**3.16. Porous silica reduction**

Reduction of porous silica is another bottom-up route to synthesize porous silicon structures [59, 60]. This preparation method is utilized when inexpensive products are required as lowcost silica feedstocks like sand, living plants, and agricultural wastes can be used. As silica reduction generally occurs by magnesium vapor at moderate temperatures, this fabrication process is called magnesiothermic reduction of silica (Eq. (10)). (*s*) <sup>+</sup> 2Mg(*g*) ⎯ ⎯

$$\text{SiO}\_2(\text{s}) + 2\text{Mg}(\text{g}) \xrightarrow{400-600^\circ \text{C}} \text{Si}(\text{s}) + 2\text{MgO}(\text{s}) \tag{10}$$

by chemical etching. Indeed, by appropriate choosing of deposition parameters, especially the deposition rate, it is possible to control the structural development of Al-Si system that occurs at the surface during the film growth at low temperatures. Accordingly, by controlling the nanoscale phase separation through deposition parameters, Fukutani et al. have successfully fabricated Al nanocylinders surrounded by an amorphous silicon matrix [66]. Removing the Al cylinders by immersing the specimen in concentrated sulfuric acid solution for 24 h, a porous amorphous silicon layer with cylindrical pores is obtained (**Figure 20**). A subsequent annealing

tion in the average size and density of the pores [67]. Further studies are needed to determine the

Laser-induced decomposition of silane has been used to prepare high porosity mesoporous structures composed of silicon nanocrystals [68]. Nanocrystals have been synthesized by pulsed

The setup consists of three vacuum chambers: a source chamber containing the flow reactor, a differential chamber encasing the chopper for size selection, and an ultrahigh vacuum (UHV) chamber that contains the time-of-flight mass spectrometer. The "source chamber" is indeed a laser-driven

conical nozzle projected into the reaction zone. The extracted nanocrystals are then skimmed into a low-pressure vacuum chamber (differential chamber); they form a "molecular beam" of noninteracting clusters. The beam passes through the slits of a rotating chopper, synchronized with the pulsed pyrolysis laser and can be vertically moved from outside. The chopper is used to preselect a small portion of the initially broad cluster pulse to narrow the cluster size distribution. Immediately behind the chopper, is a sample holder that can be moved into the cluster beam to collect the nanocrystals. Thus, silicon particles with preselected size can be deposited on the substrate. If the sample holder is moved out of the path of the beam, the silicon nanoparticles enter the UHV detector chamber equipped with a time-of-flight mass spectrometer [68, 69]. Pores are automatically formed when nanocrystals are collected by the substrate. This fabrication route has not attracted much

attention due to the complexity of the apparatus and poor adhesion of the porous layer.

Porous silicon structures can also be synthesized by galvanic deposition, also known as immer-

**Figure 20.** Fabrication of cylindrical pores by dealloying of Al-Si eutectic system and subsequent Al removal (The porous amorphous silicon structure can be crystallized by performing a thermal annealing process.) [67].

SiF6

controllability over the shape and orientation of the pores generated by this method.

atmosphere for 1 h crystallizes the porous material without any visible altera-

in a complex apparatus illustrated in **Figure 21**.

laser pyrolysis of SiH4

to dilute hydrofluoric acid concen-

. The reac-

Porous Silicon

25

http://dx.doi.org/10.5772/intechopen.72910

laser beam through a

step at 800°C in H2

**3.19. Electrodeposition**

CO2

**3.18. Laser-induced silane decomposition**

laser-induced decomposition of SiH4

CVD reactor where silicon clusters are produced by pulsed CO2

sion plating. Addition of sodium hexafluorosilicate Na<sup>2</sup>

tion products are extracted perpendicularly to both the gas flow and the CO<sup>2</sup>

However, the reduction can also takes place using other reducing agents, e.g., lithium, sodium, aluminum, and calcium [61–64]. The reaction pathway of Eq. (10) is then followed by acid leaching to remove the metal oxide byproduct and obtain porous silicon. The pore size and morphology of the synthesized porous silicon strongly depends upon morphology and moisture content of the feedstock, thermal budget of the process, and byproduct size distribution. In addition to metallothermic porous silica reduction, porous silicon synthesis has also been performed by mechanochemical reduction of SiO powder [60].

Porous silica reduction is a simple route for the realization of porous silicon. All classes of porosity can be achieved by this method. However, it is important to precisely control the process to prevent formation of magnesium silicate instead of silicon. Sintering and reintroduction of oxygen, especially in highly porous products, are also issues [63, 65].
