Surface defect modification of ZnO quantum dots based on rare earth acetylacetonate and their impacts on optical performance

Applied Surface Science 398 (2017) 97–102

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Surface defect modification of ZnO quantum dots based on rare earth acetylacetonate and their impacts on optical performance Lixi Wang a,b,∗ , Xiaojuan Yang a,b , Weimin Yang a,b , Jing Zhang c , Qitu Zhang a,b , Bo Song d , Chingping Wong d a

School of Materials Science and Engineering, Nanjing Tech University, Nanjing, 210009, Jiangsu, China Jiangsu Collaboration Innovation Center for Advanced Inorganic Function Composites, Nanjing, 210009, Jiangsu, China c China Geol Survey, Nanjing Ctr, Nanjing, 210016, Jiangsu, China d School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, 30332, GA, USA b

a r t i c l e

i n f o

Article history: Received 25 October 2016 Received in revised form 2 December 2016 Accepted 5 December 2016 Available online 6 December 2016 Keywords: Rare earth acetylacetonate Surface modification Optical properties ZnO QDs

a b s t r a c t The surface defect modification has an important effect on the application of ZnO quantum dots, and it has gained much progress in recently years, propelled by the development of additives. Our research efforts are directed toward developing a new surface modification additive RE(AcAc)3 (RE = Ce, Dy, Tb) to achieve fine ZnO QDs and adjust their surface properties. RE(AcAc)3 /ZnO QDs nanostructured materials have been designed and prepared, and particular emphasis has been given to the relation between the surface modification and optical properties. The effects of RE(III) acetylacetonate modification on the FT-IR, TEM images and photoluminescence (PL) spectra were investigated, and the surface defect modification principle and effect were discussed in details. The band gap (Eg ) was also calculated to prove the surface modification effect. For the RE(AcAc)3 /ZnO QDs complex materials, stable linkage occurs because of the affinity of COOH from acetylacetonate anionic ligand to zinc oxide surfaces, with attachment to the zinc oxide by hydrogen bonding between the protons of the hydroxyl groups on the surface of ZnO QDs and the ␲–system of acetylacetone. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Inorganic nano crystal quantum dot materials have attracted great attention for their heat, light, electricity and magnetism properties caused by quantum confinement effect and have been widely used in the fields of energy, optoelectronic devices, optoelectronics, chemical and other fields [1–4]. The zinc oxide is a typical II–VI semiconductor material, which has the characteristics of high electron affinity, high mobility and large exciton binding energy, and its low cost and simple process [5,6]. As such, it is a widely utilized, versatile material implemented in a diverse range of technological applications, particularly in electronics, optoelectronics [7,8], sensors [9], photocatalysts [10,11], electrodes for solar cells [12] and so on. Rare earth complexes have been widely used for novel lighting device [13,14]. It is well known that optimizing energy transfer processes from ligands to emitting ions was the most important

∗ Corresponding author at: School of Materials Science and Engineering, Nanjing Tech University, Nanjing, 210009, Jiangsu, China. E-mail addresses: wanglixi [email protected], [email protected] (L. Wang). http://dx.doi.org/10.1016/j.apsusc.2016.12.035 0169-4332/© 2016 Elsevier B.V. All rights reserved.

to obtain the maximum emission from 4f-4f transitions, for which the choice of ligand is essential [15]. Therefore, an effective kind of ˇ-diketonate [16], such as acetylacetone, was selected as a ligand. In this case, rare earth acetylacetonate could be used as an optical material. However, the point attracting our attention is that the rare earth acetylacetonate can combine with the groups on the surface of ZnO QDs, and it has more complex structure than many linear organic compounds. The high specific surface area and surface aggregation energy of the ZnO quantum dots greatly limited the excellent properties of quantum dots. It has been shown that in the absence of surface modifiers there is a certain tendency of nano particles to agglomerate due to the well known Ostwald ripening [17]. The necessity of utilizing organic surfactants has become crucial in order to reduce the phenomenon. One strategy to solve this problem is surface modification through organic functionalization to achieve fine particles and adjust their surface properties. Current research efforts are directed toward realizing surface modification of ZnO QDs with organic compounds such as polysiloxane [18], mercaptoacetic acid [19], silane [20], cetyltrimethyl ammonium bromide (CTAB) [21], vinyltrimethoxysilane [22], oleic acid [23,24], polyethylene glycol [25,26] and so on.

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Fig. 1. (a) Schematic illustration for fabrication of RE(AcAc)3 /ZnO QDs; (b)FT-IR spectra of ZnO QDs and RE(AcAc)3 (RE = Ce, Dy and Tb) modified ZnO QDs; (c) (d) The HRTEM images of ZnO QDs and Ce(AcAc)3 /ZnO QDs (Ce(AcAc)3 :ZnO QDs = 0.005).

In this work, we developed a simple synthetic procedure to attach RE(III) acetylacetonate to zinc oxide QDs for obtaining fine QDs and adjusting their surface properties. Linkage occurs because of the affinity of COOH from acetylacetonate anionic ligand to zinc oxide surfaces, with attachment to the zinc oxide by hydrogen bonding between the protons of the hydroxyl groups on the surface of ZnO QDs and the ␲–system of acetyl acetone. Using this method, the surface density of RE(III) acetylacetonate might be controlled by varying the molar ratios of the reactants. The effects of RE(III) acetylacetonate modification on the FT-IR, TEM images and photoluminescence (PL) spectra were investigated, and the optical band energy was calculated. The surface defect modification principle and effect were also discussed.

2. Material and methods 2.1. Materials preparation 2.1.1. ZnO QDs The ZnO QDs were synthesized by the ultrasonic sol-gel method [27]. 2.2 g (0.01 mol) Zn(CH3 COO)2 2H2 O (Zn(Ac)2 ) was dissolved in 100 mL ethanol with stirring for 30 min, and 0.84 g (0.02 mol) LiOH was dissolved in 50 mL ethanol with 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 stirring for 40 min. The LiOH solution was added into the mixed solution at last, and the ZnO QDs were obtained after continuously stirring at 60 ◦ C for 2 h.

The ZnO QDs were precipitated with oleic acid, and the sediment was centrifuged (4000 rpm, 5 min) and washed with ethanol for two or three times. 2.1.2. Re(AcAc)3 /ZnO QDs composite material The preparation of organically soluble RE(AcAc)3 /ZnO QDs were based on the modification of our previously reported method for the synthesis of Ho(AcAc)3 /ZnO quantum dots with slight modifications [28]. The rare earth acetylacetonate (RE(AcAc)3 , RE = Ce, Dy, and Tb) was dissolved in ethanol. Appropriate amount of RE(AcAc)3 solution was mixed with ZnO QDs with the ratio of n(RE(AcAc)3 ):n(ZnO QDs) = 0.0025:1, 0.005:1, 0.01:1, 0.02:1, 0.03:1 and 0.04:1. The solution was reacted under ultrasonic radiation at 10 ◦ C for 5 min. At last, the RE(AcAc)3 /ZnO QDs composite solution was obtained. The RE(AcAc)3 /ZnO QDs thin films were prepared by dispersing the obtained modified nanoparticles by sonication in isopropanol and depositing on glass substrate by spin coating (2000 rpm, 20 s). 2.2. Measurement The FT-IR spectra were characterized in the 4000–400 cm−1 range using an FT-IR spectrometer (NEXUS 670, Thermo, America). The TEM images were observed by transmission electron microscopy (JEM-2100, JOEL, Japan). The photoluminescence spectra were measured by a fluorescent spectrophotometer (Lumina, Thermo, America). The optical band gap energy (Eg) was estimated from the fundamental absorption edge of the RE(AcAc)3 /ZnO QDs

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Fig. 2. Photoluminescence properties of Ce(AcAc)3 modified ZnO QDs with different ratio of. Ce(AcAc)3 to ZnO QDs (a) excitation spectra, (b) emission spectra, (c) CIE diagram.

thin films, which were measured by a UV–vis-NIR spectrophotometer (UV-3600, Shimadzu, Japan). 3. Results and discussion 3.1. RE(AcAc)3 modified ZnO QDs RE(AcAc)3 /ZnO QDs composite has been prepared, and the formation process is presented in Fig. 1(a). It is well known that ZnO QDs possess hydroxyl groups on their surface [29]. Therefore, in the modification process, RE(III) acetylacetonate interacts with the surface of ZnO QDs by hydrogen bonding between the protons of the hydroxyl groups on the surface of ZnO QDs and the ␲ −system of acetylacetone. It also has been proved by other researcher in the formation of ZnO@c-Fe2 O3 core–shell materials [30]. Fourier transform infrared (FT-IR) analysis of ZnO QDs and RE(AcAc)3 (RE = Ce, Dy and Tb) modified ZnO QDs have been conducted to investigate surface group species of the nanoparticles and the results are shown in Fig. 1(b). Broad hydroxyl ( OH) stretching peaks at 3420 cm−1 were observed in the pristine QDs and modified QDs, and the peaks located at 1600 cm−1 were related to hydroxyl (-OH) bending peaks. The peaks located at 2920 cm−1 and 2850 cm−1 were sensitive to C H stretching, and represented −CH3 and −CH2 , respectively [31–33]. Methyl ( CH3 ) bending peaks also

exhibited at 1330 cm−1 , and peaks related to C O C was obvious at 1110 cm−1 [33]. The peaks near 1385 cm−1 was attributed to the mixture of C H peaks of ZnO QDs and RE(AcAc)3 . The peaks exhibited at 3010 cm−1 and 1590 cm−1 were referred to C C and C O, respectively. The peaks of −CH3 , −CH2 and −OH were attributed to the oleic acid in the preparation of ZnO QDs. It is clearly indicated that the surfaces of the ZnO QDs synthesized in oleic acid were modified with −CH3 and −OH functional groups. However, the peaks observed for the modification of RE(AcAc)3 at about 3010 cm−1 , 2360 cm−1 , 880 cm−1 and 620 cm−1 attributed to the modification process were different from ZnO QDs. The HRTEM images of series of RE (AcAc)3 /ZnO QDs (RE = Ce, Dy, and Tb) have been measured in our work, with similar micrographs for these three rare acetylacetonate compounds modified ZnO QDs. Therefore, Ce(AcAc)3 modified ZnO QDs was taken for an example to discuss the modification effect of rare acetylacetonate. Fig. 1(c) and (d) show the HRTEM images and electron diffraction patterns of ZnO QDs and Ce(AcAc)3 /ZnO QDs, respectively. In the HRTEM images, the black dots in Fig. 1(c) are ZnO QDs, and those in Fig. 1(d) are Ce(AcAc)3 /ZnO QDs. As is shown in Fig. 1(c) and (d), the ZnO QDs are agglomerated to a certain degree several hours after preparation without the modification of Ce(AcAc)3 . The Ce(AcAc)3 modificated ZnO QDs could obtain an average sizes of about 3.0 nm in the same condition. However, much bigger agglomerated nano

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Fig. 3. Photoluminescence properties of Dy(AcAc)3 modified ZnO QDs with different ratio of. Dy(AcAc)3 to ZnO QDs (a) excitation spectra, (b) emission spectra, (c) CIE diagram.

particles with a size of about 8–10 nm could only be obtained without the modification. The ZnO QDs without modification will aggregate with QDs nearby easily for the unstable surface atoms to form bigger particles under the force of Gibbs free energy. With the RE (AcAc)3 attached on the surface of ZnO QDs, wrapped/coordinated with ZnO QDs will be obtained. Firstly, the attached RE (AcAc)3 restricted the ZnO QDs into a limited space, resulting in the obstruction of the ZnO QDs growth. Then, the RE (AcAc)3 separated the ZnO QDs with the ones nearby, which reduced the contacting probability of ZnO QDs and possibility of formation of bigger aggregated particles. Finally, the defects modification completed, and the stable RE(AcAc)3 /ZnO QDs were obtained. The modification process has been explained in Fig. 1(a). 3.2. Photoluminescence properties Surface modification of ZnO has been found to constitute another key point in triggering a chemical modification of surface defects [34,35]. Therefore, a kind of electronic relaxation mainly results in a broad emission band exhibited by all ZnO QDs, in which the surface O2− /O− states trap the photogenerated holes. In this case, the emission effect in the visible range has been observed attributed to the recombination of the trapped photogenerated holes with oxygen vacancies [36]. The photoluminescence properties are related to the emission of the surface defects attributed to the electronic transitions of surface defects [37], therefore the

effect of surface defects modification can be used to address the photoluminescence properties diversification. Fig. 2 shows the photoluminescence properties of series of Ce(AcAc)3 modified ZnO QDs. The excitation peaks occurred at 370 nm, and the emission peaks exhibited at 540 nm. However, the Ce(AcAc)3 modification has the most obvious effect on the ZnO QDs. When a small amount of Ce(AcAc)3 was used (with the mole ratio of Ce(AcAc)3 to ZnO QDs = 0.005:1), the excitation and emission peaks disappeared. It is indicated that the Ce(AcAc)3 had obvious effect for ZnO QDs. It is shown in Fig. 2(c) that the color coordinate of ZnO QDs was (0.3546, 0.4673), which was in the region of yellow-green light. With the increase of Ce(AcAc)3 content, the color coordinate shifted to green and then blue-green region because of the decrement of the emission peaks brought by ZnO QDs near 540 nm [38]. The pristine ZnO QDs has a high emission intensity related to their abundant defect energy levels. The high area activation results in aggregation of ZnO QDs. The defect emission peak decreased distinctly with the modification of Ce(AcAc)3 , and with the increase of content of Ce(AcAc)3 it decreased gradually. It is demonstrated that the Ce(AcAc)3 attached to the surface of ZnO QDs and restricted the ZnO QDs into a limited space, resulting in the obstruction of the ZnO QDs growth. In the while, the passivation of the surface defects occurred for the formation of stable hydrogen bond between hydroxyl groups on the surface of ZnO QDs and the ␲–system of acetyl acetone. The emission intensity was related to the surface defects of ZnO QDs, suggesting that the surface defect modification will decrease the emission intensity. Therefore, the

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Fig. 4. Photoluminescence properties of Tb(AcAc)3 modified ZnO QDs with different ratio of. Tb(AcAc)3 to ZnO QDs (a) excitation spectra, (b) emission spectra, (c) CIE diagram.

disappearance of emission intensity when mole ratio of Ce(AcAc)3 to ZnO QDs was 0.005:1 indicated that the most effective wrapping completed. Dy(AcAc)3 has also been used to modify the ZnO QDs, and the photoluminescence properties of series of Dy(AcAc)3 modified ZnO QDs are shown in Fig. 3. The excitation peaks occurred at about 375 nm, and the emission peaks exhibited at 540 nm. The intensity of emission and excitation peaks decreased with the increase of Dy(AcAc)3 content, and then disappeared when the mole ratio of Dy(AcAc)3 to ZnO QDs reached 0.03:1. It is shown in Fig. 3(c) that the color coordinate of ZnO QDs was (0.3546, 0.4673), which was in the region of yellow-green light. With the increase of Dy(AcAc)3 content, the color coordinate shifted to blue green light and then near white light region because of the decrement of the emission peaks brought by ZnO QDs near 540 nm [38]. The photoluminescence properties of series of Tb(AcAc)3 modified ZnO QDs are shown in Fig. 4. The excitation peaks also located at 375 nm, and the emission peaks were at 540 nm. As is shown in Fig. 4, the emission and excitation peaks decreased with the increase of Tb(AcAc)3 content, and then disappeared when the mole ratio of Tb(AcAc)3 to ZnO QDs reached to 0.04:1. It is indicated that Ce(AcAc)3 was the most effective among three kinds of RE(AcAc)3 for ZnO QDs. With the increase of Tb(AcAc)3 content, the color coordinate (0.3546, 0.4673) shifted from yellow-green light to blue-green light, attributing to the decrement of the emission peaks brought by ZnO QDs near 540 nm [38].

The optimal mole ratios of RE(AcAc)3 to ZnO QDs for RE(AcAc)3 /ZnO QDs are different. It is shown that the effective order of the RE(AcAc)3 is Ce(AcAc)3 > Dy(AcAc)3 > Tb(AcAc)3 . The ionic radii of these three rare earth ions are different, and the ionic radius of Ce3+ (0.1034 nm) is larger than Dy3+ (0.0908 nm) and Tb3+ (0.0923 nm). Therefore, the Ce(AcAc)3 was the most convenient for warping the ZnO QDs among these three surface modification agents with more suitable warping layer. The PL properties is related to the optical band gap, thus the band gap energy has been calculated to discuss the modification effect of RE(AcAc)3 . The value of optical band gap was calculated from the formula as follows: (˛h)2 = A(h − E g )

(1)

Here A is a parameter that depends on the transition probability [39]. The absorption coefficient (␣) can be calculated by the next formula [22]:



1 ˛ = ln d

(1 − R)2 +



(1 − R)2 + 4R2 T2 2T



(2)

Here d is the film thickness, T is the transmittance and R reflectance of the ZnO QDs and RE(AcAc)3 /ZnO QDs films. The plots (˛h)2 versus low photon energy (h) of ZnO QDs (emission peaks 540 nm) and RE(AcAc)3 /ZnO QDs are shown in Fig. 5. The band gap energy of ZnO QDs was 3.583 eV, while with the

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Program Development of Jiangsu Higher Education Institutions (PAPD). References

Fig. 5. The plots (˛h)2 versus low photon energy (h) of ZnO QDs and RE(AcAc)3 /ZnO QDs (mole ratio of RE(AcAc)3 to ZnO QDs = 0.005).

modification of RE(AcAc)3 the band gap energy decreased. The band gap of ZnO QDs modified with Ce(AcAc)3 , Dy(AcAc)3 and Tb(AcAc)3 was 3.544 eV, 3.551 eV and 3.550 eV, respectively. The largest variation of band gap energy of 0.039 eV (from 3.583 eV to 3.544 eV) was obtained for Ce(AcAc)3 /ZnO QDs material. It is indicated that the Ce(AcAc)3 was the most efficient modification agent for the ZnO QDs, which was consistent with the PL spectra results. 4. Conclusions In summary, a series of RE(AcAc)3 (RE Ce, Dy and Tb) modified ZnO QDs were successfully prepared. The modification process was exhibited that RE(III) acetylacetonate interacts with the surface of ZnO QDs by hydrogen bonding between the protons of the hydroxyl groups on the surface of ZnO QDs and the ␲–system of acetylacetone. The FT-IR spectra indicated that the peaks located at 3010 cm−1 , 2360 cm−1 , 880 cm−1 and 620 cm−1 were related to the modification of RE(AcAc)3 . It is observed in the TEM images that the ZnO QDs are agglomerated to a certain degree with a size of about 8–10 nm without the modification of Ce(AcAc)3 . However, the Ce(AcAc)3 modified ZnO QDs could obtain average sizes of about 3.0 nm. The color coordinate could be shifted among yellow-green, blue-green, and green region by changing the RE(AcAc)3 ratios. In our work, Ce(AcAc)3 has great potential for the surface defect modification for ZnO QDs. The calculated band gap data also proved the efficient modification of Ce(AcAc)3 for ZnO QDs with the largest variation of band gap energy of 0.039 eV (from 3.583 eV to 3.544 eV). Acknowledgements The authors gratefully acknowledge the financial support for this work from the Project Funded by National Natural Science Foundation of China (51202111) and the Priority Academic

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