13.3 Synthesis Strategies
In past 20 years, many methods have been developed to prepare AgX photocatalytic
materials. Most of these methods are mainly based on the extremely low solubility of
AgX. Generally, these methods can be classified into three synthesis strategies:
liquid–solid precipitation, in situ oxidation transformation, and ion exchange.
13.3.1 Liquid–Solid Precipitation
As we all know, in an aqueous solution, the activities of Ag ions [Ag
+
] and X ions [X
À ]
are related to the solubility product K sp ([Ag
+
] [X
+
] ¼ K sp ). The relation between
ion concentration and precipitation process of AgX is described in Fig. 13.2, in
which C s , C
*
min , and C
*
max represented the concentration corresponding to solubility, the critical supersaturation (the minimum concentration of nucleation), and the
limiting supersaturation (the maximum concentration of nucleation), respectively.
At the initial stage of adding Ag
+ and X
À ions (Stage I), no AgX nucleus can
be formed because the concentration is below C
*
min . When the concentration
exceeds C
*
min (Stage II), with the continuous addition of Ag
+ and X
À
, the spontaneous nucleation of AgX takes place rapidly and the concentration begins to
decrease. When the concentration is below C
*
min but higher than C s (Stage III),
the nucleation of AgX ceases and the AgX crystal nuclei grow gradually with the
continuous addition of Ag
+ and X
À ions. In order to control the uniformity of crystal
size, it is important to keep the concentration lower than C
*
min after the initial
nucleation finished to prevent the occurrence of further nucleation.
Table 13.1 CB level, VB
level, and band gap of AgCl,
AgBr, and AgI(α)
Name
CB level (eV)
VB level (eV)
Band gap (eV)
AgCl
À1.20
2.10
3.30
AgBr
À0.50
2.15
2.65
AgI(α)
À0.42
2.38
2.80
Fig. 13.2 Illustration of the
nucleation and growth of
AgX crystal during
precipitation process, in
which C s is the solubility
while C
*
min and C
*
max are
the minimum and maximum
concentration of nucleation,
respectively
13.3 Synthesis Strategies
309
In past 20 years, many methods have been developed to prepare AgX photocatalytic
materials. Most of these methods are mainly based on the extremely low solubility of
AgX. Generally, these methods can be classified into three synthesis strategies:
liquid–solid precipitation, in situ oxidation transformation, and ion exchange.
13.3.1 Liquid–Solid Precipitation
As we all know, in an aqueous solution, the activities of Ag ions [Ag
+
] and X ions [X
À ]
are related to the solubility product K sp ([Ag
+
] [X
+
] ¼ K sp ). The relation between
ion concentration and precipitation process of AgX is described in Fig. 13.2, in
which C s , C
*
min , and C
*
max represented the concentration corresponding to solubility, the critical supersaturation (the minimum concentration of nucleation), and the
limiting supersaturation (the maximum concentration of nucleation), respectively.
At the initial stage of adding Ag
+ and X
À ions (Stage I), no AgX nucleus can
be formed because the concentration is below C
*
min . When the concentration
exceeds C
*
min (Stage II), with the continuous addition of Ag
+ and X
À
, the spontaneous nucleation of AgX takes place rapidly and the concentration begins to
decrease. When the concentration is below C
*
min but higher than C s (Stage III),
the nucleation of AgX ceases and the AgX crystal nuclei grow gradually with the
continuous addition of Ag
+ and X
À ions. In order to control the uniformity of crystal
size, it is important to keep the concentration lower than C
*
min after the initial
nucleation finished to prevent the occurrence of further nucleation.
Table 13.1 CB level, VB
level, and band gap of AgCl,
AgBr, and AgI(α)
Name
CB level (eV)
VB level (eV)
Band gap (eV)
AgCl
À1.20
2.10
3.30
AgBr
À0.50
2.15
2.65
AgI(α)
À0.42
2.38
2.80
Fig. 13.2 Illustration of the
nucleation and growth of
AgX crystal during
precipitation process, in
which C s is the solubility
while C
*
min and C
*
max are
the minimum and maximum
concentration of nucleation,
respectively
13.3 Synthesis Strategies
309