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Topics in Current Chemistry (2020) 378:7
This method has been found to be very effective for incorporating dopants into
the crystalline structure of TiO 2 or ZnO. Because of the high photocatalytic activity achieved by the controlled synthesis of hollow TiO 2 particles, it has attracted
much interest among the scientific community [53]. For example, Zhou et al. prepared flower-like F-doped TiO 2 hollow microspheres using a hydrothermal synthesis
method by controlling the hydrolysis of TiF 4 in a autoclave lined with Teflon at a
reaction temperature of 180 °C [54].
2.2.3 Precipitation Method
The preparation of photocatalysts through the precipitation method consists of the
chemical transformation of a highly soluble metal precursor into another substance
of lower solubility, which precipitates in solution. The conversion into the low-solubility compound (and then into the precipitate) is usually obtained by changing
(generally by increasing) the pH of the solution [55]. To avoid a rapid precipitation
in solution (that can cause a strong increase in particle size), it is better if the mixing and the generation of the precipitant are carried out separately. At the laboratory scale, this is possible through the use of a base. The precipitation method using
a base has been applied for the preparation of various catalysts. Upon increasing
the pH of the solution, the precipitation of a hydroxide is induced. The semiconductor typically prepared through this method is ZnO. In particular, the preparation involves the reaction of zinc salts such as Zn(NO 3 ) 2 , Zn(CH 3 COO) 2 , or ZnSO 4
with solutions containing NH 4 OH, NaOH, etc. [56, 57]. For the doping of ZnO with
metals in order to shift its absorption to the visible region, the precursor salt of the
doping element can be added to the solution of the zinc precursor before inducing
precipitation [9, 58, 59]. The obtained precipitate is then transformed into the ZnOdoped photocatalyst through thermal treatment.
2.2.4 Solution Combustion Synthesis
Solution combustion synthesis (SCS) is a well-known synthesis method used for
the preparation of inorganic compounds for many catalytic, photocatalytic, and
electrocatalytic applications. The method is based on the redox reactions that take
place between a fuel and an oxidant in the presence of metal cations. Oxidants are
metal precursors such as metal nitrates, while fuel is an organic material such as
glycine, urea or citric acid. The final products of this synthesis are characterized by
high purity, narrow particle distribution, and good agglomeration [60, 61]. Another
advantage is the possibility of using different precursors, both soluble and insoluble.
Solution combustion synthesis is characterized by three main steps: (1) the formation of the combustion mixture, (2) the formation of the gel, and (3) the gel combustion [62]. A schematic description of these three steps is shown in Fig. 2 [62].
The metal precursors are mixed in water solution with an organic fuel. The product
obtained at the end of the combustion process is a soft powder, typical of combustion synthesis processes whose characteristics depend on the parameters chosen for
the synthesis, such as the type of fuel.
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Reprinted from the journal
Topics in Current Chemistry (2020) 378:7
This method has been found to be very effective for incorporating dopants into
the crystalline structure of TiO 2 or ZnO. Because of the high photocatalytic activity achieved by the controlled synthesis of hollow TiO 2 particles, it has attracted
much interest among the scientific community [53]. For example, Zhou et al. prepared flower-like F-doped TiO 2 hollow microspheres using a hydrothermal synthesis
method by controlling the hydrolysis of TiF 4 in a autoclave lined with Teflon at a
reaction temperature of 180 °C [54].
2.2.3 Precipitation Method
The preparation of photocatalysts through the precipitation method consists of the
chemical transformation of a highly soluble metal precursor into another substance
of lower solubility, which precipitates in solution. The conversion into the low-solubility compound (and then into the precipitate) is usually obtained by changing
(generally by increasing) the pH of the solution [55]. To avoid a rapid precipitation
in solution (that can cause a strong increase in particle size), it is better if the mixing and the generation of the precipitant are carried out separately. At the laboratory scale, this is possible through the use of a base. The precipitation method using
a base has been applied for the preparation of various catalysts. Upon increasing
the pH of the solution, the precipitation of a hydroxide is induced. The semiconductor typically prepared through this method is ZnO. In particular, the preparation involves the reaction of zinc salts such as Zn(NO 3 ) 2 , Zn(CH 3 COO) 2 , or ZnSO 4
with solutions containing NH 4 OH, NaOH, etc. [56, 57]. For the doping of ZnO with
metals in order to shift its absorption to the visible region, the precursor salt of the
doping element can be added to the solution of the zinc precursor before inducing
precipitation [9, 58, 59]. The obtained precipitate is then transformed into the ZnOdoped photocatalyst through thermal treatment.
2.2.4 Solution Combustion Synthesis
Solution combustion synthesis (SCS) is a well-known synthesis method used for
the preparation of inorganic compounds for many catalytic, photocatalytic, and
electrocatalytic applications. The method is based on the redox reactions that take
place between a fuel and an oxidant in the presence of metal cations. Oxidants are
metal precursors such as metal nitrates, while fuel is an organic material such as
glycine, urea or citric acid. The final products of this synthesis are characterized by
high purity, narrow particle distribution, and good agglomeration [60, 61]. Another
advantage is the possibility of using different precursors, both soluble and insoluble.
Solution combustion synthesis is characterized by three main steps: (1) the formation of the combustion mixture, (2) the formation of the gel, and (3) the gel combustion [62]. A schematic description of these three steps is shown in Fig. 2 [62].
The metal precursors are mixed in water solution with an organic fuel. The product
obtained at the end of the combustion process is a soft powder, typical of combustion synthesis processes whose characteristics depend on the parameters chosen for
the synthesis, such as the type of fuel.
231
Reprinted from the journal