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Nanofoams are a class of nanostructured, porous materials (foams) containing a significant population of pores with diameters less than 100 nm. Aerogels are one example of nanofoam. [1]
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, nickel, gold, and platinum. [2] Metallic nanofoams may offer certain advantages over alternative polymer nanofoams; structurally, they retain the electrical conductivity of metals, offer increased ductility, as well as the higher surface area and nano-architecture properties offered by nanofoams. [2]
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, cobalt, nickel, copper, silver, and palladium have been prepared through this technique. These materials exhibit densities as low as 11 mg/cm3, and surface areas as high as 258 m2/g. These foams are effective catalysts [3] and electrocatalyst supports. [4] Also, metal nanofoams can be made by electrodeposition of metals inside templates with interconnected pores, such as 3D-porous anodic aluminum oxide (AAO). [5] [6] [7] Such method gives nanofoams with an organized structure and allows to control the surface area and porosity of the fabricated material. [8] [9] [10]
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. [11]
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. [12]
Metallic nanofoams have seen a broad variety of applications, including catalysts, [13] hydrogen storage, [14] as well as fuel cells. [15] Additionally, applications of metallic nanofoam as 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. [16] Applications to the clean energy industry, specifically for lithium-ion batteries and other fuel cells, have been discussed as well. [11]
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. [17] In these instances, biopolymers such as collagen and gelatine, [18] chitosan, [19] and pure curcumin [17] have been used to varying degrees.
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. [17] 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. [19]
Additionally, an iron-nitrogen co-doped carbon nanofoam was purposed to be fabricated through the acile salt-assisted pyrolysis process of chitooligosaccharides. [20]
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. [17] 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. [17]
Silver 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 alloys of silver and another metal such as aluminum. [21] 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.
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. [21]
Ag foams with hierarchical porous structures are prepared by the following three-step method: [21]
(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.
Silver ions have been shown to have potent antibacterial activity, and have been shown to affect the growth of Gram-positive and Gram-negative bacteria. This is due to their ability to form ligand complexes with proteins or enzymes in bacterial cells. [21] 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. [21]
These silver nanofoams have also been used as electrocatalysts for the reduction reaction of CO2 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. [22]
Carbon nanofoam is an allotrope of carbon discovered in 1997. [23] 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/cm3 (0.0012 lb/ft3), which is among the lightest materials to date. [23] [24] [25] [26] [27]
There are multiple formation methods for carbon nanofoams. Pulsed Laser Deposition (PLD) has been the first technique used for the synthesis of carbon nanofoam, [23] and is considered one of the most versatile approach for the production of carbon nanofoams with controlled density and morphology. [27] The process of nanofoam growth via the Pulsed Laser Deposition has been described in terms of a "snowfall-like" mechanism: [26]
(i) Carbon nanoparticles are generated upon laser ablation 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 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 nanofibers (CNF) were constructed into nanofoams by: [28]
(i) Recycled milk container board was pretreated with deep eutectic solvent (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 agents.
A porous carbon nanofoam was created by: [29]
(i) Pitch and CaCO3 (in a 1:14 ratio) were dissolved in methylene chloride. 10mL of NaCl was added. Mixture was stirred continuously.
(ii) Sample was naturally air dried at room temperature.
(iii) Sample was carbonized at 600 °C for 2 hours. The heating rate was 2 °C per minute.
(iv) Carbonized structure is washed in 1M HCl to remove excess CaCO3 nanoparticles.
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. [29]
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 [28]
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. [30]
Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be coated acts as the cathode of an electrolytic cell; the electrolyte is a solution of a salt of the metal to be coated; and the anode is usually either a block of that metal, or of some inert conductive material. The current is provided by an external power supply.
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 fractal-like bond structure consists of sp2 graphite-like clusters connected by sp3 bonds. The sp3 bonds are located mostly on the surface of the structure and make up 15% to 45% of the material, making its framework similar to diamond-like carbon films. The material is remarkably light, with a density of 2-10 x 10−3 g/cm3 (0.0012 lb/ft3) and is similar to an aerogel. Other remarkable physical properties include the large surface area of 300–400 m2/g. 1 US gallon of nanofoam weighs about 0.25 ounces (7.1 g).
Proton-exchange membrane fuel cells (PEMFC), also known as polymer electrolyte membrane (PEM) fuel cells, are a type of fuel cell being developed mainly for transport applications, as well as for stationary fuel-cell applications and portable fuel-cell applications. Their distinguishing features include lower temperature/pressure ranges and a special proton-conducting polymer electrolyte membrane. PEMFCs generate electricity and operate on the opposite principle to PEM electrolysis, which consumes electricity. They are a leading candidate to replace the aging alkaline fuel-cell technology, which was used in the Space Shuttle.
Laser ablation or photoablation is the process of removing material from a solid surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser flux, the material is typically converted to a plasma. Usually, laser ablation refers to removing material with a pulsed laser, but it is possible to ablate material with a continuous wave laser beam if the laser intensity is high enough. While relatively long laser pulses can heat and thermally alter or damage the processed material, ultrashort laser pulses cause only minimal material damage during processing due to the ultrashort light-matter interaction and are therefore also suitable for micromaterial processing. Excimer lasers of deep ultra-violet light are mainly used in photoablation; the wavelength of laser used in photoablation is approximately 200 nm.
In materials science, a metal foam is a material or structure consisting of a solid metal with gas-filled pores comprising a large portion of the volume. The pores can be sealed or interconnected. The defining characteristic of metal foams is a high porosity: typically only 5–25% of the volume is the base metal. The strength of the material is due to the square–cube law.
In metallurgy, selective leaching, also called dealloying, demetalification, parting and selective corrosion, is a corrosion type in some solid solution alloys, when in suitable conditions a component of the alloys is preferentially leached from the initially homogenous material. The less noble metal is removed from the alloy by a microscopic-scale galvanic corrosion mechanism. The most susceptible alloys are the ones containing metals with high distance between each other in the galvanic series, e.g. copper and zinc in brass. The elements most typically undergoing selective removal are zinc, aluminium, iron, cobalt, chromium, and others.
Metal–organic frameworks (MOFs) are a class of compounds consisting of metal clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures. The organic ligands included are sometimes referred to as "struts" or "linkers", one example being 1,4-benzenedicarboxylic acid (BDC).
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 can be heterogeneous such as a platinized electrode. Homogeneous electrocatalysts, which are soluble, assist in transferring electrons between the electrode and reactants, and/or facilitate an intermediate chemical transformation described by an overall half reaction. Major challenges in electrocatalysts focus on fuel cells.
The electrochemical reduction of carbon dioxide, also known as CO2RR, is the conversion of carbon dioxide to more reduced chemical species using electrical energy. It represents one potential step in the broad scheme of carbon capture and utilization.
The lithium–air battery (Li–air) is a metal–air electrochemical cell or battery chemistry that uses oxidation of lithium at the anode and reduction of oxygen at the cathode to induce a current flow.
Surface-assisted laser desorption/ionization (SALDI) is a soft laser desorption technique used for mass spectrometry analysis of biomolecules, polymers, and small organic molecules. In its first embodiment Koichi Tanaka used a cobalt/glycerol liquid matrix and subsequent applications included a graphite/glycerol liquid matrix as well as a solid surface of porous silicon. The porous silicon represents the first matrix-free SALDI surface analysis allowing for facile detection of intact molecular ions, these porous silicon surfaces also facilitated the analysis of small molecules at the yoctomole level. At present laser desorption/ionization methods using other inorganic matrices such as nanomaterials are often regarded as SALDI variants. As an example, silicon nanowires as well as Titania nanotube arrays (NTA) have been used as substrates to detect small molecules. SALDI is used to detect proteins and protein-protein complexes. A related method named "ambient SALDI" - which is a combination of conventional SALDI with ambient mass spectrometry incorporating the direct analysis real time (DART) ion source has also been demonstrated. SALDI is considered one of the most important techniques in MS and has many applications.
Carbon nanotube supported catalyst is a novel supported catalyst, using carbon nanotubes as the support instead of the conventional alumina or silicon support. The exceptional physical properties of carbon nanotubes (CNTs) such as large specific surface areas, excellent electron conductivity incorporated with the good chemical inertness, and relatively high oxidation stability makes it a promising support material for heterogeneous catalysis.
Research in lithium-ion batteries has produced many proposed refinements of lithium-ion batteries. Areas of research interest have focused on improving energy density, safety, rate capability, cycle durability, flexibility, and cost.
Nanoclusters are atomically precise, crystalline materials most often existing on the 0-2 nanometer scale. They are often considered kinetically stable intermediates that form during the synthesis of comparatively larger materials such as semiconductor and metallic nanocrystals. The majority of research conducted to study nanoclusters has focused on characterizing their crystal structures and understanding their role in the nucleation and growth mechanisms of larger materials. These nanoclusters can be composed either of a single or of multiple elements, and exhibit interesting electronic, optical, and chemical properties compared to their larger counterparts.
Foam glass is a porous glass foam material. Its advantages as a building material include its light weight, high strength, and thermal and acoustic insulating properties. It is made by heating a mixture of crushed or granulated glass and a blowing agent such as carbon or limestone. Near the melting point of the glass, the blowing agent releases a gas, producing a foaming effect in the glass. After cooling the mixture hardens into a rigid material with gas-filled closed-cell pores comprising a large portion of its volume.
Magnesium batteries are batteries that utilize magnesium cations as the active charge transporting agents in solution and often as the elemental anode of an electrochemical cell. Both non-rechargeable primary cell and rechargeable secondary cell chemistries have been investigated. Magnesium primary cell batteries have been commercialised and have found use as reserve and general use batteries.
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 density and extremely low thermal conductivity. Aerogels can be made from a variety of chemical compounds. Silica aerogels feel like fragile styrofoam to the touch, while some polymer-based aerogels feel like rigid foams.
Irshad Hussain is a Pakistani Scientist in the field of chemistry and among the few pioneers to initiate nanomaterials research in Pakistan.
Titanium foams exhibit high specific strength, high energy absorption, excellent corrosion resistance and biocompatibility. These materials are ideally suited for applications within the aerospace industry. An inherent resistance to corrosion allows the foam to be a desirable candidate for various filtering applications. Further, titanium's physiological inertness makes its porous form a promising candidate for biomedical implantation devices. The largest advantage in fabricating titanium foams is that the mechanical and functional properties can be adjusted through manufacturing manipulations that vary porosity and cell morphology. The high appeal of titanium foams is directly correlated to a multi-industry demand for advancement in this technology.
Metal Assisted Chemical Etching is the process of wet chemical etching of semiconductors with the use of a metal catalyst, usually deposited on the surface of a semiconductor in the form of a thin film or nanoparticles. The semiconductor, covered with the metal is then immersed in an etching solution containing and oxidizing agent and hydrofluoric acid. The metal on the surface catalyzes the reduction of the oxidizing agent and therefore in turn also the dissolution of silicon. In the majority of the conducted research this phenomenon of increased dissolution rate is also spatially confined, such that it is increased in close proximity to a metal particle at the surface. Eventually this leads to the formation of straight pores that are etched into the semiconductor. This means that a pre-defined pattern of the metal on the surface can be directly transferred to a semiconductor substrate.
