Growth of PbBr₂ microrods with unique structure and surface morphology

During the last few years, a large number of studies have been performed to develop the lead halide crystals due to their interesting optoelectronic properties [1]. Interestingly, lead halides can make multidimensional crystal networks using a similar unit structure [2]. Typically, in the case of bulk crystals, the dimensions of the lead halide crystals are in the range of centimetres and offers limited flexibility for practical applications [3]. On the other hand, in the case of nanosized crystals, it is difficult to achieve good optoelectronic properties and decent atomic consistency. Also, the conventional crystal growth procedure requires a seed and a specific environment [4, 5]. Therefore, the development of simple and more economical methods for the crystal growths is vital [6].

As compared to the film analogues, crystalline structures offer more fascinating optoelectronic properties [7]. These distinctive properties result from their considerable surface-to-volume ratio, strong light absorption and facile fabrication methods [8]. A consistent regular distribution and alignment of the crystalline structures are more appropriate for the perovskite-based photodetectors applications and beyond.

In the present study, we synthesized the 1D microrods of PbBr₂ using solution-based facial growth technique. The morphological, structural, thermal and chemical properties of the microrods have been studied using various characterization tools. To the best of our knowledge, this is the first report of PbBr₂-based 1D microcrystalline rods consisting of such interesting structural and morphological characteristics.

Lead (II) Bromide (PbBr₂) power was purchased from Lumtec (Taiwan), while the di-methyl-formamide (DMF) (anhydrous, 99.8%) was obtained from Agros chemicals. To synthesize the 1D microrods, 1 M solution of PbBr₂ in dimethylformamide (DMF) was prepared by heating the DMF at 100 °C in a sealed container. Then the solution was kept in a stable place for two weeks. After 10 days, the crystals were started visualizing in the solution and they took a few more days to grow longer. Detail characterization and the schematic of the procedure are given in supplementary data file (see Fig. S1). Figure S2 shows the photos of the PbBr₂ and CH₃NH₃PbBr₃ solution during the crystal growth. Here it is important to mention that initially both the solutions were transparent; however, in the case of PbBr₂ the solution turned to coloured once the crystalline growth was started. JEOL 7600 SEM was used to study the surface morphology. TGA and DSC studies were performed using the Perkin Elmer TGA 4000 Analyzer and Perkin Elmer DSC 8500 system, respectively. Jobin-Yvon HR800 Raman spectrometer was used to record the Raman Spectra. FTIR spectra were obtained using Spectrum 2000, USA (Perkin Elmer). XRD patterns were measured by using the EMPYREAN X-ray-diffractometer. Compositional analysis was performed using AXIS Ultra DLD XPS system.

X-ray diffraction (XRD) spectra of the PbBr₂ crystalline rods (cyan circles) and crystalline powder (black lines) are shown in Fig. 1a. Both spectra exhibit an orthorhombic system (P n m a (624) space group) with a = b = 0.8059 nm and c = 0.4732 of PbBr₂ (hematite, JCPDS file No. 07-0235). However, the PbBr₂ microcrystalline rods have a very well-oriented development in the direction along the a-axis. In the case of the crystalline rods, the exciton absorption bands shifted towards the shorter wavelength (blue shift). In the case of PbBr₂ crystalline rods, the intensity of the peaks is much higher as compared to the PbBr₂ powder. The prominent peaks have been detected at 14.37°, 18.58°, 21.63°, 22.04° and 23.69° corresponding to the (101), (002), (011), (102) and (111) lattice planes of the PbBr₂. (112) and (211) lattice planes have been found at the diffraction angles of 28.76° and 30.58°, respectively. It can be noticed that the d-spacing of the PbBr₂ crystalline rods is higher as compared to PbBr₂ powder ensuing the shift of the lattice planes towards the lower diffraction angles. The shifting of the d-value may be due to the reordering of lattice positions during the growth of the rods [9]. The high-intensity XRD peaks attained at 9.5° and 12.3° represent the highly crystalline nature of the preparation phase. The existence of anonymous peaks proposes a new in-between phase, which may be due to the complex of the PbBr₂-DMF [10]. The EDS results are given in Fig. S3(a). The peaks at 2.48 and 10.5 keV denote the lead while the other two peaks (at 1.5 and 11.75 keV) can be assigned to Bromine. In the PbBr₂ microcrystalline rods, the atomic composition of Pb and Br was found to be 1:2.28, which refers to the development of pure phases.

Raman spectrum (Fig. 1b) exhibits the internal vibration due to the C–N stretching, CH₃ and NH₃ bending, and CH₃ and NH₃ stretching around 600–1100 cm^− 1, 1300–1600 cm^− 1 and 3000 cm^− 1, respectively. The FTIR results are shown in Fig. 1c demonstrating the existence of the vibrations at 860 cm^− 1 and 660 cm^− 1 corresponding to the C–N and C–O stretching. The climaxes at 1007 cm^− 1 and 1058 cm^− 1 belong to sp³ (C–H) stretching. The peaks in the range of 1250–1550 cm^− 1 matched with the C–H and N–H bending. Stretching vibration because of the C=O bond appeared at 1620 cm^− 1 and 1660 cm^− 1. The FTIR features (in the range of 500–1700 cm^− 1) are well matched to the Raman spectrum. Nevertheless, some peaks presented in the Raman spectra are absent in the FTIR spectra over the 1700 cm^− 1. This is owing to the fact that in the Raman measurements the vibrational energies are activated because of the changes in polarization while the FTIR active intensities are dependent on the dipole moment [11]. The description of the thermal characteristics (Fig. 1d) is given in the supplementary data. The Raman, FTIR (Fig. 2) and XPS (Fig. 3) results indicate the presence of CH– and NH– groups, which should not be observed in PbBr₂. The existence of CH– and NH– groups is maybe due to the solvent (DMF in this case). We had performed the experiments to develop the microrods of the both PbBr₂ and CH₃NH₃PbBr₃. In the case of CH₃NH₃PbBr₃ perovskite solution in DFM, no growth of the rods has been found (see the attached photo in the supplementary data, Fig. S2).

TGA was used to find the thermal stability (see Fig. 1d). The weight loss is detected in two temperature ranges. The first weight loss was found at ~ 115 °C due to the evaporation of moisture in the materials [12]. The second weight loss was detected at ~ 580 °C and the sequential decomposition has been found in the material [13]. Our TGA results are in a good agreement with the previously reported data [12, 14]. DSC results (in the inset of Fig. 1d) show a sharp endothermic peak at 126 °C which can be attributed to the polymorphic transformation during the heating procedure [15].

Figure 2 illustrates the structure and morphology of the crystalline rods using the FESEM images. The crystals grow in longitudinal dimension having the lengths up to 5 mm, while the diameter of the rods is in the range of 10–100 µm. The FESEM images also show that the PbBr₂ crystalline rods are consisting of porous structure with submicron size pores. The magnified FESEM images of the rods illustrate that several micro-structures are linked in the longitudinal direction to form the microcrystalline rods. As a result of the minor variation in the dimensions of the micro-structures, a small gap between the head-to-head micro-structures results in the creation of micro-pores. Nevertheless, no head-to-head cracks or any visual defect has been found on the surface of the microcrystalline rods. The HRTEM images [Fig. S3(b), supplementary data] were taken at the certain area to investigate the presence of any pores on the surface of the connected micro-structures in the microrods. However, there is no proof of any crack or nano-pores on the surface of the connected micro-structures in the microrods. More high-resolution TEM micrographs of PbBr₂ crystalline rods cannot be taken since the PbBr₂ crystals have been found to be very sensitive to high-energy electron beam, which may cause distortion in the crystal structure.

To determine the elemental composition, XPS spectra of the rods were recorded in the range of 0 to 800 eV. The elemental composition of the rods contains lead, bromide, carbon and oxygen. In C1s spectra (Fig. 3b), the intensity of carbon peaks expressively improves after crystal growth and the peaks at 284.6, 286.3 and 288.3 EV are referred to C–C, C–O–C and O–C=O bonds, respectively [16]. The peak positions are consistent with powder samples as well. In the O1s (Fig. 3c) spectra, the peaks at 529, 531.5, and 533 eV can be assigned to the metal oxides, C–O bond and C=O bond, respectively. Figure 3d shows the two distinguished peaks of Bromine at 68.5 eV and 70.1 eV correspond to the Br 3d_5/2 and Br 3d_3/2. The intensity of bromine peaks has significantly improved in the crystals as compared to lead bromide powder. The core-level spectrum of lead (Fig. S3) shows two different peaks, the 1st at 136.8 eV envisioned for Pb 4f_7/2 and the 2nd at 141.6 eV anticipated for Pb 4f_5/2 [17]. Pb 4f region has well-separated spin-orbit components with Δ = 4.87 eV (see Fig. S4, supplementary data).