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Kinetics of Hydrogen Generation from Oxidation of Hydrogenated Silicon Nanocrystals in Aqueous Solutions
Silicon has also been investigated for hydrogen generation purposes [37]. Indeed, hydrogen can be produced, for example, from the reaction between ferrosilicon, sodium hydroxide and water. Further investigations showed the high potential and economic efficiency of the silicon-based hydrogen fuel [38,39]. In strong contrast to oil and molecular hydrogen, the transport and storage of silicon are free from potential hazards and require a simple infrastructure similar to that requested for coal. Whereas the latter material is converted to carbon dioxide, silicon involved in hydrogen production is transformed in sand. Among a number of the known metalloids, silicon shows the best efficiency for the chemical splitting of water [40,41]. Moreover, photoelectrochemical applications of crystalline silicon have been investigated for solar-induced splitting of water [42,43]. Silicon nanoparticles can be used to generate hydrogen much more rapidly than bulk silicon because of their high specific surface area. Chemical or anodic etching [44–46], the beads milling method [47–49], or laser pyrolysis [50] can produce the nano-powders used for hydrogen generation from oxidation reactions of Si in water. Since the reaction rate strongly depends on pH, the addition of alkalis or ammonia is often used in order to increase the rate of hydrogen production. The fabrication of silicon nanostructures by electrochemical etching has a significant advantage over the other approaches cited above. Indeed, it results in the formation of nano-porous morphologies covered with an abundant number of silane groups (SiHX) [51–53]. The maximum specific surface of nanoporous silicon can reach the value of 800 m2 /g, while the content of hydrogen chemically bound on the surface can be as high as 60 mmol of atomic H per gram of porous Si, corresponding to the H/Si ratio ~1.8 or to the 6% mass of H [44]. The presence of this surface hydrogen can increase the output volume of H2, which releases in the reaction of porous silicon nanoparticles with water by 1.5 times [54]. General hydrogen output and release rate strongly depend on the porosity and sizes of the particles. Moreover, sufficient reaction rates are achieved even at room temperature, without additional heating or mixing [55,56]. A working prototype of cartridge generating hydrogen from PSi nanopowder was designed and coupled with a portable fuel cell [57]. Silicon nanostructures have also been considered as good candidates for photoelectrochemical and photocatalytic water splitting to produce hydrogen. Photo-electrodes based on arrays of silicon nanowires coupled with different catalysts [58–60], as well as porous silicon structures [61–63], were used for the generation of hydrogen. The surface modification of silicon nanostructures plays a considerable role in designing materials for solar-driven catalysis. The applications of Si nanostructures can be moved from photoelectrical to photochemical conversion by taking catalytic sites into account [58]. However, the chemical reactivity of silicon surface with water complicates the implementation of the photocatalytic reaction. This is especially evident for the porous nanostructures, where a higher oxidation degree of nanosilicon can result in the blocking of the nanopores with silicon oxide [64]. 2. Hydrogen Generation from Oxidation of Porous Silicon Nanopowders in Water Research work described in the current paper is devoted to the investigation of influence of various physico-chemical factors on the kinetics of hydrogen generation resulting from chemical reactions of hydrogenated porous silicon (PSi) nanopowders with water. In particular, the impact of ambient temperature, chemical nature and the concentration of used alkalis, grinding degree, and the porosity of PSi nanopowders are examined. In general, this reaction occurs as follows: SiHX + 3H2O → SiO2•H2O + 2 + X 2 H2 ↑ +Q (1) where Q is equal to 361 kJ/M and to 634 kJ/M for x = 0 and x = 2, respectively. Reaction (1) is schematically illustrated in Figure 1. Nanomaterials 2020, 10, 1413 4 of 14 Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 15 well as by the intensity of external light results in the abundant production of molecular hydrogen [44,45]. Figure 1. Transformation of partially hydrogenated PSi nanoparticles in silicon oxide in water solution. Si-H bonds covering specific surface of PSi nanoparticles are known to be the most chemically active ones. These bonds can react, for example, with the bases, as shown by reaction (a) in Figure 2, substituting surface hydrogen by –OH groups. Si–Si bonds located on the PSi surface can also react with oxidizers, according to the reactions (b) and (c) in Figure 2, leading to the formation of O3Si–H and Si–OH surface fragments. As a result, a silicon oxide film starts to appear on the PSi surface. The O3Si–H fragments are rather passive and stable. Nevertheless, they can react with strong bases, as shown by reaction (d) in Figure 2 [44,54]. Figure 1. Transformation of partially hydrogenated PSi nanoparticles in silicon oxide in water solution. As one can see, partially hydrogenated PSi nanoparticles are almost completely transformed in silicon oxide nanoparticles, as shown in Figure S1 in the Supplementary Materials. This transformation is modulated by the pH and temperature of the surrounding aqueous solution, as well as by the intensity of external light results in the abundant production of molecular hydrogen [44,45]. Si-H bonds covering specific surface of PSi nanoparticles are known to be the most chemically active ones. These bonds can react, for example, with the bases, as shown by reaction (a) in Figure 2, substituting surface hydrogen by –OH groups. Si–Si bonds located on the PSi surface can also react with oxidizers, according to the reactions (b) and (c) in Figure 2, leading to the formation of O3Si–H and Si–OH surface fragments. As a result, a silicon oxide film starts to appear on the PSi surface. The O3Si–H fragments are rather passive and stable. Nevertheless, they can react with strong bases, as shown by reaction (d) in Figure 2 [44,54]. Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 15 well as by the intensity of external light results in the abundant production of molecular hydrogen [44,45]. Figure 1. Transformation of partially hydrogenated PSi nanoparticles in silicon oxide in water solution. Si-H bonds covering specific surface of PSi nanoparticles are known to be the most chemically active ones. These bonds can react, for example, with the bases, as shown by reaction (a) in Figure 2, substituting surface hydrogen by –OH groups. Si–Si bonds located on the PSi surface can also react with oxidizers, according to the reactions (b) and (c) in Figure 2, leading to the formation of O3Si–H and Si–OH surface fragments. As a result, a silicon oxide film starts to appear on the PSi surface. The O3Si–H fragments are rather passive and stable. Nevertheless, they can react with strong bases, as shown by reaction (d) in Figure 2 [44,54]. Figure 2. Schematic representation of gradual transformation of initially hydrogenated PSi nanopowder in silicon oxide during its oxidation in water. Nanomaterials 2020, 10, 1413 5 of 14 Reactivity of the chemical bonds of PSi surface with water and other oxidants increases in the following order: O3Si–H < Si3Si–H < Si–Si. In the other words, the oxidation of the Si surface proceeds more preferably via (c)–(d), than via (a)–(b), as shown in Figure 2 [45,65,66]. When a sufficiently thick layer of hydrated silicon oxide is formed on the PSi surface, it efficiently preserves the inner parts of Si NPs from further oxidation, and the reaction stops [40]. The presence of bases in aqueous solution results in the significant acceleration of the reaction between Si and water due to an increase in the primary reaction rate of the surface bonds with water as well as due to the partial dissolution of the surface oxide layer and penetration of water molecules and OH– ions to the inner parts of the PSi nanoparticles. 3. Materials and Methods 3.1. Formation of PSi Nanopowders The PSi nanopowders used in this study were obtained by mechanical grinding of porous silicon (PSi) layers. The initial PSi layers were fabricated according to a standard procedure based on electrochemical etching [67] of monocrystalline (100)-oriented boron-doped Si wafers (1–10 cm and 0.01 cm for nano- and meso-PSi structures, respectively) at current densities of 2–340 mA/cm2 . The etching solutions of 9:1, 3:1 and 1:1 mixtures (by volume) of concentrated aqueous hydrofluoric acid (48%) and ethanol were used for nano-PSi formation and an HF/ethanol solution of 1:1 volume mixture for meso-PSi formation. The experimental conditions of the electrochemical etching mentioned above allowed for the formation of nano-PSi and meso-PSi nanopowders with various porosities, as shown in see Figures S2–S4 in Supplementary Materials, and sizes of nanoparticles constituting the porous powders, as shown in see Figure S5 in Supplementary Materials. 3.2. Structural Characterization of PSi Nanopowders Structural properties and morphologies of the PSi nanopowders were investigated by means of atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Atomic force microscopy (AFM) characterization was performed on the Digital Instruments 3100 instrument using ultra-sharp silicon cantilevers (NanosensorsTM SSS-NCH, Switzerland) with a typical curvature radius of 10 nm and nominal spring constant of 42 N/m. AFM images were acquired in a tapping mode at room temperature under ambient conditions for the particles deposited onto an atomically flat surface of electronic grade silicon wafers. Scanning electron microscope (SEM) images were taken using the ultra-high resolution Tescan MIRA 3 scanning electron microscope.