Facile synthesis, photoluminescence properties and microwave absorption enhancement of porous and hollow ZnO spheres

Highlights

• Porous and hollow ZnO spheres were synthesized by a facile method.

• Microwave absorption properties of porous and hollow ZnO spheres were investigated.

• It was found that the ZnO exhibited a better performance of microwave absorption.

• The relationship of the ZnO and the microwave absorption properties was revealed.

Abstract

Three-dimensional (3D) porous ZnO nanostructures were synthesized via one-pot solvothermal treatment. The structural, morphological and spectral properties were investigated using X-ray diffraction (XRD), N₂ sorption measurement, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Raman spectroscopy, and photoluminescence (PL) spectroscopy. It was found that the minimum reflection loss value of ZnO calcined at 500 °C reached − 5.62 dB at 16.24 GHz with the thickness of 2.5 mm, which had a superior performance of microwave absorption than those uncalcined and calcined at 600 °C. The possible mechanism for the formation of porous ZnO hollow spheres was proposed. The relationship of the ZnO microstructure and the microwave absorption properties was revealed via studying the dielectric loss and interference multi-reflection absorption in this paper as well. In addition, the photoluminescence results show that the uncalcined ZnO and porous hollow ZnO calcined at 500 °C and 600 °C show a narrow and sharp UV emission at 355.8 nm and a relatively broad visible spectra emission at around 423 nm.

Introduction

With the modern technology developing rapidly, the electromagnetic (EM) wave interference has become a serious problem and curious concern. In recent years, EM wave absorbing materials have attracted considerable attention because of their promising application in military and commercial markets. For example, the microwave absorbing materials can reduce radar reflection of target to achieve a better shielding defense system. Similarly, microwave absorber employed in daily lives can prevent human beings from harmful EM radiation [1]. Therefore, a great deal of microwave absorbents has caused enormous focus by many researchers including ferrite, carbon fibers and conductive polymers. The nanostructured ZnO materials, as an important n-type semiconductor, have been extensively applied in various fields such as ultraviolet (UV) lasers, solar cells, gas sensors, photocatalysis [2], [3], [4], [5] and so forth due to their special physical and chemical properties. Recently, many studies demonstrate that ZnO can be also used as a microwave absorption material. Complex morphology and different dimensionalities have attracted great research interest due to the fact that their advanced geometric structure and atom arrangement on the specific facets of these nanostructures can provide novel properties. Controlled synthetic method is of vital importance for advanced functional materials because it not only helps to explore material fabrication techniques, but also provides opportunities to reveal the relationship between material structure and property [6]. To fabricate efficient shielding materials, various microscopic morphologies and structures of ZnO have been studied for their microwave absorption characteristics. Chen et al. [7] showed that pure ZnO nanowire is a low-loss material, and strong microwave absorption in the X band is enhanced with the concentration of nanowire increasing in the composites. Zhuo [8] has reported that ZnO nanostructures (nanowire and nanotrees) with paraffin composites have been investigated in the frequency of 0.1–18 GHz. Excellent microwave absorption performances have been observed in ZnO nanotree composite compared to ZnO nanowire composite. The value of minimum reflection loss for the composites with 60 vol.% ZnO nanotrees is − 58 dB at 4.2 GHz with a thickness of 4.0 mm. Such strong absorption is attributed to the unique isotropic antenna morphology of the ZnO nanotrees in the composite. Li et al. [9] showed that the reflection loss for composite with 50 vol.% ZnO netlike structures is − 37 dB at 6.2 GHz with a thickness of 4.0 mm. Further investigation by Cao et al. [10] demonstrated that cagelike ZnO/SiO₂ (weight ratio 1:5) nanocomposites exhibit a relatively strong attenuation of − 10.68 dB at the frequency of 12.79 GHz to EM waves. The special morphology and distribution configuration of cagelike ZnO nanostructures resulted in enhanced absorption for EM waves. The microwave absorption of porous hollow ZnO by CO₂ soft-template was also reported [11]. And a minimum reflection loss value of wax composite with 25 wt.% porous hollow ZnO is − 36.3 dB at 12.8 GHz with a thickness of 4.0 mm. Table 1 lists corresponding microwave absorption indexes of related ZnO with different structures and morphologies. Previous reports have demonstrated that ZnO can become a strong EM absorbing material. Therefore, we try to prepare porous ZnO of much thinner thickness with better microwave absorption property.

ZnO with lightweight, semiconductive features and large scale synthesis has been a desiring highly efficiency material for microwave absorption. Furthermore, the novel properties of special ZnO nanostructure have attracted increasing interests. Despite lots of researches on ZnO absorbent, until now there have been only few reports on the microwave absorbing property with porous ZnO nanostructure. In this paper, we have successfully synthesized 3D porous ZnO spheres by one-pot simple solvothermal approach. Compared with the synthetic method using a hard-template followed by subsequent removal of the template, the template-sacrificial approach with low cost and environmentally friendly route is much more convenient for its facile and controllable synthesis process [12]. What's more, we believe that the hollow and open porous structures are desirable for the application in the shielding absorbing materials, since the 3D porous ZnO architectures can not only provide the high surface accessibility and more free space to form the multi-interface reflection but also offer point defects to cause another additional pathway for the highly-efficient steady absorption of EM waves. Importantly, the present paper also reveals the intrinsic reasons why porous nanostructured shielding materials have excellent microwave absorption performances.