Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24478−24484
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Observing the Growth of Pb3O4 Nanocrystals by in Situ Liquid Cell Transmission Electron Microscopy Wei Wei,† Hongtao Zhang,† Wen Wang,† Meng Dong,† Meng Nie,† Litao Sun,*,†,‡ and Feng Xu*,† †
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SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of the Ministry of Education, Collaborative Innovation Center for Micro/Nano Fabrication, Device and System, Southeast University, Nanjing 210096, China ‡ Center for Advanced Materials and Manufacture, Joint Research Institute of Southeast University and Monash University, Suzhou 215123, China S Supporting Information *
ABSTRACT: Understanding the growth behaviors of nanomaterials during liquid-phase synthesis will be beneficial in designing and applying many functional nanodevices. However, the growth pathways regarding the nanocrystal facet development remain largely unknown as direct observation is lacking. Herein, the in situ study of Pb3O4 nanocrystals’ growth is reported by using the liquid cell transmission electron microscopy with high spatial and temporal resolution. The findings indicate that Pb3O4 nanocrystals’ growth follows distinct trajectories with shape evolution when the growth pathways are varied. Three growth pathways are observed, including the monomer growth of Pb3O4 nanocrystals, the coalescence growth of four stationary Pb3O4 nanocrystals, and the oriented attachment growth of Pb3O4 nanocrystal pairs and multiple randomly dispersed Pb3O4 nanocrystals. It is the first observation that Pb3O4 nanocrystals with a regular quadrilateral shape are formed, in which nanocrystal facets preferentially grow along the [002] direction of Pb3O4. Theoretical analysis confirms in this study that the surface energy and physical driving force play key roles in the growth of nanocrystals in a liquid. Such understanding of the growth pathways and quantification of formation kinetics are important for the design of hierarchical nanomaterials and the control of nanocrystal self-assembly for functional devices. KEYWORDS: liquid cell TEM, Pb3O4 nanocrystals, growth pathways, surface energy, driving force
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INTRODUCTION Lead oxides, including PbO, PbO2, Pb2O3, and Pb3O4, are widely applied in industries such as luminescent glass, pigments, storage batteries, and nanoscale electronics devices, because of their excellent physicochemical properties. In particular, Pb3O4 has fascinating physical properties because of its mixed valence band and unique electronic structure,1 and thus it exhibits unique hopping conductivity phenomenon and photoresponsive performance. According to the available reports, the physicochemical properties of materials depend not only on their components, the but also on their morphology and structure, which in turn influence specific applications.2 Therefore, it is very important to understand the fundamental growth mechanism of synthesized nanocrystals in order to control the size, morphology, and structure of Pb3O4 nanocrystals, so as to adjust their physicochemical properties to match with specific applications, especially for the nanodevices.3,4 Solution synthesis has promising control on the size, morphology, and shape of different metal nanocrystals.5 The crystallization process in solution is complicated, which has involved the permutation of thousands of atoms and molecules © 2019 American Chemical Society
near the surface; besides, such a process would be further complicated by the interactions between atoms and environmental changes.6 Many growth pathways for obtaining monodisperse nanocrystals are based on nucleation and then grow through the attachment of monomers.7 Nevertheless, numerous reports indicate that the monodisperse nanocrystals can also be attained through nanocrystal coalescence, where a greater nanocrystal can be produced through agglomerating many nanocrystals,8−10 or through the oriented attachment (OA) of nanocrystals with the aligned crystal orientation. Typically, the interaction of nanocrystals plays an important role in the growth of nanocrystals, which may include the hydrophobic interaction, van der Waals (vdW) force, charge interaction, and magnetic force.11 Notably, many factors should be taken into account to expound the contribution of each individual factor during the growth of nanocrystals, as the environment is extremely complicated in a solution.12−14 Additionally, some mechanisms in the growth of nanocrystals Received: May 16, 2019 Accepted: June 19, 2019 Published: June 19, 2019 24478
DOI: 10.1021/acsami.9b08524 ACS Appl. Mater. Interfaces 2019, 11, 24478−24484
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic diagram of the liquid cell TEM observation of Pb3O4 nanocrystals. (a) Fabrication of a liquid cell: Pb2+ precursor solution is sandwiched by two TEM copper grids with the formvar stabilized carbon support films face-to-face. The sample is then left under atmosphere until the extra liquid has been volatized. (b) Schematic diagram of a liquid pocket in the process of TEM imaging of Pb3O4 nanocrystals.
Figure 2. Growth process of a single Pb3O4 nanocrystal. (a) Monomer growth of the Pb3O4 nanocrystal. (b) Sequences are corresponding filtered images, which emphasize the evolution of a single Pb3O4 nanocrystal. (c) 2D projection of Pb3O4 along the [−110] view zone axis. (d) Size evolution of the Pb3O4 nanocrystal. L1 and L2 are the lengths of the Pb3O4 nanocrystal along [002] and [220], respectively. Scale bar, 5 nm.
remain largely unknown because of the lack of direct observation. Electron microscopy of the liquid processes has become as an active research field, which allows the growth of nanocrystals in solution to be observed in real time. During the solution synthesis experiments, the electron beam is used not only for imaging but also for reducing the precursor solution. Under electron beam irradiation, the nucleation and growth processes of various metals and compounds (such as Cu,15 Pt,16 Pd,17 Ag,18 Pt3Fe,19 spinel ferrite,20 and iron oxyhydroxide21) are allowed for the real-time observation. Nonetheless, the dynamic evolution during the growth of Pb3O4 nanocrystals remains largely unclear so far. This study had utilized an ordinary carbon film-based liquid cell for the in situ transmission electron microscopy (TEM) investigation of different growth models, so as to achieve monodisperse Pb3O4 nanocrystals and to identify the atomic pathways of these nanocrystal facet development. Theoretical calculations confirmed that different surface energies between nanocrystal facets had played key roles in nanocrystal growth. Additionally, contributions to the physical driving force,
including Coulombic (electrostatic) interaction and vdW interaction, were also examined as a function of the interaction distance of nanocrystals. Such understanding of the growth pathways and quantification of formation kinetics are of great significance for the design of hierarchical nanomaterials and the control of nanocrystal self-assembly for functional devices.
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RESULTS AND DISCUSSION Recently, the rapid development of micromachining technology, thin-film technology, and imaging detectors has made it possible for nanoscale spatial resolution imaging in liquids.15 Besides, thin carbon films22,23 and graphene sheets24 have been investigated as the ultrathin membranes to encapsulate the liquid pocket in the middle. In addition, the well-developed Sibased manufacturing allows the integrating of microelectrodes and micrographics into the microchips with Si3N4 windows.15,25,26 The ultrafine designs of microchips can also be integrated with microchannels for flowing liquids.27 The fabrication of a liquid cell is presented in Figure 1a. Briefly, about 2.5 μL of the resulting solution was sandwiched by two TEM grids with carbon film-covered faces, so as to produce a 24479
DOI: 10.1021/acsami.9b08524 ACS Appl. Mater. Interfaces 2019, 11, 24478−24484
Research Article
ACS Applied Materials & Interfaces
Figure 3. Atomic-resolution snapshots of the coalescence process of four Pb3O4 nanocrystals from video. (a) Attachment of Pb3O4 nanocrystals coalescence. (b) Sequences are corresponding filtered images, which emphasize the coalescence growth process of Pb3O4 nanocrystals. Scale bar, 5 nm.
the atomic level, and nanocrystal growth at this interface was much easier than that at the smooth one. On the other hand, there were high-energy surfaces along [220], which grew at a higher rate than that of the low-energy facet during growth of the nanocrystals. The rapidly growing facets would eventually disappear, thus the forming nanocrystals were terminated by low-energy facets.29,30 This accounted for the reason why L2 was remarkably increased at the beginning. The Pb3O4 nanocrystal would not stop growing when it was surrounded by low-energy facets. On the contrary, L1, the length along [002], was only slightly increased when the surfaces were also stable and smooth along [220]. Consequently, it was speculated that the Pb3O4 nanocrystal would grow preferentially along [002] when it was surrounded by low-energy facets, until an equilibrium had been reached. This view was also confirmed by the following work. Figure 3 displays the second growth mechanism taken from Movie S2. These sequential images show the coalescence growth of four Pb3O4 nanocrystals. The coalescence process occurs only when the neighboring nanocrystals have the same crystal structure and planar orientation.31 In most cases, nanocrystals keep moving randomly in the solution.32 It is inferred in previous studies that, translation and rotation play vital roles in the coalescence growth of nanocrystals, like gold nanocrystals,32 iron oxide nanocrystals,21 and hybrid perovskite nanoparticles.33 In this study, different coalescence growth processes of Pb3O4 nanocrystals were detected. Four nanocrystals might grow in the four corners of a regular quadrilateral Pb3O4 nanocrystal, which were stable and would not rotate in the liquid; in addition, the {002} and {220} facets of every two nanocrystals were perfectly aligned to each other, with a small space (
