Ultrasonics Sonochemistry xxx (2015) xxx–xxx

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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson

Fast synthesize ZnO quantum dots via ultrasonic method Weimin Yang a, Bing Zhang a, Nan Ding a, Wenhao Ding a,b, Lixi Wang a,⇑, Mingxun Yu b, Qitu Zhang a,⇑ a b

College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China Institute 53 of China’s Ordnance Industry, Jinan 250031, China

a r t i c l e

i n f o

Article history: Received 21 July 2015 Received in revised form 13 November 2015 Accepted 13 November 2015 Available online xxxx Keywords: Zinc oxide Colloidal quantum dots Ultrasonic method

a b s t r a c t Green emission ZnO quantum dots were synthesized by an ultrasonic sol–gel method. The ZnO quantum dots were synthesized in various ultrasonic temperature and time. Photoluminescence properties of these ZnO quantum dots were measured. Time-resolved photoluminescence decay spectra were also taken to discover the change of defects amount during the reaction. Both ultrasonic temperature and time could affect the type and amount of defects in ZnO quantum dots. Total defects of ZnO quantum dots decreased with the increasing of ultrasonic temperature and time. The dangling bonds defects disappeared faster than the optical defects. Types of optical defects first changed from oxygen interstitial defects to oxygen vacancy and zinc interstitial defects. Then transformed back to oxygen interstitial defects again. The sizes of ZnO quantum dots would be controlled by both ultrasonic temperature and time as well. That is, with the increasing of ultrasonic temperature and time, the sizes of ZnO quantum dots first decreased then increased. Moreover, concentrated raw materials solution brought larger sizes and more optical defects of ZnO quantum dots. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Colloid quantum dots (QDs) attracted lots of attentions in recent years due to their amazing characteristics in optics. The energy gap of QDs could be adjusted by their sizes, which are depended on the synthesis methods. Compared with other QDs, Zinc oxide (ZnO) QDs are non-toxic, easy obtained and stable. ZnO QDs exhibited excellent UV-absorption and photoluminescence properties due to their wide band gap (3.37 eV) and large exciton binding energy (60 meV) [1–3]. Whereas it is not easy to control the sizes uniformity and defects amount of QDs by conventional sol–gel method. Due to the ultrasonic cavitation, reactions in liquid have different processes such as oxidation, reduction, dissolution, decomposition, and hydrolysis under ultrasonic radiation [4,5]. Moreover, ultrasonic radiation brings more free radicals in the system [6], which improves the reaction activity of reactants. Furthermore, ultrasonic method can also control the morphology and reduce the surface defects of QDs [7]. Many researches have been done on ZnO nanoparticles synthesized via ultrasonic method. It is reported that, ultrasonic time influenced the growth mechanism and luminescence properties of the ZnO nanoparticles [5]. The

⇑ Corresponding authors. E-mail addresses: [email protected] (W. Yang), [email protected] (L. Wang), [email protected] (Q. Zhang).

shapes of ZnO nanostructures would be controlled by the ultrasonic time, and the sizes of ZnO nanostructures are uniform [8]. However, there is still little systematic research on the photoluminescence properties of low ultrasonic temperature synthesizing ZnO QDs in ethanol solution. In this article, the effects of ultrasonic temperature and time on ZnO QDs photoluminescence properties were explored. The effect of raw materials concentration on photoluminescence properties of ZnO QDs was studied as well. Finally, the growth mechanism of ZnO QDs was discussed. 2. Material and methods 2.1. Materials Ethyl alcohol (99.7%), polyethylene glycol-400 (PEG-400, 99.0%), lithium hydroxide (LiOH, 95.0%), oleic acid (OA, 99.0%), nhexane (97.0%) from Shanghai Lingfeng Chemical company and zinc acetate (Zn(Ac)2, 99.0%) from Xilong Chemical company have been used without further purification. 2.2. Synthesis of ZnO QDs by ultrasonic method ZnO QDs samples were prepared via the ultrasonic method [2,5,9]. To gain 0.0025 mol ZnO QDs, 0.55 g (0.0025 mol) Zn(CH3COO)22H2O (Zn(Ac)2) was dissolved in 100 mL ethyl alcohol and kept ultrasonic dispersion for 30 min. 0.21 g (0.005 mol) LiOH

http://dx.doi.org/10.1016/j.ultsonch.2015.11.015 1350-4177/Ó 2015 Elsevier B.V. All rights reserved.

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Fig. 1. Photoluminescence properties of ZnO QDs synthesized in 60 °C with different ultrasonic time. (a) Emission spectra (b) excitation spectra (c) emission and excitation peaks (d) quantum yield (e) CIE diagram.

was dissolved in 50 mL ethyl alcohol and kept ultrasonic dispersion for 30 min. Then appropriate amount of PEG-400 with n (PEG): n(Zn) = 1:1 was added into the system with 40 min ultrasonic dispersion. After that, the LiOH solution was inserted. ZnO QDs were obtained after continuously ultrasonically (ultrasonic frequency = 53 kHz, ultrasonic power = 180 W) promoted reaction at 20–60 °C for 5–90 min. All the ultrasonic processes were done in a water bath ultrasonic cleaning machine. Temperature of the water bath was accuracy controlled by a low-temperature thermostat bath. Then 0.6 mL oleic acid (OA) was used to precipitate the QDs. The white sediment was centrifuged for 5 min under the condition of 4000 r/min for 2–3 times washed with excess ethanol to remove any un-reacted material. Finally, ZnO QDs were dispersed in the n-hexane.

2.3. Measurement The photoluminescence spectra were taken by a fluorescent spectrophotometer (Lumina, Thermo, U.S.A). For each sample, the emission spectrum was measured under the measured emission peak as excitation wavelength, and the excitation spectrum was measured under the measured excitation peak as emission wavelength. The XRD pattern of ZnO QDs was obtained by the Xray diffraction (Rigaku D/Max-2500, Rigaku, Japan). The FT-IR spectra were recorded in the 4000–400 cm1 region by an FT-IR spectrometer (NEXUS 670, Thermo, U.S.A) with KBr plates. The TEM micrographs were characterized by transmission electron microscopy (JEM-2100, JOEL, Japan). The absorption spectra were recorded on a UV–Vis–NIR spectrophotometer (UV-3600, Shimadzu, Japan) used n-hexane as reference. Finally the

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Fig. 2. Fluorescence lifetime of ZnO QDs synthesized in 60 °C with different ultrasonic time. (a) 5 min (b) 30 min (c) 60 min (d) 90 min (e) summary of fluorescence lifetime.

time-resolved photoluminescence decay spectra of the ZnO QDs were obtained by an ultrafast time-resolved fluorescence spectrometer (DletaFlex, HORIBA Scientific, Japan). The photoluminescence quantum yields (PL QYs) were calculated by the following equation, using Rhodamine 6G dissolved in water (QY = 95%) as reference [10]

QY sample

!    2 nsample F sample Aref QY ref ¼ F ref Asample n2ref

ð1Þ

where F is the measured fluorescence, A is the absorbance at the excitation wavelength, and n is the refractive index of the solvent.

3. Results and discussion The XRD pattern of ZnO QDs is shown in Fig. s.1. Only the pure ZnO phase could be found in the XRD pattern. The FT-IR spectra is shown in Fig. s.2. It shows that PEG-400 and OA were surely capped on ZnO QDs.

3.1. Time of ultrasonication The photoluminescence properties of ZnO QDs synthesized in 60 °C with changed times (5 min, 30 min, 60 min, 90 min) are shown in Fig. 1. Fig. 1(a) shows the emission spectra of ZnO QDs. The spectra shapes of ZnO QDs synthesized with different ultrasonic times are almost the same. Fig. 1(e) shows the CIE diagram of ZnO QDs. The change of emitting color is expediently seen in Fig. 1(e). As is known that, the visible photoluminescence of ZnO QDs is brought by their defects and ions doping. For these samples, the emission peaks were between 520 nm and 530 nm, approached to the sum of oxygen vacancy (510 nm), zinc interstitial (420 nm) and oxygen interstitial (550 nm) defects [5,11]. For the sample synthesized for 5 min, the emitting color was green, and its blue emission was stronger. It suggests that the main defects of this sample were oxygen vacancy and zinc interstitial defects. With the reaction going on, the emission peaks and emitting color red shifted from green to yellow. This change indicates that, there was a transformation between the optical defects of ZnO QDs. The amount of oxygen interstitial defects raised, while the amount of zinc interstitial and oxygen vacancy defects decreased. In another word, with the

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Fig. 3. Emission spectra of ZnO QDs synthesized with different ultrasonic temperature and time. (a) 5 min (b) 30 min (c) 60 min (d) 90 min.

Table 1 Emission peaks of ZnO QDs synthesized with different ultrasonic temperature and time. Time/min

5 30 60 90

T/°C 20

30

40

50

60

527.6 nm 515.5 nm 512.7 nm 508.9 nm

518.2 nm 503.0 nm 514.5 nm 499.0 nm

513.6 nm 508.0 nm 510.8 nm 508.9 nm

507.0 nm 518.2 nm 516.4 nm 521.0 nm

528.5 nm 521.0 nm 522.0 nm 527.6 nm

reaction going on, zinc interstitial defects and oxygen vacancy defects transformed to oxygen interstitial defects. Fig. 1(b) shows the excitation spectra of ZnO QDs. It was reported that, ZnO QDs excitation peak is related to the energy band [10–12], which is determined by the size of ZnO QDs. In another word, keeping the concentration of ZnO QDs solution all the same [13], the sizes of ZnO QDs could be reflected by the measured excitation peaks. As is shown in Fig. 1(b) and (c), with the reaction time increasing, the excitation peaks of ZnO QDs first increased from 364 nm to 367 nm. Then the excitation peaks remained at 367 nm. It does mean that, during the reaction under 60 °C, the sizes of ZnO QDs first grew up then kept unchangeable. That may be because after 30 min reaction under 60 °C, ions in the solution were all reacted completely. The sizes of ZnO QDs synthesized by ultrasonic method were almost same [7,8]. As a result, the ostwald ripening process was at chemical equilibrium. In another word, the sizes of QDs could hardly increase through ostwald ripening. As a consequence, ZnO QDs would not grow under 60 °C by extending ultrasonic time after 30 min reaction. Fig. 1(d) shows the changing trend of ZnO QDs quantum yields influenced by the ultrasonic time. It is reported that, quantum

yields of ZnO QDs are related to their optical defects. Introducing more optical defects [11] could improve the quantum yields of ZnO QDs. Enlarging the sizes of ZnO QDs would decrease the quantum yields by the reduction of optical defects [14]. As is shown in Fig. 1(b) and (c), with the reaction increasing, the sizes of ZnO QDs synthesized in 60 °C first grew up, then kept unchangeable. In theory, with the increasing of reaction time, quantum yields of these samples should first decrease then keep unchangeable. However, it is seen that the quantum yields of ZnO QDs first increased then decreased. The reason is that, quantum yields of QDs are also determined by the electronic traps [15], especially the dangling bonds defects on the surface of ZnO QDs. Passivation of the surface-state traps would improve the quantum yields of QDs as well [16]. Even the quantum yields decreased by enlarging the sizes of ZnO QDs, the decreasing of dangling bonds increased the quantum yields. As the increment was larger than the decrement, the final changing trend of quantum yields were increasing. Therefore, the first increment of quantum yields may be because of the reducing of dangling bonds defects, while the following decrement was owing to the reducing of optical defects. To find out the changing trend of defects in ZnO QDs during the reaction, the time-resolved photoluminescence decay spectra were measured. Fig. 2(a)–(d) are the measured Time-resolved photoluminescence decay spectra of ZnO QDs synthesized in 60 °C. Fig. 2(e) shows the fluorescence lifetime of ZnO QDs under different reaction time. Fig. 2(e) shows that, with the reaction going on, the fluorescence lifetimes of ZnO QDs decreased from 1.45  104 s to 8.54  105 s. It is reported that, the fluorescence lifetime is related to the nonradiative exciton. Shorter fluorescence lifetime means less defects [17]. In another word, with the reaction going on, the amount of defects in ZnO QDs reduced.

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Fig. 4. Excitation spectra of ZnO QDs synthesized with different ultrasonic temperature and time. (a) 5 min (b) 30 min (c) 60 min (d) 90 min.

Table 2 Excitation peaks of ZnO QDs synthesized with different ultrasonic temperature and time. Time/min

5 30 60 90

T/°C

Table 3 Quantum yields of ZnO QDs synthesized with different ultrasonic temperature and time. Time/min

20

30

40

50

60

354.5 nm 356.6 nm 354.5 nm 346.1 nm

352.4 nm 349.2 nm 354.5 nm 346.1 nm

358.7 nm 356.6 nm 356.6 nm 358.7 nm

355.5 nm 360.8 nm 360.8 nm 362.9 nm

364.0 nm 367.2 nm 367.2 nm 367.2 nm

Combined with the previous result, the changing trend of defects in ZnO QDs could be summarized. During the reaction, both the optical defects and the dangling bonds defects reduced with the reaction going on. The decrement of the dangling bonds defects were faster than those of the optical defects. It means that, the optical defects decreased only after the dangling bonds defects almost disappeared. 3.2. Temperature of reaction Fig. 3 shows the emission spectra of ZnO QDs synthesized with different ultrasonic temperature and time. It could be seen in Fig. 3 that, the samples synthesized with different ultrasonic temperature and time displayed similar spectra shapes. Table 1 collects the emission peaks of those ZnO QDs. All the emission peaks were between 500 nm and 530 nm, which are mainly composed of the oxygen vacancy defects and oxygen interstitial defects. With increasing of ultrasonic temperature, the defects in QDs first changed from oxygen interstitial defects to oxygen vacancy defects.

5 30 60 90

T/°C 20

30

40

50

60

5.8% 8.1% 11.2% 12.4%

11.2% 14.8% 19.8% 15.4%

10.7% 16.3% 18.0% 20.8%

27.8% 13.2% 13.3% 15.9%

12.9% 14.9% 17.9% 16.8%

Then the defects changed back to oxygen interstitial defects. For ultrasonic time, the changing trend was as same as that of ultrasonic temperature. In another word, increasing both ultrasonic time and temperature could make the defects transformed from oxygen interstitial defects to oxygen vacancy and zinc interstitial defects first. Then the defects would change back to oxygen interstitial defects again. Fig. 4 shows the excitation spectra of ZnO QDs synthesized with different ultrasonic temperature and time. Table 2 collects the excitation peaks of those ZnO QDs. As is shown in Fig. 4 and Table 2, at the ultrasonic temperature under 50 °C, the excitation peaks first decreased then increased. While at the ultrasonic temperature above 50 °C, the excitation peaks kept increasing. Since the changing trend of excitation peaks could reflect the changing trend of ZnO QDs sizes. In consequence, with the increasing of both ultrasonic temperature and time, the sizes of ZnO QDs first decreased then increased. Table 3 shows the quantum yields of ZnO QDs synthesized with different ultrasonic temperature and time. As is mentioned in Sec-

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Fig. 5. TEM and HRTEM micrographs, size distributions, electron diffraction patterns of ZnO QDs synthesized in 30 °C with different ultrasonic time. (a) 5 min (b) 30 min (c) 60 min (d) 90 min.

tion 3.1, QDs with small sizes should have large quantum yields [14]. As is seen in Tables 2 and 3, most of the samples approached to this rule but some were not. As is reported, the defects of QDs are composed of the optical defects (positive) and the dangling bonds (negative). The amount of all the defects reduced with the reaction going on. The dangling bonds defects reduced faster than the optical defects. After the particles finished growing, new layer formed. At this time, a group of new optical defects appeared, which made the quantum yields increased with the sizes rising at low ultrasonic temperature. Fig. 5 shows the TEM and HRTEM micrographs, size distributions, electron diffraction patterns of ZnO QDs synthesized in 30 °C with different ultrasonic time. In the TEM micrographs, the black dots are ZnO QDs. As is shown in the Fig. 5, the ZnO QDs are homodisperse and not agglomerated. Most of the ZnO QDs are in the size of 2.5 ± 0.5 nm. The average sizes of ZnO QDs synthesized for 5 min, 30 min, 60 min and 90 min are 2.8 nm, 2.6 nm, 2.8 nm and 2.5 nm respectively. This changing trend approached to the changing trend of excitation peaks. As a result, the changing trend of excitation peaks of ZnO QDs could reflect the changing trend of ZnO QDs sizes. Furthermore, as is seen in Fig. 5(a), most of the ZnO QDs are in small sizes. While there are several abnormal ZnO QDs particles with bigger sizes. The ZnO QDs with bigger sizes could hardly be found in the QDs synthesized after 30 min. Those bigger ZnO QDs particles could be seen as the ZnO QDs mesocrystals, which were constituted by smaller ZnO QDs particles. Because of the PEG inside, those constituted particles maybe not densified. With the reaction going on, PEG might be discharged and the particles grew densified. As a result, the sizes of ZnO QDs decreased. 3.3. Role of PEG To find out the effect of PEG on ZnO QDs photoluminescence properties. The ZnO QDs were synthesized via the same ultrasonic method at 30 °C for 5–90 min without adding PEG (called group b for short). Photoluminescence properties of the ZnO QDs are presented in Fig. 6. As is shown in Fig. 6(a) and (c), the emission peaks were changed from 500 nm to 494 nm. This trend is also connected to the change of emitting color shown in Fig. 6(e). While the excitation

peaks were about 347 nm which is shown in Fig. 6(b) and (c). This trend is different from those ZnO QDs synthesized with PEG at 30 °C (called group a for short). The excitation peaks of group b were at the same level of the sample synthesized for 60 min in group a. It means that the sizes of ZnO QDs in group b were not changed with long time reaction. It also means that, sizes of ZnO QDs synthesized without PEG would not first decrease then increase. The reason is that, for group a, during the reaction, small ZnO QDs particles clustered and become new particles with larger sizes. As the existence of PEG, these constituted particles were not densified. With reaction going, PEG were crushed out of the ZnO QDs, and the sizes of ZnO QDs decreased. Fig. 6(d) displays the quantum yields of group b. It shows that, the quantum yields first increased from 17.5% to 21.3% with 30 min reaction, then decreased to 18.0%. This trend was almost the same with those samples in group a. However the quantum yields were larger than those of group a, and reached the extremum peak earlier. As is mentioned, PEG was inside the constituted ZnO particles. It would slow down the formation of ZnO QDs. The opticals defects would also be modified with PEG. As a result, the formation of ZnO QDs went faster and the optical defects in ZnO QDs were more without PEG. 3.4. Role of ultrasonic radiation To find out the effect of ultrasonic radiation on ZnO QDs photoluminescence properties. The ZnO QDs were synthesized via a stirring method at 30 °C for 5–90 min (called group c for short). Photoluminescence properties of the ZnO QDs are presented in Fig. 7. As is shown in Fig. 7(a) and (c), the emission peaks dropped from 501 nm to 494 nm, which could also be seen in Fig. 7(e). The changing trend of excitation peaks is shown in Fig. 7 (b) and (c). The excitation peaks were increased from 340 nm to 351 nm. It means that, with the reaction going, sizes of ZnO QDs kept growing. This trend is different group a. It may be because that, for the ZnO QDs synthesized under ultrasonic, the nucleary process happened uniformly in the solution. Small ZnO QDs particles assembled and grew up. As a result, the sizes of QDs first decreased then increased. While for the reaction under stirring, the nucleary process was not happened uniformly in the solution.

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Fig. 6. Photoluminescence properties of ZnO QDs without PEG synthesized in 30 °C with different ultrasonic time. (a) Emission spectra (b) excitation spectra (c) emission and excitation peaks (d) quantum yield (e) CIE diagram.

The growth process was processed by the reaction between Zn2+ ions and OH ions on the surface of small ZnO QDs particles. So the sizes of ZnO QDs kept increasing. Moreover, the max excitation peak of group c were smaller than that of group a. It indicates that, the sizes of ZnO QDs in group c were smaller than those in group a. It also means that, ultrasonic radiation brought more energy for ZnO QDs growth, which made the synthesizing process faster. Fig. 7(d) displays the quantum yields of group c. As is shown in the figure, with the reaction going, the quantum yields of ZnO QDs increased from 11.2 to 18.3. It suggests that, with the reaction time increasing, the amount of dangling bonds defects decreased. Compared with group a, the changing trend of quantum yields is same to that of the sample with the reaction time under 60 min. It indi-

cates that, the rate of reaction under ultrasonic radiation is faster than that under stirring. 3.5. Growth mechanism The formation of ZnO QDs under ultrasound radiation was reported to be the formulas below [18]

Zn2þ þ 4OH !2ZnðOHÞ2 4

ð2Þ

2ZnðOHÞ2 4 !2ZnO þ 4H2 þ 3O2

ð3Þ

Fig. 8 displays the formation of ZnO QDs in ethanol solution during ultrasonic reaction. After the solution mixed under ultrasound

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Fig. 7. Photoluminescence properties of ZnO QDs synthesized in 30 °C with different stirring time. (a) Emission spectra (b) excitation spectra (c) emission and excitation peaks (d) quantum yield (e) CIE diagram.

Fig. 8. Formation of ZnO QDs in ethanol solution during ultrasonic reaction.

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Fig. 9. Photoluminescence properties of ZnO QDs synthesized for 90 min with different ultrasonic temperature. (a) Emission spectra (b) excitation spectra (c) emission and excitation peaks (d) quantum yield (e) CIE diagram. (The concentration of Zn(Ac)2 is 0.1 mol/L).

radiation, the ions mixed equality quickly and collided seriously. This made ZnO QDs nucleated easily. There were also many of dangling bonds defects on the surface of ZnO QDs, which brought nonradiative exciton and lowered the quantum yields. As ultrasonic radiation brought more free radicals, such as OH radicals [8,18], ZnO particles were more chemical active. Because of the ultrasonic cavitation, ZnO QDs could nucleate homogeneously in the ethanol solution. In that case, small ZnO particles could cluster, and become new particles with larger sizes. These new particles constituted by smaller particles could be seen as mesocrystals [19–23]. However, as the existence of PEG and dangling bonds defects, these constituted particles were not densified. With the reaction going on, these particles grew densified. At last, their volume shrunk, which means the sizes decreased. As a result, the optical defects increased and the dangling bonds defects decreased by this growth process. When this step finished, new layer formed by the same processes. Moreover, thanks to the uniform sizes of ZnO QDs, ostwald ripening only reduced the defects of ZnO QDs instead of enlarging the sizes of bigger particles. As dangling bonds defects were almost died away at this time, the optical defects started disappearing. As a result, the

quantum yields started decreasing. Furthermore, with the ultrasonic temperature increasing, whole the mentioned processes started ahead of time. 3.6. Concentration of raw materials The raw materials of 0.1 mol/L Zn(Ac)2 and 0.2 mol/L LiOH were used to discussed the influence of the concentration of raw materials on ZnO QDs photoluminescence properties. These ZnO QDs were synthesized via same ultrasonic method at 20–60 °C for 90 min (called group d for short). Photoluminescence properties of the ZnO QDs are presented in Fig. 9. As is shown in Fig. 9(a)–(c), the emission peaks were changed from 509 nm to 539 nm and the excitation peaks were changed from 357 nm to 374 nm. This trend is same as those ZnO QDs synthesized with 0.025 mol/L Zn(Ac)2 and 0.05 mol/L LiOH at 20–60 °C for 90 min (called group e for short). However the excitation peaks of group d were all larger than those of group e. It means that the sizes of ZnO QDs in group d were larger than those in group e. It also means that, changing the concentration of raw materials solu-

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tion is an effectively way to control the sizes of ZnO QDs. The reason is that, in the concentrated solution, the raw materials ions collided easily. So the nucleation process and growth process occurred easily. Furthermore, in the weak solution, there were fewer ions left after the first nucleation process. As a result, ostwald ripening was the main way for ZnO QDs to grow. While in the concentrated solution, there were more ions left after the first nucleation process, which would take parts in the growth process. At last, the growth process occurred easily. Fig. 9(d) displays the quantum yields of group d. It is obviously seen that, with the temperature increasing, the quantum yields decreased sharply. It indicates that the quantity of optical defects reduced with the temperature rising by enlarging the sizes of ZnO QDs [14]. However, there is something interesting when comparing group d with group a. Under same ultrasonic temperature, the sizes of ZnO QDs in group b were larger than those of group a. As reported, the quantum yields of group d should be smaller than those of group e. While the fact is that, the quantum yields of group d was larger than those of group e for each sample. As the amount of raw materials for each concentration is different, the maximum size of ZnO QDs might be different. That is concentrated solution leaded to larger maximum size, while weak solution leaded to smaller maximum size. Small maximum size means the growth process would stop early, so the ostwald ripening could start earlier. It is mentioned that, modifying the defects of ZnO QDs would instead of largen them sizes during ostwald ripening. As a result, optical defects of ZnO QDs reduced easier in weak solution than those in concentrated solution. The quantum yields of the ZnO QDs synthesized in weak solution were also smaller than those synthesized in the concentrated solution. 4. Conclusions In this work, ZnO QDs were synthesized via an ultrasonic method in ethanol solution. All the samples displayed green emission. Compared with the reaction without ultrasonic radiation, reaction under ultrasonic radiation was faster. The sizes changing trend was also different for the ZnO QDs synthesized with or without ultrasonic radiation. Both ultrasonic temperature and time have effects on the amount and type of defects and the sizes of ZnO QDs. With the increasing of ultrasonic temperature and time, the amount of all the defects decreased. The dangling bond defects were the major reducing defects. With the ultrasonic temperature and time increasing, the optical defects of ZnO QDs first changed from oxygen interstitial defects to oxygen vacancy and zinc interstitial defects. Then they changed back to oxygen interstitial defects. Sizes of ZnO QDs first decreased then increased with the increasing of ultrasonic temperature and time. As the ultrasonic cavitation, ZnO QDs could homogeneously nucleate. When the nucleation finished, ZnO mesocrystals were formed by small size ZnO QDs. While due to the existence of PEG inside, ZnO QDs mesocrystals were not densified. As a result, the sizes of ZnO QDs decreased after ZnO mesocrystals formed due to the discharge of PEG. The concentration of raw materials also affected defects and sizes of ZnO QDs. The result shows that, concentrated solution brought larger sizes and more optical defects. These ZnO QDs could be employed to improve the UV-absorption of LED package resin and enhance the photoluminescence intensity of LED device. The growth mechanism of ZnO QDs synthesized under ultrasonic radiation might also be useful in explaining the formation of other nanoparticles synthesized under ultrasonic radiation. Acknowledgements

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The authors gratefully acknowledge the financial support for this work from the Project Funded by the Priority Academic Please cite this article in press as: W. Yang et al., Fast synthesize ZnO quantum dots via ultrasonic method, Ultrason. Sonochem. (2015), http://dx.doi.org/ 10.1016/j.ultsonch.2015.11.015

Fast synthesize ZnO quantum dots via ultrasonic method.

Green emission ZnO quantum dots were synthesized by an ultrasonic sol-gel method. The ZnO quantum dots were synthesized in various ultrasonic temperat...
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