Tunable Band Engineering Management on Perovskite MAPbBr3 /COFs Nano-Heterostructures for Efficient S-S Coupling Reactions.

Efficient artificial photosynthesis of disulfide bonds holds promises to facilitate reverse decoding of genetic codes and deciphering the secrets of protein multilevel folding, as well as the development of life science and advanced functional materials. However, the incumbent synthesis strategies encounter separation challenges arising from leaving groups in the ─S─S─ coupling reaction. In this study, according to the reaction mechanism of free-radical-triggered ─S─S─ coupling, light-driven heterojunction functional photocatalysts are tailored and constructed, enabling them to efficiently generate free radicals and trigger the coupling reaction. Specifically, perovskites and covalent organic frameworks (COFs) are screened out as target materials due to their superior light-harvesting and photoelectronic properties, as well as flexible and tunable band structure. The in situ assembled Z-scheme heterojunction MAPB-M-COF (MAPbBr3 = MAPB, MA+ = CH3 NH2 + ) demonstrates a perfect trade-off between quantum efficiency and redox chemical potential via band engineering management. The MAPB-M-COF achieves a 100% ─S─S─ coupling yield with a record photoquantum efficiency of 11.50% and outstanding cycling stability, rivaling all the incumbent similar reaction systems. It highlights the effectiveness and superiority of application-oriented band engineering management in designing efficient multifunctional photocatalysts. This study demonstrates a concept-to-proof research methodology for the development of various integrated heterojunction semiconductors for light-driven chemical reaction and energy conversion.

Efficient and stable emission of warm-white light from lead-free halide double perovskites.

Lighting accounts for one-fifth of global electricity consumption1. Single materials with efficient and stable white-light emission are ideal for lighting applications, but photon emission covering the entire visible spectrum is difficult to achieve using a single material. Metal halide perovskites have outstanding emission properties2,3; however, the best-performing materials of this type contain lead and have unsatisfactory stability. Here we report a lead-free double perovskite that exhibits efficient and stable white-light emission via self-trapped excitons that originate from the Jahn-Teller distortion of the AgCl6 octahedron in the excited state. By alloying sodium cations into Cs2AgInCl6, we break the dark transition (the inversion-symmetry-induced parity-forbidden transition) by manipulating the parity of the wavefunction of the self-trapped exciton and reduce the electronic dimensionality of the semiconductor4. This leads to an increase in photoluminescence efficiency by three orders of magnitude compared to pure Cs2AgInCl6. The optimally alloyed Cs2(Ag0.60Na0.40)InCl6 with 0.04 per cent bismuth doping emits warm-white light with 86 ± 5 per cent quantum efficiency and works for over 1,000 hours. We anticipate that these results will stimulate research on single-emitter-based white-light-emitting phosphors and diodes for next-generation lighting and display technologies.

Elucidating the Unique Hot Carrier Cooling in Two-Dimensional Inorganic Halide Perovskites: The Role of Out-of-Plane Carrier-Phonon Coupling.

Two-dimensional (2D) halide perovskites represent the natural semiconductor quantum wells (QWs), which hold great promise for optoelectronics. However, due to the hybrid structure of Ruddlesden-Popper 2D perovskites, the intrinsic nature of hot-carrier kinetics remains shielded within. Herein, we adopt CsPbBr3 nanoplates as a model system to reveal the intrinsic carrier dynamics in inorganic perovskite QWs. Interestingly, we revealed an ultrafast and hot-phonon-bottleneck (HPB)-free carrier cooling in monodisperse CsPbBr3 QWs, which is in sharp contrast to the bulk and nanocrystalline perovskites. The absence of HPB was attributed to the efficient out-of-plane triplet-exciton-LO-phonon coupling in 2D perovskites because of the structural anisotropy. Accordingly, the HPB can be activated by shutting down the out-of-plane energy loss route through forming the layer-stacked perovskite superlattice. The controllable on and off of HPB may provide new possibilities in optoelectronic devices and these findings deepen the understanding of a hot-carrier cooling mechanism in 2D perovskites.

Insight into the Key Restriction of BiVO4 Photoanodes Prepared by Pyrolysis Method for Scalable Preparation.

Bismuth Vanadate (BiVO4 ) photoanode has been popularly investigated for promising solar water oxidation, but its intrinsic performance has been greatly retarded by the direct pyrolysis method. Here we insight the key restriction of BiVO4 prepared by metal-organic decomposition (MOD) method. It is found that the evaporation of vanadium during the pyrolysis tends to cause a substantial phase impurity, and the unexpected few tetragonal phase inhibits the charge separation evidently. Consequently, suitably excessive vanadium precursor was adopted to eliminate the phase impurity, based on which the obtained intrinsic BiVO4 photoanode could exhibit photocurrent density of 4.2 mA cm-2 at 1.23 VRHE under AM 1.5 G irradiation, as comparable to the one fabricated by the currently popular two-step electrodeposition method. Furthermore, the excellent performance can be maintained on the enlarged photoanode (25 cm2 ), demonstrating the advantage of MOD method in scalable preparation. Our work provides new insight and highlights the glorious future of MOD method for the design of scale-up efficient BiVO4 photoanode.

Anomalous Charge Transfer from Organic Ligands to Metal Halides in Zero-Dimensional [(C6 H5 )4 P]2 SbCl5 Enabled by Pressure-Induced Lone Pair-π Interaction.

Low-dimensional (low-D) organic metal halide hybrids (OMHHs) have emerged as fascinating candidates for optoelectronics due to their integrated properties from both organic and inorganic components. However, for most of low-D OMHHs, especially the zero-D (0D) compounds, the inferior electronic coupling between organic ligands and inorganic metal halides prevents efficient charge transfer at the hybrid interfaces and thus limits their further tunability of optical and electronic properties. Here, using pressure to regulate the interfacial interactions, efficient charge transfer from organic ligands to metal halides is achieved, which leads to a near-unity photoluminescence quantum yield (PLQY) at around 6.0 GPa in a 0D OMHH, [(C6 H5 )4 P]2 SbCl5 . In situ experimental characterizations and theoretical simulations reveal that the pressure-induced electronic coupling between the lone-pair electrons of Sb3+ and the π electrons of benzene ring (lp-π interaction) serves as an unexpected "bridge" for the charge transfer. Our work opens a versatile strategy for the new materials design by manipulating the lp-π interactions in organic-inorganic hybrid systems.

Water-Stable Nickel Metal-Organic Framework Nanobelts for Cocatalyst-Free Photocatalytic Water Splitting to Produce Hydrogen.

Development of water-stable metal-organic frameworks (MOFs) for promising visible-light-driven photocatalytic water splitting is highly desirable but still challenging. Here we report a novel p-type nickel-based MOF single crystal (Ni-TBAPy-SC) and its exfoliated nanobelts (Ni-TBAPy-NB) that can bear a wide range of pH environment in aqueous solution. Both experimental and theoretical results indicate a feasible electron transfer from the H4TBAPy ligand (light-harvesting center) to the Ni-O cluster node (catalytic center), on which water splitting to produce hydrogen can be efficiently driven free of cocatalyst. Compared to the single crystal, the exfoliated two-dimensional (2D) nanobelts show more efficient charge separation due to its shortened charge transfer distance and remarkably enhanced active surface areas, resulting in 164 times of promoted water reduction activity. The optimal H2 evolution rate on the nanobelt reaches 98 µmol h-1 (ca. 5 mmol h-1 g-1) showing benchmarked apparent quantum efficiency (AQE) of 8.0% at 420 nm among water-stable MOFs photocatalysts.

Zone-Folded Longitudinal Acoustic Phonons Driving Self-Trapped State Emission in Colloidal CdSe Nanoplatelet Superlattices.

Colloidal CdSe nanoplatelets (NPLs) have substantial potential in light-emitting applications because of their quantum-well-like characteristics. The self-trapped state (STS), originating from strong electron-phonon coupling (EPC), is promising in white light luminance because of its broadband emission. However, achieving STS in CdSe NPLs is extremely challenging because of their intrinsic weak EPC nature. Herein, we developed a strong STS emission in the spectral range of 450-600 nm by building superlattice (SL) structures with colloidal CdSe NPLs. We demonstrated that STS is generated via strong coupling of excitons and zone-folded longitudinal acoustic phonons with formation time of ∼450 fs and localization length of ∼0.56 nm. The Huang-Rhys factor, describing the EPC strength in SL structure, is estimated to be ∼19.9, which is much larger than that (∼0.1) of monodispersed CdSe NPLs. Our results provide an in-depth understanding of STS and a platform for generating and manipulating STS by designing SL structures.

Auger-Assisted Electron Transfer between Adjacent Quantum Wells in Two-Dimensional Layered Perovskites.

Two-dimensional (2D) layered perovskites are naturally formed multiple quantum-well (QW) materials, holding great promise for applications in many optoelectronic devices. However, the further use of 2D layered perovskites in some devices is limited by the lack of QW-to-QW carrier transport/transfer due to the energy barrier formed by the insulating ligands between QWs. Herein, we report an Auger-assisted electron transfer between adjacent QWs in (CmH2m+1NH3)2PbI4 2D perovskites particularly with m = 12 and 18, where the electron energy barrier (Eb) is similar to the QW band gap energy (Eg). This Auger-assisted QW-to-QW electron transfer mechanism is established by the observation of a long-lived and derivative-like transient absorption feature, which is a signature of the quantum confined Stark effect induced by the electron-hole separation (thus an internal electric field) between different QW layers. Our finding provides a new guideline to design 2D perovskites with an optically tunable QW-to-QW charge transport property, advancing their applications in optoelectronics and optical modulations.

Ultrafast and High-Yield Polaronic Exciton Dissociation in Two-Dimensional Perovskites.

Layered two-dimensional (2D) lead halide perovskites are a class of quantum well (QW) materials, holding dramatic potentials for optical and optoelectronic applications. However, the thermally activated exciton dissociation into free carriers in 2D perovskites, a key property that determines their optoelectronic performance, was predicted to be weak due to large exciton binding energy (Eb, about 100-400 meV). Herein, in contrast to the theoretical prediction, we discover an ultrafast (<1.4 ps) and highly efficient (>80%) internal exciton dissociation in (PEA)2(MA)n-1PbnI3n+1 (PEA = C6H5C2H4NH3+, MA = CH3NH3+, n = 2-4) 2D perovskites despite the large Eb. We demonstrate that the exciton dissociation activity in 2D perovskites is significantly promoted because of the formation of exciton-polarons with considerably reduced exciton binding energy (down to a few tens of millielectronvolts) by the polaronic screening effect. This ultrafast and high-yield exciton dissociation limits the photoluminescence of 2D perovskites but on the other hand well explains their exceptional performance in photovoltaic devices. The finding should represent a common exciton property in the 2D hybrid perovskite family and provide a guideline for their rational applications in light emitting and photovoltaics.

The Hole-Tunneling Heterojunction of Hematite-Based Photoanodes Accelerates Photosynthetic Reaction.

Single-atom metal-insulator-semiconductor (SMIS) heterojunctions based on Sn-doped Fe2 O3 nanorods (SF NRs) were designed by combining atomic deposition of an Al2 O3 overlayer with chemical grafting of a RuOx hole-collector for efficient CO2 -to-syngas conversion. The RuOx -Al2 O3 -SF photoanode with a 3.0 nm thick Al2 O3 overlayer gave a >5-fold-enhanced IPCE value of 52.0 % under 370 nm light irradiation at 1.2 V vs. Ag/AgCl, compared to the bare SF NRs. The dielectric field mediated the charge dynamics at the Al2 O3 /SF NRs interface. Accumulation of long-lived holes on the surface of the SF NRs photoabsorber aids fast tunneling transfer of hot holes to single-atom RuOx species, accelerating the O2 -evolving reaction kinetics. The maximal CO-evolution rate of 265.3 mmol g-1 h-1 was achieved by integration of double SIMS-3 photoanodes with a single-atom Ni-doped graphene CO2 -reduction-catalyst cathode; an overall quantum efficiency of 5.7 % was recorded under 450 nm light irradiation.

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.

Host-Guest Thin Films by Confining Ultrafine Pt/C QDs into Metal-Organic Frameworks for Highly Efficient Hydrogen Evolution.

Combining the features of host templates and guest species is an efficient strategy to optimize the photo/electrocatalytic performance. Herein, novel host-guest thin-film electrocatalysts are designed and developed with Pt doped carbon (Pt/C) confined into porphyrin-based metal-organic frameworks (MOFs). Porous MOF PCN-222 and PCN-221 thin films are used as the host templates and fabricated using vapor-assisted deposition method, and then the guest Pt/C quantum dots are encapsulated into the MOFs by loading the glucose mixed H2 PtCl6 and heating at 200 °C. Thanks to the confinement effect of MOF pores, the homogenous and ultrafine Pt/C nanowires (Pt/CNWs) and nanodots (Pt/CNDs) are confined in nanochannels of PCN-222 and nanocages of PCN-221 (Pt/CNW@PCN-222 and Pt/CND@PCN-221), respectively. The electrocatalytic study shows that the host-guest thin films have highly-efficient electrocatalytic hydrogen evolution performance under light irradiation. Furthermore, the time-resolved photoluminescent results reveal that Pt/CNW@PCN-222 has a faster charge transfer (441 ps) from PCN-222 to Pt/CNWs comparing to that (557 ps) of Pt/CND@PCN-221, indicating the guests with different shapes play an important role in the electrocatalytic performance. This work serves to present both the outstanding level of control in the precise synthesis and high potential for nanocomposite thin films in photo-electrocatalytic application.

Interfacial Engineering at the 2D/3D Heterojunction for High-Performance Perovskite Solar Cells.

Perovskite solar cells based on two-dimensional/three-dimensional (2D/3D) hierarchical structure have attracted significant attention in recent years due to their promising photovoltaic performance and stability. However, obtaining a detailed understanding of interfacial mechanism at the 2D/3D heterojunction, for example, the ligand-chemistry-dependent nature of the 2D/3D heterojunction and its influence on charge collection and the final photovoltaic outcome, is not yet fully developed. Here we demonstrate the underlying 3D phase templates growth of quantum wells (QWs) within a 2D capping layer, which is further influenced by the fluorination of spacers and compositional engineering in terms of thickness distribution and orientation. Better QW alignment and faster dynamics of charge transfer at the 2D/3D heterojunction result in higher charge mobility and lower charge recombination loss, largely explaining the significant improvements in charge collection and open-circuit voltage (VOC) in complete solar cells. As a result, 2D/3D solar cells with a power-conversion efficiency of 21.15% were achieved, significantly higher than the 3D counterpart (19.02%). This work provides key missing information on how interfacial engineering influences the desirable electronic properties of the 2D/3D hierarchical films and device performance via ligand chemistry and compositional engineering in the QW layer.

Excitation-Dependent Emission Color Tuning from an Individual Mn-Doped Perovskite Microcrystal.

Divalent manganese cation (Mn2+) doped perovskite materials are of great interest for their unique optical, magnetic, and electric properties. Herein, we report an excitation-dependent emission color tuning from an individual Mn-doped CsPbCl3 microcrystal (MC) with a wide color tuning range, reversible and continuous color change, and high photostability. We demonstrate that the Mn-doped CsPbCl3 MCs exhibit dual-color emission from both host excitons (blue) and Mn-dopants (orange) through an internal energy transfer (IET) process. By simple change of the laser excitation repetition rate or pulse intensity, the relative emission intensity between exciton (Iexciton) and Mn-dopant (IMn) can be continuously and reversibly tuned from IMn/Itotal (Itotal = IMn + Iexciton) = 0.9-0.8 to 0.1-0.2, corresponding to a color change from orange to blue. Such emission color tuning is enabled by the saturation of Mn-dopant emission at high excitation intensity and a linear dependence of exciton emission with excitation intensity. Transient spectroscopy and temperature-dependent photoluminescence (PL) measurements confirm that the exciton-to-dopant IET in Mn-doped CsPbCl3 MCs is mediated by some shallow trap states, rather than through a direct transfer pathway. Therefore, the saturation of Mn-dopant emission is caused by a bottlenecked energy transfer effect by saturating the mediating trap states at high excitation intensities. The Mn-doped MCs also exhibit a high photostability on the reversible switch of emission color between orange and blue for more than 300 cycles within a continuous operation time of 14 h. In view of the stable and color-switchable emission properties, Mn-doped perovskite MCs may find application in nanophotonic devices using a single MC.

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.

Process Optimization, Characterization and Antioxidant Capacity of Oat (Avena Sativa L.) Bran Oil Extracted by Subcritical Butane Extraction.

Oat bran is a traditional agricultural byproduct and rarely used in edible oil processing. In this paper, oat bran oil (OBO) was firstly extracted by subcritical butane extraction (SBE) and the extraction process was optimized using response surface methodology. Three variables involving liquid-to-solid ratio, extraction time and extraction temperature were studied. The optimum conditions for extraction of OBO were obtained as follows: liquid-to-solid ratio 4.30, extraction time 48.15 min, and extraction temperature 46.52 °C. Based on this, an alternative method (SBE-e) for cosolvent (ethanol) was proposed to improve SBE method. Compared to conventional hexane extraction (CHE), the SBE-e had significant effect on yield, bioactive compounds (phytosterols and phenols) and antioxidant capacity (AC) in the extracted OBO. The results indicated that the proposed methods were appropriate for OBO extraction. Additionally, OBO had the potential to be an acceptable substitute for edible oil, owing to its desirable physicochemical characteristics, a balanced fatty acids composition and high antioxidant capacity.

Long-Distance Charge Carrier Funneling in Perovskite Nanowires Enabled by Built-in Halide Gradient.

The excellent charge carrier transportation in organolead halide perovskites is one major contributor to the high performance of many perovskite-based devices. There still exists a possibility for further enhancement of carrier transportation through nanoscale engineering, owing to the versatile wet-chemistry synthesis and processing of perovskites. Here we report the successful synthesis of bromide-gradient CH3NH3PbBrxI3-x single-crystalline nanowires (NWs) by a solid-to-solid ion exchange reaction starting from one end of pure CH3NH3PbI3 NWs, which was confirmed by local photoluminescence (PL) and energy dispersive X-ray spectroscopy (EDS) measurements. Due to the built-in halide gradient, the long-distance carrier transportation was driven by the energy funnel, rather than the spontaneous carrier diffusion. Indeed, local PL kinetics demonstrated effective charge carrier transportation only from the high-bandgap bromide-rich region to the low-bandgap iodine-rich region over a few micrometers. Therefore, these halide gradient NWs might find applications in various optoelectronic devices requiring long-distance and directional delivery of excitation energy.

Observation of Internal Photoinduced Electron and Hole Separation in Hybrid Two-Dimentional Perovskite Films.

Two-dimensional (2D) organolead halide perovskites are promising for various optoelectronic applications. Here we report a unique spontaneous charge (electron/hole) separation property in multilayered (BA)2(MA)n-1PbnI3n+1 (BA = CH3(CH2)3NH3+, MA = CH3NH3+) 2D perovskite films by studying the charge carrier dynamics using ultrafast transient absorption and photoluminescence spectroscopy. Surprisingly, the 2D perovskite films, although nominally prepared as "n = 4", are found to be mixture of multiple perovskite phases, with n = 2, 3, 4 and ≈ ∞, that naturally align in the order of n along the direction perpendicular to the substrate. Driven by the band alignment between 2D perovskites phases, we observe consecutive photoinduced electron transfer from small-n to large-n phases and hole transfer in the opposite direction on hundreds of picoseconds inside the 2D film of ∼358 nm thickness. This internal charge transfer efficiently separates electrons and holes to the upper and bottom surfaces of the films, which is a unique property beneficial for applications in photovoltaics and other optoelectronics devices.

Macroscopic Wires from Fluorophore-Quencher Dyads with Long-Lived Blue Emission.

We report the formation of macroscopic wires up to centimeters in length from a series of structurally flexible, covalently tethered small-molecular fluorophore-quencher dyads (FQDs, average MW = 425 Da), comprised of carbazole, melatonin, and cyanobenzoate moieties. These FQDs are nonemissive in organic solutions but become moderately to highly luminescent (ΦF = 0.037-0.39) upon formation of wires with emission maxima in the blue region (446-483 nm). The blue photoluminescence (PL) is ascribed to a combination of singlet charge transfer, localized triplet state, and possibly delayed fluorescence emissions with intrinsic luminescence lifetimes ranging from 0.228 to 21333 µs, based on luminescence, transient absorption measurements, X-ray diffraction, and calculations.