Influence of Annealing Temperature on Structural Properties and Stability of CsPbBr3/TiO2 Core/Shell

Although all-inorganic CsPbBr3 perovskites quantum dots exhibit an excellent photophysical properties, which could be applied in various applications, they still suffer from poor structural stability owing to environment factors. Therefore, TiO2 was coated on the CsPbBr3 QDs as a protecting shell to enhance their stability. Moreover, anatase phase of TiO2 shell was obtained by annealing at more than 300°C due to its most effective electron extraction properties among other crystalline forms of TiO2. However, the CsPbBr3 QDs can be degraded under high temperature. Herein, the annealing temperatures ranging from 300-500°C were optimized to obtain the anatase TiO2 shell and good crystallinity of CsPbBr3 QDs core. The morphology and optical properties of CsPbBr3 QDs, CsPbBr3/TiOx and CsPbBr3/TiO2 were studied by using transmission electron microscope (TEM), photoluminescence (PL) measurement and UV-Visible spectroscopy, respectively. Finally, the CsPbBr3/TiO2 annealed at the optimum temperature was compared with the bare CsPbBr3 QDs to prove the enhancement of their structural stability under ambient air by using X-ray diffraction (XRD).


Introduction
CsPbBr3 quantum dots (QDs) are one of perovskite materials that have been reported with the excellent optical properties such as adjustable band gap, long diffusion lengths, ambipolar charge transport, and high carrier mobility [1]. CsPbBr3 QDs with a green emission centered at 510-518 nm presented a high photoluminescence quantum yield (PL QY) of 90-95% and high color purity with a narrow full width at half maximum (FWHM) of 16-27 nm [2]. Although their excellent properties led to the success achieved in various fields, they still suffered from the environment factors such as water and heat because of their inherent ionic nature on polar solvents and the direct decomposition under high temperature. For the enhancement of their stability, CsPbBr3 QDs have been reported with an excellent water stability for 12 weeks maintaining 75% of initial PL intensity when they were encapsulated with TiO2 as a protecting shell; furthermore, it was proved that there was charge carrier transfer from CsPbBr3 QDs core to TiO2 shell in photoelectrochemical application [3,4]. TiO2 is widely used as an electron transport layer (ETL) in photovoltaic devices such as perovskite solar cells because it can absorb light at wavelength of ultraviolet region. It can transport electrons generated from perovskite layer to prevent charge recombination and serve as the block layer to hinder direct contact between the holes and fluorine-doped tin oxide (FTO). Anatase phase of TiO2, in particular, exhibited the most effective electron extraction properties among other crystalline forms of TiO2 due to the closest conduction band minimum (CBM) to the lowest unoccupied molecular orbital (LUMO) of CsPbBr3 QDs, that these anatase TiO2 layers could easily create a quasi-ohmic contact, providing the effective electron transport pathways and enhancing electron extraction [5]. Moreover, TiO2 appeared a high chemical and thermal stability under stressed condition so it has been attracted more attention to stabilize the perovskite materials. In order to obtain the anatase phase of TiO2, the annealing temperature was required in the range of 300 to 500°C before rutile phase of TiO2 formed and grown at higher temperature. Meanwhile, the high temperature can affect the degradation of CsPbBr3 QDs, although thermogravimetric analysis (TGA) has shown the weight loss of the CsPbBr3 QDs only about 5% between 30 and 500°C [3]. In this study, the effects of this heat treatment on the structure of CsPbBr3/TiO2 core/shell were investigated. The annealing temperatures of 300, 400 and 500°C were optimized in order to obtain the highest purity and crystallinity of anatase phase of TiO2 shell while maintain the good structure of CsPbBr3 core. Morphology, structural and optical properties were investigated using TEM, XRD pattern, PL emission spectra and absorption value, respectively. The stability of CsPbBr3/TiO2 annealed at the optimum temperature was tested under ambient air to prove the enhancement of their stability.

Experimental
The experimental includes the preparation of CsPbBr3 QDs, encapsulation of CsPbBr3 QDs with TiO2 at various annealing temperatures, stability testing in ambient air and characterizations of the samples.

Preparation of CsPbBr3
QDs 16 mg of cesium carbonate (Cs2CO3), 76 mg of lead acetate trihydrate (Pb(CH3COO)2.3H2O), 5.00 mL of octadecene (ODE), 0.45 mL of oleic acid (OA) and 1.00 mL of oleylamine (OLM) were mixed in a three-neck flask, stirred at 250 rpm and heated under vacuum at 130°C for 1.40 h. Then, the solution was heated under argon flow at 170°C for 15 min. 72 µL of benzoyl bromide (C6H5COBr) was swiftly injected to form yellow colloidal solution of CsPbBr3 QDs. The solution was immediately cooled down in an ice-water bath to room temperature (25°C). The CsPbBr3 QDs solution was collected by using 5 mL of toluene and poured into the centrifuge tube. Afterwards, it was centrifuged at 4,000 rpm for 10 min and the supernatant was discarded to remove the ligands. Then the QDs in precipitates were redispersed in 5 mL of toluene. The centrifugation and re-dispersion process was repeated 1 time. The CsPbBr3 QDs were collected in toluene for further use.

Preparation of CsPbBr3/TiO2
20 µL of titanium tetraisopropoxide (TTIP) in 1 mL of toluene was dropped into 10 mL of CsPbBr3 QDs in toluene (1 mg/mL) and the solution was stirred for 3 h under 40%RH at room temperature. The solution was centrifuged at 5,000 rpm for 5 min to separate the CsPbBr3/TiOx, which were dried under vacuum for 1 h. Next, the CsPbBr3/TiOx were annealed under nitrogen flow at 300, 400 and 500°C for 1 h with heating rate of 5°C/min to transform amorphous TiOx shell to anatase TiO2 shell.

Stability testing of CsPbBr3/TiO2
The bare CsPbBr3 QDs and CsPbBr3/TiO2 were stored under ambient air (40%RH, 25°C). The samples were characterized by XRD at day 0, 2, 5 and 11 to investigate their structural stability.

Characterizations of structural and optical properties
The Fourier Transform Infrared spectrophotometer (FT-IR, FT/IR-4100, JASCO Co.) was employed to investigate the ligand removal of QDs during the process. The morphology of CsPbBr3 QDs was observed by transmission electron microscope (TEM-2000FX, JEOL). The crystallinity of CsPbBr3 QDs core and the anatase TiO2 shell were determined by X-ray diffraction (RINT2100, Rigaku Co., Ltd) with CuKα radiation source in the range of 10-60°. The optical properties were studied by using a UV-Visible spectrophotometer (V-650, JASCO Co.) at the wavelength range of 300-700 nm. Steady state PL spectra were obtained with a spectrometer (FP-6000, JASCO Co.) with excitation at 330 nm.

Results and discussion
Li group reported that the structure of CsPbBr3 QDs started to decompose at around 500°C. However, the nucleation and growth of anatase phase was initiated at high temperature. Therefore, CsPbBr3/TiO2 annealed at 400°C was the most suitable representative of the annealed sample in this study. The sample before annealing, named CsPbBr3/TiOx, and the sample after annealing at 400°C, named CsPbBr3/TiO2, were investigated to study the effects of heat treatment on morphology and optical properties. Afterwards, the samples annealed at various temperatures were performed to study the influence of annealing temperature on their structural properties.
The CsPbBr3 QDs were obtained by hot-injection method improved from M. Imran et al. [6] that was based on the dissolution of Cs + and Pb 2+ cations in fatty acids and used benzoyl halides as halide precursors. It was possible to precisely tune the composition of the final QDs. Furthermore, the QDs had a high phase purity and a high level of particle-morphology control. In order to control the growth of CsPbBr3 QDs in solution via this method, the long carbon chain organic ligands including of oleic acid (OA) and oleylamine (OLM) have been applied to surround the QDs as a stabilizing shell. However, these ligands have prevented charge transfer as an insulating shell in optoelectronic devices as well as hinder the encapsulation of TiO2 around QDs surface. Therefore, the ligands were inspected by FT-IR measurements on CsPbBr3 QDs, CsPbBr3/TiOx before annealing and CsPbBr3/TiO2 annealed at 400°C.
FT-IR spectra ( figure 1) showed that the sample exhibited characteristic modes of oleyl groups corresponding to the OA and OLM as follows: peaks at 2,924 cm -1 and 2,854 cm -1 correlating with the asymmetric and symmetric CH stretching mode in both OA and OLM, respectively, and peaks at 1,462 cm -1 and 1,376 cm -1 correlating with the CH2 and CH3 bending in OLM, respectively [7]. After coating the CsPbBr3 with amorphous TiOx, the intensity of the ligand absorption band decreased significantly compared to CsPbBr3 QDs because the ligands surrounding QDs were probably replaced with TiOx and discarded by centrifugation. However, the ligands removal possibly caused an agglomeration of CsPbBr3/TiOx due to the strong ionic nature between the uncapped QDs of neighboring particles [8] as well as between the oxide of titanium. After annealing at high temperature, all ligands were removed due to the decomposition of their structure under an elevated temperature. Moreover, the peaks from 800 to 600 cm -1 were occurred due to the presence of Ti-O bond [9] from the anatase TiO2 shell. The TEM image showed that bare CsPbBr3 QDs had the average size-particle of 11±1 nm with a cubic shape (  2a). The CsPbBr3 QDs were coated with TiO2 by using titanium tetraisopropoxide (TTIP) under 40%RH at ambient temperature. This titanium alkoxide (Ti(OR)n) was hydrolysed when there was a presence of water in an ambient air and subsequently condensed to form an amorphous oxide (TiOx.nH2O) that covered on the surface of agglomerated CsPbBr3 QDs core as seen as a gray shadow area in TEM ( (a) (b) (c) figure 2b). After annealing the sample, the particles had a larger size of 160-170 nm with a polycrystalline structure (Figure 2c). Besides the chemical bond interaction between -Ti-O-Ti-network and QDs, the change in morphology of coatings could be attributed to the densification mechanisms activated during the heat treatment leading to grain growth and increase in the particle agglomeration. PL emission spectra (figure 3a) showed that the bare CsPbBr3 QDs exhibited a sharp green emission peak at wavelength of 515 nm (2.41 eV) with a full width at half maximum (FWHM) of 18.09 nm when excited at 330 nm (3.76 eV). After coating CsPbBr3 QDs with titanium precursor, the color changed from green solution of CsPbBr3 QDs to yellow solution of CsPbBr3/TiOx because the emission peak slightly shifts to 518 nm (2.39 eV) with a FWHM of 16.95 nm. The PL intensity of the sample after annealing was very low but the sample still absorbed radiation and has a UV-Vis absorption spectrum. This red-shift of PL emission peak corresponded to the QDs agglomeration. PL quenching of CsPbBr3 QDs after coating TiO2 were possibly attributed to the delocalization of the electron wave function from CsPbBr3 core into TiO2 shell [4] since the CBM of TiO2 was lower and closer to the LUMO of CsPbBr3 QDs, which provided the effective electron transport pathways. For the absorption spectrum (figure 3b), CsPbBr3 QDs, CsPbBr3/TiOx and CsPbBr3/TiO2 had the excitonic peak at 508 nm, 509 nm and 523 nm, respectively. When compare in term of the intensity, CsPbBr3/TiOx and CsPbBr3/TiO2 possessed a strong absorption into the UV region (< 350 nm) due to the UV absorption of TiO2 shell. The effects of encapsulation and annealing temperature on the structure of CsPbBr3 QDs and CsPbBr3/TiO2 were studied by XRD (figure 4a-, and 4b). The XRD result confirmed that CsPbBr3 QDs exhibited an orthorhombic phase. For CsPbBr3/TiOx, the XRD pattern was similar to the bare CsPbBr3 QDs phase according to no phase transition in the QDs structure upon encapsulation process. After coating TiO2 and annealing at various temperatures, the XRD patterns confirmed the existence of TiO2 shell at 2θ = 25.2° assigning to the anatase planes of (101) for CsPbBr3/TiO2 at 400°C and 500°C. The largest amount of transformed anatase phase was occurred at 500°C. The peak shapes of CsPbBr3/TiO2 became sharper owing to crystal size growth, in which the crystallinity of CsPbBr3 QDs was improved by 37.2%. In addition, the peak split around 2θ = 30.5° was appeared corresponding to the (040) and (202) inter-reticular planes of Pnma orthorhombic phase. As the annealing temperature was increased, these peaks were slightly broader and the (040) peaks shifted to lower angles, which coincided to a thermal cell expansion and there might be a distortion or a preferred orientation during the process.
Although the grain growth of QDs should increase with an increase of annealing temperature, the sample annealed at 400°C showed a higher peak intensity due to its better crystallinity compared to the sample annealed at 500°C because the addition of larger amount of anatase TiO2, which annealed at 500°C, to the coating probably acted to retard the growth of QDs grains during this heat treatment, as predicted by Zener pinning. When counterbalanced between the quality of CsPbBr3 core and anatase TiO2 shell, annealing CsPbBr3/TiO2 at 400°C improved its crystallinity; meanwhile the amorphous of TiOx shell was transformed to the excellent charge-transfer crystalline form of anatase phase. To verify the stability enhancement of CsPbBr3/TiO2 annealed at 400°C with respect to bare CsPbBr3 QDs, XRD measurement was performed to observe the transformation of their structures (figure 5a-, and 5b), in which the samples were stored without an aqueous medium under ambient air with 40%RH. The normalized XRD pattern showed that CsPbBr3 QDs had a poor stability when stored under ambient air for a long time because there was a rapid decrease in crystallinity by 19.3% within 5 days. A decrease of (110) peak intensity at 2θ = 15° and a peak splitting were observed. It was possible that the environment factors such as light, oxygen and humidity have destroyed their structure as studied in previous researches [8]. On the other hand, the CsPbBr3/TiO2 annealed at 400°C showed the excellent stability for more than a week under ambient atmosphere. Additionally, the change of material composition due to the degradation was not occurred. It can be explained that TiO2 acted as a protecting shell because of its high thermal and water stability under stressed conditions, and prevented the CsPbBr3 QDs core from contacting with degradation factors [3]; at the same time, the structure of CsPbBr3 QDs still maintained a good crystallinity after heat treatment at 400°C.

Conclusion
The encapsulation process of CsPbBr3 QDs with TiO2 caused the ligand removal, which was useful for further optoelectrical applications, as well as the change of morphology and optical properties due to the particle agglomeration. Charge extraction capability can be enhanced owing to the suitable energy level of TiO2 shell with respect to the CsPbBr3 QDs. The structure of orthorhombic CsPbBr3 QDs was maintained after coating with TiO2 and annealing at elevated temperature. Furthermore, the heat treatment enhanced the crystallinity of CsPbBr3/TiO2 but distorted their structure and caused the polycrystalline particles at the same time. For annealing at 300°C, 400°C and 500°C, the sample annealed at 400°C exhibited the best crystallinity and appeared the desired anatase phase of TiO2 shell. Moreover, CsPbBr3/TiO2 annealed at 400°C showed the excellent structural stability against the bare CsPbBr3 QDs when stored under ambient atmosphere for 11 days. It has proven that encapsulation with TiO2 shell and annealing at a suitable temperature had a significant influence on both structural property and stability of the CsPbBr3 QDs.