Anchoring Single-Atomic Metal Sites in Metalloporphyrin-Based Covalent Organic Frameworks for Enhanced Photocatalytic Hydrogen Evolution.

A photoactive covalent organic framework (COF) was built from metalloporphyrin and bipyridine monomers and single-atomic Pt sites were subsequently installed. Integrating photosensitizing metalloporphyrin and substrate-activating Pt(bpy) moieties in a single solid facilitates multielectron transfer and accelerates photocatalytic hydrogen evolution with a maximum production rate of 80.4 mmol h-1 gPt -1 and turnover frequency (TOF) of 15.7 h-1 observed. This work demonstrates that incorporation of single-atomic metal sites with photoactive COFs greatly enhances photocatalytic activity and provides an effective strategy for the design and construction of novel photocatalysts.

Exfoliation of a Two-Dimensional Metal-Organic Framework for Enhanced Photocatalytic CO2 Reduction.

A two-dimensional metal-organic framework, FICN-12, was constructed from tris[4-(1H-pyrazole-4-yl)phenyl]amine (H3TPPA) ligands and Ni2 secondary building units. The triphenylamine moiety in the H3TPPA ligand readily absorbs UV-visible photons and sensitizes the Ni center to drive photocatalytic CO2 reduction. FICN-12 can be exfoliated into monolayer and few-layer nanosheets with a "top-down" approach, which exposes more catalytic sites and increases its catalytic activity. As a result, the nanosheets (FICN-12-MONs) showed photocatalytic CO and CH4 production rates of 121.15 and 12.17 µmol/g/h, respectively, nearly 1.4 times higher than those of bulk FICN-12.

Modulating the Schottky barrier of Pt/PbTiO3 for efficient piezo-photocatalytic hydrogen evolution.

Charge recombination severely restricts the photocatalytic efficiencies of materials. Loading cocatalysts on the surface of host photocatalysts is a promising strategy for charge separation, which, however, suffers from the large Schottky barrier at the cocatalyst/host interface. Herein, a series of Pt/PbTiO3 compounds were constructed as a proof-of-concept utilizing the piezoelectric field of PbTiO3 under acoustic vibrations to modulate the height of the interfacial Schottky barrier. These hybrid systems achieved highly efficient piezo-photocatalytic H2 evolution under simultaneous ultrasonication and light illumination. The manipulation of the height of the Schottky barrier by the piezoelectric effect was validated by the I-V characteristics collected from conductive AFM. It is proposed that the acoustic-wave-induced piezoelectric field increased the electron flow from PbTiO3 to Pt over the modulated Schottky barrier, which promoted the spatial separation of photo-generated charge carriers and consequently enhanced the H2 evolution. These findings will extend the fundamental understanding of the synergistic piezo-photocatalysis mechanism and provide a new opportunity toward the rational design of novel materials systems for clean energy conversion.

Alkalis-doping of mixed tin-lead perovskites for efficient near-infrared light-emitting diodes.

Substitution of lead (Pb) with tin (Sn) is a very important way to reduce the bandgap of metal halide perovskite for applications in solar cells, and near infrared (NIR) light-emitting diodes (LEDs), etc. However, mixed Pb/Sn perovskite becomes very disordered with high trap density when the Sn molar ratio is less than 20%. This limits the applications of mixed Pb/Sn perovskites in optoelectronic devices such as wavelength tunable NIR perovskite LEDs (PeLEDs). In this work, we demonstrate that alkali cations doping can release the microstrain and passivate the traps in mixed Pb/Sn perovskites with Sn molar ratios of less than 20%, leading to higher carrier lifetime and photoluminescence quantum yield (PLQY). The external quantum efficiency (EQE) of Sn0.2Pb0.8-based NIR PeLEDs is dramatically enhanced from 0.1% to a record value of 9.6% (emission wavelength: 868 nm). This work provides a way of making high quality mixed Pb/Sn optoelectronic devices with small Sn molar ratios.

Partial Metalation of Porphyrin Moieties in Hydrogen-Bonded Organic Frameworks Provides Enhanced CO2 Photoreduction Activity.

In natural photosynthesis, the architecture of multiproteins integrates more chromophores than redox centers and simultaneously creates a well-controlled environment around the active site. Herein, we demonstrate that these features can be emulated in a prototype hydrogen-bonded organic framework (HOF) through simply varying the proportion of metalated porphyrin in the structure. Further studies demonstrate that changing the metalloporphyrin content not only realizes a fine tuning of the photosensitizer/catalyst ratio, but also alters the microenvironment surrounding the active site and the charge separation efficiency. As a result, the obtained material achieves the challenging overall CO2 reduction with a high HCOOH production rate (29.8 µmol g-1 h-1 , scavenger free), standing out from existing competitors. This work unveils that the degree of metalation is vital to the catalytic activity of the porphryinic framework, presenting as a new strategy to optimize the performance of heterogeneous catalysts.

Facile Preparation of Hydrogen-Bonded Organic Framework/Cu2O Heterostructure Films via Electrophoretic Deposition for Efficient CO2 Photoreduction.

Photocatalytic CO2 reduction is one of the most cost-effective and environmentally friendly techniques of converting CO2 into high-value compounds and/or fuels. However, the performance of most current photocatalytic CO2 reduction catalysts is less than satisfactory for practical applications. Here, we synthesized a heterogeneous structure by integrating Cu2O and a porphyrin hydrogen-bonded organic framework (PFC-45), which was then fabricated into a thin-film catalyst on carbolic paper (CP) using a facile electrophoretic deposition technology. With improved electron-hole separation efficiency and visible-light-harvesting ability, this film (PFC-45/Cu2O@CP) significantly enhanced CO2-to-CO photoreduction, exceeding 2.4 and 3.2 times that of PFC-45@CP and PFC-45/Cu2O particles, respectively. Remarkably, PFC-45/Cu2O@CP also exhibited high selectivity (99%) and outstanding activity (11.81 µmol g-1 h-1) for photocatalytic CO2 reduction in pure water without any sacrificial agent. This work demonstrates a new strategy to design photocatalysts for efficient CO2 reduction.

Engineering Hierarchical Architecture of Metal-Organic Frameworks for Highly Efficient Overall CO2 Photoreduction.

Previous studies on syntheses of metal-organic frameworks (MOFs) for photocatalytic CO2 reduction are mainly focused on the exquisite control over the net topology and the functionality of metal clusters/organic building blocks. This contribution demonstrates that the rational design of MOF-based photocatalyst can be further extended to the hierarchical structure at micrometer scales well beyond the conventional MOF design at the molecular level. By taking advantage of the disparity of two selective MOFs in nucleation kinetics, a hierarchical core-shell MOF@MOF structure is successfully constructed through a simple one-pot synthesis. Besides inheriting the high porosity, crystallinity, and robustness of parent MOFs, the obtained heterojunction exhibits extended photoresponse, optimized band alignment with large overpotential, and greatly enhanced photogenerated charge separation, which would be hardly realized by the merely molecular-level assembly. As a result, the challenging overall CO2 photoreduction is achieved, which generates a record high HCOOH production (146.0 µmol/g/h) without using any sacrificial reagents. Moreover, the core-shell structure exhibits a more effective use of photogenerated electrons than the individual MOFs. This work shows that harnessing the hierarchical architecture of MOFs present a new and effective alternative to tuning the photocatalytic performance at a mesoscopic level.

Metallization-Prompted Robust Porphyrin-Based Hydrogen-Bonded Organic Frameworks for Photocatalytic CO2 Reduction.

Under topological guidance, the self-assembly process based on a tetratopic porphyrin synthon results in a hydrogen-bonded organic framework (HOF) with the predicted square layers topology (sql) but unsatisfied stability. Strikingly, simply introducing a transition metal in the porphyrin center does not change the network topology but drastically causes noticeable change on noncovalent interaction, orbital overlap, and molecular geometry, therefore ultimately giving rise to a series of metalloporphyrinic HOFs with high surface area, and excellent stability (intact after being soaked in boiling water, concentrated HCl, and heated to 270 °C). On integrating both photosensitizers and catalytic sites into robust backbones, this series of HOFs can effectively catalyze the photoreduction of CO2 to CO, and their catalytic performances greatly depend on the chelated metal species in the porphyrin centers. This work enriches the library of stable functional HOFs and expands their applications in photocatalytic CO2 reduction.

Polymerized Hybrid Perovskites with Enhanced Stability, Flexibility, and Lattice Rigidity.

The intrinsic soft lattice nature of organometal halide perovskites (OHPs) makes them very tolerant to defects and ideal candidates for solution-processed optoelectronic devices. However, the soft lattice results in low stability towards external stresses such as heating and humidity, high density of phonons and strong electron-phonon coupling (EPC). Here, it is demonstrated that the OHPs with unsaturated 4-vinylbenzylammonium (VBA) as organoammonium cations can be polymerized without damaging the perovskite structure and its tolerance to defects. The polymerized perovskites show enhanced stability and flexibility compared to regular three-dimensional and two-dimensional (2D) perovskites. Furthermore, the polymerized 4-vinylbenzylammonium group improves perovskite lattice rigidity substantially, resulting in a reduced non-radiative recombination rate because of suppressed electron-phonon coupling, and enhanced carrier mobility because of suppressed phonon scattering. 2D polymerized perovskite light-emitting diodes (PeLEDs) with strong electroluminescence at room temperature, and quasi-2D PeLEDs with an external quantum efficiency (EQE) of 23.2% and enhanced operation stability are demonstrated. The work has opened a new way of enhancing the intrinsic stability and optoelectronic properties of OHPs.

Controllable sulphur vacancies confined in nanoporous ZnS nanoplates for visible-light photocatalytic hydrogen evolution.

Controllable sulphur vacancies (Sv) confined in nanoporous ZnS nanoplates (Sv-ZnS) were prepared successfully via rapid heat treatment of ZnS(en)0.5 nanoplates. Sv with controllable concentrations originating from the in situ doping of N atoms endowed Sv-ZnS with a visible-light photocatalytic H2 production activity, having a positive linear correlation with Sv concentration.

Radiochromic Hydrogen-Bonded Organic Frameworks for X-ray Detection.

Porous materials have been investigated as efficient photochromic platforms for detecting hazardous radiation, while the utilization of hydrogen bonded organic frameworks (HOFs) in this field has remained intact. Herein, two HOFs were synthesized through self-assembly of tetratopic viologen ligand and formic acid (PFC-25, PFC-26), as a new class of "all-organic" radiochromic smart material, opening a gate for HOFs in this field. PFC-26 is active upon both X-ray and UV irradiation, while PFC-25 is only active upon X-ray irradiation. The same building block yet different radiochromic behaviors of PFC-25 and PFC-26 allow us to gain a deep mechanistic understanding of the factors that control the detection specificity. Theoretical and experimental studies reveal that the degree of π-conjugation of viologen ligand is highly related to the threshold energy of triggering a charge transfer, therefore being a vital factor for the particularity of radiochromic materials. Thanks to its convenient processibility, nanoparticle size, and UV silence, PFC-25 can be further fabricated into a portable naked-eye sensor for X-ray detection, which shows obvious color change with the merits of high transmittance contrast, good sensitivity (reproducible dose threshold of 3.5 Gy), and excellent stability. The work exhibits the promising practical potentials of HOF materials in photochromic technology.

Large-area and efficient perovskite light-emitting diodes via low-temperature blade-coating.

Large-area light-emitting diodes (LEDs) fabricated by mass-production techniques are needed for low-cost flat-panel lighting. Nevertheless, it is still challenging to fabricate efficient large-area LEDs using organic small molecules (OLEDs), quantum dots (QLEDs), polymers (PLEDs), and recently-developed hybrid perovskites (PeLEDs) due to difficulties controlling film uniformity. To that end, we report sol-gel engineering of low-temperature blade-coated methylammonium lead iodide (MAPbI3) perovskite films. The precipitation, gelation, aging, and phase transformation stages are dramatically shortened by using a diluted, organoammonium-excessed precursor, resulting in ultra-flat large-area films (54 cm2) with roughness reaching 1 nm. The external quantum efficiency of doctor-bladed PeLEDs reaches 16.1%, higher than that of best-performing blade-coated OLEDs, QLEDs, and PLEDs. Furthermore, benefitting from the throughput of the blade-coating process and cheap materials, the expected cost of the emissive layer is projected to be as low as 0.02 cents per cm2, emphasizing its application potential.

Boosting Interfacial Charge-Transfer Kinetics for Efficient Overall CO2 Photoreduction via Rational Design of Coordination Spheres on Metal-Organic Frameworks.

The recombination of electron-hole pairs severely detracts from the efficiency of photocatalysts. This issue could be addressed in metal-organic frameworks (MOFs) through optimization of the charge-transfer kinetics via rational design of structures at atomic level. Herein, a pyrazolyl porphyrinic Ni-MOF (PCN-601), integrating light harvesters, active catalytic sites, and high surface areas, has been demonstrated as a superior and durable photocatalyst for visible-light-driven overall CO2 reduction with H2O vapor at room temperature. Kinetic studies reveal that the robust coordination spheres of pyrazolyl groups and Ni-oxo clusters endow PCN-601 with proper energy band alignment and ultrafast ligand-to-node electron transfer. Consequently, the CO2-to-CH4 production rate of PCN-601 far exceeds those of the analogous MOFs based on carboxylate porphyrin and the classic Pt/CdS photocatalyst by more than 3- and 20-fold, respectively. The reaction avoids the use of hole scavengers and proceeds in a gaseous phase which can take full advantage of the high gas uptake of MOFs. This work demonstrates that the rational design of coordination spheres in MOF structures not only reconciles the contradiction between reactivity and stability but also greatly promotes the interfacial charge transfer to achieve optimized kinetics, providing guidance for the design of highly efficient MOF photocatalysts.

An easy and low-cost method of embedding chiral molecules in metal-organic frameworks for enantioseparation.

A facile method, post-synthetic exchange of modulators (PSEm), has been demonstrated here to prepare chiral metal-organic frameworks for enantioseparation. Based on this method, three chiral porous Zr-based metal-organic frameworks have been prepared through exchanging the coordinated modulators on metal clusters of MOFs with commercially available chiral carboxylic acid molecules. In addition, the obtained materials show enantioselectivity toward three different enantiomers, which presents a proof of concept for the design of MOF materials for enantioseparation by an easy and low-cost method.

Efficient All-Inorganic Perovskite Light-Emitting Diodes with Improved Operation Stability.

Stability is becoming a main issue for perovskite light-emitting diodes (PeLEDs), as their external quantum efficiency (EQE) has been boosted to above 20%. An all-inorganic perovskite, cesium lead iodide (CsPbI3), has better stability than organic-inorganic hybrid perovskites but suffers from a transition to yellow δ-CsPbI3 phase at room temperature. Herein, we report stabilization of the α-CsPbI3 phase by in situ formation of perovskite nanocrystals (NCs). By incorporation of a proper ratio of bulky organoammonium halides, 4-fluoro-phenylmethylammonium iodide (4-F-PMAI), stable α-CsPbI3 films with nanometer-sized crystals can be obtained using a one-step spin-coating approach. The PeLEDs using α-CsPbI3 NC films as emitters show a pure red emission at 692 nm and a high EQE of 14.8%. The EQE is further boosted to 18.6% using CsPbI2.8Br0.2 as the emissive layer. Furthermore, the PeLEDs show a very decent half-lifetime of over 1200 min and a shelf stability of over 2 months, much longer than that of hybrid PeLEDs.

Record Complexity in the Polycatenation of Three Porous Hydrogen-Bonded Organic Frameworks with Stepwise Adsorption Behaviors.

Hydrogen-bonded organic frameworks (HOFs) show great potential in many applications, but few structure-property correlations have been explored in this field. In this work, we report that self-assembly of a rigid and planar ligand gives rise to flat hexagonal honeycomb motifs which are extended into undulated two-dimensional (2D) layers and finally generate three polycatenated HOFs with record complexity. This kind of undulation is absent in the 2D layers built from a very similar but nonplanar ligand, indicating that a slight torsion of ligand produces overwhelming structural change. This change delivers materials with unique stepwise adsorption behaviors under a certain pressure originating from the movement between mutually interwoven hexagonal networks. Meanwhile, high chemical stability, phase transformation, and preferential adsorption of aromatic compounds were observed in these HOFs. The results presented in this work would help us to understand the self-assembly behaviors of HOFs and shed light on the rational design of HOF materials for practical applications.

A Comparison of Two Isoreticular Metal-Organic Frameworks with Cationic and Neutral Skeletons: Stability, Mechanism, and Catalytic Activity.

Although many ionic metal-organic frameworks (MOFs) have been reported, little is known about how the charge of the skeleton affects the properties of the MOF materials. Herein we report how the chemical stability of MOFs can be substantially improved through embedding electrostatic interactions in structure. A MOF with a cationic skeleton is impervious to extremely acidic, oxidative, reductive, and high ionic strength conditions, such as 12 m HCl (301 days), aqua regia (86 days), H2 O2 (30 days), and seawater (30 days), which is unprecedented for MOFs. DFT calculations suggested that steric hinderance and the repulsive interaction of the cationic framework toward positively charged species in microenvironments protects the vulnerable bonds in the structure. Diverse functionalities can be bestowed by substituting the counterions of the charged framework with identically charged functional species, which broadens the horizon in the design of MOFs adaptable to a demanding environment with specific functionalities.

Creating Chemisorption Sites for Enhanced CO2 Photoreduction Activity through Alkylamine Modification of MIL-101-Cr.

The lower CO2 utilization and poor charge conductivities have limited the application of metal-organic frameworks (MOFs) in photocatalysis. In this work, different alkylamines [ethylenediamine (EN), diethylenetriamine (DETA), and triethylenetetramine (TETA)] were successfully introduced into MIL-101-Cr by postmodification and created abundant CO2 chemisorption sites in structures. Photocatalysis reaction showed that the alkylamine modification promoted the charge separation and migration rate and enhanced the reduction potential of the electron generated by the MOF photocatalyst. Among them, the EN-modified material exhibits the highest CO generation rate of 47.2 µmol·h-1·g-1 with a high selectivity of 96.5%, much superior than the pristine MOFs MIL-101-Cr and MIL-101-SO3H, as well as the DETA- and TETA-modified products, which can be ascribed to the abundant chemisorption sites for CO2 reactants and the optimized pore size in structures. The strategy of introduction of alkylamine groups as CO2 chemisorption sites has been demonstrated to be a new pathway for the design of efficient MOF catalysts for CO2 photoreduction.

A visualizable means for verifying the manner of charge transfer in WO3-based type-II heterostructures.

Research into photocatalytic mechanisms and charge carrier transfer is of vital significance. For type-II heterostructures containing WO3, a visualizable means is proposed for the first time for verifying the manner of charge transfer via observing the photochromism of WO3. The accuracy of this visualizable means is evidenced through corresponding characterization, such as XPS and OCP. In addition, photocatalytic H2 evolution as a supporting proof is studied to prove the manner of charge transfer owing to the inactivity of WO3. If the charge transfer pathway principally follows a conventional type-II manner, the heterostructure will change color from yellow to a dark color and show lower activity compared with the individuals. However, if the charge transfer primarily follows a Z-scheme mechanism, the color won't show a noticeable change but much higher activity will be exhibited than that by the individual components. CdS-WO3 and ZnIn2S4-WO3 (ZIS-WO3) are used as examples to verify the universality of this method and exclude the impact of the crystal phase of WO3 on photochromism.

Study on water splitting characteristics of CdS nanosheets driven by the coupling effect between photocatalysis and piezoelectricity.

Ultrathin semiconductors have been proposed as an excellent platform to promote solar conversion due to their ultra-large specific surface area and unique surface structures. So far, the researchers designed and constructed some multi-component heterostructure photocatalysts, but they are still unable to avoid the recombination of photoexcited electron-hole pairs. This study introduces a built-in electric field in a one-component nanosheet to promote photo-generated carrier separation. For this reason, CdS nanosheets with both photocatalytic and piezoelectric properties were selected as research objects. The combination of these two properties renders CdS an excellent candidate for efficiently utilizing both light and vibrational energy for photocatalytic water splitting, without the need for coupling it to other materials or using an external bias. The result shows that the photocatalytic and piezoelectric coupling effect of CdS can make hydrogen production reach 633 µL h-1, which was more than twice the superposition of light and vibration. The development of this coupling effect contributes to the application of green energies, such as the use of natural sunlight and noise or vibration.