Enhanced Visible Light-Driven Photoelectrocatalytic Degradation of Paracetamol at a Ternary z-Scheme Heterojunction of Bi2WO6 with Carbon Nanoparticles and TiO2 Nanotube Arrays Electrode.

In this study, a ternary z-scheme heterojunction of Bi2WO6 with carbon nanoparticles and TiO2 nanotube arrays was used to remove paracetamol from water by photoelectrocatalysis. The materials and z-scheme electrode were characterised using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), EDS mapping, ultraviolet diffuse reflection spectroscopy (UV-DRS), photocurrent measurement, electrochemical impedance spectroscopy (EIS), uv-vis spectroscopy and total organic carbon measurement (TOC). The effect of parameters such as current density and pH were studied. At optimal conditions, the electrode was applied for photoelectrocatalytic degradation of paracetamol, which gave a degradation efficiency of 84% within 180 min. The total organic carbon removal percentage obtained when using this electrode was 72%. Scavenger studies revealed that the holes played a crucial role during the photoelectrocatalytic degradation of paracetamol. The electrode showed high stability and reusability therefore suggesting that the z-scheme Bi2WO6-CNP-TiO2 nanotube arrays electrode is an efficient photoanode for the degradation of pharmaceuticals in wastewater.

1. Introduction

Paracetamol, also known as acetaminophen, is one the most prescribed drugs used for the treatment of pain, aches and fevers [1,2,3,4]. The discharge of paracetamol into aqueous environment can reach a toxic level that is harmful to humans and aquatic life [5]. Paracetamol reaches water bodies through the disposal of effluents, discharge of faeces/urine or through hospital sewage [6]. Due to the difficulty in the removal of paracetamol from water during treatment, advanced oxidation processes have been used for its removal in water instead of conventional methods [1,7]. Photo-fenton, sonolysis and H2O2/UV are extensively used advanced oxidation processes for the removal of paracetamol in water and wastewater [8,9]. Amongst other different types of AOPs, photoelectrocatalysis is gaining popularity due to its simplicity, low maintenance, positive environmental impact, recycling and reusability [10]. This method involves the use of a semiconductor immobilized on an electrochemical substrate [11,12,13]. Researchers have reported several different photoelectrocatalytic systems including single semiconductor systems. The problem with the single semiconductor systems for the degradation of paracetamol is that it cannot possess wide absorption and strong redox abilities simultaneously [14,15]. For instance, Bi2WO6 is a single semiconductor that has been used in the PEC degradation of organic pollutants [16]. Bi2WO6 is a typical Aurivillius oxide with a perovskite layered structure and a high surface area, which means it has sufficient active sites to promote photocatalysis [14,16]. Bi2WO6 also has a narrow band gap of 2.7 eV, which means it has strong light harvesting ability compared to TiO2. However, Bi2WO6 is limited by its low separation efficiency and its inability to fully utilize solar energy [15]. To overcome the challenge of low separation efficiency and improve the photocatalytic potency of Bi2WO6 (and most semiconductors in general), z-scheme photoanodes have been constructed via two semiconductors of dissimilar/similar band gap and one electron mediator or conductor. This system mimics the photosynthesis process which includes a two-step photoexcitation [14,15,17]. In a typical mechanism, electrons of photocatalyst I are recombined with holes in photocatalyst II via an electron mediator under light. This process inhibits the recombination of photo-induced charge carriers while they keep their redox properties [14,15,17,18].

In this study, bismuth tungstate and anodised TiO2 serve as photocatalyst I and II, respectively. TiO2 is the most widely studied semiconductor with properties such as low toxicity, cost effectiveness and excellent stability [19,20,21]. In recent years, researchers have synthesized a unique morphology of highly ordered TiO2 nanotube arrays (NTA). TiO2 NTA consists of a high surface area and unique cylindrical structure with improved electron transport efficiency compared to common TiO2 nanoparticles [22]. Amongst other different synthetic routes, anodisation of Ti sheets has been widely used, as it tends to give rise to a well-aligned structure with tuneable morphology and inner diameter. Anodised TiO2 are cost effective and provide better reusability compared to powdered TiO2 photocatalysts as they tend to disperse in solution during a reaction [22]. Unfortunately, TiO2 NTA only needs UV light for activation due to its wide band gap [22]. The formation of the z-scheme heterojunction with a narrow band gap semiconductor such as Bi2WO6, will solve this problem by tuning TiO2 NTA towards solar activation. To improve the conductivity of Bi2WO6 and TiO2 NTA, carbon nanoparticles (CNP) were used as an electron mediator. Nanomaterials have been incorporated into semiconductor structures as dopants or heterostructures for the enhancement of electrochemical, photocatalytic or photoelectrocatalytic properties, especially in the visible light [13,23,24].

Bi2WO6 has been applied in the photocatalytic degradation of organic pollutants and some of these reports used the z-scheme heterojunction approach. For example, Sharma et al. used a z-scheme of Bi2WO6 and titania for the degradation of antibiotics [14]. Hu et al. added porphyrin to Bi2WO6 for a z-scheme for photocatalytic application [15]. A recent review by Khedr et al. [25] highlights various photocatalytic applications of Bi2WO6. To the best of our knowledge, our work presents the photoelectrocatalytic application of a Bi2WO6 based photoanode for the first time. While TiO2 powder has been composited with Bi2WO6, this report used a novel route of anodized titanium sheet to form TiO2 NTA as a support for Bi2WO6 in the design of the z-scheme photoanode. The challenge of catalyst recovery, usually experienced in photocatalysis, is mitigated by the immobilization of the semiconductor on an electrode (photoanode) in photoelectrocatalysis, as in this case [10,26].

Bi2WO6 was coupled with CNP forming a Bi2WO6/CNP nanocomposite. The resulting Bi2WO6/CNP nanocomposite was immobilized on the surface of TiO2 NTA, giving rise to a novel ternary z-scheme photoelectrocatalyst (Bi2WO6/CNP/ TiO2 NTA) for the degradation of paracetamol under visible light. The extent of degradation was monitored by uv/vis spectroscopy and total organic carbon measurements.

2. Materials and Methods

2.1. Chemicals

Bismuth (III) nitrate pentahydrate, sodium tungsten oxide dehydrate, acetone, ethanol, nitric acid, orthophosphoric acid, ammonium fluoride and glucose were purchased from Sigma Aldrich, Johannesburg, South Africa and were used as received. All reagents were in analytical grade and were used without further purification.

2.2. Preparation of z-Scheme Bi2WO6/CNP/TiO2 NTA

Firstly, Bi2WO6 was synthesized by Sharma et al.’s 2020 method with slight modification, using the solvothermal method. A mass of 1.455 g of bismuth nitrate pentahydrate (Bi(NO3)35H2O) was solubilized in 37.5 mL of ethylene glycol, instead of deionized water, and labelled as solution A. Next, 0.5 g of sodium tungstate dehydrate (Na2(WO4)2H2O) was dissolved in 12.5 mL of ethylene glycol and labelled as solution B. Solution B was introduced to solution A under sonication with dropwise addition. The resulting solution was stirred for an hour and then transferred into an autoclave for solvothermal treatment at 160 °C for 25 h. After the reaction, a brown precipitate was collected via centrifugation, washed with ethanol and water thoroughly and dried at 50 °C overnight.

The Bi2WO6-CNP composite was prepared by the dispersion of synthesized Bi2WO6 in absolute ethanol and sonicated for 50 min to obtain uniformity of suspension of the particles followed by the addition of 10 to 30% weight of the previously prepared CNP, using a method suggested by Tshwenya and Arotiba [27]. The mixture was further sonicated for 30 min. The mixture was placed in the oven to dry at 100 °C overnight for the complete evaporation of ethanol. The resulting composite was placed in the muffle furnace for 2 h at 500 °C.

A highly ordered TiO2 NTA was prepared using the approach reported by Koiki et al. [22]. Titanium sheets of 5 × 3 cm were used as the substrate for the fabrication of TiO2 NTA. Before the anodisation process, the sheets were sonicated in detergents acetone, ethanol, nitric acid and deionised water, respectively, and dried in the oven. The anodisation process was performed with a two-electrode electrochemical cell connected to a DC power source. A platinum foil was used as a counter electrode and was placed at a 2 cm distance from the titanium sheet anode. A potential of 25 V was applied to the electrochemical cell, which contained a mixture of 0.2 M H3PO4 and 0.3 M NH4F as the electrolyte for 2 h while being stirred magnetically. Thereafter, the prepared electrode was rinsed with deionised water to eliminate blocked ions from its surface and subsequently annealed for 2 h with a heating and cooling rate of 10 °C/min. The Bi2WO6-CNP composite was immobilized on the anodized TiO2 NTA via a simple deposition where 10% wt of nafion was used as a binder. The electrode was then placed in the oven and left to dry overnight.

2.3. Characterisation

The crystalline properties of the fabricated powders were measured by X-ray diffraction (XRD, D8 Advance, Bruker, Germany). The morphology and structure were observed by scanning electron microscopy (SEM, Supra55, Zeiss, Germany) and transmission electron microscopy (TEM, JEM-2100, JEOL, Tokyo, Japan), and the element constitution was detected by energy-disperse X-ray spectroscopy (EDS). The Brunauer–Emmett–Teller (BET) surface areas of the as-prepared materials were measured using a surface area and porosity analyzer (Quadrasorb EVO, Quantachrome, Florida, USA) by nitrogen adsorption–desorption. The optical properties of various photocatalysts were examined by UV–Vis diffuse reflectance spectra (UV–Vis DRS, UH4150, Hitachi, Japan).

2.4. Electrochemical and Photoelectrochemical Characterisation

The electrochemical and photoelectrical measurements was conducted on Autolab PGSTAT204 (Barendrecht, Netherland) potentiostat/galvonstat with a three-electrode configuration. The fabricated electrode, platinum foil and Ag/AgCl (3.0 M KCl) were used as working electrodes, counter electrode and reference electrode, respectively. An Oriel LCS-100 W solar simulator with UV cut-off (λ less than 400 nm) was used as the light source for PEC experiment. The prepared electrode was positioned vertically facing the incident light of the simulator and the distance between the photoelectrochemical cell and the light source was kept constant at 10 cm. Photocurrent measurements and linear sweep voltammetry was carried out in a 0.1 M Na2SO4 solution. Cyclic voltammetry was carried out in a 5 mM solution of [Fe(CN)6]3−/4− (prepared in a 0.1 M KCl). For the degradation experiments, paracetamol solutions were prepared in 0.1 M Na2SO4 supporting electrode. Electrochemical degradation experiments were carried out in the dark. An aliquot of the solution was collected from the reactor at certain time intervals using a disposal syringe. The concentration decay and degradation pattern of the analyte was observed using UV-visible spectrophotometer and the total organic removal was recorded using TOC. The effects of applied potential on the removal efficiency were investigated.

Figure 1 presents the schematics of the photoanode and the photoelectrochemical set-up for the degradation of paracetamol.

Figure 1

Schematic diagram of prepared Bi2WO6-CNP-TiO2 NTA z-scheme photoanode used for photoelectrocatalytic degradation of paracetamol in a three-electrode cell.

3. Results

3.1. Structural and Morphological Characterisation

The crystal structure of the synthesized Bi2WO6 and Bi2WO6-CNP composite was characterised by XRD as indicated in Figure 2a. The Bi2WO6 diffraction peaks at 2θ = 28.34°, 32.88°, 47.06°, 55.99°, 58.59°, 68.92°, 75.97° and 78.33° are attributed to the orthorhombic phase of the Bi2WO6 and are in agreement with JCPDS card no. 39-0256 (Sharma et al., 2020). However, after the addition of CNP on the surface of Bi2WO6, new diffraction peaks were observed. The sharpest peak found at 28.7° may be an influence of highly disordered carbon atoms with amorphous nature, as seen in most carbon materials. Figure 2b presents the crystal structure of synthesized TiO2 NTA and Bi2WO6-CNP-TiO2 NTA. During the synthesis of TiO2 NTA, it is important to control parameters such as the composition of the electrolyte, anodization potential and annealing temperature to achieve highly ordered tubes [22]. For instance, the use of a viscous electrolyte may lead to an excessive supply of oxygen ions than needed for oxidation [22]. The use of ethylene glycol may provide a slow anodic oxidation process and formation of glycolaldehyde and glycolic acid, making the donation of oxygen challenging [22]. Hence, the use of H3PO4 is advantageous, since it serves as a buffer and prevents rapid etching during nanotube growth [22]. Accurate anodization potential must be used, as high potential can damage the surface structure, resulting in disordered spongy porous tubes. The annealing temperature is also important as it gives rise to anatase and rutile form. At temperatures higher than 500 °C, the anatase and rutile form begins to combine and at 800 °C, they completely disappear [22]. As seen in Figure 2b the synthesized TiO2 NTA shows diffraction peaks at 2θ = 35.1°, 38.4°, 40.2°, 53°, 70.6° and 76.2°, corresponding to (100), (002), (101), (102), (103) and (112) diffraction plane of Ti metal, respectively, and are in agreement with JCPDS card no. 04-004-7631. The TiO2 NTA only showed anatase peaks at 2θ = 25.3°, 48.1°, 62.7°, and 75.1°, corresponding to (101), (200), (204) and (205) in agreement with JCPDS card no 01-075-2546. This suggested that pure TiO2 anatase was formed. After the deposition of Bi2WO6-CNP on the surface of TiO2, the peaks belonging to Bi2WO6-CNP and TiO2 NTA were observed on the XRD pattern of Bi2WO6-CNP- TiO2 NTA, confirming the successful synthesis of the ternary composite. The crystallite size of Bi2WO6 and Bi2WO6-CNP was calculated from Debye-Scherrer’s formula, given by D = 0.94 λ/β cos θ where D is the average crystallite size, λ is the wavelength of X-ray radiation, β is the broadening of the diffraction line assessed at full width half maximum value (FWHM) and θ is the Bragg’s angle. The crystallite size of Bi2WO6 and Bi2WO6-CNP are 9.36 nm and 5.32 nm, respectively. The decrease in crystallite size shows that the Bi2WO6-CNP has an increased surface area than the pure Bi2WO6. BET analysis was carried out to further confirm the increase in surface area where Bi2WO6 gave a surface area of 6.3214 m2/g and Bi2WO6-CNP gave a surface area of 15.0235 m2 g−1.

Figure 2

XRD patterns of (a) Bi2WO6 and Bi2WO6-CNP, (b) TiO2 NTA and Bi2WO6-CNP-TiO2 NTA. FESEM images of (c) Bi2WO6; (d) Bi2WO6-CNP; (e) TiO2 NTA; (f) Bi2WO6-CNP-TiO2 NTA; (g) EDS spectrum of Bi2WO6-CNP-TiO2 NTA and (h) EDS elemental mapping of Bi2WO6-CNP-TiO2 NTA electrode.

The surface morphology of the prepared Bi2WO6, Bi2WO6-CNP, TiO2 NTA and Bi2WO6-CNP-TiO2 NTA was investigated using FESEM, as seen in Figure 2c–f. The pure Bi2WO6 shows spherical aggregate morphology (Figure 2c). However, with Bi2WO6-CNP the morphology of the aggregate changed to rod-like structures, as seen in Figure 2d. Figure 2e shows pristine TiO2 NTA with highly ordered and well-oriented tubes, with an estimated diameter of 98 nm and wall thickness of 18 nm around them. After deposition, the agglomerated rod-like structures of Bi2WO6-CNP cover the surface of the TiO2 NTA as seen in Figure 2f. However, the TiO2 holes can be seen within the Bi2WO6-CNP, indicating that both materials are present on the electrode. The EDS spectrum in Figure 2g further confirms the elemental composition of Bi2WO6-CNP-TiO2 NTA with the following elements present in different proportions and quantities: Bi, Ti, W, C, and O. Other elements seen on the spectrum are due to the binder used for deposition. The EDS mapping analysis in Figure 2h validates the existence and even distribution of the elements Bi, Ti, W, C, and O, making up the Bi2WO6-CNP-TiO2 NTA electrode.

3.2. Optical Properties

UV-vis diffuse reflectance spectroscopy shows that the Bi2WO6-CNP-TiO2 NTA electrode has higher absorbance than the TiO2-NTA electrode in the visible region of 400 to 800 nm (Figure 3a). This suggests that the formation of the z-scheme heterojunction has enhanced visible light activity.

Figure 3

(a) UV-Visible diffuse reflectance spectra TiO2 NTA Bi2WO6-CNP- TiO2 NTA (b) photocurrent responses of TiO2 NTA and Bi2WO6-CNP- TiO2 NTA in 0.1 M Na2SO4 (c) schematic diagram of Z-scheme mechanism of Bi2WO6-CNP-TiO2 NTA electrode (d) Nyquist plot TiO2 NTA and Bi2WO6-CNP- TiO2 NTA photoanodes in 5 mM [Fe(CN)6]3−/4− in 0.1 M KCl (pH 7).

3.3. Electrochemical Studies

Figure 3b shows the chronoamperographs of pristine TiO2 NTA and Bi2WO6-CNP- TiO2 NTA under solar light illumination at 2 V bias potential using 0.1 M KCl solution. The two electrodes showed photocurrent responses (improved current in the presence of light than in the dark). However, the Bi2WO6-CNP-TiO2 NTA electrode exhibits a more marked photocurrent response with a higher current density of 0.789 mA cm−2 when compared to bare TiO2 NTA with a current density of 0.172 mA cm−2. The photocurrent density of Bi2WO6-CNP- TiO2 NTA is about 4.6 times higher than the TiO2 NTA. The increase could result from the z-scheme of heterojunction between Bi2WO6, CNP and TiO2 NTA, giving rise to an improved separation of electron hole pairs (Figure 3c). To further confirm the improved performance of the prepared electrodes an experiment was conducted using electrochemical impedance spectroscopy. The experiment was performed in an electrolytic solution of 5 mM [Fe(CN)6]3−/4− in 0.1 M KCl at pH 7 with external application of +0.2 V. Figure 3d shows the Nyquist plot of the fabricated photoanodes consisting of a semicircle, which represents the charge transfer process happening at the solution-electrode interface. The diameter of the semicircle represents the charge transfer resistance (Rct) at the interface of the electrode. Lower Rct values indicate a better charge transfer efficiency. The Rct of TiO2 NTA and Bi2WO6-CNP- TiO2 NTA are 92.14 Ω and 31.32 Ω, respectively. This suggested that the fabrication of the z-scheme heterojunction has improved the charge mobility of the electrode and lowered the rate of instantaneous recombination of electron-hole pairs [11].

3.4. Photoelectrochemical Studies

The photoelectrocatalytic performance of the prepared electrodes was evaluated by applying it in a solar reactor for the degradation of paracetamol with an initial concentration of 5 mg L−1. The degradation was accomplished with an applied current of 10 mA cm−2, pH 7 and irradiation with solar light. UV-visible spectrophotometry was used to monitor the degradation efficiency of the electrode at a wavelength of 243.0 nm. Figure 4a shows the decrease in the intensity of the peak at 243.0 nm as the reaction time increases, representing the reduction in the concentration of the pharmaceutical. A degradation efficiency of 84% was achieved after 3 h when using the z-scheme electrode for photoelectrocatalysis, as presented in Figure 4b. When the Bi2WO6-CNP–TiO2 NTA photoanode was applied for electrochemical oxidation of the paracetamol (that is without the stimulation of solar light), the degradation efficiency decreased to 57% (Figure 4b). In photocatalysis, where only the solar light was applied, a 42% degradation efficiency was recorded (Figure 4b). Photoelectrocatalytic (PEC) degradation of the paracetamol at the TiO2 NTA only (that is without Bi2WO6) yielded 62%. These degradation percentages show that the formation of a z-scheme enhanced the PEC process. The degradation of paracetamol using the z-scheme electrode was further confirmed by the percentage of total organic carbon removal (TOC) which was 72, 30 and 22% for PEC, electrochemical oxidation (EC) and photocatalysis (PC), respectively. EC is the same setup as PEC but without irradiation with visible light, while PC is without the application of potential. This observation confirms that the synergy between solar energy and applied bias potential is crucial for breaking down the organic pollutant. Consequently, the solar energy promotes the generation of electron–hole pairs on the surface of the electrode, while the biased potential helps with the transfer of photo-excited electrons, which reduces the recombination of electron-hole pairs.

Figure 4

(a) UV-Vis spectra of PEC degradation of paracetamol; (b) normalised concentration decay versus time plot for photocatalytic, electrocatalytic and photoelectrocatalytic degradation of paracetamol on of Bi2WO6-CNP–TiO2 NTA electrode and (c) corresponding kinetics plots; (d) normalised concentration decay versus time plot for PEC degradation of Paracetamol on of Bi2WO6-CNP–TiO2 NTA electrode. (e) Effects of current density and (f) pH on the degradation of paracetamol. (g) Reusability cycle experiments for the degradation of paracetamol on of Bi2WO6-CNP–TiO2 NTA electrode.

The kinetic of paracetamol degradation for each process was investigated using Langmuir-Hinshelwood pseudo first order kinetic equation. The degradation rate constant using the z-scheme electrode for PEC, EC and PC were 0.00987 min−1, 0.00352 min−1 and 0.00243 min−1, respectively. This reveals that the degradation of paracetamol was faster when the z-scheme electrode was applied in photoelectrocatalysis, as shown in Figure 3c. Figure 4d depicts the effect of different amounts of CNP to Bi2WO6 on the fabrication of Bi2WO6-CNP–TiO2 NTA photoanode. The results clearly indicate that above 10% wt of CNP, the degradation of paracetamol is suppressed, therefore it is important to keep the amount of CNP minimal for maximum PEC degradation efficiency. Since 10% gave the highest degradation efficiency, it was used for the preparation of the Bi2WO6-CNP–TiO2 NTA electrode. The effect of parameters such as current density and pH were investigated as shown in Figure 4e,f, respectively. The degradation efficiency increased with an increase in current density ranging from 5 mA cm−2 to 10 mA cm−2 (Figure 4e). This shows that the production of hydroxyl radicals depends on the applied current density. However, current densities should be kept at the optimum to prevent electrode leaching and the production of scavenging radicals. A high current density gives rise to oxygen evolution, which prevents the generation of hydroxyl radicals, which can suppress the degradation efficiency [12].

The surface charge of the catalyst, drug molecule, radicals and solvent molecules affects the photoelectrocatalytic reaction process under different pH conditions (Sharma et al., 2020). According to previous studies, the point of zero charge for Bi2WO6 and TiO2 are 5.5 and 6.2, respectively (Sharma et al., 2020). Therefore, at an acidic pH value, the surface of the catalyst becomes positively charged. As presented in Figure 3f under acidic conditions (pH 2), a degradation efficiency of 4% was obtained. It was also reported that paracetamol is in its protonated form under acidic conditions [28], hence the result obtained could be due to the repulsion between the surface of the catalyst and the drug molecule. At pH 10, a degradation efficiency of 18% was obtained. In alkaline conditions, more hydroxyl radicals are formed; however, paracetamol is in its deprotonated form [28], thus the repulsion between the negatively charged solution and the drug molecule could minimize the degradation efficiency. A maximum degradation efficiency of 84% was obtained at pH 7 where paracetamol is in its neutral form.

An ideal photoanode is projected to be stable and reusable. The stability and reusability of the prepared of Bi2WO6-CNP–TiO2 NTA photoanode was investigated by using the same electrode six times; after each cycle the electrode was gently rinsed with deionized water and air dried. As shown in Figure 4g, the degradation efficiency decreased by only 2% after the sixth cycle. This suggests a good stability and reusability of photoanode for PEC degradation.

3.5. Plausible Mechanism for Paracetamol Degradation

The position of the conduction band and valence band of Bi2WO6 and TiO2 was determined using the calculations from UV-DRS studies and the following equations:E E where ECB, EVB, Xe, Ee and Eg are the conduction band of the semiconductor, valence band of the semiconductor, the absolute electronegativity of the semiconductor, energy of free electrons on the hydrogen scale (4.5 eV) and the band gap energy of the semiconductor, respectively. Therefore, EVB and ECB of Bi2WO6 are 3.05 eV and 0.35 eV, while EVB and ECB for TiO2 are 3.285 eV and 0.12 eV, respectively. Based on the approximate bandgap energy values of Bi2WO6 and TiO2 NTA a multiple charge transfer mechanism is followed for the degradation of paracetamol. When the prepared Bi2WO6-CNP–TiO2 NTA is irradiated by solar light, Bi2WO6 and TiO2 NTA undergo excitation, where electrons migrate from the VB to the CB, leaving positively charged holes on the VB and electron (negative charge) on the CB. However, the electrons on the CB of Bi2WO6 migrate through the CNP to the holes on the VB of TiO2 NTA. The electron transportation that occurs through the carbon nanoparticles enhances the separation of electrons and holes of CB of TiO2 NTA and VB of Bi2WO6, respectively. The separated electrons on the CB of TiO2 NTA and the separated holes on the VB of Bi2WO6 make TiO2 NTA electron-rich and Bi2WO6 hole-rich. The carbon nanoparticles on the surface of Bi2WO6 act as a charge transfer bridge, where electrons from Bi2WO6 migrate to holes in TiO2 NTA. The increased number of electrons on TiO2 NTA react with adsorbed O2 to form superoxide radicals. The holes in the VB of Bi2WO6 can oxidize H2O to form hydroxyl radicals, which also participates in the degradation of paracetamol.

The role of the active species for photoelectrocatalytic degradation of paracetamol was investigated by conducting a radical suppressing experiment (Figure 5). The holes, hydroxyl radical and superoxides were suppressed using ethylenediaminetetraacetate (EDTA), t-butanol and p-benzoquinone, respectively. When EDTA was added to the solution, the degradation efficiency dropped significantly to 8%. On the other hand, when t-butanol and p-benzoquinone were added in the solution the degradation efficiency was 34 and 64%, respectively. The results indicate that the hole and hydroxyl radicals play a major role in the degradation of paracetamol, while superoxides play a minor role. The photoelectrocatalytic degradation of paracetamol can be projected to follow Equations (3)–(7). hv + Bi e h h

Figure 5

Scavenger studies of the photoelectrocatalytic degradation of Paracetamol on Bi2WO6-CNP-TiO2 NTA.

4. Conclusions

A visible light driven ternary z-scheme Bi2WO6-CNP-TiO2 NTA heterojunction photoanode was successfully prepared for photoelectrocatalytic application of the water treatment process. FESEM images showed that the catalyst was formed by an agglomerated rod-like structure of Bi2WO6-CNP and highly ordered TiO2 NTA, while EDS mapping supported the formation of the catalyst by showing all the elements present. The formation of the z-scheme heterojunction enhanced the visible light activity of the semiconductor heterojunctions. The formation of the z-scheme also improved the photocurrent response to about 4.6 times higher and charge carrier separation. When the z-scheme heterojunction photoanode was applied for PEC degradation of paracetamol, a degradation efficiency of 84% was obtained within 180 min. Scavenger studies revealed that the holes played a major role in the degradation process. The electrode showed excellent stability and reusability, hence the reported photoanode shows great potential for the photoelectrochemical application of organic pollutants.