Lithium Metal Anodes: Advancing our Mechanistic Understanding of Cycling Phenomena in Liquid and Solid Electrolytes.

Lithium metal anodes have the potential to be a disruptive technology for next-generation batteries with high energy densities, but their electrochemical performance is limited by a lack of fundamental understanding into the mechanistic origins that underpin their poor reversibility, morphological evolution (including dendrite growth), and interfacial instability. The goal of this perspective is to summarize the current state-of-the-art understanding of these phenomena, and highlight knowledge gaps where additional research is needed. The various stages of cycling are described sequentially, including nucleation, growth, open-circuit rest periods, and electrodissolution (stripping). A direct comparison of lessons learned from liquid and solid-state electrolyte systems is made throughout the discussion, providing cross-cutting insights between these research communities. Major themes of the discussion include electro-chemo-mechanical coupling, insights from in situ/operando analysis, and the interplay between experimental observations and computational modeling. Finally, a series of fundamental research questions are proposed to identify critical knowledge gaps and inform future research directions.

Bioinspired Zwitterionic Nanowires with Simultaneous Biofouling Reduction and Release.

Marine biofouling is a complex and dynamic process that significantly increases the carbon emissions from the maritime industry by increasing drag losses. However, there are no existing non-toxic marine paints that can achieve both effective fouling reduction and efficient fouling release. Inspired by antifouling strategies in nature, herein, a superoleophobic zwitterionic nanowire coating with a nanostructured hydration layer is introduced, which exhibits simultaneous fouling reduction and release performance. The zwitterionic nanowires demonstrate >25% improvement in fouling reduction compared to state-of-the-art antifouling nanostructures, and four times higher fouling-release compared to conventional zwitterionic coatings. Fouling release is successfully achieved under a wall shear force that is four orders of magnitude lower than regular water jet cleaning. The mechanism of this simultaneous fouling reduction and release behavior is explored, and it is found that a combination of 1) a mechanical biocidal effect from the nanowire geometry, and 2) low interfacial adhesion resulting from the nanostructured hydration layer, are the major contributing factors. These findings provide insights into the design of nanostructured coatings with simultaneous fouling reduction and release. The newly established synthesis procedure for the zwitterionic nanowires opens new pathways for implementation as antifouling coatings in the maritime industry and biomedical devices.

Atomic Layer Deposition of Cu Electrocatalysts on Gas Diffusion Electrodes for CO2 Reduction.

Electrochemical reduction of CO2 using Cu catalysts enables the synthesis of C2+ products including C2H4 and C2H5OH. In this study, Cu catalysts were fabricated using plasma-enhanced atomic layer deposition (PEALD), achieving conformal deposition of catalysts throughout 3-D gas diffusion electrode (GDE) substrates while maintaining tunable control of Cu nanoparticle size and areal loading. The electrochemical CO2 reduction at the Cu surface yielded a total Faradaic efficiency (FE) > 75% for C2+ products. Parasitic hydrogen evolution was minimized to a FE of ∼10%, and a selectivity of 42.2% FE for C2H4 was demonstrated. Compared to a line-of-sight physical vapor deposition method, PEALD Cu catalysts show significant suppression of C1 products compared to C2+, which is associated with improved control of catalyst morphology and conformality within the porous GDE substrate. Finally, PEALD Cu catalysts demonstrated a stable performance for 15 h with minimal reduction in the C2H4 production rate.

Visible-Light-Driven Photocatalysts for Self-Cleaning Transparent Surfaces.

Highly transparent photocatalytic self-cleaning surfaces capable of harvesting near-visible (365-430 nm) photons were synthesized and characterized. This helps to address a current research gap in self-cleaning surfaces, in which photocatalytic coatings that exhibit activity at wavelengths longer than ultraviolet (UV) generally have poor optical transparency, because of broadband scattering and the attenuation of visible light. In this work, the wavelength-dependent photocatalytic activity of Pt-modified TiO2 (Pt-TiO2) particles was characterized, which exhibited activity for wavelengths up to 430 nm. Pt-TiO2 nanoparticles were embedded in a mesoporous SiO2 sol-gel matrix, forming a superhydrophilic surface that allowed for water adsorption and formation of reactive oxide species upon illumination, resulting in the removal of organic surface contaminants. These self-cleaning surfaces only interact strongly with near-visible light (∼365-430 nm), as characterized by photocatalytic self-cleaning tests. Broadband visible transparency was preserved by generating a morphology composed of small clusters of Pt-TiO2 surrounded by a matrix of SiO2, which limited diffuse visible light scattering and attenuation. The wavelength-dependent self-cleaning rate by the films was quantified using stearic acid degradation under both monochromatic and AM1.5G spectral illumination. By varying the film morphology, the average transmittance relative to bare glass can be tuned from ∼93%-99%, and the self-cleaning rate can be adjusted by more than an order of magnitude. Overall, the ability to utilize photocatalysts with tunable visible light activity, while maintaining broadband transparency, can enable the use of photocatalytic self-cleaning surfaces for applications where UV illumination is limited, such as touchscreen displays.

Atomic layer deposition of metal sulfide materials.

CONSPECTUS: The field of nanoscience is delivering increasingly intricate yet elegant geometric structures incorporating an ever-expanding palette of materials. Atomic layer deposition (ALD) is a powerful driver of this field, providing exceptionally conformal coatings spanning the periodic table and atomic-scale precision independent of substrate geometry. This versatility is intrinsic to ALD and results from sequential and self-limiting surface reactions. This characteristic facilitates digital synthesis, in which the film grows linearly with the number of reaction cycles. While the majority of ALD processes identified to date produce metal oxides, novel applications in areas such as energy storage, catalysis, and nanophotonics are motivating interest in sulfide materials. Recent progress in ALD of sulfides has expanded the diversity of accessible materials as well as a more complete understanding of the unique chalcogenide surface chemistry. ALD of sulfide materials typically uses metalorganic precursors and hydrogen sulfide (H2S). As in oxide ALD, the precursor chemistry is critical to controlling both the film growth and properties including roughness, crystallinity, and impurity levels. By modification of the precursor sequence, multicomponent sulfides have been deposited, although challenges remain because of the higher propensity for cation exchange reactions, greater diffusion rates, and unintentional annealing of this more labile class of materials. A deeper understanding of these surface chemical reactions has been achieved through a combination of in situ studies and quantum-chemical calculations. As this understanding matures, so does our ability to deterministically tailor film properties to new applications and more sophisticated devices. This Account highlights the attributes of ALD chemistry that are unique to metal sulfides and surveys recent applications of these materials in photovoltaics, energy storage, and photonics. Within each application space, the benefits and challenges of novel ALD processes are emphasized and common trends are summarized. We conclude with a perspective on potential future directions for metal chalcogenide ALD as well as untapped opportunities. Finally, we consider challenges that must be addressed prior to implementing ALD metal sulfides into future device architectures.

Core-Shell CdS-Cu2S Nanorod Array Solar Cells.

As an earth-abundant p-type semiconductor, copper sulfide (Cu2S) is an attractive material for application in photovoltaic devices. However, it suffers from a minority carrier diffusion length that is less than the length required for complete light absorption. Core-shell nanowires and nanorods have the potential to alleviate this difficulty because they decouple the length scales of light absorption and charge collection. To achieve this geometry using Cu2S, cation exchange was applied to an array of CdS nanorods to produce well-defined CdS-Cu2S core-shell nanorods. Previous work has demonstrated single-nanowire photovoltaic devices from this material system, but in this work, the cation exchange chemistry has been applied to nanorod arrays to produce ensemble-level devices with microscale sizes. The core-shell nanorod array devices show power conversion efficiencies of up to 3.8%. In addition, these devices are stable when measured in air after nearly one month of storage in a desiccator. These results are a first step in the development of large-area nanostructured Cu2S-based photovoltaics that can be processed from solution.

Quantifying Geometric Strain at the PbS QD-TiO2 Anode Interface and Its Effect on Electronic Structures.

Quantum dots (QDs) show promise as the absorber in nanostructured thin film solar cells, but achieving high device efficiencies requires surface treatments to minimize interfacial recombination. In this work, lead sulfide (PbS) QDs are grown on a mesoporous TiO2 film with a crystalline TiO2 surface, versus one coated with an amorphous TiO2 layer by atomic layer deposition (ALD). These mesoporous TiO2 films sensitized with PbS QDs are characterized by X-ray and electron diffraction, as well as X-ray absorption spectroscopy (XAS) in order to link XAS features with structural distortions in the PbS QDs. The XAS features are further analyzed with quantum simulations to probe the geometric and electronic structure of the PbS QD-TiO2 interface. We show that the anatase TiO2 surface structure induces PbS bond angle distortions, which increases the energy gap of the PbS QDs at the interface.

Epitaxially aligned cuprous oxide nanowires for all-oxide, single-wire solar cells.

As a p-type semiconducting oxide that can absorb visible light, cuprous oxide (Cu2O) is an attractive material for solar energy conversion. This work introduces a high-temperature, vapor-phase synthesis that produces faceted Cu2O nanowires that grow epitaxially along the surface of a lattice-matched, single-crystal MgO substrate. Individual wires were then fabricated into single-wire, all-oxide diodes and solar cells using low-temperature atomic layer deposition (ALD) of TiO2 and ZnO films to form the heterojunction. The performance of devices made from pristine Cu2O wires and chlorine-exposed Cu2O wires was investigated under one-sun and laser illumination. These faceted wires allow the fabrication of well-controlled heterojunctions that can be used to investigate the interfacial properties of all-oxide solar cells.

Uniform doping of metal oxide nanowires using solid state diffusion.

The synthesis of one-dimensional nanostructures with specific properties is often hindered by difficulty in tuning the material composition without sacrificing morphology and material quality. Here, we present a simple solid state diffusion method utilizing atomic layer deposition to controllably alter the composition of metal oxide nanowires. This compositional control allows for modification of the optical, electronic, and electrochemical properties of the semiconductor nanowires. Using this method and a novel process for manganese oxide atomic layer deposition, we produced manganese-doped rutile TiO2 nanowires and investigated their structural and photoelectrochemical properties. A homogeneous incorporation of the Mn dopant into the rutile lattice was observed, and the local chemical environment of the Mn was determined using X-ray absorption spectroscopy. The doping process resulted in a tunable enhancement in the electrocatalytic activity for water oxidation, demonstrating that this simple and general method can be used to control the properties of one-dimensional nanostructures for use in a variety of applications including solar-to-fuel generation.

Spatial variation of available electronic excitations within individual quantum dots.

Quantum dots (QDs) allow for manipulation of the position and energy levels of electrons at sub-10 nm length scales through control of material chemistry, size, and shape. It is known from optical studies that the bandgap of semiconductor QDs increases as their size decreases due to the narrowing of the quantum confinement potential. The mechanism of quantum confinement also indicates that the localized properties within individual QDs should depend on their shape in addition to their size, but direct observations of this effect have proven challenging due to the limited spatial resolution of measurement techniques at this scale and the ability to remove contributions from the surroundings. Here we present experimental evidence of spatial variations in the lowest available electron transition energy within a series of single electrically isolated QDs due to a dome-shaped geometry, measured using electron energy-loss spectroscopy in a (scanning) transmission electron microscope [(S)TEM-EELS]. We observe a consistent increase in the energy onset of electronic excitations from the lateral center of the dot toward the edges, which we attribute purely to shape. This trend is in qualitative agreement with a simple quantum simulation of the local density of states in a dome-shaped QD.

Subtractive Patterning of Nanoscale Thin Films Using Acid-Based Electrohydrodynamic-Jet Printing.

As an alternative to traditional photolithography, printing processes are widely explored for the patterning of customizable devices. However, to date, the majority of high-resolution printing processes for functional nanomaterials are additive in nature. To complement additive printing, there is a need for subtractive processes, where the printed ink results in material removal, rather than addition. In this study, a new subtractive patterning approach that uses electrohydrodynamic-jet (e-jet) printing of acid-based inks to etch nanoscale zinc oxide (ZnO) thin films deposited using atomic layer deposition (ALD) is introduced. By tuning the printing parameters, the depth and linewidth of the subtracted features can be tuned, with a minimum linewidth of 11 µm and a tunable channel depth with ≈5 nm resolution. Furthermore, by tuning the ink composition, the volatility and viscosity of the ink can be adjusted, resulting in variable spreading and dissolution dynamics at the solution/film interface. In the future, acid-based subtractive patterning using e-jet printing can be used for rapid prototyping or customizable manufacturing of functional devices on a range of substrates with nanoscale precision.

Thermodynamics, Adhesion, and Wetting at Li/Cu(-Oxide) Interfaces: Relevance for Anode-Free Lithium-Metal Batteries.

A rechargeable battery that employs a Li metal anode requires that Li be plated in a uniform fashion during charging. In "anode-free" configurations, this plating will occur on the surface of the Cu current collector (CC) during the initial cycle and in any subsequent cycle where the capacity of the cell is fully accessed. Experimental measurements have shown that the plating of Li on Cu can be inhomogeneous, which can lower the efficiency of plating and foster the formation of Li dendrites. The present study employs a combination of first-principles calculations and sessile drop experiments to characterize the thermodynamics and adhesive (i.e., wetting) properties of interfaces involving Li and other phases present on or near the CC. Interfaces between Li and Cu, Cu2O, and Li2O are considered. The calculations predict that both Cu and Cu2O surfaces are lithiophilic. However, sessile drop measurements reveal that Li wetting occurs readily only on pristine Cu. This apparent discrepancy is explained by the occurrence of a spontaneous conversion reaction, 2 Li + Cu2O → Li2O + 2 Cu, that generates Li2O as one of its products. Calculations and sessile drop measurements show that Li does not wet (newly formed) Li2O. Hence, Li that is deposited on a Cu CC where surface oxide species are present will encounter a compositionally heterogeneous substrate comprising lithiophillic (Cu) and lithiophobic (Li2O) regions. These initial heterogeneities have the potential to influence the longer-term behavior of the anode under cycling. In sum, the present study provides insights into the early stage processes associated with Li plating in anode-free batteries and describes mechanisms that contribute to inefficiencies in their operation.

Lamination of >21% Efficient Perovskite Solar Cells with Independent Process Control of Transport Layers and Interfaces.

Transport layer and interface optimization is critical for improving the performance and stability of perovskite solar cells (PSCs) but is restricted by the conventional fabrication approach of sequential layer deposition. While the bottom transport layer is processed with minimum constraints, the narrow thermal and chemical stability window of the halide perovskite (HP) layer severely restricts the choice of top transport layer and its processing conditions. To overcome these limitations, we demonstrate lamination of HPs─where two transport layer-perovskite half-stacks are independently processed and diffusion-bonded at the HP-HP interface─as an alternative fabrication strategy that enables self-encapsulated solar cells. Power conversion efficiencies (PCE) of >21% are realized using cells that incorporate a novel transport layer combination along with dual-interface passivation via self-assembled monolayers, both of which are uniquely enabled by the lamination approach. This is the highest reported PCE for any laminated PSC encapsulated between glass substrates. We further show that this approach expands the processing window beyond traditional fabrication processes and is adaptable for different transport layer compositions. The laminated PSCs retained >75% of their initial PCE after 1000 h of 1-sun illumination at 40 °C in air using an all-inorganic transport layer configuration without additional encapsulation. Furthermore, a laminated 1 cm2 device maintained a Voc of 1.16 V. The scalable lamination strategy in this study enables the implementation of new transport layers and interfacial engineering approaches for improving performance and stability.

Atomic layer deposition of platinum catalysts on nanowire surfaces for photoelectrochemical water reduction.

The photocathodic hydrogen evolution reaction (HER) from p-type Si nanowire (NW) arrays was evaluated using platinum deposited by atomic layer deposition (ALD) as a HER cocatalyst. ALD of Pt on the NW surface led to a highly conformal coating of nanoparticles with sizes ranging from 0.5 to 3 nm, allowing for precise control of the Pt loading in deep submonolayer quantities. The catalytic performance was measured using as little as 1 cycle of Pt ALD, which corresponded to a surface mass loading of ∼10 ng/cm(2). The quantitative exploration of the lower limits of Pt cocatalyst loading reported here, and its application to high-surface-area NW photoelectrodes, establish a general approach for minimizing the cost of precious-metal cocatalysts for efficient and affordable solar-to-fuel applications.

Atomic layer deposition of lead sulfide quantum dots on nanowire surfaces.

Quantum dots provide unique advantages in the design of novel optoelectronic devices owing to the ability to tune their properties as a function of size. Here we demonstrate a new technique for fabrication of quantum dots during the nucleation stage of atomic layer deposition (ALD) of PbS. Islands with sub-10 nm diameters were observed during the initial ALD cycles by transmission electron microscopy, and in situ observations of the coalescence and sublimation behavior of these islands show the possibility of further modifying the size and density of dots by annealing. The ALD process can be used to cover high-aspect-ratio nanostructures, as demonstrated by the uniform coating of a Si nanowire array with a single layer of PbS quantum dots. Photoluminescence measurements on the quantum dot/nanowire composites show a blue shift when the number of ALD cycles is decreased, suggesting a route to fabricate unique three-dimensional nanostructured devices such as solar cells.

Durable Liquid- and Solid-Repellent Elastomeric Coatings Infused with Partially Crosslinked Lubricants.

Surfaces that are resistant to both liquid fouling and solid fouling are critical for many industrial and biomedical applications. However, surfaces developed to address these challenges thus far have been generally susceptible to mechanical damage. Herein, we report the design and fabrication of robust solid- and liquid-repellent elastomeric coatings that incorporate partially crosslinked lubricating chains within a durable polymer matrix. In particular, we fabricated partially crosslinked omniphobic polyurethane (omni-PU) coatings that can repel a broad range of liquid and solid foulants. The fabricated coatings are an order of magnitude more resistant to cyclic abrasion than current state-of-the-art slippery surfaces. Further through the integration of classic wetting and tribology models, we introduce a new material design parameter (KAR) for abrasion-resistant polymeric coatings. This combination of mechanical durability and broad antifouling properties enables the implication of such coatings to a wide variety of industrial and medical settings, including biocompatible implants, underwater vehicles, and antifouling robotics.

Integrating Structural Colors with Additive Manufacturing Using Atomic Layer Deposition.

We demonstrate tunable structural color patterns that span the visible spectrum using atomic layer deposition (ALD). Asymmetric metal-dielectric-metal structures were sequentially deposited with nickel, zinc oxide, and a thin copper layer to form an optical cavity. The color response was precisely adjusted by tuning the zinc oxide (ZnO) thickness using ALD, which was consistent with model predictions. Owing to the conformal nature of ALD, this allows for uniform and tunable coloration of non-planar three-dimensional (3D) objects, as exemplified by adding color to 3D-printed parts produced by metal additive manufacturing. Proper choice of inorganic layered structures and materials allows the structural color to be stable at elevated temperatures, in contrast to traditional paints. To print multiple colors on a single sample, polymer inhibitors were patterned in a desired geometry using electrohydrodynamic jet (e-jet) printing, followed by area-selective ALD in the unpassivated regions. The ability to achieve 3D color printing, both at the micro- and macroscales, provides a new pathway to tune the optical and aesthetic properties during additive manufacturing.

Geometric Optimization of Bismuth Vanadate Core-Shell Nanowire Photoanodes using Atomic Layer Deposition.

In this study, systematic geometric tuning of core-shell nanowire (NW) architectures is used to decouple the contributions from light absorption, charge separation, and charge transfer kinetics in photoelectrochemical water oxidation. Core-shell-shell NW arrays were fabricated using a combination of hydrothermal synthesis of ZnO and atomic layer deposition (ALD) of SnO2 and BiVO4. The length and spacing of the NW scaffold, as well as the BiVO4 film thickness, were systematically tuned to optimize the photoelectrochemical performance. A photocurrent of 4.4 mA/cm2 was measured at 1.23 V vs RHE for sulfite oxidation and 4.0 mA/cm2 at 1.80 V vs RHE for water oxidation without a cocatalyst, which are the highest values reported to date for an ALD-deposited photoanode. Electromagnetic simulations demonstrate that spatial heterogeneity in light absorption along the core-shell NW length has a critical role in determining internal quantum efficiency. The mechanistic understandings in this study highlight the benefits of systematically optimizing electrode geometry at the nanoscale when designing photoelectrodes.

Area-selective atomic layer deposition of lead sulfide: nanoscale patterning and DFT simulations.

Area-selective atomic layer deposition (ALD) of lead sulfide (PbS) was achieved on octadecyltrichlorosilane (ODTS)-patterned silicon substrates. We investigated the capability of ODTS self-assembled monolayers (SAMs) to deactivate the ALD PbS surface reactions as a function of dipping time in ODTS solution. The reaction mechanism was investigated using density functional theory (DFT), which showed that the initial ALD half-reaction is energetically unfavorable on a methyl-terminated SAM surface. Conventional photolithography was used to create oxide patterns on which ODTS SAMs were selectively grown. Consequently, PbS thin films were grown selectively only where ODTS was not present, whereas deposition was blocked in regions where ODTS was grown. The resulting fabricated patterns were characterized by scanning electron microscopy and Auger electron spectroscopy, which demonstrated that ALD PbS was well confined to defined patterns with high selectivity by ODTS SAMs. In addition, AFM lithography was employed to create nanoscale PbS patterns. Our results show that this method can be applied to various device-fabrication processes, presenting new opportunities for various nanofabrication schemes and manifesting the benefits of self-assembly.

Scanning tunneling spectroscopy of lead sulfide quantum wells fabricated by atomic layer deposition.

We report the use of scanning tunneling spectroscopy (STS) to investigate one-dimensional quantum confinement effects in lead sulfide (PbS) thin films. Specifically, quantum confinement effects on the band gap of PbS quantum wells were explored by controlling the PbS film thickness and potential barrier height. PbS quantum well structures with a thickness range of 1-20 nm were fabricated by atomic layer deposition (ALD). Two barrier materials were selected based on barrier height: aluminum oxide as a high barrier material and zinc oxide as a low barrier material. Band gap measurements were carried out by STS, and an effective mass theory was developed to compare the experimental results. Our results show that the band gap of PbS thin films increased as the film thickness decreased, and the barrier height increased from 0.45 to 2.19 eV.