3D hierarchical Nb3O7(OH) superstructures

Global warming caused by continuous emission of greenhouse gases is omnipresent. Anthropogenic CO2 resulting from the combustion of fossil fuels adds the largest mass to the scales. Therefore, new technologies for power generation and energy storage are required. This study focuses on the synthesis and characterization of novel materials to be used as photoelectrode in dye-sensitized solar cells or as photocatalyst for water splitting. Hydrothermal conditions feature the formation of 3D hierarchical Nb3O7(OH) superstructures which are composed of highly-ordered nanowire networks. Despite their complexity these superstructures form self-organized starting from amorphous hollow cubes. Advanced transmission electron microscopy is applied for the characterization of the crystallographic structure, atomic arrangement and bonding characteristics of the nanostructures. 3D reconstruction of the nanowire arrangement, based on a combination of local thickness measurements and electron tomography, indicates suitable charge transport paths. The stabilization of the superstructures is based on the nanowire junctions. Even though no complete interpenetration of the nanowires was observed these networks exhibit a very high thermal stability. The morphology remains stable for temperatures up to 850 °C despite of the phase transformation of Nb3O7(OH) to H-Nb2O5. This phase transformation was investigated in detail with ex situ and in situ experiments yielding a good understanding of the impact of temperature, atmospheric condition and electron beam on the crystal structure. The morphological and photophysical properties of the nanostructures determine their performance in functional devices and promising hydrogen production rates are observed for the superstructures. These rates can be further enhanced by the incorporation of titanium into the crystal lattice. The capacity of the Nb3O7(OH) crystal lattice to incorporate titanium is limited to about 12 at% and the formation of anatase TiO2 plates is observed for titanium excess. The presence of titanium in the crystal lattice has two main effects. It slows down the crystallization of Nb3O7(OH) leading to superstructures composed of smaller nanocrystals and furthermore it reduces the surface defects resulting in lower charge recombination rates. Therefore, the hydrogen production rate of titanium doped (5.5 at% Ti) superstructures was by a factor of two higher than the one observed for undoped Nb3O7(OH)

Additive-mediated size control of MOF nanoparticles

A fast synthesis approach toward sub-60 nm sized MOF nanoparticles was developed by employing auxiliary additives. Control over the size of HKUST-1 and IRMOF-3 particles was gained by adjusting the concentration and type of stabilizers. Colloidal solutions of the MOFs were used for the formation of optically homogeneous thin films by spin-coating

Strong structural and electronic coupling in metavalent PbS moire superlattices

Moire superlattices are twisted bilayer materials, in which the tunable interlayer quantum confinement offers access to new physics and novel device functionalities. Previously, moire superlattices were built exclusively using materials with weak van der Waals interactions and synthesizing moire superlattices with strong interlayer chemical bonding was considered to be impractical. Here using lead sulfide (PbS) as an example, we report a strategy for synthesizing of moire superlattices coupled by strong chemical bonding. We use water-soluble ligands as a removable template to obtain free-standing ultra-thin PbS nanosheets and assemble them into direct-contact bilayers with various twist angles. Atomic-resolution imaging shows the moire periodic structural reconstruction at superlattice interface, due to the strong metavalent coupling. Electron energy loss spectroscopy and theoretical calculations collectively reveal the twist angle26 dependent electronic structure, especially the emergent separation of flat bands at small twist angles. The localized states of flat bands are similar to well-arranged quantum dots, promising an application in devices. This study opens a new door to the exploration of deep energy modulations within moire superlattices alternative to van der Waals twistronics

Influence of sub-zero temperature on nucleation and growth of copper nanoparticles in electrochemical reactions.

Cu metal nanostructures have attracted wide interest of study as catalysts for CO2 reduction reaction and other applications. Controlling the structure and morphology of Cu nanostructures during synthesis is crucial for achieving desired properties. Here, we studied temperature effects on electrochemical deposition of Cu nanoparticles. We found the size, nucleation density, and crystallinity of Cu nanoparticles are strongly influenced by low temperature processing. The electrodeposition at low temperature (-20°C) results in clusters of assembled small Cu nanoparticles, which is distinctly different from the large individual highly crystalline Cu nanoparticles obtained from the room temperature process. The differences in Cu nanoparticle morphology and crystallinity are attributed to the variations in reduction reaction rate and surface diffusion. The limitation of the reaction rate promotes multiple nuclei, and low surface diffusion induces poor crystallinity. This study deepens our understanding of low-temperature effects on electrochemical processes assisting the design of diverse hierarchical catalytic materials

Lithium metal stripping mechanisms revealed through electrochemical liquid cell electron microscopy

An understanding of lithium stripping is as important as that of lithium plating to achieve significant advances in using lithium metal anodes for high-energy rechargeable batteries. However, there have been limited studies on lithium stripping compared to lithium plating. Here we report the lithium stripping mechanisms revealed through in-situ electrochemical liquid cell transmission electron microscopy (TEM). We directly observe and compare the stripping behavior of the in-situ grown lithium dendrites and lithium nanograins covered by a lithium fluoride-rich solid-electrolyte interphase (SEI). We find the sporadic lithium stripping behavior and three important modes that can describe the stripping of individual lithium deposits, regardless of their morphology: (i) symmetric stripping, (ii) surface-preferred asymmetric stripping, and (iii) interface-preferred asymmetric stripping. In addition, SEI chemical mapping with high spatial resolution shows a remarkable SEI loss at the end of the lithium metal stripping, which illustrates the importance of SEI protection in the subsequent cycles

Template-free synthesis of novel, highly-ordered 3D hierarchical Nb₃O₇(OH) superstructures with semiconductive and photoactive properties

3D hierarchical Nb3O7(OH) mesocrystals can be formed by self-organization from nanometer sized building blocks. The present study focuses on the synthesis and detailed investigation of mesocrystals, which can be achieved from a one-step, template-free hydrothermal synthesis approach. The obtained cubic superstructures consist of a periodic nanowire-network and combine a large surface area, high crystallinity, with a band gap of 3.2 eV and photocatalytic activity. Their easy processability in combination with the named excellent properties makes them promising candidates for a large number of applications. These include photochemical and photophysical devices where the Nb3O7(OH) mesocrystals can be used as electrode material since they are semiconducting and possess a large surface area. Generally the forces involved in the self-organized formation of mesocrystals are not fully understood. In this regard, the assembly of the Nb3O7(OH) mesocrystals was investigated in-depth applying transmission electron microscopy, scanning electron microscopy, UV/Vis measurements and electron energy-loss spectroscopy. Based on the achieved results a formation mechanisms is proposed, which expands the number of mechanisms for mesocrystal formation reported in literature. In addition, our study reveals different types of nanowire junctions and investigates their role at the stabilization of the networks

Heat-Induced Phase Transformation of Three-Dimensional Nb₃O₇(OH) Superstructures: Effect of Atmosphere and Electron Beam

Nanostructured niobium oxides and hydroxides are potential candidates for photochemical applications due to their excellent optical and electronic properties. In the present work the thermal stability of Nb₃O₇(OH) superstructures prepared by a simple hydrothermal approach is investigated at the atomic scale. Transmission electron microscopy and electron energy-loss spectroscopy provide insights into the phase transformation occurring at elevated temperatures and probe the effect of the atmospheric conditions. In the presence of oxygen, H₂O is released from the crystal at temperatures above 500 °C, and the crystallographic structure changes to H-Nb₂O₅. In addition to the high thermal stability of Nb₃O₇(OH), the morphology was found to be stable, and first changes in the form of a merging of nanowires are not observed until 850 °C. Under reducing conditions in a transmission electron microscope and during electron beam bombardment, an oxygen-deficient phase is formed at temperatures above 750 °C. This transformation starts with the formation of defects in the crystal lattice at 450 °C and goes along with the formation of pores in the nanowires which accommodate the volume differences of the two crystal phases

Strong Structural and Electronic Coupling in Metavalent PbS Moiré Superlattices

Moiré superlattices are twisted bilayer materials in which the tunable interlayer quantum confinement offers access to new physics and novel device functionalities. Previously, moiré superlattices were built exclusively using materials with weak van der Waals interactions, and synthesizing moiré superlattices with strong interlayer chemical bonding was considered to be impractical. Here, using lead sulfide (PbS) as an example, we report a strategy for synthesizing moiré superlattices coupled by strong chemical bonding. We use water-soluble ligands as a removable template to obtain free-standing ultrathin PbS nanosheets and assemble them into direct-contact bilayers with various twist angles. Atomic-resolution imaging shows the moiré periodic structural reconstruction at the superlattice interface due to the strong metavalent coupling. Electron energy loss spectroscopy and theoretical calculations collectively reveal the twist-angle-dependent electronic structure, especially the emergent separation of flat bands at small twist angles. The localized states of flat bands are similar to well-arranged quantum dots, promising an application in devices. This study opens a new door to the exploration of deep energy modulations within moiré superlattices alternative to van der Waals twistronics