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Article

CsPbI3 Perovskite Nanorods: Enhancing Fluorescence Efficiency and Environmental Stability via Trioctylphosphine Ligand Coordination

by
Chengqi Liu
†,
Zahir Abdalla
†,
Xiaoqian Wang
,
Manrui Liu
,
Yanhui Jiao
,
Zisheng Tang
,
Qi Zhang
and
Yong Liu
*
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, International School of Materials Science and Engineering (ISMSE), Wuhan University of Technology, Wuhan 430070, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Materials 2025, 18(7), 1518; https://doi.org/10.3390/ma18071518
Submission received: 3 March 2025 / Revised: 17 March 2025 / Accepted: 25 March 2025 / Published: 28 March 2025
(This article belongs to the Special Issue Advanced Materials in Photoelectrics and Photonics)
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Abstract

:
Metal halide perovskite nanorods hold great promise for optoelectronic applications. However, they tend to undergo phase transitions due to the instability of the crystal phase under environmental conditions, leading to a rapid decline in the fluorescence efficiency. Here, we report a method in which trioctylphosphine (TOP) directly serves as both the surface ligand and solvent to synthesize highly stable α-CsPbI3 nanorods (NRs). This approach produces monodisperse α-phase NRs with controlled sizes (1 μm and 150 nm in length, and an aspect ratio of 10:1), as confirmed by high-resolution transmission electron microscopy (TEM) and X-ray diffraction. The optimized NRs exhibit a high photoluminescence quantum yield of around 80%, as well as excellent environmental stability; after 15 days of storage, the photoluminescence quantum yield (PLQY) retention is 90%. Transient absorption spectroscopy shows that the carrier lifetime is extended to 23.95 ns and 27.86 ns, attributed to the dual role of TOP in defect passivation and hydrolysis suppression. This work provides a scalable paradigm for stabilizing metastable perovskite nanostructures through rational ligand selection, paving the way for durable perovskite-based optoelectronics.

1. Introduction

Metal halide perovskites (MHPs) are a class of materials known for their excellent optical properties and structural diversity, making them suitable for a wide range of optoelectronic applications [1,2,3,4,5,6]. The all-inorganic cesium lead iodide perovskite (CsPbI3) in its α-phase, with its nearly ideal bandgap (~1.73 eV), high carrier mobility (>100 cm2 V−1 s−1), and strong photoluminescence properties, holds significant potential for next-generation photovoltaic and nano-laser devices [2,7,8,9,10,11]. Among these materials, quasi-one-dimensional metal halide perovskite nanorods exhibit remarkable optical and electronic properties [12,13].
In recent years, research in this field has expanded rapidly, with significant progress made in understanding the growth mechanisms and exploring a wide range of potential applications [14,15,16,17,18,19]. However, the synthesis of CsPbI3 perovskite nanorods still faces considerable challenges, particularly in producing colloidal quantum dots with a high crystal quality and stability [20,21,22]. This is primarily due to the metastable nature of CsPbI3, which tends to undergo phase transitions from the black α-phase to the yellow phase under ambient temperature and humidity. These transitions result in the rapid degradation of optical activity, which remains a critical barrier to practical applications. In some CsPbI3 NR synthesis systems, significant challenges persist, such as non-radiative recombination induced by iodine vacancy defects on the surface of the one-dimensional structure. This significantly reduces the PLQY [23,24], with many reports indicating PLQY values below 50%. Furthermore, in non-vacuum environments, lattice hydrolysis accelerates phase transitions, causing a significant decay in PLQY within just seven days [25,26]. Currently, the primary approach researchers use to improve the stability of perovskites is by encapsulating perovskite nanocrystals, such as encapsulation in hydrophobic polymer matrices or borosilicate glass [27,28,29]. This method can effectively enhance the stability of perovskites. However, it not only increases the complexity of the fabrication process but also carries the potential risk that the encapsulating layer may affect the optical absorption properties of the perovskite material, thus posing certain limitations.
To address these challenges, this study developed a crystal-face-selective coordination strategy mediated by TOP, enabling the effective synthesis of nanorods with two different sizes (1 µm and 150 nm in length, with an aspect ratio of 10:1). The precise attachment of TOP to the surface of the perovskite nanorods was confirmed through energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and infrared spectroscopy. The TOP-modified α-CsPbI3 nanorods exhibited excellent optical performance and stability in ambient air, with an initial PLQY of 80%, retaining 90% of the initial PLQY after 15 days, significantly outperforming traditional ligand systems in terms of the performance degradation. Transient absorption spectroscopy measured average carrier lifetimes of 23.95 ns and 27.86 ns. This work provides a valuable reference for the precise synthesis and stability enhancement of perovskite nanostructures, with both theoretical and practical significance.

2. Materials and Methods

2.1. Materials

Cesium carbonate (Cs2CO3, 99.9%, Aladdin, Shanghai, China), oleic acid (OA, 80–90%, Aladdin, Shanghai China), lead (II) iodide (99.99% metals basis, Aladdin, Shanghai China), trioctylphosphine (TOP, 90%, Aladdin, Shanghai China), and n-hexane (Aladdin, Shanghai China, >99%) were used.

2.2. Methods

2.2.1. Preparation of CsOA Precursors

A typical synthesis method involves adding 800 mg (2.46 mmol) of Cs2CO3 and 10 mL of OA into a three-necked flask. The system is then connected to a double-row tube and dried under vacuum at 120 °C for 1 h, ensuring the complete dissolution of the powder and resulting in a clear solution. The mixture was then cooled to 110 °C for later use.

2.2.2. Synthesis of TOP-CsPbI3 NRs

As shown in Figure 1, 200 mg of PbI2 and 10, 8, and 6 mL of TOP were added to another three-necked flask and dried under vacuum at 80 °C for 30 min. The system was then purged with nitrogen gas, heated to 120 °C under a nitrogen atmosphere, and maintained until completely dissolved.
The reaction temperature was then raised to 110 °C, and 0.2 mL of cesium oleate was quickly injected. The reaction was allowed to proceed for 30 s, during which the solution turned deep red. The reaction was quenched by placing the solution in an ice-water bath. The sample was then purified by centrifugation at 7000 rpm for 3 min, and the supernatant was removed. The residue was dispersed in n-hexane for further use.

2.3. Characterization

2.3.1. Characterization of Material Morphology

TEM images were acquired using a JEOL JEM-1400 Plus transmission electron microscope (Tokyo, Japan) equipped with a thermionic electron gun at an accelerating voltage of 120 kV. HRTEM images were obtained on a Talos F200S-type field-emission transmission electron microscope (Hillsboro, OR, USA) at 200 kV. The size distribution of the NRs was determined by analyzing the TEM images via ImageJ software (Fiji).

2.3.2. Structural and Surface Characterizations

X-ray Diffraction Analysis: XRD measurements were conducted using a Bruker D8 X-ray diffractometer, equipped with Cu-Kα radiation (λ = 1.54056 Å), and operating at 40 kV and 40 mA. Perovskite NR samples for the XRD analysis were prepared by drop-casting a diluted NR suspension in n-hexane onto a SiO2/Si substrate. X-ray Photoelectron Spectroscopy: XPS analysis was performed on a Thermo Scientific (Waltham, MA, USA) Kα XPS spectrometer, featuring a monochromatic Al-Kα X-ray source. The binding energy scale was calibrated using the C 1 s peak, with a binding energy of 284.8 eV, corresponding to the C–C bond. Fourier-Transform Infrared Spectroscopy: FTIR spectra were recorded using a Nicolet 6700 FTIR spectrometer (Waltham, MA, USA). For the analysis, concentrated NR solutions were drop-cast onto a clean glass substrate.

2.3.3. Optical Characterization of Materials

The UV–vis absorption spectra were obtained using a Shimadzu UV-1800 spectrophotometer (Kyoto, Japan) with UVProbe 2.52 software. The steady-state photoluminescence (PL) spectra were measured on a Shimadzu RF-6000 spectrophotometer with LabSolutions RF software (Version 5.82). An excitation wavelength (λex) of 480 nm was used. Perovskite NR samples were prepared by diluting NR solutions in n-hexane within quartz cuvettes with a path length of 10 mm. The time-resolved photoluminescence spectra were measured at 685 nm using a 485 nm picosecond pulsed diode laser with a pulse width of ~200 ps and a pulse energy of 14 pJ (TRPL, Nano LED-C2 N-485L, HORIBA Scientific Company, Kyoto, Japan).

3. Results and Discussion

According to the existing literature, oleic acid and oleylamine are commonly used surface ligands in the synthesis of CsPbI3 perovskite. Studies have shown that organic ligands not only play a crucial role in the stability and surface modification of perovskites but also serve as key factors in the morphology control of CsPbX3 perovskites [30,31,32]. Specifically, by adjusting the type and concentration of ligands, precise control over the size, shape, and crystallinity of perovskite nanomaterials can be achieved. In this context, this study investigates the effect of using TOP as a surface ligand on the morphology of perovskite nanomaterials. The detailed experimental process is described in Section 2.
By systematically controlling the concentration of the lead precursor (PbI2), we successfully achieved the size-controllable synthesis of CsPbI3 NRs. The TEM images (Figure 2a–d and Figure S1) reveal that the PbI2 concentration significantly affects the aspect ratio of the nanorods. Specifically, when the volume of the TOP solvent is 10 mL, the nanorods have an average length of 153 nm and a diameter of 9.6 nm (Figure 2e,f). However, when the TOP volume is reduced to 8 mL, the growth size of the nanorods increases substantially. This effect is particularly pronounced in high-concentration precursor solutions, where the crystal growth rate exceeds the nucleation rate. This suggests that, once the initial crystal nuclei form, they rapidly absorb the abundant surrounding reactants, leading to faster crystal growth. Consequently, as the reactant concentration increases, the size of the perovskite nanorods also enlarges, with the length and diameter growing to 0.99 μm and 117 nm, respectively (Figure 2g,h). As the size of the NRs increases, the specific surface area (the surface area to volume ratio) decreases with the increase in the volume. When a monolayer of TOP is maintained, it effectively reduces the total amount of surface ligands required to cover each unit of the CsPbI3 NRs. This experimental observation is directly associated with the reduction in the amount of TOP used.
However, under high-concentration conditions, the supersaturation of the reaction system increases significantly, which may lead to aggregation during the nucleation stage. Under the influence of van der Waals forces, the CsPbI3 nanorods tend to aggregate and entangle along the long axis, resulting in a non-uniform diameter distribution. Moreover, when the PbI2 concentration is further increased and the TOP volume is reduced to 6 mL, the size of the nanorods grows further to 1.87 μm (length) and 300 nm (diameter). This trend, where the size of the CsPbI3 nanorods increases as the TOP dosage decreases, is still observed. However, when the TOP dosage falls below 6 mL, the current experimental conditions may not be sufficient to effectively dissolve the perovskite nanorod precursors. Therefore, the TOP dosage range in this study was set between 6 mL and 10 mL. However, at this point, impurity byproducts such as nanocubes and nanosheets also appear in the reaction system (as shown in Figure S2).
As shown in Figure 3a–e, we performed high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) characterization on the CsPbI3 nanorods synthesized under 8 mL TOP conditions. Additionally, energy-dispersive X-ray spectroscopy (EDS) was employed to analyze the elemental distribution of Cs, Pb, and I in the CsPbI3 nanorods (Figure 3b–d). The results demonstrate that Cs (red), Pb (green), and I (blue) are uniformly distributed throughout the nanorods. The measured elemental stoichiometry ratio is 13.5:15.7:38.9, which is in excellent agreement with the theoretical ratio of 1:1:3. This confirms that the synthesized CsPbI3 nanorods possess good chemical homogeneity and stability. Furthermore, the EDS analysis clearly reveals that the distribution of phosphorus (P) overlaps significantly with that of Cs, Pb, and I, indicating that TOP is uniformly distributed on the surface of the nanorods. This provides direct evidence of its role in surface modification during the nanorod synthesis process.
The XRD pattern shown in Figure 3f further reveals the crystallographic features of the sample. All of the diffraction peaks align precisely with the peak positions of the standard α-phase CsPbI3 (PDF#97-018-1288), confirming the phase purity of the synthesized product. Notably, the XRD pattern exhibits two prominent diffraction peaks around 2θ ≈ 14° and 2θ ≈ 29°, which correspond to the (001) and (002) crystal planes of CsPbI3 perovskite, respectively. This indicates that the nanorod assembly is composed of highly crystalline α-phase CsPbI3 perovskite and also reflects the preferential orientation of the sample along specific crystallographic directions. These results further validate that the synthesized CsPbI3 nanorods possess excellent crystal quality and a well-defined phase structure.
Additionally, we characterized the ligands on the surface of the nanorods using Fourier-transform infrared (FTIR) spectroscopy. As shown in Figure 4a, compared to the traditional synthesis method using OA and OAm as ligands, the nanorods synthesized with TOP as the solvent exhibit a small, broad feature peak at 1155 cm−1, corresponding to the stretching vibration of the C-P bond in TOP [33,34,35]. Moreover, a stretching vibration peak of the CH2 bond in TOP is observed at 2876 cm−1. The precise position and intensity variations in these characteristic peaks suggest that a specific chemical interaction may occur between the TOP and CsPbI3 nanorods. This result further demonstrates that TOP, in addition to serving as a solvent during the nanorod synthesis process, may also participate in surface modification through coordination, thereby influencing the structure and stability of the nanorods.
Additionally, we analyzed the surface chemical states of the CsPbI3 nanorods (CsPbI3-NRs) using X-ray photoelectron spectroscopy (XPS). As shown in Figure 4b–d and Figure S3, the XPS spectra of the TOP-treated CsPbI3 nanorods clearly display characteristic binding energy peaks corresponding to the primary elements of the perovskite nanorods (Cs, Pb, and I). Furthermore, weak characteristic peaks for phosphorus are observed in the binding energy ranges of 129–133 eV (P 2p3/2) and 189 eV (P 2s), indicating that TOP may exist in some form on the surface of the nanorods. Specifically, the I 3d orbital spectra show binding energy peaks for I 3d5/2 and I 3d3/2 at approximately 619 eV and 631 eV, respectively. In the Pb 4f orbital spectra, the binding energy peaks for Pb 4f7/2 and Pb 4f5/2 are located around 139 eV and 144 eV, respectively. These features are consistent with the chemical states of Pb2⁺ and I− [36]. Comparison with the literature data shows that the binding energy positions for Cs, Pb, and I in the CsPbI3 nanorods align with previously reported values, which are all within the experimental error range. These results suggest that the CsPbI3 nanorods treated with TOP exhibit good stability in surface chemical states, and that TOP may influence the surface properties of the nanorods through surface modification.
Subsequently, we systematically characterized the optical properties of the CsPbI3 NRs using UV–visible absorption spectroscopy (UV-vis), photoluminescence spectroscopy, the photoluminescence quantum yield, and the fluorescence lifetime. As shown in Figure 5a,b, for the CsPbI3 nanorods treated with 10 mL of TOP (10 mL TOP-CsPbI3 NRs), the absorption peak is located at 690 nm, and the photoluminescence peak is at 695 nm. In contrast, for the CsPbI3 nanorods treated with 8 mL of TOP (8 mL TOP-CsPbI3 NRs), the absorption and photoluminescence peaks are located at 697 nm and 703 nm, respectively. As the TOP concentration decreases, the PL emission peak of the 8 mL TOP-CsPbI3 NRs exhibits a 13 nm red shift. This red shift can be attributed to the quantum confinement effect: the diameter of the 10 mL TOP-CsPbI3 NRs is approximately 9.6 nm, with some nanorods having diameters smaller than 9 nm, which is close to the Bohr radius (7 nm) of CsPbI3 perovskite. This leads to an enhanced quantum confinement effect, causing a blue shift in the fluorescence peak of the nanorods [37].
Additionally, we analyzed the time-resolved fluorescence spectra of the CsPbI3 nanorods using a double-exponential fitting method (Figure 5c). The results show that the average fluorescence lifetime (Γave) of the 10 mL TOP-CsPbI3 NRs is approximately 23.95 ns, while that of the 8 mL TOP-CsPbI3 NRs is slightly longer, around 27.86 ns. The PL lifetime data Γave we provide consist of a short-lived component (Γ1) and a long-lived component (Γ2) [38,39,40]. The short-lived component is primarily associated with radiative recombination, while the long-lived component is related to non-radiative recombination. A1 and A2 represent the contributions of the radiative and non-radiative recombination processes to the overall recombination process, respectively. Detailed data can be found in Table S1. Both types of nanorods we synthesized have a dominant A1 parameter, which indicates that the surface passivation is effective, with fewer surface defects. As a result, radiative recombination dominates, leading to a high PLQY. As shown in Figure 5d, compared to the CsPbI3 perovskite synthesized using traditional ligand methods, the 10 mL TOP-CsPbI3 NRs exhibit outstanding photoluminescence performance, with an initial PLQY as high as 80%, and, after being stored in n-hexane solution at room temperature with 50% relative humidity for two weeks, it was able to retain 90% of its initial luminescent efficiency. This result demonstrates that the CsPbI3 nanorods treated with TOP not only exhibit a significant improvement in photoluminescence efficiency but also show excellent long-term stability, laying a solid foundation for the practical application of cesium lead iodide perovskite nanorods.

4. Conclusions

In summary, we have innovatively employed TOP as both the surface ligand and solvent to successfully synthesize α-CsPbI3 NRs with a high stability. This method enables the synthesis of size-controlled, monodisperse α-phase nanorods with lengths of 1 μm and 150 nm, and an aspect ratio of 10:1. These nanorods exhibit a high PLQY of approximately 80%, along with excellent environmental stability, as evidenced by a retention of 90% PLQY after 15 days of storage. Transient absorption spectroscopy reveals a carrier lifetime of up to 23.95 ns and 27.86 ns. Through the XPS and FTIR analyses, we investigated the adsorption and binding of TOP on the surface of cesium lead iodide perovskite. We present a simple, yet versatile, synthesis method for the production of highly stable α-CsPbI3 nanorods, significantly expanding their potential for a wide range of practical applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ma18071518/s1. Figure S1: TEM images of CsPbI3 nanorods synthesized at 6 mL of TOP. Figure S2: The corresponding diameter and length distribution histograms of 6 mL TOP CsPbI3. Figure S3: XPS spectra of TOP-CsPbI3. Table S1. The detailed recombination lifetimes described by the double-exponential fitting method.

Author Contributions

C.L. and Y.L. conceived the project ideas and material synthetic designs. C.L., Z.A. and Y.J. performed the material characterization and data analysis. X.W., M.L., C.L., Z.T., Q.Z. and Z.A. discussed the results and contents for this work. X.W., C.L. and Y.L. wrote the original drafts, and performed the reviews and edits. Y.L. revised all of the work. All authors discussed the results and commented on the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This work is financially supported by the National Natural Science Foundation of China (52072281, Y.L.); The Major Program of the National Natural Science Foundation of China (22293021, Y.L. and B.L.S.); the National Natural Science Foundation of China (52103285, Z.Y.H.); The National innovation and entrepreneurship training program for college students (No. S202210497011, S.L, Z.Q., and Y.L.). Y.L. gratefully acknowledges the Youth Innovation Research Fund project of State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors acknowledge the support for and help with the testing platform provided by the School of Materials Science and Engineering of Wuhan University of Technology and the State Key Laboratory of New Materials Composite Technology.

Conflicts of Interest

The authors declare no conflicts of interest.

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Materials 18 01518 g001
Figure 1. Flow diagram of the TOP-CsPbI3 NR synthesis process.
Figure 1. Flow diagram of the TOP-CsPbI3 NR synthesis process.
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Figure 2. (a,b) TEM images of CsPbI3 nanorods synthesized at 10 mL of TOP. (c,d) TEM images of CsPbI3 nanorods synthesized at 8 mL of TOP. (e–h) The corresponding diameter and length distribution histograms.
Figure 2. (a,b) TEM images of CsPbI3 nanorods synthesized at 10 mL of TOP. (c,d) TEM images of CsPbI3 nanorods synthesized at 8 mL of TOP. (e–h) The corresponding diameter and length distribution histograms.
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Figure 3. (a–e) High-resolution dark-field TEM image and elemental composition ratios. The corresponding elemental mapping images of Cs (b), Pb (c), I (d), and P (e). (f) XRD patterns of the CsPbI3 nanorods.
Figure 3. (a–e) High-resolution dark-field TEM image and elemental composition ratios. The corresponding elemental mapping images of Cs (b), Pb (c), I (d), and P (e). (f) XRD patterns of the CsPbI3 nanorods.
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Figure 4. (a) FTIR spectra of TOP-CsPbI3. (b–d) XPS spectra of TOP-CsPbI3. (b) Survey, (c) P 2p, and (d) P 2s elements for the CsPbI3 nanorods.
Figure 4. (a) FTIR spectra of TOP-CsPbI3. (b–d) XPS spectra of TOP-CsPbI3. (b) Survey, (c) P 2p, and (d) P 2s elements for the CsPbI3 nanorods.
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Figure 5. (a,b) PL and UV–vis absorption spectra of the CsPbI3 nanorods synthesized at 10 mL of TOP and 8 mL of TOP. (c) Time-resolved PL decay curves of the CsPbI3 nanorods. (d) Stability changes in the CsPbI3 perovskite PLQY synthesized in different systems.
Figure 5. (a,b) PL and UV–vis absorption spectra of the CsPbI3 nanorods synthesized at 10 mL of TOP and 8 mL of TOP. (c) Time-resolved PL decay curves of the CsPbI3 nanorods. (d) Stability changes in the CsPbI3 perovskite PLQY synthesized in different systems.
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MDPI and ACS Style

Liu, C.; Abdalla, Z.; Wang, X.; Liu, M.; Jiao, Y.; Tang, Z.; Zhang, Q.; Liu, Y. CsPbI3 Perovskite Nanorods: Enhancing Fluorescence Efficiency and Environmental Stability via Trioctylphosphine Ligand Coordination. Materials 2025, 18, 1518. https://doi.org/10.3390/ma18071518

AMA Style

Liu C, Abdalla Z, Wang X, Liu M, Jiao Y, Tang Z, Zhang Q, Liu Y. CsPbI3 Perovskite Nanorods: Enhancing Fluorescence Efficiency and Environmental Stability via Trioctylphosphine Ligand Coordination. Materials. 2025; 18(7):1518. https://doi.org/10.3390/ma18071518

Chicago/Turabian Style

Liu, Chengqi, Zahir Abdalla, Xiaoqian Wang, Manrui Liu, Yanhui Jiao, Zisheng Tang, Qi Zhang, and Yong Liu. 2025. "CsPbI3 Perovskite Nanorods: Enhancing Fluorescence Efficiency and Environmental Stability via Trioctylphosphine Ligand Coordination" Materials 18, no. 7: 1518. https://doi.org/10.3390/ma18071518

APA Style

Liu, C., Abdalla, Z., Wang, X., Liu, M., Jiao, Y., Tang, Z., Zhang, Q., & Liu, Y. (2025). CsPbI3 Perovskite Nanorods: Enhancing Fluorescence Efficiency and Environmental Stability via Trioctylphosphine Ligand Coordination. Materials, 18(7), 1518. https://doi.org/10.3390/ma18071518

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