In the quest for better batteries and catalysts, scientists are mastering the art of making ions gently touch down on surfaces.
Imagine trying to understand a conversation in a crowded, noisy room where everyone is talking at once. This is the challenge scientists face when studying the complex interfaces in energy storage devices like batteries and supercapacitors. At these interfaces, countless different molecules interact simultaneously, making it nearly impossible to understand any single one's role. Now, researchers have developed a solution: ion soft landing, a technique that allows them to select individual charged molecules and gently place them on surfaces, creating perfectly defined interfaces for study.
Ion soft landing (ISL) is a sophisticated materials preparation technique where specific charged molecules are selected from a complex mixture and deposited onto surfaces at carefully controlled energies that preserve their structural and functional integrity1 . Think of it as molecular air traffic control: just as an air traffic controller guides specific planes to designated runways, scientists use ISL to select particular ions and guide them to precise locations on a surface.
The "soft" aspect is crucial—by keeping the landing energy low (typically less than 10 electron volts), the ions maintain their structure and functionality upon impact1 .
Gently transfers ions from solution into the gas phase without fragmentation3 .
Ions are selected by their mass-to-charge ratio before being softly landed on a prepared surface7 .
Electrospray ionization transfers ions from solution to gas phase.
Mass spectrometer filters ions by mass-to-charge ratio.
Selected ions are gently deposited onto prepared surfaces.
Deposited ions are analyzed using various techniques.
The interfaces where materials meet and transform are the active centers of energy storage devices. Every battery and supercapacitor relies on molecular transformations at these electrode-electrolyte interfaces to store and release energy7 . The problem is that these interfaces are incredibly complex, with multiple chemical species interacting in ways that are difficult to untangle.
Ion soft landing simplifies this picture dramatically. It allows scientists to systematically build and study interfaces one molecule at a time, observing how each component behaves individually before understanding how they work together2 . This approach has already yielded important insights, such as the discovery that lithium-sulfur ions in advanced battery systems undergo multiple reactions centered on the reduction and oxidation chemistry of sulfur rather than lithium—a finding that helps explain why oxidized forms of sulfur exist at battery interfaces2 .
Beyond fundamental understanding, ISL enables the creation of precisely designed electrodes with superior performance. At PNNL, researchers used soft landing to create a carefully defined electrode in less than 3 hours—a process that previously took 24 hours with earlier instruments6 . These designer electrodes have shown higher performance in electrochemical energy storage devices like supercapacitors6 .
To understand how soft landing works in practice, let's examine a key experiment that revealed both the power and challenges of the technique.
In this study, researchers investigated what happens when tris(bipyridine)nickel ions ([Ni(bpy)₃]²âº)—a transition metal complex relevant to photoredox catalysis and electrocatalytic applications—are soft-landed onto surfaces3 .
Researchers prepared a solution of nickel bipyridine complexes and used electrospray ionization to transfer them into the gas phase as intact doubly charged cations3 .
The ions traveled through a mass spectrometer where the specific [Ni(bpy)₃]²⺠ions were selected, filtering out all other molecular species3 .
The selected ions were gently deposited onto a fluorinated self-assembled monolayer (FSAM) surface—a carefully prepared organic surface that serves as a model for electrode interfaces3 .
The researchers used various techniques, including mass spectrometry and electronic structure calculations, to examine what happened to the ions after deposition3 .
The experiment revealed a surprising phenomenon: the soft-landed [Ni(bpy)₃]²⺠ions underwent spontaneous ligand loss, transforming into undercoordinated [Ni(bpy)₂]²⺠species3 . This occurred due to two factors: structural reorganization as the ions adjusted to the surface, and charge reduction as the deposited ions lost some of their charge to the surface3 .
When researchers co-deposited the [Ni(bpy)₃]²⺠ions with stable [Bâ‚â‚‚Fâ‚â‚‚]²⻠anions, the extent of dissociation decreased significantly3 . The strong electrostatic interaction between the cations and anions stabilized the soft-landed complexes, preserving their structure by reducing both charge reduction and structural reorganization3 .
| Aspect Studied | Observation | Scientific Significance |
|---|---|---|
| Structural Stability | Soft-landed [Ni(bpy)₃]²⺠spontaneously lost ligands | Reveals how ion-surface interactions can drive molecular changes |
| Driving Factors | Structural reorganization and charge reduction | Identifies key factors affecting integrity of deposited species |
| Stabilization Method | Co-deposition with stable [Bâ‚â‚‚Fâ‚â‚‚]²⻠anions | Demonstrates strategy to preserve structure of landed ions |
| Theoretical Support | DFT calculations explained geometry distortion | Connects experimental observations with computational models |
Soft landing research requires specialized instruments and materials. Here are some of the essential components:
| Tool/Reagent | Function in Soft Landing Research |
|---|---|
| Electrospray Ionization Source | Gently transfers ions from solution to gas phase without fragmentation3 7 |
| Mass Filter (Quadrupole/Ion Trap) | Selects specific ions by mass-to-charge ratio for deposition7 |
| Ion Mobility Separator (SLIM) | Provides high-resolution separation by ion shape and size before deposition1 |
| Self-Assembled Monolayers (SAMs) | Serve as well-defined surfaces for deposition and study of ion-surface interactions3 |
| Transition Metal Complexes | Model systems for studying catalysis and energy storage processes at interfaces3 |
| Polyoxometalates (POMs) | Redox-active metal oxide clusters used in energy storage and memory devices7 |
While traditional soft landing selects ions by their mass-to-charge ratio, recent advances now enable selection based on molecular shape and size through ion mobility separation. Using a technology called Structures for Lossless Ion Manipulations (SLIM), researchers can separate ions with subtle structural differences before deposition1 .
In one demonstration, researchers separated a mixture of tetra-alkyl ammonium ions containing different chain lengths (C5-C8), selecting only the C6 and C7 chains for deposition1 . The deposited ions were subsequently confirmed to be the correct selected species, demonstrating the precision of this approach1 .
This mobility-based selection opens the door to isomer-selective deposition—placing molecules with the same mass but different structures onto surfaces—with potential applications in creating more selective catalysts and understanding how molecular shape affects function at interfaces1 .
| Selection Method | Basis of Separation | Key Applications |
|---|---|---|
| Mass-to-Charge Selection | Molecular weight and charge state | General purification and deposition of ionic species7 |
| Ion Mobility Selection | Molecular shape, size, and collision cross-section | Separation and deposition of structural isomers1 |
| Combined Approaches | Both mass and mobility characteristics | Highest purity deposition for complex molecular mixtures1 |
Researchers are now working on simultaneously depositing both positive and negative ions to create more realistic models of energy storage devices where different ions interact with each other and the surface2 . This approach better captures the complexity of real-world interfaces while maintaining the precision of soft landing.
Soft landing is finding applications in structural biology, where researchers are combining it with cryo-electron microscopy to determine protein structures5 . Although this application is still developing, initial results show promise for preparing structurally intact proteins for high-resolution imaging.
As instrumentation advances and our understanding of ion-surface interactions deepens, ion soft landing is poised to become an increasingly powerful tool for designing the materials that will power our future—from longer-lasting batteries to more efficient catalysts. By enabling precise control over the molecular building blocks of interfaces, this technique gives scientists the power to understand and optimize energy materials one molecule at a time.
Precise interface engineering for improved energy density and cycle life.
Tailored active sites for more efficient and selective chemical transformations.
Understanding molecular behavior at interfaces for next-generation materials.