Dark Photocatalysis: Storage of Solar Energy in Carbon Nitride for Time-Delayed Hydrogen Generation

While natural photosynthesis serves as the model system for efficient charge separation and decoupling of redox reactions, bio-inspired artificial systems typically lack applicability owing to synthetic challenges and structural complexity. We present herein a simple and inexpensive system that, under solar irradiation, forms highly reductive radicals in the presence of an electron donor, with lifetimes exceeding the diurnal cycle. This radical species is formed within a cyanamide-functionalized polymeric network of heptazine units and can give off its trapped electrons in the dark to yield H$_{2}$ , triggered by a co-catalyst, thus enabling the temporal decoupling of the light and dark reactions of photocatalytic hydrogen production through the radical's longevity. The system introduced here thus demonstrates a new approach for storing sunlight as long-lived radicals, and provides the structural basis for designing photocatalysts with long-lived photo-induced states.This work was supported by the Deutsche Forschungsgemeinschaft (project LO1801/1-1) and an ERC Starting Grant (B.V.L., grant number 639233), the Max Planck Society, the cluster of excellence Nanosystems Initiative Munich (NIM), and the Center for Nanoscience (CeNS). We acknowledge support by the Christian Doppler Research Association (Austrian Federal Ministry of Science, Research and Economy, National Foundation for Research, Technology and Development) and the OMV Group (H.K., E.R.). V.W.-h.L. gratefully acknowledges a postdoctoral scholarship from the Max Planck Society

Relaxed Current Matching Requirements in Highly Luminescent Perovskite Tandem Solar Cells and Their Fundamental Efficiency Limits

Perovskite-based tandem solar cells are of increasing interest as they approach commercialization. Here we use experimental parameters from optical spectroscopy measurements to calculate the limiting efficiency of perovskite–silicon and all-perovskite two-terminal tandems, employing currently available bandgap materials, as 42.0% and 40.8%, respectively. We show luminescence coupling between subcells (the optical transfer of photons from the high-bandgap to low-bandgap subcell) relaxes current matching when the high-bandgap subcell is a luminescent perovskite. We calculate that luminescence coupling becomes important at charge trapping rates (≤106 s–1) already being achieved in relevant halide perovskites. Luminescence coupling increases flexibility in subcell thicknesses and tolerance to different spectral conditions. For maximal benefit, the high-bandgap subcell should have the higher short-circuit current under average spectral conditions. This can be achieved by reducing the bandgap of the high-bandgap subcell, allowing wider, unstable bandgap compositions to be avoided. Lastly, we visualize luminescence coupling in an all-perovskite tandem through cross-section luminescence imaging

How reproducible are surface areas calculated from the BET equation?

Porosity and surface area analysis play a prominent role in modern materials science. At the heart of this sits the Brunauer-Emmett-Teller (BET) theory, which has been a remarkably successful contribution to the field of materials science. The BET method was developed in the 1930s for open surfaces but is now the most widely used metric for the estimation of surface areas of micro- and mesoporous materials. Despite its widespread use, the calculation of BET surface areas causes a spread in reported areas, resulting in reproducibility problems in both academia and industry. To prove this, for this analysis, 18 already-measured raw adsorption isotherms were provided to sixty-one labs, who were asked to calculate the corresponding BET areas. This round-robin exercise resulted in a wide range of values. Here, the reproducibility of BET area determination from identical isotherms is demonstrated to be a largely ignored issue, raising critical concerns over the reliability of reported BET areas. To solve this major issue, a new computational approach to accurately and systematically determine the BET area of nanoporous materials is developed. The software, called "BET surface identification" (BETSI), expands on the well-known Rouquerol criteria and makes an unambiguous BET area assignment possible

Atomic Resolution Observation of the Oxidation of Niobium Oxide Nanowires: Implications for Renewable Energy Applications

Niobium oxide exists in several crystal phases and valence states, yielding a variety of properties. Phase transitions between polymorphs significantly alter the properties of functional niobium oxide nanostructures, which are applied in many different renewable energy applications. This study uses environmental transmission electron microscopy to investigate the oxidation of NbO nanowires to T-Nb2O5. Small oxygen partial pressures (<1 mbar) suffice to initiate the oxidation which initially causes the destruction of the lattice ordering prior to the crystallization of the new phase. Different oxygen partial pressures and temperatures are used to manipulate the reaction kinetics. The crystallization is slow at room temperature and confined to the nanowire surface. Elevated temperatures accelerate the crystallization, resulting in the transformation of the complete nanowire. The transformation proceeds via dislocations, yielding the woven-like structure of the transformed nanowire. Despite the dramatic changes of the atomic composition of the material, an orientation relationship between the different phases is preserved. The phase transformation of NbO to T-Nb2O5 observed in situ complies with ex situ phase diagrams. This illustrates the importance of environmental transmission electron microscopy for the investigation of oxidation and reduction reactions of metal oxide nanostructures to identify the impact of shape and intrinsic defects

Lithium Tin Sulfide—a High-Refractive-Index 2D Material for Humidity-Responsive Photonic Crystals

Extending the portfolio of novel stimuli-responsive, high-refractive-index (RI) materials besides titania is key to improve the optical quality and sensing performance of existing photonic devices. Herein, lithium tin sulfide (LTS) nanosheets are introduced as a novel solution processable ultrahigh RI material (n = 2.50), which can be casted into homogeneous thin films using wet-chemical deposition methods. Owing to its 2D morphology, thin films of LTS nanosheets are able to swell in response to changes of relative humidity. Integration of LTS nanosheets into Bragg stacks (BSs) based on TiO2, SiO2, nanoparticles or H3Sb3P2O14 nanosheets affords multilayer systems with high optical quality at an extremely low device thickness of below 1 µm. Owing to the ultrahigh RI of LTS nanosheets and the high transparency of the thin films, BSs based on porous titania as the low-RI material are realized for the first time, showing potential application in light-managing devices. Moreover, the highest RI contrast ever realized in BSs based on SiO2 and LTS nanosheets is reported. Finally, exceptional swelling capability of an all-nanosheet BS based on LTS and H3Sb3P2O14 nanosheets is demonstrated, which bodes well for a new generation of humidity sensors with extremely high sensitivity

Cross-Linking Bi2S3 Ultrathin Nanowires: A Platform for Nanostructure Formation and Biomolecule Detection

This paper describes the use of chemical cross-linking of ultrathin inorganic nanowires; as a bottom-up strategy for nanostructure fabrication as well as a chemical detection platform. Nanowire microfibers are produced by spinning a nanowire dispersion into a cross-linker solution at room temperature. Nanomembranes with thicknesses down to 50 nm were obtained by injecting the nanowire dispersion at the crosslinker-solution/air interface. Furthermore, the sensitivity of the nanowire to amine cross-linkers allowed development of a novel sensing platform for small molecules, like the neurotransmitter serotonin, with detection limits in the picomolar regime

Tailor-Made Photoconductive Pyrene-Based Covalent Organic Frameworks for Visible-Light Driven Hydrogen Generation

Covalent organic frameworks (COFs) have emerged as a new class of crystalline porous polymers displaying molecular tunability combined with structural definition. Here, a series of three conjugated, photoactive azine-linked COFs based on pyrene building blocks which differ in the number of nitrogen atoms in the peripheral aromatic units is presented. The structure of the COFs is analyzed by combined experimental and computational physisorption as well as quantum-chemical calculations, which suggest a slipped-stacked arrangement of the 2D layers. Photocurrents of up to 6 mu A cm(-2) with subsecond photoresponse times are measured on thin film samples for the first time. While all COFs are capable of producing hydrogen from water, their efficiency increases significantly with decreasing number of nitrogen atoms. The trending activities are rationalized by photoelectrochemical measurements and quantum-chemical calculations which suggest an increase in the thermodynamic driving force with decreasing nitrogen content to be the origin of the observed differences in hydrogen evolution activities