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Abstract
This study proposes a method to improve the production efficiency and photocatalytic performance of TiO2 nanoparticles using the pulsed laser ablation in liquid (PLAL) method to optimise preparation parameters. In this study, the variation of particle size, morphology, preparation, and catalytic efficiency due to the increase in the number of pulses is studied. The mechanism of particle morphology change is analysed using thermodynamic simulation. The density functional theory (DFT) is used to calculate and characterise the reason why the special structure formed by particle breaking improves the photocatalytic performance. In addition, the influence of the law of solution height on particle breakage is summarised to obtain an optimised preparation parameter. The proposed method provides a reference for the selection of parameters in actual production.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the advancement of industrial modernisation, chemical dye products meet peoples’ needs for a better life. However, the output of dye wastewater is increasing year by year, which is difficult to biologically degrade, resulting in great challenges to the safety of our living environment [1,2]. Photocatalytic technologies using a catalyst were derived at the beginning of the 20th century, which could achieve an ideal degradation effect in a relatively short time. TiO2 nanoparticles are one of the most commonly used photocatalysts due to strong oxidation and reductive photocatalysts [3–5].
Due to the superiority of TiO2 nanoparticles, many scientists have studied its preparation methods [6–8], such as the wet chemical [9], hydrothermal [10], and precipitation [11] methods. However, the traditional preparation methods have complex processes and high preparation costs. The pulsed laser ablation in liquid (PLAL) method is a method of nanomaterial preparation with good application prospects. Compared with traditional methods, this method has the characteristics of environmental friendliness, simple operation, and low preparation costs [12–16]. In addition, the PLAL method has many advantages, such as safety, high efficiency, real-time monitoring and stable preparation parameters when using remote PC control [17,18].
However, there are many parameters that affect the characteristics of the final particles in the actual preparation process, which makes it difficult to control the size and characteristics of nanoparticles prepared by a laser [14,19–21]. Du et al. [19] found that the sizes of nanoparticles were different under the same laser parameters and different solution thicknesses of gold and silver targets. Hong et al. [22] continued the ablation of a solution containing TiO2 nanoparticles with a laser and found that smaller particles were obtained after secondary ablation. This suggests that the characteristics of nanoparticles can be changed by adjusting specific parameters. However, the law and mechanism of the morphology change of nanoparticles under laser ablation remains to be studied, and further research on the size change of nanoparticles caused by multi-parameter interaction is particularly important.
This study proposes a method to improve the production efficiency and photocatalytic performance of TiO2 nanoparticles to optimise preparation parameters. TiO2 nanoparticles were prepared using the PLAL method on a Ti target. Through experiments and simulations, the mechanism of the particle size and morphology of the nanoparticles modulated by multiple pulses was analysed and discussed, and the evolution law was obtained with an increase in the number of pulses. A photocatalytic degradation experiment of methylene blue solution was carried out with the prepared TiO2 nanoparticles. Based on the first-principle calculation, the evolution law of the photocatalytic effect and catalytic performance analysis of TiO2 nanoparticles with special morphology were obtained with an increase in the number of pulses. The influence of different solution height conditions on the characteristics of prepared particles was discussed and compared. The effects of pulse times and solution height on catalytic and production efficiencies were comprehensively discussed, and the optimised preparation parameters were obtained. It is of great significance to improve the preparation and catalytic efficiency of nanoparticles prepared using PLAL.
2. Experiment
2.1 Experimental facility
A schematic diagram of the PLAL device is shown in Fig. 1. The laser is a TEM00(Beamtech Optronics Co., Ltd) with an output wavelength of 1064 nm, pulse width of 12 ns, frequency of 3 Hz, and ablation time of 2 h. The target of the ultrasonic cleaning is a titanium metal sheet (> 99.99%) placed at the bottom of the beaker. Subsequently, it is slowly placed into a beaker with a deionised water solution. A transparent cover is placed over the beaker mouth to prevent the liquid from splashing. The laser output energy is regulated by the voltage during the experiment. The laser passes through a splitting ratio of 8:2, and a small part of the laser is reflected to the energy meter to monitor the laser energy. Cause in actual experiments, there are some fluctuations in the laser energy output, which will lead the size, morphology, structure and optical properties of NPs prepared by the PLAL method change [23–26]. This study focuses on the effect of pulse repetition frequency in laser parameters on particle characteristics, so the experimental process is real-time monitoring of the laser output energy through a spectroscope and energy meter. Most of the laser that is transmitted to the 1064 nm total reflector changes direction and finally focuses on the target surface in the beaker through a lens. The laser pulse energy is 230 mJ, spot size is approximately 2 mm2, and the energy density concentrated on the target is 11.5 J/cm2. The solution height (the distance between the water layer and target) is successively changed, ablating the target with 3, 5, 10, 15, 25, and 35 pulses. The number of pulses was carried out at one target material position and the total ablation time was 2 h. The three-dimensional displacement platform was automatically controlled to moved from one point to another, and the action time has been set simultaneously. The time for changing the ablation position is in the interval between two pulses, which ensure every laser pulse can work.
2.2 Experimental result
2.2.1 Variation of particle size
The product particles were observed using a scanning electron microscope (SEM). Under the action of continuous pulse at the same position, the size of the prepared TiO2 particles was calculated with different pulse times at a certain solution height. The observation results are as follows.
First, under the action of continuous pulses at the same position with a solution height of 2 mm, the TiO2 particle size gradually increased with the increase of the number of pulses, as shown in Fig. 2. Large particles with a proportion of more than 500 nm appeared under 10 pulses, and the number of large particles with a proportion of more than 500 nm decreased to a certain extent when the number of pulses was 15. With an increase in the number of pulses, the distribution range of the particle size gradually widened and the average particle size was steadily maintained at approximately 70 nm.
Through statistical analysis, the experimental particle size was small when the number of pulses was small (3-5 times), and the degree of preparation was lower. The particle size tended to increase with an increase in pulse times. When the pulse was continually increased, the average particle size obviously decreased. Finally, the particle size reached a dynamic equilibrium, which kept the particle size in a certain range.
2.2.2 Particle morphology
The morphology of TiO2 nanoparticles also changed with an increase in the number of pulses, as shown in Fig. 3. The particles were spherical with a small particle size (Mean: 30-50 nm) at a solution height of 2 mm when the number of pulses was low (3-5 times). In addition, the number of prepared particles was lower. This is called the production stage of small particles, as shown in Figs. 3(a) and (b).
Fig. 3. Morphology change and elemental analysis of TiO2 nanoparticles with an increase in the number of pulses. SEM images for: (a) and (b) production stage, (c)-(g) fusion stage, and (h)-(l) breakage stage.
As the pulse times increased (5-15 times), the previously received pulse from the preparation of TiO2 particles was suspended in the solution due to several secondary pulse laser plasma shock waves. However, even at this time, it appeared that particles gathered fusion. This is because the previous preparation of smaller particles was formed by the subsequent pulses from large size particles (Mean: 80-190 nm) under the action of fusion. This is called the particle fusion stage. The phenomenon of multi-particle contact fusion in a region is illustrated in the SEM images in Figs. 3(c) to (g). There are roughly three steps in the fusion process of two or more small particles into larger particles. First, particles contact each other and form a “gourd” shape, as shown in Figs. 3(d) and (e). Second, under the continuous extrusion of the shock wave caused by the subsequent pulse, the two particles continue to approach fusion, as shown in Fig. 3(f). The dividing line between the two particles is still relatively clear, and the arc of the particles on the opposite side is relatively round if one side is covered arbitrarily. This proves that the dividing line is formed by the fusion of two separate spherical particles under stress. The lower part of the lower particle is relatively flat, which is believed to be the stress mark. Third, the two particles continue to fuse under further extrusion pressure, extending the contact area. When there is one large and one small particle, a “hat-like” shape is formed, as shown in Fig. 3(g). These large nanoparticles are formed from coalescence of previously produced nanoparticles. In the previous research, Z. Aabdin et al. had made experimental observations on the grain fusion of gold nanoparticles in this process [27].
The small particles squeeze and fuse into large particles when they contact one other, which dominates for a period of time. Subsequently, the particle fusion stage reaches the threshold, and the average particle size at this time is the largest among the three stages. Under a continuous pulse action, the absorption of a shock wave increases with a larger particle size, and the particle crushing effect is enhanced. When the pulse is increased (15-35 times), the large-size particles are broken and the average particle size is obviously reduced (Mean: 70-130 nm). Moreover, some special structures appear due to the particle breakage, which is called the large-size particle breakage stage. The breakage is mainly divided into two categories, as shown in the SEM images in Fig. 3. First, the surface of the particles show cracks, holes, and a flake structure, as shown in Figs. 3(h) to (j). Second, a smaller particle from the interior of the larger particle is excited and appears to escape from the interior of the larger particle, creating a “snail” shape, as shown in Figs. 3(k) and (l).
Elements of TiO2 nanoparticles at the production stage with a crack structure generated in the breakage stage (points A, B, C, and D) were selected for elemental analysis. According to EDS analysis, the oxygen content in the outer layer of the particles with a broken structure was higher than that in the inner layer. This can be used to determine the different oxidation degrees of TiO2 nanoparticles prepared using the PLAL method.
As the number of pulses increases, the particle morphology changes can be described as the production, fusion, and breakage stages.
The fusion rate of the particles is critical to the overall catalytic degradation characteristics, and the particles morphologies has been observed to characterize the probability of particle fusion. Figure 4(a-c) is the representative SEM diagram for 3, 5, 10 pulses in 2 hours, and the fused particles were marked with yellow boxes. For 3 pulses, the NPs are sparse, the average particle size is the smallest (about 30 nm to 50 nm), and the probability of fused particles is about 11.7%. For 5 pulses, the number and size of NPs increased, and the probability of fused particles is about 25.4%. For 10 pulses, the NPs had a low dilution, and the probability is about 52.1%. It can be obtained that the fusion probability of particles increases with the action times of laser pulses as well as the laser plasma shock waves.
Fig. 4. SEM images of TiO2 nanoparticles were obtained in the case of (a)3 pulses (b) 5 pulses (c) 10 pulses at single ablation point for 2 hours.
2.2.3 Catalytic degradation rate
The nanoparticle size showed a trend of first increasing and then decreasing as the pulse number increased. Additionally, the nanoparticle size change continued this same cycle with further pulse number increases, but could no longer reach the previous number. This demonstrates that the effect of particle breakage was increasing at this time, thus the decreasing trend of particle size was reduced and the fusion and breakage stages reached dynamic equilibrium.
In order to ensure the same concentration, we collected the solution containing TiO2 nanoparticles after preparation, placed the solution in a test tube, centrifuged at 10,000 r/min for 10 minutes, and extracted and dried the nanoparticles. Under the conditions of 3, 5, 10, 15, 25 and 35 pulses, TiO2 NPs were collected in this way. In the experiment of catalytic degradation, we weighed 450 mg of TiO2 NPs collected and added it into methylene blue solution (200 mg/l, 50 ml) respectively, and placed under a UV light. The fusion effect of particles was weak when the number of pulses was low, and the prepared TiO2 nanoparticles had the smallest particle size and highest catalytic efficiency compared with other conditions. The catalytic efficiency was more than 10% higher than that of the large particles. However, in the actual preparation process, the production efficiency and output of nanoparticles prepared using few pulses were very low. Higher preparation efficiency was achieved using multiple pulses, and the catalytic efficiency remained high under the same conditions after the particle size reached the dynamic equilibrium. The changes of particle size (black curve) and catalytic efficiency (red curve) with an increase in pulses are shown in Fig. 5. The curves were obtained by fitting.
Fig. 5. Change of particle size and catalytic efficiency of TiO2 nanoparticles prepared under multiple pulses.
One of the reasons for the enhanced catalytic effect of TiO2 nanoparticles prepared with multiple pulses is that the particle size decreases to a certain extent in dynamic equilibrium. Another reason is that the rupture of previously fused large particles produces special structures that improve catalytic performance under the condition of multiple pulses, which will be discussed in more detail later.
3. Mechanism research and discussions
3.1 Particle formation process
A schematic diagram of the nanoparticle formation in a liquid environment is presented in Fig. 6. High plasma bubbles began to form from the first pulse laser beam, and its surroundings quickly produced a GPa order of magnitude shock wave. Simultaneously, an extremely high temperature and pressure environment was formed inside the bubble under the restriction of liquid environment, increasing the nanoparticles inside the bubble nucleation. The bubble then expanded, collapsed, and broke, releasing the nanoparticles into the solution. This is called the production stage. The second, third, and subsequent pulsed lasers also generated new nanoparticles through the above steps. In addition, the nanoparticles previously released into the solution fused and broke under the shock waves of several laser pulses.
3.2 Fusion stage
Simulation software was used to analyse the stress of the two particles. In the simulation example, 1 GPa stress was continuously applied on the two ends of the 100 and 500 nm particles to simulate the extrusion pressure of a shock wave formed by the particles under the action of laser pulse plasma. Two 100 nm diameter particles were subjected to a shock wave, as shown in Fig. 7. The stress transfer path is the stress change of two 100 nm particles along the y = 0 direction from left to right. A peak value was reached at 100 nm in the centre of the two particles. As the contact position of the two particles was close to a point, the stress applied on the surface reached approximately 60 MPa when it was transferred to the contact point. This extrusion pressure was favorable for the fusion of small particles, as shown for two 500 nm particles in Fig. 7(b). The stress transfer diagram shows that shock wave stress took more time to transfer in large particles and the loss was larger. The stress transfer to the contact point was much less than for small particles at only 8 MPa, proving that it was more difficult for large particles to fuse under the same shock wave conditions. This is consistent with the observed experiments.
Fig. 7. Stress transfer process of (a) 100 nm and (b) 500 nm particles subjected to a shock wave at both ends. The particle stress transfer path is the stress change of two particles along the y = 0 direction from left to right.
3.3 Breakage stage
The crushing mechanism of different nanomaterials under the action of shock waves was different. The main reason for particle breakage in metal nanoparticles is that a strong charge aggregation and oscillation effect occur at specific parts of particles when surface plasmon resonance occurs between nanoparticles and shock waves, resulting in particle breakage [20]. The main reason for breakage in metal oxide nanomaterials is the uneven internal and external materials of the particles. This is because the high degree of oxidation leads to different internal and external thermal properties. The internal heat transfer of nano metal oxide particles was faster under the heat action of a shock wave, first melting and expanding and then leading to the rupture of the oxide shells [28]. In the TiO2 particles with uneven internal and external oxidation, the internal oxidation degree is low or nonexistent. The thermal conductivity of Ti is 21.9 W /(m*k) and the melting point is 1941K. The thermal conductivity of TiO2 is 11.7 W /(m*k) and the melting point is 2113 K. The internal temperature conduction was faster and its melting point was lower under the thermal effect from the laser plasma shock wave. Therefore, it first melted and then cracked the oxide shell. The crushing mechanism is the main reason for the breakage of TiO2.
The breakage is mainly divided into two categories, as previously illustrated in the SEM images in Figs. 3(h)-(l). The two phenomena are caused by the influence of particle size on the temperature field distribution. The phenomenon that causes cracks, holes, and flake structures has a particle size above 1 µm in Figs. 3(h)-(j). The phenomenon that creates a “snail” shape has a particle size less than 1 µm in Figs. 3(k)(l). The temperature of the particles increased faster and the melting within the particles was faster when the particle size was less than 1 µm. The temperature gradient and thermal stress of small particles were larger, thus it was easy to rupture from the inside. The thermal absorption caused by the laser plasma shock wave increased with an increase of particle size, and was mostly deposited inside the particles. Additionally, the larger surface area led to faster surface heat dissipation and a slow increase in internal temperature. This resulted in a larger temperature gradient on the surface of the particles, which is more likely to rupture on the surface of the particles. A larger temperature gradient will produce a pair of thermal stresses in opposite directions, namely compressive and tensile stresses, respectively expressed as:
Fig. 8. Temperature gradient of (a) 1 µm and (b) 2 µm TiO2 particles. (c) Fusion (blue curve) and breakage (red curve) stresses with the increase of particle size.
So why does breaking occur in larger particles? It was previously determined through experiments and simulations that small size particles are more likely to fuse. With the increase of particle size, the fusion stress of particles gradually decreases, as shown in the blue curve in Fig. 8(c). The larger the particle size, the more breakage stress it receives from a shock wave, as shown by the red curve. The crushing threshold gradually decreases, and the crushing phenomenon is more likely to occur.
3.4 Analysis of photocatalytic performance
When TiO2 nanoparticles are irradiated by UV light, a catalytic reaction is generated using an electron-hole pair formula [29,30], expressed as:
Photogenerated holes have a strong ability to obtain electrons (oxidation capacity), which can directly react with hydroxyl acid to generate CO2. This adsorbed molecule oxidation formula is expressed as:
OH- and H2O molecules adsorbed on the surface of titanium dioxide can also be oxidised to hydroxyl radical ·OH using:
andAdditionally, the ·OH radical can further oxidise the adsorbed molecule using:
where R represents the organic molecule to be catalysed and R’ is the catalytic product. After a series of reactions, photogenerated electrons can also produce a hydroxyl group with strong oxidation ability using [29]:Therefore, the recombination of photocarriers should be avoided to improve the catalytic efficiency.
When the excited electrons and holes recombine and return from the conduction band to the valence band, they produce additional energy in the form of photons. Therefore, the electron hole recombination rate can be analysed using a fluorescence test. To ensure the same concentration of samples in the fluorescence test, TiO2 nanoparticles prepared under the action of different pulses were successively collected and quantity having the same weight (100 mg) were obtained, and this quantity was then added to an equal amount of deionized water and shaken by ultrasound before fluorescence testing. The sample was excited at 385 nm, and an obvious fluorescence emission peak was observed at approximately 400 nm, as shown in Fig. 9. The fluorescence intensity of TiO2 nanoparticles prepared with 3 pulses was the lowest, which increased when 30 pulses were used and was highest for 10 pulses. The corresponding electron hole recombination rates were 10, 30, and 3 pulses, respectively. X-ray diffraction (XRD) was used with increasing pulse numbers to detect the prepared TiO2 nanoparticles, and the crystal state changed from an anatase phase to a rutile phase with an increase in pulse times. This was because the mass density of anatase was smaller than that of rutile. Additionally, the rutile phase TiO2 was more stable due to its high hardness, density, dielectric constant, and refractive index. The anatase and rutile phases had prominent 101 and 110 crystal faces, respectively.
The reason for the high catalytic efficiency of the flake structure generated by the particle breakage under the action of multiple pulses is that the flake structure had a high-energy 001 crystal plane [31,32]. Therefore, based on the DFT, the Perdew-Burke-Ernzerhof parameterisation of generalised gradient approximation was adopted [33] to calculate the effective mass and energy band of electrons on the 001,101, and 110 crystal planes, as listed in Table 1.
Table 1. Effective electron mass and band gap energy of the 001,101, and 110 crystal planes.
The energy band and state density of the three crystal planes are shown in Fig. 10. The density of states (DOS) of the three crystal planes were similar, and the valence band was mainly composed of O 2p states and a small number of Ti 3d states. This indicates that there was a strong p-d hybridisation between the O 2p and Ti 3d states, forming bond states in the valence band region. There was also strong hybridisation in the conduction band between the Ti 3d and O 2p states, leading to the formation of antibond states. A small band gap in the 001 crystal plane indicated strong light absorption, electron migration, and hole formation abilities. A band gap in the 101 crystal plane was slightly larger than that of the 110 crystal plane, indicating that the energy of the photogenerated carriers was higher and the carrier migration ability was stronger under the same structure condition. This is conducive to reach the surface for an oxidation-reduction reaction.
The effective mass of the 001 crystal plane in two directions was obviously lower than that of the other two crystal planes, and the effective mass of the 110 crystal plane was the largest, which affected the mobility of the carriers. The higher the mobility, the lower the recombination rate of the photogenerated electron holes. The relationship between effective mass and mobility can be indirectly expressed as:
where V is the transfer rate of the carrier (electron or hole), h is Planck's constant, K is the wave vector, and m is the effective mass of the carrier.A two-sided 2D layer structure appeared on the broken surface of the large particles, which had a high-energy 001 crystal plane. It could shorten the migration path of the photogenerated electrons and holes to the surface of the photocatalyst, thus reducing electron-hole pair recombination and improving catalytic performance [32]. The path of flake electrons and holes to the surface was shorter compared with a spherical electron-hole disordered motion, as shown in Fig. 11(a)(b). In addition, when smaller particles melt and break, they also form a snake-like morphology connected by a rod-like structure, as shown in Fig. 11(c). These morphologic structures increase the specific surface area and provide more active contact sites. Moreover, electron holes in the rod-like structure conduct freely in the radial direction, which reduces the recombination rate of photogenerated carriers [34,35].
Fig. 11. (a) Internal photogenerated hole electron recombination of spherical TiO2 particles. (b) Photogenerated hole electrons of flake TiO2 particles reach the surface and participate in catalytic reaction. (c) Radial movement of the photogenerated hole electrons of snail-shaped TiO2 particles participating in the catalytic reaction.
Whether the photocatalytic reaction can continue depends on whether the hole electron separation reaction is smooth. If the recombination of electrons and holes occurs, it is not conducive to a series of photocatalytic reactions. From the perspective of fluorescence and crystal phase analysis, the prepared particles gradually transform into rutile with an increase in the number of pulses due to the stability of rutile, leading to the reduction of catalytic efficiency. However, the special structure in the crushing process of large particles reduces the recombination rate of electron holes, which is conducive to the continuous catalytic reaction.
4. Discussion on preparation parameters
4.1 Preparation experiment and simulation analysis of solutions with different heights using PLAL
4.1.1 Preparation experiment under the condition of increasing solution height
When the solution height was changed to 8 mm, TiO2 particle size gradually increased with an increase in the number of pulses under the action of continuous pulses at the same position. However, the amplitude of increase was less than 2 mm. Moreover, the increase of particle size induced by increasing the number of pulses lagged behind that of the solution at a lower height (2 mm). When the number of pulses was increased to 25, the overall particle size increased significantly and large particles with a proportion of more than 300 nm appeared. When the number of pulses was increased to 35, the proportion of large particles with a proportion of more than 300 nm decreased and the particle size was mostly distributed between 30-150 nm, with an average size of 69 nm. A comparison of nanoparticles prepared at different solution heights is presented in Table 2.
Table 2. Comparison of the preparation of nanoparticles for solution heights of 2 and 8 mm.
When the number of pulses was small (3-5 times) in the particle production stage, the particle sizes obtained by the 2 and 8 mm experiments were small. In the 8 mm experiment, the particle size and number of prepared particles were lower than that of 2 mm. With an increase in the number of pulses, the particle size tended to increase through fusion. In the solution with a higher height (8 mm), the increase of particle size induced by increasing the number of pulses lagged behind that of the solution with a lower height (2 mm). Additionally, the particle size at the fusion stage was smaller at 8 mm. When the pulse continued to increase, the average size of particle breakage prepared at the two solution heights decreased significantly. The fragmentation degree of large particles was small for 8 mm, as shown in Fig. 12. The final particle size reached dynamic equilibrium, maintaining the particle size within a certain range. The particle size variations at the two heights was considered to be because the laser pulse plasma shock wave was restricted to different degrees in different solutions. This is discussed in further detail in the next section.
4.1.2 Simulation analysis of the effect of laser pulse plasma shock wave on solution at different heights
At this stage, the restriction effect on the laser plasma shock wave was stronger under the condition of lower solution height. This is also the reason why the particle fusion induced by increasing the number of pulses in the solution was higher at 8 mm than at 2 mm. There was also a bigger shock wave propagation loss and more energy absorption on subsequent pulses at the higher height. Consequently, the effect on induced particle breakage was decreased.
The thermal stress of the laser pulsed plasma shock wave was always applied to the particles, and the particle size and morphology changed under the action of the shock wave. The pressure of the shock wave was affected by the propagation distance (solution height) in the solution. According to the law of momentum conservation and combined with experimental results, the expressions of pressure P1 and shock wave velocity D on the underwater shock wave front are expressed as [36]:
where K1 = 51190 m/s and K2 = 25306 m/s. ρ0, P0, and C0 are the liquid density before disturbance, initial pressure, and liquid sound velocity at room temperature and pressure, respectively. This formula applies to pressure less than 2.5 GPa. According to Eq. (10), the pressure of the shock wave front in the liquid was approximately 0 when the shock wave velocity was less than the sound velocity in the liquid. Then, the shock wave continued to propagate in the form of sound waves. The formation of the initial pressure P0 compared to the one in the preceding paragraph can be ignored. The main factor affecting the shock wave front pressure was the shock wave velocity D. Additionally, the shock wave velocity is related to its propagation distance, laser pulse induced plasma shock wave in water through the process of the formation, increase, and attenuation.A schematic diagram of the upward transfer of the shock wave pressure excited by a laser pulse at the bottom of solution under the condition of different solution thicknesses using simulation software is shown in Fig. 13. When the solution thickness was small (A in Fig. 13–1 mm), the shock wave propagated more intensely inside the solution, reflected at the top of solution, and created high pressure. When the solution thickness was large (B in Fig. 13–4 mm), the velocity of the shock wave decreased greatly with an increase of distance in the upward propagation process, and it continued to oscillate in the solution in the form of sound waves after attenuation.
The propagation curve was simulated using the pressure propagation formula (Eq. (10)) on the shock wave front under a pulse energy of 230 mJ, as shown in Fig. 13. The shock wave pressure decreased exponentially with the increase of transmission distance. The shock wave generated by the subsequent laser pulse had a stronger breakage effect on the particles in the solution when the solution height was small.
4.2 Discussion on optimum preparation parameters
The selection of optimal preparation parameters depends on the preparation efficiency and photocatalytic performance of the TiO2 nanoparticles. The main parameters in this study were solution height and pulse times, which both demonstrated an impact on the preparation efficiency and photocatalytic performance.
4.2.1 Solution height
When the solution height was higher (8 mm), there was greater absorption of laser energy, the laser ablation process of plasma was not effectively limited in the water membrane, and the height of the solution was reduced by 70%, which increased the preparation efficiency by 350% [37]. At a smaller solution height (2 mm), nearly 20% of the particles were larger than 500 nm and 6% were larger than 1 µm. This was due to the stronger limiting effect of the liquid film on the shock wave, even though the particle size obviously increased after approximately (but no more than) 10 pulses. Particles in this size range were easily broken by a shock wave, forming a special structure conducive to improving the photocatalytic performance of TiO2 nanoparticles.
4.2.2 Pulse times
First, the preparation time was high and the efficiency was low due to the transformation of the preparation position. Although the particle size prepared with less pulse times was relatively small, the photocatalytic performance was high. However, the number of particles was particularly small. At the same time, the prepared particles demonstrated fusion and breakage stages with an increase in the number of pulses at a small solution height (2 mm). The particle size first increased and then decreased, and the catalytic efficiency of the same amount of TiO2 nanoparticles first decreased and then increased. This was due to the special structure formed by the crushing of particles with an increased particle size in the fusion stage under the action of subsequent pulses. A 650 nm opening of more than 1 µm particles formed a double-sided structure with a thickness of 25 nm, which accounted for 32% of the whole. Therefore, the special structure with a certain proportion reduced the recombination rate of electron holes and greatly benefited the continuous photocatalytic reaction.
In conclusion, when selecting the parameters of TiO2 nanoparticles prepared using the PLAL method, the solution height should be reduced as much as possible (to approximately 1-2 mm) to improve the preparation efficiency and enhance the particle breaking effect. However, the height should not be so low that it prevents losing the limitation of plasma, and the number of pulses should be increased (to approximately 35) to provide optimal conditions for particle breaking.
5. Conclusion
This study proposed a method to improve the production efficiency and photocatalytic performance of TiO2 nanoparticles using PLAL to optimise preparation parameters. TiO2 nanoparticles were prepared using multiple pulses at different solution heights, which were applied to degrade a methylene blue solution. With an increase in the number of pulses, the morphology evolution of particles could be divided into three stages: production (3-5 pulse effects), fusion (5-15 pulse effects), and breakage (15-35 pulse effects). Affected by the crystal phase and morphology of the particles, the comprehensive effect of the corresponding production efficiency P and degradation efficiency D was significantly improved (P:D = 200 mg/h:77%, 450 mg/h:64%, 500 mg/h:74%). According to the principle of thermodynamics, the increase of particle size in the fusion stage was the premise to improve the degradation efficiency by controlling the pressure constraint effect with a solution height of 1-2 mm. According to the plasma shock wave effect and DFT calculations, the crystal phase (001) and morphology evolution (two-dimensional, rod-like structures) of the prepared particles were ensured. Finally, this work provided an optimised PLAL method for the preparation of TiO2 nanoparticles. The preparation efficiency and photocatalytic performance was improved by controlling only two parameters (pulse number and solution height) and avoiding a complex preparation process. The proposed method provides a reference for the selection of parameters in actual production.
Funding
National Natural Science Foundation of China (U2004162, U2030108); Sichuan Province Science and Technology Support Program (2021YFSY0027); Open-Foundation of Key Laboratory of Laser Device Technology, China North Industries Group Corporation Limited (KLLDT202113).
Acknowledgments
We thank professor Houkun Liang for fruitful discussions and guidance on the project.
Disclosures
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
The data that support the findings of this study are available from the corresponding author, [J. Han], upon reasonable request.
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