Synthesis of All-Inorganic Halide Perovskite Nanocrystals for Potential Photoelectric Catalysis Applications

Abstract

Compared with conventional semiconductors, halide perovskite nanocrystals (NCs) have a unique crystal structure and outstanding optoelectronic properties, offering wide potential for applications in optoelectronic devices such as solar cells, photodetectors, light-emitting diodes, lasers, and displays. Rational technological design is providing vital support for the development of perovskite optoelectronics. Herein, monodisperse all-inorganic halide perovskite nanocrystals with consistent morphology and cubic crystal phase were synthesized employing a modified one-pot hot injection method to independently modulate the stoichiometric ratios of three precursors involving cesium salt, lead source, and halide. In combination with an anion exchange reaction, mixing two kinds of perovskite NCs with different halogens enables a transition from violet emission to green and finally to red emission over the entire visible region. Additionally, optical and electrochemical tests suggested that the as-synthesized halide perovskite NCs are promising for photoelectric catalysis applications.

1. Introduction

Over the past years, halide perovskite materials have developed dramatically as a novel solution-processable ionic semiconductor with a direct band gap. They possess a unique structure compared to conventional semiconductors, which can be expressed as ABX₃ (X = Cl⁻, Br⁻, I⁻) [1]. The A-site is a monovalent cation with a relatively small radius, which can be Cs⁺, (CH₃NH₃⁺) MA⁺, or ([HC(NH₂)₂]⁺) FA⁺, the B-site is occupied by divalent metal cations such as Pb²⁺ and Sn²⁺, and the X-site is a halogen anion [2,3,4,5,6]. In the cubic halide perovskite crystal structure, six X⁻ and B²⁺ are connected to create a covalent top-linked [BX₆]⁴⁻ coordinated octahedron B-site ion located at the octahedral center, while the A-site cation populates octahedral vacancies, which enables charge balance [7].

The structure of halide perovskite allows it to display superior optoelectronic properties, including a high absorption coefficient, extremely large photoluminescence quantum yield (PLQY), pure color emission, and variable band gap [8,9,10]. Among these properties, the PLQY of the red-emitting CsPbI₃ is already close to 100% [11]. Owing to its soft ionic nature and strongly ionic interactions with surface ligands, it allows for facile anion exchange to occur between perovskite nanocrystals (NCs) of different halogens, which can be utilized for flexible band-gap width modulation (1.78–3.1 eV) and control of the fluorescence wavelength covering the entire visible spectral range [9,12]. In addition, the enhanced quantum confinement effect of halide perovskite can be harnessed to tailor optical absorption and luminescence performance [13]. Electrically, halide perovskite is characterized by long charge diffusion lengths and high carrier mobility [14,15]. By means of time-resolved terahertz spectroscopy CsPbBr₃ NCs have been found to possess excellent free carrier performance, with carrier mobilities of up to 4500 cm² V⁻¹ s⁻¹ and diffusion lengths greater than 9.2 μm, making for ideal intrinsic transport properties [14]. Thus, halide perovskite holds great value for optoelectronic applications and offers a wide scope of prospects in high-efficiency solar cells [16,17,18], sensitive photodetectors [19,20,21], low-threshold lasers [22,23,24], light-emitting diodes (LEDs) [25,26,27], scintillators [28,29,30], and bioimaging systems [31,32,33]. Among the multitude of applications, metal halide perovskite is particularly impressive in the field of solar cells. In the short span of about a decade, photoelectric conversion efficiency (PCE) has increased from an initial 3.8% to 25.5%, comparable to mature silicon-based solar cells [16]. As a result of these developments, halide perovskites now provide the ability to separate electron–hole pairs, and can be employed in the area of photoelectric catalysis applications [34,35,36,37,38,39,40,41,42].

The superior stability of all-inorganic CsPbX₃ perovskite materials compared to organic-inorganic hybrid halide perovskites is attributed to the sensitivity of organic molecules (such as MA⁺) to light, heat, moisture, and irradiation, which decompose or damage the structure, limiting its application in storage, fabrication, and equipment manipulation processes [43,44,45]. The choice of synthesis technique likewise has an impact on the stability of the material. Until now, methods for the direct synthesis of all-inorganic perovskite comprised both solution-processed and vapor phase methods. The vapor phase method deposits single perovskite crystals or arrays on a substrate through a process of high temperature reactions [46,47], while solution-based processes aim to regulate the size and dimension of the halide perovskites by controlling technical parameters such as precursors, the amount of surface ligands, and the chemical reaction temperature and duration [48,49]. Nanoscale perovskite materials are a prerequisite for high performance optoelectronic applications. There are numerous approaches to the synthesis of promising NCs, with the ligand-assisted reprecipitation, solvothermal, and hot injection methods being effective and proven strategies. The synthesis protocol of the solvothermal method is relatively simple and controllable [50]. The precursors are usually mixed with a nonaqueous solvent and kept in a closed container such as a stainless steel autoclave, where cesium acetate (cesium oleate) and halide lead react to produce CsPbX₃ NCs under a certain temperature and self-generated pressure of the solution [51]. The ligand-assisted reprecipitation approach is based on the recrystallization of ligand halide salts, in which the metal halide salt is dissolved in a polar solvent and subsequently dropped into a nonpolar medium in the presence of the ligand to trigger the recrystallization of NCs [52]. In general, the hot injection method involves the synthesis of NCs by injecting cesium oleate into a lead halide solution under an inert atmosphere [8]. The aforementioned approaches are restricted in that the PbX₂ is applied as the lead and halogen sources, which makes it impossible to directly adjust the stoichiometric ratios of the two elements individually. The result is that the NCs are susceptible to damage through irradiation and are less stable under transmission electron microscopy [53,54]. Hence, improved hot injection methods are essential to govern the structural stability and optoelectronic properties of all-inorganic halide perovskite NCs.

Here, we employ a modified one-pot hot injection method to synthesize monodisperse and uniformly sized CsPbX₃ perovskite nanocrystals (NCs) by independently manipulating the stoichiometric ratios of Cs⁺, Pb²⁺, and X⁻. Their cubic crystal structure and high phase purity were demonstrated by XRD analysis. The findings of Fourier transform infrared spectroscopy (FTIR) revealed the validated protonation of oleylamine on the surface of the CsPbBr₃ nanocrystals. In addition, the presence of a single narrow emission peak in the fluorescence spectra of the perovskite nanocrystals is driven by different halogen compositions, giving rise to varying band gaps and consequently differential fluorescence absorption and emission peak positions. Accompanied by the red shift of the fluorescence wavelength (narrowing of the band gap), the PL lifetime of the CsPbCl₃, CsPbBr₃, and CsPbI₃ NCs grow larger and their decay rate slows. The mixture of NCs with different halogens is susceptible to anion exchange reactions, allowing the absorption peaks of the nanocrystals to be tuned from 400 nm to 700 nm and the color of the nanocrystals to be gradually tuned from violet to red under 365 nm UV lamp excitation, covering the entire visible range. Optical and electrochemical tests revealed that the all-inorganic halide perovskite materials are applicable for photoelectric catalysis applications.

2. Results and Discussion

Synthesis of all-inorganic halide perovskite nanocrystals (NCs) is crucial for effective implementation of superior performance in optoelectronic devices. In particular, the selection of suitable precursors and reaction conditions are vital factors influencing the quality of the NCs. For the common hot injection method, cesium oleate is injected into a solution of lead halide (PbX₂) for reaction to transform into perovskite [8]. Cesium oleate is prone to solidifying at room temperature, and needs to be preheated prior to the injection process. Thus, we take the option of incorporating benzoyl halides, injecting them into salt solutions of the metal cations Cs⁺ and Pb²⁺, which provides a halogen-rich environment while facilitating the effective protonation of oleylamine (OAm). In our typical synthetic process, Cs₂CO₃ and Pb(CH₃COO)₂·3H₂O were applied as the cesium source and lead source, respectively, while oleic acid (OA) and OAm were chosen as the surface ligands and 1-octadecene (ODE) was chosen as the solvent. The reaction system was kept under vacuum by removing excess air and water by evacuation at 130 °C for 1h. Subsequently, CsPbI₃, CsPbBr₃, and CsPbCl₃ NCs were immediately synthesized by metathesis reactions via rapid injection of benzoyl halides under N₂ at 165 °C, 170 °C, and 200 °C (Scheme 1; more details are provided in the Section 3). In contrast to the conventional hot injection method, this approach made it possible to produce well-distributed perovskite NCs as a result of the independent control of the stoichiometric ratios of origin of Cs⁺, Pb²⁺, and X⁻ instead of employing PbX₂ as both the lead and halogen sources. In addition, a series of subsequent centrifugation procedures facilitated the final hexane dispersed solution of monodisperse cubic NCs.

As shown in Figure 1, TEM images of the CsPbCl₃, CsPbBr₃, and CsPbI₃ nanocrystals (NCs) display uniform and monodisperse cubic structures. Under electron microscopic irradiation, none of the three NCs instantly decomposed into white cubes and irregular black spots located in the corners, all keeping their shape and structure more integrated. Naturally, if irradiated in a continuous manner, the high-energy electron beam eventually damages the structure of the perovskite NCs; the decomposition products can include PbX₂, CsX, and CsPb [54]. It has been reported that the resistance of halide perovskite NCs to radiation exposure can be enhanced by supplementing the amount of halogen ions [55]. Our synthesis procedure is equipped with the ability to independently manipulate the stoichiometric ratio of the elements composing halide perovskite, which can effectively improve the irradiation stability of perovskite NCs. In addition to the halogen content, the temperature and electron beam dose have an influence on the morphology of all-inorganic halide perovskite. Incident electrons trigger the desorption of halogen ions from the CsPbBr₃ surface, which takes place with the evolution of the electron dose; moreover, low temperatures can be introduced to inhibit ion migration [54]. Therefore, in order to avoid or reduce radiation damage, it is possible to take measures such as changing the electronic irradiation dose and decreasing the working temperature of transmission electron microscope (or directly employing cryo-electron microscopy) when capturing TEM images [56]. Additionally, 300 NCs were selected to measure their size, and analysis of the results indicated a concentrated distribution of sizes, with the average sizes being CsPbCl₃: 9.97 ± 0.20 nm, CsPbBr₃: 9.61 ± 0.13 nm, and CsPbI₃: 15.12 ± 0.14 nm. However, the sizes of the original nanocrystal particles were estimated by the Scherrer formula, $\mathrm{D}=\mathrm{K}\mathsf{\gamma }/\mathrm{B}\mathrm{c}\mathrm{o}\mathrm{s}\mathsf{\theta },$ at 8.85 nm, 9.51 nm, and 9.95 nm, respectively. Here, D is the grain size and K is the Scherrer constant; if B is the half-height width of the diffraction peak, then K = 0.89. Moreover, γ is the X-ray wavelength, typically 1.54056 Å for Cu kα; B is the half maximum full width (FWHM) of the diffraction peak of the measured sample; and θ is the Bragg diffraction angle. The reason for the difference in the results obtained by the two approaches is that the software was chosen and calculated in different ways.

X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) are methods for characterizing the structure and surface functional groups of materials. Figure 2A–C shows the XRD patterns obtained for the three different halogens of perovskite NCs, which were analyzed and well matched to cubic perovskite crystals. The corresponding ICSD numbers for the CsPbCl₃, CsPbBr₃, and CsPbI₃ NCs are 23108, 29073, and 181288, respectively, corresponding to the space group pm$\stackrel{-}{3}$m (No. 221). The diffraction peaks in the vicinity of 2θ ≈ 15°, 20°, and 30° for all three types of perovskite nanocrystals correspond to the (100), (110), and (200) crystallographic planes. However, there is a notable divergence in the diffraction peak positions associated with the (210), (211), and (220) crystal planes of the NCs, with CsPbCl₃ at 35.8°, 39.3°, and 45.7°, respectively, whereas CsPbBr₃ is at 34.1°, 37.4°, and 43.5°, respectively, and CsPbI₃ near 31.9°, 35°, and 40.7°, respectively. The diffraction angles corresponding to the same diffraction crystal plane of the perovskite NCs change depending on the halogen component. From CsPbCl₃ to CsPbBr₃ to CsPbI₃, the diffraction peaks appear to be shifted in the direction of a small angle. Based on the Bragg equation, 2dsinθ = nλ, where d is the spacing between crystal planes, θ is the diffraction half angle, and n is the diffraction order, XRD analysis of perovskite nanocrystals with heterogeneous halogen fractions was carried out and the results did not match the appropriate phases in the currently available card library. However, as shown in Figure S2, perovskite NCs with different halogen components all present only one set of diffraction peaks, instead of diffraction peaks of CsPbBr₃, CsPbBr_xCl_3−x (0 < x < 3) (or CsPbBr_yI_3−y (0 < y < 3), and CsPbCl₃ (or CsPbI₃) at the same time. This demonstrates that only one material is ultimately present. In Figure S2A, the angle of the main diffraction peak tends to change at a smaller angle as the elemental content of Br (Cl) increases (decreases). Likewise, in Figure S2B the angle of the diffraction peak varies towards a smaller angle as the elemental content of I (Br) increases (decreases). In the FTIR spectrum of CsPbBr₃ NCs (Figure 2D), peak positions of 2925, 1379, and 2854 cm⁻¹ can be observed, indicating asymmetric and symmetric stretching vibrations of the C–H single bond, respectively, while absorption peaks of 1466 and 721 cm⁻¹ represent in-plane bending vibrations of the C–H single bond in the –CH₂– group. The peaks at 993, 909, and 801 cm⁻¹, which originate from OA and OAm, represent the bending vibration of =C–H. The peak at 1536 cm⁻¹ can be attributed to the C=O bond from the –COO– group of OA. In particular, the absorption peak located at a wave number of 1642 cm⁻¹ denotes a deformational vibration of the N–H bond corresponding to –NH₃⁺, which arises from the OAm. The appearance of differential absorption peak positions in the FTIR spectra is indicative of a variation in the vibrational mode of the functional groups, demonstrating the effective protonation of oleylamine on the surface of the CsPbBr₃ NCs.

The XPS spectra were employed to analyze the surface chemical valence states of the CsPbCl₃, CsPbBr₃, and CsPbI₃ NCs. The binding energy curves of Cs 3d and Pb 4f are recorded in Figure S2B,C. The peaks of Cs 3d_5/2 and Cs 3d_3/2 are located near 720 eV and 734 eV, respectively, and the peaks of Pb 4f_7/4 and Pb 4f_5/4 are located near 135 eV and 139 eV, corresponding to Cs⁺ and Pb²⁺. In Figure S2D–F, the coupled peaks located near 71 eV and 73 eV originate from Br 3d_5/2 and Br 3d_3/2, the strong peaks located near 194 eV and 196 eV are attributed to Cl 2p_3/2 and Cl 2p_1/2, and the peaks located at 615 eV and 627 eV originate from I 3d_5/2 and I 3d_3/2, corresponding to Br⁻, Cl⁻, and I⁻, respectively. The full XPS spectra (Figure S2A) present the chemical elemental composition of CsPbX₃ (X = Cl, Br, I) NCs. The ratios of elemental content are Cs: Pb: Cl ≈ 1: 0.9: 2.2, Cs: Pb: Br ≈ 1: 1.7: 3.7, and Cs: Pb: I ≈ 1: 0.9: 2.2, respectively.

Later, we measured the fundamental optical properties of the three NCs, including the UV-vis absorption, PL, and TRPL spectra (Figure 3). The band edge peak positions of the CsPbCl₃, CsPbBr₃, and CsPbI₃ NCs are found at 402 nm, 506 nm, and 675 nm, respectively. The absorption spectra exhibit two or three exciton leap peaks as a result of the quantum confinement effect, indicating the smaller size and better homogeneity of the all-inorganic perovskite NCs prepared by our scheme. The fluorescence emission peaks in the PL spectra are 411 nm, 521, nm and 694 nm, respectively, with the corresponding Stokes shifts due to quantum confinement effects calculated as 9 nm, 15 nm, and 19 nm, respectively. The average fluorescence lifetimes of CsPbX₃ (X = Cl, Br, I) NCs were 11.29 ns, 12.92 ns, and 49.26 ns, respectively, through a third-order exponential fitting from TRPL curves (Figure 3D). The different ionic radii of the three halogens Cl⁻, Br⁻, and I⁻ in turn become larger, leading to a variation in the volume of their vacancies in the perovskite crystal structure, which allows the optical band gap of the perovskite NCs to shift from CsPbCl₃ to CsPbBr₃ to CsPbI₃, becoming smaller in turn, with a corresponding gradual red shift in the fluorescence wavelength. Furthermore, only a narrow emission peak with a high photoluminescence quantum yield (PLQY) appears in the fluorescence spectrum. All-inorganic halide perovskite NCs possess huge potential for applications in semiconductor light-emitting devices such as light-emitting diodes and displays on account of their excellent optical properties.

In light of the ionic nature of perovskite and the variability of its surface ligands, anion exchange reactions readily occur between perovskite materials with diverse halogens. The common method adopted by researchers is to add metal halide salts of different halogens such as CuX₂ [57], KX [9], AlX₃ [58], and ZnX₂ [55] to perovskite NCs, then stir the mixture for a period of time before using centrifugal separation to obtain perovskite with different halogen components. Rather than following this approach, we instead mixed two types of perovskite nanocrystals with different halogens, achieving differences in luminescence via ion exchange. The UV-vis spectra of the solutions of perovskite NCs with different halogen contents exhibit different absorption peak positions (Figure 4A). These solutions are composed of CsPbCl₃, CsPbBr₃ and CsPbCl₃ in volume ratios of 1:3 and 3:1, CsPbBr₃, CsPbBr₃ and CsPbI₃ in volume ratios of 3:1 and 1:3, CsPbI₃ NCs, corresponding to the positions of the respective leap peaks at 402 nm, 420 nm, 484 nm, 506 nm, 520 nm, 585 nm and 676 nm. Afterwards, based on the Einstein photoelectric effect and the Kubelka-Munk equation [59], the linear region of the absorption edge in the UV-Vis spectra was extrapolated to the energy axis intercept, which was used to estimate the surface optical band gaps of perovskite NCs with different halogen components as 3.03 eV, 2.84 eV, 2.49 eV, 2.37 eV, 2.33 eV, 2.02 eV, and 1.76 eV, respectively. Certain samples were observed to feature two sharp exciton absorption peaks, while others appeared to be relatively smooth, which we attributed to the varying extent of quantum confinement effects caused by differences in size, concentration, and homogeneity of the NCs. Perovskite nanocrystal solutions mixed in varying volume ratios behaved differently when stimulated with 365 nm UV lamps. As illustrated in Figure 4B, CsPbCl₃ NCs are purple, and gradually change to blue and then green as Br^- increases when mixed with CsPbBr₃, while CsPbBr₃ NCs are green and gradually change to yellow-orange and finally red CsPbI₃ as I⁻ increases when mixed with CsPbI₃. The mixing of CsPbCl₃ with CsPbI₃ NCs results in fluorescence extinction rather than fluorescence emission. According to our investigation, a plausible explanation is that the Cl⁻ and I⁻ radii are sufficiently distinct from each other that the mixing process may cause them to enter each other’s crystal structures in such a way as to cause damage. Thus, there is no very effective means of making CsPbCl₃ coexist with CsPbI₃ while maintaining two different fluorescence emissions at the same time.

The states of the CsPbX₃ (X = Cl, Br, I) NCs in different solvents were compared, as shown in Figure S4. During these experiments, it was found that the NCs were more soluble in hexane than in water, and were more readily dispersed in hexane. The NCs dispersed in both solvents emitted light in similar colors under UV lamp irradiation. After one day, the CsPbCl₃ and CsPbBr₃ dispersed in water continued to emitted relatively strong light, while the CsPbI₃ NCs no longer emitted red light and behaved much less stably. The more stable CsPbBr₃ NCs were selected to test the photoresponse current density and electrochemical impedance spectra (EIS), as shown in Figure S5. The EIS spectra of CsPbBr₃ NCs exhibited a certain semicircular arc, indicating the presence of charge transfer resistance. In order to capture the photocurrent generated by the light signal and to exclude other influences, the photocurrent distribution is normally recorded at regular intervals when the light source is switched on. As depicted in Figure S5B, when the light source was switched off, the recorded photocurrent density was almost zero, while when the light source was switched on again after 30 s, the current increased instantaneously. When the light source was switched off every 30 s, the current decreased instantaneously. This further confirms that the CsPbBr₃ perovskite material has suitable carrier migration efficiency with charge transport properties. Thus, CsPbBr₃ NCs could be promising in the field of photoelectric catalysis. Perovskite NCs with different halogen fractions feature diverse fluorescence emission and contribute to the construction of light-emitting diodes. Moreover, analogous to conventional semiconductor NCs, halide perovskite NCs exhibit good homogeneity and monodispersity along with a well-defined cubic shape, which might be promote self-assembly for use as building blocks in ordered superstructures such as superlattices. The degree of ordering of the perovskite superstructure affects the optoelectronic properties, and as such can be used to alter the performance of functional electronic devices.

3. Materials and Methods

3.1. Chemicals

Cesium carbonate (Cs₂CO₃, Aladdin, 99.9%), lead acetate trihydrate (Pb(CH₃COO)₂·3H₂O, Aladdin, 99.99%), sodium iodide (NaI, Macklin, 99.99%), oleic acid (OA, Aladdin, AR), oleylamine (OAm, Aladdin, 80~90%), 1-octadecene (ODE, Aladdin, >90%(GC)), benzoyl bromide (C₆H₅COBr, Aladdin, 97%), benzoyl chloride (C₆H₅COCl, Aladdin, 99%), n-hexane (Aladdin, >99%). All reagents labeled Aladdin are from Aladdin Reagent (Shanghai) Co., Ltd., Shanghai, China. The reagent marked Macklin is from Shanghai Macklin Biochemical Co., Ltd., Shanghai, China.

3.2. Synthesis Scheme of All-Inorganic Perovskite Nanocrystals

3.2.1. Preparation of Benzoyl Iodine

The benzoyl iodine was synthesized following the method reported by Muhammad Imran. In a typical synthesis, sodium iodide (3 g, 0.2 mmol) and benzoyl chloride (1.4 mL) were mixed in a 20 mL vial. The mixture was stirred vigorously at 75 °C on a hot plate for 5 h. With heating and stirring, the mixture changed from white to orange red color, manifesting the successful synthesis of benzoyl iodine. Then, the mixture was gradually cooled to room temperature and 5 mL of anhydrous ODE was added to the vial. The solution was filtered by a polytetrafluorethylene membrane with a 0.45 μm pore size to obtain clear benzoyl iodine, which was stored in vacuum for further use.

3.2.2. Synthesis of CsPbX₃ (X = Cl, Br, I) Nanocrystals (NCs)

CsPbX₃ (X = Cl, Br, I) nanocrystals (NCs) were synthesized via the previously published procedure [60] with modifications. In a 50 mL three-necked round-bottom flask, 0.016 g Cs₂CO₃, 0.076 g Pb(CH₃COO)₂·3H₂O, 0.3 mL OA, 1 mL OAm, and a variable amount of ODE (5 mL for CsPbCl₃ and CsPbBr₃ NCs, 8 mL for CsPbI₃ NCs) were loaded and dried under vacuum (−0.05~0.08 MPa) at 130 °C for 1 h. When the reaction mixture had solubilized completely, the flask was heated up to 165~200 °C under N₂ flow; (the specific reaction temperatures are 200 °C for CsPbCl₃ NCs, 170 °C for CsPbBr₃ NCs and 165 °C for CsPbI₃ NCs, respectively). Subsequently, benzoyl halides (1.8 mmol benzoyl chloride, 0.6 mmol benzoyl bromide, and 0.6 mmol benzoyl iodine) were swiftly injected. The reaction mixture was cooled immediately to room temperature using an ice bath for CsPbBr₃ and CsPbI₃ NCs, while the CsPbCl₃ NCs were cooled after 20 s to obtain crude solutions.

3.2.3. Isolation of CsPbX₃ (X = Cl, Br, I) Nanocrystals (NCs)

First, 5 mL of n-hexane was added to the CsPbCl₃ and CsPbBr₃ NC crude solutions, while 2 mL solvent was added to the CsPbI₃ NC solution. Next, the solution was centrifuged at 6000 rpm for 5 min. The supernatant was discarded and the precipitate was centrifuged again at 6000 rpm for 3 min without additional solvent; residual supernatant was removed with a paper tissue. Later, the precipitate was redispersed by adding 500 μL of n-hexane and centrifuged again at 4000 rpm for 10min in order to separate larger particles. The resulting supernatants of CsPbBr₃ and CsPbI₃ NCs were collected, while the supernatant of the CsPbCl₃ NCs was discarded. Next, 2 mL n-hexane was loaded into the precipitate to redisperse the CsPbCl₃ NCs, followed by centrifuging at 4000 rpm for 10 min, and the supernatant was collected in a vial. Finally, CsPbX₃ (X = Cl, Br, I) NCs were diluted with n-hexane (500 μL, 1 mL, and 800 μL for CsPbCl₃, CsPbBr₃, and CsPbI₃ NCs, respectively).

3.2.4. Anion Exchange Reactions

In brief, CsPbBr₃ was mixed with CsPbCl₃ NCs taking different volume ratios. CsPbBr₃ was blended with CsPbI₃ NCs according to different volume ratios. When the anion exchange was complete, the solutions were transformed for 20~60 min into CsPbBr_xCl_3−x (0 < x < 3) and CsPbBr_yI_3−y (0 < y < 3) NCs, respectively.

3.3. Materials Characterization

3.3.1. Transmission Electron Microscopy (TEM) Characterization

TEM images were acquired on a JEOL JEM-1400 Plus electron microscope (JEOL, Beijing, China) equipped with a thermionic gun at an accelerating voltage of 120 kV. The samples of nanocrystals were prepared by depositing a diluted nanocrystal suspension in n-hexane onto carbon-coated copper grids. The size distribution images of NCs were calculated using Nano Measurer software version 1.02.0005.

3.3.2. X-ray Diffraction (XRD) Characterization

XRD analysis was recorded on a Bruker D8 ADVANCE X-ray diffractometer (Bruker, Karlsruhe, Germany) with Cu-Kα radiation (λ = 1.54056 Å, 40 kV and 40 mA). NC specimens for the XRD measurements were prepared by drop-casting a concentrated NC suspension in n-hexane onto a substrate of SiO₂/Si. XRD data analysis was conducted using Jade 6.5 software.

3.3.3. Fourier Transform Infrared (FTIR) Spectroscopy Characterization

Fourier transform infrared (FTIR) spectroscopy data were obtained utilizing a Nicolet 6700 Fourier transform infrared spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) at room temperature. The wave number range of the instrument was set to 400 to 4000 cm⁻¹, the scanning speed was set to 1 time/s, and the maximum resolution was 0.019 cm⁻¹. According to the working principle of interferometer interference frequency modulation, the light emitted from the lamp is turned into interferometric light by a Michelson interferometer, then the interferometric light is allowed to irradiate the sample. The receiver receives the interferometric light, which contains information about the materials, which is then converted into a Fourier transform by the computer software to obtain a spectral diagram of the sample. For the test, a highly concentrated n-hexane dispersion solution of CsPbBr₃ perovskite nanocrystals was applied to a glass slice, left to dry, and then tested.

3.3.4. X-ray Photoelectron Spectroscopy (XPS) Characterization

The XPS spectra of perovskite nanocrystals were obtained using an AXIS SUPRA X-ray photoelectron spectrometer (Kratos Analytical, a Shimadzu Group Company, Manchester, UK) equipped with a Thermo Scientific Kα spectrometer and a monochromatic Al-Kα X-ray source. The data were converted to VGD format and processed utilizing Avantage software version 5.9922. The binding energy scale was internally referenced to the C1s peak (binding energy for C−C = 284.8 eV).

3.3.5. Fluorescence Spectrum Measurements

The UV-vis absorption spectra were carried out using a Shimadzu UV-1800 spectrophotometer with UVProbe 2.52 software. The steady-state photoluminescence (PL) spectra were measured on a Shimadzu RF-6000 spectrophotometer with LabSolutions RF software, using an excitation wavelength (λ_ex) of 350 nm for CsPbCl₃ and 450 nm for CsPbBr₃ and CsPbI₃. Time-resolved photoluminescence spectra (TRPL) were collected via single photon counter. In the procedure of optical characterization, samples were prepared by diluting NCs solutions in n-hexane in quartz cuvettes with a path length of 10 mm.

3.3.6. (Photo) Electrochemistry Measurements

A Zennium electrochemical workstation (Kronach, Germany, Zahner Company) was used to obtain photocurrent and electrochemical impedance curves of halide perovskite NCs. The measurement was performed in a three-electrode setup with a working electrode of the sample, a platinum disk counter-electrode, and a reference electrode of Ag/AgCl (saturated KCl) (E_Ag/AgCl = +0.1989V vs. NHE) in the (photo) electrochemical electrolyte (0.5 M Na₂SO₄). For photocurrent measurement, the light source was a 405 nm LED, and the light intensity was tested with a Newport photometer. Electrochemical impedance spectra were measured in 0.5 M Na₂SO₄ at −0.1 V vs. NHE with an amplitude of 10 mV (frequency: 100 mHz~20 kHz).

4. Conclusions

In conclusion, monodisperse all-inorganic halide perovskite nanocrystals (NCs) with a homogeneous cubic morphology, concentrated size distribution, and cubic crystal structure were synthesized using a modified hot injection method and independently modulating the ratios of cesium, lead, and halogen precursors. Moreover, we were able to achieve relevant information on the surface functional groups via FTIR spectroscopy, which demonstrated the effective protonation of OAm acting as a surface ligand. Different halogen components contribute to variations in the band gap of the perovskite NCs, and consequently to varying emission and absorption peak positions in the respective fluorescence spectra and UV-vis absorption spectra. Furthermore, in the fluorescence lifetime curve the fluorescence lifetime is larger, with a red shift of the fluorescence wavelength (band gap) and more slow decay. Nanocrystals mixed from distinct volume ratios undergo anion exchange reactions, and consequently exhibit various absorption peaks. In addition, the solution is capable of adjusting color from violet to green and from green to red under the stimulation of a UV lamp, covering essentially the entire visible light range. Photocurrent and electrochemical impedance tests indicate that our work can be expected to advance the potential of halide perovskite materials for photoelectric catalysis applications.