9 Si Nanopowder for Photoluminescence and Hydrogen Generation Materials
371
Both the reactions generate 2 M hydrogen from 1 M Si, i.e., 1712 mL hydrogen
at room temperature from 1 g Si. Reaction (9.17) doesn’t consume OH − ions, while
reaction (9.18) does, resulting in a decrease in pH. Therefore, the reaction ratio, i.e.,
the hydrogen volume generated by reaction (9.18) divided by the total generated
hydrogen volume, can be estimated from the decrease in pH. This ratio is estimated
to be 1.0 for pH 13.4 and 13.9 solutions, 0.58 for pH 12.9 solutions, and 0.12 for
pH 12.1 solutions. By reaction (9.18), soluble H 2 SiO 4
2− ions are formed, and thus,
all Si atoms in Si nanopowder can react with OH − ions because no materials to
prevent the reaction with OH - ions are present on the Si surface. For reaction (9.17),
on the other hand, SiO 2 with low solubility is formed on the surface, and when the
SiO 2 thickness reaches a certain value, the hydrogen generation stops because of
the migration-limited mechanism. (For reaction (9.17), OH − ions are the migrating
species through SiO 2 as explained below.) We have observed that when the SiO 2
thickness reaches to ∼5 nm, the hydrogen generation reaction stops. The decrease
in the volume of generated hydrogen with a decrease in pH can be explained by this
migration-limited mechanism.
1589 mL/g hydrogen generated by the reaction with pH 13.9 solutions is 93% of
the stoichiometric value of 1712 mL/g at room temperature. This small difference
is attributable to the presence of a silicon oxide layer on Si nanopowder before the
hydrogen generation reaction, i.e., 7wt% Si is already oxidized before the reaction.
972 mL/g hydrogen is generated by the reaction with the pH 12.9 solutions,
which is equal to 61% hydrogen generated by the reaction with 13.9 solutions,
and therefore, unreacted Si is estimated to be 39% that before the reaction. The
weight of SiO 2 formed by the hydrogen generation reaction is estimated to be 1.22 g
from initial 1 g Si nanopowder. Assuming that Si nanopowder before the reaction
possesses spherical shape with the 23.4 nm diameter (i.e., the average diameter
before the reaction), the diameter of unreacted Si nanopowder is estimated to be
17.1 nm. From the diameter of unreacted Si nanopowder, its weight, and the weight
of formed SiO 2 , the thickness of formed SiO 2 is estimated to be 5.1 nm, in good
agreement with that estimated from XPS measurements, i.e., 4.8 nm.
9.2.3.2 Reaction of Si Nanopowder with Neutral Water
Si nanopowder was produced from Si powder for hydrogen generation experiments
with neutral water. The size of Si nanopowder is larger than that produced from Si
swarf, i.e., for one-step milling, the mode diameter, the median diameter, and the
average diameter are 6.6, 14.0, and 23.4 nm, respectively, and those for two-step
milling are 5.8, 9.6, and 13.8 nm, respectively.
Figure 9.17 shows the hydrogen volume generated by the reaction with water
in the neutral pH region vs. the reaction time. Even in the case of the reaction
with ultrapure water of pH 7.0, Si nanopowder generates hydrogen (plot a), but
the hydrogen generation rate is low, i.e., 0.026∼0.047 mL/min.g. The hydrogen
generation rate increases by the reaction with tap water having pH 7.4 (plot b). The
hydrogen generation rate further increases by increases of pH to 8.0 (plot c) and 8.6
371
Both the reactions generate 2 M hydrogen from 1 M Si, i.e., 1712 mL hydrogen
at room temperature from 1 g Si. Reaction (9.17) doesn’t consume OH − ions, while
reaction (9.18) does, resulting in a decrease in pH. Therefore, the reaction ratio, i.e.,
the hydrogen volume generated by reaction (9.18) divided by the total generated
hydrogen volume, can be estimated from the decrease in pH. This ratio is estimated
to be 1.0 for pH 13.4 and 13.9 solutions, 0.58 for pH 12.9 solutions, and 0.12 for
pH 12.1 solutions. By reaction (9.18), soluble H 2 SiO 4
2− ions are formed, and thus,
all Si atoms in Si nanopowder can react with OH − ions because no materials to
prevent the reaction with OH - ions are present on the Si surface. For reaction (9.17),
on the other hand, SiO 2 with low solubility is formed on the surface, and when the
SiO 2 thickness reaches a certain value, the hydrogen generation stops because of
the migration-limited mechanism. (For reaction (9.17), OH − ions are the migrating
species through SiO 2 as explained below.) We have observed that when the SiO 2
thickness reaches to ∼5 nm, the hydrogen generation reaction stops. The decrease
in the volume of generated hydrogen with a decrease in pH can be explained by this
migration-limited mechanism.
1589 mL/g hydrogen generated by the reaction with pH 13.9 solutions is 93% of
the stoichiometric value of 1712 mL/g at room temperature. This small difference
is attributable to the presence of a silicon oxide layer on Si nanopowder before the
hydrogen generation reaction, i.e., 7wt% Si is already oxidized before the reaction.
972 mL/g hydrogen is generated by the reaction with the pH 12.9 solutions,
which is equal to 61% hydrogen generated by the reaction with 13.9 solutions,
and therefore, unreacted Si is estimated to be 39% that before the reaction. The
weight of SiO 2 formed by the hydrogen generation reaction is estimated to be 1.22 g
from initial 1 g Si nanopowder. Assuming that Si nanopowder before the reaction
possesses spherical shape with the 23.4 nm diameter (i.e., the average diameter
before the reaction), the diameter of unreacted Si nanopowder is estimated to be
17.1 nm. From the diameter of unreacted Si nanopowder, its weight, and the weight
of formed SiO 2 , the thickness of formed SiO 2 is estimated to be 5.1 nm, in good
agreement with that estimated from XPS measurements, i.e., 4.8 nm.
9.2.3.2 Reaction of Si Nanopowder with Neutral Water
Si nanopowder was produced from Si powder for hydrogen generation experiments
with neutral water. The size of Si nanopowder is larger than that produced from Si
swarf, i.e., for one-step milling, the mode diameter, the median diameter, and the
average diameter are 6.6, 14.0, and 23.4 nm, respectively, and those for two-step
milling are 5.8, 9.6, and 13.8 nm, respectively.
Figure 9.17 shows the hydrogen volume generated by the reaction with water
in the neutral pH region vs. the reaction time. Even in the case of the reaction
with ultrapure water of pH 7.0, Si nanopowder generates hydrogen (plot a), but
the hydrogen generation rate is low, i.e., 0.026∼0.047 mL/min.g. The hydrogen
generation rate increases by the reaction with tap water having pH 7.4 (plot b). The
hydrogen generation rate further increases by increases of pH to 8.0 (plot c) and 8.6