Spark Ablation

Building Blocks for Nanotechnology

Andreas Schmidt-Ott

• Publisher: Jenny Stanford Publishing
• Year: 2019
• Print ISBN: 9789814800822
• eBook ISBN: 9781000730449
• Topic: Physical Sciences
• Subtopic: Biology

Spark ablation has been used worldwide for decades. However, in many fields, the special properties of nanoparticles, which come into play especially for sizes <20 nm, are just beginning to be exploited. The technique offers unprecedented flexibility regarding composition and size, and revolutions in the domains of catalysis and sensor technology, and more are to be expected. This book is the first review of spark ablation as a unique, scalable source of building blocks for nanotechnology and a powerful tool to promote this development. The introductory chapters give an overview of the technological fields that can exploit size effects, and explain the process of spark ablation in the gas phase, as well as principles of immobilizing particles to create novel products and materials. Fundamentals of the spark ablation process are then discussed, in addition to the characteristics of the particles formed. The rest of the book deals with a selection of application fields that profit from the spark ablation source from the perspective of research. With the authors' many years of experience in spark ablation and its applications, all the chapters complement one another and contain numerous cross-references in order to enable the reader to obtain a complete picture of the subject.

Chapter 1

Application Domains of <20 nm Particles and the Role of the Spark Discharge Generator

Eva A. J. Rennen,^a Maurits F. J. Boeije,^b and Andreas Schmidt-Ott^b,c

^aVSPARTICLE B.V., Molengraaffsingel 10, NL-2629 JD Delft, The Netherlands

^bFaculty of Applied Sciences, Delft University of Technology, NL-2629 HZ Delft, The Netherlands

^cEnvironment and Water Research Center, Cyprus Institute, Nicosia, Cyprus

[email protected]

The present chapter compiles application areas of nanoparticle production by spark ablation. The spark ablation technique is best applied to produce inorganic particles in the diameter range between atomic clusters (smaller than 2 nm) and 20 nm. In this range, particle properties are very different from macroscopic (bulk) materials and strongly size dependent.

1.1 Introduction

Particles close to the atomic size obey the laws of quantum mechanics, as expressed by Schrödinger’s equation, which represents the most important category of differences with respect to the bulk in the size range below 20 nm. A review by Roduner [1] explains the origin of these differences. For example, the solution of Schrödinger’s equation for the valence electrons in a small metal particle (similar to the electron-in-a-box problem) demands discrete (quantized) energy levels the electrons may occupy. This change in the electronic structure of sub-20-nm particles with respect to bulk systems changes virtually all properties. Additional influences of the small size on particle properties also governed by quantum mechanics are associated with collective electron oscillations (plasmons), the occurrence of superparamagnetism, and more. Other size effects that specifically apply to these very small particles are related to a reduced binding energy between surface atoms due to surface curvature, such as a reduced melting point and size-dependent catalytic activity. Astonishingly, nanotechnology has, until now, hardly exploited the most interesting features of this size range, where size effects are most prominent. We claim that difficulties in producing these particles in a clean, controlled, and flexible way are one reason for holding back this development. Another feature of such small nanoparticles that has practically not been exploited is to be seen in the stronger tendency to form alloys with respect to bulk material. It has frequently been observed that the phase diagrams of mixed materials allow for new mixed phases when the dimensions shrink [2]. Atomic mixing thus receives a new quality in nanoparticles and basically opens myriads of possibilities associated with new material properties.

We believe that the development of spark ablation nanoparticle production will play an important role in moving nanotechnology into the sub-20-nm size range, where exploiting new properties for applications promises to be most rewarding. This development will be aided by the feature of spark ablation to easily mix solid materials [2], from the atomic scale (alloys) to the scale of a few nanometers. This opens an exciting spectrum of new possibilities for functional materials as well as nanodevices.

In this chapter we list the basic size effects that govern the properties of sub-20-nm particles, and link them to applications that have already been found in products and those that are in the research stage. We also speculate on possible future applications that the properties seem to open up. The reader may use this overview of what is known today also as an inspiration of what may be done tomorrow.

Subsequently, features of spark ablation are briefly outlined and compared to other nanoparticle production methods. Indications of required quantities of nanoparticulate material per item are given for industrial applications, and a selection of examples illustrates the contribution of spark ablation in the development of applications listed in the overview that make use of the size effects.

1.2 Sub-20 nm Particle Properties and Their Application Domains

1.2.1 Basic Size Effects

Table 1.1 lists basic size effects that cause properties of very small particles to change with size and that are of practical interest. It groups them into three categories. Strictly speaking, the basic categories on the left are related (e.g., binding energy is also connected to quantum mechanics), but the separation is considered as illustrative. Some effects of practical interest are listed in the right-hand column. The basic properties of category B1 deal with the rather trivial, purely geometric consequences (mainly surface-to-volume ratio), which also apply to larger particles. The second category of surface curvature effects, B2, is based on the atomic structure of matter, which can be qualitatively explained by classical, non-quantum-mechanical arguments and that deal with important changes that occur when reducing particle size. Surface curvature in small particles reduces the coordination number of surface atoms, and that has some drastic consequences below 20 nm, as listed in the right-hand column of the second category. “Faster dissolution” is mentioned in both categories B1 and B2 because the kinetics of particle dissolution profits from a higher surface area (for the same volume), and additionally the weaker binding of surface atoms with rising surface curvature promotes dissolution. Category B3 deals with phenomena that can only be understood with the principles of quantum mechanics, even on a qualitative level.

Table 1.1 Basic size effects encountered in solids when going from larger to smaller particle sizes in the diameter
range below 20 nm

Basic category	Basic size effects of practical interest
B1. Geometry or surface-to-volume ratio related	B1.1 Faster mass transport to surface of particle in medium
	B1.2 Faster mass transport from surface to center of particle
	B1.3 Faster dissolution
	B1.4 Reduced strain in volume expansion
B2. Reduced binding energy between atoms due to surface curvature	B2.1 Reduced melting point
	B2.2 Increased vapor pressure
	B2.3 Faster dissolution
	B2.4 Increased chemical reactivity
	B2.5 Size-dependent catalytic activity
	B2.6 Increased adsorptive binding of atoms/molecules
	B2.7 Larger variety of crystal habits
	B2.8 Laplace pressure inside nanoparticle
	B2.9 Superplasticity
	B2.10 Reduced surface free energy
B3. Quantum-mechanical size effects	B3.1 Size-dependent plasma resonance
	B3.2 Size-tunable bandgap in quantum dots
	B3.3 Photoelectron emission enhancement
	B3.4 Superparamagnetism
	B3.5 Formation of “superatoms”
	B3.6 Strong size dependence of light scattering and absorption
	B3.7 Surface-enhanced Raman scattering (SERS) from adsorbed molecules

Application examples related to the basic categories of Table 1.1 are listed in Table 1.2. For example, catalysis is influenced by a high surface-to-volume ratio and is additionally influenced by a “coordination number effect,” that is, the changed atomic arrangement on a curved surface, leading to higher reactivity or stronger adsorptive binding with respect to the bulk. Some effects mentioned in Table 1.1 do not have any notable applications yet, which means that they are still to be found.

“Superatoms” is a term introduced in publications of the groups of Castleman and Khanna [3]. As pointed out before, valence electrons in a metal particle occupy discrete energy levels, and these particles thus mimic specific atoms in the periodic table and chemically behave like these (see also Ref. [1]). For example, Al₁₃ behaves like a halogen atom, and Al₇ behaves like an alkali metal atom. Eventually, it may be possible to design stable atomic clusters that form “cluster assembled materials” with new properties or properties of more expensive materials. This vision was already expressed in 1996 in Klaus Sattler’s book Cluster Assembled Materials, but since then there has been little progress in this field, and it remains a future perspective, which may now have become more realistic with the availability of flexible high-production-rate spark discharge generators combined with mobility classification (see Chapter 7).

1.2.2 Application Domains of sub-20 nm Particles

The basic properties described before bare the potential of many technological applications. Table 1.2 compiles applications that have been described in literature and links them to the size effects in Table 1.1. The application areas listed are examples, most of these have made use of spark ablation, and for all of them spark ablation would be an option. Chapters and sections of the present book that deal with the applications are indicated. In Section 1.4, many of these application examples are further described.

1.2.3 Requirements for Successful Applications

The effectiveness of the particles for specific applications, both in research and on an industr...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• 1. Application Domains of <20 nm Particles and the Role of the Spark Discharge Generator
• 2. Nanoparticle Production by Spark Ablation: Principle, Configurations, and Basic Steps toward Application
• 3. Spark Plasma Diagnostics
• 4. The Physics and Equivalent Circuit Modeling of Spark Ablation
• 5. Generation of Mixed Nanoparticles by Spark Ablation of Alloys and Spark Mixing
• 6. Characterization of Nanoparticles from Spark Ablation
• 7. Atomic Clusters: Potential of Spark Discharge Generation
• 8. The Use of Spark Ablation for Generating Cluster Beams: A Review
• 9. Patterned 3D Nanostructure Arrays from Charged Aerosols
• 10. Catalytic Applications of Nanoparticles Produced by Spark Ablation
• 11. Nanoparticles and Nanoparticle-Based Materials Produced by Spark Ablation for Environmental Gas Sensors
• 12. Alloy Plasmonic Nanoparticles and Their Applications
• 13. Spark Ablation for Biomedical Application
• 14. Application of a Spark Discharge Generator for Production of Combustion-Like Aerosols
• Index