Hydrogel-involved portable colorimetric sensor based on oxidase mimic Fe/Co-NC for acetylcholinesterase detection and pesticides inhibition assessment.

Herein, we synthesized a novel N-doped carbon layer encapsulated Fe/Co bimetallic nanoparticles (Fe/Co-NC), which exhibited superior oxidase-like activity due to the facilitation of electron penetration and the formation of metal-nitrogen active sites. Fe/Co-NC could catalyze the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) to blue oxTMB. Acetylcholinesterase (AChE) could catalyze the hydrolysis of thioacetylcholine to produce reducing thiocholine, which prevented TMB from oxidation. Thus, a portable hydrogel colorimetric sensor was developed for on-site and visual monitoring of AChE with the detection limit of 0.36 U L-1, and successfully applied to detect AChE in human erythrocyte samples. Furthermore, this platform was used to investigate the inhibition of triazophos on AChE activity.

Metal Site and Size-Controlled BTC-Based MOF as Cysteine Oxidase Mimic for Self-Cascade Detection of Cysteine and Hg2.

Nanozyme-mediated strategy for sensing has been widely applied nowadays, in which the construction of a nanozyme cascade platform is an effective and challenging method to simulate the complexity and multifunctionality of natural systems. Herein, a simple and convenient self-cascade sensing platform was developed for the fluorescent detection of cysteine and Hg2+ by a BTC-based MOF through screening the metal sites and crystal sizes. By the introduction of polyvinylpyrrolidone, the as-prepared Cu-BTC possessed a metal center of Cu2+ and smaller size, which exhibited both cysteine oxidase- and peroxidase-like activities. The dual enzymic characters of Cu-BTC made a self-cascade reaction occur during which cysteine was first oxidized to cystine and generated H2O2 in the presence of O2, then H2O2 was decomposed into ·OH, and finally the ·OH triggered the turn-on fluorescence of Cu-BTC. Based on the self-cascade reactions and high affinity of Hg2+ and -SH within cysteine, a fluorescent method was developed to detect cysteine and Hg2+ with a range of 0-160/0-15 µM and a limit of detection of 0.04/0.09 µM, respectively. This work reveals the important role of the Cu2+ center for mimicking cysteine oxidase and gives a feasible strategy for constructing simple self-cascade reactions.

Nanospatial Charge Modulation of Monodispersed Polymeric Microsphere Photocatalysts for Exceptional Hydrogen Peroxide Production.

Photocatalysis offers a sustainable strategy for hydrogen peroxide (H2 O2 ) production, which is an essential oxidant and emerging energy carrier in modern chemical industry. The development of polymer-based photocatalysts to produce H2 O2 has great potential but is limited by lower efficiency due to the limitation of light utilization and the low charge separation efficiency. Herein, a series of monodispersed mesoporous resorcinol-formaldehyde resin spheres (MRFS) are reported with a rational designed spatial charge distribution, exhibiting wide light absorption with a solar-to-chemical conversion (SCC) efficiency of 1.1%. Surface photovoltage microscopy (SPVM) measurements unraveled the charge separation in nanospace with uneven distribution of donor (D) and acceptor (A) sites. A density functional theory (DFT) calculation elucidated the origin of photogenerated electrons and holes. Moreover, MRFS demonstrates photocatalytic water oxidation ability. The findings in this work open a new avenue for the development of porous polymeric photocatalysts toward highly efficient solar energy conversion.

Modular Construction of Prussian Blue Analog and TiO2 Dual-Compartment Janus Nanoreactor for Efficient Photocatalytic Water Splitting.

Janus structures that include different functional compartments have attracted significant attention due to their specific properties in a diverse range of applications. However, it remains challenge to develop an effective strategy for achieving strong interfacial interaction. Herein, a Janus nanoreactor consisting of TiO2 2D nanocrystals integrated with Prussian blue analog (PBA) single crystals is proposed and synthesized by mimicking the planting process. In situ etching of PBA particles induces nucleation and growth of TiO2 nanoflakes onto the concave surface of PBA particles, and thus enhances the interlayer interaction. The anisotropic PBA-TiO2 Janus nanoreactor demonstrates enhanced photocatalytic activities for both water reduction and oxidation reactions compared with TiO2 and PBA alone. As far as it is known, this is the first PBA-based composite that serves as a bifunctional photocatalyst for solar water splitting. The interfacial structure between two materials is vital for charge separation and transfer based on the spectroscopic studies. These results shed light on the elaborate construction of Janus nanoreactor, highlighting the important role of interfacial design at the microscale level.

Defect-Induced Inhomogeneous Phase Transition in 2D Perovskite Single Crystals at Low Temperatures.

The temperature-induced phase transition in two-dimensional (2D) layered perovskites was recently found to be incomplete even if the temperature dropped to tens of kelvin. However, its intrinsic cause still remains unclear, and the information on the phase transition in individual single crystals (SCs) is also limited. Herein, we study the phase transition process in individual (n-C4H9NH3)2PbI4 SCs using a home-built photoluminescence (PL)-scanned image microscope. At 83 K, the phase transition is indeed incomplete, leading to the coexistence and inhomogeneous distributions of room-temperature and low-temperature phases. We map the distribution of phase transition degree on individual SCs at 83 K, which exhibits a strong positive (negative) correlation with the distribution of local defects (PL lifetimes) at 293 K, indicating that the phase transition is enhanced by initial defects. Our findings might provide new insight into the phase transition of (n-C4H9NH3)2PbI4 crystals, which is of potential value for applications based on 2D layered perovskites.

Photovoltaic Control of Ferromagnetism for Flexible Spintronics.

The demand for low-power flexible spintronics for sensing, communicating, and data processing applications boosts an intense search for novel ways of controlling magnetism. In this work, a photovoltaic controllable flexible spintronic device within a Kapton/Ta/Co/(PC71BM/PTB7-Th)/Pt heterostructure was demonstrated, and the magnetic anisotropy change of this flexible heterostructure as a function of the external light radiation and strain was quantitatively determined. 150 mW/cm2 white light illumination induced 489 Oe out-of-plane ferromagnetic resonance field modulation, which was attributed to the photogenerated electron doping in the cobalt film. The chemical contamination effect and the interfacial oxidation effect during the photovoltaic doping process were eliminated. Moreover, it was found that the working function of the thin-film electrodes were different from the bulk values via an ultraviolet photoelectron spectroscopy test. Our results on flexible photovoltaic spintronics systems will invigorate the research toward the development of solar-driven energy-efficient spintronics.

Trap-Enabled Long-Distance Carrier Transport in Perovskite Quantum Wells.

Layered two-dimensional (2D) hybrid perovskites are naturally formed multiple quantum well (QW) materials with promising applications in quantum and optoelectronic devices. In principle, the transport of excitons in 2D perovskites is limited by their short lifetime and small mobility to a distance within a few hundred nanometers. Herein, we report an observation of long-distance carrier transport over 2 to 5 µm in 2D perovskites with various well thicknesses. Such a long transport distance is enabled by trap-induced exciton dissociation into long-lived and nonluminescent electron-hole separated state, followed by a trap-mediated charge transport process. This unique property makes 2D perovskites comparable with 3D perovskites and other traditional semiconductor QWs in terms of a carrier transport property and highlights their potential application as an efficient energy/charge-delivery material.

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.

Unraveling the Kinetics of Photocatalytic Water Oxidation on WO3.

Understanding the reaction kinetics of photocatalytic water splitting is important for the solar energy conversion field. Particularly, identifying the main obstacle in solar water oxidation is intriguing for efforts to promote the energy conversion efficiency. Herein, we take WO3 and cesium treated WO3 as prototypical models to disclose the reaction kinetics of photocatalytic water oxidation and found that the lack of long-lived surface holes is the bottleneck in the photocatalytic process. Analysis of the kinetic barriers of the surface catalytic reactions indicates that the water oxidation on WO3 is kinetically fast, whereas surface treatment of WO3 with cesium carbonate would enlarge the reaction energy barrier but unexpectedly increase the photocatalytic water oxidation rate. A further comparison of the charge dynamics by surface photovoltage and intensity modulated photocurrent spectroscopy reveals that the increased surface hole density due to the suppression of charge recombination accounts for the improvement in the photocatalytic activity.

Sunlight Control of Interfacial Magnetism for Solar Driven Spintronic Applications.

The inexorable trend of next generation spintronics is to develop smaller, lighter, faster, and more energy efficient devices. Ultimately, spintronics driven by free energy, for example, solar power, is imperative. Here, a prototype photovoltaic spintronic device with an optical-magneto-electric tricoupled photovoltaic/magnetic thin film heterojunction, where magnetism can be manipulated directly by sunlight via interfacial effect, is proposed. The magnetic anisotropy is reduced evidenced by the out-of-plane ferromagnetic resonance (FMR) field change of 640.26 Oe under 150 mW cm-2 illumination via in situ electron spin resonance (ESR) method. The transient absorption analysis and the first-principles calculation reveal that the photovoltaic electrons doping in the cobalt film alter the band filling of this ferromagnetic film. The findings provide a new path of electron doping control magnetism and demonstrate an optical-magnetic dual controllable logical switch with limited energy supply, which may further transform the landscape of spintronics research.

Ultralong UV/mechano-excited room temperature phosphorescence from purely organic cluster excitons.

Purely organic room temperature phosphorescence (RTP) has attracted wide attention recently due to its various application potentials. However, ultralong RTP (URTP) with high efficiency is still rarely achieved. Herein, by dissolving 1,8-naphthalic anhydride in certain organic solid hosts, URTP with a lifetime of over 600 ms and overall quantum yield of over 20% is realized. Meanwhile, the URTP can also be achieved by mechanical excitation when the host is mechanoluminescent. Femtosecond transient absorption studies reveal that intersystem crossing of the host is accelerated substantially in the presence of a trace amount of 1,8-naphthalic anhydride. Accordingly, we propose that a cluster exciton spanning the host and guest forms as a transient state before the guest acts as an energy trap for the RTP state. The cluster exciton model proposed here is expected to help expand the varieties of purely organic URTP materials based on an advanced understanding of guest/host combinations.

Stable Two-Photon Pumped Amplified Spontaneous Emission from Millimeter-Sized CsPbBr3 Single Crystals.

Metal-halide perovskites are promising optical gain materials because of their excellent photophysical properties. Recently, large perovskite single crystals with phase purity, less defects, and over millimeter dimensions have been successfully synthesized. However, the optical gain effect from these large-size single crystals has not yet been realized. Herein, we for the first time report efficient two-photon pumped amplified spontaneous emission (ASE) from millimeter-sized CsPbBr3 single crystals (SCs) with a low threshold of 0.65 mJ cm-2 and an optical gain of 38 cm-1. Furthermore, the CsPbBr3 SCs also exhibit ultrastable ASE under continuous laser irradiation for more than 40 h (corresponds to 1.5 × 108 laser shots) at ambient condition. This work suggests the potential application of large-size perovskite single crystals in practical nonlinear optical devices.

Efficient photoredox conversion of alcohol to aldehyde and H2 by heterointerface engineering of bimetal-semiconductor hybrids.

Controllable and precise design of bimetal- or multimetal-semiconductor nanostructures with efficient light absorption, charge separation and utilization is strongly desired for photoredox catalysis applications in solar energy conversion. Taking advantage of Au nanorods, Pt nanoparticles, and CdS as the plasmonic metal, nonplasmonic co-catalyst and semiconductor respectively, we report a steerable approach to engineer the heterointerface of bimetal-semiconductor hybrids. We show that the ingredient composition and spatial distribution between the bimetal and semiconductor significantly influence the redox catalytic activity. CdS deposited anisotropic Pt-tipped Au nanorods, which feature improved light absorption, structure-enhanced electric field distribution and spatially regulated multichannel charge transfer, show distinctly higher photoactivity than blank CdS and other metal-CdS hybrids for simultaneous H2 and value-added aldehyde production from one redox cycle.

Tailoring vertical phase distribution of quasi-two-dimensional perovskite films via surface modification of hole-transporting layer.

Vertical phase distribution plays an important role in the quasi-two-dimensional perovskite solar cells. So far, the driving force and how to tailor the vertical distribution of layer numbers have been not discussed. In this work, we report that the vertical distribution of layer numbers in the quasi-two-dimensional perovskite films deposited on a hole-transporting layer is different from that on glass substrate. The vertical distribution could be explained by the sedimentation equilibrium because of the colloidal feature of the perovskite precursors. Acid addition will change the precursors from colloid to solution that therefore changes the vertical distribution. A self-assembly layer is used to modify the acidic surface property of the hole-transporting layer that induces the appearance of desired vertical distribution for charge transport. The quasi-two-dimensional perovskite cells with the surface modification display a higher open-circuit voltage and a higher efficiency comparing to reference quasi-two-dimensional cells.

Dynamical Transformation of Two-Dimensional Perovskites with Alternating Cations in the Interlayer Space for High-Performance Photovoltaics.

The two-dimensional (2D) perovskites stabilized by alternating cations in the interlayer space (ACI) define a new type of structure with different physical properties than the more common Ruddlesden-Popper counterparts. However, there is a lack of understanding of material crystallization in films and its influence on the morphological/optoelectronic properties and the final photovoltaic devices. Herein, we undertake in situ studies of the solidification process for ACI 2D perovskite (GA)(MA) nPb nI3 n+1 (⟨ n⟩ = 3) from ink to solid-state semiconductor, using solvent mixture of DMSO:DMF (1:10 v/v) as the solvent and link this behavior to solar cell devices. The in situ grazing-incidence X-ray scattering (GIWAXS) analysis reveals a complex journey through disordered sol-gel precursors, intermediate phases, and ultimately to ACI perovskites. The intermediate phases, including a crystalline solvate compound and the 2D GA2PbI4 perovskite, provide a scaffold for the growth of the ACI perovskites during thermal annealing. We identify 2D GA2PbI4 to be the key intermediate phase, which is strongly influenced by the deposition technique and determines the formation of the 1D GAPbI3 byproducts and the distribution of various n phases of ACI perovskites in the final films. We also confirm the presence of internal charge transfer between different n phases through transient absorption spectroscopy. The high quality ACI perovskite films deposited from solvent mixture of DMSO:DMF (1:10 v/v) deliver a record power conversion efficiency of 14.7% in planar solar cells and significantly enhanced long-term stability of devices in contrast to the 3D MAPbI3 counterpart.

Encapsulating a Ni(II) molecular catalyst in photoactive metal-organic framework for highly efficient photoreduction of CO2.

Photocatalytic reduction of CO2 to CO is a promising strategy for reducing atmospheric CO2 levels and storing solar radiation as chemical energy. Here, we demonstrate that a molecular catalyst [NiII(bpet)(H2O)2] successfully encapsulated into a highly robust and visible-light responsive metal-organic framework (Ru-UiO-67) to fabricate composite catalysts for photocatalytic CO2 reduction. The composite Ni@Ru-UiO-67 photocatalysts show efficient visible-light-driven CO2 reduction to CO with a TON of 581 and a selectivity of 99% after 20-h illumination, because of the facile electron transfer from Ru-photosensitizer to Ni(II) active sites in Ni@Ru-UiO-67 system. The mechanistic insights into photoreduction of CO2 have been studied based on thermodynamical, electrochemical, and spectroscopic investigation, together with density functional theory (DFT) calculations. This work shows that encapsulating molecular catalyst into photoactive MOF highlights opportunities for designing efficient, stable and recyclable photocatalysts.

Bromine Doping as an Efficient Strategy to Reduce the Interfacial Defects in Hybrid Two-Dimensional/Three-Dimensional Stacking Perovskite Solar Cells.

Solar-to-electricity conversion efficiency, power conversion efficiency (PCE), and stability are two important aspects of perovskite solar cells (PSCs). However, both aspects are difficult to simultaneously enhance. In the recent two years, two-dimensional (2D)/three-dimensional (3D) stacking structure, designed by covering the 3D perovskite with a thin 2D perovskite capping layer, was reported to be a promising method to achieve both a higher PCE and improved stability simultaneously. However, when reducing the surface defects of 3D perovskite, the thin 2D capping layer itself may probably introduce additional interfacial defects in a 2D/3D stacking structure, which is thought to be able to trigger trap-assisted nonradiative recombination or ion migration. Thus, efforts should be paid to reduce the interfacial defects of 2D hybrid perovskite when serving as a modification layer in a 2D/3D stacking structure PSCs. Here, we demonstrate that bromine (Br) doping of the 2D perovskite capping layer is an efficient strategy to passivate interfacial defects robustly, by which the photoluminescence lifetime is enhanced notably, whereas the interfacial charge recombination is suppressed a lot. As a result, the PCE is enhanced from 18.01% (3D perovskite) to 20.07% (Br-doped 2D/3D perovskite) along with improved moisture stability.

Photo-generated dinuclear {Eu(II)}2 active sites for selective CO2 reduction in a photosensitizing metal-organic framework.

Photocatalytic reduction of CO2 is a promising approach to achieve solar-to-chemical energy conversion. However, traditional catalysts usually suffer from low efficiency, poor stability, and selectivity. Here we demonstrate that a large porous and stable metal-organic framework featuring dinuclear Eu(III)2 clusters as connecting nodes and Ru(phen)3-derived ligands as linkers is constructed to catalyze visible-light-driven CO2 reduction. Photo-excitation of the metalloligands initiates electron injection into the nodes to generate dinuclear {Eu(II)}2 active sites, which can selectively reduce CO2 to formate in a two-electron process with a remarkable rate of 321.9 µmol h-1 mmolMOF-1. The electron transfer from Ru metalloligands to Eu(III)2 catalytic centers are studied via transient absorption and theoretical calculations, shedding light on the photocatalytic mechanism. This work highlights opportunities in photo-generation of highly active lanthanide clusters stabilized in MOFs, which not only enables efficient photocatalysis but also facilitates mechanistic investigation of photo-driven charge separation processes.

Lead-Free, Two-Dimensional Mixed Germanium and Tin Perovskites.

Hybrid two-dimensional (2D) organic-inorganic perovskites continue to draw increased attention in view of their outstanding performance in optoelectronic devices such as solar cells and light-emitting devices. Herein, for the first time, we report the synthesis and characterization of lead-free, 2D mixed Ge-Sn halide perovskites, (PEA)2Ge1- xSn xI4 (where PEA = C6H5CH2CH2NH3+), and demonstrate that the bandgaps decrease linearly with increasing Sn content. Most importantly, among them, (PEA)2Ge0.5Sn0.5I4 possesses the smallest bandgap of 1.95 eV. Density functional theory calculations confirm that Sn substitution induces a smaller bandgap and more dispersed band structure, which are desirable characteristics of light-absorbing materials. In addition, conductivity and stability of (PEA)2Ge0.5Sn0.5I4 have also been assessed.