2.3.1 Microstructure evolution of ablated target
Figure 1 show the XRD patterns of the target after thermal treatment. Only ZrTiO4, ZnAl2O4 and residual ZrO2 was detected (Fig.2.1.a). The microstructure of target ZAT1and ZAT2 were primarily composed of granular ZnAl2O4 crystal about (2 µm) and submicro prismatical ZrTiO4 crystal. In order to investigate the relationship between laser fluence and nanoparticles composition, phase transition of microstructure of ZAT1and ZAT2 target under different laser fluence was examined.
As shown in Fig.2.2 show surface evolution. It was observed that the surface evolution changes from relief structure to smooth structure while the applied laser fluence increases. Figure 2.3 shows the surface element mapping for ZAT1under 0.34 J/cm2. Mapping result disclosed prominent area contains Zr-rich and concave area has Zn-rich. It means that Zr-rich area was subjected to a lower ablation rate but a higher ablation rate was applied at Zn-rich area under 0.34 J/cm2 fluence. That is the reason why the generated fine nanoparticles contain higher Zn content and lower Zr content. In addition, Al element distribution is similar to Zn element, and Ti element could be found at the whole area. As increasing the fluence over threshold value, all oxide compounds or elements could be laser-ablated to generate nanoparticles with homologous microstructure and compositions other than Zn content.
2.3.2 Morphology of ablated nanoparticle
Prior to laser ablation for multiphase oxide, single oxide compound was first studied. I was found that ZrO2 has a higher threshold energy density of 3~5 J/cm2 and TiO2 is 1~1.5 J/cm2. The nanoparticles with an average particle size of 20~40 nm under threshold fluence can be obtained by laser ablation process. On the other hand, both ZnO and Al2O3 have lower threshold of 0.4 - 0.6 J/cm2, but 5~20 nm finer nanoparticles were generated. According to the ablation model for single oxide ceramic, it reveals that many parameters such as bonding strength, absorption property and wavelength will influence the threshold energy density. Therefore, for multiphase oxide compound, it would become more complicated because of different bonding strength or threshold fluence.
According to laser-ablation experiment for ZAT1 and ZAT2, 0.34 J/cm2 fluence only obtains
ablation rate of 0.3~0.7µg/shot at air atmosphere that was below threshold . As increasing the fluence to 1 J/cm2 and 4 J/cm2, the ablation rate of 1.9~3.2 µg/shot and 2.3~3.4µg/shot, respectively, can be reached that is near the saturated ablation condition. Therefore, three fluences of 0.34, 1 and 4 J/cm2 would be used for the laser ablation process .
Nanoparticles produced by 0.34~4 J/cm2 laser fluence at air atmosphere show amorphous-like structure [Fig. 2.1b and c] and exhibit two kind of particle size distribution with fine nanoparticles of 5~20 nm (70~90%) and large spherical nanoparticles of 40~100 nm (10~30%) as shown in Fig.2.4.a,b,c). While stronger laser fluence was applied, the number of large spherical particles becomes more (is increased). According to static method, average particle size of 8.2 nm, 10.7 nm and 11.6 nm was obtained under 0.34, 1 and 4 J/cm2 of laser fluence, respectively. The influence of laser power on the crystallization evolution of nanoparticle is also observed in Fig. 2.5 for ZAT1 and ZAT2 where both exhibit different phase evolution. However, The phase crystallization increases with a stronger fluence. The electron diffraction pattern shows better crystalline particle for high laser power (1 J/cm2) but lower laser power gives amorphous-like particle. These results may explain both gas and liquid phases existed in the ablated agglomeration. The higher laser power digs out larger particle with liquid phase on its surface. Good crystallization is also found for these large particles. When the low laser power is applied, the only excited particle is gas-like phase and will be consolidated into amorphous phase because of the quench effect of flow gas or atmosphere. Therefore, fine particle with amorphous phase is found (see Fig.2.4.b). The amorphous fine particle can be crystallized by electron beam during TEM observation or temperature treatment. Therefore, it can be summarized. The higher laser power gives larger particles with wide size distribution, while the lower ones provides fine particles with narrow size distribution. Compromise between laser power and temperature treatment can produce fine particle with narrow size distribution and good crystallization.
As the nanoparticles were synthesized in nitrogen atmospheres, the nanoparticles in Fig. 2.4.d appeared wilder particle distribution than that synthesized in air atmospheres. On the other hand, in
oxygen atmospheres, some hollow nanoparticles as shown in Fig.2.4.e.f.appear in this condition.
Gas flow to bring out particle from surface changes ablation rate. Increasing flow rate carries out more particles and makes the surrounding environment easier for the following particle generation The ablation rate varies with the change of atmosphere condition because gas flow to bring out particle from surface occurs during laser ablation and changes with atmosphere condition. It was found that oxygen has a larger density and viscosity (1.43×10-3g/cm3 and 2.08×10-5g/s cm, Poise) ‧ than air (1.32×10-3g/cm3 and 1.86×10-5g/s cm, Poise) and nitrogen (1.25×10‧ -3g/cm3 and 1.79×10-5g/s cm, Poise). Therefore, in oxygen atmospheres, the laser‧ -induced plume become small due to the short mean free path because of high viscosity and thus it causes more probability for the laser beam to irradiate the agglomerated nanoparticles to generate large hollow nanoparticles
2.3.3 Phase separation of ablated nanoparticle
Composition of the generated nanoparticles from both targets of ZAT1 and ZAT2 were inspected by TEM and EDS analysis based on a normalized molar ratio of atoms with Zr+Ti+Zn+Al=100%. Fig.2.6 show element evolution of three types of nanoparticles under different fluences in air atmosphere. Both ZAT1 target have similar result as ZAT2, but element show more close value under 1 and 4 J/cm2 laser fluence for fine nanoparticles. For fine nanoparticles (5~20nm) generated from ZAT2 target, it was found that Zn content decreases but Zr content increases while the laser fluence rises. In addition, both Al and Ti content present similar trend to Zr but littler irregular. However, in the case of large spherical nanoparticles (40~100nm), the Zn content was apparently decreased but the others increase with an increase of laser fluence. It was believed that because Zn content was easier evaporated into gas phase than the others as the fine nanoparticles were condensed from vapor phase, more Zn content would be trapped. In addition, as the larger crystalline particles were formed, the EDS shows that it was primarily composed of Zr over 70 % with little Zn.
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2.3.4 Far-infrared ray emissivity characteristion
The far-infrared ray emissivity characterization of nanoparticles based on ZrTiO4--ZnAl2O4 system was investigated with black body furnace as a reference of 1.
Table 2.1 summarized the average emissivity of more than 80﹪fine nanoparticles (wavelength in the range of 4 to 12 µm) under different condition. The average emissivity were as function of the synthesized nanoparticle size, composition and formation phase and varies upward with increasing large spherical particles and crystal phase ratio . 2.4. Summary
Laser ablation is an important and useful method for oxide nanopartcles with single or multiphase. Surface of target will be annealed by laser power and presents nearly constant ablation rate after several shots. Higher laser power creates larger particle with good crystallization, while lower ones gives fine and amorphous particles. Multiphase target suffers from phase separation due to the different laser absorption ability.
Modification of target composition may relieve this problem.
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2.5 References
[2.1]M. Choi, J. Nanoparticle Res., 3 (2001) 201-211 [2.2]K. Landfester, Adv. Mater., 13 (2001) 765-768
[2.3]Y. C. Kang, S. B. Park and Y. W. Kang, Nanostructured Mater. 5 (1995) 771-791 [2.4]Z. Paszti, G. Peto, Z. E. Horvath and A. Karacs, Appl. Surface Sci., 168 (2000) 114-117
[2.5]T. Sasaki, S. Terauchi, N. Koshizaki and H. Umehara, Appl. Surface Sci., 127-129 (1998) 398-402
[2.6]K. Tanaka and D. Sonobe, Appl. Surface Sci., 140 (1999) 138-143 [2.7]H. Takashima, Yogyo-Kyokai-Shi 89,[12](1981)655-660
[2.8]H. Takashima, K. Matsubara, Y. Nishimura and E. Kato, Yogyo-Kyokai-Shi 90,[7](1982)373-379
[2.9]Y. Kawakami, T. Seto and E. Ozawa, Appl. Physics A69[Suppl.],(1999)249-252
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Fig.2.1.a. XRD pattern of crstal phases of ZAT1 target after thermal treatment. b. XRD pattern of nanoparticles produced by 4 J/cm2 laser fluence at air atmospheres.c. XRD pattern of nanoparticles produced by 0.34 J/cm2 laser fluence at air atmospheres.
Fig.2.2 Surface evolution of ZAT1and ZAT2 target
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Fig.2.3. Surface mapping for ZAT1 under 0.34 J/cm2
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Fig.2.4.a.b.c. Nanoparticles produced by 0.34~4 J/cm2 laser fluence irradiating at air atmosphere. 2.4.d . nanoparticles synthesized at nitrogen atmospheres. 2.4..e&f.
nanoparticles synthesized at oxygen atmospheres.
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Fig.2.5. The evolution of TEM diffraction pattren of nanoparticles of ZAT1 and ZAT2 under different fluence
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Fig.2.6. element evolution of three type nanoparticles under different fluence and air atmosphere
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.
target fluence amorphous-like nanoparticle crystal phase far-infrared average emissivity J/cm2
fine nanoparticle Vol%
large spherical particles Vol% Vol%
wavelength range 4~ 12µm test at 36 test at 60℃ ℃ ZAT1 0.34 80~85% 15~20% <1% 0.85 0.87
ZAT1 1 70~80% 20~30% <2% 0.86 0.89
ZAT1 4 70~80% 20~30% <6% 0.88 0.92
ZAT2 0.34 85~90% 10~15% <1% 0.83 0.85
ZAT2 1 75~85% 15~25% <3% 0.85 0.88
ZAT2 4 70~80% 20~30% <5% 0.86 0.89
Table 2.1 summarized the average emissivity of the nanoparticles that were more than 80
﹪(wavelength range from 4 to 12µm) under different condition.
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Chapter 3
Nanoscaled TiO2/Ag catalyst and photodecomposition characteristic
3.1 Introduction
The photodecomposition of various pollutants by TiO2 has been demonstrated to be efficient under ultraviolet light [3.1-3.16]. The photocatalytic activity of TiO2 can also be enhanced by modifying its surface with noble metals and metal oxides [3.8]. These photocatalysts are utilized in many approaches. For example, TiO2 photocatalysts anchored on supporting materials with large surface areas have been developed to eliminate the shortcomings of the filtration and the suspension of fine photocatalyst particles [3.5]. TiO2 photocatalysts anchored on various substrates were prepared using a pasting treatment, an ionized cluster beam (ICB) method or a sol–gel method [3.6]. The sol-gel method is frequently adopted to prepare TiO2 thin films on supported substrates [3.1-3.7, 3.10-3.13]. However, heat treatment may cause a phase transition of TiO2 and reduce the photocatalytic activity [3.5].
This study proposes the anchoring of well-dispersed nano TiO2 on a metal carrier using a binder-free, low temperature process. Well-dispersed nano TiO2 deposited on a branch-like silver (Ag) carrier, called “nano-TiO2/Ag catalyst” was synthesized here to overcome the aforementioned shortcomings. This study discusses the preparation, morphology and reaction kinetics of photocatalytic activity of nano-TiO2/Ag catalyst.
3.2 Experimental
An Ag carrier was prepared based on the reaction, 2Ag+ + CuÆ2Ag + Cu+2. The net redox potential of the reaction is 0.5V indicating that the reaction occurs spontaneously.
In a pretest, the formation of the branch-like Ag carrier was favored in the acidic solution at high concentration [17]. Bulk copper (Cu) was placed in AgNO3 solution at a ratio of Cu: AgNO3: water =5: 3: 100 (wt %), to yield Ag particles. Nitric acid was added to give the solution a pH of 3. The solution temperature was set to 25 , which℃ was maintained for 2 hrs. Then, the precipitate was rinsed with D.I water to remove residues.
Secondly, an appropriate amount of TiO2 particles (P25, Degussa) was placed in the solution at various pH values, obtained by adjusting the amount of NH3(aq) or nitric acid
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added. The solution was treated ultrasonically for 90mins. No dispersing agent was added to the solution. The experimental design was such that 5 wt % of TiO2 was mixed with Ag carrier at various pH conditions. A critical coverage ratio of TiO2 of approximately 5% was identified: exceeding this dosage of TiO2 may result in aggregation under all test pH conditions. The mixture was stirred for 30 mins to increase the number of opportunities for contact between nano TiO2 particles and Ag carriers. Finally, a composite of nano TiO2 particles and Ag carrier was synthesized by washing, filtering and drying, in that order. Table 3.1 presents the characteristics of nano-TiO2/Ag catalyst used in this study.
Methylene blue (MB) is a representative dye that is commonly adopted to evaluate the catalytic activity of a catalyst. The test conditions were as follow; 1g of catalyst, 30 ml of 100 ppm MB solution, UV light with a wavelength of 254 nm and an illumination intensity of 4mW/cm2. The UV lamp was placed 6cm above the test sample. The mixture (MB and catalyst) was stirred gently while being irradiated. A centrifuge was used to separate the mixture after irradiation had been completed. The absorption of the MB supernatant was then determined using a spectrophotometer (Unico UV2102). A blank experiment (without a catalyst) was also performed; the results indicated that irradiation did not significantly change MB absorbance. The photocatalytic activity was defined as
C 100%
where Cinitial and Cirradiation are the absorbance of MB before and after irradiation.
The morphology of the catalysts is observed by field emission-scanning electron microscopy (FE-SEM, LEO, 1530).
3.3 Results and discussion
The SEM images of the Ag nanoparticles are shown in Figure 3.1. Figure 3.1a~3.1d shows the typical SEM images of the product obtained by self-reducing the solution with concentration from 0.1wt% to 1.5wt% AgNO3. It is apparent that Ag nanoparticles display dendritic growth while concentration>0.5wt%. We believe that the excess of silver in the solution may be favorable for the aggregation and growth into the dendritic structures of the Ag cluster. It is found that the concentration of AgNO3 plays a significant role in the formation and growth of the silver nanoparticles. When the concentration of AgNO3 is descreased to 0.2wt%, the flake-like Ag can
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also be observed as shown in Fig 3.2c, 3.2d. Rather, the Ag nanoparticles display irregular flake shapes with the size of about 1µm in diameter. It is found that the concentration of AgNO3 also plays a key role in the formation of Ag nanoparticles.
Fig. 3.2 is the XRD patterns of the as-prepared 1.5wt% AgNO3 sample, in which five strong peaks can be indexed to diffraction from the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) of face-centered cubic (fcc) Ag, and no impurity peaks from silver oxide were detected. Fig. 3.3a shows the SEM images of well-defined silver dendrites with pine-like shape. It is found that the side branches are well symmetric and the angles of them to the main branches are all about 60o, which implies that all side branches grow along the same direction. With careful observation(Fig.
3.3b and 3.3c), the side branches of these dendritic Ag are constructed by lots of well-crystallized small nanorods with diameter of 50–60 nm and length up to 200nm. The inset SAED pattern from one of the left side branch (in Fig. 3.3d) reveals that the Ag dendrite has single crystal nature with cubic phase and the side branch direction assembles along [0 1 1] direction. It may suggest that the large dendrites grow from small clusters and in many places the dendrites lack corners and arms.
In the catalyst preparation, TiO2 nanoparticles were initially suspended in solution.
The corresponding zeta potentials of the TiO2 solution at pH 11, pH 6.2 and pH 3 were measured to be -55, -2.5 and 28 mV, respectively. These results revealed that TiO2
particles tended to be negatively charged by the excess bonding of hydroxyl ion (OH-) in the alkaline solution (pH 11), and positively charged by the excess bonding of hydrogen ion (H3O+) in the acidic solution (pH 3), suggesting that the electric repulsion between TiO2 nanoparticles was very strong in both alkaline and acidic solutions, causing their
“effective-dispersion”.
When the Ag particles were mixed with TiO2 particle solution at pH 11, pH 6.2 and pH 3, the resultant composite was as shown in Fig 3.4. 1a~1c, respectively. A composite of well-dispersed nano TiO2 anchored on Ag carrier was formed in alkaline solution (pH 11, Fig. 3.4a), while nano-TiO2 aggregated in the neutral solution (pH 6.2, Fig. 3.4b).
Almost no TiO2 particles were deposited on the Ag carrier in acidic solution (pH 3, Fig.
3.4c). The results indicated that the pH status significantly affects the combination of nano TiO2 and Ag carriers. The surface of the Ag carrier is preferentially oxidized because of its extreme activity if the size of the Ag particles is reduced to the nano-scale [17]. Accordingly, the positively oxidized Ag surface spontaneously attracted negative
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TiO2 particles in the alkaline solution, yielding the composite, “well-dispersive ” TiO2
anchored on Ag carriers, as shown in Fig. 3.4a. In neutral solution (pH=6.2), the repulsive force between the nano TiO2 particles was very weak, because neutral pH was very close to the isoelectric point [18]. H2O molecules provide a “bridge” between nano TiO2 particles via hydrogen bonding, resulting in the aggregation of TiO2 particles, as shown in Fig. 3.4b. Positive TiO2 particles could not be easily anchored on positive Ag carrier, because of the electric repulsion, as shown in Fig. 3.4c. Consequently, the effective dispersion of nano TiO2 and Ag carriers is governed by electrostatic attraction, achieved by adjusting the pH status of solution.
The prepared catalysts, as shown in Fig. 3.4 at various pH values were then examined to determine the photocatalytic activity. As seen in Fig. 3.5, gradual changes in the absorbance of MB were observed from its characteristic absorption at 250nm, 290nm and 666 nm. The absorbance decreased in the order Cat. pH 11, Cat. pH 6.2 and Cat. pH 3 with an irradiation period of 20mins. This result indicates that the “pH status in the catalyst preparation” not only affects the dispersion between nano-TiO2 particles but also significantly influences the photocatalytic activity, and can be clearly distinguished from the results in Figs. 3.4 and 3.5. Cat. pH 6.2 has a higher MB-absorbance (and thus a lower photocatalytic activity) than Cat. pH 11, even though both nano-TiO2 particle loadings were identical. The photocatalytic activity was evidently reduced by the aggregation of nano-TiO2 particles. Larger aggregated nano TiO2 particles correspond to less surface area exposed to UV irradiation. Therefore, the enhancement of the photocatalytic activity of nano-TiO2/Ag catalyst depends on effective dispersion and an appropriate proportion of nano-TiO2, and can be achieved in this approach without adding a dispersive agent or binder.
Figure3. 6 plots photocatalytic activity as a function of reaction time. The photocatalytic activity also followed the order Cat. pH 11, Cat. pH 6.2 and Cat. pH 3 for various periods of irradiation. Cat. pH 11 and Cat. pH 6.2 reached a decomposition efficiency of over 90% after an irradiation time of 2hrs. Cat. pH 11 was associated with the near complete decomposition of MB after irradiation for 1 hr. Notably, the MB concentration (100ppm) tested herein is much higher than the 10ppm tested in the literature [3, 5, 10]. A high initial MB concentration was removed completely in a short period, indicating that
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the well-dispersed nano-TiO2/Ag catalyst exhibited outstanding catalytic activity. Cat. pH 3
comprised mainly the Ag carrier, which still had a decomposition efficiency of 40% after irradiation for 2hrs, suggesting that the Ag carrier also exhibited photocatalytic activity.
Table 3.2 presents the rate constant calculated from the results in Fig. 3.6. The rate constant of all nano-TiO2/Ag catalysts was determined for a first-order reaction, and was consistent with the results found in the literature [3, 5, 10]. However, the reaction rate constant of Cat. pH 11 obtained herein clearly exceeded those in the literature [3, 5, 10].
Zainal et al. [5], prepared TiO2 thin film using a typical sol-gel method, and the catalysts was accomplished by a heat treatment at 600 for 6hrs. ℃ The TiO2/glass treated at 600 ℃ contained a rutile phase TiO2. Unfortunately, the calcination period was usually several hours, to ensure strong adhesion on the substrate. The amount and crystallinity of the formed rutile increased with the calcination temperature. In this study, well-dispersed nano TiO2 particles were combined with Ag carriers in the alkaline solution. No further thermal annealing was applied in the catalyst preparation. The aggregation or phase transition of TiO2 particles did not occur in the catalyst preparation herein. Therefore, the photocatalytic activity was enhanced by the increase in the effective reactive surface area of the “well-dispersed” nano TiO2 particles.
3.4 Summary
A highly dispersed nano-TiO2/Ag catalyst is synthesized in an alkaline solution.
Nearly all of the dimethy-blue target pollutant at high concentration was removed when the photoreaction was performed in a short period. This novel nano TiO2 photocatalyst exhibits excellent photocatalytic activity because it is well dispersed. Since no dispersant or organic binder was used, this synthetic process has the advantages of low
Nearly all of the dimethy-blue target pollutant at high concentration was removed when the photoreaction was performed in a short period. This novel nano TiO2 photocatalyst exhibits excellent photocatalytic activity because it is well dispersed. Since no dispersant or organic binder was used, this synthetic process has the advantages of low