Recent Advances in Photocatalytic CO2 Reduction Using Earth‐Abundant Metal Complexes‐Derived Photocatalysts

Photochemical reduction of CO₂ with H₂O into energy‐rich chemicals using inexhaustible solar energy is an appealing strategy to simultaneously address the global energy and environmental issues. Earth‐abundant metal complexes show promising application in this field due to their easy availability, rich redox valence and tunable property. Great progress has been seen on catalytic reduction of CO₂ under visible light illumination employing earth‐abundant metal complexes and their hybrids as key contributors, especially for producing CO and HCOOH via the two‐electron reduction process. In this minireview, we will summarize and update advances on earth‐abundant metal complex‐derived photocatalytic system for visible‐light driven CO₂ photoreduction over the last 5 years. Homogeneous earth‐abundant metal complex photocatalysts and earth‐abundant metal complex derived hybrid photocatalysts were both presented with focus on efficient improvement strategy.     

Visible‐Light‐Induced Photoredox Catalysis with a Tetracerium‐Containing Silicotungstate

The development of visible‐light‐induced photocatalysts for chemoselective functional group transformations has received considerable attention. Polyoxometalates (POMs) are potential materials for efficient photocatalysts because their properties can be precisely tuned by changing their constituent elements and structures and by the introduction of additional metal cations. Furthermore, they are thermally and oxidatively more stable than the frequently utilized organometallic complexes. The visible‐light‐responsive tetranuclear cerium(III)‐containing silicotungstate TBA₆[{Ce(H₂O)}₂{Ce(CH₃CN)}₂(μ₄‐O)(γ‐SiW₁₀O₃₆)₂] (CePOM; TBA=tetra‐n‐butylammonium) has now been synthesized; when CePOM was irradiated with visible light (λ>400 nm), a unique intramolecular Ce^III‐to‐POM(W^VI) charge transfer was observed. With CePOM, the photocatalytic oxidative dehydrogenation of primary and secondary amines as well as the α‐cyanation of tertiary amines smoothly proceeded in the presence of O₂ (1 atm) as the sole oxidant.     

A Conductive Hybridization Matrix of RuO2 Two‐Dimensional Nanosheets: A Hybrid‐Type Photocatalyst

A universal methodology to efficiently improve the photocatalyst performance of semiconductors was developed by employing exfoliated RuO₂ two‐dimensional nanosheets as a conducting hybridization matrix. The hybridization with a RuO₂ nanosheet is easily achieved by crystal growth or electrostatically derived anchoring of semiconductor nanocrystals on the RuO₂ nanosheet. An enhanced chemical interaction of inorganic semiconductor with hydrophilic RuO₂ nanosheet is fairly effective in optimizing their photocatalytic activity and photostability by the enhancement of charge separation and charge mobility. The RuO₂‐containing nanohybrids show much better photocatalyst functionalities than do the graphene‐containing ones. The present study clearly demonstrates that hydrophilic RuO₂ nanosheets are superior hybridization matrices, over the widely used hydrophobic graphene nanosheets, for exploring new efficient hybrid‐type photocatalysts.     

Synthesis of Multifunctional Metal‐ and Metal Oxide Core@Mesoporous Silica Shell Structures by Using a Wet Chemical Approach

We demonstrate a facile wet chemical approach for fabricating spherical metal/metal‐oxide core@mesoporous silica shell hybrid nanoparticles with different core and shell thicknesses. Vertically aligned mesoporous silica (mSiO₂) shells were fabricated over the pre‐synthesized spherical SiO₂ nanoparticles through a three‐step strategy: 1) synthesis of core materials, 2) covering the core with an organic–inorganic composite layer, and 3) removing the organic template through calcinations in air. The mechanisms of hybrid structure formation are proposed. The multifunctional nature of the hybrid structures could be induced by incorporating guest ions/molecules, such as Ag, Mn, and TiO₂, into the pores of an mSiO₂ shell. Mn and TiO₂ cluster‐ incorporated composite structures have been tested to be antioxidizing agents and effective photocatalysts through electron spin resonance, radical scavenging tests, and the photocatalytic degradation of rhodamine B. The possibility of incorporating several hetero‐element guest clusters in these mesoporous composite particles makes them highly attractive for multifunctional applications.     

Deciphering Photoluminescence Dynamics and Reactivity of the Luminescent Metal–Metal‐Bonded Excited State of a Binuclear Gold(I) Phosphine Complex Containing Open Coordination Sites

Luminescent metal complexes having open coordination sites hold promise in the design of sensory materials and photocatalysts. As a prototype example, [Au₂(dcpm)₂)]²⁺ (dcpm = bis(dicyclohexylphosphanyl) is known for its intriguing environmental sensitive photoluminescence. By integrating a range of complementary ultrafast time‐resolved spectroscopy to interrogate the excited state dynamics, this study uncovers that the events occurring in extremely rapid timescales and which are modulated strongly by environmental conditions play a pivotal role in the luminescence behavior and photochemical outcomes. Formed independent of the phase and solvent property within ～0.15 ps, the metal–metal bonded ³5dσ*6pσ state is highly reactive possessing strong propensity toward increasing coordination number at Au^I center, and with ～510 ps lifetime in dichloromethane is able to mediate light induced C–X bond cleavage.     

Coordination Chemistry of [Co(acac)2] with N‐Doped Graphene: Implications for Oxygen Reduction Reaction Reactivity of Organometallic Co‐O4‐N Species

Hybridization of organometallic complexes with graphene‐based materials can give rise to enhanced catalytic performance. Understanding the chemical structures within hybrid materials is of primary importance. In this work, archetypical hybrid materials are synthesized by the reaction of an organometallic complex, [Co^II(acac)₂] (acac=acetylacetonate), with N‐doped graphene‐based materials at room temperature. Experimental characterization of the hybrid materials and theoretical calculations reveal that the organometallic cobalt‐containing species is coordinated to heterocyclic groups in N‐doped graphene as well as to its parental acac ligands. The hybrid material shows high electrocatalytic activity for the oxygen reduction reaction (ORR) in alkaline media, and superior durability and methanol tolerance to a Pt/C catalyst. Based on the chemical structures and ORR experiments, the catalytically active species is identified as a Co‐O₄‐N structure.     

Rational Design of MOF/COF Hybrid Materials for Photocatalytic H2 Evolution in the Presence of Sacrificial Electron Donors

Crystalline and porous covalent organic frameworks (COFs) and metal‐organic frameworks (MOFs) materials have attracted enormous attention in the field of photocatalytic H₂ evolution due to their long‐range order structures, large surface areas, outstanding visible light absorbance, and tunable band gaps. In this work, we successfully integrated two‐dimensional (2D) COF with stable MOF. By covalently anchoring NH₂‐UiO‐66 onto the surface of TpPa‐1‐COF, a new type of MOF/COF hybrid materials with high surface area, porous framework, and high crystallinity was synthesized. The resulting hierarchical porous hybrid materials show efficient photocatalytic H₂ evolution under visible light irradiation. Especially, NH₂‐UiO‐66/TpPa‐1‐COF (4:6) exhibits the maximum photocatalytic H₂ evolution rate of 23.41 mmol g^?1 h^?1 (with the TOF of 402.36 h^?1), which is approximately 20 times higher than that of the parent TpPa‐1‐COF and the best performance photocatalyst for H₂ evolution among various MOF‐ and COF‐based photocatalysts.     

Design of Advanced Functional Materials Using Nanoporous Single‐Site Photocatalysts

Nanoporous silica solids can offer opportunities for hosting photocatalytic components such as various tetra‐coordinated transition metal ions to form systems referred to as “single‐site photocatalysts”. Under UV/visible‐light irradiation, they form charge transfer excited states, which exhibit a localized charge separation and thus behave differently from those of bulk semiconductor photocatalysts exemplified by TiO₂. This account presents an overview of the design of advanced functional materials based on the unique photo‐excited mechanisms of single‐site photocatalysts. Firstly, the incorporation of single‐site photocatalysts within transparent porous silica films will be introduced, which exhibit not only unique photocatalytic properties, but also high surface hydrophilicity with self‐cleaning and antifogging applications. Secondary, photo‐assisted deposition (PAD) of metal precursors on single‐site photocatalysts opens up a new route to prepare nanoparticles. Thirdly, visible light sensitive photocatalysts with single and/or binary oxides moieties can be prepared so as to use solar light, the ideal energy source.     

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.     

Wintersweet‐Flower‐Like CoFe2O4/MWCNTs Hybrid Material for High‐Capacity Reversible Lithium Storage

CoFe₂O₄/multiwalled carbon nanotubes (MWCNTs) hybrid materials were synthesized by a hydrothermal method. Field emission scanning electron microscopy and transmission electron microscopy analysis confirmed the morphology of the as‐prepared hybrid material resembling wintersweet flower “buds on branches”, in which CoFe₂O₄ nanoclusters, consisting of nanocrystals with a size of 5–10 nm, are anchored along carbon nanotubes. When applied as an anode material in lithium ion batteries, the CoFe₂O₄/MWCNTs hybrid material exhibited a high performance for reversible lithium storage. In particular, the hybrid anode material delivered reversible lithium storage capacities of 809, 765, 539, and 359 mA h g^?1 at current densities of 180, 450, 900, and 1800 mA g^?1, respectively. The superior performance of CoFe₂O₄/MWCNTs hybrid materials could be ascribed to the synergistic pinning effect of the wintersweet‐flower‐like nanoarchitecture. This strategy could also be applied to synthesize other metal oxide/CNTs hybrid materials as high‐capacity anode materials for lithium ion batteries.     

Visible‐Light‐Induced Electron Transport from Small to Large Nanoparticles in Bimodal Gold Nanoparticle‐Loaded Titanium(IV) Oxide

A key to realizing the sustainable society is to develop highly active photocatalysts for selective organic synthesis effectively using sunlight as the energy source. Recently, metal‐oxide‐supported gold nanoparticles (NPs) have emerged as a new type of visible‐light photocatalysts driven by the excitation of localized surface plasmon resonance of Au NPs. Here we show that visible‐light irradiation (λ>430 nm) of TiO₂‐supported Au NPs with a bimodal size distribution (BM‐Au/TiO₂) gives rise to the long‐range (>40 nm) electron transport from about 14 small (ca. 2 nm) Au NPs to one large (ca. 9 nm) Au NP through the conduction band of TiO₂. As a result of the enhancement of charge separation, BM‐Au/TiO₂ exhibits a high level of visible‐light activity for the one‐step synthesis of azobenzenes from nitrobenzenes at 25 °C with a yield greater than 95 % and a selectivity greater than 99 %, whereas unimodal Au/TiO₂ (UM‐Au/TiO₂) is photocatalytically inactive.     

N‐Methyl‐2, 2'‐Dipicolylamine Complexes as Potential Models for Metal‐Mediated Base Pairs

Metal‐mediated base pairs can be used to insert metal ions into nucleic acids at precisely defined positions. As structural data on the resulting metal‐modified DNA are scarce, appropriate model complexes need to be synthesized and structurally characterized. Accordingly, the molecular structures of nine transition metal complexes of N‐methyl‐2, 2'‐dipicolylamine (dipic) are reported. In combination with an azole‐containing artificial nucleoside, this tridentate ligand had recently been used to generate metal‐mediated base pairs (Chem. Commun. 2011 , 47, 11041–11043). The Pd^II and Pt^II complexes reported here confirm that the formation of planar complexes (as required for a metal‐mediated base pair) comprising N‐methyl‐2, 2'‐dipicolylamine is possible. Two Hg^II complexes with differing stoichiometry indicate that a planar structure might also be formed with this metal ion, even though it is not favored. In the complex [Ag₂(dipic)₂](ClO₄)₂, the two Ag^I ions are located close to one another with an Ag ··· Ag distance of 2.9152(3) Å, suggesting the presence of a strong argentophilic interaction.     

Round‐the‐Clock Photocatalytic Hydrogen Production with High Efficiency by a Long‐Afterglow Material

Long afterglow materials can store and release light energy after illumination. A brick‐like, micrometer‐sized Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺ long‐afterglow material is used for hydrogen production by the photocatalytic reforming of methanol under round‐the‐clock conditions for the first time, achieving a solar‐to‐hydrogen (STH) conversion efficiency of 5.18 %. This material is one of the most efficient photocatalysts and provides the possibility of practical use on a large scale. Its remarkable photocatalytic activity is attributed to its unique carrier migration path and large number of lattice defects. These findings expand the application scope of long afterglow materials and provide a new strategy to design efficient photocatalysts by constructing trap levels that can prolong carrier lifetimes.     

Synthesis and characterization of graphene oxide‐based hybrid ligand and its metal complexes: Highly efficient sensor and catalytic properties

A new graphene oxide‐based hybrid material (HL) and its Co(II), Cu(II) and Ni(II) metal complexes were prepared. Firstly, graphene oxide and (3‐aminopropyl)trimethoxysilane were reacted to give graphene oxide–3‐(aminopropyl)trimethoxysilane (GO‐APTMS) hybrid material. After that, hybrid material HL was synthesized from the reaction of GO‐APTMS and 2,6‐diformyl‐4‐methylphenol. Finally, Co(II), Cu(II) and Ni(II) complexes of HL were obtained. All the materials were characterized using various techniques. The chemosensor properties of HL were investigated against Na⁺, K⁺, Cd²⁺, Co²⁺, Cu²⁺, Hg²⁺, Ni²⁺, Zn²⁺, Al³⁺, Cr³⁺, Fe³⁺ and Mn³⁺ ions and it was found that HL has selective chemosensing to Fe³⁺ ion. All the graphene oxide‐supported complexes were used as heterogeneous catalysts in the oxidation of 2‐methylnaphthalene (2MN) to 2‐methyl‐1,4‐naphthoquinone (vitamin K₃, menadione) in the presence of hydrogen peroxide, acetic acid and sulfuric acid. The Cu(II) complex showed good catalytic properties compared to the literature. The selectivity of 2MN to vitamin K₃ was 60.23% with 99.75% conversion using the Cu(II) complex.     

Chemosensing hybrid materials: Chalcones‐functionalized cage‐like mesoporous SBA‐16 material for highly selective detection of toxic metal ions

Highly selective and low‐cost optical nanosensors of organic–inorganic hybrid materials for heavy metal ions detection have been prepared via the functionalization of mesoporous silica (SBA‐16) with chalcone fluorescent chromophores. The successful attachment of organic chalcone moieties and preservation of original structure of SBA‐16 after the anchoring process were confirmed by extensive characterizations using various techniques like Fourier transform infrared and UV–visible spectroscopies, transmission electron microscopy, nitrogen adsorption–desorption isotherms, low‐angle X‐ray diffraction and thermogravimetric analysis. The colorimetric behaviour, selectivity and sensitivity were also investigated. The optical nanosensors respond selectively to heavy metal ions, such as Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Hg²⁺, with observable colour changes in 0.01 M Tris–HCl aqueous buffer solution. Also, the optical sensing ability of the investigated nanosensors to the mentioned metal ions was investigated using steady‐state absorption and emission techniques. Significant increase in the absorption spectra and a static quenching in the emission spectra are observed upon adding various concentrations of the studied metal ions. The spectral changes as well as the observable colour changes suggest that the investigated nanosensors are suitable for simple, economic, online analysis and remote design of these toxic metal ions with fast kinetic responses. Finally, the low detection limits for all the studied metals are in good agreement with those recommended by both the US Environmental Protection Agency and World Health Organization, except for Hg²⁺ and Cd²⁺, indicating that the investigated nanosensors have hypersensitivity, selectivity and better recognition for all the studied metal ions.     

Engineering Fast Ion Conduction and Selective Cation Channels for a High‐Rate and High‐Voltage Hybrid Aqueous Battery

The rechargeable aqueous metal‐ion battery (RAMB) has attracted considerable attention due to its safety, low costs, and environmental friendliness. Yet the poor‐performance electrode materials lead to a low feasibility of practical application. A hybrid aqueous battery (HAB) built from electrode materials with selective cation channels could increase the electrode applicability and thus enlarge the application of RAMB. Herein, we construct a high‐voltage K–Na HAB based on K₂FeFe(CN)₆ cathode and carbon‐coated NaTi₂(PO₄)₃ (NTP/C) anode. Due to the unique cation selectivity of both materials and ultrafast ion conduction of NTP/C, the hybrid battery delivers a high capacity of 160 mAh g^?1 at a 0.5 C rate. Considerable capacity retention of 94.3 % is also obtained after 1000 cycles at even 60 C rate. Meanwhile, high energy density of 69.6 Wh kg^?1 based on the total mass of active electrode materials is obtained, which is comparable and even superior to that of the lead acid, Ni/Cd, and Ni/MH batteries.     

Efficient Solar‐Driven Nitrogen Fixation over Carbon–Tungstic‐Acid Hybrids

Ammonia synthesis under mild conditions is of supreme interest. Photocatalytic nitrogen fixation with water at room temperature and atmospheric pressure is an intriguing strategy. However, the efficiency of this method has been far from satisfied for industrialization, mainly due to the sluggish cleavage of the N≡N bond. Herein, we report a carbon–tungstic‐acid (WO₃ ? H₂O) hybrid for the co‐optimization of N₂ activation as well as subsequent photoinduced protonation. Efficient ammonia evolution reached 205 μmol g^?1 h^?1 over this hybrid under simulated sunlight. Nitrogen temperature‐programmed desorption revealed the decisive role of carbon in N₂ adsorption. Photoactive WO₃ ? H₂O guaranteed the supply of electrons and protons for subsequent protonation. The universality of carbon modification for enhancing the N₂ reduction was further verified over various photocatalysts, shedding light on future materials design for ideal solar energy utilization.     

Selective Metal‐vapor Deposition on Organic Surfaces

Selective metal‐vapor deposition signifies that metal‐vapor atoms are deposited on a hard organic surface, but not on a soft (low glass transition temperature, low T_g) surface. In this paper, we introduce the origin, extension, and applications of selective metal‐vapor deposition. An amorphous photochromic diarylethene film shows light‐controlled selective metal‐vapor deposition, which is caused by a large T_g change based on photoisomerization, but various organic surfaces, including organic crystal and polymers, can be utilized for achieving selective metal‐vapor deposition. Various applications of selective metal‐vapor deposition, including cathode patterning of organic light‐emitting devices, micro‐thin‐film fuses, multifunctional diffraction gratings, in‐plane electrical bistability for memory devices, and metal‐vapor integration, have been demonstrated.     

Synthesis and electrical conductivity of some poly[4‐amino‐2,6‐pyrimidinodi‐thiocarbamate]–metal complexes

Poly[4‐amino‐2,6‐pyrimidinodithiocarbamate] was prepared from the reaction of 2‐mercapto‐4,6‐diaminopyrimidine with carbon disulfide, followed by condensation through the removal of H₂S gas. Five polymer–metal complexes of manganese, ferrous, ferric, zinc and mercury were then prepared. The polymer–metal complexes are investigated by elemental analyses, ultraviolet Fourier transform infrared and magnetic susceptibility. The DC electrical conductivity variation with the temperature in the region 298–498 K of the five polymer–metal complexes was determined. Doping with 5% ZnCl₂ increased the electrical conductivity of the polymer at all temperatures investigated. All the polymer–metal complexes showed an increase in conductivity with an increase in temperature, which is a typical semiconductor behavior. The proposed structure of the complexes is (MLX₂·mH₂O)_n. All the polymer–metal complexes are thermally stable, are insoluble in common organic solvents and have high melting points. Copyright © 1999 John Wiley & Sons, Ltd.     

Boosting the Catalytic Performance of Metal–Organic Frameworks for Steroid Transformations by Confinement within a Mesoporous Scaffold

Solid‐state crystallization achieves selective confinement of metal–organic framework (MOF) nanocrystals within mesoporous materials, thereby rendering active sites more accessible compared to the bulk‐MOF and enhancing the chemical and mechanical stability of MOF nanocrystals. (Zr)UiO‐66(NH₂)/SiO₂ hybrid materials were tested as efficient and reusable heterogeneous catalysts for the synthesis of steroid derivatives, outperforming the bulk (Zr)UiO‐66(NH₂) MOF. A clear correlation between the catalytic activity of the dispersed Zr sites present in the confined MOF, and the loading of the mesoporous SiO₂, is demonstrated for steroid transformations.