4. Results and discussion

quadrupole mass spectrometry. In the initial 20 seconds of each trial, the quadrupole mass spectrometer was operated without applying visible light irradiation, after which visible light irradiation was applied to the specimen until hydrogen and oxygen generation was detected

The specimen produced by ejector cavitation processing (as shown in Figure 11) did not generate hydrogen through water splitting in response to visible light irradiation, whereas hydrogen generation did occur upon UV irradiation. This can be explained by noting that the energy of UV light is greater than that of visible light. Because the band gap associated with water splitting by the photocatalyst is in the range 3.0–3.2 eV, the generation of holes in the valence band and the movement of electrons to the conduction band are both difficult when the increased reaction points are generated only by nanolevel ejector processing. As noted, multifunction cavitation results in both mechanical and electrochemical processing by the microjet as a consequence of the presence of hotspots and their chemical reaction field. Therefore, reductions in the band gap and the promotion of water splitting would be expected. In present study, the surface potential images of titanium oxide particles was measured by KFM

A Kelvin force microscope (KFM) was used to measure surface potential. A KFM observes specimen morphology and potential by variations in the work function and the contact potential. "Work function" is the energy required in order to extract a single electron from the surface of the substance, or, in other words, how easy it is to extract hydrogen. This is

based on their respective peaks.

60 Cavitation - Selected Issues

(Kelvin Probe Force Microscope).

Figure 13. Schematic diagram of the Kelvin force microscope.

diagrammed in Figure 13.

Chemical reactions of the ITO film on the soda-lime glass can also be induced by multifunction cavitation, as shown in Figure 14. The ITO film is composed of In2O3 and SnO2 and its melting point is in the range of 1800–2200 K, while the soda-lime glass on which the ITO film is deposited has a melting point of 1270 K and is made from a mixture of SO2, Na2CO3, and CaCO3. The microjet hot spots were found to peel the ITO film from the glass to generate particles consisting of a combination of the ITO film and the soda-lime glass.

Figure 15 presents a surface FE-SEM image of TiO2 particles supporting minute Pt particles after EC processing. Nanoscale roughness was formed on the surface of the TiO2 particles after the cavitation processing. The pressure of cavitation collapse was estimated to be approximately 1000 MPa. It is considered that the titanium surface was processed by the micro-jet of cavitation, when the cavitation approaches to the surface of the titanium particles in the ejector reactor. Because the exit of the ejector nozzle is narrow and has a small gap, the particles are very likely to encounter the cavitation and suffer the micro-jet fabrication in the ejector nozzle. Because the specific surface area of the titanium oxide particles increases, the reaction activation point changes, and the amount of gases generated should increase under ultraviolet irradiation.


Figure 14. Chemical reactions of an ITO film on soda-lime glass following multifunction cavitation (a) region at which the ITO film was not removed; (b) region at which the ITO film was removed.

is 3.0 eV at a wavelength of 413 nm, whereas the bandgap of anatase-type TiO2 is 3.2 eV at a wavelength of 388 nm. In this study, a mechanical cavitation jet was used for surface processing in an ejector reactor. If the sub-stream contains not only TiO2 but also Pt but also a chemical, mechanochemical cavitation is generated. This mechanochemical cavitation has a synergy of mechanical processing and chemical processing in the study of ballast water treatment [19, 20] and in the study of improvement of corrosion resistance. The bandgap water splitting should be

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When water is irradiated with ultrasound, acoustic cavitation occurs with the concurrent generation of active oxygen species and shock waves [21–23]. Previously, the use of ultrasound was investigated for disaggregation of agglomerated particles and surface modification of diamond nanoparticles [24]. Coating of host particles with guest nanoparticles by ultrasonic irradiation in liquid CO2 has also been studied [25, 26], as well as mixing and conjugation of nanoparticles using ultrasonic cavitation in high-pressure liquid CO2 [27, 28]. Sonodynamic therapy was applied to cancer cells based on the delivery of titanium oxide (TiO2) nanoparticles modified with avidin protein, which preferentially discriminated cancerous cells from healthy cells [29]. Figure 18 compares the amount of hydrogen, oxygen and water generated from EC-processed, ultrasonic cavitation (UC)-processed and stir-processed TiO2 particles supporting Pt (TiO2 + 9.1 wt% Pt). It can be seen that gas generation for the UC-processed particles is less than that for the EC-processed material. In this study, the EC processing technique was investigated with the aim of obtaining a new photocatalytic material, and it was shown to represent a potentially useful approach to nanoscale surface processing and synthesis of hybrids of different particles, with applications not only in the field of photoc-

It was found that the work ratio generated by multifunction cavitation could be varied by tuning the pressure and processing duration as well as the power and frequency of the ultrasonic wave. The balance between hot treatment by hot spots in the microjet and high pressure physical working by the microjet is thus determined by both the waterjet and

Figure 17. Comparison of EC processing, UC processing and stir processing with regard to amount of gas generation

decreased by the processing of mechanochemical cavitation.

atalysis but also other industrial processes (Figure 17).

from TiO2 particles supporting Pt (TiO2 + 9.1 wt% Pt) under UV irradiation.

Figure 15. FE-SEM images of TiO2 particles supported by Pt particles as a co-catalyst after cavitation processing (100,000). (a) Pt particle mixed with TiO2 and (b) minute Pt particles supporting TiO2 particles.

The Pt nanoparticles were detached from the surfaces of the original Pt particles and attached to the surfaces of the TiO2 particles, as shown in Figure 15. Such nanoparticles are thought to act as a co-catalyst to promote the photocatalytic reaction. Conventionally, Pt addition is performed using an electrodeposition method [16], in which Pt becomes attached to the titanium oxide surface by stirring Pt and TiO2 particles in a methanol/water solution under UV irradiation for about 40 hours [16]. In contrast, it takes only a few seconds to add Pt nanoparticles to the titanium oxide surface in case of EC processing. Thus, EC processing is an effective method not only for nanoscale surface processing but also for supporting one substance on another.

After treating the mixture of TiO2 and Pt particles with multifunction cavitation, nanoscale TiO2 particles were obtained and were found to agglomerate to produce porous structures, as shown in Figure 16. The microjets resulting from floating cavitation of the waterjet were not able to fabricate nanoscale TiO2 particles such as these. In addition, the hot work performed by the microjet hot spots tended to melt the TiO2 particles. It should be noted that Pt co-catalyst particles were also generated during the multifunction cavitation process and were intermingled with the molten, porous TiO2.

It may be premature to expect that surface treatment by micro-jet can enhance the photocatalytic properties in emulsified water under visible light. However, the bandgap of rutile-type TiO2

Figure 16. FE-SEM images of titanium oxide particles (a) as-received, (b) after WJ processing, (c) after MFC processing (100,000).

is 3.0 eV at a wavelength of 413 nm, whereas the bandgap of anatase-type TiO2 is 3.2 eV at a wavelength of 388 nm. In this study, a mechanical cavitation jet was used for surface processing in an ejector reactor. If the sub-stream contains not only TiO2 but also Pt but also a chemical, mechanochemical cavitation is generated. This mechanochemical cavitation has a synergy of mechanical processing and chemical processing in the study of ballast water treatment [19, 20] and in the study of improvement of corrosion resistance. The bandgap water splitting should be decreased by the processing of mechanochemical cavitation.

When water is irradiated with ultrasound, acoustic cavitation occurs with the concurrent generation of active oxygen species and shock waves [21–23]. Previously, the use of ultrasound was investigated for disaggregation of agglomerated particles and surface modification of diamond nanoparticles [24]. Coating of host particles with guest nanoparticles by ultrasonic irradiation in liquid CO2 has also been studied [25, 26], as well as mixing and conjugation of nanoparticles using ultrasonic cavitation in high-pressure liquid CO2 [27, 28]. Sonodynamic therapy was applied to cancer cells based on the delivery of titanium oxide (TiO2) nanoparticles modified with avidin protein, which preferentially discriminated cancerous cells from healthy cells [29]. Figure 18 compares the amount of hydrogen, oxygen and water generated from EC-processed, ultrasonic cavitation (UC)-processed and stir-processed TiO2 particles supporting Pt (TiO2 + 9.1 wt% Pt). It can be seen that gas generation for the UC-processed particles is less than that for the EC-processed material. In this study, the EC processing technique was investigated with the aim of obtaining a new photocatalytic material, and it was shown to represent a potentially useful approach to nanoscale surface processing and synthesis of hybrids of different particles, with applications not only in the field of photocatalysis but also other industrial processes (Figure 17).

The Pt nanoparticles were detached from the surfaces of the original Pt particles and attached to the surfaces of the TiO2 particles, as shown in Figure 15. Such nanoparticles are thought to act as a co-catalyst to promote the photocatalytic reaction. Conventionally, Pt addition is performed using an electrodeposition method [16], in which Pt becomes attached to the titanium oxide surface by stirring Pt and TiO2 particles in a methanol/water solution under UV irradiation for about 40 hours [16]. In contrast, it takes only a few seconds to add Pt nanoparticles to the titanium oxide surface in case of EC processing. Thus, EC processing is an effective method not only for nanoscale surface processing but also for supporting one

Figure 15. FE-SEM images of TiO2 particles supported by Pt particles as a co-catalyst after cavitation processing

(100,000). (a) Pt particle mixed with TiO2 and (b) minute Pt particles supporting TiO2 particles.

After treating the mixture of TiO2 and Pt particles with multifunction cavitation, nanoscale TiO2 particles were obtained and were found to agglomerate to produce porous structures, as shown in Figure 16. The microjets resulting from floating cavitation of the waterjet were not able to fabricate nanoscale TiO2 particles such as these. In addition, the hot work performed by the microjet hot spots tended to melt the TiO2 particles. It should be noted that Pt co-catalyst particles were also generated during the multifunction cavitation process and were

It may be premature to expect that surface treatment by micro-jet can enhance the photocatalytic properties in emulsified water under visible light. However, the bandgap of rutile-type TiO2

Figure 16. FE-SEM images of titanium oxide particles (a) as-received, (b) after WJ processing, (c) after MFC processing

substance on another.

62 Cavitation - Selected Issues

(100,000).

intermingled with the molten, porous TiO2.

It was found that the work ratio generated by multifunction cavitation could be varied by tuning the pressure and processing duration as well as the power and frequency of the ultrasonic wave. The balance between hot treatment by hot spots in the microjet and high pressure physical working by the microjet is thus determined by both the waterjet and

Figure 17. Comparison of EC processing, UC processing and stir processing with regard to amount of gas generation from TiO2 particles supporting Pt (TiO2 + 9.1 wt% Pt) under UV irradiation.

ultrasonication parameters. In the case of TiO2, the highest work ratio was obtained when applying a waterjet pressure of 35 MPa together with a duration of 2 min, an ultrasonication power in the range of 150–300 W (ideally 225 W) and frequencies of 28, 40, and 100 kHz (with 28 kHz being optimal).

potential reaction field of multifunction cavitation, these impurities will gather locally in the

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Figure 19 demonstrates the enhanced visible light photocatalytic properties obtained by processing with multifunction cavitation. The levels of hydrogen and oxygen that were generated following multifunction cavitation were greatly increased compared to the quantities obtained from material treated with standard waterjet processing. Interestingly, even the waterjet processing was more effective than the conventional approach, in which TiO2 and Pt particles undergo simple mixing in solution via a magnetic stirring mechanism. It should be noted that the quantities of hydrogen and oxygen produced by water splitting under visible light irradiation were in accordance with the expected stoichiometric ratio, indicating efficient

As experiments showing the effectiveness of MFC, experimental results of WJC processing, UC processing, UC processing after WJC processing, and WJC processing after UC processing

A significant increase in the amount of hydrogen generation by MFC processing compared to the other processing indicates that high-temperature and high-pressure microjet is generated, suggesting an increase in the water splitting reaction point due to the increase in surface area. It can also be realized from Figure 17(b) and (c). In addition, there is a high possibility that the band gap of water splitting by MFC treatment was reduced. Compared with other treatment methods, the amount of hydrogen generation dramatically increased in MFC treatment, and it became clear that microjet with hot spot is effective for water splitting. Also from the result of oxygen generation shown in Figure 19, it was confirmed that the amount of oxygen generation

Figure 19. Visible light photocatalytic generation of hydrogen by TiO2 following processing by multifunction cavitation

porous structure of the TiO2. In fact, NaCl crystals can be observed in Figure 16.

photocatalysis.

or waterjet cavitation.

were compared (Figures 20 and 21) [30].

dramatically increased by MFC treatment as well.

Figure 18 shows FE-SEM image and elemental maps for TiO2 particles supporting Pt particles and other compounds produced by processing with multifunction cavitation. As noted, the TiO2 powder was 99.997% pure and the Pt powder was more than 99.0% pure. Therefore, both materials contained low levels of impurities such as Al, Fe, Si, Na, Mg, and S. In the high

Figure 18. (a) FE-SEM image and elemental maps of (l) Ti, (k) Pt, and (d) O for TiO2 particles supporting Pt particles and other compounds of (b) C, (c) Na, (e) Mg, (f) Si, (g) S, (h) Al, (i) Ca, (j) Cl, produced by processing with multifunction cavitation.

potential reaction field of multifunction cavitation, these impurities will gather locally in the porous structure of the TiO2. In fact, NaCl crystals can be observed in Figure 16.

ultrasonication parameters. In the case of TiO2, the highest work ratio was obtained when applying a waterjet pressure of 35 MPa together with a duration of 2 min, an ultrasonication power in the range of 150–300 W (ideally 225 W) and frequencies of 28, 40, and 100 kHz (with

Figure 18 shows FE-SEM image and elemental maps for TiO2 particles supporting Pt particles and other compounds produced by processing with multifunction cavitation. As noted, the TiO2 powder was 99.997% pure and the Pt powder was more than 99.0% pure. Therefore, both materials contained low levels of impurities such as Al, Fe, Si, Na, Mg, and S. In the high

Figure 18. (a) FE-SEM image and elemental maps of (l) Ti, (k) Pt, and (d) O for TiO2 particles supporting Pt particles and other compounds of (b) C, (c) Na, (e) Mg, (f) Si, (g) S, (h) Al, (i) Ca, (j) Cl, produced by processing with multifunction

28 kHz being optimal).

64 Cavitation - Selected Issues

cavitation.

Figure 19 demonstrates the enhanced visible light photocatalytic properties obtained by processing with multifunction cavitation. The levels of hydrogen and oxygen that were generated following multifunction cavitation were greatly increased compared to the quantities obtained from material treated with standard waterjet processing. Interestingly, even the waterjet processing was more effective than the conventional approach, in which TiO2 and Pt particles undergo simple mixing in solution via a magnetic stirring mechanism. It should be noted that the quantities of hydrogen and oxygen produced by water splitting under visible light irradiation were in accordance with the expected stoichiometric ratio, indicating efficient photocatalysis.

As experiments showing the effectiveness of MFC, experimental results of WJC processing, UC processing, UC processing after WJC processing, and WJC processing after UC processing were compared (Figures 20 and 21) [30].

A significant increase in the amount of hydrogen generation by MFC processing compared to the other processing indicates that high-temperature and high-pressure microjet is generated, suggesting an increase in the water splitting reaction point due to the increase in surface area. It can also be realized from Figure 17(b) and (c). In addition, there is a high possibility that the band gap of water splitting by MFC treatment was reduced. Compared with other treatment methods, the amount of hydrogen generation dramatically increased in MFC treatment, and it became clear that microjet with hot spot is effective for water splitting. Also from the result of oxygen generation shown in Figure 19, it was confirmed that the amount of oxygen generation dramatically increased by MFC treatment as well.

Figure 19. Visible light photocatalytic generation of hydrogen by TiO2 following processing by multifunction cavitation or waterjet cavitation.

Figure 20. Hydrogen generation from various cavitation processed TiO2 particles supported by Pt particles.

Figure 21. Oxygen generation from various cavitation processed TiO2 particles supported by Pt particles.

Figures 22 and 23 present the results for surface potential. Figure 22 shows the results for rutile, and Figure 23, for anatase. The images on the left show the results before MFC processing, and those on the right, after processing. Table 1 also provides the measured values of the surface potential.

The results for surface potential in Figure 23 indicate that both rutile- and anatase-type TiO2 during processing by MFC decreases the surface potential. Also, as Table 1 shows, for an evaluation of the surface potential of a single particle of TiO2 powder, the mean surface

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Figure 22. Results of surface potential imaging (Rutile-type TiO2).

Figure 23. Results of surface potential imaging (Anatase-type TiO2).

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Figure 22. Results of surface potential imaging (Rutile-type TiO2).

Figure 23. Results of surface potential imaging (Anatase-type TiO2).

Figures 22 and 23 present the results for surface potential. Figure 22 shows the results for rutile, and Figure 23, for anatase. The images on the left show the results before MFC processing, and those on the right, after processing. Table 1 also provides the measured values

Figure 21. Oxygen generation from various cavitation processed TiO2 particles supported by Pt particles.

Figure 20. Hydrogen generation from various cavitation processed TiO2 particles supported by Pt particles.

of the surface potential.

66 Cavitation - Selected Issues

The results for surface potential in Figure 23 indicate that both rutile- and anatase-type TiO2 during processing by MFC decreases the surface potential. Also, as Table 1 shows, for an evaluation of the surface potential of a single particle of TiO2 powder, the mean surface


decreased to the surface potential of 0.172 V in case of rutile-type. This is a result of processing

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After processing titanium dioxide particles and platinum particles as a co-catalyst by cavitation jet using an ejector nozzle (EC) and the multifunction cavitation (MFC), the photocatalytic performance of the modified particles under ultraviolet radiation and visible light irradiation

1. The fundamental characteristics of multifunction cavitation was assessed theoretically and

2. In order to increase the temperature and pressure of MFC, the effect of additional swirl flow nozzle (SFN), which was put on a waterjet nozzle, was evaluated experimentally and

3. The amount of hydrogen, oxygen, and water released from the particles was increased by

4. Surface irregularities at the nanoscale were formed on the titanium oxide surface by EC

5. TiO2 particles were found to support Pt nanoparticles, which were detached from the

6. Materials processed by EC exhibited a higher photocatalytic performance than those

7. The application of ultrasonication to the floating cavitation of a waterjet was found to

8. Multifunction cavitation exhibited the capacity to perform nanolevel hot working at a material surface, modifying the surface morphology and the surface electrochemical con-

9. The balance between hot treatment by hot spots in the microjet and working due to the high pressure in the microjet was determined by the waterjet and ultrasonication power conditions.

10. The amounts of hydrogen and oxygen generated by titanium dioxide particles in response to visible light was remarkably increased following treatment by multifunction cavitation

11. Doping with Pt using multifunction cavitation processing lowered the surface potential. 12. The band gap energy was also reduced concomitantly with the reduction of the surface potential, and this improved the efficiency of generation of H in the dissociation of water.

compared to the results obtained following waterjet (EC) processing.

by ultra-high temperature and pressure cavitation.

was examined. The following results were obtained.

original Pt particles during EC processing.

processed by ultrasonic cavitation (UC).

produce microjets containing hot spots.

dition by hot spot melting.

5. Conclusions

experimentally.

theoretically.

EC processing.

processing.

Table 1. Measured surface potential values.

potential of a rutile-type TiO2 before MFC processing was 0.554 V, while it fell to 0.272 V after processing, a difference of �0.282 V in the contact potential. In anatase-type TiO2, the mean surface potential before MFC processing was 9.928 V, but it fell to 0.166 V after processing, a difference of �9.762 V in the contact potential. The anatase-type TiO2 showed a far higher reduction than the rutile-type TiO2. This advantage of the anatase-type TiO2 is consistent with its superior results in dissociation of water and with the already-known lower band gap of rutile-type TiO2 under visible light.

Table 2 shows how the measured volume of evolved H as measured with a QMS compared with the surface potential results. In rutile-type specimens, the surface potential fell after processing, while the volume of evolved H increased. This is consistent with the promotion of H generation by the low band gap. There was also a correlation between the difference between evolved H volume and surface potential in both rutile- and anatase-type specimens. In rutile-type specimens, the surface potential fell after processing, while the volume of evolved H increased. This is consistent with the promotion of H generation by the low band gap. The particles processed by swirling taper nozzle (inflow hole: 2 pieces) of Figure 4 further


Table 2. Relationship between surface potential and H + H2 pressure.

decreased to the surface potential of 0.172 V in case of rutile-type. This is a result of processing by ultra-high temperature and pressure cavitation.
