Interrogating solar photoelectrocatalysis on an exfoliated graphite–BiVO4/ZnO composite electrode towards water treatment

Photo-electrocatalytic based removal of acetaminophen: Application of visible light driven heterojunction based BiVO4/BiOI photoanode

The presence of organic micro-pollutants (OMPs) in wastewater treatment effluents is becoming a major threat to the water safety for aquatic and human health. Photo-electrocatalytic based advanced oxidation process (AOP) is one of the emerging and effective techniques to degrade OMPs through oxidative mechanism. This study investigated the application of heterojunction based BiVO₄/BiOI photoanode for acetaminophen (40 μg L^-1) removal in demineralized water. Photoanodes were fabricated by electrodeposition of BiVO₄ and BiOI photocatalytic layers. Optical (UV-vis diffusive reflectance spectroscopy), structural (XRD, SEM, EDX) and opto-electronic (IPCE) characterization confirmed the successful formation of heterojunction for enhanced charge separation efficiency. The heterojunction photoanode showed incident photon to current conversion efficiency of 16% (λ_max = 390 nm) at an external voltage of 1 V under AM 1.5 standard illumination. The application of the BiVO₄/BiOI photoanode in the removal of acetaminophen at 1 V (external bias) vs Ag/AgCl under simulated sunlight showed 87% removal efficiency within the first 120 min compared to 66% removal efficiency of the BiVO₄ photoanode. Similarly, combining BiVO₄ and BiOI exhibited 57% increase in first order removal rate coefficient compared to BiVO₄. The photoanodes also showed moderate stability and reusability by showing 26% decrease in overall degradation efficiency after three cycles of each 5 h experiment. The results obtained in this study can be considered as a stepping stone towards the effective removal of acetaminophen as an OMP present in wastewater.

Enhanced visible light driven photoelectrochemical degradation of tetracycline hydrochloride using a BiOI photoanode modified with MnO2 films

Removal of pharmaceuticals in wastewater has been the focus of many research due to the recalcitrant nature and hazardous effects of these compounds. The photoelectrochemical degradation process has proven to be suitable to harness solar energy for the mineralization of organic compounds in wastewater. Herein, we report the application of BiOI/MnO₂ heterostructured anode for the photoelectrochemical degradation of tetracycline hydrochloride in aqueous solution. The photoanode was prepared through electrodeposition technique and fully characterized through microscopic, spectroscopic and electrochemical techniques. The results showed that formation of p-n heterojunction between BiOI and MnO₂ in the photoanode led to improved charge separation which was evident in improved optical and photoelectrochemical properties. The FTO-BiOI/MnO₂ electrode attained a photocurrent density of 0.104 mA cm^-2 with applied potential of 1.0 V (vs Ag/AgCl) which was almost double that of pristine BiOI suggesting efficient charge separation. The heterostructured photoanode achieved 94% removal of tetracycline hydrochloride after 120 min through the PEC degradation process with 61% mineralization efficiency. The electrode showed good reusability and stability with 92% PEC removal after eight cycles. Hence, the FTO-BiOI/MnO₂ has a great potential as anode for PEC wastewater treatments.

Construction of an S-Scheme Ag2MoO4/ZnFe2O4 Nanofiber Heterojunction for Enhanced Photoelectrocatalytic Activity under Visible Light Irradiation

The removal of organic dyes and pathogenic bacteria from contaminated water remains a significant challenge. In the present study, S-type heterojunction Ag₂MoO₄/ZnFe₂O₄ (AMO/ZFO) composite nanofibers were synthesized by electrospinning and co-precipitation and fabricated into photoanodes. It is found that the constructed S-type heterojunction of AMO/ZFO composites effectively inhibits the recombination of photogenerated carriers, in addition to the benefits of more exposed active sites and a greater specific surface area. When several properties are improved, AMO/ZFO composites exhibit excellent photoelectrocatalytic performance. The results demonstrate that under visible light irradiation, the photoelectrocatalytic degradation rate of AMO/ZFO-3 to methylene blue reached 76.2% within 50 min, and the killing rate of Salmonella was 83.6% within 80 min. The enhanced photoelectrocatalytic activity was due to the synergy of both electrochemical and photocatalytic effects. More importantly, after four testing cycles, AMO/ZFO-3 still has a better ability to kill pathogenic bacteria and degrade organic dyes due to its high stability. This work provides a feasible method for oxidizing organic dyes and pathogenic bacteria.

Fabrication of novel BiVO4/Bi2O3heterostructure with superior visible light induced photocatalytic properties

Heterostructure BiVO₄/Bi₂O₃nanocomposites with enhanced visible light activity are effectively synthesized through an easiest and single step hydrothermal route, using bismuth subnitrate and ammonium meta-vanadate as main raw materials in existence of citric acid. The phase and surface structure, topography and optical properties of synthesized composites are characterized by XRD, SEM, EDX, FTIR, UV-Visible and PL spectroscopy. It was found that 5%BiVO₄/Bi₂O₃(BOBV-5) nanocomposite exhibit excellent photocatalytic performance for rhodamine B dye degradation and tetracyclic under irradiation of visible light as compared to single component i.e. BiVO₄. The increased photocatalytic activity should be ascribed for making p-n heterojunction among p-type Bi₂O₃and n-type BiVO₄. This p-n heterojunction successfully reduce the recombination of photogenerated charge carriers. Furthermore, the BOBV-5 novel photocatalyst shows good stability in constructive five cycles and photocatalytic activity is best for conquering photo corrosion of a photocatalysts. To explain charge migration route, whole photocatalytic mechanism was described in terms of energy band structures. Furthermore, the present work is helpful effort for design of new visible light photocatalytic materials with heterojunction structures.

The application of photoelectrocatalysis in the degradation of rhodamine B in aqueous solutions: a review

The pollution of the water environment by industrial effluents is an ongoing challenge due to the rate of industrialisation and globalisation. Photoelectrocatalysis (PEC), an electrochemical advanced oxidation process, has proven to be an effective method for removing organics from wastewater. Photoelectrocatalysis is environmentally benign, cost-effective and easy to operate. In this present review, we examine the recent progress in the removal of rhodamine B dye, a common constituent of textile effluent released into the environment, through photoelectrocatalytic degradation. We present a detailed discussion on the use of different kinds of unmodified and modified photoanodes that have been explored for the photoelectrocatalytic removal of this dye. More importantly, discussions are presented on the mechanisms and kinetics of the degradation of rhodamine B dye using these photoanodes. Hence, this review will be beneficial for researchers in developing future projects in the area of wastewater treatments through photoelectrocatalysis.

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.

Solar-driven photoelectrocatalytic degradation of anticancer drugs using TiO2 nanotubes decorated with SnS quantum dots

In recent years, the growing interest in applying photoelectrocatalysis (PEC) to decompose organic pollutants has resulted in the need to search for new photoelectrode materials with high activity under visible light radiation. The presented research showed an increased photoelectrocatalytic activity under sunlight of Ti/TiO₂ sensitized with SnS quantum dots, obtained by the successive ionic layer adsorption and reaction (SILAR) method. The presence of SnS caused the enhanced absorption of visible irradiation and the reduction of recombination of generated charges by a p-n heterojunction created with the TiO₂. The highest efficiency of photoelectrocatalytic degradation of anticancer drugs (ifosfamide, 5-fluorouracil, imatinib) was achieved for the SnS-Ti/TiO₂ photoelectrode with a SnS quantum dot size from 4 to 10 nm. In addition, a decrease of IF PEC degradation efficiency was observed with increasing pH and with the presence of Cl^-, NO₃^-, HCO₃^- and organic matter in the treated solution. Studies of the PEC mechanism have shown that drug degradation occurs mainly as a result of the direct and indirect action of photogenerated holes on the SnS-Ti/TiO₂ photoelectrode, and the identified degradation products allowed for the presentation of the degradation pathway of IF, 5-FU and IMB. Duckweed (Lemna minor) growth inhibition tests showed no toxicity of the drug solutions after treatment.

Enhanced photoelectrocatalytic degradation of diclofenac sodium using a system of Ag-BiVO4/BiOI anode and Ag-BiOI cathode

We report the photoelectrocatalysis of diclofenac sodium using a reactor consisting of Ag-BiVO4/BiOI anode and Ag-BiOI cathode. The electrodes were prepared through electrodeposition on FTO glass and modified with Ag nanoparticles through photodeposition. The structural and morphological studies were carried out using XRD, SEM, and EDS which confirmed the successful preparation of the materials. The optical properties as observed with UV-DRS revealed that the electrodes were visible light active and incorporation of metallic Ag particles on the surface increased the absorption in the visible light region. Presence of p-n heterojunction in the anode led to decrease in the spontaneous recombination of photoexcited electron-hole pairs as seen in the photocurrent response. The results from photoelectrocatalytic degradation experiments revealed that replacing platinum sheet with Ag-BiOI as counter electrode resulted in higher (92%) and faster removal of diclofenac sodium as evident in the values of apparent rate constants. The reaction mechanism further revealed that efficiently separated photogenerated holes played a major role in the degradation of the pharmaceutical. The prepared electrodes showed good stability and impressive reusability. The reports from this study revealed that the dual photoelectrodes system has a great potential in treating pharmaceutical polluted wastewater using visible light irradiation.

Recent advances in degradation of pharmaceuticals using Bi2WO6 mediated photocatalysis - A comprehensive review

The pollution of water bodies by residual pharmaceuticals is a major problem globally. Bismuth tungstate mediated photocatalysis has been effective in the removal of these organics from water. Bismuth tungstate (Bi₂WO₆) has proven to be an excellent visible light active photocatalyst because of its non-toxicity, low band gap energy and ease of preparation. It has been widely applied for the removal of a wide array of organic pollutants, particularly dyes, from wastewater. However, recently, much attention has been channelled to its application for the degradation of pharmaceuticals. In this present review, the recent trends in the applications of Bi₂WO₆ based photocatalysts for the removal of pharmaceuticals in wastewater are comprehensively discussed. The fabrication of Bi₂WO₆ based photocatalysts with enhanced photocatalytic performances through morphology control, doping and formation of heterojunctions are highlighted. Much discussion centres on the mechanisms and possible degradation pathways of antibiotic pharmaceuticals in wastewater. Finally, areas needing more attention and investigation on the use of Bi₂WO₆ based photocatalysts for removal of pharmaceuticals from wastewater especially towards real-life applications are presented for future research directions.

Photoelectrocatalysis on TiO2 meshes: different applications in the integrated urban water management

Recently, among AOPs, photoelectrocatalysis (PEC) on TiO2 is gaining interest. In this study, five different real waters sampled in four different points of the integrated urban water management (IUWM) system were tested with PEC and UV alone, for comparison. This work aims to verify the effect of the PEC suggesting the optimal position in IUWM system where the PEC should be located to obtain the best performance. In groundwaters (GWs), PEC effectively removed atrazine-based compounds (> 99%), trichloroethylene, and perchloroethylene (96%), after 15 min of reaction time. However, given the low concentrations of emerging compounds, the synergistic effect of UV radiation with the catalyst and with the polarization of the mesh was not visible, with very few differences compared with the results obtained with UV alone. Pharmaceutical industrial wastewater (IWW) showed a significant increase in biodegradability after 2 h, both if subjected to PEC or UV (200%), despite the absence of COD removal. The PEC applied on IWW from a sewage sludge treatment plant allowed to effectively remove the COD (39.6%) and increase the biodegradability (300%). Good results in terms of COD removal (33.9%) and biodegradability increase (+900%) were also achieved testing PEC on wastewater treatment plant effluent. Except for GWs, PEC allowed significant EEO savings respect to UV alone (76.2-99.1%).

Role of Interfacial Defects in Photoelectrochemical Properties of BiVO4 Coated on ZnO Nanodendrites: X-ray Spectroscopic and Microscopic Investigation

Synchrotron-based X-ray spectroscopic and microscopic techniques are used to identify the origin of enhancement of the photoelectrochemical (PEC) properties of BiVO₄ (BVO) that is coated on ZnO nanodendrites (hereafter referred to as BVO/ZnO). The atomic and electronic structures of core-shell BVO/ZnO nanodendrites have been well-characterized, and the heterojunction has been determined to favor the migration of charge carriers under the PEC condition. The variation of charge density between ZnO and BVO in core-shell BVO/ZnO nanodendrites with many unpaired O 2p-derived states at the interface forms interfacial oxygen defects and yields a band gap of approximately 2.6 eV in BVO/ZnO nanocomposites. Atomic structural distortions at the interface of BVO/ZnO nanodendrites, which support the fact that there are many interfacial oxygen defects, affect the O 2p-V 3d hybridization and reduce the crystal field energy 10Dq ∼2.1 eV. Such an interfacial atomic/electronic structure and band gap modulation increase the efficiency of absorption of solar light and electron-hole separation. This study provides evidence that the interfacial oxygen defects act as a trapping center and are critical for the charge transfer, retarding electron-hole recombination, and high absorption of visible light, which can result in favorable PEC properties of a nanostructured core-shell BVO/ZnO heterojunction. Insights into the local atomic and electronic structures of the BVO/ZnO heterojunction support the fabrication of semiconductor heterojunctions with optimal compositions and an optimal interface, which are sought to maximize solar light utilization and the transportation of charge carriers for PEC water splitting and related applications.

Synthesis of POMOFs with 8-fold helix and its composite with carboxyl functionalized SWCNTs for the voltammetric determination of dopamine

Although many satisfactory studies have been developed for biomolecule detection, the complexity of biofluids still poses a major challenge to improve the performance of nanomaterials as electrochemical sensors. Herein, unprecedented polyoxometalate-based metal-organic frameworks (POMOFs) with 8-fold meso-helical feature, [Ag₅(trz)₄]₂[PMo₁₂O₄₀] (PAZ), were synthesized and explored as electrochemical sensors to detect dopamine (DA). To improve the conductivity of PAZ and the binding ability with single-walled carbon nanotubes (SWCNTs), the nanocomposite of carboxyl functionalized SWCNTs (SWCNTs-COOH) with nano-PAZ (NPAZ), NPAZ@SWCNTs-COOH, was fabricated, and transmission electron microscopy (TEM) shows that NPAZ can interact stably and uniformly with SWCNTs-COOH, owing to more defect sites on the surface of SWCNTs-COOH. The electrochemical result of NPAZ@SWCNTs-COOH/GCE towards detecting DA shows that the linear range was from 0.05 to 100 μM with a detection limit (LOD) of 8.6 nM (S/N = 3). A new electrochemical biosensing platform by combining 8-fold helical POMOFs with SWCNTs-COOH was developed for enhancing detection of dopamine for the first time, exhibiting the lowest detection limit to date.

A Silver-Loaded Exfoliated Graphite Nanocomposite Anti-Fouling Electrochemical Sensor for Bisphenol A in Thermal Paper Samples

Silver nanoparticles (AgNPs) were synthesized separately and loaded onto the expanded layers of exfoliated graphite (EG) to form a silver nanoparticle-exfoliated graphite nanocomposite (AgNPs-EG). The AgNPs-EG was compressed into a pellet (0.6 cm in diameter) and used to prepare an electrochemical sensor for bisphenol A (BPA) in standard samples and in thermal paper. The synthesized materials were characterized by ultraviolet-visible spectrophotometry, X-ray diffraction spectroscopy, scanning electron microscopy, and energy-dispersive X-ray. The electrochemical behavior of BPA on the AgNPs-EG sensor was investigated by cyclic voltammetry and square wave voltammetry. Under optimized experimental conditions, the oxidation peak current was linearly proportional to bisphenol A concentrations in the range from 5.0 to100 μM, with a coefficient of determination (R2 ) of 0.9981. The obtained limit of detection of the method was 0.23 μM. The fabricated sensor was able to overcome electrode fouling with good reproducibility (RSD = 2.62%, n = 5) by mechanical polishing of the electrode on emery paper. The proposed method was successfully applied to determine bisphenol A in thermal paper samples and demonstrated good accuracy of 93.1 to 113% recovery.

Towards visible light driven photoelectrocatalysis for water treatment: Application of a FTO/BiVO4/Ag2S heterojunction anode for the removal of emerging pharmaceutical pollutants

Pharmaceuticals have been classified as emerging water pollutants which are recalcitrant in nature. In the quest to find a suitable technique in removing them from contaminated water, photoelectrocatalytic oxidation method has attracted much attention in recent years. This report examined the feasibility of degrading ciprofloxacin and sulfamethoxazole through photoelectrocatalytic oxidation using FTO-BiVO4/Ag2S with p-n heterojunction as anode. BiVO4/Ag2S was prepared through electrodeposition and successive ionic layer adsorption/reaction on FTO glass. Structural and morphological studies using XRD, SEM, EDS and diffusive reflectance UV-Vis confirmed the successful construction of p-n heterojunction of BiVO4/Ag2S. Electrochemical techniques were used to investigate enhanced charge separation in the binary electrode. The FTO-BiVO4/Ag2S electrode exhibited the highest photocurrent response (1.194 mA/cm-2) and longest electron lifetime (0.40 ms) than both pristine BiVO4 and Ag2S electrodes which confirmed the reduction in recombination of charge carriers in the electrode. Upon application of the prepared FTO-BiVO4/Ag2S in photoelectrocatalytic removal of ciprofloxacin and sulfamethoxazole, percentage removal of 80% and 86% were achieved respectively with a low bias potential of 1.2 V (vs Ag/AgCl) within 120 min. The electrode possesses good stability and reusability. The results obtained revealed BiVO4/Ag2S as a suitable photoanode for removing recalcitrant pharmaceutical molecules in water.

Self-constructed side-by-side nanofiber photocatalyst via oppositely charged electrospinning and its photocatalytic degradation of rhodamine B

Fabricating side by side (SBS) nanofibers with two distinct materials using dual spinnerets is challenging because of the formation of bulk heterojunctions, which limits the application of these nanofibers.