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: Ho(AcAc)3 modified ZnO quantum dots (QDs) were successfully prepared via a conventional method. The FTIR spectra were taken to explore the combination of Ho(AcAc)3 and ZnO QDs. The addition of Ho(AcAc)3 could lower the emission intensity of ZnO QDs by modifying the defects of ZnO QDs. Three types of ZnO QDs with different optical defects were used to composite with Ho(AcAc)3. The effect of Ho(AcAc)3 on different optical defects in ZnO QDs was distinctive. Ho(AcAc)3 modified ZnO QDs were successfully composited with the epoxy resin. With the addition of ZnO QDs, the emission intensity and UV absorption increased. Samples with Ho(AcAc)3 had higher visible light transmittance than those without Ho(AcAc)3. As such, Ho(AcAc)3 could improve the dispersity of ZnO QDs in epoxy. Thus, Ho(AcAc)3 could act as a compatibilizer between ZnO QDs and epoxy resin.
© 2016 Optical Society of America
1. Introduction
Resin was widely used in our lives. However, as the resin aging, the service life of resin is limited. Especially for the resin used in LED package [1], which is brought by the UV-radiation. Nowadays, nanoparticles were used to improve the properties of resin formulated composites. For example, ZrO2 was reported to improve the UV-absorption of epoxy resin [2]. CdSe-QD was composited with silicone nanocomposites to obtain tunable photoluminescent property materials [3].
Colloid quantum dots (QDs) have attracted much attention in the recent years due to their amazing characteristics in optics. It is easy to adjust the energy gap of QDs by changing the size of QDs. The sizes of QDs could be controlled by the synthetic methods. As is reported, QDs could enhance the UV-absorption and photoluminescence properties of the polymer, such as epoxy resin composites [4].
To increase the stability, lower the photoluminescence losses and improve the solubility with the resin, modifying ZnO QDs was widely studied. In recent years, TOPO (trioctyl-phosphine oxide) [5], mercaptoacetic acid [6, 7], OA (oleic acid) [8, 9], PVB (polyvinyl butyral) [10], PVA (polyvinyl alcohol) [11] and PEG (polyethylene glycol) [12, 13] were used to modify the QDs. It is also reported that the optical properties of CdSe QDs-epoxy composites were changed with different ligands of the additives [14]. In any case, to find a suitable ligand is necessary and advantageous.
Zinc oxide (ZnO) QDs exhibited excellent UV-absorption and photoluminescence properties owing to their wide band gap (3.37 eV) and large exciton binding energy (60 meV) [10,15]. ZnO QDs were also used to improve the optical performance of resin [4, 16, 17].
However, to search for an excellent ligand which could improve the solubility with resin is still needed. In this article, holmium acetylacetonate Ho(AcAc)3 was used to modify ZnO QDs. The optical properties of Ho(AcAc)3 modified ZnO QDs and Ho(AcAc)3 modified ZnO QDs-epoxy resin composites were studied.
2. Material and methods
2.1. Materials preparation
2.1.1 ZnO QDs
ZnO QDs samples were prepared via the ultrasonic sol-gel method [10, 18–20].
To have a 0.0025 mol ZnO QDs emission peaks near 490 nm, 0.55 g (0.0025 mol) Zn(CH3COO)2∙2H2O (Zn(Ac)2) was dissolved in 100 mL ethyl alcohol and the sample was kept stirring for 30 min. 0.21 g (0.005 mol) LiOH was dissolved in 50 mL ethyl alcohol and kept stirring for 30 min. Then an appropriate amount of PEG-400 with n(PEG): n(Zn) = 1:1, was added into the Zn(Ac)2 solution with additional 40 min stirring. After that, the LiOH solution was added to this mixture. ZnO QDs were obtained after continuously stirring at 20 °C for 2 hours.
To obtain 0.01 mol ZnO QDs emission peaks near 510 nm, 2.2 g (0.01 mol) Zn(CH3COO)2∙2H2O (Zn(Ac)2) was dissolved in 100 mL ethyl alcohol and the solution was then kept stirring for 30 min. Then 0.84 g (0.02 mol) LiOH was dissolved in 50 mL ethyl alcohol and was kept stirring for addition 30 min. Then appropriate amount of PEG-400 with n(PEG): n(Zn) = 1:1 was added into the Zn(Ac)2 solution then with 40 min stirring. After that, the LiOH solution was added. ZnO QDs were obtained after continuously stirring at 20 °C for 2 hours.
To obtain 0.01 mol ZnO QDs emission peaks near 540 nm, 2.2 g (0.01 mol) Zn(CH3COO)2∙2H2O (Zn(Ac)2) was dissolved in 100 mL ethyl alcohol and was kept stirring for 30 min. 0.84 g (0.02 mol) LiOH was dissolved in 50 mL ethyl alcohol and then stirring for addition 30 min. Then appropriate amount of PEG-400 with n(PEG): n(Zn) = 1:1 was added into the Zn(Ac)2 solution with 40 min stirring. After that, the LiOH solution was added. ZnO QDs were obtained after continuously stirring at 60 °C for 2 hours.
For the entire solutions, 1.5 mL oleic acid (OA) was used to precipitate the ZnO QDs. The white ZnO QDs sediment was centrifuged for 5 min under the condition of 4000 rpm for 2-3 times and further washed with an excess ethanol to remove any un-reacted material. Finally, ZnO QDs were dispersed in the n-hexane.
2.1.2 Ho(AcAc)3 – ZnO QDs composite material
The Holmium acetylacetonate (Ho(AcAc)3) was dissolved in ethyl alcohol for the further use. Appropriate amount of Ho(AcAc)3 solution was added to modify ZnO QDs with mixing rations of: n(Ho(AcAc)3):n(ZnO) = 0.0025:1, 0.005:1, 0.01:1, 0.02:1, 0.03:1 and 0.04:1. The mixed solution was reacted under ultrasonic radiation at 10 °C for 5 min. After that, the Ho(AcAc)3 – ZnO QDs composite solution was obtained.
2.1.3 Ho(AcAc)3 – ZnO QDs modified with an epoxy composite
The Ho(AcAc)3 – ZnO QDs modified epoxy resin were prepared by the following method. Firstly, appropriate amount of Ho(AcAc)3 – ZnO QDs was mixed with the epoxy curing agent. The mass fraction of Ho(AcAc)3 – ZnO QDs in the epoxy resin was 0wt%, 1wt%, 3wt%, 5wt% and 7wt%. Then the mixture of Ho(AcAc)3 – ZnO QDs and epoxy curing agent was added into epoxy resin. The epoxy resin was with the quality of three times weight to the epoxy curing agent. The epoxy resin and epoxy curing agent were kept stirring for 5 min to obtain a homogeneously solution. Then the resin was filled into the mould with the thickness of 3 mm. After 1 day curing, the Ho(AcAc)3 – ZnO QDs modified epoxy resin was obtained.
2.2. Measurement
The FT-IR spectra was recorded in the 4000 - 400 cm−1 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 photoluminescence spectra were taken by a fluorescent spectrophotometer (Lumina, Thermo, USA). For each sample, the emission spectrum was measured under the measured emission peak as an excitation wavelength, and the excitation spectrum was measured under the measured excitation peak as an emission wavelength. Finally the absorption spectra were recorded on a UV–Vis-NIR spectrophotometer (UV-3600, Shimadzu, Japan) used n-hexane as reference.
3. Results and discussion
3.1. Ho(AcAc)3 modified ZnO QDs
Figure 1 displays the FT-IR spectra of ZnO QDs, Ho(AcAc)3, and Ho(AcAc)3 modified ZnO QDs.
Fig. 1 FTIR spectra of ZnO QDs samples (a) ZnO QDs, (b)~(f) ZnO QDs + Ho(AcAc)3, n(Ho(AcAc)3):n(ZnO) = 0.005:1, 0.01:1, 0.02:1, 0.03:1 and 0.04:1, (g) Ho(AcAc)3.
Sample (a) is ZnO QDs capped with PEG-400 and oleic acid. The absorption peaks near 3420 cm−1 and 1030 cm−1 are attributed to -OH. The stretching peaks of ZnO are near 450 cm−1. Peaks near 2920 cm−1, 2850 cm−1 and 1430 cm−1 are for the C-H in methyl and methylene. The peak near 1110 cm−1 is arising from the C-O-C. Peaks near 3010 cm−1, 1590 cm−1 and 940 cm−1 are assigned to the C = C and C = O.
For sample (g) which is Ho(AcAc)3, peaks near 1600 cm−1, 1520 cm−1 and 920 cm−1 are approached to the C = O and C = C. Peaks near 3030 cm−1, 2920 cm−1, 1360 cm−1, 1020 cm−1, 820 cm−1 and 770 cm−1 were reported to be for the C-H in methyl and methylene.
As is shown in the figure, compared with ZnO QDs, Ho(AcAc)3 modified ZnO QDs presented the peaks near 1385 cm−1 and 1000 cm−1, which were also seen in the spectra of Ho(AcAc)3. With the increase of Ho(AcAc)3 content, these peaks became evident. That is because of the mixture of the C-H peaks of ZnO QDs and Ho(AcAc)3. The peaks of C-O-C near 1100 cm−1 enhanced, while the peaks of C = C and C = O near 3010 cm−1, 1590 cm−1 and 920 cm−1 weakened. It means that the C = C reacted with C = O for C-O-C. In another word, the C = O of Ho(AcAc)3 was reacted with the C = C of oleic acid on the surface of ZnO QDs.
Figure 2 shows the TEM and HRTEM micrographs, size distributions, electron diffraction patterns of ZnO QDs (emission peaks near 510 nm). In the TEM micrographs, the black dots are ZnO QDs. As is shown in the Fig. 2, the ZnO QDs are homodispersed and not agglomerated. Most of the ZnO QDs are in the size of 3.0 ± 0.5 nm. The average sizes of ZnO QDs are 3.0 nm.
Fig. 2 TEM and HRTEM micrographs, size distributions, electron diffraction patterns of ZnO QDs (emission peaks near 510 nm).
For ZnO QDs, the excitation peaks could reflect the sizes, with the increasing of sizes, the excitation peaks red shifted [21]. While the green photoluminescence near 500 nm of ZnO bulk (with the optical gap of 3.37 eV) is reported to be (1) the transition from deep donor level by to valence band (2.43 eV), (2) transition from near conduction band edge to deep acceptor level by or (2.24 eV)(2.38 eV) [17, 22–26].
Figure 3 displays the photoluminescence properties of Ho(AcAc)3 modified ZnO QDs (emission peaks near 490 nm) with different amount of Ho(AcAc)3. As is shown in Fig. 3, the emission intensity and excitation intensity decreased when the amount of Ho(AcAc)3 increased. When the ratio between Ho(AcAc)3 and ZnO QDs reached 0.02:1, the emission and excitation peaks were almost disappeared. With the increasing of the amount of Ho(AcAc)3, both the emitting colour, emission peaks and excitation peaks red shifted. It is highly possible that variations in ligand graft density (Ho:Zn ratio) would change ZnO cluster sizes. For the excitation spectra, the changing trend could be seen in Fig. 3(b). The excitation intensity of those spectra under 360 nm decreased due to the modifying of optical defects in ZnO QDs. On the opposite, the excitation intensity of those spectra over 360 nm increased because of the introducing of Ho(AcAc)3.
Fig. 3 Photoluminescence properties of Ho(AcAc)3 modified ZnO QDs (emission peaks near 490 nm) with different ratio between ZnO QDs and Ho(AcAc)3. (a) emission spectra, (b) excitation spectra, (c) CIE diagram.
Figure 4 displays the photoluminescence properties of Ho(AcAc)3 modified ZnO QDs (emission peaks near 510 nm) with different amount of Ho(AcAc)3. As is shown in Fig. 4, the changing trend of emission intensity and excitation intensity with the amount of Ho(AcAc)3 increasing is as same as that of the ZnO QDs with emission peaks near 490 nm. While when the ratio between Ho(AcAc)3 and ZnO QDs changed to 0.03:1, both the emission and excitation peaks almost disappeared. The positions of both emission and excitation peaks almost did not change with the increasing of the amount of Ho(AcAc)3. It means that, for the ZnO QDs with 510 nm emitting, the ZnO cluster sizes were large. As a result, the variations in ligand graft density (Ho:Zn ratio) might not change the sizes obviously. For excitation spectra, the changing trend could be seen in Fig. 4 (b). This trend is the same to that of the ZnO QDs with emission peaks near 510 nm. Figure 4 (c) shows the emitting colour of the samples. As is seen that, the emitting colour blue-shifted. That is because of the decrement of the emission intensity brought by ZnO QDs defects near 510 nm and the increment of the emission intensity brought by Ho(AcAc)3 near 430 nm, which could change the relative ratio of peaks near 510 nm and 430 nm.
Fig. 4 Photoluminescence properties of Ho(AcAc)3 modified ZnO QDs (emission peaks near 510 nm) with different ratio between ZnO QDs and Ho(AcAc)3. (a) emission spectra, (b) excitation spectra, (c) CIE diagram.
Figure 5 displays the photoluminescence properties of Ho(AcAc)3 modified ZnO QDs (emission peaks near 540 nm) with different amount of Ho(AcAc)3. As is shown in Fig. 5, the changing trend of emission intensity and excitation intensity with the amount of Ho(AcAc)3 increasing is as same as that of the ZnO QDs with emission peaks near 490 nm and 510 nm. When the ratio between Ho(AcAc)3 and ZnO QDs changed to 0.02:1, both the emission and excitation peaks were almost disappeared. While for the intensity of emission peak near 430 nm, it increased with the increasing of the amount of Ho(AcAc)3. The positions of both emission and excitation peaks did not change with the increasing of the amount of Ho(AcAc)3, which is same to the ZnO QDs with 510 nm emitting. It implies that, for the ZnO QDs with 540 nm emitting, the ZnO cluster size was large. For the excitation spectra, the changing trend could be seen in Fig. 5(b). This trend is the same to that of the ZnO QDs with emission peaks near 540 nm. Figure 5(c) shows the emitting colour of the samples. As shown, the emitting colour has blue-shifted. That is because of the decrement of the emission intensity brought by ZnO QDs defects near 540 nm and the increment of the emission intensity brought by Ho(AcAc)3 near 430 nm, which could change the relative ratio of peaks near 540 nm and 430 nm.
Fig. 5 Photoluminescence properties of Ho(AcAc)3 modified ZnO QDs (emission peaks near 540 nm) with different ratio between ZnO QDs and Ho(AcAc)3. (a) emission spectra, (b) excitation spectra, (c) CIE diagram.
All the samples showed that, with the addition of Ho(AcAc)3, the characteristic emission and excitation intensity of ZnO QDs decreased. The characteristic emission and excitation peaks of ZnO QDs did not change, while emission intensity of peaks near 430 nm increased. This was brought by the Ho(AcAc)3.
3.2. Epoxy resin composited with Ho(AcAc)3 modified ZnO QDs
Figures 6(a)-(d) shows the emission spectra of epoxy composited with Ho(AcAc)3 modified ZnO QDs. The changing trend of emission intensity was shown in Fig. 6(e). When the content of ZnO QDs was 1 wt%, the emission spectra were almost the same as those of the pure epoxy resin. While when the content reached 3 wt%, the emission spectra exhibited the characteristic of ZnO QDs. It means that, the emission at wavelength near 400 nm was brought by the epoxy resin. For the emission intensity, the changing trend was obviously seen in Fig. 6(e). With the increasing of ZnO QDs content in epoxy, the emission intensity of composite materials increased. While with the rising of Ho(AcAc)3 content, the emission intensity first increased then decreased. The emission intensity is determined by the content of ZnO QDs, the emission intensity of ZnO QDs themselves and the compatibility of ZnO QDs with epoxy resin. As is mentioned in 3.1, the emission intensity of ZnO QDs modified with Ho(AcAc)3 was smaller than those without Ho(AcAc)3. While for the ZnO QDs modified with Ho(AcAc)3 in epoxy resin, the emission intensities were as high as those of ZnO QDs. The reason is that, when the content is large, ZnO QDs would not uniformly scattered in the epoxy resin. So undispersed QDs scattered the light. Ho(AcAc)3 could enhance the dispersion uniformity of QDs, which could reduce the scattering brought by those undispersed QDs. In another word, the Ho(AcAc)3 played a role like the compatibilizer or coupling between ZnO QDs and epoxy resin.
Fig. 6 Emission spectra of epoxy resin composited with Ho(AcAc)3 modified ZnO QDs (emission peak 510 nm), n(Ho(AcAc)3): n(ZnO) = (a) 0:1, (b) 0.01:1, (c) 0.02:1, (d) 0.03:1, (e) changing trend of emission intensity.
The excitation spectra of epoxy composited with Ho(AcAc)3 modified ZnO QDs was shown in Figs. 7(a)-7(d). The changing trend of emission intensity was shown in Fig. 7(e). The changing trend of excitation intensity was almost the same to those of the emission intensity. The excitation waveforms of ZnO QDs, epoxy resin with ZnO QDs and epoxy resin with Ho(AcAc)3 modified ZnO QDs were quite different. The excitation waveforms of ZnO QDs had sharp excitation peaks. For the epoxy resin with ZnO QDs, these sharp peaks disappeared. Instead, the waveform became boardband, and the peak had blue-shifted. With the ZnO QDs content increasing, the excitation peaks had red-shifted. For ZnO QDs, the excitation peaks were connected to their sizes. Enlarging the sizes of ZnO QDs could lower their energy band, which makes the excitation peaks red shifted. For the ZnO QDs in the solution, the excitation peaks had red-shifted with the increasing of concentration [27]. This phenomenon could be obviously seen in epoxy resin/ZnO QDs system. For the epoxy resin with Ho(AcAc)3 modified ZnO QDs, the excitation waveforms were just like a triangle. Which means that, the emitting of UV-light from 300 nm to 350 nm were weakened by the modifying of Ho(AcAc)3.
Fig. 7 Excitation spectra of epoxy resin composited with Ho(AcAc)3 modified ZnO QDs (emission peak 510 nm), n(Ho(AcAc)3): n(ZnO) = (a) 0:1, (b) 0.01:1, (c) 0.02:1, (d) 0.03:1 (e) changing trend of excitation intensity.
Figure 8 shows the light transmittance performance of epoxy resin composited with Ho(AcAc)3 modified ZnO QDs. As is shown in the figure, The UV-absorption of epoxy resin increased with the ZnO content increasing. Without Ho(AcAc)3, the visible light transmission first increased then decreased with the addition of ZnO QDs. The epoxy resin with 3 wt% and 5 wt% ZnO QDs had larger visible light transmission. This is because that, ZnO QDs could fill into the micro-pore inside epoxy. As a result, the scattering brought by the pore reduced. However, when the content of ZnO QDs increased to 7 wt%, the unscattered QDs lowered the visible light transmission. When the content of Ho(AcAc)3 is n(Ho(AcAc)3): n(ZnO) = 0.01:1, the changing trend of was almost the same with those without Ho(AcAc)3. While when the content of Ho(AcAc)3 increased to n(Ho(AcAc)3): n(ZnO) = 0.02:1 and 0.03:1, the visible light transmission of epoxy resin with ZnO QDs were all larger than those of pure epoxy resin. It means that, the addition of Ho(AcAc)3 made the ZnO QDs homogeneously dispersed in the epoxy resin. In another word, the consistency of epoxy resin and ZnO QDs increased through Ho(AcAc)3. This phenomenon approached to the opinion mentioned, which is the Ho(AcAc)3 was just like a compatibilizer /coupling between ZnO QDs and epoxy resin.
Fig. 8 UV-Vis transmission spectra of epoxy resin composited with Ho(AcAc)3 modified ZnO QDs (emission peak 510 nm), n(Ho(AcAc)3): n(ZnO) = (a) 0:1, (b) 0.01:1, (c) 0.02:1, (d) 0.03:1.
4. Conclusions
Ho(AcAc)3 modified ZnO QDs were successfully prepared. The addition of Ho(AcAc)3 could lower the emission intensity of ZnO QDs by modifying the defects of ZnO QDs. The emitting colour of Ho(AcAc)3 modified ZnO QDs changed with the added amount of Ho(AcAc)3. For the ZnO QDs with 490 nm emission, with the increasing of amount of Ho(AcAc)3, the emitting colour had red-shifted. While for the ZnO QDs with 510 nm and 540 nm emission, with the increasing of amount of Ho(AcAc)3, the emitting colour had blue-shifted.
For epoxy resin with ZnO QDs, with the adding of ZnO QDs, the emission intensity and the UV absorptivity increased. For moderate amount of ZnO QDs addition, the visible light transmittance increased because the ZnO QDs could fill in the pores in epoxy resin. While when the amount of ZnO QDs is large, because of the undispersed ZnO QDs, the visible light transmittance decreased. However, appropriate amount of Ho(AcAc)3 could improve the dispersity of ZnO QDs in epoxy. This Ho(AcAc)3 addition enhanced the emission intensity and visible light transmittance of epoxy with larger amount of ZnO QDs. With the adding of Ho(AcAc)3 modified ZnO QDs, the UV absorptivity of epoxy resin could be boosted and the visible light transmittance of epoxy resin could remain at the same level. In another word, Ho(AcAc)3 could be a compatibilizer and couple between ZnO QDs and epoxy resin.
Acknowledgments
The authors gratefully acknowledge the financial support for this work from the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the National Natural Science Foundation of China (51202111).
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