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Congratulations to Daniel, Deniz and Michelle for publishing their paper on “Inkjet printing of epitaxially connected nanocrystal superlattices”, for publication in Nano Research; check out the full paper here (doi: 10.1007/s12274-021-4022-7).

Creating and studying mesoscale materials formed via directed assembly of nanoscale building blocks presents myriad exciting scientific and technological challenges. Recent advances in mesocrystal superlattices formed by the directed assembly and epitaxial attachment of colloidal nanocrystal building blocks have opened new horizons to create novel materials with properties by design. The community has been particularly intrigued by theoretically predicted emerging properties such as Dirac-type electronic structure and phenomena like topologically protected spin-polarized states. However, to date, experimental validation of these emerging properties is still lacking, which is largely due to gaps in our understanding of and control over processing-structure-property relationships.

Bringing the prospects of mesocrystals to fruition is contingent not only access to high-quality nanomaterial building blocks, but also on advances in fabrication methods. The conventional route to create mesocrystals involves assembly and attachment of colloidal nanocrystals at the interface of an immiscible fluid. This process involves an intricate choreography of several physicochemical processes which has complicated the path towards high-fidelity mesocrystal superlattices. With an eye towards addressing these fabrication challenges, the results described in our enclosed manuscript establish the foundational processing-structure relationships of epitaxially connected PbSe nanocrystalsas a model system.We systematically examined the printing parameter space successfully demonstrated how inkjet printing of colloidal nanocrystal solutions on a sessile subphase droplet can create epitaxially connected mesocrytstals at sub-millimeter length scales. Beyond the specific system of PbSe-based mesocrysals, we anticipate that insights from this work will spur on future advances to enable more mechanistic insights into the assembly processes and new avenues to create high-fidelity superlattices.

Congratulations to Jessica, Daniel and Tyler for publishing the methods paper “Fundamental Processes and Practical Considerations of Lead Chalcogenide Mesocrystals Formed via Self-Assembly and Directed Attachment of Nanocrystals at a Fluid Interface” in Chemistry of Materials

We have been intrigued by mesocrystals formed from colloidal nanocrystal building blocks and have been fortunate to contribute to the impressive recent advances in this field. In particular, epitaxially-connected lead chalcogenide superlattices present many interesting scientific and technological challenges spanning fundamental materials chemistry to emerging optoelectronic properties. Theory predicts that the underlying balance between quantum confinement and quantum coupling will give rise to exotic phenomena like topologically protected spin-polarized states; however, synthesis and fabrication methods have, so far, not yet been able to create superlattices with the requisite structural fidelity to probe and explore these predictions. The formation of the epitaxially connected superlattices involves a complex choreography of multiple physicochemical processes. Beyond assembling the constituent nanocrystals into arrangements with high spatial and orientational ordering, the process must also be coordinated to achieve the directed attachment of neighboring dots. Remarkably, the epitaxial bonding of dots involves the irreversible attachment of on the order of 10^5 dots to form a μm-scale superlattice grain. Perhaps not surprisingly, this assembly and attachment process is very sensitive to the processing conditions, choice of solvents, subphase, and preparation of the colloidal NC building blocks. We wrote this methods and protocols paper to share our best practices with the community and to provide, as far as possible, our understanding the fundamental relationship between processing conditions and mechanistic impacts leading to the formation of high-fidelity superlattices. Given the strong scientific interest in mesocrystalline materials, we hope that better understanding of the processing /structure relationships will spur on further advances in this blossoming field. Again, congrats to Jessica, Daniel and Tyler.

Congratulations to Rileigh for publishing her paper titled “Pulse Symmetry Impacts the C2 Product Selectivity in Pulsed Electrochemical CO2 Reduction” in ACS Energy Letters.

We have been fascinated with the concept of dynamic catalysis and exploiting resonances between applied modulations (e.g., electric field or strain) and microkinetic steps to impact overall catalytic activity and selectivity. We previously demonstrated that product selectivity of electrochemical CO₂ reduction can be tuned by applying a square wave pulsed potential. Beyond modulating the frequency or amplitude of the square wave, changing the waveform presents an interesting, previously unexplored, parameter to gain new mechanistic insights and further advance control over product selectivity.

Rileigh’s paper describes our systematic examination of the effects of pulse potential, duration, and profile symmetry on the resulting products from electrocatalytic CO₂ reduction. Our study provides new physical insight into the role of co-adsorbed ions, interfacial charge, and changing electric field at the reaction interface on C-C coupling. Our in-depth analysis of transient charge transfer processes provides new understanding of the role of programmed potential pulses on improved C₂ product selectivity. We show, for the first time, that symmetric pulse shapes optimize C₂ selectivity, which we attribute to balancing of microkinetic steps and optimization of the the local reaction environment for C-C coupling through electrolyte ion co-adsorption and electric field effects.

Congratulations to Rileigh and Kelsey for publishing their review/perspective paper in Joule (https://doi.org/10.1016/j.joule.2021.05.014).

Given the recent surge of interest in electrochemical CO2 reduction and in particular, recent discoveries of the curious effects of using pulsed potentials, we felt that this is a perfect time to write a review and perspective paper to share recent advances and hopefully inspire other researchers to pursue these exciting opportunities . To put the compelling prospects of pulsed methods in broader context, we provide a brief review of fundamental considerations in electrochemical CO2reduction as a segue to our forward-looking perspective on exciting emerging opportunities in this field.

Amidst growing concerns about increasing atmospheric CO2levels, electrochemical CO2reduction has emerged as a promising approach to valorize CO2under commercially relevant conditions. Intensive recent experimental and computational research efforts have significantly advanced our understanding of the role of the electrode, electrolyte, and electrolyzer design. Among this flurry of research activity, the application of pulsed potentials has led to several discoveries, primarily that the pulse profile presents a new, yet unexplored, ‘knob’ to control product selectivity. The ostensibly simple process of switching the potential on (or off) influencesthe choreography and complex interrelation of underlying physicochemical processes including hydrodynamics, ad-and desorption, and changes in the electrode structure and composition. The leitmotif of this perspective is that amidst this complexity resides a compelling opportunity for both scientific and technological advances in electrosynthetic processes beyond CO2reduction. We have organized the perspective as follows: we first discuss critical general electrochemical considerations and then review recent achievements with regards to pulsed electrochemical CO2reduction before diving into specific aspects of physical processes and the reaction mechanism. We provide an outlook to both future scientific challenges and potential technological applications

Congratulations to Rileigh, Kevin, Kelsey, Jessica, Jiyoon, and Tyler for publishing the paper “Effect of Electrolyte Composition and Concentration on Pulsed Potential Electrochemical CO2 Reduction” in ChemElecroChem (see the full paper here link).

Electrochemical CO2 reduction (eCO2R) on copper has garnered strong interest as a promising pathway to convert CO2 emissions into higher value chemicals including fuels and hydrocarbon feedstocks. We previously reported (Kimura et al. ChemSusChem (2018)) that the application of a pulsed potential during eCO2R improves selectivity toward converting CO2 to higher order products, suppresses undesirable H2 production, and maintains electrode stability. The working hypothesis which emerged from the initial study was that the pulsing-dependent product selectivity was related to dynamic changes in species adsorbed to the electrode surface. Subsequent studies have pointed to interesting dynamic processes at the solid-electrolyte interface (SEI). In the specific context of electrochemical CO2 reduction, the composition and concentration of the electrolyte are known to impact product selectivity. How the dynamic processes and electrolyte effects at the SEI play out in the case of pulsed potentials has not yet been investigated. Pulsed potential experiments thus present an intriguing opportunity to establish deeper insights into these processes.
We found that the relationship between electrolyte concentration/composition and product distribution for pulsed potential eCO2R is different from constant potential eCO2R. In the case of constant potential eCO2R, increasing KHCO3 concentration is known to favor the formation of H2 and CH4. In contrast, for pulsed potential eCO2R, H2 formation is suppressed due to the periodic desorption of surface protons, while CH4 is still favored. In the case of KCl, increasing the concentration during constant potential eCO2R does not affect product distribution, mainly producing H2 and CO. However, increasing KCl concentration during pulsed potential eCO2R persistently suppresses H2 formation and greatly favors C2 products, reaching 71% Faradaic efficiency. Collectively, these results provide new mechanistic insights into the pulsed eCO2R mechanism within the context of proton-donator ability and ionic conductivity.

Here is a link to Jen-Yu’s and Yuanze’s paper https://doi.org/10.1021/acsami.0c14827

Recent concurrent advances in the synthesis of mesoscale porous materials and digital light processing have opened up a broad opportunity space for the design of hierarchical materials and their enabled applications. One immediate application area of such materials is in the separation of small molecule impurities; an urgent challenge in this case is the separation of small molecule carcinogenic contaminants like N-nitrosodimethylamine (NDMA) which has triggered extensive recalls of ranitidine drugs for blood-pressure and heartburn. Conventional chemical separation methods have been ineffective for removal of the small molecule contaminants. Size-exclusion separation methods in particular have conventionallybeen limited to the separation of relatively large molecules. The advent of novel mesoporous materials (with programmable pore size in the Ångstrom range) combined with digital light processing strategies to create multi-scale flow structures has introduced the prospect of extending size-exclusion principle to the separation of molecules with sub-nm size differences.

In collaboration with the Millner group, we demonstrated how hierarchical control over microscopic porosity, mesoscale assembly, and macroscopic superstructures leads to new materials exhibiting efficient mass transport and size-selective adsorption of small molecule contaminants. Beyond the specific proof-of-concept and urgent example of NDMA removal, we note that the approach demonstrated in our manuscript is applicable to a broad range of small-molecule separation challenges. Moreover, we see the fabrication strategies described in our manuscript as an enabling platform for many recent advances in porous coordination polymers and metal-organic framework materials.

Please join me in congratulating Jen-Yu and Yuanze on this paper.

Congratulations to Jen-Yu and the rest of the ream for publishing their work on Porous cage-derived nanomaterial inks for direct and internal three-dimensional printing  in Nature Communications. (https://www.nature.com/collections/wdzvyhgxft/content/johannes-kreutzer)

The convergence of 3D printing techniques and nanomaterials is generating a compelling opportunity space to create advanced materials with multiscale structural control and hierarchical functionalities. While most nanoparticles consist of a dense material, less attention has been paid to 3D printing of nanoparticles with intrinsic porosity. Here, we combine ultrasmall (about 10 nm) silica nanocages with digital light processing technique for the direct 3D printing of hierarchically porous parts with arbitrary shapes, as well as tunable internal structures and high surface area. Thanks to the versatile and orthogonal cage surface modifications, we show how this approach can be applied for the implementation and positioning of functionalities throughout 3D printed objects. Furthermore, taking advantage of the internal porosity of the printed parts, an internal printing approach is proposed for the localized deposition of a guest material within a host matrix, enabling complex 3D material designs.

Congratulations to Isaiah Chen (in Paulette Clancy’s group at JHU) and the rest of the team at Cornell (Jessica, Daniel, and Michelle) for publishing the paper in ACS Nano ‘the role of dimer formation in the nucleation of superlattice transformations and its impact on disorder’ 

This paper focuses on elucidating the mechanism of the self-assembly of colloidal nanocrystals to form epitaxially connected superstructures, which researchers know needs to be practically perfect to allow coherent charge transport. The community is currently unable to create superlattices with the desired fidelity because we lack an understanding of how nanocrystals assemble and attach to form a bridged superlattice.

This paper sheds light on this mechanism using a molecular-scale computational approach to simulate the assembly process, informed by molecular-scale information from cutting-edge imaging techniques to help both in setting up the initial pre-assembled starting point and validating the computational predictions. Our two key results in this paper are to (i) uncover the significant influence of dimerization during assembly; and (ii) establish precisely and quantifiably exactly how tolerant the (essentially irreversible) attachment of nanoparticles to form a fully connected superlattice will be to positional and rotational disorder in the pre-assembled system. This new definition will assist researchers to find ways to stay within these tolerances as they pursue the goal of perfectly connected superlattices.

Congratulations to Kevin, Rileigh and the rest of the team for publishing their work in ACS Catalysis (10.1021/acscatal.0c02630)

Electrochemical CO₂ reduction (CO₂R) on copper has garnered strong interest as a promising pathway to convert CO₂ emissions into higher value chemicals including fuels and hydrocarbon feedstocks. We previously reported  (Kimura et al. ChemSusChem (2018)) that the application of a pulsed potential during CO₂R improved selectivity toward converting CO₂ to higher order products, suppressed undesirable H₂ production, and maintained electrode stability. The working hypothesis which emerged from the initial study was that the pulsing-dependent product selectivity was related to dynamic changes in species adsorbed to the electrode surface.

In this study we set out to establish deeper fundamental insights into the mechanism by applying  in-situ techniques. We used in-situ X-ray adsorption spectroscopy to identify and monitor the bulk Cu valence state remained reduced during the pulsing, but found surface hydroxides (OH_ads) were adsorbing during the anodic pulsed potential. We combined these spectroscopic insights with the recent findings by Iijima et al. (ACS Catalysis (2019)), in which in-situ attenuated total reflection surface-enhanced infrared absorption spectroscopy was used to demonstrate the effect of surface hydroxides promoting CO adsorption and preventing Cu deactivation. We concluded the pulsed mechanism favors CO₂ reduction and increases stability due to two effects: 1) proton desorption/displacement by OH_ads during the anodic potential, which suppresses H₂ production and 2) the accumulation of OH_ads, promoting CO_ads and preventing Cu deactivation.

Congratulations to Yingjie, Jen-Yu, Daniel and Yuanze on publishing the paper on “Photoinitiated transformation of nanocrystal superlattice polymorphs assembled at a fluid interface”

The directed assembly of nanoscale building blocks into complex superstructures is of widespread scientific and technological interest. Scientists and engineers have been intrigued by the prospects of tailoring self-assembly processes to create materials whose properties and function can be tuned through the interaction between constituent particles. In particular, recent reports of superlattice structure transformations leading to epitaxially connected nanocrystal superlattices with long-range atomic coherence have generated significant interest as a platform for novel, quasi 2D ‘designer materials’. To date, experimental methods to initiate this structure transformation have relied on rather coarse chemical or thermal triggers. Progress to bring the prospect of these nanocrystal solids to technological fruition is limited by the lack of processing methods that enable spatial programming of the transformation and the extent of the transformation.

In this article, we combine, for the first time, advances in nanocrystal self-assembly at fluid interfaces with photolithographic patterning. We demonstrate nanocrystal superlattice structure transformation (from 6-fold to 4-fold symmetric) can be spatially programmed by digital light processing and the addition of a photoacid generator to the fluid subphase. We describe our approach as the ‘optically-driven reorganization of colloidal assemblies’ (ORCA). Beyond the ability to spatially program the extent and 2D pattern of the transformation, our detailed structural and chemical analysis also provides new insights into the relationship between nanocrystal ligand coverage and superlattice structure. We quantified how increasing photodosage decreases nanocrystal ligand coverage, which in turn impacts the effective sphericity of the ligand shell and drives the hexagonal-to-square superlattice structure transformation.

The work described in this manuscript provides new understanding of and control over the thermodynamic stability and kinetic lability of ligand-passivated nanocryst

als. Building on these fundamental scientific insights, the method presents new opportunities to create nanocrystal superlattice structures with programmable structures; we anticipate that this ability will spur many new advances in the field of nanocrystal superlattices and their enabled technologies.