Template-Free Synthesis of Cube-like Ag/AgCl Nanostructures via a Direct-Precipitation Protocol: Highly Efficient Sunlight-Driven Plasmonic Photocatalysts

In this paper, we report that cube-like Ag/AgCl nanostructures could be facilely fabricated in a one-pot manner through a direct-precipitation protocol under ambient conditions, wherein no additional issues such as external energy (e.g., high temperature or high pressure), surfactants, or reducing agents are required. In terms of using sodium chloride (NaCl) as chlorine source and silver acetate (CH₃COOAg) as silver source, it is disclosed that simply by adding an aqueous solution of NaCl into an aqueous solution of CH₃COOAg, Ag/AgCl nanostructures with a cube-like geometry, could be successfully formulated. We show that thus-formulated cube-like Ag/AgCl nanospecies could be used as high-performance yet durable visible-light-driven or sunlight-driven plasmonic photocatalysts for the photodegradation of methyl orange (MO) and 4-chlorophenol (4–CP) pollutants. Compared with the commercially available P25–TiO₂, and the Ag/AgCl nanospheres previously fabricated via a surfactant-assisted method, our current cube-like Ag/AgCl nanostructures could exhibit much higher photocatalytic performance. Our template free protocol might open up new and varied opportunities for an easy synthesis of cube-like Ag/AgCl-based high-performance sunlight-driven plasmonic photocatalysts for organic pollutant elimination

Photoreductive Debromination of Decabromodiphenyl Ethers in the Presence of Carboxylates under Visible Light Irradiation

Polybrominated diphenyl ethers (PBDEs) have aroused global environmental concerns because of their toxicity and ubiquitousness in the biological and environmental systems. It is important to find an efficient method for their decontamination and to understand their chemical transformation in the environment. Here, we report that decabromodiphenyl ether (BDE209) undergoes efficient reductive debromination reactions under visible-light irradiation (≥420 nm) in the presence of various carboxylate anions that are common in the environmental media. The debromination reactions occur in a stepwise manner, producing a series of lower brominated PBDE congeners. Solvent-derived radials are observed by spin-trapping electron spin resonance (ESR) experiments during the photoreaction. Further experiments by the UV–vis absorption and isothermal titration calorimetry (ITC), combined with theoretical calculations, reveal a new photochemical debromination pathway based on the halogen binding interaction. According to this pathway, the formation of halogen-binding-based complex between PBDE and carboxylate enables the visible-light absorption and debromination of PBDEs, although neither PBDEs nor carboxylates have visible-light absorption. The halogen-bond-based photochemical debromination could find its application for our better understanding of the transformation process of PBDEs in the environment

Photocatalytic Degradation of Aromatic Pollutants: A Pivotal Role of Conduction Band Electron in Distribution of Hydroxylated Intermediates

The modulation of the yield distribution of intermediates formed in the photocatalytic degradation of organic pollutants is of extreme importance for the application of photocatalysis in environmental cleanup, as different intermediates usually exhibit distinct biological toxicity and secondary reactivity. In this paper, we report that the distribution of monohydroxylated intermediates (<i>m</i>-, <i>p</i>- and <i>o</i>-) formed during the photocatalytic oxidation of aromatic compounds changes with the variation of reaction conditions, such as O₂ partial pressure and substrate concentration. By detailed product analysis, theoretical calculation, and oxygen isotope labeling experiments, we show that these changes are due to the selective reduction of HO-adduct radicals (the precursors of hydroxylated intermediates) by conduction band electrons (e_cb^–) back to the original substrate, that is, <i>p</i>- and <i>o</i>-HO-adduct radicals are more susceptible to e_cb^– than the <i>m</i>- one. Our experiments give an example that, even under oxidative conditions, the yield distribution of isomeric intermediates can be modulated by e_cb^–-initiated reduction. This study also illustrates that the unique redox characteristics of photocatalysis, that is, both oxidation and reduction reactions take place on or near the surface of a single nanoparticle, can provide opportunities for the reaction control

Facile Large-Scale Synthesis of Urea-Derived Porous Graphitic Carbon Nitride with Extraordinary Visible-Light Spectrum Photodegradation

We report the large-scale synthesis of porous graphitic carbon nitride (g-C₃N₄) in a direct heat treatment process by controlling the thermal condensation temperature of the low-cost urea precursor. An excellent linear relation between the yield of the urea-derived porous g-C₃N₄ (U-g-C₃N₄) and the input urea was experimentally demonstrated, and consequently, a large-scale yield >50 g in a batch was readily achieved. A series of morphology and structure characterizations revealed the actual evolutionary process of the temperature-dependent porous architecture of U-g-C₃N₄ and its inherent superiority. Furthermore, we demonstrated the extraordinary visible-light-driven photodegradation activity of large-scale U-g-C₃N₄ toward organic pollutants such as rhodamine B, safranine T, and α-naphthol. Such superior photodegradation performance and long-term photocatalytic stability, together with a scalable preparation method, may render as-fabricated U-g-C₃N₄ as a promising candidate for practical application in environmental remediation

Determining the TiO₂‑Photocatalytic Aryl-Ring-Opening Mechanism in Aqueous Solution Using Oxygen-18 Labeled O₂ and H₂O

The molecules O₂ and H₂O dominate the cleavage of aromatic sp² C–C bonds, a crucial step in the degradation of aromatic pollutants in aqueous TiO₂ photocatalysis, but their precise roles in this process have remained elusive. This can be attributed to the complex oxidative species involved and to a lack of available models for reactions with a high yield of direct products. Here, we used oxygen-18 isotope labeled O₂ and H₂O to observe the aromatic ring-opening reaction of the model compound 3,5-di-<i>tert</i>-butylcatechol (DTBC), which was mediated by TiO₂ photocatalysis in an aqueous acetonitrile solution. By analyzing the primary intermediate products (∼75% yield), especially the seven-membered ring anhydrides that were formed, we obtained direct evidence for the oxygen atom of dioxygen insertion into a C–C bond of the aromatic ring. This indicates that molecular oxygen is the ultimate ring-opening agent in TiO₂ photocatalysis and that it undergoes single O atom incorporation rather than the previously proposed molecular oxygen 1,2-addition processes. The ratio of intradiol to extradiol products depends on the particle size of TiO₂ catalysts used, which suggests that the O₂ activation is correlated with the available coordination sites on the TiO₂ surface in the photocatalytic cleavage of the aromatic ring

Controllable Synthesis of 3D Thorny Plasmonic Gold Nanostructures and Their Tunable Optical Properties

Three-dimensional (3D) thorny plasmonic gold nanostructures were synthesized by adding Ag nanospheres to the reaction systems containing HAuCl₄ and NH₂OH at room temperature. The redox displacement between silver seeds and HAuCl₄ first yielded gold nanoseeds, and the simultaneously formed byproduct AgCl precipitates controlled the growth of the produced gold nanoseeds on the basis of the surface-catalyzed reduction of AuCl₄^– by NH₂OH. When the total amount of the reactants was fixed, the morphology of thorny gold nanostructures could be easily controlled via changing the volume of the reaction system, the reaction temperature, or the manner of introducing the growth solution, all of which altered the reaction rate and consequently the crystal growth pathway of final products. The resulting gold nanostructures exhibited a distinctive surface plasmon resonance band in the visible and near-IR region depending on their morphologies, which could be readily controlled by varying the experimental parameters. This study opens a new route for fabricating metal nanoparticles with designed optical and structural properties

Pivotal Role and Regulation of Proton Transfer in Water Oxidation on Hematite Photoanodes

Hematite is a promising material for solar water splitting; however, high efficiency remains elusive because of the kinetic limitations of interfacial charge transfer. Here, we demonstrate the pivotal role of proton transfer in water oxidation on hematite photoanodes using photoelectrochemical (PEC) characterization, the H/D kinetic isotope effect (KIE), and electrochemical impedance spectroscopy (EIS). We observed a concerted proton–electron transfer (CPET) characteristic for the rate-determining interfacial hole transfer, where electron transfer (ET) from molecular water to a surface-trapped hole was accompanied by proton transfer (PT) to a solvent water molecule, demonstrating a substantial KIE (∼3.5). The temperature dependency of KIE revealed a highly flexible proton transfer channel along the hydrogen bond at the hematite/electrolyte interface. A mechanistic transition in the rate-determining step from CPET to ET occurred after OH^– became the dominant hole acceptor. We further modified the proton–electron transfer sequence with appropriate proton acceptors (buffer bases) and achieved a greater than 4-fold increase in the PEC water oxidation efficiency on a hematite photoanode

Silver Iodide Microstructures of a Uniform Towerlike Shape: Morphology Purification via a Chemical Dissolution, Simultaneously Boosted Catalytic Durability, and Enhanced Catalytic Performances

The fabrication of microstructures/nanostructures of a uniform yet well-defined morphology has attracted broad interest from a variety of fields of advanced functional materials, especially catalysts. Most of the conventional methods generally suffer from harsh synthesis conditions, requirement of bulky apparatus, or incapability of scalable production, etc. To meet these formidable challenges, it is strongly desired to develop a facile, cost-effective, scalable method to fulfill a morphology purification. By a precipitation reaction between AgNO₃ and KI, we report that irregular AgI structures, or their mixture with towerlike AgI architectures could be fabricated. Compared to the former, the mixed structures exhibit enhanced catalytic reactivity toward the photodegradation of Methyl Orange pollutant. However, its catalytic durability, which is one of the most crucial criteria that are required by superior catalysts, is poor. We further show that the irregular structures could be facilely removed from the mixture via a KI-assisted chemical dissolution, producing AgI of a uniform towerlike morphology. Excitingly, after such simple morphology purification, our towerlike AgI displays not only a boosted catalytic durability but also an enhanced catalytic reactivity. Our chemical dissolution-based morphology purification protocol might be extended to other systems, wherein high-quality advanced functional materials of desired properties might be developed

Hydrogen-Bond Bridged Water Oxidation on {001} Surfaces of Anatase TiO₂

To gain an atomic-level understanding of the relationship among the surface structure, the interfacial interaction, and the water oxidation activity on TiO₂, we studied the adsorption of water and its photocatalytic oxidation on anatase TiO₂ with {101} and {001} exposed surfaces by in situ infrared spectroscopy, kinetic isotope effect studies, and density functional theory (DFT)-based molecular dynamics calculations. Our experimental results demonstrate that the oxidation reaction occurs exclusively on hydrogen-bonded water molecules (via surface hydroxyls) over {001} surface, whereas water molecules coordinated on the {101} surface, which are conventionally assigned to the reactive target for hole transfer, remain unchanged during the irradiation. The theoretical calculations reveal that the selective oxidation of water adsorbed on the {001} surfaces is primarily attributed to the formation of hydrogen bonds, which provides a channel to the rapid hole transfer and facilitates the O–H bond cleavage during water oxidation

Brönsted Catalyzed Hydrolysis of Microcystin-LR by Siderite

Six naturally occurring minerals were employed to catalyze the hydrolysis of microcystin-LR (MC-LR) in water. After preliminary screening experiments, siderite stood out among these minerals due to its higher activity and selectivity. In comparison with kaolinite, which is known to act as a Lewis acid catalyst, siderite was found to act primarily as a Brönsted acid catalyst in the hydrolysis of MC-LR. More interestingly, we found that the presence of humic acid significantly inhibited catalytic efficiency of kaolinite, while the efficiency of siderite remained high (∼98%). Reaction intermediates detected by LC-ESI/MS were used to indicate cleavage points in the macrocyclic ring of MC-LR, and XPS was used to characterize siderite interaction with MC-LR. Detailed analysis of the <i>in situ</i> ATR-FTIR absorption spectra of MC-LR indicated hydrogen bonding at the siderite–water–MC-LR interface. A metastable ring, involving hydrogen bonding, between surface bicarbonate of siderite and an amide of MC-LR was proposed to explain the higher activity and selectivity toward MC-LR. Furthermore, siderite was found to reduce the toxicity of MC-LR to mice by hydrolyzing MC-LR peptide bonds. The study demonstrates the potential of siderite, an earth-abundant and biocompatible mineral, for removing MC-LR from water