Nanofoams are a class of nanostructured,
porous materials
A porous medium or a porous material is a material containing pores (voids). The skeletal portion of the material is often called the "matrix" or "frame". The pores are typically filled with a fluid (liquid or gas). The skeletal material is usua ...
(
foam
Foams are materials formed by trapping pockets of gas in a liquid or solid.
A bath sponge and the head on a glass of beer are examples of foams. In most foams, the volume of gas is large, with thin films of liquid or solid separating the reg ...
s) containing a significant population of pores with diameters less than 100
nm.
Aerogel
Aerogels are a class of synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas, without significant collapse of the gel structure. The result is a solid with extremely low ...
s are one example of nanofoam.
Metal
Overview
Metallic nanofoams are a subcategorization of nanofoams; more specifically, there are nanofoams consisting of metals, often pure, that form interconnected networks of ligaments that make up the structure of the foam. A variety of metals are used, including
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
,
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
,
gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...
, and
platinum
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Platinu ...
.
Metallic nanofoams may offer certain advantages over alternative polymer nanofoams; structurally, they retain the
electrical conductivity
Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allow ...
of metals, offer increased
ductility
Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...
, as well as the higher surface area and nano-architecture properties offered by nanofoams.
Fabrication
Synthesis of metallic nanofoams may be accomplished through a variety of methods. In 2006, researchers produced metal nanofoams by igniting pellets of energetic metal bis(tetrazolato)amine complexes. Nanofoams of
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
,
cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, pr ...
,
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
,
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
,
silver
Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
, and
palladium
Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself na ...
have been prepared through this technique. These materials exhibit densities as low as 11 mg/cm
3, and surface areas as high as 258 m
2/g. These foams are effective catalysts and electrocatalyst supports. Also, metal nanofoams can be made by electrodeposition of metals inside templates with interconnected pores, such as 3D-porous anodic aluminum oxide (AAO). Such method gives nanofoams with an organized structure and allows to control the surface area and porosity of the fabricated material.
A 2016 study discussed a low temperature/pressure microwave solvothermal method for fabricating pure copper, silver, and nickel metal nanofoams. The process claims to be non-hazardous, novel, as well as facile, with an emphasis on its low-waste and low-cost method of manufacturing.
Additionally, a 2020 publication discussed successful synthesis of nanofoam films from silver, gold, copper, and palladium through the use of a modified vacuum thermal evaporation method.
Applications
Metallic nanofoams have seen a broad variety of applications, including catalysts,
hydrogen storage
Hydrogen storage can be accomplished by several existing methods of holding hydrogen for later use. These include mechanical approaches such as using high pressures and low temperatures, or employing chemical compounds that release H2 upon demand ...
, as well as
fuel cell
A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requ ...
s. Additionally, applications of metallic nanofoam as an
electrocatalyst
An electrocatalyst is a catalyst that participates in electrochemical reactions. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or, most commonly, may be the electrode surface itself. An electrocatalyst ...
have been fruitful; a nickel-iron nanofoam catalyst has proven to exhibit exceptional electrocatalytic performance, as well as
water-splitting to isolate hydrogen atoms. Applications to the clean energy industry, specifically for lithium-ion batteries and other fuel cells, have been discussed as well.
Biopolymers
Overview
Through literature discussing the fabrication of a completely porous nanofoam biopolymer is scarce, recent endeavors have resulted in the formation of nanofoam surfaces on biopolymers.
In these instances, biopolymers such as collagen and gelatine, chitosan,
and pure curcumin
have been used to varying degrees.
Fabrication
A 2008 study explored the usage of femtosecond laser irradiation to create permanent spatial arrangements in transparent materials, particularly in its usage to form a singular foamed layer upon biopolymers such as collagen or curcumin.
Foaming these surfaces results in a variety of surface modifications that may improve the material's ability for cell adhesion, permeability of fluids due to cell structure, and the formation of nanoscopic fibers.
Additionally, an iron-nitrogen co-doped carbon nanofoam was purposed to be fabricated through the acile salt-assisted pyrolysis process of chitooligosaccharides.
Applications
Foamed biopolymers have multiple purported applications in the biomedical and pharmaceuticals industry due to their modified surface properties. Gelatine films with curcumin dropped upon the surface, for instance, displayed a higher tolerance for ablation following its foaming; this tolerance is suspected to arise from curcumin's binding to proteins to protect from free radicals, as well as its anti-oxidant properties.
These findings present implications for greater cellular surgery, as well as the manufacturing of biopolymers as a whole, due to these modifications from plasma irradiation.
Silver
Overview
Silver
Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
nanofoams are specific metal nanofoams consisting of mainly silver that are uniquely regarded for their antibacterial and electrical properties. Many of these silver nanofoams are
alloy
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...
s of silver and another metal such as aluminum.
They are unique for their hierarchical porous structure are a current point of modern research and development. They have many applications in the fields of mechanical, chemical, and biomedical engineering, including filtration, air management, and use in electrical systems.
Fabrication
The underlying principle is to merge pores of different sizes into a material with a large surface area (thanks to smaller pores), which in turn allows efficient molecular transport (which requires larger pores). The process used to produce these materials is a combination of the replication method, typically used to produce large-pore foams, and the selective dissolution method, generally used to manufacture small-pore foams.
Ag foams with hierarchical porous structures are prepared by the following three-step method:
(i) Packing large spherical NaCl particles to create a hard template, with a distinct perform network of negative space. Then this network is filled with liquid Al-25Ag.
(ii) Removing the NaCl template by water dissolution to form Al−25Ag macro-porous foam.
(iii) Dissolving the Al-rich phase by a chemical attack with aqueous solutions of HCl or NaOH to form the final Ag foam. This creates the nanoscale pores of the foam.
Applications
Silver ions have been shown to have potent antibacterial activity, and have been shown to affect the growth of
Gram-positive
In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall.
Gram-positive bacte ...
and
Gram-negative
Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...
bacteria. This is due to their ability to form
ligand complexes with proteins or enzymes in bacterial cells.
Due to this unique property, these nanofoams create excellent air filters designed to filter out bacteria and other microorganisms, this level of filtration was shown to be more effective than tradition HCl analogues.
These silver nanofoams have also been used as electrocatalysts for the reduction reaction of to CO. It was found that on average silver nanofoams can maintain over 90% FECO in a wide potential window (−0.5 to −1.2 VRHE), enabling the maximum CO selective current density of 33 mA cm−2 and the mass activity of 23.5 A gAg−1, which are the highest values among recently reported metal foam-based electrocatalysts.
Carbon
Overview
Carbon nanofoam Carbon nanofoam is an allotrope of carbon discovered in 1997 by Andrei V. Rode and co-workers at the Australian National University in Canberra. It consists of a cluster-assembly of carbon atoms strung together in a loose three-dimensional web. The ...
is an
allotrope of carbon
Carbon is capable of forming many allotropy, allotropes (structurally different forms of the same element) due to its Valence (chemistry), valency. Well-known forms of carbon include diamond and graphite. In recent decades, many more allotrope ...
discovered in 1997.
Its structure consists of a cluster-assembly of carbon atoms strung together in a loose three-dimensional web, similar to an aerogel. The material has a density of 2–10 mg/cm
3 (0.0012 lb/ft
3), which is among the lightest materials to date.
Fabrication
There are multiple formation methods for carbon nanofoams.
Pulsed Laser Deposition
Pulsed laser deposition (PLD) is a physical vapor deposition (PVD) technique where a high-power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. This material is vaporized from the t ...
(PLD) has been the first technique used for the synthesis of carbon nanofoam,
and is considered one of the most versatile approach for the production of carbon nanofoams with controlled density and morphology.
The process of nanofoam growth via the Pulsed Laser Deposition has been described in terms of a "
snowfall-like" mechanism:
(i) Carbon nanoparticles are generated upon
laser ablation
Laser ablation or photoablation (also called laser blasting) is the process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser ...
of a graphite target, either directly of because of the presence of a background atmosphere
(ii) Nanoparticles stick together in micrometric-sized,
fractal-like aggregates that grow in-flight within the deposition chamber
(iii) fractal-like aggregates land on a suitable substrate, much like
snowflakes
A snowflake is a single ice crystal that has achieved a sufficient size, and may have amalgamated with others, which falls through the Earth's atmosphere as snow.Knight, C.; Knight, N. (1973). Snow crystals. Scientific American, vol. 228, no. ...
land on the ground
(iv) a void-rich, web-like nanofoam is obtained by the layering of fractal-like aggregates
Two of the most common alternatives to PLD synthesis are described below:
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
nanofiber
Nanofibers are fibers with diameters in the nanometer range (typically, between 1 nm and 1 μm). Nanofibers can be generated from different polymers and hence have different physical properties and application potentials. Examples of natural polyme ...
s (CNF) were constructed into nanofoams by:
(i) Recycled milk container board was pretreated with
deep eutectic solvent Deep eutectic solvents or DESs are solutions of Lewis or Brønsted acids and bases which form a eutectic mixture. Deep eutectic solvents are highly tunable through varying the structure or relative ratio of parent components and thus have a wide va ...
(DES) to fibrillate it.
(ii) The pretreated board was put through a simple freezing drying procedure to form a nanofoam shape.
(iii) Fibers are then modified for increased hydrophobicity and reinforced structure by
sialylation Sialic acids are a class of alpha-keto acid sugars with a nine-carbon backbone.
The term "sialic acid" (from the Greek for saliva, - ''síalon'') was first introduced by Swedish biochemist Gunnar Blix in 1952. The most common member of this g ...
agents.
A porous carbon nanofoam was created by:
(i)
Pitch and CaCO
3 (in a 1:14 ratio) were dissolved in
methylene chloride
Dichloromethane (DCM or methylene chloride, methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odour is widely used as a solvent. Although it is not miscible with ...
. 10mL of
NaCl
Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...
was added. Mixture was stirred continuously.
(ii) Sample was naturally air dried at room temperature.
(iii) Sample was
carbonized
Carbonized were a Sweden, Swedish avant-garde metal band formed 1988 in Saltsjöbaden. The band was formed by Lars Rosenberg in 1988, with Dismember (band), Dismember vocalist Matti Kärki. Joined by drummer Piotr Wawrzeniuk, the trio was compl ...
at 600 °C for 2 hours. The heating rate was 2 °C per minute.
(iv) Carbonized structure is washed in 1M
HCl HCL may refer to:
Science and medicine
* Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia
* Harvard Cyclotron Laboratory, from 1961 to 2002, a proton accelerator used for research and development
* Hollow-cathode lamp, a spe ...
to remove excess CaCO
3 nanoparticles.
Applications
Carbon Nanofoams have been shown to have great application as
solar steam generators. They possess excellent light absorption, good thermal stability, low density, and low thermal conductivity, all factors important to solar generators. In experiments done, carbon nanofoams showed superior solar photo-thermal performance with an evaporation rate of 1.68 kg m−2 h−1 achieved under 1 sun irradiation.
Additionally, carbon nanofoams have also been used to create extremely efficient
aerosol filters. Using cellulose nanofibers collected from recycled milk jugs, researchers were able to develop a carbon nanofoam that achieved a very high filtration efficacy (>99.5%) in tests run with 0.7 wt% nanofoam sample for particles smaller than 360 nm. This efficiency value even meets the standard requirements of the N95 respirator face masks. The structure of the nanofoam filter gives it an advantage in performance over normal filters when dealing with high particle bearing
Glass
In 2014, researchers also fabricated glass nanofoam via femtosecond laser ablation. Their work consisted of raster scanning femtosecond laser pulses over the surface of glass to produce glass nanofoam with ~70 nm diameter wires.
See also
*
Nanoporous material
*
Ceramic foam Ceramic foam is a tough foam made from ceramics. Manufacturing techniques include impregnating open-cell polymer foams internally with ceramic slurry and then firing in a kiln, leaving only ceramic material. The foams may consist of several ceramic ...
*
Metal foam
Regular foamed aluminium
A metal foam is a cellular structure consisting of a solid metal (frequently aluminium) with gas-filled pores comprising a large portion of the volume. The pores can be sealed (closed-cell foam) or interconnected (open-c ...
*
Carbon nanofoam Carbon nanofoam is an allotrope of carbon discovered in 1997 by Andrei V. Rode and co-workers at the Australian National University in Canberra. It consists of a cluster-assembly of carbon atoms strung together in a loose three-dimensional web. The ...
*
Ultrashort pulse
In optics, an ultrashort pulse, also known as an ultrafast event, is an electromagnetic pulse whose time duration is of the order of a picosecond (10−12 second) or less. Such pulses have a broadband optical spectrum, and can be created by ...
References
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Nanomaterials
Foams
Gels