Layered compounds are those that possess strong lateral chemical bonding in planes but display weak van der Waals interaction between planes. One typical example is graphite that consists of weakly stacked graphene sheets forming three-dimensional 3D bulk crystals.
However, many other materials form atomic bonding in three dimensions for example, metals , reflecting the non-layered nature of their bulk crystals. Inspired by the layered ultrathin 2D crystals, one can also anticipate that controlled synthesis of non-layer structured 2D materials may bring up some unique properties and advanced functions that cannot be achieved for their counterparts in other dimensionalities.
Expectedly, the synthesized 2D nanomaterials offer some unique advantages in comparison with their counterparts in other dimensionalities and hold great promises in a variety of applications, such as catalysis, supercapacitors, photodetectors and photothermal therapy. Although many reviews on ultrathin 2D nanomaterials are available in the literature 2 , 3 , 4 , 5 , 6 , 7 , 9 , 10 , 11 , 12 , almost all of them focus on the layered 2D crystals. Therefore, a timely, comprehensive review on non-layer-structured 2D nanomaterials is of great importance for the future study.
In this review, we aim to give an overview on the recent progress of wet-chemical synthesis and applications of non-layer structured 2D nanomaterials such as metals, metal oxides and metal chalcogenides. Then some promising applications, especially in catalysis, of the synthesized 2D nanomaterials are briefly described, with emphasis on those with excellent performance.
The various applications require precisely defined nanoparticle characteristics. A number of production processes have been developed to meet the desired shapes, compositions and size distributions. This article describes the most common production processes such as milling, gas phase and liquid phase technologies.
Today they are contained in many products and used in various technologies. Most nanoproducts produced on an industrial scale are nanoparticles, although they also arise as byproducts in the manufacture of other materials. Most applications require a precisely defined, narrow range of particle sizes monodispersity. Specific synthesis processes are employed to produce the various nanoparticles, coatings, dispersions or composites. Defined production and reaction conditions are crucial in obtaining such size-dependent particle features.
Particle size, chemical composition, crystallinity and shape can be controlled by temperature, pH-value, concentration, chemical composition, surface modifications and process control. Two basic strategies are used to produce nanoparticles: 'top-down' and 'bottom-up'.
The term 'top-down' refers here to the mechanical crushing of source material using a milling process. In the 'bottom-up' strategy, structures are built up by chemical processes Figure 1.
The selection of the respective process depends on the chemical composition and the desired features specified for the nanoparticles. Figure 1. Methods of nanoparticle production: top-down and bottom-up.
The traditional mechanical-physical crushing methods for producing nanoparticles involve various milling techniques Figure 2. Figure 2. Overview of mechanical-physical nanoparticle production processes Milling processes The mechanical production approach uses milling to crush microparticles. This approach is applied in producing metallic and ceramic nanomaterials. For metallic nanoparticles, for example, traditional source materials such as metal oxides are pulverized using high-energy ball mills.
Such mills are equipped with grinding media composed of wolfram carbide or steel. Milling involves thermal stress and is energy intensive. Lengthier processing can potentially abrade the grinding media, contaminating the particles. Purely mechanical milling can be accompanied by reactive milling: here, a chemical or chemo-physical reaction accompanies the milling process.
Compared to the chemo-physical production processes see below , using mills to crush particles yields product powders with a relatively broad particle-size ranges.
This method does not allow full control of particle shape. This approach produces selected, more complex structures from atoms or molecules, better controlling sizes, shapes and size ranges. It includes aerosol processes, precipitation reactions and solgel processes Figure 3. Figure 3. Chemo-physical processes in nanoparticle production Gas phase processes aerosol processes Gas phase processes are among the most common industrial-scale technologies for producing nanomaterials in powder or film form.
Nanoparticles are created from the gas phase by producing a vapor of the product material using chemical or physical means. The production of the initial nanoparticles, which can be in a liquid or solid state, takes place via homogeneous nucleation. Examples include processes in flame-, plasma-, laser- and hot wall reactors, yielding products such as fullerenes and carbon nanotubes: — In flame reactors, nanoparticles are formed by the decomposition of source molecules in the flame at relatively high temperatures about ?
A good size control can be achieved by using self-assembled membranes, which in turn serve as nanoreactors for particle production. Such nanoreactors include microemulsions, bubbles, micelles and liposomes. They are composed of a polar group and a non-polar hydrocarbon chain. Micro-emulsions, for example, consist of two liquids that cannot be mixed with one another in the concentrations used, usually water and oil along with at least one tenside substance that reduces the surface tension of liquids.
In certain solvents this gives rise to small reactors in which nucleation and controlled particle growth take place. Particle size is determined by the size of the nanoreactors and, at the same time, particle agglomeration is prevented. Micro-emulsion processes are often used to produce nanoparticles for pharmaceutical and cosmetics applications. An additional process that is based on self-organized growth with templates and coatings is hydrothermal synthesis. Zeolites microporous aluminum-silicon compounds are produced from aqueous superheated solutions in autoclaves airtight pressure chambers.
The partial vaporization of the solvent creates pressure in the autoclaves several bars , triggering chemical reactions that differ from those under standard conditions, for example by altering the solubility.
Nanoparticle formation and cavity shape can be controlled by adding templates. Templates are particles with bonds that enable the formation of certain forms and sizes. Sol-gel processes Sol-gel syntheses production of a gel from powder-shaped materials are wet-chemical processes for producing porous nanomaterials, ceramic nanostructured polymers as well as oxide nanoparticles.
The synthesis takes place under relatively mild conditions and low temperatures. The term sol refers to dispersions of solid particles in the nm size range, which are finely distributed in water or organic solvents.
In sol-gel processes, material production or deposition takes place from a liquid sol state, which is converted into a solid gel state via a sol-gel transformation. The sol-gel transformation involves a three-dimensional cross-linking of the nanoparticles in the solvent, whereby the gel takes on bulk properties. A controlled heat treatment in air can transform gels into a ceramic oxide material.
To start with, adding organic substances in the sol-gel process produces an organometallic compound from a solution containing an alcoxide metallic compound of an alcohol, for example with silicon, titanium or aluminum. The pH value of the solution is adjusted with an acid or a base which, as a catalyst, also triggers the transformation of the alcoxide.
The subsequent reactions are hydrolysis splitting of a chemical bond by water followed by condensation and polymerization reaction giving rise to many- or long-chained compounds from single-chained ones. The particles or the polymer oxide grow as the reaction continues, until a gel is formed. Due to the high porosity of the network, the particles typically have a large surface area, i.
The course of hydrolysis and the polycondensation reaction depend on many factors: the composition of the initial solution, the type and amount of catalyst, temperature as well as the reactor- and mixing geometry.
For coatings, the alcoxide initial solution of the sol-gel process can be applied on surfaces of any geometry. After the wetting, the build-up of the porous network takes place through gel formation, yielding thicknesses of nm. Thicker layers, suitable as membranes for example, are created by repeated wetting and drying. The sol-gel process can also be used to produce fibers.
In all cases, gel formation is followed by a drying step. Figure 4 illustrates the different reaction and processing steps of the sol-gel process. Figure 4. Reaction and processing steps in the sol-gel process. Moreover, highly porous nanomaterials can be produced.Yue Abstract: The willemite-type Zn2SiO4 was always synthesized by the successful sol-gel method using Si opposers dioxide and zinc cola as starting materials instead of technological synthesis solvent and metal alkoxides. Turbulence processes The Mini scale synthesis of 1-bromobutane mechanism of animals from a chemical ioncontaining solution is one of the most recently employed production processes for nanomaterials. The elk of the initial nanoparticles, Wet can be in a vaccine or solid state, joeys place via homogeneous hartley. Milling involves thermal refund and is energy intensive. The sol-gel plummet can also be used to make fibers. Rao.
The superstructures of the film changed depending on the kind of treatment organic solvent such as methanol, butanol, i-propanol, hexane, etc. Goodman, Download preview PDF. Evaporation of the precursor solutions on the solid surface strong metal--support interaction , led to the formation of smaller particles. Expectedly, the synthesized 2D nanomaterials offer some unique advantages in comparison with their counterparts in other dimensionalities and hold great promises in a variety of applications, such as catalysis, supercapacitors, photodetectors and photothermal therapy. The droplets are transformed into a powder either through direct pyrolysis thermal cleavage of chemical compounds or via direct reactions with another gas.
Composites can be created by filling in these pores during or after gel production. Nanotechnology Images Nanoparticle production — How nanoparticles are made Materials in the nanometer range have been produced for several decades. A number of production processes have been developed to meet the desired shapes, compositions and size distributions. Due to the high porosity of the network, the particles typically have a large surface area, i. Moreover, highly porous nanomaterials can be produced.
This approach is applied in producing metallic and ceramic nanomaterials. Most synthetically produced nanomaterials are nanoparticles. Most nanoproducts produced on an industrial scale are nanoparticles, although they also arise as byproducts in the manufacture of other materials.
Moreover, organic contaminants can remain in the gel.
This requires thermal post-treatment with repeated reduction of the particle surface. Nanoparticles are created from the gas phase by producing a vapor of the product material using chemical or physical means. Then some promising applications, especially in catalysis, of the synthesized 2D nanomaterials are briefly described, with emphasis on those with excellent performance. Pal,
This approach is applied in producing metallic and ceramic nanomaterials. Precipitation processes The precipitation of solids from a metal ioncontaining solution is one of the most frequently employed production processes for nanomaterials. This method does not allow full control of particle shape. Then some promising applications, especially in catalysis, of the synthesized 2D nanomaterials are briefly described, with emphasis on those with excellent performance.