Ligand-Free CsPbBr3@TiO2 Core-Shell Nanofibers with Enhanced Carrier Transport

Abstract

Cesium lead halide perovskites have attracted attention in various applications such as photovoltaics, laser gain media, photodetectors, and LEDs, owing to their following photoelectronic properties: high quantum yields, tunable absorption, and luminescence with narrow emission. Recently, various shapes of cesium lead halide perovskite nanomaterials, such as nanocubes, nanoplatelets, nanowires, and nanorods, have been designed by conventional solution-based processes under different controlled reaction conditions. Significantly, one dimensional nanostructure is one of the most attractive cesium lead halide perovskite nanomaterials because of their particular advantages, such as effective carrier transport along one direction and longer carrier diffusion length. Cesium oleate (CsOA), a commonly used precursor for synthesizing cesium lead halide perovskites, plays a crucial role in forming anisotropic structures by its selective adsorption onto a particular facet of growing perovskite nanocrystals. Oleylamine and oleic acid, as surface ligands, also promote anisotropic growth and stabilize nanocrystals. However, using hygroscopic CsOA hinders the easy handling of the process and lowers the reproductivity, and surface ligands that have long chain alkyl groups inhibit carrier transfer between perovskite nanocrystals and other components due to forming the electrical insulation layer on the surface, which lowers the performance of optoelectronic devices such as photovoltaics and light emitting diodes (LED). To solve the above problems, we prepared one dimensional nanostructured CsPbBr3 core by crystallizing it inside the nanostructured TiO2 shell. The ligand-free core-shell structure, prepared by a simple process without hygroscopic cesium salts, is expected to improve electron injection from the CsPbBr3 to TiO2 electrodes due to not only eliminating the electrical insulation layer but also the unique features of one dimensional composite formed by the in-situ process.CsPbBr3@TiO2 core-shell nanofibers were obtained by a simple solution process with hollow TiO2 nanofibers. A precursor solution of CsPbBr3 was prepared by mixing the cesium bromide (CsBr) and lead bromide (PbBr2) in equal molar ratios in dimethyl sulfoxide (DMSO). These chemicals were stable under atmospheric conditions. Hollow TiO2 nanofibers were prepared by electrospinning technique with a coaxial needle and the hollow nanofibers were mixed and stirred with the above precursor solution while heating at 100 ˚C to fill the cavities of the hollow nanofibers with the precursor. After removal of the excessive precursor by centrifugation, CsPbBr3 was crystallized by annealing at 140 ˚C for 12 hours.The morphological and structural properties of CsPbBr3@TiO2 core-shell nanofibers were examined by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM). The TEM image of core-shell nanofiber shown in Figure 1 shows clear core and shell shapes, in which a nanofiber is wrapped with a thin wall. The existence of CsPbBr3 and anatase TiO2 crystals were confirmed by the fast Fourier transform (FFT) pattern (Figure 1). Figure 2 exhibits CsPbBr3 nanorods left inside a split hollow TiO2 nanofiber, whose diameter is found to be about 200 nm, the same as the cavity diameter. Energy-dispersive X-ray spectroscopy (EDS) in scanning transmission electron microscopy (STEM) was also carried out. Distributions of CsPbBr3 elements (Cs, Pb, Br) were matched with the shapes of hollow TiO2 nanofibers and were not observed outside the nanofibers (Figure 3). These results confirmed that one dimensional nanostructure of CsPbBr3 was formed in cavities of hollow TiO2 nanofibers. Such structural arrangement may help protect the core from water or other harsh environments. To investigate the effect of core-shell structure on carrier transport between CsPbBr3 and TiO2, we measured photoluminescence spectra before and after the core-shell structure was broken by intense ultrasonic irradiation for 5 or 10 minutes. The photoluminescence intensity increased with increasing ultrasonic irradiation time (Figure 4). In other words, photoluminescence spectra exhibited strong quenching in the presence of core-shell structure, which means injections of photoexcited electrons from CsPbBr3 into TiO2 are improved by the forming of core-shell structure. It is considered that the vast contact area between the core surface and inner wall of the shell and better contact due to in-situ synthesis resulted in better electron transfer.In our method, CsPbBr3 anisotropic nanostructures were formed inside the TiO2 hollow nanofibers by a straightforward solution-based process without hygroscopic chemicals and surface ligands. This perovskite material was encapsulated in hollow nanofibers, which could improve stability by reducing exposure to water and the atmosphere. Moreover, there is an advantage that the shell is TiO2. TiO2 has a conduction band level suitable for electron transfer from CsPbBr3. In this regard, TiO2 has been widely used in photovoltaics and photocatalysts with CsPbBr3. This one-dimensional nanomaterial composed of TiO2 and CsPbBr3 has the potential to be used in various energy devices. Figure 1