Atomic Layer Deposition of Aluminum Sulfide: Growth Mechanism and Electrochemical Evaluation in Lithium-Ion Batteries

This study describes the synthesis of aluminum sulfide (AlS_<i>x</i>) thin films by atomic layer deposition (ALD) using tris­(dimethylamido)aluminum and hydrogen sulfide. We employed a suite of in situ measurement techniques to explore the ALD AlS_<i>x</i> growth mechanism, including quartz crystal microbalance, quadrupole mass spectrometry, and Fourier transform infrared spectroscopy. A variety of ex situ characterization techniques were used to determine the growth characteristics, morphology, elemental composition, and crystallinity of the resultant AlS_<i>x</i> films. This study revealed that the AlS_<i>x</i> growth was self-limiting in the temperature range 100–250 °C, and the growth per cycle decreased linearly with increasing temperature from ∼0.45 Å/cycle at 100 °C to ∼0.1 Å/cycle at 250 °C. The AlS_<i>x</i> films were amorphous in this temperature range. We conducted electrochemical testing to evaluate the ALD AlS_<i>x</i> as a potential anode material for lithium-ion batteries (LIBs). The ALD AlS_<i>x</i> exhibited reliable cyclability over 60 discharge–charge cycles with a sustainable discharge capacity of 640 mAh/g at a current density of 100 mA/g in the voltage window of 0.6–3.5 V

Kinetics for the Sequential Infiltration Synthesis of Alumina in Poly(methyl methacrylate): An Infrared Spectroscopic Study

Sequential infiltration synthesis (SIS) is a method for growing inorganic materials within polymers in an atomically controlled fashion. This technique can increase the etch resistance of optical, electron-beam, and block copolymer (BCP) lithography resists and is also a flexible strategy for nanomaterials synthesis. Despite this broad utility, the kinetics of SIS remain poorly understood, and this knowledge gap must be bridged in order to gain firm control over the growth of inorganic materials inside polymer films at a large scale. In this paper, we explore the reaction kinetics for Al₂O₃ SIS in PMMA using in situ Fourier transform infrared spectroscopy. First, we establish the kinetics for saturation adsorption and desorption of trimethyl aluminum (TMA) in PMMA over a range of PMMA film thicknesses deposited on silicon substrates. These observations guide the selection of TMA dose and purge times during SIS lithography to achieve robust organic/inorganic structures. Next, we examine the effects of TMA desorption on BCP lithography by performing SIS on silicon surfaces coated with polystyrene-<i>block</i>-poly­(methyl methacrylate) films. After etching the organic components, the substrates are examined using scanning electron microcopy to evaluate the resulting Al₂O₃ patterns. Finally, we examine the effects of temperature on Al₂O₃ SIS in PMMA to elucidate the infiltration kinetics. The insights provided by these measurements will help extend SIS lithography to larger substrate sizes for eventual commercialization and expand our knowledge of precursor–polymer interactions that will benefit the SIS of a wide range of inorganic materials in the future

Vapor-Phase Atomic-Controllable Growth of Amorphous Li₂S for High-Performance Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries hold great promise to meet the formidable energy storage requirements of future electrical vehicles but are prohibited from practical implementation by their severe capacity fading and the risks imposed by Li metal anodes. Nanoscale Li₂S offers the possibility to overcome these challenges, but no synthetic technique exists for fine-tailoring Li₂S at the nanoscale. Herein we report a vapor-phase atomic layer deposition (ALD) method for the atomic-scale-controllable synthesis of Li₂S. Besides a comprehensive investigation of the ALD Li₂S growth mechanism, we further describe the high performance of the resulting amorphous Li₂S nanofilms as cathodes in Li–S batteries, achieving a stable capacity of ∼800 mA·h/g, nearly 100% Coulombic efficiency, and excellent rate capability. Nanoscale Li₂S holds great potential for both bulk-type and thin-film high-energy Li–S batteries

Atomic Layer Deposition of Al–W–Fluoride on LiCoO₂ Cathodes: Comparison of Particle- and Electrode-Level Coatings

Atomic layer deposition (ALD) of the well-known Al₂O₃ on a LiCoO₂ system is compared with that of a newly developed AlW_<i>x</i>F_<i>y</i> material. ALD coatings (∼1 nm thick) of both materials are shown to be effective in improving cycle life through mitigation of surface-induced capacity losses. However, the behaviors of Al₂O₃ and AlW_<i>x</i>F_<i>y</i> are shown to be significantly different when coated directly on cathode particles versus deposition on a composite electrode composed of active materials, carbons, and binders. Electrochemical impedance spectroscopy, galvanostatic intermittent titration techniques, and four-point measurements suggest that electron transport is more limited in LiCoO₂ particles coated with Al₂O₃ compared with that in particles coated with AlW_<i>x</i>F_<i>y</i>. The results show that proper design/choice of coating materials (e.g., AlW_<i>x</i>F_<i>y</i>) can improve capacity retention without sacrificing electron transport and suggest new avenues for engineering electrode–electrolyte interfaces to enable high-voltage operation of lithium-ion batteries

Amorphous Metal Fluoride Passivation Coatings Prepared by Atomic Layer Deposition on LiCoO₂ for Li-Ion Batteries

Amorphous Metal Fluoride Passivation Coatings Prepared by Atomic Layer Deposition on LiCoO₂ for Li-Ion Batterie

Energy Levels, Electronic Properties, and Rectification in Ultrathin p‑NiO Films Synthesized by Atomic Layer Deposition

NiO is an attractive p-type transparent semiconductor that is being explored for a variety of applications. We report a systematic study of the electronic properties, relevant to hole-transporting materials in solar energy conversion applications, of NiO synthesized by atomic layer deposition (ALD). The acceptor concentration, flat band potential, and valence band position were determined by electrochemical Mott–Schottky analysis of impedance data in aqueous electrolytes for films less than 100 nm in thickness on F:SnO₂ (FTO)-coated glass substrates. The effects of postdeposition annealing and film thickness were studied. Oxidation of the NiO was observed at temperatures as low as 300 °C in 1 atm of oxygen. Films annealed at 400 °C and above in Ar exhibited signs of thermal decomposition. Thinner films were found to have a higher carrier concentration. F:SnO₂ and thermally evaporated Ag were both observed to form ohmic contact to ALD-synthesized TiO₂ and NiO. A p/n heterojunction diode was fabricated from the transparent ALD TiO₂ and NiO layers with the structure FTO/NiO/TiO₂/Ag that showed rectification

Combining Electronic and Geometric Effects of ZnO-Promoted Pt Nanocatalysts for Aqueous Phase Reforming of 1‑Propanol

Compared with Pt/Al₂O₃, sintering-resistant Pt nanoparticle catalysts promoted by ZnO significantly improved the reactivity and selectivity toward hydrogen formation in the aqueous phase reforming (APR) of 1-propanol. The improved performance was found to benefit from both the electronic and geometric effects of ZnO thin films. <i>In situ</i> small-angle X-ray scattering and scanning transmission electron microscopy showed that ZnO-promoted Pt possessed promising thermal stability under APR reaction conditions. <i>In situ</i> X-ray absorption spectroscopy showed clear charge transfer between ZnO and Pt nanoparticles. The improved reactivity and selectivity seemed to benefit from having both Pt-ZnO and Pt-Al₂O₃ interfaces

First-Principles Predictions and <i>in Situ</i> Experimental Validation of Alumina Atomic Layer Deposition on Metal Surfaces

The atomic layer deposition (ALD) of metal oxides on metal surfaces is of great importance in applications such as microelectronics, corrosion resistance, and catalysis. In this work, Al₂O₃ ALD using trimethylaluminum (TMA) and water was investigated on Pd, Pt, Ir, and Cu surfaces by combining <i>in situ</i> quartz crystal microbalance (QCM), quadrupole mass spectroscopy (QMS), and scanning tunneling microscopy (STM) measurements with density functional theory (DFT) calculations. These studies revealed that TMA undergoes dissociative chemisorption to form monomethyl aluminum (AlCH₃*, the asterisk designates a surface species) on both Pd and Pt, which transform into Al­(OH)₃* during the subsequent water exposure. Furthermore, the AlCH₃* can further dissociate into Al* and CH₃* on stepped Pt(211). Additional DFT calculations predicted that Al₂O₃ ALD should proceed on Ir following a similar mechanism but not on Cu due to the endothermicity for TMA dissociation. These predictions were confirmed by <i>in situ</i> QCM, QMS, and STM measurements. Our combined theoretical and experimental study also found that the preferential decoration of low-coordination metal sites, especially after high temperature treatment, correlates with the differences in free energy between Al₂O₃ ALD on the (111) and stepped (211) surfaces. These insights into Al₂O₃ growth on metal surfaces can guide the future design of advanced metal/metal oxide catalysts with greater durability by protecting the metal against sintering and dissolution and enhanced selectivity by blocking low-coordination metal sites while leaving (111) facets available for catalysis

Catalysts Transform While Molecules React: An Atomic-Scale View

We explore how the atomic-scale structural and chemical properties of an oxide-supported monolayer (ML) catalyst are related to catalytic behavior. This case study is for vanadium oxide deposited on a rutile α-TiO₂(110) single-crystal surface by atomic layer deposition (ALD) undergoing a redox reaction cycle in the oxidative dehydrogenation (ODH) of cyclohexane. For measurements that require a greater effective surface area, we include a comparative set of ALD-processed rutile powder samples. In situ single-crystal X-ray standing wave (XSW) analysis shows a reversible vanadium oxide structural change through the redox cycle. Ex situ X-ray photoelectron spectroscopy (XPS) shows that V cations are 5+ in the oxidized state and primarily 4+ in the reduced state for both the (110) single-crystal surface and the multifaceted surfaces of the powder sample. In situ diffuse reflectance infrared Fourier transform spectroscopy, which could only achieve a measurable signal level from the powder sample, indicates that these structural and chemical state changes are associated with the change of the VO vanadyl group. Catalytic tests on the powder-supported VO_<i>x</i> revealed benzene as the major product. This study not only provides atomic-scale models for cyclohexane molecules interacting with V sites on the rutile surface but also demonstrates a general strategy for linking the processing, structure, properties, and performance of oxide-supported catalysts

W:Al₂O₃ Nanocomposite Thin Films with Tunable Optical Properties Prepared by Atomic Layer Deposition

A systematic alteration in the optical properties of W:Al₂O₃ nanocomposite films is demonstrated by precisely varying the W cycle percentage (W%) from 0 to 100% in Al₂O₃ during atomic layer deposition. The direct and indirect band energies of the nanocomposite materials decrease from 5.2 to 4.2 eV and from 3.3 to 1.8 eV, respectively, by increasing the W% from 10 to 40. X-ray absorption spectroscopy reveals that, for W% < 50, W is present in both metallic and suboxide states, whereas, for W% ≥ 50, only metallic W is seen. This transition from dielectric to metallic character at W% ∼ 50 is accompanied by an increase in the electrical and thermal conductivity and the disappearance of a clear band gap in the absorption spectrum. The density of the films increases monotonically from 3.1 g/cm³ for pure Al₂O₃ to 17.1 g/cm³ for pure W, whereas the surface roughness is greatest for the W% = 50 films. The W:Al₂O₃ nanocomposite films are thermally stable and show little change in optical properties upon annealing in air at 500 °C. These W:Al₂O₃ nanocomposite films show promise as selective solar absorption coatings for concentrated solar power applications