Bettina V. Lotsch

Crystalline Carbon Nitride Nanosheets for Improved Visible-Light Hydrogen Evolution

Nanosheets of a crystalline 2D carbon nitride were obtained by ionothermal synthesis of the layered bulk material poly(triazine imide), PTI, followed by one-step liquid exfoliation in water. Triazine-based nanosheets are 1-2 nm in height and afford chemically and colloidally stable suspensions under both basic and acidic conditions. We use solid-state NMR spectroscopy of isotopically enriched, restacked nanosheets as a tool to indirectly monitor the exfoliation process and carve out the chemical changes occurring upon exfoliation, as well as to determine the nanosheet thickness. PTI nanosheets show significantly enhanced visible-light driven photocatalytic activity toward hydrogen evolution compared to their bulk counterpart, which highlights the crucial role of morphology and surface area on the photocatalytic performance of carbon nitride materials.

A hydrazone-based covalent organic framework for photocatalytic hydrogen production

Covalent organic frameworks (COFs) have recently emerged as a new generation of porous polymers combining molecular functionality with the robustness and structural definition of crystalline solids. Drawing on the recent development of tailor-made semiconducting COFs, we report here on a new COF capable of visible-light driven hydrogen generation in the presence of Pt as a proton reduction catalyst (PRC). The COF is based on hydrazone-linked functionalized triazine and phenyl building blocks and adopts a layered structure with a honeycomb-type lattice featuring mesopores of 3.8 nm and the highest surface area among all hydrazone-based COFs reported to date. When illuminated with visible light, the Pt-doped COF continuously produces hydrogen from water without signs of degradation. With their precise molecular organization and modular structure combined with high porosity, photoactive COFs represent well-defined model systems to study and adjust the molecular entities central to the photocatalytic process.

A tunable azine covalent organic framework platform for visible light-induced hydrogen generation

Hydrogen evolution from photocatalytic reduction of water holds promise as a sustainable source of carbon-free energy. Covalent organic frameworks (COFs) present an interesting new class of photoactive materials, which combine three key features relevant to the photocatalytic process, namely crystallinity, porosity and tunability. Here we synthesize a series of water- and photostable 2D azine-linked COFs from hydrazine and triphenylarene aldehydes with varying number of nitrogen atoms. The electronic and steric variations in the precursors are transferred to the resulting frameworks, thus leading to a progressively enhanced light-induced hydrogen evolution with increasing nitrogen content in the frameworks. Our results demonstrate that by the rational design of COFs on a molecular level, it is possible to precisely adjust their structural and optoelectronic properties, thus resulting in enhanced photocatalytic activities. This is expected to spur further interest in these photofunctional frameworks where rational supramolecular engineering may lead to new material applications.

Triazine‐based Carbon Nitrides for Visible‐Light‐Driven Hydrogen Evolution

A new dimension: The doping of amorphous poly(triazine imide) (PTI) through ionothermal copolymerization of dicyandiamide with 4-amino-2,6-dihydroxypyrimidine (4AP) results in triazine-based carbon nitrides with increased photoactivity for water splitting compared to crystalline poly(triazine imide) (PTI/Li(+)Cl(-), see picture) and melon-type carbon nitrides. This family of carbon nitride semiconductors has potential as low-cost, environmentally clean photocatalysts for solar fuel production.

Nanofabrication by self-assembly

The self-assembly paradigm in chemistry, physics and biology has matured scientifically over the past two-decades to a point of sophistication that one can begin to exploit its numerous attributes in nanofabrication. In what follows we will take a brief look at current thinking about self-assembly and with some recent examples taken from our own work examine how nanofabrication has benefited from self-assembly.

Dirac cone protected by non-symmorphic symmetry and three-dimensional Dirac line node in ZrSiS

Materials harbouring exotic quasiparticles, such as massless Dirac and Weyl fermions, have garnered much attention from physics and material science communities due to their exceptional physical properties such as ultra-high mobility and extremely large magnetoresistances. Here, we show that the highly stable, non-toxic and earth-abundant material, ZrSiS, has an electronic band structure that hosts several Dirac cones that form a Fermi surface with a diamond-shaped line of Dirac nodes. We also show that the square Si lattice in ZrSiS is an excellent template for realizing new types of two-dimensional Dirac cones recently predicted by Young and Kane. Finally, we find that the energy range of the linearly dispersed bands is as high as 2 eV above and below the Fermi level; much larger than of other known Dirac materials. This makes ZrSiS a very promising candidate to study Dirac electrons, as well as the properties of lines of Dirac nodes.

Poly(triazine imide) with Intercalation of Lithium and Chloride Ions [(C3N3)2(NHxLi1−x)3⋅LiCl]: A Crystalline 2D Carbon Nitride Network

Poly(triazine imide) with intercalation of lithium and chloride ions (PTI/Li(+)Cl(-)) was synthesized by temperature-induced condensation of dicyandiamide in a eutectic mixture of lithium chloride and potassium chloride as solvent. By using this ionothermal approach the well-known problem of insufficient crystallinity of carbon nitride (CN) condensation products could be overcome. The structural characterization of PTI/Li(+)Cl(-) resulted from a complementary approach using spectroscopic methods as well as different diffraction techniques. Due to the high crystallinity of PTI/Li(+)Cl(-) a structure solution from both powder X-ray and electron diffraction patterns using direct methods was possible; this yielded a triazine-based structure model, in contrast to the proposed fully condensed heptazine-based structure that has been reported recently. Further information from solid-state NMR and FTIR spectroscopy as well as high-resolution TEM investigations was used for Rietveld refinement with a goodness-of-fit (χ(2)) of 5.035 and wRp=0.05937. PTI/Li(+)Cl(-) (P6(3)cm (no. 185); a=846.82(10), c=675.02(9) pm) is a 2D network composed of essentially planar layers made up from imide-bridged triazine units. Voids in these layers are stacked upon each other forming channels running parallel to [001], filled with Li(+) and Cl(-) ions. The presence of salt ions in the nanocrystallites as well as the existence of sp(2)-hybridized carbon and nitrogen atoms typical of graphitic structures was confirmed by electron energy-loss spectroscopy (EELS) measurements. Solid-state NMR spectroscopy investigations using (15)N-labeled PTI/Li(+)Cl(-) proved the absence of heptazine building blocks and NH(2) groups and corroborated the highly condensed, triazine-based structure model.

New Light on an Old Story: Perovskites Go Solar

There are only few inorganic materials that have been shaping the progress in the solid-state sciences as much as perovskites. Although they are deceivingly simple in structure, the archetypal ABO3-type perovskites have a built-in potential for complexity and surprising discoveries. The history of perovskites is both winding and dazzling, but they have always been a major player in solid-state chemistry and physics because of their many intriguing properties. Perovskites, which were named after the Russian mineralogist Lew A. Perowski, span a large class of ternary oxides, but nitrides, halides, and other compositions are also known. Apart from notable exceptions, such as SrTiO3, perovskites commonly unfold their full potential when they gain complexity, for example, by undergoing structural distortions, by adopting complex intergrowth structures as in Aurivillius phases, or by doping in general. Subtle distortions of the octahedral sublattice give rise to magnetism and orbital ordering phenomena, non-stoichiometries paired with electron correlations boost the critical temperature in high-Tc superconductors, such as YBa2Cu3O7 x, and oxygen vacancies may increase the ionic conductivity by orders of magnitude. Owing to these and other properties, such as ferroand piezoelectricity or giant magnetoresistance, numerous perovskites have gained an indisputable rank among high-performance materials for capacitors, electrooptical switches, electrode materials, or memory devices. Ironically, a technology that has never been associated with these materials has recently sparked a renaissance of the perovskites, namely photovoltaics. Even more ironically, a small and somewhat exotic class of perovskites, which is quite different from the common rock-solid perovskite oxides, has now turned over a new leaf in solar cell research. Recently, methylammonium tin and lead halides with the general formula (CH3NH3)MX3 xYx (M = Pb or Sn; X,Y= I, Br, Cl) attracted particular attention; they are members of a family of artificial hybrid perovskites with differing dimensionality that were first discovered by Weber and later significantly advanced by Mitzi and co-workers. 2] This class of perovskites has now pushed the limits of dye-sensitized solar cell (DSSC) technologies to photoconversion efficiencies (PCEs) beyond 15 %, a benchmark value nobody would have dreamed of hardly more than a year ago. Hybrid perovskites made a re-entry into the literature in the early nineties, as they show a tunable metal-to-semiconductor transition as a function of the thickness of the perovskite blocks. Starting from the 3D perovskite ABX3 with n =1, where small A site alkylammonium ions are encased by corner-sharing MX6 octahedra, the perovskite blocks can be gradually electronically decoupled and “quantum-confined” by interspersing the perovskite matrix with organic modulation layers that are composed of bulky organoammonium cations (Figure 1). Like their closely related alkali halide counterparts (A = alkali), organometal halide perovskites may be processed in solution, which allows the low-temperature deposition of the perovskite structure by self-assembly from completely soluble precursors. Organoperovskites do not only possess modular structures and an inherently high tendency for crystallization, but also feature high molar absorption coefficients and panchromaticity, that is, excellent light-harvesting performance that spans the entire visible range, high charge-carrier mobilities, unusually large exciton diffusion lengths, and, importantly, band-level characteristics that favorably match those of common holeand electron-transport materials in DSSCs. This is a rather ideal combination of properties, which should give the longstanding concept of all-solid-state solar cells a sizable boost. The pace at which DSSC-type solar-cell technologies have been developing since their discovery by O Regan and Gr tzel in 1991 has been intimately linked to the weaknesses of such intricately optimized systems on the one hand, and benchmark discoveries that overcame these limitations on the other hand. Compared to other systems, namely thinfilm silicon, CdTe, and CIGS (CIGS = copper indium gallium selenide) solar cells, which currently reach efficiencies of up to 20%, the performance of DSSCs has been competitive, but nevertheless lower and less robust. The subtle interplay between the TiO2 photoanode, the dye, and the electrolyte requires a balance in the choice of materials that is notoriously hard to achieve. At the same time, however, DSSCs are based on inexpensive materials and feature exceptional flexibility, both with respect to materials selection and the manufacturing processes. DSSCs can be engineered into a plethora of shapes, flexible sheets, and colorful designs at low cost, which would bode well for a bright future, if it was not for their inherent weaknesses, including photobleaching [*] Prof. B. V. Lotsch Max Planck Institute for Solid State Research Heisenbergstr. 1, 70569 Stuttgart (Germany) and Chemistry Department Ludwig-Maximilians-Universit t M nchen Butenandtstr. 5–13, 81377 M nchen (Germany) E-mail: b.lotsch@fkf.mpg.de Angewandte Chemie

Photonic clays: a new family of functional 1D photonic crystals.

Clays have shown potential as intelligent optical sensing platforms when integrated into a one-dimensional photonic crystal (PC) environment. The clay component imparts intrinsic functionality to the multilayer system by combining the signature ion exchange with the tunable structural color of photonic crystals, giving rise to environmentally sensitive photonic clay architectures. We have fabricated different Laponite-based 1D PCs and clay defect PCs by simple bottom-up self-assembly methodologies and elaborate their working principles and chemically encoded optical response. Accessibility of the multilayer system to analytes is studied on the background of the barrier properties of clays and diffusion control by the mesoporous oxide layers. The time dependence of analyte uptake and the extent and driving force for analyte release are pointed out and discussed in the context of different interactions between the clay layers and analytes. We demonstrate the possibility of optical cycling associated with repeated analyte uptake and removal processes, rendering photonic clays recyclable and low cost sensing platforms with simple optical read-out.

A fluorene based covalent triazine framework with high CO2 and H2 capture and storage capacities

Porous organic polymers have come into focus recently for the capture and storage of postcombusted CO2. Covalent triazine frameworks (CTFs) constitute a nitrogen-rich subclass of porous polymers, which offers enhanced tunability and functionality combined with high chemical and thermal stability. In this work a new covalent triazine framework based on fluorene building blocks is presented, along with a comprehensive elucidation of its local structure, porosity, and capacity for CO2 capture and H2 storage. The framework is synthesized under ionothermal conditions at 300–600 °C using ZnCl2 as a Lewis acidic trimerization catalyst and reaction medium. Whereas the materials synthesized at lower temperatures mostly feature ultramicropores and moderate surface areas as probed by CO2 sorption (297 m2 g−1 at 300 °C), the porosity is significantly increased at higher synthesis temperatures, giving rise to surface areas in excess of 2800 m2 g−1. With a high fraction of micropores and a surface area of 1235 m2 g−1, the CTF obtained at 350 °C shows an excellent CO2 sorption capacity at 273 K (4.28 mmol g−1), which is one of the highest observed among all porous organic polymers. Additionally, the materials have CO2/N2 selectivities of up to 37. The hydrogen adsorption capacity of 4.36 wt% at 77 K and 20 bar is comparable to that of other POPs, yet the highest among all CTFs studied to date.