Pivotal Role of Fluorine in Tuning Band Structure and Visible‐Light Photocatalytic Activity of Nitrogen‐Doped TiO2

Synergistic interactions between N and F can be used to tune the band structures of N‐doped TiO₂ (see scheme). Reduction potential of conduction band electron is positively shifted to weaken unwanted O₂ reduction, but enhances the target reduction reaction in air. The tuning also enhances the oxidation ability of the valence band to oxidize water to O₂, which makes photocatalytic reduction proceed without any organic sacrificial reagents.

     

On the Origin of the Visible‐Light Activity of Titanium Dioxide Doped with Carbonate Species

Plane‐wave‐based pseudopotential density functional theory (DFT) calculations are used to elucidate the origin of the high photocatalytic efficiency of carbonate‐doped TiO₂. Two geometrically possible doping positions are considered, including interstitial and substitutional carbon atoms on Ti sites. From the optical absorption properties calculations, we believe that the formation of carbonates after doping with interstitial carbon atoms is crucial, whereas the contribution from the cationic doping on Ti sites is negligible. The carbonate species doped TiO₂ exhibits excellent absorption in the visible‐light region of 400–800 nm, in good agreement with experimental observations. Electronic structure analysis shows that the carbonate species introduce an impurity state from Ti 3d below the conduction band. Excitations from the impurity state to the conduction band may be responsible for the high visible‐light activity of the carbon doped TiO₂ materials.     

A Comparison Study of Rhodamine B Photodegradation over Nitrogen‐Doped Lamellar Niobic Acid and Titanic Acid under Visible‐Light Irradiation

Slip between the sheets! The intercalation properties of lamellar solid acids have a profound impact on nitrogen doping as well as on the resultant visible‐light photocatalysis, and the effects depend strongly on the protonic acidities of the samples (see figure).

     

Visible‐Light‐Induced Click Chemistry

A rapid and catalyst‐free cycloaddition system for visible‐light‐induced click chemistry is reported. A readily accessible photoreactive 2H‐azirine moiety was designed to absorb light at wavelengths above 400 nm. Irradiation with low‐energy light sources thus enables efficient small‐molecule synthesis with a diverse range of multiple‐bond‐containing compounds. Moreover, in order to demonstrate the efficiency of the current approach, quantitative ligation of the photoactivatable chromophore with functional polymeric substrates was performed and full conversion with irradiation times of only 1 min at ambient conditions was achieved. The current report thus presents a highly efficient method for applications involving selective cycloaddition to electron‐deficient multiple‐bond‐containing materials.     

Visible‐Light Photocatalytic Hydrogen Generation by Using Dye‐Sensitized Graphene Oxide as a Photocatalyst

Dye‐sensitized graphene oxide is able to generate hydrogen from water/methanol mixtures (80:20) by using visible or solar light. The most efficient photocatalyst tested contained a tris(2,2‐bipyridyl) ruthenium(II) complex incorporated in the interlayer spaces of a few layers of graphene oxide with a moderate degree of oxidation. The graphene oxide‐based photocatalyst does not contain noble metals and we have determined that it is two orders of magnitude more active than catalysts based on conventional titania.     

Photochemical Construction of Carbonitride Structures for Red‐Light Redox Catalysis

Metal‐free carbonitride(CN) semiconductors are appealing light‐transducers for photocatalytic redox reactions owing to the unique band gap and stability. To harness solar energy efficiently, CN catalysts that are active over a wider range of the visible spectrum are desired. Now a photochemical approach has been used to prepare a new‐type triazine‐based CN structure. The obtained CN shows extraordinary light‐harvesting characteristics, with suitable semiconductor‐redox potentials. The light absorption edge of the CN reaches up to 735 nm, which is significantly longer than that of the conventional CN semiconductor at about 460 nm. As expected, the CN can efficiently catalyze oxidation of alcohols and reduction of CO₂ with visible light, even under red‐light irradiation. The results represent an important step toward the development of red‐light‐responsive triazine‐based structures for solar applications.     

Visible‐Light Photochemical Reduction of CO2 to CO Coupled to Hydrocarbon Dehydrogenation

Research on the photochemical reduction of CO₂, initiated already 40 years ago, has with few exceptions been performed by using amines as sacrificial reductants. Hydrocarbons are high‐volume chemicals whose dehydrogenation is of interest, so the coupling of a CO₂ photoreduction to a hydrocarbon‐photodehydrogenation reaction seems a worthwhile concept to explore. A three‐component construct was prepared including graphitic carbon nitride (g‐CN) as a visible‐light photoactive semiconductor, a polyoxometalate (POM) that functions as an electron acceptor to improve hole–electron charge separation, and an electron donor to a rhenium‐based CO₂ reduction catalyst. Upon photoactivation of g‐CN, a cascade is initiated by dehydrogenation of hydrocarbons coupled to the reduction of the polyoxometalate. Visible‐light photoexcitation of the reduced polyoxometalate enables electron transfer to the rhenium‐based catalyst active for the selective reduction of CO₂ to CO. The construct was characterized by zeta potential, IR spectroscopy, thermogravimetry, scanning electron microscopy (SEM) and energy dispersive X‐ray spectroscopy (EDS). An experimental Z‐scheme diagram is presented based on electrochemical measurements and UV/Vis spectroscopy. The conceptual advance should promote study into more active systems.     

Synthesis of N‐Containing Heterocyclic Compounds Using Visible‐light Photoredox Catalysis

Nitrogen heterocycles represent a highly important class of compounds which are widely used in materials science, agrochemistry, and medicinal chemistry. Therefore, there is continuing interest in the development of convenient, efficient, and environmentally benign synthetic methods for the construction of nitrogen containing heterocycles. Due to its natural abundance, ease of use, and promising application in industry, the use of visible light as a driving force for chemical reactions has received considerable attention in the past few years. This account summarizes the synthesis of N‐heterocycles using visible‐light photoredox catalysis published in the last two years, according to the size and type of the formed N‐heterocyclic rings. In the context of seminal works of others in this area, a concise summary of the contributions of the authors is also offered.     

Deracemization Enabled by Visible‐Light Photocatalysis

Deracemization is an ideal but challenging strategy for the conversion of a racemic mixture into a single enantiomer. Recent studies have demonstrated that visible‐light photocatalysis could be utilized to promote selective deracemization of axially chiral allenes as well as cyclopropylquinolones and cyclic ureas with central chirality either through energy transfer or through a sequence of electron, proton, and hydrogen‐atom transfer.     

Ultrathin g‐C3N4 Nanosheets Coupled with AgIO3 as Highly Efficient Heterostructured Photocatalysts for Enhanced Visible‐Light Photocatalytic Activity

The photocatalytic activity of graphite‐like carbon nitride (g‐C₃N₄) could be enhanced by heterojunction strategies through increasing the charge‐separation efficiency. As a surface‐based process, the heterogeneous photocatalytic process would become more efficient if a larger contact region existed in the heterojunction interface. In this work, ultrathin g‐C₃N₄ nanosheets (g‐C₃N₄‐NS) with much larger specific surface areas are employed instead of bulk g‐C₃N₄ (g‐C₃N₄‐B) to prepare AgIO₃/g‐C₃N₄‐NS nanocomposite photocatalysts. By taking advantage of this feature, the as‐prepared composites exhibit remarkable performances for photocatalytic wastewater treatment under visible‐light irradiation. Notably, the optimum photocatalytic activity of AgIO₃/g‐C₃N₄‐NS composites is almost 80.59 and 55.09 times higher than that of pure g‐C₃N₄‐B towards the degradation of rhodamine B and methyl orange pollutants, respectively. Finally, the stability and possible photocatalytic mechanism of the AgIO₃/g‐C₃N₄‐NS system are also investigated.     

Improvement of the Visible‐Light Photocatalytic Performance of TiO2 by Carbon Mesostructures

An improvement in the photodegradation performance for dyes due to interaction between carbon and titania in a self‐assembled mesoporous C? TiO₂ composite catalyst, even for the difficult degradation of azo dyes, is reported herein. The dye removal process involves adsorption of the dye from water by the mesoporous carbon–titania, followed by photodegradation on the separated dye‐loaded solid. Such adsorption–catalysis cycles can be carried out more than 80 times without discernible loss of photocatalytic activity or the anatase content of the composite. In each run, about 120 mg dye per g catalyst can be degraded. The mesoporous carbon–titania catalyst also exhibits a high capacity for converting methyl orange in aqueous solution under visible light. Characterization by XRD, TEM, and N₂ sorption techniques has revealed that the self‐assembled composite catalyst has an ordered mesostructure, uniform mesopores (4.3 nm), a large pore volume (0.30 cm³ g^?1), and a high surface area (348 m² g^?1). The pore walls are composed of amorphous carbon and anatase nanoparticles of size 4.2 nm, which are well dispersed and confined. X‐ray photoelectron spectroscopy (XPS), surface photovoltage spectroscopy (SPS), and UV/Vis absorption results indicate doping of carbon into the anatase lattice and a change in the bandgap of the semiconductor. The synergistic improvement in the composite catalyst can be attributed to the following features: (1) carbon doping of the anatase lattice modifies its bandgap and enhances its activity under visible light; (2) confinement within carbon pore walls prevents aggregation of tiny anatase nanoparticles, improving their activity and stability; (3) the mesopores provide a confined space for photocatalysis; and (4) the strong adsorption ability of porous carbon for organic substances ensures that large quantities can be processed and inhibits further diffusion of the adsorbed organic substances, thereby enhancing the mineralization on anatase.     

Visible‐Light‐Induced Direct Photocatalytic Carboxylation of Indoles with CBr4/MeOH

Photocatalysis enables the cascade reactions of indoles and CBr₄ in MeOH through a C(sp²)?H functionalization/methanolysis sequence. The title reaction provides an efficient access to indole 2‐ and 3‐carboxylates in a single operation (no preinstallation of protecting as well as directing groups was required) with good yields under mild reaction conditions.     

Visible‐Light Promoted Distereodivergent Intramolecular Oxyamidation of Alkenes

The visible‐light‐promoted diastereodivergent intramolecular oxyamination of alkenes is described to construct oxazolindinones, pyrrolidinones and imidazolidones via mild generation of primary amidyl radicals from functionalized hydroxylamines. A unique phenomenon of highly diastereoselective ring‐opening of aziridines controlled by electron sacrifices was observed. Highly diastereoselective amino alcohols derivatives were obtained efficiently through this protocol in gram scales. The mechanistic studies suggested the isolatable anti‐aziridine intermediates were generated quickly from primary amidyl radicals and the diastereoselectivities were controlled by pK_a values of the electron sacrifices.     

Visible‐Light/Photoredox‐Mediated sp3 CH Functionalization and Coupling of Secondary Amines with Vinyl Azides in Flow Microreactors

Structurally diverse imidazole derivatives were synthesized by a visible‐light/[Ru(bpy)₃][(PF₆)₂]‐mediated coupling of vinyl azides and secondary amines in flow microreactors. This operationally simple and atom‐economical protocol allows the formation of three new C?N bonds through the functionalization of sp³ C?H bonds adjacent to the secondary nitrogen atom. In order to get mechanistic insight of the coupling reaction, several control experiments were carried out and discussed.     

Integrating TEMPO and Its Analogues with Visible‐Light Photocatalysis

Visible light has risen to become a very important facilitator for selective radical reactions enabled by well‐cognized photocatalysts. The renaissance of visible‐light photocatalysis on this matter partly relies on integrating it with other fields of catalysis. In parallel, 2,2,6,6‐tetramethylpiperidin N‐oxide (TEMPO), a quintessential persistent radical, has a wide range of uses owing to its exceptional redox behavior, which gives rise to its latest prominence in catalysis. Therefore, integrating the catalysis of TEMPO with photocatalysis to perform visible‐light‐induced selective reactions becomes a very convenient marriage of merits. In this context, the integration of different types of photocatalysts, including metal complexes, metal‐free organic dyes, and semiconductors, with TEMPO for outstanding organic transformations will be summarized. To expand further the catalytic repertoire, the integration of TEMPOH analogues such as NHPI (N‐hydroxyphthalimide) and NHS (N‐hydroxysuccinimide) with photocatalysis will also be discussed. Hopefully, these advances will pave the way for more breakthroughs by integrating TEMPO and its analogues with photocatalysis to lead to a valuable blueprint for visible‐light‐induced selective organic transformations.     

Controlled Trifluoromethylation Reactions of Alkynes through Visible‐Light Photoredox Catalysis

The control of a reaction that can form multiple products is a highly attractive and challenging concept in synthetic chemistry. A set of valuable CF₃‐containing molecules, namely trifluoromethylated alkenyl iodides, alkenes, and alkynes, were selectively generated from alkynes and CF₃I by environmentally benign and efficient visible‐light photoredox catalysis. Subtle differences in the combination of catalyst, base, and solvent enabled the control of reactivity and selectivity for the reaction between an alkyne and CF₃I.     

CuO Quantum‐Dot‐Sensitized Mesoporous ZnO for Visible‐Light Photocatalysis

How to extend ultraviolet photocatalysts to the visible‐light region is a key challenge for solar‐driven photocatalysis. Herein, we show that ultraviolet ZnO photocatalysts can present high visible‐light photocatalytic activity when combined with CuO quantum dots (QDs; <3 nm). Theoretical analysis demonstrates that the quantum size effect plays a key role in the photoactivity of the CuO/ZnO composite. For CuO QDs smaller than 3 nm, the separated charges could transfer from CuO QDs to the conduction bands of ZnO due to quantum splitting of the CuO energy level and phonon compensation for the difference in the conduction band minimum of CuO and ZnO; however, this process would not occur with the disappearance of the quantum size effect. Further structural analysis demonstrates that interfacial charge separation and transfer between ZnO and CuO dominate the photocatalytic processes instead of a single CuO or ZnO surface. Compared with ZnO? noble metal structures (e.g., ZnO? Ag or ZnO? Au), these ZnO? CuO QD composites present wider absorption bands, higher visible photocatalytic efficiencies, and lower costs.