations in aqueous phase under VUV irradiation. Water Air Soil Pollut 225(5):1951 References 18. 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Decolourisation of reactive black-5 at an RVC substrate decorated with PbO2/TiO2 nanosheets prepared by anodic electrodeposition

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Chemistry; Physical Chemistry; Electrochemistry; Energy Storage; Characterization and Evaluation of Materials; Analytical Chemistry; Condensed Matter Physics
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Abstract

Reticulated vitreous carbon (RVC) substrates were coated with a composite of PbO and titanate nanosheets (TiNS) by anod- ic electrophoretic depostion. The structure and morphological characteristics of the coating were evaluated by field emission scanning electron microscopy (FESEM) and Raman spectroscopy. The TiNS/PbO /RVC coating contained the anatase phase and showed a well-defined, microporous morphology with hydrophilic character along the length and thickness of the RVC struts. Electrochemical and photocatalytic activity of the coatings facilitated RB-5 dye degradation as a model organic pollutant in wastewater. The electrochemical decolourisation involves the generation of hydroxyl free radicals over the TiNS/PbO /RVC anode composite surface, whereas photocatalytic decolourisation was driven by the synergetic photocatalytic effect imparted by the photoinduced holes and free electron acceptors. The photocatalytic properties of the TiNS/PbO coating were achieved by calcination at 450 °C for 60 min in air which converted the titanate phase to anatase and modified its surface area. This enabled 98% electrochemical decolourisation of the RB-5 dye solution (measured by visible absorption at 597 nm) in a time of 60 min. . . . Keywords Anodic electrophoretic deposition PbO RVC TiNS Introduction need to develop cost-effective coatings, which operate in a similar fashion to BDD and remain stable in a filter press flow The anodic oxidation of organic compounds in wastewater has cell during the mineralisation, decolourisation, and removal of attracted great interest due to its ability to purify and clean organic compounds. Over the last decade, TiO -based mate- effluents in a rapid and controlled manner. The methodology rials and coatings have been adopted as photocatalytic mate- has demonstrated high efficiency, especially in research works rials with the aid of ultraviolet (UV) light to degrade organic for pollutant decolourisation and chemical oxygen demand compounds. The increasing interest in electrochemical (COD) removal that utilize boron-doped diamond (BDD) methods [2], such as electro-Fenton [3], anodic oxidation electrodes in filter press flow cells [1, 2]. In many cases, how- [4], electrocoagulation [5], solar photoelectro-Fenton [6], and ever, the application of BDD is limited to laboratory scale, photoassisted electrochemical methods [2], have attracted the since BDD electrodes are expensive and considerable effort attention of researchers in the field of water treatment as effi- is required to set up a treatment plant. Therefore, there is a cient, cost-effective, and environmental friendly technologies. Anodic oxidation has been reported to be effective for par- tial and complete mineralisation with colour removal of or- Highlights � Stable TiNS/PbO films on 100-ppi RVC were achieved by 2 ganic compounds by using mixed metal oxides of Ru, Ti, Sb, electrophoretic deposition. Sn or Ir, and PbO [7]. Anodic oxidation is widely employed � SEM and Raman spectroscopy showed uniform layers of PbO /TiNS. and recognized as much easier in comparison to other electro- � PbO /TiNS/RVC allowed 98% decolourisation of 100-mL RB-5 dye in 60 min. chemical technologies, such as electrochemical reduction (e.g. direct electrochemical reduction of amaranth azo dye) [2]. * C. Ponce de León Anodic oxidation produces physisorbed hydroxyl radicals capla@soton.ac.uk OH that discharge over the anode surface (M) during the electrolysis of water: Electrochemical Engineering Laboratory, Faculty of Engineering and � þ − Environment, Engineering Sciences, University of Southampton, M þ H O→ MðÞ OH þ H þ e ð1Þ Highfield, Southampton SO17 1BJ, UK J Solid State Electrochem The species M(˙OH) react with organic material until Polypyrrole/anthroquinone disulphonate composite film- mineralisation and produce carbon dioxide and water [4]. modified graphite cathodes were used for the oxidation of Metal oxide electrode material plays an important role during azo dyes and achieved up to 80% mineralisation by electro- the anodic oxidation reaction, since both chemical reactivity Fenton [29]. RVC was also employed to produce hydrogen and ability to electrogenerate M( OH) are related to the nature peroxide to form the Fenton reagent and oxidase formic acid of the electrode. Different anode materials have been investi- [30] and for removing metal ions [31, 32]. RVC substrate gated to observe the stability and efficiency in batch as well as coated with TiNT produced inexpensive novel electrodes for flow studies [8–11]. The anode materials should be selected advanced processes, such as anodic oxidation and photocata- on their high over potential for the oxygen evolution reaction lytic degradation. In another example, TiO nanotubular ar- (OER) as well as for their large surface area. On BDD elec- rays and titanium-based electrodes have showed considerable trodes for example, OER occurs at ca. + 2.3 V vs. SCE and it electrocatalytic behavior of organic compounds like ascorbic is more suitable for the production of M(˙OH) species than acid, glucose, dopamine, and alcohols [33, 34]. The use of other materials [12–14]. lower cost carbonaceous alternatives, such as carbon foam Titanate nanotube (TiNT) coating has recently emerged as PbO composite coatings with TiNT, is worth study due to an attractive alternative for wastewater remediation due to their morphology and porosity, which increase the stability their stability at high temperature, ease of preparation, low of TiNT during anodic oxidation [35]. price, and relatively high oxygen evolution potential of The inclusion of photocatalytically active titanium dioxide −1 1.8 V vs. SCE in 0.1-mol L Na SO solution. For example, produces electron-hole pairs, as depicted in reaction (2), under 2 4 TiNT/Sb-doped SnO electrodes showed complete UV light. mineralisation and decolourisation for anodic oxidation of − þ −2 TiO þ UV→ e þ h ð2Þ CB VB benzoic acid at 20 mA cm [15]. Other nanomaterials, such as nano PbO TiO and TiO nanosheets, have been used for 2/ 2 2 The adsorbed photons over the titanium dioxide decolourisation and demineralisation of methyl orange and nanoparticulate photocatalyst are initiated at an energy greater chloroethene [16, 17]. Recent research has shown that the than 3.2 eV, which corresponds to the energy necessary to preparation of PbO anodes decorated with TiO nanotubes 2 2 excite an electron from the valance to the conductive band was found effective for the complete mineralisation and with the formation of positive hole at the valance band [36]. decolourisation of reactive blue-194 at a current density of −2 This creates electron-hole pairs, and the active electrons can 150 mA cm [18]. A modified Ti/SnO -Sb/PbO electrode 2 2 −� form superoxide ions O and peroxide radicals by reacting has also being utilized for the removal of the azo dye acid 2 −2 with O as shown in reactions (3)and (4)[37, 38]. black-194 at a current density of 30 mA cm [19]. 2 Other approaches, seeing to replace expensive metals, such − − � þ e þ O →O þ H ð3Þ CB 2 as platinum [20], involve the formation of PbO inside the − � þ − TiO nanotubes formed on a titanium substrate [21], which O þ H →HO ð4Þ 2 2 show high catalytic activity and large surface areas for Such photocatalysis is reported in the literature for TiO electrodegradation. PbO acts as a conductive bridge during under UV light irradiation [39]: the anodic oxidation [22] demonstrating their suitability as electrodes for wastewater treatment [23, 24]. − − 2HO → H O þ O ð5Þ 2 2 2 2 PbO coatings on RVC or a carbon-polymer substrate ob- − � � − H O þ O → OH þ OH þ O ð6Þ 2 2 2 tained by electrodeposition in methanesulfonic acid electro- 2 þ � þ lytes have been found effective for electrochemical water H O þ h → OH þ H ð7Þ VB treatment [25]. The inclusion of TiNT films has been claimed Reaction (5) involves production of hydrogen peroxide, to be 100% efficient for the mineralisation and removal of benzoic acid from wastewater [15]. The anatase phase of while reactions (6)and (7) represent the formation of OH 2 −1 TiNT provides active surface area of 20 m g , larger than radicals. The oxidation of a water molecule forms the the traditional Degussa P25 TiO particles that have a 7- short-lived and powerful oxidant OH radical (redox poten- 2 −1 m g surface area and are 20% more efficient for the photo tial = 2.7 V vs. SCE) [40]. This oxidant is capable of decolourisation of rhodamine-B dye [26]. The potential ad- decolourisation and mineralising organic matter residue vantage of using a substrate with defined regular structure, to lower molecular weight compounds and ultimately to such as RVC, provides efficient mass transport of the effluent carbon dioxide and water [41]. over the surface, which ultimately provides efficient degra- The aim of this study is to investigate the electrochemical dation [27]. RVC possesses a honeycomb structure [28]and and photochemical performance of TiNS/PbO coating on has been employed for the electro Fenton degradation of azo RVC for the decolourisation of reactive black-5 (RB-5) dye and characterize the structural and morphological properties dyes [1]. J Solid State Electrochem of the coatings. The decolourisation of this dye is important electrical connection. The RVC electrode was placed in an due to its widely commercial usage in the leather, wool, poly- undivided electrochemical glass cell fitted with a water jacket mer, rubber, and silk industries. 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 −1 Experimental details an electrolyte solution containing 1-mol L Pb(CH SO ) and 3 3 2 −1 0.2-mol L MSA 100 mL of electrolyte. The electrodeposition Reagent grade 50% Pb (II) methanesulfonate (Pb(CH SO ) , of PbO on the RVC anode was carried out at 2.5 A at 60 °C for 3 3 2 2 70% methanesulfonic acid (MSA), tetrabutylammonium hy- 30 min with a constant stirring of electrolyte at 700 rpm with a droxide (TBAOH), cesium carbonate (Cs CO ), Degussa 0.25-cm long PTFE-coated magnetic follower [44, 45]. 2 3 TiO (P25), and reactive black-5 (RB-5) dye were purchased from Sigma Aldrich, while hydrochloric acid, sulfuric acid, Anodic electrophoretic deposition sodium hydroxide, sodium sulphate, and acetone obtained from Fischer scientific and used as received. The RVC substrate coated with PbO was used as an anode for the electrophoretic deposition of TiNS from a solution con- Synthesis of titanate nanosheets taining exfoliated TiNS with TBAOH. The cathode was graphite plate of 1.5-cm × 6-cm × 1.2-cm dimensions in a par- A single-layer of titanate nanosheets (TiNS) was developed allel plate configuration with an interelectrode gap, d, of 1 cm. using an adapted methodology from Sasaki et al. [42]. TiNS A cell potential difference of 15 V was applied between the −1 were prepared by the solid reaction between the Cs CO and electrodes, which created a potential gradient of 15 V cm . 2 3 P25 at molar ratio 1:5.3 and 800 °C for 90 min. Once cooled, The electrolyte was vigorously stirred at 700 rpm with a mag- the resultant mixture was ground into fine powder. The pow- netic stirrer, as shown in Fig. 2. der was reheated at 800 °C for two cycles of 20 h each; after each interval of high-temperature treatment, the resulting mix- Heat treatment ture was cooled overnight and ground to fine powder. The resulting white powder was lepidocrocite-like cesium titanate, The TiNS/PbO /RVC samples obtained from electrophoretic Cs Ti □ O ,where □ represents a vacancy [43]. deposition were calcined at 450 °C for 1 h. The sections of 0.7 1.82 0.175 4 Ion exchange method was used to produce smectite-like acid RVC that were not coated with TiNS/PbO film due to the fact titanate from a mixture that contained 2 g of lepidocrocite-like that material outside the electrolyte, disintegrated at such a −1 cesium titanate and 80 mL of 1-mol L HCl. The solution was high temperature, whereas those coated with the film resisted stirred with a magnetic follower over 4 days at 700 rpm. The calcination. The TiNS film calcined after the heat treatment solution wasreplacedwithafreshacid solutionevery dayto exhibited uniform coating over the RVC substrate. + −1 maintain the amount of H ions at 1 mol L for acid leaching of Cs ions. The remaining solution was vacuum filtered by Characterisation of the coated substrate employing a nylon membrane and ultimately cleaned with dis- −1 tilled water until the conductivity reached around 10 μScm . The surface morphology of the TiNS/PbO /RVC samples was The resulting smectite-like acid titanate was recovered and used characterized by field emission scanning electron microscope to produce exfoliated single-layer TiNS. This was achieved by (FESEM), using a JEOL 6500F at an accelerating voltage of stirring 0.4 g of smectite-like acid titanate with 100 mL of 20 kV. Raman spectra were obtained using a confocal micro- −1 aqueous solution of 0.0165-mol L tetrabutylammonium hy- scope (Renishaw, RM 2000) fitted with a light source of droxide (TBAOH) at 200 rpm and room temperature (25 °C) 632.8-nm wavelength to measure the peaks for the presence for 2 weeks. This process replaced the protons (H )present in of deposited species. The exposure time was 30 s with a 10% the TiNS with bulky (TBA ) ions. The exfoliated nanosheets intensity of laser radiation. were obtained as an aqueous suspension. Electrochemical experiments Electrodeposition of PbO on RVC The electrochemical experiments were performed by means of A piece of RVC substrate from ERG materials of 100 pores per a computer aided PGSTAT302 N potentiostat/galvanostat linear inch (ppi, porosity grade of RVC) of 2-cm × 2-cm × from Autolab (EcoChemie, Netherlands) by using Nova 0.15-cm dimensions was treated with 70% nitric acid (Fisher 1.11 software. The obtained coatings including calcined Scientific) at 110 °C for 1 h and thoroughly rinsed and left in TiNS/PbO /RVC, PbO /RVC, and acid treated RVC were 2 2 deionized water overnight, then dried at 90 °C overnight. A Cu washed with ultra-pure water and dried. Each electrode was wire was glued (Leit-C, Agar scientific) to the RVC to provide used to electrolyze a solution of 100 mL containing 10 ppm of J Solid State Electrochem Fig. 1 Electrodeposition of PbO from a solution containing 1- −1 mol L Pb (CH SO ) and 0.2- 3 3 2 −1 mol L 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 −1 RB-5 dye in 0.5 mol L of sodium sulphate at pH = 3 and the micrograph image of the PbO agglomerates covering the −2 25 °C at 2 mA cm . The electrolyte was stirred and main- surface of RVC at larger magnification. TiNS films were anod- tained a stable pH throughout the electrochemical studies. ically deposited on the PbO /RVC surface by electrophoretic −1 Platinum mesh and Hg/HgO (0.6-mol L NaOH) were used deposition as seen in Fig. 3c, which provides more uniform as a counter and reference electrodes, respectively. morphology than the PbO /RVC film. The more magnified SEM image as seen in Fig. 3d revealed the presence of smooth Photocatalytic experiments and elongated strands of TiNS on the top of the PbO /RVC. The TiNS/PbO /RVC coating covers the typical tetrahedron struc- The calcined TiNS/PbO /RVC sample was cut into a disc of 4- ture of the RVC (100 ppi) substrate. The high resolution image mm diameter and 2-mm thickness (0.919 g) and was fitted in a (30,000 magnification) of TiNS/PbO /RVC coating exhibit that glass tube holder which was immersed in a 10-mL solution TiNS are nanosize particles distributed over the substrate. −1 containing 10 ppm of RB-5 dye in 0.5 mol L of sodium sul- After calcination at 450 °C, the TiNS/PbO /RVC coatings phate at pH 3 in Fig. 2b). The photocatalytic experiments were remain stable as shown in Fig. 4a. The high temperature of the −2 performed under a UV lamp with 20-mW cm intensity. Prior calcination creates surface defects over the TiNS/PbO /RVC to the photocatalytic experiments, the solution with tube holder coatings. The image also shows that the nanosheets are still was kept in the dark for 30 min in order to achieve maximum attached to the PbO /RVC coating even after the calcination at adsorption of RB-5 dye on the calcined TiNS/PbO /RVC. This 450 °C. ensured that the decolourisation measurements in solution were Figure 4b is an image obtained for similar coating, which due to the UV irradiation and not to the adsorption of the dye on denotes an undulated surface with ridges and valleys of TiNS/ the composite. Following this, the UV irradiation was provided PbO /RVC coatings after the transformation of the TiNS from every 1 min to the solution. The absorbance of the RB-5 in the titanate to the anatase phase which imparts the photocata- solution was measured in a Hitachi U3010 UV-Vis spectropho- lytic active nature to the film and can be used to degrade tometer at a wavelength of 597 nm. A calibration curve was used reactive black-5 dye. The uncoated RVC (used to connect to to evaluate the concentration of RB-5 dye in the solution. the power supply during electrodeposition) oxidised at high temperature, while the carbon on the coated areas remained intact, which suggests that TiNS/PbO coating preserves the Results and discussion mechanical integrity of the RVC structure. The elemental analysis of the coating was determined by Surface characterisation of TiNS/PbO coated RVC EDX from the micrograph shown in Fig. 5a. It confirmed that the TiNS/PbO /RVC coating contained titanium, lead, and Figure 3ashows PbO layer coated over the strut of the 100-ppi carbon elements. Figure 5b–d shows the elemental maps re- RVC after electrodeposition at 2.5 A, whereas Fig. 3bshows vealing particles uniformly distributed over the substrate. J Solid State Electrochem Fig. 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 The structural characteristics related to the transformation strong interaction of the hydroxyl radicals over electrodes of the TiNS after the heat treatment were seen by Raman takes place due to oxides generation [2], which leads to the spectroscopyasshowninFig. 6. The peaks seen at 177, OER (4). Platinum is as an active anode, as it is capable of −1 200, 397, 517, 639, and 747 cm in the curve a indicate that attracting hydroxyl radicals over its surface, due to its greater heat treatment converted the film from titanate to anatase adsorption enthalpy. In the case of non-active electrodes like phase, which have been previously reported in the literature PbO , there is weak interaction of the electrodes with hydrox- −1 � [46]. The peaks observed at 279 and 464 cm in curve b for yl ( OH) radicals which are more prone to react in the electro- non-calcined coatings disappeared after calcination. lyte and oxidise organic compounds dissolved in solution [8]. Cyclic voltammetry studies using calcined TiNS/PbO / −1 Electrochemical studies of the coatings RVC and PbO /RVC electrodes in 0.5-mol L Na SO at a 2 2 4 −1 scan rate of 10 mV s are shown in Fig. 7a. The electrode The OER is an unwanted reaction during the direct electro- potential at which the OER starts to be significant (> −2 chemical anodic oxidation of organic compounds. The first 1mA cm ) on the calcined TiNS/PbO /RVC, (curve 1) was step is the formation of hydroxyl radical ( OH) from the water 2.2 V vs. Hg/HgO, whereas in the case of PbO /RVC, curve 2, oxidation at the anode substrate (Eq. (1). The following steps the oxygen evolution started at > 1.8 V vs. Hg/HgO. The depend upon the nature of electrode substrate. Two types of higher overpotential for the OER observed in calcined TiNS/ substrates can be classified for direct anodic oxidation: PbO /RVC suggests that this coating can potentially be a bet- Bactive^ and Bnon-active^ electrodes [4]. In active anodes, ter catalyst for the production of hydroxyl radicals. J Solid State Electrochem Fig. 3 FESEM images of TiNS/ PbO coatings over the RVC PbO film substrate obtained by electrodeposition and anodic electrophoretic deposition. a RVC strut coated with PbO after anodic electrodeposition. b PbO film at higher magnification. c TiNS/PbO /RVC layer showing fully covered layer of TiNS film over PbO /RVC. d TiNS/PbO / 2 2 RVC layer at higher magnification. e TiNS/PbO /RVC at 30,000 magnification at a scale of 100 nm b) a) 1m 10 m TiNS film c) d) 1m 10 m TiNS film 30,000 magnification e) 100 nm When 10 ppm of RB-5 dye was added to the solution of 0.5- of the dye was observed in comparison to curve 1 and the −1 mol L Na SO , the oxidation process on the calcined TiNS/ current density observed is significantly higher than the cal- 2 4 PbO /RVC started earlier at around 0.5 V vs. Hg/HgO but the cined TiNS/PbO /RVC after 2.5 V vs. Hg/HgO. This might be 2 2 −2 current was around 1 mA cm at 1.4 V vs. Hg/HgO. This due to the fact that by increasing the potential, the formation of suggests that the oxidation of the dye on this catalyst started the hydroxyl radical ( OH) competes with the direct oxidation before the OER as can be seen in Fig. 7b (curve 1). In the case of the dye and this oxidation is undetectable due to lower of the PbO /RVC electrode (curve 2), no significant oxidation current density recorded in the voltammograms [10, 44]. 2 J Solid State Electrochem Fig. 4 a TiNS/PbO /RVC after calcination at 450 °C. b Calcined TiNS/PbO /RVC layer having TiNS film undulated surfaces at higher magnification after calcination b) a) 10 m 100 m Electrochemical decolourisation and colour removal electrodes are shown in Fig. 8. The data of the concentration of RB-5 dye decay for the decolourisation of RB-5 dye on different elec- trodes can be fitted into the following logarithmic relationship: −2 Electrolysis at constant current of 2 mA cm was carried out in order to remove the colour from the RB-5 dye solutions c ln ¼ −kt ð7Þ using different coatings. The electrolyte consisted of 100 mL −1 of solution containing 10 ppm of RB-5 dye in 0.5-mol L Na SO at pH 3. UV-Vis spectra were used to follow the where k is the rate constant, c the concentration, and t the 2 4 colour removal of RB-5 dye, which shows a maximum ab- reaction time. The fitting suggests a pseudo-first-order reac- sorption band in the visible light region at λ =597 nm. The tion kinetics for the decolourisation. The comparison of rate max oxidation of the azo dye complex molecule leads to low mo- constant (k) is presented in Table 1. lecular weight intermediates, such as aliphatic and aromatic It can be seen that the pseudo-first-order rate constant decolourisation kinetics obtained using different anodes and organic molecules [4]. The formation of these compounds is due to the displacement of the chromophore functional group from the data from the literature depends upon the nature of the electrocatalytic activity of the coating [40]. The anode and followed by the oxidation of the organics to carbon diox- ide and organic acids (carboxylic acids) [19–21]. electrodes utilized are able to decolourate the solution < 99% The time-dependent profiles of the normalized concentra- except for RVC electrode. This may be due to the tendency of tion c/c , for the electrochemical decolourisation of the dye PbO /RVC and calcined TiNS/PbO /RVC anodes to utilize 2 2 using RVC, PbO /RVC, and calcined TiNS/PbO /RVC anode OH radicals produced from water discharge reaction and 2 2 Fig. 5 EDX images for elemental analysis of calcined TiNS/PbO / RVC obtained by anodic electrophoretic deposition. a Calcined TiNS/PbO /RVC. b Titanium elemental analysis. c Lead elemental analysis. d 1m Carbon (RVC) elemental analysis a) b) Ti Ka1 c) Pb Ma1 d) C Ka1-2 J Solid State Electrochem a) 2) 1) Fig. 6 Raman spectra of the TiNS/PbO /RVC obtained by anodic b) electrophoretic deposition a calcined at 450 °C and b non-calcined employ the radicals for the decolourisation of organic dye as shown in Eq. (8)below: RB−5 þ OH→ Intermediates→ H O þ CO ð8Þ 2 2 1) The generation of hydroxyl radicals for the decolourisation 2) of dyes over active electrodes plays an important part; how- ever, 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 elec- trodes with large overpotential for oxygen evolution like TiNS/PbO /RVC and PbO /RVC are preferred. The logarithm 2 2 of the normalized concentration of RB-5 decay vs. time when −2 a current density of 2 mA cm was applied on the anodes is Fig. 7 a Polarisation curve by using coating 1 (TiNS/PbO /RVC) as shown in Fig. 9. The comparison of these electrodes shows 2 −1 anode after calcination at 450 °C in 0.5-mol L Na SO , pH = 3.0. 2) 2 4 that using TiNS/PbO /RVC removes 98% of the colour with 2 −1 (PbO /RVC) as anode in 0.5-mol L Na SO . Experimental conditions: 2 2 4 −1 linear pseudo-first-order decolourisation and rate constant k = potential sweep rate 10 mV s , temperature 298 K. b Polarisation curve −1 − 0.060 min . by using novel coating 1 (TiNS/PbO /RVC) as anode in an electrolyte 10- −1 ppm R.B-5 dye in 0.5-mol L Na SO , pH = 3.0. 2) (PbO /RVC) as 2 4 2 Colour removal is also substantial when using PbO /RVC anode after calcination at 450 °C in an electrolyte 10-ppm R.B-5 dye in electrode as shown by the normalized concentration data in −1 0.5-mol L Na SO , pH = 3.0, Experimental conditions: potential sweep 2 4 −1 Fig. 9, which is around 98%. This behavior is due to the rate 10 mV s , temperature 298 K 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 Photocatalytic decolourisation of RB-5 dye 60 min which is lower compared to the previously mentioned electrodes. Some reported works showed only 70% conver- In order to evaluate the photocatalytic activity of the anatase sion of RB-5 dye by using photoassisted Fenton, which is phase of the calcined TiNS (TiO )/PbO /RVC coating charac- 2 2 lower than the values reported here [47]. This revealed that terized by Raman studies (Fig. 6), photocatalytic studies of the the OH radical is weakly adsorbed over the surface of non- decolourisation of RB-5 dye in Na SO andpH= 3were car- 2 4 active electrode (PbO )[8] and ultimately favors the produc- ried out. The coating showed noticeable photocatalytic activ- tion of OH radical as indicated in Eq. (1) and also subsequent ity towards decolourisation of RB-5 dye. The activity of the adsorption of the dye molecules over the active sites provided coating depends on factors like protonation of TiO nano- by positively charged TiNS under acidic conditions over the sheets in acidic media [53] and the generation of holes and PbO /RVC substrate. electrons over the nanosheet surface in the presence of UV 2 J Solid State Electrochem Fig. 9 Electrochemical remediation kinetics of RB-5 dye by (■) RVC, Fig. 8 Electrochemical remediation of RB-5 dye by (■)RVC, (□) (▲)RVC/PbO ,and (●) calcined TiNS/PbO /RVC 2 2 photoassisted Fenton using iron oxide on activated alumina support [47], (▲)RVC/PbO , and (○) calcined TiNS/PbO /RVC 2 2 −� electron acceptor like O to produce oxidizing radicals O as shownbyreaction(13)[49]. irradiation [54, 55]. In acidic solutions, the TiO nanosheets − þ become positively charged as indicated by Eq. (10) and attract TiO þ hv→ e þ h ð11Þ CB VB the negatively charged RB-5 dye [49]. − þ � þ OH þ h → OH þ H ð12Þ VB − − � þ þ e þ O → O ð13Þ TiOH þ H → TiOHðÞ acidic solution ð10Þ 2 CB 2 RB−5 þ OH→ Decompose ð14Þ Under UV light, there is the formation of electrons (e ) in the conduction band and holes (h )inthevalance band of Overall, the holes and radicals are responsible for the pho- the TiO nanosheet anatase as seen in Eq. (11). The photo- tocatalytic activity and decolourisation of RB-5 dye as indi- induced holes possess oxidative properties, which can at- cated by Eqs. (4), (5), and (6)producing OH radical, which tack the dye molecule directly leading to its decolourisation ultimately decompose the dye as shown in Eq. (14). In this or indirectly forming hydroxyl radicals by promoting the case, PbO dominates the base of the conduction band of reaction between the hydroxyl anions and water as shown TiNS (anatase TiO ), while the valance band belongs to the by Eq. (12). The hydroxyl radicals react with the organic TiNS (anatase TiO ) nanoparticle states [56]. The formation of matter. However, the electrons can also react with an the new band gap will intensify the electron-hole pair transfer Table 1 Comparison of apparent rate constants for decolourisation of RB-5 dye by RVC, calcined TiNS (TiO )/PbO /RVC, and PbO /RVC and other 2 2 2 related values from selected literature −1 Substrate − k/min Decolourisation % Time/min Reference RVC by using anodic oxidation 0.0064 36 60 This study PbO /RVC by using anodic oxidation 0.050 97 60 This study Calcined TiNS(TiO )/PbO /RVC by using anodic oxidation 0.060 98 60 This study 2 2 Calcined TiNS(TiO )/PbO /RVC by using photocatalytic decolourisation 0.383 85 5 Photocatalytic rate 2 2 in this study TiO (P25) by using photocatalytic degradation 0.038 ≈ 90 60 [48] TiO nanofiber-nanoparticle composite by using photocatalytic degradation 0.039 94.4 60 [49] TiO nanofiber composite by using photocatalytic degradation 0.026 75.5 60 [50] Photoassisted Fenton by using iron oxide on activated alumina support N.G 70 480 [47] TiO -P25 by using photocatalytic degradation N.G 98 150 [51] UV/H O by using Iron salt N.G 99.3 180 [52] 2 2 J Solid State Electrochem 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 PbO /RVC electrodes, the calcined TiNS/PbO /RVC coatings were able to decolourise RB-5 dye from wastewater more effectively. & The calcined TiNS/PbO /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 chro- mophore in the dye. The hydroxyl free radical ( OH) gen- Fig. 10 Photocatalytic remediation of RB-5 dye by (▼)RVC,(●) calcined TiNS/PbO /RVC, (■) using calcined TiNS/PbO /RVC erates on the electrode surface provides an efficient and 2 2 clean method for the decolourisation of RB-5 dye. Using upon photoexcitation. The photocatalytic decolourisation is calcined TiNS/PbO /RVC electrodes; a first-order decolourisation kinetics has been determined with higher shown in Fig. 10 and indicates that the RB-5 dye follows a pseudo-first-order reaction kinetics on the calcined TiNS/ rate of reaction in comparison with some reported values PbO /RVC coating, as revealed in Fig. 11. in the literature. & Raman results indicated the transformation of the titanate The comparison of rate constant (k) calculated by using Eq. −1 (7) for the photocatalytic decolourisation (− 0.383 min )with phase of TiNS/PbO /RVC to the anatase structure after −1 calcination at 450 °C. electrochemical decolourisation (− 0.060 min ) reveals that photocatalytic activity is higher by 84%. The annealing con- & The anatase phase in the coating also found effective in pho- tocatalytic studies and removal results revealed a first-order verts TiNS to the single-crystal anatase phase, which im- proved the photocatalytic activity when irradiated with UV kinetics decolourisation of RB-5 dye in acidic conditions. & The photocatalytic effect resulted from three mechanisms, light. The calcined TiNS/PbO /RVC decolourized the reactive black-5 solution in 15 min. The improved photocatalytic ac- which involve photoinduced holes in valance band, the tivity of a coating, i.e. TiNS/PbO /RVC ascribed due to the protonation of TiNS in acidic solutions, and the formation formation of holes and radicals by the incident UV light and of oxidative radicals evolved from the reaction between ultimately participates in the photocatalytic decolourisation of the electrons from the conduction band and the electron acceptor ( O ). the dye. This study opens up a promising approach in developing metal oxide structures decorated with TiO nanosheets over inexpensive carbon based substrate, for applications in waste- water 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 PbO and TiNS. Acknowledgements S.Z.J. Zaidi is grateful to the Faculty of Engineering and the Environment at the University of Southampton for the Rayleigh studenship, to the Bestway Foundation, Charity No. 297178, UK and to the European Commission Project H2020 CO2EXIDE, for the economic support.The authors acknowledge D.V. Bavykin for useful discussions. The raw data presented in this paper can be found at the repository of the University of Southampton: https://doi.org/10.5258/SOTON/D0530 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// Fig. 11 Photocatalytic remediation kinetics by (▼)RVC,(●) calcined creativecommons.org/licenses/by/4.0/), which permits unrestricted use, TiNS/PbO /RVC 2 J Solid State Electrochem distribution, and reproduction in any medium, provided you give appro- and its application in organic degradation. Electrochim Acta 201: priate credit to the original author(s) and the source, provide a link to the 240–250 Creative Commons license, and indicate if changes were made. 17. Biyoghe BNL, Ibondou MP, Gu X, Xu M, Lu S, Qiu Z, Mbadinga SM (2014) Efficiently synthetic TiO Nano-sheets for PCE, TCE, and TCA degradations in aqueous phase under VUV irradiation. Water Air Soil Pollut 225(5):1951 References 18. 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Journal

Journal of Solid State ElectrochemistrySpringer Journals

Published: Jun 1, 2018

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