Periodic Reporting for period 1 - TOGETHER (Towards Green Hydrogen by Layered Metal Halide Perovskite Heterostructures)
Reporting period: 2023-01-16 to 2025-01-15
The European Green Deal targets climate neutrality by 2050, emphasizing renewable energy and green hydrogen as key strategies for decarbonization. Direct solar-to-fuel conversion, where solar energy drives the reduction of protons into hydrogen using a photocatalyst, is a promising yet challenging approach that requires advances in material efficiency. Two-dimensional layered perovskites (2DLPs) offer tunable optoelectronic properties, solution processability, and enhanced stability compared to their 3D counterparts, making them strong candidates for light-driven chemical reactions. However, their high exciton binding energy limits charge separation, posing a challenge for efficient photocatalysis. Further, the layered structure of 2DLPs allows excitons and charge carriers to move more freely in the in-plane direction compared to the out-of-plane direction. This makes lateral heterostructures, where material properties vary along the in-plane direction, especially useful for efficient energy transfer, charge separation, and extraction.
TOGETHER (Towards Green Hydrogen by Layered Metal Halide Perovskite Heterostructures) tackled this challenge by integrating lateral heterostructures into 2DLPs to improve charge separation and enable directional charge carrier flow for light-driven chemical reactions. Through controlled ion exchange and sequential crystallization, the project developed tailored 2DLP heterostructures with tunable band alignments. By combining expertise in material synthesis, advanced photophysics, and material characterization, TOGETHER aimed to bridge the gap between fundamental material design and practical photocatalytic applications. The project focused on three key objectives: development of lateral heterostructures in 2DLPs with tunable band alignments, understanding their formation mechanisms and structural and optical properties, and demonstrating their versatility for light-driven applications. Overall, TOGETHER introduced novel synthesis techniques for complex lateral heterostructures in 2DLPs, provided insights into their formation and energy flow dynamics, and successfully integrated 2DLPs into a platform for osmotic power generation.
TOGETHER (Towards Green Hydrogen by Layered Metal Halide Perovskite Heterostructures) tackled this challenge by integrating lateral heterostructures into 2DLPs to improve charge separation and enable directional charge carrier flow for light-driven chemical reactions. Through controlled ion exchange and sequential crystallization, the project developed tailored 2DLP heterostructures with tunable band alignments. By combining expertise in material synthesis, advanced photophysics, and material characterization, TOGETHER aimed to bridge the gap between fundamental material design and practical photocatalytic applications. The project focused on three key objectives: development of lateral heterostructures in 2DLPs with tunable band alignments, understanding their formation mechanisms and structural and optical properties, and demonstrating their versatility for light-driven applications. Overall, TOGETHER introduced novel synthesis techniques for complex lateral heterostructures in 2DLPs, provided insights into their formation and energy flow dynamics, and successfully integrated 2DLPs into a platform for osmotic power generation.
TOGETHER was structured into three main parts: the synthesis of 2DLPs and their heterostructures, the in-depth characterization of their formation mechanisms, optical and structural properties, and the demonstration of light-driven reactions.
The project developed innovative synthesis strategies for 2DLPs and their lateral heterostructures. TOGETHER introduced three distinct synthesis techniques for fabricating regularly shaped pristine 2DLPs, achieving lateral sizes from 100 nm to 100 µm, tailored to the scale requirements of various characterization methods. To create lateral heterostructures, a solution-based anion exchange process was developed, involving the exposure of pristine 2DLPs to halide salts of the spacer cation dissolved in alcohols such as octanol. Both Br-to-I and I-to-Br exchanges were performed on PEA2PbX4 microcrystals, forming core-frame lateral heterostructures. These structures feature at least two distinct phases with unique optical properties within a single crystal. More complex heterostructures with high-quality interfaces were synthesized using a developed sequential crystallization technique, leveraging the solubility differences of various 2DLPs in polar solvents. These advancements enabled the rational design of lateral heterostructures in 2DLPs, leading to the successful fabrication of triple-halide heterostructures incorporating three distinct phases within a single crystal, as well as lateral heterostructures featuring different metal cations, such as PEA2PbBr4–PEA4AgBiBr8 and PEA4AgBiBr8–PEA2SnBr4.
In situ imaging and in situ spectroscopic techniques were combined with advanced electron microscopy to elucidate the formation mechanism of lateral heterostructures. These studies revealed that anion exchange in 2DLPs is strongly influenced by the initial halide composition of the parent 2DLP, which can be explained by the preferential occupation of halides within the octahedral framework. The stability of anion-exchanged heterostructures was assessed by storing microcrystals under ambient conditions. Both Br-to-I and I-to-Br exchanged microcrystals remained stable without any signs of interdiffusion between the two phases. However, while the iodine-rich phase in Br-to-I exchanged crystals degraded over several days, the I-to-Br heterostructures remained fully stable throughout the experiment. Time-resolved PL and exciton diffusion measurements on anion-exchanged samples revealed indications of directional energy flow toward the lower bandgap material within the lateral heterostructures. Advanced electron microscopy was used to evaluate the interface quality between the two phases after the exchange process, revealing interfaces dominated by pores and overgrowth. As a consequence of the low interface quality, a one-pot sequential crystallization technique was developed. In situ photoluminescence spectroscopy of the sequential crystallization technique showed a stepwise growth of heterostructures, driven by the varying solubility of 2DLPs in polar solvents upon antisolvent injection. Whether PEA2PbBr4 and PEA2PbI4 were mixed from the beginning or introduced at different stages of the crystallization process determined the formation of either alloyed phases or distinct pure-phase heterostructures. Structural analysis of the triple-halide heterostructures confirmed an almost strain-free epitaxial connection between the different phases.
The deposition of metal nanoparticles on 2DLPs with the bifunctional spacer molecule cysteamine was used as a benchmark reaction to demonstrate the potential of 2DLPs for light-driven chemical reactions. In line with TOGETHER’s green energy character, 2DLPs were also explored for osmotic power generation. Solid-state nanopores were fabricated using focused ion beam drilling, with pore diameters tunable by exposing 2DLP microcrystals to a precursor solution of PbBr2 and PEABr. The pore size depended on the dilution factor of the precursor solution. To enhance stability in aqueous solutions, 2DLPs were treated with halide salts of the spacer cations (e.g. PEABr). Conductivity measurements, performed as a function of salt concentration and pH value, revealed that the pore sidewalls carried a positive charge. Under a 1000× concentration gradient, the osmotic power output reached a value of nearly 6000 pW.
The project developed innovative synthesis strategies for 2DLPs and their lateral heterostructures. TOGETHER introduced three distinct synthesis techniques for fabricating regularly shaped pristine 2DLPs, achieving lateral sizes from 100 nm to 100 µm, tailored to the scale requirements of various characterization methods. To create lateral heterostructures, a solution-based anion exchange process was developed, involving the exposure of pristine 2DLPs to halide salts of the spacer cation dissolved in alcohols such as octanol. Both Br-to-I and I-to-Br exchanges were performed on PEA2PbX4 microcrystals, forming core-frame lateral heterostructures. These structures feature at least two distinct phases with unique optical properties within a single crystal. More complex heterostructures with high-quality interfaces were synthesized using a developed sequential crystallization technique, leveraging the solubility differences of various 2DLPs in polar solvents. These advancements enabled the rational design of lateral heterostructures in 2DLPs, leading to the successful fabrication of triple-halide heterostructures incorporating three distinct phases within a single crystal, as well as lateral heterostructures featuring different metal cations, such as PEA2PbBr4–PEA4AgBiBr8 and PEA4AgBiBr8–PEA2SnBr4.
In situ imaging and in situ spectroscopic techniques were combined with advanced electron microscopy to elucidate the formation mechanism of lateral heterostructures. These studies revealed that anion exchange in 2DLPs is strongly influenced by the initial halide composition of the parent 2DLP, which can be explained by the preferential occupation of halides within the octahedral framework. The stability of anion-exchanged heterostructures was assessed by storing microcrystals under ambient conditions. Both Br-to-I and I-to-Br exchanged microcrystals remained stable without any signs of interdiffusion between the two phases. However, while the iodine-rich phase in Br-to-I exchanged crystals degraded over several days, the I-to-Br heterostructures remained fully stable throughout the experiment. Time-resolved PL and exciton diffusion measurements on anion-exchanged samples revealed indications of directional energy flow toward the lower bandgap material within the lateral heterostructures. Advanced electron microscopy was used to evaluate the interface quality between the two phases after the exchange process, revealing interfaces dominated by pores and overgrowth. As a consequence of the low interface quality, a one-pot sequential crystallization technique was developed. In situ photoluminescence spectroscopy of the sequential crystallization technique showed a stepwise growth of heterostructures, driven by the varying solubility of 2DLPs in polar solvents upon antisolvent injection. Whether PEA2PbBr4 and PEA2PbI4 were mixed from the beginning or introduced at different stages of the crystallization process determined the formation of either alloyed phases or distinct pure-phase heterostructures. Structural analysis of the triple-halide heterostructures confirmed an almost strain-free epitaxial connection between the different phases.
The deposition of metal nanoparticles on 2DLPs with the bifunctional spacer molecule cysteamine was used as a benchmark reaction to demonstrate the potential of 2DLPs for light-driven chemical reactions. In line with TOGETHER’s green energy character, 2DLPs were also explored for osmotic power generation. Solid-state nanopores were fabricated using focused ion beam drilling, with pore diameters tunable by exposing 2DLP microcrystals to a precursor solution of PbBr2 and PEABr. The pore size depended on the dilution factor of the precursor solution. To enhance stability in aqueous solutions, 2DLPs were treated with halide salts of the spacer cations (e.g. PEABr). Conductivity measurements, performed as a function of salt concentration and pH value, revealed that the pore sidewalls carried a positive charge. Under a 1000× concentration gradient, the osmotic power output reached a value of nearly 6000 pW.
Throughout the project, each component contributed to advancing knowledge beyond the current state of the art. Key examples are highlighted below.
The solution-based anion exchange for 2DLPs, utilizing octanol as a non-toxic solvent, offers an accessible and scalable approach that could facilitate device fabrication for solar cells, LEDs, and transistors, extending beyond the initial focus on catalysis. Notably, solution-based anion exchange in 2DLPs has not been previously demonstrated in the literature, marking a significant advancement toward fully solution-processed devices with potential industrial applications. Furthermore, TOGETHER revealed a strong dependence of the exchange mechanism on the starting halide in 2DLPs. The developed one-pot sequential crystallization reaction enables the synthesis of high-quality, complex lateral heterostructures that were not previously reported, significantly improving control over material composition. Integrating this method into automated synthesis schemes could accelerate material discovery and the screening of new material combinations, paving the way for the rational design of heterostructures. TOGETHER fostered an interdisciplinary approach by integrating 2DLP-based solid-state nanopores into osmotic power generation, expanding the potential applications of 2DLPs beyond the original scope. The compositional tunability of these materials, combined with the developed exchange and crystallization protocols, significantly enhances the functionality of solid-state nanopores, impacting a scientific field that was not initially targeted.
The solution-based anion exchange for 2DLPs, utilizing octanol as a non-toxic solvent, offers an accessible and scalable approach that could facilitate device fabrication for solar cells, LEDs, and transistors, extending beyond the initial focus on catalysis. Notably, solution-based anion exchange in 2DLPs has not been previously demonstrated in the literature, marking a significant advancement toward fully solution-processed devices with potential industrial applications. Furthermore, TOGETHER revealed a strong dependence of the exchange mechanism on the starting halide in 2DLPs. The developed one-pot sequential crystallization reaction enables the synthesis of high-quality, complex lateral heterostructures that were not previously reported, significantly improving control over material composition. Integrating this method into automated synthesis schemes could accelerate material discovery and the screening of new material combinations, paving the way for the rational design of heterostructures. TOGETHER fostered an interdisciplinary approach by integrating 2DLP-based solid-state nanopores into osmotic power generation, expanding the potential applications of 2DLPs beyond the original scope. The compositional tunability of these materials, combined with the developed exchange and crystallization protocols, significantly enhances the functionality of solid-state nanopores, impacting a scientific field that was not initially targeted.