Elsevier

Applied Surface Science

Volume 398, 15 March 2017, Pages 97-102
Applied Surface Science

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

https://doi.org/10.1016/j.apsusc.2016.12.035Get rights and content

Highlights

  • Defects modification of ZnO QDs based on rare earth acetylacetonate.

  • Stable Ce(AcAc)3/ZnO QDs with an 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.

Abstract

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 single bondCOOH 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 π–system of acetylacetone.

Graphical abstract

RE(AcAc)3 (RE = Ce, Dy and Tb) can realize the defects modification of ZnO QDs based on the linkage occurs between the protons of the hydroxyl groups on the surface of ZnO QDs and the π–system of acetylacetone. The color coordinate could be shifted among yellow-green, blue-green, and green region by changing the RE (AcAc)3 ratios. The stable Ce(AcAc)3/ZnO QDs with average sizes of about 3.0 nm can be obtained. 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).

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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], [2], [3], [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 to obtain the maximum emission from 4f-4f transitions, for which the choice of ligand is essential [15]. Therefore, an effective kind of β-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.

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 single bondCOOH 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 π–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.

Section snippets

ZnO QDs

The ZnO QDs were synthesized by the ultrasonic sol-gel method [27]. 2.2 g (0.01 mol) Zn(CH3COO)2 2H2O (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

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 π −system of acetylacetone. It also has been proved by other researcher in the formation of ZnO@c-Fe2O3 core–shell

Conclusions

In summary, a series of RE(AcAc)3 (REdouble bondCe, 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 π–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

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 Program Development of Jiangsu Higher Education Institutions (PAPD).

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