Bifunctional photoelectrochemical process for humic acid degradation and hydrogen production using multi-layered p-type Cu2O photoelectrodes with plasmonic Au@TiO2

Design and Preparation of Heterostructured Cu2O/TiO2 Materials for Photocatalytic Applications

The extensive use of fossil fuels has sped up the global development of the world economy and is accompanied by significant problems, such as energy shortages and environmental pollution. Solar energy, an inexhaustible and clean energy resource, has emerged as a promising sustainable alternative. Light irradiation can be transformed into electrical/chemical energy, which can be used to remove pollutants or transform contaminants into high-value-added chemicals through photocatalytic reactions. Therefore, photocatalysis is a promising strategy to overcome the increasing energy and environmental problems. As is well-known, photocatalysts are key components of photocatalytic systems. Among the widely investigated photocatalysts, titanium dioxide (TiO2) has attracted great attention owing to its excellent light-driven redox capability and photochemical stability. However, its poor solar light response and rapid recombination of electron-hole pairs limit its photocatalytic applications. Therefore, strategies to enhance the photocatalytic activity of TiO2 by narrowing its bandgap and inhibiting the recombination of charges have been widely accepted. Constructing heterojunctions with other components, including cuprous oxide (Cu2O), has especially narrowed the bandgap, providing a promising means of solving the present challenges. This paper reviews the advances in research on heterostructured Cu2O/TiO2 photocatalysts, such as their synthesis methods, mechanisms for the enhancement of photocatalytic performance, and their applications in hydrogen production, CO2 reduction, selective synthesis, and the degradation of pollutants. The mechanism of charge separation and transfer through the Cu2O/TiO2 heterojunctions and the inherent factors that lead to the enhancement of photocatalytic performance are extensively discussed. Additionally, the current challenges in and future perspectives on the use of heterostructured Cu2O/TiO2 photocatalysts are also highlighted.

Hydrogen Production via Electrolysis of Wastewater

The high energy consumption of traditional water splitting to produce hydrogen is mainly due to complex oxygen evolution reaction (OER), where low-economic-value O2 gas is generated. Meanwhile, cogeneration of H2 and O2 may result in the formation of an explosive H2/O2 gas mixture due to gas crossover. Considering these factors, a favorable anodic oxidation reaction is employed to replace OER, which not only reduces the voltage for H2 production at the cathode and avoids H2/O2 gas mixture but also generates value-added products at the anode. In recent years, this innovative strategy that combines anodic oxidation for H2 production has received intensive attention in the field of electrocatalysis. In this review, the latest research progress of a coupled hydrogen production system with pollutant degradation/upgrading is systematically introduced. Firstly, wastewater purification via anodic reaction, which produces free radicals instead of OER for pollutant degradation, is systematically presented. Then, the coupled system that allows for pollutant refining into high-value-added products combined with hydrogen production is displayed. Thirdly, the photoelectrical system for pollutant degradation and upgrade are briefly introduced. Finally, this review also discusses the challenges and future perspectives of this coupled system.

Progress in Promising Semiconductor Materials for Efficient Photoelectrocatalytic Hydrogen Production

Photoelectrocatalytic (PEC) water decomposition provides a promising method for converting solar energy into green hydrogen energy. Indeed, significant advances and improvements have been made in various fundamental aspects for cutting-edge applications, such as water splitting and hydrogen production. However, the fairly low PEC efficiency of water decomposition by a semiconductor photoelectrode and photocorrosion seriously restrict the practical application of photoelectrochemistry. In this review, the mechanisms of PEC water decomposition are first introduced to provide a solid understanding of the PEC process and ensure that this review is accessible to a wide range of readers. Afterwards, notable achievements to date are outlined, and unique approaches involving promising semiconductor materials for efficient PEC hydrogen production, including metal oxide, sulfide, and graphite-phase carbon nitride, are described. Finally, four strategies which can effectively improve the hydrogen production rate-morphological control, doping, heterojunction, and surface modification-are discussed.

Nanoparticles and nanofiltration for wastewater treatment: From polluted to fresh water

Water pollution poses significant threats to both ecosystems and human health. Mitigating this issue requires effective treatment of domestic wastewater to convert waste into bio-fertilizers and gas. Neglecting liquid waste treatment carries severe consequences for health and the environment. This review focuses on intelligent technologies for water and wastewater treatment, targeting waterborne diseases. It covers pollution prevention and purification methods, including hydrotherapy, membrane filtration, mechanical filters, reverse osmosis, ion exchange, and copper-zinc cleaning. The article also highlights domestic purification, field techniques, heavy metal removal, and emerging technologies like nanochips, graphene, nanofiltration, atmospheric water generation, and wastewater treatment plants (WWTPs)-based cleaning. Emphasizing water cleaning's significance for ecosystem protection and human health, the review discusses pollution challenges and explores the integration of wastewater treatment, coagulant processes, and nanoparticle utilization in management. It advocates collaborative efforts and innovative research for freshwater preservation and pollution mitigation. Innovative biological systems, combined with filtration, disinfection, and membranes, can elevate recovery rates by up to 90%, surpassing individual primary (<10%) or biological methods (≤50%). Advanced treatment methods can achieve up to 95% water recovery, exceeding UN goals for clean water and sanitation (Goal 6). This progress aligns with climate action objectives and safeguards vital water-rich habitats (Goal 13). The future holds promise with advanced purification techniques enhancing water quality and availability, underscoring the need for responsible water conservation and management for a sustainable future.

Dual functional WO3/BiVO4 heterostructures for efficient photoelectrochemical water splitting and glycerol degradation

Dual functional heterojunctions of tungsten oxide and bismuth vanadate (WO₃/BiVO₄) photoanodes are developed and their applications in photoelectrochemical (PEC) water splitting and mineralization of glycerol are demonstrated. The thin-film WO₃/BiVO₄ photoelectrode was fabricated by a facile hydrothermal method. The morphology, chemical composition, crystalline structure, chemical state, and optical absorption properties of the WO₃/BiVO₄ photoelectrodes were characterized systematically. The WO₃/BiVO₄ photoelectrode exhibits a good distribution of elements and a well-crystalline monoclinic WO₃ and monoclinic scheelite BiVO₄. The light-absorption spectrum of the WO₃/BiVO₄ photoelectrodes reveals a broad absorption band in the visible light region with a maximum absorption of around 520 nm. The dual functional WO₃/BiVO₄ photoelectrodes achieved a high photocurrent density of 6.85 mA cm^-2, which is 2.8 times higher than that of the pristine WO₃ photoelectrode in the presence of a mixture of 0.5 M Na₂SO₄ and 0.5 M glycerol electrolyte under AM 1.5 G (100 mW cm^-2) illumination. The superior PEC performance of the WO₃/BiVO₄ photoelectrode was attributed to the synergistic effects of the superior crystal structure, light absorption, and efficient charge separation. Simultaneously, glycerol plays an essential role in increasing the efficiency of hydrogen production by suppressing charge recombination in the water redox reaction. Moreover, the WO₃/BiVO₄ photoelectrode shows the total organic carbon (TOC) removal efficiency of glycerol at about 82% at 120 min. Notably, the WO₃/BiVO₄ photoelectrode can be a promising photoelectrode for simultaneous hydrogen production and mineralization of glycerol with a simple, economical, and environmentally friendly approach.

Recent advances and challenges of photoelectrochemical cells for hydrogen production

Photoelectrocatalytic water splitting and organic reforming have recently received significant attention among researchers due to the potential opportunity to convert sunlight into hydrogen energy using efficient and low-cost photoelectrode materials under practical operating conditions. This paper discusses an overview of various aspects related to the implementation of photoelectrochemical (PEC) cells for hydrogen generation. Information on (i) reaction energies of photosplitting and photoreforming, (ii) state-of-the-art semiconductor-based materials for PEC hydrogen evolution reaction (HER) active both under UV and visible-light irradiation, (iii) PEC photo-efficiency indicators, and (iv) criteria for the standardization of photoelectrochemical reactor performances are summarized. This short review provides a fundamental background to highlight the most effective approaches developed to date in PEC research and promote future competitive large-scale systems for hydrogen generation for practical applications.

Interface-engineered Z-scheme of BiVO4/g-C3N4 photoanode for boosted photoelectrochemical water splitting and organic contaminant elimination under solar light

Although n-type bismuth vanadate (BiVO₄) is regarded as an attractive solar-light-active photoanode, its short carrier-diffusion length, sluggish oxidation kinetics, low electronic conductivity, and high recombination rate are the major intrinsic shortcomings that limit its practical application. To this end, the rational design of a solar-light-active, metal-free BiVO₄-based Z-scheme heterojunction photoanode is of great significance for achieving effective charge-separation features and maximum light utilization as well as boosting redox activity for efficient environmental treatment and photoelectrochemical water splitting. Herein, we propose a facile approach for the decoration of metal-free graphitic carbon nitride (g-C₃N₄) nanosheets on BiVO₄ to form a Z-scheme BiVO₄/g-C₃N₄ photoanode with boosted photoelectrochemical (PEC) water splitting and rapid photoelectrocatalytic degradation of methyl orange (MO) dye under simulated solar light. The successful preparation of the Z-scheme BiVO₄/g-C₃N₄ photoanode was confirmed by comprehensive structural, morphological, and optical analyses. Compared with the moderate photocurrent density of bare BiVO₄ (0.39 mA cm^-2), the Z-scheme BiVO₄/g-C₃N₄ photoanode yields a notable photocurrent density of 1.14 mA cm^-2 at 1.23 V vs. RHE (≈3-fold higher) with the promising long-term stability of 5 h without any significant photo-corrosion. Moreover, the PEC dye-degradation studies revealed that the Z-scheme BiVO₄/g-C₃N₄ photoanode successfully degraded MO (≈90%) in 75 min, signifying a 30% improvement over bare BiVO₄. This research paves the way for rational interface engineering of solar-light-active BiVO₄-based noble-metal-free Z-schemes for eco-friendly PEC water splitting and water remediation.

Surface plasmon-driven photoelectrochemical water splitting of a Ag/TiO2 nanoplate photoanode

A silver/titanium dioxide nanoplate (Ag/TiO2 NP) photoelectrode was designed and fabricated from vertically aligned TiO2 nanoplates (NP) decorated with silver nanoparticles (NPs) through a simple hydrothermal synthesis and electrodeposition route. The electrodeposition times of Ag NPs on the TiO2 NP were crucial for surface plasmon-driven photoelectrochemical (PEC) water splitting performance. The Ag/TiO2 NP at the optimal deposition time of 5 min with a Ag element content of 0.53 wt% demonstrated a remarkably high photocurrent density of 0.35 mA cm-2 at 1.23 V vs. RHE under AM 1.5G illumination, which was 5 fold higher than that of the pristine TiO2 NP. It was clear that the enhanced light absorption properties and PEC performance for Ag/TiO2 NP could be effectively adjusted by simply controlling the loading amounts of metallic Ag NPs (average size of 10-30 nm) at different electrodeposition times. The superior PEC performance of the Ag/TiO2 NP photoanode was attributed to the synergistic effects of the plasmonic Ag NPs and the TiO2 nanoplate. Interestingly, the plasmonic effect of Ag NPs not only increased the visible-light response (λ max = 570 nm) of TiO2 but also provided hot electrons to promote photocurrent generation and suppress charge recombination. Importantly, this study offers a potentially efficient strategy for the design and fabrication of a new type of TiO2 hybrid nanostructure with a plasmonic enhancement for PEC water splitting.

Recent Progress and Prospects in Catalytic Water Treatment

Presently, conventional technologies in water treatment are not efficient enough to completely mineralize refractory water contaminants. In this context, the implementation of catalytic processes could be an alternative. Despite the advantages provided in terms of kinetics of transformation, selectivity, and energy saving, numerous attempts have not yet led to implementation at an industrial scale. This review examines investigations at different scales for which controversies and limitations must be solved to bridge the gap between fundamentals and practical developments. Particular attention has been paid to the development of solar-driven catalytic technologies and some other emerging processes, such as microwave assisted catalysis, plasma-catalytic processes, or biocatalytic remediation, taking into account their specific advantages and the drawbacks. Challenges for which a better understanding related to the complexity of the systems and the coexistence of various solid-liquid-gas interfaces have been identified.

Treatment of real wastewater by photoelectrochemical methods: An overview

An inadequate and inefficient performance ability of conventional methods to remove persistent organic pollutants urges the need of alternative or complementary advanced wastewater treatments methods to ensure the safer reuse of reclaimed water. Photoelectrochemical methods are emerging as promising options among other advanced oxidation processes because of the higher treatment efficiency achieved due to the synergistic effects of combined photochemical and electrolysis reactions. Synergistic effects of integrated photochemical, electrochemical and photoelectrochemical processes not only increase the hydroxyl radical production; an enhancement on the mineralization ability through various side reactions is also achieved. In this review, fundamental reaction mechanisms of different photoelectrochemical methods including photoelectrocatalysis, photo/solar electro-Fenton, photo anodic oxidation, photoelectroperoxone and photocatalytic fuel cell are discussed. Various integrated photochemical, electrochemical and photoelectrochemical processes and their synergistic effects are elaborated. Different reactor configurations along with the positioning of electrodes, photocatalysts and light source of the individual/combined photoelectrochemical treatment systems are discussed. Modified photoanode and cathode materials used in the photoelectrochemical reactors and their performance ability is presented. Photoelectrochemical treatment of real wastewater such as landfill leachate, oil mill, pharmaceutical, textile, and tannery wastewater are reviewed. Hydrogen production efficiency in the photoelectrochemical process is further elaborated. Cost and energy involved in these processes are briefed, but the applicability of photocatalytic fuel cells to reduce the electrical dependence is also summarised. Finally, the use of photoelectrochemical approaches as an alternative for treating soil washing effluents is currently discussed.

Three-Dimensional Hierarchical Porous TiO2 for Enhanced Adsorption and Photocatalytic Degradation of Remazol Dye

Three-dimensional hierarchical mesoporous structures of titanium dioxide (3D-HPT) were synthesized by self-assembly emulsion polymerization. Polymethyl methacrylate (PMMA) and pluronic 123 (P123) were used as the soft templates and co-templates for assisting the formation of hierarchical 3D porous structures. The TiO2 crystal structure, morphology, and Remazol red dye degradation were investigated. The 3D-HPT and normal three-dimensional titanium dioxide (3D-T) presented the good connection of the nanoparticle-linked honeycomb within the form of anatase. The 3D-HPT structure showed greatly enhanced adsorption of Remazol dye, and facilitated the efficient photocatalytic breakdown of the dye. Surprisingly, 3D-HPT can adsorb approximately 40% of 24 ppm Remazol dye in the dark, which is superior to 3D-T and the commercial anatase at the same condition (approx. 5%). Moreover, 3D-HPT can completely decolorize Remazol dye within just 20 min, which is more than three folds faster than the commercial anatase, making it one of the most active photocatalysts that have been reported for degradation of Remazol dye. The superior photocatalytic performance is attributed to the higher specific surface area, amplified light-harvesting efficiency, and enhanced adsorption capacity into the hierarchical 3D inverse opal structure compared to the commercial anatase TiO2.

Charge transfer channels of silver @ cuprous oxide heterostructure core-shell nanoparticles strengthen high photocatalytic antibacterial activity

Marine biological fouling has always been a hot research topic. In this study, silver @ cuprous oxide (Ag@Cu₂O) core-shell nanoparticles were synthesized via in-situ synthesis method and developed an outstanding antibacterial activity. The bacteriostasis efficiency of Ag@Cu₂O reached to 99% and 98% against Staphylococcus aureus and Pseudomonas aeruginosa, respectively. The minimum inhibitory concentration of Ag@Cu₂O decreased from 113.6 μg/mL to 56.8 μg/mL compared with Cu₂O. Ag@Cu₂O had better antibacterial activity than Cu₂O with lower content of Cu₂O and was more environment friendly. The heterostructure formed at the interface between Ag and Cu₂O promoted the separation and diffusion of photogenerated electron-hole pairs through the charge transfer channel and promoted the generation of reactive oxygen species. The outstanding antibacterial activity of Ag@Cu₂O was strongly depended on the generation of the reactive oxygen species. Density functional theory and finite element method calculations demonstrated that the structure of core-shell improved photocatalytic efficiency. Additionally, synergetic effect of released Ag⁺ and Cu²⁺ also enhanced the bacteriostasis rate and the long-term antifouling performance in 60 days. Hence, the synthesized core-shell Ag@Cu₂O can be applied as novel antifoulants in the marine field.

Efficient and Rapid Photocatalytic Degradation of Methyl Orange Dye Using Al/ZnO Nanoparticles

ZnO and Aluminum doped ZnO nanoparticles (Al/ZnO NPs) were successfully synthesized by the sol-gel method. Together with the effect of calcination temperatures (200, 300 and 400 °C) and Al dosage (1%, 3%, 5% and 10%) on structural, morphological and optical properties of Al/ZnO NPs, their photocatalytic degradation of methyl orange (MO) dye was investigated. The calcination temperatures at 200, 300 and 400 °C in forming structure of ZnO NPs led to spherical nanoparticle, nanorod and nanoflake structures with a well-crystalline hexagonal wurtzite, respectively. The ZnO NPs calcined at 200 °C exhibited the highest specific surface area and light absorption property, leading to the MO removal efficiency of 80% after 4 h under the Ultraviolet (UV) light irradiation. The MO removal efficiency was approximately two times higher than the nanoparticles calcined at 400 °C. Furthermore, the 5% Al/ZnO NPs exhibited superior MO removal efficiency of 99% in only 40 min which was approximately 20 times enhancement in photocatalytic activity compared to pristine ZnO under the visible light irradiation. This high degradation performance was attributed to the extended light absorption, narrowed band gap and effective suppression of electron-hole recombination through an addition of Al metal.

A comparative review on clean hydrogen production from wastewaters

This paper provides a unique review of hydrogen production methods with wastewater treatment to depict a clean and sustainable approach. Various methods for hydrogen production from wastewaters are identified and discussed with recent details by discussing the critical challenges, opportunities, and future directions. Five main performance sectors are considered in detail for each hydrogen production method of the recent case studies, including economic, environmental, social, technical, and reliability. Eight hydrogen production methods are reviewed, including anaerobic method, photo fermentation, dark fermentation, electrolysis, electrodialysis, photocatalysis, photoelectrochemical methods, and super water gasification. A comparative assessment of six reviewed methods for hydrogen production, including environmental, economic, energetic, and exergetic impacts, is evaluated. The comparative assessment results indicate that dark fermentation technology is the most economical method, and it is followed by microbial electrolysis and photofermentation. The most environmentally friendly method for the lowest global warming potential (GWP) is the microbial electrolysis method, and it is followed by photocatalysis and photoelectrochemical methods. Furthermore, the highest energy and exergy efficiencies have been recorded for the microbial electrolysis to be 68% and 64.7%, respectively.