1 3
Topics in Current Chemistry (2020) 378:7
of the semiconductor. Generally, doping with metal ions is able to generate further
energy levels between the valence band and conduction band of the undoped semiconductor, shifting the absorption properties in the visible region due to the decrease
in bandgap energy of TiO 2 or ZnO. Several metals have been investigated in the synthesis of second-generation visible active photocatalysts, such as Mn, Fe, Co, Ni, Cu
for ZnO [5, 30, 31] or Mo, Cr, La, Er, Ce for TiO 2 [32]. The main drawback of this
type of doping is that metal ions could act as recombination centers of electron–hole
pairs, reducing the photocatalytic activity [33, 34].
2.1.3 Photocatalyst Doping with Non‑Metals
With together the second-generation visible active photocatalysts are of concern,
ZnO or TiO 2 doped with non-metals element (third-generation visible active photocatalysts) have been widely investigated. The doping with non-metals can significantly extend the visible light absorption of the doped-photocatalysts and minimize
photogenerated charge recombination [35]. Doping process with non-metals (mainly
anion of C, N, F, P or S) aims to replace oxygen atoms with these elements in the
semiconductor lattice. The high activity under visible light irradiation of the thirdgeneration visible active photocatalysts is due to the decrease of their bandgap by
mixing p orbital of the anions with 2p orbital of oxygen [32, 36]. Moreover, the
inclusion of these elements in the semiconductor crystalline structure causes the formation of some defects that delay the recombination of the photo-excited species
[37, 38].
2.2 Photocatalyst Preparation Methods
One of the main limitations of the application of photocatalytic processes to wastewater treatment at full scale is related to the preparation of the photocatalysts, which
significantly affects the cost of the process, particularly compared with homogenous
photo-driven AOPs [103, 150]. Unlike HPC technology, which is expensive to produce in terms of photocatalyst preparation and reactor design, UV/H 2 O 2 is easy to
implement, and H 2 O 2 is widely available commercially. In order to provide a contribution to fill this gap, the easiest and possibly most cost-effective preparation
methods for the synthesis of pure or doped semiconductors are summarized and discussed in the following subsections. A comparison summarizing the main advantages, disadvantages, and prospective applications at large industrial scale is also
proposed.
2.2.1 Sol–Gel Method
Sol–gel is the most commonly used method for the preparation of photocatalysts. A
sol is made by solid particles homogeneously dispersed in a liquid medium in colloidal form, whereas a gel is an organized three-dimensional continuous solid arrangement having sub-micrometer-sized pores in which the liquid phase is present. During sol–gel synthesis, the sol is produced from the hydrolysis and polymerization
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Topics in Current Chemistry (2020) 378:7
of the semiconductor. Generally, doping with metal ions is able to generate further
energy levels between the valence band and conduction band of the undoped semiconductor, shifting the absorption properties in the visible region due to the decrease
in bandgap energy of TiO 2 or ZnO. Several metals have been investigated in the synthesis of second-generation visible active photocatalysts, such as Mn, Fe, Co, Ni, Cu
for ZnO [5, 30, 31] or Mo, Cr, La, Er, Ce for TiO 2 [32]. The main drawback of this
type of doping is that metal ions could act as recombination centers of electron–hole
pairs, reducing the photocatalytic activity [33, 34].
2.1.3 Photocatalyst Doping with Non‑Metals
With together the second-generation visible active photocatalysts are of concern,
ZnO or TiO 2 doped with non-metals element (third-generation visible active photocatalysts) have been widely investigated. The doping with non-metals can significantly extend the visible light absorption of the doped-photocatalysts and minimize
photogenerated charge recombination [35]. Doping process with non-metals (mainly
anion of C, N, F, P or S) aims to replace oxygen atoms with these elements in the
semiconductor lattice. The high activity under visible light irradiation of the thirdgeneration visible active photocatalysts is due to the decrease of their bandgap by
mixing p orbital of the anions with 2p orbital of oxygen [32, 36]. Moreover, the
inclusion of these elements in the semiconductor crystalline structure causes the formation of some defects that delay the recombination of the photo-excited species
[37, 38].
2.2 Photocatalyst Preparation Methods
One of the main limitations of the application of photocatalytic processes to wastewater treatment at full scale is related to the preparation of the photocatalysts, which
significantly affects the cost of the process, particularly compared with homogenous
photo-driven AOPs [103, 150]. Unlike HPC technology, which is expensive to produce in terms of photocatalyst preparation and reactor design, UV/H 2 O 2 is easy to
implement, and H 2 O 2 is widely available commercially. In order to provide a contribution to fill this gap, the easiest and possibly most cost-effective preparation
methods for the synthesis of pure or doped semiconductors are summarized and discussed in the following subsections. A comparison summarizing the main advantages, disadvantages, and prospective applications at large industrial scale is also
proposed.
2.2.1 Sol–Gel Method
Sol–gel is the most commonly used method for the preparation of photocatalysts. A
sol is made by solid particles homogeneously dispersed in a liquid medium in colloidal form, whereas a gel is an organized three-dimensional continuous solid arrangement having sub-micrometer-sized pores in which the liquid phase is present. During sol–gel synthesis, the sol is produced from the hydrolysis and polymerization
229
Reprinted from the journal