Enhanced photoelectrocatalytic activity of direct Z-scheme porous amorphous carbon nitride/manganese dioxide nanorod arrays

Cascade reaction triggered colorimetric array for identification of organophosphorus pesticides congeners

A modern agriculture uses alternative pest control methods to boost productivity, leading to an accumulation of organophosphorus (OPPs) congeners. This necessitates an intuitive and quick way to identify OPPs congeners. A colorimetric sensor for detecting OPPs congeners using a double-enzyme cascade reaction has been successfully designed and constructed in this study. The OPPs regulate the color changes induced by manganese dioxide nanoflowers (MnO2 NFs) and specific alkaline phosphatases (ALP) during the etching of gold nanopyramids (Au NBPs). The ascorbic acid (AA) produced by ALP hydrolysis inhibits Au NBPs etching by MnO2 NFs oxidized 3, 3', 5, 5'-tetramethylbenzidine (TMB). By inhibiting ALP catalytic activity, OPPs prevent AA formation. In this process, Au NBPs will undergo further etching, resulting in various colors so they can be analyzed semi-quantitatively with the naked eye. It has been found that different types of OPPs inhibit enzymes differently and therefore result in varying degrees of etching of Au NBPs. Principal Component Analysis (PCA) is performed by smart devices that convert R, G, and B signals into digital signals. This colorimetric array tests various foods (tea, apple, and cabbage). Colorimetric visualization sensors combined with data analysis will be used in real-life product development.

Design strategies and mechanisms of g-C3N4-based photoanodes for photoelectrocatalytic degradation of organic pollutants in water

Emerging photoelectrocatalytic (PEC) systems integrate the advantages of photocatalysis and electrocatalysis and are considered as a promising technology for solving the global organic pollution problem in water environments. Among the photoelectrocatalytic materials applied for organic pollutant degradation, graphitic carbon nitride (CN) has the combined advantages of environmental compatibility, stability, low cost, and visible light response. However, pristine CN has disadvantages such as low specific surface area, low electrical conductivity, and high charge complexation rate, and how to improve the degradation efficiency of PEC reaction and the mineralization rate of organic matter is the main problem faced in this field. Therefore, this paper reviews the progress of various functionalized CN used for PEC reaction in recent years, and the degradation efficiency of these CN-based materials is critically evaluated. First, the basic principles of PEC degradation of organic pollutants are outlined. Then, engineering strategies to enhance the PEC activity of CN (including morphology control, elemental doping, and heterojunction construction) are focused on, and the structure-activity relationships between these engineering strategies and PEC activity are discussed. In addition, the important role of influencing factors on the PEC system is summarized in terms of mechanism, to provide guidance for the subsequent research. Finally, suggestions and perspectives are provided for the preparation of efficient and stable CN-based photoelectrocatalysts for practical wastewater treatment applications.

Controllable Anchoring of Graphitic Carbon Nitride on MnO2 Nanoarchitectures for Oxygen Evolution Electrocatalysis

The design and fabrication of eco-friendly and cost-effective (photo)electrocatalysts for the oxygen evolution reaction (OER) is a key research goal for a proper management of water splitting to address the global energy crisis. In this work, we focus on the preparation of supported MnO2/graphitic carbon nitride (g-CN) OER (photo)electrocatalysts by means of a novel preparation strategy. The proposed route consists of the plasma enhanced-chemical vapor deposition (PE-CVD) of MnO2 nanoarchitectures on porous Ni scaffolds, the anchoring of controllable g-CN amounts by an amenable electrophoretic deposition (EPD) process, and the ultimate thermal treatment in air. The inherent method versatility and flexibility afforded defective MnO2/g-CN nanoarchitectures, featuring a g-CN content and nano-organization tunable as a function of EPD duration and the used carbon nitride precursor. Such a modulation had a direct influence on OER functional performances, which, for the best composite system, corresponded to an overpotential of 430 mV at 10 mA/cm2, a Tafel slope of ≈70 mV/dec, and a turnover frequency of 6.52 × 10-3 s-1, accompanied by a very good time stability. The present outcomes, comparing favorably with previous results on analogous systems, were rationalized on the basis of the formation of type-II MnO2/g-CN heterojunctions, and yield valuable insights into this class of green (photo)electrocatalysts for end uses in solar-to-fuel conversion and water treatment.

Preparation of MnO2-Carbon Materials and Their Applications in Photocatalytic Water Treatment

Water pollution is one of the most important problems in the field of environmental protection in the whole world, and organic pollution is a critical one for wastewater pollution problems. How to solve the problem effectively has triggered a common concern in the area of environmental protection nowadays. Around this problem, scientists have carried out a lot of research; due to the advantages of high efficiency, a lack of secondary pollution, and low cost, photocatalytic technology has attracted more and more attention. In the past, MnO₂ was seldom used in the field of water pollution treatment due to its easy agglomeration and low catalytic activity at low temperatures. With the development of carbon materials, it was found that the composite of carbon materials and MnO₂ could overcome the above defects, and the composite had good photocatalytic performance, and the research on the photocatalytic performance of MnO₂-carbon materials has gradually become a research hotspot in recent years. This review covers recent progress on MnO₂-carbon materials for photocatalytic water treatment. We focus on the preparation methods of MnO₂ and different kinds of carbon material composites and the application of composite materials in the removal of phenolic compounds, antibiotics, organic dyes, and heavy metal ions in water. Finally, we present our perspective on the challenges and future research directions of MnO₂-carbon materials in the field of environmental applications.

Constructing the Sulfur-Doped CdO@In2O3 Nanofibers Ternary Heterojunction for Efficient Photocatalytic Hydrogen Production

An S-doped CdO@In₂O₃ nanofiber was successfully designed by in-situ electrospinning along and subsequent calcination treatment. Under artificial sunlight illumination, the S/CdO@In₂O₃-25 displayed a superior photocatalytic hydrogen evolution rate of 4564.58 μmol·g^-1·h^-1, with approximately 22.0 and 1261.0-fold of those shown by the S/CdO and S/In₂O₃ samples, respectively. The experimental and theoretical analyses illustrate that the unique one-dimensional (1D) nanofiber morphology and rich oxygen vacancies optimized the electronic structure of the nanofibers and adsorption/desorption behaviors of reaction intermediates, contributing to the realization of the remarkable solar-to-H₂ conversion efficiencies. Moreover, the staggered band structure and intimate contact heterointerfaces facilitate the formation of a type-II double charge-transfer pathway, promoting the spatial separation of photoexcited charge carriers. These results could inform the design of other advanced catalyst materials for photocatalytic reactions.

Carbon nitride-based Z-scheme heterojunctions for solar-driven advanced oxidation processes

Solar-driven advanced oxidation processes (AOPs) via direct photodegradation or indirect photocatalytic activation of typical oxidants, such as hydrogen peroxide (H₂O₂), peroxymonosulfate (PMS), and peroxydisulfate (PDS), have been deemed to be an efficient technology for wastewater remediation. Artificial Z-scheme structured materials represent a promising class of photocatalysts due to their spatially separated charge carriers and strong redox abilities. Herein, we summarize the development of metal-free graphitic carbon nitride (g-C₃N₄, CN)-based direct and indirect Z-scheme photocatalysts for solar-driven AOPs in removing organic pollutants from water. In the work, the classification of AOPs, definition and validation of Z-schemes are summarized firstly. The innovative engineering strategies (e.g., morphology and dimensionality control, element doping, defect engineering, cocatalyst loading, and tandem Z-scheme construction) over CN-based direct Z-scheme structure are then examined. Rational design of indirect CN-based Z-scheme systems using different charge mediators, such as solid conductive materials and soluble ion pairs, is further discussed. Through examining the relationship between the Z-scheme structure and activity (charge transfer and separation, light absorption, and reaction kinetics), we aim to provide more insights into the construction strategies and structure modification on CN-based Z-schemes towards improving their catalytic performances in AOPs. Lastly, limitations, challenges, and perspectives on future development in this emerging field are proposed.

Facile preparation of a novel modified biochar-based supramolecular self-assembled g-C3N4 for enhanced visible light photocatalytic degradation of phenanthrene

The rational design of a novel and environmentally friendly photocatalytic composite for persistent pollutant removal, energy production and catalytic applications have attracted widespread interest. In this study, the new composite composed of KOH-modified biochar and g-C₃N₄ with different morphologies was successfully prepared with facile supramolecular self-assembly and thermal poly-condensation method. The characterization results of the as-prepared composites suggested that KOH-modified biochar had been well combined with g-C₃N₄ with different morphologies. These synthesized catalysts were used to degrade phenanthrene under visible light radiation. A-BC/g-C₃N₄-D performed best and removed 76.72% phenanthrene. Its first-order reaction rate constant was 0.355 h^-1, which was 3.7 times higher than that of g-C₃N₄. A-BC/g-C₃N₄-D still exhibited a high photocatalytic activity after four cycles. Radical quenching results showed that superoxide radical (·O₂^-), hydroxyl radical (·OH) and hole (h⁺) could be used as active species in the redox reaction with phenanthrene. Based on the exploration results of gas chromatography-mass spectrometer (GC-MS), a possible reaction pathway of phenanthrene degradation was also proposed. This study provides a novel strategy for fabricating various high-performance photocatalysts and the removal of persistent organic pollutants.

In situ configuration of dual S-scheme BP/(Ti3C2Tx@TiO2) heterojunction for broadband spectrum solar-driven photocatalytic H2 evolution in pure water

Artificial photocatalysis with high-efficiency is a promising route for storing sustainable energy from water splitting. Whereas it is challenging to broaden the solar-spectrum responsive window for harvesting high level of conversion. Herein, based on the band-matching engineering theory, a design of dual S-Scheme heterojunction system is proposed and established in a BP/(Ti₃C₂T_x@TiO₂) composite photocatalyst. The complementary light response region between TiO₂ and BP realizes the extension of solar energy utilization over a broad absorption window. Furthermore, this specific band-matching configuration endows spatially long-lived charge carriers with greater accumulation on the divided sub-systems, thereby maintaining the sufficient potential energy capacity associated with excellent photocatalytic properties (H₂ evolution rate of 564.8 μmol h^-1 g^-1 and AQE of 2.7% at 380 nm in pure water). This work describes a promising protocol of designing advanced broadband light-activated photocatalytic systems for solar-chemical energy conversion applications.

Electrostatic self-assembly to form unique LiNbO3/ZnS core-shell structure for photocatalytic nitrate reduction enhancement

Photocatalytic NO₃^- reduction in water has been regarded as a promising route due to its high efficiency and green feature. Several limiting factors, such as lack of catalytic sites, insufficient light collection, and spatial charge separation capacity photocatalytic denitrification, still need to be overcome for the practical applications. Herein, an innovative LiNbO₃/ZnS heterojunction with a unilateral opening core-shell structure was constructed. ZnS was tightly anchored on the surface of LiNbO₃ by modified electrostatic self-assembly method. High nitrate removal rate (98.84%) and N₂ selectivity (98.92%) were achieved with a molar ratio of LiNbO₃ and ZnS of 1:5 (1:5L-ZS) using formic acid as a hole scavenge. The LiNbO₃/ZnS degradation kinetics of NO₃^- was corresponding to the first-order kinetics equation. The nitrate removal rate and N₂ selectivity remained stable after three cycles in such photocatalytic NO₃^- reduction. The outstanding photocatalyst performance can be ascribed to the improved surface active sites, the well-matched band structure, and the unique core-shell structure. It provides an effective strategy for controllable fabrication of core-shell photocatalyst with strong light-harvesting ability and charge separation efficiency to enhance the removal rate of nitrate in water.

Photoelectrocatalytic degradation of deoxynivalenol on CuO-Cu2O/WO3 ternary film: Mechanism and reaction pathways

A ternary film of CuO-Cu₂O/WO₃ possessing high visible-light photoelectrocatalytic (PEC) performance was prepared for degradation of deoxynivalenol (DON). In such a ternary film, the introduction of CuO-Cu₂O significantly promoted the absorption of WO₃ in the visible region and reduced the recombination of photogenerated charge carriers. As a result, the CuO-Cu₂O/WO₃ film exhibited high photoelectrochemical activity under visible light illumination. The PEC performance of CuO-Cu₂O/WO₃ film was evaluated by the decoloration of Rhodamine B. The result indicated that the CuO-Cu₂O/WO₃ film exhibited higher PEC activity than WO₃ or CuO-Cu₂O film. When the CuO-Cu₂O/WO₃ film was applied to study the removal of DON, the degradation efficiency reached 87.6% after 180-min PEC treatment. According to reactive oxygen species detected by electron spin resonance analysis, a Z-scheme and type-II PEC mechanism was proposed for this ternary film. Furthermore, the intermediates formed during the PEC degradation process of DON were separated by high-performance liquid chromatography and identified with liquid chromatography-mass spectrometry. On the basis of sixteen intermediate products identified, we proposed a degradation pathway for DON in such a PEC system.

Template-Free Synthesis of One-Dimensional g-C3N4 Chain Nanostructures for Efficient Photocatalytic Hydrogen Evolution

The development of graphite-carbon nitride (g-C3N4) photocatalyst is of great significance for various visible utilization applications. Control the nanostructures of g-C3N4 can tailor its photocatalytic performance. In this paper, one-dimensional chain-like g-C3N4 was successfully synthesized by heat-induced polymerization of melamine which was saturated in ethylene glycol. The photocatalytic hydrogen production rate (HER) of the prepared g-C3N4 chain enhanced about 3 times than that of bulk g-C3N4, increasing from 9.6 μmolh-1 to 28.7 μmolh-1. The improved photocatalytic activity of the g-C3N4 chain was attributed to the advantages of porosity and nanostructure. The extraordinary nanopores result in an enlarged specific surface area for adsorption and the production of abundantly available channels for charge transfer. The one-dimensional chain-like structure can facilitate the exposure of internal/external active sites as many as possible, and induce the directional migration of charge carriers.

A uniformly decorated and photostable polydopamine-organic semiconductor to boost the photoelectrochemical water splitting performance of CdS photoanodes

Photoelectrochemical (PEC) water splitting to produce renewable H2 fuel by storage of solar energy has attracted increasing attention as it could reduce carbon footprint and solve the global consumption growth. Herein, a photostable polymer polydopamine (PDA) was introduced to enhance the PEC performance by forming a uniform inorganic-organic hybrid heterostructure with CdS. The organic semiconductor PDA not only forms a strong coordinate bond to facilitate the transfer of electrons, but also acts as a passivation layer, contributing to improve the stability of the photoelectrode. A photocurrent density of 1.08 mA cm-2 was achieved for CdS/1PDA, which was about 2.4 times that of bare CdS at 0.28 V vs. RHE, and CdS/1PDA featured a reasonable photocurrent stability compared with bare CdS. The Co-Pi co-catalyst, as a hole acceptor, further prohibited charge recombination and promoted the water oxidation kinetics. The photocurrent density of CdS/1PDA/5Co-Pi was up to 2.68 mA cm-2 (0.28 V vs. RHE), which was 5.7 and 2.5 times higher than that of bare CdS and CdS/1PD, respectively. The strategy provides a beneficial insight to design an inorganic-organic uniform heterostructure for the enhancement in PEC performance.

Hierarchical PANI/ZnO nanocomposite: synthesis and synergistic effect of shape-selective ZnO nanoflowers and polyaniline sensitization for efficient photocatalytic dye degradation and photoelectrochemical water splitting

Hierarchical nanoflowers (NFs) of zinc oxide (ZnO) have been synthesized in the hexagonal wurtzite structure by a facile hydrothermal method. Polyaniline (PANI) has been prepared by the chemical oxidative polymerization method and incorporated with ZnO NFs by the chemisorption method. The potential of the synthesized nanostructures has been demonstrated for efficient photocatalytic degradation of methylene blue (MB) and photoelectrochemical water splitting. The PANI/ZnO nanocomposite has exhibited the enhanced photocatalytic activity which is ∼9 fold higher in comparison to pristine ZnO NFs and enhanced photocurrent density which is ∼16 fold higher than the ZnO photoanode. Importantly, ∼4 fold increment in the incident photon-to-current conversion efficiency (IPCE) is exhibited by PANI/ZnO, than that of ZnO photoanode. The remarkably enhanced photocatalytic and photoelectrochemical performance of PANI/ZnO nanocomposite is attributed to the availability of more interfacial sites facilitated by the hierarchical ZnO NFs, improved overall photoresponse due to its photosensitization with PANI and the resulting type-II heterojunction between them, which helps in the efficient separation of photogenerated charge carriers at the interface. A plausible reaction mechanism for the substantially improved performance of nanostructured PANI/ZnO towards MB degradation and water splitting has also been elucidated.

Fabrication of Dandelion-like p-p Type Heterostructure of Ag2O@CoO for Bifunctional Photoelectrocatalytic Performance

A novel three-dimensional purple dandelion-like hierarchical Ag₂O@CoO heterojunction with an appropriate redox potential was constructed by chemical precipitation of Ag₂O nanoparticle on flower-like CoO. By feat of this hierarchical structure, the Ag₂O@CoO photocathode showed significantly high photoelectroreduction activities toward p-nitrophenol (p-NP) and Cr(VI). The high performance of Ag₂O@CoO was mainly attributed to the specific structural characteristics and synergistic effect of each chemical component. This hierarchical structure could effectively increase the specific surface area, provide more exposed active edges, and be beneficial for multiple light reflection/scattering channels and light utilization efficiency. The introduction of Ag₂O optimized the composition and further improved the band structure, resulting in an improved separation of photogenerated electrons and holes. The unique photocathode achieves a removal efficiency of 86% for photoelectrocatalytic p-NP degradation after 120 min and 95% for Cr(VI) after 40 min under visible light irradiation with excellent stability. This research provided a simple way for the synthesis of photoelectrocatalytic material with potential applications in the field of environmental governance with visible light illumination.

Fabrication and high photoelectrocatalytic activity of scaly BiOBr nanosheet arrays

Bismuth oxybromide (BiOBr) nanosheet arrays (NSAs) were successfully prepared on the surface of indium tin oxide glass (hydrophilic pretreated) by solvothermal method using [C₁₆mim]Br ionic liquid as bromine source and template. The effects of different reaction temperatures on array synthesis were investigated. BiOBr NSA-160 (NSAs prepared at 160 °C for 8 h) had the best photoelectrocatalytic (PEC) activity. The removal rate of ciprofloxacin hydrochloride by BiOBr NSA-160 was 91.4% by applying a bias voltage of 0.9 V and irradiating under visible light for 180 min. Results of the analyses of the morphology, photoelectric properties, energy band structure, and degradation active species show that BiOBr NSA-160 is a p-type photocatalyst with a thickness of approximately 500 nm, a light response range of less than 440 nm, and photocurrent density of 69 μA/cm² at the optimal bias voltage is 0.9 V. The high PEC activity of BiOBr NSA-160 was deduced from two aspects: one is that the bias potential effectively improves the separation efficiency of photogenerated carrier, and the other is that the structure of the nanoarray increases light absorption and active sites. BiOBr NSAs are promising PEC material for application in pollutant removal.