Topics in Current Chemistry (2020) 378:29
1 3
photocatalytic oxidation [11]. Some additives help to enhance the basic cavitational
effects, and these same additives will be equally helpful to enhance catalyst synthesis, though the alterations in catalyst purity need to be looked at. The presence
of solid particles or gases help to provide additional nuclei due to the introduced
surface heterogeneity in the continuous liquid medium, resulting in a higher number
of nuclei. It is also important to note that there is an optimum loading as too much
presence will lead to scattering effects and hence reduced energy transfer. The use
of salts can help in pushing the organic molecules towards the site of cavity collapse, giving higher exposure to cavitating conditions and hence higher extents of
degradation in the combined operation of ultrasound and photocatalytic oxidation
[13]. Catalysts can provide both the effects of more nuclei and enhanced formation
of free radicals based on the intrinsic chemical mechanisms; for example, the presence of TiO 2 or CuO bestows surface activation together with heterogeneous nucleation, often yielding synergistic effects in combination approach. The use of radical
promoters, such as hydrogen peroxide or carbon tetrachloride or ozone, enhances
the formation of radicals due to the dissociation of the added compounds, which can
result in a higher extent of degradation [14, 15] depending on the type of pollutant.
3 Improvements in Catalyst Synthesis Based on the Use
of Ultrasound
Various methods have been conventionally developed for the synthesis of metal
nanoparticles to be used as photocatalysts. Each method has also been subjected to
variations to arrive at best conditions to control the size, shape, structure and properties of the photocatalyst. The choice and composition of the precursor is also a
very important factor in determining the characteristics of the synthesized catalyst.
Various conventional methods of synthesis include chemical vapour deposition, laser
ablation, physical evaporation, solvo-thermal processes, among others [16]. Conventional methods are often associated with several drawbacks, including long synthesis
times (up to several days), requirement for various chemicals, including solvents, and
energy-intensive conditions. In addition, it is difficult to efficiently remove the solvents and precursors used in conventional synthesis methods, as well as in maintaining the uniformity of the nanoparticles [16]. For example, the conventional method
of synthesizing mesoporous titanium photocatalysts involves the use of alkyl phosphate surfactants [17]; however, the phosphorous becomes bound to the particles and
there is subsequently great difficulty in removing it. Similarly, the other conventionally used processes, such as calcination and solvent extraction, are also ineffective in
terms of giving the best catalyst morphology and required purity. Multiple reports on
phosphorus-free mesoporous titanium catalysts have been published [18, 19], but the
use of phosphorus-free titania requires very long synthesis times (up to 15 days) [20].
Similarly, the synthesis of CdSe photocatalysts requires thermal treatment as one of
the important processing steps; this process process is highly energy intensive and
demonstrates difficulty in maintaining uniform crystalline distribution [21].
Ultrasound-assisted synthesis of photocatalysts has been receiving considerable
attention in the past few years. The local hotspots during ultrasonic cavitation are
74
Reprinted from the journal
1 3
photocatalytic oxidation [11]. Some additives help to enhance the basic cavitational
effects, and these same additives will be equally helpful to enhance catalyst synthesis, though the alterations in catalyst purity need to be looked at. The presence
of solid particles or gases help to provide additional nuclei due to the introduced
surface heterogeneity in the continuous liquid medium, resulting in a higher number
of nuclei. It is also important to note that there is an optimum loading as too much
presence will lead to scattering effects and hence reduced energy transfer. The use
of salts can help in pushing the organic molecules towards the site of cavity collapse, giving higher exposure to cavitating conditions and hence higher extents of
degradation in the combined operation of ultrasound and photocatalytic oxidation
[13]. Catalysts can provide both the effects of more nuclei and enhanced formation
of free radicals based on the intrinsic chemical mechanisms; for example, the presence of TiO 2 or CuO bestows surface activation together with heterogeneous nucleation, often yielding synergistic effects in combination approach. The use of radical
promoters, such as hydrogen peroxide or carbon tetrachloride or ozone, enhances
the formation of radicals due to the dissociation of the added compounds, which can
result in a higher extent of degradation [14, 15] depending on the type of pollutant.
3 Improvements in Catalyst Synthesis Based on the Use
of Ultrasound
Various methods have been conventionally developed for the synthesis of metal
nanoparticles to be used as photocatalysts. Each method has also been subjected to
variations to arrive at best conditions to control the size, shape, structure and properties of the photocatalyst. The choice and composition of the precursor is also a
very important factor in determining the characteristics of the synthesized catalyst.
Various conventional methods of synthesis include chemical vapour deposition, laser
ablation, physical evaporation, solvo-thermal processes, among others [16]. Conventional methods are often associated with several drawbacks, including long synthesis
times (up to several days), requirement for various chemicals, including solvents, and
energy-intensive conditions. In addition, it is difficult to efficiently remove the solvents and precursors used in conventional synthesis methods, as well as in maintaining the uniformity of the nanoparticles [16]. For example, the conventional method
of synthesizing mesoporous titanium photocatalysts involves the use of alkyl phosphate surfactants [17]; however, the phosphorous becomes bound to the particles and
there is subsequently great difficulty in removing it. Similarly, the other conventionally used processes, such as calcination and solvent extraction, are also ineffective in
terms of giving the best catalyst morphology and required purity. Multiple reports on
phosphorus-free mesoporous titanium catalysts have been published [18, 19], but the
use of phosphorus-free titania requires very long synthesis times (up to 15 days) [20].
Similarly, the synthesis of CdSe photocatalysts requires thermal treatment as one of
the important processing steps; this process process is highly energy intensive and
demonstrates difficulty in maintaining uniform crystalline distribution [21].
Ultrasound-assisted synthesis of photocatalysts has been receiving considerable
attention in the past few years. The local hotspots during ultrasonic cavitation are
74
Reprinted from the journal