Rational Design of Crystalline Covalent Organic Frameworks for Efficient CO2 Photoreduction with H2O

Solar energy‐driven conversion of CO₂ into fuels with H₂O as a sacrificial agent is a challenging research field in photosynthesis. Herein, a series of crystalline porphyrin‐tetrathiafulvalene covalent organic frameworks (COFs) are synthesized and used as photocatalysts for reducing CO₂ with H₂O, in the absence of additional photosensitizer, sacrificial agents, and noble metal co‐catalysts. The effective photogenerated electrons transfer from tetrathiafulvalene to porphyrin by covalent bonding, resulting in the separated electrons and holes, respectively, for CO₂ reduction and H₂O oxidation. By adjusting the band structures of TTCOFs, TTCOF‐Zn achieved the highest photocatalytic CO production of 12.33 μmol with circa 100 % selectivity, along with H₂O oxidation to O₂. Furthermore, DFT calculations combined with a crystal structure model confirmed the structure–function relationship. Our work provides a new sight for designing more efficient artificial crystalline photocatalysts.     

硼掺杂SiC的制备、表征及其可见光分解水产氢性能

采用原位碳热还原法制备了硼掺杂的β-SiC (B_xSiC)光催化剂,并考察了其可见光下光催化分解水制氢的性能. 利用X射线衍射仪、X射线光电子能谱、扫描电镜及紫外-可见吸收光谱等测试方法对所制备催化剂的晶型、形貌、表面性质及能带结构进行了表征. 分析结果表明,硼原子掺杂进入SiC 晶格并取代了Si 位点,在价带上方形成了浅受主能级,从而导致了带隙宽变窄. 浅受主能级作为空穴的捕获中心可抑制光生电子和空穴的复合. 因此,与SiC相比,硼掺杂SiC光催化剂在可见光下催化分解水产氢的活性大大提高. 当B/Si 的摩尔比为0.05时,硼掺杂SiC表现出最高的光催化产氢活性.     

Fabrication of Z-Scheme WO3/KNbO3 Photocatalyst with Enhanced Separation of Charge Carriers

Z-Scheme photocatalysts as a research focus perform strong redox capability and high photocatalytic performance. WO₃/KNbO₃ photocatalysts were fabricated by ball milling method, and performed higher photocatalytic activity in liquid degradation(rhodamine B, methylene blue and bisphenol A), compared with WO₃ or KNbO₃ monomer. This is due to that Z-scheme heterojunction is formed between WO₃ and KNbO₃, and the holes photo-excited in valence band of KNbO₃ are quickly combined with the electrons in conduction band of WO₃. The electrons accumulated in conduction band of KNbO₃ show high reducibility, thereby reducing O₂ to ^·O₂^-, and the holes in valence band of WO₃ show high oxidative to oxidize H₂O to ^·OH, respectively. Furthermore, it is proved by means of electron spin resonance(ESR) spectra, terephthalic acid photoluminescence probing technique(TA-PL), and UV-Vis absorption spectra of nitroblue tetrazolium. This work indicates that the fabrication of Z-scheme structure can improve the photocatalytic activity by efficiently separating the photogenerated electrons and holes in the photocatalytic reaction system, which is helpful to deeply understand the migration mechanism of photoexcited carrier(band-band transfer and Z-scheme transfer) in heterojunction photocatalysts.     

Preparation of Silver Carbonate and its Application as Visible Light‐driven Photocatalyst Without Sacrificial Reagent

Visible light‐driven photocatalyst is the current research focus and silver oxyacid salts with p‐block elements are the promising candidates. In this research, Ag₂CO₃ was prepared by a facile precipitation method and used to degrade the pollutants from waters. The results revealed that the silver carbonate with monoclinic structure quickly decomposed methyl orange and rhodamine B in less than 15 min under visible light irradiation. When it was recycled six times, the degradation of methyl orange still can reach 87% after 30 min. The calculated band gap of Ag₂CO₃ was 2.312 eV with Valence band edge potential of 2.685 eV and Conduction band 0.373 eV vs NHE, which endowed the excellent photo‐oxidation ability of silver carbonate. Photogenerated holes and ozone anion radicals were the primary active species in the photo‐oxidization degradation of dye. The generation of metallic silver resulted from photocorrosion and the consequent reduction in the ozone anion radical amount led to the performance degradation of Ag₂CO₃. The simple preparation method and high photocatalytic performance of Ag₂CO₃ increases its prospect of application in future.     

Enhancing the Photocatalytic Activity of Anatase TiO2 by Improving the Specific Facet‐Induced Spontaneous Separation of Photogenerated Electrons and Holes

Recently, it has been proven that directional flow of photogenerated charge carriers occurs on specific facets of TiO₂ nanocrystals. Herein, we demonstrate that the photocatalytic activity of anatase TiO₂ nanocrystals in both photoreduction and photooxidation processes can be enhanced by selectively depositing Pt nanoparticles on the {101} facets, which strengthens spontaneously surface‐induced separation between photogenerated electrons and holes in the photocatalysis process. An optimal ratio of the oxidative {001} facets to the reductive {101} facets exists with regard to the photocatalysis of the faceted TiO₂ nanocrystals, and this is crucial for balancing the recombination and redox reaction rates of photogenerated electrons and holes. The present work might help us gain deeper insight into the relation between the specific surface of semiconductor photocatalysts and their photocatalytic activities and provides us with a new route to design photocatalysts with high photocatalytic activity.     

Enhancement of Solar Fuel Production Schemes by Using a Ru,Rh,Ru Supramolecular Photocatalyst Containing Hydroxide Labile Ligands

Polyazine‐bridged Ru^IIRh^IIIRu^II complexes with two halide ligands, Cl^? or Br^?, bound to the catalytically active Rh center are efficient single‐component photocatalysts for H₂O reduction to H₂ fuel, with the coordination environment on Rh impacting photocatalysis. Herein reported is a new, halide‐free Ru^IIRh^IIIRu^II photocatalyst with OH^? ligands bound to Rh, further enhancing the photocatalytic reactivity of the structural motif. H₂ production experiments using the photocatalyst bearing OH^? ligands at Rh relative to the analogues bearing halides at Rh in solvents of varying polarity (DMF, CH₃CN, and H₂O) suggest that ion pairing with halides deactivates photocatalyst function, representing an exciting phenomenon to exploit in the development of catalysts for solar H₂ production schemes.     

Beyond Reduction Cocatalysts: Critical Role of Metal Cocatalysts in Photocatalytic Oxidation of Methane with Water**

Environmentally sustainable and selective conversion of methane to valuable chemicals under ambient conditions is pivotal for the development of next-generation photocatalytic technology. However, due to the lack of microscopic knowledge about non-thermal methane conversion, controlling and modulating photocatalytic oxidation processes driven by photogenerated holes remain a challenge. Here, we report novel function of metal cocatalysts to accept photogenerated holes and dominate selectivity of methane oxidation, which is clearly beyond the conventional concept in photocatalysis that the metal cocatalysts loaded on the surfaces of semiconductor photocatalysts mostly capture photogenerated electrons and dominate reduction reactions exclusively. The novel photocatalytic role of metal cocatalysts was verified by operando molecular spectroscopy combined with real-time mass spectrometry for metal-loaded Ga₂O₃ model photocatalysts under methane and water vapor at ambient temperature and pressure. Our concept of metal cocatalysts that work as active sites for both photocatalytic oxidation and reduction provides a new understanding of photocatalysis and a solid basis for controlling non-thermal redox reactions by metal-cocatalyst engineering.     

Morphology-Controlled Synthesis and Applications of Silver Halide Photocatalytic Materials

Semiconductor photocatalysis is considered to be one of the most promising technologies to solve the worldwide environmental and energy issues. In recent years, silver halide (AgX)-based photocatalytic materials have received increasing research attention owing to its excellent visible light-driven photocatalytic performances in the applications of organic pollutant degradation, H₂/O₂ generation, and disinfection. AgX-based materials used in photocatalytic fields can be classified into three categories: AgX (Ag/AgX), AgX composites, and supported AgX materials. For the AgX (Ag/AgX) photocatalysts, it has been widely accepted that the final photocatalytic performances of photocatalysts are severely dependent on their morphological structures as well as exposed crystal facets. As a result, considerable efforts have been devoted to fabricating different morphological AgX photocatalysts as well as exploring the relationship between the morphological structures and photocatalytic performances. In this review, we mainly introduce the recent developments made in fabricating morphology and facet-controllable AgX (Ag/AgX) photocatalytic materials. Moreover, this review also deals with the photocatalytic mechanism and applications of AgX (Ag/AgX) and supported AgX materials.     

Revealing the A‐Site Effect of Lead‐Free A3Sb2Br9 Perovskite in Photocatalytic C(sp3)−H Bond Activation

The lead‐free halide perovskite A₃Sb₂Br₉ is utilized as a photocatalyst for the first time for C(sp³)?H bond activation. A₃Sb₂Br₉ nanoparticles (A₃Sb₂Br₉ NPs) with different ratios of Cs and CH₃NH₃ (MA) show different photocatalytic activities for toluene oxidation and the photocatalytic performance is enhanced when increasing the amount of Cs. The octahedron distortion caused by A‐site cations can change the electronic properties of X‐site ions and further affect the electron transfer from toluene molecules to Br sites. After the regulation of A‐site cations, the photocatalytic activity is higher with A₃Sb₂Br₉ NPs than that with classic photocatalysts (TiO₂, WO₃, and CdS). The main active species involved in photocatalytic oxidation of toluene are photogenerated holes (h⁺) and superoxide anions (^.O₂^?). The octahedron distortion by A‐site cations affecting photocatalytic activity remains unique and is also a step forward for understanding more about halide‐perovskite‐based photocatalysis. The relationship between octahedron distortion and photocatalysis can also guide the design of new photocatalytic systems involving other halide perovskites.     

Vacancy‐Rich Monolayer BiO2−x as a Highly Efficient UV,Visible, and Near‐Infrared Responsive Photocatalyst

Vacancy‐rich layered materials with good electron‐transfer property are of great interest. Herein, a full‐spectrum responsive vacancy‐rich monolayer BiO_2−x has been synthesized. The increased density of states at the conduction band (CB) minimum in the monolayer BiO_2−x is responsible for the enhanced photon response and photo‐absorption, which were confirmed by UV/Vis‐NIR diffuse reflectance spectra (DRS) and photocurrent measurements. Compared to bulk BiO_2−x, monolayer BiO_2−x has exhibited enhanced photocatalytic performance for rhodamine B and phenol removal under UV, visible, and near‐infrared light (NIR) irradiation, which can be attributed to the vacancy V_Bi‐O′′′ as confirmed by the positron annihilation spectra. The presence of V_Bi‐O′′′ defects in monolayer BiO_2−x promoted the separation of electrons and holes. This finding provides an atomic level understanding for developing highly efficient UV, visible, and NIR light responsive photocatalysts.     

Curious behaviors of photogenerated electrons and holes at the defects on anatase,rutile, and brookite TiO2 powders: A review

Photocatalytic reactions are governed by photogenerated charge carriers upon band gap excitation. Therefore, for better understanding of the mechanism, the dynamics of photocarriers should be studied. One of the attractive materials is TiO₂, which has been extensively investigated in the field of photocatalysis. This review article summarizes our recent works of time-resolved visible to mid-IR absorption measurements to elucidate the difference of anatase, rutile, and brookite TiO₂ powders. The distinctive photocatalytic activities of these polymorphs are determined by the electron-trapping processes at the defects on powders. Powders are rich in defects and these defects capture photogenerated electrons. The depth of the trap is crystal phase dependent, and they are estimated to be < 0.1 eV, ∼0.4 eV and ∼0.9 eV for anatase, brookite, and rutile, respectively. Electron trapping reduces probability to meet with holes and then elongate the lifetime of holes. Therefore, it works negatively for the reaction of electrons but positively works for the reaction of holes. In the steady-state reactions, both electrons and holes should be consumed. Hence, the balance between the positive and negative effects of defects determines the distinctive photocatalytic activities of anatase, rutile, and brookite TiO₂ powders.     

Stable Heterometallic Cluster‐Based Organic Framework Catalysts for Artificial Photosynthesis

A series of stable heterometallic Fe₂M cluster‐based MOFs ( NNU‐31‐M , M=Co, Ni, Zn) photocatalysts are presented. They can achieve the overall conversion of CO₂ and H₂O into HCOOH and O₂ without the assistance of additional sacrificial agent and photosensitizer. The heterometallic cluster units and photosensitive ligands excited by visible light generate separated electrons and holes. Then, low‐valent metal M accepts electrons to reduce CO₂, and high‐valent Fe uses holes to oxidize H₂O. This is the first MOF photocatalyst system to finish artificial photosynthetic full reaction. It is noted that NNU‐31‐Zn exhibits the highest HCOOH yield of 26.3 μmol g^?1 h^?1 (selectivity of ca. 100 %). Furthermore, the DFT calculations based on crystal structures demonstrate the photocatalytic reaction mechanism. This work proposes a new strategy for how to design crystalline photocatalyst to realize artificial photosynthetic overall reaction.     

Vacancy‐Rich Monolayer BiO2−x as a Highly Efficient UV,Visible, and Near‐Infrared Responsive Photocatalyst

Vacancy‐rich layered materials with good electron‐transfer property are of great interest. Herein, a full‐spectrum responsive vacancy‐rich monolayer BiO_2?x has been synthesized. The increased density of states at the conduction band (CB) minimum in the monolayer BiO_2?x is responsible for the enhanced photon response and photo‐absorption, which were confirmed by UV/Vis‐NIR diffuse reflectance spectra (DRS) and photocurrent measurements. Compared to bulk BiO_2?x, monolayer BiO_2?x has exhibited enhanced photocatalytic performance for rhodamine B and phenol removal under UV, visible, and near‐infrared light (NIR) irradiation, which can be attributed to the vacancy V_Bi‐O′′′ as confirmed by the positron annihilation spectra. The presence of V_Bi‐O′′′ defects in monolayer BiO_2?x promoted the separation of electrons and holes. This finding provides an atomic level understanding for developing highly efficient UV, visible, and NIR light responsive photocatalysts.     

A REVIEW OF THE SPECTRAL SENSITIZATION OF SILVER HALIDES,PAST AND PRESENT*

Abstract— Some of the important historical steps leading to our present understanding of the spectral sensitization of silver halides are reviewed and a new modification of the theory of the electron transfer mechanism of spectral sensitization is discussed. It is proposed that it is not necessary for spectral sensitizing dyes to inject electrons directly into the conduction band of the silver halide in the primary step. The only energetic restriction for the primary act of spectral sensitization is suggested to be the simple reduction of surface silver ions. However, for the spectral sensitizing act to be recorded photographically, secondary processes must be favorable for the subsequent deep trapping of the electrons that arise from the ionization of single silver atoms produced from the initial spectral sensitizing act.     

AgI Microplate Monocrystals with Polar {0001} Facets: Spontaneous Photocarrier Separation and Enhanced Photocatalytic Activity

Elucidating the facet‐dependent photocatalytic activity of semiconductor photocatalysts is important in improving the overall efficiency of photocatalysis. Furthermore, combining facet control with selective deposition of oxidation and/or reduction cocatalysts on specific faces of semiconductor photocatalysts is potentially an effective strategy to synergistically optimize the functionality of photocatalysts. In the present study, high‐purity wurtzite‐type β‐AgI platelet microcrystals with polar {0001} facets were prepared by a facile polyvinylpyrrolidone‐assisted precipitation reaction. The polar‐faceted AgI microplates were used as archetypes to demonstrate preferential diametric migration (i.e., effective separation) of photogenerated electrons and holes along the c axis. Such vectorial electron–hole separation stems from the asymmetric surface structures, which give rise to distinct photoexcited reaction behaviors on the ±(0001) polar facets of wurtzite‐type semiconductors. Furthermore, on selective deposition of Ag and MnO_x (1.5<x<2) cocatalysts on the reductive (0001) and oxidative (000$\bar 1$ ) facets, respectively, photocatalytic activity of the AgI microplates in degrading organic pollutants was dramatically enhanced thanks to the broad light‐absorption range, strong dye‐adsorption ability, and effective spatial separation of photocarriers.     

Synthesis of halide anion‐doped bismuth terephthalate hybrids for organic pollutant removal

Halide anion‐doped bismuth terephthalate hybrids were synthesized using a facile solvothermal method. Four series of hybrids doped with halide anions X^? (F^?, Cl^?, Br^? and I^?) were produced by varying the molar ratios (n) of X^? relative to Bi(NO₃)₃ (n = 0.25, 0.5, 0.75 and 1) in dimethylformamide solution. The results indicated that 0.25 equiv. of different halide anion‐doped bismuth terephthalate hybrids, especially BiBDC‐Cl(0.25) and BiBDC‐Br(0.25), exhibited excellent photocatalytic activity under visible light and UV light irradiation. They also exhibited excellent adsorption performance for Rhodamine B which could be attributed to high surface areas and negative charge on the surface of the catalysts. Moreover, the degradation of Rhodamine B under visible light irradiation is a photosensitization process and ^?O₂^? is the most important active species. The halide anion‐doped bismuth terephthalate hybrids are promising photocatalysts for removal of organic pollutants. Copyright © 2016 John Wiley & Sons, Ltd.     

Enhanced Photocatalytic Activity of MIL‐125 by Post‐Synthetic Modification with CrIII and Ag Nanoparticles

NH₂‐MIL‐125, [Ti₈O₈(OH)₄(bdc‐NH₂)₆] (bdc^2?=1,4‐benzene dicarboxylate) is a highly porous metal–organic framework (MOF) that has a band gap lying within the ultraviolet region at about 2.6 eV. The band gap may be reduced by a suitable post‐synthetic modification of the nanochannels using conventional organic chemistry methods. Here, it is shown that the photocatalytic activity of NH₂‐MIL‐125 in the degradation of methylene blue under visible light is remarkably augmented by post‐synthetic modification with acetylacetone followed by Cr^III complexation. The latter metal ion extends the absorption from the ultraviolet to the visible light region (band gap 2.21 eV). The photogenerated holes migrate from the MOF’s valence band to the Cr^III valence band, promoting the separation of holes and electrons and increasing the recombination time. Moreover, it is shown that the MOF’s photocatalytic activity is also much improved by doping with Ag nanoparticles, formed in situ by the reduction of Ag⁺ with the acetylacetonate pendant groups (the resulting MOF band gap is 2.09 eV). Presumably, the Ag nanoparticles are able to accept the MOF’s photogenerated electrons, thus avoiding electron–hole recombination. Both, the Cr‐ and Ag‐bearing materials are stable under photocatalytic conditions. These findings open new avenues for improving the photocatalytic activity of MOFs.     

Enhanced Photocatalytic Activity of Thienyl-Porphyrins-TiO2 for Degradation of 4-Nitrophenol

5‐Mono‐(2‐thienyl)‐10,15,20‐triphenyl porphyrin (H₂MTP) and its Co(II), Cu(II), Zn(II) metalloporphyrins (CoMTP, CuMTP, ZnMTP) were synthesized and characterized spectroscopically. The corresponding Co(II), Cu(II), Zn(II) metalloporphyrins‐TiO₂ photocatalysts were then prepared and characterized by means of scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), UV‐vis spectra and FT‐IR. The photocatalytic activities of these photocatalysts were investigated by testing the photodegradation of 4‐nitrophenol (4‐NP) in aqueous solution under irradiation with high‐pressure mercury lamp and metal halide lamp respectively. The results indicated that these metalloporphyrins greatly enhanced the photocatalytic efficiency of bare TiO₂ in degradating the 4‐NP, and CuMTP‐TiO₂ exhibited the highest photocatalytic activity of all photocatalysts. In addition, the first order rate constants of these photocatalytic reactions were calculated.     

Tungsten and nitrogen co‐doped TiO2 nanobelts with significant visible light photoactivity

Tungsten and nitrogen co‐doped TiO₂ nanobelts (W/N‐TNBs) have been successfully synthesized via 1‐step hydrothermal method. The structure, morphology, and composition of prepared samples were characterized by X‐ray diffraction, scanning electron microscopy, and X‐ray photoelectron spectroscopy, respectively. The prominent phase of all as‐prepared samples is anatase crystal. For samples with N doping, new energy states can be introduced on top of O 2p states which reduced the band gap by 1.1 eV. The reduced band gap leads to efficient visible light activity. The 3%‐W/N‐TNBs were found to exhibit the highest activity. The photocatalytic performance of 3%‐W/N‐TNBs under visible light is about 4.8 times than that of pure TiO₂ nanobelts, which emphasizes the synergistic effect of W and N co‐doping for effectively inhibiting the recombination of photogenerated electrons and holes. In addition, our results testify the different redox potentials of the photoelectrons at different final states.     

Ternary BiOBr/TiO2/Ti3C2Tx MXene nanocomposites with heterojunction structure and improved photocatalysis performance

The rational design and construction of heterojunction structure is an effective strategy to improve the photocatalytic performance. Herein, a series of BiOBr nanosheets-immobilized TiO₂/Ti₃C₂T_x MXene hybrid materials with heterojunction structure were synthesized by a facial one-step hydrothermal method. The ternary composites show outstanding performance as photocatalysts for the degradation of rhodamine B due to the optimized synergetic effects of BiOBr, TiO₂ and Ti₃C₂T_x. The improved photocatalytic performance is remarkably attributed to the construction of a heterojunction between TiO₂ and BiOBr due to their well-matching of energy band position, which can enhance the absorption for visible light and promote the transfer of photo-generated charge carriers. Moreover, Ti₃C₂T_x acts as an electron trap to further accelerate the separation of photo-generated electrons and holes.