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The entropy-driven monolayer assembly of hexagonal prisms and cylinders was studied under hard slit confinement. At the conditions investigated, the particles have two distinct and dynamically disconnected rotational states: unflipped and flipped, depending on whether their circular/hexagonal face is parallel or perpendicular to the wall plane. Importantly, these two rotational states cast distinct projection areas over the wall plane that favor either hexagonal or tetragonal packing. Monte Carlo simulations revealed a re-entrant melting transition where an intervening disordered Flipped-Unflipped (FUN) phase is sandwiched between a fourfold tetratic phase at high concentrations and a sixfold triangular solid at intermediate concentrations. The FUN phase contains a mixture of flipped and unflipped particles and is translationally and orientationally disordered. Complementary experiments were conducted with photolithographically fabricated cylindrical microparticles confined in a wedge cell. Both simulations and experiments show the formation of phases with comparable fraction of flipped particles and structure, i.e., the FUN phase, triangular solid, and tetratic phase, indicating that both approaches sample analogous basins of particle-orientation phase-space. The phase behavior of hexagonal prisms in a soft-repulsive wall model was also investigated to exemplify how tunable particle–wall interactions can provide an experimentally viable strategy to dynamically bridge the flipped and unflipped states.

Summary Developing affordable, robust, and selective CO2 electroreduction technologies is crucial to address concerns about rising CO2 emissions. Pulsed potential electrochemical CO2 reduction (p-eCO2R) has emerged as a simple and responsive knob to increase electrolyzer durability and improve product selectivity. In this review, we summarize the recent findings of p-eCO2R on copper electrodes as a function of applied potential and pulse duration. We discuss how pulse methods present scientific and technological opportunities for electrochemical technologies beyond p-eCO2R, in particular, ones involving competing reactions or electrode deactivation.

The electrochemical reduction of CO2 presents an attractive opportunity to not only valorize CO2 as a feedstock for chemical products, but also to provide a means to effectively store renewable electricity in the form of chemical bonds. The recent surge of experimental and computational studies of electrochemical CO2 reduction (ECR) has brought about significant scientific and technological advances. Yet, considerable gaps in our understanding of and control over the reaction mechanism persist, in particular for the formation of products. Moreover, while theoretical and computational studies have proposed many candidate reaction pathways, comprehensively reconciling these models with experimental observations remains challenging and elusive. The conventional electrocatalytic analysis of catalyst activity and selectivity generally relies on steady-state measurements. In a departure from this convention, we show in this study that time-resolved measurements (i.e., chronoamperometry) provide a powerful diagnostic tool to gain valuable insights into the complex interplay of electrochemical reactions, chemical reactions, and mass transport. Specifically, we show that the initial ECR reaction sequence involves a chemical reaction interposed between two charge transfer reactions (“ECE”). We hope that the methods and insights presented in this work will inspire future studies to exploit chronoamperometric analysis to resolve outstanding questions in ECR and other multi-step electrochemical reaction pathways.

The electrochemical CO2 reduction reaction (CO2RR) can convert widely available CO2 into value-added C2 products, such as ethylene and ethanol. However, low selectivity toward either compound limits the effectiveness of current CO2RR electrocatalysts. Here, we report the use of pulsed overpotentials to improve the ethylene selectivity to 67% with >75% overall C2 selectivity on (100)-textured polycrystalline Cu foil. The pulsed CO2RR can be made selective to either ethylene or ethanol by controlling the reaction temperature. We attribute the enhanced C2 selectivity to the improved CO dimerization kinetics on the active Cu surface on predominately (100)-textured Cu grains with the reduced hydrogen adsorption coverage during the pulsed CO2RR. The ethylene vs ethanol selectivity can be explained by the reducibility of the Cu(I) species during the cathodic potential cycle. Our work demonstrates a simple route to improve the ethylene vs ethanol selectivity and identifies Cu(I) as the species responsible for ethanol production.

Epitaxially connected quantum dot solids have emerged as an interesting class of quantum confined materials with the potential for highly tunable electronic structures. Realization of the predicted emergent electronic properties has remained elusive due in part to defective interdot epitaxial connections. Thermal annealing has shown potential to eliminate such defects, but a direct understanding of this mechanism hinges on determining the nature of defects in the connections and how they respond to heating. Here, we use in situ heating in the scanning transmission electron microscope to probe the effect of heating on distinct defect types. We apply a real space, local strain mapping technique, which allows us to identify tensile and shear strain in the atomic lattice, highlighting tensile, shear, and bending defects in interdot connections. We also track the out-of-plane orientation of individual QDs and infer the prevalence of out-of-plane twisting and bending defects as a function of annealing. We find that tensile and shear defects are fully relaxed upon mild thermal annealing, while bending defects persist. Additionally, out-of-plane orientation tracking reveals an increase in correctly oriented QDs, pointing to a relaxation of either twisting defects or out-of-plane bending defects. While bending defects remain, highlighting the need for further study of orientational ordering during the preattachment phase of superlattice formation, these atomic-scale insights show that annealing can effectively eliminate tensile and shear defects, a promising step toward delocalization of charge carriers and tunable electronic properties.

Small-molecule impurities, such as N-nitrosodimethylamine (NDMA), have infiltrated the generic drug industry, leading to recalls in commonly prescribed blood pressure and stomach drugs in over 43 countries since 2018 and directly affecting tens of millions of patients. One promising strategy to remove small-molecule impurities like NDMA from drug molecules is by size exclusion, in which the contaminant is removed by selective adsorption onto a (micro)porous material due to its smaller size. However, current solution-phase size-exclusion separations are primarily limited by the throughput-selectivity trade-off. Here, we report a bioinspired solution to conquer these critical challenges by leveraging the assembly of atomically precise building blocks into hierarchically porous structures. We introduce a bottom-up approach to form micropores, mesopores, and macroscopic superstructures simultaneously using functionalized oxozirconium clusters as building blocks. Further, we leverage recent advances in photopolymerization to design macroscopic flow structures to mitigate backpressure. Based on these multiscale design principles, we engineer simple, inexpensive devices that are able to separate NDMA from contaminated drugs. Beyond this urgent model system, we expect this design strategy to open up hitherto unexplored avenues of nanomaterial superstructure fabrication for a range of size-exclusion purification strategies.

The formation of defect-free two-dimensional nanocrystal (NC) superstructures remains a challenge as persistent defects hinder charge delocalization and related device performance. Understanding defect formation is an important step toward developing strategies to mitigate their formation. However, specific mechanisms of defect formation are difficult to determine, as superlattice phase transformations that occur during fabrication are quite complex and there are a variety of factors influencing the disorder in the final structure. Here, we use Molecular Dynamics (MD) and electron microscopy in concert to investigate the nucleation of the epitaxial attachment of lead chalcogenide (PbX, where X = S, Se) NC assemblies. We use an updated implementation of an existing reactive force field in an MD framework to investigate how initial orientational (mis)alignment of the constituent building blocks impacts the final structure of the epitaxially connected superlattice. This Simple Molecular Reactive Force Field (SMRFF) captures both short-range covalent forces and long-range electrostatic forces and allows us to follow orientational and translational changes of NCs during superlattice transformation. Our simulations reveal how robust the oriented attachment is with regard to the initial configuration of the NCs, measuring its sensitivity to both in-plane and out-of-plane misorientation. We show that oriented attachment nucleates through the initial formation of dimers, which corroborate experimentally observed structures. We present high-resolution structural analysis of dimers at early stages of the superlattice transformation and rationalize their contribution to the formation of defects in the final superlattice. Collectively, the simulations and experiments presented in this paper provide insights into the nucleation of NC oriented attachment, the impact of the initial configuration of NCs on the structural fidelity of the final epitaxially connected superlattice, and the propensity to form commonly observed defects, such as missing bridges and atomic misalignment in the superlattice due to the formation of dimers. We present potential strategies to mitigate the formation of superlattice defects.

Pulsing the potential during the electrochemical CO2 reduction (CO2R) reaction using copper has been shown to influence product selectivity (i.e., to suppress the undesired hydrogen evolution react...

Spontaneous hierarchical self-organization of nanometre-scale subunits into higher-level complex structures is ubiquitous in nature. The creation of synthetic nanomaterials that mimic the self-organization of complex superstructures commonly seen in biomolecules has proved challenging due to the lack of biomolecule-like building blocks that feature versatile, programmable interactions to render structural complexity. In this study, highly aligned structures are obtained from an organic-inorganic mesophase composed of monodisperse Cd

Concurrent advances in the programmable synthesis of nanostructured materials and additive three-dimensional (3D) manufacturing have created a rich and exciting opportunity space to fabricate novel...

Periodic arrays of strongly coupled colloidal quantum dots (QDs) may enable unprecedented control of electronic band structure through manipulation of QD size, shape, composition, spacing, and assembly geometry. This includes the possibilities of precisely engineered bandgaps and charge carrier mobilities, as well as remarkable behaviors such as metal-insulator transitions, massless carriers, and topological states. However, experimental realization of these theoretically predicted electronic structures is presently limited by structural disorder. Here, we use aberration-corrected scanning transmission electron microscopy to precisely quantify the orientational disorder of epitaxially connected QD films. In spite of coherent atomic connectivity between nearest neighbor QDs, we find misalignment persists with a standard deviation of 1.9°, resulting in significant bending strain localized to the adjoining necks. We observe and quantify a range of out-of-plane particle orientations over thousands of QDs and correlate the in-plane and out-of-plane misalignments, finding QDs misoriented out-of-plane display a statistically greater misalignment with respect to their in-plane neighbors as well. Using the bond orientational order metric ψ

Structural transformations in molecules and solids have generally been studied in isolation, while intermediate systems have eluded characterization. We show that a pair of CdS cluster isomers provides an advantageous experimental platform to study isomerization in well-defined atomically precise systems. The clusters coherently interconvert over an est. 1 eV energy barrier with a 140 meV shift in their excitonic energy gaps. There is a diffusionless, displacive reconfiguration of the inorganic core (solid-solid transformation) with first order (isomerization-like) transformation kinetics. Driven by a distortion of the ligand binding motifs, the presence of hydroxyl species changes the surface energy via physisorption, which determines phase stability in this system. This reaction possesses essential characteristics of both solid-solid transformations and molecular isomerizations, and bridges these disparate length scales., Comment: Main text - 17 pages, 4 figures, SI - 65 pages, 10 figures, 6 tables

Nanowire growth from metal surfaces presents several scientifically interesting and technologically important research challenges. Scientifically, many knowledge gaps remain about the fundamental m...

The electrochemical CO

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 payed 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., 3D printing of nanomaterials generates opportunities to create advanced materials but printing of nanoparticles with intrinsic porosity has gained only little attention. Here, the authors demonstrate printing of silica nanocages using digital light processing to fabricate hierarchically porous parts with tunable internal structure and complex shapes.

Understanding the mechanism and ultimately directing nanocrystal (NC) superlattice assembly and attachment have important implications on future advances in this emerging field. Here, we use 4D-STEM to investigate a monolayer of PbS NCs at various stages of the transformation from a hexatic assembly to a nonconnected square-like superlattice over large fields of view. Maps of nanobeam electron diffraction patterns acquired with an electron microscope pixel array detector (EMPAD) offer unprecedented detail into the 3D crystallographic alignment of the polyhedral NCs. Our analysis reveals that superlattice transformation is dominated by translation of prealigned NCs strongly coupled along the11

Current research in CO2 catalytical conversion is usually conducted with single-pass lab-scale reactors. Operating conditions affecting catalyst performance optimized for these reactors were not necessarily transferrable for large scale applications. Herein we report a scalable optofluidic photoreactor based on glass waveguides coated with photocatalyst. Inside the “shell-and-tube” reactor design, tubes are replaced by internal light-guiding waveguides with specially designed scattering surfaces to enable deep and efficient penetration of the light irradiation. Using reverse water–gas shift (RWGS) as a pilot reaction, the effect of temperature, light irradiation and residence time on the photocatalytic activity of this photoreactor platform was examined.

In this work, we introduce HI-Light, a surface-engineered glass waveguide based “shell-and-tube” type photothermal reactor which is both scalable in diameter and length. We examine the effect of temperature, light irradiation, and residence time on its photo-thermocatalytic performance for CO2 hydrogenation to form CO, with a cubic phase defect-laden indium oxide, In2O3-x(OH)y catalyst. We demonstrate the light enhancement effect under a variety of reaction conditions. Notably, the light-on performance for the cubic nanocrystal photocatalyst exhibits a CO evolution rate at 15.40 mmol gcat−1 h−1 at 300°C and atmospheric pressure. This is 20 times higher conversion rate beyond previously reported In2O3-x(OH)y catalyst in the cubic form under comparable operation conditions, and more than 5 times higher than that of its rhombohedral polymorph. This result underscores that improvement in photo-thermocatalytic reactor design enables uniform light distribution and better reactant/catalyst mixing, thus significantly improving catalyst utilization.

We demonstrate a simple strategy to enhance the CO2 reduction reaction (CO2 RR) selectivity by applying a pulsed electrochemical potential to a polycrystalline copper electrode. By controlling the pulse duration, we show that the hydrogen evolution reaction (HER) is highly suppressed to a fraction of the original value ( 75 % CH4 and >50 % CO faradaic efficiency). We attribute the improved CO2 RR selectivity to a dynamically rearranging surface coverage of hydrogen and intermediate species during the pulsing. Our finding provides new insights into the interplay of transport and reaction processes as well as timescales of competing pathways to enable new opportunities to tune CO2 RR selectivity by adjusting the pulse profile. Additionally, the pulsed potential method we describe can be easily applied to other catalysts materials to improve their CO2 RR selectivity.

The combination of self-assembly and directed attachment of colloidal nanocrystals at fluid interfaces presents a scientifically interesting and technologically important research challenge. Remarkable strides have been made in the synthesis of polyhedral nanocrystals with precisely defined shapes and their self-assembly into highly ordered superstructures. We discuss the interplay of entropic and enthalpic driving forces and the kinetic aspects of interfacial self-assembly and attachment. We present in situ parallel small-angle X-ray scattering measurements and emerging insights into the complex choreography of interfacial transport processes involved in the formation of highly ordered epitaxially connected nanocrystal solids. New understanding emerging from in situ measurements provides process control and design principles for the selective formation of specific superlattice polymorphs. We discuss outstanding challenges that must be resolved to translate know-how from controlled assembly and attachment...

The formation of epitaxially connected quantum dot solids involves a complex interplay of interfacial assembly, surface chemistry, and irreversible-directed attachment. We describe the basic mechanism in the context of a coherent phase transition with distinct nucleation and propagation steps. The proposed mechanism explains how defects in the preassembled structure influence nucleation and how basic geometric relationships govern the transformation from hexagonal assemblies of isolated dots to interconnected solids with square symmetry. We anticipate that new mechanistic insights will guide future advances in the formation of high-fidelity quantum dot solids with enhanced grain size, interconnectivity, and control over polymorph structures.

This article describes the chemical kinetics of germanium nanowire growth on inductively heated copper surfaces using diphenylgermane as a precursor. Inductive heating of metal surfaces presents a simple, rapid, and contact-free method to activate the direct growth of nanowires on metal surfaces. We show the main effects of synthesis temperature, duration, precursor concentration on the morphology, and loading of the nanowire film. We describe the complex interplay of precursor degradation, nucleation, and growth in context of a multistep reaction mechanism. We studied the temporal evolution of nanowire loading and morphology to develop a kinetic model, which predicts critical thresholds that define the onset of sequential axial and radial nanowire growth modes. These results may be used to commercially scale a nanowire growth process.

Epitaxially connected assemblies of nanocrystals (NCs) present an interesting new class of nanomaterial in which confinement of charge carriers is intermediate between that of a quantum dot and a quantum well. Despite impressive advances in the formation of high-fidelity assemblies, predicted collective properties have not yet emerged. A critical knowledge gap toward realizing these properties is the current lack of understanding of and control over the formation of epitaxial interdot bonds connecting the NCs within the assemblies. In this work we demonstrate successive ionic layer absorption and reaction (SILAR) to enhance the interdot bonding within the NC assembly. SILAR treatment improved the fraction of interdot bonds from 82% to 91% and increased their width from 3.1 to 4.0 nm. Absorption spectra and charge transport measurements indicate that the effect of postassembly growth on quantum confinement in this system depends on the composition of the SILAR shell material. Increased NC film conductance following SILAR processing indicates that building and strengthening interdot bonds lead to increased electronic coupling and doping in the assemblies. The postassembly film growth detailed here presents an opportunity to repair structural defects and to tailor the balance of quantum confinement and interdot coupling in epitaxially connected NC assemblies.

Lead halide perovskites show slow (from seconds to minutes) and fast (milliseconds to submicroseconds) charge dynamics. We use scanning Kelvin probe microscopy and dissipation microscopy to probe these charge dynamics in a thin film of CsPbBr3. We demonstrate the existence of a light-intensity-dependent τfast in CsPbBr3 that exhibits a slow, activated, intensity-independent recovery in the dark. The observed τfast, while highly light-dependent, remained essentially unchanged when the light was turned off, taking 10 ± 2 s to relax at room temperature. The data presented here show direct evidence that the slow and fast charge dynamics ubiquitously seen in lead-halide perovskites have a common origin related to a highly activated charge redistribution.

Optoelectronic properties of nanoparticles are intimately coupled to the complex physiochemical interplay between the inorganic core and the organic ligand shell. Magic-sized clusters, which are predominately surface atoms, provide a promising avenue to clarify these critical surface interactions. Whereas these interactions impact the surface of both nanoparticles and magic-sized clusters, we show here that only clusters manifest a shift in the excitonic peak by up to 0.4 eV upon solvent or ligand treatment. These results highlight the utility of the clusters as a probe of ligand-surface interactions.

Magic-sized clusters (MSCs) are renowned for their identical size and closed-shell stability that inhibit conventional nanoparticle (NP) growth processes. Though MSCs have been of increasing interest, understanding the reaction pathways toward their nucleation and stabilization is an outstanding issue. In this work, we demonstrate that high concentration synthesis (1000 mM) promotes a well-defined reaction pathway to form high-purity MSCs (greater than 99.9 percent). The MSCs are resistant to typical growth and dissolution processes. Based on insights from in-situ X-ray scattering analysis, we attribute this stability to the accompanying production of a large, hexagonal organic-inorganic mesophase (greater than 100 nm grain size) that arrests growth of the MSCs and prevents NP growth. At intermediate concentrations (500 mM), the MSC mesophase forms, but is unstable, resulting in NP growth at the expense of the assemblies. These results provide an alternate explanation for the high stability of MSCs. Whereas the conventional mantra has been that the stability of MSCs derives from the precise arrangement of the inorganic structures (i.e., closed-shell atomic packing), we demonstrate that anisotropic clusters can also be stabilized by self-forming fibrous mesophase assemblies. At lower concentration (less than 200 mM or greater than 16 acid-to-metal), MSCs are further destabilized and NPs formation dominates that of MSCs. Overall, the high concentration approach intensifies and showcases inherent concentration-dependent surfactant phase behavior that is not accessible in conventional (i.e., dilute) conditions. This work provides not only a robust method to synthesize, stabilize, and study identical MSC products, but also uncovers an underappreciated stabilizing interaction between surfactants and clusters., Comment: main text - 24 pages, 8 figures. SI - 35 pages, 30 figures, 6 tables

Epitaxially connected superlattices of self-assembled colloidal quantum dots present a promising route toward exquisite control of electronic structure through precise hierarchical structuring across multiple length scales. Here, we uncover propagation of disorder as an essential feature in these systems, which intimately connects order at the atomic, superlattice, and grain scales. Accessing theoretically predicted exotic electronic states and highly tunable minibands will therefore require detailed understanding of the subtle interplay between local and long-range structure. To that end, we developed analytical methods to quantitatively characterize the propagating disorder in terms of a real paracrystal model and directly observe the dramatic impact of angstrom scale translational disorder on structural correlations at hundreds of nanometers. Using this framework, we discover improved order accompanies increasing sample thickness and identify the substantial effect of small fractions of missing epitaxial bonds on statistical disorder. These results have significant experimental and theoretical implications for the elusive goals of long-range carrier delocalization and true miniband formation.

Trap states formed at the surface of colloidal semiconductor nanocrystals can have deleterious impact on performance in emerging optoelectronic applications. To mitigate surface traps in nanocrystal thin films we investigated simultaneous surface passivation and ligand exchange for PbSe nanocrystal films via treatment with a cadmium acetate solution. We show that a kinetically limited surface cation exchange produces a thin CdxPb1 − xSe shell that effectively passivates the nanocrystal surface as confirmed by increased photoluminescence intensity and photoluminescence lifetime. Ligand exchange to acetate ligands is confirmed via Fourier transform infrared spectroscopy and grazing incidence small angle X-ray scattering. We studied the impact of the cadmium acetate treatment on interparticle coupling and found that the reduced interparticle spacing and limited shell thickness leads to increased Forster resonant energy transfer in nanocrystal films. Simultaneous cation/ligand exchange enables the production of heterostructured nanocrystal films with properties like Quasi-Type II nanocrystals synthesized in solution.

Surface states of colloidal nanocrystals are typically created when organic surfactants are removed. We report a chemical process that reduces surface traps and tunes the interparticle coupling in PbS nanocrystal thin films after the surfactant ligands have been stripped off. This process produces PbS/PbI2 core/shell nanocrystal thin films via a combined ammonium sulfide and iodine treatment. These all-inorganic nanocrystal thin films are air-stable and exhibit bright emission with optimum photoluminescence quantum yield close to that of pristine PbS nanocrystals passivated by oleate ligands. Interparticle coupling of post-treatment nanocrystal thin films is continuously tunable by varying the iodine treatment process. Optical studies reveal that this method can produce PbS nanocrystal thin films superior in both coupling and surface quality to nanocrystals linked by small molecules such as ethanedithiol or 3-mercaptopropionic acid.

Summary In this work, we introduce HI-Light, a surface-engineered glass-waveguide-based “shell-and-tube” type photothermal reactor which is both scalable in diameter and length. We examine the effect of temperature, light irradiation, and residence time on its photo-thermocatalytic performance for CO2 hydrogenation to form CO, with a cubic phase defect-laden indium oxide, In2O3-x(OH)y, catalyst. We demonstrate the light enhancement effect under a variety of reaction conditions. Notably, the light-on performance for the cubic nanocrystal photocatalyst exhibits a CO evolution rate at 15.40 mmol gcat−1 hr−1 at 300°C and atmospheric pressure. This is 20 times higher conversion rate per unit catalyst mass per unit time beyond previously reported In2O3-x(OH)y catalyst in the cubic form under comparable operation conditions and more than 5 times higher than that of its rhombohedral polymorph. This result underscores that improvement in photo-thermocatalytic reactor design enables uniform light distribution and better reactant/catalyst mixing, thus significantly improving catalyst utilization.

The ability to control reaction kinetics and double layer species during an electrocatalytic process is highly desirable, especially for electrochemical CO2 reduction (CO2R) — a complex process in which multiple reaction steps are competing on the electrode surface. Here we show evidence suggesting the double layer can be disrupted with the application of a pulsed potential during CO2R. Pulsing the potential during CO2R using copper has been shown to influence product selectivity (i.e., to suppress the undesired hydrogen evolution reaction (HER)) and to improve electrocatalyst stability compared to constant applied potential.1 However, the underlying mechanism and contribution of interfacial/surface phenomena behind the pulsed potential application remain largely unknown. To uncover this unknown we investigated the state of the copper surface during the pulsed potential electrochemical CO2R using in-situ X-ray Adsorption Near Edge Spectroscopy (XANES). We probed the surface valence of the metallic electrode and found that the Cu electrode remains metallic over a broad pulsed potential range and only oxidizes to form Cu(OH)2 in the bulk when the pulsed potential reaches a highly oxidative limit (> 0.6 V vs. reversible hydrogen electrode (RHE)). Our results suggest that the pulsed anodic potential influences the double layer on the electrode surface, i.e., the dynamic competition between protons and hydroxide adsorbates instead of bulk copper oxidation. We attribute the suppressed HER to the electro-adsorption of hydroxides, which outcompetes protons for surface sites. As shown in a recent in-situ infrared study2, adsorbed hydroxides promote CO adsorption, a crucial CO2 reduction intermediate, by preventing CO from becoming inert through a near neighbor effect. We corroborate this interpretation by demonstrating that the pulsed potential application can suppress the HER during the CO reduction just as the CO2R. Our results suggest that the pulsed potential mechanism favors CO2R over the HER due to two effects: 1) proton desorption/displacement during the anodic potential and 2) the accumulation of OHads creating a higher surface-pH environment, promoting CO adsorption. We can describe this pulsed potential dynamic double layer mechanism in a competing quaternary Langmuir isotherm model. We conclude that the active disruption of the double layer can be leveraged to tune the surface reaction environment during CO2R. Furthermore, the insights from this investigation have wide-ranging implications for applying pulsed potential profiles to improve electrocatalytic processes in general by dynamically disrupting double layer species. [1] Kimura, K. W.; Fritz, K. E.; Kim, J.; Suntivich, J.; Abruña, H. D.; Hanrath, T. Controlled Selectivity of CO2 Reduction on Copper by Pulsing the Electrochemical Potential. ChemSusChem 2018, 11 (11), 1781–1786. https://doi.org/10.1002/cssc.201800318. [2] Iijima, G.; Inomata, T.; Yamaguchi, H.; Ito, M.; Masuda, H. Role of a Hydroxide Layer on Cu Electrodes in Electrochemical CO2 Reduction. ACS Catal. 2019, 9 (7), 6305–6319. https://doi.org/10.1021/acscatal.9b00896.

One of the grand challenges in electrocatalysis is to better understand the factors that determine activity and selectivity to control the precision of electrochemical reactions.1 Electrocatalytic CO2 reduction (eCO2R) is a prototypical example of such a reaction, where control over product selectivity would completely transform electrosynthesis processes. Beyond the pursuit of fundamentally understanding electrochemical catalysis, development of eCO2R is driven by growing concerns about global CO2 emissions and the quest for valorization of captured CO2. However, product selectivity and electrocatalyst longevity persist as obstacles to broad implementation of eCO2R. One possible solution to address this challenge is to apply a pulsed potential during eCO2R, which creates a stable reduction environment and tunable product selectivity.2 We leveraged this long-term product stability of pulsed potential eCO2R to examine the relationship between electrolyte concentration and composition with product selectivity for a copper electrode. Whereas constant potential experiments suffer from quick degradation as selectivity towards CO2 reduction products lasts only on the order of one hour, pulsing the potential maintains robust selectivity over 24 hours. This stability presents a unique opportunity to vary the electrolyte parameters while keeping experimental conditions consistent thereby eliminating electrode variability. We find the relation of electrolyte concentration and composition differs greatly for constant and pulsed 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 adsorption of surface hydroxides, 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, during pulsed potential eCO2R, increasing KCl concentration suppresses H2 evolution and greatly favors C2 products, reaching 71% Faradaic efficiency. Collectively, these results provide new mechanistic insights into pulsed potential eCO2R in context of the ionic conductivity and higher presence of surface hydroxides which promote C-C bonding. More broadly, the techniques employed here can be used to understand and optimize other electrosynthesis processes. [1] Bell, A. T.; Gates, B. C.; Ray, D.; Thompson, M. R. Basic research needs: catalysis for energy; Pacific Northwest National Lab.(PNNL), Richland, WA (United States): 2008. [2] Kimura, K. W.; Fritz, K. E.; Kim, J.; Suntivich, J.; Abruña, H. D.; Hanrath, T., Controlled Selectivity of CO2 Reduction on Copper by Pulsing the Electrochemical Potential. ChemSusChem 2018, 11 (11), 1781-1786.

New methods are developed for the synthesis and surface modification of colloidal PbS nanosheets. We find that residual acetate is a crucial reagent for the formation of PbS nanosheets by existing syntheses. The amount of acetate in reaction mixtures is, however, difficult to control, which substantially reduces the reproducibility of the synthesis. To solve this problem, we develop an acetate-free synthetic method to yield colloidal PbS nanosheets with lateral dimensions of several hundred nanometers. The thickness and shape of PbS nanosheets can be readily tuned by varying the synthetic parameters. As-synthesized nanosheets have a photoluminescence quantum yield of around 6%. Time-resolved photoluminescence spectroscopy shows that the carrier decay in nanosheets is not a single exponential but involves surface defect states as well as multiple carrier interactions. The surfaces of PbS nanosheets can be modified by reaction with Cd(OA)2. Atomic resolution electron microscopy reveals the formation of PbS/...