Carbon quantum dots based charge bridge between photoanode and electrocatalysts for efficiency water oxidation

High SERS performance of functionalized carbon dots in the detection of dye contaminants

INTRODUCTION

The long-term overuse of malachite green (MG) has potential carcinogenic, teratogenic, and mutagenic effects. The functional nanocomposite is novel and challenging to construct and implement through surface enhanced Raman scattering (SERS) strategy to reveal the contributions in application.

OBJECTIVES

The novel Ag-CDs (carbon dots)-PBA (phenyl boric acid) nanocomposite was constructed by a facile route to detect toxic MG molecule with high SERS sensitivity and good uniformity.

METHODS

The enhanced substrate used for the detection of MG has been successfully constructed using PBA modulated Ag-CDs on a structured surface with rich binding sites.

RESULTS

The fabricated Ag-CDs-PBA substrate can be used to analyze various probe molecules exhibiting high sensitivity, good signal reproducibility, and excellent stability. The mechanism between components has been proved by calculations originating from the plasmonic Ag and active electronic transmission among the bridging CDs and PBA via the close spatial π-π effect. In addition, the accelerated separation of electron-hole pairs was triggered to further improve the SERS activity of the hybrid via a bidirectional charge transfer (CT) process. Significantly, the Ag-CDs-PBA system shows distinctive selectivity, in which PBA can hinder the interference of other species without specific hydroxyl groups.

CONCLUSION

Based on this deeper insight on plasmon-mediated mechanism, the SERS substrate was successfully practiced for quantitative determination in real water and fish samples. The strategy developed promises to be a new sensor technology and has great potential for environmental and food safety applications.

Fe-based Fenton-like catalysts for water treatment: Preparation, characterization and modification

Fenton reaction based on hydroxyl radicals () is effective for environment remediation. Nevertheless, the conventional Fenton reaction has several disadvantages, such as working at acidic pH, producing iron-containing sludge, and the difficulty in catalysts reuse. Fenton-like reaction using solid catalysts rather than Fe²⁺ has received increasing attention. To date, Fe-based catalysts have received increasing attention due to their earth abundance, good biocompatibility, comparatively low toxicity and ready availability, it is necessary to review the current status of Fenton-like catalysts. In this review, the recent advances in Fe-based Fenton-like catalysts were systematically analyzed and summarized. Firstly, the various preparation methods were introduced, including template-free methods (precipitation, sol gel, impregnation, hydrothermal, thermal, and others) and template-based methods (hard-templating method and soft-templating method); then, the characterization techniques for Fe-based catalysts were summarized, such as X-ray diffraction (XRD), Brunauer, Emmett and Teller (BET), SEM (scanning electron microscopy)/TEM (transmission electron microscopy)/HRTEM (high-resolution TEM), FTIR (Fourier transform infrared spectroscopy)/Raman, XPS (X-ray photoelectron spectroscopy), ⁵⁷Fe Mössbauer spectroscopy etc.; thirdly, some important conventional Fe-based catalysts were introduced, including iron oxides and oxyhydroxides, zero-valent iron (ZVI) and iron disulfide and oxychloride; fourthly, the modification strategies of Fe-based catalysts were discussed, such as microstructure controlling, introduction of support materials, construction of core-shell structure and incorporation of new metal-containing component; Finally, concluding remarks were given and the future perspectives for further study were discussed. This review will provide important information to further advance the development and application of Fe-based catalysts for water treatment.

Z-Scheme Photocarrier Transfer Realized in Tungsten Oxide-Based Photocatalysts by Combining with Bismuth Vanadate Quantum Dots

Multicomponent photocatalysts with a Z-scheme charge transfer are promising in converting solar to hydrogen fuel because of their significantly improved light absorption and restrained photocarrier recombination while keeping their redox capacity. In this work, a composite photocatalyst of BiVO₄ quantum dot-decorated WO₃ nanosheet arrays was synthesized and investigated. The existence of the Z-scheme charge transfer behavior was confirmed by the redox probe technique. Such a Z-scheme charge transfer makes it possible to generate hydrogen without bias. An optimized photocatalyst produces a hydrogen generation rate of 0.75 μmol/h without bias and a photocurrent of 1.91 mA/cm² at 1.23 V versus RHE, which is about 70% higher than that of pure WO₃. We attributed these improvements to the enhanced light absorption, extended conduction band level of BiVO₄, as well as the unique charge transfer behavior in the Z-scheme structure. This work presents a generalizable method to improve the redox capacity of a variety of semiconductors through rationally selecting the building material blocks in view of energy levels.

Solution-mediated nanometric growth of α-Fe2O3 with electrocatalytic activity for water oxidation

This paper describes a simple, low-temperature, and environmentally friendly aqueous route for the layer-by-layer nanometric growth of crystalline α-Fe2O3. The formation mechanism involves alternative sequences of the electrostatic adsorption of Fe2+ ions on the surface and the subsequent onsite oxidation to Fe3+. A combination analysis of X-ray diffraction, scanning electron microscopy, UV-Vis spectroscopy, and X-ray photoelectron spectroscopy revealed that α-Fe2O3 is directly formed without post-growth annealing via designed chemical reactions with a growth rate of ca. 1.7 nm per deposition cycle. The obtained α-Fe2O3 layer exhibits electrocatalytic activity for water oxidation and, at the same time, insignificant photo-electrocatalytic response, indicating its defective nature. The electrocatalytic activity was tailored by annealing up to 500 °C in air, where thermal diffusion of Sn4+ into the α-Fe2O3 lattice from the substrate probably provides an increased electrical conductivity. The subsequent surface-modification with Ni(OH)2 lowers the overpotential (250 mV at 0.5 mA cm-2) in a 1 M KOH solution. These findings open direct growth pathways to functional metal oxide nanolayers via liquid phase atomic layer deposition.

Blue- and green-emitting hydrophobic carbon dots: preparation, optical transition, and carbon dot-loading

In the past decade, hydrophobic fluorescent carbon dots (OCDs) have received little attention, and its potential application and light transition mechanism is seldom explored. Here we report a novel one-step approach for synthesizing blue- and green-emitting hydrophobic fluorescent carbon dots (OCD_b and OCD_g) by calcinating with the uses of citric acid and hexadecylamine as initial reactants. The optimal conditions for preparing OCD_b and OCD_g were obtained by using the Taguchi L25 (3⁵) orthogonal array. The highest quantum yield and product yield of OCDs reached 80.2% and 57.1%, respectively, larger than those from most of all the known reports. The fluorescent stability of OCD_b and OCD_g was excellent under UV irradiation (30 W) for days. The luminescent color of OCDs showed a great dependence on reaction conditions. It is easier to get OCD_g via a reaction kept at a high temperature for a long time. The optical transition mechanism was studied for the two kinds of color OCDs, and therefore proposed in combination with their optical properties and surface groups. The reason for light transition is probably related to an appropriate critical ratio and surface density of the C=O and N-H bond in the surface structure of the product. For the OCD_g, the concentration matching ratio of N-H and C=O bonds in the surface structure of the green-emitting product is approximately between d/2 and 3d/2, where d is a fixed constant. Lower than or higher than this critical ratio range, the product emits blue light. Based on their high fluorescence quantum efficiency and the advantages mentioned above, these OCDs were then respectively used for preparing hydrophobic fluorescent carbon dot-loading liposomes and acrylate films, both exhibiting a perfect performance with no fluorescence quenching.

Rational Design and Construction of Cocatalysts for Semiconductor-Based Photo-Electrochemical Oxygen Evolution: A Comprehensive Review

Photo-electrochemical (PEC) water splitting, as an essential and indispensable research branch of solar energy applications, has achieved increasing attention in the past decades. Between the two photoelectrodes, the photoanodes for PEC water oxidation are mostly studied for the facile selection of n-type semiconductors. Initially, the efficiency of the PEC process is rather limited, which mainly results from the existing drawbacks of photoanodes such as instability and serious charge-carrier recombination. To improve PEC performances, researchers gradually focus on exploring many strategies, among which engineering photoelectrodes with suitable cocatalysts is one of the most feasible and promising methods to lower reaction obstacles and boost PEC water splitting ability. Here, the basic principles, modules of the PEC system, evaluation parameters in PEC water oxidation reactions occurring on the surface of photoanodes, and the basic functions of cocatalysts on the promotion of PEC performance are demonstrated. Then, the key progress of cocatalyst design and construction applied to photoanodes for PEC oxygen evolution is emphatically introduced and the influences of different kinds of water oxidation cocatalysts are elucidated in detail. Finally, the outlook of highly active cocatalysts for the photosynthesis process is also included.

Solvothermal synthesis of phosphorus and nitrogen doped carbon quantum dots as a fluorescent probe for iron(III)

Carbon quantum dots (CQDs) doped with phosphorus and nitrogen were prepared via a hydrothermal method starting from citric acid, urea and phosphoric acid in dimethylformamide solution. The size, morphology, surface composition, energy levels, and optical properties of the CQDs were characterized. They show both green down-conversion and up-conversion fluorescence. Ferric ions (Fe³⁺) are found to quench the fluorescence. Cyclic voltammetry was used to identify the HOMO and LUMO levels of the doped CQDs. The quenching mechanism, as confirmed by energy level calculations and absorption spectra, can be attributed to the selective coordination of Fe³⁺ by the surface functional groups on the CQDs. This facilitates the photo-induced electron transfer from the CQDs to the d orbitals of Fe³⁺. The CQDs are shown to be viable fluorescent probes for determination of Fe³⁺ with high selectivity and sensitivity. The assay has a linear response in the 0.1 μM to 0.9 μM Fe³⁺ concentration range and a 50 nM as limit of detection (at a S/N ratio of 3). Graphical abstract Fluorescence probe for determination of ferric ions based on carbon quantum dot quenching via chelation facilitate photo-electron transfer.