Top down nanoparticle preparation methods presents interesting pathways to approach the nanoscale by starting with the bulk scale materials and then scaling them down to the nanometer level dimensions. These strategies involve physical breaking of the source material through high energy processes. In this page we will review some of the most frequently used top down nanomaterial synthesis pathways that are used by scientists and industry alike.
Mechanical milling is a top down approach to prepare nanomaterials and has found great acceptability especially in industrial nanomaterial preparation environments due to its simplicity, versatility of the process ( can be adapted to make many types of nanomaterials), scalability of the process and for the low cost. In this method, bulk material which is usually in the micro dimensions are grounded down to the nanoscale with strong mechanical shear forces applied by the milling technique. There are three types of attrition devices that are more frequently used than others. They are, shaker mills, planetary ball mill and attrition mill.
In the shaker mills, as name suggests material to be milled is charged in to a vial with “milling balls”, spherical balls that are made of hard material. The sample is then securely attached to the shaker and energetically swung back and forth for several thousand cycles per minute. During this shaking process, milling balls, collide on each other and with the vial wall. The high shear and impact forces produced in the process grinds the solids down and mix it thoroughly.
Planetary ball mills has gained its name from the movement of the vial in the device. These vials are attached in to a rotating disk that rotates around its axis and individual vials are also rotated in an axis of their own but in an opposite direction to the main rotating disk. The whole system is rotated several thousand rpm and strong frictional and impact forces grinds the material down to smaller sizes. Planetary ball mill is more popular among many as it can be used to grind several hundreds of material at a time.
Attrition mill is similar to a ball mill where grinding balls are loaded in to a horizontal cylinder and rotated to perform the milling action. However, in attraction mills the vertical drum is attached with series of carefully positions impellers inside. These impellers are fixed to have right angles to each other. Unlike ball mill, attrition mill is rotated at high speeds with the inside impeller working. This can produce very high shear and impact forces that are not possible to obtain with attrition mills.
The main problem with this method is contamination of the nanomaterial with the milling balls and the container. High shear and impact forces generated inside can chip small surfaces from these surfaces and incorporated in to the nanomaterial. This is the main reason, that at least in the labs, this method is slowly getting dismissed. Also, high shear forces applied in the milling process impart significant imperfections of surface and crystallographic structure of the material. The milling ball sizes, the milled material, duration of milling are typical control parameters of this process.
Laser ablation synthesis
This technique deals with generation of nanoparticles using a powerful laser beam. It’s a top down process where a high energy laser is focused on to a target in a solvent. The laser delivers, short pulses of energy which is enough to vaporize small spots of the metal target which condenses as nanoparticle in the solvent. Usually this method is used to generate noble metallic nanoparticles such as gold, silver and platinum. However, it can seamlessly be extended to make other nanomaterials such as nanoparticles of metal alloys: a unique advantage of this method.
The typical setup consist of a pulsed laser, set of focusing optics and a container containing a metal target. The container is filled with a solvent ( eg: water, ethanol) in which the metal target is immersed. The placement of the metal target is close to the focus of the laser. The size and dispersion of the nanoparticles can be tuned by the pulse duration, wavelength and the intensity of the laser pulses. Typically, the laser pulses are in extremely small time scales such as femtoseconds(1/1,000,000,000,000,000 of a second), picoseconds (1/1,000,000,000,000 of a second) or nanosecond (1/1,000,000,000 of a second). Both visible and near infrared wavelength lasers are typical in laser ablation instruments.
Use of short pulses in the process allows the ablation process to be carried out in number of different mediums, including very volatile solvents to highly reactive monomers. Since the action is primarily physical, this technology can be used on wide range of material and solvent possibilities. Another unique advantage of this method is the ability of making nanoparticles with very high purity. Since this method produce no by products or residual chemicals.
Laser ablation apparatus
Arc discharge synthesis
Arc discharge method presents another physical method of making nanomaterial. This can be classified as a top down process as nanomaterials are produced from bulk materials due to arc assisted breakdown of the bulk material.In the arc discharge process, two electrodes are kept in proximity to each other in a solution and a high voltage is applied in between them. Due to the high potential difference and the proximity, an electrical breakdown occurs in the medium, giving rise to arc discharge which produces an ongoing thermal plasma discharge. The local temperature of the ongoing plasma can reach several thousand of degrees Celsius, which effectively vaporized the electrode surfaces in the liquid medium. Vaporized metal condenses on the solvent making nanoparticles. Most of the metallic nanoparticles, nanostructures and metal oxide nanoparticles are produced in this way. The condensing solvent medium, voltage and current supplied to the electrode and electrode material are major parameters in determining the end product.The extremely high temperature produced between the electrodes can also facilitate atomic assembly pathways that can create new nanostructures that are different to the electrode materials. Carbon nanotubes are produced this way in arc discharge synthesis. The discharge vaporized the surface of one of the carbon electrodes and forms a rod shaped deposit on the other. This deposit is rich with mixture of single and multiwall carbon nanotubes.