Molecular Metallocorrole-Nanorod Photocatalytic System for Sustainable Hydrogen Production

Molecular Geometry and Electronic Structure of Copper Corroles

Copper corroles are known for their unique multiconfigurational electronic structures in the ground state, which arise from the transfer of electrons from the π orbitals of the corrole to the d-orbital of copper. While density functional theory (DFT) provides reasonably good molecular geometries, the determination of the ground spin state and the associated energetics is heavily influenced by functional choice, particularly the percentage of the Hartree-Fock exchange. Using extended multireference perturbation theory methods (XMS-CASPT2), the functional choice can be assessed. The molecular geometries and electronic structures of both the unsubstituted and the meso-triphenyl copper corroles were investigated. A minimal active space was employed for structural characterization, while larger active spaces are required to examine the electronic structure. The XMS-CASPT2 investigations conclusively identify the ground electronic state as a multiconfigurational singlet (S0) with three dominant electronic configurations in its lowest energy and characteristic saddled structure. In contrast, the planar geometry corresponds to the triplet state (T0), which is approximately 5 kcal/mol higher in energy compared to the S0 state for both the bare and substituted copper corroles. Notably, the planarity of the T0 geometry is reduced in the substituted corrole compared with that in the unsubstituted one. By analyzing the potential energy surface (PES) between the S0 and T0 geometries using XMS-CASPT2, the multiconfigurational electronic structure is shown to transition toward a single electron configuration as the saddling angle decreases (i.e., as one approaches the planar geometry). Despite the ability of the functionals to reproduce the minimum energy structures, only the TPSSh-D3 PES is reasonably close to the XMS-CASPT2 surface. Significant deviations along the PES are observed with other functionals.

Unravelling Dynamics Involving Multiple Charge Carriers in Semiconductor Nanocrystals

The use of colloidal nanocrystals as part of artificial photosynthetic systems has recently gained significant attention, owing to their strong light absorption and highly reproducible, tunable electronic and optical properties. The complete photocatalytic conversion of water to its components is yet to be achieved in a practically suitable and commercially viable manner. To complete this challenging task, we are required to fully understand the mechanistic aspects of the underlying light-driven processes involving not just single charge carriers but also multiple charge carriers in detail. This review focuses on recent progress in understanding charge carrier dynamics in semiconductor nanocrystals and the influence of various parameters such as dimension, composition, and cocatalysts. Transient absorption spectroscopic studies involving single and multiple charge carriers, and the challenges associated with the need for accumulation of multiple charge carriers to drive the targeted chemical reactions, are discussed.

Insights into the Synthesis Mechanisms of Ag-Cu3P-GaP Multicomponent Nanoparticles

Metal-semiconductor nanoparticle heterostructures are exciting materials for photocatalytic applications. Phase and facet engineering are critical for designing highly efficient catalysts. Therefore, understanding processes occurring during the nanostructure synthesis is crucial to gain control over properties such as the surface and interface facets' orientations, morphology, and crystal structure. However, the characterization of nanostructures after the synthesis makes clarifying their formation mechanisms nontrivial and sometimes even impossible. In this study, we used an environmental transmission electron microscope with an integrated metal-organic chemical vapor deposition system to enlighten fundamental dynamic processes during the Ag-Cu₃P-GaP nanoparticle synthesis using Ag-Cu₃P seed particles. Our results reveal that the GaP phase nucleated at the Cu₃P surface, and growth proceeded via a topotactic reaction involving counter-diffusion of Cu⁺ and Ga³⁺ cations. After the initial GaP growth steps, the Ag and Cu₃P phases formed specific interfaces with the GaP growth front. GaP growth proceeded by a similar mechanism observed for the nucleation involving the diffusion of Cu atoms through/along the Ag phase toward other regions, followed by the redeposition of Cu₃P at a specific Cu₃P crystal facet, not in contact with the GaP phase. The Ag phase was essential for this process by acting as a medium enabling the efficient transport of Cu atoms away from and, simultaneously, Ga atoms toward the GaP-Cu₃P interface. This study shows that enlightening fundamental processes is critical for progress in synthesizing phase- and facet-engineered multicomponent nanoparticles with tailored properties for specific applications, including catalysis.

Electronic Structure and Excited State Dynamics of Cadmium Chalcogenide Nanorods

The cylindrical quasi-one-dimensional shape of colloidal semiconductor nanorods (NRs) gives them unique electronic structure and optical properties. In addition to the band gap tunability common to nanocrystals, NRs have polarized light absorption and emission and high molar absorptivities. NR-shaped heterostructures feature control of electron and hole locations as well as light emission energy and efficiency. We comprehensively review the electronic structure and optical properties of Cd-chalcogenide NRs and NR heterostructures (e.g., CdSe/CdS dot-in-rods, CdSe/ZnS rod-in-rods), which have been widely investigated over the last two decades due in part to promising optoelectronic applications. We start by describing methods for synthesizing these colloidal NRs. We then detail the electronic structure of single-component and heterostructure NRs and follow with a discussion of light absorption and emission in these materials. Next, we describe the excited state dynamics of these NRs, including carrier cooling, carrier and exciton migration, radiative and nonradiative recombination, multiexciton generation and dynamics, and processes that involve trapped carriers. Finally, we describe charge transfer from photoexcited NRs and connect the dynamics of these processes with light-driven chemistry. We end with an outlook that highlights some of the outstanding questions about the excited state properties of Cd-chalcogenide NRs.

The Other Dimension-Tuning Hole Extraction via Nanorod Width

Solar-to-hydrogen generation is a promising approach to generate clean and renewable fuel. Nanohybrid structures such as CdSe@CdS-Pt nanorods were found favorable for this task (attaining 100% photon-to-hydrogen production efficiency); yet the rods cannot support overall water splitting. The key limitation seems to be the rate of hole extraction from the semiconductor, jeopardizing both activity and stability. It is suggested that hole extraction might be improved via tuning the rod's dimensions, specifically the width of the CdS shell around the CdSe seed in which the holes reside. In this contribution, we successfully attain atomic-scale control over the width of CdSe@CdS nanorods, which enables us to verify this hypothesis and explore the intricate influence of shell diameter over hole quenching and photocatalytic activity towards H₂ production. A non-monotonic effect of the rod's diameter is revealed, and the underlying mechanism for this observation is discussed, alongside implications towards the future design of nanoscale photocatalysts.